Preparation and characterizations of new coatings based on PTFE-anodic films for the improvement of the tribological behavior of aluminum substrate

Transcription

1 Preparation and characterizations of new coatings based on PTFE-anodic films for the improvement of the tribological behavior of aluminum substrate L. ARURAULT, J. ESCOBAR, V. TURQ, P-L. TABERNA IHAA th Technical Symposium New York, September 24-26, 2014

2 Background Anodizing : Preparation of an oxide film on a metal surface Thermal and electrical insulation Enhancement of corrosion resistance Functionalization 1960: Tefal (contraction of Teflon and aluminum) prepares non stick pan with PolyTetraFluoroEthylene (PTFE) or Teflon coatings [2,3] H. Wang et al. [4,5] performed heat treatment to incorporate melted PTFE into the pores Actual PTFE coatings: Non stick properties Limit the wear of parts Only on the top surface Objectives: Functionalize an anodic film with PTFE particles into the pores Enhance the lifetime/ limit wear of mechanical parts Limit energy dissipation Ideal microstructure [1] [1] F. KELLER et al. - Structural Features of Oxide Coatings on Aluminum, J. Electrochem. Soc., 100, 1953, 411. [2] M. GREGOIRE & HARTMANN (TEFAL) - Perfectionnement aux traitements de surfaces des métaux - FR (1959) [3] TEFAL - Produit en polytétrafluoroéthylène et son procédé de fabrication - FR (1959) [4] H. WANG et al. - Applied Surface Science, Vol.252 (2005), 1662 [5] H. WANG & H. WANG - Applied Surface Science, Vol.253 (2007),

3 Approach Substrates Al 1050 and Al 7175 T7351 Pretreatments Clean the surface Anodizing An anodic film with a thickness of about 10 µm and a pore diameter of about 200 nm Functionalization Incorporate PTFE particles into the porous structure Improved sedimentation Electrophoretic Deposition Characterizations Microstructure Tribological tests (friction and wear) 10µm Substrate Substrate 3

4 Anodizing 1050 Aluminum alloy (99.5% Al) Pretreatments (ESA notification ECSS Q70-03A) Degreasing Etching Neutralization Ethanol R a 0.3 µm PV 3 µm Na 2 CO 3 (6.2 g.l -1 ) Na 3 PO 4 (12.5 g.l -1 ) 5 min. at 93 C R a 0.6 µm PV 8 µm HNO 3 (50%v/v) 3 min. at ambient T C R a 0.6 µm PV 10 µm Anodizing Preparation of a porous anodic film Anode (Al) + - Cathode (Pb) on 1050 aluminum alloy Two antagonistic reactions : Electrochemical growth of the oxide film 2Al + 3H 2 O Al 2 O 3 + 6H + + 6e - Electrolyte (H 3 PO 4 ) Magnetic stirring bar Water circulation Chemical dissolving of the film by acidic electrolyte Magnetic stirrer Al 2 O 3 + 6H + 2Al H 2 O 4

5 Anodizing Parametric study H 3 PO 4 (0.4M), Current density 1.4 A/dm², bath temperature 20 C, Time 34 min a b 200 nm 1 µm Thickness of 10.0 ± 0.5 µm and pore diameter of about 200 ± 15 nm 5

6 PolyTetraFluoroEthylene (PTFE) aqueous dispersion Functionalization Improved sedimentation technique Size Distribution by Intensity Surfactant : alkyl polyglycol ether Zeta potential: - 45 mv Intensity (%) Intensity (%) Size (d.nm) Particle size (nm) Record 1: ptfe50_70 1 Incorporation technique Improved sedimentation technique Total evaporation Bain-marie at 60 C Hot plate PTFE dispersion Anodized aluminum Hot plate PTFE particles 6

7 Functionalization Improved sedimentation technique 1 µm 1 µm 200 nm Incorporation of PTFE particles into the pores and on the surface J.ESCOBAR et al., Improvement of the tribological behaviour of PTFE-anodic film composites prepared on 1050 aluminum substrate, Appl. Surf. Sci. 258 (2012),

8 Tribological characterizations Anodized Al and PTFE plate Tribology Sliding direction F N F T Friction coefficient µ = F T F N Anodized Al and PTFE plate Rotative mode, radius 5 mm; speed = 5.04 cm/s; alumina counterface, F N =1N Number of cycles: 5000 Optical microscopy after 5000 cycles on the anodic film Ball Track Metallic scraps on the ball Aluminum is exposed Optical microscopy after 5000 cycles on the PTFE plate Ball Track Aluminum: Friction coefficient : Anodized Al: Friction coefficient : 1.1 Lifetime: 3000 cycles PTFE plate: Friction coefficient : 0.1 PTFE scraps on the counterface Not visible track on PTFE plate 8

9 Tribology Track radius: 5 mm; Speed= 5.04 cm/s; Alumina counterface, F N =1N Tribological characterizations Improved sedimentation technique Period 2: Friction coefficient between 0.17 and 0.90 Composite PTFE/ anodic film Improvement of the anodic film lifetime by 3 PTFE surface Ball Ball PTFE inside Period 1: friction coefficient 0.17 PTFE on top surface, length depends on the PTFE coating thickness Track Track Period 3: friction coefficient = Aluminum + alumina fragments Improvement of the total lifetime by at least 20 Localized EDX on the center of the track I F I Al = = I F I Al 9

10 Functionalization Electrophoretic Deposition Electrophoretic Deposition (EPD) A voltage (or a current) is applied between two immersed electrode in a dispersion Anode + - Cathode Advantages: Higher deposition rate Homogeneity of the deposit Suitable for complex shaped surface PTFE aqueous dispersion Water At the cathode : 2H 2 O (l) + 2e - At the anode : 2H 2 O (l) H 2 (g) + 2 OH - (aq) Hydrogen evolution O 2 (g) + 4 H + + 4e - Oxygen evolution 10

11 Functionalization Electrophoretic Deposition Cathodic EPD Use of cationic surfactant to change the zeta potential Cetyl TriméthylAmmonium Bromide (CTAB) Particles size Zeta Potential Solution without CTAB 90 ± 40 nm - 45 mv Solution with CTAB 90 ± 40 nm + 55 mv 11

12 Functionalization Electrophoretic Deposition Electrophoretic Deposition Cathodic EPD: PTFE + CTAB dispersion U = -0.5 and -2 V t = 5h -0.5V - 2V No particles neither inside the pore nor on the surface Aggregated particles on the top surface Low efficiency for low voltage Gas bubbles production avoids the incorporation of the particles and can leads to an aggregation of the particle [1] [1] L. BESRA, et al., Experimental verification of ph localization mechanism of particle consolidation at the electrode/solution interface and its application to pulsed DC electrophoretic deposition (EPD), J. Eur. Ceram. Soc. 30 (2010)

13 Functionalization Electrophoretic Deposition Besra et al. found a way to make bubble free deposit by EPD with aqueous dispersion [1] Use of pulsed current on a plan surface Time T off Current T on 4 main parameters: - Voltage - Pulse frequency - Pulse duty cycle (T on / (T on +T off )) - EPD duration Objectives: - Incorporate the PTFE particles by using pulsed voltage - Improve homogeneity, efficiency and elaboration time [1] L. BESRA, Application of constant current pulse to suppress bubble incorporation and control deposit morphology during aqueous electrophoretic deposition (EPD), J. Eur. Ceram. Soc. 29 (2009)

14 Functionalization Electrophoretic Deposition Pulsed voltage EPD Voltage (V) Frequency (Hz) Duty cycle (%) Duration (min) min - Incorporation of PTFE into the pores 14

15 Conclusion/Prospects Incorporation of PTFE particles inside the porous structure by using improved sedimentation technique Reduction of the friction coefficient from 1.1 to fold improvement of the lifetime of the anodic film Drawbacks: Non-homogenous deposit, elaboration time, suitable for plan surface only Electrophoretic deposition Constant voltage leads to bubble production and avoids PTFE particles incorporation Incorporation of particles was obtained by using pulsed voltage Tribological tests on EPD tests (in progress) Tests on Al 7175 T3 (in progress) 15

16 Thank you! Pr. Laurent ARURAULT CIRIMAT, University of Toulouse, France J.ESCOBAR, L.ARURAULT, V.TURQ, Improvement of the tribological behavior of PTFE-anodic film composites prepared on 1050 aluminum substrate, Appl. Surf. Sci. 258 (2012), J.ESCOBAR, L.ARURAULT, V.TURQ, P-L.TABERNA, Study of electrophoretic deposition of PTFE particles on porous anodized aluminum, Submitted 16