Metallographic Workflow for Ceramic Matrix Composites

October 28, 2019

Follow this Robo-Met recipe to analyze ceramic matrix composite materials with Robo-Met.

Ceramic matrix composites (CMCs) are a unique class of materials that combine different compositions of ceramics to achieve optimized properties. Their density and radiopacity makes them challenging to analyze by NDE techniques such as CT. Robo-Met.3D® provides a proven workflow to analyze the structure and defect profiles of these materials.

This recipe evaluates a SiC-SiC CMC fabricated using polymer infiltration and pyrolysis. This step-by-step workflow provides a starting point that can be customized to analyze your CMCs with Robo-Met.


Read also: Metallographic Workflow for Steels and Ferrous Metals


Mounting

The sample is conventionally cold mounted in a 1 ½” mount. The imaging area is highlighted in red.

The CMC sample is conventionally cold mounted in this 1 ½” mount.

Metallographic Workflow

Most workflows can be divided into a planarizing step that is not repeated, and grinding/polishing steps for material removal and surface smoothing. In this case, we used a 15µ Dia-Grid Diamond Disk for planarization.

1) Grinding Step 1

  • Pad/Cloth/Disc: 15 µ Dia-Grid diamond disk
  • Medium: water
  • Time/RPM: 15s/150 RPM

2) Polishing Step 1

  • Pad/Cloth/Disc: Struers MD-Dac woven acetate pad
  • Medium: 3µ diamond
  • Time/RPM: 60s/150 RPM

3) Polishing Step 2

  • Pad/Cloth/Disc: Struers MD-Dac woven acetate pad
  • Medium: 1µ diamond
  • Time/RPM: 120s/150 RPM

4) Polishing Step 3

  • Pad/Cloth/Disc: Struers MD-Chem polishing cloth
  • Medium: 0.05µ colloidal silica
  • Time/RPM: 240s/150 RPM

5) Cleaning Step 1

  • Pad/Cloth/Disc: Struers MD-Chem polishing cloth
  • Medium: Water
  • Time/RPM: 10s/150 RPM

Other Parameters:

  • Prelube 10s
  • Dispense Time On 3s / Off 30s
  • Rinse 10s
  • Air Dry 10s
  • Ethanol Dip 10s after last cleaning step

Maintaining moisture on the pads is essential to successful automated metallography. The final on-pad cleaning step provides a better imaging surface.

Sectioning Results

Robo-Met.3D® Comparisons for a SiC/SiC CMC

100x Total Magnification

CMC sample at 100x total magnificaiton

  • Objective: 10x/0.25 EC Epiplan-Neofluar HD DIC
  • XY resolution: 1.3 microns
  • Depth of Field: ~8.5 microns
  • XY Field of View: 1x1mm

200x Total Magnification

CMC sample at 200x total magnificaiton

  • Objective: 20x/0.5 EC Epiplan-Neofluar HD DIC
  • XY resolution: 0.6 microns
  • Depth of Field: ~1.9 microns
  • XY Field of View: 0.5x0.5mm

Robo-Met.3D provided high resolution images using the 10x and 20x objectives described above. The 20x objective image was useful to visualize the coating on the fibers, as well as unreacted silica from the pyrolysis process. The different phases are clearly segmentable using a WEKA routine.

Raw 2D image of CMC sample, collected with the Robo-Met.3D

Raw 2D image

Image of CMC sample, collected with the Robo-Met.3D, and segmented to show features (Pores- red , fibers- green, matrix- purple, unreacted silica- yellow)
Segmented (Pores- red , fibers- green, matrix- purple, unreacted silica- yellow)


This workflow provided a linear material removal of about 3.8µ per polishing step, with an R squared value of 0.998 over 54 sections. The removal results (focal plane of section versus section number) are plotted on the above figure, which demonstrates a highly linear material removal for the CMC sample.

Figure plots Robo-Met removal results, which demonstrates a highly linear material removal for the CMC sample.

This provides essential input into our 3D reconstruction models.

Image shows 3D reconstruction of the CMC sample.

For Further Information

The wide variety of materials that we analyze for clients has given us extensive experience in developing workflows suited to materials such as aerospace alloys and polymer and additively manufactured materials, among others. Contact us to jump-start your utilization of Robo-Met. We’ll be happy to consult with you to create a custom workflow to characterize your materials.

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