莫来石陶瓷中的位错与断裂

有透射电镜和扫描电镜研究了莫来石陶瓷的亚结构特征和断裂过程。发现裂纹尖端成分布着位错列。认为这种特写位错的交互作用引起了莫来石陶瓷的解理和早期断裂，并使莫来石陶瓷在常温显示低的强度。实验证实，掺加第二相显微颗粒，能有效地钉扎位错，也提高了莫来石陶瓷的常温强度。     

原料杂质对ZTM陶瓷性能的影响及其排除

氧化锆增韧莫来石（ＺＴＭ）陶瓷可用电熔莫来石和高纯莫来石为原料制备，前者因原料中含有一定量的碱金属离子而使得陶瓷性能在高温下不象后那样呈现增长的趋势，要使工业电熔莫来石原料得到利用并改善陶瓷的性能，必面抑制或排除原料中杂质离子的作用，本文通过引入氧化硼添加剂，减弱了这些杂质离子的作用从而提高了该陶瓷的力学性能。     

常压烧结莫来石／氧化锆／碳化硅复相陶瓷的研究

本文对莫来石／氧化锆／碳化硅复相陶瓷进行了Ｎ２气氛中常压烧结的研究。实验结果表明：ＳｉＣ粒子添加量≤２０ｖｏｌ％，材料均可致密烧结并可获得均匀的微观结构。ＳｉＣ粒子的加入使材料人力学性能较莫来石／氧化锆陶瓷有明显的提高，并在ＳｉＣ含量为１０ｖｏｌ％时达到峰值，室温强度和断裂韧性分别为６０１ＭＰａ和５．８ＭＰａ＾Ｃ２，接近热压材料。     

在碳化硅陶瓷上用等离子喷涂莫来石涂层的新方法

莫来石作为一种用于高温工作环境的硅质陶瓷保护层的材料是大有前途的，在碳化硅上用传统等离子喷涂莫来石涂层，在受热循环时，易于产生裂纹和剥落。实验证实这是由于传统喷涂的莫来石中存在有非晶质莫来石所致。高温的碳化硅基质陶瓷在用新的等离子喷涂法喷涂莫来石涂层时，可消除产生的非晶质相，因此可以显著地改善涂层的性能。用此法喷涂的莫来石涂层，在从室温到１０００℃￣１４００℃的高温循环条件下，显示出优良的耐着性能     

ZTM／SiCp复相陶瓷的力学性能与显微结构分析

本文采用热压工艺制备莫来石一氧化锆一碳化硅复相陶瓷；研究了分散相ＳｉＣ粒子的添加量和粒径对氧化锆增韧莫来石陶瓷力学性能的影响。实验结果表明：ＳｉＣ添加量在１０－３０ｖｏｌ％范围之内，材料力学性能有显著提高，其断裂韧性比ＺＴＭ材料要提高８９％，达８．５ＭＰａｍ＾１／２，弯曲强度要提高９１％左右，达６８０ＭＰａ；通过对断口进行观察，细ＳｉＣ粒子强韧ＺＴＭ陶瓷主要是通过裂纹钉扎和偏转来实现的，当添加一定     

莫来石的工业应用

本文综述了莫来石陶瓷作为耐火材料、高温和工业材料、电子材料以及光学材料在工业中的应用，同时概述了莫来石在工业中的其他应用。     

Al2O3在莫来石中固溶对ZTM/Al2O3陶瓷结构与性能的影响

研究了加入氧化铝对ZTM陶瓷结构和性能的影响.发现在烧结过程,氧化铝可固溶于莫来石颗粒形成富铝型柱状莫来石,并因其产生的体积膨胀增强了基质对氧化锆颗粒的约束,使材料中的四方氧化锆相对含量增加,其强韧化效果进一步发挥,明显改善了材料的力学性能.     

微珠陶瓷材料的摩擦磨损性能

由高温摩擦磨损试验研究了复合莫来石(22.6%硅酸锆,75%莫来石,2.4%碳酸钙,质量分数)、硅酸锆和氧化铝3种陶瓷微珠材料在干摩擦和水润滑条件下的摩擦磨损性能,并对其磨损机理进行了分析.结果表明:3种材料的磨损均随着负荷的增加而加剧;在同等载荷下,水润滑条件相对于干摩擦,复合莫来石和硅酸锆的磨损都有所降低,氧化铝磨损反而加剧.在低载荷下,微珠磨损机理主要是塑性变形和微裂纹,在较高载荷下,主要磨损机理是脆性剥落和磨粒磨损.     

高纯莫来石陶瓷的工业应用

莫来石陶瓷是主晶相为莫来石（３ＡＩ２Ｏ３２ＳｉＯ２）的一类陶瓷的总称。若以合成的超细高纯莫来石粉末制备出的不含玻璃相的莫来石瓷,又称新莫来石陶瓷,亦即高纯莫来石陶瓷。１优良的性能１．１力学性能 高纯莫来石陶瓷烧结体的力学性能由ＡＩ２Ｏ３／ＳｉＯ２之比和显微结构决定,尤其是ＡＩ２Ｏ３含量为６８％的莫来石陶瓷,在１３００℃时抗弯强度达５７０ＭＰａ,断裂韧性Ｋｉｃ达５．７ＭＰａ．Ｎｍ,均比常温时高１．６倍,这种随温度升高、强度和韧性不仅不衰减反而大幅度提高,在现有的高温陶瓷材料中除ＳｉＣ外,是绝无仅有…     

凝胶注模莫来石/碳化硅复合陶瓷的显微结构与力学性能

以SiC粉和莫来石粉为原料,采用凝胶注模成型工艺制备莫来石/碳化硅复合陶瓷,研究了莫来石含量对复合陶瓷显微结构和力学性能的影响,结果表明:随莫来石含量增加,复合陶瓷气孔率先降低后升高,而力学性能呈相反变化趋势。当莫来石含量为20%(体积分数)时,材料气孔率达到最小值9.4%,线收缩率以及弹性模量达到最大值26.5%和148.6 GPa;在莫来石含量为30%时抗弯强度和断裂韧性达到最大值397.7 MPa和4.17MPa.m1/2。适量莫来石的加入明显改善了材料的力学性能,过多则由于莫来石挥发而在材料中产生缺陷,材料性能下降。     

The instability of the dislocation ensemble near the first-order phase transition points in crystals

We analyse the influence of the nucleating of the new phase along dislocations on several elements of the dislocation ensemble (DE) of a crystal which undergoes the 1st-order phase transition (PT). We have shown that some of these elements lose their stability near PT points and we have estimated the temperature interval in which the instability may take place. According to this a microplasticity of a crystal may occure in the vicinity of the 1st-order PT and anomalies of mechanical properties are possible     

Dislocations and the Tensile Strength of Magnesium Oxide

It has been shown with single crystals, bicrystals, and polycrystalline material that the strength of magnesium oxide depends on the availability of mobile dislocation sources. Mobile dislocations, introduced by mechanical contact at the surface, are responsible for the ductility normally observed in single crystals. When mobile dislocations are removed by a chemical polish, single crystals become extremely strong (>160,000 psi) and elastic. In the absence of mobile dislocations bicrystals are also extremely strong and elastic but in their presence bicrystals are relatively weak (10,000 psi) and brittle. The brittleness is due to the direct interaction of slip bands with grain boundaries to generate cracks. The strength of high-density polycrystalline magnesia is similarly sensitive to the presence of mobile dislocations. When care is taken to eliminate surface defects and to immobilize dislocations, tensile strengths of 30,000 psi can be attained, but when mobile dislocations are present the strength drops to 15,000 to 20,000 psi. Mobile dislocations can be introduced directly by mechanical contact or indirectly through the presence of pores.     

Introducing dislocations in ceramics by mechanical rolling: A first demonstration using SrTiO3 crystal

It has long been known that dislocations can be used to tune the functional and mechanical properties of ceramics. However, introducing dislocations with controllable networks and densities into ceramics is difficult. In this study, a mechanical rolling technique was proposed to introduce dislocations into ceramics. Using a hard SiC ball with a diameter of 5 mm as a roller, plastic zones and dislocations were successfully produced in a SrTiO₃ (STO) single crystal. The plastic zone area and dislocation densities were determined by the applied force (F) and number of rolling cycles. A force of 10 N produced a scalable plastic zone with an area of 140 µm × 5000 µm without crack formation after 100 rolling cycles. The dislocation density at the center of the plastic deformation zone can reach ∼10¹⁴ m², which is an order of magnitude higher than that achieved previously by others. Increasing the applied force increased the density of the introduced dislocations, for example, ∼2 × 10¹⁴ m⁻² under F = 30 and 35 N, however, lead to crack nucleation in the sample. The dislocations introduced significantly enhanced the mechanical properties of the STO crystal. The measured Vickers’ hardness and fracture toughness increased by 55%–60% and 23%–24%, respectively, compared to the crystal before rolling. This method can serve as a robust technique for engineering dislocations in ceramics, fulfilling the requirements of dislocation-tuned mechanical and functional investigations.     

Thermal-Mechanical History and the Strength of Magnesium Oxide Single Crystals: II, Etch Pit and Electron Transmission Studies

Mechanical tests show that the room-temperature flow strength of magnesia single crystals varies with heat treatment. Flow strength depends on the size, density, and distribution of precipitate particles which inhibit the motion of dislocations. The present paper describes optical and electron transmission microscope evidence for a precipitation reaction in magnesium oxide. The observations correlate fairly well with mechanical test data. Mechanical tests also show that mobile dislocations are locked by heating above 600°C. The room-temperature yield strength increases with time and temperature particularly when the aging temperature exceeds 1000°C. Studies to determine the nature of this locking mechanism are described. Etch pit studies show that dislocations aged above 600°C etch at a slower rate than fresh dislocations. This corresponds precisely with the change in mechanical behavior and both effects are attributed to impurity diffusion to dislocation lines. Electron transmission studies indicate a redistribution of point defects, left by moving dislocations, above 600°C. Eventually these result in a change in dislocation configuration which is considered responsible for the strong locking observed above 1000°C.     

Mechanical Properties of Polycrystalline TiC

The mechanical properties of fine-grained polycrystalline TiC were studied using both four-point bending and compression tests. The ductile-brittle transition (D-B) temperature in compression was determined to be =800°C and was found to depend on grain size. Yield-point behavior was observed for the first time in fine-grained TiC deformed in compression and was found to depend on grain size and test temperature. The yield stress as a function of grain size can be described by a Hall-Petch type of relation, i.e. yield stress α (grain size)-1/2. The dislocations resulting from deformation in compression at lower temperatures were predominately screw in character, with edge dipoles and dislocation loops being present. As the temperature of deformation was increased, the dipoles and loops were gradually annihilated by climb and the dislocations were observed in the form of hexagonal networks with a much-reduced dislocation density. A plot of log yield stress vs 1/T showed a change in slope, which suggests that two rate-controlling mechanisms are in operation during deformation at different test temperatures. Thermal activation analysis at T = 1050° to 1500°C suggested that the rate controlling mechanism during deformation in this temperature range is associated with cross slip.     

Dissociation of Basal Dislocations in 4H–SiC Single Crystals Deformed Around the Transition Temperature

The dislocation microstructure was studied in 4H–SiC samples plastically deformed by basal slip activation around the transition temperature (1000°C–1100°C). Dissociation of basal dislocations takes place over a wide temperature range (800°C–1300°C), but its influence on dislocation motion is different in the high‐ and low‐temperature regimes due to the difference in mobility of partials. Consequently, this material exhibits a completely different mechanical behavior below and above its transition temperature, indicating a change in the deformation mechanism. In this work, the dislocation microstructure was studied around the transition temperature at which both mechanisms are still operative, thus providing a richer number of different configurations generated by dissociation of basal dislocations. They were observed and analyzed by means of the complementary use of weak‐beam dark‐field imaging and high‐resolution transmission electron microscopy. Firstly, 3C band nucleation in the 4H–SiC matrix was identified and its appearance discussed from an energy standpoint. Secondly, the attractive interaction between partials in dipoles and the difference in mobility between the leading and the trailing partial have remarkable effects on the dissociation width, and explain the absence of work hardening above the transition temperature.     

Dislocation toughening in single-crystal KNbO3

The growing research interest in dislocation-tuned functionality in ceramics is evident, with the most recent proofs-of-concept for enhanced ferroelectric properties, electrical conductivity, and superconductivity via dislocations. In this work, we focus on dislocation-tuned mechanical properties and demonstrate that, by engineering high dislocation densities (up to 10¹⁴ m⁻²) into KNbO₃ at room temperature, the fracture toughness can be improved by a factor of 2.8. The microstructures, including dislocations and domain walls, are examined by optical microscopy, electron channeling contrast imaging, piezo-response force microscopy, and transmission electron microscopy methods to shed light on the toughening mechanisms. In addition, high-temperature (above the Curie temperature of KNbO₃) indentation tests were performed to exclude the influence of ferroelastic toughening, such that the origin of the toughening effect is pinpointed to be dislocations.     

Surface Structure in Corundum: I, Etching of Dislocations

Triangular etch pits, corresponding to the intersection of individual edge dislocations with the surface, have been observed in corundum single crystals after about 5 minutes of immersion in boiling phosphoric acid. Dislocations of both the (0001), (1120) and (1210), (1010) slip systems (where reference is made to the slip plane and slip direction, respectively) have been detected by this method. Edge dislocations of the (0001), (1120) system etched on surfaces close to and including the (1011) and {2021} planes. Dislocations of the {1210}, (1010) systems etched on the basal (0001) plane. The orientation dependence and technique of etching is described. The agreement with the Nye formula and with some of the expected properties of dislocations is cited as evidence that individual edge dislocations are actually detected. Typical photomicrographs of the etched surface in as-received, deformed, flame-polished, and polygonized crystals are shown. The dislocation structure produced in basally deformed crystals was found to depend critically on the constancy of the temperature during deformation. Crystals held at 2000°C. during bending exhibited dislocations arrayed in rows in the slip planes in good agreement with the Nye formula. Crystals bent while cooling (quench-bent) had about twice as many dislocations as predicted by the Nye formula and these were distributed randomly. The possibility of vacancy condensation as a method for dislocation generation in quench-bent crystals is discussed.     

Production of Chlorosulfonated Rubber from Postconsumer Polyethylene and Evaluation of Produced Rubber Properties

In order to convert the postconsumer polyethylene to a valuable product, chlorosulfonation of virgin and highly degraded polyethylene has been carried out in the solution phase under atmospheric pressure. Produced chlorosulfonated polyethylene samples from virgin and degraded HDPE are compounded and cured at different temperatures and their mechanical properties are determined. The results show that degradation of used polyethylene does not have noticeable negative effects on the final mechanical properties of produced chlorosulfonated rubbers. Therefore, it can be concluded that chlorosulfonation can be considered as an effective new method for polyolefin recycling.     

Thermal-Mechanical History and the Strength of Magnesium Oxide Single Crystals: I, Mechanical Tests

Various dislocation impurity interactions in magnesium oxide and their effect on mechanical properties between room temperature and 1000°C are discussed. (1) Impurity precipitate particles in single crystals can be redistributed by heat treatment. Their size and density affect the lattice resistance to the motion of fresh dislocations and thus affect the yield strength at room temperature. Above 1200°C impurities go into solid solution and the strength decreases to 0.7 times the as-received fully-precipitated value. (2) Fresh dislocations can be locked by heating above 600°C to produce yield phenomena at room temperature. The upper yield strength increases with time and temperature particularly when the aging temperature exceeds 1000°C. Locking is attributed to the diffusion of impurities and point defects to dislocations. (3) The basic temperature dependence of yield strength contains two regions: (a) Up to approximately 500°C the strength decreases rapidly, suggesting a dislocation cutting mechanism during flow. Material preheated above 1200°C shows a stronger dependence than as-received material. (b) Above 500° C strength decreases linearly at a rate equal to the decline in elastic modulus. Superimposed on this basic temperature dependence curve there is, for material preannealed above 1200° C, a hump due to reprecipitation between 500° and 1000°c.