Ti1?x Al x N coatings by plasma-assisted chemical vapour deposition using a TiCl4/AlCl3/N2/H2/Ar gas mixture

Abstract

Recently, Til-xAlxN, a new coating material for advanced protective coating on steel tools, has attracted much concern with regard to its coating process because it shows increased anti-oxidation and lifetime, and improved cutting performance compared with previous TiN coatings [1-3]. The high oxidation resistance and consequent increase of tool lifetime is known to be due to the formation of an aluminium oxide layer on top of the Tii_xAlxN coating [4], which protects the remaining Til-xAlxN layer from further oxidation. Til_~AI~N coatings have been prepared by physical vapour deposition (PVD) techniques such as sputtering [5, 6] and ion plating [7-9], and show improved cutting performance of tools. However, there has been no report on Til_xAI~N coatings prepared by plasma-assisted chemical vapour deposition (PACVD). The PACVD technique is industrially important because it makes coating possible at low temperature like PVD, it is also suitable for mass-production and it applicable to complex shaped tools because of its good stepcoverage characteristics. Very recently, PACVD was tried by Lee et al. [10]: Til_xAlx N coating was prepared by PACVD using a gas mixture of TIC14, A1C13 and NH 3 as a suitable nitriding gas. NH 3 gas thermodynamically enables nitriding of A1C13 source to A1N [11]. The A1N phase forms from a gas mixture of A1CI 3 and NH 3 above 600°C and has generally been deposited at higher temperatures for crystalline films. The CVD processes using NH 3 gas, however, has problems associated with the formation of solid compounds such as TiC14"nNH3, A1CI3"nNH3, and NH4C1, which would either clog the system lines or be incorporated into the growing film with consequent deterioration of film properties. Thus, N2 gas would be preferred to NH3 on these respects. Even though N2 gas is not suitable for the formation of A1N from a thermodynamic point of view [11], the plasma process seems to enable the formation of metastable Til_xAI~N because the gas phase deposition itself proceeds under highly non-equilibrium conditions. In this letter, we report successful coating of Til_~AI~N by PACVD using N2 gas instead of NH3. Til_~AlxN was deposited onto high-speed steel (HSS) by a PACVD technique using a gaseous mixture of TIC14, A1C13, H 2, Ar and N2 instead of NH 3 as a nitrogen source. The deposition proceeded between parallel plates in a cold-wall type vacuum chamber, which was capacitively coupled with an r.f. source. The upper electrode was connected to an r.f.