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

陶瓷超晶格是一种在纳米尺度上具有非均相陶瓷组合的层状结构，是制备高功能涂层的一种很有前途的方法。超晶格薄膜的压痕硬度和耐磨性随层压时间的延长而显著提高，有望成为耐摩擦涂层材料。然而，强度增强的机理还不清楚。为了阐明陶瓷超晶格的微观结构与力学性能之间的关系，为高功能陶瓷涂层的设计提供指导，本研究对TiN/AIN超晶格薄膜进行了不同层压周期的单轴和双轴强度测试的分子动力学（MD）计算机模拟。弧离子镀TiN/AIN陶瓷超晶格薄膜的微压痕实验结果表明，薄膜硬度随膜层时间的增加而增加，且膜层时间为2.5 nm时硬度增强最大。通过与第一性原理计算结果的拟合，提出了TiN、AiN、ZrN等具有岩盐结构的陶瓷的原子间势函数。利用原子间势函数，成功地进行了陶瓷和由陶瓷组成的超晶格薄膜的单轴和双轴强度测试的MD模拟。MD模拟表明，在具有超晶格结构的薄膜中，在AIN层和TiN层中分别观察到残余的拉应力和压应力。随着层压时间的缩短，起裂强度减小。在双轴加载下，AIN层和TiN层分别发生裂纹扩展和塑性变形。然而，断裂和变形在TiN/AIN界面上停止。超晶格膜层合时间越长，裂纹扩展时间越长。这使得超晶格薄膜的压痕硬度较低，层压周期较长。MD模拟结果表明，存在压痕硬度提高最高的最佳层压时间。MD模拟可以从原理上清楚地认识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。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