Deformation of a hard coating on ductile substrate system during nanoindentation: Role of the coating microstructure
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Deformation and fracture of a columnar-grained, ∼1.3-m-thick TiN coating on a stainless steel substrate was investigated using a spherical tipped conical indenter of 5-m nominal tip radius. Structural analysis, performed with the support of focused ion beam (FIB) milling and imaging techniques, revealed that the microstructure of the TiN coating had a strong influence on the deformation behavior of the coating. Intergranular sliding in the coating, as well as plastic flow in the ductile substrate, was found to be the predominant processes during the indentation. Neither plastic deformation, in the form of plastic flow, within the coating nor delamination of the interface was observed. Coating deformation was observed to be controlled by the intergranular shear cracking and thus by the interfacial columbic frictional stress between columnar grains. An indentation-energy based model was developed, which deconvolutes the coating behavior from that of the substrate, allowing quantification of the intergranular sliding stress.
I. INTRODUCTION
Hard materials are often engineered onto the surface of ductile materials via different processing methods to form a thin, protective layer against wear and corrosion during operation. To measure the mechanical properties of such hard, thin coatings, a depth-sensing indentation (DSI) technique, generally known as “nanoindentation,” has been developed and widely applied in the last two decades.1 Additionally, nanoindentation is a useful tool in probing contact deformation behavior of hard coating on ductile substrate systems. For example, in the load– displacement curves of hard coating/ductile substrate systems measured by nanoindentation, the elastic/plastic transition and/or discontinuities (also termed as “popins”) were frequently observed.2–8 Microscopic observation on such coating systems indented by a spherical indenter suggested that substrate plastic deformation occurred after initial elastic contact between the indenter and the coating, followed by coating fracture and/or delamination.2,3 A large number of numerical simulations have been carried out to analyze spherical indentation-induced deformation and fracture in hard coating on ductile substrate systems.2,3,9 The results of the studies showed that radial cracks tended to initiate from outside of the contact
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0050 J. Mater. Res., Vol. 21, No. 2, Feb 2006
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periphery on the coating surface where the maximum tensile stress prevailed. In the modeling study by AbdulBaqi,10 it was suggested that radial cracks in the coating material can also be initiated at the coating metal interface, directly below the contact region, along the symmetry axis, where tensile stresses are maximum. Key factors that have been considered to influence deformation and fracture processes of hard coating on ductile substrate systems include mechanical properties of both the coating and t
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