Ceramic Thick Films for MEMS and Microdevices (Micro and Nano Technologies)

Ceramic Thick Films For Mems and Microdevices

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The aim of this thesis is preparation of various fluorescent QDs in aqueous solution by novel approaches of synthesis via refluxing, hydrothermal treatment or microwave irradiation. The toxicity of QDs is the primary factor that influences their biomedical application, thus it will be essential to evaluate the cell viability, morphology, oxidative stress and DNA damage and apoptosis. As one of the key steps in the preparation of QDs-based fluorescent probes, conjugation between QD and specific molecules shows great impact on the probe performance.

Therefore, conjugation process and the labelling efficiency of selected biomolecules will be studied. The expansion of nanotechnology accelerated the development of new methods for deposition of TiO2 films. In particular, the fabrication of various titania nanostructured surfaces has got the scientists attention. The TiO2 nanostructures include mainly nanotubes, nanorods, nanowires, and nanodots as well as nanoporous films. All of these mentioned structures can be composed of pure phase or bi-phase TiO2, usually after transformation from amorphous phase depending on annealing temperature.

These nanostructures may improve electron transport through the photocatalytic film as well as provide a large surface area for the adsorption of pollutant. The topic is focused on development of numerical methods for rigorous simulation of electromagnetic wave propagation in arbitrary inhomogeneous media.

Developed techniques will applied to modeling of light scattering by selected biological samples. Experimental work is based on measurement of temperature dependence of noise using helium cryostat and study of amplitude and mean time of capture and emission as a function of electric field intensity and charge carrier concentration in channel.

These results will be used to improve generation-recombination model of noise origin and localization of traps. Since biomolecules are not as robust as classical nanostructures, such as, for example, carbon nanotubes or metal nanoparticles, special precautions should be taken to avoid deformation of the molecules during the measurement. Materials exhibiting high permittivity dielectric constant, k are needed for new applications, particularly in integrated circuits ICs using the 32 nm technology and in capacitors.

In capacitors, high-k dielectrics are used in order to attain higher energy densities in capacitors and thus to reduce the size of capacitors themselves. In ICs manufacturing, the present drive toward smaller dimensions results in the thinning of insulating layers, accompanied by an unwanted increase of leakage currents. In order to prevent this effect, higher gate thicknesses are desired which, however, because of the necessity to keep the capacitance constant, should exhibit higher dielectric constant than the pure SiO2.

Materials for ICs should be used within the current silicon technologies and, therefore, they must be able to sustain all manufacturing steps without being damaged. Suitable dielectrics are mostly transition metal oxides, e. Moreover, all materials considered must be thermodynamically stable on silicon for a long time.

In case of dielectrics for capacitors, the use of multilayer ceramic chip capacitors again necessitates the use of material that can withstand sintering temperatures. The work on this topic will require experimental work in sample preparation and design, studies of the theory of high -k dielectrics and the measurement of electrical properties of developed material systems.

Study of nano-objects behaviour is provided in level of quantum physics. Scientists are still working to understand and describe the interactions in the nanoworld therefore the research in this area belongs top topics that move knowledge toward using the phenomena in various areas and they probably become a base of future pikotechnology. The subject of the research will be dielectric properties of nanocomposites for electrical insulation. These materials are based on thermosetting resins, mostly epoxides, containing finely dispersed SiO2, TiO2, Al2O3 or WO3 microfillers and nanofillers, eventually more complex chemical formulations.

The presence of nanoparticles with dimensions of some 10 - 20 nm favorably affects the withstand capability of nanocomposites to partial discharges and electrical treeing and, hence, the breakdown strength as well as the degradation resistance. This in turn brings the possibility to manufacture electrical equipment e. An important issue concerning all nanocomposites is the presence of a large number of interfaces.

They are to due to the presence of nanoparticles with complex shapes neither planar nor spherical. These interfaces exhibit a low stability, which may later cause substantial changes of electrical properties in the course of ageing. One of the objectives of the proposed research would therefore be to study the behavior of nanocomposites in the course of accelerated ageing. The work on this topic will require experimental work in sample preparation and design, studies of the relation between microphysical structure and electrical properties and the measurement of electrical properties of developed material systems.

Equipment currently available in the Department of Physics: Cracks creation in mechanical loaded solids is accompanied by origin of electromagnetic EME and acoustic AE signals. These signals can be used for study of cracks formation evolution, their characteristics finding and their localization. EME and AE methods are usable in electrical engineering, mechanical engineering and civil engineering.

The goal will be determination of cracks primary parameters and their localization in conventional materials and in modern composite materials for structural applications. Analysis of EME and AE signals origin and propagation will be performed in these materials and models will be suggested. The methodology for determination of selected primary parameters and cracks localization will be suggested and verified. Methods, which enable displaying the entire 3D structure of the studied object in a non-destructive way are intensively studied in many scientific and industrial branches.

Presently, computed tomography i. The topics of the dissertation work include study, application and improvements of X-ray micro-CT techniques for material analysis. Advanced methods of imaging by coherence-controlled holographic microscope The topic is focused on research in the field of numerical image reconstruction in coherence-controlled holographic microscope.

Application of spectroscopic reflectometry for lubrication film study Application of spectroscopic reflectometry to the study of lubrication films to obtain their thickness and refractive index within highly loaded lubricated contact. Behaviour of nanoparticles in thin film lubrication films Study of rheological properties of thin lubrication film through the in situ observation of the particles movement within the lubricated contact. Carbon nanotubes fluorescence spectroscopy The development of the technology for the detection and characterization of the semiconductor carbon nanotubes by the laser spectroscopy techniques, such as fluorescence spectroscopy and laser-induced breakdown spectroscopy LIBS.

Development of a device and methodology for Laser-Induced Breakdown Spectroscopy LIBS Laser-Induced Breakdown Spectroscopy LIBS is a technique that utilizes high power-densities obtained by focusing the radiation from a pulsed laser to generate a luminous micro-plasma from an analyte in the focal region. Development of incoherent holographic microscopy and related techniques For detailed info please contact the supervisor. Effect of proteins on friction and wear of hip joint replacements Study of lubricant film formation between rubbing surfaces of joint implants to describe the effect of proteins on friction and wear reduction.

Fabrication and characterization of nanostructures with functional properties in the field of plasmonics For detailed info please contact the supervisor. Fabrication and characterization of nanostructures with functional properties in the field of plasmonics II For detailed info please contact the supervisor. Fabrication and characterization of nanostructures with functional properties in the field of spintronics For detailed info please contact the supervisor. Gas sensor based on MEMS cantilever resonator Gas sensor based on cantilever resonator fabricated using MEMS microelectromechanical systems technology belongs to a quite new principles for gas sensing.

Characterization and utilization of nanocoposites based on graphene Nanocomposites containing graphene sheets are already available and are used for various applications. Charge carriers transport and noise in carbon nanofibers base supercapacitors The goal is to propose the methodology for the supercapacitor lifetime prediction with respect to the attainment of 10 years life time guarantee required for the applications in the satellite systems.

Lowering the detection limits of Laser-Induced Breakdown Spectroscopy LIBS technique via innovative approaches Laser-Induced Breakdown Spectroscopy LIBS is a technique that utilizes high power-densities obtained by focusing the radiation from a pulsed laser to generate a luminous micro-plasma from an analyte in the focal region.

Modeling of functional properties of nanostructures for plasmonics For detailed info please contact the supervisor. Nanoelectronic devices with novel magnetic and electric transport properties For detailed info please contact the supervisor. Nanoelectronic devices with novel optoelectronic properties For detailed info please contact the supervisor. New methods of inhomogenity characterization by using passive nondestructive techniques Passive methods of nondestructive testing NDT enables longtime monitoring of materials and structures during service or during their loading at laboratory.

Noise spectroscopy of defects and transport of charge carriers in CdTE sensors The aim of the doctoral dissertation will be analysis of electronic noise in monocrystalline samples of CdTe radiation sensors and sensors produced on CdTe based. Porous-alumina-assisted formation of metal and metal-oxide nanostructures for use in advanced micro-devices Current generation of commercially available energy-conversion, distribution and storage microdevices use micron-scale powders or lithographically prepared materials arrays for fabricating active electrodes, limiting device performance and restricting the choices of device chemistries.

Preparation and bioconjugation of quantum dots for cellular labelling Using fluorescent nanoparticles as probes for bioanalytical application is highly promising because fluorescence-based techniques are very sensitive. Preparation of nanostructured TiO2 based surfaces with photocatalytic activity The expansion of nanotechnology accelerated the development of new methods for deposition of TiO2 films. Rigorous simulation of electromagnetic wave propagation in inhomogeneous media The topic is focused on development of numerical methods for rigorous simulation of electromagnetic wave propagation in arbitrary inhomogeneous media.

Semiconductor heterostructure nanowires with applications in nanoelectronics For detailed info please contact the supervisor. While such comparisons elucidate interfacial phenomena of single-asperity sliding contacts, it remains a great challenge in understanding how observations at the molecular scale translate onto microscale tribometry. Defining a contact area in microtribological experiments becomes somewhat more complex compared to nano- and macro- tribology. Unlike in nano- and macrotribology, where the contact consists of a single and countless asperities respectively, in microtribology only a few asperities contribute to the contact.

Due to the significant reduction in contact size compared to macrotribology, in microtribology roughness plays a larger role and therefore it is crucial consider the real contact area. The concept of third bodies, for macroscopic tribology, was initially introduced by Godet in the s [ 34 ]. Nowadays this phenomenon is widely accepted for a variety of materials, where the behavior of the third bodies plays a significant role in determining the macrotribological performance i.

For pure metals and metallic alloys it is known that the third bodies typically consist of tribofilms i. Previous studies on pure metals have shown that the formation of certain tribofilms decreases friction and wear [ 35 , 36 ]. However, inclusive correlations between the properties of the tribofilm e. In terms of transfer films and wear debris, however, the correlations to friction and wear of metals are somewhat more evident.

In their book Engineering Tribology, Stachowiak and Batchelor summarised some effects on transfer material and particles on the wear rate and friction behavior of metals [ 37 ]. One early understanding of the transfer film behavior in metals i. As the pin continues to slide, the transfer material grows larger and is eventually removed causing positive wear. This ends up being a cyclic process that is repeated throughout the sliding process.

This idea of the formation and removal of the transfer film was proven to be true from observations of the normal displacements of the tip for the case of zinc against zinc [ 39 ]. For example, in the case of MoS 2 , the third bodies are typically crystalline tribofilms with basal planes oriented parallel to the sliding direction at the worn surface and a MoS 2 transfer film on the counterface.

This phenomenon for MoS 2 based coatings has been extensively studied at the macroscopic length scales using in situ and ex situ techniques e. More recently, Scharf et al. The rheology and flows of the third bodies for macroscopic tribology has been described in detail by Descartes et al.

The authors describe the source flow of the third body i. The internal source includes material detached from the first bodies and the external source consists of artificial material. Throughout the sliding, the material trapped in between the first bodies circulates back and forward creating an external flow of material. The external flow can be further separated into material that is completely emitted from the contact and material that is re-circulated into the contact.

These concepts were largely understood through ex situ observation of tribocontact surfaces, but were more explicitly observed and quantified by in situ tribometry see Figure 2 , which will be discussed in more detail in Section 4. Optical image right panel showing the contact area of sapphire sphere sliding against Ti-MoS 2 coating obtained using an in situ tribometer left panel.

The image was captured with a microscope monitoring the contact area through the transparent sapphire counterface, as shown on the left side of the Figure. The image shows the three contact zones throughout sliding; 1 the entry zone; 2 the lateral zones; and 3 the internal zone. Understanding the rheology and flows of the third bodies in a tribological system is important.

In general, the contact, including transfer material, can be divided into three zones; the entry zone, the internal zone, and the lateral zones [ 44 ], Figure 2. The entry zone is located in front of the contact and the lateral zones are located on both sides of the contact. The entry and lateral zones consist of material e. The internal zone, on the other hand, is where most of the contact occurs.

In literature, the material in this zone is normally referred to as transfer film. The third body behavior for microtribology is not understood as well as in macrotribology. As it can be speculated, the rheology and zones of the third body in a certain tribological system are influenced by the contact geometry and applied normal forces.

Therefore, when scaling down to microscopic tribology, the flow of the transfer material may be different than macroscale. For instance, such a decrease in contact area can have a significant effect on the external source flow. It is also likely that environmental condition e. Similarly, since roughness plays a larger role in the tribological behavior i. These issues are covered in the subsequent sections in more details. However, it was later noted that due to the roughness of the surfaces, the actual area is smaller than the apparent area and consists of a large number of asperity contacts.

The single-asperity theories for nanotribology are shown in Table 1 in terms of the friction force relation to the contact area and the normal load. Summary of friction laws, adapted from reference [ 47 ].

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Improving systems with microscale sliding contacts requires a better understand of how the relationship of the friction force to normal force varies for contacts between single asperity and macroscopic length scales as well as identifying additional components that contribute to the the friction force in microtribology.

For some instances, non-adhesive laws e. Besides the deviation of macroscopic friction laws, when decreasing the contact size down to few micrometers, there are also variations in the friction values. Figure 3 illustrates the differences in the static friction for macrotribology and microtribology over a wide range of normal loads.

Various configurations for both length scales are summarized on the upper portion of the Figure and the data ranges are shown on the bottom. The data in this Figure represents the static friction i. It is observed from this figure that macroscopic friction values are generally lower compared to microscopic values. However, it should be noted for the values in Figure 3 that it is not always possible to determine the exact contact geometry e. For example, researchers using MEMS tribometers are much more confident in measuring the static friction values rather than the kinetic friction and have difficulties determining actual contact area in terms of the number of asperities in contact and their size.

This is one of the reasons Figure 3 compares macro- and microtribology using the most confident values i. Comparison of static coefficient of friction for micro- and macroscale tribology over a range of normal loads, adapted from reference [ 9 ]. The interest in studying micro- and nano- tribology became significantly greater over the last few decades due to the advances in the production of microelectromechanical systems MEMS.

Such micro-devices are possible to be produced using silicon photolithographic process as well as a number of other micromachining methods that have recently been developed such as microcutting, microdrilling, micromilling, laser machining [ 49 ]. Using such fabrication methods, numerious successful MEMS have been designed, which are currently being used commercially; pressure sensor and air bag sensor for automotive industry, TI digital mirror display [ 50 ], RF MEMS capacitive switch [ 51 ], Inkjet nozzles HP , micro-gears, etc.

There are also many potential applications for micro devices for applications in automotive, aerospace, and for medical instrumentation [ 1 ]. It is generally accepted, from the reliably perspective of MEMS, that these microdevices can be separated into different classes [ 52 , 53 ].

Class I consists of microdevices with no moving parts such as pressure sensors, accelerometers, strain gauge, etc. Class III devices are more complex in that they consist of moving parts and impacting surfaces e. The most complex microdevices Class IV are those that contain moving parts, impacting surfaces, and rubbing surfaces. Such devices include optical switches, shutters, scanners, locks, etc. This class is the most exciting one for the tribology community since there exist many opportunities for research and development.

Based on the material properties of these materials and the operating conditions to which they are exposed i. Possibly the most common and unavoidable failure mechanism of microdevices, from this list, is stiction. The main types of stiction consist of solid bridging, capillarity forces, van der Waals forces, and electrostatic forces. Solid bridging is typically formed after the rinsing and etching stage and results in permanent adhesion [ 55 ]. Common failure mechanisms of Microsystems adapted from [ 54 ].

Failure modes that have either a primary or secondary relationship with tribological interactions are underlined. Traditional macroscopic tribometers e. An early approach to span the gap between the contacts of atomic force microscopy and macrotribometers was performed by Ducker et al.

In this study, the authors describe the principle of using an atomic force microscope to measure the force between a planar surface and a colloid article. The authors used an AFM with a glass sphere as a tip, having a radius of 3. The design of this tribometer is based on a scanning force microscope developed by the authors. Their instrument overcomes many of the limitations that are observed with standard SFM e. The authors illustrate the capabilities of this nanotribometer by performing high-cycle wear tests on poly-carbonate, friction measurements on mica, and indentation tests on polyethylene [ 61 ].

Possibly the most effective way to study the friction and wear phenomena of microsystems is to build actual MEMS test platforms that are capable of measuring friction. One advantage of using MEMS as miniature tribometers over AFM based tribometers is that such systems are capable of simulating contact and sliding conditions virtually identical to real microdevices.

Another advantage of such MEMS tribometers is that the friction and wear behaviour of such devices can be monitored in situ in a desired environment. Table 3 summarizes some of the MEMS tribometers and their capabilities. For comparison, the ranges in normal loads and static frictions of these microsystems are presented in Figure 3. One of the first studies on using a MEMS system designed as a miniature tribometer was reported by Lim et al.

This structure was fabricated using a five-mask process. The normal load in this device was generated by an electrostatic attraction between an overhanging structure and an underlying electrode. The friction was measured using the restoring force of a displaced spring [ 62 ]. The authors measured the static friction of polysilicon—Si 3 N 4 and polysilicon—polysilicon interfaces using this MEMS tribometer.

The friction coefficient for both interfaces came out higher than expected i. Shortly after this publication, several other MEMS tribometers were designed for microtribological studies of materials used in micro-devices [ 63 , 64 , 65 , 66 , 67 ]. In the last few years, researchers have successfully used the Nanotractor design in order to obtain insights on asperity contacts and microtribological behaviour of MEMS materials [ 68 , 72 , 73 , 74 , 75 , 76 ].

It consists of two frictional clamps on each side spanned by an actuation plate in the middle. Using a clamping sequence with different phases, back and forward motion is achieved in 50 nm steps. Zabinski and co-workers published several articles on using a MEMS electrostatic lateral output motor for investigating tribological performance of various lubrication strategies at the micro-scale see Table 3 [ 69 , 77 , 78 , 79 , 80 , 81 ]. This MEMS tribometer is able to provide a significant amount of information on wear behaviour at the micro length scale due to the numerous contact locations.

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The slider consists of several dimples, which provide the contact with the counter surface. The cantilever is composed of a gold on polysilicon bimorph and due to the stress in the gold it is therefore naturally curled up [ 79 ]. Once voltage is applied to the system, the cantilever curls down and provides a lateral motion to the slider.

Wear information after the sliding tests can be obtained from the hinge area and also from the surface underneath the slider. The polysilicon MEMS sidewall tribometer has been extensively used for research on trying to understanding the precise physical process that causes wear of silicon at the microscopic length scale [ 70 , 82 , 83 , 84 , 85 , 86 , 87 , 88 ], Table 3. This device was developed by Senft and Dugger and was the first microdevice that was capable of measuring kinetic friction operating at realistic MEMSs contacts and velocities [ 52 ].

The motion in the orthogonal directions in the sidewall tribometer is created using two electro-static comb-drive actuators. The friction response in this device is calculated using automated image analysis of the displacements of the movable portion [ 52 ]. This method for measurements allows the sidewall tribometer to be used for friction responses of MEMSs materials for a high number of cycles.

Therefore, it has been extensively used for microtribological studies of monolayer coatings and thin hard coatings at various environmental conditions. It is different compared to the one presented by Senft in the sense that the normal force in the Leiden tribometer is generated by pushing the slide against a counter surface. In , van Spengen et al. With low normal loads i.

In addition, under these low-load conditions, no wear was observed on the sidewall or the counterface. With higher normal loads i. Conducting experiments using MEMS devices that are constructed as miniature tribometers is beneficial in the sense that they provide conditions that a real device would experience. However, MEMS tribometers typically require extensive investment in the fabrication and a great deal of care in order to obtain precise results. Simply for polycrystalline silicon, wide variations in friction values for have been reported with such MEMS microstructures i.

The differences in friction values between these studies can possibly be attributed to a variation in surface preparation, fabrication methods, contact pressures, environmental condition, and extraction of friction data from the device [ 69 ]. In any event, the scatter in the friction results obtained with MEMS tribometers makes it difficult for a MEMS designer to choose the right materials and fabrication method [ 69 ]. An additional disadvantage of MEMS tribometers is that any desired variations e. Therefore, experiments seeking to explore a range of contact conditions either become costly with MEMS tribometers or require innovative designs.

Thus, when exploring a range of experimental variables, one often turns to either commercially or custom-built microtribometers. For example, Gee et al. The authors used a lever arm design, which resulted in a tip motion that was an arc of a circle. The angular deviation of the tip, however, was not significant i. Besides being able to use different tip radii with this microtribological system, it also allows for a wide variation of normal loads i.

In addition, all materials that were used for the components of the tribometer were selected to be compatible for operation in an SEM. The authors published some preliminary microtribological results using this system on hard metals i.

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More recently, in the same laboratory, Gee et al. To validate the instrumentation, experiments were carried out on several commercially available coatings. In a later study, the same apparatus was used to simulate abrasion experiments on hardmetals [ 93 ]. The set up of this instrumentation allows for performing various track patterns such as uni-directional, cyclic, square, and intersecting. The micro-scratcher consists of a movable x-y stage and a tip holder on a balanced cantilever arm.

The normal force is applied above the tip holder using calibrated weights. In their study, the authors used a silicon cube corner tip to investigate the mechanism of wear-debris generation of polyethylener terephthalate PET. Several labs have also developed microtribometers that operate in vacuum chambers [ 95 , 96 , 97 , 98 , 99 , , ].

The main motivation of such a set up has been to perform MEMS research. For example, Kosinskiy et al. A microtriboapparatus was developed by Liu et al. Similarly, samples engagement and normal load is accomplished using a piezo in the Z- direction. The advantages of this tribometer over atomic force microscopy are that it can operate at higher velocities i. The authors presented the reliability of this microtriboapparatus by studying the surface adhesion and microtribological properties of Si , diamond-like carbon DLC , and hexadecane thiol HDT.

In their study, Liu et al. A comparable microtribometer was also used by Sawyer et al. Similarly, Bonnevie et al. Currently there exists many microtribometers designed by a variety of instrument manufacturers. The normal loads throughout the scratch tests of this instrument can be varied form 0. Similarly, Ding et al. Another commercially available technique for measuring tribological properties at the microscopic scale is the CSM nanotribometer [ ]. This tribometer is composed of three stepper motors two in the X - and Y -axis and one in Z -axis [ ]. The coefficient of friction is determined by monitoring the deflection force on the cantilever.

This nanotribometer is capable of operating in a reciprocating and a rotating mode. Using this instrument, in reciprocating mode, Barriga et al. More recently, Sahoo et al. Similarly, Zhao et al. Besides applications in metallic and ceramic tribosystems, the CSM microtribometers have also been used for biomediacal applications, such as human tooth wear [ , ]. Basalt-Must Tetra Ilmenau is another commonly used microtribometer. The applied normal load as well as the lateral forces are measured from the deflection of a double-leaf cantilever by means of a fiber-optical sensor [ ].

More detail on this microtribological instrument can be found elsewhere [ ]. This microtribometer as well as modified versions using the same cantilever has been used to study a multy-disciplinary range of topics including contact lenses [ ], dental tribology i. The Hysitron nanoindentation platform is another widely used instrument that has the capability of measuring the mechanical and tribological properties of materials at the microscopic length scale.

This instrument operates with normal and lateral force loading configurations using a patented three-plate force displacement transducer [ ]. The force in this system is applied electrostatically, which results in pulling down the center plate towards the bottom plate. The normal force is calculated from the magnitude of the applied voltage.

Usually, for mechanical properties measurements a diamond Berkovich tip is used and for microtribological a diamond spherical tip can be used. A typical image of the wear track that has been created using this instrument is shown Figure 4. A typical image of the wear track that has been created in reciprocating motion 1D using the Hysitron nanoindentation instrument. Typically, a spherical diamond or sapphire tip is used in such wear tests with the Hysitron systems.

Similarly, Schiffmann and co-workers [ ] explored different techniques for microtribological and nanomechanical testing using a Hysitron nanoindenter. The authors classified the different testing techniques by the dimensions of the lateral movement of the tip; standard nanoindentation with no lateral movement of the tip 0D , scratch and reciprocating wear tests where the lateral movement of the tip is one dimensional 1D , and scanning wear tests where the lateral movement of the tip is in the X and Y direction with a constant load 2D.

In their study, the authors performed the comparison between the different testing modes on AF45 glass, single crystal [ ] silicon, and polycrystalline aluminium. It was found that the scanning wear tests i. This was explained by two main differences in the sliding mechanisms; 1 the exact number of the tip crossing of over a single point on the wear track is much larger and more difficult to be determined in the 2D tests and 2 the piled-up material is redistributed back onto the worn surface, whereas for the 1D test it is re-deposited on both sides of the wear track.

Atomic force microscope images of the a reciprocating wear test i. In an earlier study, Schiffmann and Hieke [ ] investigated the micro-tribological properties of diamond-like carbon DLC coatings using a Hysitron Triboscope. The experimental procedure for the reciprocating wear tests that the authors used consisted of three phases: The exponent m in the ploughing component is related to the strain hardening index. The research group led by Chromik used a similar approach to the one of Schiffmann et al. The authors measured the friction coefficient using a Hysitron Ubi system with a 2D transducer and spherical diamond tips for varying normal loads.

The friction results from the microwear experiments were extracted by the Hysitron software and then analyzed using a custom-build analysis code which was written with Matlab software. The topography and shape of a nanoindentation tip can be characterized using an atomic force microscope operated in tapping mode [ 21 ]. Using a custom-built pixel-counting algorithm produced in Matlab, Stoyanov et al. The topography and shape of a nanoindentation tip is characterized using an atomic force microscope operated in tapping mode in combination with a custom-built pixel-counting algorithm produced in Matlab.

This characterization method is useful for calculating the actual contact area and consequently the contact pressure during a given sliding cycle. For certain systems, the contact pressure can then be used to calculate the interfacial shear strength using Equation 2. However, due to deviation in the Hertzian contact behaviour at the microscale [ 21 , 22 ], the pressure P is calculated using the real contact area of the tip. The contact area can be obtained from the area function of the tip i. Besides obtaining friction with the Hysitron nanoindenter, the wear behaviour of surfaces can also be evaluated by using the three phases during the microtribological tests.

A schematic representation of a typical wear test result is shown in Figure 7 in order to illustrate the three phases during the sliding process [ ]. In this Figure, D IL represents the initial loading depth, which can be calculated from the elastic-plastic depth of an indentation test or from the post analysis of the test [ ]. D RD is the residual depth and is measured from the height difference between the last cycle and the last cycle under a certain load and the post scan. Lastly, D EC is the elastic recovery and is measured from the height difference of the post scan and the last cycle.

Using these measurements and the equations presented in reference [ 22 ], one can calculate the different depth contributions. An example of the depth contributions is shown in Figure 8 for cosputtered Ti-MoS 2 coatings. Schematic representation of wear data obtained from a microtribology reciprocating wear test conducted with a Hysitron nanoindentation system, adapted from reference [ ]. An example of the depth contributions measured from a microtribology reciprocating wear test on a Ti-MoS 2 coating. The depth contributions are determined from the depths labeled in Figure 6 in the manner described in References [ 21 , ].

The process between the two surfaces in contact i. Most studies have used ex situ techniques of separated contacts to investigate the interfacial process. Similarly to a crime scene, however, many of the important aspects in the interfacial process, controlling the tribological properties, have a dynamic behaviour e. Therefore, tribologists have often developed different surface analytical tools to examine the buried interfaces throughout the sliding procedure.

Over the years, such techniques have included optical microscopy, videography, interferometry and chemical spectroscopy Raman, FTIR [ 9 ]. As recently reviewed by Chromik et al. Following these early pioneering studies, other groups began conducting in situ tribometry, including the groups of Berthier at INSA de Lyon in France [ 44 ], the group at Perdue University [ ] and the group led by Singer and Wahl at the U.

Naval Research Laboratory NRL , where the tribometer developed by the latter had the combined capabilities for visual observations and Raman spectroscopy [ 42 , , , , ]. The observations and Raman microscopy in this tribometer are performed through transparent counterfaces which are typically made out of sapphire or glass.

Even though this technique uses macroscopic contacts and while the interfacial phenomena in macrotribology may not directly translate to a microscale sliding contact, it does provide valuable insight into third body processes that likely still occur for microtribology. The objective of this in situ instrumentation was to identify what third body processes and velocity accommodations modes control the friction and wear behaviour of solid lubricants such as molybdenum disulphide, diamond-like carbon, and other nanocomposite coatings. In situ tribometry has been effective at demonstrating the importance of third bodies in controlling friction and wear, and in general, enforcing the idea that the wear process can be thought of as a tribological circuit [ ].

When third body material is retained in the sliding interface, it is often a positive influence on wear resistance, leading to like materials sliding against one another resulting in a potentially low friction interface. This is especially true for molybdenum disulfide—based solid lubricants [ 25 , 42 , , ] and some diamond-like carbon coatings [ , , ], but has even been found to be important for metals and metal matrix composites [ ]. In situ tribometry can be used in many ways to analyze third body formation, flows and retention at the sliding interface of a tribocontact.

On the upper left part of this figure is a schematic representation of the cross-section of a transfer film formed between a solid lubricant and the counterface. The top right of this figure shows an actual view of the contact through a sapphire hemisphere the slider material using an optical microscope. This image was taken before the sliding commenced and the dark circle in the center of this image is the contact area, which agrees well with calculations from the Hertzian contact model [ ].

The interference fringes are around the contact region and can be used to measure an average transfer film thickness. It is observed that the changes in transfer film thickness with respect to the cycle number correlate with the evolution of the coefficient of friction for a Ti-MoS 2 coating [ ], as seen in the lower right graph. The interference fringes are located around the contact region and can be used to quantify transfer film thickness, as shown in the bottom left of the figure.

In this study it was found that the addition of Si stabilized the transfer film compared to pure a Ti-C coating. Adapted from reference [ , ]. Another method to explore stability of thirdy bodies with in situ tribometry is by conducting an image analysis of the coverage of transfer film material during a sliding test.

Figure 9 b shows an example for Ti-C and Ti-Si-C coatings, where the transfer film coverage was measured by image processing routines written in ImageJ software. The findings in this study were that the retention of transfer film was enhanced when Si was added to Ti-C and this also was reflected level and stability of the coefficient of friction [ ]. Using in situ tribometry, the group found correlations between transfer film characterization and tribological properties i. The authors found that when sliding on Pb-Mo-S coating, a MoS 2 transfer film is formed during the first few cycles of the test which contributes to low friction coefficients in dry and humid environment.

Furthermore, in situ monitoring revealed that the dominant velocity accommodation mode was interfacial sliding for both conditions. In humid air, however, a second velocity accommodation mode was observed i. In addition to exploring the dynamic nature of transfer film thickness and coverage, in situ tribometry is also useful for determining velocity accommodation modes [ ]. With in situ tribometry, they identified the primary velocity accommodation modes to be similar to the study of Dvorak i.

However, for coatings with higher YSZ content coatings, a third velocity accommodation mode appeared—abrasive plowing that led to instability in the transfer film and friction spiking [ ]. While in situ methodologies within the contact are ideal for understanding third body behavior, they confine the material selection of the counterface to a transparent one [ ], which makes it sometimes questionable to extrapolate sapphire vs. Generally, the differences in adhesion of transfer film material to sapphire versus the adhesion to steel are what determine the degree to which one can quantify third body processes by in situ tribometry for engineering applications.

Another limitation for the in situ tribometry technique is for study of opaque materials, like metals.

While valuable information can still be gained about transfer films stability and third body flows for metals [ ], the sliding interface is not directly observed and most information on the dynamic processes occurring for metallic wear cannot be observed, even with in situ tribometry. Nevertheless, third body processes do occur in much the same way for any tribological contact and insights from the in situ tribometry methods are a useful starting point for understanding their behavior.

A complementary in situ methodfor the study of metallic friction and wear was developed by a group led by Martin Dienwiebel. The instrument includes atomic force microscopy and holographic microscopy to monitor topographical changes of the worn surface of sliding systems. The most significant advantage of this tribometer is the capability to perform surface topography at the micro- and nanoscale with online measurement of wear and with in situ lateral forces detection [ ].

Figure 10 shows the concept of this instrument, which consists of an optical microscope, pin, and an AFM. The image to the right in Figure 10 shows the path for the AFM, optical microscope, and the force sensor and pin. This motion path is designed in this way for the purpose to allow all devices to have access on a specific part of the path i. The pin is attached to the force sensor and is capable of performing smooth cycles without losing contact with the counter surface. The other two sensors i. Thus, surface topographical images can be obtained after every cycle with a very short pause of the motion.

In their first study, the authors performed a preliminary study using this instrument on pure iron samples with polyslphs olephim PAO as a lubricant. This study showed promising results in terms of the wear and friction behaviour and the authors concluded that this tribometer can also be used for more complex systems.

The concept of this instrument which consists of an optical microscope triangle , pin square , and an atomic force microscopy AFM circle. On the right, the travel path for the three components is indicated as AFM dashed line , optical microscope dotted line and pin solid line.

In order to also capture the dynamic process within the buried interfaces in metallic sliding contacts, in a set of resent studies [ , ], the experimental results of the on line tribometer were linked to atomistic simulations using realistic bond order potentials. This approach for studying the interfacial process is captured in Figure It is reasonable to argue that such a comparison between atomistic simulations and macroscopic experiments is not possible due to the differences in the environmental conditions.

Using the same approach in the second study [ ], the topographical changes of dry and lubricated sliding contacts were evaluated to identify the different velocity accommodation modes which lead to fluctuations in the friction and wear. An approach of studying dynamic interfacial processes leading to the variations in the friction and wear. This concept involves linking on line experiments to atomistic simulations with realistic bond order potentials.

In a more recent study, the two approaches i. This unconventional approach showed that the evolution of the roughness followed the coefficient of friction trend closely, with initially low values followed by higher roughness during steady state. Similarly, the transfer film behavior correlated well with the roughness of the worn surfaces and the subsurface microstructure of the worn surfaces, as revealed by the in situ technique.

Schematic representation of combining in situ and on-line methods for studying the interfacial processes in sliding couples [ ]. In situ and on line tribometry are powerful tools for understanding interfacial phenomena at the macroscopic length scale, but the working principles of these instruments are difficult and, in some cases, impossible to implement at the microscopic length scale.

One of the few examples of in situ methods applied to microtribology was accomplished by Krick et al. They developed an in situ micro-tribometer that allows for optical observations of the contact size, geometry and topography. In order to validate their instrumentation, the group performed a study on a rough rubber sphere sliding against a glass counterface with varying normal loads between 1 and 50 mN.

The results of the loading-unloading experiments revealed a strong hysteresis of the measured real contact area plotted against the normal load. When sliding however, a distortion was observed in the contact geometry as well as an increase in contact area during steady state.

In addition to the experiments, the authors used Persson contact hardness mechanics theory [ ] to calculate the changes in contact area with respect to the magnification. A limitation of this technique is that information on third body flows and transfer film behavior is still unable to be observed. It has been designed with the primary goal of determining the real contact area. While it is generally understood that many of the velocity accommodation modes and third body flows presented above for macrotribology also occur for microtribology, all studies of these processes for microtribology have been primarily with traditional post analysis, where the contact is separated and analyzed after the test is complete.

Therefore, there still remains an incomplete understanding of the differences in velocity accommodation modes for solid lubricants, metals and coatings mentioned above when decreasing the contact area to a few micrometers. For instance, the transfer film at the microscale is likely to differ in its dimensions; roughness of the contacting surface may play a more prominent role in determining transfer film stability and environmental effects may differ from macroscopic tribology.

One such example of a study of microtribology with separated contacts was conducted by the tribology group at McGill University. The interfacial process were studied for solid lubricants due to interest for their potential applications in microdevices. In this study, Stoyanov et al. The microtribological test in this study were performed using a Hysitron nanoindenter, whereas the macrotribological experiments were performed using an in situ tribometer. At the micro-scale, on the other hand, the transfer films were analyzed ex situ on the counterface i.

Both length scale under dry conditions formed transfer films which contribute to a decrease in friction and wear and the main velocity accommodation mode was interfacial sliding [ 25 ]. However, the presence of humidity led to different velocity accommodation modes for both length scales; transfer film shearing and transfer film removal for the macro- and micro- scale respectively. Transfer film analysis for macro- and micro- tribology using in situ and ex situ techniques.

Figure 13 a shows AFM analysis of the transfer films for the microscale experiments and Figure 13 b shows the in situ analysis for the macroscale experiments. Both length scale under dry conditions formed transfer films which contribute to a decrease in friction and wear and the main velocity accommodation mode was interfacial sliding Adapted from reference [ 25 ]. Exploring the origin of friction and wear properties at the atomic level and the development in the fabrication technology of microsystems were initially the two major drives for studying tribological properties at the small length scale.

Understanding these miniaturized properties of biological materials is useful in solving most challenges in further development of complex microdevices [ ] as well as a requirement in the bio-inspired design of tribological systems.

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In addition, for certain biological applications, the microtribological approach tends to be a more simple and robust tool compared to conventional macroscopic set ups. For example, Scherge et al. Recent studies have further verified the convenience of microtribology in dental application by investigating human tooth enamel and artificial hydroxyapatite [ , ]. A similar approach was also adopted by Erickson et al. A more detailed review on micro- and nano- biotribology can be found in reference [ ].

Geckos are probably one of the most interesting creatures from the adhesion point of view of biological systems. Their ability to climb up smooth vertical surfaces is due to a combination of adhesive forces and the increased surface-to-volume ratios. The authors mentioned that the values of their adhesive force measurements indicate that the individual seta operate by van der Waals forces. The authors in this study focused on measuring the influence of atmospheric condition and surface chemistry on the adhesion of a gecko. The results in this study clearly showed that the adhesive forces of a single gecko spatula are strongly affected by humidity on a nanoscopic level.

Therefore, the authors suggested that when using this biological mechanism for the design of artificial attachment systems, one should take into consideration the influence of the atmospheric conditions. Besides the great scientific interest in researching the adhesive and tribological properties of geckos and flies, there have also been a large number of studies on other biological surfaces at the microscopic level. Microtribometers have been shown to be a very useful tool for measuring friction properties of contact lenses due to their challenging contact and sliding conditions [ ].

For instance, the research group led by Sawyer at the University of Florida, studied the friction coefficient of soft contact lenses using a microtibometer with dual glass flexures to apply and measure mN-range forces [ ]. In this study the authors obtained a friction coefficient in the range between 0. The results indicated that the main contributions to the friction forces include viscoelastic dissipation, interfacial shear, and viscous shearing.

The authors also developed a model that takes these three contributions into account and was consistent with the experimental data. It was suggested that the main contributions to the friction force was due to viscoelastic dissipation of the contact lens material and interfacial shearing. Sawyer led a different study on tribology properties of biological materials using the same microtribological apparatus as in the study above [ ]. The purpose of this research was to measure friction of a single layer of arterial endothelial cells and to obtain a better understanding of the biological response to the tribological stresses e.

Using a normal force of 0. It was also observed that with the lower friction values nearly no cells were removed, whereas with the higher limit of the coefficient of friction many cells were detached. Further investigations with the same dual flexure-based microtribometer were also performed by Dunn et al. The interest of studying tribological properties of Si and SiO 2 surfaces at the microscopic length scales is quite different compared to biomaterials. The authors used scanning and transmission electron microscopy to examine the worn surfaces and local temperature changes were monitored using advanced infrared microscopy.

Debris particles were observed, which were created by adhesion and fracture through the silicon grains. Initially, the dominant wear mechanism was adhesion, but subsequently to the formation of the debris particles, plowing tracks were observed on the worn surfaces indicating a second wear mechanism. Using these devices, the authors showed that friction and wear behaviour of silicon at the microscopic length scale have a strong dependence on the surface condition prior to testing. For example, the highest wear coefficients were seen with surfaces that were cleaned with oxygen plasma prior to testing.

On the other hand, surfaces that were exposed to air before testing showed negligible wear and relatively low friction values. Such observations in this study indicated that some element present in air or in water attach to the surface of the silicon during air exposure and act as lubricants i. Previous studies have also shown that the surface condition of silicon, prior to testing, can have a significant effect on the microtribological properties. In terms of roughness effects, the authors found that if at least one of the surfaces is relatively flat, the friction behaviour is strongly influenced by capillary forces.

With the rougher surfaces, on the other hand, the roughness effect showed to decrease the friction. Therefore, it was concluded that the surface conditions e. Sliding on silicon typically leads to the formation of damaging debris particles, which are formed mainly due to breakage of asperities or fractures in the grains. Using SEM imaging of the worn surface, I.

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The authors suggested, based on previous literature [ ], that the sliding causes the silicon surface to oxidize, which when removed leads to the formation of amorphous silicon oxide debris particles. The volume of these debris particles have also shown to increase with a decrease in relative humidity [ ]. The evidence of grooves on the worn surface in the study of Ku et al. High sliding speed leads to a reaction between the silicon oxide on the Si surface with water molecules and therefore results in the formation of a Si OH 4 layer. This Si OH 4 tribofilm layer is known to have low shear strength and thus decreases the friction force.

More recently, a detailed study on the wear mechanisms of polycrystalline silicon was performed using the sidewall tribometer [ ]. During the running in period, the authors suggested that the wear process was dominated by adhesive forces leading to a removal of the monolayer coating and the native silicon oxide layer. Subsequently, adhesive wear occurs on the freshly exposed silicon surface, creating debris particles by fracture in the grains and changing the wear mechanism to abrasive wear, as also suggested by Ku et al.

Once the steady state stage is reached, the agglomerates are broken down in to smaller pieces and consequently decreasing the surface roughness. Debris particles are a common problem in microdevices and can lead to premature failure of the system. However, this causes many constraints in the design and therefore is not very efficient.

A more effective solution would be to use coatings e. One of the most commonly tested coatings at the micro scale is diamondlike carbon DLC [ 79 , , , , , , , , , , , , , , ]. The main drive for examining DLC coatings at the micro-scale is due to its desirable properties e. DLC coatings are generally harder compared to other solid lubricants or even hard metals [ ]; hardness values of DLC coatings have been reported up to 27 GPa measured with nanoindentation instruments [ 49 , ].

The main purpose of their study was to identify the fundamental wear and friction behavior of diamond-like carbon coatings using new experimental methods and analysis. The authors investigated DLC coatings on various materials currently used in hard drives and microdevices. Similarly, the coefficient of friction for the DLC on Si sample was 0. Using the same nanoindentation instrument and experimental methods, Schiffmann et al. The authors showed that with the lower load regimes the coefficient of friction was dominated by the elastic component and after a critical load the plowing component increases.

This was explained by the fact that with the lower normal loads the tip is mostly sliding on the surface and with the higher normal loads the tip is plowing through the coating. Using the same equation, Schiffmann was also able to show that the coefficient of friction is mainly dominated by the plowing component in the first few cycles of the test, whereas with the higher cycle number, the elastic component increases and eventually dominates, as shown in Figure In terms of the coatings performance in this study, the Si doped DLC coating showed the lowest coefficient of friction i.

Coefficient of friction vs. The data is fitted according to Equation 1. It is observed that for the higher cycle numbers the friction remains relatively constant for the higher load regimes, Figure 14 b; For the first few cycles on the other hand, the friction increases with the higher load regime, which resulted in higher plowing exponent m , Figure 14 a Adapted from reference [ ].

As a MEMS device, the authors used the electrostatic lateral output motor which is shown in Figure This MEMS device provides many opportunities for characterizing the wear behaviour due to the numerous contact locations e. The first wear analysis in this study was performed on the wear track and the slider, as shown with the SEM image in Figure 15 , and the second analysis on the hinge region of the MEMS.

Comparing the different conditions, slight wear tracks were observed for the uncoated MEMS device that was run in air, whereas the DLC coated system revealed nearly no signs of wear. Most debris particles for all conditions were observed in the hinge region, where the greatest number of contact is present and also where catastrophic failure was observed. In air, the DLC coated systems revealed a six times increase in performance over the uncoated systems, whereas in vacuum the DLC coated MEMS performed up to three hundred times better over the uncoated ones.

Similarly to the study with the nanoindentation instrument by Kuster et al. SEM of the slider of the electrostatic lateral output motor after the tests for a uncoated in air; b uncoated in vacuum; c DLC coated in air; d DLC coated in vacuum; and e an electrostatic lateral output motor as well as a cross sectional schematic figure of the microsystem Adapted from reference [ 79 ].

Not surprisingly, the high wear resistance of the DLC coating in this study was also consistent with earlier studies on microtribological behaviour for DLC [ , , , ]. This was also confirmed with the study performed by Liu et al. Although DLC coatings have a great potential for applications in MEMS due to their low adhesion and friction forces, there have been some setbacks. One of these issues is with the deposition processes, which are not compatible on certain microdevices.

In addition, due to the small sizes and design constraints of MEMS, coatings applied to these systems are prefered to be relatively thin. However, the tribological properties of DLC coatings with a thickness below nm can be influenced by the substrate materials and the topography. Keeping the thickness issues of these coatings in mind, researchers have recently focused on graphene as an alternative to DLC for applications in microsystems. Using a Basalt Must tribometer, Marchetto et al. The authors found that while the graphene layer is damaged during reciprocating sliding, the coefficient of friction is still lower compared to graphite and five times lower compared to hydrogen etched SiC.

More recently, other materials besides DLC have also become of interest in microtribological applications. Researchers have demonstrated how the atomic-layer deposition technique can be used to successfully depositing chalcogenides onto microdevices [ , ] and therefore have raised the interest of studying the microtribological properties of solid lubricants such as WS 2 and MoS 2. Similarly to DLC, the behaviour of molybdenum disulphide based coatings at the micro-scale have been studied intensively due to their low friction values at the macro- length scale [ 40 , 41 , 42 , 43 , , , , ].

This was explained by the adhesion properties of the particles and their ability of the particles to stay in the contacts. In addition, from the micro-scale friction results in this study, it was observed that the coefficient of friction decreased with increasing the normal load and contact pressure. Lower friction coefficient with higher normal loads has also been observed at the micro-scale with MoS 2 based thin film coatings of thicknesses between and nm [ 21 ]. Examples of the coefficient of friction vs. The friction results in these plots were obtained for a variation of normal loads i.

A summary and analysis of the result in this study are shown in Table 4. The velocity accommodation parameter i. The S o value was found to decrease with the addition of metal content, which correlated with a decrease in wear volume and friction coefficient, Table 4. In terms of the relationship between the friction force and the normal force i. It was concluded that the improved tribological performance with the metal doped MoS 2 coatings was attributed to an increase in mechanical properties, a decrease in surface adhesion, and a decrease in surface adhesion.

Average coefficient of friction vs. The coefficient of fricition decreases with the addition of Ti and Au to MoS 2 for all normal loads tested. The lower friction values with the metal doped coatings is attributed to an increase in mechanical properties, a decrease in surface adhesion, and a decrease in surface adhesion adapted from reference [ 21 ]. Microtribological properties summary for the experiments shown in Figure 16 for a diamond spherical tip sliding against pure MoS 2 and metal doped MoS 2.

The addition of Ti and Au to MoS 2 drastically increases the wear resistance and the interfacial shear strength, which is calculated using Equation 2. The authors were able to provide a visualization of the tribofilm on the worn surface of the Au-MoS 2 nanocomposite film, as shown in Figure The tribofilm consisted of crystalline MoS 2 with the basal planes parallel to the sliding direction, which is consistent with literature on tribological properties of Pb-MoS 2 and PbO-MoS 2 coatings at the macroscopic scale. It was concluded in this study that the tribofilm formation at the nanoscale is responsible for the low friction and high wear resistance of MoS 2 based coatings.

The image was produced using an atomic force microscope and shows the high -resolution crystal structure of the tribofilm. The lattice structure corresponds to that of a single crystal MoS 2 with the basel planes parallel to the surface adopted from reference [ ]. The evolution of the crystalline MoS 2 tribofilm with respect to the friction behaviour, at the small length scale, was further explored using Raman spectroscopy [ 21 , 22 , 25 ].

Previous macrotribological studies [ 42 ] have shown Raman spectroscopy to be an effective analysis technique for identifying crystalline orientation of MoS 2 based films. Thus, ex situ micro-Raman analysis on worn surfaces, which were created using a microtribological instrument, were performed metal-doped MoS 2 coatings with varying metal content. For a variety of contact pressures, Raman peaks that are consistent with crystalline MoS 2 were observed on the worn surfaces and were not seen on the as deposited amorphous coatings.

This was an indication that a MoS 2 tribofilm, similar to the one observed by Kim et al. These MoS 2 tribofilms have thicknesses of a few nanometers and typically have their basal planes parallel to the sliding direction [ 40 , 43 ]. Gold is a noble material and is known for excellent corrosion resistance, great electrical conductivity and thermal properties [ 49 ]. It is currently a widely used material in contact switches, where adequate electrical properties are crucial [ 3 , , ].

Dugger proposed [ ] that possibly the largest market for microdevices in the near future is with microswitches e. These devices offer essential advantages over conventional diode-type switches [ ], including lower power loss, which can increase the battery life in certain systems e. Failure mechanisms in these microswithes devices typically include adhesion, melting, and increase in electrical resistivity due to coating i. Therefore, there have been numerous studies on the mechanical and tribological properties of Au with the purpose to improve its reliability in microsystems.

Such a study on the mechanical and tribological behaviour of gold and copper coatings for applications in RF MEMS switches was performed by Barriga et al. The authors concluded that for Cu-Cu contacts the coefficient of friction at the microscale was affected by relative humidity i. The increase in the friction coefficient for the Cu-Cu contact was explained due to capillary forces at the higher relative humidity level. One successful lubrication strategy with the purpose to improve the mechanical and tribological properties of gold was developed by Lince [ , ], who cosputtered Au with a small amount of MoS 2.

In a macrotribological study, the authors reported that the best performance i. Therefore, this coating showed the most promise for applications in microswitches and was further investigated with instruments that simulate real MEMS contacts [ 22 ]. The mechanical and tribological performance of the Au-MoS 2 coating were compared to pure Au coatings with varying contact sizes and pressures. The small addition of MoS 2 to Au was found to increase the wear resistance significantly for all normal loads, as shown in Figure The higher wear resistance with the Au-MoS 2 coating compared to the pure Au coating was attributed to a decrease in surface adhesion and differences in velocity accommodation modes.

This article concluded that the small additions of MoS 2 to Au could be a helpful for microcomponent and microswitch applications with sliding interfaces. The higher wear resistance with the Au-MoS 2 coating compared to the pure Au coating was attributed to a decrease in surface adhesion and differences in velocity accommodation modes adopted from reference [ 22 ]. Another approach to address reliability issues in microswitches was proposed with the addition of a MoS 2 sacrificial layer on top of Au coatings [ ].

In addition, this layer would promote the formation of a transfer film on the counterface which would further increase the wear resistance. These coatings were tested using three different microwear experiments classified as 1D and 2D tests and were compared to pure Au coatings. The results of this study showed that the bilayer coatings i. While the silicon micromachining processes are currently well developed, they are expensive, time consuming, and require special facilities [ , ].

thick film

In addition, silicon-based bioMEMS have revealed challenges with moving components and therefore limits the design opportunities. Polymeric materials, overcoming most of these challenges, have therefore received a great amount of attention for bioMEMS [ ]. Besides being more costly beneficial, the machining and manufacturing process of polymeric materials is significantly easier compared to silicon.

Moreover, polymers are ideal for bioMEMS due to their increased wear and corrosion resistance, compared to silicon. The wide variety of polymeric materials makes it also easier to select a material with the desired mechanical properties for a specific application. Most literature on studying the friction and wear properties of polymeric material has used macro-scale tribological equipment [ ] mostly because of the interest for biomedical joint applications. However, it is certainly possible that the wear behavior of polymers also varies with decreasing the contact size down to a few micrometers.

Therefore, more studies have recently focused on investigating the tribological properties of polymers at the microscopic length scales in order to understand the scale effects [ ] and their range of applications in bioMEMS. The authors showed that the friction values were smaller with the polymers compared to the silicon. In addition, PDMS and PMMA showed no dependence of the holding time and relative humidity on the adhesive forces, while adhesion force of Si was strongly dependent on the rest time and humidity levels. Therefore, the obtained results in this study suggested that these polymeric materials are ideal for applications in bioMEMS, where significant variations in environmental conditions and rest times are present.

Microscale abrasive wear for a wide range of polymeric materials was also studied by Shipway et al. One of the conclusions of this study was that the wear rate has a strong dependence on the polymer type and its mechanical properties e. It was pointed out that the polymers with small hardness values experienced a low wear coefficient value and the polymers with high strain to failure showed less indention induced wear. While most of the attention on polymer brushes has been given in studying the nano- [ , , ] or macro- scale [ , , ] tribological properties, some studies have also been recently performed at microscopic length scales [ ].

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