Cui Wei Li

Tribo-Chemical Reaction in Bulk Ti3SiC2 under Sliding Friction

The Ti3SiC2 samples with a second phase TiC, prepared by hot-pressing progress route, were rubbed against low carbon steel disk with a sliding speed of 20 m/s under normal pressure 0.8 Mpa in atmosphere on a block-on-disk type friction tester. The morphology was observed by scanning electron microscope (SEM) and meanwhile the composition was checked by energy dispersion spectroscopy (EDS). X-ray diffraction (XRD) patterns show some impurity phases containing Ti, Si and Fe oxides in the samples. The possible tribo-chemical reaction mechanism on surface layer of Ti3SiC2 was suggested.

Fabrication of Porous Mullite Ceramics with High Porosity Using Foam-Gelcasting

In this paper, porous mullite ceramics with an apparent porosity up to 81 % were fabricated by foam-gelcasting using mullite powder as raw material with solid loading of 40 vol.%. The monomers content and sitering temperature have obvious effect on the properties of porous mullite ceramics. The apparent porosity of the prepared samples was in the range of 75~82%, compressive strength, was in the range of 3.0~16.02 MPa, and thermal conductivity was between 0.14 and 0.47 W/(m•K). A complex porous microstructure was formed, where large spherical pores contained small cellular pores on their internal walls.

Flexure Creep Behaviors of Si3N4/BN Laminated Ceramic Composites Produced by Rolling

Si3N4/BN laminated ceramics were fabricated via a processing o f rolling, coating and hot pressing. Their flexure creep behaviors were examined at 1100 ∼ 3 °C in air, and then compared with those of monolithic Si 3N4 ceramics. The Si 3N4/BN laminated ceramics exhibited a better creep resistance than their monolithic counterparts. It was supposed that th e incorporation of BN interlayers improved the creep resistance of the composites by producing incompatible deformation and confining the distribution of glassy phase to planar continuity. Further mor , the BN interlayers could influence the creep resistance of the composites by “purifyin g” Si3N4 grain boundaries of Si3N4 layer. Compared to those with (Al 2O3+Y2O3+MgO) addition, the composites using (Al 2O3+Y2O3+La2O3) system as sintering aids showed an improved creep resistance . O the basis of experimental results, the primary mechanisms of creep deforma ti n could be mainly attributed to viscous flow of the glassy phase and grain boundary sliding.

Fabrication and Properties of Porous Anorthite⁄Mullite Ceramics

Porous anorthite/mullite composite ceramics with different mullite content were fabricated by foam-gelcasting, using CaCO3, SiO2, α-Al2O3 as raw material for anorthite phase and mullite powder for mullite phase. Effects of mullite powder content on bulk density, porosity, compressive strength and thermal conductivity of the porous composite ceramics were researched. It has been shown that mullite powder content has great effect on microstructure and properties of the porous anorthite⁄mullite composite ceramics. The open porosity of the prepared porous anorthite⁄mullite composite ceramics is in the range of 58.7 %~77.5 %, the compressive strength is between 4.2 and 30.9 MPa, and the thermal conductivity is in the range of 0.18 ~1.47 W⁄(m·K).

Synthesis of Ti3Si0.4Al0.8C1.8 Solid Solution

In this study, free Ti/Si/Al/C powder mixtures with molar ratio of 3:0.4:0.8:1.8 were heated in Argon with various schedules, in order to reveal the possibility for the synthesis of Ti3Si0.4Al0.8C1.8 solid solution powder. X-ray diffraction (XRD) was used for the evaluation of phase identities of the powder after different treatments. Scanning electron microscopy (SEM) was used to observe the morphology of the Ti3Si0.4Al0.8C1.8 solid solution. XRD results showed that predominantly single phase samples of Ti3Si0.4Al0.8C1.8 was prepared after heating at 1400oC for 5 min in Argon and the lattice parameters of Ti3Si0.4Al0.8C1.8 lay between those of Ti3SiC2 and Ti3AlC2.

Synthesis and Properties of Ternary Carbide Ti3AlC2 Ceramics

Polycrystalline bulk samples of ternary carbide Ti3AlC2 ceramics were fabricated by reactively hot-pressing a mixture of Ti, Al, and graphite powders with and without Sn additive. The effects of sintering temperature, time and addition of Sn on the purity, mechanical properties and microstructure of Ti3AlC2 were investigated. The result showed that the TiC content was strongly influenced by sintering temperature for the Ti3AlC2 samples without Sn additive, and the most suitable sintering temperature to create the lowest TiC content was 1400°C. The addition of Sn additive led to a distinct decrease in TiC content. The flexural strength of the testing materials had close relation with the TiC content and sintering time. A certain content of TiC second phase and longer sintering time were helpful to improving the flexural strength. The sample sintered at 1400°C for 2 h possessed the highest flexural strength.

Study of Ti3Si1.2-xAlxC2 (x = 0~1.2) Solid Solution via X-Ray Diffraction

A search for Ti3Si1.2-xAlxC2 (x=0~1.2) solid solution was undertaken using precise X-ray diffraction measurements. The samples covering the whole concentration range were studied. Except very ends of the concentration range, the samples contained two phases, identified as Ti3Si1.2-xAlxC2 solid solution and TiC respectively. Lattice parameter, a increased, c increased, c/a increased, and cell volume increased with the increasing of Al concentration.

Influence of Toughening Method on Microstructures and Mechanical Properties of Alumina-Matrix Composites

Various toughening methods, i.e. partially stabilized zirconia transformation toughening, transformation- SiC whisker reinforcing and transformation-SiC particle reinforcing were used to improve the mechanical properties of alumina ceramic. Influence of various toughening methods on microstructure and mechanical properties of the alumina-matrix composites were studied. On the basis of transformation toughening, by which the strength and toughness of Al2O3 ceramic improved simultaneously, the addition of SiC whisker substantially enhanced the toughness, whereas the addition of SiC particle increased both toughness and strength to a certain degree. Mechanical properties of the testing materials were closely related with their morphologies of fracture surfaces. Toughening mechanisms of the composites were also studied. In the transformation-whisker reinforced composite or the transformation-particle reinforced composite, the two toughening methods affected with each other and produced a cooperative toughening effect.

Mechanical Properties and Oxidation Behavior of Si3N4/BN Laminated Ceramics

Laminated ceramics with high mechanical properties were fabricated in the Si3N4/BN system. The mechanical properties at elevated temperatures were tested, and the oxidation behavior during tested procedure was studied at the same time. The flexure strength of the Si3N4/BN laminated ceramics changed a little below 1000°C. The displacement-load curves appeared non-linear characteristic even at high temperature. During testing procedure at high temperature, oxidation behavior of silicon nitride and silicon carbide happened, and no oxidation product of boron nitride was found. The silicon nitride layers were oxidized to form a protective silicate scale, which prevented oxidation of the boron nitride interlayers. The stability of boron nitride was beneficial to the boron nitride interlayer to partition the silicon nitride matrix layers at high temperature.

Influence of Loading Conditions on Cyclic Compressive Fatigue Behavior of an Al2O3 Matrix Composite

An Al2O3 matrix composites, i.e. partially stabilized zirconia toughening alumina (ZTA) reinforced by SiC particle (ZTA-SiCP), was prepared by hot pressing (HP). Fatigue behavior of ZTA-SiCP under cyclic compressive loads was investigated on different loading conditions. The application of cyclic compressive loads to a notched specimen led to a stable crack growth along the notch plane in a direction normal to the far-field compressive axis. Irreversible damages in the main form of microcrack were induced at the stress concentration zone during compression loading, and it led to high residual tensile stresses ahead of the notch root upon unloading. Nucleation and growth of a model I fatigue crack were caused by the residual tensile stresses at the notch root. Along with propagation of the fatigue crack, a gradual decrease in crack growth rate was shown due to the crack closure caused by accumulating of debris particles within the wake of growing crack, and thus led to the crack arrested at last. The fatigue crack length was investigated as a function of notch length, the maximum compressive stress, stress range and load frequency.