Influence of the Measuring Method for Crack Length on the Fracture Toughness of Silicon Nitride Cera

Table 1.Effect of Composition and Sintering Condition on the Bulk Densities of the Si 3N 4Ceramics g cm 3

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Journal of the Ceramic Society of Japan 114[9]787–790(2006)

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Hiroyuki MIYAZAKI,Hideki HYUGA,Kiyoshi HIRAO and Tatsuki OHJI

National Institute of Advanced Industrial Science and Technology AIST ,2266–98,Anagahora,Shimo-shidami,Moriyama-ku,Nagoya-shi 463–8560

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The dependence of fracture toughness estimated by indentation fracture technique on measuring method was studied using several Si 3N 4samples with various microstructures.The crack length at the indentation load of 19.6N was measured by optical microscopy and SEM and fracture toughness was calculated for each case.The consistency of results between the two methods was obtained within the accuracy of the data for any microstruc-ture.It was revealed that in the case of silicon nitrides indented at low load,the IF technique was not so equip-ment sensitive,indicating that the accuracy of IF method by OM is not necessarily inferior to one by the SEM.It was considered that the IF method with optical microscopy would become useful test for fracture toughness if the testing procedure is properly controlled to minimize the source of the measuring error.

=Received March 31,2006;Accepted July 20,2006?

Key-words :Indentation fracture technique,Silicon nitride,Fracture toughness,Optical microscopy,SEM

1.Introduction

Silicon nitride ceramics have been applied extensively as tribological parts such as bearings for hard disks and mechani-cal seals in the last decade,because of their excellent tolerance during the usage in sever conditions,etc.and there have been many attempts to enhance their wear resistance by designing the microstructure.For such applications,evaluation of fracture toughness from real parts themselves is necessary.However,sizes of tribological parts such as bearings are gener-ally limited,making it difficult to attain the exact fracture toughness using the standard testing methods with large speci-mens,such as SEPB and chevron-notched beam technique.Therefore,the global market of silicon nitride bearing balls is now demanding the international standardization of test method for fracture toughness of small ceramic samples.Indentation fracture IF method has been widely used for evaluation of fracture toughness of ceramics since it has been proposed by Lawn and his co-workers.1 This method has the advantage of applicability to small materials and is the most suitable technique for determining the fracture toughness of small ceramic parts.However,IF method has been accepted as the substandard test method in the foreign countries and regarded as the subsidiary technique in the Japanese industrial standard JIS R 1607 since it contains some week points to be solved.The most critical and practical issue is that between-laboratory consistency is poor,which was revealed by the round-robin tests conducted in order to standardize the fracture-toughness test for ceramics such as VAMAS 2 –4 and the preliminary investigations for standardization of fine ceramics.5 Although these problems of IF method have been pointed out,few attempts have been made to eliminate the source of scattering,leading to the need for examination of the factors affecting the accuracy.One of the major origins of experimental error is the difficulty in measuring the exact crack tip.The scattering in crack length measured by several

laboratories was suspected to arise from the limitation of the equipment's resolution and the subjectivity of the opera-tor.2 –5 However,there have been a few reports about the sensitivity of the equipment.6 ,7 In this study,fracture tough-ness of ten kinds of silicon nitride ceramics with different microstructures was evaluated by IF technique.The effect of measuring method on the toughness was examined by compar-ing the results from optical microscopy OM and scanning electron microscopy SEM .In order to magnify the crack length uncertainty,indentation load was fixed at as low as 19.6N.

2.Experimental procedure

Several kinds of Si 3N 4ceramics,whose microstructures were varied from fine and uniform to coarse and elongated one,were fabricated by altering both the amount of sintering additives and the sintering condition using the same starting powders.The starting powders were a -Si 3N 4 SN–E10,Ube Industries,Ltd. ,Al 2O 3 AKP–50,Sumitomo Chemical,Ltd. and Y 2O 3 Shin-Etsu Rare Earth,Ltd. .The composi-tions of samples are shown in Table 1.The additions of Al 2O 3and Y 2O 3were equal in each composition and were changed from 1mass !to 10mass !.These powders were mixed in ethanol using nylon-coated iron balls and a nylon pot for 24h.

Fig.1.SEM micrographs of Si 3N 4sintered with Al 2O 3and Y 2O 3as sintering additives.The sintering condition and the amounts of addi-tives are shown at the left top corner for each graph.For example,10A10Y denotes 10mass !Al 2O 3and 10mass !Y 2O 3.Note that fine and uniform microstructure for the lower sintering temperature,whereas some coarse and elongated grains in the sample sintered at the higher sintering temperature.

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Influence of the Measuring Method for Crack Length on the Fracture Toughness of Silicon Nitride Ceramics Obtained by the Indentation Fracture Technique

The slurry was dried,and then passed through 125mesh sieve.

The powders were hot-pressed at 1950c

C for 2h with an applied pressure of 40MPa in a 0.9MPa N 2atmosphere.In the case of the two compositions of 2.5mass !Al 2O 3 2.5mass !Y 2O 3and 5mass !Al 2O 3 5mass !Y 2O 3,the samples with much fine microstructures were also fabricated by employing the sintering conditions of 1750c C 1h and 1850c C 1h Table 1 .Densities of the sintered bodies were measured using the Archimedes technique.Relative densities were calculated on the assumption that the density of the grain boundary phase was almost the same as that of the composites composed of alumina 3.99g cm 3 and yttria 4.84g cm 3 .The machined samples were polished and plasma etched in CF 4gas before microstructural observations with SEM.Young's modulus was measured by the ultrasonic pulse echo method.Vickers indentations were made on the polished surface perpendicular to the hot-pressing axis using a micro-hardness tester at the load of 19.6N.The length of the impres-sion diagonals and surface cracks were measured with an optical microscope furnished with the microhardness tester immediately after the indentation.The magnifications of the objective and eyepiece were 40and 10,respectively.For the observations of cracks with SEM,the surfaces of the indented samples were coated with Au using an ion-sputtering coater ?15min after the optical measurements to avoid moisture-as-sisted subcritical growth of the as-indented cracks.The proce-dure for measuring the crack length with SEM is as follows. 1 The left tip of a crack was roughly centered in the monitor with a low magnification of 100. 2 The tip of the crack was magnified by a factor of 10k for the clear identification. 3 The sample stage was moved to adjust the tip to an mark at the center of the monitor. 4 The location of the sample stage was recorded by the accuracy of 1m m. 5 To locate the right side tip of the crack with the same accuracy,the procedure 1 to 4 was conducted as well. 6 The length of the crack was measured from the shift of the sample stage.In this study,the type of cracks was hypothesized as Palmqvist one as described bellow,and the fracture toughness,K IC ,was calculated with the Niihara's equation 8 as follows:

3K IC HVa 1 2 HV 3E 2 5 0.035 c a a 1 2

1

Where E is the Young's modulus and a is the half of impres-sion diagonal measured optically,HV is the Vickers hardness calculated with a and the indentation load,and c is the half-length of as-indented surface crack measured with OM and SEM.The number of the measured impressions,N ,for most samples was 4,but N 3for three samples whose configura-tion of the cracks did not satisfy the specification of JIS R 1607.The average and standard deviation of fracture tough-ness was calculated for both,respectively.

3.Results and discussion

Table 1shows the effect of composition and sintering con-dition on the bulk densities of Si 3N 4.The bulk density increased with the amount of sintering additives and changed little by the sintering condition.The relative densities of sam-ples were above 98!,excluding the case of the largest amount of sintering additives which value was 97.1!,indicating that these ceramics were densified almost fully.Figure 1shows microstructures of the samples.The sample sintered with 2.5mass !Al 2O 3and 2.5mass !Y 2O 3at 1750c C for 1h consists of fine ?0.5m m and uniform grains,whereas some of the grains in the sample of the same composition grew unidirectio-naly after the sintering at 1950c C for 2h and their size exceed-

ed 2m m.In the sample sintered with 10mass !Al 2O 3and 10

mass !Y 2O 3at 1950c

C for 2h,more elongated grains 6m m exist occasionally.The tendency of the growing size of grain with increasing sintering temperature and additives was also confirmed from the micrographs of other samples.

Figure 2shows an example of the Vickers indentation observed with OM.Only indentations whose 4primary cracks emanated straightforward from each corner were accepted.Indentations whose horizontal crack length differed by more than ?10!from vertical one were rejected.The other specifi-cations for the configuration of indentation of JIS R 1607excluding the numerical condition of the ratio of c a were

also

Fig.2.Optical micrograph of a representative Vickers indentation in Si3N4sintered with5mass!of Al2O3and5mass!of Y2O3at 1850?C for1h.Solid circle indicates the aria observed by SEM Fig.4 .

https://www.360docs.net/doc/fc18159126.html,parison of the fracture toughness obtained by the IF technique using the crack length measured with SEM and optical microscopy OM for Si3N4with various microstructures.Each sym-bol indicates the sintering condition :1750?C for1h, :1850?C for1h, :1950?C for2h .Solid line indicates K IC OM K IC SEM .The pair of figure by every point shows the mass!of Al2O3

and Y2O3in the sample.Fig.4.SEM micrograph of the crack tip induced by the Vickers indentation in Si3N4sintered with5mass!of Al2O3and5mass!of Y2O3at1850?C for1h,which corresponds to the aria indicated by the solid circle in the optical micrograph Fig.2 .

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Hiroyuki MIYAZAKI et al.Journal of the Ceramic Society of Japan114[9]2006

used for the selection of the adequate indentations.The lengths of cracks,2c,were90?110m m,whereas those of the impressions,2a,were?50m m.It is generally accepted that the crack type transits from median type to Palmqvist one when the ratio of c a becomes smaller than2.5,whereas the other study using the same materials implied the median type from the analysis of the load dependence of the crack length. Although it is difficult to decide the crack type at this stage, the fracture toughness was estimated on the assumption that the crack type was Palmqvist one.

Figure3shows the fracture toughness attained from the crack length measured using SEM and OM for Si3N4with vari-ous microstructures.The X axis in the figure stands for K IC from optical measurements and the Y axis from SEM.By comparing the samples sintered at1950c C,it can be found that K IC from OM increased with increasing the amounts of additives except for the points of 2.5mass!Al2O3 2.5 mass!Y2O3and3.34mass!Al2O3 3.34mass!Y2O3.The

K IC from OM for the samples with the same composition increased with sintering temperature.The K IC from SEM exhibited the same tendency with both the composition and the sintering conditions.The variation in fracture toughness of those samples is attributable to the microsturctural evolu-tion due to the sintering condition and the amounts of addi-tives,since it has been widely known that the elongated rod-like grains are effective for the improvement in fracture tough-ness of Si3N4.9 –11

When the width of the standard deviation is taken into account,most of the data fell on the straight line,suggesting that K IC from OM was nearly equal to the value from SEM. However,there is some tendency that most of the plots lie right side of the line.This results arose from the shorter crack length 0–2m m for the optical measurements,with the excep-tion of the data plots apparently away from the line.This phenomenon can be rationalized by the difference in the reso-lution between SEM and OM.Figure4shows an example of the crack tip observed with SEM,which corresponds to the region indicated by the solid circle in Fig.2.In Fig.4,the crack propagated from left to right and the opening of the crack decreased significantly as it approached its end,making its contrast very faint at its front portion ?1m m from the end .By comparing Figs.2and4,it is clear that such a faint contrast in the SEM micrograph could be hardly discriminated optically.It is rather surprising that the accuracy of OM was so high to detect the position only?1m m inside the real crack tip since optical micrograph does not have such high resolu-tion.This may be attributed to the superior operator's sen-sitivity that could detect subtle changes of the color and or contrast near the crack tips,which could not be captured by a digital camera mounted to the optical microscope due to its limited resolution.Thus,the shorter crack length for the opti-cal measurements seems reasonable.Although most of the optical measurements missed the small portion of the real tip of cracks,the standard deviation of K IC for each data was revealed to be predominant over the error of OM.Therefore, one can safely conclude that the accuracy of the optics in this study is sufficient for determining the fracture toughness by IF method for Si3N4with various

microstructures.

790Influence of the Measuring Method for Crack Length on the Fracture Toughness of Silicon Nitride Ceramics Obtained by the Indentation Fracture Technique

The results in this study were consistent with the Ponton's

report which indicated that the difference in cracks length

measured using SEM and OM was no more than?4!.6

Their sample was glass ceramics and the indentation load of

103–152N which was more than5times higher than one used

in our study.However,Petrovic suggested that the difference

became as large as15!when another glass ceramics were

indented with low load of9.8N.7 The reason of much smaller

difference observed in our study may be attributed to the fact

that the crack tips in Si3N4were relatively easy to measure

as compared by other ceramics such as ZrO2 Al2O3compo-sites,3 ,4 Al2O3and SiC.5

From the practical point of view,OM has an advantage over

the SEM since the efficiency of OM is superior to that of

SEM.For instance,the optical measurements can be succes-

sively followed by the indentation,and vice-versa.It takes

only?2min to complete the measurement of an impression in

the case of OM.By contrast,much longer time is needed for

SEM,?20min,since many steps such as coating with conduc-

tive film and evacuation process etc.are necessary before

SEM.It is usually recommended to measure many indenta-

tions to attain a precise data point e.g.N 5for JIS R

1607 .Then the total measurement time for SEM becomes

markedly large so that SEM can not be used practically.If the

level of accuracy needed for crack-length measurements is

satisfied with OM,optical measurement is more suitable for

determining the fracture toughness by the IF method.It has

been known that the condition of sample surface,such as the

degrees of mirror finish,porosity,transparency and color,has

great influence on the detection of crack tips.3 –5 Therefore,

these factors affecting the precision of optical measurement

should be clarified quantitatively for other structural ceramics

in the future work to secure the acceptable accuracy.

4.Conclusion

Si3N4ceramics with various microstructures were synthe-

sized to evaluate the influence of measuring method on the

calculated fracture toughness by IF technique.Optical micro-

scopy and SEM was used to measure the crack length at the

indentation load of19.6N.The fracture toughness was calcu-

lated for both measurements and compared to detect the error

in the measuring procedure.Despite the difference in magnifi-cation,almost the same K IC value was obtained for any microstructures.Thus,it was revealed that the IF technique was not so equipment sensitive as expected when the indenta-tion load was19.6N.It was concluded that the precision of IF method with optical microscopy was not inevitably inferior to that with SEM in the case of Si3N4and has a potential to become a useful means if the mechanism affecting the meas-urements error is understood and the testing procedure is properly specified.

Acknowledgment This work has been supported by METI, Japan,as part of the international standardization project of test methods for rolling contact fatigue and fracture toughness of ceramics for ball bearings.

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