*Article* **Microstructure Observation and Nanoindentation Size E**ff**ect Characterizationfor Micron-**/**Nano-Grain TBCs**

#### **Haiyan Liu 1, Yueguang Wei 1,\*, Lihong Liang 2,\*, Yingbiao Wang 3, Jingru Song 3, Hao Long 1 and Yanwei Liu 1**


Received: 21 February 2020; Accepted: 31 March 2020; Published: 2 April 2020

**Abstract:** Microstructure observation and mechanical properties characterization for micron-/nano-grain thermal barrier coatings were investigated in this article. Scanning electron microscope images demonstrated that both micron-grain coating and nano-grain coating had micrometer-sized columnar grain structures; while the nano-grain coating had the initial nanostructures of the agglomerated powders reserved by the unmelted particles. The mechanical properties (hardness and modulus) of micron-/nano-grain coatings were characterized by using nanoindentation tests. The measurements indicated that the nano-grain coating possessed larger hardness and modulus than the micron-grain coating; which was related to the microstructure of coatings. Nanoindentation tests showed that the measured hardness increased strongly with the indent depth decreasing; which was frequently referred to as the size effect. The nanoindentation size effect of hardness for micron-/nano-grain coatings was effectively described by using the trans-scale mechanics theory. The modeling predictions were consistent with experimental measurements; keeping a reasonable selection of the material parameters.

**Keywords:** micron-/nano-grain coatings; microstructure; nanoindentation size effect; trans-scale mechanics theory
