Within the large range of magnetic recording media presently available on the market, most utilise a thin protective coating of some kind on their read/write surfaces. In the case of magnetic hard disk drives, for example, the protective coating is usually DLC with a thickness of only a few nanometres. On the other hand, Minidisk surfaces have a thin polymeric coating as their protection against scratches incurred during production and subsequent use. The Minidisk surface tested in this example had a polycarbonate surface film of thickness 1.2 μm and was characterised using the Nano Scratch Tester and the pin-on-disk Tribometer.
By using two different instruments to characterise such a surface enables adhesion data (scratch testing) to be combined with frictional and lifetime data (pin-on-disk testing) which together give a better overall picture of how the coating will perform in service conditions. Fig. 1 shows typical Tribometer traces for polycarbonate thin films where a rapid rise in friction can be taken as a measure of coating breakdown under load. Nano Scratch data is presented in Fig. 2 where two distinct critical failure points can be measured both from the optical micrographs and from the friction and depth traces.
Figure 1. Friction coefficient vs. distance traces for Minidisk surface. Tests were carried out with a 100Cr6 steel ball as the static partner, applied load of 1 N and speed 20 cm/s.
Figure 2. Nano Scratch Tester results for a 1.2 μm polymer coating on the surface of a Minidisk. Complete results are shown in (c) including the frictional force, friction coefficient, penetration depth and residual depths. The first critical failure point (Lc1) corresponds to an applied load of 19.6 mN (a) whereas total delamination (Lc2) occurs at 38.7 mN (b). A spherical diamond indenter of radius 2 μm was used with a loading rate of 150 mN/min. over the range 0 - 50 mN.