Reaction-bonding Process Research Articles

The Effect of Bond Phase Additive and Sintering Temperature on the Properties of Mullite Bonded Porous SiC Ceramics

Currently, porous SiC ceramics have been a focus of interesting research in the field of porous materials due to their excellent structural properties, high strength, high hardness, and superb mechanical and chemical stabilities even at high temperatures and hostile atmospheres. Porous SiC ceramics have been considered as suitable candidate materials for catalyst supports [1-2], hot gas or molten metal filters [3], high temperature membrane reactors [4], thermal insulating materials [5], gas sensors [6] etc. Porous SiC ceramics are fabricated by various methods including partial sintering [7], carbothermal reduction [8-9], replication or pyrolysis of polymeric sponge [10-12], reaction bonding [13] etc. In all these methods SiC needs to be sintered which requires a very high temperature due to the strong covalent nature of the Si-C bond, selective sintering additives, expensive atmosphere, costly and delicate instrumentation. Processing of porous SiC ceramics at low temperature using a simple technique thus becomes necessary. Bonding of SiC can be done at low temperatures by use of different oxide and non-oxide secondary phases. They include silica, mullite, cordierite, silicon nitride, etc. Various sintering additives are used for the formation of variety of secondary oxide bond phases for formations for porous SiC [14-19] Choice of mullite as a bond for SiC has many advantages. Mullite possesses a high melting point (Tm= 1850°C) and a low oxygen diffusion coefficient (5.6 x 10-14 m2/sec at 50°C). It has a matching thermal expansion coefficient with SiC (CTEmullite= 5.3 ×10-6/K; CTESiC = 4.7 ×10-6/K at RT-1000 °C) and a high strength that can be retained up to a very high temperature. Different sources of aluminum, such as Al2O3, Al, AlN, and Al (OH)3 powders were used for the formation of mullite bonded porous SiC ceramics (MBSC) [20-21]. However, the mullitization temperature of 1550o C is still necessary. In this work, mullite bonded porous SiC ceramics were fabricated by an in situ reaction-bonding process; the mixture of clay and CaCO3 were chosen as sintering additives to lower the mullitization reaction between Al2O3 and oxidation-derived SiO2. The effect amount of alumina, sintering temperature and other sintering aids on material property such as porosity/pore size distribution mechanical and micro structural properties of porous oxide bonded SiC ceramics were studied.

Performance optimization of complicated structural SiC/Si composite ceramics prepared by selective laser sintering

Combining reaction-bonded (RB) process and selective laser sintering (SLS) method is an effective approach to prepare ceramic components with complex shapes. The purpose of this paper is to find efficient ways to improve the performance of SiC/Si composites prepared by SLS/RB technologies. Effects of epoxy resin binder on the performance and microstructure of preforms and sintered bodies were studied first. Then, based on the above results, graphite with low reactivity was used as an alternative slow-release carbon source to promote sintering densification process and improve the carbon density of preforms. When the added amount of graphite increased, clusters of nanometer-sized SiC grains formed and the silicon phase transformed into pieces, which reduced the content and dimension of silicon phase. Furthermore, by applying a two-step sintering method, the residual silicon content of SiC/Si composites decreased further and the flexural strength at high temperatures increased.

Microstructural development and factors affecting the performance of a reaction-bonded silicon carbide composite

Abstract Due to its near net-shape forming characteristic, reaction-bonded silicon carbide (RBSC) is an ideal sintering material to prepare complicated structural SiC-based ceramics when combined with selective laser sintering (SLS). With this understanding, various factors affecting RBSC composites and the performance control methods were analyzed in this work. By studying the microstructural development and reaction mechanism during the reaction-bonded process, the main factor affecting the performance was found to be the carbon density of SiC/C preform. The residual silicon content decreased with the increase in the carbon density of preform, thereby improving the performance. When the carbon density of preform exceeded the maximum value, some unreacted carbon existed in the sintered body, resulting in the degradation of performance. In order to break the limit of maximum carbon density of the preform, two effective approaches, namely the slow-release carbon source and new-style structure capillary channel were proposed in this work.

Fabrication of β-SiAlONs by a Reaction-Bonding Process Followed by Post-Sintering

A cost-effective route to synthesize β-SiAlONs from Si mixtures by reaction bonding followed by post-sintering was investigated. Three different z values, 0.45, 0.92 and 1.87, in Si 6-z Al z O z N 8-z without excess liquid phase were selected to elucidate the mechanism of SiAlON formation and densification. For RBSN (reaction-bonded silicon nitride) specimens prior to post-sintering, nitridation rates of more than 90% were achieved by multistep heating to 1400℃ in flowing 5%H₂/95%N₂; residual Si was not detected by XRD analysis. An increase in density was acquired with increasing z values in post-sintered specimens, and this tendency was explained by the presence of higher amounts of transient liquid phase at larger z values. Measured z values from the synthesized β-SiAlONs were similar to the values calculated for the starting compositions. Slight deviations in z values between measurements and calculations were rationalized by a reasonable application of the characteristics of the nitriding and post-sintering processes.

Preparation of HA/Al2O3 Functionally Gradient Coating on Titanium Substrate Using a Combination of Electrophoretic Deposition and Reaction Bonding Process

The major difficulty in fabricating hydroxyapatite (HA) coatings on titanium substrate using the electrophoretic deposition (EPD) process is the full densification and crack prevention of the coating under the restricted sintering temperature. First, HA/aluminum oxide composite coatings were fabricated on titanium substrate using the combination of electrophoretic deposition and reaction bonding process. Results show that the reaction bonding process enhances the coating's densification degree and significantly increase its bonding strength. Then, HA/Al2O3 functionally gradient coating (FGC) were also fabricated on titanium. Results show that, the preparation of FGC prevents cracking of the coating with a thickness about 35um. Chemical composition and microstructure of the HA/Al2O3 FGC exhibit obvious gradient variation. The HA/Al2O3 FGC also exhibits much higher bonding strength than that of HA coating with the same thickness.

Processing of micro-components made of sintered reaction-bonded silicon nitride (SRBSN). Part 1: Factors influencing the reaction-bonding process

In order to establish a process for the manufacturing of injection moulded micro-components of sintered reaction-bonded silicon nitride (SRBSN) several process parameters were investigated with regard to their influence on the reaction-bonding step. One question to be answered was how the sintering aids affect the nitridation behaviour of a silicon green body. For the processing of micro-components it was of special interest to study, how a decreasing sample size and wall thickness would influence the rate of Si 3N 4 formation. By varying the added amounts of the sintering aids, it was found that increasing the Y 2O 3 and MgO contents both improved the nitridation rate, whereas an increase of Al 2O 3 content resulted in reduced nitridation rates. Within the investigated range of sample dimensions (0.2–4.0 g) the unexpected observation was made, that with decreasing sample weight the nitridation rate also decreased. This was explained by the exothermic nature of the reaction between Si and N 2 and the fact that small samples with a large surface-to-volume ratio attain thermal equilibrium with their environment better than large samples which may be subject to local overheating.

Fabrication of pressureless sintered dense β-SiAlON via a reaction-bonding route with ZrO 2 addition

A reaction-bonding and post-sintering process was applied to fabricate pressureless sintered β-Si 6− Z Al Z O Z N 8− Z ( Z = 1–3) ceramics with monoclinic ZrO 2 added to the starting powder. Samples with ZrO 2 showed enhanced nitridation and achieved near theoretical density following post-sintering, although this could not be achieved in the samples produced without ZrO 2. Thus the ZrO 2 is effective in both enhancing the nitridation of Si and as a sintering additive for densification of β-SiAlON. Part of the added monoclinic ZrO 2 was transformed to tetragonal ZrO 2, and the ZrN phase was also formed due to reaction with nitrogen during the reaction-bonding process. After the post-sintering process, the ZrN phase remained only in the Z = 3 composition. In the other compositions the ZrN reacted with SiO 2 to form both tetragonal and monoclinic ZrO 2 phases. These differences are explained in terms of the increasing densification and grain growth for Al 2O 3 rich compositions and the ZrN being trapped inside such grains in the case of the Z = 3 sample.

Preparation of HAp/Al2O3 Functionally Gradient Coating on Titanium by a Combination of Electrophoretic Deposition and Reaction Bonding Process

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Effect of the amount of excess oxides on the densification of α-SiAlON fabricated via a reaction-bonding process

Y-α-SiAlONs with elongated grains were fabricated via a reaction-bonding process using starting compositions containing excess oxides in the form of Y2O3 and SiO2. The density of post-sintered specimens reached a maximum value with compositions containing 2 mass% excess oxides, although conventionally sintered materials of this composition were not fully dense. However for compositions containing smaller amounts of excess oxides, the density of the specimen fabricated via a reaction-bonding process was lower than that via a conventional process. In these samples it is thought that liquid phase sintering was difficult to be achieve during the post-sintering process, since the amount of SiO2 contained in the starting powder was lower and the Al2O3 in the starting composition was consumed for the production of β-SiAlON during nitridation. There was also a decrease in density of the reaction-bonded materials with further increases in the amounts of additional oxides. For these samples the compacts could not be densified uniformly, because of non-uniformity of the phase composition in the nitrided compacts. The dense reaction bonded α-SiAlON with elongated grains, fabricated from compositions with 2 mass% excess oxides exhibited both high fracture toughness and high hardness.

Post-Hot-Pressing and High-Temperature Bending Strength of Reaction-Bonded Silicon Nitride-Molybdenum Disilicide and Silicon Nitride-Tungsten Silicide Composites

A hot-pressing technique was used for the further densification of reaction-bonded silicon nitride-molybdenum disilicide and silicon nitride-tungsten silicide (Si3N4-MoSi2 and Si3N4-WSi2, respectively) compacts that were prepared via a presintering step and a nitriding process from silicon-molybdenum or silicon-tungsten powders. After hot pressing was performed at 1650°C (25 MPa for 1 h), most of the alpha-Si3N4 that formed during the reaction-bonding process was transformed to β-Si3N4 and, moreover, a very small amount of Mo5Si3 (W5Si3) was formed in addition to MoSi2 (WSi2). Three- and four-point bend tests were performed at room temperature (25°C), 1000°C, 1200°C, and 1400°C. The bend strength of the Si3N4-WSi2 composite increased slightly from room temperature up to 1000°C, whereas the Si3N4-MoSi2 composite showed a more-pronounced increase up to 1200°C. Microstructural analysis was performed on the fracture surfaces of both composites that were tested at different temperatures.

Fabrication of Elongated α-SiAlON via a Reaction-Bonding Process

A reaction-bonding process, which offers low sintering shrinkage and is a low-cost process, was applied to fabricate Y–α-SiAlON ceramics. The green compacts composed of Si, Y2O3, Al2O3, and AlN were nitrided and subsequently postsintered. Dense single-phase Y–α-SiAlON with elongated grain morphology could be achieved in the specimen postsintered at 1900°C. The material exhibited high hardness (1850 HV10) and high fracture toughness (5.1 MPa·m1/2).

Fabrication and mechanical property of high crystalline SiC fiber reinforced SiC matrix composite by reaction-bonding process

Fabrication and mechanical property of high crystalline SiC fiber reinforced SiC matrix composite by reaction-bonding process

Shrinkage-free ZrSiO 4-ceramics: Characterisation and Applications

Dense, shrinkage-free ZrSiO 4-ceramics can be produced by a reaction-bonding process using ZrSi 2, ZrO 2, and a polysiloxane as starting materials. Sinter shrinkage is compensated by the volume increase during oxidation of ZrSi 2. In addition, the use of a Si-containing so called low-loss-binder [polymethylsilsesquioxane (PMSS)] reduces shrinkage further. Near net-shape ceramic compacts can be produced by an embossing process. As the green bodies are extremely dense but not brittle, they can be mechanically machined. The reaction-bonding process has to be controlled in a way that first the pyrolysis of PMSS, followed by the oxidation of ZrSi 2, and finally the formation of ZrSiO 4 and the sintering to dense compacts take place. Due to the absence of shrinkage and the good mechanical properties obtained, these ceramics open up new applications in fields requiring high dimensional accuracy such as microsystem engineering or dentistry. ©

Fabrication of reaction-bonded Cr 2O 3 ceramics

Reaction-bonding to form Cr 2O 3 can be achieved by gaseous oxidation of a Cr phase. The reaction-bonding process is best conducted by complete oxidation before sintering. Below ≈800 °C, the activation energy for oxidation is 220 kJ mol −1, indicating the predominance of Cr 3+ outward diffusion along high diffusivity paths, e.g., grain boundaries and dislocations. At higher temperatures, the activation energy is reduced to 52 kJ mol −1 as a result of oxygen transport along lower-energy paths, e.g., along microcracks, and the internal and external surfaces. In spite of the decrease in activation energy, the access of oxygen to the inside of the powder compact is hindered by the progressive densification of the oxidizing powder compacts. Maximum densification is achieved for fully oxidized Cr/Cr 2O 3 compacts when the oxygen partial pressure is close to that of the Cr-Cr 2O 3 equilibrium. 0.1 wt% MgO addition increases the density and reduces the grain size of the reaction-bonded Cr 2O 3 samples due to the possible formation of the spinel phase MgCr 2O 4. ZrO 2 and MgO additions improve the fracture strength and toughness of conventionally sintered Cr 2O 3 and change its fracture mode from intergranular to intragranular. For reaction-bonded Cr 2O 3 samples with or without MgO addition, their fracture strength and toughness data are roughly the same as those of sintered Cr 2O 3 doped with ZrO 2 and MgO and their fracture surfaces are predominantly intragranular.

Controlled Firing of Reaction‐Bonded Aluminum Oxide (RBAO) Ceramics: Part I, Continuum‐Model Predictions

The reaction‐bonded aluminum oxide (RBAO) process is a novel, reaction‐forming technique for producing monolithic, alumina‐based ceramics. Although there has been extensive work on the RBAO process, it is often difficult to reproduce the process and avoid sample cracking. To solve the problems that are associated with the RBAO process, it is necessary to have a fundamental understanding of the reaction‐bonding process and the effects of various processing parameters on the reaction behavior. To gain some insight into the process, a continuum model has been developed. The model, which considers the interaction between the macroscopic material and energy balances, is used to predict conditions under which RBAO bodies may be fired in a controlled manner, i.e., avoiding the runaway reaction. In particular, the effects of the oxygen content of the atmosphere, the heat loss by convection and radiation, the heating cycle, and scale (sample size) have been investigated. For small sample sizes, model predictions indicate that the reaction may be controlled by reducing the oxygen content of the atmosphere, increasing the heat loss, and/or incorporating an isothermal hold into the heating cycle at a temperature just below the ignition temperature. For larger sample sizes, model predictions indicate the need for multiple low‐temperature holds at increasing temperatures. It is believed that firing RBAO bodies in a controlled manner will allow one to avoid sample cracking. Part II of this work presents a complementary experimental study that investigates the reaction behavior and structural integrity of samples that have been fired under the predicted conditions.

Fabrication and Mechanical Properties of Reaction-Bonded Carbon Fiber/Si/SiC Composites.

Two types of low-density carbon/carbon preforms which had different fiber volume fractions and fiber orientations, i.e., a carbon woven fabric (≈55vol%)/carbon and a chopped carbon fiber (≈40vol%)/carbon composites, were reaction-bonded with a silicon melt at 1750°C in vacuum to fabricate dense carbon fiber/Si/SiC composites. The reaction-bonding process increased the density to -2.1g/cm3 from 1.6 and 1.15g/cm3 for carbon woven and chopped carbon preforms, respectively. All of the composites fractured with extensive fiber pullout. The higher the density, the higher the stiffness and proportional limit stress. The carbon woven fabric/Si/SiC composites with a density of 2.06g/cm3 exhibited a -120MPa ultimate strength and a -80MPa proportional limit in bending test.

Formation of Mullite from Filled Siloxanes

Monolithic mullite with low sintering shrinkage was synthesized from polymer/filler blendsMthat is, siloxane/alumina (siloxane/Al2O3) (and siloxane/aluminum (siloxane/Al)) mixtures. The synthesis was based on a reaction‐bonding process of amorphous silica, which formed when the siloxane was oxidized, with Al2O3 (or oxidized aluminum filler) at temperatures >1250°C. Thermodynamic calculations were used to calculate the phase composition at equilibrium. Thermoanalytical, infrared‐spectroscopic, and microscopic techniques were applied to reveal the microstructural evolution. The corresponding volume changes were used to evaluate the linear shrinkage, based on the quantitative phase assemblage.

Preliminary results on a novel fabrication route for α-Al 2O 3 single crystal monofilament-reinforced reaction-bonded mullite (RBM)

Owing to their excellent properties, continuous-fibre reinforced mullite-matrix composites are good candidates for applications in which oxidation resistance and damage tolerance at high temperatures (> 1000 °C) are required. To avoid fibre damage, near net-shape fabrication techniques of the composite are required. This has been achieved by using the reaction-bonding process which benefits the oxidation of metal powders producing volume expansion, and hence fully or partially compensating for the sintering-induced shrinkage. Starting materials include Al-Si alloy (80:20), Si metal, α-Al 2O 3 and mullite precursor powders. Due to the variety of starting compounds with different reaction and sintering kinetics, composite fabrication becomes a complex process. Differential scanning calorimetry (DSC) measurements, scanning electron microscopy (SEM) observations, and X-ray diffractometry (XRD) data show that effective milling of metal powder leads to a high degree of mullite formation (≈ 84%) at temperatures as low as 1500 °C, although densification of the ceramic compacts remains rather low (≈ 45% of theoretical density). Single crystal α-Al 2O 3 monofilaments were used to reinforce the reaction-bonded mullite (RBM) matrix. Although no intense reaction between the matrix and the fibres was observed at process temperature, strong bonding develops between uncoated fibres and the matrix. In order to produce a weaker fibre-matrix interface, which is necessary for improvement of the damage tolerance, the fibres were coated with ZrO 2 by means of high frequency sputtering. Microstructural observations of the fibre surfaces before and after the reaction-bonding process indicate that thick coatings (> 10 μm) produce very weak bonding, insufficient for matrix-fibre load transfer due to shrinkage of the low density ZrO 2 layers. Thinner layers (1 μm) produce a better interfacial relation with suitable pull-out of fibres.

Secondary operations

Owing to their excellent properties, continuous-fibre reinforced mullite-matrix composites are good candidates for applications in which oxidation resistance and damage tolerance at high temperatures (> 1000 °C) are required. To avoid fibre damage, near net-shape fabrication techniques of the composite are required. This has been achieved by using the reaction-bonding process which benefits the oxidation of metal powders producing volume expansion, and hence fully or partially compensating for the sintering-induced shrinkage. Starting materials include Al-Si alloy (80:20), Si metal, α-Al2O3 and mullite precursor powders. Due to the variety of starting compounds with different reaction and sintering kinetics, composite fabrication becomes a complex process. Differential scanning calorimetry (DSC) measurements, scanning electron microscopy (SEM) observations, and X-ray diffractometry (XRD) data show that effective milling of metal powder leads to a high degree of mullite formation (≈ 84%) at temperatures as low as 1500 °C, although densification of the ceramic compacts remains rather low (≈ 45% of theoretical density).Single crystal α-Al2O3 monofilaments were used to reinforce the reaction-bonded mullite (RBM) matrix. Although no intense reaction between the matrix and the fibres was observed at process temperature, strong bonding develops between uncoated fibres and the matrix. In order to produce a weaker fibre-matrix interface, which is necessary for improvement of the damage tolerance, the fibres were coated with ZrO2 by means of high frequency sputtering. Microstructural observations of the fibre surfaces before and after the reaction-bonding process indicate that thick coatings (> 10 μm) produce very weak bonding, insufficient for matrix-fibre load transfer due to shrinkage of the low density ZrO2 layers. Thinner layers (1 μm) produce a better interfacial relation with suitable pull-out of fibres.

Quantitative kinetic analysis of silicon nitridation

Kinetic analysis of silicon nitridation requires intrinsic single-particle behaviour to be isolated from global or compact effects that typically manifest during the reaction-bonding process. These effects arise from the influence of adjacent particles, which modify the macropore structure as the reaction proceeds. Much of the variation in the published kinetic data can be attributed to compact effects, particle shape, and size distribution, resulting in a myriad of models being reported, each only applicable to the nitridation conditions in which the data were obtained. Our work clearly demonstrates that the intrinsic single-particle nitridation behaviour is well described by a sharp-interface model, with diffusion control (E a = 301.5–310.0 kJmol−1) through an expanding Si3N4 product layer developing on the individual grains. For the nitridation of silicon compacts, the reaction-bonding process can be divided into three fundamental stages: (1) initial devitrification/nucleation, (2) massive nitridation, and (3) termination by further sintering, densification, and coarsening of the Si3N4 product. Factors influencing and controlling each stage are summarized.