Advancing M50 steel performance through plasma immersion ion implantation: A brief review of current progress and future outlook

M50 steel is a critical material in aerospace bearing systems owing to its high strength and fatigue resistance. Nevertheless, it remains highly susceptible to surface degradation under extreme operational conditions involving flash temperatures above 800°C and contact stresses exceeding 4 GPa. Conventional surface treatments are frequently limited by inadequate modification depth and low thermal stability. Plasma immersion ion implantation (PIII) has recently arisen as a promising technology to overcome these challenges. High-dose nitrogen implantation promotes the formation of nitride layers through CrN precipitation and dislocation pinning, resulting in achieving a remarkable nanohardness increase to 16.3 GPa. Similarly, gradient carbon implantation fosters architectured carbide coatings that reduce abrasive wear rates. Recent advances in process control, particularly through machine learning-assisted regulation and Langmuir probe diagnostics, have further enhanced implantation uniformity to over 98.7%, thereby extending full-scale bearing fatigue life. Despite these promising developments, critical challenges remain in understanding the multiphysics interactions during extreme surface damage and in optimizing the mechanical performance under combined thermo-mechanical loads. This review provides a systematic assessment of recent progress in PIII technologies for M50 steel and identifies the major scientific and technical gaps that remain unresolved. By summarizing current limitations and emerging research directions, it outlines potential pathways for advancing surface engineering strategies that could facilitate the transition from laboratory studies to practical application.