| Due to its unique combination of high strength to weight ratio, superior corrosion resistance and good biocompatibility, titanium alloys has been applied in aeronautical, chemical and biomedical industries widely. However, it has been limited used in key tribological components due to its low hardness, poor wear resistance and insufficient high temperature oxidation resistance. Laser cladding has become one of technology to improve surface engineering of components because the coating is compact structure, fine grain size and metallurgical bond with base material.In order to improve wear and high-temperature oxidation resistance of titanium alloy simultaneously, the new materials system of composite coating has been studied. NiCr-40%Al-10%Si, NiCr-40%Al-20%Si and NiCr/Cr3C2-40%Al-20%Si compound powders have been chosen as raw materials and composite coating has been synthesized on Ti6Al4 V alloy by laser cladding. The microstructure, phase and chemical composition analysis had been studied by means of XRD, SEM and EDS. The micro-hardness was measured by Vickers micro-hardness tester; The tribological properties of composite coatings and Ti6Al4 V alloy substrate had been measured by a ball-on-disk high temperature dry sliding wear tester at difference temperature(25℃ and 600℃), and the wear mechanism was analyzed by means of SEM/EDS. The high temperature(800℃) oxidative properties of composite coatings and Ti6Al4 V alloy had been measured by an electric resistance furnace and an electronic balance, and the oxidative mechanism is analyzed.The results indicated that NiCr, Al and Si compound powders have been preplaced on Ti6Al4 V alloy and composite coatings have been fabricated by laser cladding. The thickness of coating is about 1.5 mm, the bonding of composite coating and Ti6Al4 V alloy substrate is metallurgical, the coating was mainly consists of primary Ti5Si3/Al3Ni2 reinforcement particles and the inter-primary Al3Ti/NiTi intermetallic compounds matrix. Coating N1(NiCr-40%Al-10%Si) has highest hardness(1100 HV0.5), which is three times that of Ti6Al4 V alloy. However, the composite coating N1 has high dense Al3Ni2 ball hardness particle, which has high room temperature brittleness. The microhardness of coating N2(NiCr-40%Al-20%Si) is 870 HV0.5, which is a half of two times that of Ti6Al4 V alloy. Coating N2 has a high hardness and good toughness. The composite coating N3(NiCr/Cr3C2-40%Al-20%Si) have been fabricated by laser cladding. The matrix is γ-Ni and Al8Cr5 and the reinforcement phase is TiC and Ti5Si3. The microhardness of coating is 750 HV0.5, which is twice that of Ti6Al4 V alloy.The tribologiacl results indicated that the coating N1 has poor wear due to its high room temperature brittleness, the wear mechanism is three body abrasive wear. Coating N2 has better wear resistance, which is twice that of Ti6Al4 V alloy. At 600℃, the friction coefficient and wear rate of coating N2 are reduced due to its oxide scale that it exhibited partly better self-lubrication. The wear resistance of composite coatings N3 is 2.2 times that of Ti6Al4 V alloy at room temperature. At 600℃, the wear resistance of composite coating N3 is slightly poorer than Ti6Al4 V alloy because the hardness particles has broken off due to high coefficient of thermal expansion.The high temperature oxidation results indicated that the Ti6Al4 V alloy has poor oxidation resistance and the loose and porous Ti O2 oxide scale has broken off at 800℃ in air. The oxide scale of coating N2 is dense Al2O3, NiO and SiO2 oxide scales. Coating N2 exhibits excellent high temperature oxidation resistance, which is nine times that of Ti6Al4 V alloy. The oxide scale of composite coatings N3 is Al2O3, NiCr2O4, TiO and SiO2 oxide scales. Coating N3 exhibits excellent high temperature oxidation resistance, which is seven times that of Ti6Al4 V alloy. |
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