Interface graphitization of carbon-carbon composites by nanoindentation

The present study explores mechanical behavior of carbon-carbon composites using low-load nanoindentation. Indentations are performed using flat-punch and Berkovich tips indenting normal to the cross-section of a single fiber-matrix filament. A systematic set of tests are carried out employing multiple load functions to critically assess loading behavior and load-unload irreversibility at the filament scale. Flat-punch indentation response clearly shows irrecoverable displacements and an energy loss, accumulating over multiple cycles. This is compared against the indentation response of single crystal Aluminum where the unload-reload behavior is linear-elastic. The mechanical test is coupled with Raman micro-spectroscopy to correlate energy dissipation with structural changes in the carbon constituents. Three microstructurally distinct regions are identified using the Raman microprobe. First order spectra were tracked close to the interface revealing a reduction in the width of the defect peak. Constraining the nanoindentation footprint via a sharp-Berkovich indentation, the same correlation was confirmed. An incremental load-unload response was analyzed using the Oliver-Pharr model to reveal a consistent drop in indentation modulus with increasing maximum load. All observations suggest that graphitization could be the possible mechanism of plastic deformation in the material. The implications are discussed in relevance to toughness in structural and tribological applications. (C) 2019 Elsevier Ltd. All rights reserved.