Friction, Lubrication, and Wear

  • basic
    • Learning Objectives
      • Understand surface properties that affect friction and wear behavior.
      • Review basic concepts of static and kinetic friction.
      • Understand the different types of lubrication and wear, and where they are commonly found.
      • Identify areas of concern for friction, lubrication, and wear in vivo.
      • Know important design factors from the perspective of friction and wear concerns.
    • Design Inquiry - Total Knee Replacement
      • You are designing a total knee replacement, a high-load device for which wear is a major concern.
      • Contact stresses of 40 MPa.
      • 1 million fatigue cycles per year.
      • Selecting a material that meets these demands and still maintains low wear is a team challenge.
      • Key material-selection concerns
        • low coefficient of friction
        • low wear debris generation
        • fatigue resistance under repeated joint loading
        • biocompatibility of both bulk material and debris
        • surface finish and lubricant compatibility
    • Clinical Motivation
      • Wear and Tear Arthritis
        • Osteoarthritis (OA)骨关节炎 is the most common form of arthritis.
        • It occurs most frequently in the hands, hips, and knees.
        • With OA, the cartilage within a joint begins to break down and the underlying bone begins to change.
        • 图 - OA links cartilage wear, bone change, and joint pain
      • Wear Debris Cause Severe Outcome
        • A patient with metal-on-metal hip replacement presented tremor, fatigue, forgetfulness, hip swelling, and stiffness.
        • Cobalt levels
          • 23.5 ppb in urine
          • 5.1 ppb in blood
          • 440 ppb in joint fluid
        • Where does the Cobalt come from?
        • 图 - pelvis radiograph with metal-on-metal hip replacement and cobalturia article
    • Tribology摩擦学
      • Tribology is the field of friction, lubrication, and wear.
      • When designing devices that contact other materials, it is necessary to consider friction, lubrication, and wear.
      • Orthodontic implants正畸植入物
        • 比如牙套braces
        • A smooth surface to arch wires is very important.
        • Frictional forces can reduce the amount of force applied to teeth by 50% or more.
      • Interface characterization determines friction and wear behavior
        • composition of the articulating materials
        • surface finishes
        • lubricant, if any
  • Bulk and Surface Properties
    • basic
      • Surface texture is the repetitive and/or random deviations from the nominal surface of an object.
      • surface profile = error of form + waviness + roughness
        • roughness粗糙度
          • S < 1 mm
          • small, finely-spaced deviations from nominal surface
          • determined by material characteristics and processes that formed the surface
        • waviness波纹度
          • 1 ≤ S ≤ 10 mm
          • deviations with much larger spacing
          • caused by work deflection, vibration, tooling, etc.
        • Roughness is superimposed on waviness.
        • spacing S and height Z describe local topography.
    • Asperities微凸体 and Real Area of Contact
      • As two surfaces come into contact, the actual contact comes at asperities, small peaks on each imperfect surface.
      • Asperities are commonly modeled as hemispherical elastic contacts
      • real area of contact, Ar 真实接触面积
        • Ar differentiates asperity contacts from the apparent contact area.
        • A surface with high roughness can have a smaller Ar than a smoother surface.
        • Ar can change during sliding contact as asperities undergo deformation and/or wear.
        • For hard rough surfaces under moderate load, Ar may be only 15-20% of the apparent contact area.
    • Roughness Metrics
      • A curve from a single roughness measurement
      • average roughness Ra平均粗糙度
        • formula
        • yi is the distance from the profile to the mean line.
        • The mean line is defined so that the area between the profile and the line is equal above and below.
      • RMS roughness RRMS均方根粗糙度
        • formula
        • RRMS is weighted by the square of the peak heights.
        • RRMS can be more useful because Ra can have the same value for different topographies due to averaging.
      • Surface topography should use roughness values together with waviness measurements.
      • RRMS ≥ Ra
  • Friction
    • Definition
      • Friction is the resistance to relative motion between two surfaces.
      • It is normally expressed as the coefficient of friction, μ
      • μ expresses the ratio between lateral and normal force in a simple friction test.
      • For general cases
        • μ = FS / FN
        • FS is shear force.
        • FN is normal force.
    • Static and Kinetic Friction
      • static friction静摩擦
        • μs
        • friction between non-moving surfaces
      • kinetic friction动摩擦
        • μk
        • friction between moving surfaces
      • The static coefficient of friction is higher than the kinetic coefficient of friction.
    • Friction Mechanisms
      • Friction can be thought of as a combination of adhesion and deformation.
      • adhesion粘着
        • asperities have become attached
      • deformation变形
        • asperities from one surface must slide past asperities of the opposing surface
      • Coefficient of friction is proportional to the normal force applied on the object.
      • It is generally independent of apparent contact area and velocity in the simplified model.
  • Lubrication
    • Definition and Benefit
      • Lubrication is the use of a film or layer to separate opposing surfaces during relative motion.
      • The primary benefit is to reduce the opportunity for damage by preventing direct contact between surfaces.
      • Lubricants can be
        • solid lubricants, such as graphite
        • liquid lubricants, such as synovial fluid or lubricating oil
        • gas lubricants, such as air bearings
    • Hydrodynamic Lubrication流体动压润滑
      • most common type of lubrication
      • Relative motion between the two surfaces draws the lubricant into the contact area.
      • The surface separation is usually 10^-3 - 10^-4 cm.
      • The surfaces are completely separated with no asperity contact.
      • This lubrication scheme is considered ideal because the potential for surface damage is minimal.
      • 图 - moving surface pulls lubricant into a wedge-shaped contact gap
    • Elastohydrodynamic Lubrication弹流润滑
      • characterized by 10^-4 - 10^-5 cm separation.
      • The normal load is transmitted through the lubricant.
      • At least one surface undergoes elastic deformation.
      • This process may result in localized fatigue over long-term use.
      • Because film thickness is small, it may be on the same order of magnitude as surface roughness.
      • 图 - loaded contact deforms elastically while a thin lubricant film carries load
      • lambda parameter λ 膜厚比
        • 由于弹流润滑中的膜厚非常小,有必要质疑它是否与表面粗糙度处于同一数量级,这个参数就是λ
        • formula
        • h0 is the film thickness in meters.
        • RRMS is the RMS roughness for materials A and B.
        • λ compares lubricant film thickness with combined surface roughness.
    • Squeeze Film Lubrication挤压膜润滑
      • In squeeze film lubrication, a highly viscous lubricant has its own elastic response to loading.
      • It appears during transient overloads.
      • Example - sudden increase in load in the human knee.
      • 突然一压，液体来不及完全流走，就短时间帮忙承载
    • Boundary Lubrication边界润滑
      • Boundary lubrication is demonstrated when the lubricant is a solid or gel coating on one or both surfaces.
      • It differs from simply having a low-shear fluid interface. Boundary lubrication is found in high-pressure, low-velocity situations.
      • Surface separation is usually less than 10^-5 cm.
      • Boundary coating protects asperities when fluid film is too thin.
    • Mixed Lubrication混合润滑
      • Mixed lubrication occurs when multiple lubrication models are present simultaneously.
      • A primary lubrication mode such as hydrodynamic lubrication is supplemented by additional protection from coating or other factors.
      • Example - articular cartilage = boundary lubrication + elastohydrodynamic lubrication + squeeze film + coating
      • 现实关节通常不是单一机制，而是 fluid pressure + surface coating 一起工作
    • Synovial Fluid滑液
      • Synovial fluid, also known as joint fluid, is a thick liquid located between joints.
      • The fluid cushions the ends of bones and reduces friction when joints move.
      • It is rheopectic, meaning that it becomes less viscous during shear motion.
      • The viscosity of synovial fluid decreases as a result of disease or trauma.
      • Lower viscosity synovial fluid leads to higher rates of cartilage-on-cartilage contact and joint damage.
      • Molecular Lubricants
        • Hyaluronic Acid (HA)透明质酸
          • A naturally occurring glycosaminoglycan in synovial fluid.
          • Acts as a lubricant and shock absorber for joints.
          • Contributes to the viscoelasticity of synovial fluid.
          • Protects cartilage from friction-induced damage.
        • Lubricin润滑素
          • A glycoprotein that works alongside HA to reduce friction between cartilage surfaces.
        • Phospholipid磷脂
          • Provides boundary lubrication by forming a thin film on cartilage surfaces.
  • Wear
    • basic
      • Wear is the unwanted removal of material from solid surfaces by mechanical action.
      • Three stages of wear
        • 图
        • running in跑合期
          • initial period where surfaces adapt to each other
          • wear rates may be higher
        • steady-state wear稳定磨损期
          • period of stable wear rates
          • device functions within expected parameters
        • catastrophic wear灾难性磨损
          • sudden, significant wear or failure
          • often leads to device malfunction or breakdown
    • Adhesive Wear 粘着磨损
      • accounts for nearly half of all wear.
      • Fragments from one surface are pulled off and adhered to the second surface.
      • Particles tend to be 10-100 μm in diameter.
      • Adhesive wear volume, V
        • formula
        • V is adhesive wear volume.
        • k is Archard’s coefficient.
        • psoft is the Vickers hardness of the softer material, the softer material is more likely to lose volume to wear debris
        • x is total sliding distance.
    • Abrasive Wear 磨粒磨损
      • the second most common type of wear
      • It occurs when hard asperities remove material from a softer surface.
      • Main processes
        • plowing犁削
        • cutting切削
        • cracking开裂
      • Abrasive wear particles are typically 2-3 orders of magnitude larger than adhesive wear particles.
      • Abrasive wear can be beneficial in polishing.
      • two-body abrasion二体磨损
        • only the two counterfaces are involved
        • 仅涉及两个接触面
      • three-body abrasion三体磨损
        • two counterfaces plus unconstrained grains of material
        • loose wear debris can become abrasive particles
        • 两个接触面加上不受约束的材料颗粒
    • Corrosive Wear 腐蚀磨损
      • 图
      • 2 types
        • A brittle oxide corrosion layer is removed from its corresponding surface, resulting in pitting点蚀
        • Gradual removal of a coherent corrosion layer, this leads to general topography change.
      • Corrosive wear can be controlled by careful attention to lubricant and environment.
    • (Rolling)Contact Fatigue (滚动)接触疲劳
      • Contact fatigue, or rolling contact fatigue, is material removal due to near-surface alternating stresses induced through rolling.
      • This type of wear is common in bearings and gears.
      • The contact can be conforming or non-conforming.
      • Near-surface stress field in elastic materials is often approximated using Hertzian contact theory.
      • Contact fatigue can take place in addition to other wear mechanisms.
    • Erosive Wear 冲蚀磨损
      • Erosive wear involves removal of material from a surface bombarded轰击 by liquid or solid particles.
      • Removed volume depends on
        • particle velocity
        • particle shape
        • particle mass
        • impingement angle冲击角
      • Erosive wear is maximized at 30-90° to the surface, depending on the material.
      • It is commonly seen in high-velocity flow situations such as valves.
      • It is not generally an issue in vivo.
    • Fretting Wear 微动磨损
      • Fretting wear is material removal when two contacts under load also experience minute reciprocating or vibrating motion.
      • Sliding motion can range from 20-200 μm.
      • Wear debris from fretting can contribute to further material degradation as abrasive wear particles.
      • Fretting is usually a combination of corrosive and abrasive wear.
    • Delamination Wear 分层磨损
      • Delamination wear occurs under conditions somewhat similar to contact wear
      • During loaded rolling or sliding, subsurface stresses develop in a material.
      • A void or imperfection grows into a crack parallel to the surface.
      • Ultimately, this piece of material delaminates分层剥离
      • Delamination wear is characterized by large, thin wear particles.
      • It is more common in layered materials, but can also occur in homogeneous objects.
  • Case Study
    • Biomimetic Cartilage-Lubricating Polymers
      • Nature Biomedical Engineering article.
      • Biomimetic cartilage-lubricating polymers regenerate cartilage in rats with early osteoarthritis.
      • DOI: 10.1038/s41551-021-00785-y
      • The case links lubrication polymer design with cartilage repair and friction reduction.
    • Natural Cartilage Lubrication Layer
      • Normal joint has synovial fluid and a lubrication layer on cartilage.
      • Layer components shown in the figure
        • lubricin
        • phospholipid
        • HA
        • fibronectin
        • Coll II
      • Brush-like lubrication complex
        • polymer brushes project from HA backbone
        • hydrated brush layer reduces friction between cartilage surfaces
      • 图 - natural joint lubrication layer uses HA, lubricin, phospholipid, and collagen-associated matrix
    • Synthetic Strategy
      • HA/PA and HA/PM polymers are designed to form brush-like lubrication complexes.
      • Solution of HA/PA, HA/PM, or both is injected into a knee joint in a rat model of early OA.
      • The polymers assemble into a lubrication layer on cartilage surface.
      • Treated knee joint aims to recover a low-friction interface.
      • 图 - injection into rat knee, assembled lubrication layer, treated knee joint
    • Friction Test Setup
      • Cartilage samples are immersed in lubrication solution in a Petri dish.
      • A parallel plate applies load and rotational sliding.
      • Stainless baseplate supports the sample.
      • The setup measures friction coefficient of treated and normal cartilage.
      • pre-sliding time
        • 1.2 s
        • 12 s
        • 120 s
        • 1,200 s
      • Groups compared
        • PBS
        • HA
        • HA/PA
        • HA/PM
        • HA/PA+HA/PM
    • Graph Reading
      • Treated cartilage kinetic friction
        • PBS is highest, around 0.02-0.025.
        • HA is lower, around 0.013-0.015.
        • HA/PA, HA/PM, and HA/PA+HA/PM are lower, roughly around 0.005-0.009.
        • HA/PA+HA/PM gives one of the lowest kinetic friction levels.
      • Treated cartilage static friction
        • PBS and HA are higher than polymer-brush groups.
        • HA/PA, HA/PM, and HA/PA+HA/PM keep static friction lower across pre-sliding times.
        • Longer pre-sliding time tends to increase static friction in several groups.
      • Normal cartilage friction
        • Normal cartilage kinetic friction is lower overall than treated cartilage PBS.
        • Polymer groups approach the low-friction range of normal cartilage.
        • For normal cartilage static friction, HA/PA+HA/PM is generally among the lowest groups.
      • Main conclusion
        • Biomimetic brush-like polymers reduce both kinetic and static friction compared with PBS and HA alone.
        • Combined HA/PA+HA/PM treatment provides strong lubrication performance.
  • Design Factors for Low-Wear Implants
    • Material pair selection
      • avoid high wear coefficient material pairs under high load
      • consider softer material hardness psoft because adhesive wear volume scales inversely with psoft
      • wear debris biocompatibility matters as much as bulk material biocompatibility
    • Surface engineering
      • control roughness and waviness together
      • avoid high asperity contact and small real contact area concentration
      • surface coatings can provide boundary lubrication under high-pressure, low-velocity conditions
    • Lubrication design
      • maintain enough fluid film thickness relative to RMS roughness
      • use λ to compare film thickness and combined surface roughness
      • in vivo joints often need mixed lubrication instead of a single ideal lubrication mode
    • Wear-risk control
      • reduce adhesive and abrasive wear through lubrication and surface finish
      • consider fatigue and delamination under repeated cycles
      • prevent fretting at modular interfaces where small vibration under load can occur
  • Take-Home Message
    • What are roughness and waviness?
      • Roughness: S < 1 mm, small, finely-spaced deviations from nominal surface.
      • Waviness: 1 ≤ S ≤ 10 mm, larger-spacing deviations caused by work deflection, vibration, tooling, etc.
      • Roughness is superimposed on waviness.
    • How to calculate Ra and RRMS?
      • Ra = 1/n Σ|yi|
      • RRMS = sqrt(1/n Σ yi^2)
      • Ra averages absolute profile deviations; RRMS weights high peaks more strongly.
    • Describe different types of lubrication.
      • Hydrodynamic lubrication: relative motion draws lubricant into contact; 10^-3 - 10^-4 cm separation; no asperity contact.
      • Elastohydrodynamic lubrication: 10^-4 - 10^-5 cm separation; load transmitted through lubricant; elastic surface deformation.
      • Squeeze film lubrication: viscous lubricant responds during transient overloads.
      • Boundary lubrication: solid or gel coating; high pressure and low velocity; separation less than 10^-5 cm.
      • Mixed lubrication: multiple modes coexist, as in articular cartilage.
    • Describe different types of wear.
      • Adhesive wear: fragments pulled from one surface and adhered to another; particles 10-100 μm.
      • Abrasive wear: hard asperities remove softer material by plowing, cutting, or cracking.
      • Corrosive wear: corrosion layer removal causes pitting or topography change.
      • Contact fatigue: rolling-induced alternating near-surface stresses remove material.
      • Erosive wear: liquid or solid particle bombardment removes material.
      • Fretting wear: small reciprocating or vibrating motion under load; sliding 20-200 μm.
      • Delamination wear: subsurface crack grows parallel to surface and a thin sheet peels off.
    • Why does tribology摩擦学 matter in vivo?
      • OA and implant wear show that surface damage can create tissue degeneration, debris, inflammation, and systemic metal ion exposure.
      • Low-wear design must combine material properties, surface topography, lubricant behavior, and biological response.