MATERIAlS SCIENCE & ENCINEERINC ELSEVIER
Materials Science and Engineering A209 (1996) 270-276
A
Fracture toughness and thermal resistance of polycrystalline diamond compacts D. Miess, G. Rai Smith MegaDiamond, Provo, UT, USA
Abstract Polycrystalline diamond compacts (PCD) are being used increasingly for oil and gas drilling and in machining of ceramics and hard non-ferrous materials. Average diamond grain size and its distribution are used as one of the means to tailor properties of PCD compacts. The diamond sintering process requires use of a tungsten carbide cobalt disc placed onto diamond powder followed by high pressure and high temperature conditions. During this process pseudo-eutectic, WC-Co liquid from the tungsten carbide disc is infiltrated into diamond powder providing a liquid phase to facilitate inter-grain diamond bonding. The amount and chemical composition with respect to carbon content of the liquid phase are dependent on average diamond grain size and its distribution. Finer diamond sizes tend to have higher sintered density than coarser sizes indicating a higher volume fraction of metallic content. The role of residual metallic content of the diamond layer in conjunction with average grain size on fracture toughness 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 grained materials with higher amounts of residual metallic phase. PCD compacts of different starting diameter grain sizes were subjected to elevated temperatures under different gas environments and examined for their thermal resistance. The results are explained in terms 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 edges for machining, mining, or petroleum drilling applications can be described as a result of predominantly two processes, namely chemical dissolution and mechanical abrasion [1]. During high speed machining [2] and mining or drilling [3], high temperatures of around 1000 °C can be attained. Chemical effects predominate and the tool material dissolves into the chip material on an atomic scale and is carried away by the flowing chip. However, mechanical or abrasive wear is believed to occur by generation of numerous micro cracks [4] at the grain boundaries or in the grains that make up the sintered layer of the tool. As the cracks grow to a critical size under the applied cutting load, the cutting edge fails by forming a chip which is an aggregate of crystals or grains of the PCD material. The later process becomes dominant over chemical wear at lower temperatures since the chemical solubility falls off expo-
nentially with temperature. The micro fracturing or chipping comprising of abrasive or mechanical wear of cutting edges, is therefore governed by the fracture toughness of the PCD layer. Fracture toughness of relatively thin PCD layers is not a well-defined property in view of the absence of any standard procedure for its determination. 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 for ceramic materials. The choice of technique applicable to PCD layers is severely limited in light of availability of only limited sizes which are very often thin discs of about 0.5 mm thickness and 50 mm in diameter. The diametral compression test [5,6] also referred as Brazilian disk test appears to be more suitable choice for PCD materials. The test specimen in the form of a disk having a notch, is loaded in compression along a diameter and the transverse tensile stress that splits the disk along the loaded diameter is used to calculate Elsevier Science S.A. SSDl 0921-5093(95)10105-5
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fracture toughness [5]. This technique was used to determine fracture toughness of PCD layers having mean grain size from 4 f.1 to 120 II. Since the PCD cutting edges are subjected to elevated temperatures during their application, the other objective of this study was to evaluate effects of grain size and such microstructure variables such as the amount and dispersion of second phase on the thermal stability of PCD layers,
r----------,I --.l t 1-----0 - - - - I I I 2. Fracture toughness evaluation of PCD Fig. 2. Fracture toughness of PCD layers as a function of relative diamond grain size.
2.1. Material and test results Most PCD tools are made using tungsten carbide substrate with a diamond layer of thickness of about 0.5 mm. A number of test specimens were chosen which were produced using a starting diamond feed powder with an average grain size ranging from 2 f.1 to about 120 f.1. The tungsten carbide substrate of all specimens were ground using a smaller grit (200 mesh) diamond wheel to obtain smooth outer diameter. Final surface preparation included lapping both sides of the PCD disk using a diamond lapping powder of about 2 II in size. This served to provide a scratch free smooth surface. The penny shaped notch was cut using a laser beam between 0.07 to 0.08 mm in diameter. The notch length was maintained at 3.2 mm. The specimen geometry is shown in Fig. 1. Mechanical testing was performed on an Instron machine. The notch was aligned along the load axis and both the load and displacements were recorded. The cross head speed was held constant at 0.05 mm per minute for all test specimens. From the load displacement data, fracture toughness K/c was calculated using the 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
3
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0
•
20
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60
80
100
120
AVERAGE GRAIN SIZE (MICRONS)
Fig. I. Schematic diagram of PCD test specimen used for diametrial compression test.
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 starting diamond grain size of PCD sintered products is shown in Fig. 2. Each point on the curve represents an average of 10 specimens tested for fracture toughness under identical conditions. The scatter in data as measured by standard deviation was less than 10(/'(1 of the mean value. It is clear that toughness of PCD layer increases with increasing grain size rapidly up to about 30 f.1 beyond which the rate of increase with respect to grain size diminishes. Lammer's [6] measurement on PCD disks showed similar trends and results considering the differences in manufacturing process used to produce PCD disks.
3. Abrasion testing Abrasion tests comprised turning logs of grainte of about 50 cm in diameter mounted on a lathe with the help of steel axles glued to the log. The tests were run using a constant surface speed of 600 SFPM (183 meters per minute) with about 0.28 mm per revolution feed rate and depth of cut of 1.02 mm. The tests were conducted using PCD rounds with a tungsten carbide substrate. The tools were always held with a negative rake angle of 30°C. The abrasion resistance of the tools was defined as ratio of volume of granite removed to the area of wear flat generated on the rake of the tool. The test results shown in Fig. 3 are normalized values of wear resistance with respect to 30 11 PCD tool and is plotted as a function of grain size of PCD tools. The choice of 30 f.1 grain size was purely abitrary. As shown in 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) 270-276
272
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0 1
10
100
1,000
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. Fig. 5. Microstructure of the carbide substrate showing the presence of a distinct reaction zone. Polarized light was used in this view to emphasize the reaction zone (top lighter band is PCD. the dark region below is the we-co substrate region).
4. Metal content of PCD layers The PCD high pressure, high temperature manufacturing technology invloves the technique of infiltrating molten tungsten carbide cobalt material from the substrate into the diamond feed powder. A pseudo binary WC-Co phase [7] diagram suggests presence of an eutectic reaction at about 1360 °C and at approximately 75% Co. The infiltrating material wets diamond particles and provides a liquid phase medium for mass transport between diamond grains of different sizes. Since the thermodynamic driving force for sintering is a minimization of surface energy, the mass transport or the bonding between diamond grains is not very rapid and tends to slow down with time. Even after prolonged exposure to time and temperature, it is unlikely that a monolithic structure of diamond grains is possible. Therefore the metallic phase in the microstructure is retained in the final product and since its properties are different than diamond, it has significant impact on the performance of the PCD compacts. Typical microstructure of a 10 f.1 PCD layer is shown in Fig. 4(a)
(a)
and (b). Fig. 4(a) shows an as polished PCD surface and Fig. 4(b) shows an as polished and leached surface respectively, to delineate inter crystalline bonding of the diamond grains. During the process of sintering, dissolution of carbon from diamond grains into the infiltrating liquid metal from the carbide substrate leads to saturation of liquid phase. This not only leads to inter grain bonding of diamond particles but also results in carbon diffusion into the carbide substrate. As sintering progresses, the cobalt phase in the carbide substrate gets saturated with respect to carbon. When the compact is allowed to cool the excess carbon in the cobalt phase is rejected creating a distinct zone in the carbide substrate as shown in Fig. 5. The nature of this precipitate was analyzed by Raman spectroscopy and found to correspond to amorphous carbon as shown in Fig. 6. The amount and distribution of this second phase material is signifi-
(b)
Fig. 4. (a) Typical microstructure of a PCD compact as polished with a grain size of approximately 10 JI. (b) Typical microstructure of a PCD compact with a grain size of approximately 10 JI leached with acid to show the extent of intergranular bonding.
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Materials Science and Engineering A209 (1996) 270-276
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5. Thermal resistance testing 5.1. Thermo -gravimetric study of diamond powder
600
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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 and any other additives that are made to the diamond feed powder. The metal content of the abrasive layer was measured by determining the density of PCD layer without the carbide substrate and then using diamond density of 3.53 gm cm - 3 the metal content in the layer can be caluculated by the following equation
The micronized diamond powder used for making PCD products is derived from high grade diamond crystals grown for rock or concrete sawing applications. The process of growing diamond crystals requires the use of catalyst material which is retained within the crystal after the process has been completed. The micronized diamond powder therefore contains certain amounts of metallic catalyst materials which vary with the size of crystals. The thermo-gravimetric analysis was carried out to determine the weight of residual catalyst in diamond powder by oxidation of powders in an oxygen environment. The temperature was continuously raised and both weight loss as well as the difference in temperature between the diamond powder specimen and platinum reference were monitored as a function of time. The process was stopped when no further reduction in weight was observed. The results are shown in Fig. 8. The only peak observed in the DTA signal relates to oxidation of diamond to form carbon dioxide. No attempt was made to analyse the residues left from the process. 5.2. Observation of thermal cracking in peD layers
Percent metal content = (dpcD
-
(2)
ddiaxtl)/ddiaxtl
where: dpcD is the density of PCD layer without carbide substrate, and ddiaxtl is the density of diamond crystal. This method is likely to produce consistent results over the metal leaching technique where accuracy can be affected by such variables as undissolved metal, loss of diamond particles during filtering and the subsequent drying operation. Consequently, the acid leaching technique tends to over estimate the metal content of PCD layers. Following the density measurement procedure and taking diamond crystal density as 3.53 gm cm - 3, the metal content of various grain size peD layers was calculated and shown in Fig. 7 as a function of starting diamond grain size. 35
~----------------,
30 25
Polycrystalline diamond layers where the carbide substrate was removed by grinding, were studied for thermal stability in different environments by exposing them to higher temperatures. Specimens of fine, medium, and coarse grades typically having starting diamond grain size of about 5, 10 and 30 j.l respectively were exposed to temperatrues from 600 to 800°C in nitrogen, hydrogen, and air. Microstructrues of these three types of specimens subjected to varying thermal treatments under nitrogen, hydrogen, and air are shown in Figs. 9 and 10, and Fig. 11 for fine, medium and coarse PCD layers respectively. All the specimens showed catalyst material extrusion on the diamond surface. The extruded metallic phase either spherodized owing to oxidation in air or spread on the diamond surface which was the case in nitrogen or hydrogen atmosphere. It appears that fine grained material degradation when exposed to air or nitrogen was caused mostly by rapid graphitization of diamond grains.
20
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6. Discussions
51L0.1
---' 1
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100
1.000
10,000
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Fig. 7. Metal content of peD layers as a function of grain size.
6.1. Diamond grain size and metal content effect on fracture toughness As shown in Fig. 2, fracture toughness increases rapidly with the increase in the grain size up to 40 j.l
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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 of PCD layers of grain size up to 140 fl. It is also evident from Fig. 7 that amount of metallic phase in PCD layers decreases with increasing grain size and the rate of decrease is insignificant for PCD layer of grain size beyond about 40 fl. It is interesting to compare the metallic phase amount and the toughness of PCD layers of 2 fl and 40 fl grain size. While the amount of metallic phase in the 40 fl PCD layer decreased by a factor of two approximately, there was a three fold increase in fracture toughness. The grain size dependence of fracture toughness has been discussed in detail by Rice and co workers [8,9] for various ceramic materials. This was explained on the basis of differences in thermal expansion of the base and the second phase material as well as elastic anisotropy. In case of PCD layers there is a large difference in thermal expansion of diamond grains and the Co- WC second phase and this is expected to create high interface stresses, so that micro-cracks might propagate relatively easily along diamond/Co- WC interfaces. In ceramics materials the grain boundaries are weaker than the grains, consequently the reduction of the grain boundary area will enhance the toughness of the material. This perhaps plays a more significant role in increasing fracture toughness 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 of environment and temperature on the behaviour of PCD layers of 4 fl, 10 fl, and 30 fl grain size respectively can be summarized as followes: (1) In a hydrogen environment, visual degradation of
PCD layers begins between 700 and 750°C. At this temperature failure is manifested by large stress relief cracks created from the thermal expansion difference.
NOT PERFORMED DUE TO THE EXTENSIVE DAMAGE AT 750 0 C (ABOVE)
f-----1
1 micron
Fig. 9. Fine (5 fl) grain size PCD showing thermal treatment effects in different atmospheres. Temperatures from top to bottom: 600; 700; 750 and 800°C, (magnification 1500 x).
D. Miess, G. Rai
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Materials Science and Engineering A209 (1996) 270-276
NOT PERFORMED DUE TO THE EXTENSIVE DAMAGE AT 750' C (ABOVE)
NOT PERFORMED DUE TO THE EXTENSIVE DAMAGE AT 750' C (ABOVE)
1-----1 1 micron
f------1
Fig. 10. Medium (10 1') grain size PCD showing thcrmal treatmcnt effects 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 phase eo- we metallic material which appears to wet rather well on the surface of the peD layer. The peD layer cracking is predominantly intergranular and changes to mixed mode with rise in temperature. (2) In a nitrogen environment, at about 600°C the second phase Co- WC material begins to seep out of the grain boundaries. This effect increases with rise in temperature and about 750°C the first sign of PCD fracturing is noticed. At 800 °e serious damage occurs on all three grain size peD layers with major fracturing observed on the 10 fl and 30 fl peD layers. (3) In an air environment on the other hand, the onset of damage appears to happen around 600°C accompanied by extensive eo- WC second phase metal extrusion out of peD layers. At this stage slight inter granular micro cracking is also observed. The extruded metallic phase is spherical in shape which appears to be non wetting primarily owing to oxidation of both metallic phase as well as diamond grains. Raman spectrographic [10] stress measurements done on 5 fl and 30 fl peD layers suggest the presence of
1 micron
Fig. 11. Coarse (30 II) grain size PCD showing thermal treatment effects 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 metal content of peD layers where the coarse grained (30 /;) peD layer has approximately one half of the metal content of fine (5 fl) grain material. Higher amounts of metallic phase is responsible for greater amount of extrusions observed in thermally treated fine grained PCD layers.
7. Summary (1) Fracture toughness of peD layers is strongly dependent on the starting grain size of diamond particles. Larger grain size peD layer have higher toughness. (2) The grain size dependence appears to be limited to about 40 fl beyond which no significant change in fracture toughness of PCD layer is observed. (3) The amount of entrapped Co- we metallic phase in PCD layers decreases with increasing grain size.
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(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 become tensile.
References [I] B.M. Kramer and N.P. Suh, Tool Wear by Dissolution: A Quantitative 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 National Laboritories, 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, Fracture Toughness 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, Birmingham, Alabama, Personal communications, 1995.