Interaction of hydroxylated PACVD silica coatings on titanium with simulated body fluid

Nano-film coatings of hydroxylated silica reacted at low-temperature (<100 °C) with titanium substrates by a plasma assisted chemical vapour deposition (PACVD) technique have been studied for their interaction with simulated body fluid (SBF; the inorganic component of human blood plasma) with the aim of growing calcium hydroxy phosphate (CHP) bioactive overlayers. This layer is the precursor to formation of crystalline calcium hydroxyapatite (HA), a growth component of bone and teeth. The in situ plasma method produces 50–100 nm thick silica films with hydoxylated surfaces using a sequence of dry air; tetraethoxysilane (TEOS) vapour mixture with air; and 30% H2O2 solution. Two types of hydroxylated silica coatings were deposited and studied: Type I coatings, deposited using high TEOS/air ratio and high deposition rates, were observed to be unstable upon interaction with simulated body fluid tending to detach from the substrate in SBF solution; Type II coatings, deposited using low TEOS/air ratio and low deposition rates, produced functionally graded, strongly adherent films which exhibited chemical and mechanical stability and the growth of CHP and HA upon interaction with amine-free SBF. The interaction of Type II coatings with amine-stabilized SBF as used by other researchers did not result in the formation of the CHP layer. The results show that control of the TEOS/air ratio and rate of deposition (operating pressure) is essential for production of the functionally graded, strongly adherent oxide/silicate/silica layer and that hydroxylation of the silica surface strongly promotes CHP and HA growth. The study shows that the surface chemistry of the PACVD silica films and their behaviour in SBF are different from those of silica gels produced by hydrolysis and polycondensation of alkoxide precursors and from bioglass formulations.The results are also cautionary in that it is possible to produce PACVD silica films without the formation of the graded interlayers and consequently with poor adhesion to the substrate. It is clearly essential to maintain control of the TEOS/air ratio and low deposition rates to produce the required sequence of reactions in the metal oxide/silicate layer and in the surface hydroxylation of the silica film without incorporation of hydrocarbons and molecular water.