Materials Science 3: Overview

Materials Science 3 Overview

• This course is Materials Science 3
• Materials Science is scheduled to be taught in January 2025
• Course content and chapter 11 covers materials science
• More information can be found at: https://www.tudelft.nl/me/over/afdelingen/materials-science-and-engineering

Course Content MAT3

• Key resources and information are available on OnderwijsOnline (OO) at https://han.onderwijsonline.nl
• It includes information, lesson presentations, assignments, and a test exam
• Focus is on materials covered within the book
• There will be material on friction and lubrication chapter 11
• There will be material on thermal properties chapter 12
• There will be material on creep chapter 13
• There will be material on electrical properties chapter 15
• There will be material on magnetism chapter 16
• There will be material on optical properties chapter 17

Learning Goals MAT3

• This course aims to provide knowledge and insight into non-mechanical material properties
• Lessons are prepared using the book and laptop with CES EduPack
• Students are encouraged to answer and ask questions
• The purpose of lessons is discussed and consolidated through tests and exercises
• There are six key lessons in the MAT3 program
• Lesson 1 covers friction and wear, corresponding to Chapter 11
• Lesson 2 covers materials and heat, corresponding to Chapter 12
• Lesson 3 covers materials at high temperatures, corresponding to Chapter 13
• Lesson 4 covers electrical conductors, insulators, and dielectrics, corresponding to Chapter 15
• Lesson 5 covers magnetic materials, corresponding to Chapter 16
• Lesson 6 covers materials for optical devices, corresponding to Chapter 17

Lesson Planning

• The lesson begins with self-study of Chapter 11 from the book Materials
• The study will include paragraphs 11.1 to 11.8 and exercise questions E11.1 to E1.13
• The lesson will further discuss friction and lubrication chapter 11;
• Topics include tribological properties, charting friction and wear, the physics of friction and wear, friction in design and friction in material processing

Tribological Properties

• Tribology studies friction, lubrication, and wear phenomena in interacting surfaces in relative motion

Coefficient of Friction

• Friction force (Fs) is directly related to the normal force (Fn)
• The relationship is expressed as: 𝐹𝑠 = 𝜇 ⋅ 𝐹𝑛
• Fs is the friction Force measured in Newtons
• Fn is the Normal force, also measured in Newtons
• 𝜇 represents the coefficient of friction, which is dimensionless
• Friction force, at a given normal force, is independent of the size of the surface
• No surface is perfectly flat, regardless of its finish
• When two surfaces are in contact, they touch only at the tops, or asperities

Static vs Dynamic Friction

• Static friction is greater than dynamic friction due to creep
• The coefficient of static friction is typically larger than the coefficient of dynamic/kinetic friction
• Dynamic friction is independent of sliding speed
• Before movement, the static friction force is equal to the applied force
• This relationship is 𝑓 = 𝐹, where 𝑓 is the static friction force and 𝐹 is the applied force
• μs indicates the coefficient of static friction.
• μk Coefficient of dynamic (kinetic) friction.
• The formula for static friction is: 𝑓 = 𝜇𝑠 ⋅ 𝐹𝑛 (before movement)
• After movement begins, the dynamic friction is described by: 𝑓 = 𝜇𝑘 ⋅ 𝐹𝑛

Wear

• Wear occurs when surfaces slide and material is lost from both surfaces
• Wear rate is defined as the volume of material removed from the contact surface divided by the distance slid
• Specific wear rate considers the wear rate per unit area
• The specific wear rate Ω and is given by: Ω = W/An = kaFn/An = kaP
• Where: W is the wear rate (m². A)
• Ω is the specific wear rate (-)
• Fn is the Normal force (N)
• An is the Nominal surface area (m²)
• ka is the Archard wear constant (MPa-1)
• P is the Bearing pressure (N/m²)
• Archard wear constant (ka) measures the tendency of a material pair to wear
• A high ka value indicates high wear
• There are limits to where the bearing pressure P is higher than the material hardness

Physics of Friction and Wear

• No surface is perfectly flat
• Contact mainly occurs at the peaks/asperities between two surfaces
• This means that the load Fn is carried only at these points
• This leads to plastic deformation in these zones
• These points flatten to form junctions with an arear Ar
• This can cause bonding of materials, leading to increased friction and stick-slip behavior

Adhesive Wear

• Typically occurs between the same/similar materials
• Tops stick together and shear off, causing wear damage

Abrasive Wear

• Typically occurs between hard-soft materials
• Tops from harder material plough through softer material
• This grinds off material, similar to sandpaper

Friction in Design: Bearings Bearings allow two components to move relative to each other

• This provides movement in one or two dimensions while constraining them in others
• Should have low friction and the capacity to support load without yielding
• Plain bearings and rolling bearings are the main types

Friction in Design: Lubrication

• Lubrication: Can be dry, boundary or hydrodynamic
• Lubricant allows easy shear, reducing the coefficient of friction to 0.001 – 0.01
• Boundary lubrication: Fn squeezes lubricant out, but additives are strong enough to resist contact
• Enough for during start-up or low load
• For continuous or high load conditions, hydrodynamic lubrication is preferable

Hydrodynamic Lubrication

• Lubricant is dragged by viscosity into wedge-shaped region
• Result is fully separated surfaces by oil film

Lubrication at High Temperatures

• Teflon (PTFE) can be used up to 250 °C
• Graphite can be used up to 300 °C
• Molybdenum disulfide (MoS2) can be used up to 500°C
• Using small holes in a metal, can also work as lubricant for example grey cast iron

High Friction Materials

• Friction linings use a coefficient of friction of around 0,4
• There are high demands on friction materials
• Must have no vibration
• Must be low wear
• Should not fade
• No stick-slip
• High temperature resistance
• High heat conduction

Waging War on Wear

• Wear increases with pressure (P) and sliding velocity (v)
• For high pressure, use bearing materials with higher hardness
• For high speed, decrease the coefficient of friction
• If the burst velocity is exceeded, there will be disintegration of bearing
• To reduce wear, consider surface hardening or coatings

Friction in Material Processing

• In many fabrication processes, friction is a limiting, negative factor
• It increases in tool/machine loads and High temperature for tools and products during processing, it also Limits section thickness and shapes
• Friction welding, on the other hand, uses friction as an advantage
• It enables joining of dissimilar materials normally not compatible for welding by other joining methods
• Creates narrow heat-affected zones, needing minimal joint preparation using a saw cut surface
• No filler material or gases required a solid-state process, with no porosity or slag inclusions

Homework Example

• The prototype disc brake homework example is:
• A prototype disc brake for a bicycle is to be mounted on the hub of the rear wheel and consists of a pair of pads of area 5 cm x 1 cm, which are pressed against opposing faces of the steel disc
• The pads are aligned with their longer dimension in the rotation direction, with the pad centers at a distance of 10 cm from the center of the disc.
• The brake is to be tested on a rotating machine to simulate the loading and wear in service
• The disc rotates at a suitable constant speed, and the brakes are applied for 10 s every minute, with each pad applying a normal load of 1 kN.
• Question 1 asks, If on application of brakes, the machines registers an increase in torque of 30 Nm, Find

Homework 1 and 2

• Question 1 solution shows:
• The friction force that is applied to each pad equates to 150N where Fs= 300 N
• 150N per brake pad equates to μ = 150/1000 = 0,15
• Question 2 solution:
• d¸= v• t = (150⋅(2𝜋⋅0,1)/60)*10 = 15,7 m
• Δχ = (2⋅10−14 ⋅1000⋅15,7)/(0,05⋅0,01) = 6,28 ⋅ 10−7 m (per minute)

Closing

• For the next class read book Materials (self-study) Ch 11
• Including Paragraphs 11.1 to 11.8, and Exercise questions E11.1 to E1.13
• Next lesson will be on Thermal properties (Ch. 12 )
• Including Paragraph 12.1 to 12.9, and Exercise questions E12.1 to E12.18