Fabrication And Properties Of Copper Matrix Composite Reinforced With Ti₃AlC₂

With increasing speed of electric locomotives, the property of pantograph needs to be continuously improved. In this study, a new Ti₃AlC₂/Cu matrix composites which will be used on pantograph were prepared by copper plating or silver plating, ball milling, hot pressing and hot extrusion processes. The effect of the chemical copper plating, silver plating, sintering temperature, hot extrusion on the microstructure and the properties of the composite were discussed. The morphology and microstructure of composite material were observed by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM), and the density, hardness, tensile strength, impact toughness, conductivity and friction and wear properties were measured.The effect of milling time on uniformity of mixed powder were discussed. The optimized milling time was 12h under conditions of ball to ratio 4:1 and milling speed of 200rpm.When the sintering temperature was 900℃, the particle sizes of Ti₃AlC₂ was about 20μm with ellipsoidal shape, and Ti₃AlC₂ were uniform distribution. By surface scanning, a large number of Al atoms of Ti₃AlC₂ were diffused into the copper. When the sintering temperature was 800℃, Ti₃AlC₂ distributed uniformly in the matrix, the relative density was 95.45% and tensile strength was 160MPa. The surface scanning results show that the Al of Ti₃AlC₂ concentrated in Ti₃AlC₂ area.The effect of the capacity of Ti₃AlC₂, temperature, pH value, formaldehyde content on copper plating were investigated. The optimum parameters of copper plating were as follows: capacity of Ti₃AlC₂: 6g/L; temperature:60℃; pH:12.5～13.0; formaldehyde: 15ml/L. Ti₃AlC₂ particles were coated with a uniform and dense copper layer with thickness of 1.3μm with the optimized parameters. The effect of the nitrate silver content, ammonia content, formaldehyde content, and ethanol content on silver plating were discussed also, the optimum parameters: temperature was 20℃, silver nitrate was 10g/L, ammonia was 100ml/L, the formaldehyde content was 15ml/L, ethanol content was 400ml/L, load capacity was 7.5g/L.The interfacial wettability of ceramic particles on copper matrix, density, hardness, tensile strength were greatly improved by copper plating or silver plating. For 10vol.%Ti₃AlC₂/Cu composite materials, the relative density of sintered composite with copper plating was 97.86% and tensile strength was 221MPa; the relative density of sintered composite with silver plating was 98.78% and tensile strength was 260MPa. The density, hardness, room temperature mechanical properties of the composites were greatly improved by hot extrusion. The relative density of extruded composite with copper plating was 99.98% and tensile strength was 306MPa; The relative density of extruded composite with silver plating was 98.99% and tensile strength was 408MPa.The effect of different load, sliding velocity, different sliding distance on coefficient friction, were volume and wear rate were discussed. Morphology of the wear were observed, friction and wear mechanisms were studied. The results show that, when the sliding velocity was 0.8m/s and sliding distance was 960m,with increasing of the load , type of wear mechanism of sintered composites were changed from a plow type abrasive wear into a micro-crack type abrasive wear. The wear rate of sintered composites with copper plating were decreased from 0.6×10^-4mm³N^-1m^-1 of 10N load to 0.50×10^-4mm³N^-1m^-1 of 40N load. The wear rate of sintered composites with silver plating were decreased from 0.7×10^-4mm³N^-1m^-1 of 10N load to 0.4×10^-4mm³N^-1m^-1 of 40N load. It was found that the plating and extruded composite were complex. The wear rate of copper plating and extruded composite were decreased from 0.55×10^-4mm³N^-1m-1 of 10N load to 0.31×10^-4mm³N^-1m^-1 of 30N load, then decreased to 0.17×10^-4mm³N-1m^-1 of 60N load. Type of wear mechanism were changed from a chisel-type cutting of abrasive wear of 10N load into a plow type of abrasive wear of 30N load, and then to a grinding type abrasive wear of 40N load. The wear rate of silver plating and extruded composite were decreased from 0.75×10^-4mm³N^-1m^-1 of 10N load to 0.42×10^-4mm³N-1m^-1 of 30N load, then decreased to 0.16×10^-4mm³N^-1m^-1 of 40N load. Type of wear mechanism were changed from a type of abrasive wear of scratch of 10N into a type adhesive wear of scratch of 30N load, and then to a scratch-type abrasive wear of 40N load. When the load was 20N load, sliding distance was 960m, with increasing sliding speed, the wear rates were gradually decreased from 0.58×10^-4mm³N^-1m^-1 of 0.4m/s sliding speed to 0.11×10^-4mm³N^-1m^-1 of 3.2m/s sliding speed, corresponding the type of wear mechanism were changed from a micro-crack type abrasive wear into a scratch-type abrasive wear.The results show that the sintered material surface prone to micro-cracks when the load were 40N, wear volume were high, because of relatively low density materials and poor mechanical properties; the extruded composite with high density, better mechanical properties, when the load were beyond 40N, the self-lubricating materials fully played out its role, wear rate decreased significantly.