Ni-W electrodeposits show a unique ensemble of properties, including high hardness, good wear resistance, excellent corrosion resistance, a smooth surface finish and also a good thermal stability. Not surprisingly a number of applications for Ni-W coatings have been envisaged; particularly, Ni-W electrodeposits are candidate for substitution of hard chromium coatings. Other possible applications, depending on alloy composition, include: micro-electroforming, barrier or capping layer in microelectronic circuitry, highly textured tape conductors for high temperature superconductors, and electrocatalytic coatings for hydrogen evolution electrodes. The literature on Ni-W electrodeposition deals almost entirely with alkaline baths. More recently electrolytes of nearly neutral pH have been developed and studied in some detail. Acid baths were studied as long ago as in the forties of the twentieth century but with the only result of raising quite a strong prejudice against the feasibility of an acid process for Ni-W electrodeposition. In fact, according to Brenner: The acid tungsten alloy plating baths are of no practicable value because sound deposits cannot be obtained from them The present work was undertaken to study the electrodeposition of amorphous Ni-W alloy from an acid electrolyte. An early work entirely devoted to the development of an acid electrolyte for Ni-W deposition was published by Nishido et al. (see ref. 9) in 1989. This remained the only paper proposing an acid formulation for a Ni-W bath, to the best of authors' knowledge, up to 2006 when a paper by Moussa et al. (see ref. 10) was published. The electrolyte by Nishido et al. -in Tab. 1- was assumed as a starting electrolyte formulation in the present work. This electrolyte was characterized in detail studying the effects of composition parameters on current efficiency and tungsten content of the alloy, with the objective of identifying critical parameters for the optimization of the electrolyte composition. A further direction of investigation was a preliminary study into the deposition of Ni-W with pulsating current, considering a limited set of pulse parameters as detailed in Tab. 2. Electrodeposition tests were carried out at current density (cd) of 0.150 A cm-2 and temperature of 70C, under stagnant conditions, using a copper substrate and a platinum coated titanium sheet as anode. Using the base electrolyte, the current efficiency was about 20% in the conditions leading to the deposition of a Ni-W30 at% alloy. The structure of this alloy appears to be amorphous according to the XRD analysis (Fig. 1); the average size of coherent scattering domains is about 1.8 nm; the surface morphology is characterised by fine globular features and defects such as pits and cracks, the latter at the boundary of the globules (Fig. 1). In order to improve the formulation of the bath, the effect of different composition parameters on current efficiency and deposits composition was studied. In the present work the following parameters were considered: theelectrolyte acidity, the molar ratio between the nickel cation Ni 2+ and the tungstate anion WO4 2- (in the following also [Ni]/[W]), and the overall metal ionic concentration. Besides, the influence of the deposition current density was studied. The results of this work can be summarised as follows: (1a) current efficiency increases above 20% when the [Ni]/[W] ratio becomes higher than 0.5 -the overall concentration of metal ions being the same as in the base electrolyte; alternatively, when the overall metal ions concentration is higher than about 0.3 M -the [Ni]/[W] ratio being the same as in the base electrolyte (Fig. 2); (1b) for the starting electrolyte formulation, current efficiency shows a maximum with the pH at about 5.5 (Fig. 3); (1c) for the same formulation, current efficiency decreases with current density increase, as shown in the graph of Fig. 3; (2a) the tungsten content is higher that 28 at.% for a [Ni]/[W] molar ratio in the electrolyte higher than about 0.33, at the overall metal ion concentration of the base electrolyte; the coating composition changes within the range 28-32 at.% as the overall metal ion concentration increases in the range 0.1 to 0.5 M, at the [Ni]/[W] molar ration of the base electrolyte (Fig. 4); (2b) the tungsten content of Ni-W coatings from the base electrolyte changes with the pH as shown in Fig. 5, on the left; a maximum of the tungsten content is found at pH 5.0-5.5; (2c) the same changes with current density as shown in Fig. 5; as cd increases above 0.100 A cm -2 tungsten content remains in the range 28-32 at.%. Electrodeposition was performed also under pulsating current. Pulse current density was kept constant at the value used in DC plating, i.e. 0.150 A cm -2. The composition of pulse plated coatings was not significantly affected compared to direct current plating when operating at 4 Hz pulse frequency, with only a small increase of the W content with duty cycle increase. On the other hand, the tungsten content appeared to increase with decreasing pulse frequency (Fig. 6). As the duty cycle decreased (in the range of pulse period values considered in the present work) the current efficiency increased. These preliminary results suggest that there is a complex relationship between duty cycle, frequency and W content. Further work is required to shed light on process sensitiveness to pulse plating parameters. The second part of this work was concerned with coating characterization and only direct current Ni-W alloy deposits were examined. The composition was determined either by ICP or EDS analysis; structure was characterized by XRD analysis; hardness by indentation measurements (50 mN maximum load). The structure of deposits is nanocrystalline or amorphous depending on composition. According to XRD analysis, the average size of coherent scattering domains becomes lower than 2 nm for tungsten content above about 18 at.%. The hardness of Ni-W coating was found to increase from about 5 to 7.3 GPa with W content increasing from 15 to 34 at.%, respectively. Standard deviation of hardness measurements was relatively high and the higher the lower was the current efficiency. The structure stability was preliminary studied by differential thermal analysis in the temperature range between 150 and 1300C at scan rate 0,33C s-1. The structure undergoes two transformations at temperature of about 740C and 1050 °C, in agreement with the findings of Yamasaki (see ref. 2). The first transformation occurred at about the Tamman temperature of the alloy and is obviously related to the amorphous to crystalline transformation of the fcc Ni-W alloy. The second transformation can be interpreted as either due to the formation of the tetragonal phase Ni4W or to the formation of complex nickel tungsten carbide (see ref 13). The thermal stability and the microstructural evolution of Ni-W 30%at. alloy coatings with thickness of 20 μm were further investigating after heat treatment at temperature varying in the range from 300 to 700C for 6 h in argon. The average size of coherent diffraction domains remains unchanged up to 450C (see Fig. 9). Microhardness changes in a sigmoidal fashion with temperature (see Fig. 10): the hardness raise occurs in the temperature range 400-600C. After heat treatment the Ni-W30%at. Coatings show extensive microcracking and display a fragile behaviour during indentation measurement at load in excess of about 50 mN. From the present work the following conclusions can be drawn: - Ni-W coatings from the acid bath show an amorphous structure for W content above about 18% at., i.e. a composition close to the solubility limit of W in Ni; - the hardness increases almost linearly with W content in the range 17-32 at.% reaching a maximum value of about 7.3 GPa; - hardness increases with lower standard deviation without apparent XRD structure modification after heat treatment at temperature up to 450 °C for 6 h; - nanocrystallization of Ni-W fcc solid solution and the precipitation of nickel- tungsten carbide (and p ossibly Ni4W intermetallic) result in a strong increase of hardness after heat treatment at T>450C, up to 20 GPa, according to indentation measurements at maximum load of 50 mN.
