Cooling dilatational stress

The volume decrease accompanying formation of intermetallic compounds produces a bulk dilation stress over the whole thickness of growing layers. Thermal expansion of the couple constituents during heating up as well as their contraction during cooling down in the course of successive anneals of the same couple produces a shear stress. These inevitably lead to the rupture of Ni-Zn and Co-Zn diffusion couples, with the latter effect being most disastrous. 

On cooling a polymer mixture of a hard and a pliant component, the hard phase will try to desegregate from the pliant phase because of the different expansion coefficients of the demixing components. The two components are bound to each other, however, when the hard component is grafted onto the pliant component. But a non-cross-linked pliant phase further shrinks on cooling, which leads to stresses being set up in the pliant phase. Formation of holes within the pliant phase releases the stresses. If the pliant phase is cross-linked, it must dilate on cooling since it is under... 

Ceramics are subject to failure by thermal shock due to the disruptive stresses that result from the differential dilation between the surface and core of a body. With the relatively slow cooling rates usually encountered in most electrical and electronic applications and with the exception of materials with very low thermal expansion coefficients, the thermal conductivity usually determines the resistance of a ceramic to failure by thermal shock. Listed in Table 2.2 are the thermal stresses or shock resistances of several ceramics as derived from the relation R" = C oIccE, where C is the thermal conductivity, o is the tensile strength, a is the coefficient of thermal expansion, and E is Young s modulus. 

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