Microstructure evaluation and strengthening mechanism of Ni–P–W alloy coatings

Abstract

Ni–P–W alloy coatings were deposited onto AISI420 steel substrates by both RF magnetron sputtering and electroless plating techniques. The coating hardness was investigated through the microhardness testing with a Knoop indenter. The strengthening mechanism of the Ni–P–W coatings in both as-deposited and heat-treated states were described with respect to compositional distribution and microstructural evolutions. In the as-deposited state, all the coatings exhibited amorphous/nanocrystalline structure. After heat treatment, the amorphous phase of Ni–P–W coatings transformed into Ni₃P precipitates and Ni(W) solid solution. The coatings were strengthened by the precipitation of Ni–P compounds and dissolving of W in the crystallized Ni matrix. With the aid of microstructure, the strengthening mechanism for the Ni–P–W coatings were proposed. Quantitative analysis for the strengthening effect of the Ni–P–W coatings was performed based on the elemental concentrations, Ni–P compound precipitation and Ni(W) matrix ratio. The heat-treated Ni_76.7P_15.9W_7.4 coating showed the highest peak hardness of 17.5 GPa when the maximum W solution in the Ni matrix was achieved.

Introduction

Nickel–phosphorous films are one of the well-known protective coating and widely adopted in versatile industrial applications owing to its merits in mechanical and chemical properties, such as uniform thickness, high hardness, corrosion and wear resistance [1], [2], [3], [4]. Ni–P coating can be fabricated by various techniques including electroplating [5], electroless deposition [1], [2], [3], [4], sputtering [6], [7], [8], [9], etc. The as-deposited Ni–P coating is an amorphous or nanocrystalline Ni matrix supersaturated with P, and under appropriate thermal history, it can be hardened by the precipitation of nickel–phosphide compounds, such as Ni₃P, Ni₅P₂ and Ni₁₂P₅ [10], [11], [12], in the crystalline Ni matrix. This feature makes the Ni–P coating a suitable material for the application as a protective layer for injection or press molding dies. However, after excessive annealing or higher temperature usage the hardness of Ni–P films is degraded due to coarsening and grain growth of Ni₃P and Ni matrix [13]. The increase of the phase transformation temperature and the enhancement on coating hardness are thus critical to assure that the Ni–P deposit retains sufficient or even superior strength at elevated temperatures.

Encouraging results in optimizing characteristics of the Ni–P system by the introduction of a third element to form a ternary Ni–P-based alloy coating have been recently proposed, including Ni–Cu–P [14], [15], Ni–W–P [10], [16], [17], Ni–Re–P [18], and Ni–Zn–P [19]. Thermal stability and surface hardness were improved by electroless Ni–Cu–P platings [7], as compared to Ni–P coating. An even superior improvement in surface hardness and phase transformation temperature was carried out by the incorporation of W instead of Cu into the Ni–P coating [8], [17]. It is believed that W, which exhibits a relatively high hardness and a high melting point of 3410 °C, retards the Ni₃P precipitation and Ni crystallization, and thus promotes the mechanical and thermal properties of the Ni–P–W coatings. However, the detailed microstructure evolution and the strengthening mechanism for ternary Ni–P–W coatings are not yet fully understood. In the present study, intensive studies on microstructure evaluation of the Ni–P–W alloy coating were performed. The strengthening mechanisms of the Ni–P–W coating system fabricated by both electroless plating and magnetron sputtering techniques was proposed according to microstructure features. Quantitative analysis on Ni₃P precipitation and Ni(W) matrix phases was conducted to correlate the hardness of the ternary Ni–P–W coating with the associated microstructure.