Oxidation behavior of thermal barrier coatings modified by laser remelting

Chang, Keh Chin; Wei, W.; Chen, C.

doi:10.1016/s0257-8972(97)00657-9

Cited by 31 publications

(17 citation statements)

References 29 publications

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“…The presence of segmented cracks and interconnected porosity due to volume shrinkage and residual stresses in the top coat [2][3][4][5][6][7] affects the mechanical properties and deteriorates the oxidation and corrosion resistance. These features are considered to be the path for molten salts and corrosive gases to attack the TBC system, especially in applications where low purity fuels are burned.…”

Section: Introductionmentioning

confidence: 99%

“…[10] verified an enhancement of also approximately fourfold in the lifetimes of plasma-sprayed TBCs by laser-glazing in high temperature corrosion tests involving V 2 O 5 salts. Latest studies [11] have shown that laser-glazing ZrO 2 -8%wtY 2 O 3 coatings improved considerably microhardness, erosion and abrasion resistance, whilst lowered strength and stiffness.Cracks, perpendicular to the surface along the densified layer are characteristic features of laser melted ceramic materials [5,6,[9][10][11][12][13][14][15][16][17][18][19][20][21] and are generated by shrinkage and relief of thermal-induced stresses [9,16,20,22]. It was shown that these cracks, induced by laser treatment, improved thermal shock resistance.…”

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confidence: 99%

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Morphological and microstructural characterization of laser-glazed plasma-sprayed thermal barrier coatings

Batista

Portinha

Ribeiro

et al. 2006

Surface and Coatings Technology

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Laser-glazing has been revealing a high potential for the improvement of plasma-sprayed (PS) thermal barrier coatings (TBCs) by reducing surface roughness, eliminating open porosity on the surface and generating a controlled segmented crack network, although the relationship of the processing parameters with the resultant properties has not yet been completely established. In this investigation, TBCs consisting of atmospheric plasma sprayed (APS) ZrO 2 -8%wtY 2 O 3 were subjected to a CO 2 continuous wave laser glazing process in order to seal its surface porosity generating an external dense layer. For that purpose, different amounts of radiation resulting from different scanning speeds were applied to the specimens as well as different track overlapping. Results have shown a significant decrease of the surface roughness after the laser treatment. All specimens presented a fully dense and porous free external layer with a polyfaceted columnar microstructure highly adherent to the plasma-sprayed coating. Controlled surface crack networks, extremely dependent on the laser scanning speed and track overlapping, were achieved for each set of processing parameters. The cracks were found to have a tendency to be oriented in two perpendicular directions, one in the direction of the laser beam travel direction, the other perpendicular to it. Moreover, the cracks parallel to the beam moving direction are found to be on the overlapping zone, coinciding with the edge of the subsequent track. The cracks are perpendicular to the surface along the densified layer and tend to branch and deviate from the vertical direction below it, within the porous PS coating. XRD results revealed mainly t' non-transformable tetragonal zirconia with a small percentage of residual monoclinic zirconia for the as-sprayed coating. All glazed coatings presented only t' non-transformable tetragonal zirconia with some variations on preferable crystal orientation. Grain sizes varied from 26 to 52 nm, increasing with an increase of laser irradiated energy, microstrain behaved inversely. Keywords IntroductionThermal barrier coatings (TBCs) have successfully entered service in the turbine section of advanced gas turbine engines in the mid-1970s [1]. Since then, several improvements on the selected materials, processing techniques and life prediction tools have been taking place to contribute for the improvement of TBC's performance as well as for the widening of the field of applications.Current thermal barrier coating systems consist of a bond coat deposited over the material to be protected and then the TBC itself is applied to the bond coat. The metallic bond coat (MCrAlY, M=Ni, Cr or Ni/Cr) acts as a corrosion resistant layer to protect the substrate material (Nickel-based superalloy), forming alumina as the principal protective scale (thermally grown oxide (TGO)). Due to mismatch of thermoelastic properties between the metallic substrate and the ceramic top coat, the bond coat is also necessary to accommodate residual stresses avoiding its...

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Section: Introductionmentioning

confidence: 99%

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confidence: 99%

Morphological and microstructural characterization of laser-glazed plasma-sprayed thermal barrier coatings

Batista

Portinha

Ribeiro

et al. 2006

Surface and Coatings Technology

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“…Laser-glazing provides a remelting and subsequent solidification of the surface resulting on a dense top layer with a new microstructure with reduced surface roughness, free from porosity but -4 -with formation of crack networks perpendicular to the surface [10][11][12][13][16][17][18][19][20][21][22][23][24][25][26][27]. Moreover, this post-treatment technique is suitable for surface treatment without structural modification of the zirconia nontransformable tetragonal phase because of the rapid solidification and subsequent cooling [10,11,16].…”

Section: Introductionmentioning

confidence: 99%

Evaluation of laser-glazed plasma-sprayed thermal barrier coatings under high temperature exposure to molten salts

Batista

Portinha

Ribeiro

et al. 2006

Surface and Coatings Technology

126

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Thermal Barrier Coating (TBC) systems are frequently used in gas turbine engines to provide thermal insulation to the hot-section metallic components and also to protect them from oxidation, hot corrosion and erosion. Surface sealing treatments, namely laser-glazing, have been showing a high potential for extending in-service lifetimes of these systems by improving chemical and thermo-mechanical resistance. In this investigation, both as-sprayed and laserglazed TBCs were exposed to hot corrosion in molten salts. The glazed coatings were obtained by scanning the surface of the plasma-sprayed coatings with either a CO 2 or a Nd:YAG laser.The hot corrosion investigation was accomplished by subjecting the specimens to an isothermal air furnace testing under V 2 O 5 and/or Na 2 SO 4 in a temperature of 1000ºC for 100 hours.Spallation has been observed in coatings in the as-sprayed condition under V 2 O 5 or -2 -V 2 O 5 +Na 2 SO 4 . Na 2 SO 4 itself had no or minimal effect on the degradation of the laser-glazed or as-sprayed condition coatings, respectively. The degradation in V 2 O 5 was accomplished by destabilization of YSZ as a consequence of depletion of yttria from the solid solution to form YVO 4 and therefore led to the disruptive transformation of the metastable tetragonal phase to the monoclinic phase. Moreover, the presence of both corrosive salts induced the formation of large high aspect ratio YVO 4 crystals that introduced additional stresses and contributed to the degradation of the coatings. The laser-glazed specimens were not efficient in avoiding the molten salt penetration along the thickness direction due to the presence of cracks on the glazed layer.However due to a reduced specific surface area of the dense glazed layer, the corrosion reaction of the molten salts with the YSZ has been lower than in coatings in the as-sprayed condition.-3 -

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“…At low repetition rates, the heating rate is high, and in addition, the thickness of molten material is small ( Table I), so that bubbles do not have time to coalesce. 17 This could explain the absence of these structures at 4 and 10 kHz. One important consequence of the slow heating rates is that thicker modified layers can be obtained (Table I).…”

Section: Discussionmentioning

confidence: 97%

“…Regarding the small holes, droplets and bubbles present at 30 kHz and at CW mode can be attributed to the release of entrapped gas during melting. 9,10,15,17 The slow heating process allows bubbles to coalesce and escape leaving small holes and droplets on the surface. At low repetition rates, the heating rate is high, and in addition, the thickness of molten material is small ( Table I), so that bubbles do not have time to coalesce.…”

Section: Discussionmentioning

confidence: 99%

Structural characterization of laser-treated Cr₃C₂–NiCr coatings

et al. 2001

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The interconnected porosity of the Cr 3 C 2 -NiCr coatings obtained by high-velocity oxy fuel spraying is detrimental in corrosion and wear resistance applications. Laser treatments allow sealing of their surfaces through melting and resolidification of a thin superficial layer. A Nd:YAG laser beam was used to irradiate Cr 3 C 2 -NiCr coatings either in the continuous wave mode or at different repetition rates in the pulsed one. Results indicated that high peak and low mean laser irradiances are not good, since samples presented deep grooves and an extensive crack network. At low peak and higher mean laser irradiances the surface was molten, and only a few shallow cracks were observed. The interconnected porosity was completely eliminated in a layer up to 80 m thick, formed by large Cr 7 C 3 grains imbedded in a NiCr matrix.

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Oxidation behavior of thermal barrier coatings modified by laser remelting

Cited by 31 publications

References 29 publications

Morphological and microstructural characterization of laser-glazed plasma-sprayed thermal barrier coatings

Morphological and microstructural characterization of laser-glazed plasma-sprayed thermal barrier coatings

Evaluation of laser-glazed plasma-sprayed thermal barrier coatings under high temperature exposure to molten salts

Structural characterization of laser-treated Cr₃C₂–NiCr coatings

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Oxidation behavior of thermal barrier coatings modified by laser remelting

Cited by 31 publications

References 29 publications

Morphological and microstructural characterization of laser-glazed plasma-sprayed thermal barrier coatings

Morphological and microstructural characterization of laser-glazed plasma-sprayed thermal barrier coatings

Evaluation of laser-glazed plasma-sprayed thermal barrier coatings under high temperature exposure to molten salts

Structural characterization of laser-treated Cr3C2–NiCr coatings

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Structural characterization of laser-treated Cr₃C₂–NiCr coatings