Influence of phase stability on the residual stress in partially stabilized zirconia TBC produced by plasma spray

Thermal barrier coatings (TBCs) of yttria-partially-stabilized zirconia were plasma sprayed on aluminium substrates under controlled atmosphere and temperature conditions (ATCS process) at 45 degrees C and 155 degrees C. Residual stress and phase composition were deeply influenced by substrate temperature and atmosphere during deposition. Spraying in Ar atmosphere promoted the formation of highly disordered and under-stoichiometric ceramics with unstable phase composition. Reducing atmosphere and low substrate temperature caused the retention of excess oxygen vacancies which stabilize a high density cubic phase (p approximate to 6.16 g cm(-3)), which transformed to the non-transformable tetragonal phase (rho approximate to 6.05 g cm(-3)) after low temperature ageing. Phase composition and residual stress field in TBCs were studied by two different techniques, X-ray diffraction (XRD) and coating length change after detaching from the substrate (CD); the two techniques gave complementary results, since the former measured surface strain, whereas the latter gave information on average bulk strain. The average in-plane stress was compressive, with a positive gradient leading to a low surface tensile stress; at higher deposition temperatures coatings were increasingly in compression, During ageing and heat treatment, the volume expansion associated with the cubic to tetragonal transformation led to a considerable increase of the compressive stress, demonstrating that high residual stresses can develop after phase transformations.