JPH05105461A - Mold for glass molded lens - Google Patents

Abstract

PURPOSE:To obtain the mold with the smooth surface and mold releasability kept over a long period by using a cubic boron nitride having specified purity and relative density, and produced by the direct conversion method from a pyrolytic boron nitride. CONSTITUTION:Boron trichloride is allowed to react with a raw gas such as ammonia at <=50Torr and >=1800 deg.C to deposit a pyrolytic boron nitride as a solid on a graphite substrate. The relative density is controlled to >=99.9% in this case. The purity can be increased to >=99.5% by paying attention to the purity of the raw gas and the furnace material of a CVD device. When the pyrolytic boron nitride is directly converted to the cubic boron nitride using this raw material, the purity of the sintered body finally obtained is kept at >=99.5%. Accordingly, since the content of the impurity easy to react with the gas is minimized when the cubic boron nitride is used to obtain a mold for a glass molded lens, the mold surface is not roughened even after the mold is repeatedly used for a long period, mold releasability is not deteriorated, and a high-performance mold having a long service life is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a glass mold lens mold suitable for manufacturing optical lenses.

[0002]

2. Description of the Related Art As a method for molding a glass lens, there is known a method in which an optical glass is heated and pressed into a desired shape. Molds are required to have such properties as chemical stability and wear resistance to high temperature glass, releasability from glass, and surface smoothness that makes post-polishing of glass after molding unnecessary. .. Materials satisfying such requirements have been studied so far, and various metals and ceramics have been proposed.

Among them, cubic boron nitride (cBN)
JP-A-63-270324 proposes to use this as an optical element molding die because it has basic characteristics satisfying many of the above. In this prior art, the molding surface of the molding die is made of a sintered body having a cBN content of 50 atm% or more, more preferably 70 atm% or more.

[0004]

[Problems to be Solved by the Invention] However, in practice cBN
There are various technical difficulties in using the sintered body on the molding surface of the molding die, and its practical application has not progressed. The main reason for this is that, first of all, cBN is a substance with a strong covalent bond and is difficult to sinter by itself, so normally, various ceramics and metals are mixed with cBN as a sintering aid, and this is This is because the sintered body is manufactured by sintering at ultrahigh pressure and high temperature. Also in Japanese Patent Laid-Open No. 63-270324, all of the examples use a sintering aid of nitride or carbide. Therefore, a substance other than cBN is present at the grain boundaries of the cBN sintered body, and the characteristics of the sintered body do not reflect the original characteristics of cBN.

When such a cBN sintered body is used as a mold for a glass mold lens, ceramics and metal parts remaining at grain boundaries are selectively corroded and abraded to roughen the mold surface, resulting in mold releasability. There is a problem that it becomes worse and cannot be used as a mold at an early stage. JP-A-3-8873
No. 2 gazette proposes to use a wurtz-type BN (wBN) sintered body having characteristics very close to cBN, but also in this case, there is a problem of adverse effects of the sintering aid remaining on the grain boundaries. Is the same as for cBN. Further, since wBN is softer than cBN, its wear resistance does not reach that of cBN, and there is a problem in terms of life.

Therefore, it has been considered to use it in the form of a thin film coating, or to grind and polish a single crystal having no grain boundaries to obtain a mold. However, with thin film coating, the thin film forming technology itself has not yet been completed,
A method for industrially forming a crystalline cBN film having high chemical stability has not been established. In addition, cBN single crystals can only be manufactured with a diameter of 10 mm or less at present, and there is a problem in terms of size when used as a molding die. Also, since a single crystal that is easily cleaved is mechanically ground and polished, considerable cost is required. It becomes high.

Another problem is that, as described in Japanese Patent Laid-Open No. 3-88732, the conventional cBN sintered body has a small thickness so that it can be used for molding a deep glass lens. That is, it is very difficult to produce the sintered body of, and the die cost increases.

The object of the present invention is to solve the above-mentioned problems of the prior art and to easily mold a deep surface mold having a small curvature,
Moreover, it is to provide a mold for a glass mold lens having a long life, in which a smooth surface is maintained for a longer period of time and mold releasability is maintained, as compared with the conventional cBN mold.

[0009]

That is, the present invention is a cubic crystal having a purity of 99.5% or more and a relative density of 99.9% or more produced by a direct conversion method using pyrolytic boron nitride (P-BN) as a raw material. A mold for a glass mold lens, which is made of a boron nitride sintered body.

The present invention will be described in more detail below.

The cBN sintered body used in the present invention is manufactured by a method known as a direct conversion method. When converting low-pressure phase hexagonal boron nitride (hBN) into cBN, Generally, it is produced by ultrahigh pressure and high temperature treatment in the absence of the action of a substance called "catalyst" or "solvent". In the production of a cBN sintered body by the direct conversion method, a method using powdery hBN having boron formed on the surface as a raw material (Japanese Patent Laid-Open No. 55-167110) and a method using a sintered body of hBN powder (Japanese Patent Laid-Open No. 3-1599
No. 64), and a method using P-BN manufactured by the CVD method (JP-A-54-33510 and JP-A-1-184033).
Are known.

The present invention adopts the latter method among them. The reason is that in the method of using the powder of the low-pressure phase BN or the sintered body of the powder as the raw material, since many voids exist in the raw material itself, the pores having a relative density of 99.9% or more suitable for the mold of the glass mold lens are formed. It is not easy to obtain a dense cBN sintered body that is almost free of pressure, and very strict ultra-high pressure and high-temperature processing conditions such as a pressure of 7 GPa or more and a temperature of 2100 ° C. or more are required. This is because it is difficult to obtain a thick sintered body due to the low relative density of the raw material at the stage of filling the high-pressure high-temperature processing apparatus, and it is difficult to form a deep glass lens molding die having a small curvature. ..

On the other hand, when P-BN is used as the raw material, P-BN itself is a compact of the low-pressure phase BN having a very high density, and since it is produced by the CVD method, it is of high purity. Therefore, it is possible to obtain a dense sintered body having almost no pores and a density of 99.9% or more, although the ultra-high pressure and high temperature treatment conditions are relatively mild.

The glass mold lens is intended for use without being polished after molding, and for that purpose, the molded surface roughness must be as smooth as possible. Specifically, Rmax = 0.5 μm. You need the following: In the case of cBN sintered body manufactured by the direct conversion method,
Such surface roughness can be achieved with a relative density of 99.9% or more.

P-BN is "FC Report" vol.4 No2
As described in pages 7 to 15 (1986), a raw material gas such as boron trichloride and ammonia, a pressure of 50 Torr or less,
It can be produced by reacting at a high temperature of 1800 ° C. or higher and precipitating it as a solid on a graphite substrate. The P-BN compact thus obtained has almost no pores as observed with an electron microscope and is extremely dense. Therefore, when this is converted to cBN by a direct conversion method. Also, a cBN sintered body having no pores, which would be a problem in the case of using a glass molding die, can be obtained, and the relative density thereof is extremely high, 99.9% or more.

Further, since P-BN is produced by the CVD method, it is easy to make it highly pure if attention is paid to the purity of the raw material gas and the furnace material of the CVD apparatus, and it is used in the production of semiconductors, for example. It is possible to maintain a P-BN purity of 4 to 6 N. In the present invention, if the P-BN purity is 99.5% or more, the objective mold for high-performance glass molded lens can be obtained.

When the above-mentioned P-BN raw material is converted to cBN by the direct conversion method, the direct conversion method itself is a method in which the contamination of impurities into cBN is very small, and thus the finally obtained c is obtained.
The BN sintered body can also maintain a very high purity of 99.5% or more. Therefore, when this is used as a mold for a glass mold lens, the impurity component that easily reacts with glass is extremely small, so even if it is repeatedly used for a long time, the surface of the mold does not become rough and the mold is not defectively released. You can get the mold. However, if the cBN purity is lower than 99.5%, the mold surface may start to get rough at a relatively early stage in molding a glass lens, which is not preferable. When using a P-BN raw material having high crystallinity as disclosed in JP-A 1-184033, a pressure on the order of 6 GPa and 2000 ° C.
Since a dense cBN sintered body can be obtained even at the following temperature, it is advantageous in terms of manufacturing cost.

The characteristic feature of the high density of P-BN is that, when the P-BN is charged as a raw material into an ultra-high pressure high-temperature processing apparatus, the limited ultra-high pressure generation space can be efficiently filled with the low-pressure phase BN, It also has an advantageous effect in producing a thick sintered body that can be applied to a deep surface forming die having a small size.

Another advantage of using P-BN as a raw material is that although P-BN is a ceramic, it is highly flexible, and it easily undergoes plastic deformation by applying a certain temperature and pressure. This means that a method of obtaining a cBN sintered body having a size and shape close to a conventional molding surface can also be adopted. In other words, the P-BN plate is placed between the molds that have been processed into almost the same shape as the molding surface, and 600
By pressing this while applying a temperature of not less than 0 ° C., it is possible to obtain P-BN which is plastically processed according to the shape of the mold as in the case of metal press molding. Although it is difficult to form a glass with a very sharp corner, the glass lens can be formed without any problem as long as it has a curvature required to form a glass lens.

As described above, the preformed P-BN is sandwiched between the molds having the same desired shape and set in the ultrahigh pressure and high temperature treatment apparatus, and the ultrahigh pressure and high temperature treatment is carried out to obtain the final product. It is possible to obtain a so-called near net shape cBN sintered body that is very close to the molding surface, and the subsequent grinding process is almost unnecessary, which is very advantageous in terms of cost.

Further, the plastic working of P-BN and its conversion into cBN can be continuously carried out in an ultrahigh pressure and high temperature processing apparatus. This is possible because P-B
N is a material with a very high orientation, and the structure is such that the c-plane of the hBN crystal is oriented in a direction parallel to the deposition surface during CVD, and the BN is very slippery in the c-plane direction. This behavior cannot be expected at all in a normal hBN sintered body in which particles are randomly oriented. When this method is adopted, it is preferable that P-BN as a raw material has a high degree of orientation because it can be easily deformed. Specifically, it is particularly preferable that the degree of selective orientation of the c-axis is 100 or less. If the value of the degree of selective orientation is larger than this,
After the plastic working of the raw material, it tends to return to its original shape, making it difficult to fill the ultra-high pressure and high temperature processing apparatus. The degree of selective orientation of the c-axis and its measuring method are described in US Pat. No. 3,578,403, Physica 139 & 140B (1986).
256-258 etc.

[0022]

EXAMPLES Next, the present invention will be described more specifically by way of Examples and Comparative Examples.

Example 1 Comparative Examples 1 and 2 A disc having a diameter of 15 mm was cut out from a commercially available P-BN plate having a thickness of 3 mm (trade name "Denka P-BN" manufactured by Denki Kagaku Kogyo KK).
This was treated at a pressure of 6.5 GPa and a temperature of 1980 ° C. for 30 minutes to prepare a cBN sintered body having a thickness of about 1.8 mm and a diameter of 15.5 mm. IC
Purity analysis by P method showed that the raw material P-BN was 9
More than 9.995%, cBN sintered body manufactured by the same method is 99.95%
% Or more. The density of this sintered body was equal to the theoretical density of cBN of 3.45 g / cm ³ , and was almost 100% densified.

This sintered body was ground by a diamond grindstone to almost finish the shape of the molding surface, and was then brazed to a pedestal made of cemented carbide with a high-temperature brazing material containing Ti. In this state, the molding surface was further polished with diamond abrasive grains to finish the surface roughness to Rmax = 0.03 μm to prepare a glass lens molding die (Example 1).

For comparison, according to the method disclosed in JP-A-63-270324, cBN85atm%, TiN11atm%, TiC
A cBN sintered body was produced using a mixed powder of 2 atm% and the balance Ni, and a molding die was produced by the same method as in Example 1 (Comparative Example 1). At that time, when the finish polishing of the molding surface was performed by the same method as in Example 1, the surface roughness was Rmax.
= 0.05 μmm, which is a relatively large value.

SF type glass (SiO ₂ -PbO type) lenses were molded using the above two molding dies, and in Example 1, there was no change in the quality of the glass even after molding 75,000 shots, and the glass could not be used continuously. It was possible. On the other hand, in Comparative Example 1, the discoloration of the glass gradually became conspicuous after the time of repeating 50000 shots, and after the time of exceeding 55,000 shots, the image sticking of the glass became remarkable and the glass became unusable.

When the same test was performed on a mold made from a sintered body of wBN (Comparative Example 2), discoloration and seizure became remarkable after 20,000 shots, and the life was reached.

Comparative Examples 3 to 4 Commercially available hBN powder (trade name "grade" manufactured by Denki Kagaku Kogyo Co., Ltd.)
GP ") was heat-treated at 2000 C for 10 minutes in a vacuum of 10 ^-3 Torr to form free boron on the surface (Comparative Example 3),
And a commercially available hBN sintered body (trade name "Grade N-1" manufactured by Denki Kagaku Kogyo Co., Ltd.) are heated in a vacuum of 10 ^-3 Torr at 1600 ° C for 2 hours, and in a nitrogen atmosphere at 2100 ° C for 2 hours. To prepare an oxygen content of 0.06% or less (Comparative Example 4).

Attempts were made to convert them into cBN sintered compacts under the same conditions as in Example 1, and the products were analyzed by powder X-ray diffraction method.
Some BN remained. Therefore, the pressure is 7 GPa and the temperature is
By increasing the temperature to 2150 ° C. and treating for 30 minutes, a cBN sintered body in which no residual hBN was observed by the X-ray diffraction method could be produced. However, the relative densities of the cBN sintered bodies were 99.6% in Comparative Example 3 and 99.8% in Comparative Example 4, which were smaller than those in Example 1.

The obtained sintered body was ground and polished in the same manner as in Example 1, but the surface roughness did not fall below Rmax = 0.05 μmm and the smoothness reached the level of Example 1. There wasn't. When the same SF type glass as in Example 1 was molded, 35000 shots were made in Comparative Example 3 and 23000 shots in Comparative Example 4.
Roughness of the molding surface began to be noticeable from around the point where the shot was crossed, making subsequent use difficult. Under this conversion condition, conversion to cBN occurs almost completely, but it seems that the level of the present invention is not reached in terms of densification and development of interparticle bond.

Example 2 A P-BN plate having a degree of selective orientation of the c-axis of 10 was machined to a thickness of 2 mm in the direction in which the c-axis was selectively oriented.
A P-BN disc having a thickness of 2 mm and a thickness of 2 mm was cut out. With graphite, make a mold that matches the curvature of the desired glass lens,
A P-BN disc is sandwiched between the molds, and the temperature is 1200 ° C, and the pressure is
Hot working was carried out at 30 MPa for 30 minutes to obtain a P-BN plate having a curvature substantially matching the molding surface of the glass lens mold.

Next, this P-BN plate was sandwiched between the same graphite molds, and the whole was set in an ultrahigh-pressure high-temperature treatment apparatus and treated at a pressure of 6.5 GPa and a temperature of 1980 ° C. for 30 minutes. When the sample was cooled and depressurized to recover the sample, a cBN sintered body having a curvature substantially matching the molding surface of the molding die was obtained. The purity of the cBN sintered body was 99.9%, the relative density was equal to the theoretical density of 3.45 g / cm ^3, and the density was almost 100%.

This was brazed to a base made of cemented carbide and polished in the same manner as in Example 1 with diamond abrasive grains to obtain a surface roughness Rmax = 0.03 μm. Unlike Example 1, grinding with a diamond grindstone was hardly necessary in this case, and the surface of the mold could be finished only by polishing, so the manufacturing cost could be reduced. When SF glass was molded in the same manner as in Example 1, it was 75
It was able to be used without problems even after repeating more than 000 shots.

[0034]

The glass mold lens molding die of the present invention has less glass adhesion and a longer life than the conventional molding die made of cBN sintered body, and can reduce the glass lens molding cost.

Claims (1)

[Claims] 1. A purity of 99.5% or more and a relative density of 99.9 produced by a direct conversion method using pyrolytic boron nitride as a raw material.
% Of a cubic boron nitride sintered body, a glass mold lens forming die.