Timothy P. Weihs

Nanoindentation mapping of the mechanical properties of human molar tooth enamel

The mechanical behavior of dental enamel has been the subject of many investigations. Initial studies assumed that it was a more or less homogeneous material with uniform mechanical properties. Now it is generally recognized that the mechanical response of enamel depends upon location, chemical composition, and prism orientation. This study used nanoindentation to map out the properties of dental enamel over the axial cross-section of a maxillary second molar (M(2)). Local variations in mechanical characteristics were correlated with changes in chemical content and microstructure across the entire depth and span of a sample. Microprobe techniques were used to examine changes in chemical composition and scanning electron microscopy was used to examine the microstructure. The range of hardness (H) and Youngs modulus (E) observed over an individual tooth was found to be far greater than previously reported. At the enamel surface H>6GPa and E>115GPa, while at the enamel-dentine junction H<3GPa and E<70GPa. These variations corresponded to the changes in chemistry, microstructure, and prism alignment but showed the strongest correlations with changes in the average chemistry of enamel. For example, the concentrations of the constituents of hydroxyapatite (P(2)O(5) and CaO) were highest at the hard occlusal surface and decreased on moving toward the softer enamel-dentine junction. Na(2)O and MgO showed the opposite trend. The mechanical properties of the enamel were also found to differ from the lingual to the buccal side of the molar. At the occlusal surface the enamel was harder and stiffer on the lingual side than on the buccal side. The interior enamel, however, was softer and more compliant on the lingual than on the buccal side, a variation that also correlated with differences in average chemistry and might be related to differences in function.

Mechanical deflection of cantilever microbeams: A new technique for testing the mechanical properties of thin films

The mechanical deflection of cantilever microbeams is presented as a new technique for testing the mechanical properties of thin films. Single-layer microbeams of Au and SiO 2 have been fabricated using conventional silicon micromachining techniques. Typical thickness, width, and length dimensions of the beams are 1.0,20, and 30 μm, respectively. The beams are mechanically deflected by a Nanoindenter, a submicron indentation instrument that continuously monitors load and deflection. Using simple beam theory and the load-deflection data, the Youngs moduli and the yield strengths of thin-film materials that comprise the beams are determined. The measured mechanical properties are compared to those obtained by indenting similar thin films supported by their substrate.

Hardness and young's modulus of human peritubular and intertubular dentine

A specially modified atomic-force microscope was used to measure the hardness of fully hydrated peritubular and intertubular dentine at two locations within unerupted human third molars: within 1 mm of the dentine enamel junction and within 1 mm of the pulp. The hardness of fully hydrated peritubular dentine was independent of location, and ranged from 2.23 to 2.54 GPa. The hardness of fully hydrated intertubular dentine did depend upon location, and was significantly greater near the dentine enamel junction (values ranged from 0.49 to 0.52 GPa) than near the pulp (0.12-0.18 GPa). A Nanoindenter was used to estimate the Youngs modulus of dehydrated peritubular and intertubular dentine from the unloading portion of the load displacement curve. The modulus values averaged 29.8 GPa for the peritubular dentine (considered to be a lower limit), and ranged from 17.7 to 21.1 GPa for the intertubular dentine, with the lower values obtained for dentine near the pulp.

Effect of intermixing on self-propagating exothermic reactions in Al/Ni nanolaminate foils

Exothermic reactions can self-propagate rapidly in multilayered foils, and the properties of these reactions depend strongly on the heat of reaction, the average atomic diffusion distance, and the degree of intermixing at the layer interfaces prior to ignition. By performing low-temperature anneals on sputter-deposited Al/Ni nanolaminate foils, the thickness of the intermixed region between layers was increased and its effects on the heats and velocities of reactions were measured. The intermixed region consisted of the metastable Al9Ni2 phase while the final phase of the foil was Al3Ni2. Analytical and empirical models were used to predict reaction velocities as a function of bilayer thickness and intermixing thickness, and the predictions are in good agreement with the experimental results. Increasing the average thickness of the intermixed region from 2.4 to 18.3 nm reduced the reaction velocity for all of the foils but was most significant for the foils with bilayer thicknesses less than 25 nm. The re...

Grain boundary accommodation of slip in Ni3Al containing boron

Abstract Experiments at room temperature have established that the addition of 750 ppm by weight (0.35 at.%) of boron to stoichiometric Ni3Al reduces the effectiveness with which grain boundaries strengthen the alloy. This effect leads to boron-induced weakening of the most finely grained aggregates (d ≲ 10 μm). The effect of boron is explained in terms of an increase in the mobility of grain boundary dislocations, and is related to boron-induced ductility.

Room-temperature soldering with nanostructured foils

Self-propagating formation reactions in nanostructured multilayer foils provide rapid bursts of heat and can act as local heat sources to melt solder layers and join materials. This letter describes the room-temperature soldering of stainless steel specimens using freestanding, nanostructured Al/Ni foils. The products, heats, and velocities of the reactions are described, and the microstructure and the mechanical properties of the resulting joints are characterized. A tensile shear strength of 48 MPa was measured for the reactive foil joints, compared to 38 MPa for conventional joints. Both numerical predictions and infrared measurements show limited heat exposure to the components during reactive joining.

Joining of stainless-steel specimens with nanostructured Al/Ni foils

We describe the joining of stainless-steel specimens at room temperature using free-standing Al/Ni foils as local heat sources for melting AuSn solder layers. The foils contain many nanoscale layers of Al and Ni that react exothermically, generating a self-propagating reaction. The heats, velocities, and products of the reactions are described, and the microstructure and the mechanical properties of the resulting joints are characterized. Increasing the foil thickness, and thereby increasing the total heat released, can improve the strength of the joints until foil thickness reaches 40 μm. For thicker foils, the shear strength is almost constant at 48 MPa, compared to 38 MPa for conventional solder joints. The higher strength is due to finer microstructures in the solder layers of reactive joints. A numerical study of heat transfer during reactive joining and experimental results suggest that the solder layers need to melt completely and remain molten for at least 0.5 ms to form a strong joint.

Modeling and characterizing the propagation velocity of exothermic reactions in multilayer foils

Combustible multilayer foils can be fabricated by sputter depositing alternate layers of materials which react exothermically during thermally induced intermixing. Current models for these reactions consider pure materials which only intermix during the self-propagating stage of the reaction, though in reality during fabrication the materials undergo partial intermixing. An analytical model dealing with the premixing is presented and compared with experimental results for Al/Ni and Al/(Ni:Cu) multilayers. The model and the results indicate that premixing lowers the propagation velocity both by slowing the rate of atomic diffusion between layers and by lowering the temperature of the reaction. The lower temperature can cause solid/liquid phase changes to dominate the reaction path. It is concluded that to use these foils in commercial and engineering applications, the method of fabrication and the phase changes occurring during the reaction must be controlled to give the desired characteristics.

Investigating the thermodynamics and kinetics of thin film reactions by differential scanning calorimetry

In this paper we demonstrate the utility of differential scanning calorimetry for investigating the thermodynamics and kinetics of a broad range of thin film reactions. We begin by describing differential scanning calorimeters and the preparation of thin film samples. We then cite a number of examples that illustrate how enthalpies of crystallization, heats of formation and enthalpies of interfaces can be measured using layered thin films of Ni/Al, Cu/Zr and Zr/Al and homogeneous thin films of Co-Si, Nb-Cu, Cr-Cu and Ge-Sn. Following these examples of thermodynamic measurements, we show how kinetic parameters of nucleation, growth and coarsening can also be determined from differential scanning calorimetry traces using layered thin films of Ni/Al, Ti/Al and Nb/Al and homogenous thin films of Co-Si and Ge-Sn. The thermodynamic and kinetic investigations highlighted in these examples demonstrate that one can characterize phase transformations that are relevant to commercial applications and scientific studies both of thin films and of bulk materials.

Reactive nanostructured foil used as a heat source for joining titanium

We have joined titanium alloy (Ti-6Al-4V) specimens at room temperature and in air by using free-standing nanostructured Al∕Ni multilayer foils to melt a silver-based braze. The foils are capable of undergoing self-sustaining exothermic reactions and thus act as controllable local heat sources. By systematically controlling the properties of the foils and by numerically modeling the reactive joining process, we are able to conclude that the temperatures reached by the foils during reaction are critical in determining the success of joining when using higher melting temperature braze layers.