Press releases
Bonded coatings as modern design elements
published in Tribologie + Schmierungstechnik, edition 4 (August) 2000
This article explains the fundamentals of bonded coatings: their effect is described as well as their composition and performance features. The article also provides useful information on the design of components to be coated. .
published in Tribologie + Schmierungstechnik, edition 4 (August) 2000
This article explains the fundamentals of bonded coatings: their effect is described as well as their composition and performance features. The article also provides useful information on the design of components to be coated. .
Bonded coatings as modern design elements
Bonded coatings are making their way into more and more technical applications. While viscous and cohesive lubricants need a certain hydrodynamic speed to build up a "separating effect", a bonded coating remains effective even when the system is standing still. A bonded coating ensures complete separation of the friction surfaces and consequently protects them against wear also under conditions that would normally lead to mixed friction. Other drawbacks of conventional lubrication such as lubricant oxidation, carbonisation, running-off from the lubrication point or other changes in the lubricant itself hardly ever occur when using bonded coatings. Their main advantages are reliability, cleanliness and hygiene.
This article shows the functional principles and the advantages and disadvantages of bonded coatings by comparing them with other dry lubrication systems as well as with oil and grease lubrication. Information is provided on how to use bonded coatings as design elements and their main performance characteristics are presented. Today we have established a proven step-by-step procedure leading from the characterisation of the technical problem to series application, so there is no more reason to regard bonded coatings as "difficult" lubricants. Their many functions, reliable quality and the highly developed application technology available today make bonded coatings preferred products in many fields.
1. Introduction
Bonded coatings are one of the numerous lubrication types classified under dry lubrication (Fig. 1). The main advantages of dry lubrication are generally known. Whenever the use of a lubricating oil or grease can be avoided, this will be the option of choice. Clothes, tools and skin contaminated with oil or grease normally have to be cleaned right away. The use of seals is no safeguard against leakages, which allow the lubricant to run off the lubricating point due to its own gravity and flow characteristics.
Of course, a dry lubricant also must provide effective protection against wear. Whether a bonded coating can meet the requirements on hand has to be decided depending on the operating conditions a component is to be exposed to. The use of bonded coatings is restricted to applications where sliding distances are not too long and the opposing bodies not too rough. This is due to the principle of transfer lubrication: when moving relative to the opposing body, small particles are worn off the bonded coating and transferred to the opposing surface. The quantity worn off very much depends on the bonded coating itself (especially the binder and the solid lubricant used) and the surface roughness of the opposing body.
Provided their capacity limits are observed, bonded coatings offer excellent characteristics for multiple applications. Their most important characteristics are listed in Fig. 2. Especially where several extreme requirements occur simultaneously, bonded coatings offer decisive advantages.
2. Bonded coatings as opposed to other wear protection techniques
2.1 Composition and function of bonded coatings
Bonded coatings are processed similar to other industrial lacquers, but they have different components:
Solid lubricant, e.g. polytetrafluoroethylene (PTFE), molybdenum disulfide (MoS2), graphite or a combination of solid lubricants
Binder (organic or anorganic resin of one or two components)
Solvent (organic or water-miscible)
Further components may be fillers (e.g. colouring agents) or additives (e.g. corrosion inhibitors).
Once it has hardened, the bonded coating forms a thin, dry lubricating layer adhering firmly to the substrate.
When friction sets in, the bonded coating is gradually worn off, which is why we speak of transfer or abrasive lubrication. This is comparable to a pencil which writes while there is still lead left.
This film acts as a separating and lubricating layer reducing friction and wear between the friction bodies in contact.
The service life of a bonded coating is "limited", depending on a large variety of product characteristics, especially the binder's resistance to wear and the coating's elasticity and adhesion to the component surface.
Fig. 3 shows a surface with bonded coating and the same surface after running-in.
2.2 Design notes for the use of bonded coatings
When designing parts that are to be coated, certain aspects should be considered:
No sharp edges.
This applies to both the friction body to be coated and the opposing body. Sharp edges of the opposing body tend to scratch / scrape the coating layer. Sharp edges of the coated parts tend to corrode more quickly due to the thin coating layer above these edges, which reduces service life. Tolerances
Permissible tolerances must be defined according to the layer thickness. Standard layers are between 7 and 15 µm. Component selection
To save costs, only one friction body is normally coated in practice, in most cases the one with the larger friction surface. Surface roughness
The minimum surface roughness of the component to be coated should be Ra @ 1 to 2 µm. The surface can be roughened for example by mechanical or chemical processes. The ideal depth of roughness of the uncoated opposing body is between Ra @ 0.2 to 0.4 µm and should not exceed 0.8 µm. Baking temperature
When selecting the component material, it should be taken into account that many bonded coatings require temperatures between 160 and 250 °C to harden. These temperatures are transferred to the component so they have to be sufficiently heat-resistant. Depending on the temperatures, hardening times are between 15 and 60 minutes.
If the component materials chosen are not temperature-resistant, air-hardening or hygrosetting bonded coatings may be used. Corrosion protection
Corrosion protection can be improved by applying an additional phosphate layer.
2.3 Characteristics of bonded coatings compared with other types of dry lubrication
As opposed to other types of dry lubrication (see Fig. 1), bonded coatings are characterised by a thin layer adhering firmly to the component surface. Wear protection is not the result of extreme hardness, as is the case for example with hard material coating, but by optimising friction coefficients and sliding properties. It is especially in comparison with hard material coatings or other thin coatings applied by PVD or CVD methods that bonded coatings appear very inexpensive. Since the layers are very thin, the shear stresses occurring within the layer are fairly low and can be transferred from the coating across the boundary to the base material without causing problems.
2.4 Characteristics of bonded coatings compared with oil and grease lubrication
The major advantages of bonded coatings and the application limits to be observed are listed in Fig. 4. It should be noted that there is a certain trend towards high-performance lubricants and bonded coatings due to ever more stringent requirements regarding performance, hygiene and environmental protection. Examples are lubricating points inside cars such as in car body seals and adjustable air inlet flaps. As convenience and hygiene are gaining in importance in such applications, oils and greases have to increasingly make way to bonded coatings and dry lubricants.
In fact there are many more applications where bonded coatings are given preference to oils and greases.
3. Evaluating the characteristics of bonded coatings
As is common in tribology and lubrication engineering, tests are required for bonded coatings to evaluate their characteristics. Besides measurements performed on original components, there are also suitable lab test systems. The following tests are frequently used for the evaluation of bonded coatings:
Oscillation friction wear test acc. to DIN E 51834 (friction and wear with different types of contact)
Tannert sliding indicator (friction coefficient and stick-slip)
Disk/pin - sliding friction test rig (friction coefficient, sliding distance)
Resistance to wear (modified Reichert method)
Salt spray test acc. to DIN 50021 (corrosion protection)
Cross-cut adhesion test acc. to DIN 53151 (surface adhesion)
Bending test acc. to DIN 53152 (elasticity)
Coating thickness measurement, e.g. acc. to DIN 50981, 50983, 50984
Resistance to media/chemicals acc. to DIN 53168 B
4. Step-by-step development of a bonded coating solution
4.1 Overview
An overview of activities from the requirements profile down to series application is given in Fig. 5. The steps listed have been optimised during the last few years to ensure efficient cooperation between the customer (design, development, components testing, purchasing), the coating company and the coating manufacturer. This allows technical as well as administrative procedures to be carried out hand in hand.
Close cooperation between customer, coating company and coating manufacturer enable us to meet quality and environmental requirements as well as to develop satisfying innovative solutions.
4.2 Steps in selecting a suitable bonded coating
The steps required in selecting a bonded coating are similar as with other lubricants. The requirements profile defines the necessary performance characteristics. These are obtained in the form of test and measurement results as described under 3. In cases where laboratory results cannot be transferred directly to practical applications, comparisons between the results for different products allow relevant conclusions. General know-how of dry lubricants and the various types of binders comes in useful in this task. This in combination with the knowledge of how the component can be coated at low cost makes for a sound basis for deciding on a certain bonded coating.
Bonded coating selection is easier, of course, if similar components with a comparable requirements profile have already been coated and tested. Such components tests following coating selection and sample coating are obligatory in particular if series application is planned. This is the safest way of determining the suitability of a bonded coating.
4.3 Steps in applying bonded coatings
The performance capabilities of a bonded coating can be fully utilised only if it is applied in the best possible way. Especially a bonded coating that is highly resistant to adhesion and abrasion will separate from the substrate within the first few minutes upon application if the components were not thoroughly degreased prior to application.
There is a variety of well-known application techniques, which are described in detail in the pertinent literature and the manufacturers' documentation. However, the procedures used should be individually and accurately adjusted to the parts geometry and their number, from the very first steps right through to quality assurance. It is therefore advisable to choose coating companies offering optimised coating performance and quality assurance. For components manufacturers wishing to do the coating themselves, the individual steps are shown in Fig. 6. Special attention should be dedicated to the pretreatment of the parts since this has a decisive influence on the coating's performance. Conscientious pretreatment in combination with carefully selected performance characteristics will enable the bonded coating to perform also under extreme operating conditions.
5. Summary
Bonded coatings today are widely accepted as lifetime and running-in lubricants in many fields of engineering. This is confirmed by a wide variety of applications where the use of dry lubricants offers decisive advantages.
If bonded coatings are used with the full knowledge of their special characteristics and the techniques employed from defining the requirements profile down to series application, they are suitable as design elements. Especially because of today's requirements in terms of consistent performance, quality, convenience, cost reduction and environmental protection, the trend towards lubrication by bonded coatings will continue.
Bonded coatings are making their way into more and more technical applications. While viscous and cohesive lubricants need a certain hydrodynamic speed to build up a "separating effect", a bonded coating remains effective even when the system is standing still. A bonded coating ensures complete separation of the friction surfaces and consequently protects them against wear also under conditions that would normally lead to mixed friction. Other drawbacks of conventional lubrication such as lubricant oxidation, carbonisation, running-off from the lubrication point or other changes in the lubricant itself hardly ever occur when using bonded coatings. Their main advantages are reliability, cleanliness and hygiene.
This article shows the functional principles and the advantages and disadvantages of bonded coatings by comparing them with other dry lubrication systems as well as with oil and grease lubrication. Information is provided on how to use bonded coatings as design elements and their main performance characteristics are presented. Today we have established a proven step-by-step procedure leading from the characterisation of the technical problem to series application, so there is no more reason to regard bonded coatings as "difficult" lubricants. Their many functions, reliable quality and the highly developed application technology available today make bonded coatings preferred products in many fields.
1. Introduction
Bonded coatings are one of the numerous lubrication types classified under dry lubrication (Fig. 1). The main advantages of dry lubrication are generally known. Whenever the use of a lubricating oil or grease can be avoided, this will be the option of choice. Clothes, tools and skin contaminated with oil or grease normally have to be cleaned right away. The use of seals is no safeguard against leakages, which allow the lubricant to run off the lubricating point due to its own gravity and flow characteristics.
Of course, a dry lubricant also must provide effective protection against wear. Whether a bonded coating can meet the requirements on hand has to be decided depending on the operating conditions a component is to be exposed to. The use of bonded coatings is restricted to applications where sliding distances are not too long and the opposing bodies not too rough. This is due to the principle of transfer lubrication: when moving relative to the opposing body, small particles are worn off the bonded coating and transferred to the opposing surface. The quantity worn off very much depends on the bonded coating itself (especially the binder and the solid lubricant used) and the surface roughness of the opposing body.
Provided their capacity limits are observed, bonded coatings offer excellent characteristics for multiple applications. Their most important characteristics are listed in Fig. 2. Especially where several extreme requirements occur simultaneously, bonded coatings offer decisive advantages.
2. Bonded coatings as opposed to other wear protection techniques
2.1 Composition and function of bonded coatings
Bonded coatings are processed similar to other industrial lacquers, but they have different components:
Further components may be fillers (e.g. colouring agents) or additives (e.g. corrosion inhibitors).
Once it has hardened, the bonded coating forms a thin, dry lubricating layer adhering firmly to the substrate.
When friction sets in, the bonded coating is gradually worn off, which is why we speak of transfer or abrasive lubrication. This is comparable to a pencil which writes while there is still lead left.
This film acts as a separating and lubricating layer reducing friction and wear between the friction bodies in contact.
The service life of a bonded coating is "limited", depending on a large variety of product characteristics, especially the binder's resistance to wear and the coating's elasticity and adhesion to the component surface.
Fig. 3 shows a surface with bonded coating and the same surface after running-in.
2.2 Design notes for the use of bonded coatings
When designing parts that are to be coated, certain aspects should be considered:
This applies to both the friction body to be coated and the opposing body. Sharp edges of the opposing body tend to scratch / scrape the coating layer. Sharp edges of the coated parts tend to corrode more quickly due to the thin coating layer above these edges, which reduces service life.
Permissible tolerances must be defined according to the layer thickness. Standard layers are between 7 and 15 µm.
To save costs, only one friction body is normally coated in practice, in most cases the one with the larger friction surface.
The minimum surface roughness of the component to be coated should be Ra @ 1 to 2 µm. The surface can be roughened for example by mechanical or chemical processes. The ideal depth of roughness of the uncoated opposing body is between Ra @ 0.2 to 0.4 µm and should not exceed 0.8 µm.
When selecting the component material, it should be taken into account that many bonded coatings require temperatures between 160 and 250 °C to harden. These temperatures are transferred to the component so they have to be sufficiently heat-resistant. Depending on the temperatures, hardening times are between 15 and 60 minutes.
If the component materials chosen are not temperature-resistant, air-hardening or hygrosetting bonded coatings may be used.
Corrosion protection can be improved by applying an additional phosphate layer.
2.3 Characteristics of bonded coatings compared with other types of dry lubrication
As opposed to other types of dry lubrication (see Fig. 1), bonded coatings are characterised by a thin layer adhering firmly to the component surface. Wear protection is not the result of extreme hardness, as is the case for example with hard material coating, but by optimising friction coefficients and sliding properties. It is especially in comparison with hard material coatings or other thin coatings applied by PVD or CVD methods that bonded coatings appear very inexpensive. Since the layers are very thin, the shear stresses occurring within the layer are fairly low and can be transferred from the coating across the boundary to the base material without causing problems.
2.4 Characteristics of bonded coatings compared with oil and grease lubrication
The major advantages of bonded coatings and the application limits to be observed are listed in Fig. 4. It should be noted that there is a certain trend towards high-performance lubricants and bonded coatings due to ever more stringent requirements regarding performance, hygiene and environmental protection. Examples are lubricating points inside cars such as in car body seals and adjustable air inlet flaps. As convenience and hygiene are gaining in importance in such applications, oils and greases have to increasingly make way to bonded coatings and dry lubricants.
In fact there are many more applications where bonded coatings are given preference to oils and greases.
3. Evaluating the characteristics of bonded coatings
As is common in tribology and lubrication engineering, tests are required for bonded coatings to evaluate their characteristics. Besides measurements performed on original components, there are also suitable lab test systems. The following tests are frequently used for the evaluation of bonded coatings:
4. Step-by-step development of a bonded coating solution
4.1 Overview
An overview of activities from the requirements profile down to series application is given in Fig. 5. The steps listed have been optimised during the last few years to ensure efficient cooperation between the customer (design, development, components testing, purchasing), the coating company and the coating manufacturer. This allows technical as well as administrative procedures to be carried out hand in hand.
Close cooperation between customer, coating company and coating manufacturer enable us to meet quality and environmental requirements as well as to develop satisfying innovative solutions.
4.2 Steps in selecting a suitable bonded coating
The steps required in selecting a bonded coating are similar as with other lubricants. The requirements profile defines the necessary performance characteristics. These are obtained in the form of test and measurement results as described under 3. In cases where laboratory results cannot be transferred directly to practical applications, comparisons between the results for different products allow relevant conclusions. General know-how of dry lubricants and the various types of binders comes in useful in this task. This in combination with the knowledge of how the component can be coated at low cost makes for a sound basis for deciding on a certain bonded coating.
Bonded coating selection is easier, of course, if similar components with a comparable requirements profile have already been coated and tested. Such components tests following coating selection and sample coating are obligatory in particular if series application is planned. This is the safest way of determining the suitability of a bonded coating.
4.3 Steps in applying bonded coatings
The performance capabilities of a bonded coating can be fully utilised only if it is applied in the best possible way. Especially a bonded coating that is highly resistant to adhesion and abrasion will separate from the substrate within the first few minutes upon application if the components were not thoroughly degreased prior to application.
There is a variety of well-known application techniques, which are described in detail in the pertinent literature and the manufacturers' documentation. However, the procedures used should be individually and accurately adjusted to the parts geometry and their number, from the very first steps right through to quality assurance. It is therefore advisable to choose coating companies offering optimised coating performance and quality assurance. For components manufacturers wishing to do the coating themselves, the individual steps are shown in Fig. 6. Special attention should be dedicated to the pretreatment of the parts since this has a decisive influence on the coating's performance. Conscientious pretreatment in combination with carefully selected performance characteristics will enable the bonded coating to perform also under extreme operating conditions.
5. Summary
Bonded coatings today are widely accepted as lifetime and running-in lubricants in many fields of engineering. This is confirmed by a wide variety of applications where the use of dry lubricants offers decisive advantages.
If bonded coatings are used with the full knowledge of their special characteristics and the techniques employed from defining the requirements profile down to series application, they are suitable as design elements. Especially because of today's requirements in terms of consistent performance, quality, convenience, cost reduction and environmental protection, the trend towards lubrication by bonded coatings will continue.
