New Plating Process for the Anti-Friction Layer of Sliding Bearings (3)

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In the plating process for anti-friction layers, the role of free boric acid (H₃BO₃) is crucial. According to literature [7, 11–24], the recommended concentration of free boric acid is between 15–40 g/L. This compound plays a key role in maintaining the stability of the plating solution by participating in several chemical equilibria. The following reactions occur in the bath: - HBF₄ + 3H₂O ⇌ H₃BO₃ + 4HF - HF ⇌ H⁺ + F⁻ - 2F⁻ + Pb²⁺ ⇌ PbF₂↓ When the concentration of boric acid is too low, these equilibrium reactions shift to the right, leading to the formation of harmful PbF₂ precipitates. This can cause contamination of the plating solution and result in defects such as pitting, roughness, or blistering in the coating. The solubility product (Ksp) of PbF₂ is 4.0 × 10⁻⁸ mol/L. Using this value, we can calculate the threshold for [F⁻] that would lead to PbF₂ precipitation: $$ [F⁻]^2 \times [Pb^{2+}] \geq K_{sp} $$ Assuming [Pb²⁺] = 0.68 mol/L (equivalent to 140 g/L), the calculation shows that when [F⁻] exceeds approximately 4.556 ppm, PbF₂ may begin to form, causing issues with the plating solution and the final coating quality. To prevent this, an adequate amount of free boric acid must be maintained in the solution. It helps stabilize the fluoroboric acid by controlling the concentration of fluoride ions, thus preventing unwanted PbF₂ buildup. However, if the boric acid content is too high, especially at lower temperatures, excessive crystal formation can occur, leading to a rough surface and poor adhesion between the coating and the substrate. Cathode current density (DK) also plays a significant role in the plating process. As DK increases, the tin content in the coating rises, and the deposition rate accelerates. However, higher DK can also affect the crystallization of the coating. At very high DK values, the dominant crystal plane orientation may shift from {111} to {100}, which changes the microstructure of the coating. This transition is influenced by factors like cathode potential, and it affects the grain size and overall quality of the plated layer. The stabilizer content in the plating solution is equally important. Oxygen dissolved in the solution can oxidize Sn²⁺ to Sn⁴⁺, which can then form insoluble compounds like Sn(OH)₄. To counteract this, antioxidants such as hydroquinone or phenol are added. These substances help reduce oxygen levels, preventing oxidation and stabilizing the solution. Additives like gelatin or peach gum are also used in the plating solution. They improve cathode polarization, refine the crystal structure, and enhance the uniformity of the coating. However, improper amounts can lead to problems—too little results in a rough or uneven coating, while too much can make the coating brittle. Temperature is another critical factor. As temperature increases, the dissolution of components in the plating solution improves, allowing for higher current densities and faster deposition rates. However, excessively high temperatures can cause rapid anode dissolution, increase sludge formation, and accelerate the evaporation of volatile components like fluoroboric acid and organic stabilizers. On the other hand, low temperatures can cause boric acid to precipitate out, making the solution unstable and resulting in a rough coating. The deposition rate slows down, reducing production efficiency. In summary, the plating process for anti-friction layers requires careful control of multiple variables, including boric acid concentration, current density, stabilizers, additives, and temperature. Each factor influences the quality and performance of the final coating, and optimizing them is essential for achieving a smooth, durable, and functional bearing surface.

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