Characterisation of Silicon Oxide Coatings using Microwave Plasma Enhanced Chemical Vapour Deposition

The drive towards net zero carbon usage has been increasingly important over the past couple of decades. The reduction of fossil fuels and the need for renewable energy resources is predominant within the automotive industry. Hydrogen fuel cells are an excellent alternative to the current internal combustion engines, however, the need to find suitable materials and coatings, which can withstand the corrosive environments encountered is becoming more prevalent. Silicon oxide (SiOx) coatings are a potential coating for the use in hydrogen fuel cells to withstand the corrosive environments and enhance the lifetime of the fuel cells. In this study, the use of microwave plasma enhanced chemical vapour deposition (MW-PECVD) has been investigated as a way to deposit SiOx coatings onto high speed steel (HSS) substrates. A matrix of coatings has been developed using different hexamethyl disiloxane (HMDSO) and oxygen (O2) ratios during the deposition processes, to characterise the mechanical, composition and microstructural properties. The corrosion behaviour of the coatings has also been established, using electrochemical techniques in a 1 molar sodium chloride (NaCl) solution. A further matrix of coatings has also been developed and characterised with a variation in the coating thickness. A Hauzer Flexicoat 850 deposition system at the University of Leeds was used to deposit 20 unique SiOx coatings using a novel MW-PECVD technique. The coatings were characterised to quantify their hardness, Young’s modulus, chemical and structural composition, and to determine the corrosion behaviour using polarisation techniques and electrochemical impedance spectroscopy (EIS).This study has shown that varying the HMDSO:O2 ratios of the SiOx coatings can affect both the mechanical, chemical and microstructural composition of the coatings, leading to coatings with different chemistries and stoichiometries. The corrosion performance of MW-PECVD deposited SiOx coatings has shown that the HMDSO:O2 ratios can affect the corrosion resistance of the coatings, and increasing the thickness of the coatings can have an increased effect on the corrosion performance of the coatings.