AT0100447 S. Morita et al. HM 18 139 15" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 2

Synthesis of Powder by Carbo-thermal Nitriding

Susumu Morita, Hideaki Shimizu, Yasumasa Sayama

Japan New Metals Co., Ltd. 1-6-64 Sennaricho Toyonaka, Osaka 561-0829, Japan

Summary

Fine chromium nitride powders were synthesized by carbo-thermal nitriding from Cr2O3 and carbon black. Thermal nitriding reaction of O2O3 and carbon black mixture was investigated by TG-DTA. The products were identified by XRD. O3C2 and O2 (CN) were formed in the early stage of the reaction, but finally they changed into CteN and CrN. Lab-scale syntheses of O2N and CrN were carried out using an electric tube furnace. Cr2N was synthesized by firing the mixed powder at 1393K for 1 hr under and hydrogen mixed gas flow, whereas CrN was synthesized by sequentially nitriding of Cr2N at 1173K. The both synthesized powders showed homogeneous morphology with narrow particle size distribution and average size of about 1|a.m. Cr2N and CrN contained 11 and 20% of nitrogen respectively, sub percents of oxygen and carbon.

Keywords chromium nitride powder, Cr2N, CrN, carbo-thermal nitriding

1. Introduction

Nitrides of transition metals belonging to Groups IV, V and VI of the Periodic Table have high melting points and hardness, just like carbides. At the same time, they have the metal-like properties of good thermal and electrical conductivity. These are widely used as an additive for improving the properties of high-speed steels and ceramics. Coating of and chromium nitride by the CVD and PVD methods remarkably improves the performance and durability of cutting tools, molds and sliding parts. Coating technology of chromium nitride has been investigated by many workers, because chromium nitride is superior in corrosion resistance to titanium nitride (1)'(2). Attempts are also being made to prepare bulk chromium nitride ceramics from chromium nitride powder, as a high performance material for 140 HM 18 S. Morita et al.

15" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 2 making mechanical parts (3) Table.1 shows the properties of chromium nitride™.

Table. 1 Properties of chromium nitride

Cr2N CrN Crystal structure type Hex. C. P Cubic Density 6.5 6.1 Melting point (K) - 1373(dec.) Micro hardness (GPa) 15.3 10.3 Heat of formation (kJ/mol) 129 125

There are two forms of chromium nitride, CrN and Cr2N. Fig. 1 shows the phase diagram of the Cr-N binary system (5). In a nitrogen atmosphere of 0.1 MPa, CrN is stable up to 1373K, whereas Cr2N under the same conditions has two coexisting phases, including the liquid phase, at 1973K.

2273

20 30 40 50 60 Composition (mol.%) Fig.1 Phase diagram of Cr-N system S. Morita et al. HM 18 141

15'" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 2

Chromium nitride powder is generally synthesized by thermal nitriding of chromium metal powder in nitrogen or gas flow(6). It is possible to synthesize highly pure powder by this method. However, direct synthesis of fine chromium nitride powder is difficult because preparing fine chromium metal powder, the starting material, is not easy. Furthermore it is difficult to synthesize fine powder, because some growth of the particle is unavoidable during reaction due to the large amount of heat generated. On the other hand, carbo-thermal nitriding from a mixture of the metal oxide and carbon powder appeared to be suitable for preparing fine powder because the heat of formation can be significantly lowered as compare with the nitriding of the metal powder. But the method is not suited for preparing highly pure nitride because of the greater likelihood of the presence of dissolved oxygen and carbon in the product. In this study, fine chromium nitride powder of high purity was synthesized by carbo-thermal nitriding using chromium oxide powder and carbon black as the starting materials. We investigated the reaction mechanism, and examined the effects of reaction temperature and atmosphere on the physical properties of the synthesized powder.

2. Experimental procedure

Cr2C>3 powder of mean particle size 0.5,um (A-1 grade, Nippon Chemical Industrial Co., Ltd.) and high purity carbon black powder (Denka Black, Denki Kagaku Kogyo) were mixed in the molar ratio of C/Cr2O3=2.8 to 3.2, and wet- mixed for 12 hours in methanol using a ball mill with the balls and pot made of resin. The mixture was then dried and sieved at 850^ to prepare the starting material. Fig. 2 shows SEM photographs of the starting materials powder. Table 2 gives the analyzed data of the two starting materials. The reaction mechanism and the effect of the atmosphere in the carbo- thermal nitriding of chromium oxide and carbon were studied by TG-DTA. Thermal analysis was carried out using a mixture having molar ratio of C/Cr2O3=3.0 at a temperature range of room temperature to 1773K by increasing of 0.1K/sec under nitrogen or a nitrogen-hydrogen mixed gas flow. For synthesizing chromium nitride, an SUS tray was charged with the powder mixture (thickness of powder layer: 5mm) and placed in an electric tube furnace with SUS core. The powder was then nitrided under nitrogen or nitrogen-hydrogen mixed gas flow by firing at the rate of 0.5K/sec and maintaining the mixture for 3600 seconds at a desired reaction temperature range of 1173-1503K. The products were characterized by XRD, SEM, chemical analysis and particle size analysis. 142 HM 18 S. Morita et al.

15" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 2

"» %,

, V

Cr2O3 C.B. Fig.2 SEM photographs of O2O3 and carbon black

Table. 2 Average particle size and chemical analyses of starting materials Cr7O3 FSSSOum) Fe(ppm) Na(ppm) Ca(ppm) Mg(ppm) S(ppm) P(ppm)

0.53 190 134 3 1 289 10 Carbon Black BET(m 2/g) Ash(%) Fe(ppm) Ca(ppm) S(ppm) 42 0.00 5 11 47

3. Results and discussion

3. 1. Thermal analysis The results of TG-DTA of the chromium oxide and carbon black powder mixture are given in Fig.3. In nitrogen gas flow, the endothermic carbo- thermal nitriding reaction started at 1473K and resulted in rapid decrease in weight, and the reaction was completed at 1503K. In the nitrogen-hydrogen mixed gas flow, however, the reaction initiation and completion temperatures S. Morita et al. HM 18 143

15* International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 2

were 1353 and 1393K respectively, showing a shift to the low temperature side. Final reactants obtained by TG-DTA were CrN and Cr2N in both the reaction systems.

CQ

-80 1323 1373 1423 1473 1523 Temperature (K) Fig.3 The results of TG-DTA of carbo-thermal nitriding from Cr2O3 and carbon black

The intermediate products in lab-scale synthesis at 1353 and 1393K under the nitrogen-hydrogen mixed gas flow were examined by XRD (Fig. 4). At 1353K, apart from the main peak of Cr2 (CN), minor peaks corresponding to O3C2 and O2O3 appeared. At 1393K, only O2N peek was observed. This result suggests that the chromium oxide transformed into the carbide and the carbonitride at the beginning of reaction and then converted into the nitride. Fig. 5 shows the change in standard free energy of chromium nitride formation when chromium oxide is the starting material (7). This graph indicates that a reaction temperature of 1450K or higher is needed for the reaction of chromium oxide and carbon in the nitrogen atmosphere. This agrees well with the results of TG-DTA. In the reaction of chromium oxide with a gas mixture of methane and nitrogen, AG had a negative value at the reaction temperature 1200K or higher, suggesting that the reaction started at a relatively low temperature. On the basis of the above results, it is assumed that when the nitrogen-hydrogen gas mixture is used as the reaction gas, the surface of the chromium oxide is first reduced by the hydrogen, and generates a lower oxide and H2O gas<8). Then hydrocarbon gas, produced by 144 HM 18 S. Morita et al.

15* International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 2 the reaction between the H2O gas and the carbon, takes part in the formation of intermediate compounds like Cr3C2, Cr2(CN), etc. It is thought that this lowered the initiation temperature of reaction needed to form chromium nitride.

1353K

1393K 20 30 40 50 60 29 (degree)

Fig.4 XRD profiles of powders fired at 1353 and 1393K in H2/N2 gas 200 ! •Cr O3+1/2C+1/2N2=Cr N+3CO 150 2 2 Ai/2Cr2O3+3/2C+1/2N2=CrN+3/2CO

OCr203+3CH4+1/2N2=Cr2N+3CO+6H2 £ 100 Ai/2Cr2O3+3/2CH4+1/2N2=CrN+3/2CO+3H2 oto 50 i r——= i ^— *H

' • —. -50 I -100 800 900 1000 1100 1200 1300 1400 1500 1600 Temperature (K) Fig.5 Standard free energy change of chromium nitrides formation S. Morita et al. HM 18 145

15th International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 2

3. 2. Synthesis of O2N powder To optimize the conditions for synthesis of O2N powder, synthesis trials were conducted, varying the C/O2O3 molar ratio in the range 2.8 to 3.2 and the H2/N2 molar ratio in the reaction atmosphere in the range 0 to 2.0. The reaction temperature used was 1393K. Thermal analysis had shown that the reaction was completed at this temperature with hydrogen also present in the atmosphere. Fig. 6 shows the results of chemical analysis of powder synthesized from starting material powder mixtures having different molar ratios, in an atmosphere with H2/N2 molar ratio 0.5. When C/O2O3 molar ratios of up to 3.0 were used, the reaction advanced more easily with increase in the molar ratio because of the lowering of oxygen content and increase in nitrogen content. However, in the range of molar ratio higher than 3.0, although there was further decrease in the oxygen content, the amount of nitrogen decreased along with increase in the content of carbon. The x-ray profiles of samples synthesized at C/Cr2O3 molar ratio 2.8, 3.0 and 3.2 are shown in Fig. 7. The main peak was of O2N in all the profiles. However, at the molar ratio of 2.8 some unreacted O2O3 was remaining and at 3.2 the O2N peak shifted towards a lower angle, suggesting increased dissolution of carbon in the product.

11.5

2.8 3.0 3.2 C/Cr2O3 (mol. ratio) Fig.6 The results of chemical analysis of powders fired mixed powder (C/Cr2O3=2.8,3.0,3.2 molar ratios) at 1393K in H2/N2 gas 146 HM 18 S. Morita et al.

15" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 2

C/Cr2O3=2.8

C/Cr2O3=3.0

I C/Cr2O3=3.2 40 29 (degree)

Fig.7 XRD profiles of powders fired mixed powder at 1393K in H2/N2 gas

11.5

0.0 0.5 1.0 1.5 2.0 H2/N2 (molar ratio) Fig.8 The results of chemical analysis of powders fired at 1393K in mixed gas (H2/N2=0~2 molar ratios)

We then studied the effect of hydrogen in the reaction atmosphere by synthesizing powders keeping the C/Cr2C>3 molar ratio in the starting material constant at 3.0 and varying the H2/N2 molar ratio in the range 0 to 2. Fig. 8 S. Morita et al. HM 18 147

15th International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 2 shows the results of chemical analysis of the powders obtained. When no hydrogen was present in the gas flow, the oxygen content in the product was as high as 1.24%. With increase in the proportion of hydrogen in the atmosphere, the residual oxygen level decreased, suggesting that the reaction had further advanced. Although the amount of residual carbon was lower at the H2/N2 ratio of 0.5 compared to when no hydrogen was present, it increased when the H2/N2 ratio was higher than 0.5. The reason for this could be that with increase in the proportion of hydrogen in the atmosphere there is an increase the partial pressure of hydrocarbon gas produced as a byproduct. This causes gas carburizing and the carbon content increases. The level of dissolved nitrogen was maximum (11.2%) when the H2/N2 molar ratio was 0.5.

3um

Powder fired at 1503K in N2 Powder fired at 1393K in H2/N2

Fig.9 SEM photographs of the Cr2N powders

Fig. 9 is a photograph showing Cr2N particles synthesized from starting material with C/Cr2C)3 molar ratio 3.0, in nitrogen atmosphere (1503K) and in nitrogen-hydrogen atmosphere (H2/N2=0.5, 1393K). Both were spherical particles with narrow particle size distributions. The chromium nitride powder synthesized in nitrogen atmosphere had a particle size of 2-3jum, but that prepared in nitrogen-hydrogen mixed gas atmosphere had a particle size of 148 HM 18 S. Morita et al.

15" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 2 about 1/vm, clearly showing that the particle growth was inhibited by the lower reaction temperature.

3. 3. Synthesis of CrN powder As shown in the phase diagram (Fig. 1), CrN is unstable at temperatures above 1373K at atmospheric pressure. Therefore, it is difficult to control the composition of product in the earlier-mentioned reaction temperature range for Cr2N. So we tried to synthesize pure CrN by further nitriding the fine particles of Cr2N synthesized from Cr2O3 and carbon, in the stable temperature range (below 1373K) of CrN. Thermal analysis was carried out with the Cr2N powder synthesized by carbo- thermal nitriding at 1393K using TG-DTA in the range of room temperature to 1773K, under nitrogen-hydrogen mixed gas flow. The results obtained are shown in Fig. 10. With increase in temperature, the weight started increasing gradually from 733K. Nitriding of Cr2N into CrN advanced up to 1300K. Beyond this temperature, the weight sharply decreased, reflecting the start of CrN decomposition. At the temperature range of 1403K to 1773K a mild decrease in weight was observed with increase in temperature. The rate of this weight reduction suggested that CrN was decomposing into Cr2N and Cr. This result suggests that it is possible to synthesize pure CrN from Cr2N by re-nitriding at a temperature between 733 and 1300K.

300

T200

+ 100

473 673 873 1073 1273 1473 1673 1773 Temperature (K)

Fig.10 TG-DTA curve of nitriding of Cr2N powder S. Morita et al. HM 18 149

15" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 2

Table 3 shows the results of analysis of a fine O2N powder synthesized by carbo-thermal nitriding at 1373K and a CrN powder obtained after heat- treating this O2N powder at 1173K for 3600 seconds under a nitrogen- hydrogen mixed gas flow. Both were fine powders with mean particle size of about 1/vm. The nitrogen content was 11.02% and 20.61%, which are respectively close to the theoretical nitrogen content of O2N and CrN. The residual oxygen and carbon, which are impurities, were each less than 1% in both cases. The CrN powder was purer, with only 0.08% carbon, which was only about 1/10th of the carbon content in the original C^N powder. Fig. 11 shows a SEM photograph and Fig. 12 shows the results of particle size distribution of the CrN powder. The CrN particles were spherical and fine like the Cr2N particles and showed a very narrow particle size distribution.

Table 3 Average particle size and chemical analyses of Cr2N and CrN powder Sample FSSS (urn) C (%) N (%) O (%)

Cr2N 1.20 0.74 11.02 0.83 CrN 1.31 0.08 20.61 0.45

Fig. 11 SEM photographs of the CrN powder 150 HM 18 S. Morita et al.

15r International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 2

100 r- 90 j | 80 ! £ 70 I 5 60 j. f 50 I 4300 I n 1 ! •f]f 20 O 10 j o L l ill 0.1 0.2 0.3 0.5 1 2 3 4 5 10 20 30 60 Particle diameter (|jm) Fig.12 Particle size distribution of the ON powder

Conclusion

The following conclusions were obtained in this study on the synthesis of chromium nitride powder by carbo-thermal nitriding.

1. Carbo-thermal nitriding from Cr2C>3 and carbon black started at 1473K in nitrogen atmosphere and produced chromium nitride powder.

2. The reaction initiation temperature was lowered to 1353K, when a gas mixture of nitrogen and hydrogen was used as the atmosphere in the reaction.

3. Chromium nitride was formed through the intermediate products of Cr3C2 and Cr2 (CN).

4. The fine Cr2N powder can be synthesized in a mixed gas atmosphere of nitrogen and hydrogen, and the pure CrN powder can be synthesized by renitriding of Cr2N powder.

5. The nitrogen contents in Cr2N and CrN powder were 11.02% and 20.60%, which were respectively close to the theoretical nitrogen values. S. Morita et al. HM 18 151

15" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 2

References

(1) G.V.Samsonov and I.M.Vinitskii: "Handbook of Refractory Compounds", (IFI/Plenum, New York, 1980), pp.371-386 (2) H.Kuwahara and JXakada: J. Japan Inst. Powder and Powder Metallurgy, 39(1992), pp.318-321 (3) H.Kuwahara, New Ceramics, Vol.11, No.9 (1998), pp.37-44 (4) H.J.Goldschmidt: "Interstitial Alloys", (Butterworths, London, 1967), pp.220 (5) M.Venkatraman and JP.Neuman, "Binary Phase Diagram", (ASM International, 1990), pp.1293-1297 (6) P.Schwarzkopf and R.Kieffer: "Refractory Hard Metals", (The Macmillan Company, New York, 1953), pp.248-250 (7) O.Kubaschewski and C.B.AIcock: "Metallurgical Thermochemistry", (Pergamon Press, Oxford, 1979), pp.378-379 (8) A.H.Sully: "Metallugy of the Rarer Metals No.1, Chromium", (Butterworth Scientific Publication, 1954), pp.52-54