13332 Damage from Machining

Machining of ceramic materials is almost always necessary at some point in the manufacture of high-performance structural ceramics. It will not be the purpose of this brief section to review ceramic machining; rather the reader is referred to several excellent references on this topic [32-34]. While many methods are under development for machining of ceramics at various stages in the green state, significant machining in the sintered state is still being done. In sintered-state grinding, a grinding wheel is in contact with the workpiece, and the grit in the grinding wheel can introduce various damage levels in the material. This is shown schematically in Fig. 13.11. The grinding wheel contains embedded grit, usually diamond, and these grit particles can induce various types of damage. Of particular interest are: (a) the radial cracks that form normal to the tool-path direction, (b) lateral cracks that form below the surface and are parallel to the material surface, and (c) the median crack that forms to a greater depth and often follows the tool path mark.

From the point of view of NDE, it is the detection of the deep median crack and the subsurface lateral crack that becomes of great concern. One method that has shown promise is the polarized laser scatter method [35, 36]. In a recent series of tests, Si3N4 ceramic materials were aggressively machined using different types of grinding wheels, different grit sizes on the grinding wheels, different feed rates (or material removal rates) and other parameters [36]. These were studied by the laser scatter method. Figure 13.12 shows a diagram of the samples and typical back scatter laser image data. Difference in the back scatter image data can be seen by observing Fig. 13.12c. This figure shows two back scatter images from two different grinding parameters: material removal rate using all other parameters constant. Clearly the main characteristic difference between these scan "images" is the presence of the varying amounts of "black" dots. By examining

FIGURE 13.11 Schematic diagram of one grinding setup and descriptions of types of induced damage: (a) grinding wheel on test piece, and (b) diagram of typical damage types of brittle materials.

FIGURE 13.11 Schematic diagram of one grinding setup and descriptions of types of induced damage: (a) grinding wheel on test piece, and (b) diagram of typical damage types of brittle materials.

FIGURE 13.12 Machining samples used to study back scatter laser methods for detecting machining damage: (a) diagram of machining process, (b) samples sectioned, and (c) typical back scatter images of surfaces.

FIGURE 13.12 Machining samples used to study back scatter laser methods for detecting machining damage: (a) diagram of machining process, (b) samples sectioned, and (c) typical back scatter images of surfaces.

the laser data as a two-dimensional array of gray-scale values and calculating the coefficient of variation, Cv, one can then plot Cv versus the strength of the material. In this test four-point bend strength was used. Figure 13.13 shows the correlation between the laser scatter data, Cv, and the four-point bend strength for two Si3N4 materials.

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