|Title of Invention||
A METHOD OF MANUFACTURE OF ARTICLES OF EGLAZED CERAMIC COMPOSITED ON METAL SUBSTRATES
|Abstract||A method of manufacture of articles of glazed ceramic composites on tnetal substrates comprising the Steps of grinding the composite coating on the said substrate to obtain a predetermined surface finish exposing the said composites to microwave irradiation of 0.9 to 18 GHZ in a charcoal environment, until glazing is complete, to obtain the said articles.|
This invention relates to articles of glazed ceramic composites on metal substrates and a method of manufacture thereof by microwave glazing.
In the material-environment configuration, mechanical components and tools are progressively facing higher performance requirements under more stringent conditions. The performance of many components is often restricted by surface properties and thermal stability that can be enhanced by applying ceramic and/or ceramic composite coatings on relatively weaker substrates. Ceramic composites are reinforced ceramic materials with higher resistance to wear and corrosion, enhanced toughness, thermal and related properties and are widely used in tribological applications. The use of surface coatings open up the possibility to tailor-made a component to meet the specific properties demanded by the composite design. Atmospheric plasma spraying is one of the state-of-the-art technologies representing a cheap and easier route to develop thick coatings on industrial components. In spite of significant advancements in the plasma-spray process, coatings developed through this technique exhibit anisotropic structure, including voids and unmelted particles, which are undesirable in aggressive environments. Moreover, plasma-sprayed coatings tend to have a rough surface and poor bonding qualities. Published literatures in the field, while recommend ceramic or ceramic composite coatings for improved wear, corrosion and thermal as well as mechanical properties, however, emphasize on the scope to further enhance their as-sprayed properties including improvement of surface finish, reduction of porosity, microstructure refinement, enhanced bonding etc. These call for developing suitable post processing methods/techniques to modify the metallurgy, chemistry and the topography of the coated surfaces. To achieve this, different techniques including laser beam technology, are being tried to produce smooth ceramic-coated surfaces with a view to improve tribological as well as mechanical properties.
The proposed method of microwave glazing is a simple, clean and cost-effective novel method for post processing of plasma sprayed coatings that shows significant enhancement in as-sprayed characteristics.
According to this invention, the method of manufacture of articles of glazed ceramic composites on metal substrates, comprises the steps of grinding the composite coating on the \ said substrate to obtain a predetermined surface finish; exposing the said composites to microwave irradiation of 0.9 to 18 GHz in a charcoal environment, until glazing is complete, to obtain the said articles.
The single Figure in the accompanying drawings illustrates, by way of example, and not by way of limitation, an article of ceramic composite coating on a metal substrate located in a microwave oven, in a charcoal environment.
In the drawings, the charcoal is indicated by C, the ceramic composite metal substrate article by M.
Microwave processing is fundamentally different from other conventional heating techniques. In microwave processing, microwave energy is directly applied to the material. High frequency microwaves (>1GHz usually) penetrate into the bulk of the material and the volumetric interaction of the electromagnetic fields with the material results in dielectric heating. This enables higher heating efficiency and rapid processing. Ceramics are transparent to microwaves at low temperatures, however, start absorbing microwaves at higher temperatures leading to microstructure refinement and change in material characteristics.
In the method proposed herein, alumina-titania (Al203-13%Ti02) ceramic composite coating of 0.9mm thickness is developed using Plasmadyne atmospheric plasma spraying facility on steel substrates with 0.1mm thick interfacial layer of nickel-aluminum. The as-sprayed coating was diamond ground and subsequently subjected to microwave radiation of 2.45GHz in activated charcoal environment to facilitate hybrid heating. The metallic substrate was masked to avoid microwaves being reflected by it. The masking was carried out by placing the metallic substrate in a slot/cavity of thick walled carbon block such that, only the ceramic composite coating is exposed to microwaves. To ensure effective glazing of the surface, the specimens were irradiated for 30 minutes. A conventional, multi-mode, cavity type microwave heating equipment was used.
During microwave irradiation of the spray deposited ceramic composites, microwaves penetrate into the bulk of the material leading to volumetric heating. This results in reverse gradient of heating. On prolonged irradiation, progressive absorption of microwaves gives rise to higher order of surface heating resulting in further transformation of α -alumina into ductile Y-alumina facilitating material flow. This induces material glazing with marginal stressing of the material as observed in the typical XRD profiles. The material flow owing to the volumetric heating of the ceramic composites on continued exposure to microwave radiation also results in partial filling of the surface defects of the as-sprayed samples leading to improvement in surface finish. Relatively faster rate of cooling experienced by the top surface of the irradiated samples further leads to the microstructural refinement forming a thin amorphous layer with glazed appearance, while, relatively slow rate of cooling at the middle of the ceramic layer facilitates epitaxial growth in sprayed splats resulting reduction in porosity (densification) of the coatings with observed enhancement in microhardness The increased microhardness also influences the tribological characteristics of the glazed ceramic composite coatings. The sliding and erosion wear resistance of the glazed surfaces is increased.
As stated earlier, plasma spray deposited ceramic and ceramic composites are widely used in tribological applications. With enhanced characteristics, the microwave glazed coatings would definitely be better candidates for such applications. Possible healing of surface defects (micro cracks, voids etc.) and microstructural refinement during glazing will undoubtedly enhance the life of the coatings/components. There exists a possibility to extend the concept to powder metallurgy products too, where densification and surface finish are important criteria.
Microwave glazing is a new approach employed successfully for processing of spray deposited alumina-titania ceramic composite. The enhancement of as-sprayed characteristics is significant in the following:
1. Surface finish of the glazed surface improves.
2. Improvement in microhardness.
3. Material flow facilitates partial closing of micro cracks/voids.
4. Densification of the coatings through microstructure refinement.
5. Wear resistance (in sliding as well as erosion) improves.
6. Porosity reduces.
l.A method of manufacture of articles of glazed ceramic composites on metal substrates comprising th e steps of grinding the composite coating on the said sub8trate to obtain a
predetermined surface finish exposing the said composites to
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'microwave irradiation of 0.9 to 18 GHz in a charcoal environment until glazing is complete, to obtain the said articles.
2.The method as claimed in Claim 1 wherein the metal substrate is m asked.
3. The method as claim ed in Claim lor Claim 2 wherein the microwave irradiation is carried out in a multi-mode cavity type microwave heating equipment. 4.The method of manufacture of articles of glazed ceramic composites on metal substrates substantially as herein described and illustrated with reference to the Example and the accompanying drawings.
5.ArticIes of glazed ceramic composites on metal substrates whenever manufactured by a method as claimed m anyone of the preceding Claims. Dated this the 9th July 2001 INDIAN INSTITUTE OF TECHNOLOGY
|Indian Patent Application Number||563/MAS/2001|
|PG Journal Number||27/2006|
|Date of Filing||09-Jul-2001|
|Name of Patentee||INSTITUTE OF TECHNOLOGY,|
|Applicant Address||IIT . P.O., CHENNAI -600036 , TAMIL NADU , INDIA|
|PCT International Classification Number||C04B35/00|
|PCT International Application Number||N/A|
|PCT International Filing date|