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Combustion synthesis of chromium nitrides by SHS of Cr powder compacts under nitrogen pressures
Journal of Alloys and Compounds 426 (2006) 131–135
Combustion synthesis of chromium nitrides by SHS of Cr powder compacts under nitrogen pressures C.L. Yeh ∗ , E.W. Liu Department of Mechanical and Automation Engineering, Da-Yeh University, 112 Shan-Jiau Road, Da-Tsuen, Changhua 51505, Taiwan Received 16 January 2006; accepted 30 January 2006 Available online 10 March 2006
Abstract An experimental study on the preparation of chromium nitrides by self-propagating high-temperature synthesis (SHS) was conducted with powder compacts under nitrogen pressures of 0.45–4.24 MPa. The SHS process is characterized by the steady propagation of a self-sustained combustion front, followed by prolonged afterburning reactions. Nitrogen pressure was demonstrated to play an important role in the combustion characteristics as well as in the phase composition of synthesized products. Experimental evidence indicated that combustion temperature and flame-front propagation velocity were substantially increased by increasing the nitrogen pressure. It was found that multiphase products consisting of two nitride phases Cr2 N and CrN were yielded by the self-sustained reactions conducted in this study. The combustion product contained only about 10 mol% of CrN when the reaction was performed under a low nitrogen pressure of 0.79 MPa. However, the CrN content was found to increase significantly with nitrogen pressure. Namely, the mole fraction of CrN in the final product increases up to about 54% at 1.83 MPa of nitrogen, and finally approaches an asymptotical value around 80% in nitrogen of 4.24 MPa. © 2006 Elsevier B.V. All rights reserved. Keywords: Chromium nitride; CrN; Cr2 N; SHS; Nitrogen pressure
1. Introduction Chromium nitride (CrN) has been recognized as an excellent coating material and a potential substitute to titanium-based coatings for both wear and corrosion applications [1–6]. CrN is also a promising candidate for the replacement of electroplated hard chromium as a tribological coating [3,6]. The chromium–nitrogen system contains two nitride compounds, a cubic phase CrN and a hexagonal phase Cr2 N which possesses higher hardness than that of CrN [7]. However, the single-phase CrN coating exhibits better wear resistance especially when (2 2 0) preferred orientation is developed [8]. Chromium nitrides are normally produced by heating chromium metals at temperatures about 1000 ◦ C in a stream of ammonia (NH3 ), or by reactions of chromium halides or hydrides with nitrogen or ammonia [9]. These reactions are considerably time-consuming and typically require 2–3 weeks for fine powders [9]. In addition, since it is not possible to pre-
∗
Corresponding author. Tel.: +886 4 8511888x2118; fax: +886 4 8511215. E-mail address: [email protected] (C.L. Yeh).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.01.082
pare chromium nitride, as in the case of titanium nitride, by evaporating chromium in a nitrogen atmosphere, Engel et al. [5,6] obtained Crx N films through the ion-beam-assisted deposition (IBAD) which evaporated chromium under irradiation with nitrogen ions. Deposition of a CrN coating on machine parts and tools was also achieved by means of reactive magnetron sputtering [2,3,7]. Formation of nearly 100% CrN from Cr powders was reported by Ren et al. [10] using a modified high energy milling process called mechanically activated synthesis (MAS) that mills the Cr powders in NH3 for 24 h prior to nitridation at 800 ◦ C. Recently, Cai et al. [11] employed sodium azide (NaN3 ) as the nitrogen source and the reduction reagent in a solid-state reaction with anhydrous chromium chloride (CrCl3 ) at a temperature of 600 ◦ C for 12 h to produce nanostructured CrN. With the advantages of time saving, low energy requirement, and simplicity, combustion synthesis particularly in the mode of self-propagating high-temperature synthesis (SHS) has been recognized as a promising alternative to the conventional methods of producing advanced materials, including carbides, borides, nitrides, hydrides, silicides, intermetallics, etc. [12–16]. Preparation of nitrides of transition metals, especially in the groups of IVB and VB, by SHS has been extensively investi-
132
C.L. Yeh, E.W. Liu / Journal of Alloys and Compounds 426 (2006) 131–135
gated, including TiN, ZrN, NbN, VN, and TaN [17–24]. However, studies on the production of chromium nitrides by combustion synthesis are relatively few. Hirota et al. [25] initiated the self-propagating combustion of Cr powders which were loosely placed in a carbon crucible under nitrogen pressures of 0.18–2.0 MPa and obtained nitride products containing both Cr2 N and CrN. Their results also indicated that single-phase Cr2 N and CrN were yielded in nitrogen at 0.18 and 2.0 MPa, respectively [25]. The objective of this study was to experimentally investigate the combustion synthesis of chromium nitride in the SHS mode under gaseous nitrogen with compacted samples from chromium powders. Combustion characteristics, such as the propagation of self-sustained combustion wave, flame-front velocity, and combustion temperature, were studied. Effects of nitrogen pressure, compact density, and initial sample temperature on the degree of product conversion were explored. Phase composition of synthesized products was identified by the XRD analysis. 2. Experimental procedure Chromium (Cr) powders (Cerac Incorp., −325 mesh) of 99.2% purity were used as the starting material and were pressed into cylindrical specimens having a diameter of 7 mm and a height of 12.5 mm. To retain high permeability and rigidity of the test specimen, two compact densities equal to 50 and 55% of the theoretical maximum density (TMD) of chromium were adopted. The synthesis reaction was conducted in a stainless-steel windowed chamber under a nitrogen pressure ranging from 0.45 to 4.24 MPa. The nitrogen gas used in this study had a purity of 99.999%. The sample holder in the combustion chamber was equipped with a 600 W cartridge heater used to raise the initial temperature of test samples prior to ignition. In this study, experiments were performed using samples either without preheating or at a preheating temperature of 200 ◦ C. Details of the experimental setup and measurement approach were previously given [21–24]. According to the fact that Cr particles were normally converted to either Cr2 N or CrN during the nitridation process [10,25], the reaction of the powder compact in gaseous nitrogen conducted in this study was expressed by the following equation: xCr + yN2 → (4y − x)CrN + (x − 2y)Cr2 N
(1)
In Eq. (1), the coefficient x was known from the test sample used and the y value representing the amount of nitrogen uptake was calculated from the measurement of weight change of the sample compact after combustion [17–24].
As a result, the amounts of CrN and Cr2 N yielded in the final product were obtained. The molar ratio of CrN to Cr2 N determined by Eq. (1) was verified by the measurement of relative intensity of diffraction peaks signifying CrN (2 0 0) and Cr2 N (1 1 1) in the XRD spectrum of the combustion product [25].
3. Results and discussion 3.1. Observation of combustion characteristics Fig. 1 illustrates a typical sequence of recorded images showing the SHS process of a Cr powder compact under nitrogen pressure. It is evident in Fig. 1 that upon ignition a distinct and self-sustained flame front develops instantly and advances downward from the ignited top plane. Moreover, the planar combustion wave forms a nearly parallel front propagating in a steady manner. It was also found that after the arrival of the flame front at the bottom of the sample at about 6.67 s, the burning luminosity on the sample remained for a relatively long period of time, implying a prolonged phase conversion taking place in bulk. This observation is typical of the combustion synthesis involving gaseous reagents [21–24]. It was believed that complete nitridation of the reactant to end products at the combustion front was difficult to achieve, due to the inadequate nitrogen at the reaction front. Therefore, the continuous infiltration of nitrogen gas into the porous compact to react with the solid reactant could lead to the appearance of afterburning glow on the burned sample after the passage of the flame front. 3.2. Measurement of combustion temperature and flame-front propagation velocity Fig. 2 presents the effect of nitrogen pressure on combustion front temperature and propagation velocity measured from 55% TMD powder compacts ignited without prior heating. Both combustion temperature and flame-front velocity were found to increase with increasing nitrogen pressure. This is most likely attributed to the increase of initial nitrogen concentration at the reaction front and within the porous compact, which substantially enhances the nitridation and consequently aug-
Fig. 1. Recorded combustion images illustrating SHS process of a 55% TMD powder compact ignited without prior heating under nitrogen of 3.55 MPa.
C.L. Yeh, E.W. Liu / Journal of Alloys and Compounds 426 (2006) 131–135
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to a slow temperature decline after the passage of the flame front. 3.3. Composition analysis of final products Fig. 4(a)–(c) show the XRD patterns of the combustion products synthesized from 55% TMD compacts under nitrogen
Fig. 2. Effects of nitrogen pressure on combustion temperature and flame-front propagation velocity of 55% TMD Cr powder compacts ignited without prior heating.
ments the heat generation and the reaction rate. As indicated in Fig. 2, the combustion temperature increases from 840 ◦ C at 0.79 MPa of nitrogen to approximately 1150 ◦ C at 4.24 MPa. Similarly, the flame-front velocity increases from 0.63 to slightly above 1.0 mm/s with increasing nitrogen pressure from 0.79 to 4.24 MPa. Moreover, it is useful to note that the combustion temperatures detected in this study are considerably lower than the melting points of chromium (1860 ◦ C) and chromium nitride (about 1500 ◦ C). This means no tendency to melt the metal reactant and nitride product during the SHS reaction. Two typical combustion temperature profiles measured from the compacts in nitrogen of 0.79 and 2.86 MPa, respectively, are plotted in Fig. 3. On account of the fast arrival of the flame front at the tip of thermocouple attached on the sample surface, both profiles exhibit a significant temperature rise to the maximum value. The temperature gradient stemming from the propagation of combustion wave is found to be greater for the case of 2.86 MPa than that of 0.79 MPa, due to a higher speed of the reaction front at 2.86 MPa of nitrogen. In addition, as a result of the presence of prolonged afterburning reactions, both temperature curves indicate that reactant compacts were subjected
Fig. 3. Typical temperature profiles recorded from powder compacts ignited under nitrogen of 0.79 and 2.86 MPa.
Fig. 4. XRD patterns of combustion products obtained from 55% TMD powder compacts without preheating and ignited under nitrogen pressures of (a) 0.79 MPa, (b) 1.83 MPa, and (c) 3.55 MPa.
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value for the CrN mole fraction close to 80 mol% is attained at 4.24 MPa of nitrogen. Effects of compact density and sample preheating on the extent of nitridation were investigated and the results are presented in Fig. 6. As shown in Fig. 6, the mole fraction of CrN in the final product is almost independent of the variations of compact density and initial sample temperature. This implies that the phase composition of products is mainly affected by the pressure of nitrogen gas. However, it is interesting to note that formation of single-phase Cr2 N from a reactant compact preheated at 200 ◦ C was observed at a nitrogen pressure of 0.45 MPa, under which combustion ceased to be self-propagating and was extinguished for the powder compacts without prior heating. 4. Conclusions Fig. 5. Variation of mole fractions of Cr2 N and CrN in combustion products of 55% TMD compacts with nitrogen pressure.
pressures of 0.79, 1.83, and 3.55 MPa, respectively. It is evident that the combustion product is a mixture of two nitride phases Cr2 N and CrN. Formation of Cr2 N in addition to CrN is primarily attributed to the deficiency of nitrogen during the reaction. Therefore, as shown in Fig. 4(a)–(c), the intensity of diffraction peaks corresponding to the phase of CrN increases appreciably with the increase of nitrogen pressure, indicative of the dominancy of CrN over Cr2 N at high pressures of nitrogen and vice versa. Based upon the experimental data and Eq. (1), the relative amounts of Cr2 N and CrN formed in the final products were calculated and presented in Fig. 5 as a function of nitrogen pressure. It was found in Fig. 5 that at 0.79 MPa of nitrogen the reaction yielded a combustion product consisting of almost 90 mol% of Cr2 N. With the increase of nitrogen pressure from 0.79 to 2.17 MPa, Fig. 5 indicates that the mole fraction of CrN in the final product increases considerably and reaches about 60% at 2.17 MPa, suggesting a significant enhancement in the degree of nitridation within this pressure range. Further increase in nitrogen pressure continues to increase the CrN content but with a moderate pace. Finally, a nearly asymptotic
This study represents an experimental investigation on the preparation of chromium nitrides by combustion synthesis of Cr powder compacts in gaseous nitrogen. Experimental observations indicate that the combustion process is characterized by the planar and self-sustained combustion wave traversing the entire sample, subsequently followed by prolonged afterburning reactions taking place in bulk after the passage of the flame front. With the increase of nitrogen pressure, the nitridation of reactant compacts was greatly enhanced, thus leading to a higher combustion temperature and a correspondingly faster propagation rate of the reaction front. The XRD analysis on the burned samples identifies the formation of multiphase products consisting of two nitrides Cr2 N and CrN from the combustion of powder compacts in nitrogen. The content of CrN in the final composition increases significantly with nitrogen pressure. For the reactant compacts of 55% TMD, the mole fraction of CrN increased from around 10% at 0.79 MPa of nitrogen to 54% at 1.83 MPa, and finally arrived at a nearly asymptotic value close to 80% in nitrogen of 4.24 MPa. The contents of the CrN phase formed in the final products were found to be comparable for the reactant compacts of 50 and 55% TMD. An increase of initial sample temperature to 200 ◦ C by prior heating did not affect the degree of nitridation within the pressure range of 0.79–4.24 MPa, but resulted in the formation of a single-phase product composed of Cr2 N in nitrogen of 0.45 MPa. Acknowledgement This research was sponsored by the National Science Council of Taiwan, ROC, under the grant of NSC 94-2212-E-212-018. References
Fig. 6. Effects of nitrogen pressure, compact density, and sample preheating temperature on CrN content of final products.
[1] H. Chen, P.Q. Wu, C. Quaeyhaegens, K.W. Xu, L.M. Stals, J.W. He, J.P. Celis, Wear 253 (2002) 527–532. [2] G. Berg, C. Friedrich, E. Broszeit, C. Berger, Surf. Coat. Technol. 86/87 (1996) 184–191. [3] L. Cunha, M. Andritschky, K. Pischow, Z. Wang, A. Zarychta, A.S. Miranda, A.M. Cunha, Surf. Coat. Technol. 153 (2002) 160–165. [4] C. Liu, A. Leyland, Q. Bi, A. Matthews, Surf. Coat. Technol. 141 (2001) 164–173.
C.L. Yeh, E.W. Liu / Journal of Alloys and Compounds 426 (2006) 131–135 [5] P. Engel, G. Schwarz, G.K. Wolf, Surf. Coat. Technol. 98 (1998) 1002–1007. [6] P. Engel, G. Schwarz, G.K. Wolf, Surf. Coat. Technol. 112 (1999) 286–290. [7] P. Hones, R. Sanjines, F. L´evy, Surf. Coat. Technol. 94/95 (1997) 398–402. [8] Y. Chiba, T. Omura, H. Ichimura, J. Mater. Res. 8 (1993) 1109–1115. [9] L.E. Toth, Transition Metal Carbides and Nitrides, Academic Press, New York, 1971. [10] R. Ren, Z. Yang, L.L. Shaw, Nanostruct. Mater. 11 (1999) 25–35. [11] P. Cai, J. Zhu, Z. Yang, Y. Qian, Mater. Chem. Phys. 95 (2006) 1–4. [12] Z.A. Munir, U. Anselmi-Tamburini, Mater. Sci. Rep. 3 (1989) 277–365. [13] A. Varma, J.P. Lebrat, Chem. Eng. Sci. 47 (1992) 2179–2194. [14] A.G. Merzhanov, Combust. Sci. Technol. 98 (1994) 307–336. [15] J.J. Moore, H.J. Feng, Prog. Mater. Sci. 39 (1995) 243–273. [16] P. Mossino, Ceram. Int. 30 (2004) 311–332.
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[17] M. Eslamloo-Grami, Z.A. Munir, J. Am. Ceram. Soc. 73 (1990) 1235– 1239. [18] M. Eslamloo-Grami, Z.A. Munir, J. Am. Ceram. Soc. 73 (1990) 2222–2227. [19] C.C. Agrafiotis, J.A. Puszynski, V. Hlavacek, Combust. Sci. Technol. 76 (1991) 187–218. [20] A.G. Merzhanov, I.P. Borovinskaya, Dokl. Akad. Nauk USSK 204 (1972) 366–369. [21] C.L. Yeh, H.C. Chuang, Ceram. Int. 30 (5) (2004) 705–714. [22] C.L. Yeh, H.C. Chuang, Ceram. Int. 30 (5) (2004) 733–743. [23] C.L. Yeh, H.C. Chuang, E.W. Liu, Y.C. Chang, Ceram. Int. 31 (1) (2005) 95–104. [24] C.L. Yeh, E.W. Liu, Y.C. Chang, J. Eur. Ceram. Soc. 24 (2004) 3807–3815. [25] K. Hirota, Y. Takano, M. Yoshinaka, O. Yamaguchi, J. Am. Ceram. Soc. 84 (2001) 2120–2122.
Combustion synthesis of chromium nitrides by SHS of Cr powder compacts under nitrogen pressures C.L. Yeh ∗ , E.W. Liu Department of Mechanical and Automation Engineering, Da-Yeh University, 112 Shan-Jiau Road, Da-Tsuen, Changhua 51505, Taiwan Received 16 January 2006; accepted 30 January 2006 Available online 10 March 2006
Abstract An experimental study on the preparation of chromium nitrides by self-propagating high-temperature synthesis (SHS) was conducted with powder compacts under nitrogen pressures of 0.45–4.24 MPa. The SHS process is characterized by the steady propagation of a self-sustained combustion front, followed by prolonged afterburning reactions. Nitrogen pressure was demonstrated to play an important role in the combustion characteristics as well as in the phase composition of synthesized products. Experimental evidence indicated that combustion temperature and flame-front propagation velocity were substantially increased by increasing the nitrogen pressure. It was found that multiphase products consisting of two nitride phases Cr2 N and CrN were yielded by the self-sustained reactions conducted in this study. The combustion product contained only about 10 mol% of CrN when the reaction was performed under a low nitrogen pressure of 0.79 MPa. However, the CrN content was found to increase significantly with nitrogen pressure. Namely, the mole fraction of CrN in the final product increases up to about 54% at 1.83 MPa of nitrogen, and finally approaches an asymptotical value around 80% in nitrogen of 4.24 MPa. © 2006 Elsevier B.V. All rights reserved. Keywords: Chromium nitride; CrN; Cr2 N; SHS; Nitrogen pressure
1. Introduction Chromium nitride (CrN) has been recognized as an excellent coating material and a potential substitute to titanium-based coatings for both wear and corrosion applications [1–6]. CrN is also a promising candidate for the replacement of electroplated hard chromium as a tribological coating [3,6]. The chromium–nitrogen system contains two nitride compounds, a cubic phase CrN and a hexagonal phase Cr2 N which possesses higher hardness than that of CrN [7]. However, the single-phase CrN coating exhibits better wear resistance especially when (2 2 0) preferred orientation is developed [8]. Chromium nitrides are normally produced by heating chromium metals at temperatures about 1000 ◦ C in a stream of ammonia (NH3 ), or by reactions of chromium halides or hydrides with nitrogen or ammonia [9]. These reactions are considerably time-consuming and typically require 2–3 weeks for fine powders [9]. In addition, since it is not possible to pre-
∗
Corresponding author. Tel.: +886 4 8511888x2118; fax: +886 4 8511215. E-mail address: [email protected] (C.L. Yeh).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.01.082
pare chromium nitride, as in the case of titanium nitride, by evaporating chromium in a nitrogen atmosphere, Engel et al. [5,6] obtained Crx N films through the ion-beam-assisted deposition (IBAD) which evaporated chromium under irradiation with nitrogen ions. Deposition of a CrN coating on machine parts and tools was also achieved by means of reactive magnetron sputtering [2,3,7]. Formation of nearly 100% CrN from Cr powders was reported by Ren et al. [10] using a modified high energy milling process called mechanically activated synthesis (MAS) that mills the Cr powders in NH3 for 24 h prior to nitridation at 800 ◦ C. Recently, Cai et al. [11] employed sodium azide (NaN3 ) as the nitrogen source and the reduction reagent in a solid-state reaction with anhydrous chromium chloride (CrCl3 ) at a temperature of 600 ◦ C for 12 h to produce nanostructured CrN. With the advantages of time saving, low energy requirement, and simplicity, combustion synthesis particularly in the mode of self-propagating high-temperature synthesis (SHS) has been recognized as a promising alternative to the conventional methods of producing advanced materials, including carbides, borides, nitrides, hydrides, silicides, intermetallics, etc. [12–16]. Preparation of nitrides of transition metals, especially in the groups of IVB and VB, by SHS has been extensively investi-
132
C.L. Yeh, E.W. Liu / Journal of Alloys and Compounds 426 (2006) 131–135
gated, including TiN, ZrN, NbN, VN, and TaN [17–24]. However, studies on the production of chromium nitrides by combustion synthesis are relatively few. Hirota et al. [25] initiated the self-propagating combustion of Cr powders which were loosely placed in a carbon crucible under nitrogen pressures of 0.18–2.0 MPa and obtained nitride products containing both Cr2 N and CrN. Their results also indicated that single-phase Cr2 N and CrN were yielded in nitrogen at 0.18 and 2.0 MPa, respectively [25]. The objective of this study was to experimentally investigate the combustion synthesis of chromium nitride in the SHS mode under gaseous nitrogen with compacted samples from chromium powders. Combustion characteristics, such as the propagation of self-sustained combustion wave, flame-front velocity, and combustion temperature, were studied. Effects of nitrogen pressure, compact density, and initial sample temperature on the degree of product conversion were explored. Phase composition of synthesized products was identified by the XRD analysis. 2. Experimental procedure Chromium (Cr) powders (Cerac Incorp., −325 mesh) of 99.2% purity were used as the starting material and were pressed into cylindrical specimens having a diameter of 7 mm and a height of 12.5 mm. To retain high permeability and rigidity of the test specimen, two compact densities equal to 50 and 55% of the theoretical maximum density (TMD) of chromium were adopted. The synthesis reaction was conducted in a stainless-steel windowed chamber under a nitrogen pressure ranging from 0.45 to 4.24 MPa. The nitrogen gas used in this study had a purity of 99.999%. The sample holder in the combustion chamber was equipped with a 600 W cartridge heater used to raise the initial temperature of test samples prior to ignition. In this study, experiments were performed using samples either without preheating or at a preheating temperature of 200 ◦ C. Details of the experimental setup and measurement approach were previously given [21–24]. According to the fact that Cr particles were normally converted to either Cr2 N or CrN during the nitridation process [10,25], the reaction of the powder compact in gaseous nitrogen conducted in this study was expressed by the following equation: xCr + yN2 → (4y − x)CrN + (x − 2y)Cr2 N
(1)
In Eq. (1), the coefficient x was known from the test sample used and the y value representing the amount of nitrogen uptake was calculated from the measurement of weight change of the sample compact after combustion [17–24].
As a result, the amounts of CrN and Cr2 N yielded in the final product were obtained. The molar ratio of CrN to Cr2 N determined by Eq. (1) was verified by the measurement of relative intensity of diffraction peaks signifying CrN (2 0 0) and Cr2 N (1 1 1) in the XRD spectrum of the combustion product [25].
3. Results and discussion 3.1. Observation of combustion characteristics Fig. 1 illustrates a typical sequence of recorded images showing the SHS process of a Cr powder compact under nitrogen pressure. It is evident in Fig. 1 that upon ignition a distinct and self-sustained flame front develops instantly and advances downward from the ignited top plane. Moreover, the planar combustion wave forms a nearly parallel front propagating in a steady manner. It was also found that after the arrival of the flame front at the bottom of the sample at about 6.67 s, the burning luminosity on the sample remained for a relatively long period of time, implying a prolonged phase conversion taking place in bulk. This observation is typical of the combustion synthesis involving gaseous reagents [21–24]. It was believed that complete nitridation of the reactant to end products at the combustion front was difficult to achieve, due to the inadequate nitrogen at the reaction front. Therefore, the continuous infiltration of nitrogen gas into the porous compact to react with the solid reactant could lead to the appearance of afterburning glow on the burned sample after the passage of the flame front. 3.2. Measurement of combustion temperature and flame-front propagation velocity Fig. 2 presents the effect of nitrogen pressure on combustion front temperature and propagation velocity measured from 55% TMD powder compacts ignited without prior heating. Both combustion temperature and flame-front velocity were found to increase with increasing nitrogen pressure. This is most likely attributed to the increase of initial nitrogen concentration at the reaction front and within the porous compact, which substantially enhances the nitridation and consequently aug-
Fig. 1. Recorded combustion images illustrating SHS process of a 55% TMD powder compact ignited without prior heating under nitrogen of 3.55 MPa.
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to a slow temperature decline after the passage of the flame front. 3.3. Composition analysis of final products Fig. 4(a)–(c) show the XRD patterns of the combustion products synthesized from 55% TMD compacts under nitrogen
Fig. 2. Effects of nitrogen pressure on combustion temperature and flame-front propagation velocity of 55% TMD Cr powder compacts ignited without prior heating.
ments the heat generation and the reaction rate. As indicated in Fig. 2, the combustion temperature increases from 840 ◦ C at 0.79 MPa of nitrogen to approximately 1150 ◦ C at 4.24 MPa. Similarly, the flame-front velocity increases from 0.63 to slightly above 1.0 mm/s with increasing nitrogen pressure from 0.79 to 4.24 MPa. Moreover, it is useful to note that the combustion temperatures detected in this study are considerably lower than the melting points of chromium (1860 ◦ C) and chromium nitride (about 1500 ◦ C). This means no tendency to melt the metal reactant and nitride product during the SHS reaction. Two typical combustion temperature profiles measured from the compacts in nitrogen of 0.79 and 2.86 MPa, respectively, are plotted in Fig. 3. On account of the fast arrival of the flame front at the tip of thermocouple attached on the sample surface, both profiles exhibit a significant temperature rise to the maximum value. The temperature gradient stemming from the propagation of combustion wave is found to be greater for the case of 2.86 MPa than that of 0.79 MPa, due to a higher speed of the reaction front at 2.86 MPa of nitrogen. In addition, as a result of the presence of prolonged afterburning reactions, both temperature curves indicate that reactant compacts were subjected
Fig. 3. Typical temperature profiles recorded from powder compacts ignited under nitrogen of 0.79 and 2.86 MPa.
Fig. 4. XRD patterns of combustion products obtained from 55% TMD powder compacts without preheating and ignited under nitrogen pressures of (a) 0.79 MPa, (b) 1.83 MPa, and (c) 3.55 MPa.
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value for the CrN mole fraction close to 80 mol% is attained at 4.24 MPa of nitrogen. Effects of compact density and sample preheating on the extent of nitridation were investigated and the results are presented in Fig. 6. As shown in Fig. 6, the mole fraction of CrN in the final product is almost independent of the variations of compact density and initial sample temperature. This implies that the phase composition of products is mainly affected by the pressure of nitrogen gas. However, it is interesting to note that formation of single-phase Cr2 N from a reactant compact preheated at 200 ◦ C was observed at a nitrogen pressure of 0.45 MPa, under which combustion ceased to be self-propagating and was extinguished for the powder compacts without prior heating. 4. Conclusions Fig. 5. Variation of mole fractions of Cr2 N and CrN in combustion products of 55% TMD compacts with nitrogen pressure.
pressures of 0.79, 1.83, and 3.55 MPa, respectively. It is evident that the combustion product is a mixture of two nitride phases Cr2 N and CrN. Formation of Cr2 N in addition to CrN is primarily attributed to the deficiency of nitrogen during the reaction. Therefore, as shown in Fig. 4(a)–(c), the intensity of diffraction peaks corresponding to the phase of CrN increases appreciably with the increase of nitrogen pressure, indicative of the dominancy of CrN over Cr2 N at high pressures of nitrogen and vice versa. Based upon the experimental data and Eq. (1), the relative amounts of Cr2 N and CrN formed in the final products were calculated and presented in Fig. 5 as a function of nitrogen pressure. It was found in Fig. 5 that at 0.79 MPa of nitrogen the reaction yielded a combustion product consisting of almost 90 mol% of Cr2 N. With the increase of nitrogen pressure from 0.79 to 2.17 MPa, Fig. 5 indicates that the mole fraction of CrN in the final product increases considerably and reaches about 60% at 2.17 MPa, suggesting a significant enhancement in the degree of nitridation within this pressure range. Further increase in nitrogen pressure continues to increase the CrN content but with a moderate pace. Finally, a nearly asymptotic
This study represents an experimental investigation on the preparation of chromium nitrides by combustion synthesis of Cr powder compacts in gaseous nitrogen. Experimental observations indicate that the combustion process is characterized by the planar and self-sustained combustion wave traversing the entire sample, subsequently followed by prolonged afterburning reactions taking place in bulk after the passage of the flame front. With the increase of nitrogen pressure, the nitridation of reactant compacts was greatly enhanced, thus leading to a higher combustion temperature and a correspondingly faster propagation rate of the reaction front. The XRD analysis on the burned samples identifies the formation of multiphase products consisting of two nitrides Cr2 N and CrN from the combustion of powder compacts in nitrogen. The content of CrN in the final composition increases significantly with nitrogen pressure. For the reactant compacts of 55% TMD, the mole fraction of CrN increased from around 10% at 0.79 MPa of nitrogen to 54% at 1.83 MPa, and finally arrived at a nearly asymptotic value close to 80% in nitrogen of 4.24 MPa. The contents of the CrN phase formed in the final products were found to be comparable for the reactant compacts of 50 and 55% TMD. An increase of initial sample temperature to 200 ◦ C by prior heating did not affect the degree of nitridation within the pressure range of 0.79–4.24 MPa, but resulted in the formation of a single-phase product composed of Cr2 N in nitrogen of 0.45 MPa. Acknowledgement This research was sponsored by the National Science Council of Taiwan, ROC, under the grant of NSC 94-2212-E-212-018. References
Fig. 6. Effects of nitrogen pressure, compact density, and sample preheating temperature on CrN content of final products.
[1] H. Chen, P.Q. Wu, C. Quaeyhaegens, K.W. Xu, L.M. Stals, J.W. He, J.P. Celis, Wear 253 (2002) 527–532. [2] G. Berg, C. Friedrich, E. Broszeit, C. Berger, Surf. Coat. Technol. 86/87 (1996) 184–191. [3] L. Cunha, M. Andritschky, K. Pischow, Z. Wang, A. Zarychta, A.S. Miranda, A.M. Cunha, Surf. Coat. Technol. 153 (2002) 160–165. [4] C. Liu, A. Leyland, Q. Bi, A. Matthews, Surf. Coat. Technol. 141 (2001) 164–173.
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