Development of gradient cemented carbides with surface layers free of cubic phase via h-BN nitrogen source

This study investigates gradient cemented carbide with a cubic phase-free surface layer (CFL), commonly used as substrates for coated inserts to extend their service life. The tough gradient surface layer effectively prevents crack propagation, minimizing coating delamination and edge chipping. We propose using hexagonal boron nitride (h-BN) as a non-cubic nitrogen source for fabricating gradient cemented carbide with cubic (W,Ti)C, as a replacement for traditional cubic nitrogen sources. By employing powder metallurgy, we successfully created gradient cemented carbide with a CFL derived from h-BN. We also analyze the mechanism of gradient structure formation and characterize the microstructure, elemental distribution, and mechanical properties of the novel materials. The results demonstrate the feasibility of producing gradient cemented carbides with a CFL when h-BN is used as the nitrogen source. The crystal orientations at the gradient boundary are randomly distributed, showing no abrupt interface between the surface layer and the interior. Notably, titanium (Ti) and boron (B) are nearly absent in the surface layer, while cobalt (Co) shows higher concentration compared to the interior. The use of h-BN facilitates a nearly homogeneous titanium distribution in the subsurface zone, contrasting with the clustering seen when traditional cubic TiN is employed as the nitrogen source. Additionally, the thickness of the gradient surface layer can be effectively controlled by adjusting the h-BN levels, increasing with higher h-BN content in the raw materials. However, excessive h-BN addition facilitates abnormal WC phase growth in cemented carbides, and negatively affects transverse rupture strength despite the toughening effects of the cubic phase-free layer. Increased h-BN content also promotes the formation of TiB phases, which enhances the hardness of the cemented carbide.