Characterization and Erosion

Behavior of Plasma Sprayed

NiCrAlY and Ni-20Cr Coatings on
an Fe-based Superalloy
Degradation of materials due to solid particle erosion is encountered in a variety of
S. B. Mishra1 engineering industries, either at room temperature or elevated temperatures. Nickel-
e-mail: surya_iitr@yahoo.co.in
based coatings are commonly used in applications where wear resistance, combined with

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oxidation or hot corrosion resistance, is required. In the present work, NiCrAlY and
K. Chandra Ni-20Cr metallic coatings were deposited on an iron-based superalloy by a shrouded
plasma spray process. The coatings were characterized by scanning electron microscopy,
S. Prakash optical microscopy, microhardness testing, and x-ray diffractometry. Erosion studies were
conducted using an air-jet erosion test rig at a velocity of 40 ms−1 and impingement
Department of Metallurgical and Materials
angles of 30 and 90 deg. Scanning electron microscopy was used to analyze the eroded
Engineering,
surfaces. 3D surface roughness profiles of the eroded samples were taken using a Veeco
Indian Institute of Technology Roorkee,
Optical Profilometer. NiCrAlY coatings had slightly lower average porosity and lower
Roorkee-247 667, India
microhardness as compared to Ni-20Cr coatings. The observed erosion rate of the
NiCrAlY coatings, however was lower than that of the Ni-20Cr coatings at both 30 and
90 deg impingement angles. Ni-20Cr coating had shown higher erosion rate at 90 deg
impingement angle than that at 30 deg, whereas the effect of impingement angle on the
erosion rate is negligible for plasma sprayed NiCrAlY coating. The higher bond strength
of NiCrAlY coating might be one of the major contributing factors for lower erosion rate
of NiCrAlY coating as compared to Ni-20Cr coating under the tested conditions. Erosion
mechanisms of plasma sprayed coatings are discussed. 关DOI: 10.1115/1.2197843兴

Keywords: solid particle erosion, plasma spray coatings, Superfer 800H, NiCrAlY,
Ni-20Cr, erosion mechanism

1 Introduction antioxidation coatings to take care of the erosion and corrosion

problems in energy generation systems. The MCrAlY 共where M is
Erosion and high-temperature oxidation by the impact of fly ash
Ni, Co, or a combination of both兲 bond coats provide a rough
and unburned carbon particles are the main problems in heat ex- surface for mechanical bonding of the ceramic top coat and mini-
changer tubes and other structural materials in coal-fired boilers mize the effect of the coefficient of thermal expansion mismatch
关1兴. Gas and steam turbines operate in the environments where the between the substrate and the top coat materials 关6兴. Thermal
ingestions of solid particles are inevitable and result in erosion of sprayed 50/50 nickel-chromium alloy is usually recommended as
materials. In such environments protective coatings on the surface an erosion-corrosion protection for boiler tubes in power genera-
of superalloys are frequently used 关2兴. Coating technology is a tion applications 关7兴.
rapidly growing technology in the field of materials. A combina- With the aim of practical application, an Fe-based superalloy,
tion of the development of materials specifically designed for ero- Superfer 800H 共32Ni-21Cr-1.5Mn-1Si-0.3Ti-0.3Al-0.1C and Bal-
sion resistance and the ability to create thin film coatings of these ance Fe兲 supplied by Mishra Dhatu Nigam Limited, Hyderabad, is
newly developed erosion resistant materials can provide good en- selected as a substrate material. This superalloy finds application
gineering solutions to erosion problems. Suitable coating tech- in steam boilers, furnace equipment, heat exchangers, and piping
niques also allow for regeneration of parts that have been rendered in the chemical industry, as well as reformer and baffle plates/
unusable by erosion. Coatings are primarily used to restrict sur- tubes in fertilizer plants. This work investigates the erosion behav-
face damage of components in practice, where other requirements ior of plasma sprayed Ni-22Cr-10Al-Y and Ni-20Cr coatings on
prevent the substitution of an inherently resistant material 关3兴. substrates of Superfer 800H superalloy by using an air jet erosion
Coatings impart properties to materials which without the coating test rig.
would not be possible.
Thermal spray is a well-developed surface engineering tech- 2 Experimental Procedure
nique that produces a wide range of coatings for diverse applica-
tions. Plasma spraying is the most flexible and versatile thermal 2.1 Development of Coatings. Two types of alloy powders,
spray process with respect to the sprayed materials 关4兴. Hidalgo et NiCrAlY and Ni-20Cr, were used in the study 共Table 1兲. The
al. 关5兴 have reported the use of plasma sprayed thin antiwear and NiCrAlY is also used as a bond coat in the Ni-20Cr coating to
provide a rough surface for improved mechanical bonding of the
top coat.
1
Corresponding author. Rectangular specimens of approximate size 30⫻ 30⫻ 5 mm3
Contributed by the Tribology Division of ASME for publication in the JOURNAL OF
TRIBOLOGY. Manuscript received October 21, 2005; final manuscript received March
were used in the present work. Before plasma spraying specimens
3, 2006. Review conducted by Thierry Blanchet. Paper presented at the World Tri- were grit blasted by alumina particles of grit size 40 mesh. The
bology Congress III 共WTC2005兲, September 12–16, 2005, Washington, DC, USA. grit blasting conditions used in the present work are listed in Table

Journal of Tribology Copyright © 2006 by ASME JULY 2006, Vol. 128 / 469
 Table 1 Coating powders

Chemical composition
Powder name 共wt. %兲 Particle size Designation

Ni-22Cr-10Al-1Y 22Cr-10Al-1Y-Bal. Ni −45 ␮m + 10 ␮m NiCrAlY

共Praxair NI-343兲
Ni-20Cr 80Ni-20Cr −45 ␮m + 5 ␮m Ni-20Cr
共Praxair NI-105兲

2. The plasma spraying was carried out within one hour of grit constant rate from a conveyor-belt-type feeder in the mixing
blasting. The substrates were preheated with the plasma torch to a chamber and then accelerated by passing the mixture through a
temperature of about 250– 300° C and efforts were made to keep tungsten carbide converging nozzle of 4 mm diameter. These ac-
the substrates’ temperature within this range throughout the coat- celerated particles impacted the specimen, which could be held at
ing process. 40 kW Miller 共USA兲 Thermal Plasma Spray Appara- various angles with respect to the impacting particles using an

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tus was used to apply the coatings. Argon was used as the powder adjustable sample holder. The feed rate of the particles could be
carrying and shielding gas. All the process parameters including controlled by monitoring the distance between the particle feeding
the spray distance were kept constant throughout the coating pro- hopper and the belt drive carrying the particles to the mixing
cess. The parameters of the argon shrouded plasma spray process chamber. The impact velocities of the particles could be varied by
are listed in Table 3. varying the pressure of the compressed air. The velocity of the
2.2 Characterization of the As-Sprayed Coatings. The mi- eroding particles was determined using a rotating disc method 关8兴.
crostructural features of the coated specimens were studied using In the present study silica sand 共␳ = 2600 kgm−3; Knoop hardness
standard metallographic techniques. A scanning electron micro- 880 关9兴兲 was used as erodent. The scanning electron micrograph
scope 共SEM兲 共LEO 435VP model兲 was used for surface analysis of silica sand particles used in the present work is shown in Fig. 2.
of the as sprayed coatings. A SEM attached with a Robinson Back The erosion test conditions utilized in the present study are
Scattered Detector 共RBSD兲 was used to obtain the backscattered listed in Table 4. A standard test procedure was employed for each
electron image 共BSEI兲 along the cross section of the as sprayed erosion test. The samples were cleaned in acetone, dried, weighed
coatings. Porosity and oxide contents 共averages of five measure- to an accuracy of 1 ⫻ 10−5 g using an electronic balance, eroded in
ments兲 of the coated samples were measured on a Zeiss Axiovert the test rig for 5 min, and then weighed again to determine weight
200 MAT Inverted optical microscope fitted with imaging soft- loss. The ratio of this weight loss to the weight of the eroding
ware Zeiss AxioVision Release 4.1 共Germany兲. Microhardness particles causing the loss 共i.e., testing time ⫻ particle feed rate兲
values 共average of ten measurements at each distance兲 of the coat- was then computed as the dimensionless incremental erosion rate.
ings were measured by Leitz’s Hardness Tester Mini Load-2 This procedure was repeated until the erosion rate attained a con-
共Made in Germany兲. The x-ray diffraction 共XRD兲 analysis was stant steady-state value as per ASTM standard G76-95 共2000兲
carried out using a Bruker AXS D-8 Advance Diffractometer 关10兴.
共Germany兲 with CuK␣ radiation. The bond strength of the coatings
was measured at Anod Plasma Spray Limited, Kanpur using
UTM. The dumb-bell shaped samples of standard specifications
were prepared and desired coating powders were sprayed on them.
After coating, one coated and one uncoated dumb-bell shaped
sample were glued together. The bond strengths of the coatings
were measured on Universal Testing Machine and the strength
values in MPa have been reported in Sec. 3.1.
2.3 Room Temperature Erosion Test. Erosion testing was
carried out using a solid particle erosion test rig 共Fig. 1兲 at the
Defence Metallurgical Research Laboratory, Hyderabad, India.
The rig consisted of an air compressor, a particle feeder, an air
particle mixing chamber, and an accelerating chamber. Dry com-
pressed air was mixed with the particles, which were fed at a

Table 2 Grit blasting conditions

Material used Alumina particles

Grit mesh size 40
Pressure 共MPa兲 0.49
Angle 共deg兲 90
Specimen distance 共mm兲 90–100

Table 3 Parameters of the argon shrouded plasma spray

process

Arc current 共A兲 700

Arc voltage 共V兲 35
Spraying distance 共mm兲 90–110
Plasma arc gas pressure 共MPa兲 0.4068
Carrier gas pressure 共MPa兲 0.2757
Spray gun nozzle diameter 共mm兲 6
Fig. 1 A schematic view of an air jet erosion test rig

470 / Vol. 128, JULY 2006 Transactions of the ASME

 Fig. 3 Scanning electron micrograph showing the surface
morphology of as-sprayed plasma coatings „a… NiCrAlY and „b…
Ni-20Cr coating on Superfer 800H superalloy

phase is also observed. The ␥-Ni phase shown by XRD of the

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as-sprayed Ni-20Cr coating might be an indication of a formation
Fig. 2 Scanning electron micrograph showing silica sand of nickel solid solution matrix in the coating. Wu et al. 关15兴 have
particles also reported a similar structure of plasma sprayed NiCrAlY
coatings.
The bond strength has been calculated by taking the average of
3 Results and Discussion three readings in each case and has been reported in Table 5. The
average bond strength of the NiCrAlY coating was found to be
3.1 Characterization of the As-Sprayed Coatings. SEM 40 MPa whereas that of Ni-20Cr coating was found to be 24 MPa.
morphologies for the plasma sprayed NiCrAlY and Ni-20Cr coat-
ings on a Superfer 800H superalloy substrate are shown in Fig. 3. 3.2 Surface Macrographs of Eroded Samples. Surface mac-
Microstructures revealed are typical for a plasma spray process rographs of the eroded samples are shown in Fig. 7. In all the
consisting of splats, which are irregular shaped with distinguish- samples, the erosion starts at the center first and then proceeds
able boundaries. From the back scattered electron image, the towards the edges of the samples. At a 30 deg impact angle, ma-
NiCrAlY coating can be seen 共Fig. 4兲. The micrographs show that terial is eroded, creating an elliptical shape depression, while at a
the interlayer bonding between the substrate and bond coat, as 90 deg impact angle, material is eroded, forming a circular de-
well as bond coat and top coat, is extremely clean and continuous. pression. The macrographs of eroded samples clearly reveal three
The as-coated structure is lamellar with some oxide inclusions and zones: a central area from where most of the eroded material has
open pores. Generally lamellar structure of the coating is evident. been produced, a second zone of faint color where somewhat
For the as-sprayed Ni-20Cr coating the inner coat layer represents lesser erosion can be seen, and a third outside region where a
the bond coat of NiCrAlY and the outer layer represents the Ni negligible amount of erosion is observed.
-20Cr coating. The average thickness of the coatings measured 3.3 Erosion Rate as a Function of Impingement Angle.
from the micrographs 共Fig. 4兲 is compiled in Table 5. The porosity
Figure 8 shows the effect of impingement angle 共30 and 90 deg兲
of the NiCrAlY coating was found to be in the range of 2–5%,
on erosion rate at an impact velocity of 40 ms−1 for the plasma
whereas the porosity of the Ni-20Cr coatings was found slightly sprayed coatings. From the macrographs it can be seen that ero-
higher than that of the NiCrAlY coating and is in the range 3–6% sion is higher during the initial cycles of study, thereafter a steady-
共Table 5兲. The oxide contents of both coatings are found to be in state erosion rate is achieved at both the impact angles. The
the range of 3–5% 共Table 5兲. The measured values of porosity for Ni-20Cr coating shows a higher erosion rate between the two
the coatings are almost in agreement with the findings of Miguel coatings. A histogram 共Fig. 9兲 shows that steady-state erosion rate
et al. 关11兴 and Erickson et al. 关12兴. The microhardness values of
of the NiCrAlY coating is only weakly dependent upon impact
the coatings have been compared with those reported by Miguel et
angle and exhibits only a small difference in the erosion rate at a
al. 关11兴, Mateos et al. 关13兴, and Sampath et al. 关14兴 and are found
30 and 90 deg impact angle, whereas in the case of the Ni-20Cr
to be in similar range.
coating this difference is appreciable. Thus the NiCrAlY coating
The microhardness profiles of these plasma sprayed coatings
on Superfer 800H superalloy gives rise to a flat erosion versus
are shown in Fig. 5. Of the two coatings, the maximum value of
angle plot.
the average microhardness was achieved by the Ni-20Cr coating
The erosion rate of Ni-20Cr coating is higher than that of
of the order of 686 HV, whereas the average microhardness of
NiCrAlY coating; this might also be attributed to the slightly
NiCrAlY coating was found to be 467 HV 共Table 5兲. Figure 6
higher porosity contents of this coating. This is in agreement with
shows the XRD patterns for the Ni-22Cr-10Al-1Y and Ni-20Cr
the findings of Levy 关16兴, in which he has reported that the greater
coatings on a Superfer 800H substrate in the as-sprayed condition.
␥ 共Ni兲 was the major phase observed in the NiCrAlY and
Ni-20Cr coatings, while in the NiCrAlY coatings, ␥⬘ 共Ni3Al兲

Table 4 Erosion conditions

Erodent material Silica sand 共irregular shape兲

Particle size 共␮m兲 150–212
Particle velocity 共ms−1兲 40± 3
Air pressure 共MPa兲 0.1961
Erodent feed rate 共gmin−1兲 5 ± 0.4
Impact angle 共deg兲 30, 90
Test temperature Room temperature
Nozzle diameter 共mm兲 4 Fig. 4 Scanning electron micrograph of as-sprayed plasma
Test time 共min兲 Cycles of 5 min sprayed „a… NiCrAlY and „b… Ni-20Cr coatings on Superfer 800H
superalloy along the cross-sectional BSEI

Journal of Tribology JULY 2006, Vol. 128 / 471

 Table 5 Average values of coating thickness, porosity range, oxide content range, and micro-
hardness range of plasma sprayed coatings

Average coating thickness

共␮m兲
Oxides Average
Porosity content bond
Bond coat Outer range range Microhardness strength
Coating 共NiCrAlY兲 coat Total 共%兲 共%兲 range 共HV兲 共MPa兲

Ni-22Cr-10Al-1Y 440 ¼ 440 2–5 3–5 385–530 40

Ni-20Cr 106 288 394 3–6 3–5 638–724 24

the porosity of the coating, the easier it is for the erodent particles
to knock off pieces of exposed surface, and the greater is the

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removal rate. It can be observed from Figs. 5 and 9 that increasing
erosion rates correspond to decreasing hardness and increasing
porosity contents. Similar findings for hardness and porosity of
the ceramic thermal barrier coatings have been reported by Davis
et al. 关17兴. Also Takeda et al. 关18兴 have reported that the hardness
does not contribute directly to the improvement of the erosion
resistance. Moreover, the erosion rates observed in this study are
low and similar to those obtained by Davis et al. 关17兴 for the
ceramic thermal barrier coatings. Further, the higher bond strength
of NiCrAlY coating as compared to Ni-20Cr coating might have
also contributed to the better erosion resistance of NiCrAlY coat-
ing under the tested condition.
Fig. 5 Microhardness profiles of as-sprayed NiCrAlY and The effect of impact angle 共30 and 90 deg兲 on the erosion re-
Ni-20Cr coatings with bond coat of NiCrAlY for Superfer 800H sults obtained for plasma sprayed NiCrAlY coating in this experi-
substrate along the cross section
ment is very small, as seen from Fig. 9. Identical observations
have been reported by Hidalgo et al. 关1兴 for plasma sprayed
NiCrBSiFe coating, whereas the effect of impact angle on erosion
rate is significant in case of Ni-20Cr coating. A histogram illus-
trating the steady-state erosion rate 共Fig. 9兲 clearly indicates the
higher erosion rate of Ni-20Cr coating at 90 deg impact angle
compared to that at a 30 deg impact angle.
3.4 Surface Morphology of Eroded Material. The SEM ob-
servations were made on the as-eroded surfaces of the plasma
sprayed coatings 共Fig. 10兲. These figures reveal severe plastic de-

Fig. 7 Macrographs of plasma spray-coated superalloys

eroded at a velocity of 40 ms−1 and impingement angles of 30
and 90 deg. „a… NiCrAlY coating at 30 deg, „b… NiCrAlY coating
Fig. 6 XRD of plasma-sprayed coatings on Superfer 800H su- at 90 deg, „c… Ni-20Cr coating at 30 deg, and „d… Ni-20Cr coat-
peralloy. „a… NiCrAlY coating and „b… Ni-20Cr coating ing at 90 deg.

472 / Vol. 128, JULY 2006 Transactions of the ASME

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Fig. 9 A histogram illustrating the steady-state erosion rate
„gg−1… of the plasma-sprayed NiCrAlY and Ni-20Cr coating on
Superfer 800H at 30 and 90 deg impact angle

surface roughnesses of the eroded NiCrAlY coating were found to

be 2.88 and 2.2 ␮m at 30 and 90 deg impact angles, respectively,
whereas those of Ni-20Cr coating were found to be 2.93 and
3.4 ␮m at 30 and 90 deg impact angles, respectively. Sari and
Sinmazcelik 关21兴 have also reported the higher roughness of the
eroded surfaces resulting from higher erosion rates.
3.5 Cross-Sectional Analysis. The cross-sectional SEMs
across the central line of eroded depression in the Ni-20Cr coating
on Superfer 800H superalloy substrate at a 90 deg impact angle
have been depicted in Fig. 12. The micrographs 共Fig. 12兲 indicate
some lips or platelets extruded out of the craters as a result of sand
particles impact and are identical to those reported by Levy 关20兴.
He reported that it is the extrusion that forms the lips or platelets.
According to him the platelets do not adhere to the surfaces over
which they are extruded and forged. Rather, they are attached to
some location along the extrusion path. Further, the presence of
platelets on the eroding surfaces has been reported extensively
关22–25兴. Levy 关20兴 had suggested a sequence of steady-state ero-
sion conditions where an extrusion first forms a single platelet,
followed by its subsequent spreading due to forging, and, finally,
its removal as a result of particles striking it.
Fig. 8 Erosion rate „gg−1… against cumulative mass of erodent
of plasma-sprayed coatings on Superfer 800H. „a… 30 deg im- 4 Conclusions
pact angle and „b… 90 deg impact angle.
1. The continuous interface between the substrate and the

formation at the surfaces. Figures 10共a兲 and 10共b兲 are the micro-
graphs of the eroded surface of plasma sprayed NiCrAlY coating
on Superfer 800H at a 30 and 90 deg impact angle. Figure 10共a兲
shows that the material removal takes place by lips formation with
plastic deformation 关19,20兴. Figure 10共b兲 shows that at a 90 deg
impact angle material removal by erosion takes place by a platelet
mechanism. The SEM micrograph of eroded Ni-20Cr coating at
30 deg impact angle 共Fig. 10共c兲兲 reveals ploughing with lips for-
mation and fracture as the dominant erosion mechanism. From the
SEM micrograph of eroded Ni-20Cr coating at 90 deg impact
angle 共Fig. 10共d兲兲, it can be seen that the material removal by
erosion take place by plastic deformation and lip fracture.
The formation of platelets by plastic deformation in Fig. 10
indicates that the material removal is by ductile erosion mecha-
nism. It can be seen from the eroded micrographs of 90 deg im-
pact angle, the hard erodent particles had plastically deformed the
surface of the samples and caused material removal by platelet
mechanism. Similar erosion behavior of the plasma sprayed
Ni-Cr coating has been observed by Hidalgo et al. 关1兴.
The 3D optical profiles of the eroded NiCrAlY and Ni-20Cr Fig. 10 Scanning electron micrographs of eroded plasma
coatings can be seen in Fig. 11, which shows the higher roughness sprayed coatings „a… and „b… NiCrAlY coating, and „c… and „d…
of the eroded Ni-20Cr coating at both impact angles. The average Ni-20Cr coating, at 30 and 90 deg impact angle, respectively

Journal of Tribology JULY 2006, Vol. 128 / 473

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Fig. 11 Three-dimensional optical profile of the eroded NiCrAlY and Ni-20Cr coating on Superfer 800H superalloy „a… and „b…
NiCrAlY coating, and „c… and „d… Ni-20Cr coating, at a 30 and 90 deg impact angle, respectively. Dimension of scanned area is
591Ã 449 ␮m2.

bond coat as well as the bond coat and the top coating some perceptible influence on the erosion rate of the
have been obtained for the plasma sprayed NiCrAlY and plasma sprayed coatings.
Ni-20Cr coatings. 6. The higher bond strength of NiCrAlY coating might be
2. NiCrAlY coatings had slightly lower average porosity one of the major contributing factors for better erosion
and lower microhardness than the Ni-20Cr coatings. resistance of NiCrAlY coating as compared to Ni-20Cr
3. The observed erosion rate of the NiCrAlY coating was coating under the given conditions.
lower than that of the Ni-20Cr coating at both 30 and
90 deg impingement angles. Acknowledgment
4. Ni-20Cr coating had shown higher erosion rate at the The authors are very thankful to ASME for giving permission
impingement angle of 90 deg than that at 30 deg, to publish the full length paper by modifying our extended ab-
whereas the effect of the impingement angle on the ero- stract titled “Solid Particle Erosion Behaviour of Plasma Sprayed
sion rate is negligible for plasma sprayed NiCrAlY coat- Coatings on a Fe-Based Superalloy” published in the WTC 2005
ing. conference proceeding. The authors wish to thank the Mishra
5. It can be inferred that the porosity contents may have Dhatu Nigam Limited, Hyderabad, India for supplying the super-
alloy substrates. The support and interaction of Dr. B. Venkatara-
man, Defence Metallurgical Research Laboratory, Hyderabad, In-
dia for allowing us to use the erosion test facility during this work
are gratefully acknowledged.

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Journal of Tribology JULY 2006, Vol. 128 / 475