Preparation And Properties Of Electroless Ni-P-B₄C Composite Coatings

Electroless Ni-P alloy coatings, well known of their excellent wear and corrosionresistance and high hardness, have many applications in industries. Codeposition anothermetallic or non-metallic elements or abrasive/lubricative particles or combination of them inbinary Ni-P matrix can greatly enhance their properties. In this paper, electroless Ni-P-B₄Ccomposite coatings were prepared on low carbon steel (Q235) sheets by adopting optimumplating process. Metallurgical microscope and X-ray diffraction (XRD) were used todetermine surface morphology and microstructure of the coatings. Corrosion resistancecharacterization of the coatings was enabled by means of potentiodynamic polarization curves.The oxidation resistance of the coatings was showed by their weight gains in a muffle furnace.Hardness and dry friction and wear properties of the coatings were also tested. On the groundof former experiments, electroless Ni-P-B₄C gradient coatings were studied elementarily. Themain results gained are as follows:(1) The perferable solution compositions and plating conditions for electroless Ni-P-B₄Ccomposite coatings are as follows: nickel sulfate 25³0 g/L, sodium hypophosphite 25³0g/L, sodium citrate 10²0 g/L, sodium acetate anhydrous 10²0 g/L, suitable amount ofadditive agent A, suitable amount of additive agent B, B₄C 5²5 g/L, pH 4.1⁴.6,Temperature 85⁸7℃, Stirring rate, 150³00 rpm, Plating time 2⁴ h. The results show thatthe adhesion between Ni-P-B₄C composite coating and substrate is qualified and a relativelyuniform concentration of particles can be seen along the cross section of electroless Ni-P-B₄Ccomposite coatings. The deposition rate is up to 37μm per-hour.(2) The order of corrosion resistance of Ni-P-B₄C composite coatings during theirexposure to acid, alkali and salt aqueous solution from strong to weak is as follows:10%NaOH> 3.5%NaCl> 10%HCl. Further studies show that, corrosion resistance of Ni-P-B₄C composite coatings is independent of nickel-phosphorus molar ratio, while varieswith the contents of B₄C particles, heat treatment and additive concentrations.(3) The weight gains of Ni-P-B₄C composite coatings are high during the initial stage ofthe high temperature oxidation test, but decrease with time going on. And the weight gains ofNi-P-B₄C composite coatings increase linearly with the oxidation temperature, while theweight gains of Ni-P-B₄C composite coatings decrease linearly with the increasing of contentsof B₄C particles.(4) The hardness of Ni-P-B₄C composite coatings increases with both the increasing ofcontents of B₄C particles and heat treatment temperature. With the increasing of contents ofB₄C particles, the friction coefficient of Ni-P-B₄C composite coatings decreases while thewear rate increases. After heat treatment, both the friction coefficient and wear rate ofNi-P-B₄C composite coatings reduce.(5) Compared with Ni-P-B₄C uniform composite coatings, Ni-P-B₄C gradient coatingshave higher corrosion resistance and dry friction and wear properties.