Thermal microstresses

For most particulate composites the mismatch between the particles and the matrix is more important than the anisotropy of either component (though alumina/aluminium titanate composites provide a notable exception and are described below). The main features of the stresses can therefore be understood in terms of a simple elastic model assuming thermoelastic isotropy and consisting of a spherical particle in a concentric spherical shell of matrix with dimensions chosen to give the appropriate volume fractions. The particles are predicted to be under a uniform hydrostatic stress, ap after cooling. If the particles have a larger thermal expansion coefficient than the matrix, this stress is tensile, and vice versa. For small particle volume fractions the stress 

3 Hydrostatic stress in the SiC particles in a magnesia-10%SiC nanocomposite during a thermal cycle to 1550°C. Note the small amount of relaxation during the cycle and the good agreement with the prediction of a simple elastic model for the stresses [17]. 

As well as producing these broadly beneficial effects, thermal microstresses can also degrade the strength of composites. The tensile components of stress can help in crack initiation. In a composite with a uniform distribution of particles, the tensile components act only over distances comparable with 

All else being equal, a toughening effect occurs if the crack tilts or twists away from a planar geometry because this reduces the net crack driving force. In homogeneous materials such as glass, cracks tend to propagate in 

4 (a) Backscattered SEM micrograph of unetched alumina-3 pm SiC composite. The straight lines on the specimen surface are scratches from metallographic preparation, (b) Unetched alumina-13 pm SiC composite showing radial microcrack network in the matrix [19]. 

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Todd, R.I., Morsi, K. and Derby, B. Neutron diffraction measurements of thermal residual microstresses in ceramic particle reinforced alumina , Brit. Ceram. Proc. 57 (1997) 87-101. 

Microstresses arise inside the material as a result of the unequal thermal expansion coefficients of the glass and of the crystals. This phenomenon is characteristic of polyphase ceramics. In porcelains, the stresses arise in particular at the boundary between large quartz crystals and glass. Their formation is also contributed to by the modification inversion of quartz. The stresses sometimes bring about formation of microscopically visible cracks inside the quartz grains or around them. The structure of porcelain thus contains weak points which enhance the development of fracture on loading. 

The facts mentioned above indicate that one of the ways to increase the mechanical strength of porcelain is to change the glassy phase composition in order to increase its thermal expansion coefficient, and thus reduce the microstress at the boundary with quartz. Practical experience shows that the same effect is obtained just by reducing the quartz grain size the small quartz grain size suppresses the adverse effects of microstresses at the boundaries. 

The differences in thermal expansion coefficients of the individual phases and also their anisotropies result in non-uniform shrinkage on cooling. Tf this non-uniform shrinkage cannot be met by deformation, stresses arise restricted to short distances (microstresse.s). This phenomenon is characteristic for ceramics and influences their mechanical properties. High tensile stresses may even result in the formation of ciacks visible under the microscope, for example iji fireclay or porcelain. These cracks arc usually situated at phase boundaries. 

The residual stresses in coatings may be subdivided into microstresses within individual particles and macrostresses within the coating as a whole. Microstresses arise because of the restraint due to the thermal contraction of individual particles,... 

In a polyphase material, when there is a large difference in thermal expansion coefficients among the different phases, microstresses develop during... 

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