In selection of the composite matrix and fibre for use in this research a number of factors needed to be considered, many of which have been mentioned in the above discussion and Chapter 1. As indicated in Chapter 1., CMCs can offer
Time. Figure 2.7. Processing compromises in the fabrication of GCMCs.
considerable benefit to the gas turbine engine, but this requires the material to have high specific strength, good durability, creep resistance and complex shape capability.
These requirements and in particular those of creep resistance at high temperatures preclude the use of glass matrix composites. Complex shape capability indicates that a large diameter fibre such as Textron SCS-6 fibre is more difficult to utilise because of its maximum bend radius and so a small diameter fibre is preferable. From Table 2.1., it can be seen that a large number of fibres can be found with a small diameter, but other selection criteria for fibres need to be invoked. These criteria are
. Low density. . High strength. . High modulus. . Oxidation resistance.
. Retention of properties to high temperatures ( ~ 1000-1200°C ). . Creep resistance.
. Thermal expansion coefficient close to that of the matrix material.
These further criteria allow many of the fibres to be eliminated. Carbon fibres cannot be used since their use temperature is limited by oxidation resistance to below ~ 500°C. As stated in Section 2.2.1., oxide fibres require precoating before incorporation in the matrix, in order to achieve a low shear sliding resistance interface and problems associated with high temperature creep deformation cause problems with their use.
The nitride / carbide fibres have properties which suit CMC application. The Fiberamic fibre retains its amorphous state to 1400°C before a 25% drop in strength occurs. However, these fibres are not readily available for use and hence, because of the scale of process development required in this study, were not used. Furthermore, studies have shown that pre-synthesised interfaces may be required with this type of fibre constitution, as fabrication with an MAS matrix / HPZ fibre resulted in a S i0 2 / C phase separated interface which, although not characterised mechanically, was thought not to have the required micromechanical response (77).
Hence the fibres which fulfil the selection criteria, and to which access is available in reasonable quantities, are the SiC fibres of Nicalon and Tyranno. Both fibres are very similar in composition and specification as seen in Table 2.1., with the Tyranno fibre having a higher oxygen content and a lower residual carbon content. The Nicalon SiC fibre was selected for this research since development has continued since its first manufacture in 1976 by Yajima et al (49) (81), and its properties and degradation are characterised. For both fibres studies have indicated that the free carbon is partly dispersed as graphite microcrystals and partly dissolved in the amorphous, mixed co-ordination tetrahedra ( SiOxC4_x : 0 < x < 4 ), with the oxygen accommodated in the network (85).
The degradation in properties of Nicalon with temperature was shown by Ko (75) and evaluation in more detail was undertaken by Bender et al (117). For the
Nicalon fibre, when heat treated in argon or nitrogen for 15 minutes, strength is retained to 1200°C before significant degradation. Under identical heat treatment conditions but in air, strength degradation is seen to occur at~ 850°C (75). From Bender et al (117), a number of degradation mechanisms have been proposed including grain growth ( originally ~ 2nm (117) ), a change in Young’s modulus, microporosity formation, crystallisation, growth of pre-existing flaws and the formation of surface flaws. Studies have suggested that the microstructure and chemistry are changed by high temperature exposure due to the evolution of SiO and CO from the fibre and hence thermochemical stability is critical. The evolution of CO from the fibre is critical to strength retention, and a study has shown that using CO heat treatments could increase the fabrication temperatures used currently (117).
For this research two types of Nicalon fibre were used. The type originally utilised is the NL-201 fibre, this being a standard ceramic grade Nicalon fibre with epoxy sizing designed for general use. The second Nicalon fibre utilised is the NL- 607 fibre, which is a carbon coated ceramic grade fibre. This fibre was designed for use in CMCs and, apart from a IO-20nm pyrolytic carbon coating, shows the same properties as the standard ceramic grade fibre, and is supplied with PVA sizing.
For the selection of the matrix, as for the fibre, a number of criteria can be used and since a particular fibre has been identified the criteria relevant for this fibre need to be incorporated as well
. Composite use temperature. . High strength.
. Good chemical durability.
. Matrix thermal expansion needs to be close to that of the fibre. . Fibre / matrix interface formation.
. Creep resistance. . Oxidation resistance.
Since Nicalon only exhibits thermal stability in nitrogen or argon to ~ 1200°C, a low temperature fabrication route is required for the composite. The way this
was achieved in this work was to utilise a glass ceramic matrix.
By appropriate selection of the major phase in a glass ceramic, the composite matrix can fulfill all of the above criteria. Since a refractory matrix was required, a high temperature melting phase was selected, with a view to mechanical properties, thermal expansion, creep and oxidation resistance. The magnesium aluminosilicate ternary system was selected, with the major crystalline phase identified as cordierite. Cordierite in its high temperature polymorphic form has a thermal expansion coefficient of 2.6 x lO '^ C '1 ( very close to that of the fibre ), a flexural strength of ~ 200MPa and, from Table 2.4., a use temperature of up to 1200°C, which is the point of thermal instability in the fibre for nitrogen or argon atmospheres. Within this study compositional issues, which are discussed more fully in Chapter 4., have allowed tailoring of the matrix thermal expansion by moving the composition along a tie line between cordierite and the higher thermal expansion coefficient phase enstatite ( for enstatite cx = 7.8 x l O ^ C ).
Cordierite is a phase much used in the refractories industry and retains good high temperature strength with good thermal shock properties (51) (52). Since it is a silicate glass ceramic the fortuitous reaction described in Section 2.3. of this chapter enables the fabricated composite to have the desired interfacial micromechanical properties if the time and temperature process parameters are suitable (77).