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.     

Joining of Ti–Al–C ceramics by oxidation at low oxygen partial pressure

The joining of titanium aluminum carbides has been successfully performed at high temperature and low oxygen partial pressure. The mechanism of the bonding is attributed to the preferential oxidation of Al atoms in the titanium aluminum carbides at low oxygen partial pressure, which leads to the formation of an Al₂O₃ layer through the joint interface. The specimens joined at 1400 °C exhibit a high flexural strength of 315 ± 19.1 MPa for Ti₂AlC and 332 ± 2.83 MPa for Ti₃AlC₂, which is about 95% and 88% of the substrates, respectively, and the high flexural strength can be retained up to 1000 °C. The high mechanical performance of the joints is attributed to the similar density and thermal expansion coefficient values of Al₂O₃ to those of the Ti₂AlC and Ti₃AlC₂ substrates. It indicates that bonding via preferential oxidation at low oxygen partial pressure is a practical and efficient method for Ti₂AlC and Ti₃AlC₂.     

The relationship between microstructure and flexural strength of pressureless liquid phase sintered SiC ceramics oxidized at elevated temperatures

The oxidation behavior of pressureless liquid phase sintered SiC ceramics with Al₂O₃ and Y₂O₃ as sintering additives was investigated in the temperature range from 1000 °C to 1600 °C at the interval of 100 °C for 5 h. The relationship between residual flexural strength and microstructure was analyzed in detail. It was found that the SiC specimens suffered from mild oxidation below 1300 °C. The flexural strength of SiC specimens after oxidation at 1100 °C was the highest (90% of the original strength) due to the formation of dendritic grains, which filled pores and healed cracks. And the flexural strength was almost above 80% of the original flexural strength when the oxidation temperature was below 1300 °C. Meanwhile, the weight of specimens underwent steady increase. However, when the oxidation temperature was elevated to above 1400 °C, the specimens began to suffer from severe oxidation, which resulted in a lot of through pores and cracks on the surface, bringing about the sharp decrease of flexural strength to 30% of original strength when the oxidation temperature of 1600 °C was reached. And the weight of the specimens after huge increase began to show downtrend when the oxidation temperature was elevated to 1600 °C due to the spalling of oxidation products.     

Effect of Si additions on the Al2O3 grain refinement upon oxidation of Cr2AlC MAX phase

The effect of Si additions on the oxidation behavior of Cr₂AlC based coatings is investigated. Oxidation experiment was performed at 1120 °C in air for 4 h for Cr₂AlC and Cr₂Al_1−xSi_xC (0 < x ≤ 0.06) coatings. The crystal structure, microstructure and chemical composition of the as-deposited as well as oxidized coatings have been investigated. Alloying Cr₂AlC with up to 0.7 at.% Si causes an increase in Al₂O₃ scale thickness by up to 40 ± 17%. Electron microscopy and atom probe tomography data support the notion that the here reported Si concentration induced 40% increase in Al₂O₃ layer thickness (during oxidation at 1120 °C for 4 h) is enabled by the Si concentration induced, and hence concomitant, increase in nucleation density of Al₂O₃.     

Microstructure and mechanical properties of the spark plasma sintered Ta2C ceramics

Single phase hexagonal α-Ta₂C ceramics were synthesized by spark plasma sintering and using TaC and Ta as the starting powders. Effects of sintering temperatures and holding times on the densification process, phase formation, microstructure development, and mechanical properties of the α-Ta₂C ceramics were investigated. Densification occurred in the temperature range of 1520–1675 °C in less than 2.5 min. But completion of the Ta₂C formation took about 40 min at 1500 °C, and 5 min at 1900 °C. The materials sintered at 1500 °C consisted of fine equiaxed grains. The Ta₂C grains grew anisotropic to form an elongated self-toughening microstructure at 1700 °C. At 1900 °C, the neighboring Ta₂C individual crystals coalesced to form large Ta₂C blocks to entrap the residual pores. Although higher flexural strength and fracture toughness were reached at 1700 °C, the unstable microstructures of the Ta₂C materials indicated limited applications at high temperatures.     

Probing the oxidation behavior of Ti2AlC MAX phase powders between 200 and 1000 °C

Oxidation of commercial Ti₂AlC MAX phase powders at 200–1000 °C has been investigated by XRD, XPS, SEM, STA and TGA coupled with FTIR. These powders are a mixture of Ti₂AlC, Ti₃AlC₂, TiC and Ti_1.2Al_0.8. Oxidation at 400 °C led to disappearance of carbide phases from Ti 2p, Al 2p and C 1s XPS spectra. At 600 °C, powders changed from dark grey to light grey with a significant volume increase due to crack formation. Powders were severely oxidized by detecting rutile with minor anatase TiO₂. At 800 °C, α-Al₂O₃ was detected while anatase transformed into rutile TiO₂. The cracks were healed and disappeared. At 1000 °C, the Ti₂AlC powders were fully oxidized into rutile TiO₂ and α-Al₂O₃ with a change of powder color from light grey to yellow. FTIR detected the release of C as CO₂ from 200 °C onwards but with additional CO above 800 °C.     

The phase transformation and microstructure of TiAl/Ti2AlC composites caused by hot pressing

Two series of raw materials were adopted to form TiAl/Ti₂AlC composites: Ti/Al/TiC and Ti/Al/C. Differential thermal analysis (DTA) of starting powers and X-ray diffraction (XRD) of samples sintered at different temperatures from 600 °C to 1300 °C by hot pressing were utilized to analyze the phase transformation and the mechanism of synthesis. Scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS) was utilized to investigate the morphology characteristics of the products. The experimental results showed that Ti reacted with Al to form TiAl intermetallics below 900 °C; and above 900 °C, TiAl reacted with TiC to produce dense TiAl/Ti₂AlC composites. The products sintered at 1200 °C had fine crystals and dense fibres, and the distribution of Ti₂AlC particles in TiAl matrix was homogeneous.     

Low-temperature crystallization of tantalum pentoxide films under elevated pressure

Ta₂O₅ thin films deposited via a metal-organic decomposition method were crystallized via atmospheric pressure annealing and a high-pressure crystallization (HPC) process. Ta₂O₅ thin films started to become crystallized at 700 °C as subjected to atmospheric pressure annealing. When the HPC process was adopted and annealing at 16.5 MPa was performed, the crystallization temperature of Ta₂O₅ films was greatly dropped to as low as 350 °C. The developed HPC process considerably reduced the thermal budget and energy consumption during film processing. The crystallized Ta₂O₅ phase was found to be homogeneously distributed within the HPC-derived films. With annealing at 700 °C under atmospheric pressure, the silicon species diffused from the substrates into the Ta₂O₅ layers, thereby leading to reduced dielectric constants. The HPC process effectively suppressed the interdiffusion between the substrates and dielectric layers by lowering the required heating temperature, and also significantly increased the dielectric constants of Ta₂O₅ thin films. The HPC process was confirmed to effectively lower the crystallization temperature and improve the dielectric properties of Ta₂O₅ thin films.     

Synthesis and ultra-high temperature ablation behavior of a ZrC/Cr2AlC composite

This work reports on the fabrication and high temperature ablation property of a new ZrC/Cr₂AlC composite. The ZrC/Cr₂AlC composite was obtained by hot pressing a mixture of 15 vol% ZrC and 85 vol% Cr₂AlC powders at 1300 °C with 20 MPa for 1 h in Ar atmosphere. The composite had a flexural strength of 622 MPa, higher than 400 MPa for Cr₂AlC. The high temperature ablation behavior of the composite was investigated using the oxyacetylene torch ablation test. During oxyacetylene torch testing, the composite underwent a series of thermal decomposition and oxidation. Microstructure and composition of the synthesized composite before and after the ablation test were characterized with scanning electron microscopy and X-ray diffractometry techniques.     

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.     

Self-healing behavior in MoSi2/borosilicate glass composite

The self-healing behavior of MoSi₂/borosilicate glass composite was investigated by comparing the flexure strength of pre-scratched specimens before and after healing treatment. The post-healing strength partially recovered when healing temperature was higher than 800 °C, and the healing of the scratch was observed after healing treatment at 900 °C and 1400 °C. Two kinds of possible mechanisms were proposed on the basis of morphology and elemental analyses of healing areas. The specimen treated at 900 °C showed a porous healing area and the strength recovered 28.6% resulted from the oxidation of MoSi₂ into MoO₃ and SiO₂. By contrast, the specimen treated at 1400 °C had a dense healing area and the strength recovered 86.9% due to the viscous flow of borosilicate glass.     

Degradation of plasma-sprayed yttria-stabilized zirconia coatings via ingress of vanadium oxide

V₂O₅ reaction and melt infiltration in plasma-sprayed 7 wt% Y₂O₃–ZrO₂ (YSZ) coatings were investigated at temperatures ranging from 750 °C to 1200 °C using SEM and TEM combined with EDS. The interlamellar pores and intralamellar cracks, common in plasma-sprayed materials, provide pathway for the molten species. The microstructure of the contaminated coatings is therefore the result of the interplay between the dissolution/reaction rates of the V₂O₅ with YSZ coating and the infiltration rates of the molten species. Near the coating surface, the reaction front proceeds in a planar fashion, via dissolution of the lamella and precipitation of fine-grained reaction products composed of ZrV₂O₇ (for reactions at 750 °C and below), m-ZrO₂ and YVO₄. The thickness of this planar reaction zone or PRZ was found to increase as reaction time and temperature increased. The melted V₂O₅ was observed to infiltrate along the characteristic microstructure of plasma-sprayed coatings, i.e. the interconnected pores and cracks, and react with the YSZ. The thickness of this melt infiltrated reaction zone or MIRZ ranged from 5 μm for reactions at 750 °C for 30 min to 130 μm for reactions at 1000 °C for 90 min. At 1200 °C, only a PRZ was observed (i.e. the thickness of the MIRZ was nominally zero), suggesting that the dissolution reaction within the pores/cracks and subsequent formation of reaction products may limit infiltration. Fifty-hour heat-treatments at 1000 °C and 1200 °C prior to reaction with the V₂O₅ at 800 °C for 90 min were used to change the microstructural features of the coating, such as crack connectivity and pore size. The heat-treatment at 1000 °C was found most deleterious to the coating due to large cracks created via a desintering process that afforded deep penetration of the molten V₂O₅.     

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.     

Oxidation resistance of tantalum carbide-hafnium carbide solid solutions under the extreme conditions of a plasma jet

The oxidation behaviors of tantalum carbide (TaC)- hafnium carbide (HfC) solid solutions with five different compositions, pure HfC, HfC-20 vol% TaC (T20H80), HfC- 50 vol% TaC (T50H50), HfC- 80 vol% TaC (T80H20), and pure TaC have been investigated by exposing to a plasma torch which has a temperature of approximately 2800 °C with a gas flow speed greater than 300 m/s for 60 s, 180 s, and 300 s, respectively. The solid solution samples showed significantly improved oxidation resistance compared to the pure carbide samples, and the T50H50 samples exhibited the best oxidation resistance of all samples. The thickness of the oxide scales in T50H50 was reduced more than 90% compared to the pure TaC samples, and more than 85% compared to the pure HfC samples after 300 s oxidation tests. A new Ta₂Hf₆O₁₇ phase was found to be responsible for the improved oxidation performance exhibited by solid solutions. The oxide scale constitutes of a scaffold-like structure consisting of HfO₂ and Ta₂Hf₆O₁₇ filled with Ta₂O₅ which was beneficial to the oxidation resistance by limiting the availability of oxygen.     

Oxidation behavior of β-SiAlON powders fabricated by combustion synthesis

The present study presents the effect of addition of diluents on the crystal morphologies and the impact on the oxidation resistance of β-SiAlON powders obtained from combustion synthesis method. Pure β-SiAlON were synthesized with diluents. It was observed by SEM that the diluents resulted in the transformation from large hexagonal column-shaped crystals to fine isotropic grains. Oxidation experiments were conducted by thermogravimetric analysis (TGA) in the temperature range from 1200 °C to 1400 °C. The TGA results indicated that oxidation was controlled by mixed chemical reaction and diffusion process. The chemical reaction step was found to be rate-controlling at low oxidation temperatures (1200 °C and 1300 °C), while at high temperature (1400 °C), diffusion was found to control the reaction rate. Oxidation products were investigated by X-ray Diffraction (XRD) analysis and found to be made up of SiO₂, Al₂O₃. This research aims to provide guidance for the fabrication of β-SiAlON by combustion synthesis, thus facilitating its further application in high temperature industry.     

Modification of the oxidation behaviour of Si3N4–TiB2 composites by PECVD alumina coatings

A nearly fully dense (>98%) electroconductive silicon nitride—35 vol.% titanium diboride composite was obtained by hot isostatic pressing (HIP) in presence of a low content of sintering aids (0.5 wt.% Y₂O₃ + 0.25 wt.% Al₂O₃). To improve the oxidation behaviour of this composite material, a 3-μm thick protective coating of aluminium oxide was deposited on cubic samples (4 mm × 4 mm × 4 mm) by microwave plasma-enhanced chemical vapor deposition (PECVD) using an oxygen plasma and an organometallic precursor (trimethylaluminium). SEM images demonstrated that the coating was homogeneously distributed on the external surface of the specimens.Non-isothermal and isothermal oxidation tests were carried out with a Setaram Microbalance under pure flowing oxygen (10 L/h) on both uncoated and coated Si₃N₄–TiB₂ samples. In the case of non-isothermal oxidation of a substrate without coating, the reaction started at 600 °C. Between 1100 and 1350 °C, a plateau was observed and above 1350 °C the weight gain increased significantly. In presence of an Al₂O₃ coating, the composite started to oxidize at higher temperature (1200 °C). Isothermal kinetics recorded for 24 h, at 1350 and 1400 °C, revealed that the presence of the Al₂O₃ coating improved drastically the oxidation resistance and changed the shape of the curves from globally parabolic to almost logarithmic. An explanation of this protective behaviour, based on the characterization by XRD, SEM and EDS of the reaction products, is proposed.     

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.     

Influence of sintering temperature on dielectric properties and crystal structure of corundum-structured Mg4Ta2O9 ceramics at microwave frequencies

Microwave dielectric properties of corundum-structured Mg₄Ta₂O₉ ceramics were investigated as a function of sintering temperatures by an aqueous sol–gel process. Crystal structure and microstructure were examined by X-ray diffraction (XRD) technique and field emission scanning electron microscopy (FE-SEM). Sintering characteristics and microwave dielectric properties of Mg₄Ta₂O₉ ceramics were studied as a function of sintering temperature from 1250 °C to 1450 °C. With increasing sintering temperature, the density, ε_r and Qf values increased, saturating at 1300 °C with excellent microwave properties of ε_r=11.9, Qf=195,000 GHz and τ_f=?47 ppm/°C. Evaluation of dielectric properties of Mg₄Ta₂O₉ ceramics were also analyzed by means of first principle calculation method and ionic polarizability theory.     

Crack healing behaviour of Cr2AlC MAX phase studied by X-ray tomography

The autonomous crack-healing capability of Cr₂AlC MAX phase ceramic by surface oxidation at elevated temperatures has a huge potential for high temperature structural and protective coating applications. In this work we use time-lapse X-ray computed tomography (CT) to track the fine details of local crack filling phenomena in 3 dimensions (3D) with time. The maximum crack width that could be fully healed upon exposure to 1200 °C in air is 4.8 μm in 4 h and 10 μm after 12 h. Furthermore, during healing Cr₇C₃ phase is observed beneath the dense Al₂O₃ layer (average thickness of 1 μm on each crack surface) when the crack width exceeds 2 μm. The 3D image sequences indicated that the rate of healing is essentially independent of position along, or across, the crack faces. The crack healing kinetics of Cr₂AlC at 1200 °C broadly follows a parabolic rate law with a rate constant of 4.6 × 10⁻⁴ μm² s⁻¹. The microstructure, composition and thickness of the oxide scale in the healed crack area are characterized via post mortem SEM-EDS measurements and confirm the formation of an initial dense alumina layer on top of which a more porous layer forms. Impurity Cr particles appear to accelerate the oxidation process locally and correlative SEM imaging of the same region suggests this is by providing Cr₂O₃ nucleation sites.     

Oxidation behavior of Al2O3 reinforced MoSi2 composite coatings fabricated by vacuum plasma spraying

MoSi₂, MoSi₂–10 vol.% Al₂O₃, MoSi₂–30 vol.% Al₂O₃ (denoted as MA0, MA1, MA3, respectively) coatings were fabricated by vacuum plasma spraying (VPS), and their oxidation behavior was examined at low temperature (500 °C) and high temperature (1500 °C). The test at 500 °C showed that the addition of Al₂O₃ effectively restrained the pest oxidation of MoSi₂. The MA1 coating had satisfactory fluid surface and presented good oxidation resistance at 1500 °C. However, the MA3 coating showed worse oxidation resistant behavior compared with the MA0 coating because of mullite formation.