Glass/ferrite interactions and corrosion of gap glasses in recording heads

Certain glass compositions in common use as gap spacers in ferrite recording head sliders have been shown to interact with air at 75% R.H. The reaction proceeds at 20°C and the corrosion products appear within a few days. The products vary from shape-less lumps to long single-crystal whiskers. In the presence of 300 ppb each of SO 2 and NO 2 the products are more voluminous. While the products are more prominent at the interface between glass and ferrite, they also appear on polished bulk glass specimens in total absence of a ferrite. The effect has been traced to the diffusion of lead, barium or lanthanum to the glass-air surface where the ions react with the atmosphere. An interaction between glass and ferrite was observed to occur during glass bonding. This interaction was indicated by the appearance of prominant grain boundaries in the ferrite at the ferrite/glass interface. Transgranular grain etching was also found. It is believed that interdiffusion of glass and ferrite components occurs and that this contributes to the mobility of the metal ions in the glass.