Reaction at Standard Pressure

The transformation of c-BN into h-BN was investigated during DTA-analysis [22]. The SEM images show formation of differently textured h-BN at the surface of the c-BN crystallites, which indicates that there is no uniform mechanism of phase transition (Fig. 5). 

The DTA measurements exhibit the stability of c-BN at standard conditions. Influences of grain size and purity (oxide content) of the cubic boron nitride crystals on the conversion temperature become obvious. Fine grained samples containing boron oxide show a significantly lower conversion temperature than coarse material (conversion at 900 °C for 1.5-pm c-BN containing boron oxide and 1500 °C for 600-pm pure c-BN). 

This investigation also showed that two different routes are possible for the c-BN — h-BN transformation at normal pressure and elevated temperatures  

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