Wan Chang Sun

Effect of short carbon fibre concentration on microstructure and mechanical properties of TiCN-based cermets

Short carbon fibre (Cf) reinforced TiCN-based cermets (Cf/TiCN composites) were produced by powder metallurgy method with pressureless sintering technology. The phase evolution, microstructure and fracture morphology of Cf/TiCN composites were investigated. The results showed that TiC, TiN, WC, Cr3C2 and Mo phases disappeared gradually and diffused into core and rim phases by dissolution–reprecipitation process, finally formed new hard TiCN core phases and complex compound (Cr, W, Mo, Ti)(CN) rim phases, with the sintering temperature increasing. The added Cf did not change the ‘core–rim’ microstructure but improved the mechanical properties of TiCN-based cermets. The Cf/TiCN composite containing 3 wt-% Cf achieved the best comprehensive mechanical properties, with fracture toughness and bending strength increasing by about 14.4% and 30.8%, respectively, when compared with the composite without Cf. Toughening and strengthening mechanisms of Cf/TiCN composite were concluded as crack deflection and branch, as well as the pull-out, fracture and bridging of carbon fibres.

Microstructural Control of Al/Si Composites by Area-Selectively Liquid-Phase-Sintering Method

Area-selectively liquid-phase sintering (AS-LPS) method was developed to control the coarsening of Si particles in Al/60vol.% Si composites with the help of sol-gel-derived interfacial modification, achieved by pre-coating discontinuous Al2O3 film on Si powders. The results show that due to the local presence of less than 1 μm-thick films, Si particles can approximately preserve their as-received size and morphology after being sintered at 950 °C. Whereas, for those prepared by use of the unmodified Si powders, Si phases evolve to several hundred-micron-sized flakes, being similar to the typical eutectic Si in Al-Si casting alloys. It is assumed that the Al2O3-film-coated areas at Al/Si interface control the dissolution of Si into Al liquid, while in uncoated areas Al matrix and Si particles bond together by LPS, thus realizing AS-LPS.

Effects of Heat Treatment on Mechanical Properties of Ni-P-CNT Composite Coating

Ni-P-CNT nanocomposite coating was successfully co-deposited by electroless plating and the heat treatment was carried out at 200°C, 400°C, 600°C in nitrogen atmosphere respectively for a holding period of 1 h. The effects of heat treatment on the microstructure and mechanical properties of Ni-P-CNT composite coating were investigated. The results indicate that the heat treatment at 400°C can greatly improve the hardness and wear resistance of the composite coating. The reason is that Ni3P hard phase is greatly precipitated after the heat treatment, which played a strengthening effect. On the other hand, the precipitated Ni, Ni3P crystalline phases in the coating result in an increase of the amount of grain boundary. The increased amount of grain boundary broke the spread of shear force during friction process, and reduced the wear loss caused by friction pair. Compared with as-deposited coating, the coatings after heat treatment possess higher microhardness and wear resistance.

Microstructure and Corrosion Behavior of AZ91-0.4%Nd Magnesium Alloy

The corrosion behavior of AZ91-0.4%Nd alloys was investigated by immersion test in 3.5wt.% NaCl at 25°C. The phase compositions and microstructure of the AZ91-0.4%Nd alloy were characterized by XRD and OM, respectively. The results show that the number of Nd element in the AZ91 magnesium alloy has effect on the grain refining efficiency, and the granular or acicular Al3Nd phase precipitate in matrix.The corrosion products of the AZ91-0.4%Nd alloy mainly composed of Mg(OH)2 and Al. The addition of Nd element significantly refine the microstructure and improve the compactness of corrosion products of AZ91-0.4%%Nd magnesium alloy, as a result the corrosion resistance of alloy was improved obviously.

Microstructure of Pyrolytic Carbon Matrix in Carbon/Carbon Composites after Graphitization

The morphologies and textures of the pyrolytic carbon matrix in 2D-C/C composites after graphitization were investigated by means of polarized light microscope (PLM) and high resolution transmission electron microscope (HRTEM). The microstructure parameters of the pyrolytic carbon matrix before and after graphitization were characterized with X-ray diffraction (XRD) technology. It was found that the interplanar distance of (002) planes (d002) of pyrolytic carbon matrix decreases, and the microcrystalline stack height (LC) increases after graphitization. Graphitization treatment resulted in a coarsening of the surface texture and in the formation of circumferential cracks within the matrix. The lattice fringes of the pyrolytic carbon matrix are continuous and longer in each domain and the (002) peak spot is smaller and more intense after graphitization.

Application of Artificial Neural Networks to Optimize Processing - Properties of Ni-Tic Composite Coatings

The plating parameters for optimizing the wear and corrosion resistance of Ni-TiC composite coatings were selected by orthogonal test, mainly including the TiC particles concentration, current density, duty cycle, frequency and stirring rate. A three-layer BP (Back Propagation) neural network with Lavenberg-Marquardt algorithm was established by MATLAB, which was used to train the network and predicted orthogonal experimental data. In addition, the best parameters combination of the composite coating were predicted and verified by experiments. The results predicted through the proposed BP model are in good agreement with the experimental values, the relative error is small, and the maximum error is less than 3% and the coefficient of determination value is 0.9997.

Microstructure and High-Temperature Oxidation of Ni-Si3N4 Composite Coatings by Pulse Electrodeposition

Nickel matrix and Si3N4 micron particles were co-deposited on the aluminum alloy by pulse electro-deposition for high temperature performance. Meanwhile, the oxidation resistance was evaluated through the high temperature oxidation test. The phase structure, micrographs and components of the composite coatings were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) together with energy dispersive spectroscopy (EDS) respectively. The results indicated that Si3N4 particles were uniformly distributed across the coating and there were no pores and cracks or other defects at the coating/substrate interface. Ni-Si3N4 composite coatings are characterized by pyramidal micro-crystallite structure. The thickness of Ni-Si3N4 composite coatings were up to 80 μm for 2h. The results also revealed that the Ni-Si3N4 composite coatings presented better oxidation resistance than the pure Ni coating and aluminum alloy at high temperature. After oxidation at 673 K for 8h, the oxidation resistance of Ni-Si3N4 composite coatings presented the improved oxidation resistance behavior compared to pure Ni and the aluminum alloy, respectively.

Influence of SiC Content and Heat Treatment on the Corrosion Resis-Tance and Oxidation Resistance of Ni-P-SiC Composite Coating

Ni-P alloy and SiC micron particles were codeposited on Q235 steel by electroless plating. The composition, microstructure, micro-hardness, corrosion resistance and oxidation resistance of the composite coating were studied. The results revealed that the deposited composite coating shows dispersed SiC particles and continuous Ni-P matrix. When the content of SiC was 8g/L and the heat treatment temperature was 300°C, the corrosion potential and corrosion current of Ni-P-SiC coating were-0.292V, and 8.2×10-7 A/cm2, respectively, while those of Ni-P composite coating were-0.501V, and 4.2×10-5 A/cm2, respectively. Ni-P-SiC composite coating with high content of SiC exhibits better oxidation resistance than Ni-P coating.

Microstructure, Corrosion and Wear Resistance of Co-Ni-ZrO2 Composite Coating

Co-Ni alloy and ZrO2 submicron particles were successfully co-deposited on carbon steel substrate by direct current electrolytic deposition. The micromorphology, constituent, microhardness, corrosion and wear resistance of the composite coatings were tested, respectively. The results show that the embedded submicron ZrO2 particles are uniformly distributed in the entire Co-Ni matrix and the coating showed a good adhesion to the substrate. The hardness, friction coefficient, wear loss, and electrode voltage of Co-Ni alloy coating were 356 HV, 0.8, 1.901×10-2 mg/m, and-0.47 V, respectively, while those of Co-Ni-ZrO2 composite coating were 413 HV, 0.6, 1.174×10-2 mg/m, and-0.37 V, respectively. The data above suggested that Co-Ni-ZrO2 composite coating possesses higher microhardness, better wear and corrosion resistance.

Effects of Ni/Co Metal Binder Phase on Mechanical Properties of Ti(C,N)-Based Cermets

Ti(C,N)-based cermets were prepared by pressureless sintering using micron-sized TiC and TiN powders as main starting materials and Ni and Co as metal binders. The fracture micrographs of Ti(C,N)-based cermets were observed by SEM. The effects of metal binder phase content on porosity and mechanical properties of Ti(C,N)-based cermets were investigated. The results show that when sintering at 1450 °C for 50min, typical core/rim microstructure is observed in the cermets, which is compact and distributed homogeneously, the density and relative density are 5.9 g/cm3 and 95.6%, respectively. Intergranular fracture is the main crack propagation mode, with a few of the transgranular fracture, and there are more dimples and tearing ridges in the fracture surface. Using the same sintering process, Ti(C,N)-based cermets with metal binder content of 10%Ni5%Co has a compact microstructure and fine grains, the metal binder phase is dispersed more evenly, the best vicker hardness, bending strength and fracture toughness of Ti(C,N)-based cermets are 15.8 GPa, 885.5MPa and 9.7 MPa·m1/2, respectively. However, Ti(C,N)-based cermets with Ni replaced of high Co content will generate some brittle intermetallic compound, its toughness is inferior to the metal binder phase, which reduces the strength and toughness of Ti(C,N)-based cermet materials.