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Modern surface treatment technologies / Kaasaegsed pinnatöötlustehnoloogiad / Современные технологии обработкиповерхности

Loengukonspekt

Koostas: Andrei Surženkov

Microarc oxidation (MAO)

also known as plasma electrolytic oxidation (PEO);

the principle of the process is similar to anodizing, but in the case of MAO higher voltages (starting from 200 V) are utilized;

the metal parts, playing the role of anode, are inserted into electrolyte bath, and the voltage is applied between them and a stainless steel cathode;

relatively high voltage leads to the electrical breakdown near the surface of the metal parts; as a result, plasma forms;

plasma improves the oxidation conditions, providing harder and tougher oxide layers in comparison with the 'conventional' anodizing;

MAO is applied to aluminium, magnesium and titanium alloys.

Fig. 1. Microarc oxidation process.

Laser alloying.

laser alloying is a surface alloying technology, which consists of two major phases:

- carrying of the alloying element onto the substrate's surface; the alloying element can be in the form of a dry powder or in the form of a powder suspension (water based or organic binder based);the alloying material can be carried by hand-brushing, gas jet, etc.

- after creating of a layer of the alloying element at the substrate surface, it is melted by laser beam in the way that the carried layer is melted through together with the substrate; as the result, the intermixing of the substrate material and alloying material takes place in a thin surface layer, what further solidifies; thus the alloyed layer forms at the surface of the substrate;

only metal alloys and metal-ceramic composites can be applied as the substrates for laser alloying.

Fig. 2. Principle scheme of laser alloying.

Laser alloying. Advantages

higher usage efficiency of alloying elements;

no or minimal subsequent mechanical treatment necessary;

no subsequent thermal treatment necessary;

surface geometry unimportant;

high productivity;

high quality of the alloyed layer;

high process controllability.

Atomic layer deposition (ALD)

in principle, ALD is a form of a CVD process, where coating is deposited in two stages: firstly molecules of the first gaseous precursor adhere to the substrate's surface, secondly the molecules of the second precursor react with the adhered molecules of the first precursor, forming a coating (in a 'conventional' CVD process, the precursors react and deposit onto the substrate simultaneously);

in the case of ALD, coating is deposited by atomic layers; after each layer deposition the chamber, where the process takes place, is purged to remove the unreacted precursors and reaction by-products;

ALD is a self-terminating process (as soon as the whole surface of the substrate is covered by adhered molecules of the first precursor or all the adhered molecules have reacted with the molecules of the second precursor, the process stops automatically).

ALD. Advantages relatively inexpensive method;

dense and pinhole-free coatings; accurate and simple thickness control;

large area uniformity;

doping of coatings easily achieved;

excellent conformality;

low deposition temperature (~ 400 ºC);

gentle deposition process (for sensitive substrates);

low level of internal stresses in the coating;

amorphous or crystalline, oxide, nitride, metal, semiconductor, etc. coatings can be deposited;

the main disadvantage of the ALD technology is its slowness.

Fig. 4. Principle scheme of the ALD process.

Fig. 5. Comparison of different coating processes.

Method ALD CVD Sputte-ring

Evapo-ration

PLD

thickness uniformity +++ +++ +++ ++ ++

film density +++ +++ +++ + +++

step coverage +++ varies + + +

interface quality +++ varies + +++ varies

number of materials ++ + +++ ++ +++

low temp. deposition +++ varies +++ + +++

deposition rate ++ +++ +++ +++ +++

industrial applicability +++ +++ +++ +++ +

Comparison of vapour deposition methods.

+ - poor, ++ - fair, +++ - good

Ion implantation (II) ion implantation is the process of introduction of dopant atoms into the surface of a solid body by bombarding it with ions having the energy in the electron volt – megaelectron volt energy range;

ions are accelerated by the high voltages, applied between the cathode (implanted substrate) and the anode;

implanted ions are typically accelerated by the voltages from 10 kV to 100 kV;

the typical ion doses are in the range 1014 … 1017 ions/cm2;

the implantation depth does not exceed a few hundreds of nm, however, the ion implantation induced effects have a much larger extent (up to several μm);

mechanical, electrical, optical, magnetic, superconducting, etc. properties can be altered by ion implantation;

all existing materials (metals, polymers, ceramics, composites) can be implanted;

ion implantation is used in tooling industry, semiconductor industry, chemical industry, machinery, etc.

Conventional ion implantation

solid targets are used as the ion sources;

ions are accelerated inside the electrical field, applied between the target, made of implanted material (anode), and the substrate (cathode);

line-of-sight process (implanted ions move by a straight line);

the process temperature does not usually exceed 200 ºC;

causes practically no distortion of materials.

Fig. 6. Schematic representation of the conventional ion implantation process.

Plasma immersion ion implantation (PIII, PI3), plasma source ion implantation (PSII).

plasmas are used as the ion sources (NB! Both non-metallic and metallic!);

substrate is surrounded by glow discharge plasma, implanted ions are accelerated inside the plasma and implanted into the substrate's surface;

both PIII and PSII are 3D technologies (implanted ions, being in the plasma phase, have no distinct directions of movement);

the difference between PIII and PSII is the way of generation of plasma: in the case of the PIII, plasma is generated by a radio frequency glow discharge, in the case of the PSII, plasma is generated by high-energy electrodes, emitted from a biased filament and confined by permanet magnetic field;

PIII and PSII are usually carried out at higher temperatures and lower voltages than conventional ion implantation.

Fig. 7. Scheme of the PIII process.

Ion beam mixing

a two-stage process: at the first stage, a thin film of coating material (~ 50 nm) is deposited onto the substrate (by magnetron sputtering or a similar method), at the first stage this thin film is bombarded by highly energetic neutral ions (for example, Ar+);

the energy of the implanted ions allows them to penetrate through the film and activate the atoms of the substrate material;

activated atoms of both the deposited film and the substrate mix together in the result of atomic collisions;

ion beam mixing allows to improve the wear and corrosion resistance of the substrate.

Ion beam assisted deposition (IBAD)

during the IBAD process, the deposition of a coating material (by magnetron sputtering or a similar technique) and irradiation with ions take place simultaneously;

the ions, applied for irradiation, can be both neutral (Ar) and reactive (N, O, etc.); In the last case the process is called reactive ion beam assisted deposition (RIBAD);

the application of irradiation with ions improves the adhesion of the coating and its density, as well as surface roughness.

Fig. 8. Scheme of the IBAD process.

References

1. Лазерная и электронно-лучевая обработка материалов. Справочник под ред. Н.Н. Рыкалина, А.А. Углова, И.В Зуева, А.Н. Кокоры. Москва, “Машиностроение”, 1985.2. J.P. Celis, D. Drees, M.Z. Huq, P.Q. Wu, M. De Bonte. Hybrid processes – a versatile technique to match process requirements and coating needs. Surf. Coat. Tech., 113, 1999, 165 – 181.3. S.M. Allameh, N. Yao, W.O. Soboyejo. Synthesis of self-assembled nanoscale structures by focused ion beam induced deposition. Scripta Materialia, 50, 2004, 915 – 919.4. M. Nastasi, J.W. Mayer. Ion Implantation and Synthesis of Materials. Springer, 2006.5. Priit Kulu loengukonspekt.6. Taivo Jõgiaasa loengukonspekt.