A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 2 2 4 4 iTSSe TSS iTSSe TSS Figure 3 highlights common TBC microstructural artifacts caused during mounting. The images show several distinctive artifacts: a) a hot-mount acrylic with artificially large pores in the TC, most elongated or aligned horizontally, which is consistent with horizontal cracking in the TC that becomes emphasized in relief during polishing; b) a hot-mount phenolic with a continuous crack along the TCBC interface due to elastic-plastic strain, collapse of TC pores under compression, or overall bending forces; c) cold-mount acrylic samples have no horizontal cracking but may have localized large elongated pores near the bottom of the TC microstructure and ASTM E1920-03 Method-II[3] is the basis of all polishing. FACTORS THAT CAUSE TSC ARTIFACTS Sectioning involves applying several forces to a part/ sample, including clamping pressure, blade incursion, bending, friction, shear, and thermal expansion stresses. Specific to as-deposited TSCs that are layered structures and are not metallurgically bonded to the substrate, the saw, blade, or cutoff wheel applies directional forces on the sample. If the blade enters the substrate and exits through the coating, it applies forces that push the coating away from the substrate. If those forces exceed the local bond strength of the coating, “debonding” can occur. Debonding presents as horizontal cracks or gaps along or near the substrate interface. Also, specific to TBCs, where the TC is a less-ductile material, sectioning into the substrate and exiting the TC surface can also cause horizontal cracking within the TC. The TC cracks may appear discontinuous as the crack propagates in and out of the plane of view of the image. Debonding and TC cracking are possible artifacts of sectioning. Subsequent grinding and polishing steps may not remove the artifacts of sectioning. A unique artifact of a TBC TC initiates as horizontal cracking due to sectioning and presents in the final polished structure as large, elongated pores or strings of pores that are not randomly distributed. Misinterpretation of this structure characterizes the TC as having greater pore fraction than actual. Figure 2 shows a TBCmicrostructure with artifacts from improper sectioning, i.e., at high incursion rate, using limited coolant, and with the blade oriented to cut into the substrate and exit the coating surface. Figure 2a shows several fields of view demonstrating the crack path in the TC and loss of TC that resulted from cracking during sectioning. Figure 2b shows a higher magnification of this sample with the specific artifacts labeled. For non-porous wrought materials, sectioning is performed prior to mounting. For TBCs, the porosity in the TC is critical to the function of a TBC, and those pores represent internal surfaces that can fracture or yield under applied loads from sectioning and mounting. Because TBCs are porous and easily damaged by the forces involved in sectioning, coatings should be vacuum-infiltrated with epoxy prior to sectioning to support the pore walls during sectioning, thus minimizing the risk of artifacts. Mounting processes can be characterized broadly as hot-mount type and cold-mount type. Among the various mounting materials in either process, cold-mount materials applied using vacuum infiltration provide the lowest risk of artifacts for TBCs. All samples presented in this section were polished using ASTM E 1920-03 Method-II, non-modified. The distinctions of polishing using Method-I or Method-III are not discussed here. FEATURE 10 Fig. 2 — Artifacts in TBC caused by sectioning shown in (a) lower magnification images showing loss of TC by cracking and decohesion during sectioning and (b) high magnification image with arrows indicating additional horizontal cracking in the TC along the substrate interface in the BC. Samples and images provided by FL Institute of Technology, Center for Advanced Coatings, Melbourne, Fla., and Technetics Group, Deland, Fla.. (b) (a)
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