Multilayered Interphases Research Articles

Effect of interphase characteristics on long-term durability of oxide-based fibre-reinforced composites

Abstract Mullite based fibre-reinforced composites having double layer fibre-coatings were produced and characterised. The multi-layer interphases were produced by a CVD-process (carbon(fugitive)/ZrO2 or Al2O3) on aluminosilicate Nextel™ 720 fibres. Composites were fabricated by infiltration of coated fibres with a pre-mullite slurry and hot-pressing in argon at 1300°C. Short term heat-treatment of composites in air yielded a gap between the fibre and the oxide layer by oxidation of the carbon layer (so-called fugitive layer). The composites were statically and cyclically heat-treated at 1300°C for 1000 h in order to identify the mechanical and microstructural changes. Mechanical characterisation of the heat-treated composites was carried out by three-point bending. The effectiveness of the fugitive layer is determined by the oxide layer and its high-temperature stability in interaction with the matrix but it also depends on the loading condition. Under cyclic heat-treatment conditions the composites are found to be more stable and damage tolerant than under constant high-temperature exposure.

TEM structure of (PyC/SiC) n multilayered interphases in SiC/SiC composites

Two generations of multilayered interphases, composed of carbon and silicon carbide, have been developed to act as a mechanical fuse in SiC/SiC composites with improved oxidation resistance. Pyrocarbon is an ideal interfacial material, from the mechanical point of view, whereas SiC has a good oxidation resistance. In the multilayered interphase, the carbon mechanical fuse is split into thin sublayers, each being protected against oxidation by the neighbouring SiC-based glass former layers. A first generation of multilayers as synthesised by means of isobaric-CVI with sublayers with micrometric thickness. Then, in order to push forward the concept, pressure pulsed-CVI was involved to deposit nanometric scale sublayers. In this work, transmission electron microscopy was developed to characterise the two generations of materials. The microstructure of the layers and the influence of the fibrous preforms on the structure of the layers were studied. Examinations were then performed on the loaded samples and damaging mode characterised at nanometric scale.

Thermomechanical Behaviour of 2D-SiC/Sic Composites with Nanoscale-Multilayered (PyC/SiC)n Interphases

2D woven SiC/SiC composites, with nanoscale-multilayered (PyC/SiC)n interphases, have been made via chemical vapour infiltration of a preform of either Nicalon or Hi-Nicalon fibres. The nanoscale-multilayered (PyC/SiC)n interphases consist of n successive PyC-SiC sequences. In the present paper n = 10 and the thickness of the PyC and SiC sublayers was about 20 nm and 50 nm respectively. The influence of Hi-Nicalon fibres and multilayered interphases on the mechanical behaviour at room temperature and the lifetime in cyclic fatigue at 600°C and 1200°C was investigated. The Hi-Nicalon fibre and the multilayered interphases are shown to lead to improved 2D-SiC/SiC composites.

Hi‐Nicalon/SiC Minicomposites with (Pyrocarbon/SiC)n Nanoscale Multilayered Interphases

SiC/SiC minicomposites that comprise different pyrocarbon/silicon carbide ((PyC/SiC)n) multilayered interphases and a tow of SiC fibers (Hi‐Nicalon) have been prepared via pressure‐pulsed chemical vapor infiltration. Pyrocarbon and SiC were deposited from propane and a CH3SiCl3/H2 mixture, respectively. The microstructure of the interphases has been investigated using transmission electron microscopy. The mechanical tensile behavior of the minicomposites at room temperature exhibits the classical features of tough composites, regardless of the characteristics of the (PyC/SiC) sequences. The interfacial shear stress has been determined from the width of hysteresis loops upon unloading/reloading and from the crack‐spacing distance at saturation. All the experimental data indicate that the strength of the fiber/interphase interfaces is rather weak (∼50 MPa).

A New Method for an Inhomogeneity with Stepwise Graded Interphase under Thermomechanical Loadings

The solution for a circular inhomogeneity embedded in an infinite elastic matrix with a multilayered interphase plays a fundamental role in many practical and theoretical problems. Therefore, improved analysis methods for this problem are of great interest. In this paper, a new procedure is presented to obtain the exact stress fields within the inhomogeneity and the matrix under thermomechanical loadings, without the need of solving the full multiphase composite problem. With this short-cut method, the problem is reduced to a single linear algebraic equation and two coupled linear algebraic equations which determine the only three real coefficients of the stress field within the inhomogeneity. In particular, the average stresses within the inhomogeneity can be calculated directly from the three real coefficients. Further, the other three unknown real coefficients associated with the stress field in the matrix can be determined subsequently. Hence, the influence of the stepwise graded interphase on the stress fields is manifested by its effect on the six real coefficients. All these results hold for stepwise graded interphase composed of any number of interphase layers. Several examples serve to illustrate the method and its advantages over other existing approaches. The explicit solutions are used to study the design of harmonic elastic inclusions, and the effect of a compliant interphase layer on thermal-mismatch induced residual stresses.

Properties of Multilayered Interphases in SiC/SiC Chemical‐Vapor‐Infiltrated Composites with “Weak” and “Strong” Interfaces

The interfacial properties of SiC/SiC composites with interphases that consist of (C‐SiC) sequences deposited on the fibers have been determined by single‐fiber push‐out tests. The matrix has been reinforced with either as‐received or treated Nicalon fibers. The measured interfacial properties are correlated with the fiber‐coatingbond strength and the number of interlayers. For the composites reinforced with as‐received (weakly bonded) fibers, interfacial characteristics are extracted from the nonlinear portion of the stress‐displacement curve by fitting Hsueh's push‐out model. The interfacial characteristics are controlled by the carbon layer adjacent to the fiber. The resistance to interface crack growth and fiber sliding increases as the number of (C‐SiC) sequences increases. For the composites reinforced with treated (strongly bonded) fibers, the push‐out curves exhibit an uncommon upward curvature, which reflects different modes of interphase cracking and a contribution of fiber roughness.

(PyC/SiC)<sub>n</sub> and (BN/SiC)<sub>n</sub> Nanoscale-Multilayered Interphases by Pressure Pulsed-CVI

Pyrocarbon/silicon carbide (PyC/SiC){sub n} and boron nitride/silicon carbide (BN/SiC){sub n} nanoscale multilayers have been fabricated by pressure pulsed-CVI (P-CVI), pyrocarbon, boron nitride and silicon carbide were deposited from C{sub 3}H{sub 8}, BCl{sub 3}-NH{sub 3} and MTS-H{sub 2} gas precursors, respectively. Such nanoscale multilayers were used as interphases in SiC/SiC minicomposites, manufactured by P-CVI and reinforced by Hi-Nicalon fibres. The stress-strain behaviour under tension at room temperature, and the lifetime in static fatigue at high temperatures in air were investigated. The minicomposites exhibited improved mechanical properties and oxidation resistance in comparison to their counterparts having a single layer of pyrocarbon or boron nitride as interphase. The multilayered interphases allows the control of fibre/matrix interactions and the oxidation resistance. (orig.) 11 refs.

Processing of Ceramic Matrix Composites by Pulsed-CVI and Related Techniques

P-CVD/CVI is used to densify model pores and to fabricate Nicalon/SiC microcomposites with pyrocarbon or (PyC-SiC)/sub n/ multilayered interphases. The gaseous precursors are hydrocarbons (CH/sub 4/ or C/sub 3/H/sub 8/) for pyrocarbon and CH/sub 3/SiCl/sub 3/-H/sub 2/ for SiC. The best densification conditions for 60; 120 and 320 mum model pores, by pyrocarbon, are T=900-1000degC; P/sub 2/=1-3 kPa and t/sub R/=a few s. Even under such conditions, a thickness gradient is always present along the pore axis. Filling ratios as high as 90% are observed for the largest pores. Smooth or rough laminar pyrocarbon is deposited depending on the value of t/sub R/ and hence the maturation of the gas phase. Multilayered (PyC-SiC)/sub n/ interphases (with n=10-30; e(PyC)=3-50 nm and e (SiC)=10 or 30 nm) act as mechanical fuses and are more resistant to oxidation than pyrocarbon interphase. P-CVD/CVI can be used to elaborate highly engineered new ceramics

Multilayered Oxide Interphase Concept for Ceramic‐Matrix Composites

Hi‐Nicalon/SiC minicomposite specimens containing three oxide interphase layers (amorphous SiO2, monoclinic ZrO2, and amorphous SiO2) were prepared by chemical vapor deposition. The minicomposites exhibited graceful composite failure behavior with reasonable load‐carrying capability in room‐temperature tensile tests. Much of the composite behavior and load‐carrying capability was retained even after matrix precracking and subsequent oxidation in air at 960°C for 10 h. In both the as‐prepared and oxidized specimens, crack deflection and fiber pull‐out occurred preferentially within the multilayered interphase region. The potential merits and uncertainties associated with this multilayered oxide interphase approach were discussed in the context of designing environmentally durable interfaces for ceramic‐matrix composites.

Tensile static fatigue of 2D SiC/SiC composites with multilayered (PyC–SiC)n interphases at high temperatures in oxidizing atmosphere

The pyrocarbon (PyC) interphase, which is commonly used to monitor the fibre/matrix interactions in SiC/SiC and C/SiC composites, generally oxidizes in air at high temperatures. This phenomenon alters the performances and the load-carrying capability. Recently, SiC/SiC composites with multilayered interphases have been developed. The interphase consists of alternate sublayers of PyC and SiC deposited on the fibres by Chemical Vapour Infiltration. The composites exhibit mechanical properties (high strength, high toughness) and features (multiple crack deflection) that are interesting for high temperature applications. The mechanical behaviour and lifetime under tensile static loading (60–140 MPa) at high temperatures (700°C–1200°C) in air are reported for a 2D SiC/SiC composite with a multilayered interphase involving 4 PyC/SiC sequences ((PyC–SiC)4). The lifetime of the composite with the multilayered interphase is significantly improved with respect to that of the composite with a homogeneous PyC interphase.

Interfaces and interphases of (bio)materials: Definitions, structures, and dynamics

What exactly is an interface, and when is an interphase the more correct concept? The nature of interfaces and the phenomena occurring there are described before the role of interphases is illustrated with a series of examples taken from purely artificial systems (e.g., polymer and polymer‐metal “interfaces”) and natural and mixed artificial–natural systems (e.g., the multilayer interphase of a biomedical system such as a dental implant). The approach presented demonstrates that “interphase” or “interface” depends to some extent on the point of view.

Interfacial Properties of Multilayered Interphases in SiC/SiC CVI-Composites with Weak and Strengthened Interfaces

Interfacial Properties of Multilayered Interphases in SiC/SiC CVI-Composites with Weak and Strengthened Interfaces

Fracture Toughness of 2‐D Woven SiC/SiC CVI‐Composites with Multilayered Interphases

Relations between fracture toughness and fiber/matrix interphases were examined on various SiC/SiC composites made by chemical vapor infiltration (CVI) and reinforced with woven fiber bundles. Strong and weak fiber/matrix bondings were obtained using multilayered interphases consisting of various combinations of carbon and SiC layers of different thickness and using fibers which had been previously treated. Fracture toughness was estimated using the J‐ integral and using strain energy release rate computed with a model taking into account the presence of a process zone of matrix microcracks. Both approaches evidenced similar trends. It appeared that higher toughness was obtained with those composites possessing strong interphases and subject to dense matrix microcracking.

A fracture mechanics based numerical analysis for predicting optimum interface properties in a metal matrix composite

The interface in continuous fiber reinforced composites governs the mode and extent of load transfer between the fiber and the matrix. It also affects the damage initiation and growth mechanisms. An additional feature of the interface is that it is strongly influenced by thermal residual stresses due to the processing history of the composite. The effect of residual stresses on the stress state of the interface near a crack tip might prove to be advantageous or detrimental to the global mechanical properties of the composite. These issues are of interest for a better understanding of the mechanics of fracture of composites, which in turn will lead to the development of composites with superior properties. In this study, 2-D plane stress and axisymmetric residual stress analyses are performed for a 37% volume fraction SiC fiber (SCS-6) reinforced titanium (Ti-15V-3Cr-3Al-3Sn) composite plate with single and multilayered interphases. “Design plots” for optimum utilization of fiber and matrix strength properties are obtained. Initial debonding and subsequent debond growth at the fiber/interphase interface are predicted using quasistatic numerical analysis. The results suggest that the composite retains sufficient load bearing capability upon initial debonding to allow a graceful failure. The residual stresses due to the coefficient of thermal expansion (CTE) mismatches between the fiber and the matrix with single or multilayered interphases are also compared with the help of axisymmetric analysis of the composite. The use of the multilayered interphase effects a smooth transition of the hoop stresses between the fiber and the matrix across the interphase, thus reducing the propensity of interfacial cracking due to the residual stresses caused by the CTE mismatch.

Strong Interface in CMCs, a Condition for Efficient Multilayered Interphases

ABSTRACTA fiber treatment was used to change the bonding strength of the Nicalon NLM 202 SiC fiber from weak to strong, in a series of 2D-SiC/SiC composites with multilayered interphases. The materials with the pre-treated fibers were compared to the same materials but reinforced with as received fibers. The stress-strain behavior and the fracture toughness were examined as a function of crack patterns identified by TEM. All the materials could be grouped into two distinct families: (i) materials reinforced with untreated fibers have a weak fiber bonding and are characterized by a low strength and a low toughness and (2) materials with the pre-treated fibers have a strong fiber bonding and are characterized by a high strength and a high toughness. This latter behavior is identified by TEM. It corresponds to a new interfacial behavior with a cohesive mode of interfacial cracking, involving branching and deflection by the successive interfaces. In the former family, the adhesive interfacial failure mode corresponds to the classical debond/sliding mechanism.

Adsorption of sodium dodecylsulphate on mercury as an example of micellization within a multilayer interphase

The adsorption of sodium dodecylsulphate from aqueous electrolytic solutions on a polarized mercury electrode was studied by means of differential capacitance measurements over a wide range of concentration and potential, and in the presence of different supporting electrolytes with different ionic strengths. An interpretation of the differential capacitance vs. applied potential curves is given, based on theoretical treatments developed previously. It is shown that at concentrations below the cmc, two-dimensional aggregates are formed on the electrode surface within a polarization region which is bounded by two capacitance peaks at extreme positive and negative polarizations. This film is not particularly stable and is transformed into a compact layer at polarizations close to the potential of maximum adsorption, resulting in a capacitance pit. With increasing bulk concentration the aggregation process extends across the interphase and at least two layers of aggregates—micelles are formed at concentrations around and above the cmc. This three-dimensional aggregation is characterized by the appearance of deformed and/or split capacitance peaks which determine the polarization region wherê this phenomenon occurs.