Improving your friction control coatings

The extensive use of topcoats for fasteners began in the 1980’s. The primary need was to enhance corrosion protection and were generically known as leach and seal, due to their ability to turn yellow passivates to an almost silver colour. An improvement to leach and seal was to incorporate dry film lubricants in the coating to lower the coefficient of friction (CoF) of a fastener. With the introduction of high performance trivalent chromium passivates in 2000, the technology evolved into processes applied at room temperature. These were more sympathetic to the underlying passivates, giving high quality black finishes. One major global automotive OEM followed this evolution, moving from hexavalent passivates with leach and seal, to trivalent passivates with a thin film topcoat and lubricant combination.

Leach and Seal

Arguably the best known leach and seal process is the MacDermid JS500 system. Used alone, it reduces the CoF range of pure zinc from >0.4 to 0.22 +/- 0.08. Combined with an integral lubricant it reduces the CoF to 0.12 (+/- 0.02). This integrated process (known as JS600) provided the required protection, improvement and lubrication for the majority of their fasteners.

Change instigates higher performance requirements

Around the year 2000, the ELV directive drove many automotive companies to upgrade their existing plated fastener finishes requirements. Typically this was the new specification:

  • Higher corrosion resistance
  • Compatibility with trivalent passivation
  • CoF 0.15 with a deviation of +/- 0.02
  • New CoF requirements for different fastener innovation
  • Identification with an integral UV tracer

The answer was a new breed of topcoats. These mixed inorganic and organic compounds gave thin topcoats which adhered to and respected the underlying passivate, gave significant improvements in neutral salt spray (corrosion) resistance and a very predictable CoF of the desired 0.15. As the topcoat is so thin and transparent, its application can be verified by the presence of tracers, which maybe seen under a UV lamp. Let us review how these new topcoats achieve these performance enhancements.

Higher corrosion resistance

Three effects are taking place to increase the overall protection: (i) water resistance (ii) corrosion inhibition (iii) adhesion to the passivate layer. The first line of defence is that the coating performs as a barrier layer. The topcoat prevents water reaching the surface by providing a strong hydrophobic layer. The homogeneity of the coating also prevents premature swelling of the coating by water absorption. The second defence is the presence of corrosion inhibitors throughout the coating. These help to seal the coating in any areas where minute discontinuities in the film might occur. Thirdly the adhesion to the passivate layer is so strong that any ‘undercutting’ of the film is prevented. This is particularly important on sharp profiles (where the coating will typically be thinner).

Compatibility with trivalent chromium passivates

The original leach and seal coatings were designed around hexavalent chromium passivates. The leach process ensured exceptional adhesion by combining the passivate with topcoat layer. Trivalent passivates are homogenous layers and not so easy to leach. So the new products had to adhere to a smooth and pore / crack free coating. They also had to be compatible with various types including thin film (typically blue) and thick film (iridescent or black) passivates.

Narrow range coefficient of friction

All fasteners have a designed maximum proof load. Creating the correct torque-tension relationship achieves maximum joint security without exceeding the proof load of the fastener. Zinc and zinc alloys have a relatively high and variable coefficient of friction. This can adversely affect the torque-tension properties of fasteners. Additionally passivates offer different levels of CoF. For example, it was noted that hexavalent passivates have an average CoF of 0.4, whereas a high build trivalent could be as high as 0.5.

If fasteners are used without a friction control fluid, the increase in friction results in lower bolt tension for a given torque, resulting in a joint weakness, which leads to poor clamping, insecure joints and, possibly premature bolt fatigue failure. Conversely too much can lead to bolt fracture and thread stripping. This factor becomes even more crucial in safety critical applications, such as wheels, seat belts, steering and suspension component system.

Torque n Tension for zinc nickel coatings

Torque n Tension for zinc nickel coatings

Therefore lubricated topcoats provide both a lower friction coating than simply metal to metal joints; and also makes the relationship more predictable, avoiding too low or too high clamping forces. Returning to our OEM, they changed to the newer topcoats in order to consistently achieve this predictable surface CoF. Another consideration was to ensure that all applicators, across an increasingly global supply chain, conformed to the same standard. Incorporation of the UV tracer permits verification that the right topcoat has been applied.

New range coefficient of friction requirements

As new fastener technology is introduced, new CoF ranges are demanded, whilst still maintaining the corrosion resistance and compatibility with trivalent passivates. The most current dry film lubricant systems can be tailored to meet these new CoF demands, while still returning low variability.

Summary

Dry film lubricants have evolved from leach and seal processes, designed primarily for hexavalent chromium passivates, to non leach systems compatible with trivalent chromium passivates. Coupled with outstanding corrosion resistance, the non-leach technology delivers exceptionally predictable torque-tension relationships without interfering with the dimensional tolerances. Additionally they can be modified to meet newer demands for friction ranges. This technology allowed a major global automotive OEM to improve the effectiveness of the fastener assembly operations by consistently returning desired corrosion and coefficient of friction on zinc and zinc alloy plate and trivalent passivation systems.

http://www.industrial.macdermid.com

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Streamlining automotive coating requirements

The number of coatings available to automotive fastener engineers keeps growing. Drivers such as increased performance, friction properties, robustness in use and environmental compatibility have led to more choice; but is more choice always a good thing? How does an engineer make that choice? Does a vehicle need so many different fastener coatings? Perhaps the opposite is now more preferable, less choice and one coating which meets all the needs!

Zinc-Nickel, the coating which is gaining new advocates

High alloy (15% Ni) zinc-nickel’s popularity has grown to such an extent that an engineer will now consider it directly alongside contemporary finishes such as organic (paint) and (pure) zinc electroplate coatings. Whereas these two coatings have limitations when assessed against the primary drivers, zinc-nickel meets all the requirements.

Electroplate, long considered as the bulk sacrificial coating of choice, has a real advantage in delivering a coating which respects even the finest dimensional tolerances of fasteners. However pure zinc electroplate does not meet the higher corrosion (neutral salt spray) requirements of many modern automotive specifications.

On the other hand painted coatings, which meet the tougher neutral salt spray demands, are not usually employed when coating the smaller bolt or screw sizes due to the risk of head and thread fill.

So we come back to zinc-nickel. With a neutral salt spray resistance in excess of 1000 hours, it competes with any other commercially available mass fastener coating. Being electroplated it respects even the finest dimensional tolerance, has a high hardness (450HV) and has exceptional adhesion. The growing use of self threading fasteners means that a coating which retains adhesion during assembly process is critical. Unlike other coatings, zinc-nickel won’t gall or lift during the tapping and tightening process. Additionally it can be specified in both black or silver (without the need for a heavy painted topcoat).

Application of a thin film dry film lubricant provides friction properties which can be tailored to the OEM specification demands. This dry film (often referred to as a topcoat) also provides abrasion resistance. Good abrasion resistance is required to maintain the coating properties during vehicle assembly, where processes such as vibratory automated line feeds subject fasteners to high abrasion.  Finally being free of hexavalent chromium compounds these coatings meet ELV and RoHS directives.

 Summary

In summary some engineers are looking at zinc-nickel as being an effective way to reduce the number of finishes required to achieve high fastener service life performance and reduce coating variability by streamlining their fastener supply chain.

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