Kenneth Holmberg

1
REPORTING EXPERIMENTAL RESULTS IN TRIBOLOGY -
SUMMARY OF THE DISCUSSION ON NEW WEAR RATE UNIT

• Kenneth Holmberg
• VTT Technical Research Centre of Finland
• OECD IRG 26th Meeting, Lyon, France, 5-6.10.2006

2
BACKGROUND

• Important role of tribology in our society
  safety, reliability, energy, new products
• Tribology develops and expands number of
  meetings, conferences, articles submitted,
  published
• New and more generic and precise theories needs
  to be confirmed empirically
• How good is our reported experimental data?

3
BACKGROUND

• Experience from writing a tribology articles and
  books
• Many articles are today published that are never
  or seldom referred to - what is their
  contribution to our general knowledge?
• In tribology we are presenting scatters of
  experimental results that would not be acceptable
  in other fields of natural science!
• Two basic problems
• Specifying experimental conditions
• Reporting wear

4
1. SPECIFYING EXPERIMENTAL CONDITIONS

• 25th IRG OECD meeting in Uppsala, June 2005,
  decided to recommend a short list of parameters
  with which a tribological contact can be described

5
BASIC TRIBOEMPIRIC PARAMETERS

• Energy input speed, load
• Materials for body 1 and 2
• Physical / chemical / metallurgical specification
• H, E, Ra
• Coating h, H, E phys/chem/met specification
• Third body
• Lubricant ? chemical composition
• Contaminants material, size, H
• Environment
• Humidity, temperature

6
IRG OECD RECOMMENDATION OF MINIMUM SPECIFICATIONS
FOR EXPERIMENTAL CONDITIONS IN TRIBOLOGY ARTICLES

• Description of test device
• - a brief description of the test system or in
  case of commercially available device, its code.
• Geometry
• For both surfaces in contact
• - macrogeometry given typically as radius or
  diameter of a curved surface, or contact spot
  dimensions for flat contacts, so that stresses
  can be determined from loads,
• - microgeometry surface roughness, given as
  appropriate for the application, but also as
  Ra-value for comparison.
• Energy input
• - load (or stress),
• - speed.
• Materials
• For both surfaces in contact
• - material specification metallurgical, physical
  or chemical including processing method when
  appropriate,
• - any coating material, coating thickness and
  deposition or surface treatment method,
• - hardness and Young's modulus,
• - fracture toughness, when possible.

For abrasion and erosion - abrasive or erodent
material, size, size distribution and shape, -
speed and direction of fluid stream for
erosion. Environment - lubricant viscosity and
chemical composition/specification of fluid and
additives, - humidity, - temperature, - possible
gases, radiation etc. Results - friction as
coefficient of friction typically the steady
state value and the maximum value, - wear for
both surfaces as -- wear rate (e.g. 10-6
mm3/Nm) and/or -- wear volume per time or per
sliding distance and/or -- wear depth per time
or per sliding distance, - for fatigue wear the
endurance life time, - graphs of friction and
wear versus time when appropriate, - surface
observations of both surfaces in contact,
including observations of surface layers and
their extent, - observations of wear debris
and when possible their average size and size
distribution.
7
1. SPECIFYING EXPERIMENTAL CONDITIONS

• 25th IRG OECD meeting in Uppsala, June 2005,
  decided to recommend a short list of parameters
  with which a tribological contact can be
  described
• The list has been distributed to 15 tribology
  journals
• Response positive - already in use in Tribotest
  and Journal of Engineering Tribology, IMechE

8
TRIBOLOGY JOURNALS APPROACHED

• Wear
• Tribology International
• Tribology Transactions - STLE
• Tribology Lubrication Technology - STLE
• Journal of Engineering Tribology, Part J -
  IMechE  
• Industrial Lubrication and Tribology 
• Tribology Letters
• Tribotest
• Tribologia - Finnish Journal of Tribology
• Tribologia - Teoria i Praktyka
• Tribologie und Schmierungtechnik
• Surface and Coatings Technology
• Thin Solid Films
• Surface Engineering
• Trenie I Iznos

9
2. REPORTING WEAR

• Comparison of tribological performance often done
  based on Friction even if Wear is more important!
  
• Because µ is a good uniform parameter.
• Wear is reported in many forms
• wear volume
• wear depth
• wear rate/factor/coefficient/constant.
• wear volume/time or distance or speed..
• etc

10
WEAR PARAMETERS

• We need a more uniform way to express wear.
• What is that?
• Czichos suggested Wear Rate (
  ) where k W / (s F)
• k wear volume / input of energy into the
  contact. If hardness is included there is a loss
  of generality.
• Sometimes we need W wear volume, e.g. erosion
• Sometimes we need h wear depth, e.g. tolerances
• Sometimes we need t endurance life, e.g.
  fatigue
• Do we need more wear parameters?
• Should we use k, or COW similar to COF, or the
  Greek ? similar to µ?

11
REPORTING WEAR IN THE LITERATURE
12
COEFFICIENT OF FRICTION AND WEAR RATE
FIGURES/TRIBOLOGY/IRG Wear Rate
Friction-01-290605
13
IRG OECD UPPSALA MEETING, 2005

• Recommendation from IRG OECD meeting with about
  30 participants to introduce a new unit for wear
  rate Bowden (B) equal to

14
EXAMPLES OF WEAR RATE UNITS
15
IRG OECD UPPSALA MEETING, 2005

• Recommendation from IRG OECD meeting with about
  30 participants to introduce a new unit for wear
  rate Bowden (B) equal to
• Web http//virtual.vtt.fi/virtual/proj3/irg/discu
  ssion.htm
• Resulted in intensive discussion on the web with
  27 contributions of which
• 13 are positive
• 2 are neutral
• 4 are negative
• 12 come with new suggestions
• The suggestions were discussed in the "Cutting
  edge" column in Tribology Lubrication
  Technology by Tysoe and Spencer

16
NEW SUGGESTIONS

• Other unit name suggestions Archard (Ar), WEARON
  (WR or WE), Tabor (Ta), Czichos (Cz)
• There is a "Tabor parameter" in use to interpret
  the ratio of elastic deformation of two
  contacting bodies
• The wear unit should be expressed as
  10-6mm3/N-m
• The wear index (WI) should be used WI Wear
  volume 106 GPa / (Normal load Sliding
  distance)
• Use WIR for rotational wear, WIL for
  straight-line wear, WIF for fretting wear etc
• Use wear resistance Rabinowicz 1 R 1/B
  106 Nm/mm3 10-3 Pa

17
SCEPTIC COMMENTS

• Bowden not right name - he was more working on
  friction
• Introducing a new unit is confusing - no more
  derived units are needed
• The new unit will most likely not be adopte
  widely
• The unit would add false validity to the Archard
  equation - wear rate is totally system dependent
• Pascal (Pa) is N/m2 - thus will B 1/Pa
• Fatigue, erosive and impact wear cannot use
  Bowden
• There are gt30 important variables and 70 minor
  ones influencing on wear
• Misleading that wear is more exact subject than
  it is - wear is a statistically varying entity

18
CONTRIBUTORS TO THE DISCUSSION

• Philippe Kapsa, ECL, France
• Bo Jacobson, Lund Univ., Sweden
• Kenneth Holmberg, VTT, Finland
• Sture Hogmark, Uppsala Univ., Sweden
• Ian Hutchings, Cambridge Univ., UK
• Steve Shaffer, Battelle Memorial Inst., USA
• Staffan Jacobson, Uppsala Univ., Sweden
• Koji Kato, Sendai Univ., Japan
• Brian Briscoe, Imperial Col., UK
• Nic Spenser, Fed. Inst. Techn. Zurich,
  Switzerland
• Eddy Tysoe, Univ. Wisconsin-Milwaukee, USA
• Ken Ludema, Univ. Michigan, USA
• Michel Maillat, Lab. Dubois, Switzerland
• Steven Franklin, Philips, The Netherlands
• Yoshi Kimura, Japan
• James Kauzlarich, Univ. Virginia, USA
• Jordan Liu, Advanced Materials Technology, USA
• Per Kjeldsteen, Danfoss, Denmark

19
THREE SUGGESTIONS FOREXPRESSING WEAR

• The IRG OECD Uppsala suggestion Wear Rate
  expressed with a new unit Bowden (B) equal to
• The earlier " Czichos " suggestion Wear Rate
• Wear resistance Rabinowicz 1 R 106
  Nm/mm3

20
THE END