A Polymer Route to the Design of Thermally Stable Metal Matrix Composites: Materials Selection and In-situ Processing
The microstructural design of metal matrix composites for elevated temperature structural applications has two needs: (i) the ceramic phase must be dispersed at the nanoscale, and lie within the grain matrix, and (ii) the dispersed phase should not coarsen at high temperatures. Many ceramics, both oxides and non-oxides are currently produced from polymer precursors. The potential for incorporating these ceramics into a metal via in-situ conversion of polymers can address both the requirements. Most often the polymers are environmentally benign. These polymers are self-contained, that is all constituents of the ceramic phase are present within the organic molecules. The diversity of materials selection and processing approaches can spur further innovation. Among them, the silicon-based polymers convert into the silicon oxy-carbonitride (Si-C-N-O) phase in the temperature range of 800 – 1000 oC. The polymer can be infused into molten metal by stir-casting method, as in magnesium, or, in the solid state, by mechanical milling of the polymer and metal powders, as in copper. In this article, the recent development on the processing approaches, and mechanical properties of the polymer derived metal matrix composites are reviewed. Some of the newer results of magnesium matrix composites are also discussed. The primary advantages of silicon-based polymer and its role on enhancing the creep-rupture performance of magnesium matrix composites at ~0.8TM (450 oC) are summarized. The current processing challenges and the potential for scale-up and manufacturability of composites will be addressed.