Measuring the elastic and plastic properties with nanoindentation is predicated on the indentation not fracturing the material. In this study, an unloading curve analysis is used to identify indentation-induced fracture in brittle molecular organic crystals to define conditions, where properties measurements are accurate, and for calculating the toughness. Single crystals of cyclotetramethylene tetranitramine (HMX) and idoxuridine were indented from 1 to 300 mN with indenter probes of varying acuity to identify fracture initiation loads. Idoxuridine displayed no fracture up to and at 100 mN, with fracture occurrence then seen at an increasing rate until every indentation made induced fracture at 300 mN. HMX displayed no fracture up to and at 4 mN, with fracture then occurring at an increasing rate until every sample fractured at 8 mN. The toughness of HMX and idoxuridine is ≈0.28 ≈ 0.4–0.5 MPa/m1/2, respectively.

Organic molecular crystals are often noncubic and contain significant steric hindrance within their structure to resist dislocation motion. Plastic deformation in these systems can be imparted during processing (tableting and comminution of powders), and the defect density impacts subsequent properties and performance. This study measured the elastic and plastic properties of representative monoclinic, orthorhombic, and triclinic molecular crystalline structures using nanoindentation of as-grown sub-mm single crystals. The variation in modulus due to in-plane rotational orientation, relative to a Berkovich tip, was approximately equal to the variation of a given crystal at a fixed orientation. The onset of plasticity occurs consistently at shear stresses between 1 and 5% of the elastic modulus in all three crystal systems, and the hardness to modulus ratio suggests conventional Berkovich tips do not generate fully self-similar plastic zones in these materials. This provides guidance for mechanical models of tableting, machining, and property assessment of molecular crystals.

Teflon amorphous fluoropolymer (TAF) multi-walled carbon nanotube (MWCNT) suspensions have the potential for creating conductive coatings on insulating films for numerous applications. However, there are few studies on polymer MWCNT suspension properties and even fewer that use Teflon. To define mechanical and electrical property relationships, bilayer films of TAF-MWCNT were created with differing concentrations of MWCNTs. Nanoindentation revealed that addition of 8 wt% MWCNTs to TAF increased the elastic modulus by about 25% and hardness by about 15%. Conducting indentation showed 8 wt% MWCNT films exhibit uniform stable conductance once indentation depth exceeds several hundred nanometers. Films with lower concentrations of CNTs were insulating. The two techniques provide a unique description of structure property relationships in this suspension film system.

Localized heating of metals and alloys using a focused laser beam in ambient atmosphere produces dielectric oxide layers that have characteristic optical appearances including different colors. Nanoindentation probed the deformation and fracture of laser-fabricated oxides on 304L stainless steel. Conductive nanoindentation measured electrical contact resistance (ECR) of the same colored oxides indicating a correlation between laser exposure, conductance during loading, current-voltage (I-V) behavior at constant load, and indentation response. Microscopy and X-ray diffraction examined the microstructure and chemical composition of the oxides. Combining techniques provides a unique approach for correlating mechanical behavior and the resulting performance of the films in conditions that cause wear.

Because of their high mechanical compliance and electrical properties, the idea of using Ga and CNTs for micro electrical relay contacts has been investigated to minimize damage from switching and make good electrical contacts. Ga was electroplated into droplets on the order of 50 µm in radius on single crystal Si to create a contact for a switch that can be annealed to recover its original electrical properties after mechanical damage. CNTs were grown on Si substrates, coated with a thin Au layer, and transferred to other Si or Kapton substrates through thermocompression bonding. In the case of the Ga contact, repeated switching led to an increase in the resistance, but the resistance recovered after a thermal reflow process at 120 °C. Longer term and larger area contacts were used to measure the contact behavior under switching conditions of up to 200 A/cm2. At moderate cycling conditions (on the order of 200 cycles) the adhesion began to significantly degrade the switch. The oxidation behavior of the Ga droplets was characterized for thermal reflow, suggesting a passivating 30 nm oxide forms at 100 °C. The oxide formed by the Ga is thin and fragile as demonstrated by its use in a switch. The Ga droplets were examined with electrical contact resistance nanoindentation and the loads at fracture and the onset of electrical contact were identified. CNT turfs were also tested for making patterned electrical contacts; turfs of lateral dimensions similar to the Ga droplets were tested using electrical resistance testing during nanoindentation and as macroscopic contacts, and shown to be able to carry similar current densities. The results will be compared between the two systems, and benefits and challenges of each will be highlighted for creating compliant electrical switches and contacts.

A flexible electrode array consisting of a thin metal film on a polymer substrate has been developed for neural implantation in rats. The biocompatible arrays record cortical brain signals from awake and mobile rats in order to gather significant neurological data. Four point bend testing of the metal-Kapton system has been used to characterize the interfacial toughness, and therefore the mechanical durability, of the array. Several different adhesion layers on were evaluated using this method. Use of a titanium-tungsten interlayer increases the mixed-mode fracture toughness from approximately 1 J/m2 to approximately 2 J/m2, while a titanium interlayer provides a toughness of more than 4 J/m2. Gold-Kapton arrays were implanted in rats for periods exceeding 200 days, and neural recordings were taken frequently. The arrays exhibit excellent long-term reliability, with no decrease in signal recording capability over the course of the implantation.

When grown via chemical vapor deposition carbon nanotubes (CNTs) may take on the form of a “turf”, consisting of many CNTs with a complex interconnectedness attached to an inflexible substrate. These turfs can be formed over large areas and with a range of heights (between 1 to 100 μm), and grown on photolithographically patterned catalysts to form different aspect ratios. This study focuses on the indentation and permanent deformation of CNT assemblages under applied contact loading. Nanoindentation was conducted on CNT turfs and the properties, nominally the turf's elastic modulus and hardness, were 14.9 MPa ± 5.7 MPa and 2 MPa respectively. The onset of permanent deformation during indentation occurred at applied stresses of 2.5 MPa. The turf's collective permanent deformation under applied compressive loading was also studied. A model predicting the buckling stress of CNT turfs is also described.

A micro (∼1 cm3) dynamic heat engine, capable of producing electrical power from lowgrade heat sources, utilizes a micro-machined diaphragm with a piezoelectric element as a The electromechanical coupling of a piezoelectric diaphragm under large initial stresses and/or large deflections – in the membrane limit – is described here. A simple model is derived for electromechanical transduction of a pressurized piezoelectric membrane and an experiment is described to measure it. Electromechanical coupling initially increases as the square of the center-point deflection as the residual stress is overcome. In the limit of large pressures, the electromechanical coupling approaches a limit that is predicted by the model.

Extended abstract of a paper presented at Microscopy and Microanalysis 2005 in Honolulu, Hawaii, USA, July 31--August 4, 2005

Arrays of multi-walled carbon nanotunbe (CNT) bundles were fabricated on silicon [100] substrate with iron-nitrate sol-gel catalyst patterned via standard photolithographic techniques. Nanotube bundles with diameters ranging from 400µm to 15µm were grown in a chemical vapor deposition reactor and electrically characterized using a scanning-anode probe apparatus. Results showed a relatively low number of graphitic layers in individual nanotubes and a definite increase in field emission performance with decreasing bundle diameter. A 400µm wide matt of CNTs yielded a turn-on field of 6.7 V/µm and field enhancement of 602 while 15µm bundles performed significantly better with turn-on fields of 1.6 V/µm and field enhancement factors of 2425. The overall trend strongly suggests that the field emission character of CNT based aggregate structures such as those presented here is proportional to their aspect ratio.

Elastic modulus values of the thin films utilized in three different hard film- soft substrate systems were measured using a combination of traditional and continuous stiffness indentation. These systems were chosen to represent typical systems currently utilized in MEMS and included Si/SU8/W, Si/SiO2/Ti/Pt/PZT, and Si/SiO2/Ti/Pt. The last system was coated with either a compressive or nonstressed Cr film. By taking into account ratio between the creep due to the polymer substrate, SU8, to the unloading rate, the modulus of W was measured to be 400 GPa. The modulus of the SU8 was also determined to be 6 GPa. Comparing both the CSM and traditional indentation for the Si/SiO2/Ti/Pt/PZT system showed that the dynamic motion of the indenter caused pile-up in the PZT and resulting in the overestimation of the PZT modulus. This pile-up is a function of the sinusoidal loading frequency. Instead, the modulus of the PZT was measured by shallow depths of traditional indentation that resulted in the PZT modulus of 100 GPa. The hard film-soft substrate systems were show to follow the same trend as stressed soft-film hard-substrate systems with residual stress. The modulus was over estimated for compressively stressed Cr films.

Strength, friction, and wear are dominant factors in the performance and reliability of materials and devices fabricated using nickel based LIGA and silicon based MEMS technologies. However, the effects of frictional contacts and wear on the mechanical performance of microdevices are not well-defined. To address these effects on performance of LIGA nickel, we have begun a program employing nanoscratch and nanoindentation. Nanoscratch techniques were used to generate wear patterns using loads of 100, 200, 500, and 990 μN with each load applied for 1, 2, 5, and 10 passes. Nanoindentation was then used to measure properties in each wear pattern correcting for surface roughness. The results showed a systematic increase in hardness with applied load and number of nanoscratch passes. The results also showed that the work hardening coefficient determined from indentation tests within wear patterns follows the work hardening behavior established from tensile tests, supporting use of a nanomechanics-based approach for studying mechanical properties of wear tested material.

A new generation of sensors based on biologically inspired whisking action will help determine the presence and location of solid objects and fluid vortices similar to mechanisms used by whisker bearing animals such as rats and seals. By using nanoindentation, we demonstrate that mechanical properties are essentially uniform by cross section, but vary longitudinally from the whisker base (a 3.9 GPa elastic modulus) to the tip (a 3.1 GPa elastic modulus). Several recent studies show propagation of high frequency information through whiskers that are tuned by their physical properties. In order to fully understand and model these properties, this study demonstrates a more complex whisker structure than previously assumed.

A new generation of sensors based on biologically inspired whisking action will help determine the presence and location of solid objects and fluid vortices similar to mechanisms used by whisker bearing animals such as rats and seals. By using nanoindentation, we demonstrate that mechanical properties are essentially uniform by cross section, but vary longitudinally from the whisker base (a 3.9 GPa elastic modulus) to the tip (a 3.1 GPa elastic modulus). Several recent studies show propagation of high frequency information through whiskers that are tuned by their physical properties. In order to fully understand and model these properties, this study demonstrates a more complex whisker structure than previously assumed.

Lead zirconate titanate (PZT) films between 1 and 3 μm thick were grown using solution deposition techniques to study the effects of crystal structure and orientation on the direct piezoelectric output of these films on platinized Si membranes. By varying the chemistry of the film from Zr-rich to Ti-rich, the {100}/(111) relative intensity increased for films grown on randomly oriented Pt films. The 40:60 PZT had a tetragonal crystal structure and produced greater electrical output at a given strain than the rhombohedral film (Zr:Ti concentrations less than 50:50), while having a similar e31 constant, between 4.8 and 6.3 C/m2. Orientation and voltage output at a given strain were not strongly influenced by thickness in the ranges investigated. Defects in internal PZT/PZT crystallization interfaces were identified and include porosity on the order of tens of nm, with a corresponding depletion in Pb and accumulation of O at these interfaces. The {100} texture of rhombohedral films deposited upon (111) textured Pt films is significantly greater than the {100} texture of tetragonal films, which show both a {100} and {111} orientation on the same Pt film.

In thin film systems, failure often occurs via fracture mechanisms, with either through thickness cracking or interfacial delamination leading to failure of the device or layer. Measuring the stress at which fracture occurs in these thin film systems requires testing methods amicable to both the small scale of the films as well as the complex relationship between the mechanical properties of the film and the substrate. This paper will cover two oxide film systems, a piezoelectric ceramic (PZT) on platinum and passive oxide films (primarily Cr2O3) on stainless steels. Both will be tested with a bulk method (bulge testing in the case of PZT and circumferentially notched tensile bars for the stainless steel) and then with a nanoindentation method developed for testing fracture for hard coatings on soft substrates. The differences in stresses required for failure between the bulk and the nanoscale tests will be discussed in terms of differences in flaw population.

The mechanism by which deformation is transferred across grain boundaries and ways in which boundaries of different misorientations impact this process has been studied using indentation testing. This information could be useful in designing texture of nanocrystalline materials to maximize their mechanical properties for specific applications. Atomic force microscopy (AFM) and orientation imaging microscopy (OIM) has been combined to identify slip systems activated around indentations. When indentations are placed near grain boundaries, slip steps can be imaged on both sides of the boundary and the associated slip systems of each grain can be determined. Dislocation pile ups have been observed around indentations near boundaries which do not share a common slip direction with the active slip planes of either grain, and slip steps have been seen to traverse boundaries when these shared slip directions are present.

To improve the mechanical compliance of a piezoelectric generator membrane, alterations in PZT processing including variations in crystallization temperature, cooling rates and precursor chemistry were made to reduce the residual stress. In addition to changing the stresses, these treatments also altered the morphology of the PZT and its modulus. Nanoindentation of these films were made to determine the moduli of the PZT films, which varied from 70GPa to 90GPa. The residual stress of 40:60 2-MOE PZT taken by x-ray diffraction were shown to reduce in residual stress from 210 MPa to 154 MPa with annealing. The static pressure-deflection of membranes with these films showed correlating composite effective stresses of 110MPa and 83MPa. Adding PEG to the precursor solution also lowered the stress of the PZT films. Bulge testing showed that decreasing the crystallization temperature from 700°C to 600°C lowered residual stress from 210 MPa to 154 MPa. Raman spectroscopy showed differences in both stress and structure of PZT deposited on thin support structures (2μm) over bulk wafers (350 μm). The compliance of the generators was also increased by etching Pt in highly stressed regions.

Piezoelectric materials can convert mechanical and electrical energy, a particularly useful tool in developing micro and nanoscale systems. Characterizing the electromechanical behavior is essential to the design and optimization of the material's and device's performance. This paper examines the influence of boundary (clamping) conditions, relative thickness variations between the active one to two micron thick piezoelectric membrane and underlying passive support structure, and the electrode coverage on the electromechanical behavior. Membranes were fabricated with silicon and lead zirconate titanate (PZT) with a ratio of Zr to Ti of 40:60 that provide thickness ratios between 1:2 and 2:1 by depositing the PZT using sequential solution deposition. PZT films contain a tensile stress that accumulates during processing, therefore a compressive stressed layer of tungsten was sputtered on bulk micromachined membranes to produce a near zero net residual stress. A nonlinear finite element numerical simulation technique is utilized for the analysis of the composite thin film. A comparison between the behavioral trends determined by simulation and experimental methods will be discussed.