CMCs adopt the common reinforcement/matrix architecture. The inherent anisotropy in structure calls for controlling not only the bulk level of impurities but also their distributions. Low atomic mass elements such as B, C, N, O, Al and Si present in CMCs could outgas fairly easily during the manufacturing processes and during their service phases as compared to traditional alloys, resulting in internal voids, erosion and even corrosion of components. Hence, it is envisioned that chemical impurities and distribution, as well as outgassing, must be thoroughly understood and controlled to realize the full potential of CMC materials.
CMCs present unique challenges to conventional chemical analysis techniques. They show chemical inertness toward even the most aggressive digestion media, rendering wet chemistry-based analysis techniques ineffective, let alone not appealing due to loss of spatial distribution information.2 In that sense, direct solid sampling techniques, which eliminate the need of sample digestion altogether and have the ability to provide depth-specific distribution information, are gaining attraction. However, the complex nature of CMCs could lead to large measurement uncertainties, especially for those direct solid sampling techniques which require matrix-matched calibration standards.
Thermo Scientiﬁc introduced fast-ﬂow glow discharge mass spectrometry technique, in their Element GD model, in 2005. This instrument combines a fast ﬂow direct current GD (Glow Discharge) source with a sector ﬁeld mass analyzer. One of the main features of the Element GD is that it has very efficient direct solid sampling capability, fast data acquisition, and very high mass resolution (up to 10,000). Figure 1 illustrates the schematic of the fast ﬂow GD source. In this conﬁguration, a discharge gas ﬂow, typically argon, is directed toward the sample surface via a vertically aligned replaceable ﬂow tube. At discharge gas ﬂow rates of few hundreds of sccm/min, the sample surface sputters at very high atomization rates (μm/min) setting the prerequisites for very sensitive mass fraction determinations.3 The presently investigated GD source differs from those employed in VG9000 and Astrum GDMS models. The fast ﬂowing gas generates a jet stream that moves in the direction of the plasma region from the cathode surface. This jet stream-assisted transport of the sputtered atoms, among other effects, noticeably makes sputtering more uniform for samples having internal voids, cavities or rough surfaces, such as composites. This enhancement becomes even more pronounced when the FF-GD source voltage is modulated.4
Figure 1: Schematic illustration of the fast flow high power source, the standard GD source deployed on the FF-GDMS instruments.
Previously, we have demonstrated FF-GDMS as a very effective tool in assessing the trace impurities in isotropic nuclear grade graphite.5 In this study, FF-GDMS was evaluated for its applicability for chemical analysis of carbon fiber–reinforced carbon composites (CFRCs). Our findings demonstrate the advantages of this method for analysis of composite samples with complex architectures, especially when the source is operated in modulated mode. Our approach is very effective for full survey chemical analysis of composites, providing sensitivities down to ultra-trace levels practically for all elements in the periodic table. The technique has also shown to be extremely robust, making it ideal for daily quality or process control of a broad range of materials that are presently actively sought for advanced applications. In addition to full survey analysis of CFRC samples by FF-GDMS, we have examined the release characteristics of gas-forming elements in CFRCs. Outgassing analysis, important in its own right as a quality gauge to CFRC, also helps to validate GDMS measurements, given that outgassing molecular species in the plasma can affect the atomization and ionization processes. Hence, multiple complementary analytical approaches were studied for evaluations of gas forming elements in CFRCs.