Mechanism of Strength Enhancement of Ceramics Superlattice Coating Films and Its Application to Design of Microstructure

陶瓷超晶格具有纳米尺度的异质陶瓷组合的层状结构，是制备高性能涂层的有效途径之一。由于超晶格薄膜在特定的复合周期内具有显著的压痕硬度和耐磨性，因此有望成为一种承受较大摩擦的涂层材料。为了弄清陶瓷超晶格的微观结构与力学性能之间的关系，并为高性能陶瓷涂层的设计提供指导，本研究对不同层压周期的TiN/AlN超晶格薄膜进行了单向和双向强度测试的分子动力学(MD)计算机模拟。对电弧离子镀TiN/AlN陶瓷超晶格薄膜的微压痕实验结果表明，随着层压时间d…的增加，陶瓷超晶格薄膜的硬度增加通过与第一性原理计算结果的拟合，给出了TiN、AlN、ZrN等岩盐结构陶瓷的原子间相互作用势函数。利用原子间相互作用势函数，成功地对陶瓷和由陶瓷组成的超晶格薄膜进行了单轴和双轴强度测试的MD模拟，结果表明，在具有超晶格结构的薄膜中，AlN和TiN层分别存在残余拉应力和压应力。断裂起裂强度随层压周期的减小而降低。在双向载荷作用下，AIN层和TiN层分别发生裂纹扩展和塑性变形。然而，TiN/AIN界面上的断裂和变形被阻止。超晶格薄膜的分层周期越长，裂纹扩展越长。这导致了超晶格薄膜具有较低的压痕硬度，并具有较长的叠层周期。分子动力学模拟的结果表明，压痕硬度的最大提高存在一个最佳的层压周期。分子动力学模拟可以从原理上清楚地了解微结构与力学性能之间的关系，为陶瓷超晶格的结构设计提供有用的工具。较少

Ceramics superlattice, which has a laminated structure of heterogeneous ceramics combination in nanometer scale, is one of the promising approaches for highly functional coating films. As a superlattice film shows remarkable enhancement in indentation hardness and wear-resistance with a specified laminating period, it is expected to be a material of coating films subjected to heavy friction. However, the mechanism of strength enhancement has not been understood well.In this research, to clarify the correlation between the microstructure and mechanical properties of ceramics superlattice and to establish the guidelines for design of highly functional ceramics coating films, molecular dynamics (MD) computer simulations of uniaxial and biaxial strength testing are carried out on TiN/AIN superlattice films with various laminating periods.Experimental results of microindentation on arc ion-plated TiN/AIN ceramics superlattice films show that the hardness increases as the laminating period d … More ecreases and that the highest hardness enhancement is observed on the film with 2.5 nm laminating period.Interatomic potential functions are proposed for ceramics with rock salt structure, such as TiN, AiN are and ZrN, by the fitting with the results of first- principle calculations. Using the interatomic potential functions, MD simulations of uni- axial and biaxial strength testing of the ceramics and superlattice films composed of the ceramics are carried out successfully.The MD simulations show that residual tensile and compressive stresses are observed in the layers of AIN and TiN, respectively, in the films with superlattice structure. The strength for fracture initiation decreases as the laminating period decreases. Under the biaxial loading, crack extension and plastic deformation occur in the layers of AIN and TiN, respectively. However, the fracture and deformation are stopped on the TiN/AIN interface. The longer the laminating period of superlattice film, the longer the crack extension. This results in the lower indentation hardness on the superlattice film with longer laminating period. The results of the MD simulations suggest that there is an optimum laminating period for highest enhancement of indentation hardness.MD simulation can be an useful tool for clear understanding in principle the correlation between miorostructure and mechanical properties and for structure design of ceramics superlattice. Less