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

Abstract

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...

Hot-corrosion behaviors of overlay-clad yttria-stabilized zirconia coatings in contact with vanadate–sulfate salts

“…[12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] Laser treatment has been proved to be a highly promising method to improve the coating quality of the plasma-sprayed coatings by eliminating open pores within the coating surface and generating a controlled segmented crack network. Hot-corrosion tests in the presence of V 2 O 5 indicated that the lifetimes of the plasma-sprayed 6.1YSZ, 7.3YSZ, 12YSZ and 19.5YSZ (numbers denote wt.% of Y 2 O 3 ) coatings were largely increased In our former work, the plasma-sprayed 8YSZ coating was laser glazed and achieved remarkably improved hot-corrosion resistance against molten V 2 O 5 + Na 2 SO 4 salts.…”

Cost-effective SU-8 micro-structures by DUV excimer laser lithography for label-free biosensing

“…The rapid cooling involved in this process also eliminates the residual monoclinic phase but leaves small traces of phase tЈ. 26 All of the above contribute to a better behavior during oxidation tests in the presence of molten salt at 1000°C than that achieved with sprayed condition coatings. 27 Laser glazing transforms the metastable ␥-Al 2 O 3 present in the APS coating to ␣-Al 2 O 3 .…”

Influence of laser‐glazing on hot corrosion resistance of yttria‐stabilized zirconia TBC in molten salt mixture of V2O5 and Na2SO4

“…These segmented cracks were caused by the shrinkage of the molten material and by the relief of thermal-induced residual stresses during cooling. The segmented cracks in the coating surface are characteristic features of laser-re-melted ceramic materials [14][15][16][17][18][19][20][25][26][27].…”

“…In previous studies, a variety of methods have been conducted to seal the top coat surface, including the liquid metal impregnation, hot isostatic pressing (HIP), laser-glazing, sol-gel process, and thin chemical vapor deposition (CVD) overcoatings, to prevent the penetration of molten salts into the porous YSZ coatings [12][13][14][15][16][17][18][19][20][21][22][23]. Laser-glazing has been proved to be a highly promising method to improve the coating quality of the plasma-sprayed coating by reducing the surface roughness (Ra), eliminating open pores within the surface and generating a controlled segmented crack network.…”