Chien-Ping Ju

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.

Augmentation of tendon-bone healing by the use of calcium-phosphate cement

The healing of a hamstring graft to bone is the weak link in the reconstruction of a cruciate ligament using this donor material. We therefore investigated the augmentation of healing at the tendon-bone interface using calcium-phosphate cement (CPC). We performed semitendinosus autograft reconstructions of the anterior cruciate ligament on both knees of 22 New Zealand white rabbits. The interface between the grafted tendon and the bone tunnel for one knee was filled with CPC. Six rabbits were killed at the end of the first and second post-operative weeks in order to evaluate the biomechanical changes. Two rabbits were then killed sequentially at the end of weeks 1, 3, 6, 12 and 24 after operation and tissue removed for serial histological observation. Histological examination showed that the use of CPC produced early, diffuse and massive bone ingrowth. By contrast, in the non-CPC group of rabbits only a thin layer of new bone was seen. Mechanical pull-out testing at one week showed that the mean maximal tensile strength was 6.505 +/- 1.333 N for the CPC group and 2.048 +/- 0.950 N for the non-CPC group. At two weeks the values were 11.491 +/- 2.865 N and 5.452 +/- 3.955 N, respectively. Our findings indicate that CPC is a potentially promising material in clinical practice as regards its ability to reinforce the fixation of the tendon attachment to bone and to augment the overall effectiveness of tendon healing to bone.

Effect of humidity on the tribological behavior of carbon-carbon composites

The present work is a study of the effect of humidity on the tribological behavior of carbon-carbon composites, including two-dimensional PAN/pitch (designated TM), PAN/CVI (designated E), and pitch/resin/CVI (designated A) formulae. Results indicate that there exist close relationships among friction coefficient, wear rate, and worn surface morphology. Whenever a morphological transition has occurred, transitions in friction and wear also occur. Three different debris morphologies are identified. The smooth type I and type III debris films can lubricate, but not the powdery type II debris. The relative humidity level has a strong effect on the tribological behavior of all three composites. Low humidity and high sliding speed generally accelerate the occurrence of type I-to-type II transitions in friction, wear, and debris morphology, whereas high humidity and low speed enhance the formation of type III debris. At any humidity level, the type I morphology is always accompanied by low friction and wear. After the type I-to-type II transition, higher friction coefficient and wear rate are observed in the low humidity than in the high humidity. Under the present conditions, composite E appears to be the most sensitive to the humidity effect, while composite A appears the least sensitive.

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

Effect of doped bioactive glass on structure and properties of sintered hydroxyapatite

Abstract The effects of doped bioactive glass (BG) on the structure and properties of sintered hydroxyapatite (HA) have been studied. The results showed that a calcination treatment at 900 °C increased the degree of crystallinity and the amount of hydroxyl group in HA, but resulted in the loss of a small amount of phosphorus. The addition of BG enhanced HA phase decomposition and hydroxyl group breakdown processes, decreased HA density and microhardness, increased indentation toughness in most cases, and promoted microcracking when sintered at high temperatures. The addition of BG could either strengthen or weaken HA, depending on the BG content and sintering condition. Practically, the optimal properties were obtained from the HA doped with 2.5 wt.% BG sintered at 1250 °C.

Friction and wear of PAN/pitch-, PAN/CVI- and pitch/resin/CVI- based carbon/carbon composites

Abstract In this study we compared the tribological behavior under a high speed condition (1.7 MPa, 2000 rev min −1 ) of six different carbon/carbon composites including three two-dimensional PAN/pitch composites (TH, TM and TL), one two-dimensional PAN/CVI composite (E), one two-dimensional pitch/resin/CVI composite (A) and one three-dimensional PAN/pitch composite (T3D). Results indicated that, among the five two-dimensional composites, TM and E performed significantly better than the other three composites under the present condition. Both TM and E exhibited a reasonably low friction coefficient (both about 0.4) and a wear rate that was an order of magnitude lower than those of the other three. A transition in friction occurred for A, TH and TM, but not for E or TL. The pre-transitional friction coefficients of the three composites were 0.1–0.2, similar to those measured under the low speed condition. During transition, the initially formed thin, smooth lubricative film was suddenly disrupted and turned into a thick powdery debris layer that caused the friction coefficient to rise abruptly to 0.5–0.9. The powdery debris on TM and E was easily “ironed” into a smooth and tight lubricative film to cause both friction and wear to decline. The three-dimensional composite T3D was not suitable for high speed applications owing to extensive structural damage.

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.

Low energy tribological behavior of carbon-carbon composites

Abstract The present study compares low energy tribological behavior among five different carbon-carbon composites designated “E” (2D PAN/CVI), “A” (2D pitch/resin/CVI), “TH” (high density 2D PAN/pitch), “TL” (low density 2D PAN/pitch) and “T3D” (3D PAN/pitch). Results indicate that all composites had steady-state friction coefficients close to 0.2, except for “TL” which had a much larger initial friction peak (up to 0.6) and higher average friction coefficient (nearly 0.3). Unlike the other four composites, which had quite constant friction coefficients after the initial peaks, the friction coefficient of “TH” continued to increase up to 66 meters of sliding. Weight losses of “TL” were an order of magnitude higher than those of the other four composites, due to the high wear rate during the first few minutes of sliding. After thirtysome meters of sliding the wear rate of “TL” decreased to a level comparable to those of the other four composites. Among “E,” “A,” “TH,” and “T3D,” the composite “T3D” had the largest weight loss, whereas “A” had the smallest. The composites “E” and “TH” had nearly identical weight losses up to 66 meters. For all composites, a flat and smooth self-lubricative debris film was generated on the worn surfaces after sliding for a few meters. Compared to the other composites, the debris film covering the “TH” surface was developed at a lower rate. For the present carbon-carbon composites, the development of a steady-state friction coefficient value generally occurred earlier than the development of a steady-state wear rate.

Surface effect on braking behavior of PAN-pitch carbon-carbon composite

Abstract This work studies the braking (simulated-stop) behavior of a polyacrylonitrile (PAN)-pitch carbon-carbon composite under different surface conditions. The results indicate that broken-in (BI) specimens exhibited much higher friction coefficients and wear than those of aspolished (AP) specimens under the same braking conditions. The specimens braked from higher speed always suffered higher wear either due to their higher friction coefficients or longer braking times. The friction behavior of the present composite was found most sensitive to the debris morphology. The friction behavior of BI specimens, which had undergone a transition during the BI treatment, was more complex and harder to predict than that of AP specimens, which did not undergo transition before or during braking. The severe structural damage on the B1/2000 surface, which could not be mended (a stable lubricative debris layer could not be developed), resulted in an unstably high friction surface and large wear.