Abstract
The crack extension resistance and plastic deformation, i.e., the life of CVD-coated cemented carbide tool are greatly affected by the thickness of β(WC-TiN-XC solid solution, XC: added carbide)-free layer near the tool corner as well as near the tool flat surface, and by those ratio, i.e., the morphology of the layer. In this study, the powder compacts of WC-TiN-Co added with six kinds of carbides were firstly sintered in Ar atmosphere of 1.3 kPa at 1693 K for 3.0 ks, and the surface layer of sintered compacts were ground by 0.5 mm thickness so that the altered surface layer generated mainly in the porous state by the sintering was removed. Then, such compacts were re-sintered mainly in vacuum of 10 Pa at 1693 K for 0∼14.4 ks.
The main results were as follows. (1) The thicknesses near the flat surface and the corner were both in the order of Cr3C2 > NbC > Mo2C > ZrC > TiC > TaC and Cr3C2 > Mo2C > ZrC > TiC > NbC > TaC, respectively, irrespective of re-sintering time. Namely, the orders were different with each other. (2) The thickness near the corner for periodic table group-4 metal carbides (TiC, ZrC) and group-6 metal carbides (Cr3C2, Mo2C) was smaller than that near the flat surface at short re-sintering and/or medium time, but became larger at longer time. The thickness near the corner for group-5 metal carbides (NbC, TaC) was always smaller than that near the flat surface. However, the thickness as far as for Cr3C2 became larger near the corner than near the flat surface even at short re-sintering time, and for TaC the thickness difference between both areas became considerably smaller, when the initial nitrogen content of the specimen was increased by controlling N2 pressure in the first sintering furnace. (3) The results of the item (2), etc., suggested that the rate-determining step for the formation of β-free layer is not easy to determine: it depends on the kind of added carbide, the initial nitrogen content, sintering time (in-situ nitrogen content near the boundary of β-free layer), etc.
