J. Halling

Friction and wear of ion-plated soft metallic films

Abstract Experimental results of the tribological behaviour of soft metallic films on hard substrates are given. It is shown that the adhesion of the film to the substrate is a critical factor determining the life of the film which illustrates the value of ion-plated films. The friction mechanism is discussed and is shown to agree with the basic premises of an existing theory. The wear mechanism appears to be an initial microcutting of the film by the mating surface asperities followed by a much less severe form of fatigue wear due to asperity-substrate contacts.

A contribution to the theory of mechanical wear

Abstract An attempt is made to predict the wear behaviour, particularly of metals, when wear arises from asperity interactions alone. It incorporates the concept of fatigue failure and also of simple plastic deformation failure which is considered as fatigue failure in one cycle of loading. The analysis demonstrates that fatigue mechanisms of wear are not inconsistent with observed behaviour and indicates an approach to a better appreciation of the effects of such factors as material constants and surface topographical features.

The Hertzian contact of surfaces covered with metallic films

Abstract A solution is presented for the size of the contact zone and the degree of normal approach when an elastic sphere is in contact with a rigid substrate surmounted by an elastic film. The solution is based on the method of Popov for similar problems when the film thickness contact radius ratio is less than 0.5 and on the method of Vorovich when this ratio is greater than 1.7. Between these two values the solution is inferred by interpolation and corrections are also introduced to accommodate elastic deformation of the substrate.

The normal approach between rough flat surfaces in contact

Abstract The relationship between the normal approach and load for a variety of rough flat surfaces has been considered experimentally and theoretically. The experimental values of normal approach measured by a capacitance technique correlate well with the theoretical values based on the measured surface parameters. For smooth surfaces the texture deformation is essentially elastic whilst the rough surfaces behave plastically. Linearity between the normal approach and the surface roughness at any given load is established and is thereby in agreement with theoretical expectation.

Ceramic coatings in the war on wear

Abstract The conditions for elastic contact between the asperities on contacting surfaces are considered in this paper, and a unique relationship between the plasticity index and the non-dimensional nominal pressure is provided. If conditions of elastic contact can be maintained then mechanical wear can occur only by fatigue. It is shown that such conditions can be realistically maintained with ceramics over a wide pressure range and with polymers and rubbers at relatively low pressures. Recent developments in surface coating technology enable ceramic coatings to provide excellent solutions to many tribological problems. The effects on the tribological properties of such coatings of surface finish, effective elastic modulus, effective hardness and coating thickness are described.

The thermal behaviour of rolling bearing assemblies subjected to preload

Abstract The experimentally observed time dependent behaviour of two different rolling bearing assemblies subjected to initial preloads is described for a wide range of operating conditions. It is shown that although the assemblies exhibited very different and apparently complicated behaviour their response, for a given set of conditions, was dependent on only two factors, namely the thermoelastic properties of the system and the manner in which the rate of energy input into the system varied with time. Both assemblies exhibited well defined thermal instability which was determined entirely by these properties.

A contribution to the theory of friction

Abstract The nature of the interaction between a rigid spherical asperity and an asperity governed by the stress/strain law \ gs = B \ ge n is studied. The interfacial shear stress is defined by fτmax where 0 , τmax being the maximum allowable shear stress at the contact. By integrating the total effect of a population of such surface asperities expressions for the total frictional forces and the total load are derived. The value of the coefficient of friction is thus obtained and the special conditions for perfectly plastic and elastic behaviour are considered. In both cases the friction coefficient is seen to contain a term defined by the deformation and dependent on surface roughness and a term totally defined by ƒ.

A new method for the study of ball motion in ball bearings

Abstract In this paper apparatus which enables the study of the orientation of the magnetic axis of a ball in a complete ball bearing assembly is described. A continuous record of the axis location thereby allows the determination of the instantaneous velocity and acceleration of the ball. The location of the ball axis is determined using a coordinate system of Hall elements. Using this new method it is expected that reliable experimental studies of ball motion in complete bearings under complex loading conditions will be possible.

An experimental study of the Hertzian contact of surfaces covered with ion-plated films

Abstract The results are given of an experimental study of the normal-approachload relationship for steel spheres on steel plates covered with ion-plated lead films. It is found that the degree of plastic deformation is large at high loads for the thickest films (15 μm). With the thinnest films (1 μm) the degree of plastic deformation is of the same order as that associated with the deformation of the surface texture; this suggests that for such thin films an elastic theory for such composite materials is appropriate.

Tribological Properties of Solid Materials

Much of the material in the previous chapters has been concerned with the tribological properties of metals, and it is not necessary to reiterate it here. In particular, the effect of hardness on resistance to wear was mentioned frequently in Chapter 3 and the importance of oxide films was stressed in Section 3.5. However, in covering the basic mechanisms of wear some other properties which can have a marked effect on wear behaviour were mentioned only in passing, or not at all. The effects of these properties are described in the following sections.