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Lectures 16 and 17
New Trends in Surface Engineering and Coating
Technologies:Superhardness and
Superlubricity
Motivation for Superhard and Lubricious Coatings
• Increased durability/long life (warranty)• Conservation
– Energy– Environment– Material
• Product safety/reliability• Economic reasons/cost• Productivity
Superhardness:Recent Developments
Hardness of Materials
• Classification of materials based on their hardness:<20 GPa : Soft Materials>20 GPa : Hard Materials
(Nitrides (TiN, CrN, TiAlN) , Carbides (TiC, WC), Carbonitrides (TiCN), DLC)
>40 GPa : Super HardMaterials (Superlattice, Nanocomposite, c-BN, Diamond, DLC, a-CNx)
Hardness:Definition,
Classification
Progress in Superhard Coatings
Hardness• Hardness is the resistance of a material to plastic deformation• Dislocation movements result in plastic deformation. • As dislocation density increases, because dislocations
movements are inhibited by each other, the hardness of the material increases.
• Grain boundaries act as a barrier to moving dislocation under stresses above yield strength of materials
• Hardness of a material depends on;– the materials crystal structure (lattice geometry and bonding
energy)– microstructure of the material (point, line and surface defects)
• dislocation density• grain size• amount and strength of grain boundaries
Definition
Nanostructured, superlatticeCoatingarchitectureFor superhardness • Major improvement in coatings technology for < 10 years
• How to maximize H/E ? “elastic strain to failure”• Hardness, Toughness and Wear resistance increase• Limitation : T-induced phase transformations & diffusion• Tribological behavior / mechanisms : still open questions …
• Obstruction of dislocation glideand crack propagation
• Grain boundary sliding
Historical concept Veprek 95 Schiotz 01
Nanocomposites• nc-metal nitride / metal Musil 99• nc-metal nitride / a-nitride (silicide) Veprek 99• nc TI-B-N system Mitterer 98, Rebholz 98• nc-metal carbide / a-C Voevodin 97-99
Zhang 03 (Review)
Barnett 03
Superlattice Barnett 03, Münz 03 (Reviews)
… after annealing at 1000°C / 1h… no hardness decrease
Courtesy of C. Donnet
Hard Coatings
• Key factors that affect crystalline rigidity or resilience of solid materials and coatings
High Coordination Numbers
Covalent Bonds
Dense atomic packing
“Hardness-Bond Type” Relation
Hard materials for nanocompositecoatings in the bond triangle and changes in properties with the change in chemical bonding
Surface Engineering: Science and Technology I, The Minerals, Metals and Materials Society, 1999, pp. 207–231.H. Holleck, in: A. Kumar, Y.W. Chung, J.J. Moore, J.E. Smugeresky (Eds.)
Origin ofHardness
Grain Size – Hardness Relation• Hall – Petch Equation
(hardness – grain size relation)
H(d) = H0 + Kd-1/2
K : Material related coefficientd : Grain size of the materialH : Hardness
• Because finer grains have more grain boundaries inhibiting dislocation movements, a decrease in grain size results in an increase in material hardness
• Below a certain grain size, grain boundary sliding becomes more dominant than dislocation movement. Such a situation decreases hardness in nano-structured materials
• General assumption for this critical grain size in the literature is that Hall-Petch equation can not be applied to the materials with grains finer than approx. 10 nm.
• For such materials, common approach is to strengthen the grain boundaries.• Grain boundary sliding causes a softening in nanocomposite materials because of the
large amount of defects in grain boundaries allowing fast diffusion of atoms and vacancies under stress.
• Below a certain grain size, this relation deviates from the actual hardness of materials due to effectiveness of the grain boundary sliding process rather than dislocation movements
Origin ofHardness
Nanocomposite Coatings
• Formulations based on the uses of immiscible elements and/or phases.
• Two main categories:- Hard phase / Hard phase
• nc-MeN / nitride (e.g., a-Si3N4, a-TiB2)(a: amorphous)
– Hard phase / Soft Phase• nc-MeN / metal (e.g., Cu, Ni, Y, Ag, Co)
Nano-Composite Coatings• Miterer et al.[1] deposited nanocrystalline phases within a metal matrix, such as
TiN in Ni, ZrN in Ni, Zr–Y–N, ZrN in Cu, CrN in Cu. The hardness of these coating systems varied from 35 GPa to approximately 60 GPa.
– In these coatings, a certain chemical affinity to each other forms high strength grain boundaries.
– The dislocation and the grain boundaries increase the hardness, while the existence of a metal matrix improves toughness. However, thermal stability is still problem
• DLC, amorphous carbon nitride or other hard amorphous materials are employed as the primary phase for the amorphous matrix and nano-sized refractory nitrides, such as TiN, Si3N4, AlN, BN, etc., are used as strengthening phases.
• TiC in DLC matrix produces a nanocomposite of hardness of 32 GPa [2]• Veprek et al [3]. deposited 4–11 nm TiN crystals in amorphous Si3N4 matrix and obtained
a coating hardness of 50–70 GPa. • Zhang et al.[4] prepared TiCrCN nanocomposite coatings with hardness of 40 GPa, in
which 8–15 nm TiCrCN crystals were formed in an amorphous DLC matrix
[1] C. Miterer, et al., Surf. Coat. Technol. 120–121 (1999) 405.[2] A.A. Voevodin, J.P. O’Neill, S.V. Prasad, J.S. Zabinski, J. Vac. Sci. Technol. A 17 (1999) 986.[3] Veprek, et al., J. Vac. Sci. Technol. B 116 (1) (1998) 19.[4] S. Zhang, Y.Q. Fu, H.J. Du, Surf. Coat. Technol. 162 (2002) 42–48.[5] Also, See MRS Bulletin, March 2003 Issue
CurrentPractices
Hard phase / Hard phaseNanocomposite Coatings
– Strong bonds at grain boundaries
– All phases are hard
– One phase is amorphous
– Decreased grain sizes
Due to Veprek’ and his group (Veprek, J. Vac. Sci. Technol. B 116 (1) (1998) 19.
Coating : nc-TiN / a-Si3N4, or a- and nc-TiB2(a:Amorphous, nc:Nanocrystalline)
TiN grain size average 3-6 nm105 GPa Hardness!!!
Scientific explanation for increased hardness:Griffith theory
Nano-structured Coatings with Super-hardness
(Veprek, 1999)
These coatings, produced by a plasma-based deposition process, can potentially minimize wear-related problems in most machine elements, hence increase their reliability
Nano-composite TiN/amorphous Si3N4
Ultrananocrystalline Diamond Films
Nano-grains are heldtogether by sp2-bondedcarbon atoms (onlyone atomic layer thick) which is perhaps responsiblefor the observed superhardness
Max. Hardness is only achieved when the grain boundaries of hard nitride phase is surrounded by a few monolayers of thin soft metals.
Hard phase surroundedBy relatively thick soft phase
Soft nano-composite
Hard phase is surrounded byA very thin soft phase
Superhard nano-composite
Hard phase / Soft phaseNanocomposite Coatings
Hard phase / Soft phaseNanocomposite Coatings
J. Musil’ and his group (Musil, et al., Surface and Coatings Technology, 115(1999) 32.)
Coating parameters – Structure – Nanocomposite coating hardness(Bias voltage, Temperature, N2 pressure eg. – XRD, SEM – Microhardness)
No particular explanation for increased hardness???
23022TiN (J.L.He)160?ZrN10125Ti-Cu-N (J.L.He*)232310-15Zr-Cu-N478,19,5Al-Cu-N301,522Ti-Cu-N (J.L.He)
35170-90CrCu-N551-238Zr-Cu-N
HardnessHardness (GPa)(GPa)% Cu (at.)% Cu (at.)Average Grain SizeAverage Grain Size(nm)(nm)
CoatingCoating
*
Deformation Mechanism of Nano-Composite Coatings
Deformation
Hardness Enhancement in Superlattices
Large ∆d0
coherency strains (Cahn 1963)
"local hardnening"
Chu & Barnett, J. Appl. Phys. 77 (1995) 4403
Shinn, Hultman, Barnett,J. Mater. Res. 7 (1992) 901
10
20
30
40
50
60
0 5 10 15 20 25 30 35 40Multilayer period [nm]
Har
dnes
s [G
Pa]
∆Gshear∆do = 0
TiN / V0.6Nb0.4 N∆Gshear= 0∆do
V0.6Nb0.4N / NbN
NbN / VN ∆Gshear= 0∆do
TiN / NbN ∆Gshear∆do
TiN / VN ∆Gshear∆do
Increased hardness for:
Large ∆Gshear (Koehler 1970)hinder dislocation movement across phase boundaries
From where comes the strengthening?!
J.W. Cahn, Acta Met. 11 (1963) 1274M.Shinn, Hultman, Barnett, J.Mater.Res (1992)
Coherency strain hardening
Different shear moduli: ∆Gshear
GTiN
GNbN
Barriers for dislocation movement
Lattice mismatch: ∆d0
J.C. Koehler, Phys. Rev, B 2 (1970) 547S.L.Lehoczky, J.Appl.Phys. 49 (1978) 5479
3.6%mismatch NbN
TiN
Deformation study by nanoindentation• Slip lines propagate through the top TiN layer• Slip arrested at the superlattice interface
10 mN:
TiN/NbNTiN/NbNS.L.S.L.J. Molina, Clegg,
Joelsson, Hultman,unpubl.
Sample made by Focused Ion Beam (FIB) preparation
It takes 3.5 more load to induce the same damage in a TiN/NbN single-crystal superlattice
200 nm
L.Hultman, Engström, Odén, Surf. Coat. Technol. 133/34 (2000) 227J.Molina, Hultman, Phil.Mag.A82 (2002) 1983
∴ Dislocation glide confined to within layers in agrement with theory for Koehler-hardening !
(110)[1-10] slip in MgO
ΛΛ = 12 nm= 12 nm
Zone axis [100]
Pile-up
0.5 0.5 µµmm
Tensile CrackTensile Crack
No slip!No slip!
35 mN nanoscratchPlane rotation
Formation of dislocation walls & sub-grains
Reduction & expansion (!) of layer periods
13
TiC/DLC Nano-Composites
Low Crit Load, NHigh Crit Load, N
0
1 0
2 0
3 0
4 0
5 0
6 0
7 0
8 0
30 40 50 6 0 7 0 8 0 9 0 10 0
Carbon Content, at. %
Crit
ical
Load
,N
0
2
4
6
8
10
Peak
Con
tact
Pres
sure
, GPa
Super-Toughness Effect
TiC
TiC/DLC
DLC
50 N load1.0 µm thick on steel
Hardness 30 GPaElastic modulus 400 GPa
Mechanical behavior adaptation:•at low load – hard and stiff•at high load – ductile (by GB sliding)
J. Appl. Phys., 82 (1997) 855
13
200 nm
High toughness was achieved for TiC/DLC and WC/DLC hard composites because of: 1) nanocrack termination and2) ductility by grain boundary sliding.
WC/DLC Nano-Composites
0
10
20
30
40
50
60
40 50 60 70 80 90 100Carbon Content, at. %
Low
Crit
ical
Loa
d, N
Super-Toughness Effect
W-DLCWC
WC/DLC
J. V.S.T. A, 17 (1999) 986
Superlubricity or Frictionless Sliding:
Recent Developments
Friction: Resistance to sliding• From the very beginning mankind
has always been fascinated by friction and was often challenged to use, reduce, or control it to make life easier/more enjoyable.
• It is more common yet less understood than many other physical phenomena.
• Since the beginning, we often relied on it (or lack of it) for safety (or mobility).
10,000 B.C.
3,000 BC
Basic concepts&
shortcomingsTribo-mechanisms
Macro mechanisms
Holmberg 01
Micro mechanisms
Transfer
Tribochemistry
Nano mechanisms
Adhesion and Friction
Ff = σ.Ar
A1 A2
Ar = A1 + A2 + . . .
To achieve low friction both σ and Ar must be small.This means extreme hardness but very low shear strength.
Friction coefficient, µ = Ff / Fn
where, Fn is the normal force
Shear strength of contact spot
Adhesion and FrictionReal Surface Contour Real Contact Areas
Elastic/plastic Deformation
Adhesive Bonding
Shear and Recovery
Major Adhesive Forces
OTHERS- Ionic- Magneticπ-bonding/attraction
Capillary
Electrostatic
van der WaalsCovalent
Metallic
Friction vs Energy/Economic Losses
• Economic Losses in U.S. due to inadequate control of friction and wear
• Worldwide, it is estimated that 1/3 to 1/2 of world’s energyproduction is used to combat friction and wear (A. Z. Szeri, Tribology: Friction, Lubrication, and Wear; Hemisphere Publishing, 1980, p.2)
• Therefore, even very small improvements in energy efficiency (friction) and durability (wear) can save billions of dollars.
Loss Cost(b$)
Material 100Wear 100Friction 70
When lost-labor, down-time, cost of replacement parts added, these figures may double.
Latest Estimate: $500BP. Cummins/ORNL
1990 2000
10-3
10-2
10-1
1980
Teflon
MoS2, DLC H2S(g)MoS2
Fric
tion
Coe
ffici
ent,
<µ>
“NFC”
ULTRALOW FRICTION
LOW FRICTION
SUPERLOW FRICTION
Classification and History of Low-friction Materials and Coatings
WD-40
Debris entrapment
Transfer film lubrication
Tribochemicalfilm
Near-FrictionlessCarbon
Courtesy of I. L. Singer
??
0.15-0.60.20.20.10.1-0.6
B2O3Re2O7MoO3TiO2 (sub-stoichiometric)ZnO
Single Oxides
0.3-0.10.35-0.20.2-0.10.47-0.20.180.3-0.20.1
CuO - Re2O7CuO - MoO3PbO - B2O3CoO-MoO3Cs2O-MoO3NiO - MoO3Cs2O-SiO2
Mixed Oxides
0.2-0.350.15-0.20.2-0.30.15-0.250.2
AgPbAuInSn
Soft Metals
0.002-0.250.01-0.20.150-0.70.07-0.50.05-0.150.02-0.20.15-0.25
MoS2WS2HBNGraphitegraphite fluorideH3BO3GaSe, GaS, SnSe
Lamellar Solids
Typical range of Friction Coefficient*
Key Examples Classification
Classification of Low-friction or self-lubricating Materials
-Multiple slipsystems
-Low meltingpoint
-Rapid recovery/recrystallization
Higher ionic potential
Large difference in Their ionic potentials
*Strongly affected by test conditions, environment and temperature
What makes them low friction
A layeredCrystal
structure
0.1-0.50.05-0.15
Electroplated Ni and Cr films consisting of PTFE, graphite, diamond, B4C etc. particles as lubricantsNano-composite or multilayer coatings consisting of MoS2, Ti, DLC, etc.
Thin-Film (<50 micrometer) Composites
0.05-0.4Metal-, polymer-, and ceramic-matrix composites consisting of graphite, WS2, MoS2, Ag, CaF2, BaF2, etc.
Bulk or Thick-Film (>50 micrometer) Composites
0.1-0.20.2-0.40.15-0.250.04-0.15
Zinc steariteWaxesSoapsPTFE
Organic Materials/ Polymers
0.02-10.003-0.50.15-0.150.05-0.30.05-0.2
DiamondDiamond-like carbon Glassy carbonHollow carbon nanotubesFullerenes Composites
Carbon-Based Solids
0.2-0.40.15-0.2
CaF2, BaF2, SrF2CaSO4, BaSO4, SrSO4
Halides and Sulfates of Alkaline Earth Metals
Classification Cont’d
High chemical inertnessLow surface and/or chemicalinteractions
PTFE
Solidlubricant
Multi-layercoatings
Same mechanism as for oxides
Non-stick
single asperity or nano-contact
engineering surfaces
microsystem domain
F=σsA F=µN
experimental tools
AFM, IFM, SFA, QCM, HRTEM
atomistic coarse-grained continuum
micromachine devices
molecular-scale energy dissipation
constitutive laws for the interface
Lennard-Jones
simulation
Atomic ScaleContacts
MolecularDebris
POD, etc.
Superlubricity (a state of frictionless or near-frictionless sliding: In what scale?
Engines,Turbines, etc.
Courtesy of M. Dugger
single asperity or nano-contact
engineering surfaces
microsystem domainAtomic ScaleContacts
MolecularDebris
Vanishing friction: Is it really possible?(across the boundaries)
Virtual Lab. Real Lab. Practical Application
Yes Yes/Depends Perhaps / Maybe / No
MEMS devices Industrial Gears
1Å cm-m
AFM, FFM
Vanishing Friction: MD - Simulations and Single Asperity/Nano-Contact Experiments
In virtual world, everything (including vanishing friction etc.) seems to be possible.Theoretical Studies/ Predictions:Sokoloff, Hirano, Brenner, Robbins, etc.Work by Motohisa Hirano et al.:Superlubricity: A state of vanishing friction.Both theoretically and experimentally demonstrated frictionless sliding between Si(001) and a W (011) tip in ultra-high vacuum, (PRL, 78(1997)1448)).
Hirano’s movie
Superlubricity/State of frictionless sliding
single asperity or nano-contact
engineering surfaces
microsystem domainAtomic ScaleContacts
MolecularDebris
Vanishing Friction: Nano-scales Systems
• 0D against 1D Carbon Structures
• 1D on 1D
• 1D on 2D
• 2D on 2D
D. E. Luzzi, University of Pennsylvania
Sliding Between 0-D (C60) and 1D Carbon Structures
HRTEM
IncommensurateNo edge effectNo dangling bonds
NearlyFrictionless Sliding betweenC60 and nanotube
Luzzi’s Movie:
The Case of Nested Carbon Nanotubes (1D/1D)
Rod Ruoff - Northwestern UniversityStick-slip
Smooth Sliding
11--D Carbon Sliding Against 2D Carbon Sliding Against 2--D Carbon StructuresD Carbon Structures
Commensurate Incommensurate
Tribolever
Dienwiebel et.al.,
PRL, 92(2004)126101
µ ~ 0.001STM of one layer of
graphite
2D/2D
Dry N2
Brenner’s Movie (NC State)
Others who have also modeled/simulated/predicted frictionless sliding in graphite include:•Mate, et al., “Atomic-scale friction of a tungsten tip on a graphitesurface” PRL, 59(1987)1942.•Buzio, et al., Carbon, 40, 883 (2002).•Dienwiebel, et. al., “Superlubricity of Graphite”, PRL, 92(2004)126101.
Superlow µ in Other Materials:•Socoliuc, et al., “Entering a new Regime of Ultralow Friction”, PRL, 92(2004)134301 (Si tip over NaCl crystal)•Martin et al., Singer et al., (in-situ deposited or H2S-produced MoS2)•And others . . .
Superlow µ Citing on Graphite and Others at Nano Scale
Lattice image of graphite
STM-Graphite
Superlubricity in MoS2
• Martin et. al., 1993, (PVD-MoS2)
• Singer, et al. (In-situ H2S formed monolayers of MoS2 tested in UHV)
STM Image
N-type MoS2
Near frictionless sliding in UHV
PVD MoS2•Atomically clean & smooth surfaces
µ = 0.001
Mo
Mo
S
S
The Case ofBoric Acid
Inside Wear TrackLow Magnification High Magnification
Sliding interface
Solid Lubricant
Proposed Lubrication Mechanism for Layered Solids
Platelike crystallites are oriented in the direction of sliding.Note intercrystallite slip/shear consistent with proposed mechanism
PinOn
Disk Machine
single asperity or nano-contact
engineering surfaces
microsystem domain
F=µNconstitutive laws for the interface
Atomic ScaleContacts
MolecularDebris
Friction Vanished (When scale is right)Practical Applications
Perhaps / Maybe / No
Summary
Yes, superlow friction was indeed achieved at
atomic/nano-scale simulations/experiments
along certain sliding directions, on certain
atomically smooth crystalline materials
that are also atomically clean and
structurally intact) (no deformation, dirt,
or asperities on surface).
Is it possible to vanish friction in micro-to-macro scale systems?
Plasmas: Ions, electrons, energetic atoms, molecules, clusters, etc
High-energyIon beams
Yes, perhaps, by designing new coatings
Microwave CVD
Sun
The ultimate plasma
PECVD
Advanced plasma-based deposition technologies may be key to achieving the kinds of super-critical,non-eqlubirium chemical/physical states needed for the synthesis of new coatings with unusual
properties, such as super-low friction.
Si
Film
Synthesis of superlow friction carbon filmsAt Argonne National Laboratory
TEMSEM
AFMH-13 - 3D
PECVD
Methane – Hydrogen Plasma
Typical plasma gas composition: 25% CH4 + 75% H2
Timeline: 1989-present
Friction Experiments: Pin-on-disk Machines
LoadSapphireBall
DiskLoad: 1 - 20 NSpeed: 0.3 - 1 m/sEnvironment: Dry NitrogenBall Radius:3.175 - 5 mm
Contact geometry Operating principles
Coating
Nearly-Frictionless Carbon (NFC)
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0 1000 2000 3000 4000 5000 6000
Time (s)
Coated sapphire ball against coated sapphiredisk in dry nitrogen
0.001
Erdemir, A., et al., J. Vac. Sci. Technol., A 18(2000)1987
Load: 10 NSpeed: 0.3 m/sTemperature: 23oCBall Radius:3.175 mm
Width of WearTrack
Wear Scar (0.18 mm)
Ball Side
Disk Side
1997LoadSapphireBall
DiskLoad: 10 NSpeed: 0.3 m/sEnvironment: Dry NitrogenBall Radius:3.175 mm
0
0.005
0.01
0.015
0.02
0.025
0.03
0 1000 2000 3000 4000 5000 6000
DLC-coated SapphireDLC-coated Steel
Time (s)
Depending on Surface Roughness and/or Substrate Material, these coatings can provide friction coefficients
of 0.001 – 0.006 (in inert or vacuum environments)
Soft, less rigid, and Rough steel ball/disk
Hard, smooth and more rigid sapphireBall/disk
Fric
tion
Coe
ffici
ent Steel Substrate
Sapphire Substrate
Sliding Distance vs WearWear Volume
0.00E+00 0.00E+007.62E-06
1.22E-04
8.1E-068.0E-067.8E-06
0.0E+00
2.0E-05
4.0E-05
6.0E-05
8.0E-05
1.0E-04
1.2E-04
1.4E-04
10 50 150 250 500 5000 50000
Distance [m]
Wea
r Vol
ume
[mm
^3]
Wear TrackDisk
Ball
Ball
Load: 10 NSpeed:0.5 m/sEnvironment: Dry N2Coated Steel ballAgainst Coated Steel Disk
(Results From Naval Research Lab. Washington, D.C.)
100µm
<µ>=0.00310 100 1000
0.0000.0250.0500.0750.1000.1250.150
5.0 mm track, 3.0 mm/sec
RUN-IN
cycles
AFTERBEFORE
J. Heimberg, K. J. Wahl, I. L. Singer/NRL
µ
100µm
Third Bodies?
Evolution de f et Rc en fonction de N
0
0.05
0.1
0.15
0.2
0.25
0 500 1000 1500 2000 2500 3000
Nombre de cycles, N
Coe
ffici
ent d
e fr
otte
men
t f
0
1
2
3
4
5
6
7
Rési
stan
ce é
lect
rique
de
cont
act R
c
Frictional Performance of Argonne’s NFC Coating(Test was run at Ecole Centrale de Lyon, France)
Test Conditions: 5 N Load, 1 mm/s velocity, Steel Substrate
0.003
Courtesy of M. Bellin
Not all carbon films are created equal (at least tribologically)
0.7
0.001
0.01
0.1
1
Source Gas Chemistry
H- f
r ee
DL
C
C2H
2-G
row
n D
LC
C2H
4-G
row
n D
LC
CH
4-G
row
n D
LC
75%
CH
4+2 5
% H
2G
row
n D
LC
25%
CH
4+7 5
% H
2G
row
n D
LC
Fric
tion
Coe
ffici
ent
Test Conditions: Load, 10 N; Speed, 0.5 m/s; Temperature, 22oC; Environment, Dry N2,9.5 mm-diameter M50 Balls
H/C=0H/C=1
H/C=10
H/C=4
0.003
Plasma Discharge
PECVD
Near-FrictionlessState
Plasma gas composition has a strong influence on friction
A Model for Superlubricity of Hydrogenated Carbon Films
Super-hydrogenatedDLC
H-terminatedC atoms
+
+
WW
F
Erdemir, Surface and Coatings Technology, 146-147(2001)292
HydrogenMolecule
HydrogenAtom
BondedHydrogen
Figure 7.Courtesy of L. Curtis and P. Zapol
Sliding NFC Surfaces
single asperity or nano-contact
engineering surfaces
microsystem domain
F=µN
Atomic ScaleContacts
MolecularDebris
Friction Vanished (When test conditions are right)
Dream Reality Practicality
Yes Yes/Depends Maybe / No
SummaryYes, superlow friction was achieved on a carbonfilm in inert gas environments and under realistic/macro-scale test conditions. But, unfortunately, in practical world, we hardly use inert environmentsand the operating conditions of most everydaymachines require the use of a lubricant and they often operate at high temperatures.
Is there still any hope?
POD
0
0.05
0.1
0.15
0.2
0.25
0 100 200 300 400 500 600
Distance (m)
Limits of Carbon-film Lubricity
Fric
tion
Coe
ffici
ent
Ambient Air, 40% R.H.
Dry Air
Dry Nitrogen
SteelBall 0.01
NFCBall H2O, H3O+,OH-
NFC6_SQ11
00.020.040.060.080.1
0.120.140.16
0 2000 4000 6000 8000 10000
Number of cycles
Fric
tion
coef
ficie
nt
0
10
20
30
40
50
RH
(%)
f RH
N2 AA N2 DA N2
Test Environment vs Friction
Load
AISI 52100Steel Ball
Disk
DLC (NFC)Film
Uncoated Ball/Coated Disk N2: Dry Nitrogen; AA: Ambient Air; DA: Dry Air;
RH: Relative Humidity
Friction
RH 0.01
Load: 10N Max. Hertz Press ~ 1 GPaSpeed: 0.1 m/s
Future Directions in the Design of Superlow Friction Coatings
SmartProcesses
(hybrids, etc.)Nanostructured,
Superlattice, Gradient
Multicomponent, Multilayer
Single component
Textured, Adaptive (smart)
Nano-composite Coatings: superhard
Self-lubricating
Sculptured Coatings
1980s 1900s 2000s
Novel Coating Architectures for the 21st Century
Historical developments and new trends in tribological and solid lubricant coatings,C. Donnet and A. Erdemir, Surface and Coatings Technology,180 –181 (2004) 76–84
Superlattice and/or multi-layer coatings
Adaptative & smart coatingsExamples
Voevodin 02
Self-adjustment of YSZ/Au/MoS2/DLC vs. Temperature & Environment
• Concept of adaptative coatings = f (temperature, environment, pressure)• Smart = adaptative + reversible
TiAlN + Y, Cr : friction-induced oxide formation Savan 99WC/DLC/WS2 : cycled air / vacuum friction Voevodin 99,00CaF2/WS2 : friction-induced CaSO4 formation at high T John 98MoS2 or WS2 / ZnO or PbO : friction induced PbMO2 or ZnWO4 Walck 97
Courtesy of C. Donnet
Recommended