Densification, thermodynamics and interfaces of liquid phase sintered carbon fibres reinforced ZrB2/SiC composites

• Liquid-phase sintering enables UHTCMC densification at reduced temperature. • ZrSi 2 demonstrated the highest efficacy as sintering aid at 1500 °C. • Y 2 O 3 identified as the most effective additive against interface corrosion. • A local chemistry concept discriminates interfacial and matrix mechanisms. • Correlating interface thermodynamics with densification enables interface engineering. Zirconium diboride-based Ultra-High Temperature Ceramic Matrix Composites (UHTCMCs) are promising materials for aerospace applications. A major challenge in their fabrication is achieving full densification, which typically requires temperatures above 1900 °C. In this study, the liquid-phase sintering of ZrB 2 /SiC–Cf UHTCMCs was explored to reduce densification temperature through the addition of ZrSi 2 , Y 2 O 3 , or Si 3 N 4 . Both fibre-reinforced and monolithic systems were investigated. The introduction of a liquid phase enabled densification at significantly lower temperatures (1500–1700 °C), with ZrSi 2 proving the most effective additive, allowing densities of ∼ 83%, while Si 3 N 4 and Y 2 O 3 achieved ∼ 76–80% at 1700 °C. However, the liquid phase also interacted with the carbon fibres, affecting the fibre/matrix interface. Thermodynamic analyses were performed to elucidate reactions occurring at the interface during densification, introducing the concept of local chemistry to distinguish bulk matrix reactions from interfacial ones. By correlating densification temperature with the thermodynamic stability of interface reactions, valuable insights were obtained for the design and engineering of the fibre/matrix interface. The distinct behaviour of the liquid phases resulted in significantly different mechanical performances: fracture toughness increased from ∼ 7.5 MPa·m 0.5 (ZrSi 2 and Si 3 N 4 ) to ∼ 12 MPa·m 0.5 for Y 2 O 3 , while ZrSi 2 promoted the highest flexural strength (250 MPa at room temperature and 370 MPa at 1500 °C).