Ultrafine-grained β-Sialon fabricated by two-stage sintering and study on diffusion toughening of Ti–22Al–25Nb

The ultrafine-grained β-Sialon ceramics were fabricated by spark plasma sintering at different temperatures with inorganic Al₂O₃–Y₂O₃ and Ti–22Al–25Nb intermetallic powder as composite additives. The research showed that β-Sialon ceramics achieve two-stage sintering densification. Al₂O₃–Y₂O₃ inorganic additives promoted the synthesis and densification of β-Sialon ceramics at 1125–1215°C. Ti–22Al–25Nb intermetallic powder diffused Ti and Nb elements at 1240–1425°C, thereby improving the fracture toughness of β-Sialon ceramics. The maximum fracture toughness (∼9.69 MPa m^1/2) under 19.6 N was obtained for β-Sialon ceramics sintered at 1600°C.     

The effect of precursor composition and sintering additives on the formation of β-sialon from Al, Si and Al2O3 powders

A study was performed to investigate the effect of increasing the Al or Al₂O₃ precursor content, above the stoichiometric amount, on the formation of β-sialon by pressureless sintering of Al, Si and Al₂O₃ powders in flowing nitrogen gas. The effect of adding Y₂O₃ or Fe to the precursor mixture, on the β-sialon formation, was also studied. The phase morphology and yield produced by the various compositions were examined using X-ray diffraction (XRD). Additional Al₂O₃ decreases the β-sialon phase yield and results in a greater amount of Al₂O₃ in the final sintered material. Additional Al improved the conversion to β-sialon up to a maximum of 4 wt% Al beyond which the β-sialon:15R sialon ratio in the sintered material decreases. 1 wt% Y₂O₃ was determined to be the optimum sintering additive content, as yttrium aluminium garnet (YAG) was found to be present in materials formed from higher Y₂O₃ containing precursors. The presence of Fe in the precursor powder retards the formation of β-sialon by preferentially forming Fe silicides at low temperatures, thus depleting the reaction system of elemental Si, favouring the formation of 15R sialon.     

Preparation of O′-Sialon-based ceramics by two-stage liquid phase sintering and study on the toughening mechanism of ultrafine-grained sintered clusters

Ultrafine-grained O′-Sialon-based ceramics were prepared by two-stage sintering at 1250 °C, with large particle GH4169 superalloy powder and nano Al₂O₃–Y₂O₃ as composite sintering aids. The effects of these aids on the densification, microstructure, and mechanical properties of O′-Sialon-based ceramics during two-stage sintering were also studied. Studies have shown that the densification process of O′-Sialon-based ceramics promoted by composite sintering additives, presents with the characteristics of two-stage liquid-phase sintering. In the first stage, GH4169 formed ultrafine-grained sintered clusters in the sintered material through liquid phase diffusion. In the second stage, the uniformly dispersed nano Al₂O₃–Y₂O₃ realized the uniform sintering of the material. In the fracture process, the ultrafine-grained sintered clusters hindered the crack propagation and promoted multiple deflections of the crack around the edge of the clusters, achieving the effect of crack deflection toughening. This effect, dominated by ultrafine-grained sintered clusters, significantly improved the fracture toughness of O′-Sialon-based ceramics up to 8.52 MPa m^1/2.     

Spark plasma sintering of Al-doped ZrB2–SiC composite

This research presents the influence of Al addition on microstructure and mechanical behavior of ZrB₂–SiC ultra-high temperature ceramic matrix composite (UHTCMC) fabricated by spark plasma sintering (SPS). A 2.5?wt% Al-doped ZrB₂–20?vol% SiC UHTCMC was produced by SPS method at 1900?°C under a pressure of 40?MPa for 7?min. The microstructural and phase analysis of the composite showed that aluminum-containing compounds were formed in-situ during the SPS as a result of chemical reactions between Al and surface oxide films of the raw materials (i.e. ZrO₂ and SiO₂ on the surfaces of ZrB₂ and SiC particles, respectively). The Al dopant was completely consumed and converted to the intermetallic Al₃Zr and Al₄Si compounds as well as Al₂O₃ and Al₂SiO₅. A relative density of 99.8%, a hardness (HV5) of 21.5?GPa and a fracture toughness (indentation method) of 6.3?MPa?m^1/2 were estimated for the Al-doped ZrB₂–SiC composite. Crack bridging, branching, and deflection were identified as the main toughening mechanisms.     

Effect of Y2O3 on the physical properties and biocompatibility of β–SiAlON ceramics

To investigate the effects of Y₂O₃ on the physical properties and biocompatibility of β–SiAlON ceramics, β–SiAlON ceramics were prepared with Al, Si, and α–Al₂O₃ powders using a direct nitriding technique. As a sintering additive, Y₂O₃ helps lower the sintering temperature and forms β–SiAlON ceramics. In this study, the physical and biological properties of the prepared ceramics were investigated to evaluate their use as bone-repairing material. Experiments revealed that the main crystal composition of the sample was Si₄Al₂O₂N₆, containing small amount of additional phases Y₃Al₅O₁₂ with increasing content of Y₂O₃. The porosity and compressive strength initially decrease and then increase to their initial values, whereas the bulk density exhibits the opposite trend with an increased proportion of Y₂O₃. The proliferation of osteoblastic and angiogenic cells demonstrates that β–SiAlON and Y₃Al₅O₁₂ have good biocompatibility; however, the sample porosity has a slight effect on the cell proliferation rate. This implies that in human tissues, bone-repairing speed can be adjusted by modifying the sample porosity or material surface roughness. Therefore, Y₂O₃ can be added to β–SiAlON ceramics to regulate their microstructure, physical properties, and biological properties for tissue engineering applications.     

AlN with high strength and high thermal conductivity based on an MCAS-Y2O3-YSZ multi-additive system

The additive composition of an AlN ceramic substrate material was optimized to achieve high strength and thermal conductivity. MgO-CaO-Al₂O₃-SiO₂ (MCAS) glass and Y₂O₃ were used as basic additives for improved sintering properties and thermal conductivity, thereby allowing for AlN to be sintered at a relatively low temperature of 1600 °C without pressurization. Yttria-stabilized zirconia (YSZ) was added (0–3 wt%) to further improve the strength of the AlN ceramic. YSZ and Y₂O₃ reacted with AlN to produce ZrN, Y₄Al₂O₉, and Y₃Al₅O₁₂ secondary phases. The formation of these yttrium aluminate phases improved the thermal conductivity by removing oxygen impurities, while ZrN formed at the AlN grain boundaries provided resistance to grain boundary fractures for improved strength. Overall, the AlN ceramic with 1 wt% MCAS, 3 wt% Y₂O₃, and 1 wt% YSZ exhibited excellent thermal and mechanical properties, including a thermal conductivity of 109 W/mK and flexural strength of 608 MPa.     

The effect of alumina-based sintering aid on the microstructure,selected mechanical properties,and coefficient of friction of Cf/SiC composite prepared via spark plasma sintering (SPS) method

C_f-SiC air brake discs are being developed due to their high-temperature oxidation resistance compared to conventional C_f/C discs. The C_f-SiC air brake discs should have a coefficient of friction (COF) close to 0.4, a low wear rate, a density higher than 95% of the theoretical density, and flexural strength of more than 200 MPa. To reach the properties of C_f-SiC composite to the required characteristics of the air brake disc, different amounts of alumina-based sintering aid were used. For this purpose, first silicon carbide nanoparticles, sintering aids Al₂O₃–MgO, MgAl₂O₄, Al₂O₃–Y₂O₃, Al₂O₃–SiO₂–MgO, and carbon fiber (20 wt%) with a 5-mm length were prepared. Next, the final composite bulk was created via the SPS method at 1900 °C under a pressure of 50 MPa. The density of the sample sintered with the Al₂O₃–SiO₂–MgO sintering aid was higher than that of other sintering aids. The density value was obtained at 98% and 100% at 8 wt% and 4 wt% respectively. It was also found that the use of 4 wt% of Al₂O₃–SiO₂–MgO offered better mechanical properties compared to 8 wt%, due to the absence of Al₈Si₄O₂₀ phase at 4 wt%. The examination of mechanical properties showed that the hardness (3564 Vickers) and flexural strength (479 MPa) of the sample with the Al₂O₃–SiO₂–MgO sintering aid were higher than those of other sintering aids. The samples with the Al₂O₃–SiO₂–MgO sintering aid with 4 wt% revealed a COF of 0.41, showing the closest feature to the desired indices of aircraft brake discs.     

In situ formation of mullite strengthened SiC porous ceramics via gelcasting with high strength and good alkali resistance

Mullite-bonded porous SiC ceramics sintered in air by gelcasting are still challenges due to the high porosity induced severe oxidation of SiC, which results in the formation of large amount of detrimental cristobalite phase. Here in this work, small amounts of Y₂O₃ and CaF₂ were added in SiC and Al(OH)₃ raw materials as sintering additives for the in situ growth of mullite reinforcement. This additive system promoted the reaction between oxidation-derived SiO₂ from SiC and Al₂O₃ decomposed from Al(OH)₃ to mullite phase. Almost no cristobalite phase was detected when sintered at 1450℃/2 h with CaF₂ addition of more than 2.0 wt%. Mullite whisker reinforcement was in situ formed due to the gas reaction mechanism caused by CaF₂ addition. Thus obtained porous SiC ceramics exhibited a flexural strength of 67.6 MPa at porosity of 41.3%, which maintained exceeding 36 MPa after 8 h corrosion in 10 wt% NaOH 80℃ solution, being the best performance up to now. This high performance of porous SiC was attributed to the additive induces proper phase control and in situ formation of whisker-like mullite reinforcement.     

Mechanical and thermal properties of spark plasma sintered Al2O3-graphene-SiC hybrid composites

Monolithic Al₂O₃ and Al₂O₃-graphene-SiC hybrid composites were prepared by spark plasma sintering (SPS) under vacuum atmosphere. The results show that the hybrid composites were almost completely dense (>97%). SiC content has a significant effect on the microstructure of the composites. With the increase of SiC content, the average grain size of alumina decreased gradually. The addition of SiC to alumina changed fracture mode from inter-granular fracture to mixed fracture mode of inter-granular fracture and trans-granular fracture. The Al₂O₃-0.4 wt%graphene-5 wt% SiC hybrid composite has the highest bending strength and hardness, which were 57% and 19.22% higher than those of the monolithic alumina, respectively. The room temperature (RT) thermal conductivity of the monolithic Al₂O₃ (25.5 W/m·K) was the highest. The thermal conductivity and thermal diffusivity coefficient of the composites decreased with the increase in temperature, while the specific heat of monolithic alumina and composites increased with the increase in temperature and additives. These properties were related to the microstructure of materials and the possible transport mechanisms were discussed.     

Phase evolution mechanism of non‐oxide bonded Al–Al2O3–MgO–ZrO2 composites at 1873 K in flowing nitrogen

In flowing nitrogen, non‐oxides such as Al₄O₄C, Al₂OC, Zr₂Al₃C₄, and MgAlON bonded Al₂O₃‐based composites were successfully prepared by a gaseous phase mass transfer pathway using aluminum, zirconia, alumina, and magnesia as raw materials at 1873 K, after an Al–AlN core‐shell structure was formed at 853 K. Resin bonded Al–Al₂O₃–MgO–ZrO₂ composites after sintering were characterized and analyzed by X‐ray diffraction (XRD), scanning electron microscope (SEM) and, energy dispersive spectrometer (EDS), and the influence of the MgO content on the sintered composites was studied. The results show that after sintering, the phase composition of the Al–Al₂O₃–ZrO₂ composite is Al₂O₃, Al₄O₄C, Al₂OC, and Zr₂Al₃C₄, while the phase composition of the Al–Al₂O₃–ZrO₂ composite with the addition of MgO 6 wt% and MgO 12 wt% is Al₂O₃, MgAlON, Al₄O₄C, Al₂OC, and Zr₂Al₃C₄ as well as Al₂O₃, MgAlON, Al₂OC, and Zr₂Al₃C₄, respectively. The addition of MgO changed the phase composition and distribution for the resin bonded Al–Al₂O₃–MgO–ZrO₂ system composites after sintering. When the added MgO content is equal to or more than 12 wt%, the Al₄O₄C in the resin bonded Al–Al₂O₃–MgO–ZrO₂ system composites is unable to exist in a stable phase.     

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.     

Anisotropic properties of textured h-BN matrix ceramics prepared using 3Y2O3-5Al2O3(-4MgO) as sintering additives

Textured hexagonal boron nitride (h-BN) matrix composite ceramics were prepared by hot pressing using 3Y₂O₃-5Al₂O₃ (mole ratio of 3:5) and 3Y₂O₃-5Al₂O₃-4MgO (mole ratio of 3:5:4) as liquid phase sintering additives, respectively. During the sintering process with liquid phase environments, platelike h-BN grains were rotated to be perpendicular to the sintering pressure, forming the preferred orientation with the c-axis parallel to the sintering pressure. Both h-BN matrix ceramic specimens show significant texture microstructures and anisotropic mechanical and thermal properties. The h-BN matrix ceramics prepared with 3Y₂O₃-5Al₂O₃-4MgO possess higher texture degree and better mechanical properties. While the anisotropy of thermal conductivities of that prepared with 3Y₂O₃-5Al₂O₃ is more significant. The phase compositions and degree of grain orientation are the key factors that affect their anisotropic properties.     

Thermodynamic analysis and experimental verification of interface reactions of iron aluminide/tetragonal zirconia composite

Abstract

This paper described the thermodynamic analysis and experimental verification of interface reactions between iron aluminide intermetallic and tetragonal zirconia. Thermodynamic analysis confirmed that chemical reactions between Fe–Al intermetallic and ZrO₂ (3 mol.–%Y₂O₃ stabilised zirconia) mainly depended on the Al content in Fe–Al intermetallic. For ZrO₂(3Y)/Fe₃Al composite, the interface reactions to form Al₂O₃ and ZrAl₂ would take place when Al content was >40 at-% in Fe–Al intermetallic, while no interface reaction occurred when using Fe₃Al as toughening phase. ZrO₂(3Y)/Fe₃Al composite was synthesised by hot press sintering to further verify the thermodynamic analysis of interface reactions between iron aluminide intermetallic and tetragonal zirconia. The phase composition, morphology and interface structure of ZrO₂(3Y)/Fe₃Al were investigated by X-ray diffraction, SEM and TEM. The results show that Fe₃Al was thermodynamic stable in ZrO₂(3Y) matrix, which was in good agreement with thermodynamically analysis.     

The influence of Y2O3-containing sintering additives on the oxidation of Si3N4-based ceramics and the interfacial interactions with liquid Al-alloys

Sintering additives containing Y₂O₃ influence the microstructure and the crystalline-state of Si₃N₄-ceramics produced via pressureless sintering, and determine their response towards oxidation. Y₂SiO₅ and Y₂Si₂O₇ were formed after sintering and oxidation, respectively. The superficial layers formed after oxidation are thinner and formed faster on the surface of the compositions 90Si₃N₄–5Y₂O₃–5Al₂O₃ and 90Si₃N₄–5Y₂O₃–5AlN than on 90Si₃N₄–5Y₂O₃–2.5Al₂O₃–2.5AlN (in wt.%). The 90Si₃N₄–5Y₂O₃–5Al₂O₃/liquid Al interface features strong interfacial adhesion while mild diffusion should govern the interfacial interactions. Compounds, whose formation results from the yttria-containing sintering aids, such as yttrium aluminates, should act as diffusion barriers at the ceramic/liquid metal interface. The experimental results indicate attractive features for applications in both Al-foundry industry and production of Si₃N₄–Al composites.     

Design of a New Zirconia–Alumina–Ta Micro‐Nanocomposite with Unique Mechanical Properties

2Y‐TZP/Al₂O₃ hybrid nanoparticles prepared by CO₂ laser covaporization (CoLAVA) were wet mixed with biocompatible lamellar Ta metal particles (20 vol%) and consolidated by spark plasma sintering. The microstructure and mechanical properties of this novel ceramic–metal composite have been studied. The achieved results demonstrate that both the homogeneity of the 2Y‐TZP/Al₂O₃ nanocomposite matrix and the reinforcement with a micrometer‐sized ductile phase are prerequisites for the successful design and fabrication of ceramic–metal composites with high strength (1300 MPa), enhanced fracture toughness (16 MPa·m^1/2), and improved low‐temperature degradation resistance.     

Low-temperature pressureless sintering of Al2O3-SiC-Ni nanocermets in air environment

This paper introduces a simplified method for low-temperature pressureless sintering of Al₂O₃-Ni-SiC nanocermets in air environment. In this method, a thin and continuous Ni shell was coated on the surface of Al₂O₃ particles using electroless deposition method. The composite powders were subsequently compressed to prepare bulk specimens. By preventing the ceramic particles from direct contact during the densification of green specimens, sintering temperature of cermet materials was reduced from that of Al₂O₃ (>?1400?°C) to the range of Ni solid-phase sintering temperature. Furthermore, dissolution of a low amount of phosphorus in the composition of Ni coatings caused the further decrease of the sintering temperature to 800?°C. At such low temperatures, pressureless sintering of the cermets in the air environment was successfully performed instead of the common hot pressing process in a reducing atmosphere. Optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS) and X-ray diffraction (XRD) characterizations indicated that the microstructure of such sintered samples consists of a continuous Ni network surrounding Al₂O₃ grains, without any structural defects or Ni oxidation. Furthermore, mechanical properties of the cermet materials were improved through reinforcement of the continuous Ni network by different amounts of SiC nanoparticles. The results showed that Al₂O₃-Ni-5?wt% SiC nanocermets sintered at 800?°C obtain the highest compressive strength of 242.5?MPa, hardness of 56.8 RA, and the lowest wear weight loss of 0.04?mg/m.     

The influence of different SiC amounts on the microstructure,densification, and mechanical properties of hot-pressed Al2O3-SiC composites

The hot pressing process of monolithic Al₂O₃ and Al₂O₃-SiC composites with 0-25 wt% of submicrometer silicon carbide was done in this paper. The presence of SiC particles prohibited the grain growth of the Al₂O₃ matrix during sintering at the temperatures of 1450°C and 1550°C for 1 h and under the pressure of 30 MPa in vacuum. The effect of SiC reinforcement on the mechanical properties of composite specimens like fracture toughness, flexural strength, and hardness was discussed. The results showed that the maximum values of fracture toughness (5.9 ± 0.5 MPa.m^1/2) and hardness (20.8 ± 0.4 GPa) were obtained for the Al₂O₃-5 wt% SiC composite specimens. The significant improvement in fracture toughness of composite specimens in comparison with the monolithic alumina (3.1 ± 0.4 MPa.m^1/2) could be attributed to crack deflection as one of the toughening mechanisms with regard to the presence of SiC particles. In addition, the flexural strength was improved by increasing SiC value up to 25 wt% and reached 395 ± 1.4 MPa. The scanning electron microscopy (SEM) observations verified that the increasing of flexural strength was related to the fine-grained microstructure.     

Effects of oxide addition on structure and properties of WC–10Co cemented carbide obtained by in situ synthesized powder

Combining spray drying and in situ synthesized technology, WC–10Co cemented carbide with uniform composition was prepared by vacuum sintering. The effects of Al₂O₃ and additions of different rare-earth oxides (La₂O₃, Y₂O₃ and CeO₂) on the microstructure and mechanical properties of WC–10Co were investigated. As the Al₂O₃ content increased from .5 to 2 wt%, the hardness of the sintered sample increased, whereas the relative density and fracture toughness decreased. Compared with the addition of .5 wt% Al₂O₃, the WC–10Co alloy with .5 wt% rare-earth oxides had higher hardness. In addition, compared with the alloy without an inhibitor (.80 μm), after adding .5 wt% Al₂O₃, La₂O₃, Y₂O₃ and CeO₂, the WC grain sizes were reduced to .73, .65, .71 and .62 μm, respectively, which indicated that the addition of Al₂O₃ and rare-earth oxides could refine WC grain during sintering. Among these additives, CeO₂ had the best effect. With the addition of .5 wt% CeO₂, the hardness and the fracture toughness increased from 1299 to 1710 HV₃₀ and from 16.18 to 18.90 MPa m^1/2, respectively.     

Investigation of Characterization and Mechanical Performances of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and SiC Reinforced PA6 Hybrid Composites

In this study, the influence of different weight percentages of alumina oxide (Al₂O₃) and silicon carbide (SiC) reinforcement on the mechanical properties of Polyamide (PA6) composite is investigated. Test specimens of pure PA6, 85 wt% PA6 + 10 wt% Al₂O₃ + 5 wt% SiC and 85 wt% PA6 +10 wt% SiC + 5 wt% Al₂O₃ are prepared using an injection molding machine. To investigate the mechanical behaviors tensile test, impact test, flexural test, and hardness test were conducted in accordance with ASTM standards. Experimental results indicated that the mechanical properties, such as tensile, impact, hardness, and flexural strength were considerably higher than the pure PA6. The tensile fracture morphology and the characterization of PA6 hybrid composites were observed by scanning electron microscope and Fourier transform infrared spectroscopic method. Further, thermogravimetric analysis confirms the thermal stability of PA6 hybrid composites. The reinforcing effects of Al₂O₃ and SiC on the mechanical properties of PA6 hybrid composites were compared and interpreted in this paper. Improved mechanical and thermal characteristics were observed by the addition of small amount of Al₂O₃ and SiC simultaneously reinforced with the pure PA6.     

In-situ formation of Al2O3/Al core-shell from waste material: Production of porous composite improved by graphene

Aluminum dross produced from aluminum industry was used to fabricate Al₂O₃/Al porous composites. The dross was milled for 20?h to obtain nano powder. The milled material was examined by TEM and XRD. Graphene (up to 4?wt%) was mixed with the dross and utilized to reinforce sintered composites. The milled powders were compacted then fired at various temperatures up to 700?°C. Physical properties in terms of bulk density and apparent porosity for sintered composites were tested using Archimedes method. SEM attached by energy dispersive spectrometer (EDS) was used to inspect microstructure and elemental analysis of sintered composites. Microhardness and compressive strength were also measured. Ultrasonic nondestructive technique was utilized to examine the elastic moduli. Electrical conductivity of sintered composite was also studied. During milling up to 20?h, Al₂O₃/Al core-shell was in-situ formed with size of 65.9 and 23.8?nm, respectively. The apparent porosity of sintered composites was improved with rising graphene percent while it decreased with increasing sintering temperature. Increasing of graphene mass percent and firing temperature led to remarkable increase in all mechanical properties and electrical conductivity. The maximum compressive strength, hardness, elastic modulus and electrical conductivity were 200?MPa, 1200?MPa, 215?GPa and 1.42?×?10^?5 S/m, respectively, obtained for composite sintered at 700?°C having 4?wt% graphene.