40 Experimental Program

Task I: Heat Treatment and Characterization of Stand-alone Metallic Coatings

Objective: To heat treat and subsequently characterize the TGO scale formed on standalone metallic coatings as a function of temperature and oxygen partial pressure.

Technical Approach: Specimen (1 inch diameter by 1/8 inch thickness) coated with stand-alone metallic coatings, PtAl, Si,Hf modified MCrAlY and MCrAlY all on CMSX-4 superalloy substrate which has been supplied by Howmet International and was fabricated with current commercial processing practices that include various surface finishes: as-coated, grit blasted and media finished. In addition a very smooth laboratory polish will be used to justify improved commecial processing if appropriate. The surface morphology and roughness of the specimens has been analyzed using optical and electron microscopy and ZYGO ™ surface profilometry, capable of measuring roughness in the nano-scale. Emphasis will be given to the relationship between processing technique, surface roughness and the presence of surface defects/features. The specimens have been cut into small pieces and heat treated as a function of temperature (T = 900°, 1000°, 1100° C) and oxygen partial pressure (PO2 = 0.01, 0.2, 1.0 atm) for an hour as described in the first semiannual report. The TGO scale formed during the heat treatment has been characterized with respect to phase constituents, residual stress and morphology by x-ray diffraction, optical and electron microscopy, and by using the laser fluorescence technique. Details of specimen descriptions and heat treatment are presented in Table I.

Based on the characterization of the TGO scale formed on stand-alone metallic coatings that were heat treated, three conditions (3 out of 9 identified in Figure 11) have been selected. These three conditions will be selected so that the specimens will contain three different types of TGO scale. These three conditions are selected to demonstrate the importance of homogeneous stable a-Al2O3 scale compared to other oxide conditions. In the case of the tow variants of MCrAlY a Howment process for the removal of residual oxide is being employed on half of the samples instead of having a third surface finish or third preoxidation treatment. The selected heat treatments and the corresponding TGO scale formation will be employed for the thermal cycling tests of stand-alone metallic coatings (Task II) and TBCs (Task III).

Table I. Specimen Descriptions and Evaluation for Task I and II.

Metallic Coating

Surface Finish

Surface Defect/Features

Heat Treatment

Total Number of Specimen Required

Pt-Al

As-coated

Ridges at full height

As identified in Figure 8

3 for Heat Treatment 3 for Oxidation Testing

Gritblasted

Reduced ridge height Rough surface

As identified in Figure 8

3 for Heat Treatment 3 for Oxidation Testing

Media-finished

No ridges Smooth surface

As identified in Figure 8

3 for Heat Treatment 3 for Oxidation Testing

Pt-Al (Si,Hf)

As-coated

Ridges at full height

As identified in Figure 8

3 for Heat Treatment 3 for Oxidation Testing

Gritblasted

Reduced ridge height

Rough surface

As identified in Figure 8

3 for Heat Treatment 3 for Oxidation Testing

Media-finished

No ridges Smooth surface

As identified in Figure 8

3 for Heat Treatment 3 for Oxidation Testing

MCrAlY

Shot-peened

Rough surface

As identified in Figure 8

3 for Heat Treatment 3 for Oxidation Testing

Media-finished

Smooth surface

As identified in Figure 8

3 for Heat Treatment 3 for Oxidation Testing

Total number of specimen required: 54 stand-alone metallic coatings

Total number of specimen required: 54 stand-alone metallic coatings

Task II: Oxidation Testing of Stand-alone Metallic Coatings

Objective: To test and evaluate selected stand-alone metallic coatings using isothermal and cyclic oxidation testing.

Technical Approach: After the selected heat treatments, the stand-alone coatings will be subjected to isothermal and cyclic oxidation testing. One forth of a full disk will be used allowing more different conditions to be considered. The isothermal oxidation will be carried out at 1100° C with thermal gravimetric analysis (TGA) to examine the oxidation kinetics of the coatings. The cyclic oxidation will be carried out in CM Rapid Temperature Cyclic Furnace. The thermal cycle will consist of a 10-minute heat-up, 40-minute hold at 1100° C and 10-minute cooling. Micro structural evaluation of oxidized coatings will be carried out by x-ray diffraction, optical and electron microscopy as well as by laser fluorescence piezo-spectroscopy. Emphasis will be given to the oxidation kinetics, TGO phases, stress and adherence/spallation as a function of initial oxide phase constituents, surface roughness, surface defects/features and bond coat compositions. Mechanisms associated with spallation of TGO scale will also be assessed. Early results of this type are described in the section on preoxidation

Objective: To procure bond coated TBC specimens with selected heat treatments and to test and evaluate them as a function of thermal cycling.

Technical Approach: In accordance to the specified heat treatment process, bond coated TBCs with CMSX-4 superalloy substrate will be prepared in April by Howmet International. Thermal barrier (ZrO2-7wt.%Y2O3) layer will be deposited on heat-treated bond coats by EB-PVD. The TBC specimens will be subjected to cyclic oxidation testing using a CM Rapid Temperature Cyclic Furnace. The thermal cycle will consist of a 10-minute heat-up, 40-minute hold at 1100°C and 10-minute cooling. During thermal cycling, phase constituents and residual stress of the TGO will be monitored as a function of thermal cycle by laser fluorescence and specimens will be visually inspected frequently for any sign of spallation in order to accurately assess the lifetime of standalone metallic coatings and TBCs. Microstructural characterization of TBC specimens will be carried out prior to failure as well as after the failure to address failure mechanisms associated with various heat treatments, surface preparation and bond coat composition. Specimen descriptions and testing plans are presented in Table II.

Table II. Specimen Descriptions and Evaluation for Task III.

Metallic

Surface

Surface

Heat

Total number

or bond

finish

Defect/Features

treatment

of

boating

0 0

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