Material T8 - Degradation and Failure

Tribology

Friction, wear, lubrication (science of interaction between relative surfaces)

Asperities

Areas where surface degrades, more energy needed to overcome wear

Tribosystem

Two bodies moving against each other

Adhesion

Occurs between 2 clean surfaces in contact undergoing relative motion

Abrasion

Interaction of hard particle on a counterface, hard material scratches soft

Two-Body Sliding Abrasion

Three-Body Sliding Abrasion

Medium inbetween two bodies causing abrasion

Surface Fatigue

Fatigue and crack formation due to repetitive alternating stress

Tribochemical Reaction

Formation of reaction products due to chemical reactions of surface - wear and corrosion

Adhesive Wear Suppressed

First stage of tribochemical wear

Destruction of Film in Contact, Adhesive Wear Suppressed

Second stage of tribochemical wear

Intense Corrosion by Anodic Dissolution between Fissures

Third stage of tribochemical wear

Unchecked Adhesive Wear, Rapid Corrosion on Exposed Surface

Fourth stage of tribochemical wear

Corrosion

Deterioration of a material due to reactions with the environment

Metal Corrosion

Formation of rust

Non-Metal Corrosion

Deterioration by sunlight or UV

Inorganic Materials

More correosive than organic materials (e.g. polymers)

Corrosion Process

Involves the movement of electrons from one atom/molecule to another

Oxidation

Metals lose electrons and become positive ions, occurs at the anode

Reduction

Ions generated from oxidation reaction form new materials (corrosion products), occurs at the cathode

Wet Corrosion Cell

Anode corrodes by loss of electrons, cathode consumes electrons

Electrode Difference

Energy required to add/remove electrons from metals

Electrochemical Series

Determined under standard conditions, values absolute for each element but CANNOT be used for alloys

Galvanic Series

True only for specified conditions (in seawater), dependent on electrolyte/temperature/pressure and CAN be used for alloys

More Active (Anodic)

Lower on the electrochemical/galvanic series

More Inert (Cathodic)

Higher on the electrochemical/galvanic series

Uniform Corrosion

Occurs uniformly over material surface, measured mm/year

Pitting Corrosion

Localised corrosion occurs in materials with coating (passive oxide or painted surface)

Intergranular Corrosion

Attacks between individual grains in polycrystalline materials

Welding of Stainless Steel

Common cause of intergranular corrosion

Galvanic Corrosion

Occurs when two dissimilar materials (metals) are in contact

Crevice Corrosion

Occurs when corrosive solution is trapped under crevice and causes localised corrosion

Physical Barrier, Cathodic Protection, Corrosion Inhibitors

Methods of preventing corrosion

Physical Barrier

Separate material from environment, use different coatings, or insulate metals from each other

Cathodic Protection

Galvanising, sacrificial anode, or impressed current

Sacrificial Anode

A more anodic (active) material is used to prevent a structure from corroding

Corrosion Inhibitors

Remove active species from environment, add low concentration additives (e.g. hydrazine)

Galvanising

Supplying electrons to push oxidation reaction backwards

Counter Electrode

Completes cell circuit but does not interact (made from inert conductor, e.g. platinum)

Reference Electrode

Measures the working electrode potential, has a very stable potential

Working Electrode

Sample intended to be tested for corrosion

Fatigure

Failutre due to cyclic stress

Beach Marks

Semi-circular and outward radiating fracture surface caused by fatigue

10^7 Cycles

‘Infinite’ lifetime strength for fatigue

Crack Nucleation

Determines low cycle fatigue

Crack Propagation

Determines high cycle fatigue

Fatigue Strength

Lower plateau at high no. of cycles

Creep

High temperature phenomena relative to melting point

Creep Rate

dε/dt, change in strain with time measured in s^-1

Primary Creep

Slope (rate of creep) decreases w/ time

Secondary Creep

Steady-state (constant slope)

Tertiary Creep

Slope increases w/ time

Greater strain and earlier fracture

Effect of increasing temperature on creep

Creep Failure

Usually occurs along grain boundaries