Can high strength and moderate ductility be combined in wear resistant coatings? A fundamental plasticity study of X2BC nanolaminates (X=Hf, Mo)

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In situ synchrotron X-ray diffraction experiments and small scale mechanical studies supported by electron microscopy will be conducted to delineate the relationship between coating stress and strain with the plastic behavior of Mo2BC. We seek to identify the fundamental mechanisms that govern the plastic behavior of X2BC nanolaminates (X=Hf, Mo) and compare these results to previously published and here obtained quantum mechanical predictions. Since our initial micro mechanical data support the notion of moderate ductility predicted by the quantum mechanical calculations this project aims at answering the following questions linking microstructural characteristics with the mechanical properties: 1. Does the microstructure of X2BC nanolaminate (X=Hf, Mo) coatings influence their mechanical behavior? The microstructure in coatings is controlled by the deposition parameters such as deposition temperature, deposition rate, and degree of ionization of the film forming species. We will deposit fully amorphous, fully crystalline coatings as well as films with various amorphous to crystalline phase fractions. TEM will be employed to determine the domain sizes of the crystalline and amorphous regions and, together with XRD, also the domain size of the crystalline regions for the fully crystalline films. In situ mechanical straining will be performed for different microstructures of X2BC nanolaminate (X=Hf, Mo) coatings. The influence of internal stresses and phase fraction and domain size on the mechanical behavior will be studied. Wafer curvature measurements are performed to assess the stress state of the as-deposited coatings. XRD is used to determine the stress (strain) in the crystalline regions of the coating and the mean sizes of the crystalline domains in the coatings. 2. What deformation mechanisms are active in X2BC nanolaminates (X=Hf, Mo)? Typically, layered solids such as MAX phases are known as plastically anisotropic materials. Upon loading, such solids deform by glide of basal plane dislocations. The formation of Kink Bands (KBs) was also reported as a consequence of the plastic anisotropy of the MAX phases. Based on post mortem TEM experiments, we aim for the characterization of the dislocations structure and its interaction with other microstructural features in order to identify and understand the deformation mechanism active in amorphous and nanocomposite (nanocrystals in an amorphous matrix) and fully crystalline X2BC coatings. 3. Does B/G and the Cauchy pressure serve as predictors for the plastic behavior of X2BC nanolaminates (X=Hf, Mo) While Mo2BC was predicted to behave moderately ductile, Hf2BC is expected - based on both, Cauchy pressure and B/G to be brittle. Hence, a comparative investigation of these two nanolaminate systems will shed light on the question as to the predictive capability of B/G and the Cauchy pressure and potential limits thereof for nanolaminates.