Advanced Materials Research Vols. 105-106

Abstract: After fiber reinforced ceramics occur a crack, their fibrous position form bridging fibers, moreover a crack usually extends in the modality of similarity. In order to analyze facilely problems of fiber reinforced ceramics, bridging fiber segment is substituted for loads. A dynamic model of crack propagation is built and its fracture dynamics problems are researched by the approaches of self-similar functions. When a crack propagates at high speed its fiber continues to break. By application of the theory of complex functions, the problems dealt with can be easily translated into Remann-Hilbert problem. Using the built dynamic model and the ways of self-similar functions, analytical study of the displacements, stresses, dynamic stress intensity factor and bridging fibrous fracture velocity α under the action of a running constant force P and an running increasing load Pt, respectively, can be attained, and it is also utilized to obtain the concrete solution of the model by means of superposition theorem.

Abstract: A novel cost-effective pressureless sintering method has been developed to prepare polycrystalline cubic boron nitride (PcBN) compacts. The effect of feldspar as sintering aids was analyzed in this paper. Various amounts of feldspar from 5 to 15 wt% were added to cBN powders, and the pressureless sintering was conducted at temperatures ranging from 900°C to 1200°C under an air atmosphere. The microstructure, phase, density and microhardness of the as-obtained PcBN compacts were measured and correlated to amounts of Si added and to sintering temperatures. The sample showed superior sintering behavior in comparison to those fabricated using hot pressed sintering. The results of X-ray diffraction (XRD) and scanning electron microscopy (SEM) showed that feldspar diffused homogeneously and tightly bonded with cBN. But hBN appeared when the sintering temperature even at 900°C, which dramatically affected the property of PcBN compacts. A PcBN compact with a relative density of 91% was obtained at 1100°C and its microhardness was as high as 1000HV.

Abstract: Sm2O3 and MgO as a sintering additives to fabricated porous silicon nitride by reaction-bonded. The phase of as-produced silicon nitride characterized by XRD-diffraction. The microstructure of product was investigated by SEM. The samples were machined into test bar for flexural strength testing. Using Archimedes theory testing the porosity of porous silicon nitride. MgO have restrain effect on the growth of rod-like silicon nitride, because of produce a restrain layer which is MgO reacted with SiO2 on the surface of silicon., the microstructure of crystal is particle like, the maximum flexural strength is 48MPa with porosity of 35%. Sm2O3 can assistant the growth of high aspect ratio Si3N4 crystal at beginning temperature of 1300°C for it’s low melt point and low viscidity in liquid state, the as-product’s morphology is rod-like and the flexural strength is as high as 300 MPa when the porosity is 30%, high than the sample that of low rod-like crystal content.

Abstract: The morphology and distribution of in-situ VC ceramics in cast high speed steel with about 5-10wt.% vanadium and 1.7-3.2wt.% carbon were studied. The results show that the elementary morphology of in-situ VC ceramics has six kinds: spherical, lumpy, flower shaped, strip, short rod shaped, and vermicular. The in-situ VC in high-speed steel has three kinds of distributions: grain boundary, chrysanthemum-like and homogeneous distributions, depending on the chemical composition of high-speed steel. The in-situ VC ceramics grows up along grain boundaries if the high-speed steel is hypoeutectic, leading to grain boundary distribution of VC ceramics. On the contrary, it distributes homogeneously when the high-speed steel is hypereutectic. Whereas, the in-situ VC ceramics takes on chrysanthemum distribution if the high speed steel is just eutectic. Modification can improve the morphology of primary VC ceramics, but has not obvious effect on the morphology of eutectic VC.

Abstract: The surface microhardness and flexural strength of colored zirconia were examined. Two groups of zirconia disks (1mm thick, 20mm in diameter) within 5 disks each were shading with the same coloring liquids IL2 (Vita Classic-scale) when another group of 5 disks measured in no color. The shading time of one group was 3s and that of the other group was prolonged to 30s. The mechanical properties were tested after sintering at 1500°C. Data were evaluated using ANOVA analysis. Disks of shading 30s showed a lower strength 712  53 MPa. The value of 3s was 853  46 MPa. There were no significant difference on microhardness between the two shading time. Prolonged the shading time lowered the biaxial flexural strength of zirconia ceramic, but shading time did no effect on surface microhardness.

Abstract: Ti(C,N) solid solution were prepared at 1450°C and 1atm nitrogen pressure and the effect of the ways of adding nitrogen on the mechanical properties and microstructures of Ti(C,N)-based cermets were investigated. XRD and FESEM were employed to characterize the as-prepared Ti(C,N)-based cermets. Vickers hardness and fracture toughness were also tested. As a result, The mechanical properties of Ti(C,N)-based cermets prepared by Ti(C,N) solid solution is better than that directly prepared by TiN. The former shows the homogeneous microstructures, the same particle size of hard phases, low porosity, and good sintering character, due to the significant reducing of denitrification when using Ti(C,N) solid solution to introduce nitrogen element. The cross-section SEM images reveal that the dominant fracture behavior of Ti(C,N)-based cermets is transgranular fracture, also including intergranular rupture. The presence of Ni3Ti brittle phase will affect the fracture toughness property of the as-prepared cermets. The more the content of Ni3Ti is, the lower KIC is.

Abstract: The partial debonding interphase has large influence on the behaviors of composite ceramic. The emerging cause of partial debonding interphase is interpreted and the interaction between particles and partial debonding interphase is accounted for determining the equivalent eigen strains by using the four phase model method. Then the effective stiffness of composite ceramic is estimated. The results show the effective stiffness is associated with the stiffness of the matrix and particles, the volume fractions of particles and partial debonding interphase, the subtending angle and thickness of partial debonding interphase.

Abstract: It is necessary to know the synthesis mechanism of Fe-Si3N4 by flash-combusting FeSi75 in order to control the phase composition and microstructure of Fe-Si3N4 composites. In this paper, Fe-Si3N4 and starting material FeSi75 were analyzed with XRD, SEM and EDS. The results show that Fe-Si3N4 synthesized by flash-combusting FeSi75 (≤0.074 mm) is composed of β-Si3N4, α-Si3N4, FexSi, and SiO2, in which β-Si3N4 and α-Si3N4 are from the nitridation of metal silicon and part of the silicon in ξ phase while FexSi is from the nitridation of ξ phase; during the nitridation of ξ phase, Si content declines gradually, when Fe: Si is close to 3, the nitriding reaction tends to balance; the loose accumulation of nitriding products results in the slow heat release, which makes α-Si3N4 transform to β-Si3N4 and β-Si3N4 grow further to form rod-like β-Si3N4 crystals of high slenderness ratios; the rapid quenching of nitriding products helps to keep the proportion between α-Si3N4 and β-Si3N4 small particles, and to reduce the crystallization of rod-like β-Si3N4; The grain size and distribution of FexSi in Fe-Si3N4 are related to the particle size and distribution of ξ phase in the starting material.

Abstract: The influence of silicon addition on the microstructure and properties of corundum-mullite refractory was studied. Results show that Si addition as raw material of the matrix can accelerate mullite crystal formation. The oxidation of Si leads to nucleation of mullite via liquid phase and gas phase. The network microstructure is formed by tiny mullite crystals with few glass phase. So it enhances obviously the high temperature strength of sintered corundum-mullite refractory.

Abstract: Nanostructured surface layer was produced on Ti-6Al-4V alloy by surface mechanical attrition treatment (SMAT) technique, and the composite ceramic coatings of surface nanocrystallization/ micro-arc oxidation (SNC/MAO) were prepared on the nanocrystallized surface of Ti-6Al-4V alloy by pulsed single-polar micro-arc oxidation in NaAlO2 solution. XRD, SEM and TEM techniques were used to investigate the phase and surface morphology of ceramic coatings and the influences on the surface state of the alloys. Meantime, the mechanical properties of Ti alloys were measured by tensile test. The results show that after the SMAT treatment for a short period of time, the surface layer was refined into ultrafine grains. The influences of the SMAT technique on the surface morphology of the ceramic coatings were also studied. The ceramic coatings are mainly composed of Al2TiO5, more compact and less porous than those untreated. The mechanical properties of Ti-6Al-4V alloy by SMAT technique are improved, compared with those untreated by SMAT technique.