Fatigue and Fracture Characteristics of a

a Departament de Cie ncia dels Materials i Engenyeria Metal.lu rgica,ETSEIB,

Universitat Polite cnica de Catalunya,Barcelona08028,Spain

b Research Center Juelich,IWE-1,SOFC,Juelich52425,Germany

c National Research Council,Research Institute for Ceramics Technology,Faenza48018,Italy (Received7August1998;accepted11November1998)

Abstract

The fatigue and fracture characteristics of a partially-stabilized®ne-grained zirconia with spinel additions, (Mg,Y)±PSZ,were studied.Fracture toughness, crack growth resistance curves and fatigue crack growth(FCG)behavior,under both sustained and cyclic loading,were evaluated.Mechanical fatigue e ects were clearly evidenced by(1)remarkable crack growth rate di erences under cyclic and static loading and(2)signi®cant loading ratio e ects. Comparing the cyclic and the static FCG behavior allows to deduce a higher cyclic fatigue sensitivity of the®ne-grained(Mg,Y)±PSZ with respect to a commercial peak-aged Mg±PSZ used as a reference material.By in situ observation of crack extension under cyclic loading,the fatigue mechanisms could be resolved.Mechanical degradation of bridging ligaments,as already known for coarse-grained Mg±PSZ,is one source of cyclic fatigue.An additional source attributed to the particle dispersed micro-structure of the(Mg,Y)±PSZ is the interaction between crack faces and hard spinel particles.The sensitivity of(Mg,Y)±PSZ and Mg±PSZ to cyclic fatigue is discussed in terms of the respective micro-structures,prevalence and operativity of distinct mechanical fatigue mechanisms.#1999Elsevier Science Limited.All rights reserved

Keywords:PSZ,fatigue,racture,toughness, ZrO2.

1Introduction

Zirconia ceramics are candidate materials for a wide range of structural applications where improved and consistent performance together with high reliability are demanded.It is directly associated with their pronounced rising crack growth resistance with increasing crack size, namely R-curve behavior,mainly as a consequence of the large transformation toughening capability that they exhibit.This imparts these ceramics with the highly desirable property of`¯aw tolerance',i.e. their strength is independent of initial¯aw size or subsequent in-service damage.1,2

The above statements are particularly true for MgO±partially stabilized zirconia(Mg±PSZ).For this system,a large number of investigations con-ducted over the last two decades has led to optimal processing routes and,as a result,to signi®cant improvements in its relative fracture toughness and R-curve behavior(e.g.Refs3±6).However,similar achievements have not been accomplished when considering other important structural character-istics of Mg±PSZ,e.g.maximum long-term applica-tion temperature.With respect to this parameter,it is well known that this material shows pronounced mechanical degradation when exposed at tempera-tures above900 C,mainly as a consequence of the subeutectoid decomposition promoted in the MgO±ZrO2system.3,7±10This decomposition is highly localized on grain boundaries and may induce microcracking at them because the signi®cant thermal expansion mismatch between the cubic matrix and the eutectoid product.3,8,10

In order to overcome the above impairment of Mg±PSZ,literature results indicate the addition of yttria as an alternative,particularly operative in terms of suppressing the subeutectoid decomposi-tion up to temperatures about1250 C.11,12Fol-lowing that approach,Meschke et al.13±16have developed a zirconia partially stabilized with Y2O3 and MgO,which includes small amounts(3vol%) of MgAl2O4spinel particles.The resulting material, Journal of the European Ceramic Society19(1999)1705±1715

#1999Elsevier Science Limited

Printed in Great Britain.All rights reserved

P I I:S0955-2219(98)00281-70955-2219/99/$-see front matter

1705

*To whom correspondence should be addressed.referred to as(Mg,Y)±PSZ,is a®ne-grained(mean grain size below15"m)material that exhibits,after appropriate aging treatments,not only similar hydrothermal stability,toughening capability and corrosion resistance as conventional coarse-grained Mg±PSZ,but also better high temperature phase stability and higher¯exural strength,under peak-aged conditions,than the latter material.14±17

On the other hand,many structural applications for which(Mg,Y)±PSZ could be intended to use include variable stresses too.Therefore,for design purposes as well as for estimation of reliability,in addition to the thermomechanical and environ-mental characteristics cited above,the resistance to fatigue crack propagation may also be of primary importance.

Cyclic degradation phenomena in PSZs were®rst con®rmed about10years ago.18,19Since then, numerous investigations on the fatigue of these materials,mainly of Mg±PSZ,have been carried out experimentally and theoretically by many researchers. Extensive literature surveys on the fatigue behavior of Mg±PSZ have been presented,e.g.in Refs.20±22, and the interested reader is referred to them for detailed information.Here,only some experi-mental facts on the fatigue crack growth(FCG) characteristics of Mg±PSZ will be brie¯y reviewed. From the mechanics standpoint,it is now well known that for Mg±PSZ:(1)crack growth rates for a given maximum applied stress intensity factor (K m x)are higher under cyclic loads than under a constant one;and(2)there is a very large power-law dependence of crack growth rates of long sharp cracks on K m x(orÁK).10,18,20,23±27Further, such relationships are strongly a ected by extrinsic (environment,temperature,load ratio,etc.)24,25as well as intrinsic(phase transformation capability, microstructure,etc.)factors.10,20,24,26,27On the other hand,from the mechanisms viewpoint,several cyclic-induced phenomena have been postulated in association with cyclic fatigue of Mg±PSZ.The proposed processes include:slip line formation25 and microcracking21at the crack tip;bridging degradation,of both ligaments22,24,27,28and pre-cipitates,22,28at the crack wake;and transforma-tion toughening reversibility.21However,there is not conclusive experimental evidence for identifying any of them as consistently responsible for the mechanical fatigue e ects observed.Taking into consideration the experimental facts presented for each mechanism in the corresponding studies,it seems more concordant to describe the mechanical fatigue in these materials as the result of a complex interactive e ect of all,or at least some,of them than on the basis of a particular phenomenon.

It is the aim of this study to investigate and to establish the cyclic and the static FCG behavior,i.e.that related to subcritical crack growth by true mechanical fatigue and environmentally-assisted degradation respectively,of a®ne-grained zirconia with spinel additions,(Mg,Y)±PSZ.Of particular interest is the investigation of how microstructural characteristics a ect the relative cyclic to static FCG behavior of(Mg,Y)±PSZ,as compared to that previously reported by the authors for a con-ventional coarse-grained Mg±PSZ(FZM type, 3wt%MgO,Friatec AG,Mannheim,Germany).

2Experimental Procedure

2.1Sample preparation

The material studied was a®ne-grained zirconia, co-stabilized with magnesia and yttria,with spinel additions,(Mg,Y)±PSZ.Samples were prepared from mixtures of commercial stabilized zirconia powders(®nal stabilizer contents of0.5Mol%Y2O3 and7.8mol%MgO).An amount corresponding to 3vol%of MgAl2O4spinel particles was added to the composition.This mixture was attrition milled in ethanol in a polyethylene-lined recipient.After wet sieving and rotovap drying,the powder was sieved with a140-"m grid.

Powder compacts were prepared by uniaxial pressing at31MPa followed by cold isostatic pressing at500MPa.Green bodies with a density of55%of the theoretical one were sintered in air at 1720 C for30min,rapidly cooled to1400 C,iso-thermally held for60min,and®nally quenched to room temperature.From the4mm thick blocks obtained,rectangular prisms were cut and recti®ed.

2.2Microstructural examination

Samples for analysis by optical(OM)and scanning electron microscopy(SEM)were prepared by pol-ishing sectioned pieces of the rectangular prisms. They were then etched at room temperature in concentrated HF for4min.Microstructural features were examined using a JEOL JMS00microscope. Transmission electron microscopy(TEM)samples were also prepared.Thin foils were cut and mechanically ground to100m m.Disks(3mm in diameter)were cut from the thin foils of material using an ultrasonic drill.The disks were then dim-ple-polished to30"m center thickness and ion milled.A thin®lm of carbon was evaporated onto the foil to avoid charging while under the electron beam.A120-KV JEOL1200EXII microscope was used for TEM.

2.3Mechanical characterization

Fracture strength was measured at room tempera-ture on unnotched beams of4Â4Â30mm dimen-sions.A fully articulating four-point bend test jig

1706R.FernaÁndez et al(inner span11.5mm,outer span23mm)was used. The specimens were®rst polished on the surface which was later subjected to the maximum stress in bending.Polishing was conducted using dia-mond pastes of30and6"m and colloidal silica as a®nal step.The edges of the polished samples were slightly chamfered for the¯exural strength tests.

Fracture toughness,crack growth resistance curve(R-curve)and FCG behavior were measured under four-point bending and using single edge-notch beams(SENB)of4Â7Â30mm dimensions with a notch length-to-specimen width ratio(a a w) of0.3.The specimens were®rst pre-cracked by cyclic compression in an Instron servo-hydraulic testing machine(model1341).Loading conditions during the pre-cracking procedure included:sine waveform,load ratio(R)of10,testing frequency of20Hz and minimum applied stress between300 and380MPa.The resulting cracks were usually about50to150"m long,after105to106cycles, and were associated with extensive degradation together with an extremely rough appearance.In order to avoid possible pre-cracking and crack-tip geometry e ects,29these cracks were further pro-pagated about120"m,under far-®eld tensile loads,before carrying out any subsequent crack growth resistance testing.

Fracture toughness and R-curve behavior were determined following a direct-measurement method. Tests were run under crack opening displacement (COD)control in an Instron servo-hydraulic test-ing machine(model8511).The imposed COD ramp was of0.1"m sÀ1.Load values were compu-ter-recorded with a data acquisition rate of10 points/s.Stable crack extension was monitored in situ with an accuracy of 5"m using a long-range telescope Questar(model QM100).In order to facilitate crack extension measurement,the lateral faces of the SENB specimens were previously polished.The characteristic R-curve(K appiied versus crack length)was®nally assembled from the simultaneously collected data series of load-time and crack length-time,using the stress intensity factor expression for SENB geometry as given by Tada et al.30Fracture toughness was determined as the plateau value of the measured R-curve.

FCG under cyclic loading was evaluated at a frequency of2Hz under load control using a sine waveform and R values of0.2and0.6.For com-parison,environmentally assisted or static FCG was measured with specimens subjected to constant applied load.The load value was that correspond-ing to a resulting K applied equal to the K max value experienced by the sample under cyclic loading.It was reached following an initial ramp of100N sÀ1. As before,and for both loading conditions,stable crack growth was measured in situ using a long-range telescope.All tests were carried out at room temperature and humidity of about55%. Fractographic aspects associated with di erent loading conditions were documented either from in-situ examination,using the long-range optical microscope,or by investigating the fracture surfaces of broken samples through OM and SEM.

3Results and Discussion

3.1Microstructure

The average grain size,as obtained from OM and SEM images,was about12"m(Fig.1).Grain size di erences between the material studied and com-mercial Mg±PSZ is attributed to the addition of spinel particles(of size between1and5"m)and their e ect on hindering grain growth.A SEM and TEM examination allowed to discern a common PSZ microstructure,i.e.orthogonally oriented oblate precipitates immersed in a cubic matrix.The volume fraction of thin ellipsoidal second-phase particles was about40%.Those of size over150nm showed a twinned aspect and monoclinic symmetry, whereas precipitates smaller than100nm had a tetragonal symmetry.The precipitate size distribu-tion was highly heterogeneous,as shown in Fig.2. As clearly discerned by TEM(Fig.3.)zones close to spinel particles were often characterized by less numerous precipitates but of sizes larger than the corresponding mean values.The pronounced het-erogeneity observed for the precipitate size dis-tribution together with the existence of spinel particles in(Mg,Y)±PSZ will be two important microstructural features to consider when compar-ing the mechanical behavior of this material with that of a peak-aged Mg±PSZ previously investi-gated by the

authors.

Fig. 1.SEM image showing microstructural features of (Mg,Y)±PSZ.Black spots correspond to spinel particles.

Fatigue and fracture characteristics of a®ne-grained,(Mg,Y)±PSZ17073.2Mechanical characteristics

3.2.1Flexural strength and fracture toughness The mean¯exural strength of the material studied was of678MPa( 45MPa),about40%higher than that previously found by the authors for a commercial coarse-grained Mg±PSZ under peak-aged conditions,i.e.with maximum fracture toughness.10The higher measured strength for the (Mg,Y)±PSZ must be directly associated with the lower processing¯aw size resulting of the grain size reduction.15,16

Fracture toughness,R-curve and FCG behavior were evaluated using SENB specimens with sharp cracks obtained®rst under cyclic compression loading and subsequently extended under far-®eld tensile loads.The cyclic compression pre-cracking procedure yields residual tensile stresses at the tip of the notch upon unloading from the maximum far-®eld compression stress.31It®nally resulted, before further propagation of the obtained pre-cracks,in pronounced fatigue damage at the bottom of the notch,as clearly shown in Fig.4.Particular fractographic features were also found as related to the cyclic compression pre-cracking procedure. Figure5shows an example of the indentation-like impressions(arrow)usually found on fracture sur-faces corresponding to the precracking zone.Their morphological aspect and microstructural dimen-sions suggest such impressions as being produced as a consequence of hard spinel particles smashing against the®ne-grained structure during the cyclic compression procedure.Hence,it is proposed that wedging e ects associated with the interaction between crack faces and spinel particles may also exist,under cyclic loading,just behind the crack tip for the material studied.The relative in¯uence of such e ects on the mechanical characteristics of (Mg,Y)±PSZ will be discussed later.

Fracture toughness plateau value for the (Mg,Y)±PSZ was9.8MPa

m

p

( 0.4MPa

m

p

).It was about10%higher than that of the peak-aged Mg±PSZ studied before.Figure6shows the measured R-curve.It may be described as

very

Fig.3.Bright-®eld TEM image showing regions of low den-

sity but very large monoclinic precipitates near spinel

particles.

Fig.4.SEM image showing fatigue damage at the bottom of

the notch in a sample pre-cracked under cyclic compression

loading.

Fig.2.SEM micrograph showing details of the precipitate size

distribution within a grain of

(Mg,Y)±PSZ.

Fig.5.Fracture surface corresponding to the pre-cracked zone

of a tested sample.The indentation-like impression(arrow)is

suggested to be produced by the mechanical interaction

between crack faces and spinel particles during the pre-crack-

ing procedure.

1708R.FernaÁndez et al

pronounced and developing over a crack extension of about 1mm.In that ®gure,the R -curve of the Mg±PSZ used as reference in this investigation 32is also shown for comparative purposes.Crack growth rates during the R -curve measurements for both materials were experimentally determined to be in the range of 2Â10À5to 2Â10À4m/s À1,i.e.high enough to neglect any possible environmental assisted cracking e ects,as concluded from the sta-tic fatigue results to be presented in the next section.The qualitatively alike raising R -curves indicate that both materials exhibit similar main toughening mechanisms.Experimental observations con®rm crack shielding by transformation of the transgra-nularly distributed precipitates as the main tough-ening mechanism for both materials.The referred experimental ®ndings were:(1)a large volume fraction of highly metastable tetragonal zirconia precipitates,a feature already reported in a pre-vious work;16(2)surface rumpling during crack propagation;and (3)relatively large crack extension in which the plateau values were achieved.How-ever,other toughening mechanisms such as pre-cipitate or uncracked ligament bridging may also be active.20,22,27,28Whereas di erences with respect to toughening e ectivity are not expected for the precipitate bridging mode,because of the similar mean size value of the transforming precipitates in both materials;they could exist for the ligament mode due to the more heterogeneous microstructure

of the (Mg,Y)±PSZ.The optical resolution of the in-situ crack growth monitoring technique used in this investigation did allow to evidence the ligament bridging but not the precipitate one.Uncracked ligaments of size ranging from 5to 30"m were clearly observed during crack growth.Their occurrence was,at least from lateral surface obser-vations,more widespread in the ®ne-grained PSZ than previously found in the coarse-grained one.Following Moller et al.'s 28and Ho man et al.'s 22,27

®ndings,it is suggested that this larger pre-valence of uncracked ligaments in the ®ne-grained PSZ must be a direct consequence of the similarly higher fraction of large monoclinic precipitates in this material.These precipitates are believed to favor,under the stress ®eld associated with the incoming crack,radial cracking and then bridging ligament formation.Hence,the di erences in the R -curves between the two PSZs referred in this investigation,particularly in terms of fracture toughness plateau value,seem to be related to a more prominent e ect from uncracked ligament bridging in the (Mg,Y)±PSZ as a complementary toughening mechanism of the one resulting from the stress-induced transformation of tetragonal precipitates.

3.2.2Fatigue crack growth

FCG behavior for the (Mg,Y)±PSZ is shown in Fig.7.The crack growth rate (d a a d t )data

was

Fig.6.Experimentally determined R -curve for the ®ne-grained (Mg,Y)±PSZ.The corresponding R -curve for a peak-aged coarse-grained Mg±PSZ 32is also shown for comparative purposes.

Fatigue and fracture characteristics of a ®ne-grained,(Mg,Y)±PSZ 1709

plotted in terms of maximum applied stress inten-sity factor (K m x )in order to include results from static fatigue testing,i.e.that conducted at R =1.Crack growth thresholds were attained following an incremental loading sequence corresponding to 0.1MPa

m p steps.Threshold values were de®ned as the K m x associated with a d a a d t of 10À9m s À1.A power-law dependence,(d a a d t )=AK n m x ,i.e.a Paris-like relationship,was obtained in all cases in the range of crack growth rates between 10À8and 10À4m s À1.

Remarkable crack growth rate di erences were found under cyclic and static loading conditions.Like in a previous work for other advanced zirconia-based ceramics,10crack growth threshold and the (Paris)exponent in the power-law dependence were higher under static than under cyclic loading.The corresponding absolute values are given in Table 1for each loading condition studied.A clear load ratio e ect,for values between 0.2and 1,was measured.The trend showed by these experimental ®ndings indicates by itself the existence of true mechanical fatigue.Damage was also evident through in situ observation of degradation of bridges formed by uncracked ligaments during crack growth under cyclic loads (Fig.8).The development and subsequent rupture of these bridging ligaments was exclusively observed in the surface;however,the relationship crack extension±applied load as well as the fractographic

features allow to point out the phenomenon as occurring across the through-thickness crack front at various stages.Following previous studies on cyclic fatigue of coarse-grained Mg±PSZ 20,22,24,27,28,33±35it is stated that cyclic

fatigue

Fig.7.Crack growth rate as a function of maximum applied stress intensity factor (K max )for (Mg,Y)±PSZ and for each loading condition studied.The corresponding Paris coe cients are given by n and dropping arrows represent the highest K m x for which

crack growth was not noticed after 50000s.

Table 1.Crack growth threshold and Paris exponent in the power-law dependence,(d a a d t AK n m x ,for ®ne-grained

(Mg,Y)±PSZ and for each loading condition studied Loading ratio Fatigue crack growth Paris exponent

(R )threshold [K th (MPa m p )]

(n )

17.0380.65.7220.2

5.1

17

Fig.8.SEM image showing a typical uncracked ligament developed in (Mg,Y)±PSZ during stable fatigue crack growth.

1710R.Ferna Ández et al

e ects in PSZ are associated with degradation o

f not only ligament bridgin

g but also precipitate bridging (possibly the most important one 22,28)and localized microplasticity or microcracking at the crack tip.However,clear in-situ evidence of the latter processes was impossible to obtain within the optical microscope range used in this investigation.Transgranular fracture was always observed regardless of the applied loading condition.The fracture surface of a sample tested under cyclic loading,within the stable crack propagation zone,is shown in Fig.9.Here,polyhedral spinel particles

or holes of similar morphology are clearly seen,indicating that cracks usually went around parti-cles during stable propagation.This observation is very interesting because it points out the feasibility of a particle±crack face interaction similar to that previously found under cyclic compression loading.It is then suggested that during stable crack pro-pagation,at stress levels associated with small crack tip opening,such interaction may promote wedging e ects that could result in an additional cyclic fatigue degradation mechanism in (Mg,Y)±PSZ.On the other hand,unstable crack propaga-tion areas showed a higher fraction of cleaved particles.The fact that di erent particle±crack fracture interactions are dominant at low and high applied stress intensity factors pinpoints the relative importance of parameters such as speci®c interface fracture energy and particle size in determining the fracture characteristics of the material studied.3.3Relative cyclic to static fatigue behavior

Following the results above presented for the (Mg,Y)±PSZ,in this section it is attempted to dis-cuss the cyclic and static fatigue behavior of this material as compared to that previously found by the authors for a commercial peak-aged Mg±PSZ.10The observed trends for R values of 0.2and 1in both materials are shown in Fig.10.Clearly,environmental-assisted crack growth rates are much higher,for a given applied K m x ,for

the

Fig.9.SEM micrograph showing transgranular fracture and exposed spinel particles under stable crack propagation in

(Mg,Y)±PSZ.

Fig.10.Experimental data trends corresponding to the FCG behavior of ®ne-grained (Mg,Y)±PSZ and a coarse-grained Mg±PSZ

used as a reference,10,32both at peak-aged conditions,under cyclic (R =0.2)and constant (R =1)applied loading.

coarse-grained material than for the ®ne-grained one,pointing out the higher static fatigue crack growth resistance of the (Mg,Y)±PSZ.On the other hand,the di erences between the corresponding rates under cyclic loading conditions are much less pronounced.This observation,once the corre-sponding static fatigue e ects are considered,indi-cates a di erent cyclic to static fatigue behavior for both materials.

The fact that crack growth under cyclic loading in ceramics is a cojoint e ect of environmental action and (possible)mechanical fatigue degrada-tion leads to evaluate their fatigue behavior,parti-cularly for comparison purposes among di erent materials,in terms of rather relative cyclic to static fatigue e ects.Accordingly,in this investigation one parameter is proposed which intends to weigh the reduction in resistance to crack growth due to mechanical fatigue.It is then used for evaluating and comparing the true cyclic fatigue crack propa-gation behavior of the two di erent PSZs.

This parameter,termed as 0g ,is associated with the real mechanical fatigue e ects involved during crack growth under cyclic loading.It is de®ned as the ratio between the experimentally determined threshold value (K th Y )and that estimated consider-ing environmental-related degradation to occur exclusively under cyclic loading (K th,ce ),i.e.0C K th Y a K th Y e .The cyclic crack growth rate,assuming that there is not any mechanical fatigue,is calculated from the experimentally measured static fatigue data,following the guidelines given by Evans and Fuller.36Under these conditions,crack growth rates related to cyclic loading are estimated to diminish for a given maximum applied stress intensity factor.The decreasing amount would depend upon the exponent coe cient ®tted in the corresponding power-law dependence obtained for the static fatigue experience.

Figure 11shows the relative positions of the experimental and estimated data for (Mg,Y)±PSZ.From that ®gure,the existence of true mechanical fatigue e ects in this material is even reinforced.Similar trend is found for Mg±PSZ.The calculated 0g values for the ®ne-and coarse-grained PSZs are 0.69and 0.76respectively.Based upon the physical meaning of the parameter 0g ,the higher its value,the lower the material sensitivity to real mechanical fatigue degradation.Hence,it is concluded that the ®ne-grained material exhibits a lower intrinsic cyc-lic fatigue resistance to crack growth than the coarse-grained one,although the former has a higher static fatigue resistance to crack growth than the latter.However,the di erences are rela-tively small when compared with those determined for Y±TZP,37a material whose behavior is strongly a ected by the environment.29Such slender varia-tions between the rationalized behaviors of (Mg,Y)±PSZ and Mg±PSZ are expected,once the PSZ character of both materials is

considered.

Fig.11.Schematic graph showing relative positioning of experimental and estimated crack growth rate versus K m x data under

cyclic and static loading conditions.

The microstructure of the®ne-grained(Mg,Y)±PSZ was much more heterogeneous,in terms of both precipitate size distribution and volume fraction of large monoclinic precipitates,than that of the coarse-grained PSZ.On the other hand,the number of crack-bridges involving uncracked liga-ments was also signi®cantly higher in the former material than in the latter one.These two observa-tions,following Ho man et al.'s ideas,22are closely related by means of large monoclinic precipitates acting indirectly as seeds for developing the corre-sponding bridges.Cyclic degradation of these bridges was clearly identi®ed by in situ observation during crack growth,and as a consequence,surface crack growth rates were enhanced.Thus,the more widespread®nding of those uncracked ligaments in (Mg,Y)±PSZ than in Mg±PSZ would indicate a similarly more pronounced mechanical fatigue e ect,as observed.Although this approach has initially been postulated by Ho man et al.22in attempting to explain a higher prevalence of erratic FCG behavior in larger cubic grain size Mg-PSZs observed in a previous work,27the dependence of mechanical fatigue e ects on frequency of uncracked ligaments is di cult to discern from their experimental results,mainly because they correspond to materials with quite di erent intrinsic fracture toughness and do not include crack growth data under sustained loading for all the conditions studied.

In order to attain a®ne-grained microstructure hard spinel particles were added to the material here studied.Following experimental evidences such as those shown in Fig.6,it is suggested that those particles may interact with crack faces,and hence,provide a particular mechanical fatigue mechanism in terms of wedging e ects developing during the unloading portion of a cycle due to crack closure.Such particle-crack faces interaction may induce,as previously postulated for fatigue crack growth in silicon nitride,38nucleation and growth of microcracks at the crack tip;and thus,a more pronounced mechanical fatigue e ect in the ®ne-grained(plus hard spinel particles)structure than in the coarse-grained(without any second phase particles)one.This mechanism is then sug-gested to be a complementary reason for the higher relative cyclic fatigue degradation observed for (Mg,Y)±PSZ as compared to the conventional Mg±PSZ previously studied.

4Conclusions

The fracture and fatigue characteristics of a®ne-grained(Mg,Y)-PSZ have been investigated.On the basis of the experimental®ndings the following conclusions can be drawn:

1.The addition of spinel particles to partially

stabilized zirconia not only hinders grain growth but also strongly a ects the intrin-sic precipitate size distribution,particularly in terms of fraction of large monoclinic precipitates.

2.The®ne-grained(Mg,Y)±PSZ showed a

higher¯exural strength than that of a com-mercial peak-aged Mg±PSZ used as reference, but without a ecting the toughness-related behavior,i.e.the material exhibited similar high fracture toughness plateau value and pronounced R-curve as those usually reported for Mg±PSZ.

3.As previously found for other PSZ materials,

®ne-grained(Mg,Y)±PSZ clearly displays true mechanical fatigue e ects.Further,once environmental assisted cracking under sus-tained load is considered,they are found to be more pronounced than those determined for a reference coarse-grained Mg±PSZ.However, even if its mechanical fatigue sensivity is higher,(Mg,Y)±PSZ exhibits also higher crack growth resistance than the conventional Mg±PSZ,particularly under static fatigue conditions.

4.The higher mechanical fatigue sensitivity of

the(Mg,Y)±PSZ,as compared to that of a conventional Mg±PSZ,is related to:(1)a lar-ger prevalence of uncracked ligament brid-ging;and hence,a more pronounced e ect associated with its degradation under cyclic loading;and(2)the existence of particular wedging e ects in terms of interactions between crack faces and hard spinel particlesin the crack wake which®nally result in another operative cyclic degradation process at the crack tip.

Acknowledgements

This work was supported by the Spanish Comisio n Interministerial de Ciencia y Tecnologia(CICYT) under grant MAT94-0431.The authors gratefully acknowledge this generous support and also thank M.Marsal,J.M.Manero,and D.Casellas for experimental and analytical assistance in this work. Likewise,one of the authors(R.F.)would like to express his gratitude for the scholarship received from the Instituto de Cooperacio n Iberoamericana (ICI).

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