Erosion of diamond films and graphite in oxygen plasma
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The nature and rates of erosion of diamond, graphite, and diamond-like carbon (DLC) films exposed to oxygen plasmas were evaluated by comparison of surface morphological changes and weight losses. The RF plasma oxidation at ambient temperature caused severe etching of graphite and DLC specimens, while causing only minor damage to diamond film surfaces. The results suggest that erosion by low energy ions is very selective to the sp2 and sp states compared to the sp3 state of carbon. The selectivity of etching by oxygen plasma is significant, as compared to what has been reported for hydrogen in atomic and ionic states, or for oxidation at elevated temperatures in molecular oxygen. These observations have significant implications to the synthesis of diamond films by chemical vapor deposition (CVD) as well as to the application of diamond film coatings on graphite or other substrates for protection against energetic atomic oxygen prevailing in the low earth orbits (LEO).
1. INTRODUCTION Oxygen plays an important role in synthesis and applications of diamond and diamond-like carbon (DLC) films. In chemical vapor deposition (CVD) and gas jet processes used for synthesizing diamond, oxygen in atomic, ionic, or molecular forms is known to influence growth rates and enhance faceting. Oxygen content of the process gas or plasma has been known to affect the quality of the film, often measured by the amount of non-diamond carbon present in it. Oxygen also limits the applications of carbon and diamond films due to its high reactivity with carbon. Both diamond and graphite oxidize readily in molecular oxygen or air at temperatures above 600 °C. For materials selected for low earth orbit (LEO) applications, resistance against attack by low energy atomic oxygen is required. Graphite, which forms the basis for many high temperature and high strength carbon-carbon structural composites, has very limited resistance to atomic oxygen. If diamond films prove to offer a greater resistance against LEO oxygen than graphite, they would be attractive as protective coatings over carbon-carbon composites. A major goal of this study is to evaluate such feasibility. The role of oxygen in the synthesis of diamond films has been the subject of many recent investigations. The influence of oxygen additions in various forms such as 0 2 , H 2 O, CO, and CO 2 to the CVD process gases on the nature of diamond growth has been widely studied.1"9 In the filament activated CVD process, Kawato and Kondo 1 observed that additions of 0.4% O 2 to H 2 containing 1.6 to 4% CH4 resulted in an increase in growth rate and in reduced amount of non-diamond carbon over an O 2 free system. Chen et al2 examined the role of O 2 , CO 2 , and 1484
http://journals.cambridge.org
J. Mater. Res., Vol. 6, No. 7, Jul 1991
Downloaded: 07 Apr 2015
CO additions to H 2 + CH4 process gases in the CVD system. They note that these additions permit deposition of crystalline diamond at high CH 4 concentrations and at higher rates than otherwise possible. The use of CO + H 2 process
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