Structural and phase transformation behaviour of electroless Ni–P and Ni–W–P deposits
Introduction
Electroless nickel (EN) plating has found many applications in industry because of its deposit properties, such as its excellent corrosion and wear resistance, deposit uniformity, solderability, etc. [1], [2]. It has been shown that most of the properties of electroless nickel are structure-dependent and the structure is closely related to the phosphorus content of the coatings [3]. The phosphorus content of the deposit varies with pH of the bath [4]. Deposits plated from an alkaline solution have lower phosphorus content and are crystalline [5], [6], whereas deposits plated from an acidic bath contain high phosphorus and usually have an amorphous or a microcrystalline structure [7], [8]. On heating, the single phase electroless nickel crystallises [9] (or rapid grain growth occurs from the microcrystalline structure) to a two phase mixture of Ni and Ni3P as predicted by the Ni–P binary phase diagram [10]. At the same time, the hardness and wear resistance of the deposit increases rapidly [11], [12]. Though electroless binary Ni–P alloys have had extensive application in industry due to their excellent wear and corrosion resistance and special physical performances, some ternary electroless alloys, such as Ni–Cu–P [13], [14], Ni–Zn–P [15] and Ni–W–P [16] have also been developed to further enhance the properties of the binary system to meet more rigorous demands. Considerable work has been carried out to study the structure and phase transformation behaviour of binary electroless Ni–P alloys [17], [18], [19] while the ternary systems have been studied to a much lesser extent [20], [21], [22]. Very little work has been done to evaluate the structural and crystallisation behaviour of electroless Ni–W–P behaviour. This paper makes an attempt fill this gap.
Section snippets
Experimental details
Coating was done on mild steel coupons of size 30 mm diameter and 5 mm thickness. Coating for obtaining foils was done on stainless steel plates. Due to the presence of passive film on the stainless steel plate, coating adhesion is very weak, aiding in easy peeling off of the coating. The sample is mechanically polished and degreased using trichloroethylene and thoroughly rinsed with deionised water, before actual deposition. The chemically cleaned coupons and plates were electrolytically cleaned
Plating rate
Plating rate was calculated using the weight gain method after every hour of plating. The rate was found to decrease with time. All plating was conducted for 3 h and the plating rate was found to be in the range of 10–14 μm/h for the first hour with about 20% reduction every hour thereafter.
Chemical composition
Nomenclature and chemical composition of the coatings are given in Table 1. By varying the concentration of sodium tungstate in the bath, tungsten content in the deposit was varied.
Structural behaviour
X-ray diffraction profiles
Conclusion
Increase in tungsten content in the electroless Ni–P deposits, results in reduction of its phosphorus content. Reduced phosphorus content leads to a more crystalline deposit. Acidic Ni–P based deposits are found to have a nanocrystalline structure with crystallite size varying from 5 to 40 nm in as-plated conditions. Tungsten incorporation increases the temperature for crystallisation of the deposits. Heat treatment leads to crystallisation of all deposits and heating at high temperatures leads
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