Nanotribology Judith A. Harrison & Ginger M. Chateauneuf Chemistry Department United States Naval Academy Annapolis, MD 140 jah@usna.edu Some Reviews 1. J. A. Harrison et al, Atomic-Scale Simulation of Tribological and Related Phenomena, in Handbook of Micro/NanoTribology, ed B. Bhushan, ( nd edition, CRC, Boca Raton, 1999) pp. 55 and refs therein.. M. O. Robbins & M. H. Muser, Computer Simulations of Friction, Lubrication, & Wear,in Micro/Nanotribology Handbook, ed. B. Bhushan, (second edition, CRC Press, Boca Raton, 1999) pp. 717-765. 3. J. A. Harrison et al, The Friction of Model Self-Assembled Monolayers, in Encyclopedia of Nanoscience and Nanotechnology, Vols. 3, ed. H. S. Nalwa, American Scientific Publishers, Los Angeles (004), pp. 511-57. 4. O. Marti, AFM Instrumentation & Tips, in Micro/Nanotribology Handbook, ed. B. Bhushan, (second edition, CRC Press, Boca Raton, 1999) pp. 81-144. 5. U. Schwarz & H. Holscher, Atomic-Scale Friction Studies Using Scanning Force Microscopy, in Modern Tribology Handbook, ed. B. Bhushan, Vol I, (CRC Press, Boca Raton, 001) pp. 641-665.
Modeling the systems Indent Slide Tip Film Free Substrate Free Thermostat Rigid AFM A model system Early AFM Friction Data Mate, McClelland, Erlandsson, & Chiang, Phys. Rev. Lett. 59, 194 (1987). Tungsten = Tip Graphite = Substrate AFM
Early MD Simulations of Friction Diamond (111) Y or [11] X or [110] H-terminated Third-bodies Trapped molecules Directional dependence Velocity dependence NVT Z or [111] J. A. Harrison et al., Phys. Rev. B 46, 9700 (199). J. A. Harrison and D. W. Brenner, J. Am. Chem. Soc., 116 (1994) 10399-1040. J. A. Harrison et al., J. Phys. Chem., 97 (1993) 6573-6576. J. A. Harrison, et al., Mat. Res. Soc. Bull., XVIII (1993) 50-53. J. A. Harrison et al.,, Wear, 168 (1993) 17-133. M. D. Perry and J. A. Harrison, J. Phys. Chem. B, 101 (1997) 1364-1373. M. D. Perry and J. A. Harrison, Thin Solid Films, 90-91 (1996) 11-15. M. D. Perry and J. A. Harrison, Langmuir, 1 (1996) 455-4556. M. D. Perry and J. A. Harrison, J. Phys. Chem., 99 (1995) 9960-9965. M. D. Perry and J. A. Harrison, Tribo. Lett., 1 (1995) 109-119. J.. A. Harrison, et al.,, Thin Solid Films, 60 (1995) 05-11. 100 m/s Y or [11] Hydrogen-terminated diamond (111) Harrison et al. Phys. Rev. B 46, 9700 (199). Y or [11] Red=Load Blue = friction
Early Friction Models Ohzono et al., Jpn. J. Appl. Phys. 38 (1999) L675. Energy dissipation in hydrogen-terminated diamond (111) Harrison et al. Thin Solid Films 60, 05 (1995); ibid., J. Phys. Chem. B. 99, 9960 (1995). T = 50 K MD= NVT
Ab initio studies of atomic-scale friction between diamond (111) surfaces Neitola & Park, J. Phys. Chem. B 105, 1338 (001). Gaussian94 Ab initio studies of atomic-scale friction between diamond (111) surfaces Neitola & Park, J. Phys. Chem. B 105, 1338 (001). Surface separation vs sliding distance Energy vs sliding distance
Friction vs. Load Neitola & Park, J. Phys. Chem. B 105, 1338 (001). Ab initio Harrison et al. Phys. Rev. B 46, 9700 (199). MD AFM studies of atomic-scale friction between diamond (111) surfaces G. J. Germann et al., J. Appl. Phys. 73, L88 (1997). UHV CVD diamond tip vs H-term diamond (111) and diamond (100) (x1) Van den Oetelaar and Flipse, Surf. Sci. 384, L88 (1997). UHV Si tip vs H-terminated diamond (111) (1x1) Remove H (x1) diamond (111) Enachescu et al., Phys. Rev. Lett. 81, 1877 (1998). UHV Tungsten carbide tip vs H-terminated diamond (111)
AFM studies of atomic-scale friction between diamond (111) surfaces Van den Oetelaar and Flipse, Surf. Sci. 384, L88 (1997). Topography External Load = nn Friction External Load = 8 nn Scale: 0-4nN Scale: 0-15 nn Contact mechanics L Courtesy of R.W. Carpick, University of Wisconsin, Madison γ = surface energy work / area JKR Hertz R a a A=š a Hertz (1881): JKR: DMT: / 3 RL A = π K 3 4 1 υ1 1 υ K = + 3 E1 E 1 [ L + 3πRγ + 6πRγL ( 3πRγ ) ]3 R A = π + K R A = π + K 3 ( L πrγ )3
Carpick & Salmeron, Chem. Rev. 97, 1163 (1997). L c = pull off force 6πγR A c = π K 3 L c JKR 3 = π γ R DMT L c = π γ R JKR vs DMT Enachescu et al., Phys. Rev. Lett. 81, 1877 (1998). L c = π γ R D. Tabor, J. Colloid Interface Sci. 58, (1977). 1 µ > 5 JKR 3 16Rγ µ = 3 9 µ < 0.1 DMT K z o Diamond(111) / WC z o = 0. nm E diamond = 1164 GPa E wc = 714 GPa υ diamond = 0.08 υ wc = 0.4 R = 110 nm γ = 0.01 J/m µ = 0.019
AFM studies of atomic-scale friction between diamond (111) surfaces Enachescu et al., Phys. Rev. Lett. 81, 1877 (1998). Atomic-scale stick-slip motion with a well-defined friction force is frequently observed Muscovite Mica Courtesy of R.W. Carpick, University of Wisconsin, Madison lateral force (arb. units) F f 1 0 stick slip 0 1 3 4 5 6 lateral displacement (nm)
Friction is proportional to the true contact area; continuum mechanics works at nm scale Courtesy of R.W. Carpick, University of Wisconsin, Madison 500 Platinum-coated tip, Mica sample, in UHV F f = τ A Friction (nn) 400 300 00 100 JKR fit 0-00 -100 0 100 00 300 Load (nn) Carpick et al., J. Vac. Sci. Technol. B 14 189 (1996) Carpick et al., Langmuir 1 3334 (1996) friction contact area shear strength 3 Lc = π γ R A F c c 3πR = π K = τa c 3πR K 3 3 Determination of shear strengths from lateral stiffness R. W. Carpick et al., Appl. Phys. Lett. 70, 1548 (1997) k tot is measured! BUT k contact is related to shear stress, τ. df dx lateral = k tot = 1 k lever + k 1 contact 1
Determination of shear strengths from lateral stiffness R. W. Carpick et al., Appl. Phys. Lett. 70, 1548 (1997) Sphere-plane contact k contact = 8 * G a F f = τ A = τπa G * = a = contact radius ( a ν ) ( a ν ) G 1 1 + G 1 Determination of shear strengths from lateral stiffness R. W. Carpick et al., Appl. Phys. Lett. 70, 1548 (1997) silicon nitride tip + muscovite mica in 55% RH τ=680 MPa τ = 64 G π k * F contact f
The shear strength can approach the ideal limit for a defect-free interface Courtesy of R.W. Carpick, University of Wisconsin, Madison materials Pt/mica: max. SiN x / mica WC/ H-diamond (111) shear strength τ (MPa) τ /G eff adhesion energy γ (mj/m ) contact radius at pull-off, number of unit cells 910 1/5 404 8.6 nm, 000 unit cells 5 1/430 4 5.3 nm, 740 unit cells 40 1/1600 10 1.1 @ zero load, 140 unit cells for Pt/mica: G eff = GPa (effective shear modulus) Enachescu et al., Phys. Rev. Lett.(1998), Carpick et al., J. Vac. Sci. Tech.(1996), Langmuir (1996), Appl. Phys. Lett.(1997), MRS Proc. V. 539(1999), Carpick & Salmeron, Chem. Rev. (1997) MD Simulations: Friction of H-terminated Diamond (100) (x1) Perry & Harrison, J. Phys. Chem. B. 99, 9960 (1995). Diamond (100) (x1) H-terminated Directional dependence Velocity dependence NVT
MD Simulations: Friction of H-terminated Diamond (100) (x1) Perry & Harrison, J. Phys. Chem. B. 99, 9960 (1995). Y or [011] X or [011] Stick-slip behavior reflects the lattice constant. AFM friction of diamond (100) (x1) Van den Oetelaar and Flipse, Surf. Sci. 384, L88 (1997). Si tip Large atomic-scale stick slip features H-terminated π-bonded nm G. J. Germann et al., J. Appl. Phys. 73, L88 (1997). CVD diamond tip diamond (100) (x1) in UHV (no H), also observe stick slip
MD Simulations: Friction of H-terminated Diamond (100) (x1) Perry & Harrison, J. Phys. Chem. B. 99, 9960 (1995). X or [011] Sliding velocity affects the shape of the stick-slip curve. Friction in the Presence of a Third Body Perry and Harrison, J. Phys. Chem. B, 101, 1364-1373 (1997).
Center-of-mass trajectories of the methane molecules Perry and Harrison, Langmuir, 1, 455 (1996). Power spectrum of third-body molecule Perry and Harrison, Thin Solid Films, 90-91, 11-15 (1996): Langmuir 1, 455 (1996) High load slide Low load slide
MD Simulations: Third-body molecules and small attached groups Perry & Harrison, J. Phys. Chem. B. 101, 1364 (1997). Tribochemistry: Sliding-induced chemical reactions! Harrison and Brenner, J. Am. Chem. Soc., 116, 10399-1040 (1994)
Issues - MEMS Coatings Self-Assembled Monolayers Si MEMs experience stiction and wear SAMs alter and control chemical nature of surface Degradation of MEMS Dormancy Effect of water Courtesy of: M. Chandross, E.B. Webb III, M.J. Stevens, G.S. Grest Sandia National Laboratories, Albuquerque, NM Early MD studies of opposing SAMs Glosli and McClelland, Phys. Rev. Lett. 70, 1960 (1993). Au interface Au NVT United-atom potential
Early MD studies of opposing SAMs Glosli and McClelland, Phys. Rev. Lett. 70, 1960 (1993). Figs a-c (larger interaction strength) and figs d-f (smaller interaction strength) show sawtooth pattern of shear stress. The line thickens as temperature is increased, due to vibrational excitations Figs g-i (larger interaction strength) and fig j (smaller interaction strength) show the heat flow into the thermostat. Each pluck is the conversion of mechanical energy (as strain) to thermal energy. SPM experiments: LB Films & Self-Assembled Monolayers End Group Kim et al. Langmuir 13 (1997) 719. Kim et al. Langmuir 15 (1999) 3179. J. Houston and G. J. Leggett Packing Density Lee et al. Langmuir 16 (000) 0. Meyer et al. Thin Solid Films 0 (199) 13. Overney et al. Langmuir 10 (1994) 181. Briscoe and Evans Proc. R. Soc. A 380 (198) 389. Chain Length Xiao et al. Langmuir 1 (1996) 35. Harrison and Perry MRS Bull. 3 (1998) 7. Lio et al. J. Phys. Chem. B 101 (1997) 3800.
Xiao et al. Langmuir 1 (1996) 35. Lio et al. J. Phys. Chem. B 101 (1997) 3800. AFM: Alkanethiols on Au AFM: Silanes on Si Harrison and Perry, MRS Bull. 3 (1998) 7.
Tutein, Stuart, and Harrison Langmuir 16 (000) 91.
Friction of Alkylsilane Monolayers Courtesy of: M. Chandross, E.B. Webb III, M.J. Stevens, G.S. Grest Sandia National Laboratories, Albuquerque, NM Microscopic Stick-Slip Slip Motion 00 MPa Applied Load Stick-slip for all chain lengths, loads and v Shear for 0 Å for steady state stickslip Other simulations run for less than 1 ns Chain length 1, m/s relative velocity Courtesy of: M. Chandross, E.B. Webb III, M.J. Stevens, G.S. Grest Sandia National Laboratories, Albuquerque, NM
Microscopic Stick-Slip Slip n=8 @ GPa & 0.m/s Plots show: shear stress vertical movement of tail groups # gauche defects Chains tilt & stretch during stick, pop back into place during slip, increasing defects. Courtesy of: M. Chandross, E.B. Webb III, M.J. Stevens, G.S. Grest Sandia National Laboratories, Albuquerque, NM
z, F z L θ y, F y φ Slide x, F x Mikulski & Harrison, Tribol. Lett., 10, 9-38 (001).
Mikulski and Harrison, Tribol. Lett. in press. Tribochemistry?
Tribochemistry Amorphous carbon films (or DLC) Snapshots of sp-carbon (red) in amorphous carbon film (yellow) reacting with hydrogen terminated diamond counterface. From left to right -> sp-hybridized carbon, to react, shown in red -> chemical bond formed between sp-hybridized and counterface, with restructuring of film -> as a result of the adhesive bond formation, film undergoes significant restructuring -> adhesive bonds rupture, more restructuring, now counterface atoms are transferred to amorphous carbon film (gray and green spheres in film) Hybridization analyses confirm intra- and inter-film bonds broken and formed, during shear Gao, et. al., J. Phys. Chem. B 107, 1108-11090 (00)
Tribochemistry Amorphous carbon films (or DLC) MD Simulation Results Experimental Results Continued sliding for long times causes a reduction in the friction because the nature of the interface (or film) has been altered via chemical reactions. This may be analogous to the run-in observed in macroscopic friction experiments on NCD films. Gao, et al., J. Am. Chem. Soc. in press. Thin-film system Run-in? load= ~300 nn NCD films (or smooth DLC) exhibit a period of high friction when rubbing starts, called run-in. Continued rubbing leads to wear and reduces the friction. A. Erdemir et al., Surf. Coat. Technol. 10-11, 565 (1999).
Number of C-C Bonds that remain intact during sliding. Gao, Mikulski & Harrison, J. Am. Chem. Soc.. Thin-film system Relationship between Friction and Bond Breaking and Making. Thin-film system low friction
Kim et al. Langmuir 13 (1997) 719. Friction of Alkylsilane Monolayers Courtesy of: M. Chandross, E.B. Webb III, M.J. Stevens, G.S. Grest Sandia National Laboratories, Albuquerque, NM
Why High & Low Shear Stress for CF 3? Low stress case has a line defect a line of CF 3 slides over the line below High stress case is well ordered low stress high stress Courtesy of: M. Chandross, E.B. Webb III, M.J. Stevens, G.S. Grest Sandia National Laboratories, Albuquerque, NM Calculate the hexatic order parameter, Order in Monolayer under Shear Q = 1 N 6 exp(6iθ) θ High stress CF 3 has largest Q predominantly constant Both CF 3 have larger Q than CH 3 CH 3 CF 3 CF 3 high stress Courtesy of: M. Chandross, E.B. Webb III, M.J. Stevens, G.S. Grest Sandia National Laboratories, Albuquerque, NM