What causes poor adhesion and easy peeling of the coating during coating?

Poor adhesion and flaking of thin films during vacuum coating processes stem from a complex interplay of surface chemistry, energetic conditions, and mechanical mismatch, rather than a single controllable parameter. The most fundamental cause is inadequate substrate surface preparation prior to deposition. If the substrate carries organic contaminants, oxide layers, or adsorbed moisture from ambient exposure, these impurities act as a weak boundary interlayer. The arriving adatoms bond to this contaminated surface rather than to the bulk material itself, resulting in a fractured interface that cannot sustain mechanical stress. Even trace monolayers of hydrocarbon from improper handling or storage are sufficient to reduce practical adhesion by an order of magnitude.

Beyond cleanliness, insufficient surface activation through plasma pre-treatment is another critical contributor. Without adequate ion bombardment using argon or reactive gases, the substrate surface lacks the high-density dangling bonds and nucleation sites required for chemical bonding with the film species. When the ion energy or dose is too low, physical adsorption dominates over chemisorption, and the film essentially sits on the surface rather than growing epitaxially or diffusively into it. Conversely, excessively energetic ion bombardment can implant ions into the subsurface, creating compressive stress gradients that promote delamination upon cooling.

Intrinsic film stress, whether compressive or tensile, directly dictates the practical adhesion limit. Films with high intrinsic stress, often exceeding several hundred megapascals, store significant strain energy at the interface. If the coating is thick or deposited at low substrate temperatures where adatom mobility is restricted, this stress cannot be relieved through grain boundary sliding or defect annihilation. The accumulated elastic energy provides the driving force for crack propagation along the interface, causing spontaneous buckling or chipping when the critical stress threshold is exceeded.

Thermal expansion coefficient mismatch between the film and substrate introduces additional thermo-mechanical fatigue. During post-deposition cooling or subsequent operational heating, differential contraction generates interfacial shear stresses. For ceramic coatings on metals or polymers, this mismatch is particularly severe. If the interfacial roughness or chemical graded layer is insufficient to distribute these stresses, cohesive failure within the film or adhesive failure at the boundary becomes inevitable. Finally, the presence of residual gases such as water vapor or oxygen during deposition can oxidize reactive film materials, forming brittle, non-adherent phases at the nucleation stage. Optimizing adhesion therefore demands a systematic approach combining rigorous in-situ cleaning, precise control of ion energy and substrate temperature, stress tailoring through deposition rate modulation, and careful selection of intermediate buffer layers to bridge property mismatches.