Milivoj Konstantin Brun

Migration of grain boundaries in alumina induced by chromia addition

Abstract When alumina polycrystal is heat-treated at 1400 and 1500°C with Al2O3-Cr2O3 powder mixtures, the grain boundaries migrate, forming behind them a solid solution enriched with Cr2O3. The migration rate increases with the heat-treatment temperature and the Cr2O3 content in the mixture. The driving force for the migration is believed to raise from the coherency strain in the Cr2O3 diffusion zone in front of the migrating grain boundaries. The migrating grain boundaries develop strong faceting with flat segments meeting at sharp edges. Because of the elastic anisotropy of alumina, the diffusional coherency strain energy Gc(n) varies with the surface orientation n of the retreating grain, but the variation is smooth at the minimum in the polar plot of Gc(n). Therefore, smoothly curved boundaries meeting at sharp edges are predicted for the growth shape contrary to the observed flat facet boundaries. It is suggested that the grain boundary mobility may vary with the grain surface orientation and therefore influence the growth shape.

Rig and Gas Turbine Engine Testing of MI-CMC Combustor and Shroud Components

GE is continuing work on the development of Melt-Infiltrated Ceramic Matrix Composites (MI-CMC) for use in industrial gas turbine engine components. Long-term environmental degradation of test samples under realistic engine conditions is being determined using a unique high-pressure combustion rig apparatus. Rig testing is also being used to evaluate an F-class 1st stage shroud system incorporating an MI-CMC inner shroud component. While large, advanced engines, such as the F and H classes, offer the greatest benefits for using MI-CMC components, initial engine tests have been done using a GE-2 (2MW) machine to reduce costs and risk. Long term (1000 hours) engine testing results for single piece GE-2 shrouds are also described.Copyright

Reactive hot-pressing studies of high density oxide-carbide composite ceramics

Abstract A number of brittle-brittle composites have been synthesized by reactive hot pressing. By using one inert phase and a phase formed in situ from metal hydride and a solid nonmetal, it was possible to produce microcomposites in which the reacting phase formed a continuous matrix, and isolated the grains of the inert phase. Densification rates for the reactively hot-pressed materials were appreciably higher than for the conventionally hot-pressed materials with the same nominal composition. Composites formed by Al2O3TiC, Al2O3TiB2 and B4CTiB2 exhibited high Rockwell A hardness. Reactively hot-pressed Al2O3TiC composite was tested on inconel 718 and exhibited performance better or equivalent to the commercially available Al2O3TiC tool. B4CTiB2 tested on titanium 6–4 alloy exhibited higher uniform flank wear at short cutting times than a commercial WC 6% Co cutting tool (Carboloy grade 883). However, it continued cutting to the end of the 3-min test, while the carbide tool was limited to 2 min of cutting due to gross deformation. The volume of the tool material worn showed linear dependence on cutting times at all cutting speeds higher than 200 surface feet per minute (sfm). At those speeds, the dominant wear mechanism appeared to be chemical interaction with the workpiece, while at 200 sfm, microfracturing became more prominent.

Fracture toughness measurement by microindentation and three-point bend methods☆

Abstract The microindentation and the three-point bend test of the chevron-notched-bar methods were used to measure the fracture toughness of several common ceramic materials between room temperature and 1000 °C. The fracture toughness calculated from the maximum loads of chevron-notched beams was shown to be in good agreement with the values given in the literature. Agreement of the results obtained by the two methods was excellent at room temperature. The two methods, however, gave opposite trends for the fracture toughness of silicon nitride as a function of temperature. Because of the fewer assumptions involved in the chevron-notched-beam method, it is considered preferable despite more complicated specimen preparation.

Rig and Engine Testing of Melt Infiltrated Ceramic Composites for Combustor and Shroud Applications

GE has developed SiC fiber reinforced SiC-Si matrix composites produced by silicon melt infiltration (MI) for use in gas turbine engine applications. High temperature, high pressure combustion rig testing and engine testing has been performed on combustor liners and turbine shrouds made from such MI composites. Frame 5 sized combustor liners were rig tested under LHE diffusion flame conditions for 150 hours, including 20 thermal trip cycles, with no observed damage to the ceramic liners. Similarly, 46 cm diameter, single piece turbine shroud rings were fabricated and tested in a PGT-2 gas turbine engine. The fabrication and testing of both components are described.Copyright