Key Engineering Materials Vols. 434-435

Abstract: The influence of Lu2O3 on phase transformation and seeds morphology was investigated. The result showed that the β-Si3N4 seeds with up to 95% β phase content could be obtained with 2wt% Lu2O3 as the additive content under 1750°C for two hours. The microstructure and mechanical properties of hot-pressed Si3N4 ceramics, using 9wt.% of Lu2O3• additives were investigated by the means of MTS measurements and Vickers indentation crack size measurements, as well as XRD and SEM. It was known that the high fracture toughness of Si3N4 ceramics was attributed to the rodlike morphology of β-Si3N4 grains. And the reinforcement effect and mechanism of β-Si3N4 seed were studied. It was found that the grain size and its distribution influence the property and microstructure of Si3N4 ceramics, namely, the relative narrow distribution of grain diameter in some extent and relative wide range of bimodal distribution of grain aspect ratio could improve the property of Si3N4 ceramics. The improvement in the fracture toughness with the amount of additive was mainly attributed to elongated grain growth during the sintering process.The high temperature properties of self-reinforced Si3N4 with different additives were studied. By this method, self-reinforced Si3N4 ceramics with an increment of 10~20 percent of fracture toughness was successfully fabricated.

Abstract: Thermal stability and wear resistance of nanoceramic materials are excellent, so it is an ideal die material and has become one of the increasingly important topics in the field of die research. A new 3Y-TZP nanocomposite die material was prepared with hot pressing technique by using 3Y-TZP, CeO2, Al2O3 and nanometer sized TiC as raw materials. Effects of CeO2 on the mechanical property and microstructure of 3Y-TZP nanocomposite die material were investigated. The addition of CeO2 has obvious effect on the flexural strength and fracture toughness. When the content of CeO2 is 2mol%, the fracture toughness of the composite material reaches 11.22MPa•m1/2. With the increase of CeO2 content, the fracture toughness becomes lower, the flexural strength and hardness first increase and then decrease. When the CeO2 content is 6mol%, the maximum flexural strength reaches 866MPa. The reason for the improvement of mechanical property can be attributed to the effect of CeO2 on the phase transformation of t-ZrO2 and microstructure and the resulted phase transformation toughening mechanism.

Abstract: Three-dimensional needled carbon/silicon carbide (C/SiC) composites with pyrolytic carbon interfacial layer were fabricated by precursor pyrolysis. The microstructure and mechanical property of the three-dimensional needled C/SiC composites were investigated. A thin pyrolysis carbon layer (0.2m) was firstly deposited on the surface of carbon fiber as the interfacial layer with C3H6 at 850 °C and 0.1 MPa by chemical vapor infiltration. Polycarbosilane and divinylbenzene were selected as a precursor to silicon carbide ceramics and a cross-linking reagent for PCS, respectively. The density of the composites was 1.94 g cm-3. The flexural strength of the three-dimensional needled C/SiC composites was 135 MPa. The three-dimensional needled C/SiC composites with pyrolytic carbon interfacial layer exhibit good mechanical properties and a typical failure behavior involving fibers pull-out and brittle fracture of sub-bundle.

Abstract: CNTs/AlN ceramics were fabricated by hot-pressing and their bulk density, flexural strength, thermal conductivity were characterized. The microstructure was also investigated. The fracture surface were analyzed by SEM. TEM was used for analyzing the microstructure. It is found that the density, mechanic and thermal-conductivity properties markedly decreased as the CNTs reinforced AlN ceramic; through microstructure observation, the conglomeration of CNTs mostly exist among the AlN grain boundary, and the CNTs were scathed by the high temperature and pressure of the hot-pressing.

Abstract: B4C-TiB2 composite was prepared using hot pressure sintering. The microstructures and mechanical properties of the B4C-TiB2 composite were investigated. The B4C-TiB2 composite with 43 mass % TiB2 showed the optimized properties. The relative density, hardness, flexural strength and fracture toughness of that were 98.2 %, 25.9 GPa, 458 MPa and 8.7 MPa•m1/2, respectively. A number of toughening mechanisms, including fine grain, crack deflection and grain pull-out, were observed during microstructural analysis of the composite. The fracture mode of the B4C-TiB2 composite was greatly affected by the existence of the second phase of TiB2.

Abstract: Three dimensional carbon fiber reinforced silicon oxycarbide (3D Cf/Si-O-C) composites with low cost silicon resin as precursors and 3D Cf as reinforcement. Effects of adding SiC powder (SiCP) on the microstructure, mechanical properties and anti-oxidation properties of 3D Cf/Si-O-C composites were investigated. The results showed that adding SiCP filler could reduce the porosity and improve the interface bonding, therefore the properties of composites increased. But when the SiCP content was excessive, it was difficult to dense the matrix of composites at the further cycles and pores existed in the matrix. As a result, the mechanical properties of the composites decreased. It was found that when fabricated with 18.2 weight percent SiCP the composites exhibited highest mechanical properties, and the flexural strength and fracture toughness reached 421.3MPa and 13.0 MPa•m1/2, respectively. And the anti-oxidation properties were improved with the increase of the SiCP content. When fabricated with 25.0 weight percent SiCP the composites exhibited best oxidation resistance properties, and the composites retained 89.5% of original flexural strength.

Abstract: In this work damage micro-mechanisms of two different types of fibre reinforced composites are investigated by acoustic emission, AE. Ceramic based oxide fibre reinforced mullite matrix composite and metallic based SiC fibre reinforced titanium matrix composites exhibit different fracture mechanisms during loading and AE technique could pinpoint these damage mechanisms based on the AE responses detected simultaneously. The results show that in a ceramic matrix composite, the identification of fibre fracture and matrix cracking requires careful analysis of the AE data as both fibres and matrix break in brittle manner. Whereas the separation of fibre fracture from the ductile tearing of matrix ligaments could be easier in metallic based composites, such as titanium matrix composites.

Abstract: The sections of Φ55mm silicon nitride balls, sintered by gas pressure sintering were analyzed. The results show that temperature gradient during of the sintering process from the surface to the core of large size silicon nitride balls occurred because of the lower thermal conductivity of Si3N4. With the diameter increasing, the temperature gradient was more visible. The impurity of raw materials, such as free Si, free C and other metal oxides, like the SiO2 could produce gas, such as SiO, CO and so on, during the sintering process through the thermodynamic analysis. The producing gas exhausted more difficultly with the diameter of silicon nitride balls increasing. These factors were the most important to the defects of large size silicon nitride balls during the sintering process and made cracks and crescent on the surface of balls.

Abstract: Effects of SiO2 and Fe2O3 on morphological of mullite were investigated for a special use related to mullite containing SiO2 and Fe2O3. It was concluded that, when Al2O3/SiO2 ratio near the theoretical component of mullite, the composites cannot been sintered at 1600°C. The normal morphology of mullite presents granule form with Fe2O3 addition, while takes a long column shape with the condition of excessive. SiO2 presenting. Fe2O3 can promote sintering of mullite, as well as growing in needle-like morphology.

Abstract: SiC is a perfect reinforced material, characteristic of high hardness, high wear- and corrosion-resistant property, and low cost. SiC-reinforced iron-matrix composites show high wear resistance, high hardness, high inflexibility and high strength, with wide applications as superior wear-resistant and high temperature materials. This paper reported a heterogeneous precipitation method to coat SiC with copper particles. The vacuum hot-pressing method was used to sinter the Fe/SiC composites with Cu-coated SiC powders. The techniques of XRD and SEM were used to characterize the compositions and microstructures of the samples. The Archimedes method was used to test the density. The results showed that SiC and Cu were homogeneously mixed in the composite powders obtained by the heterogeneous deposition method, and that the composites with 5wt% of SiC (Cu) obtained at 950°C have a high relative density of 96%, a high hardness of 4121 MPa and a high bending strength of 646 MPa. The enhanced properties of Fe/SiC composites could result from the improved interfacial consistency by using Cu-coated SiC powders, which could inhibit some adverse interfacial reactions.