Transformation Toughening and Fracture Behavior of Molybdenum Disilicide Composites Reinforced with Partially Stabilized Zirconia

The effects of yttria stabilization (0–6 mol%) on the fracture toughness of molybdenum disilicide composites reinforced with 20-vol%-yttria-stabilized zirconia particles are elucidated. Fracture toughness tests were conducted under three-point bend loading using single-edge-notched specimens. The stress-assisted martensitic phase transformation of zirconia associated with the fracture process was then studied using optical interference microscopy and laser Raman spectroscopy. Stress-induced martensitic transformation from tetragonal to monoclinic phase was observed only in the plastic wake of the material stabilized with 2 mol % yttria. The degree of toughening in this composite was also predicted using micromechanical models that assess the combined effects of transformation toughening and crack deflection. However, the fracture toughness of the 2-mol%-yttria-stabilized composite was in the same range as those of the other composites that did not exhibit evidence of transformation toughening. The toughening in the other composites is explained by considering the effects of crack deflection, residual stresses, and microcracking induced by residual stresses that occur as a result of the thermal expansion mismatch between the matrix and the reinforcements.     

High-Pressure Transformation Toughening: A Case Study on Zirconia

Transformation-toughened zirconia compacts have been produced using a pressure-induced phase of zirconia as the toughening agent. The high-pressure phase is retained metastably after compaction at 8.6 GPa and sintering at temperatures as low as 250°C. High-pressure processing offers potential for new transformation-toughening phases in other ceramic materials.     

Toughening by Multiple Mechanisms in Ceramic-Matrix Composites with Discontinuous Elongated Reinforcements

The dependence of toughening mechanisms on reinforcement orientation and the toughening effect governed by multiple toughening mechanisms were characterized for ceramic-matrix composites (CMCs) with discontinuous elongated reinforcements. Two kinds of Si₃N₄-based composites, with directionally oriented and randomly oriented SiC whiskers, respectively, were tested by the three-point bending of chevron-notched bars. Based on microscopic observations and micromechanical analyses, three mechanisms were confirmed to dominate the crack-bridging behavior: (1) bridging and breaking of long reinforcements, (2) frictional pullout and breaking of short reinforcements, and (3) local matrix spalling. Both the occurrence of the multiple mechanisms and their toughening effects were proved dependent on the reinforcement orientation. The combined effect of the multiple mechanisms correlated with random orientation thus was characterized by a statistical approach to solve for the crack-bridging stress function. The theoretical model was in good agreement with the experimental results.     

聚合物基复合材料的增强增韧

介绍了聚合物基复合材料增强增韧的改性方法和机理,包括有机小分子、弹性体、刚性粒子、纤维以及碳纳米管增强增韧聚合物。并介绍了一些具有代表性的聚合物/增强增韧剂体系。并对聚合物基复合材料增强增韧的发展前景进行了展望。     

尼龙6的增强增韧研究

以环氧基树脂为增容剂,通过熔融挤出方式引发其与尼龙6的界面微交联反应,其产物在受到冲击时可产生较大塑性变形和较多银纹,从而吸收能量使体系增韧。同时,均匀分布的微交联网络结构可有效传递应力,束缚分子链运动,使复合材料的强度、刚性和热稳定性同时得到提高。     

Whisker Toughening: A Comparison Between Aluminum Oxide and Silicon Nitride Toughened with Silicon Carbide

Two whisker-toughened materials have been studied, with the objective of identifying the mechanisms that provide the major contribution to toughness. It is concluded that, for composites with randomly oriented whiskers, bending failure of the whiskers obviates pullout, whereupon the major toughening mechanisms are the fracture energy consumed in creating the debonded interface and the stored strain energy in the whiskers, at failure, which is dissipated as acoustic waves. The toughening potential is thus limited. High toughness requires extensive pullout and, hence, aligned whiskers with low fracture energy interfaces.     

Strengthening and Toughening Mechanisms of Ceramic Nanocomposites

Crack-tip bridging by particles is considered to be one of the primary strengthening mechanisms of ceramic nanocomposites. Small, brittle particulate inclusions have been shown to cause crack-tip bridging at short distances behind the crack tip. This mechanism leads to modest toughness but a very steep R -curve, and it is the latter that produces the very high fracture strength of the ceramic nanocomposite. Localized high residual stress around the particles (particularly in the case of silicon carbide-alumina material) causes the strengthening mechanism to operate effectively, even at a small volume fraction of 5%. The present study predicts the magnitude of the toughness increase and the extent of R -curve behavior for the nanocomposite.     

氧化锆(氧化钇)增韧莫来石陶瓷的组成与强韧化机理的研究

本文采用常压烧结工艺制备ZrO2增韧莫来石陶瓷(ZTM).通过调整ZrO2、Y2O3含量制得不同组成的ZTM材料,其强度随ZrO2加入量的增加经历了先上升而后下降的过程.在不同组成下,材料的强韧化机理不同.当ZrO2≤30vol%时,ZTM材料是以应力诱导相变增韧为主;ZrO2＞30vol%(如ZT M40(2Y)),材料则以微裂纹,裂纹偏转等强韧化机理为主;并且分散相ZrO2晶粒的大小及分布均匀性等对强韧化机理的发挥有很大影响.     

晶须含量和取向对Al2O3—SiCw陶瓷刀具材料的增韧特性及切削性能的影响

晶须含量和取向不同将影响到Al_2O_3 SiCw陶瓷刀具材料的力学性能,由此将进一步影响到该刀具材料的切削性能。本文测量了不同晶须含量和不同晶须取向的Al_2O_3-SiCw材料的断裂韧性,分析了晶须含量和晶须取向对该刀具材料增韧效果的影响。试验了不同晶须含量和不同晶须取向的Al_2O_3 SiCw刀具的切削性能。结果表明:晶须含量和取向对刀具材料的磨损和破损有很大的影响。因此在实际应用中应根据刀具损坏方式的不同分别选用不同晶须含量和不同晶须取向的Al_2O_3-SiCw刀具材料。以充分发挥该刀具材料的增韧补强效果。     

溶剂热合成氧化镁晶须及对聚丙烯的改性

以MgCl2·6H2O和NH3·H2O为原料,首先通过溶剂热合成纤维状的Mg2(OH)3Cl·3H2O前驱物,采取前驱物分解方法制备氧化镁晶须.利用SEM和XRD对产物进行了表征.结果表明:未加有机溶剂合成的晶须直径较小,且粗细不均;添加有机溶剂后晶须直径变大,粗细比较均匀,分散性也好,较佳的合成条件是添加乙醇和异丙醇混合溶剂.用氧化锓晶须改性聚丙烯,探讨晶须含量对复合材料力学性能的影响,结果表明:当晶须添加量为20%时,复合材料力学性能最佳.     

纳米CaCO_3增强增韧聚丙烯的研究

通过熔融共混法制备了ＰＰ／纳米ＣａＣＯ３ 复合材料。力学性能测试表明,纳米ＣａＣＯ３ 在低含量下（０．５％ ～５％ ）可以使聚丙烯冲击强度提高３～４ 倍,同时保持其拉伸强度和刚度。通过对填充复合材料的冲击断面观察及断口损伤分析证明了材料的增韧是由于基体发生了大面积屈服所致。     

Stress Relief Mechanisms in Magnesia- and Yttria-Stabilized Zirconia

Stabilized zirconias exhibit pronounced photoelastic effects when viewed with transmitted polarized light. Thin sections reveal stress concentrations around pores and inclusions and at grain boundaries. The yield point of these materials, at some high temperature, is exceeded and plastic deformation tends to relieve these stress concentrations. The microhardness measurements of magnesia-stabilized zirconia indicate both solution hardening and solute segregation and help to explain the physical behavior of this system as the amount of MgO is varied.     

Cubic-Formation and Grain-Growth Mechanisms in Tetragonal Zirconia Polycrystal

The microstructure in Y₂O₃-stabilized tetragonal zirconia polycrystal (Y-TZP) sintered at 1300°–1500°C was examined to clarify the role of Y³⁺ ions on grain growth and the formation of cubic phase. The grain size and the fraction of the cubic phase in Y-TZP increased as the sintering temperature increased. Both the fraction of the tetragonal phase and the Y₂O₃ concentration within the tetragonal phase decreased with increasing fraction of the cubic phase. Scanning transmission electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) measurements revealed that cubic phase regions in grain interiors in Y-TZP generated as the sintering temperature increased. High-resolution electron microscopy and nanoprobe EDS measurements revealed that no amorphous layer or second phase existed along the grain-boundary faces in Y-TZP and Y³⁺ ions segregated at their grain boundaries over a width of ∼10 nm. Taking into account these results, it was clarified that cubic phase regions in grain interiors started to form from grain boundaries and the triple junctions in which Y³⁺ ions segregated. The cubic-formation and grain-growth mechanisms in Y-TZP can be explained using the grain boundary segregation-induced phase transformation model and the solute drag effect of Y³⁺ ions segregating along the grain boundary, respectively.     

PP／UHMWPE合金增韧增强机理研究

采用超高相对分子质量聚乙烯（UHMWPE）对乙丙共聚型聚丙烯（PP）进行共混改性，所得合金材料的刚性和韧性同时得到显著提高，其缺口冲击强度大于72.1kJ/m^2，断裂伸长率大于900％。采用多种测试手段从共混物性能与结构关系出发，探讨了合金高性能化的深层原因。认为在适当的工艺条件下，UHMWPE能以其较高的熔体粘度和强度在PP基体中以微纤状均匀分散，并与PP形成双连续相结构。在熔体冷却过程中，UHMWPE的高分子链段与PP基体的部分PE链段形成共晶，产生一种“共晶物理交联点的互穿网络结构”，从而使合金的韧性和刚性同时得以提高。     

钼酸钠晶须的水热合成及对聚丙烯的改性

在水热条件下合成钼酸钠晶须,用XRD、SEM和IR对其结构进行表征.用其与聚丙烯进行复合,探讨钼酸钠晶须含量对复合材料力学性能的影响.结果表明:水熟合成的晶须长30～100μm,直径2～3μm,表面光滑.用它来改性聚丙烯,可以使复合材料的拉伸强度、弯曲强度和模量都增加,当晶须质量含量为20%时,复合材料力学性能最佳.     

反应性挤出尼龙66增强和增韧研究

运用反应挤出技术对尼龙66（PA66）进行增强增韧改性，采用双螺杆挤出机作为反应器，同时完成玻璃纤维增强和弹性体就地增容、增韧反应，简化了工艺，提高生产效率。通过力学性能测试及扫描电镜分析，结果表明在加工过程中PA66与弹性体发生了原位增容反应，生成了马来酸酐嵌段共聚物，增加了界面强度，改性后的PA66冲击强度提高，而且具有良好的挤出和注射成型加工性能。     

Silicon Carbide Platelet/Alumina Composites: III, Toughening Mechanisms

This is the last part of a series of papers on the processing and fracture behavior of SiC-platelet/Al₂O₃ composites. The objective of this paper was to identify the mechanisms involved in the toughening process. A hot-pressed composite with a SiC volume fraction of 0.3 was chosen as the model system for study. Based on microstructural observations, crack deflection and grain bridging were both identified as possible toughening mechanisms and were further investigated. A Modified two-dimensional crack deflection model is presented to account for the anisotropic microstructure in hot-pressed platelet composites, in which preferred platelet orientation was present. Relative toughness values were predicted for two crack propagation directions assuming crack deflection is the toughening mechanism. Fracture toughness measurements for specific crack directions were made using a bridge indentation technique. The correlation of experimental results with theoretical predictions is discussed. To distinguish the effect of grain bridging from crack deflection, an in situ observation of crack growth was conducted. The results showed no distinct rising T -curve behavior for cracks in the size range 80 to 500 μ m. Measurement of fracture surface roughness was also made and the implications are discussed. The results indicate that crack deflection is the dominant toughening mechanism in the SiC-platelet/ Al₂O₃ composites studied herein.     

晶须/纤维增强3D打印氧化锆多孔陶瓷制备及性能研究

钇稳定氧化锆（YSZ）是一种抗氧化性和耐久性优异的陶瓷,够承受高温,非常适合作热防护材料。采用乳液/泡沫模板法将其制成具有微米级孔的多孔结构,再以氧化铝晶须或氧化锆纤维作为增强相,然后结合直写成型这种3D打印成型技术,又可在毫米级孔尺度上获得设计的自由。由此制备的梯度多孔结构,不仅可以增大材料的比表面积,减小体积密度,更能大大提高多孔YSZ的力学性能。研究增强体的类型、加入量及烧结温度对多孔氧化锆陶瓷微观形貌结构的影响,分析其与抗压强度的相互作用关系。结果表明,氧化铝晶须和氧化锆纤维的加入,均能有效提高多孔氧化锆陶瓷孔的抗压强度,晶须的增强效果更好。氧化锆纤维加入量为4wt%的多孔氧化锆陶瓷孔隙率最高,抗压强度提升最小,为166.6MPa。在1500℃烧结温度下,当氧化锆纤维加入量为8wt%时,抗压强度最大,达到269.36MPa。