Mateirals In service - surface engineering

Question 1

Primary function

Answer 1

1) Mirror - has to be reflective
2) Braking disk - high friction + low wear over range of T
3) Machine tools - high hardness, abrasion + oxidation resistant
4) Semiconductor substrates - smoothness and free from defects

Question 2

Protection

Answer 2

1) corrosion resistant coatings + surface treatment
2) polishing metals for fatigue resistance
3) surface hardening for fatigue + wear resistance
4) polishing ceramics for strength
5) erosion + wear resistant layers

Question 3

Surface different from bulk ∵ processing or service

Answer 3

1) machining processes
2) oxide layers
3) work hardened layers
4) erosion/wear damage
5) fatigue imitation
6) corrosion

Could lead to failure of bulk material

Question 4

4 surface characterisation

Answer 4

1) general - appearance, shape
2) chemistry - elements, phases
3) mechanical - modulus, yield strength, toughness
4) localised defects - cracks

Question 5

How to measure surface topography

Answer 5

Stylus profilometry

= sharp point dragged across surface + up and down movement recorded

Can combine profile traces = surface picture

Question 6

Profilometry parameters

Answer 6

Avg roughness = Ra

RMS roughness = Rq

Look at slide for eqt?

Question 7

Downfall to stylus profilometry

Answer 7

No info about:

1) distribution of heights
2) length scale in surface
3) possible anisotropy

Question 8

Surface visualisation

Answer 8

1) Profilometry
2) microscopy - problems w/depth of field + resolution
3) Scanning probe methods

Question 9

How to assess overall flatness + gross waviness

Answer 9

Interferometry

Question 10

Non-contact profilometry

Answer 10

Laser-scanning

Question 11

scanning probe methods

Answer 11

1) Scanning Tunnelling Microscopy (STM)
2) Atomic force microscopy (AFM)
3) UFM

Question 12

Work hardening

Answer 12

V. Good for fatigue resistance
Eg. valve, spring wire, leaf spring, gears

1) flame hardening
2) induction hardening

Both use water quenching - spray or bath

3) laser hardening
4) e-beam hardening

Question 13

How to harden steel

Answer 13

1) heat surface into austenite range and quench

• let cold bulk quench
• extra surface cooling

Question 14

Surface melting

Answer 14

= grain refinement
(Steels can also get transformation)

Via: 1) electron beam 2) laser beam 3) TIG welding

Good for: 1) medium carbon steel 2) cast iron

3) Al alloys: Si platelets -> below critical size for fracture

Question 15

Carburising - case hardening

Answer 15

= done in austenite phase filed

Either:

1) Quench immediately + light temper

Or

2) cool slowly, machine, then heat-treat

Effects:

1) some dimensional changes (try minimise)
2) hardening (wear resistance)
3) surface compressed (fatigue resistance)

Question 16

Types of carburising

Answer 16

1) Gas carburising: 900C, atmosphere either: CO, H2, N2 or CH3OH, N2
2) case carburising: 900, pack in box w/ charcoal. CO2 produced

3) vacuum carburising: 1050C, components heated w/ low P, CH3, C5H12 added
Further diffusion heat

4) plasma carburising - same as vacuum apart from C5H12. Glow discharge deposits C on -vly charged surface
Further diffusion heat

Question 17

Carbonnitriding

Answer 17

C + N diffused into steel: quench + temper

Often oil quench ∵ = High hardness * reduces risk of cracking

BETTER WEAR RESISTANCE THAN CARBURISING

1) Gas Carbonitriding: add ammonia to gas
2) salt bath: liquid. Immerse in molten salt bath

Question 18

Nitrocarburising

Answer 18

• low allow/mild steels
• substrate hardened by N diffused in

Eg. Tuffriding

Use of salt baths

Modern processes: less toxic baths, or adapted gas/plasma nitriding

Question 19

Boronizing

Answer 19

FeB, Fe2B

Good for abrasive wear