Hot-pressed Silicon Nitride Ceramics Research Articles

Effect of Sintering Additive on the Strength and Fractography of Hot-Pressed Silicon Nitride Ceramics

Several silicon nitride ceramics with different compositions and amounts of sintering additive were fabricated by hot-press method. The effect of sintering additive on the sinterability, flexural strength and fractography was examined. The flexural strength of silicon nitrides sintered with alumina as an additive were not high at approximately 400MPa, although the sinterability was good in that bulk densities of 3.05-3.18 × 103kg m-3 were obtained. The sinterability of silicon nitrides sintered with yttria as an additive resulted in low bulk densities of approximately 2.7 × 103kg m-3, with flexural strength ranging from 370 to 650MPa. The sinterability of silicon nitrides that were sintered with both yttria and alumina additives resulted in relatively high bulk densities of approximately 3.21 × 103kg m-3 and extremely high flexural strength in the range of 900 to 1140MPa. The fractography of fracture origin was dependent on the type of sintering additive used; an inclusion size of approximately 30μm for the alumina additive, pores for the yttria additive and small secondary sintering defects for both yttria and alumina as additives were observed at the respective fracture origins.

The Relationship Between Multiple Scratch Tests and Wear Behavior of Hot‐Pressed Silicon Nitride Ceramics with Various Rare‐Earth Additive Systems

The wear behavior of Si3N4ceramics sintered with various rare earth additives was studied for nonlubricated sliding under different conditions, and scratch tests carried out in an attempt to correlate the wear behavior. When multiple scratch testing is used the results can be used to indicate the initial wear behavior under fracture‐dominated wear of the materials. The additive system used in the sintering of the Si3N4ceramics affected the specific wear rate under nonlubricated sliding conditions, and under high load conditions, where fracture is dominant, the specific wear rate was shown to increase in samples sintered with lutetium as a consequence of a strong bonding strength between the grains and grain boundary resulting in a higher degree of brittle fracture.

Microstructural characterization and high-temperature strength of hot-pressed silicon nitride ceramics with Lu2O3 additives

The microstructures of two hot-pressed Si3N4 ceramics, with 3.33 and 12.51 wt% Lu2O3 additive, have been characterized using transmission electron microscopy. The microstructures of both samples consisted of elongated β-Si3N4 grains and a secondary phase, contained in pockets surrounded by the grains, with a crystalline or amorphous form. In the 3.33 wt% Lu2O3-containing Si3N4 ceramic, all the multiple-grain junctions were completely crystalline while, in the 12.51 wt% Lu2O3-containing Si3N4 ceramic, approximately half the junctions were devitrified. A thin intergranular amorphous film present between the two-grain boundary was common; however, a film-free grain boundary was observed in the 12.51 wt% Lu2O3 sample. The film-free grain boundary was determined to be approximately 35%. Both ceramics fractured in four-point flexure between 1200 and 1600°C. Their high-temperature strength is closely associated with the nature of the grain-boundary phase formed during the sintering process.

Hot-pressed silicon nitride ceramics with Lu2O3 additives: elastic moduli and fracture toughness

Abstract The elastic moduli and fracture toughness of hot-pressed Si3N4 ceramics, using 1.68, 3.33, 6.52 and 12.51 wt.% of Lu2O3 additives, were studied by means of ultrasonic measurements and Vickers indentation crack size measurements, respectively. The elastic moduli were found to increase with the amount of Lu2O3 additive for silicon nitride which was not completely densified. However, the elastic moduli remained nearly constant independent on the amount of additive for completely dense silicon nitride. The elastic moduli of completely dense silicon nitride should reach the following average values: shear modulus G=132 GPa, Young's modulus E=338 GPa, bulk modulus B=255 GPa and Poisson's ratio ν=0.28. On the other hand, the fracture toughness showed a clear dependence of additive amount for both incompletely and completely dense silicon nitride ceramics. The fracture toughness increased with additive amount over the range 1.68 to 12.51 wt.% Lu2O3. The improvement in the fracture toughness with the amount of additive was mainly attributed to elongated grain growth during the sintering process.

Analysis of Bending Creep Behavior of Silicon Nitride Ceramics at Elevated Temperature

Three-point bending creep behavior of hot-pressed silicon nitride ceramics was investigated at 1170 °C. A unique creep constitutive equation consisting of linear and power law terms was proposed, and the numerical calculation programs for determining the parameters included in the constitutive equation and for describing the creep deformation behavior of silicon nitride ceramics at elevated temperatures were developed. The model for interpreting the time-dependent deflection rate and the change in curvature of the beam due to creep under three-point bend loading was also developed, which fitted well the experimental data.

Effects of heat treatments on the creep properties of a hot pressed silicon nitride ceramic

Effects of heat treatment in an argon atmosphere at high temperatures for varying times on the creep properties of a Y2O3-Al2O3 (8-2 wt%) doped hot pressed silicon nitride (HPSN) ceramic were investigated. It was observed from the creep measurements that higher temperature, i.e. ≥1360‡C, and longer time, i.e. ≥8 h, heat treatment in an argon atmosphere improved the creep properties, (e.g. secondary creep rate) of this material. Heat treatment at a lower temperature of 1300‡C and for a shorter time of 4 h did not change the creep behaviour. Improvement of the creep properties was related to the crystallization of an amorphous grain boundary phase by heat treatment. Secondary creep rate parameters of the as-received material: stress exponent, n (2.95–3.08) and activation energy, Q (634–818 kJ molS−1), were in the range of values found by other investigators for various hot pressed silicon nitride ceramics.

ホットプレス窒化珪素セラミックスの微細組織と破壊じん性との関係

ホットプレス窒化珪素セラミックスの微細組織と破壊じん性との関係

Indentation Characterization of Silicon Nitride

Abstract The hardness of hot-pressed silicon nitride ceramics has been measured over a wide range of applied loads, 0.02 to 300 N, for both Vickers and Knoop indenter geometries. The indentation size effect (ISE) was evident for loads less than about 1 N. The Vickers and Knoop hardness values agree provided they are compared at a given penetration of the respective indenter. The influence of the residual stresses of the ceramic surface on the indentation response in the as-machined, polished, and post-annealed state has been investigated. The results are discussed in relation to the characterization of indentation damage in brittle materials and fracture analysis models.

Ion beam induced modification of metal-engineering ceramic interfaces II. Chromium on hot-pressed silicon nitride

100 nm thick chromium layers were deposited on hot-pressed silicon nitride ceramics and irradiated at room temperature with 430 keV Si + ions to fluences ranging from 2.5 × 10 15 to 4 × 10 16 ions cm −2. RBS, TEM and XPS techniques were used for the characterization of the irradiated structures. There is no ion induced interfacial layer, but a very limited reaction takes place between the metal film and the substrate with at least one reaction product, CrN. Some preliminary results on adhesion enhancement are also presented and the role of pre-deposition sputter cleaning of the substrate is discussed.

Crack-Free Processing of Hot-Pressed Silicon Nitride Ceramics Using a Pulsed YAG Laser. Fracture Strength and Residual Stress.

This paper describes the effect of crack-free laser processing of hot-pressed silicon nitride ceramics on fracture strength and residual stress. The fracture strength of the processed test pieces was determined by three-point bending tests. The residual stress in the processed surface was characterized by X-ray diffraction measurement. The strength of the laser-processed workpieces was reduced to 90-80% compared with that of the ground workpieces because of subsurface layer damage, which included the heat-affected layer and the residual stress layer. In particular, the latter greatly affected the fracture strength because the higher compressive residual stress layer generated during grinding was released by laser processing. It is concluded that the newly developed crack-free processing technique has great industrial possibilities.

Relation between Microstructure and Color Tone of Skin Layer and Bulk Area of Silicon Nitride Ceramics

A skin layer which was generated near the surface of hot-pressed silicon nitride ceramics was studied from microscopic viewpoint. The ceramics was composed of acicular β-Si3N4 grains and glassy grain boundary which was liquid at hot-pressing temperature. The thickness of skin layer increased with an increase in the amount of sintering additive up to 8wt%. The amount and viscosity of the liquid phase and the applied pressure in the hot-pressing influenced strongly the microstructure development of the silicon nitride ceramics. These factors affected the α→β phase transformation and the formation of acicular β-Si3N4 grains. A compositional difference of glassy phases was observed between the skin layer and the bulk of Si3N4 ceramics. A mechanism of the generation of skin layer was discussed on the basis of experimental results.

Characterization of Hot-Pressed Silicon Nitride Ceramics with Alkoxide-Derived Oxide Mixtures As the Sintering Aid

ABSTRACTA homogeneous mixture of Y2O3, CeO2 and MgO with a final weight ratio of 3:1: 2 was prepared by the alkoxide method. The powder mixture was then added into Si3N4 powder in amounts ranging from 4 to 12 wt%, andconsolidated by hot-pressing. Microstructure and mechanical properties of the sintered bodies were determined and compared to those of materials prepared by the conventional route of mixing the oxide powders as sintering aids individually in essentially same composition. The β-fraction (modification ratio) in same composition was higher in thesintered bodies made through the alkoxide method than those made through the conventional one. The room temperature flexural strength was maximized with 6wt% addition of the alkoxide derived oxide, whereas, 12wt% addition of the total oxide was required to maximize the strength by conventional processing.

共有結合性セラミックスのレーザ加工に関する研究 （第２報） 破壊要因とクラックの抑制

This paper describes the fracture mechanism and the prevention of crack propagation in laser-processing of the hot-pressed silicon nitride ceramics using the pulsed YAG laser. The crack formation mechanism is successfully explained by the thermal stress and fracture strength distribution. The laser pulses controlled at below the critical duration and frequency are effective for the purpose of crack-free processing because the thermal stress is localized only in the laser-heated shallow region.

Crack-free processing of hot-pressed silicon nitride ceramics using pulsed YAG laser.

It is difficult to eliminate recast layers and cracks in laser-machining caramics. These defects significantly detract from the excellent properties of ceramics. This paper describes the feasibility of a crack-free processing method for hot-pressed silicon nitride (Si3N4) ceramics using a yttrium aluminum garnet (YAG) laser. The Q-switched YAG laser pulses controlled at below 500 ns in duration and 10 kHz in repetition rate are effective for the purpose of crack-free machining of Si3N4 ceramics because the thermal stress is most likely localized only in the laser-heated shallow area. The integrity of the crack-free surface is evaluated by the fracture strength and residual stress. The strength of laser-processed test pieces is reduced by 10 to 20% compared with diamond-ground ones because the high residual compression layer generated during grinding is removed.

Crack-Free Processing of Hot-Pressed Silicon Nitride Ceramics Using Pulsed YAG Laser. (3rd Report. Analysis of Fracture Strength and Residual Stress).

This paper describes the effect of crack-free laser processing of hot-pressed silicon nitride ceramics on the fracture strength and residual stress. The fracture strength was determined by three-point bending tests. The residual stress in the processed surface was characterized by X-ray diffraction measurement. The strength of the laser-processed test pieces is reduced to 90-80% compared with that of the ground test pieces because of surface layer damages, which are the recast layer and the residual stress layer. In particular, the latter greatly affects the fracture strength because the higher compressive residual stress layer generated during grinding is removed by laser machining.

Crack-free processing of hot-pressed silicon nitride ceramics using pulsed YAG laser. (2nd report. Analysis on transient thermal stress and crack formation mechanism in laser-processing of silicon nitride ceramics).

Thermal-stress-induced cracks are generated during the laser-processing of ceramics. This paper describes the temperature profile and thermal stress propagation for silicon nitride ceramics heated by Gaussian heat flux. The thermal stress distribution is calculated with consideration of the temperature dependence on material properties, the influence of laser pulse duration and its repetition rate using FEM. The tensile stress in circumferential, radial and axial directions induces the radial lateral and median cracks, respectively. The use of short laser pulses below 20μs with a repetition frequency under 10 kHz is experimentally recommended to reduce thermal stress and to avoid crack formation.

Effects of Grinding on Dynamic Fatigue Behavior and Strength Degradation of Hot Pressed Silicon Nitride Ceramics at Room Temperature

Four-point bending dynamic fatigue tests were carried out at five different crosshead speeds from 5×10-4 to 5×100mm/min in air at room temperature. Hot pressed silicon nitride ceramics was used for the tests and ground with 600, 400 and 200 grit resin bonded diamond wheels perpendicular to the direction of bending stress. Both the specimens ground with the 400 and 200 grit wheels showed a decrease in the fatigue parameter and fracture strength, while no decrease was observed in the specimen ground with the 600 grit wheel. It is suggested that grinding induces surface flaws and that the mean fracture strength of specimens ground with the 400 and 200 grit wheels tested at low crosshead speeds is lowered by slow crack growth of the surface flaws which are easily affected by stress corrosion.

Crack-free processing of hot-pressed silicon nitride ceramics using pulsed YAG laser.

In the laser machining of ceramics, it is difficult to avoid the formation of a recast layers and cracks. These deffects significantly spoil some excellent properties of ceramics. This paper describes the feasibility of a crack-free processing method. for hot-pressed silicon nitride (Si3N4) ceramics with an yttrium aluminum garnet (YAG) laser. It is found that, in order to attain the defect-free machining, it is necessary to process the ceramics using a laser beam pulsed below 500 ns in duration and 10 kHz in repetition rate.

Corrosion Behavior of Ceramics in High Purity Water at 290°C

Recently, advanced ceramics with superior strength and corrosion resistance have been developed. They have a possibility for various engineering applications as structural materials used in high temperature water, such as steam turbine systems or water cooled reactors. Fundamental corrosion behaviors of 14 kinds of sintered polycrystalline ceramic were examined in oxygen-free (less than 20ppb dissolved oxygen) and oxygenated (about 32ppm dissolved oxygen) pure water at 290°C using a refleshed-type autoclave system. Change in weight and surface morphology detected by X-ray diffraction and scanning electron microscopy on the samples corroded for 72 hours were discussed. Silicon carbide showed the best corrosion resistance among the present ceramics. The corrosion resistance of alumina strongly depended on its purity. 99.99% purity alumina showed high corrosion resistance. For pressure-less sintered and hot-pressed silicon nitride and sialon ceramics, the preferential dissolution of crystalline phase occurred at the surface of the sample and the grain boundary phases remained near the surface. Aluminum nitride formed a thick boehmite layer in the tests. Partially stabilized zirconia, stabilized by 3mol% yttria, flaked away by accompanying a phase transformation from tetragonal to monoclinic. Many of the non-oxide ceramics, particularly silicon carbide, showed more accelerated dissolution rates in oxygenated water than in oxygen-free water.

Creep behavior of hot-pressed silicon nitride ceramics with rare-earth oxides and alumina additives

The creep behavior of hot-pressed silicon nitride (HPSN) ceramics with rare-earth oxides and alumina additives has been studied. The creep experiments were performed under bending stress of 200–360 MPa and at temperatures of 1170–1360°C in air. The creep resistance of HPSN with rare-earth additives only is much higher than that with rare-earth oxides plus alumina. With increasing alumina content, the creep rate increases rapidly and the creep activation energy decreases. Based on the stress exponent and microstructural observations, a “mixed” mechanism of creep including both diffusion and grain-boundary sliding is suggested for HPSN with rare-earth oxides only. The creep mechanism of HPSN with rare-earth oxides and alumina is described to grain-boundary sliding accommodated by void formation.