One-step synthesis of petal-like apatite/titania composite coating on a titanium by micro-arc oxidation

Petal-like apatite/titania (TiO₂) coating was prepared on commercially pure titanium (Ti) by micro-arc oxidation in electrolyte containing calcium and phosphate for the first time. The surface morphology, crystalline structure, chemical composition and binding state of the apatite/TiO₂ composite coating were characterized. The coating consists of a double-layer (apatite layer and TiO₂ layer) structure. The average thickness of the inner TiO₂ layer and the outer apatite layer is about 6 μm and 16 μm respectively. The outer apatite layer is porous and exhibits petal-like pattern. The apatite layer consists of hydroxyapatite (HA) and carbonate-apatite and the inner TiO₂ layer consists of anatase and rutile.     

In vitro effect of magnesium inclusion in sol-gel derived apatite

Magnesium-containing apatite coatings were prepared on Ti6Al4V substrates by sol-gel dip coating method. Standard simulated body fluid (SBF) was used to evaluate the bioactivity of the coatings. A series of the coatings according to the composition (Ca_10−xMg_x)(PO₄)₆(OH)₂, where x = 0 to 2, is synthesized and immersed in the standard SBF for periods of 7 to 35 days for direct deposition of apatite layer from the SBF solution. Scanning electron microscopy (SEM) was used to examine the morphology changes of the SBF apatite layer that occurred during in vitro immersion. X-ray diffractometry, Fourier Transformation Infra-Red Spectroscopy and X-ray Photoelectron Spectroscopy were used to analyse the phases, chemical groups and composition of the sol-gel coating. Results show that as the sol-gel coating contains magnesium, this promotes deposition of apatite layer from SBF. As x ≤ 1, SBF immersion gives rise to a dense apatite layer. However, as x ? 1, selected dissolution of the deposited layer takes place, which results in serious pitting on the surface. Also, Mg ions from the dissolution of the sol-gel coating during immersion in the SBF apparently played a role in the subsequent deposition of apatite o the coating, evidence of Mg was found in the apatite layer.     

Corrosion behaviour of single (Ti) and duplex (Ti–TiO2) coating on 304L stainless steel in nitric acid medium

Sputter deposited single titanium (Ti) layer, and duplex Ti–TiO₂ coating on austenitic type 304L stainless steel (SS) was prepared, and the corrosion performance was evaluated in nitric acid medium using surface morphological and electrochemical techniques. Morphological analysis using atomic force microscope of the duplex Ti–TiO₂ coated surface showed minimization of structural heterogeneities as compared to single Ti layer coating. The electrochemical corrosion results revealed that, titanium coated 304L SS showed moderate to marginal improvement in corrosion resistance in 1 M, and 8 M nitric acid, respectively. Duplex Ti–TiO₂ coated 304L SS specimens showed improved corrosion resistance as compared to Ti coating from dilute (1 M) to concentrated medium (8 M). The percentage of protection efficiency for base material increases significantly for duplex Ti–TiO₂ coating as compared to single Ti layer coating. The oxidizing ability of nitric acid on both the coatings as well as factors responsible for improvement in protection efficiency are discussed and highlighted in this paper.     

Fabrication of highly porous titanium (Ti) scaffolds with two interlaced periodic pores

This paper reports a novel type of porous titanium (Ti) scaffolds with two interlaced periodic pores that were produced by coating the surfaces of a dual-channeled hydroxyapatite (HA) scaffold, as a supporting framework, with a titanium hydride (TiH₂) slurry followed by heat-treatment at 1200 °C for 3 h in a vacuum to convert TiH₂ to Ti metal. This method allowed the porous Ti scaffolds to mimic the original pore structure of the dual-channeled HA scaffold in a tightly controlled manner. It was observed that the Ti layer was strongly adhered to the HA layer, owing to the diffusion of P ions into the Ti layer. The fabricated sample showed a high compressive strength of 6.0 ± 0.77 MPa and a porosity of 78 vol.% due to its unique pore structure, as well as perfect interconnections between the pores.     

Study of vanadium-based chemical conversion coating on the corrosion resistance of magnesium alloy

In this study, magnesium alloy (AZ61) was immersed in vanadium containing bath with various conditions, such as the vanadium concentration, immersion time and bath temperature. The results indicate that increase of both vanadium concentration and immersion time produces a thicker conversion layer. However, when immersion time is too long, it will worsen the corrosion resistance due to the increasing of the crack density. The experimental parameter of bath temperature has no significant effect on corrosion resistance. Our results demonstrated that the better corrosion resistance coating can be obtained when the samples are submitted to an immersion in the conversion bath containing NaVO₃ with concentration of 30 g l⁻¹ for 10 min at 80 °C. The presented conversion treatment has its potential to replace the chrome-based conversion coating treatment.     

Kinetics of titanium nitride coatings deposited by thermo-reactive deposition technique

In this study, the growth kinetics of titanium nitride layer deposited on pre-nitrided AISI 1020 steel samples by thermo-reactive diffusion (TRD) techniques in a solid medium was reported. Steel was at first tufftrided and then titanium nitride coating treatment was performed in a powder mixture consisting of ferro-titanium, ammonium chloride and alumina at 1173, 1223 and 1273 K for 1-4 h. Titanium nitride layer thickness on the titanium nitride coated AISI 1020 steel ranged from 5.5 to 19.2 μm depending on treatment time and temperature. Layer growth kinetics was analyzed by measuring the depth of titanium nitride layer as a function of time and temperature. The kinetics equation of the reaction has also been determined with Arhenius equation K=K_oexp(−Q/(RT). The result showed that the diffusion coefficient (K) of the process increased with treatment temperature. Activation energy (Q) for TRD process was calculated as 187.09 kJ/mol. The diffusion coefficients (K) changed between 6.637×10⁻¹¹ and 2.097×10⁻¹⁰ cm²/s depending on the process temperature.     

Growth of highly textured aluminum films on LiTaO3 with optimized titanium intermediate layers

Ultra thin titanium films in the range of a few nanometers have been deposited on monocrystalline lithiumtantalate (LiTaO₃) followed by deposition of 400 nm pure aluminum (Al). Texture measurements by means of electron backscatter diffraction show that the thickness of the intermediate titanium (Ti) layer significantly influences texture and grain structure of the overlying Al film. Increasing the thickness of the Ti layer from 0 nm to 20 nm leads to a change of aluminums texture from unoriented polycrystalline over highly oriented in single direction to highly oriented in twin structure.     

Growth morphology and phase analysis of titanium-based coating produced by thermochemical method

In the present study, high-speed tool steel was used to coat titanium by pack cementation technique. Coatings show a growth morphology similar to that of the chemical vapor deposition method. Time and temperature of the coating affects its growth morphology. Coating obtained at low temperature (900 °C) yields morphology with growth of tiny particles while coating produced at high temperature (1000 °C) has a morphology with coarser particles. Phase structure of the layers also varies depending on the process time and temperature. Short coating duration yields TiC_0.3N_0.7 phase structure, whose composition is close to TiN whereas long coating duration combined with high temperature yields TiC_0.7N_0.3, whose composition is close to TiC. Mechanical properties such as hardness and resistance to abrasion also reflect changes in phase structures of different types of coating.     

The effects of boro-tempering heat treatment on microstructural properties of ductile iron

In this study, the effects of boro-tempering heat treatment on microstructural properties of ductile iron were investigated. Test samples with dimensions of 10 × 10 × 55 mm were boronized at 900 °C for 1, 3 and 5 h and then tempered at four different temperatures (250, 300, 350 and 450 °C) for 1 h. Both optical microscopy and scanning electron microscopy were used to reveal the microstructural details of coating and matrix of boro-tempered ductile iron. X-ray diffraction was used to determine the constituents of the coating layer. The boride layer formed on the surface of boro-tempered ductile cast iron is tooth shape form and consisted of FeB and Fe₂B phases. The thickness of boride layer increases as the boronizing time increases and tempering temperature decreases. Tempering temperature is more effective than boronizing time on the matrix structure. Boro-tempering heat treatment reduces the formation of lower and upper ausferritic matrix temperature according to classical austempering. This causes formation of upper ausferritic matrix in the sample when tempered at 300 °C. This is in contrast to general case which is the formation of lower ausferritic matrix via austempering at this temperature.     

Influence of the conversion coating on the corrosion of galvanized reinforcing steel

Galvanized reinforcing steel with a cerium conversion coating have been studied in Ca(OH)₂ saturated solution with and without chlorides. Electrochemical results reveal that cerium conversion coating provides an effective corrosion resistance compared to galvanized steel at short immersion times, 5 days without Cl⁻ ions and 2 days with Cl⁻ in the solution. The results suggest that cerium layer inhibits hydrogen evolution on the galvanized coating at early stages. At longer immersion times, galvanized steel with cerium conversion coating and galvanized steel describe similar corrosion behaviour in both electrolytes. There is not significant differences in the corrosion current density, about 5 μA/cm², due to the presence of chlorides ions in the Ca(OH)₂ saturated solution up to approximately 17 days of immersion. At longer immersion times, from 30 to 50 days, specimens in the chlorides containing solution exhibit higher corrosion activity than that recorded in the free Cl⁻ solution revealing that cerium layer cannot inhibit the localized attack promoted by chloride ions.     

Biomimetic whisker-shaped apatite coating of titanium powder

Biomimetic apatite coatings on chemically modified titanium powder have been processed and the resulting coating layers evaluated in terms of morphology, composition and structure, using TF-XRD, XPS, SEM, TEM and FTIR analysis. After 7 days immersion in a simulated body fluid (SBF), nanometer-sized fine precipitates with an amorphous whisker-like phase and a Ca/P atomic ratio of 1.94 were obtained on the external surface of the titanium particles. When the immersion time in SBF was extended to 16 days, the coating layer consisted of the whisker-like nanostructured crystals of carbonated hydroxyapatite with a atomic ratio of 3; in such a case, a double coating layer was developed. The double layer could be divided into two regions and could be clearly distinguished: an inner dense region (~200 nm in thickness) which may include hard agglomerated crystals and an outer less dense region (> 500 nm in thickness) in which crystals are loosely distributed.     

Optical properties of TiO2 thin films after Ag ion implantation

Metal plasma ion implantation has being successfully developed for improving the electronic and optical properties of semiconductor materials. Prior to deposition, a TiO₂ colloidal suspension was synthesized by microwave-induced thermal hydrolysis of the titanium tetrachloride aqueous solution. The TiO₂ thin film was optimized to obtain a high-purity crystalline anatase phase by calcinations at 550 °C. The TiO₂ coating was uniform without aggregation, which provided good photo conversion efficiency. Ag ion implantation into the as-calcined TiO₂ thin films was conducted with 1 × 10¹⁵ ~ 1 × 10¹⁶ ions/cm² at 40 keV. The peak position and intensity of the photoluminescence and UV-Vis absorption spectra are quite sensitive to Ag doping. The optical characterization showed a shift in optical absorption wavelength towards infrared ray side, which was correlated with the structure variation of the Ag⁺ implanted TiO₂. Due to the strong capability of forming compounds between the energetic silver ions and TiO₂, the photoluminescence emission and UV-Vis absorption efficiencies were improved.     

Industrially-styled room-temperature pulsed laser deposition of titanium-based coatings

Titanium-based compounds are widely used as coating materials for mechanical, tribological, electrical, optical, catalytic, sensoric, micro-electronical applications due to their exceptionally physical and chemical properties. Recently, the trend of using temperature-sensitive materials like polymers and tool steels with the highest hardness demands new low-temperature coating techniques for protective surface finishing as well as for surface functionalization, but up to now there is lack of industrially scaled vacuum coating techniques at temperatures below 50 °C. An alternative for overcoming this problem is the pulsed laser deposition (PLD) technique, which was up-scaled for industrial demands at Laser Center Leoben of JOANNEUM RESEARCH Forschungsgesellschaft mbH.The current paper summarizes the application of the industrially-scaled PLD technique on the deposition of the presently most important Ti-based coatings: metallic titanium, titanium nitride (TiN), titanium oxide (TiO₂) and titanium carbonitride (TiCN). PLD coating allows, even at room temperature, the formation of film structures of Zone-T type of Thornton's structure zone model, both on substrates aligned normal and parallel to the incident vapor flux. The high-energetic deposition conditions are revealed by the occurrence of (2 2 0) textures for the fcc TiN-based films. The dense grown structure affects advantageously the tribological behavior—generally, low wear rates and (for TiCN) very low friction coefficients were found. For TiO₂ coatings, growing as a mixture of β-TiO₂ and amorphous phases, the easily reproducible change of deposition parameters in the room-temperature PLD allows large differences in the optical transmission and electrical resistance.     

Effect of TiO2-SiO2 sol-gel coating on the cpTi-porcelain bond strength

The effects of TiO₂-SiO₂ sol-gel coating with different firing temperatures (300 °C, 500 °C, and 750 °C) on the cpTi-porcelain bond strength were investigated in the present study. Prior to applying the low-fusing dental titanium porcelain, the phase, surface morphology, surface roughness and static water contact angle of the intermediate layer were evaluated. The cpTi-porcelain bond strength was measured using the three-point flexure test according to ISO 9693 standard. Statistical analyses were made using one-way ANOVA and Dunnett-t test. Significantly higher bond strength of TiO₂-SiO₂/750 °C (specimens coated with TiO₂-SiO₂ sol-gel coating and fired at 750 °C for 1 h) when compared to the control group was observed (p < 0.05). No rutile phase was found in all the tested specimens coated with TiO₂-SiO₂ sol-gel coating. The surface morphology of the intermediate layer was apparently different with different firing temperatures. It was found that the static water contact angle of TiO₂-SiO₂/750 °C significantly decreased (p < 0.05). However, no markedly different R_a of TiO₂-SiO₂/500 °C and TiO₂-SiO₂/750 °C in comparison to that of the control group was observed (p > 0.05). The results show that the TiO₂-SiO₂ sol-gel coating fired at 750 °C for 1 h can notably improve the cpTi-porcelain bond strength and may be suitable for clinical use.     

An investigation on boriding kinetics of AISI 316 stainless steel

Boronizing was performed by using a solid medium of Ekabor powders at 1073, 1148 and 1223 K for 2, 4 and 8 h. After boronizing, the major dominant phase was found to be Fe₂B and the minors were CrB and Ni₂B. Boride coating resulted in smooth and dense feature confirmed by optical and SEM. The thickness of boride layer varied from 7 to 87 μm depending on the process time and temperature. Boride layer has a hardness of over 1700 HVN, while the substrate's hardness was about 180 HVN. The growth kinetics of boride layer was found to obey a parabolic rate demonstrating a solid diffusion limited process. The kinetic rates for different process times were plotted by using Arrhenius equation. From this measurement, the activation energy of boride growth for this study was determined as 199 kJ/mol. In addition, the possibility of predicting the iso-thickness of boride layer variation was studied and an empirical relationship between process parameters and boride layer thickness was established. EDS studies showed that Cr concentrated in the coating layer and Ni and Fe concentrated in the substrate.     

Hydroxyapatite coating by biomimetic method on titanium alloy using concentrated SBF

This article reports a biomimetic approach for coating hydroxyapatite on titanium alloy at ambient temperature. In the present study, coating was obtained by soaking the substrate in a 5 times concentrated simulated body fluid (5XSBF) solution for different periods of time with and without the use of CaO-SiO₂ based glass as a possible source of nucleating agent of apatite formation. Optical microscopic and SEM observations revealed the deposition of Ca-P layer on the titanium alloy by both the methods. Thickness of coating was found to increase with the increase in immersion time. The use of glass did not help the formation of apatite nuclei on the substrate and the coating obtained by this method was also not uniform. EDX analysis indicated that the coating consisted of Ca-P based apatite globules, mostly in agglomerated form, and its crystallinity was poor as revealed by XRD.     

Formation of chromium nitride layers on AISI 1010 steel by nitro-chromizing treatment

In this study, chromium nitride coating was realized on AISI 1010 steels by nitro-chromizing treatment. Steel samples were tufftrided at 575 °C for 2 h in the first step of the coating process, and then chromized by pack method in the powder mixture consisting of ferro-chromium, ammonium chloride and alumina at 1000 °C for 1-4 h. Samples were characterized by scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy, X-ray diffraction (XRD) analysis and Vickers micro-hardness tests. Coating layer formed on the steel samples is smooth and compact and well bonded to the steel matrix. The thickness of chromium nitride layer formed on the steel samples ranged from 5.16±1.48 to 13.45±1.73 μm, depending on the treatment time. The average micro-hardness value of the layer was 1789±59 HV_0.05. The layer consisted of Cr₂N and (Cr,Fe)₂N_(1−x) phases, according to XRD. EDS results showed that coating layer includes chromium and nitrogen.     

In situ composite coating of titania-hydroxyapatite on titanium substrate by micro-arc oxidation coupled with electrophoretic deposition processing

In situ composite coating of hydroxyapatite (HA)/TiO₂ were produced on titanium (Ti) substrate by micro-arc oxidation coupled with electrophoretic deposition (MAO&EPD) technique with different concentrations of HA particles in the 0.2 M NaOH electrolyte solution. The surface morphology and chemical composition of the hybrid coating were effected by HA concentration. The amount of HA particles incorporated into coating layer increased with increasing HA concentration used in the electrolyte solution. The corrosion behavior of the coating layer in simulated body fluids (SBF) was evaluated using a potentiodynamic polarization test. The corrosion resistance of the coated sample was increased compared to the untreated Ti sample. The in vitro bioactivity assessment showed that the MAO&EPD treated Ti substrate possessed higher apatite-forming ability than the untreated Ti. Moreover, the apatite-forming ability had a positive correlation with HA concentration. In addition, the cell behavior was also examined using cell proliferation assay and alkaline phosphatase ability. The coating formed at HA concentration of 5 g/L exhibited the highest cell ability.     

Failure analysis of a hip implant by using the FITNET fitness for service procedure

This paper analyses the failure of a hip implant, which occurred nine months after the corresponding surgery operation. The base material of the implant is a metallic matrix piece of alloy Ti-6Al-4V, coated by two surface layers; the first one is a 50 μm thick layer of pure porous titanium and the other one is a hidroxiapatite ceramic layer of 150 μm. The study uses scanning electron microscopy techniques and X-ray microanalysis in order to analyse the coatings and the state of the interphases, as well as the type of fracture present on the fracture surface. From this analysis, it is concluded that failure has been produced by a fatigue process initiated on imperfections and alumina inclusions (Al₂O₃) located in the interphase area between the base material and the titanium coating. An analysis has also been performed in order to justify the failure conditions and the life time of the implanted piece, based on the fracture mechanics approach of the FITNET structural integrity assessment procedure.     

Chromate conversion coating on Al-0.2 wt.% Fe alloy

SEM and TEM investigations revealed that the chromate coating developed rapidly over the macroscopic alloy surface. The coating thickness increased over the immersion period employed in this study and was about 50 nm and 200 nm for coatings formed after 30 s and 120 s respectively. The coating is composed of chromium compounds with aluminium compounds probably concentrated at the alloy/coating interface.