FGMs High-Heat-Flux Environments: Cost/Performance Issues
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made us realize that serious barriers to FGM use exist. Technology managers will not advocate use of a composite if small changes in composition, for example, render that composite obsolete. Figure 1 shows the relative compositional options available to the FRC and the FGM designer. The limited number of available fiber compositions places definite boundaries on FRC fabrication. For FGM manufacture, however, chemical compatibility provides the only clear compositional boundaries, producing a much larger range of material combinations. This overabundance of choices dictates that extensive modeling of the effects of microstructure, composition, and layer thickness should take priority over expanding the range of processing techniques. We need to: (1) define the needs of the application (often a moving target), (2) use well-grounded models to define the "ultimate" FGM for the application, and (3) from these restrictions, select the appropriate processing technique. Not all processing techniques can produce all types of FGMs. Defining the FGM structure defines the candidate processing techniques
Background In an FGM, composition, microstructure, and architecture (layer thickness and arrangement) can all be varied independently if the fabrication technique is flexible. The sedimentation technique we use1 at Ohio State University has this kind of flexibility. However, materials selection issues encountered at the start of our work
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Material Combinations
Figure 1. Schematic comparison of practical materials combinations fabricable from FGMs and FRCs.
and composite fabrication economics. Unfocused, random expansion of known processing techniques—simply because they are capable of producing FGMs— wastes valuable research dollars.
FGMs Versus FRCs It is difficult to imagine how the large improvements in axial strength that are theoretically possible from unidirectional fiber reinforcement can be matched by any other composite form. On the other hand, much effort has been expended on specific FRCs to reach the simple conclusion that net matrix characteristics are degraded by the presence of the very fibers meant to improve them. Substantial degradation in fiber properties due to processing also appears inevitable. Our work has shown that ceramic-intermetallic FGMs can undergo graceful failure at moderate to high temperatures (527 and 727°C).2 Multiple cracks propagate and are blunted in the intermetallic NiAl layer (Figure 2). At no point in our research was loss of the alumina layer observed, giving rise to a ceramic surface displaying graceful failure at T~500°C. Also observed was the insensitivity of this dense ceramic surface to thermal cycling stresses. Modeling estimates of internal thermal profiles indicated that the temperature remained approximately constant throughout. Therefore, ceramic thermal shock resistance is not a selection factor for properly designed FGMs. The final and possibly most inspiring reason to use FGMs is cost. In many of the processes employed, the starting materials are in powder form. C
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