ENHANCED GLASS SUBSTRATE HARDENING BY NA+/AG+ ION EXCHANGE

Very high density magnetic recording requires reduced flying heights and media surfaces of extremely low waviness and roughness. It is well known, that glass substrates are an excellent choice to meet these demands. But glass surfaces need to be made shockproof by ion exchange hardening. Na2O-CaO-SiO2-glass, known as "float glass", is a possible candidate, because it is worldwide available with reproduced quality, low price and its surface can be chemically hardened by Na+/K+ ion exchange, but with relatively moderate rise of hardness. A new process of Na+/Ag+ ion exchange, which provides considerable strength enhancement by adding AgNO3 to KNO3 melt under proper operation conditions is presented. The strength increases clearly by a factor of about 6. The interdiffusion coefficient of Na+/Ag+ is 8-10 times higher than that of Na+/K+ leading to much broader zones of enhanced strength. No noticeable degradation of surface topography has been observed.