Improved strength scaling for large ALON windows

Abstract

ALON® Transparent Ceramic (ALON) consists primarily of aluminum and oxygen, similar to that of alumina, with a small amount of nitrogen added to help stabilize its cubic phase. Importantly, materials with cubic symmetry are optically isotropic, and consequently, transparent in their polycrystalline form. This allows ALON to be manufactured by conventional powder processing methods. ALON is deployed in several Vis-MWIR Sensor Window applications and provides outstanding environmental durability. Further, ALON windows are available in very large sizes as required for Defense reconnaissance systems. The conventional approach for predicting how the strength of a ceramic material scales from the small strength coupons (~1” diameter), to full sized sensor windows, which may be orders of magnitude larger, is to use Weibull scaling as is described by Harris et al in reference 1. Weibull scaling assumes that as the window gets larger and larger, the strength controlling flaws get larger too. However, this ignores the microstructure of the window material, and the role that the microstructure may have in limiting the size of strength controlling flaws. All materials, amorphous, single crystal and polycrystalline are therefore treated as equivalent. Jeff Swab et al2 measured strength in ALON samples over a range of sizes. The largest strength samples measured were <11-in diameter. These ALON samples were purchased from Surmet with our standard commercial polish. We have the average strength and Weibull modulus data for samples produced with the same surface. Scaling the strength from coupons (~350MPa, and Weibull modulus m=3.11) to the largest samples broken by Swab et.al, predicts a strength of only 53 MPa. However, the actual strengths measured by Swab et.al. was 152+/-28 MPa, 3x higher than predicted. The potential role of ALON’s microstructure in this higher than predicted strength will be discussed, and future experiments will be proposed to determine an improved approach to scaling strength with area.