钛合金兼具高的比强度和良好的耐腐蚀、耐高温及抗氧化性能，目前已广泛应用于航空航天、石油化工等多个国家重点领域，但是，钛合金硬度低和摩擦系数大的缺点降低了其减摩耐磨性能。采用激光熔覆技术在钛合金表面制备高硬度涂层是提高钛合金表面耐磨性的有效手段，此外，激光熔覆层中添加或原位生成的固体自润滑剂能显著改善基材的减摩性，因此，设计兼具高硬度和自润滑特性的复合涂层是改善钛合金摩擦磨损性能的有效途径。本文以Ni60、TC4、Ni-MoS₂和Y₂O₃粉末为熔覆材料，采用同轴送粉、多道搭接激光熔覆技术，在钛合金表面成功制备了镍基自润滑复合涂层，分析了激光功率、Ni-MoS₂添加量和Y₂O₃添加量对涂层成形质量的影响规律，确定了最优的激光熔覆工艺参数和最佳的成分配比；研究了Ni60+TC4+Ni-MoS₂激光熔覆层的微观组织、相结构特征和力学性能；揭示了Y₂O₃对Ni60+TC4+Ni-MoS₂激光熔覆层组织演变规律及摩擦磨损性能的影响机制。本文的主要研究工作有： (1)激光熔覆Ni60+TC4+Ni-MoS₂自润滑涂层成形质量研究 设计了Ni60+TC4+Ni-MoS₂+Y₂O₃镍基复合材料体系，采用同轴送粉激光熔覆技术，在TC4钛合金基材上成功制备了镍基复合激光熔覆层。以涂层宏微观缺陷作为评价标准，确定了激光功率最优值为1100W，Ni-MoS₂最佳添加量为35wt％，Y₂O₃最佳添加量为3wt％。利用ABAQUS软件构建了熔覆层三维有限元模型，揭示了激光熔覆层中残余应力分布规律，为研究熔覆层开裂、孔隙等问题提供了理论支撑。研究结果表明，熔覆层过渡区及边缘的残余应力较为集中，极易萌生裂纹，之后裂纹向熔覆层内部扩展。 (2)Ni60+TC4+35％Ni-MoS2熔覆层组织和摩擦磨损性能研究 Ni60+TC4+35％Ni-MoS₂激光熔覆层中生成相包括金属间化合物Ti₂Ni，陶瓷增强相TiC、TiB和TiB₂，固体润滑相Ti₂SC，钛硫化合物Ti₈S₃、钛硫不定比化合物Ti〓S〓和基体α-Ti。基于Bramfitt二维点阵错配度理论，探究了涂层中TiB₂与TiC的镶嵌生长机理，研究结果证明，TiB₂与TiC的二维点阵错配度δ=0.876％， TiB₂可作为TiC的有效形核基底，其中TiB₂为初生相、TiC为次生相。涂层的平均显微硬度约为720 HV₀.₅，较基材提高了约83％，但熔覆层显微硬度分布不均匀；涂层的摩擦磨损性能显著提升，磨损机制主要为磨粒磨损。 (3)Y₂O₃对Ni60+TC4+Ni-MoS₂熔覆层组织和摩擦磨损性能的影响 Ni60+TC4+Ni-MoS₂+Y₂O₃激光熔覆层中的主要物相包括金属间化合物Ti₂Ni、陶瓷增强相TiC、固体自润滑相Ti₂SC、钛硫化合物Ti₈S₃、钛硫不定比化合物Ti〓S〓和基体α-Ti。Y₂O₃的加入抑制了涂层开裂、细化了涂层组织，抑制了涂层中钛硼化合物的反应析出，同时增加了固体自润滑相Ti₂SC的反应析出量。涂层显微硬度分布均匀性较未添加Y₂O₃时显著提升，当Y₂O₃添加量为3wt％时，熔覆层显微硬度分布均匀性最佳，磨损表面平整度最优，摩擦系数最小，涂层的平均显微硬度约为596 HV₀.₅，较基材提高约1.5倍，涂层的摩擦磨损性能显著提升，磨损机制主要为磨粒磨损。结合涂层成形质量分析，在Ni60+TC4+Ni-MoS₂+Y₂O₃熔覆材料体系中Y₂O₃最佳添加量为3wt％，但涂层中稀土氧化物Y₂O₃添加量必须有效控制，添加量不足或过量添加都不利于熔覆层力学性能的改善。 关键词：激光熔覆；钛合金；稀土氧化物；固体自润滑；摩擦磨损

 With satisfactory performance of specific strength, corrosion resistance, high-temperature resistance and oxidation resistance, titanium alloys have been widely utilized in many national key fields, such as aerospace, petrochemical industry, etc. However, the friction and wear resistance of titanium alloys is worsened by their low hardness and high friction coefficient. Thus, it's deemed as an effective way to improve the wear resistance of titanium alloys through fabricating coating with high hardness on the surface of titanium alloys by laser cladding technology. Moreover, the addition or in-situ formation of self-lubricants in the laser cladding layer can also help to improve the antifriction of the substrate. Therefore, it is an effective way to improve the friction and wear performance of titanium alloys by means of designing the composite coating with high hardness and self-lubricating properties. In this paper, with Ni60, TC4, Ni-MoS₂ and Y₂O₃ powders serving as the cladding material, Ni-based self-lubricating composite coating was successfully prepared on the surface of titanium alloy using coaxial powder feeding and multi-track overlapping laser cladding technology. Furthermore, the influence rules of the laser power and the content of Ni-MoS₂ and Y₂O₃ to the forming quality of the cladding layer were analized along with the optimal process parameters of laser cladding and the best ratio of ingredients were determined. Not only that, the microstructure, phase structure and mechanical properties of Ni60+TC4+Ni-MoS₂ laser cladding layer were studied and the mechanism by which Y₂O₃ impacted the structure and friction and wear properties of Ni60+TC4+Ni-MoS₂ laser cladding layer was revealed as well. The main research contents of this paper are as follows: (1)Studies on the forming quality of Ni60+TC4+Ni-MoS₂ self-lubricating laser cladding layer The design of the Ni60+TC4+Ni-MoS₂ and Ni60+TC4+Ni-MoS₂+Y₂O₃ Ni-based self-lubricating materials system was accomplished, based on which the Ni-based composite coatings were successfully prepared on the TC4 alloy substrates by coaxial powder feeding laser cladding technology. Taking the macro and micro defects of the coating as the criterion, the optimal laser power, the best addition of Ni-MoS₂ and Y₂O₃ were determined to be 1100W, 35wt%, 3wt% respectively. Also, the three-dimensional finite element model of the cladding layers was built by the ABAQUS, which helped to reveal the residual stress distribution in the laser cladding layers and laid a theoretical foundation for the further study of the cracking and porosity in cladding layer. The results show that the residual stress is relatively concentrated in the bonding zone and the edge of the cladding layer, leading to the initiation of cracks which would further develop to the interior of the cladding layer. (2)Studies on structure and friction and wear properties of Ni60+TC4+35%Ni-MoS₂ laser cladding layer Several formation phases, including intermetallic compound Ti₂Ni, ceramic reinforced phases TiC, TiB and TiB₂, solid lubricating phase Ti₂SC, titanium-sulfide compounds Ti₈S₃, Ti〓S〓 and matrix α-Ti, appeared in Ni60+TC4+35% Ni-MoS₂ laser cladding layer. Based on the Bramfitt's two-dimensional lattice mismatch theory, the growth mechanism of TiB₂ and TiC in the cladding layer was investigated and the result of the planar disregistry for TiB₂ and TiC was 0.876%, indicating that TiB₂ can work as an effective nucleation center for the growth of TiC. Moreover, the average microhardness of the coating was around 720 HV₀.₅, about 83% higher than that of the substrate, while the microhardness distribution in the cladding layer was not uniform. Besides, the friction and wear properties of the coating were significantly improved, and the wear mechanism was mainly abrasive wear. (3)Effects of Y₂O₃ on the structure and friction and wear properties of Ni60+TC4+Ni-MoS₂ laser cladding layer The main phases in Ni60+TC4+Ni-MoS₂+Y₂O₃ laser cladding layer included intermetallic compound Ti₂Ni, ceramic reinforced phase TiC, solid self-lubricating phase Ti₂SC, titanium-sulfide compounds Ti₈S₃, Ti〓S〓 and matrix α-Ti. The addition of Y₂O₃ brought several benefits to the laser cladding layer: less cracks, fine structure, less reaction precipitation of Ti and B compounds, and more reaction precipitation of solid self-lubricating phase Ti₂SC, etc. Also, the uniformity of microhardness distribution of the layer was remarkably improved thanks to the addition of Y₂O₃. Furthermore, the optimal uniformity of microhardness distribution, the smoothness of the wear surface and friction coefficient of the layer were obtained when the addition of Y₂O₃ was 3wt%. Besides, the average microhardness of the layer was about 596 HV₀.₅, 1.5 times higher than that of the substrate, and the friction and wear properties of the coating were significantly improved as well. The wear mechanism was abrasive wear. Thus, the optimum addition of Y₂O₃ in the system of Ni60+TC4+Ni-MoS₂+Y₂O₃ cladding material was determined to be 3wt% according to the forming quality of the layer, so due attention should be paid to the addition level of Y₂O₃ in order to achieve a satisfactory improvement in mechanical properties of the laser cladding layer. Keywords: Laser cladding; Titanium alloy; Rare earth oxide; Solid self-lubricating; Friction and wear