Reaction-bonded Silicon Nitride Research Articles

Effect of oxygen on the strength of reaction-bonded silicon nitride

When obtaining reaction-bonded Si3N4, drying of the silicon powder or the charge at ≥400°C leads to an increased oxygen content in it and can cause deterioration of the mechanical properties of the ceramics after sintering.

The sintering kinetics of reaction bonded silicon nitride by using non-isothermal technique

The sintering kinetics of reaction bonded silicon nitride (RBSN) with 4.3% of Y 2O 3 and 1.4% of Al 2O 3 were investigated under non-isothermal conditions at various constant heating rates. The activation energy was found to be ≈ 670 kJ mol −1. The density of RBSN increased from 2.55 g cm −3 to 3.09 g cm −3 and also the room temperature strength increased from 224 to 424 MN m −2 with decreasing heating rate.

Synthesis of Reaction Bonded Silicon Nitride from High Purity Si Powders

ABSTRACTSignificantly improved reaction kinetics and properties have been realized for reaction bonded Si3N4 (RBSN) by using Si powders having unusual physical and chemical characteristics combined with clean, oxygen-free processing conditions. For instance, complete nitridation can be achieved in < 1 h at 1150°C or < 100 m at 1250°C without pre-nucleation, or < 1 h at 1100°C with pre-nucleation. These compare with normal schedules of ∼100 h at 1470-1500°C. Resulting strengths (up to 870 MPa) and oxidation resistances of the RBSN essentially equal those of fully dense Si3N4 even though RBSN includes 25% open porosity. The results show that every aspect of the process, ranging from powder synthesis to < ppm levels of oxygen in the nitriding atmosphere, have demonstrable effects on the reaction kinetics and the resulting properties of the parts. With careful processing, reaction formed ceramics can compete advantageously with fully dense counterparts.

Properties of reaction bonded silicon nitride obtained from slip cast preforms

Fabrication of silicon preforms of high green density (>1·2 g/cm3) by slip casting of silicon (in aqueous medium) has been studied. The nitridation product consists of 59–85% α-Si3N4, 7–22%β-Si3N4 and 7–23% Si2N2O phase. The amounts of un-nitrided silicon were negligible. The microstructure is either granular or consists of needle-like grains (α-Si3N4) and whiskers deposited in the large pores. MOR values of the specimens are almost constant up to 1000°C or 1400°C or show slight increase up to 1000°C or 1200°C. In some cases a little dip around 1200°C, then a sharp increase in MOR up to 1400°C was observed.K ic values are almost constant up to 1000°C, and thereafter increase sharply.Pore size distribution, existence of Si2N2O phase and oxidation of RBSN at high temperatures have been considered for the explanation of the observed behaviour.

Advanced Structural Ceramics: A Round Robin

The strength of 1500 sintered alumina and reaction‐bonded silicon nitride flexure specimens was measured by seven laboratories in four countries. Consistent results were obtained only with a well‐defined standard test method. Older procedures were less consistent or gave erroneous results. A simple statistical procedure allowed a quick estimate of consistency, and outlier samples could be appraised for the likelihood of their occurrence.

Thermal effects on the mechanical properties of SiC fibre reinforced reaction-bonded silicon nitride matrix composites

The elevated temperature four-point flexural strength and the room temperature tensile and flexural strength properties after thermal shock were measured for ceramic composites consisting of 30 vol pct uniaxially aligned 142 micron diameter SiC fibers in a reaction bonded Si3N4 matrix. The elevated temperature strengths were measured after 15 min of exposure in air at temperatures to 1400 C. Thermal shock treatment was accomplished by heating the composite in air for 15 min at temperatures to 1200 C and then quenching in water at 25 C. The results indicate no significant loss in strength properties either at temperature or after thermal shock when compared with the strength data for composites in the as-fabricated condition.

The measurement of sliding friction and wear of ceramics at high temperature

A wear testing system is described which can be used to carry out sliding wear tests at temperatures up to 1500 °C. The tests can be performed in air, vacuum or controlled atmospheres under applied loads in the range of 5–100 N and at speeds up to 1.5 m s−1. The relative displacement of the two specimens and the frictional load generated between the two surfaces are monitored throughout the test. Preliminary results are given of wear tests carried out at temperatures up to 1200 °C on reaction bonded silicon nitride, sintered silicon nitride, sintered silicon carbide and yttria partially stabilized zirconia.

Role of the Interfacial Thermal Barrier in the Effective Thermal Diffusivity/Conductivity of SiC‐Fiber‐Reinforced Reaction‐Bonded Silicon Nitride

Experimental thermal diffusivity data transverse to the fiber direction for composites composed of a reaction bonded silicon nitride matrix reinforced with uniaxially aligned carbon‐coated silicon carbide fibers indicate the existence of a significant thermal barrier at the matrix‐fiber interface. Calculations of the interfacial thermal conductances indicate that at 300°C and 1‐atm N2, more than 90% of the heat conduction across the interface occurs by gaseous conduction. The magnitude of the interfacial conductance is decreased significantly under vacuum or by removal of the carbon surface layer from the fibers by selective oxidation. Good agreement is obtained between thermal conductance values for the oxidized composite at 1 atm calculated from the thermal conductivity of the N2 gas and those inferred from the data for the effective composite thermal conductivity.

Influence of a CVD carbon coating on the mechanical property stability of Nicalon SiC fiber

A thin carbon coating was applied to Nicalon NLM 102 fibers with the objective of improving both the stability of fiber strength and the matrix interface characteristics for reaction-bonded silicon nitride (RBSN) composites. Fiber strength was decreased by the 1100°C CVD coating process, but the much more severe degradation that normally results from heat treatment at 1350°C in N 2 was mitigated somewhat in the coated fibers. The carbon film was not strongly bonded to the fiber, and thus was favorable for composite toughening by debonding and pull-out. Fiber degradation was greatly reduced in the presence of silicon. Both coated and uncoated fibers heat treated in close proximity to Si powder at 1350°C in N 2, simulating reaction-bonded silicon nitride composite processing, retained mean strengths of 1·6 GPa at 1 cm gage length.

High-Temperature Tensile Properties of Fiber Reinforced Reaction Bonded Silicon Nitride

A testing technique has been developed for measuring the tensile properties of ceramic matrix composite laminates at temperatures up to 1500°C in air. Using this technique, the tensile properties of unidirectional silicon carbide (SiC) fiber reinforced reaction bonded silicon nitride (SiC/RBSN) composite specimens were measured at 25, 1300, and 1500°C. The composite consists of nearly 30 vol % of aligned 142 μm, chemically vapor deposited SiC fibers (Textron SCS-6). The specimens used were straight section flat specimens with bonded end tabs. A specially designed gripping system was used to minimize bending moment and assured that failure always occurred in the gage section. The specimen extension was measured using a high-temperature capacitive extensometer.

Fatigue of silicon nitride ceramics under cyclic loading

Abstract Cyclic fatigue behavior of normally sintered and reaction-bonded silicon nitride materials was investigated from the microstructural point of view. It has been found that in any silicon nitride material the crack propagates by cyclic loading even under the condition of K max ISCC , and that surface upheavals (microscopic lifting of the surface) and different type of cracks are produced in the region near the cyclically propagating cracks. It is supposed that asperity-contacts of crack surfaces are closely related to the fatigue damage behavior.

Effect of grinding, lapping and various surface treatments on the strength of silicon nitride

Fully dense sintered reaction-bonded silicon nitride was manufactured as test bars in a variety of surface conditions (as-sintered, commercially diamond ground to a coarse and fine grit finish in both the longitudinal and transverse directions, and lapped), and lapped specimens were further treated by annealing at 1000°C in argon, annealing at 600°C in air, thermally cycling from 600°C to ambient in air and etching in hydrofluoric acid. The nature of the surface in each condition was characterised using scanning electron microscopy and profilometry measurements, and the flexural strength measured and analysed using the Weibull function. Machining considerably increased the strength of the fired material, but the effects of the various surface and thermal treatments were relatively small. Lapping increased the strength of the ground specimens by about 10%, and both annealing and etching reduced the strength of the lapped specimens.

Laminate Behavior for SiC Fiber-Reinforced Reaction-Bonded Silicon Nitride Matrix Composites

The room temperature mechanical properties of SiC fiber reinforced reaction-bonded silicon nitride matrix composite laminates (SiC/RBSN) have been measured. The laminates contained ∼30 vol% fraction of aligned 142-μm diameter SiC fiber in a porous RBSN matrix. Three types of laminates studied were unidirectional laminates ([0]8, [10]8, [45]8, and [90]8), cross piled laminates [02/902]s, and angle plied laminates [+452/−452]s. Each laminate contained eight fiber plies. Results for the unidirectionally reinforced composites tested at various angles to the reinforcement direction indicate large anisotropy in in-plane elastic moduli and strength. In addition, strength properties of these composites along the fiber direction were independent of specimen gage length and were unaffected by notches normal to the fiber direction. Matrix crack deflection along the fiber at the notch tip appears to be the dominant crack blunting mechanism responsible for notch insensitive behavior of these composites. Inplane properties of the composites can be improved by 2-D laminate construction. Mechanical property results for [02/902]s and [+452/−452]s laminates showed that their matrix failure strains were similar to that for [0]s laminates, but their primary elastic moduli, matrix cracking strengths, and ultimate composite strengths were lower. The elastic properties of unidirectional, cross-ply, and angle ply composites can be predicted from modified constitutive equations and laminate theory. Further improvements in laminate properties may be achieved by reducing the matrix porosity and by optimizing the bond strength between the SiC fiber and RBSN matrix.

High-temperature oxidation-protection CVD coatings for structural ceramics: Oxidation behaviour of CVD-coated reaction-bonded silicon nitride

Chemical vapour deposition of Si 3N 4 by low-pressure (4–10 Torr) pyrolysis (1200–1275 °C) of tetramethssilane-ammonia-hydrogen mixtures ( N: Si = 10) was used to protect reaction-bonded silicon nitride substrates against internal oxidation at 1000–1350 °C. The results reveal the existence of a critical thickness (6 μm) of the coatings and the poorer protectiveness of amorphous deposited coatings compared with crystallized coatings. Under the optimum deposition conditions, better protection was achieved than by the use of more conventional surface modification treatments such as high-temperature flash pre-oxidation.

Influence of Crystalline Secondary Phases on the Densification Behavior of Reaction‐Bonded Silicon Nitride During Postsintering under Increased Nitrogen Pressure

The influence of reaction‐bonded silicon nitride (RBSN) characteristics, especially the crystalline secondary phases formed during the nitridation process, on the densification behavior of RBSN and the resulting microstructure of the sintered material during postsintering under increased nitrogen pressure is discussed. The type and amount of the secondary phases are controlled by the chemical characteristics of the Si starting powder and the additive composition. Three different commercially available Si powders which reveal particular differences in the Fe and Al content were investigated, and a correlation between the microstructural characteristics of RBSN and of the postsintered materials is presented. It is shown that besides micropore size distribution the crystalline secondary phases play an important part in the densification behavior during postsintering and the resulting microstructure of postsintered reaction‐bonded silicon nitride.

Influence of some parameters on the strength and fracture toughness of reaction-bonded silicon nitride composites

Strength, Young's modulus and fracture toughness data in the temperature range 30–1400°C are presented for reaction-bonded silicon nitride and its particulate composites with SiC (5–20 vol.%), BN (10 vol.%) and TiC (5 vol.%). The composites of Si 3N 4 with SiC showed the best mechanical properties of the three types of composites. The toughening mechanism and interdependence of various mechanical properties for these composites are discussed.

Interfacial Bonding and Friction in Silicon Carbide [Filament]‐Reinforced Ceramic‐ and Glass‐Matrix Composites

Interfacial shear strength and interfacial sliding friction stress were assessed in unidirectional SiC‐filament‐reinforced reaction‐bonded silicon nitride (RBSN) and borosilicate glass composites and 0/90 cross‐ply reinforced borosilicate glass composite using a fiber pushout test technique. The interface debonding load and the maximum sliding friction load were measured for varying lengths of the embedded fibers by continuously monitoring the load during debonding and pushout of single fibers in finite‐thickness specimens. The dependences of the debonding load and the maximum sliding friction load on the initial embedded lengths of the fibers were in agreement with nonlinear shear‐lag models. An iterative regression procedure was used to evaluate the interfacial properties, shear debond strength (Td), and sliding friction stress (Tf), from the embedded fiber length dependences of the debonding load and the maximum frictional sliding load, respectively. The shear‐lag model and the analysis of sliding friction permit explicity evaluation of a coefficient of sliding friction (μ) and a residual compressive stress on the interface (σ0). The cross‐ply composite showed a significantly higher coefficient of interfacial friction as compared to the unidirectional composites.

Oxidation and Fracture Strength of High‐Purity Reaction‐Bonded Silicon Nitride

Reaction‐bonded Si3N4 (RBSN) made from high‐purity Si powder is unusually resistant to degradation caused by exposures to air for up to 50 h at temperatures up to 1400°C. The weight gain during oxidation of this SiH4‐originating RBSN is approximately 10 times less than conventional RBSN. Contrary to normally observed strength degradations, room‐temperature strengths of this high‐purity, oxidized RBSN (avg = 435 MPa, max. = 668 MPa) remained at their unusually high, as‐processed levels after 1000° and 1400°C oxidizing exposures. Fracture toughness values were unaffected by oxidation (KIC= 2.3 to 2.4 MPa · m1/2). This superior oxidation resistance results from the high purity and the small diameter pore channels (0.01 to 0.06 μm) achieved in this SiH4‐originating RBSN.

Slip-cast hot isostatically pressed silicon nitride gas turbine components

The aim of the work presented is the production of gas turbine components from hot isostatically pressed reaction-bonded silicon nitride (RBSN) by aqueous slip casting of silicon metal powder followed by nitridation and post-densification of the components by hot isostatic pressing. For subsequent densification of the RBSN components the slip also must contain powders of the sintering aids yttria and alumina. In a first step the phase system silicon-water was investigated: the dependence of the rate of hydrolysis, the viscosity and the surface charge of the suspended powder particles on the pH were determined, and the change of pH by adding silicon powder to an alkaline solution was examined. Comparable investigations were done with the phase system silicon-water-sintering aids. Various deflocculants with different pH values were tested. The pH range within which castable and chemically stable slips can be prepared was found to be between 7.5 and 9.0. Therefore, alkaline deflocculation agents based on polymer polyanions and silicates were used for the preparation of castable slips. Test specimens and parts for the gas turbine were cast into plaster moulds, nitrided and hot isostatically pressed. The different processing steps will be described briefly and the properties of the components will be discussed.

Upgrading technical ceramics

Technical ceramics, in particular engineering ceramics, could benefit substantially from the elimination of surface defects. This will result in a marked enhancement of their strength and corrosion resistance. This effect can be achieved by coating these ceramics by chemical vapour deposition (CVD). By coating a ceramic material using essentially the same material applied by a suitable CVD process surface porosity and surface cracks can be virtually eliminated. In many of today's technical ceramics densification is achieved by liquid-phase sintering. The resulting grain boundary phases are, in many cases, an important factor in the corrosion damage of ceramic materials. CVD coating can effectively shield off those grain boundary phases from the corrosive medium. This markedly enhances the corrosion resistance. At the Centre for Technical Ceramics at Eindhoven University of Technology a research program was recently initiated in order to determine systematically the potential of this approach. In this presentation a literature survey is given of results obtained by other workers together with some preliminary results of our own. The latter concern the upgrading of reaction bonded silicon nitride and sintered AIN. A number of potential applications of this approach are indicated. The limitations of this upgrading of technical ceramics by CVD are also discussed.