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Nanoindentation Laboratory

Nanoindentation tester is used to investigate mechanical properties of materials by indenting the test material with a diamond tip to depths of 100 to 1000 nano-meters while measuring the force-displacement response. This tool is being used to study the relationship between microstructure and strength and toughness of materials.

The Atomic Force Microscope is used to observe and quantitatively characterize the surface topography of specimens with vertical resolutions in the range of one nano-meter. This instrument has been used to quantify surface damage in metallic specimens as a result of fatigue damage due to cyclic loading. It can be used for tribological studies and for studying corrosion behavior in metallic materials.

1.Nanoindenter
       a. MTS Nanoindentor
       b. Two indenter heads
             i. XP – 50gf max load
             ii. DCM – 10gf max load
       c. Berkovich shaped diamond indenter (E=1141 GPa; n=0.07)
             i. 3 sided pyramid similar to a Vickers
       d. MTS TestWorks 4 controller software

General Test Page
(Go to Nanoindentation Results)

Pre-test Procedure

- Turn on nanoindenter first and then the computer

- Choose NT WorkStation 4.0

- Login name Administrator, with no password (Click OK)

- Start TestWorks 4

- Login in name MTS, with no password (Click OK)

- Open one of three methods

MTS DCM Strain Rate and Tip Calibration

Strain controlled test, one load and unload using the DCM head

MTS XP Multiple Load and Unloads

Series of loads and unloads to a designated maximum force

MTS XP Strain Rate and Tip Calibration

Strain controlled test, one load and unload using the XP head

Click OK

- If using the XP head, carefully pull the pins from the indenter head. Starting with the bottom one first, followed by the top one.

- Initialize the head (Right click on the grid, choose Initialize). 

This may take a few minutes so be patient.

If after a few minutes (5-10) the initialization is not complete, you may notice that the displacement is changing rapidly and the indenter tip is visibly moving. Click Cancel and restart the initialization. 

- Run the Microscope to Indenter Calibration

Move tray to the aluminum sample. (Left click on grid on the location of the aluminum sample.  Then right click on the grid, choose Move to Target)

Turn on light (Between computer and nanoindenter)

Change the grid to microscope. (Right click on the grid, choose Nano VideoHandset)

Focus the microscope with the arrow buttons at the bottom left.  The large arrows for large focusing and the smaller arrows for fine focusing.

Move the sample tray, left click on the video screen

Choose location to run the Microscope to Indenter Calibration. Choose a large enough area to accommodate 5 indents. Make sure you are on the aluminum by first finding other indents.  The center indent will be made under the “+”.

Run the Microscope to Indenter Calibration (At the top under Tools, then Microscope to Indenter Calibration)

 “Are you sure you want to use current position under the microscope?” Click Yes. The tray should already be positioned.

When the calibration is finished, position the “+” in the center indent and click Yes.

 If there are no visible indents, it is possible that the calibration is way off.  Search the entire aluminum specimen for indents. If they are still not visible, the indenter could be indenting the epoxy, in which the indents are very difficult to find.  In this case ask Rejanah or Art for help.

Test Procedure

- Define the Mode. (At the top under Mode, chose Batch Mode)

- Choose the Define Tab above the Grid/Video

Defining a Test in Batch Mode Page

- Click Next Step until Sample Name. Input a Sample name, which will later be the output file name. Click Next Step.

- Define Surface Find Parameters: the nanoindenter will first perform a surface find by slowly lowering the indenter head until it makes contact with the surface.  The surface find is performed on the first indentation in a series and on indentations following a failed indentation. The following parameters define where the surface find will occur.

Delta X for finding surface: how many micrometers in the x direction will the surface find occur. (Default: 50 µm)

Delta Y for finding surface: how many micrometers in the y direction will the surface find occur.  (Default: 50 µm)

Allowable Drift Rate: allowable amount of thermal drift prior to testing. (Default: 0.050 nm/s, which maybe a little tight.  0.07-0.09 will work well)

- Click Next Step

- Define Surface Approach Parameters: these variables deal with how far and fast the indenter tip travels before hitting the surface.  The defaults here are pretty good.  So as a rule these do not need to be changed.

Surface Approach Distance: How far the tip will travel during its approach.  (Default: 1000nm)

Surface Approach Sensitivity: How much stiffness change indicates surface contact. (Default: 50%)

Surface Approach Velocity: How fast the indenter tip moves during the surface approach. (Default: 10 nm/s)

Approach Distance to Store: Deals with data collection.  Leave alone.

- Click Next Step

- Define Required Inputs: these variables will be used to run the indent and make output calculations.  Just click on the value to change it.

If you chose the “Strain Rate and Tip Calibration Test” the inputs are

Depth Limit: how deep in the material the indenter will go.  (Default: 2000 nm)

Strain Rate: how fast the indenter will travel inside the material. (Default: 0.050).  This is not a traditional strain rate e. This strain rate is defined as such:

Poissons Ratio (Default: 0.18 for silicon)

If you chose the “Multiple Load and Unloads”, the inputs are

Maximum Load: Maximum load to which will be placed on the indenter (Default: 50 gf)

Number of Times to Load: the number of loads and unloads that will be performed. The last one will occur at the maximum load. (Default: 8)

Poissons Ratio (Default: 0.18 for silicon)

- Click Next Step

- Define First Location: only define first point.

- Click Accept.  The x-y coordinates in mm will show up on the left.

- Click Next Step

- Enter Location Type. Chose Manual or Array.

Manual

Click Next Step

Shows same page as Define First Location

Move Tray by clicking on the video screen

Make an indent, click Accept Next

If you want to check an indent location, highlight x-y coordinates on the left and click, Go To Location.

Can not delete a location, but you can Re-assign a location.

Only way to delete location is to start over

Array

Can name a group, not necessary unless you plan on using the same configuration many times

Define the number of indents in the X direction

Define the number of indents in the Y direction

Define the spacing between the indents (in mm)

Define an angle at which the array runs at

Changes in previous steps can be made by clicking on the desired step at the left.

To start over at anytime, click on the Start folder on the left

- Click Next Step

- “Add / edit another sample” – No

- Choose the Test Tab

- Click on the big green Start button

- Start Time: Set time to start test.  Click OK.

- If you want the test to start immediately, click Resume

- To stop the test at anytime, click on the big Stop button

- As each indent is completed, click the Review tab to view the results

General Review Page

- The nanoindenter outputs two different types of results: continuous results and unloading results. The difference between the two is explained in following pages.

- The continuos hardness and modulus results are averaged over a range that can be changed on the Inputs list (maximum and minimum calculation depth).  These values are also shown on the graph as LL (Lower Limit) and UL (Upper Limit).

- If the maximum and minimum calculation depths are changed, the results need to be recalculated.

- Left clicking on the graph and choosing X-axis or Y-axis can change the continuos graph axis.  Left clicking on the graph and choosing properties can change the scale.

- The unloading results can be seen in the Completed Test Results.

Very Important

- Once the sample is complete, output the results to Excel (At the top, Excel, Output Sample to Excel)

Analyst

Analyst is a program that will run some calculations and make graphing results easier.

Adding a Graph in Analyst

- Analyst can be run after the sample has been completed or at a later time.

- To run Analyst, Start on the Windows toolbar. Go to Programs, TestWorks 4, Analyst. 

- If you have just finished a sample, Analyst will automatically find the data.  Otherwise you will have to open the sample (File, Open Project).  The data must be in an Excel format.  Analyst can only read Excel files.

-Once the sample is open, click on the Calculator to make the calculations.  This must be done fr Analyst to make any graphs.

- To make a general graph, or average of all the indents, complete with error bars, right click on Project.

- To graph individual tests, right click on the sample name or on the desired test.

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Nanoindentation Results

There are two different types of results reported by nanoindenter:

- Hardness and modulus over a defined range

- Hardness and modulus from unload

The hardness and modulus over a defined range is based on continuous stiffness and hardness readings. The hardness and modulus from unload is based on the unloading stiffness.

Continuous measurements

Continuous stiffness measurements are accomplished by applying a small oscillation to the force signal at a relatively high frequency. The amplitude of the force oscillation is small enough that it does not affect the deformation process.

There are two methods by which the continuous stiffness can be calculated:

Or

C_f = load frame compliance (~1.13 m/MN)\

K_s = column support spring stiffness (~ 60 N/m)

D = damping coefficient (~ 54 N s/m)

P_os = magnitude of force oscillation

h(w) = magnitude of resulting displacement oscillation

w = frequency of oscillation (69.3 Hz)

f = phase angle between force and displacement signals

m = mass (~4.7 gms)

Oliver-Pharr method

The Oliver-Pharr method is based on elastic solutions by Sneddon, who derived general relationships among the load, displacement, and contact area for any punch that can be described as a solid revolution of a smooth function.

The Oliver-Pharr method builds on solutions by Doerner and Nix who suggested that unloading stiffness could be computed from a linear fit of the unloading curve.  By extrapolating the linear portion of the curve to 0 load, the extrapolated depth could be used to determine contact area.

Oliver-Pharr found that unloading data is usually not linear but better described with a power law.

P = A(h-h_f)^m

The modulus from unloading is then calculated using the following equations:

Where:

S = measured stiffness of the upper portion of the unloading curve

A = projected contact area          

E_r = reduced modulus    

b = correction factor which is 1.034 for Berkovich indenter

E and n = Young’s modulus and Poisson’s ratio for specimen

E_I and n_I = Young’s modulus and Poisson’s ratio for the indenter

One major advantage to the Oliver-Pharr method is that a direct measurement of the indent is not necessary. The indenter contact area can be calculated using the following equations.

The area function for a perfect Berkovich indenter is the following:

A(h_c) = 24.5h_c²

(Other terms following the first one describe deviations in geometry due to blunting at the tip.)

Where:

P_max = peak indentation load

e = 0.75 for Berkovich indenter

S = unloading stiffness

h_max = maximum depth of indentation

The Hardness from unloading is calculated using the following equation:

H = Pmax = peak indentation load     A = projected area of hardness impression

This hardness definition may be different from conventional hardness definition. The observed hardness impression may be less than that at peak load if a portion of the contact area did not plastically deform.

Limitations

Since the analysis is based on elastic contact solutions, there is some concern on how well this method works in elastic/plastic situations. One example of this is pile-up.  The Oliver-Pharr method works well for hard ceramics, when sink in predominates but fails with soft metals which display extensive pile up.  It has been shown that pileup is significant for materials that do not work harden and for which h_f/h_max is close to 1.  When pileup is significant, the Oliver-Pharr approach underestimates the true contact area by as much as 60%. This error in contact area is then causes errors in hardness and modulus.

Fracture Toughness

In order to measure fracture toughness, radial cracking from the corners of the indent must take place.  In brittle materials this can be accomplished with a Vickers or Berkovich diamond tip.

Fracture Toughness can be calculated using the following equation:

Where

K_c = fracture toughness

c = length of radial cracks

E = modulus

H = hardness

a = empirical constant (0.040 for cube-corner)

Limitations

For Vickers and Berkovich indenters, fracture toughness indentations are typically made at loads of 1000g and crack lengths are on the order of 100 mm. These types of loads are well outside our systems.

The use of a cube-corner indenter significantly reduces the amount of loaded needed to induce cracking.

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