Question: For whom are microstructural investigations interesting?
Answer: For those who want to find out the cause of damage of their component.
Wrong!
The microstructure of a material is one of the most important parameters – if not THE most important – to be used to assess the quality of a newly developed material or component. Therefore, information about the microstructure can be obtained by porosity measurements or strength tests, since these properties are greatly influenced by the microstructure. Directly, the microstructure of e.g. technical ceramics can be studied using a wide range of methods that have been available to industry and the end-user market since the last century to complement conventional optical microscopy.
You may have wondered why one series of components works “fine” while the following series experiences increased failures even though “nothing” has been changed chemically. One possible answer is the microstructure, because it is a sum of many individual aspects interacting with each other. If the configuration of the coarse and fine grains is incorrect, defects will already occur during production. During sintering, the wrong (mineral) phases are formed, an inhomogeneous microstructure develops, and finally, internal stresses are relieved by cracks under mechanical and thermal load – a (pre-)damage that nobody wants.
Improvements in the composition of a material and the manufacturing process or the change to a larger furnace for series production automatically lead to different properties, since the microstructure is always the sum of the starting materials and the treatment. If the microstructure of the component is continuously observed, its development can be traced and the resulting properties can thus be evaluated and improved. For example, the sintering curve for technical ceramics could be modified so that the formation of a relevant mineral phase is favoured by holding times, while disadvantageous areas are passed through as quickly as possible. In this way, the microstructure can be controlled in terms of mineral composition and porosity – which in turn leads to adapted properties that better suit the intended use.
The same applies to metals. Here, too, the method of production and treatment, and the resulting properties can be explained in a different way. The ferritic-pearlitic structure formed from the melt can become martensitic or form only a transitional structure as a result of quenching. Properties such as hardness can be shifted from “soft” to more hardness without loss of elasticity or pronounced brittleness. This formation of the grains can be clearly differentiated in a good micrograph and thus the history can be reconstructed – partly up to the rolling.
The chemical composition and the resulting mineralogy also have a major influence on the service behaviour of a component. Chromia, for example, is used in refractory materials as a corrosion inhibitor for alumina against the attack by alkalis and alkaline earths. However, it is not only the proportion according to the data sheet of chromia in the refractory brick that is important. If the distribution in the brick structure is not correct, even the correct percentage in the entire brick is of no use. A fine distribution of chromium oxide is much more effective than individual grains, where the corrosion is only locally inhibited. After all, the attack of alkalis and alkaline earths does not occur in one place, but over larger areas of the stone and also penetrates the microstructure. This is a good example of reduced resistance in thermochemistry due to an unsuitable microstructure.
It is therefore advisable to examine different types of bricks prior to the installation and to compare the manufacturers so no surprises happen when the microstructure is analysed to find the cause of the damage.
However, as simple as it may sound to look at the structure and composition of a material such as a technical ceramic to assess the performance of the component, it is important to choose the right method and preparation.
This is because the results can vary significantly. For example, looking at a fracture surface provides information about the initiator and type of fracture. A section perpendicular to the fracture surface provides deeper insights:
- the influence of the components (grains, glass phase, matrix)
- the extent of damage int the macroscopically “undamaged” microstructure
- the cause of the failure
With additively manufactured components such as binder jetting, the plane to be examined must be cleverly chosen. A lateral cut makes it possible to observe the layering and the bond between individual layers. The sectioning in the layer plane allows a statement to be made about the homogeneity of the powder distribution.
In any case, the analysis of the microstructure is not only indispensable in damage analysis, but also relevant for everyone who depends on high-performance and qualitative components: both in development, production and quality assurance.
