WO2009072834A2

WO2009072834A2 - Ti3alc2 composite materials with high strength and method of manufacturing the same

Info

Publication number: WO2009072834A2
Application number: PCT/KR2008/007199
Authority: WO
Inventors: Sang Whan Park; Jae Ho Han; Sung Ho Yoon; Gyung Sun Cho
Original assignee: Korea Institute of Science and Technology KIST
Current assignee: Korea Institute of Science and Technology KIST
Priority date: 2007-12-07
Filing date: 2008-12-05
Publication date: 2009-06-11
Anticipated expiration: 2010-06-07
Also published as: KR100882924B1; WO2009072834A3

Abstract

The present invention relates to Ti3AIC2 composite material with high strength and a manufacturing process thereof, and particularly to TI3AlC2 composite material with high strength prepared using a mixture of TiCx and Al powder by hot- pressing process, where the particle size of unreacted TiCy in Ti3AIC2 matrix is controlled into nanometer size and the volumetric ratio of unreacted TiCy is adjusted by regulating the molar ratio of C in TiCx and the molar mixing ratio of TiCx/ Al, and a manufacturing process thereof. Ti3AIC2 composite material in the present invention and a sintered material prepared using the composite material as disclosed herein are similar to or even superior to the conventional Ti3AlC2 composite material in machinability, thermal, electrical and chemical properties, while remarkably improving the flexural strength of Ti3AIC2 sintered material. Thus, the material herein is useful for essential functional material in future energy industry such as an interconnector or bipolar plate for a fuel cell.

Description

[INVENTION TITLE]

TI₃ALC₂ COMPOSITE MATERIALS WITH HIGH STRENGTH AND METHOD OF MANUFACTURING THE SAME

[TECHNICAL FIELD]

The present invention relates to Ti₃AlC₂ composite material with high strength and a manufacturing process thereof, and particularly to Ti₃AlC₂ composite material with high strength prepared using a mixture of TiC_x and Al powder by a hot-pressing process, where the particle size of unreacted TiCy in Ti₃AlC₂ matrix is controlled into nanometer size and volumetric ratio of unreacted TiCy is adjusted by regulating the molar ratio of C in TiC_x and the molar mixing ratio of TiC_x/ Al, and a manufacturing process thereof.

[BACKGROUND ART] Pietzka and Schuster synthesized Ti₃AlC₂, one of layer-structured ternary carbides, in 1994 for the first time [/. phase Eψiilib., 1994, 15 (4), 392-400]. The most outstanding feature of ternary carbide-based ceramics including Ti₃AlC₂ is that they have both metal property and ceramic property at the same time, which is rare in engineering ceramics developed until now. Ti₃AlC₂ is superior in electrical and thermal conductivity, machinability and high thermal shock resistance, while showing a relatively low hardness and plastic deformation behavior at room temperature. At the same time, Ti₃AlC₂ has also general properties of ceramics such as a relatively high melting point, thermal and chemical stability, a relatively low density (4.25 g/cm³) and a relatively high elasticity. In particular, Ti₃AlC₂ is known to maintain high-temperature strength even at a temperature where super alloys cannot be used [Scr. Metall. Mater., 1997, 36, 535-541]. Moreover, T-3AIC2 sintered material is superior in heat resistance, oxidation resistance and mechanical properties, and is also machinable. Accordingly, Ti₃AlC₂ sintered material is applicable in high-temperature heat exchange, high-temperature structural material, aerospace engineering material and general oxidation-resistant industrial material, thus being commercially valuable compared to other engineering ceramics.

Pietzka and Schuster synthesized TI3AIC2 at 1300 ⁰C for 20 hours by using Ti, TiAl, AI4C3 and C powder as starting materials. However, the purity of Η3AIC2 has not been reported and the synthesis requires a relatively long time {]. phase Equilib., 1994, 15 (4), 392-400]. TIsAlC₂ sintered material is prepared using various starting materials at high temperature by various pressurizing processes. Tzeonov & Barsoum synthesized Η3AIC2 using Ti, AI4C3 and C powder as starting material by a hot isostatic pressing (HIP) method at 1400 ⁰C and 70 MPa for 16 hours. The reaction is 'AI4C3 + 6H2O → 2AI2O3 + 3CH₄ ¹. AI2O3 exists as secondary phase and the reaction requires high pressure and a relatively long synthesis time [/. Am. Ceram Soc, 2000, 83 (4), 825-832]. Recently, there have been attempts made to synthesize Η3AIC2 using Ti, Al and C powder as starting material by a self-propagating high- temperature synthesis (SHS) or by a hot pressing method. These are disclosed in Korean patent publication Nos. 10-2004-0030683, 10-2002-86867 and 10-2004- 0004091; U.S. patent Nos. 6497922, 6231969 and 6013322; and Japanese patent publication No. 2003-311627.

However, it has been known that highly pure Ti₃AlC₂ sintered material is difficult to synthesize because thus synthesized Ti₃AlC₂ contains reaction intermediates due to the reaction between starting materials. Further, the conventional Ti₃AlC₂ sintered material has been reported to comprise a large amount of micrometer-sized unreacted TiCy and secondary reactants [/. Ceram. Soc Japan, 1998, 106 (7), 676-681; /. Am. Ceram. Soc, 2000, 83 (4), 825-832; /. Mat. Sd. Let. 2001, 20, 1971-1973; Acta Mat, 2002, 50, 314-319; /. Mater. Chem., 2002, 12, 455-60; /. Europ. Ceram. Soc, 2003, 23, 567-574].

Further, the reported flexural strength of the conventional Η3AIC2 sintered material differs depending on the size of Η3AIC2 crystal grain and the amount of secondary phase contained in unreacted TiCy. Maximum flexural strength that is machinable with a conventional tool bit made of WC-Co is reported as 400-500 MPa or less. However, when considering brittle fracture characteristics of ceramics, flexural strength of TI3AIC2 is required to be improved without hampering machinability for applying T13AIC2 to the structural material in industry. However, the control of microstructure for increasing flexural strength is technically unsatisfactory until now due to formation of reaction intermediates through the reaction between reactants during the process of preparing T13AIQ.

The conventional process of preparing TIsAlC₂ sintered material uses Ti/TiAl/ALiCs/C powder, Η/AI4C3/C powder or Ti/ Al/ C powder as a starting material, and the target material is prepared by a hot pressing or pressure-less sintering method at high temperature. The use of the highly reactive reactants as starting materials causes the formation of reaction intermediates due to the reaction between reactants, which prevents the achievement of high purity TisAlC₂ sintered material. Moreover, microstructure of Η3AIQ sintered material and size and distribution of secondary phase formed in Η3AIC2 are difficult to control, thus making it difficult to achieve high strength of TIsAlC₂. Furthermore, maximum flexural strength of Ti3AlC2 sintered material containing secondary phase prepared by using thus developed starting material and manufacture process is 400-500 MPa or less.

[TECHNICAL PROBLEM]

Therefore, the present inventors have performed extensive researches to develop Η3AIC2 composite material with remarkably improved flexural strength of up to 920 MPa, while maintaining its machinability, thermal and chemical properties comparable to those of the Ti3AlC₂ composite material fabricated by a conventional process.

In an aspect, the present invention discloses Η3AIC2 composite material with high strength. In another aspect, the present invention discloses TISAIC₂ composite material with high strength, and a manufacturing process thereof. In particular, this material has a flexural strength of 7000 MPa or more as measured by a three-point bending strength test (span size: 20 mm, loading speed: 0.5 mm/min). This material is prepared by using a mixture of TiC_x and Al powder through a direct solid/ liquid phase reaction between TiC_x and Al powder without generating reaction intermediates such as unreacted TiCy and Ti₂AlC, and by hot-pressing process, where the particle size of unreacted TiC_y in Η3AIC2 matrix is controlled into nanometer size and volumetric ratio of unreacted TiCy is adjusted by regulating the molar ratio of C in TiC_x and the molar mixing ratio of TiC_x/ Al.

[TECHNICAL SOLUTION]

In still another aspect, the present invention discloses T13AIC2 composite material with high strength comprising TIsAlC₂, Ti₂AlC and TiCy (y > 0.6) as main crystalline phase where Η3AIC2 matrix is enhanced by TiCy phase with a size of 100 nm or less.

In yet another aspect, the present invention discloses TIsAlC₂ composite material with high strength where Η3AIC2 matrix comprises 10 vol% or less of Ti₂AlC secondary phase and unreacted TiC_y (y > 0.6), and 70 vol% or more of the TiCy has a size of 100 nm or less.

In yet still another aspect, the present invention discloses Ti₃AlC₂ composite material with high strength where crystal grain of the Ti₃AlC₂ has an average size of 30 μm or less.

In a further aspect, the present invention discloses Ti3AlC₂ composite material with high strength, where flexural strength is 700 MPa or more, in particular 700-920 MPa, as measured by a three-point bending strength test (span size: 20 mm, loading speed: 0.5 mm/min), and the Ti₃ AlC₂ composite material is machinable with a conventional tool bit made of WC-Co. In another further aspect, the present invention discloses a process of preparing Η3AIC2 composite material with high strength, the process comprising the steps of (a) hot-pressing a mixture of TiC_x(x=0.59-0.63) powder and Al powder mixed in a molar ratio of 3/1.0-3/1.4 up to 20-50 MPa at 1280-1320 ⁰C under vacuum (< 10'¹ torr) or inert conditions for 30 minutes - 1 hour, and (b) furnace cooling the hot-pressed material in the pressurized state.

In still another further aspect, the present invention discloses a process of preparing TIsAlC₂ composite material with high strength, where the TiC_x powder is prepared by uniaxially pressing a mixture of Ti or TiH₂ powder and a carbon black or graphite powder, elevating temperature at a rate of 2-10 °C/min, and reacting the pressed material at 1,000-1,500 °C for 5 minutes - 10 hours.

[DESCRIPTION OF DRAWINGS]

The above and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof illustrated the accompanying drawings which are given hereinbelow by way of illustration only, and thus are not limitative of the present invention, and wherein:

Figure 1 shows the X-ray diffraction pattern of the crystalline phase in Η3AIC2 composite material with high strength of the present invention; and

Figure 2 is the HR-TEM photographs showing the microstructure of nanometer-sized TiC_y (y > 0.6) particles in Η3AIC2 matrix of T13AIC2 composite material with high strength of the present invention.

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing.

[BEST MODE]

Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the drawings attached hereinafter, wherein like reference numerals refer to like elements throughout. The embodiments are described below so as to explain the present invention by referring to the figures.

Hereunder is provided a detailed description of the present invention. In the present invention, a mixture of TiC_x powder and Al powder is used as a starting material for preparing Η3AIC2 composite material, and TiC_x powder and Al powder have a particle size of 100 μm or less and 200 μm or less, respectively. The value of x in TiC_x powder is in the range of 0.59-0.63. The TiC_x powder is synthesized by using Ti or TiEb powder and carbon black or graphite powder. Ti or TiH₂ powder has a size of 100 μm or less, and carbon black powder and graphite powder have a size of 50 μm or less, respectively. The mixing ratio of Ti or TiH₂ powder and carbon or graphite powder can be selected so that TiC_x(x=0.59-0.63) powder may be prepared.

With regard to the preparation of TiC_x powder, Ti or TiH₂ powder and carbon black or graphite powder are mechanically mixed without addition of additives, and the mixture is uniaxially pressed. The pressed material is heated at a rate of 5-20 °C/min up to 1,000-1,500 ⁰C, and reacted under an inert or vacuum condition (< 10"¹ torr) for 5 minutes - 10 hours to provide TiC_x. The synthesized TiC_x is pulverized into a size of 100 μm or less.

A mixture of TiC_x and Al powder, which is used in the present invention as starting material for preparing Η3AIC2 composite material with high strength, has TiCχ/Al molar ratio of 3/0.9-3/1.3. A mixture of TiC_x/ Al powder is prepared by mechanical mixing method commercially used. A mixture of TiC_x/ Al powder is placed in a BN-coated graphite mold, and pressurized up to 5-20 MPa in a vacuum graphite high-temperature furnace. TIsAlC₂ composite sintered material was fabricated by a hot-pressing up to 20-50 MPa for 30 minutes - 1 hour at 1280-1320 ⁰C under inert or vacuum conditions (< 10⁴ torr), in which both synthesis and densification of Ti₃AlC₂ composite occur. During the hot-pressing process, temperature is elevated at a rate of 5-20 °C/min. TIsAlC₂ composite sintered material is prepared by furnace cooling the hot-pressed material at a pressurized state. The average size of Ti₃AlC₂ crystal grain in TIsAlC₂ composite sintered material herein can be controlled in the range of 5-30 μm.

Figure 1 shows the X-ray diffraction pattern of the crystalline phase in Ti₃AlC₂ composite material with high strength of the present invention. As shown in Figure 1, Ti₃AlC₂ composite material with high strength comprises Ti₃AlC₂, Ti₂AlC as secondary phase, and unreacted TiCy (y > 0.6). The amount of Ti₂AlC secondary phase and unreacted TiC_y in Ti₃AlC₂ matrix is 10 vol% or less.

Figure 2 shows the HR-TEM photographs showing the microstructure of nanometer-sized TiC_y (y > 0.6) particles in Ti₃AlC₂ matrix of Ti₃AlC₂ composite material with high strength of the present invention. As shown in Figure 2, unreacted TiCy phase with a size of 100 nm or less is incorporated in Ti₃AlC₂ matrix of Ti₃AlC₂ composite material with high strength. Seventy (70) vol% or more of the unreacted TiCy phase has a size of about 100 nm or less. No pores are found between the interfaces of unreacted TiCy phase and Ti₃AlC₂ matrix.

Ti₃AlC₂ composite sintered material with high strength of the present invention has a flexural strength of 700 MPa or more, in particular 700-920 MPa as measured by a three-point bending strength test (span size: 20 mm, loading speed: 0.5 mm/min), which is about 1.8 times higher than that of the Ti₃AlC₂ and Ti₃AlC₂ composite materials fabricated by conventional processes. Ti₃AlC₂ composite sintered material with high strength of the present invention has a Vickers hardness of 5-6 GPa, and is machinable with an ordinary hacksaw and a conventional tool bit made of WC-Co.

[EXAMPLES]

The following examples illustrate the invention and are not intended to limit the same.

Example 1

A mixture of TiC_x and Al powder was used as starting material for preparing Η3AIC2 composite sintered material with high strength. The size of TiC_x powder and Al powder is about 50 μm or less and about 200 μm or less, respectively.

TiC_x powder was synthesized by using Ti powder with a size of 100 μm or less and carbon black powder with a size of 30 nm or less. Ti powder and carbon black powder were mechanically mixed by changing the molar ratio of Ti/ carbon black to 0.57 and 0.60, and uniaxially pressed to provide a powder compact. The powder compact was reacted at 1550 ⁰C under vacuum condition (< lO"² torr) for 3 hours to provide TiC_x, and thus obtained TiC_x was pulverized into 100 μm or less.

The TiC_x/ Al molar ratio in the mixture of TiC_x and Al powder was 3/1.1, and a mixture of TiC_x/ Al powder was prepared by a mechanical mixing as commercially used. A mixture of TiC_x/ Al powder was placed in a BN-coated graphite mold, and pressurized up to 10 MPa in a vacuum graphite high- temperature furnace. Η3AIC2 composite sintered material was fabricated by hot- pressing this material up to 25 MPa at 1200, 1250, 1300, 1350 and 1400⁰C for an hour under argon atmosphere (< lO"¹ torr). During the hot-pressing process, temperature is elevated at a rate of 10 °C/min. Η3AIC2 composite sintered material is prepared by furnace cooling the hot-pressed material at a pressurized state.

The average size of Η3AIC2 crystal grain in TIsAlC₂ composite sintered material herein can be controlled in the range of 5-30 μm. It was ascertained that crystalline phase, a relative density, the size of Η3AIC2 crystal grain and mechanical property of thus prepared Ti₃AlC₂ composite sintered material varies depending on the compositions of TiC_x powder and the hot-pressing temperature.

X-ray diffraction analysis of TiC_x(x=0.57) powder shows that C is not totally dissolved in Ti, and some of Ti crystalline phase was observed. TiAl and Ti₃Al phase were found in Ti₃AlCa composite materials prepared by sintering TiC_x(x=0.57)/Al powder at all the hot-pressing temperatures. Flexural strength is 400 MPa as measured by a three-point bending strength test (span size: 20 mm, loading speed: 0.5 mm/min).

No Ti crystalline phase was found in X-ray diffraction analysis of TiC_x(X=O.60) powder. No crystalline phase other than Ti₃AlC₂, Ti₂AlC and unreacted TiCy (y > 0.6) was found in Ti₃AlC₂ composite sintered material prepared by hot-pressing TiC_x(x=0.60)/ Al powder at all the hot-pressing temperatures. With increasing hot pressing temperature to 1350 ⁰C or higher, no crystalline phase other than Ti₃AlC₂ was present in Ti₃AlC₂ composite sintered material. Moreover, the density of Ti₃AlC₂ composite sintered material fabricated by a hot pressing at 1300 ⁰C or higher reaches a theoretical density because the density of Ti₃AlC₂ sintered material increases as the hot-pressing temperature increases. Layer-structured plate-shaped Ti₃AlC₂ crystal grain began to form at 1250 ⁰C or higher. As the hot- pressing temperature increase from 1250 ⁰C to 1350 ⁰C, the average size of Ti₃AlC₂ crystal grain increase from 5 μm to 30 μm. The observation of Ti₃AlC₂ composite sintered material prepared at 1250-1400 ⁰C shows that Ti₃AlC₂ composite sintered material fabricated by a hot pressing at 1300 ⁰C exhibits the highest flexural strength, while flexural strength drastically decreases as the hot-pressing temperature increases beyond 1350 °C.

Figure 2 is the HR-TEM photographs showing the microstructure of Ti₃AlC₂ matrix of Ti₃AlC₂ composite material hot-pressed at 1300 ⁰C or lower. Unreacted TiCy particles with a size of 100 nm or less are found in Ti₃AlC₂ matrix. Ti₃AlC₂ and Ti₃AlC₂ composite material hot-pressed at 1250 ⁰C or higher has a Vickers hardness of 5-6 GPa, and is machinable with a hacksaw and a conventional tool bit made of WC-Co. Example 2

A mixture of TiC_x and Al powder was used as starting material for preparing Ti3AlC2 composite sintered material with high strength. The size of TiC_x powder and Al powder is about 100 μm or less and about 200 μm or less, respectively.

TiC_x powder was synthesized by using Ti powder with a size of 50 μm or less and carbon black powder with a size of 50 nm or less. Ti powder and carbon black powder were mechanically mixed by changing the molar ratio of Ti/ graphite to 0.62 and 0.67, and uniaxially pressed to provide a powder compact. The a powder compact was reacted at 1550 ⁰C under vacuum condition (< 10^~2 torr) for 3 hours to provide TiC_x, and thus obtained TiC_x was pulverized into 50 μm or less.

The TiC_x/ Al molar ratio in the mixture of TiC_x and Al powder was 3/1.1 and 3/1.2, and a mixture of TiC_x/ Al powder was prepared by a mechanical mixing as commercially used. A mixture of TiC_x/ Al powder was placed in a BN-coated graphite mold, and pressurized up to 10 MPa in a vacuum graphite high- temperature furnace. TIsAlC₂ composite sintered material was fabricated by hot- pressing this material up to 25 MPa at 1250, 1300 and 1350 ⁰C for an hour under argon atmosphere. During the hot-pressing process, temperature is elevated at a rate of 10 °C/min. Ti₃AlC₂ composite sintered material is prepared by furnace cooling the hot-pressed material at a pressurized state.

Crystalline phase, a relative density, the size of Ti₃AlC₂ crystal grain and mechanical property of thus prepared Ti₃AlC₂ composite sintered material varies depending on the C molar ratio in TiC_x powder and the hot-pressing temperature. In contrast, TiC_x/ Al molar ratio has an even weaker influence on crystalline phase, a relative density and TiβAlC- crystal grain size and mechanical properties of thus prepared Ti₃AlC₂ composite sintered material.

Effect of hot-pressing temperature on crystalline phase, a relative density of sintered material, size of Ti₃AlC₂ crystal grain and flexural strength of Ti₃AlC₂ composite sintered material fabricated by a hot pressing using a mixture of 3TiC_x(X=O.62)/ 1.1 Al, 1.2Al powder is similar to that of TIsAlC₂ composite sintered material fabricated by hot pressing using TiC_x(x=0.60)/Al powder as described in Example 1. In Η3AIC2 composite sintered material fabricated by hot pressing using 3TiC_x(0.62)/ 1.2 Al powder, Al layer is found on the surface of Η3AIC2 composite sintered material because excess unreacted Al liquid phase is squeezed out from Η3AIC2 composite sintered material during the hot-pressing. Η3AIC2 composite sintered material fabricated by hot pressing at 1300 ⁰C using 3TiC_x(x=0.62)/l.lAl and 3TiC_x(x=0.62)/ 1.2 Al powder has a fracture->flexural(?) strength of 920 MPa and 850 MPa, respectively. Flexural strength of T13AIC2 composite sintered material increases as the amount of C in TiC_x powder increases, and Flexural strength of Η3AIC2 composite sintered material decreases as the amount of Al in TiC_x(x=0.62)/ Al powder mixture increases. Η3AIC2 composite sintered material fabricated by a hot pressing using 3TiC_x(x=0.62)/l.lAl powder and 3TiC_x(x=0.62)/1.2Al powder at 1300 ⁰C has a Ti₃AlC₂ crystal grain size of 5-6 μm. Figure 2 is the HR-TEM photographs showing the microstructure of Η3AIC2 sintered material fabricated by a hot pressing at 1300 ⁰C or lower. Unreacted TiCy particles with a size of 100 nm or less are found in Η3AIC2 matrix. Η3AIC2 and Η3AIC2 composite material fabricated by a hot pressing at 1250 ⁰C or higher has Vickers hardness of 4-6 GPa, and is machinable with a hacksaw and a conventional tool bit made of WC-Co.

Effect of hot-pressing temperature on crystalline phase, a relative density of sintered material, size of Η3AIC2 crystal grain and flexural strength of Η3AIC2 composite sintered material fabricated by a hot pressing using a 3TiC_x(x=0.67)/l.lAl powder is similar to that of Η3AIC2 composite sintered material fabricated by a hot pressing using 3TiC_x(x=0.62)/l.lAl powder and 3TiC_x (O.62)/1.2A1 powder. However, TiCy (y > 0.6) phase is found in Η3AIC2 composite sintered material fabricated by a hot pressing using a 3TiC_x(x=0.67)/l.lAl powder at 1350 ⁰C or higher, and for Η3AIC2 composite sintered material fabricated, by a hot pressing using a 3TiC_x(x=0.67)/l.lAl powder at 1350 ⁰C or lower, the amount of unreacted TiCy (y > 0.6) Ti₃AlC₂ composite sintered material is more than 1.2 times higher in U3AIC2 composite sintered material fabricated by hot pressing using 3TiC_x(x=0.62)/l.lAl powder or 3TiC_x(x=0.62)/1.2Al powder. Ti₃AlC₂ composite sintered material fabricated by a hot pressing using 3TiC_x(x=0.67)/l.lAl powder and

powder shows the highest flexural strength when it was hot- pressed at 1300 ⁰C. The flexural strength of TIsAlC₂ composite sintered material fabricated by a hot pressing using 3TiC_x(x=0.67)/l.lAl powder and 3TiC_x(x=0.67)/1.2Al powder is 620 MPa and 560 MPa, respectively. HR-TEM and SEM microstructures of Ti₃AlC₂ sintered material fabricated by a hot pressing using 3TiC_x(x=0.67)/l.lAl powder and 3TiC_x(x=0.67)/1.2Al powder, at 1300 ⁰C or lower show not only unreacted TiCy particles with a size of 100 nm or less and TiC_y particles with a size of from a few to several tens of micrometers in Ti₃AlC₂ matrix. Ti₃AlC₂ and Ti₃AlC₂ composite material fabricated by a hot pressing at 1250 ⁰C or higher has Vickers hardness of 7-8 GPa, and machining with a conventional tool bit made of WC-Co causes chipping during the process.

Example 3

A mixture of TiC_x and Al powder was used as starting material for preparing Ti₃AlC₂ composite sintered material with high strength. The size of TiC_x powder and Al powder is about 10 μm or less and about 80 μm or less, respectively.

TiC_x powder was synthesized by using TiH₂ powder with a size of 40 μm or less and carbon black powder with a size of 50 nm or less. TiH₂ powder and carbon black powder were mechanically mixed by changing the molar ratio of Ti/ graphite to 0.61 and 0.63, and uniaxially pressed to provide a powder compact. The powder compact was reacted at 1100 ⁰C under vacuum condition for 6 hours to provide TiC_x, and thus obtained TiC_x was pulverized into 10 μm or less.

The TiC_x/ Al molar ratio in the mixture of TiC_x and Al powder was 3/1.1 and 3/0.9, and a mixture of TiC_x/ Al powder was prepared by a mechanical mixing as commercially used. A mixture of TiC_x/ Al powder was placed in a BN-coated graphite mold, and pressurized up to 10 MPa in a vacuum graphite high- temperature furnace. Η3AIC2 composite sintered material was fabricated by a hot pressing up to 25 MPa at 1270, 1320 and 1350 ⁰C for 40 minutes under argon atmosphere. During the hot-pressing process, temperature is elevated at a rate of 10 °C/min. Η3AIC2 composite sintered material is prepared by furnace cooling the hot-pressed material at a pressurized state.

Crystalline phase, a relative density, the size of T13AIC2 crystal grain and mechanical property of thus prepared Ti3AlC2 composite sintered material varies slightly depending on the C molar ratio in TiC_x powder. Moreover, as the molar ratio of C in TiC_x powder increases, size of Η3AIC2 crystal grain slightly decreases and the amount of unreacted TiC_y slightly increases in Η3AIC2 composite sintered material as described in Example 2.

In Ti3AlC2 composite sintered material fabricated by a hot pressing using 3TiCχ(x=0.61)/l.lAl powder mixture and 3TiC_x(x=0.63)/l.lAl powder mixture, the amount of unreacted TiC_y in Ti₃AlC₂ matrix decreases and size of crystal grain increases as the hot pressing temperature increases from 1320 ⁰C to 1350 ⁰C. Moreover, in Η3AIC2 composite sintered material fabricated by a hot pressing using TiC_x(x=0.61)/l.lAl powder and 3TiC_x(x=0.63)/l.lAl powder, flexural strength drastically decreases as the hot pressing temperature increases from 1320 ⁰C to 1350 ⁰C. In Η3AIC2 composite sintered material fabricated by hot pressing at 1320 ⁰C by using 3TiCχ(x=0.61)/l.lAl and 3TiC_x(x=0.63)/l.lAl powder, size of Ti₃AlC₂ crystal grain is 8 μm and 7 μm, respectively, and flexural strength is 880 MPa and 720 MPa, respectively. Figure 2 shows the microstructure of Ti₃AlC₂ composite sintered material fabricated by hot pressing at 1320 ⁰C using 3TiC_x(x=0.61)/l.lAl powder mixture. Unreacted TiCy particles with a size of 100 nm or less are found in Η3AIC2 matrix. In TIsAlC₂ composite sintered material fabricated by hot pressing using 3TiC_x(x=0.63)/l.lAl powder, a small amount of micrometer-sized TiCy (y > 0.63) particles are found in Η3AIC2 matrix besides unreacted TiCy particles with a size of

100 nm or less. Η3AIC2 composite sintered material fabricated by hot pressing at 1320 and 1350 ⁰C using 3TiC_x(x=0.61)/l.lAl and 3TiC_x(x=0.63)/l.lAl powder mixture has Vickers hardness of 5-6 GPa, and is machinable with a hacksaw and a super hard tool (WC-Co).

The amounts of unreacted TiCy particles in Η3AIC2 composite sintered material fabricated by hot pressing using 3TiC_x(x=0.61)/0.9Al and 3TiC_x(x=0.63)/0.9Al powder is more than two times higher than that in Ti₃AlC₂ composite sintered material fabricated by hot pressing using

powder and 3TiC_x(x=0.63)/l.lAl powder. As shown in Figure 2, TiCy particles with a size of from a few to several tens of micrometers are found in Ti₃AIC₂ matrix besides unreacted TiCy particles with a size of 100 nm or less. A relative density of Η3AIC2 composite sintered material is 95% or less. The amount of unreacted TiCy in T13AIC2 matrix increases as the molar ratio of C in TiC_x powder used as starting material increases. Η3AIC2 and TIsAlC₂ composite material fabricated by a hot pressing using 3TiC_x(x=0.61)/0.9Al powder and 3TiC_x(x=0.63)/0.9Al powder has flexural strength of up to 450 MPa_x and cannot be machinable with a conventional tool bit made of WC-Co.

[INDUSTRIAL APPLICABILITY]

Despite its superiority in machinability, thermal, mechanical and electrical properties, the conventional Η3AIC2 and Ti₃AlC₂ composite sintered material has flexural strength of 500 MPa or less. Therefore, the application of Ti₃AlC₂ showing brittle fracture property to the structural part industry is difficult as in the conventional engineering ceramics.

In contrast, novel starting material is used with the optimization of mixing ratio in the present invention for the development of Η3AIC2 composite material with high strength, while unreacted starting material with a size of about 100 nm is incorporated in the T-3AIC2 matrix. As a result, the flexural strength of Ti₃AlC₂ composite material is improved by 1.8 times or even higher, while machinability, thermal and electrical properties of Η3AIC2 are maintained. Η3AIC2 composite material with high strength herein and its preparation method thereof can facilitate the application of T13AIC2 composite material in industry scale structural parts.

Ti3AlC2 composite sintered material in the present invention prepared using the composite material as disclosed herein are similar to or even superior to the conventional TI3AIC2 composite material in machinability, thermal, electrical and chemical properties, while remarkably improving the flexural strength of TIsAlC₂ sintered material. Thus, the Η3AIC2 composite sintered material herein is useful for essential functional material in future energy industry such as an interconnector or bipolar plate for a fuel cell.

The invention has been described in detail with reference to preferred embodiments thereof. However, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.

Claims

[CLAIMS]

[CLAIM 1]

A Η3AIC2 composite material with high strength comprising Η3AIC2, Η2AIC and TiCy (y > 0.6) as main crystalline phase, wherein Η3AIC2 matrix is strengthened with TiCy phase with a size of 100 nm or less.

[CLAIM 2]

The Η3AIC2 composite material of claim 1, wherein the Η3AIC2 matrix comprises 10 vol% or less of Ti₂AlC secondary phase and unreacted TiC_y (y > 0.6), and 70 vol% or more of the TiC_y has a size of 100 nm or less.

[CLAIM 3]

The Η3AIC2 composite material of claim 1 or 2, wherein crystal grain of the Η3AIC2 has an average size of 30 μm or less.

[CLAIM 4]

The Η3AIC2 composite material of claim 1 or 2, wherein flexural strength is 700 MPa or more as measured by a three-point bending strength test (span size: 20 mm, loading speed: 0.5 mm/min), and the T13AIC2 composite material is machinable with a conventional tool bit made of WC-Co.

[CLAIM 5]

A process of preparing Η3AIC2 composite material with high strength, the process comprising the steps of:

(a) hot-pressing a mixture of TiC_x(x=0.59-0.63) powder and Al powder mixed in a molar ratio of 3/1.0 - 3/1.4 up to 20-50 MPa at 1280-1320 ⁰C under vacuum (< 10'¹ torr) or inert conditions for 30 minutes - 1 hour, and

(b) furnace cooling the hot-pressed material in the pressurized state. [CLAIM 6]

The process of claim 5, wherein the TiC_x powder is prepared by uniaxially pressing a mixture of Ti or TiH₂ powder and a carbon black or graphite powder, elevating temperature at a rate of 2-10 ^oC/min, and reacting the pressed material at 1,000-1,500 ⁰C for 5 minutes - 10 hours.