TITLE:
A method of depositing elevated temperature erosion resistant coating for ID fan blade
application
FIELD OF THE INVENTION:
The invention relates to a method of depositing elevated temperature (upto 250 °C) erosion
resistant coating to protect ID fan blades from high ash dust burden.
BACKGROUND OF THE INVENTION:
The Induced draft fan (ID fan) is an axial reaction type fan which comprises of an impeller
made precisely to accommodate a set of replaceable aero foil blades on blade shafts (Fig 1).
The blade shafts are placed in combined radial and axial anti-friction bearings which are
sealed to the outside. The blades are made up of GGG 40 (Nodular grey cast iron) that
undergoes severe erosion mainly at the edge where the flue gas hits the fan surface. The flue
gas temperature is in the range of 150-180°C with maximum temperature goes up to 250°C.
Indian coal exhibits high amounts of ash content which ultimately comes as fly ash after
combustion through ID system. Also, the dust particles contain silica of bigger particle size
which is very much erosive for the blade surface. Due to erosion the life cycle of the ID fan

reduces, currently less than 25000 hrs for dust burden of 250mg/Nm3. A suitable coating need
to be developed to enhance the erosion resistance of these blades and provide erosion
proofing techniques against high dust burden for the blade surface.
According to United States Patent Application 6156443, a method of onsite application of an
erosion resistant and oxidation resistant composite materials that is formed by high
temperature melting a metal possessing low affinity for oxygen with requisite proportion of
ferroboron particles of the required particle size is disclosed. Raw materials are shaped in the
form of a cored wire that is arc sprayed with air or deposited by welding techniques for
producing erosion resistant coating for components exposed to a high velocity blasts of large
particles at temperatures upto 500°C.
Another method reported in an International Patent Application No: WO 2010/044936 A1, for
erosion and impact resistant ceramic coating suitable for protecting surfaces subjected to
collisions with particles, including nominally round particles that typically inflict impact
damage and more aggressive irregular shaped particles that typically inflict erosion damage.
The ceramic coating is formed to have one of the three compositions TiAIN, CrN and TiSiCN
deposited by physical vapor deposition process to have a columnar and/or dense
microstructure and total coating thickness of up to 100µm.

According to US Patent No 6428630 there is provided a method of coating a metallic layer
comprising at least about 8% Nickel on a substrate by thermal spraying. The metallic layer
has an average density greater than about 80%. A slurry layer comprising from about 10% to
about 90% aluminum or alloy thereof is deposited on the metallic layer. Heating to a
temperature in excess of 500° C results in the formation of an intermetallic layer of NiAl to
protect the substrate from high temperature oxidation and corrosion.
Another US Patent No 5120613 provides a process for increasing the resistance to corrosion
and erosion of a vane of a rotating heat engine, consists of ferritic and/or ferritic-martensitic
base material, in that a firmly adhering protective surface layer consisting of 6 to 15% by
weight of Si, the remainder being Al, is sprayed onto the surface of the base material using the
high speed process with a particle velocity of at least 300m/s.
United States Patent Application US 5700743 disclosed the protective coating against erosion.
The layer consists of a baked and compressed inorganic lacquer and the lacquer of the lacquer
layer and of the covering layer is baked on the component below the temperature which
causes damage to the structural component of below the softening or decomposition
temperature of the component The protective layer is advantageously used for the external
components of propulsion units, such as housing, nose cones or fan blades.
M/s TLT Turbo GmbH has developed advanced anti erosive thin film coatings with trade
name of TLT-Turbo-H-101 applied by HVOF spraying method basically consists of tungsten
carbide and some special additional materials. However no further details are available.
OBJECTS OF THE PRESENT INVENTION
An object of the present invention is to propose a method of depositing elevated temperature
erosion resistant coating for ID fan blades.

Another object of the present invention is to propose a method to achieve elevated
temperature erosion resistant coating by deposition of carbide based coating.
Further object of the present invention is to propose a method to propose a method of
depositing elevated temperature erosion resistant coating through robotically programmed
high velocity oxy fuel process on GGG 40 base material.
BRIEF DESCRIPTION OF THE INVENTION:
This invention relates to a method of depositing elevated temperature erosion resistant coating
for ID fan blades comprising:
depositing erosion resistant coating using robotically programmed high velocity oxy fuel
spray process;
obtaining a coating thickness of 15-25µm per layer amounting to a total thickness of 300-350
µm.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure I: A Typical ID fan blade
Figure 2: Optical micrographs of HVOF coating @ 100x
Figure 3: Elevated temperature air jet erosion test results
Figure 4: Air jet erosion damages on test samples

DETAIL DESCRIPTION OF THE INVENTION
According to the invention, there is also provided a method for producing elevated
temperature erosion resistant coating by deposition of tungsten carbide - cobalt - chrome or
chromium carbide - nickel chrome coating through liquid fuel based robotically programmed
high velocity oxy fuel process on GGG40 base material.
These coating has been characterized for hardness, porosity, bond strength and surface
roughens. Both the coating found to be hard, dense (Porosity < 1%) and microstructure free
from any defects. Bond strength is > 10000 PSI. Elevated temperature erosion resistance
properties of coated surface was improved by around 4-6 folds as compare to base material.
Tungsten carbide (WC-10Co-4Cr) coating displayed best erosion resistance at room
temperature and operating temperature of 150oC. Chromium Carbide (Cr3C2-NiCr) coating
displayed similar erosion resistance as tungsten carbide coating at 250°C.
Surface preparation
GGG40 nodular cast iron specimens were fabricated and the surface to be coated was grit
blasted with fine alumina grit using a grit blasting machine to achieve the desired surface
roughness. The specimens were thoroughly cleaned to remove dust and embedded particles.


High Velocity Oxy-Fuel (HVOF) coating
High Velocity Oxygen Fuel (HVOF) coating is a thermal spray coating process in which
molten or semi-molten materials are sprayed onto the surface by means of the high
temperature, high velocity gas stream produced by mixing and igniting oxygen and fuel (gas
or liquid) in a combustion chamber and allowing the high pressure gas to accelerate through a
nozzle. It produces a dense spray coating which can give a very high surface finish. Tungsten
carbide and chromium carbide based cermet powder was sprayed using a liquid fuel based
High Pressure High Velocity Oxy-Fuel (HVOF) spray system. Spray parameters like Oxygen
& fuel, gun movement, distance, powder flow etc., were monitored. Coating was carried out
by mounting the HVOF gun on the six axis robot. This method is used to improve a
component's surface properties, thus extending life of the component by significantly
increasing erosion/wear resistance and corrosion protection. The spray parameters were:
Oxygen Flow: 700-900 LPM; Fuel Flow: 0.3-0.4 LPM; Chamber Pressure: > 7.2 bar; Powder
Flow- 70-100 g/min; Spray Distance: 300mm; spray velocity: 300-500 mm/s.

Characterization
These coating were characterized for hardness, porosity, bond strength and surface roughens.
The HVOF coated samples were cut in slow speed cutting machine and the cross section were
polished using a diamond lapping compound. The Vickers micro hardness of the coatings was
determined using a micro hardness tester under a load of 300 gms. Microstructure were taken
at 100x magnification using image analyzer for porosity measurement, coating integrity and
interface analysis. Bond test was carried out using pull-offtest
Air Jet erosion testing
The test has been carried out on air jet erosion test rig as accordance with the ASTM-G76
standard. The erodent used is fly ash. The initial weight of the sample is recorded. It is then
placed in the fixture suitable for particular impingement angle. The erodent is loaded in the
hopper positioned at the top of the rig. The erodent discharge rate is controlled using a motor
positioned below the hopper. The erodent flowing at high pressure impinges on the sample,
which is positioned at the required angle. The experiment is carried out for a pre-defined time
and temperature. The weight of the sample is recorded after the experiment and the weight
loss is measured.
The experimental conditions for these tests were:
• Nozzle diameter: 1.5 mm
• Nozzle to sample distance: 30 mm
Sample dimension: 24 X 20 X 6 mm
• Erodent: Fly Ash.
• Air pressure: 0.6 bar
• Particle velocity: 66.7 m/s.
• Time of experiment: 30 minutes
Temperature (3 Levels): Room Temperature, OT (150°C) & HT (250°C)
• Angle of impingement: 30°

According to the invention, the tungsten carbide (WC-10Co-4Cr) coating and chromium
carbide (Cr3C2-NiCr) coating were deposited using optimized process parameters with liquid
fuel (Kerosene) based HVOF process. The deposition was controlled by manipulating six axis
robot at optimized spray velocity in the range of 300-5OOmm/s to achieve coating thickness of
15-25 urn per layer amounting to a total thickness of 300-350 µm. All the deposition was
carried out at a fixed spray distance of 300 mm. The characterization results are given in
Table 1. Figure 2 shows the microstructure of coatings.

The Air Jet erosion test has been carried out as per the ASTM G-76 standard. For the air jet
. erosion testing, samples of size 24x20x6mm were prepared. The accelerated air jet erosion
test was carried out at dust burden of 425000 mg/Nm3 for 30 minutes. The erodent used was
actual fly ash obtained from the power plant. The fly ash was agglomerated using a binder to
make ash granules of specified size so that it can be used for testing. The test was conducted
at three temperature range; RT-Room temperature, OT-Operating temperature (150°C) and
HT- High Temperature (250"C). Weight loss has been measured after each test and total

volume loss has been calculated for coatings and base material. Elevated temperature erosion
behavior of each coating has been compared with base material and results are shown in Fig
3. The air jet erosion damages on samples are shown in Figure 4. Air jet erosion resistance of
HVOF coating is found to be manifold (4-6 times) better than base material.

WE CLAIM:
1. A method of depositing elevated temperature erosion resistant coating for ID fan
blades comprising:
depositing erosion resistant coating using robotically programmed high velocity oxy
fuel spray process;
obtaining a coating thickness of 15-25µm per layer amounting to a total thickness of
300-350 µm.
2. The method as claimed in claim 1, wherein the said erosion resistant coating is
selected from tungsten carbide (WC-10Co-4Cr) and chromium Carbide (Cr3C2-NiCr).
3. The method as claimed in claim 1, wherein me process parameters are in the range as
follows:
Oxygen Flow: 700-900 LPM;
Fuel Flow: 0.3-0.4 LPM;
Chamber Pressure: > 7.2 bar;
Powder Flow- 70-100 g/min;
Spray Distance: 300mm;
Spray Velocity: 300-500 mm/s.

4. The method as claimed in claim 1, wherein the said erosion resistant coating is
deposited on the surface of ID fan blade to achieve ~ 4 times improved erosion
resistance than base material at 250°C and - 5.2 times improved erosion resistance
than base material at 150°C against high ash dust burden.