A study on the wear resistance of electroless Ni–P/Diamond composite coatings
Introduction
Electroless nickel coatings have excellent corrosion and wear resistance, as well as other specific properties [1], [2]. They can be produced with crystalline and/or amorphous structure over a wide range of compositions [3]. With the increasing demands from industries for improved performance, development of better surface coatings has become imperative with higher wear and corrosion resistance. In this context, electroless nickel poly-alloy deposits and electroless composite coatings are being tailored more closely to customer and environmental requirements. Electroless nickel composite coatings are usually obtained by co-depositing particulate materials, such as ceramics, diamond and fluoropolymers and carbides of silicon, tungsten and chromium [4], [5]. Successful co-deposition depends on various factors, including electroless bath composition, particle size, compatibility of the particles with the metal matrix, etc. The composite coatings can significantly increase the wear resistance of the deposits and/or lower co-efficient of friction and impart lubricity [6], [7], [8]. Their applications in various industries provide components of higher quality with enhanced life.
The objective of this study was to estimate the wear resistance of electroless Ni–P–C composite coatings before and after heat treatment. It is customary to express the wear resistance in terms of specific wear rate (kg N−1 m−3) [6], [7], [8], [9]. In the case of composite coatings, this way of expressing wear resistance does not distinguish between the loss of particulate matter and matrix material. It is well known that the ratio of the integrated intensities of the diffraction lines of individual phases forming a binary system could be used as a measure of the volume fractions of the phases present. By employing this ratio as a measure of wear, a distinction between the loss of particulate matter and the matrix material could be made. Such an approach is adopted in this paper.
Section snippets
Experimental procedure
Mild steel specimens (0.39% C) of 22.5 mm diameter and 7 mm thick after mechanical polishing up to 1/0 level and subsequent electropolishing, were used in this investigation. The coating was done with alkaline hypophosphite baths having nickel chloride as the source of nickel and sodium hypophosphite as the reducing agent along with other ingredients such as sodium citrate as the stabiliser and ammonium chloride as the complexing agent. The sodium hypophosphite content in the bath was varied to
Results and discussions
The variation of the intensity ratio (IC/INi) with wear test time is presented in Fig. 1 from coatings containing small (3–6 μm), medium (6–12 μm), and large (20–40 μm) size diamond particles with the variation in the phosphorus content of the matrix in each case. It is observed that the deposits with lower phosphorus content have very little integrity with the diamond particles compared to the deposits with higher phosphorus content in the as-deposited condition. In all cases, there is a sharp
Conclusions
(1) It was observed that Ni–P–C composite coatings incorporating finer diamond particles are more wear resistant compared to those incorporating coarse diamond particles.
(2) Wear resistance increases with increase of phosphorus content of the matrix of Ni–P–C composite coating.
(3) The degree of incorporation of particles and the resultant wear resistance appears to be influenced by the size of the particles when coating thickness is held constant. Apparently, the ratio of coating thickness to
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