Microstructural evolution and properties of cemented carbides alloyed with hexagonal boron nitride (h-BN) using selective laser melting

In this study, WC-17Co-based cemented carbides reinforced with hexagonal boron nitride (h-BN) were processed to investigate the microstructural evolution and effect on the mechanical properties of cemented carbides. In order to verify the potential of h-BN in additively built cemented carbides, the study focused primarily on determining the ideal concentration and how it influences microstructure and mechanical characteristics. Commercial powders of WC-17Co composite mixed with 10 vol. % h-BN, 5 vol. % h-BN, and 3 vol. % h-BN were processed using selective laser melting (SLM). Benchmarked against an unmodified SLM-processed WC-17Co, evaluation of the h-BN composites was primarily dependent on the volume fraction of h-BN added. WC-17Co with 10 vol. % h-BN composite resulted in high levels of pores (~ 3% porosity) and microcracks. This was attributed to poor adhesive bonding between h-BN and WC-17Co, leading to the onset of pores and cracks during processing. However, high densification was observed in both 3 vol. % and 5 vol. % h-BN with the 3 vol. % h-BN attaining the highest densification (0.83% porosity) while having a density 0.07% lesser than the unmodified specimen (11.33 g/cm³). Comparing the XRD analyses of the specimens yielded two distinct findings. Unlike the unmodified WC-17Co specimen, none of the reinforced specimens contained the undesirable W₂C phase at all. It is thus possible to prevent the formation of the W₂C phase by adding hBN to WC-17Co. There were no hexagonal BN phases found in the specimens, either. Rather, phases like Co-W-B and W-B were indexed in the specimens. WC carbides, WC platelets, and a secondary phase (Co-W-B) were observed in both the 5 vol. % and 3 vol. % h-BN composites. The volume fraction of WC platelets increased as the h-BN content increased. This was attributed to the sluggish grain growth favorable for WC platelets induced during processing due to h-BN addition. Hardness increased 28.7% with 3 vol. % h-BN as opposed to the unmodified WC-17Co. This was attributed to the reduced WC carbide grain sizes observed in the WC-17Co with 3 vol. % h-BN. In addition, the 5 vol. % h-BN specimen had the highest fracture toughness value (6.97 MPa√m) due to the high-volume fraction of WC platelets in that specimen. WC platelets contain few basal planes resulting in lower possibility in stacking faults interfering with crack propagation. In all, the specimen with 3 vol. % h-BN showed better mechanical properties than its unmodified counterpart. Thus, utilizing 3 vol. % h-BN in cemented carbide could be beneficial for SLM-processed cemented carbides.