WO1989000984A1

WO1989000984A1 - SINTERED CERAMIC PRODUCT COMPRISING SILICON CARBIDE AND SiAlON POLYTYPOID

Info

Publication number: WO1989000984A1
Application number: PCT/AU1988/000271
Authority: WO
Inventors: Mark Brian Trigg
Original assignee: Commonwealth Scientific And Industrial Research Or
Priority date: 1987-07-27
Filing date: 1988-07-27
Publication date: 1989-02-09

Abstract

A sintered ceramic product having a body comprising silicon carbide and SiAlON polytypoid is provided. The body has at least 40 weight percent silicon carbide and a total of silicon carbide and SiAlON polytypoid of at least 75 weight percent, the balance if any of the body comprising not more than 10 weight percent of a phase substantially comprising silicon or silicon containing material, not more than 10 weight percent of a phase consisting of a reaction product comprising an aluminate and not more than 5 weight percent of a glassy phase. A process for densifying silicon carbide to produce the sintered ceramic product comprises intimately mixing from 40 to 98 weight percent silicon carbide with from 2 to 60 weight percent of powdered source material of aluminium, nitrogen and silicon as well as an oxide material to produce an intimate mixture, forming the intimate mixture into a compact, and heating the compact at a temperature of from 1500°C to 2500°C thereby producing a densified sintered product. In the process, the powdered source material is added to the silicon carbide in such proportions that after said heating the product contains at least 40 weight percent silicon carbide and a second crystalline phase comprising SiAlON polytypoid.

Description

SINTERED CERAMIC PRODUCT COMPRISING SILICON CARBIDE AND SIALON POLYTYPOID

TECHNICAL FIELD

The present invention relates to the formation of dense ceramic products and is particularly concerned with densifying silicon carbide.

Shaped articles comprised of polycrystalline silicon carbide are known. They are characterized by excellent physical properties such as high resistance to thermal shock, abrasion and oxidation, as well as by high levels of strength and thermal conductivity. It is this combination of properties which make silicon carbide materials leading candidates for engineering applications. However, this combination of properties only concur i high density materials.

During high temperature heat treatments of prerequisite powder compacts, a reduction in the surface energy of the system can occur through the diffusion of atoms resulting in either grain boundary diffusion and densification, or • in grain growth by surface diffusion and/or vapor transport mechanisms with virtually no densification. At the high temperatures required for the sintering of silicon carbide powder compacts, surface diffusion and/or vapor transport prevails over grain boundary diffusion and/or vapor transport. This results in coarsening of the silicon carbide grains in a powder compact with little densification taking place and resultant degradation of the potential mechanical properties.

The oldest known method for the production of silicon carbide bodies is reaction sintering. A number of techniques involving a wide range of compositions can be employed. In general, a plastic body is produced from a mixture of silicon carbide powder, graphite and a plasticizer. The plasticizer is burnt off or decomposed to form a char. Silicon metal, as a liquid or vapour, is then impregnated into the body. This reacts with the carbon to form a silicon carbide, which bonds the original silicon carbide grains together. As a consequence of the process and the requirement of silicon to infiltrate the body, the finished articles typically contain between 8 to 12 volume percent of free silicon. This limits the maximum operating temperature of this type of material to approximately 1300 to 1400°C.

In addition to the problems of surface diffusion and/or vapor transport occuring at the high temperatures required for sintering silicon carbide, it has been found always necessary to reduce the surface silica layer which occurs naturally on the silica carbide because the presence of this phase tends to halt the densification of the powder compact so that little or no overall shrinkage takes place.

It is well known that dense articles of silicon carbide can be obtained by pressure assisted and pressureless sintering with the aid of boron or boron containing and carbon additives. The use of aluminium and beryllium compounds as densification aids for both pressureless sintering and hot pressing also have been proposed. It is believed that the use of certain additives promotes grain boundary diffusion over surface diffusion and/or vapor transport. Thus, boron has been found to be effective in increasing grain boundary diffusion. In addition to boron, carbon is usually added to reduce the surface silica layer to silicon carbide and carbon monoxide. The addition of carbon is also believed to limit exaggerated grain growth during densification.

It has also been suggested that the addition of boron during the hot pressing of silicon carbide may cause exaggerated grain growth. Despite the addition of carbon the grain growth can only be inhibited by strict control of both temperature and pressure within narrow limits. Furthermore, the final product usually contains carbon particles in the microstructure which also leads to degradation of the mechanical properties of the product.

Other densifying aids have been proposed. Thus, the use of silicon nitride as a hot pressing aid for silicon carbide has been proposed in relatively large amounts (up to 10%) but it also has been found necessary for boron to be present in either its elemental form or as boron carbide. This densifying step requires strict control in regard to temperature due to the tendency of the silicon nitride to decompose. In an alternative process using silicon nitride to densify silicon carbide, dense bodies were obtained without the concomitant use of sintering aids. The disadvantage of this technique is that the powder compacts have to be sealed in gas tight containers that must deform in a plastic manner at temperatures of 1800 to 2200°C. This greatly adds to the complexity of the process under current practice, but offers potential benefits in particular applications.

It has also been proposed to densify silicon carbide with beryllium carbide or with aluminium, oxygen and carbon. In both cases it is specified as necessary to have very low levels of oxide on the precursor powders, but this requires an additional operation during or prior to the densification process or the addition of further additives, such as carbon, with associated problems as described above.

A number of proposals for the densification of silicon carbide have been made using aluminium nitride. Aluminium nitride suffers the disadvantage of dissolving in warm water if it remains free in the silicon carbide, but it has been shown to be useful in improving the electrical characteristics and corrosion resistance of silicon carbide at elevated temperatures. Generally silicon carbide and aluminium nitride alone do not sinter to high density materials. It therefore has been proposed to incorporate carbon in the powder compact to facilitate densification and aid the reduction of oxides which might otherwise remain in the finished product. It is not known whether any unreacted aluminium nitride remains in the densified product.

It is also known that dense silicon carbide materials may be produced without using silicon carbide as a starting material. Thus as disclosed in US Patent specification 3837871, a quarternary silicon aluminium oxynitride having the hexagonal phenacite crystal structure can be formed by reacting silicon oxynitride with an appreciable amount of aluminium to form a material of reported composition Si, Al ON₂. It was also claimed that a dense material can be produced consisting of predominantly silicon carbide and the product of the high temperature reaction between aluminium and silicon oxynitride with a phenacite structure. This process comprised mixing aluminium with silicon oxynitride and reacting for sufficient time to allow a substantial proportion of the aluminium to be completely reacted with the silicon oxynitride. Silicon, which is a by-product of the aluminium and silicon oxynitride reaction, is then reacted with carbon to form silicon carbide. No details regarding the properties of the materials were given.

I.B. Cutler et al in Nature Vol.275, No. 5679 Pages 434 to 435 1978 describes the production of solid solutions of silicon carbide and aluminium nitride by carbothermal reduction using silica, carbon and other oxides, particularly aluminium precipitated as the hydroxide. The solid solutions are said to be based on the 2H wurtzite-type structure although many of the materials cited as potentially forming solid solutions with silicon carbide and aluminium nitride do not have a wurtzite-type structure. No indication is given as to how the materials may be densified into end products with useful strengths.

US Patent 4141740 of Cutler et al discloses a solid solution of silicon carbide and aluminium oxycarbide with or without aluminium nitride, again starting from non-silicon carbide materials. SUMMARY OF THE INVENTION

The present invention provides an improved form of shaped articles based on polycrystalline silicon carbide, and to a process for the production of such articles.

According to the invention, there is provided a sintered ceramic body, comprising silicon carbide and SiAlON polytypoid, with the body including at least 40 weight percent silicon carbide and the total of silicon carbide and SiAlON polytypoid being at least 75 weight percent, the balance if any of the body comprising not more than 10 weight percent silicon or a silicon containing material, not more than 10 weight percent of a reaction product comprising an aluminate and not more than 5 weight percent of a glassy phase.

The SiAlON polytypoid may be substantially entirely present as a second phase. Alternatively, it may be partially present in solid solution in the silicon carbide.

The SiAlON polytypoid, in one form of the invention, is defined as

where "x" can have a value of 4, 5, 6, 7, 9 or 11 with:

⁰ -^ a ^ °'²⁵

0.75 ^ b 1-0

0.20 c °«⁵

0.50 ^d ζ 0.80, where a + b = 1 and c + d = 1. In such case, the body normally will be substantially free of aluminate and the total of silicon carbide and SiAlON polytypoid will comprise at least about 85 weight percent of the body.

Alternatively, the SiAlON polytypoid may be defined by ^(Ma^Sib^A1c⁾x⁽⁰c^Nd⁾x₊l where "x" can have a value between 4 and 11, with:

0.5

0.3

0.9

0.8

0.8 where a + b + c = 1 and d + e =1, and M can be lithium, beryllium, calcium, magnesium, scandium; yttrium or a rare earth. As indicated by this alternative polytypoid composition, there is partial substitution of silicon and/or aluminium by an element designated by M. However, it is found in practice that, at least some of these elements tend in part to form stable aluminates, which appear in the body as a secondary phase. This tendency is particularly pronounced where the element is magnesium, the addition of magnesium tending to result in formation of a spinel second phase and, hence, only partially in substitution in the SiAlON.

The present invention also provides a process for densifying silicon carbide which comprises intimately mixing from 40 to 98 weight percent of silicon carbide powder with from 2 to 60 weight percent of powdered source material of aluminium, nitrogen and silicon as well as with an oxide material to produce an intimate mixture, forming the intimate mixture into a compact, and heating the compact at a temperature of from 1500 to 2500°C thereby producing a densified sintered product, the powdered source material being added to the silicon carbide in such proportions that after said heating the product contains at least 40 weight percent silicon carbide and a second crystalline phase comprising SiAlON polytypoid.

Further according to the present invention there is provided a densified silicon carbide product when formed by the process described in the immediately preceding paragraph.

Generally, but not essentially, the oxide material may comprise the oxide impurity layer which typically is present on the silicon carbide and on other non-oxide precursor powders used. The oxide material as a liquid phase is believed to be converted into a stable secondary SiAlON polytypoid crystalline- phase of which a part can go into solid solution with the silicon carbide. The densification rate is high at the high fabrication temperatures employed, and is believed to be improved by the appearance of a liquid phase which enhances the atomic diffusion rates resulting in the more rapid elimination of porosity than if a liquid was not present. It appears that the liquid may also be transient in nature, in the sense that it is essentially consumed in the reaction to form the crystalline structure of the SiAlON polytypoid phase, as the product of the process generally is not found to be characterised by an excessive amount of a glassy phase, if any. As the oxide layer is incorporated into a crystalline SiAlON polytypoid phase, both the need to remove it from the precursor powders and the need for carbon to be added to the starting mix for this purpose are eliminated. In addition, there is believed to be an increase in the reaction rate to form a solid solution in the presence of the transient liquid. By incorporating any aluminium nitride present into a solid solution, the tendency of free aluminium nitride to hydrolyse and impair the mechanical properties of an end product is alleviated.

Polytypoids of the Si-Al-O-N and M-Si-Al-O-N types have been discussed by D.P. Thompson and co-workers (Progress in Nitrogen Ceramics, ed. F.L. Riley, Mar inus Nijhoff publishers 1983, page 61). It is stated that the structures are determined by the metal to non-metal atom ratio A:B which in the Si-Al-O-N system takes the form of Ax:Bx+1.. where

"x" has the values of 4, 5, 6, 7, 9 and 11 for the 8H, 15R, 12H, 21R, 27R and 2H (delta prime) structures. By maintaining the charge balance and the overall A:B ratio, it is said also to be possible to incorporate metal cations, such as beryllium, magnesium and scandium and possibly other elements into these wurtzite-type structures.-

Referring to the process of the invention, in the starting powder mixes, a range of silicon carbide from about 40 to 98 weight percent is found to be useful. The remainder of the starting mix and powders preferably are conveniently composed of appropriate combinations selected from alumina, aluminium nitride, silicon nitride, silica and silicon. In addition, there also can be added at least one optional metal of the group comprising lithium, beryllium, calcium, magnesium, scandium, yttrium and other elements from the rare earths group, which can with the other compounds form a SiAlON polytypoid with a wurtzite crystal structure. The non-silicon carbide powders are added to the silicon carbide such that, after densification by heating to 1500-2500°C, with or without pressure, the finished product consists principally of silicon carbide and a second crystalline phase comprising a sialon polytypoid of chemical composition (M,Si,Al) (0,N) ,, where x can take selected values from the range 4 to 11; with or without optional additions of M, which can comprise any of the elements such as lithium, beryllium, calcium, magnesium and scandium and/or the aforesaid other elements. The silicon carbide may be a solid solution formed from the silicon carbide and part of a crystalline phase in the form of a sialon polytypoid of that chemical composition (M,Si,Al) (0,N) ..

The solid solution of silicon carbide, where present, comprises either of the following assemblage of elements: Si-Al-C-O-N and M-Si-Al-C-O-N, such that the structure of the said silicon carbide is either rhombohedral or hexagonal. ^• Assuming a hexagonal unit cell, the lattice dimensions are about 3 angstroms for a and greater than about 5 angstroms for c . These modifications have been designated 2H, 4H, 6H, 8H, 19H, 27H and 15R, 21R, 27R, 33R, 51R, 75R, 84R, 87R, 141R, 174R and 393R by RWG Wyckoff, Crystal Structures, second edition, volume 1.

The silicon carbide powder may be of the alpha or beta modification or forms, or it may comprise amorphous silicon carbide. Also, the powder may comprise a mixture of any two or all three forms of silicon carbide. In terms of densification, it is believed that no significant practical difference occurs with variation in the form or forms of silicon carbide used. However, transformation from one form to another can occur, to an extent dictated by sintering conditions; partial transformation of the beta to the alpha form being most typically encountered.

As will be appreciated from the foregoing, the powdered source material and the oxide material most typically comprise reactants which, under the sintering conditions, react to form SiAlON polytypoid. However, it is to be understood that the such precursor materials are not essential, and the silicon carbide may be mixed and compacted with a powdered SiAlON polytypoid. The polytypoid may be that which is required in the end product. Alternatively, the polytypoid with which the silicon carbide is mixed and compacted may be another polytypoid, depending for example on how the polytypoid varies due to loss of its constituents during heating or by it taking up further silicon, oxygen, or both from the silicon carbide and/or aluminium, nitrogen or both from other powder, if any, with which it initially is mixed.

Where the powdered source material is not powder SiAlON polytypoid, but rather is reactants which form such polytypoid, then silicon, oxygen, or both can be derived from the oxide impurity layer on the silicon carbide and on other non-oxide precursor powders used. However, if such impurity layer is insufficient, silicon can be derived from precursor powders such as elemental sil.icon, silica, aluminium silicon carbide, SiAlON polytypoid, silicon nitride or oxynitride such as aluminium containing silicon nitride or oxynitride and aluminium silicate. Similarly, aluminium can be derived from elemental aluminium, alumina, aluminium silicon carbide, SiAlON polytypoid, aluminium containing silicon nitride or oxynitride, and aluminium silicate. Oxygen and nitrogen most conveniently are derived from relevant sources specified for silicon and/or aluminium. Other metal species, comprising lithium, beryllium, calcium, magnesium, scandium, yttrium or a rare earth such as yttrium can be^' added as, for example, the elemental metal powder, the oxide, nitride, carbide, carbonate, nitrate, sulphate or silicate, or as metal SiAlON polytypoid or its silicon nitride derivative. As will be noted, the other metal species may contribute to the total silicon, aluminium, oxygen, nitrogen content of the precursor powders or, by choice of their suitable other compounds, to the aluminium content of the precursor powders.

As indicated, the body of an article according to the invention has at least 40 weight percent silicon carbide, with a total of silicon- carbide and SiAlON polytypoid of at least 75 weight percent. The silicon carbide content preferably exceeds 75 weight percent, most preferably 90 weight percent. Preferably the silicon carbide content does not exceed 95 weight percent; while it is found that inadequate densification results if the silicon carbide content exceeds about 98 weight percent due to the SiAlON polytypoid being present at an insufficient level of not more than about 2 weight percent.

As previously indicated, formation of SiAlON polytypoid can, in part, be replaced by formation of an aluminate, such as spinel (MgO.Al₂0₃). The extent to which this can occur depends on the time and temperature of sintering. The formation of aluminate also depends on whether . sintering is conducted with or without pressure; the absence of pressure favouring the tendency for aluminate formation.

The materials can be conveniently densified in the temperature range of 1500 to 2500 C, with or without the application of pressure, in inert atmospheres such as nitrogen or argon, where the oxygen partial pressure is less than about 10 atmosphere. It is appreciated that the application of pressure can be useful in reducing the temperature at which the densification can be carried out in producing a dense body.

The silicon carbide and other .powders used in the invention preferably are equiaxed and have a particle size of less than 10 microns, such as less than 5 microns. Most preferably, the particle size is less than 1 micron, at least on the basis of mean particle size.

The end product preferably has a grain size of less than 10 microns, most preferably less than 5 microns. However some grain growth, particularly in the silicon carbide, is inevitable during heating to and holding at the sintering temperature. Allowance for such grain growth is desirable in selecting the particle size of the powders to be used. Typically, the product has a substantially equiaxed grain structure, although a minor proportion of acicular grains can be formed and, at least in some instances, can be beneficial. Also, a minor proportion of secondary phases present at grain boundaries can be amorphous and, particularly where this is the case, the formation of such acicular grains can be beneficial for enhancing fracture toughness.

The product of the invention exhibits good densification, generally in excess of 75% of the theoretical density of silicon carbide. However, densities in excess of 85% are readily achievable, while densities of 95% and higher have been achieved consistently. Such densities are obtained without any need for recourse to the use of conventional sintering aids such as boron and carbon. That is, boron need not be used at all. In the case of carbon, its addition as a sintering aid is not required. Also, while carbon can be present as a result of burn off of binder necessary for pressureless sintering, but need not be present for sintering under pressure binder burn off preferably is conducted so as to remove all residual carbon.

As indicated, sintering necessitates a temperature of from 1500°C to 2500°C. Preferably, sintering is at a temperature of from 1600 C to 2200°C, most preferably from 1900°C to 2100°C.

Sintering can be with or without pressure. However, • substantial variation occurs depending on whether or not pressure is employed. Thus, while some weight loss tends to occur in either case, the extent of weight loss is greater with pressureless sintering than with sintering under pressure. For this reason, sintering under pressure is preferred, such as by hot isostatic pressing. That is, sintering under pressure reduces weight loss, typically to not more than about 5 wt.%, and enables production of a product which in overall composition accords more closely with that of the initial powder mix. Also, sintering under pressure minimises shrinkage attributable to weight loss on sintering. Despite these matters, sintering without pressure does enable production of useful, well densified products.

The weight loss which occurs during sintering tends to result in the attainment in the product -of a SiAlON polytypoid or polytypoids different to that of the initial powder mix, as evidenced by the Examples detailed below. This tendency can be offset to a degree, if required, by increasing in the initial powder mix the proportion of those materials which are evolved during sintering. However, the components involved in weight loss are difficult to identify quantitatively, and it appears that they vary both with the composition of the initial powder constituents and with the temperature and time of sintering.

The minimum quantity of silicon carbide in the initial powder mix is 40 wt.%, with this quantity preferably being in excess of 75 wt.% and most preferably in excess of 90 wt.%. The proportion of silicon carbide in the sintered product preferably is at a similar level. However, due to weight loss, a given product can contain a higher proportion of silicon carbide than the powder mix from which it is formed, while the measured proportion of silicon carbide will be increased in so far as SiAlON polytypoid is present in solid solution in the silicon carbide. It is believed that, in general, the proportion of the polytypoid in solid solution will be minor and, at least where sintering is under pressure, the measured level of silicon carbide in the product general-ly does not significantly exceed its level in the powder mix from which the product is formed.

Where sintering is performed without pressure, weight loss and shrinkage during sintering generally is greater. There thus is a tendency for the silicon carbide content of a resultant product to exceed by from about 5 to 10 wt.% its level in the powder mix from which the product is formed. The weight loss, in general is principally from one or more of the constituents of the powder mix which form SiAlON polytypoid. The tendency for such weight loss to occur can be reduced, to a degree, by use of SiAlON powder in the powder mix rather than use of precursor powders. Also, to the extent that weight loss does occur, it tends to change the polytypoid from that of the polytypoid or precursor powder. The weight loss can be such that, in addition SiAlON polytypoid being formed in the product as a secondary phase, the product can contain elemental silicon and/or a glassy phase. If the initial powder mix includes a suitable additional element such as magnesium, a proportion of an aluminate or spinel secondary phase can occur. In general, the level of each of silicon and of aluminate or spinel does not exceed 10 wt.%, while the glassy phase, if present, generally does not exceed 5 wt.%. However, the overall tendency for weight loss in pressureless sintering can be minimised by avoidance of excessive sintering temperatures and times, with the sintering temperature for pressureless sintering preferably, being at from 1800°C to 2100°C, most preferably from 1900°C to 2050°C.

Various examples of a method in accordance with the present invention will now be described by way of example only. Comparative Example 1

Powders were prepared by mixing beta silicon carbide powder obtained from HC Starck of West Germany and known as grade BIO and aluminium nitride powder also from HC Starck (see table 1 for compositions) . The powders were mixed in a ball mill for 8 hours with iso-propanol as the milling fluid. The slurry was dried in a vacuum oven. Discs were prepared by uniaxially pressing the powder mix in a steel die followed by wet bag isostatic pressing at 210 MPa. The samples were then hot pressed in boron nitride coated graphite dies. The samples were heated to 2000 C, unless specified otherwise, and held for 60 minutes at a pressure of 35 MPa. The Vickers hardness was calculated using a 10kg load and the fracture toughness was calculated by the indentation technique. Although it was possible to produce dense bodies, analysis by XRD revealed the presence of silicon carbide and unreacted aluminium nitride the amount of which increased with increasing addition in the starting composition. This indicates the difficultly of forming solid solutions to minimise or eliminate the occurrance of aluminium nitride.

Sample

la lb 2 3

Example 2

A powder was prepared as described in Example 1 from beta silicon carbide (BIO), aluminium nitride and silicon nitride (LCIO) all from HC Starck and alumina (XA17) from Alcoa of America. . The starting composition was 79.8 weight percent silicon carbide together with the other powders listed added at 2.6 wt.% silicon nitride, 8.7 wt.% aluminium nitride and 8.9 wt.% alumina such that the overall composition of the other powders corresponded to a SiAlON polytypoid designated 8H (Si 0.5_CA1 ά . O_c0__ . O_CN . 0_c). In calculating the starting compositions, the oxides associated with the non-oxide powders were taken into account. The samples were then hot pressed as described in Example 1. After hot pressing the major crystalline phase detected by XRD was silicon carbide. Other minor X-ray diffraction peaks were observed at 2.80, 2.73 and 2.39 angstroms and these are attributed to 15R, 12H and 21R SiAlON polytypoids, respectively. This shows the advantage of the use of . SiAlON polytypoids which increased the reaction rate, such that after the hot pressing no unreacted aluminium nitride was detected. The reason for a higher than expected aluminium content of the SiAlON polytypoid is attributed to a small weight loss of approximately 5 wt.% during hot pressing.

TABLE 2

Sample Hot Press Density g/cc Hv K_τ

Temp °C Green Fires Gpa. MPa.m^0,5

4 2000 2.00 3.25 32.6 3.3

Example 3

Powders were prepared as described in Example 1 from beta silicon carbide (BIO), aluminium nitride and silicon nitride (LCIO) all from HC Starck and alumina (XA17) from Alcoa of America. The starting compositions were varying amounts of silicon carbide together with the other powders listed and added such that the overall composition of these powders corresponded to a SiAlON polytypoid designated 15R (Si,Al₄0„N₄) . In calculating the starting compos¬ itions, the oxides associated with the non_^oxide powders were taken into account. The compositions used are given in Table 3. The samples were then hot pressed as described in Example 1. After hot pressing the major phases detected by XRD were polytypes of silicon carbide. Other minor X-ray diffraction peaks were observed at 2.78 and 2.39 angstroms in sample 5 and these are attributed to 15R SiAlON polytypoid. In sample 6, minor X-ray diffraction peaks were observed at 2.79, 2.74 and 2.39 angstroms and these are attributed to 15R and 12H SiAlON polytypoids. The reason for a higher than expected aluminium content of the SiAlON polytypoids is attributed the small weight losses which were approximately 5 weight percent.

TABLE 3

Sample SiC

5 89.7

6 81.1

TABLE 4

Example 4

Powders were prepared as described in Example 1 from beta silicon carbide (B10), aluminium nitride and silicon nitride (LCIO) all from HC Starck, alumina (XA17) from Alcoa of America, and magnesium oxide (calcined at 1000°C for 3 hours) obtained from BDH of England. The starting compositions were varying amounts of silicon carbide together with the other powders listed, added such that the overall composition of these powders corresponded to a magnesium SiAlON polytypoid designated 15R (Mg,Si,Al₃0₃N₃) . In calculating the starting compositions, the oxides associated with the non-oxide powders were taken into account. The compositions used are given in Table 4. The samples were then hot pressed as described in Example 1. After hot pressing the major crystalline phases detected by XRD were polytypes of silicon carbide. Other minor XRD peaks were observed at 2.63 in sample 7 and this was attributed to 27R SiAlON polytypoid. In sample 8, other minor peaks were observed at 2.73, 2.70 and 2.39 angstroms, and these were attributed to 12H and 21R SiAlON polytypoids. A small amount of magnesium aluminate (spinel) was detected in each of samples 7 and 8. TABLE 5

Density g/cc Green Fired

1.97 3.21

1.98 3.21

Example 5

Powders were milled as described in Example 1 from beta silicon carbide (a B10 variant with a lowe alpha content) from HC Starck of West Germany, aluminium nitride (grade F) from Tokuyama Soda, silicon nitride

(LC10) from HC Starck and alumina (A16SG) from Alcoa of America. The starting composition, excluding the silicon carbide, corresponded to a SiAlON polytypoid designated 15R Si, l₄0_{2 4}. In calculating the starting composition, the oxides associated with the non-oxide powders were taken into account. The compositions used are given in Table 7. After milling the powders were spray dried to remove the milling fluid and then hot pressed, with the exception of sample 9c, as described in Example 1. In sample 9c, carbon paper was used to inhibit reaction between the graphite die and the sample in place of boron nitride-. After hot pressing the major crystalline phase detected in samples by XRD was cubic silicon carbide. X-ray diffraction peaks were observed at 2.73, 2.70 and 2.39 angstroms and these are attributed to 12H and 21R SiAlON polytypoids. These samples indicate that materials containing high levels of polytypoid addition can be produced with high bulk densities even at temperatures as low as 1800°C. In addition, sample 9c demonstrates that boron nitride or boron containing compounds are not required in the process of the present invention. Comparison of examples la and 9a reveal that lower fabrication temperatures can be employed without the unwanted presence of aluminium nitride to comprise properties.

TABLE 7 Starting Compositions (wt.%)

Sample SiC Si₃N₄ ^A12°3 A1N 9 40.5 12.2 16.0 31.3

TABLE 8 Selected Properties of Samples

Sample Hot Press Density g/cc Temp (°C) Green Fired

9a 1800 2.01 2.88 9b 1900 2.01 3.17 9c 1900 2.03 3.15

Example 6

Powders for sample 10 were milled as described in Example 1 from beta silicon carbide (B10 variant) from HC Starck of West Germany, aluminium nitride (grade F) from Tokuyama Soda, silicon nitride (LC10) from HC Starck and alumina (A16SG) from Alcoa of America. The starting composition excluding the silicon carbide, corresponded to a SiAlON polytypoid designated 21R (Si._j lg0₂Ng) . In calculating the starting composition, the oxides associated with the non-oxide powders were taken into account. The composition used are given in Table 9. After milling,, the powders were spray dried to remove the milling fluid. Sample 11 was prepared by mixing equal weight of silicon carbide and pre-reacted SiAlON polytypoid. Samples were hot pressed as described in Example 1. After hot pressing the major crystalline phase detected in samples by XRD was cubic silicon carbide. X-ray diffraction peaks were observed at 2.69, 2.64 and 2.40 angstroms and these are attributed to 21R and 27R SiAlON polytypoids. These samples indicate a trend of increasing processing temperatures with increasing aluminium content of the polytypoid to achieve the same level of densification.

TABLE 9 Starting Compositions (wt.%)

Sample SiC ^Si3^N4 ^Al2°3 ^A1N

10 50.6 7.2 8.9 33.3

TABLE 10 Selected Properties of Samples

Sample Hot Press Density g/cc

Temp (°C) Green Fired

10a 1800 1.99 2.73

10b 1900 1.98 2.96

11 1900 2.09 3.00 Example 7

Powders were milled as described in Example 1 from alpha silicon carbide (UF-15) from Lonza of Switzerland, aluminium nitride (grade F) from Tokuyama Soda and silicon nitride (LCIO) from HC Starck and alumina (A16SG) from Alcoa of America. The starting composition excluding the silicon carbide, corresponded to a SiAlON polytypoid designated 21R

(Si,Al_^O„N,.) . In calculating the starting ^v 1 6 2 6 composition, the oxides associated with the non-oxide powders were taken into account. The composition used are given in Table 11. After milling the powders were spray dried to remove the milling fluid and then hot pressed as described, in Example 1. After hot pressing the major crystalline phase detected in samples by XRD was 6H alpha silicon carbide. X-ray diffraction peaks were observed at 2.64 and 2.38 angstroms and these are attributed to the presence of 27R SiAlON polytypoid. This example indicates that both the alpha and beta forms of silicon carbide are useful as starting materials.

TABLE 11 Starting Compositions (wt.%)

Sample SiC ^si3^N4 ^Al2°3 ^A1N 12 50.6 7.2 8.9 33.3

TABLE 12 Selected Properties of Samples Sample Hot Press Density g/cc

Temp (°C) Green Fired

12 1900 1.82 3.05 In the Examples, the densities are given in terms of measured values. In terms of percentages of the theoretical density (%T.D.) for the composition of each product sample, the relationship to the measured values are as follows:

Table 13 Sample

la lb

2

3

4 5 6

7

Of theoretical density indicate good densification. Expectations, as already indicated by this and hardness values, are that the product of the invention will have good physical properties. The product also is expected to have good chemical properties, at least in terms of oxidation resistance.

Finally, it is to be understood that various alterat¬ ions, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A sintered ceramic product having a body comprising silicon carbide and SiAlON polytypoid,, the body having at least 40 weight percent silicon carbide and a total of silicon carbide and SiAlON polytypoid of at least 75 weight percent, the balance if any of the body comprising not more than 10 weight percent of a phase substantially comprising silicon or silicon containing material, not more than 10 weight percent of a phase consisting of a reaction product comprising an aluminate and not more than 5 weight percent of a glassy phase.

2. A sintered ceramic product according" to claim 1, wherein said SiAlON polytypoid is substantially entirely present as a secondary phase.

3. A sintered ceramic product according to claim 1, wherein said SiAlON polytypoid is partially present in solid solution in said silicon carbide, the remainder thereof being present as a secondary phase.

4. A sintered ceramic product according to any one of claims 1 to 3, wherein said SiAlON polytypoid is defined as

(SiaAl,o)x„(0cNQ-)x_+i-. where "x" has a value of 4, 5,

6, 7, 9 or 11, with:

0 ζ a ^ 0.25

0.75 ^ b ■ 1.0

0.20 C 0.5

0.50 ^ d ^ 0.80, where a + b = 1 and c + d = 1, and said body is substantially free of said aluminate.

5. A sintered ceramic product according to claim 4, wherein the total of silicon carbide and SiAlON polytypoid comprises at least 85 weight percent of said body.

6. A sintered ceramic product according to any one of claims 1 to 3, wherein said SiAlON polytypoid is defined as (M_aSi_bAl_c)₂.(0_dN_e)_χ+1 where "x" has a value between 4 and 11, with:

0 ^ a 0.5 0 ^. b ^. 0.3 0.3 c ^ 0.9 0.2 ^ d ^ 0.8 0.2 ^ e "^ 0.8 where a + b + c = 1 and d + e =1 and M is selected from lithium, beryllium, calcium, magnesium, scandium, yttrium and other rare earth metals.

7. A sintered ceramic product according to claim 6, wherein said body includes an aluminate at a level of not more than 10 weight percent.

8. A sintered _.ceramic product according to claim 7, wherein M is magnesium and said aluminate comprises spinel (MgO.Al₂0₃) .

9. A sintered ceramic product according to any one of claims 1 to 8, wherein said body has a grain size of less than 10 microns.

10. A sintered ceramic product according to claim 9, wherein said grain size is less than 5 microns.

11. A sintered ceramic product according to any one of claims 1 to 10, wherein said body has a density of at least 75% of the theoretical density of silicon carbide.

12. A sintered ceramic product according to claim 11, wherein said body has a density of at least 85% of said theoretical density.

13. A sintered ceramic product according to claim 1-2, wherein said body has a density of at least 95% of said theoretical density.

14. A sintered ceramic product according to any one of claims 1 to 13, wherein said body is substantially free of carbon.

15. A sintered ceramic product according to any one of claims 1 to 14, wherein said body is substantially free of boron.

16. A process for densifying silicon carbide to produce a sintered ceramic product, comprising intimately mixing from 40 to 98 weight percent silicon carbide with from 2 to 60 weight percent of powdered source material of aluminium, nitrogen and silicon as well as an oxide material to produce an intimate - mixture, forming the intimate mixture into a compact, and heating the compact at a temperature of from 1500°C to 2500°C thereby producing a densified sintered product, the powdered source material being added to the silicon carbide in such proportions that after said heating the product contains at least 40 weight percent silicon carbide and a second crystalline phase comprising SiAlON polytypoid.

17. A process according to claim 16, wherein the powdered source material is added in such proportions that the product contains a total of silicon carbide and SiAlON polytypoid of at least 75 weight percent, with the balance if any of the product comprising not more than 10 weight percent of a phase substantially comprising silicon, not more than 10 weight percent of a phase consisting of a reaction product comprising an aluminate and not more than 5 weight percent of a glassy phase but optionally being substantially free of one or more of said silicon, aluminate and glassy phase.

18. A process according to claim 16 or claim 17, wherein said powdered source material is added in such proportion that said SiAlON polytypoid is substantially entirely present as a secondary phase.

19. A process according to claim 16 or claim 17, wherein said powdered source material is added in such proportions that said SiAlON polytypoid is partially present in solid solution in said silicon carbide, the remainder thereof being present as a secondary phase.

20. A process according to any one of claims 16 to 19, wherein said powdered source material is added in such" proportions that said SiAlON polytypoid is defined as (Si Al, ) (O N,) , where "x" has a value of 4, 5,

0.25

1.0

0.5

0.80, where a + b = 1 and c + d = 1, and said body is substantially free of said aluminate.

21. A process according to claim 20, wherein the powdered source material is added in such proportions that said total of silicon carbide and SiAlON polytypoid comprises at least 85 weight percent of said product.

22. A process according to any one of claims 16 to 19, wherein said powdered source material is added _. in such proportion that said SiAlON polytypoid is defined as (M_aSi_bAl_c)_χ(O_dN_e)_χ+1 where "x" has a value between 4 and 11, with:

where a + b + c = 1 and d + e =1 and M is selected from lithium, beryllium, calcium, magnesium, scandium, yttrium and other rare earth metals derived from a constituent of said source material.

23. A process according to claim 22, wherein said powdered source material is added in such proportions that said product includes an aluminate at a level of not more than 10 weight percent.

24. A process according to claim 23, wherein M is magnesium and said aluminate comprises spinel (MgO.Al₂0₃).

25. A process according to any one of claims 16 to 24, wherein said silicon carbide is selected from alpha and beta modifications of silicon carbide, amorphous silicon carbide and mixtures thereof.

26. A process according to any one of claims 16 to 25, wherein said silicon carbide and said powdered source material are characterised by a particle size of not more than 10 microns.

27. A process according to claim 26, wherein said particle sizes are not more than 5 microns.

28. A process according to claim 27, wherein said particle sizes are not more than 1 micron.

29. A process according to any one of claims 16 to 28, wherein said powdered source material is powdered SiAlON polytypoid.

30. A process according to any one of claims 16 to 28, wherein said powdered source material comprises precursor powders for a SiAlON polytypoid.

31. A process according to any one of claims 16 to 28, wherein said oxide material comprises an oxide impurity layer on said silicon carbide and any such layer on non-oxide powder of said powdered source material.

32. A process according to claim 31, wherein said powdered source material together with said oxide material together comprise precursor material for forming a SiAlON polytypoid.

33. A process according to any one of claims 16 to 28, wherein said SiAlON polytypoid of said product has a silicon content derived at least in part from _a component of said powdered source material selected from elemental silicon, silica, aluminium silicon carbide, SiAlON polytypoid, silicon nitride, silicon oxynitride and aluminium silicate.

34. A process according to claim 33, wherein said SiAlON polytypoid of said product has an aluminium content derived at least in part from a component of said powdered source material selected from elemental aluminium, alumina, aluminium silicon carbide, SiAlON polytypoid, aluminium silicon nitride, aluminium silicon oxynitride and aluminium silicate.

35. A process according to claim 33 or claim 34, wherein said SiAlON polytypoid of said product has an oxygen content derived at least in part from a component of said powdered source material selected from silica, SiAlON polytypoid, silicon oxynitride, aluminium silicate and alumina.

36. A process according to any one of claims 33 to 35, wherein said SiAlON polytypoid of said product has a nitrogen content derived at least in part from a component of said powdered source material selected from SiAlON polytypoid, silicon nitride, silicon oxynitride, aluminium silicon nitride and aluminium silicon oxynitride.

37. A process according to any one of claims 33 to 36, wherein said SiAlON polytypoid of said product contains other metal species substituted in part for silicon or aluminium, said other metal species being selected from lithium, beryllium, calcium, magnesium, scandium, yttrium and rare earth metals derived from a component of said powdered source material.

38. A process according to any of claims 16 to 37, wherein said compact is heated under pressure to produce said densified sintered product.

39. A process according to claim 38, wherein said compact is heated under pressure by being subjected to hot isostatic pressing.

40. A process according to any one of claims 16 to 37, wherein said compact is heated under pressureless .sintering conditions to produce said densified sintered product.

41. A process according to any one of .claims 16 to 40, wherein said compact is heated to effect sintering at a temperature of from 1600°C to 2200°C.

42. A process according to claim 41, wherein said temperature is from 1800°C to 2050°C.

43. A process according to any one of claims 16 to 42, wherein said silicon carbide and powdered source material has a grain size of less than 10 microns.

44. A process according to claim 43, wherein said grain size is less than 5 microns.