Low Advances in Science and Technology of Mn+1AXn Phases

1. Auflage 2012
ISBN: 978-0-85709-601-2
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 474 Seiten

Advances in Science and Technology of Mn+1AXn Phases presents a comprehensive review of synthesis, microstructures, properties, ab-initio calculations and applications of Mn+1AXn phases and targets the continuing research of advanced materials and ceramics. An overview of the current status, future directions, challenges and opportunities of Mn+1AXn phases that exhibit some of the best attributes of metals and ceramics is included. Students of materials science and engineering at postgraduate level will value this book as a reference source at an international level for both teaching and research in materials science and engineering. In addition to students the principal audiences of this book are ceramic researchers, materials scientists and engineers, materials physicists and chemists. The book is also an invaluable reference for the professional materials and ceramics societies. - The most up-to-date and comprehensive research data on MAX phases is presented - Written by highly knowledgeable and well-respected researchers in the field - Discusses new and unusual properties

Autoren/Hrsg.

Low, I M

Leseproben

Preface
This book deals with the recent advances in the science and technology of MAX phases, which is a new class of materials that exhibit a unique combination of characters of both ceramics and metals. MAX phases are nano-layered ceramics with the general formula Mn+1AXn (n = 1–3), where M is an early transition metal, A is a group A element, and X is either carbon and/or nitrogen. These materials exhibit a unique combination of characteristics of both ceramics and metals with an unusual combination of mechanical, electrical and thermal properties. Similar to ceramics, they possess low density, low thermal expansion coefficient, high modulus and high strength, and good high-temperature oxidation resistance. Like metals, they are good electrical and thermal conductors, readily machinable, tolerant to damage, and resistant to thermal shock. The unique combination of these interesting properties enables these ceramics to be promising candidate materials for use in diverse fields, especially in high temperature applications. However, these MAX phases, Ti3 SiC2 in particular, have poor wear resistance due to low hardness (~ 4 GPa) and are susceptible to thermal dissociation at ~ 1400°C in inert environments (e.g., vacuum or argon) to form a protective surface coating of TiC. Depth-profiling by X-ray diffraction of Ti3SiC2 annealed in vacuum at 1500°C has revealed a graded phase composition with more than 90 wt% TiC on the surface and decreasing rapidly with an increase in depth. A similar phenomenon has also been observed for Ti3AlC2 whereby it decomposes in vacuum to form TiC and Ti2AlC. It follows that this process of thermal dissociation to form protective coatings of binary carbide or nitride will also occur in other MAX phases such as Cr2GeC, Ta4AlC3, Ti2AlN, and Ti4AlN3. The formation of a graded surface coating such as TiC or TiN has the potential to impart high hardness and wear-resistance to the otherwise soft but damage-resistant substrate. In spite of the intense studies, the chemistry and kinetics of the dissociation processes involved are not yet fully understood. Fundamental knowledge concerning the thermal stability of technologically important MAX phases is still very limited and the actual process of phase degradation remains unresolved. This limited understanding has generated much controversy concerning the high-temperature thermochemical stability of MAX phases. For instance, Ti3SiC2 has been reported to be thermally stable up to 1300 °C in nitrogen, but above this temperature drastic degradation and damage occurred due to surface decomposition. However, this problem has been observed by others to occur at much higher temperature whereby TiC was only observed to form on the surface of Ti3SiC2 annealed at 1600–2000 °C in vacuum. The propensity of decomposition of Ti3SiC2 to TiC has been attributed to the relatively high vapour pressure of Si at temperatures above 1400 °C. This process of surface-initiated phase dissociation was even observed to commence as low as 1000–1200 °C in Ti3SiC2 thin films during vacuum annealing. The large difference in observed dissociation temperatures between bulk and thin-film Ti3SiC2 has been attributed to the difference in diffusion length scales involved and measurement sensitivity employed in the respective studies. In contrast, other researchers did not observe decomposition of Ti3SiC2 until 1600 °C in vacuum for 24 hours and in argon atmosphere for 4 hours. They further argued that the reduced temperature at which Ti3SiC2 decomposed as observed by others was due to the presence of impurity phases (e.g., Fe or V) in the starting powders which interfered with the reaction synthesis of Ti3SiC2, and thus destabilized it following prolonged annealing in an inert environment. Mixed results have also been reported whereby Ti3SiC2 was shown to be stable in a tungsten-heated furnace for 10 hours at 1600 °C and 1800 °C in an argon atmosphere, but dissociated to TiCx under the same conditions when using a graphite heater. These conflicting results suggest that the thermochemical stability of MAX phases is still poorly understood although its susceptibility to thermal dissociation is strongly influenced by factors such as the purity of powders and sintered materials, temperature, sintering pressure, atmosphere, and the type of heating elements used. Although a few studies have been conducted on the thermal stability of 312 phases, virtually no work has been reported for the 211 or 413 phases such as Ti2AlC, Cr2AlN, Ta4AlC3 and Ti4AlN3. Recent investigations have shown 211 phases (e.g. Cr2AlC, Ti2AlC) to be more resistant to phase dissociation than 312 phases (e.g. Ti2 SiC2, Ti2 AlC2) during high- temperature vacuum annealing. The apparent activation energies for the decomposition of sintered Ti3SiC2, Ti3AlC2 and Ti2AlC have been determined to be 179.3, –71.9 and 85.7 kJ mol- 1, respectively. The Avrami kinetics of decomposition in MAX phases have also been modelled and the Avrami exponent (n) of isothermal decomposition of Ti3AlC2, Ti3SiC2, Ti2AlN, Ti4AlN3 was determined to be in the range of 0.18-0.85. The low values of n (i.e. < 1) imply that the decomposition process is driven mainly by a highly restricted out-diffusion and sublimation of high vapour pressure A element (e.g. Al, Si) from the bulk to the surface of the sample and into the vacuum, i.e. (I.1a) (I.1b) In spite of these recent studies, there remain several unresolved issues which relate to the thermal and phase stability of MAX phases: a) Phase stability of ternary carbides and nitrides in controlled atmosphere. It remains poorly understood whether the decomposition kinetics of ternary nitrides will behave as their carbide counterparts. The relative importances of various factors that control the decomposition of MAX phases are also poorly understood. This understanding is essential for the improvement of their resistance to thermal dissociation through formulation of new stabilisers. The following hypotheses can be proposed: • The vapour pressure of element A is critical to the phase stability of MAX phases. The higher the vapour pressure of element A, the more susceptible the MAX phase is to phase dissociation at elevated temperature. • The Avrami kinetics of phase dissociation is dependent on the rate of removal of vaporised element A. A dynamic atmosphere with a flowing gas or in high vacuum will facilitate the continual removal of the vaporised A and thus the continuous dissociation of the MAX phase. In contrast, a static atmosphere is expected to be most conducive for a MAX phase to resist phase dissociation. • Dissociation of a MAX phase is dependent on its phase purity, porosity and controlled atmosphere. A porous and impure sample is expected to dissociate faster than a dense high purity sample because of increased surface area and nucleation sites. Use of very high vacuum will accelerate the rate of phase decomposition because of high vapour pressure of element A (e.g. Al and Si). b) The role of microstructural modification due to phase dissociation on the mechanical performance of MAX phases. It remains unknown how microstructural changes will affect the mechanical properties. New stabilisers will be formulated to arrest the susceptibility of MAX phases to thermal dissociation at elevated temperature. TiSi2 is an effective stabiliser for Ti3SiC2. c) Development of improved models (e.g. Avrami equation) to describe the chemical processes and kinetics of phase dissociation. No such models exist currently that can adequately describe and predict the property modification, especially for the ternary nitrides. Hitherto, there is an enormous but fragmented amount of research papers on MAX phases published in various journals in recent years. A dedicated book on this topic is timely to bring all the scattered research findings into a single volume which will provide an invaluable resource for both students and researchers in this field. Fifteen peer-reviewed chapters are presented in this book. Each chapter has been written by a leading researcher of international recognition on MAX phases. This book is concerned with the synthesis, characterisation, microstructure, properties, modelling and challenges of MAX phases. The synopsis of each chapter is as follows: Chapter 1: This chapter provides a comprehensive review of current research activities on synthesizing techniques of MAX phases. The chapter begins with an overview of powder-synthesis techniques for MAX phases and is followed by the strategies for the fabrication of bulk, thin-films, coatings and composites of MAX phases. The comparison between thin-film and bulk synthesis of MAX phases is also made. Chapter 2: In this chapter, the methods to synthesize MAX phase powders, solids and films are introduced. Powders of MAX phases can be synthesized from element or compound starting materials at high temperature (1100–1500 °C) by many methods, such as self-propagating high temperature synthesis, mechanical alloy assistant synthesis, vacuum sintering or normal sintering, etc. Due to the oxidation susceptibility of starting materials and the final product, all reactions must be conducted in vacuum or argon atmosphere. The bulk solids of MAX phases can be prepared from the same starting...