Intergranular Glass Research Articles

Exploration of an atomic‐scale boron additive in SiAlON ceramics

AbstractSiAlON ceramics are of interest for high‐risk applications such as biomedical implants and combustion engine turbines. This work is part of a larger study aimed at leveraging atomic‐ or molecular‐scale additives to control the densification, microstructures, and ultimate structural properties of SiAlONs. Here, we investigate the effects of boron in silicon nitride‐based ceramics. The present work demonstrates a possible chemical method for controlling the microstructural development of SiAlONs by incorporating boric acid (H3BO3) into the starting powder blend. Raman spectroscopy and 11B solid‐state magic angle spinning nuclear magnetic resonance cooperatively indicate that after sintering, boron exists in threefold coordination with nitrogen in the turbostratic boron nitride (t‐BN) structure. The results of this work indicate that the incorporation of boron and generation of t‐BN bonding in the SiAlON system result in a narrower grain size distribution, a suppression of second phases such as yttrium aluminosilicates, and ultimately, increased flexure strength. A separate fractographic study showed that SiAlONs fabricated with 3 wt% boric acid exhibited fracture origins such as subtle surface flaws or cracks, while lower dopant levels and undoped SiAlONs typically failed from flaws such as inclusions or large grains. It is argued that the modification of the intergranular glass chemistry and resulting generation of t‐BN reduces atomic diffusion through the grain boundary phase and inhibits the crystallization of second phases as well as exaggerated grain growth that often characterizes the development of β′‐SiAlON.

In-situ study of phase changes in glass by impedance spectroscopy and scaling

Transformation in “evolving glass” in ion-conducting silicate glasses, glass ceramics or ceramics containing an intergranular glass is studied by in-situ impedance spectroscopy and scaling laws. Approaches are developed to scale frequency-dependent electrical conductivity and dielectric properties of multiphase ion-conducting materials to derive temperature-independent information on bulk phases and interfaces in presence. A material's scaled electric conductivity provides a unique response over a range of temperature, if the material remains unaltered, but changes, once modifications in the material occur. Therefore, impedance spectroscopy in combination with scaling can be used for tracking modifications in structure, organization, chemistry and microstructure in “evolving (ion-conducting) glass”.In a first part of this work, our approach is validated through application to stable silicate glasses and glass ceramics. Then “evolving glass” is studied in bulk glasses and ceramics with intergranular glass. Examples include ceramming of glass under formation of a glass ceramic, glass-glass phase separation, modification of intergranular glass in ceramics as result of chemical reactions or melt corrosion.Our approach allows to determine onset of phase separation, nucleation or reaction with high precision. It further reveals that nucleation in our exemplary lithium aluminum silicate glass starts with formation of interfaces (phase separation) prior to forming a well-organized crystal lattice. During phase separation in glass, early capacitive contributions in the impedance spectra indicate that the formation of glass-glass interface precedes the compositional changes in the bulk glass.Impedance of refractories is monitored during thermal treatments or corrosion in glass melt at temperatures up to 1600 °C, and glass characteristics of refractories with glass content as low as 3% are extracted. Conductivity and/or level and/or interconnectivity of the refractory's glass are shown to change during thermal treatments due to reactions between glass and crystalline refractory phases, due to phase transformation or, in case of melt corrosion, due to melt infiltration and ion exchange with the melt.In-situ tracking of impedance and application of scaling laws is shown to be able to reveal changes in bulk glass and intergranular glass.

Effect of nano- and micro-sized Si3N4 powder on phase formation, microstructure and properties of β′-SiAlON prepared by spark plasma sintering

The phase formation behavior of β′-SiAlON with the general formula Si6-zAlzOzN8-z was studied comprehensively for z values from 1 to 3 using spark plasma sintering (SPS) as the consolidation technique at synthesis temperatures from 1400 to 1700 °C. The samples were prepared close to the β′-SiAlON composition line: Si3N4 − 4/3(AlN·Al2O3) in the phase diagram using (A) nano-sized amorphous Si3N4 and (B) micro-sized β-Si3N4 precursors. Field-emission scanning electron microscopy (FESEM) was used for microstructural analysis. Most compositions reached almost full density at all SPS temperatures. Compared with the micro-sized β-Si3N4 precursor, the nano-sized amorphous Si3N4 precursor accelerated the reaction kinetics, promoting the formation of dense β′-SiAlON + O′-SiAlON composites after SPS at synthesis temperatures of 1400–1500 °C. This resulted in very high values of Vickers hardness (Hv10) = 18.2–19.2 GPa for the z = 1 composition related to the hardness of the O′-SiAlON component phase. In general, for samples synthesized from nano-sized amorphous Si3N4, which were almost fully dense, containing >95% β′-SiAlON, the hardness values were 13.4–13.8 GPa with a fracture toughness of 3.5–4.6 MPa m1/2. For equivalent samples synthesized from micro-sized β-Si3N4, hardness was in the range 13.9–14.4 GPa with a fracture toughness of 4.3–4.5 MPa.m1/2. These values are comparable with fully dense β′-SiAlONs, usually containing intergranular glass phase which has been sintered by HIP and other processes at much higher temperatures for longer times.

Optimized sintering and mechanical properties of Y-TZP ceramics for dental restorations by adding lithium disilicate glass ceramics

The novel dental ceramics can be fabricated at lower temperatures when sol-gel derived lithium disilicate glass ceramics (LDGC) was used as an additive for yttria stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramics. The effect of LDGC on the sintering, mechanical, and translucent properties of Y-TZP ceramics was investigated in the present study. The results showed that the LDGC additive effectively improved the densification of Y-TZP at 1100 °C, which was much lower than the sintering temperature for pure Y-TZP. When sintered at 1100 °C, the Y-TZP with 1 wt% LDGC reached a relative density of 95.45%, and prossessed a flexural strength of 482.4 MPa and a fracture toughness of 5.94 MPa·m1/2. Moreover, its translucency was also improved. While, the addition of LDGC could result in an escape of yttrium atoms from the grain lattice of zirconia, which induced the tetragonal-monoclinic transformation of zirconia and abnormal growth of monoclinic grains. The escaped yttrium atoms diffused into the intergranular glass phase. The results indicated that the novel Y-TZP-LDGC ceramics has a great potential to be used for all-ceramic restorations.

Observation of yttrium oxide segregation in a ZrO2‐SiO2 glass‐ceramic at nanometer dimensions

AbstractDopant segregation at grain boundaries (GBs) in ceramics has been widely reported, while whether similar segregation behavior occurs in glass‐ceramics remains unknown. The distribution of dopant in glass‐ceramics may be totally different due to the existence of glass phase. This study examines the distribution of Y3+ ions in a ZrO2‐SiO2 glass‐ceramic. Two samples were prepared by hot pressing, yttrium oxide‐doped, and undoped 65 mol% ZrO2‐35 mol% SiO2 nanocrystalline glass‐ceramics (NCGCs). The NCGCs had the same microstructure, that is, ZrO2 nanoparticles (NPs) embedded in an amorphous SiO2 matrix. XRD results showed that the undoped NCGC was composed of 20.9 wt% (weight percentage) monoclinic ZrO2 (m‐ZrO2) and 79.1 wt% tetragonal ZrO2 (t‐ZrO2), while the yttrium oxide‐doped NCGC was composed of 9.6 wt% m‐ZrO2 and 90.4 wt% t‐ZrO2. X‐ray energy‐dispersive spectrometry (EDS) results in scanning electron transmission microscopy (STEM) mode demonstrated that Y3+ ions segregated both on the surface of ZrO2 NPs and within the thin intergranular glass film (with a thickness of approximately 7 Å) between ZrO2 NPs in the yttrium oxide‐doped NCGC. Interestingly, no obvious Y signals were detected in the amorphous SiO2 matrix. Density functional theory calculation results showed that Y3+ ions had a strong segregation tendency in the GB area and the segregation of Y3+ ions increased the work of separation of GB layer. These findings provide new understanding of the segregation behavior of dopant in glass‐ceramics, which may offer useful guidance for other researchers to tailor the properties of glass‐ceramics through GB engineering.

Intergranular Oxynitride to Regulate Solution–Reprecipitation Process in Gas–Pressure‐Sintered SiC Ceramics with AlN–Y2O3 Additives

Core−rim structures are found as a general microstructural feature in SiC ceramics gas–pressure sintered with AlN−Y2O3 additives in different ratio, combined with a range of α/β‐SiC seeding to control the phase transformation. Using analytical electron microscopy, the authors investigate the re‐distribution of additives to detect uniform level of AlN solution into SiC rims, independent to the seeding ratio, and also not proportional to additives. The behavior of AlN solution reveals the presence of sintering melt based on nitro‐carbide to start the reprecipitation of SiC rims, while the cores are remnants from starting powders to control the phase transformation as well as the grain growth behavior. Among various intergranular phases, Y10Al2‐zSi3+zO18+zN4‐z is observed as common hence the secondary phase, while its Al level changes in an opposite trend to AlN solution in SiC rims. This oxynitride phase precipitates from the Y‐rich oxynitride melt, which extends and regulates the solution−reprecipitation process and develops into the Y−Si−O−(C) melt to create a series of residual intergranular oxides. Post‐annealing can fully devitrify the viscous intergranular glass to reach complete crystallization of ceramic body, which leads to improved creep resistance at high temperature for this SiC ceramic system.

Quenched primary melt in Ramlat as Sahmah 517 – Snapshot of ureilite anatexis in the early solar system

Ureilites are the second largest group of achondrite meteorites but consensus is still lacking on the nature of their precursors, melting processes, and the genetic relationship between monomict ureilites and brecciated ureilites. The recently found ureilite Ramlat as Sahmah 517 is of special interest in this context. This meteorite lacks shock features in its primary silicates and belongs to a rare augite- and chromite-bearing subset of the monomict ferroan ureilites. It hosts abundant intergranular glass veinlets speckled with pyroxene and metal globules. Detailed petrographic investigations show that the Si-Al-rich glass represents quenched anatectic melt that was present prior to formation of the reduced olivine rims by incomplete low-pressure equilibration (smelting) of carbon and silicates. The melt facilitated smelting which, along with rapid crystallization of secondary pyroxene, modified the originally trachyandesitic melt. Melt-silicate equilibrium preceding these events is constrained by modelling using MELTS and the first reported in-situ measurements of LREE-enriched glass that is largely complementary to the depleted mafic silicates in monomict ureilites. The inferred major element composition of the partial melt that formed in RaS 517 is similar to that of trachyandesite in Almahata Sitta but RaS 517 lacks phosphates which are abundant in the Almahata Sitta trachyandesite and in alkali-rich feldspathic clasts in polymict ureilites. The LREE-depletion in the dominant monomict ferroan ureilite population can be explained by the formation of melt fractions similar to the glass in RaS 517 after initial rapid melting of phosphates. These finds provide evidence for a genetic relationship between ferroan ureilites and lithologies similar to the Almahata Sitta trachyandesite and further suggest that these ureilites formed by partial melting of P- and alkali-rich precursors with trace element concentrations similar to equilibrated ordinary chondrites. Quenched Si-Al-rich glass also occurs in magnesian ureilites but has lower concentrations of alkalis and LREE-depleted trace element signatures which can reflect more depleted compositions at the onset of partial melting. The evidence presented here favors a scenario in which the primary ureilite differentiation was driven by gradual heating from radioactive decay with resulting temperatures (>1100 °C) being maintained until disruption of the ureilite parent asteroid.

Highly siderophile elements mobility in the subcontinental lithospheric mantle beneath southern Patagonia

Peridotite xenoliths collected from alkali basalts in the Argentinian Patagonia reveal the existence of an ancient depleted Paleoproterozoic mantle that records a subsequent multistage metasomatic history. Metasomatism is associated with carbonatite-like melts that evolved, after variable melt/rock ratio interaction, towards CO2-rich and Na-bearing Mg-rich (mafic) silicate, and volatile-rich alkali silicate melts. High degrees of partial melting produced strongly depleted mantle domains devoid of base-metal sulphides (BMS). Moderate degrees of partial melting and later unrelated metasomatism produced a range of slightly depleted, slightly enriched, and strongly enriched mantle domains that preserve different types of BMS. Thus, six different BMS populations were identified including typical residual Type 1A BMS enriched in Os, Ir, and Ru relative to Pt, Pd, and Au located within primary olivine and clinopyroxene, and metasomatic Type 2A BMS that are relatively enriched in Pt, Pd, Au occurring as interstitial grains. Reworking of these two types of BMS by later metasomatism resulted in the formation of a new generation of BMS (Type 1B and Type 2B) that are intimately associated with carbonate/apatite blebs and/or empty vesicles, as well as with cryptically metasomatised or metasomatic clinopyroxene. These newly formed BMS were re-enriched in Os, Pd, Au, Re and in semi-metal elements (As, Se, Sb, Bi, Te) compared to their Type 1A and Type 2A precursors. A third generation of BMS corresponds to Ni-Cu immiscible sulphide mattes entrained within Na-bearing silica under-saturated alkali melt. They occur systematically related to intergranular glass veins and exhibit distinctively near flat CI-chondrite normalised highly siderophile element patterns with either positive Pd (Type 3A) or negative Pt (Type 3B) anomalies. Our findings indicate that Os, Pd, Re and Au can be selectively transported by volatile-rich alkali silicate melts in the subcontinental lithospheric mantle. Moreover, the transport of sulphide mattes entrained in silicate melts is also an effective mechanism to produce HSE endowment in the SCLM and play an important role as precursors of fertile, metal-rich magmas that form ore deposits in the overlying crust.

Degradation of Nextel™ 610‐based oxide‐oxide ceramic composites by aluminum oxychloride decomposition products

AbstractNextel™ 610 alumina fibers and alumina‐YAG (yttrium‐aluminum garnet) matrices were used to make oxide‐oxide ceramic matrix composites (CMCs) with and without monazite (LaPO4) fiber‐matrix interfaces. Twelve sequential aluminum oxychloride (AlOCl) infiltrations with 1 hour heat treatments at 1100°C and a final 1 hour heat treatment at 1200°C were used for matrix densification. This matrix processing sequence severely degraded CMC mechanical properties. CMC tensile strengths and interlaminar tensile (ILT) strengths were less than 10 MPa and 1 MPa, respectively. Axial fracture of Nextel™ 610 fibers was observed after ILT testing, highlighting the extreme degradation of fiber strength. Extensive characterization was done to attempt to determine the responsible degradation mechanisms. Changes in Nextel™ 610 fiber microstructure after CMC processing were characterized by optical microscopy, SEM, and extensively by TEM. In AlOCl degraded fibers, grain boundaries near the fiber surface were wetted with a glass that contained Y2O3/SiO2 or Y2O3/La2O3/P2O5/SiO2, and near‐surface pores were partially filled with Al2O3. This glass must also contain some Al2O3 and initially some chlorine. AlOCl decomposition products were predicted using the FactSage® Thermochemical code, and were characterized by mass spectrometry. Effects of AlOCl precursors on monazite coated and uncoated Nextel™ 610 fibers tow and filament strength were evaluated. A mechanism for the severe degradation of the oxide‐oxide CMCs and Nextel™ 610 fibers that involves subcritical crack growth promoted by release of chlorine containing species during breakdown of intergranular glasses in an anhydrous environment is proposed.

Mantle xenoliths from Szentbékálla, Balaton: Geochemical and petrological constraints on the evolution of the lithospheric mantle underneath Pannonian Basin, Hungary

Pliocene alkali basalts from the Bakony-Balaton Highland Volcanic Field (BBHVF) in the western Pannonian Basin carry mantle xenoliths comprising hydrous and anhydrous spinel peridotites. The studied mantle xenoliths from Szentbékálla, near Lake Balaton, Hungary, are fine- and coarse-grained fertile to depleted spinel lherzolites, spinel harzburgites and dunites, with protogranular, porphyroclastic, and secondary protogranular and mosaic equigranular textures. Melt pockets, with shapes resembling amphibole, are common in a number of samples, whereas other samples have thin films of intergranular glass. Bulk-rock major element abundances show that the mantle lithosphere beneath this area experienced variable degrees (up to 20%) of partial melting.The clinopyroxene trace elements systematics retain a record of melt depletion and metasomatic processes attributed to subduction-related melt/fluids or to the infiltration of percolating undersaturated melts in the Pannonian lithospheric mantle. The radiogenic isotopes of Sr, Nd and Hf in clinopyroxene suggest that this metasomatism was a relatively recent event. Textural evidence suggests that the calcite filling up the vesicles in the melt pockets and in veinlets cross-cutting the constituent minerals is of epigenetic nature and not due to carbonatite metasomatism.The non-metasomatized primitive mantle normalized clinopyroxene REE abundances mimic those, but at higher values, of their bulk-rock REE patterns. Bulk-rock and clinopyroxene REE with upward and downward LREE respectively, indicate up to 2.4% host basalt infiltration.The calculated bulk composition of the melt pockets is identical to small amphibole relics found as inclusions in second generation clinopyroxene within the melt pockets, suggesting incongruent melting of amphibole, without the need for additional metasomatic melt/fluids to initiate the amphibole breakdown. The heat for the temperature increase necessary for amphibole breakdown was derived from the host basalt. The estimated time for diffusive Ca exchange between matrix olivine and olivine overgrowth in contact with the melt pockets is very short, ranging between 21 and 200days, indicating that amphibole breakdown took place immediately before or during the xenolith entrainment in the alkali basalt.

Liquid/glass immiscibility in yttria doped mullite ceramics

5wt.% yttria doped mullite ceramics were sintered using nanocomposite powders which consisted of a Y2O3–SiO2 core coated with pseudoboehmite. Firstly, the powders were heated up to 1300°C; mullite began to form at 1050°C which is ∼150°C lower than in previous diphasic precursors. Then, the powders were sintered at 1550°C for 0.5, 5 and 48h, and characterized using XRD, SEM and TEM with EDS.The phases present were mullite and two intergranular glasses comprising silica-rich droplets embedded in a Y–Si–Al–O glassy matrix, providing direct evidence of liquid-liquid phase separation at the grain boundaries during sintering. This liquid-liquid separation is facilitated by the prior existence of yttrium disilicate which moves the overall sintering liquid compositions toward the two immiscible liquids region of the Y2O3–Al2O3–SiO2 phase diagram. The presence of the silica-rich phase in the grain boundaries will effectively increase their viscosity and thus the resistance of the Y2O3 densified mullite to creep.

Influence of Borosilicate Addition on Mechanical Properties and Sinterability of 8YSZ

The influence of borosilicate (BS) addition on the mechanical properties and sinterability of yttria-stabilized zirconia (8YSZ) was investigated using scanning electron microscopy (SEM) and Vickers-hardness test. Undoped and 0.5–10 wt.% BS-doped 8YSZ powders were prepared using a colloidal process. The powders were then pelletized under a pressure of 200 MPa. The BS-doped 8YSZ specimens were sintered at 1200–1400 ◦C for 1 h. SEM results showed that intergranular glass phases were formed at the grain boundaries of 8YSZ, particularly in the 5 and 10 wt.% BS-doped 8YSZ specimens. These intergranular glass phases caused a decrease in grain size of 8YSZ. The grain sizes of the undoped and 0.5 wt% BS-doped 8YSZ specimens were 2.68 μm and 1.83 μm, respectively. The addition of BS to 8YSZ caused a decrease in the relative density. In addition, the relative density was found to be increased with higher sintering temperature. The hardness of 8YSZ decreased with the increase in BS dopant amount, due to presence at the grain boundaries of a BS glassy phase that has a lower hardness than 8YSZ.

SPS sintering of silicon nitride with fluoride additive

Abstract Silicon nitride, one of the major structural ceramics, is sintered using additives such as Al2O3+MgO to provide conditions for liquid phase sintering to full density. The final microstructure contains high aspect ratio β-Si3N4 grains and intergranular glass. In this study, Spark Plasma Sintering (SPS) of silicon nitride, using Al2O3+MgO or MgF2 as sintering additives, has been investigated in the temperature range 1400–1600 °C for 3 min since MgF2 should produce liquid phases with lower melting temperatures than with MgO and therefore should sinter at lower temperature. Densification, microstructural development and mechanical behaviour have been compared. Maximum density (3.146 g cm−3) was obtained for Si3N4+3% Al2O3+9.3% MgF2 sintered by SPS at 1550 °C. In all samples, α- to β-Si3N4 transformation was incomplete but samples with MgF2 showed higher β:α ratio than with MgO. Fluoride doped samples also exhibited higher fracture toughness than equivalent samples sintered with MgO.

Grain growth in BaTiO3 ceramics assisted by an intergranular glass film

Abstract This research demonstrates that BaTiO 3 ceramics exhibit increased grain boundary mobility ( M b ) triggered by an inter-granular glass film (IGF) as previously demonstrated in polycrystalline Al 2 O 3 ceramics conversion to single-crystal sapphire upon annealing at high temperatures. These results suggest that secondary abnormal grain growth (SAGG) during sintering of BaTiO 3 ceramics at 1365 °C, a temperature above the eutectic points in the BaTiO 3 –SiO 2 and BaTiO 3 –TiO 2 systems, is another example of faster M b caused by IGF. Higher M b magnified through an interface glass film of several nanometers thickness facilitates a faster grain growth rate that produces exceedingly large grains of ∼2.4 mm in size. The growth mechanism involving twin-plane re-entrant (TPRE) corners may not be the only route responsible for SAGG in BaTiO 3 , particularly when sintered at above eutectic temperatures. How this abnormal grain growth occurs through mechanisms involving liquid-phase-enhanced M b and the {111} twins as a chemically induced growth fault is discussed.

Silicon Nitride—Grain Boundary Oxynitride Glass Interfaces: Deductions From Glass Bulk Properties

Oxynitride glasses exist as grain boundary phases in Si3N4 ceramics. This paper provides an overview of oxynitride glasses outlining effects of composition on properties. A review of the effects of grain boundary glass chemistry on fracture resistance of silicon nitride is given. A knowledge of overall additive compositions and their quantities in Si3N4 combined with measured properties of bulk glasses allows residual stresses in the interfacial glasses to be calculated. Increase in Y:Al ratio leads to higher thermal expansion mismatch and higher residual stresses in intergranular glasses. Values are in good agreement with those obtained using micromechanical finite element analysis.

Influence of Composition on Self-toughening and Oxidation Properties of Y-α-Sialons

Y-α-sialon ceramics with different compositions were hot-press sintered. Varying the m or n value in the α-Sialon general formula, both the α-sialon grain morphology and phase assemblages present diversification, therefore, affecting the toughness and hardness. Meanwhile, varying the compositional parameters also influenced the composition and structure of the intergranular glass phase, as well as its oxidation behavior at 1100–1200 °C. The oxidation of the intergranular phase M′ in the Y-α-sialon ceramics was significantly enhanced with further rising the oxidation temperature to 1300 °C.

Effect of CaO Addition on Compressive Deformation of Silicon Nitride Ceramic with Y-Mg-Si-O-N Glassy System

Creep behavior of Si3N4 polycrystals containing Y2O3-MgO-SiO2 glass phase (with and without Calcium oxide [CaO] additive) was studied by compression tests between 1500 and 1700°C. We studied the effect of CaO additive on flow stress, microstructural evolutions, and thermal stability of the intergranular glass phase during deformation. While the addition of CaO did not affect grain size, the flow stress decreased with the amount of CaO. This result suggested that the addition of CaO reduced the viscosity of intergranular glass phase. The addition of CaO further improved the thermal stability of the glass phase by suppressing the evaporation at elevated temperatures.

Effect of frequency on high-temperature fatigue crack growth in a silicon carbide reinforced silicon nitride composite

A detailed study on a silicon nitride reinforced with silicon carbide whiskers, Si3N4SiCW, has been undertaken at elevated temperature during static and dynamic loading at increasing K and ΔK respectively. It is shown that cyclic sub-critical crack growth rates are lower than static crack growth rates. The increased crack growth rate during static far field loading is attributed to the stress relaxation of the inter-granular glass phase which allows time-dependent processes to occur ahead of the crack tip which lead to enhanced sub-critical crack growth rates. During cyclic fatigue the glass phase has insufficient time to relax and glassy ligaments are able to bridge the crack wake thereby shielding the crack tip from the full force of the applied load. Also, at particular temperatures, bridging between the surfaces of the crack wake by the inter-granular glass phase results in increased strength and fatigue retardation. The extent of ‘crack wake healing’ is shown to be time and temperature dependent. The viscosity of the glass phase is directly related to the temperature and the bonding force associated with glass phase bridging is observed to reduce with increasing temperature. The results from a previous study at room temperature are compared to those found during this investigation.

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.

Grain boundary glasses in silicon nitride: A review of chemistry, properties and crystallisation

Abstract Silicon nitride for engineering applications is densified by liquid phase sintering using oxide additives such as yttria and alumina. The oxynitride liquid remains as an intergranular glass. This paper provides a review of microstructural development in silicon nitride, grain boundary oxynitride glasses and effects of chemistry on properties. Nitrogen increases Tg, viscosities, elastic moduli and microhardness. These property changes are compared with known effects of grain boundary glass chemistry in silicon nitride ceramics where significant improvements in fracture resistance of silicon nitride can be achieved by tailoring the intergranular glass chemistry. Crystallisation of the grain boundary Y–Si–Al–O–N glass phase can improve properties. Nucleation and crystallisation of a Y–Si–Al–O–N glass, similar to that found in grain boundaries of silicon nitride densified with yttria and alumina, can be optimised to form different Y-disilicate polymorphs at different temperatures. One solution to provide a single disilicate phase over a range of temperatures is discussed.