Mechanism On Rolling Contact Fatigue Improvement Of GCr15 Bearing Steel By Ion Implantation And Deposition

To prolong the fatigue life, most of all, to extend the rolling contact fatigue life(FCF) of rolling bearing, is a very important research project for bearing staff at all times. In recent decades yeears, although the steels metallurgy quality, structure design, processing and lubrication technique made a great progress, it is difficult to increase to a great extent in practical operating condition. In the dessertation, from the point of view of the surface modification, based on the traditional processing of heat treatment, physical vapior desposition (PVD), chemical vapior desporation (CVD) and ion implantation, the plasma immersion ion implantation and deposition (PIII&D) technique is applied to synthesize titanium nitride (TiN) and a series of composite films on the GCr15 bearing steel substrate surface. It is shown that the RCF increased markedly, the fatigue life decentrality of treated samples decreased obviously according to the Weibull distribution curves. In order to analyse the RCF of treated sample, some effect facrors, such as surface micro-hardness, residual stress, substrate surface roughness, film types and film thickness are clarified. The maximum shear stress theory of Lundberg-Palmgren (L-P) and Weibull distribution theory of untreated bearing rolling contact fatigue destruction is depicted. The deficiency and limitation of these two theories in the field of rolling contact fatigue destruction mechanism after PIII&D was discussed. In addition to, based on the L-P theory and Weibull distribution theory, combined with the contact stress and strain field analysis results of ANSYS finite element software, Weibull distribution curves and fatigue destruction scanning electron microscope (SEM) photograph of film samples, the fatigue destruction five phase model of treated film bearing sample was educed.The phase compositions, surface quality and mechanical properties of TiN film bearing samples were be characterized by X-ray photoelcctron spectrum (XPS), X-ray diffraction (XRD) spectrium and atom force microscope (AFM). XRD spectra veried TiN phase exist in the film, and the (200) crystalline plane is the preferential orientation. The XPS analysis results of Ti2p and N1s in different surface depths show that TiN, TiO2 and TiNxOy exist in the surface film layer, and still a small quantity of remnant simple nitrogen. Two-dimensional and three-dimensional AFM morphology and roughness curves reveal that the surface film possess perfect crystalline, compact structure and clear grain.The nano-hardness and elastic modulus of TiN film bearing specimens are larger than that of GCr15 substrates', but the extent vary with the processing. Compared with the GCr15 steel substrate, the maximal nano-hardness is 25GPa, increased by 127.3%; the maximal elastic modulus is 350GPa, increased by 59.1%. Nano-scratching curves display that the film bearing samples underwent three phases during the scratching: elastic deformation, elastic-plastic deformation, upload crack and no load delamination. It is also shown that TiN film has good elastic comeback ablity and a strong adhesion with the GCr15 bearing steel substrate.The surface residual stress of various TiN film thickness samples were be measured by grazing incidence X-ray analysis (GIXA). Analysis results revealed that all samples treated by PIII&D can produce residual compressive stress in the surface film layer, and the compressive stress value decrease with the TiN film thickness increse. Friction coefficient curves shown that the friction and wear behaviors of TiN film bearing samples is better than that of substrate's, the minimal friction coefficient less than 0.2. In addition to, the lower of the substrate surface roughness before treated by PIII&D, the better of the mechanical property of TiN film bearing sample after PIII&D.ANSYS simulation results indicated that the maximal Von mises exist in the contact region of ball and bearing ring and colse up to the boundary corner for ordinary rolling conact under the axial force condition. The maximal shear stress generated in a certain depth under the surface. It is consistent with the maximum dynamic shear stress theory by Lundberg G. and Palmgren A.As far as TiN film bearing sample is concerned, under the identical axial force condition, the maximum Von mises generated in the film-substrate interface. The maximum shear stress initiated in the film-substrate interface, and point to the film interior. Film thickness has an important effect on the above-mentioned two kinds of stress. Under a constant film elastic modulus conditions, both the maximum Von mises and shear stress decreased with the film thickness increase. The author considered that maximal shear stress is still the main reason to induce the rolling contact fatigue failure of hard coating materials by PIII&D.The RCF life of TiN film bearing specimens tested using self-made ball-rod fatigue tester under different PIII&D processing conditions. Based on the fatigue life data and Weibull distribution theory, the different characteristic life parameters of film bearing samples were be done, and the Weibull distribution curves were be plotted.The RCF life of TiN and a series of composite film bearing specimen is prolonged under different substrate surface roughness and PIII&D implanted time conditions. All the characteristic fatigue life of TiN film bearing samples enhanced to a great extent. Thereinto, the maximum L10 , L50, La and average fatigue life of single layer TiN film bearing sample is about 9.2 times, 4.2 times, 3.5 times and 3.5 times larger than that of substrate's, respectively. The reason, fatigue life improved after PIII&D, is that the enhancement of sample surface intensity and hardness; the descent of surface roughness and friction coefficient; the optimization of surface structure and chemical components and the state of surface compressive stress. According to the slope parameters of Weibull distribution curves, the characteristic fatigue life data of all composite film bearing samples are larger than that of single layer TiN film bearings.The optical microscope(OM) and SEM morphology of TiN film bearing specimen fatigue pitches shown that the micro-defects of film-substrate interface and close up to the film interior is the inducement to produce rolling contact fatigue pitches; the action of maximum shear stress together with the lubricant oil grain formed during the rolling cycle is the exterior driving force. The five phase model revealed that the rolling contact fatigue mechanism of PIII&D TiN film bearing is micro-defects in the film to lead to small cracks, and the cracks propagation gradually during the cycle load, with the help of lubrication oil grain combination with maximum shear stress; finally, the surface film was delaminated from the substrate.