Fibre-matrix interphase

Besides the good quality of the C/SiC composites processed by chemical vapour infiltration, one of the major advantages of this manufacturing route is that is allows the control of the fibre/matrix interphase. Therefore, C/SiC composites are shortly deposited in a first step with carbon (e.g. by the deposition of methane gas, CH4) to govern the fibre/matrix bonding forces. In summary, the manufacture of C/SiC components via the gradient-CVI process comprises the following steps ... 

It has also been shown that this is the consequence of oxidation of the fibres, in the case of carbon-fibre-reinforced composites, or of degradation of the fibre/matrix interphase, which is in fact a carbon-rich nanometric interfacial layer, in the case of SiC-fibre-reinforced composites. 

Naslain, R. (1993). Fibre-matrix interphases and interfaces in ceramic matrix composites processed by CVI. Composite Interfaces, 1, 253-286. 

Tressler RE (1999) Recent developments in fibres and interphases for high temperature ceramic matrix composites. Composites A30 429-437... 

Tillie et al. (1998) examined the effect of the fibre/matrix interface on the cure of glass-fibre-filled epoxy-resin systems. They found that the introduction of a lower-Tg interphase based on hydroxylated PDMS oligomers allowed an increase in toughness without a reduction in modulus or Tg. This was due to a modification of the stress field under load due to the elastomeric interphase. 

PHYSICOCHEMICAL CHARACTERISATION OF THE FIBRE/MATRIX INTERACTION IN POLYETHYLENE FIBRE/EPOXY MATRIX COMPOSITES. II. CHARACTERISATION OF THE INTERPHASE WITH FTIR MICROSPECTROSCOPY Van Mele B Verdonck E Brussels,Free University... 

The interphase in PE fibre/epoxy resin matrix composites was studied by FTIR microspectroscopy using a set-up for investigation of the matrix as close to the fibre as a few microns or less. It was shown that moisture present on the fibre surface could influence the polymerisation reaction of the epoxy/anhydride matrix in an irreversible manner. This effect was enhanced for composites from the more hydrophilic PVAl fibre. The fibre/matrix interaction in these thermoplastic fibre composites was also studied by DSC through characterisation of the fibre melting. A decreased DSC interaction parameter was found if the composition of the interphase was changed by moisture. For a composite with an epoxy/amine matrix, on the other hand, the DSC interaction parameter was unaffected by moisture from the fibre surface. 22 refs. (Pt.I, ibid, p.83-100)... 

It has also been found that moisture absorbance of the natural fibre-polymer composite can be prevented if the fibre-matrix adhesion is optimized [15, 24]. Indeed, whereas composites based on standard PP and cellulosic fibres displayed high water content at the interphase, due to the presence of microcavities, the encapsulation of the fibres with MAPP decreased the water sensitivity of the composites in terms of both the water uptake and its diffusion coefficient [25], as shown in Fig. 19.9. 

Figure 9.13 Schematic of the interphase region between fiber and matrix. Source Reprinted with permission from Herrera-Franco PJ, Drzal LT, Comparison of methods for the measurement of fibre/matrix adhesion in composites, Composites, 23, 2,1992. Copyright 1992, Elsevier.

The influence of the presence of two different kinds of interphase on the level of stress transfer at the fibre-matrix interface is briefly examined in the next section. 

A tentative model has been proposed to relate the interfacial shear strength at the fibre-matrix interface, measured by a fragmentation test on single fibre composites, to the level of adhesion between both materials. This last quantity has been estimated from the surface properties of both the fibre and the matrix and was defined as the sum of dispersive and acid-base interactions. This new model clearly indicates that the micromechanical properties of a composites are mainly determined by the level of physical interactions established at the fibre-matrix interface and, in particular, by electron acceptor-donor interactions. Moreover, to a first approximation, our model is able to explain the stress transfer phenomenon through interfacial layers, such as crystalline interphases in semi-crystalline matrices and interphases of reduced mobility in elastomeric matrices. An estimation of the elastic moduli of these interphases can also be proposed. Furthermore, recent work [21] has shown that the level of interfacial adhesion plays a major role on the final performances (tensile, transverse and compressive strengths and strains) of unidirectional carbon fibre-PEEK composites. 

After surface oxidation, the fibres are coated with a polymeric size, which is usually an epoxy resin. Whether this resin coating can be dissolved efficiently into the matrix is unclear. This aspect of the fibre-matrix interaction in carbon-fibre composites is under researched. It is generally now accepted that an interphase region forms. This can be distinct or graded depending on the solubility of the size and subsequent phase separation on curing. 

The bonding between the fibres and the matrix is another decisive element. It depends on the quality and processes that appear in the fibre/matrix interface (interphase). In some advanced composites, but also in glass fibre-reinforced cements, the chemical interaction between these two constituents may be destructive for the composite integrity. The fibre-matrix bond is ensnred by different processes by adhesion, mechanical anchorage and by friction, depending on the chemical and mechanical properties of both phases. 

Hayes S A, Lane R and Jones F R (2001) Fibre/matrix stress transfer through a discrete interphase. Part 1 Single-fibre model composites. Composites Part A 32 379-389. 

The above CVD techniques have been widely applied and used to fabricate superconductor wires and interphase layers for fibre-reinforced metal- and ceramic- matrix composite materials. 

Therefore, moisture absorption has a larger effect on the transverse properties of a typical composite system. Despite this, the strength of a 0° composite is also affected by moisture ingress since the reloading of a broken fibre occurs through shear stress transfer Ifom the interphasal matrix. To achieve isotropy, unidirectional plies are stacked at a set of angles such as 0°, 45° and 90° to form a laminate. In this situation, moisture ingress will modify the residual stress state in the individual laminae. 

The interphase is mentioned again briefly at the beginning of Chapter 3, and the subject is treated more extensively in relation to dynamic loading in Chapter 11. Many writers still refer only to the interface and ignore the gradation of structure between fibre and matrix. 

Reinforced plastics, sometimes called polymer composites, consist of reinforcing fibres or particles embedded in a polymeric matrix. It is now recognized that a third component, called an interphase region, can exist at the interface between the fibre and resin. The properties of the interphase are probably not constant but vary to give a graded region. Thus the environmental durability of a composite material is a complex interplay between the various microstructural aspects of the material, which are ... 

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