Surface laser-glazing of plasma-sprayed thermal barrier coatings

Abstract

Atmospheric plasma sprayed (APS) ZrO 2 -8%WtY 2 O 3 thermal barrier coatings (TBCs) were subjected to a CO 2 continuous wave laser-glazing process in order to generate an external dense layer produced by different processing parameters. For that purpose, different beam scanning speeds and track overlapping were chosen.Surface roughness has been reduced significantly after laser-glazing. Despite the surface crack network, all laser-glazed specimens presented a fully dense and porous free external layer with a columnar microstructure. Surface cracks along the densified layer were found to have tendency to be oriented in two perpendicular directions, one in the direction of the laser beam travel, 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 along the densified layer are vertical and tend to branch and deviate from the vertical direction within the porous PS coating. The largest overlapping allied to the smallest amount of irradiated energy generated the most uniform layer with the shortest crack branches within the PS coating. For the as-sprayed coating, the XRD results revealed mainly t' non-transformable tetragonal zirconia with a small percentage of residual monoclinic zirconia. All glazed coatings presented only t' non-transformable tetragonal zirconia with some variations on preferable crystal orientation.

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

“…4a,b,c), XRD analyses revealed, for all three series of coatings a polycrystalline structure composed solely by the nontransformable t' tetragonal phase which indicates that both laser systems are suitable for surface treatment without structural modification of zirconia. Moreover, ZrO 2 -8wt%Y 2 O 3 plasma-sprayed coatings may contain small amounts of residual monoclinic phase that can be suppressed by laser-glazing [10,11,31]. It was also detected some minor relative peak intensity variation due to columnar grain orientation caused by the laser-glazing process.…”

“…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].…”

“…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].…”

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

“…These perpendicular cracks on the surface of a densified coating can increase the thermal shock resistance up to four times due to the accommodation of tensions. 24 Recent studies by Batista et al 25 have shown that the APS laser glazing of ZrO 2 -8 wt %Y 2 O 3 coatings with CO 2 systems reduces the defects and inhomogeneity of Y 2 O 3 caused by thermal spraying. The rapid cooling involved in this process also eliminates the residual monoclinic phase but leaves small traces of phase tЈ.…”

Effects of Thermal Fatigue on Laser Modified H13 Die Steel

“…Surface modification techniques such as plasma vapor deposition, brush plating, plasma surface metallurgy technology and chemical vapor deposition are widely used in fabricating high wear resistance, oxidation resistance and corrosion resistance coating. (Batista et al, 2005).The materials used for surface coating may show different resistances against thermal cracking as compared to reference materials and the mechanism behind the thermal fatigue failure of coating surfaces are still not fully explored. Difficulties meeting the effective coating requirement caused surface treatment are more preferable in protecting dies from premature failure (Tsai, Lee, & Chang, 2007;Xu et al, 2006).…”