以奈米壓痕法探討針葉樹木材細胞壁奈米機械性質

Abstract

隨著科技的進步，材料科學研究的方向已從大尺度往微觀科技發展，原子力顯微鏡（Atomic force microscopy, AFM）以原子間凡得瓦力（Van der Waals force）為探測媒介，可運用於奈米壓痕技術（nanoindentation），對材料的力學性質與結構做深入分析，進而了解材料的表面性質，並結合奈米壓痕之力學特性，對材料做出最佳的研究與應用，在學術與工業應用上皆具發展潛力。 本研究透過奈米壓痕測試與AFM原子力顯微鏡進行觀測，對臺灣針葉樹種的細胞壁S2層進行奈米性質研究，配合木材之生理特質如木質素與微纖維傾斜角（microfibril angle，MFA）進行比較；結果顯示，木材微觀春秋材力學性質特性存在著很高的變異性，受木材組成分與生理結構微妙變化的影響，亦可能因為壓痕試驗參數設定的影響造成塑性變形；而木材的彈性係數性質與理論模型與考量探針幾何效應所推算的趨勢相近，可以推論出，在微觀尺度下，木材的彈性係數性質和巨觀性質相同，隨MFA增加而降低，但由於尺度細微，所以趨勢緩和許多；此外，木材微觀硬度大致隨MFA增加而降低，但仍有偏離趨勢的樣本點，考量木材為非均質的生物性材料，可以推論木材之微觀硬度是同時受到基質與纖維，再加上非均質性材料特性共同影響的力學性質。 由實驗的結果可以了解，木材在微觀尺度下的性質量測需考量的變因較巨觀量測複雜許多，也不如巨觀性質有較為一致的結果，故木材的微觀性質日後仍有很大的研究潛力。

With the progress of technology, the trend of material science has shifted from macro-scale to micro-scale study. Atomic force microscopy (AFM), which probes via the Van der Waals force between atoms, can be applied in nanoindentation technology and have detailed analysis of the mechanical properties and the structure of materials. With the understanding of the surface properties and nanoindentation mechanics, we can make the best use of materials in academic research and industrial applications. It has large potential in both two fields. The S2 cell wall layer of common softwood species in Taiwan are studied through nanoindentation in our study. The micro-mechanics and biological properties such as lignin content and microfibril angles (MFA) are studied. The results indicate that there are high variability of the mechanical properties between early and latewood in micro-scale. It is affected by the differences of wood components and structures. Some plastic deformations may also exist due to the effect of indentation loading parameters. The trend of elastic modulus is close to the model calculation and the consideration of the geometry of indentation probes. It decreases with the increase of MFA, but shows a more gentle curve than macro study due to micro-scale observations. Furthermore, the hardness in micro scale is generally decrease with the increasing MFA, but some deviating samples exist. Considering the material properties of wood, we can infer that the micro hardness is affected by matrix, structure, and the inhomogenerous nature of wood. From the results of our study, the mechanical properties of wood in micro scale are much more complex than macro ones. And there is also high variability exist. The study on the mechanical properties of wood in micro scale still has large potential in the future. Our study provides a preliminary research on the material preparation and mechanical properties for the nanoindentation of Taiwanese tree species, which serves as references of further researches in the future.