JP6344844B2 - Boron carbide / titanium boride composite ceramics and method for producing the same - Google Patents

Description

The present invention relates to high-hardness ceramics, particularly boron carbide / titanium boride (B ₄ C / TiB ₂ ) composite ceramics and a method for producing the same.

Boron carbide (B ₄ C) is a lightweight (theoretical density Dx = 2.515 Mg · m ^-3 ) and known as a material with a high melting point (Tm = 2450 ° C). Diamond, cubic boron nitride (c-BN) ) (Vickers hardness Hv: 29 to 33 GPa), the thermal conductivity λ is 27 to 28.9 W / mK, and the electrical resistivity ρ is 0.3 to 0.8 Ω · cm. Such boron carbide is industrially produced by igniting a mixture of boron oxide and carbon. For example, Patent Document 1 below discloses a method of low-temperature synthesis using an amorphous precursor. ing. However, boron carbide having the above-mentioned physical properties has a small toughness value, and when it is used as a cutting tool, it cannot be said that the hardness and thermal conductivity are sufficient when the workpiece is hard.

On the other hand, in the metal boride, Hv of the hardest titanium boride TiB ₂ (Tm = 3220 ° C, Dx = 4.495 Mg · m ^-3 ) is harder than 35 GPa and B ₄ C, and the thermal conductivity λ is 36 ~ As high as 69.8 W / mK, for example, the following Non-Patent Document 1 discloses a sintering method by hot pressing. However, since the electrical resistivity ρ of titanium boride is 8.7 to 14.1 × 10 ⁶ Ω · cm and is insulative, electrical discharge machining, which is a technique for producing precise parts, cannot be performed.
Both B ₄ C and TiB ₂ are high melting point covalently bonded compounds, and there is a problem that it is difficult to produce dense ceramics due to the difficulty of sintering.

JP 2003-277039 A

Ronald G. Munro, J. Res. Inst. Stand. Technol. 105, 709-720 (2000)

An object of the present invention is to provide a method capable of producing a conductive boron carbide / titanium boride composite ceramic having a hardness of Hv 40 GPa or more and a relative density of 95% or more.
As a result of various studies, the present inventors have uniformly mixed boron B, carbon C of amorphous fine particles (particle diameter Ps about 30 nm), and TiN (Ps about 20 nm) of fine particles in a predetermined molar ratio. When a green compact is produced and this is subjected to pulsed electric-current pressure sintering (PECPS) under the conditions of 1900 ° C / 30MPa / 10min, the self-combustion synthesis reaction (SHS) occurs during PECPS heat treatment. When induced, the temperature inside the sample rises above the external heating temperature, which makes it possible to produce dense B ₄ C / TiB ₂ composite ceramics, and the amount of TiN added initially is 0.5-1.5 vol% ( When TiB ₂ is 0.67 to 2.0 vol%), it has been found that conductive high density ceramics (boron carbide / titanium boride composite ceramics) can be obtained with a hardness Hv of 40 GPa or more and a relative density of 95% or more. The present invention has been completed.

The method for producing the boron carbide / titanium boride composite ceramic of the present invention capable of solving the above problems is as follows.
A step of weighing amorphous boron and amorphous carbon to a molar ratio of B: C = 4: 1 and performing wet mixing to prepare a starting material composed of amorphous boron and amorphous carbon When,
Prepare 0.5 to 1.5 vol.% Titanium nitride internally divided into boron carbide synthesized from the starting material, add the titanium nitride to the starting material, and a solvent selected from water and alcohol A dispersion treatment in the interior and drying to obtain a mixed powder;
Molding is performed using the mixed powder to obtain a molded body having a desired shape, and the obtained molded body is subjected to cold isostatic pressing, followed by pulsed current pressure sintering to boron carbide / boron. A step of synthesizing and simultaneously sintering titanium composite ceramics.

  The present invention is also directed to a method for producing a boron carbide / titanium boride composite ceramic having the above-mentioned characteristics, wherein the pulsed current pressure sintering is a pressure of 10 to 100 MPa in a vacuum of 10 Pa or less. , 1800 to 2000 ° C. sintering temperature, and 5 to 30 minutes holding time.

Moreover, boron carbide / titanium boride composite ceramic of the present invention produced by the manufacturing method described above, the volume ratio of boron carbide / titanium boride 99.33 / 0.67 to 98.0 / 2.0 der is, a relative density of 95 % or more, Vickers hardness of 40 GPa or more, yet fracture toughness value, characterized in der Rukoto 5 MPa · m ^1/2 or more.

  The boron carbide / titanium boride composite ceramics obtained by the production method of the present invention have a high relative density and excellent mechanical properties (particularly high hardness). Suitable as a hard material tool.

It is a flowchart which shows the procedure of a preferable example in the method of this invention for producing a boron carbide / titanium boride composite ceramics. B ₄ C / TiN changing the mixing ratio of a XRD pattern of the sintered ceramic at 1900 ° C. / 10 min / 30 MPa _{conditions, B 4 C / TiN = (} a) 100/0, (b) 99.9 / 0.1, (c) 99.5 / 0.5, (d) 99/1 and (e) 98/2 vol%. (4B: 1C) / a 2 vol% TiN powder mixture is an SEM photograph of the PECPS treated B ₄ was produced _C / _2.69vol% TiB ₂ composite fracture surface, and (a) (d) is SEI image, ( b) and (c) are BEI images. The energy dispersive X-ray analysis (EDS analysis) results of the B ₄ C / 2vol% TiN composition sintered body are shown. (A) is the analysis result of the white part, (b) is the analysis result of the matrix part. is there. B is a ₄ EDS analysis results of acicular particles portion of the C / 2 vol% TiN composition sintered body. ₂ is a graph showing mechanical properties of a B ₄ C / TiB ₂ composite as a function of TiN addition amount, where (a) is a Vickers hardness Hv and (b) is a fracture toughness value K _IC .

First, each step in the method for producing a boron carbide / titanium boride composite ceramic of the present invention will be described. FIG. 1 is a flowchart showing a preferred example procedure in the production method of the present invention.
In the first step, amorphous boron and amorphous carbon are weighed so that the molar ratio is B: C = 4: 1 and wet-mixed, so that amorphous boron and amorphous carbon are homogeneous. In this case, commercially available products can be used as they are as amorphous boron and amorphous carbon, and the average particle size is 10 nm to 60 nm (BET method). It is preferable to use those having an average particle diameter (determined from the specific surface area measured in step 1).
In the wet mixing of amorphous boron and amorphous carbon, it is preferable to mix in an alcohol (for example, ethanol) for a certain period of time using an alumina mortar and pestle, but the invention is not limited to this. .

  In the next step, 0.5 to 1.5 vol.% (More preferably 0.8 to 1.2 vol.) Is calculated based on the theoretical volume of boron carbide synthesized from the mixture of amorphous boron and amorphous carbon. %) Of titanium nitride (average particle size 10 nm to 40 nm (average particle size determined from the specific surface area measured by the BET method)), and adding this titanium nitride to the starting material In a solvent selected from water and alcohol, for example, an ultrasonic homogenizer is used for dispersion treatment to uniformly disperse titanium nitride, followed by drying to obtain a mixed powder.

In the final step, molding is performed using the mixed powder obtained in the above step to obtain a molded body having a desired shape, and the obtained molded body is subjected to cold isostatic pressing (CIP) treatment, followed by pulse energization. A boron carbide / titanium boride composite ceramic is synthesized and sintered simultaneously by pressure sintering.
As a means for forming a molded body in this step, uniaxial mold molding is generally used, but is not limited thereto. Further, in the present invention, it is preferable to remove the moisture and adsorbed gas from the surface of the fine particles constituting the compact by heating the compact before being subjected to pulsed current pressure sintering in a vacuum.
In this specification, “synthetic co-sintering” refers to a dense sintered body (boron carbide / titanium boride composite ceramics) from a homogeneous mixture of starting materials (a mixture of boron and carbon containing a specific amount of titanium nitride). ) Shall be indicated.

Pulse electric pressure sintering (PECPS) in the production method of the present invention can be carried out using a commercially available pulse electric pressure sintering.
In the case of pulsed current pressure sintering, pulsed DC large current (several tens to several hundreds A) with low voltage (several V) under uniaxial pressure (10 to 100 MPa): this current value varies depending on the size of the sample To the carbon plunger mold to induce a spark discharge phenomenon in the compact, instantly generate high energy between the particles and sinter the sample. Diffusion and self-propagating high-temperature synthesis (SHS) occurs. And to obtain a dense sintered body (high density, fine crystal grain size) with suppressed grain growth by high-speed heating (50-100 ° C / min) and short-time sintering (5-30 min) under high pressure Can do.
In the present invention, a mixed powder of amorphous boron B, carbon C, and titanium nitride TiN is subjected to pulsed current pressure sintering, so that B ₄ C is formed from the element mixed powder by self-combustion synthesis at the time of heating and heating. In addition, TiN + 3B → TiB ₂ + BN, TiB ₂ particles and B ₄ C and BN are generated, and the generated heat at that time causes the internal temperature of the sample to be higher than the external heating temperature. As a result, a dense sintered body can be obtained.

  The pulse current pressure sintering in the production method of the present invention is performed under the conditions of a pressure of 10 to 100 MPa, a sintering temperature of 1800 to 2000 ° C., and a holding time of 5 to 30 minutes in a vacuum of 10 Pa or less. More preferably, the conditions of pulsed current pressure sintering are under a pressure of 10 Pa or less, a sintering temperature of 1850 to 1950 ° C., a holding time of 7 to 15 minutes, a pressing force of 25 to 35 MPa, The condition of 1900 ° C./30 MPa / 10 min is particularly preferred. At this time, if the applied pressure is less than 10 MPa, the sintering density becomes low. Conversely, if the applied pressure exceeds 100 MPa, the strength of the mold used for pulse current compression sintering has an upper limit and cannot be used. On the other hand, if the sintering temperature is less than 1800 ° C., the density is low, and if it exceeds 2000 ° C., grain growth tends to occur, which is not preferable. The holding time is sufficiently densified in 5 to 30 minutes.

In the present invention, the addition amount of TiN for boron carbide synthesized from a mixture of amorphous boron and amorphous carbon is 0.5 to 1.5 vol% at the inner split (0.67~2.0vol% as TiB _2), In this case, a conductive high-density boron carbide / titanium boride composite ceramic having a hardness of Hv 40 GPa or more and a relative density of 95% or more is obtained.
On the other hand, high-density ceramics with a relative density of 95% or more can be obtained even when low-temperature and short-time sintering similar to the above is performed using a mixture of non-sinterable B ₄ C and TiB ₂ commercial powders. It cannot be produced and the hardness does not reach 40 GPa or more.
EXAMPLES Hereinafter, although this invention is demonstrated concretely based on an Example, this invention is not limited by an Example.

[Production example of high-hardness boron carbide / titanium boride composite ceramics]
Commercially available amorphous boron (average particle size: 30 nm) and amorphous carbon (average particle size: 30 nm) were weighed so that the molar ratio was B: C = 4: 1, and the mortar made of alumina And a pestle were used for 30 minutes in ethanol to prepare a starting material.
On the other hand, a commercially available titanium nitride (average particle size: 20 nm) is divided into 0.5 vol.% By an internal ratio with respect to boron carbide B ₄ C synthesized from the mixture of amorphous boron and amorphous carbon. Weigh to 1.0 vol.%, Add this to the starting material, perform dispersion treatment in ethanol using an ultrasonic homogenizer (frequency 20 kHz, output 300 W) for 30 minutes, and dry To obtain a mixed powder. The mixed powder thus obtained was sized, then uniaxially molded (16.0 mmφ, 140 MPa), and then subjected to cold isostatic pressing (245 MPa). Thereafter, the obtained molded body was heat-treated (950 ° C./2 h / vacuum), and further, 10 Pa using a commercially available pulsed electric current pressure sintering apparatus (using SPS Syntex Co., Ltd./SPS-510A). Under the following vacuum, pulsed current pressure sintering was performed under the conditions of sintering temperature 1900 ° C, holding time 10 minutes, pressurization pressure 30 MPa, heating rate 100 ° C / min, and sintered body (B ₄ C / TiB ₂ Composite ceramic).
As a comparative product, a sintered body was manufactured using the same method as above except that the amount of titanium nitride added was 0 vol.%, 0.1 vol.%, 2 vol.%, 3 vol.%. did.

FIG. 2 shows an XRD pattern of ceramics sintered under the conditions of 1900 ° C./10 minutes / 30 MPa while changing the blending ratio of B ₄ C / TiN, (a) shows B ₄ C / TiN = 100/0 vol%, (b) is B ₄ C / TiN = 99.9 / 0.1 vol%, (c) is B ₄ C / TiN = 99.5 / 0.5 vol%, (d) is B ₄ C / TiN = 99/1 vol% and (e) are XRD patterns of ceramics obtained with a composition of B ₄ C / TiN = 98/2 vol%.
From the XRD pattern of FIG. 2, it was confirmed that when titanium nitride was added, in addition to the formation of boron carbide, titanium boride and boron nitride were generated. This is because the titanium nitride is amorphous. It is considered that titanium boride and boron nitride were synthesized by reacting with boron. In addition, it is considered that the diffusion has been promoted by the synthesis reaction, so that the densification has progressed and the characteristics have been improved.

FIG. 3 shows a scanning electron microscope (SEM) photograph (FE-SEM, B4 C / 2.69 vol% TiB ₂ composite) of a fractured surface of B ₄ C / 2.69 vol% TiB ₂ composite prepared by (4B: 1C) / 2 vol% TiN mixed powder. (Measured with JSM 7000 manufactured by JEOL), (a) and (d) are secondary electron image (SEI) images, and (b) and (c) are backscattered electron image (BEI) images. (A) 500 times, (b) 2,700 times, (c) 10,000 times, (d) 20,000 times.
From the SEM photograph of FIG. 3, (a) the ceramic structure is more homogeneous and dense than the low magnification image, and (b) the backscattered electron image contains an element with a higher atomic number than other elements. The particles appear white and the particles with the same white particle size are uniformly dispersed (this white particle is estimated to be TiB ₂ from the XRD analysis and the elemental analysis of the SEM image in Fig. 4 described later) ) From (c) and (d), the diameter of white particles was about 0.3 to 1.0 μm, and plate-like or needle-like particles were observed in the cavities visible on the surface.

In FIG. 4, a SEM photograph of the fracture surface of the B ₄ C / 2 vol% TiN composition sintered body, a white portion (a) in the SEM photograph, and a matrix portion (b) in black, The result of analysis (EDS analysis) using an energy dispersive X-ray spectrometer (EDS analysis) is shown. From the result of EDS analysis, the white part (a) is TiB ₂ fine particles, Matrix portion (b) was confirmed to be B ₄ C.
FIG. 5 shows an SEM photograph of the fracture surface of the B ₄ C / 2 vol% TiN composition sintered body, and an EDS analysis result of the acicular particle portion in the SEM photograph. From this, it was confirmed that the acicular particles were BN.

Table 1 below shows various B ₄ C / TiB ₂ composite ceramics (TiN additions: 0, 0.1, 0.5, 1.0, 2.0, 3.0 vol.) Sintered at 1900 ° C / 10 min / 30 MPa in vacuum. .%) Some characteristics are shown.
In Table 1, D _obs is a bulk density, D _x is a theoretical density, D _obs / D _x is a relative density, H _v is a Vickers hardness, and K _IC is a fracture toughness value.

The results in Table 1 above show that B ₄ C / TiB ₂ composite ceramics having a relative density (D _obs / D _x ) of 95.7% or more can be produced when the amount of TiN added is 2.0 vol.% Or less. In addition, when the amount of TiN added is 0.5 vol.% And 1.0 vol.%, B ₄ C / TiB ₂ composite ceramics with Hv = 40 GPa or higher exceeding the Vickers hardness Hv = 34.4 GPa of B ₄ C alone can be obtained. Moreover, when the TiN addition amount is 1.0 vol.%, The fracture toughness value exceeds 5 MPa · m ^1/2 .

FIG. 6 is a graph showing the mechanical properties of B ₄ C / TiB ₂ composite ceramics with respect to TiN addition based on the experimental data of FIG. 5, where (a) is Vickers hardness Hv and (b) is fracture toughness value. K _IC .
From the graph of FIG. 6, it is possible to have a Vickers hardness of 40 GPa or more by setting the amount of TiN added in the range of 0.5 vol.% To 1.5 vol.% (0.67 to 2.0 vol% for TiB ₂ ), and about 5 MPa. It can be seen that B ₄ C / TiB ₂ composite ceramics having fracture toughness values of m ^1/2 or more can be produced. From the results shown in Table 1, the relative density when the TiN addition amount is 0.5 to 1.5 vol.% Is 95% or more.
In the present invention, a more preferable range of TiN addition amount is 0.8 vol.% To 1.2 vol.%, And in this range, it has a Vickers hardness of 42 GPa or more and a fracture toughness of 5.5 MPa · m ^1/2 or more. B ₄ C / TiB ₂ composite ceramics with values can be produced.

Therefore, by using the production method of the present invention, a B ₄ C / TiB ₂ composite having a relative density of 95% or more, a Vickers hardness of 40 GPa or more, and a fracture toughness value of about 5 MPa · m ^1/2 or more. It was found that ceramics can be produced, and that the Vickers hardness and fracture toughness values are the largest in the composition where the amount of TiN added is 1.0 vol.%.
Note that the electrical resistivity ρ of the B ₄ C / TiB ₂ composite ceramics produced using the production method of the present invention described above is 0.3 to 0.8 Ω · cm, and that the ceramics have conductivity. confirmed.

  The boron carbide / titanium boride composite ceramics obtained by the production method of the present invention has high relative density and high hardness, and is useful as a superhard material tool.

Claims (3)

A method for making a boron carbide / titanium boride composite ceramic comprising:
A step of weighing amorphous boron and amorphous carbon to a molar ratio of B: C = 4: 1 and performing wet mixing to prepare a starting material composed of amorphous boron and amorphous carbon When,
Prepare 0.5 to 1.5 vol.% Titanium nitride internally divided from boron carbide synthesized from the starting material, add the titanium nitride to the starting material, and further selected from water and alcohol A step of dispersing in a solvent and drying to obtain a mixed powder;
Molding is performed using the mixed powder to obtain a molded body having a desired shape, and the obtained molded body is subjected to cold isostatic pressing, followed by pulsed current pressure sintering to boron carbide / boron. A method for producing a boron carbide / titanium boride composite ceramic, comprising a step of synthesizing and simultaneously sintering a titanium carbide composite ceramic.   The pulsed current pressure sintering is performed in a vacuum of 10 Pa or less under a pressure of 10 to 100 MPa, a sintering temperature of 1800 to 2000 ° C., and a holding time of 5 to 30 minutes. A process for producing a boron carbide / titanium boride composite ceramic according to claim 1. The volume ratio of the boron carbide / titanium boride 99.33 / 0.67 to 98.0 / 2.0 der is, a relative density of 95% or more, Vickers hardness of 40 GPa or more, yet fracture toughness is 5 MPa · m ^1/2 or more der Boron carbide / titanium boride composite ceramics.