JP7305584B2 - Method for producing sintered body of composite material - Google Patents

Description

The present invention relates to a method for producing a sintered body of a composite material of titanium or a titanium alloy and ceramics.

The present applicant has proposed that a sleeve used for die casting of non-ferrous metals such as aluminum, magnesium, zinc, tin, lead, and alloys thereof has a double structure of an outer cylinder and an inner cylinder, and is fitted into a steel outer cylinder. It has been proposed and implemented to form the inner cylinder from a sintered body of a composite material of titanium or a titanium alloy and ceramics (hereinafter referred to as "TC composite material") (see, for example, Patent Document 1).

In the past, general sleeves were made of steel such as SKD61, but since non-ferrous metals tend to react with iron, steel sleeves are prone to erosion due to contact with the molten metal to be filled, and have a short service life. I had a problem. Also, since steel has a high thermal conductivity, the temperature of the molten metal supplied into the sleeve tends to drop. If the temperature of the molten metal supplied into the sleeve drops in the sleeve before it reaches the cavity and solidified pieces are generated, defects such as peeling are likely to occur at those portions in the molded product, and the mechanical strength is reduced. There is a problem of lowering

On the other hand, the TC composite material used for the inner cylinder has low reactivity with non-ferrous metals, and is therefore excellent in erosion resistance. In addition, while SKD61 has a high thermal conductivity of 35.6 W/mK, TC composite material has a very low thermal conductivity of 7.4 W/mK, which is excellent in heat retention. It has the advantage that the temperature of the metal does not easily drop. Furthermore, when the inner cylinder is formed only from ceramics, although it is possible to improve the resistance to corrosion and heat retention, ceramics, which is a brittle material, has the disadvantage of low impact resistance. Since it is a composite material of metal and ceramics, it has the advantage of being excellent in impact resistance.

However, although the TC composite material has many advantages such as excellent erosion resistance, heat retention and impact resistance, it has the disadvantage of low hardness. In die casting, the plunger tip is inserted from one end of the sleeve and slides in the sleeve in the axial direction. If the hardness is low, the sliding of the plunger tip will wear the inner peripheral surface. When the inner peripheral surface of the inner cylinder is worn, a gap is formed between it and the plunger tip, and molten metal may leak from the gap. For this reason, conventional steel sleeves use steel that has undergone surface hardening treatments such as quenching and nitriding, but the TC composite material has a lower hardness than the hardened steel.

Therefore, the present applicant proposes nitriding an inner cylinder made of a TC composite material by heating it in an atmosphere containing nitrogen to form a nitrided layer on the inner peripheral surface of the inner cylinder. (See Patent Document 2). The nitrided layer is not only harder than TC composites, but it is also much harder than hardened steel, which makes it possible to increase the wear resistance of the inner cylinder of the sleeve.

However, in the conventional manufacturing method of heating the TC composite material in a nitrogen-containing atmosphere, the fact is that only the very surface of the TC composite material can be nitrided. If the nitrided layer is formed only on the very surface layer, the nitrided layer may be lost when processing is performed to increase the dimensional accuracy in order to bring the inner peripheral surface of the inner cylinder into close contact with the plunger tip. In addition, when the nitride layer is formed only on the very surface layer, due to the large difference in hardness at the boundary between the surface layer and the inner layer, it tends to peel off due to contact with the sliding plunger tip. There is

Furthermore, with regard to nitriding of steel, it is said that the higher the heating temperature in the nitrogen-containing atmosphere and the longer the heating time, the easier it is to form a nitrided layer and increase the hardness. There is a problem that the material is deformed by heating for a long time.

JP-A-3-142053 JP-A-4-224066

Therefore, in view of the above circumstances, the present invention provides a sintered body of a composite material that can increase the hardness of not only the surface layer but also the surface layer while suppressing deformation of the sintered body of the composite material of titanium or titanium alloy and ceramics. The object is to provide a method for producing the

In order to solve the above problems, a method for producing a sintered body of a composite material according to the present invention (hereinafter sometimes simply referred to as "manufacturing method") comprises:
"In manufacturing a sintered body of the composite material by molding the starting material of the composite material of titanium or titanium alloy and ceramics and firing it in a non-oxidizing atmosphere,
Hexagonal boron nitride is added to the starting material containing at least titanium and a ceramic raw material, and nitrogen derived from the hexagonal boron nitride is reacted with titanium to produce titanium nitride in the matrix of the sintered body. and
Hexagonal boron nitride is added to the starting material at a ratio of 0.66 to 1.98 parts by weight per 100 parts by weight of titanium .

In this production method, titanium nitride is produced by reacting nitrogen derived from hexagonal boron nitride with titanium. Conventionally, in nitriding a composite material (TC composite material) of titanium or a titanium alloy and ceramics, titanium nitride is generated only on the surface layer of the sintered body by heating the sintered body of the TC composite material in a nitrogen atmosphere. It was something. On the other hand, in this production method, when the starting material for the TC composite material is fired, titanium contained in the starting material reacts with nitrogen derived from the starting material to form titanium nitride. Titanium nitride is produced in a substantially uniform distribution. This manufacturing method of forming titanium nitride inside the sintered body can be referred to as "internal nitriding".

Therefore, since titanium nitride is not formed only on the surface layer, unlike conventional nitrides, titanium nitride is not lost by processing for improving dimensional accuracy. In addition, since the hardness of the entire matrix is increased by titanium nitride, there is the problem of easy peeling due to the large hardness difference at the boundary between the surface layer and the inner layer, which has been a problem with conventional nitrides. There is no

In addition, in conventional nitriding, since nitrogen in the atmosphere enters the sintered body from the outside, the heating temperature is increased and the heating time is increased with the intention of facilitating the formation of the nitrided layer. . On the other hand, in the present manufacturing method, during firing, the nitrogen derived from the starting material reacts with the titanium contained in the starting material for "internal nitridation", so excessive heating or long-time heating is required. do not have. Therefore, unlike conventional nitriding, the present production method does not have the problem that the sintered body is deformed due to excessive heating or long-term heating.

In addition to the above configuration, the method for manufacturing a sintered body of a composite material according to the present invention includes:
"By including molybdenum in the starting material, boron derived from hexagonal boron nitride and molybdenum are reacted to form a compound of molybdenum and boron in the matrix of the sintered body."

In this manufacturing method, the starting material of the TC composite material contains molybdenum. Molybdenum reacts with boron derived from hexagonal boron nitride to form compounds of molybdenum and boron (MoB, Mo ₂ B). That is, decomposition of hexagonal boron nitride produces nitrogen and boron, nitrogen reacts with titanium to form titanium nitride, while boron reacts with molybdenum to form a compound of molybdenum and boron. Since a compound of molybdenum and boron has a hardness equal to or higher than that of titanium nitride, the hardness of the sintered body of the TC composite material can be further increased. A compound of molybdenum and boron is also generated so as to be distributed throughout the matrix inside the sintered body, like titanium nitride. When silicon is contained in the TC composite material, a compound of molybdenum, silicon, and boron is also produced as a compound of molybdenum and boron, as will be described later.

Next, the composite material sintered body produced by the production method of the present invention is
"A sintered body of a composite material of titanium or a titanium alloy and ceramics,
It contains titanium nitride and a compound of molybdenum and boron."

This is the structure of the sintered body produced by the above production method in which molybdenum is contained in the starting material. It has high hardness because it contains a compound of molybdenum and boron in addition to titanium nitride.

As described above, according to the present invention, the sintered body of a composite material of titanium or a titanium alloy and ceramics can be suppressed from being deformed, and the hardness of not only the surface layer but also the hardness of the sintered body of the composite material can be increased. A method of manufacture can be provided.

4 is a graph comparing apparent porosity, hardness, wear width, and bending strength for sample R, samples S1 to S3, and sample E1. 4 is a graph comparing the hardness distribution in the depth direction of sample E1 with that of sample R subjected to conventional nitriding treatment. 4 is a graph comparing the thermal conductivity of sample R, samples S1 to S3, and sample E1 with steel (SKD61); FIG. 4 is a diagram showing the results of elemental analysis of sample E1; It is an X-ray diffraction pattern of sample E1. 4 is a graph comparing the apparent porosity and hardness of samples E1 to E4 with sample R. FIG.

Specific embodiments of the present invention are described below. The production method of the present embodiment includes a raw material preparation step of preparing a starting raw material for the TC composite material, a molding step of molding the starting raw material, and a step of firing the molded body.

In the raw material preparation step, a starting raw material containing at least titanium and a ceramic raw material is prepared. The proportion of ceramics in the TC composite material is desirably 1-15 parts by weight, more desirably 3-10 parts by weight, per 100 parts by weight of metal atoms. When the TC composite material contains metals other than titanium in addition to titanium as metals, "100 parts by weight of metal atoms" referred to herein is the total weight of titanium atoms and other metal atoms. Examples of ceramics include silicon carbide, silicon nitride, aluminum nitride, zirconia, magnesia, and yttria. The ceramic raw material may be a powder of these ceramics or a raw material powder from which these ceramics are produced by heating.

The starting material may further contain molybdenum. The content of molybdenum in the TC composite material is desirably 10 to 50 parts by weight of molybdenum atoms, more desirably 20 to 45 parts by weight, per 100 parts by weight of titanium atoms.

In the molding step, the starting material is molded into a desired shape to form a compact. Examples of the molding method include cold isostatic pressing, hot isostatic pressing, and hot pressing.

The firing process is performed at a temperature of 1000° C. to 1400° C. in a non-oxidizing atmosphere. Since hexagonal boron nitride decomposes at 1000° C. or higher, the firing temperature is preferably 1100° C. or higher. The non-oxidizing atmosphere can be a vacuum atmosphere or an inert gas atmosphere such as argon.

A starting material was prepared by mixing 66% by mass of titanium powder, 5% by mass of silicon carbide powder, and 29% by mass of molybdenum powder. was used as sample R. Based on the starting material of sample R, sample S1 was prepared from the starting material obtained by adding 5 parts by weight of zirconia to 100 parts by weight of the starting material of sample R, sample S2 was prepared from the starting material obtained by adding 2 parts by weight of silicon nitride, and molybdenum. Sample S3 was prepared from the starting material to which 2 parts by weight of was added, and Sample E1 was prepared from the starting material to which 2 parts by weight of hexagonal boron nitride was added. The molding conditions and firing conditions were the same for all samples R, samples S1 to S3, and sample E1.

Apparent porosity, bulk density, apparent density, hardness, wear width, and bending strength were measured for each sample. The apparent porosity, bulk density, and apparent density were measured by the Archimedes method according to JIS R2205. Hardness was measured under the same conditions using a Rockwell hardness tester (manufactured by Akashi Seisakusho). The wear width (width of wear marks) was measured using an Okoshi rapid wear tester (manufactured by Tokyo Shikenki Seisakusho). Bending strength was determined by using an electronic universal material testing machine YS2TL (manufactured by Yonekura Seisakusho) and measuring three-point bending strength according to JIS R1601. The measurement results are shown in Table 1, and the apparent porosity, hardness, wear width, and bending strength are shown in FIGS. 1(a) to 1(d), respectively.

As shown in Table 1 and FIG. 1, the sample E1 to which hexagonal boron nitride was added had an apparent porosity so small that it could not be evaluated by the Archimedes method, and was considered to be densified. Sample E1 has a Rockwell hardness that is approximately 1.6 times that of sample R, which is a standard (no additive) TC composite material, and the degree of increase in hardness is greater than that of samples S1 to S3 to which other materials are added. It was big. Moreover, sample E1 had a small wear width. The wear width was considered to be an index of wear resistance, and it was thought that the wear resistance increased with increasing hardness. Furthermore, materials with high hardness generally have low toughness, but sample E1 had high bending strength despite its high hardness.

From the above results, it was found that the hardness of the TC composite material can be increased by adding hexagonal boron nitride, so the hardness distribution in the depth direction was measured for sample E1. Hardness was measured using a micro Vickers hardness tester HMV-G21DT (manufactured by Shimadzu Corporation). For comparison, the hardness distribution in the depth direction was measured under the same conditions for the sample R subjected to the conventional nitriding treatment. Conventional nitriding treatment was performed by heating the sintered body of sample R at a temperature of 1000° C. in a nitrogen atmosphere. The results are also shown in FIG.

As is clear from FIG. 2, in the sample R subjected to the conventional nitriding treatment, the hardness increased only on the surface (0 mm in depth), but in the range of 0.1 mm to 1.8 mm in depth, the hardness increased. The hardness was almost constant at 450HV to 500HV regardless of the hardness. This hardness is believed to be that of a non-nitrided TC composite. On the other hand, in the sample E1 to which the hexagonal boron nitride was added, the hardness was high over the entire range in the depth direction. The average hardness of sample E1 was about 800 HV, which is 1.6 to 1.7 times higher than that of non-nitrided TC composite material, which is quite large. The hardness distribution from the surface to a depth of 1.8 mm was an average value ±10%. In other words, in the conventional nitriding treatment, only the surface layer was nitrided and hardened, whereas in the sample E1 to which hexagonal boron nitride was added, titanium nitride was generated inside the sintered body, so that the matrix It is possible to obtain a sintered body in which titanium nitride is generated almost uniformly and there is no significant difference in hardness distribution from the surface to the inside of the sintered body.

Here, "there is no significant difference in the hardness distribution from the surface to the inside of the sintered body" means that "the hardness distribution from the surface of the sintered body to a depth of at least 1.8 mm is within the range of the average hardness ± 15. It can be translated as 'inside'.

As mentioned above, when a TC composite material is used for the inner cylinder of a die casting sleeve, the property of low thermal conductivity is very important. Therefore, the temperature change of the thermal conductivity of the sample E1 was measured and compared with the samples S1 to S3 and the sample R. Thermal conductivity was measured by a laser flash method using a thermal conductivity measuring device LFA457 (manufactured by NETZSCH). The measurement results are shown in FIG. For comparison, the thermal conductivity of steel (SKD61) is also shown in FIG.

As is clear from FIG. 3, it was confirmed that the characteristic that the thermal conductivity of the TC composite material is smaller than that of steel is not lost by adding hexagonal boron nitride. This was the same for samples S1 to S3 to which other materials were added.

Next, FIG. 4 shows the results of elemental analysis (EPMA) of the polished surface of the cross section of the sintered body of sample E1 using an electron probe microanalyzer. Here, FIG. 4(a) is an image observed by a scanning electron microscope, and FIGS. 4(b) to 4(e) are mapping images of specific elements in the same field of view. The elements to be analyzed are titanium in FIG. 4(b), nitrogen in FIG. 4(c), molybdenum in FIG. 4(d), and boron in FIG. 4(e). In the mapping image, the more elements to be analyzed, the brighter and whitish it looks.

From FIGS. 4(b) and 4(c), it is clear that the distribution of titanium and the distribution of nitrogen match, and it is considered that titanium nitride is produced. 4(c) and 4(e), and 4(d) and 4(e), the distribution of nitrogen does not match the distribution of boron, whereas the distribution of molybdenum and it is thought that a compound of molybdenum and boron is produced. For these reasons, at least part of the nitrogen derived from the added hexagonal boron nitride reacts with titanium to form titanium nitride, while at least part of the boron derived from the hexagonal boron nitride reacts with molybdenum. It was thought that a compound of molybdenum and boron was produced as a result.

Therefore, the crystal phase was identified by measuring the X-ray diffraction pattern of the cross section of the sample obtained by cutting the sample E1. The X-ray diffraction pattern was measured using an X-ray diffractometer (Rigaku, Ultima III) with a copper tube, a voltage of 40 kV, a current of 40 mA, and a step width of 0.02 degrees and 2 degrees/min by the step scan method. was measured under the conditions of The results are shown in FIG.

As shown in FIG. 5, a titanium nitride peak was clearly observed, confirming that titanium nitride was produced. Also, peaks of compounds of molybdenum and boron (compounds of molybdenum, silicon, and boron) were observed. This compound is known for its very high hardness and high temperature strength. Therefore, the TC composite material to which hexagonal boron nitride is added constitutes the inner cylinder of the die-casting sleeve, which requires durability at high temperatures, by containing such a phase with high hardness and high high-temperature strength. It is considered to be suitable as a material for

As described above, it was found that by adding hexagonal boron nitride to the TC composite material and firing it, a nitriding reaction occurs inside the sintered body to generate titanium nitride and increase the hardness. A study was carried out by changing the amount of crystal system boron nitride added. Specifically, based on the starting material of sample R, sample E2 was prepared from a starting material in which 1 part by weight of hexagonal boron nitride was added to 100 parts by weight of the starting material of sample R, and 3 parts by weight was added. A sample E3 was produced from the starting material, and a sample E4 was produced from the starting material to which 5 parts by weight of hexagonal boron nitride was added. The molding conditions and firing conditions were the same as those of sample R and sample E1. For samples E1 to E4, the ratio of hexagonal boron nitride to 100 parts by weight of titanium is 0.66 parts by weight, 1.32 parts by weight, 1.98 parts by weight, and 3.30 parts by weight, respectively. be.

The apparent porosity, bulk density, apparent density, and Rockwell hardness of the obtained samples E2 to E4 were measured in the same manner as above. The results are shown in Table 2 together with sample E1, and the apparent porosity and Rockwell hardness are shown in FIGS. 6(a) and 6(b), respectively.

As can be seen from Table 2 and FIG. 6, samples E1, E2, and E3 have a lower apparent porosity and a higher hardness than the reference (no additive) sample R, while sample E4 has an apparent porosity. The hanging porosity is significantly increased, and the hardness is reduced to the same extent as sample R. From this, hexagonal boron nitride is effective in increasing the hardness and densification of the TC composite material even when added in a very small amount. It was considered that no effect could be obtained when reaching the part. Further, from the examination results of samples E1 to E3, if the ratio of hexagonal boron nitride is at least 0.66 parts by weight to 1.98 parts by weight with respect to 100 parts by weight of titanium, the TC composite material can be produced at a high level. It was confirmed that the effect of hardening and densifying can be sufficiently exhibited.

In the conventional nitriding treatment in which a sintered body is heated in a nitrogen atmosphere, nitriding is difficult to proceed if the porosity of the sintered body is small. It is believed that this is because the open pores serve as passages through which the nitrogen-containing gas permeates the sintered body. However, if the porosity is high, the mechanical strength of the sintered body is reduced, such as cracks extending along the pores. Therefore, in the case of the conventional nitriding treatment, there is a problem that it is difficult to nitridize the TC composite material that has been densified in order to increase the mechanical strength. On the other hand, in the manufacturing method of the present embodiment, since nitriding proceeds inside the sintered body, it is possible to meet both the demand of increasing the hardness by nitriding and the demand of increasing the mechanical strength by densification. can be done.

As described above, the present invention has been described with reference to preferred embodiments, but the present invention is not limited to the above embodiments, and as shown below, various improvements can be made without departing from the scope of the present invention. and design changes are possible.

For example, in the above, the case where the present invention is applied to increase the hardness when the TC composite material is used for the inner cylinder of the die-casting sleeve is exemplified, but the TC composite material used for other applications is not limited to this. Of course, the present invention can also be applied when increasing the hardness of .

In addition, in the present invention, it is possible to increase the hardness of the TC composite material by nitriding almost uniformly regardless of the depth from the surface by “internal nitriding”, so the conventional nitriding that heats the sintered body in a nitrogen atmosphere. It has the advantage of not needing any treatment and shortening the time required for the manufacturing process to produce a hard TC composite material. However, it is not excluded that the manufacturing process according to the invention is followed by a further conventional nitriding treatment.

Furthermore, the TC composite material of the present invention may contain other metals in addition to molybdenum. For example, the addition of nickel can make the sintered body of the TC composite material very dense. The TC composite material densified by the addition of nickel has open pores that serve as passages for permeation of nitrogen-containing gas into the sintered body in the conventional nitriding treatment in which the sintered body is heated in a nitrogen atmosphere, as described above. is extremely small, it is difficult to perform nitriding treatment. In contrast, in the present invention, since nitriding proceeds inside the sintered body of the TC composite material, the TC composite material densified by the addition of nickel to increase the mechanical strength can be sufficiently nitrided.

Claims (1)

In manufacturing a sintered body of the composite material by molding the starting material of the composite material of titanium or titanium alloy and ceramics and firing in a non-oxidizing atmosphere,
Hexagonal boron nitride and molybdenum are added to the starting material containing at least titanium and a ceramic raw material, and nitrogen derived from the hexagonal boron nitride is reacted with titanium to obtain titanium nitride in the matrix of the sintered body. and reacting boron and molybdenum derived from hexagonal boron nitride to generate a compound of molybdenum and boron in the matrix of the sintered body,
Hexagonal boron nitride is added to the starting material at a ratio of 0.66 to 1.98 parts by weight with respect to 100 parts by weight of titanium ,
By using silicon carbide as the ceramic raw material, a compound of molybdenum, silicon, and boron is produced as the compound of molybdenum and boron.
A method for producing a composite material sintered body, characterized by:

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