The Preparation Of Ni-P-Si₃N₄ Ceramic Composite Coating And Study Of Its Tribological Properties

Because of the large losses caused by the friction and wear in equipment, reducing the friction coefficient and wear between frictional surfaces has become an important research topic in the surface engineering. In order to improve the wear resistance and antifriction property of test-pieces and obtain coatings which have excellent bonding strength and corrosion resistance, composite coatings and gradient composite coatings have been prepared in this paper by electroless plating in which solid particles and matrix metal deposited on the substrate surface.To obtain the Ni-P based and Ni-Mo-P based composite coatings,particles Si₃N₄,SiC and WS₂ were selected in the conventional hard particles and lubricant particles to serve as incorporated particles. After the normal composite coatings were prepared and investigated,the author prepared the gradient composite coatings Ni-P-Si₃N₄,Ni-P-Si₃N₄-WS₂ and Ni-P-Si₃N₄-SiC-WS₂ and then investigated them as contrast to the former coatings.In the experiment, SEM, EDS, XRD, micro-hardness tester and electrochemical workstation were used to investigate the micro-morphology, constituent, hardness and corrosion resistance of coatings respectively. The friction coefficient of the researched coatings was measured by ball-disc frictional wear testing machine. After the wear resistance was valued on method of loss of weight, the wear mechanism was analysed with the wear scratch morphology obtained by SEM.It is showed that the surface of researched composite coatings is smooth and defectless. The contents of element P range from 5% to 10% in the Ni-P based composite coatings which have the structure of amorphousness and micro-crystal.With the concentration of particles in the plating bath increasing, both the deposition rate and content of particles in the coatings increase and then decrease. When the content of hard particles increases in the composite coating, the corresponding hardness rises. On the contrary, the lubricant particle WS₂ has decreased the hardness of coating. When the concentration of Si₃N₄ in the plating bath is 5g/L,the hardness of corresponding composite coating Ni-P-Si₃N₄ reaches the peak at 727HV. However,when the concentration of Si₃N₄ keeps unchanged and another addition of WS₂ is at concentration of 2g/L,the hardness of corresponding coating Ni-P-Si₃N₄-WS₂ drops to 677HV.As to gradient composite coatings, the cross section morphology obtained by optical microscope shows that the micro particles are seen incorporated in the coating and the constituent grades along the thickness. Due to the striking coating, the whole coating bonds well with the substrate surface and no obvious boundary exists. The analysis of EDS shows that the coating has incorporated all kinds of wanted particles.By contrast to the Ni-P based composite coating,incorporation of particles Si₃N₄ has reduced the friction coefficient of the composite coating. In addition, composite coating which incorporates particles Si₃N₄ and WS₂ has the lower friction coefficient than the former composite coating incorporating Si₃N₄ only, the wearing resistance also has been improved. Compared with the Ni-P-Si₃N₄, the simultaneous incorporation of particles SiC and Si₃N₄ lead to the increase of friction coefficient, but the wearing resistance become better. The weight loss of gradient composite coatings in the friction-wear test is less half than that of normal composite coatings, which proves that the gradient composite coating has a better wear resistance.