Startsida
CT-Project
Computer tomography as a method to study the structure of intergranular microcracks in brittle materials and coatings
Short Description of Måns Mångård's PhD Project, finished 12/2000
Introduction
Microcracks are an important feature in the microstructure of brittle materials and coatings. The microcracks are one of the means for the material to absorb the elastic energy released at fracture and may thereby determine the toughness of the material. The microcracks may also contribute to the compliance of a coating thereby allowing the coating to accommodate thermal and mechanical strains. To understand the origin of microcracks during the manufacturing of materials is therefore of importance.
Aim
In this project non-destructive investigation by the modern high resolution microfocal computer tomography system at Linköping University has been used. The material for the studies were thermally sprayed zirconia coatings and powder metallurgically produced steel components from an ongoing projects in the group.
The computer tomography system has been used to try to image and reveal the microcrack structure in brittle materials and coatings. Measured densities and orientations of microcracks were compared to densities found on polished surfaces of the same material. The crack densities were correlated to the manufacturing process of the materials including thermal history, stress build-up and stress relaxation.
Financing
The project was financed by The Swedish Research Council for Engineering Sciences, TFR.
CT investigation of TBC sample T327-7 (Demo1)
Below is a radiography image (shadowgraph) of the sample. The segmentation cracks are clearly visible.
CT investigation of TBC
50 slices (cross-sections) were taken. The shaded area in the image below shows where the CT slices have been taken.
- CT parameters:
- X-ray: 180 kV, 33 microampere
- Filtration: 1.2mm Sn
- Camera: 7.2 s shutter time, 1 averaged frame, 50% iris.
- Image intensifier zoom: Normal
- Angular inc. 0.185 degrees (1012 projections were collected)
- 50 slices, 1 line per slice, Shepp Logan filtration, reconstructed with backprojection.
Slice 1
The black line in the radiography image above shows where the CT slice is reconstructed.
The cracks are seen as dark (air). There is unfortunately almost no difference in color between the coating, bondcoating and superalloy.
The bondcoating can be seen most easily close to the edge of the sample. The fact that the sample is brighter close to the edges is due to a CT-imaging error (artefact) called beamhardening.
Slice 25
The black line in the radiography image above shows where the CT slice is reconstructed.
Slice 30
The black line in the radiography image above shows where the CT slice is reconstructed.
Observe that there are some pores in the bondcoating.
Slice 45
The black line in the radiography image above shows where the CT slice is reconstructed.
In this slice a crack through the whole sample can be seen. The crack density is very high in the center of this slice, close to the bottom of the hole.
Slice 50
The black line in the radiography image above shows where the CT slice is reconstructed.
This slice shows also a crack through the coating, bondcoating and superalloy.
Page responsible:
ru.peng@liu.se
Last updated: 2009-12-14
