The mechanism of coating detachment on colorful spring steel plates during fatigue testing involves the interaction of multiple factors, with the core being the combined effects of insufficient interfacial bonding strength between the coating and the substrate, stress concentration effects, and environmental corrosion. Under fatigue loads, the surface of the spring steel is subjected to alternating stress. Due to the difference in elastic modulus between the coating and the substrate, the deformation is inconsistent, making micro-delamination easily occur at the interface. If the coating has porosity or uneven thickness, the stress concentration effect will be further aggravated, forming crack initiation points. Furthermore, colorful coatings are usually deposited using chemical or electrochemical methods. If the pretreatment process is imperfect, residual oil, oxide scale, or microscopic defects on the substrate surface will significantly reduce the coating adhesion, becoming a contributing factor to detachment during fatigue testing.
The interfacial bonding strength between the coating and the substrate is a key factor affecting detachment. The bonding between the colorful coating and the spring steel substrate mainly relies on the combined effects of physical adsorption and chemical bonding. If the plating solution composition is not properly controlled, such as excessively high main salt concentration or an imbalanced additive ratio, it will lead to increased internal stress in the coating and reduced interfacial bonding strength. Meanwhile, insufficient surface roughness or inadequate cleanliness of the substrate weakens the mechanical interlocking between the coating and the substrate, making the interface a weak point for fatigue crack propagation. Under alternating stress, cracks propagate rapidly along the interface, eventually leading to overall coating peeling.
Stress concentration plays a catalytic role in coating peeling. In fatigue testing of spring steel, surface defects such as scratches, pits, or coating porosity become stress concentration sources. The stress levels at these defects are much higher than the average stress, causing premature local plastic deformation and the formation of microcracks. Abrupt thickness changes or incomplete coverage in colored coatings further exacerbate uneven stress distribution, making cracks more likely to initiate at the coating-substrate interface. With continuous cyclic loading, cracks propagate along the interface, eventually leading to coating separation from the substrate.
Environmental corrosion accelerates the coating peeling process. In humid or corrosive environments, colored coatings may degrade due to electrochemical corrosion. If the coating has porosity or cracks, corrosive media can penetrate to the interface, causing substrate corrosion. The internal stress generated by the volume expansion of corrosion products further damages the bond between the coating and the substrate, creating a corrosion-fatigue synergy that significantly shortens the coating life. Furthermore, high-temperature environments reduce the thermal expansion coefficient matching between the coating and the substrate, exacerbating interfacial stress and promoting coating detachment.
Design optimization can significantly improve the fatigue performance of spring steel plated with colorful coatings. First, the selection of substrate materials should prioritize metallurgical quality, reducing the content of non-metallic inclusions and lowering the risk of crack initiation. Second, optimizing coating process parameters, such as controlling the plating solution composition, current density, and temperature, can reduce internal stress in the coating and improve interfacial bonding strength. Simultaneously, introducing composite coating technologies, such as embedding nanoparticles or fiber reinforcement phases in the coating, can enhance coating hardness and toughness, inhibiting crack propagation. In addition, improvements to surface pretreatment processes, such as using sandblasting or chemical etching to increase substrate surface roughness, can enhance the mechanical interlocking between the coating and the substrate, improving adhesion.
Structural design optimization is also a crucial step. By reducing the stress concentration factor of spring steel, such as by optimizing the cross-sectional shape, avoiding sharp transitions, or using rounded corners, the stress level under fatigue loads can be reduced. Simultaneously, rationally controlling the geometric parameters of the spring, such as pitch, diameter, and number of turns, can improve the uniformity of stress distribution and reduce the risk of localized overload. Furthermore, surface treatment techniques such as shot peening can introduce a residual compressive stress layer on the surface of the spring steel, inhibiting crack initiation and extending the service life of the coating.
Comprehensive application of the above design strategies can significantly improve the performance stability of spring steel plated with colorful coatings in fatigue testing. Through the synergistic effect of material selection, process optimization, structural improvement, and surface treatment, a multi-layered protection system can be constructed to effectively inhibit coating peeling, meeting the requirements of high-end applications for long service life and high reliability of spring steel. In the future, with the continuous development of materials science and surface engineering technology, the design methods for spring steel plated with colorful coatings will become more refined, providing key support for the lightweighting and durability improvement of industrial equipment.
