Theoretical study on gas-phase reactions during chemical vapor deposition of TixAl1–xN from TiCl4, AlCl3, and NH3

Abstract

Fcc-Ti_xAl_1–xN (TiAlN) coatings synthesized via chemical vapor deposition (CVD) reduce cutting tool wear. Although CVD conditions reportedly influence coating quality, no quantitative guidelines are yet available. To quantitatively study the film-forming mechanism of TiAlN CVD, the gas composition over the surface must be known. Therefore, we developed a gas-phase elementary reaction model for TiAlN CVD derived from TiCl₄/AlCl₃/NH₃. First, we constructed a novel thermodynamic dataset including molecules that contained both Ti and Al, and calculated the equilibrium composition. Thermal equilibrium calculations in the gas phase showed that the most stable species were AlCl₃ and TiCl₃ rather than TiCl₄. An elementary reaction model was constructed based on the kinetics of the gas-phase species that were generated. Kinetic analysis revealed that gas-phase reactions were largely absent under our reactor conditions. The thermal equilibrium calculations indicated that TiCl₄ may have given rise to TiCl₃. Thus, other reaction pathways of TiCl₄ to TiCl₃ were explored. We calculated the reaction rate constants of 12 reactions of Ti species and added them to the model, which revealed that TiCl₄ decomposed to TiCl₃ via TiCl₃NH₂. Under our conditions, TiCl₄ and TiCl₃NH₂ are the major Ti species and AlCl₃ and AlCl₂NH₂ are the major Al species, which suggests that some of these species may form films. Unlike in the earlier reaction model, NH₂ and H radicals were produced, which may have contributed to the surface reactions. For reactors with large Surface/Volume ratio of reactor, the effects of gas-phase reactions should be considered. Our reaction model enables estimation of the partial pressures of reactor gas species and will therefore aid study of the TiAlN deposition mechanism.