Traveling-wave ultrasonic motors: reconciling friction and wear through structural-interfacial engineering

Unlike conventional electromagnetic motors, traveling-wave ultrasonic motors (TWUMs) offer high torque density, low noise, simple construction, and inherent self-locking. Despite these advantages, their industrial adoption remains limited by a persistent lack of integrated optimization strategies that simultaneously balance multiple performance metrics. This review systematically evaluates recent advances in TRUMs from three interconnected perspectives: stator/rotor architecture, contact-interface tribology, and novel operating principles. We identify critical gaps — particularly in wear control under ultrasonic vibration, multi-physics coupling (mechanical–thermal–electrical–acoustic), and the temperature stability of lead-free piezoelectric ceramics such as KNN-based materials. Notably, surface micro-texturing with optimized parameters increases the friction coefficient by up to 36.3% and reduces adhesive wear. Structural modifications — including embedded piezoelectric stacks, double-sided staggered teeth, and suspended stators — substantially improve no-load speed and stall torque. Non-contact designs eliminate mechanical wear but currently deliver limited torque (<50 mN·m), confining them to precision rather than power applications. We propose a three-level optimization framework — structural, interfacial, and operational — that unifies previously fragmented efforts. A phased roadmap is outlined: short-term material substitution (KNN for PZT) and texture standardization; mid-term multi-physics field-coupled modeling; and long-term intelligent drive with self-diagnostics. This structured synthesis provides an actionable foundation for developing high-efficiency, miniaturized, and intelligent TWUMs.