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New plating process for anti-friction layer of sliding bearing
Source: Bearing network time: 2018-06-25
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1 Foreword Sliding bearings (also known as bearing bushes) used in internal combustion engines are vulnerable parts. After machining, the nickel (Ni) barrier layer of 1~3μm thick is usually plated on the inner and outer substrates. 1~2]; followed by electroplating 15~30μm thick lead-tin-copper (PbSnCu) ternary alloy anti-friction layer [3~24]; at the end of the plating on the surface of 1~2μm thick tin (Sn) or lead tin ( PbSn) alloy protective layer [2].
The purpose of supplying the anti-friction layer on the inner and outer surfaces of the bearing bush is to improve the friction reducing, wear resistance, corrosion resistance, inlaying, compliance, running-in, seizure resistance, fatigue strength, compressive strength and load bearing of the bearing bush. Can wait; then improve its job function; extend the life of the application; end to ensure the high-function operation of the host.
Usually, the coating type is selected according to the application requirements. The bearing bush of small engine usually uses PbSn6~20 binary alloy [12~13, 16~17, 22]; also uses lead copper (PbCu), lead silver (PbAg) binary alloy. With the passage of time and the development of science and technology; about the bearing bush used in large and medium-sized diesel engines and internal combustion engines; the demand has large load and long service life; and it should have outstanding lubricity, corrosion resistance and wear resistance. It is proved by theory that a small amount of the third component element copper (Cu) can be significantly improved in the conventional lead-tin (PbSn6~20) binary alloy anti-wear coating [3, 10, 20]. (PbSn6~20) When adding 2~3% copper to the binary alloy coating; on the one hand, copper and tin have a certain affinity; in some extent, the tin is scattered to the fabric (ie, the substrate); The tin content in the coating and the stability of the metallographic layout, on the other hand; because of the participation in the copper; the hardness of the coating has improved from the original HV8~10 to HV13~15; greatly improved the wear resistance and fatigue strength of the coating; Then significantly improved the load capacity of the bearing bush; .
If the anti-friction coating is directly electroplated on the fabric metal; the tin in the coating is simply scattered to the fabric; so that the bearing is reduced to less than 6% (mass) after a period of operation; and whether it is a copper-based alloy fabric It is still an aluminum-based alloy fabric; it is rich in a certain amount of copper; the tin that is scattered into the fabric can form a brittle intermetallic compound (Cu3Sn) with copper. This not only reduces the mechanical function of the coating, but also destroys the fabric. Layout; to reduce the overall mechanical function of the bearing bush. The method of dealing with this problem is to plate a layer of nickel or nickel-based alloy barrier layer (also known as gate layer or barrier layer) between the fabric material and the anti-friction bottom layer; Fabrics are scattered [1~2].
The protective layer of tin or lead-tin alloy has a certain anti-corrosion effect; it can also make up the content of tin in the anti-friction layer by the method of dispersing at the time of bearing operation; the content of each component is relatively stable. Because this layer of protective layer contains no copper; it is relatively soft; therefore, the bearing bush can reach the outstanding running-in demand at the beginning of the operation.
This paper first investigates the electroplating process of the bearing friction layer.
2 The process of anti-friction coating in foreign countries started earlier. In 1920, J. Grooff proposed the first patent for electroplating lead-tin alloy; and used for internal and external electroplating of sailor torpedo gas cylinders; In the 1940s, it was used for the plating of bushings. In 1952, Schults proposed a patent for electroplating lead-tin-copper ternary alloys on aluminum and aluminum-silicon (AlSi) alloy substrates. Schaefe, 1953 ) A general statement on the use of lead-tin-copper alloys in bearing bushes. 1976; Jong Sang Kim; Su ιι Pyun and Hyo Geun Lee announced the paper "Pore-Orientation and Microscopic Tracing of Lead-tin-copper Electroplating Layers" [7]. 1980 Beebe proposed copper-containing 2~3% (mass) and tin 9~12% (mass); other copper-based ternary alloy electroplating production process; coating thickness is 15μm. Waterman (1982) The et al. proposed a treatment method for the replacement of copper ions (Cu2+) in ternary alloy plating solution.
The domestic research and application of anti-friction alloy coating started late. In 1960, the first research and development and application of brush-plated tin-lead alloy technology by Wuhan Data Insurance Research Institute and Hailing Top Parts Factory has been used for electroplating of speedboat engines. In the mid-1970s; Shanghai Alloy Bearing Factory and Shanghai East Shipyard made a detailed discussion on the electroplating of copper and tin alloys [12~13].1985; Harbin Institute of Technology Electrochemical Teaching and Research Institute and China National Shipbuilding Corporation The 466-600 factory jointly studied the process of electroplating lead-tin-copper ternary alloy anti-friction layer on lead bronze sliding bearing; it has been used in production. In 1989, Dusanka Radoric announced "hydroquinone in fluoroborate plating solution". (Pb-tin phenol) is an additive for lead-tin alloy plating. 16. The end of the tenth century; Fan Jiahua and Jiang Zhidong 21, 24 of Nantong Bearing Factory; Zeng Liangyu, Yang Xiangui, Wang Huiwen 8, 17 of Wuhan Data Insurance Research Institute; Qin Shengyi 9 of Guangxi Guilin Internal Combustion Engine Parts Factory; Xue Shusheng 20 of Qishuyan Locomotive and Rolling Stock Technology Research Institute has carried out different degrees of electroplating process on the anti-friction layer. Discussed; certainly laid the foundation for further improvement in the production process in use.
3 The problem of the problem The electroplating process of the lead-tin-copper ternary alloy anti-friction coating of our factory is the first in China; for many years, we have supplied a lot of bearing bushes for the mainframe parts shopping malls in China. These years; the order quantity of the bearing products of our factory has increased year by year; The product has entered the world market. Yu Xinghuan Kangmou. This is ample expression of the factory's bearing products in the fierce shopping mall competition has the right strength.
From 1989 to 1991; after our hard work and repeated experiments; it has fundamentally dealt with the problem of the fatal defects of the bearing layer such as foaming and peeling, which eliminates the problem of serious corrosion of the substrate. The batch wall thickness is out of tolerance; the process capability is improved, and the waste loss rate has always been very low; the rate of qualified products for one inspection has improved year by year.
However, the defects such as rough coating, nodule, spot, pit, and airflow stripe are still onset; sometimes the cathode current density (DK) does not reach the scale of the process.
The above-mentioned shortcomings of the anti-friction coating directly affect the quality of the bearing products. With the continuous upgrading of the main engine model, the localization of imported models and the increasingly fierce competition in the shopping malls; the demand for the quality of the bearing products is getting higher and higher. Fundamentally speaking, it is the competition of product quality. Under the situation that users are increasingly demanding the quality indicators of products; we are faced with a serious challenge to improve the plating process of the bearing bush and further improve the quality of the bearing products.
4 Factors affecting the quality of the anti-friction coating
The literature formula and process parameters of lead-tin-copper-ternary alloy anti-friction layer plating solution are as follows: [3~10, 20, 23~24] The parameters are summarized as follows:
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,
Safety agent: 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, progress in guiding 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 can 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 high current density accumulate too fast; the main salt ions in the plating solution can not be compensated; that is, from the surface dispersing 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 metal hydride formed is an unstable substance; it will differentiate when released by baking and heat release ( H2) Then make the plating episode bubbling appearance.
4.2.3 The effect of free boric acid (H3BO3) content reported in the literature [7, 11~24] is 15~40g/ι.
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; a lot of crystals will be separated under the condition of lower temperature; the plating layer is very rough when plating; and the bonding force between the bottom layer and the substrate is affected.
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 rises to the extent that it is suitable for 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 (related 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 to deal 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 Effect of additive content Participating in 0. 1~5g/ι gelatin, bismuth or peach gum in the plating solution can improve cathodic polarization; refine crystallization; improve enthalpy; also help to improve 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; improving the cathodic polarization during electroplating of the coating; improving the formation speed of the nucleus; facilitating the crystallization detailed coating. When the cathodic polarization 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 is too high. Or when the concentration of the main salt is too low; the plating will present the above serious disadvantages.
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.
5 Experimental process The optimal value of [Pb2+], free [HBF4], free [H3BO3] and process parameters DK in the plating solution was determined by L9(34) orthogonal experiment.
5.1 Experimental conditions Anode composition: PbSn10,
Temperature: 20~25°C,
Power supply: three-phase full-wave silicon rectifier,
Several scales of the test piece: 100×50×4,
The composition of the test piece fabric (ie the substrate to be plated): CuPb22Sn2,
The appearance roughness of the test piece fabric: Ra0. 4.
Fixed component content.
Sn 2+: 15g/ι, Cu: 5g/ι, stabilizer: 5g/ι, additive: 2g/ι,
Table 1 Column number, factor comparison table ------------------------------------------ --------------
L9(3< 1 2 3 5
Sup>4) column number -------------------------------------------- ------------
Factor ABCD
-------------------------------------------------- ------
Table 2 Factor, Level Table -------------------------------------------- -----------------
Horizontal/number free fluoroboron lead ion free boric acid cathode current/factor acid [HBF [H3 density (DK)
Ub>4] BO
3 sub>]
ABCD
-------------------------------------------------- -----------
1 75g/ι 250g/ι 25g/ι 4A/dm2
2 150g/ι 150g/ι 35g/ι 3A/dm2
3 300g/ι 80g/ι 15g/ι 2A/dm2
-------------------------------------------------- -----------
5.2 factors and their levels of free fluoroboric acid (HBF4) concentration (A): A1 = 70g / ι, A2 = 150g / ι, A3 = 300g / ι.
Concentration of lead ion (Pb2+) (B): B1=250g/ι, B2=150g/ι, B3=80g/ι.
Concentration of free boric acid (H3BO3) (C): C1=25G/ι, C2=35g/ι, C3=15g/ι.
Anode power density (DK) (D): D1 = 4A / dm2, D2 = 3A / dm2, D3 = 2A / dm2.
5.3 The experimental process and its results evaluation after careful and careful preparation; according to the orthogonal experimental plan (see Table 3) carefully arranged nine experiments; the experimental results were evaluated.
Table 3 Orthogonal experimental plan and its results analysis table L9 (34)
-------------------------------------------------- -----------
Experiment No./Water 1 2 3 4 Experimental Results Rating Flat/Column----------------------------------- --------------------------
1 1 1 1 1 100
2 1 2 2 2 100
3 1 3 3 3 95
4 2 1 2 3 80
5 2 2 3 1 65
6 2 3 1 2 70
7 3 1 3 2 45
8 3 2 1 3 50
9 3 3 2 1 30
-------------------------------------------------- -----------
K1 295 225 220 195
K2 215 215 210 215
K3 125 195 205 225
R 56.6 10. 0 5. 0 10. 0
-------------------------------------------------- -----------
In Table 3; K1 is the sum of the first level scores in each column, K2 is the sum of the second level scores in each column, and K3 is the sum of the third level scores in each column.
R=(Kmax Kmin)/3.
The order of influence of four factors on the quality of bearing antifriction coating is A>B=D>C. The concentration of free fluoroboric acid has the greatest influence; the concentration of lead ion and the influence of cathode current density are the second; the concentration of free boric acid has the least influence. .
Now analyze the influence of each level of each factor on the quality of the anti-friction coating.
In A: K1>K2>K3; the first level is the best.
In B: K1>K2>K3; the first level is the best.
In C: K1>K2>K3; the first level is the best.
In D: K3>K2>K1; the third level is the best.
After the above analysis; the best combination condition is A1B1C1D3. The best plating condition obtained by orthogonal experiment is HBF4 (free): 70g/ι,
Pb2+: 250g/ι,
H3BO3 (free): 25g/ι,
DK: 2A/dm2.
In the theoretical production; think about the conductivity of the bath and the loss of the elements; adjust the concentration of free fluoroboric acid and the concentration of lead ions; the former is properly adjusted; the latter is properly adjusted. From Table 3 can also It can be seen that when the DK is within the scale of 2~3A/dm2; the quality of the coating is not much different; it is considered that the production progresses; therefore, it is determined to be within the appropriate scale. When the temperature is lowered; the free boric acid is easy to separate and crystallize; Therefore, the concentration is considered to be slightly lower.
The various factors have been weighed extensively; the process of electroplating the ternary alloy antifriction layer has been improved as follows.
Pb2+ (participated in the form of Pb(BF4)2): 150~200g/ι,
Sn2+ (in the form of Sn(BF4)2): 10~20g/ι,
Cu2+ (participated in the form of Cu(BF4)2): 3~6g/ι,
HBF4 (free): 70~120g/ι,
H3BO3 (free): 20~25g/ι,
Safety agent: 3~10g/ι,
Additive: 0. 5~5g/ι,
Temperature (T): 15~35°C,
Cathode current density (DK): 2~2. 8A/dm2,
Time (t): 15~35min,
Coating thickness (δ): 15~30μm,
Anode composition: PbSn9~11,
The ratio of anode area to cathode area: 2.
6 The improved process has experienced many years of production and operation testing. The result is that the coating crystal is fine and lubricated; it eliminates the shortcomings of airflow streaks, pinholes, pits, nodules, roughness, etc. The loss rate of the shunt products is originally The reduction of about 0.5% to less than 0.1% today, the rate of qualified products in one inspection has increased from about 90% to 99% today. It can be seen that the quality of the plating products has improved significantly.
Once; each anode orientation can only be plated with a pair of bearing bushes; DK open middle and lower limits also have rough coatings, streaks and other shortcomings. After the improvement process; each anode orientation can be plated with 2~4 sub-bearings; cathode current density ratio The progress is nearly 50%; and the crystallization of the coating is still very fine. The improved process progress is 2~4 times.
The order quantity of our factory has been increasing year by year; the output value has increased year by year; the current annual output value has reached more than 10 million yuan. In the past ten years, the direct economic benefits have been tens of millions of yuan. Together with the localization of the host and imported mainframes We have supplied the right amount of high-quality bearing parts. In the past few years, some of our bearing products have entered the world market. I am fortunate to have a certain relationship with the improvement of the quality of the plating.
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