Aurustussadestatud pinded / Vapour Deposited Coatings /

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Loengukonspekt

Koostas: Andrei Surženkov

Process definition.

 vapour deposition (either physical or chemical) is a coating process, where coating material is condensed in vacuum at the substrate from vapour phase, forming a thin (≤ 10 μm in the case of physical deposition and ≤ 1000 μm in the case of chemical deposition) film. Sometimes the deposited material further reacts with the gaseous substances to form a final compound coating.

Generally as metals, as non-metals can be deposited, certain case depends on the applied metod.

Comparison of the coating processes.

Process features.

Process type

Thermochemical Chemical Electrodeposition Hot-dip Cladding Sol-gel

Material is introduced in

Solid / liquid / gaseous form Solution form Solution form Liquid melt Solid form Solution form  process is carried out in vacuum only;  low productivity (several hours);   high price; Vapor (gaseous) form

only

… but properties of the coatings (hardness, CoF, etc.), which can't be obtained by other methods.

Process application fields.

 electrical devices (LCDs, touch panels, microelectronic devices, solar cells, etc.);  magnetic devices (magnetic storage media);  optics (laser mirrors, antireflection coatings, etc.);  tool industry (plastic molds, cutting tools, blanking tools, etc.);  aerospace devices (turbine blades, etc.);  decorative purposes (watches bands, etc.).

Process limitations.

 as the coatings are very thin, they can't improve bulk properties (tensile strength, fracture toughness, etc.), that is why good mechanical properties of the substrate are needed;  as vapour deposition is mostly a high-temperature process, the ability of the substrate to withstand high temperatures is required;  vapour deposition is a low production process;  vapour deposition equipment is relatively expensive (tens of thousands euros).

Part I. Physical Vapour Deposited (PVD) Coatings

General features.

 absence of chemical reaction during the deposition process;  relatively low temperatures (250 … 600 ºC) in comparison with chemical vapour deposition (CVD) processes;  electric [and magnetic] field is applied;  the variety of PVD technologies can be classified into three types: evaporation, sputtering and arc evaporation.

1. Evaporation technology.

1.1 Thermal evaporation.

 in the simpliest variant the material of the future coating is resistance heated up to the evaporation temperature, and atoms of the material move towards the substrate by a straight trajectory and deposit on it, producing a film.

 in this case, only metals and metal alloys with a low melting points can be applied (for example, Zn, Al, etc.)  such technology is used, for instance, for metallisation of fabrics.

1. Evaporation technology.

1.2 Electron / laser beam evaporation.

 coating material is ablated* by a focused electron or laser beam at a small spot at the target, made of the coating material;  the ablated atoms / atom layers travel through the vacuum chamber and deposit at the substrate, forming the coating;  in contrast with thermal evaporation, all metals and metal alloys, as well as ceramics and polymers, can be applied as coating materials, whereas the temperature of the process can be as low as room temperature;  the main disadvantage of this method is the high price of the equipment (electron beam gun or laser);  this method is applied, for example, for magnetic data storage media production, but for ultra-high rate aluminization of fabrics and transparent barrier coatings, as well.

* Ablation – removal of material by vaporization, bypassing the liquid phase.

Fig. 1. Pulsed laser deposition

2. Sputtering.

2.1 General principles.

 the process finds place in a low vacuum (0.0005 … 0.12 torr ↔ 0.07 … 16 Pa) with a presence of inert gas atoms (usually Ar);  a high voltage (1 … 5 kV) is applied between the target, made of the sputtered material and the substrate to be coated;  due to high voltage, the inert gas atoms ionize, forming a plasma cloud near the surface of the sputtered material;  the ions become accelerated in the electric field between the substrate to be coated and the target of the sputtered material;  sputtered material is bombarded by accelerated inert gas ions from a plasma cloud, which is situated close to its surface;  as a result, atoms and atomic layers are extracted from the sputtered material, which are then directed to the substrate to be coated;  sputtered atoms form a coating.

2. Sputtering.

2.2 Sputtering technologies.

1.

direct current (DC) diode sputtering (direct voltage is applied between 2. the target (cathode) and substrate (anode)); 2. triode sputtering (heated filament is used as a source of secondary electrons to help ignite and sustain the plasma); 1. radio frequency (rf) sputtering (an alternative voltage at a high frequency (~13.6 MHz) is applied between the target and the substrate); 1. magnetron sputtering (an electron trap is formed by magnetic fields, what make electrons, extracted during the ionization and bombardment process, to arrive back at the target). If the sputtered and deposited material further reacts with gases, intentially introduced to the vacuum chamber after the deposition process, the process is called

reactive sputtering

.

2. Sputtering.

2.2 Sputtering technologies.

2.2.1 Magnetron sputtering.

 distinct advantages of magnetron sputtering in comparison with other sputtering technologies are higher available deposition rates (10 times DC diode sputtering) and lower substrate heating;  both DC and AC current sources can be applied for magnetron sputtering (in other words, it can be eiter direct current magnetron sputtering or radio frequency magnetron sputtering);  balanced and unbalanced magnetrons are used for sputtering;  magnetron sputtering is used in, for example, semiconductive industry, tool industry, etc..

Fig. 2. Magnetron sputtering process.

3. Arc evaporation.

3.1 General principle.

 a high-current low-voltage strikes at the surface of the target, made of the material of the future coating;  due to arc striking, a highly energetic emitting area (so-called cathode spot) appears at the surface of the target;  the temperature inside the cathode spot is extremely high (around 15000 ºC), causing the evaporation of the target material with the formation of a highly ionized quasi-plasma cloud;  this quasi-plasma cloud is directed towards the substrate and is deposited onto it, forming a film;  if necessary, a reactive gas is introduced after this, so that gas molecules react with the deposited film, creating a coating.

Fig. 3. Schematic representation of the arc evaporation process.

3. Arc evaporation.

3.2 Technological features.

 in comparison with other PVD technologies, arc evaporation is the most productive one; in addition to that, it casually allows to deposit denser coatings with highest mechanical properties;  also arc evaporation provides the highest target utilization among the PVD technologies;  the main disadvantage of arc evaporation is the formation of macrodroplets during the evaporation process, what leads to higher surface roughness of the coatings (highest among all PVD technologies) and lowers their adhesion to the substrate;  one possible solution to the problem of macrodroplets is seen in magnetic filtering systems, however, their application lowers the deposition range.

4. Comparison of PVD technologies.

Process Deposition rate, μm/s Pressure, Pa Particle energy, eV Adhesion

Evaporation 0.05 … 25 10 -3 < 2 + Sputtering* 0.0001 … 0.7

10 -1 ... 10 10 … 100 Arc evaporation 0.0003 … 1.85

0.5 × 10 -2 … 1.5

× 10 -2 10 … 100 ++ +++ * Unbalanced magnetron sputtering

Part II. Chemical Vapour Deposited (CVD) Coatings

General features.

 chemical reaction is applied to deposit a coating;  precursor is decomposed, and the decomposition products form a coating at the substrate;  relatively high process temperature in comparison with the PVD processes (600 … 1100 ºC);  the three main CVD technologies are thermal CVD, plasma-enhanced CVD (PECVD), and laser CVD (LCVD).

Fig. 4. Example scheme of a CVD process (TiC deposition).

Comparison of CVD processes.

Process name Precursor decomposition Decomposition process Thermal CVD In contact with hot substrate Dissociation Process temperature +++ PECVD LCVD In contact with plasma Dissociation ++ In contact with laser heated substrate Photochemical process / pyrolysis +

III Applications.

Cutting tools.

 TiN, TiAlN, AlTiN, CROMVIc® (Platit AG), ALCRONA (Balzers), etc.

Fig. 5. Comparison of uncoated and coated drills.

Punching and forging tools.

 CrN, TiAlCN, ALCRINVIc® (Platit AG), etc.

Fig. 6. Comparison of uncoated and coated forging tools.

Injection molding.

 TiCN, CrN, nACRo® (Platit AG, etc.) 1 Fig. 8. Comparison of nitrided and coated molds.

Diamond-like carbon (DLC) CVD coating.

 Low-friction hard chemically inert coating, used for coating tools, machine parts and for decorative purposes.

References.

 Coatings Technology Handbook. Ed. By A. A. Traction. Taylor & Francis Group, Boca Raton, 2006.

 Modern Surface Technology. Ed. By F.-W. Bach, A. Laarmann, T. Wenz. Wiley-VCH Verlag GmbH & Co. KgaA, Weinheim,  2006.

Platit Compendium 2012, 49 th edition. Platit AG, 2011.

 C. Donnet, A. Erdemir. Historical Developments and New Trends in Tribological and Solid Lubrication Coatings. Surf. Coat. Tech., 180 – 181, 2004, 76 – 84.

 Loengukonspekt Surface Engineering (1). Vapor Deposition. Koostajad: Priit Kulu, Eron Adoberg.

Smooth deposition!