Investigation of Microstructure Effects on Hardness and Oxidation Resistance of New Generation Ceramic Coating

This work was motivated by the demand for a new generation of materials capable for harsh environment applications, and focused on developing a better understanding of the effects of elemental composition and microstructure on the mechanical and thermal properties of coatings. The knowledge gained from this work can be used to design protective coatings with tailored properties for harsh environment applications in the future. High resolution transmission electron microscopy (HRTEM) and electron diffraction have been mainly employed in the microstructure studies of the new generation ceramics, with the assistance of other materials characterization techniques, such as x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and nano-indentation measurements.;Ceramic coatings studied in this work have been divided into two groups: 1) Transition metal based nanocomposite coatings with enhanced hardness, 2) Amorphous coatings with high oxidation resistance. In the first group, the microstructure effects and hardness enhancement mechanism of Zr-B-C-N, Hf-B-Si-C and Hf-Si-N coatings are discussed. The hardness enhancement is based on the design of nanostructure and the control of grain size during the formation of coatings. The formation of amorphous boundaries with proper thickness between crystalline structures and the well aligned crystallographic orientation were found to result in the increasing of hardness and decreasing of compressive stress of coatings. In the second group, a mechanism of extraordinary high oxidation resistance is established for the Si-B-C-N and Hf-B-Si-C-N coatings by studying the microstructure evolution as a function of being exposed to high temperatures. The results showed that the oxidation resistance enhancement of coatings was based on the suppression of crystallization. The oxidation resistance increases with increasing thermal stability of amorphous structure. Amorphous coatings containing phases can react with oxygen will hinder the ingress of oxygen and prevent further oxidation by forming barrier layers.;Two new nanostructures can result in the hardness enhancement have been discovered: 1) Grains composed of sub nano-domains with same orientation separated by semicoherent monolayer boundaries observed from the Zr41B 30C8N20 coating; 2) The columnar structures which grow on multilayers, consisting a bundle of small subdomains separated by thin boundaries observed from the Hf-Si-N coating deposited with 15% nitrogen in the gas mixture. Further studies of amorphous coatings with outstanding oxidation resistance indicated that the barrier layers consisting of grains dispersed in amorphous matrix can provide good protection for substrate against oxidation. A compromise between the oxidation resistance and hardness can be reached if nanocomposite coatings are composed of small amount of nanograins embedded in amorphous matrix.