Key Engineering Materials Vols. 368-372

Abstract: The K2SO4-Na2SO4 system was studied by differential scanning calorimetry (DSC) with the aim of developing a new phase-change thermal energy storage material. The temperature range of phase change is from 800°C to 1069°C according to the phase diagram. A new shape-stabilized phase-change material made of molten salts impregnated by capillary forces in a porous-fiber matrix was presented. These materials were characterized by X-ray diffraction analysis and differential scanning calorimetry analysis. The results indicated that the compound included 70~80% of molten salts, meanwhile the heat storage material could keep its shape without any leakage during the heating process.

Abstract: The uniaxial tension under various strain rates, creep under various sustained loads, and equalamplitude- strain loading and unloading tests are carried out at room temperature with polyamide 6/nano- SiO2 composite specimens. According to the elasticity recovery correspondence principle, the recovered elastic stresses (strains) in the case of prescribed strain (stress) history are calculated, and the instantaneous elastic constitutive equations are deduced. The nonlinear viscoelastic constitutive relations in single integral form on the basis of the instantaneous elastic constitutive equations are constructed and applied to model the current stress (strain) responses of polyamide 6/nano-SiO2 composite. The theoretic results agree well with the experimental data, which demonstrates that the single integral constitutive relations used in this work can accurately simulate the physical nonlinear viscoelastic properties of polyamide 6/nano-SiO2 composite. Finally, the creep curve at higher stress level is horizontally shifted along logarithmic timescale using a stress shift factor in terms of the time-stress superposition principle and superposed on that at relative lower stress level to form a master creep compliance curve that spans a longer timescale interval than the short-term test curve does, which suggests that TSSP provides an accelerated characterization method for the long-term creep performance of polyamide 6/nano-SiO2 composite.

Abstract: A theoretical model considering the distribution of the grain sizes and grain-boundary ledges was developed for cavity nucleation in creeping ceramics containing viscous grain boundary phase. The critical stress involving the effects of various shapes of cavities and viscosity coefficient (η) for cavity nucleation was presented. Then, the local stress concentration factor concerning different distributions of grain-boundary ledges was calculated and discussed. The results showed that various shapes of cavities and η are not the main influencing factors on the critical stress. The stress concentration factor increases with the increase of grain-boundary ledge size or the density. Therefore, it would be liable to cause cavity nucleation at gain boundary when the density of grain-boundary ledges is high and the size is large.

Abstract: The high-temperature friction and wear characteristics of different ceramics and ceramic matrix composites (CMCs) incorporated with various solid lubricants have been investigated from room temper- ature to 1000oC. The solid lubricants considered in this paper include representative precious metals, hexagonal boron nitride, graphite, fluorides, soft oxides, chromates, sulfates, and combinations of various solid lubricants. General design considerations relevant to solid lubrication were proposed on the basis of friction and wear data of self-lubricating CMCs. The self-lubricating composites incorporated with SrSO4 or/and CaSiO3 exhibits low and stable friction coefficients of 0.2 to 0.3 and small wear rates in the order of 10-6 mm3/Nm from room temperature to 800oC. The optimized composites appear to be promising can- didates for long-duration, extreme environment applications with low friction and small wear rate.

Abstract: Friction and wear tests of Al2O3 and SiC were conducted from room temperature to 1200°C both in air and in vacuum. Results show that the wear mechanism of Al2O3 is dominated by micro fracture, debris abrasive and delamination at temperatures below 600 °C, while is controlled by plastic deformation and recrystallization among 600~1200 °C, resulting in an obvious decrease of wear loss. The wear rate and surface microstructure of SiC are closely depending on the testing temperature, atmosphere and contact pressure. Oxidation of SiC at elevated temperatures plays important role on the wear rate. Self lubrication of both Al2O3 and SiC at high temperatures was observed, which is mainly depending on the formation of a specific surface layer composed of nano-particles or very thin glassy film.

Abstract: Al alloy reinforced with SiCp (size: 70-220μm) was fabricated by pressureless-infiltration. Its wear resistant property was investigated under different heat-treatment conditions, and morphology of worn surface was examined. The results showed that the composite was integrated, uniform and compact, and its wear resistant property was better than that of the unreinforced matrix alloy. It was indicated that some rigid SiCp in the abraded surface of the composite could support part of loads and replace matrix to wear-tear, which improved the wear resistant property. Compared to annealing, solution aging strengthens Al alloy matrix and cohesion with SiCp, and the wear resistant property of composites was better. Combining interface is also an important factor which influences on wear resistant property. During the wear test, the smaller SiCp size, the more interfaces, there are more SiCp falling off because of loosening combining interface, which results in more wear-tearing value. The wear rate of composite increases with decreasing SiCp size, thus, the composite with larger SiCp has better wear-resistant property than that with smaller SiCp. At last, the wear mechanism of the composite was also studied, and it showed that abrasive wear dominated in the abrasion process.

Abstract: The transition-metal carbide cemented by metal has excellent combined properties. In this study, cermets were prepared by vacuum sintering from carbide-metal composite powders. The transition- metal oxides (Cr2O3, MoO3, V2O5, Nb2O5 and TiO2), cementing-metal oxides (Co3O4 and NiO), and carbon black were used as raw materials to pre-synthesize composite powders such as Cr3C2-Co, Mo2C-Co, VC-Co, NbC-Co and TiC-Ni, by a direct reduction and carburization process in vacuum. Results show that the participation of Co3O4 and NiO as well as the vacuum circumstance were greatly propitious to the carburization of transition-metal oxides into carbides. The carbothermal condition was greatly improved by the direct reduction and carburization process.

Abstract: The chemical composition and solid state reaction of the nano-Ti(CN) base cermets in different sintering temperature were studied. The total carbon and oxygen content in compact were declined gradually with the increasing of sintering temperature, the nitrogen content in compact began to decline above 1100°C, the peak of de-gassing of N2 was formed before the emergence of liquid phase, the decomposition of N2 was arisen acutely above 1500°C. Mo2C and TaC diffused and took part in solid state reaction with Ti(CN) above 900°C, the solid state reaction was finished below 1200°C. WC diffused and took part in solid state reaction with Ti(CN) above 1100°C, it was dissolved below 1250°C, there were only two phases, Ti(CN) and Ni(Ni+Co), in the alloy.

Abstract: Pure Ti, TiN and graphite powders were mechanically alloyed to synthesize Ti(C,N) powder with nanocrystalline microstructure. The effect of milling variables on the synthesizing behavior of the powders was investigated. As a result of milling, nano Ti(C,N) phase was synthesized by mechanical alloying. After that, Ti(C,N)-based cermets were fabricated with the powders as raw ceramic materials. It was found that the grain size was affected by the content of mechanical alloyed (MAed) Ti(C,N). The grains of the cermet with suitable MAed Ti(C,N) powder were smaller than that of the typical cermets prepared from the commercial powders, and the rim phase surrounding the hard core was also complete. As a result of this, the transverse rupture strength and hardness of the cermets were improved.

Abstract: In this paper, the TiCN-based cermets were fabricated with ultrafine TiCN, metal binders and carbide additives. Different content of C and Al were separately added to eliminate the oxygen contamination. The composition of the mixture was TiCN-10Mo-20WC- 20Ni-xC/Al. Effect of the C and Al addition on microstructure and mechanical properties was investigated. Results indicated that cermets with carbon addition had higher relative density and more homogenous microstructure while the Al addition had the opposite effect. Proper addition of carbon improved the mechanical properties.