The present invention relates to a thermal barrier coating system for application on copper/working surfaces exposed to high temperature zones and a process for obtained such thermal barrier coatings. In particular the invention specifically relates to thermal barrier coating system for application on blast furnace tuyeres and a process for manufacture of blast furnace tuyeres coated with such thermal barrier coating. A blast furnace is a counter heterogeneous reactive heat transfer system involving dynamic interaction between gaseous, liquid. and solid phases. Preheated air, one of the inputs for iron making, is introduced through cast high purity copper tuyeres. The blast furnace tuyeres are important components of blast furnaces. The tuyeres, 20 in number, are double walled, high purity (-99.5%) cast copper structures, through which preheated air (750-870°C) is introduced into the blast furnace. These tuyeres are located at the intersection of bosch and the hearth of the furnace along the entire periphery. As the tuyeres are exposed to the hottest region in the blast furnace where the raceway adiabatic flame temperature, RAFT, may frequently exceed 2000°C, the tuyeres are cooled internally by circulating water. The tuyere failure may take place due to accidental contact with burning coke particles/lumps, slag or hot metal drops leading to high localised thermal load on relatively small area. The sudden tuyere failure adversely affects the stable operation of blast furnace. Thermal barrier coatings applied to the exposed/working surface of tuyeres lead to drastic reduction in the incidence of tuyere failures The high melting point, low thermal'conductivity, phase stability of ceramic material is found to beiiseful as a good thermal barrier coating material and thermal based ceramic coatings are known to be provided on subjects exposed to high temperature. The ceramic coatings provide a thermal barrier to the heat flux between the hot zone of the furnace and the tuyere members Also, the provision of a bond coat for enhancing the adhesion of ceramic top coat is known as would be evident from the following prior arts : US Patent: 5498484 discloses a thermal barrier coating system for high temperature nickel-based and cobalt-based super alloys including a hardenable metallic coating on the substrate and a ceramic top coat. The metallic coating is preferably a gamma/gamma prime hardenable nickel-based super alloy, and the ceramic top coat is preferably zirconium oxide having* from about 6 to about 20 percent yttrium oxide or from about 15 to about 40 percent cerium oxide. Optionally there may be provided between the hardenable metallic layer and the ceramic topcoat an intermetallic coating such as a nickel aluminide. US Patent 5824423 disclosed an improved bond coat of a thermal barrier coating system useful for enhancing adhesion of a ceramic topcoat to a superalloy substrate. The bond coat includes about 13 to 30 weight percent aluminum between trace and about 3 weight percent of a reactive element selected from the group consisting of yttrium, zirconium, lanthanum, and~2 top coat comprise powders with granulometric particle size of -100 + 325 mesh and -230 mesh respectively. 4. A thermal barrier coating system as claimed in anyone of claims 1 to 3 wherein the thickness of said bond coat range from 0.3 to 0.5mm and said top coat range from 0.6 to 1.0mm with said total thickness of the coat system in the range of 1.0 to 1.5 mm. 5. A thermal barrier coating system as claimed in anyone of claims 1 to 4 wherein the surface roughness of the ceramic coating range from 6.91 to 8.47 microns Ra.. 6. A process for coating substrates exposed to high tempered zones with said thermal barrier coating system as claimed in anyone of claims 1 to 5 comprising : preparing the surface to be coated ; preheating the surface to be coated to a temperature of 120-200°C ; applying the bond coat of Ni-Cr-Al alloy comprising 17-22% Cr, 2-6% Al and balance Ni and allowing the surface to1 be cooled to a temperature of 50-100°C ; depositing the top a coat of titania (TiO2) on said bond coat of Ni-Cr-Al. 7. A process as claimed in claim 6 wherein said step of preparation of the surface to be coated comprise : cleaning the surface by machining 2-4 mm on diameter with or without hand/spot grinding ; roughening the surface by grid blasting and knurling to provide mechanical anchorage; and washing the surface to degrease the same. 8. A process" as claimed in claim 7 wherein the surface is washed with acetone for degreasing. 9. A process as claimed in anyone of claims 6 to 8 wherein said Ni-Cr-Al alloy bond coat and said titania top coat are applied by plasma spraying techniques. 10. A process as claimed in anyone'of claims 6 to 9 wherein said bond coat is applied using the following plasma spray conditions : Current - 700 to 750 amp. Voltage - 33 to 43 VDC Distance of spray gun - 4 to 6 inches. 11. A process as claimed in claim 10 wherein for said bond coat the plasma spray power feed rate is preferably 3.2 rpm. and said gas pressure is preferably 40 psi. 12. A process as claimed in anyone of claims 6 to 11 wherein said plasma spray conditions for said top coat of titania comprise : Current - 700 to 900 amp. Voltage - 33 to 40 VDC Distance of spray gun - 4 to 6 inches. 13. A process as claimed in claim 12 wherein for said top coat the plasma spray power feed rate is preferably 4.5 rpm and said gas pressure is preferably 40 psi. 14. A process as claimed in anyone of claims 6 to 13 wherein the coating material for said Ni-Cr-Al bond coat and TiO2 top coat are powders with granulometric particle size of -100 +375 mesh and -230 mesh respectively. 15. A process as claimed in anyone of claims 6 to 14 wherein the said bond coat of Ni-Cr-Al is obtained of 0.3 to 0.5mm thickness preferably 0.4 mm thickness built up in four runs and the total thickness of coating (the bond coat and top coat) is obtained to be 1.0 to 1.5 mm. 16. A process as claimed in anyone of claims 6 to 15 wherein substrate . to be coated is a blast furnace tuyere. 17. A processes claimed in anyone of claims 6 to 15 wherein said substrate to be coated are blast furnace tuyeres and the tuyeres are mounted on a horizontally turn table and rotated at 3-4 rpm during the coating process. 18. A thermal barrier coating system and a process for carrying out such coating on substrate such as blast furnace tuyeres substantially as hereindescribed and illustrated with reference to the accompanying examples and figures. A thermal barrier coating system for application on copper/working surfaces exposed to high temperature zones comprising : a bond coat as a first layer of coat on the substrate obtained of Ni-Cr-A1 alloy comprising 17-22% by wt. Cr, 2-6% by wt A1, and balance Ni, and a second layer of top coat of titania TiO2 over said bond coat and a process for providing such coating system on substrates. The coat system as above is dense, adherent and mechanically stable and has a strong bonding with the basic substrate. The process of applying such coat is also simple and cost-effective. The coating system is especially suitable for coating of blast furnace tuyeres exposed to high temperature as effective thermal barriers.