WO1994025641A1

WO1994025641A1 - Forming a hard layer on a substrate

Info

Publication number: WO1994025641A1
Application number: PCT/GB1994/000881
Authority: WO
Inventors: Roger John Wedlake; Izak Le Roux Strydom; Johan Frans Prins; Jochanan Hosea Romm
Original assignee: Roger John Wedlake; Izak Le Roux Strydom; Johan Frans Prins; Jochanan Hosea Romm
Priority date: 1993-04-26
Filing date: 1994-04-26
Publication date: 1994-11-10
Also published as: AU6574594A

Abstract

A method of forming a hard layer on a substrate comprises rubbing particles of a suitable source material across the surface of the substrate at a high speed and under sufficiently high pressure to cause a hard layer to form on the surface. The source material typically comprises a hard metal carbide, or a material containing diamond and boron nitride. A mop of carbon fibre rotating at a speed of about 15 000 rpm is held against a substrate such as polished stainless steel, with a slight interference between the periphery of the mop and the substrate. Particles of source material are applied to the interface between the mop and the substrate, and the substrate is traversed relatively to the mop until a coating of the source material builds up on the surface of the substrate. The substrate can be moved back and forth underneath the mop, or can be moved uni-directionally relative to the mop.

Description

FORMING A HARD LAYER ON A SUBSTRATE

BACKGROUND OF THE INVENTION

This invention relates to a method of forming a hard layer on a substrate, and to substrates coated in accordance with the method.

SUMMARY OF THE INVENTION

According to the invention a method of forming a hard layer on a substrate comprises providing a suitable substrate; providing a source material selected from the group comprising transition metal carbides, silicon carbide, boron carbide, and materials containing covalently bonded mixtures of carbon, boron and nitrogen in any stable ratio; and nibbing the source material across a surface of the substrate at a sufficiently high speed and under sufficiently high pressure to cause a layer of hard material to adhere to the surface.

The source material may comprise a hard metal carbide.

For example, the source material may comprise titanium carbide, tantalum carbide, or tungsten carbide.

Alternatively, the source material may comprise a mixture of covalently bonded carbon and boron nitride; or a mixture of covalently bonded boron carbide, carbon and/or boron nitride in any stable ratio.

The carbon may be in the form of cubic or wurtzitic diamond.

The boron nitride may be cubic or wurtzitic boron nitride.

The layer of hard material may have a cubic, zinc blend or wurtzite structure.

Preferably, the source material is paniculate.

The source material particles are preferably less than 100 microns in size, and more preferably less than 10 microns in size. Ideally, the particles should be less than 1 micron in size, typically with a size distribution of 0 to 0.5 microns.

The particles are preferably rubbed across the substrate at a linear speed of at least 100 m/s, and preferably at least 200 m/s.

The speed may be substantially higher, up to about 360 m/s or more.

The substrate may comprise, for example, a metal such as steel or stainless steel, a carbide, a hard metal, a ceramic, or glass.

The particles may be rubbed across the surface of the substrate by means of a rotating wheel or mop, the particles being applied to the mop or to the interface between the mop and the surface of the substrate.

For example, the mop may be impregnated with the particles, or the particles may be directed at the interface between the mop and the surface of the substrate by a gas or liquid jet, or both.

Preferably, the interference between the periphery of the mop and the surface of the substrate is less than about 200 μm.

The mop and the substrate may be moved back and forth repeatedly relative to one another whilst the substrate is in contact with the mop.

Alternatively, the substrate may be moved repeatedly in a single direction relative to the mop while in contact with the mop.

The invention extends to articles comprising a substrate having a hard layer formed thereon by the method of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a simplified schematic end view of an apparatus for carrying out the method of the invention;

Figure 2 is a schematic side view of the apparatus of Figure 1, showing one method of caπying out the invention;

Figure 3 is a schematic side view of the apparatus of Figure 1 illustrating a different method of carrying out the invention;

Figure 4 is a schematic sectional side view (not to scale) of a portion of a substrate coated with a hard layer according to a method of the invention; and

Figure 5 is a schematic illustration of a practical apparatus for carrying out the method of the invention.

DESCRIPTION OF EMBODIMENTS

Figure 1 illustrates in a simplified schematic form apparatus which can be used to carry out the method of the invention. The apparatus comprises a "mop" in the form of a cloth wheel or composite buffing wheel 10 which is supported on one side by a high speed bearing 12 and which is arranged to be rotated at high speed by a motor 14. The motor 14 is preferably an air bearing turbine motor driven by compressed air which is designed for rotation at very high speeds, typically between 10 000 and 50 000 rpm or even higher speeds. The axis of the mop 10 is horizontal, and the periphery of the wheel is supported just above the surface 16 of a table or other supporting surface. Fixed to the surface 16 is a substrate 18 which is to be coated with a hard layer. The substrate 18 will typically have a planar upper surface and may comprise, for example, steel or stainless steel, a carbide, a hard metal, a ceramic, or glass. The substrate 18 could have a curved surface. The method of the apparatus is also expected to work with other substrate materials.

In order to carry out the method of the invention, the mop 10 is used to rub particles 20 of the source materials mentioned below very rapidly and under pressure across the surface of the substrate 18, resulting in a "smearing" effect, which causes the particles to bond to the surface of the substrate and to build up a hard layer on the surface of the substrate.

In a simple version of the invention, the periphery of the mop 10 is impregnated or coated with fine particles of source material, and the mop is then rotated at high speed while in contact with the surface of the substrate 18. This causes the particles to be rubbed across the surface of the substrate.

The linear speed at which the particles are rubbed across the surface of the substrate is proportional to the speed of rotation of the mop 10 and the radius of the mop, in accordance with the following equation:

v_hnβar = rpm χ 2ττ .r 6o

In the case of a mop 10 having a radius of approximately three inches (7.62 cm) rotated at a speed of 20 000 rpm, the linear speed at which the particles are rubbed across the surface of the substrate 18 will be approximately 160 m/s. A typical speed of rotation is 25 000 rpm, corresponding to a linear speed of approximately 200 m/s. Speeds of up to 45 000 rpm are envisaged, conesponding to a linear speed of approximately 360 m/s.

The basic method described above is obviously wasteful and messy, in that particles of source material will tend to be scattered from the periphery of the mop 10 as it rotates. In the arrangement shown schematically in Figure 2, a fine nozzle 24 is used to direct a thin spray of particles 20 into the interface between the periphery of the mop 10 and the surface of the substrate 18, using air or gas pressure. This minimises wastage of the particles. Instead of compressed air or gas, a jet of liquid can be used for this purpose.

The mop can be impregnated initially with the sprayed particles, with the spray being continued as the coating process progresses.

In Figure 3, an alternative method is shown. A conduit 26 feeds the source material particles 20 under pressure into the hub 28 of the mop 10, which is perforated, allowing the particles 20 to migrate outwardly through the material of the mop 10. This method provides a relatively uniform distribution of particles at the periphery of the mop 10.

The result of the above described process is shown schematically in Figure 4, which is not to scale. On the surface of the substrate 18 is formed a thin hard layer 22, typically several hundred urn thick, with a composition which depends on the source material used.

In one embodiment of the invention, the source material comprises particles of a transition metal carbide or hard metal carbide such as titanium carbide, tantalum carbide, or tungsten carbide, or silicon or boron carbide. In this embodiment, the method provides a hard carbide layer on the surface of the substrate. In another embodiment of the invention, the source material comprises a mixture of particles of covalently bonded carbon and boron nitride, or a mixture of covalently bonded boron carbide, carbon and/or boron nitride in any stable ratio. In the latter two cases, the carbon can comprise cubic or wurtzitic diamond particles, while the boron nitride comprises cubic or wurtzitic boron nitride particles.

Where the source material includes carbon, a carbon fibre mop can be employed, so that particles of carbon are included in the coating which is formed on the substrate as they break off the carbon fibres of the mop.

The source material particles are preferably less than 100 microns in size, and more preferably less than 10 microns in size. Ideally, the particles have a size distribution in the range 0 - 0.5 μm.

In a prototype version of the method of the invention, the apparatus illustrated in Figure 5 was used. The mop 10 was a carbon fibre/epoxy composite mop manufactured from multiple circular woven fibre mat sections. To manufacture the mop, the inner portions of the mops are impregnated with epoxy resin, leaving a peripheral zone of approximately 15 mm width free of resin. The fibre mats are laid down one on top of the other in a staggered arrangement and moulded by conventional techniques to form a mop of approximately 300 mm diameter and approximately 5 mm thickness. After curing of the epoxy resin, all loose fibres are removed and the free fibres at the edge of the mop are trimmed. Final finishing and rounding of the mop is carried out by running it against sandpaper on the coating apparatus, and the mop is then balanced to prevent vibrations, which at high speed can damage the bearings of the apparatus. As shown in Figure 5, the substrate 18 is supported on a substrate table 30, which is in turn mounted on a pneumatically driven platform 32. The platform is mounted on a heavy machine base 34, and its movements are controlled by a control system 36, allowing the substrate table 30 and thus the substrate 18 to be driven back and forth below the mop by a pneumatic actuator.

Fine particles of source material are supplied to the interface between the mop 10 and the substrate 18 by a powder feed system 38. A shroud 40 surrounds the mop to collect surplus powder, and is connected to a powder extraction system 42 which collects surplus powder for recycling. A data acquisition system 44 is also provided, which is connected to sensors in the substrate table 30. The sensors are ananged to measure both the normal force F_N and the friction force F_p between the mop and substrate.

In initial experiments, a stainless steel substrate was used. Specimen substrates of 50 by 30 mm were cut from a 2 mm thick sheet and were polished to a surface finish of approximately 0.02 μm Ra, giving a substantially mirror-like finish. A polished surface was preferred for experimental purposes, as this was useful in subsequent examination and characterisation of the layer formed by the method.

The specimen substrate is clamped onto the substrate table of the apparatus and the height of the compressed air turbine is adjusted so that the lowermost edge of the mop 10 contacts the surface of the substrate 18 with a degree of interference. In experiments, an interference of less than 100 μm (approximately 50 μm) was found to give good results. Significantly greater interference (greater than about 200 μm) was found to lead to damage of the substrate by the high speed mop. The turbine is started and the speed of the mop 10 adjusted to approximately 15 000 rpm by regulating the compressed air flow to the turbine. With a mop of 300 mm diameter, this corresponds to a circumferential velocity at the periphery of the mop of approximately 235 m/s.

The powder feed system is started and the particles of source material are fed to the interface between the mop and th substrate at a rate of approximately 20 to 30 mg/min.

The pneumatic actuation and control system 36 is actuated and the substrate table 30 is driven back and forth under the mop at a speed which is much slower than the speed at which the mop rubs the particles across the substrate. In experiments, the substrate table was moved at a speed of approximately 0.2 m/s. A number of passes are normally required. Typically, 10 back and forth cycles were employed. The data acquisition system 44 records the normal and friction forces between the mop and the substrate. In the above prototype method, normal forces of 8 to 12 N and friction forces of 4 to 8 N were measured. In a variation of the method, the table was driven repeatedly under the mop in a single direction (either with or against the rotation of the mop) by lowering the table on its return stroke. This technique improves the rate of deposition of the coating and its characteristics.

The coatings obtained by the above described method were subjected to examination by TEM, SEM and optical microscopy, and XPS and AES analysis techniques. Pin-on disc wear tests, using a tungsten carbide- cobalt ball sliding on the coated surface were also performed.

Microscopic examination of the hard coatings showed them to be continuous, with a flecked appearance and a wavy surface. Tests indicated that the coatings had a thickness in the range 0.3 to 1 μm. The tribological properties of the sample coatings in terms of their behaviour in a pin-on disc wear test were found to be excellent.

Experiments were carried out using tungsten carbide-cobalt or boron carbide substrates in place of stainless steel. These experiments also resulted in the deposition of continuous hard layers comprising the relevant source material.

It has been found that the hardness (or softness) of the mop 10 has a significant influence on the effect obtained in carrying out the method. When a relatively hard mop is used, the substrate is eroded until a hard layer begins to form. The erosion of the substrate results in the appearance of striations in the resulting layer. The force applied by the mop to the substrate also has an influence on the rate of formation of the layer on the substrate.

The above described examples of the method of the present invention are provided by way of example, and it will be understood that the use of paniculate source material of different particle size and/or shape, the use of different substrates and mops, and the variation of the parameters of the method will significantly affect the results obtained. However it is clear from the tests described above the that the method of the invention can be utilised to produce continuous hard coatings on the surface of a suitable substrate by the rubbing of dry, discrete particles of suitable source materials across the surface of the substrate at suitable speeds and pressures.

The invention may be used for coating wear parts, for example hydraulic/pneumatic pistons and cylinders, producing coated cutting tools, passive electronic devices such as heat spreaders and active electronic devices such as semi-conductors.

Claims

CLAIMS:

1. A method of forming a hard layer on a substrate comprising providing a suitable substrate; providing a source material selected from the group comprising transition metal carbides, silicon carbide, boron carbide, and materials containing covalently bonded mixtures of carbon, boron and nitrogen in any stable ratio; and rubbing the source material across a surface of the substrate at a sufficiently high speed and under sufficiently high pressure to cause a layer of hard material to adhere to the surface.

2. A method according to claim 1 wherein the source material comprises a hard metal carbide.

3. A method according to claim 2 wherein the source material comprises titanium carbide, tantalum carbide or tungsten carbide.

4. A method according to claim 1 wherein the source material comprises a mixture of covalently bonded carbon and boron nitride.

5. A method according to claim 1 wherein the source material comprises a mixture of covalently bonded boron carbide, carbon and/or boron nitride in any stable ratio.

6. A method according to claim 4 or claim 5 wherein the carbon is in the form of cubic or wurtzitic diamond.

7. A method according to any one of claims 4 to 6 wherein the boron nitride is cubic or wurtzitic boron nitride.

8. A method according to any one of claims 4 to 7 wherein the layer of hard material has a cubic, zinc blend or wurtzite structure.

9. A method according to any one of claims 1 to 8 wherein the source material is paniculate.

10. A method according to claim 9 wherein the source material particles are less than 100 microns in size.

11. A method according to claim 10 wherein the source material particles are less than 10 microns in size.

12. A method according to claim 11 wherein the source material particles have a size distribution in the range 0 to 0.5 μm.

13. A method according to any one of claims 9 to 12 wherein the particles are rubbed across the surface of the substrate by means of a rotating mop, the particles being applied to the mop or to the interface between the mop and the surface of the substrate.

14. A method according to claim 13 wherein the mop is impregnated with the particles.

15. A method according to claim 13 wherein the particles are directed at the interface between the mop and the surface of the substrate by a fluid jet.

16. A method according to any one of claims 13 to 15 wherein the interference between the periphery of the mop and the surface of the substrate is less than about 200 μm.

17. A method according to claim 16 wherein the interference is less than about 100 μm.

18. A method according to claim 17 wherein the interference is about 50 μm.

19. A method according to any one of claims 13 to 18 wherein the mop and the substrate are moved back and forth repeatedly relative to one another while the substrate is in contact with the mop.

20. A method according to any one of claims 13 to 18 wherein the substrate is moved repeatedly in a single direction relative to the mop while in contact with the mop.

21. A method according to any one of claims 1 to 20 wherein the source material is rubbed across the substrate at a linear speed of at least 100 m/s.

22. A method according to claim 21 wherein the speed is greater than 200 m/s.

23. A method according to any one of claims 1 to 22 wherein the substrate comprises a metal, a carbide, a hard metal, a ceramic, or glass.

24. A method according to claim 23 wherein the substrate comprises stainless steel.

25. A method according to claim 23 or claim 24 wherein the surface of the substrate is polished.