Quantitative Hardness Measurement by Instrumented AFM-indentation

The overall goal of this AFM based experiment is to quantitatively determine the nano hardness of gold thin film. This method can help on the key question in the nano mechanics of materials field regarding the strengths of materials at the nanometer scale and help to determine the mechanisms responsible for plasting deformation The advantage of this technique, how it's high special and forced resolutions. To begin, mount a stiff diamond coated cantilever with the first free resonance frequency of at least 180kHz, a quality factor of at least 300 and a bending stiffness of at least 40 N/m onto a clamping holder of the AFM.

Mount the cantilever holder onto the AFM head align the laser beam and perform a frequency sweep to determine the first free bending residence of the cantilever. Then, in the set up menu of the AFM software, select the default value of the photodiode sensitivity for the particular cantilever type. Place a smooth and non-compliant surface such as nano crystal and diamond or sapphire into the sample holder.

Next, draw the cantilever towards the reference sample surface, using the step motor of the AFM. Keep the cantilever in focus during course approach and stop the course approach before the sample surface is in perfect focus in order to avoid contacting the surface. Next, click the approach button to bring the cantilever tip into contact with the reference sample at a load of 10 nano newtons.

In the force spectroscopy menu, set the relative retraction and extension of the z scanner to 50 N/m and the z scanner retraction and extension to 0.3 micrometers per second. Click on the acquire button in the forced spectroscopy menu to record a forced distance curve on the non compliant surface. In the calibration menu of the software, fit the repulsive part of the forced distance curve with the linear function, then substitute the determined value with the default value by clicking the execute calibration button.

Mount the sample onto a magnetic sample holder and place the holder onto a scanner. Prior to measuring the sample, set the oscillation frequency slightly off resonance at 200.26 khz and the oscillation amplitude to 23.35 nm. Then, enter an oscillation set point of 5 nanometers.

Next, draw the cantilever towards the sample surface using the step motor of the AFM. Keep the cantilever in focus during course approach and stop the course approach before the sample surface is in perfect focus in order to avoid contacting the surface. Now, click on the approach button to automatically approach the foresensor.

Once the oscillation amplitude has reached its set point the tip is ready to scan the typography of the sample surface. Record a series of typography images on areas ranging from 5 micrometers by 5 micrometers to one micrometer by one micrometer. As needed, adjust the slope of the typography signal by tilting the xy scanner.

Make sure that successive images of the same area do not exhibit any sign of drift and that the z scanner position remains almost constant. While attempting this procedure it is important to remember to minimize the instrumental drift. To do so, successive imaging of an area to be indented can be performed prior to the measurements.

Once the system has stabilized in the smooth one micrometer square area has been found, click on the retract button to back the fore sensor a few micrometers from the sample surface. Then, select the fore spectroscopy mode, set the fore set point to 10 nanometers and move the fore sensor to the middle of the pre-selected one micrometer square area. Monitor the position of the z scanner until it remains constant.

Select a two by two grid of points whose center corresponds to the center of the pre-selected area and set the distance between the two next neighboring points at 500 nanometers. Also, set the relative scanner distance to go from 0 to 150 nanometers at a speed of 300 nanometers per second and then to retract over the same distance and at the same velocity. Apply a tilt correction as described elsewhere by moving the lateral scanner by z times tangent pi during a vertical scanner extension of length z where pi is the tilt angle.

At this point, press the start button to begin the acquisition of the AFM indentation data. Once the AFM indentation measurements have been completed, retract the fore sensor a few micrometers away from the sample surface. Then, perform a scan over the same 1 micrometer square surface area so as to locate the exact position of the indents.

Perform additional scans over a 500 by 500 nanometers square surface area to image the remaining indents with greater detail. Shown here are non contact AFM typography images of a gold thin film surface. The thin film surface was found to consist of grains in the micrometer range.

Each grain exhibits an atomically flat gold 111 surface consisting of large terraces and mono atomic steps. This gold 111 surface was indented four times by an AFM tip with a maximal vertical force of 7.2 micronewtons. The typographical difference between the two surfaces shows that the indents were the only major change to the surface.

The projected area of each indent can be measured by masking the area with negative typography values relative to the unmarred surface. From this information, the hardness of the material can be calculated though it is likely to be underestimated because of elastic recovery. After watching this video, you should have a good understanding of how to determine the hardness of a metallic material at the true nanometer scale in under two hours.

While attempting this procedure, it is important to remember to minimize the instrumental drift. To do so, successive imaging of an area to be indented can be performed prior to the measurements. After its development, this technique paved the way for researchers in the field of surface science to explore the atomistic mechanisms of plastic deformation in solid materials.