Alan Hywel Jones

A tribological study of cobalt chromium molybdenum alloys used in metal-on-metal resurfacing hip arthroplasty

Abstract Some modern metal-on-metal bearings have undergone early failure and have exhibited a different microstructure from early hip replacement systems that produced very long lifetimes. This has led to investigations as to why different microstructural conditions, from ostensibly the same alloy chemistry, have had differing levels of survival. This paper is concerned with the effect of thermal treatment and hence the resulting microstructure on the wear properties of coupons using a micro-abrasion test. A series of cobalt–chromium–molybdenum (CoCrMo) alloy (ASTM F-75 98, ISO 5832-4: 1996; BS 7252: Part 4: 1997) sample coupons with different combinations of thermal treatments (solution heat treatment, hot isostatic pressing or sintering) were studied. Metallographic studies have been used to determine the volume fraction of carbide present. The carbide level and morphology varied markedly between samples with respect to thermal history. This work has established that microstructure and in particular the volume fraction, size and distribution of carbides is critical to the development of a low wear rate system. As-cast materials were determined to have greater abrasive wear resistance when compared to single or multiple heat treated materials.

Mechanical properties and tribology of Si3N4–TiB2 ceramic composites produced by hot pressing and hot isostatic pressing

Ceramic matrix composites containing TiB2 as a particulate phase have been produced by hot pressing and hot isostatic pressing. The problems of the reactivity of TiB2 with the Si3N4 matrix and with the sintering environment have been successfully addressed. A novel dual atmosphere sintering profile combined with low temperature hot pressing has been used to successfully produce fully dense materials. The effect of TiB2 addition on mechanical properties has been investigated and the tribological behaviour of optimised compositions has been examined using a ball-on-disc apparatus. Wear coefficients and friction coefficients for Si3N4–TiB2 composites have been measured and compared to monolithic Si3N4 materials. Composites containing TiB2 show significant improvements in hardness, fracture toughness and wear whilst also exhibiting electrical conductivity.

Production and EDM of Si3 N4–TiB2 ceramic composites

Abstract Ceramic matrix composites containing TiB2 as a particulate phase have been produced by hot pressing. The problems of the reactivity of TiB2 with the Si3 N4 matrix and with the sintering environment have been successfully addressed. A novel dual atmosphere sintering profile combined with low temperature hot pressing has been used to successfully produce fully dense materials. The effect of TiB2 addition on mechanical properties has been investigated. Composites containing TiB2 show significant improvements in hardness and fracture toughness. The addition of TiB2 at a level of 40 vol.-% raises the conductivity of the composite to a level where electro-discharge machining (EDM) is possible. A comprehensive study of the application of EDM has been carried out and the optimum machining conditions have been identified. Under these conditions Si3 N4–TiB2 composites have been shaped with relative ease and proved to be ‘robust’ materials under EDM.

Sialon/TiB2 ceramic composites: Synthesis, properties and tribology

AbstractThe addition of titanium diboride particulates to a sialon matrix offers the possibility of ultrahard ceramic materials suitable for tribological applications (e.g. ball bearings, cutting tools, etc.). Sialon matrix composites have been fabricated by hot pressing and pressureless sintering with TiB2 added by both direct addition and in situ reaction sintering. Both processes require a dual atmosphere sintering cycle of argon followed by nitrogen at the sintering temperature. Hot pressing has been used to produce materials which approach full density independent of the amount of TiB2 present. However, the densification of pressureless sintered materials is inhibited by the presence of TiB2 . Measurements of hardness, indentation fracture toughness, microstructural examination, and tribological behaviour showed significant improvements as a result of the TiB2 addition. At >40 vol.-%TiB2 the low resistivity of these materials also allows the possibility of electrodischarge matching (EDM). A mini pin-...