MAX Phase 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.

Solid solution of zirconium on the M-site in Ti2AlC MAX phase coatings: Synthesis, structure and mechanical properties

Ti2AlC MAX phase is considered an ideal candidate for surface protective coatings in harsh environments due to its excellent resistance to oxidation and corrosion, yet poor mechanical properties are a key bottleneck for practical applications. In this work, taking the conception of solid solution, high-purity and high-hardness (Ti, Zr)2AlC coatings were synthesized on Ti-6Al-4V substrates using a multiple-target magnetron sputtering technique combined with subsequent annealing. The as-deposited Ti-Zr-Al-C coating was partially crystalline containing TiAlx phase, and MAX phases formed after annealing at 750 °C for 90 min. Comprehensive X-ray diffraction and high-resolution TEM analysis identified the formation of solid solution (Ti0.9Zr0.1)2AlC. The incorporation of Zr reduced the surface roughness and grain size of the coatings due to the increased nucleation sites and the suppressed atomic diffusion. Comparing with pristine Ti2AlC coating, the hardness of the (Ti0.9Zr0.1)2AlC coating was enhanced by 30 % from 12.8 GPa to 16.6 GPa. This enhancement can be ascribed to the synergistic strengthening from solid solution and grain refinement. However, fracture toughness slightly deteriorated in the (Ti0.9Zr0.1)2AlC coating due to the preferential crack path along the refined-grain boundaries.

Excellent oxidation resistance of rapidly crystallizing Cr2AlC coatings at high temperatures: Effect of microstructure

Two kinds of Cr2AlC coatings with different microstructures were prepared by direct crystallization during oxidation at 1000 ℃ and conventional crystallization at 700 ℃ for 1 h for amorphous Cr-Al-C coatings prepared by magnetron sputtering, respectively. The effect of microstructure on the oxidation behavior of the coatings at 1000 ℃ was investigated. The grain boundary density and dislocation density of the Cr2AlC coatings crystallized directly during oxidation were much higher, which provided much more sites for heterogeneous nucleation of α-Al2O3 and fast diffusion paths for Al. Therefore, the coating formed complete pure α-Al2O3 scales earlier with better antioxidant properties. More importantly, the present work demonstrates feasibility and advantages of directly applying amorphous Cr-Al-C coatings at high temperatures without additional crystallization treatments, which simplifies preparation process of Cr2AlC coatings, facilitates their industrial applications, and provides a new method for rapid preparation of MAX-phase coatings.

Corrosion resistance and conductivity behavior of Cr-Al-N MAX phase coatings prepared by magnetron sputtering technology

Chromium aluminum nitride (CrAlN) coatings containing Cr2AlN phase were prepared by unbalanced magnetron sputtering followed by annealing at 1000 °C. The crystal structures, X-ray diffraction patterns, and selected area electron diffraction patterns of the Cr2AlN phase were modeled using computer software and compared with experimental observations. A good agreement was found between the two sets of results, thereby confirming the presence of Cr2AlN phase in the coatings. While previous studies have demonstrated the formation of Cr2AlN phase theoretically, this study presents experimental evidence of its physical existence. The potentiodynamic polarization results and interfacial contact resistance measurements show that the CrAlN coating with Cr2AlN phase has excellent corrosion resistance (a corrosion current density of 5.35 × 10−7 A/cm2 and electrical conductivity of 5.5 mΩ·cm2) and is thus a promising candidate for the protection of fuel cell bipolar plates.

High-performance Cr2AlC MAX phase coatings for ATF application: Interface design and oxidation mechanism

Surface-modified Zr-based alloy (ZIRLO) claddings with advanced ceramic coatings are increasingly required for accident-tolerant fuel (ATF) systems in light-water reactors. Cr2AlC MAX phase coatings are promising for this purpose owing to their remarkable properties combining radiation/oxidation/corrosion resistance. However they are suffering from weak interface compatibility to ZIRLO substrate and poor structural densities for long-term services. Herein, we fabricated densely high-purity Cr2AlC MAX phase coatings with uniquely designed Cr/CrCx interfacial layers. The oxidation behavior of the coatings was focused under steam environments at 1000–1200 °C. Results showed that Cr2AlC coatings exhibited an oxidation mass gain of 8.9 mg/cm2 and an oxide thickness of 680 nm after oxidation at 1200 °C for 30 min, which were about 10% and 0.5% of ZIRLO substrate, respectively. Based on microstructural evolutions, the embedded interfacial layers significantly suppressed the rapid diffusion of Al in Cr2AlC coatings to the substrate and the premature delamination of oxidized coatings. Particularly, the formed oxides were identified as dense yet pure α-Al2O3, which endowed the protection against further oxidation and excellent resistance to high-temperature steam corrosion.

Temperature-adaptive lubrication of Ag doped Cr2AlC nanocomposite coatings

Self-lubricating nanocomposite carbides and nitrides coatings that enable to exhibit both low friction coefficient (CoF) and high wear resistance over a wide temperature range, are in high demand for mechanical components used in aerospace and marine systems. In this work, we developed high-purity Ag-doped Cr2AlC (Ag–Cr2AlC) nanocomposite coatings using a facile arc/sputter deposition technique followed by annealing. Compared to typical nanocrystalline Cr2AlC MAX phase coatings, the incorporation of Ag resulted in a unique nanocomposite structure, where Ag encapsulates elongated Cr2AlC nanograins. The dry sliding wear of this material was evaluated using the ball-on-disc method, with an Al2O3 ball as the counter body, at temperature ranging from 25 °C to 700 °C. Interestingly, Ag dopants contributed to a significant reduction in COF and wear rate at temperatures below 500 °C, while the synergistic effect of the Ag distributed at grain boundaries and the outermost oxides of Cr2O3 and Al2O3 generated during friction dominated the excellent self-lubrication at 500–700 °C. Notably, the Ag–Cr2AlC coatings promoted the in-situ formation of gradient bilayer oxides, consisting of a loosely coarse-grained inner layer and a densely refine-grained outmost layer. The combination of unique gradient oxides and a combined nanocomposite matrix allows the coating to accommodate plastic strains without extended cracks or localized brittle fracture.

Stoichiometric Cr2AlC MAX phase coatings deposited by HPPMS from composite targets using industrial deposition technology

Compositionally-graded Cr2AlC coatings were deposited from composite targets with Al concentration gradients along the long-axis of the targets, ranging from 20.0 to 30.0 at.%, in an industrial plant. Stoichiometric Cr2AlC coatings deposited by HPPMS with a peak-power density of 217 W/cm2 and at substrate bias potential of −100 V require an Al concentration in the target of 30.0 at.% to compensate for the Al-loss induced by preferential re-sputtering and thermally induced desorption at these deposition conditions. The HPPMS deposition of a stoichiometric Cr2AlC coating at floating potential can be attained from a target segment containing 26.4 at.% of Al. Hence, during synthesis at 560 °C in an industrial deposition system with two-fold rotation significant deviations between target and coating composition were observed. It was demonstrated that these ion energy and power density induced reductions in Al concentration can be compensated for by utilizing Al-rich composite targets.

Corrosion behavior of various conductive materials in Sb3Sn7 alloy at 450 °C

Sb-Sn alloys are attractive positive electrode materials for liquid metal batteries. In this work, the corrosion behavior of three steels (f/m steel T91, austenitic steels SS304 and SS316L), one Fe-Co-Ni alloy (4J29 Kovar), molybdenum, and three MAX-phase coatings (Cr2AlC, Ti2AlC, Ti3AlC2) is investigated in static exposures to liquid Sb3Sn7 alloy at 450 °C for 750 h in oxygen-poor conditions. The results reveal unacceptably high corrosion rates in the range 0.5 to 1.3 µm/h for the Fe-based alloys, while Mo and MAX-phases show promising corrosion resistance with corrosion rates below 0.1 mm/a despite a small arsenic impurity of the liquid metal.

Ti-Al-C Based MAX Phase Coating As Corrosion Protection for Ferritic Bipolar Plates

Metallic bipolar plates in fuel cells have the advantages of high electrical conductivity, low permeability to the reactants and gases, and easy manufacturability. However, resistance to corrosion under long term operation conditions limits their practical use in the fuel cell. Owing to the importance of corrosion stability, materials such as austenitic stainless steels, Ni-based alloys and coated titanium or stainless steel plates are currently being used in bipolar plates, although ferritic steels would be cost effective. A pinhole-free anticorrosion coating can help in application of the ferritic steels in fuel cells. MAX phase materials offer excellent corrosion stability and high electronic conductivity and have been successfully tested for corrosion protection of metallic bipolar plates. In the current work, Ti-Al-C-based MAX phase coatings were prepared using high power impulse magnetron sputtering (HiPIMS). The HiPIMS process was in some cases also combined with plasma immersion ion implantation (PIII) to study the influence of the high energetic ion implantation process on the structure and performance of the resulting MAX phase layer. The deposited samples were annealed at 800 °C for 2 h in Ar atmosphere. The structure and composition of the coatings produced were studied with X-ray photoelectron spectroscopy, atom probe tomography and transmission electron microscopy. The corrosion performance of the layers was studied using a scanning flow cell coupled with inductively coupled plasma mass spectroscopy.

On the formation of Ti2AlN MAX phase coatings and improvement in tool life by superimposing on tungsten carbide cutting tool for machining Ti-6Al-4V alloys

This paper presents the development of (002) textured Ti2AlN coated wear resistant tool utilizing reactive magnetron co - sputtering. The Ti2AlN coating was developed in two different Argon, Nitrogen ratios such as 64:3 and 64:4, the influence of these parameters was identified using crystallographic, microstructural, mechanical and tribological studies. Experimental studies were performed to compare the cutting force as well as the tool life of superimposed Ti2AlN tool with a commercial tool by turning titanium alloy (Ti-6Al-4V). Results from the experimental analysis revealed an 80–85 % increase in hardness, a 13–20 % decrease in wear rate and a 27 % increase in tool life for Ti2AlN coated cutting tool. According to chip morphology, there is a 12 % increase in shear angle, 43 % and 51 % decrease in friction angle and coefficient of friction respectively. The nanomechanical studies were performed which show a maximum hardness of 29.12 ± 5.4 GPa and minimum wear rate of 10.078 × 10−9 mm3/mN.m−1 for superimposed Ti2AlN.

On the determination of the thermal shock parameter of MAX phases: A combined experimental-computational study

Thermal shock resistance is one of the performance-defining properties for applications where extreme temperature gradients are required. The thermal shock resistance of a material can be described by means of the thermal shock parameter RT. Here, the thermo-mechanical properties required for the calculation of RT are quantum-mechanically predicted, experimentally determined, and compared for Ti3AlC2 and Cr2AlC MAX phases. The coatings are synthesized utilizing direct current magnetron sputtering without additional heating, followed by vacuum annealing. It is shown that the RT of both Ti3AlC2 and Cr2AlC obtained via simulations are in good agreement with the experimentally obtained ones. Comparing the MAX phase coatings, both experiments and simulations indicate superior thermal shock behavior of Ti3AlC2 compared to Cr2AlC, attributed primarily to the larger linear coefficient of thermal expansion of Cr2AlC. The results presented herein underline the potential of ab initio calculations for predicting the thermal shock behavior of ionically-covalently bonded materials.

Research and Development on Cold-Sprayed MAX Phase Coatings

Cold spraying is an attractive solid-state processing technique in which micron-sized solid particles are accelerated towards a substrate at high velocities and relatively low temperatures to produce a coating through deformation and bonding mechanisms. Metal, ceramic, and polymer powders can be deposited to form functional coatings via cold spraying. MAX phase coatings deposited via cold spraying exhibit several advantages over thermal spraying, avoiding tensile residual stresses, oxidation, undesirable chemical reactions and phase decomposition. This paper presents a review of recent progress on the cold-sprayed MAX phase coatings. Factors influencing the formation of coatings are summarized and discussions on the corresponding bonding mechanisms are provided. Current limitations and future investigations in cold-sprayed MAX coatings are also listed to facilitate the industrial application of MAX phase coatings.

Characterization and reaction mechanism of in-situ micro-laminated Cr2AlC coatings by plasma spraying Cr3C2/Al/Cr powder mixtures

Large-scale fabrication of MAX phase coatings such as Cr2AlC and Ti2AlC by plasma spraying is extremely challenging, owing to the ultra-high temperature and insufficient reaction time. In this research, the Cr2AlC coatings were produced by plasma spraying from Cr3C2@Al-Cr agglomerates. The influence of Al contents on microstructure and mechanics properties of coatings were investigated. Results showed the as-deposited coatings displayed a typical layered structure, mainly composed of Cr2AlC, Cr7C3, Cr23C6 and sub-stable (Cr, Al)Cx. Rapid deposition resulted in an imperfect reaction of raw powders, thus generated sub-stable (Cr, Al)Cx. The adequate Al and nonstoichiometric carbides CrCx were the main key to promote the presence of MAX-Cr2AlC phase. Nevertheless, excessive Al would create extra (Cr, Al)Cx, thus reduce the Cr2AlC content. Notably, the coatings retained high hardness (>10 GPa) and good fracture toughness (~2.5 MPa·m1/2). The micro-lamellar structure, nano-layered Cr2AlC and microcrack-toughening ensured the good mechanical properties of coatings.

A new strategy to fabricate Ti2AlC MAX coatings by the two-step laser method

High-content Ti2AlC MAX coatings were efficiently prepared on TC4 plates by two-step laser method (laser cladding and post-laser treatment). The microstructure and phase composition of the coatings were analyzed, and the in-situ synthesis mechanism of MAX phases was explored. According to the results, the diffusion of Al atoms in the cladded coatings led to the formation of a core-shell structure with TiC as the core and Ti2AlC as the shell. During post-laser treatment, the diffusion of C and Al atoms was intensified, which facilitated the nucleation-growth of the Ti2AlC phase, producing two types of MAX phase structures. The short rod-shaped MAX phase was precipitated directly from the coating matrix, while the blocky MAX phase gradually formed owing to atomic diffusion. Finally, the content of Ti2AlC MAX phase in as-obtained coatings after post-laser treatment at the laser power of 1.2 kW and the scanning speed of 1 mm/s reached 82.93 wt%, which was 9.03 times higher than that in the cladded coating. Therefore, the two-step laser technique proposed in this work enables the production of high-content MAX coatings, which opens up new prospects for efficient preparation of MAX phase coatings and related structures.

M-site solid solution of vanadium enables the promising mechanical and high-temperature tribological properties of Cr2AlC coating

Cr2AlC MAX phase coating exhibited excellent oxidation resistance and hot corrosion, but it typically suffered from low hardness and toughness as well as lack of lubrication at high temperature. With a solid solution design on M-site, we demonstrated that the hardness of the Cr2AlC MAX phase coating was enhanced by 34.3% when Cr was partially substituted with V (47 at.%), and the coating toughness was improved simultaneously. Furthermore, according to the high-temperature tribometer test, both the friction coefficient and the wear rate of the coatings at 900 °C against Al2O3 balls were significantly reduced at 47 at.% V. This could be attributed to the formation of a large number of molten V2O5 wrapped (Cr, Al)2O3 hard crystal grains, which not only provided a wide range of liquid-phase lubrication, but also prevented the coating from being apt to wear out. Different from the multi-phase compositing, this study suggested a promising strategy to enhance the combined mechanical and tribological performance of MAX phase coatings by M-site V solid solution for harsh applications at a high temperature of 900 °C.

Performance analysis of Ti2AlN superimposed WC cutting tool

This article presents the development of superimposed Ti2AlN MAX phase coatings on cutting tools through reactive co-sputtering technique in dry machining of Ti-6Al-4 V. The effect of varying nitrogen flow rate during the reactive co-sputtering process and its influence on cutting properties has also been discussed. The predictive modeling along with validation of experimental results were carried out using response surface methodology (RSM) and optimized condition for cutting force were obtained using composite desirability function. Machining studies using the superimposed Ti2AlN coatings reduced the cutting force by 23–35 % when compared to commercial coatings. Investigation of cutting mechanics revealed an increase of shear angle by 3–5 % and decrease in friction angle by 20 %. The superimposed MAX phase coatings also reduced the friction coefficient by 34 % during dry turning of Ti-6Al-4 V. The superimposed Ti2AlN coatings were held responsible to improve the performance of the tool by 21–30 % in terms of tool life when compared to commercial tool. It could be inferred that the superimposed Ti2AlN coatings reduced the effect of chemical affinity between the tool and workpiece, eliminating the formation of built-up edge at rake face of the tool.

Microstructure and reaction mechanism of Ti-Al-C based MAX phase coatings synthesized by plasma spraying and post annealing

This work explored the feasibility of fabricating thick Ti2AlC and Ti3AlC2 coatings by plasma spraying from Ti/Al/graphite agglomerates and post annealing. The phase transformation and microstructure evolution of the sprayed coatings annealed at 500–900 °C were proposed. The as-deposited coating exhibited a micro-laminated structure with mainly of (Ti, Al)Cx, Ti2AlC, residual Al and minor TixAly. The short reaction time of spraying process led to insufficient reaction in C@Ti-Al agglomerates, thus forming dominate (Ti,Al)Cx and limited amount of Ti2AlC. The phase in the coating changed little when annealing below 600 °C. As increasing temperature to 700 °C, both of Ti3AlC2 and Ti2AlC showed obvious growth but Ti3AlC2 was predominate. Under higher temperature (>700 °C), Ti3AlC2 in the coating decreased while Ti2AlC grew apparently. The residual Al, transition phase (Ti, Al)Cx and high temperature annealing were main factors to promote the formation of Ti2AlC phase in sprayed coatings. The as-annealed coating showed increasing hardness with elevated temperature because of the formation of Ti2AlC, reduced pores and homogenous structure. This study implied that element agglomerates would be an effective alternative of expensive MAX phase powders as feedstock for plasma spraying MAX phase coatings.

Phase formation and microstructure evolution of plasma sprayed Cr2AlC MAX phase coatings under post annealing

In this study, thick Cr2AlC coatings were first synthesized via plasma spraying of Cr3C2–Al–Cr agglomerated powders and post annealing. The microstructure evolution and mechanical properties of the Cr2AlC coatings annealed at 500–1000 °C were investigated. The as-sprayed coatings exhibited a lamellar structure, primarily consisting of Cr2AlC, Cr7C3, Cr23C6, and (Cr, Al)Cx solid solutions. The short residence time during spraying led to incomplete reactions in the Cr3C2@Al–Cr agglomerates, resulting in the formation of (Cr, Al)Cx. Post annealing provided sufficient energy for the transition of (Cr, Al)Cx → Cr2AlC. With an increase in the annealing temperature (<900 °C), gradual transition of the (Cr, Al)Cx phase led to a slight increase in the Cr2AlC content, and thus, the as-annealed coatings maintained high hardness (>1000 HV0.2) with improved fracture toughness. Higher annealing temperatures (>900 °C) promoted clear enhancement of the Cr2AlC content, thus reducing the coating hardness. The transition phase (Cr, Al)Cx and high temperature annealing were the primary factors to promoting the formation of the Cr2AlC phase in sprayed coatings. This study indicates that the Cr3C2@Al–Cr agglomerates can be effective alternatives to expensive MAX phase powders as feedstock for plasma spraying of Cr2AlC coatings.

Corrosion behavior of Al-containing MAX-phase coatings exposed to oxygen containing molten Pb at 600 °C

Four types of MAX-phase coating samples (Cr2AlC, V2AlC, Ti2AlC, and Ti3AlC2), synthesized via physical vapor deposition and post-annealing of elemental multilayer films, have been investigated toward their potential corrosion protection in 10-6 wt% oxygen-containing molten Pb for 3200 h at 600 °C. Three MAX-phase coatings (Cr2AlC, Ti2AlC, and Ti3AlC2) are corrosion resistant against the molten Pb, via formation of a protective oxide layer. Specifically, various oxide scales formed on each sample: an Al2O3 scale, containing a low amount of Cr2O3 was formed in case of the Cr2AlC coating; a mixed Al2O3 and TiO2 oxide scale was observed for both Ti2AlC and Ti3AlC2 coatings. A more complex mixed oxide layer consisting of VO2, V4O7, and Pb3(VO4)2 phases is formed on the V2AlC coating, while the MAX-phase coating was almost entirely consumed. According to their corrosion performance against oxygen containing molten Pb, the order of corrosion resistance is: Cr2AlC > Ti2AlC > Ti3AlC2 > V2AlC.

Structural-phase and tribo-corrosion properties of composite Ti3SiC2/TiC MAX-phase coatings: an experimental approach to strengthening by thermal annealing

Structural-phase and tribo-corrosion properties of composite Ti3SiC2/TiC MAX-phase coatings: an experimental approach to strengthening by thermal annealing