Tailoring of Transition Metal Silicides as Protective Thin Films on Austenitic Stainless Steel

Transition metal silicide is an important material used in microelectronic devices and a possible candidate for high temperature application. The objective of this study is trying to make use of this material in a new way as protective thin films on engineering materials. The brittle nature of the silicides can thus be compensated by the ductile bulk material and the surface corrosion resistance can be improved. The choices of thin film constituents are targeted at the transition metal elements including titanium (Ti, Z = 22), chromium (Cr, Z = 24), iron (Fe, Z = 26) and nickel (Ni, Z = 28) in combination with silicon (Si, Z = 14) to form the binary-component intermetallic compounds by means of ion-beam sputter deposition, which was carried out in an ultrahigh vacuum (UHV) chamber at 10 Pa. The chosen substrate material is AISI 304 stainless steel. Thin film characterisations were conducted by means of surface analysing instruments: X-ray photoelectron spectrometer (XPS) and grazing incidence X-ray diffractometer (GIXRD). Silicide properties, including the degree of crystallinity, the formed phases, the electronic structures, the chemical compositions and the chemical states of each characterised element, were analysed. For the films in crystalline form, silicides were well characterised by both techniques. In case the crystalline phase did not develop and the phase(s) could not be analysed through XRD, the possible silicide phases in short-range order or no ordering can be estimated in accordance with the core-level XPS binding energy state of the transition metal peak defined by the crystalline form. On the other hand, the phase formation sequence of the silicides during annealing processes can be predicted by a theoretical method of Pretorius’ effective heat of formation (EHF) model provided that the initial chemical concentrations were determined in advance. The corrosion properties of the silicide films and the uncoated stainless steel specimens were assessed and compared by means of potentiodynamic polarisation measurements in hydrochloric acid (HCl) and sulphuric acid (H2SO4), respectively. The polarisation curves of all the silicide-coated specimens show lower current densities along the measured potentials than the uncoated steel and thereby the silicides have lower reactivity. In the same test, it was found that binary-component silicides with Si content above 60 at. %, independent of the choice of transition metal element, facilitate the high integrity Si-oxide layer formation at the top of the tested specimens, and thereby a better corrosion resistivity is guaranteed. Among the four tested silicides, Ti-Si and Ni-Si films are found to be better candidates. In another study, corrosion tests were performed on the coated specimens that were thermally annealed after deposition with the purpose of promoting silicide phase development and film crystallisation. However, the polarisation results do not show significant benefit from this treatment. As a result, composition of the transition metal silicide thin films is a more important design factor compared to the film structure. When considering the silicides as protective thin films, their tribological properties such as adhesiveness and wear resistance cannot be neglected. The results from the Rockwell-C adhesion test and the reciprocal dry sliding wear test proved that the silicide films are well-adhered on the substrates and their wear properties are slightly better than the stainless steel by showing a lower specific wear rate from 10 m/N⋅m to 10 m/N⋅m. It is believed that transition metal silicides, besides deposited on stainless steels, can also be the protective thin films on other engineering materials, as far as well-adhesiveness is guaranteed.