FINITE-ELEMENT ANALYSIS OF THE INITIAL-STAGE OF THE INDENTATION ROTATION TEST FOR PHOSPHATE AND STEARATE COATINGS

Well fitted to determine the behaviour and seizure of thin coatings, t he test involves a rotating cylindrical specimen on which a small prof iled indentor exerts a non-uniform normal pressure. The normal load re mains constant as the time of rotation increases. The friction-force v alues are related to the behaviour of the coatings and when the interf ace become damaged generally, the friction force increases strongly. T he rotating time required for these observations is related to the ini tial stress and strain distribution in the vicinity of the contact sur face when the normal load is applied on the indenter, the specimen bei ng motionless. For this indentation phase, the distributions of stress and strain are obtained by means of a refined localised finite-elemen t model which takes into account the mean thickness and the mechanical properties of a typical zinc-phosphate coating first, and then of a t ypical bi-layered zinc-phosphate and zinc-stearate coating second. It is found that the slider-specimen contact area and the stress and stra in maps in the contact vicinity change significantly from the virgin s teel specimens to the bi-layered specimens. The effective stress decre ases whilst the effective strain increases, the strain maps being more perturbed by the coatings than the stress map. Particular attention i s focused on the local shear stress which is related to the performanc e of the adhesion coating.