Sintered Reaction-bonded Silicon Nitride Research Articles

Thermal conductivity prediction of sintered reaction bonded silicon nitride ceramics using a machine learning approach based on process conditions

Thermal conductivity (TC) of sintered reaction bonded silicon nitride (SRBSN) ceramics was predicted from process conditions using support vector regression (SVR) as a machine learning algorithm. A total of ten explanatory variables were selected from the process conditions for building a SVR model. The determination coefficient (R2) in the testing results of the trained SVR model reached a satisfactory level (R2 = 0.8 on average) when more than 100 data points were used as the training data. The relative importance scores of the explanatory variables revealed that sintering conditions (temperature and time) had the largest contribution to the TC prediction while those of oxygen impurity content in the main starting powder, types and concentrations of sintering additives and nitriding conditions were also influential to a certain extent.

Relation between crystal structure and lattice oxygen content of sintered reaction‐bonded silicon nitride

AbstractWhen reaction‐bonded silicon nitride containing MgO/Y2O3 additives is sintered at three different temperatures to form sintered reaction‐bonded silicon nitride (SRBSN), the thermal conductivity increases with sintering temperature. The β‐Si3N4 (silicon nitride) crystals of SRBSN ceramics were synthesized and characterized to investigate the relation between the crystal structure and the lattice oxygen content. The hot‐gas extraction measurement result and the crystal structure obtained using Rietveld analysis suggested that the unit cell size of the β‐Si3N4 crystal increases with the decrease in the lattice oxygen content. This result is reasonable considering that the lattice oxygen with the smaller covalent radius substitutes nitrogen with the larger one in the β‐Si3N4 crystals. The lattice oxygen content decreased with increasing sintering temperature which also correlated with increase in thermal conductivity. Moreover, it is noteworthy from the viewpoint that it may be possible to apply the lattice constant analysis for the nondestructive and simple measurement of the lattice oxygen content that deteriorates the thermal conductivity of the β‐Si3N4 ceramics.

The effect of silicon particle size on the characteristics of porous sintered reaction bonded silicon nitride

The effect of silicon particle size on the β-phase ratio, microstructure and the flexural strength of porous sintered reaction-bonded silicon nitride(SRBSN) was systematically investigated. Six kinds of raw powders containing Yb2O3 addtive (5, 10 wt%), prepared with different particle size distribution, were prepared by planetary ball-milling duration of 0, 3, 24 h. The behavior of α-to-β phase transformation of Si3N4 sintered at 1650 °C and 1750 °C, was found to be in good relation with SiO2 molar ratio over all the oxide additives. High flexural strength of SRBSN over 400 MPa was obtained when the amount of SiO2 was less than 0.7 owing to having the rod-like β-particles. Otherwise, the flexural strength of SRBSN became lower due to major occurrence of the equiaxed β-particles. This significant difference was considered to be caused from the seeding effect of β-ratio of RBSN samples as well as the different solubility of nitrogen element in the eutectic liquid phase.

Characterization of porous sintered reaction-bonded silicon nitride containing three different rare-earth oxides

The effect of rare-earth oxides on the characteristics of porous sintered reaction-bonded silicon nitride (SRBSN) was investigated. Three types of raw Si powder mixtures containing different rare-earth oxide (La2O3, Er2O3, and Yb2O3) were prepared and nitrided in the form of compacts. The nitriding profiles of the respective raw powder mixtures with elevating temperature indicated that Yb2O3 clearly promoted the nitridation of Si compacts at low temperature compared with other rare-earth oxides, and the β-Si3N4 ratio after completion of the nitriding reaction was different at which temperature the major nitridation occurred. Yb2O3 was found to be the most effective additive to achieve a strong porous SRBSN, having the flexural strength of 441 MPa. The reason why Yb2O3 promotes the nitridation reaction and has excellent mechanical properties after post-sintering is that oxygen was removed during the nitriding reaction, which was supported by the analysis of oxygen content via the laser-induced breakdown spectroscopy.

Controlling the width of particle size distribution of Si powder and properties of sintered reaction-bonded silicon nitride (SRBSN) ceramics with high thermal conductivity

Sintered reaction-bonded silicon nitride (SRBSN) for high-thermal-conductivity ceramic substrates was fabricated from waste Si scrap. To prepare homogenously sized Si powder, the width of the particle size distribution (WPSD) was selected for the first time as the main index to characterize the milled Si powder and was systematically controlled. The coarse and irregular shape of the as-received Si scrap was carefully pulverized by controlling planetary milling parameter and then processed. As the WPSD of Si powder was narrower, the thermal conductivity of SRBSN ceramics increased from 67 to 81 W/mK with enhancing grain growth under the same sintering condition. Furthermore, to fabricate SRBSN ceramics with higher thermal conductivity, the effects of the milling fluid and the amount of sintering additive were investigated. It was proven that a smaller amount of a sintering additive was effective for enhancing the thermal and mechanical properties of SRBSN. Selection of the fluid for milling Si powders was an important inflection point for the surface properties of Si powder, which greatly influence the nitridation and thermal conductivity of the SRBSN ceramics.

Microstructure and Thermal Conductivity of Sintered Reaction‐Bonded Silicon Nitride: The Particle Size Effects of MgO Additive

The particle size effect of MgO as a sintering additive on the thermal conductivity of sintered reaction‐bonded silicon nitride (SRBSN) was investigated. It was revealed that the size of MgO is critical for thermal conductivity with regard to the microstructural evolution process. That is, the abnormal grain growth promoted by an inhomogeneous liquid‐phase distribution led to higher thermal conductivity when coarser MgO was added, whereas a relatively homogeneous liquid‐phase distribution induced moderate grain growth and lower thermal conductivity when finer MgO was added.

High throughput hybrid laser assisted machining of sintered reaction bonded silicon nitride

Silicon nitride is a high-performance ceramic reserved for the most demanding high-temperature structural applications due to its elevated strength, fracture toughness, corrosion resistance and hardness. These aspects also make it extremely difficult to machine, leading to component costs that are prohibitive in many fields where its physical properties could nonetheless provide improvements in performance, efficiency and lifespan. In the present work, precision two-step laser assisted machining of sintered reaction bonded silicon nitride is performed to reduce grinding forces and improve tool service life. Laser surface treatment is first undertaken at 15mm/s with a 980nm wavelength 3.3kW diode laser focused to a 32mm×2mm rectangular spot, inducing thermal cracks to a depth of approximately 615μm with 2.5–3kW laser power, corresponding to an energy dose of 4.7–5.6J/mm2. Grinding tests are then performed on the treated areas, confirming a reduction in peak and average machining forces of approximately 26–27% where thermal cracks are present. A numerical simulation is developed to provide insight into the failure mechanisms leading to thermal cracking and afford a relatively simple method of selecting laser parameters for real mechanical components.

Effect of Si addition on the mechanical and thermal properties of sintered reaction bonded silicon nitride

Advanced silicon nitride (Si3N4) ceramics were fabricated using a mixture of Si3N4 and silicon (Si) powders via conventional processing and sintering method. These Si3N4 ceramics with sintering additives of ZrO2+Gd2O3+MgO were sintered at 1800°C and 0.1MPa in N2 atmosphere for 2h. The effects of added Si content on density, phases, microstructure, flexural strength, and thermal conductivity of the sintered Si3N4 samples were investigated in this study. The results showed that with the increase of Si content added, the density of the samples decreased from 3.39g/cm3 to 2.92g/cm3 except for the sample without initial Si3N4 powder addition, while the thermal diffusivity of the samples decreased slightly. This study suggested that addition of Si powder, which varied from 0 to 100%, in the starting materials might provide a promising route to fabricate cost-effective Si3N4 ceramics with a good combination of mechanical and thermal properties.

Improved thermal conductivity of sintered reaction-bonded silicon nitride using a BN/graphite powder bed

Sintered reaction-bonded silicon nitride (SRBSN) with improved thermal conductivity was achieved after the green compact of submicron Si powder containing 4.22wt% impurity oxygen and Y2O3-MgO additives was nitrided at 1400°C for 6h and then post-sintered at 1900°C for 12h using a BN/graphite powder bed. During nitridation, the BN/10wt%C powder bed altered the chemistry of secondary phase by promoting the removal of SiO2, which led to the formation of larger, purer and more elongated Si3N4 grains in RBSN sample. Moreover, it also enhanced the elimination of SiO2 and residual Y2Si3O3N4 secondary phase during post-sintering, and thus induced larger elongated grains, decreased lattice oxygen content and increased Si3N4-Si3N4 contiguity in final SRBSN product. These characteristics enabled SRBSN to obtain significant increase (∼40.7%) in thermal conductivity from 86 to 121 W∙m−1∙K−1 without obvious decrease in electrical resistivity after the use of BN/graphite instead of BN as powder bed.

Effects of microstructure and intergranular glassy phases on thermal conductivity of silicon nitride

In this study, the binary sintering additives Y2O3-Sc2O3, were first applied to the Si3N4 system to investigate their effects on microstructure and thermal conductivity. The microstructure and thermal conductivity of both sintered silicon nitride (SSN) and sintered reaction-bonded silicon nitride (SRBSN) were found to be significantly dependent on the additive composition. Among various combinations of Y2O3 and Sc2O3, 1mol% Y2O3−3mol% Sc2O3 prominently enhanced thermal conductivity, and the enhancement could not be attributed to any difference in microstructure or lattice defects. TEM observation revealed that this composition was more liable to devitrify the glassy phase with a lower degree of stress accumulation, and to possibly produce a grain boundary that was cleaner or with a higher order of atomic arrangement. A microstructure model for thermal conductivity was proposed which took the thermal resistance of the grain boundaries into account. The grain boundary state exerted a remarkable influence on the thermal conductivity of fine microstructures, and the experimentally measured thermal conductivity values were consistent with those given by the proposed model.

Sintered reaction-bonded silicon nitride foams with a high level of interconnected porosity

Sintered reaction-bonded silicon nitride (SRBSN) foams with very high porosity levels of up to 85 vol% and pore interconnectivity have been fabricated in this study. SRBSN foams were fabricated based on gel-casting of Si-PMMA suspensions followed by nitriding and sintering. The addition of MgO and Y2O3–Al2O3 as sintering additives resulted in significant changes in the foam microstructure. Small amounts of MgO completely prevented the whisker-forming reactions during nitridation while the addition of Y2O3–Al2O3 could not prevent whisker formation during nitridation. It was observed that regardless of the sintering aid system, SRBSN foams were whisker-free and revealed high pore interconnectivity after sintering. The SRBSN foam struts were only composed of β-Si3N4 grains embedded in an intergranular amorphous phase.

The effect of fabrication parameters on the mechanical properties of sintered reaction bonded porous Si3N4 ceramics

Porous silicon nitride ceramics were prepared via sintered reaction bonded silicon nitride at 1680 °C. The grain size of nitrided Si3N4 and diameter of post-sintered β-Si3N4 are controlled by size of raw Si. Porosity of 42.14–46.54% and flexural strength from 141 MPa to 165 MPa were obtained. During post-sintering with nano Y2O3 as sintering additive, nano Y2O3 can promote the formation of small β-Si3N4 nuclei, but the large amount of β-Si3N4 (>20%) after nitridation also works as nuclei site for precipitation, in consequence the growth of fine β-Si3N4 grains is restrained, the length is shortened, and the improvement on flexural strength is minimized. The effect of nano SiC on the refinement of the β-Si3N4 grains is notable because of the pinning effect, while the effect of nano C on the refinement of the β-Si3N4 grains is not remarkable due to the carbothermal reaction and increase in viscosity of the liquid phase.

Effects of Impurity Iron Content on Characteristics of Sintered Reaction‐Bonded Silicon Nitride

The effects of impurity iron content on characteristics of sintered reaction‐bonded silicon nitrides were examined by adding iron powder to a high purity raw Si powder. Powder compacts of the raw Si powder doped with 2 mol% Y2O3 and 5 mol% MgSiN2 as sintering additives and Fe as impurity (0 mass%, 0.1 mass%, 1.0 mass% and 5.0 mass%) were nitrided at 1400°C for 8 h under a N2 pressure of 0.1 MPa, followed by post‐sintering at 1900°C for 6 h under a N2 pressure of 0.9 MPa. All the SRBSN (Sintered Reaction‐Bonded Silicon Nitride) specimens had about the same 4‐point bending strength of 730–770 MPa. The fracture toughness of the specimens was gradually decreased with increasing Fe additive amount due to the inhibition of development of rodlike β‐Si3N4 grains by SiFex particles formed during nitridation process. The thermal conductivity was also decreased with an increase in Fe amount. It seems that the increasing oxygen in grain‐boundary phase caused by the oxidation of Fe during milling resulted in the increase in lattice oxygen of β‐Si3N4 grains, which caused phonon scattering and thereby decreased thermal conductivity of β‐Si3N4. There was little change in the dielectric breakdown strength of the specimens: 24, 22, 22, and 21 kV/mm for the specimens without Fe, and with 0.1 mass%, 1.0 mass% and 5.0 mass% Fe, respectively. The surface resistivity of the specimens with 0 mass%, 0.1 mass% and 1.0 mass% Fe was in the range of 1013 Ω, but the specimen with 5 mass% Fe was about one order lower than the others.

Cutting tool material selection using grey complex proportional assessment method

In today’s metalworking industry, many types of materials, ranging from high carbon steel to ceramics and diamonds, are used as cutting tools. Because of the wide range of conditions and requirements, no single cutting tool material meets all the needs of machining applications. Each tool material has its own properties and characteristics that make it best for a specific machining application. While evaluating a cutting tool material for a machining operation, the applicability is dependant on having the correct combination of its physical properties. Thus, it is extensively important to select the most appropriate cutting tool material with the desired properties for enhanced machining performance. This paper considers an exhaustive list of 19 cutting tool materials whose performance are evaluated based on ten selection criteria. The grey complex proportional assessment (COPRAS-G) method is then applied to solve this cutting tool material selection problem considering grey data in the decision matrix. Synthetic single crystal and polycrystal diamonds emerge out as the best two choices. Oil quenched tool steel (AISI O2) and powder metal tool steel (AISI A11) may also be used as the suitable cutting tool materials. Sialon and sintered reaction bonded silicon nitride are the worst chosen cutting tool materials.

Processing of micro-components made of sintered reaction-bonded silicon nitride (SRBSN). Part 2: Sintering behaviour and micro-mechanical properties

This second part of the report deals with, how the sintering additives Y 2O 3, Al 2O 3, and MgO influence the sintering behaviour of SRBSN. Paraffin-based feedstocks with varying sintering aid compositions and silicon grain size were used for moulding macro- and micro-scale samples. It was observed that compositions with smaller Si grain size (with correspondingly high SiO 2 content) and containing Al 2O 3 as sintering additive exhibit higher shrinkage and lower residual porosity when sintered at 1700–1800 °C after nitridation. The mechanical properties determined for micro-scale samples were obtained by three-point bending tests, with the resulting characteristic strength values σ 0 ranging from 500 MPa up to 1200 MPa. Surprisingly residual porosity did not play the role of a strength limiting factor; rather it was observed that the presence of crystalline secondary phases – mainly Y 2Si 3O 3N 4 – was responsible for reducing the micro-bending strength. As micro-samples exhibit a large surface-to-volume ratio they are in particular affected by decomposition of Si 3N 4 and volatilization of SiO 2 which is considered to be responsible for the occurrence of secondary phases preferred at the sample surface. The powder bed condition was also found to play a prominent role in the development of the secondary phases during liquid phase sintering.

Surface strength of silicon nitride in relation to rolling contact performance

Silicon nitride material has been traditionally used as bearing material due to its superior performance against bearing steel. Its successful application as a bearing element has led to the development of rolling contact applications in turbomachinery and automotive industries. In the case of latter, this is especially true for the engine manufacturing industry where its excellent rolling contact performance can make significant savings on warranty cost for engine manufactures. In spite of these advantages, the remaining limitation for their broader application is the high component machining cost. Further understanding of rolling contact performance of silicon nitride in relation to its surface integrity will enable engine manufacturers to produce components that meet the design requirements while at the same time reduce the machining cost. In the present study, the relationship between the C-sphere strength of a silicon nitride and its rolling contact fatigue life is investigated. The C-sphere test is used here to compare the strengths of three batches of sintered reaction-bonded silicon nitride (SRBSN) specimens with different subsurface quality induced by variation of machining parameters. In parallel, the rolling contact fatigue (RCF) performance of those machining conditions is studied on a modified four-ball tester. The results show that the most aggressively machined specimens have the weakest C-sphere strength and the shortest RCF life. This positive relationship can give component manufacturers a valuable reference when they make selections of candidate material and finishing standards.

Processing of micro-components made of sintered reaction-bonded silicon nitride (SRBSN). Part 1: Factors influencing the reaction-bonding process

In order to establish a process for the manufacturing of injection moulded micro-components of sintered reaction-bonded silicon nitride (SRBSN) several process parameters were investigated with regard to their influence on the reaction-bonding step. One question to be answered was how the sintering aids affect the nitridation behaviour of a silicon green body. For the processing of micro-components it was of special interest to study, how a decreasing sample size and wall thickness would influence the rate of Si 3N 4 formation. By varying the added amounts of the sintering aids, it was found that increasing the Y 2O 3 and MgO contents both improved the nitridation rate, whereas an increase of Al 2O 3 content resulted in reduced nitridation rates. Within the investigated range of sample dimensions (0.2–4.0 g) the unexpected observation was made, that with decreasing sample weight the nitridation rate also decreased. This was explained by the exothermic nature of the reaction between Si and N 2 and the fact that small samples with a large surface-to-volume ratio attain thermal equilibrium with their environment better than large samples which may be subject to local overheating.

Sintered Reaction‐Bonded Silicon Nitride with High Thermal Conductivity and High Strength

Sintered reaction‐bonded silicon nitride (SRBSN) materials were prepared from a high‐purity Si powder doped with Y2O3 and MgO as sintering additives by nitriding at 1400°C for 8 h and subsequently postsintering at 1900°C for various times ranging from 3 to 24 h. Microstructures and phase compositions of the nitrided and the sintered compacts were characterized. The SRBSN materials sintered for 3, 6, 12, and 24 h had thermal conductivities of 100, 105, 117, and 133 W/m/K, and four‐point bending strengths of 843, 736, 612, and 516 MPa, respectively. Simultaneously attaining thermal conductivity and bending strength at such a high level made the SRBSN materials superior over the high‐thermal conductivity silicon nitride ceramics that were prepared by sintering of Si3N4 powder in our previous works. This study indicates that the SRBSN route is a promising way of fabricating silicon nitride materials with both high thermal conductivity and high strength.

Processing and properties of sintered reaction-bonded silicon nitride with Y 2O 3–MgSiN 2: Effects of Si powder and Li 2O addition

The effects of Si powder and Li 2O addition on the processing, thermal conductivity and mechanical properties of sintered reaction-bonded silicon nitride (SRBSN) with Y 2O 3–MgSiN 2 sintering aids were studied. Addition of Li 2O provides a less-viscous liquid phase that results in a more uniform and finer pore structure in RBSN with the coarser Si powders, but the pore structure plays a less important role in the densification of RBSN. The thermal conductivity of SRBSN without porosity decreases with increased Al impurity content and also decreases with the Li 2O addition regardless of the Si purity. The impurest coarse Si powder produces the lowest thermal conductivity (93 W m −1 K −1) but the highest four-point bending strength (∼700 MPa) and a higher fracture toughness (∼10 MPa m 1/2). However, the purer fine Si powder produces the highest thermal conductivity (119 W m −1 K −1) and highest toughness (∼11 MPa m 1/2) but the lowest strength (∼500 MPa).

Processing and Thermal Conductivity of Sintered Reaction‐Bonded Silicon Nitride. I: Effect of Si Powder Characteristics

This paper will present sintered reaction‐bonded silicon nitride (SRBSN) material with a high thermal conductivity of 121 W·(m·K)−1, which has been successfully prepared from a coarse Si powder with lower levels of oxygen and aluminum impurities, using a mixture of Y2O3 and MgSiN2 as sintering additives, by nitriding at 1400°C for 8 h and subsequent post‐sintering at 1900°C for 12 h at a nitrogen pressure of 1 MPa N2. This thermal conductivity value is higher than that of the materials prepared from high‐purity α‐Si3N4 powder (UBE SN‐E10) with the same additive composition under the same sintering conditions. In order to study the effects of Si powder characteristics on the processing, microstructure, and thermal conductivity of SRBSN, the other type of fine powder with higher native oxygen and metallic impurity (typically Al and Fe) contents was also used. The effects of Si particle size, native oxygen, and metallic impurities on the nitriding process, post‐sintering process, and thermal conductivity of the resultant SRBSN materials were discussed in detail. This work demonstrates that the improvement in thermal conductivity of SRBSN could be achieved by using higher purity coarse Si powder with lower levels of oxygen and aluminum impurities. In addition, this work also shows that the nitriding temperature has no significant effect on the microstructure and thermal conductivity of SRBSN during post‐sintering, although it does affect the characteristics of RBSN formed during nitridation.