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1986 65: 993J DENT RESR. Morena, G.M. Beaudreau, P.E. Lockwood, A.L. Evans and C.W. Fairhurst

Fatigue of Dental Ceramics in a Simulated Oral Environment

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Fatigue of Dental Ceramics in a Simulated Oral Environment

R. MORENA, G.M. BEAUDREAU, P.E. LOCKWOOD, A.L. EVANS, and C.W. FAIRHURST

Dental Materials Division, School of Dentistry, Medical College of Georgia, Augusta, Georgia 30912

Fatigue in ceramics refers to the subcritical growth of cracks, aidedby the combined influence of water and stress. The dynamic fatigue(constant stressing rate) method was used to obtain subcritical crackgrowth parameters for three dental ceramics: afeldspathic porcelain,an aluminous porcelain, and a fine-grain, polycrystalline core ma-terial. The constant stressing rate experiments were carried out at37"C for all three ceramics in distilled water, and, for the feldspathicporcelain, in artificial saliva as well. Considerable differences werefound in the value of the crack growth exponent (n) among the threeceramics. The feldspathic porcelain was lowest in n-value, while thefine-grain ceramic had the highest n-value. No differences werefoundfor the feldspathic porcelain with respect to n measured in water andin the artificial saliva. Lifetime prediction curves in 37C water, con-structed from the n-values and inert strengths, showed that fatiguefailure within five years is a good possibility forfeldspathic porcelainspecimens at stress levels which can reasonably be anticipated tooccur in the oral environment. Little likelihood of failure was per-ceived for the fine-grain ceramic. The aluminous porcelain was in-termediate between these two materials with respect to failureprobability.

J Dent Res 65(7):993-997, July, 1986

Introduction.

It has long been known that the presence of water will degradethe strength of silicate glasses and many other ceramic mate-rials (Shand, 1958; Mould, 1959). Exposure to an aqueousenvironment has also been found to affect the mechanical per-formance of dental ceramics. Sherrill and O'Brien (1974) dem-onstrated that the fracture stress of feldspathic and aluminousdental porcelains decreased by nearly 30% when the sampleswere broken in water. Southan and Jorgensen (1974) showedthat the ability of a dental porcelain to sustain a static load inwater decreased as the duration of load application increased.The effect of aqueous exposure and other aspects of dentalporcelain mechanical behavior have been the subject of anextensive review by Jones (1983), who also lists decreasingstrength with decreasing stress/strain rate as further evidencefor the detrimental role played by water.

The process of strength degradation of ceramics in aqueousenvironments is termed fatigue and is believed to be causedby a stress-corrosion process involving the stable growth ofsmall, pre-existing flaws (Hillig and Charles, 1965; Michalskeand Freiman, 1983). The effect of water is so pronounced thatstress-corrosion has been observed in silicate glasses at mois-ture levels as low as 0.017% relative humidity (Wiederhom,1967). Failure, generally after long loading times and often atstresses substantially less than the measured short-term fracturestress, will ensue once these flaws attain a size equivalent tothat needed to initiate fast fracture.

The oral environment would appear to have all the factorsnecessary for fatigue failure to occur in ceramic-based dentalprostheses. Water is, of course, the primary chemical species

in saliva. A dental restoration would also be exposed to waterfrom a cementing agent as well as from the dentin tubules.Stresses, both masticatory-related and also associated withthermal expansion mismatches between the various compo-nents of the restoration, would be present to provide the drivingforce for fatigue. Flaws in a dental ceramic may be introducedby grinding during occlusal adjustment or may be intrinsic andrelated to microstructural features (Jones and Wilson, 1975;Corbitt et al., 1985). Finally, the range of temperatures thatwould be encountered by a restoration during service in theoral cavity is sufficiently elevated from ambient as to exacer-bate any slow crack growth process (Ritter et al., 1985).

Failure of dental ceramic restorations caused by fatigue doesappear to be a possibility, as judged by several studies reportedby Lehman (1967) on long-term failures of feldspathic por-celain jacket crowns. McLean (1983) lists a failure rate of15.2% within three years following placement in the mouthfor platinum-bonded, aluminous porcelain crowns in molar sites.Similar studies documenting failures for porcelain-fused-to-metal(PFM) crowns and bridges could not be found, although War-peha and Goodkind (1976) list several types of persistent clin-ical failures observed with PFM crowns that were all associatedwith the porcelain component. Delayed failures which oc-curred up to six weeks after firing have been observed byWalton and O'Brien (1985) for model PFM crowns and wereattributed to a probable fatigue interaction. The large numberof commercial in vivo porcelain repair materials currentlyavailable provides indirect testimony for the existence of long-term failures in PFM restorations (Ferrando et al., 1983).

Although the several studies cited have given evidence forthe fatigue susceptibility of various dental porcelains, no datacould be found in the literature relating to the fundamentalmaterial parameters which determine the kinetics of slow crackgrowth for dental ceramics. In the past 10 years, the disciplineof fracture mechanics has become firmly established for ana-lyzing the fracture properties of ceramics and predicting theirfatigue behavior (Wiederhorn, 1974; Ritter, 1978). Slow crackgrowth is described in terms of a fracture mechanics analysisby a fundamental parameter the crack growth exponent, n

which is a material constant for a given environment. As-suming a power law expression for subcritical crack growth(i.e., the rate of crack growth x K, , where K, is the openingmode stress intensity factor), the failure time of a ceramiccomponent experiencing slow crack growth-induced fatigue isgiven (Pletka and Wiederhorn, 1982) by:

t = BSn -2t-an (1)

where tf = failure time under a constant applied stress, Ua;B = constant dependent on n, and also on crack ge-

ometry and the fracture toughness of the material;and

S = inert or initial strength in the absence of subcrit-ical crack growth.

Note that of two materials with comparable values of initialstrength that are candidates for a given application in which astress must be sustained without failure over a long time pe-riod, the material with the higher n value will be more likelyto survive.

993

Received for publication October 31, 1985Accepted for publication April 10, 1986This investigation was supported in part by USPHS Research Grant

DE07045 from the National Institute of Dental Research, NationalInstitutes of Health, Bethesda, MD 20892.

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994 MORENA ET AL.

It was the objective of this study to determine the crackgrowth exponent using standard fracture mechanics tests for anumber of restorative ceramics, including a polycrystalline corematerial. Additionally, predicted lifetime curves, constructedfrom the fatigue data using Eq. (1), will be presented so thatthe survivability of various dental ceramics can be comparedunder different levels of applied stress and fatigue conditions.In general, lifetime prediction curves are constructed based onvalues of n and B measured in controlled laboratory fatiguetests, and with S, the inert strength of the component, obtainedon a geometry as representative as possible of the actual end-use.

Materials and methods.The fatigue data were obtained by the dynamic or stressing

rate technique, in which fracture stress in the environmentunder study is measured as a function of stressing rate. Thematerials used in the study consisted of a feldspathic porcelain(Ceramco Gingival, Ceramco, Inc., East Windsor, NJ), analuminous porcelain (Vitadur-N, Vident Corp., Baldwin Park,CA), and a fine-grain polycrystalline "shrink-free" ceramic(CerestoreT, Johnson & Johnson, East Windsor, NJ). Theselatter two materials are intended for use as the core componentin all-ceramic replacement crowns. All materials were obtainedfrom regular commercial lots supplied by the vendor.

Specimens of both the feldspathic and aluminous porcelainswere vibration-condensed into thin disks by means of a splitbrass mold. Unfired dimensions of these disks were typically15 mm in diameter and 2.0 mm in thickness. Following re-moval from the mold, the specimens were permitted to air-dryfor at least two hours prior to being fired. Firing was carriedout in a commercial porcelain furnace (System 8, J.M. NeyCo., Bloomfield, CT), following the firing schedule recom-mended by the respective porcelain manufacturer.The fine-grain, polycrystalline core material was processed

into thin disks by a variant of the lost wax technique, withcommercial equipment supplied by the vendor. The time/tem-perature schedule recommended by the manufacturer was fol-lowed in firing the test specimens using the programed furnacesupplied as part of the processing unit.

Following firing, all specimens were ground and polishedon both surfaces using a sequence of steps culminating in 15-pum diamond paste on a nylon cloth-backed bronze wheel. Onesurface of each specimen received an additional polishing se-quence through 1-pLm diamond paste. Final dimensions of thepolished disks for all three ceramics were typically 12.5 mmin diameter and 1.0 mm in thickness. For all three materialsin this study, a sample size of 10-12 specimens was used foreach stressing rate and for the determination of inert strengths.All specimens were tested such that the finest polished surfacewas the tensile surface during fracture.

Prior to the strength-testing procedure, a single Vickers in-dentation was made in the center of the 1-pRm diamond pol-ished surface of each specimen of the aluminous porcelain andthe fine-grain, polycrystalline core material. This was done toensure that a single dominant flaw would be present in thematerial to minimize any variation in strength caused by thepresence of random handling damage. The load selected forthe indentation (4.9 N for the aluminous porcelain, 9.8 N forthe fine-grain ceramic) was the minimum necessary to producea significant decrease in strength (p < 0.05 by Student's t test)relative to that of unindented specimens. No such indentationwas made for the feldspathic porcelain, since a previous study(Corbitt et al., 1985) had indicated that the strength of thismaterial was controlled by large, intrinsic microcracks.

Evans (1974) has shown that the dependence of strength on

stressing rate, caused by subcritical crack growth, can be de-scribed by:

af = afo (&/6 )1/n+ l (2)where of= fracture stress determined at a given stressing

rate,(Ufo = a constant for a given environment,& = stressing rate, and&O = a normalizing constant (1 MPa/s).

The parameters n and coy were obtained from the stressingrate experiments by fitting the dynamic fatigue data to a ln-lnrelationship, and carrying out a linear regression analysis. Alldata points were used in these calculations. The value of nobtained from Eq. (2) was corrected for the two materialswhich had been indented by multiplying by a constant term,1.31, to correct for residual stress effects associated with theindentation (Lawn et al., 1981). The standard error of n wasdetermined from a statistical analysis of the linear regression,using the approach derived by Pletka and Wiederhorn (1982).The residual stress correction factor was included in this erroranalysis.The B term in Eq. (1) can be obtained (Ritter, 1978) from:

BUn+lIi- (3)n+l()

where Si = inert or initial strength of laboratory specimensin the absence of subcritical crack growth.

The strength values at each stressing rate were obtained witha constant displacement rate mechanical testing machine (ModelTT-B, Instron Corp., Canton, MA). Test specimens were bro-ken in bi-axial flexure by means of the piston-on-3-ball tech-nique described by Wachtman et al. (1972). This techniquewas selected since it eliminates spurious edge failures (whichoccur in flexure tests) associated with stress concentrations atcontact points. This is because tensile stress in the bi-axialflexure apparatus decays rapidly with increasing radial distancefrom the center of the disk. The disks were concentricallysupported on three ball bearings (each 1.6 mm in diameter)that were evenly spaced around a support circle having a radiusof 3.5 mm. The specimen-holder is shown in Fig. la. Thespecimens were loaded to failure by a compressive load appliedby a right circular cylinder with a diameter of 1.6 mm. Thefracture stress was calculated (Wachtman et al., 1972) fromthe load at failure by:

of3P

(I+v) l + 2 Ina

47r t2 LbI + V b2 4a2l

+ lII+v S2a2 2

(4)

where P = failure loadt = thickness of specimenD = diameter of specimenv = Poisson's ratio of specimen (assumed to be 0.25)a = radius of support circleb = radius of right circular cylinder

All stressing rate tests were carried out in a simulated oralenvironment consisting of distilled water maintained at 370C+ 0.50C by a thermostatically-controlled liquid circulator (ModelFE2, Haake Instruments Co., Saddle Brook, NJ). Specimenswere allowed to reach thermal equilibrium in the liquid me-

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FATIGUE OF DEINTAL CERAMICS

that had received a relatively coarse final polish of 30 vm inorder to duplicate the actual surface condition of a ceramiccomponent in a dental restoration. All specimens for inertstrength determination were chosen from the same sample pop-ulation from which test specimens for the stressing rate ex-periments were selected.

Fig. I B3i-axial flexure apparatus: A. Specimen-holder; B. Environ-mental chamber.

Results.The dynamic fatigue curves are shown in Fig. 2 for the three

ceramics tested in water. Dynamic fatigue curves are comparedin Fig. 3 for the feldspathic porcelain evaluated in both waterand the artificial saliva environments. The error bars shown inthese Figs. for fracture stress correspond to the Mean + 1S.D. No error bars are shown for stressing rate, since thecoefficient of variation (S.D./mean) was typically 5% or less.Also shown in both Figs. is the coefficient of determination,r2, for each test situation.The results of the linear regression analyses on the dynamic

fatigue data are listed in Table 1. The value of the crack growthexponent, measured in distilled water, varied from approxi-mately 14.5 for the feldspathic porcelain to nearly 81 for thefine-grain core material. The differences in n-value for thethree dental ceramics were found to be highly significant (p 0.50 by Student's t test) were found in the crack growth ex-ponent for the feldspathic porcelain when tested in the waterand artificial saliva environments. This agrees with stress-cor-rosion studies of other bioceramics where no differences wererecorded in the n-value between water and other biologic me-dia, such as Krebs-Ringer solution and bovine serum (Rockarand Pletka, 1978). The higher strength values measured at eachstressing rate for the feldspathic porcelain tested in water rel-ative to the artificial saliva media are mirrored in the higher

dium for 10 min prior to breaking. The environmental chamberwith the specimen-holder in place is shown in Fig. lb. Datawere also obtained with an artificial saliva solution (Salivart,Westport Pharm., Westport, CT), held at 370C + 0.50C for asecond set of feldspathic porcelain specimens to determinewhether differences in crack growth kinetics occur between thetwo aqueous media. The stressing rates were determined di-rectly from the chart recorder of the mechanical testing ma-chine and spanned nearly four decades, from almost 0. 10 toapproximately 85 MPa/sec.The inert strengths used to obtain the B term in Eq. (3) and

to predict lifetime behavior from Eq. (1) were obtained in bi-axial flexure following the procedure outlined by Ritter et al.(1985). Test specimens were broken in mineral oil at highstressing rates ( 100 MPa/sec) with a thin layer of plasticplaced within the support cycle on the tensile surface to min-imize further the possibility of exposure to water. Note thattwo different measures of inert strength are used in Equations(I) and (3). Si (Eq. 3) refers to the inert strength of laboratorytest specimens, which, for this study, were finely-polished disksthat, where specified, had received indentations on their tensilesurfaces. S (Eq. 1) relates to the inert strength of engineeringspecimens which, ideally, should represent the actual size,shape, manufacturing conditions, stress state, etc., of the com-ponent (Pletka and Wiederhorn, 1982). Since the stress distri-bution in a restoration such as a replacement crown is complexeven in relatively simple loading situations (Farah and Craig,1975), lifetime curves for comparison of the three dental ce-ramics were constructed with S measured on unindented disks

0.20 1.04 8

4.6

ffi 4A42

- 42c;4.

4.01

Stressing Rate, MPals10 100

3.8-

3.6

Fig. 2-in water.

+4 00 +2.0In (Stressing Rate, MPals)

120

100

80

-0

996 MORENA ET AL.

TABLE 1DYNAMIC FATIGUE RESULTS

n-value(mean + S.E.) ufo (MPa)

In WaterFeldspathic Porcelain 14.6 + 1.4 44.0Aluminous Porcelain 28.9 + 5.8 78.7Fine-grain Ceramic 80.8 20.7 105In Artificial SalivaFeldspathic Porcelain 14.9 1.8 41.1

TABLE 2INERT STRENGTH VALUES (Mean -+ S.D.)

Si (MPa) S(MPa)Surface Polished through Polished through

Condition 1-pum diamond 30-pum diamondFeldspathic Porcelain 72.7 + 8.2 72.9 + 5.3Aluminous Porcelain 147.9 + 18.2* 147.1 + 11.3Fine-grain Ceramic 131.6 8.2* 151.4 + 9.9*Indented prior to being broken.

Stressing Rate, MPals

1.0

-2.0 0 2.0In (Stressing Rate, MPa/s)

4.0

1o01o

2

Applied Stress(103psi)5

Feldspathic Porcelain

\ \~

\\ \\

1 08

en

0E

0h-._v(U

-1 yr.\

-1 mo.

106

--1 day

104_

102

Fig. 3 Dynamic fatigue curves at 37TC for feldspathic porcelain inwater and in artificial saliva.

(tf0 value obtained for water. This effect is presumably due toa larger initial flaw size for the set of feldspathic porcelainspecimens evaluated in the artificial saliva.The various values of inert strength obtained on the test

materials are listed in Table 2. Lifetime curves, constructedfrom the inert strengths and n-values using Equations (1) and(4), are presented in Fig. 4. Also shown for each curve is the90% confidence band, represented by dashed lines, and con-structed according to a propagation of errors technique outlinedby Pletka and Wiederhorn (1982).

Discussion.Crack growth exponents. - The widely different n-values

obtained in water for the three dental ceramics indicate thatthese materials behave quite differently with respect to theeffect that an aqueous environment has on subcritical crackgrowth and mechanical degradation:

(a) The low value obtained for the feldspathic porcelain in-dicates a material whose mechanical performance would beappreciably adversely affected when exposed to water. This n-

-1 hr.

10

Aluminous Porcelain F

\ I\ k

k I

k k

I

K \ \ \~~~~~~~~~~~~~~~~~

\

k k

\ \ ~~~~~~~~~~~~IA

\

10 20

Fine Grain Ceramic

II1'I'

lI

II

I

II'IIA

Il

I IIl

I , I,I\I '. I1

25 50 75 100Applied Stress(MPa)

Fig. 4 Lifetime curves for the three dental ceramics in 370C water.

value is comparable with that of silicate glasses (Wiederhornand Bolz, 1970), which are among the most fatigue-susceptibleof all ceramics. The close agreement with respect to n-valuebetween feldspathic porcelain and silicate glasses also pertainsto other aspects of mechanical performance, such as strengthand fracture toughness (Morena et al., 1984). These similari-ties indicate that the mechanical properties of a feldspathicporcelain are controlled largely by the matrix glass and not byany crystalline phase(s).

(b) The very high value measured for the fine-grain, "shrink-free" ceramic core material agrees with data cited on fine-grain, ot-AI203 ceramics (Pletka and Wiederhorn, 1982). Thismaterial should be relatively unaffected by the oral environ-ment except in instances where a high tensile stress is main-tained for a long time period.

(c) The middle value recorded for the aluminous porcelainplaces this material at a position intermediate between the feld-spathic porcelain and the fine-grain core material with respectto resistance to subcritical crack growth. The aluminous por-celain, despite a glassy content comparable with that of feld-spathic porcelain (30-40 vol %), has a crack growth exponentsignificantly higher than that of a silicate glass. This indicatesthat direct interaction is occurring between cracks and the dis-persed phase (a-A1203), a behavior manifested in other frac-ture properties as well (Morena et al., 1985).

Lifetime curves. - As noted, the lifetime curves apply strictlyto thin disks that are being subjected to a static applied stress.The slopes of these curves are governed by the n-value, which

I I~~~~~

I I I I

J Dent Res July 1986

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FATIGUE OF DENTAL CERAMICS

is a material constant for a given environment. However, therelative position of each curve is dependent on the inert strength,S. Thus, if anything, the lifetime curves in Fig. 4 may wellunderestimate actual fatigue degradation. This is because theinert strength of a ceramic in a dental restoration would ac-tually be less than that obtained on the laboratory specimens.Design factors, such as stress concentrations at margins, andinterfacial stress - arising, for example, from differences inthermal contraction between the components of a restoration

all would be present in a ceramic prosthesis to lower strength.The usefulness of the lifetime curves can be illustrated by

employing them to predict the level of applied stress that wouldcause failure within five years for the three dental ceramics.Using the regression analyses for the lifetime curves, we findthat the five-year failure stress for these ceramics is:

feldspathic porcelainaluminous porcelainfine-grain ceramic

13.0 MPa (1900 psi)42.1 MPa (6100 psi)95.2 MPa (13,800 psi)

Stress of the magnitude needed to cause failure within fiveyears may well exist for PFM restorations in the oral environ-ment. Finite element analyses of two-dimensional PFM modelcentral incisors indicate conservative estimates of masticatorystress, ranging from 6.5 to 17.5 MPa on the upper incisor edge(Farah and Craig, 1975; Anusavice et al., 1985). In addition,residual mismatch stresses, arising from thermal contractiondifferences between porcelain and metal in a PFM restoration,can be substantial and attain a value as high as 50 MPa, asshown by Anusavice et al. (1982) for model PFM crowns withporcelain facial coverage.The lifetime curves for the aluminous porcelain and fine-

grain ceramic core materials both suggest that long-term failureshould not be a problem for anterior applications. Althoughno values of masticatory stress could be found in the literaturefor posterior crowns, preliminary data (Anusavice, personalcommunication) indicate a value of approximately 30 MPa.Thus, should even moderate levels (15 MPa) of residual ther-mal mismatch stress exist for the aluminous porcelain, fatiguefailure could result. It is possible that the platinum backingused in aluminous porcelain crowns could minimize moisturecontact, thus improving the fatigue resistance of aluminousporcelain restorations. The fine-grain ceramic would appear tobe relatively immune to fatigue failure, even for posteriorplacement. However, the steep slope of the lifetime curve couldpose a potential problem for restorations where a large tensilevalue of residual mismatch stress exists (caused perhaps by agross compositional error for the overglaze), since rapid fa-tigue failure would occur once stresses much exceeded 95 MPa.

Future work. - Future investigations on fatigue will utilizecyclic stressing to simulate mastication and will also determinewhether aging or crack tip blunting can occur at low stresslevels to mitigate, to some extent, the fatigue process.

Acknowledgments.The authors wish to thank the Johnson & Johnson Company

for providing commercial lots of their dental ceramics. Thanksare also expressed to Skelley Dental Arts, Inc., Aiken, SC,for their assistance in fabricating the fine-grain ceramic corespecimens. Appreciation is also expressed to S.W. Twiggs forassistance with statistical analyses.

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