In-situ Observation of h-BN Formation on the Surface of Weld Dissimilar Joint Steels

In-situ observation of h-BN formation by surface precipitation on the surface of joined dissimilar steels is presented. Because the substrate consists of two different types of steels, different growth behaviors can be seen on different sides and also in the middle of the weld interface. This observation demonstrates that formations of 2D materials can occur on surfaces of steels under suitable conditions e.g. temperature, microstructures and concentrations of impurities. Characterizations by electron microscopy and synchrotron spectroscopy technics confirm that h-BN crystals that appear on the surface after annealing are of similar quality to those prepared by other methods such as chemical vapor deposition. Moreover, real-time observation during sample temperature swing above and below the phase transition temperature of Fe shows that h-BN islands reversibly form and dissociate on the surface. The results show that the formation of h-BN on steels is reversible and the analysis suggests that the process is likely affected by structural change of the steels near the phase transition temperature, which in-turn drives the diffusions of B and N atoms back and forth between surface and bulk.