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Corrosion Effect On Hardness Using Nanoindentation

The mechanical properties of materials deteriorate during the corrosion process. For example, lepidocrocite (γ-FeOOH) and goethite (α-FeOOH) form in the atmospheric corrosion of carbon steel. Their loose and porous nature results in absorption of moisture and in turn further acceleration of the corrosion process. Akaganeite (β-FeOOH), another form of iron
oxyhydroxide, is generated on the steel surface in chloride containing environments. Nanoindentation can control the indentation depth in the range of nanometers and microns, making it possible to quantitatively measure the hardness and Young’s modulus of the corrosion products formed on the metal surface. It provides physicochemical insight in corrosion mechanisms involved so as to select the best candidate material for the target applications.

Corrosion Effect On Hardness Using Nanoindentation

Drywall Texture & Pitting Using 3D Profilometry

Drywall texture and roughness is critical in the final products quality and appearance. A better understanding of the effect of surface texture and consistency on the moisture resistance of the coated drywall allows selecting the finest product and optimizing the painting technique to obtain the best result. Quantifiable, fast and reliable surface inspection of the coating surface is in need for quantitative evaluation of the surface quality. The Nanovea 3D Non-Contact Profilometers utilizes chromatic confocal technology with unique capability to precisely measure the sample surface. The line-sensor technique can finish scanning a large drywall surface in minutes.

Drywall Texture & Pitting Using 3D Profilometry

Cyclical Nanoindentation Stress-Strain Measurement

Importance of Nanoindentation

Continuous stiffness measurements (CSM) obtained by nanoindentation reveals the stress-strain relationship of materials with minimally invasive methods. Unlike traditional tensile testing methods, nanoindentation provides stress-strain data at the nanoscale without the need of a large instrument. The stress-strain curve provides crucial information on the threshold between elastic and plastic behavior as the sample is subject to increasing loads. CSM gives the capability to determine the yield stress of a material without dangerous equipment.

 

Nanoindentation provides a reliable and user-friendly method to quickly investigate stress-strain data. Furthermore, measuring stress-strain behavior on the nanoscale makes it possible to study important properties on small coatings and particles in materials as they get more advanced. Nanoindentation provides information on elastic limit and yield strength in addition to hardness, elastic modulus, creep, fracture toughness, etc. making it a versatile metrology instrument.

The stress-strain data provided by nanoindentation in this study identifies the elastic limit of the material while only going 1.2 microns into the surface. We use CSM to determine how mechanical properties of materials develop as an indenter travels deeper into the surface. This is especially useful in thin film applications where properties can be depth dependent. Nanoindentation is a minimally invasive method of confirming material properties in test samples.

The CSM test is useful in measuring material properties versus depth. Cyclical tests can be performed at constant loads to determine more complex material properties. This can be useful to study fatigue or eliminate the effect of porosity to obtain true elastic modulus.



Measurement Objective

In this application, the Nanovea mechanical tester uses CSM to study hardness and elastic modulus versus depth and stress-strain data on a standard steel sample. Steel was chosen for its commonly recognized characteristics to display the control and accuracy of the nanoscale stress-strain data. A spherical tip with a 5-micron radius was used to reach high enough stresses beyond the elastic limit for steel.

 



Test Conditions & Procedures

The following indentation parameters were used:

 



Results:

 

Increase in load during oscillations provide the following depth versus load curve. Over 100 oscillations were conducted during loading to find the stress-strain data as the indenter penetrates the material.

 


We determined stress and strain from the information obtained at each cycle. The maximum load and depth at each cycle allows us to calculate the maximum stress applied in each cycle to the material. Strain is calculated from the residual depth at each cycle from the partial unloading. This allows us to calculate the radius of the residual imprint by dividing the radius of the tip to give the strain factor. Plotting stress versus strain for the material shows the elastic and plastic zones with the corresponding elastic limit stress. Our tests determined the transition between the elastic and plastic zones of the material to be around 0.076 strain with an elastic limit of 1.45 GPa.






Each cycle acts as a single indent so as we increase load, we run tests at various controlled depths in the steel. So, hardness and elastic modulus versus depth can be plotted directly from the data obtained for each cycle.






As the indenter travels into the material we see hardness increase and elastic modulus decrease.



Conclusion





We have shown the Nanovea mechanical tester provides reliable stress-strain data. Using a spherical tip with CSM indentation allows for material property measurement under increased stress. Load and indenter radius can be changed to test various materials at controlled depths. Nanovea mechanical testers provide these indentation tests from the sub mN range to 400N.


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