FIELD OF THE INVENTION:
The present invention relates to a method of preparing and also applying a novel dip, ceramic coating with silicon oxy carbide and PMHS with triethylenediamine, which is applicable to the material for high temperature application.
BACKGROUND OF THE INVENTION:
In a typical coal fired thermal power plant coal of different grades are being used. The coal has different quality with high ash content and higher moisture content. With this coal of different sources and properties are either added or blended to feed in to the furnace. Due to varying nature of the fuel, behaviour of the fuel firing is different and not as per design. This sort of unsure nature of the coal, ash deposition, clinkering and other problem prop up. One such problem the ash deposition problem is very persistent in many boilers. A boiler which can accept any type, grade or impurities of fuel is required. Ni alloy coating is viable method to prevent the ash deposition on the evaporator tubes but the economics warrants a technical advancement of coating with economical significant. Hence there is a need to find out a method of coating on the evaporator tubes required to prevent the ash deposition while firing added or blended fuel.
Hence, there is always a long-felt need to provide a novel and improved ceramic coating which can reduce the quantity of ash deposition substantially yet very economic and simple.
The present invention meets the above-mentioned need.
SUMMARY OF THE INVENTION:
A novel method for ceramic coating on boiler tube comprises the steps of: i) preparing a composition of ceramic coating PMHS is taken as precursor solution with SiOC as passive filler and toluene as solvent; ii) thermolysis of cross linked polymethylhydrosoloxane (PMHS) with triethylenediamine (TEDA), where thermolyzing takes place in a tubular furnace at different suitable

temperatures in argon atmosphere at suitable heating rate of (5°C to 8°C/min); iv) grinding of the thermolyzed material in agate mortar and pastle and milling through a high energy ball mill in toluene medium for a suitable duration at a 300rpm speed; v) drying of milled powder at room temperature for 48 hour for complete evaporation of toluene; vi) tilled powder was sieved through a mesh; vii) preparing of slurry where suitable quantity of filler SiOC was added in a solvent in a suitable ratio; and viii) adjustment of viscosity of the slurry by varying quantity of solvent and weight percentage of catalyst by keeping the dipping speed, holding and drying time as constant; ix) applying of the coating by a single dip coater after cleaning the steel surfaces; x) measuring the thickness of the coating by scanning electron microscope and the coating were subjected to severe thermal cycling in air for estimating their stability under sever thermal loads.
OBJECTS OF THE INVENTION:
It is therefore, the primary object of the present invention to provide a method for dip ceramic coating which will reduce the ash deposition substantially on the wall of the furnace, super heater and other heat transfer surfaces of a boiler.
Another object of the present invention to provide a method for dip ceramic coating, which prevent the slagging or fouling on boiler heat transfer surfaces by the coating on boiler tube steel.
Another object of the present invention to provide a method for dip ceramic coating, where adherent crack free ceramic coating takes place on steel.
Yet another object of the present invention to provide a method for dip ceramic coating, where weight percentage of triethylenediamine was varied to alter the cross linking time and which eventually results the modification of thickness of the coating.
Further object of the present invention to provide a method for dip ceramic coating, where the different parameters of coating such as withdrawal speed

and number of cycles were optimized to obtain a smooth and uniform polymeric coating of thickness of about 2mm.
Another object of the present invention to provide a method for dip ceramic coating, where the effect of solvent in slurry controls the coating thickness and influence of catalyst facilitates for obtaining the uniform coating.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING:
It is to be noted, however, that the appended drawings illustrate only typical embodiments of the present subject matter and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments. The detailed description is described with reference to the accompanying figures. Some embodiments of system or methods in accordance with embodiments of the present subject matter are now described, by way of example, and with reference to the accompanying figures, in which:
Fig. 1 illustrates the various steps/stages involved in dip coating process: (step 1) dipping (step 2) holding (step 3) withdrawal (step 4) drying operations carried out one by one for water wall tube materials coupons.
Fig. 2 illustrates contact angle measurements of sandblasted uncoated sample.
Fig. 3 illustrates contact angle measurements of ceramic coated samples thermolyzed at 600˚C.
Fig. 4 illustrates contact angle measurements of ceramic coated samples thermolyzed at 700˚C.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:
The subject matter of the present invention relates to a novel method of dip ceramic coating provided on boiler steel tube to reduce the quantity of ash deposition on the wall of the furnace.

The method using polymethylhydrosoloxane (PMHS) as a precursor solution and silicon oxy carbide (SiOC) as passive filler to compensate for shrinkage and porosity occurs during thermolysis.
A novel method for ceramic coating on boiler tube comprises the steps of:
i) preparing a composition of ceramic coating PMHS is taken as precursor solution with SiOC as passive filler and toluene as solvent;
ii) thermolysis of cross linked polymethylhydrosoloxane (PMHS) with triethylenediamine (TEDA), where thermolyzing takes place in a tubular furnace at different suitable temperatures in argon atmosphere at suitable heating rate of (5°C to 8°C/min);
iv) grinding of the thermolyzed material in agate mortar and pastle and milling through a high energy ball mill in toluene medium for a suitable duration at a 300rpm speed;
v) drying of milled powder at room temperature for 48 hour for complete evaporation of toluene;
vi) tilled powder was sieved through a mesh;
vii) preparing of slurry where suitable quantity of filler SiOC was added in a solvent in a suitable ratio; and
viii) adjustment of viscosity of the slurry by varying quantity of solvent and weight percentage of catalyst by keeping the dipping speed, holding and drying time as constant;
ix) applying of the coating by a single dip coater after cleaning the steel surfaces;
x) measuring the thickness of the coating by scanning electron microscope and the coating were subjected to severe thermal cycling in air for estimating their stability under sever thermal loads.
The dip coating was employed to coat the steel, using the slurry containing PMH Sand SiOC (as filler), with triethylenediamine as catalyst and toluene as

solvent. The weight % of triethylenediamine was varied to alter the cross linking time, which will eventually modify the coating thickness. The material is applicable in high temperature application, since the materials have a low surface energy compared to the parent material, which are carbon steel and alloy steel. Various processing parameters such as dip coating parameters and thermolysis parameters were altered. Dip-coating parameters such as withdrawal speed and number of cycles were optimized to obtain a smooth, uniform polymeric coating of thickness ~2 mm. The polymer and filler were characterized using thermo-gravimetric analysis to identify the thermolysis temperature and detect the occurrence of any chemical reaction respectively.
PMHS was cross linked with 10% of TEDA and thermolyzed at various temperature of 600, 700, 800 and 900°C at a heating rate of 5°C to 8°C/min in argon atmosphere to obtain a uniform, adherent and crack free ceramic coating.
The thermolyzed material was then finely ground using agate mortar and pestle and ball-milled using high energy ball mill. Milling was carried out in toluene medium for 10 h at 300 rpm using tungsten carbide vials and balls, with ball-to-powder weight ratio of 10:1. The milled powders were dried at room temperature for 48 h to ensure complete evaporation of toluene. After drying, the filler powder was sieved and passed through 170 mesh (particle size < ~ 90 µm).
The coating thickness was measured using scanning electron microscope and was observed to reduce with increasing thermolysis temperature. The influence of substrate surface property on coating adhesion was studied by altering the substrate surface roughness prior to coating. Ceramic coating thickness was observed and adhesion mechanism was understood from scanning electron micrographs of coated samples. Surface properties such as surface roughness, surface morphology and surface energy were studied.
The slurry was prepared by taking PMHS and SiOC as filler with triethylenediamine (TEDA) as catalyst and toluene as solvent.

The viscosity of the slurry was adjusted by varying the quantity of the solvent and the wt. % of the catalyst (from 0.5 to 10 wt. %). The catalyst was added to promote cross-linking at room temperature, as the amount of catalyst added determines the cross-linking time and thereby, the viscosity of the slurry. The solvent was subsequently added to prepare uniformly mixed slurry and to adjust the viscosity of the slurry. Viscosity of the polymer increases with time as a result of catalytic action leading to cross-linking of the polymer and its resulting thixotropic behaviour.
The various ceramic coating parameters are dipping speed, holding time, withdrawal speed, drying time and the number of cycles.
The dipping speed and holding-drying time was kept constant at 50 mm/min and 10 min respectively during the experiments.
The only variable parameter of the process is withdrawal speed and number of cycles.
The method of dipping ceramic coating was carried out at a temperature of 25°C. The filler SiOC used in the slurry is 10wt %.
All the coating experiments were carried out after cleaning the steel surfaces in acetone via ultrasonication for 1 h to remove any contamination present. The dip-coating was carried out as shown in Fig. 1 using a single dip-coater. All the polymer-coated samples with filler were thermolyzed at 600, 700 and 800°C in Argon atmosphere at a heating rate of 8°C/min with constant holding time.
All the parameters of the novel method of preparing and applying ceramic coating are given below:
1. Catalyst - 0.5 wt. % of TEDA
2. Solvent – Toluene
3. Solute (Polymer with or without addition of filler)-to-solvent ratio – 1:1
4. Dipping and Withdrawal speed – 50 mm/min
5. Holding and drying time: 10 min

6. Heating rate for ceramization: lower heating rate (8 °C/min)
Example:
PMHS + 0.5 wt. % TEDA + 10 wt. % Si-O-C filler was dissolved in toluene (ratio of polymer + filler to solvent as 1:1), complete dispersion of filler until the completion of cross-linking was observed due to its low density and smaller particle size. Coating on steel using this slurry at a withdrawal speed of 50 mm/min resulted in a smooth and uniform polymeric coating. The polymer coating (without filler), on thermolysis at a heating rate of 8 °C/min in Ar atmosphere, was observed to crack and spall off. However, polymer coating (with filler), after thermolysis at the heating rate of 8 °C/min in Argon atmosphere, resulted in a uniform, smooth and crack-free coating. When the thermolysis was carried out at temperatures 600 °C, 700 °C and 800 °C the coating thickness was found to decrease with increase in temperature from ~ 40 to ~ 30 µm.
The ceramic coating thickness (~ 35 µm) achieved is the highest ever reported in PDC without any bond coat. The coatings were thermolyzed at varying temperatures, heating rate and atmosphere to understand their influence in obtaining a uniform, adherent and crack-free ceramic coating. The influence of substrate surface property on coating adhesion was studied by altering the substrate surface roughness prior to coating. Ceramic coating thickness was observed and adhesion mechanism was understood from scanning electron micrographs of coated samples. Surface properties such as surface roughness, surface morphology and surface energy were studied. The coatings were subjected to severe thermal cycling in air to study their stability under severe thermal loads and characterized for their adhesion strength, both before and after thermal cycling. The sandblasted surface was observed to be hydrophobic i.e. having low surface energy (as illustrated in Fig 2).
Fig 3 and Fig 4 show the ceramic coating showed traces of hydrophobicity, which was observed to decrease with an increase in thermolysis temperature.
The non-limiting advantages of the present invention are given below:

1. processing of adherent crack-free ceramic coating on steel without a bond coat.
2. understand the effect of solvent in the slurry on coating thickness and understanding the influence of catalyst in obtaining uniform coating.

3. effect of thermolysis parameters on obtaining crack-free ceramic coating.
4. influence of substrate surface roughness on adhesion of ceramic coating

5. the effect of thermal cycling on coating adhesion.
6. optimization of thermolysis parameters for crack-free, adherent coating
Although embodiments for the present subject matter have been described in language specific to structural features, it is to be understood that the present subject matter is not necessarily limited to the specific features described. Rather, the specific features are disclosed as embodiments for the present subject matter. Numerous modifications and adaptations of the method of the present invention will be apparent to those skilled in the art, and thus it is intended by the appended claims to cover all such modifications and adaptations which fall within the scope of the present subject matter.

WE CLAIM:
1. A novel method for ceramic coating on boiler tube comprises the steps of:
i) preparing a composition of ceramic coating polymethylhydrosoloxane (PMHS) is taken as precursor solution with silicon oxy carbide (SiOC) as passive filler and toluene as solvent;
ii) thermolysis of cross linked polymethylhydrosoloxane (PMHS) with triethylenediamine (TEDA), where thermolyzing takes place in a tubular furnace at different suitable temperatures in argon atmosphere at suitable heating rate;
iii) grinding of the thermolyzed material in agate mortar and pastle and milling through a high energy ball mill in toluene medium for a suitable duration at a 300rpm speed;
iv) drying of milled powder at room temperature for 48 hour for complete evaporation of toluene;
v) tilled powder was sieved through a mesh;
vi) preparing of slurry where suitable quantity of filler SiOC was added in a solvent in a suitable ratio;
vii) adjustment of viscosity of the slurry by varying quantity of solvent and weight percentage of catalyst by keeping the dipping speed, holding and drying time as constant;
viii) applying of the coating by a single dip coater after cleaning the steel surfaces;
vi) measuring the thickness of the coating by scanning electron microscope and the coating were subjected to severe thermal cycling in air for estimating their stability under sever thermal loads.
2. The method for ceramic coating on boiler tube as claimed in claim 1, wherein
the triethylenediamine (TEDA) was used as catalyst in 0.5 to 10% wt.

3. The method for ceramic coating on boiler tube as claimed in claim 1, wherein the different temperatures of thermolyzing are 600, 700, 800 and 900°C at a heating rate of 5°C to 8°C /min.
4. The method for ceramic coating on boiler tube as claimed in claim 1, wherein the most prepared temperature for thermolyzing is 900°C.
5. The method for ceramic coating on boiler tube as claimed in claim 1, wherein milling was carried out for 10 hour by using a tungsten carbide vials and bulls, where the weight ratio of ball to powder is 10:1.
6. The method for ceramic coating on boiler tube as claimed in claim 1, wherein solvent is toluene, where the ratio of filler to solvent is 1:1.
7. The method for ceramic coating on boiler tube as claimed in claim 1, wherein SiOC is added in 10 weight %.
8. The method for ceramic coating on boiler tube as claimed in claim 1, wherein the dipping and withdrawal speed is 50mm/min and holding and drying time is 10 minutes.
9. The method for ceramic coating on boiler tube as claimed in claim 1, wherein the cleaning the steel surface takes place in acetone through ultrasonication for one hour for complete removal of all contamination.
10. The method for ceramic coating on boiler tube as claimed in claim 1,
wherein the process of dip ceramic coating takes place at room temperature i.e,
25°C.