ELSEVIER Materials Science and Engineering A209 (1996) 270-276

MATERIAlSSCIENCE &

ENCINEERINC

A

Fracture toughness and thermal resistance of polycrystallinediamond compacts

D. Miess, G. RaiSmith MegaDiamond, Provo, UT, USA

Abstract

Polycrystalline diamond compacts (PCD) are being used increasingly for oil and gas drilling and in machining of ceramics andhard non-ferrous materials. Average diamond grain size and its distribution are used as one of the means to tailor properties ofPCD compacts. The diamond sintering process requires use of a tungsten carbide cobalt disc placed onto diamond powderfollowed by high pressure and high temperature conditions. During this process pseudo-eutectic, WC-Co liquid from the tungstencarbide disc is infiltrated into diamond powder providing a liquid phase to facilitate inter-grain diamond bonding. The amountand chemical composition with respect to carbon content of the liquid phase are dependent on average diamond grain size andits distribution. Finer diamond sizes tend to have higher sintered density than coarser sizes indicating a higher volume fractionof metallic content. The role of residual metallic content of the diamond layer in conjunction with average grain size on fracturetoughness of the diamond layer was investigated. The fracture toughness was determined using a diametral compression test.Larger grain PCD compacts having lower amounts of matallic content were found to have a higher toughness than fine grainedmaterials with higher amounts of residual metallic phase. PCD compacts of different starting diameter grain sizes were subjectedto elevated temperatures under different gas environments and examined for their thermal resistance. The results are explained interms of total metal content of the diamond layer in conjunction with the development of inter-grain diamond bonding.

Keywords: Fracture toughness; Thermal resistance; Polycrystalline diamond compacts

1. Introduction

The wear of polycrystalline diamond cutting edgesfor machining, mining, or petroleum drilling applica­tions can be described as a result of predominantly twoprocesses, namely chemical dissolution and mechanicalabrasion [1]. During high speed machining [2] andmining or drilling [3], high temperatures of around 1000°C can be attained. Chemical effects predominate andthe tool material dissolves into the chip material on anatomic scale and is carried away by the flowing chip.However, mechanical or abrasive wear is believed tooccur by generation of numerous micro cracks [4] at thegrain boundaries or in the grains that make up thesintered layer of the tool. As the cracks grow to acritical size under the applied cutting load, the cuttingedge fails by forming a chip which is an aggregate ofcrystals or grains of the PCD material. The later pro­cess becomes dominant over chemical wear at lowertemperatures since the chemical solubility falls off expo-

nentially with temperature. The micro fracturing orchipping comprising of abrasive or mechanical wear ofcutting edges, is therefore governed by the fracturetoughness of the PCD layer. Fracture toughness ofrelatively thin PCD layers is not a well-defined propertyin view of the absence of any standard procedure for itsdetermination. Different procedures for fracture tough­ness evaluation such as double cantilever, double tor­sion, single edge notched beam, chevron notched beam,etc, have been used to determine this property forceramic materials. The choice of technique applicableto PCD layers is severely limited in light of availabilityof only limited sizes which are very often thin discs ofabout 0.5 mm thickness and 50 mm in diameter.

The diametral compression test [5,6] also referred asBrazilian disk test appears to be more suitable choicefor PCD materials. The test specimen in the form of adisk having a notch, is loaded in compression along adiameter and the transverse tensile stress that splits thedisk along the loaded diameter is used to calculate

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D. Miess, G. Rai Materials Science and Engineering A209 (1996) 270 276 271

1-----0

fracture toughness [5]. This technique was used todetermine fracture toughness of PCD layers havingmean grain size from 4 f.1 to 120 II. Since the PCDcutting edges are subjected to elevated temperaturesduring their application, the other objective of thisstudy was to evaluate effects of grain size and suchmicrostructure variables such as the amount and dis­persion of second phase on the thermal stability ofPCD layers,

2. Fracture toughness evaluation of PCD

2.1. Material and test results

Most PCD tools are made using tungsten carbidesubstrate with a diamond layer of thickness of about0.5 mm. A number of test specimens were chosen whichwere produced using a starting diamond feed powderwith an average grain size ranging from 2 f.1 to about120 f.1. The tungsten carbide substrate of all specimenswere ground using a smaller grit (200 mesh) diamondwheel to obtain smooth outer diameter. Final surfacepreparation included lapping both sides of the PCDdisk using a diamond lapping powder of about 2 II insize. This served to provide a scratch free smoothsurface. The penny shaped notch was cut using a laserbeam between 0.07 to 0.08 mm in diameter. The notchlength was maintained at 3.2 mm. The specimen geome­try is shown in Fig. 1.

Mechanical testing was performed on an Instronmachine. The notch was aligned along the load axis andboth the load and displacements were recorded. Thecross head speed was held constant at 0.05 mm perminute for all test specimens. From the load displace­ment data, fracture toughness K/c was calculated usingthe following equation:

K{( = l.012P(k/(1 - k»

(I-0.6038k+ 1.672k 2 -1.698k 3 )t(3.14R)12 (I)

6~

i s

I ..

i 3

•20 20 .4Q 60 80 100 120

AVERAGE GRAIN SIZE (MICRONS)

Fig. I. Schematic diagram of PCD test specimen used for diametrialcompression test.

--.lr----------,I t

----III

Fig. 2. Fracture toughness of PCD layers as a function of relativediamond grain size.

where: P is the fracture load; t is the specimen thick­ness; R is the radius of specimen and k is the ratio(2a /D) of notch length (2a) to the specimen diameter(D).

Fracture toughness as a function of the startingdiamond grain size of PCD sintered products is shownin Fig. 2. Each point on the curve represents an averageof 10 specimens tested for fracture toughness underidentical conditions. The scatter in data as measured bystandard deviation was less than 10(/'(1 of the meanvalue. It is clear that toughness of PCD layer increaseswith increasing grain size rapidly up to about 30 f.1beyond which the rate of increase with respect to grainsize diminishes. Lammer's [6] measurement on PCDdisks showed similar trends and results considering thedifferences in manufacturing process used to producePCD disks.

3. Abrasion testing

Abrasion tests comprised turning logs of grainte ofabout 50 cm in diameter mounted on a lathe with thehelp of steel axles glued to the log. The tests were runusing a constant surface speed of 600 SFPM (183meters per minute) with about 0.28 mm per revolutionfeed rate and depth of cut of 1.02 mm. The tests wereconducted using PCD rounds with a tungsten carbidesubstrate. The tools were always held with a negativerake angle of 30°C. The abrasion resistance of the toolswas defined as ratio of volume of granite removed tothe area of wear flat generated on the rake of the tool.

The test results shown in Fig. 3 are normalized valuesof wear resistance with respect to 30 11 PCD tool and isplotted as a function of grain size of PCD tools. Thechoice of 30 f.1 grain size was purely abitrary. As shownin the chart, the wear resistance decreases with increas­ing grain size of PCD sintered layer rather significantly,and then tapers off beyond about 10 f.1.

D. Miess, G. Rai I Materials Science and Engineering A209 (1996) 270-276272

300

! 250!:j

I 200

150~3 100:!

~ 50

" 01 10 100 1,000

_. me 1j...._~rOH!I!ti,loI~_J~........=••__..........._ ...• ....j~_d_,.lIIIrt.··.ty'IIlI?1li••X

AVERAGE PARTICLE SIZE (MICRONS)

Fig. 3. Abrasion resistance of PCD layer as a function of grain size.The 30 micron PCD layer was taken as reference.

4. Metal content of PCD layers

The PCD high pressure, high temperature manufac­turing technology invloves the technique of infiltratingmolten tungsten carbide cobalt material from the sub­strate into the diamond feed powder. A pseudo binaryWC-Co phase [7] diagram suggests presence of aneutectic reaction at about 1360 °C and at approxi­mately 75% Co. The infiltrating material wets diamondparticles and provides a liquid phase medium for masstransport between diamond grains of different sizes.Since the thermodynamic driving force for sintering is aminimization of surface energy, the mass transport orthe bonding between diamond grains is not very rapidand tends to slow down with time. Even after pro­longed exposure to time and temperature, it is unlikelythat a monolithic structure of diamond grains is possi­ble. Therefore the metallic phase in the microstructureis retained in the final product and since its propertiesare different than diamond, it has significant impact onthe performance of the PCD compacts. Typical mi­crostructure of a 10 f.1 PCD layer is shown in Fig. 4(a)

(a) (b)

Fig. 5. Microstructure of the carbide substrate showing the presenceof a distinct reaction zone. Polarized light was used in this view toemphasize the reaction zone (top lighter band is PCD. the darkregion below is the we-co substrate region).

and (b). Fig. 4(a) shows an as polished PCD surfaceand Fig. 4(b) shows an as polished and leached surfacerespectively, to delineate inter crystalline bonding of thediamond grains.

During the process of sintering, dissolution of carbonfrom diamond grains into the infiltrating liquid metalfrom the carbide substrate leads to saturation of liquidphase. This not only leads to inter grain bonding ofdiamond particles but also results in carbon diffusioninto the carbide substrate. As sintering progresses, thecobalt phase in the carbide substrate gets saturated withrespect to carbon. When the compact is allowed to coolthe excess carbon in the cobalt phase is rejected creat­ing a distinct zone in the carbide substrate as shown inFig. 5. The nature of this precipitate was analyzed byRaman spectroscopy and found to correspond to amor­phous carbon as shown in Fig. 6. The amount anddistribution of this second phase material is signifi-

Fig. 4. (a) Typical microstructure of a PCD compact as polished with a grain size of approximately 10 JI. (b) Typical microstructure of a PCDcompact with a grain size of approximately 10 JI leached with acid to show the extent of intergranular bonding.

D. Miess, G. Rai Materials Science and Engineering A209 (1996) 270-276 273

Fig. 6. Raman spectroscopy lines of carbide zone indicating precipita­tion of amorphous carbon from super-saturated cobalt phase.

cantly affected by the starting diamond crystal size andany other additives that are made to the diamond feedpowder.

The metal content of the abrasive layer was measuredby determining the density of PCD layer without thecarbide substrate and then using diamond density of3.53 gm cm - 3 the metal content in the layer can becaluculated by the following equation

Percent metal content = (dpcD - ddiaxtl)/ddiaxtl

5.2. Observation of thermal cracking in peD layers

5.1. Thermo -gravimetric study of diamond powder

5. Thermal resistance testing

The micronized diamond powder used for makingPCD products is derived from high grade diamondcrystals grown for rock or concrete sawing applications.The process of growing diamond crystals requires theuse of catalyst material which is retained within thecrystal after the process has been completed. The mi­cronized diamond powder therefore contains certainamounts of metallic catalyst materials which vary withthe size of crystals. The thermo-gravimetric analysiswas carried out to determine the weight of residualcatalyst in diamond powder by oxidation of powders inan oxygen environment. The temperature was continu­ously raised and both weight loss as well as the differ­ence in temperature between the diamond powderspecimen and platinum reference were monitored as afunction of time. The process was stopped when nofurther reduction in weight was observed. The resultsare shown in Fig. 8. The only peak observed in theDTA signal relates to oxidation of diamond to formcarbon dioxide. No attempt was made to analyse theresidues left from the process.

(2)

20001800200 '---~_~_~-C-L_.---~.

1000 1200 1400 1600Raman Shift (em )

300

600

l!J:§ 500

e<~

~ 400

~

Fig. 7. Metal content of peD layers as a function of grain size.

35~----------------,

Polycrystalline diamond layers where the carbidesubstrate was removed by grinding, were studied forthermal stability in different environments by exposingthem to higher temperatures. Specimens of fine,medium, and coarse grades typically having startingdiamond grain size of about 5, 10 and 30 j.l respectivelywere exposed to temperatrues from 600 to 800°C innitrogen, hydrogen, and air. Microstructrues of thesethree types of specimens subjected to varying thermaltreatments under nitrogen, hydrogen, and air are shownin Figs. 9 and 10, and Fig. 11 for fine, medium andcoarse PCD layers respectively. All the specimensshowed catalyst material extrusion on the diamondsurface. The extruded metallic phase either spherodizedowing to oxidation in air or spread on the diamondsurface which was the case in nitrogen or hydrogenatmosphere. It appears that fine grained material degra­dation when exposed to air or nitrogen was causedmostly by rapid graphitization of diamond grains.

As shown in Fig. 2, fracture toughness increasesrapidly with the increase in the grain size up to 40 j.l

6. Discussions

6.1. Diamond grain size and metal content effect onfracture toughness

10,0001 10 100 1.000

AVERAGE PARTICLE SIZE (MICRONS)

0.1

30

25

20

::[51L- ---'

where: dpcD is the density of PCD layer without carbidesubstrate, and ddiaxtl is the density of diamond crystal.

This method is likely to produce consistent resultsover the metal leaching technique where accuracy canbe affected by such variables as undissolved metal, lossof diamond particles during filtering and the subse­quent drying operation. Consequently, the acid leach­ing technique tends to over estimate the metal contentof PCD layers. Following the density measurementprocedure and taking diamond crystal density as 3.53gm cm - 3, the metal content of various grain size peDlayers was calculated and shown in Fig. 7 as a functionof starting diamond grain size.

274 D. Miess, G. Rai / Materials Science and Engineering A209 (1996) 270 276

1500 5500.--------....-------r-----.---------, 7.3

-22

START NEl6HT: 68.26711030 e/MIN RAMP

T6A

40001100

1080.4 e22.112 .In

700 25001954.0 uY -51.3

U >:::l

"0.X "" l!lW ....

l-I- 0

300 1000 -80.61347.8 e43.97 .In-118.8 I

lit 10••DTA

-100 -500 109.90 10.99 21.99 32.98 43.97

TIME min

Fig. 8. Thermal gravametric run performed using diamond crystals of size 230/270 mesh in oxygen.

and then there is no significant increase in toughness ofPCD layers of grain size up to 140 fl. It is also evidentfrom Fig. 7 that amount of metallic phase in PCDlayers decreases with increasing grain size and the rateof decrease is insignificant for PCD layer of grain sizebeyond about 40 fl. It is interesting to compare themetallic phase amount and the toughness of PCD lay­ers of 2 fl and 40 fl grain size. While the amount ofmetallic phase in the 40 fl PCD layer decreased by afactor of two approximately, there was a three foldincrease in fracture toughness. The grain size depen­dence of fracture toughness has been discussed in detailby Rice and co workers [8,9] for various ceramic mate­rials. This was explained on the basis of differences inthermal expansion of the base and the second phasematerial as well as elastic anisotropy. In case of PCDlayers there is a large difference in thermal expansion ofdiamond grains and the Co-WC second phase and thisis expected to create high interface stresses, so thatmicro-cracks might propagate relatively easily alongdiamond/Co- WC interfaces. In ceramics materials thegrain boundaries are weaker than the grains, conse­quently the reduction of the grain boundary area willenhance the toughness of the material. This perhapsplays a more significant role in increasing fracturetoughness of large grain over the fine grain PCD mate­rials.

6.2. Thermal resistance of peD layers

From Figs. 9 and 10, and Fig. 11, the effect ofenvironment and temperature on the behaviour of PCDlayers of 4 fl, 10 fl, and 30 fl grain size respectively canbe summarized as followes:

(1) In a hydrogen environment, visual degradation of

PCD layers begins between 700 and 750°C. At thistemperature failure is manifested by large stress reliefcracks created from the thermal expansion difference.

NOT PERFORMED DUETO THE EXTENSIVEDAMAGE AT 750 0 C

(ABOVE)

f-----11 micron

Fig. 9. Fine (5 fl) grain size PCD showing thermal treatment effectsin different atmospheres. Temperatures from top to bottom: 600; 700;750 and 800°C, (magnification 1500 x).

D. Miess, G. Rai Materials Science and Engineering A209 (1996) 270-276

NOT PERFORMED DUETO THE EXTENSIVEDAMAGE AT 750' C

(ABOVE)

1-----11 micron

275

NOT PERFORMED DUETO THE EXTENSIVEDAMAGE AT 750' C

(ABOVE)

f------11 micron

Fig. 10. Medium (10 1') grain size PCD showing thcrmal treatmcnteffects in different atmospheres. Temperatures from top to bottom:600; 700; 750 and 800 DC, (magnification 1500 x ).

This is also accompanied by extrusion of second phaseeo-we metallic material which appears to wet ratherwell on the surface of the peD layer. The peD layercracking is predominantly intergranular and changes tomixed mode with rise in temperature.

(2) In a nitrogen environment, at about 600°C thesecond phase Co-WC material begins to seep out ofthe grain boundaries. This effect increases with rise intemperature and about 750°C the first sign of PCDfracturing is noticed.

At 800 °e serious damage occurs on all three grainsize peD layers with major fracturing observed on the10 fl and 30 fl peD layers.

(3) In an air environment on the other hand, theonset of damage appears to happen around 600°Caccompanied by extensive eo-WC second phase metalextrusion out of peD layers. At this stage slight intergranular micro cracking is also observed. The extrudedmetallic phase is spherical in shape which appears to benon wetting primarily owing to oxidation of bothmetallic phase as well as diamond grains.

Raman spectrographic [10] stress measurements doneon 5 fl and 30 fl peD layers suggest the presence of

Fig. 11. Coarse (30 II) grain size PCD showing thermal treatmenteffects in differcnt atmospheres. Temperatures from top to bottom:600; 700: 750 and 800 DC, (magnification 1500 x ).

compressive stresses in the layer. The fine grain (5 fl)peD layer was found to have higher compressive stress(1.9 GPa) vs. 0.5 GPa for the coarse grained (30 fl)layer. This seems to follow the second phase metalcontent of peD layers where the coarse grained (30 /;)peD layer has approximately one half of the metalcontent of fine (5 fl) grain material. Higher amounts ofmetallic phase is responsible for greater amount ofextrusions observed in thermally treated fine grainedPCD layers.

7. Summary

(1) Fracture toughness of peD layers is stronglydependent on the starting grain size of diamond parti­cles. Larger grain size peD layer have higher tough­ness.

(2) The grain size dependence appears to be limitedto about 40 fl beyond which no significant change infracture toughness of PCD layer is observed.

(3) The amount of entrapped Co-we metallic phasein PCD layers decreases with increasing grain size.

276 D. Miess, G. Rai / Materials Science and Engineering A209 (1996) 270-276

(4) The increase in fracture toughness of peD lay­ers also corresponds to decrease in metal content.

(5) Increasing amounts of metallic second phase 10

peD layers leads to lower thermal resistance.(6) The internal stresses in peD layers as measured

by Raman spectroscopy are compressive in nature,however, upon thermal treatment they tend to becometensile.

References

[I] B.M. Kramer and N.P. Suh, Tool Wear by Dissolution: AQuantitative Understanding, Trans. ASME, J. Eng,for Industry,102 (1979) 303-312.

[2] M.e. Shaw, Temperatures in Cutting, Proc. Special Prod. Eng.ConI, ASME, Special volume, November 1988.

[3] D.A. Glowka, Geothermal Research Dept., Sandia NationalLaboritories, Personal communications, 1994.

[4] J. Gary Baldoni and Sergej T. Buljan, Ceramics for Machining,Ceramic Bull, 67 (2) (1988) 381-387.

[5] D.K. Shetty, A.R. Rosenthal and W.H. Duckworth, FractureToughness of Ceramics Measured by a Chevron-Notch Diame­tral Compression Test, Am. Ceram. Soc., 68 (1985) 325-327.

[6] A. Lammer, Mechanical Properties of Polycrystalline Diamonds,Mater. Sci. Technol., 4 (1988) 948-956.

[7] H.E. Exner, Physical and Chemical Nature of Cemented Car­bides, Int. Met. Rev. 4 (1979) 149-170.

[8] R.W. Rice, S.W. Freiman and P.F. Becher, 1. Am. Ceram. Soc.,64 (6) (1981) 345-354.

[9] R.W. Rice, J. Mater. Sci., 19 (1984) 1267-1271.[10] Y. Vohra, Dept. of Physics, University of Alabama, Birming­

ham, Alabama, Personal communications, 1995.