Breviary Technical Ceramics


      From Powder to Part

 Ground Surfaces

The characteristic of grinding is the overlapping of a large number of scraping movements on fixed paths taken by grinding grains in one particular direction parallel to the surface of the workpiece. The scraping movement of the cutting grains directly induces micro-cracks, brittle breakage and/or plastic flow, and hence the ductile separation of ceramic particles.

Figure 67 shows machining tracks which illustrate the mechanisms by which material becomes separated, with areas of primarily plastic deformation, of primarily brittle fracture and of a crack may clearly be distinguished. The crack area is characterised by microscopic axial, radial and lateral cracks.

Surfaces whose creation has primarily been influenced by plastic deformation procedures typically display scoring, rounding, stratification, flaking and a relatively small number of break-outs. The process is called "ductile cutting".
In contrast to this, break-outs, pile-ups, facetting and spalling are typical of "brittle cutting".
Grinding a surface usually results both in ductile and in brittle cutting. The dominant mechanism of material separation depends on the local stress conditions on the individual grains of the ceramic microstructure during the scraping process. The changeover from brittle to ductile cutting depends in particular on exceeding a critical cutting thickness at the individual grain, but also on the micro-geometries of the cutting grains.

Bild 67: Unterschiedliche Werkstofftrennmechanismen bei geschliffenem Siliciumnitrid

Figure 68: Ceramic surfaces subjected to brittle, mixed and ductile grinding

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