Preparation And Mechanical Properties Of Ni-P-Al₂O₃ Electroless Composite Coatings

As a new method to prepare composite materials,electroless composite plating has been rapidly developed in our country.Al₂O₃ particles with high hardness,good chemical stability,cheap accessible were pushed into electroless Ni-P alloy coating at first.However,the deposition efficiency of particles was the bottleneck problem of composite plating.Because the amount of composite particles was restricted,the mass fraction of particles in the coating was generally lower than 20%.The application and researches of high performance alumina composite coatings were hindered.To solve this problem,Al₂O₃?size 1.5?m?content of 34.7wt%of the composite coating has been prepared by self-made electroless plating device.The material foundation was established about the research of high content of alumina composite coatings.At first,the dependence of Ni-P alloy deposition rate with different process parameters was analyzed.The best electroless plating formula was decided.And then adding different amount of alumina particles to the plating bath,Alumina mass fraction about 10%³4.7%of Ni-P-Al₂O₃ composite coating was prepared.The hardness of composite coatings was tested by a Vickers microhardness tester.The tribological properties of the coatings were evaluated by a ball-on-disc wear instrument and the wear mechanisms were also discussed.The morphology,microstructure and chemical composition of the coatings and the counterpart balls were characterized by scanning electron microscopy?SEM?,optical microscopy?OM?,energy disperse spectrum?EDS?,and X-ray diffraction?XRD?technique.The results show that:with the increase of nickel sulfate and sodium hypophosphite concentration in the electroless bath,the deposition rate increases at first and decreases later.The best concentration is 25g/L and 30g/L.With the increase of temperature and pH value,the deposition rate is linearly increased.But,when the temperature and p H value are high,the stability of bath is bad.So,the best temperature is 85⁹0?a nd the best pH value is 4.4⁴.8.The dependence between surfactant type and deposition rate is not big.But with increase of surfactant concentration in the bath,the deposition rate decreases.With increase of Al₂O₃ concentration in the bath,the Al₂O₃ content of coatings increases and deposition rate decreases.The needle shaped holes gradually increase in surface morphology of the coatings.The Al₂O₃ diffraction peak in the composite coating of Al₂O₃ content?34.7wt.%?was found,the phosphorus content of the coatings decreases greatly,and the Ni-P matrix of the composite coatings remain amorphous.With the increase of Al₂O₃ content in the coatings,the friction coefficient decreases at first and increases later at as-plating and after annealed at 400?,the dependence of the coating's friction coefficient with alumina content exhibits“V”type.The lowest friction coefficient corresponded alumina mass fraction is 30.2wt.%?after 400?heat treatment?and 25.2wt.%?as-plating?.The micro-hardness of the coatings linearly increases from 502 HV to 764 HV?as-plating?;after 400?heat treatment,the micro-hardness of the coatings linearly increases from 853 HV to 1250HV.and the wear rate of the coatings monotonously decreases,wear rate from 11.58×10^-15 m³/?Nm?dropped to 3.18×10^-15 m³/?Nm?at as-plating,and wear rate from 5.83×10^-15 m³/?Nm?dropped to 1.63×10^-15 m³/?Nm?after 400?heat treatment.And the major wear mechanism of the coatings changes from adhesive wear to abrasive wear.