Al2O3/TiC纳米陶瓷刀具材料的抗热震性能及断裂机理研究

用强度衰减法对Al2O3／TiC纳米陶瓷刀具材料的抗热震性能进行了详细的研究。热震实验表明该种材料的临界热震温差为340℃。当温差达到350℃时，强度有明显的下降。与微米级的材料相比，纳米材料的抗热震性能反而有所降低。通过对热震后试样表面的显微观察和表面热应力的估算发现，材料在烧结后冷却过程和热震过程中产生的热应力导致试样表面产生裂纹，从而导致了材料强度的下降。     

含锆莫来石-刚玉质陶瓷的抗热震性与断裂参数

研究了MZ-3、MZ-6和MZ-8三种含锆莫来石-刚玉质陶瓷的断裂参数(断裂韧性K_(10)、断裂功γ_f和固有强度σ_f)与其组分和组成相之间的关系,分析了三种材料热震后强度的衰减趋势。 实验表明,MZ-3材料的抗热震临界温差△T_o为200℃左右,而斜锆石含量较高的MZ-6和MZ—8材料的抗热震临界温差△T_o比MZ-3材料提高了约100℃;经过温差△T=300℃的热震处理后,MZ-6材料在强度与热震温差的关系曲线上出现极大值。热震后三种材料的强度衰减均随热震温差的增加而加剧。当温差从1000℃增大到1200℃时,粘滞态的玻璃相在淬冷瞬间吸收了部分能量,缓和了热震裂纹的扩展,除MZ-3材料因严重开裂造成强度继续衰减外,其它两种材料的强度有所提高。三种材料的断裂韧性K_(10)和断裂功γ_f随温度变化的情况与强度σ_f随温度变化的情况相同。 在实验的基础上,根据热弹性理论,讨论了三种含锆莫来石-刚玉质陶瓷的抗热震断裂特征,根据断裂力学的能量平衡观点,分析了材料的抗热震损伤能力。     

用声发射监测陶瓷材料的热震损伤

介绍了声发射技术的基本原理，用声发射技术监测了莫来石－钛酸铝陶瓷材料急冷时热震损伤强度衰减及裂纹的扩展过程，结果表明：由热应力引起的微裂纹稳态扩展和失稳扩展的声发射特性与该试样热震损伤后的残余强度变化趋势是一致的。     

多孔SiC基耐火材料的热震损伤

本文从抗热震性是材料的力学和热学性能的综合反映这观点出发,研究了SiC基耐火材料的组分-显微结构-力学性能之间的关系,从而剖析材料的抗热震能力。首先,探讨了粘土结合剂含量不同的三种SiC基材料的强度随热震温差△T的增大而下降的趋势;多次热震导致的强度衰减规律;分析了材料的热震裂纹扩展行为;确定了该系列材料具有“准静态热震裂纹扩展”的属性,亦即其抗热震能力主要由“裂纹稳定性系数”R_(at)-[γf/Eα~2]~(1/2)来表征;从而说明断裂功rf的提高,是改善该系列材料抗热震性的最重要的力学因素。在系统实验的基础上得出:在600℃≤△T≤1000℃的热震温差范围内,材料的强度不仅随着温差△T的增大而下降,亦随着热震次数的增多而衰减。因此,这是热震损伤问题较为突出的阶段,当△T≥1000℃,由于SiC颗粒间界的铝硅酸盐在高温下出现了粘滞态的玻璃相,在淬冷瞬间吸收了部分能量,削弱了热震裂纹的扩展趋势,相应缓和了材料的热震损伤倾向。     

不同形态ZrO2复合Al2O3陶瓷的抗热震性设计与表征

采用湿化学方法制备了具有不同团聚度及稳定度的氧化锆陶瓷粉体,并将其复合到氧化铝基体中的结构微气孔,以同时提高材料的抗热震性能与强度。研究表明：水洗复合材料的抗热震性能更为优越。其原因在于团聚氧化锆的形成,这也得到显微结构证实。运用函数构造建立了含裂纹脆性材料的具有普话意义的热震方程,进而探讨了几类复合材料的抗热震行为的表征和模拟。     

氮化硅陶瓷材料高温抗热震性能及其衰减规律研究

氮化硅陶瓷作为先进陶瓷材料具有耐高温、抗腐蚀等优异性能,因此被广泛应用于航空航天领域的强热冲击环境。热压烧结制备的Si3N4复合材料的抗弯强度较高,但抗热震性能随温度升高显著降低,热压烧结工艺在提升抗热震性能方面尚有不足。本文提出了使用二次热处理烧结方式来提高Si3N4陶瓷的抗热震性能,通过热压烧结-气压烧结二次热处理的烧结方式获得更致密、抗热震性能更好的Si3N4陶瓷材料。测试结果显示,常规热压方式制备的氮化硅陶瓷,随着热震温度的升高、次数的增加,材料内部产生微裂纹的概率增大,热震后试样抗弯强度逐渐降低,1200℃时平均强度下降率达23.48%。而经过二次热处理后氮化硅陶瓷抗弯强度略有降低,但抗热震性能得到明显改善,随着热处理时间增加,二次热处理后氮化硅陶瓷显微结构更加致密,抗热震性能将明显提高,热震后强度下降率明显减小,1200℃热震10次后强度下降率为12.25%。本文提出了提高Si3N4陶瓷的抗热震性的方法,探讨了氮化硅陶瓷在1200℃高温下的抗热震性能及其衰减规律,为改善氮化硅陶瓷器件高温性能提供了参考。     

碳化钛烧结复合材料的抗热震性

利用残余强度方法研究了碳化钛烧结金属陶瓷复合材料的抗热震性。发现抗热震性及残余强度变化系数与材料中金属结合的化学成分及碳化物相的体积比之间存在着强烈的依存关系。     

不同铝基原料对MgO-Al-C材料性能和显微结构的影响

以烧结镁砂骨料、电熔镁砂细粉、Al粉、N220炭黑为原料,酚醛树脂为结合剂,制备MgO-Al-C材料,研究了三种铝基原料(造粒氧化铝微粉、氧化铝空心微珠、六铝酸钙)对其常温抗折强度、高温抗折强度、抗热震性及抗氧化性的影响,并借助XRD和SEM对其物相组成及显微结构进行分析。结果表明,造粒氧化铝微粉是多孔结构,可吸收热应力,加入量为1%(质量分数,下同)时,可提高材料的常温抗折强度和抗热震性,明显改善材料的抗氧化性。氧化铝空心微珠是中空结构,可缓冲热应力,加入量为3%时,可明显提高材料的常温抗折强度,并具有较高的抗热震性和抗氧化性。六铝酸钙的热膨胀系数较低,可赋予材料较好的韧性,加入量不超过5%时,样品具有较好的抗热震性。     

氧化铝陶瓷与不锈钢钎焊的接头强度与热震抗力

研究了钎焊温度对氧化铝陶瓷与１Ｃｒ１８Ｎｉ９Ｔｉ不锈钢接头强度的影响，探讨了钎焊接头的热震抗力。结果表明，由于陶瓷／钎料界面反应和界面残余应力的综合影响，随着钎焊温度的增加，接头强度先增加然后逐渐降低，８５０℃钎焊时接头强度最高。在低于６００℃温度范围内加热后空冷或加热空冷循环热震时对钎焊接头强度损伤较小，而淬水热震，特别是在３００℃以上温度时对接头强度损伤较大，严重者出现开裂现象，这主要是界面附近存在较高的残余热应力的缘故。     

氧化物-非氧化物复合耐火材料高温性能的研究

对氧化物-非氧化物复合材料(如ZCM-SiC,ZCM-BN,O’-Sialon-ZrO2,β-Sialon-Al2O3等)的高温性能(强度、抗热震性、抗氧化性等)进行了研究。结果表明(1)所研究的氧化物-非氧化物复合材料的高温强度明显优于碳结合材料的高温强度。(2)在氧化物基质中引入非氧化物,可提高材料的抗热震性。(3)在非氧化物基质中引入氧化物可明显改善材料的抗氧化性。     

Acoustic Emission of Thermally Cycled Refractories

Acoustic emission (AE) technique was applied to continuously monitor property degradation during standard thermal shock fatique testing of three distinct types of refractories. AE results were correlated with respective strength losses of analyzed materials after a defined number of heating and cooling cycles. The complex nature of the AE signals was revealed by AE amplitude and duration analysis indicating that not all of the signals that were registered during thermal shock tests could be correlated with strength changes. On this basis, it was postulated that simple counting of a number of AE events should not be used as a measure of the degree of damage in refractories under thermal shock.     

Thermal Shock and Size Effects in Castable Refractories

To study accurately the effects of thermal shock on castable refractories, it is necessary to model the fracture process zone. The fictitious crack model, which is often used for the analysis of cementitious materials, is a suitable way to include the fracture process zone in thermal shock analysis. The analysis reinforces the importance of the thermal shock resistance parameter, R "", which has the units of length and is very similar to the characteristic length, l _ch, used as a size scale in studies of the fracture of cementitious materials. Whether a cementitious bar behaves in a brittle or ductile fashion depends on the size of the bar relative to its characteristic length. Therefore, the behavior of refractories after they have been quenched is dependent on their size relative to R "". It is also shown that the critical temperature to cause strength degradation by thermal shock is dependent on the relative size of the bar.     

Role of fatigue in damage development of refractories under thermal shock loads of different intensity

Refractories insulation of industrial furnaces often fail under repetitive thermal shock. Degradation of silica refractories under thermal shock loads of different intensity was studied. The load variation was achieved by utilisation of geometrically similar samples of different dimensions. Finite element method modelling predicted loads developing during the test. Resulting damage was determined by the ultrasound velocity and crack patterns. Tests involving up to 150 cycles demonstrated the role of fatigue in enabling sub-critical crack formation and countering the crack arrest. Repetitive cycles reduce crack wake friction and intensify loading due to crack debris re-location. Damage saturation, sigmoidal and near-exponential damage growth was typical for low, intermediate and high loads, respectively. Similar trends of damage accumulation were observed in mechanical displacement controlled cyclic fatigue tests performed in wedge splitting set-up. Strain and strain energy based criteria of thermal shock intensity seem to have complimentary value in predicting the crack formation and growth. Thermal shock damage after the first cycle seems to be an effective parameter to predict overall resistance to the degradation in the sample. Load reduction due to previous crack formation related to the fatigue potential for subsequent crack development can explain the crack size variation typically observed in refractories after multiple thermal shocks. For thermal shock tests, the variation of sample size, instead of the temperature interval, is a suitable alternative for refractories with strongly temperature dependant material properties.     

The microstructure evolution and mechanical properties of MgO-C refractories with recycling Si/SiC solid waste from photovoltaic industry

In the present work, the recycling of Si/SiC solid waste from photovoltaic industry for MgO-C refractories preparation has been introduced. The influence of solid waste powders as antioxidant additive on microstructure evolution, mechanical properties and thermal shock resistance of MgO-C refractories has been investigated systematically. With 4?wt% Si/SiC rich solid waste addition, the MgO-C refractories exhibited the highest strength (4.39?MPa) and residual Young's modulus (7.86?GPa) after firing at 1400?°C, compared to only Si or SiC-addition. The presence of iron in the solid waste also promoted the formation of MgO and Mg₂SiO₄ whiskers via catalyst-assisted method. Moreover, a dissolution-saturation-precipitation growth mechanism was used to explain the formation process of the whiskers. The improvements in strength as well as thermal shock resistance can be attributed to the microstructural evolution.     

钛酸铝对刚玉质耐火材料性能的影响

探讨了在刚玉质耐火材料的基质中引入TiO2 原位形成钛酸铝对其性能的影响。结果表明 :钛酸铝的形成可显著提高刚玉质耐火材料的热震稳定性及热态强度 ,从而有望生产出性能更加优良的刚玉质耐火材料。     

Fracture behavior of low carbon MgO–C refractories using the wedge splitting test

The fracture behavior of low carbon MgO–C refractories containing various carbon sources were investigated by means of the wedge splitting test and microscopic fractographic analysis to evaluate quantitatively their thermal shock resistance in the present work. The results showed that the addition of various nanocarbons in MgO–C specimens can lead to more tortuous crack propagation path during the wedge splitting test and much better thermal shock resistance compared to the specimen with flaky graphite as carbon source; particularly, the specimen containing carbon nanotubes had the most outstanding thermal shock resistance. Also, it was suggested from the correlation analysis that the increase of the specific fracture energy and interface crack propagation as well as the decrease of the modulus of elasticity, coefficient of thermal expansion and transgranular crack propagation can contribute to an improvement of thermal shock resistance of MgO–C refractories.     

Mechanical behavior and thermal shock resistance of MgO-C refractories: Influence of graphite content

The mechanical and thermo-mechanical properties of MgO-C refractories are of major importance in the industrial applications, and highly depend on the optimization of their microstructural design. In the present work, the influence of flaky graphite content on mechanical behavior and thermal shock resistance of such refractories was investigated with the aid of the wedge splitting test, fractal and microscopic fractographic analysis. The results showed that the increase of graphite content in the specimens led to an enhanced non-linear fracture behavior, a reduced nominal notch tensile strength (σ_NT), and a higher specific fracture energy (G_f), characteristic length (l_ch) and thermal shock resistance parameter (R_st). The fractal analysis of the crack propagation path of the specimens after the wedge splitting test indicated that increasing graphite content in the refractories can enhance their irregularity of the crack propagation path during fracture. Also, it was suggested from microscopic fractographic analysis that the improvement of thermal shock resistance of MgO-C refractories was positively correlated with the increase of interface crack propagation.     

Effect of ceramic bonding phases on the thermo-mechanical properties of Al2O3–C refractories

Ceramic bonding phases of non-oxide whiskers can enhance the hot strength and the thermal shock resistance of Al₂O₃–C refractories. In this paper, the effect of different metals on the microstructure and thermo-mechanical properties of Al₂O₃–C refractories has been investigated. Thermodynamic calculation of Al–Si–O–C–N systems shows that Al₄C₃, AlN, SiC and β-Sialon are stable at elevated temperature. AlN with the shape of short column can be generated in Al₂O₃–C refractories with metallic Al, which leads to high hot modulus of rupture (HMOR) and poor resistance to thermal shock. SiC whiskers formed in Al₂O₃–C refractories with metallic Si give rise to low HMOR and good resistance to thermal shock. When metallic Si and Al are added together in the refractories, β-Sialon (z=2) with plane structure can be generated under the action of catalyst (nano-sized Ni). The existence of the catalyst promotes the diffusion of Al and O in Si₃N₄ crystals and contributes to the generation of plane-shaped β-Sialon. The corresponding HMOR and residual cold modulus of rupture respectively increase to about 20 MPa and 10.3 MPa. The plane-shaped β-Sialon can significantly enhance both hot strength and thermal shock resistance of Al₂O₃–C refractories.     

Combination of Brazilian test and digital image correlation for mechanical characterization of refractory materials

Flexibility of refractories is an interesting property to improve thermal shock resistance of refractories. It can often be obtained by promoting a nonlinear mechanical behaviour which reduces brittleness. Among industrial refractory materials developed in this aim, magnesia hercynite is of a particular interest for cement industry. As linings for rotary kilns, magnesia hercynite can be submitted to tensile and compressive stresses. Since tensile strength is usually much lower than compressive one for brittle materials, the mechanical characterization is, in such case, more significant in tension than in compression. To overcome difficulties involved by direct tensile test, an indirect tensile test (Brazilian test) has been applied here and combined to digital image correlation in order to measure kinematic fields on the surface of the sample during loading. This combination has allowed to accurately measure initial elastic properties (Young’s modulus and Poisson ratio), to detect crack initiation and to analyze fracture process.