| The surfaces of titanium and of all commercially produced alloys of titanium have relatively poor wear resistance. In particular, titanium surfaces in contact with each other or with other metals readily gall under conditions of sliding contact or fretting. Even with light loading and little relative movement, complete seizure of surfaces can occur. This situation is caused by adhesive wear in which microscopic asperities on the metal surfaces come into contact as a result of relative sliding and they tend to weld together forming a bond at the junction which can have a rupture strength greater than the strength of the underlying metal. Fracture then takes place at one of the asperities causing metal to be transferred from one surface to the other. The debris so formed gives rise to the accelerated wear that occurs with titanium. In view of this situation, should it be necessary to use titanium in circumstances where galling or fretting could be a problem, it is essential that use be made of one of a number of protective coatings that are available for the material. Enhanced Surface Films The thickness of the naturally occurring oxide film on titanium can be artificially increased to provide a coating that will prevent galling in situations of sliding contact involving light loads and low speeds. The most common method of producing an enhanced oxide film on titanium is by anodising in either acid or alkaline electrolytes. Anodising in acid solution results in a film the thickness of which is dependent upon the voltage applied. This technique is used widely with titanium fasteners to prevent seizure in the screwed threads. By use of an alkaline electrolyte, a thick porous oxide can be produced within which conventional solid lubricants such as molybdenum disulphide can be incorporated. While not suitable for applications where there are high loads applied, conventional enhanced oxide film coatings have the advantages that they are relatively cheap to apply, they are carried out at room temperature and so do not result in any distortion of the component being treated, and they do not have any adverse effect on the fatigue resistance of the titanium. More recently, work has been carried out on the enhancement of oxide films on titanium by controlled heating of the material. This can have a substantial beneficial effect on wear resistance and allows titanium to be used under much more severe conditions than the conventionally produced oxide films. Conversion Coatings Numerous materials can be diffused into the surface of titanium by relatively high temperature heat treatments to provide a hard surface layer on the material. An example of this is carbo-nitriding by immersion in a molten cyanide bath at around 800°C. Unfortunately, while this type of coating does substantially improve wear resistance, the temperature at which the treatment is carried out can cause distortion problems and also most conversion coatings result in a marked drop in the fatigue properties of the titanium. Alternative forms of conversion coating which have been applied to titanium either experimentally or as commercial processes are borodising, carburising, cyaniding and diffusion treatments of aluminium, copper, chromium and tin. The latest version of these processes is laser or electron beam surface alloying. Here a coating on the titanium surface is melted in and the molten layer is rapidly cooled. Electrolytic and Electroless Plating Processes It is difficult to plate metals onto titanium because of the presence of a stable oxide film on its surface. However, techniques have become available which will allow adherent coatings to be deposited and certain of these such as hard chrome plating provide wear resistant coatings on titanium. Similarly, electroless nickel plating can be beneficial particularly if PTFE or ceramics are incorporated into the coating. Any deterioration in fatigue properties can largely be restored by shot peening of the component prior to the plating operation. Flame, Plasma, and Detonation Gun Sprayed Coatings A considerable amount of work has been carried out to examine the characteristics of detonation gun and plasma arc sprayed coatings on titanium and its alloys. The technique is widely accepted by the aircraft industry both in the USA and in the UK and satisfactory adherence between the substrate and the coating can be obtained. A wide range of materials from pure metals to cermets may be applied but tensile strength and fatigue resistance of the titanium are normally reduced by these coatings. The surface characteristics are clearly those of the coating and can largely be selected according to application because of the versatility of the processes. Flame and plasma spraying of molybdenum has been satisfactorily carried out to build up and protect titanium surfaces in sliding contact, for example, on the stems of motor car poppet valves and the slide faces of titanium alloy connecting rods. Adhesion of coatings was originally poor but further development has largely overcome this problem. Nitride Coatings Titanium nitride coatings developed for the treatment of tool steels can have a beneficial effect on the wear resistance of titanium itself. The physical vapour deposition (PVD) technique involves the production of titanium nitride in a chamber by the reaction of low pressure nitrogen gas with titanium atoms or ions from an auxiliary source. The TiN so formed is then deposited onto the titanium workpiece. In plasma nitriding, the titanium surface is activated by plasma in either a nitrogen or ammonia atmosphere. In both the PVD and plasma nitriding processes, films are produced on the titanium surface having hardnesses of around 2000 HV and low frictional coefficients. In each case there is some reduction in mechanical properties, principally fatigue resistance, but this can be overcome by shot peening. Other techniques for introducing nitrogen into the titanium surface include ion implantation and high pressure gas nitriding. Shot Peening Shot peening in itself can be effective in reducing fretting and galling of titanium and can be very beneficial in restoring the fatigue properties of material which has been treated by other techniques. Essentially, the process involves the introduction of compressive stresses into the titanium surface by controlled bombardment with media such as glass beads, ceramic balls, or chopped stainless steel wire. Shot peening would normally be carried out prior to the coating operation but may have to be post coating should the process involve heating of the titanium which would anneal out the stresses introduced during shot peening. |