| Wear resistance is one of the most important performances of all kinds of gears and some other properties such as high corrosion resistance and high temperature oxidation resistance are required for some gears worked in the special environment. At present, gear surface strengthening methods can not completely meet the requirements of gears used in advanced machinery, especially the gears used for special purpose. The layer with certain thickness and high hardness can’t be acquired simultaneously whatever PVD, CVD or TD technology is used. Therefore, composite strengthening applied to prepare gradient coating on gear surface for strengthening has been became a hot topic. For the absence of gradient coating with good bonding performance and ultra-fine grain on surface with a single traditional process, two kinds of gear coatings which can be used in special occasion are prepared with composite technology of chemical heat treatment on the basis of theoretical analysis of modifying effects generated in the process of HCPEB on the gradient coating of gears,and the study of corresponding process, microstructure and properties are carried out.Combining with HCPEB modification process, Cr/CrN coatings are prepared using PVD technology. Results show that the affected depth of mechanical properties is up to 450μm. Besides, appropriate HCPEB parameters can maintain a certain thickness of gradient coating and obtain a certain thickness of diffusion layer and modified layer which can make the bonding force of coating increase 2 times for the metallurgical bonding. In addition, the wear and friction property of coating material modified by HCPEB is influenced by many factors, such as the hardness relationship between coating and substrate, the thickness of coating, surface roughness and wear debris. The hardness, thickness of the coating or the modified layer and their binding force are the most important factors while the roughness within a certain range doesn’t play a decisive role on the wear process of coating material. Furthermore, the coexistence of oxidation wear and fatigue wear are the main wear forms of Cr/CrN before and after it was modified with HCPEB.Considering the following characteristics: first, modified layer whose thickness is up to mm level can be obtained by traditional process, such as carburizing and nitriding; besides, the coating which is prepared with electrodeposition has high wear resistance and high corrosion resistance, moreover, HCPEB has high energy, and can act on gear surface which has composite strengthening coating in a very short time, consequently, a series of physical and chemical reactions, such as coating surface melting, vaporization, and stress wave, shock wave and increased diffusion, are momentarily triggered, one composite process route which combining HCPEB with carburizing and nitriding, electrodeposition Cr is designed. The results coming from it’s research of microstructure and properties show that equiaxed nano-grain structure of Cr coating whose thickness is up to 100 μm is obtained eventually. Besides, the interfaces which are on the both sides of carburized and nitrided layer have diffused elements coming from adjacent coating, therefore, the coating possesses the characteristics of slow gradient transited section hardness, fracture without obvious boundary and transited from cleavage fracture at surface to dimple fracture in the interior, excellent wear resistance and corrosion resistance. In addition, combining with the super high hardness and high red hardness that the boronized layer possesses, another composite process route which combing HCPEB with carburizing and nitriding, boronizing is designed again. The results coming from it’s research of microstructure and properties show that the thickness of modified layer is up to 305μm, the section hardness is gradient transited to about 700 HV in interior from 1400 HV on the surface, mutual diffusion of elements between layers make them combine more closely so that the morphology of fracture is dimple, cleavage, quasi cleavage and dimple in turn from surface to core, which further proof that this kind of coating has excellent mechanical properties.A new type of gear surface composite coating process which consists of electron beam(EB),PVD, thermal diffusion technology and chemical deposition technique is put forward. Specifically, three process routes of composite coating on gear surface are realized, including HCPEB and Cr/CrN coating prepared with PVD;HCPEB, carburizing and nitriding and electroposition Cr, HCPEB, carburizing and nitriding, boronizing. Moreover, the characteristics of the gear material, including surface morphology, surface hardness, surface roughness, coating bonding force and tribological behavior, are studied. Ultra-fine and gradient transited of mechanical properties of gear coating are achieved as well. |
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