DHR/AR: Setting Up a Tribology Test Procedure and Performing a Tribo-Rheometry Test

Overview

Tribology procedures can be set up by combining conditioning steps with existing test modes. Predefined test procedures are also available for setting up typical tribology tests and can be accessed from the Procedure bar during the Experiment setup.

Two basic tribology procedures are available for the ARES-G2 rheometer: Tribo-Rheometry Rate Sweep and Tribo-Rheometry Rate Ramp. The procedures include a Conditioning Option block as the first step to control the axial load (normal force) during the tribo-rheometry test.

• NOTE: Only Flow and Transient (step) tests can be used in a procedure when tribology geometry is setup.

Suggested Use

The tribology test procedures are used to measure the coefficient of friction (CoF) between two substrates as a function of the sliding speed under dry or lubricated conditions.The applications for these tests range from understanding the performance of lubricants and greases to personal products and designing novel biological components such as prosthetic devices.

Procedure Setup

To select a tribology test procedure, see Using Experimental Procedures and the help files for the individual test mode and conditioning steps used for detailed instructions.

1. Basic tribo-rheometry procedures can be loaded from the procedure bar using the ‘Predefined Procedures’ icon.
2. Select the Tribo-rheometrydirectory and choose the appropriate test from the list of tribology test procedures available for the current instrument.

DHR/AR Tribology Test Procedures

Select one of the predefined tribology test modes: Tribo-Rheometry Rate Sweep or Tribo-Rheometry Rate Ramp. The test procedure block automatically populates with the required testing and conditioning steps, as shown below.

Note that the procedure contains 2 steps:

• Conditioning Option step
• Flow Ramp or Flow Sweep test mode

It is recommended to apply the axial load (normal force) from the Control Panel before the experiment is started. Accordingly, the Conditioning Option step is set to inherit the current axial force settings. Adding the Conditioning Option step is necessary to continue axial force control during the test at the same conditions that were set through the Control Panel prior to starting the test. The main test parameters are specified in the Flow Sweep or Flow Ramp steps.

Detailed settings for the tribo-rheometry procedure are given below. The online help for the individual steps used in the tribology procedures provides additional information about the settings available in each step.

Conditioning Options

Set the Axial force adjustment mode to Inherit. This is done to ensure that the axial load (normal force) settings specified through the Control Panel prior to starting the test are maintained during the tribo-rheometry test.

Flow Ramp

(See also Setting up a flow ramp for additional details.)

Select the following environmental control parameters:

• Temperature: This is the temperature to maintain throughout the test. Enter the desired test temperature, or select Inherit set point to maintain the previously-specified temperature at the start of this step.
• Wait for temperature: Select this option to wait until the entered temperature is reached before beginning data collection.
• Soak time: This is the amount of time to delay data acquisition at the start of the step, typically to allow for temperature equilibration. If the Wait for temperature option is selected, the soak time will only begin after the desired test temperature has been reached.

Set the following test parameters for tribology tests:

• Duration: Total time of the ramp.
• Logarithmic ramp: The logarithmic ramp uses the entered values as the starting and ending range of the test, with intermediate points spaced logarithmically. Logarithmic ramps can be run in ascending or descending order. For tribology tests, it is recommended to program velocity as the ramp parameter.
• Linear ramp:The linear ramp uses the entered values as the starting and ending range of the test, with intermediate points calculated by adding or subtracting the increment until the final value is reached. . For tribology tests, it is recommended to program velocity as the ramp parameter.
• Select inherit rate/stress only if more than one ramp is used in the procedure and the last value of the previous ramp should be the first value of the following ramp.

• In Controlled Rate Advanced, set the Motor Mode to Medium and select Oversampling for rate and stress controlled modes.

Use the default data acquisition settings:

• Save image:When using the camera accessory, this option stores images of the sample within the data file– the images can be subsequently viewed along with the rheology data.
• End of Step: Maintain final torque/velocity for tribology tests.

Flow Sweep

(See also Setting up a flow sweep for additional details.)

Select the following environmental control parameters:

• Temperature: This is the temperature to maintain throughout the test. Enter the desired test temperature, or select Inherit set point to maintain the previously-specified temperature at the start of this step.
• Wait for temperature: Select this option to wait until the entered temperature is reached before beginning data collection. If the Wait for temperature option is selected, the soak time will only begin after the desired test temperature has been reached.
• Soak time: This is the amount of time to delay data acquisition at the start of the step, typically to allow for temperature equilibration.

Three types of sweep tests can be run. Choose the desired method from the list below. The available range is dependent on your instrument type and configuration. It is recommended to program velocity or torque as the sweep parameters for tribology tests.

• Logarithmic sweep: The logarithmic sweep uses the entered values as the starting and ending range of the test, with intermediate points spaced logarithmically. Logarithmic sweeps can be run in ascending or descending order.
• Linear sweep: The linear sweep uses the entered values as the starting and ending range of the test, with intermediate points calculated by adding or subtracting the increment until the final value is reached.
• Discrete sweep: The discrete sweep takes a measurement at each frequency in a list of frequency values, with up to 10 discrete frequencies being specified. The test frequencies in the list can be run in any order (i.e., do not have to be monotonic).
• Set the equilibration and averaging time required to obtain a steady data point. For tribology tests, suggested values are 20 s for equilibration and 40 s for data averaging. Do not use Steady state sensing and scaled time average for tribology tests.

• In Controlled Rate Advanced, set the Motor Mode to Medium.

The following data acquisition options can be adjusted to control how data is obtained and what additional information is collected during the measurement:

• Save image: When using the camera accessory, this option stores images of the sample within the data file – the images can subsequently be viewed along with the rheology data.

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Running a Tribology Test Procedure

The default tribology test procedure requires the substrates to be installed and the appropriate axial force (load) to be applied prior to starting the experiment. The manual test setup is different for ARES-G2 and DHR.

Running the default test procedure on the DHR

After the desired tribology test geometries have been installed, the sample substrate (if any) attached/ loaded and the lubricant (if any) loaded, proceed as follows.

1. With the upper stage in the loading position, manually zero the normal force from the Axial Force Control tab on the Instrument panel.
2. Close the gap until the upper geometry is in contact with the lower geometry i.e. the sample substrate. From the Axial Force Control tab on the Instrument panel, select the desired axial force, sensitivity and upper/lower displacement and press the Apply button to start axial force control.
3. Set up the test procedure according to the guidelines in the Procedure setup above.

• NOTE: After the sample has been loaded (suitable test substrates with or without a lubricant), it is not necessary within a test series to reset the normal force to zero prior to each experiment. It is sufficient to change the axial force if desired. However when the previous test has been unsteady (wobbling of the upper geometry), it is recommended to separate the upper and lower geometry, re-zero the normal force and apply the new load.
• NOTE: The soft coupling works well up to a load of 20 N. The stiff coupling is preferred for higher loads and can be used from 6–50 N.

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Tribology Test Variables

In a tribology test system parameters and not material parameters are measured. When a tribology geometry is loaded, the following new variables are calculated and reported:

• Friction force ( FF)
• Axial load or normal force ( FL)
• Sliding speed ( vs) and sliding distance (ds)
• Work (W)

Note that the typical rheology variables such as stress and strain are not calculated in tribo-rheometry tests with the exception of the Ring on Plate geometry (see Tribo-Rheometry Test Geometries). However, the relevant variables and all the dependent variables are still shown, but without numerical data.In such cases, you can populate these variables by editing the stress and normal stress geometry constants in the geometry file. It is up to you to enter correct and meaningful geometry constants.

Modifying an Existing Tribology Test Procedure

The pre-defined tribology test procedures can be modified and saved under a new name as user procedures. The tribo-rheometry geometries can be combined with all flow and step tests on the DHR rheometers.

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