Residual stress in plasma sprayed Y2O3-PSZ coatings on piston heads

Yttria—partially-stabilized zirconia (Y—PSZ) thermal barrier coatings (TBCs) were deposited on Al alloy disks under different plasma spraying conditions, using both air plasma spraying (APS) and atmosphere and temperature controlled spraying (ATCS) processes. X-ray diffraction (XRD) was used to make a non-destructive residual stress analysis (RSA) on the surface of coatings, either as-sprayed and after a burner rig test. Regardless of the deposition process the surface of as-prepared coatings was always in a low tensile state (10–35 MPa), with an increases of the stress near the coating edge. The stress intensification in this region, where mechanical failure is supposed to start when the coated components are subjected to thermal and mechanical solicitations, was confirmed by a finite element modelling. The burner rig test validated this picture, showing an interfacial crack propagating from the border towards the center as the main failure mechanism. As a consequence of the thermal cycling, the surfaces stress changed from tensile to compressive. In particular, the stress was high (−150 MPa) in the central region still well-adherent after the test and decreasing towards the edge where the coating was partially detached. The role played by ceramic sintering, Al substrate creep, and Y—PSZ phase instability during the burner rig test, is also discussed.