On the fracture behavior of Cr2AlC coatings

Abstract

Bulk MAX phase materials were investigated heavily in the last decades due to their advantageous combination of metallic and ceramic properties. In recent years, MAX phases also gained the interest of the protective coatings community. Cr2AlC is a very promising material, since the crystalline MAX phase can be deposited at comparatively low (550 °C) substrate temperatures. Another advantage of the Cr2AlC MAX phase is its self-healing ability. The goal of this investigation was to characterize the fracture toughness of Cr2AlC protective coatings using in situ SEM micro-cantilever tests and to determine the influence of different microstructures on the fracture behavior. Surprisingly, the fracture toughness is only moderately affected by the microstructure of the crystalline samples investigated here, which reveal a fracture toughness ranging from 1.8 ± 0.1MPam1/2 to 2.4 ± 0.2MPam1/2.In contrast to that, it could be shown that there is a significant increase in fracture toughness for the amorphous coating with identical chemical composition (4.1 ± 0.5MPam1/2) of almost twice the fracture toughness compared to the crystalline coatings. The detrimental influence of grain boundaries in the crystalline coating and the lack of grain boundaries in the amorphous sample might explain the formidable fracture toughness.