Showing papers on "Sialon published in 2002"

Silicon Nitride Ceramics

Abstract: Silicon nitride ceramics is a generic term for a variety of alloys of Si3N4 with additional compounds necessary for a complete densification of the Si3N4 starting powder. They are heterogeneous, multicomponent materials characterised by the inherent properties of the crystalline modifications α and β of Si3N4 and the significant influence of the densification additives. With a view to ability of the α and β modification to form solid solutions α-Si3N4 (αss) and β-Si3N4 (βss) solid solutions can be distinguished. Each group contains engineered materials with interesting properties for special applications. Phase relations and micro-structures determine the properties decisively. Composition of the phases, the distribution of the grains, their aspect ratio and the grain boundary phase are pronounced microstructural features. The formation of the microstructure strongly depends on the one hand on the quality of the Si3N4 starting powders, which closely is related to the chemistry of the production process, and on the other on the liquid phase sintering as the most important step in the densification route. The interrelation between pure Si3N4, the densification of the powder including the role of sintering additives, microstructural engineering, physicochemical properties of the sintered Si3N4 ceramics (SSN, GPSN, HPSN, HIP-SSN, HIP-SN) are described in more detail and compared to reaction bonded Si3N4 ceramics (RBSN), which are produced by nitridation of silicon powders.

Oxynitride phosphor activated by a rare earth element, and sialon type phosphor

Abstract: A sialon type phosphor in the form of a powder comprising at least 40 wt % of α-sialon represented by the formula (Ca x ,M y )(Si,Al) 12 (O,N) 16 (where M is at least one metal selected from the group consisting of Eu, Tb, Yb and Er, 0.05<(x+y)<0.3, 0.02

The effect of processing conditions, amount of additives and composition on the microstructures and mechanical properties of α-SiAlON ceramics

Abstract: A series of samples with yttrium α-SiAlON compositions and different amounts of additive has been fabricated from α-Si 3 N 4 , AlN, Al 2 O 3 and Y 2 O 3 starting powders, using gas pressure sintering and three different sintering procedures. One series of samples was heated up to 1825°C and then held for 3 h, another group of samples was held at a lower temperature (1500 or 1600°C) for 1 h and then heated up to 1825°C and held for 3 h. The results of investigations using scanning electron microscopy showed the effect of composition and sintering procedure on the morphology of α-SiAlON grains. It was found that the amount of elongated grains increased with increasing amount of liquid phase. The mechanical tests showed that all of the samples exhibited HV10 values in the range of 1800–1976 kg/mm and K IC values in the range of 3.9–6.3 MPam 1/2 .

Influence of starting material composition and carbon content on the preparation of Mg-α SiAlON powders by carbothermal reduction-nitridation

Abstract: This paper investigated the synthesis of Mg–α SiAlON from Talc (Mg 3 (Si 2 O 5 ) 2 (OH) 2 ) and halloysite clay (Al 2 Si 2 O 5 (OH) 4 ). Talc, halloysite and carbon black as a reducing agent were used as the starting materials, and 3 wt.% α-Si 3 N 4 was added as the seed in all cases. The mixtures were heated in flowing N 2 (gas) to synthesize Mg–α SiAlON powders. The chosen molecular ratios of talc to halloysite were 1.0, 1.5, 2.0 and 2.5. The influences of various reaction parameters such as the carbon content, temperature and holding time at the top temperature were studied. The results showed that the synthesized powders were composed of α-SiAlON, β-SiAlON, β-SiC, 15R-AlN polytypoid and AlN phases; the phases and the particle morphology greatly depended on starting material composition and synthesis parameters. A larger amount of talc was needed to compensate for Mg evaporation loss in the starting composition. Higher carbon content seemed to retard the reaction rate, resulting in coarse particle size with an irregular grain shape. The highest content of Mg–α SiAlON, 90 wt.%, was achieved at 1480 °C for 4 h at talc to halloysite ratios of 1.5 and 2.0.

Thermal shock properties of β-sialon ceramics

Abstract: An indentation–quench method based on Vickers cracks for measuring thermal-shock properties has been applied to β-sialon materials. The thermal-shock properties have been correlated with the morphology of the β-sialon grains, the z value in the β-sialon solid solution Si6−zAlzOzN8−z and the amount of residual intergranular glass phase. z Values in the range 0.6–3.0 were tested, and the amount of residual yttrium-containing glass phase was varied between 0 and 20 vol.%. The best thermal-shock resistance was found at low z values, and was further improved by adding an intergranular glass phase. The poorest resistance to thermal shock was found for the highest z value where the presence of glass had no measurable influence. One composition (z=1.5, 10 vol.% glass) was selected for studying the influence of the microstructure on the thermal-shock properties. The microstructure was varied by applying different sintering conditions. An improvement of the thermal-shock properties and the fracture toughness was found in samples containing elongated β-sialon grains formed in situ. In general, in-situ reinforced β-sialon materials with low z values and containing an intergranular glass phase exhibited the best thermal-shock resistance and improved fracture toughness (K1c>4 MPa m 1 2 ).

Corrosion-resistant ceramics

Abstract: Anti-corrosion ceramics comprising a substrate of at least one kind of silicon-containing ceramics selected from a silicon nitride, a silicon carbide and Sialon, and a surface protection layer formed on the surface of the substrate, wherein the surface protection layer comprises a zirconium oxide stabilized with an element of the Group IIIa of periodic table, and the total amount of Al and Si in the surface protection layer is suppressed to be not larger than 1% by mass. Particularly, the surface layer has a thickness of from 5 to 200 μm and a porosity of 5 to 30%. The anti-corrosion ceramics exhibits a high resistance against the corrosion due to the water vapor of high temperatures in a region of not lower than 1000° C., and can be preferably used as parts of internal combustion engines such as parts of gas turbine engines, like a turbine rotor, nozzles, a combustor liner and a transition duct.

Synthesis of (Ca,Mg)-α-sialon from slag by self-propagating high-temperature synthesis

Abstract: The synthesis of (Ca,Mg)-α-sialon powders using slag as a starting material by self-propagating high-temperature synthesis (SHS) is reported for the first time. The only crystalline phase detected in the synthesized products was α-sialon. The characters of the synthesized powders were observed and compared with that of conventionally hot-pressed materials. More metal cations were absorbed into the α-sialon structure of the SHS synthesized powders. Fully densified sialon ceramics can be obtained using the SHS powders by hot-pressing. The hardness and fracture toughness of the hot-pressed α-sialon ceramics reached 16 GPa and 5.1 MPa m−1/2 respectively.

Stability of intergranular phases in hot-pressed Si3N4 studied with mechanical spectroscopy and in-situ high-temperature XRD

Abstract: The impulse excitation technique (IET) and high temperature X-ray diffraction (HTXRD) were used to investigate the intergranular glass phase and its crystallisation behaviour in four hot-pressed silicon nitrides. The internal friction or damping peak height measured with IET near the glass transition temperature, T g , is used as a qualitative indicator for the amount of residual intergranular amorphous phase after sintering. Silicon nitride powder was hot-pressed with different sintering additives. The silicon nitride containing 4 wt.% Al 2 O 3 does not reveal an internal friction peak at T g , i.e. it does not contain a significant amount of intergranular glass phase. Three other silicon nitrides, containing either 8 wt.% Y 2 O 3 , 6 wt.% Y 2 O 3 +2 wt.% Al 2 O 3 , or 2 wt.% Y 2 O 3 +4 wt.% Al 2 O 3 +2 wt.% TiN, do show an internal friction peak near T g . This “ T g -peak” is nearly unaffected by heating up to 1400 °C in the silicon nitride with Y 2 O 3 +Al 2 O 3 +TiN sintering aids, whereas the amount of intergranular glass in the ceramics containing either Y 2 O 3 +Al 2 O 3 or Y 2 O 3 as a sintering aid is strongly reduced by subsequent heating. As observed from HTXRD, the onset temperature of crystallisation of the intergranular glass in the ceramic containing Y 2 O 3 +Al 2 O 3 sintering aids is about 1100 °C, with the formation of Y–N-apatite (Y 20 N 4 Si 12 O 48 ) and O-sialon (Al 0 . 04 Si 1 . 96 N 1 . 96 O 1 . 04 ). The O-sialon phase in the yttria and alumina containing ceramics, formed either during sintering or during heat treatment, is not stable at elevated temperatures and dissolves in the intergranular glass phase between 1300 and 1400 °C. The O-sialon phase in the ceramic without Y 2 O 3 sintering additive, however, is thermally stable. The presence of Ti 4+ ions in the intergranular glass phase is suggested to inhibit its crystallisation, resulting in a stable high temperature damping behaviour.

Formation of in-situ reinforced microstructure in α–sialon ceramics I: Stoichiometric oxygen-rich compositions

Abstract: The abnormal grain growth in α–sialon ceramics was investigated. The preparations had stoichiometric compositions on the oxygen-rich phase boundary, and they were stabilized by Y, Nd, Sm, Dy, and Yb, respectively. Specimens were prepared from α–Si3N4 as precursor powder by applying conventional hot pressing and a novel rapid consolidation process, namely spark plasma sintering (SPS). Single-phase α–sialon ceramics with in situ reinforced bimodal microstructure, i.e., large elongated grains embedded in a matrix consisting of small equiaxed grains, were obtained above 1750 °C in all systems compacted by SPS and above 1800 °C in systems stabilized by Nd and Sm but not Dy, Y, or Yb by a two-step hot-pressing procedure. It was observed that the formation of abnormally grown α–sialon grains was strongly temperature-dependent, indicating that it was encouraged by the formation of a transient liquid phase that stimulated the dissolution of any remaining nitride precursors and early formed small α–sialon grains and sequentially facilitated supersaturation by the α–sialon constituents. The presence of elongated grains improves fracture resistance in the obtained materials.

Formation of in situ Reinforced Microstructures in α-sialon Ceramics : Part II. In the Presence of a Liquid Phase.

Abstract: In situ reinforced microstructures with well-dispersed elongated grains, up to 10 μm in length, embedded in matrices consisting of submicron equiaxed grains, were developed by hot pressing Y-, Yb-, ...

In-Situ Reaction Synthesis of Non-Oxide Boron Nitride Composites

Abstract: Ceramic composites reinforced by second-phase particles demonstrate excellent mechanical properties, corrosion/oxidation resistance, thermal shock resistance, or their combination compared with monolithic materials. In this communication the authors describe the preparation of high-strength in-situ non-ox ide boron nitride (NOBN) composites such as AlN-BN, Alon-BN, and Sialon-BN having fine and homogeneous microstructures with fine and closed pores. These materials were obtained by nitridation of AlB 2 .

Sialon sintered compact

Abstract: The present invention proposes a technique for obtaining a sialon sintered body at low cost by sintering a beta sialon powder directly synthesized by a combustion synthesis method. Ultra-fine beta sialon powder is directly synthesized using a combustion burner for the purpose of further reducing manufacturing costs for use as a general-purpose material for automobile applications. [Solution] 1. By performing simple post-treatment on fine beta sialon powder obtained by direct combustion synthesis of low-price raw materials using a combustion burner, ultra-fine powder with a particle size of several hundreds of nanometers was obtained at a low price. 2. The sialon sintered body manufactured from the ultrafine powder showed a characteristic value higher than that of the existing silicon nitride sintered body. 3. According to the present invention, a sialon sintered body can be produced at a price applicable to automobile applications and the like.

Structure and bonding in a cubic phase of SiAlON derived from the cubic spinel phase of Si3N4

Abstract: The structure and electronic bonding in the spinel SiAlON (Si6−zAlzOzN8−z, z=1) derived from the cubic c-Si3N4 are studied by a first-principles density functional method. Al prefers the octahedral site of the spinel lattice. The small energy difference between the four possible structural configurations indicates that the real SiAlON may be a random solid solution. The lowest energy configuration of c-Si5AlON7 is a semiconductor with a direct LDA band gap of 2.29 eV.

Preparation and Luminescence Spectra of Calcium- and Rare-Earth (R: Eu, Tb, and Pr)-Codoped α-SiAlON Ceramics.

Abstract: Rare-earth-doped oxynitride or nitride compounds have been reported to be luminescent and may then serve as new phosphors with good thermal and chemical stabilities. In this work, we report the photoluminescence (PL) spectra of europium-, terbium-, and praseodymium-doped Ca-α-SiAlON ceramics. The highly dense ceramics were prepared by hot pressing at 1750°C for 1 h under 20 MPa in a nitrogen atmosphere. Europium-doped Ca-α-SiAlON displayed a single broad emission band peaking at λ = 550-590 nm depending on the europium concentration. The emission bands in the spectra of europium-doped Ca-α-SiAlONs were assigned to the allowed transition of Eu 2+ from the lowest crystal field component of 4f 6 5d to 8 S 7/2 (4f 7 ) ground-state level. The emission spectra of terbium- and praseodymium-doped Ca-α-SiAlON ceramics both consisted of several sharp lines, which were attributed to the 5 D 4 → 7 F j (j = 3, 4, 5, 6) transitions of Tb 3+ and 3 P o → 3 H j (j = 3, 4, 5) transitions of Pr 3+ , respectively. In particular, the terbium-doped α-SiAlON ceramics showed a strong green emission among these phosphors.

Multication doped alpha-beta sialon ceramics

Abstract: This invention is about SiALON ceramics comprising an alpha SiA1ON, a beta SiA1ON and an intergranular amorphous and/or crystalline phase. Said alpha SiALON phase was prepared from a multi-cationic mixture including the element of calcium, at least one of yttrium and/or a rare earth element with atomic number greater than 62 and at least one of a rare earth element with atomic number equal or smaller than 62. The second phase, beta SiALON contains elements of Si, AL, O, N. The third phase, amorphous and/or crystalline intergranular phase contains in addition to elements of Si, AL, O, N, element of calcium, at least one of yttrium and/or a rare earth element with atomic number greater than 62 and at least one of a rare earth element with atomic number equal or smaller than 62. This material with its toughness, high fracture resistance and high temperature resistance is useful for cutting tool applications and components in machinery and engines.

Grain Growth of α‐SiAlON in the Calcium‐Doped System

Abstract: Two calcium-doped α-SiAlON compositions (Ca 0.6 Si 10.2 Al 1.8 -O 0.6 N 15.4 and Ca 1.8 Si 6.6 Al 5.4 O 1.8 N 14.2 ) were prepared by hot pressing at 1600° and 1500°C, respectively, for complete phase transformation from α-Si 3 N 4 to α-SiAlON. Both samples were subsequently fired at different temperatures for different periods of time to study the grain growth of α-SiAlON. Elongated α-SiAlON grains were developed in both samples at high temperatures. The kinetics of grain growth was investigated based on the variations in length and width of the α-SiAlON grains under different sintering conditions. Different growth rates were found between the length and width directions of the α-SiAlON crystals, resulting in anisotropic grain growth in the microstructural development.

Microstructures and Fracture Characteristic of Si3N4-O’SiAlON Composites using Waste-Si-Sludge

Abstract: Si 3 N 4 -O'SiAlON composites using waste-Si-sludge was fabricated by gas-pressure-sintering (GPS) process at 1950°C for 3.5 h. The percent nitridation of waste Si compacts showed a lower value than that of using commercial Si powders caused by the high contents of oxygen in the waste Si particles. Some amounts of Si 2 N 2 O and Y 2 Si 2 O 7 phases were also detected in the reaction-bonded Si 3 N 4 (RBSN) although main components were α-Si 3 N 4 , β-Si 3 N 4 and residual Si phases. In the post-sintered body by GPS, no residual Si, α-Si 3 N 4 , Si 2 N 2 O and Y 2 Si 2 O 7 phases were detected except the β-Si 3 N 4 and O'SiAlON. The grain growth of rod-like Si 3 N 4 grains was inhibited by the dispersion of fine O'-SiAlON particles. The fracture toughness and strength of Si 3 N 4 -O'SiAlON composites were 5.6 MPa.m 1/2 and 456 MPa, respectively.

Sliding wear of various materials in molten zinc

Abstract: A full journal bearing tester was built to study wear behaviour of materials sliding in molten zinc. Various material combinations, including cobalt based superalloys, cermet coatings (on 316L stainless steel substrates), and ceramics were evaluated at conditions similar to continuous galvanising line (CGL) operations. Metallographic and chemical analyses were conducted on tested specimens to characterise the wear. Cobalt based superalloys not only suffered considerable wear but also reacted with molten zinc to form intermetallic compounds. Cermet coatings and ceramics experienced noticeable wear although they were non-wettable and did not react with zinc baths. The study showed that MgO stabilised ZrO2 suffered the most wear when running against laser clad WC coated 316L while the combination of SiAlON and laser clad WC coated 316L showed the least amount of wear. The composite nature of hard particles imbedded in a soft matrix, posing resistance against both abrasive and surface fatigue wear, is...

Material for build-up welding having excellent corrosion resistance, wear resistance and seizure resistance and composite tool

Abstract: PROBLEM TO BE SOLVED: To provide a material for build-up welding which has excellent corrosion resistance, wear resistance and seizure resistance in hot and cold working, and a composite tool including a roll and a roller whose surface layer has been subjected to build-up welding using the material. SOLUTION: The material for welding or thermal spraying has a composition containing, by mass, 1.0 to 4.5% C, <=0.5% Si, <=1.0% Mn, 15.0 to 30.0% Cr, 1.0 to 20.0% Co and 5.0 to 25.0% V, and the balance Fe with inevitable impurities. In the composite tool having excellent corrosion resistance, wear resistance and seizure resistance, the surface layer part has been subjected to build-up welding using the above material.

Rapid Synthesis of Finely Dispersed Sialon Powder by Carbothermal Nitridation of Kaolin

Abstract: A powder based on β-sialon is prepared by carbothermal nitridation. A scheme for rapid synthesis of β-sialon with controlled dispersity at high temperature is developed. The critical size range of β-sialon particles is determined. Experimental data on phase composition and particle size of the synthetic sialon powders are in agreement with theory.

Low cost process of synthesizing sialon ceramic powder

Abstract: The method for synthesizing sialon ceramic powder material by low cost is characterized by using metallurgical furnace slag or limestone or fly ash industrial waste material or cheap natural mineral as raw material, adding metal silicon powder, aluminium powder and partial crystal seed through the processes of ball grinding, drying, high-temp. self spreading and other treatment to obtain the invented single-phase alpha-sialon powder material. Said powder material possesses excellent sintering property, can be sintered without pressure at 1600-1800 deg.C, its density is up to 3.07g/se.cm, hardness is 15.53 GPa, and its toughness is 4.72 Mpa.m(1/2), as compared with Al2O3, ZrO2 and SiC ceramics its erosion resistance is excellent.

Study on oxidation behavior of (Nd,Y)- and (Nd,Yb)-α-Sialon

Abstract: The oxidation behavior of multi-cation α-Sialons containing Nd and Y or Yb has been investigated for the compositions (Nd0.18Y0.18)Si10.38Al1.62O0.54N15.46 and (Nd0.18Yb0.18)Si10.38Al1.62O0.54N15.46 respectively in the temperature range of 1200°C to 1400°C in air. The grains of silicate containing Nd and Y as well as Nd and Yb were observed in preferred orientation on the surface of the materials oxidized at 1200°C or 1300°C for 20 h for (Nd,Y)- and (Nd,Yb)-α-Sialon respectively. By increase of oxidation temperature from 1300°C to 1400°C, bubble, which was caused by softening of silicate oxidation layer, occurred and glassy phase then appeared obviously. The phases formed on the surfaces of multi-cation α-Sialons during the oxidation were also discussed in this paper.

Silicon nitride-based sintered compact and cutting tool using it

Abstract: PROBLEM TO BE SOLVED: To provide a silicon nitride-based sintered compact with high wear resistance and a cutting tool using it SOLUTION: This silicon nitride sintered compact consists of a main crystalline phase which consists of α- and/or β-silicon nitride or a sialon, and a boundary phase which contains at least one kind of sintering additives consisting of metal oxide Content of at least one kind of sintering additives on the top surface of the sintered body is lower than that of the lower part (a depth of 1000 μm) by volatilizing the additives in firing The lower ratio of the content of the additives on the top surface to that of the depth of 1000 μm (>=080) gives the higher wear resistance The cutting tool made from the silicon nitride sintered compact is used for cutting a cast iron