Wide application of plasma sprayed coatings to shafts and rotating components has led to concern at the effect that such sprayed coatings have on the fatigue properties of the coated parts. Previous work has been inconclusive as to the effect of such coatings and the mechanism by which they affect the fatigle properties. In the present work throe high strength steels typical of those used industrially for shafts and cams have been sprayed with molybdenum, aluminium and alumina. Pure molybdenum markedly reduced the fatigue limit of the three steels, whilst the self fusing molybdenum containing low melting point constituent did not have this effect. The reasons advanced to account for this difference lay in the pattern of die residual stresses at the coating/substrate interface. It has been shown that although the pure molybdenum coatings contained cracks, initiation of fatigue, occurred at the substrate interface. This is due to the unfavourable tensile residual stresses present on the surface of the substrate which favour crack initiation and a shorter Stage I growth period thereby reducing the fatigue limit. Self fusing molybdenum which contracts over a narrower temperature range than the pure molybdenum coating produces tensile residual stresses at the substrate surfaces which are less severe and do not appear markedly to affect the fatigue behaviour of the substrate material. It was further established that both flame and arc sprayed molybdenum coatings also reduce the fatigue limit of the substrate material. In the case of arc sprayed molybdenum the fatigue life at the finite life portion of the S-N curve was reduced by 40%. This reduction is attributed to the formation of a highly stressed region beneath the martensitic zone formed at the interface on the surface of the substrate as a result of arc sprayed molybdenum coatings. The reduction in fatigue limit in the case of flame sprayed molybdenum coatings is believed to be due to a similar mechanism to that proposed for plasma spraying. No marked effect on the fatigue behaviour of the steel was observed after spraying with either aluminium or alumina. This is believed to be due to the presence of less harmful interfacial stresses because of the lack of metallurgical bonding.
|Date of Award||1977|