Investigation of techniques for determining the residual stresses in WC-17Co thermal sprayed coatings and studying the effect of residual stress on its abrasion resistance

摘要

The aim of this project was to investigate the techniques for determining the residual stressesudin WC-17Co thermal sprayed coatings and to study the effect of residual stress on abrasionudresistance on different substrates. The choice of the substrate was due to their differentudcoefficients of thermal expansion (aluminium, super-invar, 304L stainless steel, mild steeludand brass). Coatings of about 200μm were successfully deposited on all the substrates.udCoatings were deposited by a high velocity oxyl-fuel spraying system (HVOF), andudcharacterized by evaluating the coating phases and wear resistance, and the residual stressesudwere determined via non-destructive methods. Investigations involved both as-sprayed andudtheir annealed counterparts to ascertain the effect of heat treatment.udNon-destructive determination of residual stresses in the WC-Co coated systems wasudexceptionally challenging in that the coatings were only 200 microns thick. The best suitedudtechniques for investigation of WC were diffraction-based strain scanning using penetratingudradiation such as thermal neutrons (most penetrating), high energy synchrotron X-rays (100udkeV enables 20 micron penetration) and laboratory X-rays (limited to 5 micron penetration).udLaboratory X-rays (Necsa, using Co radiation), thermal neutrons (ANSTO, Australia) and Xrayudsynchrotron (ESRF, France) were successfully employed to resolve the stress conditions.udThe neutron investigations enabled two approaches for the determination of the in-surfaceudstresses, direct measurements (good results for the low neutron attenuation substrates), andudindirect determination using stress balance conditions inferred from the through thicknessuddepth profiles measured in the substrates (applicable to all the higher neutron attenuatingudsubstrates). Investigations were expanded to the study of the influence of annealing at 40% ofudthe respective substrate melting temperatures. For each substrate, the through thickness stressudprofile differences between the grit-blasted reference material (final before the HVOFudcoating) and the grit-blast coated samples were used to determine the elastic contributionsudpurely ascribed to the coating process. This required exceptional positional resolution neutronuddiffraction investigations (positional accuracies better than 0.01 mm). There were both smalludcompressive and low tensile stresses on the as-sprayed coated samples. After annealing, theudstresses became substantially more compressive. The near-surface trends of the grit-blastedudsubstrates were completely relaxed after annealing, with thermal stresses being the dominantudmechanism for residual stress induced due to the large difference in the coefficients ofudthermal expansion (CTE) between the WC coatings and the substrates.udThe three-body abrasive tests were carried out on the parent materials (substrate), grit blastedudsubstrates, as-sprayed coatings and heat treated coatings. The contributions of each coatingudprocess were established. There was about 45-50% reduction in the wear of grit-blastedudsamples compared to the substrates, whilst 80-95% reduction in the wear of the coatingsudcompared to substrates was found. The wear resistance is due to many different contributions,udbut there was a strong correlation between the residual stress and wear resistance of the coating.