GB2232411A

GB2232411A - Process for the manufacture of green compacts and sintered moldings from silicon carbide

Info

Publication number: GB2232411A
Application number: GB9010793A
Authority: GB
Inventors: Rainer Hamminger; Hartmond Kruener; Fritz Aldiner
Original assignee: Hoechst AG
Current assignee: Hoechst AG
Priority date: 1989-05-16
Filing date: 1990-05-14
Publication date: 1990-12-12
Anticipated expiration: 2010-05-14
Also published as: GB2232411B; GB9010793D0; JPH035368A; DE3915914A1

Description

- I- Process for the manufacture of green compacts and sintered moldings

from silicon carbide The invention relates to a process for the manufacture of green compacts from silicon carbide that is sinterable without pressure. In particular, the invention relates to a process in which a-or fi-silicon carbide powder with a specific surface area of at least 10 M2/ g, a source of an element for assisting sintering and a carbon source are mixed and the mixture is shaped to form a green compact. The invention further relates to a process for the manufacture of a molding from silicon carbide sintered without pressure.

Among the non-oxidic high-perf ormance ceramics, dense one-phase silicon carbide represents a particularly important and promising material. Because of the combination of outstanding material properties such as hardness, abrasion resistance, high-temperature resistance and resistance to oxidation and corrosion, a high use potential in apparatus engineering and machine construc- tion can be expected in the near future. The compaction of pure silicon carbide can only be carried out by a pressure sintering process. On the other hand, if small amounts of certain sinter-activating additives are used, preforms of silicon carbide powder, called green com- pacts, can be compacted by pressureless sintering, that is, sintering without mechanical compression of the compact, under a protective gas or under vacuum to form highstrength polycrystalline workpieces. The resulting product is known as one-phase silicon carbide sintered without pressure (SSiC).

_: 0 S inter- activating additives which can be used are, in particular, boron and compounds thereof, carbon being added at the same time. By virtue of these additions, a favorable relationship between the grain boundaries and the surface energy is established and the sintering process is thereby activated. Carbon can be added in the form of graphite or carbon black or by the pyrolysis of carbon-producing resins and is necessary to reduce the layers of silicon dioxide adhering to the primary powder particles of silicon carbide.

US-PS 3 993 602 describes a process for the manufacture of an electrically conductive, polycrystalline silicon carbide molding, in which process a submicron-size powder of j6-silicon carbide, a boron additive, beryllium carbide and a carbon-containing additive, which decomposes at temperatures of about 50C to 1000C to form free carbon, are mixed, the mixture is shaped to form a green compact and the green compact is sintered without pressure at temperatures of about 1850C to 2300C. High-molecular aromatic compounds, e.g. phenol- formaldehyde condensation resins, can be used as carbon-containing additives.

DE 31 16 768 C2 has disclosed the use of aromatic resins, including phenol-formaldehyde condensation products, for the manufacture of reaction-sintered moldings from silicon carbide and silicon (SiSiC). The aromatic resins are said to improve inter alia the workability of the moldings, especially in the coked state. These mixtures used f or the manuf acture of SiSiC moldings dif f er in many respects from mixtures for the manufacture of SSiC moldings sintered without pressure. The components of the starting mixture have a larger particle size. Sinter aids are not added. So that a sufficient amount of carbon is available for the formation of secondary silicon carbide during the reaction- sintering step, the mixtures always contain graphite and/or carbon black, if appropriate in combination with a compound which produces carbon on pyrolysis. In the coked state, the moldings manufactured from these mixtures may already be strong enough to allow mechanical working.

In the manufacture of SSiC moldings, a linear shrinkage of between 14 and 19%, based on the dimensions of the green compact, occurs during the pressureless sintering process. This shrinkage has to be controlled in the mass production of components and is regarded as disadvantageous on account of necessary oversizing. The components are shaped by compression or casting, allowing for the expected sintering shrinkage. As i s known, however, only geometrically simple components can be manufactured by these shaping processes. As the green compacts are not strong enough for mechanical working, the definitive shaping is carried out on the sintered moldings, to the final geometrical dimensions, by laborious and cost-intensive grinding with diamond tools.

An object of the present invention is to provide a process, of the type mentioned at the outset, for the manufacture of green compacts which are strong enough for mechanical working. -It is a further object to provide a process for the manufacture of a molding from silicon carbide sintered without pressure, by which process moldings with complicated geometries can be manufactured, the hitherto conventional cost-intensive grinding in the sintered state being reduced to a minimum or avoided altogether.

The present invention provides a process for the manufacture of mechanically workable green compacts from silicon carbide that is sinterable without pressure, in which process a- or fi- silicon carbide powder with a specific surface area of at least 10 m2/g, a source of an element for assisting sintering and a carbon source are mixed and the mixture is shaped to form a green compact, wherein, as the carbon source, a phenol-formaldehyde resol is incorporated into the mixture in an amount of 0.7 to 12% by weight, based on the total mixture of solids, and the green compact is hardened. To manufacture the molding from sintered silicon carbide, the hardened green compact is mechanically worked and the worked green compact is sintered without pressure at 18500C to 22000C.

Through the addition of a phenol- formaldehyde resol in k an amount of 0.7 to 12% by weight, based on the total mixture of solids, and hardening of the green compact, the green compact becomes strong enough to allow mechanical Forking, e.g. by turning, drilling or milling with machine tools known in the metal-working industry. This working brings the green compacts at least to a shape approximating to the final dimensions. By allowing for the expected sintering shrinkage, it is possible at least to reduce the laborious and cost-intensive finishing to a minimum, especially in the case of parts with rotational symmetry. The hardened green compacts manufactured according to the invention have a dry flexural strength (DIN 51030) of 8 MPa or more.

The phenol-formaldehyde resol serves a dual purpose. After hardening, it gives the green compact the strength required for mechanical working. Also, it acts as a carbon source in the sintering step, reducing the layers of silicon dioxide adhering to the primary powder particles of silicon carbide.

As the source of an element for assisting sintering, it is possible to use one or more of the following: boron, - aluminum, beryllium, rare earth metals, yttrium, hafnium, scandium, niobium and lanthanum, in each case in elemental form or in the form of compounds. It is preferred to use boron or boron compounds. For boron and compounds thereof, the amount is preferably 0.01 to 5.00% by weight, calculated as the element and based on the total mixture of solids.. and most preferably 0.1 to 3.0% by weight. For the other elements or compounds thereof, the amounts are generally in the range from 0.1 to 10.0% by weight. calculated as the element and based on the total mixture of solids.

Commercially available a- or p-silicon carbide powder with a specific surface area of at least 10 m2/g can be used as the silicon carbide. This is mixed in any suitable manner with the source of an element for assisting 1 1 sintering and the phenol-formaldehyde resol.

As phenol- formaldehyde resols, it is preferred to use those which harden at temperatures above room temperature and up to 2000C. Especially preferred resols are those which harden at temperatures above SOOC and up to 1000C.

The phenol-formaldehyde resol is preferably incorporated into the mixture in the form of a solution or suspension. Examples of suitable solvents or suspending media are methanol, ethanol, isopropanol, various glycols and water. It is preferred to use an aqueous solution of the phenolformaldehyde resol. This permits aqueous processing of the ceramic mass on mixing, which is advantageous for reasons of cost and environmental protection. Furthermore, a drying step and a change of solvent are then superfluous if the slip casting process is used for shaping. The phenolformaldehyde resol is preferably used in the form of a highly concentrated aqueous solution with a resol content of about 65 to 75% by weight. The phenol-formaldehyde resol is used in an amount of 0.7 to 12% by weight, based on the total mixture of solids. The preferred range is 2 to 8% by weight, based on the total mixture of solids. The minimum amount is most preferably 4% by weight, based on the total mixture of solids.

After the silicon carbide powder, the source of an element for assisting sintering and the phenol-formaldehyde resol have been mixed, shaping to form the green compacts can be carried out by any process suitable for shap- ing mixtures containing silicon carbide. The green compacts are preferably manufactured by compression or casting.

The hardening of the green compacts does not require special procedures, but can be carried out in air at temperatures above room temperature and up to about 200C. The hardening temperature applied in each case depends on the resol used and the desired hardening time. Preferred hardening temperatures are in the range from 60 to 1000c. A temperature range of 700C to 80C is particularly suitable. The time required depends on the temperature level and the dimensions of the green compact. It will be appreciated that the conditions of the hardening step will be chosen such that phenol-formaldehyde resol in the green compact is hardened, while decomposition of the resol is substantially avoided.

After hardening, the green compacts have a dry flexural strength of 8 HPa or more. They can be worked with conventional machine tools and, by allowing f or the expected sintering shrinkage, they can be brought to the desired geometrical dimensions with exacting tolerance. After mechanical working, the green compacts are sintered in conventional manner at temperatures of 1850 to 2200"C without pressure, under a protective gas, e.g. argon, or under vacuum. Coking of the phenol- formaldehyde resol takes place during the sintering process. A special coking step is not necessary since the green compacts already have a good strength before coking.

The invention is illustrated in greater detail with the aid of the following Examples and the drawing. The drawing shows a shaft manufactured by mechanical working from a hardened cylindrical green compact. The resol used in the Examples was an aqueous resol based on a phenol-formaldehyde condensation product having the following properties:

Resin content DIN 16916-02-Hl 71 1% by weight Density at 200C DIN 53217 part 2 1.23-1.24 g1Cm3 Viscosity at 20C DIN 53015 300-500 mPa.s pH DIN 16916-02-E 6.7-7.0 Free formaldehyde DIN 16916-02-6.15:51% Free phenol DIN 16916-02-Ll:s4% Acid reactivity DIN 16916-02-F 90-1000c Water content 15-18% k A 1 Methanol content <5% Flash point DIN 53213 700C Chloride content DIN 53474 <1% Ash:s2% The weight given in Example 1 for the phenol- formaldehyde resol is based on the aqueous resol indicated above.

Example I The starting material used was commercially available silicon carbide powder with a specific surface area of >10 M 2/ g and a mean particle size of <1 Am. The powder was homogenized in a planetary ball mill for 60 min with 0.4% by weight of amorphous B, based on the total mixture of solids, 5.5% by weight of the aqueous resol indicated above (corresponding to 2.5% by weight of carbon after pyrolysis) and water in the amount necessary to bring the solids content to 50% by weight. The mass was dried in a spray tower. The formulated powder was then subjected to cold isostatic compression under a pressure of 300 MPa to form cylindrical compacts of 200 mm in length and 30 nun in diameter. The green compact was hardened for 24 h in air at 80C in a drying cabinet, after which the dry flexural strength (DIN 51030) of the hardened green compacts was between 8 and 10 MPa. The hardened cylindrical-green compacts were then mechanically worked with a conventional universal lathe, allowing for the expected sintering shrinkage of 14.5%; as shown in the drawing, different diameters of between 12 and 28 mm and various radii and phases were produced in this process. The green compacts worked in this way were then sintered without pressure, under argon, at a temperature of 2150"C. After sintering, the required diameter tolerance of 0.1 mm had been maintained over the entire length of the silicon carbide shaft. The sintered shafts had a density of 97% of the theoretical density and were characterized by a homogeneous fine- grained texture of the 2-5 Am crystallites. The 4-point room temperature bending strength of the shafts produced (e.g. to be used for high-speed spindle drives) was over 450 MPa.

Example 2

The ceramic mass was prepared and dried as in Example 1. The formulated powder was then axially compressed under a pressure of 200 MPa to form annular compacts. The green compacts were hardened as in Example 1, affording a dry flexural strength of 8 to 10 MPa as in the case of the hardened green compacts of Example 1. The hardened annular green compacts were mechanically worked with a universal lathe, allowing for the expected sintering shrinkage of 15.0%; different radii, phases and grooves were produced in this process. The green compacts worked in this way were sintered without pressure, under argon, at 2150C. The sintered components had the properties indicated in Example 1.

Example 3

The ceramic mass was prepared as in Example 1. A slip with a solids content of 65% by weight was formed and cast into plaster molds. The green compacts released from the mold were hardened in air at a temperature of 80C over 5 h. The dry flexural strength of the hardened green compacts was 9 MPa. The hardened green compacts were mechanically worked with a universal lathe, allowing for the expected sintering shrinkage of 17.0%; different radii and phases were produced in this process and grooves of different widths and depths were produced with a universal milling machine. The green compacts worked in this way were sintered without pressure, under argon, at 21500C. The sintered components had the properties indicated in Example 1.

ExamT)le 4 The ceramic mass was prepared according to Example 1. The shaping and hardening were carried out as indicated in Examples 1 to 3. After the green compacts obtained in each case had been worked and sintered, the sintered components were mechanically reground and/or polished according to the required tolerance (e.g. 0.05 mm).

1 Example 5

The starting material used was commercially available fi-silicon carbide powder with a specific surface area of,,10 M2/ g and a mean particle size of <1 pm. The ceramic mass was prepared as in Example 1. The green compacts were produced, hardened and mechanically worked as indicated in Examples 1 to 3. The moldings of #-silicon carbide obtained in each case after sintering under argon at a temperature of 20500C had a density of 96.8% of the theoretical density. The crystallite sizes were 2 to 7,um and the 4-point room temperature bending strengths were between 340 and 390 MPa. The sintered p-silicon carbide moldings were mechanically reground and/or polished as described in Example 4.

Claims

Claims:

1. A process for the manufacture of mechanically workable green compacts from silicon carbide that is sinterable without pressure, in which process a- or 8-silicon carbide powder with a specific surface area of at least 10 m2/g, a source of an element for assisting sintering and a carbon source are mixed and the mixture is shaped to form a green compact, wherein, as the carbon source, a phenol-formaldehyde resol is incorporated into the mixture in an amount of 0.7 to 12% by weight, based on the total mixture of solids, and the green compact is hardened.

2. A process as claimed in claim 1, wherein the resol is selected from those phenol-formaldehyde resols which harden at temperatures above room temperature and up to 200'C.

3. A process as claimed in claim 2, wherein the resol is selected from those phenol-formaldehyde resols which harden at temperatures above SO'C.

4. A process as claimed in any one of claims 1 to 3... wherein the phenolformaldehyde resol is incorporated into the mixture in the form of a solution or suspension.

5. A process as claimed in claim 4, wherein the phenolformaldehyde resol is incorporated into the mixture in the form of an aqueous solution.

6. A process as claimed in any one of claims 1 to 5, wherein the phenolformaldehyde resol is incorporated into the mixture in an amount of 2 to 8% by weight, based on the total mixture of solids.

7. A process as claimed in claim 6, wherein the phenolformaldehyde resol is incorporated into the mixture in an amount of 4 to 8% by weight, based on the total mixture of solids.

111

8. A process as claimed in any one of claims 1 to 7, wherein boron or a boron compound is used as the element for assisting sintering.

9. A process as claimed in any one of claims 1 to 8, wherein the green compact is hardened at a temperature above room temperature and up to 20CC.

10. A process as claimed in claim 9. wherein the green compact is hardened at between 60 and 1000C.

11. A process as claimed in claim 8, wherein 0.01 to 5.00% by weight of boron or boron compound, calculated as the element and based on the total mixture of solids, is incorporated into the mixture.

12. A process as claimed in claim 11. wherein 0.1 to 3.0% by weight of boron or boron compound, calculated as the element and based on the total mixture of solids, is incorporated into the mixture.

13. A process as claimed in any one of claims 1 to 12, wherein 0.1 to 10. 0% by weight of one or more metals and/ or metal compounds selected from aluminum, beryllium, rare earth metals, yttrium, hafnium, scandium, niobium, lanthanum and compounds of those metals, calculated in each case as the element and based on the total mixture of solids, is incorporated into the mixture as the element for assisting sintering.

14. A process as claimed in any one of claims 1 to 13, wherein the hardened green compact has a dry flexural strength of at least 8 MPa.

15. A process for the manufacture of a molding from sintered silicon carbide, wherein the hardened green compact manufactured by the process as claimed in any one of claims I to 14 is mechanically worked and the worked k green compact is sintered without pressure at 18500c Lo 22000C.

16. A process as claimed in claim 15, wherein the sintering is carried out under argon.

17. A process for the manufacture of a silicon carbide green compact, the process being substantially as described in any of Examples 1 to 5 herein.

18. A silicon carbide green compact that has been manufactured by a process as claimed in any one of claims 1 to 17.

19. A sintered silicon carbide molding that has been prepared by a process substantially as described in any of Examples 1 to 5 herein.

Pubashed 1990 at The Patent Of.,;ce. State House, 6671 High Holborn. London WClR4TP. Flurther copies maybe obtained from The Patent Office. Sales Branch. St Maxy Cray. Orpington. Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con. 1/87