High Velocity Oxyfuel Spray Coatings for Wear and Corrosion Protection a Review

International Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (October 5-7, 2012)

Punjab Technical University, Jalandhar-Kapurthala Highway, Kapurthala, Punjab-144601 (INDIA) 649

HIGH VELOCITY OXY FUEL SPRAY COATINGS FOR WEAR AND

HOT CORROSION PROTECTION: A REVIEW

Harkulvinder Singh a*, Sukhpal Singh Chatha

a,Hazoor Singh Sidhu

a

aYadavindra College of Engineering, Punjabi University Guru Kashi Campus, Talwandi Sabo, Punjab, India-151302

(Email: [email protected])

*Corresponding author email-id [email protected]

ABSTRACT Materials operating at high temperatures fail due to erosion-corrosion, wear, oxidation, and hot corrosion. In the

recent years, there has been a growing interest in the use of thermal spray coatings onto the surfaces of engineering

components to allow them to function under extreme conditions. Among the available HVOF process provides the

coatings with have high density, increased thickness capability, smoother surface finish, lower oxide levels, low

porosities, less effect of the environment during the spray process and have better corrosion and wear resistance

properties. This study is done with the aim of putting together the performance capabilities and applications of

HVOF process.

Keywords :HVOF, wear, hot corrosion, Coatings

1. INTRODUCTION

Coating provide a way of extending the limits of use

of materials at the upper end of their performance

capabilities by allowing the mechanical properties of the

substrate materials to be maintained while protecting

them against wear and erosion. The high velocity oxy

fuel process belong to the family of thermal spraying technologies and are capable of producing coatings with

lower porosity, higher hardness, superior bond strength,

and lower decarburization than many other thermal

spraying methods ( Sidhu et al, 2005). In the HVOF

thermal spray technology, oxygen and liquid fuel are

combusted under high pressure in a chamber and the

combustion products are accelerated through a

converging-diverging nozzle. The powder fed into the

hot stream of gases, is heated and then is accelerated on

the substrate at very high speed (650-850 m/s) Fig 1.

Shows the schematic diagram of HVOF process (Kaur et al, 2009). HVOF flame spraying has been recognized

as the most significant development in the thermal spray

industry during the last 15 years. Since the initial use of

tungsten carbide-cobalt, the range of powders has

expanded to include a large variety of other carbides as

well as metallic and ceramic materials ( Dobler et al,

2000). Thermally sprayed cermets coatings are widely

used in many engineering applications for their high

levels of wear resistance. Such cermet coatings include

WC-Co and Ni(Cr)Cr3C2 used for wear protection(

Jones et al, 2001).

1.2 Wear Wear involves the physical removal of material from

a solid surface by another surface or material. When a

hard surface with asperities slides on a softer surface

and removes material by gouging or plowing, the

process is called machining wear (Donachie et al, 2002).

Generally the level of abrasive wear depends on the

difference between the hardness of the abrasive particles

and that of counter materials. Adhesive wear is initiated

by the interfacial adhesive junctions that form if solid materials are in contact on an atomic scale. Regardless

of surface finish, every surface has hills and valleys, so

when these surfaces slide against each other more and

more material will be plastically fractured from the

softer material and small fragments of metal are torn

away ( Lal and Vineet, 2010).

1.3 Hot corrosion failure Metals and alloys sometimes experience accelerated

oxidation when their surfaces are covered with a thin

film of fused salt in an oxidizing gas atmosphere at elevated temperatures. This is known as high

temperature or ‘hot’ corrosion where a porous non

protective oxide scale is formed at the surfaces and

sulphides in the substrate (Singh et al, 2007). Hot

corrosion is an accelerated form of oxidation that occurs

at higher temperature in the presence of salt

contaminants such as Na2SO4, NaCl,V2O5 that combine

to form molten deposits, which damage the protective

oxide layer (Eliaz et al, 2002).



2. Coating for wear

Sidhu et al, 2010 examined wear behavior of the

HVOF deposited Cr3C2–NiCr and WC–Co coatings on

Fe-base (ASTM-SA213-T22) steels by the pin-on-disc

mechanism. HVOF sprayed WC–Co coating showed the

higher wear resistance as compare to Cr3C2–NiCr

coating, because of high hardness, uniform and dense

microstructure. Yang et al, 2003 investigated WC–Co coating was

deposited by HVOF system. Dry sliding friction and

wear tests using sintered alumina (Al2O3) as the mating

material were performed. The specific wear rate of the

coatings was very low 10−6 mm3/(Nm) and increased

with increasing carbide grain size.

Zhao et al, 2004 examined the influence of spray

parameters on the particle in-flight properties and

coating properties during HVOF-spraying of WC-CoCr

powder using on-line particle monitoring. The wear

behavior of the coatings was evaluated both by rubber

wheel tests and by pin-on-disk tests. It was found that the particle velocity was more sensitive to the spray

parameters than the particle temperature. In general, the

coating hardness increased with increasing the particle

temperature and velocity and the coating porosity

decreased. Under the experimental conditions, the total

gas flow rate showed more influence than the powder

feed rate, which again had more influence than the spray

distance.

Asl et al, 2006 investigated WC-17Co coating

deposited onto ST37 mild steel substrate using HVOF

spray technique and then heat treated at different temperatures in a vacuum chamber. The coatings were

then evaluated in the as sprayed and heat treated

conditions. SEM/ XRD indicated that some brittle eta

(η) phases were produced at high temperature heat

treatments. Generation of these phases increased the

coating’s hardness and decreased fracture toughness of

the

coating. Wear test results showed that as sprayed

deposit had the best wear resistance and its wear

mechanism was sharp cutting abrasion.

Machio et al, 2005 investigated WC–Co and WC–

VC–Co coatings deposited on stainless steel substrates

using a high velocity oxy-fuel process. Both have been

tested under identical conditions in order to compare

their resistance to abrasion and slurry erosion. Wear

and erosion testing results show that the WC–VC–Co

coatings powders exhibit higher abrasion resistance than

commercial WC–Co coatings. In slurry erosion, the best performance of the VC-containing coatings is as good

as that of the commercial WC–Co coatings

Richert, 2011 investigated WC-Co-Cr, CrC

coatings deposited on fun blades by HVOF and Plasma

Spray techniques respectively. The results shows that

HVOF sprayed coats show more uniform and fine

grained microstructure than plasma sprayed coats. The

microhardness of the WC-Co carbide coating has been

found to be better than CrC coating.

The wear resistant strongly depends on the internal

micro-structure of coatings. The nanometric features

contributes to the increase of surface smoothness of coatings and increase the resistance against the wear. C

lima et al, 2003 investigates WC-Co and CrC coatings

produced on low carbon steel substrates by using two

types of equipment: a high pressure HVOF model JP-

5000 and a portable HVOF model TJ-4000. SEM/ XRD,

hardness and wear testing results shows WC-12%Co

coatings by two different HVOF JP-5000 and TJ-4000

have presented similar results with relationship to micro

hardness, morphology, microstructure and abrasive wear

resistance. Cr3C2–25 NiCr coatings sprayed by TJ-4000

have presented 64 % higher average volume loss than similar WC-Co system in three bodies’ abrasion wear

test. Jones et al, 2001 Examined behavior of FeCr–TiC

and NiCrCr3C2 coatings deposited by high velocity oxy-

Fig-1 Schematic representation of HVOF system (Karagöz M et al, 2011)



fuel (HVOF) spraying on mild steel substrates. Abrasive

wear was examined using a modified DSRW technique

and microstructure of coating reveals by SEM/XRD

techniques. It was found that the abrasive wear

resistance of the FeCr–TiC coatings better than the

NiCrCr3C2 coating sprayed from blended powder. Sahraoui et al, 2003 investigated the microstructure,

wear resistance and potentials of HVOF sprayed Cr3C2–

NiCr and WC–Co coatings for a possible replacement of

hard chromium plating in gas turbine components

repair. Friction and wear tests show the Coatings exhibit

high hardness with a high volume fraction of carbides

being preserved during the HVOF spraying process.

Hardness and wear resistance of the WC–Co coatings

were better than those of the Cr3C2–25NiCr coatings.

3. Coatings for hot corrosion Cr3C2-NiCr coating was deposited on SAE-347H

boiler steel by HVOF spray process and investigated at 700ºC for 50 cycles in Na2SO4-Fe2(SO4) molten salt, as

well as air environments by Kaur et al, 2009. The

results of HVOF spray Cr3C2-NiCr coating was found to

be successful in maintaining its adherence in both the

environments. The formation of chromium rich oxide

scale might have contributed for the better hot

corrosion/oxidation resistance in the coated steel.

Aalamialeagha et al, 2003 reveals that NiCr alloy

gas and water atomized powders sprayed by high

velocity oxy fuel with a gaseous propylene fuel and with

liquid fuel (kerosene) on mild steel substrates. The characterization and corrosion resistance of the coatings

was evaluated by use of a salt spray chamber and

potentiodynamic tests. Scanning electron microscopy

(SEM) and X-ray diffraction (XRD), results shows that

greatest corrosion protection to the steel substrate is

given by coatings produced from gas atomized NiCr

powders when sprayed by the liquid fuelled HVOF

system.

Sundararajan et al, 2004 investigated 80Ni-20Cr and

50Ni-50Cr coatings deposited by HVOF process and

APS (Air plasma spray) on 9Cr-1Mo steel substrate respectively. Steam oxidation test was carried out at

650°C for 100, 1000 and 3000 hours. SEM/EDAX and

XRD analysis shows HVOF coatings of both 80Ni-20Cr

and 50Ni-50Cr have good protection till 750°C by

forming Cr oxide as protective layer as compared to

APS.

Sidhu et al, 2006 investigated ASTM-SA210 boiler

tube steel, after depositing the Cr2O3–NiCr, WC–12Co

and stellite-6 powder and Ni–20Cr wire coating with

HVOF process by using oxygen and LPG as the fuel

gases. Cyclic oxidation was performed in molten salt

(Na2SO4–60% V2O5) at 900°C for 50 cycles. The studies were performed for uncoated and coated samples

for the purpose of comparison. The results of XRD,

EDAX and EPMA analysis shows the NiCr coating has

provided the protection to the base steel, which may be

due to the formation of protective oxides like NiO,

NiCr2O4 and Cr2O3. In dense layered structure, oxygen

has to travel a long distance along the grain boundary to attack substrate steels, which is believed to increase the

corrosion resistance.

Sidhu et al, 2006 formulated NiCr and Stellite-6

coatings on SA-210, T-11 and T-22 boiler tube steels by

HVOF technique using LPG as fuel gas. These coatings

have been examined for characterization by

SEM/EDAX and XRD techniques for describes the

transformations that take place during HVOF spraying.

The results of Stellite-6 coating were better than NiCr

coatings for low value of porosity and surface

roughness. Microhardness of the Stellite-6 coating has

higher hardness as compared to the NiCr coating, although both coatings have high hardness values

compared to the substrate steels.

Mahesh et al, 2010 investigated oxidation of T11 and

T22 boiler tube steels after depositing NiCrFeSiBalloy

coating with HVOF. The standard testing shows

microstructure of coatings has a dense and layered

structure with porosity less than 0.5%. The superior

performance of NiCrFeSiB coating can be attributed to

continuous and protective thin oxide scale of amorphous

SiO2 and Cr2O3 formed on the surface of the oxidized

coatings. Kaushal et al, 2010 examined Ni-20Cr coating

deposit by HVOF on 347H boiler steel specimens and

the samples with and without coating were exposed to

the super heater zone of a thermal power plant boiler at

a temperature of 973 K (700ºC) under cyclic conditions

to ascertain their erosion-corrosion (E-C) behavior.

Examination of samples revealed that coating were

found to have significant resistance to its oxide scale

spallation during cyclic oxidation exposures; moreover,

the coating was found to have retained its continuous

contact with the substrate steel during these thermal

cycles. This indicates that the coating has good adhesion strength.

Ramesh et al, 2009 HVOF sprayed Ni–5Al coatings

on Ni- and Fe-based superalloy substrates were

characterized to assess the microstructural features and

strength in the as deposition condition for their

applications in high-temperature corrosive environment

of gas turbine. SEM/EDAX, XRD and mapping results

shows coatings with less porosity and inclusions were

produced using HVOF process. Diffusion of alloying

elements from the substrate into the coating has

occurred in all the three superalloy substrates. Seong et al, 2000 evaluated corrosion resistance of

HVOF-sprayed Cr3C2NiCr coatings on heat exchanger

pipes of recuperators of steel mills. Three kinds of

corrosion tests under cyclic conditions were carried out



high-temperature oxidation tests in air, cyclic oxidation

tests in an SO2 environment, and a molten-salt corrosion

test. In this experiment, a platinum catalyst was used for

the fast transformation of SO2 into SO3. It was found

that Cr3C2NiCr coatings exhibited excellent corrosion

resistance in the molten salt as well as in the oxidation environment.

Conclusions: - • HVOF process provides dense coatings, which

are suitable for wear and high temperature

applications. The better adhesion strength,

lower porosity and high hardness of the HVOF coatings is attributed to the better mechanical

interlocking of the sprayed droplets with the

substrate due to the high kinetic energy

experienced by the impinging particles

• Carbide and cermet coatings are successfully

deposited with HVOF. Hence HVOF process

can be thought of engineering solution to

enhance surface against wear & corrosion

degradation and other surface phenomena.

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High Velocity Oxyfuel Spray Coatings for Wear and Corrosion Protection a Review

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