An interfacial study of sol-gel-derived magnesium apatite coatings on Ti6Al4V substrates

Magnesium apatite coatings on Ti6Al4V substrate were synthesized by the sol-gel dip-coating method. Magnesium was incorporated in the coating according to the formula (Ca_10−xMg_x)(PO₄)₆(OH)₂, where x = 0, 0.50, 1.00, 1.50 and 2.00. Approximately 2-μm-thick apatite coatings were derived after five cycles of dip-drawing-drying-firing process. A transitional region (R_t) was formed between substrate and coating during the firing process. Adhesion tests show that the adhesion strength between substrate and apatite coating is enhanced by the incorporation of magnesium in the coating. The quantity of magnesium incorporated appeared to correspond to the Mg-Ti-O chemical bonds formed in the transitional region, which contributed to the adhesion strength of the coatings.     

In vitro biocompatibility study of calcium phosphate glass ceramic scaffolds with different trace element doping

Porous calcium phosphate based glass ceramics (CaO-P₂O₅-Na₂O) containing different trace elements (2.0 mol% Mg, Sr and Zn respectively) were prepared by coating polyurethane foams with sol-gel derived glass slurry. After heat treatment at suitable temperatures, main phase catena hexaphosphate (Ca₄P₆O₁₉) and minor phase calcium pyrophosphate (β-Ca₂P₂O₇) crystallized from the glass matrix. These scaffolds were soaked in simulated body fluid (SBF) to determine the solubility and apatite formation, and mouse MC3T3-E1 cells were used to investigate the bioactivity and biocompatibility. The Sr doped scaffold showed a higher degradability than those samples containing Zn or Mg, inducing the formation of an apatite layer with a high (Sr + Ca)/P molar ratio of 1.64, whereas only some discontinuous CaP layers and spare apatite agglomerates were found on the scaffolds doped with Mg ((Mg + Ca)/P = 1.12) and Zn ((Zn + Ca)/P = 1.55) respectively. In vitro cell culture, a high degree of cell adhesion and spreading was achieved on the samples containing Sr or Zn, while only a few cells adhered to the Mg doped sample. These results implied that the bioactivity and biocompatibility of the scaffolds were not only strongly associated with the apatite forming ability, but also related with the Ca/P molar ratios of the deposits.     

Bioactivity of calcium phosphate coatings prepared by electrodeposition in a modified simulated body fluid

The purpose of this study was to investigate bioactivity of calcium phosphate coatings prepared by electrodeposition in a modified simulated body fluid (SBF). Calcium phosphates were electrodeposited on commercially pure titanium substrates in the modified SBF at 60 °C for 1 h maintaining the cathodic potentials of − 1.5 V, − 2 V, and − 2.5 V (vs. SCE). Subsequently, the calcium phosphate coatings were transformed into apatites during immersion in the SBF at 36.5 °C for 5 days. The apatites consisted of needle-shaped crystallites distributed irregularly with different grain sizes. As the coatings were electrodeposited at higher cathodic potential, the crystallite of the apatites got denser and the grain sizes of the apatites became bigger during subsequent immersion in the SBF. However, as the coatings were electrodeposited at higher cathodic potential, the coatings were transformed into apatites with lower crystallinity and the Ca/P atomic ratio of the apatites got higher than 1.67, that of stoichiometric hydroxyapatite, after subsequent immersion in the SBF. In addition, CO₃²⁻ ions contained in the modified SBF were incorporated in the calcium phosphate coating during electrodeposition and had an influence on transforming the calcium phosphate into bonelike apatite during subsequent immersion in the SBF showing that CO₃²⁻ incorporated in the apatites disturbed crystallization of the apatites. These results revealed that the coating electrodeposited at − 2.0 V (vs. SCE) in the modified SBF containing CO₃²⁻ ions was the most bioactive showing transformation into carbonate apatite similar to bone apatite.     

Effects of substrate bias voltage and target sputtering power on the structural and tribological properties of carbon nitride coatings

Effects of substrate bias voltage and target sputtering power on the structural and tribological properties of carbon nitride (CN_x) coatings are investigated. CN_x coatings are fabricated by a hybrid coating process with the combination of radio frequency plasma enhanced chemical vapor deposition (RF PECVD) and DC magnetron sputtering at various substrate bias voltage and target sputtering power in the order of −400 V 200 W, −400 V 100 W, −800 V 200 W, and −800 V 100 W. The deposition rate, N/C atomic ratio, and hardness of CN_x coatings as well as friction coefficient of CN_x coating sliding against AISI 52100 pin in N₂ gas stream decrease, while the residual stress of CN_x coatings increases with the increase of substrate bias voltage and the decrease of target sputtering power. The highest hardness measured under single stiffness mode of 15.0 GPa and lowest residual stress of 3.7 GPa of CN_x coatings are obtained at −400 V 200 W, whereas the lowest friction coefficient of 0.12 of CN_x coatings is achieved at −800 V 100 W. Raman and XPS analysis suggest that sp³ carbon bonding decreases and sp² carbon bonding increases with the variations in substrate bias voltage and target sputtering power. Optical images and Raman characterization of worn surfaces confirm that the friction behavior of CN_x coatings is controlled by the directly sliding between CN_x coating and steel pin. Therefore, the reduction of friction coefficient is attributed to the decrease of sp³ carbon bonding in the CN_x coating. It is concluded that substrate bias voltage and target sputtering power are effective parameters for tailoring the structural and tribological properties of CN_x coatings.     

Apatite formation from simulated body fluid on various phases of TiO2 thin films prepared by filtered cathodic vacuum arc deposition

Hydroxyl (OH⁻)-free TiO₂ thin films with amorphous and crystalline phases were deposited onto (100) silicon substrates using filtered cathodic vacuum arc deposition in order to investigate the in vitro apatite formation in simulated body fluid (SBF). The surface morphology, composition and structure of the TiO₂ thin films were characterized. The X-ray photoelectron spectroscopy results confirmed the presence of calcium and phosphorus on all TiO₂ thin film surfaces after immersion in SBF at 37 °C. Fourier transform infra red results showed the presence of carbonated apatite on the surface of these films. Amorphous structured TiO₂ thin film showed poor ability to form apatite on its surface in SBF. Apatite formation was more pronounced on the surfaces of the anatase films in comparison to those of rutile. The carbonated apatite deposition rate increased significantly when the TiO₂ film was illuminated with UV light prior to immersing in the SBF. In particular, the UV-treated anatase and rutile films showed increased rates of carbonated apatite formation on their surfaces in comparison to samples not treated with radiation. The increase in hydrophilicity due to UV treatment appears beneficial for the apatite growth on these surfaces.     

Improved stability of plasma-sprayed dicalcium silicate/zirconia composite coating

Dicalcium silicate/zirconia composite coatings were produced on Ti-6Al-4V substrates using atmospheric plasma spraying. Different weight ratios of zirconia (50 wt.%, 70 wt.%, 90 wt.%) were mechanically blended with dicalcium silicate (C₂S) powders as feedstocks. The composite coatings were immersed in a simulated body fluid (SBF) and a Tris-HCl solution for the in vitro appraisement of stability and long-term performance in a biological environment. The ion concentration changes of Ca, Si, and P in SBF and Tris-HCl solution were monitored using inductively-coupled plasma atomic emission spectroscopy (ICP-AES). Compared to the pure C₂S coating, our results show that the dissolution rate of the composite coatings is effectively reduced and the stability is improved by the addition of zirconia. The high content of zirconia in the coatings ensures the long-term performance in biological environment, while dissolution of C₂S in the coatings results in a higher Ca ion concentration in SBF and rapid precipitation of bone-like apatite on the composite coating surfaces indicating good bioconductivity of the coatings.     

Sol-gel prepared β-TCP/FHA biphasic coatings

β-tricalcium phosphate/fluoridated hydroxyapatite (β-TCP/FHA) biphasic coatings were prepared on titanium alloy substrate by means of sol-gel method. The coatings combine the initial dissolution of β-TCP with the long-term stability of FHA to create a high quality bioactive coating. Ca(NO₃)₂, P₂O₅ and HPF₆ were dissolved in ethanol respectively and mixed in designed sequence and Ca : P : F ratios to form a sol. After the sol was refluxed for 24 h, the as-refluxed sol was used for FHA coating. β-TCP powders were dispersed into the sols to form colloidal sols for β-TCP/FHA biphasic coatings. The as-refluxed sols with different Ca : P : F ratios only resulted in apatite coatings with low F content. Biphasic coatings were prepared with the colloidal sols. The β-TCP contents of the coatings could be tailored by varying the amount of the powders in the colloidal sols. The surface morphology of the coatings becomes rougher with increasing amount of the powders, which favors cell attachment. However, excessive amount of powders results in powder agglomeration, leading to more cracks in the coatings. Fine powders and good dispersion are essential factors for good biphasic coatings.     

Preparation and in vitro bioactivity of zinc incorporating tricalium silicate

Zinc incorporating tricalcium silicate (Zn-C₃S) at various concentrations (0.80-8.00 wt% Zn) were prepared and investigated as compared with the pure C₃S. The incorporation of Zn promotes the formation of C₃S phase during the calcining process, and inhibits the phase transformation and decomposition of C₃S during the quenching process. The excessive incorporation of Zn (≥ 4.80 wt%) results in the disappearance of the main heat libration during the hydration of Zn-C₃S. The incorporation of Zn decreases the pH value of SBF solution after soaking, and increases the ionic dissolution concentrations. The deposition of apatite on Zn-C₃S paste surface is inhibited during the first 12 hours of soaking. However, the apatite layer could deposit on the surface of Zn-C₃S paste after soaking for 3 days. Our results indicate that the incorporation of Zn significantly improves the formation and stable of C₃S, and alters the bioactive behaviors and ion releases to the SBF solution.     

Aerosol deposition of silicon-substituted hydroxyapatite coatings for biomedical applications

Silicon-substituted hydroxyapatite (Si-HA) coatings on commercially pure titanium (Ti) were prepared by aerosol deposition using Si-HA powders. Si-HA powders with the chemical formula Ca₁₀(PO₄)_6 − x(SiO₄)_x(OH)_2 − x, having silicon contents up to x = 0.5 (1.4 wt.%), were synthesized by solid-state reaction of Ca₂P₂O₇, CaCO₃, and SiO₂. The Si-HA powders were characterized by X-ray diffraction (XRD), X-ray fluorescence spectrometry, and Fourier transform infrared spectroscopy. The corresponding coatings were also analyzed by XRD, scanning electron microscopy, and electron probe microanalyzer. The results revealed that a single-phase Si-HA was obtained without any secondary phases such as α- or β-tricalcium phosphate for both the powders and the coatings. The Si-HA coating was about 5 µm thick, had a dense microstructure with no cracks or pores, and showed a high adhesion strength ranging from 28.4 to 32.1 MPa. In addition, the proliferation and alkaline phosphatase activity of MC3T3-E1 preosteoblast cells grown on the Si-HA coatings were significantly higher than those on the bare Ti and pure HA coating. These results revealed the stimulatory effects induced by silicon substitution on the cellular response to the HA coating.     

MgxZn1−xO (x = 0-1) films fabricated by sol-gel spin coating

Mg_xZn_1−xO films were deposited onto the glass substrate by a sol-gel spin coating method. The drying and annealing temperatures were 300 and 500 °C in air. As x varies from 0 to 1, it was observed that the crystal structure is changed from wurtzite ZnO to cubic MgO. The morphology characterizations of these films were observed by scanning electron microscope. The randomly oriented hexagonal nanorods were gown on the glass surface when x = 0 and 0.25, which became disappeared with increasing Mg contents. The optical properties of these films were investigated by room-temperature photoluminescence (PL) and UV-vis absorption spectra, which show that the optical band gap and photoluminescence in the visible and UV regions can be ideally tuned by varying the Mg contents in the Mg_xZn_1−xO alloy films.     

Improve corrosion resistance of magnesium in simulated body fluid by dicalcium phosphate dihydrate coating

A dicalcium phosphate dihydrate (DCPD) coating composed of bar-shaped crystals was deposited on the surface of magnesium in order to slow down the corrosion rate of the substrate. The corrosion resistance of the DCPD-coated specimens was evaluated in a simulated body fluid (SBF) with uncoated specimens as a control. Time-dependent characteristics of specimens and the corresponding SBF were analyzed at 3, 5, 7, 14 and 21 days of immersion. Less weight loss and pH increase were observed for the coated group than the uncoated group. The coating was intact after 3 days of immersion although its dissolution was manifested by XRD examination. Noticeable DCPD dissolution occurred at the 5th day accompanied by a temporary increase in Ca and P concentrations in SBF which otherwise kept decreasing. Despite the dissolution of the coating, some DCPD particles were still observed on the surface of the substrate after 21 days of immersion. In contrast to the coated specimens, a porous layer of Mg(OH)₂ was formed on the surface of uncoated specimens at the 5th day of immersion. It was found that the corrosion rate of the coated group was substantially lower than that of the control.     

In vitro behavior of fluoridated hydroxyapatite coatings in organic-containing simulated body fluid

A dense and pure hydroxyapatite [HA, Ca₁₀(PO₄)₆(OH)₂] coating and a fluoridated HA [Ca₁₀(PO₄)₆(OH)_0.67F_1.33] are deposited on Ti6Al4V substrates by sol-gel dip coating method. Glucose and bovine serum albumin have been added in standard simulated body fluid (SBF) to form organic-containing SBF in simulation of the physiological blood plasma. The HA and the fluoridated HA coatings are immersed in the standard and modified SBF for time periods of 2, 4, 7, 14 and 28 days at 37 ± 0.1°C. After soaking, the coating surface is examined for nucleation and growth of apatite using SEM morphological observation. The post-soaking SBF solutions are analyzed via Inductively Coupled Plasma spectroscopy for calcium ion concentration. The results show that at concentration of 40 g/L, bovine serum albumin has significant retardation effect on apatite precipitation from SBF onto pure or fluoridated HA coatings; Fluorine-incorporation in HA has positive bio-activation effect in both standard SBF and organic-containing SBF. However, glucose addition in SBF does not generate significant influence on the bioactivity of HA and fluoridated HA.     

Nanostructure transition in Cr-C-N coatings deposited by pulsed closed field unbalanced magnetron sputtering

Cr-C-N coatings with different compositions, i.e. (C + N)/Cr atomic ratios (x) of 0.81-2.77, were deposited using pulsed closed field unbalanced magnetron sputtering by varying the chromium and graphite target powers, the pulse configuration and the ratio of the nitrogen flow rate to the total gas flow rate. Three kinds of nanostructures were identified in the Cr-C-N coatings dependent on the x values: a nano-columnar structure of hexagonal closed-packed (hcp) Cr₂(C,N) and face-centered cubic (fcc) Cr(C,N) at x = 0.81 and 1.03 respectively, a nanocomposite structure consisting of nanocrystalline Cr(C,N) embedded in an amorphous C(N) matrix at x = 1.26 and 1.78, and a Cr-containing amorphous C(N) structure at x = 2.77. A maximum hardness of 31.0 GPa and a high H/E ratio of 1.0 have been achieved in the nc-Cr(C,N)/a-C(N) nanocomposite structure at x = 1.26, whereas the coating with a Cr-containing amorphous C(N) structure had a minimum hardness of 10.9 GPa and a low H/E ratio of 0.08 at x = 2.77. The incorporation of carbon into the Cr-N coatings led to a phase transition from hcp-Cr₂(C,N) to fcc-Cr(C,N) by the dissolution into the nanocrystallites, and promoted the amorphization of Cr-C-N coatings with the precipitation of amorphous C(N). It was found that a high x value over 1.0 in the Cr-C-N coatings is the composition threshold to the nanostructure transition.     

Microstructure and mechanical properties of CrAlN/SiNx nanostructure multilayered coatings

Multilayered CrAlN and SiN_x films were deposited periodically by radio frequency reactive magnetron sputtering. In the CrAlN/SiN_x multilayered coatings, the thickness of CrAlN layer was fixed at 4 nm, while that of SiN_x layer was adjusted from 4 nm to 0.3 nm. The dependence of the SiN_x layer thickness on the preferred orientation, crystalline behavior and mechanical properties of multilayered coatings were discussed with the aid of XRD patterns and HRTEM. It was demonstrated that amorphous SiN_x layer transformed to a crystallized one when the thickness decreased from 4 nm to 0.3 nm. The crystalline SiN_x layer grew epitaxially, formed the coherent interface with the CrAlN layer, and the columnar structure was exhibited. The critical layer thickness for the transition from amorphous SiN_x to a crystallized one was found to be around 0.4 nm, and maximum hardness of 33 GPa was revealed.     

Optical emission studies of the gas phase during the silicon suboxide deposition by reactive r.f. magnetron-sputtering

Silicon oxide (a-SiO) is one of the most used silicon-based materials in optoelectronic and microelectronic technology. It is well-known that the electronic properties are linked to the material structure, which depends on the deposition technique and on the details of the deposition. Silicon suboxide (a-SiO_x 0 < x < 2) layers were prepared by r.f. magnetron-sputtering from a polycrystalline silicon target in a well defined oxidation environment.Optical Emission Spectroscopy (OES) was employed to study the plasma used in SiO_x depositions. Theoretical calculations performed in the frame of electron impact excitation mechanism for argon and atomic oxygen corroborated with electron impact dissociation of the molecular oxygen have shown the strong influence of the electron temperature on the rate coefficients of photons' production.Correlation functions between the OES signals assigned to silicon and oxygen atoms from plasma (gas phase) and the SiO_x layer composition (infrared and energy dispersive X-ray investigated) have been found. Based on these functions, the OES plasma monitoring is proposed as a tool to control in-situ and in direct time the SiO_x layer composition.     

Effect of grain size on mechanical properties in CrAlN/SiNx multilayer coatings

Both CrAlN and SiN_x coatings were deposited sequentially by RF magnetron sputtering. During sputtering, thickness of SiN_x layer was set to be 1 nm, while that of CrAlN layer was controlled to be 4, 20, 40, 100, and 200 nm. According to XRD results, it was revealed that grain size of the CrAlN coatings increased from 3.6 nm to 24.2 nm with the increasing thickness. From HRTEM images, the variation on grain size was attributed to the amorphous SiN_x layer, which significantly retarded the continuous growth of CrAlN layer. Hardness of the CrAlN/SiN_x coatings with various bilayer thicknesses was measured by nanoindentation. The relationship between grain size and hardness could be interpreted by the Hall-Petch equation, and an improved hardness around 32 GPa was achieved.     

微波辅助法制备镁合金的植酸镁/羟基磷灰石复合涂层及其耐蚀性能

采用微波辅助法在AZ31镁合金表面制备了植酸镁/羟基磷灰石(PA/HA)复合涂层。利用FESEM、EDS、XRD和电化学性能测试等方法表征涂层的表面形貌、物相组成以及耐蚀性能,探究了植酸溶液的pH值对PA/HA复合涂层形貌及耐蚀性能的影响,并通过浸泡实验研究了镁合金及PA/HA复合涂层在模拟体液(SBF)中的降解矿化行为。结果表明:在植酸预处理中,植酸溶液的pH=5.0时制备得到的PA/HA复合涂层表面均匀、无裂纹,与镁合金基底的界面结合良好;并且在此pH值下PA/HA复合涂层包覆镁合金样品的交流阻抗最大,自腐蚀电流密度最小,说明其耐蚀性最好。在SBF中,PA/HA复合涂层能够快速诱导磷灰石的生成,并显著提高镁合金基底的耐蚀性能。     

Deposition of nano-scaled CrTiAlN multilayer coatings with different negative bias voltage on Mg alloy by unbalanced magnetron sputtering

In a magnetron sputtering system, the negative substrate bias voltage has been used as a basic process parameter to modify the deposition structure and properties of coatings. In this paper we report the effect of bias voltage ranging from −40 V to −90 V on nano-scaled CrN/TiN/CrN/AlN (CrTiAlN) multilayer coatings synthesized on a Mg alloy by a closed-field unbalanced magnetron sputtering ion plating system in a gas mixture of Ar + N₂. The technological temperature and atomic concentration in the multilayer coatings were controlled by adjusting the current density of different metal magnetron targets and the plasma optical emission monitor. The composition, crystallographic structure, deposition model and friction coefficient of multilayer coatings were characterized by X-ray photoelectron spectrometry (XPS), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and ball-on-disc testing. The experimental results show that the deposition model and friction coefficient of nano-scaled CrTiAlN multilayer coatings were significantly affected by the negative bias voltage (V_b). The nitride species in multilayer coatings mainly involve CrN, AlN and TiN, and XRD analysis shows that the crystallographic structure was face-centered cubic. Under different bias voltage conditions, the multilayer coating composition shows a fluctuation, and the Al and Cr concentrations respond in the opposite sense to the bias voltage, attaining their greatest values at V_b = −70 V. The surface and cross-sectional morphology shows deposition model change from a columnar model into non-columnar model with the increase in negative bias voltage. The friction coefficient of the nano-scaled multilayer coatings at V_b = −55 V stabilize after 10 000 cycles.     

Anodic TiO2 nanotubes from stirred baths: hydroxyapatite growth & osteoblast responses

Self-organized porous nano-tubular TiO₂ was anodically formed on titanium from electrolyte containing 1 M Na₂SO₄ and 0.5 wt.% NaF. The oxidation was carried out for 2 h at 20 V and the electrolytes were stirred using (i) magnetic pellet and (ii) ultrasonic waves. The nanotubes have single-pore diameter of 50–90 nm under the magnetic pellet stirring and 75–110 nm under the ultrasonic agitation. The coatings were amorphous in this condition. Amorphous coatings cannot nucleate apatite easily and require crystallization heat-treatments for inducting apatite. The current nanotubes were heated for 2 h at 500 °C. This treatment converted them to crystalline mixed anatase and rutile. These heat-treated coatings were then immersed in simulated body fluid (SBF) for 5, 168 and 504 h to nucleate apatite. Apatite produced on ultrasonically obtained nanotube contained finer crystals.     

Improvement in crystallinity of apatite coating on titanium with the insertion of CaF2 buffer layer

In the apatite coatings on Ti the heat treatment process is necessary to crystallize the apatite structure for improved chemical stability and biological properties. However, the heat treatment normally degrades the mechanical strength of the coating layer associated with thermally induced stress. In this study, we aimed to improve the crystallization of apatite coating by using calcium fluoride (CaF₂) as a buffer layer. The insertion of a thin layer of CaF₂ (0.2–1 μm) between apatite and Ti significantly improved the crystallization behavior of apatite. Moreover, this crystallization was more enhanced as the thickness of CaF₂ was increased. When a 1 μm-thick CaF₂ was inserted, the crystallization of apatite initiated at a temperature as low as 320 °C, being a dramatic improvement in the crystallization when considering the crystallization initiation temperature of a bare apatite coating on Ti was ∼450 °C. As a result of this crystallization enhancement, the dissolution behavior of CaF₂-inserted apatite coatings was more stable than that of the bare apatite coating, showing much reduced initial-burst effect. Preliminary cellular assay showed the CaF₂-inserted apatite coating provided a substrate for cells to spread and grow favorably, as being similar to the bare apatite coating. This novel way of apatite coating on Ti using CaF₂ buffer layer may be useful in the coating systems particularly requiring low temperature processing and increased crystallinity with high chemical stability.