Study On The Preparation And Properties Of Ni-P Electroless Composite Coating Containing Potassium Titanate Whisker

Nickel-phosphorus (Ni-P) composite coating containing potassium titanate whiskers (PTWs) were prepared via electroless plating process. PTWs served as the strengthening phase are high-performance inorganic fibre materials. In this dissertation, the optimal operation parameters of electroless Ni-P-PTWs composite coating preparation were obtained according to experiments and theoretic analyse. Moreover, a new codeposition mechanism was presented based on the classical adsorbable model. Otherwise, the chemical properties, tribological properties and mechanical properties of the composite coating were comprehensively studied.The orthogonal method was used to optimize the codeposition technology of Ni-P-PTWs composite coating. The optimum preparation process was as follows: plating temperature at 86℃, the mass fraction of PTWs was 10 g·L^-1, agitation velocity was 160 r·min^-1 and pH value was 4.9. The obtained composite coatings were of inlaid microstructure investigated by scanning electron microscope.Generally, the Zeta voltage of PTWs in acidic nickel plating solution is negative. Thus, the PTWs particles are adsorbed and codeposited with Ni-P alloys in virtue of mechanical force. The critical adsorption strength was used to classify the adsorption into weak adsorption and strong adsorption. Only when the adsorption strength exceeds the critical adsorption strength, can the PTWs particles codeposite with the Ni-P matrix. The kinetic mathematical model deduced in this dissertation was validated experimentally.The results of corrosion performance testing indicated that the corrosion resistance of the electroless Ni-P-PTWs composite coating was inferior to that of the electroless Ni-P coating. And the corrosion resistance of the composite coatings was reduced with the increase of the PTWs content in the matrix alloy.The results of the high temperature oxidation-resistance testing indicated that the mass loss curve of the Ni-P-PTWs composite coating was of exponential characteristic. Because the codeposition of PTWs reduced the integrity of the oxide film, the high temperature oxidation-resistance of Ni-P matrix alloy was reduced.The stable microstructure of the Ni-P-PTWs composite coating should consist of Ni, Ni₃P and PTWs. Owing to the soften effect of PTWs, the hardness of composite coating usually was lower. A maximum hardness was produced in the electroless Ni-P-PTWs composite coating with the increase of heat treatment temperature. In addition, the reduction in the friction coefficient and abrasion mass loss of the Ni-P-PTWs composite coatings was due to the whisker's self-lubrication ability.The effects of different pH values and heat treatment temperatures on the residual stress of the composite coatings were studied by XRD. The results showed that with the increase of pH value, the residual stress of the composite coating transitioned from compressive stress to tension stress. The increasement of heat treatment temperature could lead to an increase of the tension stress. Moreover, the residual stress of the composite coating gradually reduced with the increase of the thickness of the composite coating.The tensile testing results indicated that the toughness of the composite coating was improved with the PTWs addition, but the rupture of the composite coating was still brittleness rupture. And the tensile strength and elasticity modulus would decrease with the increase of the PTWs content, while the elongation, area reduction and the crack density of the fracture side of Ni-P-PTWs composite coatings samples increased. Nono-indentation experiments indicated that the hardness and elasticity molulus of the composite coatings decreased with the increase of the PTWs content. The results also indicated that the hardness and elasticity molulus of the composite coatings increased with the increase of the indentation depth.