TABLE OF CONTENTS Background in the fundamental concepts of thermodynamics, kinetics of reactions, diffusion, crystal structure, phase equilibrium, and phase diagrams of alloys is essential in order to understand the selection, processing, and behavior of high-temperature coatings. These concepts are discussed briefly in this chapter.
The principles of thermodynamics are used to determine the spontaneity of many chemical and metallurgical phenomena and the direction in which they proceed. A few examples of practical applications of thermodynamics include processes for extraction of metals from ores, formation of alloys, precipitation and grain growth, and degradation of materials in service. In order to understand these processes, a number of thermodynamic parameters need to be defined. The concepts, discussed next, will be specifically helpful in understanding the processes of oxidation and corrosion, phase and microstructure changes of high-temperature coatings, and their interactions with substrate alloys.
Enthalpy H is the measure of the combination of internal energy E and the product of pressure P and volume V of a system. Thus H = E + PV. For example, if the system consists of an inflated balloon made of a thermally insulating rubber, the internal energy is the energy of the gas molecules inside the balloon, and the pressure is that of the ambient atmosphere. If a small amount of heat dQ is added to this system, it will increase internal energy by dE and induce a change in volume dV while the pressure remains constant...
