Thermally sprayed ceramic coatings are commonly used in the manufacturing industry in environments, where high temperatures, chemicals and mechanical wear is present. The drawback of ceramic coatings is their brittleness, which translates to even a small scale impact with enough energy being able to cause component failure and subsequent machine shutdown. Damage tolerance is a property that cannot be deduced from other coating properties, and until now there has not been a method to its determination.
“The measurement of damage tolerance is well-know with traditional material groups, such as ceramics, metals and polymers. What makes coatings challenging is their small scale. When the thickness of a coating is roughly 0.3 mm on top of a base materials 10-100x thicker, we must be careful what we are measuring”, Kiilakoski reveals.
In the thesis, measurement methods were evaluated for different mechanical stresses: high-energy; high-velocity impacts, slow bending of the coating inside an electron microscope and small-energy micro-impact fatigue. All of the methods aimed at filling a gap between laboratory tests and the requirements of real-life conditions.
“In R&D, for example, it takes courage to jump directly from lab results into manufacturing a pilot-scale component. The connection between the conditions of the two is rarely linear and, for now, there hasn’t been many measurement methods in the middle for this property”, states Kiilakoski.
The aim of the study was to develop ceramic coatings with improved damage tolerance after finding a suitable measurement method. A large European collaboration network was utilized in both developing the measurement methods and the coatings to ensure that the work would benefit from the latest knowledge in the field. Liquid feedstocks were utilized in producing ceramic coatings with improved damage tolerance, which allowed the implementation of nanostructured areas in the coatings. These areas can act as crack arrestor, slowing down the failure of the coating.
“The novel feeding system for liquid feedstock in to the high-velocity oxy-fuel spray system was a result of brainstorming with our collaborators and it is a big leap forward in the process know-how of this technology. Even though work remains to be done, the information sharing between the companies that have sponsored the work has strengthened the collaborative industrial network in the field of thermal spray.”
Jarkko Kiilakoski is originally from Lahti and works currently for Saint-Gobain Coating Solutions in Avignon, France, in the business development of thermal spray equipment and consumables.
The doctoral dissertation of M.Sc. Jarkko Kiilakoski in the field of materials science Damage Tolerance of Thermally Sprayed Oxide Coatings will be publicly examined in the Faculty of Engineering and Natural Sciences of Tampere University on Friday 18.12.2020 starting at 12 in the lecture hall K1702 of the Konetalo building, Korkeakoulunkatu 6. Professor Christopher Berndt from Swinburne University of Technology, Australia, and professor Jari Koskinen from Aalto University will act as opponents. Professor Petri Vuoristo from Tampere University will act as the custos.
The event can be followed via remote connection (MS Teams).
The dissertation is available online at http://urn.fi/URN:ISBN:978-952-03-1505-4
Photo: Mika Kanerva