Autonomous high‐temperature healing of surface cracks in Al2O3 containing Ti2AlC particles

In this work, the oxidation‐induced crack healing of Al₂O₃ containing 20 vol.% of Ti₂AlC MAX phase inclusions as healing particles was studied. The oxidation kinetics of the Ti₂AlC particles having an average diameter of about 10 μm was studied via thermogravimetry and/or differential thermal analysis. Surface cracks of about 80 μm long and 0.5 μm wide were introduced into the composite by Vickers indentation. After annealing in air at high temperatures, the cracks were filled with stable oxides of Ti and Al as a result of the decomposition of the Ti₂AlC particles. Crack healing was studied at 800, 900, and 1000°C for 0.25, 1, 4, and 16 hours, and the strength recovery was measured by 4‐point bending. Upon indentation, the bending strength of the samples dropped by about 50% from 402 ± 35 to 229 ± 14 MPa. This bending strength increased to about 90% of the undamaged material after annealing at 1000°C for just 15 minutes, while full strength was recovered after annealing for 1 hour. As the healing temperature was reduced to 900 and 800°C, the time required for full‐strength recovery increased to 4 and 16 hours, respectively. The initial bending strength and the fracture toughness of the composite material were found to be about 19% lower and 20% higher than monolithic alumina, respectively, making this material an attractive substitute for monolithic alumina used in high‐temperature applications.     

Crack-healing behaviour of MoSi2 dispersed Yb2Si2O7 environmental barrier coatings

MoSi₂ doped Yb₂Si₂O₇ composites were designed to extend the lifetime of Yb₂Si₂O₇ environmental barrier coatings (EBCs) via self-healing cracks during high-temperature applications. Yb₂Si₂O₇–Yb₂SiO₅–MoSi₂ composites with different mass fractions were prepared by applying spark plasma sintering. X-ray diffraction results confirmed that the composites consisted of Yb₂Si₂O₇, Yb₂SiO₅, and MoSi₂. The thermal expansion coefficients (CTEs) of the composites increased with an increase in the MoSi₂ content. The average CTE of the 15 wt% MoSi₂ doped Yb₂Si₂O₇ composite was 5.24 × 10^?6 K^?1, indicating that it still meets the CTE requirement of EBC materials. After being pre-cracked by using the Vickers indentation technique, the samples were annealed for 0.5 h at 1100 or 1300 °C to evaluate the crack-healing ability. Microstructural studies showed that cracks in 15 wt% MoSi₂ doped Yb₂Si₂O₇ composites were fully healed during annealing at 1300 °C. Two mechanisms may be responsible for crack healing. First, the cracks were filled with SiO₂ glass formed by MoSi₂ oxidation. Second, the formed SiO₂ continued to react with Yb₂SiO₅ to form Yb₂Si₂O₇, which can cause cracks to heal owing to volumetric expansion. The Yb₂Si₂O₇ formation with smaller volume expansion is more beneficial.     

Reducing the self-healing temperature of Ti2AlC MAX phase coating by substituting Al with Sn

In an effort to overcome the property degradation of Ti₂AlC MAX phase coating used in harsh environments, we fabricated a solid solution Ti₂(Al_0.6Sn_0.4)C coating with amount of Ti₅Sn₃ (20 wt.%) by a combined technique composing of magnetron sputtering and post-heat treatment. The cracks induced by Vickers indentation on coating surface were self-healed at 700 ℃, which is the lowest self-healing temperature among the Al-based MAX phase coatings till now. The structural evolution and kinetic diffusion revealed that the formation of SnO₂ is the key factor to achieve the crack self-healing at such a low temperature for Al-based MAX phase coatings. Additionally, the self-healed Ti₂(Al_0.6Sn_0.4)C coating exhibited better oxidation resistance compared to the unhealed one at 800 ℃. The results provide a novel and facile strategy to develop the protective MAX phase coatings with high performance at high temperature by partially substituting Al with Sn.     

Healing performance of Ti2AlC ceramic studied with in situ microcantilever bending

In situ microcantilever bending tests were carried out to evaluate the healing efficiency of pre-notched Ti₂AlC ceramic after annealing at 1200 °C for 1.5 h. Microcantilevers of different orientations were fabricated with focused ion beam method at different locations, i.e. in a singular Ti₂AlC grain, at a grain boundary or at the Ti₂AlC–Al₂O₃ interface after healing. Ti₂AlC microcantilever shows an anisotropic bending strength (ranging between 9.6 GPa and 4.6 GPa depending on the precise crystallographic orientations) that is closely related to the different atomic bonds in the layered structure. After healing, the Ti₂AlC–Al₂O₃ microcantilevers exhibit almost the same strength of about 5.2 GPa, i.e. slightly higher than the cleavage strength (4.6 GPa) of the initial Ti₂AlC microcantilevers. It suggests that the orientation of the matrix grain has no significant influence on the strength of healed microcantilevers. Furthermore, it turns out that the strength of the microcantilever with a healed grain boundary is at least twice the strength of the initial Ti₂AlC cantilever with a grain boundary. It is concluded that the oxidation dominated self-healing mechanism of Ti₂AlC ceramics can result in a perfect recovery of mechanical performance. The paper shows that the in situ microcantilever bending test provides a quantitative method for the evaluation of the strength of self-healing ceramics.     

Autonomous crack healing ability of SiC dispersed Yb2Si2O7 by oxidations in air and water vapor

Yb₂Si₂O₇ is a popular environmental barrier coating; however, it decomposes into Yb₂SiO₅ in high-temperature steam environments. The thermal mismatch between Yb₂Si₂O₇ and Yb₂SiO₅ leads to the cracking and failure of the disilicate coating via oxidation. Dispersing SiC nanofillers into the Yb₂Si₂O₇ matrix is suggested to maintain the Yb₂Si₂O₇ matrix and promote crack self-healing. This study is aimed at clarifying the effect of water vapor on the self-healing ability of such composites. X-ray diffraction analysis and scanning electron microscopy were used to monitor the surface composition and the crack formation, respectively, in 10 vol% SiC-dispersed Yb₂Si₂O₇ composites. Annealing at temperatures higher than 750 °C in air or in a water vapor rich atmosphere led to strength recovery and the self-healing of indentation-induced surface cracks owing to volume expansion during the oxidation of SiC. The self-healing effect was influenced by the oxidation time and temperature. Rapid diffusion of H₂O as an oxidizer into the SiO₂ layer promoted self-healing in a water vapor rich atmosphere. However, accelerated oxidation at temperatures higher than 1150 °C formed bubbles on the surface. Fabricating composites with a small amount of Yb₂SiO₅ will be a solution to these problems.     

SiC modified self-healing ytterbium disilicate materials for potential environmental barrier coating application

Environmental barrier coatings (EBCs) are crucial to the reliability and durability of SiC_f/SiC composite components seeking applications in hot sections of next-generation advanced aero-engines. The cracks initiated and developed in EBCs owing to various reasons during service greatly undermine their lifespans. To address this problem, in this work, silicon carbide (SiC) in the forms of particles and whiskers with various amounts have been introduced to ytterbium disilicate (Yb₂Si₂O₇), the mainstream EBC topcoat materials, so as to gain some self-healing potential. The results reveal that, the SiC inclusions in Yb₂Si₂O₇ in the presence of ytterbium monosilicate (Yb₂SiO₅) can trigger the following reactions. Specifically, SiC self-healing agents are oxidized to form viscous SiO₂, which actively reacts with Yb₂SiO₅ upon encountering it, forming Yb₂Si₂O₇. This has brought twofold beneficial effects including ① silicon supplementation of disilicate topcoat, whose silicon element tends to be “dragged out” by water vapor, leading to the deterioration of thermal mismatch; as well as ② crack self-healing resulting from the volume expansion induced by the above reactions. Then the two aspects of self-healing agents, namely the “promptness” and “sustainability,” have been discussed in detail. The former is unveiled to be more pertinent to the repairing of large cracks, whilst the latter is more relevant to the self-healing of tiny cracks at initiation or early stage of propagation. The current work sheds some lights on the design and development of more durable and robust EBCs with self-healing capability.     

Effect of Yb2SiO5 addition on the physical and mechanical properties of sintered mullite ceramic as an environmental barrier coating material

In this study, ytterbium monosilicate (Yb₂SiO₅)-added sintered mullite ceramics are prepared as candidate materials for environmental barrier coatings (EBCs). The effect of adding Yb₂SiO₅ on the physical and mechanical properties of the sintered mullite ceramics is investigated. The Yb₂O₃–SiO₂–Al₂O₃ ternary phase diagram indicates that adding Yb₂SiO₅ to the mullite goes beyond simply mixing; instead, liquid sintering occurs. Therefore, when we add Yb₂SiO₅ to the mullite, the sintered body possesses a denser microstructure and faster densification rate than does pure mullite. The density rapidly increases with the addition of 6 wt% Yb₂SiO₅ in the mullite, and almost full densifications are achieved with the addition of 12 wt% and 18 wt% Yb₂SiO₅. In this study, mullite ceramic that contains 12 wt% Yb₂SiO₅ exhibits the smallest plastic deformation and the highest elastic modulus among ceramics containing 6, 12, and 18 wt% Yb₂SiO₅, according to Hertzian indentation results. The results suggest that 12 wt% Yb₂SiO₅-doped mullite may be expected to act as a potential EBC material based on its excellent elastic properties, dense microstructure, and appropriate coefficient of thermal expansion.     

Design and strengthening behaviour of Ti2AlC/TiAl composite by low-temperature hot-pressing process

In situ Ti₂AlC/TiAl composite was first fabricated by reactive hot-pressing technique at low temperature of 1150°C for 2?h using Ti₃AlC₂ and Ti–Al alloy powders. The composite with fine-grained structure consisted of TiAl, Ti₃Al and Ti₂AlC phases. The Vickers hardness, flexural strength and fracture toughness of the Ti₂AlC/TiAl composite reached 5.2?GPa, 937.7?MPa and 7.7?MPa?m^1/2, respectively. The action mechanism for the composite was mainly attributed to the grain refinement, the uniform distribution of the dispersed Ti₂AlC particles, transgranular cracking, crack deflection, crack bridging and pull-out of Ti₂AlC.     

Thermal shock resistance of tri‐layer Yb2SiO5/Yb2Si2O7/Si coating for SiC and SiC‐matrix composites

A new tri‐layer Yb₂SiO₅/Yb₂Si₂O₇/Si coating was fabricated on SiC, C/SiC, and SiC/SiC substrates, respectively, using atmospheric plasma spray (APS) technique. All coated samples were subjected to thermal shock test at 1350°C. The evolution of phase composition and microstructure and thermo‐mechanical properties of those samples before and after thermal shock test were characterized. Results showed that adhesion between all the 3 layers and substrates appeared good. After thermal shock tests, through microcracks which penetrated the Yb₂SiO₅ top layer were mostly halted at the Yb₂SiO₅‐Yb₂Si₂O₇ interface and no thermal growth oxide (TGO) was formed after 40‐50 quenching cycles, implying the excellent crack propagation resistance of the environmental barrier coating (EBC) system. Transmission electron microscopy analysis confirmed that twinnings and dislocations were the main mechanisms of plastic deformation of the Yb₂Si₂O₇ coating, which might have positive effects on crack propagation resistance. The thermal shock behaviors were clarified based on thermal stresses combined with thermal expansion behaviors and elastic modulus analysis. This study provides a strategy for designing EBC systems with excellent crack propagation resistance.     

Lamellar structure and effect of Ti2AlC on properties of prepared in-situ TiAl matrix composites

TiAl matrix composites, reinforced with varying Ti₂AlC content levels, were synthesized by vacuum arc melting by use of Ti, Al and graphite powders as raw materials. The effects of varied Ti₂AlC content from 0 to 100 mol% on the phases, microstructures, fracture behaviors and mechanical properties of resulting products were analyzed. Phase transformation was characterized by X-ray diffraction (XRD) and microstructures were measured by scanning electron microscopy (SEM), equipped with energy-dispersive spectroscopy (EDS). Results show that the products are composed primarily of TiAl, Ti₃Al, Ti₂AlC and small amounts of TiC. The unreacted TiC phase is due mainly to evaporation of Al. The Vickers hardness value of TiAl matrix composite was increased by adding Ti₂AlC, while the TiAl/Ti₂AlC composite fracture behaviors were improved in terms of crack deflection, trans-lamellar cracking and extraction of carbide reinforcements.     

The synthesis and mechanical properties of high pure Ti2Al(Sn)C solid solution

High pure Ti₂Al_(1?x)Sn_xC (x = 0‐1) powders were synthesized using Ti, Al, Sn, and TiC powders as raw materials by pressureless sintering method. The influence of sintering temperature and raw material ratio on the purity of Ti₂AlC and Ti₂Al_0.8Sn_0.2C powders were investigated. The results show that pure Ti₂AlC and Ti₂Al_0.8Sn_0.2C powders were obtained from the mixed raw materials ratio of Ti:1.1Al:0.9TiC and Ti:0.9Al:0.2Sn:0.9TiC at 1450°C, respectively. Subsequently, fully dense Ti₂AlC and Ti₂Al_0.8Sn_0.2C bulks were prepared using mechanically alloying and hot pressed sintering method. The Vickers hardness of Ti₂AlC and Ti₂Al_0.8Sn_0.2C approaches approximately about 6 GPa and 4 GPa, the flexural strength was measured to be 650 ± 36 MPa and 521 ± 33 MPa, respectively. Microstructural analysis reveals that grain delamination, kink bands, and crack deflection occurred around the indentation area and at the fracture surface.     

Characterization of Yb2Si2O7–Yb2SiO5 composite environmental barrier coatings resultant from in situ plasma spray processing

Plasma spraying of multicomponent materials produces shifts in coating composition associated with differential vaporization of constituent elements within the strong thermal gradients of the process. This effect is quite noticeable in rare-earth silicates which are now widely being employed as Environmental Barrier Coatings (EBCs) for SiC based ceramic components of turbine engines. Of particular interest is the preferential volatilization of SiO₂ during thermal plasma spraying Yb₂Si₂O₇ (ytterbium disilicate) coatings which leads to the deviation from stoichiometry of the desired disilicate composition resulting in a mixed phase coating consisting of Yb₂Si₂O₇ plus Yb₂SiO₅ (ytterbium monosilicate). Recent work has shown that presence of monosilicate can be beneficial as its evolution from amorphous, metastable to stable crystalline phase can lead to crack healing during high temperature exposure, however, careful control of the chemistry and architecture may be needed. In this work a 50/50 mol% Yb₂Si₂O₇–Yb₂SiO₅ composite coating has been targeted through in situ decomposition during plasma spray from stoichiometric Yb₂Si₂O₇ powder. The as sprayed amorphous coating reverts to crystalline upon thermal treatment passing through a metastable state identified by XRD and Raman spectroscopy. The transition to the final stable phases results in a mixed phase coating comprising of 46/54 mol% Yb₂Si₂O₇–Yb₂SiO₅ composite that is thermo-mechanically stable with the underlying bond coated silicon coated SiC substrate.     

Reaction synthesis of layered ternary Ti2AlC ceramic

Reactive sintering of 8Ti:Al₄C₃:C powder mixtures to form the ternary carbide Ti₂AlC is studied in the temperature range 570–1400 °C. After sintering at 1400 °C for 1 h, only the MAX phase Ti₂AlC and some TiC are produced. A series of intermediate phases, such as TiC, Ti₃Al, Ti₃AlC are detected during the reactive sintering process. From X-ray diffraction (XRD) and scanning electron microscopy (SEM) characterizations, a reaction path is proposed for the intermediate phases and Ti₂AlC formation. Results show that reaction kinetics may play an important role in the understanding of the reaction mechanisms.     

Nb2AlC-particle induced accelerated crack healing in ZrO2–matrix composites

The oxidation induced crack healing behavior of pre-cracked Nb₂AlC particle loaded ZrO₂-matrix composites was explored by annealing in air at 1200?°C for short periods of 10 and 20?min. Composites loaded with 0, 6.5, 13, and 19.5?vol% Nb₂AlC powder dispersed in 3Y-TZP matrix powder were manufactured by spark plasma sintering (SPS) at 1300?°C. Semi-elliptical artificial surface cracks with a length exceeding 220?µm were produced by Vickers indentation. The modulus of rupture of virgin, indented and annealed samples was measured in three-point bending mode. Compared to single phase 3Y-TZP strength recovery of the Nb₂AlC loaded composite upon annealing at 1200?°C in air is accelerated and reaches >?60% of the initial strength after a short healing period of 10?min only. A semi-empirical oxidation cohesive zone healing model was derived which describes the crack microstructure evolution as a combined effect of 3Y-TZP-matrix healing superimposed by Nb₂AlC particle oxidation induced healing.     

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.     

Multiple crack healing of a Ti2AlC ceramic

A highly attractive self-healing material would be one which combines excellent mechanical properties with a multiple healing capability. Self-healing ceramics have been studied for over 40 years to obtain some performance recovery and to prevent material failure during service, but so far only materials with the capability of single healing event per damage site have been realized. Here we report on a self-healing Ti₂AlC ceramic capable of repeatedly repairing damage events. The Ti₂AlC ceramic achieves at least seven healing cycles after repeated cracking at a given location. The main healing mechanism at high temperature is the filling of the cracks by the formation well adhering α-Al₂O₃ and the presence of some rutile TiO₂. For healed samples, the flexural strength returned or even slightly exceeded the virginal strength. The fracture toughness recovery has been quantified for multiple healing cycles.     

Infiltration behavior of Cu and Ti fillers into Ti2AlC/Ti3AlC2 composites during tungsten inert gas (TIG)brazing

Herein we study the infiltration behavior of Ti and Cu fillers into a Ti₂AlC/Ti₃AlC₂MAX phase composites using a TIG-brazing process. The microstructures of the interfaces were investigated by scanning electron microscopy and energy dispersive spectrometry. When Ti₂AlC/Ti₃AlC₂ comes into contact with molten Ti, it starts decomposing into TiC_x, a Ti-richandTi₃AlC; when in contact with molten Cu, the resulting phases are Ti₂Al(Cu)C, Cu(Al), AlCu₂Ti and TiC. In the presence of Cu at approximately 1630 °C, a defective Ti₂Al(Cu)C phase was formed having a P63/mmc structure. Ti₃AlC₂ MAX phase was completely decomposed in presence of Cu or Ti filler-materials. The decomposition of Ti₂AlC to Ti₃AlC₂ was observed in the heat-affected zone of the composite. Notably, no cracks were observed during TIG-brazing of Ti₂AlC/Ti₃AlC₂ composite with Ti or Cu filler materials.     

Oxidation-induced crack healing and erosion life assessment of Ni–Mo–Al–Cr7C3–Al2O3 composite coating

The present investigation focuses on the effect of Cr₂AlC MAX phase addition on erosion and oxidation-induced crack healing behavior of Ni–Mo–Al alloy. For this, Ni–Mo–Al and 20 wt% Cr₂AlC-blended Ni–Mo–Al powders were coated by Air Plasma Spray (APS). For oxidation-induced crack healing studies, the samples were heat treated at 500, 800, and 1100°C in the air for 5 hours. The heat-treated samples were analyzed by X-Ray Diffraction (XRD) analysis, Scanning Electron Microscopy (SEM), and Energy Dispersive Spectroscopy (EDS) for the phases, morphology, and composition. Erosion behavior studies were carried out at 30, 250, 500, 800, and 1000°C temperatures. The average hardness was obtained to be 400 ± 10 HV for Ni–Mo–Al coating and 580 ± 10 HV for 20 wt% Cr₂AlC-blended Ni–Mo–Al coating. The addition of Cr₂AlC MAX into Ni–Mo–Al matrix reduces the overall erosion rate and improved the crack healing ability. This was attributed to the presence of in-situ-formed Cr₇C₃ and Al₂O₃ phases.     

Multifunctional performance of Ti2AlC MAX phase/2D braided alumina fiber laminates

The processing and characterization of laminates based on Ti₂AlC MAX phase, as matrix, and triaxial alumina braids, as reinforcing phase, are presented. Ti₂AlC powders with a mean particle size below 1 µm are synthesized, while commercial 3M Nextel 610 alumina fibers are braided in a three-stage process consisting of spooling, braiding with an angle of 0° and ±60° and the separation to single-layer fabric. The laminates are processed by layer-by-layer stacking, where 3 two-dimensional alumina braids are interleaved between Ti₂AlC layers, followed by full densification using a Field-Assisted Sintering Technology/Spark Plasma Sintering. The multifunctional response of the laminates, as well as for the monolithic Ti₂AlC, is evaluated, in particular, the thermal and electrical conductivity, the oxidation resistance, and the mechanical response. The laminates exhibit an anisotropic thermal and electrical behavior, and an excellent oxidation resistance at 1200℃ in air for a week. A relatively lower characteristic biaxial strength and Weibull modulus (i.e., σ₀ = 590 MPa and m = 9) for the laminate compared to the high values measured in the monolithic Ti₂AlC (i.e., σ₀ = 790 MPa and m = 29) indicates the need but also the potential of optimizing MAX-phase layered structures for multifunctional performance.     

Oxidation and Crack Healing Behavior of a Fine‐Grained Cr2AlC Ceramic

This work reports the oxidation and crack healing behavior of a fine‐grained (~2 μm) Cr₂AlC MAX phase ceramic. The oxidation behavior was investigated in the temperature range 900°C–1200°C for times up to 100 h. The material showed a good oxidation resistance, owing to the formation of a dense and thin α‐Al₂O₃ layer. The microstructure, composition and thickness of the oxide scale were characterized. Its oxidative crack healing behavior as a function of temperature, healing time, and initial crack size was studied systematically. The material showed excellent healing behavior. The main crack healing mechanism is the filling of the crack by oxides well adhering to the crack faces. The crack geometry before and after healing was characterized by X‐ray tomography. Three‐point bend tests showed the dependence of strength recovery at 1100°C as a function of initial crack length and healing time.