Coating processes – Coating by vapor – gas – or smoke – Carbon or carbide coating
Patent
1986-04-28
1990-07-17
Childs, Sadie
Coating processes
Coating by vapor, gas, or smoke
Carbon or carbide coating
427255, 4272552, 4274197, C23C 1632, C23C 1652
Patent
active
049420623
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a process for the production of boron carbide coatings on the surface of a substrate; it includes the coatings produced.
The invention applies in particular to obtaining cutting tools and wear pieces with excellent properties of hardness and resistance to abrasion and erosion.
It is known to coat pieces of various materials and shapes by means of ceramic coatings (TiC, TiN, Ti(CN), Al.sub.2 O.sub.3) in order to increase their resistance to erosion and abrasion. Nevertheless, the hardness of these coatings remains inadequate for certain applications, in particular in the case of drilling tools.
As well as this it is known to use sintered boron carbide B.sub.4 C for production of items resistant to erosion; the hardness of these large pieces is significantly improved compared to the coatings previously mentioned, but these pieces are extremely fragile and their resilience (characterizing their resistance to shock) is incompatible with their use as cutting tools.
Elsewhere certain laboratory work discloses the preparation of compounds of the boron/carbon system by their deposition in the vapour phase by reaction between a compound of boron and a compound of carbon, in the gaseous state. (A M Moore, H F Volk: Final report AMMRC CR 69-10 (1969) Union Carbide Corporation carbon products division, Parma OHIO; A A Cochran, J B Stephenson. J G Donaldson J. of Metals, 22 (1970) pp 37-42; K PLOOG: J Less Common Metals 35 (1974) pp 115-145; L Vandeubulcke et al; J. Less Common Metals 82 (1981) pp 49-56; 80 (1981) pp 7-22).
In these experiments the deposit is obtained in a rapid gas flow (above 1.times.10.sup.-3 g.cm.sup.-2.s.sup.-1), most often at atmospheric pressure and a high temperature; their aim is a study of the boron-carbon system or the preparation of a boron carbide whose composition approaches B.sub.4 C.
A recent application of this type of coating (B.sub.4 C) is described in U.S. Pat. No. 4,287,259 and consists of making the deposit on graphite fibre fabrics in order to improve their chemical inertness and their resistance to corrosion with the object of using them in the nuclear industry. In this application, the vaporous deposit obtained is made up of an amorphous form of which the atomic carbon content is about 30%.
In certain technical areas and particularly in the drilling industry it is desirable, because of the application envisaged, to get the best compromise between hardness and resilience so that the coating benefits from an increased hardness (better than that of known coatings) while showing a resistance to shock adapted to the application.
Consequently the present invention seeks to provide a new coating of boron carbides on a large substrate. The hardness of the coating, notably superior to that of known industrial coatings, being in the range 3200-5100 kg/mm.sup.2 (micro-hardness Knoop determined by an "OLP" micro-hardness meter equipped with a Knoop pyramid under a load of 100g. in future we will always use this definition of hardness which will be called HK.sub.0.1)
An essential objective of the invention is to allow a precise adjustment of the hardness of the coating in this range according to the intended application, so as to get the best compromise between hardness and resilience for that application.
To achieve this, the process of the invention consists of depositing on the surface of the substrate one of or a mixture of the following crystalline forms: tetragonal form B.sub.50 C.sub.2); rhombohedral form (B.sub.13 C.sub.2). Such a deposit has a hardness HK.sub.0.1 within the range 3200 to 5100 kg/mm.sup.2. To get the desired hardness in this range one adjusts the atomic ratio C/B of the deposit, in a range which runs from 0.04 to 0.22, to a value which is directly proportional to the desired hardness.
The process, which is preferably carried out by chemical deposit in the vapour phase, consists of reducing a gaseous mixture containing at least one compound of boron, preferably boron trichloride, and at least one compound of carbon, preferably methane; it is ch
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patent: 4225355 (1980-09-01), Galasso et al.
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patent: 4540596 (1985-09-01), Nimmagadda
Chemical Abstracts, vol. 74, 1971, 131438z.
Vandenbulcke et al., "Composition & Structural Changes of Boron Carbides Deposited by Chemical Vapour Deposition Under Various Conditions of Temperature & Supersaturation", J. of the Less Common Metals, vol. 82, pp. 49-56, 1981.
Basse Alain G.
Cazajous Didier P.
Ducarroir Michel R.
Male Germain J.
Nadal Marcel F.
Childs Sadie
Coating Development Societe Anonyme
Dutton, Jr. Harold H.
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