Fabrication and Corrosion Properties of Nickel Silicides Thin Films

Licentiatavhandling, 2009

Surface engineering is a vital issue in engineering materials design. The enhancement of surface properties extends the materials lifetime and improves the materials performance without changing the bulk properties. Thin film deposition is one of the most commonly applied techniques in this field. In the present study, nickel silicides (Ni-Si), binary component materials mostly used in microelectronics, are chosen as protective layers on widely used AISI 304L austenitic stainless steel, which can suffer from localised corrosion attack in aggressive environment. A series of Ni-Si films of different compositions (Ni, Ni0.66Si0.33, Ni0.40Si0.60, Ni0.20Si0.80 and Si) is fabricated on the 304L steel plates by means of ion-beam sputter (IBS) deposition. The surface compositions of the thin films are determined by means of X-ray photoelectron spectroscopy (XPS). High resolution core level spectra of Ni 2p3/2 and Si 2p are curve-fitted to characterise the silicides formed. The fitted results agree with the evaluation of the thermodynamic Pretorius’ effective heat of formation (EHF) model. Potentiodynamic polarisation measurements of the as-deposited thin films are conducted in a three-electrode electrochemical cell. Electrochemistry parameters such as corrosion potential (Ecorr), corrosion current density (icorr) and polarisation resistance (Rp) are determined. The chemical compositions of corroded specimens and the chemical states of each element after polarisation tests are then characterised by means of XPS. The comparative studies among the Ni-Si films show that the Ni0.40Si0.60 film has the best corrosion resistance while the Ni0.66Si0.33 film has the worst. Nonetheless, the corrosion resistivity of the Ni0.66Si0.33 film can be improved simply by heat treatment at 200°C in air. For all corrosion resisting specimens, the development of the SiO2 layer at the top is crucial to inhibit the materials deterioration underneath. Morphological examination by means of scanning electron microscopy (SEM) shows that minor pitting corrosions occur on all non-heated specimens. The amount and size of pits reduce as the Si content increases. Meanwhile, the heat-treated Ni-rich specimen shows no pits. In all cases, the Ni-Si thin films show improved corrosion resistance compared to the 304L stainless steel in the hydrochloric acid (HCl) solution of pH 1.3.