Coating Characteristics - Wettability

In addition to the chemical reactivity of a surface, the surface tension, or surface energy may be of importance. This is frequently measured as the contact angle between the air and the liquid of a droplet on a surface, which defines the wettability of the surface.

This has been recognized as sometimes being an important factor influencing its tribological characteristics. It influences the behaviour of any liquid lubricants present. It can affect thermal transfer, such as in heat exchanger systems, and increasingly it is being studied in terms of the ability of surfaces to ‘self-clean’.

Although usually simply determined by the contact angle of a droplet of liquid in contact with a solid surface, in practice the wetting angle is a thermodynamic variable which is given by the adhesion energy of the solid/liquid interface minus the liquid surface tension. The wettability is influenced by the surface state, e.g. the roughness, its degree of cleanliness and the degree of oxidation, and it can be assumed that both friction and wear can be affected by changes in the wettability or hydrophobicity of a surface.

Especially, since the surface energy of a material depends on the nature of the medium on either side of the material boundary, it is clear that the wettability of a surface is subject to changes in theoperating environment.

When considering the subject of wettability in tribology, there is a tendency to think of liquid/solid contacts at the non-elevated temperatures experienced by lubricants.

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Coating Characteristics - Composition

There are many methods available which provide chemical composition and other information about coatings. However, simple chemical composition is not a sufficient indication of a coating’s properties to enable its tribological behaviour to be predicted with a satisfactory degree of confidence. This is especially true in the case of ceramic coatings.

Coatings can be composed of grains, i.e. they can be polycrystalline, either random or textured and have a preferred orientation, and they may even be epitaxial.

The appropriate analysis technique for a particular coating will thus depend on its morphological nature, as well as aspects such as its thickness, surface finish and actual composition.

In tribology, increasing attention is being focused on tribochemical reactions, and the choice of surface’s analytical technique will also be determined by its ability to detect and evaluate the reaction products and transfer layers which often control contact behaviour.

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Coating Characteristics - Adhesion

Adhesion is the ability of a coating to remain attached to the substrate under the required operating conditions and this is clearly vital. Many use the words ‘adhesion’ or ‘bond strength’ to describe this ability, but the subject is much more complex than the description of a single strength parameter might suggest. The adhesion between the coating and the substrate should not be mixed up with the adhesion between the coating top surface and the sliding counterface, which is related to adhesive friction and adhesive wear.

Failure of coating adhesion to the substrate results in interfacial de-bonding which for brittle materials is often called cracking. The crack nucleation and propagation to larger surface failure is due to local load levels and directions, resulting stress conditions, strain and deformation. Sometimes a failure that is referred to as adhesive may in fact be a cohesive failure when the cracking occurs in the coating or in the substrate.

Indeed, the complexity is such that it is unlikely that any one measure or test can satisfactorily differentiate between the ability of different coatings to stay bonded to different substrates under different application conditions.

From a scientific point of view, it is preferable to think of adhesion as a fundamental property which can be quantified according to known data about atomic binding forces. Breaking the bonds between the coating and the substrate will produce a lateral crack. 

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Coating Characteristics - Thickness

This, apparently simple property has a criticality which many people have failed to recognize, especially in tri-biological applications. There is a complex interplay between effects such as the intrinsic stress within a coating, the hardness of the coating and the substrate and the adhesion to the substrate which must be optimized, essentially by the appropriate choice of thickness.

It can also be represented by concepts such as ‘mass thickness’, which seeks to recognize that films have varying densities and porosities. Thus simple step-height measurements of thickness could in many applications be meaningless without a quantification of density or porosity.

When dealing with surface treatments, such as nitriding or ion implantation, the definition of thickness or treatment depth is even less clear. Usually, in the former case the depth is taken to be that at which the layer hardness falls to a level one-third above the core hardness of the bulk material.

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Coating Characteristics - The surface

The surfaces of coatings, like the surfaces of bulk materials, have particular attributes which may differ from those of the material within the coating. For example, a chromium coating will generally have an oxide film at the surface. Similarly, as with a bulk material, a coating which has been subjected to a final mechanical treatment, such as polishing, may exhibit the effects of near-surface deformation. It is important to be aware of these localized variations in the nature of coatings, especially since this is where the immediate contact with the counter face occurs.

Effects such as wettability, adsorption and chemisorptions are controlled by the immediate surface, or more correctly the interface between the coating and its counter face – be that solid, liquid or gas. Equally important are the geometrical properties of surfaces and especially their topography. With coatings, the subject takes on a new significance, since the deposition method may sometimes significantly influence the surface topography. Some coating methods, such as electroplating, can smooth the surface. Other methods such as some thermal spraying techniques can lead to a roughening, while others may impart a specific and distinct type of topography, such as the faceted type exhibited by CVD diamond films.

Given this range of topographies it is particularly important to utilize appropriate surface roughness parameters when characterizing coated surfaces. Indeed, for certain types of coatings, such as those used in optical devices, it has been necessary to develop new techniques for the investigation of surface topography, even on an atomic scale, and these are referred to later.

ABOUT SGI

Sheth Group of Industries believes in providing quality services to our customers in right time and cost effective manner. They expertise in all types of surface preparation and thermal coating techniques and also provide shop and/or field services to both industries and large commercial customers in and out of India. SGI since 1980, specialize in all types of coatings and surface preparation.

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