Chapter 12 - Smart Coatings — Multilayered and Multifunctional in-situ Ultrahigh-temperature Coatings

Abstract

This chapter reveals that to develop ultrahigh-temperature structural materials, advanced smart coatings, which provide excellent oxidation resistance, are required. Several functions that are also required for practical coatings are self-healing, equivalent thermal expansion, thermal barrier, phase stability, and diffusion barrier. The basic concept in improving oxidation resistance is to reduce the oxygen permeation through the coating to the substrate material. Iridium (Ir) possesses the lowest oxidation permeability of all known materials. Therefore, Ir is expected to be applicable for the oxygen diffusion barrier (ODB), which is the most important part of ultrahigh-temperature coatings. A serious problem with Ir is that the oxidation resistance itself is not sufficient at high temperature. In order to improve the high-temperature oxidation resistance of Ir, artificial multilayered coatings, such as Ir-Ag and Ir-Al2O3, were previously proposed for carbon-carbon composites and were fabricated by deposition techniques. Ag and Al2O3 layers act as oxidation-resistant coatings that protect the Ir layer. The chapter reviews the basic idea and design of the smart coatings, the fabrication, structure, mechanical properties, oxidation of Ir and IRAl, and effects of ternary additions are described, particularly for ternary Co addition to IRAl.