Debond coating requirements for brittle matrix composites

The cause of improved fracture toughness in Y₂O₃-coated niobium-toughened TiAl relative to either uncoated niobium or Al₂O₃-coated niobium was examined. Reactively sputtered Y₂O₃ coatings, 1–2 m thick, were deposited on to rock salt (NaCl), polished single-crystal (0001) Al₂O₃, and polished polycrystalline niobium. Sputtered niobium coatings, 1–2 m thick, were also deposited on to polished single-crystal Y₂O₃ substrates for comparison. The oxide coating was characterized and consisted of stoichiometric bcc Y₂O₃ witha ₀=1.0602 nm. Indentation tests were performed to correlate the fracture toughness and debond characteristics of as-deposited Y₂O₃ coatings on Al₂O₃ and polycrystalline niobium, and niobium coatings on single-crystal Y₂O₃, to that found in TiAl/Nb and Al₂O₃/Al₂O₃ laminates. The calculated fracture toughness of sputtered Y₂O₃ on sapphire was similar to reported values for bulk Y₂O₃. However, a wide variation in interfacial fracture toughness was obtained by indentation methods, and is attributed to the microstructure of as-deposited coatings and to weak bonding between as-deposited yttria and the sapphire substrate. These results are related to factors that affect debonding and fracture toughness of brittle matrix composites. Reactive and non-reactive metal/ceramic systems were reviewed in an effort to understand why Y₂O₃ coatings perform well. It is postulated that yttrium oxide coatings applied to niobium have an atomically sharp interface that has a lower fracture energy compared to Nb/Al₂O₃, resulting in improved interfacial debonding and composite fracture toughness.     

Fracture mechanics of brittle matrix ductile fiber composites

A model to predict the increase in critical flaw size or stable crack growth potential which can occur by the inclusion of ductile fibers in a brittle matrix is considered. The model is based upon the super-position of two known stress intensity solutions; one for the crack opening mode resulting from a remotely applied stress and the second, an opposing stress intensity that results from a crack closing force exerted by unbroken fibers spanning the crack surfaces. The extent of stable growth possible is computed at the ultimate stress of the brittle phase as functions of fiber strength and of volume fraction for various amounts of fiber rupture. A hot pressed beryllium matrix is used as an example. The crack surface displacement over which a given fiber is capable of deforming without rupture is found to be sensitive to the fiber-matrix interface strength. The factors leading to maximum crack surface displacement without rupture are a high strain hardening capability of the fiber and an interface designed to fail at fiber stresses between yield and ultimate strengths.     

Mechanistically based fatigue-damage evolution model for brittle matrix fibre-reinforced composites

An analysis of the fatigue-damage evolution process through prediction of stiffness drop in brittle matrix unidirectional composites reinforced with continuous stiff fibres is presented. The drop in stiffness of the composite is calculated by partitioning the total damage between the components of the composite, namely the matrix, the fibre, and the interface. Predictions of drop in stiffness are validated for different fatigue test conditions in borosilicate glass-ceramic matrix-Nicalon fibre-reinforced composites. In addition, fatigue test results from other composite systems, such as LASII-Nicalon and aluminosilicate-Nicalon, are examined in the light of this model.     

Influence of residual stresses and interfacial shear strength on matrix properties in fibre-reinforced ceramic matrix composites

Zircon matrix composites, uniaxially reinforced with a variety of SiC fibres were fabricated in order to create composites with different interfacial properties. Interfacial properties were varied by changing the nature of fibre coatings. The effect of changes in interfacial shear strength on important matrix properties, such as hardness and fracture toughness, was studied on a micro-scale using the microindentation technique. In addition, the relative orientation of the indented cracks with respect to the fibres was varied to investigate the existence of anisotropic behaviour of the matrix. The results indicated that the crack growth in the matrix was influenced by the presence of residual radial and axial stresses, such that relatively higher crack lengths were seen in certain directions in the matrix with respect to other directions. This asymmetric nature of the crack formation upon indentation was the reason for the observed anisotropic fracture toughness of the matrix. The residual stresses also led to anisotropic hardness and a critical load for crack initiation in the matrix.     

The physics and mechanics of fibre-reinforced brittle matrix composites

This review compiles knowledge about the mechanical and structural performance of brittle matrix composites. The overall philosophy recognizes the need for models that allow efficient interpolation between experimental results, as the constituents and the fibre architecture are varied. This approach is necessary because empirical methods are prohibitively expensive. Moreover, the field is not yet mature, though evolving rapidly. Consequently, an attempt is made to provide a framework into which models could be inserted, and then validated by means of an efficient experimental matrix. The most comprehensive available models and the status of experimental assessments are reviewed. The phenomena given emphasis include: the stress/strain behaviour in tension and shear, the ultimate tensile strength and notch sensitivity, fatigue, stress corrosion and creep.Nomenclature a _i Parameters found in the paper by Hutchinson and Jensen [33], Table IV - a _o Length of unbridged matrix crack - a _m Fracture mirror radius - a _N Notch size - a _t Transition flaw size - b Plate dimension - b _i Parameters found in the paper by Hutchinson and Jensen [33], Table IV - c _i Parameters found in the paper by Hutchinson and Jensen [33], Table IV - d Matrix crack spacing - d _s Saturation crack spacing - f Fibre volume fraction - f _l Fibre volume fraction in the loading direction - g Function related to cracking of 90 ° plies - h Fibre pull-out length - l Sliding length - l _i Debond length - l _s Shear band length - m Shape parameter for fibre strength distribution - m _m Shape parameter for matrix flaw-size distribution - n Creep exponent - n _m Creep exponent for matrix - n _f Creep exponent for fibre - q Residual stress in matrix in axial orientation - s _ij Deviatoric stress - t Time - t _p Ply thickness - t _b Beam thickness - u Crack opening displacement (COD) - u _a COD due to applied stress - u _b COD due to bridging - v Sliding displacement - w Beam width - B Creep rheology parameter _o/ _o ⁿ - C _v Specific heat at constant strain - E Young's modulus for composite - E _o Plane strain Young's modulus for composites - Unloading modulus - E _* Young's modulus of material with matrix cracks - E _f Young's modulus of fibre - E _m Young's modulus of matrix - E _L Ply modulus in longitudinal orientation - E _T Ply modulus in transverse orientation - E _t Tangent modulus - E _s Secant modulus - G Shear modulus - G Energy release rate (ERR) - G _tip Tip ERR - G _tip ^o Tip ERR at lower bound - K Stress intensity factor (SIF) - K _b SIF caused by bridging - K _m Critical SIF for matrix - K _R Crack growth resistance - K _tip SIF at crack tip - I _o Moment of inertia - L Crack spacing in 90 ° plies - L _f Fragment length - L _g Gauge length - L _o Reference length for fibres - N Number of fatigue cycles - N _s Number of cycles at which sliding stress reaches steady-state - R Fibre radius - R R-ratio for fatigue (_max/_min) - R _c Radius of curvature - S Tensile strength of fibre - S _b Dry bundle strength of fibres - S _c Characteristic fibre strength - S _g UTS subject to global load sharing - S _o Scale factor for fibre strength - S _p Pull-out strength - S _th Threshold stress for fatigue - S _u Ultimate tensile strength (UTS) - S _* UTS in the presence of a flaw - T Temperature - T Change in temperature - t Traction function for thermomechanical fatigue (TMF) - t _b Bridging function for TMF - Linear thermal coefficient of expansion (TCE) - _f TCE of fibre - _m TCE of matrix - Shear strain - _c Shear ductility - _c Characteristic length - Hysteresis loop width - Strain - _* Strain caused by relief of residual stress upon matrix cracking - _e Elastic strain - _o Permanent strain - _o Reference strain rate for creep - Transient creep strain - _s Sliding strain - Pull-out parameter - Friction coefficient - Fatigue exponent (of order 0.1) - Beam curvature - Poisson's ratio - Orientation of interlaminar cracks - Density - Stress - _b Bridging stress - ¯_b Peak, reference stress - _e Effective stress = [(3/2)s _ijs_ij]^1/2 - _f Stress in fibre - _i Debond stress - _m Stress in matrix - _mc Matrix cracking stress - ^o Stress on 0 ° plies - _o Creep reference stress - _rr Radial stress - ^R Residual stress - _s Saturation stress - _s ^* Peak stress for traction law - Lower bound stress for tunnel cracking - ^T Misfit stress - Interface sliding stress - _f Value of sliding stress after fatigue - _o Constant component of interface sliding stress - _s In-plane shear strength - ¯_c Critical stress for interlaminar crack growth - _ss Steady-state value of after fatigue - ^R Displacement caused by matrix removal - _p Unloading strain differential - _o Reloading strain differential - Fracture energy - _i Interface debond energy - _f Fibre fracture energy - _m Matrix fracture energy - _R Fracture resistance - _s Steady-state fracture resistance - _T Transverse fracture energy - Misfit strain - _o Misfit strain at ambient temperature     

The interfacial properties of fibrous composites

The dependence of the magnitude of the interfacial parameters for a glass-fibre-reinforced polypropylene on the thickness of the applied silane layer on the glass-fibre surface was investigated. The interfacial parameters studied included the interfacial-shear strength, the interfacial coefficient of friction, the interfacial-frictional stress and the shrinkage pressure. These parameters were evaluated from pull-out data using a recent model. The results indicate that the maximum interfacial-shear strength is obtained at a critical thickness of the silane layer on the treated fibre. Both the interfacial-frictional stress and the interfacial coefficient of friction decreased with increased thickness of the silane coat.     

烷基硫醇自组装铜镀层及其树脂基复合材料界面性能研究

为改善金属转移法制备的铜镀层与碳纤维增强氰酸酯树脂基复合材料的界面粘附性能,选用4种烷基硫醇偶联剂自组装膜(SAMs)改性铜镀层.通过表征改性前后镀层表面形貌、表面极性变化、烷基硫醇与铜镀层之间的化学键合情况,研究不同烷基硫醇对铜镀层与复合材料界面结合强度的影响.结果表明：4种烷基硫醇不同程度地提高了铜镀层与复合材料的界面结合强度,具有反应活性基团和长链结构的11巯基-十一烷酸(MUA)和11巯基-十一烷醇(MUOL)因与复合材料产生化学键连接和分子链缠结作用,形成铜镀层-SAM-复合材料的界面结构,使界面结合强度提高超过70%.     

Prediction of first matrix cracking in micro/nanohybrid brittle matrix composites

The classical Aveston–Cooper–Kelly shear-lag model for predicting the first matrix cracking strength in a brittle matrix composite is extended to the case of a hybrid brittle matrix composite containing both micro-scale and nano-scale fibers. First, closed-form solutions for the stresses in the two types of fibers and the matrix are derived. These are then used along with an energy analysis to predict the matrix cracking stress as a function of relevant material parameters. The analysis is applied to a typical Nicalon-SiC/CVI-SiC ceramic matrix composite containing additional nanofibers, for a wide range of nanofiber properties. A few volume percent of small diameter, moderate-stiffness nanofibers is predicted to provide significant strengthening and reduced crack opening while maintaining acceptable post-cracking fiber stresses. Various issues in the design of such micro/nanohybrid composites are then discussed.     

The effect of surface treatment on the interfacial properties in carbon fibre/epoxy matrix composites

Carbon fibres with different degrees of surface oxidation, as well as epoxy-sized fibres, were used to prepare epoxy composites in order to compare the effects of the fibres surface chemistry on the interfacial properties. X-ray photoelectron spectroscopy, water vapour adsorption measurements and contact angle examination were applied to characterize the carbon fibre surfaces. A correlation was found between the content of primary adsorption sites on the fibre surface and interlaminar shear strength (ILSS) of the composites. Higher values of ILSS obtained for the oxidized fibres containing composites are proposed to be due to the higher concentration of carboxylic groups created on the oxidized fibres surface and to the creation of chemical bonds at the fibre/epoxy matrix interface. Enthalpy of cure, reaction peak temperature and glass transition temperature of the composites were determined by differential scanning calorimetry.     

镁基复合材料界面调控研究进展EI北大核心

界面是影响镁基复合材料综合性能的关键因素,如何进行界面调控一直是镁基复合材料的研究热点。本文围绕镁基复合材料三种界面结构类型(共格界面、半共格界面和非共格界面),针对影响界面性能的两个关键问题(界面润湿性和界面反应),综述了界面优化方案的研究进展,提出了实现良好界面结合的界面结构设计与调控准则:良好润湿性与轻微界面反应。针对镁基复合材料的界面性能提升,可以考虑添加稀土元素,起到净化界面、改善润湿性的作用;根据工程需要选择基体和增强体,得到某方面性能优异的复合材料;开发新的增强体表面涂层,充分提高界面结合能力;通过第一性原理等计算模拟方法,深入探究界面结构与界面性能之间的关系。     

On the optimal direction of short metal fibres in brittle matrix composites

In the composite materials considered in this paper the fibres bridge the cracks in the matrix and control their propagation. The ability to sustain large cracks before complete failure is of primary importance in several applications. This quality formulated as the fracture energy is chosen as an objective function for optimization. Five different components of the fracture energy are expressed by simplified formulae, derived from the assumed behaviour of fibres in the cracked matrix. The angle of the orientation of the parallel fibres system is the only design variable. The optimization problem is solved by derivation with respect to that angle. An element subjected to axial tension is considered for the maximum of fracture energy and the optimum angle of fibres is determined. Several examples for steel fibres reinforced cements are calculated and discussed. The proposed approach may be further developed using adequate formulae and assumptions for various kinds of fibre reinforced materials.     

Stochastic models of fragmentation of brittle fibers or matrix in composites

The present paper compares the fragmentation strength distributions predicted using various models including:

–

The well-known Monte Carlo simulation method based on chain-of-segments model and fiber strength distribution. This model has been widely used to simulate fragmentation of fibers in polymer or metal matrix.     

A dual-phase-lag diffusion model for interfacial layer growth in metal matrix composites

The time dependence of a reinforcement-matrix interfacial layer growth (RMILG) in most metal matrix composites (MMC's) is not t^1/2 as described by Fick's law. Moreover, the RMILG vs. t^1/2 could be linear for relatively short and long times, and an anomaloustransition behavior exhibits in between. In this paper, a dual-phase-lag diffusion (DPLD) model is proposed to characterize the RMILG kinetics. Unlike Fick's law, it accounts for the two lagging times required for the processes of interdiffusion and chemical reaction. This unique feature empowers the DPLD model to capture the multiple stages response over the entire RMILG history. Model validation is verified with the experimental results of seven different MMC systems.     

Thermochemical modelling of interfacial reactions in molybdenum disilicide matrix composites

The thermal and environmental stabilities of molybdenum disilicide have been evaluated using thermochemical modelling. The chemical reactivity of molybdenum disilicide with oxygen indicates that various molybdenum compounds and silica are formed, depending on oxygen pressures. The structure and properties of the silica films play an important role in the oxidation reaction and the reactions of water vapour (moisture) with molybdenum disilicide at high temperatures. The thermodynamic stabilities of various potential reinforcements, e.g. carbon, silicon carbide, silicon nitride, alumina, and some refractory compounds (borides, carbides, and oxides of titanium, zirconium and hafnium) in molybdenum disilicide matrix have been evaluated. Based on the results of thermochemical computations, SiC, Si₃N₄, TiC, ZrC, HfC, TiB, TiB₂, ZrB₂, HfB₂, ZrO₂ and HfO₂ were found to be stable, but carbon and TiO₂ were found to be unstable in MoSi₂.The Al₂O₃/MoSi₂ system was found to be stable below 1800 K. At temperatures above 1800 K, significant mass losses could occur due to the high vapour pressures of gaseous species (Al₂O, SiO). These thermodynamic predictions are in agreement with available experimental data.     

Microindentation tests as a tool for the estimate of mechanical properties and the modeling of the interfacial behavior of ceramic matrix composites

This article has the scope of presenting the usefulness of microindentation tests for the estimate of in situ mechanical properties of the matrix and fibers as well as the assessment of interfacial behavior in ceramic matrix composites. For this concern, we have choosen the example of Hi-Nicalon–BN–silicon nitride ceramic matrix composite. Using the multiple unloading procedure, which has been described in detail elsewhere [Drissi-Habti M, Nakano K. Comput Sci Technol 1997;57 (in press)], the longitudinal Young's modulus of the fiber and the matrix have been estimated. The values are in good agreement with previous results [Drissi-Habti M, Nakano K. Comput Sci Technol 1997;57 (in press)]. Using the same technique, the interfacial properties have been checked. When varying the maximum applied load and considering a constant interfacial shear stress, the presence of Poisson's effect has been identified and a model, based on a Coulomb law of friction, has been proposed to derive frictional stress values adjusted for the contribution of the expansion of the fibers. From the results presented herein, the main point which should be noted is the wide range of applications permitted by the microindentation technique for the micromechanical characterization of ceramics and ceramic matrix composites.     

The effect of fabrication processes on the mechanical and interfacial properties of SiCf/Cu–matrix composites

Three groups of SiC_f/Ti/Cu composites were prepared under conditions of 650 °C + 105 min (sample 1#), 750 °C + 85 min (sample 2#) and 840 °C + 50 min (sample 3#), respectively, by foil-fiber-foil method (FFF), and their room temperature tensile strengths were established. The aim is to model the reactive bonding states between Ti and SiC fiber and between Ti and Cu when Ti is used as interfacial adhesion promoters in SiC_f/Cu–matrix composites. The fracture surfaces, SiC_f/Ti interfaces and Ti/Cu interfaces were investigated by scanning electron microscopy (SEM), optical microscopy and energy dispersive spectroscopy (EDS). The tensile tests show that the tensile strengths of samples 1# and 2# are not obviously enhanced due to the weak bonding strength between SiC fiber and Ti, while those of sample 3# are achieved above 90% of ROM (the rule of mixtures) strength because of excellent bonding between SiC fiber and Ti. However, there are distinct Ti/Cu interfacial reaction zones after the three processes, which are approximately 5.4, 9.0 and 13.3 μm thick, respectively. The Ti/Cu interfacial reaction products are mainly distributed in four layers. In samples 1# and 2#, the products are predicted to be Cu₄Ti, Cu₃Ti₂, CuTi and CuTi₂ according to their chemical compositions determined by EDS, while in sample 3#, the products are Cu₄Ti, Cu₄Ti₃, CuTi and CuTi₂. Additionally, the relationships between the thickness of Ti interlayer and its reaction with C and Cu are also discussed, and an optimal thickness of Ti is introduced.     

Influence of interfacial microcracks on the elastic properties of composites

A model is established to quantify the influence of interfacial microcracks on the elastic properties of a particulate composite using a combination of theoretical and finite element analysis. A unique way to construct physical models which could accommodate both crack size and crack density is proposed. Based on energy principles, the influence of a dilute concentration of interfacial microcracks is first studied. The case of a finite concentration of microcracks is solved subsequently by combining the dilute concentration solutions and the differential scheme. Both cases agreed well with existing composite theories for the limiting condition of complete decohesion. The final model predicts the effective elastic properties as functions of both crack size and microcrack density.     

The tensile failure of brittle matrix composites reinforced with unidirectional continuous fibres

The tensile failure strength of ceramic composites can be measured by tests in bending or in tension, but care must be exercised over the experimental conditions. The strength values obtained are dependent on the test method and specimen size. It is shown that differences between strengths measured in bend and tensile tests can be understood in terms of the statistical distribution of the strengths of individual fibres.     

Effect of composite layering on the fracture toughness of brittle matrix/particulate composites

Glass matrix/Ni particulate composites, ranging from 0 to 25% particulate phase, were prepared both as single-volume-fraction composites and as multi-volume-fraction layered composites. Fracture toughness (K_c) measurements were made on all composites using the applied moment double cantilever beam technique. The measured toughness values for the layered composites were found to be equivalent to those of the single-volume-fraction composites. The fracture toughness measured for the layered composites was found to be dependent on the volume of composite phase tested and ultimately on the number of crack-particle interactions which occurred. R-curve like behaviour was observed in the layered composites.