Performance evaluation of thermal barrier coating on heat shield for electrical turbo compounding system / Muhammad Syazani Farhan Zaini

Thermal Barrier Coating (TBC) is usually deposited on nickel-based superalloy substrates and widely used in extremely high temperature applications in order to improve the performance and efficiency of their systems. In this study however, TBC is deposited on medium carbon steel for heat shield to protect electrical motor in electrical turbo compounding system. During the operation, high temperature heat generated could damage the electrical motor. The purpose of this research is to study the characteristic of TBC deposited on medium carbon steel substrate with different top coat materials. Two different top coat material which are 8 wt% yttria stabilized zirconia (8YSZ) and mullite were deposited by using air plasma spraying (APS) method at different voltage and current which is 50 and 70 V for voltage and 400 and 600 A for current. There are 8 samples produced with different deposition parameters which are Sample 1, 3, 5 and 7 using 8YSZ as top coat and Sample 2, 4, 6 and 8 using mullite. Sample 1, 2, 3 and 4 using 50 V as the top coat deposition voltage while Sample 5, 6, 7 and 8 using 70 V. The current for top coat deposition for Sample 1, 2, 5 and 6 is 400 A and 600 A for Sample 3, 4, 7 and 8. The microstructure, porosity, microhardness, phase and element, thermal cycle life and thermal conductivity of the deposited TBC are investigated. From the SEM image, both the 8YSZ and mullite coatings are uniformly deposited on medium carbon steel substrates where the porosity and the microhardness of the 8YSZ coatings is lower than of the mullite coatings. The element composition of the TBC layer is of the element from that material itself. Besides that, the phases appeared at the peak of the XRD patterns for 8YSZ and mullite samples are zirconium yttrium oxide (ZrY2O5) phase and mullite (3Al2O3.2SiO2) phase, respectively. In addition, the lifetime of 8YSZ coatings is higher than of the mullite coating where its longest cycle lifetime is Sample 7 with 64 cycles while its shortest lifetime goes to Sample 4 with only 12 cycles before TBC failure. The thermal conductivity of 8YSZ and mullite samples are lower compared to the uncoated samples. Consequently, in using medium carbon steel as a substrate, the applicable top coat material for TBC application is the 8YSZ rather than the mullite in terms of better thermal cycle life and lower thermal conductivity.