The Donald Julius Groen Prize Lecture

Citation

(1999), "The Donald Julius Groen Prize Lecture", Industrial Lubrication and Tribology, Vol. 51 No. 3. https://doi.org/10.1108/ilt.1999.01851cab.013

The Donald Julius Groen Prize Lecture

The Donald Julius Groen Prize Lecture

This lecture was given by Professor Tom Bell of the University of Birmingham, the subject being "The tribological enhancement of titanium alloys". The presentation ended a most interesting and enjoyable afternoon at the Institution of Chemical Engineers, being preceded by the Mission of Tribology Research ­ seven presentations and awards. Much of the subject matter of this presentation was covered in the 1998 Issue 6 of Industrial Lubrication and Tribology in an article entitled "Designer surfaces for titanium components" by Research Fellow Hanshan Dong and Professor Bell.

Keywords Alloys, Titanium, Tribology

Titanium is the fourth most abundant structural metal in the world. Titanium and titanium alloys have been increasingly used since the pure metal first became commercially available in 1948. This is primarily because of their combination of outstanding properties in terms of high strength to weight ratio, exceptional resistance to corrosion, and excellent bio-compatibility. These attractive properties of titanium alloys make them suitable for a wide range of applications, ranging from aerospace through defence, chemical, petrochemical, marine and offshore. On the other hand, titanium alloys, especially in sliding situations, are characterised by poor tribological properties, including high and unstable friction coefficients, severe adhesive wear, susceptibility to fretting wear, and a strong tendency to seize. As a result, the use of titanium alloys has generally been restricted to non-tribological applications.

However, with the end of the "cold war", defence spending has been reduced, resulting in cutbacks in the aerospace industry which used to consume over half of the production of titanium alloys. At the same time, there is ever-increasing interest in the applications of titanium alloys in such sectors as biomedical, automotive, performance sports, power generation and general engineering, in which tribological behaviour is often a major concern. Therefore, how to overcome the tribological limitations of titanium alloys, thus realising the full benefit of titanium materials in friction and wear application, is a timely task, which presents a major challenge to surface engineers and tribologists from both a scientific and technological point-of-view.

Surface engineering research centred at the University of Birmingham for several years has been focused on enhancing the tribological properties of titanium based materials. Significant progress towards titanium designer surfaces has been made recently through the invention of a cost-effective titanium oxide thin film technique and a novel duplex system combining DLC coating with deep oxygen case hardening. Such advances thus provide scope for designer titanium components for general engineering applications, usually as direct replacements for steel components.

The poor tribological characteristics of titanium and titanium alloys is due to a number of factors. The fact that the electron structure of titanium has a low d-bond structure results in an active surface which will readily form alloys with other materials. Also, alpha titanium, although having a hexagonal structure, has an axial ratio c/a of 1.588, which is less than ideal for close packing. The resultant increase in blocking of dislocations and strain hardening results in higher friction coefficients. The ineffectiveness of conventional lubricants when used on titanium is thought to be due to the fact that little effective adsorption of the lubricant molecules takes place on titanium surfaces, also the low heat conductivity of titanium results in higher surface temperatures, resulting in desorption of boundary lubricant.

Great progress towards titanium designer surfaces has been made recently through the invention of the cost-effective thermal oxidation (TO) technique and the novel duplex system combining DLC coating with BDOX deep case hardening. Although it was observed in the early 1950s that the surface of titanium was effectively hardened when heated in the range 850 to 1,000°C in air at a pressure between 10^-3 and 10^-2mm Hg, little interest was shown at the time since, due to the formation of excessive scale, the process was not considered to be commercially feasible. However, treatment of titanium alloys by the TO method, where a thin compound rutile layer of about 2µm on top of a diffusion zone was formed, resulted in surfaces with reduced friction values and lower wear rates. Such surfaces dramatically improved the tribological behaviour of most titanium alloys under light to moderate loads, especially under oil lubricated conditions. However, the load carrying capacity of TO treated components was not high enough to withstand the high stresses encountered in bearings and gears, and further development was indicated. The depth of the modified surface was increased by a boost diffusion oxidation (BDOX) process, and, although this resulted in improved abrasive resistance, sliding wear resistance was only marginally improved. Consequently, a novel duplex system combining low friction, high wear resistance diamond-like coating with BDOX deep case hardening was developed, where amorphous hydrogenated carbon a-C:H or DLC was sputtered onto the BDOX-treated surface using an rf-reactive sputtering system. Adhesion problems of DLC were overcome by introducing a graded intermediate layer, Ti/TiN/TiCN/TiC between the substrate and the DLC layer. This resulted in a high-hardness surface with an extremely low friction coefficient, although the exact mechanisms involved are still the subject of some debate.

Such advances have provided scope for designer titanium components for general engineering applications, usually as direct replacements for steel components. The autosport and off-shore industries are currently realising the potential of such advanced surface-engineered titanium alloys.