Depth-sensing nanoindentation is a new but well-established technique for the mechanical characterization of nanometer-scale materials. The purpose of this article is to provide a brief overview of the nanoindentation technique as it applies to ceramic materials.
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Ceramics are defined as inorganic, non-metallic, polycrystalline carbide, nitride, or oxide materials. Some of their examples are Tungsten carbide, Silicon carbide, Zinc oxide, Titanium carbide, Zirconia, Alumina, and Magnesia.
Typically, ceramic materials are ionic or covalently bonded. In ceramics, ionic and covalent bonds are responsible for many of their unique properties.
For example, high hardness and melting point make them excellent refractory materials; high electrical resistivity makes them suitable as an insulator, and low mass density is useful for space applications. Good chemical resistance also makes them durable.
Need for Nanoindentation
Understanding the mechanical characteristics of ceramic materials is crucial for evaluating the quality of products as well as for comprehending their failure mechanism. In addition, the mechanical property, particularly the modulus of elasticity, is a crucial aspect in the design of structural materials.
Traditional mechanical testing methods, such as bending and tensile tests, are utilized to determine the elastic modulus and fracture strength of ceramic materials. These tests are appropriate when an adequate volume of homogenous solid material is available but not when ceramic samples are very thin.
Nanoindentation: Basic Overview and its Importance in Ceramic Study
Nanoindentation is especially useful for small ceramic samples or thin ceramic coatings. This technique can quantify mechanical properties such as elastic modulus and hardness, yield strength, residual stress, fracture toughness and adhesive strength of coatings.
In general, the indentation test consists of penetrating a hard indenter at the surface of a material at a given indentation load. For this purpose, the indenter can be of various geometrical shapes, such as pyramidal, conical, and spherical. Sharp pyramidal indenters have been widely investigated as depth-sensing indentation, determining cracking resistance and hardness of ceramic materials, whereas spherical indenters are primarily used to determine bulk modulus and tensile properties.
A known geometry indenter is penetrated into a material, and the penetration depth (h) is constantly recorded against the applied load (P) for both the loading and unloading cycles. The unloading curve (generally associated with elastic deformation) differs from the loading curve (elasto-plastic deformation), indicating permanent deformation with a hardness imprint.
Nanoindentation of Glass Ceramics
Many ceramics experts do not consider amorphous (non-crystalline) materials (such as glass) to be ceramics, despite the fact that their mechanical properties are comparable to ceramic materials. Heat treatments, on the other hand, can transform glass into a semi-crystalline material known as glass-ceramic. Glass-ceramics are composed of small grains encased in a glassy phase and have properties that are intermediate between glass and ceramics. They have numerous applications in biomedical devices and consumer electronics.
For example, Li2O-2SiO2 glass-ceramics ((LS2) are appealing materials for smartphone applications. Sharp contact damage resistance is essential for determining the viability of such applications. The growing demand for crack and scratch resistance cover materials emphasizes the importance of understanding contact deformation mechanisms in glass ceramics. Corning Research and Development Corporation researchers used molecular dynamics simulations to understand atomistic deformation mechanisms of glass-ceramics under nanoindentation.
In this study, the sharp intender tip with a radius of 1 nm is used to increase the likelihood of crack initiation events in the model samples. The indenter is steadily pushed into the sample to a depth of 25 nm, nearly one-third of the sample height, to simulate the indentation process. Following that, the indenter is removed at the same speed after briefly resting at the peak position. Nanoindentation provided detailed information on the effect of LS2 nanocrystal on crack system implications. Overall, nanoindentation could contribute to an improved understanding of the most commonly observed glass-ceramic surface damage behaviors.
Nanoindentation of Supercrystalline Materials
Supercrystalline materials are gaining popularity in the fields of materials science and nanoengineering. They are made up of nanoparticles arranged in ordered structures known as superlattices. Their established applications include optoelectronics, catalyst, energy storage materials, and sensors.
Researchers from Hamburg University of Technology used nanoindentation to investigate the properties of supercrystalline ceramic-organic nanocomposites. Moreover, cracking is more likely to occur in supercrystalline materials, which worsens when a sharper tip is used. As a result, scientists used the Berkovich indenter to assess elastic modulus and hardness and the cube-corner tip to investigate fracture toughness.
According to nanoindentation analysis of fracture toughness, the intrinsic fracture toughness of the supercrystals is much lower than the measured one. As a result, extrinsic contributions to the material's resistance to crack propagation, such as forming a plastic zone or microcracking, must be present in these crosslinked supercrystals.
Nanoindentation of Ceramic Coatings
The key difference while conducting nan indentation on a ceramic coating is that at a given penetration depth, the response will be provided not only by the first indented layer but also by the substrate layers. It is widely accepted that nanoindentation with a maximum depth of less than 10% of the coating thickness will yield the coating's hardness without influencing the substrate. As a result, using spherical indenters rather than sharp indenters allows for characterizing the elastic modulus at much lower penetration depths.
Challenges and Future Direction
Once correctly calibrated, nanoindentation is a powerful technique for extracting material properties at very small scales. Because of ceramic materials' high hardness and stiffness, special care must be taken in data acquisition and analysis. When indenting ceramic laminates, the underlying material's influence must be considered.
References and Further Reading
Xiao, H., et al. (2021). Theoretical model for determining elastic modulus of ceramic materials by nanoindentation. Materialia, 17, 101121. https://doi.org/10.1016/j.mtla.2021.101121
Bumgardner, C. et al. (2021). Probing the local creep mechanisms of SiC/SiC ceramic matrix composites with high temperature nanoindentation. Journal of Materials Research, 36(12), 2420-2433. https://doi.org/10.1557/s43578-021-00128-2
Deng, B., et al. (2021). Toward revealing full atomic picture of nanoindentation deformation mechanisms in Li2O-2SiO2 glass-ceramics. Acta Materialia, 208, 116715. https://doi.org/10.1016/j.actamat.2021.116715
Bor, B., et al. 2019). Nanoindentation-based study of the mechanical behavior of bulk supercrystalline ceramic-organic nanocomposites. Journal of the European Ceramic Society, 39(10), 3247-3256. https://doi.org/10.1016/j.jeurceramsoc.2019.03.053
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