Nanoindentation-Induced Deformation of DLC Coatings

The deformation behaviour of diamond-like carbon (DLC) coatings on silicon substrates induced by indentation under different loading conditions has been investigated. Silicon is a commonly used substrate for DLC coatings. However, it is known that during loading, diamond cubic silicon (Si-I) transforms to the metallic (Si-II) phase, and on unloading to either amorphous silicon (a-Si) or complex crystalline phases, termed Si-III and Si-XII, depending on the applied load and unloading rate. A number of hydrogenated amorphous carbon films (a-C:H) were deposited on (100) oriented single crystal silicon substrates by a plasma assisted chemical vapour deposition technique. The total sp3 content of these coatings (including C-H bonds) was estimated by Raman spectroscopy to be in the range of 35 – 40 %. The coatings were subjected to nanoindentation over a range of loads from 100 to 500 mN with a 5 µm spherical indenter. Following indentation, the surface and subsurface morphology of the indented regions were examined using various techniques, including atomic force microscopy, focused ion beam (FIB) microscopy and cross-sectional transmission electron microscopy (XTEM).
 
 
Figure 1: Bright field XTEM image of DLC-coated Si indented to a load of 150 mN
 
Figure 1 shows a bright field XTEM image of the coating-substrate after indentation to a 150 mN load. It shows both planar defects (marked 1, 2) on the {113} and dislocations (marked X, Y) on {111}, both indicative of deformation by slip and associated processes. Figure 2 is a dark field XTEM image of the coating-substrate after indentation to a 300 mN load showing, in addition to crystalline defects, regions of amorphous silicon and complex crystalline silicon phases. Compared to uncoated silicon, the deformation zone in the coated material occurs over a much larger volume, and higher stresses are required to induce phase transformations.
 
 
Figure 2: Dark field XTEM image of DLC-coated Si indented to a load of 300 mN
 
Ayesha Haq
Paul Munroe
Mark Hoffman
Phil Martin and Avi Bendavid (CSIRO Industrial Physics)
 
This work is sponsored by the Australian Research Council.