Key Engineering Materials Vols. 336-338

Abstract: In the present study, a Fe-Cr-C hypereutectic alloy was prepared from industry-grade materials and subjected to modification and fluctuation, through which new types of particle reinforced composites, hypereutectic in-situ composite, was generated. The structures of the composite modified or not with the range of fluctuation addition from 0% to 2.8wt.%, were investigated. The primary carbides were refined with the addition of modifying agents and fluctuations. Increasing the amount of fluctuation resulted in finer primary carbides. At 1380oC, with the addition of modifying agents and 2.8wt.% fluctuation addition, the structure was well modified.

Abstract: There has been a great progress in the development of heat-resistant silicon carbide ceramics, owing to the better understanding of composition-microstructure-properties relations. Based on the progress, it has been possible to fabricate heat-resistant SiC ceramics with improved fracture toughness. In this paper, three rare-earth oxides (Re2O3, Re=Er, Lu, and Sc) in combination with AlN were used as sintering additives for a β-SiC containing 1 vol% α-SiC seeds. The effect of intergranular phase, using Re2O3 and AlN as sintering additives, on the microstructure and mechanical properties of liquid-phasesintered, and subsequently annealed SiC ceramics were investigated. The microstructure and mechanical properties were strongly influenced by the sintering additive composition, which determines the chemistry and structure of IGP. The strength and fracture toughness of the Lu2O3-doped SiC were ∼700 MPa at 1400oC and ∼6 MPa.m1/2 at room temperature, respectively. The beneficial effect of the new additive compositions on high-temperature strength was attributed to the crystallization of the intergranular phase.

Abstract: The TiB2-40Cu-8Ni composite prepared by combustion synthesis is investigated. Both 2D digital radiography (DR) and 3D computed tomography (CT) are used to test the specimens nondestructively. The result shows that ceramic phase aggregation is the main defect. Then the mechanical properties of samples are assessed by means of three-point-bend test. It appears that mechanical properties are related with the distribution of TiB2 phase and Cu phase. The scanning electron microscope (SEM) is also used to observe the crack growth of the bending test sample.

Abstract: Aluminum titanate ceramics (Al2TiO5) is a synthetic ceramic material of potential interest for many structural applications. A critical feature, which greatly limits the mechanical properties of polycrystalline Al2TiO5, is considerable intergranular microcracking, which occurs due to the high thermal anisotropy of individual grains. In this study, the temperature dependencies of mechanical properties were discussed along with the microstructure observation. Both of fracture strength and fracture toughness increased considerably with increasing the temperature. These phenomena were explained on the basis of the stress redistribution and unique microscopic feature on the fracture surface of aluminum titanate ceramics. The experimental results also revealed that the repeated heat treatments resulted in the change of fracture strength and fracture toughness due to the stress redistribution in the Al2TiO5 matrix.

Abstract: A new interpretation model, instead of classical Kohlrausch-Williams-Watt (KWW) equation, was applied to interpret stress relaxation behavior of Si3N4-Y2Si2O7 ceramics. Results revealed that the new model could obtain reasonable relaxation plastic viscosity and viscoelastic viscosity under testing temperature range of 1300°C~1575°C. From the plotted curve of viscosity vs 1/RT, an activation energy change occurred around 1500°C was found for both plastic viscosity and viscoelastic viscosity curves. This change, which indicated the microstructure change, was in good agreement with the significant decrease of high temperature strength retention property.

Abstract: In this study, Al2O3-TiC composite was prepared at 1600°C for 1 h by hot-pressing using the Al2O3-TiC composite powder synthesized from self-propagating high-temperature synthesis (SHS) process. Mechanical properties of the composite were investigated, such as bulk density, Vickers hardness, fracture toughness and bending strength. The microstructure of the composite was also investigated by scanning electron microscopy (SEM).

Abstract: A series of BN-SiO2 composite ceramics with high relative densities have been fabricated by hot pressing of a mixture of hexagonal boron nitride powder and silica sol. X-ray diffraction (XRD) results show that silica are distributed in the BN matrix as noncrystalline state. Microstructure of as-sintered composite ceramics is observed by scanning electron microscopy (SEM). The influence of the secondary phase on the mechanical properties is studied by varying the silica content from 10 wt% up to 40 wt%. Dielectric properties of composite ceramics are also measured from 1 MHz to 2.0 GHz.

Abstract: Spark plasma sintering is employed to synthesize a variety of self-lubricating ZrO2(Y2O3)- Al2O3 matrix nanocomposites by tailoring the chemical compositions and by adjusting the sintering parameters. Different additives are incorporated into the nanocrystalline ceramics of ZrO2(Y2O3)- 20wt.% Al2O3 to evaluate their potentials as effective high temperature solid lubricants from room temperature to 800oC by using a high temperature friction and wear tester in sliding against alumina ball in air. The density, microstructure, hardness and tribological properties of the sintered nanocomposites have been investigated, as contrasted with the unmodified ceramics, to obtain a better understanding of lubrication mechanisms over a wide temperature range. The ZrO2(Y2O3)-Al2O3-SrSO4 composite exhibits steady-state friction coefficients of less than 0.2 and wear rates in the order of 10-6 mm3/Nm over a broad temperature range from room temperature to 800oC.

Abstract: Al/Ti3SiC2 composite samples were prepared by pressless-sintering route with high purity of polycrystalline Ti3SiC2 and aluminum powders. As yttria Y2O3 being additives during sintering process, the interesting change is that impurities Al4C3, Al4SiC4 and Al3Ti phase which are familiar in products of reactions between Ti3SiC2 and aluminum disappeared and that is valuable to stability of Al/Ti3SiC2 composite in atmosphere due to hydrolyzation of Al4C3. Then the tribological properties of 50Al/ 45Ti3SiC2/5Y2O3 and 50Al/50Ti3SiC2 were investigated by sliding the composites block dryly against low carbon steel disk for the sliding speed 20 m/s and the normal pressure of 0.2~0.8MPa. It was found that with load higher, the friction coefficient of 50Al/45Ti3SiC2/5Y2O3 increased from 0.21 to 0.57 and then reduced to 0.48, which is a little higher than 50Al/50Ti3SiC2 on large scale of pressure except under 0.2 ~ 0.3 MPa, but meanwhile it is remarkable that its rate of wear maintained a nearly steady value about 1.40 × 10-5 mm3/N·m comparing with 50Al/50Ti3SiC2, which shows a valuable tribological properties called non-pressure dependence to frictional materials.

Abstract: The tribological behavior of a new cermet Ti3AlC2/Cu was experimentally investigated. The results showed that the Ti3AlC2/Cu was a good tribological material sliding against the low carbon steel, especially for a high sliding speed. The friction coefficient was as low as 0.13 ~ 0.15, and the Ti3AlC2/Cu wear rate was only 3.4×10-6 mm3/Nm, for the sliding speed of 60 m/s and the normal pressure of 0.8 MPa. The forming of a frictional film consisted of Ti, Al, Cu and Fe oxides on the friction surfaces could be a fundamental cause.