Application Note | Indentation Mapping
Microindentation Mapping with Acoustic Emission on Glass
Hardness, Modulus and Crack Initiation Across a Glass Surface
Research & Experimental Testing
Duanjie Li & Jorge Ramirez
Visual Design & Editorial
Andrew Shore
Introduction
Hardness mapping has been widely used to assess the variation of mechanical properties across a large surface area. However, in the study of brittle materials such as glass, the crack initiation and propagation is of particular interest. Acoustic emission works as a useful tool to monitor the crack initiation and growth during the indentation process.
Importance of Microindentation with Acoustic Emission
Microindentation mapping has proven to be a critical tool for surface mechanics related studies. Fracture has typically been evaluated by measuring the dimension of the cracks of the indent, and the use of acoustic emission during testing has been an overlooked and valuable tool. Microindentation with the measurement of Acoustic Emissions (AE) provides a reliable and user-friendly method to track the fracture behavior and intensity during loading and unloading process.
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Measurement Objective
In this application, the NANOVEA Mechanical Tester, in Microindentation mode, performs a mapping of the mechanical properties such as the hardness, Young’s Modulus and the acoustic emission of a glass sample.
NANOVEA PB1000 Large Platform
Mechanical Tester
Test Conditions
The NANOVEA Mechanical Tester was used to perform a series (4×4, 16 indents in total) of micro indentations on a glass sample using a Vickers indenter. The load, depth and acoustic emission (AE) were measured and recorded during the indentation. All indentations were performed to a maximum load of 1 N. The test conditions are summarized in Table 1.
| Test parameter | Value |
|---|---|
| Maximum force | 1 N |
| Loading rate | 2 N/min |
| Unloading rate | 2 N/min |
| Poisson ratio | 0.3 |
| Computation method | Oliver & Pharr [1] |
| Indenter type | Vickers |
| Mapping | 4 by 4 indents |
Table 1: Test parameters of the indentation mapping.
Results and Discussion
The load-depth curve and evolution of the acoustic emission during the indentation are shown in Fig. 2 and Fig. 3, respectively. The image of the indentation map is displayed in Fig. 4. The high-precision position control of the sample stage allows users to select the target area for mechanical properties mapping.
The acoustic emission during the indentation provides the insight of the development of the cracks during the loading period. It can be observed that the acoustic emission signals start to be detected at a load of ~0.35 N, indicating the initiation of the crack at this point. As the indenter progressively penetrates into the glass, acoustic emission of higher intensity occurs, caused by the propagation of the crack. The NANOVEA Mechanical Tester allows quantification of AE in terms of energy released, unlike other systems that only provide a percentage of the intensity with no reference. At 1 N load the maximum AE value is 5 × 10⁵ attoJ (0.5 picoJ) during the loading. It is worth noting that the system can measure energy levels orders of magnitude lower than what is shown here and orders of magnitude higher.
The hardness and Young’s Modulus are summarized and compared in Fig. 5 and Fig. 6. The glass sample exhibits homogeneous mechanical properties: it possesses an average hardness of ~6.6 GPa and Young’s modulus of ~56.7 GPa throughout the test area. Fig. 6 provides a useful tool to observe the distribution of hardness and Young’s modulus across the tested sample surface. Such a 4×4 indentation matrix was finished within 20 minutes with superior precision and repeatability, thanks to the accurate position control and the direct load/displacement measurement of the NANOVEA Mechanical Tester.
The mapping mode in this study enables speedy quantitative mapping of mechanical properties including Young’s modulus and hardness, which is critical for quality control of a variety of materials.
Fracture Toughness
The fracture toughness of the glass can also be calculated based on the length of the cracks at the corners of the indents as shown in Fig. 4. Fracture toughness is calculated using the following equation:
KC = α (E/H)1/2 (P / c3/2)
where α is an empirical constant for a specified indenter, α = 0.016 for the Vickers tip, E is the Young’s Modulus, H is the hardness, P is the applied load, and c is the crack length.
Conclusion
In this study, we have demonstrated that the NANOVEA Mechanical Tester, in Microindentation mode, provides reproducible and precise mapping of mechanical properties across a glass sample. The sample possesses homogeneous hardness and elastic modulus in the measured area. The evolution of acoustic emission recorded during the indentation works as an important indication of the fracture process.
The Nano, Micro or Macro modules of the NANOVEA Mechanical Tester all include ISO and ASTM compliant indentation, scratch and wear tester modes, providing the widest and most user friendly range of testing available in a single system. NANOVEA’s unmatched range is an ideal solution for determining the full range of mechanical properties of thin or thick, soft or hard coatings, films and substrates, including hardness, Young’s modulus, fracture toughness, adhesion, wear resistance and many others. In addition, optional 3D non-contact profiler and AFM Module are available for high resolution 3D imaging of indentation, scratch and wear track in addition to other surface measurements such as roughness.
References
Oliver, W. C.; Pharr, G. M., Journal of Materials Research, Volume 7, Issue 6, June 1992, pp. 1564-1583.
Frequently Asked Questions About Microindentation of Glass
Why add acoustic emission to indentation of glass?
For brittle materials crack initiation and propagation are of particular interest. Acoustic emission tracks fracture behavior and intensity during loading and unloading, rather than relying only on measuring crack dimensions afterwards.
When did the glass start to crack?
Acoustic emission signals started at a load of about 0.35 N, indicating crack initiation; higher-intensity emission followed as the indenter penetrated further and the crack propagated.
How is acoustic emission quantified?
In terms of energy released rather than a percentage with no reference: at 1 N the maximum AE was 5 × 10⁵ attoJ (0.5 picoJ), and the system measures energies orders of magnitude lower and higher.
How uniform was the glass?
Across the 16 indents the glass was homogeneous, with an average hardness of about 6.6 GPa and Young’s modulus of about 56.7 GPa.
How long does a 4 × 4 map take?
The 16-indent matrix to 1 N with a Vickers tip was completed within 20 minutes, thanks to accurate position control and direct load/displacement measurement.
Can fracture toughness be calculated?
Yes, from the crack length at the indent corners: Kc = α (E/H)^½ (P/c^3/2), with α = 0.016 for a Vickers tip, E the modulus, H the hardness, P the load and c the crack length.
