Zelf-herstellende keramische oppervlakken Bijeenkomst: Falen of presteren van deklagen het slijtageproces 27 april 2011

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1 Zelf-herstellende keramische oppervlakken Bijeenkomst: Falen of presteren van deklagen het slijtageproces 27 april 2011 Matthijn de Rooij University of Twente Laboratory for Surface Technology and Tribology

2 Contents Introduction Crack healing Surface healing Design of self healing materials Conclusions

3 Introduction Self Healing Materials: idea Make materials stronger ( damage prevention) Make materials more damage tolerant (damage tolerance) Make materials self healing (damage restoration) Needed for self healing Detection of damage Transport of healing agent Healing of damage (autonoumous or non-autonomous) Recovery of strength (100 % or less) Many times! IOP Self Healing materials

4 Introduction Technical Ceramics Crystalline material Oxides, nitrides or carbides Ceramic types Silicium carbide Alumina 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) Zirconia At first sight not suitable ate for self healing

5 Typical production method Milling Mixing Forming of the shape nm s m s Grinding Cutting etc. Sintering ( o C) There are other methods (Extrusion, Injection mouding, hot isostatic pressing (HIP)) Also coating techniques Vacuum plasma spraying Electron Beam PVD (EB-PVD) Pulsed Laser Deposition (PLD)

6 General characteristics of ceramic materials Good news Superior properties at high temperatures Chemically inactive Erosion resistant ( in case flow almost parallel to the surfaces) Much stronger in compression than in tension Main issues: Worries about structural integrity (brittle failure = catastrophic failure) Properties are prone to variation from batch to batch

7 General characteristics of ceramic materials Bad news Failure by tensile cracking Sensitive for flaws, inclusions etc. act as stress concentrators Ceramics: strength is controlled by defects. Strength controlling defects can be a few microns or tens of microns in size. Below limit of detection of (most) detection techniques Causes (surface) cracks Pre-existing voids etc. Machining marks Formed during operation Contact events. (Impact, erosion, corrosion and wear) Thermal events» (differences thermal expansion e.g. coatings, thermal shock)

8 Introduction Indicative properties Technical ceramics Al 2 O 3 ZrO 2 (PSZ) SiC Si 3 N 4 St-37 Main component Alumina Zirconia Silicon Carbide Silicon Nitride Construction steel Density [g cm -3 ] E modulus [GPa] Hardness [GPa] Fracture toughness [MPa m 0.5 ] Tmax [ o C] <<1000 Thermal expansion [10-6 K -1 ] Thermal conductivity [W m -1 K -1 ]

9 Introduction Application areas Interesting application fields: HT applications Wear / chemical resistant applications Abrasive applications Vacuüm applications Suppose self healing Longer lifetime? Increase in the reliability Decrease inspection Decrease in the maintenance costs Example applications Thermal Barrier Coatings (TBC s) Ceramic / hybrid bearings Tools Mechatronics (precision parts) Clamping Nozzles Cutting & forming Pumps / valves Abrasives Mechanical face seals

10 Damage tolerance approach: Ceramic composites / Damage tolerant ceramics Directed at improving the toughness Added Materials: Dispersion of hard, non-oxide phases such as SiC, TiB, TiC Dispersion of oxide phases ( Al 2 O 3, TiO 2 ) Solid lubricants (graphite, soft metals) Shapes: Whisker and short-fiber, and continuous-fiber composites Homogeneity: processing is important Damage- Tolerant Ceramics dissipation of strain energy strain energy by sliding of hard bricks with mortar layers in between

11 Ceramic engines: Friction reduction required Metal engines Limited combustion temperature (< 600 o C) 30 % heat loss to surroundings (metals are good conductors) Wear Ceramic Engine 1980 s Toyota, Ford Adiabatic ceramic engine Running temperature 3000 o C Lower weight Higher efficiency However: Production problems, High friction - > Thermal blow-up

12 Valve seats / gas turbines Wear reduction / reliability increase required Valve seats of truck engines Thermal barrier coatings in gas turbines

13 Topic 1 Self healing ceramics Crack healing

14 Self healing ceramics: Self healing Mechanisms Ceramics have no reserve and considered not so reliable Ceramics could benefit a lot from self healing or increase in toughness Two directions for improvement 1. Increasing toughness (composites) ( damage tolerance ) 2. Healing cracks ( damage healing ) Some ceramics show natural self healing Three important questions: behaviour under 1. Does the occurrence of a crack trigger healing? service conditions 2. Can a crack be healed under service conditions (T, time environment)? 3. Does the healed sample keep its strength?

15 Self healing ceramics: Self healing Mechanisms Healing mechanisms Diffusion Oxidation (Re-sintering) Requirements oxidation products Mechanically strong products filling the crack Crack should be completely filled Strong bonding between product and crack wall

16 Self healing ceramics: Materials Materials where (surface) crack healing has been observed (sometimes under mechanical stress) Al 2 O 3 SiC Composites of the above: -Al 2 O 3 /SiC, Si 3 N 4 /SiC, Zr 2 O 3 /SiC, ZrB 2 /SiC, ZrB 2 /SiC Important role for oxides: SiO 2 -Al 2 O 3 (TiO 2 ) Ternary carbide ceramics Penetrate and blunt /heal cracks Zr 2 Al 4 C 5 Volume increase Ti 3 AlC 2

17 Example 1 (Surface) Crack healing of Si 3 N 4 / SiC by oxidation Si 3 N 4 / SiC particles composites Crack become filled with oxides (SiO 2 ) Volume expansion because of crack surface oxidation (~80 %) Crack-healed zone of the Si 3 N 4 /SiC has same fatigue strength as original material Also crack healing under (cyclic) mechanical stress K. Ando et al. / Journal of the European Ceramic Society 22 (2002)

18 Example 1 (Surface) Crack healing Si 3 N 4 /SiC by oxidation Crack healing function of crack dimensions (width, length) Healing at typical service conditions (T 1000 o C, cyclic mechanical stress, environment) Embedded cracks and microstructural flaws cannot be easily healed 2SiC + 3O 2 = 2SiO 2 + 2CO Crack healing by means of oxidation

19 Example 2 Thermal Barrier Coatings (TBC) Partially Yttria Stabilized Zirconia (PSZ) Al 2 O 3 Oxide layer Aluminizing or NiCoCrAlY overlay coating Ni based superalloy Ni based superalloy at high T: High strength Low hot corrosion resistance Self Healing Materials project TU Delft started 2006 Acta Materialia Volume 57, Issue 9, May 2009, Pages

20 Example 2 Thermal Barrier Coatings (TBC) Oxide scale forms at high T Mismatch between the coëfficiënts of thermal expansion Compressive stresses (~5 GPa) at cooling Crack growth -> Delamination at thermal cycling α-al 2 O 3 oxide layer shows self healing behaviour at re-heating TBC not (so far) Important for high temperature oxidation protection: Stable oxide scale. Additional elements (Cr, Pt, Y)? Self Healing Materials project TU Delft started 2006 Reactive Crack healing by means of oxidation with the environment

21 Topic 2 Self healing ceramic surfaces

22 Self healing ceramic surfaces Ceramic contacts interesting in: Vacuum applications (Positioning mechanisms) High temperature applications Lubricants not possible / 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 F N surface 1 p,t, type environment v 1 v 2 surface 2 (lubricant)

23 Ceramics: Typical tribological properties Vacuum Air Ceramic [-] k [mm 3 N -1 m -1 ] [-] k [mm 3 N -1 m -1 ] Alumina x x 10-7 Ductile metals in vacuum 0.5 8x 10-6 Silicium (unlubricated) 0.84 virtually Close to oxidized metals 4.9 x in air (unlubricated) carbide unapplicable PSZ Silicium nitride x x x x x x x 10-6

24 Ceramics in tribological applications: Wear reduction Nanoscale ceramics

25 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 Lifetime Load carrying capacity Low friction depends on thin soft layer, which, if present, tends to wear out (liquid or solid) Stability regime v F W F N Examples: Polymer metal systems Bronze steel Hydrodynamic lubrication

26 Friction reduction in the case of ceramics Generation of thin soft layers on hard 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: A HA r Elastic contact: A F N E' H 2 3 F N v F W s 1 fp

27 Needed for generation of soft layer A supply mechanism Gradual Reacts to (potential) damage, e.g. high friction A soft phase Pre-exisiting in body Formed in reaction with environment or counterbody Requirements soft phase Low shear strength Should stay in the contact Processing: Oxide? v OK Not OK

28 Some possibilities for thin soft layers 1. Hard Coatings: With friction reducing top layer (High H containing DLC in dry vacuum / helium: =0.005, H+ on the surface!) Some oxides (Cr 2 O 3 microfilms on Cr, CuO) 2. Solid lubricants (e.g. Graphite, MoS 2, soft metals) 3. Fluids as lubricants: Boundary lubrication with oil ( = 0.12) EP additives

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

30 Friction reducing oxides Friction reduction by (a combination of) oxides is currently state of the art at HT Examples of useful oxide combinations are 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. There is a relation between the shear strength (and the coefficient of friction) and the difference in ionic potential of the two oxides Processing advantage: No sintering under reducing conditions required

31 Example 1: 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) Self Healing Materials Project University of Twente

32 Example 1: 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

33 f [-] Example 1: Experimental results: High temperature CuO doped TZP TZP v F N T [oc] Collection of CuO by mechanical action MoO 3. interesting? melting point ~795 C

34 Example 2: Metals YSZ Ag Mo nanocomposite coatings Low T coating deposition -> Metastable solid solution of silver and molybdenum in nanocrystalline zirconia + Silver grains At heating ->diffusion of Ag

35 Example 2: Metals YSZ Ag Mo nanocomposite coatings Complex combination of processes: Filtered vacuum arc, magnetron sputtering, PLD ( Pulsed laser deposition) Noble metals (Au, Ag) Large diffusion coefficient and high mobility at elevated temperatures. TiN barriers to lower transport to the surface

36 Example 2: Metals YSZ Ag Mo nanocomposite coatings Low friction from 25 to 700 o C Multilayer PSZ Ag Mo coatings and TiN diffusion barriers Below 500 o C: Friction reduction by silver diffusion to surfaces Silver isolates Mo from oxygen Above 500 o C: Silver was worn off the contact surface. Friction reduction by Mo oxides in wear tracks (MoO 3 ) J.J. Hu et al. / Composites Science and Technology 67 (2007)

37 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 soft layer wear Friction force Tailor materials for certain system Stability regime?

38 Modelling soft layer formation mechanisms Part of Self Healing project University of Twente Plastically deforming inclusion Elastic ball on flat Wear Volume 268, Issue 9-10, 25 March 2010, Pages

39 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

40 Some results Effect of load Effect of coefficient of friction

41 Valve valve seat contact?

42 Model valve valve seat dynamics

43 Contact pressure (MPa) Roughness effects R q = 0.2 = 0, = 0 R=2 q = 0, 0.4 = 0/2 =3 0, = 0/3 R q = 0.6 m m m F N Time(s),1e -5 This can be used for formation of the soft layer.

44 Conclusions Self healing ceramics - Crack healing at elevated temperatures (T> 1000 o C) by - Oxidation (suitable oxides SiO 2, -Al 2 O 3 ) - Diffusion - Self healing in ceramics - Bad initial material characteristics - Low Mobility - Very little damage tolerance by nature - Typically good service conditions - High temperature - Oxidative environment - Healing under service conditions

45 Conclusions Self healing ceramic (surfaces) 1. Infinite external supply (lubrication with fluids) 1. Hydrodynamic or hydrostatic lubrication 2. Service conditions need to allow lubrication (e.g. high v, environment) 2. (Re-)creation soft layer by reaction with other system element (environment and/ or counterbody) 1. Needed: System elements which allow formation soft phase (e.g. alumina/water or oxidative environment) 3. (Re-)creation by transport from subsurface 1. Already incorporated soft 2 nd phase or transformation metastable phase 2. Needed: mobility e.g. stress gradient (mechanical squeezing out) Suitable soft phases at high T: (Graphite, MoS 2, Some oxides, soft metals)