Plastic and nonmetallic article shaping or treating: processes – Carbonizing to form article
Patent
1997-02-12
1998-02-24
Derrington, James
Plastic and nonmetallic article shaping or treating: processes
Carbonizing to form article
264 297, 264670, 264671, C01B 3136
Patent
active
057209111
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to Boron Carbide articles and in particular to a method capable of producing such articles weighing several kilograms (kg)
BACKGROUND OF THE INVENTION
Boron Carbide is an extremely hard substance, with a hardness exceeded only by diamond and cubic boron nitride. It has, therefore, many uses. However it has hitherto proved difficult to utilise the full potential of the material as a result of the difficulties met in manufacturing shaped articles.
Of the known prior art methods of forming articles from boron carbide: equipment, and has proved practical for the manufacture of only small articles of simple geometrical shape: Gestzmassigkein bei Erzeugung des Hochdicen Borcarbids durch Sinterung", by P S Kislyi and B L Grabtschuk. 4th European Powder Metallurgy Symposium (Grenoble, France) 1975, INIS-mf-2082; Chemical Abstracts 87 (1977) 140 112) requires sintering in the immediate premelt zone at about 2300.degree. C. Very accurate temperature control, which is very difficult in this region, is necessary as any incursion into the melting zone results in unacceptable glazing of the final product. A further disadvantage of this method is the fact that the finished articles have a strength 15 to 20% lower than that of similar hot-pressed articles; and 4,005,235) requires the use of a very toxic material and results in a product which contains impurities which precludes its use in the nuclear industry, where boron carbide articles have uses as neutron absorbers.
Recent advances in boron carbide technology, as described in U.S. Pat. No. 4,195,066 and GB-A-2,014,193, involve sintering boron carbide particles mixed with amorphous carbon or with a material which yields carbon when heated. In a typical process, as described in U.S. Pat. No. 4,195,066, boron carbide having a B:C atomic ratio within the range 4.5:1 to 3.5:1 and a particle size not exceeding 1 .mu.n is formed into a homogeneous mixture with amorphous carbon, or with an organic material which can be coked to form amorphous carbon at a temperature not exceeding 1000.degree. C., in an amount corresponding to 0.5 to 10% by weight carbon. The mixture (optionally also including a solvent) is molded and shaped into a green body having a density of at least 50% of the theoretical density (TD) of the boron carbide, any solvent involved and not being lost during this process then being removed, and the mixture is then sintered in an atmosphere inert to boron carbide at a temperature within a range of 2100.degree. to 2200.degree. C. Examples given in this U.S. Patent show that much higher percentage TDs were obtained when sintering with carbon than without.
GB-A-2,014,193 uses a similar method, sintering at 2390.degree. C., and gives examples where TDs of 98.5% and 98% were achieved.
The articles produced in the examples of U.S. Pat. No. 4,195,066 were 40 mm long and 8 mm wide and typically weighed of the order of 100 g, whilst the size of the examples in GB-A-2014.066 was not specified.
It has been found in practice that the use of the above mentioned boron carbide/carbon mixture processes for large articles, such as articles suitable for use in body armour, is not easy.
SUMMARY OF THE INVENTION
According to the present invention a method of making a boron carbide article by sintering includes the steps of mixing boron carbide particles with an epoxidised resin dissolved in a carrier solvent to form an homogenised mixture; drying the mixture; granulating the dried mixture; forming the granules into a required shape; and sintering at a temperature of between 2230.degree. C. and 2295.degree. C. the sintering step being preceded by a carbonisation cycle wherein the formed shape is maintained at constant temperatures for predetermined times, there being at least two temperatures, rates of temperature rise being at controlled rates.
The method may be carried out continuously in a sintering furnace. However, some sintering furnaces do not have extraction systems capable of dealing with the decomposition gases produced in the carbonisat
REFERENCES:
patent: 4005235 (1977-01-01), Prochazka
patent: 4195066 (1980-03-01), Schwetz
Ceramic Engineering And Science Proceedings, vol. 6, No. 7/8, Columbus, Ohio, USA, pp. 1151-160, Dole et al, Densification and Microstructure Development in Boron Carbide, 1985.
Crane Jonathan
Fletcher Roy
Fry Michael
Sant John H.
Taylor Richard N.J.
Derrington James
The Secretary of State for Defence in Her Britannic Majesty's Go
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