adhesion testing of thermally sprayed and laser deposited coatings 2004 surface and coatings...

Surface and Coatings Technology 184(2004) 208–218

0257-8972/04/$ - see front matter� 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2003.11.008

Adhesion testing of thermally sprayed and laser deposited coatings

Anders Hjornhede, Anders Nylund*¨

Department of Materials Science and Engineering, Chalmers University of Technology, SE – 412 96 Goteborg, Sweden¨

Received 3 June 2003; accepted in revised form 3 November 2003Available Online 27 February 2004

Abstract

Commercial coatings were deposited on low-alloyed steel tubes(Fe1Cr0.5Mo) by arc spray, HVOF(high velocity oxy fuel)and laser cladding. The adhesion strength was tested with two methods: acoustic emission and a combination of four pointbending and metallography. The agreement between the results obtained from the two different experimental techniques is verygood. Laser coatings showed no delamination for strains up to 15%, while coatings deposited with the arc spray and HVOFprocesses delaminated in the strain intervals 1.4–1.9% and 0.8–1.8%, respectively. The suggested delamination mechanism is theinitial formation of a radial crack in the coating after which the coatingysubstrate interface comes under an increased tension loadand fractures. Arc sprayed coatings of Metcoloy 2(Fe13Cr) mixed with the binder 80Ni20Al show a strongly improved adhesionstrength if the splat size is sufficiently large. The delamination interval increases to 10.5–11.5%. However, for small splats theeffect is eliminated.� 2003 Elsevier B.V. All rights reserved.

Keywords: Thermal spraying; Laser coating; Acoustic emission; Adhesion testing

1. Introduction

Components in combustion fluidised beds are sub-jected to erosion–corrosion due to erosive bed particlesand corrosive species in the gas phase. The degradationrate of low–alloyed steels in such environments is notacceptable. The use of thicker equipment walls andhigh–alloyed steels prolongs the lifetime of the com-ponents but at the expense of a high physical weightand cost. An alternative solution is to cover low–alloyedsteels with an erosion–corrosion resistant coating. Tra-ditionally, thermal spray methods like electric wire arcspray have been used due to the simple processing andlow cost w1x. The technique also has the advantage thatit makes the possibility of in-situ coating during serviceand repair possible. A more novel method is highvelocity oxygen fuel(HVOF) where the use of powderresults in denser coatings, lower oxide contents and awider range of possible coating materials, includingcarbides and ceramicsw2x. As a third alternative, laser

*Corresponding author. Tel.:q46-31-7721263; fax:q46-31-7721313.

E-mail addresses: [email protected](A. Nylund),[email protected](A. Hjornhede).¨

cladding has been introduced since lasers have becomemore sophisticated, smaller and cost effective. The mainadvantages of the laser coatings are their strong adhesionto the substrate and their low porosity and oxide contentw3x.To obtain the optimum quality of an applied coating,

the degradation rate in the chemical environment mustbe minimised and the adhesion strength to the substratemaximised. Due to different thermal expansion coeffi-cients for the coating and substrate, material stresses areinduced when coated components are used in hightemperature applications. The stresses are intensified bytemperature gradients originating from the temperaturedifference between the temperatures of the fire and thesteam sides of the coated tubes. Further, in the case ofpower plants temperature fluctuations during service,expose the coated surfaces to thermally induced strainswith an increased risk of delamination. The adhesionstrength of coatings is traditionally evaluated by tensiletesting, e.g. ASTM C 633 or bending tests followed bymetallographyw1x. However, these methods have thedrawback of requiring optical investigations to identifythe induced damages. An option would be the use ofacoustic emission(AE) where the initiation and devel-

209A. Hjornhede, A. Nylund / Surface and Coatings Technology 184 (2004) 208–218¨

Table 1The composition of the coatings used in this study

Coating Composition(wt.%)

Arc sprayMetcoloy 2 Fe12.4Cr0.6Ni0.4Mn0.4Si0.36C(Air or N , small splat size)2

80Ni20Al-binder, 80Ni20Al(Air, small splat size)

Metcoloy 2q80Ni20Al. (Air 50 (Metcoloy 2)q50 (80Ni20Al)or N , large splat size)2

Metcoloy 2q80Ni20Al-binder. 50(Metcoloy 2)q50 (80Ni20Al)(Air, small splat size)

HVOFMetcoloy 2 Fe12.4Cr0.6Ni0.4Mn0.4Si0.36CAmperit 526 83WC17CoMetco 3007 80Cr C –16Ni4Cr3 2

LaserMetcoloy 2 Fe12.4Cr0.6Ni0.4Mn0.4Si0.36CInconel 625 Ni21.5Cr9.0Mo3.6Nb2.5Fe0.3Si0.05CDuroc 5177 Ni26.8Cr8.4Mo1.7Fe1.40C0.9Nb0.7SiDuroc 17=1% C Ni25.0Cr8.8Mo1.9Fe1.9Nb1.00C0.7SiStellite 6 Co28.5Cr4.2W1.1C1.0Si-2.0Ni-1.5FeStellite 21qTiC 85(Co27.0Cr5.5Mo2.8Ni0.9Si0.25C-2.0Fe)q15(Ti19.4C)Duroc 5177qTiC 85(Ni26.8Cr8.4Mo1.7Fe1.40C0.9Nb0.7Si)q15(Ti19.4C)Duroc 17=1% CqTiC 85(Ni25.0Cr8.8Mo1.9Fe1.9Nb1.00C0.7Si)q15(Ti19.4C)Stellite 6qTiC 85(Co28.5Cr4.2W1.1C1.0Si-2.0Ni-1.5Fe)q15(Ti19.4C)

opment of cracks is continuously monitored during thetensile testing. The technique has been successfullyapplied to plasma sprayed coatings, carbon fibre mate-rials, composites, thermal barrier coatings and hard metalcoatingsw4–9x. The aim of this paper is to further applythe method of acoustic emission and use it for analysingthe adhesion of metallic coatings on a substrate of low-alloyed steel tubes. The results are compared with thosefrom traditional testing techniques.

2. Experimental

2.1. Raw materials and coating production

The adhesion strength of coatings deposited with laser,HVOF and arc spray(air or N as carrier gas) was2

tested. The thermally sprayed coatings were depositedin accordance with the recommendations given by thesuppliers, with the exception of the large splat coatings,where the parameters were slightly altered. The deposi-tion parameters used in the laser process are not allowedto be published. The coatings and their chemical com-positions can be seen from Table 1.Metcoloy 2 (wire) and Metco 3007(powder) are

standard products manufactured by Sulzer Metco. Thepowder with the same composition as Metcoloy 2 butused for the HVOF and laser deposition techniques weremanufactured by Hoganas AB. The Amperit 526, Stellite¨ ¨6 and 21, Inconel 625 and Duroc materials were manu-factured by H.C. Starck GmbH, Deloro Stellite, Special

Metals Corp. and Duroc Energy AB, respectively. Thesize grades for the powder used with laser and HVOFwere 63–150mm and 5–63mm, respectively. Thethicknesses of the coatings were in the range 0.7–1.2mm for laser, 0.4–0.6 mm for arc spray and 0.2–0.4mm for HVOF. As substrate, steel tubes of a typenormally used in high-pressure applications,Fe0.1C1Cr0.5Mo0.5Mn0.2Si, were chosen. The surfaceroughness of the arc sprayed and HVOF depositedsubstrate tubes was 5–7mm and 10mm, respectively(R -values). The Metcoloy 2 and 80Ni20Al materialsa

were supplied as the two separate electrodes in the arcspray process, resulting in a coating consisting of 50%of each material. Two different arc spray droplet sizes,50 and 200mm, were examined for the Metcoloy 2q80Ni20Al-coating.The adhesion of the coatings to the substrate was

tested with the three methods as described below; aspecially designed tensile test, a four-point bending testfollowed by metallography and a four-point bending testcombined with acoustic emission(AE).

2.2. Tensile testing

The adhesion of the coatings to the substrate wastested according to a tensile testing method developedfrom the ASTM 633 test. The modified design was usedin order to simulate the tube geometry. A 10=15 mmstring coating was deposited on the tubes(externaldiameter 33.7 mm, wall thickness 3.6 mm and length

210 A. Hjornhede, A. Nylund / Surface and Coatings Technology 184 (2004) 208–218¨

Fig. 1. Adhesion testing; experimental set-up for the tensile test.

Fig. 2. (a) Adhesion testing; experimental set-up for the four point bending and acoustic emission tests.(b) Scooped supports used in the four-point bending test.

approx. 20 mm). A cylinder (external diameter 8.0 mm)with the same curvature as the envelope surface of thetube was glued on top of the coated string, Fig. 1. Thetensile strength of the glue used, FM-1000 , is 69 MPa�

after hardening. Peeling forces caused by misalignedattachment of the cylinder were reduced by using abrace designed as a hook and equipped with a sphericalprong. A total of 54 samples were tested.

2.3. Four-point bending test combined with metallogra-phy and acoustic emission analysis

2.3.1. Experimental setupThe experimental set-up is illustrated in Fig. 2a. The

dimensions of the tubes used in this test were the sameas in the tensile test except for the length, which was300 mm. A 100=10 mm wide coating was depositedin the axial direction of the tubes and then subjected tofour-point bending at a displacement speed of 0.25 mmymin. The displacement was measured with a positiongauge centred on the coating surface. The distances

between the supports and the load applicators were 250mm and 25 mm, respectively. All cylindrical supportsand load applicators were scooped to the same radiusas the tubes, Fig. 2b, and MoS was supplied in the2

scoops before the tests in order to reduce the fretting.Due to the tube geometry the test is not a pure four-point bending test. Rather the stress load is described asa parabola with its maximum between the two loadapplicators.

2.3.2. Strain calibrationIn order to determine the strain as a function of the

displacement, five strain gauges were glued on uncoatedtubes in the same area as the coating. One strain gaugewas positioned at the centre of the assumed coatingarea, two at a distance of"5 mm off centre in theradial direction, and two at a distance of"15 mm offcentre in the axial direction. The tubes were then bentto different displacements. The largest strain was alwaysmeasured in the central position. This was then used toestablish a calibration curve from which the displace-ments applied to the coated tubes were translated intostrain. The presence of a coating on the surface willinfluence the strain distribution somewhat. In the caseof thermally sprayed coatings, the influence is limiteddue to the relatively low adhesion strength. Further, ina comparative study, like this, the deviation is about thesame among the coatings. The influence on the lasercoatings is larger, but due to the complex deformationin the coated area of the tube, no adjustments weredone. The absolute displacement among the experimentswas adjusted in order to identify the strain at whichcoating delamination was initiated, meaning that severaltests on each material were needed. After bending, an80-mm long section was cut from the centre of thedeformed tube and divided into two pieces at theposition corresponding to the centre line of the coating,Fig. 3. Each piece was then polished on emery paper


Fig. 3. The sectioning procedure of the bent tube for delaminationanalysis.

Table 2Summary of the ASTM 633 tensile tests on the different coating types

Coating method Tensile strength

Laser Various coatings. The tensile strength exceeds 69 MPa(4 tests)HVOF Amperit 526 and Metco 3007: 55"10 MPa(5 tests and 4 tests, respectively)

Metcoloy 2: 61"7 MPa(4 tests)Arc spray Various coatings. 38"12 MPa.(23 tests)

and diamond paste after which the delamination lengthwas measured with optical microscopy. A total of 69tubes were tested.

2.3.3. Acoustic emission analysisTwo piezo electrical acoustic emission sensors(Phys-

ical Acoustic Corporation(PAC), R15 resonance fre-quency 150 kHz) were attached at the ends of the coatedtubes with the full active area joined to a brass device,pinched on the tubes and tightened with a screw, Fig.2a. Dow Corning high vacuum grease was used as�

couplant between the sensor and the pinch device. Thesensor positions were chosen for minimising acousticemission emerging from deformation of the tube. Thefrequency response of the sensors is roughly 50–520kHz for transient(burst) signals. This overlaps with theresults, which have shown that 90% of the acousticemission emerging from material deformation is withinthe frequency band of 10–550 kHzw9x. The sensors areconnected to an amplifieryfilter with a gain of 40 dB.The amplifiers are in turn connected to an eight bit deepsampling resolution ISA AyD-converter card. One event,or 16 384 samplesychannel(a waveform) is sampledwhenever a trigger event occurs and then transferred tothe computer, giving a dead time of approximately 20ms. The sampling rate was set to 5 MHz. The energyof the acoustic emission for one event was calculated

as (U; voltage generated in the piezoelectric16384

2Es Ui8i

crystal due to the emitted acoustic emission) and thenaccumulated event by event. Software written in Lab-

View recorded the events as a function of the displace-ment at the centre of the tube. No analyses wereperformed in real time.In order to obtain the acoustic emission originating

from deformation of the tube, the supports, the loadapplicators, the tensile testing machine and other uni-dentified sources, uncoated tubes were tested and usedas reference. Only signals reaching both sensors wereanalysed and calculations made from the signal travel-ling time showed that the vast majority of the acousticemission recorded originated from the volume belowthe two load applicators.

2.4. Microstructural investigations

Polished cross-sections of the coatings were examinedin an optical microscope after the testing. The coatinghardness was measured with Vickers method. The com-position of selected areas on some coatings was deter-mined by Auger spectroscopy(PHI 660), from whichalso the SEM-imaging capabilities were used.

3. Results

3.1. Tensile testing

The results obtained from the tensile testing aresummarised in Table 2 and the failure mechanisms areschematically illustrated in Fig. 4.In the tests performed with laser coatings, the glue

fractures show that the adhesion strength of the coatingexceeds 69 MPa. In contrast, coatings deposited withthe HVOF technique delaminated at the coatingysub-strate interface. Arc sprayed coatings did not delaminatedue to internal fracture at a stress of approximately 38MPa, implying that in this case the adhesive strengthexceeds the cohesive strength. No significant differenceswere noted among the various arc sprayed coatings. Theresults from this study clearly show that adhesion failureis only obtained for the HVOF coatings. Thus, thetensile testing method is not applicable for laser and arcspray coatings.

3.2. Bending test in combination with metallography

Fig. 5 shows a cross section of the Metcoloy 2 coatingdeposited with laser. The microstructure is very uniform


Fig. 4. Bonding failure mechanisms of the three different coating typesas obtained with the tensile testing.

Fig. 5. Cross section of laser coated Metcoloy 2(optical microscopy).

with a low porosity and oxide content. No signs ofdelamination were seen even after bending to strains aslarge as 15%. However, radial cracks occurred on asmall number of samples after the bending procedure.These cracks cannot be linked to the composition orthickness of the coating.A dozen bending tests were performed on Metcoloy

2 coatings arc sprayed in air to find the strain at whichdelamination is initiated. The length of any delaminationwas measured for both the sectioned sample pieces.Summarising gives the maximum delamination lengthas 160 mm. Fig. 6a shows the microstructure of asample bent to a strain of 0.5%. No cracks or signs ofdelamination are seen. The coating consists of manylayers of overlapping essentially lamellar particles, drop-lets, which are sometimes called splats. Compared tothe laser coating the oxide content and degree of porosityis much largerw10x. The same material at a strain of1.25% is shown in Fig. 6b. A radial crack has formedin the splat boundaries and isolated areas of delaminationhave nucleated at the coatingysubstrate interface. Thetotal delamination length is 2 mm in this case. At astress of 1.9% a relatively large radial crack and amassive delamination(32 mm in this case) is seen, Fig.6c. Delamination without radial cracks perpendicular tothe coatingysubstrate interface has not been observedon the thermally sprayed coatings. However, coatingswith only small radial cracks and no delamination havebeen observed.The delamination process is similar for HVOF

sprayed coatings. Fig. 6d shows the microstructure ofan HVOF-sprayed Metcoloy 2 coating after bending to

a strain of 2.1%. For this type of coating, partiallydeveloped radial cracks are never observed and radialcracks without delamination are very rare. This impliesthat delamination occurs immediately after formation ofa radial crack.Figure 7 shows the delamination length as a function

of strain for some selected materials and coating meth-ods. Metcoloy 2 arc sprayed in air starts to delaminateat a strain of approximately 1.4%. From the same coatingmaterial but arc sprayed in nitrogen gas, only threepoints are available. It is seen that delamination hasoccurred at a strain of 1.8%. Mixing the bond coatmaterial 80Ni20Al into the Metcoloy 2 coatings increas-es the adhesion strength dramatically, if the splat size issufficiently large. Application of strains as large as 10–15% just gives a minor delamination. However, it isclearly seen that the effect is negligible for small splats.The HVOF sprayed coating,(Metco 3007) shows worseadhesion compared to the arc sprayed ones. Delamina-tion is initiated already at a strain of approximately 1%.Metcoloy 2 coatings deposited with laser do not delam-inate at all, not even for strains up to 15%. The resultsshow that, with the exception of the Metcoloy 2q80Ni20Al coatings with a large splat size, the delami-nation rate is very high as soon as delamination isinitiated.In Fig. 8, the strain interval at which delamination is

initiated is shown for all thermally sprayed coatings.The lower end of each column corresponds to the largeststrain where no signs of delamination have beenobserved and the higher end corresponds to the loweststrain at which delamination has been observed. TheAmperit 526 coatings were subjected to a maximumstrain of 1.5% and did not delaminate while the othertwo HVOF coatings, Metco 3007 and Metcoloy 2,delaminated at a strain of 1.0–1.5% and below 2%,respectively. Delamination of the Metcoloy 2 coatings


Fig. 6. Cross section of Metcoloy 2 coating, arc sprayed in air at(a) 0.5%,(b) 1.25% and(c) 1.9% strain.(d) Cross section of HVOF sprayedMetcoloy 2 coating at 2.1% strain(optical microscopy).

Fig. 7. The delamination length as a function of strain for selectedcoatings.

deposited with arc spray starts in the same strain regionas for the Metco 3007 coating; between 1.1–1.25% forthose sprayed in air and 0.5–2.1% for those sprayed innitrogen gas. Addition of 80Ni20Al clearly increases thedelamination region for Metcoloy 2 coatings to thestrain interval 2.2–10%. However, if the splats do nothave a sufficient size the effect of the binder disappearsas seen from columns 8 and 9 in the figure.

3.3. Bending test in combination with acoustic emissionmonitoring

During all bending tests the acoustic emission wassimultaneously recorded. The results from the coatingswere normalised by the same amplifying factor as forthe reference. In Fig. 9, the normalised accumulatedenergy is shown as a function of strain for some selectedcoatings. Results from a reference test on an uncoatedtube shows an initial sharp rise in acoustic emission ata strain of 0.1–0.25% after which the curve has acontinuous slope with a small inflexion at a strain ofapproximately 2%, Fig. 9c. The sources of acousticemission in this case are deformation of the tube, frettingagainst the supports and load applicators and noise from

the bending machine and should be considered whenevaluating the tests performed with coatings. The behav-iour of an Inconel 625 laser coating is similar to the


Fig. 8. The strain intervals where delamination is initiated on thermally sprayed coatings. The figures in the columns indicate the number oftested samples for each coating quality. The number on thex-axis corresponds to the one given in the legend denoting the coating materials.

Fig. 9. (a) Accumulated acoustic emission energy vs. strain.(b) Magnification of the region surrounded by a dashed ellipse in Fig. a.(c)Magnification of the region surrounded by a continuous ellipse in Fig. b.

reference tube, Fig. 9c, with a sharp increase in energylevel at 0.1% strain and an inflexion at 2% strain. Thisindicates that the acoustic emission activity originatesfrom deformation of the tube and not from the lasercoating. The coating did not delaminate for strains up

to 15% and the energy increase at larger strains istherefore only due to further deformation of the tube,Fig. 9a. However, in the case when radial cracks occur(only in a few laser coatings) an instant rise in theaccumulated energy level takes place(not shown here).


Fig. 10. The strain at which delamination is initiated as determinedby acoustic emission. The number on thex-axis corresponds to theone given in the legend denoting the coating materials.

Bending of Metcoloy 2 coatings deposited withHVOF or arc–sprayed with nitrogen or air, respectivelyinitially shows the same acoustic emission behaviour asthe reference tube. However, in the strain interval 1.2–2% a sharp increase in accumulated energy level isobserved, Fig. 9b,c. The delamination length for thecoating arc sprayed in air at 1.9% strain, where theexperiment was terminated, is 32 mm, corresponding to21% delamination. Sectioning of the sample depositedwith the HVOF-technique showed that the coatingdelamination was complete. The sharp increase in accu-mulated energy therefore indicates the formation ofradial cracks and subsequent delamination. The smallsteps in energy increase approximately 1.4, 1.6 and1.75% strain as seen in Fig. 9c might indicate formationof radial cracks. The Metcoloy 2 coating arc sprayed innitrogen gas was run to complete delamination duringstraining to 15%, Fig. 9a. In this case, the stepwisebehaviour is not observed. Instead, the increase inacoustic emission activity starts at a strain of 1.9% anddecays at a strain of approximately 13%, indicating theinitiation and termination of the delamination process.The Metcoloy 2q80Ni20Al coating arc sprayed in

air is bent to a strain of 11% after which the totalmeasured delamination length is 3 mm. Up to a strainof approximately 6% the pattern is similar to thatrecorded from the reference tube and Inconel 625 coat-ing, Fig. 9b,c. Above a strain of 6% there is a stepwiseincrease in energy level, which is interpreted as theformation of cracks and a subsequent delamination. Theaccumulated energy level does not deviate from thereference tube and laser coating until a strain of 9.5%is reached. The initial steeper rise in accumulated energyin the strain interval 0.1–0.25% is due to equalisationefforts made for adopting the curve to the Inconel 625coating at higher strains.The strains at which delamination is initiated as

interpreted from the acoustic emission measurements aresummarised in Fig. 10. The carbide containing HVOF-coatings, Amperit 526 and Metco 3007, show the weak-est adhesion strength and delamination is initiated at thestrains 1.0% and 0.9%, respectively. The performanceof the Metcoloy 2 coating deposited with the samemethod is somewhat better and delamination is notrecorded for strains below 1.8%. The strains at whichdelamination has been recorded for the same coating inthe arc sprayed condition are in the same range 1.4%and 1.9% with air and nitrogen as carrier gases, respec-tively. The effect of the 80Ni20Al addition to Metcoloy2 is clearly seen from the fact that the delaminationstrain increases to approximately 10%(columns 6 and7), which reflects an increased adhesion strength. How-ever, a small droplet size in the coating immediatelyeliminates the positive effect as seen from columns 8and 9.

3.4. Microstructural investigations

The microstructure of the Metcoloy 2 arc sprayed inair and mixed with the 80Ni20Al bond coat was furtherinvestigated due to the differences in adhesion strengthrecorded. The splat size, which is the same as thatobtained with normal spraying parameters, is approxi-mately 50mm, Fig. 11a. Auger spectroscopy shows thatthe only element present in the light areas whichoriginates from the 80Ni20Al bond coating material(points 1, 3 and 5) is Ni. Al was not detected at all. Inan other study, it has been suggested that Al hasevaporated during the arc spraying processw11x. A moresignificant net material loss than usual was also notedduring the coating production. The dark droplets origi-nate from the Metcoloy 2 material(points 2, 4 and 6)and Fe, Cr and Ni are recorded. Thus, Ni has diffusedinto the Metcoloy 2 splats. Fig. 11b shows the samematerial mixture as in Fig. 11a, but with a droplet sizeof 200 mm. From Auger spectroscopy it is concludedthat a very limited intermixing between the Metcoloy 2and 80Ni20Al phases has taken place in this case. Onlya minor concentration of Ni from the bond coat wasdetected in the Metcoloy 2 phase, while Al evaporatedduring the processing. On comparison image analysisshows that the total splat boundary length is three timessmaller than for coatings built of small splats. In eachof the cases, the splat size was the same throughout thecoatings.The Vickers hardnesses of the HVOF coatings Amper-

it 526, Metco 3007 and Metcoloy 2 are 800, 550 and350, respectively. Arc spraying of Metcoloy 2 gives acoating with Vickers hardness 320. Despite the differ-ences, delamination is initiated at about the same strain.Addition of 80Ni20Al into the Metcoloy 2 coatingresults in a further hardness decrease to 220 HV for thesmall splats quality and 150 HV for the large splatsquality.


Fig. 11. Cross-sections of Metcoloy 2q80Ni20Al coatings(a) small splats(b) large splats(SEM microscopy).

Table 3Correlation between delamination as determined by metallography and acoustic emission

Coating Number of coatings which Number of coatings whichdelaminated according to delaminated according tometallography acoustic emission

Amperit 526, HVOF 0 1Metco 3007, HVOF 2 3Metcoloy 2, HVOF 3 3Metcoloy 2, arc spray, air 7 7Metcoloy 2, arc spray, N2 3 3Metcoloy 2q80Ni20Al, arc 2 2spray, air, large splats

Metcoloy 2q80Ni20Al, arc 2 2spray, N large splats2,

80Ni20Al, arc spray, air, small splats 3 3Metcoloyq80Ni20Al, arc spray, air 3 3small splats

4. Discussion

Adhesion tests have been performed on coatings,deposited on low-alloyed steel tubes with the arc spray,HVOF and laser techniques. Tensile tests only gavedelamination for the HVOF deposited coatings at atensile strength of 55–61 MPa, while the arc sprayedcoatings suffered from internal fracture at 38 MPa. Thelaser coatings did not delaminate at all, but fractured inthe glue joint. The results obtained on the arc sprayedcoatings are in accordance with earlier ASTM 633 testsw12x, which showed internal fracture on Metcoloy 2coatings at stresses of 34"12 MPa. However, in thesame study HVOF deposited, Metco 3007 coatingsfractured in the glue joint at a stress of 69 MPa. Thediscrepancy in the results is explained by the tubegeometry used in our study, where the surface curvatureinduces additional peeling forces at the coatingyglueinterface.

The traditional way of testing the degree of delami-nation is tensile testing combined with optical micros-copy. The method has the drawback that it requiresseveral experiments where coated tubes are bent to aspecific strain and then metallographically analysed inorder to determine any crack formation at the coatingysubstrate interface. Using acoustic emission makes itpossible to continuously monitor the crack initiation andgrowth during delamination. Table 3 shows the correla-tion between delamination as determined by metallo-graphic analysis and the evaluation of acoustic emission.For all coatings, the number of experiments wheredelamination is detected by metallographic analysis isnoted. This is compared with the number of bendingtests where analysis of the acoustic emission has indi-cated delamination on the same coating. Only twosamples both deposited with HVOF show a discrepancywhere delamination was detected by acoustic emissionanalysis and not by optical microscopy. The explanation


may be in the sample preparation of the tested coatings.During sectioning of the bent tube a 1.6-mm widematerial strip is cut away and thereby any delaminationsand cracks in this area have vanished. Further, largestress concentrations and weak bonding in off-centreareas of the coating can induce small cracks. These arenot detected during the metallographic analysis, whichis only performed along the centreline of the coating.Contrary, the method of using acoustic emission recordsall the deformation during the bending independent ofits location. However, the correlation between the twomethods is very good and the conclusion must thereforebe that acoustic emission is a suitable technique forevaluation of coating adhesion.In this study, arc sprayed coatings with small splats

and HVOF deposited coatings usually start to delaminatein the strain interval 0.5–2.0%. Metallography revealedthat delamination is always accompanied by radialcracks perpendicular to the coatingysubstrate interface.When a radial crack has developed, the coating isunloaded and the coatingysubstrate interface comesunder increased tension, resulting in crack growth anddelamination. In this region, a minimum of materialmixing takes place during thermal spraying and thecoating is thus bonded to the substrate surface bymechanical interlockingw2x. The statement is supportedby the low adhesion strengths as measured with thetensile test and the swiftness of the delamination processafter initiation. The laser coatings did not delaminate atall, not even for strains up to 15%. The reason lies inthe laser process, where a small portion of the substratesurface is melted and mixed with the coating material.The result is a thin zone of metallic bondingw3x, whichstrongly increases the adhesion strength compared withthe thermally deposited coatings.Separate electrodes of Metcoloy 2 and the bond

coating material 80Ni20Al were arc sprayed into coat-ings with two different splat sizes,;50 mm and;200mm. The adhesion strength of the coating with the largersplats is superior to that with the smaller ones. In fact,addition of the bond coating material does not influenceon the adhesion strength at all, if the splat size is thesame as that for the pure Metcoloy 2 coatings. However,the latter quality was only available with the smallersplat size.It has been shownw13x that the adhesion strength for

coatings made of Al and SUS308 steels arc sprayed inair increases with the droplet size in the molten state.The larger droplets impinging on the surface during thespraying process have a higher kinetic energy. It hasbeen suggested that the impact introduces a peeningeffect with accompanying compressive residual stressesin the coatingw13x. Thus, the adhesion strength of thecoating is increased. In the case of HVOF deposition,the effect is larger due to the only partially melted

droplets where the smaller contact area results in ahigher impinging pressurew14x. However, the influenceof the coating microstructure was not taken into consid-eration in any of the studies referred.The present study shows that in the coatings com-

posed of large splats the splat boundary length is onlyapproximately 1y3 compared to the small size splatcoatings. Since the splat boundaries are enriched inoxides and pores, their contribution to the weakening ofthe coating is large and radial cracks are preferablyformed in these regions during tensile loading. Therebyif the number of splat boundaries is minimised, theradial cracking probability is reduced and the adhesionstrength improved.Despite the large differences in hardness among the

coatings, delamination is initiated at about the samestrain. The coating hardness therefore seems to be ofminor importance for the delamination behaviour. Thehardnesses of the Metcoloy 2 coatings mixed with the80Ni20Al bond coat are very low for both splat sizes.In both cases, evaporation of Al during processingcreates an almost pure ductile Ni phasew11x. Thus, theimprovement in adhesion strength is due to the formationof larger splats and not the presence of the bond coatingmaterial.

5. Conclusions

The adhesion among coatings deposited on low-alloyed steel tubes with arc spray(air or nitrogen ascarrier gas), HVOF and laser techniques was comparedwith a modified four point bending test and acousticemission. The conclusions are as follows:

● Acoustic emission can be used for estimating thestrain at which coating delamination is initiated.

● The delamination is always initiated by the formationof a radial crack.

● Coatings deposited with the laser technique show nodelamination for strains below 15%.

● Coatings deposited with HVOF start to delaminatein the strain interval 0.8–1.8%.

● The adhesion strength of the arc sprayed coatings isdependent on the splat size. Small splats give delam-ination in the strain interval 1.4–1.9% while largesplats give delamination in the strain interval 10.5–11.5%.

● Mixing a bond coating material into the arc sprayedcoating has no effect on the adhesion strength.

Acknowledgments

Financial support from the KME(Consortium forMaterial Technology directed towards Thermal EnergyProcesses) is gratefully acknowledged. Duroc AB and


Midroc Metalock are acknowledged for deposition ofthe coatings and Hoganas AB for provision of the¨ ¨powders. Finally, the tubes were supplied by KvaernerPulping AB which is also acknowledged.

References

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