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

在原位同步x射线衍射实验和电子显微镜支持下的小尺度力学研究将描述涂层应力和应变与Mo2BC塑性行为的关系。我们试图确定控制X2BC纳米层合物（X=Hf, Mo）塑性行为的基本机制，并将这些结果与先前发表的和这里获得的量子力学预测进行比较。由于我们最初的微观力学数据支持量子力学计算预测的中等延展性的概念，本项目旨在回答以下将微观结构特征与力学性能联系起来的问题：X2BC纳米层合（X=Hf, Mo）涂层的微观结构是否影响其力学行为？镀层的微观结构受沉积温度、沉积速率和成膜物质的电离程度等沉积参数的控制。我们将沉积全非晶，全晶涂层以及各种非晶到晶相分数的薄膜。透射电镜（TEM）和x射线衍射仪（XRD）将测定结晶区和非晶区的畴大小，并测定全晶膜的结晶区畴大小。对不同微观结构的X2BC纳米层合（X=Hf, Mo）涂层进行原位力学应变试验。研究了内应力、相分数和畴尺寸对合金力学性能的影响。晶圆曲率测量是为了评估沉积涂层的应力状态。利用x射线衍射仪测定了涂层结晶区的应力（应变）和涂层中结晶区的平均尺寸。2. 在X2BC纳米层合材料（X=Hf, Mo）中活跃的变形机制是什么？通常，层状固体如MAX相被称为塑性各向异性材料。加载后，这种固体由于基面位错的滑动而变形。扭结带（KBs）的形成也被报道为MAX相的塑性各向异性的结果。基于事后TEM实验，我们的目标是表征位错结构及其与其他微观结构特征的相互作用，以识别和理解非晶和纳米复合材料（非晶基体中的纳米晶体）和全晶X2BC涂层中的变形机制。3. B/G和柯西压力是否可以作为X2BC纳米层合材料（X=Hf, Mo）塑性行为的预测因子？虽然预测Mo2BC具有中等延展性，但基于柯西压力和B/G，预计Hf2BC具有脆性。因此，对这两种纳米层叠材料的比较研究将有助于阐明纳米层叠材料的B/G预测能力、柯西压力及其潜在极限。

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.