Study On The Influence Of Interfacial Oxidation To Failure Behavior Of Plasma Sprayed Thermal Barrier Coatings

To ensure the protective performance of hot components in aero engine and industrial gas turbine engine, thermal barrier coatings(TBCs) were prepared by high velocity oxygen fuel spray(HVOF) and air plasma spray(APS). An influence of metallic powder composition of bond coat on the growth of thermal grown oxides(TGO) was investigated. Additionally, an influence of pre-oxidation treatment was also studied on TGO microstructures, oxidation rate and behavior of the TBCs. After understanding the TGO evolution, thermal cycle and thermal shock test were adopted to evaluate the thermal shock resistant property of the TBCs. The initiation and propagation of cracks and the variation of residual stress within the TBCs were then evaluated. Failure behavior of the TBCs was discussed, simultaneously. The main conclusions are as follows:Dense bond coatings were prepared by HVOF. The TGO thicken rate of TBCs at high temperatures was controlled by the element diffusion rate. Through a vacuum heat treatment(VHT) of TBCs prior to test, α-Al2O3 film of coarse grains formed and resulted in an improved oxidation resistance of TBCs. The continuous spinel film and cracks formed in the TBCs with bond coat of low aluminum concentration, but the oxidation resistance was improved through VHT process. The VHT had a more significant effect on the oxidation resistance of bond coat than that of the aluminum concentration.The residual compressive stress, induced in the TGOs as a result of thermal expansion mismatch, had a significant relationship with the CTE of bond coat. With the thickening of TGOs, the main mismatch changed from top coat/bond coat to bond coat/TGO. The residual stress in the isothermal oxidation did not vary significantly, but it decreased gradually with increasing cracks during the thermal cycle and thermal shock test. The residual stress value indirectly revealed the amount of cracks, and evaluated the TBCs’ life.With the consumption of β aluminum-rich phase, the aluminum depleted zone appeared in the bond coat. The thickness of aluminum depleted zone was directly proportional to the square of time and the TGO thickness. The thicken rate of aluminum depleted zone was reduced after the VHT process or with increasing the aluminum concentration in bond coat. However, the thicken rate was increased in the thermal cycle test. The oxidation resistance of TBCs can be characterized by the consumption rate of β phase. The β phase transformation led to tensile stress, hole or even internal oxidation inside the bond coat.Many types of cracks formed near the TGO, and the propagation of cracks led to the prerelease of ceramic coating. The failure occurred at the TGO/ceramic coating interface near the side of ceramic coating. The shape had a more significant effect on the TBCs life, The deformation of one-face coated thin substrate accelerated the propagation of cracks, and led to an earlier failure.The oxidation mechanism of TBCs and, initiation and propagation process of cracks in the isothermal oxidation, thermal cycle test and thermal shock test were similar. The thermal cycle samples got a higher TGO thicken rate, more spinel structures and cracks, resulting in an earlier failure and a poorer oxidation resistance. The thermal cycle test is a better method to evaluate TBCs.