Preparation of Samples with Both Hard and Soft Phases for Electron Backscatter Diffraction: Examples from Gold Mineralization
Abstract Preparation of high-quality polished sample surfaces is an essential step in the collection of microanalytical data on the microstructures of minerals and alloys. Poorly prepared samples can yield insufficient or inconsistent results and, in the case of gold, potentially no data due to the “beilby” layer. Currently, preparation of ore samples is difficult as they commonly contain both hard and soft mineral phases. The aim of our research is to produce suitably polished sample surfaces, on all phases, for electron backscatter diffraction analysis. A combination of chemical–mechanical polishing (CMP) and broad ion-beam polishing (BIBP) was used to tackle the problem. Our results show that it is critical to perform CMP first, as it produces a suitable polish on the hard mineral phases but tends to introduce more damage to the soft mineral surfaces. BIBP is essential to produce a high-quality polish to the soft phases (gold). This is a highly efficient method of sample preparation and is important as it allows the complete quantification of ore textures and all constituent mineral phases, including soft alloys.
Advantages of Broad Ion Beam (BIB) Processing Compared with Focused Ion Beam (FIB) Technology for 3D Investigation of Heterogeneous Solids
Ion beam processing of heterogeneous samples has proved as a versatile method for 3D investigation of microstructures. Broad ion beams are used for conventional ion etching, polishing, thinning, depth profiling and also for cutting [1]. With introduction of liquid metal ion sources the FIB technology was developed especially for TEM sample preparation but also for SEM observations where both technologies are complementary. BIB processing has essential advantages compared with FIB technology concerning the productivity, the versatility especially in multi-step processing, the variety in the fields of application and the simplicity in practice and therefore lower costs of the available technique. FIG. 1 shows the characteristics of broad ion beam processing for 3D investigation of microstructures. With ion beam direction IB 1 slope cutting is performed with typically 5 to 10 keV inert gas ions over some millimetres in length and up to the required depth (e.g. up to 100 μm). Optionally rocking of the sample can be used to avoid ion beam induced furrows. The 10000 times larger cut area and the application of inert gases with relatively low energies are some of the advantages compared with the FIB technology where 30 keV Ga ions are applied to cut small areas of typically 10 μm laterally and vertically. In BIB applications additional ion beam processing steps can be used for polishing of the slope or selective etching to reveal the internal microstructure not only of the slope cut itself but also of the initial surface and the sputtered region. In cases of insulating samples the third step of sputter coating is performed with the same ion beam. These capabilities will be demonstrated with the investigation of NdYAG laser irradiated (Ti,Al)N layers on WC/Co substrates. The laser modified region was cut by broad ion beam slope cutting with the modified Gatan PECS and in selected micro-regions with FIB (FIB FEI 200). The PECS allows combined processing of cutting, etching, and coating [2]. The information gain with both technologies is seen in FIG. 2 to FIG. 5. FIG. 2 shows the crack system produced by the laser irradiation and selectively etched by 8 keV Kr ions. FIG. 3 shows the same surface together with the cutting line and a FIB cut crater in the SEM. The grooves are formed by physical sputtering. On this way the crack system can be observed by optical microscopy with automated image analysis. The cut through the entire (Ti,Al)N layer reveals the crack development into the third dimension (cutting under 60° to the initial surface). By the FIB technology (FIG. 4) only a single crack can be observed and the FIB cut shows projection artefacts in the ion beam direction. This can be misinterpreted as an influence of the crack into the substrate. The broad ion beam cut reveals the system vertically (hundreds of cracks simultaneously) through the entire laser spot and allows exact interpretation. Typical cracks are seen in FIG. 5a,b. The inspection showed no influence of the cracks into the underlying substrate.
Composite monolayer fabrication by an arc-spray process
A single layer (monotape) technique for fabricating complex high-temperature tungsten-fiber-reinforced superalloy composites is proposed. The fabrication of sheets of arc-sprayed monotape 38 cm wide and 122 cm long has been demonstrated. Composites fabricated using the method are shown to have equal tensile strength and a cleaner matrix than composites fabricated from powder metal cloth monotapes, and the present technique is less expensive than the powder metal fabrication techniques.