Measurement On High Temperature Oxidation Behavior And Residual Stress Of Thermal Barrier Coatings

Thermal barrier coatings （TBCs） are commonly used to improve the reliabilityand durability of hot section components as well as enhance the engine performanceand the efficiency of aero-engines and gas turbines. And TBCs can be applied to thecombustion chamber of diesel engine insulated by ceramic coatings—Low HeatRejection （LHR） Engine. In this paper, high velocity oxy-fuel （HVOF） spraying andair plasma spraying （APS） were used to prepare the CoNiCrAlY bond coat （BC） and8YSZ top coat （TC） respectively.After2.5,8,15,50and100h isothermal oxidation, the growth of thermallygrown oxides （TGO） at the interfaces of both TC/BC and BC/Substrate, phasetransformation process and elements diffusion were studied by using scanningelectron microscope, energy-dispersive X-ray spectroscopy and X ray diffraction.Therefore, it was found that the development of TGO at the interface of TC/BC by aparabola trend, and it was mainly composed of （chromium, spinel and nickel oxide）CSN layer and Al₂O₃layer. The TGO in TC/Substrate interface was lamellar ormassive with a composition of Al₂O₃. The oxidation kinetics divided into three stages:initial oxidation, stable oxidation and complex oxidation.Furthermore, it could be suggestted that the residual stresses of TGO wereclosely related to the thermal mismatch as well as the phase transformation of topceramic coat. Meanwhile, the mathematical model of the residual stress of TGO wasestablished, and the result showed that the macroscopic stress was compressive stressand decreased slightly with the thickening of TGO. Subsequently, the stresses in TGOexperienced different oxidation time were detected by using Raman spectraltechniques. It could be shown that the transformation of transient θ-Al₂O₃into stable-Al₂O₃occurred according to the weakening of the characteristic peak of θ-Al₂O₃.Stress distribution of thick ceramic coat was also measured, indicating that the stressnear vertical through cracks and mixed cracks zone were released to some extent. Therelease of the residual stress in the area of top ceramic coat could be attributed to theformation and propagation of the microcracks because the presence of the phasetransformation of8YSZ could not be found in terms of the Raman spectra.