Int. Journal of Refractory Metals & Hard Materials 28 (2010) 508–515

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Int. Journal of Refractory Metals & Hard Materials

journal homepage: www.elsevier .com/locate / IJRMHM

Meso-scale mechanical testing methods for diamond composite materials

R. Morrell a,b,*, R. Danzer b, P. Supancic b, W. Harrer b, S. Puchegger c, H. Peterlik c

a National Physical Laboratory, Teddington, Middlesex TW11 0LW, UKb Institute for Structural and Functional Ceramics, University of Leoben, Peter-Tunner-Strasse 5, A8700 Leoben, Austriac Institute for Materials Physics, University of Vienna, Strudlhofgasse 4, A1090 Wien, Austria

a r t i c l e i n f o

Article history:Received 17 November 2009Accepted 15 February 2010

Keywords:PCDFlexural strengthFracture toughnessElastic propertiesEdge chipping

0263-4368/$ - see front matter Crown Copyright � 2doi:10.1016/j.ijrmhm.2010.02.009

* Corresponding author. Address: National PhysMiddlesex TW11 0LW, UK.

E-mail addresses: roger.morrell@npl.co.uk (R. Moben.ac.at (R. Danzer), herwig.peterlik@univie.ac.at (H

a b s t r a c t

Diamond composite materials prepared as thin layers on tungsten carbide substrates for applicationssuch as rock cutting present characterisation challenges. As a consequence of the production route, theavailability of only small pieces of diamond composite material has required some ingenuity in devisingtesting methods which are appropriate but which also yield accurate data to enable differences betweengrades of material to be characterised. A number of additional problems associated with their hardnessand stiffness, coupled with difficulties of accurate machining, have to be overcome. This paper reportssome of the techniques attempted to measure flexural strength, fracture toughness, elastic modulus, fati-gue behaviour and edge chip resistance, and assesses their success.

Crown Copyright � 2010 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Diamond composite materials for applications such as miningtool bits are manufactured by high-pressure technology as layerstypically 2 mm in thickness onto tungsten carbide hardmetal form-ers. They provide a hard, wear resistance surface for impactingupon rock in drilling processes. However, the optimisation of per-formance requires optimisation of the material manufacturing pro-cess, which can only be assessed through effective means fordetermining the relevant mechanical properties. Because the typi-cal size of such pieces is often less than or about 20 mm in diam-eter, test methods that have been standardised for brittlematerials such as advanced technical ceramics are not appropriate,and methods of evaluation using miniaturised test specimens haveto be found.

In addition, the high hardness makes conventional machining ofregular-shaped test-pieces impossible. Cost-effective processes arelimited to either laser or electro-discharge machining (EDM), butdamage introduced by such methods is of unknown extent or ef-fect. Such processes also have limitations in the achievable dimen-sional tolerances, flatness of surfaces and parallelism of test facescompared with conventional machining methods on ceramicsand hardmetals, such as surface grinding.

010 Published by Elsevier Ltd. All

ical Laboratory, Teddington,

rrell), robert.danzer@unileo-. Peterlik).

This paper reviews the problem and provides some solutions tothe testing problem. There remain some limitations which onlyspecialist purpose-built equipment could overcome.

2. Miniaturised flexural strength testing

The requirements of good practice in flexural strength testingare well-understood and are embodied in the existing standardsfor testing ceramic materials. Whether the testing facility is de-signed for three-point flexure or four-point flexure, the key factorsare a self-aligning ability to ensure equalisation of forces on thesupport rollers (and loading rollers in four-point flexure) and min-imisation of friction effects at roller contacts. Significant errors candevelop if the rollers are not free to roll as the test-piece bends.

Small bar test-pieces, of minimum size 16.5 � 3–4 � 1.5–2 mmcould be machined using EDM from the diameter of discs cut fromthe tungsten carbide cylinders on which they are formed. Two dif-ferent approaches were investigated. Firstly, for room-temperatureflexural strength (and fracture toughness) evaluation, a miniatur-ised semi-articulating four-point flexure jig was constructed usingfreely rotating rollers of case-hardened steel with a 13 mm outerspan, 4.33 mm inner span, incorporating a positioning fixturewhich allowed the component parts to be accurately positioned(Fig. 1a). This arrangement allowed accurate flexural strengthand fracture toughness tests to be performed, limited primarilyby the parallelism of opposing faces of the test-pieces, and by therelative dimensional accuracy of the jig parts, which becomespoorer the small the geometry [1].

rights reserved.

Fig. 1. Flexural strength testing (a) using a semi-articulating room-temperaturefixture with 13 mm outer span, and (b) using a semi-articulating high-temperaturefixture with 15 mm outer span, 5 mm inner span, manufactured from nimonic 105with alumina ceramic rollers, with a notched test-piece in position.

R. Morrell et al. / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 508–515 509

The second arrangement was of a semi-articulating monolithicstyle of fixture (Fig. 1b) in which the test-piece support rollerswere places in guidance slots in a slotted jig to give a 15 mm span.The loading rollers were loosely held in captive locations on theend of a centralised steel pusher block to give a 5 mm inner span.This arrangement was primarily intended for the acquisition ofhigh-temperature data, so in order to achieve a testing capabilityto 700 �C in air whilst retaining sufficient strength and hardnessin the rollers and the jig body, the jig body was fabricated from animonic alloy (Inconel proved too soft at 700 �C), and the rollersfrom a high-strength alumina ceramic.

In both cases the degree of articulation was thought to be suffi-cient to overcome some out of parallelism of the test-piece facesand edges, which were otherwise reasonably flat. Restricting theinner rollers in the high-temperature jig has to be viewed as a com-promise to allow ease of test-piece insertion and convenience inheating to high temperature whilst maintaining all parts in the cor-rect location.

The issues of miniaturisation of flexural strength testing (orflexural fracture toughness testing) have therefore been overcomeby scaling down the size of testing jig whilst retaining adequate jigfunction to allow for imprecise shaping of the test-pieces. For ele-vated temperatures, it has been found that a nimonic alloy jig withceramic rollers has performed well, with only occasional rollerfailure.

3. Miniaturised biaxial strength testing

In recent years the ball-on-three-balls biaxial flexural strengthtest has been developed. Rather than using the more conventionalring-loaded, ring-supported geometry, which suffers from fric-tional effects and ring damage from test-piece fracture, this newtest provides a friction-free method of assessment, and has beendemonstrated over a wide test-piece size range. It has a key advan-tage of not requiring perfectly flat and parallel-faced test-pieces.The principal disadvantage is that the stress solution is not analyt-ical, and has to be established using finite element analysis and aninput value of the elastic properties of both test-piece and balls. Inaddition, the peak fracture stress under the central loading ball is anon-linear function of force, as a consequence of Hertzian ball flat-tening. However, a simple formulation for given elastic propertiescan be derived from linear elastic FEA analyses which allow thenominal biaxial flexural stress at fracture to be derived from theapplied force [2].

In the case of testing performed on PCD, disc samples19.1 ± 0.05 mm diameter and 2.00 ± 0.01 mm thickness were sup-plied with an EDM surface finish. The fracture jig was made with6.5 mm steel balls retained in loose contact by the jig arrangement(Fig. 2a), and centralised on the test-piece axis. The fourth loadingball was also centralised by the jig arrangement.

The FEA was performed specifically for these materials (Fig. 2b)with input elastic properties shown in Table 1, and with severallevels of applied force, assuming a typical material strength of1 GPa and fracture force of 3 kN. A typical stress distribution isshown in Fig. 2b. The factor f relating the applied force F to thebiaxial stress rmax:

rmax ¼ f ðFÞ � Ft2 ð1Þ

where t is the thickness, is a weak function of force and of Poisson’sratio, as is also shown in Table 1. The approximate straight-line fitpermits the fracture stress of each test-piece to be readily computedwithout recourse to further stress analysis.

The test was successfully applied to a set of 27 disc test-pieces(Fig. 3a), and the data were evaluated for two values of Poisson’sratio and plotted in Fig. 3b. The Weibull parameters determinedby the maximum likelihood method (CEN EN 843-5) are shownin Table 1. The mean strength is higher than that obtained by flex-ural strength testing (about 1000 MPa), mainly as a consequence ofthe small effective volume.

In summary, the circular shape of the as-manufactured speci-mens makes them ideal for biaxial testing. The most robust meth-od for testing high-strength high-stiffness materials is the four-balltest, although it became necessary to extend the previous range offinite element evaluation for the specific properties of PCD. Havingdone this, the test could be made repeatably, and a good statisticalanalysis could be made. The average biaxial strength was similar tothat of the uniaxial flexural strength determined on the samegrade.

4. Fracture toughness testing

Previously published efforts to make fracture toughness mea-surements on diamond materials have mostly involved the diame-tral compression of a small thin disc with a chevron-ended centralslot introduced by laser or EDM machining (e.g. [3–6]). Load is ap-plied parallel to the slot, and fracture toughness is computed fromthe fracture load and the notch length. This test has a number ofuncertainties associated with it, and requires the correct diameterto thickness ratio to avoid buckling, as well as high quality edge

Fig. 2. The ball-on-three-balls test arrangement (a) shown as a schematic, (b) finite element stress analysis of one-third of the disc for the particular elastic conditionsinvolved with PCD.

510 R. Morrell et al. / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 508–515

surfaces for loading. There are also concerns about the use of abluntly cut notch to represent a sharp pre-crack.

In the present work, approaches more akin to currentstandardised testing for brittle ceramic materials have been att-empted.

4.1. Single-edge V-notch beam test [7,8]

A transverse notch with a sharp tip is honed into the narrowface of a flexural beam test-piece using a razor blade and diamondpaste. This method has been standardised in ISO 23146. In the

experiments on PCD, a V-notch was made in the centre of sometest-pieces using a laser method to a depth of about 0.4 mm. Thenotch was aligned in a notch polishing machine, and further honedto a depth of about 0.6 mm using diamond paste. This process wasslow; it took some hours for each test-piece. However, a notch tipradius of less than 10 lm was achieved in most test-pieces (Fig. 4),which was considered to be sufficiently sharp to be equivalent to asharp crack. Test-pieces were then fractured in flexure and thenotch depth measured using a microscope. The fracture toughnesswas computed using the conventional equation for notched or pre-cracked beams (see ISO 23146):

Table 1Ball-on-three-balls analysis and test data.

Material properties Balls Test-piece

Young’s modulus 210 GPa 1000 GPaPoisson’s ratio 0.3 0.07 or 0.088

FEA analysis Poisson’s ratio 0.070 Poisson’s ratio 0.088

Factor f for F = 3000 ± 2000 N f ðFÞ ¼ 1:611� 0:0082ðF�30001000 Þ f ðFÞ ¼ 1:633� 0:0092ðF�3000

1000 ÞExperimental results:

Characteristic strength (MPa)a 1434 (1401–1468) 1453 (1420–1488)Weibull modulusa 14.5 (11.2–18.8) 14.5 (11.2–18.8)

a The limits of the 90% confidence interval are shown in parenthesis.

1000 1200 1400 1600 18000.67

1.81

4.86

12.66

30.78

63.21

93.40

99.94

-5

-4

-3

-2

-1

0

1

21. evaluation ν=0.0702. evaluation ν=0.088

Ln

Ln 1

/ (1-

F)

Prob

abili

ty o

f fai

lure

F

Characteristic strength , MPa

, %a

b

Fig. 3. Ball-on-three-balls tests: (a) three examples of the biaxial fractures from the test, and (b) Weibull distributions for 27 test-pieces showing slight sensitivity to Poisson’sratio values of 0.070 and 0.088.

R. Morrell et al. / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 508–515 511

K Ic;sevnb ¼ rYffiffiffiap¼ Pmax

BffiffiffiffiffiffiWp So � Si

W3ffiffiffiap

2ð1� aÞ3=2 Y ð2aÞ

where

Y ¼ 1:9887� 1:326a� ð3:49� 0:68aþ 1:35a2Það1� aÞð1þ aÞ2

ð2bÞ

and where Pmax is the peak force, Si,o are the inner and outer spans,respectively, B is the specimen width, W is the specimen thickness,

and a is the relative notch tip depth. On one particular product, tentest-pieces were notched, with the outcome of KIc = 9.9 ± 0.6MPa m1/2.

4.2. Chevron notch test

A transverse notch is cut into two adjacent corners of a beamtest-piece using EDM to leave a sharp tipped ligament of materialto fracture in a flexural test. Several standardised procedures are

Fig. 4. SEVNB test-piece with the EDM-cut pre-notch deepened and sharpenedusing the razor-blade honing technique to give a notch root radius �10 lm.

Post fatigue, 482 °C

0

20

40

60

80

100

120

140

0 5 10 15 20 25 30 35 40

Time, s

Forc

e, N

a

b

Fig. 5. (a) Flexural test-piece chevron notched using EDM and fractured in four-point flexure, and (b) a typical force/time plot at 482 �C showing marginal testvalidity according to the usual criteria.

512 R. Morrell et al. / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 508–515

available, but the method used in CEN EN 14425-3 was employedbecause flexibility was needed in the calculation equation for thespecific test-piece and notch dimensions available. One of thedrawbacks of the method is that at the outset it is not clearwhether the test material will show ‘valid’ behaviour, i.e. whetherthere will be smooth crack propagation from the notch tip, and notfast, uncontrolled fracture, considered ‘invalid’ and potentiallyleading to an overestimate of fracture toughness. The chances arebest if the test facility has high stiffness, and therefore a minimumof stored energy to drive the crack at the onset of propagation. Thechoice of chevron depth and shape is somewhat arbitrary, but thebest chances of obtaining ‘valid’ behaviour are if the notch is quitedeep, making the test-piece compliant, but still within the validityrange of the calculation equation. In the current tests, the notcheswere made into the 2 mm faces of 2 � 4 � 16 mm test-pieces, leav-ing the chevron tip 1.4 mm below the surface (Fig. 5a).

For room-temperature tests, the semi-articulating monolithictype four-point flexural strength test facility was employed in astiff loading facility in order to maximise the chances of obtainingvalid behaviour. The force time curves recorded suggested that thetests were ‘marginally valid’ (Fig. 5b, at higher T, �500 �C) in thatthere was generally a small degree of non-linearity before a sharpload drop, indicating that crack growth had commenced stably. Inmost cases a slight creaking noise was heard immediately beforethe load drop, suggesting that in fact, crack initiation had occurredsubcritically.

After fracture, the chevron notch dimensions were measuredusing a measuring microscope, and the fracture toughness KI,cnb

was computed using the relationships in EN 14425-3:

K I;cnb ¼ Y�minPmaxðSo � SiÞ

BW3=2

� �ð3aÞ

where

Y�min ¼ ð3:08þ 5:00a0 þ 8:33a20Þ 1þ 0:007

SiSo

W2

� �1=2 !

a1 � a0

1� a0

� �

ð3bÞ

and where a0 is the relative depth of the chevron tip, and a1 is theaverage relative side depth of the chevron.

For a range of PCD grades, average values between 8.0 and9.5 MPa m1/2 were obtained, and with one exception the standarddeviations were less than 0.4 MPa m1/2. For the grade also tested bythe SEVNB method there was reasonable equivalence of result.

Tests were also made at higher temperatures, although thesecould not be performed in the stiff facility. The longer loading trainpassing through the furnace system meant that there was lesschance of valid results. Nevertheless, the behaviour was similarto that at room temperature, and the creaking sound before fastfracture could still be heard. Assuming all tests were still valid,the average results for a number of material grades ranged be-tween 8.7 and 10.2 MPa m1/2 at nominally 500 �C, and between8.5 and 9.9 MPa m1/2 at nominally 700 �C.

Although the detailed microstructures of the test materialswere not investigated as part of this work, making direct compar-isons with previously published fracture toughness investigationsinappropriate, the values obtained are similar to those presentedby other authors using the diametral compression method. Themethods employed in our work had the advantage of being moreeconomical in test material and less prone to problems in testing.

In conclusion, because PCD is so difficult to pre-crack effectivelyas a result of its very high modulus, especially with small test-pieces, the usual techniques for valid fracture toughness measure-ment could not be employed. However, both the single edgeVee-notch beam and the single chevron notched beam methodswere found to give results which more or less agreed, and theCNB method could distinguish between different grades of thematerial. The latter tests were described above as ‘marginally valid’because the stiffness of the test material means it is almostinevitable that the test system cannot be sufficiently stiff to makea truly valid controlled crack growth test. Low forces are beingused, and test machines tend to have a rather higher complianceat low forces than at higher forces where contact surfaces and link-ages have been pushed together.

Table 2The first 10 measured and calculated eigenfrequencies of a PCD disc sample.

R. Morrell et al. / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 508–515 513

5. Fatigue testing

At the outset of our testing programme it was unclear whetherPCD underwent a fatigue process in a similar manner to other me-tal composite materials such as WC/Co. There has been some pub-lished evidence that long cracks can be made to propagate in acontrolled fashion [9], in that case using disc tests with a diametralnotch, like a compact tension specimen. However, the behaviour ofessentially uncracked material failing from typical small surfacedefects has not been studied.

Using conventional flexural strength test-pieces, fatigue testswere conducted at room temperature using an Instron 8872 servo-hydraulic machine and the monolithic four-point flexural strengthjig. The peak force was set initially to be 90% of the minimum fractureforce previously recorded in flexural strength tests, i.e. 1 kN, or about1 GPa test-piece stress. A series of tests were made with a sinusoidalload profile at 10 Hz, and with an R value (maximum to minimumforce ratio) of 0.1. No failures were obtained up to more than2,000,000 cycles. At higher peak force levels, there was instanta-neous failure. No clear fatiguing behaviour could be determined.

In order to make tests at elevated temperature and to promotefracture it was decided to use chevron notched test-pieces whichhave a better defined fast fracture force. However, these required amuch lower fatiguing force (�100 N) than could be well controlledby a conventional servohydraulic machine. The experiments weretherefore set up in an Instron 4505 universal testing machine oper-ating under load control with a four-second cycle period. The peakforce was set at nominally 80% of the mean chevron notch fractureforce, although because of limitations of machine control whenoperated in this way, the peak force occasionally reached 90% of thisvalue. The test was set for 100,000 load cycles with R = 0.1 operatedat 580 �C.

Only one test-piece failed during the load cycling. All other test-pieces of a range of material grades survived this treatment. Imme-diately on completion of load cycling each surviving test-piece wassubjected to a slow loading cycle to fracture as a CNB fracturetoughness specimen. It was found that the effective fracture tough-ness was similar to that determined in a straight CNB test withoutprior load cycling. The shape of the force displacement curves wassimilar to those for the straight CNB test, i.e. still marginally valid,indicating that the highly stressed tip of the chevron remainedsubstantially undamaged by the fatiguing process.

Overall, it can be concluded that the subcritical growth of fatiguecracks from EDM damaged surfaces is negligible, and that the mate-rial class essentially does not suffer from fatigue in the conventionalsense from normal surface defects, which in the present case arethose derived from EDM surface preparation. Instead, it tends to frac-ture once a certain stress level is reached. With miniature specimens,low forces are required, and these are difficult to control with con-ventional equipment. A purpose-built low force, low inertia, high-frequency system is needed for a more systematic study. If our initialconclusion is correct, it can be deduced that the residual metalliccomponent provides no weakening effect in fatigue, in contrast towhat has been seen in WC/Co hardmetals [10], and further, that thestrength and fatigue resistance are controlled by a contiguous dia-mond grain network which is difficult to damage cumulatively.

Mode Measured frequency (kHz) Calculated frequency (kHz)

1 91.7 91.52 93.1 91.53 130.1 129.44 200.2 200.15 200.9 200.16 287.2 287.17 287.8 287.18 330.8 330.9

6. Elastic modulus testing

The high stiffness and small size of available specimens meanthat conventional techniques are rather limited. The optimummethod for the available disc shapes was resonant ultrasoundspectroscopy (RUS)1; see e.g. [11]. This procedure, which is similar

1 Undertaken at the Department of Material Physics, University of Vienna, Austria

9 331.3 330.910 425.7 425.3

.

to that described in ASTM 1279 Annex for the similar impact excita-tion method, employs the measurement of at least the first two fun-damental vibration modes (symmetrical diaphragm mode and afolding mode) of a disc supported on its nodal circle. The ratio ofthese two frequencies provides Poisson’s ratio. Either of the two fre-quencies and Poisson’s ratio gives Young’s modulus. If the material isconsidered to be isotropic, shear modulus can simply be calculatedfrom Young’s modulus and Poisson’s ratio. In practice, in RUS onPCD, a wider frequency range was scanned (up to 0.5 MHz) andthe eigenfrequencies of about 30 resonant vibrations were detected.These frequencies were best-fitted to a model from which the elasticproperties are calculated (Table 2 shows the fit for the first 10peaks). The principal sources of error in this measurement werethe roundness and face parallelism of the test-piece.

Using this technique on two 19 mm diameter, 2 mm thick discs,Young’s modulus was found to be 1016 ± 10 GPa, shear modulus467 ± 10 GPa, and Poisson’s ratio 0.088 ± 0.026 (compared withpreviously reported values of E = 1000 GPa and m = 0.07). Therewere some differences between the measured frequencies andthose computed by the best-fit model. In particular, there wassome peak splitting, which is due to the test-piece not being per-fectly round or parallel faced and free from edge chips. The overalluncertainties are therefore due to the dimensional imperfections inthe test-pieces. However, considerable further efforts would beneeded to provide a ‘perfect’ specimen in PCD.

For tests to elevated temperatures a small furnace system wasdevised to surround the test-piece and provide a protective envi-ronment. Results from this system on a PCD disc are shown inFig. 6.

It appears that RUS can be satisfactorily used to obtain elasticproperties of such a high modulus material available in only smallpieces. It was possible to provide a good match between modelledeigenfrequencies for a resonating disc and those obtained in exper-iments, despite the high frequencies involved (up to 0.5 MHz). Infact it should be possible to make valid tests on even smallertest-pieces with the described facility provided that the geometryis accurate.

7. Edge chip testing

It is well understood that the resistance of brittle materials todamage at edges is related to fracture toughness or, more specifi-cally, to fracture energy. Since one of the principal applications ofPCD is in rock drilling where edge impacts occur, understandingthe resistance of PCD to edge chipping is desirable. In this test,the edge of a test-piece is loaded by an indenter, typically Rockwellor Vickers, and the load required to flake the edge is related to thedistance of indentation from the edge, and the slope of this rela-tionship is a measure of fracture energy or ‘edge toughness’. Theliterature on this test is well-documented [12–15]. The methodhas previously been applied to single-crystal diamonds [16] withsome success.

ν

EG

0 200 400 600 800 1000420

460

440

940

980

960

1020

1000

1040 0.012

0.009

0.010

0.011

0.008

ν

EGν

EG

0 200 400 600 800 1000420

460

440

940

980

960

1020

1000

1040 0.012

0.009

0.010

0.011

0.008

ν

EG

420

460

440

940

980

960

1020

1000

1040 0.012

0.009

0.010

0.011

0.008

Shea

r m

odul

us, G

PaYo

ung‘

s m

odul

us, G

Pa

Temperature, °C

Pois

son‘

srat

ioν

EGν

EG

420

460

440

940

980

960

1020

1000

1040 0.012

0.009

0.010

0.011

0.008

Fig. 6. Elastic properties as a function of temperature using the RUS method.

0

1000

2000

3000

4000

0 0.2 0.4 0.6 0.8 1.0d, mm

F, N

a

b

c

Fig. 7. Edge chipping experiments on PCD showing (a) the flattening of the tip of aconical WC/Co indenter during a test, (b) the form of flake that was produced asseen from the indentation face, and (c) a plot of edge flaking force as a function offlake thickness, showing more or less linear behaviour.

514 R. Morrell et al. / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 508–515

The main problem encountered in attempting to apply this testto PCD is that it is more difficult to damage than the diamond ind-enters, which fracture due to the very high localised forces in-volved. Consequently, a novel approach was employed in which aconical WC/Co indenter was used. This underwent plastic deforma-tion and some tip crushing in each test (Fig. 7a), but the tip couldreadily be reground for the next test using diamond machining.Thus the test could be repeated without excessive cost. Effectively,a ‘soft’ indenter was being used, which probably produced initiallya partial ring crack followed by flaking crack propagation (Fig. 7b).Two batches of material were tested with good consistency in out-come. Fig. 7c shows an example of a series of tests on one materialat different locations of loading relative to the edge. Generally, un-like in most brittle materials, only the edge flaking crack was pro-duced. There were very few cases in which additional radial cracksresulted from the loading location. This is a further indication thatthe material is difficult to crack in a progressive or controlled fash-ion, but only single fast fractures occur.

Mostly, such characterisation has been performed on brittlematerials with a blunt indenter, such as a Rockwell diamond. Forthe first time in this work this has been attempted using a rela-tively soft indenter made of WC/Co, the tip of which deforms to be-come effectively a pressure pad. From the shape of the edge flakingdamage zone it seems likely that the initiation of edge fracture hasprobably come from an initially Hertzian type ring crack, whichhas continued to propagate towards the edge. This mechanismseems to have reproduced an approximately linear scaling of frac-ture force with distance from the edge similar to that seen in otherbrittle materials subject to hard indentation, but which generallydisplay initial radial cracking from the indentation site, rather thanring cracking. In this respect the test seems validly to representnear edge contact with softer materials, such as that as seen in ser-

vice in a rock drill, and thus should represent a useful simple toolfor material differentiation. It is also noteworthy that the ‘edgetoughness’ derived from the slope of the plot shown in Fig. 7c ismore than double that determined on single crystals by [16].

8. Conclusions

The principal issue that limits the ability to provide accurateengineering data on PCD materials is the limited size of piece that

R. Morrell et al. / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 508–515 515

can be cut from the WC/Co support after manufacture, the difficul-ties of preparing accurately shaped test-pieces, and the highstrength and hardness of the material in relation to conventionaltesting facilities. These challenges have been successfully ad-dressed with the following achievements:

� Small-scale flexural strength and fracture toughness testingusing miniaturised beam testing methods. Both the SEVNB andCNB methods have produced sensible and consistent resultsfor fracture toughness commensurate with extant data usingother methods.

� Flexural fatigue testing with both straight and chevron-notchedtest-pieces has demonstrated that effectively there is no fatigu-ing effect from surface or near-surface flaws in the versions ofPCD evaluated, presumably because of diamond grain contiguityand the difficulty of inducing subcritical damage into thismicrostructure.

� Elastic modulus testing via the resonant ultrasound spectros-copy (RUS) method has been successful in the accurate determi-nation of all elastic parameters on small disc samples.

� Edge chipping damage has been successfully simulated using adeformable ‘soft’ indenter, and has produced edge flaking resultssimilar in nature to those seen in less-hard brittle materialsusing a hard indenter, with the ‘edge toughness’ parameterbeing more than double that reported for single-crystaldiamond.

Acknowledgements

The authors are grateful for the support and interest of ElementSix Pty, Springs, South Africa, particularly Mr. Stephen Masete, inthis work, and for permission to publish this paper.

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