Zinc alloy belt buckles, used as everyday clothing accessories, rely heavily on the adhesion of their surface coatings, directly impacting the product's durability and aesthetics. Sandblasting, through a combination of physical and chemical processes, significantly enhances the bond strength between the zinc alloy surface and the coating. Its core mechanisms can be analyzed from six dimensions: improved cleanliness, optimized roughness, enhanced mechanical properties, stress dispersion, surface activation, and process compatibility.
The primary function of sandblasting is to thoroughly remove contaminants from the zinc alloy surface. During die casting, the mold is coated with mineral oil as a mold protectant, and parts become contaminated with processing oil after demolding. If these greases are not thoroughly removed, they form a barrier between the coating and the substrate, leading to decreased adhesion. Sandblasting, using high-speed jets of abrasive materials (such as copper ore sand or quartz sand), not only removes oil but also removes stubborn impurities like oxide scale and rust, exposing the clean, metallic surface of the substrate. This deep cleaning provides an ideal foundation for subsequent coating adhesion, preventing the risk of coating peeling due to contaminants.
Sandblasting significantly increases the contact area between the coating and the substrate by altering the surface morphology. Zinc alloy surfaces are naturally smooth, making it difficult for coatings to form a mechanical anchoring effect. After sandblasting, the surface develops uniformly distributed micro-textures, significantly increasing roughness. This microscopic roughness allows the coating to penetrate into these pits, forming a "mechanical interlock" after curing, greatly enhancing adhesion. For example, an unsandblasted zinc alloy surface coating may only adhere through intermolecular forces, while a sandblasted surface uses both physical anchoring and chemical bonding to fix the coating, significantly improving peel resistance.
Sandblasting also improves the mechanical properties of zinc alloys, indirectly enhancing coating adhesion. When high-speed abrasive impacts the surface, a compressive stress layer forms on the substrate surface. This compressive stress can offset some of the tensile stress generated by external loads, reducing the initiation and propagation of fatigue cracks. Simultaneously, the compressive stress layer also increases the hardness and wear resistance of the substrate, making the coating less prone to cracking due to substrate deformation during long-term use. Furthermore, the micro-texture of the sandblasted surface can store lubricating oil, improving friction conditions and further extending coating life.
During die casting or heat treatment, zinc alloys may develop residual stress on their surface due to uneven cooling or phase transformation. These stress concentration areas are prone to becoming the initiation point for fatigue cracks, leading to simultaneous failure of the coating and the substrate. Sandblasting, by uniformly impacting the surface, can disperse residual stress, making it more evenly distributed. This stress redistribution not only reduces the probability of crack initiation but also allows the coating to transmit stress more evenly under load, avoiding coating delamination caused by excessive local stress.
Sandblasting also activates the zinc alloy surface, enhancing its chemical bonding with the coating. High-speed abrasive impact breaks down the oxide film on the substrate surface, exposing fresh metal atoms. These active atoms can react with chemical components in the coating (such as polar groups in resin) to form chemical bonds. For example, when spraying epoxy resin coatings, the activated zinc alloy surface can form covalent bonds with hydroxyl or carboxyl groups in the resin, significantly improving adhesion. This synergistic effect of chemical bonding and mechanical anchoring results in a higher level of bonding strength between the coating and the substrate.
The process parameters for sandblasting (such as abrasive type, blasting pressure, and blasting angle) can be flexibly adjusted according to the specific needs of zinc alloy belt buckles. For example, for decorative belt buckles requiring a fine surface finish, gentle sandblasting with low-hardness abrasives such as glass beads can be used to improve adhesion while preserving surface gloss; for industrial belt buckles requiring high wear resistance, deep sandblasting with high-hardness abrasives such as diamond can be used to create a rougher surface and enhance coating adhesion. This process adaptability allows sandblasting to meet the surface treatment needs of zinc alloy belt buckles in different scenarios.
