Atomic Force Microscopy Cantilever-Based Nanoindentation: Mechanical Property Measurements at the Nanoscale in Air and Fluid

Understanding the mechanical properties of materials is one of the most fundamental and essential tasks in engineering. For the analysis of bulk material properties, there are numerous methods available to characterize the mechanical properties of material systems, including tensile tests¹, compression tests², and three- or four-point bending (flexural) tests³. While these microscale tests can provide invaluable information regarding bulk material properties, they are generally conducted to failure, and are hence destructive. Additionally, they lack the spatial resolution necessary to accurately investigate the micro- and nanoscale properties of many material systems that are of interest today, such as thin films, biological materials, and nanocomposites. To begin addressing some of the problems with large-scale mechanical testing, mainly its destructive nature, microhardness tests were adopted from mineralogy. Hardness is a measure of the resistance of a material to plastic deformation under specific conditions. In general, microhardness tests use a stiff probe, usually made from hardened steel or diamond, to indent into a material. The resulting indentation depth and/or area can then be used to determine the hardness. Several methods have been developed, including Vickers⁴, Knoop⁵, and Brinell⁶ hardness; each provides a measure of microscale material hardness, but under different conditions and definitions, and as such only produces data that can be compared to tests performed under the same conditions.

Instrumented nanoindentation was developed to improve upon the relative values obtained via the various microhardness testing methods, improve the spatial resolution possible for the analysis of mechanical properties, and enable the analysis of thin films. Importantly, by utilizing the method first developed by Oliver and Pharr⁷, the elastic or Young's modulus, E, of a sample material can be determined via instrumented nanoindentation. Furthermore, by employing a Berkovich three-sided pyramidal nanoindenter probe (whose ideal tip area function matches that of the Vickers four-sided pyramidal probe)⁸, direct comparison between nanoscale and more traditional microscale hardness measurements can be made. With the growth in popularity of the AFM, AFM cantilever-based nanoindentation began receiving attention as well, particularly for measuring the mechanical properties of softer materials. As a result, as depicted schematically in Figure 1, the two most commonly employed techniques today to interrogate and quantify nanoscale mechanical properties are instrumented nanoindentation (Figure 1A) and AFM cantilever-based nanoindentation (Figure 1B)⁹, the latter of which is the focus of this work.

Figure 1: Comparison of instrumented and AFM cantilever-based nanoindentation systems. Schematic diagrams depicting typical systems for conducting (A) instrumented nanoindentation and (B) AFM cantilever-based nanoindentation. This figure was modified from Qian et al.⁵¹. Abbreviation: AFM = atomic force microscopy. Please click here to view a larger version of this figure.

Both instrumented and AFM cantilever-based nanoindentation employ a stiff probe to deform a sample surface of interest and monitor the resultant force and displacement as a function of time. Typically, either the desired load (i.e., force) or (Z-piezo) displacement profile is specified by the user via the software interface and directly controlled by the instrument, while the other parameter is measured. The mechanical property most often obtained from nanoindentation experiments is the elastic modulus (E), also referred to as the Young's modulus, which has units of pressure. The elastic modulus of a material is a fundamental property relating to the bond stiffness and is defined as the ratio of tensile or compressive stress (σ, the applied force per unit area) to axial strain (ε, the proportional deformation along the indentation axis) during elastic (i.e., reversible or temporary) deformation prior to the onset of plastic deformation (equation [1]):

    (1)

It should be noted that, because many materials (especially biological tissues) are in fact viscoelastic, in reality, the (dynamic or complex) modulus consists of both elastic (storage, in phase) and viscous (loss, out of phase) components. In actual practice, what is measured in a nanoindentation experiment is the reduced modulus, E*, which is related to the true sample modulus of interest, E, as shown in equation (2):

    (2)

Where E_tip and ν_tip are the elastic modulus and Poisson's ratio, respectively, of the nanoindenter tip, and ν is the estimated Poisson's ratio of the sample. The Poisson's ratio is the negative ratio of the transverse to axial strain, and hence indicates the degree of transverse elongation of a sample upon being subjected to axial strain (e.g., during nanoindentation loading), as shown in equation (3):

    (3)

The conversion from reduced to actual modulus is necessary because a) some of the axial strain imparted by the indenter tip may be converted to transverse strain (i.e., the sample may deform via expansion or contraction perpendicular to the direction of loading), and b) the indenter tip is not infinitely hard, and thus the act of indenting the sample results in some (small) amount of deformation of the tip. Note that in the case where E_tip >> E (i.e., the indenter tip is much harder than the sample, which is often true when using a diamond probe), the relationship between the reduced and actual sample modulus simplifies greatly to E ≈ E*(1 - v²). While instrumented nanoindentation is superior in terms of accurate force characterization and dynamic range, AFM cantilever-based nanoindentation is faster, provides orders of magnitude greater force and displacement sensitivity, enables higher resolution imaging and improved indentation locating, and can simultaneously probe nanoscale magnetic and electrical properties⁹. In particular, AFM cantilever-based nanoindentation is superior for the quantification of mechanical properties at the nanoscale of soft materials (e.g., polymers, gels, lipid bilayers, and cells or other biological materials), extremely thin (sub-µm) films (where substrate effects can come into play depending upon indentation depth)^10,11, and suspended two-dimensional (2D) materials^12,13,14 such as graphene^15,16, mica¹⁷, hexagonal boron nitride (h-BN)¹⁸, or transition metal dichalcogenides (TMDCs; e.g., MoS₂)¹⁹. This is due to its exquisite force (sub-nN) and displacement (sub-nm) sensitivity, which is important for accurately determining the initial point of contact and remaining within the elastic deformation region.

In AFM cantilever-based nanoindentation, displacement of an AFM probe toward the sample surface is actuated by a calibrated piezoelectric element (Figure 1B), with the flexible cantilever eventually bending due to the resistive force experienced upon contact with the sample surface. This bending or deflection of the cantilever is typically monitored by reflecting a laser off the back of the cantilever and into a photodetector (position sensitive detector [PSD]). Coupled with the knowledge of the cantilever stiffness (in nN/nm) and deflection sensitivity (in nm/V), it is possible to convert this measured cantilever deflection (in V) into the force (in nN) applied to the sample. Following contact, the difference between the Z-piezo movement and the cantilever deflection yields the sample indentation depth. Combined with the knowledge of the tip area function, this enables calculation of the tip-sample contact area. The slope of the in-contact portions of the resulting force-distance or force-displacement (F-D) curves can then be fit using an appropriate contact mechanics model (see the Data Analysis section of the discussion) to determine the nanomechanical properties of the sample. While AFM cantilever-based nanoindentation possesses some distinct advantages over instrumented nanoindentation as described above, it also presents several practical implementation challenges, such as calibration, tip wear, and data analysis, which will be discussed here. Another potential downside of AFM cantilever-based nanoindentation is the assumption of linear elasticity, as the contact radius and indentation depths need to be much smaller than the indenter radius, which can be difficult to achieve when working with nanoscale AFM probes and/or samples exhibiting significant surface roughness.

Traditionally, nanoindentation has been limited to individual locations or small grid indentation experiments, wherein a desired location (i.e., region of interest [ROI]) is selected and a single controlled indent, multiple indents in a single location separated by some waiting time, and/or a coarse grid of indents are performed at a rate on the order of Hz. However, recent advances in AFM allow for the simultaneous acquisition of mechanical properties and topography through the utilization of high-speed force curve-based imaging modes (referred to by various tradenames depending on the system manufacturer), wherein force curves are conducted at a kHz rate under load control, with the maximum tip-sample force utilized as the imaging setpoint. Point-and-shoot methods have also been developed, allowing for the acquisition of an AFM topography image followed by subsequent selective nanoindentation at points of interest within the image, affording nanoscale spatial control over nanoindentation location. While not the primary focus of this work, specific selected application examples of both force curve-based imaging and point-and-shoot cantilever-based nanoindentation are presented in the representative results, and can be used in conjunction with the protocol outlined below if available on the particular AFM platform employed. Specifically, this work outlines a generalized protocol for the practical implementation of AFM cantilever-based nanoindentation on any capable AFM system and provides four use case examples (two in air, two in fluid) of the technique, including representative results and an in-depth discussion of the nuances, challenges, and important considerations to successfully employ the technique.