Residual stresses in HVOF sprayed ceramic coatings

In this paper, the residual stress state of thermally sprayed ceramic coatings was examined by combiningdifferent experimental and analytical techniques, in order to provide a thorough characterisation of throughthickness stress profiles and a cross-verification of results. HVOF-sprayed ceramics, manufactured usingcommercial and nanostructured Al2O3 powders and commercial Cr2O3 powders, and atmospheric plasma sprayed (APS) ceramics, manufactured using commercial Al2O3 and Cr2O3 powders, were investigated. The near-surface stress was measured by X-ray diffraction. The through-thickness profile and the intrinsic quenching stresswere analytically computed by the Tsui–Clyne iterative model, using the X-ray measurement result as input, and results were validated by the substrate chemical removal method. Further verification was achieved by applying the in-situ curvature technique to the deposition of HVOF-sprayed Al2O3 coating. HVOF-sprayed Al2O3 coatings deposited using both conventional and nanostructured powders feature a similar, almost equibiaxial tensile stress on the top surface (116.5 MPa and 136.5 MPa, respectively) and amoderate through-thickness gradient (about 12 MPa and 20 MPa, respectively). Their intrinsic quenchingstresses were analytically estimated to be 184 MPa and 205 MPa, respectively. APS Al2O3 possesses higher topsurface stress (220 MPa) and quenching stress (311 MPa). However, it shows a less pronounced stress gradient (≈3 MPa) than HVOF-sprayed Al2O3-based coatings, because cracks, pores and weak lamella boundaries in the APS coating can accommodate the deformations induced by the bending moments arising both during coating deposition and during cooling.The model-derived quenching stress of the conventional HVOF Al2O3 coating was validated by the in-situ curvature measurement technique.Cr2O3-based coatings are significantly different. They display a lower residual stress in the near-surface region: 20 MPa in the APS coating, 27.5 MPa in the HVOF one. The HVOF coating also exhibits a very large stress gradient of ≈77 MPa. Machining and sliding processes (like polishing and dry sliding tribological testing) change their surface residual stresses to compressive ones.