| Because of the exceptional chemical and high temperature stability,glass-ceramics without grain boundaries can be used as high temperature protective coatings for refractory metals.However,they can be easily blown away from the substrate because of the very low viscosity as a result of softening at high temperatures.Adding borides into the glass-ceramic matrix can improve the resistance to thermal erosion without compromising the oxidation resistance.In this investigation,novel SiO2-Al2O3-ZnO-CaO-ZrO2-TiO2-based composite coating materials modified by particles of TiB2 and ZrB2 were prepared via pressureless sintering method,respectively.Furthermore,their oxidation kinetics was obtained in terms of thickness of oxide scale versus oxidation time.For the G90T,its oxidation kinetics at 1200? followed a para-linear law.At the initial oxidation stage,the borosilicate glass scale forming in-situ protected the glass-ceramic based composite substrate from the oxygen effectively.However,lots of micro-cracks formed within the oxide scale after oxidation for 30h,which led to a much higher oxidation rate due to the greatly increased short diffusion circuit diffusion paths for oxygen.Oxidation tests of the G90Z was carried out at temperatures of 1000?,1200?and 1400?,respectively,while those of the 70wt.%ZrB2 modified one were conducted at 1200 ? and 1400?,respectively.At the same oxidation temperature,the oxidation resistance of the two kinds of glass-ceramic based composites modified by ZrB2 particles was much better than that modified by TiB2.The oxide scale of the glass-ceramic based composites incorporating ZrB2 exhibited layered structure,and was mainly composed of borosilicate,ZrO2 and ZrSiO4.The formation of this layered structure was closely related to the content of silicate glass within the composite as well as the oxidation temperature and time.Furthermore,the oxidation kinetics of the 70wt.%and G90Zs followed the para-linear law at both 1200? and 1400?,while that of the G90Z just followed parabolic law at 1000?.Moreover,the oxidation resistance of the G70Z was better than that of the glass-ceramic based composite with 90wt.%ZrB2 at the temperature of 1200?,while the situation was just opposite at 1400?.In addition,the oxidation rates of the two kinds of ZrB2 modified glass-ceramic based composites increased with the increasing oxidation temperature.Based on the understanding of the oxidation behavior of borides modified glass-ceramic based composites,30wt.%ZrB2 modified glass-ceramic coating was prepared on TC4 Ti-based alloy via slurry method,and its isothermal oxidation resistance was tested at 900? in static air.It was revealed that the oxidation rate of the ZrB2 modified glass-ceramic coating was much lower than that of the bare TC4 substrate,and its oxidation kinetics followed parabolic law. |
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