Ti2AlC Coatings Research Articles

Towards developing Ti2AlC coatings with improved oxidation resistance via Nb solid solution

Solid solution MAX phase coatings of (Ti1-x, Nbx)2AlC, fabricated using a hybrid technique combining high-power impulse magnetron sputtering (HiPIMS) and DC magnetron sputtering (DCMS), were extensively evaluated for their oxidation resistance at 750 ℃. Compared to Ti2AlC, the oxidation rate of (Ti1-x, Nbx)2AlC significantly decreased, primarily due to the inhibition of non-protective TiO2 oxide growth. A thin, single-layer oxide scale consisting of TiO2 and Al2O3 formed on the (Ti1-x, Nbx)2AlC coatings, while the Ti2AlC coatings were nearly completely consumed after 24 h oxidation. The substitution of Ti4+ with Nb5+ played a crucial role in reducing the number of oxygen vacancies and enhancing the oxidation resistance of Ti2AlC.

Transforming the amorphous Ti-Al-C coatings to high-purity Ti2AlC MAX phase coatings by prolonged annealing at 550 °C

Ti2AlC MAX phases have been considered as the promising protective coating materials in nuclear energy and other harsh environments due to their unique properties. However, high preparation temperatures of Ti2AlC coatings, generally ≥700 °C, are the main barriers to their further application on temperature-sensitive substrates. Taking the concept of surface dynamics during prolonged annealing at low-temperatures, herein we reported a quite facile strategy to fabricate high-purity Ti2AlC coatings by transforming amorphous Ti-Al-C coatings at 550 °C. The transformation mechanisms of Ti2AlC MAX phase were also discussed in terms of their microstructural evolutions.

Diffusion-controlled intercalation approach to synthesize the Ti2AlC MAX phase coatings at low temperature of 550 °C

Mn+1AXn (MAX) phase materials have unique nano-laminated structures and exhibit interesting hybrid properties with ceramics and metals. However, high synthesis temperatures, e.g. typically ≥700 °C for Ti2AlC coatings, are the key barriers to their wide application on various substrates. In this study, Ti2AlC MAX phase coatings were firstly fabricated by annealing the pre-designed α-TiC0.65/Al multilayer coatings at temperatures as low as 550 °C till now, which is the lowest synthesis temperature compared to already-reported literatures. Different with the common used short-time annealing (1–3 h), noted that the long-time annealing from 15 h to 400 h was particularly conducted based on the sluggish diffusion kinetics at lower temperature. Moreover, the dependence of structural evolution upon annealing proposed that Al diffusion-controlled intercalation process played the key factor to reduce the the formation temperature of Ti2AlC MAX phase. The results provide a strategy to fabricate high-quality Ti2AlC MAX phase coatings at 550 °C for wide application of temperature-sensitive substrates.

Deposition, characterization and high-temperature steam oxidation behavior of single-phase Ti2AlC-coated Zircaloy-4

Oxidation of single-phase and dense Ti2AlC coatings with or without a 500 nm TiC diffusion barrier deposited on Zircaloy-4 by annealing of nanoscale multilayer stacks between 800 °C and 1200 °C in high-temperature steam was investigated. Coatings without TiC barrier formed a duplex scale: outer θ-Al2O3 rich layer mixed with TiO2 and inner porous TiO2 layer; correspondingly, a triple-layered scale (θ-Al2O3 + TiO2/θ-Al2O3/TiO2) grew on coatings with barrier at 800 °C. The TiC barrier suppresses the rapid diffusion of Al into the substrate, contributing to improved performance and longer life of Ti2AlC/TiC coatings. However, both coatings demonstrated low protection effect from 1000 °C in steam.

Kerosene-fuelled high velocity oxy-fuel (HVOF) spray of Ti2AlC MAX phase powders

Kerosene-fueled high velocity oxy-fuel (HVOF) spray was employed to deposit commercial Ti2AlC MAX phase powders onto Inconel 600 substrates. After optimizing three process parameters including flow rates of kerosene and oxygen as well as spray distance, a dense Ti2AlC coating with a very low decomposition was obtained. Using Rietveld phase analysis of the X-ray diffraction pattern, the weight percentage of Ti2AlC in the coating was determined to be 53.0%, which is not only far better than the coating deposited by plasma spray, but also is comparable with the optimum Ti2AlC coatings sprayed by H2-fueled HVOF and cold spray. This optimized Ti2AlC coating was further analyzed by a variety of techniques to probe its surface chemistry, morphology, cross-sectional microstructure, mechanical properties and salt fog corrosion resistance.

MAX-phase coatings produced by thermal spraying

This paper presents a comparative study on the Ti2AlC coatings produced by different thermal spray methods, as Ti2AlC is one of the most studied materials from the MAX-phase family. Microstructural ...

Synthesis and characterization of Ti2AlC coatings by magnetron sputtering from three elemental targets and ex-situ annealing

MAX phase materials have demonstrated great potential applications both as bulk and coating due to their unique properties. Deposition of phase-pure polycrystalline MAX phase coatings often remains a challenge. In this study, Ti-C-Al elemental nanolayer stacks deposited by non-reactive magnetron sputtering followed by ex-situ annealing was adopted to prepare Ti2AlC coatings on two different substrates, Si and SiO2/Si. No Ti2AlC phase was formed on Si substrate due to significant interdiffusion between the coating and the substrate during annealing. Phase-pure, i.e. single-phase Ti2AlC coatings with 3μm thickness on SiO2/Si were obtained after 800°C annealing in argon for 10min. The crystalline transition from the non-MAX competing phases to Ti2AlC MAX phase was found to appear in the region of 600°C–700°C. The microstructural evolution of the coatings during annealing from 600°C to 900°C was systematically investigated by X-Ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM). The reaction path for the formation of Ti2AlC is suggested as follows: first, aluminum reacted with titanium to form a Ti-Al intermetallic phase (i.e. Al3Ti), and the diffusion between the residue titanium and carbon produced non-stoichiometric TiCx; then, the reaction between the Al3Ti intermetallic and TiCx yielded the formation of Ti2AlC MAX phase.

Cold spray deposition of Ti2AlC coatings for improved nuclear fuel cladding

Coatings of Ti2AlC MAX phase compound have been successfully deposited on Zircaloy-4 (Zry-4) test flats, with the goal of enhancing the accident tolerance of LWR fuel cladding. Low temperature powder spray process, also known as cold spray, has been used to deposit coatings ∼90 μm in thickness using powder particles of <20 μm. X-ray diffraction analysis showed the phase-content of the deposited coatings to be identical to the powders indicating that no phase transformation or oxidation had occurred during the coating deposition process. The coating exhibited a high hardness of about 800 HK and pin-on-disk wear tests using abrasive ruby ball counter-surface showed the wear resistance of the coating to be significantly superior to the Zry-4 substrate. Scratch tests revealed the coatings to be well-adhered to the Zry-4 substrate. Such mechanical integrity is required for claddings from the standpoint of fretting wear resistance and resisting wear handling and insertion. Air oxidation tests at 700 °C and simulated LOCA tests at 1005 °C in steam environment showed the coatings to be significantly more oxidation resistant compared to Zry-4 suggesting that such coatings can potentially provide accident tolerance to nuclear fuel cladding.

The scaling behavior and mechanism of Ti2AlC MAX phase coatings in air and pure water vapor

The scale behavior of Ti2AlC coating at 750°C in air and pure water vapor was investigated. A four-layered scale, a thick TiO2 and Al2O3 mix oxide outer layer, followed by a thin α-(Al, Cr)2O3 sublayer, a thick Fe2O3 and TiO2 mix oxide mid-layer and a thin Al2O3-rich oxide inner layer in sequence, developed on the Ti2AlC coatings in air. Whereas internal oxidation occurred, no distinct oxide scale formed on the Ti2AlC coating in the case of the oxidation in pure water vapor. The Ti2AlC coating improved the oxidation resistance of 316LSS in air, especially in wet air.

Cold-spray deposition of Ti2AlC coatings

Ti2AlC coatings have been fabricated by cold-spray deposition. The microstructure evolution as a function of basic spray parameters temperature and pressure onto AA6060 aluminium alloy and 1.0037 steel substrates has been studied. Adherent and dense 50–80 μm thick Ti2AlC coatings were deposited on soft AA6060 substrates under gas temperature and pressure of 600 °C and 3.4 MPa, respectively, whilst comparable results were obtained on harder 1.0037 steel by using higher temperature (800 °C) and pressure (3.9 MPa).

Annealing of Thermally Sprayed Ti2AlC Coatings

Phase transformations during annealing of coatings sprayed with the high-velocity oxy-fuel technique using Ti2AlC powder have been investigated by in situ X-ray diffraction. The as-deposited coatings, consisting of Ti2AlC, Ti3AlC2, TiC, Ti–Al, and oxides, are stable up to 500°C. Ti3AlC2 forms above 550°C and Ti2AlC forms above 700°C by intercalation of Al into TiCx. For temperatures between 900°C and 1100°C, Ti3AlC2 and Ti2AlC decompose by losing Al to the surrounding matrix resulting in TiCx and Al2O3. The thermal expansion coefficient between ambient and 700°C is 11.9 × 10−6 K−1. The thermal diffusivity at room temperature is 10.5 × 10−6 m2/s.

Oxidation of Ti 2AlC bulk and spray deposited coatings

The oxidation behaviour of Ti 2AlC bulk and high velocity oxy-fuel spray deposited coatings has been investigated for temperatures up to 1200 °C. X-ray diffraction and electron microscopy show that bulk Ti 2AlC forms a continuous layer of α-Al 2O 3 below a layer of TiO 2 at temperatures as low as 700 °C. Oxidation of the Ti 2AlC coatings is more complex, and also involves the phases Ti 3AlC 2, TiC, and Ti x Al y , formed during the spraying process. α-Al 2O 3 is observed, however, it is unevenly distributed deep into the material, and does not form a continuous layer essential for good oxidation resistance.

Microstructure of high velocity oxy-fuel sprayed Ti2AlC coatings

The microstructure formation and phase transformations in Ti2AlC-rich coatings deposited by High Velocity Oxy-fuel spraying of Maxthal 211(A (R)) powders is presented. High resolution electron microscopy analysis, using both scanning and transmission electron microscopy with energy dispersive spectrometry and energy filtering, combined with X-ray diffraction reveals that the coatings consist of Ti2AlC grains surrounded by regions of very small TiC grains embedded in Ti (x) Al (y) . The composition of the Ti (x) Al (y) depends on its surrounding and varies with size and distribution of the adjacent TiC grains. Impact of spray parameters on coating microstructure is also discussed. Two spray parameters were varied; powder size distribution and flame power. They were found to greatly affect the coating microstructure. Increasing powder size and decreasing flame power increase the amount of Ti2AlC, but produces thinner coatings with lower cohesion. Larger powder size will also decrease oxygen incorporation.

Ti 2AlC coatings deposited by High Velocity Oxy-Fuel spraying

High Velocity Oxy-Fuel has been utilized to spray coatings from Ti 2AlC ( MAXTHAL 211 ®) powders. X-ray diffraction showed that the coatings consist predominantly of Ti 2AlC with inclusions of the phases Ti 3AlC 2, TiC, and Al–Ti alloys. The fraction of Ti 2AlC in coatings sprayed with a powder size of 38 μm was found to increase with decreasing power of the spraying flame as controlled by the total gas flow of H 2 and O 2. A more coarse powder (56 μm) is less sensitive to the total gas flow and retains higher volume fraction of MAX-phase in the coatings, however, at the expense of increasing porosity. X-ray pole figure measurements showed a preferred crystal orientation in the coatings with the Ti 2AlC (000 l) planes aligned to the substrate surface. Bending tests show a good adhesion to stainless steel substrates and indentation yields a hardness of 3–5 GPa for the coatings sprayed with a powder size of 38 μm.

Towards Large Area MAX Phase Coatings on Steel

MAX phases are promising materials for protective coatings on steel due to their unique combination of properties like corrosion and oxidation resistance, good electrical conductivity, low friction coefficient, damage tolerance, and high temperature stability. Here the deposition of large area MAX phase coatings on steel was investigated by magnetron sputtering of a Ti2AlC compound target. It was found that the chemical composition of the produced films was different from the composition of the target. The deposited films contained about half as much Ti as the target. Consequently the dependence of the film composition on temperature and pressure was investigated. The film composition prooved to be independent of pressure but the lack of Ti increased with increasing temperature. The stoichiometry of the films was then adjusted by adding Ti to the growing coating from a separate plasma source. This method provides a pathway to stoichiometric Ti2AlC coatings with an equilibrium volume that is in excellent agreement with our ab initio calculations.