Oxidation of diamond windows
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Jean M. Bennett Physics Branch, Research and Technology Division, Naval Air Warfare Center Weapons Division, China Lake, California 93555-6001
Melvin P. Nadler Chemistry and Materials Branch, Research and Technology Division, Naval Air Warfare Center Weapons Division, China Lake, California 93555-6001 (Received 6 August 1994; accepted 27 June 1995)
Diamond windows were heated at 700-900 °C in air for short times to better define the limits of application at high temperature. The infrared (IR) transmission of polished chemical vapor deposition (CVD) diamond windows was reduced after heating in a furnace at 800 °C for 75 s, while heating at 700 °C for 75 s produced little change. The 800 °C heating caused increased visible light scatter, and increased scatter appears to be mainly responsible for the reduced IR transmission. The forward scatter at 10.6 ^ m for the CVD diamond samples was 0.8% before heating, 2.8% after heating at 800 °C for 75 s, and 6.2% after heating at 800 °C for 255 s. Single crystal (110) type Ha diamond exhibited little change in IR transmission when heated at 800 °C in air for 255 s, while heating for 555 s at 800 °C caused a significant drop in IR transmittance (6-12%). A slight drop in IR transmittance (1-5%) occurred for a type Ila diamond when heated at 900 °C for 45 s. The etched surfaces were characterized by differential interference contrast optical microscopy, scanning electron microscopy, and Talystep surface profiles. Etched features are concentrated at grain boundaries (for the CVD samples) and at sites with residual damage from mechanical polishing that could not be seen before the heat treatment. Deep etch pits formed at grain boundaries that extend into the bulk of samples.
I. INTRODUCTION
II. EXPERIMENTAL
While diamond is generally inert to chemical reaction at room temperature, at elevated temperatures certain reactions proceed readily, and may limit the use of diamond in some high temperature applications. Air oxidation of diamond has an onset at 480 °C and proceeds at a significant rate at or above 600 °C. 2 ^ Under reduced pressures of O 2 , graphitization of the diamond surface occurs together with oxidation, while in an inert atmosphere graphitization does not begin until approximately 1500 °C.5'6 Graphitization can also occur at about 800 °C when diamond is in contact with certain metals, such as iron, cobalt, nickel, manganese, and chromium. 78 Diamond can also dissolve in these metals under similar conditions.9 For some applications diamond may be heated to a high temperature in air for short periods of time. Although bulk oxidation may not occur at short times, etching of the diamond surface may degrade the diamond for certain applications, such as optics. In this study polished CVD and type Ila diamond samples were heated in air for brief periods of time to determine the limits of application of infrared windows. The oxidation process that caused optical degradation was characterized by examining surface microstructure.
Heating experiments were conducted on samp
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