Oxide Sintering Additives Research Articles

Environmental impact on the strength of subcritical cracking AlON material

Subcritical crack growth is often observed in oxide ceramics. This effect is ascribed to the interaction between the strained chemical bonds at the crack tip and water molecules present in the working environment of the material. Owing to this reaction, combined with stresses at the crack tip, slow cracking of the material and relevant strength reduction may occur. In non-oxide ceramics (i.e., without significant oxide sintering additives), no subcritical crack growth was observed. The present work demonstrates slow crack propagation in an aluminum oxynitride (AlON) material. AlON samples were prepared via sintering in nitrogen at 1800 °C. This resulted in dense translucent samples. Constant stress rate tests were carried out in both air and water to assess the influence of water on material strength. The strength results were used to further evaluate the effects of subcritical cracking. A clear decrease in strength was observed at low stress rates. Measurement points below 10 MPa/s were used to determine the lowest stress rate for inert strength. The slope of the fitting line indicated the value of subcritical cracking parameter n.

Improved additive manufacturing of silicon carbide parts via pressureless electric field‐assisted sintering

High solids loading silicon carbide (SiC)-based aqueous slurries containing only .5 wt. % organic additives were utilized to create specimens of various geometries via an extrusion-based additive manufacturing (AM) technique. Pressureless electric field-assisted sintering was performed to densify each specimen without deformation. The combination of these techniques produced parts with >98% relative density despite containing only 5 wt.% oxide sintering additives. After sintering, specimens contained only the α-SiC and yttrium aluminum perovskite phases. This suggests the evolution of a nonequilibrium yttrium aluminate phase, as well as transformation from β-SiC to α-SiC. The fabrication method presented in this work has advantages over other AM techniques commonly used with SiC, because it does not require significant organic additives nor additional postprocessing steps such as chemical vapor infiltration or polymer impregnation and pyrolysis.

High-Temperature Ceramic Composites (SiC/SiCw)

The prospects for using ceramic composite materials in place of metallic materials in heat-loaded units in advanced aircraft are examined. The manufacture of silicon carbide based ceramic composite materials, reinforced with filamentary crystals (whiskers), by means of hot-pressing and spark plasma sintering using oxide and nitride sintering additives was investigated. A complex of physical-mechanical and thermal properties of composites was investigated.

Preliminary study of sintering zero‐rupture Fully Ceramic Microencapsulated (FCM) fuel

AbstractThe demonstration of industrially feasible zero‐rupture Fully Ceramic Microencapsulated (FCM) fuels has been achieved via layers of fuel particles vertically stacked and consolidated by Pulsed Electric Current Sintering. Uniaxial shrinkage, a key component of fuel arrangement, was analyzed via SEM, XRD and computational analyses, using cylindrical geometries from 0.25 to 1.5 “height‐to‐diameter” (h/d) aspect ratio. Bulk densities of ~3.1 g/cc were achieved on pellets at h/d ~0.25, using a ramp rate of 100°C per min to 1825°C and holding for 10 minutes with 10 MPa applied pressure. Using the same parameters, this value reduced to ~2.8 g/cc at h/d ~1.5. More extensive percolation of oxide sintering additives and earlier closure of interconnected pores in extreme ends of the pellet was attributed to higher temperatures in those regions. The top and bottom of the pellet densify before the center, also resulting reduced displacement between the fuel layers in these regions. Parametric studies of sintering with different temperature and time, with support of computational analyses, was used to map expected temperature distribution and sintering behavior. These tools will be used for further optimization of FCM fuel fabrication.

Effect of heat treatment on the creep behavior of α/β SiAlON sintered with multication oxide sintering additives

AbstractSiAlONs are important materials for high‐temperature applications and creep properties of SiAlONs are largely controlled by the amount and type of sintering additives. It has been established that heat treatment can reduce the amount of amorphous intergranular phase through crystallization. However, there is no study on the creep behavior of heat‐treated SiAlON ceramics containing multication sintering additives. Therefore, the aim of the study was to investigate the effect of heat treatment on the creep properties of multication containing (Y‐Sm‐Ca oxides) α/β‐SiAlON ceramics. The heat treatments of the sintered samples were carried out at 1600°C for 2 hours. The creep tests were carried out in the range 1300‐1400°C under different loads (50‐150 MPa). The existing phases and the microstructures of samples before and after creep were investigated using XRD and SEM techniques. It was found that heat treatment resulted in a better creep performance compared to as‐sintered samples. The activation energy and stress exponent for heat‐treated SiAlONs were also calculated as 708 ± 45 kJ/mol and 1.4, respectively. Compared to the sintered sample values, the results suggested that the acting creep mechanism of grain‐boundary sliding and cavitation was reduced with the heat treatment.

Oxidation resistance of SiC ceramics prepared by different proceessing routes

The oxidation behaviour at 1350–1450°C/0–204h of SiC ceramics was investigated as a function of the processing route. SiC ceramics were prepared by rapid hot-pressing of the granulated SiC powder without any oxide sintering additives. This method allows decreasing the sintering temperature around 200°C than that conventionally used. Additive free rapid hot pressed SiC show better oxidation resistance compared to hot pressed liquid phase sintered SiC (the parabolic oxidation rates are about three orders of magnitudes lower). This new way prepared SiC ceramics show an interesting combination of properties like high Vickers hardness of 27.4GPa, good fracture toughness of 3.42MPam1/2, together with excellent oxidation resistance (4.91×10−5 mg2/cm4h at 1450°C).

Neutron Irradiation Effects of Oxide Sintering Additives for SiCf/SiC Composites

Abstract Silicon carbide fiber-reinforced silicon carbide matrix composites (SiC f /SiC) are considered as one of the candidates for blanket materials in future fusion reactors. Generally, densification of SiC needs sintering additives due to the covalent nature of Si-C bonding, and sintering additives such as Al 2 O 3 , Y 2 O 3 , and YAG are frequently added to SiC. However, neutron irradiation effects of sintering additives on SiC f /SiC composite properties are still unclear. In this study, oxide sintering additives such as Al 2 O 3 , Y 2 O 3 , YAG and an Al 2 O 3 -Y 2 O 3 -CaO mixture (abbreviated as AYC), were irradiated up to the neutron fluency of 2.0∼2.5×10 24 n/m 2 (E>0.1MeV) at 333∼363K in the BR2 reactor. Macroscopic swelling and change in lattice parameter due to the irradiation were measured. Length changes of the Y 2 O 3 and Al 2 O 3 specimens were ∼0.2% but that of YAG specimen was very large, 3.86%. Macroscopic length swelling of the AYC was at the midpoint of both figures. Crystallinity of the YAG specimen was greatly reduced and many micro cracks were observed after the irradiation. Lattice parameter change of the YAG specimen was large (1.41%), however it was still smaller than the macroscopic length change. The relative correlation of macroscopic length increase and those of lattice parameters of the Y 2 O 3 and Al 2 O 3 specimens was high.

Reactive and non-reactive preparation of B6O materials by FAST/SPS

Abstract B6O materials were prepared by a reactive and a non-reactive sintering routine using FAST/SPS and sintering temperatures of 1850 °C. Their sintering behavior, phase and microstructure formation and the mechanical properties are comparatively discussed. We demonstrate that the reactive sintering of B/B2O3-mixtures with oxide sintering additives is a promising and cost-effective alternative to the densification of pre-synthesized B6O with a resulting hardness of up to 22 GPa (HV5) and a fracture toughness of up to 3 MPa m (SEVNB) similar to non-reactive prepared B6O materials. The flexure strength is up to 360 MPa and 540 MPa for the reactive and non-reactive sintered materials, respectively, and substantially affected by the amount of B2O3 in the sintering liquid.

Thermal shock resistance of Si3N4 and Si3N4–SiC ceramics with rare-earth oxide sintering additives

The influence of various rare-earth oxide additives and the addition of SiC nanoparticles on the thermal shock resistance of the Si3N4 based materials was investigated. The location of SiC particles inside the Si3N4 grains contributed to a higher level of residual stresses, which caused a failure at the lower temperature difference compared to the composites with a preferential location of the SiC at the grain boundaries. A critical temperature difference increased with an increasing ionic radius of RE3+ for both the composites and the monoliths. The critical temperature difference for the composite (580 °C) and the monolith (680 °C) sintered with La2O3 was significantly higher compared to the composite and the monolith doped with Lu2O3 (430 °C). A good agreement was found between the results of the critical temperature difference estimated by the indentation quench test and that obtained by the strength retention method.

Solid-state reactive sintering mechanism for proton conducting ceramics

Abstract The recently developed solid-state reactive sintering (SSRS) method significantly simplifies the fabrication process for proton conducting ceramics by combining phase formation, densification, and grain growth into a single high-temperature sintering step. The fabrication simplicity provided by SSRS greatly enhances the potential for deployment of proton conducting ceramics in a number of electrochemical devices. Nevertheless, the mechanisms behind SSRS are still poorly understood. In this report, the SSRS mechanism is clarified through a systematic study of the effect of a suite of metal oxide sintering additives on the phase formation and densification of prototypical proton conducting ceramics BaCe 0.6 Zr 0.3 Y 0.1 O 3 − δ (BCZY63), BaCe 0.8 Y 0.2 O 3 − δ (BCY20), BaZr 0.8 Y 0.2 O 3 − δ (BZY20), and BaZr 0.9 Y 0.1 O 3 − δ (BZY10). Sintering additives with metal ions having a stable oxidation state of + 2 and an ionic radius similar to Zr 4 + (which easily occupy the Zr 4 + site of the BaZrO 3 perovskite structure, resulting in the formation of large amounts of defects) produce the best sinterability. Sintering additives with metal ions having multiple stable oxidation states (2 +, 3 +, 4 + etc.) and ionic radii near to Zr 4 + (which also readily occupy the Zr 4 + sites of BaZrO 3 perovskite structure, but form fewer amounts of defects) can result in partial sintering by the formation of mechanically stable highly-porous microstructures with limited grain sizes. Sintering additives with metal ions having stable oxidation states ≥ 3 + or ionic radii far from Zr 4 + are unlikely to form a solid solution with BaZrO 3 and yield no effect on sintering behavior (virtually identical to the control sample without any additives). These findings provide a useful guide for the selection of sintering additives to engineer optimal microstructures in proton conducting ceramics and may have potential consequences in other ceramic systems as well.

Fatigue Crack Growth Behavior of Silicon Nitride: Roles of Grain Aspect Ratio and Intergranular Film Composition

The role of microstructure in affecting the fatigue crack growth resistance of grain bridging silicon nitride ceramics doped with rare earth (RE = Y, La, Lu) oxide sintering additives was investigated. Three silicon nitride ceramics were prepared using MgO‐RE2O3 and results were compared with a commercial Al2O3‐Y2O3‐doped material. Decreasing stress intensity range (ΔK) fatigue tests were conducted using compact‐tension specimens to measure steady‐state fatigue crack growth rates. Specimens doped with MgO‐RE2O3 additives showed a significantly higher resistance to crack growth than those with Al2O3‐Y2O3 additives and this difference was attributed to the much higher grain aspect ratio for the MgO‐RE2O3‐doped ceramics. When the crack growth data were normalized with respect to the total contribution of toughening by bridging determined from the monotonically loaded R‐curves, the differences in fatigue resistance were greatly reduced with the data overlapping considerably. Finally, all of the MgO‐RE2O3‐doped silicon nitrides displayed similar steady‐state fatigue crack growth behavior suggesting that they are relatively insensitive to the intergranular film.

Electrically Conductive Silicon Carbide without Oxide Sintering Additives

ABSTRACTThis work deals with the preparation of dense SiC based ceramics with high electrical conductivity without oxide sinteringadditives. SiC samples with different content of conductive Ti-NbC phase were hot pressed at 1850 °C for 1 h in Ar atmosphereunder mechanical pressure of 30 MPa. The conductive phase is a mixture of Ti-NbC in weight ratio of Ti/NbC 1:4. Compositewith 50% of conductive Ti-NbC phase showed the highest electrical conductivity of 30.6 × 10 3 S·m − 1 , while the good mechanicalproperties of SiC matrix were preserved (fracture toughness 4.5 MPa·m 1/2 and Vickers hardness 18.7 GPa). The obtained resultsshow that use of NbC and Ti as sintering and also electrically conductive additives is appropriate for the preparation of SiC-based composite with sufficient electrical conductivity for electric discharge machining. Key words : SiC, Hot-pressing, Composites, Electrical conductivity, Hardness 1. Introduction ilicon carbide (SiC) is of major interest in variousbranches of science and technology due to its excellentoxidation, corrosion and creep resistance, extreme hard-ness, thermal conductivity and thermal shock resistance aswell as its electronic and optical properties.

Sintered-reaction Bonded Silicon Nitride Densified by a Gas Pressure Sintering Process - Effects of Rare Earth Oxide Sintering Additives

Reaction-bonded silicon nitrides containing rare-earth oxide sintering additives were densified by gas pressure sintering. The sintering behavior, microstructure and mechanical properties of the resultant specimens were analyzed. For that purpose, Lu₂O₃-SiO₂ (US), La₂O₃-MgO (AM) and Y₂O₃-Al₂O₃ (YA) additive systems were selected. Among the tested compositions, densification of silicon nitride occurred at the lowest temperature when using the La₂O₃-MgO system. Since the Lu₂O₃-SiO₂ system has the highest melting temperature, full densification could not be achieved after sintering at 1950℃. However, the system had a reasonably high bending strength of 527 ㎫ at 1200℃ in air and a high fracture toughness of 9.2 ㎫·m 1/2 . The Y₂O₃-Al₂O₃ system had the highest room temperature bending strength of 1.2 ㎬.

SiAlON Bulk Glasses and Their Role in Silicon Nitride Grain Boundaries: Composition-Structure-Property Relationships

SiAlON glasses are silicates or alumino-silicates, containing Mg, Ca, Y or rare earth (RE) ions as modifiers, in which nitrogen atoms substitute for oxygen atoms in the glass network. These glasses are found as intergranular films and at triple point junctions in silicon nitride ceramics and these grain boundary phases affect their fracture behaviour. This paper provides an overview of the preparation of M-SiAlON glasses and outlines the effects of composition on properties. As nitrogen substitutes for oxygen in SiAlON glasses, increases are observed in glass transition temperatures, viscosities, elastic moduli and microhardness. These property changes are compared with known effects of grain boundary glass chemistry in silicon nitride ceramics. Oxide sintering additives provide conditions for liquid phase sintering, reacting with surface silica on the Si₃N₄ particles and some of the nitride to form SiAlON liquid phases which on cooling remain as intergranular glasses. Thermal expansion mismatch between the grain boundary glass and the silicon nitride causes residual stresses in the material which can be determined from bulk SiAlON glass properties. The tensile residual stresses in the glass phase increase with increasing Y:Al ratio and this correlates with increasing fracture toughness as a result of easier debonding at the glass/β-Si₃N₄ interface.

Silicon Nitride Grain Boundary Glasses: Chemistry, Structure and Properties

Silicon nitride is recognised as a high performance material for both wear resistant and high temperature structural applications. Oxide sintering additives such as yttrium oxide and alumina are used to provide conditions for liquid phase sintering, during which the additives react with surface silica present on the Si3N4 particles and some of the nitride to form an oxynitride liquid which allows densification and transformation of - to -Si3N4 and on cooling remains as an intergranular oxynitride glass. This paper provides an overview of liquid phase sintering of silicon nitride ceramics, grain boundary oxynitride glasses and the effects of chemistry and structure on properties. As nitrogen substitutes for oxygen in oxynitride glasses, increases are observed in glass transition and softening temperatures, viscosities, elastic moduli and microhardness. These property changes are compared with known effects of grain boundary glass chemistry in silicon nitride ceramics.

Wear resistance of hot-pressed Si3N4/SiC micro/nanocomposites sintered with rare-earth oxide additives

The wear resistance of hot-pressed silicon nitride/silicon carbide micro/nanocomposites as well as monolithic silicon nitrides sintered with the same rare-earth oxide sintering additives (La2O3 ,N d 2O3 ,Y 2O3, Yb2O3 and Lu2O3) has been investigated under dry sliding conditions. The friction coefficient decreased with a decreasing ionic radius of rare-earth elements in both the monoliths and the composites. The friction coefficient of composites was always lower in comparison with that of Si3N4 monoliths. Similarly, the specific wear rate significantly decreased with a decreasing ionic radius of rare-earths either in monolithic or composite materials. The composites always exhibited lower specific wear rate compared to the monoliths. Mechanical wear (micro-fracture) and tribochemical reaction were found as the main wear mechanisms in all investigated materials. The higher bonding strength in the case of materials sintered with additives of a smaller ionic radius restricts dropping of the individual silicon nitride grains during wear experiments. This high bonding strength and the high fracture toughness are the reasons why the ceramics doped by Lu exhibited the best wear resistance. The Evans and Marshall lateral-crack chipping model based on the fracture toughness and hardness values well describes the wear rate of the investigated micro/nanocomposites.

Liquid Phase Sintering of SiC-Ceramics

In view of considerable attention in the development of liquid phase sintered SiC, a comprehensive study of the data on processing, structure and properties seems highly relevant. This article provides a detailed and critical overview of liquid phase sintered silicon carbide ceramics with primary emphasis of grain-boundary/secondary phase evolution, their structure, distribution on the final properties of the sintered materials. The roles of individual additives in developing boundary microstructures will be identified and demonstrated to be critical in optimizing the mechanical properties, including fracture toughness, flexural strength and creep resistance. Numerous methods of structure-properties modification, like in-situ-toughening, -SiC phase transformation, volume of liquid phase, partial/full crystallization of grain-boundary and/or secondary phases are conclusively discussed. Apart from conventional pressureless sintering of SiC, enhanced spark plasma sintering with different oxide and non-oxide sintering additives are also discussed in terms of phase evolution, microstructure and their structure mechanical properties are correlated.

Effect of Aluminium on High-Energy Ball Milling and Spark Plasma Sintering of Silicon Nitride

A commercial silicon nitride powder with oxide sintering additives was ground with high-energy ball mill to obtain nano-sized powder. Metallic aluminium powder was added as a grinding additive. Effect of high-energy ball milling was evaluated by X-ray diffraction analysis. After milling, height of background increased and peak height of silicon nitride decreased in XRD chart, which suggested that vitrification and/or decrease in grain size of silicon nitride occurred. The milled powders were sintered by spark plasma sintering system. Aluminium nitride was formed during sintering by reaction of aluminium and atmospheric nitrogen. Dense nano-ceramics, which were composed of silicon nitride and sialon, were obtained by sintering at 1550 oC.

Effect of rare-earth species on the wear properties of α sialon and β silicon nitride ceramics under tribochemical type conditions

The wear properties under low loads of β Si3N4 and α sialon materials sintered with different rare-earth oxide sintering additives have been studied under dry sliding conditions using block-on-ring wear tests. All the worn surfaces showed an absence of fracture and smooth surfaces with the presence of an oxygen-rich filmlike debris indicating tribochemically induced oxidation of the surfaces. Extensive grain boundary removal was observed on the worn surfaces thought to be due to adhesion between this silicate phase and the tribochemically oxidized surfaces. The resistance to such oxidation and the properties of the residual grain boundary phase are thought to be important parameters affecting the wear behavior under the present testing conditions. For both the β Si3N4 and α sialon materials, there was an increase in wear resistance with decreasing cationic radius of the rare earth, thought to be due to improved oxidation resistance, and this was more remarkable in the case of the sialon materials where the incorporation of the sintering additives into the Si3N4 structure results in a lower amount of residual boundary phase.

Effects of heat treatment and sintering additives on thermal conductivity and electrical resistivity in fine-grained SiC ceramics

The effects of heat treatment and sintering additives on the thermal conductivity and electrical resistivity of fine-grained SiC materials were investigated. The thermal conductivity and the electrical resistivity of dense SiC materials were measured at room temperature by a laser flash technique and a current-voltage method, respectively. The results indicated that the thermal conductivity and electrical resistivity of the SiC materials were dependent on the sintering additives and the resultant microstructure. Annealed materials with oxide additives developed microstructures consisting of elongated grains of various α/β–SiC polytypes. In contrast, annealed materials with oxynitride additives had microstructures consisting of fine equiaxed grains entirely of β–SiC phase. For the annealed materials with oxide additives the observed thermal conductivity was over 110 W/mK. For the annealed materials with oxynitride additives the observed value was 47 W/mK. The electrical resistivity of a hot-pressed material with oxide sintering additives decreased after annealing. For annealed materials with oxynitride additives, the electrical resistivity was even lower. High-resolution electron microscopy revealed a thin amorphous phase along the grain boundaries. Energy dispersive x-ray spectroscopy results showed that there was segregation of both Al and O atoms and a very small amount of Y atoms at grain boundaries. The results indicated that the chemistry and structure of the grain boundary has significant influence on thermal and electrical properties in SiC.