Chengyu Zhang

Microstructures of melt-spun Cu100-x-Crx (x=3.4-25) ribbons

Abstract Cu 100− x –Cr x ( x =3.4–25) ribbons were prepared by melt-spinning. Examination revealed that a supersaturated solid solution and a few very fine Cr-rich particles can be obtained for the Cu 96.6 Cr 3.4 ribbon. When x is ≥9, a microstructure with refined Cu- and Cr-rich phases can be obtained by melt spinning. The analyses indicated that the Cr-rich phase in the ribbons may be formed in two ways. One is the direct solidification from the liquid alloys during melt spinning, the diameter range of the Cr-rich particles formed lying between 30 and 200 nm. The other is the solid-phase separation of a Cu-rich supersaturated solid solution, the diameter of Cr-rich particles formed being only several nanometers.

Tribological Behaviour and Cutting Performance of PVD-TiN Coating/Substrate System with Discontinuous Surface Architecture

Abstract TiN coating was deposited by arc evaporation PVD (physical vapor deposition) onto tool steel. A netted screen made of the stainless steel was placed between substrate and vaporizer in order to get discontinuous TiN coatings. Three kinds of surface condition (uncoated, continuous and discontinuous TiN coatings) were prepared and examined for their performance. Tribological behavior was investigated by means of dry and lubricated sliding tests at room temperature and 200 °C, on a disk-on-block and sphere-on-flat tribometer. The results show that the discontinuous TiN coating significantly decreases both the wear and the wearing speed of tool steel under sliding tests, and reduces friction under conditions of bidirectional sliding. In the three kinds of surface condition, the discontinuously coating has superiority for high speed cutting owing to its lifetime increasing compared with uncoated and continuously coated.

Cathode spot movements along the carbon fibres in carbon/carbon composites

The cathode spot movements on a polyacrilonitrile (PAN)-based carbon felt reinforced C/C composite and a three dimensional PAN-based carbon fibre reinforced C/C composite (3D-C/C) were investigated by a scanning electron microscope and a digital high-speed video camera. It was found that the carbon fibres have a higher ability to withstand the vacuum arc erosion than the carbon matrix. The cathode spot walks on the matrix, rather than on the carbon fibres. The cathode spot motion is controlled by the architecture of carbon fibres in C/C. The cathode spots move along the carbon fibres by a step-by-step manner rather than a random walk. The cathode spot tracks spread over a wide zone on the 3D-C/C surface parallel to the carbon fibre. The average arc spreading velocity is estimated to be about 0.9 m s−1 and the transient arc spreading velocity is in the range of 0.54–4.5 m s−1.

LOW ELECTRODE EROSION RATE OF NANOCRYSTALLINE CuCr-50 ALLOY IN VACUUM

The CuCr-50 alloy with grain size of less than 50 nm was prepared by the mechanical alloy and hot pressing technique. Its microstructure was examined by XRD, SEM and TEM. The electrode erosion rates of nanocrystalline and microcrystalline CuCr-50 were investigated. It is found that the electrode erosion rate of nanocrystalline CuCr-50 is 24.8 μg/C, much lower than that of microcrystalline CuCr-50. The main cause is that the vacuum arc shows diffusional features on the nanocrystalline electrode, decreasing the heat flux, current density and the dimensions of the droplets emmited from the cathode.

Interlaminar shear fatigue of a two-dimensional carbon fibre reinforced silicon carbide composite

Abstract Interlaminar shear fatigue properties of a two-dimensional carbon fibre reinforced silicon carbide composite were investigated at room temperature (RT) and 900°C in air. The interlaminar shear strength (ILSS) of the survived specimens was determined to reveal the damage mechanisms. The composite presents excellent resistance to the fatigue at RT. The fatigue limits at 900°C are much lower than that at RT. Moreover, ILSS can be enhanced for the survived composite to some extent. The damage involves the matrix cracking and interfacial debonding. Such damage offers the channels for the oxidation of the pyrolytic carbon interface and carbon fibres, and leads to the decrease in the fatigue limits at 900°C.

Thermal shock resistance of a 2D-C/SiC composite and its damage mechanisms

Abstract The present work investigates the thermal shock properties of a two-dimensional carbon fibre reinforced silicon carbide composite (2D-C/SiC) in air. The 2D-C/SiC specimens were thermally shocked up to 60 cycles between 900 and 300°C. The thermal shock resistance was characterised by the residual ultimate tensile strength (UTS) and interlaminar shear strength (ILSS). The surface morphology and microstructure of the thermally shocked specimens were examined by a scanning electron microscope and an X-ray diffractometer. It is found that ILSS is more sensitive to the damage caused by thermal shock. The composite retains its UTS within 20 thermal shock cycles. However, the ILSS of 2D-C/SiC decreases gradually with increasing thermal shock cycles. The damage mechanisms involve matrix cracking, weakening of the bonding strength of coating/composite and/or fibre/matrix interface, as well as oxidation of PyC interface and carbon fibres.

Cathode erosion of graphite and Cu/C materials in airarcs

Cathode erosion of graphite and Cu/C was studied in direct current arcs, which were ignited between two electrodes comprised of two kinds of carbon materials and a tungsten anode in air. The arced zones on the cathode surface were investigated by a scanning electron microscope. Also, the cathode erosion rates of the investigated materials were measured. The results show that two distinct zone can be seen on both cathodes. The eroded area was located at the zone just opposite to the anode and surrounded by a white zone. The arced surface on the Cu/C containing 9.3 % Cu is rougher than that of the pure graphite. Many particles with various sizes distributed on the Cu/C. The vaporization of Cu can lower the surface temperature and reduce the cathode erosion. Therefore, the cathode erosion rate of the Cu/C is lower than that of the pure graphite.

Oxidation Behaviors and Mechanisms of C/Si–C–N with a Mullite Interlayer

Carbon fiber reinforced Si–C–N matrix composite with a mullite interlayer (C/Mullite/Si–C–N) was fabricated via CVI and PIP process. The oxidation behaviors of C/Mullite/Si–C–N were investigated in air by comparison with that of C/PyC/Si–C–N using thermogravimetry and SEM technique. The results indicate that the weight loss of the C/Mullite/Si–C–N increases with the increase of the temperature below 800°C, decreases in the range of 800–1000°C, and increases again above 1000°C. The increase of the weight loss below 800°C results from the increase of the oxidation temperature. The closure of the matrix microcracks at 800–1000°C leads to the decrease of the weight loss, and the reopening of microcracks results in the increase above 1000°C. The oxidation resistance of C/Mullite/Si–C–N was improved greatly as compared to C/PyC/Si–C–N within the temperature range from 600°C to 1200°C. The mullite interphase changes the oxidation mode of C/Si–C–N and leads to the non-uniform oxidation of carbon fibers in the composite. At 600°C, the oxidation curve of C/Mullite/Si–C–N shows obvious non-linearity, which mainly results from the non-uniform oxidation of carbon fibers.

Measurement of In-Plane Shear Strength of Carbon/Carbon Composites by Compression of Double-Notched Specimens

The compression of a double-notched specimen was used to determine the in-plane shear strength (IPSS) of a carbon/carbon composite in the paper. The effects of the notch distance (L), thickness (T), and notch width (W) and supporting jig on the IPSS of the double-notched specimens were investigated numerically and experimentally. The fracture surfaces were examined by a scanning electron microscope. It was found that the IPSS varied with L. Thin specimen yielded low strength. W has little effect on IPSS. The main failure modes include the matrix shear cracking, delamination, fracture and pullout of fibers or fiber bundles. Meanwhile, a supporting jig can provide lateral support and prevent buckling, therefore lead to the failure in a shear mode.

The influence of high temperature exposure to air on the damage to 3D-C/SiC composites

3D-C/SiC composites, exposed in air at 600, 900, and 130℃for 0 to 15 h, were investigated by three point bend tests at room temperature, SEM, and energy dispersive spectroscopy. The results show that the damage curves, expressed as a relative change of elastic modulus, of the composites for a 15h exposure, could be divided into a sharply increasing stage (stage Ⅰ) and a steady increasing stage (stage Ⅱ). Stage I may be caused by a direct oxidation of the carbon fibers and interface carbon layers by the oxygen in air, and stage Ⅱ may be caused by a diffuse controlled oxidation of the inner part of the composites. The matrix micro-cracks, induced by a difference of coefficients of thermal expansion between matrix and carbon fibers in the cooling process after composite preparation act as oxygen diffuse paths and arc where the oxidation takes place. The fact that the damage decreases with temperature for the same exposure time may be caused by the crack shrinking at high temperature. which decreases the oxidizable surface area and inhibits the diffusion of oxygen into the composites.