Megan J. Cordill

Recent developments in thin film adhesion measurement

Interfacial fracture energies of thin films may be calculated using many different techniques. Nanoindentation and stressed overlayers are by far the most common and more reliable of the testing techniques. They depend on mechanics-based models to calculate the interfacial fracture energy of an interface using only the site specific material properties and the dimensions of the delaminated region, either in spontaneous buckle or indentation-induced blister form. This study will focus on four adhesion measurement techniques: spontaneous buckles, stressed overlayer-induced buckles, and nanoindentation-induced blisters with and without stressed overlayers, to demonstrate that the techniques will produce similar results for the measurement of adhesion energy. Films of tungsten (W), platinum (Pt), and titanium (Ti) on SiO/sub 2/ (amorphous glass) substrates are examined and values of interfacial fracture energies reported. Results of interfacial fracture energy calculated from spontaneous buckles and indentation-induced blisters compare well to one another and values are reported for the aforementioned films.

Effects of dynamic indentation on the mechanical response of materials

Dynamic indentation techniques are often used to determine mechanical properties as a function of depth by continuously measuring the stiffness of a material. The dynamics are used by superimposing an oscillation on top of the monotonic loading. Of interest was how the oscillation affects the measured mechanical properties when compared to a quasi-static indent run at the same loading conditions as a dynamic. Single crystals of nickel and NaCl as well as a polycrystalline nickel sample and amorphous fused quartz and polycarbonate have all been studied. With respect to dynamic oscillations, the result is a decrease of the load at the same displacement and thus lower measured hardness values of the ductile crystalline materials. It has also been found that the first 100 nm of displacement are the most affected by the oscillating tip, an important length scale for testing thin films, nanopillars, and nanoparticles.

Geometry and surface state effects on the mechanical response of Au nanostructures

A study of ultra-thin gold films and thin-walled nanoboxes has confirmed that length scales in terms of dislocation spacing can predict flow stress. Initial stages of deformation conform to linear hardening with average dislocation spacing controlled by the number of geometrically necessary dislocations in a pile-up. Later stages of deformation exhibit parabolic behavior with Taylor hardening interpreted in terms of a dislocation density described by the total line length of prismatic loops per unit volume. Comparisons of 20 and 40 nm thick planar films could be made to 205 nm high hollow gold nanoboxes with a wall thickness of 24 nm. These highly constrained, ultra-thin planar films demonstrated increased hardness from about 2 to 10 GPa with strains of 20 percent while less constrained nanoboxes increased from 0.8 to 4 GPa for the same strain magnitude.

An Energy Balance Criterion for Nanoindentation-Induced Single and Multiple Dislocation Events

Small volume deformation can produce two types of plastic instability events. The first involves dislocation nucleation as a dislocation by dislocation event and occurs in nanoparticles or bulk single crystals deformed by atomic force microscopy or small nanoindenter forces. For the second instability event, this involves larger scale nanocontacts into single crystals or their films wherein multiple dislocations cooperate to form a large displacement excursion or load drop. With dislocation work, surface work, and stored elastic energy, one can account for the energy expended in both single and multiple dislocation events. This leads to an energy balance criterion which can model both the displacement excursion and load drop in either constant load or fixed displacement experiments. Nanoindentation of Fe-3% Si (100) crystals with various oxide film thicknesses supports the proposed approach.

Length-scale-based hardening model for ultra-small volumes

Understanding the hardening response of small volumes is necessary to completely explain the mechanical properties of thin films and nanostructures. This experimental study deals with the deformation and hardening response in gold and copper films ranging in thickness from 10 to 400 nm and silicon nanoparticles with particle diameters less than 100 nm. For very thin films of both gold and copper, it was found that hardness initially decreases from about 2.5 to 1.5 GPa with increasing penetration depth. Thereafter, an increase occurs with depths beyond about 5–10% of the film thickness. It is proposed that the observed minima are produced by two competing mechanisms. It is shown that for relatively deep penetrations, a dislocation back stress argument reasonably explains the material hardening behavior unrelated to any substrate composite effect. Then, for shallow contacts, a volume-to-surface length scale argument relating to an indentation size effect is hypothesized. A simple model based on the superposition of these two mechanisms provides a reasonable fit to the experimental nanoindentation data.

Thermal Plasma Chemical Vapor Deposition of Superhard Nanostructured Si-C-N Coatings

A triple torch plasma reactor was used to synthesize Si—C—N composite films via the thermal plasma chemical vapor deposition process. The argon-nitrogen plasma provided atomic nitrogen to carbon- and silicon-based reactants, which were injected through a central injection probe and ring configuration. Films were deposited with variations of the total nitrogen flow through the torches (1.5-4.5slm), reactant mixture (silicon tetrachloride and acetylene or hexamethyldisilazane) and substrate material (silicon and molybdenum). Micro X-ray diffraction was used to determine that both α-Si 3 N 4 and β-Si 3 N 4 were dominant in most of the depositions. Composites of silicon nitride and silicon carbide were synthesized on molybdenum. The bonding of amorphous phases was investigated using Fourier transform infrared spectroscopy, which indicated the presence of N—H, CH x and CΞ in various films. Indentation tests on the polished film cross-sections determined that large variations in hardness and elastic modulus existed for minor changes in film composition. Correlations between indentation results and scanning electron and optical microscope images showed that the mechanical properties greatly depend on the film morphology; the denser, smoother, and more crystalline films tended to display enhanced mechanical properties.

Modeling of friction-induced deformation and microstructures.

Frictional contact results in surface and subsurface damage that could influence the performance, aging, and reliability of moving mechanical assemblies. Changes in surface roughness, hardness, grain size and texture often occur during the initial run-in period, resulting in the evolution of subsurface layers with characteristic microstructural features that are different from those of the bulk. The objective of this LDRD funded research was to model friction-induced microstructures. In order to accomplish this objective, novel experimental techniques were developed to make friction measurements on single crystal surfaces along specific crystallographic surfaces. Focused ion beam techniques were used to prepare cross-sections of wear scars, and electron backscattered diffraction (EBSD) and TEM to understand the deformation, orientation changes, and recrystallization that are associated with sliding wear. The extent of subsurface deformation and the coefficient of friction were strongly dependent on the crystal orientation. These experimental observations and insights were used to develop and validate phenomenological models. A phenomenological model was developed to elucidate the relationships between deformation, microstructure formation, and friction during wear. The contact mechanics problem was described by well-known mathematical solutions for the stresses during sliding friction. Crystal plasticity theory was used to describe the evolution of dislocation content in the worn material, which in turn provided an estimate of the characteristic microstructural feature size as a function of the imposed strain. An analysis of grain boundary sliding in ultra-fine-grained material provided a mechanism for lubrication, and model predictions of the contribution of grain boundary sliding (relative to plastic deformation) to lubrication were in good qualitative agreement with experimental evidence. A nanomechanics-based approach has been developed for characterizing the mechanical response of wear surfaces. Coatings are often required to mitigate friction and wear. Amongst other factors, plastic deformation of the substrate determines the coating-substrate interface reliability. Finite element modeling has been applied to predict the plastic deformation for the specific case of diamond-like carbon (DLC) coated Ni alloy substrates.

Size Effects on Yield Instabilities in Nickel

Dislocation events are seen as excursions, or pop-in events, in the load-displacement trace of nanoindentation experiments. When indenting single crystal metals these events occur frequently during quasi-static and dynamic loading. A single crystal of Ni (111) has been indented quasi-statically using three different loading rates (10, 100, and 1000 μN/s) as well as with three different radii diamond indenter tips (1000 nm cone, 300 nm Berkovich, and 50 nm cube corner) to examine the occurrences of excursions. As expected, excursions at higher loads have larger displacements, and that initial loading follows Hertzian behavior up to the point of yield. Also, as the tip size is reduced the excursion loads are reduced. The excursion events depend mostly on the statistical distribution of surface sources and substructure dislocation arrangements.

The Role of Adhesion and Fracture on the Performance of Nanostructured Films

Nanostructured materials are the basis for emerging technologies, such as MEMS, NEMS, sensors, and flexible electronics, that will dominate near term advances in nanotechnology. These technologies are often based on devices containing layers of nanoscale polymer, ceramic and metallic films and stretchable interconnects creating surfaces and interfaces with properties and responses that differ dramatically from bulk counterparts. The differing properties can induce high interlaminar stresses that lead to wrinkling, delamination, and buckling in compression [1,2], and film fracture and decohesion in tension. [3] However, the relationships between composition, structure and properties, and especially adhesion and fracture, are not well-defined at the nanoscale. These relationships are critical to assuring performance and reliability of nanostructured materials and devices. They are also critical for building materials science based predictive models of structure and behavior.

Interactions of Constrained Flow and Size Scale on Mechanical Behavior

Coupled effects between constrained flow, increased strength as a function of decreased sample size, and resulting high stresses affect both modulus and fracture toughness. For submicron size crystalline spheres [1,2], boxes [3], and cubes [4], we have recently shown that dislocation by dislocation events can be followed using a combination of AFM/nanoindentation. This has led to at least three proposed strengthening mechanisms for hardening of small constrained volumes under compression[4]. With the increased stresses, this can produce increased moduli of elasticity in confined volumes small in three dimensions. With increased constrained plasticity this produces increased strength in volumes small in three, two or one dimensions.