Ming Xing Ai

Unusual Microstructures and Strength Characteristics of Cu/Ti3AlC2 Cermets

Cu/Ti3AlC2 cermets prepared by pressless sintering a mixture of Ti3AlC2 and copper powders were investigated. It was found that the Cu/Ti3AlC2 possesses an unusual microstructure made up of sub-micro-sheet layered Ti3C2 and Cu-Al alloy within one Ti3AlC2 particulate. The fracture strength measured by the three-point-bending manner is increased but the deformation rate is reduced with increase in the volume content of Ti3AlC2 from 30 % to 90 %. The highest fracture strength reached to as higher as 983.9 MPa, corresponding to an extreme strain of 2.64 %. The fracture in mode was changed from brittle to ductile with reduce in the content of Ti3AlC2. The higher fracture strength can be attributed to a stronger interface bond between Ti3AlC2 and Cu-Al phase. A significant network feature formed by the Cu-Al alloy surrounding Ti3AlC2 particulates was observed from the fracture face.

Interformational Exfoliation of Ti3Al2 Induced by Cu

An interformational exfoliation behavior of the layered Ti3AlC2 induced by copper was firstly investigated via a “Cu-Ti3AlC2-Cu” sandwich sample infiltration-sintered at 1100oC to 1200oC. It was found that the molten Cu accelerates Ti3AlC2 to decompose, induces the interformational exfoliation to generate, and consequently forming a sub-micro-layered structure making up of TiC0.67 layers and Cu-Al alloy layers within a Ti3AlC2 grain. This interformational exfoliation behavior can be attributed to a topotactic mechanism due to the outward diffusion of Al the entering of Cu.

Preparation of Composites from Al and Ti3AlC2 and its Tribo-Chemistry Reactions against Low Carbon Steel

Al/Ti3AlC2 composites containing 50vol% Al were prepared with high purity of polycrystalline Ti3AlC2 and aluminum powders by pressureless-sintering route at temperatures of 700°C~ 800°C The tribological properties of the composites were investigated by sliding the composites block dryly against low carbon steel disk under high sliding speed. Before and after friction test, the morphology and phase analysis were observed by scanning electron microscope (SEM) and X-ray diffraction (XRD), separately. A definite tribo-glazing layer was found over the worn surface of the composite block, which was the results of tribo-chemical oxidation reaction and the cause forming it could be the high frictional temperature and the mechanical catabolism between the surface of Al/Ti3AlC2 and low carbon steel during sliding friction. The effect of Ti3AlC2 on tribological properties of Al/Ti3AlC2 composite and the possible tribo-chemical reaction mechanism on surface layer of Al/Ti3AlC2 were suggested.

Pressureless Sintering and Properties of Cu/Ti3AlC2 Composites

The Cu/Ti3AlC2 composites were fabricated by pressureless sintering a mixture of Ti3AlC2 and copper powders. Their microstructures and properties were investigated. It was found that the molten Cu accelerating the decomposition of Ti3AlC2, inducing the interfacial exfoliation to generate, and forming a sub-micro-layered structure making up of TiCx layers and Cu-Al alloy layers within a Ti3AlC2 grain. The flexural strength of the composites is reduced with the increase of the volume content of Ti3AlC2 from 50 % to 90 %. The highest flexural strength reaches to as high as 915 MPa. The fracture mode was changed from ductile to brittle with increase in the content of Ti3AlC2. The higher flexural strength can be attributed to a stronger interface bond between TiCx and Cu-Al phase. The electrical resistivity and Vickers hardness of the composites were also measured.

A New Ti3AlC2/Cu Cermet Exhibiting Excellent Tribological Properties

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.

Effect of Y2O3 on Properties of Al/Ti3SiC2 Composite

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.

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.

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.

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.