TRIBOLOGICAL BEHAVIOR OF SPARK PLASMA SINTERED ZrB2-SiC BASED COMPOSITES

Abstract

Zirconium diboride (ZrB2) ceramics are candidates for high temperature structural and aerospace applications due to their exceptional combination of properties such as high melting temperature (> 3000 °C), relatively low density (6.1 g/cc), high thermal conductivity (60‐120 W/m‐K), high hardness (17-23 GPa) and thermal shock resistance. Nonetheless, poor resistance against oxidation at high temperature and poor mechanical properties at high temperature restrict the use of ZrB2 ceramics in high temperature structural applications. The addition of silicon carbide (SiC) in ZrB2 enhances sinterability as well as mechanical properties viz. hardness, fracture toughness and strength. ZrB2-SiC composites also exhibit superior ablation resistance and oxidation resistance due to the formation of protective SiO2-rich layer at higher temperature. It is reported that critical components like heat shields, leading edges and nose-cones of supersonic aircraft flying at an altitude around 15 km with a speed around 3 Mach are exposed to high temperatures around 800 °C and stresses above 30 MPa because of severe aerodynamic heating and high stagnation pressure. Besides high temperature strength degradation and oxidation, it is likely that the critical components of aircraft undergo inelastic deformation or abrasion by atmospheric debris in take-off, landing or re-entry stages. Further, high temperature applications like nozzles, molten metal crucibles, furnace electrodes, bearings, mechanical seals, cutting tool inserts, extrusion dies, etc., essentially need materials with superior thermostructural- wear behavior. Thus, ZrB2-SiC composites are recommended for various applications where the synergetic effect of high temperature and wear is a major concern. However, ZrB2-SiC composites applications in high temperature wear conditions are limited mainly due to (i) challenges in processing highly dense and microstructurally tailored composites and (ii) poor understanding of friction and wear behavior of the composites in high temperature wear contacts. The available literature indicates considerable understanding of processing, mechanical properties, thermal properties and oxidation resistance of ZrB2-SiC composites whereas the tribological potential has not been thoroughly investigated. In the present research study, a systematic investigation was conducted to understand the effect of SiC addition on densification by spark plasma sintering, the resultant microstructure and mechanical properties of the sintered ZrB2-SiC composites. The tribological behavior of the composites in extreme conditions of erosive and sliding wear is systematically studied as a function of material composition (SiC content) and wear parameters (erosive wear test temperature, angle of incidence of erodent and sliding counterbody). An attempt was also done to densify ZrB2 ceramics with dual morphology SiC reinforcement (SiC particles and SiC fibers) ii by spark plasma sintering. The effects of both sintering parameters (sintering temperature and time) and sintering additives (MoSi2 and AlN) on densification and mechanical properties of the sintered ZrB2-SiC-SiCf composites are thoroughly investigated. Furthermore, the effects of sintering additives and counterbody materials (SiC, WC-Co, Al2O3, ZrO2) on friction and sliding wear behavior of ZrB2-SiC-SiCf composites are systematically studied. Thus, the research study in the present thesis is conducted in terms of (i) processing dense ZrB2 composites with SiC particles reinforcement and SiC particle-SiC fiber reinforcement by spark plasma sintering and (ii) understanding mechanisms of material removal of the sintered composites in erosive and sliding wear conditions. For the first part of the thesis investigation, ZrB2-(10, 20, 30 vol.%) SiC powder batch compositions were sintered by two stage spark plasma sintering (at 1400 °C for 6 min and 1600 °C for 2 min) in an argon atmosphere to obtain dense (>97%) composites. The microstructures of sintered composites reveal uniformly distributed SiC particles in ZrB2 matrix. A combination of maximum elastic modulus of 398 GPa, maximum indentation toughness of 5.3 MPa m1/2 and maximum hardness of 23 GPa was attained for the composites prepared with large amount (30 vol.%) of SiC particle reinforcement. The reinforcement of SiC particles improved composite indentation toughness by crack bridging and deflection. As ZrB2-SiC composites are preferred for high temperature erosive wear resistance applications like nozzles, thrusters, etc., erosive wear behavior as a function of temperature (RT, 400 °C, 800 °C), angle of incidence (30°, 60°, 90°) of erodent and SiC content (10, 20, 30 vol.%) was subsequently investigated. Results obtained from erosive wear tests indicate a large variation in erosion rate from 2.13 to 75.45 mm3/kg with a change in temperature, angle of incidence of erodent and SiC reinforcement content. Erosion rate reduced with increase in temperature, decrease in angle of incidence of erodent and increase in SiC content. With an increase in SiC content from 10 to 30 vol.%, a maximum reduction of 78% in erosion rate obtained at shallow incidence of erodent and 800 °C, while a maximum reduction of 68% in erosion rate was obtained at shallow incidence and room temperature. SEM-EDS analysis and XRD analysis reveal that the formation of B2O3 and SiO2-rich protective surface is responsible for high temperature erosive wear resistance of ZrB2-SiC composites. In view of the intended use in sliding wear resistance applications such as cutting tool inserts, extrusion dies, bearings, metal crucibles, furnace electrodes etc., a systematic investigation of friction and wear behavior of ZrB2-SiC composites as a function of SiC content (10, 20, 30 vol %) and counterbody materials: SiC (120 W/m-K, 22 GPa), WC-Co (100 W/m-K, 15 GPa), Al2O3 (30 W/m-K, 18 GPa), ZrO2 (25 W/m-K, 12.5 GPa) was further carried. The iii coefficient of friction (COF) varied in the range of 0.49-0.69 and wear volume in the range of 0.006-0.151 mm3 with varying SiC reinforcement content and counterbody. Against a given counterbody, low COF and wear volume is observed for the composite with large SiC content, owing to high hardness and indentation toughness. Results also indicate that the wear of the investigated composites is significantly affected by thermal conductivity of the counterbody. The composites showed lower wear against counterbody with higher thermal conductivity. The worn surface analysis of composites reveals pull-out, fracture, deformation as well as tribo-oxidation. Pull-out and fracture decreased with increase in SiC reinforcement content or thermal conductivity of the counterbody. In order to understand the dual morphology reinforcement effect on wear resistance of the composites, ZrB2-SiC-SiCf composites were prepared by spark plasma sintering and subjected to sliding wear against different counterbodies in the last part of the investigation. In particular, the effects of addition of SiC fiber (10 vol.%) and sintering additives (5 vol.% MoSi2 or 5 vol.% AlN) on the densification and sliding wear of ZrB2-20 vol.% SiC composites against counterbodies (SiC, WC-Co, Al2O3, ZrO2) were studied. The fiber reinforcement in ZrB2-SiC composites resulted in poor densification owing to the porosity, while additives assisted in achieving higher density (98.5-100%) by increasing the mass transfer at interface of ZrB2 and SiC fibers in the matrix. The additives significantly improved the densification by the formation of liquid phase. ZrB2-20 vol.% SiC-10 vol.% SiCf composites with 5 vol.% MoSi2 or AlN additive were spark plasma sintered to achieve high density (>98 %) at a maximum sintering temperature of 1650 °C. Further, the friction and wear of ZrB2-SiC-SiCf composites are primarily affected by the additives and thermal conductivity of counterbody. The ZrB2-SiC-SiCf composite with MoSi2 additive exhibited superior wear resistance compared to ZrB2-SiC-SiCf composite with AlN additive or without additive. The ZrB2-SiC-SiCf composite with MoSi2 additive composite demonstrated the least COF and wear volume against SiC counterbody due to the formation of stable tribo-chemical layer. Lower wear volume obtained for the composites worn against counterbody with higher thermal conductivity Overall, the contributions of the present research study in understanding the tribology of ZrB2-SiC composites in terms of material design for superior resistance in erosive wear and sliding wear conditions are highlighted. Results obtained from the series of experiments in the present research study illustrate the significant role of composition, mechanical properties and wear conditions on the tribological performance of ZrB2-SiC based composites. In the backdrop of the present experimental research, it can be concluded that the addition of 30 vol.% SiC particles to ZrB2 ceramics is recommended for high temperature erosive wear resistance applications and sliding wear resistance applications. The experimental results also indicate that the addition of 10 vol.% SiC fiber reinforcement and 5 vol.% MoSi2 additive in ZrB2-20 vol.% SiC composites is recommended for superior wear resistance in sliding contacts.