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Microstructure and tribo-mechanical properties of ultrafine Ti(CN) cermets
International Journal of Refractory Metals & Hard Materials 20 (2002) 207–211 www.elsevier.com/locate/ijrmhm
Microstructure and tribo-mechanical properties of ultrafine Ti(CN) cermets E.T. Jeon, J. Joardar, S. Kang
*
School of Materials Science and Engineering, College of Engineering, Seoul National University, Kwanak-ku, Seoul 151-742, South Korea Received 18 June 2001; accepted 30 November 2001
Abstract Role of Ti(CN) particle size on the microstructure of the Ti(CN)–WC–Ni cermets has been evaluated. The systems containing ultrafine grade Ti(CN) shows better structural homogeneity and a large volume fraction of rim phase in the core-rim structure when compared to the coarse Ti(CN) cermets. Measurable shift in the lattice parameter of the Ti(CN) core occurs in the ultrafine grade cermet possibly due to the diffusion of W and/or Ni. The improved structural features in the ultrafine grade led to considerably enhanced tribo-mechanical properties of the cermets. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Ti(CN); Cermets; Ultrafine; Microstructure; Wear
1. Introduction Development of Ti(CN) cermets has received significant research thrust in the field of cutting tool materials in recent years. The excellent wear resistance of Ti(CN), accompanied by good chemical stability at elevated temperature ensures enhanced cutting efficiency and tool life [1–3]. The Ti(C1 x Nx ) hard phases have been used invariably, with WC, Mo2 C and other secondary carbides to improve the sinterability, abrasion resistance and mechanical properties. In addition, Co, Ni, Fe, or a mixture of them, have been incorporated as binder for providing adequate toughness to the tool inserts [4–7]. The dissolution and reprecipitation of these carbides in binder phases lead to the formation of a typical core-rim structure [1,8–11], which is envisaged to be critical in controlling the material properties. The role of NbC, HfC, TaC, WC and other carbides in the Ti(CN) cermets has been studied in this regard in previous works [11–14]. However, the size effect of ultrafine Ti(CN) on the formation of the core-rim structure in Ti(CN)–WC– Ni cermets, and its influence on the mechanical properties is not known yet. The present work investigates into the role of sub-micron Ti(C0:5 N0:5 ) particles on the evolution of the microstructure in Ti(CN)–WC–Ni cer*
Corresponding author. Tel.: +82-2-880-7167; fax: +82-2-884-1578. E-mail address: [email protected] (S. Kang).
mets containing 0.4 lm WC particles, and its impact on the tribo-mechanical properties. 2. Experimental The various compositions investigated in the present study are enlisted in Table 1. Three different sets of samples, with Ti(CN) particle sizes of 0.7–0.95 (H.C. Starck GmbH), 1.4 and 3–5 lm (Kennametal Inc.) were blended with WC (Nanodyne Inc.; 0.4 lm) and Ni (INCO; 4.1 lm) in a horizontal ball mill at a ballto-powder weight ratio of 5. Milling was performed for 24 h under acetone using WC–Co balls. The milled powders were dried and compacted under uniaxial load of 100 MPa and were subsequently sintered in vacuum for 1 h at 1783 K. The sintered samples were examined for Vickers hardness, Hv , under indentation load of 20 kg and the fracture toughness was calculated from Hv using the expression derived by Shetty et al. [15]. Relative density of the sample was determined by Archimedes principle to assess the degree of porosity. The microstructural features were studied using SEM (JEOL JSM-5600) in back scattered mode and the volume fractions of the various phases were estimated by image analysis. X-ray diffraction (XRD) studies using Cu Ka radiation in M18XHF (MacScience Co.) system was also performed to monitor the phase evolution and/ or change in lattice constants during sintering. The wear
0263-4368/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 4 3 6 8 ( 0 2 ) 0 0 0 0 4 - 5
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Table 1 Compositions studied in the present work Sl. no.
Ti(C0:5 N0:5 ):WC
Ni (wt.%)
1 2 3
75:5 65:15 55:25
20 20 20
resistance of the samples was ascertained by sliding ballon-flat type wear test at 873 K temperature, using Si3 N4 ball indenter of 0.25 cm diameter under a fixed load of 28 N. The specimens were subjected to wear cycle of 10 mm s 1 for 1 h. The wear depth profile was analyzed using TENCOR alpha step-200 system.
3. Results and discussion 3.1. Microstructural evolution Fig. 1(a)–(c) illustrates comparative microstructural features evolved on sintering (at 1783 K for 1 h) of Ti(CN)–5WC–20Ni blends with varying Ti(CN) particle size. The microstructure shows a Ti(CN) core enveloped by a rim structure. Such structural development has been reported earlier [1,16] and has been attributed to the dissolution and subsequent reprecipitation of the saturated solutes on the undissolved Ti(CN). The solidstate diffusion of WC into Ti(CN) is known to play little
in the formation of a similar rim structure. The figure shows extensive rim formation in the ultrafine grade Ti(CN) (Fig. 1a) when compared to the coarse grades. This manifests fast dissolution of carbides apparently due to large surface area in the ultrafine Ti(CN) and WC particles [15]. Considering the rim and the enclosed core as single Ti(CN) particle, it appears that there is substantial coarsening in the ultrafine grade (0.7–0.95 lm), with the particle size reaching 1.5–2 lm for Ti(CN)– 5WC–20Ni (Fig. 1a). However, the resulting particle size is finer than that in coarse grades (Fig. 1b and c). It may be noted that the coarse 3–5 lm Ti(CN) shows insignificant growth under identical sintering conditions. Such behavior is in accordance with the surface energy minimization effect in the ultrafine particles. Furthermore, the ultrafine grade Ti(CN) seems to ensure a uniform distribution of the Ti(CN) core of the size near to the initial Ti(CN) particle size (0.7–0.95 lm). Such microstructure is expected to show improved and isotropic properties. An increase in the WC fraction in the hard phase shows a pronounced effect on the microstructure. As evident from Figs. 1(a)–(c) and 2(a)–(c), the formation of a two-layered rim structure with considerable thinning of the outer rim occurs with the increase in the WC content. The higher amount of WC also causes the development of discrete core-rim interface. These observations are due to the higher relative dissolution rate of WC when compared to that of Ti(CN), and to the stable
Fig. 1. Back scattered SEM images of Ti(CN)–5WC–20Ni with Ti(CN) particle size of (a) 0.7–0.95 lm (b) 1.4 lm and (c) 3–5 lm.
Fig. 2. SEM microstructures of Ti(CN)–25WC–20Ni with different Ti(CN) particle size of (a) 0.7–0.95 lm, (b) 1.4 lm and (c) 3–5 lm.
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209
Fig. 3. (a) XRD profiles of Ti(CN)–xWC–20Ni with ultrafine (0.7–0.95 lm) grade Ti(CN) on sintering at 1783 K for 1 h and (b) variation in the lattice parameter of core and rim fraction with change in WC level as estimated from the 200 peaks.
rim phase of high W-content, respectively [11]. Interestingly, XRD investigation (Fig. 3) indicates higher lattice mismatch between the core and rim phases with the increase in the WC content in the cermet. This is possibly caused by the existence of a W-rich inner rim layer in the cermet as reported in previous work [11,16,17]. Further studies based on convergent beam electron diffraction would be necessary to establish the precise change in the lattice parameters. The XRD profile also reveals a measurable shift in the lattice parameter of the Ti(CN) core with the increase in WC (Fig. 3). This was not observed in the coarse grade Ti(CN) in previous reports [11,12,14]. Such shift in the lattice parameter possibly reflect the diffusion of W and/ or Ni in the Ti(CN) core for submicron grade Ti(CN) cermet as found in a separate study with nano-WC–Co alloys [18]. However, a detailed investigation is warranted to establish this proposition. In general, the ultrafine grade Ti(CN) tends to have more fraction of solid solution rim phase, providing a uniform microstructure (Fig. 1a). Thus, it becomes more prone to coarsening. The addition of higher WC appears to restrict the grain coarsening in the ultrafine grade Ti(CN). As for example, the grain size reaches about 1– 1.2 lm in Ti(CN)–25WC–20Ni (Fig. 2a) when compared to 1.5–2 lm in Ti(CN)–5WC–20Ni (Fig. 1a). It is because extensive coarsening takes over after the fast completion of WC dissolution. The WC phase, which has a higher dissolution rate, suppresses the dissolution of Ti(CN). 3.2. Mechanical and tribological properties Table 2 shows the Vickers hardness, fracture toughness and the relative density values of the sintered Ti(CN) cermets. The ultrafine grade demonstrates enhanced hardness values when compared to the coarse grades. The coarse cermets were prone to severe fracture
Table 2 Mechanical properties of Ti(CN)–x(WC)–20Ni cermets sintered at 1783 K Ti(CN):WC
Particle size of Ti(CN) (in lm)
Hv (GPa)
KIC (MPa m1=2 )
Relative density
75:5 65:15 55:25 75:5 65:15 55:25
0.7–0.95 0.7–0.95 0.7–0.95 3–5 3–5 3–5
13.6 14.5 14.2 7.0 7.5 7.0
7.25 8.00 8.80 – – –
5.70 6.11 6.50 5.39 5.72 5.84
making it difficult to estimate the fracture toughness from the multiple crack paths. Such observation reflects the inferior toughness of the coarse cermets and is apparently due to its poor microstructural homogeneity (ref. Figs. 1 and 2) and high porosity. It may, however, be noted that the cermet compositions studied in the present work do not represent actual commercial cutting tool grades, which usually contains additional secondary carbides. Therefore, the hardness values are considerably low. Nevertheless, the present investigation provides preliminary estimates on the possibility of retaining the ultrafine Ti(CN) structure during sintering and comparative study on the dissolution–reprecipitation induced structural changes in the coarse and ultrafine cermets, and its influence on the cermet properties. The fact that the softer version with 5%WC shows poor fracture toughness compared to the harder version (25%) could be due to the porosity level as represented by the relative density values indicated in Table 2. The improved properties of the ultrafine Ti(CN)based products may be correlated to the well-developed rim structure and a uniformly dispersed fine-grained structure retained after sintering (Figs. 1a and 2a). The SEM image analyses of the cermet microstructures (Fig. 4) clearly indicate a notable increase in the rim fraction
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Fig. 4. Fraction of rim, core and binder phases in different Ti(CN)– xWC–20Ni samples.
for 0.7–0.95 lm grade when compared to the coarse 3–5 lm Ti(CN). Moreover, an increase in the binder phase fraction with the increase in WC (Fig. 4) is also likely to influence the hardness and fracture toughness of the cermets. A higher volume fraction of the binder phase in the samples with higher WC level is evident from the XRD patterns (Fig. 3). This is due to the high solubility limit of W (30 wt.% at 1500 °C) in the Ni binder. It may be noted that the coarse grades exhibit higher binder fraction than the ultrafine grade (Fig. 4). In spite of the relatively higher binder fraction in the coarse grades, the presence of higher porosity, as represented by lower relative density values (ref. Table 2), leads to poor properties. The extent of wear of the samples was estimated from the depth profile analysis of the wear zone produced during the wear test. As evident from Fig. 5, the ultrafine grade Ti(CN) cermet with 25%WC shows considerably higher wear resistance (lower wear depth) when compared to the corresponding coarse grade. However, at lower WC level (5%WC), the coarse grade shows relatively better wear properties under identical test conditions. The figure also shows the difference in the wear surfaces with a smooth and rough surface profile in the ultrafine and coarse grades, respectively. This suggests uniform wear rate over the entire wear zone in the former due to highly homogeneous distribution of the fine particles. As evident from Figs. 4 and 5, in spite of lower core fraction in the ultrafine grade at 25%WC, it shows better wear resistance than the corresponding coarse grade. The ultrafine grade has, however, higher rim fraction. On the other hand, lower core fraction along with a higher rim in the ultrafine grade at 5%WC
Fig. 5. Wear depths in different grades of Ti(CN)–xWC–20Ni samples. Inset shows the typical difference in the wear surface in the different grades as viewed by optical microscopy.
shows inferior wear behavior when compared to the corresponding coarse grade. Thus, the result implies that the phase fractions influence little on the wear behavior while the hard phase size and its distribution play an important role in wear of Ti(CN) cermets. Fig. 5 corroborates that the ultrafine grade cermets have comparable wear properties irrespective of the WC level. This is possibly because of similar degree of microstructural homogeneity (ref. Figs. 1a and 2a).
4. Conclusions 1. The use of ultrafine grade Ti(CN) led to a higher volume fraction of rim structure when compared to coarse-grained Ti(CN). The increase in the surface area of the Ti(CN) and WC particles resulted in extensive formation of (Ti,W)(CN) solid solution rim. 2. Substantial coarsening of the Ti(CN) particles occurred on sintering of the ultrafine Ti(CN)-based cermets containing small amount of WC. However, the coarsening tendency diminished as the WC content was increased. In general, the ultrafine grade system led to a much more homogenous microstructure. 3. Detectable shift in the lattice parameter of the Ti(CN) core in the ultrafine grade cermet was observed possibly due to W and/or Ni diffusion to the Ti(CN) cores. 4. Considerably improved hardness as well as fracture toughness was observed in the ultrafine grade Ti(CN) cermets when compared to conventional coarse Ti(CN)-based materials.
E.T. Jeon et al. / International Journal of Refractory Metals & Hard Materials 20 (2002) 207–211
5. The ultrafine Ti(CN), with >15wt.% WC of 0.4 lm in size, showed improved wear resistance at elevated temperature.
[5] [6] [7] [8] [9]
Acknowledgements This research was funded by the basic research program of the Korea Research Foundation (KRF) vide grant no. KRF-00-0422-E00133. One of the authors (JJ) is thankful to the Brain Korea 21 fellowship (Ministry of Education, Republic of Korea) for financial support. A special thanks goes to Bayer Korea Ltd. (H.C. Starck GmbH) for the supply of Ti(CN) powders and to Prof. D.S. Lim at Korea University in Seoul for the experimental assistance.
[10]
[11]
[12] [13] [14]
References [15] [1] Ettmayer P, Kolaska H, Lengauer W, Dreyer K. Ti(C,N) Cermets––Metallurgy and properties. Int J Refract Met Hard Mater 1995;13:343–51. [2] Prengel HJ, Pfouts WR, Santhanam AT. State of the art in hard coatings for carbide cutting tools. Surf Coat Tech 1998;102:183– 90. [3] Kang S. Stability of Ni in Ti(CN) solid solutions for cermet applications. Powder Metall 1997;40:139–42. [4] Oskarson RG et al. US patent 5503653, 1996.
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Grab GP et al. US patent 5976707, 1999. Rollander U et al. US patent 6004371, 1999. Heinrich HW et al. US patent 6024776, 2000. Ettmayer P, Lengauer W. The story of cermets. Powder Metall Int 1989;21:37–8. Rynemark M. Investigation of equilibria in the Ti–W–C–N system at 1750° C. Int J Refract Met Hard Mater 1991;10:185– 93. Doi A, Nomura T, Tobioka M, Takahashi K, Hara A. In: Bildstein H, Ortner HM, editors. Proceedings 11th International Plansee Seminar ’85. Reuttre, Triol, Austria, vol. 1. 1995. p. 825– 43. Ahn SY, Kang S. Formation of core/rim structures in Ti(C,N)– WC–Ni cermets via a dissolution and precipitation process. J Am Ceram Soc 2000;83:1489–94. Qi F, Kang S. A study on microstructural changes in Ti(CN)– NbC–Ni cermets. Mater Sci Eng A 1998;251:276–85. Mun S, Kang S. Effect of HfC addition on microstructure of Ti(CN)–Ni cermet system. Powder Metall 1999;42:251–6. Park YJ, Kim SW, Kang S. The formation of a solid solution band at the surface of Ti(CN)-based cermets. Mater Sci Eng A 2000;291:198–206. Shetty D, Wright I, Mincer P, Clauer AH. Indentation fracture of WC–Co cermets. J Mater Sci 1985;20:1873–82. Gee MG, Reece MJ, Roebuck B. High resolution electron microscopy of Ti(C,N) cermets. J Hard Mater 1992;3:119–42. Yoshimura H, Sugizawa T, Nichigaki K, Doi H. Reaction occurring during sintering and the characteristics of TiC–20TiN– 15WC–10TaC–9Mo–5.5Ni–11Co cermet. Int J Refract Hard Met 1983;2:170–4. Seo O, Kang S, Lavernia EJ. Nano-WC–Co alloys, unpublished work.
Microstructure and tribo-mechanical properties of ultrafine Ti(CN) cermets E.T. Jeon, J. Joardar, S. Kang
*
School of Materials Science and Engineering, College of Engineering, Seoul National University, Kwanak-ku, Seoul 151-742, South Korea Received 18 June 2001; accepted 30 November 2001
Abstract Role of Ti(CN) particle size on the microstructure of the Ti(CN)–WC–Ni cermets has been evaluated. The systems containing ultrafine grade Ti(CN) shows better structural homogeneity and a large volume fraction of rim phase in the core-rim structure when compared to the coarse Ti(CN) cermets. Measurable shift in the lattice parameter of the Ti(CN) core occurs in the ultrafine grade cermet possibly due to the diffusion of W and/or Ni. The improved structural features in the ultrafine grade led to considerably enhanced tribo-mechanical properties of the cermets. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Ti(CN); Cermets; Ultrafine; Microstructure; Wear
1. Introduction Development of Ti(CN) cermets has received significant research thrust in the field of cutting tool materials in recent years. The excellent wear resistance of Ti(CN), accompanied by good chemical stability at elevated temperature ensures enhanced cutting efficiency and tool life [1–3]. The Ti(C1 x Nx ) hard phases have been used invariably, with WC, Mo2 C and other secondary carbides to improve the sinterability, abrasion resistance and mechanical properties. In addition, Co, Ni, Fe, or a mixture of them, have been incorporated as binder for providing adequate toughness to the tool inserts [4–7]. The dissolution and reprecipitation of these carbides in binder phases lead to the formation of a typical core-rim structure [1,8–11], which is envisaged to be critical in controlling the material properties. The role of NbC, HfC, TaC, WC and other carbides in the Ti(CN) cermets has been studied in this regard in previous works [11–14]. However, the size effect of ultrafine Ti(CN) on the formation of the core-rim structure in Ti(CN)–WC– Ni cermets, and its influence on the mechanical properties is not known yet. The present work investigates into the role of sub-micron Ti(C0:5 N0:5 ) particles on the evolution of the microstructure in Ti(CN)–WC–Ni cer*
Corresponding author. Tel.: +82-2-880-7167; fax: +82-2-884-1578. E-mail address: [email protected] (S. Kang).
mets containing 0.4 lm WC particles, and its impact on the tribo-mechanical properties. 2. Experimental The various compositions investigated in the present study are enlisted in Table 1. Three different sets of samples, with Ti(CN) particle sizes of 0.7–0.95 (H.C. Starck GmbH), 1.4 and 3–5 lm (Kennametal Inc.) were blended with WC (Nanodyne Inc.; 0.4 lm) and Ni (INCO; 4.1 lm) in a horizontal ball mill at a ballto-powder weight ratio of 5. Milling was performed for 24 h under acetone using WC–Co balls. The milled powders were dried and compacted under uniaxial load of 100 MPa and were subsequently sintered in vacuum for 1 h at 1783 K. The sintered samples were examined for Vickers hardness, Hv , under indentation load of 20 kg and the fracture toughness was calculated from Hv using the expression derived by Shetty et al. [15]. Relative density of the sample was determined by Archimedes principle to assess the degree of porosity. The microstructural features were studied using SEM (JEOL JSM-5600) in back scattered mode and the volume fractions of the various phases were estimated by image analysis. X-ray diffraction (XRD) studies using Cu Ka radiation in M18XHF (MacScience Co.) system was also performed to monitor the phase evolution and/ or change in lattice constants during sintering. The wear
0263-4368/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 4 3 6 8 ( 0 2 ) 0 0 0 0 4 - 5
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E.T. Jeon et al. / International Journal of Refractory Metals & Hard Materials 20 (2002) 207–211
Table 1 Compositions studied in the present work Sl. no.
Ti(C0:5 N0:5 ):WC
Ni (wt.%)
1 2 3
75:5 65:15 55:25
20 20 20
resistance of the samples was ascertained by sliding ballon-flat type wear test at 873 K temperature, using Si3 N4 ball indenter of 0.25 cm diameter under a fixed load of 28 N. The specimens were subjected to wear cycle of 10 mm s 1 for 1 h. The wear depth profile was analyzed using TENCOR alpha step-200 system.
3. Results and discussion 3.1. Microstructural evolution Fig. 1(a)–(c) illustrates comparative microstructural features evolved on sintering (at 1783 K for 1 h) of Ti(CN)–5WC–20Ni blends with varying Ti(CN) particle size. The microstructure shows a Ti(CN) core enveloped by a rim structure. Such structural development has been reported earlier [1,16] and has been attributed to the dissolution and subsequent reprecipitation of the saturated solutes on the undissolved Ti(CN). The solidstate diffusion of WC into Ti(CN) is known to play little
in the formation of a similar rim structure. The figure shows extensive rim formation in the ultrafine grade Ti(CN) (Fig. 1a) when compared to the coarse grades. This manifests fast dissolution of carbides apparently due to large surface area in the ultrafine Ti(CN) and WC particles [15]. Considering the rim and the enclosed core as single Ti(CN) particle, it appears that there is substantial coarsening in the ultrafine grade (0.7–0.95 lm), with the particle size reaching 1.5–2 lm for Ti(CN)– 5WC–20Ni (Fig. 1a). However, the resulting particle size is finer than that in coarse grades (Fig. 1b and c). It may be noted that the coarse 3–5 lm Ti(CN) shows insignificant growth under identical sintering conditions. Such behavior is in accordance with the surface energy minimization effect in the ultrafine particles. Furthermore, the ultrafine grade Ti(CN) seems to ensure a uniform distribution of the Ti(CN) core of the size near to the initial Ti(CN) particle size (0.7–0.95 lm). Such microstructure is expected to show improved and isotropic properties. An increase in the WC fraction in the hard phase shows a pronounced effect on the microstructure. As evident from Figs. 1(a)–(c) and 2(a)–(c), the formation of a two-layered rim structure with considerable thinning of the outer rim occurs with the increase in the WC content. The higher amount of WC also causes the development of discrete core-rim interface. These observations are due to the higher relative dissolution rate of WC when compared to that of Ti(CN), and to the stable
Fig. 1. Back scattered SEM images of Ti(CN)–5WC–20Ni with Ti(CN) particle size of (a) 0.7–0.95 lm (b) 1.4 lm and (c) 3–5 lm.
Fig. 2. SEM microstructures of Ti(CN)–25WC–20Ni with different Ti(CN) particle size of (a) 0.7–0.95 lm, (b) 1.4 lm and (c) 3–5 lm.
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209
Fig. 3. (a) XRD profiles of Ti(CN)–xWC–20Ni with ultrafine (0.7–0.95 lm) grade Ti(CN) on sintering at 1783 K for 1 h and (b) variation in the lattice parameter of core and rim fraction with change in WC level as estimated from the 200 peaks.
rim phase of high W-content, respectively [11]. Interestingly, XRD investigation (Fig. 3) indicates higher lattice mismatch between the core and rim phases with the increase in the WC content in the cermet. This is possibly caused by the existence of a W-rich inner rim layer in the cermet as reported in previous work [11,16,17]. Further studies based on convergent beam electron diffraction would be necessary to establish the precise change in the lattice parameters. The XRD profile also reveals a measurable shift in the lattice parameter of the Ti(CN) core with the increase in WC (Fig. 3). This was not observed in the coarse grade Ti(CN) in previous reports [11,12,14]. Such shift in the lattice parameter possibly reflect the diffusion of W and/ or Ni in the Ti(CN) core for submicron grade Ti(CN) cermet as found in a separate study with nano-WC–Co alloys [18]. However, a detailed investigation is warranted to establish this proposition. In general, the ultrafine grade Ti(CN) tends to have more fraction of solid solution rim phase, providing a uniform microstructure (Fig. 1a). Thus, it becomes more prone to coarsening. The addition of higher WC appears to restrict the grain coarsening in the ultrafine grade Ti(CN). As for example, the grain size reaches about 1– 1.2 lm in Ti(CN)–25WC–20Ni (Fig. 2a) when compared to 1.5–2 lm in Ti(CN)–5WC–20Ni (Fig. 1a). It is because extensive coarsening takes over after the fast completion of WC dissolution. The WC phase, which has a higher dissolution rate, suppresses the dissolution of Ti(CN). 3.2. Mechanical and tribological properties Table 2 shows the Vickers hardness, fracture toughness and the relative density values of the sintered Ti(CN) cermets. The ultrafine grade demonstrates enhanced hardness values when compared to the coarse grades. The coarse cermets were prone to severe fracture
Table 2 Mechanical properties of Ti(CN)–x(WC)–20Ni cermets sintered at 1783 K Ti(CN):WC
Particle size of Ti(CN) (in lm)
Hv (GPa)
KIC (MPa m1=2 )
Relative density
75:5 65:15 55:25 75:5 65:15 55:25
0.7–0.95 0.7–0.95 0.7–0.95 3–5 3–5 3–5
13.6 14.5 14.2 7.0 7.5 7.0
7.25 8.00 8.80 – – –
5.70 6.11 6.50 5.39 5.72 5.84
making it difficult to estimate the fracture toughness from the multiple crack paths. Such observation reflects the inferior toughness of the coarse cermets and is apparently due to its poor microstructural homogeneity (ref. Figs. 1 and 2) and high porosity. It may, however, be noted that the cermet compositions studied in the present work do not represent actual commercial cutting tool grades, which usually contains additional secondary carbides. Therefore, the hardness values are considerably low. Nevertheless, the present investigation provides preliminary estimates on the possibility of retaining the ultrafine Ti(CN) structure during sintering and comparative study on the dissolution–reprecipitation induced structural changes in the coarse and ultrafine cermets, and its influence on the cermet properties. The fact that the softer version with 5%WC shows poor fracture toughness compared to the harder version (25%) could be due to the porosity level as represented by the relative density values indicated in Table 2. The improved properties of the ultrafine Ti(CN)based products may be correlated to the well-developed rim structure and a uniformly dispersed fine-grained structure retained after sintering (Figs. 1a and 2a). The SEM image analyses of the cermet microstructures (Fig. 4) clearly indicate a notable increase in the rim fraction
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Fig. 4. Fraction of rim, core and binder phases in different Ti(CN)– xWC–20Ni samples.
for 0.7–0.95 lm grade when compared to the coarse 3–5 lm Ti(CN). Moreover, an increase in the binder phase fraction with the increase in WC (Fig. 4) is also likely to influence the hardness and fracture toughness of the cermets. A higher volume fraction of the binder phase in the samples with higher WC level is evident from the XRD patterns (Fig. 3). This is due to the high solubility limit of W (30 wt.% at 1500 °C) in the Ni binder. It may be noted that the coarse grades exhibit higher binder fraction than the ultrafine grade (Fig. 4). In spite of the relatively higher binder fraction in the coarse grades, the presence of higher porosity, as represented by lower relative density values (ref. Table 2), leads to poor properties. The extent of wear of the samples was estimated from the depth profile analysis of the wear zone produced during the wear test. As evident from Fig. 5, the ultrafine grade Ti(CN) cermet with 25%WC shows considerably higher wear resistance (lower wear depth) when compared to the corresponding coarse grade. However, at lower WC level (5%WC), the coarse grade shows relatively better wear properties under identical test conditions. The figure also shows the difference in the wear surfaces with a smooth and rough surface profile in the ultrafine and coarse grades, respectively. This suggests uniform wear rate over the entire wear zone in the former due to highly homogeneous distribution of the fine particles. As evident from Figs. 4 and 5, in spite of lower core fraction in the ultrafine grade at 25%WC, it shows better wear resistance than the corresponding coarse grade. The ultrafine grade has, however, higher rim fraction. On the other hand, lower core fraction along with a higher rim in the ultrafine grade at 5%WC
Fig. 5. Wear depths in different grades of Ti(CN)–xWC–20Ni samples. Inset shows the typical difference in the wear surface in the different grades as viewed by optical microscopy.
shows inferior wear behavior when compared to the corresponding coarse grade. Thus, the result implies that the phase fractions influence little on the wear behavior while the hard phase size and its distribution play an important role in wear of Ti(CN) cermets. Fig. 5 corroborates that the ultrafine grade cermets have comparable wear properties irrespective of the WC level. This is possibly because of similar degree of microstructural homogeneity (ref. Figs. 1a and 2a).
4. Conclusions 1. The use of ultrafine grade Ti(CN) led to a higher volume fraction of rim structure when compared to coarse-grained Ti(CN). The increase in the surface area of the Ti(CN) and WC particles resulted in extensive formation of (Ti,W)(CN) solid solution rim. 2. Substantial coarsening of the Ti(CN) particles occurred on sintering of the ultrafine Ti(CN)-based cermets containing small amount of WC. However, the coarsening tendency diminished as the WC content was increased. In general, the ultrafine grade system led to a much more homogenous microstructure. 3. Detectable shift in the lattice parameter of the Ti(CN) core in the ultrafine grade cermet was observed possibly due to W and/or Ni diffusion to the Ti(CN) cores. 4. Considerably improved hardness as well as fracture toughness was observed in the ultrafine grade Ti(CN) cermets when compared to conventional coarse Ti(CN)-based materials.
E.T. Jeon et al. / International Journal of Refractory Metals & Hard Materials 20 (2002) 207–211
5. The ultrafine Ti(CN), with >15wt.% WC of 0.4 lm in size, showed improved wear resistance at elevated temperature.
[5] [6] [7] [8] [9]
Acknowledgements This research was funded by the basic research program of the Korea Research Foundation (KRF) vide grant no. KRF-00-0422-E00133. One of the authors (JJ) is thankful to the Brain Korea 21 fellowship (Ministry of Education, Republic of Korea) for financial support. A special thanks goes to Bayer Korea Ltd. (H.C. Starck GmbH) for the supply of Ti(CN) powders and to Prof. D.S. Lim at Korea University in Seoul for the experimental assistance.
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