Microstructure analysis of a C/C composite using argon ion etching (1)

The microstructure of C/C composites obtained by chemical vapor infiltration of a carbon fiber felt was comparatively studied by reflection light microscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM) and laser scanning confocal microscopy (LSCM). Ar + ion etching was used to reveal and distinguish structural units of the pyrolytic carbon matrix. Mechanically polished samples, polished and subsequently ion etched samples and fractured samples were compared. The values of surface roughness and surface height obtained after polishing and subsequent etching determined by atomic force microscopy and laser scanning confocal microscopy correlate well with the degree of texture of the matrix layers obtained by polarized light microscopy (PLM) and selected area electron diffraction (SAED). The carbon matrix is composed of structural units or “cells”, which contain a carbon fiber and a sequence of several differently textured layers around each fiber. Within high-textured layers columnar grains are well-recognizable using polarized reflection light microscopy and confocal microscopy. The size depressions within high-textured carbon layers which are found by atomic force microscopy after ion etching correlates well with the size of differently tilted domains detected by both transmission electron microscopy and scanning electron microscopy.

A comparative study of an infiltrated carbon fiber felt was presented recently which allows a more detailed understanding of the complex interplay between mass-thickness and Bragg contrast in TEM images of differently textured pyrolytic carbon layers. In this case, Ar+ ion etching was used as a routine thinning step for obtaining thin electron transparent samples for TEM. Here, we demonstrate that one can also take advantage of this non-uniform etching: we show that Ar+ ion etching can be used as a technique for revealing structural units within the matrix of infiltrated carbon fiber felts.

Experimental:

Chemical vapor infiltration: Samples synthesis was performed by chemical vapor infiltration of a carbon fiber felt. The carbon fibers produced from polyacrilonitrile (PAN) precursor fibers had a mean diameter of 12 um and were randomly oriented in the felt before infiltration. The mass density of the fibers was 1.76 g/cm3. The infiltration of the fiber felt was carried out at a temperature of 1100C in a hot-wall reactor using methane as carbon source. Further details of the infiltration procedure are described elsewhere.

Sample preparation: For the preparation of the TEM sample, a double-sided dimpling was applied using 3 um and 0.25 um diamond pastes. Two argon ion guns operating for 30 minutes at 4 kV and a current of 12 mA were used for the final thinning of the sample from both sides. The ion beam was focused on the center of the sample at an angle of 4° relative to the sample surface. The sample was continuously rotated with respect to the ion beam during the etching process to avoid preferential etching. To ensure comparability, PLM, LSCm and AFM studies were performed on the TEM sample as well as on flat sample that were cut from the same piece of infiltrated carbon fiber felt as the TEM sample. The flat samples were prepared following the same procedure as for the TEM sample, but instead of dimpling, the flat surfaces were polished with different diamond pastes.

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