Micro- and nano-mechanical approach to fracture toughness determination of borided layer produced on Nimonic 80A-alloy

Abstract

Borided layer produced on Nimonic 80A-alloy by plasma paste boriding ensure the high hardness and wear resistance. However, the high hardness and Young’s modulus of this layer contribute to its susceptibility to cracking. The fracture toughness was investigated using two approaches: micro-mechanical (crack-length-based method) and nano-mechanical (crack-energy-based method). In the case of the crack-length-based method, there was a strong correlation between the length of cracks produced from the corners of Vickers indent and the applied load. In order to characterize the susceptibility to cracking without loading, the KC0 coefficient was determined from the plot representing KC vs. indentation load. This parameter ranged from 1.5324 MPa·m1/2 to 3.6880 MPa·m1/2, depending on the applied model (Palmqvist, Casellas, Shetty, Niihara, Lankford and Laugier). The use of the crack-energy-based method resulted in a higher (even three times) fracture toughness compared to the crack-length-based method. It was found, that the KC values determined by Berkovich nanoindentation represents the absolute fracture toughness independent of the applied load and not related to crack formation.