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.