Yttria stabilized zirconia (YSZ) is the state of the art ceramic top coat material used for TBC applications. The desire to achieve a higher engine efficiency of agas turbine engine by increasing the turbine inlet temperature has pushed YSZ toits upper limit. Above 1200°C, issues such as poor phase stability, high sinteringrates, and susceptibility to CMAS (calcium magnesium alumino silicates) degradation have been reported for YSZ based TBCs. Among the new materials,gadolinium zirconate (GZ) is an interesting alternative since it has shown attractive properties including resistance to CMAS attack. However, GZ has a poor thermo-chemical compatibility with the thermally grown oxide leading to poor thermal cyclic performance of GZ TBCs and that is why a multi-layered coating design seems feasible.This work presents a new approach of depositing GZ/YSZ multi-layered TBCs by the suspension plasma spray (SPS) process. Single layer YSZ TBCs were also deposited by SPS and used as a reference.The primary aim of the work was to compare the thermal conductivity and thermal cyclic life of the two coating designs. Thermal diffusivity of the YSZ single layer and GZ based multi-layered TBCs was measured using laser flash analysis (LFA). Thermal cyclic life of as sprayed coatings was evaluated at 1100°C, 1200°C and 1300°C respectively. It was shown that GZ based multi-layered TBCs had a lower thermal conductivity and higher thermal cyclic life compared to the single layer YSZ at all test temperatures. The second aim was to investigate the isothermal oxidation behaviour and erosion resistance of the two coating designs. The as sprayed TBCs were subjected toisothermal oxidation test at 1150°C. The GZ based multi-layered TBCs showed a lower weight gain than the single layer YSZ TBC. However, in the erosion test,the GZ based TBCs showed lower erosion resistance compared to the YSZ singlelayer TBC. In this work, it was shown that SPS is a promising production technique and that GZ is a promising material for TBCs.

Yttria stabilized zirconia (YSZ) is the state-of-the-art ceramic top coat material used for thermal barrier coating (TBC) applications. Demand for higher efficiency in gas turbine engines has led to a continuous increase in the gas in let temperature. However, this increase in temperature has pushed YSZ to its upper limit. Above1200 °C, issues such as poor phase stability, high sintering rate, and susceptibility to CMAS (Calcium Magnesium Alumino Silicates) degradation limit the durability of YSZ based TBCs. Among the new top coat materials suggested for high temperature TBC applications, gadolinium zirconate (GZ) is an interesting alternative to YSZ since it has shown attractive properties which include a better resistance to CMAS attack. However, GZ has poor thermo-chemical compatibility with the thermally grown oxide (alumina), leading to poor thermalcyclic fatigue performance of single layered GZ TBCs. Therefore, a multi-layered GZ/YSZ based TBC design seems promising. This work presents a new approach of depositing multi-layered (double and triple layered) GZ/YSZ TBCs using the recently developed suspension plasma spray(SPS) process. SPS was employed in this work because of its capability to mimic the electron beam physical vapour deposition (EB-PVD) process in terms of producing columnar microstructured TBCs. Single layer YSZ TBCs were also deposited by SPS process and used as a reference for comparing the functional performance of multi-layered GZ based TBCs. The primary aim of this work was to improve the durability of GZ based multilayered TBCs at high temperatures. Durability tests were performed in the temperature range 1100 °C - 1400 °C under different thermal cyclic test conditions (with and without thermal gradient). The results indicate that multilayered GZ based TBCs improve durability compared to the single layer YSZTBCs at all the test temperatures. Failure analysis of the multi-layered GZ/YSZTBCs revealed spallation within the GZ layer close to GZ/YSZ interface and the reason was believed to be the inferior fracture toughness of GZ. In order to improve the fracture toughness in the region of failure, a composite approach comprising multi-layered GZ+YSZ based TBC was considered. It was shown that the composite GZ+YSZ based TBCs did not improve the thermal cyclic lifetime, although improvement in fracture toughness was observed. As a further extension of this work, the influence of YSZ layer thickness on the durability of GZ/YSZTBCs was investigated. It was shown that an increase in YSZ layer thickness in the GZ/YSZ TBC led to poor durability. Additionally, the other important performance criteria for TBCs, i.e. thermal conductivity, was measured experimentally and compared with the single layer YSZ TBC. It was shown that the GZ based TBCs had lower thermal conductivity than YSZ. The second aim was to investigate and compare the erosion performance of multi-layered GZ based TBCs and single layered YSZ TBCs. In the erosion testconducted at room temperature, the GZ based TBCs showed lower erosion resistance compared to the single layer YSZ TBC. The main reason for this difference was attributed to the inferior fracture toughness of GZ. In case of the composite multi-layered GZ+YSZ based TBC, an improvement in erosion resistance was observed compared to the multi-layered GZ based TBC. Based on the results obtained, this work has demonstrated that SPS is a promising processing technique to produce columnar microstructured TBCs irrespective of the composition (GZ, YSZ, GZ+YSZ). It was also shown that GZ/YSZ multilayered TBCs are promising for high temperature TBC applications due to theirl ow thermal conductivity and high thermal cyclic fatigue lifetime. However, low erosion resistance for certain applications might be an issue for the GZ basedTBCs.