Guest Editorial -For Plateworld.com                                                 

 Don Baudrand, Don Baudrand Consulting,   e-mail:donwb@tscnet.com

 

ELECTROLESS NICKEL DESIGN FOR PTFE COMPOSITE PLATING

Introduction

Much has been published about the favorable frictional properties, wear resistance and applications of electroless nickel-PTFE composite coatings. Little appears in the literature about the nature of the electroless nickel plating used with PTFE suspensions.

Functional properties are discussed by Paul Ebdon1,2,3, D. Baudrand4, J. S. Hadley et al.5 and others. Tribological assessments are found in papers by N. Wal16, P. Ebdon7, R. Duncan8, and N. Kanani 9. W. J. Schumacher discussed the types of wear and methods of testing10,11,12,13. Applications and deposit characteristics are discussed in most of the
above-referenced articles. J. Henry discusses comparison of PTFE and CFx deposits14

The PTFE Suspension

PTFE (sometimes called TeflonR) particles suspended in surfactant systems form colloidal sols. These suspensions must be stable and compatible with electroless nickel plating solutions. The surfactants must be capable of wetting the PTFE particles which, because of the hydrophobic nature of PTFE, make the polymer difficult to wet. The particles range in size from 0.2 - 0.5 microns in diameter, smaller than a cigarette smoke particle. They must be extremely well cleaned before introducing the surfactants to form a usable suspension. The surfactants must not have a negative effect on the deposit.

The suspensions can coagulate into globules which fallout of suspension. Further, the delicate balance of particle size and surfactant system can be disturbed in a number of ways causing "fall-out", no co-deposit ion with electroless nickel, or unsatisfactory deposits. "Fall-out" is defined as separation of the PTFE from the suspension in the plating solution.

The science of forming a suspension suitable for use with electroless nickel is a separate subject not included in this discussion. The influence of the nature of the electroless nickel solution and operating conditions will be discussed.

Commercial PTFE suspensions are made by using a single surfactant, two surfactants, and three or more surfactant systems. When these suspensions are added to an electroless nickel plating solution the system is complete. Some suspensions require extreme dilution, e.g., at least 3 parts water to 1 part suspension. Others can be added without further dilution. Factors
influencing the stability of the suspension are dilution, the nature and amount of surfactant in the suspension, the type of electroless nickel solution, the temperature of the electroless nickel solution at which additions are made, the amount and type of agitation during the additions. Ideally, the suspension should be capable of being added at and during operating conditions of the plating solution.

The Electroless Nickel

Many electroless nickel solution compositions will cause immediate separation of the PTFE from suspension and "fall-out" occurs. Others tested will maintain the suspension for from a few hours to several days before dropping out the PTFE. With some suspensions. if agitation is stopped. the PTFE will drop to the bottom of the tank. In most cases. the
PTFE cannot be re-suspended to produce satisfactory deposits by agitating the plating solution.

Not all factors influencing drop out of PTFE are known. But we know that only a very few electroless nickel solutions work well with various PTFE solutions. Over 100 different electroless nickel plating solutions were tested. and several different types of PTFE suspensions were tested.

In general. the following observations are made. The type of complexor system used in the plating solution has an influence on the success of the system. The type of buffer used in the bath formulation also has an influence. Most
electroless nickel plating solutions operate in about the same pH range. i.e . 4.5 - 5.0. The higher the total salt concentration. the less stable will be the suspension. The more soluble the salts are. the better the performance. By salt concentration. we mean nickel, sulfate and/or other anions of nickel compounds. sodium or potassium. hypophosphite anions. and orthophosphite anions.

The performance of the PTFE - electroless nickel plating solution is also influenced by the type and amount of stabilizer compounds in the solution. The plating rate is also influenced to some extent by the stabilizer type and concentration. The stabilizer should be adjusted to provide the maximum rate of which they are capable.

Operating conditions also influence performance. For example, very high operating temperature can cause "fall-out". Agitation of the solution has a negative influence on different PTFE suspensions. Some require vigorous agitation; one required moderate agitation; and one, very mild agitation for best performance. Over-agitation is some cases can cause "fall-out", and "edge effect", sometimes called passination.

With proper selection of PTFE suspension and attention to operating conditions of the electroless nickel solution, smooth adherent deposits containing a uniform distribution of PTFE particles throughout the deposit can be achieved. This uniform particle distribution can be maintained over all surfaces including such irregularities as slots inside holes, threads, etc.

PTFE particles in the elctroless solution tend to stay suspended without continuous agitation. Diamond silicon carbide and other heavier inert particles require vigorous agitation and tend to be occluded on horizontal surfaces.

The excellent wear and low friction characteristics of deposits of  PTFE in nickel-phosphorous alloy deposits offer numerous additional applications for electroless nickel composite coatings. The coatings provide a significant margin of safety where there is danger of loss of  lubricants. Dry friction applications are possible using EN-PTFE. Mold release characteristics are outstanding. Excellent release and long life is realized by electroless nickel-PTFE plating the cavities of plastic injection molds. Rubber molds plated with EN/PTFE release easily and are easier to clean. Extrusion dies plated with EN/PTFE show reduced wear, lower operating temperature and faster extrusion rates.

To achieve the most benefit from the deposits, sufficient PTFE must be in the deposit. Approximately 25-30% by volume is ideal. Less PTFE results in lower performance where low coefficient of friction and long wear life is required. For certain higher load applications or where a tight seal is necessary, lower quantities of PTFE are sometimes preferred. The distribution of PTFE must be uniform throughout the deposit for best results.

The stability of the suspension, the uniformity of distribution, the length of the plating bath life, and hardness of the nickel-phosphorous deposit depend on the chemistry of the electroless nickel solution as well as the type of suspension used. Fransaer, Celis and Roos discuss the various models and theory of particle codeposit ion in a recent article15. The discussion is aimed at electrodeposition, but likely applies to electroless systems as well. These processes are patented. Licenses must be obtained to use this technology 16,17,18,19,20

Some examples of PTFE composite applications:

Brass spray-nozzles, water jets, air and paint spray-nozzles, and water line valves are composite plated to prevent sticking of hard water salts and paint.

Solenoid sleeves - to prevent sticking and wear.

Yarn processing equipment - provides longer life and improved yarn quality.

Spindle for a planet-wheel in a gear box solved a seizing problem.

Submersible propulsion units were seizing or returned for maintenance after 200 hrs.

After plating, the life was extended as much as 7000 hrs.

Fluid power equipment: automatic and semi-automatic pneumatics components used in foundries, steel mills, ship and submarine building were plated to increase useful life and reduce friction.

Control and check valves for oil well controls. Composite coatings insure smooth operation, long spring life, and corrosion protection against oil contaminants, dirt, and sea water.

Auto seat belt mechanisms are plated to provide low wear and smooth reliable operation.

Sleeve liners for clutch hydraulic actuation systems plated to reduce pedal effort, friction and wear, and provide longer service life.

REFERENCES

1. Paul Ebdon, "Recent Advances in the Electroless Nickel-PTFE Composite Process".

2. P. R. Ebdon, "Electroless Nickel/PTFE Composite Plating", Proceedings of 71st Annual Technical Conference, AES.

3. P. R. Ebdon, "'Niflor' - A New Generation Approach to Self-Lubricating Surfaces", Materials and Design, vol. VI no. 1.

4. Donald Baudrand, "Electroless Nickel/PTFE Codeposits", Products Finishing, April, 1992.

5. J. S. Hadley et al., "Properties of Electroless Nickel PTFE Composite Coatings", Proceedings of 71st Annual Technical Conference, AES.

6. Dr. N. Wall, "Tribological Charactristics of Niflor".

7. P. R. Ebdon, "Composite coatings with lubricating properties", Trans IMF, 1987.

8. Ronald N. Duncan, "Hardness and Wear Resistance of Electroless Nickel-Teflon Composite Coatings", Metal Finishing, Vol. 87, no. 9, September, 1989.

9. N. Kanani, "Tribological Behaviour of Electroless Nickel", 1991 IMF Annual Technical Conference.

10. W. J. Schumacher, "Metals for Nonlubricated Wear", Machine Design, March ll, 1976.

11. W. J. Schumacher, "Wear and galling can knock out equipment", Chemical Engineering, May 9, 1977.

12. W. J. Schumacher, "Adhesive Wear Resistance of Engineering Alloys", Metal Progress, Vol. 114, No.6, November, 1978.

13. W. J. Schumacher, "A stainless steel alternative to cobalt wear alloys", Chemical Engineering, Sept. 21, 1981.

14. Jim Henry, "A New Fluorinated Electroless Nickel Codeposit", Metal Finishing, Vol. 88 No. 10, Oct., 1990.

15. J. Fransaer, J. P. Celis and J. R. Roos, "Mechanisms of Composite Electroplating", Metal Finishing, Vol. No. , June, 1993.

16. Feldstein, U.S. Patent No. 5,145,517, Sept 8, 1992

17. Feldstein, U. S. Patent No. 4,997,686, March 5, 1991

 18. Henry, U. S. Patent No. 4,830,889

 19. Hella, U. S. Patent No. 4,302,374.

 20. Hella, U. S. Patent No. 1,525,158 .


RTeflon is a registered trade mark of E.I. DuPont de Nemours, Inc.

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