plating
Electroless plating is very important in surface treatment technology. Electroless plating is the use of suitable reductant make the solution of metal ions on the surface of the catalyst active selectively reducing precipitation into a chemical treatment method of metal coating. Under the available type said:
M2 + + 2 e (provided by reducing agent) -- - > M
In chemical plating of metal ions in the solution is to rely on to get the required electronic and back into the corresponding metal. For example, in acidic electroless nickel plating solution using hypophosphite as reducing agent, its REDOX reaction process is as follows:
Ni2 + + 2 e - > Ni (reduction)
+ H2O (H2PO2) -- - > (H2PO3) - + 2 + 2 h + e (oxide)
Two type additive, got all the reduction oxidation reaction:
Ni2 + + + H2O (H2PO2) -- - > (H2PO3) - + Ni + 2 h +
Effective the reductant can use its standard oxidation potential to infer. By above knowable, phosphate is a kind of strong reducing agent, can produce a positive standard of oxidation reduction potential. But should not excessively rely on 擄 E value, because in the actual application, because of different ions in the solution activity, super potential and similar factors, can make E 擄 values have very big difference. But the calculation of oxidation and reduction potential still help to estimate in advance how effective different reducing agent. If all the standard REDOX potential is too small, or negative, the metal reduction is unlikely to happen.
The composition of electroless plating solution and its corresponding working conditions must be to limit the response have catalytic effect on the surface, and the solution itself should not happen spontaneously oxidation reduction, natural decomposition, lest solution resulted from solution quickly. If the plating metals such as nickel, palladium itself is the catalyst reaction, the chemical plating process is automatic catalysis, make the above reaction continuously, at this time, the coating thickness also increases gradually, obtain certain thickness. In addition to the nickel, cobalt, rhodium, palladium has automatic catalysis, etc.
For not have automatic catalytic surface parts, such as plastic, glass, ceramics etc. Nonmetal, usually through special pretreatment, make its have the catalysis, surface activation and electroless plating can proceed.
Chemical plating and electroplating, has the following advantages:
1) don't need additional dc power equipment.
(2) the coating density, pore less.
(3) there is no uneven distribution of power, the influence of the geometric shape of complex plating parts, also can obtain uniform coating thickness;
(4) in the metal, nonmetal, semiconductor, etc various plating on the base material.
Chemical plating and electroplating, compared to the solution of stability is poorer, and the solution of the maintenance, adjustment and regeneration is more troublesome, material cost is higher.
Electroless plating technology has important position in the electronics industry. Due to using different kinds of reducing agent of the chemical plating coating performance have significant differences, therefore, on the chosen solution formula, carefully consider the efficiency of plating solution and the characteristics of the coating.
At present, the chemical plating nickel, copper, silver, gold, cobalt, palladium, platinum, tin and electroless plating alloy and chemical composite plating, have been used in industrial production.
How to carry out chemical nickel plating
Electroless nickel plating is a method of electroless plating applied most widely, the reducing agent used once phosphate, hydrazine, sodium borohydride and dimethyl amine borane, etc.
Most of the current domestic production using sodium hypophosphite as a reducing agent, sodium borohydride and dimethyl amine borane because of more expensive, only a small amount of use.
1. The purpose of the coating
The crystallization of the chemical plating nickel layer of meticulous, low porosity, high hardness, coating uniformity, good weldability, possesses deep plating ability is good, high chemical stability, has been widely used in electronics, aerospace, aviation, machinery, precision instruments, daily hardware, electric and chemical industries.
More and more application of electroless nickel plating on nonmetal material, especially plastic products after electroless nickel plating can be according to the conventional plating methods on the metal coating, plating with the appearance of the metals. Plastic electroplating products has been widely used in electronics, electrical appliances, daily industrial products, etc.
Electroless plating nickel in the atomic energy industry, such as in the production of nuclear fuel system components and containers, and the rockets, missiles, jet engine components have been adopted.
Chemical equipment of compressors and other parts for anti-corrosion, anti-wear, and using chemical nickel plating layer is very beneficial.
Chemical nickel plating layer can improve the welding performance of aluminum, copper, stainless steel materials, reduce the abrasion of rotational parts, reduce the stress corrosion of stainless steel and titanium alloy.
Requirements for coating size accurate precision parts and complex geometry parts of deep holes, blind holes, the inner surface of the cavity, with outer surface by chemical plating nickel could get the same thickness of coating.
To require high hardness, wear-resistant parts, can be substituted for electroless nickel plating hard chromium plating.
2. The composition and properties of coating
< 1 > the composition of plating
Using hypophosphite as reducing agent of electroless nickel plating solution in the plating coating containing 4% ~ 15% of phosphorus, is a kind of nickel phosphorus alloy. With boron hydride or amino borane as reductant is pure nickel plating layer, nickel content can reach more than 99.5%. Just came out of deposition layer of electroless nickel plating is amorphous, a type of amorphous thin plate structure.
Phosphorus content in the coating is mainly depends on the pH of the solution, with lower pH, phosphorus content increase. Conventional acid electroless nickel deposition coating of phosphorus content in the solution is 7% ~ 12%, and deposition layer of nickel phosphorus content in alkaline solution is 4% ~ 7%. In addition, the solution composition and the content of each component and their relative ratio, as well as the solution of the working temperature and so on all has certain influence on phosphorus content.
< 2 > characteristics of the coating
(1) hardness
Chemical nickel plating layer is much taller than the hardness of nickel plating layer, and more wear-resisting. Nickel plating layer's hardness is only HV160 ~ 180, and chemical nickel plating layer's hardness is commonly HV300 ~ 500.
With heat treatment method can greatly improve the hardness of electroless nickel plating layer, heating at 400 鈩?for 1 hour after the hardness peak of around of HV1000. If continue to improve the heat treatment temperature, such as increased to 600 鈩? the hardness decreased to HV700 instead.
Before the heat treatment of electroless plating nickel layer is crystal structure of the amorphous structure, is transformed into crystalline type organization after heat treatment, have Ni3P phase in the formation of coating. Ni3P phase precipitation increases with the rising of quantity as the heat treatment temperature, the maximum amount of precipitation is determined by the coating of phosphorus.
In order to improve the coating hardness, suitable heat treatment regulation is: temperature 380 ~ 400 鈩? time of 1 hour. To prevent coating color, had better have protective atmosphere or use the vacuum heat treatment. When does not have protective atmosphere conditions, appropriate to reduce the heat treatment temperature (280 鈩? and extend the processing time, can also improve the hardness value.
When coating has the maximum hardness and brittleness also increases, thus not suitable under the condition of high load or shock to use. Select the appropriate heat treatment conditions, can make the coating has both a certain hardness and ductility.
General steel workpiece chemical nickel plating layer 2 hours at 200 鈩?temperature treatment, can improve the bonding strength and eliminating stress. The aluminum workpiece to stay under 150 ~ 180 鈩?for 1 hour more appropriate.
(2) the magnetic
Magnetic chemical nickel plating layer can decide on phosphorus content and heat treatment temperature. Phosphorus content more than 8% of the coating is weak magnetism; Phosphorus content in more than 11.4%, no magnetic; Phosphorus content less than 8% of the coating is magnetic, but its magnetic smaller than electroplating nickel layer, magnetic can significantly improve after heat treatment.
For example, in the coating obtained in alkaline chemical nickel plating liquid without heat treatment when the magnetic coercive force H0 = 160 for a/m, 350 鈩?heat for 1 hour H0 = 8800 a/m.
(3) resistivity
Chemical nickel plating layer resistivity is associated with phosphorus content and phosphorus content in general, the higher, the resistivity. In alkaline solution obtained by electroless nickel plating layer and the resistivity of about 28 ~ 34 mu 惟 路 cm. In acidic solution obtained by electroless plating nickel layer and the resistivity is about 51 ~ 58 mu 惟 cm, several times than the nickel plating layer (resistivity of pure nickel 9.5 mu 惟 路 cm). Chemical nickel plating layer resistivity will significantly decreased after heat treatment. The phosphorus content of 7%, for example, electroless nickel plating layer, after 600 鈩?heat treatment, the resistivity from 72 mu 惟 路 cm down to 20 mu 惟 路 cm. Boron content 1.3% ~ 4.7% of the chemical plating nickel boron, its resistivity is 13 ~ 15 mu 惟 路 cm. With dimethylamine borane reduction of nickel coating, the boron content was 0.6%, the resistivity of 5.3 mu 惟 cm, low resistivity than pure nickel.
(4) thermal expansion coefficient and density
The thermal expansion coefficient of general chemical nickel plating layer for 13 x 10-6 鈩?- 1.
Chemical nickel plating layers generally is about 7.9 g/cm3, the density of the density of chemical nickel plating layer with phosphorus content increase and decrease.
The comprehensive performance of chemical nickel plating layer are shown in table 4 to 24:
Table 4-24 for the comprehensive performance of chemical nickel plating layer for the comprehensive performance of chemical nickel plating layer phosphorus content (8% 10%) of ni-p alloy coatings
Hardness (HV) heat treatment before 500
400 鈩?heat treatment after 1000
Density (g/cm3) 7.9
Melting point (890 鈩?
Resistivity (mu 惟 路 cm) 60 ~ 75
The thermal expansion coefficient (鈩?- 1) 13 x 10-6
Thermal conductivity 5.02 W/(m 路 k)
Elongation (%) 3 ~ 6
Reflection coefficient (%) of 50 (approximation)