The Design Of High-Performance NiCr-Al₂O₃ Matrix Lubricating Materials At Elevated Temperature Basedonthe Solid State Reaction

Aiming at the urgent demand for developing new lubricating materials at high-temperature in the fields of the aerospace,the nuclear technology and the equipment manufacturing and the design issues of high-temperature lubricating materials,high-performance NiCr-Al₂O₃ matrix lubricating materials at elevated temperature based on the solid state reaction was investigated.In view of the method for the formation of the lubricating phase by solid state reaction,using powder metallurgy vacuum hot-pressing sintering,through the regulation of SrCO₃ contents and sintering temperature,a series of NiCr-Al₂O₃ matrix high lubricating materials containing different stoichiometric strontium aluminate were prepared according to decomposition characteristics of SrCO₃ and Al₂O₃-SrO binary phase diagram.The effect of SrCO₃ contents and sintering temperature on the phase compositions,microstructure,density,hardness and tribological properties of NiCr-30Al₂O₃-SrCO₃（NC30A-SrCO₃）high temperature self-lubricating composites over a wide temperature range（Room temperature⁸00°C）was evaluated.The intrinsic relationship between the composition of the friction layer and the tribological properties of the sintered composites at different temperatures was clarified in order to reveal the lubrication mechanism.The main results were as follows:1.At 800°C,NiCr alloy exhibited the excellent tribological performance,which was attributed to the formation of the glaze layer consisting of NiO,Cr₂O₃ and NiCr₂O₄ on the worn surface.2.As reinforcement,the incorpation of Al₂O₃ provided the improved wear rate of NiCr alloy over the temperature range from room temperature to 800°C.3.With the increase in SrCO₃ contents,the density and hardness of NC30A-SrCO₃ high temperature self-lubricating composites sintered at 1100°C demostrated the decreasing trendency.As for NC30A-8SrCO₃ self-lubricating composite at elevated temperature,the formation of monoclinic-SrAl₄O₇phase during hot-pressing process resulted in excellent friction coefficient and wear rateat range from room temperature to 800°C.4.ForNC30A-SrCO₃ high temperature self-lubricating composites sinteredat 1200°C,thenewphasessuchasSr Al₁₂O₁₉,Sr₄Al₁₄O₂₅andSr₄Al₂O₇wereformedintheprocessof hot-pressing.It was found that the density and hardness of thesinered specimenwere reduced by adding different amounts of SrCO₃.NC30A-4SrCO₃ composite exhibited satisfactory tribological properties(the friction coefficient was below 0.34 and the wear rate was in order of 10^-5mm³/Nm)above 400°C,which was ascribed to the formation of SrAl₁₂O₁₉.When the content of SrCO₃ ran up to 8wt.%,the synergistic lubrication effect of SrAl₁₂O₁₉ and Sr₄Al₁₄O₂₅ endowed NC30A-8SrCO₃ composite with low friction coefficient and wear rate above 600°C.Ascompared to NC30A-8Sr CO₃ composite,the wear resistance of NC30A-12SrCO₃ composite was improved due to the formation of the synergistic lubricating films composed of SrAl₁₂O₁₉,Sr₄Al₁₄O₂₅ and Sr₄Al₂O₇ in the temperatue range from 600°C to800°C.