Domande d'esame Surface Engineering

Surface engineering - Exam questions

Electroless plating

Pd, what reaction does it catalyze and why? It catalyzes the oxidation reaction, in this way the production of Ni happens only on the surface and not in the bulk solution, stabilizing it without waste Ni metal.

How do you maximize adhesion in the case of plating on plastics? What process do you use? Doing a chemical etching on the polymer, using an aggressive solution like hexavalent Cr that dissolves part of the polymer. The roughness is increased, and the adhesion of the metallic coating is enhanced with the mechanical interlocking mechanism. Note that if we increase too much the roughness, we reach a point where adhesion starts to decrease because the coating can’t perfectly match the substrate. Regarding Pd clusters, they adhere to the polymer substrate due to dispersion forces (physical adhesion).

Phosphorous: why is it in the coating, and what effect does it have on the characteristics of the material? P is present because it comes from the decomposition of the reducing agent. It allows the formation of intermetallic compounds and the Ni-P amorphous phase, providing corrosion resistance, low magnetism, low hardness but improvable with heat treatments, low solderability, low porosity.

What happens when you undergo post-treatments (intermetallic) on Ni-P? Why do you do it? With a heat treatment, we have the formation of NiP intermetallic compounds and achieve a crystalline structure. Effects: higher hardness (higher wear resistance), increase in magnetism, lower corrosion resistance, increase in adhesion of the layer due to interdiffusion with the substrate, avoid hydrogen embrittlement.

Thermal spray

Pros/cons: why do we use this technique?

• Pros: High thickness in a short time, so high corrosion protection and wear resistance, in situ, composite and ceramic materials can be deposited, no pretreatment of the substrate, also complex shapes and selective coating (different zones of the part).
• 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 the formation of oxides.

What are the main contributions to adhesion?

• Chemical: Metallic bonding in the case of two 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.