We are very grateful to Professor J. Vlček from University of West Bohemia;
Department of Physics;
Czech Republic;
for offered opportunity to use the microhardness tester Fischerscope H100 and M. Kormunda for his help with microhardness measurements. The present work was supported with the Ministry of Education of the Czech Republic;
contract J07/98:143100003;
COST 527.20 and by the Grant Agency of the Czech Republic;
contracts;
202/00/PO37;
and;
202/01/1110;