AFM Probe-Based Data Recording Technology

AFM Probe-Based Data Recording Technology Prof. Bharat Bhushan bhushan.2@osu.eduh Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 1

Introduction Objectives and Approach Outline Experimental Tip shape characterization Friction and wear measurements Mechanical properties measurements Experimental samples Results Pt-coated tips Other noble metal-coated tips Nanotribological characterization Nanomechanical characterization Role of lubricants, scanning velocity, operating environment Surface treated tips Nanotribological characterization Nanomechanical characterization Electrical and surface characterization Conclusions Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 2

Introduction Background Magnetic and optical recording technologies are used for nonvolatile data storage. These technologies are reaching limit of data recording density of 500 Gb/in 2 and 100 Gb/in 2, respectively. Battle: Flash vs. hard drives Both flash memories and micro-disk drives have begun to replace each other mostly in portable but also in some fixed drives which require low storage capacity. Probe-based based recording technology It has the potential of extremely areal recording density of several Tb/in 2 or higher. Based on thermomechanical recording, IBM has developed a technology which uses an array of 1024 silicon cantilevers (Millipede). Phase-change memory (PCM) uses chalcogenide alloys. A third technique is ferroelectric memory with typically lead zirconate titanate (PZT) medium. Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 3

Intergrated tip heaters consist of tips of nanoscale dimension. Thermomechanical recording is performed on an about 40- nm thick polymer medium on Si substrate. Heated tip to about 400 o C contacts with the medium for recording. Wear of the heated tip is an issue. 32 x 32 tip array (http://www.ibm.com) Probe-based NEMS data storage based on thermomechanical recording Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 4

Schematic of the electrical probe storage system using phase-change media (Wright et al., 2006). In phase-change media, electric current of different magnitudes are passed to a chalcogenide material (Ge 2 Sb 2 Te 3 ) using a conducting Pt-coated AFM probe Local joule heating is used to change the structure. Binary recording: generating difference levels of high and low resistance on chalcogenide medium High current (through electrode probe) heating of medium to more than 630 o C cooling to amorphous state high resistance ( 1 ) Low current (through electrode probe) heating of medium to less than 630 o C cooling to crystalline state low resistance ( 0 ) Wear of the tip and medium at 630 o C is an issue. Probe-based NEMS data storage based on phase change memory B. Bhushan and K. Kwak, Nanotechnol. 18, 345504 (2007) Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 5

Ferroelectric material, typically lead zirconate titanate (PZT) Electrical current switches between two different polarization states by applying short voltage pulses (~10 V, ~100 μs), resulting in recording. Temperature rise on the order of 80 o C is expected. Piezoresponse force can be read out by applying an AC voltage of 1 V. Wear of the tip and medium at 80 o C is an issue, but to a lesser degree. Furthermore, the tip does not need to be in contact with medium during readback. Probe-based NEMS data storage based on ferroelectric recording B. Bhushan and K. Kwak, J. Phys.: Condens. Matter 20, 225013 (2008) Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 6

Issues For fast data rate, the cantilever array needs to be moved at high velocities on the order of 100 mm/s. Wear of the tip and medium is an issue. In order to achieve high wear resistance and long lifetime, a high surface hardness of metal-coated tip with high electrical conductivity is essential. Durability is a concern for soft metals such as pure Au. An alloy such as AuNi 5 of a hard contact material, used in relays, is of interest. Lubricant coating on substrate surface should be optimized for friction and wear protection. Tip wear mechanism in nanoscale region is not well-understood, especially with various top metal layers on the probe tip. B. Bhushan and K. Kwak, J. Phys.: Condens. Matter 20, 225013 (2008) Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 7

Objectives Objectives and Approach For comprehensive investigation of friction and wear of the tip sliding against PZT film, perform a wear study at a range of loads, distances and temperature to compare wear resistance of metal-coated top layers with various noble metals and their alloys. Evaluate tribological performance of lubricants applied on PZT Evaluate mechanical properties of noble metal coatings and PZT Approach Use silicon grating and software to deconvolute tip shape in order to characterize the change in the tip shape and evaluate the tip radius and its wear volume. Adhesive force and coefficient of friction measurement to evaluate lubricant tribological performance. Surface potential and resistance mapping after wear test on PZT. Nanoindentation and nanoscratch to evaluate hardness, elastic modulus, creep and scratch resistance. B. Bhushan and K. Kwak, J. Phys.: Condens. Matter 20, 225013 (2008); M. Palacio and B. Bhushan, Nanotechnology, 19, 105705 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 8

Tip shape characterization Experimental (a) A schematic of grating (with array of sharp tips) on silicon wafer surface, (b) illustration of tip characterization and calculation of tip radius, and (c) illustration of calculation of the wear volume. B. Bhushan and K. Kwak, Nanotechnol. 18, 345504 (2007); Ibid., J. Phys.: Condens. Matter 20, 225013 (2008) Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 9

Schematics and photographs of a triangular V-shaped and rectangular cantilevers with metal-coated layer B. Bhushan and K. Kwak, J. Phys.: Condens. Matter 20, 225013 (2008) Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 10

Friction and wear measurements The friction force experiments were carried out by scanning the sample along an axis perpendicular to the long axis of the cantilever, at a scan velocity of 1 μm/s using a scan rate of 0.5 Hz at 1 to 80 nn normal loads. The measured friction force was plotted as a function of normal load. The data could be fitted with a straight line which suggests that friction force is proportional to normal load. The coefficient of friction was obtained by calculating the slope of the line. For Pt wear experiments, the tip was slid on a PZT film sample for 1 m at a normal load of 50 nn, followed by 1 m at 100 nn in contact mode at velocities ranging from 0.1 to 100 mm/s. Scan direction was parallel to the long axis of the cantilever beam. In order to obtain the tip profile to calculate wear volume, the tip was scanned before sliding and after wear test on the grating sample in tapping mode and in a direction perpendicular to the long axis of cantilever beam. Scanning was performed on 2 x 2 μm 2 scan area with a velocity of 1 μm/s. For other noble metal-coated tips, the tip profiles were obtained before and after 1, 10 and 100 m sliding at 10 mm/s, and 300 m sliding at 100 mm/s and at 100 nn. To evaluate the effect of lubrication on PZT wear, a diamond tip was used to create a 5 x 5 μm 2 wear scar. Afterwards, the surface potential and contact resistance image of the worn surface was imaged with a conducting AFM tip. B. Bhushan and K. Kwak, J. Phys.: Condens. Matter 20, 225013 (2008); M. Palacio and B. Bhushan, J. Vac. Sci. Technol. A, in press (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 11

Mechanical properties p measurements Hardness and elastic modulus were evaluated using a NanoIndenter II (MTS) in the continuous stiffness mode (CSM) equipped with a diamond Berkovich tip. The maximum indentation displacement was controlled to 250 nm for Pt, Pt-Ni and Au-Ni, and 50 nm for Pt, Pt-Si and PZT. Creep experiments were performed using CSM with the diamond Berkovich indenter tip penetrating the coatings at a rate of 100 μn/s. The tip was held for 600 s at the maximum load. Scratch experiments were carried out using a conical diamond indenter tip with 1 μm radius of curvature and 90 o included angle. Scratches made were 500 μm long, and the scratch-induced damage was evaluated by scanning electron microscopy (SEM). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 12

Experimental samples Details of noble metal-coated probes Tip Noble metal Stiffness (N/m) and initial 2-D tip radius (nm) Thickness of metal films (nm) PZT flat sample Pt Au-Ni ~2 N/m, 74 nm (CSC21, MikroMasch) ~2.8 N/m, 65 nm Au-Ni (PPP-FM, Nanosensors) 185 nm/15 nm Pt/Cr (Sputter deposition) 65 nm/10 nm Au-Ni/Cr (Sputter deposition with Au-Ni alloy, and sputtering with Cr target, respectively) -15 nm/50 nm PbZr 0. 2 Ti 0.8 O 3 /SrRuO 3 film on 0.5 mm c-axis SrTiO 3 (Pulse laser deposition) Pt-Ir ~2.8 N/m, 53 nm Pt-Ir (PPP-EFM, Nanosensors) 25 nm/3 nm Pt-Ir/Cr (Sputter deposition) Pt-Ni ~2.8 N/m, 61 nm Pt-Ni (PPP-FM, Nanosensors) 95 nm/10 nm Pt-Ni/Ni (Co-sputtering with separate Pt and Ni targets, and sputtering with Ni target, respectively) Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 13

Physical properties of materials used in probe tips Material Crystal structure Density (g/cm 3 ) Melting point ( o C) Electrical resistivity at 0 o C (mωcm) Coeff. of linear thermal expansion Tensile strength (MPa) (10 (x10-6o C -1 ) Noble metals Pt fcc a 21.45 a 1769 a 9.85 a 9.1 a 207-241 (asworked) a 124-165 (annealed) Elongation in 50 mm (%) 1-3 (asworked) a 30-40 (annealed) Hardness (GPa) 0.91 (asworked) a 0.36 (annealed) 0.42 (as-cast) Elastic modulus (GPa) Poisson s ratio 171 a 0.39 a Au fcc a 19.32 a 1064 a 2.06 a 14.2 a 207-221 221 (as- 4(as-worked) a 0.56 (as- 77 a 0.42 a worked) a 39-45 worked) a 124-138 (annealed) (annealed) 0.26 (annealed) Ir fcc a 22.65 a 2447 a 4.71 a 6.8 a 2070-2480 (hotworked) a 1103-1241 15-18 (hotworked) a 20-22 0.33 (as-cast) 6.4 (as-worked) a 2.2 (annealed) 2.2 (as-cast) 517 a 0.26 a (annealed) (annealed) Noble metal alloys Pt-Ni fcc b - - - - - - - - - Au-Ni (sp.) fcc b - - - - - - 7 c 130 c - Other materials Ni fcc a 8.90 a 1445 a 6.8 a 13.3 a 462 (annealed) e 47 e 0.45 (annealed) e 204 a 0.31 e Si(100) diamond 2.33 d 1420 d - 42 d 130 e - 13 f 180 f 0.28 e e a Davis (1998) e Kabo et al. (2004) b Hultgren et al. (1963) f Davila et al. (2007) c Baker and Nix (1994) sp. - sputtered d Bhushan and Gupta (1991) Alloys of noble metals have increased hardness and elastic modulus relative to pure noble metals. Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 14

Physical, thermal and electrical properties of BMIM-PF 6 and Z-TETRAOL 1-Butyl- Z-TETRAOL 3-methylimidazolium hexafluorophosphate (BMIM-PF 6 ) Cation C 8 H 15 N + 2 - Anion PF - 6 - Molecular weight (g/mol) 284 a 2300 b T melting ( o C) 10 c - T decomposition ( o C) 300 c ~320 b Density (g/cm 3 ) 137 1.37 a 175 1.75 b Kinematic viscosity (mm 2 /s) 281 a (20 o C) 2000 b (20 o C) 78.7 d (40 o C) Pour point ( o C) <-50 e -67 b Specific heat (J/g K) 1.44 f (25 o C) ~0.20 b (50 o C) Thermal conductivity at 25 o C 0.15 g ~0.09 b (W/m-K) Dielectric strength at 25 o C - ~30 b (kv/mm) Volume resistivity 714 ~10 13 b (Ω cm) Vapor pressure at 20 o C (Torr) <10-9 ~10-12 b Wettability on Si moderate c - Water contact angle 95 oh 102 oh Miscibility with isopropanol Total a - Miscibility with water - - a Merck Ionic Liquids Database, Darmstadt, Germany e Kabo et al. (2004) b Z-TETRAOL Data Sheet, Solvay Solexis Inc., Thorofare, NJ f Kabo et al. (2004) c Kinzig and Sutor (2005) g Frez et al. (2006) d Reich et al. (2003) h Palacio and Bhushan (2008); For comparison, the contact angle of PZT is 88 o. Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 15

Selected physical and thermal properties of bulk PZT and polycrystalline diamond PZT Poly. diamond Physical Elastic modulus, E (GPa) 200 1140 a Hardness, H (GPa) 13 80 a,b Poisson s ratio, ν 0.25 c 0.07 a Density (kg/m 3 ) 7.8x10 3 - Thermal Thermal conductivity, k (W/m K) 1.60 d (@ 227 o C) 400 a Thermal diffusivity, κ (m 2 /s) 0.60x10-6,d (@ 227 o C) - Specific heat at constant 034 0.34 d (@ 227 o C) 052 0.52 a (@ 27 o C) pressure, c p (kj/kg K) a Field (1992) b Bhushan and Gupta (1991) c assumed d Morimoto et al. (2003) A polycrystalline diamond tip was used to create wear scars on the PZT surface. Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 16

Pt-coated tips Effect of tip coating Results The wear volume increases with an increase of velocity, typical of adhesive and abrasive wear modes. At higher velocities, wear could be caused by the adhesive wear and periodical high velocity impact on the PZT film surface. The wear volume increases as the logarithm of velocity up to between 0.1 and 1 mm/s and then levels off. This wear behavior at lower sliding velocities is associated with thermally activated atomicscale stick-slip. K. Kwak and B. Bhushan, J. Vac. Sci. Technol. A 26, 783 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 17

From SEM images, significant wear is observed in the case of the higher sliding velocity as compared to those of the lower velocities. The mechanism for tip wear is adhesive and abrasive wear. At higher velocity, impact wear could be caused by periodic high velocity impact of asperities on the PZT film. K. Kwak and B. Bhushan, J. Vac. Sci. Technol. A 26, 783 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 18

At 0.1 mm/s sliding velocity, wear scars are observed on the PZT film. Black arrows are used to identify wear scars and white arrows indicate significant damage. The line profile shows wear depth in the range of 1.5-2.2 nm after sliding at 100 nn. At a velocity of 100 mm/s, wear scars are not observed on the PZT film. Here, the sliding cycles are calculated to be 500 cycles lower than that at lower velocities. Wear scar on the PZT film is not distinguishable due to this small number of the sliding cycles. K. Kwak and B. Bhushan, J. Vac. Sci. Technol. A 26, 783 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 19

At both loads of 50 nn and 100 nn, the wear volume is higher at 80 o C than at 20 o C after sliding on the unlubricated sample. The increase is associated with the degradation of the mechanical properties of the Pt coating. K. Kwak and B. Bhushan, J. Vac. Sci. Technol. A 26, 783 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 20

Other noble metal-coated tips Nanotribological characterization A normal load of 100 nn can be used with measurable wear and was selected from the baseline experiment. This procedure was used for tests t on other noble metal-coated t tips. Reduction in height indicates tip blunting resulting from wear and is seen in all cases. B. Bhushan and K. Kwak, J. Phys.: Condens. Matter 20, 225013 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 21

The wear of Pt-coated tip is higher than other metal-coated tips. The Pt-coated tip surface is significantly softer than the PZT film surface. The other noble metal-coated tips are harder so they exhibit less wear compared to the Pt-coated tip. Pt-Ir-coated tip shows highest wear resistance and will be used for further studies on lubrication of PZT. B. Bhushan and K. Kwak, J. Phys.: Condens. Matter 20, 225013 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 22

SEM images show plastic deformation of the tip, which is indicative of adhesive wear. Brittle Pt-coated silicon asperities can fracture when sliding against the film surface and produce particles. These particles stay between the contacting surfaces and could accelerate the abrasive wear. At high velocities, wear is caused by periodic impact of asperities on the PZT surface in all cases. B. Bhushan and K. Kwak, J. Phys.: Condens. Matter 20, 225013 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 23

Nanomechanical characterization PZT has H and E of about 13 and 200 GPa, respectively. Scratch deformation of PZT is a combination of plastic and brittle modes. M. Palacio and B. Bhushan, J. Vac. Sci. Technol. A, 26, 768 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 24

The alloying of Pt has a significant effect on improving its modulus and hardness. M. Palacio and B. Bhushan, Nanotechnology, 19, 105705 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 25

The noble metal coatings creep at the nanoscale. Alloyed coatings exhibit less creep compared to Pt. M. Palacio and B. Bhushan, Nanotechnology, 19, 105705 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 26

Deformation of the noble metal coatings from scratch is mainly plastic, and the Pt alloys exhibit less damage compared to Pt film. M. Palacio and B. Bhushan, Nanotechnology, 19, 105705 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 27

Role of lubricants, scanning velocity, operating environment The increase in the tip radius after sliding leads to a larger contact area leading to higher adhesion, which increases the friction force and the measured value of coefficient of friction. Adhesion, friction and wear data for the Pt-Ir tips against the Z-TETRAOLlubricated PZT film are the lowest, followed by the BMIM-PF 6 -lubricated PZT film. This shows that lubricants provide wear protection on both Pt-Ir and PZT surfaces. Z-TETRAOL-lubricated film exhibited the best performance and was used for velocity, temperature and relative humidity studies B. Bhushan and K. Kwak, J. Phys.: Condens. Matter, 20, 325240 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 28

The exposed underlayer of the Si part is observed in the tip against the unlubricated film. The tip is plastically deformed during wear; therefore, the mechanism for tip wear is adhesive. This shows that both lubricants provide wear protection on Pt-Ir. B. Bhushan and K. Kwak, J. Phys.: Condens. Matter, 20, 325240 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 29

Effect of velocity Wear volume initially increases as a logarithm of sliding velocity at two loads, and then it increases with sliding velocity at a slower rate with a velocity exponent in the range of 0.06 0.11. The initial logarithm dependence for both friction and wear is based on the thermallyactivated stick-slip mechanism. At higher velocity, impact wear could be caused by periodic high velocity impact of asperities on the PZT film. B. Bhushan and K. Kwak, J. Phys.: Condens. Matter, 20, 325240 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 30

Effect of temperature At higher test temperatures, wear and friction increase for both unlubricated and lubricated PZT because of the degradation of the mechanical properties of PZT. Lubricant reduces friction and wear at a given temperature. B. Bhushan and K. Kwak, J. Phys.: Condens. Matter, 20, 325240 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 31

Effect of humidity Surface water molecules aggregate with mobile lubricant fractions of Z- TETRAOL and form a large meniscus. Increase of meniscus force results in increase in wear volume and friction at 80% RH. B. Bhushan and K. Kwak, J. Phys.: Condens. Matter, 20, 325240 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 32

Surface water molecules are expected to form a large meniscus. Increase in meniscus thickness results in an increase of meniscus force and an increase in wear volume and friction at 80% RH. B. Bhushan and K. Kwak, J. Phys.: Condens. Matter, 20, 325240 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 33

Role of lubricants on PZT Adhesion and friction of PZT Adhesive force and coefficient of friction decreased upon application of Z-TETRAOL and BMIM-PF 6. After 100 cycles, lubricated surfaces exhibit a small change in coefficient of friction, i.e., low surface wear, in contrast to PZT. M. Palacio and B. Bhushan, J. Vac. Sci. Technol. A, 26, 768 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 34

Surface potential of PZT The change in surface potential is most distinct on the PZT surface because it experienced the most wear from sliding with a diamond tip. Built-up charges during the sliding did not get dissipated and remained on the PZT. For lubricated surfaces, change in surface potential is minimal, as Z-TETRAOL and BMIM-PF 6 provided adequate wear protection. M. Palacio and B. Bhushan, J. Vac. Sci. Technol. A, 26, 768 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 35

Contact resistance of PZT PZT did not show resistance change, indicating that the tip did not penetrate the entire thickness of the film during the wear test. Both lubricated surfaces did not show any observable bl resistance change, indicating that some lubricant may still be present on the surface and the substrate is not fully exposed. M. Palacio and B. Bhushan, J. Vac. Sci. Technol. A, 26, 768 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 36

Surface-treated tips Nanotribological characterization Formation of silicide in the Pt- Si interface results in decrease in the wear volume and the coefficient of friction. B. Bhushan, et al., Acta Mater., 56, 4233 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 37

Mechanism for tip wear is adhesive because the tip is plastically deformed. Blunting of the Pt-Si tip occurred to a lesser extent as observed earlier. B. Bhushan, et al., Acta Mater., 56, 4233 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 38

Nanomechanical characterization The increase in the H and E of the thermally-treated film indicates elemental composition changes such as formation of silicide. Pt film exhibits plastic deformation, while Pt-Si film exhibits brittle failure, which accounts for higher load to failure for the latter. B. Bhushan, et al., Acta Mater., 56, 4233 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 39

Electrical and surface characterization Electrical resistivity Measured resistivity of the thermally-treated film using four-point probe is in good agreement with reported resistivity values for platinum silicide. B. Bhushan, et al., Acta Mater., 56, 4233 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 40

Auger electron spectroscopy Surface elemental composition as a function of sputter time indicates that Si is diffusing through the Pt film, which may lead to formation of platinum silicide. AES data confirm results from electrical resistivity measurements. B. Bhushan, et al., Acta Mater., 56, 4233 (2008); B. Bhushan et al., J. Phys. Condens. Matter 20, 365207 (2008). Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 41

Conclusions Wear rate increases with load when Pt-coated tips are slid over the PZT film at the velocity range of 0.1 to 100 mm/s. Wear of noble metal-coated tips is primarily adhesive and abrasive wear with some impact wear. Wear and friction increase as a logarithm of sliding velocity in lower velocity range, which is due to a thermally-activated stick slip mechanism. At higher test temperatures, wear and friction increase for both unlubricated and lubricated PZT because of the degradation of the mechanical properties of PZT. The PZT film exhibits considerable scratch resistance and exhibits plastic and brittle deformation modes. Alloyed noble metal coatings have better hardness, elastic modulus and creep resistance, with Pt-Ir exhibiting the best characteristics. Their primary scratch deformation mode is plastic. Scratch results show comparable tends with wear experiments. Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 42

Silicide formation was accomplished by thermal treatment of a Pt film on a Si AFM probe, and confirmed by electrical resistivity measurements and Auger electron spectroscopy. This Pt-Si film has better hardness and elastic modulus, and exhibits brittle deformation during scratch testing. The thermal treatment makes the Pt-Si probes more wear resistant, electrically conducting, and therefore suitable for probe-based data storage. Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 43

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