Tribology And Maintenance Engineering (ME-802 (B)) - Unit 2 Short Notes

UNIT 2: TRIBOLOGY - CORE CONCEPTS & APPLICATIONS

1.0 Introduction and Historical Context

Significance of Tribology:

• Study of friction, wear, and lubrication.

• Industrial Impact: Directly affects energy efficiency, machine reliability, maintenance costs, and product quality.

• Economic Loss: Global economic loss due to wear and inadequate lubrication is estimated at 1-4% of GDP for industrialized nations.

Historical Developments:

• Ancient: Use of lubricants (water, animal fats) in chariots, mills (c. 2400 BC).

• Renaissance: Leonardo da Vinci (1452-1519) formulated first laws of friction.

• Modern Foundation: Amontons (1699) & Coulomb (1785) established classical laws.

• 1960s (The "Jost Report"): Peter Jost's 1966 report for the UK government formally coined the term "Tribology" and quantified massive economic losses from neglect, establishing it as a distinct scientific and engineering discipline.

• Post-Jost: Development of elastohydrodynamic lubrication (EHL) theory, surface engineering techniques, and advanced computational tribology.

[!TIP] Exam Focus: Be prepared to state Jost's contribution (coining the term & quantifying economic impact) and list at least two ancient applications.

2.0 Contact Mechanics and Friction

Contact Types

Hertzian Contact Theory

• Purpose: Calculates contact area size and pressure distribution for non-conforming elastic contacts.

• Assumptions:

  1. Both materials are homogeneous, isotropic, and obey Hooke's law.

  2. Surfaces are smooth, frictionless, and initially separated.

  3. Deformation is small (strain < 0.005).

  4. Bodies are semi-infinite (elastic half-spaces).

  5. Contact area is small compared to body dimensions.

• Key Result: Contact area is elliptical. Maximum pressure at center:

$$P_{max} = \frac{3F}{2\pi ab}$$

where $F$ is load, $a$ and $b$ are semi-axes.

• Elastic Half-Sphere: A theoretical model where a sphere deforms on a rigid flat (or vice-versa) under load, forming a circular contact patch. Central to Hertzian analysis.

[!TIP] Common Pitfall: Hertzian theory is only for elastic, non-conforming contacts. It does not account for plasticity, friction, or surface roughness initially.

Friction Fundamentals

Amonton's Laws:

1. First Law: Force of friction is directly proportional to the normal load ($$\displaystyle F_f \propto N $$).

2. Second Law: Force of friction is independent of apparent contact area.

3. Third Law (Coulomb): Kinetic friction is independent of sliding velocity (for moderate speeds).

Exceptions to Amonton's Laws:

• Very low loads (atomic scale).

• Very high speeds (frictional heating).

• Very low speeds (stick-slip regime).

• Presence of boundary lubricants or adhesives.

• Deformation-dominated friction (e.g., rubber).

Bowden & Tabor's Theory (Adhesive Theory)

• Core Idea: Actual contact occurs at microscopic asperity junctions. Friction arises from shearing these adhesive junctions.

• Equation: $$\displaystyle F_f = \tau \cdot A_a $$

  • $$\displaystyle F_f $$: Frictional force

  • $\tau$: Shear strength of the junction material

  • $$\displaystyle A_a $$: Real area of contact (much smaller than apparent area).

• Key Insight: $$\displaystyle A_a $$ is proportional to $N$ (load) because asperities deform plastically. Hence, $$\displaystyle F_f \propto N $$, explaining Amonton's First Law.

Factors Affecting Friction

1. Material Pair: Intrinsic adhesion and deformation properties.

2. Surface Roughness: Can increase mechanical interlocking or reduce real contact area.

3. Environment: Humidity, temperature, corrosive media.

4. Normal Load: Affects real contact area and deformation.

5. Sliding Velocity: Influences temperature, surface films, and stick-slip.

6. Lubrication: Presence and type of lubricant film.

Stick-Slip Phenomenon

• Definition: Alternating "stick" (no relative motion) and "slip" (rapid relative motion) during sliding.

• Cause: Difference between static friction coefficient ($$\displaystyle \mu_s $$) and kinetic friction coefficient ($$\displaystyle \mu_k $$), where $$\displaystyle \mu_s > \mu_k $$. System elasticity stores energy during "stick" and releases it during "slip".

• Consequences: Vibration, noise, surface damage, inaccurate motion (e.g., in machine tools).

• Reduction: Increase damping, use compliant elements, apply lubricants, control system stiffness.

Methods to Reduce Adhesive Component of Friction

1. Lubrication: Introduce a low-shear-strength film between surfaces.

2. Surface Coatings: Apply hard, low-adhesion coatings (e.g., DLC, PTFE).

3. Surface Texturing: Create micro-dimples to trap debris and reduce real contact area.

4. Material Selection: Use dissimilar materials to reduce adhesion.

5. Environmental Control: Reduce humidity (for metals).

3.0 Wear Mechanisms

Definition: Wear is the progressive loss of material from a solid surface due to relative motion.

Classification of Wear:

Factors Influencing Wear:

• Material Properties: Hardness, toughness, ductility.

• Load: Higher load increases contact stress and wear rate.

• Sliding Distance/Velocity: Directly proportional to material loss.

• Environment: Corrosive media, temperature, presence of abrasives.

• Lubrication: Type and effectiveness of lubricant film.

• Surface Finish: Roughness can promote abrasive wear or aid lubrication retention.

4.0 Lubrication Regimes & Theory

$$\frac{\partial}{\partial x}\left( h^3 \frac{\partial p}{\partial x} \right) + \frac{\partial}{\partial y}\left( h^3 \frac{\partial p}{\partial y} \right) = 6\mu U \frac{\partial h}{\partial x}$$

|

| Elastohydrodynamic (EHL) | $$\displaystyle \lambda > 3 $$ (but elastic deformation & pressure-viscosity are critical) | High pressure in contacts (e.g., rolling elements) causes elastic deformation and dramatic viscosity increase. | Modified Reynolds equation + elastic deformation equation (Hertz contact). | | Mixed | $$\displaystyle 1 < \lambda < 3 $$ | Partial asperity contact; fluid film supports most load. | No simple equation; often modeled via Stribeck curve. | | Boundary | $$\displaystyle \lambda < 1 $$ | Surfaces in close proximity; friction/wear controlled by surface films (adsorbed layers, additives). | Friction governed by shear strength of boundary film. | | Elasto-dynamic | N/A (often considered under EHL) | Focuses on dynamic effects (vibrations, shocks) in lubricated contacts, especially under impact loads. | Coupled solution of Reynolds, elasticity, and dynamic equations. |

Reynolds Equation Assumptions:

• Newtonian fluid.

• Inertia and body forces negligible.

• Pressure constant across film thickness.

• Isoviscous (constant viscosity) - often relaxed in EHL.

• Rigid surfaces - relaxed in EHL.

• No slip at boundaries.

[!TIP] Exam Ready: Know the Stribeck Curve (friction coefficient vs. Hersey number) which visually defines the regimes. $$\displaystyle \lambda = \frac{\text{Minimum Film Thickness}}{\text{RMS Surface Roughness}} $$.

5.0 Bearings

Classification by Applied Load

• Full Journal Bearings: 360° wrap-around; supports radial load; requires hydrodynamic film.

• Partial Arc Bearings: <180° arc; simpler but less stable; used in high-speed applications.

• Thrust Bearings: Support axial loads (e.g., collar bearings, tilting pad thrust bearings).

Classification by Film Thickness

• Hydrodynamic: Self-acting, rely on relative motion to generate pressure.

• Hydrostatic: Externally pressurized oil feed; can support load at zero speed.

• Boundary-Lubricated: Operate with thin films; rely on additives/surface films.

• Dry/Solid Lubricated: No liquid lubricant; use PTFE, graphite, MoS₂.

Rolling Element Bearings

• Types:

  • Ball Bearings: Point contact; moderate radial & thrust loads; low friction.

  • Cylindrical Roller Bearings: Line contact; high radial load capacity, low thrust capacity, high speed limit.

  • Tapered Roller Bearings: Line contact; support combined radial & thrust loads.

  • Spherical Roller Bearings: Line contact; self-aligning; high radial & moderate thrust loads.

  • Needle Roller Bearings: High length/diameter ratio; compact radial section.

• Cylindrical Roller Bearings - Advantages & Disadvantages:

  • Adv: Very high radial load capacity, low friction, high speed capability.

  • Disadv: Very low axial load capacity (except with flanges), sensitive to misalignment.

• Bearing Life (Basic Concept): L₁₀ life is the rating life at which 90% of a bearing population will survive.

$$L_{10} = \left( \frac{C}{P} \right)^p$$

where $C$ = dynamic load rating, $P$ = equivalent dynamic bearing load, $p$ = 3 for ball bearings, 10/3 for roller bearings.

6.0 Surface Engineering & Coatings

Objectives of Surface Modification

• Reduce wear (abrasive, adhesive, fatigue).

• Reduce friction.

• Enhance corrosion and oxidation resistance.

• Improve fatigue strength.

• Provide specific functional properties (thermal barrier, electrical).

Coating Techniques

Physical Vapour Deposition (PVD):

• Process: Physical vaporization of source material (e.g., by sputtering, arc evaporation) in vacuum, followed by condensation on substrate.

• Sketch:

  DiagramSEARCH: "PVD coating process diagram sputtering arc"
  Shows vacuum chamber, target (cathode), substrate (anode), plasma, and coating deposition.

• Features: Line-of-sight deposition, lower temperature (<500°C), dense, adherent coatings (TiN, DLC, CrN).

Chemical Vapour Deposition (CVD):

• Process: Chemical reaction of gaseous precursors on heated substrate surface to form solid coating.

• Features: Conformal coating, higher temperatures (700-1100°C), coatings like TiC, SiC, diamond.

Hard Facing:

• Process: Welding a hard, wear-resistant alloy onto a substrate surface.

• Sketch:

  DiagramSEARCH: "hard facing welding process diagram"
  Shows welding torch, hard-facing rod, and deposited bead on base metal.

• Methods: Oxy-fuel welding, plasma transferred arc (PTA), laser cladding.

Electroplating:

• Process: Electrolytic deposition of metal ions onto conductive substrate (cathode) from an electrolyte solution.

• Advantages: Precise thickness control, good adhesion, can be decorative, low cost for thin coatings.

• Disadvantages: Only for conductive parts, environmental concerns (cyanide, chromium VI), hydrogen embrittlement, thickness non-uniform on complex shapes.

Other Methods:

• Thermal Spraying: (Flame, plasma, HVOF) - Spraying molten particles onto surface.

• Shot Peening: Inducing compressive residual stresses to improve fatigue life.

Coating Selection for Environments

• High-Temperature Oxidation/Corrosion:

  • Thermal Barrier Coatings (TBCs): YSZ (Yttria-Stabilized Zirconia) via APS/EB-PVD.

  • Oxidation-Resistant: Aluminide coatings (Al₂O₃ scale), MCrAlY alloys.

• Acidic Environments:

  • Inert Coatings: PTFE, PFA, certain ceramics (Al₂O₃).

  • Corrosion-Resistant: Noble metals (gold, platinum), certain carbides/nitrides.

Microstructural Treatments (Any Five)

1. Carburizing: Diffuse carbon into low-carbon steel surface at ~900°C; forms hard, high-carbon case.

2. Nitriding: Diffuse nitrogen into surface at ~500-550°C; forms hard nitrides (ε, γ') without quenching.

3. Induction Hardening: Rapid surface heating by induction followed by quenching; forms martensite.

4. Flame Hardening: Localized heating with oxy-fuel flame followed by quenching.

5. Laser Hardening: High-energy laser beam for rapid surface melting/quenching; precise, minimal distortion.

6. Carburizing, Nitriding, Induction Hardening (as listed).

Geometrical Parameters of Coatings (Any Three)

1. Thickness: Measured in µm; critical for load support and fatigue life.

2. Surface Roughness ($$\displaystyle R_a $$): Affects friction, wear, and sealing.

3. Adhesion Strength: Bond strength between coating and substrate (measured by scratch test, pull-off test).

4. Porosity: Volume fraction of voids; high porosity reduces load capacity and corrosion resistance.

5. Hardness: Typically measured by micro/macro-indentation (HV, HRC).

7.0 Friction & Wear Measurement & Characterization

Friction Measuring Equipment

Pin-on-Disc Tester (Detailed):

• Setup: Stationary pin (flat or spherical) pressed against a rotating disc under constant load.

• Measurement: Friction force transducer measures tangential force; coefficient of friction $$\displaystyle \mu = F_f / N $$.

• Wear: Wear scar on pin measured (volume or linear dimensions) or disc wear track profilometry.

• Advantages: Simple, versatile, simulates sliding wear, widely standardized (ASTM G99).

• Limitations: Simplified geometry, may not replicate complex contact stresses.

Other Equipment:

1. Reciprocating Wear Test Rig: Simulates sliding with reversal (e.g., engine piston/liner).

2. Four-Ball Tester: Measures wear scar diameter and friction under point contact; used for lubricant evaluation (ASTM D4172).

3. Ball-on-Cylinder: For rolling/sliding contacts.

Wear Measurement Techniques

• Weight Loss Method: Most direct; measure mass before/after test.

• Profilometry: Stylus or optical profilometer traces wear scar/track to determine volume or depth.

• Volumetric Methods: Using coating thickness loss or 3D scanning.

Standard Test Methods

• ASTM International & ISO: Provide standardized procedures for comparability.

• Examples:

  • Friction/Wear (Unlubricated): ASTM G99 (Pin-on-Disc), ASTM G133 (Reciprocating).

  • Lubricated Wear: ASTM D4172 (Four-Ball Wear), ASTM D2266 (Four-Ball EP).

  • Hard Coatings Adhesion: ASTM C1624 (Scratch Test).

  • Coating Thickness: ASTM B568 (X-ray fluorescence), ASTM D7091 (magnetic induction).

[!TIP] Exam Tip: When asked to "explain any one," choose Pin-on-Disc or Four-Ball and clearly state setup, measured parameter (µ, wear scar), and a key standard (ASTM number).