Structure characterisation and mechanical properties of crystalline alumina coatings on stainless steel fabricated via sol-gel technology and fibre laser processing

The surface modification of stainless steel by coating with alumina (Al ₂O ₃) was carried out using sol-gel coating technology in combination with laser processing. Alumina coatings have been synthesised via a sol-gel route and deposited on stainless steel substrates by dip coating. The coated substrates were then treated with pulsed ytterbium fibre laser radiation (λ=1064nm) in continuous wave mode with different specific energies. The composition and structure of the coated surfaces after laser processing were characterised by ATR-FTIR, XRD, SEM and contact angle measurements, whilst the mechanical properties of modified surfaces were determined using nano-indentation. The results showed that the alumina xerogel films coated on the substrates are successfully converted into crystalline alumina ceramic coatings by the laser irradiation, the structure of resulting coatings being dependent on the irradiation conditions, with increase of laser specific energy leading to the formation of initially γ-Al ₂O ₃ with increasing amounts of α-Al ₂O ₃ at higher energy. Nano-indentation results reveal that the laser processing results in significant improvement in hardness and Young's modulus of the alumina-coated surface and, at optimum, can achieve the mechanical properties at the same level as pure α-alumina ceramic, much higher than those of the as-dried xerogel coating.