Wear Mechanisms When Machining Grey Cast Iron with Ceramic Tools

Abstract

Three cutting tool materials, Coromant ceramic grades CC 620 (based on alumina×zirconia), CC 650 (based on alumina and titanium carbonitride), and CC 680 (based on silicon nitride), were used for evaluation of tool life and wear mechanisms when facing pearlitic flake graphite cast iron without cast skin. Tests were performed at three different cutting speeds, 300, 600, and 800 m min−1, at a feed rate of 0.3 mm rev−1. At 600 m min −1 feed rates of 0·1 and 0·5 mm rev−1 were also used. CC 680 had the highest feed capability and was the only material that could withstand a feed rate of 0·5 mm−1 without fracturing. Tool life was determined by flank wear at all cutting conditions tested except when the highest feed rate was used. CC 620 had the longest tool life and CC 680 the shortest. The main wear mechanism for CC 620 was superficial plastic deformation. In addition, chemical reactions with adhering oxidic layers had influence on the wear rate. CC 650 behaved in a similar way to CC 620. However, the Ti(C, N) phase within CC 650 was worn by diffusion/solution into the work material and also by chemical attack from adhering Fe-Mn silicates. CC 680 was worn almost entirely by a solution/ diffusion wear mechanism. Immersion experiments indicated that composition and thickness of the layers adhering to the flank face could be critical to relative performance ranking of these tool materials. Adherent silicate layers decreased the wear rate of CC 680 due to obstruction of solution/diffusion, whereas the wear rate of alumina based tools increased due to direct dissolution of alumina into the layers. In addition, the wear due to superficial plastic deformation increased as a result of the lower flow stresses of reacted surface layers. Usually, crater wear did not occur due to formation of protective silicate and sulphide layers on the rake face.