Abstract
Investigating the structure of a material cannot be accomplished without distorting it in some way. This is to some minor extend the case, if a material is observed by using electromagnetic waves. An examination method is considered as non-destructive, if the excited matter of the sample under inspection returns in its original state after being examined. X-ray diffractometry is capable of sensing structural perturbations at the surface of an object being inspected. Data collected by an X-ray diffraction can indicate subtle heat caused differences (at the material´s surface) due to the vibration of atoms and smearing of their electron clouds, at which the diffraction of the X-rays principally occur. This elevates the X-ray diffractometry to a superior position among other investigation methods. Depending on the absorbance ratio of the material, X-rays can normally penetrate for a few tenths of millimetre into the material matrix. Therefore a plausible consequence of such an investigation would be the shortcoming information regarding the interior condition of the object of interest. The physical limitation of an X-ray collimated beam is overcome due to the external heat-supply. The method for NDT presented here comprises of a two sections. The material excitation section is an infrared-laser beam. The sensing section for observing the material´s structure alternating phenomena contains an X-ray source and X-ray beam diffraction detector. Optical heating excitation section as well as collimated X-rays will be simultaneously applied to the surface of the sample. The excitation infrared beam can be directed next to the X-ray beam (at the object´s surface), where, when it is absorbed, it provokes a local temperature modulation. Local thermal diversity is characterised through homogeneous temperature field on the surface as well as in the interior of the object of interest. In opposite, any impurities on the object´s surface or within it´s matrix, influence the heat propagation and are registered as a different diffraction pattern by an area detector which in turn provides information about the condition of material being inspected. The equipment used in this non-destructive testing (NDT) of surface and sub-surface areas of a heat-resistant material combines the OHCD (Optical Heating and Crystallisation Device) as optical heating a source of an infra-red laser-light and a powder diffractometer, specifically the GADDS (General Area Diffraction Detection System). These parts of experimental site are used to excite the material, to produce a local temperature gradient as well as to observe the heat-caused changes, respectively. This approach to non-destructive examining of materials´ structure by simultaneous application of the X-ray diffractometry and the optical heating has been proven successfully. Roughly 500 experiments were performed under various conditions with different materials, varying the laser-intensities, the distances and the exposure-duration demonstrating that the applied instrumentation is reliable and giving reproducible results. There are however certain limitations for the NDT presented here: samples that are to be examined, must consist of an infrared-light absorbing, heat-resistant material, which posses certain heat conductivity. The sample under investigation must provide discrete X-ray diffraction pattern. Further, the X-ray diffractometry must be perceptible for heat-caused structural changes in the chosen material. Besides these pre-experimental boundary conditions, during the experiment itself turn up some supplementary limitation which, if not applied, would lead to false interpretations. The whole method is relatively slow, restricted by a sensing and read-out time of an area diffraction detector. The further boundary condition is based on a new phenomenon in the field of the research of high-temperature X-ray diffractometry (HTXRD). This restricts the free choice of the position of the IR-laser beam and X-ray beam relatively to each other. Nevertheless impurities, dislocations, gas bubbles as well as other perturbations within the material bulk, which are invisible at the surface of the object, can be detected with the method presented here. Comparing to other established methods for non destructive examining, based on eddy current, ultra-sound or thermo-optics and relying on one of the precise exploring technique in scientific analytics, this method offers a good alternative for non-invasive detection of inhomogenities.