Setting the Sample Up for Testing

Once an array of VACNT pillars has been grown using the methods described in Section 2.2.1.1 or milled using the FIB as described in Section 2.2.1.2, the pillar plus substrate is mounted on a stiff ‘sample puck’ using carbon paint (PELCO Colloidal Graphite, Ted Pella, Inc.) and loaded into the G200 sample tray. Typical microscope to indenter calibration, microscope focal length/sample height, and surface find techniques as described in the G200 user’s manual and applied in the default testing methods are not applicable here due the stickiness and compliance of VACNTs and the geometry of the diamond flat punch indenter tip.

The procedure for bringing the sample to the appropriate height for testing differs from the manual as follows. To get the sample at a height suitable for mechanical testing (i.e., such that testing occurs within the well-behaved range of raw displacement values deter- mined in Section 2.3), a soft polymer sample is first mounted on either a separate puck or the same puck as the VACNT sample. Both PDMS and nail polish were used for this other sample which will subsequently be refered to as the calibration sample. The calibration sample must be soft so that during a microscope to indenter calibration, the large flat punch makes a discernible mark to calibrate against. In the case where the calibration sample is

Cut-off frequency: 50 Hz

Figure 2.11: A plot of the phase angle versus the frequency of load excitation indicating the location of the cutoff frequency.

mounted on the same puck as the VACNT sample, the surface of both sample and cali- bration sample must be as close as possible to the same height. In the case that they are mounted separately, the microscope is brought into focus over the fused silica reference sample as described in the G200 user’s manual [29] before moving the calibration sample under the microscope. The sample is brought into focus manually (without using the mi- croscope motor). A Microscope to indenter calibration reveals the ‘Raw Displacement’ of the surface of the calibration sample. If this is a large number (>100µm) the calibration sample must be raised ‘Microscope to Indenter Calibration’ and the process repeated until the ‘Raw Displacement’ when the surface is found is under 100 µm but greater than20

µm. (The first calibration is likely to occur at a very large raw displacement if the flat punch described in Section 2.2.1.3 is used, as it is much shorter than a standard tip.) Now the calibration sample is near zero ‘Raw Displacement’, the correct height for testing. The microscope focus motor is used to bring the microscope into fine focus on the calibration sample before moving the test sample under the microscope. With the test sample visible under the microscope, the sample puck is raised and lowered manually until it is in focus. Now the sample is at approximately the correct height for performing mechanical tests. The reasons for this procedure are that the calibration sample and the test sample must be at the same height in order for the calibration to be sufficiently accurate and both must be near zero ‘Raw Displacement.’

Now that the sample is in place, we perform an accurate identification of the surface of the pillar. Without this point, there is no reference for determining the correct load on the sample or displacement into the sample. Traditional Testworks surface find test segments do not work with VACNTs due to their high compliance (surface is ‘found’ much later than physical contact is made with the surface due to lack of sensitivity) and stickiness (VACNTs can be transfered onto the indenter tip when in contact and can be carried along from the rough initial surface find to interfere with the actual testing position of interest). From the procedure in the preceedign paragraph, it is known that the surface of the sample to be tested is in the ‘Raw Displacement’ range between 0 and 100µm. A series of tests is begun by manually seting a surface approach displacement of 0µm. The surface is approached at a slow to intermediate speed (on the order of 100 nm/s). As the surface is approached, a

small oscillation is applied to the indenter column (ph =10µN) and the harmonic contact

stiffness is monitored by the software. The harmonic contact stiffness measures the elastic part of the dynamic stiffness, Ch, and is corrected within the software for the machine

contribution using a set of tables determined during calibration and setup. Therefore, it hovers near 0±15N/m typically and depending on the amplitude of the oscillation can easily detect changes on the order of 50 N/m, which is the threshold we used for surface determination in these experiments. Because of the relatively high speed of the first test, it must be discarded. However, now the ‘Raw Displacement’ of the VACNT sample is known and is entered as the surface approach starting point along with a slow surface approach speed for all subsequent tests. Note that if the ‘Raw Displacement’ is > 100 µm both the test and calibration samples are raised and the ‘Microscope to Indenter Calibration’ process is repeated to avoid misalignment between the tip and pillar. Further details of the test method are given in Appendix A.