T.A. Stolarski

Wear: mechanisms, manifestations and measurement. Report of a workshop

OBJECTIVE The purpose of this workshop was to bring together a tribologist (T.A.S.), a clinician (L.H.M) and a dental materials scientist (R.W.V) to discuss the fundamental mechanisms of wear and how these relate to the manifestations and measurement of wear in dentistry. Eighty delegates contributed to a valuable discussion led by the workshop chairman (C.H.L.). KEY POINTS Wear is the net result of a number of fundamental processes: abrasion, adhesive effects, fatigue and corrosive effects which act in different combinations on the various classes of materials. In ceramics, sliding compression and surface corrosion greatly increase wear, whereas the ability to deform either plastically or elastically influences the wear of polymers. The filler particle distribution is a major variable influencing the wear of composites. In the mouth, wear can be considered in terms of its site, timing and mechanism. The latter may involve direct contact between surfaces or result from the action of slurries; both of which are affected by surface corrosion (erosion). Although wear can be categorized at the chairside, its precise measurement involves the use of replica models and surface contouring. Laboratory simulation is useful to study fundamental wear mechanisms; but it is not able to predict clinical wear. CONCLUSIONS The management of clinical wear requires a proper understanding of the underlying mechanisms. This can only be achieved through close co-operation between all the disciplines which seek to understand and manage wear. The attendance of so many delegates at this workshop indicates the willingness to participate in this process.

The friction and wear of poly(tetrafluoroethylene)-poly (etheretherketone) composites: An initial appraisal of the optimum composition☆

Abstract In this paper an experimental study of the friction and wear of a number of poly(tetrafluoroethylene)-poly(etheretherketone) (PTFE-PEEK) composites over a wide composition range is described. The inclusion of PTFE reduces the friction of PEEK although there is a small loss in wear resistance. However, the friction reduction significantly improves the load-bearing capacity at high sliding velocities. The inclusion of the PTFE also reduces the magnitude of certain mechanical properties such as the hardness. A sufficient concentration of PTFE is approximately 10% although the precise value may depend on factors such as the fabrication and operating conditions.

Tribology of polyetheretherketone

Abstract A comprehensive discussion of tribological performance of polyetheretherketone is presented. It is based on experimental results obtained in a series of studies carried out in order to assess the potential of the polymer as a material for sliding and rolling contacts.

Failure modes of ceramic elements with ring-crack defects

Abstract A hybrid ceramic/steel angular contact ball bearing is experimentally modelled using a modified four-ball machine. Ceramic ball surfaces are artificially damaged with ring pre-cracks. Rolling contact fatigue failure modes are studied after testing with high contact stresses and speeds. The ceramic ball surface and subsurface are analysed in detail using electron microscopy and a dye-penetration technique.

Rolling contact wear of polymers: A preliminary study

Abstract The wear of three different engineering polymers under rolling contact conditions has been investigated. A subsequent analysis of the results has been carried out to assess the suitability of these polymers for their application in rolling contact bearings.

Boundary lubrication of thermoplastic polymers in model fluids

Abstract Organic polymers, particularly thermoplastics, have traditionally been used in unlubricated contacts because of what is termed their ‘self lubricating’ character. Significant improvement in bearing performance may often be obtained by conventional liquid lubrication but a number of subtle interfacial phenomena are sometimes noted some of which are deleterious to good operation. A particular type of problem arises when the fluid migrates to a significant depth into the matrix of the polymer and causes a premature form of scuffing failure under intense levels of energy dissipation. This paper reviews the scuffing behaviour of several commercial polymeric matrices and speculates upon the fluid-polymer interactions which can occur at the onset of lubrication failure.

The effect of the test machine on the failure mode in lubricated rolling contact of silicon nitride

The experimentally assessed performance of technical ceramics in lubricated rolling contact is presented. Special emphasis is on the failure modes resulting from the test conditions encountered in the model rolling contact used.

Failure Modes of Ceramics in Rolling Contact

A comprehensive experimental study has been carried out to ascertain the prevailing mode of failure of concentrated rolling ceramic contacts subjected to cyclic loading conditions. The findings of the studies presented in this paper confirm that good quality silicon nitride balls in non-conforming contact do not fail in a catastrophic mode and thus can be considered for important engineering applications such as rolling contact bearings for gas turbines. The results of extensive testing carried out on perfect and artificially pre-cracked balls under high compressive cyclic stress showed that the prevailing mode of failure is delamination. Scanning electron microscope (SEM) observations enabled the delamination failures to be classified in terms of location of crack initiation and subsequent crack propagation. By using X-ray diffraction, residual stresses on delaminated surfaces were measured and results showed that under certain conditions plastic deformation of the ceramic can take place. Results of supplementary chemical analysis of failure surfaces suggest that changes in the level of silicon, nitrogen and oxygen may take place on delaminated surfaces. Experiments with artificially pre-cracked balls revealed the role played by a lubricating medium in the process of fatigue failure. All findings are illustrated with SEM micrographs and the physical aspects of failure mechanics are discussed.

Adhesive wear of lubricated contacts

Abstract A model has been developed to describe adhesive wear of lubricated contracts. It was then used to predict wear and the predictions were tested experimentally

Transfer wear of polymers during combined linear motion and load axis spin

Abstract A series of experiments are described where wear properties of a number of polymers are tested under combined apparent and imposed rotations of a specimen, using a pin-on-disc configuration. Wear of polytetrafluoroethylene (PTFE) and high density polyethylene is sensitive to the rotation of the contact area and is a marked function of Δφ, the angle of pin rotation during linear travel equal to the diameter of the apparent contact area. This effect is not observed for polymers such as low density polyethylene and polymethylmethacrylate. It is believed that the special wear behaviour of PTFE and linear polythenes arises from rotation of the contact area which results in a continuous disruption of natural orientation processes which are normally produced during linear motion.