Structure-Property Relations in Ultra-High Temperature CeramicsDr. Bill Fahrenholtz, DMR 0346800

Department of Materials Science and Engineering, University of Missouri-Rolla

Fundamental microstructure-property relations were studied for ZrB2-SiC. For most structural ceramics (e.g., Si3N4, Al2O3), microstructural changes that increase strength decrease fracture toughness. In contrast, the strength and toughness of ZrB2-SiC were increased simultaneously. Processing conditions were varied to produce ceramics with controlled grain sizes. Measured strength and toughness values were then used to calculate the size of strength-limiting flaws. Analysis revealed that the maximum size of SiC grains was nearly identical to the predicted size of strength-limiting flaws. Thus, the strength of ZrB2-SiC increased as the size of the SiC grains decreased. Likewise, crack deflection modeling showed that the energy required to propagate cracks in ZrB2-SiC increased as the SiC particle size decreased.

ZrB2-SiC ceramics exhibit a synergistic increase in both strength and toughness as SiC particle size decreases.

Calculated critical flaw size is nearly identical to the size of the largest SiC particles. In addition, ZrB2-SiC shows transgranular crack propagation with crack deflection near SiC particles.

SiCZrB2

5 µm

Structure-Property Relations in Ultra-High Temperature CeramicsDr. Bill Fahrenholtz, DMR 0346800

Department of Materials Science and Engineering, University of Missouri-Rolla

A collaboration has been initiated between the UMR team and the group of Dr. Jochen Marschall at SRI International to investigate the evolution of damage due to high velocity impacts in ultra-high temperature ceramics. UMR student Stewart Henderson has prepared ZrB2-SiC disks that will be impacted with WC spheres at SRI by summer intern Kara Mann. Stewart will test retained strength and compare it to output of his model that predicts damage evolution. Once validated, the model will allow us to understand, for the first time, which of fundamental material properties (i.e., elastic constants, mechanical strength, hardness, fracture toughness, or some combination) influence on damage evolution during high velocity impacts.

A first generation finite element model has been developed to predict damage evolution during high velocity impacts. Modeling suggests that a threshold impact energy exists. Below the threshold, (a) strength will not be affected significantly by the elastic collision. However, above it, (b) the impact energy is transferred to the substrate producing both cone and radial cracks. Initial results have shown that complex physical processes (e.g., the full stress tensor response to the applied compressive load) dominate the response to high strain rate impacts. Subsequent models will be expanded to better predict the location and extent of damage as well as its influence on properties.

(a) (b)