Atomic hydrogen accumulates in the parts of a component that are subject to particular stresses, such as at welding seams or in areas under tension. Hydrogen embrittlement then becomes a problem, especially in components that are exposed to high operating temperatures.
Physicist Lukas Gröner, Fraunhofer Institute for Mechanics of Materials IWM, MicroTribology Centrum µTC, developed and tested special coatings for steel components that virtually prevent the penetration of atomic hydrogen. These are so-called MAX-phase materials, which have been the subject of international research for over ten years.
MAX-phases, like ceramics, are insensitive to attack by oxygen and very heat-resistant. At the same time, they are electrically conductive like metals. Unlike pure ceramics, they are not brittle, so they do not break. Lukas Gröner has now succeeded in producing thin MAX-phase coatings that protect steel very well against corrosion and hydrogen embrittlement.
In a vacuum chamber, he first deposited very precisely alternating layers of aluminum nitride, an aluminum-nitrogen compound, and titanium on a steel surface using physical vapor deposition (PVD). This sandwich structure, which is only about three micrometers thick, was then heated to form a very thin MAX-phase layer of titanium, aluminum and nitrogen (Ti₂AlN). The challenge was to control the deposition of titanium and aluminum nitride in such a way that parallel Ti₂AlN platelets were formed during subsequent heating.
Gröner also investigated how the MAX-phase coating behaves when it is intensively heated – as could be the case in future gas turbines or fuel cells. To simulate normal operating conditions, he heated the material to 700 degrees and left it in the furnace for up to 1.000 hours. This created a thin layer of a special aluminum oxide on the top side of the coating – α -Al₂O₃. As was shown in the further course of the investigations, this thin aluminum oxide coating considerably increases the barrier effect of the protective layer against hydrogen.
_{(source: Fraunhofer IWM)}