| 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. |
