SiAlON bonded silicon carbide material

Abstract

A silicon carbide based material exhibiting high strength, good thermal shock resistance, high resistance to abrasion and being chemically stable to harsh environmental conditions is described. The carbide Ball Hill ceramic comprises a -SiAlON bonding phase in which sintering is facilitated by at least one rare earth oxide sintering agents incorporated within the Vibrating Sieve batch admixture as starting materials. The residual rare earth sintering aid being chosen so as to impart good mechanical and refractory properties.

Claims

1. A ceramic material comprising: an -silicon carbide phase of 60 to 80% by weight having at least two mean grain sizes of i) 70 to 250 m and ii) 0.5 to 50 m; a -SiAlON phase of 20 to 40% by weight; and an intergranular phase at least partially surrounding the -silicon carbide and -SiAlON phases of trace to 3% by weight comprising at least one rare earth element.

2. The material as claimed in claim 1 wherein the mean grain sizes comprise i) 110 to 200 m and ii) 1 to 25 m.

3. The material as claimed in claim 1 wherein the mean grain sizes comprise i) 115 to 155 m and ii) 1 to 15 m.

4. The material as claimed in claim 1 further comprising an -silicon nitride phase of trace to 40% by weight.

5. The material as claimed in claim 1 wherein the intergranular phase further comprises iron of trace to 10% by weight.

6. The material as claimed in claim 1 further comprising silicon metal.

7. The material as claimed in claim 1 wherein the rare earth element comprises gadolinium and/or lanthanum.

8. The material as claimed in claim 1 wherein the material comprises ytterbium and/or yttrium.

9. The material as claimed in claim 1 wherein the intergranular phase comprises at least one of the following set of: Sc; Y; La; Ce; Pr; Nd; Pm; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu.

10. The material as claimed in claim 1 wherein the intergranular phase is formed as a crystalline phase.

11. The material as claimed in claim 10 wherein the crystalline phase is a garnet phase.

12. The material as claimed in claim 1 wherein the intergranular phase is formed as a glass phase.

13. The material as claimed in claim 11 wherein the garnet phase is represented by the formula: (RE/RE).sub.3Al.sub.5O.sub.12 where RE and RE each comprise any one of the set of: Sc; Y; La; Ce; Pr; Nd; Pm; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu.

14. The material as claimed in claim 1 wherein the -SiAlON phase is represented by Si.sub.6-ZAl.sub.ZO.sub.ZN.sub.8-z wherein z is in the range 0.25 to 4.

15. The material as claimed in claim 14 wherein z is in the range 0.6 to 2.0.

16. The material as claimed in claim 1 further comprising a surface oxide layer at a surface of the material containing the intergranular phase and a silicon oxide phase.

17. The material as claimed in claim 16 wherein the surface oxide layer comprises predominantly a rare earth disilicate and the silicon oxide phase comprises silicon oxide cristobalite or the phases silicon oxynitride and/or O-SiAlON.

18. The material as claimed in claim 1 wherein the intergranular phase further comprises at least one or a combination of the elemental constituents: aluminium, oxygen, nitrogen and/or silicon.

19. A process for producing a ceramic material comprising: preparing a powdered batch of an admixture comprising: -silicon carbide at 60 to 80% by weight having at least two mean grain sizes of i) 70 to 250 m and ii) 0.5 to 50 m; powdered silicon metal at 15 to 20% by weight; -alumina at 2 to 6% by weight; and at least one rare earth oxide at trace to 3% by weight; processing the powdered batch to create a body; and heat treating the body under a nitrogenous atmosphere to form: an -silicon carbide phase; a -SiAlON phase and an intergranular phase containing at least one rare earth element of the rare earth oxide as part of the ceramic material.

20. The process as claimed in claim 19 wherein the mean grain sizes comprise i) 115 to 200 m and ii) 1 to 15 m.

21. The process as claimed in claim 19 wherein the powdered staring materials further comprise: iron (III) oxide trace to 10% by weight.

22. The process as claimed in claim 19 wherein the -silicon carbide is present at: 70 to 250 m mean grain sizes at 30 to 40% by weight 0.5 to 50 m mean grain sizes at 30 to 40% by weight.

23. The process as claimed in claim 19 wherein the rare earth oxide comprises an oxide of gadolinium and/or lanthanum.

24. The process as claimed in claim 19 wherein the rare earth oxide comprises an oxide of ytterbium and/or yttrium.

25. The process as claimed in claim 19 wherein the intergranular phase comprises at least one of the following set of: Sc; Y; La; Ce; Pr; Nd; Pm; Sm; Eu; Gd; Tb; Dy; Ho; Er; Tm; Yb; Lu.

26. The process as claimed in claim 19 wherein the rare earth oxide comprise any one of the set of: Y.sub.2O.sub.3 and Yb.sub.2O.sub.3; Y.sub.2O.sub.3 and Gd.sub.2O.sub.3; Y.sub.2O.sub.3 and La.sub.2O.sub.3; CeO.sub.2 and La.sub.2O.sub.3; La.sub.2O.sub.3 and Gd.sub.2O.sub.3.

27. The process as claimed in claim 19 wherein the step of heat treating the body comprises heat treating at a temperature in the range 1300 to 1750 C.

28. The process as claimed in claim 27 comprising heat treating the body at a first processing temperature in the range 1400 to 1500 C.

29. The process as claimed in claim 28 comprising heat treating the body at a second processing temperature in the range 1550 to 1750 C.

30. The process as claimed in claim 29 wherein the step of heat treating the body at the first processing temperature comprises heat treating in a flowing nitrogenous atmosphere; and the step of heat treating the body at the second processing temperature comprises heat treating in a substantially static nitrogenous atmosphere.

31. The process as claimed in claim 30 comprising heat treating the body at the second processing temperature for three to five hours.

32. The process as claimed in claim 31 comprising heat treating the body at a third processing temperature in the range 1100 C. to 1400 C.

33. The process as claimed in claim 32 wherein the step of heat treating the body at the third processing temperature is configured to promote creation of a surface oxide layer comprising a rare earth disilicide, a cristobalite, a silicon oxynitride and/or O-SiAlON.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

(2) FIG. 1 is a flow diagram for a slip production process according to one embodiment of the present invention;

(3) FIG. 2 is a graph of a first firing schedule for kiln furniture according to one embodiment of the present invention;

(4) FIG. 3 is a graph of a firing schedule for body armour according to one embodiment of the present invention;

(5) FIG. 4 is a graph of an oxide firing schedule for the kiln furniture and the body armour according to one embodiment of the present invention;

(6) FIG. 5 shows XRD data of a material with a phase composition of a SiAlON bonded SiC material using 1.7 wt % Y.sub.2O.sub.3 as a sintering aid after processing at 1450 C.;

(7) FIG. 6, shows XRD data of a material with a phase composition of a SiAlON bonded SiC material using 1.7 wt % Y.sub.2O.sub.3 as a sintering aid after processing at 1450 C. followed by processing at 1650 C.;

(8) FIG. 7 shows XRD data of a material with a phase composition of a SiAlON bonded SiC material using 1.7 wt % La.sub.2O.sub.3 as a sintering aid after processing at 1450 C.;

(9) FIG. 8 shows XRD data of a material with a phase composition of a SiAlON bonded SiC material using 1.7 wt % La.sub.2O.sub.3 as a sintering aid after processing at 1450 C. followed by processing at 1650 C.;

(10) FIG. 9 shows XRD data of a material with a phase composition of a SiAlON bonded SiC material using 1.7 wt % CeO.sub.2 (98%) as a sintering aid after processing at 1450 C.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION

(11) A specific embodiment of the present invention is described with reference to the following examples.

Example 1

(12) Table 2 details typical starting materials according to the subject invention comprising two rare earth oxides as sintering aids in the manufacture of a SiAlON bonded silicon carbide refractory material.

(13) TABLE-US-00002 TABLE 2 Starting materials formulation Raw Material Grade SC (%) Green Silicon Carbide - Coarse 100/F 40.0 Green Silicon Carbide - Fine FCP 36.0 Silgrain Standard - Silicon Metal Powder 0-53 m 20.0 Reactive Alumina alpha 4.0 Iron (III) Oxide - Fe.sub.2O.sub.3 Red Calcined +0.5 Yttrium Oxide (Y.sub.2O.sub.3) Purity >99% +0.8 Ytterbium Oxide (Yb.sub.2O.sub.3) Purity >99% +0.8 Sodium Hydroxide 1M +0.4 De-ionised Water +11.5-14.0

(14) Table 3 lists the aqueous slip properties of the resulting body based on the starting materials of table 2.

(15) TABLE-US-00003 TABLE 3 Slip properties pH 7.0-8.0 Slip Density (g/cm.sup.3) >2.45 Viscosity (deg swing) 180-220 Thixotropy (5 min) Nil Dried Green BD =>2.45

(16) The Viscosity was measured by a Gallenkamp Viscometer with 11/16 bob and 30 swg wire. The linear Firing Shrinkage (Dry to Fired) was found to be 0.10-0.15%

Experimental

Raw Materials

(17) Two grades of silicon carbide, were used including firstly an acid washed green silicon carbide 100/F having physical properties: Loose PD=1.42 g/cm.sup.3; and pH=7.0:

(18) TABLE-US-00004 TABLE 4 Silicon carbide sieve analysis (%) +150 m +106 m +75 m +45 m 45 m <4 35-45 50-60 70-85 <3

(19) TABLE-US-00005 TABLE 5 Silicon carbide chemical analysis (%) SiC C (free) Si (free) Al.sub.2O.sub.3 CaO Magnetic Iron 99.5 <0.2 <0.2 <0.03 <0.01 <0.02

(20) Possible suppliers include: ESK-SIC GmbH, Frechen, Germany with a quality: SiCgrn 100/F (SiC green 100/F); St Gobain Ceramic Materials Lillesand, Norway with a quality: 100/F HD and Navarro SIC SA, Madrid, Spain with a quality: SiC green 100/F.

(21) A second grade silicon carbide was used being a green silicon carbide, fine ceramic powder/FCP having physical properties:

(22) TABLE-US-00006 TABLE 6 Green silicon carbide physical properties D10 (m) D50 (m) D90 (m) SSA (m.sup.2/g) pH <10% 1.8-2.8% <10% 3.5-5.5 7.0

(23) TABLE-US-00007 TABLE 7 Chemical analysis (%) of green silicon carbide SiC C (free) Si (free) Al.sub.2O.sub.3 CaO Fe.sub.2O.sub.3 >97 <0.5 <0.7 <0.2 <0.01 <0.2

(24) Possible suppliers include: ESK-SIC GmbH, Frechen, Germany with a quality: NF 2/2025; St Gobain Ceramic materials, Lillesand, Norway with a quality: FCP 07 and Pacific Rundum Co Ltd, Toyama, Japan with a quality: GNF 6S.

(25) Acid leached silicon metal powders from Elkem A/S, Kristiansand S, Norway having Si grain standard of 0-53 m were used with a sieve analysis (%) of +53 m, 5 (max).

(26) TABLE-US-00008 TABLE 8 Chemical analysis (%) of silicon metal powder Si Fe Al Ca Ti 99.3 (typical) 0.15-0.35 0.2-0.35 0.02-0.05 0.015-0.035

(27) A reactive alumina powder from Alcan International Network UK Ltd, UK was used as follows: ball milled calcined alumina without any organic additives; super ground multimodal aluminium oxide (mainly -phase) with a very low soda and a low water demand. The physical properties include: particle size distribution (Sedigraph) D.sub.50 2.46 m (typical); BET Surface Area 2.35 m2/g (typical); fired density (1670 C./1 hr); 3.8 g/cm.sup.3; water absorption 14 ml/100 g.

(28) TABLE-US-00009 TABLE 9 Chemical analysis of reactive alumina powder Al.sub.2O.sub.3 Na.sub.2O CaO SiO.sub.2 Fe.sub.2O.sub.3 99.7% 300 ppm 205 ppm 880 ppm 150 ppm (typical) (typical) (typical) (typical) (typical)

(29) Red calcined iron (III) oxide from P J Colours Ltd, Flint, UK or Fahrenheit Laboratory Services Ltd, UK was used with the following physical properties: sieve residue (0.045 mm sieve)<0.06%; D.sub.50 0.22 m; pH 4.0-8.0; moisture content <0.5%. The chemical analysis was: Fe.sub.2O.sub.3 96-97%; and SiO.sub.2+Al.sub.2O.sub.3<4.0%.

(30) Yttrium Oxide Powder (Y.sub.2O.sub.3) from ABSCO Materials, Haverhill, UK; or Treibacherindustrie AG, Althofen, Austria was used at 99.99% purity with the following physical properties: particle size distribution (Microtrac) D.sub.50 2-10 m; BET Surface Area 2-12 m.sup.2/g; Loss on Ignition at 1000 C.<1.0%.

(31) TABLE-US-00010 TABLE 10 Chemical Analysis of Yttrium Oxide Powder (Y.sub.2O.sub.3) (ppm) Fe.sub.2O.sub.3 CaO Na.sub.2O SiO.sub.2 Al.sub.2O.sub.3 K.sub.2O <20 <30 <30 <250 <150 <30

(32) Ytterbium Oxide (Yb.sub.2O.sub.3) D.sub.50=1.48 m from ABSCO Materials, Haverhill, UK; or Treibacherindustrie AG, Althofen, Austria was used at 99.99% purity. Chemical Analysis: CaO=31 ppm, Co.sub.3O.sub.4=<2 ppm, Fe.sub.2O.sub.3=3 ppm, NiO=<2 ppm, Loss on Ignition=0.31%

(33) A pH adjuster was used comprising sodium hydroxide (NaOH) 1M solution in water as supplied by Fahrenheit Laboratory Services Ltd, UK.

(34) Two types of plaster from BPB Formula Ltd, Newark, UK were used for moulds. Firstly Ultramix a hard dense plaster for models case moulds; plaster:water ratio 3.03:1 (or 100:33); initial Vicat setting time 14 min; dry compressive strength 44.0 MPa; maximum linear expansion, 2 hours after setting 0.3%. Secondly, Keramicast was used (for casting moulds); slow casting plaster; plaster:water ratio 1.82:1 (or 100:55); knife setting time 8 min; dry compressive strength 14.0 MPa; maximum linear expansion, 2 hours after setting 0.3%. The mix composition was varied from 100% Keramicast to 50:50 Keramicast:Ultramix, depending on the mould size and water absorption rate required.

(35) A natural graphite powder from James Durrans and Sons Ltd, Penistone, UK was used according to the identificationH300# GGR-8040-15-080. Finally, all water used was standard de-ionised water.

(36) Equipment

(37) The following equipment was used within the experimental set-up. weighing scales with an accuracy of 1 g; ball mill either with a light charge of alumina grinding media (1 kg balls:5 kgdry charge); plaster moulds fitted with plastic feeder and header tubes and filling funnels; variable temperature drying chamber; nitrogen atmosphere kiln with upper temperatures of 1450 C. for kiln furniture and 1600 C. for body armour; suitable air atmosphere kiln for second oxidation firing as required; suitable casting benches and tables for stripping, cleaning and assembling the moulds; fettling tools.

(38) The major characteristics of the present cast SiAlON bonded silicon carbides are that they are high strength, high bulk density and relatively low porosity. However, to achieve the excellent oxidation resistance, which contributes to the long service life in oxidising environment, it is advantageous that the starting materials are intimately mixed, the green density is high and the pore sizes are small. Small pore sizes will ensure that any oxidation will occur slowly and, as it occurs, the sizes of the pores are further reduced. To achieve this, two stages are including in the manufacturing process. Firstly, by ball milling controlled conditions, a slip is produced, which has a very even and a consistent particle size distribution. It is also homogeneous, since the components become very intimately mixed. Secondly, the moisture content of the plaster moulds, in the case of slip casting, has to be controlled in order to moderate the rate of water extraction from the body. If it is too rapid, then segregation of the mix composition can occur as a result of migration of the smaller powder particles to the surface.

(39) Ball Mill Data for Milling the Slip

(40) Rubber ball mills or ceramic lined ball mills were used, since metallic lined ball mills may contaminate the slip.

(41) TABLE-US-00011 TABLE 11 Ball mill characteristics for various ball mill sizes with no media Ball Mill Charge Diameter Length Charge Alumina Media Water (mm) (mm) (kg) (kg) (kg) 300 300 15.0 3.0 1.76 600 600 200.0 40.0 23.0 900 900 325.0 65.0 38.0

(42) Water addition was based on 11.7%.

(43) Production Procedure and Techniques

(44) General

(45) According to one embodiment using the moulds as described herein, the more important processing steps characteristics include: i) high solids content of the aqueous slip; ii) low viscosity of the slip which is controlled by the pH and water addition; iii) slow drying rate of the cast body i.e. the rate of water extraction by the mould; and iv) a controlled nitriding of the silicon, which is strongly exothermic, and (v) a complete reaction between the formed Si.sub.3N.sub.4, the alumina and the sintering aid to form -SiAlON and an intergranular phase or amorphous or crystalline material.

(46) Water Content and Viscosity

(47) The amount of water required for a usable slip was observed to be influenced by a number of factors including in particular: A) pH of the slipthe most suitable deflocculant was found to be sodium hydroxide and the lowest viscosity was obtained in the range pH=7.5-8.5. Additionally, Silicon metal reacts in acidic (pH<7.0) or strongly alkaline (pH>9.0) conditions with evolution of gases. It is, therefore, important to control the pH. B) Electrical conductivity of the raw materials. Acid washed silicon metal was found to give better results than unwashed silicon. C) Fineness of the silicon metal powder. Although the finer grade of silicon is more reactive, it requires more water to produce the required viscosity.
Production Procedure
Slip Production

(48) A slip production process according to one embodiment is summarised in FIG. 1. Firstly, it was ensured that the ball mill and the alumina balls were dry. A small charge of alumina balls was added to help to break up any agglomerates of powder. However, the balls were not required for grinding, since the composition was determined for maximum packing density. All the components were weighed. The water and sodium hydroxide were premixed and added to the ball mill followed by the dry components. The ball mill was sealed and operated for 16-24 hours. The pH of the slip was measured, which should be in the range, pH=7.5-8.5. The viscosity of the slip was measured, which should be in the range 180-220 swing (Gallenkamp Torsion Viscometer fitted with 11/16 bob and 30 swg wire). The slip density was measured with a container of known volume and weighing scales, and should be a minimum of 2.45 g/cm.sup.3. Adjustments were made by adding de-ionised water to bring them into the required range. The quantity of slip required for casting was removed and the ball mill restarted to keep the slip homogeneous.

(49) Mould Preparation

(50) To work efficiently, a mould should contain some moisture to set up an equilibrium so that evaporation from the outside surface draws water through the walls and extracts it from the slip. If the mould was found to be too dry, the sections of the mould were submerged in water for approximately 5 minutes. If the mould was judged to be wet, the mould was assembled and fastened with straps and place in an oven at 30-40 C. for 10-15 hours. The mould was opened and the working faces cleaned thoroughly. The working faces of the mould were dampened and coated with a fine graphite powder using a sponge. Excess graphite was wiped off and the moulds assembled. The straps, pipes and filling funnels were fitted. If casting into small moulds they were set in the vertical position. Alternatively, large moulds were set at approximately 30 to the horizontal.

(51) Casting and Stripping

(52) Flatware and Body Armour

(53) The slip was poured into the mould in one steady and continuous operation. The mould and slip were left to stand for 0.5-3 hours. During this time the slip was de-watered, i.e., the moisture content of the green body reduces to approximately 7%. The mould is then stripped and the piece is carefully removed. Green bodies were dried at 30-40 C. until they reached a steady weight.

(54) Firing

(55) Kiln furniture and beams were fired in a nitrogen atmosphere to 1380-1450 C. with soaks at 1380 C. for 14 hours and 1450 C. for 6 hours. The nitriding temperature and time was varied with the size and efficiency of the kiln. In kilns with large loads the exothermic reaction of the silicon reacting with the nitrogen can create a runaway heat effect which can ruin the product. This is controlled by means of monitoring the kiln temperature. If a rise in temperature of the kiln above that programmed is detected then an injection of Ar gas can be admitted to the furnace to dilute the available nitrogen and slow the exothermic reaction. Once the temperature falls to the desired level then the Ar slow can be stopped and the reaction allowed to proceed. A continuous monitoring of the kiln temperature is required during the reaction phase.

(56) Some items of kiln furniture require a second firing in air at 1250-1350 C. for 1 hour to seal the outer surface with the formation of a dense oxide and increase the oxidation resistance. Body armour items were fired to 1380 C. in a nitrogen atmosphere and held for 14 hours as for other products. A further separate high temperature firing was then carried out at 1650 C. for a further 4 hours under static nitrogen atmosphere to fully convert the SiAlON bond to the beta phase and to maximise bonding and sintering. The first firing schedule for the Kiln furniture is shown in Table 12. The second oxide firing schedule for the Kiln furniture and the body armour is shown in Table 13.

(57) TABLE-US-00012 TABLE 12 Firing Schedule for Kiln Furniture - 2 cubic meter kiln Temperature Rate Soak Segment Time Nitrogen Flow ( C.) ( C./hr) (hr) (hr) Rate (l/min) 20-740 60 12 150 740-1100 60 6 250 1100-1400 25 12 300 1400 16 16 300 1400-1450 25 2 300 1450 4 4 300 1450-850 100 6 300 850-370 60 8 200 370-130 60 4 100 Total 70 16.1 10.sup.3

(58) TABLE-US-00013 TABLE 13 Oxide Firing Schedule for Kiln Furniture and Body Armour Temperature Rate Soak Segment Time ( C.) ( C./hr) (hr) (hr) 20-1300 65 20 1300 4 4 1300-1000 75 4 1000-100 45 20 Total 48
Performance Data
Abrasion Resistance

(59) The abrasion resistance of the material measured using the method described in BS 1902-4.6:1985, where the volume loss of the sample is measured after abrasion with brown fused alumina is given below. The results are expressed as an abradability index, m, which is calibrated against standard carbon block (with m=100). Average abradability index, m, without sintering aid=17 Average abradability index, m, with Y.sub.2O.sub.3 sintering aid=14 Improvement in abrasion resistance due to use of sintering aid=20%
Performance Characteristics

(60) In binary rare earth oxide mixtures the eutectic temperature is rarely lowered below 2000 C. but in the presence of SiO.sub.2 then the mixtures form liquids at much lower temperatures, typically 300-400 C. lower. For example, the system Y.sub.2O.sub.3Al.sub.2O.sub.3SiO.sub.2 exhibits liquid formation at about 1370-1400 C. when 1.7 wt % SiO.sub.2 is present [U. Kolitsch, H. J. Seifert, T. Ludwig and F. Aldinger (1999), Phase equilibria and crystal chemistry in the Y.sub.2O.sub.3Al.sub.2O.sub.3SiO.sub.2 system, Journal of Materials Research, 14, pp 447-455. doi:10.1557/JMR. 1999.0064]. For Yb.sub.2O.sub.3 the temperature is reported as being 100-150 C. higher [Murakami, Y. and Yamamoto, H., J. Ceram. Soc. Jpn., 1994, 102, 231-236.]. SiO.sub.2 is invariably present in the materials as a surface oxide on the SiC powders and also on the Si powder. 1.7% SiO.sub.2 is possible with just a few tens of nanometers of oxide on a fine powder of 5-10 microns in size.

(61) On slow cooling, or by using isothermal heat treatment at about 1000-1300 C., the liquid phase can be crystallised forming an intergranular network of the garnet phase RE.sub.3Al.sub.5O.sub.12. Alternatively under some cooling conditions other crystalline phases can form such as RE-N-Melilite and J-phase. The refractory nature of the crystalline phases gives the end product better high temperature strength compared to a material with a purely amorphous residual sintering aid phase although some reduction in oxidation resistance may be experienced.

(62) Where the end product contains an amorphous phase (e.g. where full crystallisation has not occurred due to the use of a rapid cooling rate) then the replacement of the larger cations such as Y.sup.3+ with smaller rare earth cations such as Lu.sup.3+ can increase the glass transition temperature and thus also improve the refractory properties [J. E. Shelby and J. T. Kohli, Rare-Earth Aluminosilicate Glasses, J. Am. Ceram. Soc., 73 [1] 39-42 (1990); Y. Murakami and H. Yamamoto, Properties of Oxynitride Glasses in the LnSiAlON Systems (Ln Rare Earth), J. Ceram. Soc. Jpn., 102 [3] 231-36 (1994)]. Fully replacing all the Y oxide containing sintering aid with Lu oxide sintering aid, for example, may not be desirable due to the resulting increase in the temperature at which sintering occurs and an associated decrease in the energy efficiency so a mixture of Y and Lu can provide a compromise.

(63) By using mixtures of RE.sub.2O.sub.3 oxides the nature of the liquid phase formed on heating and the nature of the crystalline phase or amorphous phase formed on cooling can be modified such that the liquid formation temperature is low enough to allow sintering to occur at relatively low temperatures but that on cooling a refractory crystalline or amorphous phase is formed in the end product. In the garnet structure the RE cations can replace each other in many cases forming a single phase containing both cations, e.g. (Y,Yb).sub.3Al.sub.5O.sub.12 while in the amorphous phase the rare earth cations can replace Y and its coordination sites. For example, in a 1750 C. hot pressed SiC composite with 30% Si.sub.3N.sub.4 present and Yb.sub.2O.sub.3 used as the sole sintering aid the onset of densification was 1345 C. but subsequent densification was slow with the maximum rate being achieved at 1620 C. When Y.sub.2O.sub.3 replaced half the Yb.sub.2O.sub.3 then the densification began at 1335 C. but increased more quickly and its maximum rate was achieved at 1590 C. Thus a more rapid densification was possible at a lower temperature by using mixed oxides as the sintering aid. When produced by pressureless sintering and cooled slowly the materials containing mixed YYb sintering aids exhibited the formation of a crystalline garnet structure (Y/Yb).sub.3Al.sub.5O.sub.12 by XRD.

(64) When fired (optionally via a third firing stage) to form a surface oxide, the use of rare earths promote the formation of a surface oxide layer containing the rare earth phase, for example yttrium disilicide (Y.sub.2Si.sub.2O.sub.7) as well as a silicon oxide phase (SiO.sub.2, low cristobalite). Yttrium disilicide is a more refractory phase than pure SiO.sub.2 [W. Y. E. M. Levin, C. R. Robbins, and H. F. McMurdie: Phase Diagrams for Ceramists1969 Supplement (The American ceramic Society, Inc., Columbus, Ohio, 1969), FIG. 2388, p. 76] and accordingly provides a surface layer which is more resistant to high temperatures. Advantageously it is possible to generate surface oxide layers having refractory characteristics that can be selectively adjusted or modified using mixtures of rare earths that have respectively different refractory properties (e.g. melting points).

Example 2

(65) Table 14 details the starting materials according to a further example of the subject invention comprising Yttrium Oxide (Y.sub.2O.sub.3) as a sintering aid in the manufacture of a SiAlON bonded silicon carbide refractory material.

(66) TABLE-US-00014 TABLE 14 Starting materials formulation Raw Material Grade SC (%) Green Silicon Carbide - Coarse 100/F 40.0 Green Silicon Carbide - Fine FCP 36.0 Silgrain Standard - Silicon Metal Powder 0-53 m 20.0 Reactive Alumina alpha 4.0 Iron (III) Oxide - Fe.sub.2O.sub.3 Red Calcined +0.5 Yttrium Oxide (Y.sub.2O.sub.3) Purity >99% +1.7 Sodium Hydroxide 1M +0.4 De-ionised Water +11.5-14.0

Example 3

(67) Table 15 details the starting materials according to a further example of the subject invention comprising Ytterbium Oxide (Yb.sub.2O.sub.3) as a sintering aid in the manufacture of a SiAlON bonded silicon carbide refractory material.

(68) TABLE-US-00015 TABLE 15 Starting materials formulation Raw Material Grade SC (%) Green Silicon Carbide - Coarse 100/F 40.0 Green Silicon Carbide - Fine FCP 36.0 Silgrain Standard - Silicon Metal Powder 0-53 m 20.0 Reactive Alumina alpha 4.0 Iron (III) Oxide - Fe.sub.2O.sub.3 Red Calcined +0.5 Ytterbium Oxide (Yb.sub.2O.sub.3) Purity >99% +0.8 Sodium Hydroxide 1M +0.4 De-ionised Water +11.5-14.0

Example 4

(69) Table 16 details the starting materials according to a further example of the subject invention comprising Ytterbium Oxide (Yb.sub.2O.sub.3) as a sintering aid in the manufacture of a SiAlON bonded silicon carbide refractory material.

(70) TABLE-US-00016 TABLE 16 Starting materials formulation Raw Material Grade SC (%) Green Silicon Carbide - Coarse 100/F 40.0 Green Silicon Carbide - Fine FCP 36.0 Silgrain Standard - Silicon Metal Powder 0-53 m 20.0 Reactive Alumina alpha 4.0 Iron (III) Oxide - Fe.sub.2O.sub.3 Red Calcined +0.5 Ytterbium Oxide (Yb.sub.2O.sub.3) Purity >99% +1.7 Sodium Hydroxide 1M +0.4 De-ionised Water +11.5-14.0

Example 5

(71) Table 17 details the starting materials according to a further example of the subject invention comprising Gadolinium Oxide (Gd.sub.2O.sub.3) as a sintering aid in the manufacture of a SiAlON bonded silicon carbide refractory material.

(72) TABLE-US-00017 TABLE 17 Starting materials formulation Raw Material Grade SC (%) Green Silicon Carbide - Coarse 100/F 40.0 Green Silicon Carbide - Fine FCP 36.0 Silgrain Standard - Silicon Metal Powder 0-53 m 20.0 Reactive Alumina alpha 4.0 Iron (III) Oxide - Fe.sub.2O.sub.3 Red Calcined +0.5 Gadolinium Oxide (Gd.sub.2O.sub.3) Purity >99% +1.7 Sodium Hydroxide 1M +0.4 De-ionised Water +11.5-14.0

Example 6

(73) Table 18 details the starting materials according to a further example of the subject invention comprising Lanthanum Oxide (La.sub.2O.sub.3) as a sintering aid in the manufacture of a SiAlON bonded silicon carbide refractory material.

(74) TABLE-US-00018 TABLE 18 Starting materials formulation Raw Material Grade SC (%) Green Silicon Carbide - Coarse 100/F 40.0 Green Silicon Carbide - Fine FCP 36.0 Silgrain Standard - Silicon Metal Powder 0-53 m 20.0 Reactive Alumina alpha 4.0 Iron (III) Oxide - Fe.sub.2O.sub.3 Red Calcined +0.5 Lanthanum Oxide (La.sub.2O.sub.3) Purity >99% +1.7 Sodium Hydroxide 1M +0.4 De-ionised Water +11.5-14.0

Example 7

(75) Table 19 details the starting materials according to a further example of the subject invention comprising Lanthanum Oxide (La.sub.2O.sub.3) and Gadolinium Oxide (Gd.sub.2O.sub.3) as a sintering aids in the manufacture of a SiAlON bonded silicon carbide refractory material.

(76) TABLE-US-00019 TABLE 19 Starting materials formulation Raw Material Grade SC (%) Green Silicon Carbide - Coarse 100/F 40.0 Green Silicon Carbide - Fine FCP 36.0 Silgrain Standard - Silicon Metal Powder 0-53 m 20.0 Reactive Alumina alpha 4.0 Iron (III) Oxide - Fe.sub.2O.sub.3 Red Calcined +0.8 Gadolinium Oxide (Gd.sub.2O.sub.3) Purity >99% +0.8 Lanthanum Oxide (LA.sub.2O.sub.3) Purity >99% +0.8 Sodium Hydroxide 1M +0.4 De-ionised Water +11.5-14.0

(77) Table 20 details the raw La and Gd materials used in the performance testing described here.

(78) TABLE-US-00020 TABLE 20 Raw material specification for La and Gd Mate- Total other LOI Fe.sub.2O.sub.3, CaO, rial TREO REO RE oxides (1000 C.) Co.sub.3O.sub.4, Cr.sub.2O.sub.3 La.sub.2O.sub.3 Min Min Max <0.1% <1750 ppm total 99.9% 97% 99.9% 0.1% La.sub.2O.sub.3 Gd.sub.2O.sub.3 Min Min Max Max 1% <40 ppm total 99.99% 99.0% 99.99% 100 ppm Gd.sub.2O.sub.3 TREO = total rare earth oxide, REO = rare earth oxide, LOI = loss at ignition (at 1000 C.)

(79) Typical particle size, D50=2.5 m

(80) Further Performance Characteristics

(81) The mechanisms, phase behaviour and findings detailed with respect to example 1 are also applicable to further examples 2 to 7. Additionally, these properties and advantageous effects extend to further specific examples that comprises the stated rare earth oxide either used alone or in combination with each other and additional rare earth oxide such as ytterbium and yttrium.

(82) Bulk Density & Porosity

(83) Density was measured using the Archimedes method and the boiling method for porous samples. Typical standard deviations on density measurements are +/0.02 g.Math.cm.sup.1

(84) Strength

(85) Results from three-point bend testing (modulus of rupture, MOR) on test bars with cross-sectional area 12.5 mm25 mm and test span 125 mm with load rate 3N/s are shown in table 20. The base composition is the SiAlON bonded SiC material with nominally 70 to 75 wt % SiC and 25 to 30 wt % in total of alpha Si.sub.3N.sub.4, beta SiAlON and sintering aid. Typical values are from 4 to 6 with standard deviations typically from 5 to 20 MPa. The results are rounded accordingly to the nearest 5 Mpa.

(86) TABLE-US-00021 TABLE 21 Three-point bend modulus of rupture and density data for the nominal SiAlON bonded silicon carbide composition with the additives listed. Density after MOR (MPa) Density after MOR (MPa) 1450 C. after further further 1650 C. after 1450 C. Processing 1650 C. Processing Composition Processing (g .Math. cm.sup.1) Processing (g .Math. cm.sup.1) No Sintering Additive 140 2.75 130 2.75 +1.7 wt % Y.sub.2O.sub.3 175 2.79 185 2.84 +1.7 wt % La.sub.2O.sub.3 185 2.80 220 2.83 +1.7 wt % Gd.sub.2O.sub.3 200 2.76 200 2.80 +0.8 wt % Yb.sub.2O.sub.3 150 2.78 205 +1.7 wt % Yb.sub.2O.sub.3 175 2.80 185 +1.7 wt % CeO.sub.2 (98%) 70 2.68 +1.7 wt % (CeO.sub.2 60% + 85 2.56 La.sub.2O.sub.3 40%)

(87) FIGS. 5 to 9 detail XRD data of various rare earth sintering aid materials. The data is evidence for the advantageous affects associated with gadolinium and/or lanthanum based materials. This data is complementary to the strength and density data for such materials as presented in table 21.

(88) Referring to FIG. 5, the XRD data confirm the phase composition of a SiAlON bonded SiC material made using 1.7 wt % Y.sub.2O.sub.3 as a sintering aid after processing at 1450 C. Quantification shows 76% SiC, 8% alpha Si.sub.3N.sub.4, 15% beta SiAlON and 1% ytrria alumina garnet (YAG, Y.sub.3Al.sub.5O.sub.12). It is noted the unidentified peaks are x-ray tube line artefacts.

(89) Referring to FIG. 6, the XRD data confirm the phase composition of a SiAlON bonded SiC material made using 1.7 wt % Y.sub.2O.sub.3 as a sintering aid after processing at 1450 C. followed by 1650 C. Quantification shows 77% SiC, 21% beta SiAlON and 1% ytrria alumina garnet (YAG, Y.sub.3Al.sub.5O.sub.12). It is noted the unidentified peaks are x-ray tube line artefacts.

(90) Referring to FIG. 7 the XRD data confirm the phase composition of a SiAlON bonded SiC material made using 1.7 wt % La.sub.2O.sub.3 as a sintering aid after processing at 1450 C. Quantification shows 77% SiC, 4% alpha Si.sub.3N.sub.4, 16% beta SiAlON and 2% silicon oxynitride (Si.sub.2N.sub.2O). Si.sub.2N.sub.2O is formed on the surface under some processing conditions and is not part of the bulk composition but incorporated in to the sample when it is crushed to a powder for XRD.

(91) Referring to FIG. 8 the XRD data confirm the phase composition of a SiAlON bonded SiC material made using 1.7 wt % La.sub.2O.sub.3 as a sintering aid after processing at 1450 C. and subsequently at 1650 C. Quantification shows 78% SiC and 22% beta SiAlON.

(92) Referring to FIG. 9 the XRD data confirm the phase composition of a SiAlON bonded SiC material made using 1.7 wt % CeO.sub.2 (98%) as a sintering aid after processing at 1450 C. Quantification shows 75% SiC, 12% alpha Si.sub.3N.sub.4 and 13% beta SiAlON. Despite a phase content similar to other materials this material had a very low strength.

(93) The results as presented in FIGS. 5 to 9 and table 21 confirm that additions of La.sub.2O.sub.3 and Gd.sub.2O.sub.3 are as least as effective at producing dense and strong materials with the correct phase compositions as the additions of yttria (Y.sub.2O.sub.3) or ytterbia (Yb.sub.2O.sub.3). This is surprising as yttria is generally regarded by those skilled in the art as the most effective rare earth for sintering due to its propensity to form a liquid phase with Al.sub.2O.sub.3 and SiO.sub.2 at typical sintering temperatures. The liquid formed with Y.sub.2O.sub.3 has a low viscosity and is able to flow between particles allowing an even distribution and rearrangement of particles whilst also having a good solubility of silicon and nitrogen from the alpha silicon nitride phase and thereby promoting the transformation from the alpha to the beta silicon nitride or SiAlON phase.

(94) However, although the above is confirmed for silicon nitride and SiAlON systems, the present results show that the use of gadolinium and lanthanum oxides are equally as effective and in some aspects superior to yttrium oxide or ytterbium oxide, when used in conjunction with silicon carbide as the major phase and combined with the processing conditions described herein. Sintered density, phase composition and three-point bend strength are all comparable if not better than yttria based systems referring to FIGS. 7, 8 and table 21.

(95) The physical and mechanical properties of the present material comprising elemental La and/or Gd in the intergranular phase are unexpected as the results indicate that the use of a more refractory rare earth oxide i.e., Yb.sub.2O.sub.3, does not significantly improve performance with regard to density. Other less refractory rare earths such as CeO.sub.2 (98%) and mixed rare earths such as 60% CeO.sub.2 and 40% La.sub.2O.sub.3 produce materials which have approximately half the strength of the material with no additives despite having a phase composition by XRD which would appear to be comparable with good performing sintering aids such as La or Gd. Accordingly, the use of Ga and/or La oxides, or these rare earth oxides in conjunction with other rare earths oxides such as Y.sub.2O.sub.3 or Yb.sub.2O.sub.3, can produce a material with properties that are surprising.

(96) Furthermore, the subsequent high temperature heat treatment to 1650 C. for 4 hours in a nitrogen atmosphere is shown to promote the formation of 100% beta SiAlON phase in Gd and La containing materials which is further surprising given the accepted view that they are less effective than yttria or ytterbia as sintering aids and less likely to produce enough liquid phase with low enough viscosity and high enough solubility to allow the full conversion of all alpha Si.sub.3N.sub.4 to beta SiAlON at the levels at which they are added and the temperature applied.