JPH0764636B2 - High hardness, high toughness and high strength titanium carbide (Tic) / silicon carbide (Sic) whisker-composite sintered body and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a TiC / SiC whisker composite sintered body having high hardness, high toughness and high strength, and a method for producing the same.

(Prior art and problems) TiC sintered body has many advantages as an electric conductor, high hardness, heat resistance (vacuum or reducing atmosphere) as structural ceramics, but low oxidation resistance and fracture toughness. Therefore, it has not been put to practical use.

Since TiC is a substance with a strong covalent bond, it is difficult to sinter, which is one of the reasons why TiC ceramics have not been studied so much.

TiC is very easy to oxidize, and its strength is greatly reduced by TiO ₂ which is already formed on the surface and has already started to oxidize at around 800 ° C.
In addition, TiC sinter has extremely low fracture toughness and is a substance that easily causes micro-destruction even from small defects. When considering TiC as a structural ceramic material, its advantages (high hardness, electrical conductivity, etc.) And the disadvantages (oxidation resistance, fracture toughness, etc.) are large, that is, the advantages are very good, but the disadvantages are also very large.

Therefore, in order to use TiC as a practical material for structural ceramics, it is necessary to pay attention to the following two points.

(1) Limit product areas where TiC-based ceramics are applied because oxidation resistance is low to areas where oxidation is not a problem.

(2) To increase fracture toughness by some method.

Focusing on these points, TiC ceramics is considered to be the most suitable application field for tools such as drawing dies, drills, and cutter blades where oxidation resistance is not a serious problem.

The present invention has been made in view of the above points, and a TiC matrix has an S content for the purpose of application to a ceramic tool.
Ti with high hardness, high toughness and high strength by dispersing iC whiskers
It is to provide a C / SiC whisker composite sintered body and a manufacturing method thereof.

(Means for Solving Problems) The first invention of the present invention is to provide SiC whiskers of 3 to 45% by volume,
A sintering aid of B and C (however, based on the total weight of SiC whiskers and TiC) 0.2 to 4% by weight, the balance being substantially TiC, a high hardness, high toughness and high strength TiC / SiC whisker composite firing The second invention is a SiC whisker 3 to 45% by volume,
B as a metal boron and C as a novolac resin sintering aid (however, based on the total weight of SiC whiskers and TiC) 0.2 to
A step of kneading the raw material powder containing 4% by weight and the balance of TiC by an ultrasonic dispersion method or the like, a step of shaping the kneaded raw material powder into a green body, and hot pressing or atmospheric pressure sintering the green body. The present invention relates to a method for producing a TiC / SiC whisker composite sintered body having high hardness, high toughness and high strength, which comprises steps.

(Operation) The first invention will be described below.

Matrix TiC has a Vickers hardness of about 3200 at room temperature and is the hardest material next to diamond and boron carbide. Further, it has good electric conductivity, and electric discharge machining is also possible. Furthermore, it is a property of TiC that high temperature strength is high in vacuum and in a reducing atmosphere.

Since the demand for TiC raw material powder for the improvement of cemented carbide material has increased in recent years, its performance has been greatly improved and the particle size is 1
Those having a size of μm or less are commercially available. Purity is
98% or more is preferable, but about 95% can also be used. If it is less than 95%, the oxides (Al ₂ O ₃ , MgO) mixed as impurities become a liquid phase at the sintering temperature and intervene in the grain boundaries to extremely reduce the strength. The finer the TiC grain size, the better. The particle size should be less than 5 μm. When it is 5 μm or more, the TiC matrix crystal grain size after sintering also becomes 5 μm or more, and the fracture toughness of the matrix is extremely lowered. In addition, the particle shape should be close to spherical. The sinterability decreases as the shape is other than spherical.

Dispersion of SiC whiskers in TiC increases fracture toughness. The reason for this is a paper written by the inventors; a journal of ceramics [Ceramics Association, Vol.21 (7), 1986.p621-].
According to the method, dispersion of SiC whiskers in the matrix is shown in Figs. 1 and 2 on the crack surface which is the origin of the fracture.
As shown in the figure, the crack deflection and the whisker extraction effect are produced, and the fracture toughness is significantly increased. However, it is a condition that the linear expansion coefficient α _M of the matrix is larger than the linear expansion coefficient α sic whiskers of the whiskers. In the present invention, αTiC 8 × 10 ⁶ (1 / ° C.), αsic whiskers = 5 × 10 ^-6
(1 / ° C) and αTiC> αSiC whiskers (1), so this condition is satisfied.

The SiC whiskers must be whiskers and single crystals. Therefore, SiC whiskers must be distinguished from polycrystalline SiC fibers such as Nicalon (long fibers of SiC). There are α type with a hexagonal structure and β type with a cubic structure in SiC, but ordinary SiC whiskers are a mixture of α and β. As long as the SiC whiskers are single crystals, α or β does not have much relation to the strength of the SiC whiskers. (PF Becher and GCWei, J.Am.Ceram.So
c., 67, c267 (1984)] Therefore, as long as it is a SiC whisker, the crystal structure may be α or β, or a mixture thereof.

Next, the diameter of the SiC whiskers needs to be 5 μm or less. Since the linear expansion coefficient of TiC is larger than that of SiC whisker in the process of cooling to room temperature after sintering, tensile stress is generated in the TiC matrix, and when the diameter of SiC whisker is 5 μm or more, this tensile stress causes
This is because microcracks are generated in the TiC matrix. However, if the whisker diameter is too small,
The whisker pull-out effect (proportional to the perimeter of the whisker cross section), which is an important toughening mechanism of this system, is reduced, and high toughness cannot be expected. A suitable whisker diameter is about 2 to 3 μm.

The whisker length should be on the order of 10-100 μm. Ten
Aspect ratio (ratio of whisker length and diameter) below μm
Is too small, the crack deflection and the effect of pulling out the whiskers cannot be fully utilized, and when it is 100 μm or more, defects due to the entanglement of whiskers are likely to occur.

This is because when the composite amount of SiC whiskers is 3 vol% or less, the toughening effect due to the whiskers is hardly present, and when it is 45 vol% or more, defects due to excessive addition of whiskers such as entanglement due to whiskers become very large.

TiC is a carbide with a strong covalent bond and is therefore difficult to sinter. For this reason, it is necessary to use a sintering aid in order to manufacture a complex shaped sintered product in the atmospheric pressure sintering method.
Further, also in the hot pressing method and the HIP method, the use of the sintering aid enables low temperature firing and enables production of a fired body having a fine structure. Due to the similarity between TiC and SiC, B- and C-based sintering aids are effective. However, the total amount of B + C is 4% by weight.
Should not be exceeded. On the other hand, although it is possible to use an oxide as an auxiliary material, it is important to carry out solid phase sintering at a melting point of the oxide or a eutectic point of the oxide and TiC or less. If the temperature is higher than this temperature, the liquid phase generated by the reaction of Al ₂ O ₃ of the sintering aid, TiO ₂ existing on the surface of TiC powder and SiO ₂ existing on the surface of SiC whiskers with SiC whiskers exists at the grain boundaries. It will become extremely weak.

As described above, compounding SiC whiskers with TiC can significantly improve the fracture toughness.
It is possible to greatly improve the fracture resistance, which is a weak point of C matrix ceramics tools. On the other hand, when considering a ceramic tool, which is a problem of oxidation resistance, the tool temperature is constant at a certain temperature because it is used under the condition of heating by heat of wear and air cooling even when it is exposed to high temperature for a long time. It cannot rise above the above temperature, and it is unlikely that this temperature will exceed 800 ° C, except for a very small portion. Therefore, SiC whisker reinforced TiC composite ceramics are considered to be very promising materials for application to ceramic tools.

The second invention will be described below.

The above-mentioned TiC raw material powder, sintering aid and SiC whiskers are kneaded. When whiskers are included as one component of the complex, special attention must be paid to the kneading. Basically, the following two methods can be considered.

(1) Ultrasonic kneading method: This is a method of kneading by mixing the raw material powder to be the matrix and the whiskers together with the mixed solvent and utilizing ultrasonic agitation. The advantage of this method is that it can be kneaded without damaging the whiskers,
It is possible to maintain the whisker length as it is up to the sintered body. However, for this method to be effective, (a) the whiskers must not be strongly entangled with each other before kneading. In other words, loosen with ultrasonic dispersion.

(B) The whisker length is uniform before kneading. In the ultrasonic dispersion method, there is no effect that the whisker lengths become uniform during dispersion. Therefore, if there are variations in the whisker length before kneading, the variations are directly introduced into the sintered body, which causes variations in strength and toughness.

(C) The diameter of the raw material powder other than the whiskers is much smaller than the whisker diameter, and the cohesiveness of the raw material powder is low.
The fact that the whiskers and the raw material powder are well kneaded means that
It means that the raw material powder is well dispersed around the whiskers. If the raw material powder particles are larger than the whisker diameter, the probability that the raw material powder exists between the whiskers is low and the dispersibility does not increase. Even if the raw material powder particle size is small, if they are strongly aggregated, it is the same as the presence of large particles, and the dispersibility is not improved.
The above three conditions are indispensable.

(2) Ball mill method: This is a kneading method in which raw material powder and whiskers are put in a container together with a mixed solvent and balls and stirred and mixed in the solvent.

The advantages of this method include the following.

(A) The device is very inexpensive and the method is also very simple.

(B) Even if the whisker length is not uniform, the length becomes uniform by kneading with a ball.

(C) Even if the whiskers are somewhat entangled, it is easy to loosen them by kneading them with a ball.

(D) The raw material powder size and the cohesiveness are not so important with respect to the dispersibility of the whiskers. Whiskers are well dispersed by the balls, and the cohesiveness of the raw material powder is also improved by the balls. However, the drawbacks of this method are as follows.

(A ') Since there is always contamination from the balls used and the mixing container, it is necessary that the material of the balls and the container is the same as that of the sintered body or harmless even if contained in the sintered body. There is.

(B ') Since whiskers are kneaded for a long time, the whisker length becomes considerably short. The uniform length of the whiskers is an advantage, but the shorter length reduces the effect of pulling out crack deflection whiskers, which is a toughening mechanism.

The above two kneading methods were compared, but they are used in this system.
TiC has a diameter of about 1 μm, which is almost the same as the whisker diameter. In addition, whiskers currently available in Japan are not uniform in length, and it is a very complicated work to classify only whiskers of uniform length. Taking these things into consideration, the kneading method is preferably a ball mill method. However, if it becomes possible to industrially obtain TiC raw powder of ultrafine powder (particle size of 0.1 μm order) and whiskers of uniform length in the future, the ultrasonic dispersion method should be adopted.

When a B or C type sintering aid is used in the ball mill method, the kneading container must be made of one or more of TiC, SiC, B and C. Also, SiC or Si ₃ N ₄ balls should be used for the balls. As the kneading medium, inorganic liquids such as distilled water as well as methyl alcohol and ethyl alcohol can be used. When kneading, ceramic raw material 200g, kneading medium 500m per container
Insert 50 balls with a diameter of 20 mm. If the volume of the container changes, it may be proportional to the above proportion.

The kneading time should be 10 hours or more. This is because if the kneading time is less than 10 hours, the auxiliary material and the whiskers will not be sufficiently dispersed.

For drying after kneading, a microwave oven or a spray dryer can be used. However, when a microwave oven is used, it is necessary to dry-mix again to remove the kneading medium. However,
As will be described later, if the material is evacuated at a predetermined temperature prior to heating to the sintering temperature during sintering, the drying can be made more reliable.

The molding may be any of press molding, injection molding and slip cast molding.

The sintering may be any of hot pressing, pressure sintering, HIP method, and atmospheric pressure sintering.

As mentioned above, prior to heating to the sintering temperature, it is necessary to remove the organic substances and water (particularly the residual content of the kneading medium) adhering to the ceramic particles within a temperature range from room temperature where surface diffusion of the particles does not occur. It is desirable to evacuate.

In the production of TiC matrix and SiC whisker-based ceramics sintered body, 400 to 100 when using hot pressing
Hold in the temperature range of 0 ° C for 10 minutes and reduce the pressure to 10 ^-1 Torr. In the case of pressure and atmospheric pressure sintering, for example, polyvinyl butyl alcohol used as a binder is present together with water, and therefore the holding time is set to 30 minutes or more to remove it.

The sintering temperature is in the range of 1800 to 2300 ° C. If the sintering temperature is less than 1800 ° C, the density of the sintered body will not increase, and if it exceeds 2300 ° C, the crystal grains will be coarsened and the mechanical strength will be reduced.

Density 95% or more, crystal grain size 3 μm or less (particularly preferably 2
It is preferable to set the sintering temperature and the sintering time so as to be (μm or less).

(Example) The TiC raw material powder is produced by the Menstrum method, and the chemical composition and particle size are as shown in Table 1. Used SiC
Whiskers have a diameter of 0.8-1.0 μm and an aspect ratio of 10-20
It is 0. As the sintering aid, B and C type materials were used. As B, a metal boron having a purity of 99% or more and 325 mesh under is used. In addition, C used a novolac resin as a supply source. Novolac resin is a condensate of phenol and formaldehyde.

The mixing ratio is 0% of the volume ratio of SiC whiskers to TiC,
It was changed to 3, 5, 15, 25, 35, 45% by volume. The sintering aid is 1% by weight of B based on the total weight of TiC and SiC whiskers,
Novolac resin was added at 1.5% by weight.

200 g of the raw material ceramics were mixed by a ball mill method.
Since the C type is a sintering aid, the kneading container was the polyethylene container of 1. 350 Si ₃ N ₄ balls having a diameter of 10 mm were put in this, 500 ml of ethyl alcohol was added, and then mixed for 48 hours. After mixing, it was dried in a microwave oven, granulated with a sieve, and the powder under the 60-mesh sieve was used as a raw material powder for sintering.

Sintering was performed by hot pressing and a program as shown in Fig. 3 was adopted. Prior to the main firing, the temperature was 700 ° C. for 30 minutes. Fig. 4 shows the optical microscope structure of TiC / 35% by volume SiC whiskers, which is considered to have the worst sinterability among the compounding ratios. As can be seen in FIG. 4, slightly fine voids are observed, but the sintering is almost completed. Also, as can be seen from FIG. 5, the dispersion of SiC whiskers is also very good.

A test piece for hardness measurement of 3 × 4 × 40 mm was sampled from the thus obtained sintered body using a diamond grindstone and a diamond blade, and the surface was mirror-finished using a diamond polishing liquid. The bending test was carried out by a three-point bending test method with a fulcrum distance of 30 mm and a crosshead speed of 0.5 mm / min, and hardness and fracture toughness were measured by a Vickers hardness meter with a load of 10 kg. Fracture toughness K _IC can be calculated by According to the formula. Here, H is the Vickers hardness, a is the diagonal length of the indentation, and c is the length of the median crack generated from the tip of this diagonal line. The test results are also shown in Table 2. In the table, is the average value, n is the number of measurements, and σ is the standard deviation.

From Table 2, TiC / SiC whiskers showed very high flexural strength at the (85:15) volume ratio. With respect to fracture toughness, the addition of SiC whiskers resulted in an improvement of 15% by volume or more, which is more than double that of TiC alone which does not contain SiC whiskers. This means how the addition of SiC whiskers improves the fracture toughness and bending strength of TiC. It is a good example of how effective it is. Moreover, K _IC is more than 15%.
The value is 6.5 or more, which is comparable to Si ₃ N ₄ known as a high toughness ceramic material. Moreover, the hardness is 2
It is considered that the TiC / SiC whisker-based composite ceramics having a very high value of 3 GPa and having high hardness, high toughness and high strength expected by the inventor were obtained.

FIG. 6 shows an optical micrograph showing the interaction between the cracks generated by micro-Vickers and the matrix. As shown in the photograph, the cracks are well-biased in a zigzag pattern, which is considered to be evidence that the crack deflection is effective. Figure 8 shows the SEM of the fracture surface of a flexural strength containing 15 vol% SiC whiskers with a bending strength of 86.7 kgf / mm ^2.
Show a photo. As shown by the arrow in the figure, the withdrawal traces of the whiskers and the withdrawn whiskers are clearly observed. In addition, the matrix part other than the whiskers is very undulating, and it is considered to be evidence that the crack deflection effectively acts in the matrix part due to the presence of SiC whiskers.

As described above, in the TiC matrix / SiC whisker ceramic composite, when the SiC whiskers are 15%, the hardness is about 23G.
Pa, fracture toughness 6.54MPa A composite ceramic with a high bending strength of 72.4 kgf / mm ^{2 and} high hardness, toughness, and strength was obtained. It is considered that this material is effectively applied to ceramics tools such as ceramics drills and drawing dies where oxidation is not a serious problem.

(Comparative Example) As a comparative example, a sintering aid containing 5% by weight of Al ₂ O ₃ added in place of the B and C systems was hot pressed under the conditions of 1800, 1900, 2000 ° C x 30 minutes. However, in this case the amount of SiC whiskers is 15
The volume% was used, and all other manufacturing conditions were the same as in the example.

The hardness, fracture toughness, and bending strength of the obtained sintered body were measured in the same manner as in Example 1, and Table 3 shows the results.

In the comparative example, the hardness and bending strength were low because the densification was not reached at the hot pressing temperature of 1800 ° C.

On the other hand, since the hardness is increased at the hot pressing temperatures of 1900 ° C and 2000 ° C, liquid phase sintering obviously occurs, and free carbon is taken into the liquid phase to promote densification. However the flexural strength respectively 30.5 kg / mm ^2, as low as 31.5 kg / mm ^2. This is because the liquid phase generated during hot pressing deteriorated the SiC whiskers and the existence of the liquid phase acted as a driving force to coarsen the structure.

(Effect) The present invention is a high hardness, high toughness and high strength TiC matrix / SiC whisker sintered body in which SiC whiskers are dispersed in a TiC matrix, and a manufacturing method thereof. It can now be applied to tools.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a crack deflecting image by whiskers, FIG. 2 is a schematic diagram of a whisker pull-out phenomenon, FIG. 3 is a graph showing temperature rise and pressure program in hot press, and FIG. The figure shows a 50X optical micrograph showing the grain structure of the TiC / 35 volume% SiC whisker sintered body. Figure 5 shows TiC / 35.
400 times optical micrograph showing the grain structure structure of the volume% SiC whisker sintered body, Fig. 6 is a 400 times optical microscope photograph showing the particle structure around the hardness measurement point of the TiC / 35 volume% SiC whisker sintered body. , FIG. 7 is an SEM photograph showing the grain structure of a TiC / 15 volume% SiC whisker sintered body. In the reference numerals shown in the drawings, 1, 7, 12, ... SiC whiskers, 2 ... crack surface when no deflection occurs, 3 ... crack surface when deflection occurs, 4 ... crack 5 ……
Whisker cutting points, 6, 9, 10, 11 ... Whisker pull-out mark, 8 ... Crack deflection.

Claims (4)

[Claims] 1. Silicon carbide (SiC) whiskers 3 to 45% by volume
And the balance is substantially 0.2 to 4% by weight of a sintering aid with respect to the total weight of titanium carbide (TiC), the sintering aid being boron (B) and carbon (C). A titanium carbide (TiC) / silicon carbide (SiC) whisker composite sintered body with a certain high hardness, high toughness and high strength. 2. Silicon carbide (SiC) whiskers 3 to 45% by volume
And a step of kneading 0.2 to 4% by weight of boron (B) and a carbon (C) -based sintering aid with respect to the total weight of the raw material powder consisting of titanium carbide (TiC), and the kneaded raw material powder A method for producing a titanium carbide (TiC) / silicon carbide (SiC) whisker composite sintered body having high hardness, high toughness, and high strength, which comprises a step of forming a formed body and a step of sintering the formed body. 3. High hardness, high toughness and high strength titanium carbide (TiC) / silicon carbide (SiC) whisker composite sintering according to claim 2, wherein metallic boron and novolac resin are used as sintering aids. Body manufacturing method. 4. A high hardness, high toughness and high strength titanium carbide (TiC) / silicon carbide (SiC) whisker composite firing according to claim 2 or 3, wherein kneading is carried out using an ultrasonic dispersion method. A method for producing a bound body.