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PURPOSE To evaluate the effect of ionizing irradiation on human enamel and zirconia after chewing simulation. METHODS Twenty enamel and twenty translucent Yttria-stabilized zirconia (Y-PSZ) specimens were divided in 4 groups: Co (control) - no irradiation on enamel cusps/opposing zirconia slabs; E70 - irradiated (70 Gray) enamel cusps/opposing irradiated enamel slabs; Z70 - irradiated zirconia cusps/opposing irradiated zirconia slabs; EZ70 - irradiated enamel cusps/opposing irradiated zirconia slabs. Cusps were abraded against slabs in a chewing simulator (CS - one million cycles, 80 N, artificial saliva, 37˚C). Wear and roughness of zirconia and enamel were analyzed using a stylus profilometer. The abraded enamel was analyzed by Electron probe micro-analyzer (EPMA) and zirconia was characterized by nanoindentation and X-ray diffraction. One-way analysis of variance (ANOVA) and Tukey test were used for analysis of wear, Repeated Measures and Bonferroni test for roughness, and hardness and modulus values were compared using Wilcoxan Mann Whitney rank sum test (overall 5% significance). RESULTS Significantly higher volume loss was presented by cusps in the E70 group (p

The authors demonstrate that multicomponent metallic alloy nanofoams can be synthesized by the polymeric templating method. The present approach enabled alloy compositions not accessible via commonly used dealloying or co-deposition methods. The authors report the synthesis of a Cu50Ni50 alloy nanofoam using electrospinning polymeric templating, which exhibits distinct polycrystallinity, process-driven segregation, and enhanced mechanical strength over pure Cu nanofoams. Transmission electron microscopy revealed microscopic grain formation and their variable compositions. The processing method is applicable to the synthesis of a wide range of multicomponent metal porous materials, creating new research opportunities for noble alloy foams not available through wet electrochemical routes.

Oxygen in titanium is a potent alpha stabilizer that provides solid solution strengthening. This study presents a method of exploiting surface hardening of titanium particulate media. Hardening was measured on particulate matter and compared to a bulk specimen with nanoindentation. Thermal exposure parameters increased surface hardness from ≈ 3 GPa to over 8 GPa as measured via nanoindentation. This process produced solid solution strengthening without creating a distinct oxide layer or significant sintering of particles.

Dual phase (DP) steels are broadly deployed by the automotive industry to enhance strength and ductility for light weighting. As DPs micro-constituents’ grain size is frequently on the same order of sizes probed with nanoindentation, an inverse technique was combined with nanoindentation load-depth curves to extract the stress-strain behavior of ferrite and martensite in DP1180. Determining the appropriate depth to evaluate the properties of the micro-constituent phases enabled computation of the flow behavior of each individual phase. Post indentation microscopy correlated the microstructure to the spatially resolved indentation arrays. The post-indentation impression was characterized to eliminate indentation size effects and probe bluntness influences. The yield strength of ferrite, martensite and DP1180 were determined to be 370, 948 and 836 MPa respectively while the corresponding strain hardening exponents (n) were computed via an inverse method as 0.175, 0.025 and 0.11. Mechanical properties obtained from the indentation method exhibited good agreement with the experimentally measured macroscopic yield stress (922 MPa) and hardening exponent (0.11) of DP1180.

Correlations between a materials microstructure and mechanical behavior are important for materials development. As materials characterization methods must consider instrument accessibility, sample dimensions and economical aspects, developing functional techniques in order to obtain better understanding of materials behavior in micro and nano scale is crucial. Procedures for assessing and interpreting the mechanical responses at small scales, combined with investigating the microstructure, are considered as significant steps to design and develop the effective frameworks for evaluating bulk properties. This research demonstrates how fundamental understanding of microstructures can assist interpreting of mechanical performance of bulk materials. Testing of materials at small scales is very important because the mechanical failure of any bulk material starts with the formation, extension, or local accumulation of initially small defects, leading finally to a catastrophic fracture by an expanding crack. Thus, any bulk material profits from an in-depth understanding of its deformation and mechanical phenomena at the nano- and micrometer length scale.This thesis shows how the micro constituents’ interactions and grain boundaries reactions to dislocations in alloys and thin films contribute to understanding material flow behavior and differences in the mechanical properties of these materials in a wide range of material systems with variations in appropriate sizes which need to be probed. Among other things, this work shows that sources of variation can be specified and quantified as predictive tools for designing materials. Several examples are presented. First, the strength and strain hardening of martensite and ferrite in a dual phase steel with a grain size less than 5 μm were determined using an inverse technique. The yield strength of the ferrite and martensite phases are obtained as 370 MPa and 950 MPa respectively. The calculated hardening exponent of the alloy was exactly the same as experimental tensile test results (0.11). The constraint phenomena was effective in restricting deformation of this elastic-plastic alloy. Secondly, the differences in hardness and pop-in behavior were used to understand of the influences of different types of grain boundaries, high density dislocations, and twins in Al thin films before and after plasticity. The third example assesses the strength of several species of diatom frustules for the first time using a combination of indentation techniques. Lightweight materials with densities well below 1000 kg/m3 demonstrated strengths on the order of 100’s of MPa. Finally, conditions for laser grown oxides and laser shock peening on a commercial steel which lead to an optical marking without a change in strength around the marking have been identified.

Graphene reinforced metal composite has been a promising materials with superior mechanical properties. Currently the mechanical properties of the additively manufactured metals are limited by the undesired microstructures and residual stress developed during the laser heating process. Strengthening in graphene/metal composite is limited to the intrinsic strength of graphene and its ability to block dislocation from propagation, which make it very difficult to introduce dislocation hardening and twin boundary strengthening. Here, a hybrid manufacturing process combining layer by layer laser deposition of graphene/metal nanocomposites and laser shock peening has been investigated through modeling and experiments. Strengthening of graphene/metal composites is introduced by the strong interactions between shock wave and selective laser sintered graphene/metal composite. Instead of constraining the dislocation motion, graphene acts as a shock-loading transferor to allow shock wave to pass through and bounce back between them, resulting in high density dislocations and nanotwinning structures around graphene/metal interface. Molecular dynamics (MD) simulation shows that the shock interaction with the graphene/metal interface generates dislocations pile-up in front of graphene and large stress intensity around the interface. Wave-like nanowrinkles in graphene are generated after laser shock loading because of interference wave propagation. Mechanical testing results showed that the laser shock treated graphene/metal composites enable ultra-stability of strength and compressive residual stress, and excellent fatigue performance. MD simulation revealed that shock wave strengthened graphene/metal interface significantly reduces the crack propagation rate and provides strong resistance to fatigue of metal/graphene composites.

A polycrystalline Cu foam with sub-micron ligament sizes was formed by creating a non-woven fabric via electrospinning with a homogeneous mixture of polyvinyl alcohol(PVA)-and copper acetate(Cu(Ac)2). Thermogravimetric measurement of the electrospun fabric of the precursor solution is reported. Oxidizing the precursor fabric at 773K formed an oxide nano-foam; subsequent heating at 573K with a reducing gas transformed the CuO nano-foam to Cu with a similar ligament and meso-scale pore size morphology. A cross-section prepared by focused ion beam lift-out shows the polycrystalline structure with multi-scale porosity. The mechanical property of the Cu nano-foam is measured by nano-indentation. The load-depth curves and deduced mechanical properties suggest that additional intra-ligament pores lead to unique structure-property relations in this non-conventional form of metal.

Composite nanostructured foams consisting of a metallic shell deposited on a polymeric core were formed by plating copper via electroless deposition on electrospun polycaprolactone (PCL) fiber mats. The final structure consisted of 1000-nm scale PCL fibers coated with 100s of nm of copper, leading to final core-shell thicknesses on the order of 1000-3000 nm. The resulting open cell, core-shell foams had relative densities between 4 and 15 %. By controlling the composition of the adjuncts in the plating bath, particularly the composition of formaldehyde, the relative thickness of copper coating as the fiber diameter could be controlled. As-spun PCL mats had a nominal compressive modulus on the order of 0.1 MPa; adding a uniform metallic shell increased the modulus up to 2 MPa for sub-10 % relative density foams. A computational materials science analysis using density functional theory was used to explore the effects pre-treatment with Pd may have on the density of nuclei formed during electroless plating.

Density functional theory was used to predict the diffusion and precipitation of Cr in Cu. The energy barrier of Cr diffusion in Cu is comparable to the self-diffusion of Cr, and higher than the energy barrier of Cu self-diffusion. The simulations support the experimentally measured hardness of 30 nm thick nanolaminate layers of Cr/Cu-3.4%Cr. As-deposited films had a hardness of 6.25 GPa; annealing at 373 K decreased hardness to 5.95 GPa, while annealing at 573 K increased the hardness to 6.6 GPa. Transmission electron microscopy indicates there is significant local strain due to precipitation, in agreement with theoretical predictions.

In the present article, α-Al2O3 nano powder was synthesized by a simple aqueous sol–gel method by using AlCl3 as precursor. It was shown that the gel calcined at 1000°C, 1100 °C and 1200 °C resulted in the formation of a crystalline α-Al2O3 nano powder. In continue TEOS and saccharose was used to prepare SiO2 xerogels containing carbon nano particles. The conversion of the gel to β-SiC nano powders was accomplished by carbothermal reduction at 1500°C for 1 h in argon atmosphere. In second pace alumina matrix composites with nano particles of 5 vol% SiC were prepared with the addition of TiO2 as sintering aid and densified by pressureless sintering method at 1600°C and 1630°C for 2 h in nitrogen atmosphere. Green pellets were obtained by uniaxial pressing of 137 MPa. Maximum density (97.3%) was achieved at 1630°C. Vickers hardness was 16.5 GPa after sintering at 1630°C. Scanning electron microscopy revealed that the SiC particles were well distributed throughout the composite matrix. The precursors and the resultant powders were characterized by X-ray diffraction (XRD), thermal analysis (STA) and scanning electron microscopy (SEM).

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 50 nm), the multilayers demonstrate limited strain rate sensitivity; at relatively large layer thickness (above 100 nm), 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.