Influence of oxidation on the high-temperature mechanical properties of zirconia/nickel cermets

1 INFLUENCE OF OXIDATION ON THE HIGH-TEMPERATURE MECHANICAL PROPERTIES OF ZIRCONIA/NICKEL CERMETS A. Mo ales-Rod íguez a,*, A. B a o-León a, G. Rich e b, M. Rühle b, A. Domínguez-Rod íguez a, M. Jiménez-Melendo a a Depa amen o de Física de la Ma e ia Condensada, Uni e sidad de Se illa, Spain b Max-Planck-Ins i u ü Me all o schung, S u ga , Ge many ABSTRACT The in luence o an oxidizing a mosphe e on he high- empe a u e plas ici y o zi conia/nickel ce me s has been s udied by conduc ing c eep es s in ai . The esul ing mic os uc u e has been cha ac e ized by scanning, con en ional and high- esolu ion elec on mic oscopy. Despi e he la ge mic os uc u al changes, he composi es do no exhibi mechanical deg ada ion. Keywo ds: ce me , zi conia, nickel, oxida ion, plas ici y. 1. INTRODUCTION Nowadays ce amic-me al composi es (ce me s) based on zi conia and nickel a e in ensi ely s udied due o hei high- alue added and mul i unc ional applica ions as senso s, he mal ba ie s and pa icula ly anodes o solid oxide uel cells (SOFCs) [1- 2 3]. These cells ope a e a high empe a u es in a educing en i onmen . Acciden al oxida ion o he me al phase in he anode can occu o di e en easons (sys em shu down, seal leakage, uel supply in e up ion) and lead o mic os uc u al and mechanical changes ha may ha e signi ican e ec s on he in eg i y and pe o mance o anode-suppo ed SOFC. The mechanical p ope ies o oxidized zi conia/nickel ce me s ha e al eady been in es iga ed a oom empe a u e by 4-poin bending es s [4]. These au ho s ound an inc ease in he bending s eng h o samples oxidized a empe a u es below 650 ºC and c ack de elopmen a highe empe a u es. This di e en beha io was explained in e ms o educ ion o po osi y esul ing om he olume expansion du ing oxida ion o he me allic Ni o NiO and he s esses gene a ed in he zi conia ma ix. Howe e , no mechanical da a a he uel cell ope a ing empe a u es a e a ailable. The aim o he p esen pape is hus o in es iga e he high- empe a u e mechanical p ope ies o zi conia/nickel ce me s de o med in oxidizing a mosphe e and he esul ing mic os uc u e. The esul s a e compa ed wi h p e ious s udies on he same ma e ials ca ied ou in a gon a mosphe e which p e en s he oxida ion o he nickel phase [5,6]. 2. EXPERIMENTAL PROCEDURE The ce me s used in his s udy we e ab ica ed by ICMM (Ins i u o de Ciencia de Ma e iales-CSIC, Mad id, Spain); de ails o he p ocessing ou e can be ound elsewhe e [7]. Two se s o high-pu i y 3 mol% Y2O3-s abilized e agonal zi conia/Ni ce me s wi h ela i e nickel con en s o 20 and 40 ol% (he ea e e e ed o as Ni20 and Ni40, espec i ely) we e p oduced in educing a mosphe e a a inal sin e ing 3 empe a u e o 1430ºC. X- ay analysis con i med ha he as- ecei ed composi es consis only o me allic Ni and e agonal Z O2. The densi y o bo h ba ches was abou 85% o he heo e ical alues de e mined by he ule o mix u es. P isma ic specimens wi h dimensions 4x3x3 mm3 we e cu om he as- ecei ed ce me s o mechanical es s. Comp essi e expe imen s we e ca ied ou unde cons an load a empe a u es T be ween 1000 and 1250 ºC and nominal s esses  be ween 15 and 100 MPa; ai was supplied as oxidan om he beginning o he expe imen s. Iden ical es s bu in a gon a mosphe e o p ese ing me allic Ni a e discussed elsewhe e [5]. Expe imen al da a, leng h o he specimens s ime, we e plo ed as log   -  cu es (c eep cu es), wi h   and  he s ain a e and ue s ain, espec i ely. Da a we e analyzed using he s anda d high- empe a u e powe law o s eady s a e de o ma ion [8]: nQ A exp RT        (1) whe e A is a cons an , n he s ess exponen , Q he ac i a ion ene gy o low and R he gas cons an . The alues o n and Q, which a e ela ed o he de o ma ion mechanism, we e deduced om sudden  o T changes du ing a c eep es . Mic os uc u e cha ac e iza ion o as- ecei ed and de o med samples was ca ied using a Philips XL-20 SEM and a Philips CM-200 TEM (Mic oscopy Se ice, Uni e sidad de Se illa, Spain). TZP/TZP g ain bounda ies and TZP/Ni in e aces we e cha ac e ized a a omic le el by using a JEOL JEM-ARM 1250 HRTEM wi h a poin o poin esolu ion o 0.12 nm; he chemical composi ion o hese bounda ies was analyzed in a VG HB 4 501 UX STEM wi h a hin windows (NORAN) ene gy-dispe si e x- ay spec ome e (Max-Planck Ins i u e o Me al Resea ch, S u ga ) [9]. 3. RESULTS AND DISCUSSION Figu e 1 is a SEM mic og aph illus a ing he mic os uc u e o he as- ecei ed Ni20 composi e. The nickel inclusions (b igh phase) a e ounded, isola ed om each o he and a he uni o mly dis ibu ed h oughou a po ous zi conia ma ix (g ay phase), wi h an a e age pa icle size o abou 1 m. Zi conia g ains ha e also an equiaxed shape bu a much ine mic os uc u e wi h an a e age g ain size o 0.13 m. Ni40 exhibi s iden ical ea u es excep o some con ac be ween nickel pa icles due o he ac ha i s me al con en is abo e he pe cola ion limi ( ound o be close o 34 ol% om elec ical measu emen s [7]). A e de o ma ion in ai , he samples exhibi a g een colo dis inc o nickel oxide, in con as o as- ecei ed and a gon-de o med samples which we e black. SEM obse a ions on c oss-sec ions e ealed a deg ee o oxida ion close o 100% o e he en i e olume o he samples. This esul is consis en wi h he mog a ime ic in es iga ions in ai on massi e zi conia/nickel ce me s pe o med by S a his e al. [4], which ha e shown ha he nickel oxida ion was comple ed a abou 700ºC. No e idence o mac oscopic ailu e o he samples was obse ed e en o he la ge s ains a ained ( = 60 %). Figu e 2 shows he mic os uc u e o ai -de o med Ni20 ce me (now NiO pa icles appea g ey and he zi conia ma ix is b igh in SEM). NiO pa icles ha e an equiaxed 5 shape, de eloping in ag anula po es occasionally in con as wi h dense Ni inclusions in he samples de o med in a gon [5] ha exhibi ed an elonga ed shape wi h he la ges diame e o ien ed pe pendicula o he loading axis in ag eemen wi h hei low c eep esis ance. The inal densi y o he samples a e de o ma ion emained oughly cons an due o a balance be ween educ ion o he o iginal po osi y by c eep and o ma ion o in ag anula po osi y inside he oxidized pa icles. The zi conia ma ix g ains do no expe ience any signi ican change in shape and size in bo h ai and a gon c eep condi ions, as could be expec ed om he supe io duc ili y (i.e., g ain bounda y sliding [10]) and he insensi i i y o c eep s eng h on es ing a mosphe e [11] exhibi ed by e agonal zi conia. I mus be poin ed ou ha no c ack de elopmen by ca i y o mic oc ack coalescence was obse ed in he oxidized specimens, despi e he la ge nominal olume expansion accompanying he Ni oxida ion. Waldbillig e al. [12] ha e ecen ly shown ha 8 mol% Y2O3-s abilized zi conia/nickel ce me s wi h a coa se mic os uc u e expe ienced negligible olume changes upon educ ion and eoxida ion. These esul s sugges ha he exis ing po es helped o accommoda e he olume expansion, a oiding he o ma ion o c acks and ailu e o he specimens. The imp o ed ac u e oughness and s eng h o he e agonal zi conia ma ix is likely o con ibu e also o elax he s esses caused by he Ni oxida ion. TEM obse a ions showed he absence o disloca ions inside bo h he zi conia and he NiO g ains, as ound in as- ecei ed and A -de o med ce me s [6]. Howe e , HRTEM obse a ions (Fig. 3) e ealed a s iking ea u e: he de elopmen o a glassy ilm o abou 1 nm hick along TZP-TZP g ain bounda ies and TZP-NiO in e aces. This 6 amo phous laye has also been obse ed a mul iple g ain junc ions. Such a ilm is no p esen a all in as- ecei ed and A -de o med samples [6], in which la ice inges o wo neighbo ing zi conia g ains a e con ac ed di ec ly each o he a he bounda y wi hou any seconda y g ain bounda y phase in HRTEM. The e a e no esul s abou o ma ion o e na y compounds in he TZP/NiO sys em, e en a ex eme condi ions o high p essu es and empe a u es [13]. Jun e al. [14] ha e ecen ly shown ha NiO we s zi conia a e hea ea men a empe a u es up o 1000ºC. This adhe ence, non-exis en in he TZP/Ni sys em [13,15], could be esponsible o he o ma ion o he i eous in e phase de ec ed along zi conia/NiO in e aces. The p esence o his phase a zi conia/zi conia g ain bounda ies could be due o he g ain neighbo swi ching ha occu s du ing de o ma ion by g ain bounda y sliding [8]. A glassy ilm may ha e de imen al e ec s on he ionic conduc i i y o hese ma e ials [16]. EDXS mic oanalysis pe o med along zi conia g ain bounda ies and in adjacen g ains e idenced a signi ican y ium seg ega ion owa ds he bounda ies, wi h an en ichmen ac o close o 2 wi h espec o he o e all y ia con en . This alue is simila o hose epo ed in as- ecei ed and A -de o med ce me s [6] and monoli hic y ia-s abilized zi conia [10,17]. I is concluded he e o e ha ca ion seg ega ion is no a ec ed by he oxida ion p ocess. Rega ding he c eep s eng h o he composi es, igu e 4 shows he c eep cu es o Ni40 de o med in ai and a gon a simila le els o s ess and empe a u e. I can be seen ha he o me specimen is much mo e c eep esis an han he la e by a ac o close o 100; he di e ence in s eng h dec eases abou a ac o o 20 o Ni20 ce me s. The p esence o well de ined s eady s a es (Fig. 4a) indica es again ha he samples e ained hei s uc u al in eg i y a e he nickel oxida ion. 7 P e ious s udies de o ed o in es iga ing he high- empe a u e mechanical beha io o TZP/Ni ce me s in a gon a mosphe e [5] showed ha he de o ma ion p ocess was mainly go e ned by he ce amic ma ix due o he e y low c eep esis ance o he me allic phase a hose empe a u es. A quan i a i e analysis o he c eep beha io o oxidized ce me s equi es he knowledge o he mechanical p ope ies o monoli hic zi conia and NiO. While e agonal zi conia has been ex ensi ely s udied in he pas ew yea s [10], a ailable da a o NiO a e e y sca ce [18] and p e en s a de ailed analysis. An ex apola ion o he c eep da a o NiO a he p esen es ing condi ions (wi h n = 1 and Q = 390 kJ/mol [18]) shows ha zi conia and nickel oxide exhibi simila c eep esis ances when co ec ed wi h hei espec i e olume ac ions; hus bo h phases would con ibu e o he o e all c eep beha io o he composi es, which is consis en wi h he ha dening exhibi ed by he oxidized ma e ials compa ed o he o iginal TZP/Ni ce me s (Fig. 4). 4. CONCLUSIONS 3 mol% Y2O3-s abilized e agonal zi conia/Ni ce me s wi h 20 and 40 ol% o me al con en ha e been de o med in comp ession a high empe a u es in ai o in es iga e he e ec o nickel oxida ion. The composi es e ain hei s uc u al in eg i y e en a he la ges s ains s udied (60%). I is belie ed ha he la ge olume expansion associa ed o he oxida ion o nickel is accommoda ed by he p eexis ing po es and he supe io oughness o he zi conia ma ix. HRTEM obse a ions e ealed he p esence o a glassy ilm along g ain bounda ies and in e aces, which is absen in he as- ecei ed and 8 he a gon-de o med ce me s. The c eep a es o oxidized composi es a e d as ically educed espec o hei alues unde a gon a mosphe e. AKNOWLEDGEMENTS This wo k has been possible hanks o he inancial suppo o he Minis e io de Ciencia y Tecnología h ough he p ojec s MAT2000-1117 and MAT2003-04199-C02-02 (Spain) and he Max-Planck-Gesellscha h ough he esea ch p og am PMT-CT-2000- 00051 (Ge many). The au ho s a e also g a e ul o S. López-Es eban o ma e ial p ocessing and o J. Thomas and M. Kelsch o echnical assis ance. REFERENCES [1] Reschke S, Bodganow C. Key Eng. Ma e ials 1999; 175-176:1. [2] Sundeen JE, Buchanan RC. Senso . Ac ua o s A-Phys. 1997; 63:33. [3] A una ST, Mu hu aman M, Pa il KC. Solid S a e Ionics 1998; 111:45. [4] S a his G, Simwonis D, Tie z F, Mo opoulou A, Naoumides A. J. Ma e . Res. 2002; 17:951. [5] Mo ales-Rod íguez A, B a o-León A, Domínguez-Rod íguez A, López-Es eban. S, Moya JS, Jiménez-Melendo M. J. Eu . Ce am. Soc. 2003; 23: 2849. [6] Mo ales-Rod íguez A, Rich e G, Rühle M, B a o-León A, Domínguez-Rod íguez A, Jiménez-Melendo M. Submi ed o J. Eu . Ce am. Soc. [7] Pecha omán C, López-Es eban S, Ba olomé JF, Moya JS. J. Am. Ce am. Soc. 2001; 84:2439. [8] Poi ie JP. C eep o c ys als. UK: Camb idge Uni e si y P ess; 1985. 9 [9] Dehm G, E ns F, Maye J, Möbus G, Müllejans H, Phillipp F, Scheu C, Rühle M. Zei sch i ü Me allkunde 87. München: Ca l Hansen Ve lag; 1996. p. 898. [10] Jiménez-Melendo M, Domínguez-Rod íguez A, B a o-León A. J. Am. Ce am. Soc. 1998; 81-2761. [11] B a o-León A, Jiménez-Melendo M, Domínguez-Rod íguez A, Chokshi AH. J. Ma . Sci. Le . 1994; 13:1169. [12] Waldbillig D, Wood A, I ey DG. Solid S a e Ionics 2005; 176:847. [13] Ha el S, Ma io JM, Hague CF. Su ace Science 1992; 269/270:1167. [14] Jun L, Yiyong W, Dianlong W, Xinguo HJ. Ma e . Sci. 2000; 35:1751. [15] So i opoulou D, Ladas S. Su ace Science 1998; 408:182. [16] Badwall SPS, Ciacchi FB, Giampie o KM. Solid S a e Ionics 2005 ; 176:169. [17] G emilla d L, Epicie T, Che alie J, Fan ozzi G. J. Eu . Ce am. Soc. 2005; 25:875. [18] Jiménez-Melendo M, Domínguez-Rod íguez A, Má quez R, Cas aing J. Phil. Mag. A 1987; 56:767.