Dense Si3N4 Ceramics Research Articles

Microstructure and mechanical properties of highly dense Si3N4 ceramics with ZrN–AlN–Re2O3 (Re=Yb, Y, Nd, Ce, La) additives

The Si3N4 ceramics with ternary ZrN–AlN–Re2O3 (Re=Yb, Y, Nd, Ce, La) and ZrN–AlN system as sintering additives were prepared by hot-pressed sintering with vacuum environment at 1750°C-30min–30MPa. Subsequently, the effect of the ternary additives system with 2 wt% Re2O3 on phase transformation, grain boundary phases, microstructure, and mechanical properties of the Si3N4 ceramics were investigated firstly. All samples achieved the high β-Si3N4 phases transformation (≥98.86%), while the grain boundary phases RexZr1-xO(4-x)/2 and Yb2Si2O7 were detected in the sintered Si3N4 ceramics with Re2O3 additives. Additionally, compared with Si3N4 ceramic not contain Re2O3 additives, the Si3N4 ceramics added Re2O3 additives presented the textured microstructure with more elongated rodlike β-Si3N4 grains based on the XRD and SEM results. Consequently, the sample S-Ce with added CeO2 additive shows the highest relative density of 99.85% and optimal fracture toughness of 8.75 ± 0.36 MPa m1/2. The sample S–Nd with added Nd2O3 additive presented a highest bending strength of 1061.78 ± 41.76 MPa. Sample S–Yb with added Yb2O3 additive presented the maximum Vickers hardness of 16.35 ± 0.07 GPa. The results of this study can provide a helpful reference for the preparation of high-performance Si3N4 ceramics.

Effect of Ultra-High Pressure Sintering and Spark Plasma Sintering and Subsequent Heat Treatment on the Properties of Si3N4 Ceramics.

In this study, coarse Beta silicon nitride (β-Si3N4) powder was used as the raw material to fabricate dense Si3N4 ceramics using two different methods of ultra-high pressure sintering and spark plasma sintering at 1550 °C, followed by heat treatment at 1750 °C. The densification, microstructure, mechanical properties, and thermal conductivity of samples were investigated comparatively. The results indicate that spark plasma sintering can fabricate dense Si3N4 ceramics with a relative density of 99.2% in a shorter time and promote α-to-β phase transition. Coarse β-Si3N4 grains were partially fragmented during ultra-high pressure sintering under high pressure of 5 GPa, thereby reducing the number of the nucleus, which is conducive to the growth of elongated grains. The UHP sample with no fine α-Si3N4 powder addition achieved the highest fracture strength (822 MPa) and fracture toughness (6.6 MPa·m1/2). The addition of partial fine α-Si3N4 powder facilitated the densification of the SPS samples and promoted the growth of elongated grains. The β-Si3N4 ceramics SPS sintered with fine α-Si3N4 powder addition obtained the best comprehensive performance, including the highest density of 99.8%, hardness of 1890 HV, fracture strength of 817 MPa, fracture toughness of 6.2 MPa·m1/2, and thermal conductivity of 71 W·m−1·K−1.

Fused deposition modeling of Si3N4 ceramics: A cost-effective 3D-printing route for dense and high performance non-oxide ceramic materials

Dense Si3N4 ceramics were prepared by fused deposition molding method accompanied by gas pressure sintering. In this study, the surface steps, inter layer bonding and microstructure evolution were characterized and dense Si3N4 ceramics without obvious defects were obtained. It was verified that layer thickness and nozzle diameter have little impact on the density and flexural strength of both green and sintered parts. As to the filling strategy, contour offset path was more effective to obtain sintered part with higher flexural strength than parallel lines and grid path, which was due to the possible voids appeared at the intersection of print paths with different directions. The highest flexure strength 824.74 ± 85 MPa was obtained with layer thickness 0.15 mm, nozzle size 0.6 mm and contour offset path. The reliability of the obtained Si3N4 ceramics was also investigated and complex shaped Si3N4 ceramic parts with good shape keeping was prepared successfully.

Microstructure and mechanical properties of dense Si3N4 ceramics prepared by direct coagulation casting and cold isostatic pressing

Complex shaped dense Si3N4 ceramics were produced by using direct coagulation casting technique via dispersant reaction method of Si3N4 suspension, followed by gas pressure sintering. The effects of solid content of the suspension, additional cold isostatic pressing of the cast parts, and sintering behaviour and on the mechanical reliability of silicon nitride ceramics were investigated. It was observed that all slurries exhibited rheological properties suitable for casting in the range of 44–50 vol.% solid concentrations. Nevertheless, higher solid concentration suspensions resulted in smaller floc size and thus better green microstructures. Parts shaped by direct coagulation casting at all the solid loadings had relatively low strength and reliability after sintering. However, application of additional cold isostatic pressing to the cast parts increased the strength and, particularly, reliability. Dense Si3N4 ceramics with relative density above 99.5%, average bending strength 760 ± 39 MPa and Weibull module 23.5 had been obtained with 50 vol.% solids content after DCC + CIP process.

Joining dense Si3N4 to porous Si3N4 using a novel glass-ceramic interlayer with precipitated β-LiAlSi2O6/Mg2SiO4

In this work, we developed a new β-LiAlSi2O6/Mg2SiO4 glass-ceramic interlayer to join dense Si3N4 and porous Si3N4 ceramics for high-temperature applications. The microstructure and phases of the joint were investigated using systematic analysis methods and the mechanism for the interfacial bonding was proposed. The double crystalline phases of β-LiAlSi2O6 and Mg2SiO4 precipitated in the joint seam during the joining process, forming a novel glass-ceramic interlayer with high crystallinity. Due to the good wettability of liquid glass on porous Si3N4, a gradient and dense infiltration layer was formed in porous Si3N4 and reinforced it, which improved the mechanical strength of the whole joint. In this case, the shear strength of the joint reached 78 MPa, and it could still maintain 74 MPa up to 850 °C, which fully demonstrated the high-temperature resistance of the joint.

Densification mechanism of stereolithographical dense Si3N4 ceramics with CeO2 as sintering additive by field assisted sintering

Densification mechanism of stereolithographical dense Si3N4 ceramics with CeO2 as sintering additive by field assisted sintering

Novel silicothermic reduction method to obtain Si3N4 ceramics with enhanced thermal conductivity and fracture toughness

The development of bimodal microstructure in Si3N4 ceramics was crucial to the thermal conductivity and fracture toughness. Silicothermic reduction treatment (SRT) combined with gas-pressure sintering (GPS) were conducted to promote the development of bimodal microstructure. SRT leads to a reduced amount of liquid phase and an increased N/O ratio in the liquid phase, resulting in a faster phase transformation rate relative to densification and a promoted grain growth during GPS. Consequently, large β-Si3N4 grains were well developed in a porous fine β-Si3N4 matrix skeleton without grain impingement. Dense Si3N4 ceramics with exaggerated bimodal microstructure was obtained after sintered at 1900 °C for 4 h. Summarily, the addition of Si leads to the improvement of thermal conductivity and fracture toughness simultaneously.

Fabrication and properties of pressure-sintered reaction-bonded Si3N4 ceramics with addition of Eu2O3–MgO–Y2O3

Dense pressure-sintered reaction-bonded Si3N4 (PSRBSN) ceramics were obtained by a hot-press sintering method. Precursor Si powders were prepared with Eu2O3–MgO–Y2O3 sintering additive. The addition of Eu2O3–MgO–Y2O3 was shown to promote full nitridation of the Si powder. The nitrided Si3N4 particles had an equiaxial morphology, without whisker formation, after the Si powders doped with Eu2O3–MgO–Y2O3 were nitrided at 1400 °C for 2 h. After hot pressing, the relative density, Vickers hardness, flexural strength, and fracture toughness of the PSRBSN ceramics, with 5 wt% Eu2O3 doping, were 98.3 ± 0.2%, 17.8 ± 0.8 GPa, 697.0 ± 67.0 MPa, and 7.3 ± 0.3 MPa m1/2, respectively. The thermal conductivity was 73.6 ± 0.2 W m−1 K−1, significantly higher than the counterpart without Eu2O3 doping, or with ZrO2 doping by conventional methods.

Wetting and joining of porous Si3N4 and dense Si3N4 ceramics with in-situ formed β-spodumene/spinel glass-ceramic interlayer

Efficient joining materials and techniques are of critical significance for the integration of ceramics in high performance structures. Porous Si3N4 has been successfully joined to dense Si3N4 by using a novel glass-ceramic based on the Li2O-MgO-Al2O3-SiO2 system. In terms of elucidating the microstructure of joints, systematic approaches were made, including scanning electron microscopy, transmission electron microscopy and X-ray diffraction, to characterize the glass/ceramic interface and to determine the influence of joining temperature on the microstructural evolution. In addition, the extent to which temperature the joint could withstand has been determined. It is suggested that the glass shows good wettability and thermomechanical compatibility with both porous and dense Si3N4 substrates. The micron-size β-spodumene, along with the submicronic spinel are identified as the main crystallization of the glass-ceramic interlayer forming an intergranular network structure. The infiltration layer, consisting of glass and rod-like Si3N4 grains, shows increased elastic modulus and microhardness, which helps improve the stress tolerance of the joint. Accordingly, this joint exhibits an extraordinary high room-temperature shear strength (112 ± 10 MPa) and a high-temperature resistance.

Effect of rare earth oxides addition on the mechanical properties and coloration of silicon nitride ceramics

The aim of this study was to evaluate the mechanical properties and coloration of silicon nitride ceramics in the presence of RE2O3 (RE = Nd, Eu or Dy). Dense Si3N4 ceramics were prepared by gas pressure sintering at 1800 °C for 2 h. XRD analysis confirmed the complete transformation of α-Si3N4 to β-Si3N4. The fracture toughness and flexure strengths were 11.93 ± 0.56 MPa·m1/2, 667 ± 40.98 MPa with the addition of Eu2O3 (SE). Base on the SEM image, the pull-out, bridging and deflection of large grains were observed and contributed to the increase in mechanical properties. The chromaticity of sintered bodies was measured using a spectrophotometer. The color difference of the ceramics is due to the formation of different color developing compounds according to the EDS. Results showed that high-toughness and colorful Si3N4 ceramics can be prepared using YAG:Ce3+ as sintering additive and RE2O3 as the colorant.

Stereolithographical fabrication of dense Si3N4 ceramics by slurry optimization and pressure sintering

Photocurable gray-colored Si3N4 ceramic slurry with high solid loading, suitable viscosity and high curing depth is critical to fabricate dense ceramic parts with complex shape and high surface precision by stereolithography technology. In the present study, Si3N4 ceramic slurry with suitable viscosity, high solid loading (45 vol %) and curing depth of 50 μm was prepared successfully when surface modifier KH560 (1 wt%) and dispersant Darvan (1 wt%) were used. The slurry exhibits the shear thinning behavior. Based on the Beer-Lambert formula, Dp(the attenuation length) and Ec (the critical energy dose) of Si3N4 ceramic slurry with solid loading of 45 vol % were derived as 0.032 mm and 0.177 mJ/mm2, respectively. Si3N4 ceramic green parts with complex shape and high surface precision were successfully fabricated by stereolithography technology. After optimizing the debinding and sintering process for green parts, dense Si3N4 ceramics with 3.28 g/cm3 sintering density were fabricated. The microhardness and fracture toughness of as-sintered Si3N4 ceramics are ~14.63 GPa and ~5.82 MPa m1/2, respectively, which are comparable to those of the samples by traditional dry-pressed and pressureless sintering technology. These results show that ceramic stereolithography technology could be promising to fabricate high performance ceramics, especially for gray-colored monolithic Si3N4 ceramics.

Study on the Effect of Bonding Process of Silicon Nitride-Based Ceramics with Yb-β/Glass Composite Solders on the Structure and Properties of Joints

Dense Si3N4 ceramics, porous/dense ceramics, SiC reinforced Si3N4 ceramics and other materials have been successfully bonded with Yb-β/glass composite solders. The effect of bonding process on the structure and properties of joints was studied.

Joining of dense Si3N4 ceramics with tape cast Lu-Al-Si-O-N interlayer

Lu-Al-Si-O-N tapes with different thickness were used to join gas pressure sintered Si3N4 ceramics. The microstructure of the joints and the influences of the joint thickness and joining temperature on the bonding strength of the as-joined Si3N4 ceramics have been investigated. The highest bonding strength about ~ 300MPa of the joined specimens was achieved by using 450µm interlayer at 1450°C. The existence of Si3N4 nanowires was beneficial for the improvement of the bonding strength by interweaving the oxynitride glass matrix in the joint region.

Microstructure and Properties of Porous Si3N4/Dense Si3N4 Joints Bonded Using RE–Si–Al–O–N (RE = Y or Yb) Glasses

The joining of porous Si3N4 to dense Si3N4 ceramics has been successfully performed using mixed RE2O3 (RE = Y or Yb), Al2O3, SiO2, and α-Si3N4 powders. The results suggested that the α-Si3N4 powders partly transformed into β-SiAlON and partly dissolved into oxide glass to form oxynitride glass. Thus, composites of glass/β-SiAlON-ceramic formed in the seam of joints. Due to the capillary action of the porous Si3N4 ceramic, the molten glass solder infiltrated into the porous Si3N4 ceramic side during the joining process and formed the “infiltration zone” with a thickness of about 400 μm, which contributed to the heterogeneous distribution of the RE–Si–Al–O–N glasses in the porous Si3N4 substrate. In-situ formation of β-SiAlON in the seam resulted in a high bonding strength. The maximum bending strength of 103 MPa and 88 MPa was reached for the porous Si3N4/dense Si3N4 joints using Y–Si–Al–O–N and Yb–Si–Al–O–N glass solders, respectively.

Improvement of strength of carbon nanotube-dispersed Si3N4 ceramics by bead milling and adding lower-temperature sintering aids

Studies on the dispersion of carbon nanotubes (CNTs) in silicon nitride (Si3N4) ceramics to provide the latter with electrical conductivity have been carried out in recent years. The density and the strength of Si3N4 ceramics were degraded, however, because the CNTs prevented Si3N4 from densifying. The CNTs disappeared after firing at high temperatures owing to the reaction between CNTs and Si3N4 or SiO2, or both Si3N4 and SiO2. In order to improve the density and suppress the reaction, sintering aids for lower-temperature densification of Si3N4 are needed. In this study, we added HfO2 as a sintering aid to a Si3N4–Y2O3–Al2O3–AlN–TiO2 system to fabricate CNT-dispersed Si3N4 ceramics at lower temperatures. Furthermore, bead milling was applied to disperse the CNTs homogeneously. Agglomerates of CNTs were pulverized by bead milling without obvious changes in morphology to eliminate larger fracture origins in CNT-dispersed ceramics. As a result of both the addition of HfO2 and bead milling, we successfully fabricated dense CNT-dispersed Si3N4 ceramics with high strength and electrical conductivity.

Potential use of only Yb2O3 in producing dense Si3N4 ceramics with high thermal conductivity by gas pressure sintering

Yb2O3 is an efficient sintering additive for enhancing not only thermal conductivity but also the high-temperature mechanical properties of Si3N4 ceramics. Here we report the fabrication of dense Si3N4 ceramics with high thermal conductivity by the gas pressure sintering of α-Si3N4 powder compacts, using only Yb2O3 as an additive, at 1900 °C under a nitrogen pressure of 1 MPa. The effects of Yb2O3 content, sample packing condition and sintering time on the densification, microstructure and thermal conductivity were investigated. Curves of the density plotted against the Yb2O3 content exhibited a characteristic ‘N’ shape with a local minimum at 3 mol% Yb2O3 and nearly complete densification below and above this concentration. The effects of the sample packing condition on the densification, microstructure and thermal conductivity strongly depended on the Yb2O3 content. The embedded condition led to more complete densification but also to a decrease in thermal conductivity from 119 to 94 W m-1 K−1 upon 1 mol% Yb2O3 addition. The sample packing condition had little effect on the density and thermal conductivity (102–106 W m−1 K−1) at 7 mol% Yb2O3. The thermal conductivity value was strongly related to the microstructure.

Thermal Conductivity Improvement by Heat-Treatment in Si<sub>3</sub>N<sub>4</sub> Ceramics Using SiO<sub>2</sub>-MgO-Y<sub>2</sub>O<sub>3</sub> Additive System

Silicon nitride (Si3N4) ceramics have been interested for electrical substrate applications, because the ceramics can be made highly mechanical strength, fracture toughness, electrical resistivity and high thermal conductivity. Generally, relatively large amount of additives are required to obtain dense Si3N4 ceramics. During sintering, additives react with SiO2 including surface oxide of Si3N4 raw powder to form a liquid phase. Most of liquid phase changed into glassy phase during cooling down. In this study, Si3N4 ceramics were fabricated by gas pressure sintering. Yttrium oxide (Y2O3), silica (SiO2), and magnesia (MgO) were used for liquid-phase-enhanced sintering process. Dense materials were sintered by this process, but their thermal conductivities were not so high (30-40 W/m·K). Therefore, post-sintering heat-treatment process was performed to reduce the excess amount of glassy phase. An additive system (3 mass% SiO2 with 3 mass% MgO and 1-5 mass% Y2O3) was selected as the sintering aid. These ceramics could be sintered to almost full density at relatively low temperature as 1650oC for 2 h under 0.1 MPa-N2 without packing powder. The resulting materials have high bending strength, about 1 GPa, when 5mass% of Y2O3 was added. Based on the creation of low temperature pressureless sintering without packing powder, a novel two-step sintering (once firing) was proposed. The two-step sintering conducted by sintered at 1650oC under 0.1 MPa-N2 for 2 h for densification in the first step. Followed by heated up to and kept at 1950oC for 8 h under 1.0 MPa-N2 in the second step. The Si3N4 ceramics could be fabricated with relatively high thermal conductivity of 90 W/m·K. Mass loss, microstructure, mechanical properties, oxygen content and chemical composition were discussed.

Erosion Wear Characteristics of Si<sub>3</sub>N<sub>4</sub> Fabricated from Low-Cost Powders

High producing cost is one of the important reasons that silicon nitride ceramics can’t be widely used today. In this paper, one kind of commercial low-cost silicon nitride powder was used as raw material. High density samples were obtained through pressureless sintering with alumina, yttria and magnesia as sintering aids. The Si3N4 material was tested and the erosion wear rate was measured. It was found that the low cost and dense Si3N4 ceramics exhibited a better erosion wear resistance than the hot-press ceramics produced with the UBE Si3N4 powder and much better then the Cr-26 cast iron. The worn surface was observed with the SEM and the erosion wear mechanisms were analyzed.

Microwave Sintering and Infiltration of Highly Porous Silicon Nitride Ceramics to Form Dense Ceramics

A new processing route for porous b-Si3N4 –ceramics suitable as substrates for Si3N4 functionally graded materials, FGM`s with an environmental barrier coating, EBC, is described. Such materials could be useful for operation at temperatures up to 1650°C in combustion engines, where oxidation and corrosion under hydrothermal conditions cause severe damage of known dense Si3N4-ceramics. The new process is based on microwave sintering of a-Si3N4–Yb-silicate green parts to obtain up to 60% open porous b-Si3N4 pre-forms equilibrated at high temperature with respect to the RE-silicate content, followed by Si-melt infiltration. A sufficient wetting by molten silicon is achieved by coating the inner surface of the porous pre-form with amorphous carbon prior to infiltration. The influence of different green processing parameters and different heating methods - microwave heating and conventional heating – on the porosity of the pre-form is reported. Infiltration is performed by microwave heating of the carbon coated pre-forms, whereas for nitridation both heating processes are combined.

Long Crack R‐Curve of Aligned Porous Silicon Nitride

Long crack R‐curve of a porous Si3N4 with aligned fibrous grains was investigated, using a chevron‐notched beam technique. A crack was constrained to propagate normal to the grain alignment. The crack growth resistance of aligned porous Si3N4 was much larger compared with that of dense Si3N4 ceramics. Microstructure observations showed that pullouts of fibrous grains in aligned porous Si3N4 markedly increased during crack propagation relative to those of dense Si3N4, due to the existence of pores. The efficient grain pullouts in porous Si3N4 increased the bridging stress at the crack wake.