Effects of HfB2 and HfN Additions on the Microstructures and Mechanical Properties of TiB2-Based Ceramic Tool Materials
The effects of HfB2 and HfN additions on the microstructures and mechanical properties of TiB2-based ceramic tool materials were investigated. The results showed that the HfB2 additive not only can inhibit the TiB2 grain growth but can also change the morphology of some TiB2 grains from bigger polygons to smaller polygons or longer ovals that are advantageous for forming a relatively fine microstructure, and that the HfN additive had a tendency toward agglomeration. The improvement of flexural strength and Vickers hardness of the TiB2-HfB2 ceramics was due to the relatively fine microstructure; the decrease of fracture toughness was ascribed to the formation of a weaker grain boundary strength due to the brittle rim phase and the poor wettability between HfB2 and Ni. The decrease of the flexural strength and Vickers hardness of the TiB2-HfN ceramics was due to the increase of defects such as TiB2 coarse grains and HfN agglomeration; the enhancement of fracture toughness was mainly attributed to the decrease of the pore number and the increase of the rim phase and TiB2 coarse grains. The toughening mechanisms of TiB2-HfB2 ceramics mainly included crack bridging and transgranular fracture, while the toughening mechanisms of TiB2-HfN ceramics mainly included crack deflection, crack bridging, transgranular fracture, and the core-rim structure.
High-strength TiB2–TaC ceramic composites prepared using reactive spark plasma consolidation
Abstract This study demonstrates that reactive consolidation achieved via spark plasma sintering (SPS) is an effective strategy to enhance the high-temperature flexural strength of TiB 2 -based ceramics. Monolithic ceramics of TaC and TiB 2 were consolidated by non-reactive SPS and were used as reference samples. The microstructural development during room- and high-temperature strength tests was analyzed. The room-temperature strength of TiB 2 ceramics was analyzed as a function of the grain size. The high-temperature strength and associated microstructural development of monolithic TaC ceramics are reported for the first time. The TiB 2 –TaC ceramic system was then consolidated by reactive SPS at 2000 °C. The flexural strength of the 10 wt% TaC–TiB 2 ceramic composites reached 533 MPa at room temperature and 480 MPa at 1600 °С. The development of the (Ti,Ta)(C,B) phase was considered the main reason for the preservation of the high strength up to 1600 °С. Both bulk TaC and TiB 2 exhibited drastic decreases in their bending strengths at 1600 °С because of the activation of plastic deformation associated with high-temperature creep, and their strength did not exceed 210 MPa.
Coarsening Induced Phase Transformation of Hafnia in Polymer-Derived Si-Hf-C-N-O Ceramics
Coarsening induced reverse phase transformation of hafnia in polymer-derived Si–Hf–C–N–O ceramics was studied experimentally. X-ray diffraction and high resolution transmission electron microscopy were used to obtain information regarding nanocrystalline structure. Hafnia crystallites precipitated out from the amorphous ceramic matrix on heat treatment were observed to remain as tetragonal hafnia below a critical crystallite radius of 4 nm. Prolonged heat treatment has resulted in coarsening, as explained by Lifshitz–Slyozov–Wagner model, resulting in reverse phase transformation from tetragonal to monoclinic hafnia.