Mechanical properties of a hybrid cemented carbide composite

Mechanical properties of a hybrid cemented carbide composite

International Journal of Refractory Metals & Hard Materials 19 (2001) 547±552 www.elsevier.com/locate/ijrmhm Mechanical properties of a hybrid cement...

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International Journal of Refractory Metals & Hard Materials 19 (2001) 547±552 www.elsevier.com/locate/ijrmhm

Mechanical properties of a hybrid cemented carbide composite Xin Deng a, B.R. Patterson a,*, K.K. Chawla a, M.C. Koopman a, Z. Fang b, G. Lockwood b, A. Gri€o b a

Department of Materials and Mechanical Engineering, University of Alabama at Birmingham, BEC 254, 1530 3rd Avenue South, Birmingham, AL 35294-4461, USA b Smith International, Inc., 16740 Hardy Street, P.O. Box 60068, Houston, TX 77205-0068, USA Received 7 March 2001; accepted 21 September 2001

Abstract Microstructural e€ects on the mechanical properties of a hybrid metal matrix composite, double cemented (DC) carbide, have been investigated. DC carbide contains granules of WC/Co cemented carbide in a matrix of cobalt. Overall composite hardness increases with decreased granule cobalt content as well as with decreased intergranular matrix fraction of cobalt. High-stress abrasive wear resistance also increases with decreased granule cobalt content and matrix fraction. Fracture toughness of the composite increases with increased cobalt matrix fraction and to a lesser extent with increased granule cobalt content. Increased granule size increases both fracture toughness and wear resistance. DC carbide exhibits a superior combination of fracture toughness and high-stress wear resistance than conventional cemented carbide. The combination of toughness and wear resistance in the composite improves with increased granule hardness. Ó 2001 Published by Elsevier Science Ltd. Keywords: Cemented carbide; Hybrid composite; Wear resistance; Toughness; Hardness

1. Introduction Double cemented (DC) carbide is a novel hybrid composite composed of granules of cemented carbide embedded in a metal matrix, such as cobalt [1±3]. This material can be described as a ``composite within a composite'' since the granules, containing WC particles within a cobalt binder, are bonded within a metal matrix (Fig. 1). DC carbide was developed to achieve greater toughness than conventional cemented carbide while maintaining adequate wear resistance. The initial application has been oil well drill bit inserts where failure by sudden fracture, rather than wear, can cause expensive downtime. It was expected that the large mean free path through the metal matrix between the granules would provide this toughness increase [2]. DC carbide has more degrees of freedom for microstructural design than most two-phase composites, including granule size, hardness/toughness and volume fraction as well as matrix strength and toughness ± all these variables enable wide variations in mechanical *

Corresponding author. E-mail address: [email protected] (B.R. Patterson).

properties. Granule properties are controlled by the grade of cemented carbide granules used and matrix properties are varied by alloying or heat treating, the latter when using a steel matrix. In particular, these independent means for controlling microstructure and properties can enable excellent combinations of properties. For example, it has been found that large, hard (low cobalt) granules contained in a relatively large amount of matrix give a better combination of fracture toughness and high-stress abrasive wear resistance than conventional cemented carbide. The primary goal of this study has been to determine the e€ects of these microstructural variations on several di€erent mechanical properties of DC carbide. Properties investigated here include hardness, high-stress abrasive wear resistance and fracture toughness. The observed property trends are explained in terms of microstructure.

2. Experimental procedures The DC carbide materials in this study were produced from conventional spray dried cemented carbide

0263-4368/01/$ - see front matter Ó 2001 Published by Elsevier Science Ltd. PII: S 0 2 6 3 - 4 3 6 8 ( 0 1 ) 0 0 0 6 0 - 9

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Fig. 1. Microstructure of (a) DC carbide, and (b) conventional cemented carbide comprising the DC carbide granules.

granules with constant WC carbide particle size, 3 lm, but containing di€erent percentages of cobalt binder, 10±25 vol% (6±16 wt%). Table 1 lists the granule compositions and properties, i.e., from materials containing only granules, with no matrix, analogous to conventional cemented carbide. As with cemented carbide, lower cobalt levels produce higher hardness while the higher cobalt levels give better toughness. The e€ect of matrix volume fraction was studied by varying the matrix cobalt level from 0 to 30 vol%. The granule mean intercept was measured metallographically to be 110 lm. For studying the e€ect of granule size on properties, granules with 18 vol% (11 wt%) cobalt were sieved into 5 size classes with mean granule intercepts ranging from 60 to 130 lm (see Tables 2 and 3). Granules were dewaxed at 500 °C and presintered at 1200 °C in hydrogen prior to wet mixing with cobalt powder in heptane and vacuum drying. The above presintering treatment also strengthened the granules to prevent disintegration during mixing. Plates of DC carbide were hot-pressed from the granule/binder mixture at 1250 °C under 35 MPa pressure for 2 h. Fig. 2 shows the microstructures of the specimens with constant granule size containing 10, 20, and 30 vol% cobalt matrix. The intergranular mean free path increases with increasing matrix volume fraction. Fig. 3 shows the microstructures of specimens containing 30 vol% cobalt matrix but with 130, 90 and 60 lm mean intercept granule sizes. It can be seen that the mean free Table 1 Mechanical properties of granules Granule Co content wt%

vol%

6 11 16

10 18 25

Hardness (HV100)

Wear number KIc …MPam1=2 † (krev=cm3 )

1660 1340 1140

10.9 13.4 16.7

15 4.6 2.5

Table 2 Mechanical properties of DC carbide Granule Co (vol%)

Matrix (vol%)

KIc …MPam1=2 †

10 10 10 10 18 18 18 18 25 25 25 25

0 10 20 30 0 10 20 30 0 10 20 30

10.9 17.6 23.5 34.5 13.4 19.9 27.7 35.7 16.7 22.7 31.5 37.9

HV100 1620 1250 1050 856 1350 1080 899 732 1100 930 804 667

Wear number …krev=cm3 † 14.8 7.3 4.7 4.3 4.6 3.5 2.7 2.0 2.5 1.8 1.5 1.4

Table 3 Mechanical properties of DC carbide with di€erent granule sizes (All contain 18 vol% cobalt granules in 30 vol% cobalt matrix.) Granule size …lm† 55 71 91 106 133

Mean free path …lm†

KIc …MPam1=2 †

HV100

Wear number …krev=cm3 †

12 17 21 26 32

29.4 32.1 32.9 34.6 38.2

743 754 736 725 728

1.5 1.7 1.9 2.0 2.3

path also increases with granule size at constant matrix volume fraction. Test specimens, including wear coupons and chevronnotched fracture toughness bars, were electro-discharge machined from the hot-pressed plates. The chevron notch was oriented such that crack growth was in the hot pressing direction, i.e., perpendicular to the plane of ¯attening of the granules. High-stress wear tests were typically performed on the back and front of the same coupon and the fracture toughness tests were performed

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Fig. 2. Microstructure of DC carbide with (a) 10, (b) 20, and (c) 30 vol% metal matrix. Intergranular mean free path increases with increasing matrix volume fraction.

on sets of ®ve bars from the same plate. The results of the various tests are given below.

3. Results and discussion Fig. 4 shows the Vickers hardness (100 kgf) of the di€erent DC carbide compositions as a function of volume fraction of intergranular cobalt matrix. It is apparent that the hardness of the overall composite decreases as the fraction of softer matrix material in-

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Fig. 3. Microstructure of DC carbide with 30 vol% metal matrix, but di€erent average granule size: (a) 130, (b) 90, and (c) 60 lm. Intergranular mean free path increases with granule size at constant matrix volume fraction.

creases, for each di€erent granule type. The hardness values at zero matrix represent hot-pressed granules alone, with no matrix addition (i.e., properties in Table 1). The curves for the di€erent granule types show that the granule hardness has a signi®cant e€ect on the overall composite hardness, with a greater e€ect at lower matrix additions. As the matrix volume fraction increases, the di€erence between the DC carbide compositions with di€erent granule hardness decreases. Fig. 5 illustrates the e€ect of matrix content on highstress wear resistance, expressed as wear number, determined by ASTM B611 [4]. The highest hardness

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Fig. 4. E€ect of granule type and metal matrix content on the hardness of DC carbide. Overall hardness decreases with increased matrix volume fraction and increased granule cobalt content. The e€ect of granule type is greatest at low matrix level.

Fig. 5. E€ect of granule type and metal matrix content on the wear resistance of DC carbide. Wear resistance is greatest at low matrix volume fraction and with hard granules. The e€ect of granule hardness is greatest at low matrix level.

granules, i.e., with the least cobalt, show the greatest wear resistance at all matrix levels, with the wear resistance decreasing with increased matrix level. Again, the variation in wear resistance with granule type decreases with increasing matrix. Fig. 6 illustrates the e€ect of granule type and matrix fraction on the fracture toughness of DC carbide. The toughness shows a strong increase with increasing matrix fraction for each granule type, due to the increased mean free path through the intergranular cobalt matrix. Granule type also has a signi®cant e€ect on the overall toughness, with the higher cobalt granules yielding higher toughness composites. The e€ect of granule size on fracture toughness and wear resistance is illustrated in Fig. 7. Here, it is apparent that increased granule size has a marked bene®cial e€ect on both of these properties. The improvement in toughness with increased granule size is most likely due to the resulting increase in mean free path through the matrix, similar to the e€ect of in-

Fig. 6. E€ect of granule type and metal matrix content on the toughness of DC carbide. Fracture toughness increases markedly with matrix volume fraction and also with granule cobalt content.

creased matrix fraction. The increase in high-stress wear resistance results from the greater protrusion of the larger hard granules from the matrix with increased granule size. Fig. 8 shows increasing wear resistance with increase in hardness for the di€erent grades of DC carbide. The curves for each granule type increase with decreasing matrix cobalt level, from 30 to 0 vol%. The curve representing conventional cemented carbide is obtained by ®tting the data from specimens containing only hotpressed granules or mixtures of WC and cobalt powders, with a wide range in cobalt content. The DC materials all show superior wear resistance at comparable hardness to the conventional structured material. The DC materials with the hardest granules, i.e., least cobalt, show the greatest wear resistance. The decrease in fracture toughness with increasing hardness for the above DC and conventional carbide structures is illustrated in Fig. 9. The curves for each granule type show decrease in toughness with decreasing matrix fraction from 30 to 0 vol%. Again, the conventional carbide line is ®tted through the data for the zero matrix and other hot-pressed particulate structured specimens. At comparable hardness the DC materials have higher toughness, with the hardest granule material showing the highest toughness values due to having the highest matrix volume fractions and the greatest matrix mean free paths. Perhaps the most valuable aspect of the DC carbide material is the superior combination of properties that are achievable compared to conventional cemented carbide. Fig. 10 illustrates that for wear resistance values similar to any particular conventional cemented carbide, the addition of the cobalt matrix between the granules in DC carbide yields greater toughness. Also, for toughness values similar to any conventional carbide, the DC carbide shows greater wear resistance.

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Fig. 7. E€ect of granule size and mean free path of metal matrix on (a) toughness, and (b) wear resistance of DC carbide. Fracture toughness is probably aided most strongly by increased matrix mean free path while wear resistance is increased mostly by granule protrusion, due to size.

Fig. 8. Relationship between wear resistance and hardness of DC carbide. At comparable overall hardness, DC carbides with harder granules have greater wear resistance. Fig. 10. Relationship between fracture toughness and wear resistance of DC carbide. DC carbides with the hardest granules have the best combination of toughness and wear resistance.

Fig. 9. Relationship between fracture toughness and hardness of DC carbide. Fracture toughness decreases with increased hardness and decreased matrix content. At comparable overall hardness, DC carbides with harder granules have greater fracture toughness, since these materials have greater matrix volume fractions and greater mean free paths.

Interestingly, this plot shows that the best combination of toughness and wear resistance is achieved with the granules having the lowest cobalt content. This result

can be explained by the fact that the matrix fraction has the greater e€ect on fracture toughness while granule type typically has the greater e€ect on wear resistance, as seen in Figs. 5 and 6. These observations lead to the combined result, Fig. 10, that DC carbide with harder (lower cobalt content) granules still has good fracture toughness at any matrix fraction, but has higher wear resistance than the softer granule material. Thus, grades of DC carbide can be produced with superior combinations of properties that have direct in¯uence on performance for many applications.

4. Summary and conclusions The above study has shown the following microstructure/property relationships for DC carbide.

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1. The hardness and high-stress wear resistance of DC carbide increases with decreasing granule cobalt content and intergranular matrix fraction. The e€ect of matrix fraction is greatest for hard granules with low cobalt content. The e€ect of granule cobalt content on overall hardness is greatest at low matrix fractions. 2. The fracture toughness of DC carbide increases strongly with intergranular matrix fraction, due to the increased mean free path through matrix metal. Toughness increases to a lesser extent with increased granule cobalt content. 3. Larger cemented carbide granules impart greater toughness and high-stress wear resistance at constant granule cobalt content and matrix fraction. The increase in toughness with granule size is due to the increased matrix mean free path. Improved wear resistance is probably due to greater protrusion of larger hard granules from the wear surface. 4. The great increase in toughness of DC carbide with matrix content, with lesser decrease in wear resistance, yields a superior combination of toughness and wear resistance in DC carbide than in conventional cemented carbide. 5. DC carbide shows both greater wear resistance and fracture toughness than conventional cemented carbide with comparable hardness.

Acknowledgements The authors gratefully acknowledge ®nancial support from National Science Foundation grant no. DMR9904352, Program Managers Dr. B.A. MacDonald and Dr. K.L. Murty. Financial and cooperative research support from Smith International, Inc., is also acknowledged. They are also grateful to Mr. Jon Bitler of Kennametal, Inc. and Mr. Dave Houck of Osram Sylvania for supplying the cobalt powders and cemented carbide granules, respectively. Valuable technical support from Ms. Cheri Moss and Dr. Robin Grin is also appreciated. References [1] Fang Z, Sue JA. Double cemented carbide composites, US Patent No. 5 880 382. [2] Fang Z, Lockwood G, Gri€o A. A dual composite of WC±Co. Metall Mater Trans A 1999;30:3231±8. [3] Deng X, Patterson BR, Chawla KK, Fang Z. Double cemented carbide composites-structure property relationships. In: Marquis FDS, editor. Powder materials: current research and industrial practices, TMS, Warrendale, PA, 1999. p. 307±14. [4] ASTM Standard B611 Standard test method for abrasive wear resistance of cemented carbide. In: Allen RF, editor. Annual book of ASTM standards, 02.05. Philadelphia: ASTM; 1999. p. 328±9.