Dye impregnation method for revealing machining crack geometry

The palladium nitrate dye penetrant method for revealing surface microcrack s was investigated and applied to display the geometry of machining cracks in silicon nitride flexure test specimens. This method used elemental mappi ng with an electron probe microanalyzer to detect the presence of the dye a nd, thereby, display the crack geometry. A previously used bending method a nd a method developed in this study in which the specimen surface is expose d to the dye under pressure were used to facilitate dye penetration. Prior to applying the method to study machining cracks, carefully controlled Knoo p indentation cracks introduced into flexure specimens were used to verify penetration of the dye to the crack tip. During these experiments it was fo und that the palladium nitrate dye resulted in a reduction in flexure stren gth, which, on further study, was attributed to the dilute nitric acid solu tion used to formulate the dye. Exposure to carbon tetrafluoride plasma etc hing prior to applying the pressurized dye method also resulted in a detect able decrease in flexure strength. Although there was clear evidence that e xposure to dye and plasma etching resulted in a small but measurable decrea se in flexure strength for the silicon nitride material studied, there was no detectable change in observed crack geometry. The reduction in flexure s trength was apparently caused by a decrease in resistance to initiate crack propagation. It was concluded that the palladium nitrate dye method is an accurate and useful means for determining the geometry of small, otherwise difficult to observe surface microcracks, Nevertheless, caution should be e xercised with the use of this method during strength measurements. When app lied to machining cracks, the complex nature of these shallow, elongated, s ometimes joining cracks was unambiguously revealed.