MAX Phase Thin Films As Electrical-Conductive Corrosion-Protective Coatings with Self-Healing Abilities
Authors: Grieseler Rolf , Camargo Magali Karina , Hopfeld Marcus , Diaz Tang Isabel , Bund Andreas , Schaaf Peter 
Published: February 2020

Nanolaminate Mn+1AXnphases (short MAX) are materials, that show interesting properties [1]. They exhibit high hardness as well as good thermal and chemical stability, which is commonly a ceramic like behavior. On the other hand, they are good conducting materials in the range of metals. MAX phases are a class of ternary carbides and nitrides, where M is an early transition metal, A is an element belonging to the groups IIIA to VA and X is either carbon or nitrogen.
One of their properties which is similar to a ceramic material is their good chemical stability and therefore, they are very well suitable as corrosion protective coating. Up till now the corrosion behavior was investigated for bulk materials in different corrosive media [2] or at elevated temperatures [3,4]. Nevertheless, there is still an ongoing discussion about the mechanisms behind the corrosion protective properties. In most of the cases it is attributed to the crystalline structure of these materials. MAX phases show a hexagonal structures build of MX-octahedrons with intermediate A-element layers between them. Some attribute the corrosion protection properties to the passivation of the A element others to the passivation of the M element.
In the presented work, the focus lies on the investigation of the corrosion protective properties of MAX phase thin films. Therefore, multilayer thin films of the three elements were deposited onto the substrates using PVD magnetron sputtering. The formation of the MAX phase was achieved using a subsequent rapid thermal annealing [5,6].
Afterwards the anodic behavior of the deposits was investigated electrochemically in 1M HCl and H2SO4 electrolytes. A good stability of the films in this media could be shown. To prove the self-healing behavior of this thin films, the deposits where mechanically scratched and afterwards annealed at 1000°C in air. A healing of the films could be shown attributed to the formation of oxides during the annealing.
All obtained properties show that MAX phases are very well suitable as corrosion protective coatings in aggressive medias and damage can be repaired taking advantage of the self-healing properties of the films.
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