⚙️Friction and Wear in Engineering Unit 10 – Engineering Applications of Friction and Wear

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