Normand Aubin C, Charrier Anne M, Arnould Olivier, Lereu Aude L
Aix Marseille Univ, CNRS, CINaM, Marseille, France.
LMGC, Université de Montpellier, CNRS, Montpellier, France.
Sci Rep. 2021 Mar 11;11(1):5739. doi: 10.1038/s41598-021-84994-0.
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.
由于在宏观尺度下木材的力学性能与亚微米尺度下细胞壁的力学性能之间已建立起相关性，基于原子力显微镜（AFM）的技术已广泛应用。特别是峰值力敲击技术，它能够区分不同的层，已成为木材细胞壁纳米力学表征的新标准。然而，其应用需要完全弹性压痕，对探针刚度有充分了解，并假设探针具有已知半径（球形）或角度（锥形）的完美尖端形状。对于像木材这样复杂、纳米结构且坚硬又有粘性的材料，这些严格的假设可能会导致提取参数时出现较大误差。在这项工作中，我们通过改进用于AFM悬臂和尖端的奥利弗和法尔纳米压痕处理及校准程序，提出了一种基于AFM力-体积压痕的可靠且互补的替代方法。引入的面积函数校准（AFC）方法能够大幅减少这些误差，并提供半定量测量。该方法无需事先了解尖端形状和悬臂刚度，且通过一个定性指标来研究粘塑性。结果表明，压痕参数的变化会影响所得测量结果，即压痕模量、粘塑性指数、粘附力和能量。将AFC方法应用于绘制张力木区域，可提供每个区域非常稳定的力学参数特征，这使得该方法在植物细胞壁研究中具有很高的价值。
