Transcript
• • Properties, Processing, and
Use in Design
Second Edition, Revised and Expanded
Contents Prel." to the Second Edilion Preface to the First Edition Introd.dion Pad I
STRtlCfllRES AND PROPERTIES
1 2 3 4
II(
I
3
162
6
204
EI.drirl' Behayio[
PROCF,sSING OF CERAMICS
32
71 123
1S1 313
373
10 Shape·FonnlllR Processes
374 418
J1
519
9 Powder Processing
pad
xi
Atomic: Bonding, and Crystal Structure Crystal Chemistry and Speci6c Crystal Structures Phase EguUibria and Phue Equilibrium Diagrams PbysicaJ and }bennal Behavior 5 Mrtblola! Bcbuior and Measurement
1 Dieledrict Magnetic. and Optical Behavior 8 Time, Temperature, and Environmental Elred! on PropeJ1Jes Part II
• vii
D'nqfinlinn
12 Final Macbining
5%
13 Quality Assurance
6ZO
DESIGN WITH CERAMICS
649
14 DesI&n Considerations 15 Deslp Approaches
651
16 FaOure Analysis
680
17 TougbeDing of Ceramics 18 AppUalions: Material Selection
731
662
Glossary
808 833
EWed:ive Ionic Radii (or CalioD' aod AniOBS
843
periodic Table of the Elements
lode x
..
851
4
Chapter 1
The second shell has eight electrons. two in s orbitals and six in p orbitals. All have higher energy than the two electrons in the first shell and are in
orbitals farther from the nucleus. (For instance . the s orbitals of the second shell of lithium have a spherical probability distribution at about 3 A radius.) The p orbit als are not spherical. but have dumbbell-shaped probability distributions along the orthogonal axes, as shown in Fig. 1.1. These p electrons have sl ightly higher energy th an s electrons of the same shell and are in pairs with opposite spins alo ng each axis when the shell is full. The third quantum shell has d o rbitals in additio n to sand p orbitals. A full d orbital contains 10 electrons. The fourth an d fifth shells contain f orbitals in add ition to s. p. and d orbitals. A full f orbital contains 14 e lectrons. A simple notation is used to show the electron configurations within shells. to show the relative energy of the electrons, and thus to show the order in which the electrons can be added to or removed from an atom during bonding. This notation can best be illust rated by a few exa mples.
Example 1.1 Oxygen has eight e lectrons and has the electron notation Is'2s'2p'. Th e I and 2 preceding the sa nd p designate the qu antum shell. the sa nd p designate the subshe ll wi thin each quantum she ll . and the superscripts designate the total number of electrons in each subshell. For oxygen the Is and 2s subshells are both full . but the 2p subshell is two electrons short of being full.
Example 1.2 As the atomic number and the number of electrons increase . the energy difference between electrons and between shells decreases and overl<.lp between quantum groups occurs. For example . the 45 subshell of iron lills before the 3d subshe ll is full. This is shown in the electron notation by
Figure 1.1 E lectron probability distributions for p orbital s. The hi ghest probability electron positions are along the ort hogonal axes. Two electrons. each with opposite spin. are associated with each axis. resulting in a total of six p elect rons if all the p orbitals in th e shell are filled .
Chapter 3
90
.,
.
•
Figure 3.18 Transmission electron micrograph showing an example of liquid immiscibility. (Courtesy of D. Uhlmann. University of Arizona .)
Polymorphism Polymorphic transformations are also shown on phase equilibrium diagrams. Figure 3.20a is a schematic of a binary eutectic diagram with no solid solution and with three different polymorphs of the A composition.
The different polymorphs are usually designated by letters of the greek alphabet. Figure 3.20b is a schematic of a binary eutectic diagram with three A polymorphs. each with partial solid solution of B. Figure 3.21 illustrates a real binary system with polymorphs. Polymorphic transformations are also present in Fig. 3.19.
Three-Component Systems A three-component system is referred to as a tertiary sysfem. The addition of a third component increases the complexity of the system and of the phase equilibrium diagram. The phase rule becomes F = 3 - P + 2 = 5 - P. As with binary ceramic systems. diagrams are usually drawn with pressure as a constant (condensed system). The phase rule for the con-
174
Chapter 5
Figure 5.S Scanning electron photomicrographs of fracture surfaces of reactionbonded silicon nitride containing nearly spherical pores resulting from air entrapment during processing. Arrows outline flaw dimensions used to calculate fracture stress.
Chapter 5
178
)<", 1- -
-
~
Figure 5.7 Typical ceramic tensile test specimen configuration.
Another method of obtaining tensile strength of a ceramic material is known as the theta test [18]. The configuration is shown in Fig. 5.6c. Applicaton of a compressive load to the two arches produces a uniaxial tensile stress in the crossbeam. Very little testing has been conducted with this configuration owing largely to difficulty in specimen fabrication.
Compressive Strength Compressive strength is the crushing strength of a material, as shown in Fig. 5.6f. It is rarely measured for metals . but is commonly measured for ceramics. especially those that must support structural loads. such as refractory brick or building brick . Because the compressive strength of a ceramic material is usually much higher than the tensile strength, it is often beneficial to design a ceramic component so that it supports heavy loads in compression rather than tension . In fact. in some applications the ceramic material is prestressed in a state of compression to give it increased resistance to tensile loads that will be imposed during service. The residual compressive stresses must first be overcome by tensile stresses before additional tensile stress can build up to break the ceramic, Concrete prestressed with steel bars is one example . Safety glass is another example.
192
Chapter 5
Screw dislocation
Figure 5.16 Simple schematic ill ust rating a screw dislocation. (From Ref. 7. p. 92 . )
the struct ure is distorted and under localized stress even when the ove rall material is no t under an app lied stress. This residual stress state can be visua li zed by examining Fig. 5.17. The dislocation ]jne extends into the structure perpendicular to the su rface of the page. Note that the structure is distorted so as to fill in the space of the missing half-plane of atom s. This results in a state of residual tens ile stress just below the ext ra plane of ato ms ba lanced by compressive stress in the re gio n above the di slocation. The presence of the disloca ti ons and the associated residual st re ss allows slip to occur a lo ng atom planes at a fraction of the £ /20 value that
Zone of compressive stress ~ Zone of tensile stress
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Figure 5.17 Schematic of the residual st ress state showing compressive stress above the dislocation and tensile stress below the dislocation. (CI ASM In te rna· tional. )
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:g
Electrical Behavior
243
Figure 6.23 Example of the Meissner effect showing the levitation of a magnet at liquid nitrogen temperature by YBa!Cu.t01., ceramic superconductor. (Courtesy Ceramatec. Inc .)
The response of the superconductive material to the amount of current
being carried or to an applied magnetic field is also very important. Too high a current density or magnetic field can destroy the superconductive behavior. Each material has a different response.
Evolution of Superconductor Materials Figure 6.24 shows th e historical progression in discovery of superconductive materials with higher T,. Progress was extremely slow up to 1986, averaging
about 4 K per decade . Initial materials identified to be superconductive were elemental metals (Hg, Pb, Nb), followed primarily by solid solutions (NbTi) and intermetallics (Nb,Sn , V,Si , Nb,Ge). Until the early 1960's , relatively few materials had been identified with superconductive behavior. Superconductivity was thought to be an anomalous property . Since 1960, techniqu es have been avai lable to achieve temperatures closer to absolute
zero (on the order of 0.0002 K) and to simultaneously apply high pressure. Under these conditions many more elements, solid solutions, intermetallics, and ceramics have been demonstrated to have superconductivity. Several ceramic compositions were identified to be superconductive. These included tungsten, molybdenum, and rhenium "bronze" composi-
tions A,WO" A,MoO . . and A,RhO" where A was Na, K, Rb, Cs, NH" Ca, Sr, Ba, etc.; oxygen-deficient SrTiO J and LiTi0 3 ; and BaPb, _.Bi.O J .
Dieleclric. Magnelic, Optical Behavior
275
equal probability of shifting in six directions toward one of the corners of the octahedron. As a result. the tetragonal crystal contains some dipoles in one portion of the crystal pointing in one direction, whereas others in another portion may point in a direction 900 or 1800 away from the first. A region of the crystal in which the dipoles are aligned in a common direction is called a domain. An example of BaTiO.1 with a ferroelectric
domain with aligned dipoles is illustrated in Fig. 7.18. Le t us return now to Fig. 7.16 and describe what happens in a ferroelectric crystal such as tetragonal BaTiO, when an electric field is applied. The ferroelectric domains are randomly oriented prior to application of the electric field, that is, at E = 0, the net polarization equals zero (P,,, = 0).
As we apply an electric field and increase the electric field, the domains begin to move in the BaTiO.\ and align parallel to the applied field . This results in an increase in net polarization along line OA. The polarization reache s a saturation value (8) when all the domains are aligned in the direction of the field. If we now redu ce the electric field to zero, many of the domains will remain aligned such that a remanent polarization (P,) exists. Interpolation of the line 8e until it intersects the polarization axis gives a value PJ , which is referred to as the spontaneous polarization. If we now reverse the electric field, we force domains to begin to switch direction. When enough domains switch, the domains in one direction balance the domains in the opposite direction and result in zero net po-
Figure 7.18 TEM image of 180 ferroelectric domains in a single grain of BaTiO,. (Courtesy of W. E. Lee, University of Sheffield.) 0
Dielectric, Magnetic , Oplical Behavior
285
Another import ant wave-generation application is the sonic delay line. A delay line consists of a solid bar or rod of a sound-transmitting material
(glass , ceram ic, metal) with a transducer attached to each end . An electric signal that is to be delayed is input to the first transducer. The signal is converted to a sonic wave impulse that travels along the sound-transmitting "waveguide." The sonic impul se is then converted back to an electrical impulse by the second transducer. The delay results because a sonic wave travels much more slowly than electrons passing through a wire . The time
of delay is controlled by the length of the waveguide. Delay lines are used extensively in military electronics gear and in color te levision sets. One example is radar systems to compare informatio n from o ne echo with the
next echo and for range calibration. The wave-generation applications discussed so far involve acoustic
waves transmitted through bulk media. Additional freedom exists in the
Figure 7.25 Piezoelectric ceramics and assembli es for a variety of applicatio ns. (Courtesy EDO Corporat io n.)
324
Chapler 8
-------.
Figure 8.7
Hot-pressed
Si l N~
specimen deformed by creep under a load of 276
MPa (40.000 psi) at llOOOC (-2200' F) for 50 hr.
mechanisms available for crack growth. Crack growth is relatively easy if the grain boundaries of the material are coated with a glass phase. At high temperature, localized creep of this glass can occur, resulting in grain boundary sliding . Figure 8.8(a) shows the fracture surface of an NC-132
hot-pressed Si JN4 specimen that fractured after 2.2 min under a static bending load of 276 MPa (40,000 psi) al - llOO' C (- 2000' F). The initial flaw was probably a shallow (20 10 40 pm) machining crack. It linked up with cracks formed by grain boundary sliding and separation and pores formed by triple-point cavitation to produce the new Haw or structurally weakened region seen in Fig. 8.8 as the large semicircular area extending inward from the tensile surface. This was the effective flaw size at fracture.
Time, Temperature, Environmental Effects on Properties
325
Figure 8.8 Comparison of a slow crack growth fracture versus a normal bend fracture for hot-pressed Si.1N•. (From Ref. 9.)
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(d) (e) Figure 8.11 Surfaces of hot-pressed Si)N. before and afte r oxidation. (a) As_machined surface, 32O-gril di amond; (b) oxidized in ai r for 50 hr al 98O"C (l8OO"F); (c) oxidized in air fo r 24 hr at 12()(rC (22OO"F); and (d) oxidized in air for 24 hr at 137O"C (25OO"F). (C ASM International.)
348
Chapter 8
I.) ~.
Figure 8.23
Reaction-bonded
SilN~
after exposure in a combustion rig with 5 ppm sea salt addition for 25 cycles of
1.5 hr at 900°C, 0.5 hr at 1120°C, and a 5-min air-blast quench. (a) , (b), and (c) show the fracture surface at increasing magnification and illustrate the glassy buildup in the region of combustion gas impingement. (From Ref. 9.)
as fouling, A thin buildup can protect the surface from corrosion and erosion and in some cases can even result in a local temperature reduction. All three of these factors can increase the life of a component. especially a metal. However, a thick buildup reduces the airflow through the engine and decreases efficiency.
Fouling is an inherent problem in the direct burning of coal. A variety of approaches have been or arc being studied to resolve this problem:
L
Intermittent removal of buildup by thermal shock, melt-off, or passing abrasive material (such as nutshells) through the system
388
Chapter 9
Figure 9.4 Si.,NJ grinding media showi ng one of the common configurations. Spheres are also commonl y used. (Court esy Ke maNord.)
wear-resistant linings and have been used successfull y with dry millin g and with water as a milling fluid. However . some milling is cond ucted with organic fluids that may att ack ru bber or po lyurethane. Very hard grinding media can reduce contamination because they wear mo re slowly. we is good for some cases because its high hardness reduces wear a nd its high specific gravity minimizes millin g time. If contamination from the media is a n especially critical consideration, milling can be conducted with medi a made of th e sa me compos iti o n as the powder being milled. Another approach is to mill wit h steel media and remove the contamination by acid leaching. Milling can be conducted either dry o r wet. The advantages and disadva ntages are listed in Table 9.5. Dry milling has the advantage that the resulting powde r does not have to be separa ted from a liquid . The major concern in dry milling is that the powde r does not pack in th e corners of
402
Chapter 9
Figure 9.10 Transmission electron microscope image of ultra fine L
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Chapter 16
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701
Failure Analysis
•
Figure 16.12 SEM photomicrographs of abnormal fracture-initiating material flaws in RBSN traceable to improper processing prior to nit riding. (a) Large pore in slip-cast RBSN resulting from inadequate de-airing . (b) Crack in greenware prior to nitriding. (c) and (d) Low-density regions in slip-cast RBSN resulting from agglomerates in the slip.
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figure 16.13 SEM photomicrographs of abnormal fraclUre-in iliating material flaws in RBSN t raceable 10 the nilriding process.
(a) and (b) Porous aggregate rich in Cr and Fe, resulting from reaction of the silicon during the nilriding cycle with stainless steel contamination picked up d uri ng powder processing; energydispe rsive x-ray analysis shown. (e) Large aggregate of un reacted silicon resulting from localized melting due to local exothermic
overheating. Arrows identify the fracture origins.
704
Chapter 16
Failure Analysis
705
Figure 16.14 SEM photomicrographs comparing normal and abnormal material flaws in sintered SiC. (a) and (b) Typical microstructure of high-strength material. (c) Large pore resulting from powder agglomeration during powder preparation and shape forming. (d) Large grains resulting from improper control of temperature during sintering.
706
Chapter 16
Failure Analysis
707
II
Figure 16.15 SEM photomicrographs showing fractures initiating at transverse machining damage. (a) The fracture surface of a tensile specimen of hot·pressed silicon nitride that had been machined circumferentially. (b) The intersection of this fracture surface with the machined surface. illustrating that the fracture origin is parallel to the grinding grooves. (c) and (d) The same situation for reaction· sintered silicon nitride.
Fig. 16.16 SEM photomicrograph showing a typical featureless thermal-shock fracture surface. (top) Overall surface at low magnification. (bottom) Fracture origin at higher magnification. (Courtesy Garrett Turbine Engine Company, Phoenix. Ariz., Division of Allied-Signal Aerospace.)
Fig. 16.17 SEM photomicrograph of a thermal-shock fracture initiating at a material flaw. (a) Overall surface at low magnification. (b) Preexisting crack at fracture origin. (Courtesy Garrett Turbine Engine Company, Phoenix, Ariz., Division of Allied-Signal Aerospace.)
715
Failure Analysis
..........-
.• -
FRACTURE SURFACE
SPECIMEN SURFACE
Figure 16.18 Typical Hertzian cone crack resu lting from impact and acting as the flaw that resulted in fracture under subsequent bend load. Shown at increasing magnification from (a) to (c). (Courtesy Garrett Turbine Engine Company, Phoe· nix, Ariz., Division of Allied Signal Aerospace.)
716
Chapter 16
Figure 16.18 (Continued)
(Courtesy Garrett Turbine Engine Company, Phoenix,
Ariz., Division of Allied Signal Aerospace.)
case, the objective is to identify the mechanism of attack and find a solution. In other cases, especially where the oxidation or corrosion is isolated along
grain boundaries, the presence and source of degradation may be more difficult to detect. In this case, the degree of attack may only be determined by strength testing, and the cause may be ascertained by controlled environment exposures and lor sophisticated instruments such as Auger spectroscopy , which can detect slight chemical variations on a microstructural level. Let us first examine some examples of oxidation and corrosion in which
visible surface changes have occurred. Figure 16.22 shows the surface and fracture surface of NC-132 hot-pressed Si,N.' after exposure in a SiC resistance-heated, oxide-refractory-lined furnace for 24 hr at llOO°C (2012°F) [25J. Figure 16.22(a) shows the complete cross section of the test bar. The fracture origin is at the surface on the left side of the photo and is easily located by the hackle marks and the fracture mirror (the dark
·Manufactured by the Norton Company. Worcester. Mass.
Failure Analysis
717
Figure 16.19 Impact fracture of a ceramic rotor blade showing Hertzian cone crack. (Courtesy Garrett Turbine Engine Company, Phoenix, Ariz., Division of Allied-Signal Aerospace.)
Failure Analysis
719
Figure 16.20 (a) and (b) Surface cracks resulting from relative movement between two contact surfaces under a high normal load and with a high coefficient of friction.
720
Chapter 16
Figure 16.20 (Continued) (c) Typical multiple chipping resulting from contact loading and visible on a fracture surface.
furnace lining had contacted the specimen during exposure. The EDX analysis included in Fig. 16.23 was taken in the glassy region at the base of the pit, showing that AI, Si, K, Ca, and Fe were the primary elements present and again indicating a propensity for Si,N. to be corroded by alkali silicate compositions. However, it should be noted that the size of the pit is much smaller than in the prior example and resulted in only a small strength decrease. Figures 16.24 and 16.25 show examples of more dramatic corrosion of hot-pressed and reaction-bonded Si,N. [25], resulting from exposure to the exhaust gases of a combustor burning jet fuel and containing a 5-ppm addition of sea salt. Exposure consisted of 25 cycles of 899'C (1650'F) for 1.5 hr, 1121'F (2050'F) for 0.5 hr, and a 5-min air quench. At 899'C (1650'F), Na,SO. is present in liquid form and deposits along with other impurities on the ceramic surface. The EDX analyses taken in the glassy surface layer near its intersection with the SiJN .. document the presence of impurities such as Na. Mg, and K from the sea salt, S from the fuel, and Fe, Co, and Ni from the nozzle and combustor liner of the test rig. An EDX analysis for the Si,N. on the fracture surface about 20 j>m beneath the surface layer is also shown in Fig. 16.25. Only Si is detected (nitrogen
721
Failure Analysis
WITNESS MARK
-~ CONTAINING Co, Fe, Ni, Cr
_ _ FRACTURE SURFACE
Figure 16.21 (a) Witness mark on the surface of the ceramic adjacent to the fracture origin , suggesting fracture due to contact loading. (b) Multiple cone features resulting from a contact fracture.
722
Figure 16.21 (Continued)
Chapter 16
(c) Multiple cone features resulting from a contact
fracture.
and oxygen are outside the range of detection by EDX), indicating that the corrosion in this case resulted from the impurities in the gas stream plus the surface oxidation. The strength of the hot-pressed Si,N, exposed to the dynamic oxidation with sea salt additions decreased to an average of 490 MPa (71,000 psi) from a baseline of 669 MPa (97,000 psi). The reaction-bonded material decreased to 117 MPa (17,000 psi) from a baseline of 248 MPa (36,000 psi). Repeating the cycle with fresh specimens and no sea salt resulted in an increase to 690 PMa (100,000 psi) for the hot-pressed Si,N, and only a decrease to 207 MPa (30,000 psi) for the reaction-bonded Si,N •. The examples presented so far for oxidation and corrosion have had distinct features that help distinguish the cause of fracture from other mechanisms, such as impact or machining damage. Some corrosion-initi-
ated fractures are more subtle. The corrosion or oxidation may only follow the grain boundaries and be so thin that it is not visible on the fracture surface. Its effects may not even show up in room-temperature strength
testing since its degradation mechanism may only be active at high temperature. How do we recognize this type of corrosion? The following sug-
Failure Analysis
Figure 16.22 SEM photomicrographs of the fracture surface of hot·pressed ShNJ exposed to static oxidation for 24 hr at 1100°C (20l2°F). (a) Overall fracture surface showing hackle marks and fracture mirror (the irregular dark spots on the fracture surface are artifacts). (b) Higher magnification showing the fracture mirror with an oxidation corrosion pit at the origin. (c) Higher magnification showing the nature of the pit and the surface oxidation layer. Specimen size 0.64 x 0.32 cm. (From Ref. 14.)
•
OJl:ID1ZEO SURFACE
FRACTURE SUR fACE
Figure 16.23 SEM photomicrograph of the fracture-initiating oxidalion-corrosion pit on the surface of rcaction-bonded Si~ •. The EDX graph shows the relative concentralion of chemical elements in the glassy region at the base of the pi!. (Courtesy Garrett Turbine Engine Company, Phoenix, Ariz., Division of AUiedSignal Aerospace.)
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Co
N,
fOX Of SURFACE LAVER
Figure 16.24 SEM photomicrograph of hot-pressed SiJl"l. that was exposed to combustion gases with ~a sail additions. showing that fracture initiated at the base of the glassy surface buildup. EDX analysis shows the chemical elements detected in the glassy material adjacent to the Si,N•. (Courtesy Garrett Turbine Engine Company. Phoenix, Ariz., n;,,;~;n .. "f Al1 i f"rl_~;l>n:ll Aerosoace.)
" Co
Co
EOX OF SURfACE LAYER
EOX OF BASE R8SN
Figure 16.25 SEM photomicrograph of reaction-bonded SiJN. that was exposed to combustion gases with sea salt additions, showing that fract ure initiated at the base of the glassy surface buildup_ EDX analysis shows the chemical elements dete<:ted in the glassy material adjacent to the Si~ •. (Courtesy Garret! Turbine Engine Company, Phoenix, Ariz.• Division of Allied-Signal Aerospace.)
Chapter 16
728
• ••
/ .
•
Figure 16.26 SEM photomicrograph of the fracture surface of a low-purity Si.'N~ material sintered with MgO and showing slow crack growth. Region of slow crack growth identified by arrows.
equation, may not be good approximations for the material under slowcrack-growth conditions. There are other limitations to the information available from the fracture surface. The size of the slow-crack-growth region provides no infor-
mation about the time to failure, the rate of loading, or the mode of loading (cyclic versus static). 16.2 SUMMARY Fractography is a powerful tool to the engineer in helping to determine the cause of a component or system failure. Well-defined features usually present on the fracture surface of a ceramic provide the engineer with useful information regarding the place where fracture initiated, the cause
of fracture, the tensile stress at the point of failure , and the nature of the surrounding stress distribution. This information helps the engineer to determine if the failure was design- or material-initiated and provides direction in finding a solution. It can also help in achieving process or product
improvement. Finally, it can help determine legal liability for personal or property damage.
Toughening or Ceramics
737
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Figure 17.3 Optical pho tomicrograph or polished cross section showing unidirectional SiC filamenrs in a metal matrix. (Courtesy Textron Specialty Materials, Lowell, Mass.)
reasonable level of bond or friction between the fibers and matrix . Tooweak a bond can result in shear at the fiber-matrix interface and reduce the amount of modulus transfer. Prestressing Ceramics generally fracture in tension, i.e., in a crack-opening stress mode. Prestressing involves placing a portion of the ceramic under a residual compressive stress. A crack cannot start or extend as long as the ceramic is prestressed in compression . Tensile fracture will only occur after a large enough load is applied to exceed the compressive prestress and to build up a tensile stress large enough to initiate a crack at a critical flaw. A compressive prestress can be achieved by many approaches [4J . One approach is to place the surface in compression by quenching, ion exchange,
Toughening of Ceramics
745
Figure 17.7 Transmission electron micrograph of optimally aged, transformation~ toughened Zr02-MgO showing the oblate spheroid precipitates of tetragonal Zr0 2 in a MgO-stabilized Zr02 cubic matrix. (Courtesy A. H. Heuer, Case Western Reserve University.)
However, such a transformation involves an increase in volume, as shown in Fig. 4.17. If the grain or precipitate size is small enough (less than about 0.5 j..lm) , the strength of adjacent grains prevents the transformation from occurring by preventing the necessary volume expansion. When a stress is applied to the zirconia and a crack tries to propagate, the metastable tetragonal zirconia grains adjacent to the crack tip can now expand and transform to the stable monoclinic crystal form. This is illustrated in Fig. 17.8. Precipitates that have transform ed to monoclinic can be distinguished from untransformed precipitates in the TEM photomicrograph by the presence of twinning. Note that only the precipitates near the crack have transformed . This martensitic transformation is accompanied by a 3% volume increase of these grains or precipitates adjacent to the crack, which places the crack in compression and stops it from propagating. To extend the crack further requires additional tensile stress. The result is a ceramic that is very tough and strong and that has been appropriately referred to as "ceramic steel." Pure ZrOl does not have transformation-toughening behavior. Additives are required to stabilize such behavior. These additives are CaO, MgO, Y,O" CeO" and rare earth oxides. Too much addition fully stabilizes the Zr02 in a cubic crystal structure, which also does not have transformation-toughening behavior because it does not go through the tetragonalto-monoclinic transformation. Toughening requires the presence of the
Chapter 17
746
02 /1'""Figure 17.8 Dark field transmission electron micrograph of optimally aged, transformation-toughened ZrOl-MgO showing twinned monoclinic precipitates adjacent to a crack and tetragonal precipitates away from the crack. (Courtesy A. H. Heuer, Case Western Reserve University.)
metastable tetragonal state. The range of addition to achieve the metastable tetragonal state and toughening is shown for various ZeOz-based compositions in Fig. 17.9. The peak of the curve for each material corresponds to the maximum tetragonal content. Monoclinic content increases to the left of the peak and cubic to the right of the peak, each resulting in a decrease in toughness and strength. PSZ in Fig. 17.9 stands for partially stabilized zirconia. DCB, ICL, and NB identify the method that was used to measure fracture toughness. DCB stands for the double-cantelever beam, ICL the indentation crack length, and NB the notched beam. Note that the composition zones for achieving peak toughness are relatively narrow. Transformation toughening is not limited to Zr02_ Very small grains of Zr02 can be added to another ceramic such as Al 20, and be retained as tetragonal during cooling. These grains will then transform near a crack tip and inhibit crack propagation. Several criteria are necessary before transformation toughening can be achieved by addition of partially stabilized Zr02 particles to a host ceramic: (1) Zr02 particles not dissolved by host; (2) particle size of the Zr02 typically under 0.5 iJ.m; and (3) host
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~
Unrectionally
.:. ::. ::. :;. ::.
..: :..: :.:: :.:: ::.
Figure 17.10 Classification or transfonnation·toughened ceramics based on microstructural featcs. (From Rcf. 19.)
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Chapter 17
