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 °C and 155 °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 (ρ ≈ 6.16 g cm−3), which transformed to the non-transformable tetragonal phase (ρ ≈ 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.