Elsevier

Surface and Coatings Technology

Volume 206, Issue 7, 25 December 2011, Pages 1771-1779
Surface and Coatings Technology

Understanding the catalytic effects of H2S on CVD-growth of α-alumina: Thermodynamic gas-phase simulations and density functional theory

https://doi.org/10.1016/j.surfcoat.2011.09.018Get rights and content

Abstract

The catalytic effect of H2S on the AlCl3/H2/CO2/HCl chemical vapor deposition (CVD) process has been investigated on an atomistic scale. We apply a combined approach with thermodynamic modeling and density functional theory and show that H2S acts as mediator for the oxygenation of the Al-surface which will in turn increase the growth rate of Al2O3. Furthermore we suggest surface terminations for the three investigated surfaces. The oxygen surface is found to be hydrogenated, in agreement with a number of previous works. The aluminum surfaces are Cl-terminated in the studied CVD-process. Furthermore, we find that the AlClO molecule is a reactive transition state molecule which interacts strongly with the aluminum and oxygen surfaces.

Introduction

Protective coatings of aluminum oxide (Al2O3) on cutting tool inserts have been one of the most important materials for metal machining applications for many years [1], [2], [3]. The deposition method for Al2O3 coatings is predominantly chemical vapor deposition (CVD) [4], [3], [5], although other methods such as physical vapor deposition (PVD) [6], [7], [8], [9] or atomic layer deposition (ALD) [10], [11] are also viable. In their patent from 1985, Lindström et al. [12] showed for the first time that H2S could be used as a catalyst to increase the growth rate of Al2O3, and furthermore to suppress the “dog-bone” effect, resulting in a more homogenous growth on flanks and edges of a cutting tool insert. Later patents have also included the usage of H2S as a catalyst for Al2O3 growth [13], [14]. In fact, the introduction of H2S leads to a considerable increase in growth rate by a factor of about 5 [1]. Also other compounds such as SF6[14], H2O3[15], ZrCl4[1], SiCl4[16] and TiCl4[17] have been found to improve the growth rate. Already in the 70th's, the strong interaction of H2S with metal surfaces [18], [19] as well as with Al2O3[20], [21], [22], [23], [24] was shown in a number of studies, due to the possible usage of Al2O3 as a catalyst. Results indicate that H2S will dissociate irreversibly to HS and H on different metal surfaces [20], [22] and that H2S primarily adsorbs to strong Lewis acid sites such as bare Al on γ-Al2O3[21]. Large efforts have been made to control the growth rate and phase composition of Al2O3 for coatings on cemented carbide tools. Although H2S has been used as a catalyst for 25 years, the underlying mechanism is not yet understood. A few studies have tried to explain the large impact of H2S on the CVD process of Al2O3[25], [26]. The common conclusion from these studies is that H2S shifts the rate determining step in the Al2O3 growth from the gas phase to the surface. However, the results do not provide a complete picture, and require further investigations.

In later years, there are several extensive studies of the Al2O3 (0001)-surfaces, mainly in the context of PVD. Rosén et al. [6] studied adsorption of Al+ on α-Al2O3 by 0 K DFT calculations and ab initio molecular dynamics. Wallin et al. [8], [9] studied both Al- and O-coordinated surfaces and their relaxations as well as diffusion of Al on these surfaces. Wallin et al. concluded that the main obstacle for α-Al2O3 growth at low to moderate temperatures is the limited surface diffusivity. This result may however not be applied to CVD of α-Al2O3, where the growth temperature is considerably higher. Hinneman et al. [27] made a detailed analysis of the adsorption of Al, O, Hf, Y, Pt and S on the single Al-terminated α-Al2O3 (0001)-surface. Hinneman et al. [27] found that the sulfur atom preferentially adsorbs to an oxygen, binding weakly to the surface. They also found that S-adsorption resembles the adsorption of O, both in geometry and in electronic structure. Starting with the clues that we have from previous theoretical and experimental studies, we apply a combined approach with thermodynamic modeling and density functional theory (DFT) and show that it is possible for H2S to act as mediator for the oxygenation of the Al-surface which will in turn increase the growth rate. Furthermore we suggest surface terminations for the three investigated surfaces. As far as we know, this is the first study from First Principles, where the adsorption of the gas species present in a typical chemical vapor deposition of Al2O3 for cutting tool inserts is studied in the literature. In addition we find that the reactive AlClO molecule will most likely be a transition state in one or several surface reactions during growth of Al2O3.

Section snippets

First principles modeling

In our work, the first-principles calculations were performed by using the projected augmented wave (PAW) method [28] as implemented in the Vienna ab initio simulation package (VASP) [29], [30]. The exchange-correlation interaction was treated by the Perdew–Burke–Ernzerhof (GGA-PBE) exchange correlation functional [31]. This functional is one of the most commonly used, along with the Perdew–Wang functional from 1991 (PW91) [32]. For all geometry optimizations an energy cutoff of 750 eV was used,

Crystal structure

The crystal structure of Al2O3 can be described by a rhombohedral unit consisting of 10 atoms, 4 Al and 6 O, which is the primitive unit cell. Alternatively three of these units are connected giving the hexagonal cell of 30 atoms, whereof 12 Al and 18 O. In Fig. 1 the hexagonal unit cell of α-Al2O3 is displayed. First we verified that the structural properties of our bulk structure were in agreement with experimental findings. In Table 1, the lattice parameters and Wyckoff positions of the

Results and discussion

The simulation of the CVD deposition of α-Al2O3 was divided into two parts: gas-phase modeling and surface modeling. The multiple approach is visualized Fig.2. For the gas phase modeling in ThermoCalc, experimental input gas contents of a standard CVD deposition method were used (Table 2). The gas species formed at thermodynamic equilibrium were then modeled by density functional theory, and were used as input for nudge elastic band calculations of the kinetic barriers in surface reactions on

Conclusions

The following may be concluded from the thermodynamical modeling of the gas phase and ab initio modeling of the molecules in gas phase and reconstruction and kinetics of the different α-Al2O3 surfaces,

  • 1.

    The AlO-surface is thermodynamically and dynamically stabilized as it is terminated by chlorine, present in the CVD gas phase. The surface undergoes considerably less reconstruction as it is chlorated. We find good agreement for the bare AlO-surface with previous results [8], [43], [44].

  • 2.

    The

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