Sintered Silicon Nitride Research Articles

Defect-free grinding of silicon nitride at high material removal rate

Owing to the extreme hardness and toughness of sintered silicon nitride (Si3N4), the material is used in high stress and/or temperature applications such bearings, turbines, and combustion engines. Unfortunately, the same properties which make it ideal for use also make it particularly difficult to machine -- microcracks, inclusions and spalling are all common. While prior research has shown that it is possible to grind sintered Si3N4 without inducing surface damage so long as material is removed entirely under ductile flow, but grind forces associated with ductile Si3N4 material flow are so small as to render the material removal rate (MRR) impractical. Prior researchers have attempted to solve the MRR problem through laser-assisted machining. Laser ablation, by inducing a steep thermal gradient, weakens material through surface and subsurface cracks. Grinding of fractured weakened Si3N4 has been done at upwards of 50 % higher MRR. There are, however, issues with laser ablation, which prevent its widespread use. Laser ablation severely disrupts the microstructure of Si3N4. Because cracks propagate along and through grain boundaries, the irregular morphology makes accurately predicting crack growth from ablation and during subsequent grinding highly problematic. In this proof-of-concept work, researchers determined that it is possible to irradiation weaken Si3N4 without cracking it, and the material can be ground defect-free at a highly productive MRR. Findings suggest present laser-assisted machining methods which fracture weaken Si3N4 prior to grinding may not be the best way to maximize MRR.

Effect of sintering additive composition on microstructure, thermal and mechanical property of multi-step spark plasma sintered silicon nitride

Dense silicon nitride composites were fabricated by multi-step spark plasma sintering (SPS). As the induced phase, 6 wt% Mo was added and related with the unadulterated β-Si3N4 seeds produced by the in-situ reaction during the sintering process. Si3N4 samples sintered with 7 mol% CeO2 and Yb2O3 by multi-step SPS exhibited a higher mechanical property, namely, a bending strength of 981 MPa and a fracture toughness of 8.85 MPa m1/2, while the thermal diffusivity was lower than that for Si3N4 samples sintered with 7 mol% CeO2 (27.77 mm2/s). The result was attributed primarily to the fully phase transformation and the abnormal growth of large β-Si3N4 grains, resulted from a longer time of solution-reprecipitation process during SPS.

Effect of interfacial microstructure evolution on the peeling strength and fracture of silicon nitride/oxygen-free copper foil joints brazed with Ag-Cu-TiH2 filler

Gas-pressure sintered silicon nitride (Si3N4) ceramic was brazed to oxygen-free copper (OFC) foil using 20.22 ± 0.93 mg/cm2 of Ag-Cu-TiH2 filler at 875 ◦C for 0.5 h. The effect of TiH2 content on the 3D morphology, including the cross-section microstructure and etched surface morphology of Si3N4 ceramic/OFC foil joints, was analyzed. An evolution model of the interfacial microstructure for joints was proposed based on 3D morphological analysis. The reaction between the brazing alloy and Si3N4 ceramic formed the interfacial reaction layer and the inside reaction zones during brazing. The growth mechanism of the interfacial reaction layer was discussed in detail. The peeling test was used to evaluate the bonding strength of the Si3N4 ceramic/OFC foil joints, and the optimum peeling strength of 25.1 N/mm was achieved at 5 wt% TiH2 content. The interfacial microstructure of the joints changed with TiH2 content, leading to different main fracture mechanisms and corresponding peeling strengths.

Comment on “Lubauer J, Lohbauer U, Henrich M, Munz M, Lube T, Belli R. Intricacies involving the evaluation of fracture toughness obtained by the surface‐crack‐in‐flexure method. J. Am. Ceram. Soc., 2022;105(12) 7582–7599”

AbstractWe have evaluated over fifty materials using small semi–elliptical controlled surface flaws with the Newman–Raju factors. Although occasionally there were nuances and peculiarities, the results were sound and comparable to other methods. So, despite the lengthy discussions and numerous plots in Lubauer et al.’s paper, what is evident is that if one simply follows the guidelines in ASTM C 1421 and the other standards for most ceramics including the SL200B sintered silicon nitride, and polish off the recommended 4.5 to 5 h material, one will obtain the correct results. Excessive indentation forces and excessive material removal to obtain sharp corner, shallow surface cracks are unwise. Removing more than 5 h should only be done to remove lateral cracks. In such cases the Strobl et al. solutions may be useful. These solutions are an interesting alternative to the reputable Newman–Raju factors, but much more experience and verification is needed before they can be accepted. They and the extension of their analysis for precrack angles χ < 70° need to be vetted in a major engineering journal.

Dielectric constant and flexural strength of micro porous slip cast partially sintered silicon nitride

Slip casting of commercial Si3N4 powder dispersed in de-ionized water with Y2O3 and Al2O3 as sintering aids was carried out using optimum slurry conditions arrived through zeta potential and viscosity measurements. The cast specimens were sintered in graphite furnace in the temperature range of 1600 °C to 1800 °C under 0.15 MPa static N2 pressure. The sintered Si3N4 show density variation from 59% to 82% of theoretical value in the sintering temperature range studied. The 3-point flexural strength varied between 80 MPa and 320 MPa and dielectric constant 3.9 to 5.7 as a function of decreasing porosity from 39% to 17%. The samples exhibit uniform distribution of micro porosity in the range 0.5 μm–2.0 μm and grain size of 0.5 μm–3.0 μm. The microstructure changed from equiaxed and platelet shaped α-Si3N4 grains to needle shaped β- Si3N4 with increase in sintering temperature from 1600 °C to 1800 °C. The dielectric and flexural strength properties found promising for high temperature radome applications.

An investigation on the thermal and microstructural properties of low-cost pressureless sintered silicon nitride (Si3N4) ceramics incorporated with Y2O3-SiO2-MgO

An investigation on the thermal and microstructural properties of low-cost pressureless sintered silicon nitride (Si3N4) ceramics incorporated with Y2O3-SiO2-MgO

Effect of rotary ultrasonic machining parameters on surface integrity of advanced ceramics

The commercial use of advanced ceramics has been hampered by the lack of an efficient, low-cost machining process. Two of the most important requirements for making machining of ceramics economically viable are: i) obtaining high enough material removal rates and ii) developing a machining process that does not cause surface and subsurface damage to the ceramic component. Therefore, it is important to study, in-depth, machining processes that have the potential to machine advanced ceramics efficiently and reliably. In this study, two-level full factorial experiments were designed and run to determine the effect of rotary ultrasonic machining process parameters (grit size, vibration amplitude, and feed rate) on the process outputs (cutting force, surface roughness, and surface compressive residual stresses). The materials under investigation were sintered silicon nitride and sintered reaction-bonded silicon nitride. The experiments reveal the main effects as well as the interaction effects of the process parameters on the process outputs such as cutting force, surface roughness, and surface compressive residual stresses. Using a larger grit size has been found to decreases the tool axial force. Samples that had a rougher surface finish also showed the highest surface residual compressive stresses. Surface compressive residual stresses were present in all samples after rotary ultrasonic machining irrespective of the conditions used. The presence of surface residual compressive stresses was found to be independent of the direction of machining.

Thickness dependence of dielectric breakdown strength for silicon nitride substrate

Silicon nitride ceramics have attracted increasing attention as insulated heat-dissipating substrates for power modules due to their high thermal conductivity and mechanical strength. However, there are very few reports on their dielectric breakdown strength, which was only evaluated for the substrates with thicknesses between 250 and 640 µm, though thinner substrates are preferable for attaining better performance of the module. In this work, dielectric breakdown of sintered silicon nitride substrates with thicknesses ranging from 285 to 15 µm was evaluated for the first time. Average breakdown strength increased from 36.38 to 103.80 kV/mm with decreasing thickness from 285 to 15 µm. It should be noted that the silicon nitride specimen had very high dielectric breakdown voltage of 1.5 kV even with a thickness as small as 15 µm.

Sintering and properties of materials based on nanosized silicon nitride powder

A high-density material (ρ = 3.28 g / cm3, porosity 1.4% by volume) based on nanosized silicon nitride powder was obtained by the method of liquid-phase sintering. The effect of compacting pressure on the density of sintered silicon nitride materials is shown. The process of material structure formation during sintering has been studied. The main physical and mechanical characteristics were determined: density, porosity and grain size of the solid phase, elastic modulus, flexural and compressive strength, crack resistance coefficient, Vickers hardness.

Effect of Mo particle size on microstructure and mechanical properties of spark plasma sintered silicon nitride

Silicon nitride ceramic composites were densified by spark plasma sintering with different-sized Mo particles. In-situ synthesis of Mo-Si compound can accelerate the nucleation of β-Si3N4 grain, and the number of β-Si3N4 grain nucleation increases with decrease of Mo particle size during spark plasma sintering. The effect of Mo particle size on the grain growth of β-Si3N4 is significant, while that for the phase composition of Si3N4 samples is negligible. Si3N4 ceramic composites with 6 wt% nano-sized Mo addition are shown to have the highest bending strength (922.56 MPa) and fracture toughness (8.94 MPa m0.5), which is attributed primarily to more elongated β-Si3N4 grains obtained by more β-Si3N4 grain nucleation.

Blue-violet emission of silicon carbonitride thin films prepared by sputtering and annealing treatment

In the study of the blue-violet emission properties of SiCN thin films, the emission intensity depends on the crystal structure of thin films. The annealing process is widely used in the preparation process of thin films as one of the effective means to remove defects and improve crystal quality. In this study, SiCN thin films were prepared by RF magnetron sputtering method utilizing sintered silicon nitride and graphite targets on Si/SiO2 substrates. Then, the samples were separately annealed from 600 to 1000℃ with an argon ambient for 30 min to optimize the photoluminescence performance of the blue-violet emission. The characterization results of XRD spectra show that the crystallinity of the SiCN sample with 900 °C annealing temperature is the best. The chemical bonds, and the surface morphology of the samples changed greatly after annealing. The RMS roughness of the SiCN samples increased from 1.9 nm to 20.19 nm before and after annealing at 900 °C. Also, the results of PL spectra show that the intensity of blue-violet emission at 365 nm reinforced 28.6 times after annealing at 1000 °C. It makes the SiCN film be a candidate of phosphor for blue-violet LEDs.

Tough negative temperature coefficient diamond thermistors comprising tungsten carbide ohmic contacts

This work presents a novel methodology for the fabrication of planar diamond film NTC (Negative Temperature Coefficient) thermistors on sintered silicon nitride (Si3N4) ceramic substrates. Such devices comprise a temperature sensitive diamond coating on one of the substrate surfaces and tungsten carbide ohmic contacts on the opposite side. Hot filament chemical vapor deposition (HFCVD) technique was used for the fabrication of both temperature sensitive flat diamond surface and WC ohmic contacts. The doping source for the semiconducting diamond layers was boron oxide dispersed in ethanol, and was added to the hydrogen/methane gas mixture using argon gas. The fabrication of WC contacts was done by first evaporating the oxide layer of the tungsten filaments used for activating the gaseous species. Hydrogenation and carburization steps were then used to produce the carbide layer. Using this approach, NTC diamond thermistors were obtained and their electrical resistance dependence with temperature was plotted and fitted to the Steinhart-Hart equation, giving β values between 1200 and 2000 K, from room temperature up to 420 °C. The WC-diamond contacts ohmic character was confirmed within the same temperature range, which highlights the potential application of such devices in precise in situ temperature measurements, at high temperature and under mechanically tough environment conditions.

Cutting performance and wear mechanism of spark plasma–sintered silicon nitride ceramics tool in dry turning of 41Cr4 hardened steel

Cutting performance and wear mechanism of spark plasma-sintered Si3N4 (SN91) and Si3N4/WC (SW10) ceramic tools in the dry turning of 41Cr4 hardened steel are studied. SN91 tool shows better wear resistance than SW10 tool and commercial ceramic tool SW500 and cemented carbide tool YG8 under the same cutting conditions. The better cutting performance of SN91 can be attributed to its excellent high-temperature flexural strength. The most significant factor affecting the tool life of SN91 is the cutting speed. A higher material removal volume can be achieved at a low cutting speed, large depth of cut, and large feed rate. A long cutting distance/time can be achieved at a low cutting speed, small depth of cut, and feed rate. At a cutting speed of 103.62 m/min (ap = 0.1 mm, and f = 0.05 mm/rev), the wear mechanisms of SN91 are adhesion, oxidation, and diffusion, and no crater wear is observed on the rake face. At a cutting speed of 214.31 (ap = 0.3 mm and f = 0.05 mm/rev) and 314 m/min (ap = 0.2 mm and f = 0.05 mm/rev), the failure forms of SN91 tool are mainly rake face crater wear and flank wear; the wear mechanisms of rake face are diffusion, abrasion, and oxidation; and the wear mechanisms of flank face are abrasion, adhesion, and oxidation. The failure forms of SW10 tool are mainly rake face crater wear and flank wear, and the wear mechanisms are diffusion, abrasion, adhesion, and oxidation.

Fracture Toughness of Silicon Nitride Measured by the Surface Crack in Flexure (SCF) Test Method

The surface crack in flexure (SCF) is a method for the evaluation of the fracture toughness of advanced ceramics. Conventionally is practiced by using a Knoop indenter to make a very small pre-crack. Removal on indent and the plastically deformed zone is required before the fracture test. The purpose of this removal is to eliminate residual stresses under the Knoop impression and to obtain a semi elliptical pre-crack shape. In this work fracture toughness values by the SCF method are compared with those measured using the SEVNB (single edge V-notched beam) method. The material chosen for this purpose was gas pressured sintered silicon nitride (Si3N4) containing 3wt. % Al2O3 and 3 wt. % Y2O3 (SL200B, Ceram Tec, Plochingen, Germany). The varied parameters the amount of material removed from the surface. Short specimens with size 3 x 4 x >25 mm were prepared for this purpose. The fracture toughness of specimens with a surface removal in the range suggested from ASTM C 1421 were found to agree with the results obtained from SEVNB. A surface removal below the recommendation resulted in low values of fracture toughness. Increasing the amount of surface removal moderately was found to still fit with results obtained from SEVNB. Surface removal of much more from the recommended amount leads to failure from natural flaws.

New method of free silicon determination in pressureless sintered silicon nitride by Raman spectroscopy and XRD

Abstract Formation of silicon affects different physical properties of silicon nitride ceramics. Decomposition of Si3N4 and formation of free Si are highly important processes and depend on many factors. The proposed method of combined nano-Raman spectroscopy and X-ray diffraction (XRD) allows quantitative analysis of Si in silicon nitride. Raman spectroscopy enables the determination of atomic bonds and rapid and easy identification of free silicon. Further analysis of the crystalline phases by XRD enables the calculation of the amount of free silicon. The proposed complex method allows the characterization of such complicated processes as silicon nitride decomposition, microstructure formation and, in particular, the formation of the nanoscale grain boundary phase because Si nanosized precipitates are the nucleants of secondary phases during crystallization. Strong 522.8 cm−1 mode and 943.1–984.3 cm−1 transverse optical modes of free Si were clearly observed in the investigated silicon nitride that was subjected to pressureless sintering at 1800 °C. Reported ceramics demonstrated typical microstructures with elongated grains and relatively high microhardness and Young's modulus. It was shown that the aspect ratio depended linearly on the microhardness and Young's modulus. High values of the Young's modulus (more than 290 GPa) and microhardness (more than 1800 HV) were shown for reported silicon nitride produced by hot pressing and pressureless sintering via cold isostatic pressing with a higher quantity of sintering agent. The features of molecular structure of the reported Si3N4 ceramics were clearly described and discussed in detail and were found to be in good agreement with the microstructure and phase composition of these ceramics.

Effect of nano-sized sintering additives on microstructure and mechanical properties of Si3N4 ceramics

Nano-sized sintering additives such as MgO, Al2O3, Y2O3 and La2O3 are added to process Si3N4 based ceramics. The effects of such sintering additives on the phase transformation, mechanical properties and microstructure of silicon nitride were investigated. XRD analysis of sintered silicon nitride with these sintering additives have shown the complete transformation of α-Si3N4 into β-Si3N4. The silicon nitride ceramics sintered with nano-sized additives (4MgO-4Al2O3–5La2O3) have exhibited the highest fracture toughness. This is not only due to the weak glass|grain interface but also due to the formation of high aspect ratio grains (β-Si3N4). Superior mechanical properties of Si3N4 can be obtained by way of adding small and large RE3+ radii RE2O3 (Y2O3 and La2O3) together than the addition of single RE2O3.

The effect of silicon carbide on the mechanical and thermal behavior of spark plasma sintered silicon nitride ceramics with Al2O3 and Y203 additives

Silicon nitride-silicon carbide composite ceramics were spark plasma sintered with yttrium oxide and aluminium oxide as additives while varying the percentage weight of silicon carbide from 3 to 12%. Relative density together with Vickers hardness and fracture toughness of the material were measured. Furthermore, holes were drilled using Nd;YAG laser to investigate the thermal stability of the materials. From the results, it was observed that the relative density of the material depended highly on the silicon carbide composition with 99.997% relative density obtained at 12% silicon carbide composition. The Vickers hardness and the fracture toughness slightly increased with increase in the silicon carbide composition from 1943 HV and 8.5 MPa·√m at 3% to 1971 HV and 8.991 MPa·√m at 12%. A huge impact of the silicon carbide composition was observed with thermal behavior in a manner that as the silicon carbide content increased, the material was becoming more thermally stable in high laser energy parameters.

Effect of Sintering Temperature and Yttrium Composition on the Densification, Microstructure and Mechanical Properties of Spark Plasma Sintered Silicon Nitride Ceramics with Al2O3 and Y2O3 Additives

Three types of silicon nitride ceramics powder blends were sintered by spark plasma sintering at different temperatures and pressures. The relative densities, microstructures, hardness and toughness of the material were investigated with the sintering behaviour. It was observed that the relative density depends on the sintering temperature, the sintering pressure and the yttrium composition. The densification of the sintered ceramics increased with increasing sintering temperature, application of step pressure and yttrium composition, facilitating necking of grains. This in turn affected the microstructure and the mechanical properties of the materials in a manner that the highly dense material induced full compacted particle deformation observed in the microstructures and also high mechanical properties of as high as 1841 HV and 8,87 MPa·√m with the relative density of 99%.

Electrical resistivity of silicon nitride produced by various methods

It has been shown that the grain growth and amount of the glass phase influence the electrical resistivity of pressureless sintered and spark plasma sintered silicon nitride. Sintering additives strongly affect the impurity conductivity of pressureless sintered silicon nitride and slightly influence the intrinsic conductivity due to the longer sintering process as compared with the spark plasma sintering. It was demonstrated that Al2O3-Y2O3 lead to decrease in the electrical resistivity of SPSed silicon nitride due to increase in the band gap width as opposed to Al2O3-MgO. Effect of the sintering additive on the impurity conductivity is practically absent but there is a strong dependence of the sintering temperature for reported spark plasma sintered silicon nitride. However, intrinsic conductivity of SPSed silicon nitride is affected by both sintering temperature and sintering additive. It was also shown that electrical resistivity of produced ceramics is linearly depends on the content of β-Si3N4 and microhardness. Electrical resistivity of manufactured silicon nitride varied from 3.16·109 to 1.73·1011 Ω m. It has been observed strong influence of the sintering additive and sintering temperature on the electrical properties of SPSed and pressureless sintered silicon nitride.

The Effect of Indentation Load, Tilted Angle, and the Amount of Surface Removal to the Fracture Toughness of Silicon Nitride Measured by the Surface Crack in Flexure (SCF) Test Method

The surface crack in flexure (SCF) is a method for the evaluation of the fracture toughness of advanced ceramics. Conventionally is practiced by using a Knoop indenter to make a very small precrack. Removal on indent and the plastically deformed zone is required before the fracture test. The purpose of this removal is to eliminate residual stresses under the Knoop impression and to obtain a semi elliptical precrack shape. In this work the influence of the variation of several experimental parameters is investigated. Fracture toughness values by the SCF method are compared with those measured using the SEVNB (single edge V-notched beam) method. The material chosen for this purpose was gas pressured sintered silicon nitride (Si3N4) containing 3wt.%Al2O3 and 3 wt.%Y2O3 (SL200B, Ceram Tec, Plochingen, Germany). The varied parameters were indentation load, orientation of the indentation crack with respect to the bending axis and the amount of material removed from the surface. Additional investigations were performed to determine the crack geometry for various indentation loads. A procedure was developed to facilitate the location and measurement of the precrack size on the fracture surfaces. The effect of measurement accuracy of the results is evaluated. The fracture toughness of specimens with a surface removal in the range suggested from ASTM C 1421 were found to agree with the results obtained from SEVNB. A surface removal below the recommendation resulted in low values of fracture toughness. Increasing the amount of surface removal moderately was found to still fit with results obtained from SEVNB. Surface removal of much more from the recommended amount leads to failure from natural flaws. Observations from serial sectioning revealed that precracks obtained from HK20 and HK30 have an irregular precrack shape with large lateral cracks. It is suggested to use HK5 and HK10 to produce semi-elliptical surface cracks.