Cracks in some metal-ceramic materials spontaneously fill at high temperatures. Now, researchers have revealed the process with x-ray tomography in Nature Scientific Reports.
It took them four visits to the Paul Scherrer Institute in Switzerland of a couple of days each. The practical challenges were manifold: Putting a prepared ceramic sample in the X-ray microscope, driving a wedge into it to force cracks and heating it evenly to 1,500 degrees Celsius with infrared lasers. All that proved harder than expected. “When we pulled the wedge back, the sample remained stuck on it,” said Dr. Wim Sloof, lead author of the article. Eventually, they did succeed in recording in unprecedented detail the spontaneous healing of three cracks in a metal-ceramic material.
Dr. Sloof, associate professor at the Faculty of Mechanical, Maritime and Mechanical Engineering, worked together with Professor Sybrand van der Zwaag (Faculty of Aerospace Engineering) and with researchers from Manchester University, the Paul Scherrer Institute and the Research Complex at Harwell (UK). Their publication ‘Repeated crack healing in MAX-phase ceramics revealed by 4D in situ synchrotron X-ray tomographic microscopy’ was published in Nature Scientific Reports on March 14, 2016.
“For a material to spontaneously self-heal, it needs to contain metal atoms that are mobile enough to come to the surface and oxidate,” Sloof explained. Ordinary ceramics can withstand heat, but they do not self-heal. A special class of ceramics does contain metals. Called MAX-phase ceramics, they even include two types of metals bound to carbon (carbides) or nitrogen (nitrides). One of these metals, aluminium, for example, oxidises on the fractured surface at high temperatures. Not only does the aluminium oxide fill the crack, but it also binds the material stronger than it was before.
MAX materials combine the best of ceramics (resistant to high temperatures and corrosion) and metals (toughness). Plus they self-heal cracks. These properties make them attractive for, for example, turbine blades. So much was evident about eight years ago. In his archive, Sloof keeps microscope photos showing a complete fill of a long crack. So what’s new in the published measurements?
“The news is that we now know how the process takes place, and how long it takes to heal a crack,” said Sloof. For a four millimetre long break of 10 microns wide, healing took about 30 minutes.
He went on to explain that mathematical models can now be set up to describe the self-healing process. These models could predict the self-healing properties of similar materials with a different composition regarding temperatures and duration.
–> Nature Scientific Reports | DOI:10.1038/srep23040
Comments are closed.