The coating in general is termed to as an interface between substrate and environment, which is considered to have functional, decorative or both purpose .Functional role of it includes improving surface properties of substrate such as appearance, adhesion, wetability corrosion resistance, wear resistance and scratch resistance. In some cases particularly in printing process and semiconductor device (where substrate is a wafer) the coating forms essential part of finished products.
Nan coatings are coatings produced usage of some components at nanoscale to obtain desired properties. It can be categorized as nanocrystalline, multilayer coatings with individual layer thickness of nanometers, and nanocomposites. Nanostructure coatings offer great potential for various applications due to their superior characteristics that are not typically found in conventional coatings.
DIFFRENCE BETWEEN CONVENTIONAL AND NANOCOATINGS
Micron scale structure
Nano scale stucture
High contact tension between water drop of and coating layer
Contact tension reduced (water repellence)
Moisture can penetrate housing
Moisture penetration is minimized
Surface roughness is 5 ?m due to larger particle size
Surface roughness reduce to 1 nm for better dirt repellence
Physical, chemical, mechanical and thermal properties
Improved the properties of conventional coating
NANOCOATING: TECHNIQUES OF PRODUCTION
Many synthesis techniques for production of nanostructure coatings have been developed
Such as sputtering, laser ablation, sol/gel technique, chemical vapour deposition, gas-condensation, plasma spraying and electrode deposition.
1. Chemical vapour deposition involves chemical reaction of input materials in the gas phase and deposition of product on the surface. The process called physical vapour deposition involves transforming the materials into gaseous phase and deposition on surface
2. The impact of an atom or ion on a surface produces sputtering from the surface, unlike many other vapour phase techniques there melting of the material. Sputtering is done at low pressure on cold substrate
3. In laser aberration, pulsed light from an exciter laser is focused onto a solid target in vacuum to boil off a plum of energetic atom. A substrate will receive a thin film of the target material. these is used for Nano layered coating for cutting tools results in production of super hard wear resistant
4. The sol-gel process is well adapted for ceramics and composites at room temperature.
5. The electrochemical deposition is the major process employed in production compared to other convectional means. it is due to rapidity, low cost, high Purity, production of free-standing parts with complex shapes, higher deposition rates, the Production of coatings on widely differing substrates. In addition, ability to produce structural Features with sizes ranging from nanometre to micrometre, and ability to produce compositions unattainable by other techniques. This method also provides for cost-effective production of freestanding forms such as ultrathin foil, wire, sheet, and plate, as well as complex shapes.
CLASSIFICATION OF COATING PROPERTIES
In general there are two classification
2.Self assempled nanophase coating
The term ‘functional coatings’ describes systems which represent other than the classical properties of a coating (decoration and protection). Functional coating come up with additional functionality. This functionality depend upon the actual application of a coated substrate. Examples of fuctional coating includes
• Easy-to clean (anti-graffiti)
• Soft feel
With which desired properties such as Durability, Reproducibility, Easy application, cost effectiveness, tailored surface morphology and Environmental friendliness needs to be attained.
Functional coatings perform by means of physical, chemical, mechanical and thermal properties. Chemically active functional coatings perform their activities either at:
a. Film–substrate interfaces (anticorrosive coatings),
b. In the bulk of the film (fire-retardant or intumescing coatings)
c. Air–film interfaces (antibacterial, self-cleaning)
GENERAL TYPES OF FUNCTIONAL COATINGS
A. ANTICORROSIVE COATINGS :-
Organic coatings are applied onto metallic substrates in order to avoid the detrimental effect of corrosion. The anticorrosive performance of the coating depends upon several parameters, including: adhesion to metal, thickness, permeability, and the different properties of the coating. In most cases, the primer is mainly responsible for protecting the metallic substrate and adhering to other coating layers. In this context, surface preparation is essential in order to provide good adhesion of the primer to the metallic substrate. Mechanism by which this coatings provides corrosion protection includes
I SACRIFICIAL PROTECTION
In the case of sacrificial protection highly electronegative element is used as anode sacrificing itself protecting cathode . The use of a sacrificial anode such as zinc to protect steel is a long standing and well-known industrial practice. The zinc layer on galvanized steel degrades when exposed to an adverse environment, and this protects the underneath surface. Using a similar approach, both inorganic and organic resin based, zinc-rich coatings have been developed to protect a variety of metal substrates.
Fig1: sacrificial protection illustration
II BARIER PROTECTION
In the case of barier protection inert elements are coated on the surface of subsrate to ensure corrosion resistance.for instance
(i) Polymeric coatings are applied to metallic substrates to provide a barrier against corrosive species. They are not purely impermeable. Moreover, defects or damages in the coating layer provide pathways by which the corrosive species may reach the metal surface, whereupon localized corrosion can occur.
(ii) Pigments having lamellar or plate-like shapes (e.g., micaceous iron oxide and aluminum flakes) are introduced to polymeric coatings; this not only increases the length of the diffusion paths for the corrosive species but also decreases the corrosion.
(iii) The orientation of the pigments in the coating must be parallel to the surface, and they should be highly compatible with the matrix resin to provide a good barrier effect.
(Iv) Layered clay platelets such as montmorillnite may also be introduced into organic resin system to increase the barrier effect towards oxygen and water molecules, thereby enhancing the anticorrosive performance of the coating.
III IMBIBITION PROTECTION
Primers containing metallic phosphate, silicate, titanate or molybdate compounds are available as compounds used as corrosion inhibitors to formulate anticorrosive primers for metallic substrate.These pigments form a protective oxide layer on the metallic substrates, and often also form anticorrosive complexes with the binder.
Fig2: Inhibitors forming a protective water repellent film
B. HIGH- THERMAL AND FIRE RESISTANT COATING :
These type of coating with nanoparticles provides high temperatue and fire resistance due to bulk property of material
i. High thermal-resistant coatings are required for a wide variety of metallic substrates, including nonstick cookware, barbecues and boilers.
ii .Fluorine or silicon-based products are used for the products. Fluorinated coatings are not suitable for high-temperature applications as they degrade above ~300 ºC and produce toxic by products. Silicon-containing polymers offer better thermal resistance due to the high energy required to cleave silicon bonds compared to carbon bonds in analogous molecules.
iii .Phosphorus containing compounds function by forming a protective layer as a glassy surface barrier.
iv. Expandable graphites also used as fire retardant; these contain chemical compounds, including an acid, entrapped between the carbon layers. Upon exposure to higher temperatures, exfoliation of the graphite takes place and this provides an insulating layer to the substrate.
V. Intumescing Coatings a classification of high thermal resistant form an expanded carbonaceous layer which acts as a protective barrier against heat transfer and hinders the diffusion of combustible gases and melted polymer to the site of combustion.
FIG 3 SEM micrograph of intumescing char obtained on an organic coating
C. SCRATCH & ABRASION RESISTANT COATINGS:
Consumer prefers to retain the aesthetic appearance of coated materials and for this reason clear coats used on automobiles must have good scratch and abrasion resistance. Scratch resistance can be obtained by incorporating a greater number of cross links in the coating’s binder but highly cross linked (hard) films have poor impact resistance due to less flexibility. A less-cross linked (softer) film will show better performance with regard to other properties