Thermo-mechanical behaviour of plasma sprayed yttria stabilized zirconia thermal barrier coatings

Thermally sprayed ceramic coatings deposited from agglomerated feedstock powder have often demonstrated improved properties relative to coatings produced from conventional powders. These types of coatings have been reported to exhibit better wear resistance and higher adhesion strength compared to conventional deposits. In order to apply these types of coatings on turbine blades and diesel engine combustion chambers, the thermo-mechanical properties of plasma sprayed yttria stabilized zirconia coating were investigated. Creep strains and creep rates were measured using free standing thick coatings (3 mm) loaded in the four-point bend configuration at a range of temperatures (from 800 to 1200°C in air) and loads (27 to 49 N). Creep exponents and activation energy of the coatings were measured and the results were compared. Under the same test conditions the coating produced from agglomerated feedstock showed a lower creep strain than that produced from conventional powders. The creep strain of deposits was correlated to the microstructure of the coating. A model was developed to predict creep behaviour of coatings based on the thickness of splats. The results showed that the creep resistance is directly proportional to the splat thickness; the thicker the splat the higher is the creep resistance. The effect of splat interface on physical and mechanical properties of zirconia, such as thermal conductivity and elastic modulus of coatings, was investigated, too. It was shown that the high density of splat interfaces results in a lower thermal conductivity and elastic modulus.