Kenneth Holmberg VTT Technical Research Centre of Finland

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: η + chemical composition • Contaminants: material, size, H • Environment: • Humidity, temperature

6. IRG OECD RECOMMENDATION OF MINIMUM SPECIFICATIONS FOR EXPERIMENTAL CONDITIONS IN TRIBOLOGY ARTICLES 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/N·m) 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. 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.

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 κsimilar toμ?

11. REPORTING WEAR IN THE LITERATURE

12. v Fμ Fn s1 t0 t1 Friction = a force resisting motion Wear = a process resulting in a volume of detached material Fμ = μ· Fn => μ = Fμ / Fn W = κ· Fn · s => κ = W / ( Fn· s ) • μ at a certain • moment t1 • κ at a certain time • interval t0 - t1 ~ s1 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/discussion.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^-6mm^3/N-m • The wear index (WI) should be used: WI = Wear volume • 10^6 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 = 10^6 N·m/mm^3 = 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/m^2 - thus will B = 1/Pa • Fatigue, erosive and impact wear cannot use Bowden • There are >30 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 = 10^6 N·m/mm^3

20. THE END