Study of Disk Wear Using Olympus HDI Thermal Asperities Mapping

This paper illustrates the possibility and advantage of use of thermal asperities (TA) mapping for study of degradation of disk surfaces. As shown below, TA mapping appears to be a very sensitive method for evaluation of disk wear, in comparison to traditional measurements of friction and acoustic emission. The TA maps help to find exact locations of the TA events and reasons of surface degradation.
All presented measurements were performed using CETR Olympus HDI Reliability Test System, which measures TA, as well as friction, acoustic emission (AE) and track average amplitude (TAA). Very low internal electromagnetic noise and precision motion control ensure sensitive and accurate TA mapping.
The experimental results below compare TA mapping with other methods of disk wear investigation, namely radial profiles of friction, AE and TAA on the disk before and after wearing. Disk wearing was done by fast sweep motion of the head between disk radii 0.9” and 1.2” for 3 days, at 60°C. 

The picture below illustrates the location of the wear zone on the disk (between the dashed lines).

Figures 1 and 2 below represent both top and 3-D views of TA data. Height and color correspond to different detection thresholds of TA events. (Olympus system also provides TA count and repeatability). 

Data in Fig.1 was taken before wearing and show a small number of uniformly distributed TA events.

Yellow arrow shows the direction of disk rotation; small yellow ball shows zero mark (index) position. 

In Fig. 2, a greatly increased number of TA events on the edges of the wear zone is evident. Two rings with large numbers of asperities developed on the disk as a direct result of surface wear and debris accumulation.

The radial profiles of friction and acoustic emission data are represented in Figures 4 and 5. Both pre-wear and post-wear values are close to zero.

Fig. 6 below represents a TAA radial profile, which shows just a slight increase in TAA due to disk burnishing. 

Conclusion

In the presented data, dramatic changes in TA events distribution due to disk surface degradation are evident, when other methods do not show significant signal changes. 

This means, that TA mapping on low-noise precision Olympus HDI testers may be used as the most sensitive means for both detection of tiny degradation changes in disk surfaces and finding exact locations of these changes.