The effect of microstructure and mechanical properties of YSZ/Al2O3 thermal barrier coatings depending on Al2O3 addition

Keywords: Thermal barrier coatings, plasma spray coating, ZrO2/Al2O3 coating powders, hardness, density, thermal shock resistance The thermal barrier coating (TBCs) material used most commonly is yttria-stabilized zirconia (YSZ) because of its low thermal conductivity and relatively high thermal expansion coefficient. The usage of YSZ coatings are normally deposited together with a bond coat, is the most preferred way in developing TBC applications in recent years. NiCrAlY or NiCoCrAlY is employed as an oxidation-resistant metallic bond coat. The metallic bondcoat must be resistant to both thermomechanical fatigue and oxidation since YSZ coating is essentially a porous layer that is transparent to oxygen penetration. Thermally grown oxide scales can provide high-temperature oxidation protection if they are slow growing, sound and adherent to the substrate. The objective of the present work is to produce YSZ and Al2O3 powders and coatings which are used for jet engines, gas turbines and diesel engines. Starting powders were YSZ, and Al2O3 powders. The coating powders were mixed in the compositions of YSZ ? 10, 20, 30, 50 and 80 wt % Al2O3. The mixtures were ball milled for 2h by using ZrO2 balls to provide homogenous mixtures. The coatings were formed using a METCO 3MB plasma spray gun on stainless steel substrates. Electron microscopy and optic microscopy were used to analyze the microstructures of powders and coated samples. The alumina contents in coatings? effects on the coatings?; hardness, relative density, surface roughness and microstructures were investigated. According to the results; alumina contents increases the hardness of the coatings and decreases the surface roughness. In addition to this, relative density values increase until certain coating composition, the additive alumina content decreases the thermal shock resistance and the most high relative value is obtained in YSZ- wt% 30 Al2O3 coatings.