Cost-effective fabrication of porous α-SiAlON bonded β-SiAlON ceramics

Porous sialon ceramics have been cost-effectively prepared by pressureless sintering from a mixture of elongated SHS β-sialon powders with an α-sialon precursor composition composed of α-Si₃N₄, AlN, Y₂O₃ and Al₂O₃. The obtained porous sialon ceramics exhibited low shrinkage and homogeneous pore size distribution. The results of X-ray diffraction and scanning electron microscopy showed that a uniform microstructure with elongated and intermingled β-SiAlON grains, which were strongly connected by α-SiAlON phase, has been obtained.     

Light element ceramics

An overview of a few structurally important light element ceramics is presented. Included in the overview are silicon nitride, sialon, aluminium nitride, boron nitride, boron carbide and silicon carbide. Methods of preparation, characterization and industrial applications of these ceramics are summarized. Mechanical properties, industrial production techniques and principal uses of these ceramics are emphasized. Contribution No. 76 from the Materials Research Laboratory     

Pressureless sintering of Y2O3-CeO2-doped sialons

Various sialon materials have been prepared by pressureless sintering at 1775 and 1825 °C using Y₂O₃ and/or Ce0₂ as sintering aids. Constant molar amounts of the oxide mixtures were added in the ratios Y₂O₃/CeO₂: 100/0, 75/25, 50/50, 25/75, 0/100 corresponding to 6.0 and 9.25 wt% for the pure Y₂O₃ and pure CeO₂, respectively. Only one of the compositional series reached full density at 1775 °C with cerium replacing yttrium, whereas at 1825 °C all compositional series except one became dense. The samples sintered showed that yttrium but not cerium stabilizes the sialon phase in these ceramics. The dense cerium-sialon ceramics sintered at 1825 °C have as good hardness and indentation fracture toughness as the corresponding yttrium-sialon ceramics, or even higher for the sialon type of materials. For the mixed - sialon materials the hardness decreased as the amount of a sialon phase decreased by increasing cerium-doping.     

Creep properties of (Si–Al–O–N)SiC whisker composites

(Si–Al–O–N) (sialon)–SiC whisker (SiC_w) composites containing up to 10 mass% SiC_w were prepared by hot isostatic pressing. The strengths and the fracture toughness of composites remained relatively unchanged with the amount of SiC_w. The addition of SiC_w enabled us to improve the creep properties of sialon ceramics. The total creep strain and steady-state creep rate at 1473 K under a stress of 400–500 MPa decreased with increasing the amount of SiC_w. The experimental creep exponent values of monolithic sialon and sialon–SiC_w composites were nearly 1. It is supposed that the creep of both monolithic sialon and sialon–SiC_w composites are dominated by the viscous flow of the interglanular glassy phase.     

BN／Sialon复相陶瓷的显微结构及Sialon晶粒Z值的不确定性

采用热压烧结工艺开发了高抗热震性及无明显各向异性的ＢＮ（Ｐ）／ＳＩＡＬＯＮ复相陶瓷，观察分析了显微结构特征，发现ＳＩＡＬＯＮ晶粒Ｚ值随晶粒结晶形态不同及晶粒内位置不同而呈现不确定的现象。     

氮化硅及Sialon陶瓷基材上金刚石膜的沉积

本文用热丝 CVD 法在氮化硅复合陶瓷及 Sialon 陶瓷上沉积了金刚石薄膜,用 X 射线衍射、拉曼光谱、扫描电子显微镜、表面形貌仪、划痕实验仪对所形成的膜及基体进行了分析。初步探讨了膜与基材的附着性影响因素。     

Differing Effects of Mn and Mg Oxide Sintering Aids on Sialon Creep and Fracture

This study is based on two sialon ceramics hot-pressed with small amounts of two different sintering additives, MgO and Mn₃O₄. The grain boundary interfaces were characterized by high resolution electron microscopy and Auger electron spectroscopy.The major microstructural difference between the two ceramics was the occurrence of microscopic regions of triple junction silicate glass in the Mn containing ceramic. High temperature creep studies of this ceramic showed cavitation induced deformation. In contrast there was no detectable interfacial silicate phase in the Mg containing ceramic and no cavitation was observed during high temperature deformation.Furthermore, microstructural observations revealed that long duration heat-treatment of the Mn containing ceramic resulted in reduction of triple junction glass beyond detection. This desegregation of impurities led to non-cavitating deformation and hence resulted in marked improvement in creep and slow fracture resistance.     

New heat treatment methods for glass removal from silicon nitride and sialon ceramics

Sialon ceramics were discovered simultaneously (but independently) in late 1971 at Newcastle University and also at the Toyota Research Laboratories in Japan. During the 30 years since their original discovery, the Newcastle laboratory has made a significant contribution to current understanding of the science and technology of these materials. Sialons are of interest as engineering materials for high temperature (>1000°C) applications because they can be pressureless-sintered to high density and be designed to retain good mechanical properties even up to 1350°C, whereas competing metallic materials are weaker and prone to corrosion. A characteristic disadvantage of all nitrogen ceramics is that an oxide additive is always included in the starting mix to promote densification, and this remains in the final product as a glassy phase distributed throughout the grain boundaries of the final microstructure. Since the glass melts at 1000°C, the high temperature properties of the final ceramic are in fact determined by the properties of the grain-boundary glass. The most common method of improving high-temperature performance is to heat-treat the material at temperatures of 1100–1350°C in order to devitrify the glass into a mixture of crystalline phases. More specifically it is desirable to convert the glass into a sialon phase plus only one other crystalline phase, the latter having a high melting point and also displaying a high eutectic temperature (max 1400°C) in contact with the matrix sialon phase. Previous studies have shown that there are a limited number of possible metal-silicon-aluminium-oxygen-nitrogen compounds which satisfy these requirements. The present paper gives an overall review of this subject area and then summarises recent work at Newcastle aimed at total removal of residual grain boundary glass. This has been achieved by: (1) a post-preparative vacuum heat treatment process to remove the grain boundary glass from silicon nitride based ceramics in gaseous form, (2) above-eutectic heat-treatment (AET) of sialon-based ceramics to crystallize grain-boundary liquid into five-component crystalline sialon phases.     

Some observations on the wetting and bonding of nitride ceramics

Several series of experiments have been conducted to gain information about the wettability of AlN, BN, Si₃N₄ and two sialon ceramics by potential braze materials. It was possible to achieve wetting of all five ceramics using aluminium, copper-titanium alloys, and a Ag-28Cu-2Ti alloy. Wetting by aluminium and the Ag-28Cu-2Ti alloy was usually good. Both wetting and non-wetting alloys containing titanium reacted to form TiN and it is argued that the achievement of wettability is associated with a certain degree of hypostoichiometry. While aluminium should also have reacted, no clear evidence was obtained. In supplementary experiments it was found that bonds formed by brazing with aluminium at 1000 °C could have shear strengths as great as 60MPa. Although the experimental work was preliminary in nature, it suggested that good brazing systems could be developed.     

The microstructure of hot-pressed sialon polytypes

The sialons-phases in the Si-Al-O-N and related systems — form the basis of a new group of engineering ceramics, the successful application of which depends on their chemical stability and resistance to microstructural changes at elevated temperature. In the Si-Al-O-N system a new series of polytypes exists in which the structure is determined by the metal: non-metal atom ratio. The polytypes coexist in extensive two-phase regions which are important in determining the way that microstructure and properties develop during the fabrication and subsequent use of the material. In an electron microscopic investigation of some sialon polytypes, diffraction contrast and direct lattice imaging are used to follow changes in microstructure during hot-pressing, and to determine the nature of lattice imperfections and stacking arrangements.     

升温速率对Y-α-sialon陶瓷致密化及微观结构的影响

根据Y-Si-Al-O-N相图设计Y2O3掺杂α-sialon陶瓷的组成,采用热压烧结方法在1900℃保温30min制备了设计成分为Y0.4Si9.8Al2.2O1.0N15的sia-lon陶瓷,研究了升温速率对陶瓷致密化过程、物相组成以及微观结构的影响规律。结果表明,所制备的陶瓷相组成均为α-sialon;不同升温速率陶瓷的致密化过程曲线变化趋势相同,但随升温速率增大向高温区平移,收缩的起止温度和峰值收缩速率均随升温速率增大而增大;快速升温有利于提高晶核密度,抑制柱状晶发育,柱状晶的尺寸和长径比随升温速率增大而减小。     

Hardness,indentation fracture toughness and compositional formula of X-phase sialon

The hardness and indentation fracture toughness,K _IC of sintered X-phase sialon, produced by simultaneous carbothermal reduction and nitriding of kaolinitic clays, are determined. By comparing the values obtained for the sialon with those for a commercially pure mullite, it is suggested that X-phase sialon is a material with potential for similar applications to those in which mullite is currently used. On the basis of the oxygen/nitrogen ratios of the sialon determined by the inert gas fusion method, the analysis of the energy of the dispersed X-rays (EDAX) from thinned samples in the transmission electron microscope (TEM) and the aluminium/silicon ratios, also determined by the TEM EDAX method, a modified compositional formula is proposed for X-phase.     

Reaction chemistry at joined interfaces between silicon nitride and aluminium

Joined interfaces of HIPed additive-free silicon nitride ceramics/aluminium braze bonded at a low temperature of 1073 K for 18 ks or at a high temperature of 1473 K for 1.8 ks in vacuum of 1.3 mPa and of β silicon nitride powders/aluminium powders bonded at the low temperature for 1.8 ks or 18 ks in the same vacuum are identified by analytical transmission electron microscopy and X-ray diffraction method. Mullite, some small crystals and β′-sialon are detected at the interface of the ceramics/aluminium braze bonded at the low temperature and 15R AIN-polytype sialon, β′-sialon, aluminium nitride, mullite and silica-alumina noncrystalline are detected at that bonded at the high temperature. At the interface of the two kinds of powders, aluminium nitride and silicon are also detected besides β′-sialon and silica-alumina noncrystalline even though the bonding was conducted at the low temperature. The interfacial reactions of the joints are influenced not only by bonding temperature but also by the oxide formed at the interface before bonded.     

Strength of β-sialon/Si3N4 layered composites

The strength of layered composites consisting of -sialon and Si3N4 layers, which were prepared by hot pressing, was investigated. The strength increased as the thickness of the sialon (outer layer) decreased, and reached almost the same level of Si3N4 (inner layer) when the sialon thickness was 250–300 m. No specific fracture morphologies were recognized around the interface of sialon and Si3N4. The aluminium concentration changed sharply around the interface, while the yttrium tended to diffuse deeper than aluminium. This tendency was remarkable in the samples hot-pressed at higher temperature (1900°C). The existence of compressive residual stress in the surface sialon layer was revealed and the residual stress increased as the sialon thickness decreased down to 250–300 m. The increase of strength with the decrease of sialon thickness was discussed based on the mechanical calculations in which the residual stress was considered. This calculation approximately agreed with the results of the samples hot-pressed at lower temperature (1800°C). However, the strength of the samples hot-pressed at 1900°C was much higher than the prediction in the thin range of the sialon thickness. The deep diffusion of yttrium into the sialon layers was thought to be one of the causes of this unpredictable effect.     

Damage behaviour of silicon nitride for automotive gas turbine use when impacted by several types of spherical particles

Spherical particle impact damage and strength degradation phenomena of silicon nitride by several types of spheres were analysed in comparison with chipping fracture behaviour reported in the literature. It was found that steel and partially stabilized zirconia (PSZ) particles caused Hertzian cone cracks, resulting from the elastic response of the material in accordance with the Hertzian cone crack theory. In contrast, alumina and sialon particles induced both median-radial crack systems at low impact velocity range and Hertzian cone cracks at high impact velocity range. The critical impact velocity for strength degradation,V _c, correlated with the hardness of the spheres and target ceramics, andV _c, for Hertzian cracks and median cracks were higher than that for chipping fracture.     

Interface reactions in the system sialon-Cu-Cu2O

The interface reactions between an /gb-sialon ceramic and Cu, Cu₂O or a Cu-Cu₂O mixture have been studied. A fully dense sialon ceramic material prepared by pressureless sintering at 1775 ° C with 6 wt% Y₂O₃ as sintering aid, were coldpressed together with Cu, Cu₂O or Cu-Cu₂O mixtures into cylindrical tablets. These samples were heat treated at 700, 850 and 1000 ° C in evacuated silica tubes. The reaction zones formed between the sialon and the powder compacts were studied in a SEM equipped with an EDS system. No reaction between copper and sialon ceramic could be detected in spite of prolonged heat treatment at 1000 ° C. Cu₂O reacted with the ceramic at 850 and 1000 ° C to form a glass containing copper and all the other sialon components. The interaction between the sialon material and the Cu-Cu₂O powder compacts was characterized by a redox reaction. The sialon was thus oxidized to SiO₂ and N₂ while Cu₂O was reduced to copper. A glass phase containing silicon, aluminium, yttrium and copper was also formed in the reaction.     

Joining of sialon ceramics by Sn-5 at % Ti based ternary active solders

The effect of a third element, such as silver, copper, indium, nickel or aluminium, on the joining of sialon ceramics with tin-5 at % titanium based ternary active solders was investigated. The content of the third element in the Sn-based solders was varied from 5–40% for Cu, from 5–10% for Ag and Al from 5–20% for In and from 1–5% for Ni. The joining was carried out in vacuum at 1100 K for 20 min. The four point bend testing of a butt joint of a ceramic/ceramic structure with dimensions 40 mm long, 3 mm wide and 4 mm high was used to study the bond strength between the ceramic and the Sn-based solders. The results show that the bond strength of the Sn-based solder with the sialon ceramic varied from 54–103 MPa. Small additions of Cu or Ag (about 5–10%), In (about 5–10%), or Ni (about 1–3%) to the solder is beneficial, but too much Ni (more than 5%) or In (more than 10%) is detrimental. On the other hand, Al in the active solder considerably decreased the bond strength of the solders with the ceramic. Suggestions are made for the selection of the third element in order to improve the bond strength of the soft solders with the ceramic. These include a high surface energy, improving the wetting of the solder on the ceramic and strengthening of the solder. This revised version was published online in November 2006 with corrections to the Cover Date.     

Sintering Behavior of Hot-Pressed Ca–αSialon Ceramics

On the basis of phase relationships in the Ca–Si–Al–O–N system, a Ca––sialon ceramic was synthesized using the hot-pressing technique. The reaction sequences and densifications of the Ca––sialon vs. firing temperatures have been characterized in detail. The present experiments reveal a reaction sequence as follows: at 1250°C the reactant mixture started to soften, at 1300°C a gehlenite phase was produced, at 1500°C the gehlenite phase was resolved into a liquid phase and a Ca––sialon started to form, and at 1600°C the formation of Ca––sialon was complete. The product was stable and almost entirely single phase Ca––sialon. Accompanying to the above sequences, densification also proceeded via a liquid-phase sintering, particle rearrangement, solution–reprecipitation, and grain growth process. In the final microstructure elongated grains of Ca––sialon were obtained, improving the fracture toughness of this Ca––sialon ceramic.     

Microstructural investigation of alpha-beta yttrium sialon materials

Three yttrium sialon materials have been manufactured under similar conditions, but with varying amounts of the constituent phases. Scanning and transmission microscopy, electron probe microanalysis and X-ray diffraction have been used to characterize the microstructure. Precipitation of the alpha-sialon phase both increases significantly the hardness at all temperatures investigated and decreases the thermal diffusivity. The sialon particles frequently contained a core of unreacted Si₃N₄ raw material. This was always found to be alpha-Si₃N₄ in alpha-sialon particles and beta-Si₃N₄ in beta-sialon particles; thus the unreacted Si₃N₄ raw materials act as nuclei for sialon particles.