Process for producing a dense ceramic product

A process for the production of dense polycrystalline silicon carbide shaped articles; includes (a) heating a powder compact containing silicon carbide and alumina or a precursor thereof to an intermediate temperature, and maintaining the said intermediate temperature for an extended dwell, and then (b) heating the product of step (a) to a higher temperature for sufficient time to produce a dense polycrystalline silicon carbide product.

This invention relates to a process for the production of dense 
polycrystalline silicon carbide shaped articles and the superior shaped 
articles produced by the process. In one aspect the invention provides a 
new firing cycle for the production of dense polycrystalline silicon 
carbide shaped articles. In accordance with this firing cycle, powder 
compacts containing silicon carbide and alumina or a precursor thereof are 
first heated to an intermediate temperature, as will be explained in more 
detail below. After an extended dwell at the intermediate temperature, the 
temperature is raised in a second stage to a higher temperature to 
complete the densification process. 
In the second stage, magnesia or a precursor is thereof may optionally be 
present as a sintering assist, and the atmosphere is essentially carbon 
monoxide. It is preferred that the original compact contains magnesia, or 
a precursor thereof, but the magnesia may be introduced during the firing 
cycle. Depending upon the particular conditions, it may also be 
advantageous to have a controlled amount of carbon present, as will be 
explained in more detail below. In a preferred embodiment of the 
invention, the first stage is carried out in an inert atmosphere such as 
argon, and the atmosphere is changed after the intermediate temperature 
dwell, for example by purging the furnace chamber and introducing carbon 
monoxide for the remainder of the firing cycle. 
BACKGROUND OF THE INVENTION 
Shaped articles comprising high density polycrystalline silicon carbide are 
well known. They are characterised by excellent physical properties such 
as high resistance to thermal shock, abrasion and oxidation together with 
high levels of strength and thermal conductivity. It is this combination 
of properties which makes silicon carbide materials leading candidates for 
engineering applications. However, the production of satisfactory high 
density materials has been fraught with difficulties. 
Early workers (eg. Alliegro, Coffin and Tinkepaugh J. Amer. Ceram. Soc., 39 
11! 386-89 1956!) showed that silicon carbide could be hot pressed to 
high density bodies with the aid of sintering aids such as aluminium and 
iron, and aluminium plus one of the metals zirconium, boron or iron. It 
was further disclosed that for the hot pressing of silicon carbide, 
magnesium additions and magnesium and aluminium additions were 
ineffective, and impaired the densification process as compared to a 
control sample of silicon carbide, hot pressed under identical conditions 
with no additives or additions. Lange (J. Mater. Sci. (10 1975!314-320) 
disclosed the hot pressing of silicon carbide using alumina as the 
densification aid. The limitations of hot pressing for the attainment of 
dense bodies are well known. 
The selection of suitable densification aids for the sintering of silicon 
carbide has been considered by Negita (J. Am. Ceram. Soc. 6912!C308-10 
1986!). Using thermodynamic arguments it was found that metal additives 
such as boron, aluminium, iron, nickel and cobalt could be effective 
densification aids. Using these principles, alumina, beryllia, yttria, 
hafnia and rare earth oxides are considered to be potential densification 
aids as they do not decompose silicon carbide during sintering. Metal 
oxides including zirconia, calcia, magnesia are not considered suitable as 
they tend to decompose silicon carbide to metallic silicon. In addition 
the use of carbon with metal oxide additions was reported to be beneficial 
for oxides such as alumina, beryllia, ytrria, rare earth oxides, calcia, 
zirconia, and hafnia. It is stated that the carbon is added to react with 
the said oxides to form the corresponding metal carbide and silicon metal. 
The formation of the metal carbides was seen as desirable. In the process 
according to the present invention, the formation of such metal carbides 
was not observed. Furthermore, in contrast to the work of Negita, in the 
current work it has been found that the reaction of carbon with the metal 
oxide densification aids is undesirable and impairs the densification of 
the bodies. This indicates that the role of carbon in the present work is 
different to that proposed by Negita and others. In addition, given the 
unstable nature of metal carbide phases, for some even in air at room 
temperature, the formation of such phases is seen as undesirable and are 
avoided in the present invention. This aspect will be discussed in greater 
detail for the calcia system. 
The work of Cutler and Miller (U.S. Pat. No. 4,141,740) describes a process 
for a refractory product based on silicon carbide containing at least 1% 
by weight of aluminium nitride and at least 1% by weight aluminium 
oxycarbide. The presence of metal impurities (other than aluminium and 
silicon) were seen to be detrimental to the process and are limited to 0.1 
percent by weight or less. No indication was given as to the properties of 
such bodies and the ease at which they can be made into dense bodies with 
desirable physical properties and the commercial utility of the process. 
Further work in this system was described by Virkar et al in International 
Patent application WO87/01693, where the pressureless sintering of silicon 
carbide-aluminium nitride-aluminium oxycarbide containing materials was 
described. A major drawback of the process as disclosed is that the 
materials must be heated at very rapid rates to minimize volatilisation of 
the active densification species. This could pose problems for the 
production of large parts in which differential sintering as a result of 
thermal gradients can lead to distortion and ultimately to micro cracking 
due to thermal stresses inevitably present as a result of the described 
firing cycles. This would make the maintenance of the desired physical 
properties difficult. In addition, the undesirable presence of aluminium 
oxycarbides in the final body may prove difficult to avoid. 
In Suzuki et al. (U.S. Pat. No. 4,354,991) the use of aluminium oxide to 
densify silicon carbide is described. In the process as described the use 
of non-oxidative atmospheres is taught. These include nitrogen, carbon 
monoxide, helium and argon. It is taught that argon or helium are 
preferable and that the atmosphere should preferably contain aluminium, 
silicon or carbon. In one method, it is proposed that mixtures of these 
gases be fed into the reaction chamber with a carrier gas such as 
nitrogen, argon and helium. In another method, the use of a powder bed or 
sintered product capable of generating the gases around the silicon 
carbide article to be densified was disclosed. It was a teaching of the 
document that it is unnecessary to remove the silica present on the 
surface of the silicon carbide. In fact it was stated that it is feasible 
to add silica as a raw material. This is in contrast to the present 
invention where the presence of this phase has been found to exert a 
deleterious effect on the densification behaviour at high temperatures and 
will be explained later on. The fired bulk densities obtained were 
inferior to those achieved by the present invention. In addition, the 
sintering times were much longer. For a continuous process for the 
densification of bodies, the significantly longer reaction times for 
densification would result in lower production rates. 
In the work of Fuentes U.S. Pat. No. 4,876,226 the use of alumina and 
calcia as densification aids for silicon carbide was disclosed. It was a 
requirement of the invention to form liquid phases comprising aluminium 
oxycarbides at the sintering temperatures to promote densification. The 
addition of calcia was to increase the amount of the aluminium oxycarbide 
liquids and enhance densification. It was further disclosed that the 
addition of free carbon is preferred. It is believed that in the system 
described by Fuentes, the addition of free carbon is to react with 
aluminium containing phases to produce or further enhance the formation of 
the oxycarbide phases which are a requirement for the process. The level 
of free carbon additions were prefereably greater than 0.4% by weight. No 
indication was given to an upper limit for the carbon addition. This is in 
contrast to the teachings of the present invention, where the reaction of 
carbon with the aluminate phase is believed to be detrimental to the 
densification process. It is claimed that the technique excludes the use 
of rare earths but no reference was given for their deletion. Indeed, in 
the work of Omori et al (U.S. Pat. No. 4,569,921) the use of calcia and 
precursors for the oxides of aluminium and rare earth elements for the 
pressureless sintering of silicon carbide was disclosed with excellent 
results. In addition, it has been reported by Foster et al (J. Am. Ceram. 
Soc. 391!-111956!) that aluminium carbide and aluminium oxycarbide, the 
latter at least required for the process as outlined, are very unstable 
towards both moisture and oxygen. They taught that these materials should 
not be used in applications where these species are likely to be 
encountered. In the process as disclosed, such aluminium oxycarbide 
species are a key element of the process. The presence of such species is 
expected to greatly degrade the performance and severely limit the 
suitability of the said materials. In the present invention, aluminium 
oxycarbides, such as Al.sub.4 O.sub.4 C and Al.sub.2 OC, have not been 
observed and their presence is not a prerequisite for the process 
disclosed herein. Thus the process and product as disclosed herein 
overcomes significant disadvantages of the process as disclosed by 
Fuentes. 
The use of rare earths and alumina as sintering assists for silicon carbide 
has been disclosed (eg see Mulla and Krstic Bull. Amer. Ceram. Soc. 
703!439-443 1991!). In order to obtain high density bodies, the 
components had to be encapsulated in closed threaded graphite crucibles 
sealed with graphite foil. It was revealed that bodies could be produced 
with over 95% of theoretical density and weight losses of less than 1 
percent. When the same experiments were carried out without encapsulation, 
the resulting bodies obtained less than 80 percent of theoretical density 
and weight losses up to 20 percent were recorded. Culter and Jackson (pp 
309-318 in Ceramics Materials and Components for Engines, Proceedings of 
the Third International Symposium, Las Vegas Nev. 1988) also disclosed the 
use of yttria and alumina for the sintering of silicon carbide. Although 
high density bodies could be produced, the recorded weight losses were 
high and increased with increasing temperature. The decomposition 
reactions between the sintering assists and the silicon carbide were cited 
as a major problem. As in the case of Mulla and Krstic only very short 
times were used, typically of 5 minutes duration at the maximum 
temperature. The requirement to subject samples to minimum times at high 
temperatures is considered difficult to carry out on a commercial scale 
especially for the manufacture of larger components or where large furnace 
loads are used. 
This can lead to large thermal gradients giving rise to problems such as 
differential sintering, leading to distortion of the fired bodies. This 
would greatly reduce the application utility of the process. 
DESCRIPTION OF THE INVENTION 
This invention provides a dense silicon carbide product and a method of 
producing the same without the use of pressure assisted processes such as 
hot pressing or hot isostatic pressing, the use of boron or boron 
compounds and carbon, the use of powder beds or the requirement for sealed 
containers. It is an object of the present invention to overcome the 
difficulties of existing technologies and produce useful products based on 
silicon carbide. This is achieved by the addition of sintering assists, 
and by providing an environment in terms of temperature and atmosphere 
that is conducive to densification. It is believed that the densification 
of the powder compacts is a result of a liquid phase sintering mechanism 
(LPS). 
In the sintering of silicon carbide by techniques such as LPS, it is 
believed that it is crucial to maintain the effective sintering aids 
within the body. In the temperature range at which densification occurs, 
these additives react to form a liquid phase into which silicon carbide 
has some degree of solubility. In the initial stages, the liquid which 
forms at high temperatures allows densification by particle rearrangement. 
This is usually followed by a solution precipitation step followed by a 
stage characterised by grain growth. If, as appears to be the case, it is 
the liquid phase that is facilitating the densification, the premature 
loss of sintering aids is to be avoided. The use of excessive amounts of 
sintering aids to compensate for the loss thereof is also to be avoided as 
this results in increased cost, and can also result in the deposition of 
unwanted species in the cooler parts of the furnace. In addition, the loss 
of sintering assists can result in compositional gradients. These species 
can be unstable with regard to the atmosphere and can result in damage to 
the furnace and make the manufacture of components hazardous. Thus, it is 
important in the efficient manufacture of components to minimise the loss 
of raw materials. 
The use of powder beds in the manufacture of LPS silicon carbide is known. 
The role of a powder bed is to provide a suitable environment. However, 
there are major drawbacks to the use of powder beds. These are: 
Additional cost of the powder bed. 
Additional costs associated with the increased number of handling 
operations such as loading and (after the firing) unloading the samples 
from the powder bed. 
Poor surface finish especially where the samples are in prolonged contact 
with the powder bed. 
The powder bed by providing active species for the densification can result 
in warping or distortion of the bodies as a result of gradients of 
densification aids therein, and densification gradients which can also 
lead to warping and distortion of the bodies. 
The use of sealed containers to encapsulate the bodies to avoid the loss of 
volatile constituents also imposes limitations for large scale production 
of bodies, as a result of additional costs associated with increased 
number of unit operations required to produce components, such as the 
requirement to load the samples into the sealed containers. 
DETAILED DESCRIPTION 
Although it will be clearly understood that we do not wish to be limited by 
any postulated or theoretical mechanism for the observed beneficial 
results of the process of the present invention, we do offer the following 
discussion of what is believed to be the underlying chemistry involved. 
According to the present invention a dense product is produced which 
contains at least 65 weight percent silicon carbide with the remainder 
substantially an aluminate. Part or all of the aluminate may also 
optionally contain magnesium in the form of spinel. The spinel phase is 
not necessarily stoichiometric with respect to the magnesium to aluminium 
ratio. The preferred composition range, expressed as the equivalent amount 
of oxide, for the product is alumina 3 to 35 weight percent; and magnesia 
0.01 to 5 weight percent. In addition, the product may optionally contain 
silicon, aluminium or glassy phases or a combination of the said phases. 
The presence of alumina in the .alpha.-corundum form is typical when the 
aluminium to magnesium ratio in the samples is high and or the firing 
temperatures in the upper part of the specified range are employed. 
Useful materials can be produced when the sintering aids are added to 
finely divided silicon carbide powder and the resultant mixture can be 
processed using traditional ceramic processing techniques to form 
consolidated powder compacts. The said materials are heated using a 
two-stage firing cycle and densification of the body results. The 
materials can be conveniently densified in the temperature range of 
1700.degree. C. to 2200.degree. C., with or without the application of 
pressure. It is appreciated that the application of pressure can be useful 
in reducing the temperature at which the densification is carried out in 
order to produce a dense body, but is not a prerequisite for the process. 
It is thought that the additives interact to form a liquid which promotes 
densification by a liquid phase sintering process. The use of magnesium 
and aluminium (both in the form as either oxides or their precursors; or 
alternatively as a magnesium spinel or a mixture of the aforementioned 
species, facilitates densification and the ultimate formation of an 
aluminate grain boundary phase. Depending on the starting compositions and 
the firing cycle employed, the aluminate may be present in total or part 
as a magnesium aluminate (or magnesium spinel). Furthermore, magnesium 
spinel can exist as a solid solution (Mgo).sub.x.(Al.sub.2 O.sub.3), where 
x is less than or equal to 1. Thus the grain boundary phase can 
accommodate shifts in chemical composition without the formation of 
additional phases. This is an important consideration for a high 
temperature reaction such as the pressureless sintering process, where the 
control and maintenance of volatile species (such as magnesium and 
aluminium) could prove difficult and expensive to carry out in practice. 
At the higher temperatures used to promote densification the said magnesium 
spinel which forms in the body or is deliberately added, may undergo 
further change leading to an increase in the aluminium to magnesium ratio. 
This may continue to the point where the magnesium is essentially lost 
from the body. It is also possible that aluminium may be lost from the 
bodies during the firing cycle. If the process continues still further, 
ultimately aluminium metal or aluminium alloy may be detected in the body. 
The attainment of a suitable environment is a requirement for the 
successful densification of silicon carbide by the method of the present 
invention. It is an aim of the present invention to provide conditions 
conducive for the densification of the bodies. It is considered that the 
formation of suitable liquids is a requirement of the process to produce 
high density bodies. In this regard it is important that the active 
species, responsible for the densification of the body, are retained in 
the body until such times as the densification is completed or reaches 
such levels that other mechanism(s) can lead to the attainment of a high 
density bodies. This is common to liquid phase sintering techniques where 
it is a requirement that the liquid be stable. In addition, it is a 
condition that unwanted components are removed by such techniques as 
volatilisation or decomposition and formation of phases that assist or do 
not interfere with the densification processes. 
There are number of conditions that must be satisfied in order to obtain 
useful products. These conditions relate to ensuring that the silicon 
carbide and the liquid phase responsible for densification, are stable at 
the densification temperature. At the high temperatures required for 
densification, the solid and liquid phases require vapour pressure of 
elements, sub oxides and other vapour species to remain stable. The bodies 
generate their own stable atmosphere through partial decomposition of the 
said phases, but this decomposition should not be such that the generation 
of the stable atmosphere depletes the liquid phase to such an extent that 
densification is so retarded or inhibited that it is not possible to make 
dense bodies. 
It is advantageous to maintain a favourable effective sample volume to 
furnace volume, thus limiting the amount of decomposition of the solid and 
liquid phases to gaseous species and ensuring an adequate level of 
additives remain in the body to obtain a dense body. This coupled with a 
carbon monoxide atmosphere in the second stage of the process, are 
successfully used to suppress the amount of decomposition of these phases 
without the need to resort to other means, such as powder beds or the 
introduction of active species into the hot zone from external sources. 
The importance of this aspect will be illustrated with some of the 
findings of this investigation. 
For the production of bodies with thicker cross sections, significant 
differences in fired bulk density can occur on the inside with respect to 
the outside of the body (coring). This can ultimately lead to cracking of 
the body after fabrication. It is believed this behaviour is the result of 
the presence of phase(s) that interfere with the densification processes 
by either changing the nature of the liquid phases or which decompose at 
the middle to higher temperatures used. The densification of the bodies is 
such that it is difficult to eliminate cores from the interior of the 
thicker bodies. From the foregoing it can be appreciated that the critical 
thickness above which the formation of cores is a problem will depend on a 
number of factors such as heating rate, stacking geometry of samples in 
the furnace, effective gas flow rate, level of porosity and pore size of 
the body. 
The use of argon for the sintering of samples results in the formation of 
dense bodies. However, an unwanted competing reaction is the decomposition 
of the silicon carbide. It is considered that this is a consequence of the 
reaction of the sintering assists with the silicon carbide. This leads to 
the formation of silicon which condenses on the bodies with a 
deterioration of the surface finish. It can also lead to the condensation 
of silicon on the contact points of the samples with furnace furniture or 
other bodies making the removal or separation of the bodies extremely 
difficult. It is anticipated that the generation of silicon is the result 
of the unwanted reaction of alumina with the silicon carbide (for example 
see reaction 1): 
EQU 2SiC+Al.sub.2 O.sub.3 .fwdarw.2Si.sub.(l) +Al.sub.2 O(.sub.g) +2CO.sub.(g)( 
1) 
The condensation of the silicon can be avoided by the use of an atmosphere 
containing carbon monoxide. This leads to a greatly improved surface 
finish. To suppress reaction (1), at the upper end of the temperature 
range used for the densification, a partial pressure of carbon monoxide 
greater than one atmosphere is required. This temperature is estimated to 
be of the order of 2100.degree. C. It has been found that at low 
temperatures there are unfavourable reactions between the powder compacts 
and the cabon monoxide furnace atmosphere. It is anticipated that below 
roughly 1550.degree. C. the following reaction is thermodynamically 
feasible and can proceed left to right. 
EQU SiC+2CO.fwdarw.SiO.sub.2 +3C (2) 
Above roughly 1550.degree. C. in one atmosphere of carbon monoxide reaction 
2 goes from right to left (see reaction 3). 
EQU SiO.sub.2 +3C.fwdarw.SiC+2CO (3) 
In addition, it is also possible that some of the silica formed by reaction 
2 can react according to reaction 4. 
EQU SiC+SiO.sub.2 .fwdarw.2SiO.sub.(g) +C (4) 
Further, the free carbon present can react with the aluminate phase leading 
to its decomposition and impairing the densification process (see reaction 
5). 
EQU Al.sub.2 O.sub.3 +2C.fwdarw.Al.sub.2 O.sub.2 O.sub.(g) +2CO(5) 
It must be appreciated, that as a raw material, the silicon carbide powder 
is inevitably covered by an oxide layer of silica. As silica is 
detrimental to the densification of bodies, it must be removed. This can 
be achieved by decomposition to gaseous species such as silicon monoxide 
(see reaction 6). 
EQU 2SiO.sub.2 .fwdarw.2SiO.sub.(g) +O.sub.2(g) ( 6) 
For thinner bodies this reaction can proceed with the silica being removed 
by a decomposition reaction such as reaction 6. However, for thicker 
bodies the time required to remove this phase is appreciable. An 
alternative is the deliberate addition of carbon to react with the silica 
phase (reaction 3) to produce silicon carbide in the lower to mid 
temperature ranges of the firing cycle. Depending on the thickness of the 
components, stacking geometry and the furnace configuration the use of 
dwells can prove beneficial to ensure that this phase is removed. The 
carbon addition should be added such that it is sufficient to remove the 
silica phase only to prevent unwanted reactions such as reaction 5. The 
dwells or slow heating rates are also important to ensure that the amount 
of gas liberated, does not cause a pressure build up greater than the 
cohesive strength of the compact, leading to the formation of cracks. The 
use of slow heating rates is in to contrast with much of the teachings of 
the prior art. 
Thus the presence of either or both the silica or carbon at the higher 
temperatures at which densification occurs, react to produce an unstable 
phase or phases. It is believed that internal stresses are generated 
within the body when the internal pores are closed off from the outside 
atmosphere. At high temperatures these stresses are sufficient to overcome 
the cohesive strength of the compact, resulting in the rupture of the 
body. This manifests itself in the bodies after firing exhibiting macro 
cracks. 
The use of nitrogen for the densification of bodies is not suitable as a 
result of unfavourable reaction of the constituents of the bodies with the 
atmosphere to form stable compounds. These compounds reduce the amount of 
liquid present at high temperatures available for densification. It is 
considered that this is the result of the reaction of alumina with the 
silicon carbide in a nitrogen atmosphere to produce aluminium nitride. The 
formation of stable aluminium nitride is accompanied by a corresponding 
decrease in the amount of liquid phases necessary to allow densification 
to proceed by liquid phase sintering. A possible reaction is as follows: 
EQU 3SiC+2Al.sub.2 O.sub.3 +2N.sub.2(g) .fwdarw.4AlN+3SiO.sub.(g) +3CO.sub.(g)( 
7) 
This demonstrates the importance of maintaining the presence of the liquid 
phase at least until densification is essentially completed. 
It has been found that the use of an inet atmosphere below 1550.degree. C. 
and preferably at least 1600.degree. C., but below the temperatures at 
which the onset of the unwanted reactions such as reaction 1 occur, is 
beneficial for the production of useful products. Thus at the temperatures 
at which there are unfavourable reactions between the silicon carbide 
based bodies and the carbon monoxide atmosphere, the use of an inert 
atmosphere such as argon or possibly nitrogen can be used. Above this 
temperature the furnace chamber is purged and carbon monoxide is 
introduced into the hot zone for the remainder of the firing cycle. The 
temperature in the reaction zone is such that the unwanted and detrimental 
reaction which leads to cracking of the body does not take place. In 
addition, the unwanted formation of silicon on the body is avoided. 
An alternative is to use reduced pressure in the furnace chamber below 
1550.degree. C. Thus the unwanted reactions of carbon monoxide with the 
compacts is avoided. Above this temperature, carbon monoxide is introduced 
into the reaction chamber to prevent the unwanted formation of silicon at 
high temperatures. 
When it is the intention to produce bodies with thicker cross sections, it 
has been found that it is advantageous to take steps to deliberately 
remove the silica from the system as this phase, or the oxide-based 
reaction product formed from the said phase with other phases present, 
interferes with the uniform densification of the said bodies. For such 
bodies it can prove difficult to remove this phase by processes such as 
decomposition and diffusion. This phase as described can persist at higher 
temperatures. It has been found that carbon added according to equation 
(3) to react with the silica present on starting materials is beneficial 
in eliminating the presence of low density cores in the fired samples. 
Care has to be taken to ensure that the evolution of gaseous species 
expected to be carbon monoxide does not lead to the cracking of the 
bodies. Further to this it has been found beneficial to hold the sample at 
a temperature at which the reaction (3) is feasible and can proceed left 
to right at an appreciable rate to ensure that the reaction is essentially 
complete before heating to a higher temperature but the rate of release 
does not lead to cracking of the bodies. Note, it is thought that reaction 
3 should be essentially completed before heating above the equilibrium 
temperature for the reaction in one atmosphere of carbon monoxide. Under 
these conditions the generation of carbon monoxide can exceed one 
atmosphere and produce conditions which are condusive to cracking. Adding 
too much carbon according to reaction (3) for the amount of silica present 
leads to an observed decrease in the fired bulk density. It is believed 
this is as a result of the unwanted reaction of the residual carbon with 
oxides phases. This is in contrast to the work reported by Fuentes where 
the reaction of carbon with aluminium containing phases to produce 
oxycarbides is beneficial. 
It is hypothesised that when carbon monoxide atmospheres are used 
exclusively, the use of extended dwells above the 1600.degree. C. may 
overcome the problems associated with the use of this atmosphere at low 
temperatures that manifests itself in the macro cracking of the samples. 
It is the intention that the use of an interrupted firing cycle with a 
dwell above that temperature at which the carbon monoxide adversely reacts 
with the bodies but below the temperatures at which densification occurs 
leading to sealing off of the internal pores from the surface, thereby 
trapping volatile products in the bodies could prove advantageous. It is 
speculated that the use of temperature dwells above about 1550.degree. C. 
say 1600.degree. C., but below the onset of extensive densification, in 
particular before sealing of porosity of the bodies, estimated to be 
around 1900.degree. C. could prove to be effective in preventing cracking. 
The firing cycle would be continued after such times as the unwanted 
species have been removed and the sample would then be heated to the final 
sintering temperature to densify the powder compact into a dense useful 
body. 
The importance of the requirement to retain the densification aids has been 
discussed. At the high temperatures used for the densification of the 
bodies, the vapour pressures of the various elements and other species can 
be significant. A consequence of this is to limit the unwanted reactions. 
In a static environment, the vapour pressure of a species will be 
determined by thermodynamic and kinetic considerations. At the 
temperatures used, the rates of the reactions are typically rapid and thus 
the main consideration are the thermodynamics of the reactions. The amount 
of material in the hot zone is an important variable. If insufficient 
material is fired, the amount of decomposition (reflected in the weight 
loss after firing) will be unacceptable and in extreme cases densification 
of the body will not be achieved. The decomposition is the result of the 
generation of the vapour species in the atmosphere. The actual partial 
pressure of the species can be predicted using thermodynamic calculations. 
The vapour pressure is thus limited to either the equilibrium partial 
pressure or until all the phase in question is consumed in trying to 
establish the equilibrium partial pressure. Depending on parameters such 
as the effective furnace volume and its construction, as outlined 
previously, it is possible to limit the amount of decomposition and 
generation of a stable partial pressure of volatile species without 
significantly changing the starting composition of the samples and 
adversely affecting the densification of the body. Furthermore, by 
incorporation of some of the gaseous reaction products, specifically 
carbon monoxide, into the furnace atmosphere from external sources it is 
possible to minimise decomposition of the samples using Le Chatelier 
principle. 
A method of forming a dense silicon carbide product is disclosed without 
the use of boron, or boron containing compounds and carbon associated 
problems of uncontrolled grain growth degrading the physical properties or 
alternatively the use of powder beds or the use need to introduce active 
densification aids into the furnace chamber via the furnace atmosphere. 
The present invention will be further illustrated by examples in a non 
limiting manner.

EXAMPLES 
In the following examples all quantities are expressed in parts by weight, 
unless otherwise specified. 
Example 1 
The raw materials used were a silicon carbide from Lonza known as grade 
UF10; alumina from Alcoa known as grade A16SG; and magnesia from Ajax 
(Analytical Grade). The powders were weighed and ball milled using silicon 
carbide milling media. The conditions used for this operation are shown in 
table 1. 
TABLE 1 
______________________________________ 
Conditions used for ball milling operation 
______________________________________ 
Time 16 hours 
Powder 300 g 
Milling Media 1500 g 
Fluid 600 ml iso-propanol 
Binder 2 wt % 
______________________________________ 
After milling, the milling media was separated from the slurry which was 
subsequently spray dried. The powder was unaxially pressed using a steel 
die and punch set and cold isostatically pressed at a pressure of 150 MPa 
into a 70 mm square tile. The sample was then heated in air to 400.degree. 
C. and held for 5 hours to remove the binder. 
The sample was placed in a graphite work box with a loose fitting lid. The 
work box was heated in a graphite resistance furnace in an atmosphere of 
argon. At 1600.degree. C. the furnace was evacuated and backfilled with 
carbon monoxide (see table 2). The specimen after firing had a smooth 
surface finish. The starting compositions and results are given in table 
3. 
TABLE 2 
______________________________________ 
Firing Cycle 
______________________________________ 
Heat at 1400.degree. C. min.sup.-1 in Ar 
Hold for 60 minutes 
Heat to 1600.degree. C. at 50.degree. C. min.sup.-1 
Hold for 30 minutes 
Evacuate and backfill with CO 
Heat to 1900.degree. C. at 5.degree. C. min.sup.-1 
Heat to 2030.degree. C. at 2.5.degree. C. min.sup.-1 
Hold for 60 minutes 
Cool at 10.degree. C. min.sup.-1 
______________________________________ 
TABLE 3 
______________________________________ 
Sample Details 
Example 1 2 3 4 
______________________________________ 
SiC 87.8 87.8 87.8 87.8 
Al.sub.2 O.sub.3 
10.7 10.7 10.7 10.7 
MgO 1.4 1.4 1.4 1.4 
Atmosphere Ar/CO Ar CO N.sub.2 
Mass (g) 164 174 179 147 
GBD(g .multidot. cc.sup.-1) 
1.70 1.69 1.72 1.75 
FBD(g .multidot. cc.sup.-1) 
3.16 3.08 3.16 2.07 
Wt Change % -5.9 -8.7 -8.2 -10.7 
Comments 1 2, 3 4 
______________________________________ 
Where 
1 Good surface finish 
2 Silicon on surface 
3 Poor surface finish 
4 Cracked 
5 Sample porous 
GBD Green Bulk Density 
FBD Fired Bulk Density 
Comparative Example 2 
The specimen preparation and the firing cycle were the same as for Example 
1 with the exception that there was no evacuation step with backfill at 
1600.degree. C. and an argon atmosphere was used exclusively. After firing 
the specimen had surface deposits of silicon (see table 3). 
The surface finish of the specimen was rough in comparison the specimen 
prepared according to example 1. 
Comparative Example 3 
The specimen preparation and the firing cycle were the same as for Example 
2 with the exception that carbon monoxide was used for the complete firing 
cycle. 
After the firing operation, the surface finish of the specimen was the same 
as example 1, however, the sample was badly macro cracked and distorted 
(see table 3). 
Comparative Example 4 
The specimen preparation was the same as Example 2 except that a disc 65 mm 
in diameter was used. The firing cycle was the same as Example 2 except 
that nitrogen was used. After the firing the sample had not densified to 
any appreciable extent (see table 3). 
From the results in table 4 it can be seen the advantage of the two stage 
firing cycle. This results in a higher fired bulk density than the use of 
argon or nitrogen atmospheres and eliminates the macro cracking observed 
with the use of carbon monoxide only atmospheres. In addition, it is 
possible to use relatively slow firing cycles and still produce useful 
components. This is important for the mass production of samples using 
large furnace loads or the manufacture of large components where it is not 
often not practical or feasible to use rapid heating rates. 
Comparative Examples 5-8 
Samples were prepared according to Example 1 with the exception that discs 
65 mm in diameter were used. The firing cycle used was identical to that 
used for Example 3 except for the dwell time at the maximum temperature. 
The results of the firings are given in table 4. 
TABLE 4 
______________________________________ 
Sample Details 
Example 5 6 7 8 
______________________________________ 
SiC 87.8 87.8 87.8 87.8 
Al.sub.2 O.sub.3 
10.7 10.7 10.7 10.7 
MgO 1.4 1.4 1.4 1.4 
Atmosphere CO CO CO CO 
Mass (g) 147 147 147 147 
GBD(g .multidot. cc.sup.-1) 
1.73 1.78 1.78 1.77 
Time (min) 0 30 45 60 
FBD(g .multidot. cc.sup.-1) 
2.86 3.02 3.07 3.13 
Wt Change % -4.8 -6.2 -7.5 -8.7 
Comments Cracked 
______________________________________ 
From the results listed in table 4, cracking only occurs at the later 
stages of the densification. Furthermore, it is thought that this 
behaviour occurs after the sample has reached the closed porosity stage. 
It is well known that there is a general inverse relationship between the 
level of porosity and strength for such materials. The inability to 
produce crack free bodies with low levels of porosity would greatly limit 
the application of such materials. 
Comparative Examples 9-13 
The samples were prepared according to the method of Example 1, with the 
exception that 25 mm discs were used. For samples heated to 1400.degree. 
C. and higher in the atmospheres indicated, the samples were held at this 
temperature for 60 minutes prior to heating the maximum temperature. 
Samples 9, 11 and 12 were heated to the maximum temperature and cooled. 
The results of the firing are given in table 5. 
TABLE 5 
______________________________________ 
Sample Details 
Example 9 10 11 12 13 
______________________________________ 
SiC 87.8 87.8 87.8 87.8 87.8 
Al.sub.2 O.sub.3 
10.7 10.7 10.7 10.7 10.7 
MgO 1.4 1.4 1.4 1.4 1.4 
Atmosphere 
CO CO CO CO Ar 
Max. Temp (.degree.C.) 
1100 1400 1600 1900 1409 
GBD(g .multidot. cc.sup.-1) 
1.72 1.74 1.75 1.73 1.72 
FBD(g .multidot. cc.sup.-1) 
1.72 1.82 1.81 2.09 1.72 
Wt Change % 
-0.0 +6.2 +3.8 -2.8 -0.9 
Colour Grey Black Black Grey Grey 
______________________________________ 
From the results of Examples 10 and 11 (see table 5), it can be seen that 
heating the samples in a carbon monoxide atmosphere at 1400.degree. C. 
results in a weight gain. It is thought that the colour change of the 
samples from grey in the as pressed state to black, was the result of 
carbon formation within the samples. Furthermore, the formation of carbon 
occurs by oxidation of the silicon carbide according to reaction 2. From 
the results of Example 9, the weight change behaviour is not the result of 
the deposition or cracking of the carbon monoxide within the samples. 
Heating of the samples to higher temperatures results in a weight loss and 
colour change from black to the original grey colour (see Example 12). 
This behaviour is attributed to the carbothermal reduction involving the 
carbon and silica formed at lower temperatures according to reaction 3. It 
is thought that the presence of the silica has an adverse effect on the 
densification of silicon carbide. For a sample heated in an argon 
atmosphere (Example 13), there was no weight increase or colour change, 
indicating no reaction between the sample and the atmosphere. 
Comparative Examples 14-17 
The effect of additives on the oxidation of silicon carbide in a carbon 
monoxide atmosphere was studied. The results are given in table 6. 
TABLE 6 
______________________________________ 
Effect of composition on weight change behaviour in CO at 
1400.degree. C. for 60 minutes 
Example 14 15 16 17 
______________________________________ 
SiC 100 90.0 98.6 87.8 
Al.sub.2 O.sub.3 
0 10.0 0 10.8 
MgO 0 0 1.4 1.4 
Mass(g) 5 5 5 5 
GBD(g .multidot. cc.sup.-1) 
1.65 1.72 1.67 1.68 
FBD(g .multidot. cc.sup.-1) 
i.66 1.73 1.72 1.80 
Wt Change % 
+0.8 +0.9 +5.4 +9.2 
______________________________________ 
Silicon carbide powder, with and without the addition of alumina is 
relatively stable under these conditions as evidenced by the negligible 
weight changes. By contrast, the addition of magnesia results in a 
significant increase in weight being observed after firing. It is believed 
that this behaviour is the result of the reaction of the additives with 
the silica layer on the silicon carbide. This provides a rapid transport 
mechanism for the oxygen through the said layer to the silicon carbide. 
Examples 18-20 
The effects of residual silica, with and without the presence of free 
carbon, on the densification behaviour was studied. The samples were 
prepared as outlined for Example 5. The resulting powder slurries were 
spray dried and green discs (65 mm diameter) were unaxially pressed at 35 
MPa and wet bag CIP at 150 MPa. 
The samples were fired in an atmosphere of carbon monoxide atmosphere at 
1400.degree. C. to form silica and carbon (Example 18-a and 19-a). Example 
18-a was subsequently heated to 600.degree. C. in air to remove the 
carbon, and the product is designated Example 18-b. 
TABLE 7 
______________________________________ 
Sample Details 
Example 18-a 19-a 18-b 
______________________________________ 
SiC 87.8 87.8 87.8 
Al.sub.2 O.sub.3 
10.7 10.7 10.7 
MgO 1.4 1.4 1.4 
Atmosphere CO CO Air 
Max Temp (.degree.C.) 
1400 1400 600 
Mass (g) 177 178 187 
Wt Change % +5.4 +5.4 -2.8 
Comments 1 1 2 
______________________________________ 
Where 
1 Colour Black 
2 Light Grey 
The results show that firing in a carbon monoxide atmosphere resulted in an 
increase in the weight of the sample. In addition the sample changed 
colour from a light grey to a black colour. This change in colour was 
attributed to the formation of carbon in the sample. After heating at 
600.degree. C. in air (Example 18-b) there was a decrease in the weight of 
the sample and the colour changed back to its original colour. However, 
there was a net weight gain observed from the sample after the carbon 
monoxide and air firing. This was attributed to the formation of silica 
from the silicon carbide. 
These samples (18-b and 19-a) were heated using the firing cycle given in 
table 8, and the products are designated 18-c and 19-b respectively. 
TABLE 8 
______________________________________ 
Summary of Firing Cycles 
______________________________________ 
Heat to 1400.degree. C. at 10.degree. C. min.sup.-1 in Ar 
Hold at 1400.degree. C. for 60 minutes 
Heat 1400.degree. C. to 1600.degree. C. at 5.degree. C. min.sup.-1 
Hold at 1600.degree. C. for 30 minutes 
Evacuate to 1 mm Hg and fill with CO 
Heat 1600.degree. C. to 1900.degree. C. at 5.degree. C. min.sup.-1 
Heat 1900.degree. C. to 2030.degree. C. at 2.5.degree. C. min.sup.-1 
Hold at 2030.degree. C. for 60 minutes 
Cool to room temperature at 10.degree. C. min.sup.-1 
until the natural cooling rate takes over. 
______________________________________ 
The results are given in table 9. 
TABLE 9 
______________________________________ 
Sample Details 
Example 18-c 19-b 20 
______________________________________ 
SiC 87.8 87.8 87.8 
Al.sub.2 O.sub.3 
10.7 10.7 10.7 
MgO 1.4 1.4 1.4 
Mass (g) 182 188 178 
FBD (g .multidot. cc.sup.-1) 
2.94 3.16 3.19 
wt Change % 
-11.5 -10.0(-5.1).sup.2 
-5.2 
Comments 1 
______________________________________ 
Comments 
Where 
1 Badly distorted 
2 weight loss for sample before CO firing. 
The results demonstrate the adverse effects of silica on the densification 
of silicon carbide. Example 18-c (oxidised in a carbon monoxide atmosphere 
and subsequently heated in air to remove the carbon), underwent a large 
weight loss after densification at high temperature. The final bulk 
density was relatively low and the microstructure of the sample was 
characterised by porous regions. Example 19-b (fired in carbon monoxide 
atmosphere to 1400.degree. C. and refired using the two stage firing 
cycle), also exhibited a large weight loss. By contrast, the fired bulked 
density obtained after firing was comparable to the two stage firing. For 
Example 19-b, it is believed that the silica and carbon formed during the 
first firing cycle to 1400.degree. C., reacted to form SiC and carbon 
monoxide in the argon atmosphere in the second firing. It should also be 
noted that the overall weight change (for both firings) for Example 19 was 
very similar to that for Example 20 fired using the two stage firing 
cycle. This demonstrates the adverse effect of silicon carbide on 
densification and the its is possible to produce high density samples in a 
carbon monoxide atmosphere if the said phases which is inevitable formed 
at lower temperatures is removed from the body prior to the high 
temperature densification step. 
Examples 21-25 
To confirm silica that has an adverse effect on the densification of 
silicon carbide, free carbon was added to react with the silica. The same 
source of silicon carbide and magnesia as used for Example 1 were used to 
produce the samples. The alumina used was obtained from Alcoa was known as 
grade A1000 and the carbon source was a phenolic resin. The powders were 
batched and ball milled using silicon carbide milling media (see table 
10). 
TABLE 10 
______________________________________ 
Milling Conditions 
______________________________________ 
Time 16 hours 
Powder 350 g 
Balls 1500 g SiC 
Fluid 700 ml iso propanol 
______________________________________ 
The compositions of the batches are given in table 12. Note the ratio of 
Si:Al:Mg was kept constant for each mix. The weight loss after pyrolysis 
for the phenolic resin was 45.6%. Assuming a surface oxide layer of 3 
weight percent for the silicon carbide, 1.8 g of carbon would be required 
for each 100 g of silicon carbide for conversion of the said silica layer 
to silicon carbide. 
TABLE 11 
______________________________________ 
Composition in Parts 
Example 21 22 23 24 25 
______________________________________ 
SiC 89.0 89.0 89.0 89.0 89.0 
Al.sub.2 O.sub.3 
10.0 10.0 10.0 10.0 10.0 
MgO 1.0 1.0 1.0 1.0 1.0 
Resin 0.0 1.6 3.2 4.9 6.5 
(C) (0.0) (0.9) (1.8) (2.6) 
(3.5) 
______________________________________ 
The resulting powder slurries were spray dried and green discs (65 mm 
diameter and mass 250 g) were unaxially pressed at 35 MPa and wet bag CIP 
at 150 MPa. The samples were heated in nitrogen to pyrolyse the resin and 
form free carbon (see table 12). 
TABLE 12 
______________________________________ 
Curing and Pyrolysis Conditions in Nitrogen 
______________________________________ 
Heat to 100.degree. C. at 30.degree. C. h.sup.-1 
Hold for 1 h 
Heat to 150.degree. C. at 30.degree. C. h.sup.-1 
Hold for 1 h 
Heat to 600.degree. C. at 50.degree. C. h.sup.-1 
Hold for 1 h 
Cool at 200.degree. C. h.sup.-1 
______________________________________ 
Details of the firing cycle are given in table 13. 
TABLE 13 
______________________________________ 
Summary of Firing Cycles 
______________________________________ 
Heat to 600.degree. C. at 5.degree. C. min.sup.-1 in Ar 
Heat 600.degree. C. to 1400.degree. C. at 10.degree. C. min.sup.-1 
Heat 1400.degree. C. to 1550.degree. C. at 5.degree. C. min.sup.-1 
Hold at 1550.degree. C. for 30 minutes 
Heat 1550.degree. C. to 1650.degree. C. at 5.degree. C. min.sup.-1 
Hold at 1650.degree. C. for 30 minutes 
Evacuate to 1 Hg and fill CO gas 
Heat 1650.degree. C. to 1900.degree. C. at 5.degree. C. min.sup.-1 
Hold at 1900.degree. C. for 60 minutes 
Heat 1900.degree. C. to 2030.degree. C. at 2.5.degree. C. min.sup.-1 
Heat 2030.degree. C. to 2060.degree. C. at 1.5.degree. C. min.sup.-1 
Hold at 2060.degree. C. for 30 minutes 
Cool to room temperature at 10.degree. C. min.sup.-1 
until the natural cooling rate takes over. 
______________________________________ 
The results of the firing are listed in table 14. 
TABLE 14 
______________________________________ 
Effect of free carbon content on densification 
using a two stage firing 
Example 21 22 23 24 25 
______________________________________ 
SiC 89.0 89.0 89.0 89.0 89.0 
Al.sub.2 O.sub.3 
10.0 10.0 10.0 10.0 10.0 
MgO 1.0 1.0 1.0 1.0 1.0 
Resin 0.0 1.6 3.2 4.9 6.5 
Mass (g) 245 248 247 247 246 
GBD(g .multidot. cc.sup.-1) 
1.69 1.70 1.70 1.71 1.72 
Carbon Content 
0 0.9 1.8 2.6 3.5 
FBD(g .multidot. cc.sup.-1) 
3.13 3.20 3.19 3.12 3.04 
Wt Change % 
-6.3 -5.8 -5.7 -6.3 -6.3 
Comments 1 2 2 2 2 
______________________________________ 
Comments 
Where 
1 Low density core 
2 Uniform 
From the results in table 14, can be seen that the addition of small 
amounts of carbon greatly enhances the level of fired bulk density 
obtained (Examples 21 and 22). For higher levels of carbon addition, there 
is a decrease in fired bulk density (Examples 24 and 25). It is believed 
that this behaviour is the result of the unfavourable reaction of the 
carbon at high temperatures. Thus there exists an optimum range for the 
carbon content. It is important to note that for thinner samples the 
addition of carbon was not found to be necessary in order to produce high 
fired bulk density products. It is believed that for these samples the 
silica layer is able to decompose and can be removed from the powder 
compact. This becomes more difficult with increasing thickness of the 
samples, as it requires considerable time to decompose and remove the 
unwanted oxide from the powder compact. 
Example 26 
The reaction of the minor secondary phases was studied to determine the 
decomposition behaviour in regards to the liberation of gaseous phases 
such as carbon monoxide. The raw materials used to produce the sample were 
silica from Pennyslvania Glass Sand Corporation known as Min-u-Sil grade 5 
micron; alumina from Alcoa was known as grade A1000, magnesia from Ajax 
Chemicals (analytical grade) and the carbon source was a phenolic resin. 
The oxide only powders were batched and ball milled. The resin was added 
as a separate step to the pre milled and spray dried oxide powders. A 
green discs (38 mm diameter and mass 60 g) was unaxially pressed to form a 
compact. The starting composition used was formulated to correspond to the 
expected oxides and free carbon present in Example 22 and is given in 
table 15. 
TABLE 15 
______________________________________ 
Composition of Example 26 
Expected Oxide Composition 
Component Parts (Example 22) 
______________________________________ 
SiO.sub.2 17.5 2.7 
Al.sub.2 O.sub.3 
65.4 10.0 
MgO 6.5 1.0 
Resin 10.6 -- 
(Carbon) (5.7) (0.9) 
______________________________________ 
The resin in the sample was pyrolysed to carbon according to the method 
outlined in Example 22. Details of the firing cycle are given in table 16. 
The sample was fired in an open crucible in a graphite tube furnace. The 
net gas flow rate and the CO content were continuously measured. 
TABLE 16 
______________________________________ 
Summary of Firing Cycle 
______________________________________ 
Heat to 600.degree. C. at 10.degree. C. min.sup.-1 in Ar gas 
Heat 1400.degree. C. to 1650.degree. C. at 5.degree. C. min.sup.-1 
Hold at 1650.degree. C. for 30 minutes 
Cool to room temperature at 10.degree. C. min.sup.-1 
until the natural cooling rate takes over. 
______________________________________ 
The results obtained during the firing are shown in FIG. 1. The results 
indicate a small amount of carbon monoxide was liberated at 700.degree. C. 
with a corresponding increase in the net gas flow rate. The main out 
gassing started at 1300.degree. C. and was rapid above 1500.degree. C. The 
reaction was completed in a short time. The composition before and after 
firing is given in table 17. 
TABLE 17 
______________________________________ 
Atomic Ratios for Example 26 
Element Initial Ratio 
Final Ratio 
______________________________________ 
Al 1.00 1.00 
Si 0.23 0.20 
Mg 0.13 0.12 
C.sup.1 0.42 0.01 
______________________________________ 
Notes 
(1) Free Carbon 
The results indicate that the major change is the loss of the free carbon 
as carbon monoxide. There was also a small loss of silicon. This indicates 
that extreme care should be taken with the use of carbon to ensure that 
the samples are not damaged by excessive out gassing. The use of a slow 
heating rate or a dwell in this critical temperature range (of roughly 
1300.degree. C. to 1600.degree. C.) can be used to overcome any adverse 
affects of this phenomena. The use of instruments such as gas flow meters 
or carbon monoxide detectors can be used to monitor the outlet gas streams 
to prevent rapid out gassing in the components being fired by interrupting 
the firing cycle at the critical points. 
Examples 27-29 
The effect of firing cycle was also examined for thick specimens. The 
composition and the sample preparation was the same as Example 22. The 
firing cycles used are summarised in table 18 and the results of the 
firing in table 19. 
TABLE 18 
______________________________________ 
Summary of Firing Cycles I II 
______________________________________ 
Heat room temperature to 600.degree. C. at 
Y Y 
5.degree. C. min.sup.-1 in Ar 
Heat 600.degree. C. to 1400.degree. C. at 10.degree. C. 
Yin.sup.-1 
Y 
Heat 1400.degree. C. to 1550.degree. C. at 5.degree. C. 
Yin.sup.-1 
Y 
Hold at 1550.degree. C. for 30 minutes 
Y N 
Heat 1550.degree. C. to 1650.degree. C. at 5.degree. C. 
Yin.sup.-1 
Y 
Hold at 1650.degree. C. for 30 minutes 
Y Y 
Evacuate to 1 mm Hg and fill 
Y Y 
Introduce CO gas Y Y 
Heat 1650.degree. C. to 1900.degree. C. at 5.degree. C. 
Yin.sup.-1 
Y 
Hold at 1900.degree. C. for 60 minutes 
Y N 
Heat 1900.degree. C. to 2030.degree. C. at 2.5.degree. C. 
Yin.sup.-1 
Y 
Heat 2030.degree. C. to 2060.degree. C. at 1.5.degree. C. 
Yin.sup.-1 
Y 
Hold at 2060.degree. C. for 30 minutes 
Y Y 
Cool to RT at 10.degree. C. min.sup.-1 until the 
Y Y 
natural cooling rate takes over. 
______________________________________ 
TABLE 19 
______________________________________ 
Effect of firing cycle 
Example 27 28 29 
______________________________________ 
SiC 89.0 89.0 89.0 
Al.sub.2 O.sub.3 
10.0 10.0 10.0 
MgO 1.0 1.0 1.0 
Carbon 0.9 0.9 0.9 
Mass (g) 248 248 70 
GBD (g .multidot. cc.sup.-1) 
1.70 1.68 1.71 
Firing Cycle 
I II II 
FBD (g .multidot. cc.sup.-1) 
3.20 3.18 3.20 
Wt Change % -5.8 -5.9 -8.0 
Comments 2 1 2 
______________________________________ 
Comments 
Where 
1 Low density core 
2 Uniform 
From the results it can be seen that the use of dwells in the middle 
temperature range is beneficial in increasing the fired bulk density of 
thick samples. In addition, it has been shown that a dwell or slowing down 
of the heating rate is beneficial as a result of the reaction of carbon 
with the silica inevitably present on the silicon carbide. The elimination 
of this silica is crucial to the successful densification of bodies. It 
will be appreciated that for thin bodies the deliberate elimination of the 
silica is not required as it is evolved during the heating cycle (Example 
29). For thicker bodies, the reliance on the diffusion and elimination of 
the silica from the body is not practical. In addition, the firing of 
multiple samples can also increase the amount of time required for the 
elimination of silica or silicate phases from the samples as a result of a 
build up of silicon containing vapour species in the furnace atmosphere, 
stabilising the said silicon containing phases. The effect of furnace 
design could also be a factor in this process. 
Examples 30-39 
The effect of alumina and magnesia additions on densification were examined 
for a fixed firing cycle for thick specimens. The carbon addition was 
based on the amount of silicon carbide in the starting mix. The mass ratio 
of carbon to silicon carbide was 0.015. The sample preparation and the 
firing cycle used was the same as Example 22. The results of the firings 
are summarised in table 20. 
TABLE 20 
______________________________________ 
Effect of composition on densification 
Example 30 31 32 33 34 
______________________________________ 
SiC 95.0 93.4 91.9 89.9 87.0 
Al.sub.2 O.sub.3 
4.0 6.3 6.3 10.0 12.0 
MgO 1.0 0.3 1.7 0.1 1.0 
Carbon 1.4 1.4 1.4 1.3 1.3 
Mass (g) 249 248 249 248 248 
GBD (g .multidot. cc.sup.-1) 
1.68 1.70 1.71 1.70 1.71 
FBD (g .multidot. cc.sup.-1) 
3.00 3.11 3.08 3.20 3.23 
% TD (calc) 
92.7 92.7 95.5 97.6 98.4 
Wt Change % 
-3.9 -4.0 -5.4 -4.5 -5.8 
______________________________________ 
Example 35 36 37 38 39 
______________________________________ 
SiC 87.0 86.0 82.0 80.6 79.0 
Al.sub.2 O.sub.3 
12.0 12.0 17.7 17.7 20.0 
MgO 1.0 2.0 0.3 1.7 1.0 
Carbon 1.3 1.3 1.2 1.2 1.2 
Mass (g) 248 248 248 248 248 
GBD (g .multidot. cc.sup.-1) 
1.72 1.73 1.74 1.74 1.77 
FBD (g .multidot. cc.sup.-1) 
3.22 3.23 3.29 3.27 3.30 
% TD (calc) 
98.3 98.4 99.0 98.7 99.1 
Wt Change % 
-5.9 -5.8 -5.2 -5.6 -6.1 
______________________________________ 
From the results it can be seen that in the range examined, increasing the 
alumina content had a greater effect on fired bulk density than the 
magnesia content. Low levels of addition of magnesia were effective in 
promoting densification. By contrast, to achieve greater than 95% of the 
theoretical density, alumina had to be added in excess of 6 weight 
percent. It can also be seen that there is good repeatability between 
firings (compare examples 34 and 35). 
Example 40 
The effect of alumina addition, in the upper range was examined for thick 
specimens. The carbon addition was based on the amount of silicon carbide 
in the starting mix. The mass ratio of carbon to silicon carbide was 
0.015. The sample preparation used was the same as Example 22. The firing 
cycle is shown in table 21. The results of the firings are summarised in 
table 22. 
TABLE 21 
______________________________________ 
Summary of Firing Cycle 
______________________________________ 
Heat to 600.degree. C. at 5.degree. C. min.sup.-1 in Ar gas 
Heat 600.degree. C. to 1400.degree. C. at 10.degree. C. min.sup.-1 
Heat 1400.degree. C. to 1550.degree. C. at 5.degree. C. min.sup.-1 
Hold at 1550.degree. C. for 30 minutes 
Heat 1550.degree. C. to 1650.degree. C. at 5.degree. C. min.sup.-1 
Hold at 1650.degree. C. for 30 minutes 
Evacuate to 1 mm Hg and fill CO gas 
Heat 1650.degree. C. to 1900.degree. C. at 5.degree. C. min.sup.-1 
Hold at 1900.degree. C. for 60 minutes 
Heat 1900.degree. C. to 2000.degree. C. at 2.5.degree. C. min.sup.-1 
Hold at temp for 60 minutes 
Cool to room temperature at 10.degree. C. min.sup.-1 
until the natural cooling rate takes over. 
______________________________________ 
TABLE 22 
______________________________________ 
Effect of composition on densification 
Example 40 
______________________________________ 
SiC 64.8 
Al.sub.2 O.sub.3 
35.0 
MgO 1.8 
Carbon 1.0 
Mass (g) 248 
GBD (g .multidot. cc.sup.-1) 
1.82 
FBD (g .multidot. cc.sup.-1) 
3.27 
% TD (calc) 
97.1 
Wt Change % 
-4.3 
______________________________________ 
It can be seen that even in the upper limits of alumina addition, dense 
bodies can be produced. It is thought that high levels of alumina result 
in considerable levels of residual stresses as a consequence of the 
thermal expansion mismatch between alumina and silicon carbide. 
It will be clearly understood that the invention in its general aspects is 
not limited to the specific details given hereinbefore.