Laura Silvestroni

Transient liquid phase bonding of HfC-based ceramics

Transient liquid phase (TLP) bonding enables joining at lower temperatures than traditional bonding techniques and preserves the potential for high-temperature applications, making it particularly attractive for joining ultra-high-temperature ceramics (UHTCs) such as carbides and borides. The feasibility of a TLP joint between “pure” carbides has been recently demonstrated. The present study examines the interactions that occur between undoped HfC or MoSi2-doped HfC and a Ni/Nb/Ni multilayer interlayer during TLP bonding. Bonding is performed at 1400xa0°C for 30xa0min in a high-vacuum furnace. SEM–EDS characterization shows that the reaction layer formed at the interlayer/ceramic interface contains mixed carbides and depending upon the ceramic, Ni–Nb–Hf, or Ni–Nb–Hf–Si, or Ni–Nb–Si alloys. Nanoindentation tests traversing the reaction layer between the bulk ceramic and Nb foil midplane also show a clear transition zone across which the indentation modulus and hardness vary. Crack-free joints have been obtained with undoped HfC. The addition of 5xa0vol% MoSi2 introduces small (<5xa0μm long) isolated cracks within the reaction layer, whereas with 15xa0vol% MoSi2 added, cracking was pervasive within the reaction layer. When the reaction layer exceeds a critical thickness, as in the case of the bond obtained with HfC doped with 15xa0vol% MoSi2, residual stresses become sufficiently large to cause extensive cracking and bond failure. The results suggest a need to characterize and balance the positive role of additives on sintering with the potentially deleterious role they may have on joining.

Indentation grid analysis of nanoindentation bulk and in situ properties of ceramic phases

Composites properties are often derived from the proper-ties of the constituent phases measured in bulk forms.However, in situ properties can be different from thosemeasured in bulk as a consequence of material processing[1–3]. In ceramic composites, for example, spurious phasescan form due to the chemical interaction of differentpowders. The knowledge of in situ properties would allowa better characterization and tailoring of composites per-formances. Many ceramic composites are particle-reinforced composites so that the evaluation of in situproperties involves measurements in very small volumes.For some mechanical properties, this can be accomplishedby nanoindentation tests. By nanoindentation, singlemicrostructural elements can be tested as grains in poly-crystals [4, 5] or single phases in composites [3, 6–8]. Inthis work, a comparison between nanoindentation bulk andin situ properties of some ceramic phases will be presented.Generally, in situ properties are evaluated by imaging theindentation marks, for example using a scanning electronmicroscope (SEM), to detect which phase was indented.Besides this traditional technique, which can be timeconsuming especially when indentations are tiny as ithappens in hard phases like advanced ceramics, in situproperties will be estimated applying a new type of anal-ysis to nanoindentation data [9–11]. According to this newanalysis, the mechanical properties of the constituentphases of a composite can be easily derived from a sta-tistical analysis of nanoindentation results without havingto image where indentation marks were placed. Basically,the analysis consists in fitting the experimental dataaccording to a proper number of statistical distributionswhose central values correspond to the specific propertiesof each phase. Constantinides et al. [10] called this analysisindentation grid (IG). In this work, IG will be applied toparticle-reinforced ceramic composites, some based onwell-known phases, such as MoSi

Advances in Transient-Liquid-Phase Bonding of Ultra-high Temperature ZrC Ceramics

Abstract Full exploitation of the many attractive engineering properties of ultra-high temperature ceramics (UHTCs) requires that they can be joined. This paper explores progress in identifying joining strategies based on the use of transient liquid phases (TLPs). Wetting studies are used to explore the suitability of specific liquids for joining, while bonding studies provide the ultimate test. Sintering aids in the UHTC provide a major potential obstacle to successful joining, and dissolved impurities in the TLP can also complicate the joining process. Nonetheless, we show that well-bonding interfaces can be achieved when ZrC ceramics are bonded at 1673 K using a Ni/Nb/Ni multilayer interlayer.

Nanoindentation characterisation of HfC-based composites

Two hot-pressed HfC-based composites with 10 and 20 vol% of MoSi2 were characterised by nanoindentation tests. The hardness and the Youngs modulus of the constituent phases were measured at low peak loads. They resulted independent on the considered composite. With higher peak loads, the same properties were measured and attributed to the composites. In this peak load range, the Youngs modulus and hardness decreased by increasing the peak load. Inserting the values of the constituent phases in simple composites models, the upper and lower bounds for the composites properties were calculated and compared to the experimental results.