Pressureless Densification of Zirconium Diboride with Boron Carbide Additions

Zirconium diboride (ZrB₂) was densified (>98% relative density) at temperatures as low as 1850°C by pressureless sintering. Sintering was activated by removing oxide impurities (B₂O₃ and ZrO₂) from particle surfaces. Boron oxide had a high vapor pressure and was removed during heating under a mild vacuum (∼150 mTorr). Zirconia was more persistent and had to be removed by chemical reaction. Both WC and B₄C were evaluated as additives to facilitate the removal of ZrO₂. Reactions were proposed based on thermodynamic analysis and then confirmed by X-ray diffraction analysis of reacted powder mixtures. After the preliminary powder studies, densification was studied using either as-received ZrB₂ (surface area ∼1 m²/g) or attrition-milled ZrB₂ (surface area ∼7.5 m²/g) with WC and/or B₄C as a sintering aid. ZrB₂ containing only WC could be sintered to ∼95% relative density in 4 h at 2050°C under vacuum. In contrast, the addition of B₄C allowed for sintering to >98% relative density in 1 h at 1850°C under vacuum.     

Oxidation of ZrB2–SiC: Influence of SiC Content on Solid and Liquid Oxide Phase Formation

The effect of SiC concentration on the liquid and solid oxide phases formed during oxidation of ZrB₂–SiC composites is investigated. Oxide-scale features called convection cells are formed from liquid and solid oxide reaction products upon oxidation of the ZrB₂–SiC composites. These convection cells form in the outermost borosilicate oxide film of the oxide scale formed on the ZrB₂–SiC during oxidation at high temperatures (≥1500°C). In this study, three ZrB₂–SiC composites with different amounts of SiC were tested at 1550°C for various durations of time to study the effect of the SiC concentration particularly on the formation of the convection cell features. A calculated ternary phase diagram of a ZrO₂–SiO₂–B₂O₃ (BSZ) system was used for interpretation of the results. The convection cells formed during oxidation were fewer and less uniformly distributed for composites with a higher SiC concentration. This is because the convection cells are formed from ZrO₂ precipitates from a BSZ oxide liquid that forms upon oxidation of the composite at 1550°C. Higher SiC-containing composites will have less dissolved ZrO₂ because they have less B₂O₃, which results in a smaller amount of precipitated ZrO₂ and consequently fewer convection cells.     

Zirconia Transport by Liquid Convection during Oxidation of Zirconium Diboride–Silicon Carbide

During high-temperature oxidation of ZrB₂–SiC composites, a multi-layer oxide scale forms with a silica-rich borosilicate liquid as the surface oxide layer. Here, a recently proposed novel mechanism for the high-temperature oxidation of ZrB₂–SiC composites is further investigated and verified. This mechanism involves the formation of convection cells in the oxide surface layer during high-temperature oxidation of the composite. The formation of zirconia deposits found in the center of the convection cells is proposed here to be the consequence of liquid transport. The nature and deposition mechanism of the zirconia is reported in detail, using calculated phase equilibrium diagrams and microstructure observations of a ZrB₂-15 vol% SiC composite tested at 1550° and 1700°C in ambient air for various times. The calculated phase equilibrium diagrams for the binary ZrO₂–B₂O₃ system as well as the ternary B₂O₃–SiO₂–ZrO₂ system at 1500°C are reported here to interpret these results.     

Rapid Oxidation Characterization of Ultra-High Temperature Ceramics

Here, a novel method for testing ultra-high-temperature ceramics (UHTC) at a high temperature, rapidly, at a low cost is introduced. A self-supported, self-heated ribbon specimen is used with a table-top apparatus to achieve the necessary high temperatures. This method enables a large temperature–time–composition parameter space to be covered by rapidly producing a large set of postoxidation samples for analysis. The complex oxide scale known to form during oxidation of UHTC materials is shown to be easily reproduced using this method. A ZrB₂–SiC (15 vol%) UHTC material was tested at 1700° C for 15 min. The oxide scale consists of a thin outermost silica (SiO₂) layer and a zirconia (ZrO₂) columnar layer with small amounts of SiO₂ embedded between the ZrO₂ columns. A region of SiC-depleted zone was observed between the unreacted core and the ZrO₂ layer. The measured thickness of the oxide scale was 102 μm and ∼120 μm for the SiC-depleted zone.     

New Route to Synthesize Ultra-Fine Zirconium Diboride Powders Using Inorganic–Organic Hybrid Precursors

Ultra-fine zirconium diboride (ZrB₂) powders have been synthesized using inorganic–organic hybrid precursors of zirconium oxychloride (ZrOCl₂·8H₂O), boric acid, and phenolic resin as sources of zirconia, boron oxide, and carbon, respectively. The reactions were substantially completed at a relatively low temperature (∼1500°C). The synthesized powders had a smaller average crystallite size (<200 nm), a larger specific surface area (∼32 m²/g), and a lower oxygen content (<1.0 wt%), which were superior to some commercially available ZrB₂ powders. The thermodynamic change in the ZrO₂–B₂O₃–C system was mainly studied by thermogravimetric and differential thermal analysis. The crystallite size and morphology of the synthesized powders were characterized by transmission electron microscopy and scanning electron microscopy.     

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 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 and Characterization of ZrB2-Based Ceramic Using Synthesized ZrB2–LaB6 Powder

ZrB₂–LaB₆ powder was obtained by reactive synthesis using ZrO₂, La₂O₃, B₄C, and carbon powders. Then ZrB₂–20 vol% SiC–10 vol% LaB₆ (ZSL) ceramics were prepared from commercially available SiC and the synthesized ZrB₂–LaB₆ powder via hot pressing at 2000°C. The phase composition, microstructure, and mechanical properties were characterized. Results showed that both LaB₆ and SiC were uniformly distributed in the ZrB₂ matrix. The hardness and bending strength of ZSL were 17.06±0.52 GPa and 505.8±17.9 MPa, respectively. Fracture toughness was 5.7±0.39 MPa·m^1/2, which is significantly higher than that reported for ZrB₂–20 vol% SiC ceramics, due to enhanced crack deflection and crack bridging near SiC particles.     

Oxidation Resistance of Fully Dense ZrB2 with SiC, TaB2, and TaSi2 Additives

Specimens of ZrB₂ containing various concentrations of B₄C, SiC, TaB₂, and TaSi₂ were pressureless-sintered and post-hot isostatic pressed to their theoretical densities. Oxidation resistances were studied by scanning thermogravimetry over the range 1150°–1550°C. SiC additions improved oxidation resistance over a broadening range of temperatures with increasing SiC content. Tantalum additions to ZrB₂–B₄C–SiC in the form of TaB₂ and/or TaSi₂ increased oxidation resistance over the entire evaluated spectrum of temperatures. TaSi₂ proved to be a more effective additive than TaB₂. Silicon-containing compositions formed a glassy surface layer, covering an interior oxide layer. This interior layer was less porous in tantalum-containing compositions.     

Effects of Phase Change and Oxygen Permeability in Oxide Scales on Oxidation Kinetics of ZrB2 and HfB2

A wide range of experimental data on the oxidation of ZrB₂ and HfB₂ as a function of temperature (800°–2500°C) is interpreted using a mechanistic model that relaxes two significant assumptions made in prior work. First, inclusion of the effect of volume change associated with monoclinic to tetragonal phase change of the MeO₂ phases is found to rationalize the observations by several investigators of abrupt changes in weight gain, recession, and oxygen consumed, as the temperature is raised through the transformation temperatures for ZrO₂ and HfO₂. Second, the inclusion of oxygen permeability in ZrO₂ is found to rationalize the enhancement in oxidation behavior at very high temperatures (>1800°C) of ZrB₂, while the effect of oxygen permeability in HfO₂ is negligible. Based on these considerations, the significant advantage of HfB₂ over ZrB₂ is credited to the higher transformation temperature and lower oxygen permeability of HfO₂ compared with ZrO₂.     

Homogeneous ZrO2–Al2O3 Composite Prepared by Nano-ZrO2 Particle Multilayer-Coated Al2O3 Particles

ZrO₂–Al₂O₃ nanocomposite particles were synthesized by coating nano-ZrO₂ particles on the surface of Al₂O₃ particles via the layer-by-layer (LBL) method. Polyacrylic acid (PAA) adsorption successfully modified the Al₂O₃ surface charge. Multilayer coating was successfully implemented, which was characterized by ξ potential, particle size. X-ray diffraction patterns showed that the content of ZrO₂ in the final powders could be well controlled by the LBL method. The powders coated with three layers of nano-ZrO₂ particles, which contained about 12 wt% ZrO₂, were compacted by dry press and cold isostatically pressed methods. After sintering the compact at 1450°C for 2 h under atmosphere, a sintered body with a low pore microstructure was obtained. Scanning electron microscopy micrographs of the sintered body indicated that ZrO₂ was well dispersed in the Al₂O₃ matrix.     

Microstructure of Sintered Mullite-Zirconia Composites

Fused mullite, pure and with 10 to 25 vol% ZrO₂ added, was milled in an attritor and sintered in air at temperatures near 1600°C to a dense fine-grained ceramic. The ZrO₂ promoted densification and retarded grain growth of the mullite phase. Transmission electron microscopy showed that compositions containing ZrO₂ were substantially free of glass, whereas pure mullite bodies contained a glassy phase. The ZrO₂ formed inter granular, isolated particles .     

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.     

Effects of Annealing Temperature on Microstructural Development at the Interface Between Zirconia and Titanium

The interfacial reaction layers in the Ti/ZrO₂ diffusion couples, isothermally annealed in argon at temperatures ranging from 1100° to 1550°C for 6 h, were characterized using scanning electron microscopy and transmission electron microscopy, both attached with an energy-dispersive spectrometer. Very limited reaction occurred between Ti and ZrO₂ at 1100°C. A β'-Ti(Zr, O) layer and a two-phase α-Ti(O)+β'-Ti(Zr, O) layer were found in the titanium side after annealing at T ≥1300°C and T ≥1400°C, respectively. A three-phase layer, consisting of Ti₂ZrO+α-Ti(O, Zr)+β'-Ti (O, Zr), was formed after annealing at 1550°C. In the zirconia side near the original interface, β'-Ti coexisted with fine spherical c- ZrO_{2− x} , which dissolved a significant amount of Y₂O₃ in solid solution at T ≥1300°C. Further into the ceramic side, the α-Zr was formed due to the exsolution of Zr out of the metastable ZrO_{2− x} after annealing at T ≥1300°C: the α-Zr was very fine and dense at 1300°C, continuously distributed along grain boundaries at 1400°C, and became coarsened at 1550°C. Zirconia grains grew significantly at T ≥1400°C, with the lenticular t -ZrO_{2− x} being precipitated in c -ZrO_{2− x} . Finally, the microstructural development and diffusion paths in the Ti/ZrO₂ diffusion couples annealed at various temperatures were also described with the aid of the Ti–Zr–O ternary phase diagram.     

Microstructure and Mechanical Properties of ZrB2-Based Composites Reinforced and Toughened by Zirconia

The present work reported the effect of addition of ZrO₂ on the microstructure and mechanical properties of ZrB₂-based ceramic composites by means of hot-pressed sintering. Observation of microstructure and systematic testing results of mechanical properties were carried out. Through X-ray diffraction analysis and calculation of the volume fraction of ZrO₂ phase transformability, the toughening mechanism of the present composites was explored. The phase transformation toughening by ZrO₂ additive played an important role in improving the fracture toughness of ZrB₂-based composites.     

Effect of Inclusions on Grain Growth of Oxides

Effects of 10 vol% Cr, Mo, W, Mn, Ni, Ti, BN, and ZrB₂ on the grain growth of Al₂O₃, HfO₂, MgO, and ZrO₂ were investigated by vacuum heat-treating previously vacuum hot-pressed specimens containing a layer of the cermet and a layer of the corresponding oxide. Most of the metals and compounds inhibit grain growth of the four oxides investigated. Some of the additions enhance grain growth, whereas others react with the matrix oxide. In most systems, the oxide layer adjacent to the cermet showed exaggerated grain growth at the interface. No correlation was found between reactivity, melting point, and volatility of the metal addition and grain growth of the matrix oxide.     

Polymer-Derived SiOC/ZrO2 Ceramic Nanocomposites with Excellent High-Temperature Stability

Polymer-derived SiOC/ZrO₂ ceramic nanocomposites have been prepared using two synthetic approaches. A commercially available polymethylsilsesquioxane (MK Belsil PMS) was filled with nanocrystalline zirconia particles in the first approach. The second method involved the addition of zirconium tetra( n -propoxide), Zr(OⁿPr)₄, as zirconia precursor to polysilsesquioxane. The prepared materials have been subsequently cross-linked and pyrolyzed at 1100°C in argon atmosphere to provide SiOC/ZrO₂ ceramics. The obtained SiOC/ZrO₂ materials were characterized by means of X-ray diffraction, elemental analysis, Raman spectroscopy as well as transmission electron microscopy. Furthermore, annealing experiments at temperatures from 1300° to 1600°C have been performed. The annealing experiments revealed that the incorporation of ZrO₂ into the SiOC matrix remarkably increases the thermal stability of the composites with respect to crystallization and decomposition at temperatures exceeding 1300°C. The results obtained within this study emphasize the enormous potential of polymer-derived SiOC/ZrO₂ composites for high-temperature applications.     

Titanium Zirconate Thick Films on Rutile Substrates by Reactive Coating of Zircon

A mechanically stable thick layer (∼SO pm) of ZrTiO₄ on a rutile substrate has been obtained at 1600°C by reactive coating of zircon. This layer is constituted by ZrTiO, grains of ∼10 μm average size embedded in a silica-rich glassy matrix. The results are discussed in terms of the ZrO₂-SOi₂-TiO₂, equilibrium diagram.     

Pressureless Sintering of Zirconium Diboride: Particle Size and Additive Effects

Zirconium diboride (ZrB₂) was densified by pressureless sintering using <4-wt% boron carbide and/or carbon as sintering aids. As-received ZrB₂ with an average particle size of ∼2 μm could be sintered to ∼100% density at 1900°C using a combination of boron carbide and carbon to react with and remove the surface oxide impurities. Even though particle size reduction increased the oxygen content of the powders from ∼0.9 wt% for the as-received powder to ∼2.0 wt%, the reduction in particle size enhanced the sinterability of the powder. Attrition-milled ZrB₂ with an average particle size of <0.5 μm was sintered to nearly full density at 1850°C using either boron carbide or a combination of boride carbide and carbon. Regardless of the starting particle size, densification of ZrB₂ was not possible without the removal of oxygen-based impurities on the particle surfaces by a chemical reaction.     

Strengthening of Oxide Ceramics by Transformation-Induced Stress

Alumina-zirconia composites were fabricated by isostatic pressing and sintering of powder mixtures in such a way that bar-shaped specimens consisted of three layers. The outer layers contained A1₂O₃ and unstabilized ZrO₂ while the central layer contained A1₂O₃ and partially stabilized ZrO₂ (with 2 mol% Y₂O₃). When cooled from the sintering temperature, some of the zirconia in the outer layers transformed to the monoclinic form while zirconia in the central layer was retained in the tetragonal form. The transformation of zirconia in the outer layers led to the establishment of surface compressive stresses and balancing tensile stresses in the bulk. The existence of surface compressive stresses was verified by a strain gauge technique and bending strength measurements on samples with varying thickness of the outer layers. The layered composites exhibited greater strength in comparison with monolithic Al₂O₃-ZrO₂ specimens. Further, variation of strength in bending with outer layer thickness (for a fixed total thickness) indicated that failure occurred from internal flaws. Scanning electron microscopy of fracture surfaces revealed that strength-limiting flaws were voids located in the central layer near the interface separating the central and the outer layers.