Showing papers on "Sialon published in 1998"

Thermal Stability of Calcium α-sialon Ceramics

Abstract: The lack of thermal stability of some rare earth α-SiAlON phases has recently raised serious doubts concerning the high temperature behaviour of such materials. As information on the Ca–SiAlON system is minimal this work investigates the thermal stability behaviour of Ca α-SiAlON (α ′). Compositions near the α′ plane in the Ca–Si–Al–O–N system were prepared by hot-pressing. These materials were subsequently heat treated at temperatures between 1100 and 1500°C for 24 h and for an extended duration, 200 h, at 1450°C. The addition of glass prior to hot-pressing to various compositions enabled the investigation of its effect on the initial phase assemblage and the thermal stability behaviour. It was found that neither the excess glass nor the heat treatment devitrification products, destabilised the Ca α′ phase. The presence of β′ grains appeared to facilitate the transformation behaviour of the Ca α′, however a single crystalline phase Ca α′ with a high value of m was thermally stable under extended heat treatment at 1450°C.

α-Sialon ceramics synthesised from a clay precursor by carbothermal reduction and nitridation

Abstract: Simultaneous carboreduction and nitridation (CRN) of Y2O3 doped SiO2 /clay or elemental Si/clay mixes at atmospheric pressure were used to prepare low-cost powders of α-sialon composition. The synthesised products contained crystalline phase mixtures of α- and β-sialon and Y3Si6N11 . Subsequent hot pressing of the CRN powders at 1800 °C resulted in poorly densified materials from mixtures with low Y2O3 , whereas high Y2O3 gave dense ceramics. The addition of SiO2to the starting mix resulted in ceramics containing substantial amounts of very fine SiC particles distributed in a matrix of β-sialon and glass. Solid state MAS NMR indicates that the crystalline SiC observed by XRD after hot-pressing is also present in an XRD-amorphous form in the pre-pressed powders, and that hot-pressing causes the Al–N units to become overall more oxygenated, with the conversion of β-sialon to α-sialon and the formation of small amounts of polytypoid sialons. The hardness,HV10 , and fracture toughness, K1C , of the densified materials were ca. 18 GPa and 3 MPa m1/2 , respectively. The use of elemental Si produced less SiC, the major phase in these ceramics being α-sialon and a residual glassy phase observable by electron microscopy. The hardness and toughness of these densified materials was ca. 20 GPa and 3 MPa m1/2 , respectively.

Elongated α-sialon grains in pressureless sintered sialon ceramics

Abstract: Calcium α-sialons, containing elongated grains, were developed by pressureless sintering. Preliminary electron microscopy analyses revealed the high aspect ratio of the α′ grains resulted from preferred crystal growth in the [001] direction. Such grain morphologies offer hope for in-situ toughening of α-sialon materials via the promotion of crack deflection and bridging during failure.

The physical properties of sintered X-phase sialon prepared by silicothermal reaction bonding

Abstract: A single-step method is described for forming X-phase sialon bodies by silicothermal synthesis from kaolinite, γ-alumina, elemental silicon and 3 wt% Y 2 O 3 , with simultaneous densification at 1500 °C for 8 h. The mechanical properties of this material (Young's modulus, Poisson's ratio, Vickers hardness, strength and fracture toughness) compare well with those of pre-synthesised silicothermal X-phase sialon sintered in a separate step at 1620 °C. The mild reaction conditions of the reaction-sintered material are reflected by its microstructure which contains large (50 μm) lath-like X-phase sialon crystals embedded in a matrix of smaller yttrium-containing grains and a few large pores. The mechanical properties of silicothermal X-phase sialon bodies compare favourably with those reported for X-phase bodies sintered at 1650–1720 °C and are also comparable with other sialons, silicon nitrides and structurally related aluminosilicate (mullite) ceramics.

Mechanism of the formation of β–Sialon by self-propagating high-temperature synthesis

Abstract: β–Sialon has been prepared by SHS (Self-propagating High-temperature Synthesis) with Si, Al, Al2O3, and β–Si3N4 under high nitrogen pressure. Rod-like crystals of b–Sialon with an aspect ratio of 4–15 and whiskers with various aspect ratios were obtained in the process. Considering the morphologies of the products, it is concluded that VLS (vapor-liquid-solid) is the predominant mechanism of the formation of rod-like crystals and whiskers.

High resolution electron microscopy investigations of interface and other structure defects in some ceramics

Abstract: Interface, grain boundary, and other structure defects are the most important structural factors to affect the properties of ceramics materials. The present paper shows the relationship between the properties and those structure features such as grain boundaries, phase boundaries, interfaces, twins, intergrowths, dislocations, point defect aggregates, order-disorder, and other structure defects in different kinds of ceramics materials. At present this research covers: C60, sialon-based ceramics (α-sialon/SiC(w) composite, Y-α-sialon/β-sialon composite), high Tc superconductors (YBa2Cu3O7, YBa2Cu4O8, Bi2Sr2CaCu2O8, Bi2Sr2Ca2Cu3O10), and bioceramics (hydroxyapatite, chlorapatite) and so on. The structure features mentioned above were characterized by high-resolution electron microscopy; so the structure details are at an atomic level and the related physical, chemical, engineering, even biological phenomena can be understood at an atomic and molecular level. Microsc. Res. Tech. 40:177–186, 1998. © 1998 Wiley-Liss, Inc.

In-situ toughened alpha prime-sialon-based ceramics

Abstract: An in situ toughened ceramic having a relatively high proportion of an alpha prime SiAlON phase and including an elongated rod shape grain structure of that phase.

Electron spectroscopic imaging and fine probe EDX analysis of liquid phase sintered ceramics

Abstract: This paper is focussed on high resolution imaging and microanalysis of two classes of liquid phase sintered ceramic materials, α and duplex α/β sialons and α-SiC ceramics. The microstructures were characterized in a FEGTEM equipped with surrounding interactive instrumentation for EDX and EELS analysis and electron energy filtering, and special attention was paid to the intergranular microstructure and the variation in local chemical composition. The α and duplex α/β sialon ceramics had been fabricated from balanced starting powder mixtures corresponding to the α-sialon composition R 0·4 Si 10·2 Al 1·8 O 0·6 N 15·4 (R=Sm, Dy or Yb) and the β-sialon composition Si 5·4 Al 0·6 O 0·6 N 7·4 . The α-SiC ceramics had been pressureless sintered or hot isostatically pressed with smaller additions of Al 2 O 3 and Y 2 O 3 . Combined high resolution analytical and spatial information was obtained from electron spectroscopic images recorded around the C K, N K, O K, Al L 2,3 and rare earth element N 4,5 edges in the electron energy loss spectrum. Residual glassy grain boundary films rich in O and cations originating from the metal oxide/nitride additives were present at all characterized grain boundaries. High resolution imaging and elemental maps computed from the electron energy filtered images showed intergranular film thicknesses in the range 1·5 to 2·3 nm. The results imply that the intergranular film thickness in the sialon microstructures is dependent upon the particular grain boundary and the local chemistry as well as the α′ stabilizing cation. Elemental concentration profiles obtained by stepwise fine probe EDX analysis across α′/α′, α′/β′ and β′/β′ sialon grain boundaries revealed significant variations in the local α′ and β′ substitution levels, both within and between analysed grains. Concentration gradients within the sialon and α-SiC grains, associated with the different grain boundaries, were not detected.

A rule-of-mixtures model for sintering of particle-reinforced ceramic-matrix composites

Abstract: A model is described for the densification of ceramics and particle-reinforced ceramic-matrix composites when the degree of shrinkage is large. A rule of mixtures model can be used to describe the degree of densification in cases where there is no interfacial reaction between the reinforcement and the matrix. These models are illustrated by the reaction sintering of sialon and of sialon with titanium nitride, and also by the sintering of clay containing silicon carbide. The extent of linear shrinkage and also the rate of densification decrease with increasing volume fraction of reinforcement and these effects are explained on the basis of the model. The correspondence between the data and the model allow the prediction of shrinkage in particle-reinforced composite systems where there is no interfacial reaction during the densification process.

Eu-doped α-sialon and related phases

Abstract: Rare-earth (Re) oxides are widely used, alone or in combination with AlN and=or Al2O3, as sintering aids in densifying Si3N4-based ceramics. Although the added Re ions cannot be accommodated in the structures of â-Si3N4 and â-sialon, Si6yzAlzOzN8yz, they may, depending on their ionic size, be incorporated in the structure of a-sialon, Rex Si12y(m‡n)Al(m‡n)OnN16yn. The largest cations that ®t into the interstices of the a-sialon structure have a radius of 1.0 AE , and therefore Ce3‡, with r ˆ 1:03 E A, is the largest Re cation that has been found to enter that structure [1, 2]. Eu3‡, with an ionic radius of 0.95 AE , is thus expected to stabilize asialon, but previous work seems to indicate that the Eu-doped a-sialon phase is not formed [3 and references therein]. It has been reported that trivalent rare-earth cations with nearly ®lled or half-4f electron con®gurations, e.g., Yb3‡, Eu3‡ and Sm3‡, are easily reduced to the divalent state during the sintering of Si3N4-based ceramics [4, 5]. Because the Eu 2‡ ion has a radius of 1.16 AE , it is too large to be accommodated in the a-sialon structure, but Eu3‡, with a radius of 0.95 AE should ®t, and it does. We have con®rmed the formation of the Eu3‡-doped asialon phase by X-ray diffraction (XRD) and electron microscope studies combined with element analysis. In addition, we have observed the formation of two new phases that most probably contain divalent Eu ions. A sample with an overall composition of Eu0:48 Si9:227Al2:703O1:178N14:701, i.e., an a-sialon composition RexSi12y(m‡n)Al(m‡n)On N16yn with x ˆ 0:48, m ˆ 1:44 and n ˆ 1:3 was prepared by hotpressing a powder mixture of Si3N4 (UBE, SNE10), AlN (H.C. Starck-Berlin, grade A), and Eu2O3 (99.9%, Johnson Matthey Chemicals Ltd.) at 1800 8C for 2 h under 35 MPa pressure in a graphite resistance furnace and in a nitrogen atmosphere (the oxygen content of the nitride starting powders has been accounted for). The prepared sample was characterized by its XRD pattern. Its microstructure was observed in a scanning electron microscope (SEM, Jeol JSM 880) and in a transmission electron microscope (TEM, Jeol 200CX), each equipped with an energy dispersive spectrometer (EDS). The obtained XRD pattern, given in Fig. 1, con®rms the formation of Eu-doped a-sialon and shows that the prepared sample consists of a mixture of a-sialon, â-sialon and two other new phases (see below). The unit cell dimensions of the Eu-a-sialon phase (a ˆ 7:7874, c ˆ 5:6590 E A) are typical for a rare-earth stabilized a-sialon phase with a composition close to the a±â phase boundary [6]. The SEM micrograph obtained in backscattering mode, given in Fig. 2, reveals the presence of four phases. EDS point analysis of individual grains (cf. Table I) suggests that the comparatively small and black grains are â-sialon, while the dark grey grains are asialon. The light grey and white grains are two new, unidenti®ed phases, and due to the contrast difference, the latter must contain more Eu than the former. The z-value of the â-sialon phase, Si6yzAlzOzN8yz, is 0.5 according to the SEM=EDS analysis, which is in fair agreement with z ˆ 0:4,

Sialon ceramic with gradient microstructures

Abstract: In order to combine the high hardness of α-Sialon and high toughness of β-Sialon, a graded Sialon ceramic was developed in this work. The microstructures and the properties of the graded Sialon ceramic change gradually from spherical α-Sialon grains on the surface (HV10 = 19 GPa, HV0.1 = 34 GPa, and K1c = 4.3 MPam1/2) to elongated β-Sialon grains in the inner core (HV10 = 15.7 GPa, HV0.1 = 24 GPa and K1c = 7.3 MPam1/2). Mixtures of the α-Sialon grains and the β-Sialon grains were observed in the transitional zone with hardness HV0.1 = 24 ~ 30 GPa. The graded Sialon ceramic was produced in situ by using a green compact starting from a homogeneously mixed powder mixture.

Study of the synthesis of sialon phases from silica-aluminium powder mixture using microwave energy

Abstract: Using a domestic microwave oven, pellets of a silica–aluminium powder were ignitedand a self-propagating high-temperature synthesis (SHS) reaction occurred to produce Si+AlN+Al2O3 as the resultant phases. Silicon AlN and Al2O3 were subsequently nitrated to synthesize sialon phases under a nitrogen atmosphere without cooling. Thus both the SHS process and the nitration were finished within one-step process, which could save processing time and energy. © 1998 Kluwer Academic Publishers

Slip casting of sialon for pressureless sintering

Abstract: The properties of aqueous slips of sialon were studied. Ammonium polyacrylate was used as a deflocculant. It was shown that the apparent viscosity for slips with solid content 40 vol % was low and the slip resulting from this is almost Newtonian. This slip proved sufficiently fluid for casting. However, the apparent viscosity for slips with solid content 45 vol % increased significantly. The slips resulting from this exhibited dilatant flow and were difficult to cast. The viscosity, fluidity, and pH of the slips were studied and tiles were cast and fired in nitrogen at 1740 °C for 3 h to a bulk density of 3.20 g cm−3.

Oxidation resistance of Nd-Si-Al-O-N glasses and glass-ceramics

Abstract: Glasses containing up to 20% (e.o.) nitrogen were prepared in Nd-Si-Al-O-N system. Compositions, which crystallise in single and multiphase materials, were heat-treated under various conditions, and oxynitride glasses were found to be self-nucleating. The crystalline phases formed depend on both the composition of the parent glass and the heat-treatment conditions. X-ray investigations show that U-phase (Nd 3 Si 3 Al 3 O 12 N 2 ) is stable at 1150 °C for many crystallisation durations. The microstructural changes were studied by scanning electron microscopy (SEM) and image analysis. The reactivities in air of different glasses and glass-ceramics were compared in the temperature range of 975–1250 °C. Sialon glasses oxidised rapidly in air at a temperature above the glass transition temperature by forming a porous oxide scale, due to the evolution of gaseous species at the internal interface. The nature and morphology of oxidised samples were characterised by optical microscopy, X-ray diffraction (XRD), and SEM. The nature and the concentration of the crystalline phases present in the glassceramics were found to control the oxidation behaviour of the resulting materials.

Cutting tools and wear resistant articles and material for same

Abstract: A ceramic having a relatively high proportion of an alpha prime SiAlON phase and exhibiting high hardness and toughness In a particularly preferred embodiment, a cation of Gd is used as a modifying cation

Method of producing a surface modified sialon composite

Abstract: A sialon composite and fabrication method thereof in which the phase of a surface layer thereof is modified differently from that of the bulk. The surface layer is modified to α-sialon or α-β sialon composite but the bulk is β-sialon. Also, the surface layer is modified to β-sialon or α-β sialon composite but the bulk is α-sialon.

Silicon nitride sintered body, method of preparing the same and nitrided compact

Abstract: A silicon nitride sintered body prepared through a nitriding reaction of Si, consists of crystal grains mainly composed of silicon nitride and/or SIALON and a grain boundary phase. The grain boundary phase includes a first component including at least one element selected from a group of Na, K, Mg, Ca and Sr and a second component including at least one element selected from a group of Y and lanthanoid series elements. The molar ratio of the first component to the second component is in the range of 1:1 to 6:1 in terms of oxides. The mean breadth and the mean length of the crystal grains are not more than 0.1 μm and not more than 3 μm respectively, and the standard deviation of the mean length in the sintered body is within 1.5 μm. Especially, the mean breadth of the crystal grains is at least 0.4 μm and not more than 0.9 μm.

Use of Rare Earth Concentrate as a SiAlON Sintering Additive

Abstract: The sintering additive used in SiAlON containing 5, 10, 15 and 20 eq% Al was a concentrate of rare earth elements, with 89.5% Y 2 O 3 , 3.9% Er 2 O 3 , 3.8% Dy 2 O 3 , 1.8% Lu 2 O 3 and other rare earth elements at lower concentrations. The sintering variables studied were: Al content in solid solution with SiAlON, heating rate, sintering time and temperature. The results show that an increase in Al concentration leads to higher densities and α→ β transformation, and smaller grains with more spherical shape. An increase in both sintering time and temperature and heating rate lead to the formation of larger grains with smaller aspect ratio. Samples sintered at 1700°C/30min had higher density values. Samples with 99% of theoretical density were obtained by using the concentrate of rare earth elements.