Self Healing Ceramic (Surfaces)

Transcription

1 Self Healing Ceramic (Surfaces) KIVI NIRIA Jaarcongres 23 november 2011 Matthijn de Rooij University of Twente Laboratory for Surface Technology and Tribology

2 Introduction Technical Ceramics Crystalline material Oxides, nitrides or carbides Ceramic types Oxide ceramics: Al 2 O 3 (alumina), ZrO 2 (zirconia), PSZ Non-oxide ceramics:,carbides, Borides, Nitrides, Silicides (SiC, Si 3 N 4, BC) Composites: Particulate reinforced, combinations of oxides and nonoxides (Glassy ceramics (Zerodur / Macor) Silicium carbide Zirconia Alumina

3 Introduction Application areas Application fields: High Temperature applications Wear / chemical resistant applications Abrasive applications Vacuüm applications Examples: Thermal Barrier Coatings (TBC s) Ceramic / hybrid bearings Cutting & forming tools Mechatronics (precision parts) Clamping Nozzles Pumps / valves Abrasives Mechanical face seals

4 Self healing ceramic surfaces Ceramic contacts interesting in: Vacuum applications (Positioning mechanisms) High temperature applications Lubricants not possible / not allowed Coefficient of friction in ceramic - ceramic contacts: Unstable: Small change (e.g. humidity) can have large effect Typically very high (~0.8) Stick-Slip Reducing friction by self healing / self regenerating soft layer on the ceramic surface environment F N surface 1 v 1 v 2 surface 2 p,t, type (lubricant)

5 Friction reduction in the case of ceramics Generation of soft layers Generation of soft layers on ceramics surfaces, in the ideal case Should work in vacuum and / or HT applications Supplied in the correct amount v F W F N Lifetime Load carrying capacity Low friction depends on thin soft layer, which, if present, tends to wear out (liquid or solid) Stability regime

6 Friction reduction Generation of thin soft layers onhard substrate Needed: Thin enough soft layer on hard substrate About same real contact area (load effectively carried by bulk) Then also reduction mechanical stresses on ceramic (contact pressure / frictional stress) There is an optimal layer thickness for lowest f (you can calculate it) f = f F F = W N Plastic contact: = τa HA r = Elastic contact: A τ F N 1 3 τ H 3 4E' 2 3 F N v F W fp s 1

7 Requirements for self restoring / self healing soft layer ω OK v Not OK A supply mechanism Gradual Reacts to (potential) damage, e.g. high friction A soft phase Generated in reaction with environment (oxidation, water) Generated in reaction counterbody Transport from subsurface Already incorporated soft 2nd phase or transformation metastable phase Needed: mobility e.g. stress gradient (mechanical squeezing out) Requirements soft phase Low shear strength, should stay in the contact, processing: Oxide?

8 Some possibilities for thin soft layers Hard Coatings: With friction reducing top layer / composite coatings (High H containing DLC in dry vacuum / helium: f=0.005, H + on the surface!) Nanocomposite coatings Polymers UHMWPE PTFE Some oxides (Cr 2 O 3 microfilms on Cr, CuO) Solid lubricants graphite, MoS 2, soft metals (Ag, In, Sn etc.) BN, Boric Acid (B(OH) 3 ). Colloidal suspensions of nanoparticles of boric acid dissolved in petroleum or vegetable oil, Self-lubricating H 3 BO 3 films HT: e.g CaF 2 EP additives

9 Friction by thin soft layers: nothing new. Boundary Lubrication Mixed LubricationHydrodynamic Lubrication Coefficient of Friction, μ [-] BL Liquids and greases ML Stribeck curve: (E)HL ηu Lubrication Number, L= P nom R a environment Stribeck curve nowadays be calculated for many situations F N surface 1 v 1 v 2 surface 2 p,t, type (lubricant)

10 Full film lubrication as ideal self restoring soft layer v F N Full film lubrication: autonomous self healing liquid coating Hydrodynamic action: 1. Sensing of coming contact (pressure increase in inlet zone by converging gap) 2. Acting by formation fluid film, separating the surfaces - Helping factor: viscosity increases with increasing pressure 3. Replenishing of surface when the ball has passed Time dependent process Wetting of metal by oil as driving force of replenishment

11 Vacuum Pressure Temperature Electrical conductivity Wear resistance Thermal conductivity Compatibility Resistance against aggressive Solid lubricants Some solids lubricate well and have low vapor pressure Good Relatively insensitive Some are electrically conductive Good resistance is fretting, wear Lifetime may be limited due to material loss from the contact Metals: Good Most inorganic and layered solids : poor Good for hard to lubricate materials (Al, Ti, stainless steel, ceramics Relatively insensitive for chemical solvents, acids, bases Liquids and greases Most liquids evaporate (not PFPE and PAO) Good in EHL May need additives in BL May solidify at low T, decompose at high T, viscosity reduction at high T Mostly isolating Anti wear additives for BL (low speeds) No contract and negligible wear under HL / EHL Good No well suitable for non-ferrous and ceramics May be affected

12 Friction reducing oxides Friction reduction by (a combination of) oxides is currently state of the art under high temperature conditions. Examples of useful oxide combinations: CuO-Re 2 O 7, CuO-MoO 3 and NiO-MoO 3 (Extended T - range). Impregnation of pores / cells with solid lubricants. Wear of the matrix material is often allowed in order to activate new solid lubricant deeper below the surface. Processing advantage: No sintering under reducing conditions required.

13 Friction reducing oxides Shear strength dependent on ionic potential (φ) of an oxide φ = Z/r (Z: cationic charge and r is the radius of the cation) Oxides with lower ionic potentials (such as Al 2 O 3, Fe 2 O 3, and MgO) are very strong and difficult to shear A. Erdemir, Surface and Coatings Technology, Volume 200, Issues 5-6, 21 November 2005, Pages

14 IOP Self Healing Materials project Mechanical and thermal activated self healing surfaces made of composite ceramics for mechanical components SELFSURF University of Twente TNO DAF Trucks IMPCO BERU Technologies

15 Materials & Samples CuO Doped oxide ceramics Powder 3Y-TZP, 30~40 nm (Tosoh, Japan) CuO, <74 µm (Alfa Aesar, Germany) Undoped TZP Processing Mixing, uniaxial pressing, cold isotropic pressing, sintering Grinding, polishing TZP doped 5 wt.% CuO

16 CuO Doped oxide ceramics Doped oxide ceramics Al 2 O 3 and ZrO 2 and their composites CuO (few wt %) Thin CuO layer is present on the surface after sintering and machining When sliding against Al 2 O 3 f 0.8 for the undoped ZrO 2 f for doped ZrO 2 Surface layer is self restoring! (up to 10 km sliding distance ball on disk)

17 Experimental results: Room temperature 3Y-TZP doped with CuO sliding against Al 2 O 3 ball Mechanism: Aluminum hydroxide surface layer reduces friction. f for doped sliding against alumina (RT) Self healing in sliding contacts

18 Experimental results: High temperature CuO doped TZP TZP f [-] v F N T [oc] Collection of CuO by mechanical action

19 Properties of Cu 2 O Relatively low melting point (~1200 o C) Plastic behaviour at elevated T MoO 3. interesting? melting point ~795 C

20 Design of self healing ceramic surfaces F N Tribological system environment surface 1 v 1 v 2 p,t, type (lubricant) main effects (isothermal for simplicity) surface 2 wear Thickness Interfacial layer generation friction

21 Modelling soft layer formation mechanisms Part of Self Healing project University of Twente Plastically deforming inclusion Time dependent stress field required for squeezing out Elastic ball on flat Wear Volume 268, Issue 9-10, 25 March 2010, Pages

22 Modelling squeezing out F N F N F N V V V F T F T FT Squeezing Smearing Time dependent stress field: Volume difference of compressed inclusion comes out

23 Some results Effect of load Effect of coefficient of friction

24 Valve valve seat contact?

25 Force- time relation

26 Model valve valve seat dynamics

27 Model valve valve seat contact Valve seat Seat insert Valve lift : h 0 Start of impact Valve velocity: V 1 Initial separation: d 0 Closing situation?

28 First few moments of impact Effect of friction

29 Roughness effects Contact pressure (MPa) R q = 0.2 µm σ=σ0,β=β0 σ=2σ0,β=β0/2 σ=3σ0,β=β0/3 R q = 0.4 µm R q = 0.6 µm F N Time(s),1e -5 This can be used for formation of the soft layer.

30 Summary Self healing ceramic (surfaces) 1. Ceramics: friction reduction by thin soft self healing / self restoring layers 2. Experimental results on CuO doped zirconia 3. Model development on formation of thin soft layer on surface