Friction and Wear in Engineering

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

Friction 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)
  • 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


© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
Glossary
Glossary