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New plating process for anti-friction layer of sliding bearing (2)
Source: China Bearing Network Time: 2018-01-28
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Pb2+ (participated in the form of Pb(BF4)2): 80~333g/ι,
Sn2+ (in the form of Sn(BF4)2): 5~33.3g/ι,
Cu2+ (in the form of Cu(BF4)2): 2~11g/ι,
HBF4 (free): 40~300g/ι,
H3BO3 (free): 15~40g/ι,
Stabilizer: 2~12g/ι,
Additive: 0. 1~5g/ι,
Cathode current density (DK): 1~8A/dm2,
Temperature (T): 15~30°C,
Time (t): 15~35min,
Coating thickness (δ): 15~30μm,
The composition of the anode: PbSn8~11.
4. 2 The relevant factors affecting the quality of anti-friction coating can be seen from the above formula; whether the composition content is still the process parameter; its scale is too wide, it is the customary production demand; it is necessary to further optimize; Before the necessary analysis of the relevant factors affecting the quality of anti-friction coatings; to determine the feasible range of each factor level in the orthogonal experiment.
4.1.2 Influence of main salt ion concentration The main salt ions in the plating solution are Pb2+, Sn2+, Cu2+. The content of Sn2+ and Cu2+ may be adjusted according to the percentage content of Sn and Cu in the alloy plating layer; Can satisfy the user's demand for the content of the coating composition. Therefore, for the main salt ion; only comment on the influence of the Pb2+ content in the plating solution on the coating quality.
The Pb2+ in the plating solution is the primary component for the alloy plating; the reported content in the literature is 80~333g/ι. If the concentration is higher, it is allowed to use a higher cathode current density; the stacking speed is faster, but the enthalpy can be reduced; The loss is large. If the concentration is low, the dispersion is better; but the deposition speed is slower. If the content is too low, the concentration polarization of the plating solution is too large; the current does not rise; the coating is easy to present the airflow stripe. And the pyramidal microscopic metallographic layout; visually manifested as rough coating. If the content is too high, on the one hand, the plating solution will be lost and increased; adding cost, on the other hand, the boric acid (H3BO3) is easy to occur when the temperature is low. The appearance of the additive is precipitated; then the coating is rough. The suitable content is DK rising to the upper limit of the process rule; and the plating is crystallized in detail, when the temperature drops below 15 °C; the plating solution should be free of the appearance of boric acid and additives.
4.2.2 The effect of free fluoroboric acid (HBF4) concentration is to promote the normal dissolution of the anode, avoid the oxidation of ferrous tin (Sn2+) and the hydrolysis of the primary ions (Pb2+, Sn2+, Cu2+); Stability, progressive conductivity and scatter, refine crystallization.
The content reported in the literature is 40~300g/ι.
When the content of free fluoroboric acid is too low; it has a low concentration of hydrogen ions (H+) which is resolved; the following hydrolysis reaction may occur in the plating solution.
Pb2++2H2O<==>Pb(OH)2↓+2H+
Sn2++2H2O<==>Sn(OH)2↓+2H+
Cu2++2H2O<==>Cu(OH)2↓+2H+.
They all form hydroxide deposits and are suspended in the plating solution. When electroplating; they adhere to the surface of the substrate or are contained in the plating layer; the bonding force between the plating layer and the substrate is reduced; and the plating layer is brittle, rough, and spotted. Then the coating's wear resistance and fatigue strength and other functions are significantly reduced.
When the content of free fluoroboric acid in the plating solution is too high; at the high current density of the plated part; that is, the local or sharp edges of the bearing bush are pulverized, and the end face has hydrogen gas. The result is that airflow streaks are generated on the bearing layer of the bearing pad and Pinhole defects. Together; because the edge effect and tip discharge make the deposition at high current density too fast; the main salt ions in the plating solution can not be compensated; that is, from the surface dispersion or nucleation manipulation to liquid phase mass transfer control; The increase in the inside and outside of the bearing shell (cathode) occurs as follows:
2H++2e<==>H2↑
It can be seen from the above reaction; when the hydrogen ion (H+) concentration (ie, the concentration of the corresponding free fluoroboric acid) is increased; the equilibrium shifts to the right; the hydrogen gas (H2) is promoted. The result of hydrogen evolution not only causes the coating to exhibit airflow streaks. And pinholes and other shortcomings; and because of the initial ecological hydrogen (H is hydrogen radicals) impregnated into the interior of the coating to form a metal hydride and lattice distortion and thread misalignment. If the scanning electron microscope (SEM) to investigate the coating section Microscopic depiction; it can be found that its crystal is in the shape of a large pyramid [7]; intuitively, the coating is rough. On the other hand, the formed metal hydride is an unstable substance; it will differentiate when released by baking and heat to release hydrogen ( H2) Then make the plating episode bubbling appearance.
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