Influence of force volume indentation parameters and processing method in wood cell walls nanomechanical studies

Aubin C Normand,Anne M Charrier,Olivier Arnould,Aude L Lereu

doi:10.1038/s41598-021-84994-0

Abstract

Since the established correlations between mechanical properties of a piece of wood at the macroscopic scale and those of the cell wall at the submicron scale, techniques based on atomic force microscopy (AFM) have become widespread. In particular Peak Force tapping, allowing the differentiation of various layers, has become the new standard for wood cell wall’s nanomechanical characterization. However, its use requires fully elastic indentation, a good knowledge of stiffness of the probe and assumes a perfect tip shape of known radius (sphere) or angle (cone). Those strong hypotheses can result in large approximations in the extracted parameters for complex, nanostructured, and stiff and viscous materials such as wood. In this work, we propose a reliable and complementary alternative based on AFM force-volume indentation by refining the Oliver and Pharr nanoindentation processing and calibration procedure for AFM cantilever and tip. The introduced area-function calibration (AFC) method allows to considerably reduce these approximations and provides semi-quantitative measurements. No prior knowledge of the tip shape and cantilever stiffness are required and viscoplasticity is investigated through a qualitative index. Indentation parameters variations are shown to impact the resulting measurements, i.e., indentation modulus, viscoplasticity index, adhesion force and energy. AFC method, applied to map regions of tension wood, provides very stable mechanical parameters characteristic of each region, which makes this method of high interest for plant cell wall studies.

Highlights

Since the established correlations between mechanical properties of a piece of wood at the macroscopic scale and those of the cell wall at the submicron scale, techniques based on atomic force microscopy (AFM) have become widespread
We propose an alternative procedure, complementary to Peak force quantitative nanoscale mechanical characterization (PF-QNM), based on AFM Force-Volume (FV) indentation mode with a data processing method adapted from the one traditionally used for nanoindentation[13,29,30,31] coupled with a method of calibration of the tip that we named area-function calibration method (AFC) using a reference sample
The areafunction calibration (AFC) method makes it possible to determine the indentation modulus independently from the knowledge of the cantilever stiffness and avoid the need to know the real shape of the tip beforehand, it allows the use of atypical ones

Summary

Introduction

Since the established correlations between mechanical properties of a piece of wood at the macroscopic scale and those of the cell wall at the submicron scale, techniques based on atomic force microscopy (AFM) have become widespread. Mechanical measurements made using FV mode and evaluated with Oliver and Pharr method have already been performed on several materials such as pulp fibers[32] or metal nanosheets[33] for example They are highly affected by various parameters such as surface roughness (induced, among others, by the sample surface preparation process), cantilever calibration or the model used to describe the geometry of the tip. The AFC method makes it possible to determine the indentation modulus independently from the knowledge of the cantilever stiffness and avoid the need to know the real shape of the tip beforehand, it allows the use of atypical ones It consists in building a tip/surface area function, i.e., the tip/sample contact area versus the indentation depth function, using a reference sample of known indentation modulus with a low surface roughness. We focus on the comparison of these two computing methods and on the interpretation of such measurements applied to wood cell walls

Methods

Results

Discussion

Conclusion