Abstract
The investigation of microhardness of RE-Si-Mg-O-N glasses (RE = Y, La, Sm, Yb and Lu) containing 20-26 eq.% of nitrogen revealed a possibility of a control of the microhardness via modification of rare-earth element additives and by the control of nitrogen content. Lu-containing glasses exhibit hardness, which is almost of 15 % higher than that of La-glasses. Four-percent increase in nitrogen content would result in almost the same increase in microhardness. Microhardness of RE-containing glass follows linear dependence on ionic radius or cationic field strength (CFS) of the corresponding cation added in the glass network. Yttrium, which does not belong among lanthanides, does not fit into the linear dependence. Its deviation from linearity can be rationalized in terms of the change of the coordination number from CN = 6 to CN = 7 without change in valency.

Key words
microhardness, oxynitride glass, lanthanides, nitrogen

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