Sintered Silicon Nitride Ceramics Research Articles

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.

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%.

The influence of sintering additives on the microstructure and material properties of silicon nitride ceramics investigated by advanced simulation tools

In multi‐phase bulk ceramics, the separate influence of individual phases on the overall macroscopic properties can often hardly be studied by purely experimental observations. However, once a realistic microstructure model exists, computer simulations allow for that by varying the properties of individual phases and, thus, for studying their role independently. In the present article, the microstructure of liquid phase sintered silicon nitride ceramics was created using a voxel‐based structure generator. These generated microstructures have been meshed with a special tool so that thermal conductivities and elastic properties of these ceramics could be calculated using standard finite element simulations. Variations of the properties of individual phases were considered in a realistic range for the experimental optimization of silicon nitride ceramics and, therefore, their respective influence on macroscopic properties could be studied. It turns out that a high volume fraction of the primary ‐SiN ‐phase has the largest positive impact on both stiffness and thermal conductivity, whereas the effect of grain shape and anisotropic material properties of the ‐SiN ‐grains is rather low. However, the thermal conductivity of the secondary phase plays a significant role in the overall thermal conductivity, provided that it drops beyond 10 W mK−1. Nevertheless, experimentally the thermal conductivity of silicon nitride ceramics is dominated by the thermal conductivity of the ‐SiN ‐grains, where the latter is controlled by impurities which can be incorporated into the silicon nitride lattice during sintering. Thus, inverse simulation can be used as a powerful tool to investigate individual phase properties in multiphase ceramics, for example, estimating a decreased thermal conductivity of the primary phase (‐SiN ‐grains).

Comparative Study on Electrical and Dielectric Properties of Sintered Nano and Micro Silicon Nitride Ceramics

In the present work we have studied the electrical conductivity, dielectric constant and dielectric loss of Sintered Silicon Nitride ceramics. In this study it was found that the grain size has great impact on electrical conductivity and dielectric properties of Sintered Silicon Nitride Ceramics. The result shows more efficiency of electrical and dielectric properties with nano sized grains. The sintering was performed in a programmable furnace at 950 K. The dc conductivity measured in the temperature range 300 K to 900 K. At higher temperature (T > 800 K), the dc conductivity increases exponentially with temperature for both of the investigated samples. Dielectric constant and loss are measured in the temperature range 300 K to 900 K with frequency range 1 KHz to 1 MHz. To confirm the grain size, the samples are characterized by the Scanning Electron Microscope (SEM). These types of samples can be used as a high temperature semi-conducting materials.

A new way of fabricating Si3N4 ceramics by aqueous tape casting and gas pressure sintering

Silicon nitride (Si3N4) ceramic was fabricated by aqueous tape casting and gas pressure sintering at 1850 °C for 60 min under a nitrogen atmosphere of 1.5 MPa. Effects of slurry properties on powder compaction and densification of the green tapes were investigated. The slurry of the Si3N4 powder with polyacrylic acid as dispersant showed shear-thinning rheological behavior. Casting of the slurry on glass plate produced flexible green tape with uniform powder distribution after dried in open air. The green tapes had relative high density, small pore size and homogeneous pore size distribution. Due to high compaction of the ceramic particles and homogeneous distribution of the pores in the green tapes, the sintered silicon nitride ceramics reached a relative density up to ∼99%. Elongated growth of β-Si3N4 grains with a small bimodal distribution, gave good flexural strength, elastic modulus and fracture toughness up to 764 MPa, 285 GPa and 7.05 MPa m1/2, respectively, to the silicon nitride (Si3N4) ceramic.

Experimental Study on Laser Assisted Milling of Sintered Silicon Nitride Ceramics

Laser assisted milling (LAML) is a potential method for machining difficult-to-machine materials such as superalloys and ceramics, which uses a high power laser to focally heat a workpiece prior to material removal with a traditional cutting tool. A laser assisted milling experimental system was set up to investigate the cutting process of LAML of sintered silicon nitride. Effects of operating parameters on cutting forces, chip morphology, surface roughness and subsurface damage were investigated. The results showed the feasibility and advantages of LAML of ceramics including good surface, decreasing cutting force and tool wear, and no micro-crack zones.

Structural and Electrical Characterization of Sintered Silicon Nitride Ceramic

The electrical conduction phenomena, dielectric response and microstructure have been discussed in sintered silicon nitride ceramics at different temperature and frequencies. Microstructure and phase of the sintered samples was investigated by Scanning Electron Microscope (SEM) and X-ray diffractometer (XRD). The electrical conductivity, dielectric constant and dielectric loss increases exponentially with temperature greater than 600 K. The dielectric constant and loss have been measured in the frequency range 100 Hz to 1 MHz. The a.c. conduction studies in the audio frequency range 500 Hz to 1 MHz indicates that the conduction may be due to the electronic hopping mechanism. Silicon Nitride ceramics became dense after sintering. The effect of grain size and role of phase on electrical and dielectric properties have been discussed. These types of samples can be used as a high temperature semi conducting materials for device packaging.

Spark Plasma Sintered Silicon Nitride Ceramics with High Thermal Conductivity Using MgSiN2 as Additives

Silicon nitride ceramics were prepared by spark plasma sintering (SPS) at temperatures of 1450°–1600°C for 3–12 min, using α‐Si3N4 powders as raw materials and MgSiN2 as sintering additives. Almost full density of the sample was achieved after sintering at 1450°C for 6 min, while there was about 80 wt%α‐Si3N4 phase left in the sintered material. α‐Si3N4 was completely transformed to β‐Si3N4 after sintering at 1500°C for 12 min. The thermal conductivity of sintered materials increased with increasing sintering temperature or holding time. Thermal conductivity of 100 W·(m·K)−1 was achieved after sintering at 1600°C for 12 min. The results imply that SPS is an effective and fast method to fabricate β‐Si3N4 ceramics with high thermal conductivity when appropriate additives are used.

Effect of tellurium on electrical and structural properties of sintered silicon nitride ceramics

The effects of tellurium (Te) additives on electrical conductivity, dielectric constant and structural properties of sintered silicon nitride ceramics have been studied. Different amounts of Te (10% and 20%) were added as sintering additives to silicon nitride ceramic powders and sintering was performed. Microstructure and composition were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrical conductivity and dielectric constant ( ε ′) increase exponentially with temperature greater than 800 K. The electrical conductivity and dielectric constant increase but activation energy decreases from 0.72 to 0.33 eV with the increase of Te concentration. However, the conductivity increases five orders of magnitude at the concentration of 10% of Te in Si 3 N 4 . As the Te concentration increases the sintered silicon nitride ceramics become denser. These types of samples can be used as high temperature semiconducting materials.

Vacuum Heat Treatment of MgO -Densified Pressureless Sintered Silicon Nitride Ceramics

Silicon nitride samples were pressureless sintered with up to 5 w/o MgO to give densities in the range 98-99% of theoretical. After pressureless sintering, selected samples were placed in a vacuum heat treatment furnace surrounded by a carbon bed in a carbon crucible at a pressure of less than 4x10-4 mbar, and vacuum heat treated at different temperatures and times to remove grainboundary glass. The results showed that this was substantially achieved at 1575oC for 3h and that increasing the time to 5 hours gave still further improvement. SEM images, EDX analysis and oxidation tests provided additional evidence for the removal of Mg from the samples.

The Effect of Dielectric Properties of Sintering Additives on Microwave Sintered Silicon Nitride Ceramics

Silicon nitride requires the use of susceptive additives for microwave liquid phase sintering due to the material’s low dielectric loss. In this article, we report the effect of complex dielectric properties of two compositions of sintering aids on 2.45 GHz microwave sintered Si3N4 with respect to power absorption, temperature distribution and densification behavior. The temperature dependent dielectric properties were measured from 25°C to 1400°C using a conventional cavity perturbation technique. Finite Difference Time Domain (FDTD) electromagnetic simulations coupled with a thermal solver was used to predict the microwave power absorption and the corresponding temperature evolution inside the samples.The additive with higher dielectric loss (4 wt% MgO, 6 wt% Y2O3 and 2.5 wt% ZrO2) produces a greater sintered density than the lower loss additive (4 wt% MgO and 6 wt% Y2O3) or pure Si3N4. Although microwave loss at temperatures below 600°C is insignificant with or without the additives, the loss begins to increase at higher temperatures when the additives are present and has a strong upward trend above 1000°C. Above 1200°C the sample containing ZrO2 exhibited the greatest loss. Numerical simulations at the peak sintering temperature show greater microwave power absorption and higher temperature in the sample with the highest loss additive. The simulation results correlate to the difference in densification behavior observed. The simulation was also useful because the material temperature was not accurately provided by optical pyrometer measurements of the crucible sample holder.

Re-sharpenable thick CVD diamond-coated Si 3N 4 tools for hardmetal turning

Sharpening and re-sharpening ability of CVD diamond-coated ceramic tools for turning operations is addressed in this work. Thick CVD diamond films (150 μm) were deposited by MPCVD on pressure-less sintered silicon nitride ceramics. These disc-shaped tools could be sharpened to the desired geometry using grinding wheels without film detachment. The tools were tested in the dry turning of three types of hardmetal differing in WC grain size (2–6 μm) and Co content (18–27 wt.%). The lower Co content hardmetal grade (GD40) is more aggressive than GD50 or GD60 to the cutting edge, always resulting in faster wear and larger cutting forces for the same machining parameters. The depth of cut was fixed at 0.2 mm for the three hardmetal grades while the speed and feed were reduced from 30 m min − 1 and 0.15 mm rev − 1 (GD50 and GD60) to 20 m min − 1 and 0.1 mm rev − 1 for GD40. Wear changes the cutting edge geometry and increases the cutting force to values above which the tool fails ( F d ∼ 700 N). Wear of the tool occurs by microchipping of the CVD diamond, resulting in abrasion of the rake and flank faces by diamond debris. Up to 2000 m dry machining length per tool could be achieved before the re-sharpening operation has to be performed. When a cutting fluid is used, increased tool life is easily achieved due to reduction of Co adhesion and enhancement of diamond debris removal from the cutting edge.

Influence of the substitution of Y2O3 for CeO2 on the mechanical and microstructural properties of silicon nitride

This work investigated the substitution of Y2O3 for CeO2 in liquid-phase sintered silicon nitride ceramics. Cost reduction as well as good physical, mechanical and microstructural properties are the main objectives of the present study. Two powder mixtures were prepared, varying the contents of alpha-Si3N4, Al2O3, AlN, Y2O3 and CeO2. The mixtures were homogenized in ethanol, dried in a rotating evaporator and kiln, respectively, and then uniaxially (100 MPa) and cold isostatically pressed (300 MPa). The samples were sintered at 1850ºC for 1 h in a graphite resistive furnace under nitrogen atmosphere. After sintering the density of the samples was higher than 97% of the theoretical value. The fracture toughness and hardness were higher than 5.28 MPa.m½ and 17.12 GPa, respectively. Phase analysis by X-ray diffraction and scanning electron microscopy revealed the presence of alpha-SiAlON and beta-Si3N4.

Relevance of internal structures to the fracture strength of injection-molded and sintered silicon nitride ceramics

Abstract The densification behavior and fracture strength of silicon nitride ceramics were investigated for samplesmade through the injection-molding process in comparison with thosemade by powder granule compaction. Although no clear difference was observed in the density, phase composition and microstructure of sintered bodies between the processes, the fracture strength was clearly higher for the samples made through the injection-molding route. A relatively uniform internal macrostructure was considered responsible for the improved fracture strength of the injection-molded and sintered silicon nitride.

Tribological properties of silicon nitride ceramics modified by ion implantation

Surface modification by ion implantation has been carried out in order to improve the tribological properties of gas-pressure sintered silicon nitride ceramics. B +, N +, Si + or Ti + ions were implanted into the silicon nitride ceramics with a fluence of 2×10 17 ions/cm 2 at an energy of 200 keV. To evaluate the tribological properties of Si 3N 4, Block-on-Ring wear tests were conducted without lubricant, using the ion implanted Si 3N 4 as block specimens and commercially supplied Si 3N 4 as ring specimens. The specific wear rate of the ion-implanted Si 3N 4 could be drastically reduced to a value of 1.6×10 −9 mm 2/N, accompanied by a decrease in the friction coefficient in the initial stage. According to surface analyses it was considered that the high wear resistance and low friction coefficient are attributed to the amorphization and the increase of surface hardness of the ion implanted layer.

Inelastic Analysis of Notched Components in Silicon Nitride Ceramics at Elevated Temperature

The displacement rate-controlled tensile tests were performed for smooth and circumferentially notched specimens of the sintered silicon nitride ceramics in argon-gas atmosphere at 1300°C, and then the axial load-displacement response was measured. It was shown from the tests that inelastic deformation was greatly dependent on displacement rate and easier to generate at the lower displacement rate. The constitutive equation was constructed on the basis of tensile stress-strain curves in the smooth specimen. Then inelastic FEM analysis was performed for the smooth and notched specimens, using this equation. The calculated load-displacement responses quantitatively agreed quite well with the measured ones in the smooth and notched specimens, showing the validity of the proposed analysis. The development of the inelastic deformation in loading process was shown from the calculation around the notch root.

Push‐Pull Fatigue of Sintered Silicon Nitride Ceramics at Elevated Temperatures

The fatigue tests under push‐pull completely reversed loading and pulsating loading were performed for silicon nitride ceramics at elevated temperatures. Then the effects of stress wave form, stress rate, and cyclic understressing on fatigue strength, and cyclic straining behavior, were examined. The cycle‐number‐based fatigue life is found to be shorter under trapezoidal stress wave loading than under triangular stress wave loading, and to become shorter with increasing hold time under the trapezoidal stress wave loading. Meanwhile, the equivalent time‐based life curve, which is estimated from the concept of slow crack growth, almost agrees with the static fatigue life curve in the short and intermediate life regions, showing the small cyclic stress effect and the dominant stress‐imposing period effect on cyclic fatigue life. The fatigue strength increased in stepwise stress amplitude increasing test, where stress amplitude is increased stepwise every given number of stress cycles, at 1100° and 1200°C. Occurrence of cyclic strengthening was proved through a gradual decrease in strain amplitude during a pulsating loading test at 1200°C in this material, corresponding to the above cyclic understressing effect on fatigue strength.

Mechanical properties of gas pressure sintered Si3N4/SiC nanocomposites

Abstract The mechanical properties of gas pressure sintered Si3N4/SiC nanocomposite materials were studied at room temperature and in the range of 1400–1600 °C. Fracture strength at room temperature was found to be between 640 and 840 MPa and thus within the range of data reported in the literature for other gas pressure sintered silicon nitride ceramics. At elevated temperature the creep and stress rupture behaviour were determined in four-point bending. The creep behaviour is characterised by primary and pronounced secondary creep, very high activation energies for creep in the range of 1050–1450 kJ/mol, and two different stress dependencies. At stresses below a characteristic transition stress in the range 170–200 MPa the stress dependency is described by a stress exponent of around two. Above the transition stress, stress rupture was observed to be controlled by subcritical crack growth with stress rupture exponents of 30–40. Annealing of the as-sintered materials promotes crystallisation of the intergranular glassy phase resulting in an increase in creep strength and resistance against subcritical crack growth. An extensive analysis of the available literature data revealed that the materials tested in this study belong to the most creep resistant silicon nitride ceramics prepared by gas pressure sintering.

Creep behaviour of two sintered silicon nitride ceramics

Abstract The high temperature mechanical properties of two sintered silicon nitride—based ceramic materials with different microstructural features, labelled N7202 and N3208, are presented. The mechanical behaviour was examined by creep tests in compression, at temperatures between 1450 and 1700 °C in argon atmosphere. The results, analysed in terms of the creep equation, yield n =0.6±0.1 and Q =470±20 kJ/mol for N7202, and n =0.6±0.1 and Q =530±10 kJ/mol for N3208. The microstructural observations allow identify a possible microscopic deformation mechanism compatible with the set of creep parameters obtained experimentally.