The effect of heat treatment on the microstructure of electroless Ni–P coatings containing SiC particles
Introduction
Current trends of coating techniques involve composite coatings, such as multilayer or multiphase coatings, which are expected to have tailor-made properties for some specific applications. Recent progress in electroless plating is the co-deposition of solid particles into coatings, although electroless Ni–P coatings have been widely used in industry during the past 20 years for wear and corrosion protection. Consequently, functional composite coatings with highly specific characteristics can easily be produced by choosing suitable particulate materials. These solid particles can be hard materials (such as SiC, Al2O3 and diamond) [1], [2], [3] to enhance the hardness and/or wear resistance of the deposits, or can be dry lubricants (such as MoS2, PTFE and graphite) [4], [5], [6] to impart lubricity and reduce the coefficient of friction.
Among the particulate materials used for reinforcement, SiC is the most frequently studied and applied. Broszeit [7] found that mechanical properties, such as hardness, strength and elastic modulus can be increased with increasing content of SiC particles in the composite coating. Xinmin and Zongang [8] suggested that the SiC particles can increase the hardness of a composite coating and improve the resistance to abrasion, but a hard and stable matrix is necessary to support them. Unfortunately, the high temperature application of Ni–SiC composite coatings is limited by the thermal decomposition of SiC particles in the nickel matrix at approximately 500 °C [7]. Pan and Baptista [9] established that the nickel silicide, which is thermodynamically more stable than SiC, makes SiC unstable. The chemical instability of SiC in presence of nickel at high temperature results in uncontrollable mechanical properties of the material and limits the application of Ni–P–SiC composite coatings. Microscopically, what really happens to the coating matrix and the particles is not well understood and needs further investigation.
In this paper, an attempt was made to incorporate SiC particles into a Ni–P alloy matrix by electroless plating. The purpose of this work was to study the effect of heat treatment on microstructural changes of electroless Ni–P–SiC composite coatings by X-ray diffractometry (XRD) and transmission electron microscopy (TEM).
Section snippets
Experimental
Tool steel JIS SKD61 specimens with a thickness of 2 mm and a diameter of 1.5 mm were used as the substrate material. The specimens were ground and then surface polished with 1 μm alumina powder. Before electroless plating, the specimens were degreased and ultrasonically cleaned in a dilute hydrochloric acid solution. The Ni–P plating and Ni–P–SiC composite plating were carried out in a beaker heated by a thermostatically controlled bath. The substrates were vertically positioned as illustrated
Results and discussion
The cross-sectional SEM micrographs of the deposited Ni–P and composite coatings in Fig. 2 show that the SiC particles are uniformly distributed in the entire Ni–P film matrix. The thicknesses of the Ni–P and Ni–P–SiC coatings are approximately 15 and 20 μm, respectively. The composition of the deposited Ni–P and composite coatings measured by GDOS in Fig. 3 indicates that SiC concentration in the composite coating increases with increasing SiC concentration in the plating bath. Phosphorus
Summary
SiC particles were successfully incorporated and uniformly distributed in a Ni–P alloy matrix by the electroless composite plating method. The SiC content in the composite coating increased with increasing SiC powder concentration in the plating bath. Heat treatment changed the structure of composite coatings. The structure of Ni–P–SiC composite coatings was similar to that of Ni–P coatings in both the as-deposited and the annealed below 400 °C states. During annealing above 450 °C, the nickel
Acknowledgements
The authors wish to thank the National Science Council of Taiwan, ROC for financial support under contract ‘NSC89-2218-E-006-017’.
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