JPH0463005B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for producing hexagonal boron nitride powder having lubricity. More specifically, by using both pressure molding and heat treatment, a molded body that can be easily crushed while growing crystallites is obtained, and by crushing this molded body, it has lubricating properties. The purpose is to easily produce boron nitride powder. [Prior Art] Hexagonal boron nitride is a white powder that has a hexagonal layered structure similar to graphite, and has various properties. In particular, it has excellent thermal conductivity, electrical insulation, corrosion resistance, lubricity, heat resistance, machinability, etc., and is used in a wide variety of applications by taking advantage of these properties. Applications as a powder include plastic additives and lubricants, and applications as molded bodies and composite materials include jigs, electrical insulation materials, and mold materials. In order to use boron nitride (hereinafter referred to as BN) as a powdered lubricant with a wide range of uses, it is necessary to develop crystals to make the hexagonal layered structure regular. At this time, the criteria for judging lubricity are as shown in Hajime Saito, Masumi Ushio: "Study on the synthesis of hexagonal boron nitride" Journal of the Ceramics Association 77 [5] 1969 P151 Materials of the JSPS Carbon Materials 117 Committee. Determined from X-ray diffraction
It can be determined using Lc, which indicates the average thickness of the stacked layers of BN crystallites (number of layers constituting one crystal grain x layer spacing). For example, as a standard for lubricity, if a coefficient of friction of 0.25 or less is considered to be lubricious,
From the measurement results, if Lc is 700 Å, it can be said that BN has lubricity. Methods for producing BN with well-developed crystals include a method in which alkaline earth metals are added to undeveloped BN to develop crystals (Japanese Patent Publication No. 44-12017), a method in which sodium borate and potassium borate are added ( Tokukai Akira
48-5689), a method of adding magnesium borate (Japanese Patent Publication No. 44-1697), a method of adding lithium borate (Japanese Patent Publication No. 51-13119), etc. are known. A method of growing Lc by hot pressing (Japanese Patent Publication No. 49-37093) is also known. [Problems to be solved by the invention] In the conventional method of adding alkaline earth metals and borates, additives other than BN are removed with acid after heat treatment, or additives other than BN are removed with an acid, or additives with low vapor pressure like alkali metals are added. If the content is high, the method is to evaporate it and separate it from BN. With acid treatment, it is difficult to completely remove impurities because highly crystallized BN is difficult to wet with water. Further, there are drawbacks such as the process being complicated by the acid treatment step. In the method of removing alkali metals by evaporation,
Since evaporated alkali metals precipitate in the cooling section and react with the furnace walls and reaction vessels, special treatment methods are required, such as using materials that do not react with the evaporation products. Furthermore, there is a drawback that it is difficult to completely remove additives even by methods such as washing and evaporation. Furthermore, the method of growing Lc by hot pressing has problems in terms of production cost and production volume because it is limited by the size of the pressurizing device. The present invention is fundamentally different from these conventional methods in that the BN powder, which has undeveloped crystals and no lubricity, is pressure-molded to increase the packing density, and then heat-treated to control the growth of crystallites. This is what I am trying to do. In addition, as a result of experiments under various conditions, we found that CIP (cold isostatic press) (hydrostatic press)
By heat-treating a sample that had been molded at room temperature to improve its density, we were able to expect the same effect as the hot pressing method. In this way, we have developed a technology that allows the production of highly lubricating BN at low cost without being subject to the limitations of pressurizing equipment. [Means for Solving the Problems] In order to achieve the above object, the present invention uses boron nitride powder that does not have lubricity as a compact with a packing density of 0.8 g/cm ³ or more, and in an oxidizing atmosphere
This is a method for producing boron nitride powder having lubricating properties, which comprises heat-treating at a temperature of 1200 to 1850°C and then pulverizing the obtained compact. BN has a hexagonal layered structure and is scaly. Therefore, in order to increase the crystallite size Lc, it is better to bring the scales into contact with each other as much as possible by increasing the packing density, etc., so that the crystal growth will be more remarkable. The present inventors have made efforts to develop an alternative method since the conventional method of growing crystals using compounds of alkali metals and alkaline earth metals has drawbacks such as mixing of impurities. While stacking the layers, I came to the idea that it would be preferable for the particles to be in contact with each other as much as possible in order for the crystals to grow in the layer direction. Furthermore, as a result of repeated studies on how to bring the particles into contact under optimal conditions, we found that the smaller the particles are, the less they grow.
It has been found that it is suitable for promoting grain growth of crystals. The results are shown in Figure 1. Lc is
It has become clear that particles of 500 Å or less are more favorable for crystallite growth when heat treated under the same conditions. Figure 2 shows the results of heat treating molded bodies with different filling densities (method 1) in order to clarify the effect of packing density. A value of ³ or more is effective for crystallite growth. Note that the packing density in non-pressure filling is usually about 0.2 to 0.3 g/cm ³ . In order to effectively grow Lc, the Lc of BN powder raw material is
However, if Lc is small, the purity is low due to the BN synthesis conditions. For example, when Lc is about 25 Å, the purity is about 90%. When this powder is heat-treated, most of the impurities are B ₂ O ₃ and therefore exist in a molten state, so in order to improve crystallite growth and purity at the same time, it is better not to increase the packing density too much. preferable. Furthermore, in order to produce a highly purified product, performing the heat treatment under reduced pressure rather than normal pressure was extremely effective in removing B ₂ O ₃ present as an impurity.

〔Example〕

A product obtained by mixing 1 kg of borax and 2 kg of urea, treating it in an ammonia atmosphere at 900°C for 2 hours, and then washing with water to remove the sodium component (BN purity 91.3, Lc =
93Å) was cooled, crushed, pressure-formed using CIP (cold isostatic pressing) to a packing density of 1.5 g/cm ³ , and then heated in a tungsten resistance furnace in an N ₂ atmosphere at normal pressure using an alumina crucible. When the mixture was heated at 1600°C for 1 hour, a product with high lubricity was obtained. At this time, the purity of BN was 96.1%, and Lc was 845 Å. Furthermore, when the firing atmosphere was _N2 at 10 ^-1 Torr, the purity of BN increased to 97.2% and Lc increased to 982 Å. As a comparative example, the same sample was packed at a packing density of 0.3g/
It was filled into an alumina crucible to a concentration of cm ³ and fired under the same conditions as in the above example, but the obtained BN had a purity of 97.5% and an Lc of 405 Å. The experimental results are shown in Table 1.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between the crystallite size of raw material boron nitride and the crystallite size after heat treatment, and FIG. 2 is a graph showing the relationship between the packing density and the crystallite size after heat treatment.

Claims (1)

[Claims] 1 Filling density of boron nitride powder with no lubricity
Nitriding with lubricity characterized by forming a compact of 0.8 g/cm ³ or more, heat-treating it at a temperature of 1200 to 1850°C in a non-oxidizing atmosphere below normal pressure, and then pulverizing the obtained compact. Method for producing boron powder.