Abstract
Coatings are added to components to provide enhanced protection from the surroundings or to recondition them after the surfaces have been damaged. But the change in conditions at the surface will often lead to a change in the resistance to fatigue loading. This investigation is focused on the behaviour of shouldered shafts electroplated with nickel, chromium and a cobalt/chromium carbide composite. Changes, in fatigue strength are demonstrated. Each of the materials has a unique effect on the behaviour of a shaft, although there is a similarity which can be utilised in design. Each can reduce the fatigue strength by a substantial amount, but it is possible to optimize the design of a new shaft so that its strength is not significantly different to that of an uncoated version. The composite coating has the least effect. Explanations for these effects are given, based on differences in the mechanical properties of the substrate and coating materials, the distribution of residual stresses within the materials, and the interaction of these factors with the applied stress profile. Recommendations and methods of analysis which can be used in the design of coated components are presented. The contribution of the manufacturing processes is discussed, and grinding is highlighted as a dominant factor. Gentle grinding procedures are shown to produce ideal conditions by inducing large compressive residual stresses in the surfaces. Radial plots of the residual stresses have been produced. These were obtained by techniques based on Sachs' method, using electrochemical machining and an extended analysis developed for use with coated parts. The stress distributions were not affected by fatigue limit loading, indicating that stress relief methods using amplitudes of this order are ineffective and unsafe.