KR100447289B1 - Titanium carbide/tungsten boride coatings - Google Patents

Abstract

The present invention provides a new class of titanium carbide / tungsten boride coating films having excellent wear resistance and corrosion resistance. These coatings comprise hard ultra-titanium carbide particles and tungsten boride precipitates dispersed in the metal matrix, with two phases constituting about 30 to 80% by volume of the coating film and the remainder being the metal matrix. The metal matrix comprises one or more metals selected from the group consisting of nickel, cobalt and iron. The coating film is a step of depositing a mechanically mixed powder mixture consisting of individual components comprising a first component containing tungsten carbide, a second component containing boron, and at least one metal selected from the group consisting of nickel, cobalt and iron And it may be prepared by a process comprising the step of heat-treating the coating film of the deposited state, wherein the powder mixture comprises titanium in the first component or the second component or in a separate third component, the first component, At least one of the bicomponent or third component has a melting point of less than about 1200 ° C. The heat treatment causes a diffusion reaction between the deposited elements resulting in the formation of ultrafine titanium carbide particles and tungsten boride precipitates dispersed in the metal matrix. Such coatings may be deposited on a substrate using any conventional deposition technique.

Description

Titanium Carbide / Tungsten Borosilicate Coating Film {TITANIUM CARBIDE / TUNGSTEN BORIDE COATINGS}

The present invention relates to a titanium carbide / tungsten boride coating film having excellent corrosion resistance and abrasion resistance and a method for producing such coating film. In particular, the present invention relates to a hard, dense, low porosity, corrosion resistant and wear resistant coating film containing ultrafine titanium carbide particles and tungsten boride precipitates dispersed in a metal matrix. The present invention further relates to a method for producing such coatings by thermal spraying and diffusion reaction techniques.

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

Cutting tools are usually made of tungsten carbide-cobalt alloys. These alloys are very hard, strong and strong, exhibiting good wear under most conditions of use. However, a problem with these alloys is that tungsten carbide is susceptible to oxidation at temperatures above about 540 ° C. When operated at such high temperatures for a certain period of time, cutting tools made of these alloys are subject to reduced wear and frequent cracks, spars or chips.

Chemical vapor deposition techniques have been used to improve the wear and oxidation resistance of tungsten carbide-cobalt cutting tools by depositing a thin layer of titanium diboride (TiB ₂ ) on the surface of the part. Due to the interaction between TiB ₂ and WC / Co at high temperatures, a thin thin film of less than 30 microns is formed on the surface of the cutting tool containing CoWB and TiC compounds. Titanium carbide is more stable because of higher oxidation resistance than tungsten carbide. Thus, the formation of a film comprising these compounds increases the wear resistance of the cutting tool.

Deposited films containing CoWB and TiC are particularly limited in use with only a few substrates, in particular with tungsten carbide-cobalt alloys. Therefore, it is advantageous to develop a TiC / WCoB coating film that can be applied to various substrate materials.

Figure 1 is a 220X magnified micrograph showing a coating in a typical deposited state in accordance with the present invention.

2 is a 440 × magnified micrograph showing a heat treated coating film according to the present invention.

3 is a 3500 × magnified micrograph of a scanning electron microscope (SEM) showing in greater detail the microstructure of a typical coating film according to the present invention.

4 is a group of curves comparing the weight increase of the coating film prepared according to the present invention and the conventional coating film when exposed to an oxidizing environment.

According to the present invention, there is provided a class of titanium carbide / tungsten boride coating films having excellent abrasion resistance and corrosion resistance and which are compatible with various substrate materials. These coatings comprise hard ultra-titanium carbide particles and tungsten boride precipitates dispersed in the metal matrix, and the two phases constitute about 30 to 80% by volume of the coating film. Such coatings have a hardness of about 700 to 1200 DPH ₃₀₀ (HV.3) and can withstand temperatures up to about 800 ° C.

The coating film of the present invention is mechanically mixed with individual components, comprising at least one of a first component containing tungsten carbide, at least one of a second component containing boron, and at least one metal selected from the group consisting of nickel, cobalt and iron. The powder mixture may be prepared by a method comprising depositing a powder mixture on a substrate and heat-treating the coating film in the deposited state, wherein the powder mixture is applied to the first component or the second component, or to a separate third component. Titanium, and at least one of the first, second, or third components has a melting point of less than about 1200 ° C. The heat treatment causes a fusion reaction between the deposited elements to form titanium carbide and tungsten boride of ultrafine particles dispersed in the metal matrix. The coating film may be deposited on the substrate using any conventional deposition technique or similar technique already mentioned above.

In a preferred embodiment of the invention, coating films containing titanium carbide and tungsten boride precipitates are applied to various substrate materials.

Description of the preferred implementation

The coating film of the present invention is suitably applied to a substrate using a thermal spray process. In this process, ie in plasma spraying, an electric arc is achieved between the non-consumable electrode and the second non-consumable electrode spaced therefrom. The gas passes in contact with the non-consumable electrode and contains an arc. The arc containing gas is compressed by the nozzle to form a high heat containing effluent. A powdered coating film material is sprayed onto the high heat containing effluent and deposited on the surface to be coated. This process and the plasma arc torch used therein are disclosed in US Pat. No. 2,858,411. The plasma spray process forms a rigid, dense, adhesively deposited coating film on the substrate. In addition, the deposition coating film is composed of irregular microscopic splats or ribs entangled and mechanically bonded to each other and to the substrate.

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, such as 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 explosion wave heats up to or above the melting point, accelerating the powder to about 2400 ft / sec (730 m / sec). After the powder is released from the barrel, a pulse of nitrogen purges the barrel, waiting 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. The circular coating is about 1 inch (25 mm) in diameter and tens of thousands of inches (ie, several microns) thick. Each circular coating film consists of many overlapping ultra-fine splats corresponding to individual powder particles. Overlapping splats are intertwined with one another and with the substrate substantially without alloying at their interfaces. The circular position in coating film deposition is precisely controlled to form a smooth coating film of uniform thickness and to minimize substrate heat and residual stress in the applied coating film.

In general, the powder coating material used in the thermal spray process will have substantially the same composition as the coating applied. However, for some thermal spray equipment, changes in composition can be expected. In this case, the powder composition will be adjusted accordingly to achieve the desired coating film composition.

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

According to the present invention, the wear resistant and corrosion resistant coating film contains individual components, including a first component containing tungsten carbide, a second component containing boron, and at least one metal selected from the group consisting of nickel, cobalt and iron. Is applied onto the metal substrate by plasma spraying the mechanically mixed powder mixture. The powder mixture also comprises titanium in the first or second component, or in a separate third component. The first component, second component or third component, preferably the second component containing boron, should have a melting point of less than about 1200 ° C. Thereafter, the deposited coating film is heat treated at a high temperature sufficient to melt the components in the powder mixture. At these temperatures, diffusion and chemical reactions occur between the thin overlapping splats deposited by the plasma spray process, some of which contain or are titanium containing tungsten carbide components, and others that contain or are not titanium. Containing boron-containing alloys, and some containing titanium when none of the above-mentioned components contain titanium. These diffusion and chemical reactions replace tungsten with titanium on the carbide and cause a reaction between tungsten and boron to form boride precipitates because the affinity of carbon for titanium is greater than for tungsten. Titanium carbide particles and tungsten boride precipitates are uniformly dispersed in the metal matrix. Substantially no reaction occurs between the powder particles during deposition, so the splats prior to heat treatment maintain the initial powder composition.

The coating film of the present invention uses a two-component system, that is, a first tungsten carbide component and a second boron-containing alloy component, wherein either the first component or the second component contains titanium or both contain titanium. It may be prepared, or alternatively, a multicomponent system may be used. This multicomponent system is used when titanium is not used in either component. Multicomponent systems can also be used in situations where it is desirable to include additional elements in the alloy matrix.

The formation of a coating film containing titanium carbide and tungsten boride precipitate may proceed according to one of the following formulas:

(1) (W, Ti) C-M ₁ + (M ₂ -B) → WM ' ₁ B + TiC + (M ₂ -M ₁ ")

(2) WC-M ₁ + Ti + (M ₂ -B) → WM ' ₁ B + TiC + (M ₂ -M ₁ ")

(3) WC-M ₁ + Ti-B + (M ₂ -B) → WM ' ₁ B + TiC + (M ₂ -M ₁ ")

Wherein M ₁ and M ₂ are at least one metal selected from the group consisting of nickel, cobalt and iron, and optionally other metals or metal alloys,

B is boron,

M ₁ is M ′ ₁ + M ₁ ″.

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

The proportion of titanium, tungsten, carbide and boron used in the powder mixture determines the volume fraction of both titanium carbide and tungsten boride that precipitates in the metal matrix. In general, the ratio of tungsten to boron should be maintained in the range of about 0.4 to 2.0. The ratio of titanium to carbon is about 1.0.

For best wear and corrosion, the volume fraction of titanium carbide and tungsten boride precipitates in the coating film should be maintained in the range of about 30 to 80% by volume. Typically, the volume fraction of the titanium carbide particles is about 15-30 vol% and the volume fraction of the tungsten boride precipitates is about 30-50 vol%.

If the coating film is maintained in the following weight ratios of the elements in the boron-containing alloy, that is, from about 3% to about 20% by weight of boron, from 0 to about 10% by weight of molybdenum, from 0 to about 20% by weight of chromium, from 0 to About 5 weight percent manganese, 0 to about 5 weight percent aluminum, 0 to about 1 weight percent carbon, 0 to about 5 weight percent silicon, 0 to about 5 weight percent phosphorus, 0 to about 5 weight percent It has been found that in the case of copper and 0 to about 5% by weight of iron, the remainder of nickel, cobalt or iron or combinations thereof, it can be produced in a volume fraction of tungsten boride within the above-mentioned ranges.

Most boron containing alloys can be used to make coatings according to the present invention as long as these alloys meet the requirements of the diffusion reaction. Particularly suitable alloys for use in preparing the coatings according to the invention are described in Table I below.

Table 1

Boron-containing alloys

Alloy No. Composition (% by weight)

Ni Cr Si B Fe C

1 Rest 13-17 3-5 2.75-4 3-5 0.6-0.9

2 remainder 6-8 3-5 2.5-3.5 2-4 0.5-Max

3 Remainder 3-4 3-5.4 8.8-10.8 2-3.5 0.32-Max

4 remaining 4.5-6 2.3-4 5.6-7 1.5-3.8 0.41-Max

5 remaining 3.2 2.5 6

It goes without saying that if the titanium containing alloy or compound is bonded with boron, for example TiB ₂ , the boron containing alloy may not be needed.

In practicing the present invention, it is important to heat-treat the coating film in the deposited state at a sufficiently elevated temperature, typically at 1000 ° C., in which the boron-containing alloy can become a fluid sufficient to promote the diffusion reaction. The heat treatment temperature may be considerably higher than 1000 ° C., for example 1200 ° C. if necessary, but this temperature should not be high enough to adversely affect the substrate. The coating film in the deposited state should be maintained at the heat treatment temperature for a time sufficient to promote reaction and / or diffusion between the components of the coating film. Although limited, significant diffusion reactions also occur in the substrate.

Heat treatment of the coating film is generally carried out in a vacuum furnace or an inert gas furnace. Alternatively, the heat treatment may be applied to surface deposition processes such as electron beams, laser beams, transfer plasma arcs, induction heating, or other techniques, as long as the time at high temperatures is sufficiently short or a protective atmosphere is provided that no significant degree of oxidation occurs. Is achieved by

An advantage of the present invention is that it can be successfully applied to various substrate types using conventional deposition techniques or similar techniques as described above. However, the substrate must be able to withstand the effects of heat treatment without deleterious effects. Suitable substrate materials which can be coated according to the invention include, 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 film of the present invention is most advantageously applied to substrates of carbon steel, stainless steel and superalloys (eg iron, nickel and / or cobalt based alloys).

The thickness of the coating film prepared according to the present invention is generally about 0.005 to about 0.040 inches (1.3 × 10 ² to 1.0 × 10 ³ ㎛).

The microstructure of the coating film of the present invention is somewhat complicated and not fully understood. However, studies to date have shown that the coating film essentially comprises two separate hard phases, including ultrafine titanium carbide particles and tungsten boride precipitates dispersed in the metal matrix. The metal matrix is in fact crystalline, fairly dense and softer than the hard phase and has low permeability.

The size of the titanium carbide particles and the tungsten boride precipitate will depend on several factors including the heat treatment temperature and time. However, the average particle size is usually less than microns, typically about 0.5 to about 3.0 microns.

In general, the hardness of the coating film is directly proportional to the volume fraction of the hard phase. For example, by varying the atomic ratio of tungsten to boron in the powder mixture, the hardness value can be tailored to a specific range. In general, the hardness of the coating film is about 800 DPH ₃₀₀ (HV.3).

An important advantage of the present invention is that the diffusion reaction between tungsten and boron containing alloys takes place at significantly lower heat treatment temperatures, for example about 1000 ° C. The exact reason for this phenomenon is unknown, but is believed to be due to the formation of dislocations and high internal stresses inside the ribs or overlapping splats deposited on the substrate by thermal spraying. In contrast, metal borides and carbides are generally formed by conventional coatings or hot pressing at very high temperatures, i. Their higher temperatures are usually not beneficial for most rivers. With the low heat treatment temperatures required in this coating process, these substrates can now be coated without any deleterious effects.

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

Example 1

A plurality of TiC / W ₂ CoB ₂ coating 3/4 × 1/2 × 2-3 / 4 inchi (19 × 13 × 70mm) of AISI 1018 steel (low carbon steel (about 0.18C and the remainder Fe) Im) onto the specimen A powder mixture of tungsten carbide cobalt alloy, titanium diboride (TiB ₂ ) and Alloy No. _{2 at} a thickness of about 0.020 inch (0.5 mm) was prepared by plasma spraying. The mixing formula was 50% by weight of (WC-10 Co) + 10% by weight of TiB ₂ + 40% by weight of Alloy No. 2. The atomic ratio of W to B was about 1. The polished cross section of the coating film in the deposited state is shown in FIG. The coating film has a layered structure composed of irregular shaped splats firmly attached to each other and to a steel substrate. The splat was formed by impinging WC-Co, TiB ₂ and Alloy No. 2 on a substrate in a molten or semi-fused state. Thereafter, the deposited coating film was heat-treated under vacuum or argon at a temperature of about 1000 to 1075 ° C. for one hour. 2 shows the structure of the coating film after the heat treatment. As shown, the coating film is composed of a main coating film and an intermediate diffusion region formed by a diffusion reaction between the coating film material and the substrate. The intermediate diffusion region has a finger-like iron-boride phase scattered along the diffusion region / substrate interface and is about 50 to 60 탆 in width. The main coating film contains a plurality of fine particles uniformly distributed in the Ni-Cr-Si-Fe matrix. Particles were identified as TiC and W ₂ CoB ₂ phases by X-ray diffraction and EDX analysis. TiC and W ₂ CoB ₂ phases are formed by substituting W in Ti and reacting W and Co with B during diffusion and chemical reaction between WC-Co and TiB ₂ splats because of the affinity of Ti for C This is because the affinity of W for C is greater. In FIG. 3, scanning electron micrographs show TiC and W ₂ CoB ₂ with a particle size of less than 1 μm. The W ₂ CoB ₂ phase exhibits characteristic bright contrast, while the TiC phase exhibits dark contrast dispersed in the matrix.

Metallurgical examination of the coating film showed that the apparent porosity was 0.5 to 0.75% and the hardness was 700 to 1100 DPH ₃₀₀ (HV.3).

The frictional wear of the TiC / W ₂ CoB ₂ coating film prepared above was measured using the standard dry sand / rubber wheel friction test described in ASTM Standard G 65-80, Procedure A. In this test, the coated specimen was mounted by a lever arm to a rotating wheel having a chlorobutyl rubber rim around the wheel. A friction wear material (ie 50-70 mesh Ottawa Silica Sand) was placed between the coating film and the rubber wheel. The wheel was rotated in the flow direction of the friction wear material. Test specimens were weighed before and after the test and the weight loss was recorded. Due to the large difference in the density of the different materials tested, the mass loss is usually converted to volume loss to evaluate the relative rank of the materials. The mean volume loss for a given coating film specimen was about 1.4 mm ^3/1000 rotation.

In addition, a corrosion test was performed on the TiC / W ₂ CoB ₂ coating film. This test was performed according to standard procedures using alumina particles having a particle velocity of about 91 m / sec and a size of 27 microns at two impact angles of 90 and 30 degrees. Corrosion rates were found to be about 128 and 26 μm / g, respectively. The friction and corrosion resistance of the present coating film is considered to be superior to other conventional coating films.

In another series of tests, two TiC / W ₂ CoB ₂ coatings of different composition were subjected to oxidation measurements and compared with conventional tungsten carbide based coatings. The amount of oxidation of each sample coating film was determined by measuring the weight increase (µg / cm ² ) and the exposure time in the oxidizing environment. The test results are shown in FIG. For example, it can be seen that conventional WC coating films exhibited significant weight gains in most cases when heated to a temperature of 650 ° C. as shown by curve A. FIG. TiC / W ₂ CoB ₂ prepared from powder mixing formula P1 comprising 50 wt% (WC-10 Co) + 8 wt% TiB ₂ + 42 wt% Alloy No. ₂ as shown by curve B The coating film showed a slightly lower weight gain. However, TiC / W ₂ prepared from powder mixing formula P2 comprising 50 wt% (WC-10 Co) + 10 wt% TiB ₂ + 40 wt% Alloy No. ₂ as shown by curve C. There was little weight increase in the CoB ₂ coating. This coating had little increase in weight even when exposed to temperatures above about 900 ° C.

Example 2

A number of TiC / WCoB coatings are tungsten carbide cobalt alloy with a thickness of about 0.020 inches (0.5 mm) on 3/4 × 1/2 × 2-3 / 4 inches (19 × 13 × 70 mm) AISI 1018 steel specimens. The powder mixture of titanium diboride (TiB ₂ ), cobalt and alloy No. 2 was prepared by plasma spraying. Additional cobalt was used in the powder mixture to favor the formation of WCoB rather than W ₂ CoB ₂ , as in Example 1. The mixing formula was 50 wt% (WC-10 Co) + 10 wt% TiB ₂ + 20 wt% Alloy No. 2 + 20 wt% Co. The atomic ratio of W to B was about 1. The deposited coating film was heat-treated under vacuum or argon at a temperature of about 1050 to 1075 ° C. for one hour. After the heat treatment, the coating film was cooled and irradiated. The coating film had a layered structure of splats containing TiC and WCoB precipitates dispersed in a Ni—Cr—Si—Fe matrix. The size of the precipitate was less than about 1 micron.

The TiC / WCoB hardness ranged from 700 to 1100 DPH ₃₀₀ (HV. 3).

Friction and corrosion properties of the coatings were measured using the same test procedure described in Example 1. Sand frictional wear rate of the coating film thereof was about 1.9 mm ^3/1000 rotation. Corrosion wear rates for alumina particles at two impact angles of 90 degrees and 30 degrees were found to be about 30 and 130 μm / g, respectively. The frictional abrasion and corrosiveness of these coating films were considered to be good or excellent.

Example 3

Multiple TiC-rich TiC / W ₂ NiB ₂ / WC / WC ₂ coatings are approximately 0.020 inch on 3/4 x 1/2 x 2-3 / 4 inch (19 x 13 x 70 mm) AISI 1018 steel specimens A powder mixture of tungsten-titanium carbide-nickel alloy and alloy No. 5 to a thickness of (0.5 mm) was prepared by plasma spraying. The mixing formula was Alloy No. 5 of 40 wt% of (W, Ti) C-Ni + of 60 wt%. The deposited film was heat-treated under vacuum or argon at a temperature of about 1045 ° C. for one hour and then cooled. The coating film had a layered structure of fine precipitates of TiC and W ₂ NiB ₂ dispersed between WC or WC ₂ particles in a Ni—Cr—Si—Fe matrix.

The hardness of the coating film was about 900 DPH ₃₀₀ (HV.3).

In addition, the tribological properties of the coating films were measured using the standard dry sand / rubber wheel test described in Example 1. The average wear rate was approximately 1.3 mm ^3/1000 rotations.

Corrosion wear rates for alumina particles (feed rate of 1.2 g / min, speed of 91 m / sec) at two impact angles of 90 degrees and 30 degrees were about 30 and 126 μm / g, respectively. The corrosiveness of these coating films is also considered to be very good to excellent.

According to the present invention, a titanium carbide / tungsten boride coating film having excellent abrasion resistance and corrosion resistance is provided. In addition, coating films may be applied to various types of substrates using conventional deposition techniques.

Claims (10)

A hard titanium carbide particle and tungsten boride precipitates dispersed in the metal matrix, wherein the volume fraction of the titanium carbide particles and tungsten boride precipitates is about 30 to 80% by volume of the coating film, the remainder being a metal matrix. Abrasion and corrosion resistant coatings on substrates. The coating film of claim 1 wherein the titanium carbide particles comprise about 15 to 30 volume% of the coating film and the tungsten boride precipitate constitutes about 30 to 50 volume% of the coating film. The coating film of claim 1 wherein the atomic ratio of tungsten to boron in the coating film is about 0.4 to 2.0 and the atomic ratio of titanium to carbon is about 1.0. The coating film according to claim 1, wherein the average size of the titanium carbide particles and the tungsten boride precipitates is about 0.5 to 3.0 microns. The coating film of claim 1 wherein the hardness is about 700 to 1200 DPH ₃₀₀ (HV. 3). The coating film according to claim 1, wherein the metal matrix is composed of at least one metal selected from the group consisting of nickel, cobalt and iron. The method of claim 1, wherein the substrate is selected from the group consisting of steel, stainless steel, iron-based alloys, nickel, nickel-based alloys, cobalt, cobalt-based alloys, chromium, chromium-based alloys, titanium, titanium-based alloys, refractory metals, and refractory metal-based alloys. Coating film, characterized in that the selected material. The coating film according to claim 1, wherein the tungsten boride precipitate is W ₂ CoB ₂ . The coating film according to claim 1, wherein the tungsten boride precipitate is W ₂ NiB ₂ . The coating film according to claim 8, wherein the tungsten boride precipitate is WCoB.