Characterization of Coated Surfaces by Indentation

Scratch testing and instrumented indentation are widely used for mechanical characterization of surfaces. Advanced instruments like the Ultra Nanoindentation Tester (UNHT) and the Nano Scratch Tester (NST) use high-tech measurement techniques to offer precise information.

Oliver & Pharr Method

“Oliver & Pharr Method” is generally used to examine indentation data. This method uses the unloading-part of the load-displacement curve to determine the mechanical properties of materials such as hardness and elastic modulus.

However, this method presumes the sample to be a monolithic halfspace and does not consider the impact of the substrate in the collective response of the coated surface to the load-induced deformations and stresses, and hence the results are effective values of the entire sample.

Thus, it is rather difficult to get the precise mechanical characteristics of the coating through this "Oliver & Pharr Method". Conversely, the “Oliver & Pharr extended for coatings” model enables a physical examination of indentation measurements as it determines the exact values of generic material properties of the coating’s layer like the elastic modulus.

It also helps in analyzing the yield strength as it determines the total elastic contact field at the point of initial unloading.

This model can also be applied to scratch test data, since it considers the extra load component and measurement effects. Hence, one can now obtain generic and more complete material properties, for instance the critical stresses of each fracture mode (I, II, and III).

Von Mises stress distribution of an assumed monolithic halfspace without considering additional loads and measurement effects

Figure 1. Von Mises stress distribution of an assumed monolithic halfspace without considering additional loads and measurement effects

Von Mises stress distribution

Figure 2. Von Mises stress distribution

The above figure shows the effect on the Von Mises stress distribution of those variations between the standard and extended model, although the standard "Oliver & Pharr Method" does not allow one to determine the entire elastic field and its stress-strain components.

This application note demonstrates how this model can be utilized to analyze indentation measurements as well as scratch tests on different surface structures, such as a 250 nm thin optical anti-reflex (AR) coating on a polymer substrate, and a 10 pm thick double-layer tribological coating on a Tungsten Carbide (WC) substrate.

Table 1. Known sample parameters

Sample TR

Structure (top down)

Compostion

Thickness[pm]

E [GPa]

Y [GPa]

Layer 1

Al1.4Cr0.6O3

3.3

-

-

Layer 2

Al0.7Ti0.3N

6.7

310.2

30

Substrate

WC

-

564

22

Sample AR

Structure (top down)

Compostion

Thickness[pm]

E [GPa]

Y [GPa]

Layer 1

SiO2

0.25

-

-

Substrate

PMMA

-

4

12

The elastic moduli, layer thicknesses, and material compositions of the substrates have been calculated beforehand. These substrates are different with respect to their mechanical structure and coating thicknesses.

Therefore, it is presumed that the substrates variably affect the mechanical parameters acquired through the "Oliver & Pharr Method".

Dimensioning of Indentation Measurements

Prior to an indentation measurement on a coated substrate, it has to be appropriately dimensioned so as to gain optimum data from the sample constituent of interest. Since the same principle applies for the following scratch test, this measurement process can be summed up as a scheme as shown below.

A flow chart of the procedure of mechanical characterization and optimization of arbitrary structured surfaces

Figure 3. A flow chart of the procedure of mechanical characterization and optimization of arbitrary structured surfaces

It should, however, be noted that the coating’s mechanical properties are assumed to be unknown in this flowchart, as the process starts with a non-dimensioned indentation within the coated substrate so as to determine the first values of its mechanical parameters EC1 and YC1.

These values are used for the initial dimensioning of the final indentation measurement. In case this dimensioning displayed that the sensitivity of the initial measurement is focused on the coating and the vagueness of the results is considerably low, then the initial phase of this mechanical characterization process is over.

Otherwise, values of elastic modulus and Poisson's ratio can be obtained from literature so as to begin with a rough dimensioning prior to beginning the initial indentation measurement. If the actual properties of the coating are close to the literature values, one can eliminate a non-dimensioned measurement.

Physical Investigation of Indentation Measurements

Since an indentation measurement was carried out using the UNHT, measurement information that includes the results of the standard "Oliver & Pharr Method" will be given in the "Oliver & Pharr for coatings” project file format.

In this format, measurement data can be easily opened by the FilmDoctor O&PfC and ISA software, as shown in the figures below, to begin the physical analysis. Both measurements appear to be well dimensioned for the coating of interest.

Measurement data of the AR sample (a) and TR sample (b) as exported by UNHT loaded into the FilmDoctor Studio to start the physical analysis

(a)

Measurement data of the AR sample (a) and TR sample (b) as exported by UNHT loaded into the FilmDoctor Studio to start the physical analysis

(b)

Figure 4. Measurement data of the AR sample (a) and TR sample (b) as exported by UNHT loaded into the FilmDoctor Studio to start the physical analysis

Then, the surface structures need to be defined since the software FilmDoctor must consider the exact structure of the sample so as to measure the impact of the other structure constituents on the measurement data.

Information on this other material is calculated beforehand. For example, the substrate’s elastic modulus has been determined on a massive sample of the substrate material, while the thickness of the layer has been measured using the CSM Instruments Calotest.

Finally, the software FilmDoctor determines the exact elastic modulus EIT of the layer in question by using the measurement data and the previously defined material structure data. The results are provided in the table below.

Table 2. Summary of measured properties for each sample

Sample TR Layer 1 (Al1.4Cr0.6O3)

Sample AR Layer 1 (SiO2)

Eeff [GPa]

261.7

39.5

EC1 [GPa]

240.9

70.3

Y [GPa]

26.4

5.4

Heff [GPa]

22

5.2

HC1 [GPa]

21.1

5.7

Conclusion

The application note highlights the benefits of using scratch testing and instrumented indentation procedures for mechanical characterization of coated surfaces.

Next-generation instruments like the Ultra Nanoindentation Tester (UNHT) and the Nano Scratch Tester (NST) employ sophisticated measurement techniques to deliver precise information.

This information has been sourced, reviewed and adapted from materials provided by Anton Paar TriTec SA.

For more information on this source, please visit Anton Paar.

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