Ultra-High Temperature Refractory Ceramic Matrix Composites for Rocket Thruster

Abstract

Space exploration depends on materials that can withstand extreme conditions, particularly for rocket thrusters. Ceramic matrix composites (CMCs) like carbon-carbon (C/C) and carbon-silicon carbide (C/SiC) have been widely used for rocket nozzle applications ,due to their thermal and mechanical properties. However, the demand for materials capable of higher temperature tolerance and reusability has shifted focus to ultra-high temperature ceramics (UHTCs) and UHTC matrix composites (UHTCMCs). Among UHTCs, ZrB₂ stands out for its excellent mechanical properties, oxidation resistance, and lower cost compared to HfB₂. To overcome ZrB₂'s inherent brittleness and enhance properties like densification and oxidation resistance, additives such as SiC and carbon fibers are introduced. SiC enhances densification, controls grain growth, and improves oxidation resistance via the formation of a SiO₂ protective layer, while carbon fibers improve mechanical strength, oxidation resistance, and reduce density. Spark Plasma Sintering (SPS) is a preferred fabrication method for its ability to rapidly densify materials while maintaining fine microstructures.
Although ZrB₂-SiC composites are extensively studied for high-temperature applications, the relationships between sintering parameters (e.g., temperature, pressure, and dwell time) and densification, microstructure, and mechanical properties remain underexplored. This study investigates these correlations using SPS and examines the impact of milling methods—high-energy milling with WC balls versus regular milling with ZrO₂ balls—on final material properties. The feasibility of incorporating short carbon fibers into the ZrB₂-SiC matrix is also assessed, focusing on the effects of preparation techniques and fiber length.
Fabrication insights revealed that increased sintering temperature generally improved densification due to enhanced atomic diffusion, grain boundary migration, and mass transport. High-energy WC milling achieved superior densification compared to ZrO₂ milling, with ZSW samples reaching a maximum relative density of 99.2%, versus 96.5% for ZSZ samples under similar conditions. ZSW samples, however, developed a secondary ZrO₂ phase due to more intense abrasion and oxygen diffusion during milling, while ZSZ samples maintained a finer microstructure, with an average grain size of 2.65 μm compared to 2.91 μm for ZSW. Attempts to incorporate 35 vol% short carbon fibers were unsuccessful under current sintering conditions, but improvements in fiber distribution were achieved with a rotary evaporator. Shorter fibers showed better structural integrity by reducing stress concentrations.
Mechanical properties were strongly influenced by sintering temperature. Higher temperatures caused grain coarsening, leading to reductions in hardness, flexural strength, and fracture toughness. For instance, ZSW hardness decreased from 14.33 GPa at 1950°C to 13.92 GPa at 2050°C, flexural strength declined from 407 MPa to 384 MPa, and fracture toughness dropped from 3.71 MPa·m¹/² to 3.58 MPa·m¹/². Milling methods also played a critical role; ZSW samples showed lower hardness and toughness due to the softer ZrO₂ phase and coarser grain sizes, with a maximum fracture toughness of 3.76 MPa·m¹/² compared to 3.97 MPa·m¹/² for ZSZ samples. However, ZSW samples exhibited comparable or higher flexural strength (384–516 MPa) due to ZrO₂’s transformation toughening effect, while ZSZ samples ranged from 317 to 476 MPa.