GB2154614A - Densified coatings by application of direct fluid pressure - Google Patents

Abstract

By applying a coating layer to a substrate in a manner that avoids significant surface-communicating subsurface porosity, for instance by suitably controlled flame-spraying or plasma-spraying, the coating may be densified and, if required, diffusion bonded to the substrate, by the application of heat and direct isostatic fluid pressure. Single and multiple coatings may be applied and densified and bonded by this method. The method is applicable to the coating of substrates to impart required surface properties and also to the repair or healing of substrates having surface defects, e.g., resulting from mechanical damage. The method enables the production of high-performance friction brake elements by providing the working surfaces of, e.g., a light alloy substrate with a densified wear-resistant surface of required frictional properties.

Description

SPECIFICATION Densified coatings This invention relates to the preparation of coatings of metal or other materials on a substrate to protect and/or improve the performance of the latter. The invention is especially concerned with the preparation of coatings by spraying techniques such as flame spraying and plasma spraying.

To perform their required function in many instances, applied coatings need to be non-porous.

Unfortunately, convenient spraying techniques, particularly those used for the preparation of metal and high-performance coatings, tend to produce coatings that exhibit superficial and/or subsurface porosity.

With the objective of obtaining denser, less porous, coatings, two techniques have been evolved: these are high velocity spraying such as plasma spraying with the use of a high-velocity plasma gun; and fusion of a specially formulated coating after this has been spray-applied by conventional techniques. While, especially, plasma spraying can lead to a dense coating with low residual porosity, nonetheless the coating tends to exhibit porosity in isolated regions but, more importantly, the plasma spraying technique used to obtain such dense coatings does not lead to desirably strong adhesion of the coating to the substrate.

The alternative technique of fusing a spray-applied coating requires the use of specially formulated coating materials that may not lead to the required properties in the eventual coating. Moreover, in this technique, the spray-applied coating is remelted or "fused" to the substrate and the reversion to a liquid phase entailed in this procedure promotes entrapment of gas and leads to shrinkage porosity. While diffusion effects at the substrate/coating interface are rapid and lead to good bonding of the coating to the substrate, the reactions occurring during fusion lead to a number of major disadvantages.Thus, during the fusion reaction the coating and substrate often become depleted in elements that are important to their respective ultimate functions: for instance, strengthening agents in the substrate tend to be removed from the latter by diffusion into the coating, thereby reducing the strength of the substrate at the interface, while conversely, removal of, for example, chromium from a coating into the substrate by diffusion can greatly reduce the oxidation-resisting characteristics of the coating. Additionally the conversion of the coating to a liquid film promotes the rapid dissolution of second phase particles - for example tungsten carbide or silicon carbide within the coating. Such particles are often included in the coating to enhance its wear-resistance and this property tends to be reduced as a consequence of the fusion reaction.Even if second phase particles are not dissolved, there is a tendency for these to move by gravitational effects and thus to reduce the uniformity of their dispersion within the coating matrix. Furthermore, the application of the necessary elevated temperature to effect remelting of a coating may significantly affect the properties of the substrate, such as its grain size. Finally, the creation of a localised liquid film promotes high surface stress during cooling; and in complex shapes there is a danger of producing a coating of non-uniform thickness as a result of the coating material flowing, while liquid, under the influence of gravity and other forces.

An object of the present invention is to provide for the preparation of suitably dense coatings, using techniques that avoid the disadvantages discussed above and that can also lead to improvements in the properties of the substrate to which such coatings are applied, and the possibility of important new uses for coated substrates.

It has been proposed, in, for example, EP-A-0,005,285 and EP-A 0,023,733, to apply a dense, hard, wear-resistant layer of cermets or cemented carbides to a metal object by spraying a powder material consisting of hard particles of cermets or cemented carbides and a binder metal - for instance by flame spraying or plasma spraying - to produce a porous layer, and then to consolidate and compact the sprayed-on layer at high temperature and under isostatic pressure. As so proposed, however, the isostatic consolidating and compacting pressure was to be derived indirectly from gas pressure in a hot isostatic press, by (in the case of EP-A-0,005,285) enclosing the coated object in a gas-tight, thin-walled holder filled with a particulate solid pressure-transmitting medium and subjecting this holder to the gas pressure.The fineness of the grain size of the pressure-transmitting medium determines the surface finish obtained on the consolidated coating layer. In the proposal of EP-A-0,023,733, the object with its sprayed-on layer, or at least that part of the object coated with such a layer, is directly covered by a thin metal foil to isolate the porous coating layer from the gas by which the isostatic pressure is developed in the hot isostatic press.

In both these proposals it is apparently assumed that the sprayed-on powder layer will inevitably be so porous, and contain so many sub-surface pores communicating with one another and with the surface of the layer, that the application of gas (or indeed any fluid) pressure directly to the surface of the layer would not lead to useful and uniform densitification of the layer, the indirect application of gas presurevia a deformable holder or foil being stipulated to overcome this difficulty.

However, we have discovered that by proper control of the procedure by which the coating layer is initially applied to the metal object or substrate the layer can be formed with sufficiently restricted porosity such that compaction thereof can be satisfactorily achieved by the direct application of fluid, e.g. gas, pressure in a hot isostatic press.

Thus, in accordance with the present invention a coating is applied to a substrate by any suitable deposition technique that leads to a coating layer of limited surface-communicating subsurface porosity, such as to permit compaction by applied fluid pressure, and the coated substrate is thereafter subjected to the application of heat and direct isostatic fluid pressure to effect compaction thereof.

It has been found that the hot isostatic pressing of the coated substrate by directly-applied fluid pressure not only can substantially eliminate residual porosity in the coating but it also can promote the formation of a strong diffusion bond between the substrate and the coating. Accordingly, coating application techniques such as plasma spraying that, because they tend to produce a poor bond between the coating and the substrate, have hitherto been rejected for many applications, may advantageously be used in the process of the present invention, as a consequence of the improved coating-to-substrate bond that results from the hot isostatic pressing step.

In preferred practice of the invention a laminar coating structure is produced by spray-deposition of coating material on a substrate, using a high-velocity spraying system, such as inert gas plasma spraying, or high-velocity fuel burning flame spraying, that leads to a laminar structure in the deposited coating as a consequence of deformation and overlapping of the sprayed powder particles. In such a coating the subsurface pores are substantially isolated one from another and not interconnected with the surface, and the coating is firmly mechanically attached to the substrate.

The coated substrate is then subjected to hot isostatic pressing, using appropriate values for the variables of temperature, pressure and time to achieve densification of the coating by elimination of residual porosity therein and to achieve controlled diffusion of constituents of both the coating and substrate at the interface therebetween so as to produce a diffusion bond thereat.

It should be understood that the hot isostatic pressing step will be conducted within a temperature range, having regard to the constituents of the substrate and coating, such that the liquefaction of any constituent is minimised: that is to say, as far as possible and with due regard to densification characteristics, the consolidation effects are developed in the solid state, thereby avoiding grain growth in either the coating or substrate and also avoiding dissoution and/or grain growth of any second phase particles in the coating.

However, we have found that with certain coating compositions, densification characteristics are improved if the hot isostatic pressing step is conducted within a temperature range that overlaps the solidus temperature of the composition to a small extent, so that at some period during the pressing step the solidus temperature is slightly exceeded. In general, the solidus temperature should not be exceeded to an extent and for a duration such that more than about five percent of the composition, by volume, liquefies.

When, as is preferred, the coating is applied by plasma spraying or by a high-velocity fuel burning flame spraying system so that the applied coating has relatively low residual porosity, the change in density of the coating during the hot isostatic pressing step is relatively small. This makes it possible to provide the coating, when required, with a high total volume fraction of second phase particles entrapped in the coating, and to produce new dense coating compositions that are impossible to produce by conventional spray-coating techniques.

The method of the invention may be applied to the densification of single or multiple coating layers on a substrate. When applied to the densification of multiple layers, both or all of these may be applied in a manner leading to minimum surface-communicating subsurface porosity, or only the outer coating layer may be so applied to enable the direct application of fluid pressure to effect densification of that layer and the underlying layer or layers.

To illustrate the possibilities afforded by the invention the following Examples may be considered.

EXAMPLE 1 A titanium carbide-cobalt composition is often applied by plasma spraying to provide a substrate with a coating of high temperature antifretting material. Such a coating normally fails in service by reason of the growth of oxides at the coating/substrate boundary. This oxide growth occurs because the coating is only mechanically attached to the substrate so that expansion/contraction effects encountered in high temperature service promote cavitation at the interface, allowing oxygen to reach the substrate by permeation through the residual porosity and the induced cavities in the coating.

By subjecting the substrate having such a coating to hot isostatic pressing so as to eiiminate the residual porosity in the coating and to produce a diffusion bond between the coating and the substrate, the possibilities for oxide growth are restricted, thus enhancing the coating life in service.

A preferred TiCxCo coating of this type may be provided by plasmaspraying a powder consisting of particles, typically having a size range -44 to +20 v, containing 55% TiC by weight and 45% Co by weight, the powder mixture exhibiting a melting range of 1455-2870"C.

In preferred practice of the method of the invention, such a powder mixture is spray-applied to a metal substrate using a METCO 7M plasma spray gun fitted with a 2-port GE nozzle and operated in accordance with the following settings: Arc gas (argon) flow 150) ) (flow control settings Secondary gas (hydrogen) flow 80 ) ) per calibrations thereof) Powder carrier gas (argon) flow 37 ) Powder feed rate 2.90 kg/hr (6.4 lbs/hr) Arc current 500 A Arc voltage 45-55 V Spray distance 50-75 mm (2-3 inches) A coating layer built up to a thickness of 1-3 mm in this way may be effectively consolidated by hot isostatic pressing, using direct application of gas pressure to the coated substrate in a hot isostatic press, by maintaining the coated substrate under pressure in the range 100-140 MPa for a period of 120-180 minutes, while the coated substrate is at a temperature within the range 1150-1250"C.

EXAMPLE 2 Fusible coatings of Ni - Cr - B - Fe - Si are normally applied to roll surfaces and the like in conjunction with tungsten carbide particles to provide high wear resistance. Because of the difficulties of accomplishing fusion of such a coating when it has a high content of tungsten carbide, the tungsten carbide content is typically limited to a maximum of 50% by weight.

However, by applying such coatings by high-velocity spraying such as plasma spraying and subsequently accomplishing densification and diffusion bonding of the coating to the substrate by hot isostatic pressing in accordance with the invention, substantially larger amounts of tungsten carbide may be incorporated in the coating as second phase particles, to provide much enhanced wear resistance.

Thus by the practice of this invention, a fusible coating powder having the composition, by weight, Cr 15.5%, C 0.8%, Si 4.3%, Fe 4.0 /O, B 3.5% and balance nickel, a melting range of 964-1003"C and a particle size range -63 to +38 ffi may be combined with a WC/Co composite powder having the composition, by volume, WC83%, Co 17%, size range -50 to +10 in proportions to give an overall powder composition containing at least 50% WC by volume, and the combined powders spray-applied to a roll or iike surface, using a METCO 7M plasma spray fitted with a 2-port GP nozzle and operated at the following settings:: Are gas (argon)flow 100) ) (flow control settings Secondary gas (hydrogen) flow 10 ) per calibrations thereof) Powder carrier gas (argon) flow 37 ) Powder feed rate 2.72 kg/hr (6.0 Ibs/hr) Arc current 500A Arc voltage 70-80 V Spray distance 125-150 mm (5-6 inches) A coating layer built up to a thickness of 1-3 mm in this way may be effectively consolidated by hot isostatic pressing, using direct application of gas pressure, by maintaining the coated substrate under pressure in the range 100-140 MPa for a period of 120-180 minutes while the coated substrate is at a temperature within the range 850-980"C. It will be noted that this temperature range overlaps the lower limit (964"C) of the melting range of the powder composition and exemplifies the possibility of conducting the hot isostatic pressing step at a temperature that slightly exceeds the solidus temperature for the coating composition.

EXAMPLE 3 The method of the invention may also be applied to the repair of castings having surface defects such as porosity or damage, and to the densification of powder metallurgy products having surface-connected porosity. In this application of the invention, a coating layer or envelope of limited surface-communicating subsurface porosity is deposited on the casting or product as the case may be, e.g. to cover the surface defects in the case of a defective or damaged casting, and the coated article then subjected to isostatic fluid pressure to compact both the coating and the underlying surface layers of the article.

An example of such an application of the invention would be the repair of defects in the surface regions of nickel-base superalloy casting. In such an application, a cobalt-base overlay coating is spray-applied to the casting by spraying a powder having, for instance, particles generally less than about 50,a and a composition, by weight, Ni 32%, CR 21%, Al 8%, Y 0.5%, balance Co, using a METCO 7M plasma spray gun fitted with a 2-port GH nozzle and operated at the following settings:: Arc gas (argon) flow 100) ) (flow control settings Secondary gas (hydrogen) flow 5) ) per calibrations thereof) Powder carrier gas (argon) flow 37 ) Powder feed rate 3.63-4.54 kg/hr (8-10 lbs/hr) Arc current 400A Are voltage 60 V Spray distance 100-125 mm (5-6 inches) An overlay coating built up to a thickness of about 2 mm is appropriate. The coated casting may then be subjected to hot isostatic pressing by direct gas pressure to close the surface defects of the casting.

Dependent upon the composition of the superalloy casting, the pressing may be conducted at temperatures within the range 1080 - 1230"C, at pressures in the range 100 - 140 MPa and for duration of 60-180 minutes.

Another example of this application of the method of the invention is the repair of the defects that can arise in use of certain high performance alloys, such as refractory alloys of molybdenum. Because such alloys exhibit recrystallisation and embrittlement under temperature cycling, components formed from these alloys and subjected to temperature cycling become susceptible to cracking damage by mechanical handling.

The method of the invention may be applied to the repair of such damage, a suitable molybdenumcontaining powder being spray-applied into a crack and/or over a cracked area and then densified by direct application of fluid pressure (70 to 105 MPa) at temperatures in the range 1300 to 1400"C. This produces a "seal" for the crack that avoids the risk of further damage such as can occur when traditional repair techniques are employed. It has been observed that a molybdenum alloy component that has been repaired in this way withstands numerous and varied thermal cycles without any deteriotation of the repaired area.

EXAMPLE 4 Another important application of the method is in the fabrication of brake elements having improved performance in friction braking systems for, e.g., aircraft and high-performance road vehicles, or for other heavy duty purposes such as in mine winding equipment. In such systems, frictional engagement between brake elements such as brake shoe(s) or pad(s) and a drum or disc is employed to derive a braking effort in relation to relative motion of the brake elements. In operation, the kinetic energy destroyed by the braking effort appears in the form of heat energy that tends to raise the temperature of the brake elements.Apart from mechanical and material constraints that limit the temperature to which brake elements may be allowed to rise, brake performance can be affected by the temperature of the brake elements, the braking effort generated for a given loading of the engaged elements varying with the temperature of the latter and giving rise to such phenomena as "fade" and "grab". For this reason friction brakes for heavy duty and particularly for high performance vehicle applications involve design compromises and expedients such as forced ventilation to limit temperature rise of the engaged brake elements.

Friction brake drums and discs are typically constructed of steel or cast iron, the material being chosen for its structural strength and for its resistance to surface damage and wear from loaded engagement by the other element or elements of the brake. However, this material has the disadvantage of relatively low thermal conductivity so that the heat energy generated at its surface by engagement with the other brake element(s) does not readily diffuse into the mass of the drum or disc to be dissipated, but instead leads to very high surface temperatures exacerbating the problems of variable performance with changing temperature of the engaged surfaces of the brake elements, and leading to temperature gradients in the drum or disc that tend to result in distortion to the detriment of braking performance.

Further disadvantage of the use of ferrous materials for brake drums and discs is that such materials are dense so that the drum or disc has relatively high inertia that both adds to the kinetic energy needed to be dissipated by the generated braking effort and also has other undesirable implications in, especially, a braking system for a high performance road vehicle.

These and other considerations point to the use of light alloys such as aluminium-based materials.

However, such materials in general do not have surface properties appropriate to friction brake duties and a light alloy brake element such as a drum or disc would need to be provided with a suitable high friction, wear-resistant, layer on its working surface. Amongst other criteria to be met by such a layer are high melting point and a thermal expansion coefficient approximating to that of the light alloy substrate.Layer materials meeting these criteria are rare and such attempts as have been made to apply wear resistant coatings to light alloy discs have failed to achieve a practical brake element of reliable and consistent performance over a useful lifespan, the main problems exhibited by experimental brake elements of this configuration being inconsistent performance and lack of durability consequent upon spalling of the coating layer from the substrate as a consequence of thermal cycling.

We have discovered that a friction brake element such as a friction brake disc of consistent high performance may be produced with good reproducibility by employing the method of the invention to apply to a light alloy substrate a coating layer containing silicon carbide dispersed in a metal matrix, and to densify the applied layer by the direct application of isostatic fluid pressure to the coated substrate at a temperature such as to achieve a diffusion bond between the matrix and the substrate. The matrix metal must be amenable to forming such a bond and should, for reasons indicated, have a thermal expansion coefficient comparable with that of the light alloy substrate.

The preferred matrix metals are copper and copper-base alloys since these have the appropriate melting point and thermal expansion coefficient, are capable of diffusion bonding to light alloys and have the additional advantage of high thermal conductivity to promote flow of heat into the substrate without the generation of excessive thermal gradients in the coating layer.

The layer containing silicon carbide is preferably applied as an overcoat on a primer coat of the matrix metal or of an alloy of the matrix metal, the primer coat being diffusion bonded both to the substrate and to the matrix of the overcoat. A cupronickel alloy is a preferred primer coat material.

The hot isostatic pressing procedure may be so organised as to constitute part of any heat treatment that the substrate may require to develop desired physical properties. Thus, for instance, the temperature and duration chosen for the hot isostatic pressing operation may be selected having regard to a soaking requirement of the heat treatment of the substrate, the pressing operation being terminated with a controlled cooling of the element so that thereafter it is not only necessary to conduct an ageing treatment to develop required physical properties in the light alloy substrate.

Thus the method of the invention has been applied experimentally to the production of a brake disco H15 aluminium alloy for suitable for use in an unventilated, inboard, rear brake installation of a high-performance go-kart and the following description of the production of such an experimental brake disc illustrates this application of the invention.

In fabricating such a brake disc, a disc of H1 5 alloy was first degreased, using conventional condensing solvent vapour equipment or a solvent such as "Genklene N". Thereafter the working surfaces of the disc were grit blasted using G47-55 chilled iron particles in an airstream motivated by a pressure of 140 kPa (20psig). Surface dust was then removed by blasting with dry air or inert gas.

A primer coat of 62/38 copperinickel alloy was then applied to the working surfaces by spraying to deposit a layer having a thickness in the range 0.25 - 0.30mm (0.010 - 0.012 inches). A high-velocity spraying system was used in order to deposit a coating layer having minimal surface-connected subsurface porosity. A suitable high-velocity spraying system for this primer coat may use either a powder feed with a spray gun such as a Metco 7M or 3M, or it may use a wire feed with an appropriate spray gun such as a Metco 1 OE or 12E. The spraying system may be of the inert gas plasma spraying type or of the high-velocity fuel burning flame spraying type, either of which are capable of depositing a coating that has a laminar structure as a consequence of deformation and overlapping of the sprayed particles.

In one experiment the primer coat was applied by plasma spraying cupronickel powder of the required composition, using a Metco 7M spray gun fitted with a two-port GH nozzle and meter wheel S. The cupronickel powder was fed at the rate of about 2.7kg/hr. The arc current was 500 amps at a voltage in the range 60-70 V. Both the arc gas and the powder carrier gas were argon while the secondary gas was hydrogen. The flow control settings (per their calibrations) were: arc gas 80; secondary gas 15; powder carrier gas 37. The spraying distance was 64mm.

In another experiment the primer layer was applied by high-velocity flame spraying of cupronickel wire, using a Metco 1 0E spray gun fitted with a 1 OE-7A 3mm (1/8") nozzle and EC air cap and standard wire feed gears. The cupronickel wire was fed at a rate of 10.4 kg/hr: air, oxygen and acetylene were supplied at pressures of 380,205, and 105 kPa with flow control settings (per their calibrations) of: air 51; oxygen 44; acetylene 40. The spray distance was 125mm and the "burn-off" position was set at 3mm in front of the air cap.

The primer coat was then covered by an overcoat of silicon carbide particles in a copper matrix so as to produce a layer having a thickness in the range 0.64 - 0.76mm (0.025 - 0.030 inches) with a silicon carbide content of about 15% by weight. This overcoat was applied by high-velocity flame spraying using a wire feed with a Metco 1 0E spray gun generally as above described for the flame-spraying of the primer coat.

Following coating with the silicon carbide containing layer the brake disc was then subjected to hot isostatic pressing with directly applied gas pressure at a temperature within the range 450 to 590"C (typically 520"C) at a pressure in the range 70 to 140 MPa (typically 105 MPa) for a duration of 100 to 200 minutes (typically 180 minutes).

Under these conditions the cupronickel primer coat forms a diffusion bond both with the substrate aluminium alloy and with the overcoat copper matrix and residual porosity in both layers is substantially eliminated so that in effect both layers become densified and integrated with the respective working surfaces of the substrate disc.

The temperature and duration combination chosen for the isostatic pressing step is also appropriate to the desired heat treatment of the H1 5 alloy to develop the required strength in the final product. Typically the required heat treatment is concluded by terminating the isostatic pressing step by controlled cooling at the rate of 40-50QClmin,followed by ageing at 1 650C for eight hours and air cooling to ambient.

Brake discs fabricated by the foregoing method have been repeatedly tested under track conditions and have been found to have extremely high braking efficiency with a performance equal to the most advanced ventilated steel disc brake available for the same application; low brake fade characteristics with no distortion of the disc during operation; minimal wear of the cooper/silicon carbide wear resistant working surfaces; and no spalling of the coating from the substrate.

Claims (15)

1. A method of coating a substrate, comprising depositing coating material thereon to form a coating layer of limited surface-communicating subsurface porosity capable of compaction by applied fluid pressure, and thereafter subjecting the coated substrate to heat and direct isostatic fluid pressure to effect compaction thereof.

2. A method according to claim 1, wherein said coating material is applied to the substrate by high-velocity spraying thereby to deposit a coating having a laminar structure with isolated pores therein.

3. A method according to claim 2, wherein said coating material is applied by an inert gas plasma spraying system.

4. A method according to any one of claims 1 to 3, wherein said heat and isostatic pressure are so applied to the coated substrate as to effect densification of the coating and diffusion bonding of the coating to the substrate substantially without liquefaction of any coating material constituent.

5. A method according to any preceding claim wherein the coating material contains a second phase constituent that in the deposited coating becomes dispersed in a matrix of other constituent(s) of the material.

6. A method according to claim 5, wherein the deposited coating comprises particles of a carbide dispersed in a metal or alloy matrix.

7. A method of coating a substrate substantially as described.

8. Coated substrates whenever produced by the method of any preceding claim.

9. A method of fabricating a friction brake element such as a brake disc, comprising applying to a light alloy substrate at least on the working surfaces(s) thereof a layer of limited surface-communicating subsurface porosity containing silicon carbide particles dispersed in a matrix metal of suitable melting point and thermal expansion coefficient, and capable of compaction by applied fluid pressure, and thereafter effecting diffusion bonding of such layer to the substrate and densification of such layer by subjecting the coated substrate to heat and direct isostatic fluid pressure.

10. A method according to claim 9, wherein the said layer is applied to the substrate by spraying techniques such as flame spraying and plasma spraying.

11. A method according to claim 9 or 10, wherein said diffusion bonding is accomplished by isotatic pressure at temperature and for a duration apposite to heat treatment of the substrate.

12. A method according to claim 11, wherein said pressure is applied at a temperature and for a duration corresponding to a soaking requirement of the heat treatment of the substrate, and is terminated by controlled cooling to permit aging to directly follow such cooling.

13. A method of fabricating a disc brake element, substantially as described.

14. A disc brake element fabricated by the method of any one of claims 9 to 13.

15. Every novel feature and every novel combination of features disclosed herein.