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/*728*90 created on 2018/5/16*/ var cpro_id = "u3440131"; Home>Bearing knowledge>New plating process for sliding bearing antifriction layer (3) New plating process for anti-friction layer of sliding bearing (3)
4.2.3 The effect of free boric acid (H3BO3) content reported in the literature [7, 11~24] is 15~40g/ι. /*250*250 was created on 2017/12/25*/ var cpro_id = 'u3171089';
The following chemical balance exists in the bath:
HBF4+3H2O<==>H3BO3+4HF
HF<==>H++F-
2F-+Pb2<==>PbF2↓
When the content of boric acid in the plating solution is too low; the above three balances all move to the right; the end leads to the formation of harmful PbF2 accumulation. Therefore, the quantitative free boric acid acts to stabilize the free fluoroboric acid.
The solubility product of PbF2 is KSP=4. 0×10-8moι/ι. According to the solubility product principle; when [F-]2[Pb2+]≥KSP=4.0×10-8moι/ι; in the plating solution It is possible to generate PbF2 deposits. [Pb2+] in the plating solution is 0. 68moι/ι (ie 140g/ι); the values ​​of [Pb2+] and KSP are substituted into the above formula:
[F-] 2 × 0. 68 ≥ 4. 0 × 10-8moι / ι.
Calculated by:
[F-]≥10-3.62moι/ι=0.0002399moι/ι
=0.0045556moι/ι
= 4.556mg/ι
4.556ppm
It can be known from the above calculation; when the free [F-] in the plating solution is greater than 4.556ppm; it will be deposited as PbF2; making the plating solution dirty. When plating is performed in such a plating solution; the plating layer will have pitting and pits. Shortcomings such as roughness and blistering.
It can be seen that a sufficient amount of free boric acid in the plating solution is dissociated by the free fluoroboric acid according to hydrazine; the free [F-] is manipulated to a degree below the PbF2 accumulation; then the plating solution is stabilized in one aspect.
When the content of free boric acid in the plating solution is too high; in the case of lower temperature, a lot of crystals will be separated; the plating will make the coating very rough; and affect the bonding between the bottom layer and the substrate.
4.2.4 The influence of cathode current density (DK) is followed by the increase of DK; the tin content in the coating is added; and the stacking speed is accelerated; the production power is improved. When DK is lowered; it is beneficial to the refinement of coating crystallization.
In the theoretical production; in order to accelerate the progress; expect to raise the DK higher, but after all, the DK is raised to the extent of the main salt ions in the plating solution; especially the lead ion (Pb2+) content.
According to the literature [7] reported; when the [Pb2+] in the plating solution is as high as 333g / ι; DK can rise to 4A / dm2; and the layout coefficient of the reflective surface {111} is still as high as 1.42; When the [Pb2+] in the plating solution is 222g/m; the layout coefficient of the reflective surface {111} is 1.47; the DK can only rise to 1A/dm2, if DK rises to 2A/dm2; the reflective surface The layout factor of {111} is reduced to 1.14; at this moment, the crystallization of the coating is also more detailed. When the [Pb2+] in the plating solution is reduced to 111g/ι; if the DK is raised to 2A/dm2; the layout of the reflecting surface {111} The coefficient is only 0.63; the coating at the moment is not exhaustive at the microscopic level; to reach a more detailed coating; DK can only rise below 1. 5A/dm2. The literature [7] claims; the rise of DK leads to the cathode The rise of the potential. When the cathode potential exceeds 560mV (relative to the full calomel electrode); no matter how the concentration and other process parameters change; the dominant crystal plane orientation changes from {111} to {100}; that is, the crystal of the coating is fine Thickening. This change is related to the formation of pyramidal crystal faces; it is the process of nucleation and crystal growth. The speed control process is changed from surface dispersing or nucleation manipulation to liquid phase mass transfer control. Results.
4.2.5 The effect of the stabilizer content The oxygen (O2) in the air has a certain solubility in the bath; it will oxidize a certain amount of divalent tin ions (Sn2+) in the bath to tetravalent tin ions (Sn4+). Sn4+ can form Sn(OH)4 with very low solubility even in a solution with high acid concentration; then, after losing one molecule of water, a colloidal suspension of stannic acid (H2SnO3) is formed to affect the quality of the coating.
The method for dealing with the above problems is to participate in 2~12g/ι of hydroquinone (p-benzenediol), meta-benzenediol or phenol and other antioxidants in the plating solution. The basic principle is that they react with oxygen to form oxidation state. When energized; this oxidation state can reduce the cost of the material at the cathode. This repetition; greatly reduces the oxygen content in the plating solution; delays the oxidation of Sn2+; stabilizes the plating solution.
4.2.6 The effect of additive content in the plating solution to participate in 0. 1 ~ 5g / ι gelatin, tantalum or peach gum can improve cathode polarization; refine crystallization; improve the ability to disperse; also help to improve the tin in the coating Content. Too little additive will make the coating loose, rough, black, and too much will make the coating brittle.
The electroplating of the anti-friction coating of the bearing bush is composed of five processes of liquid phase mass transfer, pre-conversion, charge transfer, surface dispersal or nucleation, and crystallization. When forming the plated crystal, it is divided into two processes that are carried out together; The formation and growth process of the crystallization center (nucleation). The speed of these two processes determines the thickness of the crystallization of the coating. If the nucleation rate is faster, and the growth rate after nucleation is slower, it is crystallized. The number of grains is larger; the grains are finer. On the contrary, the grains are coarser. That is to say, when the rate of formation of crystal nuclei is greater than the growth rate of crystal nuclei during electroplating, a detailed and well-plated coating can be obtained. The formation rate of the nucleus is greater than the growth rate of the nucleus; the more detailed and strict the crystallization of the coating.
The crystallization is arranged in a finer coating; its protection, functionality and appearance quality are more ambition. The theory indicates; the cathodic polarization effect of the electroplating of the electroplated layer is improved; the formation speed of the crystal nucleus can be improved; and the detailed crystallization plating layer can be obtained. When the cathodic polarization effect exceeds a certain scale; it will cause a lot of hydrogen to be separated; then the coating will become porous, rough, loose, charred; or even powdery; the quality will decrease. The additive content and free fluoroboric acid content are too high. Or when the concentration of the main salt is too low; the plating will present the above severe drawbacks.
4.2.7 The effect of temperature follows the increase of temperature; the dissolution of each component in the plating solution; its concentration can take the upper limit. This is beneficial to the increase of DK; then the accumulation speed is accelerated. However, if the temperature is too high, the anode dissolves. Fast; the anode mud is increased; the plating solution is dirty, the coating is nodulated. The evaporation of free fluoroboric acid, organic stabilizer and the like is accelerated; the addition of odor and environmental deterioration.
When the temperature is too low, the free boric acid is easily separated; the plating solution is dirty, the coating is rough, and the DK does not rise up; the stacking speed is slow; the production power is low.
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Source: China Bearing Network Time: 2014-01-22