Research On Effect Of High-temperature Treatment On Laser Cladding Self-lubricating Wear-resistant Composite Coatings

Titanium and its alloys, due to excellent corrosion, high toughness, exceptional strength-to-weight ratio and biocompatibility, have been widely used in the chemical,aeronautical and biological industries. However, the low surface hardness and poor wear resistance of titanium alloys limited severely their application in many industry fields. In the last 30 years, considerable attempts have focused on the application of various surface modification techniques to prepare reinforced self-lubricating wear-resistant composite coatings to improve surface properties of titanium alloys. Surface modification techniques were often used to improve the wear resistance of titanium alloys before. Nevertheless,these methods have their limitations, such as the poor bonding strength between coating and substrate. In recent years, laser cladding of ceramic-metal composite coatings reinforced by in situ synthesized reinforcements onto Ti-alloy substrates have been proved to be economical and environmental. The technology of laser cladding can improve the corrosion, oxidation and wear resistance of the surface of materials.In this paper, The composite Ni Cr/Cr3C2+10%Ca F2, Ni60 and Ni60+10%h-BN powders are used to fabricate the demanding high-temperature wear-resistant coatings on Ti6Al4 V alloy by laser cladding. In order to study the high-temperature stability of the coatings, the fabricated coatings were kept different hours at 600 ℃. The phase composition, microstructure, microhardness and tribological properties of the different coatings before and after high-temperature treatment were examined by X-ray diffractometer(XRD), scanning electron microscope(SEM), energy dispersive spectrometer(EDS) and ball-on-disc wear tester.The results show the γ-Ni Cr Al Ti/Ti C/Ca F2 clad coating is mainly composed ofγ-Ni Cr Al Ti solid solution, Ti C hard phase and Ca F2 self-lubricating phase. After the coating is kept at 600 ℃ for 24, 48, 96 hours, respectively, it can be found that the phasesof the coating are not decomposed and changed, the average microhardness of the coating descends from 816 HV0.2 to 706 HV0.2, which is still much higher than the Ti6Al4 V alloy(360 HV0.2). γ-Ni/Ti C/Ti B2 clad coating is mainly composed of γ-Ni solid solution and Ti B2 or Ti C hard phases, While γ-Ni/Ti C/Ti B2+Cr B+Ni3B/h-BN high-temperature self-lubricating wear resistant composite coating is also composed of Cr B, Ni3 B borides and h-BN self-lubricating phase. After both laser clad coatings are kept at 600 ℃ for 10,30, 50 hours, respectively, it can be found that the phase composition are also not decomposed and changed. After both coatings are kept at 600 ℃ for 50 hours, the microhardness of both coatings descend to 750 HV0.2 and 910 HV0.2, respectively, which are much higher than the microhardness of the Ti6Al4 V alloy.Friction and wear test show that before high-temperature treatment, γ-Ni Cr Al Ti/Ti C/Ca F2 clad coating shows outstanding tribological properties. With the increase of the processed time, the friction coefficient and wear rate of the coating have minor fluctuation.After the processing time reaches 96 hours, the tribological properties of the laser clad coating have a slight fall and are still much better than the Ti6Al4 V alloy. Before high-temperature treatment, γ-Ni/Ti C/Ti B2+Cr B+Ni3B/h-BN composite coating presents good friction-reducing and wear-resistant properties, while γ-Ni/Ti C/Ti B2 composite coating only holds good wear-resistant property. It can be found from the high-temperature treatment experiment that both the friction coefficient and wear rate increase with the increase of the processing time. When the processing time reaches at 50 hours, the average friction coefficient of the γ-Ni/Ti C/Ti B2 and γ-Ni/Ti C/Ti B2+ Cr B+Ni3B/h-BN composite coatings is about 0.45 and 0.375, respectively. The wear rate of the γ-Ni/Ti C/Ti B2 andγ-Ni/Ti C/Ti B2+Cr B+Ni3B/h-BN composite coatings is about 18.12×10-6 mm3/Nm and5.35×10-6mm3/Nm. It can be concluded that both γ-Ni/Ti C/Ti B2 and γ-Ni/Ti C/Ti B2+Cr B+Ni3B/h-BN composite coatings hold excellent wear-resistant property after they are kept at 600 ℃ for 50 hours.