⚙️Friction and Wear in Engineering Unit 11 – Manufacturing Tribology: Friction and Wear

Key Concepts and Definitions

• Tribology studies the interaction of surfaces in relative motion, including friction, wear, and lubrication
• Friction is the resistance to motion when two surfaces slide against each other, converting kinetic energy into heat
• Wear is the progressive loss of material from a surface due to mechanical action, leading to reduced performance and eventual failure
• Lubrication involves using a substance (lubricant) to reduce friction and wear between surfaces in contact
• Coefficient of friction ($\mu$) quantifies the ratio of the friction force to the normal force between two surfaces, with lower values indicating less friction
• Wear rate measures the amount of material removed per unit distance or time, often expressed as volume loss per unit sliding distance
• Tribosystem consists of the interacting surfaces, lubricant (if present), and the surrounding environment, all of which influence tribological behavior
  • Includes factors such as surface roughness, material properties, contact pressure, sliding speed, and temperature

Fundamentals of Tribology

• Tribology is an interdisciplinary field that combines principles from mechanical engineering, materials science, chemistry, and physics
• Surface topography plays a crucial role in tribological behavior, with rougher surfaces generally exhibiting higher friction and wear
  • Asperities (microscopic surface irregularities) can interlock and deform, increasing friction and wear
• Real area of contact is typically much smaller than the apparent area due to surface roughness, leading to high local pressures at asperity contacts
• Adhesion contributes to friction when strong bonds form between contacting surfaces, requiring force to shear these junctions
• Plowing occurs when a harder surface penetrates and displaces material from a softer surface, increasing friction and wear
• Deformation of surfaces can be elastic (reversible) or plastic (permanent), depending on the applied load and material properties
• Frictional heating can significantly impact tribological behavior, altering material properties and potentially leading to thermal softening or oxidation
• Tribochemical reactions involve chemical changes at the interface due to friction-induced energy input, forming protective films or abrasive particles

Types of Friction in Manufacturing

• Dry friction occurs when two unlubricated surfaces slide against each other, characterized by high friction and wear
  • Common in applications such as brakes, clutches, and mechanical seals
• Boundary lubrication involves a thin film of lubricant (a few molecules thick) separating surfaces, reducing friction and wear compared to dry conditions
  • Lubricant additives (friction modifiers, anti-wear agents) play a critical role in forming protective films
• Mixed lubrication is a transition regime between boundary and full-film lubrication, with intermittent contact between surface asperities
  • Occurs when lubricant film thickness is comparable to surface roughness, leading to a combination of fluid-film and boundary lubrication effects
• Hydrodynamic lubrication separates surfaces with a thick, pressurized lubricant film, minimizing friction and wear
  • Requires sufficient sliding speed and lubricant viscosity to generate a load-bearing film
• Elastohydrodynamic lubrication (EHL) is a form of hydrodynamic lubrication that accounts for elastic deformation of surfaces under high contact pressures
  • Prevalent in rolling-element bearings and gears, where concentrated contacts experience significant deformation
• Solid lubrication uses materials with low shear strength (graphite, molybdenum disulfide) to provide lubrication in extreme environments
  • Effective in high temperatures, vacuum conditions, or when liquid lubricants are not suitable

Wear Mechanisms and Their Impact

• Abrasive wear occurs when hard particles or protrusions slide against a softer surface, causing material removal through cutting or plowing
  • Can be two-body (rough surface against smooth surface) or three-body (loose abrasive particles between surfaces)
• Adhesive wear involves the formation and shearing of strong bonds between contacting surfaces, leading to material transfer and surface damage
  • More severe when surfaces are clean, unlubricated, and have high chemical affinity
• Fatigue wear results from repeated cyclic loading, causing the formation and propagation of surface or subsurface cracks
  • Prevalent in rolling-element bearings, gears, and rail-wheel contacts
• Corrosive wear combines mechanical wear with chemical reactions, leading to the formation of abrasive oxide particles or the removal of protective films
  • Accelerated in high-temperature, humid, or chemically aggressive environments
• Erosive wear is caused by the impact of solid particles or liquid droplets on a surface, leading to material removal and surface deformation
  • Common in fluid handling systems, pipelines, and turbomachinery
• Fretting wear occurs due to small-amplitude oscillatory motion between contacting surfaces, causing surface damage and oxidation
  • Problematic in bolted joints, splines, and press-fitted components
• Wear can lead to increased clearances, loss of precision, and premature failure of components, necessitating regular monitoring and maintenance
  • Proper material selection, surface treatments, and lubrication can mitigate wear and extend component life

Lubrication Principles and Techniques

• Lubrication aims to reduce friction and wear by separating surfaces with a lubricant film, which can be liquid, solid, or gaseous
• Viscosity is a key property of liquid lubricants, representing resistance to flow and shear
  • Higher viscosity generally provides better film formation and load-carrying capacity but may increase friction at low speeds
• Viscosity index (VI) measures the change in viscosity with temperature, with high VI lubricants maintaining stable viscosity over a wide temperature range
• Additives enhance lubricant performance by providing additional functions such as wear protection, corrosion inhibition, and friction modification
  • Common additives include zinc dialkyl dithiophosphate (ZDDP), molybdenum disulfide (MoS2), and graphite
• Grease is a semi-solid lubricant consisting of a base oil, thickener, and additives, providing extended lubrication and sealing in rolling-element bearings and gears
• Solid lubricants (graphite, PTFE, MoS2) offer lubrication in extreme temperatures, vacuum conditions, or when liquid lubricants are not suitable
• Lubricant selection depends on factors such as operating conditions, material compatibility, environmental regulations, and cost
  • Synthetic lubricants (polyalphaolefins, esters) offer superior performance and stability compared to mineral oils in demanding applications
• Lubrication systems ensure proper delivery and distribution of lubricants to tribological interfaces
  • Include splash, forced circulation, oil mist, and oil jet lubrication methods, depending on the application requirements

Materials Selection for Tribological Applications

• Material properties such as hardness, elasticity, thermal conductivity, and chemical reactivity influence tribological behavior
• Hard materials (ceramics, hardened steels) are resistant to abrasive and adhesive wear but may cause increased wear of the counterface
  • Often used in bearings, gears, and cutting tools
• Soft materials (polymers, soft metals) can conform to surface irregularities and provide low friction but have limited load-carrying capacity
  • Suitable for low-load, low-speed applications or as solid lubricants
• Elastomers (rubbers) offer low elastic modulus and high elasticity, making them ideal for sealing and vibration damping applications
• Coatings and surface treatments can modify surface properties and improve tribological performance
  • Examples include diamond-like carbon (DLC), titanium nitride (TiN), and plasma electrolytic oxidation (PEO) coatings
• Self-lubricating materials contain solid lubricants (graphite, MoS2) embedded in a matrix, providing low friction and wear without external lubrication
  • Useful in space applications, high-temperature environments, or where lubricant contamination is a concern
• Composite materials combine the properties of multiple constituents to achieve desired tribological characteristics
  • Examples include metal-matrix composites (MMCs), polymer-matrix composites (PMCs), and ceramic-matrix composites (CMCs)
• Material pairs should be selected based on their compatibility, considering factors such as adhesion, chemical reactivity, and thermal expansion mismatch
  • Dissimilar materials are often preferred to minimize adhesive wear and material transfer

Testing and Measurement Methods

• Tribological testing evaluates the friction, wear, and lubrication behavior of materials and components under controlled conditions
• Pin-on-disc test measures the friction and wear of a stationary pin sliding against a rotating disc, providing data on friction coefficient and wear rate
  • Allows control of load, speed, temperature, and lubrication conditions
• Reciprocating wear test assesses the tribological behavior under linear, oscillating motion, simulating conditions in piston rings, bearings, and seals
• Four-ball test evaluates the extreme pressure (EP) and anti-wear properties of lubricants, using three stationary balls and one rotating ball under load
  • Measures wear scar diameter and seizure load to characterize lubricant performance
• Timken test determines the load-carrying capacity of lubricants, using a rotating test cup pressed against a stationary test block
• Falex test measures the anti-wear and extreme pressure properties of lubricants, using a rotating pin and two stationary v-blocks under increasing load
• Surface profilometry techniques (stylus, optical, atomic force microscopy) quantify surface roughness parameters and wear depth
  • Provide information on surface topography, asperity distribution, and wear mechanisms
• Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analyze surface morphology, wear debris, and chemical composition
  • Help identify wear mechanisms, material transfer, and tribochemical reactions
• Online condition monitoring techniques (vibration analysis, oil analysis, acoustic emission) detect tribological failures and optimize maintenance intervals
  • Enable predictive maintenance and reduce unplanned downtime in industrial applications

Industrial Applications and Case Studies

• Automotive engines rely on effective lubrication and wear control in critical tribological interfaces such as piston rings, cylinder liners, and valve trains
  • Advanced surface treatments (plateau honing) and low-friction coatings (DLC) improve fuel efficiency and durability
• Rolling-element bearings support rotating shafts in various machines, experiencing high contact stresses and requiring effective lubrication
  • Proper bearing selection, mounting, and lubrication practices are essential for optimal performance and life
• Cutting tools encounter severe friction and wear during machining operations, leading to tool wear and reduced surface quality
  • Coated tools (TiN, TiAlN) and optimized cutting fluids enhance tool life and productivity
• Mechanical seals prevent leakage in rotating equipment (pumps, compressors) by maintaining a thin lubricating film between seal faces
  • Selection of seal materials (silicon carbide, tungsten carbide) and lubrication methods (barrier fluids, gas seals) depends on the application requirements
• Wind turbine gearboxes transmit high torques and operate under variable loading conditions, leading to micropitting, scuffing, and bearing failures
  • Synthetic gear oils with advanced additives and condition monitoring systems improve reliability and extend service intervals
• Sheet metal forming processes (stamping, drawing) involve high contact pressures and relative motion between the workpiece and tooling
  • Lubricants with extreme pressure additives and coated tools (diamond-like carbon) reduce friction, wear, and galling
• Rail-wheel contacts in the railway industry experience high contact stresses, rolling-sliding motion, and environmental exposure
  • Friction modifiers, gauge face lubrication, and laser cladding of rails mitigate wear, noise, and rolling contact fatigue
• Artificial hip and knee joints are subject to complex tribological conditions, requiring biocompatible materials with low friction and wear
  • Advanced bearing materials (cobalt-chromium alloys, ceramics) and surface treatments (diamond-like carbon) improve implant longevity and patient outcomes