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Wear Testing Procedure

The following is a procedure for wear testing; the Procedure for Nanoindenting and Interpreting Hardness Data are covered in separate sections. It is assumed that the user is familiar with operation of TappingMode and Contact Mode imaging. If not, refer to the TappingMode AFM and Contact AFM sections of the Experiment Guide and practice the engaging and imaging procedures using standard imaging cantilevers.

Set Up and Engage in Contact Mode

 
  1. Set up the system for Contact Mode imaging.
    • See Basic Contact Mode Operation for instructions.
    • Be sure to use one of the Indentation Probes
    • When aligning the laser, center the laser spot on the photodetector to zero the vertical and horizontal deflection signals. For wear tests, it is critical to set the vertical deflection as close to zero as possible, since it will affect the force calculation (see Wear the Sample Surface).
  2. Set the Scan size to 0 nm, so that the tip does not begin to scan immediately after engaging.
  3. Set the Deflection setpoint to ~0.1–0.15 V, which will minimize the engage force, but should ensure proper engagement.
  4. Set the Integral gain to 10 and the Proportional gain to 20.
  5. Set the SPM safety to at least 200 mm or the tip may be damaged during engagement.

Figure 1: Default SPM parameters panel settings for indentation
  1. Execute the Engage command.
 
  1. Immediately lower the Deflection setpoint by about 1–2 V to lift the tip off the surface. This will prevent damaging the surface prior to the wear test.

To verify that the tip is off the surface, check that the Z piezo is retracted by looking at the image window—the Z center position should move to the retracted side of the indicator located on the Realtime status bar and the word “Limit” should appear in place of the Z center voltage value. If this does not occur, the Z piezo is not fully retracted and the tip may still be on the surface; decrease the Deflection setpoint by 1 V increments until Z is retracted.
NOTE: In general, engaging in contact mode using indentation cantilevers will result in a small indentation made at the first point of contact. If desired, the user should offset to a fresh location on the sample to perform the test.

Wear the Sample Surface

 
  1. Once engaged on the surface, set the Scan size to the desired size of the wear pattern.
  2. With reference to the Scan size, set the Scan rate for the desired tip velocity.

The tip velocity (μm/s or nm/s) is calculated by multiplying the Scan size (μm or nm) by twice the Scan rate (Hz).

  1. The Lines parameter should be set to obtain the desired resolution.

Since the Lines parameter determines the number of scan lines made during the test, its value will effect the outcome of the wear test.

NOTE: When performing wear tests on multiple samples, to compare properties, the following parameters should remain fixed for all tests:
  • Scan rate
  • Scan size
  • Lines
  • Integral gain
  • Proportional gain
  • Setpoint

  1. Increase the Deflection setpoint to the required value and immediately select the Frame Up or Frame Down buttons from the toolbar.

As soon as the Deflection setpoint is increased by a sufficient amount, the tip will return to the surface and start to wear. Thus, it is important to execute the Frame Up/Down immediately, to begin a fresh scan. The force applied normal to the sample is set using the Deflection setpoint. In Contact Mode, the Deflection setpoint is a measure of the cantilever deflection when scanning the surface. The force also depends on the free-air vertical deflection, which should be set near zero prior to engaging. For further information on force calculations for wear testing, refer to Interpreting Hardness Data.
  1. Execute the Withdraw command immediately after one scan is complete (or when the desired number of full or partial scans are complete). This prevents more wear from occurring than is desired.
  1. Re-engage the surface in TappingMode and image the worn area.

Refer to Procedure for Nanoindenting for TappingMode engage guidelines using indentation cantilevers.

Do not reset the X and Y offsets or the tip will engage in a different location. Once engaged, set the Scan size to a value greater than the value used for the wear test, and image the new features.

Comments on Wear Testing

If the application requires non-standard indentation cantilevers with lower spring constants than standard indentation cantilevers, imaging the worn area may require switching to a standard TappingMode or Contact Mode probe. If this is the case, it may be necessary to reference the location of the worn areas using a specific feature on the sample prior to performing the test. This is necessary as the tip position relative to the sample will not be the same after changing probes. If the worn area is visible through the optics used to view the sample a reference point is not necessary.

Switching to another probe may also be necessary if the indentation cantilever is not adequately imaging the worn area. This sometimes occurs when the wear procedure creates a large amount of debris in and around the worn area. The debris is sometimes pushed around or picked up by the stiff indentation cantilever, resulting in poor image quality.

Before switching to other imaging probes the user should attempt to image using the indentation cantilever in TappingMode. This is best accomplished by using the lowest imaging force possible. Use the maximum Amplitude setpoint possible while still tracking the surface. Also, increasing the Integral gain may improve the image quality by increasing the feedback response. It may help to scan the worn area in the same scan direction (up/down) in which the wear test was performed, since most of the debris is usually located at the end of the scan. This will prevent debris from ruining the entire scan because the tip will not contact the debris until the end of the scan.

 

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