Volume 31, Issue 3 (May 2003)
Erosive Boundary Layers of Ni 200 in Vibratory Cavitation Erosion Tests
The test surface of a specimen for cavitation erosion testing is prepared by several machining operations. These operations induce alterations on the surface and in the material beneath it, thereby forming an affected material zone. The properties of this zone affect the mechanical and metallurgical properties of the material. As a consequence the erosive properties are affected too, but are not easily detected on the cumulative erosion-time curves.
The cumulative erosion-time curves of Ni 200 erosion tests were modeled by the Weibull cumulative distribution function. The equivalent Weibull plot consists of two intersecting straight lines, thereby providing a bimodal plot with a decreasing slope. Each line describes a different erosion mode. The first mode, from test onset to the intersection point, describes the erosion process of the specimen's boundary layer. Since this layer is removed by means of the erosion process, it is designated as the erosive boundary layer.
The second mode, from the intersection point onwards, describes the erosion process of the homogeneous material. Analysis of the Weibull plots of Ni 200 erosion tests enables determination of the erosive boundary layer thickness and erosion time; it further facilitates the evaluation of the erosive boundary layer effect on the entire erosion process. The Weibull method enables construction of plots describing the erosion process of an ideal specimen, with an erosive boundary layer of 1 μm thickness. Plots of this kind may be used to characterize Ni 200 and to compare erosion properties of various materials.