Are Diamonds a MEMS' Best Friend?

Next-generation military and civilian communication systems will require technologies capable of handling data/ audio, and video simultaneously while supporting multiple RF systems operating in several different frequency bands from the MHz to the GHz range [1]. RF microelectromechani-cal/nanoelectromechanical (MEMS/NEMS) devices, such as resonators and switches, are attractive to industry as they offer a means by which performance can be greatly improved for wireless applications while at the same time potentially reducing overall size and weight as well as manufacturing costs.     

In-plane contributions to phase contrast in intermittent contact atomic force microscopy

Contrast in the phase response of intermittent-contact atomic force microscopy (IC-AFM) reveals in-plane structural and mechanical properties of polymer monolayers. This result is unexpected, as IC-AFM has previously only been considered as a probe of out-of-plane properties. Until now, AFM measurements of nanoscale in-plane properties have employed contact mode techniques. In-plane property measurements are possible with intermittent contact AFM because there is a small but significant component of tip motion parallel to the sample surface. This in-plane component of tip displacement is virtually universal in AFM, implying that oscillating-tip techniques generally are sensitive to in-plane material properties. We present a simple Hertzian model of intermittent-contact AFM that includes such an in-plane displacement.     

Atomic Friction Modulation on the Reconstructed Au(111) Surface

Friction between a nanoscale tip and a reconstructed Au(111) surface is investigated both by atomic force microscopy (AFM) and molecular statics calculations. Lateral force AFM images exhibit atomic lattice stick–slip behavior with a superstructure corresponding to the herringbone reconstruction pattern. However, the superstructure contrast is not primarily due to variations in the local frictional dissipation (which corresponds to the local width of the friction loop). Rather, the contrast occurs primarily because the local centerline position of the friction loop is periodically shifted from its usual value of zero. Qualitatively, similar behavior is reproduced in atomistic simulations of an AFM tip sliding on the reconstructed Au(111) substrate. In both simulations and experiments, this centerline modulation effect is not observed on unreconstructed surfaces. Similarly, using a topographically flat surface as a hypothetical control system, the simulations show that the centerline modulation is not caused by variations in the reconstructed surface’s topography. Rather, we attribute it to the long-range variation of the local average value of the tip-sample interaction potential that arises from the surface reconstruction. In other words, surface atoms located at unfavorable sites, i.e., in the transition between face-centered-cubic (FCC) and hexagonal-close-packed (HCP) regions, have a higher surface free energy. This leads to a varying conservative force which locally shifts the centerline position of the friction force. This demonstrates that stick–slip behavior in AFM can serve as a rather sensitive probe of the local energetics of surface atoms, with an attainable lateral spatial resolution of a few nanometers.     

Measurement of the Length and Strength of Adhesive Interactions in a Nanoscale Silicon–Diamond Interface

The adhesive interactions between nanoscale silicon atomic force microscope (AFM) probes and a diamond substrate are characterized using in situ adhesion tests inside of a transmission electron microscope (TEM). In particular, measurements are presented both for the strength of the adhesion acting between the two materials (characterized by the intrinsic work of adhesion W_adh,int) and for the length scale of the interaction (described by the range of adhesion z₀). These values are calculated using a novel analysis technique that requires measurement of the AFM probe geometry, the adhesive force, and the position where the snap‐in instability occurs. Values of W_adh = 0.66 J m⁻² and z₀ = 0.25 nm are extracted using this technique. This value of work of adhesion is 70% higher than the work of adhesion calculated if one uses a conventional paraboloidal asperity model. Comparing to literature, the work of adhesion obtained using the new method is significantly higher than most experimental and simulation values for similar material pairs. The discrepancy is attributed to nanoscale roughness, which was not accounted for previously. Furthermore, the value of the range of adhesion is comparable to previously reported values, but is significantly larger than the commonly assumed value of the interatomic spacing.     

Abrasion of Steel by Ceramic Coatings: Comparison of RF-DLC to Sputtered B4C

The abrasion rates of steel balls sliding against a very smooth diamond-like carbon (DLC) coating and a rough boron carbide (B₄C) coating are compared. The initial abrasiveness of the B₄C coating is about 2 orders of magnitude greater than that of the DLC coating. Both coatings exhibit a rapid decrease in their abrasiveness with sliding distance, but the details of the abrasion kinetics of these coatings are quite different. The abrasiveness of B₄C falls according to a simple power law, while the abrasiveness of the DLC remains constant for a duration that depends on the load and then switches rather suddenly to zero. An explanation for this different behavior is proposed. During the abrasion process the asperities on the B₄C are smoothed to a startling extent.     

Wear‐Resistant Nanoscale Silicon Carbide Tips for Scanning Probe Applications

The search for hard materials to extend the working life of sharp tools is an age‐old problem. In recent history, sharp tools must also often withstand high temperatures and harsh chemical environments. Nanotechnology extends this quest to tools such as scanning probe tips that must be sharp on the nanoscale, but still very physically robust. Unfortunately, this combination is inherently contradictory, as mechanically strong, chemically inert materials tend to be difficult to fabricate with nanoscale fidelity. Here a novel process is described, whereby the surfaces of pre‐existing, nanoscale Si tips are exposed to carbon ions and then annealed, to form a strong silicon carbide (SiC) layer. The nanoscale sharpness is largely preserved and the tips exhibit a wear resistance that is orders of magnitude greater than that of conventional silicon tips and at least 100‐fold higher than that of monolithic, SiO‐doped diamond‐like‐carbon (DLC) tips. The wear is well‐described by an atom‐by‐atom wear model, from which kinetic parameters are extracted that enable the prediction of the long‐time scale reliability of the tips.     

Small amplitude reciprocating wear performance of diamond-like carbon films: dependence of film composition and counterface material

Small amplitude (50 μm) reciprocating wear of hydrogen-containing diamond-like carbon (DLC) films of different compositions has been examined against silicon nitride and polymethyl-methacrylate (PMMA) counter-surfaces, and compared with the performance of an uncoated steel substrate. Three films were studied: a DLC film of conventional composition, a fluorine-containing DLC film (F-DLC), and silicon-containing DLC film. The films were deposited on steel substrates from plasmas of organic precursor gases using the Plasma Immersion Ion Implantation and Deposition (PIIID) process, which allows for the non-line-of-sight deposition of films with tailored compositions. The amplitude of the resistive frictional force during the reciprocating wear experiments was monitored in situ, and the magnitude of film damage due to wear was evaluated using optical microscopy, optical profilometry, and atomic force microscopy. Wear debris was analyzed using scanning electron microscopy and energy dispersive spectroscopy. In terms of friction, the DLC and silicon-containing DLC films performed exceptionally well, showing friction coefficients less than 0.1 for both PMMA and silicon nitride counter-surfaces. DLC and silicon-containing DLC films also showed significant reductions in transfer of PMMA compared with the uncoated steel. The softer F-DLC film performed similarly well against PMMA, but against silicon nitride, friction displayed nearly periodic variations indicative of cyclic adhesion and release of worn film material during the wear process. The results demonstrate that the PIIID films achieve the well-known advantageous performance of other DLC films, and furthermore that the film performance can be significantly affected by the addition of dopants. In addition to the well-established reduction of friction and wear that DLC films generally provide, we show here that another property, low adhesiveness with PMMA, is another significant benefit in the use of DLC films.     

An In Situ Method for Simultaneous Friction Measurements and Imaging of Interfacial Tribochemical Film Growth in Lubricated Contacts

Tribological investigations of macroscopic lubricated sliding contacts are critical for a wide range of industrial applications including automotive engines, gears, bearings, and any other contacting surfaces in relative motion. However, the inability of existing techniques to access buried sliding interfaces with high spatial resolution inhibits the development of fundamental insights into the tribological processes at play. Here we demonstrate a novel and general in situ method, based on atomic force microscopy (AFM), in which micrometer-scale spherical probes are attached to a standard microfabricated AFM cantilever which is then slid over a substrate while immersed in a liquid lubricant. In this case, steel colloidal probes and steel substrates were used, and the contact was immersed in a commercial polyalphaolefin oil with zinc dialkyl dithiophosphate (ZDDP) additive at both room temperature and 100 °C, but the method can be used for a broad range of material combinations, lubricants, and temperatures. We demonstrate that the in situ measurements of friction force and the morphological evolution of the tribochemical films on the substrate can be simultaneously achieved with nanometer-level spatial resolution. In addition, we demonstrate that the sliding zone is readily accessible for further characterization with higher spatial resolution using standard AFM probes with nanometer-scale tip radii. Ex situ characterization of the micrometer-scale probe and the sample is also feasible, which is demonstrated by acquiring high-resolution AFM topographic imaging of the final state of the probe.