FIELD OF INVENTION
The present invention relates to a silicon carbide coated graphite composite material and process for preparing the same. In particular, the invention relates to means of achieving a uniform and defined silicon carbide coating on the graphite base. The invention also relates to products obtained using the composite material prepared by the invented process exhibiting excellent elastic property, lubrication property and resistance towards corrosion, thermal shock resistance and abrasion resistance. This has compressibility at micron level and can also be press fitted into fluid handling equipment like bronze housing.
BACKGROUND OF INVENTION
Silicon carbide [SiC] ceramics show excellent properties of high thermal stability and mechanical properties, such as high stiffness, high hardness, high thermal conductivity, good thermal shock resistance, corrosion and wear resistance. The properties of SiC make the coatings on ceramics highly attractive for many industrial applications. Due to this reason coatings made from SiC comes out as a new and cost-effective option to most hard coating materials on the market. Various methods used for the production of SiC are Chemical Vapour Deposition, CVI, polymer pyrolysis, reaction bonding, sol-gel processing etc. However, high manufacturing cost and limited applicability of these techniques restrict their application to high-tech products only.
Further composites obtained from chemical vapor deposition techniques suffer from disadvantages wherein the external coating volume of the object increases as the coating increases. Furthermore as the thickness of the coating increases, problems of bonding the coating to the substrate increase. Therefore difficulties are encountered while coating on large or complex-shaped objects.
Therefore despite what has been known in the prior art, a need exist for a SiC structures with uniform and stable coating with excellent properties and a

economically effective method for producing such large structures with complicated size and shapes.
Accordingly it is an object of the invention to provide novel silicon carbide coated graphite composite material with excellent properties of compressibility, high hardness, high corrosion resistance, thermal shock resistance, high abrasion resistance etc.
A further object of the invention is to provide a process for preparing a silicon carbide coated graphite composite material.
SUMMARY OF THE INVENTION
In one aspect, the present application provides a silicon carbide coated graphite composite material comprising silicon carbide coating forming a hard outer phase; graphite in a soft inner phase; wherein the outer phase has a uniform thickness ranging preferably from 100 to 1200 µm.
In another aspect of the present application, there is provided a process for preparing a silicon carbide coated graphite composite material comprising the steps of: a) providing a graphite preform; b) preparing a Si- rich powder/granules of 50 to 100% of Si, powder resin and a solvent preferably organic in which hexamethylenetetramine present in phenol-formaldehyde resin may dissolve and disperse to form a Si-mix; c)passing the Si-mix through sieves of certain mesh sizes (preferably in the range of 20 to 100 mesh) to form free flow powders followed by moulding to provide moulded-Si; d) placing the graphite preform of step(a) and moulded-Si obtained in step (c) in a vacuum furnace and subjecting to firing at high temperature in the range of 1500 to 2200°C under vacuum or in inert gas atmosphere like argon to facilitate in-situ reaction between moulded-Si and graphite preform; e) subjecting the fired product to a grinding process for product surface finish.

The advantageous aspect of the present invention includes forming an uniform and maximum thickness of the SiC coating over the graphite preform. Further the uniqueness of the product lies in the presence of hard phase over the soft phase graphite in inner side occurring in-situ simultaneously. The hard phase has the property of high hardness, high corrosion resistance, high thermal shock resistance and high abrasion resistance. The combination of hard and soft phases results in better lubrication, thereby improved load bearing capabilities over metallic based materials in pump parts.
The ceramic component obtained from the present invention which has property of elasticity under stress may be pressed during the final fitment of the assembly along with the steel. The excellent physical shock resistance of the material exhibits during this assembling process.
DESCRIPTION OF THE FIGURES
FIGURE 1 shows a microstructure of the SiC-Coated Graphite
FIGURE 2 shows the XRD of the SiC-Coated Surface
DETAILED DESCRIPTION OF THE INVENTION
For the purposes of the following detailed description, it is to be understood that the invention may assume various alternative variations and step sequences, except where expressly specified to the contrary. Moreover, other than in any operating examples, or where otherwise indicated, all numbers expressing, for example, quantities of ingredients used in the specification are to be understood as being modified in all instances by the term "about". It is noted that, unless otherwise stated, all percentages given in this specification and appended claims refer to percentages by weight of the total composition.
Thus, before describing the present invention in detail, it is to be understood that this invention is not limited to particularly exemplified systems or method parameters that may of course, vary. It is also to be understood that the terminology used herein is for

the purpose of describing particular embodiments of the invention only, and is not intended to limit the scope of the invention in any manner.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In the case of conflict, the present document, including definitions will control.
It must be noted that, as used in this specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the content clearly dictates otherwise. Thus, for example, reference to a “particle” may include two or more such particles.
The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention.
As used herein, the terms “comprising” “including,” “having,” “containing,” “involving,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to.
In one aspect, the present application provides a silicon carbide coated graphite composite material comprising silicon carbide coating forming a hard outer phase; graphite in a soft inner phase; wherein the outer phase has as uniform thickness ranging from 100-1200 µm. In one embodiment of making of the composite of the present invention there is provided a graphite preform which is produced by conventional moulding, iso-static pressing or by extrusion process. In a particular embodiment, the graphite preform is produced by iso-static pressing. The dense

graphite preform is machined by using PCD tools (poly crystallized diamond tools) to make desired shapes and sizes on all the surfaces as per the final desired requirements on the lathe machine. In a given embodiment, the graphite preform has a bulk density in the range of 1.60 g/cc to 1.90 g/cc with a particle size of 2 µm to 1200 µm.
It is believed that any converted graphite material having an acceptable amount of apparent porosity to allow for suitable silicon infiltration may also be used as a starting material for subsequent siliconization. The porosity of the converted graphite material can be in the range of 1.0 vol % and 20.0 vol %.
A silicon-rich powder used in the preparation of the SiC coated composite is prepared by mixing 50-100 % by weight of Si, a powder resin, and an organic solvent. The powder resin employed in the process is preferably phenyl formaldehyde resin. In a particular embodiment, hexamethylenetetramine is added along with phenol formaldehyde resin for cross-linking during heat-treatment. In a preferred embodiment, the amount of hexamethylenetetramine ranges from 5 to 20 wt% of phenol formaldehyde resin. These materials are mixed thoroughly for homogeneity to provide a Si-mix. In a particular embodiment, powder hBN (hexagonal boron nitride) is added while mixing the above components. The addition of boron nitride acts as a non-wetting agent and facilitates the capillary flow of molten silicon metal. The boron nitride is preferably >99% pure having a particle size in the range of 1 to 20 µm. The organic solvent employed in this process is an organic solvent selected from aliphatic ketone and alcohol.
The prepared Si-mix is passed through the sieves for having free flow powders and this step is usually referred to as the granulation process. The free flow powders are compacted/molded in a uniaxial pressing or isostatic pressing. In this process, the powder is filled in a required shape of the die and pressure is applied in uniaxial direction with the help of hard punches to get the compacted shape. The compaction results in molded-Si with a green density of 1.40-1.90 g/cc. The molded Si thus

obtained is either in the form of Si-wafers or Si-metal granules. In a specific embodiment, the Si-metal granules has a particle size between 1 mm to 50mm.
The graphite preforms and molded-Si are placed in the vacuum furnace and subjected to high temperatures in the range of 1500 to 2200 °C under vacuum or in Argon gas atmosphere. In a particular embodiment, the molded-Si contains a mixture of silicon powder and hBN powder. The firing process is usually performed for a period of 10 to 24 hours. During this firing process that occurs above the melting point of silicon, the liquid silicon infiltrates through the graphite preforms where hBN helps in creating the capillarity action for easy flow of liquid silicon into the preform. In the process of firing, the in-situ reaction between silicon and graphite forms the silicon carbide. The liquid silicon reacts with graphite and form secondary silicon carbide. The SiC is either α-SiC, β-SiC or a combination of both. The uniformity of the formation of secondary silicon carbide on the outer surface is achieved by maintaining the fired product at a soaking time and at the soaking temperature of 1500 to 2200°C for 60 min to 300 minutes. The fired product is subjected to the grinding process for product surface finish by using resin/metal/ceramic bonded diamond wheels.
The SiC coated graphite composite material prepared by the above mentioned process has a hard outer phase that comprises 86 to 94% by weight of SiC and 8-16% by weight of free Si metal. In a given embodiment, the SiC particles have a particle size ranging from 2 µm to 1500 µm. The composite material so obtained by the invented process has no more than 5 vol% porosity and a density in the range of 1.8 g/cc to 2.25 g/cc.
The following examples are provided to better illustrate the claimed invention and are not to be interpreted in any way as limiting the scope of the invention. All specific materials, and methods described below, fall within the scope of the invention. These specific compositions, materials, and methods are not intended to limit the invention, but merely to illustrate specific embodiments falling within the scope of the invention. One skilled in the art may develop equivalent materials, and methods without the

exercise of inventive capacity and without departing from the scope of the invention. It is the intention of the inventors that such variations are included within the scope of the invention.
EXAMPLES Example 1:
The graphite preforms in the form iso-static grade which was processed by iso-static pressing. The preform has the bulk density of 1.80 g/cc with particle size of 5-10 µm and also has the properties like shore hardness of 55-60 HSD, flexural strength of 40-50 MPa and compressive strength of 100-110 MPa. The graphite preforms has machined by using PCD tools to make desired shapes and sizes. The silicon-rich powder has been prepared by conventional mixing of silicon 80-90%, powder resin and organic solvent initially. The mix is passed through the sieves for having better mixing and free flow powders. The free flow powders are moulded conventionally by uniaxial pressing to have silicon-rich wafers with 1.5-1.9 g/cc. The graphite preforms and silicon wafers are placed in the vacuum furnace and subjected to high temperatures of 1700 °C under vacuum. In the process of firing, the in-situ reaction between silicon and graphite forms the silicon carbide. The uniformity of the silicon carbide on the outer surface is maintained by the soaking time at the elevated temperatures of 1700 °C. The fired product is subjected to the grinding process for desired surface finish. The final product achieved a density of 1.90-1.98 g/cc with and coating thickness of 800-1200 µm uniformly on all the surfaces.

Example 2:
The graphite preforms in the form moulded grade which was processed by iso-static pressing. The preform has the bulk density of 1.90 g/cc with particle size of 5-10 µm

and also has the properties like shore hardness of 70-80 HSD, flexural strength of 60-80 MPa and compressive strength of 150-200 MPa. The graphite preforms has machined by using PCD tools to make desired shapes and sizes. The silicon-rich powder has been prepared by conventional mixing of silicon 80-90%, powder resin &organic solvent initially. The mix is passed through the sieves for having free flow powders. The free flow powders are moulded by isostatic pressing to have silicon-rich wafers density in the range of 1.5-1.7 g/cc. The graphite preforms and silicon wafers are placed in the vacuum furnace and subjected to high temperatures of 2050 °C in Ar atmosphere. In the process of firing, the in-situ reaction between silicon and graphite takes place to form silicon carbide. The soaking time at the peak temperature (2050 °C) is maintained for 180 minutes. The fired product is subjected to the grinding process for desiredsurface finish. The final product has achieved a density of 1.95-2.10 g/cc and coating thickness of 400-600 µm uniformly on all the surfaces.
S. No Attribute Unit Value
1 Graphite Preform Density g/cc 1.90

uniformity of the silicon carbide on the outer surface is maintained by the soaking time at an elevated temperature of 1600°C. The fired product is subjected to the grinding process to desired surface finish. The final product has achieved a density of 2.00-2.20 g/cc and coating thickness of 100-200 µm uniformly on all the surfaces.


We Claim:
1. A silicon carbide coated graphite composite material comprising
- silicon carbide coating forming an hard outer phase
- graphite in a soft innerphase
wherein the outer phase has as uniform thickness ranging from 100-1200 µm.
2. The composite material as claimed in claim 1 wherein the hard outer phase comprises of 86 to 94% by weight of SiC and 8 - 16% by weight of free Si metal.
3. The composite material as claimed in claim 1 wherein the hard outer phase comprising SiC has SiC particles of size ranging from 2µm to 1500 µm.
4. The composite material as claimed in claim 1 wherein the SiC is α-SiC, β-SiC or combination of both.
5. The composite material as claimed in claim 1 wherein the composite has no more than 5 vol % porosity.
6. The composite material as claimed in claim 1 wherein the density of the composite is 1.80 g/cc – 2.25 g/cc.
7. A process for preparing a silicon carbide coated graphite composite material comprising the steps of:

a) providing a graphite preform;
b) preparing a Si- rich powder obtained by mixing 50 to 100%of Si, powder resin and an organic solvent to form a Si-mix;
c) passing the Si-mix through sieves to form free flow powders followed by moulding to provide molded-Si;

d) placing the graphite preform of step(a) and molded-Si obtained in step(c) in a vacuum furnace and subjecting to firing at high temperature in the range of 1500 to 2200 °C under vacuum or in Argon gas atmosphere to facilitate in-situ reaction between molded-Si and graphite preform;
e) subjecting the fired product to a grinding process for product surface finish.

8. The process as claimed in claim 7 wherein hexagonal boron nitride is added in step (b) of the process.
9. The process as claimed in claim 7 wherein molded Si is in the form of Si wafers or Si-metal granules.
10. The process as claimed in claim 7 wherein the graphite preform is produced by conventional moulding, iso-static pressing or by extrusion process.
11. The process as claimed in claim 7 wherein the graphite preform has a bulk density in the range of 1.60 g/cc to 1.90 g/cc.
12. The process as claimed in claim 7 wherein the graphite preform has a particle size of 2 µm to 1200 µm.
13. The process as claimed in claim 7 wherein the powder resin in step (b) is phenyl formaldehyde resin.
14. The process as claimed in claim 7, wherein hexamethylenetetramine is added along with phenol formaldehyde resin.
15. The process as claimed in claim 7 wherein the solvent is step (b) is an organic solvent selected from aliphatic ketone and alcohol.

16. The process as claimed in claim 7 wherein the molded-Si has a density of 1.40 g/cc to 1.90 g/cc.
17. The process as claimed in claim 7 wherein the step of (d) is performed for a period of 10 to 24 hrs.
18. The process as claimed in claim 7 wherein the fired product obtained in step (d) is maintained at a soaking time for 60 min to 300 minutes at the high temperature in the range of 1500 to 2200°C.