| In order to overcome the problems of turbine efficiency decrease, output reduction, shortening life span, frequency replacement and maintenance, which was reduced by cavitation and erosion damage of turbine blade at work. Plasma spraying technology was used to make gradient coating on the surface of 45 steel. The microstructure, composition and phase of the gradient coating were ananlysed by means of SEM, XRD, XRF and EDS. The binding intensity and microhardness of gradient coatings were analyzed by means of electron universal testing and Vickers hardness tester, respectively. The erosion resistance property of gradient coatings were tested by means of air blast type erosion method.The optimal gradient coating structure designed in this thesis could be described as: base metal-Ni/Al underlayer-(2:1) mixed powder coating-(1:1) mixed powder coating-(1:2) mixed powder coating- Al2O3 composite coating, 2:1, 1:1 and 1:2 referred to the quality ratio of Ni/Al composite powder and Al2O3 composite powder, the powder mixed by ball milling was better than manual mixing. The main composed phases of Ni/Al underlayer were Ni3 Al, a small amount of Al2O3 and Ni-O compound; Al2O3 composite layer was composed of many flat piled organization, the main composed phases of Al2O3 composite layer were Al2O3 and Ti O2, but also a small amount of Ti3O5; the main composed phases of transition layer were Ni3 Al, Al2O3, Ti O2 and Al2 Ti O5. The microstructure and composition of transition layer made by using manual mixing powder distributed unevenly, layered phenomenon was very obvious. While the ball milling powder was used to made transition layer, the components were distributed homogeneously, each phase was distributed staggered. The microstructure of transition layer presented microscopic continuous distribution. The gradient coating achieved a smooth transition. The internal pores of gradient coating were small. The bonding between each coating formed similar “mechanical interlocking” connectivity.Performance test results of gradient coating could be described as: When three transition layers(mixed powder using ball milling) made between Ni/Al underlayer and Al2O3 composite layer, the obtained gradient coating had the highest bonding strength, achieved 36.80 MPa. It was significantly higher than the samples without gradient coating(21.67MPa). The results of microhardness test of gradient coating was that the hardness of Ni/Al underlayer was the lowest. With the increasing of Al2O3 composite powder in the transition layer, the microhardness gradually increased. The Al2O3 composite layer had the highest microhardness. The microhardness of gradient coating presented gradient rendering. The erosion resistance of gradient coating was better than 45 steel. When the erosive angle was 90°, the erosion ware rate of gradient coating was highest. It presented brittle material erosion characteristic. The weight loss was mainly caused by the micro-cutting and grinding. When the erosive angle was larger, the erosive surface also had impact craters and flaky debris. When the erosive angle was 30°, the erosive ware rate of 45 steel reached a maximum. The erosion characteristics was typical shaping material. The weight loss was caused mainly by plowing and micro-cutting.Gradient coating developed in this thesis was successfully used into the turbine blades. The anti-abrasion performace could satisfy the requirement of use. After run 1 years later, the coating without large area falled off. It's service life was higher than 0Cr13Ni5 Mo turbine blades. |
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