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Multilayer coatings by continuous detonation system spray technique

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Page 1: Multilayer coatings by continuous detonation system spray technique

Ž .Thin Solid Films 317 1998 259–265

Multilayer coatings by continuous detonation system spray technique

I. Fagoaga a,), J.L. Viviente a, P. Gavin a, J.M. Bronte b, J. Garcia b, J.A. Tagle b

a INASMET-CNPlasma, Gabiria 82-84, Irun, Spain´b IBERDROLA, Gardoki 8, Bilbao, Spain

Abstract

Surface protection of high-temperature components for power generation has become one of the most advanced fields of modernengineering. In this area, a multilayer coating of chromium oxiderchromium carbide for erosion–corrosion protection of furnace

Ž .boiler-walls has been produced by thermal spray continuous detonation system CDS technique. These composite coatings present amicrostructure made by alternate phases of chromium carbide cermet and oxides resulting from preferential oxidation of chromiumcompounds. Mechanisms for the generation of such mixed structure are discussed. Produced coatings have been characterized by electronprobe microanalysis, X-ray diffraction and ultramicrohardness techniques, combined with traditional procedures of metallographicpreparation, quantitative image analysis, and microhardness testing. q 1998 Elsevier Science S.A.

Keywords: Continuous detonation system; Chromium oxide; Chromium carbide; Multilayer coating; Surface protection

1. Introduction

Power station boiler-walls and other utility parts ofcoal-fired plants are subjected to frequent degradation byerosion–corrosion problems relevant to the reliability and

w xeconomics of these installations 1–6 . The environment ofthe furnaces is characterised by high-temperature condi-tions together with aggressive atmospheres, leading tocorrosive deposits adhered to the walls and to erosionprocesses due to the ash particles.

During the last years, this problem has been the focusof several research groups, which have generated theknowledge for the improvement of the usually used materi-als, and for the development of new structural materialsand coatings capable of withstanding these aggressive

w xconditions 5,7 . Presently, boiler walls are made of steeltubes welded in flat panels, which are frequently coatedwith thermal-sprayed aluminium. The field experience hasshown several practical limitations of this solution; there-fore, the need for enhanced coating systems.

High chromium compounds are specially suitable forcorrosive environments due to their ability to produce achromium oxide protective layer. As an example, the useof Ni50Cr or high chromium steel coatings for the protec-tion against corrosion in the cited boiler walls is well

) Corresponding author.

w xknown 5 . However, the presence of erosive environmentssuggests that the application of harder, wear-resistant coat-ing would perform better.

ŽCermet coatings, mainly carbide type coatings WC–Co.and Cr C –NiCr , have shown an outstanding perfor-3 2

w xmance in different industrial areas 8 . These coatings arecomposed of carbide particles reinforcing a metallic ma-

Ž .trix, combining the properties of ceramic- carbide typeŽ .materials high hardness , and the toughness and ductility

of metals. Cermets based in WC are used in applicationsŽ .working at a relatively low temperature -5008C and in

non-corrosive environments, while the Cr C –NiCr formu-3 2

lations are used in applications to aggressive atmospheresŽ . w xandror high-temperature -9008C conditions 9,10 .

These cermet coatings are industrially produced by topquality, high velocity spraying techniques, such as detona-

Ž .tion, high-velocity oxygen fuel HVOF or plasma Gator-Gard processes. In fact, their final performance is in strong

w xrelation with the employed spraying technology 11 .HVOF spray processes, such as the continuous detona-

Ž .tion system CDS , are frequently used to apply wear-re-sistant coatings. Their main advantage is the ability toform coatings with improved characteristics over thosesprayed with more traditional processes, like plasma, flameor arc spraying, and with equivalent characteristics inmany aspects to those produced by detonation spraying.This thermal spray technology uses the kinetic energy and

0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved.Ž .PII S0040-6090 97 00524-5

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( )I. Fagoaga et al.rThin Solid Films 317 1998 259–265260

the output of a supersonic flow of burned gases, to softenand to propel the spray powder, producing dense, very lowporosity and well-bonded coatings.

Due to these properties, Cr C –NiCr cermets applied3 2

by CDS spraying have been selected as one of the poten-tial materials to coat boiler furnace walls. In the industrialmarket, the application of such material produces coatingsof very different and complex microstructures composedof several carbides, metallic phases and oxides resultingfrom the interaction of the powder components during thespraying process. The presence of oxide at the lamellae’s

w xboundaries has been described as specially important 4,9 .They are mainly generated during the interaction of thesprayed particles with the atmosphere, but they can also beproduced by means of heat treatments after coating deposi-tion, thus increasing the hardness and the performance ofsuch coatings against erosive environments. The layeredmicrostructure formed has been reported as one of themain factors influencing the erosion resistance of thecoating due to the synergetic behaviour of different materi-als displayed into layered composite coatings. Hard phasesshould provide protection against low angle impact andmetallic phases should act as tough materials to preventhigh-angle erosion.

In this context, this work is focused on the productionof heterogeneous layered microstructures by CDS spraydeposition, specifically adapted to environments with parti-cle erosion processes. With this aim, different Cr C –NiCr3 2

powders have been sprayed looking for enhanced oxida-tion by in situ spraying procedures and by heat treatmentsafter deposition. The produced coatings have been evalu-ated studying their microstructure and the mechanicalproperties.

2. Experimental procedure

Two different types of Cr C –25NiCr commercial3 2

powders have been used in this work. Their main charac-teristics and composition are listed in Table 1. The powderMetco 81 VF consists of a heterogeneous, mechanicallyblended mixture of Cr C particles and Ni20Cr alloy3 2

particles, and the powder Amperit 584.072 is formed byhomogeneous, round, agglomerated–sintered particles ofCr C –NiCr. Crystallographically, both powders are com-3 2

posed of highly crystalline Cr C and cubic Ni phases.3 2

HVOF spraying has been performed using a commer-Ž .cial CDS-100 system Sulzer-Metco modified to introduce

Table 2CDS spray parameters

Ž .Spray parameters Propane C3H8

Ž .Oxygen SLPM 450Ž .Propane SLPM 55

Ž .Additional N SLPM y2Ž .Carrier gas N SLPM 202

Ž .Feed rate % 15Ž .Spray distance mm 300

y1Ž .Gun velocity mm s 10

an auxiliary natural-gas fuel line. This dual system offersthe possibility of working with two types of combustion

Ž .gases propane–propylene or natural gas , thus allowing adirect comparison between the coatings produced withthem. Oxy-propane is always used for ignition purposes,following the application of the selected functional gasflows. Nitrogen has been used as carrier gas to introducethe spray powders into the gun.

The substrate material consisted of 25 mm diameterA-213-T2 steel tubes currently employed in the manufac-ture of the flat panels for furnace walls. Previously to becoated, the substrates were cleaned and grit blasted with

Ž .white corundum FEPA 24 size .The CDS spray parameters employed are included in

Table 2, corresponding to the standard values for this typeof materials. Early tests showed that the use of natural gasas combustion gas reduced the level of oxidation in thecoatings, whatever the coating composition was. For thisreason, it was discarded, and all samples were sprayedwith propane flame. The kinematic conditions for sprayingŽ .distance, gun movement, etc. were kept constant for allthe materials. Also, substrate cooling by air jets has beenused in all the cases to avoid the risk of overheating andthermal stress failures.

After spraying, all the coated specimens were heat-treated in air at 9008C for 24 h to enhance cohesivenessand bonding, and to increase the oxidation of the coating.

Metallographic sections were prepared from all thecoated specimens by standard metallographic proceduresfor Cr C cermets. Optical and SEM microscopy, quantita-3 2

Ž .tive metallography Quantimet 550 and EPMA microanal-Ž .ysis JEOL 8600 equipped with EDS-WDS , were used for

microstructuralrcompositional evaluation. The crystallo-graphic phases of the coatings and the powders wereanalysed by XRD in a Siemens 500 diffractometer usingCu-K a monochromatized radiation. A scanning speed of

Table 1Cr C –NiCr cermet powders3 2

Commercial name Nominal composition Size Morphology and fabrication Major XRD phases

Metco 81VF Cr C –25NiCr 10–45 mm Mechanical blended Cr C , Ni3 2 3 2

Amperit 584.072 Cr C –25NiCr 10–36 mm Agglomerated–sintered Cr C , Ni3 2 3 2

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( )I. Fagoaga et al.rThin Solid Films 317 1998 259–265 261

y1 Ž .0.058 s was performed between 20–808 2u with anintensity of 40 mA and 30 kV.

Microhardness of the coatings was determined by stan-dard 300 g Vickers indentations in the cross-sections of themetallographic samples. Ten measurements were taken ineach coating, calculating the average value. Ultramicroin-

Ž .dentation Fisherscope was also employed for the me-chanical characterisation of the coatings.

3. Results and discussion

All cermet coatings are composed of hard reinforcingphases in a ductile metallic matrix; however, their mi-

crostructure changes in a wide range depending on theoriginal powders and the spray technique applied.

In this case, the use of blended powders produces aŽ .microstructure Fig. 1 with clearly different phases due to

the different behaviour of each type of particles during thespraying process. It shows a typical lamella structure ofsprayed coatings, with white phases corresponding to themetallic compound, dark phases corresponding to oxides,and grey phases corresponding to carbides of both roundedŽ .unmelted and flattened aspect.

When agglomerated–sintered powder is used, the mi-Ž .crostructure shows a more uniform aspect Fig. 2a . In

fact, it is difficult to establish a clear optical difference

Ž . Ž . Ž .Fig. 1. Metallographic section of CDS sprayed Cr C –NiCr coating-blended powders Mr81VF =1000 a as sprayed b after heat treatment.3 2

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( )I. Fagoaga et al.rThin Solid Films 317 1998 259–265262

Ž . Ž . Ž .Fig. 2. Metallographic section of CDS sprayed Cr C –NiCr coating-agglomerated–sintered powders Ar584.072 a as sprayed =200 b after heat3 2Ž .treatment =500 .

between the phases present in the coating, suggesting thatall the powder particles behave in a common way.

The crystallographic XRD analysis of the coatings pro-duced with the Cr C –NiCr powders shows, as general3 2

Žrule, a complex structure of chromium carbides Cr C ,7 3. Ž .Cr C and oxides Cr O into a semiamorphous Ni3 2 2 3

Ž .matrix with solution elements Fig. 3 . On the other hand,the degree of crystallinity of the coatings produced withagglomerated–sintered powders is lower than those pre-sented by the coatings produced with blended powders. Itseems that the high specific surface of the agglomerated–sintered powders produces more intensive melting of the

sprayed particles, specially of the metallic compound thatcoats the carbide grains in each powder particle, leading toamorphous coatings during the rapid solidification of thesemelted products.

Additionally, the behaviour of the Cr C compound in3 2

both types of powders is different. In the agglomerated–sintered powders, the carbide grains are protected by themetallic NiCr coat, reducing, in this way, the interaction ofthe carbides with the CDS flame, and therefore the subse-quent generation of low-carbon carbides and oxides. Thisexplains their homogeneous microstructure, the lack ofimportant oxide presence, and the rounded aspect of the

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( )I. Fagoaga et al.rThin Solid Films 317 1998 259–265 263

Ž . Ž . Ž .Fig. 3. XRD pattern of a powders; b CDS sprayed coatings; c heat-treated samples.

carbides observed in the SEM. When these coatings areŽaged, their microstructure remains visually unchanged Fig.

.2b , but a recrystallization process takes place, transform-ing the amorphous phase to a crystalline structure wherethe XRD peaks corresponding to Cr C are clearly identi-7 3

fied, including those corresponding to the original Cr C3 2

carbide, together with the usual patterns of Ni andchromium oxides.

Comparatively, the Cr C particles present in the3 2

blended-type powder are directly exposed to the sprayingenvironment. This produces an important decarburizationand oxidation of the carbide particles that can be observed

Ž .in the microstructure Fig. 4 , composed of alternate layersŽ .of oxides, flattened melted carbides and round isolated

Ž .particles unmelted carbides , together with the flattenedparticles of the metallic alloy. The XRD analysis of thesecoatings shows that the peaks corresponding to Cr C are3 2

practically not present in the spectra of the coating, noteven after the thermal treatment. This suggests that anintense interaction occurs between the carbides and theenvironment during the spraying process.

Ž .The EPMA analysis Table 3 of this microstructureconfirms the presence of a Ni metallic phase with Cr from

the original Ni20Cr particles. Carbides are present as somerounded unmelted particles of Cr C and flattened parti-3 2

cles of decarburized carbides forms as Cr C , always with7 3

some oxide content. Finally, dark oxides are present asCr O , which results mainly from the total decarburization2 3

of the original carbide particles. Mapping micrographsŽ . ŽFig. 4 show the distribution of the different elements Ni,

.Cr, O, C in the microstructure. The low level of diffusionof the C and the O into the metallic NiCr phase revealsthat the main transformations are related to the originalcarbide phases present in the powders.

The experimental Vickers hardness measurements showdifferent tendencies in agglomerated and in the blendedpowders, as well. The first type of powders producescoatings with higher microhardness in the as-sprayed con-

Ž .dition 961 HV0.3 , and it also remains relatively stableŽ .933 HV0.3 after heat treatments in the evaluated condi-tions. In turn, the blended powders produce coatings with

Ž .relatively low hardness 713 HV0.3 , which increases no-Ž .tably after ageing 883 HV0.3 .

The microstructural differences are responsible for thedifferent behaviour of both types of coatings. The homoge-neous coatings produced with the agglomerated–sintered

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( )I. Fagoaga et al.rThin Solid Films 317 1998 259–265264

Fig. 4. EPMA microanalysis of Cr C –NiCr coating-blended powders Mr81VF. Mapping of Ni, Cr, O, C elements.3 2

powders, present a large carbide retention into a finelydistributed amorphous metallic matrix. The large retentionof carbides together with the high cohesiveness of these

Table 3EPMA; EDS analysis of the different phases present in the coating-blendedpowders

Dark phases Cr 60% Ni 5% O 35%Flattened grey phases Cr 77% Ni 4% CrO 19%Round grey phases Cr 79% Ni 4% CrO 17%White phases Cr 18% Ni 82% y y

coatings are responsible for the high microhardness mea-sured. The high-temperature stability of this microstruc-ture, which does not show large transformations undertemperature exposure, explains the microhardness stabilityafter thermal treatment.

The coatings produced with the blended powder presentlow hardness metallic islands in the microstructure of thecoating, thus reducing the overall Vickers hardness. Ultra-

Ž .microindentation measurements 100 mN load reveal aŽ y2 .composite structure of hard 12 696 N mm ceramic

Ž y2 .phases and soft 3486 N mm metallic phases. The

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( )I. Fagoaga et al.rThin Solid Films 317 1998 259–265 265

effect of the heat treatments on such coatings is an in-Ž . Ž .creased oxidation q16% of the microstructure Fig. 1b ,

producing a harder coating. As an example, ultramicroin-dentation measurements with 1000 mN load show a hard-ness change from 4565 Nrmmy2 to 8215 N mmy2 afterthe oxidation treatment.

Despite the initial presumptions, and without consider-Žing other kind of factors like the cost of powder, for

.example , technically both types of coatings could beappropriate for the protection against erosion–corrosion ofthe boiler-walls. Presumably, a higher brittleness can beexpected in the coatings produced with agglomerated–sintered powders than in those sprayed with blended pow-ders, whose lamella structure could, in principle, offerbetter erosion resistance. In any case, in order to obtain afinal selection, testing in service conditions are presently

Žbeing carried out in a coal power station Iberdrola Power.Station at Lada, Asturias, Spain .

4. Conclusions

The HVOF-CDS sprayed Cr C –NiCr coatings present3 2

dense, high-quality microstructures with high potentialityfor surface protection of boiler walls against erosion–cor-rosion wastage.

The microstructural characteristics of the Cr C –NiCr3 2

sprayed coatings depend essentially on the type of spraypowder used.

Agglomerated–sintered powders produce highly homo-geneous, hard and stable coatings in a large range ofworking temperatures. These characteristics are mainly dueto the cohesiveness and the large carbide content of suchcoatings. In principle, the erosion behaviour of this mi-crostructures will presumably perform in a typical brittlemanner.

Blended powders produce coatings with a compositestructure of alternate layers of carbides and oxides, andisolated metallic lamellae. The microhardness of these

coatings depends not only on the carbide content, but alsoon the percentage and structure of the oxides produced,both during the spray process or during the post-sprayedheat treatments. The behaviour of these coatings againsterosion will presumably offer better results at high angleconditions, outperforming brittle coatings.

The decarburization of carbides and their oxidationappears as the main source for the formation of chromiumoxide in the coatings.

Although both agglomerated–sintered and blendedpowders seem to be adequate for protection of boiler wallsagainst erosion–corrosion, a conclusion will not beachieved until the in-service tests at Iberdrola-Lada powerstation have been finished.

Acknowledgements

This work has been conducted within the framework ofPlasmatek Project No. 32.0095.0 and was financed byIBERDROLA.

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