Ke-Wei Xu

Structure and in vitro bioactivity of titania-based films by micro-arc oxidation

Abstract Titania-based films on titanium were formed by micro-arc oxidation in electrolytic solutions containing sodium carbonate, sodium phosphate, acetate monohydrate and β-glycerophosphate disodium salt pentahydrate using a pulse power supply. The morphology, elemental composition and phase components of the films were investigated as a function of the electrolytes composition and the applied voltage (in the range of 200–500 V). In vitro bioactivity of the films was evaluated in a most commonly used simulated body fluid as proposed by Kokubo et al. The results showed that the films were porous with 1–8 μm pores and nano-crystallized, without apparent interface to the titanium substrates. The phase components of the films could be anatase, rutile, CaTiO 3 , β-Ca 2 P 2 O 7 and α-Ca 3 (PO 4 ) 2 , strongly depending on the electrolytes composition and the applied voltage. The pore size and the content of Ca and P tended to increase with the applied voltage. Among the prepared titania-based films, only the film containing CaTiO 3 , β-Ca 2 P 2 O 7 and α-Ca 3 (PO 4 ) 2 could induce an apatite layer on its surface, exhibiting bioactivity. The bioactive response of the micro-arc oxidized films to the structural factors and the apatite-induced mechanism were discussed.

UV-enhanced bioactivity and cell response of micro-arc oxidized titania coatings

Using ultraviolet (UV) irradiation of micro-arc oxidized (MAO) titania coating in distilled water for 0.5 and 2h, we have achieved an enhanced bioactivity and cell response to titania surface. The MAO coating appears porous and predominantly consists of nanocrystallized anatase TiO(2). Compared with the MAO coating, the UV-irradiated coatings do not exhibit any obvious change in surface roughness, morphology, grain size and phase component; however, they have more abundant basic Ti-OH groups and become more hydrophilic because the water contact angle decreases significantly from 17.9+/-0.8 degrees to 0 degrees . In simulated body fluid (SBF), bonelike apatite-forming ability is significantly stronger on the UV-irradiated coatings than the MAO coating. SaOS-2 human osteoblast-like cell attachment, proliferation and alkaline phosphatase of the cell are greater on the UV-irradiated coatings relative to the MAO coating. UV irradiation of titania results in the conversion of Ti(4+) to Ti(3+) and the generation of oxygen vacancies, which could react with the absorbed water to form basic Ti-OH groups. The enhanced bioactivity and cell response of the UV-irradiated coatings are proven to result from abundant Ti-OH groups on the coating surfaces. After storing the UV-irradiated coatings in the dark for two weeks, the basic Ti-OH groups on the coatings slightly decrease in amount and can induce apatite formation after a short period of SBF immersion, and show relative long-term stability.

Porous nanocrystalline titania films by plasma electrolytic oxidation

Nanocrystalline titania films were prepared by plasma electrolytic oxidation of a titanium alloy at 200–350 V in a Na2CO3 electrolytic solution using a pulsed power supply. XRD, EDS and Field Emission (FE) SEM were employed to characterize the phase, composition, and microstructure of the films. Vickers indentation, nanoindentation and adhesion–tension test were used to evaluate the mechanical properties of the films. The phase, pore size and thickness of the films strongly depend on the applied voltage consistent with the previous reports. The films prepared at 350 V were porous with ∼1 μm pores and the pore walls were composed of 10–20 nm rutile crystallites. The hardness, Youngs modulus and bond strength of the film were 0.9±0.2 GPa, 32±4 GPa and 37±3 MPa, respectively. The film exhibited a significant plasticity and ductility compared to the conventional coarse-grained titania ceramics.

A study of the process and kinetics of electrochemical deposition and the hydrothermal synthesis of hydroxyapatite coatings

Hydroxyapatite (HAp) coatings were prepared using electrochemical deposition and post-hydrothermal synthesis. The composition and morphology of coatings at each processing step was studied through the application of scanning electron microscopy (SEM), X-ray diffraction (XRD) and infra-red spectroscopy (IR). The mechanism and kinetics of hydrothermal synthesis were considered in particular, and the influnce of the temperature and time on the HAp formation rate was also investigated. The results show that the electrochemical deposition coatings are composed of CaHPO42H2O crystals which are converted into needle-like HAp crystals after post-hydrothermal treatment. The HAp content of the coatings increases with the treatment temperature and time. The synthesis rate also increases with the pH value of the water. The formation of HAp coatings is considered to be a combination of several reactions. An Arrhenius relationship was found between the HAp formation rate and the temperature, and an apparent activation energy of 94.4 KJ/mol was obtained by calculation.

Characterization and stability of hydroxyapatite coatings prepared by an electrodeposition and alkaline-treatment process.

Hydroxyapatite (HA) coatings on titanium alloy substrates were prepared by an alkaline treatment of electrodeposited precursors. The structure, residual stress, and bond strength of the coatings were investigated. Test results showed that the coatings processed in this study exhibited fairly low tensile residual stress, high crystallinity, and were free of an amorphous phase. The bond strength of the coatings increased with the decrease of current density in the range of 0.2-15 mA/cm(2), and reached 14 MPa at 0.2 mA/cm(2). Evaluation of the coatings was performed together with the evaluation of the plasma-sprayed HA coatings immersed in distilled water. It was revealed that the dissolution and bond strength degradation of the coatings were much lower than those of the plasma-sprayed HA coatings.

Improvement in fretting wear and fatigue resistance of Ti–6Al–4V by application of several surface treatments and coatings

Application of surface modification methods is expected to be an ideal solution to mitigate fretting damage. In this study, our aim was to improve the fretting wear and fretting fatigue resistance of titanium alloys by using several types of surface treatments and thin films, including shot-peening, ion-beam-enhanced deposition (IBED) CrN films, shot-peening+IBED CrN films as well as IBED CuNiIn films. Results showed that with the application of all the above surface coatings and treatments, the fretting wear and fretting fatigue resistance of Ti–6Al–4V were improved. However, the mechanisms and effects of several surface modification methods to mitigate the fretting damage were quite different. IBED CrN film exhibited the best fretting fatigue performance while the duplex treatment by shot-peening/IBED CrN film exhibited the highest fretting wear resistance. There are four mechanisms which can be used to explain the different fretting performance of these surface treatments and coatings: (1) to induce a compressive residual stress; (2) to decrease the coefficient of friction; (3) to increase the hardness; (4) to increase the surface roughness.

Synthesis of nanocrystalline titania films by micro-arc oxidation

Abstract Nanocrystalline titania films were synthesized by micro-arc oxidation of titanium substrates in an electrolytic solution using a pulsed power supply. X-ray diffraction (XRD) indicated that the deposited films consisted of a high crystalline anatase phase. Field emission scanning electron microscopy (FESEM) showed that the films were macro-porous with 1–2 μm pores and the matrix was composed of 10–20 nm grains. The adhesion–tension test showed that the films had an adhesive strength of 20±2MPa. Such firmly adhesive, porous and nanocrystalline anatase films are expected to have significant applications as orthopaedic/dental implants and catalysts.

Nano-scratch and fretting wear study of DLC coatings for biomedical application

Abstract The generation of wear particles is now considered one of the most important failures in total arthroplasty. It is especially needed to study damage process of the particles at slight movement and lower load for long-term service of implanted materials. The wear and abrasion behavior of particles on Ti–6Al–4V coated with diamond-like carbon (DLC) were appropriately simulated in the present paper by fretting wear and nano-scratch test. The coatings with a thickness of 2.5 μm were deposited with an r.f.-plasma enhanced chemical vapor deposition method. The analytic techniques employed to assess DLC coatings characterization include SEM, AFM, Raman and IR spectroscopy. Assessment of hardness, elastic modulus, abrasion and adhesion of the coatings were performed using a Nano Indenter XP system with attachments of continuous stiffness measurements (CSM) and lateral force measurements (LFM). Tribological properties of the coatings were evaluated by a fretting wear test. It was concluded that the DLC coatings had a smooth surface morphology with a rms of 6.8 nm and possessed good adhesion to the substrate, they recovered fully elastic at lower load (60 mN) and were ploughed and delaminated only partially at higher load (400 mN). Nevertheless, there was wear debris concurrently generated during the nano-scratch and fretting test, which was more evident at high loads for a definite coatings, indicating an essential necessity of the work to improve the wear resistant of the coatings to avoid the generation of wear debris. The friction coefficient of DLC coatings against corundum obtained by fretting test decreased with the increase of relative humidity, it was below 0.1 in an aqueous condition, this may be beneficial to their biomedical applications in body fluids.

The composite of nitrided steel of H13 and TiN coatings by plasma duplex treatment and the effect of pre-nitriding

Abstract In order to improve the adhesion behavior of TiN coatings deposited on H13 steel, a layered composite structure has been developed by plasma nitriding and plasma-enhanced CVD in the present studies. Effects of the nitriding process on the microstructure, adhesion and microhardness, as well as the residual stress of composite nitrided H13/TiN coatings were investigated. Experimental results showed that the adhesion of TiN coatings to substrate could be remarkably enhanced by an optimized plasma nitriding process conducted at a 25% flow ratio of N2/(H2+N2) for 1 h. The formation of a new compound layer during a nitriding process at a 50% flow ratio of N2/(H2+N2) deteriorates the adhesion of TiN coatings due to premature brittle fracture between the coating and the substrate. The surface hardness of nitrided H13/TiN coatings and compressive residual stress in the diffusion interlayer increase with increasing pre-nitriding time, but the adhesion of the coatings decreases to some extent.

Morphology and composition of hydroxyapatite coatings prepared by hydrothermal treatment on electrodeposited brushite coatings

Highly pure brushite CaHPO4 · 2H2O) coatings on porous Ti6Al4V substrates were prepared by electrodeposition from aqueous electrolytes. The influence of hydrothermal treatment parameters on brushite-to-hydroxyapatite conversion and the morphology and phase composition of hydroxyapatite (HAP) coatings was studied. It was found that the content, Ca/P atomic ratio, grain size and pore size of HAP in coatings increase with increasing hydrothermal treatment temperature, and that increasing the pH value can promote brushite-to-HAP conversion and reduce the grain size of HAP. Under optimal conditions, highly pure HAP coatings with needle-like crystals and non-stoichiometric form, which are similar to those of calcium phosphate in human bone, can be obtained.