The following points highlight the four main techniques involved in the modification of surface in coating of materials. The techniques are: 1. Ion-Beam Based Techniques for Surface Modification 2. Thin-Film Vapour Phase Deposition Techniques 3. Plasma Treatments 4. Laser Treatment for Surface Modification by Surface Alloying.
1. Ion-Beam Based Techniques for Surface Modification:
The useful life of many materials is determined as to how their surfaces react with environment. Thus surface finish and treatment deserve serious attention. Surface characteristics can be modified by ion beam-based techniques at low temperatures without altering tolerances.
In ion-beam based techniques, high energy ions provide energy (not heat) for reactions to occur and thus form new compounds or alloys. Ion implantation (shallow penetration of ions to modify surface) and ion-assisted coatings are used on non-electronic materials to improve surface properties.
Ion implantation process is carried out in a vacuum (10-6 torr) in an ion implanter which contains an ion source (ions created from a plasma) and a chamber in which items to be treated are housed. The ions are fired as a beam from source to the object to be treated.
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Ions being in the form a beam, the process are line-of-sight. When an ion enters the surface, it collides with the atoms of the solid and keeps moving in, blazing a trail ahead, behind, and to the side of its path as it knocks atoms away.
The displaced atoms collide with several others for about 10-11 sec. This is known as a collision cascade and often visualised as very localised spike of thermal energy which causes very fast heat and quench of surface in a region of 0.1 micron long and 0.02 micron diameter. The effects of this rapid heat-quench cycle together with the new atoms lodged in the original material give the ion implantation process some of its unique properties.
Since ions become integral part of the material, the implanted layer can’t flake or peel off. Thus implanted ions can combine with atoms of the solid or with each other, to form conventional alloys or compounds. Implated material is buried beneath surface and can’t come off.
There is no measurable change in surface dimensions after implantation. Controllable addition of desired ions for desired surface characteristics is possible. Use of multi-energy implants can offer tailored depth profile.
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Three types of ion implanters in use are:
i. Mass analysed implanters,
ii. Nitrogen implanters,
iii. Plasma source ion implanters.
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In ion-assisted coating process, ion beam is used for chemical vapour deposition. Ion beam can also be used to mix the coating into the substrate to improve coating adhesion. Ion beam being easier to define, control, and use, it is more readily acceptable to different substrate material and geometries than plasma.
Ion-assisted coatings are generally used for high-precision surface treatments. Because of their adaptability, reproducibility, and low temperatures, they offer a relatively rapid and controllable method of developing new coatings or treating new types of materials.
2. Thin-Film Vapour Phase Deposition Techniques:
Surface properties of substrates may be modified by a wide variety of thin film vapour phase deposition techniques. Fig. 8.2 shows in schematic form the process of vapour phase deposition system.
This system basically consists of effective generator of vapour flux, effective capture of vapour flux, effective establishment of surface conditions to control film growth from the vapour flux, and effective establishment of surface condition to control film growth from the vapour flux.
Vapour source is a solid feed stock which is connected to a power source. The feedstock supply rate and the power level are used in conjunction with the chamber pressure control system to vary the composition, flux, and level of excitation of the vapour during the process.
The substrate carrier is electrically excited and temperature controlled so that the desired film growth conditions are established. Overall deposition system pressure is controlled by the chamber pressure control system.
The three surface modification techniques under this category are:
(i) Glow discharge deposition,
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(ii) Planar magnetron sputtering, and
(iii) Ion beam sputtering.
All these processes have the following common aspects which have important influence on the quality of surface modification:
(i) Control of the composition, flux, and degree of excitation of the vapour,
(ii) Positioning of the substrates to intercept the correct vapour flux composition to achieve the desired condensation rate on the surface,
(iii) Control of the substrate surface conditions which influence the growth of the coating.
In glow discharge deposition techniques, the vapour fluxes are created by dissociation and excitation of feedstock gases by glow discharges. The planar magnetron sputtering and ion beam sputtering deposition techniques use glow discharges as a source of high energy noble gas (like argon) ions, which in turn excite the vapour fluxes by sputtering atoms from feed stock targets.
Sputtering excites material into the gas phase by momentum transfer. In the sputtering technique, high-energy ions or neutral atoms supplied by the glow discharge collide with the surface of the feed stock target and transfer their momentum to the target material; and some of the feedstock material is ejected from the surface of the target as a result of the momentum transfer.
The composition of the excited vapour is controlled by the composition of the feedstock target.
The flux is varied by controlling the vaporisation rate of the feed stocks and the area of the feedstock target. The shape and area of the feedstock target influence the shape and total flux density in the space between the source and the substrate.
The degree of excitation of vapour is controlled by the sputtering ion’s incident energy and the energy lost by collisions with the background gas as the sputtered atom moves to the substrate. Glow discharge deposition is like chemical vapour deposition with free radicals formed in the discharge. This process takes place at lower operating temperature of 400°C because of the highly excited state of the feedstock gases in the glow.
In planer magnetron sputtering technique, the glow discharge is an ion source and a means of dissociation and excitation of reactive gases. The glow is ignited by applying an r.f. or d.c. potential between the cathode and an anode surrounding it. The electron confinement is achieved with magnets.
In ion beam deposition system, the substrate is completely separated from the glow discharge and thus the environment of the substrate can be independently controlled. The glow discharge comprises of ions for the sputtering process. The d.c. glow is ignited between the hot filament cathode and the anode in about one milli torr of argon. Ions are extracted from the glow through the holes in the screen grid and accelerator grid.
The deposition rate on the substrate is controlled by the cathode erosion rate and the substrate collection efficiency. The target erosion rate is controlled by the ion current voltage and the orientations of the target and ion beam. The condensation rate is controlled by the orientation location of the substrate with respect to the vapour flux from the feedstock target.
3. Plasma Treatments:
Plasma surface treating produces diffusion coatings (of nitrogen, carbon, boron, etc.) which are quite distinct from overlay coatings. It uses the phenomena of electrical glow discharge to activate the gas species. It permits better control over final work surface composition, structure and properties.
Electrical glow discharge permits faster deposition rates at lower surface temperatures. It can produce the required active species in a low pressure gas mixture. Proper plasma surface treatment includes both vacuum degassing and sputter cleaning steps before the addition of desired species.
Nitriding, carburising and boriding with positive ions derived from the plasma of an electrical glow discharge is an effective surface treatment for cast iron and alloyed steel.
The glow discharge used for plasma treating occurs by application of voltage between two electrodes positioned within a gas mixture at some suitable partial pressure.
Work surface forms cathode and a vaccum retort forms the anode for an electrical glow discharge. A regulated voltage pulse is applied between cathode and an ode to produce a positive ion glow of some selected chemical species to the surface of the workpiece. The pulse power supply permits regulation of ion concentration gradient at work surface.
4. Laser Treatment for Surface Modification by Surface Alloying:
Engineering failures at surfaces occur due to fatigue, corrosion, friction and wear because stresses are highest at surfaces and they are exposed to the environment. Best solution is therefore to provide material with surface properties which are different from those of the bulk.
Laser can be used to surface harden steels and cast irons to enhance fatigue, wear, and corrosion properties. Laser surface modification processes are designed to alter the compositions and microstructures of surface layers.
Laser parameters can be controlled to obtain the desired surface alloy compositions with unusual microstructures. Laser surface modification processes could be either laser surface alloying type or laser cladding type.
The benefits of laser treatment are conservation of strategic and expensive alloying elements, formation on non- equilibrium crystalline and amorphous phases, refinement of grains, homogenisation of microstructures, increased solid solubility of alloying elements, and modification of segregation patterns.
With laser technique, it is possible to control power density precisely, ensuring heating to a controlled depth and thereby reducing distortion. Laser technique can also be used to alloy inaccessible and localised areas. Considerable changes in chemical and microstructural features occur due to rapid heating and subsequent quenching rates.
Large temperature gradients (of the order of 106 – 108 °C/s) exist across the surface layer and the underlying substrate. Substrate by itself acts as heat sink and thus no additional quenching medium is required.
Laser cladding can be accomplished either by prior placing of the coating in the form of loose powder or by pneumatically injecting the powder into the melt pool during laser processing. The powder is carried by the stream of argon. Powder flow rate is one of the key factors affecting the shape of cladding, porosity, dilution, and adherence of the coating.
Laser cladding enables applying of cladding alloys of high melting point on low-melting point workpieces, controlled dilution, localised application of coating, good fusion bond, fine microstructures, minimal heat affected zone, and homogeneous and flawless coatings.