SUPER HARDENING OF W/NbN NANOLAYERS UNDER SHALLOW NANOINDENTATION
Superlattice materials are nanocomposites that exhibit a hardness at small bilayer repeat periods which exceeds the hardness predicted by the rule of mixtures for composites. The objective of this investigation was to utilize the experimental data obtained from nanoindentations and image scanning to examine the behavior of the superlattice material, W/NbN. Nanoindentations and in situ surface imaging were conducted over a range of applied loads on samples of W/NbN with two different bilayer periods (¼=5.6 nm and ¼=10.4 nm), and monolithic samples of the niobium nitride (NbN) ceramic and the tungsten (W) metal which comprise the superlattice material. Additional shallow nanoindentations were made to a depth equal to the individual layer thicknesses of the nanocomposites. The mechanical properties were determined using the Oliver and Pharr method and compared for all the samples. The load versus displacement curves were also compared. The energies of indentation were calculated. The characteristics of the material pile-up resulting from the nanoindentations are determined from the scanned surface images. The experimental results are discussed to evaluate the influence of the different factors to the increase in hardness. The results indicate that the elastic modulus does not influence the hardness of the superlattice materials. The hardness and load versus displacement curves for the shallow indentations show little difference in behavior between NbN sample and the two superlattice materials. However, an increase in hardness is observed in the superlattice materials at deeper indentation depths. The results indicate that this increase in hardness is related to the nature of the interface between the layers in the superlattice materials.
Advisor:Dr. Scott X. Mao; Dr. Michael R. Lovell; Dr. William S. Slaughter
School:University of Pittsburgh
School Location:USA - Pennsylvania
Source Type:Master's Thesis
Date of Publication:12/02/2002