National Physical LaboratoryNational Physical Laboratory

Mark Gee

The National Physical Laboratory

• UK’s National Measurement Institute

• Founded in 1900

• 600 people

• Scientists

• Engineers

• Support staff

• 75% of funding from DTI

• GOCO since 1995

• Occupying new building

Expertise at NPL spans the physical, materials and life sciences

• Photometry/Colour • Pressure• RF and Microwaves• Radioactivity• Radiometry• Scientific Software• Time • Physical-chemical measurements for

life sciences• Biometrology• Nanotechnology

• Acoustics• Dimensional• Electromagnetics• Ionising Radiation• Environmental Air Quality• Lasers• Mass and force• Materials• Thermal• Optical communications• Photonics

Tribology at NPL

• Sliding wear– Effect of machine dynamics– Effect of tribochemistry– Measurement of low wear

• Abrasion– WC/Co– Wear-Corrosion

• Microscale tribology

Before Wear

After Wear

Before Wear

After Wear

After Wear

Correlated and Subtracted Images

Profile from above

3D Optical Microscopy• Similar to confocal microscopy• Height data combined with image

data• Dataset correlation• Real volume measurement

Modified ASTM G65 Test System

Results for WC/Co Hardmetals

600 800 1000 1200 1400 1600 1800 2000 2200 2400

0.1

1

10

100

W

ea

r V

olu

me

, mm

3

Dry Rubber Wet Rubber Wet Steel

Hardness, HV30

Results for WC/Co Hardmetals

• Archard law (wear is proprotional to load) is not obeyed for all abrasives

• Wear does not always stop when hardness of sample is greater than hardness of abrasive (cf sand results)

• Effect of abrasive size

800 1000 1200 1400 1600 1800 20000.01

0.1

1

10

100

1000

0.01

0.1

Vo

lum

e L

oss

, mm

3

Hardness, HV30

20N Sand 20N Alumina 200N Sand 200N Alumina

800 1000 1200 1400 1600 1800 20004

6

8

10

12

14

16

18

20

22

24

26

Rat

io o

f Wea

r, 2

00 N

to 2

0 N

Tes

ts

Hardness, HV30

Sand Alumina

Conformal or Non-conformal Geometry

• Try to get flat on flat– Alignment an issue

– Need careful design to obtain self-aligning samples

• Non-conformal (typically ball on flat) – contact conditions and hence mechanisms change with time

0 2000 4000 6000 8000 10000

0

20

40

60

80

100

Dis

pla

cem

en

t, m

Time, s

10 N (0.1ms-1)

2 N (0.1ms-1)

5 N (0.1ms-1)

0 2000 4000 6000 8000 100000.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Mean Friction Coefficient at 0.1ms-1

Fri

ctio

n C

oe

ffic

ien

t

Time, s

10 N 5 N 2 N

Mild Steel on Bearing Steel

Evaluation of Wear

Brass Disc Bearing Steel Disc

Effect of Humidity – Reaction with Environment

Alumina with water vapour

SEI

BEI

Effect of Test System Dynamics

• Changed loading system in tests on alumina• 50 N load, 0.1 ms-1, pin on disc

Effect of Test System Dynamics

Micro Load Scratch Tester

Features

•Designed to be cost effective•Uses off the shelf high technology components where possible

–Flexure bearings for no friction artefacts–2 mm recirculating ball screw linear slide–6 mm servo motor+gearbaox

•Designed for in-situ SEM operation•Single and multiple repeated scratch•Operation down to ~0.3 mN load •Friction measurement Future Vision

•Tool for characterisation of performance of nanostructured and low friction materials•Foremost EU project•Mechanisms of wear in WC/Co hardmetals

Context

•Measurement of micro-scale contacts in MEMS applications•Measurement of single asperity contact in conventional macroscopic tribological contacts

Coarse Grained Hardmetal, 300 mN 25 m Radius Indenter

Single Scratch

Triple Scratch

Same Area

Knowledge Transfer

• IAG

• Organise meetings

• Paper output, particularly Good Practice Guides

• Direct interaction with industry in projects

Good Practice Guides

• Unlubricated sliding wear

• Monolithic abrasive wear

• Ball cratering wear

• Gas erosion

• Coatings

• Wear-Corrosion

Standards Participation

• CEN 184 WG5 ceramic coatings– Micro-scale abrasion, now published– Reciprocating wear– Pin-on-disc wear– Scratch testing

• ISO TC 206 ceramics and ceramic coatings– Micro-scale abrasion of ceramic coatings– Pin-on-disc of ceramics

• ASTM G99– Many standards

• ISO TC 199/ SC4– Abrasion of hardmetals

• Wear standards database, 1995, 400 standards

CEN 352 Nanotechnology

• Techniques for evaluation of tribological performance of sliding contacts

• Not SPM (AFM, FFM)

• Loads between 100 N and 100 mN

• Contact size between few nm and 10 m

• Participants from Spain, Portugal, Belgium (2), UK (2), Netherlands

• Also on ISO TC 229 work programme – participants from China, Denmark, France, Spain, USA