Oxide coating presents a promising methodology to enable reactive wetting of solid-state garnet electrolyte by molten lithium metal. The resulting intimate contact allows effective lithium-ion migration at the garnet/lithium metal interface. Although changing in coating composition by the interfacial reaction has been previously revealed, the transition in chemistry, structure, topography and mechanical performance of the contact coating area remain unclear. Here, a combination of impedance spectroscopy, scanning electron microscope, time-of-flight secondary ion mass spectrometry and nanoindentation is devoted to illustrating and discussing the effect of the interfacial reaction in altering the properties of atomic layer deposition alumina coating layer. These findings provide unique insight into the mechanism of the reactive wetting of oxide coating by molten lithium metal, thereby, enabling design of more effective lithium metal-solid electrolyte interfaces.