THE FRACTAL GEOMETRY OF SI3N4 WEAR AND FRACTURE SURFACES

Good mechanical properties and chemical stability at high temperatures make silicon nitride a good candidate as an advanced engine material. Much research has been done to characterize the mechanical strength a nd resistance of crack propagation in this material. In this paper, we use fractal analysis to study the geometry of Si3N4 fracture and wear surfaces. We found that the geometries of the failure surfaces as cha racterized by the fractal dimensional increment, D, under different f ailure stress states are similar for the same brittle material, but di fferent for different brittle materials. The similar D in an identica l brittle material implies that the failure process in the material is the same regardless of loading mode, i.e., mode I or mixed-mode stres s. The fractal technique is shown to be useful for correlating the fra ctal dimension to the material properties and fracture-surface topogra phy.