Preparation of Titanium-Chromium Nitride by Gas Nitriding

of the Explosion Products of Cr-Coated Ti Wire

Sujeong Lee1, Wonbaek Kim1;*, Chang-yul Suh1, Sung-wook Cho1,Taegong Ryu1, Je-shin Park2 and In-Jin Shon2

1Mineral Resource Research Division, Korea Institute of Geoscience and Mineral Resources,92 Gwahang-no, Yuseong-gu, Daejeon 305-350, Korea2Division of Advanced Materials Engineering, Chonbuk National University, Chonbuk 561-756, Korea

Titanium-chromium nitride nanopowders were prepared by the electrical explosion of the Cr-coated Ti wires in argon and nitrogen gasesand subsequent nitriding treatments. The explosion product of the Cr-coated Ti wire in argon gas consisted of �-Ti, TiCr2 and Ti-rich � phases.They transformed into single phase (Ti,Cr)N by the nitriding treatment at 1100�C. On the other hand, the explosion product of Cr-coated Ti wirein nitrogen gas consisted of TiN, Cr2N, and a few nanometer-sized (Ti,Cr)N particles. The particles transformed into the mixture of TiN and(Ti,Cr)N by the nitriding treatment at 1100�C. In this case, the pre-existing TiN in the explosion product was stable during nitriding andremained intact to coexist with newly-formed (Ti,Cr)N. The (Ti,Cr)N particles prepared by nitriding of the explosion product of Cr-coated Tiwire in nitrogen gas had 34.21 at% Ti, which was somewhat lower than 41.27 at% Ti of the (Ti,Cr)N produced by nitriding of the explosionproduct of Cr-coated Ti wire in argon gas. This is probably because high thermal stability of the pre-existing TiN in the explosion product of Cr-coated Ti wire in nitrogen gas creates a local titanium deficiency in the formation of (Ti,Cr)N particles. [doi:10.2320/matertrans.M2010284]

(Received August 30, 2010; Accepted December 1, 2010; Published January 25, 2011)

Keywords: electrical wire explosion, (titanium,chromium)nitride, nanoparticle, gas nitriding

1. Introduction

Titanium-chromium nitride is one of the most oftenstudied ternary nitride materials in an attempt to improvethe mechanical, optical and physical properties of TiN.1–5)

Most of the previous studies have focused on preparing(Ti,Cr)N as thin films by using various methods. However,little effort to produce (Ti,Cr)N as a powder has been made.In 2006, Chinese researchers produced Cr1�xTixN powdersthrough a high-temperature nitridation of the mixture ofCr2O3 and TiO2, which were synthesized by the homoge-neous precipitation method.6) They reported that (Ti,Cr)Npowders could be fully densified even at a low temperatureof 1100�C by spark plasma sintering.

Recently, we have successfully synthesized nanoparticlesof alloys, intermetallics and nitrides by the electrical wireexplosion (EWE) process.7–11) In our previous work, weproduced the mixture of �-Ti and TiCr2 particles along withmeta-stable Ti-rich � phase by exploding a Cr-coated Ti wirein argon.10) Later on, we produced the mixture of TiN, Cr2N,and a few nanometer-sized fine particles, which are presum-ably (Ti,Cr)N, by exploding a Cr-coated Ti wire in nitro-gen.12) In this study, we aimed to synthesize (Ti,Cr)Nnanoparticles from two different explosion products obtainedin our previous work of Refs. 10) and 12). The feasibility ofproducing single phase (Ti,Cr)N nanoparticles via two dif-ferent ways was examined by the heat-treatment in nitrogen.

2. Experimental Procedure

The Ti-Cr wire for EXE was prepared by electrodepositionof Cr on a 0.289mm diameter Ti wire. Specific detailsregarding the continuous electrodeposition set-up is describ-

ed in Refs. 7) and 8). The Cr-coating solution consisted of2.47mol/L of chromic trioxide and 0.03mol/L of sulfuricacid. The temperature for the deposition was maintained at40�C. After washing and drying, the Cr-coated Ti wire wasput into a chamber for explosion experiments. Explosion wasconducted in a 30-liter airtight chamber, which was evac-uated preliminarily and filled with pure argon or nitrogen gas.The wire was exploded under the nitrogen or argon pressureof 0.1MPa. The capacitance of the exploding circuit was3.5 mF. The applied voltage across the 20mm-long wire was11.4 kV. The total number of explosions with each conditionwas about 600. After the explosion, the powders werestabilized overnight and filtered through 125 mm sieve toremove some misfired portions. The heat treatment and gasnitriding were conducted in a tube furnace with a heating rateof 10�C/min under flowing nitrogen gas. In some cases,samples were also annealed in vacuum furnace for compar-ison purpose. The reaction products were examined by field-emission transmission electron microscopy (FE-TEM, JEM2100F, Jeol Ltd., Japan) installed at Korea Basic ScienceInstitute, and X-ray diffraction (XRD, D/MAX 2200, RigakuCorp., Japan) with Cu K� radiation.

3. Results and Discussion

We previously reported that the explosion products of theCr-coated Ti wire in argon consisted of equilibrium phasesof �-Ti and TiCr2 along with meta-stable Ti-rich � phase.10)

The meta-stable � phase decomposes to �-Ti and TiCr2 bylow-temperature annealing at 600�C in vacuum.10) In thisstudy we heated the explosion products, that is, the mixtureof �-Ti, TiCr2 and Ti-rich � phases at various temperaturesunder flowing nitrogen gas to produce single phase (Ti,Cr)Nparticles. Figure 1 shows the X-ray diffraction patterns of thenitriding products at various temperatures. The most notable*Corresponding author, E-mail: wbkim@kigam.re.kr

Materials Transactions, Vol. 52, No. 2 (2011) pp. 261 to 264#2011 The Japan Institute of Metals RAPID PUBLICATION

effect of nitriding on the mixture of �-Ti, TiCr2 and Ti-rich �phases at 800�C was the complete disappearance of Ti-rich �phase (Fig. 1(b)). It is probably because Ti-rich � phase is notan equilibrium phase, which was formed by the fast coolingafter the explosion. Therefore, the first reaction in thenitriding process is the decomposition of Ti-rich � phase to�-Ti and TiCr2. The increase of �-Ti peak intensities alsosubstantiated the decomposition of Ti-rich � phase to �-Tiand TiCr2 (Fig. 1(b)). However, it should be also noted thatan �-Ti peak at around 40� shifted to lower angle astemperature increases. This implies that TiN lattice wasexpanded by the incorporation of nitrogen into TiN structure.According to the Ti-N phase diagram, the solubility ofnitrogen in �-Ti is about 10 at%. With higher nitrogencontents, the equilibrium solid phases of the Ti-N system are�-Ti and Ti2N. Therefore, it is interesting to observe theintensities of Ti2N peak (JCPDF 17-0386) increase withconcurrent decrease of �-Ti peak intensities. This suggeststhat Ti2N phase grows at the expense of super-saturated �-Tiphase. At 1100�C all other phases except (Ti,Cr)N disap-peared (Fig. 1(e)). The peaks of the newly-formed phase at1100�C are located between the peaks of TiN and CrN, whichimplies that the newly-formed phase is certainly (Ti,Cr)N.The lattice parameter of (Ti,Cr)N was calculated to be 0.4188 nm, which lies between that of TiN and CrN havinglattice parameters of 0.4242 nm and 0.4140 nm, respectively.

Figure 2 is typical TEM micrographs showing the explo-sion products of the Cr-coated Ti wires in argon gas and thereaction products at the temperature of 1100�C. The particlesproduced by nitriding treatment were cube-shaped, reflectingthe cubic symmetry of (Ti,Cr)N phase (Fig. 2(b)), while theas-exploded particles were mostly spherical (Fig. 2(a)).Some of (Ti,Cr)N particles were analyzed by EDS (Table 1).The average Cr content of (Ti,Cr)N particles prepared bynitriding of the explosion products of the Cr-coated Ti wirein argon gas was 8.3 at%.

It was demonstrated that single phase (Ti,Cr)N particlescould be synthesized by two-stage process, that is, the EWEof the Cr-coated Ti wire in argon gas, then followed bynitriding treatments. The second way of the synthesis of(Ti,Cr)N particles was also tried as a two-stage method. We

30 40 50 60 70 80

Diffraction angle, 2 θ/degree

(a)

(b)

(c)

(d)

(e)

TiN (JCPDF 38-1420)CrN (JCPDF 11-0065)

β (Ti,Cr)α-TiTi2NTiCr2TiN(Ti,Cr)N

Inte

nsity

, arb

. uni

t

Fig. 1 X-ray diffraction patterns of the explosion products of Cr-coated Ti

wire in Ar before and after subsequent nitriding heat treatment. The

explosion products were nitrided at various temperatures for 1 h in flowing

N2. The peak positions of TiN and CrN are marked with solid and dotted

lines on the figures; (a) As exploded particles, and the nitriding products at

(b) 800�C, (c) 900�C, (d) 1000�C, and (e) 1100�C.

Fig. 2 FE-TEM micrographs of the explosion products of Cr-coated Ti

wire in Ar; (a) As-exploded particles and (b) the cube-shaped (Ti,Cr)N

particles prepared by nitriding at 1100�C for 1 h in flowing N2 gas.

Table 1 EDS results of (Ti,Cr)N nanoparticles prepared by nitriding of two

different starting materials. The values are the average of concentrations in

six isolated particles in each specimen.

Starting material Ti (at%) Cr (at%) N (at%)

EWE product of the

Cr-coated Ti wire in Ar41:27� 2:80 8:27� 1:40 50:46� 3:50

consisting of �-Ti, TiCr2

and Ti-rich � phases

EWE product of the

Cr-coated Ti wire in N234:21� 3:50 10:55� 5:40 55:24� 3:30

mainly consisting of

binary nitrides

262 S. Lee et al.

have obtained the explosion products, the mixture of TiN,Cr2N and the clusters of very fine grains by exploding theCr-coated Ti wire in N2 gas.12) This mixture was heated at1100�C in vacuum or under flowing nitrogen gas. Figure 3(a)shows the X-ray diffraction pattern of the as-explodedmixture consisting of mainly binary nitrides, that is, TiNand Cr2N. It is evident that Cr2N phase decomposed intometallic Cr and nitrogen gas by heat treatment in vacuum(Fig. 3(b)). Note that the peak positions of TiN phase in theX-ray diffraction patterns remained unchanged by the heattreatment both in vacuum and nitrogen (Figs. 3(b) and (c)).This suggests that TiN is thermally stable at 1100�C in N2

and even in vacuum. Therefore, the pre-existing TiN inthe explosion product did not probably participate in theformation process of (Ti,Cr)N phase. Then (Ti,Cr)N synthe-sized by the second way should have higher Cr content thanthat produced by the first way.

Figure 4 is the X-ray diffraction patterns of the nitridingproducts from the mixture of TiN, Cr2N and (Ti,Cr)Nparticles. It is clear that the (220) and (311) peak positions ofTiN were not changed by nitriding treatment (Figs. 4(a), (b)).This suggests that any appreciable amount of nitrogen or Cris not incorporated into the TiN lattice during nitriding.Therefore, (Ti,Cr)N phase would be formed by the reactionbetween phases other than pre-existing TiN. Conclusively,single phase (Ti,Cr)N was not obtained by nitriding of binarynitrides, due to the thermal stability of pre-existing TiNphase. The lattice parameter of (Ti,Cr)N particles wascalculated to be 0.4192 nm. Figure 5 presents the TEMmicrographs of reaction product before and after nitriding.The explosion product consists of cube-shaped TiN, sphericalCr2N, and fine particles that are presumed to be (Ti,Cr)N.12)

The fine particles were not observed after nitriding(Fig. 5(b)). It is probably because the fine particles wereconsumed in the reaction with Cr2N to form (Ti,Cr)N.

30 40 50 60 70 80

Diffraction angle, 2 θ/degree

Inte

nsity

, arb

. uni

t

TiN (JCPDF 38-1420)CrN (JCPDF 11-0065)

(a)

(b)

(c)

Cr2NCr(Ti,Cr)NTiN

Fig. 3 X-ray diffraction patterns of the explosion products of Cr-coated Ti

wire in N2 before and after subsequent heat treatment at 1100�C for 1 h in

vacuum and N2; (a) As exploded particles in N2, (b) annealing products in

vacuum, and (c) annealing products in N2.

60 65 70 75 80

Diffraction Angle, 2 θ/degree

(220)TiN

(220)(Ti,Cr)N

(311)TiN

(311)(Ti,Cr)N

(222)TiN

(222)(Ti,Cr)N

(a)

(b)

Inte

nsi

ty, a

rb.u

nit

Fig. 4 X-ray diffraction patterns of the explosion products of Cr-coated Ti

wire in N2 before and after subsequent nitriding at 1100�C for 1 h in

flowing N2; (a) As exploded particles and (b) nitriding products. The peak

positions of TiN are marked with dotted lines on the figures.

Fig. 5 FE-TEM micrographs of the explosion products of Cr-coated Ti

wire in N2 before and after subsequent nitriding at 1100�C for 1 h in

flowing N2; (a) As exploded particles and (b) nitriding products.

Preparation of Titanium-Chromium Nitride by Gas Nitriding of the Explosion Products of Cr-Coated Ti Wire 263

The EDS data of individual (Ti,Cr)N particles revealedthat the average Cr content of (Ti,Cr)N particles by nitridingof binary nitrides was higher than that of (Ti,Cr)N producedby nitriding of the explosion products of the Cr-coated Tiwire in argon gas (Table 1). Because the pre-existing TiN inthe explosion product does not participate in the formationof (Ti,Cr)N from binary nitrides, newly formed (Ti,Cr)Nparticles have lower Ti content than those synthesized bynitriding of the explosion products of the Cr-coated Ti wirein argon gas. As a result, the Cr content in the (Ti,Cr)Nproduced by nitriding of binary nitrides is relatively highcompared with (Ti,Cr)N synthesized by nitriding of theexplosion products of the Cr-coated Ti wire in Ar.

To summarize, (Ti,Cr)N nanopowders were synthesizedsuccessfully by gaseous nitrding treatments of two differentwire-explosion products up to 1100�C. The first nitridingstarting material was the explosion products of the Cr-coatedTi wire in argon, and the second one was the explosionproducts of the Cr-coated Ti wire in nitrogen. The nitriding ofthe first nitriding starting material resulted in single phase(Ti,Cr)N while that of the second nitriding starting materialproduced the mixture of TiN and (Ti,Cr)N. The Cr content of(Ti,Cr)N, however, was relative higher when (Ti,Cr)N wasproduced from the second starting material. It is presumedthat the pre-existing TiN in the second starting material doesnot participate in the formation of (Ti,Cr)N, which resulted inlower Ti and higher Cr contents in (Ti,Cr)N, compared withthe (Ti,Cr)N produced from the first starting material.

Acknowledgement

This research was supported by the Basic Research Projectof the Korea Institute of Geoscience and Mineral Resources(KIGAM) funded by the Ministry of Knowledge Economy ofKorea.

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