Application of design of experiment technique in the nickel-chromium electroplating process on WC-Co substrate prior HFCVD diamond coating

Abstract

Cemented tungsten carbide (WC-Co) cutting tools are widely used in metal cutting industry. These tools wear rapidly when machining abrasive alloys and glass-epoxy composites. In order to enhance the overall machining effectiveness of WC-Co, the coating of diamond on WC-Co has been attractive by virtue of its excellent hardness and low coefficient of friction. Poor adhesion of diamond coating to WC-Co is however observed because of the weak interface bonding resulting from graphite film formation during low-pressure diamond deposition due to presence of the cobalt binder. The use of interlayer between WC-Co and diamond coating has proved to overcome and reduce the negative effect of cobalt binder. In the current study, the coating of nickel-chromium on the cemented carbide (WC-Co) substrate via electroplating process has been investigated in order to obtain a thin layer with a good adhesion. Nickel was first coated on WC-Co substrate in experiments/trials involving three variable parameters; bath temperature, current density and plating time. Subsequently, chromium was coated on the nickel surface in experiments/trials involving two variable parameters; bath temperature and current density. Two levels factorial design of experiments involving nickel and chromium coatings on WC-Co substrates enabled the identification of current density and plating time as the two important variable parameters affecting the thickness of nickel and chromium coating. Temperature did not appear to have a significant influence on the thickness of coating. However, it had notable effects on the adhesion of nickel and chromium coatings. The current density had a slight effect on the adhesion in comparison with the temperature. A current density of 4 Amp/dm2, bath temperature of 58 ºC, and plating time of 26.31 minutes were identified as suitable electroplating parameter conditions for producing a thin, 6.3 µm, nickel layer with good adhesion on the WC-Co substrate whilst a current density of 12 Amp/dm2 , bath temperature of 42 ºC, and plating time of 15 minutes were identified as suitable electroplating parameter conditions for producing a thin, 2.4µm, chromium layer with good adhesion on the nickel coated WC-Co substrate.