What do you need to know about the fracture toughness of ceramics?

When describing the properties of technical ceramics, we primarily talk about strength, or more precisely, flexural strength. Ceramics generally have a so-called “catastrophic failure behavior”, which means that ceramics break suddenly and without “warning” when subjected to mechanical stress. This behavior can be well illustrated by determining the flexural strength in a bending test, as Anika Braun has described thoroughly on our homepage.

The fracture of a ceramic occurs spontaneously. If one tests metals or plastics in this test arrangement, one sees that the specimens are very clearly deformed before they fail.

If you compare the strength and the force-deformation curve of different ceramics, you can see very clear differences: A ceramic with relatively low strength (approx. 50 MPa) is, for example, cordierite, a technical ceramic which, due to its very low thermal expansion, is often used for applications subject to temperature changes (e.g. catalyst carriers in cars). A ceramic with significantly higher strength is aluminum oxide (approx. 350 MPa), which is used, for example, as a sealing washer or electrical insulator. Another significantly higher strength is achieved with a zirconium oxide ceramic, bending strengths of over 1200 MPa are possible. It is frequently used in dental ceramics (crowns and bridges) or implants.

Especially when it comes to dynamic loading, measuring the bending strength is no longer sufficient. In reality, components are often loaded to a certain extent and then unloaded again many thousands or millions of times. If we take the example of an implant for human hip joints into account, there is an impact load on the joint head with every step. None of these impacts exceeds the strength of the ZrO2 ceramic, but calculated over a period of e.g. 20 years and a daily number of steps of 4000, the load is over 29 million “impacts”. Here, a new characteristic value must be used, for which crack toughness has proven itself in the field of ceramics.

The destruction of ceramics occurs through cracks in their interior. The starting point are flaws created, for example, by molding, drying or sintering. When ceramics are loaded, these cracks grow. Loads can be chewing movements in dental ceramics, walking with hip implants or temperature changes of ceramics, e.g. in waste incineration plants. Under defined conditions, such loads are simulated in a bending test, in which the force is increased until the ceramic breaks. If only a force is applied that does not completely destroy the ceramic, this is referred to as a subcritical load. On closer inspection, however, it can be seen that such loads cause existing cracks to grow further. This is referred to as subcritical crack growth. There is a threshold value above which the crack length leads to destruction of the ceramic, this value is called the kIC value. Other terms for this are the “critical stress intensity factor” or the “crack toughness”. This value is given by a number that typically ranges from 0.5 to about 20, and the unit is MPa m-0.5. Ceramics that have high crack toughness are ZrO2 or silicon nitride. Fiber-reinforced ceramics have even higher fracture toughness due to their structure.

There are several options to choose from for testing crack toughness:

A very simple but also very imprecise method to determine the kIC value is to measure the Vickers hardness. This involves pressing a pyramid-shaped diamond syringe into the surface of a specimen. The hardness is derived from the cross-section of the indentation. To obtain a statement about the crack toughness, one can measure the crack lengths in the corners of the indentation and calculate the kIC value via a formula. The following figure shows such a hardness indentation in an Al2O3 ceramic: it is well visible that the cracks at the 4 corners of the indentation are of different lengths and the determination of their correct length is very difficult. Thus, the measurement error is large. Moreover, different formulas are available to convert the crack lengths into the kIC value.

Source: Mussler, B., Swain, M. V. und Claussen, N. (1982) „Dependence of Fracture Toughness of Alumina“, Journal of the American Ceramic Society, 65(11), S. 566–572.

WZR uses a slightly more complex but many times more precise method to determine the fracture toughness. The measured variables are the hardness, the modulus of elasticity, the strength and the strength after a defined damage to the specimen. Especially for this “defined damage of the specimen” there are again several possibilities. A great amount has been described in the literature on this subject as well. In the past, WZR has carried out series of tests to evaluate the fluctuation of the measured values with the various damage methods. Very consistent results at comparatively low cost could be obtained by damaging the specimen with a Vickers indentation before measuring the flexural strength. This method is abbreviated to ISB, which stands for “Indentation/Strength in Bending”.

If we compare the kIC values of different ceramics (see following table), we see that brittle ceramics such as aluminum oxide have a relatively low kIC value, but highly stressable ceramics such as zirconium oxide or silicon nitride have very high kIC values. In principle, the higher the kIC value, the more damage-tolerant the material. So if damage tolerance is an important characteristic value for a component (as in the example of the hip joint), a suitable material can be identified by comparing the kIC values of different ceramics.

Source: Kollenberg, W. (2009): Technische Keramik, Vulkan Verlag

Literature data and values in the manufacturers’ data sheets are always based on the respective raw materials, production parameters and sintering conditions. If something changes in these parameters, e.g. an alternative raw material is used or the production is changed from injection molding to axial pressing, this will have an effect on the crack toughness. WZR offers testing of materials as an independent service provider certified according to DIN ISO 9001:2015. In addition to determining measured values, we also offer our customers support in interpreting the measured values.

Finally, the measured values are sometimes outside the expected range using the same raw material or show an unexpected scatter. Here, we are happy to support you in finding the cause in the manufacturing process and help to optimize the processes in order to achieve an improvement in the fracture toughness.

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