Residual Stresses in HVOF- and APS-Deposited AI₂O₃ and Cr₂O₃ Coatings

Residual stresses in plasma-sprayed Al₂O₃ and Cr₂O₃ coatings, deposited using commercial powders, and in HVOF-sprayed ceramic coatings, deposited using conventional Al₂O₃ and Cr₂O₃ feedstock and nanostructured- Al₂O₃ feedstock, were studied by combining X-ray diffraction, substrate chemical removal technique and analytical modelling. The in-situ curvature technique was also employed for HVOF-sprayed Al₂O₃ coatings, for further verification. Both HVOF-sprayed Al₂O₃-based coatings display similar, tensile residual stresses (≈120 MPa) near the top surface and possess moderate through-thickness stress gradients (≈10 - 20 MPa). Plasma-sprayed Al₂O₃ possesses a smaller through-thickness stress gradient and a larger near-surface stress (≈220 MPa): this latter result seems to be due to higher quenching stresses in APS Al₂O₃, as determined by analytical computation The analytical model is validated by its fairly good agreement to the experimental results obtained both by substrate chemical removal and by in-situ curvature. Cr₂O₃-based coatings possess a lower near-surface residual stress (≈20 MPa); the HVOF one also exhibits a very large stress gradient (≈80 MPa). Machining and sliding processes (like polishing and dry sliding tribological testing) change their surface residual stresses to compressive ones.