烧结温度对氮化硅陶瓷球显微结构和力学性能的影响

以α-Si3N4粉为原料,纳米级Y2O3和Al2O3为烧结助剂,采用气压烧结工艺制备氮化硅陶瓷球,研究了烧结温度对陶瓷球显微结构及力学性能的影响.结果表明,随着烧结温度的升高,陶瓷球的维氏硬度和压碎强度先提高后降低,断裂韧性不断提高.烧结温度为1780℃的陶瓷球综合力学性能最佳,其相对密度达到了99％,维氏硬度、断裂韧...     

Surface strength of balls made of five structural ceramic materials evaluated with the Notched Ball Test (NBT)

In high performance hybrid bearings the balls are conventionally made of silicon nitride ceramics. There are some disadvantages such as costs or the higher stiffness of silicon nitride compared to steel. Therefore, alternative materials are under investigation. The surface strength is one of the most important criteria for the qualification of the spherical components in the application. It has to be evaluated for each new material (or new surface finish).The recently standardised Notched Ball Test (NBT) enables one to determine the surface strength (tensile strength) of balls, which is strongly influenced by the surface finish and volume flaw populations. In this paper the NBT strength of five candidate materials for rolling elements (silicon carbide, silicon nitride, alumina, zirconia and zirconia toughened alumina) is investigated. Fractography is performed to evaluate the direct correlation between the defects found at the surface and the measured strength.     

一步法制备纳米SiO_2/环氧树脂杂化材料的研究

以正硅酸乙酯(TEOS)为无机前驱体、三乙烯四胺(TETA)为固化剂和γ-氨丙基三乙氧基硅烷(KH-550)为硅烷偶联剂,采用溶胶-凝胶一步法制备SiO2/EP(环氧树脂)杂化材料。研究了TEOS和丙酮含量对杂化材料的力学性能、热性能等影响,并对杂化材料的微观相态结构和性能进行了表征和分析。结果表明:当w(TEOS)=3%(相对于改性EP质量而言)时,不加丙酮的杂化材料具有相对最好的综合性能,其断面形貌呈韧性断裂,热变形温度和玻璃化转变温度均高于纯EP体系,生成的SiO2粒径为20 nm左右且分布较均匀;加入丙酮后的杂化材料透明性变好,但其力学强度和热性能均随丙酮含量增加而降低。     

Fracture toughness of silicon nitride balls via thermal shock

A new method for fracture toughness determination of ceramic balls is presented. The starter crack is introduced into the surface of the ball by a Knoop indentation followed by grinding off the deformed zone. The loading through surface tensile stresses is realized by water quenching, i.e. dropping the heated ball into water. The temperature difference is stepwise increased to find the critical temperature difference for the initiation of crack growth. The geometric factor is calculated in a parametric finite element study, whereas the temperature distribution in the ball was previously determined by using the Biot concept. Combining experimentally measured critical temperature differences for different cracksizes and ball diameters with numerical results of the geometric factor, the fracture toughness of the silicon nitride balls is evaluated. For the evaluation, the knowledge of several material properties (e.g. the CTE) and other parameters is necessary, which have influence on the precision of the measurement. The overall measurement uncertainty is estimated to be about ± 10 %, what roughly corresponds to the value determined with standard measurement procedures. There is an excellent agreement with published fracture toughness results of these balls determined by the modified Surface Crack in Flexure procedure.     

Ultrasonic machining of alumina-based ceramic composites

In ultrasonic machining (USM), the material is removed primarily by repeated impact of the abrasive particles, and the material removal rate (MRR) and surface integrity are influenced by various factors including the material parameters of the workpiece materials. In this study, effect of the properties and microstructure of the workpiece materials on the MRR in ultrasonic machining of alumina-based ceramic composites was investigated. The distributions of strength of the ultrasonic machined specimens were used to evaluate the surface integrity. Results showed that fracture toughness of the ceramic composite played an important role with respect to MRR. In USM of whisker-reinforced alumina composites, the MRR depended on the whisker orientation. Studies of strength distributions of alumina-based ceramic composites machined by USM demonstrated that the flexural strength varied narrowly from the mean value, and the composites with high fracture toughness showed higher Weibull modulus.     

SEM studies of cold-rolled spherulitic films of poly(tetramethylene terephthalate)

By using the method of scanning electron microscopy, a visual study was made of the macro-deformation of spherulitic deformation during the cold rolling of poly(ethylene terephthalate) (PET) and poly(tetramethylene terephthalate) (PTMT). By rolling solution cast films to different extension ratios, it was found that the surface material (nearest the rollers) appeared to deform first by the shearing action. With higher extension, the material toward the core of the film began to deform. In all cases, the deformation process tended to flatten the spherulites yet, in general, the spherulitic boundaries were maintained. It was noted that the same general behavior was observed for both PET and PTMT.     

Influence of Test Methods on Fracture Toughness of a Dental Porcelain and a Soda Lime Glass

The aim of this study was to investigate the influence of the test method on fracture toughness of a dental porcelain and a soda lime glass. Three methods were used to determine fracture toughness: the indentation strength (IS) by bending, chevron-notched beam (CNB), and the single-edge-notched beam (SENB). In the IS method, the ratio of elastic modulus to hardness ( E / H ) in the formula was determined by two methods: individual measurement for E and H (IS_M) as well as direct estimation from Knoop's indentation method (IS_K). The tested materials were a dentin porcelain, a traditional feldspar-based leucite-reinforced glass ceramic (Carrara Vincent), and a soda lime glass. Carrara Vincent showed a higher toughness ( P <0.01) than glass with all three test methods. The toughness values manifested significant differences between the methods used ( P <0.01). The two-way analysis of variance suggested that the materials tested and the test methods used had interaction effects, which statistically means that differences in materials and methods influenced the comparability of the toughness result. In this study, a first step was made to compare different toughness test methods by testing the toughness of a traditional feldspar-based leucite-reinforced glass ceramic and a soda lime glass that has a homogeneous microstructure. An interaction effect of the method and the material used was shown. As a consequence, none of the methods tested is suitable as a universal fracture toughness test method. Further research is needed to investigate more extensively the influence of material composition on the fracture toughness test methods' comparability.     

Effects of carbon nanofiller functionalization and distribution on interlaminar fracture toughness of multi-scale reinforced polymer composites

Carbon nanofillers with different surface functional groups and aspect ratios, including carboxyl carbon nanotubes, un-functionalized carbon nanofibers (CNFs), glycidyloxypropyl-trimethoxysilane carbon nanotubes (GPS-CNTs) and nanofibers were evaluated for their potential for increasing the interlaminar fracture toughness of an S2-glass fiber/epoxy composite. The fillers were added in the matrix of the fiber reinforced plies, in the resin interlayer between plies, or in both regions. Comparisons were made based on mode I and mode II interlaminar fracture toughness. For composites made with CNTs dispersed in the matrix, fracture toughness was largely unaffected except for a slight increase seen with long GPS-CNTs. However, adding a CNF or CNT modified resin interlayer significantly increased the fracture toughness, with the highest improvement over the baseline material achieved by adding long GPS-CNTs in the interlayer (79% and 91% for mode I and mode II onset toughness, respectively). Important material parameters identified for improving interlaminar fracture toughness are the nanofiller aspect ratio and concentration at the fracture plane. Based on microscopic evaluations of the fracture surfaces, a high density of high aspect ratio nanofillers causes the best entanglement between the filler and glass fibers and effectively obstructs interlaminar crack propagation.     

Microstructure design and preparation of Al2O3/TiC/TiN micro-nano-composite ceramic tool materials based on properties prediction with finite element method

The objective of this paper was to improve the accuracy of semi-empirical method used to design ceramic cutting tool materials. The mechanical properties were predicted by employing finite element model of material microstructure, so as to design microstructure and prepare new ceramic materials. Based on the Voronoi and randomness method, the microstructure model representing the complexity and randomness of micro-nano-composite ceramic material microstructure was established. Combining the representative volume element (RVE) of ceramic material microstructure with mechanical tests, the simulations of mechanical tests were conducted to acquire the flexure strength, fracture toughness and hardness of materials. The microstructure models with various parameters were designed and the material properties were predicted to determine the optimal microstructure parameters. Then, The ceramic cutting tool materials possessing the optimal microstructure parameters were developed for machining ultra-high strength steels. The results showed that the mechanical properties of ceramic materials first improved and then declined as the nano-scale TiC volume fraction increased. To obtain the best comprehensive mechanical properties, the contents of micro-scale TiN, TiC and nano-scale TiC were set as 20%, 10% and 10%, respectively. The prepared ceramic materials possessed the flexure strength of 881.4 MPa, the fracture toughness of 7.8 MPa m^1/2, and the Vickers hardness of 20.8 GPa. This research is beneficial to the development of cutting tool design theory and the improvement of the tool life.     

Role of fabrication route and sintering on wear and mechanical properties of liquid-phase-sintered alumina

Liquid-phase-sintered Al₂O₃ (LPS) fabricated by slip casting, tape casting, isopressing, uniaxial pressing, piston and auger extrusion showed substantial differences in wear due to differences in morphology as observed in image analyses of SEM micrographs. The abrasive wear was low in the case of uniaxial pressing and high in the case of tape casting in the ‘dry sand and rubber wheel’ test. The wear surface of the tape cast specimen exhibited extensive microcracking possibly due to orientation of Al₂O₃ platelet (major face) parallel to the abraded surface whereas some degree of perpendicular orientation in extruded surface resulted in lower wear loss. In wet-milling wear test, the isopressed balls of a 95–97 wt% LPS derived from reactive powder (<1 μm) showed 25% lower wear loss than that of the extruded balls of a 91–94 wt% LPS derived from coarse powder (70–100 μm). Sintering at a lower temperature with longer duration and batch milling of the composition in between 12 and 16 h resulted in low wear loss. Flexural strength also improved by longer sintering time but did not show any improvement by increasing milling time. However, the variation in flexural strength was minimized by isopressing the extruded specimen. A high indentation fracture toughness at 49.03 N test load was associated with (i) large elongated reinforcement grains in a fine-grained microstructure with overall elongated morphology and (ii) with an intergranular fracture.     

1Yb-2Sm-TZP,a new co-stabilized zirconia material with high toughness and low temperature degradation resistance

Partially stabilized zirconia materials are well known for their combination of high strength and toughness. In this study a new ytterbia samaria co-stabilized zirconia is manufactured by intensive co-milling of zirconia and stabilizer oxides with subsequent consolidation by hot-pressing. Evolution of microstructure, mechanical properties and phase composition are studied with respect to sintering temperature. The material exhibits a transformation dominated fracture behavior and a combination of high strength of up to 1050 MPa, a fracture toughness exceeding 12 MPa√m and an excellent resistance to low temperature degradation.     

Microstructural Design of Silicon Nitride with Improved Fracture Toughness: I, Effects of Grain Shape and Size

The use of self-reinforcement by larger elongated grains in silicon nitride ceramics requires judicious control of the microstructure to achieve high steady-state toughness and high fracture strength. With a distinct bimodal distribution of grain diameters, such as that achieved by the addition of 2% rodlike seeds, the fracture resistance rapidly rises with crack extension to steady-state values of up to 10 MPam^1/2 and is accompanied by fracture strengths in excess of 1 GPa. When the generation of elongated reinforcing grains is not regulated, a broad grain diameter distribution is typically generated. While some toughening is achieved, both the plateau (steady-state) toughness and the R -curve response suffer, and the fracture strength undergoes a substantial reduction. Unreinforced equiaxed silicon nitride exhibits the least R -curve response with a steady-state toughness of only 3.5 MPam^1/2 coupled with a reduced fracture strength.     

Mechanical and tribological properties of alumina-MWCNTs composites sintered by rapid hot-pressing

Alumina – MWCNTs composites were prepared using a novel approach. This process comprises functionalization of MWCNTs and stabilization of alumina-MWCNTs dispersion with subsequent freezing, which resulted in formation of granulated powders with homogeneous distribution of MWCNTs. The granulated powders were sintered by rapid hot pressing (RHP) at 1550 °C. Relative densities, microstructural analysis, tribological properties, fracture toughness and bending strength of prepared composites were investigated to reveal the effect of MWCNTs. Compared to pure alumina, bending strength and fracture toughness of dense alumina-5 vol.% MWCNTs composites decreased about 37% and 18%, respectively. At higher MWCNT contents, strength remained almost constant and fracture toughness slightly increased. Thus, the positive effect of CNTs on fracture toughness was demonstrated despite their counteracting effect on the refinement of the microstructure.     

基体改性对碳纤维增韧碳化硅复合材料结构与性能的影响

采用化学气相浸渗法对2D C/SiC复合材料进行基体改性,制备了二维碳纤维增韧碳-碳化硅二元基复合材料(two dimensional carbon fiber reinforced C-SiC binary matrix composites,2D C/C-SiC).2D C/C-SiC复合材料的基体为热解碳和碳化硅交替叠层的多层基体.研究了2D C/C-SiC复合材料的微观结构,比较了2DC/SiC复合材料和2DC/C-SiC复合材料的力学性能及断口形貌.结果表明:2DC/C-SiC复合材料可在基本保持2DC/SiC复合材料抗弯强度的基础上,其断裂韧性得到显著提高.基体改性的效果明显.纤维的逐级拔出是断裂韧性提高的原因.     

Impact strength of polymers 2: The effect of cold working and residual stress in polycarbonates

The influence of cold working on the toughness improvement in glassy amorphous polycarbonates was studied. Cold working processes, namely rolling and. Steckel rolling were used to produce thickness reductions up to 40 percent in flat-strip specimens. The notched Izod impact strength and tensile properties were measured as a function of strip thickness reduction. It was shown that the toughness enhancement in polycarbonates cold worked to low thickness reductions was due to the residual stress state present as opposed to molecular orientation which becomes significant at higher degrees of cold work. Residual stress measurements were made by using the layer removal technique. Residual tensile stresses as high as 2100 psi were present in 1/4-in. cold-rolled polycarbonate at the surface. The maximum stress in the center of the specimen was 1100 psi in compression. The residual stresses at the surface decreased with increasing thickness reduction. The residual stress state for Steckel rolled. 1/2-in. polycarbonate was also measured and found to be more complex than for the thinner samples, The results demonstrated that surface tensile stresses and interior compressive stresses can produce large values of impact strength if the notch is to be machined after cold working. Thus, the values of impact strength measured from the notch Izod specimen are sensitive to the residual stress state in the polymer. This behavior is in contrast to earlier studies on thermally quenched material in which the material was quenched after notching. The thermal quenching produced surface compressive stresses which were also present at the notch tip. The presence of compressive residual stresses at the center of the notch suppressed the formation of a craze leading to toughness enhancement in cold worked polycarbonate strips. It is shown that by control of residual stresses in polycarbonate, strips at least 1/2 in. in thickness can be made to exhibit ductile failure in the notched Izod impact test.     

Synthesis of Oil-Swelling Material and Evaluation of Its Self-Healing Effect in Cement Paste

This study adopted a synthetic oil-swelling material (OSM) as a novel additive in oil well self-healing cement paste (SHCP) to achieve self-healing effect initiated by oil phase when cracks already formed. The OSM with oil absorption rate of 12.78 g/g in kerosene. The mechanical properties of uniaxial tensile fracture elongation up to 208.8% and tensile fracture toughness up to 25.7 kPa, uniaxial compressive fracture toughness up to 1.956 MPa. The results indicated that permeability of SHCP gradually alleviated due to swelling of OSM, and compressive strength obviously increased under the optimal dosage compared to the blank specimen.     

Relationship between microstructure and mechanical properties of fired Ecuadorean clay

Abstract

Hardness and fracture toughness measurements are reported for individual phases and interfacial composites present in fired specimens of an Ecuadorian clay mineral. Each investigation used a half disc from strength tests previously done and reported elsewhere. The aim was to rationalise trends in the macroscopic strength and density to quantify contributions from the important components of the microstructure. This was achieved through application of an etching and gold coating procedure prior to microindentation testing. The coating technique improved the clarity of the indents and made the diagonal and radial crack measurements more accurate. Two peaks in the strength versus firing temperature curve are shown to arise from opposite trends in the toughness of the quartz grains and the matrix phase, together with a no-linear build up of the volume of interfacial composite material which confines any cracks initiated in the residual quartz crystals at high firing temperatures.     

High-Strength and High-Fracture-Toughness Ceramics in the Al2O3/LaAl11O18 Systems

Control of microstructure in the Al₂O₃/LaAl₁₁O₁₈ system was performed. Elongated alumina grains were formed by doping with small addition of silica, and 20 vol% lanthana- luminate was formed in situ by the reaction of LaAlO₃- A1₂O₃ in an alumina matrix. Strengths of over 600 MPa and a high fracture toughness (6 MPa.m^1/2) were achieved in the material with both elongated A1₂O₃ grains and LaAl₁₁O₁₈ platelets. Generally antagonistic properties such as strength and fracture toughness have been made compatible in the same ceramic system.     

Fracture Toughness of Plasma‐Sprayed Thermal Barrier Ceramics: Influence of Processing,Microstructure, and Thermal Aging

Fracture toughness of thermal barrier coatings (TBCs) has gained significant interest in recent years as one of the dominant design parameters dictating selection of materials and assessing durability. Much progress has been made in characterizing and understanding fracture toughness of relevant TBC compositions in their bulk form, but it is also apparent that the toughness is significantly affected by process‐induced microstructural defects. In this investigation, a systematic study of the influence of coating microstructure on the fracture toughness of atmospheric plasma‐sprayed TBCs has been carried out. Yttria partially stabilized zirconia (YSZ) coatings were fabricated under different process conditions inducing different levels of porosity and defect densities. Fracture toughness was measured on free‐standing coatings in as‐processed and thermally aged conditions using the double torsion technique. Results indicate significant variance in fracture toughness among coatings with different microstructures including changes induced by thermal aging. Comparative studies were also conducted on an alternative composition, Gd₂Zr₂O₇ which, as anticipated, shows significantly lower fracture toughness compared to YSZ. The results not only point toward a need for process and microstructure optimization for enhancing TBC performance, but also a framework for establishing performance metrics for promising new TBC compositions.     

Relationship Between Microstructures and Material Properties of Novel Fibrous Al2O3–(m-ZrO2)/t-ZrO2 Composites

Novel fibrous Al₂O₃–(m-ZrO₂)/t-ZrO₂ (m, monoclinic; t, tetragonal) composites having a core/shell structure were fabricated by multi-extrusion, and their microstructures and material properties were investigated depending on the number of extrusions. The composites acquired a homogeneously fine fibrous structure as the number of extrusions increased. The bending strength and fracture toughness increased remarkably as the number of extrusions increased. In the fracture surface of the second passed composite, an Al₂O₃–(m-ZrO₂) core region appeared, flat type, although some local regions existed with an intergranular fracture. However, the fracture mode of the t-ZrO₂ region was of intergranular type having a sharp and rough surface. In the composite made by the fifth passed extrusion, the fracture strength and toughness values were high at about 665 MPa and 9.6 MPa·m^1/2, respectively. The main fracture mode was a typical intergranular mode having a rough fracture surface, and the main multi-toughening was because of mechanisms such as crack bridging, microcracking, and phase transformation.