An article published in the journal Forests demonstrates improved Berkovich quasistatic nanoindentation methods to improve measurement accuracy in complex polymeric materials such as wood cell walls.
To study: Best Practices for Quasistatic Berkovich Nanoindentation of Wood Cell Walls. Image Credit: Digital Photo / Shutterstock.com
Traditional methods assume that the specimens are rigidly supported, homogeneous and semi-infinite. However, for accurate results, the newly improved Berkovich quasistatic nanoindentation is desirable because it provides a means to correct and detect errors related to surface detection and structural conformities emerging from dirty probes, nanoindentor performance and displacement drift.
Two design examples of transducers used in commercial nanoindenters. (a) Bruker’s Hysitron (Minneapolis, MN, USA) Standard Transducer TriboIndenter is a three-plate capacitive force / displacement transducer. The probe is attached to the central plate, which is suspended between the two outer plates by leaf springs. To measure the displacement, an AC signal 180_ Phase shifted is applied to the two outer plates, creating an electric field potential between the two outer plates which is zero at the center. The voltage measured from the middle plate is calibrated to measure displacement. Force is applied to the center plate by applying a large DC offset to the bottom or top plate, which creates an electrostatic attraction with the center plate which can be calibrated as a force. The transducer is attached to a piezo scanner, which allows the nanoindenter probe to be used as an imaging stylus as well. (b) The Nano Indenter G200 from KLA-Tencor Corporation (Milpitas, CA, USA) uses an electromagnetic force transducer and capacitance gauge to measure displacement. The probe assembly is also supported by leaf springs. Image Credit: Jakes, JE and Stone, DS
From traditional nanoindentation to enhanced nanoindentation
Performing fused silica calibration tests to calculate nanoindentation machine compliance and probe zone function is a common part of traditional nanoindentation study. These calibrations are then used to calculate hardness and modulus of elasticity by analyzing the load-depth trace of the test material.
The key to the improvements is to determine multi-charge nanoindentations, in which the mechanical characteristics are evaluated based on the size of the nanoindentation at each nanoindentation point.
A substitute for improvisation may be to rely on continuous stiffness measurements (CSM), but additional effort is required to make the CSM reliable.
Basics of nanoindentation
One of the basic elements includes instrumentation to determine load and displacement separately, an actuation mechanism to perform a predefined loading function, and the ability to accurately locate nanoindentation in the sample.
In addition, the basic contact mechanics and the load depth trace are analyzed to determine elastic properties such as Young’s modulus of wood. In addition, structural compliance VSs The method is applied to study the results on the nanoindentation of the bending of neighboring edges at the sample scale.
Multicharge nanoindentations are performed to assess accuracy VSs. In addition, to acquire charge (P) and depth (h) as accurately as possible, the user has to construct pre-nanoindentation charge functions and experimental techniques. It is also necessary to select a completely flat, clean wood sample placed perpendicular to the direction in which the nanoindentation probe is driven into the material.
Diagrams illustrating potential sources of structural conformity in nanoindentation of the wood cell wall, including (a) bending at the scale of the sample represented by deformations in the cellular structure of the wood, (b) a free edge, for example with an empty light, and (vs) a heterophasic interface, such as between the CCML secondary cell wall and S1. Image Credit: Jakes, JE and Stone, DS
Materials and methods
The sample, endwood loblolly pine, is prepared under ambient conditions. During studies, the temperature is not actively regulated and fluctuates between 24 and 26 degrees Celsius. Additionally, residual fingerprints are captured with an atomic force microscope (AFM) in contact mode using advanced surface microscopy.
Analysis algorithm
To ensure the order of the process, it is necessary to perform an analysis algorithm that allows the data to be deleted or improved by making changes.
First, fused silica calibrations are performed to determine the VSm, the function of the probe area and the imperfections of the tip of the Berkovich probe. Second, images of the residual nanoindentation imprints are used to verify the placement and quality of the nanoindentation. Scanning probe microscopy (SPM) or AFM are commonly used to obtain these images.
Third, a preliminary charge depth trace is checked to identify abnormal behavior of the results obtained. If only a small percentage of the nanoindentations in a dataset behave abnormally, they should be removed before the analysis method continues.
Next, a pre-nanoindentation liftoff analysis is performed to detect dirty probe tips. In the next step, the displacement drift is measured and corrected. Then, a preliminary analysis is performed to investigate whether unloading segments correspond to nanoindentations with a nanoindentation diameter, Ao ½ > 0.266 µm. Finally, area errors are detected, then the results are compared to independent area measurements.
Diagram illustrating how to design an experiment by taking advantage of a row of daughter cells to study the effects of a treatment, such as an adhesive or a coating, on the properties of the cell wall with high sensitivity. Image Credit: Jakes, JE and Stone, DS
Study results
To highlight the main results of this study, only nanoindentations with at least three unloading segments corresponding to the size criterion were initially included. Moreover, the constant characteristics for Ao ½ > 0.266 µm also indicated that structural conformities had been correctly taken into account and that no significant depth and surface detection error had occurred.
Finally, the most important result was that the properties do not depend on the size of nanoindentation Ao ½ > 0.266 µm. In the improvements, it was advised to perform a single experiment for all treatments and to include as much information as possible in the data.
Nanoindentation – A valuable tool
Nanoindentation has proven to be an effective method for determining the mechanical characteristics of complex materials at the micrometer scale such as cell walls in wood. The accuracy of quasistatic measurements of Berkovich hardness and modulus of elasticity in complex materials, such as wood cell walls, has been improved using nanoindentation techniques and an analysis program proposed in this research.
Keep reading: https://www.azonano.com/article.aspx?ArticleID=5890.
Further reading
Jakes, JE and Stone, DS (2021) Best Practices for Quasistatic Berkovich Nanoindentation of Wood Cell Walls. Forests, 12(12). Available at: https://www.mdpi.com/1999-4907/12/12/1696
