a b Processing pore Carbonization Crack pore 200um 10um (c (d) Filled Semifilled Empty pore pore 40um 50μm Figure 7.3 Polarized-light micrographs of cross sections of unidirectional C-C composites showing different types of pores:(a)(b)undensified;and(c)-(d)densified composites.(See Color Plate IV) where pp is the density of the impregnant at room temperature,and p(0)the bulk density of the empty C-C composite.This expression can be generalized for the n-th step of impregnation as follows: Y(m)= △W(n)p(n-1) (2) 0(n-1)Pp Several factors,related not only to the experimental conditions used but also to the characteristics of the network of pores developed in the composite (Fig.7.3a)and the characteristics of the impregnant,affect the values of this parameter (Oh and Park,1994; Granda et al.,1998). The density of the carbon matrix of the final composite depends on the experimental con- ditions used in the carbonization step and may differ somewhat from the density obtained when the matrix precursor is carbonized alone under the same conditions.This is because in the composite the thermally induced stresses tend to promote additional graphitization and densification of both fiber and matrix(Rellick,1990). Another aspect that should be mentioned is the relationship between the weight gain of the composite in the impregnation and the absolute increase in weight due to the n-th cycle of densification.One might expect the ratio of these magnitudes to be the carbon yield of the matrix precursor under the processing conditions.However,this is not completely true, because when carbonization takes place after impregnation,the phenomenon of matrix bloating may occur as a consequence of the release of volatiles.As a result,matrix yields 2003 Taylor Francis
where p is the density of the impregnant at room temperature, and (0) the bulk density of the empty C–C composite. This expression can be generalized for the n-th step of impregnation as follows: (2) Several factors, related not only to the experimental conditions used but also to the characteristics of the network of pores developed in the composite (Fig. 7.3a) and the characteristics of the impregnant, affect the values of this parameter (Oh and Park, 1994; Granda et al., 1998). The density of the carbon matrix of the final composite depends on the experimental conditions used in the carbonization step and may differ somewhat from the density obtained when the matrix precursor is carbonized alone under the same conditions. This is because in the composite the thermally induced stresses tend to promote additional graphitization and densification of both fiber and matrix (Rellick, 1990). Another aspect that should be mentioned is the relationship between the weight gain of the composite in the impregnation and the absolute increase in weight due to the n-th cycle of densification. One might expect the ratio of these magnitudes to be the carbon yield of the matrix precursor under the processing conditions. However, this is not completely true, because when carbonization takes place after impregnation, the phenomenon of matrix bloating may occur as a consequence of the release of volatiles. As a result, matrix yields Yi (n) Wi (n) (n 1) (n 1) p Figure 7.3 Polarized-light micrographs of cross sections of unidirectional C–C composites showing different types of pores: (a)–(b) undensified; and (c)–(d) densified composites. (See Color Plate IV.) Crack 200 µm Carbonization pore Processing pore Semifilled pore Filled pore Empty pore 40 µm 50 µm 10 µm (a) (c) (b) (d) © 2003 Taylor & Francis
will be lower than anticipated knowing carbon yields from the impregnant,when carbonized alone.The physico-chemical properties of the precursor,the number of pores,the structure of the pore networks in the composite (size,shape,and orientation),and the experimental conditions used are the main controlling factors. Studies of the densification efficiency of C-C composites with pitches (Oh and Park, 1994;Granda et al.,1998)have led to the conclusion that impregnation efficiency increases, as the process is repeated due to a decrease in porosity,while retention efficiency follows an inverse trend due to the changes in the shape of the pores,as densification proceeds. Overall densification efficiency depends on impregnation efficiency and the effective carbon yield of the impregnating pitch.Pitch fluidity is the main factor controlling impreg- nation efficiency.Additionally,the tortuosity of the pores also plays an important role dur- ing the impregnation process.The use of pressure during carbonization reduces the release of volatiles and pitch bloating,leading to an increase in the effective carbon yield (Granda et al.,1998).It also reduces pore closure,thereby improving densification effi- ciency.It has also been shown that closed porosity may be transformed into open porosity via graphitization (Savage,1993b). 4 Matrix precursors The matrix of a C-C composite acts as a binder by maintaining the alignment and position of the fibers,and protects them from damage.It also distributes stresses,transferring the external load to single reinforcing filaments.The structural characteristics of the matrix and their interaction with the fibers contribute significantly to the properties of the composite. These characteristics depend on the chemical composition and physical properties of the precursor,as well as on the processing conditions.As the preparation of C-C composites involves heat treatment in an inert atmosphere to transform the matrix precursor into carbon form,the chemical and rheological behavior of the matrix precursor on pyrolysis is of con- siderable importance for selecting the optimum processing conditions.Generally speaking, a good matrix precursor should have a high carbon yield while at the same time retaining its fluidity and ability to wet the fibers.Moreover,low volumetric contraction is necessary in order to avoid fiber damage and matrix shrinkage cracks.It is not easy to find a product which fulfills all of the requirements at the same time.As an example,a high carbon yield is usually associated with high organic precursor viscosity,which may impede infiltration and wetting.On the other hand,a high carbon yield may be accompanied by an adequate viscosity but also by considerable matrix bloating on carbonization.Two main types of matrix precursors are currently used in the preparation of C-C composites by liquid impreg- nation:one is based on resins and polymers and the other is based on pitches. 4.1 Resin precursors In this group,phenolic and epoxy resins are the most commonly used and both are thermoset and need curing prior to carbonization(Fig.7.4a).Apart from the above mentioned matrix requirements,it is necessary that the resin cures rapidly at low temperature(without the evo- lution of volatiles).The carbon yield in commercially available resins usually ranges from 50 to 70 wt%,depending on the type of resin and the processing conditions(Murdie et al., 1992).These yields cannot be increased by the application of pressure during carbonization. New resin precursors(Fig.7.4b)have been developed with carbon yields of up to 85 wt% (800C)(Savage,1993c),but the price of these products is extremely high and in some ©2003 Taylor&Francis
will be lower than anticipated knowing carbon yields from the impregnant, when carbonized alone. The physico-chemical properties of the precursor, the number of pores, the structure of the pore networks in the composite (size, shape, and orientation), and the experimental conditions used are the main controlling factors. Studies of the densification efficiency of C–C composites with pitches (Oh and Park, 1994; Granda et al., 1998) have led to the conclusion that impregnation efficiency increases, as the process is repeated due to a decrease in porosity, while retention efficiency follows an inverse trend due to the changes in the shape of the pores, as densification proceeds. Overall densification efficiency depends on impregnation efficiency and the effective carbon yield of the impregnating pitch. Pitch fluidity is the main factor controlling impregnation efficiency. Additionally, the tortuosity of the pores also plays an important role during the impregnation process. The use of pressure during carbonization reduces the release of volatiles and pitch bloating, leading to an increase in the effective carbon yield (Granda et al., 1998). It also reduces pore closure, thereby improving densification efficiency. It has also been shown that closed porosity may be transformed into open porosity via graphitization (Savage, 1993b). 4 Matrix precursors The matrix of a C–C composite acts as a binder by maintaining the alignment and position of the fibers, and protects them from damage. It also distributes stresses, transferring the external load to single reinforcing filaments. The structural characteristics of the matrix and their interaction with the fibers contribute significantly to the properties of the composite. These characteristics depend on the chemical composition and physical properties of the precursor, as well as on the processing conditions. As the preparation of C–C composites involves heat treatment in an inert atmosphere to transform the matrix precursor into carbon form, the chemical and rheological behavior of the matrix precursor on pyrolysis is of considerable importance for selecting the optimum processing conditions. Generally speaking, a good matrix precursor should have a high carbon yield while at the same time retaining its fluidity and ability to wet the fibers. Moreover, low volumetric contraction is necessary in order to avoid fiber damage and matrix shrinkage cracks. It is not easy to find a product which fulfills all of the requirements at the same time. As an example, a high carbon yield is usually associated with high organic precursor viscosity, which may impede infiltration and wetting. On the other hand, a high carbon yield may be accompanied by an adequate viscosity but also by considerable matrix bloating on carbonization. Two main types of matrix precursors are currently used in the preparation of C–C composites by liquid impregnation: one is based on resins and polymers and the other is based on pitches. 4.1 Resin precursors In this group, phenolic and epoxy resins are the most commonly used and both are thermoset and need curing prior to carbonization (Fig. 7.4a). Apart from the above mentioned matrix requirements, it is necessary that the resin cures rapidly at low temperature (without the evolution of volatiles). The carbon yield in commercially available resins usually ranges from 50 to 70 wt %, depending on the type of resin and the processing conditions (Murdie et al., 1992). These yields cannot be increased by the application of pressure during carbonization. New resin precursors (Fig. 7.4b) have been developed with carbon yields of up to 85 wt % (800 C) (Savage, 1993c), but the price of these products is extremely high and in some © 2003 Taylor & Francis