KR20160083194A - SiC-TiN ceramic composites - Google Patents

Abstract

The present invention relates to a SiC-TiN ceramic composite which is prepared by adding TiN such that the content of TiN does not exceed 10 Volume% in the total volume of the initial composite. According to the present invention, while the amount of added TiN is minimized, Y_2O_3 and C_2O_3 are used as sintering aids such that SiC-TiN ceramic composite may be prepared, of which the fracture toughness and flexural strength are improved. In addition, to prepare SiC-TiN ceramic composite which can satisfy all the thermal and mechanical material properties, the content of Ti-N may be optimized and the combination and the content ratio of suitable sintering aids are optimized, thereby obtaining the SiC-TiN ceramic composite.

Description

SiC-TiN ceramic composites {SiC-TiN ceramic composites}

The present invention relates to a SiC-TiN ceramic composite, and more particularly, to a SiC-TiN ceramic composite excellent in thermal and mechanical properties.

One approach that can be approached for tailoring properties in any material is to combine the properties of other materials. Silicon carbide is a very important engineering ceramics for structural applications because of its properties such as excellent thermal conductivity, oxidation stability, corrosion resistance, high temperature strength and abrasion resistance.

TiN is an attractive material for functional applications such as high melting point, high electrical conductivity, radiation damage resistance, and golden color. Also, as a new approach, a method of compounding TiN with a metal oxide additive in SiC has received attention.

Previous techniques for complexing TiN into SiC can be summarized as follows:

1. Complexing TiN with SiC significantly reduces the electrical resistance of SiC. The electrical resistance of the vacuum cold spray coated SiC-TiN composite was reported to be 182 Ω · cm. SiC ceramics with temperature-dependent resistance characteristics are prepared by mixing Y ₂ O ₃ -Sc ₂ O ₃ additive with SiC-TiN powder mixture. In the initial composition, the TiN content shows the same characteristics as the semiconductor up to 10% Above the volume percentage, the metal-like characteristics were exhibited.

2. When 10 wt% of Y ₂ O ₃ -Sc ₂ O ₃ additive was added to 5 wt% of nano-TiN into SiC, the fracture toughness of SiC was maximally increased. Maximum fracture toughness is in _{Y 2 O 3 -Sc 2 O 3} TiN composite of the SiC-5% by weight was added to 10% by weight of sintering additive was reported to 7.8 MPa.m ^1/2.

3. The nanomultilayers of SiC / TiN exhibited superhardness at room temperature (r.t.). The maximum hardness of the nano-multilayer structure of SiC / TiN was reported to be 60 GPa when the thicknesses of TiN and SiC layers were 4.3 nm and 0.6 nm, respectively.

4. Addition of TiN to the SiC matrix results in a gradual reaction with the SiC and / or sintering aid during sintering, resulting in a secondary phase. The following reaction formulas (1) to (4) summarize the reaction of SiC-TiN in the sintering process.

(Scheme)

_{TiN + 2B → TiB 2 + 1/2} N 2 ↑ (1)

_{4TiN + 7SiC + 4Al 2 O 3} → 4TiC + 4AlN + 7SiO ↑ + 2Al 2 O ↑ + 3CO ↑ (2)

_{TiN + 2SiC + 3Y 2 O 3} → TiC + 2SiO ↑ + 6YO ↑ + CO ↑ (3)

_{4TiN + 2SiC → 4TiC 0.5 N 0.5} + 2Si (liquid) + N 2 ↑ (4)

In the above reaction formulas (1) to (4), B, Al ₂ O ₃ , and Y ₂ O ₃ are added as sintering aids and react with TiN and SiC to produce TiB ₂ , TiC, or AlN.

       The SiC-TiN composite can be synthesized by the following reaction formula (5):

_{3TiC + Si 3 N 4 → 3TiN} + 3SiC + 1/2 N 2 ↑ (5)

Japanese Unexamined Patent Publication No. 1994-263540 discloses a composite sintered body excellent in fracture toughness and impact resistance and / or a composite sintered body capable of being subjected to electric discharge and capable of electric discharge, wherein the matrix phase is made of silicon nitride-silicon carbide, Silicon carbide grains having a microstructure in which silicon carbide grains having a particle diameter of several nanometers to several hundreds of nanometers are dispersed in silicon nitride grains and the dispersed phase is a carbide or nitride of Group 4A, , A boride, and a silicide, or a solid solution of the compound and a matrix phase.

However, in the above-mentioned patent, in the case of using silicon nitride-silicon carbide as a matrix, at least one kind of material selected from carbides, nitrides, borides and silicides of Group 4A, 5A and 6A group, By volume. However, when the content of the dispersed phase material is large, mechanical properties such as fracture toughness may be improved, but thermal properties such as thermal conductivity may be deteriorated.

Japanese Patent Application Laid-Open No. 1994-263540

 K. J. Kim, K. M. Kim, and Y. ?. Kim, " Highly Conductive SiC ceramics containing Ti2CN. &Quot; Eur. Ceram. Soc., 34, 1149-54 (2014).

The thermal and mechanical properties of SiC-TiN composites have not been investigated so far, and only the electrical properties and manufacturing methods have been focused on.

Therefore, in the present invention, SiC-TiN composites were prepared by varying the initial TiN content in the composite composition, and the thermal and mechanical properties were investigated to find SiC-TiN ceramic composites having various physical properties.

Accordingly, an object of the present invention is to provide a SiC-TiN ceramic composite excellent in thermal and mechanical properties by optimizing the TiN content added in the entire composition and mixing an appropriate sintering auxiliary agent.

The SiC-TiN ceramic composite according to the present invention is characterized in that the content of TiN in the initial composite composition is added so as not to exceed 10% by volume of the entire composition.

The TiN content is preferably 0.1 to 5% by volume of the total composition.

The SiC-TiN ceramic composite is SiC-TiC _(1-x) N _x , wherein x is 0.5.

The composite is characterized in that Y ₂ O ₃ -Sc ₂ O ₃ additive is added as a sintering aid and hot-pressed.

The Y ₂ O _3: Sc ₂ O ₃ composite is preferably mixed at a molar ratio of 1: 1.

The Y ₂ O ₃ -Sc ₂ O ₃ additive is preferably added in an amount of 0.1 to 2% by volume of the total composite composition.

The relative density of the SiC-TiC _0.5 N _0.5 complex may be ≥98.5%.

The SiC-TiN ceramic composite is obtained by press-sintering a mixture of SiC, TiN, and Y ₂ O ₃ -Sc ₂ O ₃ sintering aids at a temperature of 2000 to 2100 ° C and a pressure of 30 to 40 MPa for 3 to 4 hours It may be manufactured.

According to the present invention, it is possible to manufacture a SiC-TiN ceramic composite having excellent thermal conductivity and improved fracture toughness and bending strength by using Y ₂ O ₃ - Sc ₂ O ₃ as a sintering auxiliary agent while minimizing the amount of TiN added have.

In addition, in order to produce a SiC-TiN ceramic composite capable of satisfying both thermal and mechanical properties, there is an effect of optimizing the content of TiN and providing a SiC-TiN ceramic composite having an optimum combination of sintering aids and a content ratio .

1 is an XRD pattern of a sintered SiC-based composite comprising single-phase SiC and 2 to 35 volume% TiN,
Fig. 2 shows the microstructure of single-phase SiC (STNO)
3 shows the microstructure of STN2,
Figure 4 shows the microstructure of STN 35, with the arrows and circles showing the core / rim structure in SiC and TiC _0.5 N _0.5 grain, respectively,
5 shows a SEM photograph and a map of each element in the STN35 composition. (A) SEM photograph, (b) silicon, (c) titanium, (d) yttrium, (e) scandium, (f) carbon, and (g) nitrogen,
Fig. 6 shows the thermal conductivity and phonon mean free path of single-phase SiC and SiC-TiC _0.5 N _0.5 composites,
7 is an HRTEM micrograph of an SiC-SiC interface having an amorphous grain boundary film of 1.13 nm in thickness,
8 is an HRTEM micrograph of a SiC-TiC _0.5 N _0.5 interface without an amorphous grain boundary film,
9 shows the flexural strength and fracture toughness of single-phase SiC and SiC-TiC _0.5 N _0.5 composites according to TiN content,
10 is a SEM photograph showing a crack path from a Vickers indentation test in the STN0 specimen,
11 is a SEM photograph showing the crack length from the Vickers indentation test in the STN35 specimen,
Figure 12 shows a typical fracture profile of the STN0 specimen,
13 shows a typical fracture profile of the STN35 specimen,
Figures 14-16 show the fracture origin in (a) STN10, (b) STN20, and (c) STN35 in SiC-TiC _0.5 N _0.5 composites, respectively,
17 shows the Vickers hardness of single-phase SiC and SiC-TiC _0.5 N _0.5 composites according to TiN content.

Hereinafter, the present invention will be described in more detail.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a,""an," and "the" include singular forms unless the context clearly dictates otherwise. Also, " comprise "and / or" comprising "when used herein should be interpreted as specifying the presence of stated shapes, numbers, steps, operations, elements, elements, and / And does not preclude the presence or addition of one or more other features, integers, operations, elements, elements, and / or groups.

The present invention relates to a SiC-based ceramic composite excellent in thermal and mechanical properties.

In particular, in the present invention, SiC-TiN ceramic composites having thermal and mechanical properties were prepared by optimizing the content of nitride in SiC ceramics. Titanium nitride (TiN) is most preferably used as the nitride according to the present invention.

The SiC-TiN ceramic composite according to the present invention is characterized in that the TiN content in the initial composition is added so as not to exceed 10% by volume of the entire composition.

The SiC-TiN ceramic composite may be represented by SiC-TiC _(1-x) N _x , where x is preferably 0.1 to 0.5.

If the content of TiN exceeds 10 vol% in the initial composition of the SiC-TiN ceramic composite according to the present invention, the thermal conductivity and the Vickers hardness decrease, which is not preferable. Therefore, the preferable content of TiN is 0.1 to 5% by volume of the total composition in the initial composition.

In the present invention, when the content of TiN exceeds 10 vol% in the initial composition of the SiC-TiN ceramic composite, the thermal conductivity decreases. First, TiC _(1-x) N _x has lower thermal conductivity . The second reason is that as the content of TiN increases by more than 10% by volume, the grain size of SiC decreases and phonon scattering increases at the phonon-grain boundary.

In addition, the SiC-TiN ceramic composite according to the present invention is characterized in that Y ₂ O ₃ -Sc ₂ O ₃ additive is added as a sintering aid and pressure sintering is performed.

The combination of the Y ₂ O ₃ -Sc ₂ O ₃ additive is selected because it can make the SiC-TiN ceramic composite to have a high thermal conductivity.

It is generally known that the lower the amount of additive, the higher the hardness in SiC-based composites. Therefore, in the present invention, the SiC-TiN ceramic composite having improved hardness was prepared by minimizing the additive content to 0.1 to 2% by volume of the total composition and having a finer microstructure than the conventional SiC-based ceramic composite .

The sintering assistant according to the present invention is preferably mixed with Y ₂ O _3: Sc ₂ O ₃ in a molar ratio of 1: 1 in order to improve hardness and thermal conductivity.

Therefore, the Vickers hardness of the TiN-SiC ceramic composite according to the present invention I have a range of 20~25 GPa, these values using the Y ₃ Al ₅ O ₁₂ as a sintering aid to 10% by weight or, Al ₂ O ₃ -Y ₂ O ₃ is 10% by weight based on the total weight of the SiC-based ceramic composite. From these results, it can be seen that even though Y ₂ O ₃ -Sc ₂ O ₃ additive is used as a sintering aid as in the present invention and its content is minimized to 1/5 of the content of the previous sintering auxiliary, SiC-TiN ceramic composite.

Also, the inclusion of TiN as an additive in the present invention can improve the density of the SiC-TiC _0.5 N _0.5 composite. That is, all specimens including TiN can be densified up to ≥99.3% when pressurized and sintered under nitrogen atmosphere at 2000 ℃ and 40MPa for 3 hours.

These results show that Y-Sc-Si oxide is formed by reacting with SiO ₂ , which is an oxide film inherent in SiC, in a process of pressurizing and sintering using a Y ₂ O ₃ -Sc ₂ O ₃ additive as a sintering aid in single-phase SiC specimens . Also, Y-Sc-Si oxycarbonitride (OCN) is formed from the dissolution of nitrogen and SiC from the sintering environment as the temperature increases. The Y-Sc-Si OCN liquid phase acts to densify the specimen through the liquid-phase sintering mechanism, thereby contributing to the improvement of the density of the finally produced SiC-TiN ceramic composite body.

Therefore, the relative density of the SiC-TiC _0.5 N _0.5 composite according to the present invention has a value of ≥98.5%.

It is preferable that the SiC-TiN ceramic composite according to the present invention is prepared by mixing the TiN content and the sintering additive of Y ₂ O ₃ -Sc ₂ O ₃ and press-sintering the mixture.

In the present invention, the pressing and sintering may be performed in a milder atmosphere than the conventional process conditions. For example, at a temperature in the range of 2000 to 2100 캜, a pressure of 3 to 4 Lt; / RTI >

In addition, the mechanical properties of the SiC-TiC _0.5 N _0.5 composite according to the present invention show somewhat different results depending on the TiN content. That is, as the content of TiN increased, the flexural strength gradually decreased from 10 vol%. However, fracture toughness increased gradually with increasing TiN content.

Therefore, the production of the SiC-TiN ceramic composite which can satisfy both the thermal and mechanical properties by using the Y ₂ O ₃ -Sc ₂ O ₃ additive as in the present invention and optimizing the initial TiN content in the entire composition It is possible.

Hereinafter, preferred embodiments of the present invention will be described in detail. The following examples are intended to illustrate the present invention, but the scope of the present invention should not be construed as being limited by these examples. In the following examples, specific compounds are exemplified. However, it is apparent to those skilled in the art that equivalents of these compounds can be used in similar amounts.

Examples 1-5

TiO 2 (Grade C, ~ 1.0 탆, HC Starck, Berlin, Germany), Y ₂ O ₃ (99.99%, Kojundo Chemical Lab Co., Ltd.), β- SiC (~0.5 μm, BF-17, HC Starck, Berlin, Germany). , Ltd., Sakado-shi, Japan), and Sc ₂ O ₃ (99.9%, Kojundo Chemical Lab Co., Ltd., Sakado-shi, Japan).

The powder mixture was mixed in five batches containing ethanol and mixed for 24 hours using a SiC ball and a polypropylene jar. Sc ₂ O ₃ : Y ₂ O ₃ molar ratio was 1: 1, and the content of the additive (Sc ₂ O ₃ , Y ₂ O ₃ ) was fixed at 2% by volume in all batches as shown in Table 1 below. The milled slurry was dried and sieved, and subjected to a pressure sintering process in a nitrogen atmosphere at 2000 DEG C and 40 MPa for 3 hours

Psalter Batch composition (vol%) Theoretical density
(g / cm3) Relative density
(%) specific heat
(Room temperature, J / (g 占))) Thermal conductivity
[Room temperature, W / (mK))] STN0 98.00% SiC + 1.13% Y ₂ O ₃ + 0.87% Sc ₂ O ₃ 3.24 98.9 0.667 ± 0.000 199.6 ± 2.7 STN2 96.00% SiC + 2.00% TiN + 1.13% Y ₂ O ₃ + 0.87% Sc ₂ O ₃ 3.262 99.9 0.702 占 .000 224.1 ± 1.4 STN10 88.00% SiC + 10.00% TiN + 1.13% Y ₂ O ₃ + 0.87% Sc ₂ O ₃ 3.457 99.3 0.667 ± 0.000 131.0 ± 0.8 STN20 78.00% SiC + 20.00% TiN + 1.13% Y ₂ O ₃ + 0.87% Sc ₂ O ₃ 3.674 99.5 0.698 ± 0.000 102.8 ± 1.4 STN35 63.00% SiC + 35.00% TiN + 1.13% Y ₂ O ₃ + 0.87% Sc ₂ O ₃ 4.000 99.7 0.628 ± 0.000 90.3 ± 1.2

Experimental Example 1: Microstructure Identification

Relative density of pressure - sintered specimens was measured by Archimedes' method. The theoretical density of each specimen was calculated according to the rules of the mixture, as shown in Table 1 below.

To measure the relative density of the monolithic SiC sample (STNO), a submicrometer size β- SiC and 2 volume% Y ₂ O ₃ -Sc ₂ O ₃ additive were added to prepare a sample having a relative density of 98.9%. All other specimens containing TiN were able to increase densities up to ≥99.3% when hot-pressed for 3 hours at 2000 ° C and 40 MPa in a nitrogen atmosphere. These results suggest that the addition of TiN can improve the density of SiC-TiC _0.5 N _0.5 composites. In the monolithic SiC specimen, the Y ₂ O ₃ -Sc ₂ O ₃ additive reacts with the SiO ₂ of the SiC-specific oxide film during the pressure sintering process to form the Y-Sc-Si oxide liquid phase. Also, Y-Sc-Si oxycarbonitride (OCN) is formed from the dissolution of nitrogen and SiC from the sintering environment as the temperature increases. The Y-Sc-Si OCN liquid phase acts to densify the specimen through liquid phase sintering. Likewise, the Y ₂ O ₃ -Sc ₂ O ₃ additive reacts with SiO ₂ of the SiC inherent oxide film and TiO ₂ on the TiN surface to form a Y-Sc-Ti-Si oxide liquid phase during the pressure sintering process. As the temperature increases, Y-Sc-Ti-Si-OCN liquid phase is formed by dissolving nitrogen, SiC and TiN from the sintering environment. The formation of the Y-Sc-Ti-Si-OCN liquid phase can be confirmed from the following EDS results. The viscosity of the Y-Sc-Ti-Si-OCN liquid phase is lower than that of the Y-Sc-Ti-Si oxide liquid phase, and such a liquid phase having a lower viscosity can improve the sintering density of the composite.

Experimental Example 2: Confirmation of XRD structure

The crystal phase of SiC was measured according to X-ray diffraction (XRD; D8 Discover, BrukerAXS GmbH, Karlsruhe, Germany) and the results are shown in FIG. The lattice constant of TiC (1-x) Nx phase was calculated using XRD data.

Referring now to FIG. 1, there is shown an XRD pattern of a specimen composed of 2% by volume of ? -SiC and TiN. In the case of specimens containing more than 10 volume% (STN10) of TiN, new peaks were observed along the beta -SiC peak (see the dotted line in Figure 1). It can be seen that the peaks agree well with the TiC (1-x) Nx peak. The calculated lattice constants of TiC (1-x) Nx were 0.42863 nm in STN20 and 0.42859 nm in STN35. Both values are close to 0.42860 nm, the lattice constant of TiC _0.5 N _0.5 (JCPDS 071-6059).

From the XRD results, 1) has been converted in the sintering process to β -SiC TiC _0.5 N _0.5 in combination with a carbon atom of the particles that are neighboring TiN particles, 2) β → α phase changes in the pressure of the SiC during the sintering process It can be seen that it is completely suppressed.

A possible mechanism for TiC _0.5 N _0.5 phase formation is the chemical reaction between TiN and SiC in the pressure sintering process, as suggested by equation (4) above. In the reaction formula (4), liquid Si and gaseous N ₂ are dissolved in the Y-Sc-Ti-Si-OCN liquid phase formed in the pressure sintering process.

The second possible mechanism is the precipitation of the TiC _0.5 N _0.5 phase from the Y-Sc-Ti-Si-OCN liquid phase formed in the pressure sintering process and the subsequent cooling process. In a previous study, in the case of a high temperature conductivity SiC ceramic was pressed and sintered at 2050 ℃ with Y ₂ O ₃ -Sc ₂ O ₃ 1% by volume of additive, a small amount of α-SiC as a result of β → α phase transition after the pressure sintering process the .

The suppression of the phase transition from SiC to β to α phase in the above specimen can be explained as two reasons, namely, a decrease in the pressure sintering temperature and an increased stability of β- SiC in the presence of the OCN liquid phase.

Figures 2 to 4 below illustrate typical microstructures of single-phase SiC and SiC-TiC _0.5 N _0.5 composites. The characteristics can be summarized as follows with reference to Figures 2 to 4:

(1) The SiC grain size decreases as the TiN content increases in the starting material composition. That is, it can be seen that the role of TiN acts as a grain growth inhibitor. (2) The grain size of TiC _0.5 N _0.5 was ~ 1 탆 in the STN2 sample, and increased to ~ 5 탆 in the STN35 sample. From this, grain growth of TiC _0.5 N _0.5 occurs in the hot-press process. (3) The position of TiC _0.5 N _0.5 phase depends on its size. That is, when the grain size is as small as <1 μm, most of the particles are trapped in the SiC grain. However, when the grain size is as large as ≧ 1 μm, the grain boundary is located at the interface of the grain or at the multigrain junction. (4) The core / rim structure was observed in both SiC and TiC _0.5 N _0.5 grains as shown in FIG. 4, indicating that the solution-re-precipitation process occurred in the specimen as in the Ti (C, N) have. (5) TiC _0.5 N _0.5 The shape of the grain varies depending on its position. For example, TiC _0.5 N _0.5 grains as small as <1 μm are trapped inside the SiC grain to form a spherical shape, whereas large grains In equiaxed or irregular form at the interface of the grain or at the junction between the grains.

From these results, it can be seen that small TiC _0.5 N _0.5 grains precipitate and form from the liquid phase and are trapped inside the growing SiC grain. Also, it can be seen that large TiC _0.5 N _0.5 grains are formed by the reaction formula (4) and are deformed by the growth of SiC grains. Like TiN, TiC _0.5 N _0.5 has ductility at the pressure sintering temperature.

Experimental Example 3: SEM image confirmation

The pressure-sintered specimens were polished and etched with CF ₄ plasma containing 10% oxygen. The etched microstructure and fracture surface were measured with an SEM (S4300, Hitachi Ltd., Hitachi, Japan), and the results are shown in Fig.

As shown in the results of FIG. 5, the dot mapping of Si, Ti, Y, Sc, C and N by EPMA in the SEM image and the STN35 sample shows the position of each atom in each sample.

The microstructure of the STN35 sample consisted of nitrogen-doped SiC grain, TiC _0.5 N _0.5 grain, and Y-Sc-Ti-Si-OCN, which could be confirmed in subsequent grain- .

5, the circled area represents the core of large TiC _0.5 N _0.5 grains composed mostly of Si. It can be seen that Si, one of the reaction products of Scheme (4), remains partially in the core of TiC _0.5 N _0.5 grain.

Experimental Example 4: Measurement of thermal conductivity

Thermal conductivity was measured by laser flash method. Thermal conductivity (LFA 447, NETZSCH GmbH, Selb, Germany) and differential scanning calorimetry (DSC, Model Q200, TA Instrument Inc., New Castle, DE) And the heat capacity ( C _p ) were respectively measured.

The hot-pressed sample was cut into a size of a sample for thermal conductivity measurement (10 mm x 10 mm x 4 mm) and a sample for heat capacity measurement (2.84 mm x 2.84 mm x 1 mm) and polished. The large surface of the specimen was aligned on the opposite side of the direction of pressure sintering, and the thermal conductivity was measured according to the following equation (1).

(1)

κ = α ρ C _p

Where rho is the density of the sample.

The average phonon free distance (ι) was calculated according to the following equation (2).

(2)

Where κ is the thermal conductivity and V is the average sound velocity in the composite.

The heat capacity of each specimen measured at room temperature is shown in Table 1 above. The sound velocities of SiC and TiN at room temperature were 11820 m / s and 9357 m / s, respectively. Since the sound velocity at TiC _0.5 N _0.5 is not valid in the literature, the sound velocity in TiN was used to calculate the phonon mean free distance. The sound velocity in the composite was calculated using the mixture rule. Therefore, the average free distance calculated here can be regarded as an approximate approximate value, not an exact value.

The thermal conductivity of the monolithic SiC and SiC-TiC _0.5 N _0.5 complexes is shown in FIG. 6 below.

The thermal conductivity of the single-phase SiC was 199.6 W / m · K, and the maximum value was 224.1 W / m · K when TiN was added at 2 vol% and 90.3 W / m · K when the TiN content was increased to 35 vol% · K gradually decreased.

Typically, in SiC and TiN ceramics, heat is mostly driven by electrons, partly by phonons. In polycrystalline ceramics, phonon migration is scattered by chemical and structural defects (impurities). Interactions include phonon-impurity scattering and phonon-grain boundary scattering. In the case of SiC ceramics, oxygen is a major impurity in the lattice that affects the thermal conductivity of the non-oxidized ceramics.

The average phonon mean free path at room temperature for STN0, STN2, STN10, STN20, and STN35 are 24.0, 25.0, 14.9, 10.7, and 9.9 nm, respectively. The change in thermal conductivity corresponds to the change in phonon mean free path in each sample.

The mean grain sizes of SiC and TiC _0.5 N _0.5 were 1.9 ~ 5.3 ㎛, respectively, as shown in Figs. Since SiC is the main phase (continuous phase) in the composite containing <20 vol% of TiN in the starting composition, the thermal conductivity is mainly affected by the nature of the SiC grains in the composite. Since the phonon average free distance is 1/100 to 1/2 of the grain size of SiC, the thermal conductivity is primarily affected by phonon-oxygen scattering in the SiC lattice and secondarily by phonon-interfacial scattering in the SiC lattice. Conversely, in a composite containing TiN ≥ 20% by volume, a three-dimensional network of TiC _0.5 N _0.5 phase is observed. Thus, a high contribution of the conductive electron's thermal conduction path can be expected in the composite.

In previous studies, the thermal conductivity of the sintered single-phase SiC containing Y ₂ O ₃ -Sc ₂ O ₃ at 1 vol% was 234.2 W / mK. However, the thermal conductivity of the single-phase SiC in the present invention was 199.6 W / mK, which is lower than the previous value. This difference contributes to the difference in microstructure and phonon mean distance between the single-phase SiC of the present invention and the SiC ceramic in the previous study. (1) The grain size of the SiC of the present invention was ~ 5.3 탆, whereas it was ~ 12 탆 in the past; (2) The average phonon average free distances in the present invention and previous studies were 24 nm and 26 nm, respectively.

The difference in grain size is due to the difference in process conditions and additive content. SiC ceramics with a thermal conductivity of 234.2 W / m · K were hot-pressed at 2050 ° C. for 6 hours. On the other hand, the single-phase SiC of the present invention is hot-pressed at 2000 ° C. for 3 hours. In the previous study, the additive content was 1 vol% but in the present invention, the additive composition was the same, but the additive content was 2 vol%. By adding 2 vol% of TiN, the thermal conductivity was improved from 199.6 to 224.1 W / m · K. The higher thermal conductivity of the STN2 specimen compared to the reference STN0 specimen indicates the following:

(1) lack of solubility of Ti in the SiC lattice; (2) absence of phase transition in SiC; (3) extraction (separation) of oxygen from the SiC surface or lattice by addition of TiN; And (4) higher electrical conductivity (5.8 × 10 ^-3 Ω cm) in the STN2 specimen compared to STN0 (1.0 × 10 ^-1 Ω cm)

In semiconductors, conductive electrons can contribute to phonon as well as thermal conduction. The formation of the α / β interface in the SiC grain is due to the phase transition of β → α induced by increased phonon scattering at the interface, which reduces the thermal conductivity.

Therefore, as shown in FIG. 1, the absence of the phase transition has the advantage of increasing the thermal conductivity. Zhou et al. Reported that oxygen in the lattice plays an important role in the thermal conduction of SiC ceramics (Y. Zhou, K. Hirao, K. Watari, Y. Yamauchi, and S. Kanzaki, "Thermal Conductivity of Silicon Carbide Densified with Rare-Earth Oxide Additives, " J. Eur. Ceram. Soc., 24 , 265-70 (2004)). The extraction of oxygen from the lattice or surface of SiC can be understood as follows: (1) The free energy of Gibbs due to the formation of TiO ₂ (rutile) is higher than that of SiO ₂ (cristobalite) Is lower through a range of up to; (2) Formation of Y-Sc-Ti-Si-OCN liquid phase in STN2 sample.

Experimental Example 5: HRTEM image

SiC disk (diameter 3 mm) was prepared according to standard method for high-resolution transmission electron microscopy (HRTEM) measurement. HRTEM (400 kV, JEM-4010, JEOL, Tokyo, Japan) was used for the observation of SiC-SiC and SiC-TiC _0.5 N _0.5 phase interfaces. A 15-nm diameter electron probe was used using energy dispersive spectroscopy (EDS).

Typical HRTEM images at SiC-SiC and SiC-TiC _0.5 N _0.5 interfaces are shown in FIGS. 7 to 8 below. In the SiC-SiC interface with a thickness of ~1.13 nm, an amorphous grain boundary film was observed, but a clean interface was observed at the SiC-TiC _0.5 N _0.5 interface. The composition of 13 spot of SiC-SiC grain boundary was observed with EDS. The average composition of STN2 sample was 0.6 at.% Y, 0.8 at.% Sc, 0.8 at.% Ti, 55.1 at. Si, 2.8 at.% O, 38.9 at.% C, and 1.0 at.% N.

Since the size of the probe for EDS analysis is greater than the thickness of the intergranular film, the data may contain some influence from neighboring SiC grains. However, the atoms present in the grain boundary region can be confirmed qualitatively through the obtained data. The intergranular film consists of Y-Sc-Ti-Si-OCN in the STN2 sample.

These results clearly show that the Y-Sc-Ti-Si-OCN liquid phase is formed in the SiC-TiC _0.5 N _0.5 complex sintered with Y ₂ O ₃ -Sc ₂ O ₃ . From this, a portion of the added TiN is melted into the glass phase during sintering, and oxygen is released from the SiC particles. In the SiC-SiC interface of the SiC ceramics with a thermal conductivity of 234.2 W / m K, a crystallized or clean interface was observed from the previous observation using HRTEM, which means that there is no amorphous film between the SiC-SiC interfaces. Comparing the present invention with previous studies, it can be seen that the incorporation of Ti into Y-Sc-Si-OCN glass inhibits the crystallization of the amorphous film at the SiC-SiC interface during cooling after sintering.

As the content of TiN increases from 2 to 35% by volume, the thermal conductivity gradually decreases from 224.1 to 90.3 W / mK due to the following reasons: 1) the addition of TiC _0.5 N _0.5 Low thermal conductivity; And (2) the reduction of the grain size of SiC from ~ 5.3 to 1.9 mu m increases phonon-grain boundary scattering as the TiN content increases to 2 to 35% by volume. It can also be supported that the phonon average free distance is reduced from 25 nm in STN2 to 9.9 nm in STN35.

Experimental Example: Measurement of mechanical properties

Samples for measuring flexural strength were cut and polished into rod-shaped samples measuring 2 mm x 2.5 mm x 25 mm. The tensile surface of the rod was polished with a 6-micron diamond paste and rounded off of the tensile edge to avoid large dege defects and stress build up caused by sectioning. The bending strength test was carried out at a crosshead speed of 0.5 mm / min using a 4-point bending strength measurement method with internal and external diameters of 10 mm and 20 mm, respectively.

Fracture toughness was measured according to ASTM C1421-10. The size of the test specimen was 3 × 4 × 25 mm. Knoop indenter was used to make an indentation at the midpoint of the polished surface of the test specimen. The indentation force was 49 N, and the holding time at the maximum load was 15 seconds. Prior to testing, the residual stress damage zone was removed by mild grinding. The Vickers hardness was measured using a Vickers indenter under conditions of 9.8 N and a holding time of 10 seconds.

9, the flexural strength and fracture toughness of single-phase SiC and SiC-TiC _0.5 N _0.5 composites vary greatly depending on the initial TiN content. Fracture toughness increased with increasing TiN content and maximum value of 6.2 MPa m ^1/2 at 35 volume% TiN. The increase in fracture toughness as the content of TiN increases is due to crack deflection by TiC _0.5 N _0.5 grain as shown in FIG.

Also, as shown in Figs. 12 to 13, the grain boundary fracture tendency was increased as the initial TiN content was increased. In addition, the fracture modes of SiC exhibited mixed modes of intergranular fracture and transgranular fracture.

On the contrary, the failure mode of STN35 is intergranular fracture due to the weak interface, which is mainly caused by the difference in the thermal expansion coefficient between the liquid phase and the SiC and / or TiC _0.5 N _0.5 grain during the cooling process after pressure sintering. The increase in grain boundary fracture tends to be due to the large difference in thermal expansion (Δα = 5 × 10 ^-6 / ° C) between SiC and TiC _0.5 N _0.5 grains. This can be supported by observation of the crack path by Vickers indentation as shown in Figs. 10 to 11 below. These results show that the incorporation of Ti into Y-Sc-Si-OCN liquid phase improves intergranular fracture and increases fracture toughness.

단일상 SiC와 SiC-TiC_0.5N_0.5 복합체의

굴곡강도와 파괴 인성은 TiN 함량에 따라 달라진다. 에러 막대(error bar)는 두 데이터에서 평균값에 대하여 표준편차를 나타낸다. 반복 횟수는 굴곡강도와 파괴 인성 값을 계산하기 위하여 각각 5, 7회로 진행하였다.

Single-phase SiC and SiC-TiC _0.5 N _0.5 composite

Flexural strength and fracture toughness are dependent on the TiN content. The error bar represents the standard deviation of the mean value from the two data. The number of repetitions was 5 and 7 for the flexural strength and the fracture toughness, respectively.

Some researchers have reported the fracture toughness of SiC-TiN composites with oxide additives: 4.0 MPa m ^{1 for} SiC-30 wt% TiN composites sintered with 10 wt% Al ₂ O ₃ -Y ₂ O ₃ ^{/ 2} ; In the SiC-5 wt% TiN nanocomposite with 10 wt% Y ₃ Al ₅ O ₁₂ , it was 7.8 MPa m ^1/2 .

In the TiN-SiC composite, which is the main phase of TiN, a lower fracture toughness value is reported, which is 5.5-6.0 MPa m ^1/2 in the TiN-25.3 vol% SiC composite prepared by reaction sintering from TiC and Si ₃ N ₄ powder, And 3.6 MPa m ^1/2 for TiN-5 wt% SiC composite.

Therefore, the fracture toughness value of 6.2 MPa m ^1/2 obtained from SiC-35 vol% TiC _0.5 N _0.5 composite sintered with 2 vol% Y ₂ O ₃ -Sc ₂ O ₃ was reported from a composite composed of SiC and TiN And is somewhat lower than the value obtained in the SiC-5 wt% TiN composite. From these results, it can be seen that the Y ₂ O ₃ -Sc ₂ O ₃ system is more effective as an additive to improve the toughness of the SiC-TiC _0.5 N _0.5 composite.

The flexural strength of the SiC-TiC _0.5 N _0.5 composite according to the TiN content is shown in FIG. Higher flexural strength values were obtained in all composites except STN35 than single-phase SiC (546 MPa) produced under the same conditions. (STN2-588 MPa, STN10-599 MPa, STN20-575 MPa) The critical flaw sizes of single-phase SiC and SiC-TiC _0.5 N _0.5 composites were calculated using the following equation (3) Surface defects.

(3)

K _IC = 1.35? C ^1/2 ,

Where K _IC is the fracture toughness and σ is the strength.

The calculated critical defect size (c) of STN0, STN2, STN10, STN20, and STN35 were ~33, ~38, ~43, 54, and 106 um, respectively. It can be seen that the c value is larger than the average grain size (1.9 to 5.3 m for SiC and 1 to 5 m for TiC _0.5 N _0.5 ) in all specimens, which can be attributed to on-fracture in external defects .

Figures 14-16 show typical fracture origins in the STN10, STN20, and STN35 specimens, respectively, which are surface pores in most of the specimens. The pore sizes of the STN10, STN20, and STN35 specimens are ~ 44, ~ 54, and ~ 106 micrometers, respectively, and are similar to the calculated critical defect size. It can be seen that the process of SiC-TiC _0.5 N _0.5 composites containing a large amount (~ 35 vol%) of TiN is more vulnerable to process defects as compared to composites comprising single-phase SiC and small amounts of TiN, It can be seen as creating a large critical defect size.

The lower strength (477 MPa) is associated with the presence of larger critical defect sizes in STN35 than in other specimens. The increase in strength in the STN2, STN10, and STN20 specimens can partially contribute to improving the density and fracture toughness of the composite. The flexural strength of SiC-5 wt% TiN nanocomposites is reported to be 545 MPa. Therefore, the flexural strength values obtained in the present invention are at least as good or better than the reported values.

As shown in FIG. 17, the Vickers hardness of the SiC-TiC _0.5 N _0.5 composite containing 2 vol% Y ₂ O ₃ -Sc ₂ O ₃ additive increased from 0 to 35% by volume of TiN And it decreases from 28.5 to 19.8 GPa. This is because the hardness of TiC _0.5 N _0.5 is lower than that of SiC. The hardness for bulk TiC _0.5 N _0.5 is not reported, but it can be seen that the hardness of TiC _0.5 N _0.5 is similar to the hardness value (13-22 GPa) of TiN in bulk state. This is because both materials have the same crystal structure and the same metal bond.

SiC-TiN와 TiN-SiC 복합체의 경도는 19.0∼26.7　GPa인 것으로 보고되었다. 버커스 경도 값인 19.0와 26.7　GPa은 각각 10 wt% Y₃Al₅O₁₂ 포함된 SiC-5　wt% TiN 나노복합체와, 10　wt% Al₂O₃-Y₂O₃ 포함된 SiC-5　wt% TiN 나노복합체에서 얻어진 것이다. TiN-5　wt% SiC 나노복합체에서는 버커스 경도값이 20 GPa, TiN-25.3　vol% SiC 나노복합체에서는 24.8　GPa의 경도 값을 나타냈다. 따라서, SiC-2　vol% TiC_0.5N_0.5 복합체에서 얻어진 경도 값인 25.5　GPa 는 상기 보고된 경도 값과 적어도 비슷한 것을 알 수 있다. 통상, 첨가제의 양이 적을수록 SiC에 기초한 복합체에서 높은 경도를 제공하는 것으로 알려져 있다. 따라서, 본 발명의 복합체에서 개선된 경도는 적은 양(2 부피%)의 산화물 첨가제만으로도 가능하며, 보고된 물질에 비해 더 미세한 미세 구조에 기인할 수 있다.

The hardness of SiC-TiN and TiN-SiC composites was reported to be 19.0-26.7 GPa. The SiC-5 wt% TiN nanocomposite with 10 wt% Y ₃ Al ₅ O _{12 and} the SiC-5 wt% SiC-5 wt% TiN nanocomposite with 10 wt% Al ₂ O ₃ -Y ₂ O _3, TiN nanocomposite. In the TiN-5 wt% SiC nanocomposites, the hardness values of the Berkers hardness value were 20 GPa and the TiN-25.3 vol% SiC nanocomposites had hardness values of 24.8 GPa. Therefore, it can be seen that the hardness value of 25.5 GPa obtained in the SiC-2 vol% TiC _0.5 N _0.5 composite is at least similar to the reported hardness value. It is generally known that the lower the amount of additive, the higher the hardness in the composite based on SiC. Thus, the improved hardness in the composites of the present invention is possible with only a small amount (2 vol%) of the oxide additive and can be attributed to finer microstructure than reported materials.

κ = thermal conductivity
α = thermal diffusion coefficient
C _p = heat capacity
ι = phonon mean free distance
V = average sonic velocity in the composite
K _IC = fracture toughness
σ = flexural strength
c = critical defect size

Claims (9)

SiC-TiN ceramic composite prepared by adding TiN in an initial composite composition such that the content of TiN does not exceed 10% by volume of the total composition.
The method according to claim 1,
Wherein the TiN content is 0.1 to 5% by volume of the total composition.
The method according to claim 1,
Wherein the SiC-TiN ceramic composite is SiC-TiC _(1-x) N _x , wherein x is 0.5.
The method of claim 3,
Wherein the composite is a sintering aid and a Y ₂ O ₃ -Sc ₂ O ₃ additive is added and sintered under pressure.
5. The method of claim 4,
Wherein the Y ₂ O _3: Sc ₂ O ₃ composite is mixed at a molar ratio of 1: 1.
5. The method of claim 4,
Wherein the Y ₂ O ₃ -Sc ₂ O ₃ additive is added in an amount of 0.1 to 2% by volume.
The method of claim 3,
Wherein said SiC-TiC _0.5 N _0.5 composite has a relative density of ≥98.5%.
The method according to claim 1,
The SiC-TiN ceramic composite is obtained by press-sintering a mixture of SiC, TiN, and Y ₂ O ₃ -Sc ₂ O ₃ sintering aids at a temperature of 2000 to 2100 ° C and a pressure of 30 to 40 MPa for 3 to 4 hours &Lt; RTI ID = 0.0 &gt; SiC-TiN &lt; / RTI &gt;
The method according to claim 1,
Wherein the SiC-TiN ceramic composite has a thermal conductivity of 190-230 W / mK at room temperature, a flexural strength of 500-600 MPa, and a Vickers hardness of 19-27 GPa. .

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