Kathryn J. Wahl

Nanoindentation and contact stiffness measurement using force modulation with a capacitive load-displacement transducer.

We have implemented a force modulation technique for nanoindentation using a three-plate capacitive load-displacement transducer. The stiffness sensitivity of the instrument is ∼0.1 N/m. We show that the sensitivity of this instrument is sufficient to detect long-range surface forces and to locate the surface of a specimen. The low spring mass (236 mg), spring constant (116 N/m), and damping coefficient (0.008 Ns/m) of the transducer allows measurement of the damping losses for nanoscale contacts. We present the experimental technique, important specimen mounting information, and system calibration for nanomechanical property measurement.

Quantitative imaging of nanoscale mechanical properties using hybrid nanoindentation and force modulation

In this article, we present a quantitative stiffness imaging technique and demonstrate its use to directly map the dynamic mechanical properties of materials with nanometer-scale lateral resolution. For the experiments, we use a “hybrid” nanoindenter, coupling depth-sensing nanoindentation with scanning probe imaging capabilities. Force modulation electronics have been added, enhancing instrument sensitivity and enabling measurements of time dependent materials properties (e.g., loss modulus and damping coefficient) not readily obtained with quasi-static indentation techniques. Tip–sample interaction stiffness images are acquired by superimposing a sinusoidal force (∼1 μN) onto the quasi-static imaging force (1.5–2 μN), and recording the displacement amplitude and phase as the surface is scanned. Combining a dynamic model of the indenter (having known mass, damping coefficient, spring stiffness, resonance frequency, and modulation frequency) with the response of the tip–surface interaction, creates maps o...

Characterization of the Adhesive Plaque of the Barnacle Balanus amphitrite: Amyloid-Like Nanofibrils Are a Major Component

The nanoscale morphology and protein secondary structure of barnacle adhesive plaques were characterized using atomic force microscopy (AFM), far-UV circular dichroism (CD) spectroscopy, transmission Fourier transform infrared (FTIR) spectroscopy, and Thioflavin T (ThT) staining. Both primary cement (original cement laid down by the barnacle) and secondary cement (cement used for reattachment) from the barnacle Balanus amphitrite (= Amphibalanus amphitrite) were analyzed. Results showed that both cements consisted largely of nanofibrillar matrices having similar composition. Of particular significance, the combined results indicate that the nanofibrillar structures are consistent with amyloid, with globular protein components also identified in the cement. Potential properties, functions, and formation mechanisms of the amyloid-like nanofibrils within the adhesive interface are discussed. Our results highlight an emerging trend in structural biology showing that amyloid, historically associated with disease, also has functional roles.

Wear behavior of Pb–Mo–S solid lubricating coatings

Abstract Amorphous Pb–Mo–S coatings 200 to 510 nm thick were deposited by dual ion-beam deposition (IBD) onto steel and Si substrates. Coating wear studies were performed using ball-on-flat reciprocating sliding with steel ball counterfaces in dry air. Tests were run between 1 and 100,000 sliding cycles, and wear depths measured by interference microscopy. Morphology and chemistry of the as-deposited coatings and worn surfaces were investigated with optical microscopy, micro-Raman spectroscopy and cross-section high resolution transmission electron microscopy (HRTEM). Pb–Mo–S coatings were found to be quite wear resistant; no more than 25% of the coating thickness was removed by 10,000 sliding cycles. Two wear mechanisms were identified. At the nanometer scale, wear proceeded in a two-part process: transformation of the coating surface to MoS 2 , then layer-by-layer removal of MoS 2 . At the micrometer scale, wear occurred by plowing. The long endurance of Pb–Mo–S coatings was attributed to slow wear of the coatings, with lubricant redistribution processes playing a minor role.

Quantification of a Lubricant Transfer Process that Enhances the Sliding Life of a MoS2 Coating

A lubricant transfer process that enhanced the wear life of a MoS2 coating has been identified and quantified. A steel ball sliding against a coated steel flat in reciprocating motion produced reservoirs at the turnaround part of the track ends, then emptied them, to provide replenishment similar to what is expected of liquid lubricants. The dynamics of the process were inferred from measurements of material loss and/or buildup in the track and on the ball; measurements were performed with Michelson interferometry and energy dispersive X-ray spectroscopy.

Barnacle cement: a polymerization model based on evolutionary concepts

SUMMARY Enzymes and biochemical mechanisms essential to survival are under extreme selective pressure and are highly conserved through evolutionary time. We applied this evolutionary concept to barnacle cement polymerization, a process critical to barnacle fitness that involves aggregation and cross-linking of proteins. The biochemical mechanisms of cement polymerization remain largely unknown. We hypothesized that this process is biochemically similar to blood clotting, a critical physiological response that is also based on aggregation and cross-linking of proteins. Like key elements of vertebrate and invertebrate blood clotting, barnacle cement polymerization was shown to involve proteolytic activation of enzymes and structural precursors, transglutaminase cross-linking and assembly of fibrous proteins. Proteolytic activation of structural proteins maximizes the potential for bonding interactions with other proteins and with the surface. Transglutaminase cross-linking reinforces cement integrity. Remarkably, epitopes and sequences homologous to bovine trypsin and human transglutaminase were identified in barnacle cement with tandem mass spectrometry and/or western blotting. Akin to blood clotting, the peptides generated during proteolytic activation functioned as signal molecules, linking a molecular level event (protein aggregation) to a behavioral response (barnacle larval settlement). Our results draw attention to a highly conserved protein polymerization mechanism and shed light on a long-standing biochemical puzzle. We suggest that barnacle cement polymerization is a specialized form of wound healing. The polymerization mechanism common between barnacle cement and blood may be a theme for many marine animal glues.

Low-friction, high-endurance, ion-beam-deposited Pb-Mo-S coatings

Abstract Thin solid lubricating coatings of PbMoS were deposited on steel substrates via ion-beam deposition. Coating endurance and friction coefficients under dry air sliding conditions were monitored with ball-on-disk tests; additional tribological testing was performed using a ball-on-flat reciprocating test rig to investigate intermediate sliding distances (100–32000 cycles). Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD), scanning Auger microscopy and micro-Raman spectroscopy were used to examine the structure, composition and chemistry of the coatings. Worn surfaces were characterized by optical microscopy and micro-Raman spectroscopy. The average endurance (at 1.4 GPa stress) of ion-beam-deposited (IBD) PbMoS coatings (thickness, 160–830 nm) containing 4–26 at.% Pb was 160000 revolutions, more than twice that of MoS2 coatings obtained by ion-beam-assisted deposition. In addition, the IBD PbMoS coatings had friction coefficients between 0.005 and 0.02, similar to the MoS2 coatings obtained by ion-beam-assisted deposition. Friction coefficients were monitored as a function of the contact stress and found to obey the hertzian contact model; measured interfacial shear strengths (S0 ≈ 12 MPa) were similar to those observed for MoS2 coatings. Although XRD and micro-Raman spectroscopy indicated that the IBD PbMoS coatings were initially amorphous, micro-Raman spectroscopy showed that crystalline MoS2 was produced both in the wear tracks on coatings and in the transfer films on balls after as few as 100 sliding cycles. The wear resistance and low-friction properties of IBD PbMoS coatings are attributed to the combination of dense, adherent coatings and the formation of easily sheared, MoS2-containing sliding surfaces.

A triboscopic investigation of the wear and friction of MoS2 in a reciprocating sliding contact

Reciprocating sliding tests with ball-on-flat geometry were performed on a duplex coating at low speeds in moist air (RH=60%). The coating, 55 nm MoS2 on 35 nm of TiN, was deposited by ion-beam assisted deposition onto a steel substrate. Friction coefficient ( μ) and electrical contact resistance (Rc) measurements were recorded at ~2 μm intervals along the track; these spatially resolved measurements were compared to the more commonly presented cycle-averaged values. The last-cycle tracks of several runs were also analyzed by a variety of microscopies and spectroscopies to identify compositions and determine thicknesses of films on the tracks and balls. Rc measurements, both averaged and spatially resolved, were more sensitive to coating damage and loss than μ measurements. In the averaged data, fluctuations in R c were observed before fluctuations in µ. Spatially resolved data showed that local drops in R c could be detected as early as 20% of life. Additionally, recovery of both high µ and low R c regions, interpreted as healing of damage in the contact, occurs. Friction coefficient data were insensitive to changes in MoS 2 coating thickness; conversely, Rc followed wear track thickness and consequently may provide an in situ method of monitoring coating wear.

Measurement of Contractile Stress Generated by Cultured Rat Muscle on Silicon Cantilevers for Toxin Detection and Muscle Performance Enhancement

Background To date, biological components have been incorporated into MEMS devices to create cell-based sensors and assays, motors and actuators, and pumps. Bio-MEMS technologies present a unique opportunity to study fundamental biological processes at a level unrealized with previous methods. The capability to miniaturize analytical systems enables researchers to perform multiple experiments in parallel and with a high degree of control over experimental variables for high-content screening applications. Methodology/Principal Findings We have demonstrated a biological microelectromechanical system (BioMEMS) based on silicon cantilevers and an AFM detection system for studying the physiology and kinetics of myotubes derived from embryonic rat skeletal muscle. It was shown that it is possible to interrogate and observe muscle behavior in real time, as well as selectively stimulate the contraction of myotubes with the device. Stress generation of the tissue was estimated using a modification of Stoneys equation. Calculated stress values were in excellent agreement with previously published results for cultured myotubes, but not adult skeletal muscle. Other parameters such as time to peak tension (TPT), the time to half relaxation (½RT) were compared to the literature. It was observed that the myotubes grown on the BioMEMS device, while generating stress magnitudes comparable to those previously published, exhibited slower TPT and ½RT values. However, growth in an enhanced media increased these values. From these data it was concluded that the myotubes cultured on the cantilevers were of an embryonic phenotype. The system was also shown to be responsive to the application of a toxin, veratridine. Conclusions/Significance The device demonstrated here will provide a useful foundation for studying various aspects of muscle physiology and behavior in a controlled high-throughput manner as well as be useful for biosensor and drug discovery applications.

Ion Beam Deposited Cu-Mo Coatings as High Temperature Solid Lubricants

Thin coatings of Cu-Mo were deposited on alumina substrates via ion-beam deposition (IBD). Structure and composition of the coatings were examined using X-ray diffraction, X-ray photoelectron spectroscopy and micro-Raman spectroscopy. Sliding tests of coated and uncoated substrates were performed in air under reciprocating sliding conditions against alumina ball counterfaces; mean Hertzian contact pressures were 1.0 and 1.6 GPa, and test temperatures were between 25 and 650 °C. The as-deposited coatings were metallic and amorphous, but after exposure to temperatures ±530 °C, they converted to oxides containing CuMoO4 and MoO3. With increasing temperature, the friction coefficients of the IBD Cu-Mo coatings decreased from ∼0.5 to ∼0.2. At high temperature, the coatings were capable of sustaining low friction sliding (μ0.2) for 3000 cycles. At high load and temperature, the ball wear coefficient against the IBD Cu-Mo-coated alumina was 20–100 times less than that of uncoated alumina. The low friction observed at high temperatures was attributed to transformation of the coatings to crystalline oxides, which are known to be lubricious at elevated temperatures.