Composites, Joining

A general prerequisite for the existence of a stable interface between two phases is that the free energy of formation of the interface be positive were it negative or zero, fluctuations would lead to complete dispersion of one phase in another. As implied, thermodynamics constitutes an important discipline within the general subject. It is one in which surface area joins the usual extensive quantities of mass and volume and in which surface tension and surface composition join the usual intensive quantities of pressure, temperature, and bulk composition. The thermodynamic functions of free energy, enthalpy and entropy can be defined for an interface as well as for a bulk portion of matter. Chapters II and ni are based on a rich history of thermodynamic studies of the liquid interface. The phase behavior of liquid films enters in Chapter IV, and the electrical potential and charge are added as thermodynamic variables in Chapter V. 

Let us now consider a case in which components 1 and 2 form an intermediate crystalline compound (C) with precise, invariant stoichiometry (e.g., 60% of component 2 and 40% of component 1). If the chemical composition of the intermediate compound is fixed, it behaves as a mechanical mixture with respect to its pure components (i.e., zero miscibility). The presence of the intermediate compound subdivides the compositional join into two fields mechanical mixture 2-C (y") and mechanical mixture C-1 (y ). The resulting crystallization path may assume two distinct geometrical configurations, as shown in figure 7.8. 

Figure 7. Phases present along the compositional join muscovite (Mu)— MgFe +. celadonite (Ce) at 2Kb pressure. MU = muscovite-phengite Mica = dioctahedral mica of unidentified composition CE = celadonite mica ...

It is important to note also that the extent of this ion-for-ion solid solution is diminished at higher temperatures. There is a gradual exsolution, in the compositional join studied, to form the mixed layered... 

Figure 23. Phase relations in the muscovite-MgAl celadonite compositional join, 2Kb pressure. (Velde, unpublished data.) M = dioctahedral mica ...

VELDE (B.), 1969. The compositional join muscovite-pyrophyllite at moderate temperatures and pressures. Bull. Soc. Fr. Min. Crisf.92 360-68. 

A similar analysis applies to the triclinic-monoclinic (/T -72/c) transition that occurs at 298 K across the compositional join CaAl2Si208-SrAl2Si208 near 85 mol % Sr, except that the order parameter varies with composition with a form similar to Equation 7 (Phillips et al. 1997). With increasing Sr-content, the Si MAS-NMR spectra (Fig. 17) clearly show a decrease in the number of peaks that corresponds to a change in the number of crystallographically distinct Si sites from four (/1) to two (72/c). The order-parameter could be related to the difference in chemical shift between the Tlo site of the 72/c phase (-85.4 ppm) and the peak for the Tlmz site of the 71 samples, which is well-resolved and moves from -89.5 to -86.7 ppm with increasing Sr-content. These results yielded a critical exponent P = 0.49+0.2, consistent with the second-order character of the transition. 

Fig. 1 A 3-D C-C composite joined to Cu-clad-Mo using (a) Ticusil and (b) (c) Cusil-ABA. Copious infiltration of inter-fiber channels by molten brazes, and dissolution of Cu cladding have occurred.

Composite materials can be prepared for bonding in several ways. Common procedures include mechanical abrasion of the cured composite surface and the use of a peel ply. This is a removable layer that is applied prior to curing the composite and then stripped to reveal a textured surface prior to adhesive bonding abrasion is sometimes used in addition to remove residues that may have been left by the peel ply (see Fibre composites - joining). There are also other surface treatments available, but aU aerospace bonding and pretreatment processes will require high standards of control. 

Several articles are included on different aspects of Fibre-matrix adhesion in these materials. Special techniques are required for Fibre composite joining. 

Some of the main constituents nsed in these materials are discussed under Fibre composites - matrices and fibres and practical aspects are considered under Fibre composites - joining and Fibre composites - processing techniques. 

Some other practical aspects of the use of these materials are discussed under Fibre composites - joining. 

A selection of these techniques is also applicable to polymer composites (see Fibre composites - joining). However, since composites gain their superior properties from the reinforcement, the weld is inevitably the weak point in the system as the fibre will be discontinuous across the joint interface. All the welding processes are governed by the parameters time, temperature and pressure. In order to achieve high-quality welds, careful optimization of the welding parameters is required for each application. 

Fibre composites - joining K B ARMSTRONG Thermoset matrices, peel-ply, abrasion thermoplastic matrices, corona discharge... 

Plastics and Composites Joining Laboratory, Welding Engineering Program, Department of Materials Science and Engineering, Ohio State University, Columbus, OH, United States... 

Schwartz, M. M., Joining Polymer Matrix Composites and Resin Matrix-Composites, Joining of Composite Matrix Materials, (ISBN 0-87170-514-1), ASMInt., 1995. 

Processing - microstructure - mechanical properties correlations Ceramics composites joining and testing NDE of ceramic components... 

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