Ammonite Fossil Mechanical Properties Using Nanoindentation

As the remaining traces of plants, animals and other organisms that once lived on the ancient earth, fossils preserve information on the evolution of life. The oldest fossils can be traced back as far as the Archaean Eon, about 3.48 billion years old^{1, 2}. Paleontology studies the formation mechanism and evolutionary relationships of fossils across geological time. The fossilization process includes several major mechanisms depending on the tissue type and external conditions, such as permineralization, casts and molds, authigenic mineralization, replacement and recrystallization.

Fossils possess different mechanical properties determined by the stage and mechanism of the fossilization process. For instance, shells and bones can be replaced with different types of minerals when they are exposed to various environments. At different stages of the recrystallization process, the hardness of the fossil changes as well.

Nanoindentation precisely measures the mechanical behaviors of materials at small scales^{3, 4}. The high-resolution load-displacement curves from the nanoindentation measurement can provide a variety of physico-mechanical properties, including hardness, Young’s modulus, creep, fracture toughness and many others. A mechanical property mapping of the fossil sample can play a key role in determining the mechanism and stages of the fossilization process. The small indentation force applied using the nanoindentation technique provides an ideal non-destructive mechanical testing solution to prevent damage to fossils under test.

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The Nano Module of the NANOVEA Mechanical Tester was applied to perform a series (3 × 3, spaced 0.08 mm apart, 9 indents in total) of nanoindentations on the top of an ammonite fossil sample using a Berkovich indenter. All indentations were performed to a maximum load of 50 mN. The distribution of the indentations is illustrated in Fig. 2 and the test conditions are summarized in Table 1.

The images of the 3 × 3 indentation matrix in the dark and light areas are displayed in Fig. 2. Two representative load-displacement curves of the indentations on the dark and light areas are shown in Fig. 3, and the corresponding hardness and Young’s modulus calculated using the Oliver and Pharr method⁵ are summarized and compared in Fig. 4. The light area possesses an average hardness of ~4.9 GPa and Young’s modulus of ~90.4 GPa, compared to ~4.1 GPa and ~87.2 GPa, respectively, for the dark area.

Such a 3 × 3 indentation matrix was carried out with superior accuracy and repeatability, thanks to the high-precision piezo load control and the direct load/displacement measurement by the NANOVEA Mechanical Tester. The excellent position control of the sample stage allows users to precisely pinpoint the target area for mechanical properties mapping. This makes mapping of the mechanical properties substantially less time-consuming and user friendly. It provides a solution for detecting different materials, phases and fossilization stages at different locations of the fossil sample.

We welcome further discussion and comments on the correlation of the hardness/Young’s modulus and the stage of fossilization and recrystallization, which will be a useful tool that provides insight into the mechanism of fossil formation and the fossil’s age.

In this study, we showcased the capacity of the NANOVEA Mechanical Tester in performing nanoindentation mapping on a fossil sample. The precise position control and accurate measurement of hardness and Young’s modulus enable identification of the mechanical properties of the materials at different locations on the fossil surface, which may be related to the stage of fossilization and recrystallization, as well as the fossil’s age. The tested ammonite fossil sample possesses varied hardness from 4.1 to 4.9 GPa at different locations.

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. 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.

1. Noffke, N.; Christian, D.; Wacey, D.; Hazen, R. M., Astrobiology, 13 (12), pp. 1103–1124.
2. Borenstein, S., Oldest fossil found: Meet your microbial mom, Associated Press, 15 November 2013.
3. Oliver, W. C.; Pharr, G. M., Journal of Materials Research, Volume 19, Issue 1, January 2004, pp. 3–20.
4. Schuh, C. A., Materials Today, Volume 9, Issue 5, May 2006, pp. 32–40.
5. Oliver, W. C.; Pharr, G. M., Journal of Materials Research, Volume 7, Issue 6, June 1992, pp. 1564–1583.