Abstract
Electroless nickel (EN) deposit is frequently employed for many industrial applications due to its various excellent properties. The electroless Ni-P deposit can be strengthened by the precipitation of Ni-P compounds after heat treatment. Nevertheless, the hardness of Ni-P films degrades with excessive annealing. It is thus critical to increase the crystallization temperature so that the Ni-P deposit can withstand sufficient or even superior hardness at elevated temperatures. To enhance the thermal stability, the addition of a third element in the Ni-P coating to form a ternary Ni-P-based coating is put into practice. It is found that thermal stability of electroless Ni-P-W deposit can be enhanced by the co-deposition of W as compared to binary electroless Ni-P films. The ternary Ni-P-W alloy coating is also fabricated by rf magnetron sputtering technique with dual targets of Ni-P/Cu and pure W. A fixed P/Ni ratio and linear W content dependence on input power is revealed in the ternary Ni-P-W coating, indicating a well control in composition of the coating through sputtering technique. Thermal stability analysis shows that the introduction of W in the Ni-P coating by co-sputtering retards the Ni3P precipitation and retains the strengthening effect to a higher temperature of 550°C. Microstructure evolution indicates that all coatings in the as-deposited state show amorphous structure. The precipitation of Ni3P accompanied with W dissoluted Ni matrix is revealed to be the final product of the phase transformation in Ni-P-W coatings after thermal treatment. Results in microhardness test show that the surface hardness can be engineered by the controlling the composition and microstructure in the Ni-P-W coating. After heat treatment, the coating is strengthened by the precipitation of Ni-P compounds and solutioning of W in the crystallized Ni matrix. Quantitative analysis for the strengthening effect of the Ni-P-W coatings is performed based on Ni-P compound precipitation and Ni(W) matrix ratio. Both electroless Ni78.4P18.3W3.3 and sputtered Ni80.0P15.3W4.7 coatings exhibit a hardness around 1600 HK due to Ni3P precipitation and W solutioning hardening in Ni matrix to a W/Ni(W) ratio of approximately 12at.% after heat treatment. A higher microhardness of 1790 HK is measured in the sputtered Ni76.7P15.9W7.4 coating. Through quantitative analysis, the effect of strengthening in Ni-P-W ternary coatings under heat treatment can be clearly demonstrated. From surface analysis, the nodular nature of the electroless coatings is responsible for the rougher surface profile as compared to the sputtered coatings. With the co-sputtering of W, the early stage Ni-P compounds is suppressed, according to X-ray and surface morphology analysis. After heat treatment, the surface morphology and roughness of both electroless and sputtered Ni-P-W films remain identical to those in the as-deposited state, indicating a stable surface characteristic under thermal treatment.