����������������� � ����������������������� ����� �!�"���#�"$������%� � &���'���#�����(�)��*+���,��&"�,��-.�&/*��#�&����(�����

M. Tului1, L. Bertamini1, F. Casadei1, T. Valente2 and F. Carassiti3

1Centro Sviluppo Materiali S.p.A., Rome, Italy 2University of Rome “La Sapienza”, Rome, Italy

3University of Rome 3, Rome, Italy

��021436587:9;3Among surface engineering technologies, thermal spray methods offer the possibility to realise functional surfaces with metallic or ceramic coatings, from hundred microns up to few millimetres in thickness. Some examples of advanced applications in energy production plants are presented and discussed. Selected areas are: 1) protection of gas turbines components against hot gases degradation, by spraying ceramic coatings with a dedicated deposition technique; 2) thick ceramic coatings for innovative combustion chambers; 3) coatings for erosion protection of components for coal fired plants, where erosion is caused by pulverised coal and ashes in the flue gases; 4) plasma spray forming of heat exchanger components, by using materials able to withstand high temperature and severe corrosion environments. Demonstrative tubular samples produced by spraying different materials are presented. <>=�?:@�A 58B21

: plasma spray, TBC, HVOF, erosion protection, spray forming, heat exchanger.

CED ��FG365 A B2H29;36I A FThermal spray is a generic term for a group of technological processes suitable for

materials surface modifications, including the possibility to fabricate near net shape or self-

standing components. With these processes metallic, ceramic, cermet and some polymeric

materials in the form of powders or rod are fed to a torch, then heated to near or somewhat

above their melting points. The resulting droplets of materials are accelerated in a gas or

plasma stream and projected against the surface to be coated (substrate). On impact the

droplets flow into thin lamellar particles adhering the surface, overlapping and interlocking

as they solidify. Good adhesion values, in terms of interfacial strength, can be achieved

and obtainable coating thickness is in the range from 60 µm up to few millimetres. Many

variants of such kind of technologies exist and, among them, the present paper is focussed

on coatings deposited by either plasma spray or high velocity oxy-fuel (HVOF), techniques

which can produce high quality and reliable coatings in terms of specific properties

(adhesion, cohesion, hardness, chemico-physical characteristics, etc.)

Plasma spray is based on a plasma jet thermal source, temperatures as high as 30000 K (or

more) and particle velocity up to 550 m/s are characteristic features. Materials with high

melting point, like ceramics or refractory metals, can be deposited providing that they melt

coherently. Coating adhesion and cohesion properties, as well as porosity and density,

strongly depend on particle velocity before impact (strictly related to plasma-particle

interactions), on proper preparation of substrate surface and on deposition temperature.

Different process are available with modern equipments: APS (air plasma spray), VPS or

LPPS (vacuum/low pressure plasma spray), RPS (reactive plasma spray) and HPPS (high

pressure plasma spray). Proper selection of starting materials (powders) and of plasma

spray processing parameters are essential for the resulting coating quality.

HVOF technology is based on combustion process where fuel and oxygen are burnt in a

combustion chamber of a spraying torch. A Venturi nozzle, just after the combustion

chamber, accelerates combusted gas to very high velocity values and speed of injected

particles in the gas stream can reach values up to 1.000 m/s. Powder particles are partially

melted and impact the substrate with a very high kinetic energy, resulting in high density

coatings (more than 99% respect to theoretical value).

The described technologies can be used to deposit a wide variety of materials, furthermore

by controlling substrate temperature, a wide range of substrate materials (ranging from

superalloys to the plastic materials) can also be coated. Functional surfaces with improved

resistance to wear, corrosion and high temperature degradation phenomena can be realised.

J D LK2HMION2P = FG361Among the plasma spraying systems, a relevant role for R&D activities is played by the so

called Q2R�S�T , which stands for Controlled Atmosphere Plasma Spray system (fig. 1).

Fig. 1. Controlled atmosphere plasma spraying system (CAPS)

Actually in Italy only one example is available1 and it is managed together by University

of Rome "La Sapienza" and Centro Sviluppo Materiali SpA. The Q2R�S�T is a multi-function

system working from 10 to 4000 mbar in inert or reactive environment as well as in air. All

the spraying processes known as APS, VPS/LPPS, IPS, RPS, HPPS can be carried out in

the same chamber, at pressure level selected inside the above mentioned operative range. A

detailed description of the system can be found in Ref. [1].

HVOF1 equipment used was a TAFA JP5000 (fig. 2). It uses kerosene as fuel for the

combustion process, instead of acetylene or hydrogen. As a consequence the maximum

flame temperature is lower (up to 2500°C), but the process is less expensive.

1 Equipment installed at Centro Sviluppo Materiali Spa - Rome (Italy)

Metallographic specimens as well as large components with complex shape can be coated

with the two methods. By a proper selection of spraying parameters, Q2R(S�T can also allow

spray forming processes, resulting in near net shape or self-standing components.

Chemically or mechanically removable substrates are used in the last case.

Fig. 2. HVOF torch during deposition

U D ��NMNGVOIW9X7Y36I A F21ZY[]\E[_^Y` a;bdc#e f2ghe:bdb�iWa;b(jlkYenmoiqpErGs�tu^lv Q:wThe deposition of a thermal barrier coating (TBC) on the airfoil of the first stage vanes or

blades of a gas turbine, allows an increase of turbine inlet temperature (TIT), thus

improving engine efficiency [2,3]. One of the problem related with the deposition process

is the poor thermal shock resistance of TBC’s. An innovative methodology to deposit

ceramic TBC on gas turbine blades and vanes by plasma spraying, was recently developed

[4,5]. Such a methodology is based on the control of the temperature of the substrate

during the deposition, by impinging a cloud of liquid Ar on the substrate. Cooling effects

generate a thermal shock on the external ceramic layers being deposited, so inducing an

homogeneous growth of microcracks, without producing any segmentation cracks. This

pattern of microcracks improves significantly the thermal shock resistance of the coating.

An experimental study was carried out to optimise the plasma spraying parameters and

also the relative movement torch-substrate, to minimise any temperature gradients along

the substrate surface and within the coating. A selected number of specimens,

representative of a real component, were coated by using different robot programs. After

deposition, metallographic polished cross-sections were investigated by optical and by

scanning electron microscopy to assess coating homogeneity and microstructure (fig. 3).

Fig. 3. Micro-segmented PSZ (8%Y2O3-ZrO2)

With the optimised parameters (tab.1) four vanes, taken from the first stage of a land based

gas turbine, were coated on the whole airfoil with a 150 µm thick layer of NiCoCrAlY, as

a bond coat, and a 300 µm thick layer of partially stabilised 8% Y2O3-ZrO2 as a top coat.

Tab.1. TBC deposition parameters Four vanes land based gas turbine � = N A 1)Io36I A FxN25 = 1d1)HM5 =

Ar - 900 mbar � = N A 1)Io36I A Fx� = P>N = 5;7Y3yHM5 =60 °C �GN2587 ? IWF2z�B2IW143y7:FM9 =

100 mm Ar - 40 SPLM � VW7:1)Px7�z:7:1 = 1H2 - 10 SPLM � Ah@�= 5LIWF2N2HG3 3 A N{VO7:1)Px7�3 A 589l|

35 kW ��7:585;I = 5}z:7:1Ar - 2.5 SPLM

The four vanes were submitted to a cyclic oxidation test in a burner rig, simulating the

operative conditions of a gas turbine. They were exposed to a gas flow with the same

composition, temperature and velocity of the inlet gas as in a real gas turbines. The surface

temperature was monitored by an optical pyrometer and the internal temperature was

monitored by thermocouples. A temperature gradient higher than 100°C was measured

between the coating surface and the metallic substrate and after 550 hours cycles, the four

vanes did not evidence any sign of damage (fig. 4).

Fig. 4 Coated turbine vanes after 550 cycles of burner rig testing

TBC’s are also candidate materials for combustion chambers: to obtain higher inlet gas

temperature in the turbine, temperature in combustion chamber has to be increased. Major

industrial companies are carrying out R&D activities to develop components able to work

at temperature higher than 1450°C. To withstand such an operative condition, walls of the

combustion chamber should be protected by very thick plasma sprayed TBC (more than 2

mm). This requirement is a critical item, since residual stresses increase with coating

thickness and can result in very poor coating quality and unacceptable in-service

performance. To achieve the same heat insulation with lower coating thickness, porosity

content has to be increased and at the same time particular attention must be paid to

prevent any detrimental effect on cohesion properties of the deposited ceramic material.

Ongoing R&D work is actually carried out to fabricate coatings with controlled porosity

and satisfactory mechanical performance.

ZY[q~ Q kYenmoiqpErGsE�:kYba;b�kYs�iWkYp,��b�k moaljEmqiWkYpIn coal fired plants, several components are exposed to erosion attack due to pulverised

coal and ashes transported by the flue gases. Critical parts are those of the feeding system

(coal to burner), burner components, tubes and boiler walls directly facing the combustion

flame [6-10]. At present the erosion problem is faced by employing bulk components

realised with metallic materials alloyed with elements such as Ni, Co, Cr or with expensive

welding overlay procedures. Related costs are high including also maintenance

requirements. Prevention or reduction of damages, with prolonged service life, by means

of protective coatings is an attractive possibility.

Several materials are being experimented for erosion protection: metals, ceramics and

CerMet [11-15]. The latter is a composite material containing an hard ceramic phase, such

as tungsten or chromium carbide, in a metallic matrix (Co or NiCr alloys) to increase

materials toughness. Fig. 5 shows a representative structure of a chromium carbide-NiCr

coating: clear areas correspond to the NiCr alloy, dark areas to the carbides.

Fig. 5. Microstructure of chromium carbide in NiCr matrix coating

Comparisons of tribological behaviour of APS and HVOF coatings with respect to bulk

material have been investigated by erosion tests and adhesive wear tests. Metallic coatings

evidenced lower erosion resistance than the correspondent bulk materials. Ceramic

coatings presented higher erosion resistance, but prolonged testing time caused surface

cracks. The best results were achieved by using CerMet (300 µm thickness of chromium

carbide in NiCr matrix) deposited by HVOF, also under high temperature testing

conditions (tab. 2 and 3).

Tab. 2. Erosion test results (particles: Al2O3; angle of impact = 30°; distance from the target = 100 mm; time = 1 hour) ��7Y3 = 58IW7YV - = IWz:|G3 V A 1d1��d���

Chromium carbide-NiCr coating 0.23 AISI 309 0.68

SiC based brick 6.12

Tab. 3. High temperature adhesive wear test results (reciprocating motion between a coated sample

and an AISI 52100 carbon steel; stroke = 2 mm; frequency = 50 Hz;

temperature = 350 °C; contact load = 30 N; time = 30 minutes) - = IWz:|G3 V A 1d1���;� �� J2CE�:��d���- = IWz:|G3,z:7:IWF9l|25 A PxIOHMP�9l7:580MIOB =}� ��IO��5$�������9 A 7Y3yIWF2z��d���

0,220% 0,003% The CerMet coating shown weight gain due to materials transfer from the steel

counterbody (fig.6).

Fig. 6. Wear trace on a chromium carbide-NiCr coating: EDS line analysis for iron after testing

ZY[OZ}��a;enm�a��Yj8`:e:pEr2a;bLmo� gYa8sVery high temperature heat exchangers are going to become critical systems in future

innovative plants for energy production, which requires heat exchanger components able to

withstand very high working temperatures and severe corrosion environments. To this

purpose, bulk ceramics, CMC’s (Ceramic Matrix Composites) and refractory metals have

been proposed. Plasma spray forming (fig. 7) is an effective production technology for

such a kind of components.

Fig. 7 Plasma spray forming process for tubular components

Thick layers of selected material are deposited onto removable substrates to obtain self-

standing components. This methods is very suitable for cylindrical parts. Demonstrative

tubular components have been fabricated, spraying refractory metals like Mo and W [16],

and ceramics (ZrO2, Al2O3) onto graphite bars. A further advantage of the spray forming

technique is the possibility to fabricate pipes with a protective layer on the internal surface,

e.g. to prevent corrosion, or with a graded materials composition. In fig. 8 four tube-

prototypes made of refractory metals are shown, their main characteristics are reported in

tab.4. On these components, finite element analysis was performed prior to the fabrication,

to establish spraying guidelines. Internal stresses under operating conditions (outside and

inside gas temperature, 1600°C and 1300°C respectively), influence of coating thickness

on thermal exchange behaviour, inside and outside skin temperature were assessed. Some

calculation results for a 4mm thick W tube are reported in fig. 8.

Fig. 8. Heat exchanger tube-prototypes fabricated by spray forming technology

Tab.4. Heat exchangers tube-prototypes fabricated by spray forming ��7Y3 = 58IW7YVO1 � = F2zY36|�uP>P+� -�7YVoV 36|2IW9X�GF = 1d1�

µP+�

�����L CW 400 3000 �����L J

W-Ni 400 3000 �����L UW + protective

ceramic layers on the external and internal

surfaces

400

3000 (W)

300 (ceramic) �����L!�

W-Ni + protective ceramic layers on the external and internal

surfaces

400

3000 (W-Ni) 300 (ceramic)

Thermal exchange (y = kcal/h) due to the convection, in relation to the external diameter of the tube (x = m).

Fig. 9. Von Mises equivalent stresses for a thick (4 mm) W tubes

� D � A FM98VWH21)I A F21The applications of thermal spray technologies are extremely varied, even though the

largest categories of use are to enhance wear and/or corrosion resistance, also for high

Material = W sprayed with plasma (E = 70 GPa; α = 4,4*10-6 K-1) Wall thickness = 4 mm External skin temperature= 1350 °C Internal skin temperature = 1450 °C Internal pressure = 1,3 bar External pressure = 1 bar

Material = W External gas temperature = 1600 °C Interna gas temperature = 1300 °C Internal diameter of the tube = 0.035 m

temperature applications. Due to unique lamellar microstructure and porosity, the thermal

conductivity of thermal sprayed coatings is usually anisotropic and significantly less than

that of their wrought or sintered counterparts. This property makes selected coatings

suitable to be used as thermal barrier for gas-turbine combustors, shroud and vanes as well

as for internal combustion cylinders and valves. Plasma sprayed coatings and plasma

forming technologies are concurrent technologies with respect to sintering based processes

especially for refractory metals and graded ceramic components. They can have a

significant impact on components and performance of next generation energy production

plants. For more conventional problems, as erosion protection, HVOF cermet coatings

appears to be a more realistic choice due to higher achievable coating density which

develops suitable engineering resistance to abrasive, erosive and adhesive wear.

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