The microstructure of highly adhesive CVD coated α-Al2O3 films on cemented carbide cutting tool inserts have been investigated. These inserts are coated with a TiN layer, a MT(moderate temperature)-Ti(C, N) layer, a bonding layer, and a α-Al2O3 layer by using CVD.
SEM, TEM, and TEM-EDX were used to characterize the coatings, and following characteristics were found.
(1) The bonding layer is composed of Ti, C, N, and O or of Ti, C, and O. Its crystal structure is a face centered cubic one with a lattice parameter of 0.431nm.
(2) Each grain in the bonding layer continues crystallographically from the MT-Ti(C, N) layer grains.
(3) There are crystalline planes running between a grain of the bonding layer and a grain of the a-Al2O3 layer.
(4) Vertical-plane-shape projections are formed on the surface of bonding layer.
The last three characteristics are considered to be the cause for improved α-A12O3 layer adherence and longer tool life.
A crystallographical model shows that the above mentioned microstructure is consistent to X-ray diffraction result which shows the (110) plane of α-Al2O3 layer is oriented to the parallel of the insert's surface.
