Surface Treatment and Bonding Strength

Surface treatment of bonding materials is one of the most critical steps in the entire bonding process and a key factor determining whether the bond will succeed or fail. Since bonding primarily relies on the adhesive's ability to adhere to the surfaces of the bonded materials, surface treatment of these materials can become the primary determinant of the strength and durability of the bonded joint. However, during various stages of processing, transportation, and storage, the surfaces of bonding materials often accumulate varying degrees of oxides, rust, oil stains, adsorbed substances, and other impurities, all of which directly affect the bonding strength.

         Bonding materials and their surfaces are highly diverse. They include both metallic and nonmetallic materials; surfaces can be clean or contaminated; they may be smooth, rough, or porous and loosely structured. From a thermodynamic perspective, surfaces are classified as either high-energy or low-energy; from a chemical-structural standpoint, they are further divided into active surfaces and inert surfaces.

         To obtain adhesive joints with high bonding strength and excellent durability, the surface layer prepared must be firmly bonded to both the base material and the adhesive, and this bond should be unaffected or minimally affected by environmental conditions.

         The main functions of surface treatment are as follows:
         (1) Remove surface contaminants and loose layers that may interfere with bonding;
         (2) Increase surface energy;
         (3) Increase surface area.

　　The quality of surface treatment directly affects the bonding strength of adhesive materials. The primary factors influencing this are cleanliness, roughness, and surface chemical structure—each of which will be discussed separately below.

         I. Cleanliness
         To achieve good bonding strength, it is essential that the adhesive fully wets the surface of the bonded materials. Typically, pure metal surfaces have high surface free energy, whereas most organic adhesives are polymeric compounds with low surface free energy. According to thermodynamic principles, these two types of materials can wet each other effectively. However, in practice, the metals used are rarely perfectly clean; their surfaces often harbor layers of rust, oxides, and organic or inorganic contaminants adsorbed during manufacturing, cutting, forming, heat treatment, and other processing steps. The cohesive strength of the contamination layer formed by these pollutants is very low, and their presence generally reduces the bonding strength.

         To achieve good adhesive strength, the contact angle on the surface of the bonding material should be small or even zero. For example, in the case of aluminum, once contaminants are removed from the surface, the contact angle drops dramatically—approaching zero. At this point, it can be considered that the hydrophobic contaminants previously covering the aluminum surface have been replaced by an adsorbed layer with a higher surface free energy. Consequently, the smaller the contact angle, the higher the adhesive strength. Therefore, using the measurement of contact angle to characterize the relationship between cleanliness and adhesive strength provides valuable reference for selecting optimal surface treatment conditions (as shown in Table 1).

         Table 1 Contact angles and bonding strength before and after surface treatment

         II. Roughness
         For a long time, it has been known that mechanical grinding can enhance the bonding strength of metals. Whether using sandpaper or sandblasting to treat bonding materials, appropriately roughening the surface can improve bonding strength. However, the degree of roughness must not exceed a certain threshold; excessively rough surfaces can actually reduce bonding strength. This is because overly rough surfaces cannot be adequately wetted by the adhesive, and air pockets remaining in the microscopic depressions on the surface are detrimental to the bond.

         Moreover, the bonding strength is not only related to surface roughness but also closely linked to the different surface geometries produced by various roughening methods. For example, the bonding strength achieved after sandblasting is higher than that obtained after polishing followed by mechanical roughening; similarly, bonding strength is greater when using sharp abrasive particles compared to that achieved with spherical abrasive particles.

         The reason why surface roughening of adhesive materials enhances bonding strength is, first, that the mechanical roughening process also undoubtedly cleans the surface; second, because it alters the surface’s physicochemical state, creating a new surface layer; and finally, differences in surface roughness can affect stress distribution at the interface, thereby achieving better bonding strength.

         III. Surface Chemical Structure
         The chemical composition and structure of the surface of adhesive materials significantly influence adhesion performance, durability, thermal aging resistance, and other properties. Typically, the impact of surface structure on adhesion performance is achieved by altering factors such as the cohesive strength, thickness, porosity, reactivity, and surface free energy of the surface layer. Among these factors, the surface chemical structure can both induce changes in the surface’s physicochemical properties and affect the cohesive strength of the surface layer, thereby exerting a pronounced impact on adhesion performance.

         For example, after being subjected to thermal aging at 288℃ for 50 minutes and 100 minutes respectively, stainless steel and aluminum bonded joints glued with phenolic resin adhesive still exhibited good stability in the aluminum joint, whereas the stainless steel joint had almost completely lost its bonding strength. This is because a solid-phase redox reaction occurred on the surface of the stainless steel, significantly degrading its high-temperature thermal aging performance. However, if a layer of zinc cycloalkanoate is applied to the steel surface, the thermal aging performance of the bonded joint can be greatly improved. Therefore, modifying the surface atomic properties that accelerate polymer degradation has a significant impact on the heat-oxidation resistance of steel joints.

         For another example, polytetrafluoroethylene is an inert polymer material with extremely low surface energy, making it difficult for most adhesives to achieve strong bonding. However, after treatment with a sodium-naphthalene-tetrahydrofuran solution, the tetrafluoroethylene undergoes chain scission, causing some of the surface fluorine atoms to be removed and leaving behind a very thin, dark-brown carbon layer on the surface. This process not only alters the chemical structure of the surface but also increases its surface free energy, thereby significantly improving the adhesive performance.

         For another example, titanium and titanium alloys treated by different methods exhibit vastly different adhesion strengths and durability performance. Surfaces suitable for bonding should have a stable, rough, and tightly adherent oxide layer. If a small amount of a reducing agent such as sodium sulfide is added to the treatment solution, the durability can be improved by more than five times.