Domande d'esame Surface Engineering

Cons:

Porosity (that can be exploited as a thermal/physical barrier), high roughness that can be tuned with the distance target-gun (HVOF and plasma spray → smaller particles so smaller roughness), inert atmosphere to avoid formation of oxides.

What are the main contributions of adhesion?

Chemical: metallic bonding in case of 2 metals, covalent bonding, interdiffusion layer, absent in polymers

Physical: intermolecular forces (Van der Waals, London, Debye or dipoles forces), electrostatic forces, important for small particles (Pd cluster)

Mechanical interlocking mechanism: depending on roughness and morphology, due to interpenetration of valleys and asperities.

Which is the typical thickness?

mm in thermal spray

2. conv. Coating - phosphating

Is conversion coating spontaneously obtained?

Yes, they are spontaneous

Why is it used as an undercoat for paint over Zinc?

Because they provide better adhesion, particularly for phosphating with

that is porous, as well as corrosion protection thanks to high thickness. - Mechanic interlocking mechanism and chemical contribution (metallic bonding), paintis made of metallic particles. - Cr: gel, self-healing, low friction coefficient, good solderability, good abrasionresistance, toxicity caused by presence of hexavalent Cr (with water), blue, yellow,iridescent, green olive, black (thicker and best corrosion resistance) P: crystalline structure, high porosity, insulator, thin, flexible and uniform coating,powder free, grey, high corrosion resistance - Exploited for the mechanical deposition in Zinc flakes. 5. - Difference between chromium plating and chromium conversion coating?Conv. Coating: Hexavalent Cr conv coating is toxic and gel like, self-healing, differentcolors, spontaneous Electroplating: required an external current and sulfur ions to catalyze reaction,

it is compact and very hard and has high wear resistance. It has an oxide layer on the external surface and a low friction coefficient. It may have microcracks and is available in grey and black colors.

The chromium ions are attracted by the cathode because the sulfur catalyst forms a complex with the Cr ions, which are then attracted by the cathode and reduced to metallic Cr.

At the cathode, metallic chromium is deposited, along with two side reactions: the evolution of hydrogen and the production of trivalent species. At the anode, there is oxygen evolution, passivation of lead, and oxidation of trivalent species to hexavalent chromium (self-regulating reactions).

The material used for the anode is lead or lead alloys because it is inert and has a dark passivation film that prevents the anode from being consumed.

The chromium electroplating coating has a morphology and features that include the presence of microcracks (due to hydrogen embrittlement), the same roughness as the substrate, and a possible external oxide layer for anti-corrosion purposes.

up to 100 microns, veryhard(wear)- Contributions to adhesion in coating and examplesChemical bonding (Ni-Fe metallic bonding), physical bonding (Pd cluster on plastic),mechanical interlocking (mechanical Zinc, thermal spray, Ni on polymer) Boundary lubricationλ<1(minimum thickness of lubricant/roughness), so we don’t prevent mechanicalinterlocking because the thickness of the lubricant is similar to the height of theasperities, but we avoid chemical and physical interactions, which work at shortdistances. The molecules of the lubricant are chemically adsorbed on the asperitiesof both surfaces, so when they get closer and closer, they start to repulse each otherpreventing the contact, also with high load the lubricant is always present, separatingthe surfaces, but it act as a solid transmitting the load causing plastic deformation.Liquid lubricants and solid lubricants(graphite) are used, lubricant must have also alow epsilon in order to increase the electricalresistance limiting physical interaction.

Difference between hydrostatic and hydrodynamic

They are both thick film lubrications (λ>3), but the hydrostatic one requires the use of a pump that keep the lubricant at the interface also when the system is still. While in the hydrodynamic condition the lubricant is present only when there is a relative motion between the two surfaces, so usually the hydrostatic is preferable.

Why Phosphating is good as final layer?

Corrosion protection and porous that act as reservoir for lubricants, thin and uniform coating with Ca

Which are the autocatalytic redox reactions?

The redox reactions are the oxidation of the reducing agent and the reduction of the metal at the surface

automotive paint shop

1. Electrocoating: Paint particles are attracted by the substrate which can act as anode or cathode, the latter is the most used because it doesn't decrease corrosion protection (mainly caused by presence of metallic ions)

In the paint that increase corrosion) Why is phosphating applied on cars before painting? Phosphating is applied on cars because it provides corrosion protection and high porosity (good for mechanical interlocking of the following layers) and it provides a uniform coating even on multilayers, it is insulant, so it avoid dispersive current (opposite of Cr)

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anodizing- Changes in voltage and current density in anodizing Voltage is gradually increased because pure Al is more conductive than its oxide, so it allows to get a conformal layer, so the voltage is increased together with the thickness of the oxide, till we reach a plateau in conductivity. Talking about the current density, it reduces the porosity, so the higher the current density, the harder the layer deposited, with sulfuric acid a more porous layer is obtained, while with boric acid a less porous film is deposited.

- Why do you have to be careful on cleaning Al alloys before

thickness. Higher temperatures also increase the film thickness due to the increased dissolution rate of the Al oxide. - Role of electrolyte composition in anodizing The composition of the electrolyte plays a crucial role in the anodizing process. Different electrolytes can result in different film properties. For example, sulfuric acid electrolyte produces a porous film, while boric acid electrolyte produces a compact film. The concentration of the electrolyte also affects the film thickness and quality. - Importance of sealing in anodizing After the anodizing process, the pores in the oxide film need to be sealed to improve corrosion resistance and enhance the appearance of the surface. Sealing is typically done by immersing the anodized part in hot water or a sealing solution. This process helps to close the pores and prevent the penetration of contaminants. - Applications of anodizing Anodizing is widely used in various industries for its ability to enhance the surface properties of aluminum. It provides corrosion resistance, improves wear resistance, and allows for coloring options. Common applications include architectural components, automotive parts, electronic devices, and consumer goods. Remember to use appropriate HTML tags to format the text.