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Investigation into the sintered behavior and properties of nanostructured WC–Co–Ni–Fe hard metal alloys
Materials Science & Engineering A 606 (2014) 150–156
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Investigation into the sintered behavior and properties of nanostructured WC–Co–Ni–Fe hard metal alloys Shih-Hsien Chang n, Po-Yu Chang Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei 10608, Taiwan, ROC
art ic l e i nf o
a b s t r a c t
Article history: Received 15 February 2014 Received in revised form 17 March 2014 Accepted 25 March 2014 Available online 1 April 2014
Nanomaterials normally possess high strength, high hardness and excellent ductility and toughness. In the research, various vacuum sintering temperatures (1250 1C, 1300 1C, 1350 1C and 1400 1C) were explored in order to investigate the optimal parameters of micro- and nano-WC sintered composites; and further to compare the sintered behavior and properties of two different sizes of WC materials. All specimens were fabricated by using vacuum sintering of the powder metallurgy technique. The experimental results show that micro- and nano-WC specimens generated a good liquid-phase sintering at 1350 1C sintered for 1 h, and thus, exhibited excellent mechanical properties. The porosities were decreased to 0.36% and 0.8%, the hardness was enhanced to 90.1 and 91.4 HRA and the TRS was increased to 1441.62 and 1540.56 MPa, respectively. Since there is an effect upon the grain refinement, nano-WC obviously possesses better performance than the micro-WC hard metal alloys. Meanwhile, the KIC value for sintered nano-WC dramatically increased to 12.71 MPa m1/2. & 2014 Elsevier B.V. All rights reserved.
Keywords: Sintering Nano-WC Composite TRS and KIC
1. Introduction Tungsten carbides have been found wide applications in cutting tool manufacture, and WC–Co hard metals with high mechanical properties account for more than 90% of hard metals production [1,2]. In the past two decades, there were a great number of works evaluating the possibility of fabrication of nanostructured WC–Co hard metals from tungsten carbide (WC) nano powders. Cemented tungsten carbide with nanocrystalline grain structure has potential for improving the mechanical properties of these materials [1–3]. In addition, the electrochemical tests also corroborate there is an increase of 350% in corrosion resistance for the nanostructured sample compared with the conventional sample [4]. The major constituents of cemented carbides consist of fine tungsten carbide particles, which are hard and brittle, and the minor constituent binder metal cobalt, which is relatively soft and ductile [3]. Although cobalt wets tungsten carbide well and has good mechanical properties, the corrosion resistance of conventional tungsten carbide/cobalt hard metals is less than satisfactory in certain applications in the chemical and food industries. The ductile phase is selected from the group consisting of Co, Ni and Fe alloys, which are sometimes added to modify the binder n Correspondence to: Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, 1, Sec. 3, Chung-Hsiao E. Rd., Taipei 10608, Taiwan, ROC. Tel.: þ 886 2 27712171x2766; fax: þ886 2 27317185. E-mail address: [email protected] (S.-H. Chang).
http://dx.doi.org/10.1016/j.msea.2014.03.096 0921-5093/& 2014 Elsevier B.V. All rights reserved.
composition. The most important aim of this modification with Ni is to improve the corrosion resistance of the hard metal. Previous study pointed out the Ni-containing hard metals showed a shift of the anodic curve of metal dissolution after electrochemical tests, indicating a decrease of the corrosion current density with growing Ni content in the binder [5]. Moreover, it has a great improvement in toughness of the tungsten carbide when the binder phase is Co–Ni–Fe. In addition, the substitution of part or all of the cobalt for nickel or nickel and iron has been investigated in recent years in an attempt mainly to improve the properties of the binder and at the same time to reduce costs associated with the short supply and prevailing high market price of cobalt powder [3,6,7]. Presently, the use of nanostructured WC powders for producing tungsten carbide materials with finer microstructures represents one of the most active fields of research in the hard metal industry, because the nanostructured tungsten carbide hard metal alloys possess more excellent mechanical properties such as high hardness, strength and toughness [8]. Moreover, the increase in strength with decreasing WC grain sizes is based on the preconditions of new quality standards for the starting powders, e.g. grain size, purity, homogeneity, etc [9]. Due to the properties of tungsten carbide depend primarily on cobalt content and grain sizes of WC. Typical cemented tungsten carbide (WC–Co) composites contain 3–30 wt% cobalt, and the grain sizes of WC range from submicron levels to a few microns [10,11]. Therefore, the cobalt content and grain sizes of tungsten carbide hard metal alloys play an important role on the mechanical properties. Furthermore, there is a general trend in the modern hard metal industry to produce WC–Co hard
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metals with WC mean grain size as small as possible with the target of achieving the range of nanomaterials [12]. However, the mechanical properties of nanostructured WC added Co–Ni–Fe binder phases are still not clear, and they need to further be examined. Powder metallurgy (P/M) is a good method for fabrication of high melting material with better mechanical properties. Sintering is a useful method for manufacturing parts from powders, by heating the material until its particles adhere to each other. However, sintering temperature cannot exceed the melting point of the sintered based materials [13,14]. In addition, conventional sintered P/M-parts usually have more than 5% porosity. Enhanced sintering techniques can be applied to obtain higher densities and improved porosity in the sintered parts [15,16]. In the present research, micro-WC–Co and nano-WC–Co–Ni–Fe hard metal alloys were produced via vacuum sintering using the P/M technique. Meanwhile, the effect of grain sizes on properties is also a chief concern. The micro-WC–Co specimens are designated as “M-WC”, and the nano-WC–Co–Ni–Fe specimens is designated as “N-WC”, respectively. Moreover, the research carried out a series of experimental tests to explore the behavior and properties of various sintering temperatures on tungsten carbide hard metal alloys. Effects of the microstructural features on mechanical properties were of particular interest. In addition, the feasibility of commercial manufacturing of nano-WC–Co–Ni–Fe cement carbides through vacuum sintering was evaluated.
underwent a sintering process in which the sintering temperatures were 1250 1C, 1300 1C, 1350 1C and 1400 1C. The vacuum was maintained at 1.33 10 3 Pa and the soaking time was 60 min. To evaluate the microstructure and mechanical properties of the M-WC and N-WC hard metal alloys via different sintering processes, the porosity, hardness, transverse rupture strength (TRS) tests, fracture toughness KIC and microstructure inspections were performed. Microstructural features of the specimens were examined by optical microscopy (Nikon Eclipse Lv150) and scanning electron microscopy (Hitachi-S4700). Porosity tests followed the ASTM B311-08 and C830 standards. The hardness of the specimens was measured by Rockwell indenter (HRA, Indentec 8150LK) with loading of 60 kg, which complied with the CNS 2114 Z8003 standard methods. The Hung Ta universal material test machine (HT-9501A) with a maximum load of 25 t was used for the TRS tests (ASTM B528-05). Meanwhile, Rbm was the transverse rupture strength, which is determined as the fracture stress in the surface zone. F was maximum fracture load, L was 30 mm, k was chamfer correction factor (normally 1.00–1.02), b and h were 5 mm in the equation Rbm ¼ 3FLk/2bh2, respectively. The specimen dimensions of the TRS test were 5 5 40 mm3. Moreover, it needs to slightly grind the surface of the specimen and tests at least three pieces. The toughness of the specimens can be also expressed in the term of the crack resistance, which is obtained by the applied load (294.3 N) through the micro-hardness tester (VMT-XT) and is calculated the sum of the dividing crack lengths. The fracture toughness KIC can be calculated by the following equation [17]:
2. Experimental procedure
K IC ¼ 0:15√ðHV30=Σ lÞ
Tungsten carbide imparts the necessary strength and wear resistance to an alloy, whereas nanomaterials of an alloy possess high strength, high hardness and excellent toughness. In the research, various sintering temperatures were examined in order to find the optimal parameters of sintered micro-WC–Co (13.5 wt% Co) and nano-WC–Co–Ni–Fe (13.5 wt% Co–Ni–Fe) alloys, as well as to compare the different properties of two binders (Co and Co–Ni– Fe) in WC materials. Moreover, the mean particle sizes (micro- and nano-structure) of various hard alloys are another concern. In this study, the micro-WC–Co and nano-WC–Ni–Fe–Co tungsten carbide alloys, will be designated as M-WC and N-WC, hereafter. The MWC and N-WC powders showed an irregular shape and rough surface. The powders also displayed a significant agglomeration phenomenon, as shown in Fig. 1. The mean particle sizes were 1.01 70.03 μm and 196.877 0.05 nm, respectively. During the forming process, the M-WC and N-WC alloy powders were put into an alloy steel mold (6 6 40 mm3) and a vertical force from a hydraulic press was applied to the mold. The pressure rate was 3.33 MPa/s and was maintained at 235.3 MPa for 5 min; then, they
where the HV30 is the hardness (N/mm2), lengths (mm).
Σl is the sum of crack
3. Results and discussion Fig. 2 shows the volume shrinkage rate and porosities of M-WC and N-WC hard metal alloys after various sintering temperature treatments. Fig. 2a shows that both the M-WC and N-WC alloys possessed significant volume shrinkage rates after sintering temperatures over 1300 1C. The volume shrinkage rate increased rapidly as the sintering temperature increased, but it presented a slow increasing trend after 1350 1C sintering. The highest volume shrinkage rate (46.9%) appeared in N-WC alloys after sintering at 1350 1C for 1 h. Further analysis of the high volume shrinkage rate of N-WC sintered alloys showed that the wax (2%) existing in the original powder was one of the main reasons. The wax can be completely melted through the full liquid-phase produced, and then, effectively improve sintering characteristics during the good
N-WC
Wax
Fig. 1. SEM photographs of the surface morphology: (a) M-WC, and (b) N-WC composite powders, respectively.
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tends to provide much higher energy particles to move and diffuse. Consequently, the dissolution and re-precipitation effects resulted in the reduction of porosity, and optimal densification was obtained. The porosity of M-WC alloys (17.45%) was obviously higher than for N-WC specimens (1.22%) at 1250 1C sintering. Significantly, the particle size of N-WC alloys has a dramatic effect on the sintering temperature of 1250 1C. It is reasonable to suggest that the smaller size of WC powders is able to produce a large surface area, which leads to a better wet-ability and LPS results at a lower sintering temperature. When the sintering temperature increased to 1300 1C, the porosity of M-WC and N-WC alloys was below 1%. Significantly, there is a positive relationship between the relative density and sintering temperature. As a result, the relative density increases and internal pores obviously decrease as the sintering temperature of M-WC and N-WC alloys increased. This demonstrates that these specimens have a much better liquid-phase generation at this temperature; thus, a good sintering density can be obtained. The result also agrees with our previous finding [17]. In this study, the main factors which affect the mechanical properties of M-WC and N-WC hard metal alloys include both the porosity and grain sizes. Fig. 3 shows the hardness and TRS test results of both N-WC and N-WC specimens after various sintering temperatures. Fig. 3a shows that the hardness of N-WC specimens reveals a higher value at the same sintering temperature compared with the M-WC because the mean particle size of original
Fig. 2. Comparison of the sintered properties of M-WC and N-WC hard metal alloys after various sintering temperature treatments: (a) volume shrinkage rate, and (b) porosity.
liquid-phase sintering (LPS) process. In the research, when the sintering temperature was increased to 600 1C, the wax easily evaporated from the green body. If the sintering temperature was continuously increased, the volatilization of wax caused the volume shrinkage rate to be enhanced. In addition, a large number of pores exist between the powder particles as the liquid-phase sintering is generated. The binder (Co or Co–Ni–Fe) can sufficiently fill these pores resulting in a good sintering effect. Therefore, the porosities obviously decreased, which made the volume shrinkage rate rise quickly. Further, when compared with the volume shrinkage rate of M-WC alloys (31.6%), the N-WC specimens possessed a relatively high value (45.1%) at 1250 1C sintering for 1 h. Consequently, the N-WC specimens can produce a better LPS result at a lower temperature; simultaneously, a relatively higher densification can be achieved as well. Fig. 2b shows the porosity level of M-WC and N-WC hard metal alloys after various sintering temperature treatments. These results show that the porosity gradually decreases and approaches zero as the sintering temperature is increased. The minimum value of porosity was 0.11%, which appeared in N-WC specimens after 1400 1C sintering for 1 h. Fig. 2b also indicates that the densification of the sintered specimens rapidly increases. Since the sintering temperature increases, the liquid binding phase (Co or Co–Ni– Fe) can fill the pores between the two grains much easier and
Fig. 3. Comparison of the mechanical properties of M-WC and N-WC hard metal alloys after various sintering temperature treatments: (a) hardness, and (b) TRS.
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N-WC powders is significantly smaller than M-WC powders. The mean grain size of the N-WC was relatively smaller after optimal sintering. Generally, the material's property can be enhanced by the fine-grained strengthening mechanism to achieve the desired results [17]. Thus, the reduction of materials in grain sizes can improve their strength and toughness. Besides, deformation and fracture of materials are owing to being dislocated by movement. To hinder the slip by dislocation, the mechanical properties can normally be further improved. On the other hand, the grain boundary of materials is like an obstacle that hinders slipping. This will cause more difficulty for movement, which is advantageous to the mechanical properties. Moreover, when the grain refinement phenomenon appeared in an alloy, the unit volume of grains increased. This is also a fine-grained strengthening mechanism [17], which can effectively improve the mechanical properties of material. The result agrees with our findings. Fig. 3a also shows that the hardness of both M-WC and N-WC alloys rose to a maximum value and then began to decline as the sintering temperature increased. As mentioned previously, the mechanical properties of tungsten carbide are related to porosities. As the sintering temperature increases, the porosity decreases and leads to better mechanical properties. When the sintering temperature increased to 1300 1C, the porosity of all specimens was less than 1%. Meanwhile, the hardness was more than 88.0 HRA. However, when the temperature kept on increasing to 1400 1C, the hardness began to decline slightly. The hardness trend is consistent with the porosity. It can be suggested that the porosity of sintered specimens plays an important role in affecting their hardness. In addition, the literature indicates that if the sintering temperature is too high, excessive grain coarsening will reduce the fine-grain strengthening effect, which causes poor mechanical properties [18,19]. Therefore, it is reasonable to suggest that the decrease in hardness of M-WC or N-WC alloys was due to excessive grain coarsening. This will be proved in the subsequent microstructure observation.
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Fig. 3b shows that the TRS value rapidly increased as the sintering temperature increased. The optimal TRS levels of MWC or N-WC alloys were 1441.62 and 1540.56 MPa after sintering at 1350 1C for 1 h, respectively. Owing to the lower sintering temperature (1250 1C), the sintering behavior and LPS effects of tungsten carbide are not complete. The specimens cannot achieve the good densification, which is disadvantageous to TRS. When the sintering temperature was gradually enhanced, the sintering behavior was more effective in eliminating porosities. The porosities of M-WC or N-WC alloys were less than 0.5% after sintering at 1350 1C for 1 h which is helpful in improving the TRS value. Actually, internal pores in the matrix easily generated the stress concentration phenomenon, which resulted in a lower strength. However, M-WC and N-WC alloys sintered at 1350 1C reached almost full densification, which is effective in reducing the strain points along the rupture mechanism and increasing the TRS value. The experimental results show that the TRS increases with the decrease in porosity, but when the TRS reached the highest value, it generated a downward trend. The TRS value of M-WC or N-WC alloys started to decline slightly as the sintering temperature was raised to 1400 1C. It was possible to judge that the sintering temperature of M-WC or N-WC alloys is slightly higher (1400 1C), which resulted in the excessive grain growth and the decrease in strength. Fig. 4 shows the fractographs' morphology observations of MWC specimens after TRS tests. It clearly shows that the internal black pores of the M-WC specimens were significantly reduced as the sintering temperature increased. Fig. 4a shows that there are many internal pores (17.45%) obviously existing in the M-WC specimens after 1250 1C sintering, which can be ascribed to the incomplete LPS since the internal pores can affect and reduce the fracture toughness of material. Thus, the TRS value of M-WC specimens only had 887.13 MPa after sintering at 1250 1C for 1 h. With the increase in sintering temperature to 1300 1C, the black pores clearly decreased to less than 1%. However, there were still
Fig. 4. Fractographs morphology observations of M-WC specimens by various sintering temperature treatments after TRS tests: (a) 1250 1C, (b) 1300 1C, (c) 1350 1C, and (d) 1400 1C.
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a few pores remaining in the specimens, as shown in Fig. 4b. The porosities obviously decreased (0.86%) which is effective in improving the TRS (1153.96 MPa) after 1300 1C sintering. When the sintering temperatures were raised to 1350 1C or more, the porosity was less than 0.5%, as shown in Fig. 4c and d. Because the sintering temperature reached complete LPS, they both show only a few pores. A comparison of Fig. 4 with Fig. 3b showed that decreasing porosity was useful in improving the TRS value. In addition, Fig. 4c and d indicated that the uniform distribution and sizes of cobalt binders appeared in the 1350 1C and 1400 1C sintered M-WC alloys. As a result, the highest TRS value of MWC specimens was 1441.62 MPa after sintering at 1350 1C for 1 h. Fig. 4a also revealed that only some precipitated WC grains appeared after 1250 1C sintering. The literature indicates that the precipitated WC grains were mostly plate-like or triangular prism columns [20]. In this study, more of the plate-like or triangular prism columns of precipitated WC grains can be observed after 1350 1C sintering. Moreover, the distribution of the binder phase is more uniform as the sintering temperature is increased. However, high temperature sintering (1400 1C) seems to easily result in the grain-coarsening phenomenon [17,21,22], as shown in Fig. 4d. An increase in the sintering temperatures is useful for the densification of the sintering mechanism, while high temperature enough to produce large amounts of the liquid-phase to fill the pores during the sintering process [17]. However, the more sufficient thermal energy was given, the easier grain-coarsening phenomenon appeared at 1400 1C sintering, which was disadvantageous to TRS value. This result can lead to the decrease in the TRS value (1421.30 MPa) of a M-WC specimen. Fig. 5 shows the fractographs' morphology of N-WC specimens after TRS tests. Fig. 5a shows that the internal pores obviously decreased to 1.22% after 1250 1C sintering, as compared with Fig. 4a. Increasing the sintering temperature to 1300 1C or 1350 1C, the porosity kept on decreasing to less than 1%. It can
also be found that no obvious pores exist in the microstructure, as shown in Fig. 5b and c. As a result, N-WC specimens easily achieve higher densification at a lower sintering temperature due to the smaller particle size. In the research, mean particle size reduction has two advantages: first, grain size reduction can increase the driving force of the sintering mechanism, which results in decreasing the sintering temperature. Second, it can increase the contact area among the particles, shortening the diffusion path and increasing the diffusion rate at a relatively lower sintering temperature. Furthermore, there are many plate precipitated WC grains existing in the N-WC specimens at 1250 1C sintering. This shows that reducing the particle size of WC powder is effective in lowering the sintering temperature. As the sintering temperature is raised, the N-WC specimens produce a greater bonding phase throughout the tungsten carbide grains. Meanwhile, the uniform distribution of the binding phase (Co–Ni–Fe) also causes N-WC specimens to have better toughness performance, as shown by arrows in Fig. 5b and c. The result also agrees with previous finding [10]. As to the above-mentioned hardness and TRS value, both of them showed a downward trend after 1400 1C sintering. The microstructure observation displayed a slight variation in grain size, as shown in Figs. 4 and 5. By using the linear intercept Table 1 Comparison of the grains size of M-WC and N-WC hard metal alloys after various sintering temperature treatments.
Non-sintering 1250 1C Sintering 1300 1C Sintering 1350 1C Sintering 1400 1C Sintering
M-WC (μm)
N-WC (nm)
1.017 0.03 1.047 0.03 1.127 0.05 1.30 7 0.05 1.487 0.06
196.87 7 0.05 385.517 0.35 535.267 0.55 590.38 7 0.55 835.787 0.75
Fig. 5. Fractographs morphology observations of N-WC specimens by various sintering temperature treatments after TRS tests: (a) 1250 1C, (b) 1300 1C, (c) 1350 1C, and (d) 1400 1C.
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method (ASTM E112-12), the research further to measure the grain size of M-WC and N-WC alloy specimens, as shown in Table 1. As the temperature is increased, a higher energy can be obtained and particles diffusion is also enhanced, which results in accelerating the grains' growth. As a result, the specimens seem to have a proportional relationship between the grain size and the sintering temperature. According to the literature, the WC grain growth behavior is not easy to be properly controlled during the sintering [8]. Significantly, the 1400 1C sintered M-WC and N-WC specimens possessed the maximum grain sizes, and they were 1.48 μm and 835.78 nm, respectively. Increasing the sintering temperature of nanostructured N-WC specimens easily resulted in a rapid growth of the grain size. Consequently, the effects of grain growth at hightemperatures are more obvious in the N-WC hard metal alloys. The grains significantly increased 4.2 times (196.87 nm-835.78 nm) after 1400 1C sintering. As a result, the grain size of tungsten carbides increases as the sintering temperature is enhanced which is detrimental to mechanical properties. Moreover, the porosity of specimens also affects the mechanical properties directly. When the specimens were sintered at 1400 1C, the porosity clearly decreased to less than 1%. Therefore, the porosity seems to have less effect on the mechanical properties. But the grain still grows and the excessive grain coarsening easily leads to a decline in mechanical properties. The fracture toughness KIC is based on fracture mechanics as a theoretical basis, which mainly measures the material's ability to resist cracks' continuous growth. Fig. 6 shows the OM images of the 1350 1C sintered M-WC and N-WC hard metal alloys after KIC tests. Significantly, the total crack lengths of N-WC specimens were shorter than the M-WC specimens. Moreover, the indentation area of the N-WC specimen is obviously smaller than the MWC specimen. This result is consistent with the hardness of the specimens where N-WC specimens were higher than the M-WC specimens. The literature indicated that the toughness value of nano-tungsten carbide reached a critical value and then decreased [18]. Comparing Fig. 6 with Figs. 4 and 5, the fracture toughness decline can be ascribed to the excessive grain coarsening effect. Fig. 7 shows the value of fracture toughness; it can be clearly seen that the fracture toughness of N-WC is better than for the M-WC specimens after various sintering temperatures. The result can be further compared with the high-magnification SEM observation after KIC tests, as shown in Fig. 8. Although WC grain-growth behavior is difficult to control during the sintering process [13,17], the main difference between both is the original grain-size. Significantly, the refined grain size of tungsten carbide plays an important role in improving the fracture toughness. The optimal KIC value was 12.71 MPa m1/2, which appeared in the N-WC hard metal alloys sintered at 1350 1C for 1 h. while, the KIC value of MWC specimens was 12.10 MPa m1/2.
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As mentioned previously, the fracture behavior and mechanisms of tungsten carbide were damaged mainly along the grains. Normally, cracks along the grain boundaries happen to expand and absorb the fracture energy through the grain boundaries after KIC tests. Compared with the M-WC and N-WC specimens, the crack extension of the M-WC specimen is relatively straight, as shown in Fig. 8a. Conversely, since the grain size of N-WC specimens decreased, the arrangement direction of the grain boundary is more complex, whereby the cracks' forward path is relatively curved, as show in Fig. 8b. This can retard the crack propagation and absorb more energy which is advantageous to the fracture toughness. Consequently, the N-WC specimens possess a higher KIC value than M-WC specimens do. According to the above results and discussion, Ni–Fe additives seem not to have a direct effect on the mechanical properties of tungsten carbide alloys. However, both grain size and sintering temperature play an important role in controlling mechanical properties.
4. Conclusions In the research, M-WC hard metal alloys possessed a lower porosity (0.41%) after sintering at 1350 1C for 1 h. Significantly, it obtained a higher hardness (90.1 HRA) and optimal TRS (1141.62 MPa). In addition, the N-WC hard metal alloys provided good densification and relatively lower porosity (0.35%). As a result, it has the highest hardness (91.4 HRA) and TRS (1540.56 MPa) after sintering at 1350 1C for 1 h.
Fig. 7. Comparison of the fracture toughness of the various temperature sintering M-WC and N-WC hard metal alloys after KIC tests.
Fig. 6. OM images of the 1350 1C sintered M-WC and N-WC hard metal alloys after KIC tests: (a) M-WC, and (b) N-WC alloys.
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Fig. 8. SEM observations of the 1350 1C sintered M-WC and N-WC hard metal alloys after KIC tests: (a) M-WC, and (b) N-WC alloys.
In addition to the sintering temperature, the refined grain size of tungsten carbide plays an important role in improving the fracture toughness. N-WC hard metal alloys possessed the highest KIC value (12.71 MPa m1/2) after sintering at 1350 1C for 1 h. Compared with the M-WC alloys, the N-WC specimens can achieve a better liquidphase sintering at a lower sintering temperature (1250 1C). However, if the sintering temperature is too high (over 1350 1C), the excessive grain growth and coarsening effects easily result in their mechanical properties' decline. Consequently, M-WC and N-WC hard metal alloys are effective in improving the microstructural and mechanical properties through the optimal vacuum sintering process. Acknowledgments This research is supported by the ASSAB Steels Taiwan Co., Ltd. and the National Science Council Taiwan under Grant #NSC-102– 2221-E-027-014. Reference [1] R. Yigit, E. Celik, F. Findik, S. Koksal, Int. J. Refract. Met. Hard Mater. 26 (2008) 514–524. [2] R. Yigit, E. Celik, F. Findik, S. Koksal, J. Mater. Process. Technol. 204 (2008) 80–88.
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Investigation into the sintered behavior and properties of nanostructured WC–Co–Ni–Fe hard metal alloys Shih-Hsien Chang n, Po-Yu Chang Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei 10608, Taiwan, ROC
art ic l e i nf o
a b s t r a c t
Article history: Received 15 February 2014 Received in revised form 17 March 2014 Accepted 25 March 2014 Available online 1 April 2014
Nanomaterials normally possess high strength, high hardness and excellent ductility and toughness. In the research, various vacuum sintering temperatures (1250 1C, 1300 1C, 1350 1C and 1400 1C) were explored in order to investigate the optimal parameters of micro- and nano-WC sintered composites; and further to compare the sintered behavior and properties of two different sizes of WC materials. All specimens were fabricated by using vacuum sintering of the powder metallurgy technique. The experimental results show that micro- and nano-WC specimens generated a good liquid-phase sintering at 1350 1C sintered for 1 h, and thus, exhibited excellent mechanical properties. The porosities were decreased to 0.36% and 0.8%, the hardness was enhanced to 90.1 and 91.4 HRA and the TRS was increased to 1441.62 and 1540.56 MPa, respectively. Since there is an effect upon the grain refinement, nano-WC obviously possesses better performance than the micro-WC hard metal alloys. Meanwhile, the KIC value for sintered nano-WC dramatically increased to 12.71 MPa m1/2. & 2014 Elsevier B.V. All rights reserved.
Keywords: Sintering Nano-WC Composite TRS and KIC
1. Introduction Tungsten carbides have been found wide applications in cutting tool manufacture, and WC–Co hard metals with high mechanical properties account for more than 90% of hard metals production [1,2]. In the past two decades, there were a great number of works evaluating the possibility of fabrication of nanostructured WC–Co hard metals from tungsten carbide (WC) nano powders. Cemented tungsten carbide with nanocrystalline grain structure has potential for improving the mechanical properties of these materials [1–3]. In addition, the electrochemical tests also corroborate there is an increase of 350% in corrosion resistance for the nanostructured sample compared with the conventional sample [4]. The major constituents of cemented carbides consist of fine tungsten carbide particles, which are hard and brittle, and the minor constituent binder metal cobalt, which is relatively soft and ductile [3]. Although cobalt wets tungsten carbide well and has good mechanical properties, the corrosion resistance of conventional tungsten carbide/cobalt hard metals is less than satisfactory in certain applications in the chemical and food industries. The ductile phase is selected from the group consisting of Co, Ni and Fe alloys, which are sometimes added to modify the binder n Correspondence to: Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, 1, Sec. 3, Chung-Hsiao E. Rd., Taipei 10608, Taiwan, ROC. Tel.: þ 886 2 27712171x2766; fax: þ886 2 27317185. E-mail address: [email protected] (S.-H. Chang).
http://dx.doi.org/10.1016/j.msea.2014.03.096 0921-5093/& 2014 Elsevier B.V. All rights reserved.
composition. The most important aim of this modification with Ni is to improve the corrosion resistance of the hard metal. Previous study pointed out the Ni-containing hard metals showed a shift of the anodic curve of metal dissolution after electrochemical tests, indicating a decrease of the corrosion current density with growing Ni content in the binder [5]. Moreover, it has a great improvement in toughness of the tungsten carbide when the binder phase is Co–Ni–Fe. In addition, the substitution of part or all of the cobalt for nickel or nickel and iron has been investigated in recent years in an attempt mainly to improve the properties of the binder and at the same time to reduce costs associated with the short supply and prevailing high market price of cobalt powder [3,6,7]. Presently, the use of nanostructured WC powders for producing tungsten carbide materials with finer microstructures represents one of the most active fields of research in the hard metal industry, because the nanostructured tungsten carbide hard metal alloys possess more excellent mechanical properties such as high hardness, strength and toughness [8]. Moreover, the increase in strength with decreasing WC grain sizes is based on the preconditions of new quality standards for the starting powders, e.g. grain size, purity, homogeneity, etc [9]. Due to the properties of tungsten carbide depend primarily on cobalt content and grain sizes of WC. Typical cemented tungsten carbide (WC–Co) composites contain 3–30 wt% cobalt, and the grain sizes of WC range from submicron levels to a few microns [10,11]. Therefore, the cobalt content and grain sizes of tungsten carbide hard metal alloys play an important role on the mechanical properties. Furthermore, there is a general trend in the modern hard metal industry to produce WC–Co hard
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metals with WC mean grain size as small as possible with the target of achieving the range of nanomaterials [12]. However, the mechanical properties of nanostructured WC added Co–Ni–Fe binder phases are still not clear, and they need to further be examined. Powder metallurgy (P/M) is a good method for fabrication of high melting material with better mechanical properties. Sintering is a useful method for manufacturing parts from powders, by heating the material until its particles adhere to each other. However, sintering temperature cannot exceed the melting point of the sintered based materials [13,14]. In addition, conventional sintered P/M-parts usually have more than 5% porosity. Enhanced sintering techniques can be applied to obtain higher densities and improved porosity in the sintered parts [15,16]. In the present research, micro-WC–Co and nano-WC–Co–Ni–Fe hard metal alloys were produced via vacuum sintering using the P/M technique. Meanwhile, the effect of grain sizes on properties is also a chief concern. The micro-WC–Co specimens are designated as “M-WC”, and the nano-WC–Co–Ni–Fe specimens is designated as “N-WC”, respectively. Moreover, the research carried out a series of experimental tests to explore the behavior and properties of various sintering temperatures on tungsten carbide hard metal alloys. Effects of the microstructural features on mechanical properties were of particular interest. In addition, the feasibility of commercial manufacturing of nano-WC–Co–Ni–Fe cement carbides through vacuum sintering was evaluated.
underwent a sintering process in which the sintering temperatures were 1250 1C, 1300 1C, 1350 1C and 1400 1C. The vacuum was maintained at 1.33 10 3 Pa and the soaking time was 60 min. To evaluate the microstructure and mechanical properties of the M-WC and N-WC hard metal alloys via different sintering processes, the porosity, hardness, transverse rupture strength (TRS) tests, fracture toughness KIC and microstructure inspections were performed. Microstructural features of the specimens were examined by optical microscopy (Nikon Eclipse Lv150) and scanning electron microscopy (Hitachi-S4700). Porosity tests followed the ASTM B311-08 and C830 standards. The hardness of the specimens was measured by Rockwell indenter (HRA, Indentec 8150LK) with loading of 60 kg, which complied with the CNS 2114 Z8003 standard methods. The Hung Ta universal material test machine (HT-9501A) with a maximum load of 25 t was used for the TRS tests (ASTM B528-05). Meanwhile, Rbm was the transverse rupture strength, which is determined as the fracture stress in the surface zone. F was maximum fracture load, L was 30 mm, k was chamfer correction factor (normally 1.00–1.02), b and h were 5 mm in the equation Rbm ¼ 3FLk/2bh2, respectively. The specimen dimensions of the TRS test were 5 5 40 mm3. Moreover, it needs to slightly grind the surface of the specimen and tests at least three pieces. The toughness of the specimens can be also expressed in the term of the crack resistance, which is obtained by the applied load (294.3 N) through the micro-hardness tester (VMT-XT) and is calculated the sum of the dividing crack lengths. The fracture toughness KIC can be calculated by the following equation [17]:
2. Experimental procedure
K IC ¼ 0:15√ðHV30=Σ lÞ
Tungsten carbide imparts the necessary strength and wear resistance to an alloy, whereas nanomaterials of an alloy possess high strength, high hardness and excellent toughness. In the research, various sintering temperatures were examined in order to find the optimal parameters of sintered micro-WC–Co (13.5 wt% Co) and nano-WC–Co–Ni–Fe (13.5 wt% Co–Ni–Fe) alloys, as well as to compare the different properties of two binders (Co and Co–Ni– Fe) in WC materials. Moreover, the mean particle sizes (micro- and nano-structure) of various hard alloys are another concern. In this study, the micro-WC–Co and nano-WC–Ni–Fe–Co tungsten carbide alloys, will be designated as M-WC and N-WC, hereafter. The MWC and N-WC powders showed an irregular shape and rough surface. The powders also displayed a significant agglomeration phenomenon, as shown in Fig. 1. The mean particle sizes were 1.01 70.03 μm and 196.877 0.05 nm, respectively. During the forming process, the M-WC and N-WC alloy powders were put into an alloy steel mold (6 6 40 mm3) and a vertical force from a hydraulic press was applied to the mold. The pressure rate was 3.33 MPa/s and was maintained at 235.3 MPa for 5 min; then, they
where the HV30 is the hardness (N/mm2), lengths (mm).
Σl is the sum of crack
3. Results and discussion Fig. 2 shows the volume shrinkage rate and porosities of M-WC and N-WC hard metal alloys after various sintering temperature treatments. Fig. 2a shows that both the M-WC and N-WC alloys possessed significant volume shrinkage rates after sintering temperatures over 1300 1C. The volume shrinkage rate increased rapidly as the sintering temperature increased, but it presented a slow increasing trend after 1350 1C sintering. The highest volume shrinkage rate (46.9%) appeared in N-WC alloys after sintering at 1350 1C for 1 h. Further analysis of the high volume shrinkage rate of N-WC sintered alloys showed that the wax (2%) existing in the original powder was one of the main reasons. The wax can be completely melted through the full liquid-phase produced, and then, effectively improve sintering characteristics during the good
N-WC
Wax
Fig. 1. SEM photographs of the surface morphology: (a) M-WC, and (b) N-WC composite powders, respectively.
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tends to provide much higher energy particles to move and diffuse. Consequently, the dissolution and re-precipitation effects resulted in the reduction of porosity, and optimal densification was obtained. The porosity of M-WC alloys (17.45%) was obviously higher than for N-WC specimens (1.22%) at 1250 1C sintering. Significantly, the particle size of N-WC alloys has a dramatic effect on the sintering temperature of 1250 1C. It is reasonable to suggest that the smaller size of WC powders is able to produce a large surface area, which leads to a better wet-ability and LPS results at a lower sintering temperature. When the sintering temperature increased to 1300 1C, the porosity of M-WC and N-WC alloys was below 1%. Significantly, there is a positive relationship between the relative density and sintering temperature. As a result, the relative density increases and internal pores obviously decrease as the sintering temperature of M-WC and N-WC alloys increased. This demonstrates that these specimens have a much better liquid-phase generation at this temperature; thus, a good sintering density can be obtained. The result also agrees with our previous finding [17]. In this study, the main factors which affect the mechanical properties of M-WC and N-WC hard metal alloys include both the porosity and grain sizes. Fig. 3 shows the hardness and TRS test results of both N-WC and N-WC specimens after various sintering temperatures. Fig. 3a shows that the hardness of N-WC specimens reveals a higher value at the same sintering temperature compared with the M-WC because the mean particle size of original
Fig. 2. Comparison of the sintered properties of M-WC and N-WC hard metal alloys after various sintering temperature treatments: (a) volume shrinkage rate, and (b) porosity.
liquid-phase sintering (LPS) process. In the research, when the sintering temperature was increased to 600 1C, the wax easily evaporated from the green body. If the sintering temperature was continuously increased, the volatilization of wax caused the volume shrinkage rate to be enhanced. In addition, a large number of pores exist between the powder particles as the liquid-phase sintering is generated. The binder (Co or Co–Ni–Fe) can sufficiently fill these pores resulting in a good sintering effect. Therefore, the porosities obviously decreased, which made the volume shrinkage rate rise quickly. Further, when compared with the volume shrinkage rate of M-WC alloys (31.6%), the N-WC specimens possessed a relatively high value (45.1%) at 1250 1C sintering for 1 h. Consequently, the N-WC specimens can produce a better LPS result at a lower temperature; simultaneously, a relatively higher densification can be achieved as well. Fig. 2b shows the porosity level of M-WC and N-WC hard metal alloys after various sintering temperature treatments. These results show that the porosity gradually decreases and approaches zero as the sintering temperature is increased. The minimum value of porosity was 0.11%, which appeared in N-WC specimens after 1400 1C sintering for 1 h. Fig. 2b also indicates that the densification of the sintered specimens rapidly increases. Since the sintering temperature increases, the liquid binding phase (Co or Co–Ni– Fe) can fill the pores between the two grains much easier and
Fig. 3. Comparison of the mechanical properties of M-WC and N-WC hard metal alloys after various sintering temperature treatments: (a) hardness, and (b) TRS.
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N-WC powders is significantly smaller than M-WC powders. The mean grain size of the N-WC was relatively smaller after optimal sintering. Generally, the material's property can be enhanced by the fine-grained strengthening mechanism to achieve the desired results [17]. Thus, the reduction of materials in grain sizes can improve their strength and toughness. Besides, deformation and fracture of materials are owing to being dislocated by movement. To hinder the slip by dislocation, the mechanical properties can normally be further improved. On the other hand, the grain boundary of materials is like an obstacle that hinders slipping. This will cause more difficulty for movement, which is advantageous to the mechanical properties. Moreover, when the grain refinement phenomenon appeared in an alloy, the unit volume of grains increased. This is also a fine-grained strengthening mechanism [17], which can effectively improve the mechanical properties of material. The result agrees with our findings. Fig. 3a also shows that the hardness of both M-WC and N-WC alloys rose to a maximum value and then began to decline as the sintering temperature increased. As mentioned previously, the mechanical properties of tungsten carbide are related to porosities. As the sintering temperature increases, the porosity decreases and leads to better mechanical properties. When the sintering temperature increased to 1300 1C, the porosity of all specimens was less than 1%. Meanwhile, the hardness was more than 88.0 HRA. However, when the temperature kept on increasing to 1400 1C, the hardness began to decline slightly. The hardness trend is consistent with the porosity. It can be suggested that the porosity of sintered specimens plays an important role in affecting their hardness. In addition, the literature indicates that if the sintering temperature is too high, excessive grain coarsening will reduce the fine-grain strengthening effect, which causes poor mechanical properties [18,19]. Therefore, it is reasonable to suggest that the decrease in hardness of M-WC or N-WC alloys was due to excessive grain coarsening. This will be proved in the subsequent microstructure observation.
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Fig. 3b shows that the TRS value rapidly increased as the sintering temperature increased. The optimal TRS levels of MWC or N-WC alloys were 1441.62 and 1540.56 MPa after sintering at 1350 1C for 1 h, respectively. Owing to the lower sintering temperature (1250 1C), the sintering behavior and LPS effects of tungsten carbide are not complete. The specimens cannot achieve the good densification, which is disadvantageous to TRS. When the sintering temperature was gradually enhanced, the sintering behavior was more effective in eliminating porosities. The porosities of M-WC or N-WC alloys were less than 0.5% after sintering at 1350 1C for 1 h which is helpful in improving the TRS value. Actually, internal pores in the matrix easily generated the stress concentration phenomenon, which resulted in a lower strength. However, M-WC and N-WC alloys sintered at 1350 1C reached almost full densification, which is effective in reducing the strain points along the rupture mechanism and increasing the TRS value. The experimental results show that the TRS increases with the decrease in porosity, but when the TRS reached the highest value, it generated a downward trend. The TRS value of M-WC or N-WC alloys started to decline slightly as the sintering temperature was raised to 1400 1C. It was possible to judge that the sintering temperature of M-WC or N-WC alloys is slightly higher (1400 1C), which resulted in the excessive grain growth and the decrease in strength. Fig. 4 shows the fractographs' morphology observations of MWC specimens after TRS tests. It clearly shows that the internal black pores of the M-WC specimens were significantly reduced as the sintering temperature increased. Fig. 4a shows that there are many internal pores (17.45%) obviously existing in the M-WC specimens after 1250 1C sintering, which can be ascribed to the incomplete LPS since the internal pores can affect and reduce the fracture toughness of material. Thus, the TRS value of M-WC specimens only had 887.13 MPa after sintering at 1250 1C for 1 h. With the increase in sintering temperature to 1300 1C, the black pores clearly decreased to less than 1%. However, there were still
Fig. 4. Fractographs morphology observations of M-WC specimens by various sintering temperature treatments after TRS tests: (a) 1250 1C, (b) 1300 1C, (c) 1350 1C, and (d) 1400 1C.
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a few pores remaining in the specimens, as shown in Fig. 4b. The porosities obviously decreased (0.86%) which is effective in improving the TRS (1153.96 MPa) after 1300 1C sintering. When the sintering temperatures were raised to 1350 1C or more, the porosity was less than 0.5%, as shown in Fig. 4c and d. Because the sintering temperature reached complete LPS, they both show only a few pores. A comparison of Fig. 4 with Fig. 3b showed that decreasing porosity was useful in improving the TRS value. In addition, Fig. 4c and d indicated that the uniform distribution and sizes of cobalt binders appeared in the 1350 1C and 1400 1C sintered M-WC alloys. As a result, the highest TRS value of MWC specimens was 1441.62 MPa after sintering at 1350 1C for 1 h. Fig. 4a also revealed that only some precipitated WC grains appeared after 1250 1C sintering. The literature indicates that the precipitated WC grains were mostly plate-like or triangular prism columns [20]. In this study, more of the plate-like or triangular prism columns of precipitated WC grains can be observed after 1350 1C sintering. Moreover, the distribution of the binder phase is more uniform as the sintering temperature is increased. However, high temperature sintering (1400 1C) seems to easily result in the grain-coarsening phenomenon [17,21,22], as shown in Fig. 4d. An increase in the sintering temperatures is useful for the densification of the sintering mechanism, while high temperature enough to produce large amounts of the liquid-phase to fill the pores during the sintering process [17]. However, the more sufficient thermal energy was given, the easier grain-coarsening phenomenon appeared at 1400 1C sintering, which was disadvantageous to TRS value. This result can lead to the decrease in the TRS value (1421.30 MPa) of a M-WC specimen. Fig. 5 shows the fractographs' morphology of N-WC specimens after TRS tests. Fig. 5a shows that the internal pores obviously decreased to 1.22% after 1250 1C sintering, as compared with Fig. 4a. Increasing the sintering temperature to 1300 1C or 1350 1C, the porosity kept on decreasing to less than 1%. It can
also be found that no obvious pores exist in the microstructure, as shown in Fig. 5b and c. As a result, N-WC specimens easily achieve higher densification at a lower sintering temperature due to the smaller particle size. In the research, mean particle size reduction has two advantages: first, grain size reduction can increase the driving force of the sintering mechanism, which results in decreasing the sintering temperature. Second, it can increase the contact area among the particles, shortening the diffusion path and increasing the diffusion rate at a relatively lower sintering temperature. Furthermore, there are many plate precipitated WC grains existing in the N-WC specimens at 1250 1C sintering. This shows that reducing the particle size of WC powder is effective in lowering the sintering temperature. As the sintering temperature is raised, the N-WC specimens produce a greater bonding phase throughout the tungsten carbide grains. Meanwhile, the uniform distribution of the binding phase (Co–Ni–Fe) also causes N-WC specimens to have better toughness performance, as shown by arrows in Fig. 5b and c. The result also agrees with previous finding [10]. As to the above-mentioned hardness and TRS value, both of them showed a downward trend after 1400 1C sintering. The microstructure observation displayed a slight variation in grain size, as shown in Figs. 4 and 5. By using the linear intercept Table 1 Comparison of the grains size of M-WC and N-WC hard metal alloys after various sintering temperature treatments.
Non-sintering 1250 1C Sintering 1300 1C Sintering 1350 1C Sintering 1400 1C Sintering
M-WC (μm)
N-WC (nm)
1.017 0.03 1.047 0.03 1.127 0.05 1.30 7 0.05 1.487 0.06
196.87 7 0.05 385.517 0.35 535.267 0.55 590.38 7 0.55 835.787 0.75
Fig. 5. Fractographs morphology observations of N-WC specimens by various sintering temperature treatments after TRS tests: (a) 1250 1C, (b) 1300 1C, (c) 1350 1C, and (d) 1400 1C.
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method (ASTM E112-12), the research further to measure the grain size of M-WC and N-WC alloy specimens, as shown in Table 1. As the temperature is increased, a higher energy can be obtained and particles diffusion is also enhanced, which results in accelerating the grains' growth. As a result, the specimens seem to have a proportional relationship between the grain size and the sintering temperature. According to the literature, the WC grain growth behavior is not easy to be properly controlled during the sintering [8]. Significantly, the 1400 1C sintered M-WC and N-WC specimens possessed the maximum grain sizes, and they were 1.48 μm and 835.78 nm, respectively. Increasing the sintering temperature of nanostructured N-WC specimens easily resulted in a rapid growth of the grain size. Consequently, the effects of grain growth at hightemperatures are more obvious in the N-WC hard metal alloys. The grains significantly increased 4.2 times (196.87 nm-835.78 nm) after 1400 1C sintering. As a result, the grain size of tungsten carbides increases as the sintering temperature is enhanced which is detrimental to mechanical properties. Moreover, the porosity of specimens also affects the mechanical properties directly. When the specimens were sintered at 1400 1C, the porosity clearly decreased to less than 1%. Therefore, the porosity seems to have less effect on the mechanical properties. But the grain still grows and the excessive grain coarsening easily leads to a decline in mechanical properties. The fracture toughness KIC is based on fracture mechanics as a theoretical basis, which mainly measures the material's ability to resist cracks' continuous growth. Fig. 6 shows the OM images of the 1350 1C sintered M-WC and N-WC hard metal alloys after KIC tests. Significantly, the total crack lengths of N-WC specimens were shorter than the M-WC specimens. Moreover, the indentation area of the N-WC specimen is obviously smaller than the MWC specimen. This result is consistent with the hardness of the specimens where N-WC specimens were higher than the M-WC specimens. The literature indicated that the toughness value of nano-tungsten carbide reached a critical value and then decreased [18]. Comparing Fig. 6 with Figs. 4 and 5, the fracture toughness decline can be ascribed to the excessive grain coarsening effect. Fig. 7 shows the value of fracture toughness; it can be clearly seen that the fracture toughness of N-WC is better than for the M-WC specimens after various sintering temperatures. The result can be further compared with the high-magnification SEM observation after KIC tests, as shown in Fig. 8. Although WC grain-growth behavior is difficult to control during the sintering process [13,17], the main difference between both is the original grain-size. Significantly, the refined grain size of tungsten carbide plays an important role in improving the fracture toughness. The optimal KIC value was 12.71 MPa m1/2, which appeared in the N-WC hard metal alloys sintered at 1350 1C for 1 h. while, the KIC value of MWC specimens was 12.10 MPa m1/2.
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As mentioned previously, the fracture behavior and mechanisms of tungsten carbide were damaged mainly along the grains. Normally, cracks along the grain boundaries happen to expand and absorb the fracture energy through the grain boundaries after KIC tests. Compared with the M-WC and N-WC specimens, the crack extension of the M-WC specimen is relatively straight, as shown in Fig. 8a. Conversely, since the grain size of N-WC specimens decreased, the arrangement direction of the grain boundary is more complex, whereby the cracks' forward path is relatively curved, as show in Fig. 8b. This can retard the crack propagation and absorb more energy which is advantageous to the fracture toughness. Consequently, the N-WC specimens possess a higher KIC value than M-WC specimens do. According to the above results and discussion, Ni–Fe additives seem not to have a direct effect on the mechanical properties of tungsten carbide alloys. However, both grain size and sintering temperature play an important role in controlling mechanical properties.
4. Conclusions In the research, M-WC hard metal alloys possessed a lower porosity (0.41%) after sintering at 1350 1C for 1 h. Significantly, it obtained a higher hardness (90.1 HRA) and optimal TRS (1141.62 MPa). In addition, the N-WC hard metal alloys provided good densification and relatively lower porosity (0.35%). As a result, it has the highest hardness (91.4 HRA) and TRS (1540.56 MPa) after sintering at 1350 1C for 1 h.
Fig. 7. Comparison of the fracture toughness of the various temperature sintering M-WC and N-WC hard metal alloys after KIC tests.
Fig. 6. OM images of the 1350 1C sintered M-WC and N-WC hard metal alloys after KIC tests: (a) M-WC, and (b) N-WC alloys.
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Fig. 8. SEM observations of the 1350 1C sintered M-WC and N-WC hard metal alloys after KIC tests: (a) M-WC, and (b) N-WC alloys.
In addition to the sintering temperature, the refined grain size of tungsten carbide plays an important role in improving the fracture toughness. N-WC hard metal alloys possessed the highest KIC value (12.71 MPa m1/2) after sintering at 1350 1C for 1 h. Compared with the M-WC alloys, the N-WC specimens can achieve a better liquidphase sintering at a lower sintering temperature (1250 1C). However, if the sintering temperature is too high (over 1350 1C), the excessive grain growth and coarsening effects easily result in their mechanical properties' decline. Consequently, M-WC and N-WC hard metal alloys are effective in improving the microstructural and mechanical properties through the optimal vacuum sintering process. Acknowledgments This research is supported by the ASSAB Steels Taiwan Co., Ltd. and the National Science Council Taiwan under Grant #NSC-102– 2221-E-027-014. Reference [1] R. Yigit, E. Celik, F. Findik, S. Koksal, Int. J. Refract. Met. Hard Mater. 26 (2008) 514–524. [2] R. Yigit, E. Celik, F. Findik, S. Koksal, J. Mater. Process. Technol. 204 (2008) 80–88.
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