Optimizing Thermal Shock Resistance

Finally, it is important to mention the effect of porosity. Since the thermal conductivity of air is negligible compared to the solid phases, the addition of large (>25 percent) volume fractions of pores can significantly reduce Ath. This approach is used in the fabrication of firebrick. As noted above, the addition of large-volume fractions of porosity has the added advantage of rendering the firebricks thermal-shock-tolerant. Note that heat transfer by radiation across the pores, which scales as has to be minimized. Hence for optimal thermal resistance, the pores should be small and the pore phase should be continuous. 

It is therefore apparently futile, with regard to relationships [8.74] and [8.75], to try to optimize simultaneously the resistance to sudden mpture by thermal shocks (parameters R or R ) and the damage resistance (parameters R and R )- In other words, the optimization of the resistance to a severe shock leads to materials which are likely to be damaged rapidly by microcracking and vice versa. A choice has therefore to be made, according to the apphcation in view. 

---