Thermal shock resistance and fracture toughness of liquid-phase-sintered SiC-based ceramics

The effect of the heat treatment on the toughness and thermal shock resistance of the silicon carbide–silicon nitride composites prepared by liquid-phase-sintering was investigated. The fracture toughness has been estimated using the indentation method and the thermal shock resistance was studied using the indentation-quench method. The results were compared to those obtained for a reference silicon carbide material, prepared by the same fabrication route. The indentation toughness increased from 2.88 to 5.39 MPa m^1/2 due to the toughening mechanisms (crack deflection, mechanical interlocking and crack branching) occurring in the heat-treated materials during the crack propagation. Similarly the thermal shock resistance increased after the heat treatment of the experimental materials.     

串接石墨化温度的计算

提出一种简单的估算石墨化过程任意时刻温度的方法,以便于时时了解石墨化过程的温度,这对于处理好生产效率与成品率之间的矛盾具有很大的经济意义。     

Improving the fracture toughness and the cyclic-fatigue resistance of epoxy-polymer blends

Relatively tough epoxy-blend polymers are now commercially available for use as adhesives and as the matrices for fibre composites. Nevertheless, another failure property which may be of equal, or even of greater, importance in some applications is the resistance of the epoxy polymer to cyclic-fatigue loading. However, the cyclic-fatigue behaviour of epoxy polymers has not been studied in great detail, especially for epoxy polymers where the material has been modified by forming a polymer blend in order to increase its toughness under quasi-static test rates or impact test rates. Therefore, a major aim of the present work has been to undertake a novel investigation of a range of rubber and thermoplastic materials to modify an epoxy polymer to study whether both a relatively high toughness and a significantly improved cyclic-fatigue behaviour can be simultaneously achieved in a given formulation. The unmodified epoxy-polymer possessed a value of the fracture energy, G_Ic, of 495 J/m² and a value for the threshold value of the maximum strain-energy release rate in a fatigue cycle, G_th, (below which no significant crack growth occurs) of 155 J/m². Several epoxy-polymer blends have been identified which do show major increases in these values and probably the best combination of such properties were for the epoxy-polymers modified with a poly(polypropylene-glycol)-based polyurethane (PU) modifier: either when used by itself or as a ‘hybrid’ polymer-blend in combination with core–shell rubber (CSii) particles, based upon a styrene-butadiene rubber core. For these PU-based epoxy polymers the values of G_Ic and G_th were found to increase to values of about 2475 J/m² and 445 J/m², respectively. The mechanisms of toughening that were induced by the addition of the polymer-blend modifier revealed that the presence of a multiphase in the epoxy-blend polymer was a critical requirement in achieving relatively high values of G_Ic and G_th. This was due to the second-phase particles initiating plastic deformation of the epoxy-matrix phase, which was the major source of energy dissipation and toughening. In turn, the extent of energy dissipated by the plastic deformation of the epoxy-matrix phase is clearly greatly influenced by the degree of ductility exhibited by this phase of the epoxy-blend polymer. Thus, another important feature of the degree of toughening observed is the effect that the modifier has upon the yield stress and plastic failure strain of the epoxy-matrix phase.     

BN-O’-SiAlON复合材料抗热震性能研究

采用热压烧结合成了BN-O’-SiAlON复合材料,通过XRD、SEM、TEM对材料的显微结构进行了表征,并采用水冷法研究其在不同热震温差(400～1 500℃)下的抗热震性能。研究表明,柱状的O’-SiAlON晶粒的弥散强化和桥联作用等增韧机制,材料内部存在的亚临界自发微裂纹,以及无规则取向的BN和O’-SiAlON晶粒相互交叉生长构成的网状结构,均有利于改善BN-O’-SiAlON复合材料的抗热震性能。虽然理论计算近似得出在热震过程中BN-O’-SiAlON复合材料中存在的热性能失配将导致较大的热应力,但实际上,在低于900℃的热震温差下材料的强度并没有明显降低,而在高于900℃的热震温差下才有较明显下降,直到达到1 500℃的热震温差时,其强度仍然保持在原来的40%左右,这说明BN-O’-SiAlON复合材料具有良好的抗热震性能。     

3Y-TZP ceramics with improved hydrothermal degradation resistance and fracture toughness

Different factors such as the way of incorporating the Y₂O₃ stabilizer, alumina addition and sintering temperature were assessed with the goal to improve the low temperature degradation (LTD) resistance of 3Y-TZP without compromising on the mechanical properties. The degradation of hydrothermally treated specimens was studied by X-ray diffraction, micro-Raman spectroscopy and scanning electron microscopy.Decreasing the sintering temperature decreased the LTD susceptibility of 3Y-TZPs but did not allow to obtain a LTD resistant 3Y-TZP with optimized mechanical properties. Alumina addition along with the use of Y₂O₃ stabilizer coated starting powder allowed to combine both an excellent toughness and LTD resistance, as compared to alumina-free and stabilizer co-precipitated powder based equivalents. Transmission electron microscopy revealed that the improved LTD resistance could be attributed to the segregation of Al³⁺ at the grain boundary and the heterogeneously distributed Y³⁺ stabilizer.     

纳米BN复合SiC材料的抗热震性研究

将分析纯H3BO3、CO(NH2)2按物质的量比为1∶2.5溶于无水乙醇中,搅拌过程中按质量分数为80%加入平均粒径0.2μm的β-SiC,在850℃氮气中(纯度99.99%,压力为0.92~0.93 MPa)反应15 h制得纳米BN复合SiC粉体,然后在0.92~0.93 MPa N2气氛中以30 MPa轴向压力于1 750~1 800℃保压0.5~1 h热压烧结上述粉体,制成纳米BN复合SiC试样,采用三点弯曲及透射电镜、扫描电镜等方法研究了纳米BN复合SiC材料的抗热震性。结果表明:在SiC材料中引入纳米BN,一方面可以降低材料的弹性模量,有利于抗热震性的提高。另一方面由于基体SiC与第二相六方氮化硼(h-BN)的热膨胀系数相差较大,热失配作用导致h-BN晶粒发生晶间脱层,在复相陶瓷材料内产生许多微孔,这些微孔的存在可以有效缓解由于高温引起的热膨胀作用,从而极大地改善材料的抗热震性。     

Evaluation of the thermal shock fracture toughness of reactor graphites by arc discharge heating

This paper presents a new technique to determine the thermal shock fracture toughness by a basic analysis of stress intensity factors of the disk with an edge crack. The thermal shock fracture toughness is defined as $\text{K}{}_{\mathrm{IC}}\text{k}\text{Eα}$ (K_IC standing for fracture toughness value of the mode I; k for thermal conductivity; E for Young's modulus; α for thermal expansion coefficient) and is determined en bloc by measuring the threshold electric power of the arc discharge heating produced when an edge crack propagates in the disk. This is intimately corelated to the thermal shock resistance presented previously. Experimental data for four kinds of reactor graphite are shown.     

Thermal shock resistance and fatigue of Zircon-Mullite composite materials

Zircon-Mullite (ZrSiO₄-3Al₂O₃·2SiO₂) composites are refractory materials widely employed in many industries. Thermal shock behaviour of these materials must be considered because it is sometimes its failure mechanism.In this work both thermal shock resistance (TSR) and fatigue (TFR) of Zircon-Mullite composites with different compositions and microstructure configurations were experimentally evaluated by a non destructive measurement of the elastic modulus (E) and compared with the prediction made from the theoretical parameters (R, R? and R_ST).A typical solid brittle material behaviour was found; a simple mathematical expression facilitated the TFR analysis. Although the microstructural configurations studied differed, the experimental behaviour of this group of materials was almost equal. This fact was satisfactorily predicted by the theoretical parameters (R and R?) showing the importance and potential of the evaluation mechanical evaluation of the ceramic material that define these two parameters. On the other hand the slight difference evaluated in the TFR was correlated with the only parameter that takes into account the fracture toughness (R_ST) showing that the significance of this property in a more deep characterization of the ceramic materials.     

锆石/SiC复合物的抗热震性

测试锆石/碳化硅(30%)复合试样的抗热震性需要在高于1 000℃的温度下进行,并与气孔率值几乎相同的纯锆石试样相比较.结果证实了淬火试样的断裂应力未受到合成SiC微粒对淬火温度升高的影响.另一方面,发现重要的温差△T在复合物中要高于纯锆石试样,低于它时材料保持其初始强度.X-射线衍射分析显示了复合试样表面层的锆石分解并生成了由内核和外壳组成的试样.     

Thermal shock resistance of zircon/SiC composite

Abstract

In the present work, thermal shock tests of zircon/silicon carbide (30 wt-%) composite specimens were evaluated up to 1000°C and compared with pure zircon specimens at nearly the same porosity content. Results confirmed that the fracture strength of the quenched specimens was not affected with the increase in quenching temperatures by incorporating SiC particles, indicating resistance to crack propagation. On the other hand, the critical temperature difference ΔT, below which material maintains its original strength, was found higher in composite rather than pure zircon specimens. X-ray diffraction investigations showed that zircon on the surface layer of composite specimens decomposes and produces a specimen comprising core and shell.     

滑板材料的抗热震性评价

介绍了滑板材料的抗热震性方面的一些评价方法,主要有模拟试验观察裂纹、基于断裂力学的测试、有限元分析等方法,并指出在滑板材料的评价手段方面还要进一步完善,如利用有限元模拟技术,特别是细观尺度有限元对滑板使用过程进行模拟。     

低碳MgO-C材料的抗热震性研究

以电熔大结晶镁砂、天然鳞片石墨、纳米炭黑、酚醛树脂、铝粉等为主要原料制备w(C)=3%的低碳MgO-C材料,以其抗热震性为考核指标,选取颗粒级配、复合抗氧化剂、石墨粒度和复合结合剂4个因素,进行了四因素三水平正交试验。结果表明:在本试验范围内,颗粒级配是影响低碳MgO-C材料抗热震性的主要因素,复合抗氧化剂次之,石墨粒度和复合结合剂的影响基本相当;通过极差分析确定,镁砂颗粒级配(3～1、1～0.088和≤0.088mm的镁砂的质量比)采用50:23:27,复合抗氧化剂采用Al2.5+Mg-Al0.5+B4C0.5,石墨粒度采用10μm的,复合结合剂采用炭黑N220+沥青+酚醛树脂,可制备出抗热震性最佳的低碳MgO-C材料。     

Thermal shock resistance of polycrystalline cubic boron nitride

The effect of thermal shock on the flexural strength has been investigated experimentally. It was found that the variation in flexural strength with quench temperature was influenced by the CBN grain size. Polycrystalline material containing small CBN grains showed a discontinuous drop in measured flexural strength above a material dependent critical quench temperature difference, ΔT_c. The sharp decrease in measured strength is accompanied by unstable crack propagation. Material containing a significantly larger CBN grain size exhibited a gradual decrease in strength above the critical quench conditions. The experimental observations agreed with an established theory developed for thermal shock of alumina. The theoretically calculated critical temperatures agree well with the observed experimental data for each material when a flaw size equal to the CBN grain size is employed.     

反应烧结碳化硅材料的抗热震性能研究

通过对比不同温差热震后材料的残余强度 ,对反应烧结碳化硅材料的抗热震性能进行了研究。结果表明 :反应烧结碳化硅材料的抗热震性能与显微组织密切相关 ,低游离硅含量与小粒径的反应烧结碳化硅材料具有较好的抗热震断裂性能 ,而高游离硅含量或大碳化硅粒径的材料具有相对优异的抗热震损伤性。对反应烧结碳化硅材料的抗热震性与显微组织的关系进行了探讨。     

Thermal shock resistance of flat plastic integrated circuit packages

Flat packages (FPs) were formed from epoxy molding compounds with various physical properties using a transfer molding machine. The compounds were prepared by changing kinds and amounts of additives and addition methods. The thermal shock test was carried out by the following procedures. The plastic package was soaked alternately in liquid nitrogen (?196°) and in liquid solder (200°) in the cycle of 140s. The median life to crack initiation was defined to be the cycles when half of the specimens exhibited crack initiation. According to linear fracture mechanics, the following expression was obtained relating the median life N, thermal stress σ_t, and strength σ_b; N = C/σ·(σ_b/σ_t)^m. We found the linear relation between logarithm of Nσ and logarithm of σ_b/σ_t for various packages, and estimated the values of C and m as 5 × 10⁴ MPa² and 5.5, respectively. The value of m was the same as that obtained for a dual-in-line package.     

Hardness and fracture toughness of moissanite

Disparities prevail among the reported hardness and fracture toughness values for hard and brittle materials. A better understanding of the physical nature of hardness and fracture toughness and a standardized technique for reliable measurements of these quantities is urgently needed. We strongly recommend the use of the measured hardness after the bend in the hardness versus load (H−F_Load) curve, when the hardness approaches its asymptotic value. The present work reports a systematic study of hardness and fracture toughness on moissanite (single crystal hexagonal silicon carbide, 6H-SiC) samples. The measurements were performed over a broad load range from 0.49 to 294 N with the direct indentation method. Asymptotic values of Knoop hardness of H_K = 19 GPa and Vickers hardness of H_V = 22 GPa were reached at a high load between 50 N and 100 N. A consistent fracture toughness of K_IC = 1.8 MPa·m^1/2 was obtained across the entire load range. Our study presents experimental results for the hardness and fracture toughness of moissanite in the asymptotic-hardness region, and it raises concern regarding the application of moissanite single crystals as anvil material under shear/fracture conditions.     

Polymer toughness and impact resistance

This report is a review of the factors that influence the impact resistance of high polymers, with an emphasis on crystalline polymers. The phenomenology of polymeric fracture is examined, including brittle and ductile failure, and the ductile-brittle transition temperature (DBTT). An in-depth discussion of the effects of crystalline morphology follows, with special attention given to the influence of spherulite size, fillers, processing conditions, transitions/relaxations, and multilayer coextrusion. Next, rubber phase addition is considered, including mechanisms, morphology, rubber type and particle size, and test conditions. Finally, common impact test methods are surveyed, including pendulum, falling weight, tensile impact, and tensile elongation tests.