GB2062600A

GB2062600A - Silicon carbide bodies

Info

Publication number: GB2062600A
Application number: GB8035341A
Authority: GB
Original assignee: Morganite Special Carbons Ltd
Current assignee: Morganite Special Carbons Ltd
Priority date: 1979-11-05
Filing date: 1980-11-04
Publication date: 1981-05-28
Also published as: GB2062600B

Abstract

A pump seal counterface or other shaped body made predominantly of silicon carbide but having free silicon inclusions, is characterised in that a surface layer of the inclusions where exposed at the surface of the body has been converted to silicon carbide.

Description

SPECIFICATION Silicon carbide bodies The invention relates to silicon carbide bodies, particularly but in no way exclusively counterface rings for pump seals.

In one method of making such bodies a porous compact of silicon carbide and carbon is contacted with a compact of excess elemental silicon and finely divided carbon. The assembly is heated to melt the silicon a portion of which then reacts with the carbon with which it has been compacted, forming a porous friable matrix of silicon carbide.

The matrix then functions to guide the remaining molten silicon into the porous silicon carbide/carbon compact, where it converts the carbon to further silicon carbide, the excess remaining as elemental silicon. Finally, after cooling, the friable matrix and remaining silicon are removed and grinding or other machining to the precise size and surface finish desired is carried out.

The method has the advantage that the original degree of porosity of the compact is not critical, and since the silicon is in excess, exact control of the amount of carbon in the compact and the amount of silicon used is not necessary. However the finished compact has inclusions of elemental silicon, visible at the surface if the compact is held to the light. These spots are a weakness, lacking the chemical inertness and extreme hardness of the predominant silicon carbide, and we have sought to eliminate them without losing the advantages of the above process of manufacture.

The invention provides a pump seal counterface or other shaped body predominantly of silicon carbide but having free silicon inclusions, in which a surface layer of the inclusions where exposed at the surface of the body has been converted to silicon carbide.

The thickness of the surface layer is not critical, once it is sufficient to give an unreactive surface to the body and to allow for such wear as may take place in normal service. The requirements for given circumstances can readily be found by test.

Less than 5y is insufficient, and over 250,(z unlikely to be attained. A range of 1 0y or preferably 1 5y up to 1 25y is preferred.

The free silicon content of the body is also not critical. Less than 5% by weight is not likely to give a reactivity problem, and a body containing over 40% free silicon is not a body predominantly of silicon carbide to which the invention applies.

More usually the range will be 10 or 12% up to 25 or 30%.

The invention further provides a method of making a pump seal counterface or other shaped body wherein a body of silicon carbide having free silicon inclusions is heated in a non-oxidising atmosphere at a temperature below the melting point of silicon (14100C) and in contact with particulate carbon or other carburising medium, to convert a surface layer of the inclusions, where exposed at the surface of the body, to silicon carbide.

One context in which the invention shows to great advantage is where the body is machined to desired dimensions and surface finish before carburising, the carburising optionally being followed by surface finishing within the thickness of the converted surface layer. Usually further finishing other than light lapping is unnecessary.

In a further aspect of the invention the heating below the melting point of silicon is followed by stages of further heating, for a first period just above said melting point and for a second period substantially above said melting point, then by slow cooling to below said melting point and preferably below 1 0000 C, both the heating and the cooling being carried out in a nitrogen atmosphere and in contact with the carburising medium, whereby the free silicon content of the body as a whole is reduced.Said first period may for example be at up to 1 5500C and said second period at a temperature higher than the first and preferably at 1650 to 20000 C. Such treatment, which may for example reduce an initial free silicon content of 1 5 to 25%, more usually 1 8 to 21%, to 8 to 12%, improves the properties of the body as a whole as well as producing a surface layer as already described. The surface finish of the final body may not be quite as good as that of bodies subjected only to the heating below 14100C, but any finishing operations required will, by reason of the reduction of the silicon content of the body as a whole, at worst expose material itself substantially improved in resistance compared with the material of the original untreated body.If this exposure in unacceptable the body can be re-treated below 141 00C to ensure that the surface is entirely silicon carbide.

The invention is particularly suitably applied to bodies initially made by contacting a porous compact of silicon carbide and carbon with a compact of elemental silicon and carbon wherein the silicon is in excess of the amount required to react with the carbon, and maintaining contact at a temperature above the melting point of silicon until the excess silicon in the second said compact has impregnated the first said compact, converting the carbon therein to silicon carbide and filling the pores with elemental silicon.

Thus in the preferred process we have found that, surprisingly, silicon carbide bodies having inclusions of silicon can be given a surface of silicon carbide throughout if they are first formed to finished size then heated in a carburising medium, in a non-oxidising atmosphere (including vacuum) until a surface layer of the inclusions has been converted to silicon carbide. The depth required is slight, since for example with a pump counterface it is the other part not the counterface that wears. The dimensional change during carburising is very small or even negligible, and at most a light lapping or polishing insufficient to remove the carburised layer, is required to restore the finished dimensions. The bodies provided have elemental silicon inclusions within the body but a surface of silicon carbide throughout.

The process of carburisation is suitably carried out in a sagger with the bodies embedded in carbon powder, but there is not restriction to any one method. A time of 3 to 5 hours in carbon powder under an argon atmosphere and at a temperature a little below the melting point of silicon (141 00C) is for example suitable. The temperature is preferably not below 13000 C. A nitrogen atmosphere is also suitable and any nitriding that accompanies the carburising not harmful. The silicon carbide of the surface so formed simply contains nitride as an impurity.

The treatment can be influenced as to the kinetics of the reaction by the use of additives and by the type of carbon used. It is preferred to use powdered charcoal. Carbon black gives a slower reaction than charcoal whilst petroleum coke leads to some pitting of the surface.

It is desirable to achieve a depth of carburisation of at least 25,u which can readily be achieved in periods of up to 3 hours at for example 1 3500C. Deeper penetration can be achieved with prelonged heating for example up to 125,u and most ideally 250y.

The procedure by which the silicon carbide bodies to which the invention is applied are themselves made is not part of the invention but a full description of a suitable procedure is given in U.S. Patent Specification No. 4 154 787 to W. G.

Brown, to which reference may be made.

EXAMPLE 1 Silicon carbide powder of maximum particle size 20,u was mixed with 15% of finely ground carbon. A carbonisable organic binder such as the synthetic resins well known in the art was added to the mix from which a disc was then formed. The disc was heated and impregnated with liquid silicon in an inert atmosphere. Upon cooling the disc contained approximately 1 5% free silicon. It was ground to give an even surface and one face was lapped to give a surface finish of 0.1-0.1 5 C.L.A.

The disc was now surface carburised by being placed in a bed of ground charcoal powder and heated under nitrogen to 1 3500C during a period of 90 minutes. It was held at this temperature for 1 hour.

Upon cooling the disc was cleaned ultrasonically and re-lapped. No free silicon was visible on the surface. A mixture of hydrofluoric and nitric acid was placed on the surface and there was no visible reaction.

Thereafter the disc was gound to remove a thickness of 250,u, thereby to ensure complete removal of all carburised surface. Free silicon was now evident. When the mixture of hydrofluoric and nitric acid was placed on the surface, a vigorous reaction was observed.

EXAMPLE 2 The following example is of the full treatment according to the invention. A body made generally in the way set out in Example 1 but containing 19% free silicon was taken through the melting point of silicon up to 1 4500C over a period of one hour, then held for 1 hour, then taken slowly to 1 8000C and held for 30 minutes before finally cooling slowly to below 1 4000C then down to below 10000C, the whole process being carried out under a nitrogen atmosphere and in contact with the carburiser. The final body was lapped to finished dimensions and showed excellent corrosion and wear resistance. Its free silicon content was 9%.

Claims

1. A pump seal counterface or other shaped body predominantly of silicon carbide but having free silicon inclusions, in which a surface layer of the inclusions where exposed at the surface of the body has been converted to silicon carbide.

2. A body according to claim 1 wherein the thickness of said surface layer is 5y to 250y.

3. A method of making a pump seal counterface or other shaped body wherein a body of silicon carbide having free silicon inclusions is heated in a non-oxidising atmosphere at a temperature below the melting point of silicon (14100C) and in contact with particulate carbon or other carburising medium, to convert a surface layer of the inclusions, where exposed at the surface of the body, to silicon carbide.

4. A method according to claim 2 wherein the thickness of said surface layer is 5y to 250y.

5. A method according to claim 3 or 4 in which the body is machined to desired dimensions and surface finish before carburising, the carburising optionally being followed by surface finishing within the thickness of the converted surface layer.

6. A method according to claim 3 in which the heating below the melting point of silicon is followed by three stages of further heating, for a first period just above said melting point and for a second period substantially above said melting point, then by slow cooling to below said melting point and preferably below 10000 C, both the heating and the cooling being carried out in a nitrogen atmosphere and in contact with the carburising medium, whereby the free silicon content of the body as a whole is reduced.

7. A method according to any one of claims 3 to 6 wherein the initial body has been made by contacting a porous compact of silicon carbide and carbon with a compact of elemental silicon and carbon wherein the silicon is in excess of the amount required to react with the carbon, and maintaining contact at a temperature above the melting point of silicon until the excess silicon in the second said compact has impregnated the first said compact, converting the carbon therein to silicon carbide and filling the pores with elemental silicon.

8. A pump seal counterface or other shaped body predominantly of silicon carbide, according to claim 1, substantially as herein described in Example 1 or2.

9. A method of making a pump seal counterface or other shaped body predominantly of silicon carbide, according to claim 3, substantially as herein described in Example 1 or2.