Adhesion Enhancement of Diamond Coating on WC-Co Substrates Through Interlayer Design

Abstract

Diamond coating with sufficient adhesion on WC-Co cutting tools is expected to significantly increase their cutting performance. However, the adhesion is always limited by the formation of graphitic soot in the interface due to the catalytic effect of Co on graphite formation. Moreover, the low nucleation density and the high thermal stress in the coatings also result in poor adhesion. The introduction of interlayer is one of the available approaches to enhance the coating-substrate interfacial adhesion. The goal of this project is to improve the adhesion through the optimization of interlayer design. The Al2O3 and Ta mono-interlayer, Al-Al2O3, Al-AlN, Al2O3-Ta and Al-Ta duplex interlayer systems have been developed in this study. These interlayer materials were prepared using a magnetron sputtering method, and diamond coating were deposited on them using microwave plasma enhanced chemical vapor deposition. In addition, different diamond seeding conditions have been studied to increase the diamond nucleation density. Grazing incident X-ray diffraction was carried out to determine the phase components in the Al-Al2O3 and Al-AlN interlayers. Raman spectroscopy and scanning electron microscopy were used to evaluate the quality, morphology and microstructure of the deposited diamond coatings. Rockwell C indentation testing was performed to evaluate the adhesion of the coatings. To elucidate the coating failure mechanism, the compositions around the delaminated spots of diamond coatings after indentation were identified by Energy-dispersive X-ray spectroscopy. To evaluate the tribological properties of the diamond coatings, the diamond coated WC-Co sheets were rubbed against steel and alumina balls respectively. The results show that continuous diamond coatings were achieved on Al2O3, Al-Al2O3, Al-AlN and Al-Ta interlayered substrates, whereas a graphite layer was still formed with the Ta monolayer or Al2O3-Ta duplex layer accompanied by an easy spallation of diamond coatings. The Al interlayer has played an important role in obtaining high purity diamond by in-situ forming an alumina barrier layer. Especially, the diamond coating deposited with an Al-AlN interlayer exhibits superior interfacial adhesion in comparison with all the other interlayers. Meanwhile, seeding with nano-diamond particles is more efficient than micro-diamond particles for improving the diamond nucleation density on Al-AlN interlayered substrates. Furthermore, the diamond coated WC-Co sheets possess lower coefficient of friction and wear rate than bare sheets when rubbing against either steel or alumina balls.