Plasma Spraying Al₂O₃-13wt%TiO₂ Thermal Barrier Coating And Study Of Its Properties

Thermal barrier coating (TBC) is a coating system which is composed of material bond coating and top ceramic coating. The use of TBC can not only improve the ability of high-temperature resistance of substrate, but also increase the usage temperature of alloy material. TBC which is prepared by Al2O3-13wt%TiO2 (AT-13) has the performance of wear-resistance, antifriction, corrosion- resistance and insulation, so that it has wide application now. Because there are just a few studies about thermal properties and failure mechanism of AT-13 TBC, thermal shock and thermal oxidation experiment of AT-13 TBC are done from the angle of macroscopic view and microscopic view in order to compare and study their thermal properties. The failure mechanism of thermal shock and thermal oxidation is also discussed for the purpose of supplying supportive theory for the further study and optimization of the processing parameters.At first, GP-80 atmospheric plasma spraying equipment (with primary plasma gas Ar, secondary plasma gas H2) was used to prepare bond coatings NiCrAl and NiCrCoAlY on the substrate steel Q235, respectively. According to the detection result of thermal shock and thermal oxidation experiment, the bond materials were compared and then chosen. Second, the processing parameters of spraying conventional Al2O3-13%TiO2 (C-AT13) and nanostructured Al2O3-13%TiO2 (N-AT13) were optimized by orthogonal test, and then C-AT13 and N-AT13 TBC were prepared on the substrate steel Q235 with the optimized processing parameters, respectively. At last, XRD, SEM and EDS were used to analyze the phase composition, microstructure, chemical composition and elements distribution. The microhardness, thermal shock resistance and thermal oxidation resistance of those two kinds of TBCs were detected and compared.The results were as follows: the thermal shock resistance and thermal oxidation resistance of NiCrAl plasma sprayed coating was better than that of NiCrCoAlY, so NiCrAl was chosen as the bond coating of the TBC in this paper. Optimum processing parameters of C-AT13 coating were voltage 55V, current 600A, main gas flow rate 50L/min, while that of N-AT13 were 60V, 450A and 40L/min. The tensity of C-AT13 plasma sprayed coating was lower than N-AT13 plasma sprayed coating. N-AT13 coating had bimodal distribution microstructure while C-AT13 had not. According to the numbers of thermal shock, there were just a few differences between C-AT13 and N-AT13 TBC in thermal shock resistance. But the microstructure of failed C-AT13 and N-AT13 TBC after thermal chock experiment showed that N-AT13 TBC had better thermal shock resistance than C-AT13 TBC. N-AT13 TBC also had better thermal oxidation resistance than C-AT13 TBC. The analysis of AT-13 TBC samples after thermal shock and thermal oxidation experiments showed that the factors which affected the failure of TBC after thermal shock or thermal oxidation experiment were as follows: pores remain in the coating during the course of spraying, cracks caused by heat stress, thermal growing oxide (TGO) generated between substrate, transition coating and top coating and the volume change associated with the phase transformation during the course of plasma spraying.