Intergranular Glass Phase Research Articles

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.

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.

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.

Microstructure and mechanical properties of glass-infiltrated Al2O3/ZrO2 nanocomposites

This work was to investigate the effect of zirconia nanoparticles content on microstructure and mechanical properties of glass-infiltrated alumina/zirconia composites (AZGs). A series of slip-cast zirconia-toughened alumina (ZTA) compacts containing 10, 20, 30 wt% nano-zirconia, respectively, were partially sintered at 1,250°C for 2 h, then infiltrated with lanthanum borosilicate glass of lower thermal expansion at 1,180°C for 4 h. A porosity ranging from 21 to 25% mainly with submicron pore size was demonstrated in the partially-sintered ZTAs. Homogeneous distribution and micro-crystallization of intergranular glass phase was showed in the AZGs. The mechanical strength and fracture toughness of AZGs increased with zirconia content, the maximum (633.5 ± 41.7, 6.7 ± 0.6 MPa m(0.5)) were obtained in 30 wt% zirconia group, which were significantly higher than those in 10 wt% zirconia group (P < 0.05). The improved mechanical performance of AZGs containing 30 wt% zirconia was attributed to their larger zirconia content as well as thinner intergranular glass film.

Distinct element modeling of laser assisted milling of silicon nitride ceramics

Laser assisted milling (LAMill) of ceramics shows some complicated characteristics such as discontinuous chips, crack formation, propagation and coalescence. In this paper, numerical simulation is conducted to explore the machining mechanism of LAMill. The distinct element method (or discrete element method, DEM) is applied to model the microstructure of a β-type silicon nitride ceramic. Clusters are used to simulate the real grain shape of the silicon nitride ceramic and parallel bonds are employed to represent the connections between intergranular glass phase and grains. Numerical tests (compression, bending and fracture toughness tests) are performed to evaluate the macroproperties of the synthetic material, thus matching the corresponding physical properties of the real silicon nitride. Moreover, a temperature-dependent synthetic DEM specimen is created and then used in simulations of LAMill. The DEM model is validated through comparing the simulation results with the experimental ones in terms of the cutting forces and subsurface damage under different depths of cut. It is shown that the model can successfully predict the subsurface damage in LAMill.

High-Temperature Compressive Deformation of SiAlON Polycrystals Prepared without Additives

The high-temperature deformation of dense nano-sized Ca-α SiAlON and submicron- sized β-SiAlON polycrystals were investigated by compression tests. These polycrystals were fabricated from Ca-α SiAlON and β-SiAlON nano powders respectively, and contained almost no intergranular glass phase. Both SiAlONs exhibited large plastic deformations with the stress exponent n ~1 in the higher stress region, and n ~2 in the lower stress region. Shear-thickening creep (n &lt; 1), which has been reported in liquid contained SiAlONs, was not observed.

Silicon Nitride Ceramics

Silicon nitride is one of the major structural ceramics that has been developed following many years of intensive research. It possesses high flexural strength, high fracture resistance, good creep resistance, high hardness and excellent wear resistance. These properties arise from the processing of the ceramic by liquid phase sintering and the development of microstructures in which high aspect ratio grains and intergranular glass phase lead to excellent fracture toughness and high strength. The glass phase softens at high temperature and controls the creep rate of the ceramic. The purpose of this review is to examine the development of silicon nitride and the related sialons and their processing into a range of high-grade structural ceramic materials. The development of knowledge of microstructure–property relationships in silicon nitride materials is outlined, particularly recent advances in understanding the effects of grain boundary chemistry and structure on mechanical properties. This review should be of interest to scientists and engineers concerned with the processing and use of ceramics for structural engineering applications.

High-temperature compressive deformation of β-SiAlON polycrystals containing minimum amount of intergranular glass phase

The deformation of equiaxed dense β-SiAlON polycrystals with an average grain size of 0.83 μm was investigated by compression tests at temperatures ranging from 1823 K to 1973 K. The β-SiAlON ceramics were fabricated from β-SiAlON nano-powder (Si 3Al 3O 3N 5) using the Spark-Plasma Sintering technique without additives. The sintered body contained almost no intergranular phase. The deformation was characterized by stress exponent n ∼ 1 in the higher stress region and n ∼ 2 in the lower stress region. The observed values of stress exponent and activation energy, Q ∼ 940 kJ/mol, in the higher stress region were close to those of a SiAlON, which was fully crystallized by annealing for a long time. The possible deformation mechanism of β-SiAlON was attributed to diffusion-related mechanisms, such as grain boundary sliding accommodated by diffusional creep.

Effect of Chemical Composition of Intergranular Glass on Superplastic Deformation of β-Silicon Nitride

In this study, the effect of composition of intergranular glass on superplastic compressive deformation of -Si3N4 has been studied by compression tests. Oxide additives were used to form Y2O3-Al2O3-SiO2 melt and, with increasing temperatures, an oxynitride melt (Y-Al-Si-O-N) by dissolving Si3N4. The relation between flow stress and glass composition qualitatively corresponded to the effect of chemical composition on viscosity of Y2O3-Al2O3-SiO2 glass. However, the rate of increase of the flow stress was not proportional to the viscosity of Y2O3-Al2O3-SiO2 glass, probably because the composition of intergranular glass phase had changed by dissolving Si3N4 and by crystallization of Si2N2O.

Isothermal and Thermal‐Mechanical Fatigue of Alumina

AbstractIsothermal four‐point‐bending tests and uniaxial tension‐compression tests with different mean stresses at 25 °C and 900 °C as well as thermal mechanical fatigue (TMF) test with Tmin = 400 °C and Tmax = 900 °C were carried out in laboratory air on an alumina ceramic with a small amount of intergranular glass phases resulting from sintering aids (MgO, SiO and CaO). From four‐point‐bending tests characteristic strength date were determined. The fatigue tests were performed at different mean stresses. From the resulting lifetime data Haigh diagrams were evaluated. The fatigue strength for mean stress below about 180 MPa at T = 900 °C is higher than at T = 25 °C because of stress relaxation processes in the failure critical sites like triple points, grain boundaries and micro‐pores. The thermal‐mechanical fatigue (TMF) behaviour was investigated using the “out‐of‐phase” test procedure with a triangle shape temperature time curve and total mechanical strain control. Under TMF loading conditions the number of cycles to failure decreases with increasing mechanical total strain amplitude. The maximal stress induced in the particular cycles within a particular test increases with increasing number of cycles. This is the more pronounced the higher the total mechanical strain amplitude is. On average, failure of the specimen take place when the induced maximal stress reaches 150 MPa.

Effect of chemical composition of intergranular glass on superplastic compressive deformation of β-silicon nitride

The effect of chemical composition of Y 2O 3–Al 2O 3–SiO 2-based intergranular glass on superplastic deformation of β-Si 3N 4 was studied by compression tests at 1873 K. All hot isostatically pressed Si 3N 4 materials had essentially the same microstructure and the same amount of glass phase, which was different in composition only. The relation between flow stress and glass composition qualitatively corresponded to the effect of chemical composition on viscosity of Y 2O 3–Al 2O 3–SiO 2 glass. However, the flow stress was not proportional to the viscosity of Y 2O 3–Al 2O 3–SiO 2 glass, probably because the composition of intergranular glass phase had changed by dissolving Si 3N 4. The strain hardening (increase of flow stress with deformation) was dependent on the chemical composition of intergranular glass. Actually, the apparent strain hardening was not proportional to the strain but was proportional to time. The crystallization of Si 2N 2O was also proportional to time, and was dependent on the chemical composition of the intergranular glass in a similar way to the strain hardening. Thus, it was suggested that the crystallization of Si 2N 2O reduced the amount of the intergranular glass, thereby increasing flow stress.

Electron microscopy study on the high-temperature oxidation of Si 3N 4–TiN ceramics: in situ and ex situ investigations

The high-temperature oxidation of Si 3N 4–TiN particulate composites with different amounts of the glass forming sinter additives Al 2O 3 and Y 2O 3 has been studied in order to reveal the oxidation mechanism with its different reaction steps and kinetics and especially identify the role of the glass phase in the course of oxidation. The initial stages of oxidation have been observed in situ in an environmental scanning electron microscope while exposing the materials to dry or humid oxidation environment at temperatures between 600 and 1100 °C. For the characterization of the later oxidation stages, materials were oxidized ex situ for longer times. The oxidation scales were characterized by X-ray diffraction, field emission scanning electron microscopy and transmission electron microscopy. Oxidation of the composites starts at 650 °C, when TiN surface particles begin to oxidize and form on their exposed surface islands of nanocrystalline TiO 2. At around 950 °C, the glass transition temperature of the intergranular glass phase, these nanocrystals start to grow laterally on the surface. At the same time, oxidation progresses into the depth of the material, forming thereby several distinguished oxidation subscales. The intergranular glass plays a crucial role for the oxidation in the temperature range between 950 and 1100 °C. Depending on the glass quantity in presence, different reaction mechanisms dominate; the oxidation kinetics are strongly controlled by the transport within the intergranular glass.

Influence of the temperature on the strength and the subcritical crack growth rate of alumina

Abstract Dynamic four-point-bending tests between 20 and 900 °C were carried out in laboratory air on an alumina ceramic with a few intergranular glass phases (MgO, SiO 2 , CaO). The inert strength decreases with increasing temperature from 25 to 900 °C. In this range the temperature has no influence on the values of the corrected Weibull modulus m k . This means, that the strength distribution of the alumina investigated did not depend on the test temperature. The subcritical crack growth parameter A increases with increasing temperature from 25 to 500 °C and then decreases with further increasing temperature up to 900 °C. In contrast to this behaviour, for the parameter n the opposite temperature dependence turns out; n decreases between 25 and 500 °C and then increases. From d a /d t - K I plots it can be seen that the subcritical crack growth behaviour changes at a temperature of about 500 °C. For a constant crack propagation rate K I decreases with increasing temperature from 25 to 500 °C and then increases with further increasing temperature.

Synthesis of zinc oxide varistors with a breakdown voltage of three volts using an intergranular glass phase in the bismuth–boron–oxide system

Zinc oxide (ZnO) crystals joined by using an intergranular glass phase were investigated in order to develop surge filters for low-voltage applications and to clarify the current transport mechanism through the junction. The junctions having a ZnO/glass/ZnO sandwich structure were synthesized by using an interfacial glass phase of the Bi–B–O system. A highly nonlinear current–voltage curve was obtained for the ZnO/glass/ZnO junction with Co-doped ZnO single crystals, and the current through the junction at the breakdown voltage was proportional to the 30th power of the bias voltage. Dielectric measurements revealed that a double-sided depletion layer was formed at the interface, and the observed high nonlinearity was ascribed to the corresponding potential barrier formed at the interface.

Characterisation of porous silicon nitride materials produced with starch

Porous silicon nitride is gaining interest for a number of applications including metal–ceramic thermal engineering components, biomaterials and catalyst supports. This paper describes the fabrication of porous silicon nitride ceramic materials using a fugitive additive, corn starch, which allows samples to be produced with different volume fractions of porosity from ∼0 to 0.25. The initial composition consisted of 92 wt.% Si 3N 4, 6 wt.% Y 2O 3 and 2 wt.% Al 2O 3. Sintering was carried out at 1800 °C for 2 h under nitrogen. Relative density as a function of the fugitive additive content has been measured. Microstructural analysis reveals a dense matrix of elongated β-Si 3N 4 grains surrounded by intergranular glass phase and containing large pores and cavities. Pore size, geometry and grain size have been measured for certain compositions. Young's modulus and modulus of rupture have been determined as a function of the volume fraction of porosity. The Young's modulus–porosity relationship has been compared with previous work in the literature and it was found that this dependency is close to that for a model for spherical pores in cubic stacking arrangement.

TiO 2 additives for in situ formation of toughened silicon nitride-based composites

This paper is focussed on the reinforcing phases developing in a Si 3N 4-based composite microstructure when sintering a powder mixture of 74 wt.% Si 3N 4, 20 wt.% TiO 2, and 6 wt.% Y 2O 3. Micron- and submicron-sized as well as Al 2O 3 and Al 2O 3+SiO 2-coated submicron-sized TiO 2 powders were used to prepare these in situ composites consisting of Si 2N 2O, α-Si 3N 4, β-Si 3N 4, and TiN. The impulse excitation technique was used to evaluate the intergranular glass phase, which affects crack growth and high-temperature deformation. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to investigate the reinforcing phases. The influence of the TiO 2-additive powder characteristics on the microstructure and properties of the ceramics is reported.

Thermal shock properties of β-sialon ceramics

An indentation–quench method based on Vickers cracks for measuring thermal-shock properties has been applied to β-sialon materials. The thermal-shock properties have been correlated with the morphology of the β-sialon grains, the z value in the β-sialon solid solution Si 6− z Al z O z N 8− z and the amount of residual intergranular glass phase. z Values in the range 0.6–3.0 were tested, and the amount of residual yttrium-containing glass phase was varied between 0 and 20 vol.%. The best thermal-shock resistance was found at low z values, and was further improved by adding an intergranular glass phase. The poorest resistance to thermal shock was found for the highest z value where the presence of glass had no measurable influence. One composition ( z =1.5, 10 vol.% glass) was selected for studying the influence of the microstructure on the thermal-shock properties. The microstructure was varied by applying different sintering conditions. An improvement of the thermal-shock properties and the fracture toughness was found in samples containing elongated β-sialon grains formed in situ. In general, in-situ reinforced β-sialon materials with low z values and containing an intergranular glass phase exhibited the best thermal-shock resistance and improved fracture toughness ( K 1 c >4 MPa m 1 2 ).