Thermal Stability of Nanocrystalline Diamond Films Grown by Microwave Plasma Chemical Vapor Deposition

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Thermal Stability of Nanocrystalline Diamond Films Grown by Microwave Plasma Chemical Vapor Deposition Mevlut Bulut, Shane A. Catledge, Yogesh K. Vohra, and Renato P. Camata Department of Physics, University of Alabama at Birmingham, Birmingham, AL 35294-1170, U.S.A.

ABSTRACT In this work, the open-air thermal stability of nanocrystalline diamond films grown on mirror-polished titanium alloy substrates by the Microwave Plasma Chemical Vapor Deposition (MPCVD) technique was studied. The results of this investigation show that nanocrystalline diamond films are highly stable in air up to 600°C with no significant change in mechanical properties. Samples annealed between 600°C and 650°C, however, exhibit values of hardness lower by as much as 40% compared to asgrown samples. Above 650°C serious delamination effects were observed in the coatings.

INTRODUCTION Open-air thermal stability of diamond coatings is crucial for some technologically important applications, like industrial abrasives and cutting tools, which are subjected to significant heating under operational conditions. Although various groups have reported nanocrystalline diamond coatings with good mechanical properties [1-3] and acceptable thermal behavior under inert atmosphere [4], the open-air thermal stability of these nanophase materials has not yet been investigated. In this study we have investigated the open-air thermal stability of nanocrystalline diamond films. The long-term goal of this research project is to elucidate the failure mechanism of these nanostructured coatings at high temperature and devise strategies to improve their thermal stability in oxidizing environments. Our expectation is that new carbon-based hard materials operable at high temperatures may be developed from this basic understanding.

Nanocrystalline Diamond Our process for the synthesis of nanocrystalline diamond films has been reported in detail elsewhere [3]. Briefly, the films are produced in a 6-kW Microwave Plasma Chemical Vapor Deposition (MPCVD) system at pressures of 30-125 Torr using methane (CH4) and hydrogen (H2) as feed gases. Previous investigations have established that microcrystalline diamond films with tailored mechanical properties and variable tetrahedral amorphous content (ta-C) may be grown in our system using feed-gas mixtures with CH4/H2 volume ratios ranging from 0.5% to 15% [5]. Moreover, by adding nitrogen gas (N2) to the plasma, up to a N2/CH4 ratio of 0.10, diamond grain size can be reduced significantly, converting the produced film into a truly nanostructured material that we refer to as nanocrystalline diamond. The films produced with N2 addition

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Figure 1. Scanning electron microscopy image of a typical nanostructured diamond film grown using a N2/CH4 ratio of 0.10. The CH4/H2 ratio was fixed at 15% and the operating pressure was125 Torr. consist of diamond nanocrystallites embedded in a tetrahedral amorphous carbon network [3]. Figure 1 shows a scanning electron microscopy image of a typical nanocrystalline diamond film gro

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