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1 A Round Robin to evaluate the degradation of materials in sliding contacts operated in aqueous solutions Jean-Pierre Celis - Pierre Ponthiaux Katholieke Universiteit Leuven - Ecole Centrale Paris (Jean-Pierre.Celis@ mtm.kuleuven.be - Pierre.Ponthiaux@ecp.fr) 1 Content of the lecture What is a Round Robin Action? Test protocol Case study: tribocorrosion of a passivating CoCrMo biomaterial Electrochemical aspects Role of surface film on friction and wear Synergism in tribocorrosion 2 1

2 What is a Round Robin Action? In experimental methodology, a round robin test is a test (analysis, measurement, or experiment) performed independently several times. The purpose is the verification of a new method of analysis! When a new method of analysis is developed, a round robin test involving proven methods would verify whether the new method produces results that agree with established ones. modern term dates from the 17th Century French ruban rond (round ribbon) = the practice of signatories to petitions appending their names on a document in a non-hierarchical circle or ribbon pattern. 3 What is a Round Robin Action? This involves multiple independent scientists performing the same test method in different equipment, or a variety of methods and equipment. A true round robin only occurs in the specific case that all participants evaluate or test the exact same test object. - a test methodology described as accurate as possible, - reporting by each participant according a given template, - an in-depth analysis by the co-ordinator, - an improvement of the methodology eventually elaborated for the new round. 4 2

3 Starting point: need for appropriate testing tools Preliminary approach: identifying existing testing methodologies (literature search, standards like ASTM, AFNOR, DIN, BS, ) identifying main process mechanisms (physical, mechanical, chemical) identifying main process parameters (temperature, pressure, environment, materials, ) Development of a test protocol (guidelines) 5 Tribocorrosion process Corrosion Passive film substrate Corrosion Corrosion Corrosion accelerated by wear Repassivation Corrosion products Wear accelerated by corrosion 6 3

4 Experimental fact: a synergism ('W) between corrosion and wear processes! W tribocorrosion = W corrosion + W mechanical + 'W Ways to determine 'W : ASTM G Determining Synergism between wear & Corrosion drawback: not applicable on passivating materials New protocol required valid for passivating materials: A methodology for the assessment of the tribocorrosion of passivating metallic materials N. Diomidis, J.-P. Celis, P. Ponthiaux, F. Wenger, Lubrication Science 2009; 21: Objective of the new test protocol: To allow a fast evaluation of the potential and risks of new materials in sliding contacts operated under wet conditions (tribocorrosion) simulating biological systems Approach used in the test protocol: Combination of electrochemical measurements and sliding tests to decouple material losses originating from corrosion (W corrosion ) and wear (W wear ) 8 4

5 Protocol for passivating metallic materials is based on: - behavior of passivating materials at open circuit potential without sliding (phase I) Î passivation kinetics - corrosion and wear of materials at open circuit potential under continuous sliding (phase II) Î mechanical and corrosive components of total wear - repassivation rate of mechanically activated material under intermittent sliding (phase III) Î influence of repassivation on wear 9 Behavior of CoCrMo at open circuit potential without sliding (phase I) Expt1 Expt2 Expt Expt 1 Expt 2 Expt 3 Potential (V vs. SHE) Potential (V vs. SHE) Time (s) PBS ph PBS + albumin ph Time (s) - good repeatability of Eoc in presence of albumin - albumin causes a faster stabilization of open circuit potential 10 5

6 Behavior of CoCrMo at open circuit potential without sliding (phase I) PBS ph 7.4 Reproducibility? 11 Behavior of CoCrMo at open circuit potential without sliding (phase I) E (V vs SHE) PBS ph 7.4 INSTITUTION Reproducibility? 12 6

7 Corrosion-wear of at OCP under continuous sliding (phase II) open-circuit potential (V vs. SSE) loading with ball AISI 304L vs. corundum in Ringer s solution ph 6.6 sliding on Competition between depassivation and repassivation events during sliding time (s) sliding off before after sliding sliding E oc = -0.6 V vs. SSE without sliding E = 170 mv E oc = V vs. SSE during sliding Sliding parameters: 5 N, 1 Hz 13 Corrosion-wear of CoCrMo at OCP under continuous sliding (phase II) Potential (V vs. SHE) Expt1 Expt2 Expt Time (s) Potential (V vs. SHE) Expt 1 Expt 2 Expt 3 PBS ph 7.4 PBS + albumin ph Time (s) - Repeatability is even better in presence of albumin 14 7

8 Al 2 O 3 E corr 1 contact Al 2 O 3 E corr TiN passivation oxide cracking E corr E corr Al 2 O 3 Al 2 O 3 t ox repassivation wear particle TiN mechanical activation 15 Physical approach: galvanic coupling between worn and unworn areas!! Potential E E c E mix E * a cathode (a) log i anode cathode Potential E E * c E mix E a anode (b) log i cathode anode unloaded loaded unloaded unloaded unloaded loaded * * 16 8

9 Physical approach: galvanic coupling between worn and unworn areas!! AISI316 in 0.5 M H 2 SO 4 under continuous unidirectional sliding (120 rpm = 0.5 s) cathodic and anodic currents wear track 40 µa/cm 2 Counterbody: ZrO 2 Normal load: 5 N micro-electrode tip 17 Corrosion-wear of CoCrMo at OCP under continuous sliding (phase II) Coefficient of friction Time (s) Expt1 Expt2 Expt3 Coefficient of friction PBS ph 7.4 PBS + albumin ph Time (s) Expt 1 Expt 2 Expt 3 Wear volume in PBS + albumin = 1.70 x 10-7 cm 3 +/ x 10-7 Wear volume in PBS = 1.02 x 10-7 cm 3 +/ x

10 Repassivation rate under intermittent sliding (phase III) Wear depth (µm) Wear scars on 316L stainless steel after fretting tests done at different holding times in ambient air of 50% RH holding time = toff Scan by profilometer (µm) 19 Repassivation rate under intermittent sliding (phase III) Wear scars on 316L stainless steel after fretting tests done at different holding times in ambient air of 50% RH holding time 2 s holding time 8 s 20 10

11 Repassivation rate under intermittent sliding (phase III) OCP during continuous (T r = 0.5 s, t off = 0 s) and intermittent (T r = 0.5 s, t off = 4.5 or 49.5 s) unidirectional sliding of AISI316 against zirconia at a normal force of 5 N 0.5 M H 2 SO 4 ph Repassivation rate of mechanically activated Al-based alloys under intermittent sliding (phase III) Open Circuit Potential Eoc (V/SSE) phosphate ph 7 Emax Emax Emin Emin Toff Al 71 Cu 10 Fe 9 Cr 10 Al 71 Cu 10 Fe 9 Cr 10 increase of T off improves passivation Al 3 Mg 2 Al 3 Mg 2 increase of T off increases corrosion Continous sliding Intermitent Sliding 22 11

12 Outcome of test protocol (phases I + II + III) W tribocorrosion = W corrosion + W mechanical + 'W 'W W mechanical W corrosion Latency time 23 Tribocorrosion of passivating metallic materials results in a change in friction and wear due to a modification of surfaces: - friction: affected by removal/re-growth of oxide in sliding contact - wear: affected by galvanic coupling between worn /unworn areas Synergism in tribocorrosion can be analyzed by : - in-situ electrochemical measurements, and - appropriate test protocol for passivating materials! Functionalizing the surface of passivating materials is one attractive way to by-pass degradation by tribocorrosion! 24 12

13 Test protocol on biotribocorrosion Think tank How would you explain the lowering of material degradation in these different wet media? 25 Main conclusions and challenges Round Robin tests are useful to evaluate repeatability and reproducibility Major attention to be given to possible variations in test conditions at different labs. Therefore: literature data must be analyzed and used critically! Tribocorrosion = emerging field of scientific interest Industrial recognition is growing (implants in health sector, food industry, electrical connectors in transportation, MEMs technology, environmental issues.) 26 13

14 Main literature for further reading J.-P. CELIS, P. PONTHIAUX, F. WENGER Tribo-corrosion of materials: Interplay between chemical, electrochemical, and mechanical reactivity of surfaces, Wear, 261 (9), (2006) S. ACHANTA, D. DREES, J.-P. CELIS Friction from nano to macroforce scales analyzed by single and multipleasperity contact approaches Surface and Coatings Technology, 202, (2008) N. DIOMIDIS, J.-P. CELIS, P. PONTHIAUX, F. WENGER A methodology for the assessment of the tribocorrosion of passivating metallic materials Lubrication Science, vol 21, issue 2, Febr. 2009, pp