Influence of Different BN Grades on Sintering Behaviour, Microstructure and Properties of Machinable Si3N4‐MoSi2‐BN Composites

Medri, Valentina; Fabbriche, Daniele Dalle; Melandri, Cesare; Guicciardi, Stefano; Bellosi, A.

doi:10.1002/adem.200500228

Cited by 3 publications

(1 citation statement)

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“…The grain size is estimated from the full width at half maximum (FWHM) by the Scherrer equation [16] and confirmed by Transmission electron microscope (TEM, JEM1200EX). The volume fraction of the monoclinic phase (Vm) was determined by the empirical formula: [17] V m I m 111 I m 11 1 I m 111 I m 11 1 I t 111 2…”

Section: Methodsmentioning

confidence: 99%

Preparation of Y‐TZP Nanoceramics by Gelcasting‐Pressing Method and Pressureless Sintering

Zhang

Chen

et al. 2006

Adv Eng Mater

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There is worldwide interest in the Y-TZP (tetragonal zirconia polycrystals stabilized by yttria) nanoceramics, which are believed to have some special characteristics such as superplasticity at low temperature. [1] However, fabrication of dense bulk nanocrystalline ceramics is very challenging since inevitable grain growth occurs at relatively high sintering temperatures needed for densification. The fabrication process usually involves forming and sintering. Duran and other researchers [2][3][4][5][6] have shown that green compacts with homogeneous microstructure, small pore size and narrow pore size distribution are necessary to ensure the low temperature sintering. Therefore, one main objective in a forming process is to produce uniform green body with homogeneous microstructure, small pore size and narrow pore size distribution. Previous studies have indicated that microstructural defects such as delamination, microcracks and large pores in the product are caused at drying forming. Recently, colloidal forming processes such as gelcasting [7] and direct coagulation casting (DCC), [8] in which ceramic powder is dispersed in a water-based suspension and held in a gel network, have attracted great interest. Among these methods, the gelcasting has significant advantages over the other processes, in terms of dimensional accuracy, the uniform structure and high strength of the green bodies, simplicity, and as well reduced manufacturing cost. Gelcasting has rapidly developed in the past decade as a promising colloidal in situ forming technique. It has been utilized in the forming of many sorts of ceramic material systems. [9][10][11][12] However, few reports about the use of this method for nanoceramics are found in the literature.Though gelcasting forming processes can improve the uniformity of green bodies and reduce structural defects but because of some characteristics of the nano powders itself, it is very difficult to preparation of a concentrated suspension with high solid volume loading in gelcasting, therefore the density of the green body is unsatisfactory. An optimal approach would be a combination of gelcasting and dry pressing processes, bringing together their advantages. Based on this concept, a new ceramics-forming process, gelcastingpressing, is developed by the authors.In addition, lots of methods, including hot-pressing, sinter-forging, hot-isostatic pressing and spark plasma sintering, have been used to fabricate nano-ceramics. [13][14][15] Among these methods, pressureless sintering is still a most promising one because of its low cost and easy technique. In this work, Y-TZP nanoceramics were prepared by gelcasting-pressing and pressureless sintering, while the microstructure, the pore distribution and sintering behavior of the green body were studied.Characteristics of powders and green bodies. Characteristics of nano-powders: The X-ray diffraction (XRD) patterns of nanopowders calcinated at different temperatures for 2 h are shown in Figure 2. It is seen that there is no diffraction peaks and t...

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Section: Methodsmentioning

confidence: 99%

Preparation of Y‐TZP Nanoceramics by Gelcasting‐Pressing Method and Pressureless Sintering

Zhang

Chen

et al. 2006

Adv Eng Mater

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Effects of Testing Temperatures and Thermal Treatments on Flexural Strength of Si₃N₄‐MoSi₂‐BN Composites

et al. 2006

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bridge S360) and energy dispersive spectroscopy (EDS, INCA Energy 300, Oxford Instruments, UK) on polished surfaces. For this purpose, the specimens were polished with diamond paste down to 0.25 lm.Vickers microhardness (HV1.0) was measured with a load of 9.81 N, using a Zwick 3212 tester. Young's modulus (E) was measured by the resonance frequency method on 28 × 8 × 0.8 mm 3 specimens using a Hewlett Packard gainphase analyzer. Fracture toughness (J Ic ) was evaluated using the chevronnotched beam (CNB) in flexure. The test bars, 25 × 2 × 2.5 mm 3 (length × width × thickness, respectively), were notched with a 0.08 mm diamond saw; the chevron-notch tip depth and average side length were about 0.12 and 0.80 of the bar thickness, respectively. The flexural tests were performed on a semiarticulated silicon carbide four-point jig with a lower span of 20 mm and an upper span of 10 mm on a universal screw-type testing machine (Instron mod. 6025). The specimens were deformed with a crosshead speed of 0.05 mm/min. The "slice model" equation of Munz et al. [25] was used to calculate J Ic . On the same machine and with the same flexural jig, the flexural strength (r), up to 1300°C in air, was measured on chamfered bars 25 × 2.5 × 2 mm 3 (length × width × thickness, respectively), using a crosshead speed of 0.5 mm/min. For the high-temperature tests, a soaking time of 18 min was set to reach thermal equilibrium. Five specimens were used for each temperature point.The electrical resistivity measurements were carried out by a four-probe DC method at room temperature, inducing a current in bar specimens of 2 × 2.5 × 25 mm 3 . The current and the voltage reading were detected at the same time in two different digital high-resolution multimeters. The resistivity values were determined from the electrical resistance measurement, taking into account the distance between the test leads and the cross-sectional area of the samples.[Verborgen]Si 3 N 4 -based ceramics are among the most important materials for structural applications due to a unique combination of properties such as strength, hardness and thermal stability. However after sintering, these materials are very difficult to be machined in complex shapes because of their brittleness and high hardness. Hexagonal BN (h-BN) can be used to improve machinability by the self-lubricating effects due to its plate-like structure. [1][2][3][4] To obtain high density Si 3 N 4 -BN composites, pressure-assisted techniques are COMMUNICATIONS ADVANCED ENGINEERING MATERIALS 2006, 8, No. 10

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Effects of different BN grades on the machinability of Si3N4–MoSi2–BN composites

Guicciardi

Calzavarini

Medri

et al. 2007

Materials Science and Engineering: A

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Influence of Different BN Grades on Sintering Behaviour, Microstructure and Properties of Machinable Si₃N₄‐MoSi₂‐BN Composites

Cited by 3 publications

References 17 publications

Preparation of Y‐TZP Nanoceramics by Gelcasting‐Pressing Method and Pressureless Sintering

Preparation of Y‐TZP Nanoceramics by Gelcasting‐Pressing Method and Pressureless Sintering

Effects of Testing Temperatures and Thermal Treatments on Flexural Strength of Si₃N₄‐MoSi₂‐BN Composites

Effects of different BN grades on the machinability of Si3N4–MoSi2–BN composites

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Influence of Different BN Grades on Sintering Behaviour, Microstructure and Properties of Machinable Si3N4‐MoSi2‐BN Composites

Cited by 3 publications

References 17 publications

Preparation of Y‐TZP Nanoceramics by Gelcasting‐Pressing Method and Pressureless Sintering

Preparation of Y‐TZP Nanoceramics by Gelcasting‐Pressing Method and Pressureless Sintering

Effects of Testing Temperatures and Thermal Treatments on Flexural Strength of Si3N4‐MoSi2‐BN Composites

Effects of different BN grades on the machinability of Si3N4–MoSi2–BN composites

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Influence of Different BN Grades on Sintering Behaviour, Microstructure and Properties of Machinable Si₃N₄‐MoSi₂‐BN Composites

Effects of Testing Temperatures and Thermal Treatments on Flexural Strength of Si₃N₄‐MoSi₂‐BN Composites