Abstract:

Multilayer ceramin coatings appear to offer the best oxidation protection for carbon-carbon composites that make up the structure of future hypersonic spcae vehicles.

Abstract:

Zirconium diboride and boron carbide particles were used to improve the ablation resistance of carbon-carbon (C-C) composites at high temperature (1500°C). Our approach combines using a precursor to ZrB2 and processing them with B4C particles as filler material within the C-C composite. An oxyacetylene torch test facility was used to determine ablation rates for carbon black, B4C, and ZrB2-B4C filled C-C composites from 800 to 1500°C. Ablation rates decreased by 30% when C-C composites were filled with a combination of ZrB2-B4C particles over carbon black and B4C filled C-C composites. We also investigated using a sol-gel precursor method as an alternative processing route to incorporate ZrB2 particles within C-C composites. We successfully converted ZrB2 particles within C-C composites at relatively low temperatures (1200°C). Our ablation results suggest that a combination of ZrB2-B4C particles is effective in inhibiting the oxidation of C-C composites at temperatures greater than 1500°C. © 2010 Elsevier Ltd.

Using an organic solvent-based formulation, flexible and homogeneous ZrB2 tapes were cast for potential use as advanced aerospace exploration vehicles. Dispersant concentrations were optimized for attrition-milled ZrB2 powder using gravitational sedimentation and viscosity measurements. Tape cast slurry formulations with varying amounts of binder (6-8 vol%), plasticizer (7-11 vol%), and solids loading (20-23 vol%) were used to optimize the casting slurry. An optimal slurry formulation was determined based on cast tape uniform particle distribution and flexibility without cracking. Thermal gravimetric analysis of the optimized tape was used to create a binder burnout schedule that did not alter the tape structure or particle distribution. Our organic solvent-based tape casting approach results in minimum oxygen contamination after colloidal processing and robust cast tapes with a thickness of 280 mu m and green density of similar to 41% theoretic density after binder burnout.

Investigating the powder processing effects on a ZrB(2)25 similar to vol% SiC ceramic composite densified using spark plasma sintering (SPS) allows for identification of densification mechanisms and enables a reduction in sintering temperature to a minimum of 1650 degrees C. Attrition milling (AM) and ball milling (BM) were investigated as processing methods to produce a fine and coarse powder densified using SPS with or without a tube furnace preheat treatment. Ceramics formed from AM and BM powders contain 1.66 similar to wt% oxygen contamination, primarily ZrO2 and SiO2, and 0.35 similar to similar to wt% oxygen contamination as SiO2, respectively. Heat treatment slightly reduces oxygen contamination but has significant impacts on the densification mechanisms. Without heat treatment, powder coarsening dominates the initial sintering process in the SPS inhibiting densification until similar to 1350 degrees C. After heat treatment, sintering and densification is enabled at low temperature, 1000 degrees C1100 degrees C. The densification of ZrB2SiC composites can be broken into a two-step process with phase 1 as the sintering step based on powder surface area reduction and phase 2 as a forging step where high-temperature creep and pressure eliminate porosity after the primary grains have formed. A timetemperature-density plot illustrates the change in densification mechanism used to fully densify ZrB2SiC composites in SPS.