Using Plasticity Values Determined From Systematic Hardness Indentation Measurements for Predicting Impact Behavior in Structural Ceramics: A New, Simple Screening Technique

Report No. ARL-RP-268
Authors: James W. McCauley; Trevor E. Wilantewicz
Date/Pages: September 2009; 16 pages
Abstract: In general, it has long been known that the hardness of ceramics correlates with gross impact performance, however, not to a degree useful for materials development. Wilkins, Cline and Honodel, 1969, were the first to point out the apparent importance of ceramic "plasticity" or inelastic deformation mechanisms in BeO and AlN in impact performance. More recently, Lundberg et al., 2000, have made compelling arguments that the compressive yield strength (related to hardness) augmented by the amount of "plasticity" in ceramics correlates well to transitional velocities (dwell), i.e. the velocity (or impact pressure) where penetration begins. However, a direct measure of plasticity has not been determined. Hardness comparisons between materials are problematic since the values vary with the applied load, however, the full hardness-load curve can provide much more information on material behavior than hardness alone measured at a single load. In this work, several methods for curve fitting hardness-load data have been compared for both Knoop and Vickers hardness on several ceramic materials: aluminum oxynitride (AlON), silicon carbide, aluminum oxide and boron carbide. A power-law equation (H = kFc) is shown to fit the Knoop data quite well. A plot of log10 (HK) vs. log10 (F) yielded easily comparable straight lines, whose slope and intercept data might be useful parameters to characterize the materials. It is shown on a series of hot pressed SiC variants that the absolute value of the reciprocal of the slope is a measure of plasticity and that the sum of this value with the calculated Knoop hardness at 1 N is a useful parameter to predict impact transitional velocity.
Distribution: Approved for public release
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Last Update / Reviewed: September 1, 2009