β-TCP coatings on zirconia bioceramics: The importance of heating temperature on the bond strength and the substrate/coating interface

β-tricalcium phosphate (β-TCP) coatings were synthesized on tetragonal zirconia (Y-TZP) discs by heating the apatite coating between 800 °C and 1200 °C. The study results suggest that heating temperature has a strong influence on the coating bond strength and microstructure of the substrate/coating interface. The β-TCP coatings fired at 800 °C and 900 °C exhibited excellent tensile bond strength (∼50 MPa) while heating at 1100 °C and 1200 °C led to decreased bond strength (∼30 MPa) as the result of substantial structural and microstructural changes: diffusion of Y3+ from the zirconia substrate in the coating resulting in partial crystal transformation (t-m) of zirconia, formation of surface uplifts and nanoporosity in zirconia, as well as generation of large residual thermal stresses leading to microcracking of the β-TCP coatings. However, these structural changes did not have any measurable effect on the flexural strength of the bulk zirconia substrates.