LI Yong ¹, LIAO Dahai ², HU Yang ², LYU Ruixuan ², WANG Jianfang ¹, HU Chen ¹, MA Chengwen ¹

(1. Jiangxi Changxing Aviation Equipment Co., Ltd., Jingdezhen 333400, Jiangxi, China;

2. School of Mechanical and Electronic Engineering, Jingdezhen Ceramic University,

Jingdezhen 333403, Jiangxi, China)

Extended abstract:[Background and purposes] To explore the subsurface damage evolution during nanoindentation of C_f/SiC (carbon fiber/silicon carbide) composite materials with different SiC coating thicknesses, molecular dynamics (MD) simulations were employed. Carbon fiber composites are widely used in aerospace and other high-performance applications, due to their low density, high specific strength and excellent thermal resistance. However, their intrinsic brittleness and weak interfacial bonding limit structural reliability under extreme loading. Applying SiC coatings can improve interfacial strength and thermal stability, but the effects of coating thickness on nanoscale deformation and subsurface damage remain poorly understood. This study was aimed to study how SiC coating thickness influenced mechanical behavior, stress distribution and atomic-scale damage evolution during nanoindentation, thereby providing theoretical insights for optimizing composite design and enhancing nanomechanical performance.[Methods] A molecular dynamics nanoindentation model of C_f/SiC composites was established with a diamond indenter interacting with a carbon fiber substrate covered by SiC coatings of different thicknesses. Four interatomic potential functions, Tersoff, AIREBO, Morse and Lennard-Jones (LJ), were combined to accurately describe interactions among Si, C and interfacial atoms. The simulations were conducted with consistent indentation parameters to ensure comparability across the models. Multiple analytical techniques were applied to reveal the underlying mechanisms of subsurface deformation and damage, including load–displacement response, stress distribution, radial distribution functions (RDFs) and atomic bond breakage analysis. The OVITO visualization tool was used to track atomic motion and microstructural evolution throughout the indentation process, allowing detailed observation of crack initiation, interfacial failure and damage accumulation.[Results] The SiC coatings significantly enhanced the stiffness and load-bearing capacity of the carbon fiber substrate. With increasing coating thickness, the maximum indentation load also increased, indicating greater overall mechanical strength. Specifically, the peak load reached the highest value, when the coating thickness was 1.0 nm, corresponding to a main RDF peak intensity of 70.9. As the thickness increased to 1.5 nm and 2.0 nm, the main peak intensities decreased to 65.0 and 58.2, respectively, suggesting progressive weakening of local structural order. This reduction in RDF peak height reflects a higher degree of atomic bond breakage and greater subsurface damage accumulation. Stress contour analysis further revealed that moderate SiC coatings effectively distributed stress and delayed crack initiation, while too thick coatings caused localized stress concentration at the interface. These localized regions became preferential sites for microcrack formation and interfacial debonding. The Si–C bonding network in thinner coatings remained more uniform, while in thicker coatings, mismatched stiffness between the coating and substrate resulted in stress localization and atomic-level damage accumulation. Moreover, the load–displacement curves showed distinct elastic–plastic transition behaviors under different coating thicknesses. The slope of the elastic region increased with coating thickness, confirming the enhanced stiffness. However, the plastic deformation region exhibited more pronounced fluctuations in thicker coatings, indicating unstable subsurface fracture events. The atomic bond analysis also revealed that Si–C bonds near the interface were most susceptible to breakage as coating thickness increased, leading to partial interfacial failure and promoting subsurface crack propagation.[Conclusions] A comprehensive atomic-level understanding of the nanoindentation response and subsurface damage mechanisms of C_f/SiC composites with various SiC coating thicknesses was provided. The SiC coatings can effectively improve the composite's stiffness and load-bearing capacity, but excessive coating thickness introduces stress concentration and accelerates local damage evolution. An optimal coating thickness (about 1.0 nm in this study) achieves a balance between stiffness enhancement and damage resistance, improving overall mechanical reliability. The integration of multiple interatomic potentials, Tersoff, AIREBO, Morse and LJ, enables accurate description of interfacial behavior and provides a realistic simulation of deformation and fracture processes. The use of radial distribution and bond breakage analyses further clarifies the relationship between coating thickness, structural stability and atomic-scale damage accumulation. These findings not only offer theoretical support for the design and optimization of coated composite materials but also demonstrate the capability of molecular dynamics simulations to reveal nanoscale deformation and failure processes that are difficult to capture experimentally. The insights gained from this work contribute to a better understanding of the mechanical response patterns of C_f/SiC composites and provide a valuable foundation for tailoring coating structures to achieve superior nanomechanical performance for high-temperature and high-stress applications.

Key words: aerospace C_f/SiC composite materials; SiC coating thickness; molecular dynamics indentation simulation; subsurface damage