Accepted Practice for Metallographic Preparation of NiCrAl/Bentonite Abradable Coatings

Abradable coatings (such as Ni-4Cr-4Al/bentonite) are used throughout jet engines, primarily as sacrificial coatings into which moving components wear. These coatings generally consist of a metallic and a non-metallic phase, and they contain relatively high porosity levels (up to 40%). Figure 1 shows typical areas of application for abradable coatings (yellow highlights) within a jet engine. Typical locations for abradable application include the fan, low pressure compressor, and high pressure compressor sections.

Bentonite abradable coatings are primarily used within the higher operating temperature regions of a gas turbine (up to 815 °C. or 1500 °F). Therefore, this coating is most commonly found on steel and/or titanium components within the compressor section of an engine.

For processing abradable coatings, tight control over spray booth parameters is critical. While bentonite is stable at temperatures commonly seen during thermal spray processing, the nonmetallic phases used in other abradable coatings (such as graphite in nickel/graphite or polyester in AlSi/polyester) can be burned off if too much heat is used during application of the coating. As a result, the coating may be metal-rich, leading to excessive hardness and a loss of the coating’s abradable properties. Instead of blade tips (or other moving parts) wearing into the coating, the coating hardness will cause the actual component to wear.

Due to their composite nature, abradable coatings present unique challenges from a metallography standpoint. Abradable coatings are generally quite thick (> 0.040 in., or 1 mm) and very porous. Due to the elevated porosity of these coatings, the nonmetallic phases are often loosely bonded and subject to “pull-out” during metallographic preparation.

Round robin testing was performed on Ni-4Cr-4Al/bentonite coating samples by the member laboratories of the TSS Accepted Practices Committee on Metallography (see the “Acknowledgments” section in this document). Each laboratory was provided with a single 1 × 3 in. coupon containing the Metco 312NS coating. The participating laboratories were instructed to prepare a minimum of one cold mount and one hot mount sample from this coupon. Preparation recipes were collected from each laboratory, and the collective findings of these laboratories are outlined in this document.

• The majority of the laboratories identified vacuum impregnation of a castable epoxy as a critical step to ensure coating integrity during preparation. These labs indicated that hot mounted samples exhibited elevated porosity relative to their cold mounted counterparts. One laboratory also suggested that the hot mounted sample appeared to be compacted as a result of the heat and pressure associated with hot mounting.

• For those samples impregnated by a cold mount epoxy, the coating did not appear to be particularly sensitive to preparation recipe.

• One laboratory did not observe a difference between hot mount and cold mount samples.

• Due to thickness and porosity considerations, vacuum impregnation with a low-viscosity cold mount epoxy is the recommended mounting method. To facilitate impregnation with “fast cure” epoxies (which are typically more viscous than “slow cure” epoxies), the resin can be heated (to ~150 °F, or ~66 °C) prior to mixing with the hardener. Holding the epoxy at elevated temperature for 15-20 minutes should result in a significant improvement in the viscosity of the epoxy.

• As with the preparation for all thermal spray coatings, sufficient material must be removed during the planar grinding stage to ensure that all sectioning damage has been removed. While all laboratories indicated a planar grinding step, one laboratory went as far as to specifying material removal of 0.060 in. (1.5 mm).

• Preparation recipes supplied by the member laboratories contained a range of recipes ranging from predominately SiC papers (120-4000 grit), to strictly resin bonded diamond discs and polishing cloths. Comparable results were generated in each case. As a result, this coating does not appear to be sensitive to the preparation recipe employed. Examples of these recipes are provided in Table 1.

See Fig. 2–4 for reference optical micrographs of this coating.

This Accepted Practice is intended to be used as a baseline, but it does not replace local test or laboratory instructions. Additional requirements may apply based on the available equipment, testing materials, customer requirements, and other criteria.

This document was prepared in 2008 by the ASM Thermal Spray Society Accepted Practices Committee on Metallography. The Accepted Practice outlined in this document is based on tests by the ASM Thermal Spray Society Round Robin Laboratories:

• Buehler Ltd.

• Chromalloy Gas Turbines

• Deloro Stellite

• Goodrich Power Systems

• HEICO Aerospace

• IMR Test Lab

• Praxair TAFA

• Pratt & Whitney Aircraft

• Standard Aero

• Struers

Selected OEM specifications for abradable coatings include GE B50TF232 Class A, Honeywell Allied Signal FP 5045AB Type XXV, Pratt & Whitney PWA 1393, Rolls-Royce MSRR 9507/45, Volvo PM 819-54.