Preparation,microstructure and mechanical properties of MoAlB-0.15% Si ceramics with superior strength and toughness

MoAlB has been regarded as a promising high-temperature structural ceramic, but the strength and toughness are still insufficient in the practical application. In this work, MoAlB ceramic bulk with superior hardness, strength and toughness has been fabricated by adding 0.15 mol. % Si. The MoAlB-0.15Si bulk is composed of Si-doped MoAlB, Mo(Al, Si)₂ and ultrafine Al₂O₃. The Vickers hardness ranges from 14.2 to 12.5 GPa with the tested load increasing from 10 to 200 N. The Vickers indentation remains the intact tetragonum in spite of the appearance of corner cracks, indicating the excellent damage tolerance. The flexural strength, fracture toughness and compressive strength of MoAlB-0.15Si are 518.46 MPa, 7.01 MPa m^1/2 and 2.62 GPa, respectively, obviously superior to the present MoAlB polycrystalline bulk. Si doping, grain refinement, strengthening effect of ultrafine Al₂O₃ and phase transformation from Al₈Mo₃ to Mo(Al, Si)₂ jointly account for the improvement of comprehensive properties of MoAlB bulk.     

Fabrication of Mo5SiB2-based composites by combustion synthesis involving aluminothermic reduction of MoO3

Fabrication of Mo₅SiB₂–Al₂O₃ composites with a broad range of the Mo₅SiB₂/Al₂O₃ ratio was conducted by thermite-based combustion synthesis in the SHS mode. Thermite reagents of MoO₃ +?2Al were introduced into two Mo?Si–B ternary systems adopting amorphous boron and MoB as their respective boron source. The boron-containing samples were more energetic and were applied to produce composites with Mo₅SiB₂/Al₂O₃ from 1.25 to 2.5, beyond which combustion was extinct. The composites with Mo₅SiB₂/Al₂O₃ from 0.8 to 1.3 were prepared from less exothermic MoB-based samples. Besides causing a decrease in combustion velocity and reaction temperature, the increase of the Mo₅SiB₂/Al₂O₃ proportion led to a transformation in combustion wave propagation from a steady to pulsating mode. For the samples featuring a pulsating combustion wave, the reaction time at peak combustion temperatures was extended and then the evolution of Mo₅SiB₂ from intermediate phases (Mo₃Si and MoB) was significantly improved. The deduced activation energy suggests a lower kinetic barrier for thermite-based combustion synthesis to fabricate Mo₅SiB₂-based composites. The microstructure of synthesized composites indicates that quadrangular Mo₅SiB₂ grains with an average size of 10–15?µm are tightly packed and irregular Al₂O₃ grains are randomly dispersed.     

Ultra-high temperature ablation behavior of MoAlB ceramics under an oxyacetylene flame

The ultra-high temperature ablation of a polycrystalline, fully dense, predominantly single phase MoAlB ceramic discs under an oxyacetylene flame is examined. The linear ablation rate decreases from 1.3 μm/s during the first 30 s to - 0.7 μm/s after 60 s when the surface temperature reached about 2050 °C (with a flame temperature around 3000 °C). Up to 60 s, the MoAlB is ablation resistant due to the formation of a protective and viscous surface Al₂O₃ layer. As the ablation time is prolonged, the protective Al₂O₃ scale becomes porous and is almost fully destroyed at the central ablation region after 120 s. This accelerates the formation of large amounts of volatile species (mainly B and Mo oxides), resulting in a reduction in the ablation resistance.     

Crack healing behavior of a MAB phase: MoAlB

Self-healing property of ceramics is highly attractive in meeting the safety and durability requirements of high temperature structural components. This work reports on the crack healing behavior of MoAlB, together with its oxidation resistance. The MoAlB material showed a good oxidation resistance due to the formation of a dense and thin Al₂O₃ scale in the temperature range 1200–1300 °C for 10 h. The crack healing behavior as a function of temperature and healing time was studied. For the healed samples, the strength recovery almost returned to the initial strength. Healing is accomplished by the formation of only Al₂O₃ in the site of damage. The microstructure and the phase composition of samples before and after healing were characterized with scanning electron microscopy and x-ray diffraction method, respectively.     

Parametric effects on femtosecond laser ablation of Al2O3 ceramics

This paper investigates the effects of parameters on femtosecond laser ablation of Al₂O₃ ceramics. Ceramic alumina (Al₂O₃) is an important substrate used in hybrid circuits. Generally it is a difficult-to-cut material by conventional drilling and cutting procedures. Laser is suitable for precise form processing on hard or brittle ceramic materials, and with the increasing availability of femtosecond pulse lasers, the interest in using femtosecond laser ablation to produce precise, well-defined micrometer-sized structures in the ceramic materials is increasing. The laser parameters and material properties play an important role in the quality and efficiency of ceramics processing. This work develops a model to investigate parametric effects on femtosecond laser ablation of Al₂O₃ ceramics. The ablation depth and squared crater diameter per pulse predicted by this study are in agreement with the available experimental data. The effects of laser parameters and material properties on femtosecond laser ablation of Al₂O₃ ceramics are also discussed.     

Stability of intergranular phases in hot-pressed Si3N4 studied with mechanical spectroscopy and in-situ high-temperature XRD

The impulse excitation technique (IET) and high temperature X-ray diffraction (HTXRD) were used to investigate the intergranular glass phase and its crystallisation behaviour in four hot-pressed silicon nitrides. The internal friction or damping peak height measured with IET near the glass transition temperature, T_g, is used as a qualitative indicator for the amount of residual intergranular amorphous phase after sintering. Silicon nitride powder was hot-pressed with different sintering additives. The silicon nitride containing 4 wt.% Al₂O₃ does not reveal an internal friction peak at T_g, i.e. it does not contain a significant amount of intergranular glass phase. Three other silicon nitrides, containing either 8 wt.% Y₂O₃, 6 wt.% Y₂O₃+2 wt.% Al₂O₃, or 2 wt.% Y₂O₃+4 wt.% Al₂O₃+2 wt.% TiN, do show an internal friction peak near T_g. This “T_g-peak” is nearly unaffected by heating up to 1400 °C in the silicon nitride with Y₂O₃+Al₂O₃+TiN sintering aids, whereas the amount of intergranular glass in the ceramics containing either Y₂O₃+Al₂O₃ or Y₂O₃ as a sintering aid is strongly reduced by subsequent heating. As observed from HTXRD, the onset temperature of crystallisation of the intergranular glass in the ceramic containing Y₂O₃+Al₂O₃ sintering aids is about 1100 °C, with the formation of Y–N-apatite (Y₂₀N₄Si₁₂O₄₈) and O-sialon (Al₀.₀₄Si₁.₉₆N₁.₉₆O₁.₀₄). The O-sialon phase in the yttria and alumina containing ceramics, formed either during sintering or during heat treatment, is not stable at elevated temperatures and dissolves in the intergranular glass phase between 1300 and 1400 °C. The O-sialon phase in the ceramic without Y₂O₃ sintering additive, however, is thermally stable. The presence of Ti⁴⁺ ions in the intergranular glass phase is suggested to inhibit its crystallisation, resulting in a stable high temperature damping behaviour.     

High temperature oxidation behavior of porous MoAlB ceramic from 800 °C to 1100 °C in air

MoAlB possesses the characteristics of both metals and ceramic materials, which has attracted extensive attention due to its excellent high-temperature oxidation resistance. For this reason, porous MoAlB is considered applicable to the practice of filtration under harsh environment. In this study, the high-temperature oxidation behavior of porous MoAlB ceramics is systematically studied at the temperatures ranging from 800 to 1100 °C. According to the results, the porous MoAlB exhibits good oxidation resistance at a maximum temperature of 1000 °C. The oxidation kinetics of porous MoAlB can be divided into three stages, and the estimated activation energies of the three stages are 253.83 kJ·mol⁻¹, 367.48 kJ·mol⁻¹ and 317.84 kJ·mol⁻¹, respectively. In the stable stage at 1000 °C, the quadratic mass gain per unit area shows linearity over time, and the oxidation rate of porous MoAlB reaches 37.31 mg²·cm⁻⁴·h⁻¹. As revealed by the analysis of the composition and microstructure of oxide layers, the main components of the oxide layer include MoO₃, MoO₂, Al₂O₃, B₂O₃. With the extension of oxidation time, the content of Al₂O₃ in the oxide films increases. The average pore size, permeability and open pore porosity of porous MoAlB show a trend of first decreasing and then tending to be stable. In addition, a discussion is conducted on the high-temperature oxidation mechanism of porous MoAlB.     

Synthesis,structure and properties of MAB phase MoAlB ceramics produced by combination of SHS and HP techniques

The kinetics and stages of phase formation in the combustion wave of the MoAlB mixture are studied. The phase diagrams in the Mo-Al-B system were built using the AFLOW and Materials Project databases. The time-resolved X-ray diffraction analysis demonstrate that the MoAlB phase crystallizes from the melt without formation of any intermediate compounds. The structure of the synthesized ceramics was MoAlB lamellar grains 0.4 µm thick and ~2–10 µm long. Compact samples characterized by homogeneous structure and low residual porosity were obtained by hot pressing of SHS powders. The ceramic MoAlB has a layered structure, which is consistent with the morphology of the synthesis products. Mechanical and thermophysical properties are measured for samples obtained under optimal HP conditions at 1300 °C. The calculated value of the oxidation rate for MoAlB at 1200 °C for 30 h was 2.21?10^?5 mg/(cm²?s). Oxide layer ~14 µm thick consists of elongated polygonal Al₂O₃ grains.     

Ablation behavior and mechanism of bulk MoAlB ceramic at ∼1670–2550 °C in air plasma flame

In this work, MoAlB samples for plasma exposure test were condensed by spark plasma sintering at 1200 °C for 10 min. Ablation resistance of MoAlB ceramic was investigated in a plasma torch facility for about 30 s at high temperature range of ～1670?2550 °C, which provided a quasi-real hypersonic service environment. The results showed that the linear ablation rate was increased from 0 μm/s at ～1670 °C to 86.4 μm/s at ～2550 °C. At ～1670 °C, the ablated surface of MoAlB ceramic was covered by Al₂O₃ layer, presenting excellent ablation resistance. At ～2220 °C, the macroscopic cracks were induced by thermal stress, which opened up channels for the inward diffusion of oxygen and deteriorated the ablation resistance of the substrate. Above ～2400 °C, the volatile MoO₃ and B₂O₃ and the erosion of viscous oxides by the high shearing force of plasma stream were the main ablation mechanisms.     

Grain boundary phase in AlN ceramics fired under reducing N2 atmosphere with carbon

An AlN ceramic was prepared with a dopant Y₂O₃ under a reducing nitrogen atmosphere with carbon at 1900 °C for 20 h. The AlN ceramic had thermal conductivity, 220 W/m°C, which contained crystalline Y₂O₃ and an amorphous intergranular film. The intergranular phase decreased during the isothermal hold period by the migration of a liquid phase that consisted of Y₂O₃, Al₂O₃, and AlN. The liquid phase composition was maintained during the firing process. Comparison of the microstructures of the ceramics prepared with different isothermal hold times revealed that the lower the quantity of intergranular phase, the higher the thermal conductivity attained.     

Intergranular film thickness of self-reinforced silicon carbide ceramics

The intergranular film of self-reinforced SiC ceramics prepared by hot pressing and further annealing with SiO₂–Y₂O₃ and SiO₂–Al₂O₃ as sintering additives was observed by high-resolution transmission electron microscopy. The film thickness of SiC ceramics with SiO₂–Y₂O₃ was 1.2 nm whereas that of ceramics with SiO₂–Al₂O₃ was 0.8 nm. Based on the refined continuum model, an explanation on the variation of thickness with sintering additives is given. It seems that the behavior of intergranular glassy film of SiC ceramics is akin to that of Si₃N₄ ceramics.     

High toughness TiB2–Al2O3 composite ceramics produced by reactive hot pressing with fusible components

A manufacturing method of high toughness TiB₂–Al₂O₃ ceramic composites by reactive hot pressing via exothermic reactions with Al, B₂O₃, C and TiB₂ has been developed. The usage of fusible initial components (Al and B₂O₃) in the hot pressing procedure allowed forming dense ceramics with homogeneous fine structure at 1900°C and 20 MPa (after 8 min exposure) without any grinding of the initial powders. The hot pressed TiB₂–Al₂O₃ composites exhibit the fracture toughness of 9 MPa m^1/2, which is much higher than that of both single phase TiB₂ (6 MPa m^1/2) and Al₂O₃ (4 MPa m^1/2) ceramics.     

Making high-performance MoAlB ceramics by hot pressing combustion-synthesized powders

It is one of the most concerning issues to achieve high purity, low cost and excellent performance in MAB phases. Herein, the MoAlB powders were synthesized by an ultra-fast combustion synthesis (CS). The reaction mechanism was analyzed by changing components of raw materials. The high-purity MoAlB powders were successfully obtained with the mole ratio of Mo, Al and B of 1:1.3:1. After hot-pressing sintering, the powders were densified and formed compact MoAlB ceramics. Together with phase composition, microstructure, mechanical properties, toughening mechanisms, and oxidation resistance properties were collaboratively outlined. The flexural strength, fracture toughness and Vickers hardness of MoAlB ceramic hot-pressed at 1300 °C reach up to 640 ± 22.37 MPa, 6.82 ± 0.64 MPa m^1/2 and 13.82 ± 0.41 GPa, which far surpass these values of ternary layered compounds prepared by other methods. The oxidation resistance was investigated up to 1200 °C, where a passivating Al₂O₃ layer inhibited further oxidation even when treatment sample for 12 h. This work provides a guidance to industrially produce MAB phases with high performance.     

Densification of Yttria Transparent Ceramics: The Utilization of Activated Sintering

Highly transparent 0.5 at.% Tm:Y₂O₃ ceramics were prepared by using solid‐state reaction combined with vacuum sintering method, with ZrO₂ and Al₂O₃ as sintering aids. Doping amount of ZrO₂ was fixed at 1 at.%, while the effect of Al₂O₃ on densification, microstructure evolution, and transmittance of the Y₂O₃ ceramics was carefully studied. It was found that the addition of Al₂O₃ was very effective in improving densification of Y₂O₃, due to the formation of an Al‐rich eutectic phase Y₄Al₂O₉ (YAM) during the sintering process. As the content of Al₂O₃ was increased from 0 to 81.8 wt ppm, porosity of the ceramics was decreased and transmittance was increased. However, when the content of Al₂O₃ was increased to 137 wt ppm, a secondary phase began to segregate at grain junctions. Further increase in the amount of Al₂O₃ led to an increase in both amount and size of the secondary phase. At the optimized content of Al₂O₃ with 81.8 wt ppm, the Tm:Y₂O₃ ceramics sintered at 1860°C for 13 h exhibited an in‐line transmittance of 83.0% at 2000 nm and 76.5% at 600 nm. It is expected that this finding can be readily applied to other transparent ceramics.     

High strength and hardness BNNSs/Al2O3 composite ceramics prepared by hot-press sintering of in-situ composite BNNSs/Al2O3 powder

In this paper, BNNSs/Al₂O₃ composite powder was prepared by in-situ reaction using borate nitridation method and BNNSs/Al₂O₃ composite ceramics were prepared by hot-pressing sintering. This method achieves uniform mixing of BNNSs and Al₂O₃ ceramic matrix and reduces the introduction of impurities in the processing process. The BNNSs/Al₂O₃ composite ceramics have excellent bending strength (549.4 MPa), fracture toughness (5.18 MPa m^1/2) and hardness (21.3 GPa). The high hardness of composite ceramics is attributed to high grain boundary strength and density. The reinforcing mechanisms of ceramics include BNNSs pull-out, BNNSs bridging, crack deflection as well as the transgranular fracture and intergranular fracture of Al₂O₃ matrix.     

SiAlON–Al2O3 ceramics as potential biomaterials

When used as implants, Al₂O₃ is unable of directly achieving good chemical bonding with soft and hard tissues. To overcome this problem, SiAlON–Al₂O₃ ceramics were prepared in this study by direct nitridation. Phase composition, porosity, bulk density, and compression strengths were examined, and biological properties were evaluated by cell culture on ceramic surface. Major phase of SiAlON–Al₂O₃ ceramics was identified as Si₄Al₂O₂N₆, formed by reaction of Si, Al and Al₂O₃ under nitrogen atmosphere at high temperature. As Al₂O₃ content increased, porosity and compressive strength decreased. Therefore, Si₄Al₂O₂N₆ phase could improve sintering, leading to formation of composites with better properties. The porosity and compression strength were found suitable for requirement of biomaterials. Cell culture experiments showed that cells could proliferate and survival well on ceramic surface, indicating good biocompatibility of Si₄Al₂O₂N₆ phase in SiAlON–Al₂O₃ ceramics. Overall, these data look promising and might provide novel strategies for development of future SiAlON–Al₂O₃ bioceramics.     

Microstructure and microwave dielectric properties of Al2O3 added Li2ZnTi3O8 ceramics

Recently, the rapid development of advanced communication systems increasingly strongly demands high-performance microwave dielectric ceramics in microwave circuits. Among them, Li₂ZnTi₃O₈ ceramics have been one of the most widely investigated species, due to its high quality factor, moderate firing conditions and low cost. However, the dielectric constants of the already reported Li₂ZnTi₃O₈ ceramics are fixed in a narrow range, limiting their wider applications. To adjust the dielectric constant of the Li₂ZnTi₃O₈ based ceramics, in this work Li₂ZnTi₃O₈ ceramics added with different amounts of Al₂O₃ (0–8?wt%) were prepared by conventional solid-state reaction. The microstructure and microwave dielectric properties of the samples were investigated. Due to the addition of Al₂O₃, the sintering temperature of the ceramics would be increased somewhat. Some Al³⁺ ions could substitute for Ti⁴⁺ ions in Li₂ZnTi₃O₈, and the added Al₂O₃ would react with ZnO to produce a ZnAl₂O₄ phase accompanying with the formation of TiO₂ phase, which would inhibit the growth of Li₂ZnTi₃O₈ grains. The dielectric constant of the finally obtained ceramics would be reduced from 26.2 to 17.9, although the quality factors of the obtained ceramics would decrease somewhat and the temperature coefficient of resonant frequency would deviate further from zero.     

Effect of intergranular phase chemistry on the sliding-wear resistance of pressureless liquid-phase-sintered α-SiC

The effect was investigated of the intergranular phase chemistry on the sliding-wear resistance of pressureless liquid-phase-sintered (PLPS) α-SiC densified with 10 vol.% 5Al₂O₃ + 3RE₂O₃ (RE = La, Nd, or Yb) additives. It was found that the sliding-wear behaviour of these ceramics is similar to what is observed in other polycrystalline ceramics: initial mild, plasticity-controlled wear followed by severe, fracture-controlled wear, with a well-defined wear transition. Most importantly, the sliding-wear resistance of PLPS SiC is found to increase with decreasing size of the RE³⁺ cation in the rare-earth oxide additive, with a lower susceptibility to mild and severe wear and a delayed transition to severe wear. Underlying this effect is likely the hardening of the intergranular phase resulting from the increase in the field strength of the RE³⁺-O²⁻ bonds as the size of the RE³⁺ cation decreases. Tailoring the intergranular phase chemistry via the selection of RE₂O₃ sintering additives with cations as small as possible thus emerges as a potentially interesting approach to improving the sliding-wear resistance of PLPS SiC ceramics.     

Cast ceramics by metallothermic SHS under elevated argon pressure

Cast ceramic composites were prepared by metallothermic SHS under elevated Ar pressure (5 MPa). Variation in green composition was used to affect the phase composition of combustion products. Under some optimized conditions, resultant ceramics can be obtained in the form of Al₂O₃–Cr₂O₃ solid solutions, Al₂O₃–Cr₂O₃ ? хZrO₂ composites or single-phase Al2MgO4. Such materials seem promising for use in jewelry, process engineering (casting molds, cutting tools), and aerospace industry.     

Direct-bonded Al/Al2O3 using a transient eutectic liquid phase

Directed bonding with Al and Al₂O₃ was achieved using a transient liquid phase (TLP) method after annealing at the low melting point of Al, which deposited Ni, Cu, Ge, and Si on the Al₂O₃ substrate. Al/Al₂O₃ microstructures were evaluated using a scanning electron microscopy and transmission electron microscopy. A reaction layer was absent at the Al/Al₂O₃ interface, and all deposited metal films dissolved into the Al foil and reacted with Al to form an eutectic liquid phase near the interface to wet and join with the Al₂O₃. Al₉Fe₂ and Al₃Fe intermetallic compounds were formed in the Al substrate because of Fe precipitation, which is an impurity of Al foil, and the reaction with Al at the grain boundaries of Al. The bonding area percentage, shear strength, and thermal conductivity for Al and Al₂O₃ were assessed using scanning acoustic tomography (SAT), the ISO 13 124 shear strength test, and the laser flash method, respectively. The Al/Al₂O₃ specimen deposited with the Ni film had the highest shear strength (33.74 MPa), thermal conductivity (42.3 W/mK), and bonding area percentage (96.78%). The Al/Al₂O₃ specimens deposited with Ge and Si exhibited relatively poor bonding because of the oxidation of Ge and Si at the surface of Al₂O₃ before bonding with Al.