P. Pallav, PhD

Introduction

In engineering science, arbitrarily several types of wear are distinguished, some of which will be briefly described here.

The many different wear mechanisms rarely occur separately. In reality the various mechanisms usually act simultaneously and / or sequentially and often influence each other in a very complex way, resulting in the general tendency of wear processes to be very difficult to predict.

The first two types, abrasive and adhesive wear, can be seen as fundamental consequences of the contact hypothesis as formulated by Bowden and Tabor (1950). Simply speaking, abrasive wear is caused by normal (=vertical) forces and adhesive wear is caused by tangential (=horizontal) forces at a contact.

In vivo wear of composites

About composites might be said that in many respects these should be viewed as resin materials with an extremely high inclusion count, which renders them highly susceptible to fatigue effects. This is complicated by the fact that filler and matrix consist of fundamentally different materials, the relative influence of which varies greatly with the wear type. With erosion, the relatively soft matrix allows digging out of filler particles, while with direct contact, abrasive and perhaps adhesive wear is located at the peaks of protruding filler particles.
If however, the filler is also relatively soft (micro-filled pre-polymer particles), then the antagonist may be smoothened by material transfer and in this way, wear may be reduced. The wear rate generally decreases at increasing strength of the filler and a higher amount of filler.
A finer filler usually also reduces wear. However, in abrasion processes, the filler particles should be large enough with respect to the antagonist's roughness asperities, to provide for sufficient support by the matrix.
At in vivo OCA, the wear facets observed on protruding filler particles and occasional fractured filler particles are caused by a mainly abrasive process in direct contact with the antagonist. In spite of the apparently hermetical occlusion of these facets during the process, the wear here is quite sensitive to the presence of many substances. In these direct contact regions frequently subsurface damage is detected in the form of a stainable subsurface region or microscopically visible cracks.

Impact wear

In engineering science the term impact wear is used for phenomena which cannot be explained by a temporarily increased load or pressure. Studies into impact wear usually deal with impact velocities greater than, or of the same order of magnitude as the velocity of sound in the substrate(s).

When two cusps repeatedly hit each other with a certain speed, surface fatigue is more likely to occur when the impact velocity is relatively high. This is because each time at the moment of impact, the force at the point of contact is temporary very high. However, the effect of the impact isn't much different than that of a temporarily increased load. A case like this would be classified as surface fatigue and not as impact wear.  

In dental science, this term is used loosely for wear processes where the velocity at which the upper and lower teeth hit each other seems to work as a source of pressure and velocity.

Bowden FP and Tabor D (1950): The Friction and Lubrication of Solids. Clarendon Press, Oxford.