Chapter 10 Surface Treatments

Question 1

Surface treatments can either be

Answer 1

Functional or aesthetic

Question 2

What can surface treatments be designed to enhance ( 5 things )

Answer 2

Thermal or electrical properties
Fatigue life
Wear Resistance 
Corrosion Resistance 
Oxidation Resistance

Question 3

Where does fatigue crack take place

Answer 3

At the surface of components

Question 4

Where does wear, corrosion and oxidation occur

Answer 4

At the surface of components

Question 5

Where is a thermally or electrically insulating surface desirable

Answer 5

Where the components contacts others or an operator

Question 6

Given an example where changing the bulk material to improve the properties increases the cost

Answer 6

Replacing mild steel by stainless steel or titanium (improve corrosion resistance)

Question 7

Changing the bulk to improve properties but may lead to undesirable properties in bulk

Answer 7

A hard material may improve fatigue and wear resistance but hard materials are always brittle

Question 8

What does surface modification do

Answer 8

Gives the bulk of the component the best economy and load bearing characteristics ( strength, modulus, and toughness ). The surface leads to improved behaviour to surface - specific degradation mechanisms or it can be aesthetically pleasing

Question 9

Printing

Answer 9

Used for decorative or aesthetic purposes
Involves the application of a thin layer of paint/ink to the surface component
The layer offers no function in an engineering capacity (except for when screen printing is used to apply metallic pads for interconnects in thick film circuits)

Question 10

Coating

Answer 10

Apply a relatively thick layer to the surface of a component.
Coating is used for technical reasons (protect the component from corrosion or increase wear resistance0

Question 11

Painting

Answer 11

Acts as a barrier between the component and the environment and gives corrosion resistance

Question 12

What are the two types of paints

Answer 12

Solvent-based (environmentally unfriendly) - pigments are suspended in a resin in a volatile organic solvent. The paint dries by solvent evaporation

Water based - synthetic resins and pigments are dispersed in water. The water evaporates.

Question 13

Powder coating

Answer 13

The pigment is mixed with a binder to form a powder with no solvent. The binder is melted to build up a film, this allows the film to flow continuously, then cooling allows the component to solidify.

The power is then applied to the surface of a component by spraying or immersing it into a fluidised bed containing the powder. The component must be able to sustain the temperature required for melting the polymer.

Question 14

Enamelling

Answer 14

Involves the application of a powdered glass to the surface of a metallic component (by spraying or dipping) then heating the glass to form a continuous and thin film.
Gives hardness, corrosion and wear resistance but retains the strength and ductility

Question 15

Electroplating

Answer 15

A current is passed through a solution of a metal in salt , the solution decomposes and the metal plates out on the electrodes. Used in the deposition of high melting point metals on the surface of metallic components.

Question 16

Electroless plating

Answer 16

Like electroplating but no a current is not passed through the solution. Chemical decomposition takes places only on the surfaces.

Question 17

Anodising

Answer 17

Creating a wear resistant oxide coating on the surface of a component by immersing it in a conductive solution and then passing a current through the solution

Question 18

Which metals are anodised

Answer 18

Aluminium, titanium, magnesium, zirconium, and zinc

Question 19

How is the anodised film better than the underlying metal

Answer 19

The oxide is far harder than the underlying metal which leads to an improved wear resistance and corrosion resistance

Question 20

What is hot dipping

Answer 20

When metals of low melting points are dipped in a bath of molten metal, so a thicker coating against corrosion resistance can be applied

Question 21

What is vapour deposition

Answer 21

A coating is built up on the surface of a component from the vapour. This can either be by transfer from a source to component through the gas phase at low pressure.

Question 22

What is surface modification

Answer 22

This is where the microstructure or the composition is changed at the surface to increase the hardness and therefore the wear resistance and fatigue lifetime.

Question 23

How can you change the microstructure of a component without changing the composition

Answer 23

Giving the surface of the component a different heat treatment to the bulk of the component ( in steel the surface is heated to a high temperature then cooled rapidly into water )

Question 24

How would you change the composition of a component

Answer 24

Diffuse an element into the surface of the component which greatly increases the hardness of the parent metal. Case hardening

Question 25

What is polishing and why is it important

Answer 25

Applying smaller abrasive particles which gives scratches to the surface too fine to be observed by the eye. It is important for improving fatigue resistance since it removes scratches which can initiate fatigue failure

Question 26

What is etching ( of glad by hydrofluoric acid )

Answer 26

Used to produce opaque finish either globally or locally to give a pattern or lettering

Question 27

What is texturing used for

Answer 27

Imparting a desirable surface finish by sand blasting or brushing with a wire to give a random or patterned rough texture into the the surface of the component