Synthesis of Ultra Fine Titanium-Tungsten Carbide Powder from Titanium Dioxide and Ammonium Metatungstate

Materials Transactions, Vol. 50, No. 1 (2009) pp. 187 to 191 #2009 The Japan Institute of Metals EXPRESS REGULAR ARTICLE

Gil-Geun Lee1 and Gook-Hyun Ha2

1Division of Materials Science and Engineering, Pukyong National University, San 100, Yongdang-Dong, Nam-Gu, 608-739, Korea 2Korea Institute of Materials Science, 531 Changwondaero, Changwon, Kyungnam, 641-831, Korea

In the present study, the focus is on the synthesis of ultra ﬁne titanium-tungsten carbide powder by the carbothermal reduction process. The starting powder was prepared by the combination of drying and calcination methods using titanium dioxide powder and an aqueous solution of ammonium metatungstate to obtain a target composition of 60 mass%TiC-40 mass%WC, i.e., (Ti0:83W0:17)C. The synthesized oxide powder was mixed with carbon black, and this mixture was then heat-treated under a ﬂowing argon atmosphere. The changes in the phase structure, thermal gravity and particle size of the mixture during heat treatment were analyzed using XRD, TG-DTA and SEM. The synthesized oxide powder has a mixed phase structure of anatase-TiO2 and WO3 phases. This composite oxide powder was carbothermally reduced to titanium-tungsten carbide by solid carbon through three steps with increasing temperature; the reduction of WO3, the reduction of TiO2 and formation of tungsten carbides, and the formation of titanium-tungsten carbide. The synthesized (Ti1xWx)Cy powder at 1673 K for 10.8 ks has an average particle size of about 200 nm. [doi:10.2320/matertrans.MER2008271]

(Received August 13, 2008; Accepted October 27, 2008; Published December 17, 2008) Keywords: titanium-tungsten carbide, hard metals, carbothermal reduction, ultra ﬁne powder

1. Introduction the other carbides (TaC, NbC and (Ti,W)C, etc.), however, no commercial process has been established for the synthesis Hard metal based on WC/Co has been widely used for of such ultra fine particles. wear-resistant machine parts or tool materials because it has Titanium-tungsten carbide, (Ti1xWx)Cy, has been used as superior mechanical properties, such as wear resistance and a dispersed particle in the WC/TiC/Co system hard metals high temperature strength, etc.1) In order to increase the for increasing high temperature hardness and decreasing mechanical properties and resistance to ‘‘cratering’’ at the reaction with steel alloy during machining. Generally, elevated temperatures, TiC is added to WC/Co hard metal. titanium-tungsten carbide was commercially manufactured WC/TiC/Co system hard metals were devised for machining by carbothermal reduction using metallic tungsten powder, steels and other ferrous alloys, the purpose of the TiC content titanium dioxide powder and carbon black as the raw being to resist the high-temperature diffusive attack that materials.1) This process requires a temperature range of causes chemical breakdown and ‘‘cratering’’. WC/TiC/Co 2073–2473 K for the commercial production of the titanium- system hard metals have two distinct carbide phases, angular tungsten carbide with a solid solution or mixed-crystal, crystals of almost pure WC and rounded TiC/WC ‘‘mixed because of the difficulty of reduction of titanium dioxide. crystals’’.2–4) Because of differential diffusion effects, the Therefore, it is difficult to produce fine particles using this latter often show a ‘‘cored’’ structure. The third phase is the process due to the agglomeration of the particles at the high cobalt binder. A solid solution or ‘‘mixed crystal’’ of WC in synthesis temperature. Recently, a new mechano-chemical TiC (TiC is almost insoluble in WC) retains the anti-cratering process was proposed for the synthesis of ultra fine WC/TiC/ property to a great extent.1–4) Co composite powder.10) This process could be used to The mechanical properties of the hard metals depend not synthesize ultra fine WC/TiC/Co composite powder at about only on the chemical composition but also on their micro- 1673 K by the combination of spray conversion and direct structure.1) The hardness, fracture toughness and wear carburization processes using a metallic salts solution as the properties of the hard metals were strongly influenced by raw material. However, this process has an environmental the size of the carbide particles, the degree of homogeneity of pollution problem due to the chemical processing based on the microstructure and purity of the initial powders. These the spray conversion of the metallic salts, especially titanium mechanical properties increased simultaneously with de- chloride. The titanium chloride, TiCl3, dissolved into creasing particle size and increasing homogeneity. To titanium and chloride ions in distilled water during prepara- manufacture high performance hard metal with an ultra fine tion of the starting solution for spray conversion. The microstructure, raw powder materials with an ultra fine chlorine ions in the solution were transformed into chlorine particle size should be used. Recently, several methods have gas during spray conversion. It become generally known that been proposed for the synthesis of ultra fine particles.5–8) In this chlorine gas causes serious environmental pollution the case of WC and TiC, it was possible to synthesize the problems. Recently, we proposed a new carbothermal WC/Co and TiC/Co composite powders with an average reduction process for the synthesis of ultra fine TiC and particle size under 100 nm and 200 nm, respectively.8,9) TiC/Co powders.9,11,12) Because this new process used ultra These composite powders were manufactured by the combi- fine titanium dioxide powder instead of chloride as the nation of spray conversion and carburization processes. For titanium source, it could possibly decrease the environmental 188 G.-G. Lee and G.-H. Ha pollution problem. Also, it decreased the reduction temperature of the titanium dioxide and titanium carbide formation temperature compared to the conventional processes. In the present study, the focus is the synthesis of titanium- tungsten carbide powder by the carbothermal reduction process to investigate the synthesis possibilities of ultra fine titanium-tungsten carbide using titanium dioxide powder as the titanium source without the chemical processing based on the spray conversion of the chloride solution.

2. Experimental Procedure

The starting slurry was prepared by suspending the titanium dioxide powder (an average particle size of 50 nm Fig. 2 SEM micrograph of ball-milled mixture of calcined composite oxide powder and carbon black. and an anatase structure) in an aqueous solution of ammonium metatungstate (AMT, (NH4)6(H2W12O40)4H2O), to obtain a target composition of 60 mass%TiC-40 ray diffraction peaks related to tungsten. This means that the mass%WC, i.e., (Ti0:83W0:17)C. Drying was performed using dried powder from the starting slurry has a homogeneously an ultrasonic generator with a frequency of 20 kHz at about mixed state of the titanium dioxide, tungsten-based salt and 358 K. The precursor powder obtained by the drying was moisture. However, the calcined powder shows diffraction decomposed at 1073 K for 7.2 ks in air to remove the salt peaks of anatase-TiO2 and WO3. By heating the precursor components and form a titanium-tungsten-oxygen-based powder in air atmosphere, the volatile components (such as composite oxide powder. The calcined composite oxide NHx,NOx and H2O) were evaporated by decomposition of powder was mixed with carbon black (mean particle size: the tungsten-based salt. The anatase-TiO2 phase was less 0.5 mm) for 86.4 ks using a tumbler-ball mill with a ball-to- stable than the rutile-phase-structured one.13) In general, powder mass ratio of 10 : 1. The mixture was placed in a titanium dioxide of the metastable anatase phase transforms graphite crucible and then heated in the temperature range into the stable rutile phase during heating. In this study, the from 823 K to 1673 K for 1.8 ks in a tube furnace under a phase transformation of the titanium dioxide was not found flowing stream of argon atmosphere. After heat treatment, the during calcination at 1073 K for 7.2 ks. No chemical reaction samples were analyzed by X-ray diffraction using Cu-K occurred between the titanium dioxide and the tungsten radiation and observed by SEM. The changes in the thermal component during calcination. gravity of this powder mixture during heat treatment from Figure 2 shows an SEM micrograph of the ball-milled room temperature to 1673 K in flowing argon was recorded mixture of the calcined composite oxide powder and carbon by TG-DTA at a heating rate of 8:3 102 K/s. black. The synthesized starting composite oxide powder from the drying had an irregular morphology with an average 3. Results and Discussion particle size of about 2 mm. After milling, the particle morphology changed to a globular shape, and the average Figure 1 shows the X-ray diffraction patterns of the dried particle size decreased to below about 0.5 mm. The composite precursor powder and calcined powder. The diffraction peak oxide particles were homogeneously mixed with carbon of the precursor powder prepared by the drying method black, and there were no visible differences in the X-ray shows only the anatase-TiO2 phase. There are no visible X- diffraction patterns of the powder before and after the ball- milling. Figure 3 shows the mass change in the mixture of the calcined composite oxide powder and carbon black as a TiO (anatase) WO 2 3 function of the temperature during heating in the flowing argon gas. The theoretical quantities of the carbon for the carbothermal reduction of the calcined composite oxide powder are estimated by:

WO3ðsÞ þ TiO2ðsÞ þ 6CðsÞ ! (Ti,W)CðsÞ þ 5COðgÞ dried precursor powder In this study, 10% excess carbon was used for the reaction. The theoretical mass fraction of the reaction was 58.83% after complete carbothermal conversion of the titanium- Intensity (arb.units) tungsten-oxygen-based composite oxide powder of 60 mass%TiC-40 mass%WC target composition by the reaction. calcined powder As shown in Fig. 3, the value of the mass fraction of the 20°°°°°°°30 40 50 60 70 80 mixture at 1673 K was higher than 58.83%, which means that 2θ the present titanium-tungsten-oxygen-based oxide in the mixture could not be completely converted to titanium- Fig. 1 X-ray diﬀraction patterns of the dried and calcined powders. tungsten carbide during the analysis by TG-DTA. The mass Synthesis of Ultra Fine Titanium-Tungsten Carbide Powder from Titanium Dioxide and Ammonium Metatungstate 189

100 1203 WO3 TiO2 (anatase) TiO2(rutile) W O magneli Ti O Ti O 1123 18 49 2 3 3 5 90 W C W (Ti W )C 1273 2 1-x x y

80 (111) (200) (220) 70 (311) (222) (102) Mass Fraction (%) 60 1513 1673K

50 300 500 700 900 1100 1300 1500 1700 1573K Temperature, T / K

Fig. 3 Change in the mass fraction of the mixture of the calcined composite oxide powder and carbon black versus temperature. 1473K of the mixture powder started to decrease around 1123 K and 1273K showed a first remarkable decrease in the mass fraction at units) Intensity (arb. about 1203 K. A second significant mass loss occurred at about 1273 K, and the mass fraction continuously decreased with increasing temperature until the heat-treatment temper- 1223K ature reached about 1513 K. These mass losses mean that the carbothermal reactions remarkably occur at these temper- 1123K atures. Figure 4 shows the X-ray diffraction patterns of the 1073K mixture of the calcined composite oxide powder and carbon 20° 30° 40° 50° 60° 70° 80° black heat-treated in the tube furnace under a flowing stream of argon at a specified temperature for 1.8 ks. The heat- 2θ treatment temperature was determined from the TG curve (Fig. 3). Below 1073 K, the X-ray diffraction patterns Fig. 4 X-ray diffraction patterns of the mixture of the calcined composite showed anantase-TiO and WO phases as the calcined oxide powder and carbon black heat treated in the tube furnace under a 2 3 flowing stream of argon at a specified temperature for 1.8 ks. composite oxide powder. The diffraction pattern changed to the mixed state of the anatase-TiO2, rutile-TiO2 and W18O49 phases at 1123 K. At 1223 K, there were two kinds of phases steps; i.e., reduction of TiO2 to form TinO2n1 (n > 3) in the heat-treated powder; i.e., the rutile-TiO2 and W. This phases, formation of titanium oxycarbide and deoxidization means that the titanium dioxide had undergone phase of the titanium oxycarbide to titanium carbide.12,15) The transformation from the metastable anatase phase structure titanium oxycarbide, TiCxOy, is a solid solution of TiC and 16,17) to the stable rutile phase structure above 1073 K. The WO3 in TiO. Generally, the initially formed carbide from the the calcined powder was preferentially reduced to tungsten titanium dioxide by the solid carbon is not pure titanium below 1223 K. Therefore, this indicates that the first carbide. Thermodynamic calculation predicts that the initial remarkable decrease in the mass fraction at about 1203 K is composition of TiCxOy is dependent on the temperature and the preferential carbothermal reduction of the WO3 in the partial pressure of CO and will be between TiC0:6O0:4 and 17) calcined composite oxide powder by the solid carbon. The X- TiC0:7O0:3. However, no diffraction peaks related to the ray diffraction pattern of the heat-treated powder at 1273 K titanium oxycarbide are found in the Fig. 4. At 1673 K, there showed the coexistence of W2C, W and magneli phases. The were two kinds of phases in the heat-treated powder; i.e., the magneli phase is the lower oxide in the Ti-O system, which is (Ti1xWx)Cy and a small amount of W2C. No different oxide 4) written as TinO2n1 (n > 3). This means that the carburiza- phases were found in the cases of heat-treated powders below tion of the tungsten and reduction of the titanium dioxide 1573 K. The diffraction peaks related only to tungsten almost simultaneously occurred at about 1273 K. Therefore, the disappeared in the diffraction pattern compared to other second significant decrease in the mass fraction at about X-ray diffraction patterns below 1573 K. However, the 1273 K indicates the reduction of the titanium dioxide in the diffraction peaks related to titanium-tungsten carbide more calcined composite oxide powder. At 1473 K, there were four clearly appeared in the diffraction pattern. This means that kinds of phases in the diffraction pattern: Ti3O5,W2C, W and the tungsten carbide and/or tungsten dissolved into the Ti2O3. The X-ray diffraction pattern of the heat-treated carburized titanium phase. powder at 1573 K showed W2C, W, Ti2O3 and (Ti1xWx)Cy. No chemical reaction occurred between the titanium and The titanium-tungsten carbide, (Ti1xWx)Cy, is a solid tungsten components during reduction of the oxide phases in solution of TiC and WC with a maximum solubility of about the calcined composite oxide powder. It was considered that 72 mass%WC.14) In the case of the carbothermal reduction of the reduction reactions of the titanium oxides and tungsten pure titanium dioxide powder by solid carbon, the titanium oxides in the titanium-tungsten-oxygen-based composite dioxide was carbothermally reduced to TiC through three oxide powder by the solid carbon occurred independently. 190 G.-G. Lee and G.-H. Ha ) y )C

x 1

W 1573K 1-x 1673K 0.1 C / (Ti 2

0.01

0.001

0 5 10 15 20 25

Relative Intensity of XRD (W Relative Heat Treatment Time, t / ks

Fig. 5 Change in the relative intensity of the XRD peaks of tungsten carbide to (Ti1xWx)Cy versus heat-treatment time at 1573 K and 1673 K.

The carburization reaction of the tungsten component occurred at lower temperature than that of the titanium component. The dissolving reaction of the tungsten carbide in the titanium component simultaneously occurred with the carburization reaction of the titanium component. Contrary to the titanium-oxygen-based oxide powder, no titanium oxycarbide, TiCxOy, was found. It was considered that the carburization of the titanium component and the formation of Fig. 6 SEM micrographs of synthesized (Ti1xWx)Cy powder at 1673 K for (a) 10.8 ks and (b) 21.6 ks by the carbothermal reduction using (Ti1xWx)Cy simultaneously occurred above 1473 K. For titanium-tungsten-oxygen-based composite oxide powder. verification of the reaction mechanism during heat treatment, however, fundamental research is needed concerning the crystal structure and chemical composition in the future. using a titanium-tungsten-oxygen-based composite oxide Figure 5 shows the change in the relative intensity of the powder. This composite oxide powder with a target compo- XRD peaks of the W2C to that of (Ti1xWx)Cy in the samples sition of 60 mass%TiC-40 mass%WC (or (Ti0:83W0:17)C) can heated at 1573 K and 1673 K. The peak intensities for the be synthesized by the combination of drying and calcination W2C and (Ti1xWx)Cy corresponding to the (102) and (111) processes using titanium dioxide powder and an aqueous reflections, respectively, are used. As shown in Fig. 5, the solution of ammonium metatungstate. The synthesized amount of the retained W2C decreased with the increasing composite oxide powder has a mixed phase structure of heat treatment time. In spite of 21.6 ks heat treatment, a small anatase-TiO2 and WO3 phases with an average particle size amount of W2C still remained in the case of the 1573 K of about 0.5 mm. This composite oxide powder is carbother- sample. At 1673 K, however, no diffraction peak related to mally reduced to the titanium-tungsten carbide through the W2C phase was found above the heat-treatment time of three steps with increasing temperature; i.e., the reduction 10.8 ks. The heat-treatment time needed for complete of WO3, the reduction of TiO2 and formation of tungsten dissolution of the retained W2C to (Ti1xWx)Cy for the final carbides, and the formation of titanium-tungsten carbide. (Ti0:83W0:17)C composition was at least 10.8 ks. The reduction reactions of the titanium and tungsten Figure 6 shows SEM micrographs of synthesized components in the composite oxide powder by the solid (Ti1xWx)Cy powders at 1673 K for 10.8 ks and 21.6 ks. No carbon occurred independently. The carburization of the difference was found in particle size, morphology and degree tungsten component occurred at a lower temperature than of agglomeration with the heat-treatment time. The mean that of the titanium component. The formation of the particle size was less than 200 nm and agglomeration degree titanium-tungsten carbide occurred by the simultaneous was very low. It was known that ultra fine (Ti1xWx)Cy reaction of the carburization of the titanium component and powder can be synthesized by the carbothermal reduction the dissolution of tungsten carbide. The synthesized process using titanium dioxide powder and an ammonium (Ti1xWx)Cy powder at 1673 K for 10.8 ks has an average metatungstate. For verification of the crystal structure of particle size of about 200 nm. the synthesized particles, however, fundamental research is needed concerning the lattice constant and chemical compo- REFERENCES sition in the future. 1) K. J. A. Brookes: Word Directory and Handbook of Hardmetals and 4. Conclusion Hard Materials, 6th ed., (International Carbide Data, Marsh Barton, 1996) pp. 9–102. 2) ASM International Handbook Committee: ASM Handbook, Vol. 7, The present study focused on the synthesis of titanium- Powder Metal Technologies and Applications ed. by ASM Inter- tungsten carbide powder by a carbothermal reduction process national Handbook Committee, (ASM International, Materials Park, Synthesis of Ultra Fine Titanium-Tungsten Carbide Powder from Titanium Dioxide and Ammonium Metatungstate 191

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