SYNTHESIS AND PROCESSING
2.04 Processing of Silicon Carbide-Based Ceramics
2.04.4 Silicon Carbide-Based Ceramics
As already mentioned in the introduction of this article many processing approaches have been developed for silicon-based ceramics in the past. The complex variety of different processing methods of SiC-containing Figure 10 Hi-NicalonÔSfiber fabric. The copyright of these images is held by the author of this article.
Table 3 Characteristics of some continuous SiCfibers (manufacturer information)
Trade name Manufacturer Composition Diameter (mm) Tensile strength (GPa) Young’s modulus (GPa)
SCS-6 Textron SiC/C 150 3.92 406
NicalonÔNL Nippon Carbon Si(C, O) 14 3.0 220
Hi-NicalonÔS Nippon Carbon SiC 12 2 .45 390
Hi-NicalonÔ Nippon Carbon SiC 14 3.02 273
Tyranno Ube Industries Si, Ti, C, O 8 2.7 206
Tyranno SA Ube Industries >99% SiC 8 3.0 290
Sylramic Dow Chemical >99% SiC, 0.5% O 10 3.0 400
Processing of Silicon Carbide-Based Ceramics 101
ceramics should be systemized for a better understanding of their technologies. The consolidation is the pro- cessing step where the microstructure of a ceramic is completed. Hence through the consolidation step a ceramic material is actually generated. The respective consolidation mechanism is most important for the properties and thus for the application profile of the corresponding material. This statement particularly applies to SiC ce- ramics (Kriegesmann, 2005a). If one classifies the SiC-based ceramics according to their fundamental consol- idation mechanisms, three different groups can be distinguished as shown inFigure 11.
Silicate bonded SiC ceramics refer to porous ceramics with relatively coarse particles bonded by a large amount of amorphous silicate matrix. The processing occurs via a sintering process in air, which may be compared with sintering process of traditional ceramics. The main sintering effect is the contact formation of a silicate melt with the SiC grains. Thus shrinkage is low or not existent.
The second group comprises either dense or porous SiC ceramics without or at most low-impurity contents.
They are sintered in protective atmospheres like argon and/or vacuum, and utilize solid-state, liquid-phase or vapor-phase (evaporation–condensation) mechanisms (Kriegesmann, 2005a). The sintering process may also be pressure assisted through axial hot pressing (HP) or hot isostatic pressing (HIP).
In general sintering means high-temperature consolidation of a bulk material through surface or interface reactions and atomic transport phenomena (diffusion). Thus sintering is a physical–chemical process occurring within the material and should not be confused withfiring, a technological step occurring within a kiln which may only cause sintering. The sintering reactions are only atomic site changes and thus “one-component reactions”(Kriegesmann, 2005b) in contrast to normal chemical reactions, which can be regarded as“two- components reactions”.
It was discovered in the 1970s that the consolidation of ceramics can also be achieved through normal chemical reactions (Moulson, 1979). This special consolidation has been called“reaction bonding”. SiC-based ceramics can also be consolidated through reaction bonding. Some of them are consolidated by reaction action alone, whereas for others the reaction bonding is combined with the infiltration of liquids. Both types of related materials can be referred to as the third group of silicon-based ceramics.
In the subsequent subsections, the processing phenomena of the three groups are described and the corre- sponding ceramics are characterized.
2.04.4.1 Silicate Bonded Silicon Carbide-Based Ceramics
The most striking properties of the compound SiC are its high hardness and its high thermal stability. Therefore it is not astonishing that thefirst attempts to develop SiC-based ceramics were focused on SiC grinding tools and refractories more than 100 years ago. Because of the already mentioned nonreactivity of covalently bonded compounds, it was not strived for achieving a self-bonding of the SiC particles. Rather, it was preferred to interconnect the particles via an aluminosilicate bond which could be compared best with the well-known microstructural conditions in traditional clay ceramics. The attempts of creating ceramics, in which the SiC grains were consolidated via a so-called ceramic bond, were successful since the surfaces of the SiC grains can be easily wetted by aluminosilicate melts.
2.04.4.1.1 Silicon Carbide Grinding Tools
The first SiC-based class of materials treated here, the grinding tools, belongs only partially to ceramics. All grinding tools are open-pored in order to be able to capture swarf. The higher the open porosity the better is the grinding yield, but the rougher will be the ground surface. The grinding tools consist of abrasive grains embedded in a binder phase matrix. Each abrasive grain can be regarded as an undefined cutting edge; thus, a Figure 11 Different groups of silicon carbide-based ceramics.
102 Processing of Silicon Carbide-Based Ceramics
multiple cutting edge tool is formed through the surface abrasive grains interconnected by the matrix. In grinding tools with ceramic bond the abrasive particles like diamond, CBN, corundum (a-Al2O3) or SiC are interconnected by a vitreous or a glass–ceramic matrix and thus silicate bonded. It should be mentioned that grinding tools can also have resin, sintered metal, galvanic and rubber bonds, but those tools cannot be regarded as ceramics and hence they are not treated here.
Diamond and CBN are the hardest compounds of all and thus the corresponding ceramic bonded grinding tools constitute a quite special and expensive category of materials. On the other hand, the cheaper corundum and SiC grinding tools are often compared with each other. SiC is harder abrasive than the corundum grinding tools, but the latter have a larger market share, particularly because the range of different qualities is wider for corundum grinding tools. Without previous testing, it is difficult to predict which grinding materialfits best for a specific assignment. However, there is a trend of using corundum grinding tools for machining of steel, whereas SiC tools are applied for more brittle materials like glass, ceramics, gray iron and nonferrous metals.Figure 12 shows a tool for grinding alumina ball for hip joints.
Since the process engineering for the manufacture of ceramic bonded SiC grinding tools will be treated below together with the refractory bricks, only some aspects concerning the recipe are covered here. For the production of SiC grinding tools both the green and the black SiC raw material powder grades are used. The SiC particles of more expensive green grade exhibit higher hardness but lower toughness than the SiC particles of the cheaper black grade. It should be taken into account that each SiC raw material particle is coated by a thin silica layer because of previous oxidation. Thus the components of the binding system for the SiC grains should be composed in such a way that the silica layer of SiC particles can be wetted through the components and interconnect with them through mutual dissolution processes. Since those processes are highly temperature- dependent, the binding systems should exhibit a high thermal stability. In order to avoid shortening the firing temperature range, which is advantageous for the production process, the recipes of binding systems should not contain valence and coordination number changes. Thus the batches of the binding systems should not include transition elements like iron or boron. Whereas the binding systems for corundum grinding tools are indicated as vitreous by the tool manufacturers, the binding systems for SiC grinding tools are considered to be porcelain-like. For this reason binder batches contain clay components like kaolin or bentonite.
Too highfiring temperatures might lead to the risk of foaming, which originates from the oxidation reaction of SiC:
2SiCþ4O2 4 2SiO2þ2CO2 ðat temperatures<1100CÞ (5) 2SiCþ3O2 4 2SiO2þ2CO ðat tempetatures>1100CÞ (6) Figure 12 SiC grinding tool for hip joints. This photo was kindly transmitted from Tyrolit Schleifmittelwerke Swarovski K.G., Schwaz, Austria.
Processing of Silicon Carbide-Based Ceramics 103
and the fact that the viscosity of binding composition will be reduced at higher temperatures. If the viscosity is lower, oxygen can diffuse easier through the binder phase to the binder/SiC interface. Through oxygen attack on SiC according toEqns (5) and (6)gaseous components CO and CO2are released, respectively, which have got relatively high vapor pressure and hence high space requirements. The above-mentioned porcelain-like binding system with high thermal stability turns out to be advantageous in order to avoid the foaming as well.
At lower temperatures the binder system is still porous so that gases can escape. The risk of foaming is the reason that the SiC grinding tools are more porous compared to some relatively dense corundum grinding tools (Frank, 1998).
The raw material composition for the binding system consists of plastic materials (kaolin, clay, and bentonite),fluxes (feldspar, nepheline syenite, and frit) and additives (quartz sand and soda). The raw materials composition should be constituted such that in the sintered ceramic tool the mismatch between thermal expansion coefficients of the SiC grains (4.5106K1) and the binder phase should be low. It had been discovered that the strength of a grinding tool ceramic is strongly dependent on the match between the thermal expansion coefficients of the abrasive grains (here SiC particles) and the aluminosilicate matrix (Kriegesmann, Rasch, & Büchler, 2002). Normally the thermal expansion coefficient of the binding phase is higher than that of the SiC if the above-mentionedfluxes are used. The coefficient can be reduced by replacing feldspar or nepheline syenite with lithium-containing minerals like spodumen or petalite.
Thefiring is conducted using oxidizing atmospheres (usually air). The maximumfiring temperature varies between 1100 and 1350C. The microstructural changes during the sintering process are principally com- parable with those occurring during the sintering of refractory silicate bonded SiC bricks, even though the firing temperature of the latter materials are generally higher. The SiC refractory bricks are treated in the next section.
The hardness of the grinding tool, i.e. the measure for the ultimate power of aluminosilicate matrix to resist cracking, is an important property according to the grinding experts (Frank, 1998). However, the specifications are not defined precisely and the measuring methods differ between the grinding tool suppliers. Some use destructive and others nondestructive measuring techniques. The destructive techniques are based on the sandblasting principle regularly. The nondestructive method seems to prevail more and more. Sonic methods allow measuring the resonance frequency, from which the Young’s modulus can be calculated. Many grinding tool suppliers believe that the Young’s modulus is the best measure of hardness. This seems to be reasonable since if we suppose that grinding tool materials (like all brittle materials) have nearly the same breaking strain (0.1%), then the strength of the grinding tool material should linearly depend on its Young’s modulus (Kriegesmann, Glagovsky, Moskovenko, & Schmid, 1993).
Figure 13 shows the fracture surface of an SiC grinding tool. The fracture mode is mainly intergranular (Section 2.04.4.2). Practical application has shown that this is advantageous for grinding since the grains break
Figure 13 SEM image of a typical fracture surface of an SiC grinding tool. This photo was kindly transmitted from Tyrolit Schleif- mittelwerke Swarovski K.G., Schwaz, Austria.
104 Processing of Silicon Carbide-Based Ceramics
off. This means that SiC grains break off as a whole and thus the corresponding grinding tool remains sharp for the further use.
2.04.4.1.2 Silicon Carbide Refractory Bricks
Refractory bricks should also be porous but for reasons different from those indicated for grinding tools. Pores decrease the crack propagation rate during thermal stressing. Thus porosity improves the thermal shock resistance of ceramics. The open porosity and the pore sizes should, however, be small because otherwise gases or melts can easily penetrate into the brick during the thermal application in practice and thus attack or even destroy its microstructure. Additionally it should be noted that the bending strength and the thermal conductivity of a ceramic increase with decreasing porosity. The microstructure should be designed in such a way to ensure volume stability at high temperature exposure; a porous refractory brick which is shrinking during its sintering process also tends toward shrinking during the application, since sintering and application temperatures of refractory bricks are in the same range. Firing shrinkage can be suppressed by using so-called tiered grain size distributions in the raw material batch. The single grain fraction itself should have a narrow grain size distribution and should be only slightly overlapped by the priorfiner and the next coarser fraction, respectively. In this way the overall configuration of particles provides a multimodal grain size distribution.
These kinds of distributions give rise to high densifications during the shape-forming step since the finer grainsfit into the gaps formed by the coarser ones. The subsequent sintering step is not affected by shrinkage if the coarsest grain size fraction of the distribution is coarse enough. Hence, getting back to SiC-based refractory bricks (Wecht, 1977), the particle sizes of the multimodal grain size distribution of SiC particles should reach from the millimeter down to the micrometer range. Since the SiC grains are less rounded (often called
“compact”) but rather splintery, the proper grain size distribution cannot be calculated properly. A practical empirical approach of multimodal grain size distribution, however, had been established a long time ago (Litzow, 1930), which is still up to date even for many other refractory recipes. Though the green SiC powder grade seems to be better than the black one because of its higher purity, in practice, this difference implies minor effects on the properties of the SiC-based ceramics. Thus in most cases the black grade has been favored because of its lower price.
The SiC content of refractory silicate bonded SiC bricks ranges from 40 to 90 wt.%. For the silicate bond the recipe contains mostly the so-called refractory clays with a high alumina content (up to 42 wt.%), which usually consist of fire clays, which are imperfectly structured kaolinites. It should be noticed that the clay not only is responsible for the interconnection of SiC grains but also increases the plasticity of the raw material composition. If the SiC content is high, the plasticity of raw material mixture is too poor for the shape-forming technique of pressing. In this case bentonite can be added (not more than 1 wt.%) to improve the plasticity. If the SiC content is low, some of the clay content may be replaced by grog (also called chamotte) in order to reduce plasticity. These recipes may also contain organic polymers as binders like polyacrylates, polyvinyl alcohols or polyvinyl acetates. For achieving better refractoriness, natural highly alumina-containing minerals like sillimanite or artificial additives like fused alumina may replace clays.
The firing is conducted using oxidizing atmospheres (air with added oxygen). The maximum firing tem- peratures vary between 1350 and 1500C according to the composition. They are higher regularly than the maximumfiring temperatures applied for SiC grinding tools. The reason is the higher thermal stability that is required for a refractory material.
As mentioned above, the microstructural changes in silicate bonded SiC bricks during the sintering process are in principle comparable with changes occurring in silicate bonded SiC grinding tools, even though the corresponding reaction temperatures may be shifted a bit to higher temperature ranges. At about 1000Cfirst silicate melt droplets may occur depending on the alkaline impurity content in raw material mixture. Mullite (3 Al2O3$2 SiO2) is formed between 1200 and 1400C. If mullite originates directly from the clay minerals via solid-state reactions, the shape of the crystals is spherical (primary mullite). On the other hand, if the mullite is formed via solution of clay relicts (e.g. meta kaolinite, Al2O3$2 SiO2) in the liquid phase followed by reprecipitation, the shapes of the crystals are needle-like (secondary mullite). The mullite/glass phase ratio in the interparticle space affects the properties of silicate bonded SiC brick notably. Thus it can be expected that a higher ratio provides better refractoriness, strength, creep resistance and thermal shock resistance for the brick (Eckert, Kara, & Kerber, 1994). Particular problems might arise from cristobalite, which usually is generated through crystallization of the early oxidized surface of the SiC particles due to the oxygen-rich Processing of Silicon Carbide-Based Ceramics 105
atmosphere of thefiring stage. The phase transformation of cristobalite at 240C causes a tremendous volume change, thus in a long run the corresponding brick might be destroyed through periodic temperature cycling in the practice.
The processing principles for the production of silicate bonded SiC bricks and for the production of corre- sponding grinding wheels resemble each other widely. Thus the processing of those two SiC-based ceramic groups should be treated together.
The compound conditioning depends on the kind of shape-forming techniques. If the shape forming is implemented through slip casting, the different SiC particle fractions and ceramic binder components should be wet stirred (15–30% water) in a counterflow mixer for 1/2 and 6 h depending on the grain size distribution. The resulting slurry is cast in a porous plaster mold, dried after demolding first in indoor air later in drying chambers. The mixture may contain components, which increase the thixotropy of the slurry, like magnesia (Frank, 1998), in order to reduce the water content of the slurry; the casting of thixotropic slurries requires a vibrational motion of the molds. Through slip casting technique it is possible to fabricate homogeneous parts, but the variation of the slip composition, especially if the recipe contains gas-forming substances (see below) is limited. Furthermore the slip casting technique is time-consuming and thus is only used for producing complex parts.
The standard shape-forming method for silicate bonded SiC-based ceramics is the uniaxial pressing tech- nique, for which a dry formable material mass is needed. The components of the mass are mixed in a compulsory mixer. Apart from SiC particles and the above-mentioned inorganic binder constituents, they contain pulverulent temporary binders like dextrin, urea resin, phenolic resin, wax or their aqueous solutions and water. Sometimes it is necessary to increase the pore sizes. In this case the components contain the so-called pore formers, which could be artificial granulated sublimating substances like naphthalene as well as natural opening substances like coke or nutshells.
The uniaxial pressing is carried out in hydraulic, friction, toggle, excentric and vibratory presses. The highest green densities can be achieved with the latter technique because of superposition of static and dynamic loads. A press mold should consist of either hardened tool-steel or cemented carbide because of extreme abrasion of the SiC grains. The contact pressure per unit area ranges from 500 to 4000 MPa. The so-called hand mold-shaped bodies are compressed with vibratory pneumatic hammers.
The moisture of the shaped bodies range from 2 to 4% for pressed, from 5 to 8% for hand mold and up to 20% for cast bodies, respectively (Wecht, 1977). Too rapid drying may induce overpressure in the pores, which may cause bursting of the bodies. Thus the drying of the shaped bodies should be proceeded with caution. Large bodies have to be dried through warm air drying (up to 50C)first. Smaller bodies can be put also directly into drying chambers with temperatures of 110C. A thorough drying process is needed for the silicate bonded SiC-based ceramics because of the risk of“black cores”, given by residual moisture promoting the decompo- sition of SiC (see below).
Thefiring is conducted in gas- or oil-heated continuous tunnel kilns or in intermittent shuttle and hood kilns.
During the heat-up stage the residual moisture should be expelled completely and both the organic polymers and the gas-forming substances have to be burned out. It should be noted that at this stage an oxidizing at- mosphere is necessary in order to prevent the formation of free carbon, which can cause the“black cores”.
Simply shaped bricks and small grinding tools (with diameters of up to 300 mm) can be left unmachined.
However, larger cutting tools and those for special applications, such as threaded and profiled disks, need thoroughfinal machining.
Figure 14shows the course of the processing procedure for the silicate bonded SiC grinding tools and refractory bricks. Thefinal machining is displayed with dotted lines since it is only necessary for some special applications. The course of processing of the silicate bonded SiC-based ceramics is largely comparable to the processing of many traditional ceramics, which is understandable, since the manufacturing routes the silicate bonded SiC-based ceramics had been developed out of the knowledge of the processing of traditional ceramics.
The uses of silicate bonded SiC bricks are too multifaceted (Wecht, 1977) to be treated in this survey.
Nevertheless the demand of the silicate bonded SiC bricks has slowed down tremendously before 2010.
Generally it can be stated that the loss of significance is attributed to the fact that lots of new SiC-based ceramics have been developed in the meantime, thus for some applications the bricks were replaced by new materials. Particularly worth mentioning are the SiC-containing“unshaped refractory materials”(Krebs, 2008). The roots of this new material class derive from the procedure of repairing damaged parts of firing assemblies. First it had been demonstrated that it is possible to repair damaged parts on location in such a 106 Processing of Silicon Carbide-Based Ceramics