Mechanical Characterization of Ultra-Thin, Hard-Disk Overcoats Using Scratch Testing and Depth-Sensing Indentation
- PDF / 1,049,629 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 9 Downloads / 223 Views
ABSTRACT Two series of five diamond-like carbon (DLC) coatings were sputtered under nominally identical conditions, but to different film thicknesses of 20 nm and 105 nm. First, the hardness of each sample was determined by depth-sensing indentation. Hardness measurements were substrate-affected to some extent for all samples but especially so for the 20 nm coatings. Two types of scratch tests were performed in an attempt to isolate and characterize the top coatings. The first was a wear test, which consisted of moving the sample back and forth repeatedly under a small constant load. The residual damage was inconsistent, but appeared to be a function of the composite, or substrate-affected hardness. The second test was a single-pass scratch in which the normal load was ramped linearly. For all samples, the friction coefficient was approximately constant as a function of load. Furthermore, samples with the same top coats yielded similar friction coefficients, regardless of the coating thicknesses. Friction coefficient decreased with hydrogen content and to some extent, increased with hardness, as measured on the 105 nm samples. The friction coefficient measured during a ramp-load scratch offers an alternative for characterizing ultra-thin films, when indentation alone yields measurements that are significantly affected by the substrate.
INTRODUCTION Amorphous carbon films of less than 30 nm are widely used as protective overcoats on magnetic recording media; however, mechanical characterization of such coatings using indentation has proven extremely difficult. The indentation depth must exceed that which is required for full plasticity, and yet remain a "small" fraction of the film thickness to avoid substrate interference. Furthermore, since the maximum shear occurs below the surface in Hertzian contact, it is very possible for yield to begin in the substrate, while the deformation in the hard overcoat remains elastic. One of the attractions of scratch testing is the hope that it may used to isolate the properties of such thin coatings by shifting the position of maximum shear stress towards the surface. For this experiment, two series of five diamond-like carbon (DLC) coatings were sputtered under nominally identical conditions, but to two different film thicknesses: the first set of five coatings were 20 nm and the second set were 105 nm. Within each series of five coatings, the hydrogen content was increased. The purpose of this work was to identify a scratch experiment that would 1) yield the same measurement for two coatings that were the same, regardless of the coating thickness and 2) differentiate between coatings by hydrogen content. One promising option was a reciprocating-wear test recently developed at the University of Alabama [1, 2]. In such a test, the sample is moved back and forth repeatedly under a diamond indenter, while subject to a low, constant load. Coating performance is assessed by measuring the change in surface profile as a result of the wear test. A second option was a single-pass, ramp
Data Loading...
