Coating 101

An increasing number of components in technical systems are subjected to surface loading that leads to the operating limits of most metallic materials being breached.
The use of technologies from the field of industrial surface technology – in this case, mainly coating – makes it possible to refine the surfaces in such a way that the operating limits can be considerably expanded in comparison with their untreated original condition. Furthermore, it is possible to achieve functional integration or additional characteristics. In the selection and use of surface technology, the decisive factor for success is a holistic approach that takes the entire technical system into consideration.

Technical components are used to fulfil particular functions within a higher level machine or installation. The component comprises a particular material and has a corresponding geometry as well as a production history.

The geometry can be further subdivided into the component volume and the component surface. These perform various subfunctions: while the component surface in the form of an active area supports the external loads and transmits these to the interior regions, the actual load-bearing function is performed preferably by the component volume.

The life of technical components is frequently determined not only by the strength but also by fatigue or wear of the surface. Since these phenomena take place on the surface, it is necessary for logical reasons to mainly address this area in order to solve the rating life issue.

The component surface generally represents the area subjected to the heaviest loading. This is where normal and frictional forces or heat flows are introduced. Electric potentials build up here or electric currents are transmitted. In many cases, wear or corrosion on the component surface determine the life of the entire component. In industrial and mobile applications, this surface loading originates essentially from the following categories:

Since the surfaces of components are of enormous significance for different functions. Coatings for the refinement of surfaces therefore can be considered in terms of their different composition and application as design elements.

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For the various types of surface loading, a wide range of different coatings is available.

Surface technology offers a large quantity of coatings as well as process engineering approaches to solutions for the production of the different coatings. The selection of the coating methods to be used is influenced by the material characteristics of the substrates and coatings, the geometry of the components and economic aspects.

In essence, the coating methods that are used for the production of coatings against tribological, corrosive or electrical surface loading can be subdivided into two groups:

1. Methods for the modification and transformation of the surface zone of the substrate
2. Methods for the creation of overlay coatings.

From an industrial perspective, the following coating methods are relevant especially for the production of large quantities in volume processes:

• thermochemical diffusion methods
• conversion methods
• chemical/electrochemical methods
• PVD method (Physical Vapor Deposition) or PACVD method (Plasma Assisted Chemical Vapor Deposition), also known as: PECVD method (Plasma Enhanced Chemical Vapor Deposition)
• thermal spraying
• painting.

The coating materials that can be produced for the aforenamed areas of application by means of the above methods are explained, in greater detail and in conjunction with the applications, see below.

Anti-corrosion coatings

In contact with water or humidity, steels tend to undergo corrosion.
In many cases, corrosion-resistant high grade steels cannot be quenched and tempered to the requisite hardnesses or lose their corrosion-inhibiting properties during the hardening process. In this case, assistance is possible using anti-corrosion coatings, mainly based on zinc and zinc alloys.

Modular coating concept

For the selection of coatings for different problem areas, Schaeffler has developed a modular coating concept.
The modular coating concept shown is intended to give easier selection of suitable coatings.

Schaeffler has developed suitable coatings for various applications.The resulting recommendations are presented below.

Friction reduction and wear protection The quality of a rolling bearing is determined to a significant extent by its smooth running and wear resistance. A low friction coefficient reduces not only energy consumption but also the requirement for lubricant.

This is associated with lower mechanical wear, while the operating life of the bearing increases. The different wear types (abrasive, adhesive, tribochemical) require different measures.

Protection against abrasive wear:

• High surface quality (high hardness, small roughness peaks) necessary.
• Protection of contact partners by particularly hard coatings (hardness in excess of 2 000 HV) that are applied by means of PVD (Physical Vapor Deposition) or PACVD (Plasma Assisted Chemical Vapor Deposition).
• High friction can be reduced by means of friction-reducing coatings such as DLC (Diamond Like Carbon) or PTFE (polytetrafluoroethylene).
• This is also possible using electroplated coatings such as chromium or NiP, since their hardness is greater than that of the base material.
  

Protection against adhesive wear:

• Occurs principally in contact partners with similar bonding characteristics, such as metal/metal.
• In order to prevent this wear (typically: slippage damage), it is sufficient to change the type of bonding close to the surface by the coating of one contact partner.
• Remedy by targeted oxidation of the metal surface by means of black oxide coating.
• Through coating with an amorphous carbon layer, a covalent bonding character can be achieved on the surface.

Protection against wear by means of tribochemistry:

• Solutions similar to those for the prevention of adhesive wear.
• Chemical reactions can be suppressed by means of suitable coatings. For example, phosphating of a surface.
• In order to improve the sliding friction contacts, bearing cages are coated by electroplating means with silver or copper. This also makes it possible to prevent fretting corrosion.

The following table shows Durotect coating systems for wear protection and friction reduction.

The following table shows Triondur coating systems for surfaces subjected to high tribomechanical loading.

Bearing parts with corrosion – as a result of contact with water or humidity – can in the case of standard bearings lead to malfunctions, lower efficiency and premature failure. Corrosion-resistant rolling bearing steels provide a remedy here but are expensive. The most economical variant under moderate corrosion conditions is therefore the combination of a standard rolling bearing steel with an appropriate coating.

The following coatings have proved effective: ■ zinc phosphating with application of oil (for low requirements) ■ extremely thin zinc alloy coatings, applied by electroplating ■ columnar thin dense chromium coating as an anti-corrosion coating resistant to wear and overrolling ■ nickel-phosphorus coatings (deposited by electroless methods) for highly corrosive media such as acids and alkalis.

The following table shows Corrotect coating systems for corrosion protection.

In order to prevent rolling bearing failures as a result of current passage, the cylindrical surfaces and end faces of the bearing rings can be provided with ceramic insulating coatings, see Figure 42.
Current insulation is achieved by means of plasma spray coating of the outside diameter and the lateral faces on the outer ring or the bore and lateral faces of the inner ring. The insulation coating comprises aluminium oxide, in which the pores are sealed with resin to give protection against the ingress of moisture.

The following table shows Insutect coating systems for current insulation.

Advantages of coated bearings:

• High level of insulation, even in a damp environment, due to a special

sealing process

• The external dimensions of the bearing correspond to the dimensions

in accordance with DIN 616 and are thus interchangeable with standard bearings

• The puncture strength of thin layers is up to 500 VDC, while the puncture

strength of thick layers is guaranteed to be at least 1000 VDC.

Take the overview table as a short list to find the best coating for your design.