Surface Technology - 230331

Question 1

Thermal spray process:

Answer 1

Shot peened, then thermal spray: powder heaten to a molten state, hot gas stream kinetic energy, controlled cooling to avoid excesive heat build up. Cool and contract, mechanical bond with the substrate. Layered.
Molibdenum: No abrasion in highly agressive env.
Circonian based materials: heat environments.
Chrome oxide and alumina, zinc: corrosion protection

Question 2

Types of spray coatings:

Answer 2

kinetic energy, radiation,
Electrical discharge in gas (most common), Plasma: atmospheric plasma spray (APS), vacuum plasma spray (VPS): avoid producing oxide, works with any material as long as it can be pulverized, but it uses a lot of energy.
flame spraying.

Question 3

Kind of materials

Answer 3

Aerospace: NiCrAlY: gas turbine blades, adherent material (haft-schicht). Then ZrO2 (Deck-schicht). High temperature applications. Aircraft engines.

Question 4

Flame spray process

Answer 4

Acetylene is used as fuel, the wire is then molten and it coats the part.

Question 5

Plasma spray coating

Answer 5

He, H2 N2 is ionized and the temperature rises to 16000 K, the coating is fed by passing powder. You must apply it very quickly, because otherwise it burns, the energy is too high.
Electricity is needed to feed the plasma.

Question 6

Homogeneity of a coating

Answer 6

The mechanical properties are then uniform.

Question 7

What kind of materials can you use with the coatings?

Answer 7

Powder flame spraying, high velocity oxygen fuel flam (HVOF) spraying: all materials (ferrous non ferrrous, self fluxing alloys, ceramics, ceramets).

Question 8

Welding point of metal

Answer 8

1200 degrees celsius around.

Question 9

What’s the use of porosities in the material?

Answer 9

the porosities absorb the variation in the volume, since every phase change will generate a change in the volume, and with ZrO2, you have only one phasechange.

Question 10

What is vapor deposition process?

Answer 10

Electrical conductivity, increases the temperature resistance. Titanium coating on steel for brocas and other cutting tools. Titanium, Nickel, Aluminum.

Question 11

Hardness increase in terms of the coating process:

Answer 11

Electro chemicall deposition, thermal spraying, PVD, CVD (the hardness increases in this sequence) ME-DLC (diamond like coatings, extreme hardness and abrasion resistant, very low friction, almost 10 times less than for steel)

Question 12

Types of corrosion

Answer 12

Wet corrosion: Water based, oil, biological.
Dry corrosion: Hot air, fretting.
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Question 13

Factors for corrosion

Answer 13

High temperature, velocity of fluid in system (increase oxygen contact with the system –> creates oxides), keep the parts as dry as possible, oil is very good against corrosion. Avoid salt.
Always dirty, frequent use. Water without salt, as in Dubai the water was salty and the corrosion was very high, the problem is that the car gets dirty always.
Welding destroys the zinc coating.

Question 14

Forms of corrosion

Answer 14

Uniform (steel), local (stainless steel, aluminum), galvanic, de-alloying, environmental, erosion

Question 15

Types of corrosion

Answer 15

1: inside sheet, 2: between the part, 3: surrounding of the bolts, 4: bolt head, 5: outside sheet

Question 16

What is pitting?

Answer 16

Located in heat exchange areas, like the exhaust of a car, stainless steel.

Question 17

Intergranullar corrosion

Answer 17

Aluminum, never in steel, because of the welding you are changing the phases –> anode and cathode, corrosion
In steel you usually have corrosion in the surroundings of weldments, because the heat evaporates the zinc coating.

Question 18

Cracks in stainless steel

Answer 18

Surface has a lot of cracks always. The corrosion originates here, in the inside of the crack, and then the corroded part starts to “flow out” of the material and it looks “red”
Only use it inside, not outside. A lot of different types of stainless steel.

Question 19

Corrosion testing:

Answer 19

It’s not difficult to generate corrosion, but it is different too determine how good is the part. Standards from a company, would set a guideline to determine what is acceptable or not. Measure how many um is a material corroted.

Question 20

Disadvantages of testing

Answer 20

Accelerated tests are not actually reliable, since they cannot replicate appropriately the real working conditions.

Question 21

Good characteristics of testing

Answer 21

Must cover all the working conditions, salt and without salt, extreme conditions and normal conditions.

Question 22

Corrosion under paint

Answer 22

Very fast corrosion, delamination under the paint, it covers a big area in few time. The corrossion inside of the material is too slow and it can take years to develop. Corrosion under the painting if the surface was not clean.