Investigation On Processes, Microstructures And Properties Of Electroless Ni-W-P Alloys

In recent years, many studies were carried out to develop the electroless Ni-W-P alloy for numerous interests such as uniform thickness, high hardness, good corrosion resistance, easy operation and low cost. However, some problems of the present reported electroless Ni-W-P process have to be solved urgently, such as low deposition rate, low stability of the plating bath and great chang of pH during plating. In the study, the electroless Ni-W-P plating was stabilized using complexing agents, i.e. sodium citrate and lactic acid. Moreover, the electroless Ni-W-P process was optimized by orthogonal experiment and the optimal operating conditions were obtained. The surface morphologies, chemical composition, microstructures and properties of the coating were investigated and the relationships among them were analyzed. The effect of heat treatment on the microstructures and properties of the coating was also studied. From above studies, the conclusions can be drawn as follows:1. Based on the orthogonal experiment, the optimal electroless Ni-W-P alloy bath is obtained:NiSO₄·6H₂O 35g/LNa₂WO₄·2H₂O 40 g/LNaH₂PO₄·H₂O 25 g/LNa₃C₆H₅O₇·2H₂O 100 g/LCH₃CH (OH) COOH 5ml/L(NH₄)₂SO₄ 30 g/LpH: 9Temperature: 80℃2. The ternary Ni-W-P deposit in the as-plated exhibits coarser nodular microstructure. The size of the "cell" obviously increases with the increase of the W content in the coatings.3. The result of the XRD showes that the peak of the Ni (111) shifted to the low angle, which shows the electroless Ni-W-P alloy, is a supersaturated solid solution of P and W dissolved in Ni matrix. The as-plated Ni-W-P alloy with single or mixed microstructures of amorphous, microcrystalline and nanocrystalline is in the state of the metastability thermodynamically. Recrystallization and the grain refinement occur after heat treatment. As temperature rises, the stable Ni₃P precipitates out. 4. The P content was decreased with the increase of the W in the Ni-W-P coatings. When the structure of the coatings was amorphous, the P content in the Ni-W-P coatings was lower than that in the Ni-P deposits, and the P content was decreased with the increase of the W content in the Ni-W-P coatings for this microstructure.5. The thermal stability is markedly improved due to the higher depositing temperature of the stable phase-Ni₃P which is attributed to the W co-deposition. The grain growth coarsening is suppressed when the W content reached to the definite amount.6. The increasing hardness of the coatings was induced by solid solution Ni (W), grain refinement and precipitated phase Ni₃P. There was an intimate relationship between the coating hardness and the W content. The hardness of the coatings was increased with the increase of the W content. When the W content reached to 6.28wt%, the hardness of the coatings was 991HK₁₀. Meanwhile, the hardness of the coating was obviously affected by heat treatment. The hardness increased with heat treatment temperature below 400℃but decreased with elevated temperature over that point because of the grain growth coarsening. At 400FFC, the grain size comes down to the minimum and the stable Ni₃P precipitates out, which result in the maximum hardness of the coating.7. The corrosion resistance of the coatings is related to its density and micros tinctures greatly. To some extent, some defects (grain boundaries) due to the increase of the W content result in the density of the coatings downgraded, so the corrosion resistance of the coatings become worse. The corrosion resistance is increased with heat treatment temperature below 400℃but decreased with elevated temperature.8. The binding force is improved after heat treatment, which is properly because the diffusion layer is formed between the coating and the substrate.In addition, the wear resistance of the coating is improved after suitable heat treatment.