Jou nal o he Eu opean Ce amic Socie y 43 (2023) 3486–3497
A ailable online 2 Feb ua y 2023
0955-2219/© 2023 The Au ho s. Published by Else ie L d. This is an open access a icle unde he CC BY-NC-ND license (h p://c ea i ecommons.o g/licenses/by-
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R-cu e e alua ion o 3YTZP/g aphene composi es by indi ec
compliance me hod
C. L´
opez-Pe nía
a
,
b
,
*
, C. Mu˜
noz-Fe ei o
a
,
b
,
c
, J. P ada-Rod igo
d
, P. Mo eno
d
, H. Re e on
c
,
J. Che alie
c
, A. Mo ales-Rod íguez
a
, R. Poya o
a
,
b
, ´
A. Galla do-L´
opez
a
a
Uni e sidad de Se illa-ICMS (CSIC), Depa amen o de Física de la Ma e ia Condensada, Apdo. 1065, 41080 Se illa, Spain
b
CSIC-Uni . de Se illa, Ins i u o de Ciencia de Ma e iales de Se illa (ICMS), A da. Am´
e ico Vespucio 49, 41092 Se illa, Spain
c
Uni e si ´
e de Lyon, INSA Lyon, MATEIS UMR CNRS 5510, 7 A enue Jean Chapelle, F-69621 Villeu bane Cedex, F ance
d
G upo de In es igaci´
on en Aplicaciones del L´
ase y Fo ´
onica (ALF-USAL), Uni e sidad de Salamanca, Plaza de la Me ced s/n, 37008 Salamanca, Spain
ARTICLE INFO
Keywo ds:
3YTZP
G aphene-based nanoma e ials (GBN)
Composi es
R-cu e, compliance me hod
ABSTRACT
This wo k add esses he c ack g ow h esis ance o 3 mol% Y ia-doped Te agonal Zi conia Polyc ys alline
(3YTZP) spa k-plasma sin e ed (SPS) composi es con aining wo ypes o g aphene-based nanoma e ials (GBN):
ex olia ed g aphene nanopla ele s (e-GNP) and educed g aphene oxide ( GO). The c ack g ow h esis ance o he
composi es is assessed by means o hei R-Cu e beha io de e mined by h ee-poin bending es s on single
edge “V” no ched beams (SEVNB), in wo di e en o ien a ions o he samples: wi h he c ack pa h pe pendicula
o pa allel o he p essu e axis du ing he SPS sin e ing. The sha p edge no ches we e machined by ul asho
lase pulsed abla ion (UPLA). The compliance and op ical-based me hods o e alua ing he c ack leng h a e
compa ed on he basis o he expe imen al R-Cu e esul s in composi es wi h 2.5 ol% GO es ed in he
pe pendicula o ien a ion. Mo eo e , he ac i a ion o ein o cemen mechanisms is e alua ed by bo h he
ac u e su ace inspec ion by Scanning Elec on Mic oscopy and a compliance analysis. I is shown ha he
indi ec compliance me hod is ele an and eliable o calcula ing he R-Cu e o 3YTZP/GBN composi es. The
e ec o he ype and con en o GBN on he c ack g ow h esis ance o he composi es is also discussed.
1. In oduc ion
Recen ly, G aphene-Based Nanoma e ials (GBN), such as G aphene
Nano-Pla ele s (GNP) o Reduced G aphene Oxide ( GO), ha e a ac ed
a conside able scien i ic in e es due o hei po en ial o imp o e he
mechanical p ope ies o ce amic composi es. In 2011, Walke e al.
epo ed o 235% enhancemen in he ac u e oughness (K
IC
) o Si
3
N
4
ce amics when inco po a ing 1.5 ol% GNP [1]. Since hen, many
s udies on he ac u e oughness o se e al ce amic ma ix composi es
con aining g aphene-based ma e ials o ca bon nano ubes ha e been
ca ied ou [1–8]. Howe e , he esul s ob ained up o da e a y s ongly
wi h di e en pa ame e s such as he ype o GBN used as a ille , i s
con en and i s dis ibu ion h oughou he ce amic ma ix o he
me hod employed o he ac u e oughness de e mina ion. A good
o e iew o he in luence o he ype and con en o he GBN inco po-
a ed in ce amics is gi en by Mi anzo el al. [3], making special emphasis
on hei mechanical, elas ic and wea p ope ies. Despi e he a iabili y
o he esul s in he li e a u e, he ela i e enhancemen o K
IC
o he
composi es when compa ed o he monoli hic ce amic ma ices has been
epo ed o be highe o GO as ille han o GNP. Mo eo e , he
ein o cemen e ec is gene ally achie ed o lowe GO con en s han
o GNP. The p ocessing o ce amic/GBN composi es is a c ucial s ep in
de eloping inc eased mechanical pe o mance. I no op imized, adding
GBN ma e ials o he ce amic could be a he de imen al.
Ano he key aspec explaining he a iabili y o he K
IC
alues e-
po ed in he li e a u e is he me hod o de e mine K
IC
. The inden a ion
me hod (IM) is one o he mos equen ly used due o i s simplici y[9,
10]. I consis s in he indi ec de e mina ion o K
IC
om he di ec
measu emen o he leng h o c acks eme ging om he co ne s o
Vicke s inden a ion p in s. Howe e , he alidi y o he inden a ion
me hod in ce amic/GBN composi es is s ongly ques ioned since he
ac u e oughness is es ima ed om a small egion o he sample which
is subjec ed o e y complex s ess ields [11–14]. Mo eo e , he
mic os uc u al aniso opy associa ed wi h hese ypes o composi es
could lead o he absence o c acks in some di ec ions o he sample,
hampe ing he co ec de e mina ion o K
IC
[4,15]. O he me hods like
* Co esponding au ho a : Uni e sidad de Se illa-ICMS (CSIC), Depa amen o de Física de la Ma e ia Condensada, Apdo. 1065, 41080 Se illa, Spain.
E-mail add ess: [email p o ec ed] (C. L´
opez-Pe nía).
Con en s lis s a ailable a ScienceDi ec
Jou nal o he Eu opean Ce amic Socie y
jou nal homepage: www.else ie .com/loca e/jeu ce amsoc
h ps://doi.o g/10.1016/j.jeu ce amsoc.2023.02.002
Recei ed 2 No embe 2022; Recei ed in e ised o m 30 Janua y 2023; Accep ed 1 Feb ua y 2023
Jou nal o he Eu opean Ce amic Socie y 43 (2023) 3486–3497
3487
h ee- and ou -poin bending es s o single-edge no ched beams (SENB)
allow measu ing oughness o hese ma e ials wi h be e eliabili y
[11]. Howe e , he ac u e oughness measu ed om hese me hods is
s ongly limi ed by a c i ical no ch oo adius [16]. Abo e i , he
calcula ed K
IC
could be highe han he ue ac u e oughness o he
ma e ial. Thus, he oo adius o he no ch should be sha p enough o
ob ain eliable K
IC
esul s. Simila o SENB, he Single-edge V-no ched
beams (SEVNB) consis ed on he ec angula ba s whose no ch is
machined wi h a saw cu . Then he no ch ip is machined in “V” by using
a azo blade wi h a diamond suspension. Mo e ecen ly, i has been
epo ed ha he azo blade can be eplaced by machining a shallow
sha p no ch by ul a-sho pulse lase abla ion (UPLA)[17].
In mos o he s udies, he mechanical ein o cemen o GBN/ce amic
composi es has been associa ed wi h ene gy dissipa ing mechanisms
o igina ed by GBN ha hinde c ack p opaga ion. The mos epo ed
mechanisms a e GBN pull-ou and c ack de lec ion, b anching and
b idging. Despi e he undeniable in luence o hose mechanisms on he
ac u e esis ance o hese composi es, de ailed s udies dedica ed o he
c ack p opaga ion beha io a e e y sca ce, especially in ela ion o
zi conia-based ce amic ma ix composi es [6,18,19]. In ans o ma ion
oughened ce amics like y ia-s abilized e agonal zi conia poly-
c ys als (YTZP), he e agonal o monoclinic phase ans o ma ion
abili y (mainly a ec ed by he g ain size o he ce amic ma ix and he
Y
2
O
3
con en ), also con ibu es o he a iabili y o epo ed ac u e
oughness alues [20–23]. Mo eo e , in his ype o composi es, he
ein o cemen e ec due o g aphene is mo e di icul o assess.
Se e al ce amics show R-cu e beha io which is cha ac e ized by
an inc ease in c ack g ow h esis ance (K
R
) wi h inc easing c ack leng h.
The s udy o he R-cu e o a gi en ma e ial allows he e alua ion o i s
c ack g ow h esis ance since a ising R-cu e is indica i e o he
de elopmen o oughening mechanisms. Despi e he ac ha mos
s udies on GBN/ce amic composi es epo an inc ease in ac u e
oughness, he e a e only a ew ha examine hei R-cu e beha io [6,
16,17]. Mo eo e , mos o hem measu e hese p ope ies only when he
GBN main a-b plane is o ien ed pe pendicula o he c ack p opaga ion
pa h. In mos GBN/ce amic composi es he GBN end o align pe pen-
dicula ly o he comp ession o ce applied du ing sin e ing p o iding a
s uc u e simila o pla ele -like o nac e-like ma e ials. In a ecen
s udy, Liang e al. [24] add essed he excellen mechanical p ope ies
and he ising R-Cu e o Al
2
O
3
ce amics wi h highly o ien ed g aphene
nanolaye s a anged in a nac e-like laye -by-laye s uc u e. Howe e ,
GBN/ce amic composi es a e highly aniso opic so ques ions abou he
in luence o o he o ien a ions o GBN ille wi hin he ce amic ma ix
do s ill a ise. To he bes o ou knowledge only he wo k om
G´
omez-G´
omez e al.[18] conside ed he R-cu e beha io o he ma e-
ials o wo o ien a ions o he sample.
Fo he R-cu e de e mina ion, a es specimen wi h a c ack is loaded
in o de o ha e a s able o con olled c ack p opaga ion. The main
di e ence be ween a ious R-cu e me hods is he p ocedu e o
measu ing he c ack leng h du ing c ack ex ension. The R-cu e is o en
e alua ed om load-displacemen cu es by c ack leng hs es ima ed
indi ec ly by he compliance me hod which conside s he inc ease in he
compliance o he ma e ial o calcula e he c ack leng h [25,26].
Al hough his me hod s ands ou o i s simplici y, i may no be p ecise
o ma e ials wi h a s ong R-cu e, as he compliance could be a ec ed
by b idging s esses a he c ack bo de s [27,28]. Thus, he eal c ack
ex ension could be unde es ima ed. In hose cases, i becomes necessa y
o check i s eliabili y by using o he me hods o a di ec (op ical) c ack
leng h de e mina ion. These me hods may equi e he obse a ion o he
eal c ack leng h a he su ace o he specimen wi h an op ical mic o-
scope and e en, he use o a ideo- eco ding uni o a close s udy.
In-si u measu emen s o he c ack leng h du ing he mechanical es
di ec ly allow s ess in ensi y ac o s calcula ion bu hey a e di icul o
se up, especially when conside ing nanos uc u ed ce amics.
Load-unload displacemen es s could be also implemen ed o measu e
he c ack leng h ex ension a di e en loads, by using op ical o
scanning elec on mic oscopes.
To ge a be e unde s anding o he mechanical beha io o zi conia
ce amic/GBN composi es, his wo k is conduc ed wi h he ollowing
objec i es: (i) o alida e he indi ec compliance me hod o he R-cu e
calcula ion in zi conia/GBN composi es, by compa ing he esul s wi h
he di ec op ical measu emen o c ack ex ension and (ii) o e alua e
he in luence o he inco po a ion o wo di e en GBN ma e ials
(ex olia ed G aphene Nano-Pla ele s (e-GNP) and educed G aphene
Oxide ( GO)) and hei o ien a ion on c ack g ow h esis ance o spa k
plasma sin e ed composi es.
2. Expe imen al p ocedu e
2.1. Ma e ials p epa a ion
2.1.1. Powde p ocessing and sin e ing
Comme cial 3YTZP powde wi h 40 nm pa icle size (Tosoh Co po-
a ion, Tokyo, Japan) was annealed in o de o emo e o ganic addi i es
[29] a 850 ºC o 30 min in ai be o e p epa ing he composi e powde s.
GNP ( e . N006-P, la e al dimension <5 µm and 10–20 nm hickness)
and GO ( e . N002-PDE, la e al dimension ~ 7 µm and 2–3 nm hick-
ness) powde s we e acqui ed om Angs om Ma e ials (Day on, Ohio,
USA) and used o p epa e 3YTPZ-based composi es.
Composi e powde s con aining 2.5, 5 and 10 ol% o ex olia ed GNP
(e-GNP) we e p epa ed using a plane a y ball mill (Pul e ise e 7,
F i sch, Ge many) in d y condi ions as desc ibed in [30]. App oxima ely
4 g o he GNP and 3YTZP powde s we e placed in a 45 mL zi conia ja
oge he wi h se en 15 mm-diame e Z O
2
balls. The powde s we e
milled o 4 h a 350 pm. Finally, composi e powde s we e homoge-
nized in an aga e mo a .
Composi e powde s con aining 2.5 ol% GO we e p epa ed using a
p ocedu e ha in ol es he combina ion o ul asonic dispe sion and
high ene gy plane a y ball-milling as desc ibed in [31]. Fi s , he GO
was dispe sed in e hanol using a KT-600 ul asonic p obe (Kon es Inc.,
Vineland, NJ) a 20 kHz and 95% ampli ude o 15 min, in ime in e als
o 5 min o a oid he hea ing o he suspension. Then, he 3YTZP powde
was added o he GO suspension and sonica ed o an addi ional 5 min.
Then, he suspension was milled in he plane a y ball mill in we con-
di ions du ing a sho ime (15 min) and a slow speed (250 pm) o
a oid damage o he GO laye s. Finally, he powde s we e d ied in a
o a y e apo a o and homogenized in an aga e mo a . Pu e 3YTZP
e e ence ce amics we e p epa ed om he 3YTZP powde a e he
annealing ea men .
All composi e and pu e 3YTZP powde s we e consolida ed by Spa k
Plasma Sin e ing (SPS Model HP D25, FCT Sys em GmbH, Ge many) in
acuum a mosphe e a 1250 ºC o 5 min wi h an applied uniaxial
p essu e o 75 MPa. Disc samples (30 o 40 mm diame e and ~4 mm
hick) we e ob ained. The GO educ ion du ing SPS di ec ly p oduces
GO-like ille s.
2.1.2. SEVNB specimen p epa a ion
The R-cu e beha io o he composi es and he 3YTZP benchma k
was assessed on single-edge V no ched beams (SEVNB). F om each
sin e ed disc, ec angula ba s o size 25 ×4 x 3 mm
3
(L x W x B wi h
L=leng h, W=wid h and B= hickness) we e cu and machined in
acco dance wi h he ASTM C1421–10 s anda d es me hod. A p e-no ch
o 2 mm dep h was pe o med mechanically on he 25 ×3 mm
2
ace
wi h a diamond coa ed saw o 0.3 mm hick. In o de o use a sha p
no ch ha will ensu e s a ing a lowe K
I
and hus, ob ain a mo e
p ecise R-Cu e[32,33], ul asho pulsed lase abla ion (UPLA) was
used o p oduce an addi ional sha p no ch o ~ 100 µm dep h a he ip
o he p e-no ch wi h almos negligible he mal load on he subs a e ou
o he abla ed zone [17,18,34]. The lase no ch was p oduced by a Ti:
Sapphi e sys em (Tsunami, Spec a Physics, USA) deli e ing pulses o
du a ion 120 s a λ =795 nm wi h pulse ene gy 34 µJ and a epe i ion
a e o 1 kHz. The ull no ch leng h (a
0
) was es ima ed as he addi ion o
C. L´
opez-Pe nía e al.
Jou nal o he Eu opean Ce amic Socie y 43 (2023) 3486–3497
3488
he p e-no ch and he lase no ch. A a io a
0
/W ~ 0.5 was achie ed o
all he specimens. An example o he sha p no ch ip is shown in Fig. 1a.
In o de o e alua e he e ec o a possible GBN main plane p e e -
en ial o ien a ion wi h espec o he SPS p essing z-axis ha would
a ec he c ack esis ance beha io o hese ma e ials, he es s we e
pe o med on samples in which he no ch was pe o med on he su ace
pe pendicula (┴) o pa allel (
||
) o he SPS p essu e z-axis. The es
con igu a ion is schema ically shown in Fig. 1b and c, aking as an
example he composi e con aining 2.5 ol% GO. In he pe pendicula
con igu a ion he main a-b plane o he GO shee s is pe pendicula o
he c ack plane. In he pa allel con igu a ion he c ack pa h aces he
edges o he GO shee s.
Six SEVNB specimens o each ype o composi e we e p epa ed.
Th ee o hem we e p epa ed in o de o be es ed in he pe pendicula
(┴) con igu a ion and he o he h ee in he pa allel (
||
) one. He eina e
composi e samples a e labeled acco ding o he ype o GBN, i s con en
and he es ed con igu a ion as xGBN⊥o xGBN‖(being “x” he GBN
con en ). Finally, h ee SEVNB specimens o 3YTZP we e es ed in pa -
allel (
||
) con igu a ion and labeled 3YTZP.
2.2. Ma e ials cha ac e iza ion
The densi ies o he sin e ed composi es we e measu ed using he
A chimedes’ me hod, wi h dis illed wa e as he imme sion medium.
The heo e ical densi ies we e calcula ed based on he ule o mix u es
conside ing densi ies o 6.05 g/cm
3
o 3YTZP and 2.2 g/cm
3
o GNP
and GO.
The semi-quan i a i e analysis o he c ys allog aphic phases p esen
in he sin e ed composi es and he e e ence monoli hic 3YTZP sample
was ca ied ou by X-Ray Di ac ion (D8 Ad anced A25 X-Ray di ac-
ome e , (B uke Co Massachuse s, USA).
The GBN dis ibu ion in he 3YTZP ma ix as well as he mo phology
o he ce amic g ains we e cha ac e ized by Scanning Elec on Mic o-
scopy (SEM) pe o med on ac u ed and polished c oss-sec ion su aces
(FEI Teneo, FEI, USA). Back-sca e ed elec ons (BSE) mode was used o
imaging GBN dis ibu ion h oughou he ce amic ma ix. To cha ac-
e ize zi conia g ain sizes, polished su aces we e he mally e ched in ai
a 1150 ºC o 15 min in o de o e eal he g ain bounda ies. The g ain
size was calcula ed as he equi alen plana diame e d =2(a ea/
π
)1/2
and he g ain shape ac o as F=4
π
a ea/(pe ime e )2. Mo e han 500
g ains measu emen s we e aking in o accoun o each sample.
Raman spec oscopy was used o assess he p esence and s uc u al
in eg i y o he GBN in he composi es a e he sin e ing p ocess. In he
composi e con aining 2.5 ol% GO, he spec a we e also acqui ed on
he as-p ocessed composi e powde s and compa ed o he ones ob ained
on sin e ed ce amics o accoun o he in-si u educ ion o GO du ing
he sin e ing p ocess [31,35]. In o de o cha ac e ize he ans-
o mabili y on 3YTZ and he composi es, ac u ed su aces a e usually
analyzed by XRD o Raman spec oscopy. In he p esen wo k, Raman
spec oscopy was used o assess whe he he -m ans o ma ion ook
place on he SEVNB es ed specimens [36]. Six o eigh spec a om each
specimen we e acqui ed on ac u ed su aces using a dispe si e mi-
c oscope (Ho iba Jobin LabRam HR800, Kyo o, Japan) equipped wi h a
He-Ne g een lase (532.14 nm) a 20 mW. Since he -phase may no
ans o m homogeneously on he en i e ac u e su ace [37], se e al
spec a we e acqui ed in di e en and andom loca ions o he ac u e
su aces.
The Young’s modulus o all ma e ials was measu ed a oom em-
pe a u e using a G indoSonic equipmen (MK6-A G indoSonic BVBA
Belgium), ins umen based on he impulse exci a ion echnique in
which 25 ×4 x 3 mm
3
samples we e subjec ed o an ini ial de o ma ion
by means o a ligh mechanical impulse. Immedia ely, he objec ac as a
sp ing-mass sys em and p oduce a ansien mechanical ib a ion. The
equency o his ib a ion depends on he mass o he sample and i s
s i ness, which was de e mined ollowing he ASTM E 1876–01 s an-
da d es me hod.
2.3. R-cu e s udy
SEVNB specimens we e es ed in a h ee-poin ix u e (suppo span
21 mm) using an Ins on 8500 machine (No wood, USA). The suppo
and uppe cylinde s we e made o alumina and had a diame e o 8 mm.
Du ing he es , he specimen de lec ion was eco ded using a Linea
Va iable Displacemen T ansduce (LVDT) placed nea he sample
no ch. The R-Cu es o he ma e ials we e buil om he load-
displacemen (P-δ) da a ob ained om he h ee-poin bending es s
acco ding o wo di e en me hods. In mos cases, R-Cu es we e ob-
ained using he indi ec compliance me hod [26]. In addi ion, and in
o de o alida e his indi ec me hod, he R-Cu e o he composi e wi h
he highes c ack esis ance beha io (2.5 GO┴ composi e) was also
es ima ed om he di ec measu emen o c ack g ow h by op ical mi-
c oscopy (load-unload h ee-poin bending es ).
2.3.1. R-cu e de e mina ion by he indi ec compliance me hod
The ins an aneous applied load (P
i
) and loading poin displacemen
(δ
i
) da a we e eco ded un il he ailu e. The specimens we e loaded in
ai wi h a cons an low speed o 10 µm/min. The R-cu e, showing he
s ess in ensi y ac o (K
IR
) as a unc ion o he c ack ex ension (Δa =a
i
–
a
0
), was ob ained by es ima ing he c ack leng h and he s ess in ensi y
ac o . Fo his, K
IR
was calcula ed by using he ollowing exp ession:
Fig. 1. (a) Op ical images showing an example o he p e-no ch (up) and he lase no ch (down) pe o med on he samples. Schema ics o he es con igu a ion o
he GO samples. The no ch is sawn in he su ace (b) pe pendicula o (c) pa allel o he SPS p essing axis. The GO shee s a e “no mal” o “edge-on”, espec i ely, o
he plana c ack on .
C. L´
opez-Pe nía e al.
Jou nal o he Eu opean Ce amic Socie y 43 (2023) 3486–3497
3489
KIR =g⋅[PiS0
BW3/2]⋅[3(ai/W)1/2
2(1−ai/W)3/2](1)
whe e P
i
is he applied load, S
0
is he span, B is he hickness o he
sample, W is he wid h o he sample, a
i
is he c ack leng h and g is a
unc ion depending on he geome y o he sample. The g unc ion was
calcula ed acco ding o ASTM C1421–10 s anda d es me hod [38].
An i e a i e me hod was used o calcula e he ins an aneous c ack
leng h (a
i
) o each pai o expe imen al P
i
- δ
i
alues as desc ibed by
Munz and Fe [26]. Fi s , he compliance (C
i
) o each pai o da a is
de ined as:
Ci=δi
Pi
(2)
Then a
i
was es ima ed om he measu ed change in he compliance
using he ollowing equa ion:
ai=a(i−1)+[W−a(i−1)
2⋅Ci−C(i−1)
Ci](3)
2.3.2. Valida ion o he indi ec compliance me hod
2.3.2.1. R-cu e de e mina ion by he op ical me hod. Di ec R-cu e
de e mina ion consis s on he op ical measu emen o he c ack ex en-
sion du ing he SEVNB es . Loading-unloading sequences on he SEVNB
specimens we e pe o med in ai wi h a cons an speed o 5 µm/min and
load-displacemen cu es we e eco ded. K
IR
was s ill calcula ed using
Eq. (1) wi h P as he load a he maximum δ o each load-de lec ion
cu e. He e, he c ack-leng h (a
op
) was measu ed di ec ly by op ical
mic oscopy a e each load-unload cycle o cons uc he R-cu e which
was compa ed wi h he one ob ained by he indi ec compliance
me hod.
The compliance was calcula ed om he load-displacemen poin (P-
δ) acco ding o Eq. (2) and i s e olu ion as a unc ion o he c ack leng h,
C(a), was i ed o a linea unc ion:
C(a) = Ma +N(4)
This unc ion was used o es ablish an empi ical exp ession o es i-
ma e he c ack leng h as a unc ion o he measu ed changes in he
compliance:
a=C−N
M(5)
This exp ession was used o ecalcula e he R-cu e ob ained by he
indi ec compliance me hod in Sec ion 2.3.1. This is, he co ec ed R-
cu e was buil wi h K
IR
calcula ed om he load-displacemen da a o
he i s expe imen acco ding o Eq. (1) bu subs i u ing he c ack
leng h calcula ed om Eq. (5).
2.3.2.2. Re-no ching es and compliance analysis. In o de o simula e
he p opaga ion o a c ack in an ideal ma e ial ee o b idging o
ein o cemen mechanisms, a hi d s udy was pe o med. Du ing his
expe imen , he no ch leng h o he specimen es ed was successi ely
ex ended ( e-no ching) wi h a diamond coa ed saw. The e-no ching
ope a ion was ca ied ou a e loading-unloading sequences, pe -
o med in ai and a 5 µm/min. The applied load was s opped igh
be o e he c ack could s a ed o p opaga e, so no c ack p opaga ion was
enhanced. The machined no ch simula ed a ull c ack wi hou b idging
and i s leng h was measu ed a e being sawn o each load-unload es
by using op ical mic oscopy. The compliance was calcula ed as he slope
o he linea pa o he P-δ cu e ob ained o each no ch leng h. Based
on his e-no ching expe imen , he compliance o e-no ched sample
was compa ed o he e olu ion o he compliance o he c acked spec-
imen subjec ed o he es desc ibed in he p e ious sec ion. This ga e
ise o an es ima ion o c ack b idging s esses [39].
3. Resul s and discussion
3.1. Ma e ials cha ac e iza ion
Well densi ied composi es ha ing ela i e densi ies ≥98.6% o he
heo e ical densi y (Table 1) we e ob ained a e he SPS sin e ing
p ocess. The addi ion o GBN ille s sligh ly a ec ed 3YTZP densi ica-
ion. The 3YTZP e e ence ce amic showed a ela i e densi y o 99.4%
while i anged om 99.4% o 97.9% (2.5% and 10 ol% e-GNP,
espec i ely) and eached 98.6% in 2.5 ol% GO composi es. This sligh
dec ease in densi y is in ag eemen wi h p e iously published s udies
[30,31,35].
The p esence o he GBN phase in sin e ed composi es was con i med
by Raman spec oscopy as he cha ac e is ic bands D, G and 2D loca ed
om ~1350 o ~2700 cm
-1
and ela ed o g aphi ic ma e ials we e
obse ed in all he Raman spec a (no shown he e). We ound ha
du ing he SPS p ocess he GO phase was in-si u educed o GO, as
con i med by he a ia ion in he in ensi y and wid h o D, G and 2D
bands [31]. Mo eo e , ega ding composi es wi h e-GNP, we also
co obo a ed he absence o GNP de e io a ion a e he SPS p ocess [30,
35].
The c ys allog aphic phases p esen in monoli hic 3YTZP and he as-
sin e ed composi es we e checked by a semi-quan i a i e XRD analysis
(no shown he e). The only phase de ec ed in all samples was he
educed e agonal zi conia (Z O
1.95
). The educ ion o he e agonal
phase is a consequence o he educing condi ions du ing he SPS since
he powde s a e placed inside a g aphi e mold and he sin e ing p ocess
akes place unde acuum [30,31]. Al hough some au ho s ha e e-
po ed ha he e agonal o monoclinic zi conia phase ans o ma ion
could be induced a e sin e ing in g aphene con aining composi es [29,
40], he main XRD monoclinic zi conia peaks loca ed a 2θ =28.2 y 31.4
º we e no obse ed in any o he samples p epa ed in he p esen wo k.
Fig. 2 shows he dis ibu ion o he g aphene phase in he pe pen-
dicula o he SPS p essing axis c oss-sec ions o he composi es. Due o
he di e en a omic numbe o he wo main phases, hey can be
dis inguished by BSE-SEM inspec ion since he da k phase will co e-
spond o GO o e-GNP while he clea one will be 3YTZP. A ema kable
di e ence in he GBN size and dis ibu ion is obse ed in composi es
con aining GO compa ed o e-GNP. The GO phase appea s as la ge
shee s wi h hei main a-b plane aligned pe pendicula ly o he SPS
p essing axis (Fig. 2a). In con as , he e-GNP phase exhibi s a smalle
size (Fig. 2b o d) and he pla ele s a e andomly dis ibu ed h oughou
he ce amic ma ix, wi hou any p e e en ial o ien a ion. This dis ibu-
ion ea u es a e obse ed in all e-GNP composi es, independen ly o he
amoun o g aphene.
Table 1 shows he es ima ed g ain sizes in sin e ed, polished and
he mal-e ched su aces. No g ain e inemen was obse ed in he
composi e con aining 2.5 ol% GO which exhibi ed a g ain size simila
o ha o 3YTZP (0.24 and 0.23 µm, espec i ely). Howe e , o com-
posi es con aining e-GNP and independen ly o he con en o g aphene
phase (i.e. 2.5–10 ol%), a sligh ly lowe g ain size (~ 0.18 µm) was
ob ained. Ce amic g ain g ow h inhibi ion by g aphene nanoshee s has
been p e iously linked o he ac ha nanopla ele s su ounding
ce amic g ains ac as di usion ba ie s hinde ing he mo emen o he
g ain bounda ies du ing sin e ing [41–43]. In 3Y-TZP/e-GNP
Table 1
Absolu e and ela i e densi ies, g ain size (d
plana
) and elas ic modulus (E) o he
monoli hic 3YTZP and he GBN/3YTZP composi es.
Samples
ρ
exp
(g/cm
3
)
ρ
el
(%) d
plana
±s.d. (µm) E (GPa)
3YTZP 6.01 ±0.01 99.4 ±0.1 0.23 ±0.10 198
2.5 ol% GO 5.87 ±0.04 98.6 ±0.3 0.24 ±0.13 185
2.5 ol% e-GNP 5.93 ±0.02 99.4 ±0.2 0.18 ±0.08 200
5 ol% e-GNP 5.80 ±0.01 98.7 ±0.3 0.17 ±0.07 191
10 ol% e-GNP 5.58 ±0.01 97.9 ±0.2 0.18 ±0.08 155
C. L´
opez-Pe nía e al.
Jou nal o he Eu opean Ce amic Socie y 43 (2023) 3486–3497
3490
Fig. 2. BSE-SEM images om he c oss sec ions pe pendicula ly o he SPS p essing axis o he sin e ed composi es wi h (a) 2.5 ol% GO, (b) 2.5 ol% e-GNP, (c)
5 ol% e-GNP and (d) 10 ol% e-GNP.
Fig. 3. Load-displacemen cu es unde con inuous loading a 10 µm/m o he monoli hic 3YTZP and he composi es con aining (a) 2.5 ol% GO, (b) 2.5 ol% e-
GNP, (c) 5 ol% e-GNP and (d) 10 ol% e-GNP, in he pa allel (
||
) and pe pendicula o ien a ions (┴).
C. L´
opez-Pe nía e al.

Jou nal o he Eu opean Ce amic Socie y 43 (2023) 3486–3497
3491
composi es, he highe hinde ing e ec may be associa ed wi h smalle
(and he e o e mo e nume ous) g aphene pla ele s which a e mo e ho-
mogeneously dis ibu ed. Thei small size was p omo ed by he use o
he plane a y ball milling unde d y condi ions [30]. Thus, he e is a
highe p opo ion o e-GNP su ounding a highe numbe o zi conia
g ains and hinde ing hei g ow h.
A dec ease in he elas ic modulus (E) o 3YTZP-based composi es
a e GBN addi ion was obse ed in 2.5 ol% GO, 5 ol% e-GNP and
10 ol% e-GNP (Table 1). Mo eo e , E dec eased as he e-GNP con en
inc eased. Al hough po osi y can play a c ucial ole o dec easing he E
o he composi es, a simila densi ica ion deg ee was achie ed o all he
composi es so his beha io can be ela ed o he inco po a ion o mo e
elas ic g aphene phases in o he igid ce amic ma ix [4,44,45]. Despi e
o he high Young’s modulus expec ed in a mechanically ex olia ed
monolaye o g aphene (~ 1.0 TPa [46]), he elas ic modulus o a
monolaye o GO is only ~ 0.25 TPa and i dec eases abou one o de o
magni ude in GO consis ing o mo e han 3 laye s [47]. The elas ic
modulus o GNP is also conside ably lowe han he one o g aphene
[48]. This would he e o e explain he dec ease o E when inco po a ing
he GO and e-GNP in 3YTZP zi conia.
3.2. E ec o GBN addi ion on he c ack esis ance beha io o 3YTZP
Fig. 3 shows he h ee-poin bending load-displacemen cu es o
no ched samples o zi conia (3YTZP) and 3YTZP composi es ( GO and
e-GNP) un il ac u e. Rega ding 3YTZP/2.5 ol% GO composi es, he
lexu al de lec ion and maximum load achie ed be o e ac u e was
highe in he pe pendicula o ien a ion (┴) compa ed o he pa allel
o ien a ion (
||
) and he monoli hic 3YTZP (Fig. 3a). On he o he hand,
he inco po a ion o e-GNP o he ma ix had no signi ican e ec in he
load-displacemen cu es o 2.5 and 5 ol% e-GNP o he wo es ed
o ien a ions (Fig. 3a and b). Mo eo e , he maximum load be o e ailu e
dec eased in 3YTZP/10 ol% e-GNP composi es (bo h di ec ions,
Fig. 3d).
Rela ed R-cu es o composi es and monoli hic 3YTZP calcula ed by
he compliance me hod a e displayed in Fig. 4. I can be no iced ha
monoli hic 3YTZP p esen ed a e y weak R-cu e. The ini ial s ess in-
ensi y ac o (K
IR0
) necessa y o he onse o s able c ack g ow h was o
~3.3 MPa m
1/2
and i inc eased du ing c ack ex ension up o a pla eau
alue o ~ 3.6 MPa m
1/2
. This K
IR0
alue is e y close o he one ob-
ained by Eichle e al. [49] in 3YTZP ce amics ha ing simila g ain size.
Such a weakly ising 3YTZP R-cu e can be explained by he ela i ely
small g ain size which also in luences he ac u e oughness o Y-TZP
ma e ials [50,51]. In gene al, ce amic c ack g ow h esis ance inc eases
wi h inc easing g ain size. In he case o zi conia-based ce amics, highe
g ain size means a s onge abili y o s ess-induced e agonal o
monoclinic phase ans o ma ion [50,52]. Due o he singula s ess ield
nea a c ack ip, e agonal zi conia g ains can unde go a s ess-induced
ma ensi ic ans o ma ion c ea ing a ans o ma ion zone ahead o he
c ack ip. As he olume o monoclinic cell is highe han ha o he
e agonal one, a olume ic expansion s ain o abou 4–5% occu s and
comp essi e s esses a e applied along he c ack leng h. The comp es-
si e s esses lead o a shielding s ess in ensi y ac o which mus be
o e come du ing c ack p opaga ion, in o he wo ds, he applied s ess
in ensi y ac o mus be inc eased o main ain s able c ack g ow h.
Clea ly, he low alues o K
IR0
and o he pla eau alue a e linked o he
small g ain size o he SPS-sin e ed 3YTZP. This would indica e a lowe
p opensi y o phase ans o ma ion oughening han 3YTZP sin e ed a
highe empe a u e, unde con en ional p ocesses, as discussed below.
The inco po a ion o 2.5 ol% GO clea ly modi ies he ac u e
beha io o 3YTZP ma ix (Fig. 4a). Highe c ack esis ance was ach-
ie ed when he plana c ack on was pe pendicula ly o ien ed o he
GO main a-b plane (Fig. 1a). Al hough his composi e showed an ini ial
K
IR0
alue o ~3.2 MPa m
1/2
simila o ha ob ained in 3YTZP, he
Fig. 4. R-Cu es (s ess in ensi y ac o , K
IR
, as a unc ion o c ack ex ension, Δa) ob ained by he indi ec compliance me hod o he monoli hic 3YTZP and he
composi es con aining (a) 2.5 ol% GO, (b) 2.5 ol% e-GNP, (c) 5 ol% e-GNP and (d) 10 ol% e-GNP, in he pa allel (
||
) and pe pendicula (┴) o ien a ions.
C. L´
opez-Pe nía e al.
Jou nal o he Eu opean Ce amic Socie y 43 (2023) 3486–3497
3492
s ess in ensi y ac o inc eased up o ~ 4.3 MPa m
1/2
. In he pa allel
o ien a ion, he ob ained R-cu e was qui e simila o ha o monoli hic
3YTZP ( he s ess in ensi y ac o sligh ly inc eased om 3.1 MPa m
1/2
o 3.4 MPa m
1/2
). This aniso opic R-cu e beha io is in good ag ee-
men wi h mic os uc u al ea u es obse ed by BSE-SEM in his com-
posi e (Fig. 2a) since s onge ein o cemen is expec ed when he c ack
on is unning pe pendicula o GO plane.
The e ec o inco po a ing e-GNP on he R-cu e o he composi es is
qui e di e en (Fig. 4a, b and c) and e en was nega i e wi h espec o
he mechanical beha io . Fo 2.5 and 5 ol% e-GNP, K
IR
alues we e
sligh ly lowe han hose obse ed on monoli hic zi conia. Al hough he
mic os uc u al obse a ions did no e eal any p e e en ial o ien a ion
o e-GNP, smalle di e ences be ween he wo es ed o ien a ions could
be ela ed o some deg ee o aniso opy o p e e en ial o ien a ion o he
pla ele s. Acco dingly, in 2.5e-GNP and 5e-GNP composi es K
IR
alues
inc eased so ly up o ~3.4 MPa m
1/2
when es ed in he pe pendicula
con igu a ion while hey only eached 3.1 MPa m
1/2
in he pa allel
con igu a ion. When inc easing he con en o e-GNP up o 10 ol%, he
c ack g ow h esis ance ema kably dec eased compa ed o ha o he
composi es wi h 2.5 and 5 ol% e-GNP and he monoli hic 3YTZP.
Mo eo e , mic os uc u al aniso opy is p obably mino as he s eady-
s a e K
IR
alues we e simila o he wo con igu a ions
(~2.8 MPa m
1/2
). In addi ion, unlike he beha io obse ed in mono-
li hic 3YTZP and composi es wi h a lowe e-GNP con en , he R-cu e o
10e-GNP┴ ose ab up ly om he s a o c ack ex ension (Fig. 4d),
ecalling he beha io al eady discussed o 2.5 GO┴ (Fig. 4a).
To assess he e ec o he g aphene phase on he R-cu e beha io o
he composi es, h ee ac o s should be conside ed. The i s one is he
e ec o he s ess-induced e agonal o monoclinic ( -m) zi conia phase
ans o ma ion a he c ack ip, he second one is he e ec o he g ain
size and he las one is he in e ac ion o he GBN ille wi h he c ack
p opaga ion (b idging e ec s).
3.2.1. S ess-induced -m phase ans o ma ion and zi conia g ain size
e ec
Fig. 5 shows he Raman spec a acqui ed on he ac u ed su aces o
monoli hic 3YTZP and composi es, in he ange whe e he main
e agonal and monoclinic zi conia peaks appea ed (100–800 cm
-1
). The
six heo e ically p edic ed Raman peaks o e agonal zi conia [36,53]
a 160, 266, 327, 470, 612 and 647 cm
-1
we e clea ly de ec ed in all he
ac u ed su aces while he wo monoclinic main peaks expec ed o
appea a 178 and 189 cm
-1
we e no obse ed. The absence o mono-
clinic peaks sugges s a null o e y weak -m ans o ma ion du ing
c ack p opaga ion. This lack o phase ans o ma ion is ela ed o he
ine zi conia g ain size de eloped by SPS [50,54]. I is also consis en
wi h he low alues o K
IR0.
The e o e, he c ack esis ance beha io o
he SPS sin e ed composi es de eloped in his wo k is mainly expec ed o
be d i en by he g aphene ille s.
3.2.2. F ac u e mechanisms in luenced by GBN ille s
F ac u e su aces ea u es a e he SEVNB es s e ealed a combi-
na ion o in e - and in ag anula ac u e in all he in es iga ed ma e-
ials (Fig. S1 in he Supplemen a y Sec ion). Howe e , he s ongly
in e g anula ac u e mode obse ed in 3YTZP/e-GNP composi es may
explain he lowe K
IR
alues, as g ain bounda ies a e a eas mechanically
weake han gains whe e c acks can easily p opaga e [55]. In addi ion,
as he la e al size o he GBN in as-p epa ed composi es was no ably
small, hey p obably did no p o ide signi ican ein o cemen o he
3YTZP ma ix.
F ac u ed su aces o composi es con aining e-GNP a e shown in
Figs. 6 and 7 and S1. The la e geome y and he smalle size o he e-
GNP compa ed o GO is clea ly no iced, co obo a ing wha was p e-
iously s a ed in he BSE-SEM images o he Fig. 2. The ac u e su ace
oughness inc eases wi h he e-GNP con en (Fig. 6). Thus, al hough he
ob ained R-cu e beha io seems o be simila o ha o 3YTZP, he
sligh ly lowe K
IR
alues could be ela ed o e-GNP ha can ac as laws
o de ec s a o ing c ack p opaga ion. When inc easing he e-GNP
con en , he amoun o nanos uc u es ac ing as laws inc eases, so a
dec ease o he ac u e esis ance would be expec ed, as obse ed.
Howe e , he composi e wi h 2.5 ol% e-GNP p esen s a simila R-cu e
o he one wi h 5 ol% e-GNP, in bo h o ien a ions (Fig. 4b and c). The
examina ion o he ac u e su aces o he 5e-GNP┴ and he 5e-GNP
||
specimens show some ea u es ha could be ela ed o some oughening
mechanisms. Se e al sphe ical-shaped oids o c a e s can be ound
h oughou he whole ac u e su ace indica ing a lowe ene gy pa h o
c ack de lec ion (Figs. 6b and 7a o c). These ea u es can be clea ly
ound in bo h o ien a ions o he composi e and also o 10 ol% e-GNP.
A close SEM examina ion sugges s ha he e-GNP a e i mly bonded o
he zi conia g ains (Fig. 7d), which could induce he c ack o seek a
lowe ene gy pa h and de lec . I is, in his case, along he e-GNP-zi -
conia in e aces. The sphe ical shape o hese pa hs is no ela ed o
po es bu o he dis ibu ion o he e-GNP h oughou he ce amic ma ix
since he e-GNP seem o be su ounding some egions o zi conia
(Fig. 2). These egions may be ela ed o he p esence o some zi conia
agglome a es o med du ing he powde p ocessing s ep. As i can be
obse ed in Fig. 7c, he in e ace be ween he e-GNP and he zi conia
may be he mos a o able pa h o he c ack o p opaga e. The c ack
p opaga es along he weak e-GNP-zi conia bounda ies c ea ing a
o uous pa h and a oughe su ace han monoli hic 3YTZP. To he bes
o ou knowledge, his is he i s ime ha he sphe ical de lec ion o he
c acks is obse ed on ce amic ma ix composi es. Al hough he mic o-
scopic obse a ions also e eal he occu ence o he pulling ou o he e-
GNP (Fig. 7d), he size o he e-GNP is no enough o p o ide c ack
b idging. In Fig. 7c he b anching o he c ack can also be obse ed,
howe e he ene gy dissipa ion o hese mechanisms is compa a i ely
lowe han he con ibu ion o b idging o he oughening. Thus, lowe
c ack esis ance han monoli hic 3YTZP a e ob ained o hese com-
posi es. These esul s a e in good ag eemen wi h p e ious wo ks [12,
56] ha epo ed ha he GNP p esen a high esis ance o be pulled ou
con ibu ing o he dissipa ion o ene gy associa ed wi h c ack p opa-
ga ion by c ea ing a mo e o uous pa h.
Fo he composi e wi h 10 ol% e-GNP, he p esence o hese
Fig. 5. E alua ion o he e agonal phase by Raman spec oscopy in he
ac u e su ace o he composi es and he monoli hic 3YTZP. Do ed and solid
lines co espond o he specimens es ed in he pa allel (
||
) and pe pendicula
(┴) con igu a ion, espec i ely.
C. L´
opez-Pe nía e al.
Jou nal o he Eu opean Ce amic Socie y 43 (2023) 3486–3497
3493
sphe ical oids is inc eased leading o a e y ough ac u e su ace
(Fig. 6c). This sligh ly inc ease in he c ack o uosi y sugges s ha a
ce ain small p opaga ion inhibi ion e ec was exe ed by e-GNP.
Mo eo e , c ack de lec ion is also e idence in Fig. 7e, as shown by he
ab up change in c ack di ec ion. Howe e , he low K
IR
alues ob ained
o his composi e sugges ha he e ec o hese mechanisms o ene gy
dissipa ion is coun e balanced by he p esence o mo e e-GNP ha a o
he c ack p opaga ion [12]. As in he composi es wi h lowe con en o
GNP, he c ack p opaga ion akes place along he GBN-zi conia in-
e aces, which gi es a o uous pa h and a oughe su ace han
monoli hic 3YTZP bu a lowe c ack esis ance o e all. This would
explain also why, despi e he e y low K
IR
alues o his composi e, i s
R-cu e ose e y s eeply a he beginning, o low c ack displacemen s
(Fig. 4d).
Fu he SEM examina ion o he ac u ed specimens e eals ha he
GO laye s in e ac wi h he p opaga ing c acks p o iding di e en
mechanisms o enhance he ac u e esis ance o he ma e ial in he
pe pendicula o ien a ion. Figs. 6 and 8 shows he ac u e su ace o
he 2.5 GO┴ composi e. The p e e en ial o ien a ion o he GO shee s,
wi h hei a-b main plane pe pendicula o he SPS p essing axis is
clea ly obse ed. Figs. 6 and 8 a e clea examples o he mul iple
oughening e en s occu ing as he c ack p opaga es. The phenomena o
c ack b anching as expe ienced du ing c ack p opaga ion is highligh ed
in Fig. 8a. Figs. 6 and 8b e idences he de lec ion o he c ack when i
encoun e s he g aphene plane. The ac ha se e al p o uded GO
shee s a e no iced on he ac u e su ace sugges s he pulling ou o he
GO shee s, which indica e he po en ial o c ack b idging being
in ol ed. I is likely ha c ack b idging is he main sou ce o oughening
o he 2.5 GO┴ composi e. The GO shee s also show an undula ion
shape ha has been also p oposed as a po en ial ein o cemen mecha-
nism [57]. These obse a ions would explain he s ong R-cu e and he
high K
IR
pla eau alue ob ained o his composi e. The e ec o he GO
laye s on he enhancemen o he c ack esis ance beha io is signi ican
only when hei main a-b plane is pe pendicula ly o ien ed o he plana
c ack on . Fig. 6e shows ha he de lec ion o he c ack signi ican ly
dec eases when he c ack on is acing he GO laye edges (pa allel
o ien a ion). Acco ding o G´
omez-G´
omez e al.[18], he eason o his
beha io is he lowe “e ec i e” a ea o g aphene ma e ial in e sec ing
he c ack on compa ed o wha happens when he c ack aces he a-b
plane o GO.
Fig. 6. F ac u e su aces o he monoli hic 3YTZP and he composi es.
C. L´
opez-Pe nía e al.
Jou nal o he Eu opean Ce amic Socie y 43 (2023) 3486–3497
3494
3.3. Valida ion o he R-cu e de e mina ion by he indi ec compliance
me hod
As i can be no ed in Fig. 4, he addi ion o a 2.5 ol% GO clea ly
modi ies he ac u e beha io o monoli hic 3YTZP in he pe pendicula
con igu a ion. I is he s onges R-cu e beha io de eloped in his
wo k, wi h he highes K
IR
imp o emen wi h c ack ex ension and he
only one wi h a highe pla eau compa ed o ha o he monoli hic
3YTZP. Fo his eason, he alida ion o he compliance me hod in
ob aining he R-cu e o 2.5 GO┴ composi e was assessed. Le us ecall
he e ha he compliance me hod can unde es ima e he eal c ack
leng h i la ge b idging in e ac ions ope a e du ing he c ack p opaga-
ion, in pa icula o la ge c ack ex ensions[58]. This beha io is ela ed
o b idging e ec s because a pa ially b idged c ack wi h unb oken
ligamen s leads o a highe s i ness o he c acked specimen han a c ack
wi hou b idging in e ac ions.
Fig. 9a shows he R-cu e ob ained by he di ec op ical measu e-
men o he eal c ack leng h on a side su ace o he 2.5 GO┴ es
specimen (blue/black ci cles). The R-cu e ob ained by he compliance
me hod (g een ci cles) is also included o compa ison as well as he
co ec ed one (emp y ci cles). The e olu ion o he compliance (C) wi h
he eal c ack leng h i ed o a linea unc ion acco ding o Eq. (2) is
shown in Fig. 9b. The ob ained pa ame e B (slope) in his i ing was
B
eal c ack
=(4.0 ±0.3) x10
-4
N
-1
. Based on his i ing, he c ack leng h
calcula ed by he compliance me hod was co ec ed and hus, he R-
cu e ob ained by he indi ec compliance me hod. The compliance
calcula ed in a se o samples wi h a i icial c ack leng hs imposed by
successi ely e-no ching o he 2.5 GO┴ composi e is also shown in
Fig. 7. Sphe ical ea u es ound in bo h (a) pe pendicula and (b) pa allel o ien a ions o he composi e wi h 5 ol% e-GNP. (Comp ession axis du ing SPS is
indica ed by yellow a ows). F ac u e su aces e ealing he oughening mechanisms ound in he composi es wi h 5 ol% e-GNP: (c) c ack de lec ion and b anching
and (d) e-GNP pull-ou s. (e) Toughening mechanisms ound in he pe pendicula composi e wi h 10 ol% e-GNP.
C. L´
opez-Pe nía e al.