Characteristics of ZrO2-Dispersed Si3N4 without Additives Fabricated by Hot Isostatic Pressing

Dense, ZrO₂-dispersed Si₃N₄ composites without additives were fabricated at 180 MPa and ∼1850° to 1900°C for l h by hot isostatic pressing using a glass-encapsulation method; the densities reached >96% of theoretical. The dispersion of 20 wt% of 2.5YZrO₂ (2.5 mol% Y₂O₃) in Si₃N₄ was advantageous to increase the room-temperature fracture toughness (∼7.5 MPa˙m^1/2) without degradation of hardness (∼15 GPa) because of the high retention of tetragonal ZrO₂. The dependence of fracture toughness of Si₃N₄–2.5YZrO₂ on ZrO₂ content can be related to the formation of zirconium oxynitride because of the reaction between ZrO₂ and Si₃N₄ matrix in hot isostatic pressing.     

Microstructure and Properties of Spark Plasma-Sintered ZrO2–ZrB2 Nanoceramic Composites

In a recent work, ¹ we have reported the optimization of the spark plasma sintering (SPS) parameters to obtain dense nanostructured 3Y-TZP ceramics. Following this, the present work attempts to answer some specific issues: (a) whether ZrO₂-based composites with ZrB₂ reinforcements can be densified under the optimal SPS conditions for TZP matrix densification (b) whether improved hardness can be obtained in the composites, when 30 vol% ZrB₂ is incorporated and (c) whether the toughness can be tailored by varying the ZrO₂–matrix stabilization as well as retaining finer ZrO₂ grains. In the present contribution, the SPS experiments are carried out at 1200°C for 5 min under vacuum at a heating rate of 600 K/min. The SPS processing route enables retaining of the finer t -ZrO₂ grains (100–300 nm) and the ZrO₂–ZrB₂ composite developed exhibits optimum hardness up to 14 GPa. Careful analysis of the indentation data provides a range of toughness values in the composites (up to 11 MPa·m^1/2), based on Y₂O₃ stabilization in the ZrO₂ matrix. The influence of varying yttria content, t -ZrO₂ transformability, and microstructure on the properties obtained is discussed. In addition to active contribution from the transformation-toughening mechanism, crack deflection by hard second phase brings about appreciable increment in the toughness of the nanocomposites.     

Pressureless Sintering and Mechanical and Biological Properties of Fluor-hydroxyapatite Composites with Zirconia

Fluor-hydroxyapatite (FHA) fabricated by a reaction between fluorapatite (FA) and hydroxyapatite (HA) was mixed with ZrO₂ to produce FHA–ZrO₂ composites. When the relative amount of FA to HA increased, the decomposition of the composite was decreased gradually because of the formation of thermally stable FHA solid solutions. With such suppression of decomposition, the FHA–ZrO₂ composites retained fully densified bodies. As a result, significant enhancements in mechanical properties, such as hardness, flexural strength, and fracture toughness, were achieved as the relative amount of FA to HA increased. The highest values in strength and toughness were 220 MPa and 2.5 MPa·m^1/2, respectively, with FHA–40 vol% ZrO₂ composites. In vitro proliferation of osteoblast-like cells (MG63) on the composites showed behavior similar to that observed on pure HA and FHA. Alkaline phosphatase (ALP) activity of the growing cells (HOS) on the composites was slightly down-regulated compared with that on pure HA and FHA at prolonged periods.     

Processing and Mechanical Behavior of CrN/ZrO2(2Y) Composites

CrN powder consisting of granular particles of ∼3 μm has been prepared by self-propagating high-temperature synthesis under a nitrogen pressure of 12 MPa using Cr metal. Dense pure CrN ceramics and CrN/ZrO₂(2Y) composites in the CrN-rich region have been fabricated by hot isostatic pressing for 2 h at 1300°C and 196 MPa. The former ceramics have a fracture toughness ( K _IC) of 3.3 MPa ·m^1/2 and a bending strength (σ_b) of 400 MPa. In the latter materials almost all of the ZrO₂(2Y) grains (0.36–0.41 μm) are located in the grain boundaries of CrN (∼4.6 μm). The values of K _IC (6.1 MPa · m^1/2) and σ_b (1070 MPa) are obtained in the composites containing 50 vol% ZrO₂(2Y).     

Spark Plasma Sintering of Alumina

A systematic study of various spark plasma sintering (SPS) parameters, namely temperature, holding time, heating rate, pressure, and pulse sequence, was conducted to investigate their effect on the densification, grain-growth kinetics, hardness, and fracture toughness of a commercially available submicrometer-sized Al₂O₃ powder. The obtained experimental data clearly show that the SPS process enhances both densification and grain growth. Thus, Al₂O₃ could be fully densified at a much lower temperature (1150°C), within a much shorter time (minutes), than in more conventional sintering processes. It is suggested that the densification is enhanced in the initial part of the sintering cycle by a local spark-discharge process in the vicinity of contacting particles, and that both grain-boundary diffusion and grain-boundary migration are enhanced by the electrical field originating from the pulsed direct current used for heating the sample. Both the diffusion and the migration that promote the grain growth were found to be strongly dependent on temperature, implying that it is possible to retain the original fine-grained structure in fully densified bodies by avoiding a too high sintering temperature. Hardness values in the range 21–22 GPa and fracture toughness values of 3.5 ± 0.5 MPa·m^1/2 were found for the compacts containing submicrometer-sized Al₂O₃ grains.     

Direct Synthesis and Sintering of Silicon Nitridenitanium Nitride Composite

Electrical discharge machining is an attractive machining technique which requires electrically conductive ceramic materials. Silicon nitride based composites with 35–40% of TiN possess electrical conductivity high enough to use this machining technique. The composite powder was prepared by combustion synthesis and densified by pressure sintering (sintering aids 6 wt% of Y₂O₃ and 4 wt% of A1,0₃). High fracture toughness, K _c 10.5 MPa^.m^1/2, is typical for the composite. It can be expected that this material can compete with pure silicon nitride in applications requiring complex machining.     

Mechanical Properties of Hot Isostatically Pressed Zirconia-Toughened Alumina Ceramics Prepared from Coprecipitated Powders

Amorphous Al₂O₃–ZrO₂ composite powders with 5–30 mol% ZrO₂ have been prepared by adding aqueous ammonia to the mixed solution of aqueous aluminum sulfate and zirconium alkoxide containing 2-propanol. Simultaneous crystallization of γ-Al₂O₃ and t -ZrO₂ occurs at 870°–980°C. The γ-Al₂O₃ transforms to α-Al₂O₃ at 1160°–1220°C. Hot isostatic pressing has been performed for 1 h at 1400°C under 196 MPa using α-Al₂O₃– t -ZrO₂ composite powders. Dense ZrO₂-toughened Al₂O₃ (ZTA) ceramics with homogeneous-dispersed ZrO₂ particles show excellent mechanical properties. The toughening mechanism is discussed. The microstructures and t / m ratios of ZTA are examined, with emphasis on the relation between strength and fracture toughness.     

Reaction Bonding and Mechanical Properties of Mullite/Silicon Carbide Composites

Based on the RBAO technology, low-shrinkage mullite/SiC/ Al₂O₃/ZrO₂ composites were fabricated. A powder mixture of 40 vol% Al, 30 vol% A1₂O₃ and 30 vol% SiC was attrition milled in acetone with TZP balls which introduced a substantial ZrO₂ wear debris into the mixture. The precursor powder was isopressed at 300–900 MPa and heattreated in air by two different cycles resulting in various phase ratios in the final products. During heating, Al oxidizes to Al₂O₃ completely, while SiC oxidizes to SiO₂ only on its surface. Fast densification (at >1300°C) and mullite formation (at 1400°C) prevent further oxidation of the SiC particles. Because of the volume expansion associated with the oxidation of Al (28%), SiC (108%), and the mullitization (4.2%), sintering shrinkage is effectively compensated. The reaction-bonded composites exhibit low linear shrinkages and high strengths: shrinkages of 7.2%, 4.8%, and 3%, and strengths of 610, 580, and 490 MPa, corresponding to compaction pressure of 300, 600, and 900 MPa, respectively, were achieved in samples containing 49–55 vol% mullite. HIPing improved significantly the mechanical properties: a fracture strength of 490 MPa and a toughness of 4.1 MPa^.m^1/2 increased to 890 MPa and 6 MPa^.m^1/2, respectively.     

High-Toughness Silicon Carbide as Armor

Silicon carbide, with single-edge precracked beam (SEPB) toughness greater than 7 MPa·m^1/2, was made by hot-pressing using Al–B–C (ABC) or Al–Y₂O₃ (YAG) as additives. The hardness of SiC processed with a liquid phase was always less than SiC densified without a liquid phase despite having a similar or finer grain size. With increasing Al content, the ABC system changed from trans- to intergranular fracture with a drop in hardness and a two- to threefold increase in SEPB toughness. Strength and Weibull modulus for materials processed with a liquid phase were higher than those of solid-state densified SiC. Ballistic testing, however, did not show any improvement over SiC densified with B and C additives. Depth of penetration was controlled by hardness of the SiC-based materials, while V ₅₀ values for 14.5 mm WC–Co cored projectiles were in the range of 720–750 m/s for all materials tested.     

Effect of Agglomeration on Mechanical Properties of Porous Zirconia Fabricated by Partial Sintering

Porous ZrO₂ ceramics were fabricated by compacting a fine ZrO₂ powder, followed by pressureless sintering. Two unidirectional pressures of 30 and 75 MPa were used to prepare the green compacts. The strength and the fracture toughness of porous ZrO₂ specimens sintered from the compacts prepared by 75 MPa were substantially higher than those by 30 MPa, especially for the specimens with low porosity. However, the corresponding Young's moduli were identical. This caused the strain to failure of these porous bodies to increase significantly with increasing compaction pressure. Microstructural analyses showed that a number of voids and small flaws existed in the green compacts prepared by the lower pressure, due to the agglomeration of fine ZrO₂ grains. It was revealed that the ZrO₂ agglomeration resulted in a localized nonuniform shrinkage and degraded the mechanical properties of porous ZrO₂ ceramics.     

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).     

Effect of Aqueous Processing Conditions on the Microstructure and Transformation Behavior in Al2O3─ZrO2(CeO2) Composites

Aqueous processing of Al₂O₃─ZrO₂ (123 mol% CeO₂) composites, combined with sintering conditions, was used to control the microstructure and its influence on the martensitic transformation temperature of t -ZrO₂ and the transformation-toughening contribution at room temperature. The resultant ZrO₂ grain sizes in the dense composites were related to the transformation-toughening behavior of t -ZrO₂. The data show that (1) the best processing conditions exist when the electrophoretic mobilities of the two solids are positive, adequately high to ensure colloidal stability, efficient packing,and uniform ZrO₂ distribution but differ greatly in magnitude, (2) the colloidal stability of ZrO₂ controls the overall stability and the rheological and processing behavior of this mixture, (3) the grain size distribution in dense pieces sintered for 1 h at 1500°C is comparable to the particle size distribution of the powders, (4) the martensite start temperature for the tetragonal to-monoclinic transformation in Al₂O₃ containing 20 and 40 vol% ZrO₂ increases and can approach 0°C with increasing average ZrO₂ grain size, and as a result, (5) the fracture toughness values at room temperature are raised from 4–5 MPa.m^1/2 to 9–12 MPa.m^1/2 for these two compositions.     

ZrO2-Transformation-Toughened Glass-Ceramics Prepared by the Sol-Gel Process from Metal Alkoxides

Glasses of composition 3ZrO₂O · 2SiO₂ were prepared by the sol-gel process from metal alkoxides. Tetragonal ZrO₂ was precipitated by appropriate heat treatment at 1000° to 1200°C. The fracture toughness of these glass-ceramics increased with increasing crystallite size of the tetragonal ZrO₂, reaching ∼5.0 MN/m^3/2 at a size of ∼40 nm. The higher fracture toughness was attributed to tetragonal → monoclinic ZrO₂ transformation toughening.     

Synthesis of Nanocrystalline Zirconium Nitride Powders by Reduction–Nitridation of Zirconium Oxide

Nanocrystalline ZrN powder was synthesized by reduction–nitridation of nanosized ZrO₂ powder in ammonia gas with magnesium as the reducing agent. The effects of nitridation temperature, holding time, and Mg:ZrO₂ mole ratio on the powder properties were investigated. Cubic phase ZrN powder with a 30–100-nm particle size was synthesized at 1000°C for 6 h, under a Mg:ZrO₂ mole ratio of 10:1.     

Mechanical and Microstructural Characterization of Mullite and Mullite-SiC-Whisker and ZrO2-Toughened-Mullite—SiC-Whisker Composites

High-purity mullite-SiC-whisker composites and mullite-ZrO₂-SiC-whisker composites were fabricated in situ by hot-pressing using a matrix prepared by the alkoxide process. Varying degrees of ZrO₂ stabilization were achieved by varying amounts of Y₂O₃ or MgO addition. Microstructural characterization was accomplished using SEM and energy dispersive analysis. Room-temperature flexural strength and fracture toughness were determined as a function of SiC-whisker content (0% to 30%) and ZrO₂-stabilizer content. The flexural strength of mullite varied with composition and was increased ∼50% by the addition of ∼30% ZrO₂ phase. The flexural strength of mullite and mullite + 30% ZrO₂ was increased ∼50% for 30% SiC-whisker additions. The fracture toughness of mullite + 30% ZrO₂ was nearly twice that of mullite. For a 30% SiC-whisker addition, the fracture toughness of mullite was doubled, and the fracture toughness of mullite + 30% ZrO₂ was increased 25% to 50%.     

Microstructure and Fracture Toughness of Hot-Pressed Zirconia-Toughened Sialon

Zirconia-toughened sialon composites have been fabricated using conventional hot-pressing techniques. The fracture toughness and microstructure were determined for CeO₂-and Y₂O₃-stabilized ZrO₂ additives and also as a function of volume percent ZrO₂. The yttria system showed a linear increase in fracture toughness with increasing volume fraction zirconia content while the ceria-stabilized system exhibited a peak in fracture toughness at 20 vol % ZrO₂ content. The fracture toughness at 800°C was measured and correlated with the microstructure. High-temperature stability was determined and it was found that the deleterious nitride phases of zirconium could be precluded from the microstructure.     

Properties of (Y)ZrO2–Al2O3 and (Y)ZrO2–Al2O3–(Ti or Si)C Composites

The effect of Al₂O₃ and (Ti or Si)C additions on various properties of a (Y)TZP (yttria-stabilized tetragonal zirconia polycrystal)–Al₂O₃–(Ti or Si)C ternary composite ceramic were investigated for developing a zirconia-based ceramic stronger than SiC at high temperatures. Adding Al₂O₃ to (Y)TZP improved transverse rupture strength and hardness but decreased fracture toughness. This binary composite ceramic revealed a rapid loss of strength with increasing temperature. Adding TiC to the binary ceramic suppressed the decrease in strength at temperatures above 1573 K. The residual tensile stress induced by the differential thermal expansion between ZrO₂ and TiC therefore must have inhibited the t - → m -ZrO₂ martensitic transformation. It was concluded that a continuous skeleton of TiC prevented grain-boundary sliding between ZrO₂ and Al₂O₃. In contrast, for the ternary material containing β-SiC in place of TiC, the strength decreased substantially with increasing temperature because of incomplete formation of the SiC skeleton.     

Development and Characterization of Y2O3-Stabilized ZrO2 (Y-TZP) Composites with TiB2, TiN, TiC, and TiC0.5N0.5

Up to 50 vol% of TiB₂, TiC_0.5N_0.5, TiN, or TiC was added to Y₂O₃-stabilized tetragonal ZrO₂ polycrystals (Y-TZP) and hot pressed under vacuum. The influence of the type of secondary phase on the microstructure and mechanical properties was studied, as a function of the hot-pressing temperature. The influence of the secondary-phase content on the mechanical properties was studied by varying the TiB₂ content up to 50 vol%. Fully dense Y-TZP-based composites with very high toughness (up to 10 MPa·m^1/2), excellent bending strength (up to 1237 MPa), and increased hardness, with respect to ZrO₂ (Vickers hardness up to 1450 kg/mm²), were obtained.     

Reaction Sintering and Properties of Silicon Oxynitride Densified by Hot Isostatic Pressing

Silicon oxynitride ceramics were reaction sintered and fully densified by hot isostatic pressing in the temperature range 1700°C to 1950°C from an equimolar mixture of silicon nitride and silica powders without additives. Conversion to Si₂N₂O increases steeply from a level around 5% of the crystalline phases at 1700°C to 80% at 1800°C, and increases a few percent further at higher temperatures. α -Si₃N₄ is the major residual crystalline phase below 1900°C. The hardness level for materials containing 85% Si₂N₂O is approximately 19 GPa, comparable with the hardness of Si₃N₄ hot isostatically pressed with 2.5 wt% Y₂O₃, while the fracture toughness level is around 3.1 MPa. m^1/2, being approximately 0.8 MPa.m^1/2 lower. The three-point bending strength increased with HIP temperature from approximately 300 to 500 MPa.     

Nd2O3-Doped Sialons with ZrO2/ZrN Additions Formed by Sintering and Hot Isostatic Pressing

β-sialon and Nd₂O₃-doped α-sialon materials of varying composition were prepared by sintering at 1775° and 1825°C and by glass-encapsulated hot isostatic pressing at 1700°C. Composites were also prepared by adding 2–20 wt% ZrO₂ (3 mol% Nd₂O₃) or 2–20 wt% ZrN to the β-sialon and α-sialon matrix, respectively. Neodymium was found to be a fairly poor α-sialon stabilizer even within the α-phase solid solution area, and addition of ZrN further inhibited the formation of the α-sialon phase. A decrease in Vickers hardness and an increase in toughness with increasing content of ZrO₂(Nd₂O₃) or ZrN were seen in both the HIPed β-sialon/ZrO₂(Nd₂O₃) composites and the HIPed Nd₂O₃-stabiIized α-sialons with ZrN additions.