Synthesis and Corrosion Behavior of Electroless Ni–P– Si3N4 Composite Coatings

Abstract

The ability to codeposit ®ne particulate matter with an electroless nickel matrix is a major step in the development of processes aimed at the engineering of surfaces. The prime objective of incorporating the particulate matter is to improve wear and corrosion resistance. The most frequently studied systems are electroless Ni±P±SiC, Ni±P±Al2O3, Ni±P±diamond, Ni±P±PTFE and Ni±P±Cr3C2. Very little information is available on electroless Ni±P±Si3N4 composite coatings. Since Si3N4 has high hardness, good oxidation resistance and good chemical stability, it is worthwhile to study this combination. In the present study we codeposit Si3N4 with electroless nickel on medium carbon steel and evaluate its hardness and corrosion resistance. A proprietary high phosphorus electroless nickel plating bath was used. The pH of the bath was 4.8. Medium carbon steel coupons of 30 mm diameter and 5 mm thickness were surface ground (Ra ˆ 0:4 im) and suspended in the plating bath. The plating bath was maintained at a constant temperature of 88 1 8C. The coupons were plated for 2 h. For the preparation of composite coatings, the Si3N4 powder (size ,10 im) of a calculated quantity was mixed thoroughly with 10 ml of the plating solution using a mortar and pestle and then transferred to the plating bath. This process ensures the distribution of Si3N4 paticles throughout the bath. To obtain a uniform dispersion of Si3N4 particles in suspended form in the bath, the plating solution was stirred using a magnetic stirrer. The electroless plated samples were heat treated for 1 h at temperatures of 200, 300, 400, 500 and 600 8C to determine the change in hardness upon heating to different temperatures. In order to represent common industrial practice no special atmosphere was maintained during heat treatment. Hardness measurements were carried out using a Leitz Micro Vickers Tester employing a load of 0.1 kg. Five readings were taken on each deposit and the values were then averaged. Potentiodynamic polarization studies were carried out using EG&G PAR (Model 362) scanning potentiostat. The electrolyte used was non-deaerated 3.5% NaCl solution. The specimens were masked with lacquer so that only a 1 cm area was exposed to the electrolyte. A saturated calomel electrode (SCE) was used as a reference electrode. The corrosion potential (Ecorr) and corrosion current (icorr) were obtained from the polarization curves using the Tafel extrapolation method. Both the Ni±P and Ni±P±Si3N4 coatings did not exhibit any porosity when subjected to the ferroxyl test. Both of them were found to be adherent when tested by the bent test method, following ASTM B 571 and the thermal shock method. The thickness of the deposit was in the range of 22±25 im in all cases after 2 h of deposition. Fig. 1 shows that cross-section of the electroless Ni±P±Si3N4 deposits when viewed by optical microscopy. The incorporation of the Si3N4 particles in the deposit is clearly visible. Fig. 2 shows the weight percentage of Si3N4 particles in the electroless nickel deposit as a function of its concentration (g=l) in the plating bath. It can be deduced from the ®gure that the percentage incorporation of Si3N4 particles (the percentage of Si3N4 represents only the weight percent of Si3N4 throughout this communication unless otherwise stated) increases with an increase in concentration and reaches a maximum at a concentration of 10 g=l. Beyond this concentration of 10 g=l, a slight decrease in the percentage incorporation is observed. A decrease in the percentage of incorporation of the second phase particles was observed earlier in the electroless Ni±P±Al2O3 system by Rajagopal [1]. The increase in percentage incorporation of Si3N4 particles in the electroless Ni±P matrix with increase in its concentration in the plating bath can be