Film hardness: Enhancing durability & protection in thin paints & coatings

Film hardness can be defined as the characteristic of a paint film that sustains and resists impact or mechanical stress like:
 

• scratching,
• impacting, or
• even penetration by a hard object.

In other words, it is the resistance of a film to damage or deformation. For a coating on a hard substrate, the hardness or resistance to deformation will increase with a decrease in film thickness. During the process, all parameters like curing time, application conditions, and other relatable parameters are kept constant. This means that the systems, namely substrate and coating are more damage resistant. Thus, the hardness is better at thin film thickness as compared to thicker ones.

 

Classification of thin film coatings
 

1. Amorphous thin films
2. Polycrystalline thin films
3. Epitaxial thin films
   
    

Key benefits of hard films
 

• It improves durability. It also protects the substrate and itself. It does so by protecting it from wear and other mechanical stresses.
• It offers excellent barrier properties against external factors. These factors include moisture, chemicals, temperature, dust/dirt, etc.
• It can add other functional performance properties. This depends on the thickness of the film. These properties include electrical, optical, cracking, and corrosion resistance.

Generally, all the thin film coating processes have three steps:
 

• Coating application,
• Solvent evaporation, and
• Hardening or curing of the film.

Based on the nature of coating and substrate, various processes are widely used for thin film coating. Some of the highly commercial thin-coating application processes are as follows:

 

I. Roll coating for enhanced coating uniformity
 

The main benefit of surface coating using rolls (calendaring) is the effective coating material utilization and uniformity. A great degree of film thickness control is also a great advantage of this coating. It is applied in various industries like:
 

• coil coating,
• paper, paperboard, plastic coatings,
• cellulose thin films,
• metal foils, etc.

The coating accuracy and control can be increased or decreased by the addition or reduction of rollers.


 

II. Dip coating: An affordable method for thin films
 

Dip coating is one of the easiest and most affordable thin film coating processes. Here the substrate is submerged in a solution of coating material. Various factors influence the degree and effectiveness of the dip-coatings. These include:
 

• immersion duration,
• surface charges on substrates,
• number of dip cycles,
• coating drying mechanism, etc.

Dip coating finds major application areas in optical coatings.

 

III. Spin coating: An accurate process for thin films

It is the most accurate process for thin and highly uniform coating films. The coating thickness range can be from a few nanometers (nm) to microns (μm). These methods are applicable for small-size substrates ranging from a few millimeters to meters. A specific amount of coating material is placed on the substrate. It is uniformly distributed on the surface by a high-speed spinning disk. Later the solvents evaporate and a thin hard film is attained.
 

Spin Coating Process for Thin Film Coating

IV. Blade/knife coating: A two-stage technique for continuous sheets

This coating technique is used for the coating of flat and usually continuous sheet material. For e.g., paper or plastic film. Initially, the excess coating is applied by any suitable technique, for e.g., roller coating. Later in the second stage a blade or knife is used to control the coating thickness by passing the web. The viscosity of such coatings is generally high.
 

Blade Coating Process for Thin Film Coating

V. Optimizing spray coatings for effective surface coverage

Spray coatings are effective in covering surfaces. The type and nature of the spray and coating must be optimized to get the best-coated film. Their major application industry is fabric, automobile, and industrial.
 

Spray Coating Process for Thin Film Coating

VI. Commercial vapor phase coatings

Common vapor deposition techniques that are used commercially are:
 

• Chemical vapor deposition (CVD) — It is a deposition method where a volatile compound of a pre-established substance is introduced into a reactor. This is done usually with an inert gas. This induces a chemical reaction. It leads to the production of a solid thin film onto a substrate at an elevated temperature.

• Physical vapor deposition (PVD) — It essentially consists of removing growth species from a source or target material. This happens via evaporation. This vapor is then transported to the surface of the substrate. Eventually, it solidifies on the surface, forming the film. The evaporation is generally carried out under a vacuum to avoid impurities in the film formation. These are produced due to collisions between vapor particles and residual gas particles.

Apart from the above coating methods, there are some less commercial thin coating application processes. They are as follows:
 

• Sputtering- radio frequency (RF) or magnetron sputtering
• Flame hydrolysis deposition (FHD)
• Atomic layer deposition (ALD)

a) Optimum cure of thin film coating or paint

Optimum curing of the film includes various factors like:
 

• drying time,
• rate of evaporation of solvents, and
• optimum film thickness.

Optimization of all these factors results in excellent film hardness.

 

b) Application process and conditions

This involves choosing the right film coating process. It allows uniform film thickness and conditions. For e.g., drying, humidity, temperature, etc.

 

c) Nature of substrate
 

The nature of substrate relates to factors like:
 

• substrate surface morphology,
• surface energy,
• coating compatibility, and
• thermal expansion of coating material.

If the substrate is not balanced, it leads to inadequate adhesion. Finally, improper and inadequate substrate preparation may also hamper film hardness.

 

d) Composition or recipe of coating

The recipe plays a critical role in film hardness. The desired film hardness of the coating is a result of the following factors:
 

• polymer selection,
• presence of cross-linkers,
• presence of nano-emulsions, or nano-materials like silica, TiO₂, or
• other hardness rendering components

Some additives can negatively affect the hardness of paint film. This is a highly undesirable property change. It can give an adverse effect on any kind of coating or paint. The factors that reduce the film hardness include:
 

• Excess use of solvents and co-solvents
• Use of low Tg polymer emulsions
• Excess dosage of wetting and dispersing agent
• High particle size waxes

To increase the film hardness, the formulator must make the right additive selection. They can do this by adjusting the formulation recipe and application process.

 

Formulation and recipe control
 

This is the most important strategy to increase the film hardness of a coating. Adding a controlled mixture of additives to your formulation recipe is crucial. The key criteria include:
 

• Selection of high Tg (Glass Transition Temperature) emulsion or polymer.
   
• Functional silanes, siloxanes, polyurethane dispersions, and fluorosurfactants (CF) as adhesion promoters.
   
• Modified micronized waxes and selection and optimization of coalescent.
   
• Selection of hard extenders or filler particles or functional ceramic microspheres. Fillers provide high Mohs hardness. Some examples include quartz, silica, nepheline syenite, and dolomite.
   
• Selection of catalysts or accelerators in 2-pack systems like epoxies and polyurethane paints. They increase reaction rates of 2-part epoxy resins.
   
• The use of crosslinkers like polyfunctional aziridine can dramatically increase hardness.
   
• The incorporation of metal salts or driers accelerates the drying process. This in turn increases the hardness of alkyd systems. For example, metals like cobalt or manganese.
   
• Adding functional glycol ethers also increases the hardness and block resistance of coatings.
   
• Silicones polymers such as epoxy, hydroxyl, or primary amine for 2-pack systems.
   

Application process and conditions
 

Several application processes and conditions that impact film hardness are as follows:
 

• The application techniques resulting in a uniform film thickness and paint delivery will give better film properties. For e.g., higher hardness, better blocking, and uniformity.
• The temperature and humidity during the application.
• The substrate preparation and substrate selection according to the coating.

After application, the paint must be allowed to cure at favorable conditions. For e.g., there should be no dirt and dust in the environment. The curing should be done at a suitable temperature and commodity as well. Curing a paint film is a very critical process. This helps in attaining the maximum hardness and functional properties of the film.

 

Test methods to evaluate film hardness
 

• ASTM D3364 and ISO 15184 — These are the internationally accepted test methods for film hardness. These are convenient, inexpensive, and quite accurate test methods. Here many grades of pencils from soft to hard, are used to evaluate the film hardness. Ratings are given in the form of pencil ratings.
   

• ASTM D4060 and ISO 7784 — These methods can also be used to evaluate the wear or abrasion resistance of paint film. Here the test results are expressed in the milligrams (mg) lost on abrasion per 1000 cycles.
   

Analytical methods to evaluate film hardness
 

• Atomic force microscopy (AFM) — This instrument is used to test advanced thin film coating. AFM enables the imaging of almost any type of surface. This includes polymers, ceramics, composites, glass, and biological samples. AFM is used to measure and localize many different forces. These forces can be adhesion strength, magnetic forces, and mechanical properties. This includes surface tensile strength, hardness, or elasticity.
   

Atomic Force Microscopy Principle

• Shore hardness scales — The shore hardness scale is also known as the durometer gauge. It measures the flexural strength of a coating. This is in terms of flexibility (softness) and stiffness (hardness).
   

Shore Hardness Scale for Coatings and Materials