Surface modification of titanium by nano-TiO2/HA bioceramic coating

A nano-TiO₂/hydroxyapatite composite bioceramic coating was developed and applied to the surfaces of pure titanium discs by the sol-gel method. A TiO₂ anatase bioceramic coating was utilized in the inner layer, which could adhere tightly to the titanium substrate. A porous hydroxyapatite (HA) bioceramic coating was utilized in the outer layer, which has higher solubility and better short-term bioactivity. Conventional HA coatings and commercially pure titanium were used as controls. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to characterize the crystallization, surface morphology, and thickness of the coatings. The bioactivities of the coatings were evaluated by in vitro osteoblast cultures. Results showed that the nano-TiO₂/HA composite bioceramic coating exhibited good crystallization and homogeneous, nano-scale surface morphology. In addition, the nano-TiO₂/HA coating adhered tightly to the substrate, and the in vitro osteoblast cultures exhibited satisfactory bioactivity.     

The structure and mechanical properties of thick rutile-TiO2 films using different coating treatments

The hardness and Young's modulus of thick rutile-TiO₂ films were determined using a continuous stiffness measurement (CSM) technique in this study. Pure rutile-TiO₂ nanopowders (TH₂O, TFeSO₄ and TCuSO₄) were prepared using a modified homogeneous-precipitation process at low temperature (MHPPLT) method. The TiO₂ films were prepared from sols using 3% (w/w) of the prepared-TiO₂ suspension solution coated onto silicon wafers. After dip-coating was completed, the coatings were further treated by natural air-drying, water-vapor exposure, and calcination, respectively. An ellipsometry with a monochromator was used to measure the thickness and refractive index of the TiO₂ films, and a scanning electron microscopy (SEM) to determine their morphology. Three coatings of TH₂O, TFeSO₄ and TCuSO₄ demonstrated their refractive indexes of around 1.60 under three treatments. Volumetric expansion and thickness of the coatings should influence their refractive index. Furthermore, the continuous stiffness measurement (CSM) technique was used to perform nanoindentation testing on the hardness and Young's modulus of prepared rutile-TiO₂ coatings. The mean hardness and Young's modulus of three coatings increased with preparation temperature. In addition, the TH₂O coatings demonstrated greater hardness and modulus than those of TFeSO₄ and TCuSO₄ coatings in the natural air-drying condition. Surface cracking observed on the calcinated TFeSO₄ should be the reason why an obvious decrease of the mean hardness and Young's modulus appeared. Finally, two mechanical properties and related nanoindentation depth of the coatings were discussed in detail.     

Hydroxyapatite/titania composite bioactivity coating processed by sol-gel method

Hydroxyapatite/titania composite material was coated onto a titanium (pure Ti) substrate by sol-gel method. The hydroxyapatite (HA) and titania (TiO₂) sol were made from precursor and mixed together. The insertion of TiO₂ enhanced the chemical affinity and the physical consistency between HA and Ti substrate. The HA/TiO₂ composite coating adhered tightly to the Ti substrate. Owing to the insertion of TiO₂, the crystallinity of HA has been delayed. The specimens with HA/TiO₂ composite coatings were soaked into SBF, and displayed good bone-like apatite forming ability. The bioactivity of the composite HA coatings were tested in vitro by cell culture.     

Microstructure and anti-oxidation property of CrSi2-SiC coating for carbon/carbon composites

To protect carbon/carbon (C/C) composites from oxidation, a new type of oxidation protective coating has been produced by a two-step pack cementation technique. XRD and SEM analysis show, the coating obtained by the first step pack cementation was a porous β-SiC structure, and a new phase of CrSi₂ was generated in the porous SiC coating after heat-treatment according to the second step pack cementation process. Oxidation test shows that, the weight loss of the SiC coated C/C is up to 11.26% after 5 h oxidation in air at 1773 K, and the weight loss of the CrSi₂-SiC coated C/C composites is only 4.15% after oxidation in air at 1773 K for 34 h. The oxidation of C/C composites was primarily due to the reaction of C/C matrix and oxygen diffusing through the penetrable cracks in the coating.     

Electrodeposited nickel-cobalt composite coating containing MoS2

Ni-Co/MoS₂ composite coatings were prepared by electrodeposition in a Ni-Co plating bath containing nano-sized MoS₂ particles to be co-deposited. The polarization behavior of the composite plating bath was examined on a PAR-273A potentiostat/galvanostat device. The friction and wear behaviors of the Ni-Co/MoS₂ composite coatings were evaluated with UMT-2MT test rig in a ball-on-disk contact mode. The morphologies of the original and worn surfaces of the composite coatings were observed on scanning electron microscope (SEM). It was found that the introduction of MoS₂ nano-particulates in the electrolyte caused the shift towards larger negatives of the reduction potential of the Ni-Co alloy coating, and the co-deposited MoS₂ showed no significant effect on the electrodeposition process of the Ni-Co alloy coating. However, the co-deposited MoS₂ led to changes in the surface morphology and structure of the composite coating as well. Namely, the peak width of the Ni-Co solid solution for the composite coating is broader as compared to that of the Ni-Co alloy coating. The co-deposited MoS₂ particulates were uniformly distributed in the Ni-Co matrix and contributed to increase tribological properties of the Ni-Co alloy coating.     

Determination of silicone coating Young's modulus using atomic force microscopy

The polymerisation degree of thin polymer coatings was checked by following the variation of their local mechanical properties. Atomic force microscope (AFM) was used in an indentation mode to investigate the mechanical characteristics of silicone coatings on polycarbonate substrates. The evolution of Young's modulus of the silicone coatings was determined as a function of the polymer annealing time. We have used a relative method to measure Young's moduli, which involves a calibration step with a set of reference polymers. No variation was observed for the modulus of silicone coatings annealed during more than 40 min at 130 °C. This result indicates that over-heating does not modify the mechanical properties of the coating.     

Electrochemically assisted co-deposition of calcium phosphate/collagen coatings on carbon/carbon composites

Calcium phosphate (CaP)/collagen coatings were prepared on the surface of carbon/carbon (C/C) composites by electrochemically assisted co-deposition technique. The effects of collagen concentration in the electrolyte on morphology, structure and composition of the coatings were systematically investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transformed infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The adhesive strength of the coatings was also evaluated by scratch tests and tensile bond tests. It was demonstrated that the coatings of three-dimensional collagen network structure was formed on the C/C composites from the electrolyte containing collagen. The surface of the collagen network was covered by uniform CaP aggregates. The coatings were actually composites of CaP and collagen. Hydroxyapatite (HA) was a favorable composition in the coatings with the increase of the collagen concentration in the electrolyte. The formed collagen network increased the cohesive and adhesive strength of the coatings. The adhesive strength between the coatings and substrates increased as the collagen concentration in the electrolyte increased. The coatings prepared at the collagen concentration of 500 mg/L in the electrolyte were not scraped off until the applied load reached 32.0 ± 2.2 N and the average tensile adhesive strength of the coatings was 4.83 ± 0.71 MPa. After C/C coated with composite coatings (500 mg/L) being immersed in a 10⁻³ M Ca (OH)₂ solution at 30-33 °C for 96 h, nano-structured HA/collagen coatings similar to the natural human bone were obtained on the C/C.     

Characterization and corrosion behavior of hydroxyapatite/zirconia composite coating on NiTi fabricated by electrochemical deposition

NiTi alloy is used as biomaterial due to its unique properties, but the high content of Ni (about 50 at.%) in biomedical NiTi is concerned. Hydroxyapatite and hydroxyapatite/zirconia composite coatings were directly electrodeposited on NiTi alloy surface. The coated samples were characterized using X-ray diffraction, scanning electron microscopy, infrared spectroscopy, bonding strength test, polarization and EIS. Results showed that when ZrO₂ was added into the electrolyte, morphology of HAP was changed from thin flake-flower-like crystals to needle-flower-like crystals, and coating was denser. Besides, HAP crystal grains in the coating were preferentially arranged in the [0 0 1] direction. Addition of ZrO₂ could improve the bonding strength between the coating and the substrate. Corrosion resistance of NiTi in the simulated body fluid at 37 °C was significantly improved by almost 60 times by electrodeposition of the hydroxyapatite/zirconia composite coating.     

Texture structure and ablation behavior of TaC coating on carbon/carbon composites

TaC coatings with hybrid, (2 0 0) and (2 2 0) texture structure were prepared on carbon/carbon (C/C) composites by isothermal chemical vapor deposition with TaCl₅-Ar-C₃H₆ system. The residual stress, hardness and ablation behaviors of the different coatings were characterized by Raman spectra, nano-indentation and oxyacetylene flame ablation machine respectively. Results shown tensile stress exists in the TaC coatings and increases when texture orientation turns from hybrid to (2 2 0) and (2 0 0), while nano-indentation hardness of the coatings also obeys the same trend. The deposited coatings could improve the ablation-resistance properties of C/C composites effectively. The texture structure also had great effects on the ablation properties and ablation morphologies of the coatings. The mass ablation rate obviously decreases when the texture structure changes from hybrid orientation to (2 0 0) and (2 2 0) orientations. The hybrid orientation and (2 0 0) texture coatings exhibit coarse oxide morphologies with crater or some breakage existed; while the (2 2 0) texture coating shows dense, molten oxide morphology. The main ablation behaviors of the hybrid, (2 0 0) and (2 2 0) texture TaC coatings are oxidation and particle denudation and block denudation, oxidation and block denudation, oxidation and mechanical erosion and block denudation, respectively.     

Influence of preparation technology on the microstructure and anti-oxidation property of SiC-Al2O3-mullite multi-coatings for carbon/carbon composites

Oxidation protective SiC-Al₂O₃-mullite multi-coatings for carbon/carbon (C/C) composites were prepared with a two-step pack cementation process. The influence of preparation temperature and SiO₂/Al₂O₃ ratio of the pack powder on the phase, microstructure and oxidation resistance of the multi-coatings were investigated. It showed that the multi-coatings that contained mullite could be produced at 1700-1800 °C. A denser coating surface was acquired with the decrease of SiO₂/Al₂O₃ ratio in the pack chemistries while a little damnification to the interface of the coating and C/C substrate. The as-prepared coating could effectively protect C/C composites from oxidation at 1600 °C for 81 h.     

Preparation and ablation properties of ZrC-SiC coating for carbon/carbon composites by solid phase infiltration

A novel ZrC-SiC coating was prepared on carbon/carbon (C/C) composites surface by solid phase infiltration and the ablation properties of the ZrC-SiC coated C/C composites under oxyacetylene flame were studied. The results show that the coating prepared on the condition of optimum process parameters exhibits dense surface and outstanding anti-ablation ability. After ablation for 20 s, the mass ablation rates of the coated C/C composites can be lowered to 2.36 × 10⁻³ g/s, 37.1% reduction compared with uncoated C/C composites. The oxide layer composed of ZrO₂ and SiO₂ acts as oxygen diffusion barrier and the evaporation of ZrO₂ and SiO₂ absorbs a great amount of heat from the flame and reduces the erosive attack on the coating.     

Bioactive coating on titanium implants modified by Nd:YVO4 laser

Apatite coating was applied on titanium surfaces modified by Nd:YVO₄ laser ablations with different energy densities (fluency) at ambient pressure and atmosphere. The apatites were deposited by biomimetic method using a simulated body fluid solution that simulates the salt concentration of bodily fluids. The titanium surfaces submitted to the fast melting and solidification processes (ablation) were immersed in the simulated body fluid solution for four days. The samples were divided into two groups, one underwent heat treatment at 600 °C and the other dried at 37 °C. For the samples treated thermally the diffractograms showed the formation of a phase mixture, with the presence of the hydroxyapatite, tricalcium phosphate, calcium deficient hydroxyapatite, carbonated hydroxyapatite and octacalcium phosphate phases. For the samples dried only the formation of the octacalcium phosphate and hydroxyapatite phases was verified. The infrared spectra show bands relative to chemical bonds confirmed by the diffraction analyses. The coating of both the samples with and without heat treatment present dense morphology and made up of a clustering of spherical particles ranging from 5 to 20 μm. Based on the results we infer that the modification of implant surfaces employing laser ablations leads to the formation of oxides that help the formation of hydroxyapatite without the need of a heat treatment.     

Influence of remaining C on hardness and emissivity of SiC/SiO2 nanocomposite coating

SiC/SiO₂ nanocomposite coating was deposited by electron beam-physical vapor deposition (EB-PVD) through depositing SiC target on pre-oxidized 316 stainless steel (SS) substrate. High melting point component C remained and covered on the surface of ingot after evaporation. When SiC ingot was reused, remaining C had an effect on the composition, hardness and emissivity of SiC/SiO₂ nanocomposite coating. The composition of ingot and coating was studied by X-ray photoelectron spectroscopy (XPS). The influence of remaining C on hardness and spectral normal emissivity of SiC/SiO₂ nanocomposite coating was investigated by nanoindentation and Fourier transform infrared spectrum (FTIR), respectively. The results show that remaining C has a large effect on hardness and a minor effect on spectral normal emissivity of SiC/SiO₂ nanocomposite coating.     

A SiC whisker-toughened SiC-CrSi2 oxidation protective coating for carbon/carbon composites

A SiC whisker-toughened SiC-CrSi₂ oxidation protective coating was prepared on the surface of C/C composites by a two-step technique of slurry and pack cementation. The oxidation protective ability and thermal stress resistance of the coating exhibit the trend of increase first and decrease afterwards as the SiCw content increases from 0 to 20 wt.%. The compound effect of SiCw and CrSi₂ on the oxidation protective ability of SiC coating is better than their individual ones.     

Adhesive and cohesive properties by indentation method of plasma-sprayed hydroxyapatite coatings

Adhesive and cohesive properties of the plasma-sprayed hydroxyapatite (HA) coatings, deposited on Ti-6Al-4V substrates by varying the plasma power level and spray distance (SD), were evaluated by an indentation method. The crystallinity and the porosity decreased with increasing both of these two parameters. The microhardness value, Young's modulus (E) and coating fracture toughness (K_C) were found to increase with a combinational increase in spray power and SD. The Knoop and Vickers indentation methods were used to estimate E and K_C, respectively. The critical point at which no crack appears at the interface was determined by the interface indentation test. This was used to define the apparent interfacial toughness (K_Ca) which is representative of the crack initiation resistance of the interface. It was found that K_Ca reaches to a maximum at a medium increase in both spray power and SD, while other mechanical properties of the coatings reaches to the highest value with further increase in these two plasma parameters. The tensile adhesion strength of the coatings, measure by the standard adhesion test, ISO 13779-4, was shown to alter in the same manner with K_Ca results. It was deduced that a combinational increase in spray power and SD which leads to a higher mechanical properties in the coatings, does not necessarily tends to a better mechanical properties at the interface.     

Preparation and properties of electroless Ni-P-SiO2 composite coatings

Dispersible SiO₂ nanoparticles were co-deposited with electroless Ni-P coating onto AISI-1045 steel substrates in the absence of any surfactants in plating bath. The resulting Ni-P/nano-SiO₂ composite coatings were heat-treated for 1 h at 200 °C, 400 °C, and 600 °C, respectively. The hardness and wear resistance of the heat-treated composite coatings were measured. Moreover, the structural changes of the composite coatings before and after heat treatment were investigated by means of X-ray diffraction (XRD), while their elemental composition and morphology were analyzed using an energy dispersive spectrometer (EDS) and a scanning electron microscope (SEM). Results show that co-deposited SiO₂ particles contributed to increase the hardness and wear resistance of electroless Ni-P coating, and the composite coating heat-treated at about 400 °C had the maximum hardness and wear resistance.     

One-step solvothermal preparation of TiO2/C composites and their visible-light photocatalytic activities

Preparation of anatase type titania nanoparticles and their carbon modification were synchronously achieved by the solvothermal method with glycerol as the carbon source. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and diffuse reflectance spectra (DRS). It was found that the glycerol/ethanol ratio affected significantly the morphology and properties of TiO₂/C composites. The TiO₂/C composite which was obtained in the solution with the glycerol/ethanol ratio of 5/75, contained 1.2 mass% carbon and exhibited both superior adsorption capability and visible-light photocatalytic activity. Contrary to this, samples prepared in the solution with higher glycerol/ethanol ratio, exhibited lower photocatalytic activity similar to that of the titania without carbon modification. It was suggested that excess addition of glycerol might contribute to large amounts of carbonaceous species and severe aggregation of the as-prepared samples, and thus reduced the surface area. As a result, the adsorption capability and visible-light photocatalytic activity increased at first and then decreased with the increase of glycerol addition. Present study provided a facile one-step method to obtain TiO₂/C composites with a controllable carbon content and photocatalytic performance under mild temperature.     

Surface modification of pure titanium by hydroxyapatite-containing composite coatings

Micro-arc oxidation (MAO) is commonly applied to modify the surface of titanium (Ti)-based medical implants with a bioactive and porous Ti oxide (TiO₂) coating. The study reports a new method of incorporating hydroxyapatite (HA) within the TiO₂ coating by MAO and alkali heat treatment (AHT) in the solution containing Ca ion and P ion. The morphology, composition and phase composition of the coatings were analyzed with scanning electron microscopy with energy-dispersive X-ray spectrometer and X-ray diffraction. Surface topography and roughness of the coatings were investigated by atomic force microscopy operated in the tapping mode. The results showed that TiO₂-based coatings were obtained on pure Ti by MAO with an electrolyte containing Ca ion and P ion; the prepared MAO coatings were mainly composed of Ca, P, O and Ti. AHT transformed Ca and P to HA crystals. In conclusion, the TiO₂/HA composite coatings can be obtained on the surface of pure Ti by MAO and AHT, and the addition of Ca ion and P ion to the AHT solution contributed to the formation of HA.     

Fabrication and characterization of rod-like nano-hydroxyapatite on MAO coating supported on Mg-Zn-Ca alloy

The poor corrosion resistance of magnesium alloys is a dominant problem that limits their clinical application. In order to solve this challenge, micro-arc oxidation (MAO) was used to fabricate a porous coating on magnesium alloys and then electrochemical deposition (ED) was done to fabricate rod-like nano-hydroxyapatite (RNHA) on MAO coating. The cross-section morphology of the composite coatings and its corresponding energy dispersion spectroscopy (EDS) surficial scanning map of calcium revealed that HA rods were successfully deposited into the pores. The three dimensional morphology and scanning electron microscopy (SEM) image of the composite coatings showed that the distribution of the HA rods was dense and uniform. Atomic force microscope (AFM) observation of the composite coatings showed that the diameters of HA rods varied from 95 nm to 116 nm and the root mean square roughness (RMS) of the composite coatings was about 42 nm, which were favorable for cellular survival. The bonding strength between the HA film and MAO coating increased to 12.3 MPa, almost two times higher than that of the direct electrochemical deposition coating (6.3 MPa). Compared with that of the substrate, the corrosion potential of Mg-Zn-Ca alloy with composite coatings increased by 161 mV and its corrosion current density decreased from 3.36 × 10⁻⁴ A/cm² to 2.40 × 10⁻⁷ A/cm² which was due to the enhancement of bonding strength and the deposition of RNHA in the MAO pores. Immersion tests were carried out at 36.5 ± 0.5 °C in simulated body fluid (SBF). It was found that RNHA can induce the rapid precipitation of calcium orthophosphates in comparison with conventional HA coatings. Thus magnesium alloy coated with the composite coatings is a promising candidate as biodegradable bone implants.     

Double-ceramic-layer thermal barrier coatings based on La2(Zr0.7Ce0.3)2O7/La2Ce2O7 deposited by electron beam-physical vapor deposition

Double-ceramic-layer (DCL) thermal barrier coatings (TBCs) of La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3) and La₂Ce₂O₇ (LC) were deposited by electron beam-physical vapor deposition (EB-PVD). The composition, interdiffusion, surface and cross-sectional morphologies, cyclic oxidation behavior of DCL coating were studied. Energy dispersive spectroscopy and X-ray diffraction analyses indicate that both LZ7C3 and LC coatings are effectively fabricated by a single LZ7C3 ingot with properly controlling the deposition energy. The chemical compatibility of LC coating and thermally grown oxide (TGO) layer is unstable. LaAlO₃ is formed due to the chemical reaction between LC and Al₂O₃ which is the main composition of TGO layer. Additionally, the thermal cycling behavior of DCL coating is influenced by the interdiffusion of Zr and Ce between LZ7C3 and LC coatings. The failure of DCL coating is a result of the sintering of LZ7C3 coating surface, the chemical incompatibility of LC coating and TGO layer and the abnormal oxidation of bond coat. Since no single material that has been studied so far satisfies all the requirements for high temperature applications, DCL coating is an important development direction of TBCs.