Academic literature on the topic 'ZrC/SiC composites'

Ces travaux de thèse ont été axés sur l’étude de l’impact de l’ajout d’un polycarbosilane comme source de SiC dans l’élaboration des composites ZrC/SiC par frittage SPS. L’enjeu principal de cette étude réside dans l’obtention de composites denses avec une distribution fine et homogène de la phase SiC au sein de la matrice ZrC. Pour ce faire, deux systèmes ZrC/polymère et ZrC fonctionnalisée/polymère ont été préparés en utilisant une voie dite « hybride ». Dans les deux cas de figure, les caractérisations microstructurales (MEB, MET) et chimiques (XPS) ont montré des similitudes avec la présence du polymère qui enrobe de manière homogène les particules de ZrC. La densification des deux systèmes a été entreprise par frittage SPS. Cette étape du procédé a nécessité la mise en place d’un traitement thermique de conversion/frittage spécifique qui a permis, après optimisation, d’obtenir des composites denses (ρrelative &gt; 96%) a relativement basse température (1700 °C). L’impact de l’utilisation d’un polycarbosilane a été mis en exergue à travers une étude comparative avec un composite préparé à partir d’un mélange de poudres ZrC et SiC. Afin de compléter ces travaux, une étude à haute résolution des joints de grains par MET a montré des disparités suivant la fonctionnalisation ou non de la surface des poudres de ZrC<br>This work focused on studying the impact of the addition of a polycarbosilane as SiC source in the production of ZrC / SiC composites by SPS sintering. The main challenge of this study lies in producing dense composites with a fine and homogeneous distribution of the SiC phase within the ZrC matrix. For this, two systems ZrC/polymer and ZrC functionalized/polymer were prepared using a so-called "hybrid" route. In both cases, the microstructural (SEM, TEM) and chemical (XPS) characterizations showed similarities with the presence of ZrC particles coated homogeneously by the polymer. The densification of the two systems was undertaken by SPS sintering. This step of the process required the implementation of a specific conversion/sintering heat treatment which, after optimization, made it possible to obtain dense composites (ρrelative &gt; 96%) at relatively low temperatures (1700 ° C). The impact of using a polycarbosilane was highlighted through a comparative study with a composite prepared from a mixture of ZrC and SiC powders. To complete this work, a high resolution study of grain boundaries by TEM showed disparities depending on the functionalization or not of the surface of ZrC powders

Le présent document obéit à un plan strict inhérent à tous les manuscrits de thèsepassée en Validation des Acquis de l’Expérience (VAE). Après un CV détaillé ledocument présente tout d’abord un retour réflexif sur le parcours professionnel c'està-dire, une synthèse sur les taches effectuées d’un travail de type projet vers uneimplication de plus en plus forte vers un travail de recherche à proprement parlé. Aprèsce retour réflexif qui permet d’avoir une vue d’ensemble de la progression du parcours,une synthèse est proposée, non pas de la totalité des travaux, mais de trois domainesbien précis et représentatifs du parcours de recherche. Ce choix s’est fait en cherchantun fil d’Ariane qui est tout simplement la nature chimique de la céramique étudiée ;dans le présent document il s’agit de carbures et plus précisément de SiC, TiC, ZrC,HfC. Tout d’abord le travail sur les céramiques monolithiques pour les applicationsnucléaires est abordé puis, les applications spatiales avec la mise au point deprotections contre l’oxydation à partir de poudres revêtues enfin, le document s’achèvepar des travaux d’infiltration de céramiques à partir d’un matériau intermétallique oucomment il est possible de faire des céramiques ultra réfractaires à basse température.Ces travaux étant originaux ils ont fait l’objet de brevets et de publications qui serontabordés dans la troisième partie<br>This document obeys a strict plan inherent in all PhD manuscripts passed in Validationof the Assets of Experiment (VAE). After a detailed resume this document first of all,presents a reflexive return on the career i.e., from a work of type project towards anincreasingly strong implication to a research task. After this reflexive return whichmakes it possible to have an overall picture of the progression of the course, asynthesis is proposed, not of total work, but of three fields quite precise andrepresentative of the course of research. This choice was done by seeking a wire ofARIANE which is the chemical nature of the studied ceramics; in this document it isabout carbides and more precisely about SiC, TiC, ZrC, HfC. First of all monolithicceramics for the nuclear applications is approached then, the space applications withthe elaborating of protections against oxidation made by core shell powders finally, thedocument is completed by ceramics infiltrations from an intermetallic material or howit is possible to make ultra refractory ceramics at low temperature. As these works areoriginal they were the object of patents and publications which will be approached inthe third part

SiC whisker reinforced (Ca_0.6,Mg_0.4)Zr₄(PO₄)₆ (CMZP) matrix composites containing 10, 20, and 30 vol % whiskers were produced using a glass encapsulated hot isostatic pressing (HIPing) technique. The best HIPing temperature, pressure, and time conditions to optimize composite density and strength were determined to be 1050°C, 103 MPa, and 0.25 h.<br>Ph. D.

Amélioration de la mouillabilité des fibres de SiC par les alliages d'aluminium grace à un traitement de surface par une solution aqueuse saturée de K₂ZrF₆ à température proche de l'ébullition. Analyse des phénomènes responsables des cette amélioration et de leur influence sur la résistance à la rupture des fibres.

Zirconium- and titanium- based compounds (borides, carbides and nitrides) are of importance because of their attractive properties including: high melting temperature, high-temperature strength, high hardness, high elastic modulus and good wear-erosion-corrosion resistance. The ultra high temperature ceramics (UHTCs) - zirconium diboride (ZrB2) and zirconium carbide (ZrC) in combination with SiC are potential candidates for ultra-high temperature applications such as nose cones for re-entry vehicles and thermal protection systems, where temperature exceeds 2000°C. Titanium nitride (TiN) and titanium diboride (TiB2) composites have been considered for cutting tools, wear resistant parts etc. There are problems in the processing of these materials, as very high temperatures are required to produce dense composites. This problem can be overcome by the development of composites through reactive hot processing (RHP). In RHP, the composites are simultaneously synthesized and densified by application of pressure and temperatures that are relatively low compared to the melting points of individual components. There have been earlier studies on the fabrication of dense ZrB2-ZrC, ZrB2-SiC and TiN-TiB2 composites by the following methods: Pressureless sintering of preformed powders at high temperatures (1800-2300°C) with MoSi2, Ni, Cr, Fe additions Hot pressing of preformed powders at high temperatures (1700-2000°C) with additives like Ni, Si3N4, TiSi2, TaSi2, TaC Melt infiltration of Zr/Ti into B4C preform at 1800-1900°C to produce ZrB2-ZrC-Zr and TiB2-TiC composites RHP of Zr-B4C, Zr-Si-B4C and Ti-BN powder mixtures to produce ZrB2-ZrC, ZrB2-SiC and TiN-TiB2 powder mixtures at 1650-1900°C Spark plasma sintering of powder mixtures at 1800-2100°C There has been a lack of attention paid to the conditions under which ceramic composites can be produced by simple hot pressing (~50 MPa) with minimum amount of additives, which will not affect the mechanical properties of the composites. There has been no systematic study of microstructural evolution to be able to highlight the change in relative density (RD) with temperature during RHP by formation of sub-stoichiometric compounds, and liquid phase when a small amount of additive is used. The present study has been undertaken to establish the experimental conditions and densification mechanisms during RHP of Zr-B4C, Zr-B4C-Si and Ti-BN powder mixtures to yield (a) ZrB2-ZrC, (b) ZrB2-SiC, (c) ZrB2-ZrC-SiC and (d) TiN-TiB2 composites. The following reactions were used to produce the composites: (1) 3 Zr + B4C → 2 ZrB2 + ZrC (2) 3.5 Zr + B4C → 2 ZrB2 + 1.52rCx- 0.67 (3) (1+y) Zr + C → (1+y) ZrCx- 1/ (1+y) (y=0 to 1) (4) 2 Zr + B4C + Si → 2 ZrB2 + SiC (5) 2.5 Zr + B4C + 0.65 Si → 2 ZrB2 + 0.5 ZrCx + 0.65 SiC (6) 3.5 Zr + B4C + SiC → 2 ZrB2 + 1.5 ZrCx + SiC (5 to 15 vol%) (7) (3+y) Ti + 2 BN → (2+y) TiN1/(1+y) + TiB2 (y=0 to 0.5) (a) ZrB2-ZrC Composites: The effect of different particle sizes of B4C (60-240 μm, <74 μm and 10-20 μm) with Zr on the reaction and densification of composites has been studied. The role of Ni addition on reaction and densification of the composites has been attempted. The effect of excess Zr addition on the reaction and densification has also been studied. The RHP experiments were conducted under vacuum in the temperature range 1000-1600°C for 30 min without and with 1 wt% Ni at 40 MPa pressure. The RHP composites have been characterized by density measurements, x-ray diffraction for phase analysis and lattice parameter measurements, microstructural observation using optical and scanning electron microscopy. Selected samples have been analyzed by transmission electron microscopy. The hardness of the composites has also been measured. The results of the study on the effect of different particle sizes B4C and Ni addition on reaction and densification in the stoichiometric reaction mixture as follows. With the coarse B4C (60-240 μm and <74 μm) particles the temperature required are higher for completion of the reaction (1600°C and above). The microstructural observation showed that the material is densified even in the presence of unreacted B4C particles. The composite made with 10-20 μm B4C and 1 wt% Ni showed completion of the reaction at 1200°C, whereas composite made without Ni showed unreacted B4C (∼3 vol%) and the final densities of both the composites are similar (5.44 g/cm3). Increase in the temperature to 1400°C resulted in the completion of the reaction (without Ni) accompanied with a relative density (RD) of 95%. The composites produced with and without Ni at 1600°C had similar densities of 6.13 g/cm3 and 6.11 g/cm3 respectively (~97.3% RD). The Zr-Ni phase diagram suggests that the addition of Ni helps in formation of Zr-Ni liquid at ~960°C and leads to an increase in the reaction rate up to 1200°C. Once the reaction is completed, not enough Zr is available to maintain the liquid phase and further densification occurs through solid state sintering. The grain sizes of ZrB2 and ZrC phases after 1200°C are 0.4 μm and 0.3 μm, which are much lower than those reported in literature (2-10 μm), and may be the reason for reducing the densification temperature to 1600°C for stoichiometric ZrB2-ZrC composites. The effect of excess Zr (0.5 mol), over and above the stoichiometric Zr-B4C powder mixture, on reaction and densification of the composites is as follows. The formation of ZrB2 and ZrC phases with unreacted starting Zr and B4C is observed at 1000°C and with increase in temperature to 1200°C the reaction is completed. Since microstructural characterization reveals no indication of free Zr, it is concluded that the excess Zr is incorporated by the formation of non-stoichiometric ZrC (ZrCx-0.67). This observation is supported by lattice parameter measurements of ZrC in the stoichiometric and non-stoichiometric composites which are lower than those reported in the literature. X-ray microanalysis of ZrC grains in the stoichiometric and non-stoichiometric composites using transmission electron microscopy confirmed the presence of carbon deficiency. The composite produced at 1200°C showed the density of 6.1 g/cm3 (~97% RD), whereas addition of Ni produced 6.2 g/cm3 (~99% RD). The reduction in densification temperature for the non-stoichiometric composites is due to the presence of ZrCx even in the absence of Ni. The mechanism of densification of the composites at 1200°C is attributed to the lowering of critical resolved shear stress with increasing non-stoichimetry in the ZrC, which leads to plastic deformation during RHP. An additional mechanism may be enhanced diffusion through the structural point defects created in ZrC. The hardness of the composites are 20-22 GPa, which is higher than those of reported in literature due to the presence of a dense and fine grain microstructure in the present work. In order to verify the role of non-stoichiometric ZrC the study was extended to produce monolithic ZrC using various C/Zr ratios (0.5-1). Here again, stoichiometric ZrC does not densify even at 1600°C, whereas non-stoichiometric ZrC can be densified at 1200°C. (b) ZrB2-SiC Composites: Since ZrB2 and ZrC do not have good oxidation resistance unless they are reinforced with SiC, the present study has been extended to produce ZrB2-SiC (25 vol%) composites using Zr-Si-B4C powder mixtures. The samples produced at 1000°C showed the formation of ZrB2, ZrC and Zr-Si compounds with unreacted Zr and B4C and as the temperature is increased to 1200°C only ZrB2 and SiC remained. A fine grain (~0.5 μm) microstructure has been observed at 1200°C. During RHP, it was observed that the formations of ZrC, Si-rich phases and fine grain size at low temperatures was responsible for attaining the high relative density at a temperature of ~1600°C. The relative densities of the composites produced with 1 wt% Ni at 40 MPa, 1600°C for 30 min is 97% RD, where as composites without Ni showed a small amount of partially reacted B4C; extending the holding time to 60 min eliminated the B4C and produced 98% RD. The hardness of the composites is 18-20 GPa. (c) ZrB2-ZrC-SiC Composites: Since ZrC plays a crucial role in densification of ZrB2-ZrC and ZrB2-SiC composites, the study has been extended to reduce the processing temperature for ZrB2-ZrCx-SiC composites by two methods. In one of the methods, Si is added to the non-stoichiometric 2.5Zr-B4C powder mixture which is resulted in ZrB2-ZrCx-SiC (15 vol%) composites with ~98% RD at 1600°C. In another method, SiC particulates are added to the non-stoichiometric 3.5Zr-B4C powder mixture to yield ZrB2-ZrCx-SiCp (5-15 vol%) composites at 1400°C. The density of the 5 vol% SiC composite is 99.9%, whereas addition of 15 vol% SiC reduced the density to 95.5% RD. The mechanisms of densification of the composites are similar to those observed in ZrB2-ZrC composites. The hardness of the composites is 18-20GPa (d) TiN-TiB2 Composites: ZrB2, ZrC, TiB2, and TiN are members of the same class of transition metal borides, carbides and nitrides; however, their densification mechanisms appear to be different. In earlier work, the RHP of stoichiometric 3Ti-2BN powder mixtures yielded dense composite at 1400-1600°C with 1 wt% Ni addition, whereas composites without Ni required at least 1850°C. The major contributor to better densification at 1600°C (with Ni) appeared to be the formation of local Ni-Ti liquid phase at ~942°C (Ti-Ni phase diagram). The present work explores the additional role of non-stoichiometry in this system. It is shown that Ti excess can lead to a further lowering of the RHP temperature, but with a different mechanism compared to the Zr-B4C system. Excess Ti allows the transient alloy phase to remain above the liquidus for a longer time, thereby permitting the attainment of a higher relative density. However, eventually, the excess Ti is converted into a non-stoichiometric nitride. Thus, the volume fraction of a potentially low melting phase is not increased in the final composite by this addition. The contrast between these two systems suggests the existence of two classes of refractory materials for which densification may be greatly accelerated in the presence of non-stoichiometry, either through the ability to absorb a liquid-phase producing metal into a refractory and hard ceramic structure or greater deformability. Conclusions: The study on RHP of ZrB2-ZrC, ZrB2-SiC, ZrB2-ZrC-SiC and TiN-TiB2 composites led to the following conclusions: • It has been possible to densify the ZrB2-ZrC composites to ~97 % RD by RHP of stoichiometric Zr-B4C powder mixture with or without Ni addition. The role of B4C particle size is important to complete both reaction as well as densification. • Excess Zr (0.5 mol) to stoichiometric 3Zr-B4C powder mixtures produces dense ZrB2-ZrCx composite with 99% RD at 1200°C. The densification mechanisms in these non-stoichiometric composites are enhanced diffusion due to fine microstructural scale / stoichiometric vacancies and plastic deformation. • In the case of ZrB2-SiC composites, the formation of a fine microstructure, and intermediate ZrC and Zr-Si compounds at the early stages plays a major role in densification. • Starting with non-stoichiometric Zr-B4C powder mixture, the dense ZrB2-ZrCx-SiC composites can be produced with SiC particulates addition at 1400°C. • Non-stoichiometry in TiN and ZrC is route to the increased densification of composites through enhanced liquid phase sintering in TiN based composites that contain Ni and through plasticity of a carbon-deficient carbide in ZrC based composites.