Tribology Notes
Tribology
Tribological System and Concepts
Tribology: The study of interacting surfaces in relative motion, encompassing friction, wear, and lubrication. It is a multidisciplinary science that incorporates aspects of materials science, engineering, chemistry, and physics to understand and optimize the performance and lifespan of mechanical components.
Friction: Resistance to movement, releasing energy (usually heat) between surfaces.
External Friction: Occurs at the boundary surfaces of solid bodies. It is influenced by factors such as surface roughness, material properties, and the presence of interfacial media.
Internal Friction: Chemical bonds of molecules damaged. This type of friction involves energy dissipation within the material itself, often related to its microstructure and composition.
Wear: Material loss from the surface due to friction, leading to degradation of mechanical components. Different types of wear include abrasive wear, adhesive wear, and fatigue wear, each with unique mechanisms and characteristics.
Lubrication: Techniques to reduce friction and wear, prevent corrosion, and dissipate heat. Effective lubrication is crucial for ensuring smooth operation and longevity of machinery by minimizing direct contact between surfaces.
Tribological System Components:
Base Body: Primary component in contact, serving as the reference for analyzing tribological interactions. It is typically the larger or more structurally significant component in the system.
Counter Body: Opposing surface in contact. Its properties and behavior directly influence the friction and wear characteristics of the tribological system.
Load Conditions: Pressure, temperature, and mechanical stress experienced by the contacting surfaces. These conditions play a critical role in determining the severity of friction and wear.
Surrounding Medium: Air, water, dirt, or other substances present around the tribological system. These can impact the tribological behavior through corrosion, contamination, or other environmental effects.
Interfacial Medium: Lubricant, air, or vacuum between the surfaces, affecting friction and wear. The choice and properties of the interfacial medium are vital for optimizing tribological performance.
Environment: Components outside the tribological system that can indirectly influence its behavior. This includes factors such as humidity, atmospheric pressure, and external vibrations.
Impact of Surrounding Medium: Can negatively influence system behavior.
Example: Warm, high-humidity air degrading lubricating oil, leading to increased friction and wear.
Key Parameters:
Material Properties: Hardness, toughness, and corrosion susceptibility of the materials involved. These properties determine the resistance to wear and deformation under tribological conditions.
Surface Roughness: Structured (e.g., machined) or stochastic (e.g., grit-blasted) surface textures. Surface roughness significantly affects the contact area and friction behavior.
Interfacial Medium Properties: Acidity, alkalinity, temperature, and particle content of the lubricant or other medium. These properties determine the effectiveness of lubrication and the potential for corrosion or abrasion.
Environment Properties: Acidity, alkalinity, temperature, and particle content in surrounding conditions. These environmental factors can influence corrosion rates and the ingress of contaminants.
Loading Conditions: Normal force, sliding speed, temperature, and duration of the tribological interaction. These parameters dictate the stress levels and heat generation during operation.
Tribological Testing Techniques
Objectives of Tribological Testing:
Assess impact of wear on machinery functionality and performance. Understanding wear mechanisms helps in predicting component lifespan and maintenance needs.
Verify wear conditions of equipment to detect anomalies and potential failures. Condition monitoring through wear analysis is crucial for preventative maintenance.
Optimize components for wear limits, ensuring they meet performance and durability requirements. Wear testing aids in identifying optimal materials and designs for specific applications.
Generate data for maintenance and lifecycle management, enabling predictive maintenance strategies. Tribological data informs decision-making on replacement intervals and maintenance schedules.
Support material and lubricant selection by evaluating their performance under relevant conditions. Testing facilitates the identification of the most suitable materials and lubricants for specific applications.
Ensure quality control through standardized testing to meet industry standards and regulatory requirements. Quality control measures are vital for ensuring consistency and reliability of components.
Enable wear testing of final assemblies to validate overall system performance. Testing complete systems helps in identifying potential issues that may not be apparent at the component level.
Classification of Tribological Tests:
Simplified Structure, Basic Loading Conditions:
Model Testing:
Focuses on fundamental friction and wear mechanisms using simplified test setups. Fundamental testing provides insights into the underlying tribological phenomena.
Uses specialized test samples under controlled, constant loading to isolate specific parameters. Controlled conditions are essential for accurate and repeatable results.
Defines friction/wear processes on standardized specimens, allowing for comparisons across different materials and conditions. Standardized tests enable benchmarking and material selection.
Example: Coating analysis with an oscillating tribometer to assess wear resistance. Oscillating tribometers are used to evaluate coating performance under sliding conditions.
Specimen Testing:
Tests under conditions close to real-world loading using specimens resembling actual components. Realistic testing provides more relevant data for specific applications.
Simulates operational wear in a controlled environment to evaluate the performance of materials and designs. Controlled environments ensure repeatability and comparability of test results.
Example: Cylinder liner coating tests using specimens machined from real engine blocks. Using actual component materials ensures accurate simulation of real-world conditions.
Tools: Simplified tribometers that apply precise normal forces and controlled relative motion and measure frictional force and other parameters. These tools allow for precise control and measurement of tribological parameters.
Complex Structure, Simplified Loading Conditions:
Component Testing:
Testing on original or simplified components to assess their tribological behavior. Component testing bridges the gap between simple specimen testing and full assembly testing.
Example: Floating Liner Engine – Study of the cylinder liner-piston ring contact under controlled conditions. This setup allows for detailed analysis of specific component interactions.
Component Assembly Testing:
Tests on original equipment under near-real operating conditions to evaluate overall system performance. Assembly testing helps in identifying interactions and system-level effects.
Example: Friction measurement on an engine test bench – Full engine analysis (without combustion) to evaluate friction losses. This provides a comprehensive assessment of friction within the entire engine.
Required Equipment: Advanced test rigs like friction dynamometers. Advanced test rigs enable precise control and monitoring of various parameters.
Advantages: Simultaneous control and monitoring of multiple parameters, closer simulation of real-world scenarios. These features ensure accurate and relevant test results.
Complex Structure, Realistic Loading Conditions:
Test Bench Evaluation:
Testing on complete, original tribological systems to replicate real-world operating conditions. This type of testing provides the most realistic assessment of system performance.
Examines complex component assemblies proven viable in near-operational tests. Focuses on components and assemblies that have shown promise in earlier stages of testing.
Methods: Vehicle test bench (chassis dynamometer), standard performance test bench, combined with friction power measurement. These methods provide comprehensive data on system performance.
Operational Testing:
Tests full, original tribological system under real-world conditions to validate performance. Operational testing is crucial for ensuring reliability in actual use.
Controlled lab environments or test tracks are used to monitor performance. Controlled environments allow for detailed analysis and data collection.
Most expensive and time-consuming, but provides most accurate performance data for real applications. The high cost is justified by the comprehensive and realistic data obtained.
Technical Surfaces and Contact Processes
Technical Surfaces: Geometrically defined boundaries of a component. These surfaces play a critical role in tribological interactions.
Material specifications generally valid for the component, but surface chemical composition may differ due to various factors.
Environmental effects can alter material composition, leading to adsorption and other surface modifications.
Technical Surfaces:
External Boundary Layer:
Contamination from external sources.
Adsorption layer of gases or liquids.
Oxide layer formed through chemical reactions.
Internal Boundary Layer:
Base material of the component.
Factors Influencing Tribological Properties:
Material properties: Hardness, elasticity, and chemical stability.
Surface roughness: Texture and topography influencing contact.
Temperature: Affecting material properties and lubricant viscosity.
Coatings: Protective layers modifying surface characteristics.
Oxide layers: Influencing friction and wear behavior.
Contaminants: Abrasive particles or reactive substances.
Load: Applied force affecting contact stress.
Sliding speed: Relative velocity influencing friction and heat generation.
Contact Processes: Atomic, molecular, and mechanical interactions at contacting surfaces. These interactions determine the friction and wear characteristics.
Adhesion:
Occurs when perfectly smooth surfaces come into contact, leading to strong bonding.
Molecular interactions involve ionic, covalent, metallic, and Van der Waals bonding.
Contact Geometry and Mechanics:
Contact occurs at discrete micro-contact points due to surface microgeometry. Understanding the geometry and mechanics of these contact points is crucial.
These deform under normal load, affecting friction and wear.
Contact Types by Geometry:
Conformal (Surface): Large area contact, providing even stress distribution. Examples include taper fits and piston ring-cylinder interfaces.
Line (Counterformal): Narrow linear contact, concentrating stress. Examples include gears and roller bearings.
Point (Counterformal): Highly localized contact, leading to high stress concentrations. Examples include ball bearings.
Surface Roughness and Parameters
Nominal Surface: Ideal geometrical surface defined in technical drawings, representing the intended design.
Real Surface: Differs from nominal due to manufacturing disturbances such as vibrations, deformations, and wear.
Deviations of Real Surface: Classified by magnitude to quantify surface imperfections.
Form Error: Deviations between real and nominal surface form, including irregularities in shape and size.
Waviness: Long wavelength, shallow-depth periodic irregularities resulting from machine instability or material deformation.
Roughness: Small-spaced, patterned repeating irregularities due to cutting tool marks or material grain structure.
Micro Roughness: Irregularities in crystal lattice structure due to physical and chemical effects at the atomic level.
Surface Roughness Parameters:
Profile Height: Sum of highest roughness peak and lowest roughness valley, providing an overall measure of surface texture.
Maximum Roughness Depth: Deepest valley within the evaluated length, indicating the severity of surface irregularities.
Average Surface Roughness: Average distance of profile points from mean line over sampling length, calculated as the absolute mean of peaks and valleys (in micrometers). This is a commonly used parameter for assessing surface finish.
Mean Roughness Depth: Average difference between the 5 highest peaks and 5 deepest valleys over the sampling length, providing a more robust measure of surface irregularities.
Abbott-Firestone Curve Parameters:
Core Roughness: Central "bearing" zone of the surface, representing the area most stable under load.
Reduced Peak Height: Height above core profile, indicating wear-prone peaks that are likely to make initial contact and wear down rapidly.
Reduced Valley Depth: Depth below core profile, revealing oil-retaining valleys critical for lubricant retention and friction reduction.
Abbott-Firestone Curve (Bearing Ratio Curve):
*Which peaks wears first (steep initial slope)
*Oil reservoir capacity (deep valleys)Measurement Methods:
Contact Profilometry: Measures surface texture by dragging a stylus across the surface.
Optical Profilometry: Uses light interference to measure surface topography without physical contact.
Stribeck Curve and States of Friction
Stribeck Curve: Describes the transformation of the friction coefficient based on relative speed and lubrication state. It illustrates the different regimes of lubrication.
-> dry: Represents direct contact between surfaces without any lubrication.
-> boundary layer: Molecular layer deposition of lubricant between surfaces.
-> mixed: Combination of dry and fluid friction, with partial separation of surfaces.
-> fluid-friction: Surfaces completely separated by a fluid film.
States of Friction:
Dry: Two bodies in direct contact, resulting in high friction and wear.
Example: Disk brakes, where heat removes surface contamination, leading to high friction.
*Example: Cold Starting (Lubricated surfaces have very little lubricating oil.Establishing proper lubrication can take several seconds after starting.)
Boundary Layer: Molecular layer deposition of lubricant occurs between two bodies, reducing friction.
Example: Piston ring-cylinder wall at deadlock, where minimal lubricant film exists.
Example: Road-wheel contact with contamination on the road surface.
Mixed: Dry and liquid friction occurring simultaneously, leading to moderate friction and wear.
Example: Road-wheel contact in rainy weather, where the tire surface is not in full contact with the road.
Liquid: Surfaces completely separated by hydrostatic or hydrodynamic liquid layer, resulting in low friction and wear.
Example: Aquaplaning, where a water layer separates the tire and road.
Gas Friction: Surfaces completely separated by aerostatic or aerodynamic layer, used in specialized applications where very low friction is required.
Why Friction Coefficient Decreases with Increasing Lambda Values:A thin lubricating deposit layer forms between surfaces, allowing easier relative movement.
Why Stribeck Curve Has a Minimum: Liquid deposit exists but is not sufficient to affect internal friction yet.
Why Friction Loss Increases: Liquid has larger inner friction.
Lubricants: Tasks, Composition, and Stribeck Curve
Functions of Lubricating Oils:
Lubrication: Reduces friction and wear between moving parts.
Cleaning: Removes contaminants and debris from the system.
Cooling and heating: Dissipates heat and maintains optimal operating temperatures.
Damping: Absorbs vibrations and reduces noise.
Sealing: Prevents leakage of fluids and gases.
Corrosion protection: Protects surfaces from rust and corrosion.
Hydraulic fluid and medium: Transmits power and provides hydraulic functions.
Composition of Lubricants: Typically composed of 70–90% base oil, 5–20% additives, and 5–10% viscosity modifiers.
Base Oils: Provide lubrication, act as solvent for additives, serve as a heat-transfer medium, and ensure proper flow properties.
Categorized by type (mineral, synthetic) and grouped by viscosity grade.
Groups I-V, with increasing performance and cost, reflecting different levels of refinement and purity.
Base Oil Properties:
Paraffinic & aromatic content: Affecting viscosity and thermal stability.
Color & odor: Indicators of purity and processing.
Viscosity: Resistance to flow, critical for lubrication.
Pour point: Lowest temperature at which the oil can flow.
Flash point: Temperature at which the oil vapors ignite.
Evaporation loss: Measure of oil volatility.
Sulfur content: Affecting corrosion and oxidation stability.
Additives: Provide structure to the lubricating layer and enhance performance.
Active Additives: Move in oil and stick to surfaces, providing specific functions:
Anti-wear additives: Reduce wear by forming protective films.
Viscosity modifiers: Improve viscosity stability over a range of temperatures.
Extreme pressure (EP) additives: Protect surfaces under high loads and temperatures.
Antioxidants: Prevent oxidation and degradation of the oil.
Corrosion inhibitors: Protect metal surfaces from corrosion.
Dispersants: Keep contaminants suspended in the oil.
Detergents: Clean surfaces and neutralize acids.
Friction modifiers: Reduce friction and improve fuel economy.
Passive Additives: Remain dissolved in the oil and provide general benefits:
Pour point depressants: Inhibit paraffin crystal agglomeration.
Viscosity modifiers: Improve viscosity characteristics.
(De)emulsifiers: Control the formation of emulsions.
Foam inhibitor: Prevents foam formation.
Tackiness improver: Enhances oil adhesion.
Solubilizer: Improves additive solubility.
Biocide: Controls microbial growth.
Pour Point Depressants: Inhibit paraffin crystal agglomeration, ensuring oil flow at low temperatures.
Impact of Highly Additivated Oil:
Reduces friction and wear when lubricant film thickness is insufficient, providing added protection.
The friction coefficient decreases, improving energy efficiency.
The wear rate decreases, extending component life.
Viscosity of Lubricants and Classification
Viscosity: Resistance of a fluid to flow, a critical property for lubrication.
Factors Influencing Viscosity:
Friction force (): Force required to move one layer of fluid over another.
Surface area of the layers (): Area of the contacting surfaces.
Relative velocity difference between the surfaces (): Speed difference between the layers.
Distance between the layers (): Separation between the layers.
Viscosity Index: Indicates how viscosity changes with temperature, with higher values indicating better stability.
Dynamic Viscosity: Resistance caused by internal friction when an external force tries to displace a fluid, measured in Pascal-seconds (Pa·s).
Kinematic Viscosity: Ratio of dynamic viscosity to density, measured in square meters per second (m²/s) or centistokes (cSt).
Classification of Engine Oils: Using SAE J 300 standard (e.g., 5W30) to define viscosity grades.
5W ("winter"): Specifies low-temperature viscosity and minimum pumpability temperature for cold starting.
30 (“summer"): Specifies kinematic viscosity at 100°C and dynamic viscosity at 150°C (under high loads) for high-temperature performance.
150°C: Temperature the oil must withstand under high shear conditions.
Measuring Methods:
CSS (Cold Cranking Simulator): Simulates cracking a shaft in a precise plain bearing with clearance at low temperature to assess cold starting performance.
MRV (Mini Rotary Viscosimeter): Measures oil's resistance to flow with rotational motion at low temperatures, evaluating pumpability.
HTHS (High Temperature High Shear): Simulates high temperature and shear stress conditions in engine bearings to measure viscosity under extreme conditions.
Institutions Defining Performance Levels:
API (American Petroleum Institute):
“S” series (gasoline engines) – spark ignition engines.
“C” series (diesel engines) – compression ignition engines.
ACEA (European): Ax, Bx, Cx classifications specifying performance requirements for different engine types.
OEM (Car manufacturers): Setting their own specific requirements for lubricants used in their vehicles.
Hydrodynamic Lubrication
Hydrodynamic Lubrication: Requires pure fluid friction (\lambda > 3), where surfaces are completely separated by a lubricant film.
Contacting surfaces are completely separated, with a load-bearing oil film formed by the motion of the surfaces.
Friction coefficient determined by fluid film friction, which depends on the lubricant's viscosity and the system's operating conditions.
Conditions for Pure Fluid Friction:
Oil Adhesion: Lubricant must adhere to both contacting surfaces to create a continuous film.
Converging Geometry: Surfaces have a tapered gap (narrowing in the direction of motion) to generate hydrodynamic pressure.
Relative Motion: Velocity difference must exist between the contacting pairs to drive the formation of the lubricant film.
Advantages of Hydrodynamic Lubrication:
Ensures low friction and minimal wear, extending service life of components.
Frictional losses governed by fluid viscosity, which depends on external factors such as temperature and pressure.
Critical Viscosity Influences:
Cold oil viscosity can be 1.5-2x higher than at operating temperature, leading to increased drag and energy consumption.
Cold Starts: High viscosity increases drag, making it harder to start the engine and increasing wear.
Stable Operation: Optimal viscosity at working temperature balances film strength and energy losses, providing effective lubrication without excessive drag.
Design Requirement: Systems must account for thermal expansion and viscosity gradients to maintain optimal lubrication.
Wear Curve and Mechanisms
Wear Curve Phases:
Run-in Phase (Running-In Wear): Initial period of high wear as surfaces conform to each other.
Normal Wear Phase (Mild Wear): Stable period of low wear as surfaces become smoother and more compatible.
End-of-Life Wear Phase (Drastic Material Removal): Rapid increase in wear as components reach their wear limits and begin to fail.
Wear Mechanisms (7 Types):
Abrasive: Wear due to hard particles scratching or cutting a softer surface.
Adhesive (or galling): Wear due to transfer of material between surfaces under high pressure.
Oxidative: Wear due to the formation and removal of oxide layers.
Fatigue: Wear due to repeated loading and unloading, leading to crack formation and material removal.
Erosive: Wear due to the impact of particles carried by a fluid or gas.
Frictional oxidation (fretting): Wear due to small-amplitude, high-frequency oscillatory movements.
Cavitation: Wear due to the formation and collapse of bubbles in a liquid.
Abrasive Wear:
Two-body abrasion: Harder surface peaks scratch the softer surface directly.
Three-body abrasion: Abrasive material gets between surfaces and causes wear.
Example: Contacting tooth surfaces of gears where hard particles can cause abrasion.
Adhesive (or Galling) Wear:
Occurs under high surface pressure between slowly moving surfaces in the absence of fluid lubrication or oxide film.
Harder metal particles tear out material from the softer surface, leading to material transfer.
Example: On lubricated surfaces when the lubricant film breaks during startup or shutdown, causing direct contact.
Oxidative Wear:
Metal oxide compounds form on the surface and detach, leading to material loss.
Slow wear, accelerated by temperature and water, as these promote oxidation.
Example: Everywhere, but more common at high temperatures where oxidation rates are higher.
Fatigue Wear:
Repeated load fatigues the surface layer, leading to microcracks and flaking.
Example: In rolling friction (roller bearing, gear) where repeated stress cycles cause fatigue.
Erosive Wear:
Particles of a flowing medium hit a solid body, tearing out material.
High impact speed = high wear, as the kinetic energy of the particles increases erosive effects.
Example: At flow separation in turbine blades where high-speed particles cause erosion.
Frictional Oxidation (Fretting) Wear:
Caused by low-amplitude, high-frequency oscillatory movements, leading to surface damage.
Produces black, reddish-brown deposits, which are characteristic of fretting wear.
Example: In all mechanical equipment under vibration (e.g., vehicle bodies, engine blocks, roller bearings).
Cavitation Wear:
Develops on surfaces of solid bodies in contact with flowing liquids, where bubbles form and collapse.
Gases dissolved in the liquid form bubbles → collapse → cause strong fluid impacts on the solid surface, leading to material removal.
Can create crater-like holes with up to 15,000 bar pressure due to the implosive forces.
Example: On impellers of cooling water pumps where cavitation is common due to pressure variations.
Technical Solutions to Improve Wear Resistance
Wear-Reducing Methods:
Proper Material Selection:
Pairing hard and soft materials (metal + plastic = GOOD, soft + soft = BAD) to reduce adhesive wear.
Coarse-grain vanadium- and chromium-carbide steel for abrasive wear resistance.
Choice of Wear-Resistant Structural Materials:
Wear-resistant steels, cast irons, ceramics, and composites.
Alloying:
e.g.: Manganese, chromium, molybdenum to enhance hardness and wear resistance.
Even a small amount significantly improves properties, making it cost-effective.
Heat Treatment:
Significant wear resistance, but becomes brittle if not properly tempered.
Sintering, Followed by Heat Treatment:
Sintering of metal and ceramic powders with reinforcing materials (graphite, diamond, boron…) to improve hardness and wear resistance.
Coating, Surface Layers:
Ensuring good sliding, wear-resistant surface through various coating techniques.
Protective layer (galvanization, metal spraying, powder coating, chemical surface treatment) to prevent corrosion and wear.
Overlay Welding:
Welding rods of similar material to wear-resistant materials to build up worn surfaces.
Spraying (Electrical):
Similar to overlay welding but uses an electrical arc to deposit material.
Lower bond strength compared to welding.
e.g.: Plasma spraying for ceramics to create a hard, wear-resistant layer.
Wear Measurement Methods
Mass Measurement: Measure component mass before and after tests to quantify material loss.
Advantage: Simple and straightforward.
Disadvantages: No wear location information; contaminants can affect results, leading to inaccuracies.
Mechanical Measurement: Measure geometric dimensions before and after tests to determine wear extent.
Advantage: Wear location can be determined, providing insights into wear patterns.
Disadvantage: Time-consuming, especially for complex components, making it less efficient for large-scale testing.
Optical Measurement: Digitize component surface before and after tests and compare to assess wear.
Indentation Method
Used with a Vickers hardness tester to determine local wear.
*Disadvantages: bumps form at the edges of the indentation, which disturbs the accuracy of the measurement, the material hardens at the indentation, the contact area size is reduced
2.Microgeometrical measurement methods for cylindrical surfaces:Difficult to measure,Custom setup must be allowed for these measurements. Microscope, interferometer, scattered light detection. Making cross-sections
3.Silicone imprint.Flexible material and can be measured optically- Advantages is that it archives the initial point of the surface
Chemical Analysis: Examine metal content of detached particles in oil to assess component wear.
Advantage: No need to disassemble components, allowing for non-destructive testing.
*Analyse wear particles with a microscope (shape, color, size, discoloration temperature).
Radioactive Wear Measurement: Activate surface of parts to be examined using radioactive isotopes to track material loss.
Advantages:
Enables online wear measurement for real-time monitoring.
Continuous wear rate measurement provides detailed wear data.
Extremely sensitive, allowing for detection of small amounts of wear.
Small and localized wear can also be measured well.
Advantages- Thanks to online wear measurement, the engine does not need to be continuously disassembled and reassembled
Disadvantages:
Radiation hazard requiring safety precautions.
Expensive due to the need for specialized equipment and handling.
Radionuclide Technique (RNT):
*The assignment of particle hit counts to wear depth. Radiation energy is measured.Methods:
*Filter-concentration