FIELD OF THE INVENTION:
This invention relates to a method of increasing the surface hardness and
erosion resistance of HVOF coatings over a base coat of self fluxing alloy results
in the diffusion of the alloy material into the coatings. The density and hardness
of the HVOF coating is increased, thereby enhancing the erosion resistance of
the coatings.
BACKGROUND OF THE INVENTION:
Thermals spray coatings are being adopted to overcome various surface related
problems facing industry. These coatings are easy to apply and economically
viable. The properties of the surface can be modified without changing the bulk
material properties. Worn out parts can be refurbished by coatings without
changing the costly components. The property of the surface can be modified
suitably to increase resistance against wear, erosion, corrosion and thermal
attacks. Various thermal spray techniques such as wire/powder spray, twin wire
arc spray, plasma spray and HVOF are adopted depending on the applicability.
HVOF thermal spray coatings are widely used for wear and corrosion resistant
applications. The coatings are used both for room temperature and high
temperature applications. HVOF coating are primarily given by carbide based
powder of tungsten carbide and Chromium carbide composition and metal matrix
binders. These coating have high hardness, good bond strength with the
substrate and are dense having porosity of less than 0.5%. Depending on the
nature and severity of the application, the life of coated component can be
increased considerably.
The wear resistances of these coatings are optimized and further increase is
difficultly with the present feedstock powder material and HVOF coating systems.
Hence for very severe wear applications these present HVOF coatings cannot be
provided.
OBJECT OF THE INVENTION:
An object of this invention is to propose a method of increasing the hardness of
HVOF coatings;
Another object of this invention is to propose a method of increasing the erosion
resistance of HVOF coatings over a base coat of self fluxing alloy;
Still another object of this invention is to propose a method to improve the
coating properties;
Further, object of this invention is to propose a method to improve the wear
resistance for component.
DETAILED DESCRIPTION OF THE INVENTION:
According to this invention there is provided a method for increasing the surface
hardness by using laser treatment of multilayer HVOF coatings comprising:
fabricating the surface to be coated to achieve desired surface roughness;
spray coating the surface with nickel based self fluxing alloy;
spraying tungsten carbide based cermet powder;
subjecting the coated sample to the step of laser treatment.
HVOF thermal spray coating is an established process for changing the surface
properties to improve the wear resistance of a component. However, the wear
properties are limited by the type of coating material, density of the coating
achieved and coating process. Unmelted particles, oxide layer at the splat
boundaries, porosity and cracks exist to some extent in these HVOF sprayed
coatings.
The use of lasers for surface treatment such as hardening, cladding, welding
etc., is increasingly being adopted. It is found that laser treatment of HVOF
coating given over a base coat of self fluxing nickel base alloy improves the
coating properties such as hardness, density, and wear resistance significantly.
The laser heating of HVOF coatings given over a base coat of nickel based self
fluxing alloy to enhance the wear properties is a novel technique. The laser
heating results in instantaneous heating of the coating. This results in melting
and uniform distribution of the nickel based alloy into the HVOF coating, reducing
the porosity levels. The instantaneous heating and cooling of the coating also
increases the hardness of the we material in the coating. It was observed that
coating hardness increased by 25% and slurry erosion resistance improved by
more than 200% over conventional HVOF coatings.
Surface preparation
Stainless steel 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.
Plasma spray coating
Plasma spray coating of a nickel based self fluxing alloy was given on the
surface prepared stainless steel specimen using a high velocity high energy
plasma spray gun. Plasma spray parameters like current, voltage, gun
movement, distance, powder flow etc., were monitored. The sample was held in
a fixture and the plasma spray gun was mounted on 6 axis robot. Coating speed,
distance, number of passes etc., were programmed and coating carried out.
High Velocity Oxy-Fuel (HVOF) coating
Over the plasma sprayed base coat, a tungsten carbide based cermet powder
was sprayed using a High Pressure High Velocity Oxy-Fuel (HP-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 as above.
High powdered diode laser (HPDL) surface treatment
The laser treatment was carried out on the HVOF coated sample surface using a
robotic high powdered diode laser system. Laser beam power was controlled in a
closed loop by a pyrometer system and uniform surface temperature was
maintained during laser treatment process. The laser scan speed, surface
temperature, distance, beam width etc., were monitored.
Micro hardness of the HPDL treated HVOF coatings
The HPDL processed HVOF coated samples were cut in slow speed cutting
machine and the cross section 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.
Slurry erosion testing of HPDL treated HVOF coating
The test facility consists of a closed looped chamber in which the test sample is
placed. Known quantity of sand and water is taken in the closed looped system
and circulated through a pump and the slurry exits through a nozzle at high
velocities to impinge the sample. The samples diamond ground to a mirror finish
are kept at various angles of impingement. The test is carried out for a
predetermined period. The material worn out is determined by measuring the
weight of the sample both before and after the test using a precision weighing
balance and determining the volume loss.
EXAMPLE:
Micro-scale abrasive wear behaviour of HVOF sprayed and laser-remelted
conventional and nanostructure WC-Co coatings
Chen h.; xu c.; zhou q. hutchings Lm.; shipway p. h.; Iiu j.;
Wear ISSN 0043-1648
WC-Co coatings were deposited by HVOF spraying from two different feedstock
powders, one with a conventionally sized WC grains and one with nanoscale WC
grains. The coatings were sUbsequently laser remelted in an attempt to realize
the benefits observed by other workers, namely a decrease in porosity and an
increase in hardness. It was found that under conditions of micro-scale abrasion,
the fine WC grain size in the nanostructured material resulted in rapid pullout of
the hard phase and thus to high wear rates. Moreover, laser remelting resulted in
a decrease in hardness of the coatings and to a corresponding increase in
abrasive wear rate.
The present work carried out is related to high velocity oxy fuel coatings of
tungsten carbide 0NC) based material with a base coat of self fluxing alloy by
plasma spray process followed by laser treatment using a high powered diode
laser (HPDL). The properties studied are micro hardness and slurry erosion
behavior of coatings.
WE CLAIM:
1. A method for increasing the surface hardness by using laser treatment of
multilayer HVOF coatings comprising:
fabricating the surface to be coated to achieve desired surface roughness;
spray coating the surface with nickel based self fluxing alloy;
spraying tungsten carbide based cermet powder;
sUbjecting the coated sample to the step of laser treatment.
2. The method as claimed in claim 1, wherein the said step of fabrication is
performed by grit blasting the surface with fine alumina grit.
3. The method as claimed in claim 1, wherein spray coating of nickel alloy is
done by using a high velocity high energy plasma spray gun.
4. The method as claimed in claim 1, wherein said tungsten carbide is sprayed
using high pressure high velocity oxy-fuel(HP-HVOF) spray system.
5. The method as claimed in claim 1, wherein said laser treatment is done using
a robotic high powdered diode laser system.