role of rare earth elements in thermal spray coatings 2015

TEQIP-II Sponsored National Conference on Latest Developments in Materials, Manufacturing and Quality

Control on 19-20th February, 2015 (ISBN 978-93-5196-055-3)

Department of Mechanical Engineering, Giani Zail Singh Punjab Technical University Campus, Bathinda Page i

TEQIP-II Sponsored National Conference on Latest Developments in Materials, Manufacturing and Quality

Control on 19-20th February, 2015 (ISBN 978-93-5196-055-3)

Department of Mechanical Engineering, Giani Zail Singh Punjab Technical University Campus, Bathinda Page ii

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ISBN of Proceedings 978-93-5196-055-3

TEQIP-II Sponsored National Conference on Latest Developments in Materials, Manufacturing and Quality Control on 19-20th February, 2015 (ISBN 978-93-5196-055-3)

Department of Mechanical Engineering, Giani Zail Singh Punjab Technical University Campus, Bathinda Page 294

ROLE OF RARE EARTH ELEMENTS IN THERMAL SPRAY COATINGS

Harkulvinder Singh1*, Sukhpal Singh Chatha

2, Buta Singh Sidhu

3

1,2Yadavindra College of Engineering, Punjabi University Guru Kashi Campus, Talwandi Sabo, Punjab, India-151302

3Punjab Technical University, Jalandher, Punjab, India-151302

*Corresponding author email-id [email protected]

Abstract: Metals are unable to meet requirement for both

the high temperature strength and the high temperature

corrosion resistance, simultaneously, so protective

coatings are used to counter the latter. The primary aim

of the coating/surface treatment is the ability to

produce a stable,slow-growing surface protective oxide

providing a barrier between the coated alloy and the

environment. The oxidation resistance of alloy coatings

failures caused by the stresses generated in protective

oxide scales.Therefore, the processes of scale cracking

and spalling are the key factor to influence the lifetime

of coatings. In order to improve the adherence &

oxidation resistance of coating rare earth elements (La,

Zr, Ce, Y etc.) are added in the coating composition.It

is concluded that the scale nucleates at the reactive

element oxide particles on the surface; blocks short-

circuit diffusion paths by segregating reactive element

ions and reduces the stresses in oxide scale by altering

the microstructure.

1. INTRODUCTION

With the advancement in science and technology, the

expectations regard the life of the component in

working conditions also increased. Most of the

materials used in the modern industrial components

and systems often subjected to take failure at a

premature stage of their life, when component

subjected under high temperature environment (Singh

& Singh, 2014). Metals are unable to meet requirement

for both the high temperature strength and the high

temperature corrosion resistance, simultaneously, so

protective coatings are used to counter the latter (Sidhu

et al, 2006). Among the available alternatives for

metal surface protection in aggressive environments,

the thermal spray process has been widely researched

and used in recent years. (Brandolt et al, 2014). The

primary aim of the coating/surface treatment is the

ability to produce a stable,slow-growing surface

protective oxide providing a barrier between the

coated alloy and the environment (Chavla et al,

2013). The oxidation resistance of alloy coatings

failures caused by the stresses generated in protective

oxide scales. The stresses mainly consist of growth

and thermal stresses, due to which oxide scale

cracking and spallation take place. Therefore, the

processes of scale cracking and spalling are the key

factor to influence the lifetime of coatings (Yedong

et al, 2013).

The addition of rare-earth (RE) compounds in metals

realizes multiple functions, such as purification,

modification and alloying, and thus can improve a

range of properties of metals to various extents

(Zhang et al, 2008).

In order to improve the adherence & oxidation

resistance of coating rare earth elements (La, Zr, Ce,

Y etc.) are added in the coating composition.Reactive

element act as vacancy sinks to suppress void

formation at the interface of alloy & scale, formation

of oxide pegs at alloy-scale interface, segregation of

reactive element to the alloy-oxide interface to form

a graded seal which strengthens the alloy-scale bond

(kumar et al, 2014). It is concluded that the scale

nucleates at the reactive element oxide particles on the

surface; blocks short-circuit diffusion paths by

segregating reactive element ions and reduces the

stresses in oxide scale by altering the microstructure

(Seal et al, 2007). In this paper, beneficial effects of

rare earth elements discussed in order to better

understand the role of RE in the corrosion process.

2. RARE EARTH COATINGS

Ma et al, 1994 studied the effect of rare earth (RE)

oxides (Y2O3, Gd2O3) on hot corrosion of NiAl

coating deposited on M38G alloy material exposed to

Na2SO4+25wt.%K2SO4 fused salt at 850C. It was

found that the RE oxide addition in coating improves

the corrosion resistance and also lighten corrosion

degree of sulfides on the coating through the

formation of stable RE-oxygen sulfides. In another

study Bottino et al, 1995 examined that the oxidation

behaviour of CeO2 coating on AISI 347 grade stainless

steel subjected to non-isothermal and isothermal

oxidation tests at 1273K in dry air in a vertically placed

quartz tube reactor.SEM, EDS, EPMA and XRD results

shows CeO2 significantly improves the oxidation rate

by outward migration of cations to the ingress of

oxidant species and scale adherence to the alloy

substrate due to change grain size of the oxide scale by

pegging mechanism. Bonnet et al, 1996 used CVD

technique to deposit thin oxide films of Cr2O3, Al2O3,



Cr2O3+Nd2O3, Al2O3+ Nd2O3 and Sm2O3+A12O3-

elements on F17Ti steel substrate and exposed tohigh-

temperature oxidation in air at 1273 K for 40 cycles.In

cyclic thermal conditions, the Al2O3 coating appeared to

be better than the Cr2O3 coating. Moreover, it was

confirmed that the presence of a rare earth oxide has a

positive effect on the alloy corrosion resistance, even if

it is not large. Seal et al, 1998 investigated that the low

and high Cr steel samples coated with cerium oxide by

dipped into slurry of CeO2 powder dispersed in ethyl

alcohol and exposed to high temperature oxidation in

dry air at 923 K for 24 h. Outcome results showed that

the presence of Ce on the surface facilitates the

formation of an early Cr2O3 protective layer. Thus, the

ion migration has shifted from outward cation migration

to inward anion ingress and Ce allows transition metal

oxides to be more covalent bond, which improves the

oxide layers ability for slower oxygen uptake to

provide a more protective barrier. In another study

Nacan et al, 2003 observed that TiN coatings with

addition of rare earth element (0.4wt% Ce) deposited

on W18Cr4V high speed steel by means of vacuum

arc ion plating and exposed to Nacl solution at

600C. The results reveal that the microhardness of

the coating decreases slightly with the addition of

cerium. Cerium is beneficial to improving oxidation

resistance of the coating, therefore the oxygen atoms

cannot easily pass through the crystal defects or pore

space to contact the substrate.This may prevent the

forming of inner oxidized zone on the substrate

surface.

FeAl based coatings containing various amounts of

CeO2 (2 to 8 wt.%) were deposited onto carbon steel

by HVOF spraying, and exposed to H2SH2O2Ar

environment at 700C for 300 h.SEM, EDS and XRD

techniques result showed that the sulfidation

resistance of FeAl coating is improved by an addition

of 25 wt.% CeO2, which inhibits the outward

diffusion of Fe, acts as traps for sulphur in the splat

boundaries, and slows down the depletion of Al in

the coating (Chen & Xiao et al, 2006).Huiming et al,

2007 evaluated that the isothermal and cyclic

oxidation behaviors of chromium substrate with and

without nanometric CeO2 coating at and subjected to

cyclic oxidation at 900C in air. SEM, TEM and

HREM results found that improvement in oxidation

resistance of chromium is believed mainly due to that

ceria coating. CeO2 coating greatly reduced the

growth speed and grain size of Cr2O3. This fine

grained Cr2O3 oxide film might have better high

temperature plasticity. Meanwhile, ceria application

reduced the size and number of interfacial defects

and enhanced the adhesive property of Cr2O3 oxide

scale formed on Cr substrate.

Hot corrosion behaviour of Superfer 800H, Superco

605 and Superni 75 has been investigated after TSC

of Y2O3 in a Na2SO460%V2O5 environment at

900C for 50 cycles. The results revealed that the

Y2O3 coating provide better adhesion of the scale in

all the alloys due to dispersed oxide phases that act

as heterogeneous nucleation sites for oxide grains

thereby reducing the inter-nuclear distance, which

allows more rapid formation of a continuous chromia

film and produces a linear oxide grain size (Singh et

al, 2009). In another work Kamal et al, 2010

investigated that hot corrosion resistance of

detonation-gun-sprayed NiCrAlY + 0.4 wt.% CeO2

coatings on superalloys, namely, superni 75, superni

718, and superfer 800H in molten 40% Na2SO4-60%

V2O5 salt environment at 900C for 100

cycles.Coated superfer 800H alloy showed the

highest corrosion resistance among the examined

superalloys.Better performance of coated superfer

800H might be due to uniform, dense, thick scale

formed on the surface mainly consisting oxides of

Cr, Ni, Al, and the spinels of NiCr2O4 and NiAl2O4.

Presence of CeO2 with vanadium across the coating

depicts the formation of CeVO4, which might have

further contributed in reducing hot corrosion attack

as shown in fig.1.

Fig.1. Schematic diagram showing proposed hot corrosion

mechanism of the NiCrAlY + 0.4 wt.% CeO2 coated

superfer 800H at 900C in Na2SO4 + 60% V2O5 after 100

cycles (Kamal et al, 2010).

Yttrium was found to be segregated along the grain

boundaries of A12O3 and lowers the scale growth

rates. RE segregate to oxide grain boundaries, where

they can significantly reduce the outward transport of

Al, hence decrease the rate of oxidation and

contributed to the improved scale adherence and

reduced interfacial void formation.



Mahesh et al, 2010 observed that the high temperature

oxidation behaviour of HVOF sprayed ceria (0.4 wt.%)

added NiCrAlY coatings has been studied for bare and

coated Superni 76, Superni 750 and Superfer 800

superalloys in air at 900C.The NiCrAlY0.4 wt. %

CeO2 coated specimen showed negligible microspalling

of the scale. During the initial period of exposure,

oxygen penetrates into the coating through the open

pores and splat boundaries. As the oxidation process

continues, the top surface of the oxidized scale consists

of oxides of nickel, chromium, aluminum and spinel of

nickel and chromium. Oxides of aluminum and

chromium prevent the permeation of the oxidising

species into the coating and substrate superalloy.

Streaks of cerium oxide are found along the splat

boundaries, which assist in enhancing the oxidation

resistance of coatings. Cu-14Al-4.5Fe bronze coating

with and without addition of 0.6wt % Ce were

deposited with the help of atmospheric plasma spraying

on medium-carbon 45# steel substrate.The effects of Ce

on the coating interface bonding strength, coatings and

bonding interface microstructure were investigated by

tensile machine, XRD, SEM and EPMA analysis. The

results showed that small amount of Ce (0.6%) into Cu-

14Al-4.5Fe coating could improve diffusion between

the coatings and substrate, and refined the

microstructure of the coating. The addition of 0.6wt %

Ce tended to improve the metallurgical bonding

between the coatings and the commercial carbon 45#

steel substrate (Wensheng et al, 2011). in another

research Hussein et al, 2012 find out the effect of Al

with and without addition of cerium ( 0.5 wt%) were

simultaneously co-deposited on austenitic stainless

steel (AISI 316L) substrates by pack-cementation

process and exposed to 50wt.%

NaCl+50wt.%Na2SO4 salt environment at 750C for

120h. The results showed that both coated systems

reveal good cyclic oxidation resistance as compared to

uncoated one. Also, it was evident that cerium

improved the hot corrosion resistance of the silicon

modified aluminide coated 316L substrates. Gond et al,

2012 worked on NiCrAlY(bond coat) and Yttria-

Stabilised Zirconia (top coat) coatings deposited on a

T-91 boiler steel with the help of plasma spray

process. Hot corrosion studies were conducted on

uncoated as well as plasma spray coated specimens

in air as well as salt (75wt. % Na2SO4 + 25wt. %

NaCl) at 900C under cyclic conditions. XRD,

SEM/EDAX results showed that resistance to

corrosion enhances significantly which can be

attributed to formation of zirconium oxides (ZrO2)

and yttrium oxide (Y2O3). Mudgal et al, 2014

examined that D-gun sprayed Cr3C2-25(NiCr)

coatings deposited on superni 718, superni 600 and

superco 605 substrates with and without the addition

of 0.4wt% ceria powder. Hot corrosion test were runs

in 40%Na2SO4-40%K2SO4-10%Nacl-10%KCl

environment at 900C for 100 cycles. FESM, EDS

and XRD techniques result shows that addition of

ceria enhanced the adherence of oxide to the coating

and reduce overall weight gain. Cr3C2-(NiCr) +0.2

wt.% zirconium powder was sprayed on Superni 718

alloy by D-gun technique. The bare and coated alloys

were tested under Na2SO4 + K2SO4 + NaCl + KCl

and Na2SO4 + NaCl environment. It was found that

Cr3C2-NiCr coating proves to be beneficial in providing

better corrosion resistance to Superni 718 under molten

salt environment. Further Addition of 0.2wt.%Zr in

Cr3C2-25%(NiCr) coating greatly reduced the

oxidation rate as well as improved the adherence of

oxide scale to the coating surface during the time of

corrosion (Mudgal et al, 2014).

CONCLUSIONS

It can be concluded that rare earth elements play an

important role in the microstructual properties of the

coating and consequently in its oxidation resistance.

Rare earth elements can improve the apparent thermal

expansion coefficient of the coating and mitigate the

thermal expansion mismatch between the coating and

substrate. Hence, it can decrease the thermal stress, and

thereby improve the spallation resistance and the

durability of coating in high-temperature service. This

can lead to the improvement of fracture toughness and

tolerance to cracking and spallation of coating.

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role of rare earth elements in thermal spray coatings 2015

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