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Influence of nitrogen Gas pressure on the miscibility Gap in the Ti–Zr carbonitride system
Int. Journal of Refractory Metals and Hard Materials 32 (2012) 11–15
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Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Influence of nitrogen Gas pressure on the miscibility Gap in the Ti–Zr carbonitride system Ida Borgh a, Susanne Norgren b, Annika Borgenstam a, John Ågren a,⁎ a b
Department of Materials Science and Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden Sandvik Mining and Construction R&D Competence Centre Hard Materials, SE-126 80 Stockholm, Sweden
a r t i c l e
i n f o
Article history: Received 18 April 2011 Accepted 26 December 2011 Keywords: Cemented carbides TiC ZrC Miscibility gap
a b s t r a c t The microstructure of cemented carbides with a gradient structure at the surface consists of WC, cubic carbonitrides and a binder phase. The carbonitrides can, for example, consist of Ti(C,N)–Zr(C,N) where it is reasonable to believe that there is a miscibility gap with Ti-rich and Zr-rich carbonitrides. In the present work, the effect of the N2-gas pressure on the equilibrium composition of the miscibility gap in the (Ti,Zr)(C,N) system has been investigated. In the study, the carbonitride system is in equilibrium with: WC, liquid binder, graphite and, N2-gas of different pressures. Both Fe and Co are used as binder phase to study the effect of the binder phase. The results verify that there is a miscibility gap in the carbonitride system and that the region of the miscibility gap will change when N is introduced. There is a critical N2-gas pressure lower than 0.1 bar and above that pressure the compositions of the carbonitride are rather constant as a result of the formation of a surface rim. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Cemented carbides are a composite material with excellent hardness and toughness commonly used for turning inserts. However, the requirements on the material properties differ depending on the application, and sometimes an enhanced hardness is needed. To achieve a higher hardness, the resistance to plastic deformation at high temperatures needs to be increased. Gradient sintering can be used to form gradients at the surface. According to Schwarzkopf et al. [1] the gradient is formed due to a connected diffusion between N and cubic-carbonitride-forming elements, and the gradient structure results in a material with a hard bulk and a ductile surface. The extension of the gradient can be controlled by, for instance, the sintering atmosphere. The microstructure then consists of WC, cubic carbonitrides, and a binder phase most commonly Co. Hence, the equilibrium composition of the carbonitrides is of interest for the production of cemented carbides. The cubic carbonitrides can dissolve many different alloying elements, and in most cases with full mutual solubility if the temperature is high enough. However, in some systems the solubility is limited, and a miscibility gap appears. Kieffer et al. [2] found a miscibility gap forming below a temperature of approximately 2373 K in the (Ti,Zr)C system. The (Ti,Zr)C system has been evaluated by Markström et al. [3] and their results also predicted a miscibility gap. Information for the (Ti,Zr)(C,N) is lacking, but due to observations in the
⁎ Corresponding author. Tel.: + 46 8 790 91 31; fax: + 46 8 10 04 11. E-mail address: [email protected] (J. Ågren). 0263-4368/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2011.12.014
carbide system, it is reasonable to believe that there is a miscibility gap extending to very high temperatures also in this system, resulting in Ti-rich and Zr-rich carbonitrides. In order to facilitate the reaction and the rearrangement of the elements, a liquid binder phase is added. It is known that sintering in an N2-gas atmosphere affects the surface zone of the Ti-rich and Zr-rich carbonitrides; thus, they can be used as gradient formers in cemented carbides. The C and N balance will affect the equilibrium composition, and to be able to predict the gradient, these effects must be known. Thus, in the present work, the effect of the N2-gas pressure on the equilibrium composition of the miscibility gap in the (Ti,Zr)(C,N) system has been investigated. Normally, when sintering cemented carbides the carbonitrides, are in equilibrium with WC, and therefore WC is included also in this study. However, the carbonitrides dissolve WC, and therefore the (Ti,Zr,W)(C,N) system is studied. The solubility of W is partly determined by the N activity, and according to Chen et al. [4] a low N activity gives a high solubility of W in the cubic carbonitride. In this study, equilibrium with graphite is also desired, in order to obtain a fixed carbon activity of unity. The N2-gas pressure is varied, and the effect of the C and N balance on the equilibrium composition has been studied. The work is a continuation of work by Crozet [5], who studied the effect of two different N2-gas pressures on the microstructure of samples with a similar composition as those studied in the present work. That study showed two different cubic carbonitride phases, and indicated that the average composition is inside the miscibility gap. In the present work, longer sintering times are used in order to reach equilibrium, and the effect of a third N2-gas pressure is studied.
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I. Borgh et al. / Int. Journal of Refractory Metals and Hard Materials 32 (2012) 11–15
Table 1 Sample composition for a total weight of 100 g. Composition
TiC [g]
ZrC [g]
C [g]
Fe [g]
Co [g]
W [g]
1. 2. 3. 4.
9.44 9.44 16.90 –
16.53 16.53 – 38.40
1.58 1.57 0.20 0.20
8.00 – 6.00 6.00
– 8.07 – –
Bal. Bal. Bal. Bal.
Ti–Zr–W–C–N–Fe Ti–Zr–W–C–N–Co Ti–W–C–N–Fe Zr–W–C–N–Fe
The present results are compared with a recent study of the miscibility gap in the (Ti,Zr,W)C system, with Co as binder phase, by Markström and Frisk [6].
Table 3 Average values from the EDS-analyses of the Ti and W content for the control samples. The Ti contents are shown with W as balance in the carbonitride phases. The analyses were performed at the surface of the samples. Composition
P [bar]
Ti–K [at.%]
Ti–K [at.%]
1. TiC60N40
2. TiC40N60
0.1
70.29
75.63
5.30
0.1
76.27
77.28
1.01
1 10
47.701 94.20
49.67 95.23
1.97 1.03
Control sample Sintering time 96 h
Difference [at.%]
Sintering time 120 h
Sintering time 96 h
2. Experimental work Table 1 shows the selected compositions of the investigated (Ti,Zr, W)(C,N) system. The total weight of each composition was 100 g. The effect of the binder phase on the equilibrium composition of (Ti,Zr,W) (C,N) was studied and two different elements were chosen, Fe and Co. For Fe, the Fe–C–N phase diagram is well known, and the solubility of C and N is higher than in Co, indicating that a faster rearrangement may occur. However, since Co is the most common binder phase in cemented carbides, it was included. To study the (Ti,Zr,W)(C,N) equilibrium phase composition and the effect of the binder phase, compositions 1 and 2 had equal compositions, except for the liquid binder. However, for this equilibrium it was assumed that the solubility of the liquid binder can be negligible in the carbonitrides in equilibrium with graphite. Compositions 3 and 4 were included in order to verify the absence of a miscibility gap for carbonitrides without both Ti and Zr being present. The samples were produced from TiC, ZrC, WC, Fe, Co and carbon black powders. The powders were milled for 8 h, dried and compacted to cylindrical small tablets. The sinterings were performed in a high-pressure furnace at 1500 °C. During sintering, the cubic carbides react with N2-gas forming carbonitrides. In order to study the effect of N, three different N2-gas pressures, 0.1, 1 and 10 bar, were used. Table 2 shows the sintering parameters. In order to establish that equilibrium with graphite was reached during sintering, an excess of C was added to get a carbon activity equal to one. Besides this, all samples were sintered on graphite foils and plates. After sintering, the samples were mounted in bakelite and polished with 9 and 1 μm diamond suspension in consecutive steps. To verify that C and N had been equilibrated, two control samples were included in all three sinterings. The two control samples contained WC, Fe as binder phase and Ti-carbonitrides, with different C and N compositions, TiC60N40 and TiC40N60. It was assumed that when the C and N contents were equal in the two Ti-carbonitrides, C and N are equilibrated in the Ti-carbonitrides, and thus also in the investigated (Ti,Zr,W)(C,N) systems. Since the W solubility in the Ti-carbonitride depends on the C and N content, it was used to determine if equilibrium was reached or not. The W and Ti contents were analyzed at the surface of the samples, and a limit of a maximum deviation of 2 at.% between the W and Ti contents between the two control samples was set. The approach was similar to the one used in an equilibrium study of Ti(C,N) performed by Frisk et al. [7].
The sintering times were selected, according to Frisk et al. [7], at 96 h for N2-gas pressures of 1 and 10 bar. The control samples showed a composition within the deviation limit indicating that equilibrium was reached. However, for the samples sintered at 0.1 bar it was found that the sintering time had to increase to 120 h in order to reach equilibrium. The compositions for the control samples from the different sinterings are shown in Table 3. To study the microstructure and to establish that all expected phases were present, Light Optical Microscopy and Scanning Electron Microscopy (SEM)(Zeiss Supra 55VP) were used. In the SEM, a quantitative analysis of the metallic elements was obtained using Energy Dispersive Spectroscopy (EDS) where both the surface and the center of the samples were analyzed. The EDS-analysis at the surface was performed approximately 50 to 100 μm from the outer surface. At both positions, a point analysis was performed in five different grains. An accurate quantitative analysis of all elements, including C and N, can normally be done using Wavelength Dispersive Spectroscopy analysis. However, because of an overlap of the wavelengths of Ti and N, this is difficult and has not been done in this work. 3. Results All expected phases, cubic carbonitride, WC, liquid binder and graphite, were observed in all samples. Fig. 1 a) and b) shows typical microstructures of the samples with compositions 1 and 2, respectively. Two separated homogeneous cubic carbonitride phases, one light and one dark gray, were observed. The white prismatic grains were WC, and the black areas were the binder phase, graphite or pores. This was in agreement with the studies by Crozet [5] and Markström and Frisk [6].
Table 2 Sintering parameters; temperature (T), time (t), and minimum (Min P), maximum (Max P) and mean (Mean P) values of the measured pressures in the furnace. T [°C]
P [bar]
t [h]
Max P [bar]
Min P [bar]
Mean P [bar]
1500 1500 1500
0.1 1 10
120 96 96
0.131 1.051 10.3
0.0768 0.888 9.9
0.102 0.999 10.0
Fig. 1. SEM image of a sample with a) composition 1 (Ti,Zr,W)(C,N) + Fe and b) composition 2 (Ti,Zr,W)(C,N) + Co sintered at 0.1 bar N2-gas pressure. Four different phases are seen where the white prismatic phase is WC, the dark gray phase is a (Ti, Zr,W)(C,N) with a high Zr content, the light gray phase is a (Ti,Zr,W)(C,N) with high contents of Ti and W, and the black areas are the binder phase or pores.
I. Borgh et al. / Int. Journal of Refractory Metals and Hard Materials 32 (2012) 11–15
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Fig. 4. SEM image of a sample with a) composition 3 (Ti,W)(C,N) + Fe and b) composition 4 (Zr,W)(C,N) + Fe sintered at 0.1 bar N2-gas pressure. Three different phases are seen where the white prismatic phase is WC, the gray phase is in a) (Ti,W)(C,N) and in b) (Zr,W)(C,N) + Fe. The black areas are the binder phase or pores.
Fig. 2. SEM image of the rim at the surface of the samples from composition 1 (Ti,Zr,W) (C,N) + Fe sintered at a) 0.1, b) 1 and c) 10 bar N2-gas pressure. Four different phases are seen where the white prismatic phase is WC, the dark gray phase is a (Ti,Zr,W) (C,N) with a high Zr content, the light gray phase is a (Ti,Zr,W)(C,N) with high contents of Ti and W and the black areas are the binder phase or pores.
All samples had a rim at the surface. In Figs. 2 and 3 the surface rim of the samples with compositions 1 and 2, respectively, is shown to illustrate the different thickness and appearance. Samples with compositions 3 and 4 show only one homogeneous carbonitride phase, as can be seen in Fig. 4, indicating the absence of miscibility gap in the carbonitride system if both Ti and Zr were not present. Table 4 shows Ti and Zr contents from the EDS-analyses of the carbonitride phases, with W as balancing element. EDS-analyses were performed on both the dark and the light gray carbonitride phases, for compositions 1 and 2. The analyses show that the dark gray carbonitride phase had a high Zr content, but a low W content. The light gray carbonitride phase had a high content of both Ti and W but a
low Zr content. Hereafter, the two carbonitride phases will be referred to as Zr-rich and Ti-rich, respectively. It was assumed that all reactions occurred during sintering. The measured values, of the content of the metallic elements, were plotted in isothermal sections at 1500 °C. The values were plotted as ufactions, which is the number of moles normalized by the number of the substitutional moles. In the isothermal sections, results for the three different N2-gas pressure were included for the surface and in the center, as shown in Fig. 5 a) and b), respectively. Both measured values for the (Ti,Zr,W)C, at 0 bar N2-gas pressure, and calculated limiting points for the miscibility gap in the (Ti,Zr)C, at 0 at.% W from Markström and Frisk [6] were also included in Fig. 5 a) and b). The sample with composition 2 sintered at 1 bar failed and could not be analyzed.
4. Discussion As expected, the results show a microstructure with two different cubic carbonitride phases in the (Ti,Zr,W)(C,N) system, which verify the presence of a miscibility gap. Fig. 1 a) and b), for compositions 1 and 2, shows no large differences of the microstructure evolution. This result from the present work is in agreement with results from a study by Crozet [5], who studied the microstructure evolution where similar compositions as in the present work were studied after being sintered at 1500°C in 1 and 10 bar N2-gas pressures.
Table 4 Average values from the EDS-analyses of the Ti, Zr and W content. The Ti and Zr contents are shown with W as balance in the carbonitride phases, both at the surface and in the center of the samples. Composition Surface Dark gray phase
Light gray phase
Center Dark gray phase Fig. 3. SEM image of the rim at the surface of the samples from composition 2 (Ti,Zr,W) (C,N) + Co sintered at a) 0.1 and b) 10 bar N2-gas pressure. Four different phases are seen where the white prismatic phase is WC, the dark gray phase is a (Ti,Zr,W)(C,N) with a high Zr content, the light gray phase is a (Ti,Zr,W)(C,N) with high contents of Ti and W and the black areas are the binder phase or pores.
Light gray phase
P [bar]
Ti–K [at.%]
Zr–L [at.%]
Ti–K [at.%]
Zr–L [at.%]
1. Ti–Zr–W–C–N–Fe
2. Ti–Zr–W–C–N–Co
0.1 1 10 0.1 1 10
6.57 7.29 6.78 50.83 48.88 51.11
82.87 81.46 80.13 9.96 11.70 11.92
6.52 – 7.14 49.85 – 51.17
81.37 – 83.66 10.39 – 9.93
0.1 1 10 0.1 1 10
8.37 7.87 7.49 46.77 46.72 54.098
72.58 73.44 80.64 13.34 13.10 13.668
7.68 – 8.42 46.68 – 46.354
75.12 – 73.81 12.08 – 13.512
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I. Borgh et al. / Int. Journal of Refractory Metals and Hard Materials 32 (2012) 11–15
a)
W
b)
W
1.0
1.0
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0 Zr 0
0 Zr 0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Ti
u-fraction Ti
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Ti
u-fraction Ti
Comp. 1, (Ti,Zr ,W)(C,N) +Fe, sintered at 0.1 bar N2-gas pressure
Comp. 2, (Ti,Zr,W )(C,N) +Co, sintered at 0.1 bar N2-gas pressure
Comp. 1, (Ti,Zr,W )(C,N) +Fe, sintered at 1 bar N2-gas pressure
Comp. 2, (Ti,Zr,W )(C,N) +Co, sintered at 10 bar N2-gas pressure
Comp. 1, (Ti,Zr,W )(C,N) +Fe, sintered at 10 bar N2-gas pressure
Markström and Frisk [6]. Measured values for (Ti,Zr,W )C Markström and Frisk [6]. Calculated values for (Ti,Zr )C
Fig. 5. Isothermal section at 1500 °C of the metallic elements in the carbonitride phase at a) the surface and at b) the center of the samples. The values are plotted as u-fractions, which is the number of moles normalized by the number of the substitutional moles. The results are compared with the result from Markström and Frisk [6], in the (Ti,Zr,W)C system at 0 bar N2-gas pressure and calculated limiting points for the miscibility gap in the (Ti,Zr)C system.
Comparing the EDS-analyses reveals a small difference between the compositions in the carbonitride phases at the surface and in the center. The difference is, however, so small that it should be reasonable to assume that equilibrium has been reached in the whole sample. This is the case for all samples except for the sample with composition 1 (Ti–Zr–W–C–N–Fe) sintered at 10 bar N2-gas pressure, which deviates. The reason for this will be discussed later but will first be excluded from the discussion. Aside from the small composition differences between the surface and center, a similar content of the metallic elements, achieved from the EDS-analyses, is observed by comparing samples sintered at different N2-gas pressures. Consequently, the composition seems to be independent of the N2-gas pressure. However, a difference is seen when the results are compared with the results from Markström and Frisk [6] for the (Ti,Zr,W)C, i.e. at 0 bar N2-gas pressure. However, the results show that the region of the miscibility gap will change when N is introduced. Due to the similar results for the different N2-gas pressures in this work, it is suggested that over a critical N2gas pressure the composition of the carbonitrides will not change, due to the formation of a rim at the surface. This can be understood by realizing that when the rim is formed, an equilibrium between the carbonitrides, WC and the N2-gas is reached, and consequently the compositions of the carbonitrides will not be changed. The lowest pressure in the present work is 0.1 bar, so consequently, the critical pressure must be even lower. Before the critical pressure is reached there will thus be no surface rim. The present observations can be compared with the results from an earlier work by one of the present authors [8], where the microstructure of the same samples was studied after a sintering temperature of 1850 °C. For samples with compositions 1 and 2, the surface rim has just started to form at a N2-gas pressure of 10 bar. Thus, we may conclude that the critical pressure is higher at a higher temperature, which probably will change the region of the miscibility gap. The observed surface rim formed during sintering might have affected the resulting composition of the carbonitride phases. Whether there is an effect depends on how the rim is formed. One explanation can be that the critical N2-gas pressure is reached at an early stage of sintering. At this stage, the rim starts to form as a layer with an open porosity, and the composition in the whole sample is given by the equilibrium between the carbonitrides, WC, liquid, graphite and N2gas. Consequently, equilibrium is reached prior to the formation of a
solid layered rim. The rim consists of a mixed cubic carbonitride, and grows by a connected diffusion of N and the cubic-carbonitrideforming elements, Ti and Zr. The diffusion through the surface rim is a rather slow process, while the diffusion in the bulk takes place in the liquid binder, which is a rather fast process, leading to equilibrium in the bulk. If the microstructures of the surface rim for the two samples sintered at 10 bar N2-gas pressure, as shown in Fig. 2 c) and Fig. 3 b), are compared, one can see that they have different appearance. For composition 1, with Fe as binder phase, the surface rim consists of a solid layer. However, for composition 2, with Co as binder phase, the layer seems more porous and seems to have formed a solid layer at a later stage during the sintering. So, an explanation for the deviating results for the sample with composition 1, exposed to 10 bars of N2-gas and with Fe as binder phase, may be a more rapid formation of a solid surface rim. This can be explained by an increased driving force for nitride formation due to the high pressure and a higher solubility of N in Fe than in Co. This can delay or even prevent the diffusion of N in the bulk of the sample, and the critical N2-gas pressure is not achieved. Figs. 2 and 3 show a change of the composition of the rim. This is probably caused by the fact that when a closed porosity, i.e. a solid layer, is reached, the entire surface region consists of the WC, and the carbonitride phases and the interior of the sample is no longer in equilibrium with the surrounding gas. The system changes from an open to a closed system. Therefore, the surface rim will start to equilibrate with the gas, and approach the local equilibrium between the carbonitrides and the N2-gas or between the carbonitrides, N2-gas and WC, determined by the present phases in the surface. The equilibrium is not invariant, and the composition of the cubic carbonitride can thus vary. However, after a while, due to grain growth of the carbonitrides, a solid layer of WC is formed. As there is no phase where the diffusion is rapid, in the surface rim, the carbonitrides facing the gas start to dissolve more N, given by the local equilibrium between the N2-gas and the carbonitride. The local equilibrium with the gas is then quicker to achieve than the equilibrium in the whole sample. By comparing compositions 1 and 2, one can also see that the different binder phase elements had no significant effect on the composition of the cubic carbonitrides, except for the sample with composition 1 (Ti–Zr–W–C–N–Fe) sintered at 10 bar N2-gas pressure. Since neither Fe nor Co is detected in the two cubic carbonitride
I. Borgh et al. / Int. Journal of Refractory Metals and Hard Materials 32 (2012) 11–15
phases, it seems that the assumption to neglect the solubility of the liquid binder elements in the carbonitrides is appropriate for this equilibrium. As mentioned, a difference is seen when the results of the carbonitride composition are compared with results from Markström and Frisk [6]. One can see that the tie lines in the isothermal section are shifted. In the present work the Ti-rich phase has a higher content of W and Zr, and the Zr-rich phase has a slightly higher content of Zr than reported by Markström and Frisk [6]. Both Ti and Zr have a high affinity to N and the solubility should be high. However, in agreement with the results from Chen et al. [4] a high solubility of N should lower the W content in the sample, since there is no attraction between these two elements. A contradiction between the present results and the results by Markström and Frisk [6] is that the carbide in their case contains a lower W content, while the absence of N should lead to a higher W content. The reason for the contradiction is not clear. Results from the EDS-analyses of the two carbonitride phases, both in the surface and in the center, show reasonable agreement with the calculated limiting points for the miscibility gap from Markström and Frisk [6], see Fig. 5. It should finally be emphasized that no spinodal-like structures have been observed. In all cases the formation of different compositions should be governed by relatively fast diffusion in the binder. 5. Conclusion The effect of the N2-gas pressure on the equilibrium composition of the miscibility gap in the (Ti,Zr)(C,N) system has been investigated. More work is needed to finally determine how N, below a critical N2gas pressure, affects the composition of the miscibility gap. However, some important experimental information is presented and some conclusions may be drawn: • For the (Ti,Zr,W)(C,N) system, a miscibility gap is observed with two cubic carbonitride phases. • There is no miscibility gap in the carbonitride system except when both Ti and Zr are present, since only one homogeneous carbonitride phase then is observed.
15
• The region of the miscibility gap will change when N is introduced. • There is a critical N2-gas pressure lower than 0.1 bar. At a pressure higher than the critical the composition of the carbonitride phases are rather constant as a result of the formation of a surface rim. • The selected binder phase element has no significant effect on the carbonitride composition, except for the case with N2-gas pressure of 10 bar and Fe as binder phase.. • No spinodal-like structure is observed in the two carbonitride phases.
Acknowledgments The authors would like to acknowledge personnel at Sandvik Tooling AB that have contributed to the work. The work is a part of the diploma work by the author I Borgh [8] and was sponsored and performed at Sandvik Tooling AB. The manuscript had been written within the frames of the VINN Excellence Center Hero-m, financed by VINNOVA, the Swedish Governmental Agency for Innovation Systems, Swedish industry, and KTH Royal Institute of Technology. References [1] Schwarzkopf M, Exner HE, Fischmeister HF, Schintlmeister W. Kinetics of compositional modification of (W, Ti)C–WC–Co alloy surfaces. Mater Sci Eng, A 1988:105–6 225-231. [2] Kieffer R, Nowotny H, Ettmayer P, Freudhofmeier M. Über die Beständigkeit von Übergangsmetallcarbiden gegen Stickstoff bis zu 300 at. Monatsh Chem 1970;101:65–82. [3] Markström A, Andersson D, Frisk K. Combined ab-initio and experimental assessment of A1âˆ'xBxC mixed carbides. Calphad 2008;32:615–23. [4] Chen L, Lengauer W, Dreyer K. Advances in modern nitrogen-containing hardmetals and cermets. Int J Refract Metals Hard Mater 2000;18:153–61. [5] Crozet C. Equilibrium studies of N-containing cemented carbides, Diploma work. Stockholm, Sweden: KIMAB; 2007. [6] Markström A, Frisk K. Experimental and thermodynamic evaluation of the miscibility gaps in MC carbides for the C–Co–Ti–V–W–Zr system. Calphad 2009;33:530–8. [7] Frisk K, Zackrisson J, Jansson B, Markström A. Experimental investigation of the equilibrium composition of titanium carbonitride and analysis using thermodynamic modelling. Metallkd Mater Res Adv Tech 2004;95:987–92. [8] Borgh I. Impact of nitrogen pressure on the carbonitride composition. Diploma work. Stockholm, Sweden: KTH Royal Institute of Technology; 2009.
Contents lists available at SciVerse ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Influence of nitrogen Gas pressure on the miscibility Gap in the Ti–Zr carbonitride system Ida Borgh a, Susanne Norgren b, Annika Borgenstam a, John Ågren a,⁎ a b
Department of Materials Science and Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden Sandvik Mining and Construction R&D Competence Centre Hard Materials, SE-126 80 Stockholm, Sweden
a r t i c l e
i n f o
Article history: Received 18 April 2011 Accepted 26 December 2011 Keywords: Cemented carbides TiC ZrC Miscibility gap
a b s t r a c t The microstructure of cemented carbides with a gradient structure at the surface consists of WC, cubic carbonitrides and a binder phase. The carbonitrides can, for example, consist of Ti(C,N)–Zr(C,N) where it is reasonable to believe that there is a miscibility gap with Ti-rich and Zr-rich carbonitrides. In the present work, the effect of the N2-gas pressure on the equilibrium composition of the miscibility gap in the (Ti,Zr)(C,N) system has been investigated. In the study, the carbonitride system is in equilibrium with: WC, liquid binder, graphite and, N2-gas of different pressures. Both Fe and Co are used as binder phase to study the effect of the binder phase. The results verify that there is a miscibility gap in the carbonitride system and that the region of the miscibility gap will change when N is introduced. There is a critical N2-gas pressure lower than 0.1 bar and above that pressure the compositions of the carbonitride are rather constant as a result of the formation of a surface rim. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Cemented carbides are a composite material with excellent hardness and toughness commonly used for turning inserts. However, the requirements on the material properties differ depending on the application, and sometimes an enhanced hardness is needed. To achieve a higher hardness, the resistance to plastic deformation at high temperatures needs to be increased. Gradient sintering can be used to form gradients at the surface. According to Schwarzkopf et al. [1] the gradient is formed due to a connected diffusion between N and cubic-carbonitride-forming elements, and the gradient structure results in a material with a hard bulk and a ductile surface. The extension of the gradient can be controlled by, for instance, the sintering atmosphere. The microstructure then consists of WC, cubic carbonitrides, and a binder phase most commonly Co. Hence, the equilibrium composition of the carbonitrides is of interest for the production of cemented carbides. The cubic carbonitrides can dissolve many different alloying elements, and in most cases with full mutual solubility if the temperature is high enough. However, in some systems the solubility is limited, and a miscibility gap appears. Kieffer et al. [2] found a miscibility gap forming below a temperature of approximately 2373 K in the (Ti,Zr)C system. The (Ti,Zr)C system has been evaluated by Markström et al. [3] and their results also predicted a miscibility gap. Information for the (Ti,Zr)(C,N) is lacking, but due to observations in the
⁎ Corresponding author. Tel.: + 46 8 790 91 31; fax: + 46 8 10 04 11. E-mail address: [email protected] (J. Ågren). 0263-4368/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2011.12.014
carbide system, it is reasonable to believe that there is a miscibility gap extending to very high temperatures also in this system, resulting in Ti-rich and Zr-rich carbonitrides. In order to facilitate the reaction and the rearrangement of the elements, a liquid binder phase is added. It is known that sintering in an N2-gas atmosphere affects the surface zone of the Ti-rich and Zr-rich carbonitrides; thus, they can be used as gradient formers in cemented carbides. The C and N balance will affect the equilibrium composition, and to be able to predict the gradient, these effects must be known. Thus, in the present work, the effect of the N2-gas pressure on the equilibrium composition of the miscibility gap in the (Ti,Zr)(C,N) system has been investigated. Normally, when sintering cemented carbides the carbonitrides, are in equilibrium with WC, and therefore WC is included also in this study. However, the carbonitrides dissolve WC, and therefore the (Ti,Zr,W)(C,N) system is studied. The solubility of W is partly determined by the N activity, and according to Chen et al. [4] a low N activity gives a high solubility of W in the cubic carbonitride. In this study, equilibrium with graphite is also desired, in order to obtain a fixed carbon activity of unity. The N2-gas pressure is varied, and the effect of the C and N balance on the equilibrium composition has been studied. The work is a continuation of work by Crozet [5], who studied the effect of two different N2-gas pressures on the microstructure of samples with a similar composition as those studied in the present work. That study showed two different cubic carbonitride phases, and indicated that the average composition is inside the miscibility gap. In the present work, longer sintering times are used in order to reach equilibrium, and the effect of a third N2-gas pressure is studied.
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I. Borgh et al. / Int. Journal of Refractory Metals and Hard Materials 32 (2012) 11–15
Table 1 Sample composition for a total weight of 100 g. Composition
TiC [g]
ZrC [g]
C [g]
Fe [g]
Co [g]
W [g]
1. 2. 3. 4.
9.44 9.44 16.90 –
16.53 16.53 – 38.40
1.58 1.57 0.20 0.20
8.00 – 6.00 6.00
– 8.07 – –
Bal. Bal. Bal. Bal.
Ti–Zr–W–C–N–Fe Ti–Zr–W–C–N–Co Ti–W–C–N–Fe Zr–W–C–N–Fe
The present results are compared with a recent study of the miscibility gap in the (Ti,Zr,W)C system, with Co as binder phase, by Markström and Frisk [6].
Table 3 Average values from the EDS-analyses of the Ti and W content for the control samples. The Ti contents are shown with W as balance in the carbonitride phases. The analyses were performed at the surface of the samples. Composition
P [bar]
Ti–K [at.%]
Ti–K [at.%]
1. TiC60N40
2. TiC40N60
0.1
70.29
75.63
5.30
0.1
76.27
77.28
1.01
1 10
47.701 94.20
49.67 95.23
1.97 1.03
Control sample Sintering time 96 h
Difference [at.%]
Sintering time 120 h
Sintering time 96 h
2. Experimental work Table 1 shows the selected compositions of the investigated (Ti,Zr, W)(C,N) system. The total weight of each composition was 100 g. The effect of the binder phase on the equilibrium composition of (Ti,Zr,W) (C,N) was studied and two different elements were chosen, Fe and Co. For Fe, the Fe–C–N phase diagram is well known, and the solubility of C and N is higher than in Co, indicating that a faster rearrangement may occur. However, since Co is the most common binder phase in cemented carbides, it was included. To study the (Ti,Zr,W)(C,N) equilibrium phase composition and the effect of the binder phase, compositions 1 and 2 had equal compositions, except for the liquid binder. However, for this equilibrium it was assumed that the solubility of the liquid binder can be negligible in the carbonitrides in equilibrium with graphite. Compositions 3 and 4 were included in order to verify the absence of a miscibility gap for carbonitrides without both Ti and Zr being present. The samples were produced from TiC, ZrC, WC, Fe, Co and carbon black powders. The powders were milled for 8 h, dried and compacted to cylindrical small tablets. The sinterings were performed in a high-pressure furnace at 1500 °C. During sintering, the cubic carbides react with N2-gas forming carbonitrides. In order to study the effect of N, three different N2-gas pressures, 0.1, 1 and 10 bar, were used. Table 2 shows the sintering parameters. In order to establish that equilibrium with graphite was reached during sintering, an excess of C was added to get a carbon activity equal to one. Besides this, all samples were sintered on graphite foils and plates. After sintering, the samples were mounted in bakelite and polished with 9 and 1 μm diamond suspension in consecutive steps. To verify that C and N had been equilibrated, two control samples were included in all three sinterings. The two control samples contained WC, Fe as binder phase and Ti-carbonitrides, with different C and N compositions, TiC60N40 and TiC40N60. It was assumed that when the C and N contents were equal in the two Ti-carbonitrides, C and N are equilibrated in the Ti-carbonitrides, and thus also in the investigated (Ti,Zr,W)(C,N) systems. Since the W solubility in the Ti-carbonitride depends on the C and N content, it was used to determine if equilibrium was reached or not. The W and Ti contents were analyzed at the surface of the samples, and a limit of a maximum deviation of 2 at.% between the W and Ti contents between the two control samples was set. The approach was similar to the one used in an equilibrium study of Ti(C,N) performed by Frisk et al. [7].
The sintering times were selected, according to Frisk et al. [7], at 96 h for N2-gas pressures of 1 and 10 bar. The control samples showed a composition within the deviation limit indicating that equilibrium was reached. However, for the samples sintered at 0.1 bar it was found that the sintering time had to increase to 120 h in order to reach equilibrium. The compositions for the control samples from the different sinterings are shown in Table 3. To study the microstructure and to establish that all expected phases were present, Light Optical Microscopy and Scanning Electron Microscopy (SEM)(Zeiss Supra 55VP) were used. In the SEM, a quantitative analysis of the metallic elements was obtained using Energy Dispersive Spectroscopy (EDS) where both the surface and the center of the samples were analyzed. The EDS-analysis at the surface was performed approximately 50 to 100 μm from the outer surface. At both positions, a point analysis was performed in five different grains. An accurate quantitative analysis of all elements, including C and N, can normally be done using Wavelength Dispersive Spectroscopy analysis. However, because of an overlap of the wavelengths of Ti and N, this is difficult and has not been done in this work. 3. Results All expected phases, cubic carbonitride, WC, liquid binder and graphite, were observed in all samples. Fig. 1 a) and b) shows typical microstructures of the samples with compositions 1 and 2, respectively. Two separated homogeneous cubic carbonitride phases, one light and one dark gray, were observed. The white prismatic grains were WC, and the black areas were the binder phase, graphite or pores. This was in agreement with the studies by Crozet [5] and Markström and Frisk [6].
Table 2 Sintering parameters; temperature (T), time (t), and minimum (Min P), maximum (Max P) and mean (Mean P) values of the measured pressures in the furnace. T [°C]
P [bar]
t [h]
Max P [bar]
Min P [bar]
Mean P [bar]
1500 1500 1500
0.1 1 10
120 96 96
0.131 1.051 10.3
0.0768 0.888 9.9
0.102 0.999 10.0
Fig. 1. SEM image of a sample with a) composition 1 (Ti,Zr,W)(C,N) + Fe and b) composition 2 (Ti,Zr,W)(C,N) + Co sintered at 0.1 bar N2-gas pressure. Four different phases are seen where the white prismatic phase is WC, the dark gray phase is a (Ti, Zr,W)(C,N) with a high Zr content, the light gray phase is a (Ti,Zr,W)(C,N) with high contents of Ti and W, and the black areas are the binder phase or pores.
I. Borgh et al. / Int. Journal of Refractory Metals and Hard Materials 32 (2012) 11–15
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Fig. 4. SEM image of a sample with a) composition 3 (Ti,W)(C,N) + Fe and b) composition 4 (Zr,W)(C,N) + Fe sintered at 0.1 bar N2-gas pressure. Three different phases are seen where the white prismatic phase is WC, the gray phase is in a) (Ti,W)(C,N) and in b) (Zr,W)(C,N) + Fe. The black areas are the binder phase or pores.
Fig. 2. SEM image of the rim at the surface of the samples from composition 1 (Ti,Zr,W) (C,N) + Fe sintered at a) 0.1, b) 1 and c) 10 bar N2-gas pressure. Four different phases are seen where the white prismatic phase is WC, the dark gray phase is a (Ti,Zr,W) (C,N) with a high Zr content, the light gray phase is a (Ti,Zr,W)(C,N) with high contents of Ti and W and the black areas are the binder phase or pores.
All samples had a rim at the surface. In Figs. 2 and 3 the surface rim of the samples with compositions 1 and 2, respectively, is shown to illustrate the different thickness and appearance. Samples with compositions 3 and 4 show only one homogeneous carbonitride phase, as can be seen in Fig. 4, indicating the absence of miscibility gap in the carbonitride system if both Ti and Zr were not present. Table 4 shows Ti and Zr contents from the EDS-analyses of the carbonitride phases, with W as balancing element. EDS-analyses were performed on both the dark and the light gray carbonitride phases, for compositions 1 and 2. The analyses show that the dark gray carbonitride phase had a high Zr content, but a low W content. The light gray carbonitride phase had a high content of both Ti and W but a
low Zr content. Hereafter, the two carbonitride phases will be referred to as Zr-rich and Ti-rich, respectively. It was assumed that all reactions occurred during sintering. The measured values, of the content of the metallic elements, were plotted in isothermal sections at 1500 °C. The values were plotted as ufactions, which is the number of moles normalized by the number of the substitutional moles. In the isothermal sections, results for the three different N2-gas pressure were included for the surface and in the center, as shown in Fig. 5 a) and b), respectively. Both measured values for the (Ti,Zr,W)C, at 0 bar N2-gas pressure, and calculated limiting points for the miscibility gap in the (Ti,Zr)C, at 0 at.% W from Markström and Frisk [6] were also included in Fig. 5 a) and b). The sample with composition 2 sintered at 1 bar failed and could not be analyzed.
4. Discussion As expected, the results show a microstructure with two different cubic carbonitride phases in the (Ti,Zr,W)(C,N) system, which verify the presence of a miscibility gap. Fig. 1 a) and b), for compositions 1 and 2, shows no large differences of the microstructure evolution. This result from the present work is in agreement with results from a study by Crozet [5], who studied the microstructure evolution where similar compositions as in the present work were studied after being sintered at 1500°C in 1 and 10 bar N2-gas pressures.
Table 4 Average values from the EDS-analyses of the Ti, Zr and W content. The Ti and Zr contents are shown with W as balance in the carbonitride phases, both at the surface and in the center of the samples. Composition Surface Dark gray phase
Light gray phase
Center Dark gray phase Fig. 3. SEM image of the rim at the surface of the samples from composition 2 (Ti,Zr,W) (C,N) + Co sintered at a) 0.1 and b) 10 bar N2-gas pressure. Four different phases are seen where the white prismatic phase is WC, the dark gray phase is a (Ti,Zr,W)(C,N) with a high Zr content, the light gray phase is a (Ti,Zr,W)(C,N) with high contents of Ti and W and the black areas are the binder phase or pores.
Light gray phase
P [bar]
Ti–K [at.%]
Zr–L [at.%]
Ti–K [at.%]
Zr–L [at.%]
1. Ti–Zr–W–C–N–Fe
2. Ti–Zr–W–C–N–Co
0.1 1 10 0.1 1 10
6.57 7.29 6.78 50.83 48.88 51.11
82.87 81.46 80.13 9.96 11.70 11.92
6.52 – 7.14 49.85 – 51.17
81.37 – 83.66 10.39 – 9.93
0.1 1 10 0.1 1 10
8.37 7.87 7.49 46.77 46.72 54.098
72.58 73.44 80.64 13.34 13.10 13.668
7.68 – 8.42 46.68 – 46.354
75.12 – 73.81 12.08 – 13.512
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I. Borgh et al. / Int. Journal of Refractory Metals and Hard Materials 32 (2012) 11–15
a)
W
b)
W
1.0
1.0
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0 Zr 0
0 Zr 0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Ti
u-fraction Ti
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Ti
u-fraction Ti
Comp. 1, (Ti,Zr ,W)(C,N) +Fe, sintered at 0.1 bar N2-gas pressure
Comp. 2, (Ti,Zr,W )(C,N) +Co, sintered at 0.1 bar N2-gas pressure
Comp. 1, (Ti,Zr,W )(C,N) +Fe, sintered at 1 bar N2-gas pressure
Comp. 2, (Ti,Zr,W )(C,N) +Co, sintered at 10 bar N2-gas pressure
Comp. 1, (Ti,Zr,W )(C,N) +Fe, sintered at 10 bar N2-gas pressure
Markström and Frisk [6]. Measured values for (Ti,Zr,W )C Markström and Frisk [6]. Calculated values for (Ti,Zr )C
Fig. 5. Isothermal section at 1500 °C of the metallic elements in the carbonitride phase at a) the surface and at b) the center of the samples. The values are plotted as u-fractions, which is the number of moles normalized by the number of the substitutional moles. The results are compared with the result from Markström and Frisk [6], in the (Ti,Zr,W)C system at 0 bar N2-gas pressure and calculated limiting points for the miscibility gap in the (Ti,Zr)C system.
Comparing the EDS-analyses reveals a small difference between the compositions in the carbonitride phases at the surface and in the center. The difference is, however, so small that it should be reasonable to assume that equilibrium has been reached in the whole sample. This is the case for all samples except for the sample with composition 1 (Ti–Zr–W–C–N–Fe) sintered at 10 bar N2-gas pressure, which deviates. The reason for this will be discussed later but will first be excluded from the discussion. Aside from the small composition differences between the surface and center, a similar content of the metallic elements, achieved from the EDS-analyses, is observed by comparing samples sintered at different N2-gas pressures. Consequently, the composition seems to be independent of the N2-gas pressure. However, a difference is seen when the results are compared with the results from Markström and Frisk [6] for the (Ti,Zr,W)C, i.e. at 0 bar N2-gas pressure. However, the results show that the region of the miscibility gap will change when N is introduced. Due to the similar results for the different N2-gas pressures in this work, it is suggested that over a critical N2gas pressure the composition of the carbonitrides will not change, due to the formation of a rim at the surface. This can be understood by realizing that when the rim is formed, an equilibrium between the carbonitrides, WC and the N2-gas is reached, and consequently the compositions of the carbonitrides will not be changed. The lowest pressure in the present work is 0.1 bar, so consequently, the critical pressure must be even lower. Before the critical pressure is reached there will thus be no surface rim. The present observations can be compared with the results from an earlier work by one of the present authors [8], where the microstructure of the same samples was studied after a sintering temperature of 1850 °C. For samples with compositions 1 and 2, the surface rim has just started to form at a N2-gas pressure of 10 bar. Thus, we may conclude that the critical pressure is higher at a higher temperature, which probably will change the region of the miscibility gap. The observed surface rim formed during sintering might have affected the resulting composition of the carbonitride phases. Whether there is an effect depends on how the rim is formed. One explanation can be that the critical N2-gas pressure is reached at an early stage of sintering. At this stage, the rim starts to form as a layer with an open porosity, and the composition in the whole sample is given by the equilibrium between the carbonitrides, WC, liquid, graphite and N2gas. Consequently, equilibrium is reached prior to the formation of a
solid layered rim. The rim consists of a mixed cubic carbonitride, and grows by a connected diffusion of N and the cubic-carbonitrideforming elements, Ti and Zr. The diffusion through the surface rim is a rather slow process, while the diffusion in the bulk takes place in the liquid binder, which is a rather fast process, leading to equilibrium in the bulk. If the microstructures of the surface rim for the two samples sintered at 10 bar N2-gas pressure, as shown in Fig. 2 c) and Fig. 3 b), are compared, one can see that they have different appearance. For composition 1, with Fe as binder phase, the surface rim consists of a solid layer. However, for composition 2, with Co as binder phase, the layer seems more porous and seems to have formed a solid layer at a later stage during the sintering. So, an explanation for the deviating results for the sample with composition 1, exposed to 10 bars of N2-gas and with Fe as binder phase, may be a more rapid formation of a solid surface rim. This can be explained by an increased driving force for nitride formation due to the high pressure and a higher solubility of N in Fe than in Co. This can delay or even prevent the diffusion of N in the bulk of the sample, and the critical N2-gas pressure is not achieved. Figs. 2 and 3 show a change of the composition of the rim. This is probably caused by the fact that when a closed porosity, i.e. a solid layer, is reached, the entire surface region consists of the WC, and the carbonitride phases and the interior of the sample is no longer in equilibrium with the surrounding gas. The system changes from an open to a closed system. Therefore, the surface rim will start to equilibrate with the gas, and approach the local equilibrium between the carbonitrides and the N2-gas or between the carbonitrides, N2-gas and WC, determined by the present phases in the surface. The equilibrium is not invariant, and the composition of the cubic carbonitride can thus vary. However, after a while, due to grain growth of the carbonitrides, a solid layer of WC is formed. As there is no phase where the diffusion is rapid, in the surface rim, the carbonitrides facing the gas start to dissolve more N, given by the local equilibrium between the N2-gas and the carbonitride. The local equilibrium with the gas is then quicker to achieve than the equilibrium in the whole sample. By comparing compositions 1 and 2, one can also see that the different binder phase elements had no significant effect on the composition of the cubic carbonitrides, except for the sample with composition 1 (Ti–Zr–W–C–N–Fe) sintered at 10 bar N2-gas pressure. Since neither Fe nor Co is detected in the two cubic carbonitride
I. Borgh et al. / Int. Journal of Refractory Metals and Hard Materials 32 (2012) 11–15
phases, it seems that the assumption to neglect the solubility of the liquid binder elements in the carbonitrides is appropriate for this equilibrium. As mentioned, a difference is seen when the results of the carbonitride composition are compared with results from Markström and Frisk [6]. One can see that the tie lines in the isothermal section are shifted. In the present work the Ti-rich phase has a higher content of W and Zr, and the Zr-rich phase has a slightly higher content of Zr than reported by Markström and Frisk [6]. Both Ti and Zr have a high affinity to N and the solubility should be high. However, in agreement with the results from Chen et al. [4] a high solubility of N should lower the W content in the sample, since there is no attraction between these two elements. A contradiction between the present results and the results by Markström and Frisk [6] is that the carbide in their case contains a lower W content, while the absence of N should lead to a higher W content. The reason for the contradiction is not clear. Results from the EDS-analyses of the two carbonitride phases, both in the surface and in the center, show reasonable agreement with the calculated limiting points for the miscibility gap from Markström and Frisk [6], see Fig. 5. It should finally be emphasized that no spinodal-like structures have been observed. In all cases the formation of different compositions should be governed by relatively fast diffusion in the binder. 5. Conclusion The effect of the N2-gas pressure on the equilibrium composition of the miscibility gap in the (Ti,Zr)(C,N) system has been investigated. More work is needed to finally determine how N, below a critical N2gas pressure, affects the composition of the miscibility gap. However, some important experimental information is presented and some conclusions may be drawn: • For the (Ti,Zr,W)(C,N) system, a miscibility gap is observed with two cubic carbonitride phases. • There is no miscibility gap in the carbonitride system except when both Ti and Zr are present, since only one homogeneous carbonitride phase then is observed.
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• The region of the miscibility gap will change when N is introduced. • There is a critical N2-gas pressure lower than 0.1 bar. At a pressure higher than the critical the composition of the carbonitride phases are rather constant as a result of the formation of a surface rim. • The selected binder phase element has no significant effect on the carbonitride composition, except for the case with N2-gas pressure of 10 bar and Fe as binder phase.. • No spinodal-like structure is observed in the two carbonitride phases.
Acknowledgments The authors would like to acknowledge personnel at Sandvik Tooling AB that have contributed to the work. The work is a part of the diploma work by the author I Borgh [8] and was sponsored and performed at Sandvik Tooling AB. The manuscript had been written within the frames of the VINN Excellence Center Hero-m, financed by VINNOVA, the Swedish Governmental Agency for Innovation Systems, Swedish industry, and KTH Royal Institute of Technology. References [1] Schwarzkopf M, Exner HE, Fischmeister HF, Schintlmeister W. Kinetics of compositional modification of (W, Ti)C–WC–Co alloy surfaces. Mater Sci Eng, A 1988:105–6 225-231. [2] Kieffer R, Nowotny H, Ettmayer P, Freudhofmeier M. Über die Beständigkeit von Übergangsmetallcarbiden gegen Stickstoff bis zu 300 at. Monatsh Chem 1970;101:65–82. [3] Markström A, Andersson D, Frisk K. Combined ab-initio and experimental assessment of A1âˆ'xBxC mixed carbides. Calphad 2008;32:615–23. [4] Chen L, Lengauer W, Dreyer K. Advances in modern nitrogen-containing hardmetals and cermets. Int J Refract Metals Hard Mater 2000;18:153–61. [5] Crozet C. Equilibrium studies of N-containing cemented carbides, Diploma work. Stockholm, Sweden: KIMAB; 2007. [6] Markström A, Frisk K. Experimental and thermodynamic evaluation of the miscibility gaps in MC carbides for the C–Co–Ti–V–W–Zr system. Calphad 2009;33:530–8. [7] Frisk K, Zackrisson J, Jansson B, Markström A. Experimental investigation of the equilibrium composition of titanium carbonitride and analysis using thermodynamic modelling. Metallkd Mater Res Adv Tech 2004;95:987–92. [8] Borgh I. Impact of nitrogen pressure on the carbonitride composition. Diploma work. Stockholm, Sweden: KTH Royal Institute of Technology; 2009.