Failure analysis of ParaPost drills that fractured in service: a retrieval analysis study

The aim was to determine the fracture mechanism of two clinically failed ParaPost drills. First, the fracture planes were analyzed by scanning electron microscopy (SEM). The drill end of one of the fractured pieces of each drill was then embedded in resin and after being metallographically ground and polished, was chemically etched. The microstructure and elemental composition were then examined by SEM/EDS analysis while hardness was determined with a Vickers testing device. Fractographic analysis revealed that both drills failed in a brittle manner and showed a pattern characteristic of a quasi-cleavage fracture mode. SEM and EDS analysis revealed a random distribution of a second phase enriched in Mo, W, and V, probably appended to (Mo, W, V)×C carbides, while the alloy composition is similar to M3 tool steel, a high-speed molybdenum tool steel. The microhardness of a ParaPost Drill #1 was found to be HV 862±29 and that for a Drill #2 was 846±16, with no significant differences (p>0.05). In both cases, fracture originated from surface points acting as stress concentrators and facilitating brittle fracture in the quasi-cleavage mode indicating that failure rate might be further minimized by a better instrument design.