Femtosecond laser processing of cemented carbide for selective removal of cobalt

Abstract

Diamond coating of tungsten carbide-cobalt (WC-Co) cemented carbide tools using chemical vapour deposition (CVD) requires removal of the cobalt (Co) binder phase from the WC-Co surface in order to improve the coating adhesion. Laser ablation is shown to be a promising technique for the selective phase removal of Co, which is based on the difference in melting and boiling temperatures between the different phases. However, laser ablation with conventional nanosecond (ns) laser pulses typically induces surface remelting, incomplete Co removal, and metallurgical changes in the WC phase due to the large heat-affected zone (HAZ) involved. Ultrashort pulsed lasers have emerged as an alternative due to minimized HAZ and thermal stresses, enabling cold ablation. This paper reports on the effect of femtosecond (fs) laser ablation on the selective removal of Co from polished WC-Co surfaces. The influence of several process variables including laser pulse energy, number of pulses, and scanning speed on the surface morphology of the fs-laser ablated surfaces was investigated. Two different WC-Co grades with 6% and 10% wt Co binder and different WC grain sizes were examined in this study. A pulsed ytterbium fiber laser (250 fs, 1030 nm) was used for the laser ablation of different morphologies such as spots, lines, and square regions. Field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray (EDX) analysis were used to characterize the microstructure and chemical composition of the processed surfaces. The experimental results indicated that fs laser processing at optimized processing parameters could achieve successful removal of the Co phase selectively from the WC-Co surface with minimized morphological changes or thermal effects on the WC grains.