J.H. Chern Lin

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.

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.

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.

Effect of load on tribological behaviour of carbon-carbon composites

The effect of load on the tribological behaviour of three different two-dimensional carbon-carbon composites, including polyacrylonitrile (PAN) fibre-pitch matrix, PAN fibre-chemical vapour infiltrated (CVI) matrix, and pitch fibre-resin/CVI hybrid matrix composites has been compared. Results indicated that friction and wear rate variations with slide distance depended on load and the type of composite. The worn surface morphology was categorized into three types (I, II and III). The friction coefficient, wear rate and temperature all increased sharply when the transition from type I to type II occurred. When the powdery type II debris was compacted to form the lubricative type III debris, the friction coefficient and wear rate declined, although never approached their initial type I levels. For all composites, a higher load can accelerate the transition from type I to type II, but impedes the transition from type II to type III. Under 2.4 MPa, the type II morphology was never observed to transform into type III morphology in this study.

Process, microstructure and properties of squeeze-cast short-carbon-fibre-reinforced aluminium-matrix composites

Two types of high-modulus short-carbon-fibre-reinforced commercially pure aluminium-matrix composites were fabricated in-house using a home-made squeeze caster. The type-I composites were fabricated from short-fibre preforms in which fibres exist as dispersed bundles. The type-II composites were fabricated from preforms in which individual fibres were uniformly dispersed. The detailed processes are described in the text. A three-point-bending strength of higher than 200 M Pa was obtained for the type-1 composite with 17 vol% of fibre. When more fibre was incorporated, both the strength and the ductility decreased due to inadequate infiltration. However, a bending strength of greater than 240 MPa was recorded on a hot-rolled type-I composite with a fibre content as high as 28 vol%. This significant improvement in the mechanical properties is explained by a hot-rolling-inducedvoid-healing effect. The type-II composites, with lower fibre volume fractions than those of the type-I due to their different preforms, exhibited bending strengths up to 166 MPa. Scanning electron microscopy fractography shows that the two types of composites fracture in distinctive manners. Transmission electron microscopy results featured thermal-stress-induced dislocations at carbon-aluminium interfaces as well as submicrometre-sized aluminium carbide, the reaction product, which nucleated from the interface and grew into the matrix interior.

Biocorrosion behavior of hydroxyapatite/bioactive glass plasma sprayed on Ti6A14V

Biocorrosion behavior and changes in morphology and phases of hydroxyapatite (HA), bioactive glass (BG) and HA/BG coatings in Hanks physiological solution is investigated. Results showed that as-coated BG had the roughest surface, while as-coated HA had the smoothest surface. The HA/BG coating was comprised of both bubble-shaped particles of BG and small irregular-shaped particles of HA. Low intensity X-ray diffraction (XRD) peaks of CaO, β-TCP and Ca4P2O5, besides the major apatite peaks, were observed in HA and HA/ BG coatings. BG coating had an essentially amorphous structure. When HA and HA/BG were immersed in Hanks solution for 30 days, most of the minor phases dissolved. When BG coating was immersed, both apatite and SiO2 peaks gradually rose. A sudden drop and recovery in OCP occurred to BG and HA/BG but not to HA. Anodic polarization showed that passivation and film breakdown occurred to all three coatings. Passivation started approximately from 200, 300 and 400 mV(SCE) for HA, HA/BG and BG coatings, respectively. The three coatings had similar critical current densities. The released calcium ion concentration was found the highest in HA-tested solution, while the concentrations of sodium and silicon were the highest in BG-tested solution. The drop-and-rise phenomenon was also observed in the FT-IR spectra of immersed HA and HA/BG coating. These FT-IR results are consistent with the XRD results reported earlier.

Process and wear behavior of monolithic SiC and short carbon fiber-SiC matrix composite

The process and wear behavior of monolithic SiC and 10 vol. % short carbon fiber-SiC matrix (C-SiC) composite have been studied. The results indicate that, among ethyl alcohol, acetone, n-hexane and n-octyl alcohol, n-octyl alcohol was the most effective dispersing agent in dispersing both SiC powder and short carbon fiber. Among AlN, Al2O3, B4C, graphite, AlN/B4C, AlN/graphite, B4C/graphite and Al2O3/B4C, the most effective sintering aid for the fabrication of SiC and C-SiC composite was a mixture of 2 wt% AlN and 0.5 wt% graphite. The monolithic SiC hot-pressed at 2100°C exhibited higher density but lower flexural strength than those hot-pressed at 2000°C due to a grain growth effect. For the C-SiC composite, both density and strength of the composite hot-pressed at 2100°C were generally higher than those hot-pressed at 2000°C. The density and strength of C-SiC composite were lower than those of monolithic SiC under the same hot pressing conditions due to a higher porosity level in the composite. When monolithic SiC slid against C-SiC composite, the weight losses of SiC and the composite were each less than that of self-mated SiC or self-mated C-SiC. In the self-mated SiC tribosystem, a mechanically stable film could not be established, resulting in an essentially constant wear rate. When sliding against C-SiC, a thin, smooth and adherent debris film was quickly formed on the SiC surface, resulting in a lower wear.

Electron microscopic study of worn PAN-pitch based carbon-carbon composite

Abstract Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM)/microdiffraction techniques have been used to characterize the microstructure of a polyacrylonitrile (PAN)-based fiber-reinforced mesophase pitch-based matrix carbon-carbon (Cue5f8C) composite before and after sliding at two different speeds. Emphasis was placed on the study of wear-induced amorphization and graphitization phenomena. The results indicated that both phenomena were observed in the debris film of the high speed-slid specimens. The wear-induced amorphous film was identified by the diffuse rings/halos in SAD patterns, while the numerous graphitized particles were confirmed by bright and dark field images, SAD and microdiffraction patterns. The wear-induced graphitization phenomenon suggests that the asperity temperatures should be high enough for graphitization to take place at local spots, particularly in the more graphitizable pitch-based matrix.

Microstructure and Segregation Behavior of Palladium in Silver-Copper-Palladium Alloys:

It has been reported that the addition of palladium can modify the microstructure and improve the properties of Ag-Cu eutectic alloy as well as admixed Cu-rich amalgam. The purpose of this work was to study the microstructure and segregation behavior of palladium in a series of Ag-Cu-Pd alloys. All microstructural and microchemical results consistently indicated a strong tendency for palladium to form the ordered Cu3Pd superlattice in the copper-rich phase of the present ternary alloys. Transmission electron microscopic examination indicated that, in addition to the large Cu-rich particles, numerous small (typically tens of nanometers) Cu-rich particles were distributed in the Ag-rich phase. In the alloys containing 10 and 15 wt% Pd, the Cu3Pd superlattice had an L1 2-type crystal structure. In the alloy containing 20 wt% Pd, the Cu 3Pd had a periodic (regular) APB structure. The solubility of palladium in the Cu-rich phase was always much larger than that in the Ag-rich phase. The ratio of the palladium concentration in the copper-rich phase to that in the silver-rich phase decreased with the overall palladium content.

Simulated-stop tribological behavior of pitch—resin—CVI carbon—carbon composite

Abstract This work studies the braking (simulated-stop) behavior of a mesophase pitch fiber-reinforced phenolic resin plus chemical vapor infiltrated (CVI) hybrid matrix carbon—carbon (C-C) composite under different surface conditions. The results indicate that friction and wear behavior is sensitive to sliding surface condition, and the initial surface condition has a significant effect on tribological behavior of the composite. AP/1400, the only condition that does not experience a transition, shows much lower friction coefficient and wear than the other conditions. Under the same test conditions, BI specimens exhibit higher friction coefficients and wear than those of AP specimens. Of all the conditions, BI/2000 induces the highest temperature rise. Specimens braked from higher speed always suffer higher wear, but the stopping time is dependent more on the initial surface condition than the initial speed. As soon as a type I-to-type II transition occurs, the friction and wear rise abruptly. A V-shaped variation is exhibited in post-transitional friction coefficient of the composite. The severe structural damage during the final stage is responsible for the final increase in friction coefficient. Collection of large wear particles is consistent with the observed heavily delaminated surface film during this stage.