Hardness control in electroplating products requires comprehensive regulation from multiple dimensions, including electroplating process, coating composition, pretreatment, post-treatment, and optimization of process parameters. The electroplating process is the core factor affecting hardness, with different coating materials exhibiting significantly different effects on hardness enhancement. For example, nickel plating hardness can be altered by adjusting the phosphorus content; low-phosphorus plating offers higher hardness but greater brittleness, while medium-phosphorus plating balances hardness and toughness. Chromium plating offers high hardness, especially hard chrome plating, which is often used in wear-resistant applications, but its brittleness can lead to cracking. Alloy plating, such as nickel-tungsten alloys, can have its hardness and wear resistance optimized through compositional ratios; for example, increasing the tungsten content can significantly enhance hardness, but this requires balancing process complexity and cost.
Precise control of the coating composition is crucial for hardness adjustment. Taking electroless nickel plating as an example, phosphorus content directly affects the coating structure: low-phosphorus coatings (phosphorus content below 3%) have high hardness due to the formation of a high-density nickel lattice, suitable for applications with stringent wear resistance requirements; medium-phosphorus coatings (phosphorus content 6%-9%) have moderate hardness and good toughness, suitable for parts like stamped springs that need to withstand repeated deformation; high-phosphorus coatings (phosphorus content above 10%) have lower hardness but excellent corrosion resistance, and are mostly used in corrosive environments. By adjusting the concentration of the phosphorus source (such as sodium hypophosphite) in the plating bath and the reaction conditions, precise control of the coating composition can be achieved, thereby meeting different hardness requirements.
The pretreatment process is crucial to the adhesion between the coating and the substrate and the uniformity of hardness. Thin stainless steel stamped springs may have residual oil, oxide layers, or work-hardened layers on their surface. If these are not thoroughly removed, it will lead to decreased coating adhesion or even localized peeling, thus affecting the consistency of hardness. Conventional pretreatment processes include degreasing (organic solvent cleaning, chemical degreasing, or electrochemical degreasing), acid pickling activation (removing the oxide film with dilute sulfuric acid or hydrochloric acid solution), and surface roughening (sandblasting or chemical etching to increase surface roughness). For example, in one case, sandblasting improved the adhesion of the coating on the spring sheet and reduced the fluctuation range of the hardness test value, indicating that surface roughening can enhance the mechanical interlocking between the coating and the substrate, thereby improving hardness stability.
Post-treatment processes play a decisive role in the final hardness performance of the coating. Heat treatment is a common method for electroplating products. By controlling the temperature and time, internal stress in the coating can be eliminated and grain recombination can be promoted, thereby improving hardness. For example, the hardness of an electroless nickel plating layer can be significantly improved after holding it at 400°C for 1 hour. This is because the irregular supersaturation of phosphorus in the coating dissolves in nickel and transforms into highly dispersed Ni₃P or nickel microcrystals containing Ni-P compounds, increasing the resistance to plastic deformation. However, it should be noted that excessively high heat treatment temperatures may lead to a decrease in substrate hardness or coating oxidation; therefore, process specifications must be developed based on the characteristics of the substrate and coating materials.
Optimization of process parameters requires consideration of both the coating material and equipment performance. Current density directly affects the deposition rate and grain size: excessively high current density leads to grain coarsening and reduced hardness; excessively low current density results in a slow deposition rate, potentially causing a porous coating. The plating bath temperature is equally critical; higher temperatures accelerate ion migration, but excessively high temperatures can cause plating bath decomposition or coating embrittlement. For example, when plating hard chrome, a combination of a slightly higher temperature (55℃) and a lower current density (30A/dm²) can yield a finely crystalline coating with uniform hardness, while avoiding burrs or scorching caused by edge discharge.
The uniformity of coating thickness and hardness distribution needs to be achieved through process control. For thin spring sheets, due to high dimensional accuracy requirements, fluctuations in coating thickness can cause localized stress concentration, leading to discrete hardness test values. Adjusting the plating bath circulation speed, the anode-cathode distance, and the placement of the workpieces can improve the uniformity of current distribution, thereby controlling coating thickness deviations. For example, a company optimized the flow field design of the plating bath, reducing the thickness deviation of the coating at different parts of the spring sheet and lowering the standard deviation of hardness test values, significantly improving product consistency.
A quality inspection and feedback mechanism is the closed-loop guarantee for hardness control. A microhardness tester should be used to test the coating hardness at multiple points (at least 5 points), and the average value and fluctuation range should be recorded. If the hardness value deviates from the target range, the cause needs to be analyzed and process parameters adjusted. For example, if the coating hardness is found to be too low, it may be due to excessive phosphorus content or insufficient heat treatment. In this case, the concentration of phosphorus source in the plating solution needs to be reduced or the heat treatment time extended. Through continuous monitoring and feedback, the process can be gradually optimized to achieve stable hardness control.
