PROGRESS IN EBC DEVELOPMENT FOR SILICON-BASED, NON-OXIDE CERAMICS

C.A. Lewinsohn, H. Anderson, J. Johnston

Ceramatec Inc.
Salt Lake City, UT, 84119

Dongming Zhu

NASA Glenn Research Center
Cleveland, OH

Hydrothermal corrosion is a lifetime-limiting mechanism for silicon-based, non-oxide ceramics in combustion environments. Many desirable materials for use as protective coatings are physically or chemically incompatible with the non-oxide substrate materials. A unique method of engineering bond-coats and coating systems for non-oxide systems has been developed and shown to improve the hydrothermal corrosion resistance of silicon nitride and silicon carbide-based materials. Progress in work to investigate the effect of additions of oxidation resistant filler materials to polymer-derived bond coats for environmental barrier coatings will be discussed. In the current work, additional data will be provided showing that the bond coat system can be adapted to composite silicon carbide. Initial results on the high-temperature durability of these coatings will be presented.

INTRODUCTION

Higher turbine inlet temperatures are one way of making turbine engines consume less fuel, which reduces both operating costs and emissions. Currently, however, turbine inlet temperatures are limited to values below which downstream components can survive. Typically, creep of metallic components at elevated temperatures and pressures limit the conditions at which turbines can be operated. Therefore, for many years, there has been a desire to introduce into turbine engine hot sections ceramic materials that are more resistant to creep. Even after sufficient strength, creep behavior, and reliability had been demonstrated, however, the lifetime of the candidate materials was found to be below desired values due to corrosion in the presence of water vapor1, which is referred to as hydrothermal corrosion.

The ceramic materials that have sufficient strength, creep behavior, and reliability to withstand turbine engine conditions are typically silicon based ceramics, such as silicon nitride and silicon carbide, and their composites. Silicon carbide (SiC) has high strength and good thermal conductivity, but it suffers from low fracture toughness and, hence, reliability. Therefore, components consisting of silicon nitride (Si3N4), which can be manufactured with higher values of fracture toughness than silicon carbide, and silicon carbide- or silicon nitride-matrix composites are currently under development for components that will be subject to appreciable stresses in operation. These materials are stable under purely oxidizing conditions, due to the formation of passivating oxide layers. In processes known as hydrothermal corrosion, however, these materials be corroded significantly by H2O and CO, which are common components in gas turbine systems2,3,4.

Extensive research has identified several oxide materials with low silica-activity that are relatively resistant to hydrothermal corrosion. These materials do not possess the strength, creep behavior and reliability required to act as the structural component, however they could be used as coatings for turbine engine hot section components. These oxides include ytterbium silicate (Yb2Si2O7)8,5,6, lutetium silicate (Lu2Si2O7)6, yttria-stabilised zirconia (8mol% yttria + 92mol% ZrO2, 8YSZ)5, strontium-stabilised celsian ((1-x)BaO-xSrO-AlO2-SiO2, 0<x<1), BSAS)9, and mullite (3Al2O3-2SiO2).7

The oxide materials that exhibit good resistance to hydrothermal corrosion typically have much higher coefficients of thermal expansion than silicon nitride8 or silicon-based materials, such that unacceptably high residual stresses develop in the substrate or coating that subsequently lead to failure after processing or during operation. One approach to mitigate these residual stresses has been to insert materials with intermediate properties between the coating and the substrate.9 The interlayer material has been limited by the requirements that it adheres to both top coat and substrate materials, has good high-temperature stability, does not exhibit any deleterious reactions with either the top coat or substrate, and has acceptable thermoelastic properties. Over the past decade, development of amorphous, non-oxide ceramics derived from preceramic polymers (polymer-derived ceramics, PDC) has led to materials with remarkable oxidation stability and mechanical properties at elevated temperatures.10 Furthermore, these materials show excellent adherence to a wide range of materials, including non-oxide ceramics, oxide ceramics, and metals. Therefore, efforts at Ceramatec, Inc. have been focused on developing coating systems with PDC interlayers and low silica-activity, outer environmental barrier coatings (EBC). This paper will describe recent progress in the development of these coating systems and their application to silicon-based ceramics, including silicon carbide, fiber-reinforced, silicon carbide matrix composites (SiCf/SiCm).

METHODS

A simplified flow chart illustrating the overall process to coat substrates, is shown in Figure 1. This flow sheet illustrates the process used to apply bond coat layers to samples at Ceramatec, some of which were then sent to the NASA Glenn Research Center for application of the EBC layer. In earlier work, Ceramatec has also developed a method of cosintering bond coat layers with EBC layers. The latter method may be more desirable since the issue of sintering material onto a non-densifying substrate only occurs once during processing. Regardless, it is desirable to have a low modulus layer in the coating system to accommodate property mismatches, so ideal coating systems have dense, outer EBC layers and bond coat layers with residual porosity.

Figure 1 Simplified process flow chart for applying bond coat(s) and outer coatings, as required.

Surface preparation is dependent on the nature of the surface of the substrate being coating, but at a minimum it involves cleaning and degreasing the surfaces to be coated and at a maximum involves sandblasting the surface to create a new one. The bond coat can be applied as several layers, with the same or different composition, or as a single layer. At Ceramatec, efforts have focused on dip-coating the coating layers. Although dip coating has the advantage that it is not a line of sight process and is relatively simple, more sophisticated methods, such as spray deposition, may produce more uniform coatings or may have other benefits in manufacturing.

Coatings were applied to bend bars made from various commercial types of silicon nitride or silicon carbide, fiber-reinforced, silicon carbide matrix composites. In some cases the silicon nitride bend bars had as-processed surfaces, but in most cases the surfaces were machined and ground. The surfaces of the SiCf/SiCm bend bars were cleaned prior to coating, but no other treatment was applied to them.

Bond coat materials consisted predominantly of allyl-hydrido-polycarbosilane (aHPCS, Starfire Corp.), a preceramic polymer. A portion of the preceramic polymer was partially pyrolysed to convert it from liquid form to solid. The partially pyrolysed material was blended with liquid precursor and filler materials. Initial work at Ceramatec to produce co-fired coating systems utilized silicon nitride and EBC powder as filler material. For bond coats applied to silicon nitride at Ceramatec and sent to NASA for application of the EBC, silicon nitride powder and EBC powder was also used as fillers, however, removing the EBC powder from the bond coat provided the best results in high temperature thermal cycling. For SiCf/SiCm, five different blends of filler powder were evaluated separately: partially-pyrolysed preceramic precursor and silicon carbide, HfO2 and silicon carbide, Yb2Si2O7 and silicon carbide, HfO2-Y2O3-GdO-Yb2Si2O7 and silicon carbide, and silicon carbide nanotubes. The ratio of experimental filler/silicon carbide filler was kept constant, as was the volume fraction of the experimental filler. Non-aqueous mixtures, using toluene as a solvent, of the liquid preceramic precursor, partially pyrolysed precursor, and filler materials were prepared such that the viscosity of the mixtures was in the range of 50 – 100 cP. Samples were attached to a labscale dip-coating apparatus that immersed and withdrew the samples from the mixture at a constant rate, 2 cm/min. The samples were approximately 4 mm-wide, 3 mm-thick, and 50 mm-long.

The flexural strength of the bars was tested according to ASTM C-1160. Hydrothermal corrosion testing was performed in an environment of 90% H2O, 10% O2 flowing at 2.2 cm/sec. Thermal cycles were performed by shuttling the specimens in and out of the hot zone of a furnace held at the test temperature, i.e. 1300°C or 1382°C. The specimens were cycled between room temperature and the test temperature with a heating time of 20 seconds to temperature, a 1 h hold, a cooling time of several minutes, and a 20 minute hold at room temperature. The specimens were contained in platinum crucibles that were contained within an alumina furnace tube. The testing was performed at the NASA Glenn Research Center.

RESULTS AND DISCUSSION

Bond coats were applied to silicon nitride by dip-coating, Figure 2. The original bond coat formulation was derived for silicon nitride made by Saint-Gobain, Inc. (now owned by CoorsTek) and is referred to as “NT-154.” When the same formulation was applied to another vendor’s silicon nitride, extensive cracking occurred. The second silicon nitride was made by Kyocera, Inc. and is referred to as “SN-282”. Modifications were made to the bond coat...