Michael Down

Finite Element Model of a Bilayered Gold-Coated Carbon Nanotube Composite Surface

Vertically aligned multiwalled carbon nanotubes (MWCNTs), with a gold (Au)-coated surface, have been shown to provide a stable contact resistance for electrical contact switching applications under low force conditions (micronewton to millinewton), with the MWCNT surface providing a compliant support for the conducting Au layer. In this paper, nanoindentation results are used in the development of a finite element contact model for the composite, referred to as Au/CNT. The results show that the surface is best modeled as a bilayered structure, in which the top layer is modeled as an elastic-plastic layer of the Au/CNT mixed material and the under layer as a CNT forest. The resultant model matches the experimental results for a range of samples with different thickness configurations.

A nano-indentation study of the contact resistance and resistivity of a bi-layered Au/multi-walled carbon nanotube composite

A bi-layer metal-carbon nanotube composite has been developed as a potential low-force electrical contact surface, for application in micro-electromechanical systems switching devices. The samples consist of a vertically aligned forest of multi-walled carbon nanotube (MWCNT), sputter coated with a layer of Au. The effect of varying the components and composition are investigated by means of a modified nano-indenter. By measuring the contact resistance of the composites under various loading conditions, the electrical properties and performance can be evaluated. The composites are shown to have homogenous properties, with each of the layers influencing the total electrical characteristics of the samples. The internal structure of the sample, the MWCNT height and penetration of gold into the forest is shown to directly influence the performance and characteristics of the samples. By analyzing the samples as bulk, the effective resistivities of the composites are also determined to have values from 303 nΩ m down to 54 nΩ m, depending on the composition of the samples.

The Effect on Switching Lifetime of Chromium Adhesion Layers in Gold-Coated Electrical Contacts under Cold and Hot Switching Conditions

Gold is commonly used for electrical contacts due to its many desirable electrical and mechanical properties. Throughout the switch lifetime, the contacts are required to survive a large number of opening and closing cycles and therefore it is important to understand the failure mechanisms. Adhesion layers (e.g. chromium or titanium) can be deposited to increase the adhesion of the gold layer to the contact surface. In this work, the inclusion of a chromium adhesion layer shows an improvement of the switching lifetime of gold-coated electrical contacts under cold and hot switching conditions. These testing conditions further the understanding of the failure mechanisms (e.g. fine transfer, etc.). The mechanism of failure when no chromium adhesion layer was used is attributed to delamination of the gold layer from one contact to the other. This failure mechanism is different in the cases where a chromium adhesion layer is included. We present a model which was developed in line with experimental results. These describe the effect of load current on material transfer between gold contacts and the contact failure.

Mechanical characterization of a Au coated carbon nanotube multi-layered structure

Multiwalled Carbon Nanotube (MWCNT)-coated surfaces have been proved to be able to provide stable resistance for the electrical contact under low force conditions since they can offer a compliant support for the conducting gold layer. However, the contact mechanics of the Au/MWCNTs composite have not been understood. In this study, a finite element multilayered contact model was developed, in which the top layer was modeled as a composite and under layer was modeled as CNT forest. The study shows that this complex surface is best modeled as a multi-layered structure. The model is optimized and validated with nano-indenter data. The model can help to better understand the configuration and material properties of Au/CNTs surfaces, and can provide guidance to optimize the surface in terms of contact resistance performance in MEMS switches.

Investigating the benefits of a compliant gold coated multi-walled carbon nanotube contact surface in micro-electro mechanical systems switching

The application of a metal-carbon nanotube composite consisting of Au sputtered coated multi-walled carbon nanotubes as an electrical contact has been shown to greatly improve the lifetime and switching characteristics. A benefit of this contact surface is shown here to be the compliance of the composite, which allows the contact to conform the shape of its opposite counterpart and increase apparent contact area. The improvement of the lifetime is shown to be affected by the contact force applied, with a limit at 4.5 mN where the composite begins to fail under “hot switching” conditions (0.4 W) at only 1.5  × 106, an order of magnitude lower than at 4 mN.

A comparison of the predictions of a multiscale model and optical real area of contact measurements

Rough surface contact is difficult to model effectively due to the existence of multiples scales of geometrical features (i.e. asperities). There are many different methodologies in the literature to theoretically predict the real area of contact, but few of these have been compared directly to experimental data. This is because it is very difficult to measure the real area of contact between surfaces due to an obstructed line-of-sight and the very small scale of the surface features that are in contact. This work presents the predictions of a multiscale rough surface contact model in comparison to measurements made of the real area of contact between a transparent glass surface pressed against a metallic surface. A laser profilometer is used to scan the deformed profile of the surface and therefore the contact area can be measured. The multiscale model is able to account for the resolution of the measurement by neglecting smaller scales of features. The multiscale model compares well with the measurements at low loads, but not at high loads. The real contact pressure predicted by the multiscale model and measured are both much higher than conventional hardness that is often used to predict the real area of contact.

An experimental analysis of the real contact area between an electrical contact and a glass plane

The exact contact between two rough surfaces is usually estimated using statistical mathematics and surface analysis before and after contact has occurred. To date the majority of real contact and loaded surfaces has been theoretical or by numerical analyses. A method of analysing real contact area under various loads, by utilizing a con-contact laser surface profiler, allows direct measurement of contact area and deformation in terms of contact force and plane displacement between two surfaces. A laser performs a scan through a transparent flat side supported in a fixed position above the base. A test contact, mounted atop a spring and force sensor, and a screw support which moves into contact with the transparent surface. This paper presents the analysis of real contact area of various surfaces under various loads. The surfaces analysed are a pair of Au coated hemispherical contacts, one is a used Au to Au coated multi-walled carbon nanotubes surface, from a MEMS relay application, the other a new contact surface of the same configuration.