Low-Temperature Processing and Mechanical Properties of Zirconia and Zirconia–Alumina Nanoceramics

The 1.5- to 3-mol%-Y₂O₃-stabilized tetragonal ZrO₂ (Y-TZP) and Al₂O₃/Y-TZP nanocomposite ceramics with 1 to 5 wt% of alumina were produced by a colloidal technique and low-temperature sintering. The influence of the ceramic processing conditions, resulting density, microstructure, and the alumina content on the hardness and toughness were determined. The densification of the zirconia (Y-TZP) ceramic at low temperatures was possible only when a highly uniform packing of the nanoaggregates was achieved in the green compacts. The bulk nanostructured 3-mol%-yttria-stabilized zirconia ceramic with an average grain size of 112 nm was shown to reach a hardness of 12.2 GPa and a fracture toughness of 9.3 MPa·m^1/2. The addition of alumina allowed the sintering process to be intensified. A nanograined bulk alumina/zirconia composite ceramic with an average grain size of 94 nm was obtained, and the hardness increased to 16.2 GPa. Nanograined tetragonal zirconia ceramics with a reduced yttria-stabilizer content were shown to reach fracture toughnesses between 12.6–14.8 MPa·m^1/2 (2Y-TZP) and 11.9–13.9 MPa·m^1/2 (1.5Y-TZP).     

Study of Fracture Behavior of Very Fine-Grained Silicon Carbide Ceramics

Dense, fine-grained silicon carbide (SiC) ceramics were fabricated by a hot-pressing technique using pyrolyzed polycarbosilane powders. Hot-isostatic pressing treatments were also applied to some of these hot-pressed samples. The grainsize range of the obtained sintered bodies was from 0.2 to 1.4 μm, which was much finer than that of ordinary sintered SiC ceramics. Relationships among sintering conditions, microstructures, and fracture toughness of the obtained ceramics were investigated. A clear grain-size dependence of fracture toughness was observed in this very fine-grain region (0.2 to 1.4 μm). Fracture toughness showed its maximum (5.1 MPa.m^1/2) at the average grain size of ∼0.7 μm. Also, the fracture toughness of the samples having similar grain sizes increased with increasing relative density.     

Effect of Poling Direction on R-Curve Behavior in Lead Zirconate Titanate

R -curves of lead zirconate titanate (PZT) have been measured with compact tension (CT) specimens for different poling conditions and grain sizes. Depending on poling direction the plateau value of the R -curves ranged from 1.13 to 1.54 MPa·m^1/2 for a grain size of 6.4 μm and from 1.14 to 1.30 MPa·m^1/2 for a grain size of 5.2 μm. Poling in the thickness direction yielded the material with the highest fracture toughness while the direction parallel to the loading direction led to the lowest fracture toughness.     

Relationship between Microstructure, Toughening Mechanisms, and Fracture Toughness of Reinforced Silicon Nitride Ceramics

Different microstructures in Si₃N₄ ceramics containing Y₂O₃and Al₂O₃ as sintering additives were prepared by two-step sintering. Pull-out and elastic bridging were most frequently observed as the toughening mechanisms in samples with fine-grained microstructures having needlelike β-Si₃N₄grains with diameters of <1 μm. Crack deflection was the main toughening mechanism observed in samples with coarse-grained microstructures having grains with diameters of >1 μm. The values of fracture toughness were varied from 6.1 to 8.2 MPa·m^1/2 with respect to the microstructural characteristics, characterized by the volume fraction of needlelike grains and their diameter.     

Processing and Mechanical Properties of Zirconium Diboride-Based Ceramics Prepared by Spark Plasma Sintering

Zirconium diboride (ZrB₂) reinforced by nano-SiC whiskers has been prepared by spark plasma sintering (SPS). Of most interest is the densification of ZrB₂–SiCw composites accomplished by SPS at a temperature as low as 1550°C. The relative density of ZrB₂–SiCw composites could reach to 97% with an average grain size of 2–3 μm. Both flexural strength and fracture toughness of the composites were improved with increasing amount of SiCw. Flexural strengths ranged from 416 MPa for monolithic ZrB₂ to over 545 MPa for ZrB₂–15 vol% SiCw composites. Similarly, fracture toughness also increased from 5.46 MPa·m^1/2 to more than 6.81 MPa·m^1/2 in the same composition range. The relative density of ZrB₂–SiCw composites could be further improved to near 100% by adding some sintering aids such as AlN and Si₃N₄; however, the effects of different sintering additives on the mechanical properties of the composites were different.     

Microwave-Sintered MgO-Doped Zirconia with Improved Mechanical and Tribological Properties

This paper reports the results of microwave sintering (without post-sintering annealing) on the microstructure, phase assemblage, and properties of 8 mol% (PSZ—partially stabilized zirconia), and 16 mol% (FSZ—fully stabilized zirconia) MgO-alloyed zirconia. For the PSZ samples sintered at 1585°C, a maximum densification of ∼98%ρ_th, along with a hardness of ∼10.6 GPa and a fracture toughness of ∼6.8 MPa·m^1/2, was obtained. The results of tribological experiments on some selected samples revealed that a good combination of a lower coefficient of friction of 0.35 and a wear rate of 10⁻⁷ mm³/N m can be obtained with the optimally sintered Mg-PSZ.     

Mechanical Properties and Microstructure of Alumina-Glass Composites

Alumina blanks were prepared via isostatic pressing and sintering at 1400°C for 2 h, whereas alumina-glass composites were prepared by infiltrating the molten glass into the partially sintered alumina compacts. The samples had a high bending strength (340 MPa) and high fracture toughness (3.91 MPa·m^1/2) and were free of shrinkage. Concurrently, zirconia (5 wt%) was used as an additive to alumina to improve the mechanical properties of both partially sintered alumina and alumina-glass composites. Results show that zirconia notably improves the mechanical properties of the partially sintered alumina but increases that of the composites by no more than 10%.     

Compaction and Pressureless Sintering of Zirconia Nanoparticles

Four nanometer-sized zirconia powders stabilized by 3 mol% Y₂O₃ were used for the preparation of dense bulk ceramics. Ceramic green bodies were prepared by cold isostatic pressing at pressures of 300–1000 MPa. The size of the pores in ceramic green bodies and their evolution during sintering were correlated with the characteristics of individual nanopowders and with the sintering behavior of powder compacts. Only homogeneous green bodies with pores of <10 nm could be sintered into dense bodies (>99% t.d.) at a sufficiently low temperature to keep the grain sizes in the range <100 nm. Powders with uniform particles 10 nm in size yielded green bodies of required microstructure. These nanoparticle compacts were sintered without pressure to give bodies (diameter 20 mm, thickness 4 mm) with a relative density higher than 99% and a grain size of about 85 nm (as determined by the linear intercept method).     

Mechanical Properties of Dry-Pressed Powder Compacts: Case Study on Alumina Nanoparticles

Young's modulus and fracture toughness of dry-pressed powder compacts of nanocrystalline alumina powders have been determined for different relative densities and average grain size. The powder with a grain size of 40 nm yielded a maximum Young's modulus of 15 GPa and a peak fracture toughness of 0.12 MPa·m^1/2. These high values are rationalized using a model based on contact flattening between particle spheres, which allows a discussion of the influence of grain size and particle packing on the mechanical properties of green bodies.     

Relationship between Microstructure and Fracture Toughness of Toughened Silicon Carbide Ceramics

Different microstructures in SiC ceramics containing Al₂O₃, Y₂O₃, and CaO as sintering additives were prepared by hot-pressing and subsequent annealing. The microstructures obtained were analyzed by image analysis. Crack deflection was frequently observed as the toughening mechanism in samples having elongated α-SiC grains with aspect ratio >4, length >2 μm, and grain thickness ( t ) <3 μm (defined as key grains 1). Crack bridging was the dominant toughening mechanism observed in samples having grains with thickness of 1 μm < t < 3 μm and length >2 μm (key grains 2). The values of fracture toughness varied from 5.4 to 8.7 MPa·m^1/2 with respect to microstructural characteristics, characterized by mean grain thickness, mean aspect ratio, and total volume fraction of key grains. The difference in fracture toughness was mainly attributed to the amount of key grains participating in the toughening processes.     

Pressureless Sintering of High-Density ZrB2–SiC Ceramic Composites

Ultra-high-temperature ceramic composites of ZrB₂ 20 wt%SiC were pressureless sintered under an argon atmosphere. The starting ZrB₂ powder was synthesized via the sol–gel method with a small crystallite size and a large specific surface area. Dry-pressed compacts using 4 wt% Mo as a sintering aid can be pressureless sintered to ∼97.7% theoretical density at 2250°C for 2 h. Vickers hardness and fracture toughness of the sintered ceramic composites were 14.82±0.25 GPa and 5.39±0.13 MPa·m^1/2, respectively. In addition to the good sinterability of the ZrB₂ powders, X-ray diffraction and scanning electron microscopy results showed that Mo formed a solid solution with ZrB₂, which was believed to be beneficial for the densification process.     

Pulse Electric Current Sintering of Al2O3/3 vol% ZrO2 with Constrained Grains and High Strength

The pulse electric current sintering technique (PECS) was demonstrated to be effective in rapid densification of fine-grained Al₂O₃/3Y-ZrO₂ using available commercial powders. The composites attained full densification (>99% of TD) at 1450°C in less than 5 min. The composites sintered at a high heating rate had a fine microstructure. The incorporation of 3 vol% 3Y-ZrO₂ substantially increased the average fracture strength and the toughness of alumina to as high as 827 MPa and 6.1 MPa·m^1/2, respectively. A variation in the heating rate during the PECS process influenced grain size, microstructure, and strength, though there was little or no variation in the fracture toughness.     

SiC/ZTM Nanocomposite with Needlelike Mullite Grains and Enhanced Mechanical Properties

Zirconia-toughened mullite (SiC/ZTM) nanocomposites were prepared by a chemical precipitation method. The samples showed good sinterability and could be densified to >98.7% of the theoretical density at 1350°–1550°C. Because of the addition of mullite seeds in the starting powder and the pinning effects of ZrO₂ and SiC particles on mullite grain growth, a fine-grained microstructure formed. Mullite grains were generally equiaxed for the sample sintered at 1400°C; whereas, for the sample sintered at 1550°C, most mullite grains took a needlelike morphology, and SiC particles were primarily located within mullite grains. The strength and toughness increased with the increasing sintering temperature, and reached their respective maximum of 780 MPa and 3.7 MPa·m^1/2 for the sample sintered at 1550°C.     

Pressureless Sintering of Zirconium Diboride Using Boron Carbide and Carbon Additions

The synergistic roles of boron carbide and carbon additions in the enhanced densification of zirconium diboride (ZrB₂) by pressureless sintering have been studied. ZrB₂ was sintered to >99% relative density at 1900°C. The combination of 2 wt% boron carbide and 1 wt% carbon promoted densification by removing surface oxide impurities (ZrO₂ and B₂O₃) and inhibiting grain growth. Four-point bending strength (473±43 MPa), Vickers' microhardness (19.6±0.4 GPa), fracture toughness (3.5±0.6 MPa·m^1/2), and Young's modulus (507 GPa) were measured. Thermal gravimetry showed that the combination of additives did not have an adverse effect on the oxidation behavior.     

Fabrication of Elongated α-SiAlON via a Reaction-Bonding Process

A reaction-bonding process, which offers low sintering shrinkage and is a low-cost process, was applied to fabricate Y–α-SiAlON ceramics. The green compacts composed of Si, Y₂O₃, Al₂O₃, and AlN were nitrided and subsequently postsintered. Dense single-phase Y–α-SiAlON with elongated grain morphology could be achieved in the specimen postsintered at 1900°C. The material exhibited high hardness (1850 HV10) and high fracture toughness (5.1 MPa·m^1/2).     

Spark Plasma Sintering of Magnesia-Doped Alumina with High Hardness and Fracture Toughness

The effect of grain size of magnesia and its content as well as spark plasma sintering conditions on the density, grain size, strength, hardness, and toughness of alumina was investigated. Spark plasma sintering conditions were optimized at 1150°C/5 min/175°C/min. Addition of 100 nm magnesia gave higher density levels (99.5%), while better strength (600 MPa), hardness (25 GPa), and fracture toughness (4.5 MPa·m^1/2) were obtained with 15 nm magnesia. The good strength and hardness is attributed to the submicrometer grain size of the matrix, and the improved toughness to the presence of Mg-rich nanoparticles and nanopores at grain boundaries.     

Effect of Rare-Earth Dopants on Mechanical Properties of Alumina

We report here about the effect of rare-earth dopants on the improvement of room-temperature mechanical properties of alumina. Rare-earth ions (RE = Yb³⁺, Er³⁺, and La³⁺) of different ionic radii in a minimum concentration of 1000 ppm were added as dopants individually to high-purity alumina and densified by pressureless sintering. High strength of about 700 MPa was attained for Yb-doped alumina sintered at 1400°C. And, high toughness of about 7.0 MPa·m^1/2 was attained for Er- and La-doped alumina samples.     

Precoarsening to Improve Microstructure and Sintering of Powder Compacts

MgO and Al₂O₃ were sintered by two types of processes: a conventional isothermal sintering and a two-step sintering consisting of an initial low-temperature precoarsening treatment before conventional isothermal sintering. The final microstructure from two-step sintering can be more uniform and finer than that of compacts sintered conventionally. A narrow-size-distribution alumina powder was sintered under constant-heating-rate conditions, with and without a precoarsening treatment, and the results were compared. The differences between two-step and conventional processing were clarified by experiments on precoarsened and as-received ZnO powders. These compacts were precoarsened at 450°C for 90 h with virtually no increase in the overall density. The resulting grain size was 1.7 times the starting one, but the standard deviation of the precoarsened powder size distribution was smaller than that of the asreceived powder. Precoarsened compacts sintered to nearly full density showed improved homogeneity. The sintering stress of the precoarsened ZnO was approximately 0.8 that of the as-received one. A computational model has been used with two components of coarsening to describe the differences in pore spacing evolution between the precoarsened and the as-received system. The benefit of two-step sintering is attributed to the increase in uniformity resulting from precoarsening. The increased uniformity decreases sintering damage and allows the system to stay in the open porosity state longer, delaying or inhibiting additional coarsening (grain growth) during the final stage of densification. Two-step sintering is especially useful for nonuniform powder systems with a wide size distribution and is a simple and convenient method of making more uniform ceramic bodies without resorting to specialized powders or complicated heat schedules.     

Mechanical Properties of Textured Alumina Made by High-Temperature Deformation

The mechanical properties of a textured alumina made by high-temperature deformation of normal-purity sintered alumina have been investigated. The textured alumina shows very high bending strength and extremely high fracture toughness. Fracture toughness of more than 10 MPa·m^1/2 was measured by the single-edge precracked beam method, and even using the single-edge V-notched beam method, toughness of over 8 MPa·m^1/2 was obtained. This high fracture toughness was attributed to a large number of aligned small platelike grains of the textured structure enhancing the grain bridging effect.     

Mechanical Properties of La1-xSrxCo0.2Fe0.8O3 Mixed-Conducting Perovskites Made by the Combustion Synthesis Technique

This paper examined the room-temperature mechanical properties of a mixed-conducting perovskite La_{1– x} Sr _x Co_0.2Fe_0.8O₃ ( x = 0.2–0.8). Powders were made by the combustion synthesis technique and sintered at 1250°C in air. Sintered density, crystal phase, and grain size were characterized. Young's and shear moduli, microhardness, indentation fracture toughness, and biaxial flexure strength were determined. The Young's and shear moduli slightly increased with increasing strontium content. Young's modulus of 151–188 GPa and shear modulus of 57–75 GPa were measured. Biaxial flexure strength of ∼160 MPa was measured for lower strontium content batches. Strength greatly decreased to ∼40 MPa at higher strontium concentrations ( x = 0.6–0.8) because of the formation of extensive cracking. Indentation toughness showed a higher value (∼1.5 MPa·m^1/2) for low strontium ( x = 0.2) content and a lower value (∼1.1 MPa·m^1/2) for the other batches ( x = 0.4–0.8). Materials with fine and coarse grain size were also tested at various indent loads and showed no dependence of toughness on crack size. In addition, fractography was used to characterize the critical flaw and fracture mode.