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Adhesion testing of thermally sprayed and laser deposited coatings
Surface and Coatings Technology 184 (2004) 208–218
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 2003 Available 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 point bending and metallography. The agreement between the results obtained from the two different experimental techniques is very good. Laser coatings showed no delamination for strains up to 15%, while coatings deposited with the arc spray and HVOF processes delaminated in the strain intervals 1.4–1.9% and 0.8–1.8%, respectively. The suggested delamination mechanism is the initial formation of a radial crack in the coating after which the coatingysubstrate interface comes under an increased tension load and fractures. Arc sprayed coatings of Metcoloy 2 (Fe13Cr) mixed with the binder 80Ni20Al show a strongly improved adhesion strength if the splat size is sufficiently large. The delamination interval increases to 10.5–11.5%. However, for small splats the effect is eliminated. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Thermal spraying; Laser coating; Acoustic emission; Adhesion testing
1. Introduction Components in combustion fluidised beds are subjected to erosion–corrosion due to erosive bed particles and corrosive species in the gas phase. The degradation rate of low–alloyed steels in such environments is not acceptable. The use of thicker equipment walls and high–alloyed steels prolongs the lifetime of the components but at the expense of a high physical weight and cost. An alternative solution is to cover low–alloyed steels with an erosion–corrosion resistant coating. Traditionally, thermal spray methods like electric wire arc spray have been used due to the simple processing and low cost w1x. The technique also has the advantage that it makes the possibility of in-situ coating during service and repair possible. A more novel method is high velocity oxygen fuel (HVOF) where the use of powder results in denser coatings, lower oxide contents and a wider range of possible coating materials, including carbides and ceramics w2x. As a third alternative, laser *Corresponding author. Tel.: q46-31-7721263; fax: q46-317721313. E-mail addresses: [email protected] (A. Nylund), ¨ [email protected] (A. Hjornhede).
cladding has been introduced since lasers have become more sophisticated, smaller and cost effective. The main advantages of the laser coatings are their strong adhesion to the substrate and their low porosity and oxide content w3 x . To obtain the optimum quality of an applied coating, the degradation rate in the chemical environment must be minimised and the adhesion strength to the substrate maximised. Due to different thermal expansion coefficients for the coating and substrate, material stresses are induced when coated components are used in high temperature applications. The stresses are intensified by temperature gradients originating from the temperature difference between the temperatures of the fire and the steam sides of the coated tubes. Further, in the case of power plants temperature fluctuations during service, expose the coated surfaces to thermally induced strains with an increased risk of delamination. The adhesion strength of coatings is traditionally evaluated by tensile testing, e.g. ASTM C 633 or bending tests followed by metallography w1x. However, these methods have the drawback of requiring optical investigations to identify the induced damages. An option would be the use of acoustic emission (AE) where the initiation and devel-
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.11.008
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Table 1 The composition of the coatings used in this study Coating Arc spray Metcoloy 2 (Air or N2, small splat size) 80Ni20Al-binder, (Air, small splat size) Metcoloy 2q80Ni20Al. (Air or N2, large splat size) Metcoloy 2q80Ni20Al-binder. (Air, small splat size)
Composition (wt.%) Fe12.4Cr0.6Ni0.4Mn0.4Si0.36C 80Ni20Al 50 (Metcoloy 2)q50 (80Ni20Al) 50 (Metcoloy 2)q50 (80Ni20Al)
HVOF Metcoloy 2 Amperit 526 Metco 3007
Fe12.4Cr0.6Ni0.4Mn0.4Si0.36C 83WC17Co 80Cr3C2 –16Ni4Cr
Laser Metcoloy 2 Inconel 625 Duroc 5177 Duroc 17=1% C Stellite 6 Stellite 21qTiC Duroc 5177qTiC Duroc 17=1% CqTiC Stellite 6qTiC
Fe12.4Cr0.6Ni0.4Mn0.4Si0.36C Ni21.5Cr9.0Mo3.6Nb2.5Fe0.3Si0.05C Ni26.8Cr8.4Mo1.7Fe1.40C0.9Nb0.7Si Ni25.0Cr8.8Mo1.9Fe1.9Nb1.00C0.7Si Co28.5Cr4.2W1.1C1.0Si-2.0Ni-1.5Fe 85(Co27.0Cr5.5Mo2.8Ni0.9Si0.25C-2.0Fe)q15(Ti19.4C) 85(Ni26.8Cr8.4Mo1.7Fe1.40C0.9Nb0.7Si)q15(Ti19.4C) 85(Ni25.0Cr8.8Mo1.9Fe1.9Nb1.00C0.7Si)q15(Ti19.4C) 85(Co28.5Cr4.2W1.1C1.0Si-2.0Ni-1.5Fe)q15(Ti19.4C)
opment of cracks is continuously monitored during the tensile testing. The technique has been successfully applied to plasma sprayed coatings, carbon fibre materials, composites, thermal barrier coatings and hard metal coatings w4–9x. The aim of this paper is to further apply the method of acoustic emission and use it for analysing the adhesion of metallic coatings on a substrate of lowalloyed steel tubes. The results are compared with those from 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 N2 as carrier gas) was tested. The thermally sprayed coatings were deposited in accordance with the recommendations given by the suppliers, with the exception of the large splat coatings, where the parameters were slightly altered. The deposition parameters used in the laser process are not allowed to be published. The coatings and their chemical compositions can be seen from Table 1. Metcoloy 2 (wire) and Metco 3007 (powder) are standard products manufactured by Sulzer Metco. The powder with the same composition as Metcoloy 2 but used for the HVOF and laser deposition techniques were ¨ ¨ AB. The Amperit 526, Stellite manufactured by Hoganas 6 and 21, Inconel 625 and Duroc materials were manufactured by H.C. Starck GmbH, Deloro Stellite, Special
Metals Corp. and Duroc Energy AB, respectively. The size grades for the powder used with laser and HVOF were 63–150 mm and 5–63 mm, respectively. The thicknesses of the coatings were in the range 0.7–1.2 mm for laser, 0.4–0.6 mm for arc spray and 0.2–0.4 mm for HVOF. As substrate, steel tubes of a type normally used in high-pressure applications, Fe0.1C1Cr0.5Mo0.5Mn0.2Si, were chosen. The surface roughness of the arc sprayed and HVOF deposited substrate tubes was 5–7 mm and 10 mm, respectively (Ra-values). The Metcoloy 2 and 80Ni20Al materials were supplied as the two separate electrodes in the arc spray process, resulting in a coating consisting of 50% of each material. Two different arc spray droplet sizes, 50 and 200 mm, were examined for the Metcoloy 2q 80Ni20Al-coating. The adhesion of the coatings to the substrate was tested with the three methods as described below; a specially designed tensile test, a four-point bending test followed by metallography and a four-point bending test combined with acoustic emission (AE). 2.2. Tensile testing The adhesion of the coatings to the substrate was tested according to a tensile testing method developed from the ASTM 633 test. The modified design was used in order to simulate the tube geometry. A 10=15 mm string coating was deposited on the tubes (external diameter 33.7 mm, wall thickness 3.6 mm and length
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between the supports and the load applicators were 250 mm and 25 mm, respectively. All cylindrical supports and load applicators were scooped to the same radius as the tubes, Fig. 2b, and MoS2 was supplied in the scoops before the tests in order to reduce the fretting. Due to the tube geometry the test is not a pure fourpoint bending test. Rather the stress load is described as a parabola with its maximum between the two load applicators.
Fig. 1. Adhesion testing; experimental set-up for the tensile test.
approx. 20 mm). A cylinder (external diameter 8.0 mm) with the same curvature as the envelope surface of the tube was glued on top of the coated string, Fig. 1. The tensile strength of the glue used, FM-1000䉷, is 69 MPa after hardening. Peeling forces caused by misaligned attachment of the cylinder were reduced by using a brace designed as a hook and equipped with a spherical prong. A total of 54 samples were tested. 2.3. Four-point bending test combined with metallography and acoustic emission analysis 2.3.1. Experimental setup The experimental set-up is illustrated in Fig. 2a. The dimensions of the tubes used in this test were the same as in the tensile test except for the length, which was 300 mm. A 100=10 mm wide coating was deposited in the axial direction of the tubes and then subjected to four-point bending at a displacement speed of 0.25 mmy min. The displacement was measured with a position gauge centred on the coating surface. The distances
2.3.2. Strain calibration In order to determine the strain as a function of the displacement, five strain gauges were glued on uncoated tubes in the same area as the coating. One strain gauge was positioned at the centre of the assumed coating area, two at a distance of "5 mm off centre in the radial direction, and two at a distance of "15 mm off centre in the axial direction. The tubes were then bent to different displacements. The largest strain was always measured in the central position. This was then used to establish a calibration curve from which the displacements applied to the coated tubes were translated into strain. The presence of a coating on the surface will influence the strain distribution somewhat. In the case of thermally sprayed coatings, the influence is limited due to the relatively low adhesion strength. Further, in a comparative study, like this, the deviation is about the same among the coatings. The influence on the laser coatings is larger, but due to the complex deformation in the coated area of the tube, no adjustments were done. The absolute displacement among the experiments was adjusted in order to identify the strain at which coating delamination was initiated, meaning that several tests on each material were needed. After bending, an 80-mm long section was cut from the centre of the deformed tube and divided into two pieces at the position corresponding to the centre line of the coating, Fig. 3. Each piece was then polished on emery paper
Fig. 2. (a) Adhesion testing; experimental set-up for the four point bending and acoustic emission tests. (b) Scooped supports used in the fourpoint bending test.
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View recorded the events as a function of the displacement at the centre of the tube. No analyses were performed in real time. In order to obtain the acoustic emission originating from deformation of the tube, the supports, the load applicators, the tensile testing machine and other unidentified sources, uncoated tubes were tested and used as reference. Only signals reaching both sensors were analysed and calculations made from the signal travelling time showed that the vast majority of the acoustic emission recorded originated from the volume below the two load applicators. Fig. 3. The sectioning procedure of the bent tube for delamination analysis.
and diamond paste after which the delamination length was measured with optical microscopy. A total of 69 tubes were tested. 2.3.3. Acoustic emission analysis Two piezo electrical acoustic emission sensors (Physical Acoustic Corporation (PAC), R15 resonance frequency 150 kHz) were attached at the ends of the coated tubes with the full active area joined to a brass device, pinched on the tubes and tightened with a screw, Fig. 2a. Dow Corning䉷 high vacuum grease was used as couplant between the sensor and the pinch device. The sensor positions were chosen for minimising acoustic emission emerging from deformation of the tube. The frequency response of the sensors is roughly 50–520 kHz for transient (burst) signals. This overlaps with the results, which have shown that 90% of the acoustic emission emerging from material deformation is within the frequency band of 10–550 kHz w9x. The sensors are connected to an amplifieryfilter with a gain of 40 dB. The amplifiers are in turn connected to an eight bit deep sampling resolution ISA AyD-converter card. One event, or 16 384 samplesychannel (a waveform) is sampled whenever a trigger event occurs and then transferred to the computer, giving a dead time of approximately 20 ms. The sampling rate was set to 5 MHz. The energy of the acoustic emission for one event was calculated 16384
as Es 8 Ui2(U; voltage generated in the piezoelectric
2.4. Microstructural investigations Polished cross-sections of the coatings were examined in an optical microscope after the testing. The coating hardness was measured with Vickers method. The composition of selected areas on some coatings was determined by Auger spectroscopy (PHI 660), from which also the SEM-imaging capabilities were used. 3. Results 3.1. Tensile testing The results obtained from the tensile testing are summarised in Table 2 and the failure mechanisms are schematically illustrated in Fig. 4. In the tests performed with laser coatings, the glue fractures show that the adhesion strength of the coating exceeds 69 MPa. In contrast, coatings deposited with the HVOF technique delaminated at the coatingysubstrate interface. Arc sprayed coatings did not delaminate due to internal fracture at a stress of approximately 38 MPa, implying that in this case the adhesive strength exceeds the cohesive strength. No significant differences were noted among the various arc sprayed coatings. The results from this study clearly show that adhesion failure is only obtained for the HVOF coatings. Thus, the tensile testing method is not applicable for laser and arc spray coatings. 3.2. Bending test in combination with metallography
i
crystal due to the emitted acoustic emission) and then accumulated event by event. Software written in Lab-
Fig. 5 shows a cross section of the Metcoloy 2 coating deposited with laser. The microstructure is very uniform
Table 2 Summary of the ASTM 633 tensile tests on the different coating types Coating method
Tensile strength
Laser HVOF
Various coatings. The tensile strength exceeds 69 MPa (4 tests) Amperit 526 and Metco 3007: 55"10 MPa (5 tests and 4 tests, respectively) Metcoloy 2: 61"7 MPa (4 tests) Various coatings. 38"12 MPa. (23 tests)
Arc spray
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Fig. 4. Bonding failure mechanisms of the three different coating types as obtained with the tensile testing.
with a low porosity and oxide content. No signs of delamination were seen even after bending to strains as large as 15%. However, radial cracks occurred on a small number of samples after the bending procedure. These cracks cannot be linked to the composition or thickness of the coating. A dozen bending tests were performed on Metcoloy 2 coatings arc sprayed in air to find the strain at which delamination is initiated. The length of any delamination was measured for both the sectioned sample pieces. Summarising gives the maximum delamination length as 160 mm. Fig. 6a shows the microstructure of a sample bent to a strain of 0.5%. No cracks or signs of delamination are seen. The coating consists of many layers of overlapping essentially lamellar particles, droplets, which are sometimes called splats. Compared to the laser coating the oxide content and degree of porosity is much larger w10x. The same material at a strain of 1.25% is shown in Fig. 6b. A radial crack has formed in the splat boundaries and isolated areas of delamination have nucleated at the coatingysubstrate interface. The total delamination length is 2 mm in this case. At a stress of 1.9% a relatively large radial crack and a massive delamination (32 mm in this case) is seen, Fig. 6c. Delamination without radial cracks perpendicular to the coatingysubstrate interface has not been observed on the thermally sprayed coatings. However, coatings with only small radial cracks and no delamination have been observed. The delamination process is similar for HVOF sprayed coatings. Fig. 6d shows the microstructure of an HVOF-sprayed Metcoloy 2 coating after bending to
a strain of 2.1%. For this type of coating, partially developed radial cracks are never observed and radial cracks without delamination are very rare. This implies that delamination occurs immediately after formation of a radial crack. Figure 7 shows the delamination length as a function of strain for some selected materials and coating methods. Metcoloy 2 arc sprayed in air starts to delaminate at a strain of approximately 1.4%. From the same coating material but arc sprayed in nitrogen gas, only three points are available. It is seen that delamination has occurred at a strain of 1.8%. Mixing the bond coat material 80Ni20Al into the Metcoloy 2 coatings increases the adhesion strength dramatically, if the splat size is sufficiently large. Application of strains as large as 10– 15% just gives a minor delamination. However, it is clearly seen that the effect is negligible for small splats. The HVOF sprayed coating, (Metco 3007) shows worse adhesion compared to the arc sprayed ones. Delamination is initiated already at a strain of approximately 1%. Metcoloy 2 coatings deposited with laser do not delaminate at all, not even for strains up to 15%. The results show that, with the exception of the Metcoloy 2q 80Ni20Al coatings with a large splat size, the delamination rate is very high as soon as delamination is initiated. 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 largest strain where no signs of delamination have been observed and the higher end corresponds to the lowest strain at which delamination has been observed. The Amperit 526 coatings were subjected to a maximum strain of 1.5% and did not delaminate while the other two 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. 5. Cross section of laser coated Metcoloy 2 (optical microscopy).
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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 sprayed Metcoloy 2 coating at 2.1% strain (optical microscopy).
deposited with arc spray starts in the same strain region as for the Metco 3007 coating; between 1.1–1.25% for those sprayed in air and 0.5–2.1% for those sprayed in nitrogen gas. Addition of 80Ni20Al clearly increases the delamination region for Metcoloy 2 coatings to the strain interval 2.2–10%. However, if the splats do not have a sufficient size the effect of the binder disappears as seen from columns 8 and 9 in the figure.
the bending machine and should be considered when evaluating the tests performed with coatings. The behaviour of an Inconel 625 laser coating is similar to the
3.3. Bending test in combination with acoustic emission monitoring During all bending tests the acoustic emission was simultaneously recorded. The results from the coatings were normalised by the same amplifying factor as for the reference. In Fig. 9, the normalised accumulated energy is shown as a function of strain for some selected coatings. Results from a reference test on an uncoated tube shows an initial sharp rise in acoustic emission at a strain of 0.1–0.25% after which the curve has a continuous slope with a small inflexion at a strain of approximately 2%, Fig. 9c. The sources of acoustic emission in this case are deformation of the tube, fretting against the supports and load applicators and noise from
Fig. 7. The delamination length as a function of strain for selected coatings.
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Fig. 8. The strain intervals where delamination is initiated on thermally sprayed coatings. The figures in the columns indicate the number of tested samples for each coating quality. The number on the x-axis corresponds to the one given in the legend denoting the coating materials.
reference tube, Fig. 9c, with a sharp increase in energy level at 0.1% strain and an inflexion at 2% strain. This indicates that the acoustic emission activity originates from deformation of the tube and not from the laser coating. The coating did not delaminate for strains up
to 15% and the energy increase at larger strains is therefore 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 the accumulated energy level takes place (not shown here).
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.
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Bending of Metcoloy 2 coatings deposited with HVOF or arc–sprayed with nitrogen or air, respectively initially shows the same acoustic emission behaviour as the reference tube. However, in the strain interval 1.2– 2% a sharp increase in accumulated energy level is observed, Fig. 9b,c. The delamination length for the coating arc sprayed in air at 1.9% strain, where the experiment was terminated, is 32 mm, corresponding to 21% delamination. Sectioning of the sample deposited with the HVOF-technique showed that the coating delamination was complete. The sharp increase in accumulated energy therefore indicates the formation of radial cracks and subsequent delamination. The small steps in energy increase approximately 1.4, 1.6 and 1.75% strain as seen in Fig. 9c might indicate formation of radial cracks. The Metcoloy 2 coating arc sprayed in nitrogen gas was run to complete delamination during straining to 15%, Fig. 9a. In this case, the stepwise behaviour is not observed. Instead, the increase in acoustic emission activity starts at a strain of 1.9% and decays at a strain of approximately 13%, indicating the initiation and termination of the delamination process. The Metcoloy 2q80Ni20Al coating arc sprayed in air is bent to a strain of 11% after which the total measured delamination length is 3 mm. Up to a strain of approximately 6% the pattern is similar to that recorded from the reference tube and Inconel 625 coating, Fig. 9b,c. Above a strain of 6% there is a stepwise increase in energy level, which is interpreted as the formation of cracks and a subsequent delamination. The accumulated energy level does not deviate from the reference tube and laser coating until a strain of 9.5% is reached. The initial steeper rise in accumulated energy in the strain interval 0.1–0.25% is due to equalisation efforts made for adopting the curve to the Inconel 625 coating at higher strains. The strains at which delamination is initiated as interpreted from the acoustic emission measurements are summarised in Fig. 10. The carbide containing HVOFcoatings, Amperit 526 and Metco 3007, show the weakest adhesion strength and delamination is initiated at the strains 1.0% and 0.9%, respectively. The performance of the Metcoloy 2 coating deposited with the same method is somewhat better and delamination is not recorded for strains below 1.8%. The strains at which delamination has been recorded for the same coating in the arc sprayed condition are in the same range 1.4% and 1.9% with air and nitrogen as carrier gases, respectively. The effect of the 80Ni20Al addition to Metcoloy 2 is clearly seen from the fact that the delamination strain increases to approximately 10% (columns 6 and 7), which reflects an increased adhesion strength. However, a small droplet size in the coating immediately eliminates the positive effect as seen from columns 8 and 9.
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Fig. 10. The strain at which delamination is initiated as determined by acoustic emission. The number on the x-axis corresponds to the one given in the legend denoting the coating materials.
3.4. Microstructural investigations The microstructure of the Metcoloy 2 arc sprayed in air and mixed with the 80Ni20Al bond coat was further investigated due to the differences in adhesion strength recorded. The splat size, which is the same as that obtained with normal spraying parameters, is approximately 50 mm, Fig. 11a. Auger spectroscopy shows that the only element present in the light areas which originates from the 80Ni20Al bond coating material (points 1, 3 and 5) is Ni. Al was not detected at all. In an other study, it has been suggested that Al has evaporated during the arc spraying process w11x. A more significant net material loss than usual was also noted during the coating production. The dark droplets originate from the Metcoloy 2 material (points 2, 4 and 6) and Fe, Cr and Ni are recorded. Thus, Ni has diffused into the Metcoloy 2 splats. Fig. 11b shows the same material mixture as in Fig. 11a, but with a droplet size of 200 mm. From Auger spectroscopy it is concluded that a very limited intermixing between the Metcoloy 2 and 80Ni20Al phases has taken place in this case. Only a minor concentration of Ni from the bond coat was detected in the Metcoloy 2 phase, while Al evaporated during the processing. On comparison image analysis shows that the total splat boundary length is three times smaller than for coatings built of small splats. In each of the cases, the splat size was the same throughout the coatings. The Vickers hardnesses of the HVOF coatings Amperit 526, Metco 3007 and Metcoloy 2 are 800, 550 and 350, respectively. Arc spraying of Metcoloy 2 gives a coating with Vickers hardness 320. Despite the differences, delamination is initiated at about the same strain. Addition of 80Ni20Al into the Metcoloy 2 coating results in a further hardness decrease to 220 HV for the small splats quality and 150 HV for the large splats quality.
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Fig. 11. Cross-sections of Metcoloy 2q80Ni20Al coatings (a) small splats (b) large splats (SEM microscopy).
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 gave delamination for the HVOF deposited coatings at a tensile strength of 55–61 MPa, while the arc sprayed coatings suffered from internal fracture at 38 MPa. The laser coatings did not delaminate at all, but fractured in the glue joint. The results obtained on the arc sprayed coatings are in accordance with earlier ASTM 633 tests w12x, which showed internal fracture on Metcoloy 2 coatings at stresses of 34"12 MPa. However, in the same study HVOF deposited, Metco 3007 coatings fractured in the glue joint at a stress of 69 MPa. The discrepancy in the results is explained by the tube geometry used in our study, where the surface curvature induces additional peeling forces at the coatingyglue interface.
The traditional way of testing the degree of delamination is tensile testing combined with optical microscopy. The method has the drawback that it requires several experiments where coated tubes are bent to a specific strain and then metallographically analysed in order to determine any crack formation at the coatingy substrate interface. Using acoustic emission makes it possible to continuously monitor the crack initiation and growth during delamination. Table 3 shows the correlation between delamination as determined by metallographic analysis and the evaluation of acoustic emission. For all coatings, the number of experiments where delamination is detected by metallographic analysis is noted. This is compared with the number of bending tests where analysis of the acoustic emission has indicated delamination on the same coating. Only two samples both deposited with HVOF show a discrepancy where delamination was detected by acoustic emission analysis and not by optical microscopy. The explanation
Table 3 Correlation between delamination as determined by metallography and acoustic emission Coating
Number of coatings which delaminated according to metallography
Number of coatings which delaminated according to acoustic emission
Amperit 526, HVOF Metco 3007, HVOF Metcoloy 2, HVOF Metcoloy 2, arc spray, air Metcoloy 2, arc spray, N2 Metcoloy 2q80Ni20Al, arc spray, air, large splats Metcoloy 2q80Ni20Al, arc spray, N2, large splats 80Ni20Al, arc spray, air, small splats Metcoloyq80Ni20Al, arc spray, air small splats
0 2 3 7 3 2
1 3 3 7 3 2
2
2
3 3
3 3
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may be in the sample preparation of the tested coatings. During sectioning of the bent tube a 1.6-mm wide material strip is cut away and thereby any delaminations and cracks in this area have vanished. Further, large stress concentrations and weak bonding in off-centre areas of the coating can induce small cracks. These are not detected during the metallographic analysis, which is only performed along the centreline of the coating. Contrary, the method of using acoustic emission records all the deformation during the bending independent of its location. However, the correlation between the two methods is very good and the conclusion must therefore be that acoustic emission is a suitable technique for evaluation of coating adhesion. In this study, arc sprayed coatings with small splats and HVOF deposited coatings usually start to delaminate in the strain interval 0.5–2.0%. Metallography revealed that delamination is always accompanied by radial cracks perpendicular to the coatingysubstrate interface. When a radial crack has developed, the coating is unloaded and the coatingysubstrate interface comes under increased tension, resulting in crack growth and delamination. In this region, a minimum of material mixing takes place during thermal spraying and the coating is thus bonded to the substrate surface by mechanical interlocking w2x. The statement is supported by the low adhesion strengths as measured with the tensile test and the swiftness of the delamination process after initiation. The laser coatings did not delaminate at all, not even for strains up to 15%. The reason lies in the laser process, where a small portion of the substrate surface is melted and mixed with the coating material. The result is a thin zone of metallic bonding w3x, which strongly increases the adhesion strength compared with the thermally deposited coatings. Separate electrodes of Metcoloy 2 and the bond coating material 80Ni20Al were arc sprayed into coatings with two different splat sizes, ;50 mm and ;200 mm. The adhesion strength of the coating with the larger splats is superior to that with the smaller ones. In fact, addition of the bond coating material does not influence on the adhesion strength at all, if the splat size is the same as that for the pure Metcoloy 2 coatings. However, the latter quality was only available with the smaller splat size. It has been shown w13x that the adhesion strength for coatings made of Al and SUS308 steels arc sprayed in air increases with the droplet size in the molten state. The larger droplets impinging on the surface during the spraying process have a higher kinetic energy. It has been suggested that the impact introduces a peening effect with accompanying compressive residual stresses in the coating w13x. Thus, the adhesion strength of the coating is increased. In the case of HVOF deposition, the effect is larger due to the only partially melted
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droplets where the smaller contact area results in a higher impinging pressure w14x. However, the influence of the coating microstructure was not taken into consideration in any of the studies referred. The present study shows that in the coatings composed of large splats the splat boundary length is only approximately 1y3 compared to the small size splat coatings. Since the splat boundaries are enriched in oxides and pores, their contribution to the weakening of the coating is large and radial cracks are preferably formed in these regions during tensile loading. Thereby if the number of splat boundaries is minimised, the radial cracking probability is reduced and the adhesion strength improved. Despite the large differences in hardness among the coatings, delamination is initiated at about the same strain. The coating hardness therefore seems to be of minor importance for the delamination behaviour. The hardnesses of the Metcoloy 2 coatings mixed with the 80Ni20Al bond coat are very low for both splat sizes. In both cases, evaporation of Al during processing creates an almost pure ductile Ni phase w11x. Thus, the improvement in adhesion strength is due to the formation of larger splats and not the presence of the bond coating material. 5. Conclusions The adhesion among coatings deposited on lowalloyed steel tubes with arc spray (air or nitrogen as carrier gas), HVOF and laser techniques was compared with a modified four point bending test and acoustic emission. The conclusions are as follows: ● Acoustic emission can be used for estimating the strain at which coating delamination is initiated. ● The delamination is always initiated by the formation of a radial crack. ● Coatings deposited with the laser technique show no delamination for strains below 15%. ● Coatings deposited with HVOF start to delaminate in the strain interval 0.8–1.8%. ● The adhesion strength of the arc sprayed coatings is dependent on the splat size. Small splats give delamination in the strain interval 1.4–1.9% while large splats give delamination in the strain interval 10.5– 11.5%. ● Mixing a bond coating material into the arc sprayed coating has no effect on the adhesion strength. Acknowledgments Financial support from the KME (Consortium for Material Technology directed towards Thermal Energy Processes) is gratefully acknowledged. Duroc AB and
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Midroc Metalock are acknowledged for deposition of ¨ ¨ AB for provision of the the coatings and Hoganas powders. Finally, the tubes were supplied by Kvaerner Pulping AB which is also acknowledged. References w1x J. Clare, D. Crowmer, Thermal spray coatings, ASM Handbook – Friction, Lubrication and Wear Technology, 18, ASM International, 1992, pp. 361–374. w2x R. Tucker, Thermal spray coating, ASM-Handbook-Surface Engineering, 5, ASM International, 1994, pp. 497–509. w3x R. Vilar, Solid freeform fabrication, laser cladding, Int. J. Powder Metall. 37 (2) (2001) 31–48. w4x D. Almond, M. Moghisi, H. Reiter, The acoustic emission testing on plasma-sprayed coatings, Thin Solid Films Metall. Prot. Coat. 108 (1983) 439–447. w5x Y. Mizutani, K. Nagashima, M. Takemoto, K. Ono, Fracture mechanism characterization of cross-ply carbon-fiber composites using acoustic emission analysis, NDT&E Int. 33 (2000) 101–110. w6x G. Qi, A. Barhorst, J. Hashemi, G. Kamala, Discrete wavelet decomposition of acoustic emission signals from carbon fiberreinforced composites, Compos. Sci. Technol. 57 (1997) 389–403. w7x G. Qi, Wavelet-based AE characterization of composite materials, NDT&E Int. 33 (2000) 133–144.
w8x X.Q. Ma, S. Cho, M. Takemoto, Acoustic emission source analysis of plasma sprayed thermal barrier coatings during four-point bend test, Surf. Coat. Technol. 139 (2001) 55–62. w9x Characterisation ´ ´ ´ par emission acoustique de l’adherence et de ˆ ˆ l’endommagement d’un revetement: cas d’un revetement WCCo sur acier, D. Dalmas, S. Benmedakhene, C. Richard, A. ´ ´ Laksimi, G. Beranger, T. Gregorie, Comptes rendus de l’academie des sciences serie il fascicule chimie 4(5) 2001 345– 350. w10x A. Hjornhede, ¨ I. Olefjord, Erosion corrosion of thermally sprayed Fe12Cr and Cr3C2 –NiCr coatings, in: R. Streiff, I.G. Wright, R.C. Krutenat, M. Caillet, A. Galerie (Eds.), Proceedings of fifth International Symposium on High-Temperature Corrosion and Protection of Materials, Les Embiez, France, May 22–26, 2000, (Trans Tech. Publications Ltd, Switzerland, 2001), pp. 507–514. w11x A. Nafari, A. Nylund, Field Study on Superheater Tubes in the Loop Seal of a Wood Fired CFB Plant, Licenciate Thesis, ¨ Chalmers, Goteborg, 2003. w12x P. Tarkpea, Ytbelaggningar ¨ ¨ erosionsskydd i fluidbaddar, ¨ for ¨ Varmeforsk Service AB, Stockholm, 1998. w13x Y. Arata, A. Ohmori, R. Kawase, Zu den faktoren welche die ¨ adhasion lichtbogspritzer schichten beeinflussen, Trans. JWRI 13 (2) (1984) 193–199. w14x C.-J. Li, Y.-Y. Wang, A. Ohmori, Effect of melting state of spray particles on the adhesive strength of HVOF nickel-based alloy coating, in: Christopher C. Berndt (Ed.) Proceeding of ´ Quebec, ´ 1st International Thermal Spray Conference, Montreal, Canada, May 8–11, 2000, (ASM International, USA) Thermal Spray, Surface Engineering via Applied Research.
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 2003 Available 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 point bending and metallography. The agreement between the results obtained from the two different experimental techniques is very good. Laser coatings showed no delamination for strains up to 15%, while coatings deposited with the arc spray and HVOF processes delaminated in the strain intervals 1.4–1.9% and 0.8–1.8%, respectively. The suggested delamination mechanism is the initial formation of a radial crack in the coating after which the coatingysubstrate interface comes under an increased tension load and fractures. Arc sprayed coatings of Metcoloy 2 (Fe13Cr) mixed with the binder 80Ni20Al show a strongly improved adhesion strength if the splat size is sufficiently large. The delamination interval increases to 10.5–11.5%. However, for small splats the effect is eliminated. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Thermal spraying; Laser coating; Acoustic emission; Adhesion testing
1. Introduction Components in combustion fluidised beds are subjected to erosion–corrosion due to erosive bed particles and corrosive species in the gas phase. The degradation rate of low–alloyed steels in such environments is not acceptable. The use of thicker equipment walls and high–alloyed steels prolongs the lifetime of the components but at the expense of a high physical weight and cost. An alternative solution is to cover low–alloyed steels with an erosion–corrosion resistant coating. Traditionally, thermal spray methods like electric wire arc spray have been used due to the simple processing and low cost w1x. The technique also has the advantage that it makes the possibility of in-situ coating during service and repair possible. A more novel method is high velocity oxygen fuel (HVOF) where the use of powder results in denser coatings, lower oxide contents and a wider range of possible coating materials, including carbides and ceramics w2x. As a third alternative, laser *Corresponding author. Tel.: q46-31-7721263; fax: q46-317721313. E-mail addresses: [email protected] (A. Nylund), ¨ [email protected] (A. Hjornhede).
cladding has been introduced since lasers have become more sophisticated, smaller and cost effective. The main advantages of the laser coatings are their strong adhesion to the substrate and their low porosity and oxide content w3 x . To obtain the optimum quality of an applied coating, the degradation rate in the chemical environment must be minimised and the adhesion strength to the substrate maximised. Due to different thermal expansion coefficients for the coating and substrate, material stresses are induced when coated components are used in high temperature applications. The stresses are intensified by temperature gradients originating from the temperature difference between the temperatures of the fire and the steam sides of the coated tubes. Further, in the case of power plants temperature fluctuations during service, expose the coated surfaces to thermally induced strains with an increased risk of delamination. The adhesion strength of coatings is traditionally evaluated by tensile testing, e.g. ASTM C 633 or bending tests followed by metallography w1x. However, these methods have the drawback of requiring optical investigations to identify the induced damages. An option would be the use of acoustic emission (AE) where the initiation and devel-
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.11.008
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Table 1 The composition of the coatings used in this study Coating Arc spray Metcoloy 2 (Air or N2, small splat size) 80Ni20Al-binder, (Air, small splat size) Metcoloy 2q80Ni20Al. (Air or N2, large splat size) Metcoloy 2q80Ni20Al-binder. (Air, small splat size)
Composition (wt.%) Fe12.4Cr0.6Ni0.4Mn0.4Si0.36C 80Ni20Al 50 (Metcoloy 2)q50 (80Ni20Al) 50 (Metcoloy 2)q50 (80Ni20Al)
HVOF Metcoloy 2 Amperit 526 Metco 3007
Fe12.4Cr0.6Ni0.4Mn0.4Si0.36C 83WC17Co 80Cr3C2 –16Ni4Cr
Laser Metcoloy 2 Inconel 625 Duroc 5177 Duroc 17=1% C Stellite 6 Stellite 21qTiC Duroc 5177qTiC Duroc 17=1% CqTiC Stellite 6qTiC
Fe12.4Cr0.6Ni0.4Mn0.4Si0.36C Ni21.5Cr9.0Mo3.6Nb2.5Fe0.3Si0.05C Ni26.8Cr8.4Mo1.7Fe1.40C0.9Nb0.7Si Ni25.0Cr8.8Mo1.9Fe1.9Nb1.00C0.7Si Co28.5Cr4.2W1.1C1.0Si-2.0Ni-1.5Fe 85(Co27.0Cr5.5Mo2.8Ni0.9Si0.25C-2.0Fe)q15(Ti19.4C) 85(Ni26.8Cr8.4Mo1.7Fe1.40C0.9Nb0.7Si)q15(Ti19.4C) 85(Ni25.0Cr8.8Mo1.9Fe1.9Nb1.00C0.7Si)q15(Ti19.4C) 85(Co28.5Cr4.2W1.1C1.0Si-2.0Ni-1.5Fe)q15(Ti19.4C)
opment of cracks is continuously monitored during the tensile testing. The technique has been successfully applied to plasma sprayed coatings, carbon fibre materials, composites, thermal barrier coatings and hard metal coatings w4–9x. The aim of this paper is to further apply the method of acoustic emission and use it for analysing the adhesion of metallic coatings on a substrate of lowalloyed steel tubes. The results are compared with those from 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 N2 as carrier gas) was tested. The thermally sprayed coatings were deposited in accordance with the recommendations given by the suppliers, with the exception of the large splat coatings, where the parameters were slightly altered. The deposition parameters used in the laser process are not allowed to be published. The coatings and their chemical compositions can be seen from Table 1. Metcoloy 2 (wire) and Metco 3007 (powder) are standard products manufactured by Sulzer Metco. The powder with the same composition as Metcoloy 2 but used for the HVOF and laser deposition techniques were ¨ ¨ AB. The Amperit 526, Stellite manufactured by Hoganas 6 and 21, Inconel 625 and Duroc materials were manufactured by H.C. Starck GmbH, Deloro Stellite, Special
Metals Corp. and Duroc Energy AB, respectively. The size grades for the powder used with laser and HVOF were 63–150 mm and 5–63 mm, respectively. The thicknesses of the coatings were in the range 0.7–1.2 mm for laser, 0.4–0.6 mm for arc spray and 0.2–0.4 mm for HVOF. As substrate, steel tubes of a type normally used in high-pressure applications, Fe0.1C1Cr0.5Mo0.5Mn0.2Si, were chosen. The surface roughness of the arc sprayed and HVOF deposited substrate tubes was 5–7 mm and 10 mm, respectively (Ra-values). The Metcoloy 2 and 80Ni20Al materials were supplied as the two separate electrodes in the arc spray process, resulting in a coating consisting of 50% of each material. Two different arc spray droplet sizes, 50 and 200 mm, were examined for the Metcoloy 2q 80Ni20Al-coating. The adhesion of the coatings to the substrate was tested with the three methods as described below; a specially designed tensile test, a four-point bending test followed by metallography and a four-point bending test combined with acoustic emission (AE). 2.2. Tensile testing The adhesion of the coatings to the substrate was tested according to a tensile testing method developed from the ASTM 633 test. The modified design was used in order to simulate the tube geometry. A 10=15 mm string coating was deposited on the tubes (external diameter 33.7 mm, wall thickness 3.6 mm and length
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between the supports and the load applicators were 250 mm and 25 mm, respectively. All cylindrical supports and load applicators were scooped to the same radius as the tubes, Fig. 2b, and MoS2 was supplied in the scoops before the tests in order to reduce the fretting. Due to the tube geometry the test is not a pure fourpoint bending test. Rather the stress load is described as a parabola with its maximum between the two load applicators.
Fig. 1. Adhesion testing; experimental set-up for the tensile test.
approx. 20 mm). A cylinder (external diameter 8.0 mm) with the same curvature as the envelope surface of the tube was glued on top of the coated string, Fig. 1. The tensile strength of the glue used, FM-1000䉷, is 69 MPa after hardening. Peeling forces caused by misaligned attachment of the cylinder were reduced by using a brace designed as a hook and equipped with a spherical prong. A total of 54 samples were tested. 2.3. Four-point bending test combined with metallography and acoustic emission analysis 2.3.1. Experimental setup The experimental set-up is illustrated in Fig. 2a. The dimensions of the tubes used in this test were the same as in the tensile test except for the length, which was 300 mm. A 100=10 mm wide coating was deposited in the axial direction of the tubes and then subjected to four-point bending at a displacement speed of 0.25 mmy min. The displacement was measured with a position gauge centred on the coating surface. The distances
2.3.2. Strain calibration In order to determine the strain as a function of the displacement, five strain gauges were glued on uncoated tubes in the same area as the coating. One strain gauge was positioned at the centre of the assumed coating area, two at a distance of "5 mm off centre in the radial direction, and two at a distance of "15 mm off centre in the axial direction. The tubes were then bent to different displacements. The largest strain was always measured in the central position. This was then used to establish a calibration curve from which the displacements applied to the coated tubes were translated into strain. The presence of a coating on the surface will influence the strain distribution somewhat. In the case of thermally sprayed coatings, the influence is limited due to the relatively low adhesion strength. Further, in a comparative study, like this, the deviation is about the same among the coatings. The influence on the laser coatings is larger, but due to the complex deformation in the coated area of the tube, no adjustments were done. The absolute displacement among the experiments was adjusted in order to identify the strain at which coating delamination was initiated, meaning that several tests on each material were needed. After bending, an 80-mm long section was cut from the centre of the deformed tube and divided into two pieces at the position corresponding to the centre line of the coating, Fig. 3. Each piece was then polished on emery paper
Fig. 2. (a) Adhesion testing; experimental set-up for the four point bending and acoustic emission tests. (b) Scooped supports used in the fourpoint bending test.
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View recorded the events as a function of the displacement at the centre of the tube. No analyses were performed in real time. In order to obtain the acoustic emission originating from deformation of the tube, the supports, the load applicators, the tensile testing machine and other unidentified sources, uncoated tubes were tested and used as reference. Only signals reaching both sensors were analysed and calculations made from the signal travelling time showed that the vast majority of the acoustic emission recorded originated from the volume below the two load applicators. Fig. 3. The sectioning procedure of the bent tube for delamination analysis.
and diamond paste after which the delamination length was measured with optical microscopy. A total of 69 tubes were tested. 2.3.3. Acoustic emission analysis Two piezo electrical acoustic emission sensors (Physical Acoustic Corporation (PAC), R15 resonance frequency 150 kHz) were attached at the ends of the coated tubes with the full active area joined to a brass device, pinched on the tubes and tightened with a screw, Fig. 2a. Dow Corning䉷 high vacuum grease was used as couplant between the sensor and the pinch device. The sensor positions were chosen for minimising acoustic emission emerging from deformation of the tube. The frequency response of the sensors is roughly 50–520 kHz for transient (burst) signals. This overlaps with the results, which have shown that 90% of the acoustic emission emerging from material deformation is within the frequency band of 10–550 kHz w9x. The sensors are connected to an amplifieryfilter with a gain of 40 dB. The amplifiers are in turn connected to an eight bit deep sampling resolution ISA AyD-converter card. One event, or 16 384 samplesychannel (a waveform) is sampled whenever a trigger event occurs and then transferred to the computer, giving a dead time of approximately 20 ms. The sampling rate was set to 5 MHz. The energy of the acoustic emission for one event was calculated 16384
as Es 8 Ui2(U; voltage generated in the piezoelectric
2.4. Microstructural investigations Polished cross-sections of the coatings were examined in an optical microscope after the testing. The coating hardness was measured with Vickers method. The composition of selected areas on some coatings was determined by Auger spectroscopy (PHI 660), from which also the SEM-imaging capabilities were used. 3. Results 3.1. Tensile testing The results obtained from the tensile testing are summarised in Table 2 and the failure mechanisms are schematically illustrated in Fig. 4. In the tests performed with laser coatings, the glue fractures show that the adhesion strength of the coating exceeds 69 MPa. In contrast, coatings deposited with the HVOF technique delaminated at the coatingysubstrate interface. Arc sprayed coatings did not delaminate due to internal fracture at a stress of approximately 38 MPa, implying that in this case the adhesive strength exceeds the cohesive strength. No significant differences were noted among the various arc sprayed coatings. The results from this study clearly show that adhesion failure is only obtained for the HVOF coatings. Thus, the tensile testing method is not applicable for laser and arc spray coatings. 3.2. Bending test in combination with metallography
i
crystal due to the emitted acoustic emission) and then accumulated event by event. Software written in Lab-
Fig. 5 shows a cross section of the Metcoloy 2 coating deposited with laser. The microstructure is very uniform
Table 2 Summary of the ASTM 633 tensile tests on the different coating types Coating method
Tensile strength
Laser HVOF
Various coatings. The tensile strength exceeds 69 MPa (4 tests) Amperit 526 and Metco 3007: 55"10 MPa (5 tests and 4 tests, respectively) Metcoloy 2: 61"7 MPa (4 tests) Various coatings. 38"12 MPa. (23 tests)
Arc spray
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Fig. 4. Bonding failure mechanisms of the three different coating types as obtained with the tensile testing.
with a low porosity and oxide content. No signs of delamination were seen even after bending to strains as large as 15%. However, radial cracks occurred on a small number of samples after the bending procedure. These cracks cannot be linked to the composition or thickness of the coating. A dozen bending tests were performed on Metcoloy 2 coatings arc sprayed in air to find the strain at which delamination is initiated. The length of any delamination was measured for both the sectioned sample pieces. Summarising gives the maximum delamination length as 160 mm. Fig. 6a shows the microstructure of a sample bent to a strain of 0.5%. No cracks or signs of delamination are seen. The coating consists of many layers of overlapping essentially lamellar particles, droplets, which are sometimes called splats. Compared to the laser coating the oxide content and degree of porosity is much larger w10x. The same material at a strain of 1.25% is shown in Fig. 6b. A radial crack has formed in the splat boundaries and isolated areas of delamination have nucleated at the coatingysubstrate interface. The total delamination length is 2 mm in this case. At a stress of 1.9% a relatively large radial crack and a massive delamination (32 mm in this case) is seen, Fig. 6c. Delamination without radial cracks perpendicular to the coatingysubstrate interface has not been observed on the thermally sprayed coatings. However, coatings with only small radial cracks and no delamination have been observed. The delamination process is similar for HVOF sprayed coatings. Fig. 6d shows the microstructure of an HVOF-sprayed Metcoloy 2 coating after bending to
a strain of 2.1%. For this type of coating, partially developed radial cracks are never observed and radial cracks without delamination are very rare. This implies that delamination occurs immediately after formation of a radial crack. Figure 7 shows the delamination length as a function of strain for some selected materials and coating methods. Metcoloy 2 arc sprayed in air starts to delaminate at a strain of approximately 1.4%. From the same coating material but arc sprayed in nitrogen gas, only three points are available. It is seen that delamination has occurred at a strain of 1.8%. Mixing the bond coat material 80Ni20Al into the Metcoloy 2 coatings increases the adhesion strength dramatically, if the splat size is sufficiently large. Application of strains as large as 10– 15% just gives a minor delamination. However, it is clearly seen that the effect is negligible for small splats. The HVOF sprayed coating, (Metco 3007) shows worse adhesion compared to the arc sprayed ones. Delamination is initiated already at a strain of approximately 1%. Metcoloy 2 coatings deposited with laser do not delaminate at all, not even for strains up to 15%. The results show that, with the exception of the Metcoloy 2q 80Ni20Al coatings with a large splat size, the delamination rate is very high as soon as delamination is initiated. 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 largest strain where no signs of delamination have been observed and the higher end corresponds to the lowest strain at which delamination has been observed. The Amperit 526 coatings were subjected to a maximum strain of 1.5% and did not delaminate while the other two 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. 5. Cross section of laser coated Metcoloy 2 (optical microscopy).
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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 sprayed Metcoloy 2 coating at 2.1% strain (optical microscopy).
deposited with arc spray starts in the same strain region as for the Metco 3007 coating; between 1.1–1.25% for those sprayed in air and 0.5–2.1% for those sprayed in nitrogen gas. Addition of 80Ni20Al clearly increases the delamination region for Metcoloy 2 coatings to the strain interval 2.2–10%. However, if the splats do not have a sufficient size the effect of the binder disappears as seen from columns 8 and 9 in the figure.
the bending machine and should be considered when evaluating the tests performed with coatings. The behaviour of an Inconel 625 laser coating is similar to the
3.3. Bending test in combination with acoustic emission monitoring During all bending tests the acoustic emission was simultaneously recorded. The results from the coatings were normalised by the same amplifying factor as for the reference. In Fig. 9, the normalised accumulated energy is shown as a function of strain for some selected coatings. Results from a reference test on an uncoated tube shows an initial sharp rise in acoustic emission at a strain of 0.1–0.25% after which the curve has a continuous slope with a small inflexion at a strain of approximately 2%, Fig. 9c. The sources of acoustic emission in this case are deformation of the tube, fretting against the supports and load applicators and noise from
Fig. 7. The delamination length as a function of strain for selected coatings.
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Fig. 8. The strain intervals where delamination is initiated on thermally sprayed coatings. The figures in the columns indicate the number of tested samples for each coating quality. The number on the x-axis corresponds to the one given in the legend denoting the coating materials.
reference tube, Fig. 9c, with a sharp increase in energy level at 0.1% strain and an inflexion at 2% strain. This indicates that the acoustic emission activity originates from deformation of the tube and not from the laser coating. The coating did not delaminate for strains up
to 15% and the energy increase at larger strains is therefore 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 the accumulated energy level takes place (not shown here).
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.
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Bending of Metcoloy 2 coatings deposited with HVOF or arc–sprayed with nitrogen or air, respectively initially shows the same acoustic emission behaviour as the reference tube. However, in the strain interval 1.2– 2% a sharp increase in accumulated energy level is observed, Fig. 9b,c. The delamination length for the coating arc sprayed in air at 1.9% strain, where the experiment was terminated, is 32 mm, corresponding to 21% delamination. Sectioning of the sample deposited with the HVOF-technique showed that the coating delamination was complete. The sharp increase in accumulated energy therefore indicates the formation of radial cracks and subsequent delamination. The small steps in energy increase approximately 1.4, 1.6 and 1.75% strain as seen in Fig. 9c might indicate formation of radial cracks. The Metcoloy 2 coating arc sprayed in nitrogen gas was run to complete delamination during straining to 15%, Fig. 9a. In this case, the stepwise behaviour is not observed. Instead, the increase in acoustic emission activity starts at a strain of 1.9% and decays at a strain of approximately 13%, indicating the initiation and termination of the delamination process. The Metcoloy 2q80Ni20Al coating arc sprayed in air is bent to a strain of 11% after which the total measured delamination length is 3 mm. Up to a strain of approximately 6% the pattern is similar to that recorded from the reference tube and Inconel 625 coating, Fig. 9b,c. Above a strain of 6% there is a stepwise increase in energy level, which is interpreted as the formation of cracks and a subsequent delamination. The accumulated energy level does not deviate from the reference tube and laser coating until a strain of 9.5% is reached. The initial steeper rise in accumulated energy in the strain interval 0.1–0.25% is due to equalisation efforts made for adopting the curve to the Inconel 625 coating at higher strains. The strains at which delamination is initiated as interpreted from the acoustic emission measurements are summarised in Fig. 10. The carbide containing HVOFcoatings, Amperit 526 and Metco 3007, show the weakest adhesion strength and delamination is initiated at the strains 1.0% and 0.9%, respectively. The performance of the Metcoloy 2 coating deposited with the same method is somewhat better and delamination is not recorded for strains below 1.8%. The strains at which delamination has been recorded for the same coating in the arc sprayed condition are in the same range 1.4% and 1.9% with air and nitrogen as carrier gases, respectively. The effect of the 80Ni20Al addition to Metcoloy 2 is clearly seen from the fact that the delamination strain increases to approximately 10% (columns 6 and 7), which reflects an increased adhesion strength. However, a small droplet size in the coating immediately eliminates the positive effect as seen from columns 8 and 9.
215
Fig. 10. The strain at which delamination is initiated as determined by acoustic emission. The number on the x-axis corresponds to the one given in the legend denoting the coating materials.
3.4. Microstructural investigations The microstructure of the Metcoloy 2 arc sprayed in air and mixed with the 80Ni20Al bond coat was further investigated due to the differences in adhesion strength recorded. The splat size, which is the same as that obtained with normal spraying parameters, is approximately 50 mm, Fig. 11a. Auger spectroscopy shows that the only element present in the light areas which originates from the 80Ni20Al bond coating material (points 1, 3 and 5) is Ni. Al was not detected at all. In an other study, it has been suggested that Al has evaporated during the arc spraying process w11x. A more significant net material loss than usual was also noted during the coating production. The dark droplets originate from the Metcoloy 2 material (points 2, 4 and 6) and Fe, Cr and Ni are recorded. Thus, Ni has diffused into the Metcoloy 2 splats. Fig. 11b shows the same material mixture as in Fig. 11a, but with a droplet size of 200 mm. From Auger spectroscopy it is concluded that a very limited intermixing between the Metcoloy 2 and 80Ni20Al phases has taken place in this case. Only a minor concentration of Ni from the bond coat was detected in the Metcoloy 2 phase, while Al evaporated during the processing. On comparison image analysis shows that the total splat boundary length is three times smaller than for coatings built of small splats. In each of the cases, the splat size was the same throughout the coatings. The Vickers hardnesses of the HVOF coatings Amperit 526, Metco 3007 and Metcoloy 2 are 800, 550 and 350, respectively. Arc spraying of Metcoloy 2 gives a coating with Vickers hardness 320. Despite the differences, delamination is initiated at about the same strain. Addition of 80Ni20Al into the Metcoloy 2 coating results in a further hardness decrease to 220 HV for the small splats quality and 150 HV for the large splats quality.
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Fig. 11. Cross-sections of Metcoloy 2q80Ni20Al coatings (a) small splats (b) large splats (SEM microscopy).
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 gave delamination for the HVOF deposited coatings at a tensile strength of 55–61 MPa, while the arc sprayed coatings suffered from internal fracture at 38 MPa. The laser coatings did not delaminate at all, but fractured in the glue joint. The results obtained on the arc sprayed coatings are in accordance with earlier ASTM 633 tests w12x, which showed internal fracture on Metcoloy 2 coatings at stresses of 34"12 MPa. However, in the same study HVOF deposited, Metco 3007 coatings fractured in the glue joint at a stress of 69 MPa. The discrepancy in the results is explained by the tube geometry used in our study, where the surface curvature induces additional peeling forces at the coatingyglue interface.
The traditional way of testing the degree of delamination is tensile testing combined with optical microscopy. The method has the drawback that it requires several experiments where coated tubes are bent to a specific strain and then metallographically analysed in order to determine any crack formation at the coatingy substrate interface. Using acoustic emission makes it possible to continuously monitor the crack initiation and growth during delamination. Table 3 shows the correlation between delamination as determined by metallographic analysis and the evaluation of acoustic emission. For all coatings, the number of experiments where delamination is detected by metallographic analysis is noted. This is compared with the number of bending tests where analysis of the acoustic emission has indicated delamination on the same coating. Only two samples both deposited with HVOF show a discrepancy where delamination was detected by acoustic emission analysis and not by optical microscopy. The explanation
Table 3 Correlation between delamination as determined by metallography and acoustic emission Coating
Number of coatings which delaminated according to metallography
Number of coatings which delaminated according to acoustic emission
Amperit 526, HVOF Metco 3007, HVOF Metcoloy 2, HVOF Metcoloy 2, arc spray, air Metcoloy 2, arc spray, N2 Metcoloy 2q80Ni20Al, arc spray, air, large splats Metcoloy 2q80Ni20Al, arc spray, N2, large splats 80Ni20Al, arc spray, air, small splats Metcoloyq80Ni20Al, arc spray, air small splats
0 2 3 7 3 2
1 3 3 7 3 2
2
2
3 3
3 3
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may be in the sample preparation of the tested coatings. During sectioning of the bent tube a 1.6-mm wide material strip is cut away and thereby any delaminations and cracks in this area have vanished. Further, large stress concentrations and weak bonding in off-centre areas of the coating can induce small cracks. These are not detected during the metallographic analysis, which is only performed along the centreline of the coating. Contrary, the method of using acoustic emission records all the deformation during the bending independent of its location. However, the correlation between the two methods is very good and the conclusion must therefore be that acoustic emission is a suitable technique for evaluation of coating adhesion. In this study, arc sprayed coatings with small splats and HVOF deposited coatings usually start to delaminate in the strain interval 0.5–2.0%. Metallography revealed that delamination is always accompanied by radial cracks perpendicular to the coatingysubstrate interface. When a radial crack has developed, the coating is unloaded and the coatingysubstrate interface comes under increased tension, resulting in crack growth and delamination. In this region, a minimum of material mixing takes place during thermal spraying and the coating is thus bonded to the substrate surface by mechanical interlocking w2x. The statement is supported by the low adhesion strengths as measured with the tensile test and the swiftness of the delamination process after initiation. The laser coatings did not delaminate at all, not even for strains up to 15%. The reason lies in the laser process, where a small portion of the substrate surface is melted and mixed with the coating material. The result is a thin zone of metallic bonding w3x, which strongly increases the adhesion strength compared with the thermally deposited coatings. Separate electrodes of Metcoloy 2 and the bond coating material 80Ni20Al were arc sprayed into coatings with two different splat sizes, ;50 mm and ;200 mm. The adhesion strength of the coating with the larger splats is superior to that with the smaller ones. In fact, addition of the bond coating material does not influence on the adhesion strength at all, if the splat size is the same as that for the pure Metcoloy 2 coatings. However, the latter quality was only available with the smaller splat size. It has been shown w13x that the adhesion strength for coatings made of Al and SUS308 steels arc sprayed in air increases with the droplet size in the molten state. The larger droplets impinging on the surface during the spraying process have a higher kinetic energy. It has been suggested that the impact introduces a peening effect with accompanying compressive residual stresses in the coating w13x. Thus, the adhesion strength of the coating is increased. In the case of HVOF deposition, the effect is larger due to the only partially melted
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droplets where the smaller contact area results in a higher impinging pressure w14x. However, the influence of the coating microstructure was not taken into consideration in any of the studies referred. The present study shows that in the coatings composed of large splats the splat boundary length is only approximately 1y3 compared to the small size splat coatings. Since the splat boundaries are enriched in oxides and pores, their contribution to the weakening of the coating is large and radial cracks are preferably formed in these regions during tensile loading. Thereby if the number of splat boundaries is minimised, the radial cracking probability is reduced and the adhesion strength improved. Despite the large differences in hardness among the coatings, delamination is initiated at about the same strain. The coating hardness therefore seems to be of minor importance for the delamination behaviour. The hardnesses of the Metcoloy 2 coatings mixed with the 80Ni20Al bond coat are very low for both splat sizes. In both cases, evaporation of Al during processing creates an almost pure ductile Ni phase w11x. Thus, the improvement in adhesion strength is due to the formation of larger splats and not the presence of the bond coating material. 5. Conclusions The adhesion among coatings deposited on lowalloyed steel tubes with arc spray (air or nitrogen as carrier gas), HVOF and laser techniques was compared with a modified four point bending test and acoustic emission. The conclusions are as follows: ● Acoustic emission can be used for estimating the strain at which coating delamination is initiated. ● The delamination is always initiated by the formation of a radial crack. ● Coatings deposited with the laser technique show no delamination for strains below 15%. ● Coatings deposited with HVOF start to delaminate in the strain interval 0.8–1.8%. ● The adhesion strength of the arc sprayed coatings is dependent on the splat size. Small splats give delamination in the strain interval 1.4–1.9% while large splats give delamination in the strain interval 10.5– 11.5%. ● Mixing a bond coating material into the arc sprayed coating has no effect on the adhesion strength. Acknowledgments Financial support from the KME (Consortium for Material Technology directed towards Thermal Energy Processes) is gratefully acknowledged. Duroc AB and
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Midroc Metalock are acknowledged for deposition of ¨ ¨ AB for provision of the the coatings and Hoganas powders. Finally, the tubes were supplied by Kvaerner Pulping AB which is also acknowledged. References w1x J. Clare, D. Crowmer, Thermal spray coatings, ASM Handbook – Friction, Lubrication and Wear Technology, 18, ASM International, 1992, pp. 361–374. w2x R. Tucker, Thermal spray coating, ASM-Handbook-Surface Engineering, 5, ASM International, 1994, pp. 497–509. w3x R. Vilar, Solid freeform fabrication, laser cladding, Int. J. Powder Metall. 37 (2) (2001) 31–48. w4x D. Almond, M. Moghisi, H. Reiter, The acoustic emission testing on plasma-sprayed coatings, Thin Solid Films Metall. Prot. Coat. 108 (1983) 439–447. w5x Y. Mizutani, K. Nagashima, M. Takemoto, K. Ono, Fracture mechanism characterization of cross-ply carbon-fiber composites using acoustic emission analysis, NDT&E Int. 33 (2000) 101–110. w6x G. Qi, A. Barhorst, J. Hashemi, G. Kamala, Discrete wavelet decomposition of acoustic emission signals from carbon fiberreinforced composites, Compos. Sci. Technol. 57 (1997) 389–403. w7x G. Qi, Wavelet-based AE characterization of composite materials, NDT&E Int. 33 (2000) 133–144.
w8x X.Q. Ma, S. Cho, M. Takemoto, Acoustic emission source analysis of plasma sprayed thermal barrier coatings during four-point bend test, Surf. Coat. Technol. 139 (2001) 55–62. w9x Characterisation ´ ´ ´ par emission acoustique de l’adherence et de ˆ ˆ l’endommagement d’un revetement: cas d’un revetement WCCo sur acier, D. Dalmas, S. Benmedakhene, C. Richard, A. ´ ´ Laksimi, G. Beranger, T. Gregorie, Comptes rendus de l’academie des sciences serie il fascicule chimie 4(5) 2001 345– 350. w10x A. Hjornhede, ¨ I. Olefjord, Erosion corrosion of thermally sprayed Fe12Cr and Cr3C2 –NiCr coatings, in: R. Streiff, I.G. Wright, R.C. Krutenat, M. Caillet, A. Galerie (Eds.), Proceedings of fifth International Symposium on High-Temperature Corrosion and Protection of Materials, Les Embiez, France, May 22–26, 2000, (Trans Tech. Publications Ltd, Switzerland, 2001), pp. 507–514. w11x A. Nafari, A. Nylund, Field Study on Superheater Tubes in the Loop Seal of a Wood Fired CFB Plant, Licenciate Thesis, ¨ Chalmers, Goteborg, 2003. w12x P. Tarkpea, Ytbelaggningar ¨ ¨ erosionsskydd i fluidbaddar, ¨ for ¨ Varmeforsk Service AB, Stockholm, 1998. w13x Y. Arata, A. Ohmori, R. Kawase, Zu den faktoren welche die ¨ adhasion lichtbogspritzer schichten beeinflussen, Trans. JWRI 13 (2) (1984) 193–199. w14x C.-J. Li, Y.-Y. Wang, A. Ohmori, Effect of melting state of spray particles on the adhesive strength of HVOF nickel-based alloy coating, in: Christopher C. Berndt (Ed.) Proceeding of ´ Quebec, ´ 1st International Thermal Spray Conference, Montreal, Canada, May 8–11, 2000, (ASM International, USA) Thermal Spray, Surface Engineering via Applied Research.