On deposition of Waspaloy coatings by cold
spray
S. Vezzù*1, S. Rech1, E. Vedelago1, G. P. Zanon2, G. Alfeo2, A. Scialpi2 andR. Huang3
The deposition of pore free and highly adhered Ni and Co superalloy coatings is of great interestfor engine design and gas turbine applications, both in case of maintenance repair and overhaul
operations as well for mechanical and chemical protection purposes in aeronautics and energy
applications. This study would like to give a wide overview about the capability of cold spraytechnology on this topic: two different commercially available deposition systems, Sulzer-CGT
Kinetiks 4000 and Plasma Giken PCS-1000, were compared, and deposition processes with both
nitrogen and helium as carrier gas have been explored. Microstructural investigation,
microhardness and adhesion results are reported to depict a preliminary scenario of coating
properties. Fully dense coatings with thickness .1 mm and adhesion .50 MPa are obtainedusing helium, while quite porous, 20 MPa adhered coatings are obtained using nitrogen. Finally,
general considerations about the potential applicability of those coatings for repair purpose inaerospace applications are pointed out.
Keywords: Nickel alloys, Waspaloy, Coating microstructure, MRO, PCS-1000, Kinetiks

This paper is part of a special issue on cold spray technology

Introduction
Superalloys are traditionally and extensively employed
for the realisation of gas turbine parts and hightemperature engines in aeronautic and energy applications thanks to their excellent mechanical, thermal andcorrosion resistance properties.1 These characteristics
are responsible at the same time for the high efforts
necessary for manufacturing and machining these parts,
leading, as a consequence, to high time and costs for
production. For these reasons, maintenance repair andoverhaul (MRO) operations and coating deposition
technolgies able to restore damaged parts are of greatinterest in this field.2 Nowadays, nickel and cobalt based
coatings used in industrial applications are mainly
deposited by thermal techniques: electric arc,3 air plasma
spray,4 high velocity oxy-fuel,5,6 laser,7 electrospark
deposition8 and joining by diffusion induced isothermal
solidification of a liquated insert metal9 are the widely
diffused methods to process high strength alloy coatings.
The major drawbacks of thermal spray techniques are
related to the thermal input during the deposition:10 for
example, on steel substrates, the deposition of the firstlayers of coating could involve the damage or increase inbrittlness at the substrate/coating interface where sometimes long discontinuity lines along the coating/substrateinterface may occur. On the other side, the occurrence of1
2
3

Veneto Nanotech scpa, Venice, Italy
AVIO Group, Rivalta di Torino, Brindisi, Italy
Plasma Giken Co. Ltd, Tokyo, Japan*Corresponding author, simone.vezzu@venetonanotech.it342

ß 2014 Institute of Materials, Minerals and Mining
Published by Maney on behalf of the InstituteReceived 27 June 2013; accepted 21 August 2013
DOI 10.1179/1743294413Y.0000000199

heat affected zones on precipitation alloys such as themost part of superalloys itself could lead to thesolubilisation or recrystallisation or simply grain growth
generally combined with a loss of mechanical properties
and strength. Furthermore, porosity and residual
stresses developed in the coating during the cooling
down from deposition temperature to room temperature
could allow pitting and subsequently promote blisteringand coating spallation in corrosive environments.11 It isclear that in this scenario the possibility to benefit from a
low temperature deposition process can be seen withgreat interest, and among the low temperature deposition
processes, cold spray (CS) has been receiving increased
attention during the last years for the deposition of
metallic and ceramic–metallic coatings. Cold spray is afully solid state powder deposition process, which allows
to produce coatings with reduced oxide content, lowresidual stresses and high efficiency.12 The reduced
porosity and oxide content in CS layers are the key
factor to suggest CS for the obtainment of high thickness
coatings with improved corrosion and oxidation resistance. Looking at the deposition of high strength
materials by CS, the main critical aspects are related tothe high plastic deformation of impinging particles
required for an optimised deposition. The extremely highyield strength, combined with the high melting temperature, is the main reason responsible for the high criticalvelocity for cold spraying of superalloys.13 Furthermore,
the quality of a coating (where quality means thecompactness of the microstructure with lack of porosity
and the mean mechanical properties, in particularcohesion and adhesion) increases progressively with theSurface Engineering

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difference between impact velocity and critical velocity.14
The increase in this difference that is the tool to maximise the coating quality can be obtained by two different
ways: the increase in the impact velocity and the
reduction of the critical velocity. Regarding the firstway, generally it is obtained by increasing as much as
possible the gas velocity, which is the reason of the effort
for the production of deposition system with higher
operating pressure and temperature. In this sense, it must
be taken into account that both pressure and temperautere influence the gas velocity, and in particular, theincrease in the inlet temperature results in larger gas
velocity after expansion through the de Laval nozzle. The
increase in the gas velocity can be also obtained bychanging the carrier gas, and in this sense, it is well known
that the use of helium is enourmosly beneficial.15
Regarding the second way, a general relation for criticalvelocity that can be considered is as follows14

1=2
As
zBcp ðTm {T Þncrit ~
r
where s is the yield strength of the sprayed material at the
impact temperature T, r is the density, cp is the specificheat and Tm is the melting temperature. A and B are
fitting costant determined empirically. Starting from thisrelation, it is clear that the reduction of critical velocity
can be obtained once again by the increase in the impact
temperature, T, which has the twofold effect to lower thecontribution (T–Tm) as well to lower the yield strength by
the phenomenon generally expressed as thermal softening
effect.16,17 An increase in the impact temperature could
also be obtained by heating the substrate18 or more
efficiently by heating the spraying particles either by
preheating the feeder or by increasing the stagnation timeusing longer heating chambers. Despite these possibilities,
considering the window of temperatures that could beconsidered in CS and that typically the YS at 500uC isroughly the 85–95% of YS at room temperature for atypical superalloy, we can state that the thermal softening
effect in CS of a high strength alloys is close to zero. Thus,
summarising, increasing as much as possible the impact
velocity seems to be the more effective and achievable
way to obtain high quality superalloy coatings by CS.
Nowadays, the deposition of superalloy coatings by CS is
still at research and development level, and someexamples are already reported in literature: for example,
the deposition and the characterisation of high temperature behaviour of Stellite-6 coatings are reported by Cinca
and Guilemany.19 Inconel 625 coatings have been
deposited onto steel substrate for oxidation resistancepurposes, and the effect of postdeposition laser remeltingon oxidation resistance has been explored by Poza et al.20
Structural analysis of cold sprayed nickel based metallic
and metallic–ceramic coatings is reported by Koivuluoto
and Vuoristo, where the enhancement of microstructurecompaction is obtained by optimising the powder feedstock and the spray parameters.21 A 6 mm thick Inconel
718 has been recently produced by Wong et al.22 for
exploring the opportunity to consider CS for rapid
manufacturing purposes, and the effect of postdeposition
thermal treatment on structural mechanical properties
has been evaluated. Ogawa and Seo deeply investigate theeffectiveness of CS technique for the repair of turbine
blades focusing the attention on Inconel 738 LC alloy.23
A leit motiv of these studies is that to deposit highDeposition of Waspaloy coatings by cold spraystrength materials and obtain high quality coatings, using
CS is necessary to force the deposition system at
maximum operating conditions in terms of gas pressure
and temperature, sometimes (when possible) also using
helium rather than oxygen to enhance the impact velocity
as much as possible. Moreover, in many cases, a
postdeposition treatment such as laser remelting or
thermal annealing is performed to enhance the mechanical and microstructural characteristics of as depositedcoatings generally characterised by only intermediate
strength and mechanical resistance.
This study would like to make clearer the situation
about the effectiveness of using CS technology for thedeposition of superalloy coatings. Waspaloy has been
selected as base and coating material because of its highmechanical properties and wide employment for the
manufacturing of aeronautic and industrial gas turbines;
two different commercially available deposition systems,
Sulzer-CGT Kinetiks 4000 and Plasma Giken PCS-1000,
were compared as well as the deposition processes withboth nitrogen and helium as carrier gas has been
explored. Only as deposited coatings have been considered in order to focus the attention on pure CS
performances.

Experimental
Powders
Commercially available gas atomised Waspaloy powders
have been employed in this study supplied by LPW
Technology Ltd. Certified size distribution was
245z15 mm evaluated by laser scattering (Mastersizer
2000, Malvern, UK). The powders exhibit spherical shapewith the presence of some small satellites stitched to theexterior surface as shown in scanning electron micrographs reported in Fig. 1. Powder microhardness wasmeasured by microindentation (Leica VMHT) with 15
and 25 gf resulting to 363¡29 and 328¡23 respectively.

Deposition
The coatings have been deposited using different
deposition system and process conditions. Firstly,
Kinetiks 4000 (Sulzer Metco Management AG,
Winterthur Switzerland) and PCS-1000 (Plasma Giken
Co. Ltd, Tokyo Japan) deposition systems have been
compared by optimising the deposition processes of asingle layer 0?6 up to 1?3 mm thick coating obtainedwith nitrogen as carrier gas. Then, the employment of
helium has been considered only with PCS-1000 system
for the deposition of a full thickness 1?2 mm single layer
coating and also a bilayer coating consisting of a,0?2 mm thick bond coat followed by a 1?0 mm top
coat deposited respectively with helium and nitrogen as
carrier gases. Deposition parameters close to the
maximum allowed by the respective plants have beenused in order to achieve the best coating performances.
Kinetiks 4000 system has been equipped with air cooledsilicon carbide nozzle (supplied by Impact Innovations
GmbH, Ampfing, Germany) that is able to enhance theprocess reliability delaying noticeably the clogging
occurrence. On the other hand, PCS-1000 deposition
system is equipped with standard water cooled nozzle.
Considering the substrates, the whole deposition plan
has been performed onto Waspaloy substrates in order
to simulate a general repair procedure of an aeronautic
turbine part. 5063063 mm plates obtained from

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Deposition of Waspaloy coatings by cold spray

1 Scanning electron micrographs of Waspaloy feedstock powders

Waspaloy AMS 5544 sheet has been employed forprocess optimisation and microstructural investigation,
while Ø25?4 mm substrates obtained from AMS 5708
bar has been employed for adhesion samples. Substrate
microhardness was 444¡96 Vickers (500 gf). Two
different substrate preparations were considered:machined and sandblasted (16 mesh corundum) finishing. Mean roughness, Ra, ,1 mm was measured in themachined surface, while ,8 mm was reported in the
sandblasted one. The layout of process conditions forthe most representative processes and produced specimens is reported in Table 1.

load was measured by a 50 kN load cell (MTS).
Averages and standard deviations of three measurements per set were calculated. Fractography investigation by scanning electron microscopy (SEM) was
performed on selected samples in order to haveinformation about the particle–particle bond. The
samples were prepared first by removing the substrateobtaining a free standing coating, then performing asmall cut on the back side of the coating to induce thefracture and then bending the free standing specimen up
to the rupture.

Coating characterisation

Results and discussion

The deposited coatings W0–W5 have been investigatedin order to depict the main microstructural andmechanical properties. The microstructure observation
was carried out by means of light optical microscope(Leica DM6000M) and scanning electron microscopy(FEI FIB-SEM dual beam NOVA 600NANO LAB,
Hillsboro, OR, USA). Porosity, as well particle deformation and qualitative aspect ratio, has been determinedby cross-section observation. The mechanical characterisation consisted of the evaluation of the coating
microhardness and coating adhesion. The Vickers
microhardness (Leica VMHTAUTO microhardness
tester) has been measured on as deposited mechanicallypolished up to 1 mm with water based diamondsuspension (Presi, Diamond Suspension Reflex LDP)
cross-section samples; indentation loads of 25 and
500 gf have been employed. The evaluation of coating
adhesion has been performed according to the norm
ASTM C633. The pull off apparatus was an MTS
mechanical testing machine (MTS, 14000 Technology
Drive, Eden Prairie, MN) under force control, and theMicrostructure
A set of Waspaloy coatings of thickness ranging from
0?7 to 1?5 mm have been sprayed onto Waspaloy
substrate, and five different coatings, W1–W5, according
to Table 1, have been selected and devoted to microstructural investigation. The coating W0 deposited with
N2 boosted Kinetiks 4000 onto relatively smooth surfaceof machined substrate exhibited no adhesion, and justafter the deposition, it is easily removed, leaving a brittle
free standing coating. Coating adhesion is one of the key
properties to validate a repair procedure, and for thisreason, the process and coating W0 has been excluded
from the characterisation plan. Figure 2 shows acollection of cross-section light optical micrographs ofsamples W1–W5. Both as deposited coatings andchemically etched (Beattie and Hagel, 5 mL H2SO4,
3 mL HNO3 and 92 mL HCl) are reported. The
characterisation has been focused taking into accounttwo main guidelines according to the coating preparation plan. First, a comparison between the performances

Table 1 Process conditions and set of samples investigated in this study

344

Sample iD

W0

Dep System
Gas
Temperature
Pressure
Powders
Substrate
Sub. Prep.

Kinetiks-4000
Nitrogennitrogen
800
500
800
3.5
3.5
3.5
Waspaloy LPW (245z15 mm)
Waspaloy
Machined
Sand blasted (corundum 16 mesh)

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W3

W4

PCS-1000
NitrogenHelium
1000
1000
5.0
3.0
Waspaloy LPW (245z15 mm)
Waspaloy
Machined

W5

HezN2
1000
3.0

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2 Light optical micrographs of cross-sectioned W1–W5 coatings; whole view, magniﬁcation 650 (left) and detail of substrate/coating interface after chemical etching, magniﬁcation 6200 (right)

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Deposition of Waspaloy coatings by cold spray

3 Total porosity of W1–W4 coatingsand characteristics of coatings was obtained, employing
nitrogen as carrier gas and different deposition systems,
coating W1–W2 and W3 respectively with Kinetiks and
PCS-1000 systems. Secondly, a comparison between
performances and characteristics of coatings was
obtained, employing nitrogen, helium and heliumznitrogen as carrier gas with PCS-1000 deposition systems,
W3, W4 and W5 coatings respectively. The first remark
concerns a noticeable difference in terms of coating
porosity as reported in Fig. 3; starting from the bottom,
PCS-1000 helium deposited coating, W4, exhibits
porosity ,0?4% (areal on cross-sectioned coating), while

the coatings deposited using nitrogen as carrier gas
exhibit porosity of ,1?3 and 0?8% in case of PCS-1000(W3) and Kinetiks (W2) deposition system respectively.
It must be taken into account that the evaluation of thewhole porosity is affected by the coating cohesion andpreparation procedure. During the grinding, someparticles weakly bonded, or rather characterised bylow cohesive energy, can be pulled out from the crosssection surface, leaving an apparent pore. This does not
represent real porosity but rather induced porosity andcannot be distinguished from the real one by the
micrographic observation. Thus, the measured porosity
is generally higher with respect to real porosity, and thisis in particular observed when coating characterised bylow cohesive energy is characterised. In this case, bothcoatings deposited with nitrogen are generally affectedby this problem. For this reason, the implemented
procedure was to perform automatic mechanical polishing of the whole set of investigated samples simultaneously in order to allow a correct comparison, in
relative, among the whole set. The coating W5 realisedby the superposition of a bond coat (He) and a top coat(N2) can be considered by the microstructural point ofview as a perfect bilayer, where the whole coating
consists of the sum of two separate layers with featuresand characteristics of W4 (pure helium) and W3 (purenitrogen) coatings. This is highly evident from the crosssection micrograph, where a sharp interface between the
two layers is observed.

4 Scanning electron micrograph of cross-sectioned coatings deposited with nitrogen as carrier gas with Kinetiks 4000
and PCS-1000 deposition system; substrate/coating interfaces of a W1, b W2 and c W3; coating microstructure of dW2 and e W3

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Concerning the comparison of coatings deposited
with nitrogen, a representative selection of cross-section
SEM images is shown in Fig. 4. The substrate/coatinginterface is shown in Fig. 4a–c in case of samples W1–
W3, while a detail of coating microstructure is emphasised in Fig. 4d and e in case of samples W2 and W3
respectively. The porosity is highlighted by white arrows
on the micrograph; sample W1 shows noticeable
porosity in particular at the interface with the substrateand very weak particle–particle deformation and bonding, and the contours of the particles are clearly visible
confirming weak cohesion and diffused porosity at theparticle–particle boundaries. Samples W2 and W3 show
no significant difference in coating porosity, microstructure and particle deformation as reported in Fig. 3 andFig. 4b–e. Slighter higher porosity is observed in sample
W3; its shape is mainly elongated and it is distributedand condensed along the contours of the particles. The
mean size of the porosity ranges from 1 to 10 mm inlength, while the width is mainly ,1–2 mm. The
observed particle deformation seems to be slightly
higher in W2 even if it is qualitatively low for both
samples; the presence of particles scarcely deformed isobserved, and in some cases, quasi-embedded particles
can be also observed as shown for example by the white
dotted line in case of coating W2 and W3 in Fig. 4d ande respectively. Among the important differences to point
out, first of all, the behaviour at the interface with thesubstrate emphasises the higher effectiveness of PCS1000 process that allows the deposition on machined
substrate, while the Kinetiks process requires thepreliminary sand blasting preparation to increase thesubstrate roughness and hence mechanical interlockingof deposited particles. In this sense, it must be noticed
that the PCS-1000 deposition process considered in thisstudy uses higher gas temperature and pressure than
K4000 (z200uC and z1?5 MPa respectively), which
could be one of the reasons of these variations and leavean open window in terms of further process optimisation. However, the general coating quality of bothcoatings is modest, the coatings delaminate and produce
some debris during grinding and machining, and somecracks often appear close to the interface with thesubstrate after shock impact or severe cutting operations
more or less empirically applied. Thus, the conclusion isthat this coating quality cannot be considered acceptable
for the MRO of aeronautical components. In this sense,
analysing the CS process, the critical velocity in case of
Waspaloy and superalloys in general, is very high mainly
due to the extremely high and stable yield strength also
at intermediate temperature; for example, the yieldstrength of Waspaloy (AMS5544) ranges between
795 MPa (at room temperature) to 725 MPa and675 MPa respectively at 540uC and 760uC, confirminga reduction of the YS of 9 and 16% only.24 Moreover,
540uC and in particular 760uC are abundantly out of theachievable particle temperature with a CS deposition
system even if special design with longer stagnation time
would be considered in order to promote the thermalisation between gas and particle temperature. As aconsequence, an increment of the particle–particle
cohesion and compaction of the microstructure led by
the plastic deformation of impacting particle due tothermal softening effect, as generally obtained withmaterials exhibiting superior reduction of the yield

Deposition of Waspaloy coatings by cold spraystrength versus temperature such as aluminium alloys,
can be neglected in this case. Furthermore the value ofyield strength of Waspaloy at room temperature is very
high too, in particular if compared to other metallic
materials traditionally applied by CS such as pure nickel
and copper. In fact, the YS of Waspaloy is 10 times
higher with respect to pure annealed nickel and 20 times
higher with respect to pure annealed copper well
depicting the complexity of processing Waspaloy andsuperalloys in general by CS. As a matter of fact, these
considerations confirm that to improve the growthmechanism and coating quality of Waspaloy, it ispossible to act only on the increase in particle impact
velocity and kinetic energy contribution. This statement
is the basis to understand the key role played by helium
for the obtainment of high quality coatings.
Concerning the comparison between W4 and W3
coatings, there are macroscopic differences to notice:first of all, the close to zero porosity of W4 as reported inFig. 3 both at the interface and in the whole crosssection. Then, the plastic deformation at the interface
with the substrate that is excellent in case of using
helium, very small and isolated porosity and no cracks
are observed and good mechanical interlock that is asignificant element to determine a good adhesion
strength. Plastic deformation is also observed at theparticle/particle interface with thin layered compact
microstructure due to the severe shear between particles
upon impact as highlighted by white arrows in Fig. 5aand b. The bilayered W5 coating showed the characteristics of W3 and W4 coatings as reported above.
Figure 5c and d shows a detail of the substrate tobondcoat interface (interface 1) and the interface
between bondcoat and topcoat (interface 2) respectively.
Interfaces 1 and 2 are also emphasised by the related
arrows in Fig. 5c and d. First, the substrate microstructure is totally unaffected by CS deposition as shownin Fig. 5c that is one of the key factor to appreciate CS
technology for coating and repair purposes. Then, aninteresting element is the observation of the intermediate
interface between the bond coat sprayed with He and the
top coat sprayed with N2 shown in Fig. 5d, where some
porosities and cracks are present in accordance with thegrowth of coatings sprayed with nitrogen, even if muchless severe than at the interface with substrate asreported in W1–W3 coatings. Thus, CS adhesion seems
to be more effective on CS coating rather than on bulk
substrate of the same material. This can be partially
unexpected considering that, set the same material, CS
produces hard and stiff coatings exhibiting low plastic
deformation behaviour. On the other side, CS leads tohigh substrate roughness and low surface oxidation thatcan be effective on promoting mechanical interlocking
with subsequent deposited coating, making CS suitablefor bond coat deposition. Finally, W4 coating emphasises once again the extremely different characteristics
achievable using either helium or nitrogen, and it could
represent a promising solution by the point of view ofthe process design because it combines the adhesion
strength requirements with a cost effective process,reducing the consumption of helium.

Microhardness
The Vickers microhardness of the whole set of samples isreported in Fig. 6. The hardness values obtained withapplied load of 25 gf is independent of the deposition

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5 Scanning electron micrograph of cross-sectioned a, b W4 and c, d W5 coatings deposited with helium as carrier gas
and PCS-1000 deposition system; detail of coating microstructure to emphasise particle–particle bonding on a helium
deposited coating and interfaces between c substrate and bondcoat and d bondcoat and topcoat on W5 sample

conditions and system employed. The highest value is579¡38 HV0?025 reported for W2 samples, while thelowest is 538¡41 HV0?025 for W1 sample, as expected
due to the low carrier gas temperature used. The
indentation imprints have been appositely positioned
inside a single particle avoiding the indentation on
particle–particle contours. The scattering of the data isquite constant for all samples and ,7%, revealing agood homogeneity of the hardness when investigated atthe ‘microscale’. Despite the high strength of Waspaloy,
the hardness increase due to the cold working induced
by deposition is noticeable, taking into account that thehardness of the powders is 328¡23 HV0?025 and theincrement is .40%. On the other hand, the hardness
values obtained with applied load of 500 gf behave very
differently. All coatings deposited with nitrogen as
carrier gas show a drop of the hardness and at the sametime a noticeable increase in the scattering of the data; inparticular, the hardness of W2 and W3 samples resulted
in 489¡92 and 457¡82 HV0?5 respectively. This dropis due to the low cohesion and particle–particle bondingin agreement with microstructural observation. In fact,
the 500 gf imprint is wide enough to cover moreparticles and a determinate portion of edges andsometime pores. Then, upon indentation, some cohesive
failures, crack formation and propagation, local sliding
or shearing of particles along the non-bonded particle–
particle edges happen as shown by the observation of theimprint shown for sample W3 in Fig. 7. In addition, thecoating deposited with helium confirmed the dataobtained with low indentation load rather with a further
slight increase in the hardness and reduction of thescattering of the data. Furthermore, the imprint shownin Fig. 7 confirmed also the high particle cohesion.

Adhesion

6 Vickers microhardness of W1–W4 coatings

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The coating adhesion has been measured according to
ASTM-C633 procedure, and the results are summarisedin Table 2. W0 coating show no adhesion to thesubstrate, and in general, the coating grown withKinetiks 4000 deposition system needs to be deposited
onto sand blasted substrates in order to be handled andmachined preventing delamination. Then, the adhesion

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7 Vickers imprint obtained with 500 gf indentation load on W3 and W4 coatingsis ,,5 MPa for the coating deposited at 500uC, while itreaches ,20 MPa in the case of coating deposited at
800uC. On the other hand, sample W3 obtained withnitrogen as carrier gas but with PCS-1000 CS showsadhesion ,20 MPa onto a machined substrate, which,however, is even too low to be considered acceptable formost application. The samples deposited with helium as
carrier gas confirm the microstructural observations andshow adhesion strength certainly higher than 50 MPa(the tensile strength of the FM1000 glue is reported to bearound this range), which is a significant value forconsidering this procedure in some selected aeronauticalapplications. A further feature of interest is the fact thatthe bilayered W5 specimen exhibits a behaviour,
regarding the adhesion to the substrate, close to thefully compact W4 specimen, giving the rupture at
the first interface between substrate and bond coat.
The second interface between bond coat (He) and top
coat (N2) that certainly represents a weak point for thecoating is, however, stronger with respect to the firstone. Furthermore, this also gives an information about
the cohesive strength of nitrogen deposited Waspaloy
coating that is still higher than 50 MPa; otherwise, thecohesive rupture of the top coat should be observed.

Fractography
Fractography investigation has been performed on
samples W2–W4 and is shown in Fig. 8. The results
confirm microstructural and mechanical investigations.
No significant difference is observed between coatings
W2 and W3 obtained with nitrogen as carrier gas
respectively with Kinetiks and PCS-1000 systems. The
cohesion is relatively low, and the fracture happened
along the particle–particle boundaries, revealing scarcecohesive features. On the other side, W4 sample deposited
with helium shows a compact microstructure with a
noticeable presence of dimples and particle–particle
cohesive bonds as highlighted by white arrows in Fig. 8d.

Conclusion
The deposition of Waspaloy coatings on Waspaloy
substrates has been performed by CS comparing theperformances of Sulzer-CGT Kinetiks 4000 and Plasma
Giken PCS-1000 deposition systems and comparing theuse of nitrogen or helium as carrier gas. The coating
microstructure, adhesion and hardness have beencharacterised.
1. The coatings deposited using N2/CS show quite
compact microstructure with porosity ,1%. However,particle deformation is limited, coating adhesion is,20 MPa and the weak particle–particle cohesion ispointed out by the crack propagation and coating failure
observed close to high load indentation imprints.
Kinetiks deposition system produces slightly morecompact coatings characterised by a hardness value
higher by ,15 Vickers even if the coating adhesion
seems to be more difficult and sand blasting substratepreparation is necessary to have sufficient adhesion toallow handling and machining of the specimen.
2. The coating deposited using He/CS shows excellent
characteristics: very compact microstructure with porosity ,0?4% and good particle–particle mechanicalinterlocking. The interface with the substrate is properly
deformed, and the coating adhesion resulted in certainly
higher than 50 MPa. Microhardness is ,560 Vickers
and shows no variation as a function of indentationload, confirming a good compactness of the microstructure and particle–particle cohesion.
3. A promising solution is proposed consisting of thedeposition of a bilayered coating composed by a fullycompact bond coat realised with helium as carrier gas
and a topcoat realised with nitrogen as carrier gas. The
microstructure and properties of each layer are inagreement with the related coatings deposited under
the same conditions; however, this process allows
obtaining high adhesion strength that is one of the main

Table 2 Coating adhesion measured following ASTM C633 tests
SampleConditions

Adhesion

W0
W1
W2
W3
W4
W5

Kinetiks 4000 – 4?0 MPa@800uC – nitrogen [substrate machined]
Kinetiks 4000 – 4?0 MPa@500uC – nitrogen [substrate sandblasted corundum 16 mesh]
Kinetiks 4000 – 4?0 MPa@800uC – nitrogen [substrate sandblasted corundum 16 mesh]
Plasma Giken PCS-1000 – 5?0 MPa@1000uC – nitrogen (substrate machined)
Plasma Giken PCS-1000 – 3?0 MPa@1000uC – helium (substrate machined)
Plasma Giken PCS-1000 – W3zW4 (substrate machined)

No adhesion/MPa
,5
20
20
.50
.50

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8 Scanning electron micrographs reporting fractographic investigation of W2, W3 and W4 coatings

characteristic required for MRO applications with a costsaving process.
Regarding the opportunity to implement the employment of CS for MRO of Waspaloy parts of aeronautic
gas turbines, certainly, the performances and characteristics of coatings obtained with helium as carrier gas canbe comparable to other thermal spray technologies.
However, process costs must be considered carefully due
to the well known economic impact of using high amountof helium. Today, CS with nitrogen is considered not yet
competitive by the qualitative point of view with current
performances obtained with thermal spray.25 Finally,
deposition of bilayer coating with He bond coat and N2
top coat could represent a promising compromise tobalance coating and process performances with operating
costs and could represent a suitable alternative to otherhigh velocity deposition processes.

Acknowledgements
This work has been carried out within the project LPT
Waspaloy Casing Repair co-funded by the P.O. PUGLIA
2007-2013 CONTRATTI DI PROGRAMMA REGIONALI - FE6.102933. The authors would like to thank
Alessandro Surpi for technical support and fruitful
discussion about coating characterisation.

References
1. R. R. Reed: ‘The superalloys: fundamentals and applications’;
2006, Cambridge, Cambridge University Press.
2. M. Okazaki: ‘High temperature strength nickel alloy coatings’, Sci.
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