Search

Cutting performances of silicon nitride (Si3N4) ceramic cutting tools with and without boride additive (2.5 vol% ZrB2 or TiB2) prepared by hot‐pressing at 1500°C were investigated. Due to the α‐ to β‐Si3N4 phase transformation and low densification temperature, boride‐containing Si3N4 ceramics with high hardness and high toughness were obtained. The turning tests showed that the effective cutting lengths of the Si3N4–2.5 vol% TiB2 ceramic (∼2480 m) and Si3N4–2.5 vol% ZrB2 ceramic (∼2200 m) were higher than the monolithic Si3N4 ceramic (∼1780 m). As the toughness was improved while maintaining relative high hardness, the cutting performances of the boride‐containing Si3N4‐based inserts were improved by adding 2.5 vol% ZrB2 or TiB2. The improved cutting performance indicated that the boride‐containing Si3N4 ceramics are expected to be used in the field of ceramic cutting tools. [ABSTRACT FROM AUTHOR]

The influences of different contents ranging 0–15 wt% of high‐entropy boride (HEB) (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 on the mechanical properties of SiC‐based ceramics using Al2O3‐Y2O3 sintering additives sintered by spark plasma sintering process were investigated in this study. The results showed that the introduction of 5 and 10 wt% (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 could facilitate the densification and the grain growth of SiC‐based ceramics via the mechanism of liquid phase sintering. However, the grain growth of SiC‐based ceramics was inhibited by the grain boundary pinning effect with the addition of 15 wt% (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2. The SiC‐based ceramics with 15 wt% (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 showed the enhanced hardness (21.9±0.7 GPa) and high toughness (4.88±0.88 MPa·m1/2) as compared with high‐entropy phase‐free SiC‐based ceramics, which exhibited a hardness of 16.6 GPa and toughness of 3.10 MPa·m1/2. The enhancement in mechanical properties was attributed to the addition of higher hardness of HEB phase, crack deflection toughening mechanism, and presence of residual stress due to the mismatch of coefficient of thermal expansion. [ABSTRACT FROM AUTHOR]

The influences of different contents ranging 0–15 wt% of high‐entropy boride (HEB) (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 on the mechanical properties of SiC‐based ceramics using Al2O3‐Y2O3 sintering additives sintered by spark plasma sintering process were investigated in this study. The results showed that the introduction of 5 and 10 wt% (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 could facilitate the densification and the grain growth of SiC‐based ceramics via the mechanism of liquid phase sintering. However, the grain growth of SiC‐based ceramics was inhibited by the grain boundary pinning effect with the addition of 15 wt% (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2. The SiC‐based ceramics with 15 wt% (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 showed the enhanced hardness (21.9±0.7 GPa) and high toughness (4.88±0.88 MPa·m1/2) as compared with high‐entropy phase‐free SiC‐based ceramics, which exhibited a hardness of 16.6 GPa and toughness of 3.10 MPa·m1/2. The enhancement in mechanical properties was attributed to the addition of higher hardness of HEB phase, crack deflection toughening mechanism, and presence of residual stress due to the mismatch of coefficient of thermal expansion. [ABSTRACT FROM AUTHOR]

• Highly c -axis textured β-silicon nitride (β-Si 3 N 4) ceramics without seeding was prepared by a new method of hot extrusion using spark plasma heating at 1800 °C in 10 min. • The (002) pole figure showed a characteristic of center rotational symmetry and an average cline angle between hot extruding direction and [ 00l ] direction of β-Si 3 N 4 grains as low as 8.7°. • Higher hardness was achieved mainly due to the finer grain size compared to that of c -axis textured Si 3 N 4 ceramics with seeding. Highly c -axis textured β-silicon nitride (β-Si 3 N 4) ceramic with fine grains was prepared by a new method of hot extrusion for the first time. The (002) pole figure on the section plane vertical to extruding direction showed a characteristic of center rotational symmetry. The average cline angle between elongated β-Si 3 N 4 grains and hot extruding direction was about 14.4°. The degree of c -axis texturing by hot extrusion was comparable to that achieved by rotating magnetic field. The hardness and toughness anisotropy in different direction was apparent and relatively higher hardness was achieved in the present work mainly due to the finer grain size. Therefore, many different compositions of c -axis aligned Si 3 N 4 -based ceramics with tailored mechanical properties could be achieved by the strategy of hot extrusion. [ABSTRACT FROM AUTHOR]

High toughness and high hardness Si 3 N 4 ceramics with fine and bimodal microstructure was prepared by combination of HfB 2 additive and spark plasma sintering (SPS). HfB 2 -free SPSed Si 3 N 4 ceramics at 1600 °C exhibited major α-Si 3 N 4 phase, a fine and equiaxed grain microstructure, high Vickers hardness (21.3 ± 0.2 GPa), and low fracture toughness (3.7 ± 0.3 MPa m1/2). The introduction of 2.5 vol% HfB 2 into Si 3 N 4 ceramics at 1600 °C promoted α-to β-Si 3 N 4 phase transformation, produced a fine and bimodal microstructure, significantly improved the toughness (7.7 ± 0.6 MPa m1/2) but slightly decreased the hardness (20.4 ± 0.6 GPa) which was higher than that of traditionally hot-pressed or pressurless sintered Si 3 N 4 ceramics (~14–18 GPa). [ABSTRACT FROM AUTHOR]

Abstract: Based on the previous work on Si3N4–ZrB2 [Wu et al. J Eur Ceram Soc;2017,37:4217], the influence of ZrB2 addition on the phase and microstructure evolution of Si3N4–ZrB2 composites was emphatically investigated, and the mechanical properties were compared with pure Si3N4 ceramics. It was revealed that the ratio of β‐ to (α+β)‐Si3N4 significantly increased from 14.3% in pure Si3N4 ceramics to 39.8% in Si3N4 with 15 vol% ZrB2 addition, indicating that the introduction of ZrB2 promoted α‐ to β‐Si3N4 phase transformation. As a consequence, the microstructure of the composite showed the bimodal distribution, containing both elongated and equiaxed Si3N4 grains. For the pure Si3N4, Vickers hardness, fracture toughness and flexural strength was 22.8 GPa, 7.6 MPa m1/2, and 334.5 MPa, respectively. In contrast, the composite of Si3N4–30 vol% ZrB2 simultaneously possessed an excellent combination of mechanical properties: 19.5 GPa in hardness, 9.8 MPa m1/2 in toughness and 702.0 MPa in strength. Present study suggested that Si3N4‐based ceramics with high hardness, high toughness, and high strength could be obtained by the combination of appropriate ZrB2 content and low hot‐pressing temperature. [ABSTRACT FROM AUTHOR]

Graded Si 3 N 4 ceramics with sandwich-like microstructure were fabricated by the combination of hot-pressing, spark plasma sintering and β-Si 3 N 4 seeds. Phase compositions, microstructures, mechanical properties, and wear behaviors were investigated. Main α-Si 3 N 4 phase were detected in the outer layers, and only β-Si 3 N 4 phase were observed in the inner layers. The outer layer with ultra-fine equiaxed grains were well bonded to the inner layer with a distinct bimodal grain size distribution. Vickers hardness of outer layer (∼21.2 GPa) was much higher than that of inner layer (∼16.1 GPa), whereas fracture toughness of outer layer (∼3.5 MPa m 1/2 ) was much lower than that of inner layer (∼5.9 MPa m 1/2 ), indicative of the hard surface and tough core. Due to the ultra-fine microstructure and high hardness of outer layer, the graded Si 3 N 4 ceramics exhibited superior wear resistance with low wear rate. [ABSTRACT FROM AUTHOR]

Textured AlN-based ceramics with improved mechanical properties were prepared by hot pressing using Si3N4 and Y2O3 as additives. The introduction of Si3N4-Y2O3 into AlN matrix led to the formation of secondary Y3AlSi2O7N2 and fiber-like 2Hδ AlN-polytypoid phases, the partial texture of all crystalline phases, and the fracture mode change from intergranular to transgranular. Consequently, Vickers hardness, fracture toughness and flexural strength of AlN-based ceramics by the replacement of Y2O3 by Si3N4-Y2O3 increased significantly from 10.4±0.3 GPa, 2.4±0.3 MPa m½ and 333.3±10.3 MPa to 14.2±0.4 GPa, 3.4±0.1 MPa m½ and 389.5±45.5 MPa, respectively. [ABSTRACT FROM AUTHOR]

Phase compositions, densification, microstructures, thermal conductivities, and mechanical properties of sintered reaction-bonded Si 3 N 4 (SRBSN) and sintered Si 3 N 4 (SSN) using the same additives and procedure were compared. The present results showed that the density of SRBSN with MgO-Y 2 O 3 additives was much lower than that of SSN with the same additives. The Eu 2 O 3 -MgO-Y 2 O 3 additives inhibited the densification, while the ZrO 2 -MgO-Y 2 O 3 additives promoted the densification. The SRBSN with relative density of 99.5% and thermal conductivity of 66.5 Wm −1 K −1 was rapidly prepared using ZrO 2 -MgO-Y 2 O 3 as aids. The SSN with relative density of 99.0% and thermal conductivity of 56.8 Wm −1 K −1 was fabricated using ZrO 2 -MgO-Y 2 O 3 additives. The comparable density and higher thermal conductivity made the SRBSN prepared rapidly from low cost Si starting powder superior over the SSN. This study suggested that the SRBSN route with a combination of nitridation catalyst and densification aids provided a promising route to fabricate cost-effective Si 3 N 4 ceramics. [ABSTRACT FROM AUTHOR]

Chemical reactivity of hot-pressed Si 3 N 4 –ZrB 2 ceramics at 1500–1700 °C under N 2 and Ar atmosphere is investigated by X-ray diffraction, thermodynamic calculation, and phase diagram. Results show that there are chemical reactions in Si 3 N 4 –ZrB 2 ceramics. The chemical reactions accelerate the decomposition and phase transformation of Si 3 N 4 . The reaction products include ZrN, BN, ZrSi 2 , SiC or Si, which strongly depend on the atmosphere or temperature. The source of N 2 for ZrN and BN products is both atmosphere and decomposition of Si 3 N 4 at 1600 °C or above, whereas that is only atmosphere at 1500 °C. There is no reaction in Si 3 N 4 –ZrB 2 ceramics at 1500 °C under Ar. The MgO–Lu 2 O 3 sintering adds lead to the significant increase of relative density of Si 3 N 4 –ZrB 2 from 48% to 94% at 1500 °C in Ar. Therefore, it is possible that dense Si 3 N 4 –ZrB 2 ceramics with no reaction could be achieved at 1500 °C or below in Ar. [ABSTRACT FROM AUTHOR]

Graded Si 3 N 4 ceramics with hard surface and tough core were prepared by two-step hot pressing with the homogenous starting composition. The inner Si 3 N 4 layer was firstly hot-pressed at 1800 °C, subsequently covered with Si 3 N 4 powders on both sides, and finally hot-pressed at 1600 °C. After two-step hot pressing, the resulting ceramics exhibited a zoned microstructure, differentiated by the phase assemblage of Si 3 N 4 and grain size. The outer layers were well bonded to the inner layer. The outer layer exhibited bimodal and fine-grained microstructure, whereas the inner layer exhibited bimodal and coarse-grained microstructure. Vickers hardness of outer and inner layers were 18.1±0.2 GPa and 16.0±0.2 GPa, respectively, and fracture toughness were 4.2±0.1 MPa m 1/2 and 5.5±0.2 MPa m 1/2 , respectively. [ABSTRACT FROM AUTHOR]

Phase composition, microstructures, and mechanical properties of silicon nitride (Si 3 N 4) ceramics were investigated with ZrB 2 and B additives. Results showed that the addition of ZrB 2 and/or B in 2.5 and 5 vol.% promoted the phase transformation of α - to β -Si 3 N 4 phase and the formation of bimodal microstructure after hot-pressing at 1500 °C. With the introduction of 2.5 vol.% (ZrB 2 -B) binary additives, fracture toughness and strength of Si 3 N 4 ceramics increased significantly from 5.2 MPa m1/2 and 384 MPa to 7.2 MPa m1/2 and 675 MPa, respectively. However, the hardness of ceramics decreased slightly from 23.5 GPa to 21.3 GPa, which was still higher than typical values reported on Si 3 N 4 ceramics (15˜17 GPa). [ABSTRACT FROM AUTHOR]

In this study, braze joining of SiC ceramics using a Ti–Si as the interlayer was first obtained at 1500 °C under pressureless condition in a conventional graphite heater furnace, and the shear strength of the resulted SiC joint was only 2.7 ± 1.2 MPa. Results showed that the joining condition could be improved by adding metallic infiltration of Fe, Co, and Ni to Ti–Si interlayer and the shear strength was significantly increased to 73.7 ± 11.7 MPa under the same processing conditions employed in this study. The joining mechanism was then proposed based on thermodynamics calculation, elements mapping as well as the microstructural analysis. In a graphite vapor environment, decomposition of the interlayer (Ti 3 SiC 2) was readily promoted as compared to tungsten or molybdenum vapor environment. The decomposed products (Si) would easily dissolve into the liquid phase of Fe, Co, and Ni. Therefore, the infiltration of the above metallic elements would promote the decomposition of the Ti 3 SiC 2 phase by decreasing the partial pressure of Si. However, the pores, which was produced by the graphite environment as well as the infiltration of the above metallic elements, provided a route for the diffusion of Fe, Co, and Ni elements. Results of this study showed that a robust SiC joint could be obtained by the decomposition of the interlayer first and then the densification of the interlayer under the same process with the metallic infiltration. [ABSTRACT FROM AUTHOR]

Dense pressure-sintered reaction-bonded Si 3 N 4 (PSRBSN) ceramics were obtained by a hot-press sintering method. Precursor Si powders were prepared with Eu 2 O 3 –MgO–Y 2 O 3 sintering additive. The addition of Eu 2 O 3 –MgO–Y 2 O 3 was shown to promote full nitridation of the Si powder. The nitrided Si 3 N 4 particles had an equiaxial morphology, without whisker formation, after the Si powders doped with Eu 2 O 3 –MgO–Y 2 O 3 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% Eu 2 O 3 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 Eu 2 O 3 doping, or with ZrO 2 doping by conventional methods. [ABSTRACT FROM AUTHOR]

Silicon nitride (Si 3 N 4) ceramic cutting tool with excellent cutting performance was successfully achieved through the control of phase composition and microstructure. The hardness of the tool with 49.7 wt% and 4.9 wt% α-Si 3 N 4 is ~20.1 GPa and ~17.5 GPa, respectively. In continuous cutting of cast iron, the tool life significantly increases from ~1200 m to ~2400 m when α-Si 3 N 4 content increases from 4.9 wt% to 49.7 wt%, respectively. The longer tool life is attributed to a better abrasive and adhesive wear resistance of α-Si 3 N 4 phase. In addition, the chemical reaction between Si 3 N 4 ceramic tool and cast iron would lead to the formation of an amorphous oxide layer on wear surface. It was noted that the higher α-Si 3 N 4 content would lead to greater probability of chipping due to lower fracture toughness. Therefore, high-performance Si 3 N 4 ceramic tool could be achieved using an α-Si 3 N 4 matrix ceramic reinforced with elongated β-Si 3 N 4 phase. [ABSTRACT FROM AUTHOR]

In this paper, Hot Pressing Flowing Sintering (HPFS) was successfully applied to the consolidation of silicon nitride ceramics at low sintering temperature. Fine-grained α/β-Si 3 N 4 with high density was fabricated by HPFS at 1550 °C. The low temperature densification process was based on the hot pressing flow enhanced particle rearrangement mechanism. The shear stress which generated in the flowing process helped enhance the particle rearrangement with the lubrication of the liquid phase, which lead to collapse of the pores. The densification process was finished with minimum grain growth. Meanwhile, the rod-like β-Si 3 N 4 nuclei was textured during the flowing process. The (101)/(210) ratio of β-Si 3 N 4 was 0.09 on the plane perpendicular to the hot-pressing direction. The Vickers hardness and flexural strength of the sintered sample was 19.59 ± 0.36 GPa and 1022 ± 138 MPa, respectively. [ABSTRACT FROM AUTHOR]