Suitable mechanical property is an important to look at in the selection of materials for any industrial application. In particular, the hardness of a material can be used to assess the material’s mechanical properties. This is because many material properties such as yield strength and wear can be effectively correlated with hardness. Therefore, hardness can also be used in quality control.
Material hardness is generally determined through indentation using indenters. Hard metal spheres, diamond pyramids, and truncated cones are just but a few examples of indenters used in hardness tests. These indenters have been effectively used in determining hardness for a variety of homogenous material. Unlike homogenous materials, no hardness test standards have been developed for ceramic matrix composite materials since they contain phases of different hardness. In ceramic matrix composites, hardness indentations have been noticed to be either much smaller or larger than the microstructural features thus requiring micro-hardness tests and micro-hardness tests respectively. However, precision measurements of indentation diameters have remained difficult due to the optically inhomogeneous surface of the composites.
Recently, German Aerospace Center researchers: Dr. Yuan Shi and Dr. Dietmar Koch together with Achim Neubrand from the Fraunhofer Institute for Mechanics of Materials explored the feasibility of using instrumented indentation tests as an alternative method for determining indentation diameters of ceramic matrix composites. By using large diamond spherical indenters, indentation diameters were determined indirectly from the depth of the indentation. Apart from determining the hardness, the authors also demonstrated how to extract the indentation modulus of isotropic materials. Their work is currently published in the journal, Advanced Engineering Materials.
In brief, a liquid silicon infiltrated process was used to fabricate composite plates of different thicknesses. The stiffness and hardness in out of plane orientation of these composites were investigated. Next, a hardness test was carried out. It involved the evaluation of the residual indentation using a hemispherical indenter to generate a load-unload curve. The curve was finally used to determine the hardness and stiffness. Investigation of hardness and stiffness through microscopic images and instrumented indentation tests were compared and contrasted.
The feasibility of using large diamond spheres-based indenters for determining indentation hardness in ceramic matrix composites was confirmed. The extended damaged region with ill-defined edges rendered the use of microscopy unsuitable method for direct indentation measurements due to the difficulty in determining the diameter of the contact area. This challenge was however addressed by instrumented indentation test. Therefore, by properly interpreting the load-displacement curves, reproducible values of the indentation hardness were determined.
The measured hardness was not depended on the thickness of the composite plates and indentation loads. However, this could be observed by maintaining meaningful limits of indentation loads and thickness. On the other hand, the results showed lower indentation modulus as compared to the measured out-of-plane modulus. According to Dr. Yuan Shi and his colleagues, the presented indentation hardness tests can be extended to other types of ceramic matrix composites. This would be of great significance in material characterization considering that hardness is generally economically viable and can be corelated with many material properties.
Shi, Y., Neubrand, A., & Koch, D. (2019). Characterization of Hardness and Stiffness of Ceramic Matrix Composites through Instrumented Indentation Test. Advanced Engineering Materials, 21(5), 1800806.Go To Advanced Engineering Materials