Hard, anti-wear and (or) low friction coatings Dr Tomasz Suszko - PowerPoint PPT Presentation

Hard, anti-wear and (or) low friction coatings Dr Tomasz Suszko

What is the object of our studies? Coatings having : • thickness >50nm and <100µm • ceramic or metallic (not polimeric-like, liquid crystals etc.) • obtained mainly with PVD methods HARD - what is hardness? - what compounds can be hard? ANTI-WEAR - what is wear? LOW FRICTION - where friction come from? COATINGS - how do properties of a coating differ from bulk material 2 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

What is hardness, actually? Is it a physical quantity? On the Mohs scale (1812) a pencil lead has a hardness of 1 a fingernail has hardness 2.5 a copper penny, about 3.5 a knife blade, 5.5 window glass, 5.5 Digression: Extended Mohs scale has additional 5 stages for hardest materials 10 - fused zirconia ZrO 2 11 - fused alumina Al 2 O 3 12 - silicon carbide SiC 13 - boron carbide B 4 C 14 - boron nitride BN 15 - diamond No, it isn't in any way - comparative, highly nonlinear 3 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

What is hardness, actually? „Resistance of materials to plastic deformation, usually by indentation” Rockwell's hardness HR = E - e It also isn't any physical quantity 4 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

Stress vs strain depndence F F F F steel Stress aluminium ultimate strength tensile strength yield strength strain necking not always hardening sharply defined ceramics very low ductility! Strain 5 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

Vicker's hardness It looks like a physical quantity 6 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

Brinell's hardness It looks like a physical quantity too 7 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

What is hardness, actually? Different indentation geometries plastically deformed Brinell (load - kilonewtons) elastically deformed (no real border) undisturbed Vickers (load - newtons) shape of the plastically deformed zone Knoop (load - decimals of newtons) Berkovich (load - mili and micronewtons) 8 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

What is hardness, actually? • It seems like a method for obtaining compression strength • In fact the material is compressed, stretched, shared, fractured • Hardness is not precisely defined superposition of given material's elasticity, strength (against compression, tension, sharing and fracture) • Hardness is mainly expressed in units of stress (GPa, kG/mm 2 ) 9 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

Hardness conversion Indentation hardness is not a single fundamental property but a combination of properties, and varies with the type of test. The modulus of elasticity and the depth of indentation influence conversions. Therefore separate conversion tables are necessary for different materials. 10 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

Depth-sensing (Oliver-Pharr) method of hardness measurements 11 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

12 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

Where can we look for hard compounds? • Diamond, HV 8000-10000 • Cubic boron nitride HV 5000 • Boron carbide HV 4000 • Silicon carbide HV 2600 • Silicon nitride HV 1700 • Titanium diboride HV 3000 • Titanium carbide HV 2800 • Titanium nitride HV 2300 • Tungsten carbide HV 2300 • Aluminum oxide HV 2100 • Molibdenum nitride HV 2000 • Chromium nitride HV 1800 • Iron tricarbide HV 1300 • Chromium carbide HV1300 13 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

Lets cover a tool with TiN What have we obtained? A monocristal :-) ? Is the structure important? 14 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

As an example - very common material - copper 15 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

As an example - very common material - copper Metals can be made into nanocrystalline materials that perform better than regular metals. How to do it? Roll copper at the temperature of liquid nitrogen. Then, heat to around 450K Result: – Structure with micrometer sized grains and nanocrystalline grains – Increased strength and hardness of metal because of the nanocrystalline grains – High ductility Source: J. Schiotz et al., Nature, 391 (1998) 561 16 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

Increasing Copper Strength • Plastic deformation of copper introduces work-hardening (copper gets stronger) and reduces the grain size • Hall-Petch relation predicts materials get stronger as grain size decreases: σ y = σ 0 + K HP d -1/2 Material Yield Strength Cold Worked Copper 393 MPa 400 nm Copper 443 MPa 100 nm Nanograin Copper 900 MPa 10 nm Nanograin Copper 2.9 GPa Can we do in endlessly? It's a pity - no. 17 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

TiN 2600 HV50 (26 GPa) TiN/TiAlN 4800 HV50 (48GPa) TiAlN (not presented) 3000 HV30 (30GPa) 18 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

Digression - balI cratering source: http://www.pvd-coatings.co.uk/ 19 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

Digression - balI cratering 20 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

Digression - arc deposition European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

Digression - arc deposition magnetron arc European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

Another example CrN/CrCN film European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

Hardness depends not only on elemental or phase composition but also on structure. So, we came to nanocomposities Is it something new? Damascenian steel - do you know it? When we google „nanocomposite”... European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

Superhard materials Binary multilayer CrN/TiAlN hindered crack propagation hindered crack propagation Binary nc-TiN/a-Si 3 N 4 TiN grains ≈ 10 nm 1 ML a-Si 3 N 4 "films" 5 nm source: Euroschool 2008, Ljubljana; EPF, Lausanne European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

Examples of nanocomposite coatings for tribological aplications  n-MeN/a-nitride (nMeN/a-Si 3 N 4, where: Me=Ti, W, V)  n-MeN/n-nitride; for example: n-TiN/n-BN  n-MeC/a-C or a-C:H; for example: TiC/DLC; TiC/a-C:H, Mo 2 C/a-C:H  n-MeN/metal, for example: ZrN/Cu, CrN/Cu, Mo 2 N/Cu, Mo 2 N/Ag  n-WC + n-WS 2 /DLC  n-MeC/a-SiC, for example: TiC/a-SiC/a-C:H How can we obtain such composite coatings? European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

The carbon-hydrogen system Hybridizations of carbon sp 3 : four identical σ-bonds, single bonds in hydrocarbons (e.g. ethane), elemental as diamond H 3 C–CH 3 sp 2 : three identical σ-bonds, one extra electron (π-bonds), double bonds in hydrocarbons (e.g. ethylene), elemental as graphite H 2 C=CH 2 sp : two identical σ-bonds, two extra electrons (π-bonds), triple bonds in hydrocarbons (e.g. acethylene), elemental does not exist HC≡CH European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

DLC - diamond like carbon 28 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

Composition-structure dependence in TiN/SiN nanocomposite Source: J. Patscheider, Surf. Coat. Technol. 146-147 (2001) 201 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

Lets come back to the hardness measurements - how to measure hardness of a coating? Thick film / small load Thin film / large load film deformed zone deformed film zone substrate substrate H composite = f (L, H film , H substrate , t film ) low L, high t film ⇒ H composite = H film high L, low t film ⇒ H composite = H substrate There is no general formula! 30 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

How to measure hardness of a coating? 31 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl

TiC/a-C:H nanocomposite thin films from our lab 32 European Summer School PPST Koszalin August 2008 Tomasz Suszko tomasz.suszko@tu.koszalin.pl