Advances in Science and Technology of Mn+1AXn Phases

I M Low

• Publisher: Woodhead Publishing
• Year: 2012
• Print ISBN: 9781845699918
• eBook ISBN: 9780857096012
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

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

Methods of MAX-phase synthesis and densification – I

X.K. Qian, Xi’an University of Architecture and Technoloǵy, PR. China

Abstract:

Ternary layered and machinable MAX phases have attracted ever-growing interest because they exhibit unique chemical, physical, electrical, and mechanical properties. Potential applications of MAX phases are many and diverse. This chapter provides a review of current research on synthesis techniques for MAX phases. We begin with an overview of powder-synthesis techniques for MAX phases, followed by the synthetic strategies used for the fabrication of bulk MAX phases. Then we concentrate on the thin-film and coating processing of MAX phases. Moreover, we also discuss the synthetic methods for fabricating MAX-phase composites. At the end of this chapter, we briefly compare the thin-film and bulk synthesis of MAX phases.

Key words

synthesis methods

powder synthesis

densification

thin-films

coatings

composites

mechanical alloying

SHS

SPS

magnetron sputtering

vapor deposition

1.1 Introduction

Recently a large family of ternary layered ceramics - with the general formula M_n+1AX_n phases (n = 1, 2, 3, 4, 5 and 6), where M is a transition metal, A is an A-group element, and X is C or/and N – has attracted extensive attention and intensive study [1–4]. The M_n+1 AX_n phases (abbreviated as MAX phases hereafter) crystallize in hexagonal structures with space group of P6₃/mmc. Up to now, over 60 members of MAX phases have been discovered to be thermodynamically stable. Table 1.1 lists all the MAX phases to date together with their lattice parameters. Depending on the value of n, MAX phases have been categorized into 6 groups so far: i.e. M₂AX (211 phase), M₃AX₂ (312phase), M₄AX₃ (413 phase), M₅AX₄ (514 phase), M₆AX₅ (615 phase) and M₇AX₆ (716 phase).

Table 1.1

Summary of MAX phase compounds known up to now [1–4]. The a and c lattice parameters (nm) are in brackets

Typical members such as Ti₂AlC, Ti₃SiC₂, V₄AlC₃ and Ta₆AlC₅ are well investigated. Figure 1.1 demonstrates the crystal structure of 211 phase, 312 phase and 413 phase [5]. In each case, the near close-packed M layer is interleaved by the A-element layer, with the X-atom filling the octahedral sites between the former. The M₆X octahedral are edge sharing and are identical to those in the rock salt. The main difference in structures shown in Figure 1.1 is the number of M layers separating the A layers: in the 211 phase, there are two; in the 312 phase, there are three; and in the 413 phase, there are four. The rest of 514 phase, 615 phase and 716 phase can be deduced by analogy. This layering structure results in some outstanding properties for MAX phases.

Figure 1.1 Crystal structure of 211 phase, 312 phase and 413 phase [5]

Considerable interest in MAX phases originates from the fact that they combine some salient properties of both ceramics and metals. Similar to ceramics, they present a high strength, high melting point and thermal stability, and good oxidation resistance. Like metals, they have high thermal and electrical conductivities, are easily machined by conventional tools without lubrication, are fatigue-resistant and are resistant to thermal shock. Therefore, potential applications for MAX phases are quite diverse. At this moment, MAX phases have potential applications in the following areas: high-temperature materials [6], protective coatings [7–9], materials for lead-cooled fast reactors [10,11] and electrical contact materials [12–14]. Here, we would like to mention that the Chinese scientists have successfully fabricated Ti₃SiC₂-based composites as a contact strip for high-speed trains. Although MAX phases have numerous potential applications, obtaining monolithic materials is a prerequisite before they can be used in industry. Moreover, monolithic MAX phases are also crucial to characterizing their intrinsic properties. Synthesis of pure MAX phase is of great importance during the research history of MAX phases, synthesis of monolithic MAX phases used to be a great challenge; however, a breakthrough in synthesis occurred recently and the characterization of their properties has become much easier.

The purpose of this chapter is to summarize synthesis techniques for MAX phases in detail. Synthesis techniques are divided into four parts: techniques for powders; bulk materials; thin films and coatings; and composites.

1.2 Synthesis methods

1.2.1 Synthesis of powder MAX phases

Synthesis of pure MAX-phase powders is becoming more and more important because powders are essential for fabricating complex shapes and composite bulk materials. So far, there are three different methods for the synthesis of MAX-phase powders. They are pressureless sintering, mechanical alloying and self-propagating high-temperature synthesis.

Pressureless sintering (PS)

PS is a conventional powder metallurgy route, which sinters materials from a green compact of powders without applying mechanical pressure. Pietzka and Schuster [15] first carried out sintering of Ti₃AlC_1-x, Ti₃AlC_2–x, Ti₂AlC_1-x in a tungsten furnace under H₂-atmosphere for 20 hours. Due to the weak bonding between TiC slabs and interleaved Al atoms in the structure of Ti₃AlC_2-x and Ti₂AlC_1-x, Al atoms can easily migrate and evaporate. Therefore, it is difficult to fabricate highly pure Ti₃AlC_2-x and Ti₂AlC_1-x from naturally stacked and mixed powders. Sunetal et al. [16] made efforts to synthesize Ti₃SiC₂ powder by PS. They obtained single phase Ti₃SiC₂ by heating the powder mixtures consisting of Ti, Si, and TiC with a composition of Ti/1.10Si/2TiC. It was found that adding 10 per cent excess Si is essential for preparing single phase Ti₃SiC₂ because Si evaporates at high temperature. Peng et al. [17] synthesized highly pure Ti₃AlC₂ powder by heating 2TiC/Ti/Al (molar ratio) mixture between 1300 °C and 1400 °C for 15–30 minutes in a flowing argon atmosphere. The synthesized samples can be easily ground into powders with a mean particle size of 4.9 μm. Importantly, this method was very reproducible in a scale from 5 g to 1000 g. Ai et al. [18] investigated the effect of tin on the synthesis of powders. It was demonstrated that the addition of tin greatly decreased the synthesis temperatures.

Mechanical alloying (MA)

MA is a convenient and effective method to fabricate powders, and the processing parameters have a great influence on the fina...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of figures
• List of Tables
• Preface
• About the editor and contributors
• Chapter 1: Methods of MAX-phase synthesis and densification – I
• Chapter 2: Methods of MAX-phase synthesis and densification – II
• Chapter 3: Consolidation and synthesis of MAX phases by Spark Plasma Sintering (SPS): a review
• Chapter 4: Microstructural examination during the formation of Ti3AlC2 from mixtures of Ti/Al/C and Ti/Al/TiC
• Chapter 5: Fabrication of in situ Ti2AlN/TiAl composites and their mechanical, friction and wear properties
• Chapter 6: Use of MAX particles to improve the toughness of brittle ceramics
• Chapter 7: Electrical properties of MAX phases
• Chapter 8: Theoretical study of physical properties and oxygen incorporation effect in nanolaminated ternary carbides 211-MAX phases
• Chapter 9: Computational modelling and ab initio calculations in MAX phases – I
• Chapter 10: Computational modeling and ab initio calculations in MAX phases – II
• Chapter 11: Self-healing of MAX phase ceramics for high temperature applications: evidence from Ti3AlC2
• Chapter 12: Oxidation characteristics of Ti3AlC2, Ti3SiC2 and Ti2AlC
• Chapter 13: Hydrothermal oxidation of Ti3SiC2
• Chapter 14: Stability of Ti3SiC2 under charged particle irradiation
• Chapter 15: Phase and thermal stability in Ti3SiC2 and Ti3SiC2/TiC/TiSi2 systems
• Index