Tungsten Oxide Sputtering Targets
Tungsten oxide which has the chemical formula of WO3, is a chemical compound containing oxygen and the transition metal tungsten. Tungsten oxide is obtained as an intermediate in the recovery of tungsten from its minerals.
Tungsten oxide is a wide band gap metal oxide semiconductor having outstanding properties for a variety of applications including electrochromic materials and gas sensors. Tungsten oxide films also possess hydrophilic, gasochromic and photochromic properties. Tungsten oxide is one of the most widely studied electrochromic materials used as films. A promising application for these coatings is a switchable glazing for a smart window, which can save energy for air conditioning. Recently, tungsten oxide films have been considered for use as dielectric layers in multilayer heat mirror coatings on building and automotive glasses. For most of these applications, thin films of tungsten oxide with nanometer thickness are usually deposited onto glass or polymeric substrates. A variety of conventional deposition methods have been used to prepare tungsten oxide films, such as dc or radio frequency (rf) magnetron sputtering, sol–gel methods, plasma spraying method, chemical vapour deposition methods, electron beam evaporation, thermal evaporation and hydrothermal methods.
Sputter deposition technique is popular to produce adherent and uniform film over wide areas with better stoichiometric control. Until today most of the studies investigated electrochromic and gas sensing properties of tungsten oxide films. As the films are exposed to adverse environments, scratch and wear resistance play a significant role for their mechanical stability and durability. Few reports are available in the literature on the hardness, scratch resistance and wear behaviour of tungsten oxide films. These studies attempted to evaluate mechanical and tribological properties of tungsten oxide films deposited mostly on metal substrates prepared by a variety of deposition techniques. Few reports are found to study tribological behaviour of tungsten oxide films deposited on metallic substrates using ball on plate or pin on disc tests. These studies evaluated mechanical and/or tribological properties for tungsten oxide films of micrometre scale thickness using micro- and macroscale measurements. The adhesion critical loads were reported for tungsten oxide films deposited on high speed steel using dc reactive magnetron sputtering. In these works, a conventional scratch testing machine with an indenter tip radius of 200 mm was employed to evaluate adhesion critical loads of micrometre scale films. Evaluation of mechanical properties of such thin films requires measurement techniques with control capability. Besides, a majority of the transparent functional tungsten oxide films are necessarily deposited on glass substrates as thin films. The investigations dealing with the mechanical properties of tungsten oxide thin films on glass substrates are scarce in the literature. From the past literatures, the mechanisms of wear and coating failures using SEM observations are also found scarce for tungsten oxide films. Hence, in order to predict the durability of thin tungsten oxide films on glass substrate and a coating/substrate system of great practical interest, it is important to have adequate data on their mechanical properties.