Preparation and Microstructural Analysis of High ... and Microstructural Analysis of High-Performance Ceramics ... and a basically brittle nature. ... rial should be either very hard

Preparation and MicrostructuralAnalysis of High-Performance CeramicsUlrike Taffner, Veronika Carle, and Ute Schafer, Max-Planck-Institut fur Metallforschung, Stuttgart, GermanyMichael J. Hoffmann, Institut fur Keramik im Maschinenbau, Universitat Karlsruhe, Germany

Fig. 2 Light micrograph of a cubic stabilized zirconia(CSZ) with 12 mol% Y2O3, thermally etched in

air at 1300 �C (2730 �F). The large cubic grains show inter-and intragranular porosity.

Fig. 1 Tetragonal zirconia polycrystals (TZP) with 2mol% Y2O3, thermally etched in air at 1300 C

(2730 �F). The scanning electron micrograph shows a fine-grained microstructure. Pores appear black.

IN CONTRAST TO METALS, high-perfor-mance ceramics have higher hardness, lowerductility, and a basically brittle nature. Othergeneral properties to note are: excellent high-temperature performance, good wear resistanceand thermal insulation (low thermal conductiv-ity), as well as high resistance to corrosion andoxidation. However, the full advantage that thesematerials can provide is strongly dependent oncomposition and microstructure.

Most high-performance ceramics are based onhigh-purity oxides, nitrides, carbides, and bo-rides with carefully controlled compositions. Ce-ramic engineering components are usually pro-duced by powder metallurgical methods. Therequired properties of a specific part are opti-mized by selecting parameters associated withthe powder mixture and the pressing and sinter-ing operations to obtain the desired microstruc-ture.

High-performance ceramics can be dividedinto two main categories; structural and func-tional ceramics. While optimization of structuralceramics is directed toward improved mechani-cal properties, the performance of componentsproduced from functional ceramics is controlledby electrical, magnetic, dielectric, or opticalproperties. Therefore, restrictions with respect tomechanical properties can be tolerated. Typicalstructural ceramics are aluminum oxide (Al2O3),zirconium dioxide (ZrO2), silicon nitride(Si3N4), and silicon carbide (SiC). However,Al2O3-, ZrO2- and SiC-based ceramics are alsooften used as functional ceramics. Other func-tional ceramics of technological interest are bar-ium titanate (BaTiO3) and lead zirconate titanate(Pb(Ti,Zr)O3).

Due to the large variations in microstructure,different ceramographic preparation techniquesare applied to achieve the surface quality desiredso structural details are revealed under the mi-croscope. For ceramographic preparation, it isnot sufficient to know that a sample is Al2O3.The manufacturing conditions must also beknown because they provide important infor-mation regarding expected porosity, grain size,

and impurities. These microstructural variableshave a strong influence on the method selectedfor preparation. An example for two differentZrO2 ceramic materials is illustrated in Fig. 1 and2. Figure 1 shows the microstructure of tetrag-onal ZrO2 (TZP, or tetragonal zirconia polycrys-tals). This is a high-strength structural ceramicused for room-temperature applications (e.g.,knives and scissors). Tetragonal zirconia poly-crystals have a grain size less than 1 lm, an ex-tremely high bending strength ranging from 800to 2400 MPa (115 to 350 ksi), and fracturetoughness (KIc) between 6 and 15 MPa (5.5m�and 15.5 ksi .), which renders this materialin�resistant to pullout during preparation.

The microstructure of cubic ZrO2 (CSZ, or cu-bic stabilized zirconia) is shown in Fig. 2. Themechanical properties of this material are con-siderably poorer than TZP, with a bendingstrength of 200 MPa (29 ksi) and a fracturetoughness of 2 to 3 MPa (1.8 to 2.7 ksi ).m in� �The microstructure is characterized by a high in-tragranular porosity and a grain size of approx-imately 30 to 50 lm. These materials are verysensitive to mechanical shock. Applications ofTZP and CSZ are focused on their high ion con-ductivity (e.g., mobility of O2 ions across the

cubic ZrO2 lattice). Cubic stabilized zirconia isalso used in as k-sensors for automobile catalyticconverters and for p(O2) measurement in liquidmetals.

Because of these differences in mechanicalproperties and microstructure, the ceramo-graphic preparation of TZP and CSZ is quite dif-ferent. The tough, fine-grained TZP requireslonger polishing times for the fine-polishing stepwith 1 and 0.25 lm diamond, while CSZ needslonger polishing times for the coarser polishingwith 6 and 3 lm diamond compounds.

Depending on the type of ceramic or ceramiccomponent, the mechanical properties (e.g., frac-ture toughness and strength) may vary consid-erably, and therefore the ceramographic prepa-ration procedures have to be adjustedaccordingly.

Specimen Preparation

Similarly to metallographic preparation, se-quential steps have to be performed to prepareceramics for microstructural investigations (Ref1–3). Careful selection of sectioning, mounting,grinding, polishing, and etching procedures is re-

© 2004 ASM International. All Rights Reserved.ASM Handbook Volume 9: Metallography and Microstructures (#06044G)

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1058 / Metallography and Microstructures of Ceramics, Composite-Metal Forms, and Special-Purpose Alloys

quired, and each step must be optimized for eachtype of ceramic. However, due to the brittleness,porosity, and chemical resistance of ceramics itis quite often difficult to polish them in the sameway as metals. Automated sample preparation isrecommended. The capability to adjust polishingpressure and the use of special grinding diskswith diamond as the abrasive material is alsopreferred. With this equipment, a flat surface thatdisplays an undistorted “true” microstructuremay be prepared in a reasonable time.

Sectioning. Generally, ceramics are cut witha lubricated (water or a special cutting fluid), ro-tating diamond cutting wheel on a bench-typelab machine or on a precision cutting machine.The cutting speed (low-speed cutting machine:25 to 500 rpm; high-speed cutting machine: 500to 5000 rpm) and the cutting pressure should beoptimized for the properties of a given material.A slow cutting speed and low pressure produceless cutting and surface damage for most ceram-ics, although some ceramic materials require theopposite (e.g., TZP-ZrO2). Diamond cuttingwheels are either metal bonded or resin bonded,and normally metal-bonded cutting wheels areselected. However, for very brittle and sensitiveceramics, resin-bonded cutting wheels are rec-ommended. These wheels are softer and willgenerally produce a better cut-surface finish thana metal-bonded diamond wheel, but their weakerbond shortens service life. Additional criteria forselection of cutting wheels also include: the con-centration of abrasive, the grain size of abrasive,and the wheel thickness. Generally, low abrasiveconcentration blades are used, because the lowerthe abrasive loading, the higher the contact stresson each abrasive particle and therefore the higherthe cutting rate. The grain size of a diamondblade is usually 94 lm, but for delicate samples,it may be advisable to cut with a diamond bladeof a finer grit to avoid unnecessary damage tothe material. The thickness of the diamond cut-ting wheel is dependent on the thickness of thesample. For samples with limited dimensions itis advisable to use a thinner wheel.

Mounting. For automatic sample preparation,specimens can be mounted or may be glued di-rectly onto a sample holder. The two possibilitiesfor metallographic mounting are hot mounting,with compression and heat, and cold mounting.It should be noted that sensitive, small, and verybrittle ceramics specimen are susceptible to dam-age and cracking when using hot mounting, be-cause of the high pressure and temperature thatis needed for this process. The mounting mate-rial should be either very hard or have good abra-sion resistance. Additionally, before the sampleis mounted consideration must be given to theetching technique that is to be used. This willguide the selection of a mounting material mostsuited to the complete sample preparation. Forexample, when using thermal etching techniquesor molten-salt etching techniques the sampleshould be removed from the mount before etch-ing. When etching in a boiling chemical solu-tion, the mount should be of a material that willnot be attacked by the acid mixture.

Mechanical Preparation (Grinding andPolishing). It is preferable to perform the grind-ing and polishing procedures with an automaticor semiautomatic machine. The structure of eachand every ceramic product has been specificallyadjusted to exhibit required properties, and thuseach material will exhibit a unique behavior dur-ing preparation. Table 1 contains preparationstandards for structural ceramics (e.g., Si3N4 andAl2O3) as well as AlN, and Table 2 provides thepreparation standards for functional ceramics(e.g., BaTiO3 and PZT). These tables should beused as a rough guide only; the parameters willneed to be adjusted according to the preparationrequirements of specific ceramics.

In general, resin-bonded diamond disks areemployed for grinding. In individual cases, sili-con carbide paper is used. For example, this typeof paper would be selected for the functional ce-ramics.

The surface damage generated during section-ing and grinding has to be removed during finegrinding, or lapping and polishing. Fine grindingand/or lapping retains the plane of the specimensurface, and no further damage is introduced.Complete removal of the damaged surface musttherefore be achieved by subsequent polishingsteps. Polishing should be performed on hardcloths. The highest removal rates will occur dur-ing steps with the application of 6 and 3 lm di-amond grain size. Polishing with 1 lm diamondremoves a minimal amount of material. Allbreakouts and scratches should be removed dur-ing these steps. In general, fine polishing with aSiO2 suspension is only applied if small and finescratches have to be removed. In some cases, thisfinal step can also produce a slight relief on thesample surface, which may be beneficial for mi-croscopy.

Microscopic Examination

For the investigation of ceramic microstruc-tures and the identification of flaws and defects,the use of light optical microscopy (LOM) orscanning electron microscopy (SEM) are mostcommon. Since most of the ceramics are electri-cal insulators, samples for SEM investigationshave to be coated by an electrical conductivelayer such as carbon, gold, or gold-palladium al-loys. Metals are used for simple microstructuralanalysis, while carbon is used for simultaneouschemical analysis, for example, energy-disper-sive x-ray (EDX) analysis. Standard scanningelectron microscopes are normally equippedwith different detectors. The backscatter detectoris useful for multiphase materials, when the dif-ferent phases reveal a strong mass contrast. Inthis case, no etching is required. Secondary elec-tron detectors are sensitive to small differencesin height of a polished and etched surface. Themicroscopic examination of ceramic specimensin the as-polished state has proved useful. Anevaluation of the number of pores, their distri-bution, and possible pullouts can only be as-sessed in this state. Evaluation of inclusions,contamination, and cracks should also be madebefore etching.

In order to reveal grain boundaries, phases,and other microstructural details, ceramic spec-imens have to be etched. Ceramic microstruc-tures, examined under the LOM (�200�) willshow a low contrast and a milky image. Ceram-ics permit light rays to penetrate the surfacewhere scattering and internal reflection occurs.To eliminate light scattering and to improve thereflectivity, coating the surface with a reflectivelayer is recommended. Such a layer should be

Table 1 Standard preparation conditions for structural ceramics (e.g., Si3N4, AlN, SiC andAl2O3), semiautomatic preparation

Process BaseDiamondgrade, lm Lubricant

Speed,rpm

Pressure(a) Time,minN lbf

Grinding Diamond disk 65 Water 300 �180 �40 Until flatDiamond disk 20 Water 300 �180 �40 5–10

Fine grinding Composite disk 6 Alcohol-based 300 �100 �23 5–15Polishing Pellon cloth 6 Alcohol-based 150 �150 �34 15–120

Hard synthetic cloth 3 Alcohol-based 150 �120 �27 15–120Hard silk cloth 1 Alcohol-based 150 �90 �20 5–10

Fine polishing Short napped, fiber cloth Silicon dioxide suspension 150 �50 �11 0.5–5

This table should be used as a rough guide only, and parameters will need to be adjusted according to the preparation requirements of specific ceramicsamples. (a) Pressure specification for a sample holder with six 25 mm (1 in.) diam specimens

Table 2 Standard preparation conditions for functional ceramics (e.g., BaTiO3, PZT, andZnO), semiautomatic preparation

Process BaseDiamondgrade, lm Lubricant

Speed,rpm

Pressure(a) Time,minN lbf

Grinding Silicon carbide paper 320 Water 300 �150 �34 Until flatLapping Hard synthetic cloth 6 Lubricant 150 �100 �23 2–15Polishing Hard synthetic cloth 3 Lubricant 150 �120 �27 5–30

Hard silk cloth 1 Lubricant 150 �90 �20 5–15Fine polishing Short napped, fiber cloth Silicon dioxide suspension 150 �50 �11 0.5–1

This table should be used as a rough guide only, and parameters will need to be adjusted according to the preparation requirements of specific ceramicsamples. (a) Pressure specification for a sample holder with six 25 mm (1 in) diam specimens



Preparation and Microstructural Analysis of High-Performance Ceramics / 1059

between 5 and 10 nm thick and can be appliedby sputter coating with either gold or aluminum.

Thin-Section Polarized Light Microscopy.In general, all nonmetallic materials are suitablefor examination by transmission polarized lightmicroscopy, including the high-performance ce-ramics (Ref 4). The only limitations are the grainsize of the material and its mechanical behavior.Many ceramics have a very small grain size (�1lm), which makes LOM examination impossi-ble. Because resolution depends on the wave-length of light, the smallest particle size that canbe observed is 1 lm. In order for a thin sectionto be mechanically stable it needs to be about 5to 30 lm thick. At this thickness, however, thegrains in fine-grained materials lie over one an-other, and this multioverlayering results in for-mation of a diffuse image. This can cause diffi-culty in differentiating between certain opticaleffects. It is for this reason that a grain size of atleast 5 lm is preferred.

The preparation of thin sections requires theuse of precision cutting, grinding, and polishingmachines. It should be remembered that severalsquare centimeters of specimen must be removedby cutting, grinding, and polishing to achieve aspecimen of uniform 5 to 30 lm thickness witha smooth, polished surface on either side, free ofsurface breakouts.

In the first stage of preparation, the specimensare treated in the same way as polished sectionsfor incident light microscopy. The ceramic is cutand mounted and then ground and polished usinga semiautomatic grinding and polishing ma-chine. A roughened glass slide is then bonded tothe polished surface of the specimen with a dropof epoxy resin (Fig. 3). To prevent the formationof pores and air bubbles between the specimenand the glass slide, this operation should be car-ried out in a vacuum infiltration chamber. Oncethe specimen has fully hardened, which gener-

ally takes about two days, the specimen is slicedoff to a thickness of about 500 lm. Thereafter,the specimen is ground down to a thickness ofabout 80 to 100 lm using a diamond grindingdisk. Because there is always a danger of deeppullouts and cracking, which then have to beeliminated by the following stages of prepara-tion, the specimen should not be ground anythinner than 80 lm. Depending on the materialbeing prepared, the specimen is either manuallyor semiautomatically fine ground and then pol-ished. As the thin section nears the end of itspreparation, it must be handled extremely care-fully and must be repeatedly checked with themicroscope under polarized light. When the tar-get thickness is achieved—that is, when all thegrains, depending on their double refraction, ex-hibit the colors gray, yellow-brown, or blue-green—a thin cover glass is bonded to the sur-face of the specimen for protection, usingthinned epoxy resin.

Ceramographic Etching

Figure 4 presents a review of ceramographicetching methods (Ref 1, 5). Three etching meth-ods can be distinguished: optical, electrochemi-cal, and physical contrasting. The most fre-quently used etching methods forhigh-performance ceramics are solution etching,thermal etching, and plasma etching. Plasmaetching works only in case of silicon-base ce-ramics (SiC, Si3N4). Details of the etching pro-cedure are described below. Thermal etching ismostly used for oxide ceramics. Typical etchingtemperatures are 150 �C (270 �F) below sinteringtemperature in air. Etching time can vary be-tween 15 min and some hours, depending on thestructure and composition of the grain boundarythat evaporates during the heat treatment. Ther-mal etching of nonoxide ceramics requires aninert atmosphere or an encapsulation of the sam-ple in a quartz tube. Due to the chemical resis-tance of many ceramics, solution etching of pol-ished specimens requires the application of very

aggressive chemicals at elevated temperatures,or the application of molten-salt reagents. Forthe etchants given in Table 3, only general detailscan be given in regard to the concentration andetching time required for a given sample. Thedesired level of etching on a sample must be de-termined by trial and error.

Oxide Ceramics. Apart from etching withvery aggressive chemical solutions, thermaletching in air is also an important method fordeveloping the microstructures of Al2O3 andZrO2 ceramics (Ref 6, 7). The polished and “de-mounted” samples are placed in an air furnace.Exposure temperature and time are selected sothat the heat treatment does not alter the micro-structure. The temperature should be approxi-mately 100 to 150 �C (180 to 270 �F) below thesintering temperature. Temperatures that are toohigh or etching times that are too long may resultin grain growth.

Generally, chemical solutions are used to de-velop microstructures, grain boundaries, and thedomain structure of other oxide ceramics such asBaTiO3 and PZT (see Table 3). Thermal etchingin air can also be used to show the grain bound-aries, but it will not develop the domain struc-ture. If the grain size exceeds 5 lm, the micro-structure of the ceramic material can also berevealed with a thin section under polarizedlight. An example is shown in Fig. 19 forBaTiO3.

Nitride Ceramics. When Si3N4 is preparedwith solutions containing concentrated phos-phoric acid or molten NaOH, the intergranulargrain-boundary phase between the Si3N4 grainsis dissolved away by chemical attack (Fig. 5–7)(Ref 8). One disadvantage of these two etchantsis their poor reproducibility. This is because theoptimal etching time is extremely dependent onboth the microstructure and the composition ofthe glass phase, which changes with the com-position of the additives used.

In contrast to solution etching for Si3N4 ma-terials, plasma etching attacks the ceramic-ma-trix grains and not the grain-boundary phase(Fig. 8 and 9) (Ref 9). Plasma etching is carried

Fig. 4 Etching techniques used in ceramographyFig. 3 Ceramographic specimen preparation sequence

from top to bottom for a thin section




out inside a special chamber, which provides agaseous mixture of CF4 and O2. A radio fre-quency generator emits high-frequency electro-magnetic oscillations and thus produces fluorineradicals inside the reaction chamber. These rad-icals then react with the silicon contained in thematrix of the ceramic (Si3N4). The intergranulargrain-boundary phase remains unattacked by theplasma etching process. The etchants for AlN,given in Table 3, are proven for chemical solu-tion etching and give reproducible results (Ref10).

Carbide Ceramics. Several suitable etchantsfor different SiC materials exist (Table 3). Mol-ten salt, thermal etching, hot chemical etchants,and a number of Murakami solutions with dif-ferent modifications reveal the microstructure byattacking the grain-boundary phases (Ref 11).For SiC with 5 to 15% oxide additions, theplasma etching technique (see above for Si3N4)can be successfully applied. In this case the SiCgrains are attacked, leaving the grain-boundaryphase untouched.

Classification and Microstructureof Frequently Used Ceramics

Details of the microstructure of commonstructural and functional ceramics along withtypical applications follows.

Aluminum oxide (Al2O3) is the most com-mon ceramic and, depending on the adjustedproperties, is either used as a structural or a func-tional ceramic material (Ref 12). Applications ofhigh-temperature-resistant alumina are cruci-bles, tubes, and rods for furnaces. Sealing disks,pumps, thread guides, and other specializeditems are made of alumina, due to its excellentwear and corrosion resistance. It is used as anelectrical insulator in spark plugs and, taking ad-vantage also of its high thermal conductivity, assubstrate material for integrated circuits (Ref13).

Fully dense and coarse-grained alumina be-comes optically transparent and is used as thelamp tube for high-temperature sodium vaporstreet lamps. It is also biocompatible; therefore,many prosthetic devices are either made fromalumina or contain alumina components.

The densification of alumina powder com-pacts is achieved by a sintering process, whichcontrols density and grain size (Ref 12). Theseparameters determine the strength characteristicsof the final product. Figure 10 shows a cross sec-tion of a fluorescent tube with isotropicallygrown grains with 15 lm average grain size anda porosity of less than 0.5%. Figure 11 illustratesa microstructure characterized by anisotropicallygrown grains with an average size of 2 to 4 lm.The resulting texture is caused by a preferredorientation of alumina seed crystals during in-jection molding. The microstructure of a ceramicsealing disk with a typical residual porosity of 3to 4%, a bimodal grain size distribution, and aglassy phase at the grain boundaries (liquid-

Table 3 Etchants for ceramics

Etchant composition Conditions Comments Uses

Oxide Ceramics

Thermal etch in air 15–20 min at 1300–1400 �C(2370–2550 �F) (Etchtemperature is approximately150 �C (300 �F) belowsintering temperature.)

Grain-boundary etchant,also small grains

Al2O3, pure and withadditives

H3PO4 (85%) 5 s to 3 min at 250 �C (480 �F) Grain boundaries are notuniformly revealedSpinel and glass phasesare dissolved.

Al2O3, pure and withadditives

100 mL alumina10 mL Murakami’s

solution

10–15 min etch polishing on ahard fiber synthetic cloth

Identification of the spinalphase, glass phase andsecondary phases

Al2O3 with additives

Thermal etch in air 10 min at 1460 �C (2660 �F) Grain-boundary etchant Na b-Al2O3

HBF4 (35%) 1–2 min at 110 �C (230 �F) Grain-boundary etchant Na b-Al2O3

10 mL glycerol (87%) 1 min Grain-boundary etchant Al2TiO5

10 mL HNO3 (65%)10 mL HF (40%)95 mL distilled H2O 5 s to 2 min Grain-boundary etchant BaTiO3

3 mL HCl (32)2 mL HF (40%)

Thermal etch in air 30–60 min at 1200 �C (2190 �F) Grain-boundary etchant BaTiO3

95 mL distilled H2O 5 s to 2 min Grain-boundary etchant PZT (PbZrTiO)4 mL HCl (32%)1 mL HF (40%)100 mL distilled H2O 10 s to 5 min Grain-boundary etchant SrTiO3

25 mL HF (40%)20 mL distilled H2O 5 s to 5 min Grain-boundary etchant SrTiO3, MgTiO3

20 mL HNO3 (65%)10 mL HF (40%)20 mL distilled H2O Seconds to minutes Reveals twins ZnO1 mL glacial acetic acid100 mL distilled H2O Seconds to minutes Grain-boundary etchant ZnO10 g NaOH10 g K3Fe(CN)6

Thermal etch in air Minutes to 1 h at 1300–1400 �C(2370–2550 �F) (according tosintering temperature)

Grain-boundary etchant ZrO2-TZP, -PSZ, -CSZ,-ZTC

H3PO4 (85%) 3 s to 2 min at 250 �C (480 �F) Grain-boundary etchant ZrO2-TZP, -PSZ, -CSZ,-ZTC

Nitride Ceramics

50 mL distilled H2O 1–3 min at 100 �C (212 �F) Secondary phase remain AlN-La2O3

50 mL saturated aqueousNaOH

50 mL cold saturatedaqueous picric acid

10 mL distilled H2O 40–60 min at 100 �C (212 �F) Grain-boundary etchant AlN-Y2O3

10 mL glacial acetic acid10 mL HNO3

50 mL distilled H2O 30 min at 100 �C (212 �F) Grain-boundary etchant AlN-Al2O3

10 mL 10% aqueousNaOH solution

10 mL 10% aqueouspotassium ferricyanidesolution

(Modified Murakami’setchant)

Plasma etching 1–5 min, 60–80 W Secondary phase remain Si3N4

CF4 gas and O2 gas (2:1)H3PO4 (85%) 5–30 min at 250 �C (480 �F) Grain-boundary etchant Si3N4

Molten NaOH (free of water) 20 s to 3 min at 300–350 �C(570–660 �F), 2–3 samples permelt

Grain-boundary etchant Si3N4

Carbide Ceramics

Plasma etchingCF4 gas and O2 gas (1:1)

3–5 min, 60–80 W Secondary phase remain SiC with 5–15% oxideAdditions

60 mL distilled H2O 5–30 min at 110 �C (230 �F) Alpha/alpha (�/�) grainboundaries. Alpha/beta(�/b) phase boundaries

SSiC

30 g K3Fe(CN)6

3 g NaOH20 mL distilled H2O 5–10 min Grain-boundary etchant SSiC doped with B10 g NaOH10 g K3Fe(CN)6

(continued)



phase sintering) is given in Fig. 12. Figure 13shows the microstructure of a spark plug withanisotropic grains, a glassy grain-boundaryphase, and large pores. The effect of differentetching techniques is demonstrated in Fig. 14and 15. After thermal etching of the magnesium-doped sample at 1500 �C (2730 �F) in air,

equiaxed grains with black gaps along the grainboundaries can be seen. The gaps are pores andvoids formed by an evaporated spinel phase(MgAl2O4) (Fig. 14). The microstructure shownin Fig. 15 is from the same material, but chem-ically etched in phosphoric acid at 270 �C (520

�F). The spinel phase is dissolved, and the grainboundaries look broader.

Zirconium dioxide (ZrO2) based ceramicsbelong to those materials that are characterizedby having both good mechanical properties andexceptional electrical properties (Ref 14, 15).The thermomechanical and electrical propertieshave led to a wide range of applications. Tough,wear-resistant, and refractory ZrO2 is being de-veloped for applications as extrusion dies, ma-chinery wear parts, and piston heads. Zirconiumdioxide is a good ion conductor and, as a solidelectrolyte, is used in oxygen sensors and insolid oxide fuel cells (SOFC). Zirconium dioxideis also used for furnace elements. The low ther-mal conductivity of ZrO2 makes it important asa thermal barrier coating in aerospace enginecomponents and land-based gas turbines. Thedifferent kinds of application require differentproperties that are realized by four basic types ofmicrostructure: cubic stabilized zirconia (CSZ),partially stabilized zirconia (PSZ), tetragonal zir-conia polycrystals (TZP), and zirconia tough-ened ceramics (ZTC).

The CSZ microstructure is a solid solution ofZrO2 and a stabilizing oxide such as Y2O3, MgO,or CaO. These additives maintain the cubicstructure down to room temperature. Cubic sta-


Fig. 5 Scanning electron micrograph of a sintered Si3N4ceramic (SSN) after etching in phosphoric acid

at 250 �C (480 �F) for 30 min. The intergranular glassyphase between the Si3N4 grains has been dissolved by theetchant.

Fig. 7 Scanning electron micrograph of hot isostaticallypressed Si3N4 (HIPSN) etched in molten NaOH

at 320 �C (610 �F) for 2 min. The intergranular glassy phasebetween the Si3N4 grains has been attacked by the etchant.It is not possible to identify intergranular porosity using thisetching technique.

Fig. 9 Scanning electron micrograph of hot isostaticallypressed Si3N4 (HIPSN) after plasma etching.

HIPSN ceramics show more equiaxed grains and less grain-boundary phase compared to GPSN (see Fig. 8).

Table 3 (continued)

Etchant composition Conditions Comments Uses

Carbide Ceramics (continued)

80 mL distilled H2O 10–20 min Grain-boundary etchant SSiC doped with Al10 g NaOH10 g K3Fe(CN)6

90 g KOH 5–10 min at 480�C Beta/beta (b/b) grainboundaries

SSiC

10 g KNO3 Melt Preheat the samples.100 mL distilled H2O 30–40 min at 60 �C Grain-boundary etchant SiC with B4C10 mL HNO3 (65%)10 g K3Fe(CN)6

100 mL distilled H2O Seconds to minutes, boiling Grain-boundary etchant B doped SiC10 mL HNO3 (65%)10 g K3Fe(CN)6

60 mL distilled H2O 8–15 min Use boiling Grain-boundary etchant SiSiC3 g NaOH30 g K3Fe(CN)6

Fig. 6 Scanning electron micrograph of a hot-pressedSi3N4 ceramic (HPSN) etched for 30 min. in

phosphoric acid at 250 �C (480 �F). The volume fraction ofthe intergranular grain boundary phase is lower in com-parison to SSN (Fig. 5).

Fig. 8 Scanning electron micrograph of a plasmaetched gas-pressure sintered Si3N4 (GPSN) ce-

ramic with typical acicular Si3N4 crystallites. The micro-structure shows gray Si3N4 matrix grains and a lighter grain-boundary phase.

Fig. 10 Light micrograph of translucent alumina. Iso-tropically grown grains with 15 lm average

grain size and a porosity of less than 0.5%. Thermallyetched at 1500 �C (2730 �F)




bilized zirconia is manufactured from powdermixtures or prealloyed powders and homoge-nized completely during sintering in the homo-geneous cubic region at temperature between1600 and 1800 �C (2910 and 3270 �F). The mi-crostructure of stabilized ZrO2 consists of cubicgrains together with an intergranular glassyphase (Fig. 2). As a result of the high processingtemperatures, a comparatively large grain size inthe range 10 to 150 lm is formed. The inter-granular glassy phase contains various impuri-ties originating from the manufacturing processand the raw materials, but these are mainly SiO2.

Partially stabilized zirconia is a ceramic with5 to 15 mol% of stabilizer additives (Y2O3,MgO, CaO). It is sintered in the cubic homoge-neity range. The PSZ microstructural develop-ment requires a special program of sintering. Ini-tially, the ceramic is compacted at hightemperatures (�1700 �C) in the cubic solid-so-lution region. A coarse microstructure with cubicgrains and an intergranular glassy phase forms.By heat treating in the two-phase field (tetrago-

nal and cubic), small lenticular particles of thetetragonal phase are precipitated. These particlesare not resolved in the LOM. They are coherentwith the cubic {100} planes. The precipitates areshown in the electron microscope image insertin Fig. 16.

Tetragonal zirconia polycrystals contains only2 to 3 mol% Y2O3 (or 10 to 20 mol% CeO2) asstabilizer and crystallizes in a metastable tetrag-onal form. However, due to the high activationenergy of the transformation, the tetragonalstructure is retained down to room temperatureand can be activated for a strain-induced trans-formation to the stable monoclinic structure. Thetetragonal and monoclinic crystallites (grains)both have the same morphology and cannot bedistinguished by LOM or SEM, but require x-ray diffraction. An example of the TZP micro-structure, taken in the SEM, is shown in Fig. 1.

Zirconia toughened ceramics consist of a ce-ramic matrix (e.g. Al2O3) in which 3 to 15 vol%of ZrO2 particles are embedded (Fig. 17). Thedifference between the thermal expansion coef-

ficients of the matrix and the ZrO2 particlescauses stresses during cooling, which is essentialto the stability of the dispersoid. The increase intoughness results from the strain-induced trans-formation and microcrack formation (Ref 16).

Piezoelectric Ceramics. BaTiO3 and PZT(Pb(Ti,Zr)O3) are the most widely used piezo-electric materials, in electronic devices. Theproperties can be varied over a wide range bydonor or acceptor dopants to achieve the re-quired properties for a given application. BaTiO3

is mainly used in capacitors because of its highdielectric strength as well as the ability to havea positive or negative temperature coefficient ofthe dielectric by dopant addition and processprocedures. Pb(Ti,Zr)O3 exhibits a high piezo-electric constant and is used for sensors and ac-tuators and for ultrasonic wave generation (Ref17, 18).

The starting powders with the correspondingdopants are usually prepared by a mixed-oxideprocess. The required oxides and/or carbonates

Fig. 11 Light micrograph of a dental prosthesis. Tex-tured alumina with anisotropically grown

grains, with an average size of 2–4 lm and a porosity of�0.5%. Thermally etched at 1500 �C (2730 �F)

Fig. 13 Spark plug alumina insulator. The light micro-graph shows the microstructure with aniso-

tropic grains, a glassy grain-boundary phase, and largepores. Etch polished with alumina and Murakami’s solution

Fig. 15 Light micrograph of the same sample shown inFig. 14, but chemically etched in phosphoric

acid at 270 �C (520 �F) for 3 min. The spinel phase is dis-solved, and grain boundaries look broader.

Fig. 12 Sealing disk of alumina. The light micrographshows typical residual porosity of 3–4%, a bi-

modal grain size distribution, and a glassy phase at thegrain boundaries (liquid-phase sintering). Thermallyetchedat 1400 �C (2550 �F)

Fig. 14 Light micrograph of magnesium-doped alu-mina after thermal etching at 1500 �C (2730 �F)

in air for 40 min. Equiaxed grains with black gaps alongthe grain boundaries can be seen. The gaps are pores andvoids formed by an evaporated spinel phase (MgAl2O4).

Fig. 16 Partially stabilized zirconia (PSZ) with 7 mol%Y2O3, thermally etched in air at 1300 �C (2730

�F). The light micrograph shows a coarse-grained micro-structure with pores (black dots). The transmission electronmicroscope image insert shows tetragonal precipitateswithin the cubic grains.



are first milled and subsequently calcined to ob-tain a single-phase powder. Consolidation of theprereacted powders is achieved by either press-ing or tape casting. Sintering is performed in airor in an oxygen atmosphere at a temperaturerange of 1000 to 1350 �C (1830 to 2460 �F). BothBaTiO3 and Pb(Ti,Zr)O3 crystallize above theCurie temperature in the cubic perovskite struc-ture and transform into a lower symmetry struc-ture during cooling. The phase transformation isaccompanied by lattice distortion, typicallyabout 1% for BaTiO3 and 1.0 to 2.5% forPb(Ti,Zr)O3, depending on the zirconium-to-ti-tanium ratio. The lattice distortion generates in-ternal stresses, which are reduced by the for-mation of domains.

The phase transformation from the paraelec-tric cubic phase to the tetragonal ferroelectricphase occurs for BaTiO3 at 130 �C (265 �F). Be-low 0 �C (32 �F) other phase transformations oc-cur, but they have no technological interest. Thedomain configuration consists of 90� and 180�domains based on the different possibilities forthe orientation of the spontaneous polarizationvector. The domain structures can be readily ob-

served by chemical etching of polished BaTiO3

ceramics samples (Fig. 18) or by using the thin-section technique (Fig. 19).

Paraelectric PZT is a solid solution betweenlead zirconate and lead titanate with a cubic crys-tal structure. Below the Curie temperature ittransforms into a ferroelectric tetragonal phasefor titanium-rich PZT and ferroelectric rhom-bohedral phase for zirconium-rich PZT. The Cu-rie temperature is not well defined and dependson the zirconium-to-titanium ratio and theamount of additives. The maximum piezoelec-tric constants and coupling factors are achievedfor compositions with approximately equal vol-ume fractions of tetragonal and rhombohedralPZT (morphotropic PZT). The domain structuresof these materials are more complex than inBaTiO3 because both crystallographic modifi-cations can coexist within one grain, and theyhave different possible directions for the spon-taneous polarization vector. Tetragonal PZTforms have only 90� and 180� domains, butrhombohedral PZT also has 71� and 109� do-

mains. A typical microstructure of a PZT ce-ramic is shown in Fig. 20. After chemical etch-ing, the grain boundaries, as well as the domainstructures within the grains, are revealed in thisSEM image.

Aluminum Nitride (AlN). High-purity AlNhas a high thermal conductivity of more than 200W/m • K together with a low thermal expansioncoefficient. These, and other useful properties,make AlN a very suitable material for applica-tion as a substrate in the electronics industry.Aluminum nitride has an hexagonal crystalstructure. Pressureless sintering is impossible forpure AlN powder, because of its covalent bond-ing. Densification is achieved by liquid-phasesintering with additives. Commonly used addi-tives are Y2O3 (Fig. 21), rare earth oxides (Fig.22), or fluorides such as CaF2 and YF3. The ther-mal conductivity of AlN is affected by the pres-ence of both secondary phases and impurities atthe grain boundaries.


Fig. 17 Scanning electron micrograph of a zirconiatoughened ceramic (ZTC), thermally etched in

air at 1300 �C (2730 �F). Lighter, tetragonal ZrO2 grains aredispersed in the Al2O3 matrix. The parallel arrays of facetswithin the grains are related to the ZTC crystallography.

Fig. 19 Transmission optical micrograph of a thin sec-tion of a BaTiO3 ceramic with crossed polars

and a 530 nm filter. The parallel stripes represent 90� do-main walls.

Fig. 18 Light micrograph of a chemically etched sur-face of a BaTiO3 ceramic showing the grains

and domain patterns within the grains

Fig. 20 Scanning electron micrograph of a lanthanum-doped PZT ceramic. The grain boundaries as

well as the domain structure are visible after chemical etch-ing.

Fig. 21 Scanning electron micrograph of an AlN ce-ramic, densified with 10 wt% Y2O3. The AlN

grains appear dark gray. Yttrium-aluminum-garnet parti-cles, formed by reaction between Y2O3 and Al2O3 in thestarting powder mixture, appear as the light gray phase.

Fig. 22 Scanning electron micrograph of an AlN ce-ramic, densified with 2.5 wt% La2O3. Etching

attack of AlN grains depends on crystallographic orienta-tion. A secondary phase (LaAlO3) is retained in the micro-structure and appears bright.




Silicon nitride (Si3N4) is a structural ceramicwith excellent mechanical properties, good oxi-dation resistance, and good thermal shock be-havior at ambient and high temperatures (Ref 19,20). Si3N4 ceramics can be processed by two dif-ferent methods: reaction-bonded silicon nitride(RBSN) and (pressureless) sintered silicon ni-tride (SSN). Other variants of the latter areknown by the following abbreviations, accord-ing to their method of manufacture: gas-pressuresintered, GPSN; hot-pressed, HPSN; and hotisostatically pressed silicon nitride, HIPSN.

Porous Si3N4 ceramics produced from inex-pensive raw materials are used as refractories.Dense materials with high strength in combina-tion with a high wear resistance are developedfor cutting tools to machine cast iron. The high-temperature-resistant Si3N4-based ceramics arebeing studied for applications in gas turbines en-gines and for components in motor vehicles(e.g., valves).

RBSN ceramics are made from a silicon pow-der compact, which is converted into a Si3N4 ce-

ramic by nitriding at temperatures between 1200and 1400 �C (2190 and 2550 �F) in the presenceof H2/N2 gas. Because iron is required as a cat-alyst, iron silicides can also be formed. They arevisible in the LOM as metallic inclusions. Al-though the chemical reaction is combined witha volume expansion, fully dense ceramics are notobtained. The porosity can vary between 5 and15%. Figure 23 shows a typical micrograph of aRBSN ceramic in the unetched condition.

All high-performance silicon nitride ceramiccomponents are densified by liquid-phase sinter-ing. The most common sintering additives areMgO, Al2O3, Y2O3, and RE2O3 (rare earth ox-ides) or mixtures of these materials. A typicalpolished surface of a sintered silicon nitride withsome residual porosity and a few silicide inclu-sions is shown in Fig. 24. The end use of theSi3N4 and the selected sintering conditions de-termine the amount and type of sintering addi-tives. After densification, the sintering additivesform an intergranular phase that significantly af-fects the properties of the material. Dependingon the sintering parameters, the material pro-duced is SSN, HPSN, GPSN, or HIPSN. Figures5 and 6 compare the microstructure of an SSNmaterial with that of an HPSN material afteretching in phosphoric acid. The different amountof sintering additives in the SSN and HPSN ma-terials becomes clearly visible when the grainboundaries are dissolved by chemical attack(large black areas between the grains). In con-trast, Fig. 8 and 9 compare the microstructure ofa GPSN material with that of an HIPSN materialafter plasma etching, where the Si3N4 matrixgrains are attacked, not the grain-boundaryphase. At higher magnification, the HIPSN mi-crostructure (Fig. 25) shows that the etching at-tack is also sensitive to the crystallographic ori-entation. Some Si3N4 grains are deeper etchedthan others. The same sample etched in moltenNaOH at 320 �C (610 �F) for 2 min is shown inFig. 7. A comparison of both etching techniquesreveals that molten NaOH attacks the grainboundary, whereas plasma etching attacks thematrix. After plasma etching, the grain bound-

aries remain intact and the evidence of residualporosity is not lost.

Silicon carbide (SiC) is the most widely usednonoxide ceramic material with superior high-temperature abrasion and corrosion resistance(Ref 21). Dense SiC ceramics exhibit a high ther-mal conductivity. This is due to its closelypacked wurtzite structure. The electrical prop-erties depend on the impurities and vary fromthe high electrical resistivity of pure silicon car-bide to an electrically conductive material whenthe impurity content increases. Silicon carbideceramics can be used under high mechanical andthermal stresses in corrosive atmospheres and inaggressive media. Important silicon carbideproducts in the area of chemical and mechanicalengineering are wear parts, such as sealing disks,and components for pumps. Other types of ap-plication include kiln supports, heat exchangers,or burner nozzles. Electrically conductive SiCceramics are used as heating elements for fur-naces for operation up to 1400 �C (2550 �F) inair.

Four types of SiC ceramics can be produced:recrystallized SiC (RSiC), silicon-infiltrated SiC(SiSiC), sintered SiC (SSiC), and liquid-phasesintered SiC (LPS-SiC). They have significantlydifferent manufacturing routes and therefore dif-ferent microstructures and properties. Recrystal-lized SiC is produced by a powder compact witha bimodal grain size distribution. During heattreatment, the fine-grained powder evaporatesand condenses at the points of contact with thelarge SiC particles. Because no densification oc-curs during heat treatment, a higher volume frac-tion of porosity remains, and this results in arelatively low strength. Figure 26 shows a typi-cal microstructure of an RSiC ceramic.

Silicon-infiltrated SiC is produced by an infil-tration of a SiC/C powder compact with liquidsilicon. Part of the silicon reacts with the carbonto form fine-grained secondary silicon carbide.A larger volume fraction of silicon remains un-reacted and reduces the oxidation and corrosionresistance of SiSiC, but increases the strength incomparison to RSiC. A typical microstructure of

Fig. 23 Light micrograph of a reaction-bonded Si3N4(RBSN). The unetched specimen contains iron

silicides (arrow) and characteristic high porosity (black).

Fig. 25 Scanning electron micrograph of hot isostati-cally pressed Si3N4 (HIPSN) after plasma etch-

ing, shown at a magnification sufficient to reveal micro-porosity.

Fig. 24 Light micrograph of an unetched sintered Si3N4(GPSN) specimen. The white inclusions are

iron silicide, and porosity is black. The porosity is muchlower compared to Fig. 23 (RBSN).

Fig. 26 Light micrograph of an RSiC ceramic. Gray ar-eas are �-grains on which b-SiC has recrystal-

lized, dark areas are pores, and white areas are silicon.Etchant: modified Murakami’s solution



the multiphase ceramic SiSiC with the primaryand secondary SiC and the unreacted silicon isshown Fig. 27.

Densification of SiC ceramics can be achievedby solid-state sintering with boron, B4C, or alu-

minum as sintering additives. The microstruc-tures can be designed over a wide range, whichenables components to withstand a variety of dif-ferent conditions. This is demonstrated forhighly loaded wear parts. The grain shape is in-fluenced by sintering aids; equiaxed grains canbe generated by the presence of boron (Fig. 28)or as plates in the presence of aluminum (Fig.29). Anisotropic grain growth results in a stronginterlocking of the grains and reduces the risk ofbreakout compared to a microstructure withglobular grains. Small grains increase strength,but large grains have proportionally fewer grainboundaries and the material becomes less sus-ceptible to grain-boundary corrosion. Smallgrains can be pulled out and ground into the wearsurface, which can lead to hydrodynamic grain-boundary corrosion. The presence of pores re-duces the strength and reliability of the compo-nent. However, these pores can also serve aslubricant reservoirs under certain wear and slid-

ing conditions and so improve the material prop-erties under abnormal stress conditions. A pre-determined porosity level can be introducedduring sintering, as shown in Fig. 29. The ma-terial behavior under abnormal stress can be fur-ther improved by introducing graphite as a sec-ondary phase (Fig. 30).

Silicon carbide powder compacts can also bedensified by liquid-phase sintering (LPS) in thepresence of oxides and nitrides (LPS-SiC). Themost common additives are similar to the siliconnitride ceramics: Y2O3, rare earth oxides, Al2O3,and AlN. After complete densification, most ofthe liquid phase forms an oxide-rich grain-boundary phase, which may be either amorphousor crystalline. This grain-boundary phase leadsto a weaker bonding between the grains com-pared to SSiC ceramics. The weaker interfaceresults in an increase in fracture toughness aswell as strength, but the strength at higher tem-perature as well as the creep resistance is re-duced. Figure 31 shows a microstructure of LPS-SiC after plasma etching. Two phases are visible:the bright, unattacked grain-boundary phase andthe SiC grains. Some of the SiC grains reveal adarker region within them. This is the remnantof the original SiC seed. The different etchingattack between the seed material and the rest ofthe grain is attributed to a small difference inchemical composition (i.e., solid-solution differ-ences between SiC and AlN).

ACKNOWLEDGMENT

The authors would like to thank Chris Bagnallfor the helpful discussions and the critical reviewof the manuscript.

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Fig. 29 Light micrograph of a pressureless-sinteredSSiC ceramic with a bimodal grain size distri-

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platelet microstructure consisting of �-SiC grains. The largedark spherical particles, approximately 50 lm in diameter,are graphite (arrow). The smaller dark spots within the SiCgrains are intragranular pores. Etchant: modified Murak-ami’s solution

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