Samantha K. Lawrence

Enhanced hardness in epitaxial TiAlScN alloy thin films and rocksalt TiN/(Al,Sc)N superlattices

High hardness TiAlN alloys for wear-resistant coatings exhibit limited lifetimes at elevated temperatures due to a cubic-AlN to hexagonal-AlN phase transformation that leads to decreasing hardness. We enhance the hardness (up to 46 GPa) and maximum operating temperature (up to 1050 °C) of TiAlN-based coatings by alloying with scandium nitride to form both an epitaxial TiAlScN alloy film and epitaxial rocksalt TiN/(Al,Sc)N superlattices on MgO substrates. The superlattice hardness increases with decreasing period thickness, which is understood by the Orowan bowing mechanism of the confined layer slip model. These results make them worthy of additional research for industrial coating applications.

Effect of accelerated aging on dental zirconia-based materials

OBJECTIVE To investigate the effect of aging on phase transformation and mechanical properties of yttria-tetragonal zirconia polycrystal (Y-TZP). MATERIALS AND METHODS Fully-sintered Y-TZP slabs, IPS E-max ZirCAD (ZC - Ivoclar) and Z-5 ceramic (Z5 - C5 Medical Werks), were artificially aged in autoclave for: 0, 30, 60 or 90min. Flexural strength (FS), crystalline changes (X-ray diffraction analysis - XRD) and surface topography were analyzed. 0 and 90min-aged samples were evaluated by nanoindentation to measure hardness and modulus, and results were compared using Wilcoxan Mann Whitney rank sum test (p≤0.05). FS results were compared using two-way ANOVA and Tukey HSD (α=0.05). RESULTS Material factor had significant effect (p=0.001) on flexural strength (Z5=966.95MPa; ZC=847.82MPa), but aging did not. Nanoindentation showed incidence of typical load/depth curves combined with some exhibiting features compatible with cracking. When typical curves were considered, aging had no effect on the modulus and hardness, but hardness was dependent on material type. A steady increase in the m phase related to aging time was observed for ZC samples. The maximum incidence of m phase was 6.56% for Z5/60min. SIGNIFICANCE Flexural strength is not affected by surface transformation in dental Y-TZP. Hydrothermal aging has an effect on m content and surface topography of different zirconia brands, but mechanical tests that can precisely characterize surface changes in aged Y-TZP are still missing.

Statistical Quantification of the Impact of Surface Preparation on Yield Point Phenomena in Nickel

Nanoindentation was used to evaluate the effect of three surface preparation techniques—mechanical polishing, electropolishing, and ion polishing—on experimental measurements of incipient plasticity in commercially pure Ni 200. Surface preparation techniques are linked to defect densities, estimated with image quality (IQ) and kernel average misorientation (KAM) data obtained from electron backscatter diffraction patterns and the Taylor relation. Minimum yield pressures are insensitive to surface preparation, while mean yield pressure depends on dislocation density, and the maximum yield pressure is likely influenced by defects other than dislocations. KAM coupled with IQ may be a useful non-destructive parameter to relate surface defect density to the resulting changes in the spatial variability of incipient plasticity during a nanoindentation experiment. This analysis makes the assumption that geometrically necessary dislocation density is proportional to total dislocation density; in cases where this condition is not satisfied, the KAM analysis may not be valid.

Investigating Dislocation-Twin Boundary Interactions in Nickel using Diffraction Contrast Scanning Transmission Electron Microscopy

The interaction of dislocations with and near grain boundaries plays an important role in the strain response of metals. Whether a dislocation is blocked or transmitted at a grain boundary is dependent on a number of inter-related factors including the alignment of slip systems, the inclination of the interface, and the nature of the defect transition states within the interface itself. In this presentation we consider the morphology and character of dislocations interacting with {111} and {112} twin boundary facets in Ni. Our work is motivated in part by interest in explaining the influence of twin boundaries on the fracture behavior of metals exposed to hydrogen environments. For instance, Bechtle et al. have reported a reduced susceptibility to intergranular hydrogen embrittlement in Ni processed to have a high fraction of Σ3 twins and related Σ3 special boundaries [1]. However, analyses have also shown that although they are resistant to crack propagation, twins can still be sites of preferential crack initiation during loading in the presence of hydrogen [2], raising questions regarding the precise mechanisms of strain localization at such interfaces and how these are changed with exposure to hydrogen.