Tài liệu Handbook of Micro and Nano Tribology P7 pptx

For microscale scratching, microscale wear, nanoscale indentation hardness measurements, and fabrication/nanomachining, a three-sided pyramidal single-crystal natural diamond tip with an

Bhushan, B “Microscratching/Microwear, Nanofabrication/Nanomachining ”

Handbook of Micro/Nanotribology

Ed Bharat Bhushan Boca Raton: CRC Press LLC, 1999

Microscratching/

Microwear, Nanofabrication/ Nanomachining, and Nano/Picoindentation

Using Atomic Force Microscopy

Bharat Bhushan

7.1 Introduction7.2 Experimental Techniques

AFM for Microscratching/Microwear and Nanoindentation • Nano/Picoindenter

7.3 Microscratching/Microwear Studies7.4 Nanofabrication/Nanomachining Studies7.5 Nano/Picoindentation

7.6 ClosureReferences

7.1 Introduction

Wear of sliding surfaces can occur by one or more wear mechanisms, including adhesive, abrasive, fatigue,impact, corrosive, and fretting The wear rate, a measure of wear, generally needs to be minimized(Bhushan, 1996) As the dimensions of components and loads used continue to decrease (such as inmicroelectromechanical systems or MEMS), scratching/wear and mechanical properties at the micro- tonanoscales become very important With the advent of the newly developed scanning probe microscopes(SPMs), particularly the atomic force microscope (AFM), it is possible to study the interfacial phenomena

at a small scale and light load The AFM/FFM tip simulates a sharp single asperity traveling over a surface.Scratching and wear processes at different normal loads are studied in AFMs by using a sharp diamondtip This tip can also be used for nanofabrication/nanomachining AFMs, in conjunction with specialsensors, are used for measurement of mechanical properties on nano- to picoscales

This chapter presents an overview of microscratching/microwear, nanofabrication/nanomachining,and nano/picoindentation using AFM and related instrumentation.

7.2 Experimental Techniques

7.2.1 AFM for Microscratching/Microwear and Nanoindentation

Commercial AFMs are commonly used to conduct microscratching/microwear and nanoindentation.Special sensors may be used in conjunction with AFMs for nano/picoindentation studies

For microscale scratching, microscale wear, nanoscale indentation hardness measurements, and fabrication/nanomachining, a three-sided pyramidal single-crystal natural diamond tip with an apexangle of 80°and a radius of about 100 nm mounted on a stainless steel cantilever beam with high normalstiffness of about 25 N/m is used at relatively high loads (1 to 150 µN); see Chapter 1 for further details(Bhushan et al., 1994a) For scratching and wear studies, the sample is generally scanned in a directionorthogonal to the long axis of the cantilever beam (typically at a rate of 0.5 Hz) so that friction forcecan be measured during scratching and wear The tip is mounted on the beam such that one of its edges

nano-is orthogonal to the long axnano-is of the beam; therefore, wear during scanning along the beam axnano-is nano-is higher(about 2 to 3 times) than that during scanning orthogonal to the beam axis For wear studies, typically

an area of 2 × 2 µm is scanned at various normal loads (ranging from 1 to 100 µN) for a selected number

of cycles For nanofabrication/nanomachining, the nanoscratching operation is extended

For nanoindentation hardness measurements, the scan size is set to zero and normal load is applied

to make the indents (Bhushan et al., 1994a,d) During this procedure, the diamond tip is continuouslypressed against the sample surface for about 2 s at various indentation loads Nanohardness is calculated

by dividing the indentation load by the projected residual area of the indents

Sample surface is scanned before and after the scratching, wear, or indentation to obtain the initialand the final surface topography, at a low normal load of about 0.3 µN using the same diamond tip Anarea larger than the scratching, wear, and indentation region is scanned to observe the marks

7.2.2 Nano/Picoindenter

As stated earlier, conventional AFMs have been used for indentation studies on nanometer-scale depths

In these studies, the hardness value is based on the projected residual area after imaging the indent.Direct imaging of the indent allows one to quantify piling up of ductile material around the indenter.However, it becomes difficult to identify the boundary of the indentation mark with great accuracy Thismakes the direct measurement of the contact area somewhat inaccurate (Bhushan et al., 1994a,d) Atechnique with the dual capability of depth sensing as well as in situ imaging is most appropriate innanomechanical property studies (Bhushan et al., 1996) This indentation system is used to make load-displacement measurement and subsequently carry out in situ imaging of the indent A schematic of thenano/picoindenter system used is shown in Figure 7.1 The indentation system consists of a three-platetransducer with electrostatic actuation hardware used for direct application of normal load and a capac-itive sensor used for measurement of vertical displacement The AFM head is replaced with this transducerassembly while the specimen is mounted on the piezoelectric scanner which remains stationary duringindentation experiments The transducer consists of a three (Be–Cu) plate capacitive structure whichprovides high sensitivity, large dynamic range, and a linear output signal with respect to load or dis-placement The tip is mounted on the center plate The upper and lower plates serve as drive electrodes.Load is applied by applying appropriate voltage to the drive electrodes, thereby generating an electrostaticforce between the center plate and the drive electrodes Vertical displacement of the tip (indentationdepth) is measured by measuring the displacement of the center plate relative to the two outer electrodesusing capacitance technique The load resolution is 100 nN or better, and the displacement resolution is0.1 nm At present, a load range of 1 µN to 10 mN can be employed Loading rates can be varied bychanging the load/unload period The AFM functions as the platform providing an in situ image of theindent with a lateral resolution of 1 nm and a vertical resolution of 0.2 nm The load–displacement data

can be acquired and displayed on the display monitor Hardness value can be obtained from the placement data, as well as from direct measurement of the projected residual area of the indent afterimaging Young’s modulus of elasticity is obtained from the slope of the unloading curve.

load–dis-A three-sided Berkovich indenter with tip radius of about 100 nm is generally used for the ments, see Chapter 10 on Nanomechanical Properties in this book Sharper diamond tips with includedangle of 60 to 90° and tip radii of 30 to 60 nm are sometimes employed for shallower indentation (onthe order of 1 nm) To obtain an accurate relation between the indentation depth and the projectedcontact area, tip shape calibration needs to be done Also for surfaces with rms roughness on the order

measure-of indentation depth, the original (unindented) prmeasure-ofile is subtracted from the indented prmeasure-ofile (Bhushan

et al., 1994a,d)

In a typical indentation experiment, the tip is lowered close to the sample (ideally <100 µm) Scansize and scan rate are selected The tip is engaged to the sample surface by a stepper motor with a setpoint of 1 nA (about 1 µN) A desired image area is captured prior to indentation The feedback is set

to zero to disable the scanner; the scan size is set to zero so that the indenter will be positioned at thecenter of the image An appropriate set point for the preload condition is selected The indentation ratecan be varied by changing the load/unload period

7.3 Microscratching/Microwear Studies

By using a standard or sharp diamond tip mounted on a stiff cantilever beam, AFMs can be used toinvestigate how surface materials can be moved or removed on micro- to nanoscales, for example, inscratching and wear (where these things are undesirable) and nanofabrication/nanomachining (wherethey are desirable) (Hamada and Kaneko, 1992; Miyamoto et al., 1991; Bhushan and Koinkar, 1994b,1995a–c,e, 1997; Bhushan et al., 1994 a,c; 1995d; Bhushan, 1995a,b, 1997, 1998a,b; Koinkar and Bhushan,1997a,b) A variety of polymers and ceramics and hard coatings have been studied Many examples ofscratching/wear of magnetic recording materials have been presented in Chapter 14 on magnetic storagedevices in this book This chapter focuses on the studies with silicon material

Figure 7.2a shows microscratches made on Si(111) at various loads (Bhushan and Koinkar, 1994b)

As expected, the scratch depth increases linearly with load Such microscratching measurements can beused to study failure mechanisms on the microscale and to evaluate the mechanical integrity (scratchresistance) of ultrathin films at low loads To study the effect of scanning velocity, unidirectional scratches,

5 µm in length, were generated at scanning velocities ranging from 1 to 100 µm/s at normal loads rangingfrom 40 to 140 µN There is no effect of scanning velocity obtained at a given normal load For repre-sentative scratches profiles at 80 µN, see Figure 7.2b (Koinkar, 1997) Insensitivity to scanning velocitymay be because of a small effect of frictional heating with the change in scanning velocity used here.Furthermore, for a small change in interface temperature, there is a large underlying volume to dissipatethe heat generated during scratching (Bhushan, 1998a)

By scanning the sample in two dimensions with the AFM, wear scars are generated on the surface

Figure 7.3 shows the effect of normal load on the wear rate We note that wear rate is very small below

20 µN of normal load A normal load of 20 µN corresponds to contact stresses comparable to the hardness

of the silicon Primarily, elastic deformation at loads below 20 µN is responsible for low wear (Bhushan

et al., 1995d; Bhushan and Kulkarni, 1995f; Koinkar and Bhushan, 1997b)

Typical wear mark generated at a normal load of 40 µN for one scan cycle and imaged using AFM at

300 nN load is shown in Figure 7.4a (Koinkar and Bhushan, 1997b) The inverted map of a wear markshown in Figure 7.4b indicates the uniform material removal at the bottom of the wear mark Next weexamine the mechanism of material removal on microscale at low loads, in AFM wear experiments

Figure 7.5 shows a secondary electron image of a wear mark and associated wear particles The specimenused for the SEM was not scanned after initial wear, to retain wear debris in the wear region Wear debris

is clearly observed An AFM image of the wear mark shows small debris at the edges, swiped during AFMscanning Thus, the debris is “loose” (not sticky) and can be removed during the AFM scanning SEMmicrographs show both cutting-type and ribbonlike debris TEM studies were performed to understand

FIGURE 7.1 Schematics of (a) indentation system, (b) three-plate transducer with electrostatic actuation hardware and capacitance

FIGURE 7.1

the material removal process The TEM micrograph of the worn region in Figure 7.6 shows evidence ofbend contours passing through the wear mark The bend contours around and inside the wear marksuggest that there are some residual stresses around and inside the wear mark region There is nodislocation activity or cracks observed inside the wear track The dislocation activity and/or crackingprobably occurs at the subsurface Based on SEM and TEM studies, it is believed that the material in theexperiment described here is removed in a brittle manner without much plastic deformation (dislocationactivity).

Finally, we study evolution of wear of a diamondlike carbon (DLC) coated disk substrate, Figure 7.7

(Bhushan et al., 1994a) The data illustrate how the microwear profile for a load of 20 µN develops as afunction of the number of scanning cycles Wear is not uniform, but is initiated at the nanoscratchesindicating that the nanoscratches (with high surface energy) and nonuniform coverage of DLC at nano-scratches act as initiation sites Thus, scratch-free surfaces will be relatively resistant to wear

FIGURE 7.2 Surface plots of (a) Si(111) scratched for ten cycles at various loads and a scanning velocity of 2 µm/s (note that the x- and y-axes are in µm and the z-axis is in nm), and (b) Si(100) scratched in one unidirectional scan cycle at a normal load of 80 µN and different scanning velocities.

7.4 Nanofabrication/Nanomachining Studies

Scanning tunneling microscopes (STMs) have been used to form nanofeatures by localized heating or

by inducing chemical reactions under the STM tip (Abraham et al., 1986; Silver et al., 1987; Albrecht

et al., 1989; Utsugi, 1990; Kobayashi et al., 1993), and nanomachining (Parkinson, 1990) AFMs have alsobeen used for nanofabrication (Majumdar et al., 1992; Bhushan et al., 1994a,c; Bhushan, 1995a,b, 1998a,b;Tsau et al., 1994) and nanomachining (Delawski and Parkinson, 1992)

Figure 7.8 shows an example of nanofabrication The word “OHIO” was written on a (100) crystal silicon wafer by scratching the sample surface with a diamond tip at specified locations andscratching angles (Bhushan, 1995a) The normal load used for scratching (writing) was 50 µN and thewriting speed was 0.2 µm/s Each line is scribed manually and debris at the ends of each line is visible

single-A few lines are not connected to each other because of the PZT drift and hysteresis Sufficient time should

be given for the thermal stabilization of the PZT scanner so that the hysteresis effect is small during thenanofabrication Next, more complex patterns were generated at a normal load of 15 µN and a writingspeed of 0.5 µm/s, Figure 7.9 (Koinkar, 1997) Such a type of patterns is useful for resistor trimming (toincrease the path resistor) on a small scale The separation between lines is about 50 nm In Figure 7.9a,the variation in line width is due to the tip asymmetry A spiral pattern generated as shown in Figure 7.9b.Nanofabrication parameters — normal load, scanning speed, and tip geometry — can be controlledprecisely to control depth and length of the devices

Nanofabrication using mechanical scratching has several advantages over other techniques (Koinkar,1997) Better control over the applied normal load, scan size, and scanning speed can be used fornanofabrication of devices Using this technique, nanofabrication can be performed on any engineeringsurface Use of chemical etching or reactions is not required, and this dry nanofabrication process can

be used where use of chemicals and electric field is prohibited One disadvantage of this technique is theformation of debris during scratching At light loads, debris formation is not a problem compared withhigh-load scratching However, debris can be removed easily out of the scan area at light loads duringscanning

7.5 Nano/Picoindentation

Nanohardness measurements using conventional AFMs is covered in Chapter 14 on magnetic storagedevices In this chapter, we will limit the discussion to the application of the three-plate transducer with

FIGURE 7.3 Wear depth as a function of normal load for Si(111) after one cycle (From Koinkar, V N and Bhushan,

B (1997), J Mater Res., 12, 3219–3224 With permission.)

electrostatic actuation hardware used in conjunction with conventional AFMs (Bhushan et al., 1996;Kulkarni et al., 1996a,b, 1997; Bhushan, 1997, 1998a,b; Bhushan and Koinkar, 1997; and Koinkar andBhushan, 1997a).

Figure 7.10a shows the load–displacement curves at different peak loads for Si(100) ment data at residual depth as low as about 1 nm can be obtained Loading/unloading curves are notsmooth, but exhibit sharp discontinuities particularly at high loads (shown by arrows in the figure) Anydiscontinuities in the loading part of the curve probably result from slip of the tip The sharp disconti-nuities in the unloading part of the curves are believed to be due to formation of lateral cracks that form

Load–displace-at the base of median crack, which results in the surface of the specimen being thrust upward From theload–displacement curves in Figure 7.10 the indentation hardness of surface films with an indentationdepth of as small as about 1 nm has been measured Triangular indentations are observed for shallowpenetration depths, Figure 7.10b

FIGURE 7.4 (a) Typical gray scale and (b) inverted AFM images of a wear mark created using a diamond tip at a normal load of 40 µN and one scan cycle on Si(111) surface (From Koinkar, V N and Bhushan, B (1997), J Mater Res., 12, 3219–3224 With permission.)

Figure 7.11 shows the load–displacement curves during three loading and unloading cycles for crystal silicon The unloading and reloading curves reveal a large hysteresis, which shows no sign ofdegeneration through three cycles of deformation and the peak load displacement shift to higher values

single-in successive loadsingle-ing–unloadsingle-ing cycles Pharr et al (1989, 1990), Page et al (1992), and Pharr (1992)have also observed hysteresis behavior in silicon at similar loads using a nanoindenter The fact that thecurves are highly hysteretic implies that deformation is not entirely elastic Pharr (1992) concluded thatlarge hysteresis is due to a pressure-induced phase transformation from its normal diamond cubic form

to a β-tin metal phase

Table 7.1 summarizes the hardness and Young’s modulus of eleasticity data at various depths for crystal silicon (Bhushan et al., 1996) Comparison of nanohardness values with that of bulk hardness

single-FIGURE 7.5 Secondary electron image of wear mark and debris for Si(111) produced at a normal load of 40 µN and one scan cycle (From Koinkar, V N and Bhushan, B (1997), J Mater Res., 12, 3219–3224 With permission.)

FIGURE 7.6 Bright-field TEM micrograph showing wear mark and bend contour around and inside the wear mark

in Si(111) produced at a normal load of 40 µN and one scan cycle (From Koinkar, V N and Bhushan, B (1997),

J Mater Res., 12, 3219–3224 With permission.)

FIGURE 7.7 Surface plots of DLC-coated thin-film disk showing the worn region; the normal load and number of test cycles are indicated (Bhushan, B et al (1994), Proc Inst Mech Eng Part J: J Eng Tribol. 208, 17–29 With permission.)