Abstract
A wider utilization of ultra high temperature ceramics (UHTC) materials strongly depends on the availability of efficient techniques for their fabrication as dense bodies. Based on recent results reported in the literature, it is possible to state that Spark Plasma Sintering (SPS) technology offers a useful contribution in this direction. Along these lines, the use of two different SPS-based processing routes for the preparation of massive UHTCs is examined in this work. One method, the so-called reactive SPS (R-SPS), consists of the synthesis and densification of the material in a single step. Alternatively, the ceramic powders are first synthesized by Self-propagating High-temperature Synthesis (SHS) and then sintered by SPS. The obtained results evidenced that R-SPS method is preferable for the preparation of dense monolithic products, while the sintering of SHS powders requires relatively milder conditions when considering binary composites. The different kinetic mechanisms involved during R-SPS of the monolithic and composite systems, i.e., combustion-like or gradual solid-diffusion, respectively, provides a possible explanation. An important role is also played by the SHS process, particularly for the preparation of composite powders, since stronger interfaces are established between the ceramic constituents formed in situ, thus favoring diffusion processes during the subsequent SPS step.
Highlights
It is well recognized that Zr, Hf- and Ta- diborides and carbides based composites, the so-calledUltra High Temperature Ceramics (UHTCs), are suitable in several traditional and innovative application fields due to their high melting temperatures as well as additional attractive chemical, physical and mechanical properties [1,2,3]
The results described above indicate that the reactive Spark Plasma Sintering (SPS) approach is preferable to produce highly dense HfB2 with respect to the spark plasma sintering of previously synthesized powders
The results obtained using the following two processing routes were compared: (a) Reactive sintering, where the in situ synthesis and consolidation of the material was performed in one processing step starting from appropriate reaction promoters (b) Classical sintering, where the ceramic powders to be sintered by SPS were first synthesized by SHS using the same reactants as stated in (a)
Summary
It is well recognized that Zr-, Hf- and Ta- diborides and carbides based composites, the so-called. With respect to analogous products prepared by alternative methods, relatively higher defect concentration are generated in the obtained powders by the extreme heating and cooling rates (up to 200,000 K/min) accompanying the combustion front propagation, as a consequence of the heat liberated by the exothermic reactions taking place [3,15]. Other motivations, such as finer grain size and the formation of strong interfaces among the different ceramic phases formed in situ during the synthesis of composite materials, account for the advantages observed when the SHS method was adopted [16]. Most of the ceramics listed in this table have been fabricated starting from previously synthesized UHTC powders, products obtained following the R-SPS approach are reported [8,18,19,20,21]
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