Fracture properties of thin brittle MTM clay coating on ductile HEC polymer substrate

Abstract

Thin clay coatings can be deposited from water dispersions for the purpose of improved gas barrier properties and fire retardancy of polymeric materials. Mechanical properties of the coatings are difficult to assess, since they are very thin (≈1µm). In-situ tests using a micro tensile stage in a scanning electron microscope reveal a thickness-dependent microcracking mechanism, and Weibull parameters for coating fracture are extracted. Complex fracture events are identified, related to a weak clay coating-polymer substrate interface. A micromechanical finite element formulation provides values of 5 MPa for interfacial shear strength and 1 J/m2 for interfacial fracture toughness. From the multiple cracking behavior of the clay coating, a clay strength ≈ 225 MPa is estimated by the Weibull strength parameter from fragmentation diagrams. The method may be extended to other combinations of brittle coating-ductile substrates.