Functionalization of the NiTi Shape Memory Alloy Surface by HAp/SiO2/Ag Hybrid Coatings Formed on SiO2-TiO2 Glass Interlayer

Abstract

The surface modification of NiTi shape memory alloys is a method for increasing their multi-functionalities. In our solution, hydroxyapatite powder was mixed with a chemically synthesized silicon dioxide/silver (nSiO2/Ag) nanocomposite in a different weight ratio between components (1:1, 5:1, and 10:1) and then electrophoretically deposited on the surface of the NiTi alloy, under various time and voltage conditions. Subsequently, uniform layers were subjected to heat treatment at 700 °C for 2 h in an argon atmosphere to improve the strength of their adhesion to the NiTi substrate. A change in linear dimensions of the co-deposited materials during the sintering process was also analyzed. After the heat treatment, XRD, Raman, and Scanning Electron Microscopy (SEM) + Energy Dispersive Spectrometer (EDS) studies revealed the formation of completely new composite coatings, which consisted of rutile and TiO2-SiO2 glass with silver oxide and HAp particles that were embedded into such coatings. It was found that spalling characterized the 1:1 ratio coating, while the others were crack-free, well-adhered, and capable of deformation to 3.5%. Coatings with a higher concentration of nanocomposite were rougher. Electrochemical impedance spectroscopy (EIS) tests in Ringer’s solution revealed the capacitive behavior of the material with high corrosion resistance. The kinetics and susceptibility to pitting corrosion was the highest for the NiTi electrode that was coated with a 5:1 ratio HAp/nSiO2/Ag hybrid coating.

Innovative Bioactive Ag-SiO2/TiO2 Coating on a NiTi Shape Memory Alloy: Structure and Mechanism of Its Formation

“…According to the fitting analysis, many individual bands with a high full width at half maximum (FWHM) indicated that an atypical molecular structure had been formed due to the sintering. A similar Raman band arrangement was previously reported for the sintered hybrid Ag-SiO 2 /β-TCP system, which had been deposited on the TiO 2 /NiTi surface [40].…”

“…In turn, the appearance of the rutile bands could be associated with the occurrence of micropores in the Ag-SiO 2 coating; the low coating thickness, which was much below the depth resolution of the Raman spectrometer (~900 nm) or reorganization of the material on the border of the silver-silica coating and the amorphous titanium interlayer. The other bands were probably an effect of the silver nanoparticles’ interaction on the amorphous silica and titanium dioxide, which formed an atypical molecular structure composed of the combination SiO 2 -TiO 2 [ 40 ]. In this context, silver acting as a catalyst probably stimulated the structural reorganization of the amorphous phases by lowering the temperature required to form such an atypical composite.…”

“…In turn, EPD is relatively cheap and does not cause damage to the structure of deposited materials. The high-temperature sintering of electrophoretically deposited ceramic coatings also has advantages, such as forming new biocompatible materials with unique properties [ 40 , 41 ]. Hence, new materials and functionalization approaches have to be considered for improving the quality of an implant.…”

Key Properties of a Bioactive Ag-SiO2/TiO2 Coating on NiTi Shape Memory Alloy as Necessary at the Development of a New Class of Biomedical Materials

“…Hence, one of the completely new routes for improving the surface of a NiTi alloy using a composite nanostructure composed of silica matrix with silver nanoparticles dispersed homogeneously within the matrix has recently appeared [ 40 ]. Hypothetically, this solution should enable the formation of stable coatings that gradually release metal nanoparticles from the matrix, as well as improving the biocompatibility and bioactivity of the final material.…”

“…Thus, calcium phosphates remain the main target material for functionalizing the NiTi alloys, while the hybrid biocomposites that are built that are based on them are simultaneously being developed as a new class of coat-forming biomaterials [ 17 , 29 , 59 , 60 , 61 ]. Additionally, there is little scientific data regarding the silver-silica-based coatings and the problem of calcination, and its impact on physicochemical features of the functionalized biomaterial [ 40 ]. Moreover, the EPD approach when considered in the context of the codeposition of such materials has not yet been used.…”