Micromechanics of oxides - From complex scales to single crystals

Doctoral thesis, 2019

At some point in your life, you might have heard of a plane crash occurring due to jet engine failure or a power plant shutting down due to turbine failure. Why do these happen in these critical applications, even after careful design and taking numerous safety measures? Advanced materials have been developed that can withstand high temperatures while maintaining their mechanical strength. Still such unexpected failure can occur which is because of the conditions these materials experience during operation. The temperatures vary, so does the mechanical loads, and all this happens in a corrosive environment. Oxygen might be a lifesaver for us, but it reacts with most metals and corrodes them. This is prevented by adding elements such a chromium and aluminium, which react with oxygen to form a continuous and dense oxide layer and prevent further reaction of oxygen, the same way as a controlled burn prevents spreading of wildfire. These layers also experience the same conditions as the high temperature materials do, which can lead to their cracking and removal from the surface, since oxides are brittle. This can initiate fracture in the material as well, and thus their life is significantly reduced. Therefore, it is very important that we study the mechanical properties of such layers so that they can be included in the life estimation of these advanced materials.

However, their size is in micrometre range, i.e. they are about 1/100th the width of human hair. Therefore, it is difficult to design experiments to study their mechanical properties. Scientists over the last few decades have done such fracture studies on protective oxides, but most data available is valid for oxide layers that are thicker than 10 microns. Also, the mechanical properties measured are influenced by internal stresses and the underlying metal. Micromechanical testing is a field which, as the name suggests, enables mechanical testing at the microscale. Specialised instruments can be used to make specimens that are in the same size range as the oxide scales we need to test. In this work, we have developed a sample geometry based on a micro sized cantilever, which can be bent using high precision instruments and record how the oxide scale behaves under those conditions. This geometry has the advantage that the oxide layer is isolated from the underlying metal, so the internal stresses do not affect the properties being measured.

This method has been used successfully to demonstrate the microcantilever bending of oxide layers grown on a high temperature material used in jet engines, where plasticity was seen at room temperature, even though oxides are generally brittle. It is also possible to calculate under what conditions the scales fail, and also how they fail, which has been demonstrated with the help of a commonly used protective oxide scale, Cr2O3. The methods developed in this work can contribute information critical to developing reliable life estimation models for such materials used in high temperature applications and make them safer.