Abstract: This paper is a review of the thermal stability of nanostructured nitride coatings synthesised by reactive magnetron sputtering technique. In the last three decade, nitride based coatings have been widely applied as hard wear-protective coatings in mechanical components. More recently, a larger interest has been addressed to evaluate the thermal stability of such coatings, as their mechanical and tribological properties are deteriorated at high working temperatures. This study describes the microstructural, mechanical and compositional stability of nano-crystalline Cr-N and nano-composited Ti-N based coatings (Ti-Al-Si-B-N and Ti-Cr-B-N) after air and vacuum annealing. For Cr-N coatings annealing in vacuum induces phase transformation from CrN to Cr2N, while after annealing in air only Cr2O3 phase is present. For Ti-N based coatings, a well-definite multilayered structure was shown after air annealing. Degradation of mechanical properties was observed for all the nitride coatings after thermal annealing in air.
Abstract: Understanding of the melting temperature of nanostructures is beneficial to exploit phase transitions and their applications at elevated temperatures. The melting temperature of nanostructured materials depends on particle size, shape and dimensionality and has been well established both experimentally and theoretically. The large surface-to-volume ratio is the key for the low melting temperature of nanostructured materials. The melting temperature of almost free nanoparticles decreases with decreasing size although there are anomalies for some cases. Superheating has been reported for some embedded nanoparticles. Local maxima and minima in the melting temperature have been reported for particles with fewer atoms. Another quantity that is influenced by large surface-to-volume ratio and related to the thermal stability, is the vapour pressure. The vapour pressure of nanoparticles is shown to be enhanced for smaller particles. In this article, we have discussed the anomaly in thermal stability of nanostructured materials.
Abstract: The present chapter deals with the characterization and description of phase transitions in metallic systems with characteristic size down to the nanometer range. In particular, the chapter focuses on the solid-to-liquid transition in nanometer-sized particles. After a short introduction to classical thermodynamics and to the way it copes with the general properties exhibited by nanometer-sized systems, a rapid overview of the state of the art in the field of the solid-to-liquid transition is given. The heterogeneous melting processes taking place in mesoscopic systems are discussed in terms of both classical thermodynamic and numerical simulation approaches. In the former case, attention is focused on the case of mesoscopic Sn particles, for which a relatively large amount of consistent experimental data exists as a consequence of previous calorimetric studies. In the latter case, the behavior of mesoscopic Cu particles is discussed.
Abstract: The authors have evaluated the surface properties as well as phase diagrams of alloys on the basis of thermodynamic databases. Extending these techniques, we developed a new system to estimate phase equilibria of metals and alloys in small particle systems. The present paper describes our trial to evaluate the phase diagrams of binary alloys in nano-sized particle systems through thermodynamic databases.
Abstract: A thermodynamic study describing relative stability of different systems solid and liquid at equilibrium involved in the growth of semiconductor nanowires is reported. A number of stable and metastable phase diagrams, taking into account the size and the shape of condensed phases are calculated for the two binary systems Au-Si and Au-Ge.
Abstract: Emergence of engineering nanomaterials to render exceptional properties require understanding the thermodynamics and kinetics of grain growth and eliciting role of grain boundary mobility therein. Grain boundary mobility in alumina (Al2O3) has shown several repercussions on the evolution of microstructure to render drastic differences in the mechanical- (hardness, yield strength), optical- (transmittance), electrical- (conductivity), magnetic- (susceptibility), and electrochemical- (corrosion) properties. Consequently, the role of surface energy and the effect of temperature in equilibrating the grain shape and size are presented herewith. Several statistical or deterministic computational modeling have been attempted by researchers to elicit the dominating grain growth mechanisms. But, the limitations extend from the memory of computer and number of atoms in a simulation, or feeding the boundary conditions without incorporation of the initial microstructure to arrive at the dominating growth mechanism parameters. Contrastingly, the role of dopants in Al2O3 to either enhance or impede the grain growth is presented via various complexions responsible for transitions at the grain boundary interface. Six complexions resulting various grain boundary interface, strongly affect the grain boundary mobility, and sideline the dopant contributions in deciding the overall grain boundary mobility. It has also been presented that grain growth exponent increases with decreasing grain size, and additionally, secondary reinforcement of carbon nanotube (CNT) in Al2O3 impedes the grain mobility by as much as four times. The effect of temperature is found to be more pronounced, and has shown to enhance the grain boundary mobility by as much as six orders of magnitude.
Abstract: Synthesis plays an important role in the phase stabilization of unusual compounds. Of late, preparation of metastable compounds has gained a tremendous momentum due to unusual properties exhibited by them. In this article, we will discuss how by mere change in certain parameters of the reaction a metastable phase can be isolated using a soft chemical route. Surface energy induced stabilization are also observed wherein enhanced stability of the mixed oxides are observed in the nano-regime of the compound.