Fabrication and characterization of UHTCMCs for tribological applications

Abstract

The interest in the Ultra-High Temperature Ceramic Matrix Composites (UHTCMCs) increased over the last years due to their combination of toughness typical of CMCs with the high temperature stability of ultra-high temperature ceramics. The demand of UHTCMCs with better properties produced through more sustainable processes had led to the investigation of different approaches. This thesis deals with the fabrication and characterization of short pitch carbon fibres reinforced UHTCMCs for extreme applications, using a slurry infiltration process that avoids the problems related to the processes reported in literature. This work can be divided into four parts: In the first part, a new technique for the manufacturing of short fibre reinforced ultra-refractory ceramics via slurry infiltration is presented. This technique - based on the use of natural thickeners and planetary mixer - allowed the fabrication of solvent-free, thin (100 µm), flexible and easy to handle sheets suitable for the fabrication of homogeneous structures by overlapping and Hot-Pressing sintering. A large range of compositions - in terms of matrix and fibre volumetric content - from 0-100% was possible with this method. The short carbon fibre amount incorporated in the sheets ranged from 20-50% vol and the fibre length ranged from 3 to 5 mm. The matrix composition investigated with this technique consisted of ZrB2 90 vol. % - SiC 10 vol. %. Microstructure and mechanical properties of different samples were investigated. The microstructure observed with SEM showed a homogeneous fibre distribution, low interaction between fibre and matrix and no damage in the fibre surface due to the mixing process. From mechanical tests, increasing the fibre amount from 35 to 50 vol.% led to an improvement of the mechanical properties. In fact, the 4-point flexural strength ranged from 107 to 140 MPa, depending on the amount of carbon fibres. The same behaviour was observed during the works of fracture tests (WOF). The WOF ranged from 108 to 253 J/m2. The second part was focused on the processing and oxidation resistance study of graded materials composed with three different layers, with an amount of 0-20-50 vol. % of short carbon fibres respectively. Three different samples were fabricated: 1) ZrB2- 85 vol %, SiC- 10, vol % Y2O3- 5 vol %, 2) ZrB2- 85 vol % MoSi2- 15 vol %, 3) SiC- 95 vol %, Y2O3- 5 vol %. The analysis of the morphology showed that all samples presented a homogeneous fibre distribution and a good interaction between the layers without the presence of defects. The same samples were subjected to oxidation at 1650 °C in air and the behaviour of each layer composition was evaluated. SEM and EDS analysis were carried out to investigate the oxidation grade and the oxygen diffusion. The sample containing MoSi2 resulted the most interesting with a mass loss of – 0.56 mg/cm2 lower than the -4.51 mg/cm2 and -7.05 mg/cm2 of the others. In the third part, the preparation and properties of complex structures with changing matrix compositions and complex shapes starting from the new slurry infiltration process presented in this thesis were investigated. In detail, four different structures were analyzed. The first sample was obtained hot pressing the sheets with the pressure applied perpendicularly to the pile-up direction. The sheets with a vertical orientation sintered by Hot-Pressing and composed by a matrix of ZrB2-85 vol.%, SiC- 10 vol.%, reinforced with 40 vol.% of 3 mm long pitch carbon fibers (Z3-40-Z). It was characterized by a good fiber distribution and an interesting sheet bending that improved the mechanical properties with a value of flexural strength of 130 MPa. The second sample was a square plate composed by an external part of ZrB2-35 vol. %, SiC- 60 vol. %, Y2O3-5 vol. % reinforced with 40 vol. % of short pitch carbon fibres with a length of 3 mm (ZSY605) and an internal square part composed by a matrix of ZrB2-85 vol. %, SiC- 10 vol. %, Y2O3-5 vol. % reinforced with 40 vol. % of short pitch carbon fibres with a length of 3 mm (ZSY105). The two joined interfaces displayed good interaction, but defects originating from different stress distribution during sintering resulted in a low flexural strength of 45 MPa. To investigate the possibility to produce samples with a non-square shape, a cylindrical shape was fabricated (Z3-40-C). It was made by vertical sheets of UHTCMCs and sintered by Hot pressing, resulting in a final sample composed by a core of ZrB2-35 vol.%, SiC- 60 vol.%, Y2O3-5 vol.% reinforced with 40 vol. % of short pitch carbon fibres with a length of 3 mm and an external shell of ZrB2-85 vol.%, SiC- 10 vol. %, Y2O3-5 vol.% reinforced with 40 vol. % of short pitch carbon fibres with a length of 3 mm. In this case, the sample resulted with a good fibre distribution and good matrix interaction at the interface between the compositions. Finally, a cone was obtained by pressure-less sintering from the bending of a UHTCMC sheets composed by ZrB2-85 vol.%, SiC- 10 vol.%, Y2O3-5 vol.% reinforced with 40 vol. % of short pitch carbon fibres with a length of 3 mm, which demonstrated the feasibility to obtain complex shapes with different techniques. In the fourth part, the tribological properties of Ultra-high-temperature ceramic matrix composites (UHTCMCs) have been examined to give a better understanding of this material class for friction applications. Two pads consisting of pitch carbon fiber (Cf) reinforced ZrB2-10%SiC were prepared with long fibers arranged in 0°/90° orientation (ZL) and randomly oriented chopped fibers (ZS). Microstructure, mechanical properties and tribological behavior on a self-designed dynamometer were tested, and the friction film was analyzed to examine relevant wear mechanisms. As disc, three different compositions were used: a carbon fiber reinforced carbon−silicon carbide disc (C/C-SiC), a Steel disc and a carbon fiber reinforced carbon disc (C/C). Tests conducted against a C/C-SiC disc showed a promising stable braking performance. However, wear and mechanical stress were high and could be tolerated only with long carbon fiber reinforcement. In the case of the test conducted against the Steel disc - most specifically for the ZS material, it was observed that abraded material from the disc formed a stable friction film by fusing together harder pad particles with abraded steel dust. This reduced wear and stabilized the braking performance. Finally, the interaction between the UHTCMCs materials and the C/C disc showed an interesting low coefficient of friction without pads damage and with an operating temperature of ≈ 550 °C.