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 (similar to 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/cm(3)). 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 root 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.