SiC x Layers on Diamond by Si Implantation for Protection Against High Temperature Oxidation
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A.R. KIRKPATRICK*, S. DALLEK** and W.E. KOSIK*** *Epion Corporation, 4R Alfred Circle, Bedford, MA 01730 **Naval Surface Warfare Center, Carderock Division, Silver Spring, MD 20903 ***U.S. Army Research Laboratory, Watertown, MA 02172
ABSTRACT
Diamond is subject to oxidation if exposed to oxygen while at temperatures above approximately 600'C. A method has been demonstrated for protection of diamond against oxidation by employing Si+ ion implantation to transform a thin surface layer into silicon carbide which exhibits excellent oxidation resistance. Integral SiCx oxidation barrier layers have been formed on diamond using high Si+ dose levels and high temperatures during implantation. SiCx layers have been characterized using Rutherford backscattering, IR spectrophotometry and scanning electron microscopy. The effects of high temperature exposure to oxygen have been examined using various oxidation test procedures including thermogravimetric analysis. SiCx layers capable of providing protection of underlying diamond for periods of several minutes at temperatures beyond 1000'C have been accomplished. INTRODUCTION
Diamond exhibits a remarkable combination of outstanding mechanical, thermal and optical characteristics. Because of these characteristics, natural diamond and the diamond film and sheet materials which can now be produced by various CVD processes are candidates for a broad range of applications. Unfortunately, at high temperatures in the presence of oxygen, diamond
is subject to rapid oxidation erosion which generally precludes consideration of its use for some applications where it might otherwise be particularly well suited. Availability of a method to protect diamond against high temperature oxidation could significantly increase the range of applications for which diamond will be employed. In the absence of oxygen, diamond is stable up to approximately 1650'C before it begins to undergo conversion to graphite1 . If oxygen is present, a diamond surface at temperatures as low as 600°C is subject to oxidation to form CO or CO 2 gas2 . At higher temperatures, the oxidation can be sufficiently rapid and exothermic that the diamond can undergo thermal runaway ignition. Oxidation kinetics have been established for the major crystallographic faces of single crystal diamond2 '3 and for4, polycrystalline CVD diamond films deposited using various processes and process conditions '. On the major crystal faces of diamond, oxidation progresses most rapidly on the (111) face, slower on the (110) face, and least rapidly on the (100) face 2. The oxidation mechanism is considered to be the same in polycrystalline CVD material as in single crystal diamond, but effective erosion rates are typically somewhat lower because they are limited by the rate of attack upon the (100) planes within the polycrystalline granular microstructure 5. Other investigators have attempted to employ deposited thin film infrared optical antireflection coatings of materials such as Y20 3 to serve also as barriers to prevent oxygen from reach
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