Microstructure, mechanical and machining properties of LPS and SPS NbC cemented carbides for face-milling of grey cast iron

The effects of spark plasma sintering (SPS), NbC as a major carbide phase and Ni as a Co binder substitute on the microstructure, mechanical properties and cutting insert wear during face-milling of grey cast iron (GCI) BS1452, grade 17, were investigated. Spark plasma sintering refined microstructures increasing the hardness, but lowered the fracture toughness, due to poor binder distribution. Both SPS and liquid phase sintered (LPS) WC based samples had higher hardness than all the NbC based samples. Substitution of Co with Ni in the NbC cemented carbides significantly increased the fracture toughness (by ~ 5.5 MPa·m1/2). During face-milling using inserts produced from the NbC and WC based cemented carbides, the cutting speed (vc) was varied between 100 and 300 m/min and the depth of cuts (ap) between 0.5 and 1 mm. Face-milling was done under dry and minimum quantity lubrication (MQL) conditions. Wear was evaluated by optical microscopy and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The spark plasma sintered NbC1.0-12Co (wt%) insert had lower flank wear rates than the SPS and LPS WC-0.8Cr3C2-12Co inserts in all machining tests, due to the combination of good chemical stability, attrition and abrasion wear resistance from the refined microstructure. The lower flank wear rate was also attributed to the shorter chamfer width. Use of MQL at a vc of 300 m/min and ap of 0.5 mm, reduced the cutting temperatures, but increased the cutting force and flank wear rates of all the inserts.