SHAKEDOWN LIMITS ON COATED AND ENGINEERED SURFACES

When a hard rough surface slides repeatedly on a softer component a sy stem of protective residual stresses may be developed in the near-surf ace layers of the weaker material which enable loads sufficiently larg e to cause plastic deformation in the early cycles of loading to be ac commodated purely elastically in the later stages of the loading histo ry. This is the process known as shakedown and limits on the intensity of loading consistent with the eventual cessation of plastic deformat ion for uniform half-spaces are now well established. However, compone nts liable to face arduous tribological conditions are traditionally h eat-treated to produce a hardened thin outer layer over a softer core; this gradation of hardness or plastic strength within a material of c onstant elastic properties leads to changes in the shakedown analysis and thus in the way the results are most conveniently presented in map s or charts. More recently introduced techniques of surface engineerin g often rely on the deposition of a well-bonded hard coat on a softer substrate so that both the elastic and the plastic properties of the c oating differ from those of the underlying material; these differences present further difficulties in formulating and displaying the corres ponding shakedown limits. In addition to the values of hardness and st iffness of the coating and those of the substrate, important system pa rameters are the thickness of the elements of the coating (in comparis on to the characteristic dimension of surface roughness) the integrity of the interlayer bonds and the coefficient of friction between the c oated component and the surface against which it slides. The results o f a sequence of studies into these effects are displayed in the form o f non-dimensional shakedown charts or maps which demonstrate to the ma terials engineer or designer the potential improvement that might accr ue from optimal surface engineering. These also indicate the condition s under which, because of an excessive mismatch between the mechanical properties of the coating and the substrate, inappropriate surface tr eatments may lead to either no significant improvement or, in some cir cumstances, even a degradation in surface tribological performance.