Epitaxial growth mechanism of a bonding layer and an α-Al2O3 layer on a Ti (C, N) layer

Microstructure of a newly developed "long-tool-life" alpha -Al2O3 coated cutting tool and mechanism of epitaxial growth of the alpha -Al2O3 layer has been investigated. The tool was produced by coating cemented carbide chips with layers of a TiN layer, a Ti (C, N) layer, a bonding layer and an alpha -Al2O3 layer successively and in this order using the techniques of CVD. Cross section TEM analysis revealed the existence of a column in which alpha -Al2O3 grains had grown epitaxially around a protrusion formed around a {111} twin plane in the bonding layer twin which had in turn had grown in a similar way (epitaxially) on a surface of a Ti(C, N) twin, and also the surface of the protrusion is covered with steps on the atomic scale. In addition, SEM and TEM analysis on the surface of the bonding layer revealed that a protrusion had been formed with a vertical-plane shape along with a edge of a {111} twin plane on the surface of a bonding layer twin, and {211} planes of both the bonding layer twin and the protrusion are oriented in tangential directions to the substrate. As {211} planes of face-centered-cubic lattices in general are covered with steps on the atomic scale made up of {111} planes etc, and include the maximum allowed number of atoms on the cross-section of the {111} planes, it is reasonable to conclude that the steps covering the surface of the bonding layer may lead to promotion of the formation of the vertical-plane shape protrusions around the edge of the {111} twin planes through absorbing particles of the recant gases in CVD process at the corner of the steps, and also the steps covering the surface of the protrusion may promote the epitaxial growth of alpha -Al2O3.