GB2050433A - Sputter coating objects with a compound of a transition metal - Google Patents

Abstract

An object is sputter coated with a compound of a transition metal by sputtering the pure metal target onto the cleaned object to form a sputtered layer of the pure metal, introducing into the sputtering chamber a gas which reacts with the surface of the sputtering target to form the compound to be deposited and sputtering the compound so formed on the surface of the target to build up a film of the compound bonding with the film of pure metal previously deposited on the object. The target operates at a temperature sufficiently high to synthesize the compound to be deposited.

Description

SPECIFICATION A method of coating objects with an adherent and coherent coating of a compound of a transition metal The invention is concerned with coating objects with compounds of transition metals and in particu larthe nitrides and carbides of the elements zir conium, titanium, vanadium and hafnium.

It is often desired to coat precision mechanical parts, and in particular bearing elements and valve spools, with titanium nitride because of its hardness and low friction properties. However, it is important that the coating should be capable of withstanding high pressures and significant mechanical stresses without itself deteriorating or becoming detached from the underlying object.

Hitherto, it has been proposed to form such coat ings by chemical processes of various types but none of them has been found to be totally satisfac tory.

Physical processes including sputtering have also been proposed but these, too, have not been satis factory. For example, in coating objects of, say, stain less steel or titanium, the surface of such objects when exposed to atmosphere forms a stable oxide layer which is not capable of bonding satisfactorily with a sputtered nitride coating. Although the stain less steel and titanium can accept such a coating the fact that these materials are reactive and in atmos phere develop an oxide layer prevents a nitride layer from being applied directly to them.

It has therefore been proposed priorto depositing a layer of titanium nitride to clean the surfaces by reversing the sputtering process and sputter-etching the objects to be coated, thereby disposing of the oxide layer. Whilst this technique can be employed on flat objects it presents problems when the objects have a complicated geometry. There is a tendency for surfaces facing the effective anode structure to become sputter-etched but any surface at a right angle will not be etched and will instead acquire a sputter coating.

The present invention seeks to provide a method of coating objects even of irregular shape to provide a satisfactory adherent and coherent coating of a transition metal compound such as a nitride or a carbide.

In accordance with the present invention, there is provided a method of coating an object with a com pound of a transition metal which comprises clean ing the surface of the object, introducing the object into a sputtering chamber having a target to be sput tered formed of the pure metal to be deposited in compound form on the object, sputtering the pure metal onto the object to form a sputtered layer of the pure metal keying into the cleaned surface of the object, introducing into the sputtering chamber a gas operative to react with the surface of the sputter ing target to form on surface of the target the com pound to be deposited and sputtering the compound so formed on the surface of the target to build up a film of the compound bonding with the film of the pure metal previously deposited on the object, the target being arranged to operate at a temperature sufficiently high to synthesize the compound to be deposited.

Preferably, the target is surrounded by an anode having a significantly greater surface area than the target so that the material sputtered onto the anode can itself act as a gettering material to purify the gas within the sputtering chamber whereby during the initial phase of sputter deposition a substantially pure coating of the metal is deposited onto the object to be coated.

The invention has particular applicability in the coating of the spools of metering valves used for metering fuel in such applications as aircraft where metal surfaces are caused to slide against each other without the possibility of a lubricant being introduced between them. Such spools are subjected to significant wear in operation and require a hard coating and hitherto no satisfactory method has been found to provide the spools with an adherent nitride coating or titanium.

It is not essential that the coating material be the same as the substrate being coated since, for example, titanium can be used to coat an object made of aluminium or stainless steel. The advantage of using titanium is that it forms a coherent as well as an adherent coating by which it is meant that the coat itself has strong mechanical properties in addition to its ability to remain adherent to the substrate made of titanium or any other suitable material.

The layer deposited by the method of the invention was analysed by Bragg diffraction techniques and found to consist of a single layer of titanium nitride in its pure stoichiometric form although the diffraction analysis did display a preferred orientation of the crystals which is consistent with the physics of sputter deposition.

The invention will now be described further, by way of example, with reference to the accompanying drawing, which is a section through an apparatus suitable for performing the method of the invention.

In the accompanying drawing a sputtering electrode assembly generally designated 10 is mounted within an aperture in a sputtering chamber 14. The electrode assembly comprises an anode 16 and a cathode mount 18 including a chuck 20 serving to grip a target rod 21. The cathode mount 18 is supported on a glass sleeve 22, the other end of which rests on the anode 16. Suitable vacuum seals 24 seal the glass sleeve 22 relative to the cathode mount 18 and the anode 16 and clamping arrangements serving to clamp the glass sleeve to the anode and the cathode mount respectively. In the interests of clarity, the clamps are not shown but the glass sleeve 22 is shown to be conically tapered at each end to enable the clamps to grip the sleeve.

The anode 16 has a flange 26 which is gripped by a clamping ring 28 which is secured by means of bolts 30 to the sputtering chamber 14. The sputtering chamber 14 illustrated is one similar to those conventionally used with disc shaped electrodes although this need not necessarily be the case. The chamber is generally in the shape of a drum with an inspection window 32. Facing the aperture 12 there is a similar aperture on the other side of the drum in which there is sealingly arranged a cooling chamber 34to which there is secured a support plate 36 for the articles to be coated.

The anode 16, which in common with the chamber 14 is made of an aluminium alloy, has a surface 40 which is generally frustoconical and with an area significantly greater than the area of the target rod 21. The anode 16 also has an extension 42 which extends between the chuck of the cathode mount 18 and the glass sleeve 22. The extension serves as a heat shield to preventthe heat of the cathode from damaging the glass and also as a shield for radiofrequency radiation which could cause the glass to be heated. The gap left between the anode extension 42 and the cathode mount is sufficiently narrow as to act as a dark space within which no sputtering or discharge takes place during operation.

The arrangement described is shown with supports for two types of objects to be coated. The first is forsmall plates and these may rest on suitable stands 44 which are arranged at an angle to the target rod 21 and surrounding the target rod. The support 36 also has means for mounting rod-like objects 46 to be coated and these are connected by means of a planetary gear system 48 to a drive motor 50 which is arranged to rotate them during the sputtering operation.

In orderto perform the method of the invention, for example to coat the rods 46 with titanium nitride, a titanium target rod 21 is used and a high power radio frequency signal is applied to the cathode with the chamber held at earth potential. The chamber 14 is at first filled with an ionizable gas under low pressure, the gas being inert so as not to react with the target rod. On application of the RF voltage, the gas will be ionized and will bombard the surface of the target.

The arrangement described, having a large anode area, enables very high current densities to be developed, sufficient to raise the target temperature to approximately 1000"C. whereupn sputtering may take place at a high rate. Because of the high rate of sputtering, any oxide layer initially on the titanium rod is very rapidly sputtered away and after a short period of operation the sputter-deposited material would be substantially pure titanium. The large anode acts as a gettering surface to assist in the purification of the sputtering gas. The objects which are to be coated, having already been cleaned before insertion into the sputtering chamber, will now acquire a coating of pure titanium which will adhere strongly to the cleaned surface of the objects to be coated.

After allowing a pure coating to be deposited on the objects, a gas, for example nitrogen, is intro duced into the sputtering chamber and at the temp erature of the target rod a reaction will occurto form titanium nitride on the target rod. This titanium nit ride is now sputtered and will readily adhere to the pure titanium coating already present on the objects to be coated to form an adherent coherent layer. It is preferable to increase the nitrogen content of the gas in the ionizing chamber gradually so that the nitride deposit is phased into the deposit of pure titanium.

If an article such as a rod 46 is placed on the support 36 the surface facing the target 21 will be preferantially coated. To provide an even coating, the rods may be rotated by a suitable drive motor 50 and the planetary gear mechanism 48.

By employing the method above described, it has been found possible to coat valve spools of metering valves to improve their mechanical performances significantly by comparison with the previous methods of titanium nitride coating including chemical methods of deposition and prior art sputtering methods.

Although the invention has been described with reference to the deposition of a titanium nitride coating, the method is equally applicable to other transition metals such as zirconium, vanadium and hafnium, and compounds other than nitrides of such metals. For example, carbides can be deposited by introducing a suitable hydrocarbon into the sputtering chamber which has a target rod formed of the pure transition metal.

Claims (1)

1. CLAIM

   1. A method of coating an object with a compound of a transition metal which comprises cleaning the surface of the object, introducing the object into a sputtering chamber having a target to be sputtered formed of the pure metal to be deposited in compound form on the object, sputtering the pure metal onto the object to form a sputtered layer of the pure metal keying into the clean surface of the object, introducing into the sputtering chamber a gas operative to react with the surface of the sputtering target to form on the surface of the target the compound to be deposited and sputtering the compound so formed on the surface of the target to build up a film of the compound bonding with the film of pure metal previously deposited on the object, the target being arranged to operate at a temperature sufficiently high to synthesize the compound to be deposited.