Thermal stability and mechanical properties of SiC particulate-reinforced Si–C–N composites after heating at 2000 °C

A SiC particulate-reinforced Si–C–N ceramic composite was fabricated using the precursor impregnation and pyrolysis method, and its thermal and mechanical properties were analyzed. The weight loss of the composite was 5% after a heating at 2100 °C in Ar. The pores of the composite enlarged at and above 1700 °C in Ar due to the decomposition of the Si–C–N matrix. However, the composite retained mechanical properties such as strength and hardness after heating at 1700 °C. 88% of the original strength was remained after heating at 2000 °C for 10 h although the fabrication temperature was 1350 °C. The weight gain of the composite was 3.2% after an oxidation at 1450 °C for 30 min in air. The inner oxidation of the particulate-reinforced composites (PRC) was suppressed above 1400 °C due to the closure of the open pores by SiO₂. Consequently, the composite possessed excellent creep resistance at 1400 °C in air. The SiC/Si–C–N composite is a challenging candidate for the application at high temperature.     

An integrated assembly method of sandwich structured ceramic matrix composites

Sandwich structured composites have been widely studied and applied at ambient temperature in aeronautical, automobile and naval applications. For high temperature applications, an integrated ceramic sandwich structure could take advantage of multiple functions such as skin stiffness and core insulation. For thermo-structural applications, skins must be made of ceramic matrix composites (CMC) because of their strength, their resistance to high temperatures (beyond 1000 °C), and their low densities. Concerning foam cores, some carbides (e.g. SiC) are, for their outstanding thermo-mechanical properties, the most appropriate. These foams can withstand long oxidative exposing conditions with low material degradation. This paper presents an assembly method of SiC based sandwich structured CMC. It is performed during sandwich manufacturing in an integrated fashion and allows the production of complex shapes at low costs. Produced flat sandwich panels, characterized by three point bending tests, showed a marked toughening behaviour.     

高温设备和构件的蠕变损伤和断裂研究进展

对高温设备和构件的失效模式和影响因素进行了介绍 ,概述了高温蠕变损伤和断裂理论的研究状况 ,对高温裂纹扩展的控制参量及其应用进行了描述。为了准确地预测构件的寿命 ,保证高温装置的安全运行 ,提出应将损伤力学理论和断裂力学研究相结合、宏观断裂力学研究和微观断裂机理研究相结合 ,对高温设备损伤断裂全过程进行深入研究     

Modeling creep behavior in ceramic matrix composites

In this work, a three-dimensional viscoplasticity formulation with progressive damage is developed and used to investigate the complex time-dependent constituent load transfer and progressive damage behavior in ceramic matrix composites (CMCs) subjected to creep. The viscoplasticity formulation is based on Hill's orthotropic plastic potential, an associative flow rule, and the Norton-Bailey creep power law with Arrhenius temperature dependence. A fracture mechanics-informed isotropic matrix damage model is used to account for CMC brittle matrix damage initiation and propagation, in which two scalar damage variables capture the effects of matrix porosity as well as matrix property degradation due to matrix crack initiation and propagation. The Curtin progressive fiber damage model is utilized to simulate progressive fiber failure. The creep-damage formulation is subsequently implemented as a constitutive model in the generalized method of cells (GMC) micromechanics formulation to simulate time-dependent deformation and material damage under creep loading conditions. The developed framework is used to simulate creep of single fiber SiC/SiC microcomposites. Simulation results are in excellent agreement with experimental and numerical data available in the literature.     

Creep mechanisms in quasi-plastic silicon nitride by instrumented indentation

Quasi-plastic creep behavior of the commercial, fine-grained silicon nitride grade, ST 1, was investigated using variety of techniques with the focus on the analysis of instrumented indentation. Creep deformation in this material was characterized by high creep rates at temperatures above 1300 °C and failure strains around 20%. It was accompanied by strong oxidation, cracking of the oxide layers, excessive cavitation at multigrain junctions and slight texture formation. Instrumented indentation revealed degradation of indentation moduli in the oxide layers and enhancement of oxidation and elastic moduli degradation during creep. Because of the similarities between the mass transport processes in cavitation, diffusion processes involved in oxidation and similar activation energies, both creep and oxidation occur simultaneously, however, oxidation is enhanced by external stress. Texture formation implied from disappearance of -silicon nitride and anisotropy of indentation modulus contributes insignificantly (<5%) to total tensile strain. Creep processes in the studied material can be explained within the expanded cavitation creep model of Luecke and Wiederhorn assuming that cavitation is facilitated by low viscosity residual glass and small matrix grain size. Tertiary-like creep is attributed to the gradual increase of the applied stress resulting from the reduction of the effective cross section due to the formation of cracked oxide layers. Size and pre-oxidation effects were predicted and confirmed using creep samples with different gauge size.     

The microstructural and mechanical properties of geopolymer composites containing glass microfibres

This paper presents the effects of microfibre contents on mechanical properties of fly ash-based geopolymer matrices containing glass microfibres at 0, 1, 2 and 3 mass%. The influence of glass microfibres on the fracture toughness, compressive strength, Young's modulus and hardness of geopolymer composites are reported, as are the microstructural properties investigated using scanning electron microscopy. Results show that the addition of 2 mass% glass microfibres was optimal, exhibiting the highest levels of fracture toughness, compressive strength, Young's modulus and hardness. The results of the microstructural analysis indicate that the glass microfibres act as a filler for voids within the matrix, making a dense geopolymer and improving the microstructure of the binder. This leads to favourable adhesion of the composites, and produces a geopolymer composite with good mechanical properties, comparable to pure geopolymer. The failure mechanisms in glass microfibre-reinforced geopolymer composites are discussed in terms of microstructure.     

High temperature molybdenum matrix composites

The ability to decrease the oxidation rate of molybdenum in oxide/molybdenum composites was discovered some years ago; hence, these composites have emerged as an important family of heat-resistant materials. To this family, silicide/molybdenum composites have recently been added. Both types of composites are produced by internal crystallisation, which allows the crystallisation of a large number of oxide and silicide fibres in cylindrical channels in a molybdenum blank. This paper provides a brief review of recent findings of the author’s team, mainly concerning the strength and damage tolerance of both types of composites. Some data on the creep and oxidation resistance of the composites are also presented.     

疲劳/蠕变交互作用下塑木复合材料的断裂损伤

塑木复合材料在动态载荷作用下，其断裂损伤并非纯疲劳或纯蠕变作用的结果。利用交变载荷的试验方法，研究了在疲劳／蠕变交互作用下塑木复合材料的断裂损伤行为。结果表明，在交变载荷为破坏载荷的80％和60％时，其疲劳／蠕变断裂曲线为三段式曲线，即瞬时弹性变形阶段、延迟弹性变形阶段和加速断裂阶段；在交变载荷为破坏载荷的40％时，38h内其疲劳／蠕变曲线为两段式曲线。随着最大载荷保持时间的增加，塑木复合材料进入延迟弹性变形阶段越晚，弯曲挠度增加越快，断裂寿命降低。     

Fatigue behavior and oxidation resistance of carbon/ceramic composites reinforced with continuous carbon fibers

The aim of this work was to compare fatigue behavior and oxidation resistance of pitch-derived CC (carbon) composite with CC/ceramic (carbon/ceramic) composites obtained by impregnation of CC composite with polysiloxane-based preceram and their subsequent heat treatment. Two types of CC/ceramic composites were studied; CC/SiCO composite obtained at 1000 °C, and CC/SiC composite obtained at 1700 °C. Both types of composites show much better fatigue mechanical performance in comparison to pure CC composite. CC/SiCO composite had 3 times better fatigue properties, and CC/SiC composite 4.5 times better fatigue properties than the reference CC composite. After a fatigue test composites partially retain their mechanical properties, and normalized residual modulus in the direction perpendicular to laminates exceeds 50% for CC and CC/SiCO composites. In the other directions normalized residual modulus is higher than 80% for all composites. Oxidative tests led at 600 °C in air atmosphere indicated oxidation resistance of CC/SiC composites.     

Compressive strength degradation in ZrB2-based ultra-high temperature ceramic composites

The high temperature compressive strength behavior of zirconium diboride (ZrB₂)-silicon carbide (SiC) particulate composites containing either carbon powder or SCS-9a silicon carbide fibers was evaluated in air. Constant strain rate compression tests have been performed on these materials at room temperature, 1400, and 1550 °C. The degradation of the mechanical properties as a result of atmospheric air exposure at high temperatures has also been studied as a function of exposure time. The ZrB₂-SiC material shows excellent strength of 3.1 ± 0.2 GPa at room temperature and 0.9 ± 0.1 GPa at 1400 °C when external defects are eliminated by surface finishing. The presence of C is detrimental to the compressive strength of the ZrB₂-SiC-C material, as carbon burns out at high temperatures in air. As-fabricated SCS-9a SiC fiber reinforced ZrB₂-SiC composites contain significant matrix microcracking due to residual thermal stresses, and show poor mechanical properties and oxidation resistance. After exposure to air at high temperatures an external SiO₂ layer is formed, beneath which ZrB₂ oxidizes to ZrO₂. A significant reduction in room temperature strength occurs after 16-24 h of exposure to air at 1400 °C for the ZrB₂-SiC material, while for the ZrB₂-SiC-C composition this reduction is observed after less than 16 h. The thickness of the oxide layer was measured as a function of exposure time and temperatures and the details of oxidation process has been discussed.     

Sol–gel approach to near-net-shape oxide–oxide composites reinforced with short alumina fibres—The effect of crystallization

Near-net-shape (NNS) high alumina (alumina:silica = 96:4, in equivalent weight ratio) fibre reinforced ceramic matrix composites (CMCs) were prepared with single and bicomponent sols following sol–gel vacuum infiltration technique. The CMCs were characterized by X-ray diffraction (XRD), three-point bend test and scanning electron microscopy (SEM). Crystallization of tetragonal zirconia (t-ZrO₂) in the composite, CZY having zirconia–yttria matrix and that of gamma alumina (γ-Al₂O₃) in the composites, CAZ having alumina–zirconia matrix, CAS having alumina–silica matrix and CA having alumina matrix, enhanced the flexural strength values and pseudo-ductile character of CMCs.     

Sintering of tricalcium phosphate–fluorapatite composites with zirconia

Zirconia (ZrO₂) addition effects on densification and microstructure of tricalcium phosphate–26.52 wt% fluorapatite composites were investigated, using X-ray diffraction, scanning electron microscopy and by analysis using ³¹P nuclear magnetic resonance. The tricalcium phosphate–26.52 wt% fluorapatite–zirconia composites densification increases versus temperature. At 1300 °C, the composites apparent porosity reaches 9% with 5 wt% zirconia. XRD analysis of the composites reveals the presence of tricalcium phosphate, fluorapatite and zirconia without any other structures. Above 1300 °C, the densification was hindered by grain growth and the formation of both intragranular porosity and new compounds. The ³¹P MAS-NMR analysis of composites sintered at various temperatures or with different percentages of zirconia reveals the presence of tetrahedral P sites. At 1400 °C, XRD analysis of the tricalcium phosphate–26.52 wt% fluorapatite–20 wt% zirconia composites shows the presence of calcium zirconate and tetracalcium phosphate. This result indicated that partial decomposition of tricalcium phosphate during sintering process of composites when 20 wt% or less ZrO₂ was added. Thus, zirconia reacts with tricalcium phosphate forming calcium zirconate and tetracalcium phosphate.     

Thermal shock effects on the impact resistance,tolerance and flexural strength of alumina based oxide/oxide ceramic matrix composites

In this study the effects of thermal shock on the impact damage resistance, damage tolerance and flexural strength of Nextel 610/alumina silicate ceramic matrix composites were experimentally evaluated. Composite laminates with balanced and symmetric layup were gradually heated to 1200°C in an air-based furnace and held for at least 30 min before being removed and immersed in water at room temperature. The laminates were then subjected to low velocity impacts via a hemispherical steel impactor. The resultant damage was characterized non-destructively, following which the laminates were subjected to compression tests. Three-point bend tests were also performed to evaluate the effect of thermal shock on the flexural strength and related failure modes of the laminates. Thermally shocked laminates showed smaller internal damage and larger external damage areas in comparison to their pristine counterparts. For the impact energy and resultant damage size considered, the residual compressive strengths for the thermally shocked and pristine laminates were similar.     

木塑复合材料蠕变性能的研究进展

简要介绍了材料的蠕变特性以及木塑复合材料蠕变性能较差的原因,并从影响木塑复合材料蠕变性能的温度、应力及加载方式、湿度等因素以及抗蠕变性研究两方面对木塑复合材料蠕变性能的研究进展进行了综述,展望了今后木塑复合材料蠕变性能研究的重点及方向.     

Fracture resistance of partially pyrolysed polysiloxane preceramic polymer matrix composites reinforced by unidirectional basalt fibres

Two kinds of composite materials reinforced by long unidirectional basalt fibres varying in the matrix type were studied. The first type of matrix material was prepared by application of partial pyrolysis of polysiloxane preceramic resin at a temperature of 650 °C. The second type of matrix material was utilised from cured only polysiloxane and/or epoxy resin and served as reference materials. The advantages of partially pyrolysed composites at elevated temperatures were described recently but the direct comparison with generally used polymer-based composites was not explored deeply. Therefore, both representative materials were characterised with the aim to determine similarities and differences in the fracture processes. The microstructural, elastic and fracture properties were also examined. The fracture resistance was obtained in two typical directions, i.e. along and across the fibres. The fractographic analysis together with fracture characteristics revealed strong and weak aspects of both types of composite materials.     

High-temperature atomically laminated materials: The toughening components of ceramic matrix composites

The atomically laminated materials with high temperature resistance, including graphite, hexagonal boron nitride (h-BN) and MAX phases, have special mechanical performance compared to isotropic materials. The intrinsic mechanical responses of these layered structures can play an important role in strengthening and toughening ceramic matrix composites. In this review, the synthesis processes and applications of pyrolytic carbon (PyC) interphase, graphite nanoplates (GNPs), h-BN interphase, and h-BN nanoplates (BNNPs) and MAX phases are summarized. Besides, this review also analyzes the differences between the graphite-like structure and the MAX phase structure, in terms of modulus, toughness, anisotropy, and toughening mechanisms. These differences are based on their crystal structures. Finally, we look forward to the future development direction of high-temperature atomically laminated materials for toughening applications.     

HfB2-CNTs composites with enhanced mechanical properties prepared by spark plasma sintering

HfB₂-x vol%CNTs (x=0, 5, 10, and 15) composites are prepared by spark plasma sintering. The influence of CNTs content and sintering temperature on densification, microstructure and mechanical properties is studied. Compared with pure HfB₂ ceramic, the sinterability of HfB₂-CNTs composites is remarkably improved by the addition of CNTs. Appropriate addition of CNTs (10 vol%) and sintering temperature (1800 °C) can achieve the highest mechanical properties: the hardness, flexural strength and fracture toughness are measured to be 21.8±0.5 GPa, 894±60 MPa, and 7.8±0.2 MPa m^1/2, respectively. This is contributed to the optimal combination of the relative density, grain size and the dispersion of CNTs. The crack deflection, CNTs debonding and pull-out are observed and supposed to exhaust more fracture energy during the fracture process.     

疲劳与蠕变复合作用下PS的应变与寿命

对PS在疲劳/蠕变复合作用下应变与寿命进行了研究。结果表明：其疲劳/蠕变曲线与纯蠕变曲线十分相似。加载时间周期越短和疲劳载荷变化越频繁。结束普弹应变阶段应变越小，进入延迟弹性变形的平台应变阶段越早。在疲劳/蠕变复合作用下聚苯乙烯存在疲劳和蠕变的交互损伤，其断裂寿命比纯疲劳或纯蠕变的断裂寿命低。断裂寿命减小，疲劳/蠕变的交互损伤程度与温度密切相关，PS在较低温度的疲劳/蠕变交互损伤作用大于较高温度的交互损伤作用。随温度升高，疲劳/蠕变断裂寿命下降是疲劳和蠕变各自的单独损伤增加所致。     

Fabrication of composites with excellent mechanical properties based on cubic boron nitride reinforced with carbon nanotubes

Composites consisting of cubic boron nitride (cBN) as a matrix and carbon nanotubes (CNTs) as reinforcing additives were fabricated by high-temperature and high-pressure sintering (HTHP). Microstructures, mechanical properties, fracture modes and toughening mechanisms of these composites were investigated. Composites exhibited excellent bending strength, wear resistance, and fracture toughness. Fracture toughness of composites reached 7.02 MPa·m^1/2. Comparing to pure cBN matrix, bending strength improved from 475.27 to 600.15 MPa, and wear resistance increased by 43.23%. Such improvements of mechanical properties were mainly attributed to pullout and bridging reinforcements by CNTs. CNTs incorporation also changed fracture mode from inter-to trans-granular.     

High-temperature mechanical behavior of Al2O3/graphite composites

Uniaxial compressive creep behaviour of spark-plasma-sintered Al₂O₃/graphite particulate composites has been studied at temperature between 1250 and 1350 °C. Values of stress exponent, n, ranging from 1 to 1.4 and, activation energy, Q, of 600 ± 40 kJ/mol have been determined. With 10 vol% graphite in the composite, the creep deformation of the composite is controlled by the fine-grained Al₂O₃ matrix, where Coble creep has been identified as the dominant creep mechanism.