Cr2AlC Coatings Research Articles

Investigation of the failure mechanisms of Zr alloy with Cr2AlC coatings using in-situ bending tests: Experiments and simulations

The failure mechanisms of Cr2AlC-coated zirconium samples under different mechanical loading conditions have been investigated by combining in-situ bending tests and finite element simulations. The results of interrupted in-situ bending tests reveal that new critical cracks are mainly initiated in the Cr2AlC coating layer, followed by subsequent propagation into the Zr substrate with increasing plastic deformation. The formation of new critical cracks in Cr2AlC material is described using the maximum principal stress criterion. A stress state-dependent damage mechanics model combined with an advanced plasticity model is used to capture the ductile fracture behavior of the Zr substrate. Finite element simulations have been performed to identify the failure properties of coating and substrate materials, leading to the accurate reproduction of experimental fracture behavior.

Phase formation mechanism of Cr2AlC MAX phase coating: In-situ TEM characterization and atomic-scale calculations

Cr2AlC, a representative MAX phase, gains increasing attention for the excellent oxidation tolerance and corrosion resistance used in harsh high temperature and strong radiation environments. However, the lack of the phase formation mechanism has become the key bottleneck to the practical applications for Cr2AlC synthesis with high purity at low temperatures. In this work, we fabricated the amorphous Cr–Al–C coating by a hybrid magnetron sputtering/cathodic arc deposition technique, in which the in-situ heating transmission electron microscopy (TEM) was conducted in a temperature range of 25–650 °C to address the real-time phase transformation for Cr2AlC coating. The results demonstrated that increasing the temperature from 25 to 370 °C led to the structural transformation from amorphous Cr–Al–C to the crystalline Cr2Al interphases. However, the high-purity Cr2AlC MAX phase was distinctly formed at 500 °C, accompanied by the diminished amorphous feature. With the further increase of temperature to 650 °C, the decomposition of Cr2AlC to Cr7C3 impurities was observed. Similar phase evolution was also evidenced by the Ab-initio molecular dynamics calculations, where the bond energy of Cr–Cr, Cr–Al, and Cr–C played the key role in the formed crystalline stability during the heating process. The observations not only provide fundamental insight into the phase formation mechanism for high-purity Cr2AlC coatings but also offer a promising strategy to manipulate the advanced MAX phase materials with high tolerance to high-temperature oxidation and heavy ion radiations.

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.

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.

Effect of suspension plasma spraying power on the microstructure and properties of Cr2AlC coatings

In this work, the microstructure evolution and mechanical properties of suspension plasma sprayed Cr2AlC coatings under different spraying powers were studied, and a self-protection model during oxidation was proposed. Due to the partial decomposition of Cr2AlC and its subsequent reactions with the surrounding H2O or O2 at high temperatures, the sprayed coating exhibits a heat-resisting shell, composed mainly of Cr2Al, carbides, oxides, and graphite, wrapping the internal Cr2AlC core against further decomposition. In addition, with the increase of spraying power, the content of Cr2AlC phase and the microhardness of the coating gradually increase, while the porosity and surface roughness decrease. The results indicate that suspension plasma spraying is a promising process for preparing Cr2AlC 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.

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.

Phase orientation improved the corrosion resistance and conductivity of Cr2AlC coatings for metal bipolar plates

In view of the Mn+1AXn (MAX) phase coatings benefiting the adaptive passivation film for good corrosion resistance and high electronic density of states for excellent electrical conductivity, here, we reported the Cr2AlC MAX phase coatings with different preferred orientations by a homemade technique consisting of vacuum arc and magnetron sputtering. The dependence of surface and interface microstructural evolution upon the corrosion and electrochemical properties of deposited coating was focused. Results showed that all the Cr2AlC coatings with different phase orientations greatly improved the performance of stainless steel (SS) 316 L substrate. Specifically, the lowest value of interface contact resistance (ICR) reached to 3.16 mΩ cm2 and the lowest corrosion current density was 2 × 10−2 μA cm−2, which were much better than those of bare SS316L. The combined studies of electrochemical properties and theoretical calculations demonstrated that the Cr2AlC coatings with preferred (103) orientation were easier to form oxide passivation film on their surface to increase the corrosion resistance.

High-temperature oxidation and hydrothermal corrosion of textured Cr2AlC-based coatings on zirconium alloy fuel cladding

Alumina-forming MAX phase coatings reveal great potential for accident tolerant fuel (ATF) cladding applications due to their favorable physical and mechanical properties and excellent high-temperature oxidation resistance. The feasibility of Cr2AlC MAX phase as protective coating on zirconium-alloy fuel claddings was explored focusing on high-temperature oxidation resistance in steam and hydrothermal corrosion performance in autoclave. Single-phase and basal-plane textured Cr2AlC coatings (~6 μm thick) were synthesized on Zircaloy-4 substrate by thermal annealing of specifically designed, magnetron-sputtered Cr/C/Al elemental multilayers at 550 °C for 10 min. Additionally, a second Cr/Cr2AlC bilayer coating design was fabricated aiming to eliminate potential rapid hydrothermal dissolution of Al during normal operating conditions. Micro-cracking appeared on both annealed coatings owing to thermal expansion differences between coating layer and substrate. Growth of an adherent and dense α-Al2O3 scale during high-temperature oxidation in steam and of a thin passivating Cr2O3 layer during hydrothermal corrosion in an autoclave imply excellent combined oxidation and corrosion resistance of the textured Cr2AlC coatings on Zircaloy-4. A self-healing capability via growth of alumina filling the micro-crack (annealing-induced) gaps was observed during high-temperature oxidation. However, partial delamination was seen for both coatings after short autoclave exposure and their mechanical properties (fracture toughness and adhesion strength) need further enhancement. Overall, tailored Cr2AlC-based (multilayered) coatings can be attractive candidates as potential type of coated ATF claddings.

On the fracture behavior of Cr2AlC coatings

Bulk MAX phase materials were investigated heavily in the last decades due to their advantageous combination of metallic and ceramic properties. In recent years, MAX phases also gained the interest of the protective coatings community. Cr2AlC is a very promising material, since the crystalline MAX phase can be deposited at comparatively low (550 °C) substrate temperatures. Another advantage of the Cr2AlC MAX phase is its self-healing ability. The goal of this investigation was to characterize the fracture toughness of Cr2AlC protective coatings using in situ SEM micro-cantilever tests and to determine the influence of different microstructures on the fracture behavior. Surprisingly, the fracture toughness is only moderately affected by the microstructure of the crystalline samples investigated here, which reveal a fracture toughness ranging from 1.8 ± 0.1MPam1/2 to 2.4 ± 0.2MPam1/2.In contrast to that, it could be shown that there is a significant increase in fracture toughness for the amorphous coating with identical chemical composition (4.1 ± 0.5MPam1/2) of almost twice the fracture toughness compared to the crystalline coatings. The detrimental influence of grain boundaries in the crystalline coating and the lack of grain boundaries in the amorphous sample might explain the formidable fracture toughness.

Fabrication and Mechanical Properties of Cr2AlC MAX Phase Coatings on TiBw/Ti6Al4V Composite Prepared by HiPIMS.

The high-power impulse magnetron sputtering (HiPIMS) technique is widely used owing to the high degree of ionization and the ability to synthesize high-quality coatings with a dense structure and smooth morphology. However, limited efforts have been made in the deposition of MAX phase coatings through HiPIMS compared with direct current magnetron sputtering (DCMS), and tailoring of the coatings’ properties by process parameters such as pulse width and frequency is lacking. In this study, the Cr2AlC MAX phase coatings are deposited through HiPIMS on network structured TiBw/Ti6Al4V composite. A comparative study was made to investigate the effect of average power by varying frequency (1.2–1.6 kHz) and pulse width (20–60 μs) on the deposition rate, microstructure, crystal orientation, and current waveforms of Cr2AlC MAX phase coatings. X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) were used to characterize the deposited coatings. The influence of pulse width was more profound than the frequency in increasing the average power of HiPIMS. The XRD results showed that ex situ annealing converted amorphous Cr-Al-C coatings into polycrystalline Cr2AlC MAX phase. It was noticed that the deposition rate, gas temperature, and roughness of Cr2AlC coatings depend on the average power, and the deposition rate increased from 16.5 to 56.3 nm/min. Moreover, the Cr2AlC MAX phase coatings produced by HiPIMS exhibits the improved hardness and modulus of 19.7 GPa and 286 GPa, with excellent fracture toughness and wear resistance because of dense and column-free morphology as the main characteristic.

Influence of MAX-Phase Deformability on Coating Formation by Cold Spraying

As solid-state deposition technique avoiding oxidation, cold gas spraying is capable of retaining feedstock material properties in the coatings, but typically fails to build up coatings of brittle materials. Ceramic MAX phases show partial deformability in particular lattice directions and may thus successfully deposit in cold spraying. However, deformation mechanisms under high strain rate, as necessary for cohesion and adhesion, are not fully clear yet. A MAX-phase deposit only builds up, if the specific mechanical properties of the MAX phase allow for, and if suitable spray parameter sets get realized. To investigate the influence of material properties and deposition conditions on coating microstructure and quality, three MAX phases, Ti3SiC2, Ti2AlC and Cr2AlC, were selected. Up to ten passes under different spray parameters yielded Ti2AlC and Cr2AlC coatings with thicknesses of about 200-500 µm. In contrast, Ti3SiC2 only forms a monolayer, exhibiting brittle laminar failure of the impacting particles. In all cases, the crystallographic structure of the MAX-phase powders was retained in the coatings. Thicker coatings show rather low porosities (< 2%), but some laminar cracks. The deposition behavior is correlated with individual mechanical properties of the different MAX-phase compositions and is discussed regarding the particular, highly anisotropic deformation mechanisms.

Effect of target peak power density on the phase formation, microstructure evolution, and mechanical properties of Cr2AlC MAX-phase coatings

The applicability of Cr2AlC MAX-phases as protective coatings in energy conversion or aerospace applications requires a dense, single-phase structure. Therefore, we study the effect of target power density and substrate bias on phase formation, microstructure evolution, and mechanical properties of Cr2AlC coatings utilizing direct current (DCMS) and high-power pulsed magnetron sputtering (HPPMS). Generally, HPPMS results in coatings with superior density and hence larger elastic moduli compared to DCMS, indicating that ion bombardment by ionized film-forming species is beneficial. However, decreasing the substrate bias to −200 V for DCMS and −100 V for HPPMS favors the ion bombardment induced formation of the disordered (Cr,Al)2Cx solid solution. It is evident that there is an optimum moderate ion energy for the formation of dense Cr2AlC coatings. Too low energy results in the formation of under-dense coatings. Too high energy yields the formation of (Cr,Al)2Cx in addition to Cr2AlC.

Tribo–Mechanical Properties of HVOF-Sprayed NiMoAl-Cr2AlC Composite Coatings

The tribo-mechanical properties of NiMoAl-Cr2AlC MAX phase composite coatings on stainless steel substrate have been investigated. NiMoAl with different amounts of Cr2AlC (10, 20, 50 and 100 wt.%) were prepared by turbo-mixing and deposited by High-Velocity Oxy-Fuel (HVOF) method on stainless steel substrate. The phase composition, microstructure, chemical composition, tribological and mechanical properties of the coatings were analyzed using x-Ray Diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), Energy-Dispersive x-ray analysis (EDAX), pin-on-disk wear testing rig and nanohardness tester, respectively. The worn surfaces were analyzed by metallurgical optical microscope, FESEM and three-dimensional surface profiler to understand the wear behavior in detail. The addition of the Cr2AlC MAX phase in NiMoAl enhances the mechanical properties and reduces the surface roughness and porosity. NiMoAl-20 wt.% Cr2AlC and Cr2AlC coatings containing equal amounts of oxygen and carbon in the tribofilm show the low coefficient of friction (COF) and wear rate. The addition of the Cr2AlC MAX phase in the NiMoAl matrix up to 20 wt.% reduces the wear rate by one order of magnitude and enhances the coating life by 7000 twist fatigue cycles.

Effects of HiPIMS discharges and annealing on Cr-Al-C thin films

Cr2AlC coatings were synthesized by high power impulse magnetron sputtering (HiPIMS) from Cr2AlC compound target and subsequent thermal annealing in Ar atmosphere. The effect of HiPIMS duty cycle and substrate bias potential (UB) on the thin film composition were investigated. All initial Cr-Al-C coatings exhibit similar compositions close to the target stoichiometry, and dense and amorphous structure independently of the duty cycle. Meanwhile, Al deficiencies up to 15% are observed with UB increasing to −200 V. Based on the measured fraction of ionized metal species flux at the substrate, highly energetic bombardment of the coating with ionized inert gas and metal plasma species causes preferential resputtering of Al. Partially crystallized Cr2AlC thin films were obtained by annealing as-deposited Cr-Al-C coatings at 550 °C for 4 h. The annealed coating is made of an amorphous inner layer and a crystalline Cr2AlC outer layer. A higher annealing temperature of 650 °C led to complete transformation from amorphous phase to crystallized Cr2AlC, and to micro-cracking. These results indicate that the synthesis temperature of MAX phase could be reduced, and the annealing time increased, to obtain protective coatings of Cr2AlC on heat-sensitive components without alteration of the substrate metallurgical properties.

Textured growth of polycrystalline MAX phase carbide coatings via thermal annealing of M/C/Al multilayers

Mn + 1AXn (MAX; n = 1–3) phases are ternary layered nitride and carbide compounds featuring a combination of metallic and ceramic properties. Highly basal-plane textured and polycrystalline Cr2AlC, Ti2AlC, and Ti3AlC2 single-phase coatings have been synthesized on both amorphous and polycrystalline substrates via controlled thermal annealing of magnetron-sputtered nanoscale multilayers built by individual transition metal, carbon, and aluminum layers. Formation of substitutional solid solution carbide phases was triggered via solid-state diffusion reactions during annealing. Lower ordered Ti2AlC initially crystallized at an intermediate temperature range and was recognized as an intermediate reactant in the case of synthesizing the Ti3AlC2 312 MAX phase via annealing corresponding stoichiometric multilayers. The crystallization onset temperatures identified via in-situ high-temperature x-ray diffraction measurements were approximately 480, 660, and 820 °C for Cr2AlC, Ti2AlC, and Ti3AlC2, respectively. Contrary to the usually observed columnar structure representative of magnetron-sputtered coatings, the coatings synthesized via the current approach are composed of plateletlike, elongated crystallites. The nanoscale multilayered design stimulates the textured growth of MAX structures during thermal annealing. More specifically, the preferred crystallographic orientation relationships among the as-deposited transition metal layers, the intermediate solid solution phases, and the end-product MAX phases facilitate the growth of textured MAX phase films.

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.

High-temperature oxidation behavior of HiPIMS as-deposited Cr–Al–C and annealed Cr2AlC coatings on Zr-based alloy

Protective coatings of Zr-based claddings have been proposed for the development of Accident Tolerant nuclear Fuel (ATF). Coatings forming stable oxides at high temperature such as MAX phases are attractive candidates for these applications. In this study Cr–Al–C coatings were deposited on coupons of Zr-based alloy (Zr702) by High Power Impulse Magnetron Sputtering (HiPIMS). Cr2AlC coatings were then obtained by annealing of the as-deposited films at a temperature below metallurgical degradation of Zr alloys. The behavior of as-deposited Cr–Al–C and annealed Cr2AlC coatings with respect to high-temperature oxidation slightly differ for short oxidation times but converge for longer durations. Oxidized coatings are made of (i) an external dense, covering, adherent and thin scale of aluminum and chromium oxides, (ii) an intermediate thicker, porous layer of chromium carbide and (iii) an interdiffusion layer. Both coatings are protective in dry and wet air (up to 1473 K for 2 h in air-28% H2O for an initial thickness of 7 μm), and are thermal shock-resistant. Self-healing capability is observed for submicronic defects. The top oxide scale acts as a barrier against oxygen diffusion, thus efficiently protecting the Zr702 substrate from extended oxidation except near coupon edges. The results indicate that Cr–Al–C thin films grown by HiPIMS process and annealed Cr2AlC coatings are both promising candidates for ATF cladding coatings.