All Study Guides Friction and Wear in Engineering Unit 10
⚙️ Friction and Wear in Engineering Unit 10 – Engineering Applications of Friction and WearFriction and wear are crucial factors in engineering, affecting the performance and longevity of mechanical systems. This unit explores key concepts, mechanisms, and applications of tribology, the science of interacting surfaces in relative motion.
From bearings and gears to cutting tools and brakes, understanding friction and wear is essential for optimizing design and performance. The unit covers material selection, measurement techniques, modeling approaches, and emerging technologies in this field.
Key Concepts and Definitions
Friction the resistance to relative motion between two surfaces in contact
Caused by surface roughness, adhesion, and deformation
Quantified by the coefficient of friction (μ), the ratio of frictional force to normal force
Wear the progressive loss or displacement of material from a surface due to relative motion
Involves complex interactions between surfaces, materials, and environmental factors
Tribology the study of friction, wear, and lubrication in interacting surfaces
Encompasses the design, analysis, and optimization of tribological systems
Lubrication the use of a substance (lubricant) to reduce friction and wear between surfaces
Can be in the form of liquids, gases, or solids
Lubricants form a protective film that separates surfaces and minimizes direct contact
Surface roughness the measure of surface texture and irregularities
Influences friction, wear, and contact mechanics
Characterized by parameters such as average roughness (Ra) and root mean square roughness (Rq)
Types of Friction and Wear
Dry friction occurs between two solid surfaces without lubrication
Governed by surface roughness, material properties, and contact pressure
Fluid friction occurs when a fluid (liquid or gas) separates two surfaces
Depends on fluid properties (viscosity, density) and flow characteristics (laminar or turbulent)
Rolling friction occurs when an object rolls over a surface
Lower than sliding friction due to reduced contact area and deformation
Adhesive wear occurs when surface asperities bond and break, resulting in material transfer
Influenced by surface energy, hardness, and chemical compatibility of materials
Abrasive wear occurs when hard particles or protrusions plough through a softer surface
Caused by hard debris, rough surfaces, or surface asperities
Can be two-body (fixed particles) or three-body (loose particles) abrasion
Fatigue wear occurs due to repeated cyclic loading and unloading of surfaces
Leads to the formation and propagation of subsurface cracks
Common in rolling contact bearings and gears
Corrosive wear occurs when chemical reactions with the environment degrade surfaces
Accelerated by high temperatures, reactive environments, and tribological stresses
Friction and Wear Mechanisms
Asperity interaction the contact and deformation of surface irregularities (asperities)
Contributes to friction through adhesion, ploughing, and hysteresis
Adhesion the bonding of surface asperities due to intermolecular forces
Depends on surface energy, chemical compatibility, and real contact area
Can lead to material transfer and adhesive wear
Ploughing the penetration and displacement of material by harder asperities or particles
Contributes to friction through plastic deformation and material displacement
Can cause abrasive wear and surface damage
Delamination the subsurface fatigue and removal of surface layers due to cyclic loading
Initiated by subsurface cracks that propagate parallel to the surface
Results in the formation of wear debris and surface damage
Oxidation the chemical reaction of surfaces with oxygen in the environment
Forms oxide layers that can modify friction and wear behavior
Can provide protective films or contribute to corrosive wear
Material Properties and Selection
Hardness the resistance of a material to localized plastic deformation
Influences wear resistance, with harder materials generally exhibiting lower wear rates
Toughness the ability of a material to absorb energy and deform without fracturing
Important for resisting fatigue wear and impact loads
Elastic modulus the measure of a material's stiffness and resistance to elastic deformation
Affects contact mechanics, stress distribution, and deformation behavior
Surface energy the work required to create a unit area of new surface
Influences adhesion, wetting, and tribochemical reactions
Compatibility the chemical and physical compatibility of materials in contact
Similar materials tend to have higher adhesion and friction
Dissimilar materials can reduce adhesive wear but may promote other wear mechanisms
Coatings thin layers of materials applied to surfaces to modify tribological properties
Can provide low friction, high wear resistance, or corrosion protection
Examples include diamond-like carbon (DLC), titanium nitride (TiN), and polytetrafluoroethylene (PTFE)
Measurement and Testing Methods
Pin-on-disc a simple tribological test that measures friction and wear between a stationary pin and a rotating disc
Allows control of load, speed, and environmental conditions
Provides data on coefficient of friction, wear rate, and wear mechanisms
Reciprocating wear test measures friction and wear in a reciprocating sliding motion
Simulates conditions in reciprocating seals, bearings, and other applications
Four-ball wear test measures wear and extreme pressure properties of lubricants
Uses four balls in a tetrahedral configuration, with one ball rotating against three stationary balls
Provides data on wear scar diameter, seizure load, and weld load
Nanoindentation measures hardness and elastic modulus at small scales
Uses a diamond indenter to apply load and measure displacement
Useful for characterizing thin films, coatings, and surface layers
Profilometry measures surface roughness and topography
Uses contact (stylus) or non-contact (optical, atomic force microscopy) methods
Provides quantitative data on surface texture, wear depth, and wear volume
Engineering Applications
Bearings enable low-friction rotation or linear movement between components
Sliding bearings (bushings) rely on a thin film of lubricant to separate surfaces
Rolling element bearings (ball, roller) use rolling elements to reduce friction and support loads
Gears transmit power and motion between rotating shafts
Subjected to sliding and rolling contact, leading to fatigue, adhesive, and abrasive wear
Lubrication and surface treatments are critical for efficient and reliable operation
Seals prevent leakage and contamination between components
Sliding seals (O-rings, lip seals) rely on contact and deformation to create a seal
Mechanical seals (face seals) use a thin fluid film to separate seal faces
Brakes and clutches transmit torque through friction between rotating and stationary components
Friction materials (pads, linings) are designed for high and stable friction, wear resistance, and fade resistance
Cutting tools used for machining and material removal processes
Subjected to high stresses, temperatures, and wear rates
Material selection (carbides, ceramics, coatings) and lubrication strategies are critical for tool life and performance
Modeling and Simulation
Analytical models mathematical descriptions of friction and wear based on simplified assumptions
Examples include Coulomb friction model, Archard wear equation, and Hertzian contact theory
Provide insights into fundamental mechanisms and trends, but have limited accuracy for complex systems
Finite element analysis (FEA) numerical method for solving complex boundary value problems
Discretizes components into small elements and solves governing equations
Enables prediction of stress, strain, and deformation in tribological contacts
Multiphysics modeling combines multiple physical phenomena (mechanics, heat transfer, fluid dynamics) in a single simulation
Captures the complex interactions between surfaces, lubricants, and environment
Examples include elastohydrodynamic lubrication (EHL) and thermomechanical wear modeling
Molecular dynamics (MD) simulates the motion and interaction of atoms and molecules
Provides insights into atomic-scale friction, adhesion, and wear mechanisms
Limited to small length and time scales due to computational complexity
Tribological contact modeling predicts the contact pressure, area, and stress distribution between surfaces
Considers surface roughness, material properties, and deformation
Examples include Greenwood-Williamson model and boundary element method (BEM)
Emerging Technologies and Future Trends
Surface texturing the intentional modification of surface topography to improve tribological performance
Includes dimples, grooves, and hierarchical patterns
Can enhance lubrication, reduce friction, and trap wear debris
Biomimetic surfaces inspired by nature's solutions to friction and wear challenges
Examples include shark skin-inspired riblets for drag reduction and lotus leaf-inspired superhydrophobic surfaces
Combines multi-scale surface features, materials, and chemistry for optimal performance
Nanocomposites materials with nanoscale reinforcements (particles, fibers, platelets) dispersed in a matrix
Offer enhanced mechanical, thermal, and tribological properties compared to conventional composites
Examples include carbon nanotube (CNT) and graphene-reinforced polymers and metals
Ionic liquids (ILs) molten salts with low melting points and unique properties
Exhibit high thermal stability, low volatility, and tunable chemistry
Promising as lubricants and additives for extreme conditions and advanced applications
Triboelectric nanogenerators (TENGs) convert mechanical energy from friction into electrical energy
Utilize the triboelectric effect and electrostatic induction
Potential for self-powered sensors, wearable devices, and energy harvesting in tribological systems