poco graphite, inc. properties and characteristics of ... · pdf fileproperties and...

Edited by A. H. Rashed

©2002 Poco Graphite, Inc.POCO Graphite, Inc.

300 Old Greenwood Rd.Decatur, TX 76234www.poco.com

NoticeReproduction or recording of any part of this book is strictly prohibited without the express written consent of the copyright holder. Text, tables, diagrams,or other material may not be copied by facsimile, enlargement, or reduction; microfilmed; nor reproduced on slides, or by other means or materials.

Poco Graphite, Inc.

Properties and Characteristics of

Silicon Carbide

P r o p e r t i e s a n d C h a r a c t e r i s t i c s o f S i l i c o n C a r b i d e

PREFACE

Poco Graphite, Inc. (POCO) perfected a unique, proprietary process for producing silicon carbide that is dif-ferent from conventional silicon carbides, the properties and characteristics of which are outlined in this docu-ment.

SUPERSiC was developed as an alternative solution to the traditional molded silicon carbide components.POCO’s silicon carbide products are produced using a unique manufacturing method. This manufacturingtechnique allows unparalleled flexibility in design without prohibitive costs and lead times. Silicon carbideproducts are designed with features to reduce thermal mass while retaining high strength.

The purpose of this document is to introduce the reader to silicon carbide properties and to describe testingtechniques, which enable true comparisons between the different manufactured silicon carbides.

Reproduction or recording of any part of this book is strictly prohibited without the express written consent of the copyright holder. Text, tables, diagrams,or other material may not be copied by facsimile, enlargement, or reduction; microfilmed; nor reproduced on slides, or by other means or materials.

ii



iii

TABLE OF CONTENTS

page

Preface ii

Table of Contents iii

List of Figures iv

List of Tables v

Chapter 1 Introduction 1

1.1 History 1

1.2 The Si-C System 1

1.3 Production of SiC 2

1.4 Fabrication of SiC Products 3

1.5 POCO Process 3

Chapter 2 Physical and Chemical Properties of SiC 5

2.1 Crystal Structure 5

2.2 Density and Porosity 5

2.3 Chemical Purity 7

2.4 Oxidation Resistance 8

Chapter 3 Mechanical Properties of SiC 11

3.1 Flexural Strength 11

3.2 Tensile Strength 12

Chapter 4 Thermal Properties of SiC 15

4.1 Thermal Conductivity 15

4.2 Thermal Expansion 15

4.3 Thermal Shock Resistance 17



iv

LIST OF FIGURES

Figure Title Page

1.1 The Si-C Phase Diagram. 1

1.2 SiC Crystal Structure: (a) Zinc Blend Structure for β-SiC and (b) Wurtzite Structure for 6H α-SiC. 2

1.3 A Flowchart of POCO’s SiC Manufacturing Process. 4

2.1 XRD Spectrum of SUPERSiC-1 Material Showing β-SiC as the Only Detectable Phase Present. 5

2.2 Oxidation Behavior of SUPERSiC-1 Material Showing Weight Gainper Unit Area as a Function of Oxidation Time at 1200˚C in Dry Air. 9

3.1 Weibull Plots for the Flexural Strength of SUPERSiC-1 at Different Temperatures. 12

3.2 Weibull Plot for the Room Temperature Tensile Strength of SUPERSiC-1 Grade. 13

4.1 Thermal Conductivity of SUPERSiC-1 as a Function of Temperature Calculated from Thermal Diffusivity Measurements (Equations 4.1 and 4.2). 16

4.2 Heat Capacity Measurements of SUPERSiC-1 as Compared to Calculated Data for Pure SiC (Equation 4.2). 16

4.3 Fourth Order Polynomial Representing the Thermal Expansion of SUPERSiC-1 as a Function of Temperature with the Room Temperature (20°C) as the Reference Point. 17

4.4 Fourth Order Polynomial Representing the Mean Coefficient of Thermal Expansion of SUPERSiC-1 as a Function of Temperature with the Room Temperature (20°C) as the Reference Point. 18

4.5 Thermal Shock Resistance of SUPERSiC-1 Determined by the Water Quench Method. 19



v

LIST OF TABLES

Table Title Page

2.1 Typical Elemental Analysis Data (in ppm) of SUPERSiC-1 Material ObtainedUsing GDMS Analysis 8

3.1 Flexural Strength of SUPERSiC-1 and SUPERSiC-2 Grades Measured at Room Temperature Using the Three-Point Bend Test. 11

3.2 Flexural Strength of SUPERSiC-1 Measured at Different Temperatures Using the Four-Point Bend Test at ORNL/HTML. 11



1

1.1 History

Silicon carbide (SiC) was accidentally discovered in1890 by Edward G. Acheson, an assistant to ThomasEdison, when he was running an experiment on the syn-thesis of diamonds. Acheson thought the new materialwas a compound of carbon and alumina present in theclay, leading him to name it carborundum, a name that isstill being used on some occasions. Silicon carbideoccurs naturally in meteorites, though very rarely and invery small amounts. Being the discoverer of SiC,Acheson was the first to synthesize SiC by passing anelectric current through a mixture of clay and carbon.Today, SiC is still produced via a solid state reactionbetween sand (silicon dioxide) and petroleum coke (car-bon) at very high temperatures in an electric arc furnace.

In the past, the list of ceramics used as industrialmaterials consisted of alumina and other oxides. In recentyears, there have been strong demands for the use ofceramics as structural materials in place of metals andalloys and for use in harsh environments. Consequently,

new ceramics such as nitrides, carbides and other cova-lently bonded materials have received increased attentionbecause of their unique characteristics.

1.2 The Si-C System

The formation of SiC from the reaction between siliconand carbon can take place at temperatures below themelting point of silicon. The phase diagram of the Si-Csystem is shown in Figure 1.1. It can be seen that SiC isthe only compound of silicon and carbon to occur in thecondensed state in addition to elemental silicon and car-bon. A eutectic point between silicon and SiC exists at1402˚C and 0.75 atom % carbon. The liquidus curvebetween Si and SiC is shown up to 2600˚C and 27 atom% C. A peritectic point is located at 2540˚C and 27 atom% C under normal conditions.

There are numerous (~200) polytypes for SiC, but onlya few are common. All of the structures may be visualizedas being made up of a single basic unit, a layer of tetra-hedra, in which each silicon atom is tetrahedrally bonded

Chapter 1

Introduction

Figure 1.1 The Si-C Phase Diagram (R.P. Elliot, “Constitution of Binary Alloys”, p. 227,McGraw-Hill, New York, 1965)



2

to four carbon atoms and each carbon atom is tetrahe-drally bonded to four silicon atoms. The differencesamong the existing polytypes are the orientationalsequences by which such layers of tetrahedra arestacked. Successive layers of tetrahedra may be stackedin only one of two ways or orientations but with many pos-sible sequential combinations, each of which represent adifferent crystal polytype.

A common system of nomenclature used to describethe different crystalline polytypes assigns a number cor-responding to the number of layers in the unit cell followedby a letter suffix designating the crystal symmetry; “C” forcubic, “H” for hexagonal and “R” for rhombohedral. Themost common SiC polytypes are the 3C, 4H, 6H, 15R and9T. The cubic 3C is commonly referred to as beta siliconcarbide, β-SiC, which has the zinc blend structure, whileall other polytypes are referred to as alpha silicon carbide,α-SiC. In general, α-SiC phase is mainly 6H, which is awurtzite structure. Figure 1.2 shows an illustration of thetwo major crystal structures, zinc blend and wurtzite,exhibited by the two SiC phases. Phase transformation ofSiC occurs from β-SiC to α-SiC upon heating. Undopedβ-SiC transforms to 6H and 15R above 2000°C, with the15R being a metastable phase that transforms to 6H.Doping β-SiC with boron lowers the transformation tem-perature and results in the formation of 4H polytype. Onthe other hand, doping with nitrogen prevents the forma-tion of 4H and stabilizes the 6H. The β → α phase trans-formation is irreversible under ambient atmosphere.However, under pure nitrogen atmosphere, the transfor-mation can be reversed and β-SiC phase can be stabi-lized up to 2500°C by applying a nitrogen pressure.Under atmospheric pressure, silicon carbide does notmelt when heated to elevated temperatures rather, it sub-limes and/or dissociates. In addition, incongruent meltingof SiC was reported at 2829°C under >500 psi pressureof argon and is also possible when it is heated rapidly inan arc-image furnace at atmospheric pressure.

Silicon carbide is considered one of the few light-weight covalently bonded ceramics. The theoretical den-sity of β-SiC is only 3.210 g/cm3 and that of α-SiC (6Hpolytype) is 3.208 g/cm3. Combining it’s lightweight and,strong covalency with other properties, such as low ther-mal expansion coefficient and high thermal conductivity,strength and hardness, make SiC a promising ceramic forthe replacement of conventional metals, alloys and ionic-bonded ceramic oxides.

1.3 Production of SiC

The most common forms of SiC include powders,fibers, whiskers, coatings and single crystals. There areseveral methods to produce SiC depending on the prod-uct form desired and its application. Purity of the productimposes certain restrictions on the selection of themethod of production.

SiC powders are produced predominantly via the tra-ditional Acheson method where a reaction mixture ofgreen petroleum coke and sand is heated to 2500°C usingtwo large graphite electrodes. Due to the high tempera-tures, the Acheson process yields the alpha form of SiC,i.e. hexagonal or rhombohedral (α-SiC). The SiC product,usually in the form of a large chunk, is broken, sorted,crushed, milled, and classified into different sizes to yieldthe commercial grades of SiC powder. To produce ultra-fine SiC powder, the finest grade of the Acheson productis further milled, typically for days, and then acid-treatedto remove metallic impurities. Fine SiC powder can alsobe produced using a mixture of fine powders of silica andcarbon reacted at lower temperatures for short periods oftime followed by quenching to prevent grain growth. Theproduct, however, is agglomerates of SiC and needs to beattrition milled to break up the agglomerates and reducethe particle size to submicron range. SiO2 powder can bereplaced with SiO (silicon monoxide) powder which, whenmixed with nano-scale carbon and heated to moderatetemperatures, produces nanocrystalline SiC powder withparticle size in the range 20-100 nm. The SiC particlecharacteristics, such as size, shape and surface chem-istry, are very important for the subsequent densificationprocesses of the SiC powder. For this reason, some postprocesses may be needed, such as the addition of certainelements as sintering aids, to achieve high density duringhot pressing or pressureless sintering.

SiC fibers are produced via the pyrolysis of organosil-icon polymers, such as polycarbosilane, and are commer-cially available. Briefly, the process consists of melt-spin-ning the polycarbosilane at approximately 300°C, unfus-ing with thermal oxidation at 110-200°C, and baking at1000-1500°C under a flow of inert gas. Nicalon fibers areknown for their excellent mechanical properties when used as reinforcement in ceramic matrix composites

Figure 1.2 SiC Crystal Structures: (a) Zinc BlendStructure for β-SiC and (b) Wurtzite Structure for 6H

α-SiC. (From: W.D. Kingery et al, “Introduction toCeramics”, 2nd ed., Wiley, New York, 1976, p. 63).

(a) (b)



3

(CMC). The drawback of Nicalon fibers has been theiroxygen and free-carbon contents, which limit their hightemperature applications. Recently, however, Hi-NicalonSiC fibers have been introduced with much lower oxygencontent. At present, much of the work in the SiC fiber-reinforced CMC development is using Hi-Nicalon SiCfibers. Another method for producing SiC fibers is via theCVD method. In this process, SiC is deposited from thegas phase on a tungsten wire used as the substrate.These fibers are stronger and have higher thermal stabil-ity due to their higher stoichiometry and purity.

SiC whiskers, which are nearly single crystals, areproduced (grown) using different methods, including theheating of coked rice hulls, reaction of silanes, reaction ofsilica and carbon, and the sublimation of SiC powder. Insome cases a third element used as a catalyst, such asiron, is added to the reacting materials to facilitate the pre-cipitation of the SiC crystals. In this arrangement, themechanism for the SiC whisker growth is called the vapor-liquid-solid (VLS) mechanism. SiC whiskers are in theorder of microns in diameter and grow several hundredmicrons in length. The VLS process, developed by theLos Alamos National Laboratory to produce longer SiCwhiskers with larger diameters, did not show promise forproduction due to the extremely low yield. Currently,commercially available SiC whiskers are produced usingthe rice-hull process with the whisker growth being large-ly of VS mechanism due to the absence of a catalyst.Because of their excellent mechanical properties, SiCwhiskers are very desirable as reinforcements of metaland ceramic matrix composites for structural applicationswhere fracture toughness and strength are significantlyimproved.

1.4 Fabrication of SiC Products

In addition to the efforts and significant developmentin the production of different forms of SiC (powders, fibers,whiskers, etc.), more efforts have been devoted to the fab-rication of SiC parts as a final product with the desiredproperties. The densification of the SiC powders hasbeen the largest part of these efforts. Analogous to pow-der metallurgy techniques, SiC powders have been den-sified using hot pressing, hot isostatic pressing, and pres-sureless sintering, in addition to wet processing, such asslip casting of SiC powder slurries. Sintering of SiC pow-ders requires the addition of sintering aids and heating toelevated temperatures. The addition of boron and carbon

elements to SiC powder assists in the densification of SiCduring pressureless sintering. Carbon is added to removesurface oxygen present as a film of SiO2 on the SiC parti-cles. Boron, on the other hand, is added to prevent graingrowth at lower temperatures before reaching the sinter-ing point. A density of 97% of the theoretical density canbe reached depending on the temperature and character-istics of the SiC powder. Other sintering aids includeoxides, such as alumina, zirconia and yttria. In this sys-tem, the oxide phase melts at relatively low temperaturesenhancing SiC particle flow, resulting in shrinkage anddensification. However, due to the presence of the liquidphase, there are limits to the high temperature applica-tions of the densified parts.

For the fabrication of SiC/SiC composites, a preform isfirst prepared from continuous SiC fibers with 20-30%loading by volume. A SiC matrix can be applied via theCVI process, or slurry processing, to make SiC/SiC com-posite. The SiC matrix can also be produced by impreg-nating the SiC preform with liquid carbon precursor, suchas a resin, followed by pyrolysis and silicon infiltration toform reaction-bonded SiC/SiC composite. The SiC fiberpreforms can also be impregnated with silicon powder inthe form of slurry, followed by nitridation to form SiC-fiberreinforced Si3N4 matrix composites. SiC-fiber reinforcedceramic composites are used in high-temperature struc-tural applications due to their high strength and fracturetoughness.

The processes discussed above produce SiC materi-als that contain high levels of impurities associated withthe processing steps. Impurities, such as metal-basedsintering additives, are not acceptable in the semiconduc-tor industry. Conventional purification by high-tempera-ture chlorination results in the depletion of silicon from theSiC and carbon enrichment. In addition, the presence ofimpurities in SiC may become an issue in high tempera-ture applications depending on the type and level of theseimpurities.

1.5 POCO Process

A proprietary process developed by POCO takesadvantage of the ease of graphite machining. POCO’sprocess starts with graphite that has already beenprocessed, pre-machined into the desired part, and puri-fied, followed by the chemical conversion to SiC, withoutaltering the shape or purity of the part. This approachyields a SiC product that has superior chemical properties



4

and very high purity as compared to traditionally producedSiC products.

The conversion of the graphite parts to SiC takesplace when they are exposed to silicon-carrying species,such as silicon monoxide (SiO) gas, at high temperatures.Figure 1.3 is a flowchart showing, in sequence, all thesteps involved in POCO’s SiC conversion process. TheSiO gas is generated in-situ using a proprietary mixture ofhigh-purity silica and carbon powders in inert atmosphere.The following is a typical reaction for the generation of SiOgas:

SiO2 + C ⇔ SiO + CO (1.1)

It is essential that the graphite material have a rea-sonable open porosity for the SiO gas diffusion. POCO’sspecially-developed graphite grade, with the tailoredporosity and particle size distribution properties, meetsthe requirements of the conversion process. The gener-ated SiO is transported from the generation zone to theconversion zone, which contains the graphite parts to beconverted to SiC. POCO has engineered the process sothat SiO gas is transported efficiently from the generationzone to the conversion zone and distributed evenly toensure uniform conversion. In the conversion zone, thereaction between the SiO and graphite takes placeaccording to the following reaction:

SiO + 2 C ⇔ SiC + CO (1.2)

This reaction is a typical gas-solid reaction in whichthe rate-limiting step is the pore-diffusion resistance.Accordingly, it is essential to ensure a large SiO concen-tration gradient between the bulk gas phase and theSiC/C interface, or the reaction front. At this interface, thereaction rate is controlled by surface kinetics which isspontaneous due to the high temperatures. Therefore, it isbelieved that the “shrinking unreacted-core” model canbest describe the conversion reaction mechanism. Inother words, the conversion rate is controlled by theinward diffusion of the SiO and outward diffusion of theCO gasses through the SiC shell.

Following the chemical conversion of the net-shapegraphite part to SiC, the parts are usually grit blastedusing high purity SiC to remove any surface debris, fol-lowed by ultrasonic cleaning to remove any dust andloose particles. The as-converted SiC material is POCO’sbasic grade of SiC, denoted as “SUPERSiC-1”.

Some SiC products cannot be fabricated as a con-tiguous part due to part complexities. One example in thesemiconductor industry is the vertical wafer carrier, inwhich the individual components (rails and plates) arefabricated as described above and then assembled usinga specially developed joining material. POCO has devel-oped a joining material that is currently being used in theassembly process. The constituents and composition ofthe joining material, as well as the application procedure,is considered proprietary information. However, the join-ing material is applied as a paste at room temperatureand, after processing, the material converts to high purity,high strength SiC material. The types and characteristicsof the raw materials constituting the paste were selectedto exhibit minimum shrinkage and cracking during pro-cessing for maximum strength.

SUPERSiC-1 is subjected to additional post process-es for the purpose of sealing the surface by eliminatingsurface porosity. Two approaches being used for this pur-pose are described below:

CVD SiC Coating: POCO has developed the capability toapply a thick, dense SiC coating on its basic SUPERSiC-1 gradeusing chemical vapor deposition (CVD) technology. CVD-SiCcoating has very high density and purity and excellent uniformi-ty. In addition, POCO’s process is very flexible and controllableto meet desired properties, such as the coating thickness andmicrostructure. A coated SUPERSiC-1 grade is denoted as“SUPERSiC-2” and is available as a standard product.

PG Infiltration/Conversion/SiC Coating: To increasethe efficiency of the CVD SiC coating mentioned above, theporous SiC part, or SUPERSiC-1, is first infiltrated/coated withpyrolytic carbon using a proprietary process developed byPOCO, followed by conversion to SiC as described above. Thistreatment provides a new layer of SiC that is partially impeded inthe porous substrate. The converted SiC layer is much lessporous and the pore size is much smaller than that of theSUPERSiC-1 substrate. The part will still be coated with a CVDSiC layer as described above, as this will improve the adhesionof the CVD SiC coating and sealing efficiency. The new grade isdenoted as SUPERSiC-4.

Figure 1.3 A Flowchart of POCO’s SiC Manufacturing Process



5

Chapter 2

Physical and Chemical Properties of Silicon Carbide

2.1 Crystal Structure

2.1.1 General

Crystal structure of a material is commonly deter-mined using X-ray spectroscopy and X-ray diffraction(XRD). Other techniques have also been used such astransmission electron microscopy (TEM). Crystallinephases diffract x-rays according to the Bragg law;

λ = 2d sin θ (2.1)

where θ is the diffraction angle, (Bragg angle) for a latticespacing d and λ is the wavelength of the x-rays. Powderor polished polycrystalline specimens are used and theintensity is recorded versus 2θ. The identification of aphase is accomplished by comparing the d spacing andrelative intensities of the material with reference data forknown materials, such as the JCPDF card files. The XRDtechnique is normally used for qualitative phase analysis;however, quantitative analysis is also possible when phas-es are randomly oriented and diffraction lines of differentphases are clearly distinguished.

2.1.2 Test Methods

As discussed above, phase analysis is most common-ly determined by the XRD technique, which helps identifyall major phases present in terms of their crystal structure.POCO uses a Siemens D5000 diffractometer equippedwith a full JCPDF library.

2.1.3 Crystal Structure of POCO SiC

Figures 2.1 shows a typical XRD spectrum. It can beclearly seen that the only phase detected is β-SiC and noindication that the α-SiC phase is present. This is expect-ed since the material is processed at temperatures signif-icantly lower than those used in conventional processes,such as SiC powder production and sintering.

2.2 Density and Porosity

2.2.1 General

Density, ρ, of a material is a measure of the mass, m,per unit volume, V, and is reported in units such as g/cm3,lb/in3, etc. Factors affecting the density include the size

Figure 2.1 XRD Spectrum of SUPERSiC-1 Material Showing β-SiC as the Only Detectable Phase Present.



6

and atomic weight of the elements comprising the materi-al, the tightness of packing of the atoms in the crystalstructure, and the amount of porosity in the microstruc-ture.

The term density is general and can have differentmeanings. Accordingly, when density is reported, it isnecessary to specify which type of density is being report-ed. This includes the following:

Crystallographic density: The ideal density of a specificcrystal structure calculated from chemical compositionand interatomic distances. Other terms include specificweight, true density and X-ray density. Crystallographicdensity, ρc, is calculated as follows:

ρc = 4M/NV (2.2)

where, M is the gram formula weight of the material(M=40.09715 for SiC), N is Avogadro’s constant(6.0221367x1023/mole), and V the volume of the unit cell.

Theoretical density: The density of a material as if thereis no microstructural porosity. This is also known as theapparent density, ρa, and is calculated as follows:

ρa = m/Vs (2.3)

where, m is the mass of the material, Vs is the volumeoccupied by the solids.

Bulk density: The measured mass contained in the totalbulk volume of the material. Bulk density ρb is calculatedas follows:

ρb = m/(Vs+Vp) (2.4)

where, Vp is the pore volume.

According to published data, theoretical density of SiClies in the range from 3.166 to 3.24878 g/cm3 dependingon the polytype. Most of these measurements wereobtained using XRD data as described above.

Porosity, which is occasionally reported along withdensity, is another important physical property used toindicate the amount of free space, i.e. not occupied bysolid material. Porosity in general, open or closed, is verydetrimental to the strength of the material, which isinversely exponentially proportional to the total porosity.Open porosity reduces the oxidation resistance of thenon-oxide materials by allowing oxygen gas diffusion. Inaddition, a material with open porosity presents outgasingproblems under high vacuum conditions. Therefore, it isvery important to accurately measure total porosity anddetermine what percentage is open porosity.

2.2.2 Test Methods

Theoretical (or apparent) density is measured usingthe pycnometer method. In this technique, the test sam-ple is pulverized until the particles are so small that noclosed porosity is present. The powder is placed in aknown-volume pycnometer bottle and weighed. A liquid isadded and the pycnometer bottle is heated to remove airbubbles trapped between particles. The bottle is thenweighed again and the difference in weight gives the vol-ume of the liquid. The volume of the solids is then calcu-lated by subtracting the volume of the liquid from the vol-ume of the bottle. The theoretical density is then calculat-ed as the weight of the solids divided by their volume.

Bulk density is calculated as described above from theweight, m, and bulk volume, V, measurements, where ρb= m/V. In many cases, however, the sample dimensionscannot be accurately measured. Accordingly, for complexshapes, the bulk volume is measured using the water dis-placement method based on the Archimedes’ principle. Inthis method, the bulk volume of the specimen is calculat-ed as the difference between the weight after water satu-ration and the weight suspended in water. To ensure allopen pores are filled with water, the specimens are boiledin di-ionized (DI) water for several hours, during which thevolume of the entrapped air expands about 27% underatmospheric pressure reducing its density. This processmakes it easier for the air to escape through the water.When the water cools down, a vacuum is created in thepores and, subsequently, water is sucked in because ofthe pressure difference. This method has been adoptedby the American Society for Testing and Materials (ASTM)and was labeled as the ASTM C-373 standard testmethod. This method can also be used for the calculationof the apparent density if no closed porosity is present.

In addition to density, porosity is also measured usingthe ASTM C-373 method by dividing the pore volume bythe total (bulk) volume of the specimen. The pore volumeis calculated by subtracting the weight of the dry speci-men from its weight after being saturated with water,which is the volume of the water intruded into the openpores. Accordingly, the calculated porosity does notaccount for any closed porosity. Mercury porosimetry isanother technique used to measure open porosity incases where pore size is extremely small or the liquiddoes not wet the material. In these cases, high pressureis used to force a known volume of mercury inside thepores, hence, measuring the open pore volume. Mercuryporosimetry can also provide information regarding poresize distribution and pore surface area. The porosimetrytechnique is based on the assumption that all poresresemble the cylindrical shape model. Mercury is intrud-ed into the open pores of the specimen under pressure.



7

Mercury was selected due to its high non-wetting charac-teristic. The relationship between the applied pressure, P,and pore diameter, d, is given by the Washburn equationbelow:

P = (4γ cos θ)/d (2.5)

where, γ is the surface tension of mercury at test temper-ature, and θ is the contact angle of mercury with the solidmaterial being characterized.

2.2.3 Density and Porosity of POCO SiC

The most common method for the density and poros-ity measurements is the ASTM C-373 as describedabove. The following results are considered typical for theSUPERSiC-1 material as determined by this method:

Average bulk density (ρb): 2.55 g/cm3

Average apparent density (ρa): 3.13 g/cm3

Average open porosity: 19%

The measured apparent density is somewhat lowerthan the theoretical density for SiC indicating the pres-ence of some closed porosity. The closed porosity wascalculated using the theoretical density of SiC, i.e. 3.21g/cm3, and the result is shown below:

Average closed porosity: 2.5%

Similarly, the total porosity was also calculated and theresult follows:

Average total porosity: 20.5%

2.3 Chemical Purity

2.3.1 General

There are different techniques and processes for thesynthesis of different forms of SiC. For example, the mostcommon process for the production of SiC powder, is theAcheson process, which primarily uses a mixture of sandand coke. The purity of the produced SiC largely dependson that of the raw materials. In addition, the subsequentcrushing and grinding post processes to reduce particlesize usually introduce additional contaminants, dependingon the type of the grinding media. Purifying SiC powderis possible only if the particle size is small enough for thepurifying chemicals to come in contact with the contami-nants. High-purity SiC powders are commercially avail-able only in the submicron size range. Hot pressing, hotisostatic pressing and pressureless sintering are the mostcommon processes for the densification of most materi-als. However, for covalently bonded materials such as sil-icon carbide, it is not possible to achieve sintering withoutthe addition of sintering aids, such as an oxide mixture

(Al2O3, ZrO2, and Y2O3), for liquid-phase sintering or acombination of boron and carbon for solid-state sintering.Unfortunately, these effective sintering aids are notacceptable in many applications where purity is of greatimportance. Because of these constraints, it is not cost-effective, if possible at all, to produce SiC with purity in thedesired ppm range using these conventional methods.

POCO SiC is produced by a process that allows themanufacturing of near net-shape SiC parts without theneed for SiC powder synthesis and processing. The pre-machined graphite part is subsequently purified to below5ppm and converted to SiC using a silicon-carryingspecies without altering the dimensions. In addition, theSiC conversion process is carried out in extremely tightconditions of purity requirements, including furnace fix-tures, raw materials and gas atmosphere, producing aSiC product of high purity.

2.3.2 Test Methods

There are several methods used for purity measure-ment of materials depending on the desired level of puri-ty. A common technique for determining impurity levels ofa solid material in the ppm range is the glow dischargemass spectroscopy (GDMS) method. This analysis tech-nique is actually based on two techniques: the glow dis-charge source and the mass spectrometry. The glow dis-charge sources have principally been used as sources foroptical emission spectroscopy. The joining of the twotechniques made it possible to obtain elemental analysiswith high sensitivity by direct sampling of the ions. Theuse of a high-resolution analyzer is essential for generaltrace analysis below a few ppm.

Another highly sensitive technique is the inductivelycoupled plasma mass spectrometry (ICP-MS) technique.Laser ablation is used to generate a sample that can beintroduced to the ICP module. This is done by focusingthe laser beam on the material to be analyzed and vapor-izing a small spot into a stream of argon gas, which car-ries the vaporized specimen into the ICP module. Fromthe ICP module the ions enter the MS module where anion beam is generated and directed to a quadrupole massanalyzer, where they are detected based on theirmass/charge ratio and quantified using appropriate stan-dards. The LA-ICP-MS system can easily detect andmeasure trace elements in the ppb range.

2.3.3 Chemical Purity of POCO SiC

Elemental analysis of all SiC specimens is conductedroutinely using the GDMS technique. Typical elementalanalysis data of SUPERSiC-1, is shown in Table 2.1.

It can be seen that most elements are either non-exis-tent or exist below the detection limit of the GDMS equip-



8

ment demonstrating the extremely high purity ofSUPERSiC-1. Only a few elements, such as Al (3.8 ppm)and Ni (2.0 ppm), are present at relatively high concen-trations. This is most likely due to their presence in theraw materials used in the conversion process. It is impor-tant to note that this is typical purity data of the bulkSUPERSiC-1 material.

2.4 Oxidation Resistance

2.4.1 General

In general, SiC has excellent oxidation resistance upto 1650°C. Oxidation resistance, however, depends large-ly on the amount of open porosity and particle size, whichdetermine the surface area exposed to oxygen. The high-er the surface area the higher the oxidation rate.Kinetically, SiC is stable in air up to ~1000°C. In the1000°-1150°C range, surface oxidation is thermodynami-cally favored and results in the kinetically rapid formationof a thin film of silica (SiO2). Above 1150°C, the SiO2 film

thickens and densifies resulting in slower oxygen diffusionand thus, slower oxidation rate, hence a passive oxidationmechanism prevails. Factors affecting the stability andintegrity of the SiO2 layer as a protective film include thepresence of impurities in the material, which tend todecrease the viscosity of the SiO2 film, therefore increas-ing the mass mobility of the oxidizing agents. Above1650°C, interfacial reactions between the SiO2 film andthe SiC substrate become apparent, resulting in the for-mation of the volatile SiO and gaseous CO species.These gaseous products tend to rupture the SiO2 film,opening new channels for more oxygen to diffuse in, andoxidation continues. In this case, the oxidation mecha-nism is said to change from passive mode, where theSiO2 film is dense and stable, to active mode, where theSiO2 layer is porous and weak. The presence of watervapor in the oxidizing atmosphere also causes the SiO2film to be more porous, thus increasing the oxidation rateof SiC materials. In general, the oxidation rate of SiC isslowest in dry air and increases in higher partial pressuresof O2, CO2 and H2O gases.

Table 2.1 Typical Elemental Analysis Data (in ppm) of SUPERSiC-1 Material Obtained Using GDMS Analysis.

Element ContentAg <0.01Al 3.8As <0.05Au <0.05B 0.08Ba 0.03Be <0.005Bi <0.01Br <0.05C BulkCa 0.90Cd <0.1Ce 0.01Cl 0.24Co <0.01Cr <0.05Cs <0.005Cu <0.05Dy <0.005Er <0.005Eu <0.005F <1Fe 0.37Ga <0.05Gd <0.005Ge <0.05

Element ContentHf 0.06Hg <0.1Ho <0.005I <0.01In 0.03Ir <0.01K 0.08La 0.02Li <0.01Lu <0.005Mg <0.01Mn <0.01Mo 0.09N N/ANa <0.01Nb <0.01Nd <0.005Ni 2.00O N/AOs <0.01P <0.01Pb <0.05Pd <0.01Pr <0.005Pt <0.01Rb <0.05

Element ContentRe <0.01Rh <0.005Ru <0.01S 0.10Sb <0.05Sc <0.05Se <0.05Si Bulk

Sm <0.005Sn <0.05Sr <0.01Ta <5Tb <0.005Te <0.05Th 0.02Ti 0.25Tl <0.01

Tm <0.005U 0.01V 0.48W 0.15Y 0.05Yb <0.005Zn <0.05Zr 0.91

Total 9.68 ppm



9

2.4.2 Test Methods

Oxidation resistance of a material is measured by howmuch oxide is formed when the material is exposed to anoxidizing agent under isothermal conditions. In mostcases, dry oxygen or air is used in the test. However, theoxidation conditions can differ depending on the intendedapplication or purpose of the test. In any case, when oxi-dation resistance is reported for a particular material, thetest conditions must also be revealed. Generally, theweight gain (W) of the specimen versus oxidation time atconstant temperature and pressure is used as an indica-tion of the oxidation resistance of that material. To deter-mine the oxidation kinetics or rate constant, a thermo-gravimetric apparatus (TGA) is normally used. The rela-tionship between the sample weight gain per unit areaand the oxidation time (t) is usually a parabolic one and isusually reported as the square of the weight gain per unitarea (e.g. g2/m4) against time to yield a linear relationship.Accordingly, this parabolic behavior for the oxidation ofmost materials can be represented by the following equa-tion:

W2 = Kp ⋅ t (2.6)

where Kp is the parabolic rate constant. Using theArrhenius equation shown below, the activation energy(Ea) for the oxidation reaction can be calculated as fol-lows:

Kp = A exp (-Ea/RT) (2.7)

where A is the Arrhenius pre-exponential factor, R the gasconstant, and T the temperature.

2.4.3 Oxidation Resistance of POCO SiC

Oxidation resistance of POCO SUPERSiC was meas-ured in dry air at 1200°C for different periods of time up to30 hours. The test was performed in a muffle furnace withflowing dry air passed through a bed of drierite (anhy-drous CaSO4) to remove any moisture. The use of a muf-fle furnace, however, may have presented uncertainties incontrolling the oxidation atmosphere. Accordingly, the oxi-dation results presented here represent a worse casethan would have been obtained in a better-controlled envi-ronment. In addition, the preferred method for measuringthe oxidation resistance of a material is the continuousmonitoring of the sample’s weight while being oxidizedusing a thermogravimetric apparatus (TGA), which allowsthe measurements of the oxidation rate as well as thekinetics parameters.

Figure 2.2 shows the oxidation results at 1200°C forSUPERSiC-1 grade as weight gain per surface area of thesample versus the oxidation time up to 30 hours using themuffle furnace. The surface area of SUPERSiC-1 wasmeasured using the Brunauer, Emmett and Teller (BET)technique. As expected, the weight gain per unit areaversus oxidation time curve approximate the classical par-abolic behavior. Due to the 18-20% open porosity ofSUPERSiC-1, the material exhibits a relatively highweight gain compared to other high density SiC grades,such as CVD or hot-pressed.

Figure 2.2 Oxidation Behavior of SUPERSiC-1 Material Showing Weight Gain per Unit Area as a Function ofOxidation Time at 1200˚C in Dry Air.



11

3.1 Flexural Strength

3.1.1 General

The flexural strength is defined as a measure of theultimate strength of a specified beam in bending. Thebeam is subjected to a load at a steady rate until rupturetakes place. If the material is ductile, like most metals andalloys, the material bends prior to failure. On the otherhand, if the material is brittle, such as ceramics andgraphite, there would be a very slight bending followed bya catastrophic failure. There are two standard tests todetermine the flexural strength of materials: the four-pointtest and the three-point test. In the four-point test, thespecimen is symmetrically loaded at two locations that aresituated one quarter of the overall span between two sup-port spans. In the three-point test, the load is applied atthe middle of the specimen between two support bear-ings.

3.1.2 Test Methods

The most commonly used test method for measuringthe flexural strength of advanced ceramics at room tem-perature is the ASTM C-1161. At elevated temperatures,however, ASTM C-1211 is used. Three-point and four-point configurations are both acceptable in these standardmethods, with the difference being in the calculationmethod. POCO uses the three-point test for routinemeasurements of flexural strength of all SUPERSiCgrades at room temperature. High temperature measure-ments are carried out using the four-point bend test at the HighTemperature Materials Laboratory(HTML) facilities at Oak RidgeNational Laboratories (ORNL).

3.1.3 Flexural Strength of POCO SiC

Room temperature flexuralstrength measurements were con-

ducted in accordance with the ASTM C-1161 methodusing the three-point configuration with specimens meas-uring 4"x1/2"x1/4". The results are shown in Table 3.1 withan average of 23.0 ksi for the flexural strength ofSUPERSiC.

Additional room-temperature as well as high-tempera-ture, flexural strength measurements were carried out atORNL’s HTML facilities. Specimens measuring 50mm x4mm x 3mm were tested using the four-point bend test atroom temperature (ASTM C-1161) and at high tempera-tures (ASTM C-1211). An average of 21.3 ksi wasobtained for the flexural strength of forty specimens test-ed at room temperature, as shown in Table 3.2. Agreeableresults were obtained between the three-point and thefour-point measurements (see Table 3.1). High tempera-ture flexural strength was measured at 1000°, 1300° and1500°C, the results of which are also shown by Table 3.2.As can be seen, there is no effect of temperature on theflexural strength up to 1500°C.

Chapter 3

Mechanical Properties of Silicon Carbide

Table 3.1 Flexural Strength of SUPERSiC-1 andSUPERSiC-2 Grades Measured at Room Temperature

Using the Three-Point Bend Test.

Grade SUPERSiC-1 SUPERSiC-2Average (MPa/ksi) 159/23.0 186/27.0 Minimum (MPa/ksi) 117/17.0 125/18.2Maximum (MPa/ksi) 173/25.1 223/32.4Std. Deviation (MPa/ksi) 13.8/2.0 23.4/3.4

Table 3.2 Flexural Strength of SUPERSiC-1 Measured at DifferentTemperatures Using the Four-Point Bend Test at ORNL/HTML.

Temperature Ambient 1000°C 1300°C 1500°CAverage (MPa/ksi) 147/21.3 146/21.2 148/21.5 149/21.7Minimum (MPa/ksi) 122/17.7 133/19.2 132/19.1 125/18.1Maximum (MPa/ksi) 163/23.7 164/23.8 177/25.6 168/24.3Std. Deviation(MPa/ksi) 9.6/1.4 8.8/1.3 8.8/1.3 14.7/2.1Weibull Modulus 17.1 16.0 18.6 10.0



12

Weibull analysis was also performed for the testresults obtained at room temperature and at 1300°C, asshown in Figure 3.1. High Weibull modulii (Table 3.2)were obtained indicating excellent material uniformity.

3.2 Tensile Strength

3.2.1 General

The tensile strength of a material is defined as themaximum tensile stress sustained by the material when apulling force is applied along the length of the specimen.The tensile strength is calculated from the maximum loadduring a tension test carried to rupture and the originalcross-sectional area of the specimen. The purpose of thistest is to determine the material’s capability of load bear-ing in structural applications and other applications wherea high degree of resistance to wear and corrosion isrequired. Although flexural test methods are more com-monly used to evaluate the strength of advanced ceram-ics, the non-uniform stress distribution of the flexure spec-imen limits the volume of the material subjected to themaximum applied stress at failure. Uniaxially loaded ten-sile strength tests provide information on strength-limitingflaws from a greater volume of uniformly stressed materi-al. As in most test methods of brittle materials, samplepreparation is extremely important. Brittle materials failcatastrophically due to crack origination followed by arapid propagation. To compensate for this behavior, a sta-tistically significant number of specimens needs to betested to be able to perform statistical analysis, such asthe calculation of Weibull parameters.

3.2.2 Test Methods

As described above, the common method for measur-ing tensile strength of a material is the tensile test asdescribed in the ASTM C-1273 standard method. Thistechnique also allows for the measurement of sampleelongation using an extensiometer; from which the elasticmodulus of the material can then be calculated. The“Alumina Ceramic Manufacturers Association” for highalumina ceramic developed another method for the tensilestrength measurements, labeled as the ACMA Test No. 4.In this test method, the samples are disk-like rather thana dog-bone shape. In addition, a compressive load isapplied to the sample rather than tensile.

3.2.3 Tensile Strength of POCO SiC

A total of forty specimens of SUPERSiC-1 materialwere tested at the ORNL/HTML facilities at room temper-ature according to the ASTM C-1273 method. The testdata was analyzed using the Weibull statistics and theresults plotted, as shown in Figure 3.2. Very uniform testresults were obtained for the tensile strength with an aver-age of 129 MPa (19 ksi) and a standard deviation of 9.1MPa (1.3 ksi). In addition, a Weibull modulus of 15.8 wasobtained, indicating a good uniformity in the tensilestrength values.

Previous measurements of the tensile strength ofSUPERSiC-1 were performed using the ACMA testmethod No. 4 as described above. An average value forthe tensile strength of 15.7±0.7 ksi was obtained forSUPERSiC-1 using this test method.

Figure 3.1 Weibull Plots for the Flexural Strength of SUPERSiC-1 at Different Temperatures.



13

In addition to the tensile strength measurements, dur-ing the tensile test the elastic modulus (E) was also meas-ured using an extensiometer. An average value of 248GPa (36 msi) was obtained the SUPERSiC-1 grade with a

standard deviation of 42.1 GPa (6.1 msi). The elasticmodulus data, however, was somewhat less uniform, asindicated by the standard deviation.

Figure 3.2 Weibull Plot for the Room Temperature Measurement of the Tensile Strength of SUPERSiC-1 Grade.



15

Chapter 4

Thermal Properties of Silicon Carbide4.1 Thermal Conductivity

4.1.1 General

Thermal conductivity (κ) is the rate of heat flowthrough a material and is usually reported in SI units, i.e.W/m⋅K. The amount of heat transfer is controlled by theamount of thermal energy present, the nature of the heatcarrier in the material, and the amount of dissipation.Thermal energy is a function of the volumetric heat capac-ity: the carriers are electrons or phonons. The amount ofdissipation is a function of scattering effects and can bethought of in terms of attenuation distance for the latticewaves or the mean free path.

Due to its high thermal conductivity, silicon carbide isa very attractive material for high temperature applica-tions. From the device design point of view, the thermalconductivity of SiC exceeds that of Cu, BeO, Al2O3, andAlN. The thermal conductivity of SiC single crystal hasbeen reported as high as 500 W/m⋅K. However, mostcommercial SiC grades have thermal conductivity in therange 50-120 W/m⋅K. The high thermal conductivity ofother commercial SiC products, such as POCO’sSUPERSiC, is attributed to the absence of thermal-con-duction-inhibiting impurities on the crystal grain bound-aries. Basically, SUPERSiC is a continuous phase of SiCwith no obvious grain boundaries. This morphology is dueto the nature of the process through which SUPERSiC ismanufactured as described in Chapter 1.

4.1.2 Test Methods

Thermal conductivity can be determined using one oftwo methods. The first is known as the direct approach orthe steady-state rate measurement of the heat flow perunit area over a given temperature gradient. The secondapproach is known as the transient or indirect approach.In this approach, thermal diffusivity of the material is firstmeasured using the temperature change data of the spec-imen; thermal conductivity is calculated using the mater-ial’s bulk density and its heat capacity (Cp). For ceramics,both thermal diffusivity and heat capacity are strong func-tions of temperature. Thermal diffusivity (α) is measuredusing the laser flash technique (ASTM E-1461) method,and the thermal conductivity (κ) is then calculated usingthe following equation:

κ = α ⋅ ρ ⋅ Cp (4.1)

where κ is in W/cm⋅K, α is in cm2/sec, ρ is the bulk densi-ty (g/cm3) and Cp is in J/g⋅K.

The heat capacity of a material can be measuredusing either the calorimetry technique or, for high-puritymaterials, it can be calculated from published data as afunction of temperature (see O. Kubaschewski and C.B.Alcock, “Metallurgical Thermochemistry”, 5th Edition,Pergamon Press, New York, 1979).

For SiC, heat capacity is calculated as follows:

Cp, SiC=1.267+0.049x10-3 T– 1.227x10+5 T-2 + 0.205x10+8 T–3

(4.2)

where T is the absolute temperature.

4.1.3 Thermal Conductivity of POCO SiC

Thermal conductivity of SUPERSiC was determinedindirectly from measured thermal diffusivity and publisheddata for the heat capacity of SiC. Thermal diffusivity wasmeasured using the laser flash method at POCO. Figure4.1 shows the thermal conductivity as a function of tem-perature.

Heat capacity measurements were also conducted in-house using the differential scanning calorimetry (DSC)technique up to 1400°C. A comparison between themeasured data and that obtained from literature (Equation4.2) is shown in Figure 4.2, in which the measured data iswithin 3% of that calculated.

4.2 Thermal Expansion

4.2.1 General

Thermal expansion is a general term used to describethe change in dimensions that occurs with most materialsas temperature changes. Thermal expansion data is nor-mally reported as the linear coefficient of thermal expan-sion (CTE), α, and is defined as follows:

α = (∆e/e0)/∆T (4.3)

where, e0 is the original length of the specimen at aknown reference temperature, ∆e is the change in lengthas temperature changes by ∆T. Thermal expansion isoften reported as single data points over a given temper-ature range. However, a preferred way for presentingthermal expansion data is as a plot of ∆e/e0 and/or α ver-sus temperature. Both quantities are calculated to a ref-erence point, usually room temperature or 20°C, unlessotherwise mentioned. The quantity ∆e/e0 is dimension-less and is usually presented as percentage or µm/m,whichever is convenient. The coefficient of thermal expan-sion, α, on the other hand has a dimension of C-1 or K-1



16

and is reported as µm/m⋅K (same as µm/m⋅C), or simplyppm/K.

For ceramics, the thermal expansion depends on typeand strength of the bond, i.e. ionic or covalent. As the per-cent covalent bond and the bond strength increases, thethermal expansion decreases. SiC is covalently bondedand, therefore, has a very low thermal expansion relativeto other ceramics. In addition, for materials with cubicstructure, the thermal expansion is uniform in all threedirections, i.e. the material is said to be isotropic. An inter-esting correlation that has been observed for most close-packed metal and ceramic structures is that the product ofthe linear thermal expansion, αL, and the melting point,MP, is always a constant depending on the crystal struc-

ture. That is, the αL⋅MP product for cubic structures isabout 0.016 and that for rectilinear structures is about0.027. This is because the melting point of these materi-als itself is a function of the type and strength of the bond.For SiC, however, it is difficult to apply this relationshipbecause SiC does not melt under ambient pressure,rather, it dissociates when heated above 2700°C.

4.2.2 Test Methods

Thermal expansion is measured over a range of tem-perature using the dilatometry technique. There are sev-eral ASTM standard methods for measuring the linearthermal expansion which were developed for different

Figure 4.2 Heat Capacity Measurements of SUPERSiC-1 Compared to Calculated Data for Pure SiC(Equation 4.2.)

Figure 4.1 Thermal Conductivity of SUPERSiC-1 as a Function of Temperature Calculated from ThermalDiffusivity Measurements (Equations 4.1 and 4.2.)

Temperature (K)

The

rmal

Con

duct

ivit

y (W

/m. K

)



17

materials. The most suitable procedure for use withceramics is the ASTM E-228, which is for linear thermalexpansion measurement of solid materials using a vitre-ous silica dilatometer up to 900˚C. The method is alsovalid to higher temperatures using laboratory apparatusthat are equipped with high temperature ceramic orgraphite tubes.

4.2.3 Thermal Expansion of POCO SiC

Thermal expansion data of SUPERSiC-1 has beenobtained from different sources including POCO,ORNL/HTML, and other independent laboratories. Fourthorder polynomials, representing the average of all datacompiled, were generated for thermal expansion (∆l/lo)and the mean CTE (α), and are shown in Figures 4.3 and4.4, respectively. Using the polynomial equations givenon the plots, thermal expansion and CTE can be calculat-ed over any temperature range (in °C) from room temper-ature (20°C) to 1050°C.

4.3 Thermal Shock Resistance

4.3.1 General

When a material is unevenly heated (or cooled) suchas in fast heating (or cooling), a temperature gradient isgenerated between the surface and the core of the mate-

rial or among different points on its surfaces. These ther-mal gradients may lead to the formation of micro-cracks,which will eventually propagate and lead to fracture andfailure. This failure is known as the thermal shock. Thepeak (or critical) value of the thermal stress, sth, can beestimated using the material’s measured physical, ther-mal and mechanical properties and is expressed as fol-lows:

σth = α⋅E⋅∆T/(1-ν) (4.5)

where, α is the CTE, E is the elastic modulus, ∆T is tem-perature gradient and ν is Poisson’s ratio. Accordingly, tominimize σth, the material must have a low CTE and a highthermal conductivity.

Hasselman defined thermal stress resistance param-eters for conditions needed for crack initiation as follows:

R = σ (1-ν)/α⋅E (°C) (4.6)

where σ is the strength (flexural or tensile). Published Rfor SiC is about 230°C (see “Modern CeramicEngineering” by David W. Richerson, p. 364).

Figure 4.3 Fourth Order Polynomial Representing the Thermal Expansion of SUPERSiC-1 as a Function ofTemperature with the Room Temperature (20°C) as the Reference Point



18

4.3.2 Test MethodsThermal shock resistance of a material is evaluated by

measuring the critical temperature drop at which microc-racks will initiate. The presence of microcracks is indicat-ed by the drop in the strength of the material. Test speci-mens, such as flexural or tensile strength, are heatedslowly and uniformly to different temperatures (Ti) andthen the temperatures are suddenly dropped, normally byquenching in water at room temperature (Tr). Other mediacan be used, such as liquid nitrogen depending on theapplication. Flexural strength of the quenched specimensis then measured and plotted against the temperature dif-ference (Ti - Tr).

4.3.3 Thermal Shock Resistance of POCO SiC

The “R” parameter for SUPERSiC was calculated atabout 285°C, which is considered very good for SiC.Richerson (see above) reported 230°C for the “R” param-eter for SiC without specifying the type of SiC material.

The “R” value of SUPERSiC was calculated using itsmeasured properties; i.e. a flexural strength, σ, of 21,300psi, a Poisson’s ratio, ν, of 0.17; a coefficient of thermalexpansion, α, of about 2.0x10-6/°C at room temperatureand an elastic modulus, E, of 31x106 psi. The good “R”value for SUPERSiC is clearly due to its low CTE value atlow temperatures as described above.

Measurements of the thermal shock of SUPERSiChave been conducted in house. Flexural strength sam-ples (1/4" x 1/2" x 4") were heated slowly to different tem-peratures in a muffle furnace and quenched in a waterbath at room temperature. The samples were then testedfor their flexural strength and the results were plotted ver-sus the quench temperature difference as shown byFigure 4.5. The critical temperature drop above whichmicrocracks start forming in the SUPERSiC-1 is shown tobe in the vicinity of 275°C. This is an excellent agreementbetween the measured and calculated values of the criti-cal temperature.

Figure 4.4 Fourth Order Polynomial Representing the Mean Coefficient of Thermal Expansion of SUPERSiC-1as a Function of Temperature with the Room Temperature (20°C) as the Reference Point.

CTE = -5.926E-12 T4 + 1.701E-08 T3 - 1.824E-05 T2 + 9.791E-03 T + 1.735



19

Figure 4.5 Thermal Shock Resistance of SUPERSiC-1 Determined by the Water Quench Method.

poco graphite, inc. properties and characteristics of ... · pdf fileproperties and...

Documents