steel substrates). Moreover, low elastic modulus is more appropriate for preventing crack formation (if not for inhibiting crack growth). Musil [] proposed a design based on the concept of ceramic nanoparticles embedded in an amorphous metallic matrix. This can be produced either by co-sputtering of the elements in an argon/nitrogen gas atmosphere or by post-deposition nanocrystallization of an amorphous coating. However, correct selection of coating elemental composition and of the deposition parameters (e.g. substrate temperature and energy of vapour flux bombardment) is very important []. Musil’s design is a deviation from the ceramic/ceramic nanocomposite coating proposed by Veprek. The consideration of an amorphous metallic intergranular phase will reduce the effective elastic modulus of this type of coating, thereby making it more compliant and tough. However, the coating-substrate interface is still susceptible to corrosive attack through any defect in its ceramic phase. Leyland and Matthews proposed using the following for the design of wear-resistant nanostructured or glassy metallic coatings []: 1. Alloys of low-miscibility elements, because they have the potential for producing nanostructured and amorphous coatings. 2. Binary alloys of ‘nitride-forming’ transition metal elements (doped to supersaturation in B, C, or N) with another low-modulus, low-miscibility transition element. 3. Ternary, quaternary or higher mixtures of elements with widely differing atomic radii (including combinations not possible to achieve in ‘conventional’, cast or rapidly-solidified bulk metallic glass systems).
Owing to the constraints of sputter target material manufacture, it is difficult to use certain individual elemental sputter targets for coating deposition (for example, nickel is ferromagnetic - and thus shields the magnets which enhance the sputtering effect; silicon and - particularly - boron are extremely brittle and susceptible to thermal shock). Therefore, to achieve a wide range of coatings incorporating all the elements mentioned above, without complicating and compromising the deposition process, commercially-available (and of proven durability) Al75Ni25 and Ti28Si17B55 sputter target alloys were chosen. The Al-Ni alloy has enough aluminium to produce coatings with the desired high atomic percent of Al, while providing just enough nickel to take advantage of the glass-promoting properties of Ni in combination with Al and other metallic and/or metalloid elements (mentioned earlier). The TiSiB composite sputter target (synthesised using Self-propagating Synthesis, SHS) provides an ideal mixture of large, medium and small atoms of the 'other' candidate elements - and has been used previously in various other sputter deposition experiments by the Surface Engineering group [, , ]. As well as promoting metallic glass formation across a wide range of coating compositions, the combination adds advantages such as high hardness, thermal stability and oxidation resistance. CHAPTER 2: BACKGROUND TO THE DEPOSITION OF PVD COATINGS This chapter covers fundamental concepts and techniques in deposition of PVD coatings. The methods described here are either specific or related to this research. However, a