Relationship between fracture toughness characteristics and morphology of sintered Al2O3 ceramics

The influence of sintering temperature and soaking time on fracture toughness of Al₂O₃ ceramics has been investigated. The samples were prepared by solid state sintering at 1500, 1600 and 1700 °C for different soaking time periods. The fracture toughness of the sintered samples was determined by inducing cracks using Vickers indentation technique. Microstructural investigations on fracture surfaces obtained by three point bend test mode were made and correlated with fracture toughness. Crack deflection in the samples sintered at 1500 and 1600 °C for which ranges of fracture toughness are 5.2–5.4 and 5.0–5.6 MPa m^1/2 respectively, are found. The samples sintered at 1700 °C have lower fracture toughness ranging between 4.6 and 5.0 MPa m^1/2. These samples have larger grains and transgranular fracture mode is predominant. The crack deflection has further been revealed by SEM and AFM observations on fracture surface and fracture surface roughness respectively.     

Influence of sintering temperature and holding time on the densification, phase transformation, microstructure and properties of hot pressing WC-40 vol.%Al2O3 composites

WC-40 vol.%Al₂O₃ composites were prepared by high energy ball milling followed by hot pressing. The tungsten carbide (WC) and commercial alumina (Al₂O₃) powders composed of amorphous Al₂O₃, boehmite (AlOOH) and χ-Al₂O₃ were used as the starting materials. The phase transformation during sintering, the influence of sintering temperature and holding time on the densification, microstructure, Vickers hardness and fracture toughness and the toughening effects of WC-40 vol.%Al₂O₃ composites were investigated. The results showed that the amorphous Al₂O₃, AlOOH and χ-Al₂O₃ were transformed to α-Al₂O₃ completely during the sintering process. With the increasing sintering temperature and holding time, the relative density increased and both the Vickers hardness and fracture toughness increased initially to the maximum values and then decreased. When the as milled powders were hot pressed at 1540 °C for 90 min, a relative density of 97.98% and a maximum hardness of 18.65 GPa with an excellent fracture toughness of 10.43 MPa m^1/2 of WC-40 vol.%Al₂O₃ composites were obtained.     

Effect of Al2O3 particle size on preparation and properties of ZTA ceramics formed by gelcasting

Zirconia-toughened alumina (ZTA) ceramics were prepared using three different kinds of Al₂O₃ powders (marked PW-A average particle size: 7.53 μm, marked PW-B average particle size: 1.76 μm, marked PW-C average particle size: 0.61 μm) by gelcasting. Effect of Al₂O₃ particle size on zeta potential, dispersant dosage and solid volume fractions of ZTA suspensions as well as the mechanical properties of ZTA green bodies and ceramics were investigated. The optimum dosages of dispersant for ZTA suspensions prepared by PW-A, PW-B and PW-C are 0.4 wt%, 0.5 wt% and 0.7 wt%, respectively. The highest solid volume fractions of ZTA suspensions can reach 62 vol% (SP-A), 60 vol% (SP-B) and 52 vol% (SP-C), respectively. The green bodies show a bending strength as high as 20 MPa, which can meet the requirement of machining. The Al₂O₃ powder with fine particle size is beneficial to the improvement of mechanical properties. The ZTA ceramics prepared by PW-B Al₂O₃ powder show the highest bending strength (680 MPa) and toughness (7.49 MPa m^1/2).     

Spark plasma sintering technique for reaction sintering of Al2O3/Ni nanocomposite and its mechanical properties

Al₂O₃/Ni nanocomposites were prepared by spark plasma sintering (SPS) using reaction sintering method and the mechanical properties of the obtained nanocomposites are reported. The starting materials of Al₂O₃–NiO solid solution were synthesized from aluminum sulfate and nickel sulfate. These Al₂O₃–NiO powders were changed into Al₂O₃ and Ni phases during sintering process. The obtained nanocomposites showed high relative densities (>98%). SEM micrographs showed homogeneously dispersed Ni grains in the matrix. The 3-point strength and the fracture toughness of the composites significantly improved from 450 MPa in the monolithic α-Al₂O₃ to 766 MPa in the 10 mol% (2.8 vol.%) Ni nanocomposite and from 3.7 to 5.6 MPa m^1/2 in 13 mol% (3.7 vol.%) Ni nanocomposite. On the other hand, Young's modulus and Vickers hardness of the nanocomposites were mostly same as those of the monolithic α-Al₂O₃.     

Microstructure, growth mechanism and mechanical property of Al2O3-based eutectic ceramic in situ composites

Directionally solidified Al₂O₃-based eutectic ceramic in situ composites with inherently high melting point, low density, excellent microstructure stability, outstanding resistance to creep, corrosion and oxidation at elevated temperature, have attracted significant interest as promising candidate for high-temperature application. This paper reviews the recent research progress on Al₂O₃-based eutectic ceramic in situ composites in State Key Laboratory of Solidification Processing. Al₂O₃/YAG binary eutectic and Al₂O₃/YAG/ZrO₂ ternary eutectic ceramics are prepared by laser zone melting, electron beam floating zone melting and laser direct forming, respectively. The processing control, solidification characteristic, microstructure evolution, eutectic growth mechanism, phase interface structure, mechanical property and toughening mechanism are investigated. The high thermal gradient and cooling rate during solidification lead to the refined microstructure with minimum eutectic spacing of 100 nm. Besides the typical faceted/faceted eutectic growth manner, the faceted to non-faceted growth transition is found. The room-temperature hardness H_V and fracture toughness K_IC are measured with micro-indentation method. For Al₂O₃/YAG/ZrO₂, K_IC = 8.0 ± 2.0 MPa m^1/2 while for Al₂O₃/YAG, K_IC = 3.6 ± 0.4 MPa m^1/2. It is expectable that directionally solidified Al₂O₃-based eutectic ceramics are approaching practical application with the advancement of processing theory, technique and apparatus.     

Mechanical properties of Al2O3-Cr2O3/Cr3C2 nanocomposite fabricated by spark plasma sintering

The fine grains of Al₂O₃-Cr₂O₃/Cr-carbide nanocomposites were prepared by employing recently developed spark plasma sintering (SPS) technique. The initial materials were fabricated by a metal organic chemical vapor deposition (MOCVD) process, in which Cr(CO)₆ was used as a precursor and Al₂O₃ powders as matrix in a spouted chamber. The basic mechanical properties like hardness, fracture strength and toughness, and the nanoindentation characterization of nanocomposites such as Elastics modulus (E), elastic work (W_e) and plastic work (W_p) were analyzed. The microstructure of dislocation, transgranular and step-wise fracture surface were observed in the nanocomposites. The nanocomposites show fracture toughness of (4.8 MPa m^1/2) and facture strength (780 MPa), which is higher than monolithic alumina. The strengthening mechanism from the secondary phase and solid solution are also discussed in the present work. Nanoindentation characterization further illustrates the strengthening of nanocomposites.     

Fabrication of novel multilayer Al2O3-(m-ZrO2)/t-ZrO2 fibrous ceramics composites

A novel layered microstructure in the Al₂O₃/ZrO₂ composites system was fabricated by the multipass extrusion method. The microstructure consisted with very fine alternate lamina of Al₂O₃-(m-ZrO₂) and t-ZrO₂. The composites were designed in such a way that a small group of 7 cylindrical alternate layers of Al₂O₃-(m-ZrO₂) and t-ZrO₂ made a concentric microgroup around 40 μm in diameter, with a common boundary layer between the adjacent groups. The thickness of both layers was around 2-3 μm. The microstructure was unidirectionally aligned throughout the composites. The composite microstructure was fibrous due to the unidirectional orientation of these microgroups. Detailed microstructure of the fabricated composites was characterized by SEM. The effect of the concentric layered microstructure on mechanical behavior was discussed. Material properties such as density, bending strength, Vickers hardness and fracture toughness were measured and evaluated depending on different sintering temperatures.     

Investigation of the effect of ZrO2 and ZrO2/Al2O3 additions on the hot-pressing and properties of equimolecular mixtures of α- and β-Si3N4

In this work, hot-pressing of equimolecular mixtures of α- and β-Si₃N₄ was performed with addition of different amounts of sintering additives selected in the ZrO₂–Al₂O₃ system. Phase composition and microstructure of the hot-pressed samples was investigated. Densification behavior, mechanical and thermal properties were studied and explained based on the microstructure and phase composition. The optimum mixture from the ZrO₂–Al₂O₃ system for hot-pressing of silicon nitride to give high density materials was determined. Near fully dense silicon nitride materials were obtained only with the additions of zirconia and alumina. The liquid phase formed in the zirconia and alumina mixtures is important for effective hot-pressing. Based on these results, we conclude that pure zirconia is not an effective sintering additive. Selected mechanical and thermal properties of these materials are also presented. Hot-pressed Si₃N₄ ceramics, using mixtures from of ZrO₂/Al₂O₃ as additives, gave fracture toughness, K_IC, in the range of 3.7–6.2 MPa m^1/2 and Vicker hardness values in the range of 6–12 GPa. These properties compare well with currently available high performance silicon nitride ceramics. We also report on interesting thermal expansion behavior of these materials including negative thermal expansion coefficients for a few compositions.     

Thermal shock resistance and mechanical properties of La2Ce2O7 thermal barrier coatings with segmented structure

La₂Ce₂O₇ (LCO) is a promising candidate material for thermal barrier coatings (TBCs) application because of its higher temperature capability and better thermal insulation property relative to yttria stabilized zirconia (YSZ). In this work, La₂Ce₂O₇ TBC with segmentation crack structure was produced by atmospheric plasma spray (APS). The mechanical properties of the sprayed coatings at room temperature including microhardness, Young's modulus, fracture toughness and tensile strength were evaluated. The Young's modulus and microhardness of the segmented coating were measured to be about 25 and 5 GPa, relatively higher than those of the non-segmented coating, respectively. The fracture toughness of the LCO coating is in a range of 1.3–1.5 MPa m^1/2, about 40% lower than that of the YSZ coating. The segmented TBC had a lifetime of more than 700 cycles, improving the lifetime by nearly two times as compared to the non-segmented TBC. The failure of the segmented coating occurred by chipping spallation and delamination cracking within the coating.     

Contact damage in a Ce-TZP/Al2O3 nanocomposite

The damage tolerance of a nanocomposite based on Ce-TZP and 30 vol% Al₂O₃ has been studied under monotonic contact with a spherical indenter. The results are compared with those previously known for commercial 3Y-TZP zirconia. It is concluded that the minimum load for ring crack appearance is similar in both ceramics. However, in the nanocomposite the ring cracks penetrate much less into the bulk, because of its higher fracture toughness. Finally, the stress-induced phase transformation of the zirconia component was quantified and mapped by micro-Raman spectroscopy.     

Pressureless sintered Al2O3–SiC nanocomposites

Al₂O₃ + 5 vol% SiC composite ceramics were prepared via a conventional powder processing route followed by pressureless sintering. Commercially available Al₂O₃ and SiC powders were milled together in an aqueous suspension. The slurry was freeze granulated, and green bodies were obtained by cold isostatic pressing of the granules. Pressureless sintering was carried out in a nitrogen atmosphere at 1750 and 1780 °C. Near full density (>99%) was achieved at 1780 °C. Densification at the lower sintering temperature was promoted by smaller additions of MgO. Vickers hardness and indentation fracture toughness varied around 18 GPa and 2.3 MPa m^1/2 after sintering at 1780 °C. Transmission electron microscopy revealed that the SiC particles were located predominantly to the interior of the matrix grains and well distributed throughout the composite microstructures. The intragranular particles had sizes in the range 50–200 nm while the intergranular particles were larger, typically 200–500 nm in diameter.     

Sintering of Al2O3-SiC composite from sol-gel method with MgO, TiO2 and Y2O3 addition

Al₂O₃-SiC composite ceramics were prepared by pressureless sintering with and without the addition of MgO, TiO₂ and Y₂O₃ as sintering aids. The effects of these compositional variables on final density and hardness were investigated. In the present article at first α-Al₂O₃ and β-SiC nano powders have been synthesized by sol-gel method separately by using AlCl₃, TEOS and saccharose as precursors. Pressureless sintering was carried out in nitrogen atmosphere at 1600 °C and 1630 °C. The addition of 5 vol.% SiC to Al₂O₃ hindered densification. In contrast, the addition of nano MgO and nano TiO₂ to Al₂O₃-5 vol.% SiC composites improved densification but Y₂O₃ did not have positive effect on sintering. Maximum density (97%) was achieved at 1630 °C. Vickers hardness was 17.7 GPa after sintering at 1630 °C. SEM revealed that the SiC particles were well distributed throughout the composite microstructures. The precursors and the resultant powders were characterized by XRD, STA and SEM.     

Rapid synthesis of Ti3AlC2/TiB2 composites by the spark plasma sintering (SPS) technique

Dense Ti₃AlC₂/TiB₂ composites were successfully fabricated from B₄C/TiC/Ti/Al powders by spark plasma sintering (SPS). The microstructure, flexural strength and fracture toughness of the composites were investigated. The experimental results indicate that the Vickers hardness increased with the increase in TiB₂ content. The maximum flexural strength (700 ± 10 MPa) and fracture toughness (7.0 ± 0.2 MPa m^1/2) were achieved through addition of 10 vol.% TiB₂, however, a slight decrease in the other mechanical properties was observed with TiB₂ addition higher than 10 vol.%, which is believed to be due to TiB₂ agglomeration.     

Synthesis and characterization of MgAl2O4–ZrO2 composites

Different types of dense 5–97% ZrO₂–MgAl₂O₄ composites have been prepared using a MgAl₂O₄ spinel obtained by calcining a stoichiometric mixture of aluminium tri-hydroxide and caustic MgO at 1300 °C for 1 h, and a commercial yttria partially stabilized zirconia (YPSZ) powder as starting raw materials by sintering at various temperatures ranging from 1500 to 1650 °C for 2 h. The characteristics of the MgAl₂O₄ spinel, the YPSZ powder and the various sintered products were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), BET surface area, particle size analysis, Archimedes principle, and Vickers indentation method. Characterization results revealed that the YPSZ addition increases the sintering ability, fracture toughness and hardness of MgAl₂O₄ spinel, whereas, the MgAl₂O₄ spinel hampered the sintering ability of YPSZ when sintered at elevated temperatures. A 20-wt.% YPSZ was found to be sufficient to increase the hardness and fracture toughness of MgAl₂O₄ spinel from 406 to 1314 Hv and 2.5 to 3.45 MPa m^1/2, respectively, when sintered at 1600 °C for 2 h.     

Effect of the volume ratio of zirconia and alumina on the mechanical properties of fibrous zirconia/alumina bi-phase composites prepared by co-extrusion

Fibrous zirconia/alumina composites with different composition were fabricated by piston co-extrusion. After a 3rd extrusion step and sintering at 1600 °C, crack-free composites with a fibre width of 50 μm were obtained for all compositions. The effect of the volume ratio of secondary phase on the mechanical properties was investigated. The Young's modulus of the composites decreased linearly with increasing the zirconia content. The fracture toughness of the composites was improved by introducing fine second phase filaments into the matrix. The maximum fracture toughness of 6.2 MPa m^1/2 was attained in the 3rd co-extruded 47/53 vol% zirconia/alumina composite. The improvement in toughness was attributed to both “stress-induced” transformation of zirconia and a crack deflection mechanism due to thermal expansion mismatch between the two phases. Bending strength of the composites was almost the same as that of the monolithic alumina regardless of the composition.     

Processing and physical properties of Al2O3/aluminum alloy composites

Porous aluminum oxide (Al₂O₃) preforms were formed by sintering in air at 1200 °C for 2 h. A356, 6061, and 1050 aluminum alloys were infiltrated into the preforms by squeeze casting in order to fabricate Al₂O₃/A356, Al₂O₃/6061, and Al₂O₃/1050 composites, respectively, with different volumes of aluminum alloy content. The content of aluminum alloy in the composites was 10–40% by volume. The resistivity of Al₂O₃/A356, Al₂O₃/6061, and Al₂O₃/1050 composites decreased dramatically from 6.41 × 10¹² to 9.77 × 10⁻⁴, 7.28 × 10⁻⁴, and 6.24 × 10⁻⁴ Ω m, respectively, the four-points bending strength increased from 397 to 443, 435.1, 407.2 MPa, respectively, and the deviations were smaller than 2%. From SEM microstructural analysis and TEM bright field images, the pore volume fraction and the relative density of the composites were the most important factors that affected the physical and mechanical properties. The ceramic phase and alloy phase in Al₂O₃/aluminum alloy composites were found to be homogenized and uniformly distributed using electrical and mechanical properties analysis, microstructure analysis, and image analysis.     

Structural, thermal and mechanical properties of transparent Yb:(Y0.97Zr0.03)2O3 ceramic

Yb doped (Y_0.97Zr_0.03)₂O₃ transparent ceramics were fabricated by solid state reaction and vacuum sintering. The microstructure, thermal and mechanical properties of Y₂O₃ ceramic, as well as the effect of Yb doping concentration on these properties were investigated in detail. The lattice parameter and unit cell volume decrease with the increasing of Yb content, whereas thermal expansive coefficient increases. With Yb content increasing from 0 to 8 at.%, the mean grain size increases from 15.82 μm to 26.54 μm, and the thermal conductivity at room temperature (RT) decreases from 11.97 to 6.39 W/m/K. The microhardness decreases with Yb content, and the microhardness and fracture toughness of (Y_0.97Zr_0.03)₂O₃ transparent ceramic is 11.11 GPa and 1.29 MPa m^1/2, respectively.     

TaB2-based ceramics: Microstructure, mechanical properties and oxidation resistance

TaB₂-based ceramics were hot pressed in low vacuum with addition of 5-10 vol% MoSi₂. Temperatures in the range of 1680-1780 °C led to relative density around 90-95%. The hardness was about 18 GPa, the fracture toughness 4.6 MPa m^1/2 and the room temperature flexural strength was around 630 MPa, but abruptly decreased above 1200 °C to 220 MPa. The composite containing 10 vol% of MoSi₂ was tested in a bottom-up furnace in the temperature range 1200-1700 °C for 30 min. The microstructure appeared covered by a SiO₂ layer, whose thickness increased with the temperature, but the bulk remained unaltered up to 1600 °C. At 1700 °C the specimen vaporized. Nanoindentation was employed on the oxidized cross sections in order to detect eventual mechanical properties modification associated to chemical/microstructural change, like formation of Ta-B-O solid solutions.     

Sintering,microstrusture and hardness of different alumina–zirconia composites

Two commercial 3 mol% yttria-partially stabilized zirconia powders, 0.3 wt% Al₂O₃-doped (Al-doped Y-PSZ) and without Al₂O₃ (Y-PSZ), were used to produce alumina (Al₂O₃)-zirconia (ZrO₂) slip cast composites. The influence of the substitution of Al₂O₃ either by different Al-doped Y-PSZ contents or 50 vol% Y-PSZ on the sintering kinetic at the intermediate stage was investigated. In addition, the microstructure of Al₂O₃ and the different composites at temperatures in the range of 1100–1600 °C was studied and related to the sample hardness. An increase in the sintering rate was observed when Al-doped Y-PSZ increased from 22 to 50 vol% or when 50 vol% Y-PSZ was substituted by 50 vol% Al-doped Y-PSZ. 50 vol% ZrO₂ was the most effective concentration to reduce the rate of Al₂O₃ grain growth in the final sintering stage; the Al₂O₃ grain growth began at lower temperatures and became greater with decreasing the Al-doped Y-PSZ content. On the contrary, the ZrO₂ grain growth slightly increased with increasing the Al-doped Y-PSZ concentration. However, for 50 vol% Al-doped Y-PSZ a smaller ZrO₂ grain size distribution compared with 50 vol% Y-PSZ could be achieved. As the average Al₂O₃ grain size of the sintered samples became greater than about 1 µm a markedly decrease in the hardness was found; this occurred at temperatures higher than 1400 °C and 1500 °C for Al₂O₃ and the composite with 10.5 vol% Al-doped Y-PSZ, respectively.     

Development of input output relationships for self-healing Al2O3/SiC ceramic composites with Y2O3 additive using design of experiments

Al₂O₃/SiC ceramic composites with Y₂O₃ as an additive, was synthesized using the Taguchi method of design of experiments, so as to develop statistically sound input output relationships. The proportion of SiC was varied from 12 to 21 vol.% whereas that of Y₂O₃ was varied from 2.5 to 4 vol.%. The composites were sintered at 1500 °C for a soaking time period of 12 h in an air atmosphere. Cracks were induced on the composite surface using a Vickers indenter with a load varying between 20 and 40 kg. Fractographical analyses have been carried out using optical and/or scanning electron microscopy to investigate the surface crack propagation behavior. Thermal aging at 1300 °C in the time range of 0.5-12.5 h was applied to find optimal conditions for healing of the pre-cracked samples. The output parameters such as crack length, healed crack length, hardness and fracture toughness of the samples were correlated with appropriate inputs such as contents of SiC and Y₂O₃, crack-healing temperature, healing time, compaction pressure, indentation load using statistical analysis. Further, the extent of influence, exerted by pertinent input parameters on output parameters, was also identified.