Wear 255 (2003) 651656

Case study

Characterization of surface contact-induced fracture inceramics using a focused ion beam miller

Z.-H. Xie, P.R. Munroe, R.J. Moon, M. HoffmanSchool of Materials Science and Engineering, University of New South Wales, Sydney NSW 2052, Australia

AbstractFocused ion beam (FIB) milling and imaging are powerful techniques for evaluation of surface contact-induced crack structures and the

effect of microstructure on crack growth in ceramics. Two distinct -sialon microstructures made from the same composition were testedunder indentation, scratching and grinding conditions. Following each test, the FIB was used to analyze fracture events in both the surfaceand subsurface, and reveal the factors that control material removal during surface contact. 2003 Elsevier Science B.V. All rights reserved.

Keywords: Focused ion beam (FIB) miller; Microstructure; Surface contact-induced cracks

1. Introduction

The focused ion beam (FIB) miller is a relatively newtechnique to electron microscopy. Most FIBs are found inthe semiconductor industry, where they are used to deposit,etch, mill and image samples during defect analysis andcircuit modification. Now the FIB is being applied in itsapplication in materials science, particularly for analyzingsubsurface damage following surface contact [1,2].

The FIB uses a liquid metal ion source to emit gallium ionsin a high vacuum environment. These particles are acceler-ated by an energy of between 5 and 50 keV, forming a fine(10 nm) energetic beam of gallium ions, which is then fo-cused onto the sample surface by electrostatic lenses. Whenthe ions impact and/or implant into the specimen surface,secondary electrons, secondary ions and atoms are ejectedfrom the sample. This process can result in surface imagingand/or surface milling. In the case of surface imaging, eitherthe positive secondary ions or secondary electrons is de-tected and used to form images broadly similar to those ac-quired in a scanning electron microscope. While for milling,material removal occurs via the ejection of atoms (or neu-trals), positive and negative ions from the sample surface.Additionally, depending on the difference in the chemistryand structure between the grains and the grain boundaryphases, microstructural images of materials can be gener-ated with high contrast via the controlled scanning of ion

This paper was unable to be presented by the author at the 14thInternational Conference on Wear of Materials due to the prevailingpolitical situation at the time. Corresponding author. Fax: +61-2-9385-5956.

E-mail address: zhxie@materials.unsw.edu.au (Z.-H. Xie).

beams [2]. In practice, imaging is normally performed usinga smaller aperture where the beam current is small and min-imal milling occurs. For a situation where ion beam millingis desired, a larger aperture is inserted, and the beam currentis increased by 23 orders of magnitude. A mill box isthen drawn onto the area of interest for milling. The beamthen scans over this selected area and milling is achieved.

These features of the FIB can be applied for the charac-terization of both cracks and microstructure in ceramics. Inthe past, surface contact-induced cracks in ceramics by ei-ther indentation [3] or scratching [4] were identified mostlyfrom direct observation on the sample surface using ei-ther optical microscopy or a scanning electron microscope.In the investigation of subsurface cracks, either ceramo-graphic polishing [5] or a bonded-interface technique [6]is used for the preparation of subsurface cross-sections ofceramics. It is noted that both techniques may introducedamage to the original cracks and affect the reliability ofidentification. Furthermore, the two techniques could notreveal the interaction of microstructure and cracks, whichprovides key information for fracture analysis and design oftougher ceramics. However, using the FIB, the subsurfacecross-sections of ceramics can be prepared rapidly withminimal damage, and the area of interest can be directlyetched to reveal the effect of microstructure.

In this work, a Ca -sialon ceramic was used as a modelmaterial. The FIB was first applied to characterize theindentation-induced cracks in the sample, and highlight theeffect of microstructure on cracking and the interaction ofcracks. Then, subsurface cross-sections of the samples wereprepared using the FIB, following scratching and grinding, toreveal how cracks affected the processes of material removal.

0043-1648/03/$ see front matter 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0043-1648(03)00043-7

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Fig. 1. Microstructure of Ca -sialon (a) EQ and (b) EL. Note that thetwo distinct microstructures are made from the same nominal chemicalcomposition, using different sintering schedules. Etched in molten NaOHand imaged by FESEM [7].

2. Experimental details

Fine equiaxed (EQ) and large elongated (EL) -sialonsamples were fabricated with the same nominal chemicalcomposition defined by the formulaCaxSi12(m+n)Alm+nOnN16n, where x = m/2, m = 2.6and n = 1.3, using different sintering schedules. Process-ing is detailed in an earlier work [7]. The two distinct mi-crostructures are shown in Fig. 1, and key microstructuraland mechanical properties are given in Table 1.

Samples were mechanically polished down to 1m dia-mond paste, then ultrasonically cleaned and dried. All in-dentation, scratching and grinding tests were performed inlaboratory air with a relative humidity of 60%. Vickersindentation was conducted at loads of 0.5 and 1 kg (Micro-hardness Tester, Model M-400-H1, Akashi Co., Japan), and

Table 1Microstructure and mechanical properties of -sialon EQ and EL [7]Sampleidentification

Average graindiameter (m)

Aspect ratio Density (g/cm3) HV (GPa),load = 98 N

KICa (MPa m1/2),load = 98 N

Youngs modulusb(GPa)

EQ 0.35 1.1 3.19 12.8 0.5 3.7 0.3 311.28EL 0.70 7.2 3.21 12.0 0.2 7.5 0.3 305.88

a Obtained from Vickers indentation test [8].b Obtained from nanoindentation test with a spherically tipped indenter (5m in radius) [9].

spherical nanoindentation, 5m tip radius was undertakenat a load of 500 mN (Ultra-Micro Indentation System 2000,CSIRO, Sydney, Australia). Single-pass scratch tests wereundertaken at a constant load of 10 N and a constant veloc-ity of 1 mm/s using a pin-on-disc type tribometer (CSEMInstruments, Switzerland). The pin was a Vickers pyramidindenter (tip radius approximately 1.5m) and the slidingdirection was parallel to the pyramid diagonal. The grind-ing tests were performed using a surface grinder (ModelKGS-250AH, Kent Industrial Co. Ltd., Taiwan, ROC) with ametal-bonded 220-grit peripheral diamond wheel. The widthof the dressed wheel was 20 mm, and the diameter was150 mm. The following conditions were set for the grindingtests: wheel speed of 2850 rpm, longitudinal table velocityof 20 m/min, automatic cross feed rate of 5 mm/stroke anddownfeed increment rate of 20m/pass. At the final ma-chining step, more than 100 passes were used to establish asteady-state condition on the ground surfaces.

The FIB (FEI xP200, FEI Company, Hillsboro, OR 97124,USA) was used in two ways. Firstly, the beam was rasteredover the sample surface to expose surface structure andcracks. Secondly, subsurface cross-sections of the sampleswere prepared through cracks and observed.

3. Results and discussion

3.1. Indentation tests

Radial cracks formed in both the EQ and EL samplesduring Vickers indentation. The crack propagation and theeffect of microstructure on cracking for EL can be seen inFig. 2. The crack bridging due to the elongated grains in thismaterial can significantly reduce the propagation of cracks(Fig. 2(b)), resulting in higher indentation fracture tough-ness relative to EQ (see Table 1). No similar crack bridgingwas observed in EQ due to its fine-grained microstructure.By comparing the molten NaOH etched microstructures inFig. 1 with the ion beam etched microstructure in Fig. 2, itcan be seen, if the chemical etching were used, most of thecrack configuration in the microstructure would have beenetched away.

In addition to the Vickers indentation test, which rep-resents a situation where sharp contact occurs, a nanoin-dentation test with a spherically tipped indenter can beundertaken, supported by FIB analysis, to investigate surface

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Fig. 2. (a) Radial crack propagation and its interaction with microstructure; (b) enlarged view. The crack was created by a Vickers indenter on EL atload of 0.5 kg. Crack bridging can be seen after ion beam etching.

Fig. 3. Nanoindentation on (a) EQ and (b) EL. Spherical tipped indenter with 5m radius; load = 500 mN.

contact-induced deformation and fracture in a small volumeof material. The effect of microstructure on fracture modecreated by nanoindentation is shown in Fig. 3. Radial crackswere observed in EQ (labeled as R in Fig. 3(a)), emanat-ing from the contact edge. However, in the case of EL onlypop-up of grains occurred along the circumference, andfurther crack propagation was hampered by the elongatedgrains (Fig. 3(b)). The sizes of plastic deformation causedby nanoindentation in both EQ and EL samples are approx-imately the same (Fig. 3), suggesting that microstructurevariation has little influence on hardness in the cases ofboth EQ and EL.

Lateral cracking and its interaction with radial cracks canbe revealed via the preparation and observation of a sub-surface cross-section of the indented sample, as shown inFig. 4. Table 2 gives an explanation of nomenclatures usedin all the figures. Contrary to the conventional concept ofthe independent growth of the radial and lateral cracks, thispresent investigation indicated that the propagation directionof the lateral crack appeared to be affected by the presenceof a radial crack. Close to the radial crack, the lateral crack

has been drawn to the surface. However, away from the ra-dial crack, the lateral crack tended to propagate away fromthe surface. This observation is consistent with the recentreports of cracking induced by Vickers indentations on sili-con nitride [5] and soda-limesilica glass [10]. In addition,a secondary shallow radial crack can be seen in Fig. 4 (la-beled as r), which was first defined by Cook and Pharr [3],but has never been clearly identified from indentation tests.

Table 2Nomenclature used in describing features visible in Figs. 36

Nomenclature Feature

D Scratch debrisG -Sialon grainL Lateral crackm MicrocrackM Median crackr Secondary radial crackR Radial crackT Surface crack

Sliding direction of the indenter

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Fig. 4. Radial and lateral cracks produced by a Vickers indenter on EQ at a load of 1 kg. The subsurface cross-section was prepared by FIB milling.

It has been demonstrated that crack interaction and the ef-fect of microstructure on crack growth could be revealed bythe FIB following the indentation tests. Likewise, the FIBcan be applied for analyzing material removal processes oc-curring during sliding contacts in ceramics, such as scratch-ing and grinding as described below.

3.2. Scratch tests

A detailed examination on the crack growth induced by aVickers indenter scratching on EQ was conducted. As shownin Fig. 5, radial cracks can be seen propagating away fromthe scratch track, and penetrating at an angle forward ofthe sliding direction (Fig. 5(b)), before turning upwards and

Fig. 5. EQ following scratch testing at a load of 10 N showing (a) surface and (b) subsurface damage, in which the growth of radial and lateral crackscan be seen [2].

popping out of the surface under the tangential force of theindenter. The lateral cracks below the scratch track were ob-served at two different depths: 2m (Fig. 5(b)).Lateral cracks that formed within 1m of the surface wereconnected to the parallel surface cracks that run perpendicu-lar to the scratch track. Lateral cracks that formed at >2mbelow the surface are more significant, and lead to materialremoval along the scratch track.

This FIB observation indicates that the radial cracks, con-nected by the lateral cracks were responsible for materialremoval occurring along the sides of the scratch track andoutwards. In contrast, the lateral cracks alone were mainlyaccountable for the material removal taking place inside thescratch track.

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Fig. 6. (a) Ground surfaces of EQ and EL, and (b) subsurface cross-section of ground surfaces of EQ and EL [11].

3.3. Grinding tests

The surface and subsurface conditions of EQ and ELwere compared using the FIB after grinding tests to high-light the effect of microstructure on crack formation andsubsequent material removal. The ground surface of EQcontained a large number of pits which were much largerthan the average size of the grains (EQ of Fig. 6(a)). Across-sectional view of this ground surface revealed bothlateral and median cracks beneath the ground surface withsome lateral cracks which have propagated towards thesurface (EQ of Fig. 6(b)), leading to the formation ofpits. In contrast, on the ground surface of EL, grain-sizedparticle pullout was observed on the broken surface layer(EL of Fig. 6(a)). Subsurface microcracking was detected,but lateral and median cracks were not detected (EL ofFig. 6(b)). Therefore, this FIB observation suggests lat-eral cracks are responsible for the removal of large vol-umes of material during the grinding process. However,elongated grains can suppress the propagation of cracks,material removal is thus caused by single-grain particledislodgement and occurs at a reduced rate with less surfacedamage.

With the support of the FIB technique, the mechanismsthat control material removal in both scratching and grind-ing conditions in ceramics can be explored in-depth withhigh efficiency and flexibility, as highlighted in this presentwork. In addition, 3D FIB imaging [12] and FIB-assisted

TEM preparation techniques [1] are currently being appliedto explore the new level of ceramic fracture characteri-zation.

4. Summary

The FIB is a valuable tool in characterizing surfacecontact-induced fracture. Utilizing the FIB in this investi-gation, the following conclusions can be drawn:

(1) Elongated grains can improve the indentation fracturetoughness of ceramics via suppressing the growth ofcracks; however, hardness showed little dependence onmicrostructure variation.

(2) The propagation direction of lateral cracks was observedto be affected by the presence of radial cracks; secondaryshallow radial cracking was clearly identified.

(3) The radial cracks, connected by the lateral cracks, wereresponsible for material removal occurring along theside of the scratch track and outwards; the lateral cracksalone were mainly accountable for the material removaloccurring inside the scratch track.

(4) The lateral cracks were responsible for large-volumematerial removal during the grinding process in theequiaxed-grained material. Elongated grains can sup-press the propagation of cracks, thus reducing the levelof surface damage caused by grinding.

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Acknowledgements

This work was supported by an Australian ResearchCouncil Large Grant entitled High Temperature Wearin -Sialon Ceramics. Samples were prepared with theassistance of Prof. Y.-B. Cheng of Monash University,Melbourne, Australia.

References

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[10] B.R. Whittle, R.J. Hand, J. Am. Ceram. Soc. 84 (10) (2001) 2361.[11] Z.-H. Xie, R.J. Moon, M. Hoffman, P. Munroe, Y.-B. Cheng, J. Euro.

Ceram. Soc., in press.[12] B.J. Inkson, H.Z. Wu, T. Steer, G. Mbus, Mater. Res. Soc.

Symp. Proc. 649 (2001) Q7.7.1.

Characterization of surface contact-induced fracture in ceramics using a focused ion beam millerIntroductionExperimental detailsResults and discussionIndentation testsScratch testsGrinding tests

SummaryAcknowledgementsReferences