Aggressive answers may come from hard intermetallic new materials

technical trends

Aggressive answers may come from hard intermetallic new materials A new class of hard materials has been created by the incorporation of industrial diamonds into an intermetallic matrix. These intermetallic bonded diamond composites have also been formulated to contain metal carbides to improve their performance. [14] and Wittmer, et al [15,16], who have t is known that special intermetalrecently developed cermets containing lic-bonded (IBD) cermets possess diamonds in a Ni3A1 matrix; with or properties that make them desirable for high-temperature wear without the addition of TiC, and substiapplications, particularly for the autotuting WC for TiC (patent pending). motive and diesel industries [1-13]. Intermetallic composites have high hardness, high-temperature strength, and excellent high-temperature corrosion resistance. They can also be engineered to have a coefficient of thermal expansion compatible with many metals. One of the most promising intermetallics was found to be nickelaluminide (Ni3A1), formed by prealloying or from a solid-state reaction between NiA1 and Ni (termed reaction sintering). The temperature used for reaction sintering can be as Figure 1. SEM of Diamond V low as 1300°C-1450°C. In previously reported work, the direct pressureless sintering of intermetallic-bonded ceramic composites was accomplished in the continuous sintering furnace at Southern Illinois University Carbondale (SIUC) and by several reported techniques at Oak Ridge National Laboratory [7-13]. The addition of diamonds to cermets has been an objective for many years, but due to problems primarily related to the degradation of the diamonds, there has been little success for high-temperature binder systems. The specific limitations have been reported recently by Hiller-Piquard Figure 2. SEM of Diamond X. 80 µm 80 µm

I

0026-0657/06 ©2006 Elsevier Ltd. All rights reserved.

These cermets have been named intermetallic-bonded diamond (IBD) composites. IBD composites may offer advantages for several hard materials applications such as coal mining and processing, rock drilling, crushing and handling, gas and oil drilling, horizontal drilling and excavating. The work of the SIUC team presented in a paper at San Diego discussed the continued development related to processing, properties and microstructure of IBD composites containing WC where the intermetallic is a pre-alloyed Ni3Al (IC50) [2]. The SIUC researchers looked at two different formulations during their investigation of IBD composites, using pre-alloyed IC-50 Ni3Al, WC, and diamond. One formulation, NAW35DX35-IC50, contained 30 vol% Ni3Al, 35 vol% WC, and 35 vol% diamond, while the other formulation, NAW17DX33-IC50, contained 50 vol% Ni3Al, 17 vol% WC, and 33 vol% diamond. The formulations were processed by ball milling in isopropyl alcohol, drying, and screening, using methods previously reported [14-16]. The Ni3Al (IC-50) contained 0.022 wt% boron and 0.076 wt% zirconium [2]. Industrial diamonds of average diameter 80 µm to 100 µm from lots V and X were used. Discs 2.54 cm in diameter by about 0.65 cm thick were formed using a

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hydraulic press to 50 MPa in a steel mould. Hot pressing was used for densification, using the facilities in the university's Center for Advanced Friction Studies (CAFS) laboratory using vacuum and low pressure argon as the protective atmosphere. High-density graphite dies and punches were used to hot press stacks of four pellets, separated by Grafoil®. The hot press die was preloaded to 0.8 MPa in vacuum, purged through three cycles using argon and vacuum. The temperature was increased to 1200°C under vacuum. After holding for an hour, it was gradually increased along with the load (~2.4 MPa) until a predetermined extension or temperature maximum was reached. Three hot pressing temperatures (1325°C, 1350°C, and 1375°C) and three hold times at peak temperature and pressure (20, 40 and 60 min) were investigated. Following sintering, the samples' fired weights were obtained and then sample density and porosity were determined by Archimedes' method in distilled water. Because a true theoretical density is not known, due to alloying effects, a calculated density was determined based on the constituents and their assumed proportion. The error in these calculations could be as high as 3 per cent to 5 per cent and therefore the apparent porosity was used to give a better measure of sintering effectiveness. Scanning electron microscopy (SEM), using both secondary and backscatter imaging modes, was used, along with XRay diffraction (XRD) to confirm the presence of diamonds in the sintered IBD composites. The friction assessment screening test (FAST) was used to measure relative wear and friction coefficient against cast iron. A constant normal force is applied to the specimen that is forced into contact with a rotating cast iron wheel. Rockwell C hardness was measured on the plane-parallel surfaces of the entire series of IBD composites using a Wilson Hardness Tester. The hardness average was determined based on five measurements made in the centre of each sample in a dice pattern. Figures 1 and 2 are SEM images of the as-received V and X diamonds. The diamonds are quite irregular in morphology and contain obvious defects

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from the production process. Cleavage planes are quite visible on many of them. Figure 3 is an SEM image of the IC-50 powder that is quite spherical because it is an atomised free-flowing powder. Figure 4 is an SEM image of the as-received WC powder obtained from Robert Bosch Company, Louisville, Kentucky. The WC powder was roughly < 3 µm and was quite agglomerated. The density results, as a function of hot-pressing conditions, for NAW35DX35-IC50 and NAW17DX33IC50 are given in Tables I and II, respectively. The average density is shown to have increased as a function of both time and temperature, as expected. The optimum hot pressing conditions for both formulations to achieve the lowest average apparent porosity was 1375°C for 60 min, while the highest average densities were obtained at the same temperature for 40 min. This would suggest that there is some reaction with the diamond or oxide contamination of the surface of the powders, creating more closed porosity for longer sintering time at that temperature.

Figure 3. SEM of IC-50 powder

Figure 4. SEM of WC powder

Table I. Average density and porosity results for hot pressed NAW35DX35-IC50. Temperature Time Average Density Average (°C) (min) (% T.D.) Apparent Porosity (%) 1325 20 85.86 11.77 1325

40

83.91

12.83

1325

60

85.64

11.60

1350

20

85.88

10.98

1350

40

85.80

11.94

1350

60

89.37

7.72

1375

20

92.34

3.45

1375

40

97.96

1.00

1375

60

95.83

0.11

1325

20

86.66

10.84

1325

40

87.81

10.05

1325

60

87.88

9.07

1350

20

90.68

7.06

1350

40

91.74

5.80

1350

60

91.45

5.45

1375

20

92.74

1.27

1375

40

95.80

1.44

1375

60

94.35

0.99

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Figure 5. Results from FAST test of NAW35D35-IC50 hot pressed at 1775°C for 40 min.

NAW17DX33-IC50 achieved a higher average density than NAW35DX35-IC50 until the sintering temperature was increased to 1375°C. At 1375°C, NAW35DX35-IC50 achieved an overall higher density (~98 per cent) than NAW17DX33-IC50 (~96 per cent). The observed change in density achieved at a higher hot pressing temperature can be directly correlated to the amount of liquid phase present. Formulations with 30 vol% of liquid phase (NAW35DX35-IC50) required a higher sintering temperature than formulations containing 50 vol% liquid phase (NAW17DX33-IC50). FAST testing was performed on samples that were hot pressed sequentially

Figure 6. Results from FAST test of NAW17D33-IC50 hot pressed at 1775°C for 40 min.

with respect to time and temperature. The results for FAST testing formulations NAW35DX35-IC50 and NAW17DX33-IC50 are given in Table III and Table IV, respectively. These results show that NAW35DX35-IC50 displayed greater wear resistance for six of the nine different hot pressing conditions when compared to NAW17DX33-IC50. IBD composites that were hot pressed using the optimised parameters displayed the least amount of weight loss. However, the specimens with the highest density did not have the highest coefficient of friction. The lower density IBD composites would have lower strength, as demonstrated by the increased weight

loss, leaving the embedded diamonds more exposed during wear testing. The expected results would be an increased friction coefficient, as was observed. Both formulations displayed the greatest wear resistance when hot pressed at 1375°C, as expected due to the higher density. The FAST test results are also shown in Figures 5 and 6 for NAW35DX35-IC50 and NAW17DX33-IC50, respectively. These results show that for NAW35DX35-IC50 the friction coefficient decreases initially as cast iron is transferred to the surface of the composite. After about 2000 seconds, the friction layer breaks down and the friction

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coefficient increases again. The FAST results for NAW17DX33-IC50 are quite "noisy" with the friction layer forming and breaking down over shorter periods of time. Since there is more intermetallic in this formulation, the friction between the composite and cast iron was higher, due to adhesion effects. The transfer of metal to the friction surface during FAST testing has a significant effect on the weight loss; making conclusions based on weight loss impractical. Table V gives the hardness results for NAW35DX35-IC50. Many of the values obtained were higher than allowed by the Rockwell C scale, since diamond is assigned a value of 100 as a maximum. However, this technique did allow for comparisons to be made. The hardness was found to increase as a function of both sintering temperature and time as expected. The highest hardness (110.4) was obtained for the hot pressing conditions of 1375°C and 60 min. Table VI gives the hardness results for NAW17DX33-IC50. These values are quite high, but lower than achieved for the NAW35DX35-IC50 because of the higher volume of intermetallic (50 vol% compared with 30 vol%). Again, the hardness increased with both increased hot pressing temperature and time, as expected. Figures 9 and 10 are backscatter SEM images of NAW35DX35-IC50 and NAW17DX33-IC50, respectively, following surface grinding with a diamond wheel. In these figures the light grey phase is WC, the dark grey phase is IC50 (Ni3Al alloy), and the black phase is diamond. The volume differences for both Ni3Al and WC can clearly be seen by comparing the two images. The microstructures show that there is little to no diamond pull-out despite the aggressive machining process used, and that the diamonds are well dispersed in the IC-50 matrix. The diamonds are physically clamped in the matrix due to the difference in thermal expansion coefficient (diamond CTE ~1 x10-6 in/in·oC compared with theintermetallic CTE ~12 x10-6 in/in·oC), and there is no evidence of a chemical bond at the diamondmatrix interface. This physical clamping has been attributed to the reason that the diamonds do not convert to graphite during processing. The WC is more dispersed in the NAW35DX35-IC50 but not

Table II. Average density and porosity results for hot pressed NAW17DX33-IC50. Temperature Time Average Density Average Porosity (°C) (min) (% T.D.) Apparent (%) 1325 20 86.66 10.84 1325

40

87.81

10.05

1325

60

87.88

9.07

1350

20

90.68

7.06

1350

40

91.74

5.80

1350

60

91.45

5.45

1375

20

92.74

1.27

1375

40

95.80

1.44

1375

60

94.35

0.99

Table III. Weight loss results from FAST testing hot pressed NAW35DX35-IC50. Temperature Time Weight Loss Weight Loss Friction (°C) (min) (g) (%) Coefficient 1325 20 0.078 0.434 0.34 1325

40

0.097

0.548

0.42

1325

60

0.201

1.127

0.42

1350

20

0.062

0.356

0.35

1350

40

0.039

0.210

0.44

1350

60

0.097

0.523

0.40

1375

20

0.011

0.060

0.40

1375

40

0.008

0.043

0.37

1375

60

0.008

0.046

0.36

Table IV. Weight loss results from FAST testing hot pressed NAW17DX33-IC50. Temperature Time Weight Loss Weight Loss Friction (°C) (min) (g) (%) Coefficient 1325 20 0.171 0.987 0.31 1325

40

0.133

0.821

0.36

1325

60

0.044

0.270

0.32

1350

20

0.078

0.500

0.42

1350

40

0.034

0.196

0.37

1350

60

0.052

0.301

0.35

1375

20

0.370

2.183

0.34

1375

40

0.025

0.143

0.36

1375

60

0.011

0.042

0.35

This article is based on Intermetallic Bonded WC-Diamond Composites, a paper by Dale E Wittmer, Tad Miller, Andrew Keeney and Peter Filip of Southern Illinois University

Carbondale, Illinois, USA. It was given at the Metal Powder Industries Federation’s PowderMet2006 Conference and Exhibition in San Diego, California.

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References 1. T N Tiegs and R R McDonald, US Patent 4,919,718, "Ductile Ni3Al alloys as bonding agents for ceramic materials," April 24, 1990. 2. K Plucknett, T N Tiegs, and P F Becher, US Patent 5,015,290, "Ductile Ni3Al alloys as bonding agents for ceramic materials in cutting tools," May 14, 1991. 3. T N Tiegs, P A Menchofer, K P Plucknett, K B Alexander, P F Becher, and S B Waters, "Hardmetals Based on Ni3Al as the Binder Phase," International Conference on Powder Metallurgy, Metal Powder Industry Fed., Princeton, NJ, pp. 211-218, 1995. 4. T N Tiegs, K P Plucknett, P A Menchhofer and P F Becher, "Ni3AlBonded WC and TiC Hardmetals," Proc. of Int. Powder Metallurgy Conference, Wash. DC (1996). 5. K P Plucknett, T N Tiegs, P A Menchhofer, P F Becher and S B Waters, "Ductile Intermetallic Toughened Carbide Matrix Composites," Ceram. Eng. Proc., 17 [3] 314-321 (1996). 6. K Plucknett, T N Tiegs and P F Becher, US Patent 5,905,937, "Improved Sintered Ductile Intermetallic-Bonded Ceramic Composites," 1999. 7. D Wittmer, F Goranson, T Tiegs and J Schroeder, "Comparison of Batch and Continuous Sintering of AluminideBonded TiC," Advances in Powder Metallurgy and Particulate Materials, 1999, Volume 3, pp 237-248, 1999. 8. T Tiegs, F C Montgomery, P A Menchofer, D L Barker, F Goranson, and D E Wittmer, "Grain Refinement in TiCNiAl Composites," Advances in Powder Metallurgy and Particulate Materials, Metal Powder Industries Federation, Princeton, NJ, pp. 9-222 to 9-229 (2000).

Figure 7. XRD Pattern for NAW35DX35-IC50

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9. T N Tiegs, F Montgomery, F Goranson, P A Menchhofer, D L Barker, and D E Wittmer, "Microstructure and Properties of TiC-Ni3Al Composites With Alternate Binder Compositions," Ceram. Eng. Sci. Proc. 21[3] 721-728, Am. Ceram. Soc., (2000). 10. F Goranson, "Intermetallic Bonded Carbides Sintered by Continuous Sintering and Batch Vacuum-Pressure Sintering," MS Thesis, Department of Mechanical Engineering and Energy Processes, Southern Illinois University, Carbondale, IL, May 2000. 11. T N Tiegs, M L Santella, C A Blue, P A Menchhofer, and F Goranson, "FGM Fabrication by Surface Thermal Treatments of TiC-Ni3Al Composites," Ceramic Trans., Vol. 114, 357-363, Am. Ceram. Soc. Westerville, OH (2001). 12. T N Tiegs, J L Schroeder, P A Menchofer, F C Montgomery, D L Barker, F Goranson and D E Wittmer, "Processing and Properties of TiCNi3Al," Structural Intermetallics-2001, ISSITMS, pp. 811-818, 2001. 13. T N Tiegs, F Montgomery, P A

Menchhofer, D L Barker, F Goranson, and D E Wittmer, "Grain Growth Inhibitors in Intermetallic-Bonded TiC Composites," Ceramic Transactions, Vol. 124, Am. Ceram. Soc., Westerville, OH, pp. 67-76, 2001. 14. S Hiller-Piquard, "Processing and Properties of Intermetallic-Bonded Diamond Composites," MS Thesis, Southern Illinois University at Carbondale, June, 2005. 15. D E Wittmer, S Piquard, T Miller, P Pejcochova, and P Filip, "Processing and Friction Properties of Intermetallic-Bonded Diamond Ceramic Composites," Ceramic Engineering and Science Proceedings, Proceedings of the 29th International Conference on Advanced Ceramics and Composites - Developments in Advanced Ceramics and Composites, Jan. 22 - 27, 2006 (accepted for publication). 16. D E.Wittmer, T Miller, A Keeney and P Filip, "WC as a Constituent in Intermetallic-Bonded Diamond Composites," Tungsten Refractory & Hardmetals VI, ISBN 0-97620574-2, pp.141-148 (2006).

Table V. Average rockwell hardness results for hot pressed NAW35DX35-IC50. Hot Pressing Hot Pressing Time (min) Temperature (°C) Average Hardness (HRC) 1325 20 85.3 1325

40

86.8

1325

60

90.2

1350

20

87.8

1350

40

92.0

1350

60

100.8

1375

20

103.0

1375

40

107.9

1375

60

110.4

Figure 8. XRD Pattern for NAW17DX33-IC50

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in the NAW17DX33-IC50. This is likely due to the size difference between the starting materials and the wetting of the WC by the IC-50. X-ray diffraction patterns shown in Figures 7 and 8, were obtained for both formulations. They were then compared to XRD database files for the known constituents as well as other possible elements and compounds that may have

Figure 9. SEM of NAW35DX35-IC50

developed during the sintering process. Peak locations matched those of the database files for Ni3Al, WC, and diamond; however, a small graphite peak was also observed in both formulations. This is believed to be due to a combination of embedded Graphfoil® on the sample surface that was not removed by cleaning and/or a thin layer of graphite forming on the diamond surface.

Table VI. Average Rockwell hardness results for hot pressed NAW17DX33-IC50. Hot Pressing Hot Pressing Average Hardness Temperature (°C) Time (min) (HRC) 1325

20

83.0

1325

40

84.6

1325

60

88.2

1350

20

93.1

1350

40

95.9

1350

60

99.9

1375

20

101.1

1375

40

99.6

1375

60

103.5

Figure 10. SEM of NAW17DX33-IC50

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