The Ohio State University

Micro/Nanoscale Differential Wear and Corrosion of Multiphase Materials William W. Scott Jr. Ph.D. Dissertation, The Ohio State University, Dept. of Mech. Engin., 2001 167 pages

Abstract:

Wear of multiphase materials at the micro/nanoscale is important in devices such as magnetic tape and disk drives, where the thin-film read-write heads are multiphase. Differential wear, which is caused by differences in wear resistance among the head’s phases, causes thin-film poles to recede from the head’s bearing surface; this phenomenon is called pole tip recession (PTR). It is a problem because it increases spacing between the poles and medium, resulting in lower write density.

Here, PTR in tape heads is studied to understand micro/nanoscale differential wear. Test results suggest that three-body abrasion, which leads to primarily plastic wear, is the operative wear mode.Most of the three-body abrasive particles originate from the tape surface; the alumina head-cleaning agents (HCAs) in the tape, which function as load bearing particles at the interface, are the primary abrasives.Some particles originate from the head substrate.

PTR can be reduced by: lowering tape tension, choosing a substrate that is harder than the tape’s HCAs, choosing a pole material that is as close as possible to the hardness of the substrate, and lowering the thickness of the head’s thin-film region to a value as low as possible. Material hardness matching, i.e. choosing the substrate and pole materials such that their hardness values are close to equal, will not reduce PTR if a substrate is chosen that is less hard than the HCAs. Covering the head with a diamond-like carbon (DLC) coating reduces PTR in the short term.

An analytical model that accounts for the observed wear is presented. The model shows that each of the following leads to higher differential wear:increasing the thickness of three-body particles, increasing tension, decreasing thin-film hardness, and increasing the thin-film wear coefficient. An increase in thin-film wear coefficient can be caused by an increase in thin-film thickness or an increase in the number of particles at the interface.

Battelle Class II and elevated temperature & humidity tests have been conducted to study corrosion of tapes and heads. The addition of DLC coatings increases the corrosion resistance of heads; the coating inhibits contact between environmental pollutants and the metal surfaces.