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A PROJECT ON MICRO/NANO TRIBOLOGY
A brief introduction to Tribology and its characterization in Micro/Nano applications, brief history of the measuring techniques, their applications and evolution from macro to micro/Nano scale machining, outlined literature review on atomic force, surface physics and explanation of roughness mechanics and different contact methods and their applications.
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Explanation of physical and chemical properties of surface layers, adhesion, Friction, Wear, Lubrication. Detailed elaborate description about the forces acting between the surfaces (Adhesion) and their characteristics with examples. Measurement of static and dynamic forces between the surfaces and the instruments used to measure them.
Introduction to Nanorheology, origin and its application. Finally, role of Micro/Nano Tribology in MEMS, Stiction issues and its rectification.
Tribology ‘s industrial significance for modern machines that use sliding and rolling surfaces, tribology is essential. Examples of productive friction include brakes, clutches, wheels on trains and cars, bolts and nozzles. Examples of productive wear include pencil writing, processing, polishing. Examples of unproductive friction and wear are internal combustion engines, gears, cams, rollers and seals for aircraft. For economic reasons and long – term reliability, the importance of reducing friction and wear control cannot be overemphasized. The purpose of tribology research is, understandably, to minimize and eliminate losses resulting from friction and wear at all technological levels where surface fretting is involved. Tribology was important in many industrial applications requiring relative motion, such as railways, automobiles, aircraft and machine component manufacturing processes.
The force required to initiate and maintain in the relative motion is known as Frictional force. It is the dependent variable of normal load acting on the surface and independent of the area of the contact.
F = f*P
Where; F = Frictional force, f = friction coefficient and P = Normal Load
Tribology mainly deals with the frictional force produced between two bodies when they come in contact with each other intentionally or unintentionally. Considering mainly about the adhesive forces, or technically known as interfacial adhesive bonds, most of the theoretical and experimental results stated that they are most commonly seen on the metallic surfaces.
Theoretically, these forces are formed by the formation of strongest bonds like metallic, covalent, ionic bonds and also weaker bonds like van der Walls bonds (classified as long-range forces) between the two surfaces (independent) of the contact area.
Frictional force can also occur due to ploughing. Ploughing mainly occurs if there is an interaction between two surfaces of different hardness. In ploughing, the hardest surface tries to penetrate into the softer or smoother surface and created some tooth on the smoother surface which results in more amount of wear if the force is applied or the particle slides on this particle.
There are few types of wear as well, like;
Adhesive wear: Due to the formation of the adhesive junctions at the contact of two surfaces, Adhesive wear is caused. Where, these junction strengths lie in the physio-chemical bonding between the particles of both the surfaces.
Abrasive Wear: Abrasive is most common and dangerous wear compared to Adhesive wear. It occurs due to which if there is an interaction of two surfaces of different hardness where one of them is significantly harder. This is one of the main causes for ploughing.
Wear is more detailed in the further chapters of this paper.
Solid Surface Characterization
A solid surface, or more precisely a solid-gas or solid-liquid interface, has a complex structure and complex properties that depend on the nature of the solids, the method of preparing the surface and the interaction between the surface and the environment. The properties of solid surfaces are crucial for surface interaction because surface properties affect real contact area, friction, wear and lubrication Solid surfaces contain irregularities or deviations from the prescribed geometric form irrespective of the method of formation. In addition to surface deviations, the solid surface itself consists of several areas with physio-chemical properties typical of the bulk material itself. There is an area of hard – working or deformed material as a result of the forming process in metals and alloys. In ceramics and polymers, deformed layers are also present. These layers are extremely important because their properties are completely different from the annealed bulk material from a surface chemistry point of view. In the same way, their mechanical behavior is influenced by the quantity and depth of the surface layers.
Physio-Chemical Characteristics of Surface Layers
The metallurgical properties of a metal, alloy or ceramic surface can vary significantly from the bulk of the material as a result of the forming process the surface of material was prepared. In the deformed zone, we usually find smaller grains from the grain recrystallization. The individual crystallite or grains with interface fritting can also be adjusted to Surface. Surface. The properties of the deformed layers may differ completely from the annealed layers Material of bulk. In the same way, their mechanical behavior also depends on the quantity and the Depth of surface layers deformation.
Chemically Reacted Layer
All metals and alloys except certain noble metals (such as gold and platinum) React with oxygen and form oxide layers in air; however, they are quite common in other environments. Probably forming other layers. Aluminum oxide is an integral oxygen Part of the structure, so a layer of oxide is not expected. Polymers don’t generally form a Layer of oxide. Surface interaction with gasses does not necessarily stop with formation. A monolayer adsorbed. if a mechanism for continuous exposure of new products is available Surface interaction with the environment proceeds, leading to a thick film formation. The oxide thickness and other chemically reactive layers depend on the reactivity of Environmental materials, reaction temperature and reaction time. Type of thickness from 10 to 100 nm of these layers, although many thicker layers can be formed Oxide layers can also be produced in the process of processing or friction. One or more elementary oxides may be oxide layers
In addition to the chemically reactive layer, which forms in reactive environments on metals, Layers can form on metal or non – metallic surfaces from the environment Physisorption is an example. The molecule is attached It is shown to the surface itself as a diatomic molecule, as could occur in oxygen. In such a way One case, the two diatomic molecule oxygen atoms can bind to the already contaminated.
Methods of Characterization of Surface Layers
Numerous analytical surface techniques which can be used to characterize surfaces Layers are available commercially The metallurgical properties The deformed layer (grain structure) can be determined by sectioning the surface and examining the cross – section with a highly magnified optical microscope or scanning Microscope Electron The chemical analysis of adsorbed organic layers using surface analysis can be carried out Tools such as mass spectrometry, infrared spectroscopy transformed by Fourier (FTIR), Raman Dispersion, Magnetic Nuclear Resonance (NMR) and XPS. The techniques most commonly used Depth profiling is used for measuring organic layer (including lubricant) thickness. Ellipsometry and XPS.
Analysis of Surface Roughness
Surface texture is the random or repeated deviation from the nominal surface that forms the Three – dimensional surface topography. Texture of the surface includes: (1) roughness; (2) waviness (3) lay; and (4) defect. Nano-and microroughness is formed by short – wave surface fluctuations
Measurement of Surface Roughness
A distinction is made between atomic and microscale nanoscale evaluation methods Surface roughness features. Physicists and physical chemists need details of fine scale of surfaces and often molecular roughness details. Usually these details are provided methods like low energy diffraction of electrons, molecular beam methods, field emissions Microscopy of field-ion, microscopy of tunneling, and microscopy of atomic force. Microscopic methods, however, are sufficient for most engineering and production surfaces, generally, they are mechanical or optical methods
Mechanical Stylus Method
This method uses a tool that amplifies and records a stylus ‘ vertical motions. Displaced by the surface to be measured at a constant speed. The stylus is mechanically linked mainly to a linear transformer. optical or capacitance sensor (LVDT). The sample is loaded with the stylus arm and either the stylus is scanned with a traverse unit over the stationary sample surface A constant speed or sample shall be carried over a flat optical reference. Like the stylus or the sample moves, the stylus rides across the surface of the sample, detecting surface differences Transducer. Transducer. It produces an analog signal that matches the vertical movement of the stylus. That’s it. Amplified, conditioned, and digitized signal
SCANNING TUNNELING MICROSCOPE
SPM and their modifications can be used at extremely high magnification of 103-108 X in x-, y-, z- direction for micro to atomic dimensions with high resolution. If a potential is difference is applied to two surfaces separated by a thin insulating film, a current flow is present due to ability of electrons to penetrate a potential barrier. The working of STM is simple, A sharp is maintained a very small gap from the surface to be investigated. A small potential is applied between the surfaces. A small tunneling current is generated, this can be sensed to analyze the topography. Two measuring methods constant height mode and constant current modes.
ATOMIC FORCE MICROSCOPE
AFM is like STM and produces high resolution images of the sample surface. AFM measures ultra-small forces between the tip and the sample. A displacement very flexible cantilever beam having the tip is used to measure the forces. The AFM combines the principle of STM and stylus profiler. It senses the forces between the atoms rather than the tunneling current to determine the proximity of the tip and sample. During the contact the atom at the tip is close enough to experience a weak repulsive force due to the electronic orbital overlap with the sample. This force produces a deflection in the beam which is measured by capacitive, optical detectors. The force gradient is measured by vibrating the cantilever and detecting the shift in resonance frequency of the beam. Force modulation mode or non-contact method. In this method he tip is raised and lowered over the surface with a feedback loop maintaining a constant force. This helps determining the viscoelastic properties of the surface
Contact Between Solid Surfaces
When two flat surfaces are in contact, the roughness of the surface causes contact. Appears at discrete points of contact. The sum of all contact areas Spots is the real (true) contact area or just the contact area for most materials. This is only a small fraction of the apparent (nominal) contact area when the load is applied Contact will initially occur at only a few points during the contact of two surfaces to support Normal (force) load. As the normal load increases, the surfaces approach, A greater number of higher asperities come into contact on both surfaces and existing To support the growing load, contacts grow
When two solid surfaces are connected, adhered to or connected through the interface may occur that requires a finite normal force to pull the two solids, called adhesive force Apart. Apart. There must be a distinction between adhesion and cohesion. Cohesion stands for The associated atomic bonding forces within a material; that is, cohesion represents the forces that There is an atom to another or a molecule to another in the bulk of the material. Adhesion is the phenomenon occurring when two surfaces are jointly pressed, Pure normal strength (load) or combined normal and shear forces
Proximity of the asperities results in an adhesive joint caused by interatomic attractions Covalent Bond, Ionic or Electrostatic Bond, Metallic Bond, Hydrogen Bond, van der Waals Bond Play key roles in tribology.
Free Surface Energy Theory of Adhesion
It is difficult to calculate the Van der Waals force in detail. A simpler way to use the Free surface energy concept. If a crystalline solid is attached to its cleavage plane, two Surfaces that are highly chemical active are generated. The process of splitting causes the fracture Cohesive bonds across the cleavage interface that leave the surface of these fractured bonds A highly energetic condition. The surface atoms have such unused energy that they can interact with each other, with other bulk atoms, and with environment species. Free energy from surface Influences adhesive bonds in contact with solids, and thus friction and wear. Moreover, it Determines the interactions between lubricants and solids.
Friction is the resistance to movement during sliding or rolling, which occurs when one is solid Body moves tangentially over another body in contact with. The resistant Tangential force, acting directly in the opposite direction to the direction of motion, is called the force of friction. There are two main types of friction usually found: dry Friction and friction in fluid. Friction is not a property of materials, it is a system response. if there are two solid surfaces clean High friction occurs without chemical films and adsorbents. Surface or thin contaminants Films have an influence on friction. Rubbing forces can be good or bad. It would be impossible to walk without friction, use car tires on the road or pick up objects. In some machine applications, even for example, vehicle brakes and clutches and power transmission friction (such as belt drives) Maximizing friction.
Wear is the material’s surface damage or removal from one or two solid surfaces in one Movement of sliding, rolling, or impact relative to each other. Wear occurs in most cases. Asperity surface interactions. Material can be removed from a surface later and may result in a transfer to the mating surface or as a wear particle break loose. If the interface is transferred from one surface to the other, net volume or mass loss It is zero, even if one surface is worn. Wear is not a material property as a friction but a system response. Conditions of operation Impact wear interface. Erroneously, high friction interfaces are sometimes assumed to exhibit High rates of wear. This doesn’t necessarily apply. interfaces, for example, with solid lubricants and Polymers show relatively low friction and relatively high wear while ceramics show relatively low friction the friction is moderate but extremely low wear.
Adhesive wear occurs during sliding contact with two nominally flat solid bodies, whether lubricated or not. Adhesion (or bonding) takes place at the interface in the asperity contacts and These contacts are cleared by sliding, which can lead to a fragment being detached. One surface and the other surface attachment. Wear particles from both materials in material combinations with different materials Even though more softer material wear particles are formed and are usually larger the harder counterpart than that. Due to malfunctions and cracks in the harder material, Local regions are of low strength. If the local regions are less strong, the harder Material coincides with local high – strength regions of the softer material during strong contact, the harder material fragment is formed. Formation of hard material fragments Also the material transferred by adhesion to the harder can be produced by detachment Surface through a fatigue process due to several loading and unloading cycles.
Abrasive wear occurs if rough, hard surface or hard particles slide on a rough surface. Softer surface and plastic deformation or fracture damage the interface. In the event of Ductile materials with a high toughness of fracture (e.g. metals and alloys), hard or hard asperities Particles result in the softer material’s plastic flow. Most metal and pottery surfaces Clear evidence of plastic flow during sliding, some even for ceramic brittle materials. Even at the lightest loads, the contact asperities of metals deform plastically. In the event of Low fracture toughness brittle materials, wear occurs through brittle fractures. In such cases, the worn area consists of extensive cracking.
In the first case, the tough the surface is harder, for example, on two rubbing surfaces (two-body abrasion) operations like grinding, cutting and machining, and the hard surface in the second case is a third body, usually a small abrasive particle, caught between the other two surfaces. and tougher enough, to cover either one or both of the mating surfaces for example, in free-abrasive lapping and polishing (three – body abrasion). In a number of cases, the wear mechanism at the beginning is adhesive that produces wear particles that are trapped in the interface, resulting in abrasive three – body wear
Repeated rolling (negligible friction) and fatigue on the surface is observed Sliding, respectively, respectively. The repeated load and unloading cycles for the materials Exposed may lead to the formation of subsurface or surface cracks afterwards A critical number of cycles will lead to the surface disintegration with the formation of the Large fragments, also known as pitting, leave large pits on the surface. Chemically improved crack growth (most frequently in ceramics) As fatigue static. With tensile stress and water vapor at the crack tip in many cases Ceramics, a chemical breakdown of the crack-tip bonds that increases rapidly the speed of crack. Chemically increased deformation and fracture lead to more wear Static and dynamic surface layers (rolling and sliding).
Chemical (Corrosive) Wear
Chemical or corrosive wear occurs in a corrosive environment when sliding occurs. In the air, in the air Oxygen is the most dominant corrosive medium. Chemical wear in the air is therefore generally Oxidative wear, called wear. In the absence of sliding, the chemical corrosion products (e.g. oxides) would generally form a film that would be less than a micrometer thick on the surfaces tend to slow or even stop corrosion, but the sliding effect wears the chemical film Removed, so the chemical attack could continue. Chemical wear therefore requires both chemicals Robbing and reaction (corrosion).
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Wear of Materials
Wear process is usually quantified according to wear rate. The rate of wear is defined as volume or Mass of material removed by sliding distance per unit time or unit. There could be other forms Dimensional less, such as material depth per unit sliding distance or removed volume Apparent contact area and sliding distance per unit. The wear rate is not usually constant. Generally, the wear rate is a complex time function. Wear rate may begin to rise low and later or vice Conversely, the wear rate remains constant for a period after a certain duration and It may change if transition occurs during a wear test from one mechanism to another. The beginning Period in which changes in wear rate are known as run-in or break-in periods. Please wear during Run-in depends on the initial structure and properties of the material and such surface conditions. As finishing of surface and the nature of any film. The surface during this transition period Roughness is modified by plastic deformation to a stable-state condition. First conditions Influences the damage and duration of the transition period.
FORCES ACTING ON THE PARTICLES:
There are two types of forces acting on the particles. They are;
- Attractive Forces
- Repulsive Forces
The category of forces is common for both types of forces, but the subclasses of these forces vary for each other. This is perfectly classified and explained by Dr Bharath Bhushan in his text book of ‘Handbook of Micro/Nano Tribology’ as
For Attractive Forces;
For Repulsive Forces;
There is also one more type of force but does not occur much often which is the reason why they are not considered is,
Dynamic Interactions between the particles:
These are the Non-Equilibrium forces, and which commonly sub classed as;
Hydraulic, Viscous, Friction and Lubrication forces. These are the energy dissipation forces which will get produced when there is a friction created on the surface of the bodies or for the direct collisions. This type of force depends on the attractive force and the repulsive force.
The attractive forces which acts right before to the collision of the particles i.e., when the distance between the particles is less than D = 2nm will most of the times get balanced by the repulsive forces which occur soon after the immediate interaction between the particles. In the technical terms, called as “These attractive forces are stabilized by the hardcore repulsive forces” by Dr Bharath Bhushan.
MEASUREMENT OF STATIC AND DYNAMIC FORCES
There are two measuring force apparatuses to measure the force between the particles. They are;
- Surface Force Apparatus
- Atomic Force Apparatus
Surface Force Apparatus: Surface Force Apparatus is used to find out the surface contact forces on the particles.
Atomic Force Apparatus: Atomic Force Apparatus is used to find out the internal bond orientations and variations between the atoms or the ions of the particles. This includes the forces between the two or multiple atoms or ions.
All the forces were calculated using the expression,
Fs = Fmax = Ks . ΔD
Ks = Stiffness constant of the spring
From Fs, we calculate the interfacial energy from
ɣ = Fs/3ΠR
If there are two spheres (particle surfaces)
ɣ = Fs/3Π((1/R1 + 1/R2)
The units for the interfacial energy are J/m2
Nanorheology is a branch of science which deals with the deformation and flow of matter nano size, particularly for non-Newtonian flow in liquids and plastic flow in solids. It defines that particles if they come in direct contact with each other (collision) the amount of wear would be very less compared to the surface to surface interaction which produces high amount of friction and hence more wear. For example, consider a smooth ball hitting on a smooth surface.
Here if we observe, the microscopic view of both the surfaces, we can tell that it is not possible to find a perfectly smooth surface, and in this case, the slip occurs in between the surfaces at the zigzag location and there will not be too much wear of surfaces and friction is also produced in very less amounts.
But if we take the rupture between two surfaces, the friction is produced due to the zigzag portion of the surfaces at microscopic level and due to which the material removal will occur to the entire portion of the surface which is in contact with the other surface.
Avoiding the friction can be done through a process called lubrication. A thin oil film is prepared and applied on to the particle surfaces and by which the film occupies the gaps between the thin zigzag tooth of the particle surfaces and forms a membrane like surface around the top layer. The material of the lubricant is well chosen that it should not have any tooth after it gets settled up on the surface.
Now, due to the amount of lubricant on both of the surfaces, the particle surfaces become smooth even at the microscopic level. This will allow the particles to just slide among them but does not produce any wear.
While choosing the lubricant, the main factor to be considered is the coefficient of viscosity which is defined as the tangential force per unit area. If there is a change in the velocity per unit distance at right angles, the magnitude of velocity is always one.
The coefficient of viscosity is defined as;
µ = q/(δv/δy)
where µ = Coefficient of Viscosity
q = Shear Stress
If Φ is the shear angle in δt time, the equation can be re-written as;
q = µ (δΦ/δy)
And therefore, the units of the coefficient of viscosity will look like,
(force/area) / (rad/sec) = M/LT
TRIBOLOGY IN MICRO ELECTRO MECHANICAL SYSTEMS
In the MEMS devices, the biggest problem in designing is Stiction. As the components are micro and nanoscale, it is quite difficult to avoid the Stiction issue in MEMS. Stiction is defined as the sticking property of a freed layer to a newly approached layer after it gets detached from the parent particle or surface. Layers when come in contact with each other, the outer most layer gets carried away with the stronger layer of each other. This property is known as surface tension and is seen in many applications and cases.
When we etch the surface layer with a lubricant or some kind of reactant, that will reduce the friction but on the other hand, due to its chemical properties, it develops some attractive forces by itself with the other layer and tries to grab it from that particle by forming strong bonds between them. This will result in the depletion of layer by layer of the particles which will again result in the exposure of the normal outer surface and causes friction and then wear and adhesion. As we know until now, once the freed layer is detached from the parent layer and attached to the new layer, it is practically impossible to separate it from the new layer. To avoid these Stiction issues, Mastrangelo, the scientist in 1997 found some mechanisms.
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