| When free standing diamond films or diamond coated components have to be exposed to oxygen bearing environments at elevated temperatures, such as in the case of missile dome of high-speed flight (4~10 Mach number) , high temperature electronic devices , severe wear surface coatings, etc, the investigation on oxidation and fracture properties of CVD diamond films is of vital industrial significance.This paper investigates the oxidation of CVD diamond films in laboratory air and describes the influence of oxidation on properties of diamond films, such as IR transmission , fracture strength and thermal conductivity. The factors influencing the oxidation properties of diamond films have been studied, including the sample thickness, quality, grain orientation, etc. The kinetics of diamond film oxygen reaction and oxidation protection of diamond films were also studied. We have demonstrated that CVD diamond can be protected from oxidation for exposure (3min) in air at temperatures up to 900 ℃ by a single-layer coating of d.c. magnetron-sputtered aluminum nitride.Oxidation of CVD diamond films in air at temperatures up to 1123K was investigated by thermogravimetry. In air, diamond films begin to oxidize slowly at about 700 ℃, and the oxidation rate become appreciable near 800 ℃ .The apparent activation energy for the oxidation of the CVD diamond films, was found to be 220KJ/mol, which agreed well with that for the oxidation of natural diamond. The diamond films before and after partial oxidation were characterized by optical and scanning electron microscopy, X-ray, and Raman spectroscopy.Optical property of large area high quality free standing diamond film is investigated, which was deposited by high power DC arc plasma jet CVD system operating at gas recycling mode. Infrared (IR) transmission of high optical quality diamond films with both surfaces mechanically polished were found degraded appreciably after heating in a furnace at 780 ℃ in air for 15min, whilst heating at-780℃ for 3min only produced little change (a decrease in IR transmission less than 3% in 8 - 14nm). Although the thermal conductivity of the diamond films has been notably degraded by oxidation, however it is still far more superior to that of the any other materials. The fracture strength of diamond films have no change after heating at 780 ℃ for 20min. Thus we have demonstrated that the high quality diamond films by high power Jet operating at gas recycling mode can be safely used below800 °C. Apparently this is of vital importance for both military and civilian applications at high temperatures.The correlation of rates of oxidation and the thickness, quality, grain orientation of diamond films were studied. The influence of the nucleation side and the growth surface on rates of oxidation of diamond films was first investigated. The results showed that the rates of oxidation in nucleation side was noticeably faster than that in growth surface.The kinetics of reaction between CVD diamond films and oxygen gas was studied by thermogravimetry. Three stage mechanistic schemes are developed involving adsorption of oxygen on CVD diamond surface, surface chemical reaction, and desorption of adsorbed species to CO or CO2. Adsorption of oxygen on CVD diamond surface was assumed as the rate-controlling step. The dominant chemical reaction involves the formation of CO, while the formation of CO2 is not significant.Diamond is an ultra-durable material with high thermal conductivity and good transmission in the visible, near-infrared and far-IR (8-12(im) wavebands. Diamond is subjected to oxidation in air at temperatures higher than 780 °C, therefore, it is essential to protect the diamond surface from exposure to air. We have demonstrated that CVD diamond films can be protected from oxidation for extended exposure (3min) in air at temperatures up to 900°C by a single-layer coating of d.c. magnetron-sputtered aluminum nitride.
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