Multigrain Junctions Research Articles

Effects of the amount of magnesia on mechanical, thermal, and electrical properties of silicon nitride ceramics

Silicon nitride (Si3N4) ceramics were prepared using various amounts of magnesia (MgO) and a fixed amount of yttria as sintering additives, and their effects on the microstructures and four essential properties were evaluated: bending strength, fracture toughness, thermal conductivity, and dielectric breakdown strength (DBS). The microstructure varied with the MgO content; a smaller amount promoted grain growth, while an excess amount formed pores. The properties varied along with these structural changes. It was revealed that the four properties coexist at a high level in the range of 4–7 mol% of MgO. In addition, the large amount of MgO enhanced the electric conductivity of grain boundary and decreased the DBS. It is speculated that tips of multigrain junctions composed of a sufficiently large amount of glass modifier might work as the severe defects for the dielectric breakdown.

Enhanced thermal conductivity in Si3N4 ceramics by carbonizing polydopamine coatings

To enhance the thermal conductivity of Si3N4, a polydopamine (PDA) coating was creatively introduced into ceramics sintered with different additive contents through a two-step sintering process consisting of a first treatment at 1500 °C for 8 h followed by 12 h at 1900 °C under 1 MPa nitrogen pressure. After the first-step sintering, the PDA-coated sample exhibited a higher elimination effect of the liquid phase and an increase in the N/O ratio, which allowed the additives to directly interact with the Si3N4 grains, resulting in a microstructure with more and larger rod-shaped grains. After the second-step sintering, the densified samples of the PDA-coating attained slightly coarser rod-shaped grains, lower O content, higher N/O ratio and peak intensity (XRD) of the Y2Si3O3N4 phase, thicker grain boundary film, and secondary phases mainly residing in multigrain junctions. Consequently, the thermal conductivity of all the PDA-coated samples typically showed a 10–12% increment in comparison to the PDA-free samples for each additive content.

Improvement of fracture toughness of ZrB2–SiC composites with carbon interfaces

Toughened ZrB2–SiC composites have been prepared by an introduction of amorphous carbon spheres. After hot pressing, microstructure observation showed that the carbon phases were present in the form of thin layer at grain boundaries, or in the form of agglomerates at the multigrain junctions. The carbon spheres led to significant improvement of fracture toughness from 4.96±0.46MPam1/2 to 7.52±0.19MPam1/2. The toughening mechanism was attributed to the weak interfaces, which occurred due to the presence of carbon layer formed at grain boundaries after sintering.

Microstructure and mechanical properties of the spark plasma sintered TaC/SiC composites: Effects of sintering temperatures

Abstract TaC/SiC composites with 20 vol.% SiC addition were densified by spark plasma sintering at 1600–1900 °C for 5 min under 40 MPa. Effects of sintering temperatures on the densification, microstructures and mechanical properties of composites were investigated. The results showed the materials achieved >98% of theoretical density at a temperature as low as 1600 °C. While the TaC grains grew slightly with the sintering temperature increasing, the SiC particles in materials decreased in size. Equiaxed to elongated grain morphology transformation was observed in the SiC phase in the 1900 °C material to obtain a higher flexural strength and fracture toughness of 715 MPa and 6.7 MPa m 1/2 , respectively. Lattice enlargement of the TaC phase in the 1900 °C material suggested possible Si diffusion into TaC grains. Ta was also detected in SiC grains by energy dispersive spectroscopy. Glassy pockets present at multi-grain junctions explained the enhanced densification.

High-temperature deformation of ZrB 2 ceramics with WC additive in four-point bending

This work investigated high-temperature four-point bending deformation of hot-pressed ZrB 2 ceramics with WC additive at 1900 °C for 0–60 min in argon atmosphere under a static load of 25 MPa. After 60 min bending deformation, about 8% strain was achieved without apparent macroscopic cracks, suggestive of superplastic behavior at 1900 °C. The tensile side and the compressive side of the deformed samples exhibited different microstructural characteristics. On the tensile side, many cavities were observed at the multigrain junctions and the fracture surface was relatively flat. On the compressive side very few cavities occurred and some large cracks were present on the fracture surface. The presence of a secondary W-containing phase formed by the addition of WC, prohibited the coalescence of cavities by its plastic deformation.

The Emergence of Intergranular Precipitates in TiO2-Doped α-Al2O3Ceramics

The minor phase accumulated at intergranular regions in ceramics is usually associated with the global doping level of additives; however, such simplification often undermines the local variation in liquid composition that leads to the emergence of precipitation. In this paper the correlation between precipitation and intergranular liquid was investigated in a TiO2-doped Al2O3 ceramic system. The discrepancy between XRD and SEM in detection of the intergranular Al2TiO5 phase was rationalized by their difference in sensitivity that corresponds to a change from the casual to general mode of local liquid variation. Analytical TEM in addition revealed Ti-rich nanoprecipitates aligning along grain boundary (GB). The two intergranular phases were associated with separate processes of intergranular liquid, as clarified via a representative case that exhibited the extra grain growth from annealing. The nanoprecipitates were created from a binary liquid in local equilibration with segregation to GB, which served as transporting routes to form ternary liquid at the multigrain junctions to initiate the main intergranular phase Al2TiO5.

Development of microstructure during creep of polycrystalline mullite and a nanocomposite mullite/5 vol.% SiC

The microstructures of as-sintered and creep tested polycrystalline mullite and mullite reinforced with 5 vol.% nano-sized SiC particles have been characterized by scanning and transmission electron microscopy. The dislocation densities after tensile creep testing at 1300 and 1400 °C were virtually unchanged as compared to the as-sintered materials which indicates diffusion-controlled deformation. Mullite matrix grain boundaries bending around intergranular SiC particles suggest that grain boundary pinning, in addition to a reduced mullite grain size, contributed to the increased creep resistance of the mullite/5 vol.% SiC nanocomposite. Both materials showed pronounced cavitation at multi-grain junctions after creep testing at 1400 °C which suggests that unaccommodated grain boundary sliding, facilitated by softening of the intergranular glass, occurred at this temperature. This is consistent with the higher stress exponents at 1400 °C.

Creep mechanism and microstructure evolution in silicon nitride ceramics

Tensile creep processes in a commercial grade of silicon nitride, SN 88, are reviewed with the focus on cavitation and microstructure changes. Experimental measurements showed that cavitation at multigrain junctions is the rate controlling creep mechanism. The cavitation creep model of Luecke and Wiederhorn was extended to explain non-power law behaviour and strong creep asymmetry. Secondary phase composition changes after prolonged creep tests resulted in the formation of Yb-disilicate phase at the expense of Yb-oxynitride due to oxidation. Such secondary phase changes affect residual glass composition, which has crucial effect on the processes involved in cavitation and resulting creep behaviour.

Microstructures of Microwave‐Sintered Silicon Nitride with Zirconia as Secondary Particulates

A microwave‐sintered Si3N4–Y2O3–MgO system containing ZrO2 particulates was characterized using X‐ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Although the starting powder was α‐Si3N4 with Y2O3, MgO, and ZrO2 dopants, the sintered crystalline phases, analyzed by XRD, are mainly β‐Si3N4 with minor N‐Melilites (Y2Si3O3N4). TEM micrographs show that the N‐Melilites are distributed at the multigrain junctions, suggesting they are formed by crystallization from a liquid phase. SEM analysis shows that Si3N4 grains are interlocked with each other. A large amount of secondary phase (13 vol%), containing Si–Y–Mg–O–N elements as identified by energy‐dispersive X‐ray spectrometer is present in the multigrain junctions and grain boundaries. TEM analysis found that small ZrO2 grains (d<200 nm) are frequently embedded in Si3N4 grains, while others remain as dispersoids and grains dispersed in the matrix. The addition of ZrO2 as a susceptor for microwave sintering increases the heating rate. However, there appeared to be very minor dissolution of ZrO2 in the oxynitride liquid during sintering. Both tetragonal and cubic Zr‐rich phases were identified by selected‐area electron diffraction, which supports the existence of N‐stabilizing ZrO2. Microwave sintering favored the formation of the Si–Mg–Y oxynitride liquid phase and thus enhanced the dissolution of α‐Si3N4.

Atomic ordering at an amorphous/crystal interface

In this study, the authors report atomic-resolution images that illustrate the transition from a crystalline Si3N4 grain across the interface into an amorphous Lu–Si–Mg–N–O glassy phase. The interface is not atomically abrupt, but is comprised of sub-nanometer-scale ordered regions that resemble a LuN-like structure. These ordered clusters bind to the prismatic surface of the Si3N4 grains at specific low energy positions for Lu adsorption as predicted by first-principles calculations. The ordered regions are filamentary in nature, extending for at least two atomic layers into the amorphous pockets at multigrain junctions before disappearing.

Creep mechanisms in quasi-plastic silicon nitride by instrumented indentation

Quasi-plastic creep behavior of the commercial, fine-grained silicon nitride grade, ST 1, was investigated using variety of techniques with the focus on the analysis of instrumented indentation. Creep deformation in this material was characterized by high creep rates at temperatures above 1300 °C and failure strains around 20%. It was accompanied by strong oxidation, cracking of the oxide layers, excessive cavitation at multigrain junctions and slight texture formation. Instrumented indentation revealed degradation of indentation moduli in the oxide layers and enhancement of oxidation and elastic moduli degradation during creep. Because of the similarities between the mass transport processes in cavitation, diffusion processes involved in oxidation and similar activation energies, both creep and oxidation occur simultaneously, however, oxidation is enhanced by external stress. Texture formation implied from disappearance of α-silicon nitride and anisotropy of indentation modulus contributes insignificantly (<5%) to total tensile strain. Creep processes in the studied material can be explained within the expanded cavitation creep model of Luecke and Wiederhorn assuming that cavitation is facilitated by low viscosity residual glass and small matrix grain size. Tertiary-like creep is attributed to the gradual increase of the applied stress resulting from the reduction of the effective cross section due to the formation of cracked oxide layers. Size and pre-oxidation effects were predicted and confirmed using creep samples with different gauge size.

Grain-Boundary Wetting-Dewetting in z= 1 SiAlON Ceramic

The grain-boundary structure of a model SiAlON polycrystal with nominal composition Si5AlON7 was characterized by transmission electron microscopy (TEM) both in an equilibrium (as-processed) state at room temperature and after quenching from elevated temperature. In addition, low-frequency (1–13 Hz) internal friction data were recorded as a function of temperature, showing a pronounced grain-boundary sliding peak positioned at 1030°C. High-resolution transmission electron microscopy (HRTEM) of the equilibrated low-temperature microstructure revealed residual glass only at multigrain junctions, but no amorphous intergranular films were observed. The detection of clean interfaces in the as-processed sample contradicts the internal friction data, which instead suggests the presence of a low-viscosity grain boundary phase, sliding at elevated temperatures. Therefore, a thin section of the as-sintered material was heated to 1380°C and rapidly quenched. HRTEM analysis of this sample showed, apart from residual glass pockets, wetted grain boundaries, which is in line with the internal friction experiment. This wetting-dewetting phenomenon observed in z= 1 SiAlON is expected to have a strong impact not only on high-temperature engineering ceramics but also on geological, temperature-activated processes such as volcanic eruptions.

Grain-Boundary Viscosity of Preoxidized and Nitrogen-Annealed Silicon Carbides

Internal friction experiments were conducted on a model SiC polycrystal prepared from preoxidized (high-purity) SiC powder. This material contained high-purity SiO2 glass at grain boundaries in addition to a free-carbon phase, which was completely removed upon powder preoxidation. Comparative tests were conducted on a SiC polycrystal, obtained from the as-received SiC powder with the addition of 2.5 vol% of high-purity SiO2. This latter SiC material was also investigated after annealing at 1900°C for 3 h in a nitrogen atmosphere. Electron microscopy observations revealed a glass-wetted interface structure in SiC polycrystals prepared from both as-received and preoxidized powders. However, the former material also showed a large fraction of interfaces coated by turbostratic graphite. Upon high-temperature annealing in nitrogen, partial glass dewetting occurred, and voids were systematically observed at multigrain junctions. The actual presence of nitrogen could only be detected in a limited number of wetted interfaces. A common feature in the internal friction behavior of the preoxidized, SiO2-added and nitrogen-annealed SiC was a relaxation peak that resulted from grain-boundary sliding. Frequency-shift analysis revealed markedly different characteristics for this peak: both the magnitude of the intergranular glass viscosity and the activation energy for grain-boundary viscous flow were much higher in the nitrogen-annealed material. Results of torsional creep tests were consistent with these findings, with nitrogen-annealed SiC being the most creep resistant among the tested materials.

Single- and multigrain nanojunctions with a self-assembled monolayer of conjugated molecules.

Systematic conductivity measurements in nanoscale junctions containing a self-assembled monolayer of conjugated molecules are reported. Different conductivity mechanisms are identified depending on the granularity of the metal used as a substrate for assembling the monolayer. Unexpectedly, the energy scale controlling the dominant conductance channels is quite low in comparison with the molecular level spacing. In single-grain junctions, the dominant conductance mechanism is hopping with an energy scale of the order of 10-100 meV determined by the nature of the metal contacts. In the case of multigrain junctions, additional tunnel conductance is observed with low-energy Coulomb-blockade features.

Elementary Mechanisms behind the High‐Temperature Deformation Behavior of Lutetium‐Doped Silicon Nitride

Intergranular sliding and diffusive mechanisms behind the deformation behavior of a commercially available lutetium‐doped silicon nitride were investigated and discussed. A method of locating and separating phenomena critical for mechanical relaxation at elevated temperatures was applied; the method was based on low‐frequency forced‐vibration damping measurements. The potentiality of lutetium addition for improving the deformation resistance of silicon nitride was clearly reflected in the high‐temperature damping behavior of the investigated polycrystal. Softening of intergranular lutetium silicate phases located at multigrain junctions, which resulted in a grain‐boundary sliding peak, occurred at remarkably high temperatures (&gt;1725 K). This phenomenon, partly overlapping diffusional flow, was followed by further damping relaxation with the melting of the lutetium silicates. Subsequent grain growth was also detected at temperatures &gt;2100 K. Torsional creep results, collected up to 2100 K, consistently proved the presence of a “locking” effect by lutetium silicates with the sliding of silicon nitride grain boundaries below 1873 K.

Grain boundary devitrification of Ca α-sialon ceramics and its relation with the fracture toughness

Grain boundary devitrification was carried out on three Ca α-sialon ceramics with different grain sizes and morphologies and various amounts of grain boundary glass. The devitrified product was gehlenite in all samples, indicating that the crystallization of the Ca oxynitride glass was accompanied by a volume reduction. The volume reduction upon crystallization and the thermal expansion mismatch between the devitrified product and α-sialon grains would result in tensile residual stresses located at multi-grain junctions. These residual tensile stresses were expected to promote the crack deflection and bridging mechanism and thus to improve the material toughness. However, indentation fracture toughness measurement and scanning electron microscope study showed that there was no significant difference in fracture toughness and the fracture mode in present samples prior to and post heat treatment. This may be attributed to a change in the chemistry of the residual glass as a result of the grain boundary devitrification, which could enhance the bonding strength between the adjacent α-sialon grains. The enhanced bonding strength could have to some degree hindered the crack deflection and bridging mechanism.

Superplastic behavior of liquid-phase sintered β-SiC prepared with oxynitride glasses in an N2 atmosphere

Abstract The superplastic behavior of liquid-phase sintered β-SiC with oxynitride glasses was compared in compression and tension tests at 1973–2048 K in an N2 atmosphere. Strain hardening was observed under all experimental conditions. Stress exponents in the compression tests of SiC with different additives were about 2 in the temperature ranging from 1973 to 2023 K. Amorphous phases were observed at the grain boundaries and at multi-grain junctions in the as-sintered materials. Superplastic behavior was greatly influenced by grain growth, vaporization of grain-boundary phase and formation of crystalline phase during tensile deformation.

Tensile behaviour of gas pressure sintered silicon nitride in the 1600–1700°C temperature range

The tensile behaviour of a silicon nitride ceramic has been studied between 1600 and 1680°C under strain rates from 6×10−6 to 1.2×10−5 s−1. At low temperature and/or high strain rate, the behaviour was essentially brittle: failure occurred by cavitation along boundaries of acicular grains lying normal to the tensile axis and linking of these cavities by interfacial debonding. At 1650°C, the deformation started to be ductile: a stress peak was observed in the tensile curve and cavities formed at multigrain junctions while grain boundary sliding began to occur. As the temperature increased, the contribution of grain boundary sliding to deformation increased at the expense of the cavitational component. The stress peak is interpreted in terms of a relaxation effect that governs the competition between cavitation and grain boundary sliding.

Superplasticity of Liquid‐Phase‐Sintered β‐SiC with Al2O3–Y2O3–AlN Additions in an N2 Atmosphere

Compression and tension tests were performed on liquid‐phase‐sintered β‐SiC fabricated by hot‐pressing, using ultrafine powders, at 1973–2048 K in an N2 atmosphere. Amorphous phases were observed at the grain boundaries and at multigrain junctions in the as‐sintered material. Strain hardening was observed under all experimental conditions. Stress exponents in the compression test were 1.7–2.1 in the temperature range 1973–2023 K. A maximum tensile elongation of 170% was achieved at the initial strain rate of 2 × 10−5 s−1 at 2048 K.

Tensile creep behavior in an advanced silicon nitride

Abstract Tensile creep behavior and changes in the microstructure of the advanced silicon nitride, SN 88M, were studied at temperatures from 1250 to 1400°C to reveal the creep resistance and lifetime-controlling processes. Assuming power law dependence of the minimum strain rate on stress, stress exponents from 6 to 8 and an apparent activation energy of 780 kJ/mol were obtained. Extensive electron microscopy observations revealed significant changes in the crystalline secondary phases and creep damage development. Creep damage was classified in two groups: ‘inter-granular’ defects in the amorphous boundary phases, and ‘intra-granular’ defects in silicon nitride grains. The inter-granular defects involved multigrain junction cavities, two-grain junction cavities, microcracks and cracks. The intra-granular defects included broken large grains, small symmetrical and asymmetrical cavities, and crack-like intragranular cavities. Cavities are generated continuously during the whole deformation starting from the threshold strain of ∼0.1%, and they contribute linearly to the tensile strain. Cavities produce more than 90% of the total tensile strain, and it is concluded that cavitation is the main creep mechanism in silicon nitride ceramics. The multigrain junction cavities are considered to be the most important for generating new volume and producing tensile strain. The Luecke and Wiederhorn (L&W) creep model, based on cavitation at multigrain junctions according to an exponential law, was proven to correspond to the stress dependence of the minimum strain rate. A qualitative model based on the L&W model was suggested and expanded to include intragranular cavitation. The basic mechanisms involve a repeating of the sequence grain boundary sliding (GBS) ⇒ cavitation at multigrain junctions ⇒ viscous flow and dissolution-precipitation. The oxynitride ⇒ disilicate secondary phase transformation also occurs during creep and may result in the reduction of the residual glass content. The suggested creep model considers the effects of phase transformation, oxidation, crystallization, etc. on the basic creep mechanisms due to changes of the properties of the residual glass. Control of the amount and viscosity of the residual glass provides the possibility for the reduction of cavitation and consequently, for more creep-resistant ceramics.