Materials Science: Wear and Fatigue Mechanisms

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Questions and Answers

What is fatigue in materials engineering?

A process involving permanent structural changes due to fluctuating stresses. (correct)

A sudden loss of tensile strength in a material.

The ability of a material to withstand impact without damage.

A method to enhance the durability of materials.

What characterizes a smooth or burnished fatigue fracture?

Indicates a rapid failure of material.

Shows intergranular fracture patterns.

Result of rubbing at the crack surfaces. (correct)

Appears only in ductile materials.

What indicates a granular fatigue fracture?

It usually appears smooth in texture.

It suggests a slow propagation of cracks.

It is associated with rapid material failure. (correct)

It demonstrates signs of ductile failure.

What do 'beach marks' or 'clamshell marks' signify in fatigue fracture?

Intermittent crack growth during loading variations. Signup and view all the answers

What is generally found in the fatigue zone of a fatigue fracture?

Rubbing marks indicative of crack arrest. Signup and view all the answers

Which of the following best describes the process of crack propagation in fatigue fractures?

It can grow intermittently during changes in loading patterns. Signup and view all the answers

Which zone is associated with rapid failure in fatigue fractures?

Rupture zone. Signup and view all the answers

Why is fatigue failure significant in materials engineering?

It leads to failures without prior visible signs. Signup and view all the answers

What is the primary definition of wear?

Damage of a surface in contact with another resulting in fragments or debris Signup and view all the answers

Which of the following ranges is typical for machined surfaces?

0.1 to 10μm Signup and view all the answers

What does abrasive wear also refer to as?

Cutting wear Signup and view all the answers

In which condition does two-body abrasion occur?

Abrasive particles are fixed to a substrate Signup and view all the answers

What is the most common form of wear?

Abrasive wear Signup and view all the answers

What mechanism does three-body abrasion involve?

Abrasive particles forced against a fixed surface by a mobile third body Signup and view all the answers

Which surface damage caused by wear leads to the formation of fragments?

Abrasive wear Signup and view all the answers

What is a physical cause commonly associated with wear mechanisms?

Contact pressure between surfaces Signup and view all the answers

What is the primary purpose of galvanization in hot-dipping?

To apply a protective zinc coating Signup and view all the answers

What type of structures are created in hot-dipped coatings?

Multi-phase laminar microstructures Signup and view all the answers

How does the hot-dipped coating bond to the substrate?

Due to a chemical reaction Signup and view all the answers

Which combination of materials is commonly used in hot-dipping?

Zinc on steel and tin on steel Signup and view all the answers

What is a common characteristic of hot-dipped coatings regarding peeling?

They rarely peel off the substrate Signup and view all the answers

What processes initiate the polymerization reactions in the liquid film of paints?

Contact with atmospheric oxygen or ultra-violet light Signup and view all the answers

Which of the following serves as a liquid medium during the storage and deposition of paints?

Solvents Signup and view all the answers

What type of compounds are commonly used as organic solvents in paints?

Xylene, toluene, and halogenated compounds Signup and view all the answers

During the coating application, how is the liquid film of paint distributed over a surface?

By surface tension forces Signup and view all the answers

What is the final state of the paint after the polymerization reaction is completed?

A more durable solid film Signup and view all the answers

What role do solvents play besides providing a medium for paints?

They control viscosity Signup and view all the answers

Which of the following is NOT an example of a solvent used in paints?

Lead oxide Signup and view all the answers

What state must paints be in when first applied to a surface?

Liquid state Signup and view all the answers

What is the primary purpose of chromium plating?

For both decorative and industrial purposes Signup and view all the answers

Which of the following types of chromium can be used in chromium plating?

Trivalent or hexavalent chromium Signup and view all the answers

What is the coating formation rate during anodizing at a current density of 25 A/m2?

1 μm/min Signup and view all the answers

What is the main difference between electroplated coatings and electroless coatings?

Electroplated coatings require a power supply, whereas electroless coatings do not Signup and view all the answers

In the mechanism of electroless plating, what is deposited from a solution?

Metal ions Signup and view all the answers

What is the principle behind plasma spraying?

Heating and spraying molten or heat softened material onto a surface Signup and view all the answers

During plasma spraying, what happens to the droplets of coating material?

They remain liquid long enough to merge and form a continuous layer Signup and view all the answers

What is a key characteristic of anodizing in terms of the nature of reactions involved?

It applies anodic electrochemical reactions Signup and view all the answers

Study Notes

Solid Surfaces

Machined surfaces typically have roughness ranging from 0.1 to 10 μm.

Wear Prevention and Definition

Wear is the damage of a surface in contact with another, leading to fragments or debris.
Preventing wear is crucial in maintaining surface integrity and functionality.

Common Wear Mechanism

Abrasive wear is the most frequent form of wear encountered in materials.
Abrasive wear involves cutting furrows on a surface caused by hard particles, also referred to as cutting wear.
Two-body abrasion: Abrasive particles are adhered to a substrate.
Three-body abrasion: Abrasive particles are forced against a fixed surface by a third body.

Fatigue and Fracture

Fatigue is a progressive localized permanent structural change in materials due to fluctuating stresses and strains, potentially leading to cracks or fractures.
Fatigue fractures feature two distinct patterns:
- Smooth or burnished: Result from rubbing during crack propagation, associated with the fatigue zone.
- Granular: Indicative of rapid material failure, signifying a rupture zone.

Beach Marks in Fatigue Fracture

Beach marks, or clamshell marks, appear on parts subjected to intermittent loading and resting, highlighting the crack growth during varied loading patterns.

Discrete Coatings Overview

Discrete coatings range from traditional paints and varnishes to advanced coating technologies for material protection.

Paints and Varnishes

Organic coatings are applied in liquid form to create a film based on surface tension.
The durability of the film is enhanced through polymerization reactions initiated by environmental factors like UV light or oxygen.

Solvents in Paints

Solvents serve as the liquid medium during the storage and application of paints and can include organic fluids like water or white spirit.

Electroplating

Electroplating involves immersing substrates in metal salt solutions connected to a power supply for metal deposition.
Commonly used for decorative and industrial finishes, with chromium plating being a notable application.

Anodizing

Anodizing is an electrochemical process that forms a protective coating on metal substrates through anodic reactions.
Typical coating formation rates are about 1 μm/min at a current density of 25 A/m², with common thickness reaching up to 20 μm.

Electroless Coatings

Electroless plating does not require a power supply for metal deposition and is utilized for coating metals onto non-metallic substrates like plastics and ceramics.

Plasma Spraying Technique

Plasma spraying involves applying molten or heat-softened materials to surfaces to form protective coatings, resulting in a continuous layer from discrete molten droplets.

Hot-Dipping: Galvanization

Hot-dipping, particularly galvanization, involves immersing metals in molten baths to create protective alloyed coatings.
This process is primarily used to apply zinc coatings on steel to prevent rust and corrosion.

Structure of Hot-Dipped Coatings

Hot-dipped coatings possess a multi-phase laminar microstructure, with strong bonding to substrates arising from reactions during the dipping process, ensuring minimal peeling.

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Description

This quiz covers the essential concepts of wear mechanisms and fatigue in materials science. Learn about abrasive wear, two-body and three-body abrasion, and the implications of fatigue on structural integrity. Test your knowledge on how to prevent wear and understand its definition.