目的 氧化铝（Al2O3）陶瓷作为典型硬脆材料,其微观力学行为的准确表征对材料设计与性能评估具有重要意义。纳米压痕技术虽能直接获取硬度与弹性模量,但加载方式对测试结果影响显著,尤其在高载荷下易引发表面损伤,导致测量误差。方法 本研究通过实验与仿真相结合的方法,系统探究了50~250 mN范围内不同最大加载力对单晶Al2O3纳米压痕响应的影响。结果 100 mN为最优测试载荷,此时压痕形貌完整、力学参数稳定;载荷过低（50 mN）时压痕尺寸过小,受表面效应干扰显著;载荷过高（≥150 mN）则引起材料开裂与塑性堆积,导致接触面积计算偏差。有限元仿真进一步揭示,压痕过程中应力高度集中于压头棱线区域,该处为裂纹萌生的主要位置;高载荷下残余应力与塑性应变显著累积,加剧损伤演化。结论 本研究建立了一种融合实验与模拟的纳米压痕分析方法,不仅为单晶Al2O3的可靠力学表征提供了载荷优化依据,也为硬脆材料在微纳尺度下的损伤机理研究与测试标准化奠定了基础。
Abstract
The work aims to present a combined experimental-simulation investigation into the effect of the maximum loading force on the nanoindentation response and deformation mechanisms of single-crystal Al2O3, to determine the optimal loading range for reliable mechanical property extraction and elucidate the underlying damage initiation and evolution processes that are otherwise difficult to capture experimentally. A systematic approach integrating precise nanoindentation tests with a rigorous dynamic finite element model is implemented.
Nanoindentation experiments were performed on polished single-crystal Al2O3 substrates with a Berkovich diamond indenter. A series of maximum loads (50, 100, 150, 200, and 250 mN) were applied under load control mode, with a 10-second holding period at peak load to mitigate creep effects followed by complete unloading. Three indentations were performed for each load condition with sufficient spacing to avoid interaction. Hardness (H) and elastic modulus (E) were derived from load-displacement curves with the Oliver-Pharr method, while post-indentation surface morphology including crack patterns and pile-up features was characterized via scanning electron microscopy (SEM).
Concurrently, a three-dimensional dynamic finite element model was developed in ABAQUS/Explicit to simulate the entire nanoindentation process. The model employed a representative cylindrical Al2O3 sample (8 μm diameter, 20 μm height) and a rigid Berkovich indenter. Al2O3 was modeled as an isotropic elastic-plastic material with a Young's modulus of 340 GPa and Poisson's ratio of 0.27. A surface-to-surface contact algorithm with a friction coefficient of 0.1 was defined, and the mesh was refined in the contact region to ensure accurate stress/strain calculation. The simulation replicated the experimental loading protocol precisely.
The experimental results identified 100 mN as the optimal load for characterizing single-crystal Al2O3, yielding well-defined indentations free of major cracks and consistent mechanical properties (average hardness to 20.0 GPa, elastic modulus to 340 GPa). At 50 mN, excessive indentation size led to significant data scatter due to surface roughness and instrument noise. At loads ≥150 mN, pronounced radial cracks and edge pile-up caused erroneous contact area estimation in Oliver-Pharr analysis, resulting in artificially elevated property values.
Finite element simulations revealed extreme stress concentration along Berkovich indentation ridges, where von Mises stress exceeded Al2O3's theoretical strength above 100 mN, identifying these sites as primary crack initiation loci, which was validated by SEM observations. The model also quantified residual stress and plastic strain evolution, explaining the transition from elastic recovery at 50 mN to dominant plastic deformation and damage at 250 mN. Excellent agreement between experimental and simulated load-displacement curves was achieved in the 100-200 mN range (discrepancies <8%), while larger discrepancies at 250 mN were attributed to unmodeled extensive crack propagation and comminution.
In conclusion, this work establishes a robust framework for optimizing nanoindentation protocols for hard brittle ceramics, definitively identifying 100 mN as the upper threshold for damage-free property measurement in single-crystal Al2O3. It decouples the mechanisms of high-load property overestimation and demonstrates that integrated experimental-simulation approaches provide powerful tools for deconvoluting complex micro-mechanical responses in advanced ceramics.
关键词
单晶Al2O3 /
纳米压痕 /
有限元仿真 /
加载力 /
力学性能
Key words
single-crystal Al2O3 /
nanoindentation /
finite element simulation /
loading force /
mechanical properties
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基金
国家重点研发计划（2023YFB3812700）
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