J. H. Chern Lin

Structural characterization of pulsed laser-deposited hydroxyapatite film on titanium substrate.

Pure, crystalline hydroxyapatite (HA) films with thicknesses of roughly 10 microns have been deposited on titanium substrate using the pulsed laser deposition (PLD) technique. Experimental results indicate that the structure and properties of the PLD-HA films varied with deposition parameters. The PLD process used in the present study did not induce significant amounts of calcium phosphate phases other than apatite, or significant changes in the behaviour of hydroxyl or phosphate functional groups. Broad face scanning electron microscopy showed that HA coating was comprised of numerous essentially spheroidal-shaped particles of different sizes, while the lateral morphology indicated that columnar and dome-shaped structures both existed in the film. Many pinholes and crevices observed on coating surfaces were linked to the original substrate surface crevices/craters. The adhesion strength of the coating, mostly in the range of 30-40 MPa, was found to be closely related to the fractography of the tested specimen. The fracture surfaces of specimens with higher bond strengths were usually accompanied by a higher degree of deformation and coating-substrate debonding, while the fracture of specimens with lower bond strengths occurred more frequently within HA coatings in a more brittle manner. The energy dispersive spectroscopy-determined Ca/P ratios of raw HA powder (1.78) and sintered HA target for PLD (1.79) were very close, indicating that the sintering process used in the present study essentially did not change the Ca/P ratio of HA. After the PLD process, the Ca/P ratio of the HA film increased to 1.99. Cross-sectional scanning electron microscopy-energy dispersive spectroscopy point analysis indicated that the value of the Ca/P ratio was significantly higher in the region near the surface, particularly near the coating-substrate interface, than in the coating interior.

Structure and properties of hydroxyapatite-bioactive glass composites plasma sprayed on Ti6Al4V

Hydroxyapatite (HA)-coated Ti6Al4V has recently been used as a bone substitute in orthopaedic and dental applications because of its favourable bioactivity and mechanical properties. Studies in the literature have shown that the bioactivity of calcium phosphate bioactive glass (BG) is higher than that of HA. In an attempt to increase the bioactivity of Ha-coated Ti6Al4V and enhance the bonding strength between coating and substrate, in the present study, HA/BG composites are applied onto Ti6Al4V using a plasma spraying technique. Microstructure and phase changes of the composite coating after plasma spraying are studied. The coating-substrate bonding strength is evaluated using an Instron, following the ASTM C633 method. Results indicate that the average bonding strengths of BG, HA/BG and HA coatings are 33.0±4.3, 39.1±5.0, and 52.0±11.7 MPa, respectively. Open pores with sizes up to 50 μm are found in both BG and HA/BG coatings, which are probably advantageous in including mechanical interlocking with the surrounding bone structure, once implanted. These HA/BG composites could provide a coating system with sufficient bonding strength, higher bioactivity, and a significant reduction in cost in raw materials. The future of this HA/BG composite coating system seems pretty bright.

Morphology and immersion behavior of plasma-sprayed hydroxyapatite/bioactive glass coatings

A series of hydroxyapatite/bioactive glass (HA/BG) coatings have been plasma-sprayed on Ti6Al-4V substrate using HA/BG powders that were prepared by both sinter-granulation and direct mixing methods. The morphology and immersion behavior of these coatings in a simulated body fluid (SBF) were investigated. The results showed that in-house fabricated BG and sinter-granulated HA powders were irregularly shaped and dense. When 5 wt % or more BG was added in HA, the powder became rough and porous. X-ray diffraction (XRD) patterns showed that the presence of BG enhanced the decomposition of HA structure during fabrication of the powders. Reasonably high bond strengths were obtained from all coatings. The granulated type HA/BG coatings showed no significant differences in bond strength from the mixed type HA/BG coatings. The plasma spray process itself and the presence of BG enhanced the decomposition of apatite. Surface morphology of all sinter-granulated type coatings was similar to that of monolithic HA coating, that was comprised of patches of smooth and shiny glassy film and irregularly-shaped particles on its surface. The dissolution depth of plasma-sprayed coatings immersed in SBF was largely dependent on the type and composition of the coating. Granulated type HA/BG coatings were much less dissolvable than monolithic HA or mixed type HA/BG coatings. It seems that the presently used granulation method for the preparation of HA/BG powders plays a predominant role in determining the dissolution behavior of the plasma-sprayed coatings. ©©2000 Kluwer Academic Publishers

Structure and properties of Titanium-25 Niobium-x iron alloys.

The present work studies the effect of iron on microstructure, mechanical properties and corrosion behavior of Ti–25Nb based system with emphasis on improving strength/modulus ratio. Experimental data shows that cast Ti–25Nb–3Fe has a β phase with a entirely of dendrite morphology. The bending strength/modulus ratio is 24.6 higher than Ti–6Al–4V (17.4) by 41.4% and than c.p. Ti (9.3) by 165%. The critical anodic current density of the metal in 37 °C Hank’s solutions is lower than approximately 100 μA/cm2. Ti–25Nb–3Fe alloy has a great potential for use as an implant material.

Effect of chromium content on structure and mechanical properties of Ti-7.5Mo-xCr alloys

The present work is a study of a series of Ti-7.5Mo-xCr alloys with the focus on the effect of chromium content on the structure and mechanical properties of the alloys. Experimental results show that low hardness, strength and modulus binary Ti-7.5Mo alloy is comprised primarily of fine, acicular martensitic α′ phase. When 1 wt % Cr is added, a small amount of β phase is retained. With 2 wt % or more chromium added, the entire alloy becomes equi-axed β phase with bcc crystal structure. The average β grain size decreases with Cr content. When the alloy contains about 2–4 wt % Cr, a metastable ω phase is present. In Ti-7.5Mo-2Cr alloy appears the highest ω intensity accompanied with high microhardness, bending strength and modulus. The ω-induced embrittling effect is most profound in Ti-7.5Mo-2Cr alloy that exhibits a terrace type fracture surface covered with numerous micron-sized dimples. The alloys with higher Cr contents show normal ductile type fractography with much larger deformation dimples. The present results indicate that Ti-7.5Mo-(4–6)Cr alloys seem to be potential candidates for implant application.

Bismuth effect on castability and mechanical properties of Ti-6Al-4V alloy cast in copper mold

Abstract The present research studies the effect of bismuth addition on castability, structure and mechanical properties of Ti–6Al–4V alloy. Experimental results indicate that addition of 1 wt.% bismuth in Ti–6Al–4V enhances both castability and strength of the alloy. Larger amounts of Bi addition caused a reduction in ductility. Undoped and 1Bi-doped alloys exhibited a ductile fracture surface covered with deformation dimples, while 3Bi and 5Bi alloys showed a brittle fracture surface.

Mechanical and tribological properties of 2-D carbon/carbon composites densified through pulse chemical vapor infiltration

This paper presents the mechanical and tribological properties of 2-D carbon/carbon composites (C/C) fabricated by the Pulse Chemical Vapor Infiltration (PCVI) process. In the PCVI process, various fabrication temperatures and different reactant pressures were adopted to investigate the influence of processing condition on physical properties, microstructure and mechanical properties. In the densification process by PCVI, holding time and pulse number are two parameters which significantly affect physical properties (such as density, porosity, and weight gain) and mechanical properties (such as interlaminar strength and wear properties). It is found that the wear properties of carbon/carbon composites can be improved obviously after the densification by 1000 pulses. Effects of initial open porosity on density of the fabricated composites are also studied. In this work, tribological performance of the specimens fabricated through the Isothermal Chemical Vapor Infiltration (ICVI) process and the PCVI process under different pulse cycles are compared. Scanning Electronic Microscopy (SEM) was used to examine the morphology of worn surface. The relationship between the tribological performance and surface morphology was studied.

Multi-braking tribological behavior of PAN-pitch, PAN-CVI and pitch–resin-CVI carbon–carbon composites

Abstract This paper compares the multiple braking behavior of a polyacrylonitrile fiber-pitch matrix (PAN-pitch), polyacrylonitrile fiber-chemical vapor infiltrated (PAN-CVI) matrix and mesophase pitch fiber-phenolic resin-chemical vapor infiltrated hybrid matrix (pitch-resin-CVI) carbon-carbon (C–C) composites. Results indicate that the tribological behavior of all three composites is sensitive to their worn surface morphology. The average friction coefficient of pitch-resin-CVI composite is much higher than that of the other two composites. The pre-transitional type low friction coefficients repeatedly appear in PAN-pitch and PAN-CVI composites, but never in the pitch-resin-CVI composite once the first transition occurs. Compared to pitch-resin-CVI, the high friction coefficients of PAN-pitch and PAN-CVI are much easier to be brought back to their pre-transitional levels by subsequent braking tests. The variations in wear of the three composites have similar trends to those in friction. The higher the friction coefficient during a braking test, the larger the weight loss. Among the three composites, PAN-pitch has the lowest average weight loss and appears to have the most stable performance. The pitch-resin-CVI composite is more susceptible to sliding-induced structural damage than the other two composites.

Effect of particle configuration on structure and properties of dispersed Pd-containing dental amalgam.

The present work investigates the effect of particle configuration on the structure and properties of four dispersed type Pd-containing amalgams with the same chemical composition. The results indicate that particle configuration plays an important role not only in the structure and chemistry, but also in such properties as compressive strength, creep and mercury release rate. Compromises among the various properties of the present Pd amalgams are needed, when a decision on particle configuration is to be made. As mechanical properties or corrosion resistance is concerned, the amalgam should comprise at least one spherical alloy (matrix or dispersant). As far as mercury release is concerned, the amalgam should comprise irregular dispersant alloy. The amalgam fabricated from irregular dispersant and spherical matrix particles seems to serve all well.

Microstructure study of PAN-pitch carbon-carbon composite lubricative film

Abstract Scanning electron microscopy (SEM), polarized light microscopy, transmission electron microscopy (TEM), and scanning acoustic microscopy (SAM) techniques have been used to characterize the microstructure of a self-lubricating film of polyacrylonitrile (PAN)-based fiber-reinforced mesophase pitch-based matrix carbon–carbon composites. The results indicated that the worn surface was covered with a smooth and dense lubricative film and the basal plane orientation of the lubricative film was randomized. The wear-induced amorphous film was identified by the diffuse rings/halos in selected-area diffraction (SAD) patterns, while the numerous SAD patterns of graphite structure on the worn surface were also observed. SAM shows the lubricating film formed by some delaminated wear debris which was compacted, rolled and piled up layer by layer.