Tensile tests at 20 degrees C have been carried out on forty-four sapphire whiskers after chemical polishing in hot orthophosphoric acid. The orientations tested were < 0001 >, < 11<(2)over bar>0 >, < 10<(1)over bar>0 >, and < 10<(1)over bar>1 >. The results show that chemical polishing increases the strength of large whiskers by a factor of up to 10, but not the strength of small ones. Good correlation is obtained between fracture strength, sigma(f), and whisker diameter, d. The relevant size-strength equations, sigma(f) = Kd(m) I(n) (where I is gauge length, and K, m, and n are constants depending on whisker orientation), predict strengths in good agreement with the theoretical strength of sapphire at unit-cell dimensions and with the measured strengths of macroscopic flame-polished crystals. The observations are contrasted with those for unpolished sapphire whiskers [1]. They show a transition in the fracture nucleation mechanism of unpolished whiskers at a certain stress. In unpolished, A-type (< 11<(2)over bar>0 > and < 10<(1)over bar>0 >) whiskers, with sigma(f) < 1000 kg/mm(2), fracture initiates at surface flaws, and strength is dependent on surface area. But, for whiskers with sigma(f) > 1000 kg/mm(2), and for all polished whiskers (both A and C type), fracture is due to dislocation pile-ups or interactions, and strength is dependent only upon diameter. In unpolished, C-type (< 0001 >) whiskers, however, with sigma(f) < 800 kg/mm(2), fracture initiates at surface flaws which are related to whisker diameter; while, for sigma(f) > 800 kg/mm(2), it occurs at dislocation pile-ups or interactions and is again related to diameter. In contrast, therefore, to A-type whiskers, the strength of C-type whiskers is always diameter-dependent, although there is a clear transition in the size-strength curve at sigma(f) similar to 800 kg/mm(2).
