Understanding the Oxidation Behavior of Polymer-Derived Si(O)C and Doped NbSi2 Ceramics

Recently there has been an increased demand for high temperature structural materials whose properties exceed those of conventional Ni-based alloys and monolithic ceramics. Therefore, this project, examines doped NbSi₂ and SiC-based polymer-derived ceramics that could show improved resistance to environmental degradation at elevated temperatures. Al, Cr, and Ti-doped NbSi₂ ceramics, are be made by spark plasma sintering (SPS) elemental powders followed by annealing. For the SiC specimens, SiC powder was mixed with 1 vol% TiSi₂ powder, and pressed into pellets. The TiSi2 functions as an embedded marker to track oxygen activity. The pellets were infiltrated with an allyl-hydrido-polycarbosilane (SMP-10) polymer precursor. Subsequent pyrolysis in argon gas at 900°C transforms the infiltrated SMP-10 into amorphous Si-(O)-C. The oxidation resistance of the ceramics is explored in temperature ranges of 1100-1400°C inside a tube furnace under a constant flow of dry air. Cross-sectional Scanning Electron Microscopy (SEM) and Energy-Dispersive Spectroscopy (EDS) analysis are used to characterize the oxidation resistance in both material systems. For the NbSi₂ samples, the mass change per surface area was measured, the results yield a value near to the goal of mass gain less than 2 mg/cm². In the Si(O)C specimens, crack formation during processing was first significantly minimized. Oxidation treatments illustrated that TiSi₂ can be used to gauge the partial pressure of oxygen, which can help understand oxygen transport in Si(O)C.