Micromechanical tensile test investigation to identify elastic and toughness properties of thin nitride compound layers

The measurement of thin-layer mechanical properties, like compounds generated by thermo-chemical nitriding process, is a key issue to optimize and predict the endurance of surface treatment against contact fatigue and wear. An original FIB micro-tensile test strategy involving plain and micro-notched tensile specimens is proposed. These specimens were FIB machined in a thin epsilon(50%)- gamma' (50%) compound layer resulting from a common low pressure gaseous nitriding process (Allnit (C)) and tested using a dedicated micro-testing system. Combined with DIC analysis, such micro-tensile test strategy allows extracting both the elastic modulus and the Poisson's ratio. Additionally, testing micro-notched specimens underlines the necessity to include FE simulations in order to take into account the radius of the micro-notch tip as well as the surface roughness induced by the FIB machining process. The investigation of this epsilon - gamma' compound layer suggests a Youngs modulus E = 200 GPa, a Poisson's ratio v =0.31 and a rather low fracture toughness KIC = 0.55 MPa root m. This methodology involving FIB machining, DIC analysis, test procedures and FE post processing simulations is fully detailed and the given results are discussed regarding literature.

Automated summary

The study presents a method using FIB micro-tensile test strategy to extract the elastic modulus and Poisson's ratio of thin-layer compounds. Testing micro-notched specimens also highlights the necessity to consider the radius of the micro-notch tip and surface roughness induced by FIB machining in FE simulations.