| Cobalt-based alloy coating was widely used on the surface of aero engine components.With the improvement of the efficiency of the engine,the turbine inlet temperature and the working pressure had been increased,which had required the Cobalt-based alloy coating to be used in higher temperature and heavier load,the application properties of the coating had been faced more severe challenges.Dueing to the increasing of the temperature,there had existed the premature failure phenomenon on the coating during the service period,which had seriously affected the flight safety.Therefore,It was of great important for improving the overall performance of advanced aero engines by developing new coating materials and advanced coating technology to improve the service life of the coating at high temperature.In this paper,by increasing the Cr content and adding the active element Y on the existing cobalt-base coating two kinds of new cobalt-base alloy coating materials were designed and fabricated.The optimized HVOF spraying process was obtained by orthogonal test and the high-temperature oxidation and fretting wear behavior of the coating were systematically studied.The oxidation kinetics models of the coating in the range of 800-1000 ℃ within the time of 300 hours were established,the influence mechanism of Cr and Y on high temperature oxidation behavior of the coating was illustrated and the static high-temperature oxidation kinetics and the oxidation failure mechanism of the coatings were revealed.Specific studies and results were as follows:The Coating which had been prepared by using optimized HVOF process had compact and homogeneous microstructure and the porosity were less than 1%,there were no large pores,vertical cracks and parallel cracks in the coatings,the oxide inclusion contents were less than 1%.The increasing of Cr and the adding of Y had no badly effect on the microhardness and adhesive strength of the coating.The microhardness of coatings(HV200)was above 700 and the adhesive strength was higher than 70MPa.The porosity of the coating had an obvious influence on the hardness and adhesive strength.When the porosity was reduced from 8%to 0.5%,the microhardness of the coating increased from 507 to 705,and the adhesive strength increased from 51.5MPa to 71.9MPa.The oxidation kinetic curves’ index factor n of Co-XY-1 and Co-XY-3 coatings at 800-900℃ and Co-XY-2 coating at 800-1000℃ were equal or greater than 2,the oxidation rate of coating had been greatly decreased with time,which showed the coatings had excellent oxidation resistance at high temperature under these conditions;at 950℃ and 1000℃.due to the serious peeling of the oxide scale,the oxidation curves of Co-XY-1 and Co-XY-3 coatings were of between linear rule and square parabolic rule.The increasing of Cr could shorten the time between initial oxidation and stable oxidation,reduce the oxidation rate of the coating and improve the purity of Cr2O3 in the oxide film;the addition of Y could change the growth mechanism of Cr2O3 film,lower the critical content of Cr to form a complete Cr2O3 film and improve the adhesion properties of the oxide film,Both of which were beneficial to improve the high temperature oxidation resistance of the coating.The addition of Y not only reduced the oxidation rate of the coating,but also enhanced the anti-stripping property of the oxide scale,which meaned the improvement on high temperature oxidation resistance of the addition of Y was better than that of increasing of Cr.The Cr were depleted in different degrees on the surface of the coatings during the oxidation period,among which the Co-XY-3 coating was the most serious and the Co-XY-2 coating was the lightest.With the increase of oxidation temperature and time,the dilution degree was more serious.After the oxide film was damaged,the degree of dilution of Cr on the surface was obviously increased.The oxidation and the depletion of Cr on the surface of the coatings were the main reasons for the degradation,which eventually leaded to the failure of coating.The damage of the primary oxide film was the breakthrough of the failure,due to the depletion of Cr near the surface,the rapid oxidation of Co could not be effectively blocked,which resulted in the rapidly degrading of coating to the direction of substrate until the coating was out of work.The high temperature fretting wear resistance of the substrate material was obviously improved by adopting the coatings.Under the fretting wear condition at 600 ℃and 800℃.the abrasion loss of the Co-XY-2 coating was the smallest of the three kinds of coatings and the abrasion loss of the Co-XY-1 and Co-XY-3 were basically the same.The addition of Y could improve the adhesion between the surface oxide film and the coating,which had improved the high temperature fretting resistance of the coating,and the increase of Cr has no significant effect on the fretting wear resistance of the coating. |
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