Electrochemical synthesis of calcium carbonate coatings on stainless steel substrates

Calcite CaCO₃ has been electrocrystallized on stainless steel substrates by the galvanostatic cathodic reduction of aqueous calcium bicarbonate solutions. The deposition is controlled by pH changes occurring close to the cathode due to electrogeneration of base. The deposit morphology varies from facetted rhombs observed at low (1-20 mA cm⁻²) current densities to corner-rounded particles observed at high (40 mA cm⁻²) current densities.     

Electrodeposition of dicalcium phosphate dihydrate coatings on stainless steel substrates

Cathodic reduction of an aqueous solution containing dissolved calcium and phosphate ions results in the deposition of micrometer thick CaHPO₄·2H₂O (dicalcium phosphate dihydrate) coatings on stainless steel substrates. The coating obtained at a low deposition current (8 mA cm^???2) comprises lath-like crystallites oriented along 020. The 020 crystal planes are non-polar and have a low surface energy. At a high deposition current (12 mA cm^???2), platelets oriented along 121? are deposited. CaHPO₄·2H₂O is an important precursor to the nucleation of hydroxyapatite, the inorganic component of bones. Differently oriented CaHPO₄·2H₂O coatings transform to hydroxyapatite with different kinetics, the transformation being more facile when the coating is oriented along 121?. These observations have implications for the development of electrodeposited biocompatible coatings for metal endoprostheses for medical applications.     

The effect of hydroxyapatite thickness on metal ion release from stainless steel substrates

The corrosion behaviour of 350 stainless steel coated with hydroxyapatite, HA, by plasma spraying was studied in Hank's Balanced Salt Solution, HBSS, and compared with that of polished passivated surfaces. Two different nominal thicknesses, 50 m and 200 m, corresponding to what one might consider a thin and a thick coating, respectively, were used. Only HA coatings with a thickness of 200 m were capable of reducing the electrical charge consumed at constant potential to values lower than those measured for polished surfaces. However, no HA detachment occurred for both thicknesses, as opposed to what has been found in a previous work [1] with Ti6Al4V alloy coated with 50 m HA. No iron, chromium or nickel were detected in solution by Atomic Absorption Spectroscopy with electrothermal atomization (ET-AAS). These elements were also absent from the bulk of the HA coating, both after a 6 month immersion period and electrochemical accelerated tests. The data indicate that in spite of the relatively low corrosion resistance of stainless steel as compared to that of titanium alloys, a thin (50 m) HA coating prevents the release of metal ions, while remaining adherent to the substrate.     

Strain rate sensitivity of hydroxyapatite coatings

The strain-rate sensitivity of strength is a significant factor to evaluate the deformation mode of crystalline materials. The strain rate sensitivity of hardness is experimentally investigated here for hydroxyapatite coatings that are sputter deposited onto titanium-coated silicon wafers. These biocompatible HA coatings can provide a strong, dense interface between metal alloy implants and porous hydroxyapatite that can help in-growth of tissue. The interface to the metal alloy implant is important to transfer stress during loading. So, it is very important to know the behavior of the coating under different conditions of loading. Our dynamic test procedure now takes advantage of nanoscratch testing to measure the change in scratch hardness of the coating over a strain rate range that may well simulate the mechanical loading conditions found at the interface between implants and hydroxyapatite coatings.     

Consolidation of nanocrystalline hydroxyapatite powder

The effect of sintering temperature on the sinterability of synthesized nanocrystalline hydroxyapatite (HA) was investigated. The starting powder was synthesized via a novel wet chemical route. HA green compacts were prepared and sintered in atmospheric condition at various temperatures ranging from 900–1300°1C. The results revealed that the thermal stability of HA phase was not disrupted throughout the sintering regime employed. In general, the results showed that above 98% of theoretical density coupled with hardness of 7.21 GPa, fracture toughness of 1.17 MPa m^1/2 and Young’s modulus of above 110 GPa were obtained for HA sintered at temperature as low as 1050 1C. Although the Young’s modulus increased with increasing bulk density, the hardness and fracture toughness of the sintered material started to decline when the temperature was increased beyond 1000–1050 °C despite exhibiting high densities > 98% of theoretical value. The occurrence of this phenomenon is believed to be associated with a thermal-activated grain growth process.     

Pulsed laser deposition of silicon-substituted hydroxyapatite coatings

Thin films of Si-substituted hydroxyapatite (Si-HA) were deposited on Si and Ti substrates by pulsed laser deposition (PLD), in the presence of a water vapour atmosphere. The PLD ablation targets were made with different mixtures of commercial carbonated HA and Si powder, in order to produce the Si-HA thin films. The physicochemical properties of the coatings and the incorporation of the Si into the HA structure was studied by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The Si atoms were successfully incorporated into the HA structure, and were found to be in the form of SiO₄⁴⁻ groups, principally displacing carbonate groups off the HA structure.     

Histomorphological study of bone response to hydroxyapatite coating on stainless steel

Bone response to hydroxyapatite coating on stainless steel has not been so extensively tested in animals as it happened for other metallic substrate, like Ti6Al4V. For this reason, authors performed an in vivo histomorphological electron microscopic study of hydroxyapatite coating on duplex stainless steel cylinders, to gather further evidences on the characters of bone apposition at the interface. Sixteen HA-coated cylinders were implanted in the distal femur of New Zealand White rabbits. Comparison with uncoated controls was made. Retrieval steps were at: 4, 8, 26 and 34 weeks. Specimens were analyzed in a Jeol JSM 6301F scanning electron microscope. The response to HA-coated samples has a morphological character of tight apposition between bone and coating. Osteocytic lacunae may be found few microns close to the coating and newly formed bone is extremely interlocked with it so that even an higher magnification electron-microscopy cannot resolve any discontinuity in between. Pictures of physiological bone-turnover are distinguishable at the bone-coating interface; areas of well preserved coating may be present together with areas where local exfoliation or fragmentation has already completely exposed the metallic substrate. On the opposite in uncoated samples, despite a morphological picture of properly formed bone, the largest area of the metal has no direct apposition with it.     

The effect of manganese oxide on the sinterability of hydroxyapatite

The sinterability of manganese oxide (MnO₂) doped hydroxyapatite (HA) ranging from 0.05 to 1 wt% was investigated. Green samples were prepared and sintered in air at temperatures ranging from 1000 to 1400 °C. Sintered bodies were characterized to determine the phase stability, grain size, bulk density, hardness, fracture toughness and Young's modulus. XRD analysis revealed that the HA phase stability was not disrupted throughout the sintering regime employed. In general, samples containing less than 0.5 wt% MnO₂ and when sintered at lower temperatures exhibited higher mechanical properties than the undoped HA. The study revealed that all the MnO₂-doped HA achieved >99% relative density when sintered at 1100-1250 °C as compared to the undoped HA which could only attained highest value of 98.9% at 1150 °C. The addition of 0.05 wt% MnO₂ was found to be most beneficial as the samples exhibited the highest hardness of 7.58 GPa and fracture toughness of 1.65 MPam^1/2 as compared to 5.72 GPa and 1.22 MPam^1/2 for the undoped HA when sintered at 1000 °C. Additionally, it was found that the MnO₂-doped samples attained E values above 110 GPa when sintered at temperature as low as 1000 °C if compared to 1050 °C for the undoped HA.     

The effect of manganese oxide on the sinterability of hydroxyapatite

The sinterability of manganese oxide (MnO₂) doped hydroxyapatite (HA) ranging from 0.05 to 1 wt% was investigated. Green samples were prepared and sintered in air at temperatures ranging from 1000 to 1400°C. Sintered bodies were characterized to determine the phase stability, grain size, bulk density, hardness, fracture toughness and Young’s modulus. XRD analysis revealed that the HA phase stability was not disrupted throughout the sintering regime employed. In general, samples containing less than 0.5 wt% MnO₂ and when sintered at lower temperatures exhibited higher mechanical properties than the undoped HA. The study revealed that all the MnO₂-doped HA achieved > 99% relative density when sintered at 1100–1250 °C as compared to the undoped HA which could only attained highest value of 98.9% at 1150 °C. The addition of 0.05 wt% MnO₂ was found to be most beneficial as the samples exhibited the highest hardness of 7.58 GPa and fracture toughness of 1.65 MPam^1/2 as compared to 5.72 GPa and 1.22 MPam^1/2 for the undoped HA when sintered at 1000 °C. Additionally, it was found that the MnO₂-doped samples attained E values above 110 GPa when sintered at temperature as low as 1000 °C if compared to 1050 °C for the undoped HA.     

Superhydrophilic textured-surfaces on stainless steel substrates

Aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) is used to produce micro/nano-textured surfaces on stainless steel substrates at low temperatures for altering the wetting property of the substrates. The micro/nano-textured surfaces were characterized using scanning electron microscopy, X-ray spectroscopy, and X-ray diffraction. The wetting properties of the textured surfaces were characterized by water contact angle measurements. It was found that AIC of a-Si changes the apparent contact angles of stainless steel substrates from 90° to about 0°, measured 0.5 s after a water droplet drops on the surfaces. The study also shows that a superhydrophilic textured surface can be converted to a highly hydrophobic surface with an apparent contact angle of 145° by coating the surface with a layer of octadecyltrichlorosilane.     

Characterization of transition joints of commercially pure titanium to 304 stainless steel

Solid-state direct bonding between commercially pure titanium and type 304 austenitic stainless steel has been carried out in the temperature range of 850–950 °C, under a uniaxial pressure of 3 MPa for 1 h. The diffusion bonds have been evaluated using light microscopy, electron probe microanalysis (EPMA), X-ray diffraction (XRD) technique and tensile testing. Light microscopy shows that different intermediate layers are formed in the reaction zone, and the width of these layers increases with an increase in bonding temperature. EPMA revealed that, at any particular bonding temperature, Ti traverses a minimum distance in the 304 stainless steel side, whereas Fe, Cr and Ni travel comparatively larger distances in the Ti side. This microanalysis also indicated different step formations in the concentration profile of Ti, Fe and Cr over different composition ranges in the diffusion zone indicating formation of intermetallic phases that were detected by XRD. Brittle intermetallic phases lower the strength and ductility of the diffusion bonded couples significantly. Best room temperature tensile strength, 217 MPa, has been obtained at 850 °C processing temperature due to minimal deleterious effects.     

Deposition and dielectric properties of CaCu3Ti4O12 thin films deposited on Pt/Ti/SiO2/Si substrates using radio frequency magnetron sputtering

Polycrystalline CaCu₃Ti₄O₁₂ thin films were deposited on Pt(111)/Ti/SiO₂/Si substrates using radio frequency magnetron sputtering. The phase formation and the physical quality of the films were crucially dependent on the substrate temperature and oxygen partial pressure. Good quality films were obtained at a substrate temperature of 650 °C and 4.86 Pa total pressure with 1% O₂. The dielectric constant (∼ 5000 at 1 kHz and 400 K) of these films was comparable to those obtained by the other techniques, eventhough, it was much lower than that of the parent polycrystalline ceramics. For a given temperature of measurements, dielectric relaxation frequency in thin film was found to be much lower than that observed in the bulk. Also, activation energy associated with the dielectric relaxation for the thin film (0.5 eV) was found to be much higher than that observed in the bulk ceramic (0.1 eV). Maxwell-Wagner relaxation model was used to explain the dielectric phenomena observed in CaCu₃Ti₄O₁₂ thin films and bulk ceramics.     

Diffusion bonding between commercially pure titanium and micro-duplex stainless steel

Diffusion-bonded joints between commercially pure titanium and micro-duplex stainless steel were prepared in the temperature range of 800–950 °C for 1.5 h under 3 MPa uniaxial load in vacuum. The diffusion bonds were characterized using light and scanning electron microscopy. The composition of the reaction products were determined by energy dispersive spectroscopy. Up to 850 °C, -Fe + λ and λ + FeTi phase mixtures were formed at diffusion interface; however -Fe + λ, λ + FeTi and FeTi + β-Ti phases mixtures were formed at 900 °C and above. The presence of these intermetallics was confirmed by X-ray diffraction technique. The maximum tensile strength of 96% of Ti and shear strength of 81% of Ti along with 6.9% ductility were obtained for the diffusion couple processed at 850 °C due to the finer width of intermetallic phases. With a rise in the joining temperature the bond strength drops owing to an increase in the width of reaction products.     

Bioactive nanocomposite coatings of collagen/hydroxyapatite on titanium substrates

Collagen/hydroxyapatite (HA) nanocomposite thin films containing 10, 20, and 30 wt.% HA were prepared on commercially pure titanium substrates by the spin coating of their homogeneous sols. All of the nanocomposite coatings having a thickness of ∼7.5 μm exhibited a uniform and dense surface, without any obvious aggregation of the HA particles. A minimum contact angle of 36.5° was obtained at 20 wt.% HA, suggesting that these coatings would exhibit the best hydrophilicity. The in vitro cellular assays revealed that the coating treatment of the Ti substrates favored the adhesion of osteoblast-like cells and significantly enhanced the cell proliferation rate. The cells on the nanocomposite coatings expressed much higher alkaline phosphatase (ALP) levels than those on the uncoated Ti substrates. Increasing the amount of HA resulted in a gradual improvement in the ALP activity. The nanocomposite coatings on Ti substrates also exhibited much better cell proliferation behaviors and osteogenic potentials than the conventional composite coatings with equivalent compositions, demonstrating the greater potential of the former as implant materials for hard tissue engineering.     

Characterization of electrophoretic chitosan coatings on stainless steel

Electrophoretic chitosan deposits on stainless steel AISI 316 L were produced and characterized. The coating quality (thickness, defectiveness, corrosion protection ability) was seen to depend on the electric field used for EPD. Corrosion studies in concentrated simulated body fluid (SBF5) demonstrated that the surface characteristics of AISI 316 L can be positively influenced by the chitosan coating.     

The process of electrochemical deposited hydroxyapatite coatings on biomedical titanium at room temperature

Calcium phosphate (CaP) ceramics, especially hydroxyapatite (HA), have received much attention and have been clinically applied in orthopaedics and dentistry due to their excellent biocompatibility. Among several methods for preparing HA coating, electrochemical deposition is a relatively new and possible process. However, documented electrochemical processes were conducted at elevated temperature. In this study, uniform HA coatings have been directly deposited on titanium at room temperature. X-ray diffractometry (XRD) results demonstrated that dicalcium phosphate dihydrate (CaHPO₄·2H₂O, DCPD) was the main component of the coating deposited at lower current densities (1 and 5 mA/cm²). HA structure was obtained at current density above 10 mA/cm² and remained stable after heat treatment at 100–600 °C for 1 h. Part of HA phase was transformed into β-TCP and became a biphasic calcium phosphate coating after annealing at 700 °C. Scratch tests showed that HA coating was not scraped off until a shear stress of 106.3 MPa. Coatings deposited at room temperature exhibited stronger adhesion than those at elevated temperature. HA coating revealed a dense inner layer and rough surface morphology which could fulfill the requisition of implant materials and be adequate to the attachment of bone tissue.     

The observation of unipolar arc damage on stainless steel and TiC coatings on stainless steel

Unipolar arcing represents a plasma-surface interaction process leading to surface damage and metal impurity influx in tokamaks. It develops if the sheath potential formed in the plasma-surface contact is high enough to ignite and sustain a micro-arc. A laser-produced plasma was used for a comparison of the arc damage produced on stainless steel surfaces with that on surfaces protected with a coating of TiC a few microns thick. Smooth coatings of superior-quality TiC were produced by the activated reactive evaporation process, which is a plasma-assisted physical vapor deposition process. For a sufficiently high electron temperature in the laser-produced plasma a large number (about 300 000 cm^?2) of unipolar arc craters were observed on the stainless steel surface which had been exposed to the expanding laser-produced plasma cloud for a few hundred nanoseconds. In comparison, no similar arc craters were detected on the surfaces protected by the TiC coating which showed minimum damage limited to the laser beam impact area.     

Al particles size effect on the microstructure of the co-deposited Ni-Al composite coatings

Micrometer and nanometer Al particles were codeposited with nickel by electrodeposition from a nickel sulfate bath containng a similar content of Al particles. The effect of Al particles size on the microstructure of the electrodeposited Ni-Al composite coatings was analyzed. The results indicated that at a given Al particles content the incorporation of Al nanoparticles instead of the microscale countparts significantly increased the particle number per unit volume in the composite, led to a more homogeneous distribution of particles, and finer Ni grains due to nucleation of smaller Ni grains on the surface of Al nanoparticles, which changed the direction of the local Ni growth and caused the formation of a fine equiaxed grain structure.