Microstructure of α-Al₂O₃-coated turning inserts whose tool lives were longer than K-Al₂O₃-coated ones have been investigated. These inserts were coated successively with a TiN layer, a MT(moderate temperature)-Ti(C, N)layer, a bonding layer, and a α-Al₂O₃ layer on cemented carbide inserts by using chemical vapor deposition(CVD). X-ray diffraction, SEM and TEM were used to characterize the coatings, and following characteristics were found.
(1) Epitaxial growth was observed in a column of MT-Ti(C, N) twinned grains, bonding layer twinned grains and α-Al₂O₃ grains by cross-section TEM, where(100)-planes of the α-Al₂O₃ grains were parallel with(110)-planes of both the MT-Ti(C, N)grains and the bonding layer grains, and(003)lattice fringes of the α-Al₂O₃ grains continue almost linearly from {111} lattice fringes of the bonding layer grains.
(2) (120)-planes of the α-Al₂O₃ grains and (211)-planes of both the MT-Ti(C, N)grains and the bonding layer grains in the column were parallel to the substrate, too.
(3) Many steps at the surface of vertical-plane-shape protrusions formed along the twin boundaries on the surface of the bonding layer, might promote the epitaxial growth of α-Al₂O₃ grains on the bonding. layer.
(4) The above mentioned microstructure agrees with X-ray diffraction results on the MT-Ti(C, N) layer and the α-Al₂O₃ layer. This may mean that the epitaxial growth will be found in many columns in the α-Al₂O₃-coated turning inserts.
The above mentioned epitaxial growth and the protrusions between the bonding layer grains and the α-Al₂O₃ grains are considered to be the cause for improved α-Al₂O₃ layer adherence and longer tool life.