[China Aluminum Network] The characteristics of the metal cutting industry are characterized by rapid technological advancement. The more important trends in the market are:
(1) In order to increase productivity, higher cutting speeds are used.
(2) In order to reduce costs and environmental protection requirements, dry machining and/or lesser amount lubrication (MQL) processing are used.
(3) In order to make parts and structures lighter, use difficult-to-machine materials, such as high-strength materials.
All these trends place higher demands on the wear resistance, resistance to plastic deformation and toughness of tool coatings.
Due to its high chemical stability and favorable thermal properties, Al2O3 is an ideal tool coating material for high-speed metal cutting. It is worth emphasizing that CVD is still a technology that can economically produce high quality Al2O3 coatings.
Even though most of the published literature in the field of coating wear resistance is studying PVD, it is important to realize that significant advances in CVD technology (especially CVDAl2O3 technology) have been made in the past few years. Currently there are three Al2O3 phases (a-Al2O3, k-Al2O3, and g-Al2O3) that can be CVD deposited in a controlled manner.
The a-Al2O3 is a stable Al2O3 phase. With the heat treatment during the deposition process, heat treatment after deposition, and heat generated during the metal cutting process, the metastable k-phase and g-phase will be converted into a stable phase.
Surprisingly, it has been found that stable a-Al2O3 stable k-Al2O3 is more difficult to carry out on industrial scale CVD deposition. One of the reasons is that the nuclei formation of k-Al2O3 smoothly occurs on the unoxidized surface of the TiC, Ti(C,N), or TiN layer having the fcc structure. When nucleated k-Al2O3 is relatively stable, it can grow to a considerable thickness (>10 μm). Therefore, if the nucleation surface is TiC, Ti(C,N), or TiN (typical case in the case of cemented carbide), the nucleation and growth of a-Al2O3 using CVD is not simple. This explains to some extent the generality of k-Al2O3 as a coating material, and there are still many commercial CVDAl2O3 coatings consisting of k-Al2O3.
Newer levels of deposition of a-Al2O3 and k-Al2O3 coatings with complete nucleation control are only reaching industrial scale in nearer time. The alumina phase was controlled by nucleation prior to Al2O3 self-deposition, and all individual Al2O3 layers (k-Al2O3 and a-Al2O3) were deposited using the same process parameters. This technique allows complete control of the phase content of the CVDAl2O3 coating.
As mentioned above, k-Al2O3 is metastable and may be transformed into a stable a-Al2O3 phase during deposition and during the cutting process, especially at high cutting speeds. The volume contraction encountered by the transformation of the phase will reduce and eventually destroy the adhesion of the k-Al2O3 layer. Therefore, in terms of deposition and wear resistance (especially at high cutting speeds), a-Al2O3 is the preferred and safer alternative. Fine particle and defect-free a-Al2O3 can significantly improve wear resistance.
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