Temperature-dependent microstructural evolution of Ti₂AlN thin films deposited by reactive magnetron sputtering

Highlights

• We investigate microstructural evolution of Ti₂AlN MAX thin films with temperature.

• The film forms a mixture of Ti, Al and (Ti,Al)N cubic solid solution at 500 °C.

• The film nucleates into polycrystalline Ti₂AlN M_n+1AX_n phases at 600 °C.

• The film transforms into a single-crystalline Ti₂AlN (0 0 0 2) thin film at 750 °C.

• The mechanisms behind Ti₂AlN phase transformation with temperature are discussed.

Abstract

Ti₂AlN MAX-phase thin films have been deposited on MgO (1 1 1) substrates between 500 and 750 °C using DC reactive magnetron sputtering of a Ti₂Al compound target in a mixed N₂/Ar plasma. The composition, crystallinity, morphology and hardness of the thin films have been characterized by X-ray photoelectron spectroscopy, X-ray diffraction, atomic force microscopy and nano-indentation, respectively. The film initially forms a mixture of Ti, Al and (Ti,Al)N cubic solid solution at 500 °C and nucleates into polycrystalline Ti₂AlN MAX phases at 600 °C. Its crystallinity is further improved with an increase in the substrate temperature. At 750 °C, a single-crystalline Ti₂AlN (0 0 0 2) thin film is formed having characteristic layered hexagonal surface morphology, high hardness, high Young's modulus and low electrical resistivity. The mechanism behind the evolution of the microstructure with growth temperature is discussed in terms of surface energies, lattice mismatch and enhanced adatom diffusion at high growth temperatures.

Introduction

The M_n+1AX_n (n = 1–3) or MAX-phase materials are a group of about 60 nanolaminate ternary carbides and/or nitrides (X = carbon or nitrogen) of transition metals (M), which are constructed by vertically repeating two M_n+1X_n layers intercalated by one layer of a group 12–16 element (A) in between [1]. After their initial discovery in 1960s, this group of materials has regained significant attentions since mid-1990s upon the discovery of their unique combination of both ceramic and metallic properties. On the one hand, like ceramics, MAX-phase materials have high melting points and high temperature oxidation resistance. On the other hand, like metals, MAX-phase materials have good electrical and thermal conductivity, high ductility, easy machinability, superior thermal shock resistance and damage tolerance [2]. The distinctive combination of these properties stems from co-existence of the strong covalent-ionic M–X bonds and the weak metallic M–A bonds inside the layered hexagonal structures of MAX materials [1], [2], [3]. Their exceptional properties have led to the exploration of a variety of applications including high-temperature protective coatings on turbine blades [4], radiation-tolerant cladding material for next-generation nuclear power plants [5] and a candidate as Ohmic contact to SiC [6].

Relatively phase-pure MAX-phase materials were initially synthesized by hot isostatic pressing (HIP) of the respective starting powders (i.e., MX + AX, or MX + A, or M + A + X) under high pressure and high temperature [1]. Some well-studied MAX-phase materials include Ti₃SiC₂, Ti₂AlC and Ti₄AlN₃. Although there are presence of other minority phases as impurities in the final products, this HIP fabrication method has offered the first-hand opportunity to characterize these MAX-phase materials’ macroscopic properties including density, electrical resistivity, heat capacities, hardness, Young's modulus, oxidation resistance, etc [1]. Recently, thin-film deposition technique primarily using sputtering has been employed to grow several carbide-based MAX-phase materials including Ti₃SiC₂ [7], [8], Ti₂AlC [9], Cr₂AlC [10], [11], Cr₂GeC [12], Ti₄SiC₃ [13], Ti₃GeC₂ [13], Ti₂GeC [13], Ti₂SnC [13], V₂AlC [14] and one nitride-based MAX-phase material, Ti₂AlN [3], [15], [16], [17], [18], [19], [20], [21], [22]. The setups in these sputtering chambers can be pretty versatile to cater for different MAX-phase material systems: from individual elemental targets to compound targets and from pure Ar plasma to a mixture of Ar/N₂ plasma. The sputtering technique not only offers the possibility to grow single-crystalline MAX-phase thin films so that their microscopic properties such as atomic structure and lattice arrangement can be determined, but also offers the opportunity to engineer the stoichiometry of the thin films and to grow multi-layer structures.

Single-crystalline Ti₂AlN thin films have recently been grown epitaxially on single-crystal MgO(1 1 1) and Al₂O₃(0 0 0 1) substrates through ultra-high-vacuum (UHV) DC magnetron sputtering [3], [15], [16], [17], [18], [19], [20], [21], [22]. While carbon in the carbide-based MAX-phase material is introduced from sputtering a graphite or C₆₀ solid targets, nitrogen in a nitride-based Ti₂AlN MAX-phase material is mainly introduced from the gaseous nitrogen in the mixed Ar/N₂ plasma. Hence, considerable attention has been focused to establish a N₂ partial pressure window so that the resultant thin film can achieve an appropriate N concentration in order for the Ti₂AlN MAX phase to nucleate and grow [18], [19]. The results showed that nitrogen-deficient conditions typically yield thin films with mixture of inverse perovskite Ti₃AlN phase and intermetallic TiAl and Ti₃Al phases, while nitrogen-rich conditions yield the binary nitride TiN and/or the Ti₂AlN [18], [19]. By comparison, little efforts have been devoted to understand the influence of the substrate temperature on the microstructure of the resulting Ti–Al–N thin films [21]. Substrate temperature has been reported to exercise significant effects during growth of several carbide-based MAX-phase thin films such as Ti₃SiC₂ [7], Ti₂AlC [9], Cr₂GeC [12], Cr₂AlC [11] and V₂GeC [14]. Although the exact influences vary among different MAX-phase materials, two common observations are that high temperature (≥370 °C) is required for the nucleation of MAX-phase materials due to their large unit cell sizes (c ∼ 13 Å for M₂AX phase, c ∼ 18 Å for M₃AX₂ phase and c ∼ 23–24 Å for M₄AX₃ phase) and that an increase in deposition temperature leads to a greater loss of A elements (like Al, Sn, Ge, etc.) due to evaporation from the thin films because of their high vapor pressures [2]. For Ti₂AlN MAX-phase thin films, a growth condition (which yields an amorphous Ti–Al–N thin film in a ratio of Ti:Al:N = 4:1:3 at room temperature) has led to the formation of a layered polycrystalline Ti_n+1AlN_n structure which maintained an overall 4:1:3 stoichiometry but had both horizontal and nearly perpendicular orientations at 600 °C. By increasing the deposition temperature to 675 °C and above, the same growth condition resulted in the formation of a mixture of TiN and Ti₂AlN instead [21]. Beckers et al. [21] attributed the reason behind different phases with a change in deposition temperature to severe loss of Al to vacuum at higher temperatures. By comparing three independent studies, it is perplexed but interesting to observe that while basal planes of Ti₂AlN single-crystalline films are grown on MgO (1 1 1) substrates at 830 °C [15] and 750 °C [20] with an epitaxial relationship of Ti₂AlN{0 0 0 1} $〈\overline{1}2\overline{1}0〉$ //MgO{1 1 1}<1 1 0>, non-basal planes of Ti₂AlN single-crystalline film are developed on MgO (1 1 1) substrate by lowering substrate temperature further to 690 °C, which results in an epitaxial relationship of Ti₂AlN{$10\overline{1}2$} $〈\overline{1}2\overline{1}0〉$ //MgO{1 1 1}<1 1 0> [17]. Beckers et al. [17] explained the non-basal plane growth to be caused by different interfacial adaptation due to kinetic restrictions of incoming atoms. Hence, the growth temperature plays a significant role in the formation of high-quality Ti₂AlN thin films, but its effects are yet to be clearly understood.

Therefore, in this work, we investigate the effect of substrate temperature on the microstructure of resulting Ti–Al–N thin films deposited from 500 to 750 °C in situ by X-ray photoelectron spectroscopy (XPS) and ex situ by X-ray diffraction (XRD), atomic force microscopy (AFM) and nano-indentation. We will show that polycrystalline Ti₂AlN MAX phase starts to nucleate at as low as 600 °C. Its crystallinity further improves at 700 and 720 °C until it transforms into single-crystalline phase at 750 °C. The mechanism behind the evolution of the microstructure with temperature will be discussed by considering various factors including surface energy, lattice mismatch and enhanced adatom diffusion at high growth temperatures.