Microstructure and mechanical properties of RB-SiC/MoSi2 composite

Microstructure, high temperature strength and oxidation behaviour of reaction bonded silicon carbide, RB-SiC/17 wt% MoSi₂ composite prepared by infiltrating a porous RB-SiC bulk (after removal of free silicon) with molten MoSi₂ were investigated. There was good bonding between the SiC and MoSi₂ particle, without a significant reaction zone and microcracking caused by the thermal mismatch stresses. A thin (2 nm) layer, however, was observed at the SiC/MoSi₂ interfaces. At room temperature, the composite exhibited a bending strength of 410 MPa, which is 20% loss in comparison to that of RB-SiC alone (containing 10 wt% free silicon). However, the composite strength increased to a maximum of 590 MPa in the temperature range 1100 and 1200° C and dropped to 460 MPa between 1200 to 1400° C, after which the strength remained constant. The passive oxidation of the composite in dry air in the temperature range 1300 to 1400° C was found to follow the parabolic rate law with the formation of a protective layer of cristobalite on the surface.     

A new polymeric route to silicon carbide and silicon nitride using elementary silicon as starting material

Elementary silicon activated with copper was heated up in tetraethylene pentamine to 300°C. After drying in vacuum, an amorphous solid was obtained. Calcining this solid under argon at temperatures of up to 1200°C led to amorphous products, while at higher temperatures silicon carbide was obtained. Calcining in ammonia at temperatures of up to 1000°C also led to amorphous products. At calcination temperatures of 1200 and 1400°C crystalline silicon nitride and silicon nitride fibres respectively were obtained. The dependence of the fibre growth on the ammonia flow rate, as well as the occurrence of spherical iron-enriched particles terminating these fibres gave evidence for a vapour-liquid-solid mechanism being responsible for the fibre growth.     

Joining of silicon nitride ceramics by hot pressing

The joining of hot-pressed silicon nitride ceramics, containing Al₂O₃ and Y₂O₃ as sintering aids, has been carried out in a nitrogen atmosphere. Uniaxial pressure was applied at high temperature during the joining process. Polyethylene was used as a joining agent. Joining strength was measured by four-point bending tests. The effects of joining conditions such as temperature (from 1400 to 1600°C), joining pressure (from 0.1 to 40 MPa), holding time (from 0.5 to 8 h) and surface roughness (R _max) of the joining couple (about 0.12, 0.22 and 1.2m) on the joining strength were examined. The joining strength was increased with increases in joining temperature, joining pressure and holding time. Larger surface roughness caused lower joining strength. The higher joining strength was attributed to a larger true contact area. The area was increased through plastic deformation of the joined couple at elevated temperatures. The highest joining strength attained was 567 MPa at room temperature, which was about half the value of the average flexural strength of the original body. The high temperature strength measured at 1200° C did not differ very much from the room-temperature value.     

Phenomenology of fracture in hot-pressed silicon nitride

Fracture phenomenology in hot-pressed silicon nitride has been studied fractographically as a function of flaw size, temperature and loading rate. Surface cracks of controlled size were introduced using the microhardness indentation technique. At room temperature, the fracture stress was found to depend on initial crack size according to the Griffith relationship and extrapolation of the data indicated that inherent processing flaws of the order of 12 to 24 m are strength-controlling in virgin material. Using a simplified Griffith approach, the fracture surface energy, , at 20° C for hot-pressed Si₃ N₄ is about 22 000 erg cm^–2. Two mechanistic regimes were manifest in the temperature dependence of the fracture stress. A mixed mode of fracture consisting of transcrystalline and intergranular crack propagation occurred up to 1100° C; at 1200° C and above, subcritical crack growth (SCG) occurred intergranularly and the extent of SCG increased with increasing temperature. Similarly, the extent of SCG decreased with increasing loading rate.     

Oxidation behaviour of a pressureless sintered AlN-SiC composite

The present study aims to investigate the oxidation behaviour of an AlN-SiC composite, pressureless sintered with the addition of Y₂O₃. Two main aspects are considered: (1) the evaluation of the oxidation kinetics in the temperature range 1300–1450°C for short term tests (30 h) and (2) the degradation of the flexural strength after oxidation at temperatures from 1000 to 1400°C for 100 h, in relationship with the microstructure of the exposed surfaces.The material starts to oxidize notably at temperatures higher than 1300°C. The oxidation kinetics is parabolic in the temperature range 1350–1450°C, the oxidation products are dependent on temperature and exposure time and are mainly constituted by crystalline mullite and alumina.The surface modification induced by long term oxidation does not affect mechanical strength until 1200°C, while after oxidation at 1400°C, the residual strength is about 25% of the starting one. These results are discussed in terms of the microstructure modifications induced by oxidation.     

The structure of slow crack interfaces in silicon nitride

Fracture interfaces formed in silicon nitride at high temperatures were studied using light and electron microscopy. The structure of the fracture interface depended on the type of silicon nitride fractured. High-purity, reaction-bonded silicon nitride always formed flat, relatively featureless, fracture surfaces. Fracture occurred by a brittle mode even at the highest temperature (1500° C) studied. The critical stress intensity factor for reaction-bonded silicon nitride ( 2.2 MN m^–3/2) is relatively low and is insensitive to temperature. By contrast, hot-pressed silicon nitride gave evidence of plastic flow during fracture at elevated temperatures. Crack growth in magnesia-doped, hot-pressed silicon nitride occurs by creep, caused by grain boundary sliding and grain separation in the vicinity of the crack tip. As a consequence of this behaviour, extensive crack branching was observed along the fracture path. The primary and secondary cracks followed intergranular paths; sometimes dislocation networks, generated by momentary crack arrest, were found in grains bordering the crack interface. As a result of the high temperature, cracks were usually filled with both amorphous and crystalline oxides that formed during the fracture studies. Electron microscopy studies of the compressive surfaces of fourpoint bend specimens gave evidence of grain deformation at high temperatures by diffusion and dislocation motion.     

Characterization and properties of controlled nucleation thermochemical deposition (CNTD)-silicon carbide

The present investigation was undertaken to characterize the microstructure of controlled nucleation thermochemical deposition (CNTD)-SiC material and to evaluate the room-temperature and high-temperature bend strength and oxidation resistance. Utilizing the CNTD process, ultrafine grained (0.01 to 0.1m) SiC was deposited on W wires (0.5 mm diameter by 20cm long) as substrates. The deposited SiC rods had superior surface smoothness and were without any macrocolumnar growth commonly found in conventional CVD material. At both room and high temperature (1200 to 1380° C), the CNTD-SiC exhibited a bend strength of ~ 200 000 psi (1380 MPa), several times higher than that of hot-pressed, sintered, or CVD SiC. The excellent retention of strength at high temperature was attributed to the high purity and fine grain size of the SiC deposit and the apparent absence of grain growth at elevated temperatures. The rates of weight change for CNTD-SiC during oxidation were lower than for NC-203 (hot-pressed SiC), higher than for GE's CVD-SiC and CVD-Si₃N₄ but considerably below those for HS-130 (hot-pressed Si₃N₄). The high-purity fully dense and stable grain size CNTD-SiC material shows potential for high-temperature structural applications, however, problem areas might include scaling the process to make larger parts, deposition on removable substrates, and the possible residual tensile stress.     

Oxidation resistance and creep behaviour of a silicon nitride ceramic densified with Y2O3

The corrosion resistance and creep behaviour in air of hot-pressed materials with a composition of 70 Si₃N₄-25 SiO₂-5 Y₂O₃ (mol%) has been studied. Kinetics data and microstructural changes in the 1180 to 1650° C temperature range indicate the presence of two oxidation mechanisms. Between 1180 and 1420° C, the preferential oxidation of the intergranular phase containing nitrogen is interpreted in terms of an inward diffusion of ionic oxygen. At temperatures higher than 1420° C, the degradation of the material is the sum of many processes (solution of silicon nitride, migration of oxygen and yttrium and release of nitrogren) but the diffusion to the nitride-oxide interface of a complex combination of yttrium and nitrogen in the boundary phase seems to be the limiting step. The three-point bending creep is discussed in relation to the evolution of the secondary intergranular phases in an oxidizing environment. The creep deformation is the sum of a viscoelastic component and a diffusional component characterized by the same activation energy (720 kJ mol^–1).     

The direct bonding between copper and MgO-doped Si3N4

An application of direct bonding method for copper to silicon nitride (Si₃N₄) joining was investigated. Si₃N₄ was sintered with 5wt% MgO at 1700 ° C for 30 min in nitrogen atmosphere, and oxidized at various temperatures. The bonding was performed at 1075 ° C in nitrogen atmosphere with low oxygen partial pressure. The direct bonding was not achieved for the Si₃N₄ oxidized below 1200 ° C or nonoxidized. During oxidation, magnesium ion added as sintering aids, diffused out to the surface of Si₃N₄ and formed MgSiO₃, which seemed to have an important role in the bonding. Fracture of the bonded specimen under tensile stress took place within the oxide layer of Si₃N₄. The bonding strength was decreased with oxidation temperature and time. Maximum strength was found to be 106 kg cm^–2 for the Si₃N₄ oxidized at 1200 ° C for 1 h.     

Strengthening and proof testing of siliconised SiC

Experimental tests were conducted to show the degree of strengthening that can be produced in siliconized SiC by a 10 h or less, 1200° C, prestress that also serves as a proof test. A 1200° C, 10 h prestress at 86% of the room-temperature strength results in more than a 25% increase in the room-temperature strength. A similar strengthening effect occurs for bonded siliconized SiC butt joints. The Weibull distribution can be used to treat the strength populations including those that are truncated by the prestress/proof test or additional proof testing.     

Mixed-mode high temperature toughness of silicon nitride

Both opening-mode and mixed-mode fracture toughness tests were carried out at 1200 and 1300 °C on a sinter/HIP grade of silicon nitride. Data for pure opening loading (K _Ic) agree well with other experiments on the same material, which showed that the toughness was lower at 1000 °C than at room temperature, but increased as temperature increased above 1000 °C. The ratio of K _IIc/K _Ic was sufficiently insensitive to temperature that it can be considered to be constant. Results are discussed in the context of mechanisms that have been proposed to explain fracture toughness in silicon nitride.     

Effect of microstructure on the erosion and impact damage of sintered silicon nitride

The erosion rates and impact damage of two sintered silicon nitride materials with identical compositions but different microstructures were determined as a function of impacting particle (SiC) kinetic energy and temperature (25–1000° C) using a slinger-type erosion apparatus. The coarse-grained silicon nitride had significantly better resistance to impact damage than the fine-grained material. Crack-microstructure interactions were characterized using scanning electron microscopy and showed that crack-bridging was an important toughening mechanism in the coarse-grained material. Post-impact strength data were significantly less than those predicted from the indentation-strength data, due to impact flaws linking up prior to fracture. Consistent with its greater fracture resistance, the erosion rate of the coarse-grained material was less than that of the fine-grained material for erosion at 25 deg, and was independent of erosion temperature.     

Fracture of hot-pressed alumina and SiC-whisker-reinforced alumina composite

The flexural strength of hot-pressed alumina and SiC-whisker-reinforced alumina composite were evaluated as a function of temperature (20 to 1400° C in air environment), applied stress and time. Two mechanistic regimes were manifest in the temperature dependence of the fracture stress. A temperature-independent region of fast fracture (catastrophic crack extension) existed up to 800° C, in which the failure mode was a mixture of transgranular and intergranular crack propagation. In this region, the alumina composite showed significantly higher fracture strength and toughness compared to polycrystalline alumina. Above 800° C, both materials (alumina and alumina composite) displayed a decreasing fracture strength due to the presence of subcritical or slow crack growth which occurred intergranularly. Flexural stress rupture evaluation in the temperature range 600 to 1200° C has identified the stress levels for time-dependent and time-independent failures.     

High-temperature electrical conductivity of aluminium nitride

The electrical conductivity of hot-pressed polycrystalline aluminium nitride doped with oxygen and beryllium was measured as a function of temperature from 800 to 1200° C and over a range of nitrogen partial pressure from 10² to 10⁵ Pa. The effect of beryllium dopant, the independence of conductivity from nitrogen partial pressure, and the observed activation energy suggested extrinsic electronic species or aluminium vacancies as charge carriers. Polarization measurements made with one electrode blocking to ionic species indicated that the aluminium nitride with oxygen impurity was an extrinsic electronic conductor.     

Evaluation of strength,toughness and subcritical crack extension of a commercial HP-Si3N4 exposed to a sulphidizing environment

Structural ceramic materials, in particular silicon nitride, have attracted interest for coal gasification technology at temperatures in excess of 1200 °C. In this study, a commercially available hot pressed silicon nitride is exposed for 200 h in a 0.4% H₂S + 0.75% H₂O + H₂ (balance) atmosphere, at 1200 and 1300 °C, simulating coal gasification environments. The crack growth behaviour of the as-received material is compared to that of the exposed condition. Measurements are carried out on specimens with small natural flaws (dynamic strength tests) and on specimens containing long cracks (chevron notch), both tested in four point bending. Results of strength tests (Modulus of rupture (MOR) and Weibull modulus) as well as toughness in both conditions are also presented. In view of the significant changes in the high temperature mechanical properties caused by crystallization of the intergranular glassy phase at triple points, further investigations of the effect of controlled devitrification seem surely worthwhile.     

Tensile creep of whisker-reinforced silicon nitride

This paper presents a study of the creep and creep rupture behaviour of hot-pressed silicon nitride reinforced with 30 vol% SiC whiskers. The material was tested in both tension and compression at temperatures ranging from 1100 to 1250°C for periods as long as 1000 h. A comparison was made between the creep behaviour of whisker-reinforced and whisker-free silicon nitride. Principal findings were: (i) transient creep due to devitrification of the intergranular phase dominates high-temperature creep behaviour; (ii) at high temperatures and stresses, cavitation at the whisker-silicon nitride interface enhances the creep rate and reduces the lifetime of the silicon nitride composite; (iii) resistance to creep deformation is greater in compression than in tension; (iv) the time to rupture is a power function of the creep rate, so that the temperature and stress dependence of the failure time is determined solely by the temperature and stress dependence of the creep rate; (v) as a consequence of differences in grain morphology and glass composition between whisker-free and whisker-reinforced material, little effect of whisker additions on the creep rate was observed.     

Fracture of flash oxidized,yttria-doped sintered reaction-bonded silicon nitride

The oxidation behaviour of a slip cast, yttria-doped, sintered reaction-bonded silicon nitride after flash oxidation was investigated. It was found that both the static oxidation resistance and flexural stress rupture life (creep deformation) were improved at 1000° C in air compared to those of the same material without flash oxidation. Stress rupture data at high temperatures (1000 to 1200° C) are presented to indicate applied stress levels for oxidation-dependent and independent failures.     

As-cast microstructures and behavior at high temperature of chromium-rich cobalt-based alloys containing hafnium carbides

Hafnium is often used to improve the high temperature oxidation resistance of superalloys but not to form carbides for strengthen them against creep. In this work hafnium was added in cobalt-based alloys for verifying that HfC can be obtained in cobalt-based alloys and for characterizing their behavior at a very temperature. Three Co–25Cr–0.25 and 0.50C alloys containing 3.7 and 7.4 Hf to promote HfC carbides, and four Co–25Cr– 0 to 1C alloys for comparison (all contents in wt.%), were cast and exposed at 1200 °C for 50 h in synthetic air. The HfC carbides formed instead chromium carbides during solidification, in eutectic with matrix and as dispersed compact particles. During the stage at 1200 °C the HfC carbides did not significantly evolve, even near the oxidation front despite oxidation early become very fast and generalized. At the same time the chromium carbides present in the Co–Cr–C alloys totally disappeared in the same conditions. Such HfC-alloys potentially bring efficient and sustainable mechanical strengthening at high temperature, but their hot oxidation resistance must be significantly improved.     

Employing reactive synthesis for metal to ceramic joining for high temperature applications

Joining of dissimilar materials allows the properties of both materials to be exploited in a device or structure. The main reasons for the incorporation of dissimilar materials are to achieve function, improve efficiency and to reduce cost.Silicon nitride is an engineering ceramic that has outstanding properties but has yet to find its full commercial potential. Silicon nitride is suitable for high temperature applications, however, its incorporation into devices or structures tends to be restricted due to a lack of suitable joining techniques.This paper presents the results of joining between the high temperature and corrosion resistant iron-chromium-aluminium alloy (Fecralloy) with silicon nitride by a nickel aluminide (NiAl) interlayer. The formation of NiAl from its constituent elements (Ni-Al compact was used) by reactive synthesis is highly exothermic and this was utilised to cause partial melting of the Fecralloy interface and reactive wetting of the silicon nitride interface.Joints with average shear strength of 94.3 MPa were fabricated under optimum processing conditions (900°C, 15 min, 45 MPa). Thermal cycling at 850°C in air showed that the joints could be used at this temperature.The primary focus of this work was on the effects of process conditions upon the microstructure and mechanical properties of the joint. The reactive synthesis of NiAl was studied using differential thermal analysis (DTA), where the effects of varied heating rate were investigated.     

Strength degradation of Si3N4 exposed to simulated gas turbine environments

Silicon nitride, sintered with the aid of alumina and yttria, was exposed at 1000°C to two different simulated gas turbine environments. The composition of the reaction gas was varied by delivering either a high or low sulphur fuel into a burner rig. The characteristics of the corrosion product varied markedly with the sulphur content of the fuel. The extent of silicon nitride degradation was examined by two techniques: weight change and 4-point flexural strength. Strength measurements were conducted both at room temperature and at 1000°C. Whereas the weight gain information revealed that corrosion was enhanced in the low sulphur combustion gas, the strength of the corroded silicon nitride did not vary significantly from that of the as-received material. Scanning electron microscopy of the fracture surfaces was utilised to identify the fracture origins in the as-received and corroded samples. Strength, even after corrosion, was controlled primarily by defects introduced during manufacture.