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

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In the plating solution, the concentration of free boric acid (H₃BO₃) is a crucial factor. According to literature [7, 11–24], the recommended range is 15–40 g/L. The chemical equilibrium in the bath involves the following reactions: HBF₄ + 3H₂O ⇌ H₃BO₃ + 4HF HF ⇌ H⁺ + F⁻ 2F⁻ + Pb²⁺ ⇌ PbF₂↓ When the boric acid content is too low, all three equilibria shift to the right, leading to the accumulation of harmful PbF₂. This can cause impurities in the solution and result in defects like pitting, roughness, and blistering in the coating. The solubility product (Ksp) of PbF₂ is 4.0 × 10⁻⁸ mol/L. Using this, we can calculate the threshold for [F⁻] to prevent PbF₂ precipitation. With [Pb²⁺] at 0.68 mol/L (140 g/L), the minimum [F⁻] required is approximately 4.556 ppm. If [F⁻] exceeds this value, PbF₂ may precipitate, making the solution dirty and affecting coating quality. Free boric acid plays a key role in stabilizing the fluoroboric acid by controlling the fluoride ion concentration. When the boric acid level is too high, especially at lower temperatures, excessive crystallization may occur, leading to a rough coating and poor adhesion between layers. Cathode current density (DK) significantly affects the tin content and deposition rate. As DK increases, the tin content in the coating rises, and the deposition speed accelerates. However, higher DK also leads to coarser crystal structures. Literature [7] suggests that when [Pb²⁺] is as high as 333 g/L, DK can reach up to 4 A/dm² with a high {111} orientation coefficient of 1.42. Lowering [Pb²⁺] reduces the maximum allowable DK and alters the crystal orientation, affecting the coating’s microstructure. Oxygen (O₂) from the air dissolves in the plating solution and oxidizes Sn²⁺ to Sn⁴⁺. This forms insoluble Sn(OH)₄, which can lead to colloidal suspensions of stannic acid (H₂SnO₃), degrading coating quality. To counter this, antioxidants like hydroquinone (2–12 g/L) are added to reduce oxygen levels and stabilize the solution. Additives such as gelatin or peach gum (0.1–5 g/L) improve cathode polarization, refine grain structure, and enhance the tin content in the coating. Too little additive results in a loose, rough, or black coating, while excess can make it brittle. The electroplating process involves several steps: mass transfer, pre-conversion, charge transfer, nucleation, and crystal growth. Controlling the balance between nucleation and growth rates determines the grain size and coating quality. A higher nucleation rate compared to growth results in finer grains and a more uniform surface. Temperature also plays a vital role. Increasing temperature enhances dissolution and allows higher DK, speeding up the process. However, excessive heat can cause anode corrosion, increase sludge, and accelerate evaporation of volatile components. At lower temperatures, boric acid may precipitate, leading to a rough coating and reduced deposition speed. Overall, optimizing the plating parameters—such as boric acid concentration, current density, temperature, and additives—is essential for achieving a high-quality anti-friction layer on sliding bearings.

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