| NiCrAlY, NiCrAlY+ZrO2, NiCrAlY+Al2O3 and NiCrAlY+(ZrO2-Al2O3) oxidation resistance coatings were prepared by air plasma spraying process on the surface of 20G The microstructure and the properties of oxidation and wear resistance coatings were studied by X-ray diffraction (XRD), electron probe microanalysis (EPMA), scan electron microscopy (SEM), transmission electron microscopy (TEM). The mechanism of oxidation resistance and the main cause of the high temperature oxidation failure were discussed. The thermal shock properties of the coatings were analyzed by calculating the thermal stress at the interface between NiCrAlY and ceramic layers.The results show that plasma sprayed NiCrAlY coatings consist of Ni-based phases, Ni3Al, NiAl and many amorphous phases. The amorphous phases are located on the grain boundaries, which can improve the strength of the coatings. Many microcracks and pores exist in the coatings. A layer of Al2O3 film has formed during oxidation. Element Y can enhance the properties of oxidation resistance. Element Y is located in the Al2O3 film which can anchored with substrate, so element Y can prolong the service life of coatings. The average hardness of the coating is 490.3HV0.1, the wear mechanism of coatings is delamination and plough wear.Plasma sprayed Al2O3 coatings consist of (X-Al2O3 and γ-Al2O3 phases, α-Al2O3 grains are fine and smooth, γ-Al2O3 grains are coarse. The bond between Al2O3 phase and NiCrAlY is well. More cracks exist in Al2O3 layer after oxidation. The thermal growth oxide (TGO) is formed on the surface of NiCrAlY layer in which element Al enriches on the interface between NiCrAlY and Al2O3 layers. The cause the coatings can protect substrate from oxidation is that element Oxygen has not diffused to substrate by plane distributions. The mechanism of oxidation resistance of Al2O3 coatings is that the compact oxide can protect the substrate from oxidation after element Al enriches on the interface. The main cause of high temperature oxidation failure is the thermal stress which overpasses the bonding strength of coatings. The average hardness of Al2O3 layeris 864HV0.1. The Al2O3 coating possesses the excellent wear resistance. The predominant wear mechanism of the coatings is microcracking wear.Plasma sprayed ZrO2 coatings consist of c-ZrO2 and t-ZrO2 phases. Many cracks and pores exist in ZrO2 layer, which can improve thermal shock resistance. The bond between bond and top layers is compact. The bonding state between bond layer and substrate is mechanical combination. ZrO2 layer has columnar crystal after high temperature nucleation growth. The coarse oxide which has formed on the interface between bond layer and substrate causes to the failure of the coatings. The average hardness of ZrO2 layer is 783HV0.1. In early stage the wear mechanism is delamination, then the mechanism transforms to plough wear.Plasma sprayed ZrO2-Al2O3 coatings consist of α-Al2O3, c-ZrO2 and t-ZrO2 phases. Many cross cracks and pores exist in the top layer. ZrO2 grains have melt with Al2O3 together, whose structures are compact. The compact TGO has been formed on the interface between bond and top layers during oxidation. Element Al preferentially combines Oxegen to form TGO which can protect the substrate from oxidation. The average hardness of ZrO2-Al2O3 coatings is 772HV0.1. The wear mechanism is delamination incorporated with slight plough wear.These coatings researched in this paper all possess well property of oxidation resistance. Oxidation kinetics curves of all types of coatings accord with Wagner parabola law. The Al2O3 coatings possess best oxidation resistance. Through the calculation of thermal stress between bond and top layers, the stress in ZrO2 coatings is least. In conclusion, Al2O3 coatings prepared by APS on the surface of 20G possess the best properties of oxidation resistance, thermal shock and wear resistance. |
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