Ceramic Thermal Barrier Coatings (TBCs) provide protection to metals from degradation at high temperature. A major factor deciding the effectiveness of the coating in service is the temperature drop across the thickness of the coating. Common practice to determine the temperature drop is to subject the coating with a high heat providing flame with preset velocity by using combustible gases focused towards the coated surface, that keep the surface at desired stabilization temperature and the temperature is measured at the back side of the coating, i.e. at the metal side. The challenge is to heat the complete specimen surface using the flame and to reach an accurate stabilization temperature by using the flame as the heating source. In the present work, this challenge was overcome by using a uniform source of heat i.e. an electric heater on the entire coating surface. This paper presents the results obtained by studying the thermal barrier effects across TBCs by using the electrical heating source that provided the heat on the ceramic surface in a controlled and uniform manner, thereby establishing a newer assessment method.
The TBCs were prepared by plasma spray coating commercial 8%Yttria-Stabilized Zirconia (8YSZ) as the top ceramic coat on flat plates of Aluminium 11% Silicon alloy removed from diesel engine pistons. TBC thicknesses varied between 100µm and 600µm. Air Plasma Spray coating was employed to coat the substrates which initially were spray coated with 50-75 µm thick bond coat of Nickel Aluminide. Thermal barrier test was conducted by heating the entire coated surface uniformly and by keeping the ceramic surface temperature constant till the stabilization in the range of 300°C to 500°C. The temperature drop achieved was in the range of 46°C to 127°C depending upon the coating thickness. Details of the tests conducted and results obtained are presented.
