8/2/2019 Fracture Toughness and Thermal Shock Behavior of Silicon NitrideBoron Nitride Ceramics

1/4

Fracture Toughness and Thermal Shock Behavior ofSilicon Nitride-Boron Nitride Ceramics

D e p a r t m e n t of Mechanica l En gineer ing ,Fracture toughness behavior, stress-strain behavior, andflaw resistance of pressureless-sintered Si3N4-BN ceramicsare investigated. The results are discussed with respect tothe reported thermal shock behavior of these composites. Al-though the materials behave linear-elastic and exhibit noR-curve behavior, their flaw resistance is different from thatof other linear-elastic materials. Whereas the critical ther-mal shock temperature difference (AT,) is enhanced byadding BN, the content of BN has no influence on thestrength loss during severe thermal shocks. [Key words: sili -con nitride, boron nitride, fracture toughness, thermalshock, strength.]

I. IntroductionILICON NITRIDE-BOKON N I T R I D E (Si,N,-BN) com posite ma -S erials are attracting considerablc interest for a range ofdiverse applications such as components for automobiles torefractory nozzles, tubes, sliding gates, and break rings for thehorizontal continuous casting of steel. The development ofsuch advanced composites has taken place in the last decadeal though i t has been know n for a number of dccadcs that theaddition of BN to TiB z composi tes greatly improves the ther-mal sho ck resistance for applications such as metal evaporatorboats . ' In addi t ion, various authors have es tabl ished that thetherm al shock res is tance of many m aterials increases with thcamount of BN added.2-'Micros t ruc tura l obse rva t ions of these compos i t es have

shown that the role of BN is to increase the amount of fineporosity as well as to en ha nc e microcracking.' T h e highly an-isotropic fracture behavior of the hcxagonal-s tructured BNflakes , due to the easy cleavage perpendicular t o th c c-axis ,assists in th e form ation of microcracks. E xcellent examples ofsuch microcracking in a range of BN-containing compositeshas been presentcd by S inc la i r and S i rnmon~ .~hese authorsshow tha t BN cleaves along parallcl planes of thickness downto 0.05 p m , enabl ing s ignif icant di la tant s t rain to developwithin th e material . Sinclair and Simm ons4 point out th at thisis due to the large coefficient of thermal expansion ( a ) an -isotropy of the BN grains, with a = 40 x lO-'"C-' parallelto the c -axi s and 900"C. The ant icipated rctaineds t r e n g t h w il l b e c o m p a r e d w i t h t h c m e a s u r c d r c t a i n c dstrength obtained after a quench test in water which will beconducted for AT = 1200C.11. Experimental Procedure

The tcs ts were conductcd on pressureless-s intered Si3N,composi tes containing 10, 20 , 30, and 40 wt% BN (KawasakiSteel, Tokyo, Japan). Fabrication condition, microstructure,and mechanical propcrt ies of thcse materials are dcscribed indetail by Isomura et af.'To avoid complicated and long descriptions Si3N4plus 10,20, 30, and 40 voM BN wil l be labeled SNBIO, SNB20,SNB30, and SNB40, respectively.67

8/2/2019 Fracture Toughness and Thermal Shock Behavior of Silicon NitrideBoron Nitride Ceramics

2/4

68 Journal of the Airiericun Cerumic Society - utz and Swain Vol. 75, No . 1Th e KR-c urve cxperimcnts wcre performed us ing compacttens ion (C T) tes ts . Th e C T samples had thc dimensions 25 to27 mm x 32 mm x 5. 2 to 6 .2 mm. Onc s idc of each testpiece was polished. The samples werc prccracked by startingthe cracks from a chevronl ike prenotch. W hcn sharp , s t raight-through cracks werc achieved, the specimens wcrc renotchcdto a point as near as possible behind th c crack fronts. T h enotch d ep th: sample width ratio (a,, w) was choscn to b e 0.51to 0.61. The samples were loaded using a crosshead specd of10 pm /min. Th c crack propagat ion was measured directly us-ing a traveling microscopc. The KR - v -I Iues wcre calculated onthc basis of the load ( P ) , he rclative crack length ( a / w ) , th csample thickness (b), nd the ap propriate Y-function, namclyI4

PK H= Y ( a / w ) -bw'for a/w 5 0.85 with

2 + a /w(1 - a/w)'(a/w) = [0.886 + 4 . 6 4 ~ / ~ 1 3 . 3 2 ( ~ / ~ ) '+ 1 4 . 7 2 ( ~ / ~ ) ' S . ~ ( U /W ) ' ] ( 2 )

Stress-strain tests werc performed in three-po int bending(span of 29.5/7.9 mm) using beams with dimensions of 5.2 to6.2 mm x 2.5 to 3.1 m m x 32 mm . Thc s t rain was measuredby strain gauges attached t o the tensile surface of the samples.The flaw resistancc tests werc conducted on beams of theabove dimensions using the indentation-strength-in-bending(ISB) method." One side of each sample wa s indented usingvarious loads between 10 an d 1000 N. The rctained s t rengthwas measured in four-point bending (span of 18/6 mm).Some addi t ional thermal shock tcs ts wcre conducted tocomplement thc manufacturers reported data by mcasuringthe rctained bcnding s t rcngth after quenching samplcs from1220C into 20C water.

111. ResultsTh e stress-strain behavior of the Si7N4-BN materials is il-

lustrated in Fig. l. All materials exhibit an almost ideal linearelastic behavior with a small deviation prior to failure. Fromthe linear portion of such curv es thc elastic m odulus is dctcr-mined and found t o agree with dynam ic resonancc measure-ments of the E modulus as listcd in Tablc 1.

0 0.05 0.1 0.15Strain, %

Fig. 1 . Stress -s t ra in behavior of S N B ceramics.

Th e f law res is tance, KR-curv ebehavior, and thermal shockbehavior are shown in Fig. 2.Th e logari thm of the retained s t rength of as-indented speci-mens is plotted vcrsus the logarithm of the indentation load( F i g . 2(A)). All m a t e r i a l s s h o w a s i m i l a r b e h a v i o r . T h cstrength rcduction is quite low for indentation loads up to100 N, but is apparently more pronounced for higher loadswith a final slope of th e curves closer to the anticipa ted valueof - /3.All S N B c o m p o s i te s c x h i b i t f l a t K R - c u r v e s w i t h t h e i rtoughness decrcasing with increasing BN content (Fig. 2(B)).A thermal shock of AT = 1200C into water at room tcm-perature rcsults in a strength loss of 56% to 66% (Fig. 2(C)).Th e rctaincd s t rcngth of the SN B ceramics subjected to suchsevcre thermal shock declines in proportion to their initialstrength as the BN content increases (Table I) .IV. Discussion

Th c resul ts obtained in this study confirm and extend theobscrvations of Isomura et al.' It has becn shown that theelastic modulus and strcngth follow the same trend as previ-ously reported. Estimates of the elastic modulus from strain-gauged-flcxurc and dynamic-resonancc methods are in goodagrcemcnt. However, this work shows that the stress-straincurve is almost lincar to failure with thc inelastic componentof thc strain at failure accounting to less than 0.01%. T h ematerial behaves very differently from othcr extremely ther-mal shock resistant m aterials.Very large differences in the fracture toughness are deter-mined depending on thc t echniquc of m e a s u r e m e n t. T h emore rcliablc C T technique gives almost constant valucs overcrack extcnsions of 6 t o 7 mm of stable crack extension. Againit had becn anticipated that some R-curvc behavior may havebeen observed duc to possible crack-tip shielding effects suchas microcracking or crack-tip bridging. The present observa-tions suggcst tha t if there is an R-curve present then its rangeis very small (5100 pm ) . Th c f racture toughness as measuredby thc C T technique decreascs almost linearly with th c Emodulus . In addi t ion, both KR n d E , and thc energy releaserate, G = K:,/[E(l - v2)], dccreasc almost linearly with in-creasing porosity of thc SNB mixtures (Tablc I).Th e fracture toughness measured by the retained s t rength(a l t ) fter indentation with different loads ( P I ) an be inter-prettcd as exhibiting R-curve behavior. For the failure of abrittle material from a mcchanically introd uced flaw of sizc cund er uniform appl ied s t ress a'!, he net stress-intensity factoris the sum of K , an d K ,, the components for uniform appl iedstrcss and for localizcd loading, namely,

K = xP,/c'\' + $gdc"2 (3)where x = [ ( E / H ) ' " (Refs. 15 and 16) is a num erical constantdepending on the contact geornctry and the elas t ic iplas t icpropert ics of the material , and [ s a material-indcpendentconstant for Vickers-produced radial cracks. $ is a numericalcrack gcometry parametcr (e.g., T I ' * for an embedded linearcrack and 2/7r'" for an embedded circular crack).An expression for the ISB toughness c an be dcrived fromEq. (3) in the formI7

Hence , the re ta ined s t rength ( T ~ f bri t t le non-R-curveceramics in gcneral obeys the law( T ~ P ' ~constant ( 5 )

The toughness value obtained from ISB tests using Eq . (4)is, in other words, constant and independent of the indenta-tion load. A plot of log (rl(vs log P, should give a straight linecxhibiting the characteristic slope of -1/3. The flaw resist-ance curves in Fig. 2 are, ho wever, significantly flatter.

8/2/2019 Fracture Toughness and Thermal Shock Behavior of Silicon NitrideBoron Nitride Ceramics

3/4

January 1992 Fructure Toughness and Thermal Shock Behavior of SiiN,-BN Cerarnic.s 69Table I. Properties of Si3N4 -BN Materials

P ru p e r t y SNBIO SNB2O L SN B3 0 SNL340Porosity (95) 18" 26" 34" 3x*Th erm al expansion coefficient, a (~ 1 0 - ~C') 3.4" 3.2" 3.1" 2.7"Toughncss, K R M P a .m1'2) 2.4 1.2 0.9 0.55Bending strength, un M P a ) 212/360* 89/200" 64/90* 48/70"

Crit ical temperature diffcrence, AT, ("C) 650" 700" KO0* 850%Retained s t rength for AT = 1200C (MPa) 72 39 26 19

Poisson's ratio, v 0.21" 0.18" 0.13" 0.13"E-modulus (GPa)" 127/150* 60/58.9 " 42134.1 * 23/ 17.1 "2(c/br)'!i2=K R / u B ml'?) 0.01 1 0.013 0.014 0.011G = K /[E(1 - v')] (J/m') 47 25 20 13R' = c ~ e ( l- v2)/Ea 'C) 470/550" 450/850* 48O/1120* 76011280"

I, 0

0.80.60.L0.2

" I w r n u r a el u l '

'c-

----

Assuming that K ,, equals the average KR-curvevalue, it ispossible to calculate thc anticipated flaw resistancc curvesfor the S N B materials on the basis of Eq. (4). The resul t ingtheoretical curves with slopc -1/3 are shown in Fig. 2(A) asdotted lines. It is apparent that, indepcndcnt of the used in-dentat ion load, KI , obtained by the ISB method greatly over-es t imates the measured KR-curve.A possible explanation forthis significant difference may be the reduction in residuals t rcss about the indentat ion impress ion due to thc poros i tyand, hence. a reduction of x. Upon indentat ion the regionbeneath the contact a rea is expectcd to dcnsify. An SE M ob-servation of the d eform ation zone beneath a 100-kg indentatio n

300 i-20 i

1 10 100 I000 'P . N10 2 6 a, mrn

A SNB O

in the SNB20 material is shown in Fig. 3. In general, it isdifficult to detect cracks in thesc porous materials and to de-terminc thcir length. It seems, however, that no well-definedradial cracks have developed from th e corncrs of thc impres-sion shown in Fig. 3; rathcr, the s ize of the cracks only ex-tends slightly beyond thc densified rcgion, from which thcsample has fracturcd. Assum ing that thc crack length equalshalf of the impression diam eter, which is 1.0mm, the calcula-t ion of K,, s ing only the component K , in Eq. (3) with$ = 2/7r'" gives exactly t he KH-value of 1.2 M P a . m " . T h i smay indicatc that the contribut ion of K , to KI, an be almostneglectcd in these matcrials. How ever, it seems likely that th eQ erm in Eq. (3) is affected by the porosity.Iwasa and Kakiuchi,Ix using thc controlled surface flawtcchniquc as proposed by Fctrovic et ul." with a 5-kg loadKnoop indenter, report K,, values of -5 M Pa .rnl" with 20and 30 vol% BN. Such values are much higher than thosemeasurcd here, and the poss ible cause of this discrepancymay be due to the diff icul ty in measuring the ini t ia l f law s ize.However, thc values reported by lwasa and Kakiuchi" maywell be higher because the size of the BN grains that theyused was much larger than in this present s tudy, and thesamplcs wcre hot-pressed, generating a strong texture in thematerial . T he f iner-grained and more equiaxed BN grainsand more isotropic propcrties associated with pressurelesssintcring arc likely to produce materials of lower toughnessand absence of R-curve behavior, but highcr strength.Of particular interest in this work is th e thcrrnal shock bc-havior of SN B composites. As show n prcviously by Isomuraer al. , it is possible to prcdict reasonably well the increasc inAT , with increasing BN content on the basis of strength V H ,E-modulus , and therm al expansion coefficient ( a ) y

R ' = ~ e ( l .)/ECY = (1/2)(~/c)"~Kl~(lv2)/Ea (6)

Fig. 2. (A ) Flaw res i s tance , (B) K R - c u r v e b e h a vi o r , a n d (C) ther -mal shock behavior of S N B compos i tes . Broken l ines in (A) ar e thS;an t ic ipa ted response for th e var ious S N B mater ia l s us ing the Kdata in Table I a n d Eq . (4) . Basis for t h e o v e r e s ti m a t i o n of K , , byus ing th e ISB t echnique i s d i scussed in the tex t . Fig. 3. Vickers indenta t ion of 100-kg load in SN B20 . No cracksare observed having deve loped a round th e impression .

8/2/2019 Fracture Toughness and Thermal Shock Behavior of Silicon NitrideBoron Nitride Ceramics

4/4

70 Journal of he American Ceramic Society - ut z an d Swrrin Vol. 74, No . 12For materials such as these that behavc almost clasticallyand do not exhibit R-curve behavior, i t is possible t o usc sucha simple estirnatc. Since Tablc I shows tha t K / c L , s almostequal for a ll SN B composites , both , K R and (rR can bc usedfor the estimate. Th e estimation gives lower valucs th an tho sccalculated by Isomura et a/. but confirms the increasing AT,values with increasing BN conte nt. A comparison of the cst i-mated values in this work with thc AT, values measured byIsomura et al. shows tha t AT , has been, i n general, undcr-estimated. This may indicatc that Eq. (6) is suitable only forqualitative predictions.

V. ConclusionPressurcless-sintcrcd Si3N4-BN ceramic composi tes exhibi tincreasing p orosity an d decrcasing strength and Youngs mod-ulus as the BN content increases. Whereas the crit ical ther-ma1 shock t empe ra tu re d i f f c rencc ( A T , ) i s enhanced bya d d i n g B N , t h e c o n t c n t of B N h a s n o i n f l u e n c e o n t h est rength loss during severe thermal shocks and st rcss-st ra inand R-curve behavior. All tested materials behave l inear-elas-tically and cxhibit flat R-curves. However, their indentationflaw resistance behavior is uncharacterist ic for l inear-elasticmatcrials which show no R-curve behavior.

Acknowledgment: The au thors t h a n k Kawasaki Stccl . Tokyo, Japan .for i t s assis tance in supply ing the matc ria l s used in th is s tudy .

ReferencesR . R ice , Capabi l i t ies and Design I ssues fo r Emerg ing Tough Ceramic s.Am . Ceruvi.Sor. Bull. , 6 3 [2] 256-62 (1984).2D . Lewis and R . R ice , Thermal Shock Fat iguc of Monoli th ic Ceramicsand Ceramic-Ceramic Par t icu la te Composi tes , Cerum. Eng. Sci. Proc., 2 ,712-18 (1981).D. Goeur io t-L aunay , G. Brayet , and F. Thcvenot , Boron Nit r ide Ef fec ton the Thermal Shock Rcuista i ice of a n A lu m in a - B a se d C e r a m ic C o m -

p o s i t e , J. Murcr. Sci. i x t t . , 5 , 940-42 (1986).immons, Microst ructure and Thermal Shock Behav- J. Mu/er.Sci. Lett . . 6, h27-29 (lY87).ti. I somura , T. Fu k u d a , K. Og a sa h a r a . T . Fu n a h a sh i , a n d R. Uchimura ,

sistance , High Erosion Resistance; pp. 624-34 in U NI T ECR 89 Proceed-ings Edited hy I.. J . Troste l . Jr . Am er ican Ceram ic Socie ty , Westerv i lle , O H,10x0.A. G . Evans. Therm al Fracture in Ceram ic Mater ia ls , Proc. Br. Cerum.Soc. , 25 . 217-35 (1975).A . G . E v a n s a n d E . A . Char leb . St i uc tura l In tegr i ty in S evere Therm alE n v i r o n m e n t s , J. Am . Cerurn.SOC.. 0 11-21 22-28 (1977) .

M .V . Swa in , R - C u r v e 3 e h av io r of Magnesia-Partially-Stabilized Zirco-n ia and I t \ Signi l icance to Thermal Shock; p p . 355-70 in Fructure Mechun-i ~ s f Cc2ru~ni(.s.ol . 6 . E d i t e d b y K . C . B r a d t , A . G . E v a n s , D. P. H .I lasse lm an , and F. F. Langc. Plenum P ress , New York , 1983. H . E . Lut r , N. C laussen , and M.V. S\ca in , %-Curve Behavior of DuplexC e r a m ic s , J. Am. Cenrni. Soc.. 74 [I] 11-18 ( l Y 9 l ) . H .E. Luti , M.V. Swain . and N. Claussen , Thermal Shock Behavior ofDu p le x C e r a m ic s , . Am . Lerum. Soc., 74 [ l ] 19-24 (1901). R . F . C o o k , B . R . L a w n , a n d c. J . Fairbanks, Microst ructure-Streng thProper t ie5 in Ceramics: 1. Effect olCrack Size on Toughness, J Am . C+rum. Soc.. 68, 04-15 (1985).U. . C o o k a n d D . R . C l a r k e , F r a c t u r e S l a h i l i t y , K - C u r v e s , a n dStrcngth Variahility.A[.fa Metull.. 36 [3] 555-62 (1988).P. C h a n t ik u l , S. .I.e n n i so n , a n d B . R . L a wn , R o le of Grain S ire in theStreng th and /