Best Practices for Quasistatic Berkovich Nanoindentation of Wood Cell Walls

Joseph E Jakes,Donald S Stone

doi:10.3390/f12121696

Abstract

For wood and forest products to reach their full potential as structural materials, experimental techniques are needed to measure mechanical properties across all length scales. Nanoindentation is uniquely suited to probe in situ mechanical properties of micrometer-scale features in forest products, such as individual wood cell wall layers and adhesive bondlines. However, wood science researchers most commonly employ traditional nanoindentation methods that were originally developed for testing hard, inorganic materials, such as metals and ceramics. These traditional methods assume that the tested specimen is rigidly supported, homogeneous, and semi-infinite. Large systematic errors may affect the results when these traditional methods are used to test complex polymeric materials, such as wood cell walls. Wood cell walls have a small, finite size, and nanoindentations can be affected by nearby edges. Wood cell walls are also not rigidly supported, and the cellular structure can flex under loading. Additionally, wood cell walls are softer and more prone to surface detection errors than harder inorganic materials. In this paper, nanoindentation methods for performing quasistatic Berkovich nanoindentations, the most commonly applied nanoindentation technique in forest products research, are presented specifically for making more accurate nanoindentation measurements in materials such as wood cell walls. The improved protocols employ multiload nanoindentations and an analysis algorithm to correct and detect errors associated with surface detection errors and structural compliances arising from edges and specimen-scale flexing. The algorithm also diagnoses other potential issues arising from dirty probes, nanoindenter performance or calibration issues, and displacement drift. The efficacy of the methods was demonstrated using nanoindentations in loblolly pine (Pinus taeda) S2 cell wall layers (S2) and compound corner middle lamellae (CCML). The nanoindentations spanned a large range of sizes. The results also provide new guidelines about the minimum size of nanoindentations needed to make reliable nanoindentation measurements in S2 and CCML.

Highlights

Over the past 25 years, nanoindentation has become a valuable tool in wood science and forest products research
An advantage of nanoindentation over other mechanical testing techniques is its ability to probe micrometer-scale volumes of materials in situ with a minimal amount of sample preparation. This makes nanoindentation ideally suited for probing individual wood cell wall layers, such as the S2 cell wall layer (S2) and compound corner middle lamella (CCML) (Figure 1), and other micrometer-scale features in woodbased materials, such as wood-adhesive bondlines in laminated structures, wood-matrix interphase regions in wood-based composites, and coatings on engineered wood products
From overview images of the residual nanoindentation impressions, two S2 and three CCML nanoindentations were discarded because they overlapped another cell wall layer or were placed in an obviously damaged area of a cell wall layer

Summary

Introduction

Over the past 25 years, nanoindentation has become a valuable tool in wood science and forest products research. Elastic modulus quantifies a material’s stiffness or resistance to elastic (recoverable) deformation under stress. Hardness quantifies a material’s resistance to plastic or permanent deformations. An advantage of nanoindentation over other mechanical testing techniques is its ability to probe micrometer-scale volumes of materials in situ with a minimal amount of sample preparation. This makes nanoindentation ideally suited for probing individual wood cell wall layers, such as the S2 cell wall layer (S2) and compound corner middle lamella (CCML) (Figure 1), and other micrometer-scale features in woodbased materials, such as wood-adhesive bondlines in laminated structures, wood-matrix interphase regions in wood-based composites, and coatings on engineered wood products. Nanoindentation is the only technique suitable for studying the mechanical properties and effects of treatments in these small volumes of material

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