Coatings for Improved Vacuum Materials
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nitride is, in many respects, an idéal vacuum material. It is a fully valenced compound and therefore, like the métal oxides, has a low sticking coefficient for incident gas molécules. TiN, however, has a lower secondary électron yield than the oxides, and therefore has a lower electron-induced desorption rate. Since there is no gap in the conduction band of TiN, the long-lived electronic excitations which lead to high photon-induced desorption cross sections for métal oxides are not permitted, and the induced desorption rates as well as the static outgassing rates are low. Since TiN is a good conductor, it is also not subject to surface charging resulting from the incidence of charged particles. As a resuit of the many other applications for TiN, a number of déposition methods are available. However, most of the existing coating methods do not produce TiN films suitable for ultrahigh vacuum applications. For tribological purposes, it is sufficient that the film be hard, smooth, adhère well, and cover most of the surface. Usually the surface to be coated represents the exterior of a relatively small object. For critical vacuum use, TiN films must also be nonporous, free of pinholes, and contain no trapped gas volume. It must be possible to apply the coating after welding, and to shapes that hâve large and very complicated internai surfaces. Spme of the materials to be coated hâve low melting points or hâve been subjected to spécial heat treatments which do not permit the use of high température coating processes. Principles of TiN Coating Commercial TiN coating processes are divided into two groups: chemical vapor déposition (CVD) and physical vapor déposition (PVD). CVD requires high températures (800-1000°C), while
PVD is generally considered a low température process in that excellent coatings can be applied at 250-500°C. PVD is especially well suited to the production of UHV and XHV components because the materials used are stainless steel and/or aluminum alloys. Thèse materials make excellent substrates for TiN coating but will not withstand the high températures required by the CVD process. Three différent forms of the PVD process are employed in the production of TiN coatings; they are ail based on the use of a pure Ti target and the formation of the nitride coating by introducing nitrogen at the substrate surface during the déposition process: (1) Thermal evaporation of Ti uses a high current électron beam gun directed at the titanium target. (2) Cathodic arc déposition is initiated by volatizing the Ti with a low voltage, high current arc. This is sometimes described as spot evaporation. (3) The requisite flux of titanium atoms is produced by directing an ion beam at the titanium target. Typically, argon is used as the sputtering gas. This article focuses on the use of TiN coatings in high vacuum components and Systems, discussing the advantages derived from TiN coating and placing particular emphasis on the use of the hollow cathode discharge (HCD) ion plating process, a variant of the électron beam déposi
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