Surface technology

Applications of Vacuum Evaporation

Applications of vacuum evaporation include:

• Electrically conductive coatings, ceramic metallization (e.g. Ti/Au, Al, Cr/Au), semiconductor metallization (e.g. Al-Cu), metallization of capacitor foils (e.g. Zn, Al)
• Optical coatings, reflective and anti-reflective multilayer coatings, heat mirrors, etc.
• Decorative coatings (e.g. Al on plastics): we exploit processability of polymers and the aesthetic appearance of metals; of course there are many other processes to do this (electro/electroless deposition are much less expansive), but presently it is the only process able to deposit Al
• Moisture and oxygen permeation barriers for packaging materials (e.g. Al and SiOx on polymer webs). Al and SiOx are the one that affect less the properties of what is inside the packaging (therefore widely used in food industry); Web coater is used in this case to combine low heat load with high deposition rate.
• Corrosion resistant coatings (e.g. Al; other metals are more

following properties: high thermal expansion coefficient, low thermal conductivity, and excellent thermal shock resistance. These properties make it an ideal material for thermal barrier coatings. In addition to these specific applications, there are many other types of coatings used in various industries. Some examples include: - Anti-corrosion coatings: These coatings are used to protect metal surfaces from corrosion caused by exposure to moisture, chemicals, or other corrosive substances. They can be applied to a wide range of materials, including steel, aluminum, and concrete. - Anti-fouling coatings: These coatings are used to prevent the buildup of marine organisms, such as barnacles and algae, on the hulls of ships and other underwater structures. They typically contain biocides that inhibit the growth of these organisms. - Anti-reflective coatings: These coatings are used to reduce the amount of light reflected off the surface of a material, improving visibility and reducing glare. They are commonly used on eyeglasses, camera lenses, and solar panels. - Self-cleaning coatings: These coatings are designed to repel dirt, dust, and other contaminants, making surfaces easier to clean and maintain. They are often used on windows, building facades, and automotive surfaces. - Conductive coatings: These coatings are used to provide electrical conductivity to non-conductive materials, such as plastics and glass. They are commonly used in electronics, aerospace, and automotive applications. These are just a few examples of the many different types of coatings used in various industries. Each type of coating has its own unique properties and applications, and the choice of coating depends on the specific requirements of the application.

Limitation of a maximum working temperature of around 1200°C, so if we want to increase more the operating temperature we need to change the oxide; at present, the main alternative is HfO (again stabilized by yttrium), whose main drawback is that it is more dense then the previous, so the global weight is increased.

In this way, I create a temperature gradient, which can be maintained if the thermal barrier sufficiently insulates the blades. Usually this ceramic thermal barrier coating is not enough, especially in the corrosion resistance (this coating is usually porous and nickel alloys are sensitive to oxidation at this temperature), thus we usually interpose a layer (BOND COAT) able to improve the oxidation and to partially accommodate the thermal expansion misfit between the nickel substrate and the thermal barrier coat; this barrier is made of 4 different elements usually: MCrAlY (Cr [10%] and Al [20%] improve oxidation resistance, while Y [1%] improves resistance to cyclic oxidation) or NiAlPt.

Al is fundamental for the corrosion resistance: when hot gasses coming from the thermal barrier coating (that is partially porous) arrive to the bond coat, a very thin and compact stable layer of aluminium oxides forms (THERMALLY GROWN LAYER) able to protect the underlying layers. Thermal barrier coating can be produced mainly by 2 methods: PLASMA SPRAY: cheaper and much higher deposition rate, thus more commonly used VACUUM EVAPORATION: electron-beam [EB-PVD] and not resistive heater since we are depositing a ceramic material [temperatures should reach around 2000°C] and, in addition, the thickness of the thermal barrier coating should be relatively large [few hundreds of μm] in order to assure a good thermal protection, so a relatively fast deposition rate is needed. In principle, the depositing of the two oxides needed for the thermal barrier could be a problem, but in this case, the two vapour pressures are substantially the same (very lucky, same composition as the source).

The process produces a columnar structure with very fine grains grown vertically on the substrate. Compared to the structure produced by plasma spray, we observe higher adhesion to the underlying layer, better corrosion resistance, and better strain tolerance; the main drawback is that this structure is characterized by higher thermal conductivity than the one deposited by plasma spray (but the latter will experience densification effect during service time, so its thermal conductivity will increase over time).

The growth starts from island growth and then becomes a columnar structure; this structure has a high rigidity in the grain direction compared to the transverse one -> this structure can partially accommodate strains in the direction parallel to the substrate (good for thermal expansion misfit accommodation).

The main limitation is the higher thermal conductivity compared to the plasma spray structure: to improve this aspect, it is possible to create micro-porosities that interrupt the heat path by

periodically interrupt the deposition through a shutter mechanism. In this way, surface atoms have time to diffuse and a layered structure with higher internal porosity will form; thermal diffusivity is therefore lowered, still maintaining good erosion resistance.

PLASMA SPRAY

In the plasma spray process, pre-alloyed powder is injected along with a carrier gas through a plasma. The molten (or semi-molten) powder particles strike and flatten (parallel to the substrate, so perpendicular to the heat flux -> very good) on the object surface where they rapidly solidify.

This produces a porous coat (good for thermal barrier), but the adhesion of the particles to the substrate is mainly a mechanical bond. As a result, the coating exhibits lower mechanical strength including reduced erosion resistant properties.

Moreover, this structure tends to densify during service, undergoing a sinter-like process, loosing part of its thermal barrier property (thermal conductivity increases) 15/04/2020

WEB COATING: is

A very common configuration used to coat polymeric foils for packages in the food industry. Usually the coat is a layer of Al or SiO and its aim is to decrease permeability to O2 and H2O vapor and therefore increase the life of the product.

This is the typical web coating configuration. With this technique we can coat typically thousands of meters of polymeric foil in a single batch, then the process is stopped, the winding roll is taken off the chamber and a new roll is loaded.

The coat thickness depends of course on the exposure time (relatively short) and on the distance (relatively short to increase the deposition rate, but it heats the substrate a lot [not good for polymeric foils] → in order to compensate this large heat supply the drum must be internally cooled).

Usually in industrial machines, before exposing the polymer foil to the source, it is exposed to a plasma source in order to increase the wettability of the polymer toward the coat material.

17/04/2020

SPUTTERING

PLASMA

Glow

discharges and plasmas are frequently used in PVD processes. A plasma is a partially ionized gas composed of ions, electrons, and neutral species that is electrically neutral when averaged over all the particles contained within. Plasmas can be generated and sustained by imparting energy to electrons through an electric field. Energetic electrons collide with atoms or molecules and create ions by the emission of new electrons. In PVD plasmas, electrons originate from ion bombarded surfaces (secondary electron emission), ionizing collisions and thermo-electron emitting sources (hot cathodes). The alternative method is an automatic emission of electrons, for example in an heated filament.

Typically, the degree of ionization, i.e. the ratio between numbers of ions and neutrals, is about 10^-4 - 10^-3 (I have much more neutral species).

An effective temperature T can be associated with a given energy E by the equation T = E / k, where k is the Boltzmann constant.

In the plasmas used in PVD processes,

The particle density is low enough and the fields are sufficiently strong so that neutrals are not in equilibrium with electrons (T≫TT); the reason is that electrons can transfer much less energy in gas through collision, while atoms can transfer energy much more efficiently. Collisional energy transfer is much less efficient for electrons, compared to ions.

In low-pressure discharges, average electron energies are typically in the range 51-10 eV, corresponding approximately to temperatures of 10 -10 K. However, because there are so few electrons, their heat content is small and the chamber walls do not heat appreciably (COLD PLASMA). In other applications where we want to exploit the heating effect of plasma, the pressure is much higher.

Neutrals have energies of only ~ 0.025 eV (T ≈ 300 K), whereas ions typically have energies of ~ 0.04 eV (T ≈ 500 K). Ions have higher energies than neutrals because they can be accelerated and acquire energy from the applied electric field.

but still the values are quite comparable.

Since surfaces (e.g., targets, substrates) are immersed in the plasma, they are bombarded by the species present. Charged particle impingement results in an effective current density J given by the product of the particle flux and the charge q transported (usually equal to the elementary charge since we mainly have singly charged ions). Therefore: qn v=J 4 where n and v are the species concentration and mean velocity, respectively.

( ) 18 kT= 2v Velocity can be expressed as . The ratio between current densities of πmelectrons (J ) and ions (J ) can be expressed as e i( ) 1J q n T me e e e i 2=J q n T mi i i i e

Electrons are greatly faster than ions, because they are much lighter and have a higher effective temperature → electrons can transfer much lower energy. For Ar+ ions, with ne=ni, Ee=2 eV, Ei=0.04 eV, Je/Ji ≈2·103 The implication of this simple calculation is that an isolated surface within the plasma charges negatively initially

(FLOATING POTENTIAL since we have no means to control it). Subsequently, additional electrons are repelled and positive ions are attracted, until the electron flux equals the ion flux and there is no net steady-state current and no further variation of the floating potential, which is usually a few tens of voltage.

The application of a large external negative potential alters the situation: the negative potential around the cathode slows down the electrons, creating a net positive charged thin region. A sheath develops around each electrode with a net positive space charge. The lower electron density in the sheath means less ionization and excitation of n