DESIGNER SURFACES FOR TITANIUM COMPONENTS


INTRODUCTION Titanium and its alloys are particularly attractive for use in situations requiring a high strength to weight ratio, for example in aerospace and automotive, particularly motorsport applications. The metal and associated alloys not only have low density allied to high tensile strength, but have good resistance to corrosion under normal conditions, and excellent bio-compatibility. One may well ask why these materials are not therefore in much more widespread use than at present. This is mainly because titanium alloys are characterised by, especially in sliding situations, poor tribological properties, including high and unstable friction coefficients, severe adhesive wear, susceptibility to fretting wear, and a strong tendency to seize. This poor tribological behaviour of titanium can be attributed to the electron configuration, crystal structure and ineffectiveness of lubricants. Furthermore, titanium and its alloys can exhibit poor corrosion resistance in some aggressive environments, such as high temperature reducing acids and a susceptibility to crevice corrosion in hot chloride solutions.


However, there is ever increasing interest in the applications of titanium alloys in such sectors as chemical, off-shore, biomedical, automotive, performance sports, power generation and general engineering in which tribological and corrosion behaviour are often major concerns for titanium component designers. Much work has therefore been carried out in investigating ways of overcoming the tribological and environmental limitations of titanium alloys, thus realising their full benefit in various applications. Since the tribological and environmental limitations of titanium and its alloys are closely related to their inherent surface nature, problems may be overcome only by changing the nature of the surface i.e. surface engineering i.e. "the design and modification of the surface and substrate together of a component, as a system, to give cost effective performance enhancement of which neither is capable on its own (Bell, 1991)".


Surface engineering has proved to be a most promising way to enhance the surface-related performance of titanium and its alloys by producing designed surfaces with economically viable technically enhanced performance. This contrasts with earlier approaches which merely endeavoured to use one of a number of existing techniques to solve or alleviate problems arising from inadequate material selection or design. Developments in the surface engineering of titanium alloys during the past few years have targeted tribological property improvement, corrosion resistance enhancement, and/or achievement of a synergistic combination of both improved tribological performance and elevated corrosion resistance in an alloy.


THE THERMAL OXIDATION PROCESS


A novel surface engineering process based on thermal oxidation, designated the TO process, has recently been successfully developed. This thermochemical process effectively improves the tribological behaviour of titanium alloys under light to moderate loads without deteriorating the good corrosion resistance. A typical cross-sectional structure of Ti6AI4V specimen treated using the proprietary TO treatment comprises a thin outer rutile oxide layer (-2J.Lm), overlying the oxygen diffusion zone (-20J.Lm).


material obtained by using a we ball sliding against the Ti6AI4V disc show marked differences. The friction trace of the untreated material fluctuates widely throughout the whole testing period, indicative of the 'stick-slip' adhesive behaviour of titanium and its alloys when sliding against most engineering materials. The friction trace of the TO treated material shows significantly reduced and stable friction values.


Friction coefficient profiles of untreated and TO treated


This friction-reducing effect of TO treated material is ascribed to the following mechanisms. Firstly, it is generally accepted that plastic deformation between contacting surfaces makes an important contribution to wear and friction. Significant reduction in friction and wear is anticipated if the contacts between surfaces are predominately elastic. Previous work on characterisation of surface mechanical properties using a nano indentation technique shown that the TO treatment effectively limits the degree of plastic deformation. As a consequence, a high degree of elastic contact can be anticipated, which favours low adhesion between the contacting surfaces and hence low friction .


Secondly, slightly oxygen-deficient rutile (Ti02-x) behaves as a low shear strength, lubricious oxide due to its particular crystallographic shear (CS) plane. Work has now shown that the TO treatment induced oxide is most likely to be slightly oxygen-deficient or substoichiometric (Tio,·_gs). Thus, there is a good reason to believe that the observed low friction of TO treated material may, to some extent, be related to the slightly oxygen-deficient or substoichiometric rutile . In short, both of the above mechanisms in terms of low plastic deformation and the low shear strength of rutile formed in the TO process are involved, to a varying degree, in reducing friction of Ti6AI4V alloy.


In lubricated sliding-rolling wear tests involving TO treated and untreated Ti6AI4V, it was shown that the untreated material is characterised by a very high wear rate ( 2. 76 x1 0·3


mg m·'), which may be associated with the


preferential transfer of Ti6AI4V onto the steel counterpart. The wear surface of the untreated material was very rough,


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