Techniques to deposit α-Al2O3 and κ-Al2O3 layers with nucleation control have been introduced on an industrial scale only recently, and it has clearly been shown that α-Al2O3 is the preferred phase in most metal cutting applications.
According to the definition used in the International Tables of Crystallography, α-Al2O3 belongs to the trigonal crystal system and has a rhombohedrally centred hexagonal lattice, the space group symbol being R 3c. The crystal structure of α-Al2O3 is often described as being composed of oxygen ions (A, B) in an approximate hcp (hexagonal close-packed) arrangement ( . . . ABAB . . . ) with the aluminium anions occupying two thirds of the octahedral interstices. The aluminium cations can take three different vacancy positions in the oxygen lattice with the stacking sequence of . . . αβγαβγ . . . . These are usually referred to as cα, cβ and cγ. The unit cell of α-Al2O3 comprises six layers of O and Al can be described in the following way: AcαBcβAcγBcαAcβBcγ. The JPDS card, defined hereinbelow, uses the hexagonal system and, consequently, four axes (hkil) are used where i=−(h+k). Often, the index i is omitted as done also in this case.
It has been known in the art to use nucleation control in order to obtain various growth textures. As described in a recent publication (S. Ruppi, “Deposition, microstructure and properties of texture-controlled CVD α-Al2O3 coatings,” Int. J. Refractory Metals & Hard Materials 23 (2005) pp. 306-315) manipulation of the nucleation surfaces can be used to obtain the growth textures <012>, <104> or <003>. The commonly observed diffraction peaks from α-Al2O3 are (012), (104), (110), (113) and (116). However the diffraction peak (006), which is an indication of the <001> texture, is always missing, as indicated by its absence in XRD-patterns obtained from textured α-Al2O3 layers using known methods.
Prior to the present invention, texture has been controlled by modifying the chemistry of the nucleation surface. This approach, however, does not provide complete nucleation control. When the nucleation control is not complete, at least a portion of the produced α-Al2O3 layers are formed via κ-Al2O3α-Al2O3 phase transformation. These kinds of α-Al2O3 layers are composed of larger grains with transformation cracks. They exhibit much lower mechanical strength and ductility than textured α-Al2O3 layers composed of α-Al2O3 formed from 100% or near 100% nucleation. Consequently, there is a need to develop techniques to more precisely control the nucleation step and growth texture of α-Al2O3.
The control of the α-Al2O3 polymorph in industrial scale was achieved in the beginning of the 1990s with commercial products based on U.S. Pat. No. 5,137,774. Later modifications of this patent have been used to deposit α-Al2O3 with preferred textures. In U.S. Pat. No. 5,654,035 an alumina layer textured in the <012> direction and in U.S. Pat. No. 5,980,988 in the <110> direction are disclosed. In U.S. Pat. No. 5,863,640 a preferred growth either along <012>, or <104> or <110> is disclosed. U.S. Pat. No. 6,333,103 describes a modified method to control the nucleation and growth of α-Al2O3 along the <10(10)> direction. U.S. Pat. No. 6,869,668 describes a method to obtain a strong <300> texture in α-Al2O3 using a texture modifying agent (ZrCl4). The prior-art processes discussed above all use deposition temperatures of about 1000° C.
US 2004/0028951A1 describes a technique to achieve a pronounced <012> texture. The commercial success of this kind of product demonstrates the importance to refine the CVD process of α-Al2O3 towards fully controlled textures.