Fracture toughness particle features

As was discussed in Chapter 1, the common feature of these ceramics is that a portion of the MgO always remains unreacted. When MgO dissolves and reacts with the acidic solution, the dissolution and subsequent reaction occur on the particle surface of MgO. The reaction produces a protective coating of less soluble products on the particle surface that inhibits the reaction of the acid solution from the core of individual MgO particles. Prevalence of unreacted MgO, in fact, is good for the overall strength and integrity of the ceramic because unreacted MgO can act as a second phase that will resist crack propagation within the ceramic and improve its fracture toughness. 

The particle size distribution resulting from a milling operation is primarily determined by both the method of particle size reduction as well as the mechanical properties of the material such as fracture toughness, elastic modulus, and hardness. Thus, two extrudate samples with different mechanical properties milled under the same conditions will yield different particle size distributions. Beyond the intrinsic properties of the system, the mechanical behavior of extruded material is also affected by features of the bulk extrudate itself such as air bubbles, particle inclusions, or other defects that can increase the apparent brittleness of the material. Foamed extrudate, for example, could have different milling behavior as compared to a nonfoamed extmdate of the same composition. 

Fig. 5.81 AFM of the same specimen as in Fig. 5.80 shows the particle size correlates well with the FESEM image. The AFM image (A) [376] provides much more detail at the particle-epoxy interface which can be compared with the fracture toughness measurements. A line scan analysis (B-C) of an interesting feature shows the epoxy forming a bridge between the rubber particles. The height of the feature is about 530 nm. (From O. L. Shaffer, unpublished [376].)...

Microstructural features that are required to improve fracture toughness in thermosets have been identified however, exact requirements may be material dependent. In general, improvements in fracture toughness are dependent on the volume fraction of the rubbery particulate phase, good dispersion of the particles within the matrix, proper particle... 

There are stiU many unanswered questions as to why one LR additive more effectively toughens a particular resin than another LR additive. Microstructural features that may affect fracture behavior such as volume fraction, particle size and size distribution, particle-matrix adhesion, and the structure of dispersed phase are difficult to control independently. As a result, the importance of each has not been quantified. Nevertheless, significant improvements in the fracture toughness of rubber-modified UP and VE resins have been realized. 

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