Morphological, structural and tribocorrosion behaviour of TiO2 MOCVD coating on Ti substrates with different morphology

Titanium (cp-Ti) and its alloys have been widely used for dental implant applications due to their excellent combination of strength-to-weight ratio, excellent corrosion resistance and biocompatibility [1]. An essential surface feature of Ti is the capability to spontaneously form a thin (4-6 nm thick), stable amorphous TiO2 film under exposure to the atmosphere and/or physiological fluids. This passive film protects Ti from corrosion and promotes a favorable osseointegration [2]. During mastication in an oral environment, dental implants are exposed to mechanical (i.e. wear), chemical (i.e. corrosion), and adverse biological effects, leading to a complex process of degradation. In this way, a tribocorrosion phenomenon is characterized by the synergistic interaction of wear with corrosion, which may lead to implant failure and adverse biological reactions due to the release of wear particles and/or corrosion products into the body [3]. Various surface treatment techniques have been developed in order to grow up protective layers with improved surface characteristics, such as tribocorrosion resistance and, at the meantime, adequate biological responses [3]. Indeed, several in vitro and in vivo studies showed the positive effect of the functionalization of Ti surface: in this context deposited rutile and anatase layers compared to native TiO2 show enhanced bone-like precipitation at the surface in simulated body fluids. MOCVD technique has been applied to the growth of titanium dioxide coatings, which successfully underwent several biological tests both in vitro and in vivo [4 and ref. therein]. However, the MOCVD process has been weakly explored to improve the tribocorrosion resistance of the Ti substrates and no literature references were found on the evaluation of performance of the TiO2 MOCVD coating on substrates with specific morphology. In this regard, besides to the chemical surface composition, the roughness and the topography of the Ti surface are significant parameters that affect the rate and quality of osseointegration. In this work, three types of Ti substrates (i.e. Ti machined, sandblasted, and sandblasted/acid etched, all of commercial grade IV) with different morphology were coated with 200 nm titanium oxide films by using LP-MOCVD, operating at 350°C and 100 Pa. The TiO2 source was titanium tetraispropoxide (TTIP). The titanium oxide thickness was chosen in order to have the best compromise between the increase of the Ti corrosion resistance and the optimal coating/substrate adhesion features. The influence of the pristine substrate morphology on TiO2 crystalline structure, and morphology, on surface wettability and on tribocorrosion performance is here presented. In particular, it is shown that the specific morphology of the pristine substrate influences both the crystalline phase of the TiO2 and the crystallite size. Scanning electron microscopy analysis shows an optimal conformal coverage of the MOCVD coating for all substrates, with specific grain size as a function of the substrate morphology (Figure 1). Even the wettability depends on the Ti substrate features, demonstrating a superhydrophilic behavior for the sandblasted/acid etched samples after MOCVD TiO2 deposition (Figure 2). Finally, tribocorrosion experiments were carried out in order to evaluate the coating stability under wear and corrosion in a special electrolyte, which mimics the oral cavity corrosive medium, namely, artificial saliva (AS).