【作者】 许金沙；

【导师】 轩福贞； 张显程；

【作者基本信息】 华东理工大学 ， 动力工程及工程热物理， 2014， 博士

【摘要】 激光熔覆颗粒增强复合涂层具有金属材料的高韧性和陶瓷材料的高硬度、耐磨损、抗氧化等特性,广泛用于航空、军事、石油、化工、医疗器械等领域。本文制备了激光熔覆碳化钨(WC)颗粒增强镍(Ni)基复合涂层,研制开发了一套能实现自动监测和模拟多种工况的滚动接触疲劳试验系统,从理论分析、实验研究和数值模拟三个层面,系统研究了其在滑动摩擦、单轴拉伸、循环滚动接触三种工况下的失效行为。主要研究工作和结论如下：进行了不同WC含量的WC/Ni复合涂层摩擦磨损实验,建立了WC体积分数、涂层微观组织和抗滑动摩擦性能三者之间的对应关系。研究发现,涂层的耐磨性随WC含量的增加先增后减,在WC体积含量为6.91%时表现出最佳的抗滑动摩擦性能。WC颗粒含量决定了涂层的磨损机制,低WC含量涂层的失效主要为两体磨料磨损导致,高WC含量涂层失效表现为疲劳磨损与三体磨料磨损机制。开展了扫描电镜下WC-Ni涂层/45钢基体的原位拉伸实验研究。基于拉伸过程中涂层表面裂纹演化过程的原位观察,探讨了WC颗粒对涂层拉伸断裂行为的影响及涂层断裂失效机理。结果表明：WC颗粒导致WC/Ni涂层的脆性增加,表现为低拉伸应变下的脆性失效。微裂纹多萌生于WC增强颗粒,随载荷增加而长大,相互连接形成横贯裂纹,导致涂层失效。系统研究了滚动接触循环载荷下涂层的疲劳行为。采用间断试验,研究了涂层表面损伤及裂纹的萌生与扩展规律。研究发现：接触载荷下涂层主要表现为次表面裂纹引发的局部剥落失效,剥落坑深度与WC颗粒相关。次表面裂纹源于WC颗粒相关的剪应力,扩展路径表现为较大的随机性。这一物理过程表现为：高剪切应力诱发次表面裂纹萌生,次表面主裂纹扩展同时伴生枝状二次裂纹、表面裂纹向涂层内部扩展,与次表面二次裂纹相互连接,形成剥落失效。有限元分析了接触载荷下涂层表面、中心轴线以及涂层/基体界面等关键部位的力学响应,定量建立了涂层厚度、弹性模量等参数与应力分布的映射关系,提出了以应力强度为目标的涂层参数优化：对于陶瓷涂层等脆性材料,容易因为接触边缘较大的径向拉应力产生表面裂纹,tc/a0应控制在0.5-3范围；对于韧性金属涂层,tc/a0值应小于0.2以防止剪应力诱发的次表面裂纹。更多还原

【Abstract】 Laser cladded composite coatings effectively combine advantages of the self-fluxing metal and the ceramic particles, so they have excellent surface performances, such as high strength, good toughness, high temperature resistance and good wear resistance. Therefore, they have been widely used in many fields, such as aerospace, military defense, petrochemical and medical device. In this dissertation, WC particle reinforced Ni matrix composite coatings with different WC contents were prepared. An experiment system was developed for the rolling contact fatigue performance testing, which could monitor the surface status automatically and simulate different lubrication conditions. The fracture and fatigue behaviors of laser cladded WC/Ni composite coating under different loads including sliding friction, uniaxial tension and cyclic rolling contact were systematically investigated in terms of theoretical analyses, experiments and numerical simulations. The main contents and conclusions of this dissertation are as follows:Experiments on the sliding wear resistance of the WC/Ni composite coatings with different WC contents have been carried out. The function relationship among the WC content, coating microstructure and sliding wear resistance was established. Results showed that the sliding wear resistance firstly increased and then decreased with the increasing of the WC content. The coating with6.91vol.%WC particles exhibited the best wear resistance. The wear mechanism depended on the volume fraction of WC particle. When the WC content was low, the two-body abrasion wear was identified as the main wear mechanism. For the coating with high WC content, the main mechanisms were fatigue and three-body abrasion.Experimental research on the in-situ tensile performance of the WC-Ni coating/45steel system have been done. Based on the in-situ observation of the evolution of crack at the coating surface during tensile test, the tensile fracture mechanism and the effect of WC particle on the fracture behavior of the composite coating were discussion. Results showed that the addition of WC particles increased the coating brittleness. It led to the brittle fracture of coating at low applied strain. Most of micro-cracks were initiated from WC particles, propagated and joined with each other or with a new one with increasing the tensile load. The joined cracks propagated towards the coatings surface and throughout the coating width, resulting in fracture of the composite coating.The fatigue behavior of laser cladded WC/Ni coating under repeated rolling contact loading was studied systematically. Interrupted experiments were used to research the surface damage and the initiation and propagation of the crack at the coating surface. Experimental results showed that the coating failed by the form of spalling which was caused by the subsurface crack. The orthogonal shear stress was the driving force that controls the initiation of the subsurface crack. The depth of the spall was related to the distribution of WC particles because of the shear stress redistribution due to the presence of WC particles. A processing model was proposed to illustrate the formation of the spall, which could be described as follow:the formation of the substrate crack due to high shear stress, propagation of the main substrate crack and the formation of branched substrate crack, the propagation of surface at an angle, the joining of the surface cracks and subsurface cracks and the formation of the spall.The elastic stress distributions at key locations, such as coating surface, contact central axis and the coating/substrate interface, were obtained under normal contact load by using finite element simulation method. The function relationship between the maximum value of stress components and coating parameters such as thickness and elastic modulus was established. A parameters optimization scheme targeted to reduction of the maximum stress was proposed. For the hard coating materials, such as ceramic coating, the normalized coating thickness tc/ao should be controlled at the range of0.5-3to prevent surface crack induced by the large radial tensile stress. For the soft coating such as metal coating, the probability of substrate crack due to the high shear stress is relatively high. The normalized coating thickness tc/ao should be set to be lower than0.2.更多还原