K.A. Khor

In vitro studies of plasma-sprayed hydroxyapatite/Ti-6Al-4V composite coatings in simulated body fluid (SBF)

The bioactivity of plasma-sprayed hydroxyapatite (HA)/Ti-6Al-4V composite coatings was studied by soaking the coatings in simulated body fluid (SBF) for up to 8 weeks. This investigation was aimed at elucidating the biological behaviour of plasma-sprayed HA/Ti-6Al-4V composite coatings by analyzing the changes in chemistry, and crystallinity of the composite coating in a body-analogous solution. Phase composition, microstructure and calcium ion concentration were analyzed before, and after immersion. The mechanical properties, such as tensile bond strength, microhardness and Youngs modulus were appropriately measured. Results demonstrated that the tensile bond strength of the composite coating was significantly higher than that of pure HA coatings even after soaking in the SBF solution over an 8-weeks period. Dissolution of Ca-P phases in SBF was evident after 24h of soaking, and, a layer of carbonate-apatite covered the coating surface after 2 weeks of immersion. The mechanical properties were found to diminish with soaking duration. However, slight variation in mechanical properties was found after supersaturation of the calcium ions was attained with the precipitation of the calcium phosphate layers.

Spark plasma sintering of hydroxyapatite powders

Dense hydroxyapatite (HA) compacts have been successfully fabricated by a spark plasma sintering (SPS). The sintering behavior of HA powders at different temperatures ranging from 850 degrees C to 1100 degrees C was studied. Results showed that spark plasma sintering resulted in rapid densification to near theoretical density. The HA compact was homogeneously sintered at 950 degrees C in a short sintering duration of 5 min, while maintaining high quality and high relative density (>99.5%). The density, microhardness and Youngs modulus of HA sintered compact initially increased with the sintering temperature, reached a maximum value at around 950-1000 degrees C, then decreased with further increase in the temperature due to the decomposition of HA into beta-tricalcium phosphates. Fracture toughness results showed no significant difference with increasing temperature due to the combined influences of density and grain size. Microstructure analysis showed no noticeable grain growth under different sintering temperatures due to the short time exposure at high temperatures.

Plasma-sprayed hydroxyapatite (HA) coatings with flame-spheroidized feedstock: microstructure and mechanical properties

Flame-spheroidized feedstock, with excellent known heat transfer and consistent melting capabilities, were used to produce hydroxyapatite (HA) coatings via plasma spraying. The characteristics and inherent mechanical properties of the coatings have been investigated and were found to have direct and impacting relationship with the feedstock characteristics, processing parameters as well as microstructural deformities. Processing parameters such as particle sizes (SHA: 20-45, 45-75 and 75-125 microm) and spray distances (10, 12 and 14 cm) have been systematically varied in the present study. It was found that the increase of particle sizes and spray distances weakened the mechanical properties (microhardness, modulus, fracture toughness and bond strength) and structural stability of the coatings. The presence of inter- and intralamellar thermal microcracks, voids and porosities with limited true contact between lamellae were also found to degrade the mechanical characteristics of the coatings, especially in coatings produced from large-sized HA particles. An effort was made to correlate the effects of microstructural defects with the resultant mechanical properties and structural integrity of the plasma-sprayed hydroxyapatite (HA) coatings. The effects of different heat treatment temperatures (600, 800 and 900 degrees C) on the mechanical properties of the coatings were also studied. It was found that a heat treatment temperature of 800 degrees C does enhance the microhardness and elastic modulus of the coatings significantly (P < 0.05) whereas a further increment in heat treatment temperature to 900 degrees C did not show any discernable improvements (P > 0.1). The elastic response behaviour and fracture toughness of both the as-sprayed and heat-treated HA coatings using Knoop and Vickers indentations at different loadings have been investigated. Results have shown that the mechanical properties of the coatings have improved significantly despite increasing crack density after heat treatment in air. Coatings produced from the spheroidized feedstock of 20-45 microm (SHA 20-45 microm) sprayed at a stand-off distance of 10 cm were found to possess the most favourable mechanical properties.

Cathode Micromodel of Solid Oxide Fuel Cell

A comprehensive micromodel considering all forms of polarization in the cathode of the solid oxide fuel cell was developed which governs the complex interdependency among the transport phenomena, electrochemical processes (charge-transfer and surface diffusion), and the microstructure of the electrode and their combined effect on the cathode overpotential under different operating conditions. To make the model more generalized, we consider possible oxygen reduction mechanisms, reactions at the cathode/ electrolyte interface, grain interior and grain boundary effects on the total resistance, both ordinary diffusion and Knudsen diffusion, active three-phase boundary length as a function of ionic/electronic particle size ratio and volume fraction, the exchange current density as a function of gases concentration, etc. Incorporated with reliable experimental data, the model can be used as a tool to design a high performance cathodes.

An in vitro investigation of plasma sprayed hydroxyapatite (HA) coatings produced with flame-spheroidized feedstock

The in vitro behaviour and characteristics of plasma sprayed hydroxyapatite (HA) coatings using flame-spheroidized HA feedstock powder on titanium alloy (Ti-6Al-4V) substrates were investigated in a simulated physiological environment as an attempt to reflect the actual incubational condition of an implant in a human body system. As-sprayed and heat-treated HA coatings were immersed in a simulated body fluid with ionic concentrations comparable to that of human blood plasma for time intervals 2, 4, 6, 8 and 10 weeks. Rapid dissolution of calcium phosphate was found to occur within the first 4 weeks, and after the 5th week a retarding rate of 4.1 mM week(-1) was observed where precipitation, nucleation, and, growth of a carbonate-containing, poorly crystallized or amorphous calcium phosphate layer on the as-sprayed coatings were noted. The heat-treated coatings showed minimal or no precipitation on the surface except for the presence of calcite minerals that is due to carbonation effect. Complete dissolution of other calcium phosphate phases such as tetracalcium phosphate, tricalcium phosphate and calcium oxide was also noted after 2 weeks of immersion due to higher ionic solubility relative to HA. A declining trend in respective microhardness and elastic modulus of the as-sprayed HA coatings from 207.06 +/- 3.2 H(k300) to 131.8 +/- 5.2 H(k300) and from 31.37 +/- 1.4 to 19.81 +/- 1.6 GPa was observed after 10 weeks of immersion. Tensile bond strength of both types of coatings showed similar declining trend, with an average dip from 24.5 +/- 2.4 to 7.9 +/- 2.6 MPa. Nevertheless. the heat-treated samples showed rather reasonable mechanical stability and structural integrity of 26.7 +/-1.4 GPa in elastic modulus after soaking.

Mechanical properties of injection molded hydroxyapatite-polyetheretherketone biocomposites

Abstract Bioactive composites comprising synthetic hydroxyapatite (HA) particulate and semi-crystalline polyetheretherketone (PEEK) polymer were produced for biomedical application. HA particulates were incorporated into PEEK polymer matrix through a series of processing stages involving melt compounding, granulating and injection molding. This investigation presents the processing route employed and the mechanical properties of HA–PEEK composites. In general, Youngs modulus, compressive strength and micro indentation hardness increased with increasing amount of HA particulate. On the other hand, tensile strength and strain to failure decreased with increasing HA loading. The tensile strength and Youngs modulus of HA–PEEK composites were found to be within the bounds of bony tissue. These results suggest that the bioactive HA–PEEK composites have the potential for use as an alternative material for load-bearing orthopedic application.

Plasma spraying of functionally graded hydroxyapatite/Ti–6Al–4V coatings

Abstract Functionally graded hydroxyapatite (HA)/Ti–6Al–4V coatings were produced by plasma spray process using specially developed HA-coated Ti–6Al–4V composite powders as feedstock. The microstructure, density, porosity, microhardness, and Youngs modulus ( E ) were found to change progressively through the three-layered functionally graded coating that composed of the layers 50 wt.% HA/50 wt.% Ti–6Al–4V; 80 wt.% HA/20 wt.% Ti–6Al–4V, and HA. No distinct interface between adjacent layers of different compositions was evident from scanning electron microscope observation. X-ray diffractometry showed that the coatings composed of HA and Ti phases. Microhardness, as measured through the indentation technique, and tensile adhesion strength decreased correspondingly with increasing HA content in the single-layered composite coatings. The application of HA/Ti–6Al–4V composite powders improved the tensile adhesion strength of the coatings significantly. The Youngs modulus and fracture toughness results showed highly anisotropic elastic behavior with relatively higher E and K IC (fracture toughness) values parallel to the coating surface due to the intrinsic lamellar structure of the plasma sprayed coatings.

Development of (La, Sr)MnO3-Based cathodes for intermediate temperature solid oxide fuel cells

Conventional (La, Sr)MnO 3 (LSM) electrodes modified by ion impregnation methods showed promising potential as cathodes for intermediate temperature solid oxide fuel cells. After impregnation of ionic conducting-type Gd 0 . 2 Ce 0 . 8 (NO 3 ) x nitrite salt solution into the LSM structure, electrochemical activity of LSM electrodes for the O 2 reduction reaction was substantially enhanced. At 700°C, the electrode polarization resistance of impregnated LSM electrodes decreased to 0.72 Ω cm 2 as compared to 26.4 Ω cm 2 for pure LSM electrodes, a reduction in electrode polarization resistance by 36 times. The polarization losses were also reduced substantially. At 300 mA cm - 2 and 700°C, overpotential for the O 2 reduction was reduced from 0.79 V on pure LSM to 0.19 V on impregnated LSM electrodes, a reduction in overpotential by four times. Ion-impregnated LSM electrodes showed better performance than those of LSM/Y 2 O 3 -ZrO 2 and LSM/(Gd,Ce)O 2 composite electrodes as reported in the literature.

The production and characterization of hydroxyapatite (HA) powders

Abstract The synthesis of appropriate calcium phosphate powders for thermal spraying applications is a fundamental yet crucial stage in the production of a bioceramic coating coupled with the desired characteristics. The performance, lifespan and quality of the resultant biological coating in vivo is largely dependent on the coating morphology, phase composition, particle size and crystallinity of the spray powders. In order to achieve highly reliable coatings from thermal spray processes, spherical powders of a specified size distribution are recommended. Such spherical morphology has been proven to produce excellent heat transfer characteristics and consistent melting behaviour, which would increase the deposition efficiency and decrease coating porosity. This paper aims to provide an insight to the preparation and characterization aspect of HA powders by the spray drying process. The raw HA powders were synthesized through a chemical reaction involving orthophosphoric acid and calcium hydroxide. The precipitated spray dried powders were examined for purity, phase composition and morphology. The heat-treated powders were then flame spheroidized to produce spherical, high flowability powders to be used as feed-stock for plasma spraying.

Sintering study of 316L stainless steel metal injection molding parts using Taguchi method : final density

Abstract Sintering is a key step in the metal injection molding process, which affects the final density as well as the mechanical properties of the sintered part. To achieve a high final density, the effects of various sintering factors pertaining to the temperature–time profile and sintering atmosphere have to be characterized and optimized as well. This paper reports the use of Taguchi method in characterizing and optimizing the process factors for sintering water-atomized 316L stainless steel (of average size 6 μm). The effects of four sintering factors: sintering temperature, heating rate, sintering time and sintering atmosphere on the final density were studied. The various factors were assigned to an L9 orthogonal array. It was found that all the chosen sintering factors have significant effects on the final density. Optimum fractional final density of 96.14% of wrought material was achieved with a sintering temperature of 1250°C, a heating rate of 20°C min−1 and isothermal heating at 1250°C for 90 min in a vacuum atmosphere. Confirmatory experiments have produced results that lay within the 90% confidence interval.