Development of the phase composition and the properties of Ti₂AlC and Ti₃AlC₂ MAX-phase thin films – A multilayer approach towards high phase purity

Highlights

• Synthesis of Ti₂AlC & Ti₃AlC₂ MAX-phases by annealing of the same multilayer system.

• The multilayer system is a sequence of pure elemental layers deposited by sputtering.

• Study of temperature influence on the stoichiometry of the MAX-phase.

• Evaluation of elastic modulus and hardness due to the MAX-phase transformation.

• Effect of the grain growth and MAX-phase transformation on the mechanical properties.

Abstract

MAX phase thin films have been synthesized by thermal treatment of a Ti-Al-C multilayer system. The preparation of the multilayer system was carried out via magnetron sputtering. Based on the thickness ratio among the individual nanoscale monolayers (Ti, Al, C), the resulting MAX phase stoichiometry can be controlled. This paper describes the synthesis of both Ti₂AlC and Ti₃AlC₂ MAX phases from the same precursor multilayer system which is composed of a sequence of Ti/Al/C pure elemental single layers with thicknesses of 14, 6, and 3.5 nm, respectively. This sequence is repeated 22 times with a total thickness of around 500 nm. Rapid thermal treatment tests were performed to study the phase development. The Ti₂AlC MAX phase forms in a temperature range below 850 °C, whereas the Ti₃AlC₂ MAX phase starts to form at temperatures above 850 °C and reaches its highest phase purity at 950 °C. The thin film structures were studied by X-ray diffraction and Raman spectroscopy. Furthermore, the electrical and mechanical properties were investigated to gain more insights regarding the phase transformation and their influence on the thin film properties.

Introduction

M_n+1AX_n phases (where n = 1, 2 or 3) are a class of ternary carbides and nitrides with a laminar hexagonal crystalline structure. These materials have attracted strong scientific interest due to their particular combination of metal-like as well as ceramic-like properties. In M_n+1AX_n phases (short: MAX) “M” is an early transition metal, “A” an element of group III A or IV A and “X” is either carbon or nitrogen (e.g. Ti₂AlC, Ti₃SiC₂, and Ti₄GaC₃) [1], [2]. MAX phases exhibit a high mechanical stability, high hardness, and, at the same time, good electrical and thermal conductivities [3], [4]. The combination of these properties is ascribed to the nano-laminar structure as well as the presence of covalent, ionic and metallic bonds in their structure. Therefore, MAX phases are suitable for applications as thermal barriers, electrical contacts, wear-protective coatings and high-temperature heating elements. Furthermore, in the last years, MAX phases were discovered as a precursor for the synthesis of 2D materials called MXenes that are obtained by selective wet etching of the A element and can be used for a wide range of applications such as energy storage, gas sensors and for water splitting [5], [6], [7].

Up to now, MAX phases can be produced in the form of bulk materials and thin films. Bulk MAX phases are usually obtained by different techniques such as hot isostatic pressing (HIP) [8], hot pressing (HP) [9], [10], spark plasma sintering [11] and microwave assisted self-propagating high-temperature synthesis [12], among others.

In the case of thin films, a typical preparation process is the physical vapor deposition, mostly via magnetron sputtering. Regarding the latter technique, one possibility to produce Ti-Al-C based MAX phases is to perform the deposition using targets of Ti, Al and C elements simultaneously at elevated temperature (up to 1000 °C) [13], [14], [15] or co-deposition from a compound target of Ti₂AlC and a Ti target at 700 °C [16]. Another possibility shown in the literature is the deposition from either Ti-Al-C or Ti-Al compound targets without additional heating during the sputtering process and a subsequent annealing at 800 °C [17], [18].

Another important number of different MAX phases was obtained by sputter deposition of multilayer systems without additional heating and subsequent ex-situ annealing. The advantage of this process is that the stoichiometry is easily adjustable by choosing the correct film thickness. Furthermore, the annealing temperatures is commonly lower than for in-situ heating during sputtering as it is possible to take advantage of the short diffusion length in thin films. This was shown for example for the synthesis of Ti₃SiC₂ from Ti-Si-C multilayer [19]. In the literature, promising studies were reported based on the multilayer deposition of three individual elements such as Ti₂AlC from Ti-Al-C [20], Cr₂AlC from Cr-Al-C [21]. Other investigations reported the deposition of Ti-AlN multilayers to obtain Ti₂AlN by physical vapor deposition on silicon [22], Al₂O₃ [23] and 4H-SiC substrates [24].

A more extensive study was lately presented by Tang et al. [25]. The dependence of the phase formation on the single layer thickness and on the structural composition of the films was shown. This work as well as others reported an as-deposited stoichiometry that was quite close to the final corresponding MAX phase composition. Furthermore, Tang et al. could show how the annealing process led to the phase formation using in-situ X-ray diffraction measurements in a rather slow heating process with a heating rate of 10 to 30 K/min.

The work presented here focuses on the synthesis of two different MAX phases Ti₂AlC and a Ti₃AlC₂ from one single Ti-Al-C precursor multilayer system based on a Ti:Al ratio close to 2. In order to perform a systematic study of the phase development, heat treatments using rapid thermal processing (RTP) with high heating and cooling rates were performed. The resulting material structure was investigated and correlated to the change of mechanical and electrical properties.