Popocatepetl Ash Infiltration in Lanthanum-Gadolinium Zirconate Ceramics

Currently, the most widely used material for thermal barrier coatings (TBC) in aeronautical industry is zirconia stabilized with 6- 8% of yttria (7YSZ). However, 7YSZ transforms from tetragonal to monoclic at temperatures above 1200°C causing phase destabilization [2]. This transformation can also be accelerated by the presence of silica compounds as in molten volcanic ashes (VAs) [3]. These deposits of molten silica infiltrate throughout the coating porosity and solidify during cooling producing residual stresses that eventually generate TBC failure [4]. Rare earth zirconates (REZs) have been considered as a promising material due to phase stabilization at high temperatures, these crystalize in a typical pyrochlore structure. Compared to 7YSZ, REZs are ceramic materials that have many advantages for TBC applications: no phase transformation from room temperature to their melting temperature, considerably high sintering resistance, a very low thermal conductivity, and a lower oxygen ion diffusivity [2]. Among these REZs, gadolinium zirconate (GZO) and lanthanum zirconate (LZO) have received great attention due to their stability at temperatures above 1500°C. When LZO and GZO interact with Si-based melts, they exhibit the formation of reaction products such as apatites (Ap). The formation of Ap is considered as a mechanism against infiltration of Si-based melts because they can act as a sealing layer along the interphase preventing the infiltration mechanism [5]. The objective of this work is to analyze the behavior of ash infiltration Popocatepetl VA (Mexico) with LZO, GZO and LZGO solid solutions ((La1-xGdx)2Zr2O7) by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). La2O3, ZrO2 and Gd2O3 precursor powders were obtained by co-precipitation of zirconium oxychloride and the respective of rare earth oxides dissolved in aqueous solutions with nitric acid, which were carefully dropped over a known amount of ammonium hydroxide. The stoichiometric powders obtained were calcinated at 1300 °C to form the pyrochlore phase. These LGZO powders were milled and isostatically pressed under 194 MPa to obtain a green body samples that were sintered at 1450 °C during 10 h to obtain dense LGZO ceramics. Subsequently, the infiltration experiments were undertaken by adding 20 mg/cm2 of VA on each ceramic sample and heat treated at 1250 °C for 10 h. The temperature was chosen because of previous DSC analyses [3] to assure that VA was molten. Cross sections of the infiltrated ceramics were analyzed by SEM to measure the infiltration depth. Elemental mappings, and point EDS analysis were also performed on the observed phases following the reactions that took place during the infiltration experiment to obtain their compositions. Fig. 1a-e shows the reprecipitated phases as VA infiltrates over the LGZO ceramics. Three zones, labeled as Z1, Z2 and Z3, are shown to distinguish the morphology of the formed phases. EDS analysis allowed to match the compositions of the reprecipitated globular phase at Z1 zones with zirconia. Z2 zones are mainly conformed of a Ap layers, phase responsible to stopping the infiltration mechanism. An additional Z3 zone [6], in Fig. 1e, is only observed at the infiltrated GZO and phases embedded matches the composition of spinel phase ((MgAl2-xFex)O4, Sp) and reprecipitates at the bottom of the reaction region. Fig. 1f shows the measured distance from surface as a function of the LGZO solid solution. Table 1 shows the chemical compositions of the reprecipitated Aps for each interaction. Al-rich Aps precipitated with La-rich LGZO ceramics. In contrast, Al-depleted Aps form with Gd-rich ceramic. This leads a local region rich in Al dissolved in the molten ash which eventually reacts to precipitated Sp phase, which has been reported to be not effective to arrest infiltration. Finally, the Aps precipitated as the molten VA interacts with LGZO solid solutions seems to be more effective to stop infiltration forming a dense layer of Aps and zirconia in comparison to the pure LZO and GZO ceramics. SEM images of ceramic cross sections of infiltrated LGZO ceramics presenting two or three zones correspondingly (a) LZO, (b) LGZO25, (c) LGZO50, (d) LGZO75 and (e) GZO. (f) shows the distance from surface as a function of x in (La1-xGdx)2Zr2O7. Ap chemical compositions taken by EDS of the reprecipitated Aps. Infiltration behavior is affected by the amount of Gd3+ in solid solution of LGZO. The infiltration resistance of intermedial compositions of LGZO is due to a different kinetic of infiltration and reprecipitated phase distribution. Monoclinic zirconia precipitates with La-rich LGZO ceramics meanwhile cubic zirconia for Gd-rich ceramics. Sp phase is not an effective phase to arrest infiltration and was only observed in the infiltrated GZO. Ap composition is related to the infiltration behavior in the LGZO ceramics.