Long-term bond strength stability of novel organic-inorganic hybrid sol-gel coatings applied to titanium and cobalt-chromium alloys

Water penetration into coating and bonding layers is still considered to negatively affect long-term material stability. This is particularly true when bonding dissimilar materials in applications where water is omnipresent and metal alloys are used. Unfortunately, conventional metal primers still bear a significant risk of continuous bond strength decline, especially after long-term storage in a wet environment. This study evaluated the impact of various organic–inorganic hybrid sol–gel interlayers on the long-term shear bond strength (SBS) of a bone cement to TiAlNb and CoCrMo alloys. Three in situ hydrolyzed alkoxysilanes (3-methacryloxypropyltrimethoxysilane, 1,2-bis- (trimethoxysilyl)ethane, 3,3,3-trifluoropropyltrimethoxysilane) and their various combinations were investigated after previous application of titanium-(IV)-tetraisopropoxide (TTP) on grit-blasted metal surfaces. For each silane combination, SBS data were determined followed by statistical evaluation and fracture surface examination. The novel sol–gel coatings show markedly improved long-term bond strength stabilities after accelerated aging in water (24 h, 80 °C) and long-term immersion in saline solution (365 d, 37 °C). Previous application of TTP on grit-blasted surfaces appeared to be vital for a bond strength improvement of 245 %. Preliminary results in saline solution show an even longer bond strength stability of more than 2 years, which makes these systems particularly interesting for use in biomedical applications.