Deconvoluting residual stress effects on (Hf-Nb-Ta-Zr)C hard coatings through nanoindentation and microcantilever fracture

H.Gopalan, M.Amalraj, M.Hans, N.Cauterts, J.Schneider, C.Kirchlechner, G.Dehm

DC-magnetron sputtering was adopted for depositing (Hf-NbTaZr)C coatings on Si (SiNx) substrate at different temperatures (573,723,873 and 1023 K). X-ray diffraction methods were used to determine crystal structure, residual stresses and the thermal expansion coefficient of the coating. Extensive microstructural characterization of deposited films were carried out through scanning electron microscopy, transmission electron microscopy and atom probe tomography. Modest variation in microstructure (grain widths 10-25 nm) with columnar grain boundaries enriched with carbon was observed with increasing deposition temperature. Nanoindentation with a Berkovich tip revealed that for low deposition temperatures nearly super hard coatings were obtained (>38 GPa). The changes in the residual stress as an extrinsic factor affecting toughness in the microscale is widely recognized. Microcantilever based fracture toughness evaluation revealed that the intrinsic toughness of the coatings are highly sensitive to the microstructure. Carbon enrichment along the grain boundaries at higher deposition temperature, lead to a drop in fracture toughness from 2.8 MPa.m^0.5 to 1.8 MPa.m^0.5. Nanoindentation based stress intensity factor measurements were strongly affected by nature and magnitude of the residual stress. Furthermore, the shape of the cracks arising from the indentation imprint were determined through focussed ion beam serial sectioning. This knowledge was used to explicitly decouple the influence of residual stress on the intrinsic indentation toughness of the coatings. Excellent agreement in the true toughness values measured from nanoindentation and microcantilver bending inspires confidence in the nanoindentation based approach for rapid testing. Fracture path in the indentation and the micro-cantilever tests followed the column grain boundaries.