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Alumina coatings were deposited on 6061 aluminum alloy substrates using plasma electrolytic oxidation (PEO) in an alkaline silicate electrolyte. Different electrical parameters, including frequency, duty cycle, and current density were applied to obtain PEO coatings. Scanning electron microscopy (SEM) and microhardness tests were used to investigate the coating microstructure and properties. For each sample, the voltage–time response of the PEO process was obtained and compared with surface morphology, and coating cross sections and thicknesses. Special consideration was given to the connection between the electrical parameters, different stages of the PEO process, and the morphology and microstructure of the coatings. Applying different electrical parameters changed the duration and ratio of the PEO stages in the voltage–time response curve and affected the growth rate, surface morphology, microstructure and microhardness of the coatings. We report the new finding of a correlation between the stage of the PEO process and the microstructure and morphology of the coating.

Ceramic coatings were created on the surface of 6061 aluminum alloy using a plasma electrolytic oxidation (PEO) process employing a pulsed direct current (DC) power mode in an alkaline electrolyte. The effect of electrical parameters including frequency and duty cycle on the microdischarge behavior and coating growth was investigated at constant current. Surface features of coatings were studied using scanning electron microscopy. Energy dispersive spectroscopy was employed to investigate elemental distribution on the coating surfaces and cross-sections. Applying lower duty cycles was found to result in increased breakdown voltages and microdischarges with higher spatial density and lower intensity. Further, applying a lower duty cycle was also found to promote the uniformity of silicon distribution in the coating. Based on these new findings, a new conceptual model is proposed to explain the concentration distribution of Si on the surface of coatings prepared at different duty cycles.

Titanium/fluorapatite (Ti/FA) composites with various FA additions were fabricated by powder metallurgy. The decomposition of FA during sintering was accelerated by the presence of Ti. The main reaction products of FA and Ti were identified as CaO, Ti phosphides, and CaTiO3. The addition of FA significantly inhibited the densification of Ti. The in vitro bioactivity of the composites was evaluated in a simulated body fluid (SBF). After immersion into the SBF, all the Ti/FA composites induced nucleation and growth of bone-like carbonated apatite on the surface. Co-precipitation of CaCO3 and Mg(OH)2 was also detected on the surface of the composite with high FA addition at an early stage of immersion. Furthermore, the release of fluorine ions from the composite was confirmed, which could promote bone regeneration and retard the formation of caries in the biological environment. The in vitro behavior was attributed to multiple factors, including the surface conditions and the constituents of the composite. The results demonstrated that the Ti/FA composites were bioactive in nature even with a low FA addition and they could introduce the benefit of fluorine ions in the service. © 2013.

To improve the bioactivity of Ti6Al4V alloy for use as a load-bearing hard tissue replacement, titanium/hydroxyapatite (Ti/HA) composites were bonded to a Ti6Al4V substrate by a novel cladding method. With the aid of a silver foil as the interlayer and an external pressure during sintering, the interfaces between the composites and the substrate were free of defects. The bioactivity of the fabricated materials was evaluated in the simulated body fluid (SBF) and the results demonstrated that the materials could induce nucleation and growth of bone-like apatite in the SBF. Factors that contributed to the bioactivity of the materials were discussed. The release of Ag + ions from the materials was also detected, which is expected to impart antibacterial effect after implantation, and further enhance the functionalities of the materials.

To improve the bioactivity of Ti6Al4V alloy, an innovative cladding method has been developed to bond a Ti/fluorapatite (FA) composite onto the alloy for load-bearing applications. With the aid of a silver interlayer and external pressure during sintering, a defect-free interface between the composite and the substrate was obtained. The fabricated materials were bioactive and could induce the nucleation and formation of bone-like carbonated apatite after immersed in the simulated body fluid (SBF). Functional ions, such as Ag+ and F−, were released from the materials during immersion, which could impart favourable activities for the implant. This work demonstrated that a simple and novel method could be applied to enhance functionalities of Ti alloys for load-bearing implant applications.

Metal injection molding (MIM) is a near net-shape manufacturing technology that is capable of mass production of complex parts cost-effectively. The unique features of the process make it an attractive route for the fabrication of metal matrix composite materials. In this paper, the status of the research and development in fabricating metal matrix composites by MIM is reviewed, with a major focus on material systems, fabrication methods, resulting material properties and microstructures. Also, limitations and needs of the technique in composite fabrication are presented in the literature. The full potential of MIM process for fabricating metal matrix composites is yet to be explored.

Porous titanium (Ti) and Ti-high density polyethylene (Ti-HDPE) composite were investigated as new hip implant materials to increase the biomechanical compatibility by promoting a matching modulus of elasticity between hip stem and human bone. Surfaces of both materials were modified to increase its bioactivity and biocompatibility through electrochemical activation treatment and deposition of hydroxyapatite (HA) coating. The electrochemical activation treatment of both materials in 10 M NaOH solution created a bone-like porous nanostructure across the surfaces, thus enhancing the growth of natural bone. A top layer with nanometer-pores and TiO 2 was formed during the activation process, creating a favorable and prerequisite condition conducive to the formation of hydroxyapatite coating. Furthermore, a layer of hydroxyapatite, a bioactive and biocompatible bioceramics that is the main component of natural bone, was deposited on the porous Ti and Ti-HDPE composite through a novel chemo-biomimetic method. The formed coating was characterized through TEM as a nanometer scale crystalline.

Ceramic mould casting process produces high precision castings at a relatively low cost for the production of small number of parts, making it particularly suitable for producing moulds and dies. There is a constant demand for improving the process capabilities including dimensional accuracy and consistency. In this study, dimensional changes and variability of ceramic moulds and steel castings (P20) have been investigated using a pyramid-shaped part. It was shown that the accuracy of the ceramic mould plays a dominant role in obtaining tight casting tolerances. It was also shown that the dimension, location and orientation of a feature on the part had significant effects on the overall linear dimensional changes as well as the dimensional variability of the ceramic mould and the casting. The complex dependence of dimensional variations on feature orientation and location on the part has been discussed on the basis of asynchronous burning on the mould cavity surface at different locations during the burning-out process.

Burning-out is an important step in mould fabrication for the ceramic mould casting process to remove alcohol in the gelled ethyl silicate binder. It affects both the accuracy and stability of the ceramic mould dimensions. In this paper, results are described for length changes of ceramic mould specimens during the burning-out process after alcohol immersion or drying in air for various lengths of time. It is shown that preventing rapid evaporation/removal of alcohol from the ceramic mould specimen before burning-out by immersing the specimen in alcohol generally produces less overall length change compared to leaving the specimen to dry in air. The immersion time for the specimen in alcohol or drying in air before burning-out has a significant effect on the overall length changes in the specimen and on the characteristics of the burning-out curves. Under the conditions investigated, immersing in alcohol for 5–10 h followed by burning-out produced near zero overall length change in the specimen.

The effect of humidity, temperature and temperature ramping rate on the dimensional changes of the photo-cured stereolithography (SL) resin SL5195 was investigated. It was found that moisture absorption by the SL resin is a very slow process at ambient temperatures. Varying relative humidity (RH) between 20 and 90 per cent in the environment only produced slight changes in the sample dimensions during the time period investigated. Increasing the environment temperature caused a significant increase in the sample dimensions through thermal expansion along with accelerated moisture absorption at 50 per cent or higher RH. Increasing the temperature ramping rate reduces the moisture absorption during the thermal cycles.

Oxide coatings were produced on a 6061 aluminum alloy using a pulsed unipolar plasma electrolytic oxidation (PEO) process. The effect of electrical parameters including pulse frequency, duty cycle and current density on phase formation in the coatings was revealed using conventional and glancing angle X-ray diffraction. The results show that PEO coatings are mainly composed of γ-Al2O3. Depending on the electrical parameters employed, the coatings can also contain α-Al2O3 and mullite with varying concentrations. Higher current densities and higher duty cycle were found to favor the formation of mullite. Under the experimental conditions used, the ratio of the integrated XRD peaks for α- and γ-Al2O3 varied from 0 to about 0.6, indicating that the relative content of α-Al2O3 in the PEO coatings varied over a wide range. Longer pulse on-times and higher current densities promoted the gamma to alpha-alumina phase transformation. Depth profiling of PEO ceramic coatings using glancing angle XRD with different incident beam angles revealed that mullite was more concentrated in the top surface of the coatings. No significant variation in α-Al2O3 concentration across the coatings could be concluded in this study, unlike the results of some other studies.

This paper presents a two-step method for the fabrication of Ti/polymer biocomposites. In the first step, a porous titanium substrate was produced by powder compaction and sintering. The porosity and pore structure of the substrate were controlled by varying the sintering process parameters. In the second step, high-density polyethylene (HDPE) or polyurethane (PU) was impregnated into the porous titanium substrate to form Ti/polymer biocomposites with the assistance of a hydraulic pressure. Ti/polymer composites with the polymer volume fraction varying from 21% to 37% have been successfully fabricated. Suitable impregnation parameters such as temperature, pressure and holding time for HDPE and PU were determined.

The effect of nickel coating on the bending properties of Stereolithography (SL) parts was investigated. It was found that a thin layer of nickel coating significantly improves the flexural strength and modulus of the SL specimens. The strengthening effect of the coating is dependent on the ratio of the Young’s moduli of the coating and the substrate materials, as well as the thickness of the coating.