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)C_x 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)C_x. Post annealing provided sufficient energy for the transition of (Cr, Al)C_x → Cr₂AlC. With an increase in the annealing temperature (<900 °C), gradual transition of the (Cr, Al)C_x phase led to a slight increase in the Cr₂AlC content, and thus, the as-annealed coatings maintained high hardness (>1000 HV_0.2) 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)C_x 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.     

Influence of Al content and post-annealing on synthesis and mechanical properties of plasma sprayed Cr–Al–C composite coatings

Due to ultra-high temperature and short reaction time, it was very challenging to produce high purity MAX phase by plasma spraying. In this study, Cr–Al-graphite agglomerated powders with different Al additions (x = 0.2–1.5) was used to prepare Cr–Al–C composite coatings by atmospheric plasma spraying followed with annealing. Results showed that the as-sprayed coatings displayed typical lamellar structure, mainly composed of Cr–C binary carbides (Cr₇C₃ and Cr₂₃C₆) and residual Al. After annealing at 700 °C, the newly formed Cr₂AlC phase increased significantly in the coatings. The higher addition of Al, the more Cr₂AlC phase formed after annealing. The enhanced atomic diffusion, sufficient Al source and existence of (Cr, Al)C_x contributed to the formation of Cr₂AlC under annealing. Annealing treatment improved the hardness of the coating, but with the increase of Cr₂AlC phase content, the hardness decreased slightly. The Al content and post-annealing had a synergistic effect on the formation of Cr₂AlC phase in the sprayed coatings. This provided an effective route to control the Cr₂AlC content in sprayed Cr–Al–C composite coatings.     

Synthesis and characterization of MAX phase Cr2AlC based composite coatings by plasma spraying and post annealing

In this work, a simple two-step method was developed to produce thick Cr₂AlC based coatings. Firstly, atmosphere plasma spraying was employed to deposit Cr-A-C coatings with Cr/Al/graphite mixtures. Then Ar-annealing treatment was conducted on as-sprayed coatings to in situ achieve Cr₂AlC. Microstructure evolution and mechanical performance of composite coatings was investigated. The as-sprayed coating exhibited a lamellar feature with mainly Cr₇C₃ and residual Al. With increasing temperature, the residual Al decreased and the newly formed Cr₂AlC phase increased. Especially, high temperature annealing (>700 °C) led to remarkable increasing amount of Cr₂AlC phase due to the enhanced atom diffusion. The annealing treatment enhanced both of hardness and fracture toughness of coatings due to the formation of Cr₂AlC. However, the increasing amount of Cr₂AlC phase resulted in slight decrease of hardness. Thus, the content of Cr₂AlC phase played a significant role in mechanical performance of composite coatings, which was adjusted by post-annealing.     

Synthesis of fine and high purity Cr2AlC powders by novel method

Cr₂AlC powders using Cr/Al/C and Cr/Al/Cr₃C₂ systems as raw materials were successfully synthesised by a microwave hybrid heating method for the first time. The mixtures of Cr, Al and graphite or Cr₃C₂ with different molar ratios were used to investigate the formation of Cr₂AlC phase. For Cr/Al/C with the molar ratios of 2:(1.0–1.2):1 system, Cr₂AlC with a small amount of Cr₇C₃ and Cr₂Al was synthesised at 1000°C for 3 min, and the average particle size was ?1?μm. For Cr/Al/Cr₃C₂ with the molar ratio of 1:2:1 system, high purity Cr₂AlC powders was synthesised at 1000°C for 3 min, and the average particle size was ?1?μm. The synthesis of high purity Cr₂AlC powders for short time was attributed to the combination of the hybrid heating effect and the introduction of Cr₃C₂ as carbon source. Microwave hybrid heating is a promising method for the preparation of various other MAX phases.     

High-temperature synthesis of composite materials based on (Cr,Mn, V)–Al–C MAX phases

Composite materials based on (Cr, Mn, V)–Al–C MAX phases were obtained by self-propagating high-temperature synthesis (SHS). Regularities of synthesis of composite materials from mixtures containing chromium (III) oxide, manganese (IV) oxide, vanadium (V) oxide, calcium (IV) oxide, aluminum, and carbon powders were studied. The synthesis of 30-g blend was carried out in an SHS reactor with a volume of 3 l under Ar pressure (5 MPa). Variation in the amount of the starting reagents markedly affected the process parameters, phase composition, and microstructure of combustion products. The combustion products were characterized by XRD, SEM, and EDS analysis. For Cr–Al–C system, MAX Cr₂AlC phase in addition to chromium aluminide Cr₅Al₈ and chromium carbides (Cr₇C₃, Cr₃C₂) was detected. SEM studies showed that Cr₂AlC has a laminated structure with layer thickness varying from 3 to 20 nm. XRD pattern of Mn–Cr–Al–C composite material were found to have signals belonging (Cr_xMn_1–x)₂AlC solid solution, Mn₃AlC, and Cr₂Al. It was shown that V–Al–C composite material contains nano-layered MAX V₂AlC phase and particles VC_х, VAl₃.     

Complex optimization of arc melting synthesis for bulk Cr2AlC MAX-phase

A complex optimization of the arc melting synthetic approach was performed to enhance the phase purity of Cr₂AlC MAX-phase in the bulk form. Optimization steps included the initial ratio of 2Cr: xAl: 1C, variation through a change in Al content (i.e. x ranging from 1 to 1.5), tuning duration of the post-annealing thermal treatment and adjustment of the melting chamber pressure. The use of Cr₃C₂ as a precursor in place of Cr and C mixture has also been tried. It was found that Cr₂AlC MAX-phase forms as the predominant phase when a stoichiometric ratio 2Cr: 1.3Al: 1C is used. By keeping this stoichiometry constant, the melting chamber pressure was then adjusted to further improve the phase purity of the samples. It has been established that the use of Cr₃C₂ is not an appropriate way to improve sample purity and promote homogeneity in the carbon distribution. The establishment of an optimized arc fusion protocol for parental Cr₂AlC is a necessity for further successful mass synthesis of the substituted (Cr_1-xMn_x)₂AlC MAX-phase.     

Cold spray deposition of Cr2AlC MAX phase for coatings and bond-coat layers

Highly pure Cr₂AlC powders were synthesized and deposited for the first time by cold spray technology on stainless steel substrates. The Cr₂AlC coatings were relative dense, up to 91%, and present high purity (> 98%) since only small traces of Cr₂Al, Al₂O₃ and Cr₂O₃ were detected by XRD, SEM and EDX. The microstructure of the coatings is homogeneous, although some preferential orientation in the basal plane was observed by XRD pole figures. The adhesion between the coating and the substrate is strong, and compressive residual stresses up to 300 MPa in the coating were determined by XRD. Furthermore, a conventional YSZ Thermal Barrier Coating (TBCs) was deposited by Atmospheric Plasma Spray (APS) on top of the cold sprayed Cr₂AlC coating in order to demonstrate the processing feasibility of Cr₂AlC MAX phases as a bond-coat layer.     

Thermal stability enhancement of Cr2AlC coatings on Zr by utilizing a double layer diffusion barrier

Decomposition of Cr₂AlC deposited onto a Zr substrate and vacuum-annealed is observed at 800 °C as Al diffuses from the MAX phase into the Zr substrate. A double layer of ZrN and AlN has been predicted by CALPHAD calculations to act as diffusion barrier between the Zr substrate and Cr₂AlC. Experimental thermal stability investigations corroborate this prediction by confirming that the proposed double layer diffusion barrier coatings suppress the decomposition of Cr₂AlC for one hour at temperatures of up to 1000 °C.     

Phase formation of Nb2AlC investigated by combinatorial thin film synthesis and ab initio calculations

The phase formation of Nb₂AlC was studied by combinatorial thin film synthesis and ab initio calculations. Thin films with lateral chemical composition gradients were synthesized by DC magnetron sputtering at substrate temperatures of 710–870 °C. The lowest formation temperature for Nb₂AlC is between 710 and 750 °C. A predominantly single phase Nb₂AlC region where 99% of the X-ray diffraction intensity originate from Nb₂AlC was identified. Furthermore, selected area electron diffraction analysis reveals the local formation of single phase Nb₂AlC. The limited Al solubility in Nb₂AlC compared with Cr₂AlC can be understood by comparing the defect formation energy of Al substituting Nb and Cr in Nb₂AlC and Cr₂AlC, respectively. This methodology may serve as indicator for the magnitude of the A-element homogeneity range in M_n+1AX_n phases. The structural and elastic properties of Nb₂AlC determined experimentally are in very good agreement with the ab initio calculated data.     

Formation of MAX solid solutions (Ti,V)2AlC and (Cr,V)2AlC with Al2O3 addition by SHS involving aluminothermic reduction

MAX solid solutions (Ti,V)₂AlC and (Cr,V)₂AlC with Al₂O₃ addition were produced by solid state combustion involving aluminothermic reduction in the mode of self-propagating high-temperature synthesis (SHS). Starting materials included TiO₂/V₂O₅/Al/Al₄C₃ and Cr₂O₃/V₂O₅/Al/Al₄C₃ powder mixtures. Attempts were made to attain (Ti_1−xV_x)₂AlC and (Cr_1−yV_y)₂AlC with the V content in terms of x and y from 0.1 to 0.7. Combustion exothermicity was increased by increasing V₂O₅ for the yield of a higher proportion of V at the substitution site, which not only increased the combustion temperature and reaction front velocity, but also facilitated the evolution of solid solutions. Due to insufficient reaction exothermicity, (Ti_1−xV_x)₂AlC/Al₂O₃ in situ composites were only produced under x≥0.4. On the other hand, the formation of (Cr_1−yV_y)₂AlC/Al₂O₃ was achieved with y from 0.1 to 0.7, because reduction of Cr₂O₃ is more energetic than that of TiO₂. The laminated microstructure characteristic of the MAX ternary carbide was observed for both Al₂O₃-added (Ti,V)₂AlC and (Cr,V)₂AlC composites synthesized in this study.     

Thermal stability and selective nitridation of Cr2AlC in nitrogen at elevated temperatures

The thermal stability and selective nitridation of Cr₂AlC under a high–temperature nitrogen atmosphere were studied. Cr₂AlC began to decompose beyond 1073 K in N₂, and the activation energy was estimated to be 108.93 kJ·mol^?1. Initially, the selective nitridation led to the generation of an AlN layer, which shielded the underlying Cr₂AlC and the process was dominated by a surface reaction. The final products were AlN, Cr₇C₃ and Cr₃C₂, and the weaker Cr–Al bonds in Cr₂AlC facilitated the rapid diffusion of aluminum from the interior outwards. As the reaction proceeded, micropores were observed on the grain surface, as well as a loose structure, which facilitated the diffusion of N₂ and thus accelerated the reaction. Finally, the intensive reaction involving Cr₂AlC and N₂ could be attributed to the gas diffusion channels caused by improved temperature and continuous escape of vaporized aluminum.     

Yttrium incorporation in Cr2AlC: On the metastable phase formation and decomposition of (Cr,Y)2AlC MAX phase thin films

Herein we report on the synthesis of a metastable (Cr,Y)₂AlC MAX phase solid solution by co-sputtering from a composite Cr–Al–C and elemental Y target, at room temperature, followed by annealing. However, direct high-temperature synthesis resulted in multiphase films, as evidenced by X-ray diffraction analyses, room-temperature depositions, followed by annealing to 760°C led to the formation of phase pure (Cr,Y)₂AlC by diffusion. Higher annealing temperatures caused a decomposition of the metastable phase into Cr₂AlC, Y₅Al₃, and Cr-carbides. In contrast to pure Cr₂AlC, the Y-containing phase crystallizes directly in the MAX phase structure instead of first forming a disordered solid solution. Furthermore, the crystallization temperature was shown to be Y-content dependent and was increased by ∼200°C for 5 at.% Y compared to Cr₂AlC. Calculations predicting the metastable phase formation of (Cr,Y)₂AlC and its decomposition are in excellent agreement with the experimental findings.     

New M n +1AXn compounds by thermal explosion in the Al-Cr-Si-C system

Ternary Cr₂AlC and Cr₃SiC₂ compounds—belonging to the family of M _n+1AX_n phases (n=1, 2, 3; M is a transition metal, A is an A group element, and X = C or N)—were prepared by thermal explosion in the Cr-Al-Si-C system in air. After mixing, drying, and compaction, the green powders taken in stoichiometric ratios of Cr: Si: C=3: 1: 2 plus 20 wt % Al were thermally exploded at 1100°C in an SHS reactor. The combustion products, Cr₂AlC and Cr_5-xSi_3-z C _x+z , were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Chromium-aluminum intermetallics and Al₄C₃ were identified as intermediate products in the synthesis of Cr₂AlC. A Cr_5-xSi_3-z C_x+z compound in the system under study was found to exist as Cr₃SiC₂.     

Mechanical properties of low temperature synthesized dense and fine-grained Cr2AlC ceramics

Mechanically activated hot-pressing technology was used to synthesize a fine-crystalline Cr₂AlC ceramic at relatively low temperatures. A mixture of Cr, Al and C powders with a molar ratio of 2:1.2:1 was mechanically alloyed for 3 h, and then subjected to hot pressing at 30 MPa and different temperatures for 1 h in Ar atmosphere. The results show that a dense Cr₂AlC ceramic with a grain size of about 2 μm can be synthesized at a relatively low temperature of 1100 °C. The synthesized fine-grained Cr₂AlC has a high density of 99%, which is higher than the 95% density for the coarse-grained Cr₂AlC (grain size of about 35 μm) as synthesized by hot pressing unmilled Cr, Al and C. The flexural strength, fracture toughness and Vickers hardness of the fine-grained Cr₂AlC were determined and compared with the values for the coarse-grained Cr₂AlC.     

Synthesis and mechanical and thermal properties of (Cr,Mn)2AlC solid solutions

(Cr,Mn)₂AlC solid solutions have attracted much attention due to their magnetic property which will be enhanced with increasing Mn solubility. So far, only 3?at.-% Mn solubility in (Cr,Mn)₂AlC bulk materials has been reported. This work reports on the synthesis of (Cr,Mn)₂AlC solid solutions via a new reaction route with AlCr₂, C and Mn as starting materials. The Mn solubility increases with increasing the starting Mn content. The maximum solubility of Mn in (Cr,Mn)₂AlC was 8.3?at.-%, corresponding to a resultant solid solution (Cr_0.83Mn_0.17)₂AlC. However, increasing the Mn content in the starting mixture caused the formation of impurities in the sintered samples. A dense (Cr_0.95Mn_0.05)₂AlC ceramic has been achieved by hot-pressing the mixture of AlCr₂/C/0.1Mn. The mechanical properties and thermal expansion coefficient of (Cr_0.95Mn_0.05)₂AlC were measured.     

Synthesis and Oxidation Testing of MAX Phase Composites in the Cr–Ti–Al–C Quaternary System

According to the properties determined for the ternary end‐members, MAX phases in the quaternary Cr–Ti–Al–C system could be of interest as protective coatings for nuclear fuel cladding in the case of severe accident conditions. In this study, syntheses of 211 and 312 MAX phase compositions were attempted using pressureless reactions starting from Cr, TiH₂, Al, and C (graphite) powders. It was observed that both the Ti substitution by Cr in Ti₃AlC₂ and the mutual solubility of Ti₂AlC and Cr₂AlC are limited to a few atomic percent. In addition, the remarkable stability of the (Cr_2/3Ti_1/3)₃AlC₂ MAX phase composition was confirmed. Due to the low miscibility of MAX phases in the Cr–Ti–Al–C system, most samples contained substantial amounts of TiC_x and Al–Cr alloys as secondary phases, thus forming composite materials. After sintering, all samples were submitted to a single oxidation test (12 h at 1400°C in air) to identify compositions potentially offering high‐temperature oxidation resistance and so warranting further investigation. In addition to (Cr_0.95Ti_0.05)₂AlC, composite samples containing substantial quantities of Al₈Cr₅ and AlCr₂ formed a stable and passivating Al₂O₃ scale, whereas the other samples were fully oxidized.     

Synthesis,microstructure and mechanical properties of Cr2AlC/Al2O3 in situ composites by reactive hot pressing

Al₂O₃ particle-reinforced Cr₂AlC in situ composites were successfully fabricated from powder mixtures of Cr₃C₂, Cr, Al, and Cr₂O₃ by a reactive hot-pressing method at 1400 °C. A possible synthesis mechanism was proposed to explain the formation of the composites in which Al₂O₃ was formed by the aluminothermic reaction between Al and Cr₂O₃, meanwhile, Cr₃C₂, Al, together with Cr reacted to form Cr₂AlC in a shortened reaction route. The effect of Al₂O₃ addition on the microstructure and mechanical properties of Cr₂AlC/Al₂O₃ composites was investigated. The results indicated that the as-sintered products consisted of Cr₂AlC matrix and Al₂O₃ reinforcement, and the in situ formed fine Al₂O₃ particles dispersed at the matrix grain boundaries. The flexural strength and Vickers hardness of the composites increased gradually with increasing Al₂O₃ content. But the fracture toughness peaked at 6.0 MPa m^1/2 when the Al₂O₃ content reached 11 vol.%. The strengthening and toughening mechanism was also discussed.     

Behaviors of helium in Cr2AlC from first principles

The properties of helium incorporation and transport in Cr₂AlC have been investigated using first principles method. The results of calculation show that a single helium atom is preferred to reside at an interstitial position near the Al plane in perfect Cr₂AlC crystal, attributing to the low predicted formation energy. Helium atoms are expected to aggregate in Al layers. The interaction between helium and lattice atoms is primarily elastic due to the closed‐shell electronic structure of helium. The Doping of helium leads to a weakening of the Cr–Al bonds. Furthermore, an interstitial helium atom is likely to migrate along an indirect migration pathway from the hexahedral interstitial position on Al plane (I_AlC), passing by the nearest tetrahedral interstitial position near Al plane (I_AlCr), and finally reach to another hexahedral interstitial position (I_AlC) with an activation energy of 1.21 eV. The high activation energy suggests a relatively low migration for helium atoms in Cr₂AlC as well as a slow growth rate of helium bubbles for the early stage of helium irradiation.     

HiPIMS induced high-purity Ti3AlC2 MAX phase coating at low-temperature of 700 °C

Ti₃AlC₂, one of Ti-Al-C MAX phases, has received extensive attention due to its unique nano-laminated structure and combined properties of metals and ceramics. However, ultra-high synthesis temperature exceeding 800 °C is a critical challenge for broad application of Ti₃AlC₂ coatings on temperature-sensitive substrates. In this study, Ti-Al-C coatings were deposited on Ti-6Al-4V substrates using high-power impulse magnetron sputtering (HiPIMS) and DC sputtering (DCMS) for comparison. Different from as-deposited amorphous Ti-Al-C coating by DCMS, nanocrystalline TiAl_x compound was achieved by HiPIMS deposition due to highly ionized plasma flux with high kinetic energy. Furthermore, HiPIMS promoted the generation of dense and smooth Ti₃AlC₂ phase coating after low-temperature annealing at 700 °C, while annealed DCMS coating only obtained Ti₂AlC. In-situ XRD demonstrated such Ti₃AlC₂ phase could be early involved in crystallization at 450 °C, lowest than synthesis temperature ever reported. The mechanical properties of Ti₃AlC₂ coating were also discussed in terms of structural evolution.     

Oxidation and hot corrosion behaviors of MAX-phase Ti3SiC2, Ti2AlC,Cr2AlC

Oxidation and hot corrosion behaviours of Ti₃SiC₂, Ti₂AlC and Cr₂AlC at 750 °C were investigated in this work. Ti₃SiC₂ and Ti₂AlC showed a linear increase in mass gain and a relatively poor oxidation resistance. This might be attributed to the porous TiO₂ scale. A dense α-Al₂O₃ layer was formed during the oxidation test. Cr₂AlC exhibited the best oxidation resistance. This dense oxide scale can effectively isolate the substrate from contact with oxygen leading to excellent oxidation resistance. In contrast to the oxidation test, Ti₃SiC₂ and Ti₂AlC showed relatively better resistance to hot corrosion, while Cr₂AlC showed inferior resistance to NaCl introduced hot corrosion. The hot corrosion mechanism of the MAX phases was analyzed. Due to the formation of Na₂TiO₃, Ti containing MAX phases showed a continuous increase in the mass gain. The corrosion products of Cr₂AlC were Al₂O₃, Cr₂O₃ and Na₂CrO₄. However, due to the volatilization of Na₂CrO₄, Cr₂AlC showed a mass loss during the hot corrosion test. The chemical reaction process of the MAX phase was also analyzed.