Coatings for Improved Vacuum Materials

Abstract

For the past 10 years, reactively deposited films of titanium nitride, TiN, have been applied to cutting tools such as drills, hob cutters, and endmills. A nominal film thickness of 2–4 μm has been shown to give excellent resistance to abrasion and corrosion and to extend tool life three times or more. This is attributable to the physical properties of TiN, which include microhardness of 1,800 kg/mm2 and surface friction approximately one-third that of high-speed tool steel. Corrosion resistance is realized from the dense, fine-grain equiaxed structure of the inert TiN film. Additional applications range from decorative use based on its goldlike appearance to use as a diffusion barrier in semiconductor devices.More recently, TiN has found application as a high quality coating for components used in ultrahigh vacuum (UHV and XHV) system apparatus and especially in high energy particle accelerators. This article discusses the application of TiN coatings to ultrahigh vacuum systems and high energy particle accelerators.The native oxides which form on stainless steel and aluminum tend to be porous and trap large amounts of water vapor and other gases. These trapped gases can be partially removed by vacuum baking, although for particle beam devices in which beam-induced desorption is at least as important as the thermal outgassing rate, an extensive beam-conditioning process is required to get rid of the final vestiges of trapped gas. The oxide surfaces have low sticking coefficients for the adsorption of incident gas molecules, but the oxides have much higher secondary electron yields than the clean metals and consequently have very high beam-induced desorption rates.