- Email: [email protected]
Effects of TaC and TiC addition on the microstructures and mechanical properties of Binderless WC
International Journal of Refractory Metals & Hard Materials 82 (2019) 167–173
Contents lists available at ScienceDirect
International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Effects of TaC and TiC addition on the microstructures and mechanical properties of Binderless WC
T
⁎
Akihiro Ninoa, , Yuma Izua, Takashi Sekineb, Shigeaki Sugiyamab, Hitoshi Taimatsua a b
Department of Materials Science and Engineering, Graduate School of Engineering Science, Akita University, 1-1 Tegata-Gakuencho, Akita 010-0852, Japan Akita Industrial Technology Center, 4-11 Sanuki, Araya, Akita 010-1623, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Tungsten carbide Tantalum carbide Titanium carbide Grain growth inhibitor Hardness Fracture toughness
WC ceramics were sintered using a resistance-heated hot-pressing machine in the temperature range of 1600–1800 °C for TaC addition and at 1800 °C for TiC addition. Dense WC ceramics containing 0–2 mol% TaC at 1800 °C, 1–2 mol% TaC at 1700 °C, and 0–6 mol% TiC were obtained. A small addition of 1–2 mol% TaC at 1700 °C improved the sinterability of WC. The W2C- and TaC-type solid solutions, (W, Ta)2C and (Ta, W)C, were produced during the sintering process. The added TaC and TiC fully changed to the solid solutions of (Ta, W)C for dense TaC-added WC ceramics and (Ti, W)C for TiC-added WC ceramics. TiC inhibited the grain growth of WC in WC ceramics. The hardness of TaC-added WC ceramics sintered at 1700 °C increased from 19 GPa with no TaC addition to 25.1 GPa with 1 mol% TaC. The hardness of TiC-added WC ceramics sintered at 1800 °C differed only slightly from that without TiC addition, ~24 GPa. The relationship between the hardness of dense TaC- and TiC-added WC ceramics and WC grain size was similar to that for pure WC. The fracture toughness of the TaCadded WC ceramics at 1700 °C increased from 5.7 MPa m0.5 without TaC addition to 6.7 MPa m0.5 at 1 mol% TaC.
1. Introduction Dense binderless tungsten carbide (WC) ceramics have a much higher Young's modulus, at > 700 GPa, than other transition metal carbides, such as Cr3C2, HfC, Mo2C, NbC, TaC, TiC, and ZrC [1,2]; consequently, WC ceramics exhibit excellent elastic deformation resistance. In addition, the ceramics have a high Vickers hardness of around 25 GPa, and a higher fracture toughness of ~ 6.0 MPa m0.5 compared with those of other transition metal carbides [1–6]. Pure WC has a very high melting point; thus, it is difficult to sinter. Specifically, a high sintering temperature above 1800 °C is required to consolidate WC ceramics without binders, even via the resistance-heated hot pressing (commonly called spark plasma sintering) used for consolidation of highly sinter-resistant substances [1,7]. As such, cemented carbides used for high-performance cutting tools and abrasion-resistant parts are commonly manufactured by adding metallic binders, such as Co and Ni, to induce liquid-phase sintering in WC. The addition of metallic binders lowers the Young modulus, hardness, and corrosion and oxidation resistance of WC. WC ceramics without metallic binders are suitable materials for application to molding dies for high-precision glass molding, and for the mechanical seals and abrasive water jet nozzles used under severe corrosive environments [8].
⁎
Sintering of WC is greatly influenced by carbon addition to raw WC powder [1,9]. Because tungsten oxides, WO2 and/or WO3, form on the WC particle surface, these oxides consume carbon via a reaction with WC [1,10], WC ceramics sintered without carbon addition always contain a small amount of W2C. When a carbon amount appropriate for converting W2C to WC is added to WC, the sinterability of WC improves significantly. Carbon addition does not lower the Young's modulus of WC ceramics; the Young's modulus of fully dense W2C is 444 GPa, which is much lower than that of dense pure WC [11]. However, the purge of W2C facilitates WC grain growth largely, during sintering. The existence of W2C in WC ceramics inhibits abnormal growth of WC grains [1]. The hardness of a WC ceramic with large WC grains is low, at around 14 GPa [1]. The smaller the grains of pure dense WC ceramics, the higher the hardness, as WC ceramics obey the Hall–Petch-like relationship [1,12]. Thus, WC grain growth must be restricted to attain high-hardness WC ceramics. For WC and WC-based ceramics, C3C2, Mo2C, NbC, V8C7, and ZrC have been shown to be effective graingrowth inhibitors [13–19]. However, to date, TaC and TiC have not been examined in terms of their effects on the sinterability and mechanical properties of WC. Imasato et al. examined the effects of Cr3C2 and V8C7 addition on the mechanical properties of WC–3 mass% TiC–2 mass% TaC [8]; however, they did not clarify the effects of a single
Corresponding author. E-mail address: [email protected] (A. Nino).
https://doi.org/10.1016/j.ijrmhm.2019.04.012 Received 8 January 2019; Received in revised form 12 April 2019; Accepted 16 April 2019 Available online 17 April 2019 0263-4368/ © 2019 Published by Elsevier Ltd.
International Journal of Refractory Metals & Hard Materials 82 (2019) 167–173
A. Nino, et al.
addition of TaC or TiC on the sinterability, grain growth, or mechanical properties of WC. TaC has the second-largest Young's modulus, of 558 GPa, following WC among the transition metal carbides. Dense pure TaC ceramics can be sintered in the temperature range of 1375–1900 °C by resistanceheated hot-pressing, as described in [20]. The densification temperature of TaC ceramic is considerably lower than that of pure WC. Therefore, the addition of TaC to binderless WC ceramics maintains the Young's modulus value of the WC ceramic. Pure TiC ceramics have a Young's modulus and Vickers hardness of 454 and 19.8 GPa, respectively [2]. Both TaC and TiC are non-stoichiometric compounds and dissolve WC in the solid-state [21–23]. Industrial application of WC ceramics requires information about the effects of the addition of transition metal carbides to WC and the most suitable addition amounts. The aim of this study was to clarify the effects of TaC and TiC additions as grain growth inhibitors on the microstructure and mechanical properties of WC ceramics. TaC and TiC-added WC ceramics were sintered by resistance-heated hot pressing, and were examined for their density, microstructure, Young's modulus, hardness, and fracture toughness. 2. Experimental procedures Powders of TaC (Japan New Metals; average particle diameter: 1.09 μm; chemical composition (mass%): total C 6.25, free C 0.00, Nb 0.04, Ta balance), TiC (A.L.M.T.; average particle diameter: 0.70 μm; chemical composition (mass%): total C 19.10, free C 0.02, Fe 0.01, Co < 0.02, W < 0.1, N 0.016, O 1.24, Al 0.0083, Ca 0.0074, Cr 0.0023, Cu < 0.0030, Mg < 0.0010, Mn < 0.0010, Ni < 0.0010, Si < 0.0030, Sn < 0.0040, Ti balance) and WC (Japan New Metals; average particle diameter: 0.75 μm; chemical composition (mass%): total C 6.16, free C 0.04, Fe max. 0.050, Mo max. 0.020, W balance) were used as the starting materials. Powders were weighed to obtain compositions of WC–(0–6 mol%) TaC and WC–(0–6 mol%) TiC. The powders were mechanically mixed for 6 h in the presence of ethanol in a nylon milling pot with WC–8 wt% Co balls. A slow rotational velocity was used to prevent Co contamination from the balls. The wet mixtures were dried in a rotary evaporator after mixing. The powder mixtures were pressed in graphite dies with an inner diameter of 20 mm at an applied pressure of 50 MPa. All obtained compacts were sintered with a resistance-heated hot-pressing machine (model SPS-2080; Sumitomo Coal Mining) at a heating rate of 50 °C min−1 under an applied pressure of 50 MPa, and were maintained for 10 min at sintering temperatures of 1600–1800 °C for TaC addition and 1800 °C for TiC addition. The temperature of the die was measured using an optical pyrometer through a small hole, in which the end of the probe was positioned 5 mm away from the compact. The shrinkage coefficient of the specimen during heating was calculated from the net displacement of the pressing ram, which was corrected for the expansion of load train spacers and work punches during heating. Both sides of each sintered body were ground down by 0.75 mm to avoid carbon contamination, and then one ground surface was polished. The bulk density of the sintered bodies was measured using the Archimedean method. The reaction products were analyzed with a high-powered X-ray diffractometer (model RINT-2500VHF; Rigaku) using Cu-Kα radiation under the operating conditions of 50 kV and 300 mA. The microstructure was surveyed with an electron probe microanalyzer (EPMA; model JXA-8230; JEOL). The average WC grain size was determined with Image-Pro Plus software (ver. 7.0; Media Cybernetics) from backscattered electron micrographs by measuring the linear intercept length. Young's modulus was determined using a pulse-echo method with an elastodynamic rate-measuring system (model UMS-HL; Toshiba Tungaloy). The Vickers hardness was measured under a test force of 9.8 N at a holding time of 15 s. The Vickers hardness and fracture toughness were determined from five measurements without the maximum and minimum values. The fracture
Fig. 1. Bulk and relative densities of tungsten carbide (WC) ceramics as a function of the additive carbides: (a) TaC and (b) TiC.
toughness was estimated by the indentation fracture method according to the Evance–Davis Eq. [24].
3. Results and discussion 3.1. Density Fig. 1 shows the bulk and relative densities as a function of added TaC and TiC amounts for WC ceramics sintered at 1600–1800 °C. Because a small amount of reaction product was detected in the sintered bodies by X-ray diffraction (XRD) analysis, as shown in Section 3.2, the theoretical density is difficult to determine accurately. Hence, the theoretical density was calculated based on the volume fractions of WC, TaC, and TiC and their densities (WC, 15.669 g cm−3 [25]; TaC, 14.498 g cm−3 [26]; TiC, 4.91 g cm−3 [27]), by assuming that these carbides were mutually immiscible in the solid state. Dense pure WC ceramics having a relative density of 99% or more were obtained at 1800 °C. Although the density of TaC is lower than that of WC, the bulk density of WC ceramics sintered at 1700 °C increased with a small addition of TaC. Dense TaC-added WC ceramics containing 1–2 mol% TaC having relative density above 98% were obtained at 1700 °C. Above 2 mol% TaC at 1700 °C, the bulk and relative densities of TaC-added WC ceramics decreased sharply with increasing TaC amount. WC–1 mol % TaC ceramics were not densely sintered at 1600–1650 °C due to the low sintering temperature. Above 1700 °C, the relative density of WC–1 mol% TaC ceramics was above 99%. A small addition of TaC improved the sinterability of WC. The bulk density of TiC-added WC ceramics sintered at 1800 °C decreased with increasing TiC amount because the density of TiC is lower than that of WC. The relative density of TiC-added WC ceramics varied slightly as we increased the amount of TiC due to error in the Archimedes method. The TiC-added WC ceramics (0–6 mol% TiC) densely sintered at 1800 °C were fully dense.
168
International Journal of Refractory Metals & Hard Materials 82 (2019) 167–173
A. Nino, et al.
Fig. 2. Expanded X-ray diffraction patterns of TiC-added WC ceramics: (a) 2 mol% TiC, (b) 4 mol% TiC, and (c) 6 mol% TiC.
3.2. Reaction products and microstructure Fig. 2 presents the expanded X-ray diffraction patterns corresponding to (Ti, W)C solid solution. The (Ti, W)C solid solution diffraction peak shifted to the TiC side as we increased the amount of TiC. The reason for this peak shift was not clear. The TiC-added WC ceramics are presumed to be in a nonequilibrium state for the 10 min holding time during sintering. Fig. 3 shows the changes in relative intensity of the XRD peaks for the constituent phases of WC, TaC, W2C, W2C-, and TaC-type solid solutions for TaC-added WC ceramics (a), and WC, W2C-, and TiC-type solid solutions for TiC-added WC ceramics (b). The relative intensity of the constituent phases was calculated using the maximum diffraction peaks for the detected phases.
Relative intensity =
Ix ∑ Iall
Fig. 4. Ternary phase diagram of (a) C–Ta–W [21] and (b) C–Ti–W [22] systems at 1750 °C; a (Ta, W)C1-δ: carbon-deficient (Ta, W)C solid solution and a (Ti, W)C1-δ: carbon-deficient (Ti, W)C solid solution are shown.
in the sintered bodies was produced by the reaction between WC and surface oxides WO2 and/or WO3 covering WC particles in the starting powder [1,10,16]. The solid solutions of (Ta, W)C and (W, Ta)2C were produced in WC ceramics containing TaC. The diffraction peaks corresponding to TaC shifted to higher diffraction angles compared with those of JCPDS card data [26]. This indicates that a TaC solid solution containing W formed during sintering, because the atomic size of W is smaller than Ta. The diffraction peaks related to W2C for the TaC-added WC ceramics shifted to lower angles in comparison with those for the ceramic without TaC, because Ta has a larger atomic size than W, indicating that TaC dissolved into the W2C phase. WC ceramics in which a small amount of unreacted TaC remained for 0.5 mol% TaC and a large amount of unreacted TaC remained above 4 mol% TaC were not densely sintered at 1700 °C. For TaC additions of 1–2 mol%, the added TaC was converted completely to a (Ta, W)C solid solution. Fig. 4(a) presents a ternary phase diagram of the C–Ta–W system at 1750 °C near the sintering temperature [21]. The starting WC powder produced a small amount of W2C after sintering, as shown in Fig. 3. In the C–Ta–W phase diagram, therefore, the thermally equilibrated composition of the WC powder at high temperature resided closer to the W2C side of the WCW2C coexistence line. For a small amount of TaC addition to the WC powder [along the dotted line in Fig. 4(a)], the phase changed from WC-W2C coexistence to WC-W2C-(W, Ta)2C coexistence via TaC dissolution in W2C, as given below:
(1)
where Ix is the maximum peak intensity of a detected phase, and ΣIall is the summation of all of the maximum peak intensities for each phase. Because W2C was not detected in the starting WC powder by XRD, W2C
W2C + 2x TaC → (W1 − x Tax )2 C + 2x WC
(2)
Further addition caused the phase to change to WC-(Ta, W)2C-(Ta, W)C1-δ coexistence by WC dissolution in TaC [(Ta, W)C1 −δ : carbondeficient (Ta, W) solid solution]. WC dissolves TaC by about 15% at 1500 °C [22]. This phase change sequence is consistent with the reaction product change shown in Fig. 3(a). For the TiC-added WC ceramics, as shown in Fig. 3(b), a solid solution of (Ti, W)C was produced; however, the W2C-type solid solution was not found. Fig. 4 (b) presents a the ternary phase diagram of the CTi-W system at 1750 °C [23]. W2C hardly forms a solid solution in the C–Ti–W system, contrary to the case of the C–Ta–W system. Hence, a
Fig. 3. Relative peak intensity of the constituent phases of the WC ceramics as a function of additive carbides: (a) TaC, and (b) TiC. (ss: solid solution). 169
International Journal of Refractory Metals & Hard Materials 82 (2019) 167–173
A. Nino, et al.
Fig. 5. Microstructures of the TaC-added WC ceramics sintered at 1700 °C: (a) no TaC addition, (b) 0.5 mol% TaC, (c) 1 mol% TaC, (d) 2 mol% TaC, (e) 4 mol% TaC, and the TiC-added WC ceramics sintered at 1800 °C: (f) 6 mol% TiC. The small circular black region corresponds to pores.
consisted of fine, granular WC grains. Relatively large bright gray grains surrounded by pores [Fig. 5 (c)] correspond to the (Ta, W)C solid solution. WC ceramics containing 0–0.5 and 4–6 mol% TaC sintered at 1700 °C contain a large number of pores. The microstructures of the TaC-added WC ceramics above 4 mol% TaC consisted of very fine equiaxed granular WC grains because the sintering had not progressed sufficiently. The quantity of pores corresponding to the small circular black region decreased with the addition of 1–2 mol% TaC. The decrease in the number of pores is related to the change in the constituent phase of the TaC-added WC ceramics. The added 1–2 mol% TaC was completely converted into a solid solution. However, in the other compositions, the added TaC remained as TaC. The addition of 1–2 mol % TaC to WC ceramics improved the sinterability of WC, but it is not clear why the sinterability of the WC ceramics improved. Fig. 5(f) shows the microstructure of the TiC-added WC ceramic; the circular dark gray regions larger than the surrounding bright gray grains correspond to the (Ti, W)C solid solution. The TiC-added WC ceramics also had fine, equiaxed granular WC grains. When WC ceramics consisted of only the WC phase without the W2C phase in the carbon-added WC ceramics, WC grains grew during the sintering process [1]. Large WC grain growth was not observed in TiC-added WC ceramics without a W2C-type solid solution. Therefore, TiC inhibited WC grain growth. Fig. 6 shows the average size of WC grains in TaC- and TiC-added WC ceramics as a function of TaC and TiC amounts. The WC grain size
Fig. 6. Average grain size of TaC- and TiC-added WC ceramics as a function of additive amount.
small TiC addition along the dotted line produces a WC-(Ti, W)C1 –δ coexistence phase [(Ti, W)C1 −δ: (Ti, W)C solid solution]. This reaction sequence is consistent with the results shown in Fig. 3(b). Fig. 5(a)–(e) shows the microstructure of TaC-added WC ceramics. The small circular black regions are pores. The TaC-added WC ceramics
170
International Journal of Refractory Metals & Hard Materials 82 (2019) 167–173
A. Nino, et al.
for TaC-added WC ceramics sintered at 1700 °C decreased with increasing TaC amount, from 0.44 μm for the WC ceramic to 0.28 μm at 4 mol% TaC. Upon the addition of 1–2 mol% TaC, slightly larger grains were occasionally visible in the micrographs, but many very small WC grains were photographed. The average grain size is believed to be smaller than that of WC ceramics without TaC due to the large number of small grains. Above 4 mol% TaC, only small grains were observed in the micrograph, and the average grain size decreased. The TaC-added WC ceramics sintered at 1800 °C had larger WC grains than those sintered at 1700 °C. At 1700 °C, the addition of 1 mol% TaC to WC ceramics was most effective in densifying the sintered bodies and inhibiting the WC grain growth because the sinterability decreased considerably above 4 mol% TaC. The WC grain size for TiC-added WC ceramics showed only a slight reduction, from 0.49 μm for the WC ceramic sintered at 1800 °C to 0.45 μm for 6 mol% TiC addition. TaC addition to WC above 2 mol% and ZrC addition above 2 mol% [12] inhibits not only WC grain growth, but also densification. TiC did not hinder densification with inhibition of WC grain growth in the addition range investigated (6 mol%). Other transition metal carbides, i.e., Cr3C2 and NbC, also inhibit WC grain growth in small additions, with little to no relative density reduction for WC-based ceramics [14,15,17].
Fig. 8. Vickers hardness results for the TaC- and TiC-added WC ceramics as a function of additive amount.
was slightly lower than that calculated using the isostrain model, of 694 GPa. Fig. 8 shows the change in Vickers hardness for TaC- and TiC-added WC ceramics as a function of TaC and TiC addition. The Vickers hardness of the WC ceramic sintered at 1700 °C was lower than that of the ceramic sintered at 1800 °C due to its higher porosity. The hardness of the TaC-added WC ceramics sintered at 1700 °C increased from 19.9 GPa without TaC to 25.1 GPa with 1 mol% TaC, due to densification of the ceramic and inhibition of WC grain growth. The hardness of dense, polycrystalline WC-based ceramics and other polycrystalline ceramics increased with decreasing grain size, according to the Hall–Petch-like relationship [1,12,16,19,28]. The TaC-added WC ceramics with 1–2 mol% TaC densely sintered at 1700 °C had higher hardness values than those of pure WC and TaC-added WC ceramics densely sintered at 1800 °C, corresponding to their WC grain size. The low hardness of the TaC-added WC ceramics, with the exception of the 1–2 mol% TaC addition, was attributed to the high porosity of the samples. The hardness change with TaC addition was similar to the relative density change. The hardness of ceramics is related not only to the hardness of the material itself, but also to the relative densities of the sintered bodies and the average grain size. The relative density increased significantly upon the addition of 1–2 mol% TaC, and this increase in relative density was the main factor driving the increase in the hardness. The increase in the relative density raised the hardness of the sintered bodies for TiC [29,30], WC [31,32], and composite ceramics [33]. There was no apparent change in the hardness of WC ceramics with and without TiC sintered at 1800 °C. The TiC-added WC ceramics were densely sintered in the range of 0–6 mol% TiC; the average WC grain size changed only slightly with TiC addition. Fig. 9 shows the WC grain size dependence of the Vickers hardness for TaC- and TiC-added WC ceramics; the Hall–Petch-like relationship reported for pure WC ceramics is indicated as dotted lines in the figure. The hardness value of TaC-added WC ceramics containing many pores was located below the lines. The relationship for dense TaC- and TiCadded WC ceramics was close to that of pure WC ceramics [1,12]. The relationship for TaC-added WC ceramics with low relative densities, under 94%, is indicated by circular solid marks, and did not obey the Hall-Petch like relationship of pure WC ceramics. As reported in the case of WC and other ceramics [29–33], the decrease in the relative density of sintered bodies reduces the hardness. The hardness of the densely sintered TaC-added WC ceramics did not decrease. The deviation from the Hall-Petch like relationship for dense ceramics is largely influenced by the relative density. The abnormal growth of WC grains was not evident in TaC- and TiC-added WC ceramics; hence, the WC grain size range in TaC- and TiC-added WC ceramics was narrow. Fig. 10 shows the fracture toughness for the TaC- and TiC-added WC ceramics. The fracture toughness of dense pure WC was 5.9 MPa m0.5.
3.3. Mechanical properties Fig. 7 shows the Young's modulus values of TaC- and TiC-added WC ceramics; on XRD analysis, there was no preferred orientation in either ceramic. Young's modulus for TaC-added WC ceramics sintered at 1700 °C increased from 586 GPa for no TaC addition to 705 GPa at 1 mol% TaC. The Young's modulus of fully dense WC ceramics sintered at 1800 °C was ~710 GPa in this study and in other reports [1,2]. The Young's modulus of the WC ceramic sintered at 1700 °C without carbide addition was much lower than that sintered at 1800 °C, due to its high porosity. The Young's modulus of dense WC ceramics containing 1–2 mol% TaC above 1700 °C was comparable to that of pure WC ceramics. The Young's modulus of fully dense pure TaC is 558 GPa [20]. Given that TaC has a relatively high Young's modulus value, the Young's modulus at 1 mol% TaC was predicted to be 708 GPa using Voigt's isostrain model. The Young's modulus of TaC-added WC ceramics at 1800 °C decreased slightly from 709 GPa when no TaC was added, to 698 GPa at 2 mol% TaC. The TaC-added WC ceramics were densely sintered at 1800 °C. The slight decrease in the Young's modulus was related to the addition of TaC. The Young's modulus for TiC-added WC ceramics decreased from 709 GPa for no TiC addition to 670 GPa at 6 mol% TiC. The slight decrease in the Young's modulus with TiC addition was attributed to the Young's modulus of TiC (454 GPa) [2], which is lower than that of pure WC. The Young's modulus at 6 mol%
Fig. 7. The Young's modulus of TaC- and TiC-added WC ceramics as a function of additive amount. The dotted and dot-dash lines show the calculated lines using Voigt's isostress model. 171
International Journal of Refractory Metals & Hard Materials 82 (2019) 167–173
A. Nino, et al.
Fig. 11. Crack propagation of Vickers indentation in the WC–1 mol% TaC ceramic (backscattered electron image).
Fig. 11 shows the crack propagation in the WC–1 mol% TaC ceramic sintered at 1700 °C, which had a higher fracture toughness value. The indentation crack for the ceramic propagated both along grain boundaries and through WC grains. No remarkable crack deflection was observed in the ceramic. The reason why the fracture toughness of WC ceramics was increased by the addition of 1 mol% TaC was not elucidated in this study.
Fig. 9. Vickers hardness values of dense TaC- and TiC-added WC ceramics as a function of average WC grain size. The closed circles correspond to TaC-added WC ceramics that were not densely sintered.
4. Conclusions WC ceramics with TaC and TiC were densely sintered under conditions of 1–2 mol% TaC at 1700 °C, 0–2 mol% TaC at 1800 °C, and 0–6 mol% TiC at 1800 °C. The addition of TaC to WC ceramics produced the solid solutions of (W, Ta)2C and (Ta, W)C as reaction products. In dense WC ceramics containing 1–2 mol% TaC, the added TaC converted completely to the solid solutions. In dense WC ceramics containing 1–2 mol% TaC, the added TaC was completely converted into solid solution. The images of the microstructure show that the number of pores was reduced dramatically compared to that with no TaC. No W2Ctype solid solution formed in the TiC-added WC ceramics. The added TiC converted completely to the (Ti, W)C solid solution. No large WC grains formed in the TiC-added WC ceramics. TiC is an effective grain growth inhibitor for WC ceramics. The Young's modulus for TaC-added WC ceramics sintered at 1700 °C increased from 586 GPa without TaC to 705 GPa with 1 mol% TaC, and its value is close to that of dense pure WC. The Young's modulus of TaC-added WC ceramics at 1800 °C decreased slightly from 709 GPa for no TaC addition, to 698 GPa at 2 mol % TaC. This was due to the relatively low Young's modulus of TaC. The addition of TiC to WC ceramics caused a small decrease in the Young's modulus due to the relatively low Young's modulus of TiC. The TaC addition to WC ceramics at 1700 °C raised the hardness from 19 GPa without TaC to 25.1 GPa for 1 mol% TaC. The TiC-added WC ceramics retained a high hardness value of ~24 GPa in the range of 0–6 mol% TiC. The relationship between the hardness of only dense TaC- and TiCadded WC ceramics and the average grain size was close to that for pure WC ceramics. The relationship for TaC-added WC ceramics with low relative densities does not obey the Hall-Petch like relationship of pure WC ceramics. The fracture toughness of the TaC-added WC ceramics sintered at 1700 °C increased from 5.7 MPa m0.5 without TaC to 6.7 MPa m0.5 for 1 mol% TaC. A small addition of TaC to WC ceramics at 1700 °C improved the hardness and fracture toughness. The fracture toughness of the TiC-added WC ceramics hardly varied with the amount of TiC because (Ti, W)C has a high fracture toughness.
Fig. 10. Fracture toughness of TaC- and TiC-added WC ceramics as a function of additive amount.
WC ceramics without TaC and TiC sintered at 1700 and 1800 °C had fracture toughness values close to the reported value [1]. The fracture toughness of TaC-added WC ceramics sintered at 1700 °C increased from 5.7 for pure WC to 6.7 MPa m0.5 for 1 mol% TaC, and decreased above 1 mol% TaC. The WC–1 mol% TaC ceramics sintered at 1650 and 1700 °C had high fracture toughness values of 6.5 and 6.7 MPa m0.5, respectively. The WC ceramics containing 1.5–2 mol% TaC sintered at 1700 °C, and the WC–TaC ceramics sintered at 1800 °C, had a similar fracture toughness to that of pure WC. The WC ceramic containing 1 mol% TaC obviously had a higher fracture toughness than the other TaC-added WC ceramics. The constituent phase of TaC-added WC ceramics was the same, in the range of 1–2 mol% TaC. If the new phase (Ta, W)C has an effect, the fracture toughness of the TaC-added WC ceramics should be similar upon the addition of 1–2 mol% TaC. Only the WC ceramic containing 1 mol% TaC had a high fracture toughness value, so this result cannot be explained by the contribution of (Ta, W)C. Above 4 mol% TaC, the fracture toughness was lower than that of pure WC ceramics, due to its high porosity. The fracture toughness of WC ceramics changed slightly with the addition of TiC, ranging between 5.6 and 6.0 MPa m0.5. TiC-added WC ceramics consisted of WC and (Ti, W)C solid solution. Jung and Kang reported the fracture toughness of (Ti, W)C at various Ti and W ratios, and these values were 6.4–7.7 MPa m0.5 [34]. The fracture toughness was maintained at that of pure WC due to the formation of (Ti, W)C, which has a high fracture toughness. The fracture toughness did not increase upon the formation of (Ti, W)C due to the small amount of TiC additive.
Acknowledgement This work was partially supported by Japan Society for the Promotion of Science KAKENHI, Grants-in-Aid for Young Scientists (B), 17K14841. 172
International Journal of Refractory Metals & Hard Materials 82 (2019) 167–173
A. Nino, et al.
References
[18] A. Nino, T. Sekine, K. Sugawara, S. Sugiyama, H. Taimatsu, Effect of added Cr3C2 on the microstructure and mechanical properties of WC-SiC ceramcis, Key Eng. Mater. 656-657 (2015) 33–38. [19] A. Nino, Y. Nakaibayashi, Shigeaki Sugiyama, Hitoshi Taimatsu, Effect of Mo2C addition on the microstructures and mechanical properties of WC-SiC ceramics, Int. J. Refract. Met. Hard Mater. 64 (2017) 35–39. [20] A. Nino, T. Hirabara, S. Sugiyama, H. Taimatsu, Preparation and characterization of tantalum carbide (TaC) ceramics, Int. J. Refract. Met. Hard Mater. 52 (2015) 203–208. [21] A.E. McHale (Ed.), Figure 9024 in “Phase Equilibria Diagrams Phase Diagrams for Ceramists Vol. X Borides, Carbides, and Nitrides”, The American Ceramic Society, Westerville, OH, 1994, p. 351. [22] H.E. Exner, Physical and chemical nature of cemented carbides, Int. Met. Rev. 243 (1979) 149–173. [23] A.E. McHale (Ed.), Figure 9029 in “Phase Equilibria Diagrams Phase Diagrams for Ceramists Vol. X Borides, Carbides, and Nitrides”, The American Ceramic Society, Westerville, OH, 1994, p. 366. [24] C.B. Ponton, R.D. Rawlings, Vickers indentation fracture toughness test part 1 review of literature and formulation of standardised indentation toughness equations, Mater. Sci. Technol. 5 (1989) 865–872. [25] Powder Diffraction Files, JCPDS-International Center for Diffraction Data, Pennsylvania, 1994, pp. 25–1047. [26] Powder Diffraction Files, JCPDS-International Center for Diffraction Data, Pennsylvania, 1994, pp. 35–801. [27] Powder Diffraction Files, JCPDS-International Center for Diffraction Data, Pennsylvania, 1994, pp. 32–1383. [28] R.W. Rice, C.C. Wu, F. Borchelt, Hardness–grain-size relations in ceramics, J. Am. Ceram. Soc. 77 (1994) 2539–2553. [29] A. Babapoor, M.S. Asl, Z. Ahmadi, A.S. Namini, Effects of spark plasma sintering temperature on densification, hardness and thermal conductivity of titanium carbide, Ceram. Int. 44 (2018) 14541–14546. [30] A.S. Namini, Z. Ahmadi, A. Babapoor, M. Shokouhimehr, M.S. Asl, Microstructure and thermomechanical characteristics of spark plasma sintered TiC ceramics doped with nano-sized WC, Ceram. Int. 45 (2019) 2153–2160. [31] W. Tang, L. Zhang, J. Zhu, Y. Chen, W. Tian, T. Liu, Effect of direct current patterns on densification and mechanical properties of binderless tungsten carbides fabricated by the spark plasma sintering system, Int. J. Ref. Met. Hard Mater. 64 (2017) 90–97. [32] J. Wang, D. Zuo, L. Zhu, W. Li, Z. Tu, S. Dai, Effects and influence of Y2O3 addition on the microstructure and mechanical properties of binderless tungsten carbide fabricated by spark plasma sintering, Int. J. Ref. Met. Hard Mater. 71 (2018) 167–174. [33] W. Chen, H. Lin, P.K. Nayak, J. Huang, Material properties of tungsten carbidealumina composites fabricated by spark plasma sintering, Ceram. Int. 40 (2014) 15007–15012.
[1] A. Nino, K. Takahashi, S. Sugiyama, H. Taimatsu, Effects of carbon addition on microstructures and mechanical properties of binderless tungsten carbide, Mater. Trans. 53 (2012) 1475–1480. [2] A. Nino, A. Tanaka, S. Sugiyama, H. Taimatsu, Indentation size effect for the hardness of refractory carbides, Mater. Trans. 51 (2010) 1621–1626. [3] L.E. Toth, Transition Metal Carbides and Nitrides, Academic Press, New York and London, 1971. [4] C. Kral, W. Lengauer, D. Rafaja, P. Ettmayer, Critical review on the elastic properties of transition metal carbides, nitrides and carbonitrides, J. Alloys Compd. 265 (1998) 215–233. [5] A.T. Santhanam, Application of transition metal carbides and nitrides in industrial tools, in: S.T. Oyama (Ed.), The Chemistry of Transition Metal Carbides and Nitrides, Springer, Dordrecht, 1996, p. 29. [6] P.T.B. Shaffer, Handbook of High Temperature Materials: No. 1 Materials Index, Plenum Press, New York, 1964. [7] S.G. Huang, K. Vanmeensel, O. Van der Biest, J. Vleugels, Binderless WC and WC–VC materials obtained by plused electric current sintering, Int. J. Refract. Met. Hard Mater. 26 (2008) 41–47. [8] S. Imasato, K. Tokumoto, T. Kitada, S. Sakaguchi, Properties of ultra-fine grain binderless cemented caribide ‘RCCFN’, Int. J. Refract. Met. Hard Mater. 13 (1995) 305–312. [9] R.T. Fox, R. Nilsson, Binderless tungsten carbide carbon control with pressureless sintering, Int. J. Refract. Met. Hard Mater. 76 (2018) 82–89. [10] J. Zhao, T. Holland, C. Unuvar, Z.A. Munir, Spark plasma sintering of nanometric tungsten carbide, Int. J. Refract. Met. Hard Mater. 27 (2009) 130–139. [11] H. Taimatsu, S. Sugiyama, Y. Kodaira, Synthesis of W2C by reactive hot pressing and its mechanical properties, Mater. Trans. 49 (2008) 1256–1261. [12] H.C. Lee, J. Gurland, Hardness and deformation of cemented tungsten carbide, Mat. Sci. Eng. 33 (1978) 125–133. [13] J. Poetschke, V. Richter, T. Gestrich, A. Michaelis, Grain growth during sintering of tungsten carbide ceramics, Int. J. Refract. Met. Hard Mater. 43 (2014) 309–316. [14] J. Poetschke, V. Richter, R. Holke, Influence and effectivity of VC and Cr3C2 grain growth inhibitors on sintering of binderless tungsten carbide, Int. J. Refract. Met. Hard Mater. 31 (2012) 218–223. [15] H. Taimatsu, S. Sugiyama, M. Komatsu, Effects of Cr3C2 and V8C7 on the microstructure and mechanical properties of WC-SiC whisker ceramics, Mater. Trans. 50 (2009) 2435–2440. [16] A. Nino, Y. Izu, T. Sekine, S. Sugiyama, H. Taimatsu, Effects of ZrC and SiC addition on the microstructures and mechanical properties of binderless WC, Int. J. Refract. Met. Hard Mater. 69 (2017) 259–265. [17] A. Nino, N. Takahashi, S. Sugiyama, H. Taimatsu, Effects of carbide grain growth inhibitors on the microstructures and mechanical properties of WC-SiC-Mo2C hard ceramics, Int. J. Refract. Met. Hard Mater. 43 (2014) 150–156.
173
Contents lists available at ScienceDirect
International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Effects of TaC and TiC addition on the microstructures and mechanical properties of Binderless WC
T
⁎
Akihiro Ninoa, , Yuma Izua, Takashi Sekineb, Shigeaki Sugiyamab, Hitoshi Taimatsua a b
Department of Materials Science and Engineering, Graduate School of Engineering Science, Akita University, 1-1 Tegata-Gakuencho, Akita 010-0852, Japan Akita Industrial Technology Center, 4-11 Sanuki, Araya, Akita 010-1623, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Tungsten carbide Tantalum carbide Titanium carbide Grain growth inhibitor Hardness Fracture toughness
WC ceramics were sintered using a resistance-heated hot-pressing machine in the temperature range of 1600–1800 °C for TaC addition and at 1800 °C for TiC addition. Dense WC ceramics containing 0–2 mol% TaC at 1800 °C, 1–2 mol% TaC at 1700 °C, and 0–6 mol% TiC were obtained. A small addition of 1–2 mol% TaC at 1700 °C improved the sinterability of WC. The W2C- and TaC-type solid solutions, (W, Ta)2C and (Ta, W)C, were produced during the sintering process. The added TaC and TiC fully changed to the solid solutions of (Ta, W)C for dense TaC-added WC ceramics and (Ti, W)C for TiC-added WC ceramics. TiC inhibited the grain growth of WC in WC ceramics. The hardness of TaC-added WC ceramics sintered at 1700 °C increased from 19 GPa with no TaC addition to 25.1 GPa with 1 mol% TaC. The hardness of TiC-added WC ceramics sintered at 1800 °C differed only slightly from that without TiC addition, ~24 GPa. The relationship between the hardness of dense TaC- and TiC-added WC ceramics and WC grain size was similar to that for pure WC. The fracture toughness of the TaCadded WC ceramics at 1700 °C increased from 5.7 MPa m0.5 without TaC addition to 6.7 MPa m0.5 at 1 mol% TaC.
1. Introduction Dense binderless tungsten carbide (WC) ceramics have a much higher Young's modulus, at > 700 GPa, than other transition metal carbides, such as Cr3C2, HfC, Mo2C, NbC, TaC, TiC, and ZrC [1,2]; consequently, WC ceramics exhibit excellent elastic deformation resistance. In addition, the ceramics have a high Vickers hardness of around 25 GPa, and a higher fracture toughness of ~ 6.0 MPa m0.5 compared with those of other transition metal carbides [1–6]. Pure WC has a very high melting point; thus, it is difficult to sinter. Specifically, a high sintering temperature above 1800 °C is required to consolidate WC ceramics without binders, even via the resistance-heated hot pressing (commonly called spark plasma sintering) used for consolidation of highly sinter-resistant substances [1,7]. As such, cemented carbides used for high-performance cutting tools and abrasion-resistant parts are commonly manufactured by adding metallic binders, such as Co and Ni, to induce liquid-phase sintering in WC. The addition of metallic binders lowers the Young modulus, hardness, and corrosion and oxidation resistance of WC. WC ceramics without metallic binders are suitable materials for application to molding dies for high-precision glass molding, and for the mechanical seals and abrasive water jet nozzles used under severe corrosive environments [8].
⁎
Sintering of WC is greatly influenced by carbon addition to raw WC powder [1,9]. Because tungsten oxides, WO2 and/or WO3, form on the WC particle surface, these oxides consume carbon via a reaction with WC [1,10], WC ceramics sintered without carbon addition always contain a small amount of W2C. When a carbon amount appropriate for converting W2C to WC is added to WC, the sinterability of WC improves significantly. Carbon addition does not lower the Young's modulus of WC ceramics; the Young's modulus of fully dense W2C is 444 GPa, which is much lower than that of dense pure WC [11]. However, the purge of W2C facilitates WC grain growth largely, during sintering. The existence of W2C in WC ceramics inhibits abnormal growth of WC grains [1]. The hardness of a WC ceramic with large WC grains is low, at around 14 GPa [1]. The smaller the grains of pure dense WC ceramics, the higher the hardness, as WC ceramics obey the Hall–Petch-like relationship [1,12]. Thus, WC grain growth must be restricted to attain high-hardness WC ceramics. For WC and WC-based ceramics, C3C2, Mo2C, NbC, V8C7, and ZrC have been shown to be effective graingrowth inhibitors [13–19]. However, to date, TaC and TiC have not been examined in terms of their effects on the sinterability and mechanical properties of WC. Imasato et al. examined the effects of Cr3C2 and V8C7 addition on the mechanical properties of WC–3 mass% TiC–2 mass% TaC [8]; however, they did not clarify the effects of a single
Corresponding author. E-mail address: [email protected] (A. Nino).
https://doi.org/10.1016/j.ijrmhm.2019.04.012 Received 8 January 2019; Received in revised form 12 April 2019; Accepted 16 April 2019 Available online 17 April 2019 0263-4368/ © 2019 Published by Elsevier Ltd.
International Journal of Refractory Metals & Hard Materials 82 (2019) 167–173
A. Nino, et al.
addition of TaC or TiC on the sinterability, grain growth, or mechanical properties of WC. TaC has the second-largest Young's modulus, of 558 GPa, following WC among the transition metal carbides. Dense pure TaC ceramics can be sintered in the temperature range of 1375–1900 °C by resistanceheated hot-pressing, as described in [20]. The densification temperature of TaC ceramic is considerably lower than that of pure WC. Therefore, the addition of TaC to binderless WC ceramics maintains the Young's modulus value of the WC ceramic. Pure TiC ceramics have a Young's modulus and Vickers hardness of 454 and 19.8 GPa, respectively [2]. Both TaC and TiC are non-stoichiometric compounds and dissolve WC in the solid-state [21–23]. Industrial application of WC ceramics requires information about the effects of the addition of transition metal carbides to WC and the most suitable addition amounts. The aim of this study was to clarify the effects of TaC and TiC additions as grain growth inhibitors on the microstructure and mechanical properties of WC ceramics. TaC and TiC-added WC ceramics were sintered by resistance-heated hot pressing, and were examined for their density, microstructure, Young's modulus, hardness, and fracture toughness. 2. Experimental procedures Powders of TaC (Japan New Metals; average particle diameter: 1.09 μm; chemical composition (mass%): total C 6.25, free C 0.00, Nb 0.04, Ta balance), TiC (A.L.M.T.; average particle diameter: 0.70 μm; chemical composition (mass%): total C 19.10, free C 0.02, Fe 0.01, Co < 0.02, W < 0.1, N 0.016, O 1.24, Al 0.0083, Ca 0.0074, Cr 0.0023, Cu < 0.0030, Mg < 0.0010, Mn < 0.0010, Ni < 0.0010, Si < 0.0030, Sn < 0.0040, Ti balance) and WC (Japan New Metals; average particle diameter: 0.75 μm; chemical composition (mass%): total C 6.16, free C 0.04, Fe max. 0.050, Mo max. 0.020, W balance) were used as the starting materials. Powders were weighed to obtain compositions of WC–(0–6 mol%) TaC and WC–(0–6 mol%) TiC. The powders were mechanically mixed for 6 h in the presence of ethanol in a nylon milling pot with WC–8 wt% Co balls. A slow rotational velocity was used to prevent Co contamination from the balls. The wet mixtures were dried in a rotary evaporator after mixing. The powder mixtures were pressed in graphite dies with an inner diameter of 20 mm at an applied pressure of 50 MPa. All obtained compacts were sintered with a resistance-heated hot-pressing machine (model SPS-2080; Sumitomo Coal Mining) at a heating rate of 50 °C min−1 under an applied pressure of 50 MPa, and were maintained for 10 min at sintering temperatures of 1600–1800 °C for TaC addition and 1800 °C for TiC addition. The temperature of the die was measured using an optical pyrometer through a small hole, in which the end of the probe was positioned 5 mm away from the compact. The shrinkage coefficient of the specimen during heating was calculated from the net displacement of the pressing ram, which was corrected for the expansion of load train spacers and work punches during heating. Both sides of each sintered body were ground down by 0.75 mm to avoid carbon contamination, and then one ground surface was polished. The bulk density of the sintered bodies was measured using the Archimedean method. The reaction products were analyzed with a high-powered X-ray diffractometer (model RINT-2500VHF; Rigaku) using Cu-Kα radiation under the operating conditions of 50 kV and 300 mA. The microstructure was surveyed with an electron probe microanalyzer (EPMA; model JXA-8230; JEOL). The average WC grain size was determined with Image-Pro Plus software (ver. 7.0; Media Cybernetics) from backscattered electron micrographs by measuring the linear intercept length. Young's modulus was determined using a pulse-echo method with an elastodynamic rate-measuring system (model UMS-HL; Toshiba Tungaloy). The Vickers hardness was measured under a test force of 9.8 N at a holding time of 15 s. The Vickers hardness and fracture toughness were determined from five measurements without the maximum and minimum values. The fracture
Fig. 1. Bulk and relative densities of tungsten carbide (WC) ceramics as a function of the additive carbides: (a) TaC and (b) TiC.
toughness was estimated by the indentation fracture method according to the Evance–Davis Eq. [24].
3. Results and discussion 3.1. Density Fig. 1 shows the bulk and relative densities as a function of added TaC and TiC amounts for WC ceramics sintered at 1600–1800 °C. Because a small amount of reaction product was detected in the sintered bodies by X-ray diffraction (XRD) analysis, as shown in Section 3.2, the theoretical density is difficult to determine accurately. Hence, the theoretical density was calculated based on the volume fractions of WC, TaC, and TiC and their densities (WC, 15.669 g cm−3 [25]; TaC, 14.498 g cm−3 [26]; TiC, 4.91 g cm−3 [27]), by assuming that these carbides were mutually immiscible in the solid state. Dense pure WC ceramics having a relative density of 99% or more were obtained at 1800 °C. Although the density of TaC is lower than that of WC, the bulk density of WC ceramics sintered at 1700 °C increased with a small addition of TaC. Dense TaC-added WC ceramics containing 1–2 mol% TaC having relative density above 98% were obtained at 1700 °C. Above 2 mol% TaC at 1700 °C, the bulk and relative densities of TaC-added WC ceramics decreased sharply with increasing TaC amount. WC–1 mol % TaC ceramics were not densely sintered at 1600–1650 °C due to the low sintering temperature. Above 1700 °C, the relative density of WC–1 mol% TaC ceramics was above 99%. A small addition of TaC improved the sinterability of WC. The bulk density of TiC-added WC ceramics sintered at 1800 °C decreased with increasing TiC amount because the density of TiC is lower than that of WC. The relative density of TiC-added WC ceramics varied slightly as we increased the amount of TiC due to error in the Archimedes method. The TiC-added WC ceramics (0–6 mol% TiC) densely sintered at 1800 °C were fully dense.
168
International Journal of Refractory Metals & Hard Materials 82 (2019) 167–173
A. Nino, et al.
Fig. 2. Expanded X-ray diffraction patterns of TiC-added WC ceramics: (a) 2 mol% TiC, (b) 4 mol% TiC, and (c) 6 mol% TiC.
3.2. Reaction products and microstructure Fig. 2 presents the expanded X-ray diffraction patterns corresponding to (Ti, W)C solid solution. The (Ti, W)C solid solution diffraction peak shifted to the TiC side as we increased the amount of TiC. The reason for this peak shift was not clear. The TiC-added WC ceramics are presumed to be in a nonequilibrium state for the 10 min holding time during sintering. Fig. 3 shows the changes in relative intensity of the XRD peaks for the constituent phases of WC, TaC, W2C, W2C-, and TaC-type solid solutions for TaC-added WC ceramics (a), and WC, W2C-, and TiC-type solid solutions for TiC-added WC ceramics (b). The relative intensity of the constituent phases was calculated using the maximum diffraction peaks for the detected phases.
Relative intensity =
Ix ∑ Iall
Fig. 4. Ternary phase diagram of (a) C–Ta–W [21] and (b) C–Ti–W [22] systems at 1750 °C; a (Ta, W)C1-δ: carbon-deficient (Ta, W)C solid solution and a (Ti, W)C1-δ: carbon-deficient (Ti, W)C solid solution are shown.
in the sintered bodies was produced by the reaction between WC and surface oxides WO2 and/or WO3 covering WC particles in the starting powder [1,10,16]. The solid solutions of (Ta, W)C and (W, Ta)2C were produced in WC ceramics containing TaC. The diffraction peaks corresponding to TaC shifted to higher diffraction angles compared with those of JCPDS card data [26]. This indicates that a TaC solid solution containing W formed during sintering, because the atomic size of W is smaller than Ta. The diffraction peaks related to W2C for the TaC-added WC ceramics shifted to lower angles in comparison with those for the ceramic without TaC, because Ta has a larger atomic size than W, indicating that TaC dissolved into the W2C phase. WC ceramics in which a small amount of unreacted TaC remained for 0.5 mol% TaC and a large amount of unreacted TaC remained above 4 mol% TaC were not densely sintered at 1700 °C. For TaC additions of 1–2 mol%, the added TaC was converted completely to a (Ta, W)C solid solution. Fig. 4(a) presents a ternary phase diagram of the C–Ta–W system at 1750 °C near the sintering temperature [21]. The starting WC powder produced a small amount of W2C after sintering, as shown in Fig. 3. In the C–Ta–W phase diagram, therefore, the thermally equilibrated composition of the WC powder at high temperature resided closer to the W2C side of the WCW2C coexistence line. For a small amount of TaC addition to the WC powder [along the dotted line in Fig. 4(a)], the phase changed from WC-W2C coexistence to WC-W2C-(W, Ta)2C coexistence via TaC dissolution in W2C, as given below:
(1)
where Ix is the maximum peak intensity of a detected phase, and ΣIall is the summation of all of the maximum peak intensities for each phase. Because W2C was not detected in the starting WC powder by XRD, W2C
W2C + 2x TaC → (W1 − x Tax )2 C + 2x WC
(2)
Further addition caused the phase to change to WC-(Ta, W)2C-(Ta, W)C1-δ coexistence by WC dissolution in TaC [(Ta, W)C1 −δ : carbondeficient (Ta, W) solid solution]. WC dissolves TaC by about 15% at 1500 °C [22]. This phase change sequence is consistent with the reaction product change shown in Fig. 3(a). For the TiC-added WC ceramics, as shown in Fig. 3(b), a solid solution of (Ti, W)C was produced; however, the W2C-type solid solution was not found. Fig. 4 (b) presents a the ternary phase diagram of the CTi-W system at 1750 °C [23]. W2C hardly forms a solid solution in the C–Ti–W system, contrary to the case of the C–Ta–W system. Hence, a
Fig. 3. Relative peak intensity of the constituent phases of the WC ceramics as a function of additive carbides: (a) TaC, and (b) TiC. (ss: solid solution). 169
International Journal of Refractory Metals & Hard Materials 82 (2019) 167–173
A. Nino, et al.
Fig. 5. Microstructures of the TaC-added WC ceramics sintered at 1700 °C: (a) no TaC addition, (b) 0.5 mol% TaC, (c) 1 mol% TaC, (d) 2 mol% TaC, (e) 4 mol% TaC, and the TiC-added WC ceramics sintered at 1800 °C: (f) 6 mol% TiC. The small circular black region corresponds to pores.
consisted of fine, granular WC grains. Relatively large bright gray grains surrounded by pores [Fig. 5 (c)] correspond to the (Ta, W)C solid solution. WC ceramics containing 0–0.5 and 4–6 mol% TaC sintered at 1700 °C contain a large number of pores. The microstructures of the TaC-added WC ceramics above 4 mol% TaC consisted of very fine equiaxed granular WC grains because the sintering had not progressed sufficiently. The quantity of pores corresponding to the small circular black region decreased with the addition of 1–2 mol% TaC. The decrease in the number of pores is related to the change in the constituent phase of the TaC-added WC ceramics. The added 1–2 mol% TaC was completely converted into a solid solution. However, in the other compositions, the added TaC remained as TaC. The addition of 1–2 mol % TaC to WC ceramics improved the sinterability of WC, but it is not clear why the sinterability of the WC ceramics improved. Fig. 5(f) shows the microstructure of the TiC-added WC ceramic; the circular dark gray regions larger than the surrounding bright gray grains correspond to the (Ti, W)C solid solution. The TiC-added WC ceramics also had fine, equiaxed granular WC grains. When WC ceramics consisted of only the WC phase without the W2C phase in the carbon-added WC ceramics, WC grains grew during the sintering process [1]. Large WC grain growth was not observed in TiC-added WC ceramics without a W2C-type solid solution. Therefore, TiC inhibited WC grain growth. Fig. 6 shows the average size of WC grains in TaC- and TiC-added WC ceramics as a function of TaC and TiC amounts. The WC grain size
Fig. 6. Average grain size of TaC- and TiC-added WC ceramics as a function of additive amount.
small TiC addition along the dotted line produces a WC-(Ti, W)C1 –δ coexistence phase [(Ti, W)C1 −δ: (Ti, W)C solid solution]. This reaction sequence is consistent with the results shown in Fig. 3(b). Fig. 5(a)–(e) shows the microstructure of TaC-added WC ceramics. The small circular black regions are pores. The TaC-added WC ceramics
170
International Journal of Refractory Metals & Hard Materials 82 (2019) 167–173
A. Nino, et al.
for TaC-added WC ceramics sintered at 1700 °C decreased with increasing TaC amount, from 0.44 μm for the WC ceramic to 0.28 μm at 4 mol% TaC. Upon the addition of 1–2 mol% TaC, slightly larger grains were occasionally visible in the micrographs, but many very small WC grains were photographed. The average grain size is believed to be smaller than that of WC ceramics without TaC due to the large number of small grains. Above 4 mol% TaC, only small grains were observed in the micrograph, and the average grain size decreased. The TaC-added WC ceramics sintered at 1800 °C had larger WC grains than those sintered at 1700 °C. At 1700 °C, the addition of 1 mol% TaC to WC ceramics was most effective in densifying the sintered bodies and inhibiting the WC grain growth because the sinterability decreased considerably above 4 mol% TaC. The WC grain size for TiC-added WC ceramics showed only a slight reduction, from 0.49 μm for the WC ceramic sintered at 1800 °C to 0.45 μm for 6 mol% TiC addition. TaC addition to WC above 2 mol% and ZrC addition above 2 mol% [12] inhibits not only WC grain growth, but also densification. TiC did not hinder densification with inhibition of WC grain growth in the addition range investigated (6 mol%). Other transition metal carbides, i.e., Cr3C2 and NbC, also inhibit WC grain growth in small additions, with little to no relative density reduction for WC-based ceramics [14,15,17].
Fig. 8. Vickers hardness results for the TaC- and TiC-added WC ceramics as a function of additive amount.
was slightly lower than that calculated using the isostrain model, of 694 GPa. Fig. 8 shows the change in Vickers hardness for TaC- and TiC-added WC ceramics as a function of TaC and TiC addition. The Vickers hardness of the WC ceramic sintered at 1700 °C was lower than that of the ceramic sintered at 1800 °C due to its higher porosity. The hardness of the TaC-added WC ceramics sintered at 1700 °C increased from 19.9 GPa without TaC to 25.1 GPa with 1 mol% TaC, due to densification of the ceramic and inhibition of WC grain growth. The hardness of dense, polycrystalline WC-based ceramics and other polycrystalline ceramics increased with decreasing grain size, according to the Hall–Petch-like relationship [1,12,16,19,28]. The TaC-added WC ceramics with 1–2 mol% TaC densely sintered at 1700 °C had higher hardness values than those of pure WC and TaC-added WC ceramics densely sintered at 1800 °C, corresponding to their WC grain size. The low hardness of the TaC-added WC ceramics, with the exception of the 1–2 mol% TaC addition, was attributed to the high porosity of the samples. The hardness change with TaC addition was similar to the relative density change. The hardness of ceramics is related not only to the hardness of the material itself, but also to the relative densities of the sintered bodies and the average grain size. The relative density increased significantly upon the addition of 1–2 mol% TaC, and this increase in relative density was the main factor driving the increase in the hardness. The increase in the relative density raised the hardness of the sintered bodies for TiC [29,30], WC [31,32], and composite ceramics [33]. There was no apparent change in the hardness of WC ceramics with and without TiC sintered at 1800 °C. The TiC-added WC ceramics were densely sintered in the range of 0–6 mol% TiC; the average WC grain size changed only slightly with TiC addition. Fig. 9 shows the WC grain size dependence of the Vickers hardness for TaC- and TiC-added WC ceramics; the Hall–Petch-like relationship reported for pure WC ceramics is indicated as dotted lines in the figure. The hardness value of TaC-added WC ceramics containing many pores was located below the lines. The relationship for dense TaC- and TiCadded WC ceramics was close to that of pure WC ceramics [1,12]. The relationship for TaC-added WC ceramics with low relative densities, under 94%, is indicated by circular solid marks, and did not obey the Hall-Petch like relationship of pure WC ceramics. As reported in the case of WC and other ceramics [29–33], the decrease in the relative density of sintered bodies reduces the hardness. The hardness of the densely sintered TaC-added WC ceramics did not decrease. The deviation from the Hall-Petch like relationship for dense ceramics is largely influenced by the relative density. The abnormal growth of WC grains was not evident in TaC- and TiC-added WC ceramics; hence, the WC grain size range in TaC- and TiC-added WC ceramics was narrow. Fig. 10 shows the fracture toughness for the TaC- and TiC-added WC ceramics. The fracture toughness of dense pure WC was 5.9 MPa m0.5.
3.3. Mechanical properties Fig. 7 shows the Young's modulus values of TaC- and TiC-added WC ceramics; on XRD analysis, there was no preferred orientation in either ceramic. Young's modulus for TaC-added WC ceramics sintered at 1700 °C increased from 586 GPa for no TaC addition to 705 GPa at 1 mol% TaC. The Young's modulus of fully dense WC ceramics sintered at 1800 °C was ~710 GPa in this study and in other reports [1,2]. The Young's modulus of the WC ceramic sintered at 1700 °C without carbide addition was much lower than that sintered at 1800 °C, due to its high porosity. The Young's modulus of dense WC ceramics containing 1–2 mol% TaC above 1700 °C was comparable to that of pure WC ceramics. The Young's modulus of fully dense pure TaC is 558 GPa [20]. Given that TaC has a relatively high Young's modulus value, the Young's modulus at 1 mol% TaC was predicted to be 708 GPa using Voigt's isostrain model. The Young's modulus of TaC-added WC ceramics at 1800 °C decreased slightly from 709 GPa when no TaC was added, to 698 GPa at 2 mol% TaC. The TaC-added WC ceramics were densely sintered at 1800 °C. The slight decrease in the Young's modulus was related to the addition of TaC. The Young's modulus for TiC-added WC ceramics decreased from 709 GPa for no TiC addition to 670 GPa at 6 mol% TiC. The slight decrease in the Young's modulus with TiC addition was attributed to the Young's modulus of TiC (454 GPa) [2], which is lower than that of pure WC. The Young's modulus at 6 mol%
Fig. 7. The Young's modulus of TaC- and TiC-added WC ceramics as a function of additive amount. The dotted and dot-dash lines show the calculated lines using Voigt's isostress model. 171
International Journal of Refractory Metals & Hard Materials 82 (2019) 167–173
A. Nino, et al.
Fig. 11. Crack propagation of Vickers indentation in the WC–1 mol% TaC ceramic (backscattered electron image).
Fig. 11 shows the crack propagation in the WC–1 mol% TaC ceramic sintered at 1700 °C, which had a higher fracture toughness value. The indentation crack for the ceramic propagated both along grain boundaries and through WC grains. No remarkable crack deflection was observed in the ceramic. The reason why the fracture toughness of WC ceramics was increased by the addition of 1 mol% TaC was not elucidated in this study.
Fig. 9. Vickers hardness values of dense TaC- and TiC-added WC ceramics as a function of average WC grain size. The closed circles correspond to TaC-added WC ceramics that were not densely sintered.
4. Conclusions WC ceramics with TaC and TiC were densely sintered under conditions of 1–2 mol% TaC at 1700 °C, 0–2 mol% TaC at 1800 °C, and 0–6 mol% TiC at 1800 °C. The addition of TaC to WC ceramics produced the solid solutions of (W, Ta)2C and (Ta, W)C as reaction products. In dense WC ceramics containing 1–2 mol% TaC, the added TaC converted completely to the solid solutions. In dense WC ceramics containing 1–2 mol% TaC, the added TaC was completely converted into solid solution. The images of the microstructure show that the number of pores was reduced dramatically compared to that with no TaC. No W2Ctype solid solution formed in the TiC-added WC ceramics. The added TiC converted completely to the (Ti, W)C solid solution. No large WC grains formed in the TiC-added WC ceramics. TiC is an effective grain growth inhibitor for WC ceramics. The Young's modulus for TaC-added WC ceramics sintered at 1700 °C increased from 586 GPa without TaC to 705 GPa with 1 mol% TaC, and its value is close to that of dense pure WC. The Young's modulus of TaC-added WC ceramics at 1800 °C decreased slightly from 709 GPa for no TaC addition, to 698 GPa at 2 mol % TaC. This was due to the relatively low Young's modulus of TaC. The addition of TiC to WC ceramics caused a small decrease in the Young's modulus due to the relatively low Young's modulus of TiC. The TaC addition to WC ceramics at 1700 °C raised the hardness from 19 GPa without TaC to 25.1 GPa for 1 mol% TaC. The TiC-added WC ceramics retained a high hardness value of ~24 GPa in the range of 0–6 mol% TiC. The relationship between the hardness of only dense TaC- and TiCadded WC ceramics and the average grain size was close to that for pure WC ceramics. The relationship for TaC-added WC ceramics with low relative densities does not obey the Hall-Petch like relationship of pure WC ceramics. The fracture toughness of the TaC-added WC ceramics sintered at 1700 °C increased from 5.7 MPa m0.5 without TaC to 6.7 MPa m0.5 for 1 mol% TaC. A small addition of TaC to WC ceramics at 1700 °C improved the hardness and fracture toughness. The fracture toughness of the TiC-added WC ceramics hardly varied with the amount of TiC because (Ti, W)C has a high fracture toughness.
Fig. 10. Fracture toughness of TaC- and TiC-added WC ceramics as a function of additive amount.
WC ceramics without TaC and TiC sintered at 1700 and 1800 °C had fracture toughness values close to the reported value [1]. The fracture toughness of TaC-added WC ceramics sintered at 1700 °C increased from 5.7 for pure WC to 6.7 MPa m0.5 for 1 mol% TaC, and decreased above 1 mol% TaC. The WC–1 mol% TaC ceramics sintered at 1650 and 1700 °C had high fracture toughness values of 6.5 and 6.7 MPa m0.5, respectively. The WC ceramics containing 1.5–2 mol% TaC sintered at 1700 °C, and the WC–TaC ceramics sintered at 1800 °C, had a similar fracture toughness to that of pure WC. The WC ceramic containing 1 mol% TaC obviously had a higher fracture toughness than the other TaC-added WC ceramics. The constituent phase of TaC-added WC ceramics was the same, in the range of 1–2 mol% TaC. If the new phase (Ta, W)C has an effect, the fracture toughness of the TaC-added WC ceramics should be similar upon the addition of 1–2 mol% TaC. Only the WC ceramic containing 1 mol% TaC had a high fracture toughness value, so this result cannot be explained by the contribution of (Ta, W)C. Above 4 mol% TaC, the fracture toughness was lower than that of pure WC ceramics, due to its high porosity. The fracture toughness of WC ceramics changed slightly with the addition of TiC, ranging between 5.6 and 6.0 MPa m0.5. TiC-added WC ceramics consisted of WC and (Ti, W)C solid solution. Jung and Kang reported the fracture toughness of (Ti, W)C at various Ti and W ratios, and these values were 6.4–7.7 MPa m0.5 [34]. The fracture toughness was maintained at that of pure WC due to the formation of (Ti, W)C, which has a high fracture toughness. The fracture toughness did not increase upon the formation of (Ti, W)C due to the small amount of TiC additive.
Acknowledgement This work was partially supported by Japan Society for the Promotion of Science KAKENHI, Grants-in-Aid for Young Scientists (B), 17K14841. 172
International Journal of Refractory Metals & Hard Materials 82 (2019) 167–173
A. Nino, et al.
References
[18] A. Nino, T. Sekine, K. Sugawara, S. Sugiyama, H. Taimatsu, Effect of added Cr3C2 on the microstructure and mechanical properties of WC-SiC ceramcis, Key Eng. Mater. 656-657 (2015) 33–38. [19] A. Nino, Y. Nakaibayashi, Shigeaki Sugiyama, Hitoshi Taimatsu, Effect of Mo2C addition on the microstructures and mechanical properties of WC-SiC ceramics, Int. J. Refract. Met. Hard Mater. 64 (2017) 35–39. [20] A. Nino, T. Hirabara, S. Sugiyama, H. Taimatsu, Preparation and characterization of tantalum carbide (TaC) ceramics, Int. J. Refract. Met. Hard Mater. 52 (2015) 203–208. [21] A.E. McHale (Ed.), Figure 9024 in “Phase Equilibria Diagrams Phase Diagrams for Ceramists Vol. X Borides, Carbides, and Nitrides”, The American Ceramic Society, Westerville, OH, 1994, p. 351. [22] H.E. Exner, Physical and chemical nature of cemented carbides, Int. Met. Rev. 243 (1979) 149–173. [23] A.E. McHale (Ed.), Figure 9029 in “Phase Equilibria Diagrams Phase Diagrams for Ceramists Vol. X Borides, Carbides, and Nitrides”, The American Ceramic Society, Westerville, OH, 1994, p. 366. [24] C.B. Ponton, R.D. Rawlings, Vickers indentation fracture toughness test part 1 review of literature and formulation of standardised indentation toughness equations, Mater. Sci. Technol. 5 (1989) 865–872. [25] Powder Diffraction Files, JCPDS-International Center for Diffraction Data, Pennsylvania, 1994, pp. 25–1047. [26] Powder Diffraction Files, JCPDS-International Center for Diffraction Data, Pennsylvania, 1994, pp. 35–801. [27] Powder Diffraction Files, JCPDS-International Center for Diffraction Data, Pennsylvania, 1994, pp. 32–1383. [28] R.W. Rice, C.C. Wu, F. Borchelt, Hardness–grain-size relations in ceramics, J. Am. Ceram. Soc. 77 (1994) 2539–2553. [29] A. Babapoor, M.S. Asl, Z. Ahmadi, A.S. Namini, Effects of spark plasma sintering temperature on densification, hardness and thermal conductivity of titanium carbide, Ceram. Int. 44 (2018) 14541–14546. [30] A.S. Namini, Z. Ahmadi, A. Babapoor, M. Shokouhimehr, M.S. Asl, Microstructure and thermomechanical characteristics of spark plasma sintered TiC ceramics doped with nano-sized WC, Ceram. Int. 45 (2019) 2153–2160. [31] W. Tang, L. Zhang, J. Zhu, Y. Chen, W. Tian, T. Liu, Effect of direct current patterns on densification and mechanical properties of binderless tungsten carbides fabricated by the spark plasma sintering system, Int. J. Ref. Met. Hard Mater. 64 (2017) 90–97. [32] J. Wang, D. Zuo, L. Zhu, W. Li, Z. Tu, S. Dai, Effects and influence of Y2O3 addition on the microstructure and mechanical properties of binderless tungsten carbide fabricated by spark plasma sintering, Int. J. Ref. Met. Hard Mater. 71 (2018) 167–174. [33] W. Chen, H. Lin, P.K. Nayak, J. Huang, Material properties of tungsten carbidealumina composites fabricated by spark plasma sintering, Ceram. Int. 40 (2014) 15007–15012.
[1] A. Nino, K. Takahashi, S. Sugiyama, H. Taimatsu, Effects of carbon addition on microstructures and mechanical properties of binderless tungsten carbide, Mater. Trans. 53 (2012) 1475–1480. [2] A. Nino, A. Tanaka, S. Sugiyama, H. Taimatsu, Indentation size effect for the hardness of refractory carbides, Mater. Trans. 51 (2010) 1621–1626. [3] L.E. Toth, Transition Metal Carbides and Nitrides, Academic Press, New York and London, 1971. [4] C. Kral, W. Lengauer, D. Rafaja, P. Ettmayer, Critical review on the elastic properties of transition metal carbides, nitrides and carbonitrides, J. Alloys Compd. 265 (1998) 215–233. [5] A.T. Santhanam, Application of transition metal carbides and nitrides in industrial tools, in: S.T. Oyama (Ed.), The Chemistry of Transition Metal Carbides and Nitrides, Springer, Dordrecht, 1996, p. 29. [6] P.T.B. Shaffer, Handbook of High Temperature Materials: No. 1 Materials Index, Plenum Press, New York, 1964. [7] S.G. Huang, K. Vanmeensel, O. Van der Biest, J. Vleugels, Binderless WC and WC–VC materials obtained by plused electric current sintering, Int. J. Refract. Met. Hard Mater. 26 (2008) 41–47. [8] S. Imasato, K. Tokumoto, T. Kitada, S. Sakaguchi, Properties of ultra-fine grain binderless cemented caribide ‘RCCFN’, Int. J. Refract. Met. Hard Mater. 13 (1995) 305–312. [9] R.T. Fox, R. Nilsson, Binderless tungsten carbide carbon control with pressureless sintering, Int. J. Refract. Met. Hard Mater. 76 (2018) 82–89. [10] J. Zhao, T. Holland, C. Unuvar, Z.A. Munir, Spark plasma sintering of nanometric tungsten carbide, Int. J. Refract. Met. Hard Mater. 27 (2009) 130–139. [11] H. Taimatsu, S. Sugiyama, Y. Kodaira, Synthesis of W2C by reactive hot pressing and its mechanical properties, Mater. Trans. 49 (2008) 1256–1261. [12] H.C. Lee, J. Gurland, Hardness and deformation of cemented tungsten carbide, Mat. Sci. Eng. 33 (1978) 125–133. [13] J. Poetschke, V. Richter, T. Gestrich, A. Michaelis, Grain growth during sintering of tungsten carbide ceramics, Int. J. Refract. Met. Hard Mater. 43 (2014) 309–316. [14] J. Poetschke, V. Richter, R. Holke, Influence and effectivity of VC and Cr3C2 grain growth inhibitors on sintering of binderless tungsten carbide, Int. J. Refract. Met. Hard Mater. 31 (2012) 218–223. [15] H. Taimatsu, S. Sugiyama, M. Komatsu, Effects of Cr3C2 and V8C7 on the microstructure and mechanical properties of WC-SiC whisker ceramics, Mater. Trans. 50 (2009) 2435–2440. [16] A. Nino, Y. Izu, T. Sekine, S. Sugiyama, H. Taimatsu, Effects of ZrC and SiC addition on the microstructures and mechanical properties of binderless WC, Int. J. Refract. Met. Hard Mater. 69 (2017) 259–265. [17] A. Nino, N. Takahashi, S. Sugiyama, H. Taimatsu, Effects of carbide grain growth inhibitors on the microstructures and mechanical properties of WC-SiC-Mo2C hard ceramics, Int. J. Refract. Met. Hard Mater. 43 (2014) 150–156.
173