Microstructure of pressureless-sintered Al2O3-24 vol% ZrO2 composite studied by high-resolution electron microscopy

The microstructure of an Al₂O₃-24 vol% ZrO₂ composite prepared by pressureless-sintering was investigated by high-resolution electron microscopy. The composite was formed of homogeneously dispersed ZrO₂ and Al₂O₃ grains with average sizes of 0.3 and 0.5 m, respectively. Most ZrO₂ grains had a monoclinic structure, but a few ZrO₂ embedded in Al₂O₃ grains were a tetragonal structure. At interfaces between ZrO₂ with a lamella-type twin structure and Al₂O₃, microcracks were observed, in addition to strain fields in the Al₂O₃ matrix. Complex twin structures accompanied by dislocations were observed in ZrO₂ with a spherical shape. In in situ observations with electron-beam heating, it was found that a crack propagated along an Al₂O₃/ZrO₂ interface and stopped at the place where a tetragonal ZrO₂ had undergone a structural change to monoclinic ZrO₂.     

Effect of cerium zirconate (Ce2Zr2O7) on microstructure and mechanical properties of Ce-ZTA

Powders for ZrO₂ toughened Al₂O₃ (ZTA) composites containing 8, 11, 13.8 and 16.5 vol% ZrO₂ (stabilized with 11.5 mol% CeO₂) are prepared by a hybrid sol-gel method using Al₂O₃ powders and a sol formed from Zr-alkoxide and cerium nitrate. Besides ZrO₂, a small amount of a Ce-zirconate phase (Ce₂Zr₂O₇) forms when the powders are calcined in air. The zirconate phase persists in the sintered specimens and its amount increases from surface to centre of the specimen. Presence of higher amount of Ce₂Zr₂O₇ promotes exaggerated grain growth of Al₂O₃. Particles of Zr rich phases are found to be trapped inside the Al₂O₃ grains. Composites exhibit higher fracture toughness despite lower transformability of t-ZrO₂ during fracture when they contain high amount of zirconate. Crack bridging is shown to be an important mechanism contributing to enhancement in fracture toughness in these composites.     

Roles of alumina in zirconia-based solid electrolyte

Up to 5 mol% Al₂O₃ was added to 9 mol% Y₂O₃-stabilized ZrO₂, and the roles of Al₂O₃ were systematically studied by means of the complex impedance approach, the positron annihilation technique, SEM, TEM, and electron probe microanalysis from the following aspects: (1) the existence of forms of Al₂O₃ in ZrO₂, (2) the effects of Al₂O₃ on the microstructure of ZrO₂, (3) the effects of Al₂O₃ on the resistance of ZrO₂, (4) the microstructure and property changes of ZrO₂ with Al₂O₃ addition during ageing at 940C. Two types of grain boundary phase, crystal and amorphous, were discovered. The Al₂O₃ segregation at grain boundaries can promote the mobility of the grain boundaries and thus results in a low density, because of entrapped pores. The Al₂O₃ addition decreases the grainboundary resistance in two ways: to scavenge the SiO₂ and CaO located at grain boundaries, and to form crystal grain-boundary phases with very high crystal defect concentrations. Ordered microdomains of Zr₃Y₄O₁₂ were precipitated from ZrO₂ grains during ageing, and aluminium was found to facilitate the precipitation.     

Pressureless-sintering behavior of nanocrystalline ZrO2–Y2O3–Al2O3 system

ZrO₂–Y₂O₃–Al₂O₃ nanocrystalline powders have been synthesized using chemical coprecipitation method. Nano-powders were compacted uniaxially and densified in a muffle furnace. Densification studies showed that a fully dense pellet of ZrO₂(3Y) and a 99% relative density for 5 mol% Al₂O₃ doped ZrO₂(3Y) were obtained after sintering at 1200 °C. The presence of Al₂O₃ inhibits grain growth and suppresses the densification process. Full densification and the maximum microhardness of 17.8 GPa were achieved for the ZrO₂(3Y)/5 mol% Al₂O₃ composites sintered at 1250 °C.     

Microstructure and mechanical properties of hot pressed Al2O3–ZrO2 ceramics prepared from ultrafine powders

Abstract

The microstructure and mechanical properties of hot pressed Al₂O₃–ZrO₂ ceramics prepared from ultrafine powders were studied by means of hardness and fracture toughness tests, X-ray diffraction, scanning and transmission electron microscopy, and electron probe microanalysis. Experimental results show that Al₂O₃–ZrO₂ ceramics combine the high modulus of Al₂O₃ and phase transformation toughening of ZrO₂ and give good mechanical properties. The fracture toughness of the samples increases monotonically with increasing ZrO₂ content. When the content of ZrO₂ is low, the ZrO₂ particles are surrounded by Al₂O₃ grains and the matrix is thus strengthened, but when the content of ZrO₂ is high, the microcracks resulting from the tetragonal monoclinic phase transformation decrease the strength of the material. The relative density of the samples also increases with increasing ZrO₂ content, which is beneficial to both strength and toughness. Banded abnormal growth of the Al₂O₃ grains parallel to the hot pressing plane was found in the samples. This phenomenon is considered to be a result of the priority of Al₂O₃ grain growth and aggregation of ZrO₂ along the pressing direction in the later stages of hot pressing.

MST/1359     

Microstructure and mechanical properties of Al2O3-Cr2O3-ZrO2 composites

Al₂O₃ is a popular ceramic and has been used widely in many applications and studied in many aspects. On the other hand, zirconia-toughened alumina (ZTA) is a desirable material for engineering ceramics because of its high hardness, high wear resistance and high toughness. In the present research, Al₂O₃-Cr₂O₃-ZrO₂ composites were produced by hot-pressing in order to harden the Al₂O₃ matrix in ZTA. Its microstructure and mechanical properties were studied by SEM, ESCA, XRD, Vickers hardness and bending strength test. It was found that addition of ZrO₂ inhibited the grain growth of Al₂O₃-Cr₂O₃ and the grain growth of ZrO₂ proceeded with increasing amounts of ZrO₂ in the Al₂O₃-Cr₂O₃-Zr₂ composite. The formation of solid solution Al₂O₃-Cr₂O₃ was also confirmed by XRD, and monoclinic ZrO₂ increased on addition of Cr₂O₃. Maximum hardness was at Al₂O₃-10wt% Cr₂O₃ with 10 vol% ZrO₂ and a stress-induced transformation was confirmed on the fracture surface of the specimen after the bending test.     

Oriented eutectic microstructures in the system Al2O3/ZrO2

Oriented eutectic microstructures have been produced in the system Al₂O₃/ZrO₂ using a Bridgman-type crystal-growing furnace. Ingots consisted of elongated columnar grains or colonies. Inside the colonies a rod-type eutectic microstructure consisting of rods of ZrO₂ surrounded by an Al₂O₃ matrix was observed. The eutectic point was re-established at 63 mol % Al₂O₃/37.0 mol % ZrO₂ and 1870±5° C. Al₂O₃ is the first phase to nucleate when eutectic growth occurs.     

Superplasticity of ZrO2 and ZrO2/Al2O3 composite consisting of nano-sized grains

 Y₂O₃-stabilized ZrO₂ and Y₂O₃-stabilized ZrO₂/10 mol%Al₂O₃ composite both consisting of homogeneous nano-sized grains were successfully fabricated by a pulse electric current sintering through solution chemistry technique. The relative density was above 97% for both the obtained materials, and the grain size was less than 90 nm and 40 nm for the ZrO₂ monolith and the ZrO₂/Al₂O₃ composite, respectively. The materials showed superplastic behavior in compression at temperature of 1200°C. Received: 2 March 1998 / Accepted: 10 March 1998     

Microstructure and Mechanical Properties of ZrO2(2Y)-Toughened Al2O3 Ceramics Fabricated by Spark Plasma Sintering

Spark plasma sintering (SPS) has been performed for 5 min at 1500°C and 30 MPa using submicrometer-sized Al₂O₃/ZrO₂(2Y) composite powders in the Al₂O₃-rich region. Dense ZrO₂-toughened Al₂O₃ (ZTA) ceramics show excellent mechanical strength; the strength of 1620 MPa is achieved in the ZTA with 50 mol% ZrO₂. The grain size of Al₂O₃ in ZTA decreases from 1.5 to 0.6 m with increased ZrO₂ content. Almost all the ZrO₂ grains (0.3 m) are located in the boundaries of the Al₂O₃ grains. Mechanical properties are discussed, with an emphasis on the relation between t-/m-ZrO₂ ratios and microstructures of ZTA.     

Fabrication and microstructure of in situ toughened Al2O3/Fe3Al

Fe₃Al nano-particles and commercial purity Al₂O₃ powders were used as raw materials to fabricate in situ reinforced Al₂O₃/Fe₃Al nano/micro-composites. Densification and microstructure were studied. The Al₂O₃ matrix grains were characterized by platelet grains. The Fe₃Al particles inhibited the grain growth of Al₂O₃ grains and limited the densification of the composites. In Al₂O₃/Fe₃Al composites, the Fe₃Al particles were uniformly dispersed in the Al₂O₃ matrix. The major Fe₃Al micro-particles, about 1 μm in average size, existed at Al₂O₃ grain boundaries, and the Fe₃Al nano-particles were found embedded in the matrix grains. The grain size of the intragranular particles ranged from several to several hundred nanometers. The grain size and aspect ratio of Al₂O₃ platelet grains and distribution of intragranular Fe₃Al could be optimized by controlling the Fe₃Al contents and sintering process. The in situ formed Al₂O₃ platelet grains, as well as Fe₃Al dispersoids, were beneficial to the increase of the mechanical properties of alumina.     

Interface structure and mechanical properties of Al2O3-20 vol% ZrO2-20 vol% SiCW ceramic composite

The microstructure, mechanical properties, fracture behaviour and toughening mechanisms of Al₂O₃-20 vol% ZrO₂ (2 mol% Y₂O₃)-20 vol% SiC_W ceramic matrix composite were investigated by X-ray diffraction, scanning and transmission electron microscopies, energy dispersive analysis of X-rays, high-resolution electron microscopy techniques and three-point bending tests. The results show that the Al₂O₃ matrix is simultaneously strengthened and toughened by both ZrO₂ particles and SiC whiskers. The interfacial amorphous layers between SiC whiskers and ZrO₂, and Al₂O₃ grains were observed by both TEM dark-field and high-resolution electron microscopy techniques.     

Microstructure and mechanical properties of a directionally solidified Al2O3/Y3Al5O12/ZrO2 hypoeutectic in situ composite

Directionally solidified ternary Al₂O₃/Y₃Al₅O₁₂(YAG)/ZrO₂ hypoeutectic rod composites were successfully fabricated by the laser zone remelting technique. The microstructure and mechanical properties of the composite were investigated. The microstructure presented a complex three-dimensional network structure consisting of fine Al₂O₃ (41 vol.%) and YAG (49 vol.%) phases, with smaller ZrO₂ (10 vol.%) phases partially distributed at the Al₂O₃/YAG interfaces. The irregular growth behavior in the hypoeutectic was revealed. The hardness and fracture toughness at ambient temperature were measured to be 17.3 GPa and 5.2 MPa m^1/2, respectively. The toughness enhancement in comparison with previous binary Al₂O₃/YAG composites was mainly attributed to the refined microstructure, and crack deflection, branching and bridging. Moreover, the residual stresses, generated by different thermal expansion coefficients of the component phases, also importantly contributed to the improved toughness. Correlations between the addition of the third component ZrO₂ and the microstructure and properties were discussed as well.     

Microwave and conventional sintering of rapidly solidified Al2O3-ZrO2 powders

The Al₂O₃-ZrO₂ eutectic composition was rapidly solidified, forming amorphous and crystalline structures. The as-quenched material was crushed and pressed into pellets which were sintered conventionally or with microwaves. Conventional and microwave sintering at temperatures up to 1600 °C resulted in a microstructure where 100–200 nm ZrO₂ grains were present intergranularly in the -Al₂O₃ grains. Larger ZrO₂ grains (1 m) were found intergranularly. The as-quenched lamellar structure spheroidized during sintering at high temperatures. Boron contamination of the powders resulted in more homogeneous and dense as-fired samples but promoted the ZrO₂ tetragonal-to-monoclinic transformation, which was attributed to increased grain boundary diffusivity. Conventional sintering at low temperatures resulted in the formation of rods of an Al₂O₃-rich phase which grew from a low-melting B₂O₃-rich liquid.     

In situ growth of elongated Al2O3 grains induced by Al nanoparticles

We have reported the in situ growth of elongated grains induced by Al nanoparticles for the fabrication of high-pure and high toughness Al₂O₃ ceramics. It is found that the introduced Al nanoparticles have a profound effect on the anisotropic growth of Al₂O₃ grains. As compared to the conventional techniques by adding impurities to induce the anisotropic grain growth, Al particles could be transformed into Al₂O₃ after sintering in air, leading to the formation of high-pure Al₂O₃ ceramics without contamination. The elongated grains induced by the Al nanoparticles make noticeable improvements of bending strength and fracture toughness of Al₂O₃ ceramics. Current work might benefit the fabrication of high-pure and high toughness Al₂O₃ ceramics with in situ grown elongated grains.     

Grain boundary bonding state and fracture energy in small amount of oxide-doped fine-grained Al2O3

Grain boundary structure and chemical bonding state were characterized in high-purity Al₂O₃, 0.1 wt% MgO, 0.1 wt% Y₂O₃ or 0.1 wt% ZrO₂-doped Al₂O₃. High resolution electron microscopy (HREM) and energy dispersive X-ray spectroscopy (EDS) revealed that all samples examined have single phase structure, and that doped cations segregate along grain boundaries. Electron energy loss spectroscopy (EELS) spectra taken from grain boundaries in doped Al₂O₃ shows slight chemical shift in comparison with those from grain interior. This result suggests that the chemical bonding state in grain boundaries changes by the segregated ions. The change in chemical bonding state seems to affect the grain boundary fracture energy of Al₂O₃.     

Preparation of Y2O3 stabilised ZrO2 ceramic nanopowders by surface doping

Abstract

Weakly agglomerated nanocrystalline Y₂O₃–ZrO₂ powder was prepared by dispersion Y₂O₃ on the surface of ZrO₂ nanopowder (7·3 nm) that was derived from gas phase synthesis. The utmost dispersion capacity of Y₂O₃ on the surface of ZrO₂ was determined to be 0·16 gY₂O₃ /gZrO₂ (or 8·7 mol.-%Y₂O₃–ZrO₂ ) which suggests that 3 mol.-%Y₂O₃ would be homogeneously dispersed on ZrO₂ and no phase segregation would occur during surface doping. The results show that the tetragonal phase content in surface doped ZrO₂ increased with grain growth or heating temperatures, unlike the undoped and the bulk doped ZrO₂ . The stabilisation of the tetragonal phase resulted from the incorporation of Y ³⁺ cations from the surface into the grains of ZrO₂ . This conclusion is supported by the X-ray photoelectron spectroscopy and X-ray diffraction evidence. Surface doped powders have a strong tendency to improve the anticoarsening ability and suppress grain growth, especially at higher doping levels and at lower heating temperatures.     

Friction and wear of ZrO2-TiO2 surface-alloyed Al2O3 ceramics in unlubricated sliding contact

Slightly and highly porous Al₂O₃ ceramics were surface remelted and alloyed by adding ZrO₂ and TiO₂ using infrared CO₂ laser radiation. The resulting composite layers of thickness of about 200 m?m contained about 31 vol.% of ZrO₂- and TiO₂-rich phases which were homogeneously distributed at the grain boundaries of the alumina matrix. Microstructures and worn surfaces were analysed by electron microscopy and EDX analysis. Tribological tests were carried out unlubricated using conditions of abrasive wear and oscillating slinging wear, respectively. The results showed that the average grain size and hardness of the ceramics were reduced due to alloying. Despite decreasing hardness the wear resistance was substantially increased. Friction and wear of the untreated ceramics depended strongly on the amount of porosity which was removed or substantially reduced by laser treatment. Surface alloying of Al₂O₃ ceramics can offer an effective process for producing components showing very different surface and bulk properties and particularly improved tribological behaviour. For the alloying elements and experimental conditions used, the improvement was more pronounced in wear resistance than in friction coefficient.     

Microstructure and high-temperature strength of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Er<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>/ZrO<Subscript>2</Subscript> ternary melt growth composite

A new Al₂O₃/Er₃Al₅O₁₂(EAG)/ZrO₂ ternary MGC (Melt Growth Composite) with a novel microstructure has been fabricated by unidirectional solidification. This ternary MGC has a microstructure consisting of continuous networks of single-crystal Al₂O₃, single-crystal EAG and fine cubic-ZrO₂ phases without grain boundaries. The ternary MGC has also characteristic dimensions of the microstructure of around 2–4 m for EAG phases, around 2–4 m for Al₂O₃ phases reinforced with around 0.4–0.8 m cubic-ZrO₂ phases. No amorphous phases are formed at interfaces between phases in the ternary MGC. The ternary MGCs flexural strength at 1873 K is approximately 700 MPa, more than twice the 330 MPa of the Al₂O₃/EAG binary MGC. The fracture manner of the Al₂O₃/EAG/ZrO₂ ternary MGC at 1873 K shows the same intergranular fracture as the Al₂O₃/EAG binary MGC, but is significantly different from the transgranular fracture of the sintered ceramic.     

Electron microscopy characterization of hot-pressed Al substituted Li7La3Zr2O12

Hot-pressing was used to prepare a dense (97% relative density) cubic Al substituted Li₇La₃Zr₂O₁₂ material at temperatures lower than typically used for solid-state and/or liquid phase sintering. Electron microscopy analysis revealed equiaxed grains, grain boundaries, and triple junctions free of amorphous and second phases and no Al segregation at grain boundaries. These results suggest that Al₂O₃ and/or Al cannot act as a sintering aid by reducing grain boundary mobility. If Al₂O₃ acts as a sintering aid its main function is to enter the lattice as Al to increase the point defect concentration of the slowest moving species.     

Preliminary study of the crack healing and strength recovery of Al2O3-matrix composites

This study focused on the crack‐healing behaviour of three commercial Al₂O₃–ceramic‐matrix composites: TiC_P/Al₂O₃, ZrO₂/Al₂O₃ and SiC_W/Al₂O₃. Vickers indentation was used to introduce surface flaws with different loads of 49, 98 and 196 N. Then the cracked specimens were annealed in air for 1 h at 1000, 1200 and 1400 °C. The annealing treatment was also conducted at 1200 °C in vacuum for 1 h. Results showed that the annealing treatments increased the indentation strength, but the extent of the increase was different. When annealed in air, the main crack‐healing mechanism of TiC_P/Al₂O₃ and SiC_W/Al₂O₃ composites was chemical reaction. When annealed in vacuum, stress relaxation caused much less strength recovery. The main crack‐healing mechanism of ZrO₂/Al₂O₃ was the existence of low melting eutectic and the rearrangement of grains caused by ZrO_2(m)→ ZrO_2(t) transformation in the crack‐opening process zone. The effects of annealing temperature, atmosphere and indentation load on the degree of strength recovery were all related to the crack‐healing mechanisms.