KR20010017861A - Chromium boride coatings - Google Patents

Abstract

PURPOSE: A chromium boride coating is provided to have excellent adhesive wear and corrosion resistance and to be compatible with a number of alloy substrates. CONSTITUTION: A coating comprises hard, ultra fine, chromium boride particles dispersed in a metal matrix, the particles having an average particle size of less than one micron and constituting less than about 30 volume percent of the coating, the balance being metal matrix. The metal matrix may be composed of nickel or a nickel base alloy containing a metal selected from the group consisting of chromium, silicon and iron. The coating may be prepared by a process which comprises depositing a mechanically blended powder mixture of chromium metal or a chromium alloy or mixture of both, and a boron-containing alloy onto a substrate and then heat treating the as-deposited coating. The heat treatment effects a diffusion reaction between the deposited elements resulting in the formation of ultra fine particles of chromium boride dispersed in a metal matrix. The coating can be deposited onto the substrate using any of the known deposition techniques.

Description

Chromium boride coating {CHROMIUM BORIDE COATINGS}

The present invention relates to a chromium boride coating having excellent wear resistance and corrosion resistance and a method of making such a coating. In particular, the present invention relates to hard, dense, low porosity, wear resistant and corrosion resistant coatings containing ultrafine particles of chromium boride dispersed in a metal matrix. The invention also relates to a process for making such coatings by thermal spraying and diffusion reaction techniques.

Throughout the specification, reference is made to plasma arc spraying and detonation guns to make coatings. Conventional explosive gun techniques are described in US Pat. Nos. 2,714,563 and 2,950,867. Plasma arc injection techniques are described in US Pat. Nos. 2,858,411 and 3,016,447. Other thermal spraying techniques are known, for example, in so-called "high speed" plasma and "ultrasonic" spray methods, as well as various flame spray processes. The heat treatment of the coating is necessary and can be done by electron beam, laser, induction heat, transferred plasma arc or other techniques in a vacuum or inert gas furnace after deposition. After heat treatment, alternative deposition techniques such as slurry, filled fiber or electrophoresis are also known. Still other methods include simultaneous deposition and dissolution using plasma delivered arc, laser or electron beam surface dissolution with or without predeposition heat treatment.

In the petroleum industry, mechanical gate valves are often used to treat various corrosive liquids under high hydraulic pressure. During operation of these valves, the gate must be moved relative to the valve seat very quickly under high mechanical forces to close the valve. This condition causes severe adhesion and corrosion wear on the metal surfaces of both the gate and valve seat, which can cause initial failure of the valve.

The use of mechanical gate valves with mating metal gates and corrosion resistant and corrosion resistant coatings applied to the valve sheet surface is a common occurrence in the petroleum industry. Due to the difference in the form of the substrate material and the wear mechanism, the coating applied to the gate and valve sheet is usually different. For example, explosive dry tungsten carbide-based coatings have been successfully used to protect metal gate surfaces against adhesive wear, and valve sheets are applied Ni-Cr-B-Si-Fe applied as superposition by conventionally known welding techniques. Protected by an alloy.

The problem with the combination of these specific coatings is that the valve sheet coating is incompatible with the many heat-treated and curable alloys useful as the substrate material. For example, conventional Ni-Cr-B-Si-Fe alloys are broken by cracking or spelling after heat treatment when applied as an overlay to a valve sheet made of AISI 410 stainless steel or AISI steel. This is due to a mismatch in the expansion rate between the substrate and the coating. Thus, there is a need to develop new coatings that can be used with more diverse substrate materials.

1 shows a group of curves showing the relationship between hardness, friction and adhesive wear and volume fraction of CrB particles in a coating according to the invention.

FIG. 2 is a bar graph depicting adhesive wear of various coatings paired against a conventional explosive tungsten carbide based coating.

3 (a) and 3 (b) to 7 (a) and 7 (b) show microstructures of sections, respectively, perpendicular and parallel to the surface of a typical CrB coating of the present invention prepared at different volume fractions of the hard face. 200X magnified micrograph showing.

8 (a), 8 (b) and 8 (c) are 200 × magnified micrographs showing the microstructure of a section perpendicular to the surface of a prior art coating.

In accordance with the present invention, there is provided a new family of chromium boride coatings with excellent adhesive wear and corrosion resistance and compatible with many alloy substrates. These coatings comprise hard, ultrafine chromium boride particles dispersed in a metal matrix, the particles comprising up to about 30 volume percent of the coating, with the remainder being the metal matrix. The atomic ratio of chromium metal to boron in the coating is between about 0.8 and 1.5. The metal matrix may be composed of a nickel or nickel-based alloy comprising a metal selected from the group consisting of chromium, silicon and iron.

The coating of the present invention comprises a process comprising depositing a mechanically mixed powder mixture of a chromium metal or a chromium alloy or both and a boron-containing alloy on a substrate and then heat treating the deposited coating in that state. Can be prepared by The heat treatment causes a diffusion reaction between the deposited elements, resulting in the formation of ultrafine particles of chromium boride dispersed in the metal matrix. The coating can be deposited onto the substrate using any conventional deposition technique already mentioned above.

Description of the preferred embodiment

The coating of the present invention is suitably applied to the substrate using a thermal spray process. In this process, namely plasma spraying, an electric arc is set between the nonconsumable electrode and the second nonconsumable electrode spaced therefrom. The gas is passed in contact with the non-consumable electrode to contain the arc. The arc-containing gas is compressed by the nozzles, resulting in a high thermal content outflow. The powdered coating material is sprayed with a high heat content outflow and deposited on the surface to be coated. The process and plasma arc torch used herein are described in US Pat. No. 2,858,411. The plasma spray process produces a rigid, dense and adhesive deposition coating on the substrate. In addition, the deposition coating consists of irregularly shaped fine splats or ribs that are connected to one another and to the substrate and are mechanically bonded.

Another method of applying the coating to the substrate is by detonation gun (D-gun) deposition. A typical explosive gun mainly consists of a water cooled barrel about 1 inch in diameter and several feet in length. In operation, a mixture of oxygen and fuel gas, for example acetylene, in a specific ratio (usually about 1: 1) is fed to the barrel with a filling of the powder to be coated. The gas is then ignited and the blast wave heats near or above the melting point, accelerating the powder to about 2400 ft / sec (730 m / sec). After the powder releases the barrel, a pulse of nitrogen purifies the barrel and prepares the system for the next explosion. The cycle then repeats several times a second.

The explosion gun deposits a circular coating on the substrate with each explosion. Circular coatings are typically about 1 inch (25 mm) in diameter and tens of thousands of inches (ie, a few microns) thick. Each circular coating consists of many overlapping extremely small splats corresponding to individual powder particles. Overlapping splats connect and bind to each other and the substrate without substantially alloying at their interface. The circular position in coating deposition is precisely controlled to produce a smooth coating of uniform thickness, to minimize substrate heat and to minimize residual stress in the applied coating.

In general, the powdered coating material used in the thermal spraying process will have essentially the same composition as the applied coating. However, changes in the composition at some thermal spray equipment can be expected and in this case the powder composition will be adjusted to achieve the desired coating composition.

Although the present invention has been described with reference to a coating made by a plasma arc spray process, it should be noted that any of the deposition techniques or similar techniques already described above may also be used.

According to the invention, the wear and corrosion resistant coatings are plasma sprayed with a mechanically mixed powder mixture of chromium metal or chromium alloys or mixtures of both and boron-containing alloys or alloys, followed by elevated temperatures, e.g. For example, it is applied to a metal substrate by heat treatment at a temperature of about 900 to 1100 ℃. At these temperatures, diffusion and chemical reactions occur between the thin overlapping plates deposited by the plasma spray process, some of which contain chromium metal components and others which include boron containing alloys or mixtures of alloys. These diffusion and chemical reactions lead to the formation of chromium boride (CrB) precipitates dispersed in the metal matrix. Precipitates are usually uniformly dispersed throughout the matrix, although they may aggregate in small clusters that are evenly distributed in the matrix. In essence, no reaction occurs between the powder particles during deposition so that the splats retain the initial powder composition prior to heat treatment.

The coating of the present invention can be prepared using two component systems, namely a first chromium metal or chromium alloy component and a second boron containing alloy component or alternatively a multicomponent system can be used. These multicomponent systems can include additional chromium metals or chromium alloys and can be used, for example, where it is desirable to use chromium metal in the alloy matrix to increase corrosion resistance.

The formation of a coating containing chromium boride precipitates in the metal matrix proceeds according to one of the following equations.

(1) Cr + (M ₁ -B) → CrB + M ₁

(2) (M ₂ -Cr) + (M ₁ -B) → CrB + (M ₁ -M ₂ )

(3) Cr + (M ₁ -B) + (M ₂ -Cr) → CrB + (M ₁ -M ₂ -Cr

Wherein M ₁ and M ₂ are at least one metal selected from the group consisting of nickel and, optionally, chromium, silicon, phosphorus, aluminum, manganese, cobalt and iron,

B is boron.

As mentioned above, the purpose of the metal M ₂ is to improve the properties of the matrix, for example to include additional chromium in order to improve the corrosion resistance.

In addition to the aforementioned elements, M ₁ and M ₂ may also contain small amounts of other elements such as carbon, oxygen and nitrogen.

The ratio of chromium metal to boron used in the powder mixture determines the volume fraction of chromium boride that precipitates in the metal matrix. In general, this ratio should be maintained in the range of about 0.8 to 1.5.

For best adhesive wear properties, the volume fraction of chromium boride precipitates in the coating will be maintained in the range of about 12-30 volume percent, suitably about 15-25 volume percent.

The coating may be applied when the elements in the boron containing alloy are maintained within the weight ratios below, ie, about 2.5 wt. % To about 10 wt. % Boron, 0 to about 25 wt. % Chromium, 0 to about 2 wt. % Manganese, 0 to about 2 wt. % Aluminum, 0 to about 1 wt. % Carbon, 0 to about 5 wt. % Silicon, 0 to about 5 wt. % Phosphorus, from 0 to about 2 wt. % Copper and 0 to about 5 wt. In the case of% iron and the remaining nickel, it can be prepared in a volume fraction of chromium boride within the above-mentioned range.

Most of the boron containing alloys can be used to prepare coatings according to the invention as long as these alloys not only satisfy the reaction conditions for one of the formulas (1)-(3) but also provide the desired elements in the metal matrix. Particularly suitable alloys for use in preparing the coatings according to the invention are shown in Table I below.

Table 1

Boron-containing alloys

Alloy No. Composition (% by weight)

Ni B Cr Si Fe

1 remaining 3 7 4 4

2 remaining 7.3 3.2 2.6

3 remaining 8.9 3.0 2.2 2.7

In general, the powder mixture used to prepare the coating has a particle size of less than about 200 mesh.

In the practice of the present invention, it is important to heat-treat the deposited coating in that state at a sufficiently elevated temperature, typically above 900 ° C., to be a fluid sufficient to promote the diffusion reaction. The heat treatment temperature may be higher than 900 ° C. if necessary, for example 1100 ° C., but this temperature should not be high enough to adversely affect the substrate. The deposited coating in that state must be maintained for a sufficient time at the heat treatment temperature to promote reaction and / or diffusion between the components of the coating. A limited but significant amount of diffusion reaction also occurs in the substrate.

The heat treatment of the coating is usually carried out in a vacuum or inert gas furnace. Alternatively, the heat treatment may be performed by surface deposition processes such as electron beams, laser beams, transferred plasma arcs, induction heating or other techniques, as long as the time at elevated temperatures is sufficiently short or a protective atmosphere is provided so that significant oxidation does not occur. Is achieved.

The coating of the present invention can be successfully applied to most types of substrates using conventional deposition techniques as described above. However, the substrate must be able to withstand the effects of heat treatment without any detrimental results. Suitable substrate materials which can be coated according to the invention are, for example, steel, stainless steel, iron alloys, nickel, nickel-based alloys, cobalt, cobalt-based alloys, chromium, chromium-based alloys, titanium, titanium-based alloys, refractory metals and Refractory metal-based alloys.

The coating according to the invention is AISI 4130 (composition is 0.3C, 0.5Mn, 0.2Si, 1.0Cr, 0.2Mo, remaining Fe) / 4140 (composition is 0.4C, 0.9Mn, 0.2Si, 1.0Cr, 0.2Mo, In the example when applied to a heat treated hardenable alloy substrate such as steel), for example, the volume fraction of the hard face may be equal to or greater than 20 percent. If the coating is applied to AISI 410 (composition is 12. 5 Cr, 0.15 (max) C, remaining Fe) stainless steel, the volume fraction of the hard face will remain below about 20 percent. Coatings having a volume fraction of CrB above these levels are not flexible enough to withstand the high internal stresses imposed by the expansion of the substrate. This is particularly annoying for some alloys, such as AISI 410, which undergo thermal expansion through martensite phase transformation.

In general, the thickness of the coating prepared according to the present invention will vary from about 0.005 to about 0.04 inches (0.1 to 1.0 mm).

The microstructure of the coatings of the present invention is rather complicated and not fully understood. However, studies to date have known that the coating comprises a hard face comprising ultrafine particles of chromium boride in the metal matrix. The metal matrix is intrinsically crystalline and fairly dense and softer and harder than the hard face.

Depending on the volume fraction of the hard face in the coating, the chromium boride particles may be dispersed in a nearly uniform manner throughout the matrix or aggregate into small clusters that are usually uniformly distributed within the matrix. In general, clusters of CrB particles form in the coating when the volume fraction reaches an upper limit of about 30 volume percent.

The size of the chromium boride particles will vary depending on several factors, including the heat treatment temperature and time. In general, the average particle size is atmospheric sub-micron, typically about 1.0 to about 1.0 micron.

The hardness of the coating changes in proportion to the volume fraction of the hard face. Therefore, by changing the atomic ratio of chromium metal to boron in the powder mixture, the hardness value can be tailored to a specific range. Generally, the hardness of the coating is from about 250 to about 700 DPH ₃₀₀ (HV.3).

An important advantage of the present invention is that diffusion reactions between chromium or chromium alloys and boron containing alloys occur at significantly lower heat treatment temperatures, for example 1000 ° C. The exact reason for this phenomenon is unknown, but it is believed to be due to the occurrence of high internal stresses and dislocations inside the ribs or layered splats deposited on the substrate by thermal spraying. In contrast, chromium boride is formed by conventional casing or hot press methods at very high temperatures, ie higher than about 1150 ° C. Their higher temperatures are usually harmful to most rivers. Due to the low heat treatment temperatures required for the present coating process, these substrates can now be coated without some detrimental results.

The following examples serve to further illustrate the practice of the present invention.

Example 1

A large number of CrB coatings were tested on low carbon AISI 1018 steel (the composition is 0.18 C and the remaining Fe) specimens of 1/2 × 3/4 × 2-3 / 4 inches (13 × 19 × 70 mm), 5/8 × 1 × 2 inch (16 × 25 × 51 mm) AISI 410 steel specimens, 1/2 × 1 × 2-3 / 4 inch (13 × 25 × 70 mm) trademarked by Inconel 718 (International Nickel Company) .The composition is 19Cr, 3.0 Mo, 5.1 Nb, 0.9 Ti, 0.5 Al, 18.5 Fe, 0.08 C max, remainder Ni) superalloy specimens, on AISI 4130 alloy steel specimens of 1/2 × 1 × 2-3 / 4 inch (13 × 25 × 70 mm) Prepared by plasma spraying a powder mixture of an alloy of nickel-20 chromium and alloy No. 2 to a thickness of about 0.020 inch (0.5 mm). The Cr to B atomic ratio in the powder mixture is about one. The deposited coating in that state is either vacuum or heat treated in an arcono at a temperature of about 970 to 1020 ° C. for one hour, followed by a subsequent heat treatment depending on the substrate material. The deposited coating as it has a clear porosity of less than about 0.5 percent. In the heat treated coating, very fine CrB precipitates are uniformly dispersed throughout the Ni—Cr—Si—Fe matrix. The intermediate diffusion region of the coating / substrate has a thickness of about 30 to 40 micrometers.

A series of heat treatment experiments were carried out on coated specimens in a horizontal furnace with an oil quench unit and a 10 cfh static flow of argon gas. The heat treatment applied to the coating / substrate system is shown approximately in Table 2 below.

TABLE 2

Heat treatment plan

Coating / 410 SS

(1) heat treatment at 1000 ° C./hour/arcon, furnace cooling at 940 ° C.,

Maintained at 940 ° C / 15 min / argon, fan cooled in Arcon.

(2) Tempering, oil quenching and at 700 ° C / 45 minutes / argon

Temper, oil quench at 685 ° C / 45 minutes / argon.

Coating / Inconel 718

(1) Heat treatment at 1000 ° C./45 minutes / argon, fan cooling at argon.

(2) Aging at 700 ° C./4 hours / argon, fan cooling at argon.

Coated / 4140

(1) Heat treatment at 1000 ° C / 45 minutes / argon, quench oil.

(2) Tempering at 600 ℃ / hr / argon, oil quenching.

Or temper, oil quench at 450 ° C./hr/argon.

Or tempered, oil quenched at 350 ° C./hr/argon.

In Table 2 above, the first heat treatment step 1 promotes the diffusion reaction in the coating, while the second heat treatment step 2 obtains the desired mechanical properties of the substrate.

Metallic experiments and penetration techniques are used to show any defects in the coating or substrate after completion of the heat treatment cycle. Except for the coating on AISI 410 stainless steel which is supposed to be cracked, the coating is found to be essentially unaffected by the second heat treatment.

The following experiments show that the same coating of 410 stainless steel is made into a crack-free coating by adjusting the heat treatment plan. However, this improvement requires very precise control of the heat treatment which is not practically suitable in the manufacturing process.

Example 2

A number of CrB coatings were coated on powders of nickel-20 chromium and alloy No. 1 in a thickness of about 0.020 inch (0.5 mm) on AISI 410 stainless steel specimens of 5/8 × 1 × 2 inches (16 × 25 × 51 mm). Is prepared by plasma spraying. The mixing formula is Alloy No. 1 + 39.3 (Ni-20 Cr). All compositions are hereafter expressed in weight percent, eg, 60.7 wt.% Alloy no. 1 + 39.3 wt.% (Ni-20 Cr) Equal to 1 + 39.3 (Ni-20 Cr). The Cr to B atomic ratio in the powder mixture is about 1.4. The deposited coating in that state is either vacuum or heat treated in an arcono at a temperature of about 970 to 1020 ° C. for one hour. The coating consists of CrB precipitates uniformly dispersed throughout the Ni-Cr-Si-Fe matrix.

The volume fraction of CrB precipitates in these coatings is about 15.5 volume percent. This is less than the volume percent of precipitate in the coating of Example 1.

The coating prepared in this example is subjected to the same heat treatment plan for the AISI stainless steel substrate as shown in Table 2. After the heat treatment, the coating was tested and found to be free of cracks and defects, indicating that the particular coating is compatible with the 410 stainless steel substrate. The hardness of these CrB coatings is about 340 DPH ₃₀₀ (HV.3). This is less than the hardness of the coating prepared in Example 1, but the coating in this example is more flexible and deformation resistant.

Example 3

A number of CrB coatings are coated on chromium metal or nickel-20 chromium with a thickness of approximately 0.020 inch (0.5 mm) on 3/4 × 1/2 × 2-1 / 2 inch (19 × 13 × 64 mm) AISI 1018 steel specimens. The boron-containing alloy powder mixture was prepared by plasma spraying. The powder mixture is based on the formula of stoichiometric CrB in the coating such that the calculated chromium boride volume fraction varies from about 13.4 to 42.6 percent. The mixing formula is

(1) Alloy No. 1 + 50 (Ni-20 Cr)

(2) Alloy No. 1 + 39.3 (Ni-20 Cr)

(3) Alloy No. 2 + 56 (Ni-20 Cr)

(4) Alloy No. 3 + 35 (Ni-20 Cr) + 15 Cr

(5) Alloy No. 3 + 30 Cr

The deposited coating in that state is either vacuum or heat treated for one hour at a temperature of about 960 to 1020 ° C. in arcon and then oil quenched. The coating consists of fine CrB precipitates in the Ni—Cr—Si—Fe matrix. The calculated volume fractions of hard faces in the coatings secreted from each formula (1) to (5) are 13.4, 15.5, 19.7, 32.5 and 42.6, respectively.

The hardness of these CrB coatings varies from about 280 to 740 DPH ₃₀₀ (HV.3).

For comparison, a number of coatings were made from conventional alloy powders referred to herein as C1, braze alloy (alloy No.1) and C2 prepared by plasma spraying alloy powders on the same AISI 1018 steel specimen, and the same as described above. The coating in that state deposited by the method is heat treated. Coatings made from other conventional alloy powders (Ni-Cr-B-Si-Fe), referred to herein as C3, are applied on steel specimens using standard weld deposition techniques.

Table 3 below lists the compositions of all coatings.

As can be seen from Table 3, the compositions of the coatings prepared from the mixing formulas (2) and (3) correspond very closely to conventional coatings, in particular coating C3. Although the compositions of the coatings prepared according to the invention are similar to those of conventional coatings, the microscopic organization of these coatings differs greatly.

The friction wear properties of the CrB coatings prepared above were determined using the standard dry sand / rubber wheel friction test described in ASTM Standard G 65-80, Procedure A. In this test, the coated specimen is mounted by a lever arm to a rotating wheel with a chlorobutyl rubber rim around the wheel. A friction material (ie 50-70 mesh Ottawa Silica Sand) is sandwiched between the coating and the rubber wheel. Rotate the wheel in the direction of the friction material flow. The test specimens are weighed before and after the test and these weight losses are recorded. Due to the wide difference in density of the various materials tested, the mass loss is normally converted to volume loss and determines the relative ranking of the materials. Coating a mean volume loss according to the present invention is in the range of 5 to 50 mm ^3/1000 rotation. Volume loss is found to decrease when increasing the volume fraction of the hard face in the coating.

CrB coatings are also corrosion tested. These tests were performed according to standard procedures using alumina particles with a particle velocity of about 91 meters / sec and a size of 27 microns at two impact angles of 90 and 30 degrees. It can be seen that the average corrosion rate is about 60 to 120 and 30 to 37 μm / gm, respectively.

Dry adhesive wear resistance of both chromium boride and conventional coatings is evaluated using a block-on-ring (alpha) tester.

The coating ring with the explosion gun (W, Cr) C-Co manufactured by Union Carbide of UCAR (Union Carbide Corporation) LW-15 is rotated against a fixed block coated with a test coating. The test conditions were determined at room temperature dry air with 80 degree vibration, 2000 cycles, 164 Kg (360 1bs) normal load and 18 m / min (60 ft / min) rotation speed. Adhesive wear resistance of the coating is determined by measuring volume loss based on the measurement of wear on the block, track length and width and weight loss on the ring. In the case of a 1000 cycle test, the coating prepared with the mixing formulas (1) to (3) shows a weight loss of about 1.3 mm ³ and a conventional coating shows a weight loss of 2.0 mm ³ . In the 2000 cycle test, each weight loss is 1.4 to 1.9 and 1.8 to 3.4 mm ³ .

Table 4 shows the metallic rating, sand friction, corrosion resistance and adhesive wear resistance of all tested coatings.

The group of curves in FIG. 1 shows the relationship between the hardness, friction and adhesive wear and chromium boride volume fraction of coatings prepared according to the invention. The curve is based on the average value of the test results obtained on the various CrB coatings prepared in this example. First, it can be seen that the hardness of the coating is linearly proportional to the CrB volume fraction. The sand friction wear rate of the coating is shown by curve A. The sand friction wear rate is nonlinear and changes inversely to the volume fraction of chromium boride. Adhesive wear at 1000 cycles is represented by curve B and curve C for 2000 cycles. Adhesive friction wear increases nonlinearly with an increase in boride in the coating. The coating shows higher adhesive wear rates when tested at 2000 cycles. It can also be seen that the minimum volume loss occurs in coatings with chromium boride volume fractions between about 12 and 30 percent. Coatings with volume fractions greater than 30 percent show a large increase in volume loss.

The bar graph of FIG. 2 shows a comparison of volume loss between conventional alloy coatings and chromium boride coatings against mating UCAR LW-15 coatings. CrB coatings M2, M3 and M4, which show preparations from mixing formulas (2), (3) and (4), respectively, are superior to, or comparable to or better than, conventional alloy coatings (C1) and C2. . The volume loss of LW-15 when paired against a CrB coating is 3 to 10 times less than that paired with conventional alloy coatings.

The microstructure of the sections parallel and perpendicular to the surface of the series of chromium boride coatings made from the mixing formulas (1) to (5) is shown in FIGS. 3 (a) and 3 (b) to 7 (a) and 7 (b). ) Is shown. The volume fraction of chromium boride in the coatings prepared from these mixing formulas (1) to (5) ranges from 13.4 to 42.6%.

In all micrographs, C refers to the coating, S refers to the substrate, dark areas refer to precipitates and bright areas refer to the matrix.

The microstructure of the section perpendicular to the surface of the coating was mixed with a volume fraction of Cr 13.4, 15.5 and 19.7% Cr, as shown in FIGS. 3 (a), 4 (a) and 5 (a). In the case of coatings made from formulas (1), (2) and (3), it is shown that the precipitate of chromium boride is uniformly dispersed throughout the matrix. The microstructure of the coatings made from the remaining mixing formulas (4) and (5) is that the precipitate of chromium boride aggregates as layered clusters distributed throughout the matrix as shown in FIGS. 6 (a) and 7 (a). Is shown. These coatings have CrB volume fractions of 32.5 and 42.6 percent, respectively.

Sections of the coating parallel to the surface are generally exposed to abrasion environments. Therefore, coating microstructures in sections parallel to the surface are expected to have a significant impact on the wear behavior of certain coatings.

3 (b) to 7 (b) show the microstructure of the sections parallel to the surface of the coating made from the mixing formulas (1) to (5), respectively, and are identical to those occurring in the section perpendicular to the surface of the coating. Representation of the form is shown fundamentally. For coatings made from mixing formulas (1), (2) and (3) having a CrB volume fraction of 1.34, 15.5 and 19.7%, FIGS. 3 (b), 4 (b) and 5 (b) As shown, the precipitate of chromium boride which is substantially uniform throughout the matrix is shown. In the case of coatings made from the remaining mixing formulas (4) and (5), the precipitates aggregate into clusters that are uniformly distributed throughout the matrix as shown in FIGS. 6 (b) and 7 (b). Each of these coatings has a volume fraction of at least 30 percent.

For comparison, the microstructure of the sections perpendicular to the conventional plasma sprayed and heat treated coatings (C1) and (C2) and the weld deposited coating (C3) are shown in FIGS. 8 (a), 8 (b) and 8, respectively. shown in (c). Since these conventional alloy coatings are made using prealloyed powder, the microstructure of the sections parallel to the surface of each coating is expected to be the same as that of the sections perpendicular to the surface. For the coating (C1), considerably high boron and low chromium contents lead to formation of very fine Ni ₃ B as the main hard face. For the coating C2, the chromium boride precipitate is needle-shaped as shown in Fig. 8B. For the weld deposited coating (C3), the CrB precipitate is agglomerates with a particle size of about 3 micrometers.

The shape and particle size of the chromium boride precipitates were also tested in sections parallel to the surface of the CrB coating by scanning electron microscopy (SEM). It is confirmed that both the form and the particle size of the chromium boride precipitates depend on the formation mechanism. Coatings made of two powder components, namely low molten boron-containing nickel-based alloys and nickel-20 chromium or chromium metal, are more uniform in precipitate than coatings made of three components, namely boron-containing alloys, nickel-20 chromium and chromium metal Has a distribution. For coatings containing 13.4, 15.5 and 19.7% CrB volume fractions, the diffusion reaction between boron from low molten nickel-based alloys and chromium in Ni-20 chromium solid solution is about 0.5 micrometers in average size (length at load). Or on small plates, a rod or plate-shaped CrB precipitate.

In a coating made of two powder components using the mixing formula (5) and having a CrB volume fraction of 42.6%, the diffusion reaction between boron and pure chromium from the low melting alloy is characterized by agglomeration CrB with a particle size of 1 to 5 micrometers. Causes formation of precipitates. In coatings made of three powder components using the mixing formula (4) and having a CrB volume fraction of 32.5%, the precipitate formation is controlled by both instruments as described above. Therefore, microplate-like CrB precipitates are formed in a matrix between boride clusters containing agglomerate precipitates having a particle size of 1 to 5 micrometers.

According to the present invention, it is possible to make a chromium boride coating having excellent wear resistance and corrosion resistance.

Claims (10)

As a wear and corrosion resistant coating on a substrate, The coating comprises multiple, thin, irregularly shaped splats superimposed and bonded to each other and bonded to the substrate, the splats comprising hard, ultrafine chromium boride particles dispersed in a metal matrix, the particles A coating having a particle size less than about 1 micron of the coating and consisting of less than about 30 volume percent of the coating, the remaining of which is a metal matrix. The coating of claim 1, wherein the chromium boride particles consist of about 12 to 30 volume percent of the coating. The coating of claim 1, wherein the average size of the particles ranges from about 0.1 to 1.0 micron. The coating of claim 1 wherein the metal matrix is a nickel-based alloy containing a metal selected from the group consisting of chromium, silicon, phosphorus, aluminum, manganese, cobalt and iron. The method of claim 1, wherein the substrate is composed of steel, stainless steel, iron alloy, nickel, nickel alloy, cobalt, cobalt alloy, chromium, chromium alloy, titanium, titanium alloy, refractory metal and refractory metal alloy. Coating, which is a material selected from the group. A method for producing a wear and corrosion resistant coating on a substrate, Depositing on the substrate a mechanically mixed powder mixture of two or more components comprising a first component comprising chromium and a second component comprising a boron containing alloy, followed by diffusion reaction between the deposited elements Heat-treating the deposited coating in that state at an elevated temperature sufficient to cause the formation of chromium boride particles dispersed in the metal matrix. The method of claim 6, wherein the mechanically mixed powder mixture is deposited on the substrate by plasma spray. The boron-containing alloy of claim 6, wherein the boron-containing alloy is about 2.5 wt. % To about 10 wt. % Boron, 0 to about 25 wt. % Chromium, 0 to about 2 wt. % Manganese, 0 to about 2 wt. % Aluminum, 0 to about 1 wt. % Carbon, 0 to about 5 wt. % Silicon, 0 to about 5 wt. % Phosphorus, from 0 to about 2 wt. % Copper and 0 to about 5 wt. % Iron, the rest is nickel. As a composition for preparing a coating, The coating comprises a mechanically mixed powder mixture of two or more components comprising a first component comprising chromium and a second component comprising a boron containing alloy, wherein the atomic ratio of chromium to boron in the mixture is about A composition that is between 0.8 and 1.5. 10. The composition of claim 9, wherein the amount of chromium and boron containing alloy used in the mixture is adapted to include about 12 to 30 volume percent chromium boride particles in the coating.