Nanomechanical testing techniques have become a popular method for quantitative, small-volume mechanical property determination. Nanoindentation is an instrumented depth-sensing indentation technique that is particularly well suited for the characterization of coated and other surface-engineered surfaces. Extremely low force and displacement noise floors, along with a high degree of positioning accuracy, allow for the high spatial resolution acquisition of material properties on the micrometer to single-nanometer length scales.

Conceptually, nanoindentation is a straightforward technique where an indenter probe of a well-known geometry is pushed into and withdrawn from the material's surface while force, displacement and time are continuously recorded. Extremely low force and displacement noise floors of 100 nN and 2Å are available from select instrument manufacturers and allow for the quantitative mechanical property measurements of very thin films, treated surface layers and small structural features.

The principal properties most often extracted using nanoindentation are the elastic modulus and indentation hardness of the test specimen. Additionally, by using complimentary techniques the storage and loss modulus, yield stress, fracture toughness, creep and stress relaxation studies, strain rate sensitivity, interfacial and surface adhesion, mechanical properties temperature dependence, and electrical contact resistance can be readily measured. Lateral probe motion can be employed to investigate the tribological behavior of surfaces, including scratch and mar resistance, wear performance, and friction coefficients. Combined, these techniques allow researchers to readily investigate a broad spectrum of mechanical and tribological properties of materials at the sub-micron scale.

Background

Indentation or hardness testing has been utilized for over a century for material characterization and quality control purposes.1 Typically, such a test consists of applying a static load to an indenter probe for a certain period of time, withdrawing the probe and optically measuring the area of the residual hardness impression. The necessity of measuring the area of the residual impression imposes a lower limit on the scale and accuracy of testing, and the relatively large utilized forces produce sample material/geometry limitations, insensitivity to thin films and poor positioning accuracy.

In contrast to traditional hardness testing, nanoindentation testing simultaneously records the depth of penetration and applied load during the loading-unloading cycle (Figure 1) and can quantitatively measure mechanical properties even when the indentations are too small to be imaged conveniently. The force-displacement curve serves as a mechanical fingerprint that provides a wealth of information concerning the material's deformation behavior during the entire indentation cycle, which enables the examination of possible modes of film failure under contact-induced stresses.

The most commonly used analysis method for obtaining hardness and modulus values was first developed by Doerner and Nix2, and later improved upon by Oliver and Pharr.3 Both elastic and plastic deformation occurs during loading as the hardness impression forms, while upon unloading it is assumed only elastic displacements are recovered. The total displacement of the indenter probe is a sum of plastic contact depth, hc, and the elastic depth, hs, which is the elastic flexure of the surface while under load:

The elastic displacement of the surface at the perimeter of tip/sample contact can be obtained by Sneddon's contact solution:

where  is a tip-dependant geometric factor and S is the unloading contact stiffness or slope of the unloading curve at maximum load. The shape of the unloading curve can be well approximated by the power law relation:

where  and m are power law fitting constants and hf is the final depth of the residual hardness impression. A schematic representation of a cross section through an indentation showing deformation during loading can be found in Figure 2.

Hardness is defined as the load divided by the projected contact area of the indentation, A, or the mean contact pressure that a material will support under load:

The reduced modulus, Er, can be determined from the initial slope of the unloading curve, S, by the equation

where A is the projected area of elastic contact. The reduced modulus accounts for elastic displacements present both in the tip and sample while indenting. The measured compliance (1/S) of the sample and the indenter tip can be combined as a spring in series:

where E and  are the elastic modulus and poisson's ratio of the indenter, i, and sample, s, respectively.

This method has become the primary and possibly the most attractive technique for determining mechanical properties of small material volumes because indentation hardness and reduced modulus values can be directly determined from load and displacement measurements without having to image the residual hardness impression. Since indenter geometries are never truly ideal, a relationship between contact area to contact depth can be established by performing indents at a range of penetration depths in a well-characterized reference material and solving for contact area using Equation 5.

The relationship between the cross-sectional area of contact and the contact depth is known as the tip area function and can be fit to a multi-term polynomial function of the form:

where C0...Cn are constant coefficients determined from the curve fit.

Typically, fused silica is the reference material used for the tip area calibration because it is elastically isotropic and has a constant modulus as a function of indentation depth. Figures 3 and 4 show a topographical SPM (scanning probe microscopy) image of impressions resulting from varying depth indents performed in fused silica and the resulting force-displacement curves, respectively.

Nanomechanical Testing of Automotive Clearcoats Nanoindentation

Three different proprietary polyurethane automotive clearcoats were obtained from a leading coatings manufacturer. The coatings were 10's of micrometers in thickness and affixed to a steel substrate. All nanoindentation tests were performed using a Berkovich diamond indenter probe. The utilized indentation load function consisted of a 5-second linear loading to peak load, a 20-second holding segment at maximum load to minimize effect of creep on the testing results, and a 5-second linear unloading segment. Fifteen nanoindentation tests were performed on each clearcoat, with a maximum utilized load of 500 µN. In- situ SPM imaging was used prior to each indent to assure test placement on defect-free regions of the coating to maximize test accuracy and repeatability.

In-situ SPM imaging is a technique that utilizes the indenter probe to provide topographical images by raster scanning the probe across the sample surface. Figure 5 shows the force-displacement plots for each indent performed on Coatings A, B and C. One can see from the raw force and displacement data that there are clear differences in the mechanical behavior of the three polyurethane coatings tested.

Average hardness and reduced modulus values for each coating tested can be found in Figure 6. Substantial property differences are seen between each of the coatings. At the tested depths, Coatings A, B and C have average reduced modulus values of 3.54±0.01 GPa, 5.20±0.08 GPa and 5.84±0.05 GPa, and average hardness values of 196.6±1.6 MPa, 328.3±7.1 MPa and 424.9±9.5 MPa, respectively.

Polymeric materials near a free surface can have properties that deviate considerably from the bulk material properties.4 Nanoindentation depth profiling experiments were performed on each clearcoat to determine mechanical properties as a function of depth into the coating. Indentation loads were incrementally varied between 20 µN and 8500 µN to obtain a large range of penetration depths. Figures 7 and 8 show reduced modulus and hardness values as a function of depth from the coatings' surface, respectively. Both the hardness and reduced modulus decrease as a function of indentation depth for all three coatings. Coatings B and C have relatively constant properties up to a penetration depth of approximately 300 nm, while Coating A displays continuously decreasing properties. The ability to identify and tailor such property trends can play a critical supporting role in altering coating formulations and processing conditions to maximize product performance and reliability.

Nanoscale Scratch Testing/Mar Resistance

A significant contributor to coating appearance and reliability is the ability to resist scratching and marring. There are numerous test methods to quantify the scratch and mar resistance of coatings, but none allow for more precisely controlled scratch conditions than nanoscale scratch testing techniques. Scratch testing is fundamental to understanding the relationship between deformation, removal mechanisms and scratch morphology.5 The ability to assess the type of damage occurring and the ability to predict scratch behavior is of great importance to tribologists and polymer material scientists.

In scratch testing, a force is applied to a scratch probe in the normal direction as it is simultaneously moved laterally across the sample surface. Normal force and lateral displacement are controlled, while lateral force and normal displacement are simultaneously recorded as a function of time. From these force and displacement measurements, comprehensive information concerning the material's nanoscratch properties can be deduced.

Several ramped force scratch tests were performed on each coating sample using a 60° conical scratch probe with a 5 µm radius of curvature at the tip. Scratch probe geometry and material can be tailored to simulate real-world sliding asperity conditions. The scratch load function consisted of a linear ramping of the applied normal load from 0 mN to 50 mN over a lateral displacement of 500 µm in 50 seconds. Figure 9 shows the normal displacement of the scratch probe into the coating as a function of lateral displacement. The bottom set of curves represent the normal displacement of the probe during the actual scratch cycle, while the top set of curves shows the extent of residual surface deformation. Each curve is an average obtained from five scratch tests on each coating sample. Coatings B and C possess nearly identical initial penetration and residual scratch depths, while Coating A showed substantially larger instantaneous and residual scratch depths. Figure 10 shows representative Differential Interference Contrast (DIC) optical micrographs of the scratch morphology from scratch tests performed on the three clearcoat samples.

The percent of scratch depth recovery was calculated by taking the recovered displacement (ds- dr) and dividing by the instantaneous penetration depth during scratching, ds:

Figure 11 shows the percent penetration depth recovery as a function of lateral displacement for 50 mN ramped force scratches performed on the polyurethane coatings. At approximately 100 µm the percent of depth recovery stabilized for all three coating systems, which relates to 10 mN of applied normal force. After this point, the residual scratch track depth remains a constant proportion to the instantaneous penetration depth attained during scratching.

From Figure 10 we can see that the scratch deformation is a ductile plowing process for 50 mN ramped force scratches on all three polyurethane clearcoats. Several surface characteristics contribute to scratch deformation behavior, including the coefficient of friction. Polymers with high friction coefficients tend to exhibit crazing, cracking, debonding and cavitation types of brittle scratch damage.6 Additionally, increased friction coefficients tend to move the location of the plastic zone produced during the scratch process toward the surface of the coating. Figure 12 shows DIC optical micrographs of 100 mN ramped force scratches on each coating sample.

As can clearly be seen in Figure 12, the same set of scratching conditions produced varying degrees of deformation on each coating. Figure 13 shows the plowing friction, or the ratio of the measured lateral force to the applied normal load over the duration of the 50 mN ramped force scratch cycle.

Conclusion

Three polyurethane automotive clearcoats were tested using nanoindentation and nanoscratch testing techniques. Substantial differences were measured in both their mechanical property gradients and scratch properties, which ultimately can be linked to coating performance and reliability. Nanomechanical testing encompasses a powerful set of techniques used for quantitative small-volume mechanical property and material deformation behavior characterization. These techniques can be easily combined to allow researchers to readily investigate a broad spectrum of mechanical and tribological properties of materials on the sub-micron scale.

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