8 RevAvMaterSi 4(1) 81-85 ENHANCED INTERFACE TOUGHNESS IN BI-MODAL NANO-ALUMINA-TITANIA COATINGS Reeive: November 7, 9 Institute of Problems of Meanial Engineering, Russian Aaemy of Sienes, Bolsoj 61, Vasil Ostrov, St Petersburg, 199178, Russia Abstrat: Two ifferent moes of elamination in nano-alumina-titania oatings on steel substrates, wi are attribute to te onventional fully-melte () an te bi-moal fully-melte an partially-melte (-PM) oating, are isusse Using simple teoretial moels of te oating/substrate erfaes, typial for tese oatings, we analyze te profits of te bi-moal - PM oatings over te onventional oatings in terms of effetive speifi energy of aesion, ritial strain of elamination, an ritial tikness of te oating It is sown tat use of te bimoal -PM oatings an lea to oubling te erfae tougness an inreasing te ritial strain by 5% an te ritial tikness by more tan 13% 1 INTRODUCTION Te struture an meanial properties of nanorystalline bulk materials, films an oatings represent te subjet of ensive resear efforts involving experimental analysis, omputer simulations an teoretial moeling; see, eg, [1-1] In reent years, a partiular attention as been pai to nanorystalline erami films an oatings [1-1] wose best samples exibit superstrengt, superarness an goo wear resistane However, in te ase of oatings, tese outstaning properties an be use in pratial appliations if te tougness of te oatings an oating/substrate erfae is also ig enoug One of te ways to enane te tougness is te fabriation of so-alle bi-moal nanostrutures in te oatings For example, te reation of bi-moal nanostruture in nano-alumina-titania (Al -13wt% TiO ) plasma-spraye erami oatings on steel substrates resulte in an inrease in te erfae tougness from to 45 J m - [-4] Te first a a mirostruture onsisting primarily of fully- Corresponing autor: M Yu Gutkin, e-mail: gutkin@efipmeru; IA Ovi'ko, e-mail: oviko@nanoipmeru -Al [4] Te seon value was arateristi roune mirostrutural features [4] Te substru- m iameter) onsiste of -Al m) surroune by a TiO -ri amorpous pase Sanning an transmission eletron mirosopy observations emon- (before meanial testing), wereas te PM/steel Te failure moes in te two types of oatings were foun rater ifferent [-4]: long, wie separate raks leaing to extene elamination were row multiple raks an suppresse elamination PM regions being enrie by TiO amorpous pase serve as obstales for rak propagation in terfae Te main possible reasons are tat (i) te
Enane erfae tougness in bi-moal nano-alumina-titania oatings 83 Fig 1 Different moes of elamination of (a, b) te onventional oatings an (, ) te bi-moal - PM oatings from te steel substrate Parts (a) an () image te initial states of te oating/substrate erfaes before external loaing; parts (b) an () sow te propagation of a large erfae rak troug (b) joining te initial erfae raks or () bypassing te PM regions free surfae energy of te amorpous pase is larger to te ontat of te steel substrate wit te amorpous pase is lower tan tat for te ontat wit In te present paper, we onsier te ifferene between te two moes of failure at te nano-alumina-titania oating/substrate erfae in terms of effetive speifi energy of aesion, ritial strain of elamination [5], an ritial tikness of te oatings [18] MODEL Consier a moel rak wi propagates along te oating/substrate erfae in two ifferent ases of onventional (Fig 1a,b) an bi-moal -PM (Fig 1,) nanoerami oatings Before external loaing, te onventional oating/substrate erfae ontains many initial erfae raks (Fig 1a), wile te bi-moal -PM oating/substrate erfae (Fig 1) is free of te initial raks Uner external loaing, te proess of elamination proees troug te propagation of a large erfae rak, joining te initial raks in te first ase (Fig 1b) or bypassing te PM regions in te seon ase (Fig 1) Let te tikness of te oating be mu larger tan te arateristi size of a PM region Ten in bot te ases, at te sale of, te erfae rak looks planar, an te ritial onition of its propagation an be esribe as G, (1) were G is te energy release rate wi is arateristi for te elamination proess an is te effetive speifi energy of aesion of te oating an substrate Following Cerepanov [5], G is given by G 1 E, () were is te tensile strain in te oating, is te elasti strain ue to te misfits in eratomi spaings an termal expansion oeffiients of te oating an te substrate, E is te Young moulus of te oating an is its Poisson ratio From (1) an () yiels tat te ritial strain is 1 rit (3) E Following Cerepanov [5], te elamination proees if reaes rit Let us onsier wat appens wit terms entering formula (3) wen te onventional oating is replae by te bi-moal -PM oating First, te absolute value of iminises ue to lower tem-
84 peratures of te system fabriation [4] Sine onventional oating [4]), a erease in its moulus seems to lea to a erease in te ritial strain rit On te oter an, Bansal et al [4] notie tat a erease in te moulus of an enane te aesion between te bi-moal -PM oating an te steel substrate an result in elimination of initial erfae raks Tis ontribution an be taken o aount by means of appropriate moifiation of te effetive speifi energy of aesion Terefore, seon, one soul onsier te anges in tis latter term A moel of te erfae between te onventional oating an te steel substrate is sown in Fig a Before external loaing, te moel erfae area of square S is ompose of te bone region (sown wite in Fig a) an te initial erfae raks (sown blak in Fig a) of sum square S r Terefore te effetive square of te bone region is S = S - S eff r If uner external loaing tis bone region is transforme to a rak (elamination), te orresponing inrease in te surfae energy an be written as follows -st S, (4) eff eff er st were an er st are te speifi surfae energies of te onventional oating an te steel substrate, -st respetively, an is te speifi energy of te oating/steel erfae before external loaing Wit te surfae fration of initial erfae raks s r = S r /S, te effetive speifi aesion energy reas eff -st 1 s r er st (5) S Terefore, te ritial strain of elamination in te ase of te onventional oating is - st 1 s 1 r er st rit ( 6 ) E In te ase of te bi-m oal -P M oating, te oating/substrate erfae pratially oes not ontain initial erfae raks an is om pose of te region (sow n planar w ite in Fig b) an te P M inlusions (sow n as wite boxes of arateristi im ensions in Fig b, w ere is a im ensionless param eter) [-4] Let uner external loaing te elam ination our troug opening erfae raks between te region an te steel substrate, an between te region an te PM inlusions (see Fig 1b), as it w as observe in experim ents [-4] Terefore, te orresponing inrease in te surfae energy is given by 1 4, -PM - st P M -PM S N N ( 7 ) eff er st er er PM w ere N is te num ber of te PM inlusions per te oating/substrate erfae square S, is te speifi er -P M surfae energy of a PM inlusion, an is te speifi energy of te /PM erfae before external loaing W it te surfae fration of PM inlusions s PM = N -P M /S, te effetive speifi aesion energy takes te form -PM -PM eff -st PM -PM 1s s 14 PM er st PM er er, (8) S from wi te ritial strain of elamination in te ase of te bi-moal -PM oating yiels: - st PM -P M s s -PM 1 1 4 1 PM er st PM er er rit E -PM ( 9 ) 3 RESULTS Let us make te numerial estimates of te quantities entering formulas (5) an (8) For efiniteness, we take PM Al O -PM Al O Al O an wit 3 3 3 3 J m - [6]; st -st st b st /8 4 J m -, er er er er / er
Enane erfae tougness in bi-moal nano-alumina-titania oatings 85 were st 77 GPa an b st moulus an te eratomi istane, respetively, of te steel substrate, an usual approximation b/8 between te speifi surfae energy an te sear moulus of a metal is use Ten, for rater realisti values s r s PM 3 an, we obtain J m - -PM an 46 J m - Terefore, aoring to formula (1), G J m - an G -PM -PM 46 J m -, wi orrespon well to te experimentally measure values Jm - an 45 Jm - [-4], respetively Using te quantities G an G -PM as measures of te erfae tougness, we an onlue tat use of te bi-moal -PM oatings an result in oubling tis very important arateristi of te oating/substrate system Taking tese values on min, it is easy to estimate te gain in te ritial strain of elamination ue to replaement of te onventional oating by te bi-moal -PM oating Using formulas (5), (6), (8) an (9), an te well known relationsip E = (1 + ), we foun -PM 1 -PM -PM rit rit 1 -PM (1) -PM were = - < For -PM = =, -PM =154 GPa, = 168 GPa, an = 1-3 m [4], formula (1) gives a value from (153-11)1-3 + = 51-3 + (for = 1 m) to (88-58)1-3 + = 3 1-3 + (for = 3 m) Tus, te first term of te ritial strain of elamination inreases by 5% ue to te replaement of te onventional oating by te bi-moal -PM oating Tis gain is iminise, owever, by te seon negative term wi is ar to estimate base on available ata It is wort noting tat in te pratie of fabriating te nanoerami oatings, te situation is possible wen te oating starts to elaminate after its tikness as reae a ritial value [18] Assuming = in tis ase, from formulas (1) an () we obtain tis ritial value as 1 rit (11) Comparing te ases of te onventional oating an te bi-moal -PM oating, one an estimate te ratio -PM / as follows rit rit Fig Moels of (a) onventional oating/steel an (b) bi-moal -PM oating/steel erfae areas of square S before external loaing Blak features in (a) enote te erfae raks (elaminations) Wite boxes in (b) image te PM regions of imensions -PM -PM 1 rit -PM -PM (1) 1 rit -PM Wit te aforementione numerial values for te system parameters, formula (1) gives -PM rit rit 3 -PM (13) -PM Taking o aount tat / >1 [4], we an onlue tat -PM / > 3 rit rit 4 SUMMARY We ave isusse two ifferent moes of elamination in nano-alumina-titania oatings on steel substrates, wi are attribute to ifferent mirostrutures of te oating Te first moe, wi is te propagation of a large rak along a relatively planar erfae ontaining many small initial raks, as been observe earlier in te onventional oating/steel erfae Te seon moe is te growt of a large urvilinear rak wi as to bypass te PM regions isperse along te erfae Tis situation is typial for te bi-moal -PM oatings Using simple teoretial moels of tese erfaes, we ave estimate te profits of te bimoal -PM oatings over te onventional oatings in terms of effetive speifi energy of aesion, ritial strain of elamination, an ritial tikness of te oating It as been sown tat
86 use of te bi-moal -PM oatings an lea to oubling te erfae tougness an inreasing te ritial strain by 5% an te ritial tikness by more tan 13% Our estimate for te erfae tougness is in a goo aorane wit earlier experimental measurements ACKNOWLEDGEMENTS Te work was supporte, in part, by te Russian Founation of Basi Resear (Grants 8-1-5- a an 8--34-a), te Russian Aaemy of Naval Resear (grant N14-8-1-45), an te National Siene Founation Grant CMMI #77 REFERENCES [1] T Yamasaki, H Yokoyama an T Fukami // Rev Av Mater Si 18 (8) 711 [] Rev Av Mater Si 18 (8) 59 [3] A Swierska-Sroa, G Kalisz, B Palosz an N Herlin-Boime // Rev Av Mater Si 18 (8) 4 [4] J Dutkiewiz, W Maziarz an L Jaworska // Rev Av Mater Si 18 (8) 64 [5] E Biztek, C Branl, PM Derlet an H Van Swygenoven // Pys Rev Lett 1 (8) 3551 [6] G J Weng // Rev Av Mater Si 19 (9) 41 [7] AG Seinerman // Rev Av Mater Si 19 (9) 63 [8] // Rev Av Mater Si 19 (9) 13 [9] Rev Av Mater Si 1 (9) 139 [1] an AG Seinerman // Rev Av Mater Si 1 (9) 99 [11] RA Anrievski // Rev Av Mater Si 1 (9) 17 [1] Nanostruture Tin Films an Nanoispersion Strengtene Coatings, e by AA Voevoin, DV Stansky, EA Levasov an JJ Moore (Kluwer, Dorret, 4) [13] C Lu, Y-W Mai an Y-G Sen // J Mater Si 41 (6) 937-95 [14] PH Mayrofer, C Mitterer, L Hultman an H Clemens // Prog Mater Si 51 (6) 13 [15] Nanostruture Coatings, e by A Cavaleiro an JTM e Hosson (Springer, New York, 6) [16] RA Anrievski // Rev Av Mater Si (9) 1 [17] J Musil an M Jirout // Surf Coat Tenol 1 (7) 5148 [18] S Veprek, Strutural Nanorystalline Materials: Funamentals an Appliations (Cambrige University Press, Cambrige, 7) [19] S Veprek an MJC Veprek-Heijman // Surf Coat Tenol (8) 563 [] C Meratore an AA Voevoin // Annu Rev Mater Res 39 (9) 97 [1] Mater Pys Me 8 (9) 18 [] M Gell, EH Joran, YH Son, D Goberman, L Saw an TD Xiao // Surf Coat Tenol (1) 48 [3] H Luo, D Goberman, L Saw an M Gell // Mater Si Eng A 346 (3) 37 [4] P Bansal, NP Pature an A Vasiliev // Ata Mater 51 (3) 959 [5] GP Cerepanov, Meani of Frature of Composite Materials (Nauka, Mosow, 1983), in Russian [6] W Pompe, H-A Bar, G Gille, W Kreer, B Sultri an H-J Weiss // Curr Topi Mater Si 1 (1985) 5