WO1993008315A1

WO1993008315A1 - A method of producing a wear-resistant coating

Info

Publication number: WO1993008315A1
Application number: PCT/AU1992/000553
Authority: WO
Inventors: Harold Leroy Harford
Original assignee: Harold Leroy Harford
Priority date: 1991-10-18
Filing date: 1992-10-16
Publication date: 1993-04-29

Abstract

A method of obtaining a wear-resistant coating (10) characterized by the steps of: applying a layer (16) of a bonding material to a surface (14) of a substrate (12); applying a layer (18) of a hard material to the layer (16) of bonding material; applying a further layer (22) of the or a bonding material to the layer (18) of hard material, the further layer (22) being easily grindable; and optionally applying further sequential layers of hard material (26) and bonding material (22), wherein the or each layer (22) of bonding material acts to limit stress in the or each layer (18, 26) of hard material.

Description

TITLE "A METHOD OF PRODUCING A WEAR-RESISTANT COATING" DESCRIPTION The present invention relates to a method of producing a wear-resistant coating. FIELD OF THE INVENTION Many different processes and techniques are widely known have been extensively used in regard to obtaining coatin high hardness to render a metal substrate or more particularly, a machine part, more resistant to wear. Several limitations and problems are known to exist in re to each of the different processes and techniques. Fusion processes can be used to obtain wear-resistant coatings which are both of high hardness and relatively thick, but such processes have several disadvantages associated therewith. Fusion processes may result in a substantial dilution of the coating material with the met substrate which it is being applied to. This dilution al the microstructure of the coating material which typicall leads to a drop in the attainable hardness of the coating material. Further, due to the high heat inputs into the metal subst during fusion processes, high distortion thereof will occ Still further, fusion processes result in an increased propensity for the coating material to crack as hardness increases. Hence, the hard phase of the micro-constituen of the coating material is limited in volume. Finally, i difficult and expensive to weld or hardface many metal substrate materials, while some metal substrates such as hyperchrome are not weldable. Diffusion methods such as powder welding, pack diffusion, physical vapour deposition (PVD) , chemical vapour depositi (CVD) and many others can be used to produce wear-resistan coatings of high hardness and integrity, but are limited a to the attainable thickness of the coating material. Typically, wear-resistant coatings produced by diffusion methods are limited to thicknesses in the range of 1 mil t mils. Thermal spray processes such as powder gas spraying and electric arc spraying are advantageous in that generally lower costs are possible in applying a wear-resistant coati to a metal substrate or a machine part, and in that the thermal spray process makes it possible to be able to apply wear-resistant coating to a broad range of different metal substrate materials. However, wear-resistant coatings produced by thermal spray processes are porous to the order of 10 — 15% of the coating material, and so coatings of ver high hardness are usually not obtainable. The high velocity oxygen fuel (HVOF) and high velocity air fuel (HVAF) thermal spray processes are recent developments in regard to thermal spray processes and methods. Wear-resistant coatings having a high density, high hardnes excellent particle co-hesion, a very low porosity and good microstructure with regard to volume and hardness of the har constituents and their even distribution within the matrix can be produced using this process. The hard phase of the micro constituents of the wear-resistant coating applied using this process is typically greater in volume as compare with metal alloys used for fusion processes. This results a greater wear life of the metal substrate or part coated with the wear-resistant coating. Typically, the HVOF or HVAF thermal spray processes can be used to apply an 88/12 tungsten-carbide alloy to a metal substrate to achieve a wear coating with a diamond pyramid hardness DPH 300 of 1200 and a density of 13 to 13.7 gm/cm³. However, the HVOF and HVAF thermal spraying processes have a disadvantage in that the wear resistant coating applied to the metal substrate at this particular hardness has a thickness limitation of the order of 10 to mils. Typically, attempts to apply a wear-resistant coati with a thickness greater than 10 to 15 mils using the HVOF HVAF thermal spray processes will result in the formation cracks in the wear-resistant coating. It is thought that residual stresses in the coating are responsible for spall of the coating at thicknesses over 10 to 15 mils. These cracks essentially render the wear-resistant coating useless. A further disadvantage associated with the HVOF and HVAF technology is that a sprayed tungsten-carbide layer has a pronounced surface roughness. This roughness, in the orde of 150 rms, limits the use of the tungsten-carbide layer i the as sprayed condition. For example, if the tungsten-carbide layer is used as a bearing surface this will produce galling on the other member, such as a seal or bearing surface and seizure occu rapidly. SUMMARY OF THE PRESENT INVENTION The present invention provides a method of obtaining a wear-resistant coating which at least reduces one or more the above entioned problems of prior art methods. In accordance with one aspect of the present invention, there is provided a method of obtaining a wear-resistant coating comprising the steps of: - applying a layer of a bonding material to a surface of a substrate; - applying a layer of a hard material to the layer of bonding material; - applying a further layer of the bonding material to the layer of hard material, the further layer being easily grindable; and - optionally applying further sequential layers of hard material and bonding material, wherein the or each layer o bonding material acts to limit stress in the or each layer of hard material. Preferably, the hard material is applied to the bonding material using a high velocity spray process. In accordance with a further aspect of the present invention, there is provided a method of obtaining a wear-resistant coating comprising the steps of: - applying a layer of a bonding material to a surface of a substrate; - applying a layer of a hard material to the layer of bonding material; and - repeatedly applying the bonding material and hard materia in sequential layers until a desired thickness of the wear-resistant coating is obtained. Preferably, the wear-resistant coating is applied to a met substrate. Preferably, the bonding material and hard material are metals or metal alloys. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described, by way of example, with reference to the accompanying drawings in which:- Figure 1 is a partial sectional view of a wear-resistant coating produced in accordance with the method of the present invention; Figure 2 is a schematic diagram of the thermal spray coati method as used in accordance with the present invention; Figure 3 is a partial sectional view of a wear-resistant coating of greater thickness than that of Figure 1 produce in accordance with the method of the present invention; Figure 4 is a schematic diagram of the microstructure of a thermally sprayed coating; Figure 5 is a schematic representation of an electric arc metal spraying system; and Figure 6 is a schematic representation of a high velocity oxygen fuel (HVOF) thermal spraying system. DESCRIPTION OF THE INVENTION The method of obtaining a wear-resistant coating will be described in the following example wherein it is sought to obtain a thick tungsten-carbide coating on a metal substrate. Figures 1 and 3 show a wear-resistant coating 10 at different stages of its application to a metal substrate 2 12. The application of the coating 10 to the metal

3 substrate 12 is achieved as follows:-

4 Initially, any oils, grease, soil, dirt or grit are

5 preferably removed from the metal substrate 12 which is t

6 be coated with the wear-resistant coating 10. This is

7 typically done by way of solvent wash or appropriate acid

8 cleaning of the metal substrate 12 to be coated. Moistur

9 and oxide films are then preferably removed from the meta

10 substrate 12 which is to be coated with the wear-resistan

11 coating 10. Typically, moisture removal is achieved by

12 placing the metal substrate 12 in an oven, or heating the

13 metal substrate 12 to about 50°C using a propane torch.

14 oxide films which exist on a surface 14 of the metal

15. substrate 12 are typically best removed by grit blasting

16 using alumina grit. Further, if the surface 14 of the met

17 substrate 12 is hard and cannot be roughened by blasting, 8 the oxide coating thereon may be simply removed by way of 9 hand grinding or a similar manual operation after 0 pre-heating the surface 14 of the metal substrate 12. The 1 aforementioned technique for removing oxide from the surfa 2 14 may also be used if the surface 14 of the metal substra 3 12 is unable to undergo blasting due to the size thereof. 4 Once moisture and oxide films are removed from the surface 5 14, the metal substrate 12 may be re-heated to about 50°C 6 and sprayed with a chlorinated hydrocarbon type solvent. 7 An electric arc metal spraying system 40 as shown in Figur 8 2, 4 and 5 may be subsequently arranged to apply a porous 9 coating of a ductile bonding material 16 to the surface 14 0 of the metal substrate 12. The electric arc metal spraying system 40 generally comprises the steps of a solid or powder starting materi 30, this material being melted in an arc 32, the melted particles being accelerated in a gas stream 34, the particles splattering on the substrate 36 followed by th cooling and coalescence 38, as is shown in Figure 2. Th resulting layer 16 comprises voids, oxidized particles a unmelted particles together with fully melted and coales particles, as shown in Figure 4. The electric arc metal spraying system 40 typically comprises an arc gun 42, a power source 44, an air suppl and wire stock 48. Each of the elements of the electric metal spraying system 40 are interconnected therebetween operation in known manner. A schematic representation o this system 40 is shown in Figure 5. The operational parameters of the electric arc metal spraying system 40 are appropriately set after considera of the type of alloy that is to be applied to surface 14 For the present example, a layer of a nickel-aluminium a 16 is to be applied and the relevant operational paramet are preferably as follows:- Volts - 32 Amps - 120 Air Pressure - 50 Psi Wire Diameter - 1/16 inch The porous layer of the nickel-aluminium alloy 16 is app to the surface 14 of the treated metal substrate 12 to a thickness of, for example, 6 mils using the appropriatel arranged electric arc metal spraying system 40. It is t noted that the nature of the alloy 16 used determines th 2 extent of the bond strength to the metal substrate 12 and the resulting surface roughness of the bond material 16. The nickel-aluminium alloy coating 16 used in this example will bond to the metal substrate 12 with a tensile bond strength of approximately 7000 - 8000 psi. The nickel-aluminium alloy 16 will also provide a surface roughness of 200 - 300 micro inches. The nickel-aluminium alloy 16 provides a coating that is both porous and ductil 0 A high velocity oxygen fuel system (HVOF) thermal spraying 1 system 50 as shown in Figure 6 may be subsequently used to 2 apply a dense layer of a hard material 18 to an exposed 3 upper surface 20 of the bonding material 16. 4 The HVOF thermal spraying system 50 typically comprises a 5 HVOF torch 52, heat exchanger 54, regulated gas supply 56, 6 gas control console 58 and a powder feeder 60. Each of 7 these elements are interconnected therebetween for operatio 8 in known manner. The operational parameters of the HVOF thermal spraying system 50.are previously appropriately set, the settings dependent upon the type of hard material 18 that is to be applied to the surface 20 of the bonding material 16. For the present example, a layer of an 88/12 tungsten-carbide alloy powder 18 is to be applied and the relevant operational parameters are set out in table 1 below:-

TABLE 1 GAS FUEL INPUT PRESSURE FLOW RATE BACK PRESSURE Oxygen 160 psi 600 SCFH 140 p Propane 160 psi 125 SCFH 23 ps Hydrogen 150 psi 100 SCFH Nitrogen 140 psi 50 SCFH 38 ps Spray distance - 11 inches powder feed - 50 - 55 g/min 22mm combustion chamber (SCFH - standard cubic feet/hour)

The layer of the tungsten-carbide alloy powder 18 is preferably applied without undue delay to the surface 20 the nickel-aluminium alloy layer 16 using the appropriatel set HVOF thermal spraying system 50. The tungsten-carbide alloy powder layer 18 may be applied from a minimum thickness of 10 mils to a maximum thickness of 15 mils. During the application of the tungsten carbide alloy powde coating 18, the temperature of the metal substrate 12 shou preferably not exceed 120°C. Cooling jets and interrupted application of the HVOF thermal spraying system 50 may be used as necessary in this regard. It is to be noted that the porosity and ductility of the bonding alloy layer 16 enables the hard material 18 to be applied to the surface 20 of the bonding alloy layer 16 to thickness exceeding what is usually obtainable using the HVOF thermal spraying systems 50. A subsequent layer 22 of the bonding alloy may be now electric arc sprayed over the first hard material coating using the aforementioned operational parameters of the electric arc metal spraying system 40. Hence, a further layer 22 of nickel-aluminium alloy may sprayed over the first tungsten-carbide layer 18. The thickness of this second layer 22 may be of the order o mils. The nickel-aluminium alloy layer 22 used in the example will bond with the tungsten-carbide layer 18 wi tensile bond strength of approximately 5000 - 6000 psi the tungsten-carbide layer interface 24. This second nickel-aluminium alloy layer 22 will also provide the suitable roughness for a subsequent hard material layer that is, a second tungsten-carbide layer. The subsequent layer of hard material 26 is now applied the second bonding alloy layer 22 using the HVOF therma spraying system 50 having the operational parameters therefor set as hereinbefore detailed. Hence, a further layer 26 of tungsten-carbide alloy powder is applied to second nickel-aluminium alloy layer 22. The two processes of electric arc spraying a porous bond alloy and subsequently applying a hard material using th HVOF thermal spraying process to the previously applied bonding alloy may be repeated in sequence several times until a desired thickness of wear-resistant coating 10 h been deposited, as shown in Figure 3. Hence, the final resistant coating 10 is a composite coating obtained usi the two different density alloys or materials, the compo coating 10 comprising a laminated structure having at le 2 layers of a softer and less dense bonding material and layer of hard material. For the present example, these two processes may be repea until ten layers of tungsten carbide have been deposited. Hence, in the present example, the final laminate coating will have a total thickness of 133 mils comprising ten tungsten-carbide layers of 10 mils each, 9 layers of nickel-aluminium alloy of 3 mils each plus the original layer of nickel-aluminium alloy of 6 mils thickness. Thus, in the present example, a 100 mil thick tungsten-carbide coating has been applied to the metal substrate without the occurrence of any micro-cracking anywhere in the composite coating. Examples of the use of the composite coating in industry provided in Table 2 as examples A, B, C and D. Note shou

6 WEEKS 3 MONTHS 6 WEEKS THICK 3 MONTH

LAMINATED 1MM THICK 2MM THICK 1MM THICK 0.4MM CARBIDE THICK

AND LIFE 9 WEEKS 18 MONTHS 60 WEEKS 1 YEAR

It should be noted that the HVAF system may be substituted for the HVOF system described hereinabove.

Further, the method can be used to create a final composite coating which possesses the characteristic of being easily machinable or grindable using conventional grinding wheels and not requiring diamond wheels.

The following paragraphs describe the way in which the method of obtaining a wear-resistant coating is used to obtain an easily grindable hard material coating. This will be by way of example in which, a procedure for obtaining an easily grindable tungsten-carbide coating is described. A machine part or metal substrate which is to be coated is machined an amount equivalant to the desired thickness of the overall wear-resistant coating under the final thickness dimension of the overall part. Hence, when the wear-resistant coating is applied, the part will be of the correct final thickness dimension. For the present example, the part is machined by 20 mils under the desired thickness of the finished part.

The metal substrate of the part is cleaned and treated as hereinbefore described to prepare the surface thereof for the application of a bonding alloy layer. As hereinbefore 2 described, a nickel-aluminium alloy layer is preferably

3 applied to the surface of the metal substrate to a minimum

4 thickness of 5 mils using the electric arc metal spraying

5 system 40.

6 Without delay, a layer of hard material is preferably

7 applied to the bonding alloy layer using the HVOF thermal

8 spraying process 50. For the present example a layer of

9 88/12 tungsten-carbide alloy powder is applied via this

10 process to a thickness of 15 mils.

11_. The hard material layer is allowed to cool to below 50°C at

12 which point the thickness of the composite coating is

13. measured. For the present example, the layer of

14 tungsten-carbide alloy powder is applied until the thicknes

15 of the composite coating is within 0.5 mils to 1 mil below

16 the desired final thickness for the machine part.

17 The tungsten-carbide layer has a surface roughness in the

18 order of 150 r s making it unsuitable for use on a bearing

19 surface. 0 The electric arc metal spraying system 40 is now arranged t 1 spray a layer of a different alloy onto the hard material 2 layer and hence the operational parameters are set 3 accordingly. For the present example, a layer of chrome 4 iron alloy is to be applied and the relevant operational 5 parameters are preferably as follows:- 6 Volts - 38 Amps - 250 7 Air pressure 65 psi 8 Wire diameter-1/16 inch 9 The chrome-iron alloy layer is applied to the surface of the 0 tungsten-carbide alloy powder layer using the electric arc metal spraying system 40. Sufficient spraying is carried out to obtain a chrome-iron alloylayer of, for example 20 mils thickness. The machine part can now be machined or ground to the required size using conventional tools or grinding wheels. For the present example, the chrome-iron alloy layer is ground down so that the tungsten-carbide layer will be 0.5 mils below the surface of the finished top alloy layer. It has been observed that the layer of porous soft final alloy material does not inhibit the performance of the denser, hard material upon which it has been applied when the machine part is placed in service. By use of the present invention, it is possible to obtain a wear-resistant coating of high hardness which is able to be finished without the use of diamond grinding apparatus. Further, it is also possible to obtain a wear-resistant coating, of high hardness and thickness without loss of integrity of the coating. The bonding layer is porous and provides ductility and effectively destresses the laminate thereby preventing cracking. It is envisaged that the bonding layers 16 and 22 may also be applied using a thermal spray of molybdenum, either powder or wire stock. Further, plasma sprayed tungsten-carbide using a low speed flame and large particle sizes in stock powder can give the required porosity in the bonding layers 16 and 22. Still further, HVOF or HVAF sprayed tungsten-carbide using a low speed flame and larger particle sizes in stock powder can also give the required porosity in the bonding layers 16 and 22. In addition, the plasma sprayed or HVOF and HV sprayed tungsten-carbide under the above conditions is dense than the HVOF or HVAF sprayed tungsten-carbide described previously. Also able to be applied in the a manner and give the required porosity are (A) an alloy comprising chrome, nickel and boron, (B) a stellite allo (being one comprising chrome, molybdenum and cobolt) , (C elemental titanium, (D) an alloy of titanium, or (E) a mixture of the above for example, a chrome-nickel-boron alloy having discrete additions of tungsten carbide particles. In effect, each bonding layer is a less dense and more porous layer than the or each hard layer. It is further envisaged that the layer of hard material alternatively comprise titanium nitride and other carbid and nitrides of a similar chemical nature. However, eac should be applied using the HVOF or HVAF thermal sprayin systems. It is further envisaged that the wear-resistant coating be post-heat treated. This heat treatment may be at approximately 850°C and could heal micro-cracks in the o each layer of hard material. Modifications and variations such^" as would be apparent t skilled addressee are deemed within the scope of the pre invention.

Claims

CLAIMS 1. A method of obtaining a wear-resistant coating characterised by the steps of: - applying a layer of a bonding material to a surface of substrate; - applying a layer of a hard material to the layer of bonding material; - applying a further layer of the or a bonding material t the layer of hard material, the further layer being easil grindable; and - optionally applying further sequential layers of hard material and bonding material, wherein the or each layer bonding material acts to limit stress in the or each laye of hard material. 2. A method according to Claim 1, characterised in that the bonding material is more porous and less dense than th hard material once applied. 3. A method according to Claims 1 or 2, characterised in that the hard material is _.applied-to the bonding material using a high velocity spray process. 4. A method according to any one of the preceding claims characterised in that the substrate to which the bonding material is applied is a metal substrate. 5. A method according to any one of the preceding claims characterised in that the bonding material and hard materi are metals or metal alloys. 6. A method according to any one of the preceding claims characterised in that the bonding material is a Nickel-Aluminium alloy. 7. A method according to any one of the preceding cla characterised in that the hard material is a tungsten-carbide alloy. 8. A method according to Claim 7, characterised in th the tungsten-carbide alloy is an 88/12 tungsten-carbide alloy. 9. A method according to any one of the preceding clai characterised in that the bonding material and material the further layer is applied using an electric arc therm spray process. 10. A method according to Claim 3, characterised in th the high velocity spray process is either one of a high velocity oxygen fuel or high velocity air fuel system. 11. A method according to any one of the preceding clai characterised in that the substrate surface is first machined down from an original level an amount equivalen the desired thickness of the wear resistant coating and coating then applied, wherein a bonding material layer i applied last and the surface thereof ground until it rea the original level of the substrate surface.