David F. Bahr

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.

Wear behavior of Au–ZnO nanocomposite films for electrical contacts

Electrical contact switches require low contact resistance for efficient passage of signals, while withstanding repetitive cycling. Hard gold with alloy additions of Ni, Co, or Ag can increase the wear resistance of Au films, however, this causes a significant decrease in conductivity and alloying elements can segregate during long-term aging leading to property evolution. The current work demonstrates that Au–zinc oxide (ZnO) nanocomposites can create a hard Au coating with a uniform, stable structure under frictional loading. Addition of ZnO particles decreases the grain size and texture of the film by 35 and 40–75xa0%, respectively, indicating a change in growth behavior of the film. The nanoindentation hardness increased directly with increasing ZnO concentration. Atomic force microscopy examination of wear-tested films demonstrated morphological stability after frictional contact and thus showed the potential for these films to replace current hard Au used on contact terminals.

Effect of accelerated aging on dental zirconia-based materials

OBJECTIVEnTo investigate the effect of aging on phase transformation and mechanical properties of yttria-tetragonal zirconia polycrystal (Y-TZP).nnnMATERIALS AND METHODSnFully-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).nnnRESULTSnMaterial 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.nnnSIGNIFICANCEnFlexural 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.

Layer thickness dependent strain rate sensitivity of Cu/amorphous CuNb multilayer

Strain rate sensitivity of crystalline materials is closely related to dislocation activity. In the absence of dislocations, amorphous alloys are usually considered to be strain rate insensitive. However, the strain rate sensitivity of crystalline/amorphous composites is rarely studied, especially at nanoscale. In this study, we show that the strain rate sensitivity of Cu/amorphous CuNb multilayers is layer thickness dependent. At small layer thickness (below 50u2009nm), the multilayers demonstrate limited strain rate sensitivity; at relatively large layer thickness (above 100u2009nm), the strain rate sensitivity of multilayers is close to that of the single layer Cu film. Mechanisms that lead to size dependent variation of strain rate sensitivity in these multilayers are discussed.

Coherent Interfaces Increase Strain-Hardening Behavior in Tri-Component Nano-Scale Metallic Multilayer Thin Films

Strain-hardening in tri-component nano-scale metallic multilayers was investigated using nanoindentation and micro-pillar compression. Cu/Ni/Nb films were made in tri-layer structures as well as bi-layers consisting of an alloy of Cu–Ni/Nb. Strain-hardening increases as the layer thickness decreases, with 5u2005nm layers exhibiting higher strengths and hardening coefficients than 30u2005nm layers. The experimental evidence is described in light of the confined layer slip model, and supports the hypothesis that coherent interfaces with a modulus mismatch in the tri-layer system are responsible for additional deformation mechanisms that can lead to hardening in excess of that found in bi-layer systems.

Nanomechanical testing technique for millimeter-sized and smaller molecular crystals

Large crystals are used as a control for the development of a mounting and nanoindentation testing technique for millimeter-sized and smaller molecular crystals. Indentation techniques causing either only elastic or elastic-plastic deformation produce similar results in assessing elastic modulus, however, the elastic indents are susceptible to surface angle and roughness effects necessitating larger sample sizes for similar confidence bounds. Elastic-plastic indentations give the most accurate results and could be used to determine the different elastic constants for anisotropic materials by indenting different crystal faces, but not by rotating the indenter about its axis and indenting the same face in a different location. The hardness of small and large crystals is similar, suggesting that defect content probed in this study is similar, and that small crystals can be compared directly to larger ones. The Youngs modulus and hardness of the model test material, griseofulvin, are given for the first time to be 11.5GPa and 0.4GPa respectively.

Effect of nanoscale patterned interfacial roughness on interfacial toughness.

The performance and the reliability of many devices are controlled by interfaces between thin films. In this study we investigated the use of patterned, nanoscale interfacial roughness as a way to increase the apparent interfacial toughness of brittle, thin-film material systems. The experimental portion of the study measured the interfacial toughness of a number of interfaces with nanoscale roughness. This included a silicon interface with a rectangular-toothed pattern of 60-nm wide by 90-nm deep channels fabricated using nanoimprint lithography techniques. Detailed finite element simulations were used to investigate the nature of interfacial crack growth when the interface is patterned. These simulations examined how geometric and material parameter choices affect the apparent toughness. Atomistic simulations were also performed with the aim of identifying possible modifications to the interfacial separation models currently used in nanoscale, finite element fracture analyses. The fundamental nature of atomistic traction separation for mixed mode loadings was investigated.

Dislocation Activity Under Nanoscale Contacts Prior to Discontinuous Yield

In nanoindentation, when stresses near the theoretical strength are reached, it is commonly assumed that the volume tested is dislocation free. This study examines the case where permanent deformation occurs prior to an apparent yield point. Force-modulated creep and quasi-static (QS) nanoindentation tests were conducted on Co, W, Ir, and Pt. Statistical comparisons show that the percentage of tests displaying creep correlates with the percentage of QS tests displaying plasticity prior to pop-in. This is evident that apparent plastic behavior prior to pop-in is due to dislocation motion, and permanent deformation can and does occur at extremely small indenter displacements before pop-in.

The effects of intrinsic properties and defect structures on the indentation size effect in metals

Abstract The indentation size effect has been linked to the generation of geometrically necessary dislocations that may be impacted by intrinsic materials properties, such as stacking fault energy, and extrinsic defects, such as statistically stored dislocations. Nanoindentation was carried out at room temperature and elevated temperatures on four different metals in a variety of microstructural conditions. A size effect parameter was determined for each material set combining the effects of temperature and existing dislocation structure. Extrinsic defects, particularly dislocation density, dominate the size effect parameter over those due to intrinsic properties such as stacking fault energy. A multi-mechanism description using a series of mechanisms, rather than a single mechanism, is presented as a phenomenological explanation for the observed size effect in these materials. In this description, the size effect begins with a volume scale dominated by sparse sources, next is controlled by the ability of dislocations to cross-slip and multiply, and then finally at larger length scales work hardening and recovery dominate the effect.

Discontinuous Yield Behaviors Under Various Pre-Strain Conditions in Metals with Different Crystal Structures

Instrumented indentation was performed to determine the statistics of discontinuous yield behavior of Co and Ni pre-strained to various levels. In both materials, increasing pre-strain decreases the frequency of indentations that exhibit discontinuous yield. Yield events in Co occurred at nominally the same stress independent of pre-strain, suggesting dislocation nucleation dominates during contact loading and that the existing dislocations are strongly pinned in Co. However, in Ni yield occurred at lower loads with increasing pre-strain, suggesting that dislocation activation, rather than true nucleation, dominates the yield mechanism. GRAPHICAL ABSTRACT