RU2795437C2 - Coatings resistant to drop impact for turbine blades and other components - Google Patents

Abstract

FIELD: resistant materials.

SUBSTANCE: group of inventions relates to a material resistant to erosion and corrosion, a material resistant to drop impact erosion, a turbine blade, a pump impeller, a method for manufacturing an erosion and corrosion resistant material, methods for applying a coating resistant to drop impact erosion, and methods for applying a coating resistant to cavitation erosion. The erosion and corrosion resistant material and the drop impact erosion resistant material comprise carbon-doped tungsten deposited on a substrate and have a columnar crystalline microstructure. Carbon-doped tungsten is a tungsten matrix in which tungsten carbide nanoparticles are dispersed in the form of a homogeneous nanostructure. There is no oxygen in said matrix, except for parts of the surface of said substrate open to air or moisture.

EFFECT: turbine blades, vanes and other parts are protected from drop impact erosion, especially erosion and/or corrosion.

50 cl, 22 dwg, 4 tbl

Description

[0001] The present invention relates to coated turbine blades or blades, more specifically to turbine blades or blades of steam turbines and gas turbines used for power generation, as well as to blades and blades used in the low temperature compressor part of an aircraft engine. Certain embodiments also relate to coated pump impellers, valves, and other components that may be subject to liquid cavitation erosion.

[0002] In addition, the present invention relates to a coating applied to a portion of the airfoil of a turbine blade to improve its surface resistance to erosion, including erosion by impacts of high-velocity water droplets, as well as erosion by suctioned solids such as sand. , dust and soot. The proposed coating is intended to protect the blades from corrosion and/or the combined effect of erosion and corrosion. The inventive coating can also impart cavitation resistance, in particular to components such as pump impellers and valves exposed to liquid flows.

[0003] The present invention also relates to a chemical vapor deposition (CVD) process developed for making the aforementioned coating.

TECHNICAL FIELD OF THE INVENTION

[0004] Turbine blades are critical parts of steam turbines and gas turbines widely used for power generation, as well as aircraft turbine engines. They are highly engineered parts often made using advanced materials. In order to achieve optimum aerodynamic performance, turbines are typically shaped and airfoil-finished in an optimum manner. During turbine operation, the blades often move at or above the speed of sound and are eroded by the impact of high-velocity water droplets and solid particles, resulting in an increase in the surface roughness of the turbine blades, a significant increase in aerodynamic drag, and a decrease in the coefficient of efficiency of the turbine.

[0005] In some operating modes of gas turbines, water is introduced into the turbine inlet gas stream to increase the mass flow and increase the efficiency of the turbine. The formation of water spray is sometimes used to remove deposits that accumulate on the blades. Water droplets from the aerosol spray strike the leading edges of the first four or five rows of turbine blades and can lead to drop impact erosion (DIP) affecting primarily the leading edges of the blades, roughening the surface of the blades and increasing friction and aerodynamic drag of the blades.

[0006] In steam turbines, the rapid expansion of steam causes water to condense on the last rows of turbine blades. Droplets of condensed water can accumulate on stationary surfaces of turbine blades, forming larger droplets that can strike the leading edges of the rotating blades, thus causing AEC. This is particularly problematic near the ends of the rotating blades, where the impact velocity is greatest. It is believed that AEC cannot be prevented under steam turbine operating conditions using wet steam.

[0007] The mechanisms of AEC are not fully understood and include many factors such as impacts of high velocity water droplets causing blade material deformation, high velocity water jets, shock waves in liquid and stress waves in metal, cavitation bubbles in liquid and hydraulic intrusion. In some turbine operating conditions, these mechanical factors can also be accelerated by chemical and corrosion effects. Due to the discrete nature of impacts, fatigue conditions are applied to the metal surface, leading to the initiation, propagation, and crossing of fatigue cracks, which in turn can lead to gradual loss of blade material. The reaction of a solid surface to the impacts of water drops has an impulsive nature, which is determined by dynamic rather than static mechanical characteristics. The reaction of the metal surface to changes under the action of AEC over time, starting from the initial stage of incubation, when the metal surface undergoes deformation, mechanical hardening, the introduction of defects such as dislocations, twinning, crack initiation and the formation of surface roughness without loss of material. This usually affects the top layer of metal with a thickness of 30 to 50 microns. This is followed by an erosion acceleration stage where the removal of material is accelerated due to the propagation of subsurface cracks and intersections, which causes loss of material particles, side jets of water at velocities many times that of the impinging drop, and removal of surface roughness peaks and deformed material through a shearing action. A distinctive feature of CUE is that the eroded metal surface acquires a high roughness. The impact of a high-velocity water drop on a pre-existing crack or pit can expand the crack or pit and lead to tunneling and swelling of the metal surface by the action of the water jet and hydraulic penetration. After some time, the erosion rate reaches its maximum and the AEC rate begins to decrease during the slowing down/weakening stage, probably due to the cushioning effect of the water cushion remaining in the deep holes and cavities that form in the metal surface, but even at this stage, erosion continues, although and at a slower speed. Some brittle materials or coatings do not exhibit this AEC slowing/weakening stage and instead undergo an increase in erosion rate, resulting in a so-called catastrophic stage in which the material or coating is completely destroyed.

[0008] Different materials are exposed to CUE to varying degrees and in different ways. For example, the surface of a viscous material will undergo plastic deformation under the action of impacts, forming depressions around which the raised edges and projections of the surface roughness can be removed by the shear action of high velocity lateral water jets. Harder and more brittle materials, on the other hand, tend to develop ruptures and fatigue cracks after repeated impacts. Materials having a heterogeneous structure, such as thermal spray coatings consisting of particles of tungsten carbide in a matrix of cobalt metal, tend to initiate damage at weak points or weaker components. Pores, mechanically weak inclusions, and defects in the surface layer of a material can become mechanical stress concentrators, where subsurface cracks can be initiated. An increased concentration of pores, inclusions, and defects in the surface layer of a material can accelerate the incubation stage and the onset of material loss, making such materials less able to resist FEC. It is believed that the destruction of the material under the action of CUE at various stages may include all or most of the mechanisms mentioned above. Due to the complexity of the AEC process, there is no unified theory of the development of AEC, and there is no common approach to predicting the resistance of a material to AEC or to designing materials that can protect against AEC. Despite extensive research in this area, it is considered impossible to develop a universal approach to protection against TEC. There are only a few rules of thumb that hold true for some types of materials but not for other types. There is no single material parameter that can reliably characterize the ability of a material to resist AFC.

[0009] One factor that is most often considered important is hardness. It is usually believed that the hardness of the material significantly affects the resistance to drop impact erosion: for similar materials, erosion resistance is considered proportional to the Vickers hardness number to the power of 2 or 2.5 [Heymann, F.J.; "On the quantitative prediction of drop-impact erosion"; STP 474; ASTM; 1970; With. 212-248].

[0010] Alloying elements in steel and other metal alloys, as well as the microstructure of the alloys, are considered important factors influencing their resistance to CUE. Alloys of cobalt, chromium, and tungsten, such as Stellite®, have been found to exhibit high resistance to CEC, and this is generally considered to be a result of their microstructure.

[0011] Corrosion can attack turbine blade surfaces, and in combination with AFC and/or particulate erosion, corrosion and erosion can exhibit some synergy where the passive surface oxide layer present on many alloys is lost due to erosion, and, thus, corrosion is accelerated. Corrosion can be a problem for turbines installed in coastal areas where salt particles and/or salt water droplets are present in the air. Additives or impurities from the feed water can concentrate to form so-called "early condensates" leading to significant corrosion damage and stress corrosion cracking of turbine components. For example, corrosive organic acids can be formed from the thermal decomposition of ethanolamine (ETA) used as a pH adjuster and chelating agent.

State of the art of the present invention

[0012] Coatings are widely used on turbine blades, vanes and other components. There are several types of coverage, and each type solves a separate problem:

i) thermal barrier coatings applied to the blades of the hot zone in order to ensure more efficient operation of the turbine at elevated temperatures,

ii) abrasive coatings applied to the ends of the blades to cut off the abradable sheath to form an airtight/gastight system,

iii) corrosion resistant coatings applied to prevent corrosion of the blades, and

iv) erosion resistant coatings.

[0013] Coatings designed to solve problems (i) and (ii) are often ineffective in solving problems (iii) and (iv) due to the different nature of the coatings.

[0014] The present invention relates in particular to coatings of types (iii) and (iv) for improving the resistance of turbine parts to erosion (including drop impact erosion and particle erosion) as well as to corrosion.

[0015] US 2012/0125980 (WO 2011/009430) describes a method for coating a turbine blade with a Co-Cr alloy, where the carbide coating is made separately and then bonded to the surface of the component in a brazing process. Co-Cr alloys such as Stellite 6 and Stellite 21, in bulk material form, indeed show excellent resistance to ACC. However, their use as protective hardfacing applied by soldering or brazing increases the number of problems. During operation of the turbine, the brazed alloy may be subject to corrosion, including possible bimetallic or contact corrosion in some cases. Residual stresses in the braze joint and other mechanical stresses can further weaken the strength of the braze joint during operation of the turbine. Some turbine blade designs allow for torsional deformation of the blades during operation due to mechanical forces, and this places additional stresses on the joint between the steel substrate and the Co-Cr hardfacing. Over time, these factors can lead to separation of the carbide layer from the base of the blade. During operation of a turbine, the blades often move at speeds that approach or exceed the speed of sound in some cases, and the separation of solid heavy hard metal particles can cause catastrophic damage to the turbine.

[0016] Methods such as laser cladding and thermal spraying are sometimes used to deposit a thinner Co-Cr alloy layer. However, these methods can create tensile stresses in the layer, which over time can lead to the formation of cracks. Surface cracks and microcracks can become stress concentrators, adversely affecting the fatigue properties of turbine blades, which are important for the durability of these fast moving stressed parts.

[0017] US 2010/0266409 (WO 2007/101465) describes a thermal barrier coating applied to turbine blades. A hollow, internally cooled gas turbine blade is coated with an MCrAlY alloy-based bonding layer and a zirconium oxide ceramic heat-shielding layer on the outer side of the blade and an inner coating containing a Cr-based diffusion layer.

[0018] US 2016/0312622 describes the manufacture of a two-layer coating comprising a bonding layer made from MCrAlX; where M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element, which is yttrium (Y) and/or silicon, scandium (Sc) and/or at least one rare earth element or hafnium. The thickness of the bonding layer is from 20 µm to 50 µm. The thermal barrier coating is applied over the tie layer using electron beam physical vapor deposition (EL-VOGF) and consists of, for example, ZrO ₂ , Y ₂ O ₃ -ZrO ₂ . In other words, an unstabilised, partially stabilized or completely stabilized yttria and/or calcium oxide and/or magnesium oxide layer can also be present on the MCrAlX. The two-layer coating can then be provided by connected cooling system ducts made, for example, by selective laser melting.

[0019] RU 2588973 describes the manufacture of a thermal barrier coating by plasma spraying a yttria-stabilized zirconium oxide coating onto a prepared surface of nickel-based alloy parts. The coating can be provided with a gradient transition layer by means of two magnetrons.

[0020] The descriptions mentioned above focus on the thermal barrier coating. The heat-insulating properties of the heat-shielding coating are determined by a ceramic layer with low thermal conductivity, which is the actual protection against heat flow. The heat-shielding coating is used on turbine blades in the high-temperature part of a gas turbine. The use of these coatings allows an increase in the operating temperature of the turbine, thus increasing the efficiency. Ceramic thermal barrier coatings are advanced materials designed to exhibit a specific combination of properties such as low thermal conductivity, high melting point, phase composition and chemical resistance, compatible thermal expansion coefficients of all coating layers, high resistance to thermal fatigue and oxidation. These materials are generally brittle and lack mechanical strength and therefore lack erosion resistance properties. In addition, since heat-shielded blades are parts of a gas turbine operating at high temperature, typically 1000°C to 1600°C, droplet impingement erosion is not a problem because there will be no water droplets. Accordingly, resistance to CUE is not required.

[0021] US 2017/0009591 describes compressor vanes or vanes that are composed of a metal alloy having an aluminum diffusion zone 10 to 30 µm thick on the surface of the vane or vane as a substrate. Compressor blades or vanes are coated with a hard material that contains TiN, TiAlN, AlTiN, CrN in the form of single-layer or multi-layer ceramic materials, usually produced by physical vapor deposition (PVD). The hard material coating provides erosion resistance and the aluminum diffusion layer provides corrosion protection. However, the hard coatings obtained by FOGF generally have a relatively thin thickness of only 3 to 4 µm and do not provide sufficient erosion protection.

[0022] US 6,800,383 and US 8,043,692 describe coatings designed as a wear resistant coating for metal parts. The structure and mechanical properties of these coatings are not considered optimal for protecting turbine blades from AFC and the combined effects of erosion and corrosion.

[0023] US 6800383 describes coatings consisting primarily of single-phase tungsten carbides, mixtures thereof, and mixtures with carbon and with metallic tungsten, for example, WC+C, WC, WC+W ₂ C, W ₂ C , W ₂ C+W ₃ C, W ₂ C+W ₁₂ C, W ₂ C+W ₃ C+W ₁₂ C, W ₃ C, W ₃ C+W ₁₂ C, W ₁₂ C, WC+W, W ₂ C+W, W ₃ C+W, W ₁₂ C+W, W ₃ C+W ₁₂ C+W. In all cases, US 6,800,383 describes compositions based on tungsten carbide(s) as the main phase, with tungsten present in the mixture as an impurity, as proven by X-ray diffraction analysis. Said tungsten carbide compositions have a high hardness of up to 3500 kg/mm ² . The carbon content in these coating materials can be up to 15 wt. %, and the fluorine content can be up to 0.5 wt. %. Like most other carbides, these materials have relatively high brittleness and high mechanical stresses. Mixtures of different phases obtained in this way tend to have very high residual stresses. Impurities of mechanically weak carbon and brittle subcarbides W ₃ C and W ₁₂ C can reduce the fracture toughness and impact resistance of the coating, and these properties are important for erosion resistance. These coatings may contain excess amounts of fluorine, up to 0.5 wt. %, which can be unfavorable for the adhesive and protective properties of the coatings, in particular if the coated articles are used in the presence of water or oxygen, as in the case of steam and gas turbine blades.

[0024] US 8043692 describes tungsten alloyed with carbon in an amount of 0.01 wt. % up to 0.97 wt. %, and additionally doped with fluorine in an amount of 0.001 wt. % up to 0.4 wt. %. The material has a high Vickers hardness number of up to 2000, in some embodiments up to 2200, which is important for wear resistance, but it has been unexpectedly found that such high hardness is unfavorable for erosion resistance and in particular for resistance to wear. KUE. The method described in US 8,043,692 neither controls nor prevents the presence of oxygen or water in the coating chamber, nor their adsorption to coated parts, or their presence in gaseous precursors. The fact that even traces of oxygen and water in the CVD process chamber can affect coating properties has only recently been discovered by the inventor of the present invention. The commercial grade hydrogen used as the reductant gas in known CVD processes contains water vapor and traces of oxygen which cannot be neglected. Commercial grade hydrogen is usually produced by electrolysis and as a result contains significant amounts of water vapor and often traces of oxygen. The Applicant of the present invention has found that large CVD reactors, which may have stainless steel interior surfaces greater than 10 m ² inside the vacuum chamber, adsorb a significant amount of water vapor when the chamber is opened after the CVD process. This occurs in particular in the case of water-cooled regions of the reaction chamber, such as the vacuum-sealed cooled region, where appreciable condensation of water has been found. In addition, it has been found that stainless steel surfaces can adsorb oxygen. The Applicant of the present invention has surprisingly found that by degassing the vacuum chamber prior to each CVD cycle and using higher purity gases (particularly hydrogen as the reductant gas and alkane gas), a substantially oxygen-free coating will be obtained having significantly improved mechanical properties. Surprisingly, it has been found that even trace amounts of oxygen can form inclusions of mechanically fragile non-volatile tungsten oxyfluorides, which can become mechanical stress concentrators and/or initiation points of subsurface microcracks, and they are the main mechanisms of CEC. The materials described in US 8,043,692 can have a high carbon content (up to 0.97 wt %), which can adversely affect fracture toughness and repetitive impact resistance when subjected to particulate or drop impact erosion. These materials may contain fluorine in excess amounts of up to 0.4 wt. %, which can adversely affect the adhesive and protective properties of the coating, in particular for steam and gas turbine blades operated in the presence of water and oxygen for extended periods of time. The method described in US 8,043,692 does not allow for the control of residual stresses and porosity in the resulting coating, which are important factors for the fatigue properties of the material and its resistance to erosion, in particular AFC.

[0025] EP 2256228 describes a two-layer coating for protecting a surface from erosion by particles such as sand, dirt and dust, or by repeated impacts of a high velocity fluid such as precipitation or other fluids. It is stated that the first layer should be present in the form of a metal matrix having a ceramic reinforcement, a hardness of 10 to 20 GPa and a thickness of 75 to 500 microns. The first layer may be deposited by high velocity oxyfuel combustion (HVOF), cold spraying, or other methods suitable for deposition of a cermet material, the layer material being selected from the group consisting of WC/Co, WC/CoCr, chromium carbide - nichrome, diamond - nickel. The second layer, which is typically a ceramic material having a hardness of 19 to 40 GPa or higher and a thickness of 1 and 25 microns, is deposited by means of VOHF or CVD, the layer material being selected from the group consisting of TiN, diamond, diamond-like carbon, CrN, cubic boron nitride, boron carbide, TiC, or combinations thereof. The surface to be coated may be an aircraft propulsion component such as a helicopter propeller blade, propeller or fan blade, wind or water turbines, impellers, marine propellers, or large piping systems.

[0026] The overlays described in EP 2256228 are not considered optimal for making steam turbines heat resistant. In particular, the hard ceramic second layer is brittle, but brittle materials are known to be rapidly damaged when subjected to repeated impacts in the CEC process, typically resulting in catastrophic failure. VKTG and other coating methods used to produce the first layer, as a rule, have residual tensile stresses, and are also brittle, which is unsuitable for providing effective protection against ACC. Using VSKTG and other spray coating methods, coatings are produced having a non-uniform thickness and high surface roughness, which will require subsequent grinding or polishing of the coating to achieve the surface finish desired for turbine blades. This post-grinding operation of the coating is relatively uncomplicated for parts having a simple geometric shape, such as cylindrical or flat parts. However, post-coating grinding in the case of highly engineered turbine blades having complex three-dimensional shapes would be very difficult. The application of a harder second coating layer in vacuum by the FOHF or CVD methods imposes stringent requirements on the cleanliness of the substrate. This can be problematic because the first layer, which is a porous coating applied by HCTG, will be contaminated with cutting fluids and oils after grinding, which will make applying the second solid layer very difficult in practice.

[0027] US 4,741,975 describes a multi-layer coating on gas turbine engine blades that improves their resistance to sand and dust erosion without exhibiting a drastic reduction in fatigue life. The coating has a first viscous layer containing palladium, platinum or nickel, a second layer consisting of substantially pure tungsten, and a third erosion-resistant layer consisting of an alloy of tungsten and carbon or a tungsten metal matrix with a dispersed mixture of tungsten-carbon phases. The relatively hard outer coating has a diamond pyramid hardness of 1600 to 2400 and preferably 1900 to 2000. The coating contains 93.88 to 97.8% tungsten and 2.12 to 6.12% carbon. The units of diamond pyramid hardness (DPH) used herein are equivalent to the more commonly used Vickers hardness number (HV). This hard outer coating can be deposited by CVD or sputtering methods. If spraying is used, the layer composition ingredients will include compounds from WC to W ₃ C, in particular W ₂ C. If CVD is used, reference should be made to US 4,427,445, which is presented below.

[0028] US 4,427,445 describes a CVD process using WF6, hydrogen, dimethyl ether (CH ₃ OCH ₃ ) and nitrogen. The material obtained under specific conditions according to this patent is a two-phase mixture in which one phase is pure tungsten in an amount of from 20 to 90 wt. %, and the other phase is the structure A15, and the structure A15 is tungsten carbide or a mixture of tungsten carbide and tungsten oxide. This was confirmed by X-ray diffraction analysis. The deposited material described in US 4,427,445 has a very high Vickers hardness (1820 to 2500) and a high degree of stress and may require further heat treatment at 600 to 700°C. Such heat treatment is unacceptable for some steels, which may warp and lose certain desirable mechanical properties.

[0029] US 4,427,445 describes the use of oxygenated precursors such as methanol, ethanol, dimethyl ether, aldehydes, and ketones and recommends the use of mixtures of organic precursors to control the hardness of the resulting material.

[0030] US 4,741,975 also states that the gas mixture used for CVD contains organic compounds containing carbon, oxygen and hydrogen (column 6, lines 23-27). The use of oxygen-containing precursors leads to significant inclusions of tungsten oxides and oxyfluorides in the deposited material.

[0031] US 4,741,975 describes a coating for protecting gas turbine engines from sand and dust erosion, but does not mention steam turbines or problems associated with droplet erosion. Particulate erosion occurs according to mechanisms that are different from the FEC mechanism and are predominantly micro-cutting, and thus high surface hardness is required for particulate erosion resistance. The hard coating described in US 4,741,975 contains brittle macrophases of tungsten carbides WC, W ₂ C and W ₃ C, and also contains minor amounts of tungsten oxides and oxyfluorides. When exposed to AEC, hard coatings having these characteristics may suffer from brittle fracture and fatigue microcracks and will not provide sufficient protection.

[0032] WO 2010/044936 describes gas turbine blades (with special reference to helicopter engines) and distinguishes between leading edge impact damage and airfoil erosion caused by reflected particle impacts by particulate matter entrained in engine intake air. Turbine blades are coated with a multilayer nitride ceramic coating using the FOGF method, one of the following three compositions: TiAlN, CrN, TiSiCN. It is said to be more resistant to erosion than the tungsten carbide coating deposited by the WCTG method. It has been noted that FOGF coatings are also susceptible to cracking and peeling when bombarded with round particles. There is no consideration of the specific problem of drop impact erosion. The coatings described in this patent are brittle ceramic materials that have high residual stresses and may be prone to cracking and flaking when subjected to repeated impacts and are not suitable for providing protection against TEC.

[0033] Based on WO 2010/044936, WO 2011/025596 describes multi-layer nitride ceramic coatings deposited by FOGF for protecting gas turbine blades. Coatings have only certain areas of blade airfoils, such as only concave surfaces according to some embodiments, and concave and convex surfaces according to other embodiments. In addition, the description compares the columnar microstructure of the coatings deposited by means of FOGF and the non-column "board-like" structure of the thermally sprayed coatings. Coatings of various nitrides deposited by means of FOGF are said to be particularly effective, and the columnar microstructure is especially noted. Similar to WO 2010/044936, the coatings are not expected to provide sufficient protection against FEC due to their brittle nature and the high residual stresses of FOFF applied ceramic coatings. In addition, some FOFF coatings have significant porosity, which can provide pathways for a corrosive environment to attack the underlying substrate.

[0034] US 6,447,932 describes coating systems for gas turbine components comprising a superalloy substrate with nitrided regions, an aluminum-rich coating, and optionally a thermally insulating ceramic layer. This patent focuses on the high temperature region of a gas turbine engine and the coatings described have been designed to withstand high temperatures and high thermal cycling stresses. These coatings are not expected to be capable of protecting blades from AFC, particulate erosion or corrosion.

[0035] EP 1939318 describes carbonizing a nickel-based substrate (e.g., superalloy) in such a way as to inhibit secondary reactions with a subsequent aluminium-enriched coating, in particular with regard to components of the high temperature part of a gas turbine. The carbonizing surface treatment and coatings described in this patent do not protect the blades from FCC, particulate erosion, or water corrosion.

[0036] EP 1634976 describes methods for coating turbine components operating at high temperature and high pressure. The mixture of MCrAlY powder with abrasive powder is applied by cold gas dynamic spraying, which may be followed by heat treatment. This coating is said to be capable of protecting gas turbine high temperature region components and is not recognized as a suitable protection against AFC or water corrosion.

[0037] WO 2014/143244 (US 2014/0272166) describes coatings applied to preselected outer airfoils of a blade, such as a leading edge airfoil, or a concave side surface, or a convex side surface, or combinations thereof. A very wide variety of coatings and surface treatments are described here, such as high velocity oxygen fuel spraying, high velocity air fuel spraying, solution plasma spraying, cold spraying, chemical vapor deposition, electrospark deposition, plasma chemical vapor deposition, or air plasma spray. Coatings can be TiAlN, AlTiN, TiAlN/TiN multilayer, TiAlN/Cr multilayer, tungsten-tungsten carbide, tungsten carbide-cobalt, cobalt-chromium-tungsten carbide, chromium-nickel carbide, chromium-nickel-chromium carbide, or diamond-like carbon material. In addition, nitriding and carbonizing the material of the blade and applying a bond coating are described.

[0038] This document focuses on the protection of gas turbine blades from particulate erosion, in particular with respect to coatings that provide improved high-angle particulate erosion protection on compressor leading edge airfoils, as well as being capable of providing low-angle erosion protection. under the action of solid particles on the concave 16 and convex 18 sides of the aerodynamic surfaces. This document does not deal with the CUE and does not specify the details of the steam turbine.

[0039] In order to resist particulate erosion, the coatings described in WO 2014/143244 (US 2014/0272166) should have a high hardness of the cover layer of about 1200 to about 2000 Vickers, preferably about 1400 to about 1600. This high hardness of the coatings increases the risk of fracture from impact. In addition, neither the porosity of the coatings, which can affect their resistance to AECs, as well as their effectiveness in protecting against corrosion, nor the issue of residual stresses existing in the coatings, which can affect the resistance of coatings to the initiation and propagation of microcracks, are considered. In addition, there is no detailed analysis of the composition of the coatings.

Brief summary of the present invention

[0040] According to a first aspect, an erosion and corrosion resistant substrate material is provided, wherein the material comprises carbon-doped metallic tungsten in a substantially uniform nanostructure that is substantially free of oxygen except for portions of the surface exposed to air or moisture, and wherein the material has a columnar crystalline microstructure.

[0041] According to a second aspect, a drop impact erosion resistant substrate material is provided, wherein the material comprises carbon-doped metallic tungsten in a substantially homogeneous nanostructure that is substantially free of oxygen except for portions of the surface exposed to air or moisture, and wherein the material has columnar crystalline microstructure.

[0042] Alloying of metals is a complex physical and chemical phenomenon of considerable practical interest. For example, alloying iron with different amounts of carbon under different conditions can significantly change its mechanical and physical properties from soft iron to low carbon steel, high carbon steel and cast iron. The properties of steel, primarily its hardness and toughness, are largely dependent on the carbon content and the form in which carbon is present in the steel (for example, as free cementite Fe ₃ C or, alternatively, as a solid solution of carbon intercalation in iron) .

[0043] Alloying should be distinguished from inclusion or simple mechanical mixing of several materials. For example, inclusions of free carbon in iron can adversely affect its mechanical properties, while doping with carbon can improve the mechanical properties of iron.

[0044] As used herein, the term "carbon-doped tungsten" means a matrix of tungsten metal in which tungsten carbide nanoparticles are dispersed. In other words, this material is mainly metallic tungsten. The tungsten carbide nanoparticles are small enough to exhibit characteristic tungsten carbide peaks when examined by X-ray diffraction analysis.

[0045] The alloy of tungsten and carbon is neither tungsten carbide as a compound nor a simple mixture of tungsten and carbon. Doping should be distinguished from simple inclusions, etc. For example, inclusions of free carbon in iron tend to have an adverse effect on its mechanical properties.

[0046] After extensive experimentation and analysis in relation to the present application, the authors of the present invention found that alloying tungsten with carbon in various specified amounts can significantly change the properties of the material.

[0047] Excessively low carbon content has little or no effect on the physical properties of tungsten. On the other hand, an excessively high carbon content may cause high stresses and may lead to cracking in the tungsten-doped layer. Under certain conditions, excess carbon can cause precipitation of carbon or carbides at the micrograin boundaries, which adversely affects both mechanical properties and corrosion resistance.

[0048] In addition, the virtual absence of oxygen or oxygen compounds in the bulk material (ignoring any surface oxide layer formed as a result of exposure to air or moisture) helps to improve the toughness of the material. In particular, inclusions of non-volatile and mechanically weak tungsten oxyfluorides, such as WOF ₄ , WO ₂ F ₂ , WO ₂ F ₄ and others, can act as mechanical stress concentrators and initiation points of subsurface microcracks. Some tungsten oxyfluorides can react with oxygen and water, which further affects the mechanical and corrosion-protective properties of the material. Surprisingly, it has been found that substantially oxygen-free alloyed tungsten has improved mechanical and fatigue properties and provides improved protection against erosion, including AFC, as well as against corrosion.

[0049] Embodiments of the present invention provide a substantially uniform small scale nanostructure that can help prevent the formation of stress raisers and thus reduce the risk of crack initiation within a material or a cover layer made from the material. After extensive research and experimentation, these factors have been found to play an important role in protecting turbine blades from AFC and to some extent from particulate erosion and corrosion.

[0050] As a result of extensive experimental work, the applicant of the present invention has determined that there are a number of factors that should be taken into account in order to protect turbine blades, blades and other parts from droplet erosion, particulate erosion and / or corrosion. An ideal coating on turbine blades and similar items should have a combination of properties and characteristics, including one or more of the following:

i) increased resistance to teardrop erosion;

ii) improved resistance to particulate erosion;

iii) corrosion protection properties and low porosity to isolate the substrate material from attack by a corrosive fluid;

iv) good thickness uniformity of the deposited coating in order to maintain the three-dimensional shape of the blade airfoil without grinding after coating;

v) strong adhesive bond between the coating and the metal spatula;

vi) sufficient thickness, suitable structure and mechanical properties to resist drop impact erosion;

vii) the coating should preferably be under compressive residual stress in order to prevent crack initiation and propagation and enhance fatigue properties;

viii) compressive stress must not be excessive as this could cause horizontal cracking and cohesive failure of the coating;

ix) sufficient hardness to resist mechanical abrasion, but also increased impact strength and toughness to protect the coating from brittle fracture and microcracking during mechanical or thermal shock or under multiple impact fatigue conditions;

x) the surface of the coating in its "after deposition" condition should have a low roughness and, if necessary, be suitable for polishing for the required surface finish on the turbine blades, the roughness Ra being generally about (or less than) 0, 2 µm (this is an advantage because the complex three-dimensional shape of the airfoil of turbine blades or vanes makes it difficult to finish after coating).

[0051] The applicant of the present invention has undertaken extensive experimentation, including the preparation and analysis of various coatings having various structures and mechanical properties, and also investigated the effect of these structures and mechanical properties on the resistance of coatings to ACC, erosion by particulates, fatigue and corrosion.

[0052] Through extensive experimentation, it has surprisingly been found that optimum ACC resistance can be achieved by using coatings of moderate hardness. This is contrary to the conventional wisdom that, for similar materials, the expected erosion resistance is proportional to the Vickers hardness number to the power of 2 or 2.5.

[0053] It is possible to initiate and develop fractures in those areas where the tensile stresses of the material exceed the maximum threshold tensile stress. In order to solve this problem, the present inventor has developed methods for manufacturing a cover material in a state of compressive residual stress. This may partially compensate for propagating tensile stress waves caused by impacts of water droplets and solid particles and may thus reduce the maximum tensile stress in the wave, which in turn may reduce the risk of fracture initiation and propagation. In addition, the present inventor has determined that very high residual compressive stresses in the plane of the coating can be detrimental to FCC resistance, since they can produce tensile stresses in the direction perpendicular to the plane due to the Poisson effect, i.e. a phenomenon in which a material tends to expand in directions perpendicular to the direction of compression. Due to the Poisson effect, tensile stresses can be combined with stress waves in the metal that produce impacts from high-velocity water droplets, and the expansion phase of the stress wave, combined with the tensile stress due to the Poisson effect, can exceed the yield strength or even the tensile strength of the coating material, resulting in deformation. or destruction of the coating.

[0054] Through modeling and experimentation, the Applicant of the present invention has determined that for a coating intended for use as a protection of a turbine blade or blade against drop impact erosion, the optimal range of residual compressive stresses in the coating is from 520 MPa to 5.3 GPa, optionally from 810 MPa to 2.63 GPa. Through extensive experimentation, the applicant of the present invention found that the residual compressive stress in the optimal range can be provided by one or more of the following process conditions:

i) maintaining the temperature during the coating process in the range from 320°C to 580°C;

ii) maintaining the coating rate in the range of 3.5 to 82 µm per hour, optionally 4 to 18 µm per hour;

iii) maintaining an average cooling rate after coating in the range from 0.12°C per minute to 1.9°C per minute;

iv) polishing the turbine parts, optionally performing other finishing operations after coating to achieve a surface finish with a roughness Ra of not more than 0.2 µm.

[0055] The destructive effects of stress waves, which are caused by impacts during the AEC process, can be increased due to the presence of pores, structural defects and mechanically weak inclusions in the coating, which can interact with stress waves and turn into mechanical stress concentrators and microcrack initiation points. Through analysis, it was found that, in particular, tungsten oxides and tungsten oxyfluorides can form mechanically weak inclusions and defects in the coating structure, which can become microcrack initiation points. In addition, it has been found that these undesirable inclusions are formed due to the reactions of gaseous precursors with traces of oxygen and water, and also if oxygen-containing gaseous precursors such as methanol, ethanol, dimethyl ether or other gases containing traces of oxygen or water are involved in the process. , then under certain conditions of temperature and pressure, non-volatile and mechanically fragile oxides and oxyfluorides of tungsten are formed.

[0056] Metallic tungsten can be alloyed with carbon in an amount ranging from 0.0001 to 0.37 wt. % relative to the total weight of the material; optionally in an amount ranging from 0.0001 to 0.21 wt. % relative to the total weight of the material. Through extensive experimentation, it has been found that, under certain conditions, alloying with carbon present in elevated concentrations can lead to precipitation of tungsten carbides or free carbon at grain boundaries. When such precipitates are present in sufficient density, they can reduce the fracture toughness of the material, as well as the tensile strength, creep resistance, and other mechanical properties. In addition, it has been found that the content of dopant carbon, which exceeds the above limits, can lead to an increase in residual stresses in the material, making it prone to cohesive failure under FEC conditions.

[0057] The material may consist primarily of carbon-doped metallic tungsten, optionally additionally doped with fluorine. In other words, the bulk material (not including any surface oxide layer that may be present due to exposure to air and/or moisture) may be free of components except for trace impurities that are not carbon-doped tungsten, optionally further doped with fluorine.

[0058] The carbon-doped tungsten metal can be further alloyed with fluorine in an amount of 0.0004 to 0.31 wt. % relative to the total weight of the material; optionally in an amount ranging from 0.0014 to 0.19 wt. % relative to the total weight of the material. Through extensive experimentation, it has been found that dopant fluorine, which is present at higher concentrations, can adversely affect the adhesive and protective properties of the coating, in particular in the case of steam and gas turbine blades that are operated in the presence of water and oxygen for extended periods of time. .

[0059] The material may be substantially free of oxyfluorides.

[0060] The material may be characterized in that it does not exhibit characteristic peaks of inclusions having an A15 crystal structure, such as tungsten carbides, when examined by X-ray diffraction analysis.

[0061] There are particular and unexpected technical advantages to be gained from a tungsten metal matrix with dispersed tungsten carbide nanoparticles that are small enough to be invisible when examined by x-ray diffraction analysis.

[0062] A unique aspect of nanotechnology is the gigantic increase in surface area to volume ratio that is present in many nanomaterials, which opens up new possibilities in surface science such as catalysis. Some physical phenomena become noticeably pronounced as the size of the system decreases. These phenomena include statistical mechanical effects as well as quantum mechanical effects such as the "quantum size effect" where the electronic properties of solids change when the particle size is greatly reduced. This effect does not manifest itself in the transition from macrosize to microsize. However, it becomes predominant when reaching sizes in the nanometer range. In addition, some physical properties change compared to macroscopic systems. One example is an increase in the surface area to volume ratio of materials.

[0063] In accordance with the Scherrer formula, the width of the diffraction lines in the x-ray diffraction pattern increases with a decrease in the size of solid crystallites. Typically, crystallites having a size of less than about 10 nm and dispersed in the matrix material do not show well-defined lines in the x-ray diffraction pattern. The fact that the tungsten carbide particles forming the carbon-doped tungsten material according to the embodiments of the present invention cannot be detected by X-ray diffraction as shown in FIG. 1 (but in certain cases can be detected by high resolution electron microscopy) confirms that the deposits of tungsten carbide particles do indeed have sizes in the nanometer range. The X-ray diffraction pattern of a CVD partially carbidized tungsten coating according to certain embodiments of the present invention shows all typical lines of metallic tungsten, but none of the characteristic lines of various tungsten carbides WC, WC _1-x , W ₂ C, W ₃ C are present in the diffraction pattern of the sample The unexpected resulting quantum mechanical effects are the combination of hardness with toughness and impact resistance (low brittleness) of the material. Most hard materials are brittle, but the material of the embodiments of the present invention combines hardness and toughness in a way that has significant and unexpected practical implications.

[0064] In some embodiments, the material is or contains a single crystallographic phase of tungsten metal with dispersed tungsten carbide nanoparticles.

[0065] The tungsten carbide nanoparticles may have an average particle size of not more than 100 nm; optionally no more than 50 nm; optionally no more than 10 nm.

[0066] Unlike the material described in US Pat. 0.0001 to 0.37 wt%, compared to 2.12 to 6.12% carbon in US 4,427,445; virtually free of oxygen compared to structure A15 containing tungsten oxides and oxyfluorides), contains various phases tungsten carbides, has a different structure (nanostructure as opposed to microstructure/macrostructure), lower residual stresses and other mechanical properties (such as, in particular, hardness and toughness/brittleness).

[0067] The nanostructure according to embodiments of the present invention can be confirmed by corresponding X-ray diffraction patterns that contain only lines or characteristic peaks of tungsten, as shown in FIG. 1. The nanostructured materials of the present invention have improved toughness, improved toughness combined with sufficient hardness, and greatly improved resistance to ACC and particulate erosion. The material is alloyed with carbon to achieve increased hardness and does not require additional heat treatment. The presence of tungsten carbides in the form of microparticles or macroparticles containing carbides with the A15 structure is not desirable because these carbides have high stresses and can act as microcrack initiation points.

[0068] The material may be substantially non-porous. In some embodiments, the material has a porosity of less than 0.5 vol. %, optionally less than 0.3 vol. %, optionally less than 0.2 vol. %, optionally less than 0.15 vol. %.

[0069] The pores in the coating can act as stress concentrators and microcrack initiation points. High-velocity droplet impacts on surface and near-surface pores can deform and expand the pores through hydraulic penetration and lateral water jets, resulting in tunneling and bulging of the metal. These processes are believed to contribute significantly to drop impact erosion and thus are desirable for reducing or minimizing porosity.

[0070] The composition of the material may be present from 97.60 to 99.99 wt. % tungsten.

[0071] The material may have a hardness of 4.4 GPa to 19 GPa, optionally 8 GPa to 16 GPa. It has been found that materials that are excessively hard can undergo a brittle fracture or cracking mode when impacted at high frequency by high velocity water droplets.

[0072] The material may have a fracture toughness of at least 9 MPa⋅m ^1/2 . This may help to provide higher resistance to impact with high frequency of impacts with high speed droplets.

[0073] The material has a columnar or almost columnar crystalline microstructure. This is in contrast to materials that are produced by thermal spray processes and tend to have lamellar or "board-like" microstructures.

[0074] Preferably, the material is substantially free of pores, voids and/or inclusions that act as stress concentrators.

[0075] According to a third aspect, chemical vapor deposition of a material deposited on a substrate according to the first or second aspects is provided.

[0076] The coating material on the substrate may have a residual compressive stress of 520 MPa to 5.3 GPa, optionally 810 MPa to 2.63 GPa. Residual compressive stresses can help to improve fatigue resistance to high frequency water drop impacts, as well as help prevent cracking of the coating. It should be noted that thermal spray coatings tend to have residual tensile stresses rather than compressive stresses.

[0077] The coating may have a thickness of at least 15 microns, optionally at least 50 microns. The coating may have a thickness of not more than 200 µm, optionally not more than 100 µm.

[0078] For coating a substrate, the stress waves that result from impacts on the surface of the coating will be at least partially reflected from the boundary between the coating and the substrate. Measures shall be taken to ensure that the amplifying interference between the incident and reflected stress waves does not exceed the tensile strength of the coating, which may cause failure. Thicker coatings can dissipate stress waves better than thinner coatings. One factor is that the energy density of a spherical stress wave decreases with the square of the distance, and thus the energy density of a stress wave reaching the interface between the coating and the substrate decreases with the square of the coating thickness. However, coatings that are excessively thick can distort the carefully designed shape and profile of the turbine blade. In addition, it must be remembered that coating the turbine blade will increase the mass of the turbine blade. It would be undesirable to apply a coating of such a thickness at which an increase in mass would mean the need to change the original design of the blade and re-examine.

[0079] The coating may have a surface roughness Ra of less than 1 µm. In some embodiments, a coating deposited on a smooth substrate surface has a surface roughness Ra of less than 1 µm in its post-deposition state (without the need for any subsequent polishing). However, it should be understood that this conformal coating formed on the surface of a substrate already having some degree of surface roughness will typically also have a surface roughness due to the surface roughness of the underlying surface. Accordingly, in some embodiments, the coating is applied such that its surface roughness Ra is not more than 1 µm greater than the surface roughness of the underlying surface of the substrate.

[0080] In some embodiments, when the coating is brought into wear/sliding or rotational contact with a bearing or seal, the surface finish of the coating becomes polished and improves over time. This feature is not often present in many other coating materials, such as HCTG spray coatings consisting of hard WC grains in a soft Co, CoCr or Ni metal matrix: the soft metal matrix undergoes selective wear or corrosion, leaving hard and sharp grains protruding from it. WC, which increase the surface roughness and make the surface obtained by means of VKTG similar to sandpaper, which leads to a high degree of abrasion of seal materials and other objects in contact.

[0081] This polishability of the coating according to embodiments of the present invention may be due to the uniform structure and mechanical properties of the coating, the absence of deposits at the grain boundaries, as well as the very small scale/fine size of tungsten carbide nanoparticles, usually obtained in the form of so-called coherent deposits, which have a precipitate crystal lattice compatible with the matrix lattice. As a result, the coating wears evenly without leaving harder "islands" or surface roughness protrusions that protrude from the surface and may lead to unwanted abrasion of opposing surfaces and/or may increase the aerodynamic drag of moving turbine blades. It is believed that low roughness and the absence of surface roughness ridges are also important factors in providing resistance to ACC. This is because a high-velocity side water jet can be an essential mechanism for AEC, and such a jet can produce a tearing effect on surface roughness peaks and high shear stresses, resulting in material loss.

[0082] Obtaining a substantially uniform coating with a low roughness/smooth surface reduces the need for grinding, lapping, rolling, polishing and other finishing operations after coating, and also allows the coating of parts having complex shapes, polishing of which would be very difficult or impossible if the coating were rough.

[0083] The ability of certain coatings of the present invention to retain a good finish or even become smoother and more polished during service means that the coating remains non-abrasive to seals, bearings, and other mating objects that are in sliding or rotational contact with the coated part. This can mean less wear on seals and less leakage in hydraulic actuators, resulting in less maintenance and can increase service life.

[0084] Accordingly, in addition to being applied to turbine blades or vanes, the coatings of the present invention may find use as coatings on rotating and/or reciprocating shafts and pistons, such as hydraulic pistons and cylinders, transmission shafts, pump shafts. and other components that move in contact with seals or bearings and have a coating applied in areas of contact with moving seals or bearings where the coating is polished after application.

[0085] It is well known that the mechanical processes occurring in cavitation erosion are not different from those occurring in drop impact erosion, and that materials that exhibit good resistance to CAE are also expected to have good resistance to cavitation. Accordingly, in addition to being applied to turbine blades or blades, the coatings of the present invention may find use as coatings on components of pumps, valves, and other devices that are exposed to fluid cavitation, and as a result, service life is increased and performance is improved.

[0086] The coating may comprise at least a softer coating layer located closer to the substrate and consisting mainly of tungsten metal, optionally doped with fluorine, and a harder coating layer containing the material according to the first aspect.

[0087] The softer layer tungsten metal can be doped with fluorine in an amount of 0.0004 to 0.31 wt. % relative to the total weight of the softer layer; optionally in an amount of 0.0014 to 0.19% based on the total weight of the softer layer.

[0088] The coating may further comprise a transition layer between the softer layer and the harder layer. The carbon concentration in the transition layer may increase from the softer layer to the harder layer. The transition layer may have a thickness of at least 0.01 µm, optionally at least 0.1 µm. The ratio of the thickness of the softer layer to the thickness of the harder layer can be from 1:10 to 10:1. The total thickness of the softer and harder layers and optionally the transition layer may be from 1 to 50 microns, optionally from 1 to 100 microns, optionally from 1 to 200 microns.

[0089] According to some embodiments, multiple pairs of softer and harder layers can be formed, resulting in a multilayer structure of alternating layers. In some embodiments, 2 to 100 pairs of softer and harder layers may be present. In some embodiments, the multilayer structure may comprise a softer layer, a transition layer, a harder layer, a transition layer, a softer layer, a transition layer, a harder layer, and so on. In each of the transition layers, the concentration of carbon, for example in the form of tungsten carbide nanoparticles, can increase through the thickness of the transition layer in the direction from the softer layer to the harder layer. The formation of such transition layers can be facilitated by controlling the flow of gaseous precursors and pressure during the CVD process, and it can be beneficial to reduce the stresses between the layers.

[0090] Providing a transition layer with a carbon concentration gradient between softer and harder layers can help reduce residual stresses and prevent sharp thermal and mechanical mismatches at layer boundaries, as well as hinder crack propagation.

[0091] The substrate may be a turbine or compressor blade or vane. Embodiments of the present invention are particularly suited to applications where the blade or vane is a steam turbine or steam compressor blade or vane because said blades or vanes are impacted at high frequency by high velocity water droplets during operation and resistance to FEC is required. .

[0092] A fourth aspect provides a turbine or compressor blade or vane at least partially coated with a material according to the first or second aspect.

[0093] According to a fifth aspect, a pump impeller, propeller, valve, or other components is provided that is cavitated in a liquid and is at least partially coated with a material according to the first or second aspect.

[0094] According to a sixth aspect, a method is provided for rendering a turbine or compressor blade or vane resistant to drop impact erosion, the method comprising at least partially coating the blade or vane with a material according to the first, second, or third aspect via chemical vapor deposition.

[0095] According to a seventh aspect, a method is provided for rendering a pump impeller, propeller, valve, or other component cavitation resistant, the method comprising at least partially coating the component with a material according to the first, second, or third aspect.

[0096] According to an eighth aspect, there is provided a method for manufacturing a material according to the first or second aspect by chemical vapor deposition using a gas phase containing, in a mixture of WF ₆ , hydrogen, at least one hydrocarbon, and optionally an inert gas, the gas phase having an oxygen content , which is not more than 10 hours/million, and the water content, which is not more than 3 hours/million. Preferably, the surface of the part covered by the material according to the first aspect is also treated or modified in such a way that oxygen and water vapor are practically absent, as is inside any reaction chamber used in this method.

[0097] The Applicant of the present invention has determined that the presence of tungsten oxides and oxyfluorides can adversely affect the mechanical properties of materials and coatings produced by CVD in a CVD reaction chamber. In embodiments of the present invention that use alkanes (eg, methane, ethane, propane) as carbonaceous precursors, they do not contain oxygen. In addition, steps can be taken to prevent or reduce the formation of tungsten oxides and oxyfluorides. These measures may include one or more of the following measures:

i) Making the CVD reaction chamber and its gas system vacuum-tight so as to prevent any leakage of air into the chamber or gas system; this can be achieved by applying a vacuum-tight design of the system and components and by examining the vacuum-tightness of the flanges and seals of the chamber and other components with a helium leak detector after each opening of the reactor. The helium leakage limit can be set, for example, at 1×10 ^-9 mbar⋅l/s. If this limit is exceeded, then the reactor is opened, the flanges and seals are cleaned, the reactor is closed, and another study of helium leakage is carried out in such a way as to ensure that this limit is observed.

ii) Purging and degassing the CVD reaction chamber and loaded components prior to the coating process to remove adsorbed water and oxygen, for example by heating the chamber and loaded components under vacuum and optionally performing a series of cycles of filling the reaction chamber with an inert gas to a predetermined partial pressure before evacuating the chamber to a predetermined residual pressure in order to remove traces of adsorbed oxygen and/or water from the surfaces of the chamber and loaded components. The Applicant of the present invention has found that large CVD reactors, which may have stainless steel inner surfaces greater than 10 ^m2 inside the vacuum chamber, adsorb a significant amount of water vapor when the chamber is opened to load components prior to the CVD process. It has been found that such condensation of water affects in particular the water-cooled regions of the reaction chamber, such as the cooled region of the vacuum seal. In addition, it has been found that stainless steel surfaces can adsorb oxygen. The Applicant of the present invention has surprisingly found that degassing the vacuum chamber prior to each CVD cycle and using gases of increased purity (particularly hydrogen as the reductant gas and alkane gas) will result in a substantially oxygen-free coating having significantly improved mechanical properties.

iii) The use of gaseous precursors and process gases which are substantially free of oxygen and water, for example, to obtain a reactive gas mixture having an oxygen content of not more than 10 ppm. and a water content of not more than 3 ppm. It has been found that the purity of the precursor and process gases is particularly important when hydrogen gas is used in coating processes because commercial grade hydrogen is usually produced by electrolysis and as a result contains significant amounts of water vapor and often traces of oxygen.

iv) Backfilling the CVD reaction chamber with an inert gas substantially free of oxygen and water after the completion of the coating process, when the coated material is allowed to cool.

v) Cooling the coated parts to a temperature of not more than 200° C. before opening the reaction chamber in order to reduce subsurface oxidation of the deposited coatings.

[0098] As a result, it becomes possible to prevent the presence of oxides and oxyfluorides of tungsten and to deposit a material or coating having substantially no oxygen in its bulk.

[0099] The gas phase may be non-ionized. Alternatively, or in addition, the gas phase may remain reactive during the deposition process.

[00100] The method can be carried out at a temperature of from 320 to 580°C using a gas phase that is a mixture of WF ₆ , hydrogen, at least one hydrocarbon and optionally an inert gas such as argon, at a pressure of 0.1 to 5 kPa for at least 10 minutes.

[00101] At least one hydrocarbon may contain or be a gaseous alkane.

[00102] At least one hydrocarbon can be thermally activated by heating to a temperature of from 500 to 850°C before mixing with WF ₆ and hydrogen.

[00103] According to a ninth aspect, there is provided a method for resisting drop impact erosion of turbine or compressor blades or blades, comprising coating the blades or blades with a material according to the first or second aspect.

[00104] According to a tenth aspect, there is provided a method for resisting cavitation erosion of pump impellers, propellers, valves, or other components, comprising coating the component with a material according to the first or second aspect.

Brief description of the figures

[00105] Further, embodiments of the present invention are described herein with reference to the accompanying figures, wherein:

in fig. 1 is an X-ray diffraction pattern of a material according to an embodiment of the present invention;

in fig. 2 is a plot of weight loss versus number of drop impacts for various coated samples as determined using a test setup;

in fig. 3 is a plan view of an uncoated stainless steel sample after drop impact erosion testing using approximately 10 million water drop impacts;

in fig. 4 is a perspective view of the sample illustrated in FIG. 3;

in fig. 5 is a plan view of a first sample coated in accordance with the present invention after drop impact erosion testing using approximately 121 million water drop impacts;

in fig. 6 is a 3D scan of the first sample showing a surface height map of the first sample after drop impact erosion testing;

in fig. 7 is a plot of the height profile over the study area of the first sample after drop impact erosion testing;

in fig. 8 is a plan view of a second sample coated in accordance with the present invention after drop impact erosion testing using approximately 101 million water drop impacts;

in fig. 9 is a 3D scan of the second sample showing a map of the height of the surface of the second sample after drop impact erosion testing;

in fig. 10 is a plot of the height profile drawn over the study area of the second sample after the teardrop erosion study;

in fig. 11 is a plan view of a third sample coated in accordance with the present invention after drop impact erosion testing using approximately 74 million water drop impacts;

in fig. 12 is a 3D scan of the third sample showing a surface height map of the third sample after drop impact erosion testing;

in fig. 13 is a plot of the height profile drawn over the study area of the third sample after the teardrop erosion study;

in fig. 14 is a plan view of a fourth sample coated in accordance with the present invention after drop impact erosion testing using approximately 74 million water drop impacts;

in fig. 15 is a 3D scan of the fourth sample showing a surface height map of the fourth sample after drop impact erosion testing;

in fig. 16 is a plot of the height profile drawn over the area of interest of the fourth sample after the teardrop erosion study;

in fig. 17 is a cross-sectional view of a fifth sample coated in accordance with the present invention, with a porosity analysis performed using the software package;

in fig. 18 is a cross-sectional view of the fourth specimen illustrating resistance to microcrack propagation;

in fig. 19 is a cross-sectional view of a CVD coating used to determine porosity in accordance with ASTM E2109;

in fig. 20 is an image of an ASTM E2109-01 study illustrating the minimum porosity level used to visually determine the percentage pore area of coatings;

in fig. 21 is a cross section of a CVD coating examined with a scanning electron microscope in shadow mode;

in fig. 22 shows a cross section of a CVD coating examined by electron backscatter diffraction (EBSD) analysis.

Detailed disclosure of the present invention

[00106] In order to investigate the effects of drop impact erosion on turbine blades, an apparatus containing a rotor rotating at a high speed in excess of 5700 rpm was placed in an evacuated chamber. Samples of materials were attached to either end of the rotor and subjected to water droplet impacts using a nozzle of the selected diameter and removal distance. The study was carried out at the National Physical Laboratory (NPL) of the National Measurement Institute of Great Britain, which is the world's leading center with excellence in the development and application of the most accurate scientific and technical measurement standards available.

[00107] Various test samples were evaluated as follows:

[00108] Residual stresses in the coatings were measured using X-ray diffraction techniques, since stresses lead to changes in interatomic distances in the crystal lattices of the coating materials, which can be measured by the shift of the characteristic X-ray diffraction lines. All measurements of various coating samples representing embodiments of the present invention exhibit residual compressive stresses. Measurements of partially carbidized type 1 CVD tungsten coatings having various thicknesses and other characteristics show residual stress values from 520 MPa to 1100 MPa, and after additional grinding and finishing operations show residual stress values from 1094 MPa to 2552 MPa. Measurements of partially carbidized type 2 CVD tungsten coatings having various thicknesses and other characteristics show residual stress values from 810 MPa to 2630 MPa, and after additional grinding and finishing operations show residual stress values up to 5300 MPa.

[00109] The chemical composition of each type of coating was analyzed. The carbon content was analyzed using an Eltra® combustion product analyzer: an independent sample of the coating was placed in an induction furnace and burned in an oxygen atmosphere. During combustion, the carbon components of the sample were oxidized to carbon dioxide (CO ₂ ), and the carbon dioxide was selectively measured using infrared detectors. The readings were linearized, integrated and divided by the mass of the sample, and then output as a mass percent of the total carbon content. For samples of partially carbidized type 1 CVD tungsten coating deposited in the same cycle as samples N3 and N7, the following results were obtained:

[00110] The following results were obtained for samples of the partially carbidized type 2 CVD tungsten coating deposited in the same cycle as samples N26 and N29:

[00111] Other samples of partially carbidized tungsten CVD coating were obtained with a carbon content ranging from 0.000927 wt. % to 0.3697 wt. %.

[00112] The fluorine content was analyzed using a secondary ion mass spectrometry (SIM) method. Partially carbidized CVD tungsten coating samples were obtained with a fluorine content ranging from 0.0004 wt. % to 0.3093 wt. %.

[00113] The oxygen content was analyzed using a secondary ion mass spectrometry (SIM) method after removing the outer oxidized coating layer using an ion beam etching method. In this analysis, no measurable oxygen signal was obtained for various coating samples representing embodiments of the present invention.

[00114] FIG. 1 is an x-ray diffraction pattern of the coating on sample N22 coated with samples N26 and N29. This diffractogram represents all characteristic peaks of tungsten metal, but does not represent the characteristic peak sets of any of the tungsten carbides. This supports the assumption that the tungsten carbide particles forming the carbon-doped tungsten material according to embodiments of the present invention are indeed nanoparticles and not particulates or microparticles.

[00115] The fracture toughness of various coating samples representing embodiments of the present invention was measured by obtaining a series of cube corner indentations as well as Vickers indentations and examining the indentations for cracks extending from the indentation corners. None of the samples exhibited any cracks produced by these methods, showing a lower fracture toughness limit of the coating of at least 9 MPa⋅m ^1/2 .

[00116] Measurements were made of the dependence of mass loss on the number of rotor arm/jet leaks (interactions) over time for the entire list of samples. Samples N3 and N7 were coated with a partially carbidized tungsten type 1 CVD coating, and samples N26 and N29 were coated with a partially carbidized tungsten CVD type 2 coating. The substrate was 410 stainless steel. control samples showed rapid weight loss due to AEC.

[00117] The results were plotted on a plot of weight loss versus time as shown in FIG. 2. The results of this study were compared with the historical results of previous studies carried out on monolithic uncoated specimens of 410 martensitic stainless steel and Stellite® of three different types. Five replicates were conducted using a sample of 410 stainless steel material and two samples of each of Stellite® 6 and Stellite® 21. Stellite® 6 is a cobalt-based alloy that consists of complex carbides in an alloy matrix. The nominal composition of the alloy contains from 27 to 32 wt. % Cr, 4 to 6 wt. % W, from 0.9 to 1.4 wt. % C, with the balance being cobalt. Stellite® 21 (also known as Stellite® 8) is a cobalt-based alloy that consists of a CoCrMo alloy matrix containing dispersed hard carbides. The nominal composition of the alloy contains from 26 to 29 wt. % Cr, from 4.5 to 6 wt. % Mo, from 0.2 to 0.35 wt. % C, from 2.0 to 3.0 wt. % Ni, with the rest of the mass being cobalt.

[00118] Good reproducibility of the results of these studies can be observed in the example of 410 SS, where the results overlap in a very narrow range. All samples of partially carbidized tungsten CVD coating N3, N7, N26 and N29 show significantly less weight loss than samples 410 SS, but all of these samples show different amounts of weight loss with increasing duration of the study. Samples N3 show a very slow increase in weight loss. Samples of Stellite® 6 and Stellite® 21 show a slight increase in weight loss up to 2×10 ⁸ hits of water drops. However, the other Stellite® material exhibits a significant weight loss under the action of 1×10 ⁸ impacts of water drops. Samples of partially carbidized tungsten CVD coatings present steps on the mass loss curves, suggesting a significant loss of sample material at these steps. It should be remembered that mass loss is a relatively approximate measure of teardrop erosion. For example, some samples (e.g. N26 and N29) have been found to exhibit some flaking in edge areas outside of the main area of interest, and this flaking is likely due to mechanical deformation of the samples during repeated removal and subsequent placement on a test bench for measurements. mass between each two-hour test periods. The loss of coating material due to this peeling is represented as a significant weight loss in the graphs of FIG. 2, although the actual damage due to teardrop erosion in the area under study appears to be negligible.

[00119] An optical microscope was used to examine the surfaces of the samples. FIG. 3 and 4 are, respectively, a plan view and a perspective view of an uncoated 410 SS sample after 10 ⁷ water drop impacts. A very clear wear mark 8 mm long was observed across the entire width of the specimen.

[00120] In the following tests, an Alicona® Infinitefocus microscope was used to examine the surfaces of the wear areas of the samples. This microscope provides high quality images of the sample surface as well as 3D information in the direction of height above the surface. The first figure of each set (FIGS. 6, 9, 12, 15) represents an image, with the surface image presented in three-dimensional projection, showing the complete wear mark, and the second image (Fig. 7, 10, 13, 16) represents a plot of the height profile along the central region of each sample in a direction perpendicular to the direction of the water jet.

[00121] FIG. 5 is a top view of sample N3 after AEC testing using approximately 1.2 x 10 ⁸ water droplets. In FIG. 6 is a plan view of sample N7 after CEC testing using approximately 1.0 x 10 ⁸ water droplets. The study of AEC was carried out over the entire width of the sample, but in contrast to the control uncoated sample 410 SS, shown in Fig. 4, both samples of the partially carbidized type 1 CVD tungsten coating show only some localized damage near the corners of the samples, but there is no pronounced damage to the rest of the area under study after more than ten times the duration of exposure to AEC. For both samples N3 and N7, discoloration of the samples is observed up to the edge of the area where the water jet hit the sample. This is believed to be due to the oxidation of the sample surface due to the elevated temperature caused by the erosion process. The central area of the wear track for sample N3 in FIG. 5 represents some "darkening" which may be indicative of some damage. There is a definite depression formed at one end of the sample where water drops hit the sample.

[00122] FIG. 6 shows the Alicona® microscope scan of sample N3 after examination. The height map represents some damage near the edges of the sample in the region of interest, but the sample shows little or no damage in the middle of the region of interest, as shown in FIG. 7.

[00123] Sample N7 in FIG. 8 shows very little damage on the specimen, with some damage near the edge of the wear mark. There is probably some reduction in the visibility of scratches from the manufacture of the original sample on the surface in the center of the wear mark.

[00124] FIG. 9 shows the Alicona® microscope scan of sample N7 after examination. The height map represents some damage near the edges of the sample in the region of interest, but the sample shows little or no damage in the middle of the region of interest, as shown in FIG. 10.

[00125] Sample N29 in FIG. 11 shows damage in the form of flaking at one edge of the sample outside of the examined area. It is possible that this damage was caused by deformation of the sample during repeated removal and subsequent placement on the test bench for mass measurements between each two-hour test periods. The investigated area in the middle of the sample does not represent visible damage due to AEC.

[00126] FIG. 12 shows the Alicona® microscope scan of sample N29 after examination. The height map shows no detectable damage in the middle of the examined area, as shown in FIG. 13.

[00127] Sample N26 in FIG. 14 shows all-round damage in the form of flaking at the edges of the sample outside of the area examined, again stress or strain during repeated removal and subsequent placement on the test rig is most likely the cause. The region examined represents some branched ACE damage that extends over the specimen for approximately 2.7 mm.

[00128] FIG. 15 shows the Alicona® microscope scan of sample N26 after examination. The height map of this sample shows some surface damage to the edge of the sample, as well as some FEA damage on one side that looks branched and runs along the sample for approximately 2.7 mm. This structure appears to be stepped and has a depth of 18 µm over most of the feature area. The middle part of the examined area does not show measurable damage to the coating, as shown in Fig.16.

[00129] With the exception of sample N26, these scans represent a slight pronounced damage from teardrop erosion in the central region of the wear marks visually observed on the three-dimensional images and profiles of the samples. However, for sample N26, a tree-like branched damage is observed over a significant part of the width of the sample.

[00130] FIG. 17 is a cross-sectional view of a sample of a partially carbidized N34 CVD tungsten coating layer, with porosity analysis performed using Gwyddion image analysis software. The determined porosity was 0.00%.

[00131] FIG. 18 is a cross-sectional view of an EEC-tested sample of an N26 coating on a steel substrate in accordance with an embodiment of the present invention, the coating comprising a softer metal tungsten layer on the substrate and a harder carbon-doped tungsten layer deposited on top of the softer layer. This section represents the area with branched FA damage on the surface of the coating. It can be seen that microcracks initiated in the upper harder layer under the action of AEC tend to stop at the boundary between the upper harder layer and the lower softer layer. A cross-sectional examination of this sample using a scanning electron microscope (SEM) unexpectedly found that in the area of the branched lesion, the thickness of the remaining area, measured at a plurality of points, was approximately 10 to 12 μm, which is the thickness of the softer metal tungsten layer. This demonstrated that in the area of branched damage, the upper coating layer, made of harder carbon-doped tungsten, was significantly damaged or lost, while the softer and more ductile metallic tungsten layer closest to the substrate remained largely intact. Accordingly, the present inventor has surprisingly found that a multi-layer coating structure in which alternating layers are present, including viscous tungsten layers and harder carbon-doped tungsten layers, can provide effective protection against FEC.

[00132] FIG. 19 is a cross-sectional view of a CVD coating used to determine porosity in accordance with ASTM E2109-01 standard methods for determining percentage pore area in thermally sprayed coatings. There are practically no pores and inclusions in the coating.

[00133] FIG. 20 is an image of an ASTM E2109-01 study showing a minimum porosity level of 0.5% and used to visually determine the percentage pore area of the coatings.

[00134] FIG. 21 shows a cross section of a CVD coating examined using a scanning electron microscope in shadow mode. The columnar crystalline microstructure of the coating is clearly visible.

[00135] FIG. 22 shows a cross section of a CVD coating examined using electron backscatter diffraction (EBSD) analysis, including pseudo-color and monochrome images. And in this case, the columnar crystalline microstructure of the coating is clearly visible. The EDSD analysis is an effective method for the crystallographic study of the microstructure of materials.

[00136] Throughout the text of this document, including the description and claims, the words "include" and "comprise" and their modifications mean "including, but not limited to", and they are not intended to exclude (and do not exclude) other elements, additives, components, integral parts or steps.

[00137] Features, parts, characteristics, compounds, chemical moieties, or groups described in conjunction with a particular aspect, embodiment, or example of the present invention should be understood to apply to any other aspect, embodiment, or example described herein, unless they are incompatible. All features disclosed herein (including any claims of the accompanying claims, abstract and figures) and/or all steps of any method or process disclosed therein may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The present invention is not limited to the details of any of the above embodiments.

[00138] When reading, attention should be paid to all materials and documents that are filed simultaneously with or prior to this document in connection with this application, and which are open for public review along with this document, and the contents of all such materials and documents are included in this document. through a link.

Claims (50)

1. An erosion and corrosion resistant material containing carbon-doped tungsten and deposited on a substrate, which has a columnar crystalline microstructure, and in which the carbon-doped tungsten is a tungsten matrix in which tungsten carbide nanoparticles are dispersed, in the form of a uniform nanostructure, while there is no oxygen in said matrix, except for parts of the surface of said substrate open to air or moisture. 2. Drop impact erosion resistant material containing carbon-doped tungsten and deposited on a substrate, which has a columnar crystalline microstructure, and in which carbon-doped tungsten is a tungsten matrix in which tungsten carbide nanoparticles are dispersed, in the form of a uniform nanostructure, while in said tungsten matrix is free of oxygen, except for parts of the surface exposed to air or moisture. 3. The material according to claim 1 or 2, in which the tungsten is alloyed with carbon in an amount of 0.0001 to 0.37 wt.% in relation to the total mass of the material, for example, in an amount of 0.0001 to 0.21 wt. % relative to the total weight of the material. 4. Material according to any one of paragraphs. 1-3, which consists of carbon-doped tungsten, optionally additionally doped with fluorine. 5. Material according to any one of paragraphs. 1-3, in which tungsten is additionally doped with fluorine. 6. Material according to claim 5, in which tungsten is doped with fluorine in an amount of 0.0004 to 0.31 wt.% in relation to the total mass of the material, for example, in an amount of 0.0014 to 0.19 wt.% by relative to the total weight of the material. 7. Material according to claim 5 or 6, in which oxyfluorides are absent. 8. Material according to any one of paragraphs. 1-7, which lacks characteristic peaks of tungsten carbides having an A15 crystal structure when examined by X-ray diffraction analysis. 9. Material according to any one of paragraphs. 1-7 which is non-porous. 10. Material according to any one of paragraphs. 1-8 which has a porosity of less than 0.5% by volume, such as less than 0.3% by volume, such as less than 0.2% by volume, such as less than 0.15% by volume. 11. Material according to any one of paragraphs. 1-10, which contains in its composition from 97.60 to 99.99 wt.% tungsten. 12. Material according to any one of paragraphs. 1-11 which has a hardness of 4.4 GPa to 19 GPa, for example 8 GPa to 16 GPa. 13. Material according to any one of paragraphs. 1-12 which has a fracture toughness of at least 9 MPa⋅m ^1/2 . 14. Material according to any one of paragraphs. 1-9 and 11, 12, in which there are no pores, voids and/or inclusions that act as mechanical stress concentrators. 15. Material according to any one of paragraphs. 1-14, which is deposited on a substrate by chemical vapor deposition. 16. The material according to claim 15, which has a residual compressive stress of 520 MPa to 5.3 GPa, for example 810 MPa to 2.63 GPa. 17. Material according to claim 15 or 16, which has a thickness of at least 15 microns, for example at least 50 microns. 18. Material according to any one of paragraphs. 15-17 which has a thickness of not more than 200 µm, for example not more than 100 µm. 19. Material according to any one of paragraphs. 15-18 which has a surface roughness Ra of less than 1 µm, such as less than 0.2 µm. 20. The material according to any one of paragraphs. 15-19, which is deposited by chemical vapor deposition, has a surface roughness Ra not more than 1 µm higher than the surface roughness of the substrate, without the need for polishing after deposition. 21. Material according to any one of paragraphs. 15-20, which is made in the form of a cover layer. 22. The material of claim. 21, which further comprises at least a softer layer located closer to the substrate and consisting mainly of tungsten, optionally doped with fluorine. 23. The material of claim 22, wherein the top layer is the harder layer. 24. The material according to claim 22, in which the tungsten of the softer layer is doped with fluorine in an amount of 0.0004 to 0.31 wt.% in relation to the total weight of the softer layer, for example, in an amount of 0.0014 to 0, 19 wt.% in relation to the total weight of the softer layer. 25. Material according to claim 23 or 24, further comprising a transition layer between the softer layer and the harder layer. 26. The material of claim 25, wherein the concentration of carbon in the transition layer increases in the direction from the softer layer to the harder layer. 27. Material according to claim 25 or 26, wherein the transition layer has a thickness of at least 0.01 µm, such as at least 0.1 µm. 28. The material according to any one of paragraphs. 23-27, in which the ratio of the thickness of the softer layer to the thickness of the harder layer is from 1:10 to 10:1. 29. Material according to any one of paragraphs. 23-28, in which the total thickness of the softer and harder layers and, if necessary, the transition layer is from 1 to 50 microns. 30. Turbine blade, which is at least partially covered with a material according to any one of paragraphs. 1-14. 31. The turbine blade of claim 30, which is a steam turbine or gas turbine blade. 32. The impeller of the pump, subjected to cavitation in the liquid and at least partially covered with a material according to any one of paragraphs. 1-14. 33. A method of applying a coating resistant to drop impact erosion on a substrate in the form of a turbine or compressor blade or blade, which includes at least partial coating of the blade or blade with a material according to any one of paragraphs. 1-29 by chemical vapor deposition. 34. The method of claim 33 wherein the blade or vane is a blade or vane of a steam turbine or steam compressor or gas turbine. 35. A method of applying a cavitation erosion resistant coating to a substrate, which includes at least partially coating a component with a material according to any one of paragraphs. 1-29 by chemical vapor deposition. 36. A method of manufacturing a material according to any one of paragraphs. 1, 3-14 by chemical vapor deposition, comprising the use of a gas phase containing a mixture of WF ₆ , hydrogen, at least one hydrocarbon and, if necessary, an inert gas, and the gas phase has an oxygen content of not more than 10 h ./ppm, and a water content of not more than 3 ppm. 37. The method of claim 36 wherein the gas phase is non-ionized. 38. The method according to claim 36 or 37, wherein the gas phase is reactive during the deposition process. 39. The method according to any one of paragraphs. 36-38, which is carried out at a temperature of 320 to 580°C, using a gas phase containing a mixture of WF ₆ , hydrogen, at least one hydrocarbon and, if necessary, an inert gas, at a pressure of 0.1 up to 5 kPa, for at least 10 minutes. 40. The method according to any one of paragraphs. 36-39, wherein at least one hydrocarbon contains or is a gaseous alkane. 41. The method according to any one of paragraphs. 36-40, wherein at least one hydrocarbon is thermally activated by heating to a temperature of 500 to 850° C. prior to mixing with WF ₆ and hydrogen. 42. The method according to any one of paragraphs. 36-41, in which chemical vapor deposition occurs in the reaction chamber. 43. The method of claim 42 wherein the reaction chamber is degassed by heating under vacuum prior to introduction of the gas phase. 44. The method according to claim 42 or 43, in which the reaction chamber is filled with an inert gas, heated and then evacuated before introducing the gas phase. 45. The method according to any one of paragraphs. 42-44, in which the reaction chamber is examined for vacuum tightness each time it is closed after opening, if necessary, by means of a helium leak detector. 46. The method according to any one of paragraphs. 42-45 wherein the material is allowed to cool to 200° C. or less before opening the reaction chamber. 47. The method according to any one of paragraphs. 36-46, in which the material after deposition is cooled at an average rate of 0.12°C per minute to 1.9°C per minute. 48. The method according to any one of paragraphs. 36-47, wherein the material is deposited at a rate of 3.5 to 82 microns per hour, for example at a rate of 4 to 18 microns per hour. 49. A method of applying a coating resistant to drop impact erosion on a substrate in the form of a turbine or compressor blade or blade, including coating the turbine or compressor blade or blade with a material according to any one of paragraphs. 1-14. 50. A method of applying a cavitation erosion resistant coating to a substrate, which includes coating said substrate with a material according to any one of paragraphs. 1-14.

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