El Sayed Ali, M.

Sørensen, Ole Toft

Three techniques for fracture toughness measurements on zirconia toughened ceramics were evaluated: the notched beam (NB) technique, the indentation fracture (IF) technique and the indentation strength in bending (ISB) technique. Using these techniques comparative measurements were performed on samples prepared by pressing (uniaxial) and sintering of four commercially available powder types. These were: Toyo Soda (Japan) powders with the designations TZ3Y (2,86 mole% Y2O3), TZ3YA (2,77 mole% Y2O3, 0,1 wt% A12O3) and TZ3Y20A (2,88 mole% Y2O3, 20 wt% A12O3) and a powder supplied by viking Chemicals (Denmark) designated as YP5Z-2.5 (2.5 mole% Y2O3). The measurementsshowed that similar Klc values were obtained with the IF- andISB-techniques, which therefore are recommended for Klcmeasurements. Too high values were, however, obtained with theNB-technique which therefore cannot be recommended. Finally,the measurements showed that a high temperature annealing is recommended prior to testing for the IF-technique

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Downloaded from orbit.dtu.dk on: Dec 20, 2017Fracture Toughness Measurements on Zirconia Toughened CeramicsEl Sayed Ali, M.; Sørensen, Ole ToftPublication date:1987Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):El Sayed Ali, M., & Sørensen, O. T. (1987). Fracture Toughness Measurements on Zirconia ToughenedCeramics.  (Risø-M; No. 2622).S vu)'r7(^i/fT-\ Riso-M-2622 * UK & z 4 i Fracture Toughness Measurements on Zirconia Toughened Ceramics M. El Sayed Ali and O. Toft Sørensen Risø National Laboratory, DK-4000 Roskilde, Denmark December 19^ 6 RISØ-M-2622 FRACTURE TOUGHNESS MEASUREMENTS ON ZIRCONIA TOUGHENED CERAMICS M. El Sayed Ali* and O. Toft Sørensen Abstract. Three techniques for fracture toughness measurements on zirconia toughened ceramics were evaluated: the notched beam (NB) technique, the indentation fracture (IF) technique and the indentation strength in bending (ISB) technique. Using these techniques comparative measurements were performed on samples prepared by pressing (uniaxial) and sintering of four commerci-ally available powder types. These were: Toyo Soda (Japan) powders with the designations TZ3Y (2.86 mole% Y 20 3) r TZ3YA (2.77 mole% Y203, 0.1 wt.% Al203) and TZ3Y20A (2.88 mole% Y203, 20 wt.% AI2O3) and a powder supplied by viking Chemicals (Den-mark) designated as YP5Z-2.5 (2.5 mole% Y2O3). The measurements showed that similar Klc va]'*es were obtained with the IF- and ISB-techniques, which therefore are recommended for Klc measurements. Too high values were, however, obtained with the NB-technique which therefore cannot be recommended. Finally, the measurements showed that a high temperature annealing is recommended prior to testing for the IF-technique. Vising scientist at Risø National Laboratory Present address: Metallurgy Department, Nuclear Research Centre, Atomic Energy of Egypt, Cairo, Egypt December, 1986 Risø National Laboratory, DK-4000 Roskilde, Denmark CONTENTS Page 1. INTRODUCTION 5 2. EXPERIMENTAL 5 2.1. Notched beam (NB) measurements 6 2.2. Indentation fracture (IP) measurements 6 2.3. Indentation strength in bending (ISB) measurements 8 3. RESULTS AND DISCUSSION 9 3.1. Notched beam (NB) technique 9 3.2. Indentation fracture (IP) technique 9 3.3. Indentation strength in bending (ISB) technique .. 10 3.4. Comparison between techniques 10 4. CONCLUSIONS 11 REFERENCES 12 TABLES 13 FIGURES 14 5 1. INTRODUCTION Several methods can be used to determine the fracture toughness of ceramic materials. Among some of the most important methods can be mentioned: (1) the notched beam (NB) technique where a pre-notched sample is fractured using the 3- og 4-point bending technique (3,5) - notching is by this method performed by cutting usually with a high speed diamond saw; (2) the indenta-tion fracture (IF) technique by which the fracture toughness is determined from the length of cracks induced around a Vickers indenter (2), and finally (3) the indentation strength in bend-ing (ISB) technique where microcracks are introduced by a Vickers indenter prior to the 3- og 4-point bending measurement of the fracture strength (1). In the present work these three methods of fracture toughness determination are evaluated and compared by measurements on zirconia toughened ceramic samples prepared from two commercial powders delivered from Toy a Soda, Japan and Viking Chemicals, Denmark, respectively. 2. EXPERIMENTAL Rectangular bars (2x6x25 mm) were prepared by pressing and sintering from the following powder..: Toya Soda, Japan I TZ3Y (2.86 mole% Y203) " " " II TZ3YA (2.77 mole% Y203, 0.1 wt.% A1203) " " I I I TZ3Y20A (2.88 mole% Y 20 3, 20 wt.% A1203) Viking Chem., DK IV YPSZ-2.5 (2.5 mole% Y 20 3). All samples were sintered to a density higher than 95% TD. 6 2.1. Notched beam (NB) measurements The sintered samples were notched with a diamond saw producing a notch width of about 0.43 mm. The exact width and depth of the notch were measured with an optical microscope equipped with a micrometer eyepiece. The stress intensity factor, i.e. the Klc values, were determined for samples with different notch depths using the 4-point bending technique. These measurements were performed with 10 mm and 20 mm inner and outer spans, respec-tively, and with a crosshead speed 0.3 mm/min. The KIc values were calculated using the following empirical equation (3,5): Klc = 3 Pf La1/2/4bh2 (1.99-2.47(a/h) + 12.97(a/h)2 - 23.17(a/h)3 + 24.80(a/h)4) (1) Here Pf is the load at fracture whereas the meaning of L, a, h and b are given in Fig. 1, which shows the geometry of the notched samples. 2.2. Indentation fracture (IF) measurements The samples were polished to nearly mirror finish (diamond paste down to 3 iim-1 fim has been recommended in the literature) prior to the Vickers indentations, which were carried out with differ-ent loads in the range 50-500 N. In order to minimize slow crack growth a small drop of silicone oil was placed over the impres-sion immediately after unloading. The crack radius, c, cht inventar diameter, 2a, and the crack length 1, are specified in Fig. 2. Average figures of these parameters were determined from ten indentations with the same load in reflected polarized light using an optical microscope. The stress intensity factor, Klc, was then calculated from the 7 following equations, depending on the c/a ratio: (1) For c/a > 2.5 - Young's modulus not known (2) Klc */H a1/2 = 0.15 k(c/a)-3/2 (2) where t is the constraint factor - 3.2 and H is the hard-ness. The accuracy obtained with this equation is about 30%. (2) For c/a > 2.5 - Young's modulus known (7) (Klc 4>/H a1/2).(H/Ed))0-4 = 0.129(c/a)~3/2 (3) where E i s Young's modulus. The accuracy obta ined by t h i s equation i s about 10%. (3) For c/a < 2.5 - Young's modulus known (6) (K l c d>/H a 1 / 2 ) . ( H / E ^ ) 0 . 4 = o.035(l/a)1/2 (4) where the radio 1/a = ( c / a ) - l . Final ly , by replacing H in Eq. 2 with the empirical Vicker's hardness re la t ion: H = 0.47 P/a2 (5) where P is the load and a the half of the indenter diagonal, the following simple equation for Klc can be obtained: Klc « 0.0726 Pc"3/2 (6) This equation can be generally used for fracture toughness measurements on ceramics. The accuracy using this equation will also be about 30% as for Eq. 2. 8 2.3. Indentation strength in bending (ISB) measurements The sintered samples were indented using Vickers pyramid with different loads. Each sample was indented (only one indentation) at the center of the tensile surface. Immediately after unload-ing a drop of silicone oil was placed on the indentation site. The samples were then fractured by the 4-point bending tech-nique, the fracture strength, o^, was calculated using the equation: *f = 3Pf L/4 b h2 (7) (Pf, L, b and h are defined in section 2.1.). The corresponding Klc values were calculated from (1): Klc = nfE/H)1/8 (af • p.V3)3/4 ( 8 ) Here E is Young's modulus, H is the hardness, P^ the indentation load and i\ = 0.59±0.12. If the ratio E/H is not known the product of nfE/H)1/8 can be replaced by an average value of 0.88. This gives the equation: KIc = 0.88 (of • Pi 1/ 3) 3/ 4 (9) which can be used for "well behaved" materials with an accuracy comparable that obtained by the IF-technique (1). 9 3. RESULTS AND DISCUSSION 3.1. Notched beam (NBl technique The Klc values determined by the NB-technique are shown in Pig. 3 as a function of the a/h (notch depth/specimen thickness) ratio. As seen from this figure this technique can give con-sistent KIc-values although with some scatter up to an a/h ratio of about 0.36, whereas a large decrease in Kjc is ob-served at larger a/h-ratios. The reason for the scatter ob-served at low a/h-ratios could be variations in the notch radius (5), non-uniform notch depth or the rather large notch width (0.43 mm) used in this work. In order to get more accu-rate data by this technique a better and more reproducible notching technique should thus be developed. It should also be noted here that all samples were annealed at 1450°c before these measurements. 3.2. Indentation fracture (IF! technique Fig. 4 shows the regular cracking pattern formed by Vickers indentation at a load of 300N on a polished sintered sample (4a) and a polished and then annealed sintered sample (4b) of the material TZ3Y20A. Annealing was also in this case performed at 1450°C. Larger cracks are apparently formed in the as-polished state than when the polished sample is annealed before the measurements. This technique is thus very sensitive to surface stresses created by polishing and in ord«r to get a reliable measurement the polished samples should be annealed before the indentation. This is also clear from Pig. 8, which snows that while the Klc value in the as-polished state is only about 4 MN/m3/2 this value increases up to about 5.8 MN/m3/2 by annealing. 10 Fig. 5 shows the relation between log a (a = half indent diago-nal) and log Pi(Pi * indent load) obtained for TZ3Y20A samples. The straight line relationship obtained in this plot indicate that the Vicker's hardness is nearly constant for this material over the load range used. Using Eq. 6 a hardness of 13.635 GPa can be calculated from this line. Finally, the Klc values obtained by using Eqs 4 and 6 are com-pared in Table I. Except for TZ3Y20A the more simple Eq. 6 consistently gives only slightly smaller Xlc values than Eq. 4. The reason for the discrepancy observed for TZ3Y20A could be that the E-values used in the calculation of Kjc according to Eq. 4 were slightly overestimated - for this material the E-value specified by the supplier was used. Considering the uncertainties involved in these measurements, Eq. 6 is there-fore believed to be generally acceptable for Klc calculations from IF-data. 3.3. Indentation strength in bending (ISB) technique According to Eq. 9 the product »f'Pi^'r which is proportional to KfC, should be constant and independent of the load, Pj, used in the Vickers indentation. This was also found in the present work as shown in Fig. 6 indicating that reproducible results can also be obtained with this technique, which is more simple to perforin than the two other techniques. Finally, the SEM picture of a fracture surface shown in Fig. 7 shows that the crack start from the indentation site as it should by this technique. 3.4. Comparison between techniques The Klc values obtained by the three techniques are presented in Table I. Furthermore the Klc values determined by the IF-11 and ISB-techniques, respectively, for TZ3Y20A are shown in Fig. 8, from which it will be noted that these two techniques give similar results if the IF measurements are carried out on po-lished and annealed samles. This is also evident from Table I for the three Japanese powders (I, II and III) whereas a some-what higher Klc value is obtained with the ISB - than with the IF-technique for YPSZ-2.5 (IV - Viking Chemical). The reason for this difference in behaviour between the two powder types is not clear. Finally, it will also be noted from Table I that the NB-technique gives much higher Klc values than the other two techniques. This is in accordance with other investigations (8), which have shown that KIc values obtained with this tech-nique are overestimated because of transformations induced by the notching. Thus the NB-technique apparently is not suitable for Klc measurements. This technique could eventually be im-proved, however, by using a better notching technique. 4. CONCLUSIONS Comparative measurements of Klc values were carried out on zirconia toughened ceramics using three techniques: the notched beam (NB) technique, the indentation fracture (IF) technique, and finally the indentation strength in bending (ISB) tech-nique. These measurements showed that similar values can be obtained with the IF- and ISB-techniques, whereas too high Klc values will be determined by the NB-technique. The IF- and ISB-techniques are thus recommended for Klc measurements. In the case of the IF-technique it was further found that a high temperature annealing of the polished samples is necessary prior to testing as this technique is very sensitive towards the surface stresses created by polishing. 12 REFERENCES 1 . CHANTIKUL, P. , ANSTIS, G.R., LAWN, B.R., and MARSHALL, D.B. "A c r i t i c a l evaluation of indentation techniques for mea-suring f r a c t u r e toughness : J. Am. Ceram. Soc. £A (1981) 539-543. 2 . EVANS, A.G. and CHARLES, E.A. "Fracture toughness determi-nations by indentation" ib id 51 (1976) 371-372. 3 . GREEN, D.J., NICHOLSON, P.S. and EMBURY, J.D. "Fracture toughness of p a r t i a l l y s t a b i l i z e d Zr02 in the system CaO-Zr02» i b i d 5fi (1973) 619-623 . 4 . H0RITS, Y.M. and SOMIYA S. "Strength-toughness r e l a t i o n s in s intered and i s o s t a t i c a l l y hot-pressed Zr02-toughened A1203" i b i d fil (1986) 169-172. 5 . HU SSL ER, B., SWAIN, M.V. and CLAUSEN, N. "Dependence of fracture toughness of alumina on gra in s i z e and t e s t t e c h -nique" i b i d fii (1982) 566-579 . 6 . NIIHARA, K., MORENA, R., and HASSELMAN, D.P.H. "Evaluation of K l c of b r i t t l e s o l i d s by the indentat ion method with low crack-to- indent r a t i o s " J. of Mater. S c i . L e t t . 1 (1982) 13-16. 7 . OKAZAKI, K. "Mechanical behaviour of f e r r o e l e c t r i c cera-mics" B u l l . Am. Ceram Soc. 63 (1984) 1150-1157 . 9 . SWAIN, M.V. and CLAUSSEN, N. "Comparison of K l c values for Al203-Zr02 composites obtained from notched beam and inden-ta t ion s t r e n g t h techniques" J. Am. Ceram. Soc. fifi (1983) C27 - C29. 13 Tabel 1. Klc values measured by IF-, ISB- and NB-techniques. Material IF (300 N) ISB NB (Eg. 4) (Eg. 6) (Eg. 9) (Eg. 1) I - TZ3Y E * 200 GPa(a) 6.56 6.21 6.37 H = 12.00 GPa (m II - TZ3YA E = 210 GPa(a) 6.024 5.96 6.04 H = 12.03 GPa (m) III - TZ3Y20A(b) E = 260 GPa(*> 6.85 5.73 5.8 9.6 H = 13.64 GPa (m) IV - YPSZ-2.5 E = 190 GPa(a) 6.77 6.64 7.31 H = 9.72 GPa (m) a: assumed; m: measured; : supplier specification. b: Klc for this material (TZ3Y20A) is 6.0 MN/m3/2 as specified by the supplier. - 14 -P/2 P/2 f h ^ ^ 1 P/2 • 4 • L • i P/2 ». i Fig. 1. Sample geometry by notched beam testi ng. 2c H — 2 a — H Fig. 2. Schematic representation of impression and radial cracks produced by a Vicker's indenter. - 15 -— o — 1 1 o 1 1 o 1 — — 1 1^ 1 0.2 0.3 0.4 a/h- ratio 3. Fracture toughness determined by the notched beam technique for TZ3YA as a function of the a/h (notch depth/specimen thickness) ratio. All samples were annealed at 1450°C before testing. •••• « • "• •• I r . * . . . i . * f ' . , < * . t « • . • • • i • • 4, * 4 a * . » -*•• t-** '-. i * 4 b » «• ** • * % * »%. / • Fig. 4. Radial cracks around Vicker 's indenter in s i n t e r ed TZ3Y20A samples as pol ished (a) and as pol ished and then annealed (b). Annealing tempera ture : 1450°C. Load: 300N. - 17 -2 -logP| (PjinN) Relation between log a (a - average value of half indent diagonal) and log Pj (PA * indent load) deter-mined by Vicker's indentation measurements on TZ3Y20A. - 18 -1500 CO O Q_ 5 1000 CO or 6" 500 -200 400 600 Indentation Load P| (N) Pig. 6. Plot of »fPj1/3 versus indentation load, Pj, for ISB-measurements on samples prepared from TZ3Y (I), TZ3YA (II), TZ3Y20A (III) and YPSZ-2.5 (IV). Samples from I, II and IV were sintered at 1535°C (3 hrs) whereas samples from III were sintered at 1635°C (30 min.). - 19 -Pig. 7. SEN photograph of fracture surface of TZ3Y20A sample fractured by the ISB-technique. - 20 -1 1 1 1 - ° o 0 r\— 0 0 tf tf o I SB , as sintered _ tf IF . a s polished i i S7 a IF , annealed after polishing i i i i ! 1 — — — 200 400 600 Indentation Load Pj(N) . 8. Klc values versus indentation load determined by the IP- and ISB-techniques on TZ3Y20A (sintered at 1635°C for 30 min.). Risø National Laboratory Rise - Ai - 2622 Till* and awthorU} Fracture toughness measurements on zirconia toughened ceramics M. El Sayed Ali and 0. Toft Sørensen Visiting scientist at Risø National Laboratory. Present address: Metallurgy Department Nuclear Research Centre, Atomic Energy of Egypt, Cairo, Egypt. Pa«*« 2 0 Tables \ INwstrations g IWsiamsi <j December 1986 Department sr grown Metallurgy Department Croups M M registration nwnbsrfs) Prejoct/contiact no. ISBM 87-550-1289-2 Abstract (KU*. M M char.) Three techniques for fracture toughness measurements on zir-conia toughened ceramics were evaluated: the notched beam (NB) technique, the indentation fracture (IF) technique and the indentation strength in bending (ISB) technique. Using these techniques comparative measurements were performed on samples prepared by pressing (uniaxial) and sintering of four commerci-ally available powder types. These were: Toyo Soda (Japan) powders with the designations TZ3Y (2.86 molet Y ^ ) , TZ3YA (2.77 mole% Y203, 0.1 wt.% A1203) and TZ3Y20A (2.88 molet Y2O3, 20 wt.% AI2O3) and a powder supplied by Viking Chemicals (Den-mark) designated as YP5Z-2.5 (2.5 mole% Y2O3). The measurements showed that similar Klc values were obtained with the IF- and I SB-techniques, which therefore are recommended for Klc measurements. 1*00 high values were, however, obtained with the NB-technique which therefore cannot be recommended. Finally, the measurements showed that a high temperature annealing is recommended prior to testing for the IF-technique. Dwcriptsf« - INIS: BENDING; CERAMICS; COMPARATIVE EVALUATIONS; DOPED MATERIALS; FRACTURE PROPERTIES; HARDNESS; MEASURING METHODS; MECHANICAL TESTS; NOTCHES; SINTERED MATERIALS; YTTRIUM OXIDES; ZIRCONIUM OXIDES Aro*J»»m wsjml from W*» lMrm% *s» m u n * UHrssjry, (Wtss Wfc.-'on*, f«mu*i»iiiHi Was), PJO. Urn m, OK mm WsssOs, P I M W S . 

Fracture Toughness Measurements on Zirconia Toughened Ceramics

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Fracture Toughness Measurements on Zirconia Toughened Ceramics

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