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About 17 years ago the first electroless nickel (EN)/PTFE coatings were introduced. At that time, platers used commercially produced PTFE dispersions combined with any available electroless nickel. The target was to obtain a coating with uniform properties in all three dimensions.
The PTFE particles were not uniformly distributed throughout the EN matrix. This influenced the mechanical and tribological (friction, lubrication, bearing and hydrodynamic) properties of the coating. Areas where less PTFE is incorporated do not have the same physical and mechanical properties as in areas where the PTFE content is uniform and higher. Many EN/PTFE coatings still have the same problem. Although some say the coating is a more stable one under high-pressure conditions, according to wear measurement and theoretical knowledge this statement could not be verified. Coatings deposited at higher bath lifetimes make the problem even worse.
To achieve optimum properties of a coating, it is necessary for the particles to be evenly distributed so that anti-adhesive and wear properties of the coating show the same appearance everywhere.
Table I shows data that an EN/PTFE process should be able to fulfill to meet today's environmental and economical needs. To achieve the "perfect coating" and the required lifetime, it was necessary to modify the electroless nickel electrolyte and the manufacturing of the dispersion to develop an improved proprietary process. A nice side effect of this was that the lifetime of the plating solutions was extended to five metal turnovers or more under normal job-shop conditions. Customers using EN/PTFE coatings of the newest generation report generally the same deposition quality for five metal turnovers or more. Also, the incorporation rate of the PTFE remains at about 25 to 30% volume during the entire bath life. To incorporate higher PTFE content does not appear practical because the plating rate is reduced and the nickel matrix becomes more fragile, increasing the wear rate. Also, agglomeration of PTFE particles is hardly a problem. Table I shows some relevant data of the EN/PTFE coating system.
|TABLE I—EN/PTFE Coating Characteristics|
|PTFE incorporation||variable in between 15 - 30 vol-%|
|Particle size||0.3 - 0.5 mm|
|Bath lifetime||about 5 MTO|
|Deposition rate||5 mm/h at 28 vol-% up to 5 MTO|
|Agitation during plating||none|
Because of the results, EN/PTFE coatings can expect a bright future as solutions to many customers' problems. Hard coatings cannot cope with all wear and corrosion problems, and industry has demanded "mass tailored" and specific problem-solving coatings. Because of this, the requests for EN/PTFE coatings have grown tremendously.
A widely held prejudice is that only hard coatings can solve wear problems. That is correct for abrasive wear, but only for that specific case. For nearly all other tribological problems, a reduction of the coefficient of friction can handle it and is sometimes even better. The main reason for the success is that forces that could fatigue one or both tribological partners are reduced, so less wear is transferred into the material.
In the top view, the color is coordinated with height, and in the hardness map with hardness. Even on the surface, the PTFE is uniformly distributed so phenomena such as fretting and galling are minimized. The comparison of a heat-treated electroless nickel and hard chromium layers shows why hardness is not the main factor for low wear. Both layers have an overall hardness of about 1000 Hv (Vickers hardness) 0.1.
The EN coating is abraded more quickly than the chromium coatings, because EN has a higher coefficient of friction (Figure 1). Even though the EN/PTFE coating is much softer (320-400 Hv 0.1), the coefficient of friction is lower than that of hard chromium (about 0.07 0 0.1 for EN/PTFE and 0.12 to 0.25 for hard chromium). Therefore it wears more quickly.
At usual technical pressure ranges there is another effect that also influences the life of the EN/PTFE layers. In Figures 2 and 3 the comparison of samples with about 1.5 mm and smaller than 0.5 mm Rt (roughness measurement). The wavelength of visible light is 500 nm, so this is a mirror-like finish. In the case of the very smooth surface, the EN/PTFE coating is abraded faster. Also, the influence of the wear materials is shown.
|TABLE II—Common Applications for EN/PTFE Coatings|
|Selected Applications Industry||Market and Parts Part||Improvement|
|Air and space||connectors||corrosion, wear|
|Hydraulics (oil/water)||piston||corrosion, wear|
|Robotics||hinges||tribo-ox, wear, corrosion|
|Automotives||piston rings, valves
carburetor, oil pumps
wear, fuel management,
fuel management, energy cons.
|Electronics||switches, magnetic coils||tribo-ox, wear|
|Synthetics||molds, nozzles||anti-adhesive, time, yield|
|Textile||cutting tools, guides||noise, lifetime|
|Pharmaceutics||stamps, molds||anti-adh., yield|
|Food||plates, pots||anti-adh., time|
Because of the fatigue behavior of the coating, PTFE and EN particles are broken out of the surface. In the case of the very smooth surface, the small Ni-P particles can increase the wear on the next layer of the coating, and the PTFE balls are simply wiped away. Against that the PTFE particles remain longer in the area of wear and can be used like ball bearings on the rougher surface. The Ni-P particles are too small to increase the wear on the rough surface.
Comparison to PVD/CVD coatings
(PVD: physical vapor deposition. CVD: chemical vapor deposition.) In comparison to other wear-protective coatings such as PVD and CVD, there are some other advantages of soft dispersion coatings:
- Because the hardness of the deposited layer is similar to that of the substrate (in most cases), there is no need for a hardening process before plating the protective layer. This is necessary in a PVD or CVD system to achieve an appropriate adhesion. Similar to the PVD/CVD processes, the surface has to be purely metallic (no oxides).
- Because of PTFE's softness, the coating can store and dissipate more energy in small volumes before it is plastically deformed.
- The EN/PTFE coating can use the base material as a support structure because of the similar hardness.
- Because of the porosity-free deposition, it is possible to get another degree of freedom for the deposit's lifetime (thickness).
- The corrosion behavior is much better than that of PVD/CVD coatings (EN: 500 hrs at a plating thickness of 12mm).
- The "soft dispersion coatings" cannot increase the wear at the tribological partner in the system.
In most PVD/CVD coatings, corrosion resistance is a problem. To cope with that, some companies apply "sandwich coatings." Usually plating of 10mm of a high-phosphorus EN layer (d=10mm) below the thin ceramic layer can help with the problem (Figure 4).
|TABLE III—New Applications for EN/PTFE Coatings|
Shock absorber axles
Not only is the corrosion behavior of these sandwich layers influenced, but also the hardness and wear (Figure 5). According to some university studies, both parameters are positively increased by the underlying layer.
The data available about wear and corrosion behavior of PVD/CVD coatings are not consistent. This may be because it is possible to obtain sharp or likewise a sinus wave of porous deposits with PVD/CVD under the right conditions. Also, inappropriate pretreatment may cause a big deviation in the results.
Ceramics deposited by physical or chemical vapor deposition present one of the most wear-resistant coatings for abrasive conditions. Galling problems are covered by these ceramics because of the difference of atomic lattice constants in a normal (steel/coating) tribological system. In a corrosive atmosphere, there is again a need to protect the base material against oxidation/corrosion. Otherwise, the hard and brittle layer may fall off the surface because the base material is corroding beneath the coating.
Galling and corrosion problems are covered best by EN/PTFE dispersion layers. This is because of the anti-adhesive and hydrophobic properties of the surface and the thermal conductivity of the coating. While ceramic coatings are electrical and thermal insulators, EN/PTFE coatings have electrical and thermal conductivity.
Different applications for EN/PTFE coatings
Table II shows some of the most common applications for EN/PTFE coatings.
New improvements in the coatings have helped in many industries: drilling equipment for paper; gardening tools; gas meters; and spiral pumps. In automotive industries there are new parts tested to avoid noise and wear or reduce weight and costs (Table III).
Combining electroless nickel deposits with sub-micron PTFE particles has not yet reached its peak. That is due not only to a lack of information, but also to the quality of the solid layers. The consistency was not good enough, and the results differed too much, so that many platers did not dare apply this coating in high-tech applications. Since improving the chemistry and the deposit, most applications can now be approached successfully. Also, the life of the electrolyte convinces some of the electroplating industry customers to use this trend-setting coating. Many problems can be solved where some years ago no one thought this could be done with a soft coating. Only some physical properties have to be changed and many applications could be managed easily.