The Effect of Heat Treatment on the Oxidation Behavior of HVOF and VPS CoNiCrAlY Coatings
- PDF / 580,616 Bytes
- 8 Pages / 593.972 x 792 pts Page_size
- 67 Downloads / 204 Views
S. Saeidi, K.T. Voisey, and D.G. McCartney (Submitted October 16, 2008; in revised form February 26, 2009) Free-standing VPS and HVOF CoNiCrAlY coatings were produced. The as-sprayed HVOF coating retained the c/b microstructure of the feedstock powder, and the VPS coating consisted of a single (c) phase. A 3-h, 1100 °C heat treatment in vacuum converted the single-phase VPS coating to a two-phase c/b microstructure and coarsened the c/b microstructure of the HVOF coating. Oxidation of freestanding as-sprayed and heat-treated coatings of each type was carried out in air at 1100 °C for a duration of 100 h. Parabolic rate constant(s), Kp, were determined for free-standing, as-sprayed VPS and HVOF coatings as well as for free-standing coatings that were heat treated prior to oxidation. The observed increase in Kp following heat treatment is attributed to a sintering effect eliminating porosity from the coating during heat treatment. The lower Kp values determined for both HVOF coatings compared to the VPS coatings is attributed to the presence of oxides in the HVOF coatings, which act as the barrier to diffusion. Oxidation of the as-sprayed coatings produced a dual-layer oxide consisting of an inner a-Al2O3 layer and outer spinel layer. Oxidation of the heat-treated samples resulted in a singlelayer oxide, a-Al2O3. The formation of a thin a-Al2O3 layer during heat treatment appeared to prevent nucleation and growth of spinel oxides during subsequent oxidation.
Keywords
CoNiCrAlY, heat treatment, HVOF, oxidation, thermally grown oxide, VPS
1. Introduction MCrAlY (M = Ni, Co, or both) coatings are commonly used as overlay coatings and as bond coats for thermal barrier coatings (TBCs) composed of yttria-stabilized zirconia (YSZ) in order to protect turbines from hightemperature oxidation (Ref 1-4). These coatings owe their protective effect to the fact that aluminum forms a continuous oxide layer on the coating surface that is thermally very stable while the remaining elements (e.g., Cr) control the aluminum activity, hold the oxide in place (e.g., Y) and adapt the coating to the properties of the base material (Ref 5, 6). Premature failure of TBCs during thermal cycling is still a critical problem, which limits the lifetime of the coated components (Ref 7). This failure mainly occurs by delamination of the top coat (Ref 8, 9). The cracks leading to delamination nucleate and propagate at the topbond coat interface. Interfacial oxides, in particular, the thermally grown oxide (TGO) layer, play an important role in the cracking process (Ref 4). S. Saeidi, K.T. Voisey, and D.G. McCartney, Department of Mechanical, Materials and Manufacturing Engineering, Faculty of Engineering, University of Nottingham, Nottingham, UK. Contact e-mails: [email protected], [email protected], katy. [email protected], and [email protected].
Journal of Thermal Spray Technology
As the oxidation behavior of the thermally sprayed MCrAlY is significantly influenced by the coating process and the composition of the
Data Loading...
