Model of superlattice yield stress and hardness enhancements

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Abstract
A model is presented that explains the yield stress and hardness enhancements that have been observed in superlattice thin films. The stress required for dislocations to glide across layers with different shear moduli was calculated using an expression that accounts for core effects and all interfaces in trapezoidal or sawtooth composition modulations. The predicted strength/hardness enhancement increased with increasing superlattice period Λ, before reaching a saturation value that depended on interface widths. A second mechanism, where dislocations glide within individual layers, was important at large Λ and gave a decrease in strength/hardness with increasing Λ. The combination of these two mechanisms gives a strength/hardness maximum versus Λ in good quantitative agreement with experimental results for nitride and metal superlattices. The results indicate that superlattice strength/hardness depends strongly on interface widths and the difference in shear moduli of the two components for Λ values below the maximum, and on the average shear modulus for larger Λ.