Wear resistant and low friction nanocomposite coatings Dr Tomasz Suszko

1. Wear resistant and low frictionnanocomposite coatings Dr Tomasz Suszko

2. Lecture outline • Plasma sputtering – short description • DC-, triode-, RF-, magnetron sputtering • Nonreactive and reactive mode • Low friction nanocomposite coatings • Chosen results: Mo2N/Cu nancristaline films– structure, mechanical and tribological properties • Structure, hardness • Friction & wear mechanisms in temperature range RT-400°C

3. Plasma - the 4thstate of matter http://fusedweb.pppl.gov/CPEP

4. Pressure ~10 Pa noble gas (e.g. Ar) Voltage ~1.5 kV • Disadvantages: • Low ion current density (low sputtering rate) Cathode 10 1 Ionisation coeffcient 0.1 - 0.01 • Electron emission • Sputtering • Implantation • Defects generation • E-m radiation 10 100 + 1000 Electron energy [eV] • High working gas pressure resulting in scattering (low deposition rate) • Dielectric materials can not be sputtered • High voltage is needed Anode + substrate Fundamentals of plasma sputtering – DC sputtering(diode sputtering)

5. Fundamentals of plasma sputtering – triode sputtering • Lower working gas pressure – 0.1 Pa (higher deposition rate) • Higher ion current density (higher sputtering rate) + 0.5 kV - - 10 100 V + 1 Ionisation coeffcient 0.1 Substrate 0.01 10 100 1000 Electron energy [eV] Target • Dielectric materials can not be sputtered

6. Fundamentals of plasma sputtering– microwave assisted sputtering • Lower working gas pressure – 0.1 Pa (higher deposition rate) • Higher ion current density (higher sputtering rate) Microwave antenna + 0.5 kV 10 – 1 Ionisation coeffcient 0.1 Substrate Target 0.01 10 100 1000 Electron energy [eV] • Dielectric materials can not be sputtered

7. Fundamentals of plasma sputtering– RF sputtering Matchbox • The differce in: • mobility of electrons and ions • areas of electrodes • results in • negative target selfbias • thus, • dielectric materials can be sputtered RF Substrate

8. a v cos e a v e R a v sin L e Fundamentals of plasma sputtering– motion of the electron in electromagnetic field

11. Fundamentals of plasma sputtering – magnetron sputtering • Low working gas pressure – 0.1 Pa • Very high ion current density is possible (high sputtering rate) 10 1 Ionisation coeffcient 0.1 0.01 10 100 1000 Electron energy [eV] – unbalanced magnetron sputtering There is a possibility to control the substrate ion current and the energy of the ions as well Substrate DC or pulsed power supply

12. What materials can be sputtered and deposited? • Whatever one need? • It must be kept in mind that: • Compounds, targets are made of, are decomposed to the atomic form and only then can react again on the substrate (not always getting appropriate conditions) • Sputtered atoms are scattered along their way towards substrate (the lighter the more intense thus the stoichiometry can change) • A sputtered compound can not to easily evaporate (sufficient vacuum can not be obtain)

13. End of part one

14. From yesterday • Mean free path • Secondary electron emmision • Ion implantation • Sputtering • Charging effect • Thermoemission • Magnetic mirror and trap • Larmor frequency and radius • Magnetron source (gun)

15. Gas pressure • Gas flows • Discharge power • (Substrate bias – energy of the ions) • (Substrate ion current density) Control unit Optical signal (optical emission spectroscopy) Fundamentals of plasma sputtering– reactive sputtering Compounds of the target and gas elements Inert gas (e.g. Ar) Reactive gas (N2, O2, CH4 etc.) For poorly conducting or insulator deposits pulsed power supply is very usefull Pumping system

16. What I won’t speak about is... • Plasma enhanced chemical vapour deposition • Laser ablation • Plasma spraying • Ion implantation (clasical orplasma immersion) • Plasma nitriding orcarburazing etc.

17. Working gases: • Ar (inert gas), • N2 (for nitrides), • O2 (for oxides), • CH4, C2H2 (for carbides and DLC) • Plasma maintained by: • DC or pulsed discharge (magnetron), • Vacuum arc, RF e-m waves What we use for deposition is... • Targets made of: • Ti, Al, Mo, V, Ag, Cu • but also • Fe, Ni, Co • and • Si

18. What we develop for process control and data acquisition is... Gases Valve unit Optical signal Coils supply Spectrometer Magnetron sources Pulsed power supply Pulsed power supply Substrate bias and heating Pumping system

19. F L What we interest in is... • Continuous looking for novel anti-wear coatings and development of their deposition methods • Phenomena in the tribolgical contact between hard coated surface and a counterpart • Structure, elemental and phase composition of the coatings in the initial state (after deposition) • Stress, adhesion, hardness of the coatings • Friction during tribological tests (especially in elevated temperatures) • Tribomutation - chemical and physical changes of the „third body” – elemental and phase composition, structure etc. of that • The role of oxides in friction process

20. Where can we look for hard compounds?

21. How to obtain hard films • Chemical sythesis ( DLC, c-BN, AlMgB, C3N4 ) • Forming proper physical microstructure • Nitride or carbide multilayers(TiN/CrN, TiN/TiAlN i in.) • Compositesnc-MexN/a-Si3N4nc-MexC/a-C:H np. nc-TiN/a-Si3N4 • Composites MexN/M np. (ZrN/Cu, Cr2N/Cu, TiN/Ag)

22. Shear strength Hardness Soft materials Hard materials F F A A L small large L large small Hardness is not all - there is friction also! • Shear strength and hardness depend on each other thus friction coefficients are comparable for various izotropic materials.

23. F L Self-lubricating materials • As a result of rubbing, a thin low-shear--strengh layer should appear • The material should be hard (what ensures small contact area) • Composite materials: • guaiac wood • PTFE impregnated bronzes • bearing metals with graphite or MoS2 inclusions • ceramic/carbon fiber composites • Izotropic materils: • diamond

24. Hard coating Enviromentalgas Lubricating film RTDinfo - Mag. Europ. Res., 39, 2003 Self-lubricating FILMS

25. An attempt - Mo2N/Cu coatings Mo2N as a hard coating MoO3 as a solid lubricant Cu additive as a mean for hardness enhancement

26. Mo2N/Cu nanocrystalline films – structure, mechanical and tribological properties • Outline • Deposition method • Some remarks on the structure • Hardness of the films • Friction & wear in temperature range RT-400°C • Conclusions Suszko et al., Surf. Coat. Tech., 200, 2006, pp. 6288-6292 Suszko et al., Surf. Coat. Tech., 194, 2005, pp. 319-324

27. Deposition method:unbalanced magnetron sputtering Ar, N2 optical signal external coils Cu Mo pulsed power supply pulsed power supply sample 30 cm pumps Temperature: 200 °C Bias: -30 V

28. Structure – XRD spectra 18 Co Kα radiation ← Cu (111) ← γ-Mo2N (111) γ-Mo2N (200)→ 16 21% at. Cu Cu (200)→ 9% at. Cu 14 6% at. Cu 12 Fe (substrate) 10 1% at. Cu Intensity [a.u.] 8 6 4 2 0% at. Cu 0 40 45 50 55 60 65 Diffraction angle 2ϑ [°]

29. The influence of copper content on crystalite size Crystallite size obtained from Scherrer’s formula AFM image of the pure γ–Mo2N nitride 13 12 Mo2N (200) 11 10 9 Crystallite size [nm] 8 7 6 5 0 5 10 15 20 25 Cu content (at. %)

30. Structure 40 Load-depth sensitive method DUH 202 (FN 20 mN) 35 30 13 Traditional method (FN 100—1000 mN) 25 H (GPa) 12 Mo2N (200) 20 11 Load-depth sensitive method Hysitron (FN 2mN) 15 10 Crystallite size (nm) 10 9 0 5 10 15 20 25 Mo2N (111) Cu content (% at.) 8 7 6 5 0 5 10 15 20 25 Cu content (% at.) Crystallite size and film hardness

31. 1.0 0 % at. Cu 0.9 3 % at. Cu TiN 7 % at. Cu 0.8 22 % at. Cu 0.7 Friction coefficient 0.6 0.5 0.4 0.3 0 100 200 300 400 Temperature [°C] Friction coefficient • Ball on discconfiguration • Speed: 5 cm/s • Normal force:1 N • Fixed and scannedtemperature • Counterpart: alumina ball

32. b) 100°C 0.5 0 μm -0.5 -1 -1.5 0 100 200 300 400 500 600 700 1 μm 0.8 0.6 Friction coefficient 0.4 0.2 0 0 1000 2000 3000 4000 5000 Revolution number Wear rate coefficient - a definition Worn volume of the sample per work unit done against friction force

33. -12 10 Wear rate ( m3/J ) 400°C -13 10 300°C Wear rate for TiN RT – 0.8·10-14 200°C – 1.5·10-14 400°C – 3·10-15 -14 10 -15 10 100°C RT, 200°C -16 10 0 5 10 15 20 25 Copper content (at. %) Wear behavior: 20-400°C

34. RT: kF ~10-16 m3/ In Out 100°C: kF ~2·10-14 m3/J ! 200°C: kF ~10-16 m3/J In Out 1 1 1 In Out 0.5 0.5 0.5 0 0 0 0 200 400 600 800 1000 0 0 200 200 400 400 600 600 800 800 1000 1000 Raman shift [cm-1] Raman shift [cm-1] Raman shift [cm-1] Wear behavior – "100°C effect" Mo2N 0% Cu

35. Wear behavior – the influence of Cu addtion (100°C friction test) 0 at. % Cu 6 at. % Cu kF ~10-16 m3/J kF ~2·10-14 m3/J 9 at. % Cu 1 at. % Cu 2.5 at. % Cu 22 at. % Cu 50 mm 50 mm 50 mm 50 mm 50 mm 50 mm

36. Conclusions • Relatively low friction coefficient against alumina is observed in room temperature. • 1-3 at. % of Cu additive increases hardness of Mo2N coatings. • Low wear rate is registered in temperatures bellow 250°C. • "The 100°C effect" is observed for samples with low content of copper. This effect is eliminated when films contain >6 at. % Cu . • Coatings gradually oxidize in temperature over 300°C.