Wetting and Joining of Mullite Ceramics by Active-Metal Braze Alloys

Mullite and ZrO₂-toughened mullite (ZTM) were joined with Ag─Cu eutectic braze alloys that contain Ti or Zr as active elements. Although neither the alloy compositions nor the processing conditions were optimized, four-point bend strengths of joined bars were as high as 108 MPa. Joining reactions were studied by conducting separate sessile drop experiments with molten Ti─Ag─Cu and Zr─Ag─Cu alloys on mullite for times and temperatures that were similar those used for joining. Detailed compositional and microstructural analyses of those metal–ceramic interfaces revealed a complex reaction zone in which the oxide of the active element was a principal reaction product. Compared with the Ti-containing alloys, those with Zr were more refractory, they were less reactive with the mullite, and the Zr did not segregate as completely to the interface during heating.     

Effects of Thermal Aging on the Mechanical Properties of a Porous-Matrix Ceramic Composite

The present article focuses on changes in the mechanical properties of an all-oxide fiber-reinforced composite following long-term exposure (1000 h) at temperatures of 1000–1200°C in air. The composite of interest derives its damage tolerance from a highly porous matrix, precluding the need for an interphase at the fiber–matrix boundary. The key issue involves the stability of the porosity against densification and the associated implications for long-term durability of the composite at elevated temperatures. For this purpose, comparisons are made in the tensile properties and fracture characteristics of a 2D woven fiber composite both along the fiber direction and at 45° to the fiber axes before and after the aging treatments. Additionally, changes in the state of the matrix are probed through measurements of matrix hardness by Vickers indentation and through the determination of the matrix Young's modulus, using the measured composite moduli coupled with classical laminate theory. The study reveals that, despite evidence of some strengthening of the matrix and the fiber–matrix interfaces during aging, the key tensile properties in the 0°/90° orientation, including strength and failure strain, are unchanged. This strengthening is manifested to a more significant extent in the composite properties in the ±45° orientation, wherein the modulus and the tensile strength each exhibit a twofold increase after the 1200°C aging treatment. It also results in a change in the failure mechanism, from one involving predominantly matrix damage and interply delamination to one which is dominated by fiber fracture. Additionally, salient changes in the mechanical response beyond the maximum load suggest the existence of an optimum matrix strength at which the fracture energy in the ±45° orientation attains a maximum. The implications for long-term durability of this class of composite are discussed.     

Effects of Pores on Mechanical Properties of Plasma-Sprayed Ceramic Coatings

The effects of pore sizes, shapes, and orientations on the mechanical properties of thermally sprayed ceramic coatings are investigated. The analysis is conducted using detailed finite-element models with geometries similar to those of actual ceramic coatings containing many embedded pores. These microstructural models include many randomly placed pores of different sizes and shapes and are loaded in tension to determine their effective elastic moduli along the spray and transverse directions. We modeled coatings with statistical distributions of pore sizes and shapes that followed those of actual Al₂O₃–TiO₂ coatings. Because the pores in such a model are of different sizes and shapes, the model must be large enough to contain sufficient pores before the average modulus obtained from uniaxial loading can be identified as an effective property. Using differently sized models, we determined the variability of the average moduli. Such information is valuable when homogenized or continuum material models are used in the stress analyses of coatings. Our computed results show that a model must be large enough to contain 50–100 pores before the averaging of properties is accurate. Using the Al₂O₃–TiO₂ models, we also simulated microindentation tests. Unlike the results determined from uniaxial loading, the elastic moduli estimated from indentation possessed large variations. Apparently, the morphology of the pores immediately beneath the indentation or within the zone of influence has a significant effect on the response of the indenter and the measured modulus. The implications of these results and the computational capability to predict the mechanical properties of porous, plasma-sprayed ceramic coatings are discussed here.     

Effects of Matrix Porosity on the Mechanical Properties of a Porous-Matrix, All-Oxide Ceramic Composite

The effects of matrix porosity on the mechanical properties of an all-oxide ceramic composite are investigated. The porosity is varied through impregnation and pyrolysis of a ceramic precursor solution. Mechanical tests are performed to assess the role of the matrix in both matrix-dominated and fiber-dominated loading configurations. The results demonstrate a loss in damage tolerance and tensile strength along the fiber direction as the porosity is reduced. Concomitantly, some improvements in interlaminar strength are obtained. The latter improvements are found to be difficult to quantify over the entire porosity range using the standard short beam shear method, a consequence of the increased propensity for tensile fracture as the porosity is reduced. Measurements of interlaminar shear strength based on the double-notched shear specimen are broadly consistent with the limited values obtained by the short beam shear method, although the former exhibit large variability. In addition, effects of precursor segregation during drying on through-thickness gradients in matrix properties and their role in composite performance are identified and discussed. An analysis based on the mechanics of crack deflection and penetration at an interphase boundary is presented and used to draw insights regarding the role of matrix properties in enabling damage tolerance in porous-matrix composites. Deficiencies in the understanding of the mechanisms that enable damage tolerance in this class of composites are discussed.     

The Effects of Calcium‐to‐Phosphorus Ratio on the Densification and Mechanical Properties of Hydroxyapatite Ceramic

In this work, hydroxyapatite (HA) powders were synthesized using calcium hydroxide Ca(OH)₂ and orthophosphoric acid H₃PO₄ via wet chemical precipitation method in aqueous medium. Calcium‐to‐phosphorus (Ca/P) ratio was set to 1.57, 1.67, 1.87 that yield calcium‐deficient HA, stoichiometric HA, and calcium‐rich HA, respectively. These synthesized HA powders (having different Ca/P ratio) were characterized in terms of particle size and microstructural examination. Then, the densification and mechanical properties of the calcium‐deficient HA, stoichiometric HA, and calcium‐rich HA were evaluated from 1000 to 1350°C. Experimental results have shown that no decomposition of hydroxyapatite phase was observed for stoichiometric HA (Ca/P = 1.67) and calcium‐deficient HA (Ca/P = 1.57) despite sintered at high temperature of 1300°C. However, calcium oxide (CaO) was detected for calcium‐rich HA (Ca/P = 1.87) when samples sintered at the same temperature. The study revealed that the highest mechanical properties were found in stoichiometric HA samples sintered at 1100–1150°C, having relative density of ~99.8%, Young's modulus of ~120 GPa, Vickers hardness of ~7.23 GPa, and fracture toughness of ~1.22 MPam^1/2.     

ABS对POM结晶行为及力学性能的影响研究

考察了丙烯腈–丁二烯–苯乙烯塑料(ABS)含量和相形态对聚甲醛(POM)结晶行为及力学性能的影响。结果表明,POM球晶生长时将ABS相作为异物排除在晶格外,使其在球晶间隙间富集或囊裹在球晶中。ABS使POM结晶完善程度稍降低,但晶型及负光性未变。ABS含量及相形态影响POM结晶行为:ABS含量在0~50%时,ABS稍促进POM结晶;ABS含量在60%~90%时,强烈干扰POM结晶,呈现分步结晶现象。ABS使POM的拉伸性能和冲击强度显著下降,但弯曲性能出现协同效应。     

High-Temperature Mechanical Properties of a Ceramic Matrix Composite

The change in mechanical properties of a fiber-reinforced ceramic from notch insensitivity at room temperature to notch sensitivity at elevated temperature has been investigated. The change in behavior has been attributed primarily to a correspondingly large variation in the shear resistance of the fiber/matrix interface caused by oxidation effects at that interface. The transition in behavior has been correlated with a fracture model based on the incidence of fiber failure in the crack wake.     

Effect of ZnO and CuO on the Sintering Temperature and Piezoelectric Properties of a Hard Piezoelectric Ceramic

Hard piezoelectrics with high dielectric and piezoelectric constants are used for high-power applications. However, the sintering temperature of these ceramics is high, around 1200°C, restricting the usage of cheap base metal electrodes in fabrication of multi-layer components. This study investigates the effect of CuO and ZnO on the sintering temperature of a hard piezoelectric, APC 841, which is a MnO₂- and Nb₂O₅-modified PZT. The addition of CuO decreased the sintering temperature through the formation of a liquid phase. However, the piezoelectric properties of the CuO-added ceramics sintered at ≤950°C were lower than the desired values. The addition of ZnO resulted in a significant improvement in the piezoelectric properties. This enhancement was attributed to the formation of a homogeneous microstructure with large grains. The APC 841+0.2 wt% CuO+1.1 wt% ZnO ceramics sintered at 950°C showed excellent piezoelectric and dielectric properties with values of k _p=0.532, Q _m=750, d ₃₃=351 pC/N, ɛ₃₃/ɛ_o=1337, and T _c=280°C.     

热处理对抗冲共聚聚丙烯结晶行为和力学性能的影响

通过WXRD和SEM研究了抗冲共聚聚丙烯(HIPC)在热处理过程中的结晶行为变化,结果表明,随着热处理的进行,HIPC中所有β晶型都转化为更为稳定α晶型,结晶度和球晶尺寸最初随着热处理时间延长而增大,经过一段时间后不再变化,橡胶相含量有所减少,但分布更均匀,且橡胶网络逐步形成。研究还发现,热处理对HIPC力学性能和热性能都有不同程度的影响。     

云母粉对聚丙烯微孔发泡行为和力学性能的影响

将云母粉加入到聚丙烯(PP)中,在二次开模条件下制备微孔发泡PP/云母粉复合材料,分析了不同含量的云母粉对微孔发泡复合材料发泡行为及力学性能的影响。结果表明,当云母粉质量分数为6%时微孔发泡复合材料的泡孔尺寸最小,泡孔密度最大;随着云母粉加入量的增大微孔发泡复合材料的缺口冲击强度略有降低,拉伸强度基本保持不变。     

Mechanical Properties of a Lithium Disilicate Strengthened Lithium Aluminosilicate Glass‐Ceramic

A glass composition in the Li₂O–Al₂O₃–SiO₂ (LAS)‐ternary phase diagram is presented, which enables the crystallization of Li₂Si₂O₅ as well as LiAlSi₂O₆ upon a well‐defined heat treatment. Li₂Si₂O₅ is the minor crystalline phase in the glass‐ceramic and generates through the bulk strengthening of the LAS glass‐ceramic. A flexural strength as higher as 400 MPa is measured in the proposed glass‐ceramic formulation, thus outperforming conventional LAS‐glass‐ceramics. A combination of the presence of those two crystalline phases of very different mechanical properties and coefficient of thermal expansion as well as the microstructure are believed to contribute largely to the enhancement of the measured properties.     

Mechanical Behavior of a Laminar Ceramic/Fiber-Reinforced Epoxy Composite

The mechanical properties of a noval laminar composite are investigated. The composite consists of dense alumina sheets bonded between sheets of a uniaxial carbon-fiber-reinforced epoxy tape. The behavior of the composite in both flexural and tensile loading is characterized and the results are related to the properties of the constituents. The role of the interlaminar interface in composite behavior is also examined. Implications for the design of laminar composites with complex shapes are briefly discussed.     

Microstructural Effects on the Mechanical Properties of SiC‐15 vol% TiB2 Particulate‐Reinforced Ceramic Composites

The microstructures and mechanical properties were studied for two different SiC ceramics containing 15 vol% of TiB₂ particulates. The first was prepared from commercially available spray‐dried granules and the second by blending individual SiC and TiB₂ powders. The average TiB₂ particle sizes were 2.7 μm for the ceramic prepared from blended powders, which had a uniform distribution of TiB₂, and 2.3 μm for the ceramic prepared from spray‐dried granules, which had a nonuniform distribution of TiB₂ agglomerates. Although the two ceramics had hardness values of 26 GPa, the other properties were different. For example, the fracture toughness was 4.3 MPa·m^1/2 for the ceramic prepared from blended powders compared to 3.1 MPa·m^1/2 for the ceramic prepared from spray‐dried granules. In contrast, the Weibull modulus for the ceramic prepared from spray‐dried granules was 21 compared to 12 for the other. Calculations predicted spontaneous microcracking in the ceramic prepared from spray‐dried granules, which was confirmed by analysis of the microstructure. The presence of microcracks accounted for the higher Weibull modulus, but lower flexural strength, Young's modulus and fracture toughness for the ceramic prepared from spray‐dried granules.     

MnCO3和CuO对中高压电容瓷料介电性能的影响

研究了MnCO3,CuO对0.90(Sr0.54Pb0.26Ca0.20)TiO3-0.1Bi2O3·3.5TiO2为系统的中高压瓷介电容瓷料的介电性能的影响,实验发现掺加MnCO3能提高居里温度,使ε-t峰向正温方向移动.MnCO3能起压峰作用,使介电常数(ε)减小,同时能改善瓷料的温度性能.MnCO3可以降低瓷料的低温损耗,但室温损耗变化不大.CuO的掺加有明显的展宽作用.     

玻纤长度及其含量对BMC力学性能影响

用正交试验设计的方法研究了玻纤／填料质量比、玻纤长度及混合时间等主要因素对BMC（预制整体模塑料）力学性能的影响。在系统地分析试验数据的基础上，以BMC的力学性能为参考值，得出了以上3个因素对BMC力学性能的影响趋势曲线；用扫描电镜讨论了3因素与BMC的微观结构及力学性能的关系，并获得了优化BMC的方案，所得的BMC的弯曲和冲击强度分别达到96．35MPa和21．82kJ／m^2。     

复合型陶瓷薄板的制备及其力学性能

作为一种轻薄、低能耗的功能化产品,陶瓷薄板因强度低而应用受限,如何对其进行低成本增强成为工业领域研究热点。本文以构筑“纤维布-黏结剂-陶瓷薄板”多层复合结构作为切入点,将多种工业级纤维布、黏结剂和陶瓷薄板进行二次后加工复合,制备了兼具低成本和优异力学性能的复合型陶瓷薄板,探究了其断裂面微观形貌及断裂机理。经研究表明,“碳纤维布-环氧树脂-陶瓷薄板”复合型陶瓷薄板具有最佳界面结合强度及力学性能,其抗弯强度和承载冲击能量分别为85.26 MPa和1.45 J,与陶瓷薄板坯体相比,性能提升幅度分别高达22.98%和141.67%。“纤维布-黏结剂-陶瓷薄板”多层复合结构能够有效提升陶瓷薄板综合力学性能,陶瓷薄板内部存在微裂纹拓展、纤维偏转等多种良性强韧化机制。     

Ultra‐High Temperature Mechanical Properties of a Zirconium Diboride–Zirconium Carbide Ceramic

The mechanical properties of a ZrB₂‐10 vol% ZrC ceramic were measured up to 2300°C in an argon atmosphere. Dense billets of ZrB₂‐9.5 vol% ZrC‐0.1 vol% C were produced by hot‐pressing at 1900°C. The ZrB₂ grain size was 4.9 μm and ZrC cluster size was 1.8 μm. Flexure strength was 695 MPa at ambient, decreasing to 300 MPa at 1600°C, increasing to 345 MPa at 1800°C and 2000°C, and then decreasing to 290 MPa at 2200°C and 2300°C. Fracture toughness was 4.8 MPa·m^½ at room temperature, decreasing to 3.4 MPa·m^½ at 1400°C, increasing to 4.5 MPa·m^½ at 1800°C, and decreasing to 3.6 MPa·m^½ at 2300°C. Elastic modulus calculated from the crosshead displacement was estimated to be 505 GPa at ambient, relatively unchanging to 1200°C, then decreasing linearly to 385 GPa at 1600°C, more slowly to 345 GPa at 2000°C, and then more rapidly to 260 GPa at 2300°C. Surface flaws resulting from machining damage were the critical flaw up to 1400°C. Above 1400°C, plasticity reduced the stress at the crack tip and the surface flaws experienced subcritical crack growth. Above 2000°C, microvoid coalescence ahead of the crack tip caused failure.     

改善建筑陶瓷墙地砖强度和导热性能的试验研究

以粘土、煅烧高岭土、长石、锂辉石和煅烧氧化铝等为原料,采用半干压法压制成型,在1210～1220℃温度范围内烧成,研制了具有较高体积密度、抗压强度和导热系数的新型陶瓷墙地砖。实验研究结果表明,通过优化现有陶瓷墙地砖的工艺配方,改善陶瓷墙地砖微观结构,有利于降低陶瓷墙地砖的气孔率,提高陶瓷墙地砖的致密度和刚玉-莫来石相含量,可获得导热系数为2.0～2.4W/(m.K)、断裂模数为51～57MPa的新型陶瓷墙地砖。     

Evaluation of Interfacial Properties of Fiber-Reinforced Ceramic Composites Using a Mechanical Properties Microprobe

The application of a mechanical properties microprobe to evaluate the interfacial properties of fiber-reinforced ceramic composites is addressed. The stress–displacement relation of the embedded fiber, which is subjected to an axial loading–unloading cycle, is analyzed. The interfacial bonding, Coulomb friction at the debonded interface, Poisson's effect of the loaded fiber, and residual stresses are included in the analysis, and closed-form analytical solutions are obtained. Based on the analytical solutions, a methodology is established to extract the interfacial properties from experimental stress–displacement curves. The roles of interfacial bonding, Poisson's effect, and residual axial stresses on the residual fiber displacement after complete unloading are also addressed in the present study.