ETAL-MATRIX COMPOSITES

In addition, it has been shown (Boccaccini etal., 2001 Chlup etal., 2001) that the chevron-notched specimen flexural technique (the CN-technique) can be a reliable method of assessing fracture properties (fracture toughness, work of fracture) in thermally shocked brittle matrix composites reinforced by brittle fibres. 

As with other ceramic composites, the combination of a- and/or P-sialon with reinforcement agents results in sialon composites. This simple and obvious statement encompasses many factors which must be taken into account for successfully fabricating composites with a designed microstructure and improved properties (Prewo, 1989). For sialon matrix composites, the most important factors are physical compatibility including Young s modulus, elastic strain (Kerans and Parthasarathy, 1991) and thermal expansion coefficient (Sambell etal., 1972a, b), and chemical compatibility between sialon matrix... 

TiAl/SiC composite by interface reactions has recently been studied (Ochiai et al., 1994). The kinetics of the phase reactions is determined by interdiffusion, which has been studied in the case of TiAl-Mo (Zhang etal., 1992a). It has been shown that both the creep resistance and the toughness of a TiAl matrix composite can be improved by using coated fibers with weak fiber/matrix interfaces (Weber et al., 1993). 

Besides these developments, which are directed at specific applications, NijAl alloys are used for the development of inter-metallic matrix composites which contain reinforcing particles or fibers of borides, carbides, oxides or carbon (Fuchs, 1989 Lee et al., 1990 Tortorelli et al., 1990 Al-man and Stoloff, 1991 Kumar, 1991 McKamey and Carmichael, 1991 Muk-herjee and Khanra, 1991 Brennan etal., 1992). Apart from the mechanical properties and the necessary corrosion resistance, the chemical compatibility of the used phases is of primary importance with respect to the long-term stability. It was found that SiC, B4C, and TiBj react extensively with Nij Al alloys, whereas very little reaction has been observed with AljOj or Tie in NijAl (Fuchs, 1989 Lee etal., 1990 Brennan etal., 1992). It should be noted that NijAl alloys are used not only as the matrix material, but also as a reinforcing phase in, e.g. an Al alloy to form a metal-matrix composite (Metelnick and Varin, 1991). 

Figure 11.20 Effect of matrix composition and ageing on the strength and toughness relative to the control of sisal-cement composite normal-untreated matrix, carbonation curing of composite and silica fume slurry treatment of fibres (adopted from the data of Filho etal. [67]). Control-28 days of normal curing water-322 days in water wet/dry-46 cycles natural-outdoor exposure.

The production of moisture resistant particleboard by treatment with a maleic anhydride -glycerol mixture and using phenol formaldehyde as the matrix material has been investigated (Fujimoto etal., 1987). Boards prepared from modified wood showed considerable improvements in modulus of elasticity and internal bond strength when compared to control boards. Composites made from aspen fibres modified with SA, MA or acetic anhydride using phenol-formaldehyde (PF) or polypropylene as binder have also been studied (Clemons etal., 1992 Rowell etal., 1993b). The reaction of wood with MA was found to proceed at a slower rate than with SA. The volume increase due to modification... 

The use of maleic anhydride as a compatiblizer between wood particles and bisphenol A-based polyesters resins has been investigated (Han etal., 1991). In this study, the MA was added directly to the composition of woody matrix filler and resin rather than by pre-modification of the wood. It was found that composite properties were improved by addition of MA, probably due to esterification of the wood occurring during the kneading process. The modification of sawdust using maleic anhydride has been performed in order to provide a compatible filler for polyester resins (Marcovich etal., 1996). Modification was performed at room temperature using a solution of maleic anhydride in acetone, in some cases catalysed with sulphuric acid. It was claimed that bonding occurred under these mild conditions from IR spectroscopic evidence only. 

Mahlberg etal. (2001) studied the effect of acetylation of wood fibres (20 % WPG) upon the mechanical properties of wood fibre PP fibre composites, with and without the addition of a novalak PF. Acetylation resulted in significant improvements in MOR, MOE and IBS in a composite containing 20 % by weight of PP fibre. With a combination of PF and PP (both 10 % by weight) and acetylated wood fibre, MOR was unchanged, MOE decreased and only IBS showed a significant improvement. Acetylation also contributed to a better dispersion of the wood fibres in the PP matrix. 

Huang and Gao20 studied HP, in N2 and at 25 MPa, of MWCNT-BaTi03 composites (CNTs obtained by CCVD). They reported 1200°C as the optimal temperature (for 0.1 vol% CNT) and, for this HP temperature, a decrease of the densification from 99% to 86.5% when the CNT content increases from 0.1 to 3.0 wt%, which is correlated with a much smaller matrix grain size. Ma etal.9 studied the densification by HP at 2000°C (25 MPa, Ar atmosphere) of a mixture of 10 wt% MWCNTs, obtained by CCVD, with nanometric SiC (80 nm). The densification obtained was only 64.7%. Thus, the addition of 1 wt% B4C and the increase of the temperature up to 2200°C were necessary to reach a densification of 98.1%. These authors reported some images showing that at least some of the MWCNTs had not been destroyed by this very high... 

An assessment of the thermal shock damage of woven Nicalon /SiC (CVI) composite specimens was performed by Webb etal. (1996), the results being confirmed subsequently by Kagawa (1997). It was noted that the many pores and irregularities in the matrix inherent to this particular composite... 

---