Diffusion of Na-22 and Fe-59 in Mg-Si-Al-O-N glasses

The diffusion of iron and sodium in oxynitride glass was studied in order to understand the corrosion mechanisms which may occur when dense Si3N4 having grain boundary oxynitride liquid is in contact with iron (in cutting tool and bearing applications) or with sodium salt (in sea water environments). The diffusion coefficients (D) of sodium and iron in MgSiAlON glasses, having compositions similar to the ones present at the grain boundary of dense silicon nitride ceramics, were deduced in the temperature range from 690 to 920 degrees C by determining the concentration profile of radiotracer Na-22 and Fe-59 from the residual activity measurement. The atomic ratio Al:Mg:Si in all the glasses was maintained constant at 1:1 36:2.32. The D-Na and D-Fe values increased from 3.96x10(-13) to 6.08x10(-12) cm(2)/s and from 2.70x10(-14) to 1.02x10(-13) cm(2)/s respectively with decreasing nitrogen concentration in glass from the highest nitrogen content of 8.56 at% to zero at% N at 800 degrees C, the mid temperature of the investigation. The activation energy of sodium and iron were respectively 42.68 and 29.92 kcal/mol for the glass containing 8.56 at% N and were larger than those determined for the nitrogen free glass. Both the diffusions were governed by interstitial mechanism