EFFECT OF ADDITIVES ON SELF-HEALING OF PLASMA SPRAYED CERAMIC COATINGS

N. Sata, A. Ansar and K. A. Friedrich

German Aerospace Center (DLR), Pfaffenwaldring 38-40, 70569 Stuttgart, Germany

ABSTRACT

MgAl2O4 coatings, used for electrical insulation along the metallic seals in high temperature fuel cell stacks, tend to fail prematurely due to generated stresses during on-off thermal cycles. To enhance stress bearing capability and the durability of these coatings, self-healing was investigated by introducing additives consisting of SiC as primary additive along with a secondary additive material. Secondary additives were one of the following compounds: BaO, CaO, ZnO, Y2Q3, Al2O3, La2Q3, TiO2, GeO2, Ceo.9Gd0.1O1.95 (GDC) orTa2O5. Crack healing can be attained due to reaction between SiC, secondary additive and oxygen that was transported to the additive due to a crack. Such reaction would be associated to a phase formation in the additive material linked with mass and volume increase assisting in closure of the advancing crack. Using TGA/DSC reaction temperatures and mass gains of SiC with or without secondary additives were identified. SiC+Y2O3 and SiC+ZnO were opted as promising additive materials. 20 wt% (SiC+Y2O3) containing MgAl2O4 coatings were produced by plasma spraying. In these developed coatings, healing was demonstrated after heat treatment at 1050°C in air for 10 hour. Defect healing in spinel coating with SiC+ZnO is under investigation.

INTRODUCTION

Defects in the sealing in high temperature solid oxide fuel cells (SOFC) has been reported as the foremost cause of failure in the fuel cell stacks1,2. The sealing, traditionally made of glass or glass-ceramic composites, ensures flow of fuel gas and air in designated compartments of SOFC stack. A leakage across the seal leads to catastrophic loss in cell potential and power output Glass-based seals exhibit limited reliability which suffers further when the stack should undergo thermal transients such as during intermittent operation. In our earlier work3, an alternative approach was proposed in which Ag-based filler material is used for sealing of two consecutive cells. Despite mismatch of coefficient of thermal expansion (CTE) between filler alloy and neighboring components of stack, the high ductility and creep of the filer alloy compensate for stresses. However, as the filler alloy is electronically conductive, short circuiting between the cells is avoided by introducing an Mg-spinel (MgAl2O4) insulating coating in between. The spinel deposit is produced by plasma spraying. The schematic of sealing the approach is given in Figure 1. In spite of enhanced reliability compared to glass-based seals, the coating-braze based seals suffer from defects and cracks in the coating. These defects, associated to the manufacturing process or arise due to thermal cycling, give site for further crack nucleation and propagation, decrease the elastic modulus, yield strength and fracture energy of the coatings. At elevated temperatures crack initiation and propagation mechanism changes and failure may occur at the featureless zones, as suggested by Lowrie and Rawlings4. Overall, the increase in temperature from room temperature to 800°C caused a 23-30% reduction in flexural strength of bulk 8YSZ. Ansar et al5 have reported that the elastic modulus of plasma sprayed 8YSZ reduces from 35±2 GPa at room temperature (instead of 120 GPa for bulk material) to 16±1 GPa at 800°C The decrease in elastic properties of such a coating was almost twice to bulk material and this was associated to the intrinsic elastic modulus of the YSZ but also to the structure of the splat boundaries.

Figure 1: Schematic of the SOFC stack sealing based on active braze and insolating coating.

Catastrophic failure in these insulation layers is a major limiting factor constraining the use of fuel cell in automotive applications. One approach to address this shortcoming consists of incorporating crack healing capabilities, which can offer improved reliability and service time of ceramic components. Though reported already in 1970's6,7, this approach has limited work attributed to it to this day. Most studied self-healing ceramics are oxide ceramic matrix composites (CMCs) having 15 to 20 wt% of well distributed SiC particles such as Al2O3/SiC8,9 and mullite/SiC10,11. It was established that in SiC containing ceramic composites the healing occurs due to oxidation reaction, associated with volume expansion of new phases into the defects12:

Other studies suggested healing can be improved by promoting vitreous phase formation by using SiC+Y2O313:

As a result, cracks up to 100 µm were fully healed and the bending strength was increased by several folds.

Self-healing was also developed in other systems including ZrO2/SiC by thermal decomposition transformation to ZrSiO4 and carbon black mixture14. Additionally, crack healing ability was investigated in non-oxide ceramics such as Si3N1 by introducing SiC15,16. In all materials presence of oxidizing environment is mandatory for a healing mechanism to occur and in most cases the minimum temperature at which full crack healing takes place has been reported at above 1000°C8-16.

Incorporation of a material that can heal the defects in insulating ceramic layer in SOFC at elevated temperatures can significantly improve the stress bearing capability and fracture strength of these coatings. Insulating spinel coatings are primarily exposed to oxidizing environment which makes it realistic to achieve a crack healing mechanism. However the potential of healing capability can be best utilized if the reaction can be achieved at operating temperature of fuel cells which is around 800°C. The potential of such an approach is investigated in this paper. A range of potential self-healing additives consisting of primarily SiC along with secondary additives, BaO, CaO, ZnO, Y2O3, AI2O3, La2Q3, TiO2, GeO2, Ce0.9Gd0.1O1.95 (GDC) or Ta2O5 were tested and reported. Self-healing was demonstrated in Mg-spinel plasma sprayed insulating coatings using SiC+Y2O3 as reference additive particles at 1050°C. It was suggested that addition of other tested additive compounds with SiC+Y2O3 can effectively reduce reaction temperature well below 1000°C and in-operando healing can be potentially attained.

EXPERIMENTAL PROCEDURE

Self-healing additives

SiC and the secondary additive powders studied in this work are listed in Table 1. SiC with two different particle sizes were investigated submicronic sized with d50 of 0.65 µm and nano sized with d50 of 50-60 nm. SiC and additive materials were mixed in molar ratio using an agate mortar and pestle. In order to understand the reaction mechanism of the mixed particles and to select the healing additives, Thermal gravimetric analysis (TGA)/ Differential scanning calorimetry (DSC), X-ray diffraction and Raman spectrometry have been performed. Reaction temperatures were defined by the commencement of weight gain (associated with volume expansion) on TGA data of the additive mixtures.

Table 1: List of the powder samples.

The TGA measurements were conducted in parallel with the DSC measurements using the STA 449C Jupiter® from Netzsch (D). The samples were tested in Pt-Rh crucibles from room temperature to 1200°C (except for SiC+Ta2O5~l 100°C) with a constant heating rate of 5°K/min. Synthetic air (O2:N2=20:80) and argon (Argon 5.0) at a flow rate of 30 ml/minwere used as reaction and protection gases, respectively..

The mixed powders were analyzed by X-ray diffraction before and after the TGA/DSC measurements. The measurements were conducted using D8 Dscover GADDS, equipped with a VANTEC-2000 area detector from Bruker AXS. A tuned monochromatic and collimated X-ray beam (Cu-Ka) was used.

For selected additive mixtures, Raman spectroscopy was conducted with a confocal Raman microscope (LabRam 800, Horiba Jobin Ybon). A green laser line (wavelength= 532 nm) was used for excitation. Mxed powders have been annealed in ambient air for 1, 5 and 10 hrs for Raman measurements.

Coating Fabrication and Characterization

MgAl2O4 and MgAl2O4 with SiC+Y2O3 were sprayed using air plasma spraying (APS) with a Triplex Pro 210 gun (Oerlikon Metco, Switzerland). 70±6 µm thick coatings were produced on Fe-Cr substrates from Thyssen group, Germany. In...