Study On The Effect Of MoSi₂ On The Tribological Properties Of Cobalt Matrix Composites Over A Wide Temperature Range

Under harsh working conditions,the performance of mechanical moving parts is often reduced due to serious wear,which has a huge impact on the operation reliability and life of systems such as transportation,coal chemical industry and advanced weapons.Cobalt-based alloys in superalloys are widely used as high-temperature mechanical parts due to their strong temperature bearing capacity and excellent wear resistance.However,the current Cobalt-based alloys still cannot meet the needs of some parts,and the development of new Cobalt-based alloys with excellent performance is the key research and development direction of high-tech industry.MoSi₂ has high hardness and excellent wear resistance.Compared with other ceramic particles,it has the dual properties of metal and ceramic and has better compatibility with the metal matrix interface.Adding MoSi₂ to cobalt alloys is expected to achieve complementary characteristics and improve the tribological properties of Cobalt-based alloy over a wide temperature range.In this paper,MoSi₂ was used as the reinforcing phase,the wear-resistant CoCrNiW-MoSi₂ composites were fabricated by using powder metallurgy technology,and the effect of MoSi₂ content on the high-temperature tribological properties of Co-based composites was studied.Subsequently,the effect of load and sliding speed on the tribological properties of composites was systematically studied under dry-sliding condition,and the MoSi₂ content was further optimized.Finally,the effect of sintering temperature on the microstructure and tribological behavior of CoCrNiW-MoSi₂composites was discussed,and the preparation process of composites was optimized.The wear-resistant CoCrNiW-（0 wt.%,3 wt.%,7 wt.%and 11 wt.%）MoSi₂ composites were designed and fabricated.The composites consisted ofγ–fcc,ε-hcp,MoSi₂,Cr Si₂,Mo_1.24Ni_0.76 and MoSi_2.43W_0.211 phases,and the microstructure of composites was dense.During the high-temperature sintering process,the interface reaction between MoSi₂ and metal matrix improved the density and compactness of the materials,and the dispersed silicide hard particles made the hardness of the materials increase.The addition of MoSi₂improved the wear resistance of the material at room temperature and elevated temperatures.The wear resistance of the CoCrNiW-MoSi₂ sample was 8.2%-55.7%higher than that of the CoCrNiW sample at room temperature,and 36.1%-38.5%in the range of 800℃-1000℃%.Especially at 1000℃,the wear rate of CoCrNiW-7 wt.%MoSi₂ sample was only 4.04×10^-6mm^3.N^-1.m^-1.At the same time,MoSi₂ significantly reduced the high temperature friction coefficient of the material.In the range of 800℃-1000℃,the lowest friction coefficients of CoCrNiW-MoSi₂ sample were only 0.38-0.40,which were much lower than 0.59-0.66 of CoCrNiW.The improvement of tribological properties of materials was attributed to the high-hardness silicides and the oxide film containing solid lubricants such as Co₂Si O₄ and Cr Mo O₃ at elevated temperatures.The wear mechanism of the material at room temperature was abrasive wear and plastic deformation.With the increase of temperature,it gradually transferred to oxidative wear.In the dry friction and wear experiments,the friction coefficients decreased with the increase of load and sliding speed.The wear rates decreased with the increase of sliding speed,and the wear rates had an opposite trend with the load.The wear rates of the sample first decreased and then increased slightly with the increase of MoSi₂ content.The wear rates of the CoCrNiW-MoSi₂ sample were lower than that of CoCrNiW under all conditions.Among them,the wear rates of the samples added with 7 wt.%and 11 wt.%MoSi₂ were low and similar values,which were 22.1%-50.5%lower than that of CoCrNiW.An appropriate content of MoSi₂ had certain anti-friction,and the friction coefficients of CoCrNiW-7 wt.%MoSi₂were the lowest,ranging from 0.24 to 0.53.The hard silicide particles with high hardness and strength played a supporting and protective role in the friction process,which improved the loading capacity of the worn surface,thereby reducing the wear rates.Combined with the high-temperature tribological behavior of materials,CoCrNiW-7 wt.%MoSi₂ showed the best oil-free wear resistance.The abrasive wear and slight oxidative wear were the wear mechanism.The effect of sintering temperature（950℃,1050℃and 1150℃）on the microstructure and tribological properties of CoCrNiW-7wt.%MoSi₂ was systematically studied.The hardness of the materials first increased and then decreased with the increase of temperature,while the porosity showed an opposite trend.In high-temperature tribological experiments,the friction coefficient decreased with the increase of temperature.Due to the"pest"phenomenon of MoSi₂,the friction coefficients of the material prepared at 950℃were higher at 200℃-400℃,which were in the range of 0.57-0.66.The friction coefficients of the samples in the range of 800℃-1000℃were less than 0.4.The wear rates of the material prepared at 1050°C were always the lowest,and the wear resistance in the medium temperature range（200°C-600°C）was the most significant.In the wide temperature range,the wear rates were 38.5%-76.4%lower than that of the sample prepared at 950℃.In dry friction and wear experiments,the friction coefficients of materials prepared at 950°C and1150°C were low and similar values,and the dependence of friction coefficient on load was less than that of dependence on sliding speed,but the wear rates were much higher than that of materials prepared at 1050°C.The wear rate of the material prepared at 1050°C was at most 5.6 times and 4.5 times lower than that prepared at 950°C and 1150°C,respectively.There were obvious plastic deformation marks on the surface of the materials prepared at950°C and 1150°C.1050°C was the best sintering temperature.The appropriate sintering temperature improved the interface bonding of the material,and the low porosity and high hardness improved the loading capacity of the material,resulting in excellent anti-wear performance.In addition,the sintering temperature was more critical than MoSi₂ content on the tribological properties of Cobalt-based alloys.