ABSTRACT
In recent years, there is a great interest to synthesize thin films with enhanced properties for various industrial applications, such as cutting and machining operations, forming dies, diffusion barriers, protective coatings against oxidation, and chemical corrosion. Improved performance in next-generation hard coatings is being sought by enhancing the physical properties of the coatings, especially hardness. Such requirements have been found in hard transition metal nitride films, which are widely used in many applications as high-performance protective coatings [1-3]. However, although MeN films exhibit excellent mechanical properties, they cannot be used at high temperature in air due to their poor chemical stability [4,5]. The limitations of their usage at elevated temperatures and in hostile and aggressive environments attract intensive effort to improve performance of the MeN films by addition of various alloying elements. Among many different multi-component materials, which have been actively explored, mainly additives, such as aluminum (Al), chromium (Cr), and silicon (Si), have been frequently reported to improve the mechanical properties and overcome the oxidation problems of transition metal nitride films [6-8]. Due to an extremely high hardness reported by some researchers [9], Ti-Si-N films are attracting much interest; also other two-phase
systems of the Me-Si-N type form a superhard material with better stability at elevated temperature than the undoped MeN films [1012]. One of the representative transition metal nitride films, which have been shown to exhibit similar properties to that of TiN, is ZrN. Compared to TiN, ZrN has a higher negative free energy of formation [13]; it is formed much easily and is even more stable than TiN. Although the chemical stability of ZrN at high temperatures is not sufficient for tool applications, after the addition of Si, the Zr-Si-N nanocomposite films composed of a nanocrystalline material embedded within an amorphous matrix, exhibit improved mechanical properties and thermal stability already at relatively low Si content [14]. Many papers report on the effect of a small Si content on the structure, phase composition, or film hardness of the Zr-Si-N films [15-17]; however, there is almost no reference on the properties of those films with a high (>25 at.%) Si content. Increasing interest in this kind of material is related to unique properties of Si3N4, which is formed in the film with higher Si content in the form of an amorphous matrix covering the ZrN grains. Si3N4-based ceramics have been shown to have excellent properties of high strength, hardness and toughness, wear-corrosion, oxidation and thermal shock resistance, low thermal expansion, self-lubricating or heat, and electrical insulation [18]. It makes them interesting for many applications in tools. The origin of the hardness enhancement of the Zr-Si-N films over the hardness corresponding to the bulk ZrN has been found in the relationship between the film nanostructure, strong chemical bonding between elements of the nanocomposite, macrostress s generated in the film during its growth, or their combined action. However, many questions remain unanswered regarding the relation between the properties of the films and the size of their grains and parameters which control the thermal stability and oxidation resistance of nanocomposite films. In this chapter, detailed study of Zr-Si-N films with a high (≥25 at.%) Si content composed of two stable ZrNx and Si3N4 phases is present with the aim to answer some of these questions. Further discussion on why Me-Si-N films with X-ray amorphous structure in the as-deposited state and a high content of the Si3N4 phase can exhibit high hardness up to or even exceeding 30 GPa and excellent thermal stability and oxidation resistance above 1000°C is discussed.
