Wear mechanisms of uncoated and coated cemented carbide tools in machining lead-free silicon brass

Free-machining brass containing 2-3% of lead is a preferred industrial material as it shows excellent machinability where low cutting forces, short chips and reduced tool wear are attained. However this addition of lead, a highly toxic and hazardous material, leads to health and environmental issues. Alternative lead-free brasses are known for poor chip control and accelerated tool wear. The current study focuses on wear mechanisms of uncoated and coated cemented carbide tools when high-speed machining lead-free CuZn21Si3P silicon brass. The study shows that severe crater formation on the rake is the dominant tool failure mode. Microscopy observations indicate the diffusion wear mechanism to be driven by diffusion of cobalt binder into the chips and minor cross-diffusion of copper and zinc. Loss of the binder in cemented carbide is accompanied by adhesive pluck-out of WC grains. As a way to hinder the loss of Co, the diffusion preventing capacity of a-C:H diamond like carbon and (Ti,V,Zr,Nb,Hf,Ta)N nitride coating were tested. SEM, EDX and TEM data show that formation of amorphous SiO2 and stoichiometric β-SiAlON stable layers was observed on the nitride coating, thus preventing diffusional tool wear. O-rich and N-rich glassy amorphous layers in Si-Al-O-N system with ZnS inclusions were found on the DLC coating. Partial delamination of the DLC coating and removal of the glassy phases resulted in localized crater formation associated with diffusional wear.