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The Analysis of Ni-P Friction Composite Coating Preparation and Mechanical Properties

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Tutor: TanYuanQiang
School: Xiangtan University
Course: General and Fundamental Mechanics
Keywords: pulse electrodeposition,composite coating,friction and wearproperty,corrosion-re
CLC: TG174.4
Type: PhD thesis
Year:  2013
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Abstract:
Due to its simple process, good performance and economy, Ni-P alloy coating has been widely used in industries, such as mechanical, electronics and chemical engineering, etc. However, with the development of the science and technology, the alloy coating cannot meet the complicated environment. Thus, multifunctional Ni-P composite coating materials have been paid more and more attention to. The hexagonal boron nitride [BN(h)] particles have excellent lubrication performance, high thermal stability and corrosion resistance, so preparation Ni-P/BN(h) composite coating by adding BN(h) particles to Ni-P base plating solution, can further improve the corrosion resistant wear-resisting performance of Ni-P alloy coating, and have much significance.Ni-P/BN(h) composite coating was prepared by electroplating, evaluated by the organization structure of the coating, microhardness and deposition rate of the coating, to study the process of Ni-P/BN(h) composite coating. Results show that the best technology for pulse frequency was1500Hz, occupies empties compared was0.2, pH was4.0, electroplating temperature was50¡æ, current density was5A/dm2, the content of BN(h) particle in plating solution was20g/L, and surface active agent was poval.The effect of technology for heating processing on organization structure and microhardness of Ni-P/BN(h) composite coating was studied. Results show that as-deposited Ni-P/BN(h) composite coating was amorphous structure; a small amount of Ni5P4and Ni12P5etc. metastable phases were precipitated from coating, and the coating was still amorphous structure after200¡æheat treatment; but after300¡æheat treatment in80minutes, the precipitated phases were a lot of unsteady phases and a small amount of Ni3P stable state phase, the composite coating was amorphous structure and crystalline state mix organize; after heat treatment of400¡æin80minutes, the composite coating was crystallized to great degree. Compared to Ni-P alloy coating, BN(h) particles could lead to reduce the precipitated speed of the second phase and extend crystallization time. At given heat treatment temperature, the microhardness of Ni-P/BN(h) composite coatings increase at the beginning, then decrease with the extension of heat treatment time, which have a maximum microhardness; The time of reaching the maximum microhardness of composite coating was short, as the heat treatment temperature rises.Friction factor of Ni-P/BN(h) composite coating reduced with the suspension quantity of BN(h) increased; when suspension quantity of BN(h) was20g/L, the friction factor was minimum, about0.08, wear loss was just as a quarter of Ni-P alloy plating, Ni-P/BN(h) composite coating showed excellent tribological properties. The friction factor of composite coating increased with loads and rotation speed increased, the wear mechanism of composite coating was slight particle attrition when load and rotation speed were low. However, the wear mechanism of composite coating was fatigue wear and particle attrition synergy when load and rotation speed were high. The corrosion resistance performance of Ni-P/BN(h) composite coating was researched by SEM, tafel polarization curves and electrochemical impedance spectroscopy. The experimental results show that the corrosion resistant performance of Ni-P/BN(h) was superior to Ni-P alloy plating when immersed in3.5%NaCl and10%H2SO4solution for240h. This research concluded that corrosion resistance performance of Ni-P/BN(h) composite coating after heat treatment in3.5%NaCl solution were changed, the best corrosion resistance performance was at300¡æheat treatment; and after400¡æheat treatment of corrosion resistant performance was reduced, a little corrosion would appear, and the polarization resistance was least; And, the corrosion resistance of Ni-P/BN (h) composite coating were first increased, then decreased as for the extension of immersed time.In order to explore the mechanism of coating failure, the work had simulated scratching process on coating surface of TiN hard material deposited on strain hardening steel substrate with finite element method; next, it analyzed stress field in coating; finally, it predicted positions and modes that crack might initiate. Scratch process may be divided into three phases, i.e. a stylus tip indenting coating surface, the stylus tip slipping on coating surface and the stylus tip rising over coating surface. During former two phases, the stylus tip has coating surface formed a groove due to its acting on coating surface, and induces local stress concentration. There are residual stresses in coating after the stylus tip rises over the coating surface. Stress analyses have showed that after the stylus tip indents the coating surface, pile-up deformation appears near the edge of contact zone due to the plastic deformation of substrate, and radial stress and first principal stress reach the maximum tensile stresses at the vertex of pile-up deformation; likewise, radial stress and first and second principal stresses reach the maximum tensile stresses at the corresponding point of the substrate/coating interface underneath the contact center. When the stylus tip slips continuously on the coating surface after it indents the coating surface, the coating surface takes place the highest pile-up deformation front the stylus tip. At the vertex of pile-up deformation, the normal stresses along slipping direction and first principal stress have the maximum tensile stress. The deeper the stylus tip indents the coating surface, the heavier the substrate generates plastic deformation, the higher the coating surface take place the pile-up deformation, the larger the stresses are induced. For the coating system in which the coating material is deformed by elastic deformation and the substrate material is deformed by elastic and plastic deformation, biggish residual stresses induced during scratch process mainly occur on the coating surface and in the coating of the substrate/coating interface, and those residual stresses all are tensile.Based on the analysis of stress distributing rules in the scratch process, the crack initiating in the coating may mainly occur on the surface of the pile-up zone front stylus tip and on the coating/substrate interface underneath the contact region between stylus tip and coating. In the zones, the maximum stresses induced by scratching are tensile; accordingly, the cracks induced by the maximum stresses are mode. Moreover, because there are biggish residual stresses in coating after scratching, they may still induce fresh crack generation and original crack propagation.
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