Degradation of multi-layer CVD-coated cemented carbide in finish milling compacted graphite iron

Abstract

Stricter regulations on emissions and higher demands on engine performance drive automotive industries to replace conventional gray cast iron with compacted graphite iron (CGI). CGI has a higher wear resistance, strength, elastic modulus, and almost double fatigue strength as to gray cast iron, yet the same positive properties make CGI more difficult to machine. This study focuses on finish milling CGI EN-GJV-450 with current industrial standard of tooling: cemented carbide with multi-layer CVD-coating of Ti(C,N)–Al2O3. At cutting speed of 150 m/min a total of 3126 cm3 of material after 190 passes was removed, while 1645 cm3 was removed after 100 passes in case of 250 m/min. Studying the wear evolution at different engagement times demonstrated an accelerated chemical wear of the Al2O3 top coating due to its reaction with MgO-containing inclusions that creates a softer reaction product of (Mg,Fe,Mn)Al2O4 spinel which is abraded and removed with the chip flow. Ti(C,N) coating layer experiences diffusional and mechanical (abrasion, debonding, micro-fracture) wear mechanisms. Exposure of cemented carbide substrate resulted in its rapid wear in the form of cratering and massive material loss on the flank where the latter eventually causes tool failure. The adherence of CGI to WC-Co facilitates diffusional loss of carbon and tungsten from WC grains. Carbon reacts with iron at the interface forming iron carbide, while residual tungsten alloys it thus forming (Fex,W1-x)3C. This phase can reduce the wear rate as it acts as a diffusion barrier, but its high brittleness enables its periodic removal by adhesive wear. Outward cobalt diffusion was also an active wear mechanism that facilitates further diffusion of carbon and tungsten but also weakens the WC-grain bonding, further facilitating adhesive wear by CGI flow. Deposition of oxide inclusions to exposed WC-Co work as an anti-stick for CGI adhesion and thus reduces the wear rate.