Surface treatments — anodising, plasma spraying, vapour deposition

1. Anodising — growing a protective oxide on aluminium

WHY do we do it? Bare aluminium already self-passivates with ~2–3 nm of oxide, but that is too thin to resist wear/corrosion. Anodising grows it to 5–25 µm, hard and porous (so it can be dyed and then sealed).

HOW — derive the thickness from charge. The oxidation half-reaction is $2\mathrm{Al} + 3\mathrm{H_2O} \rightarrow \mathrm{Al_2O_3} + 6H^+ + 6e^-$

So 6 electrons liberate 1 mole of $\mathrm{Al_2O_3}$ . From Faraday's law the moles of oxide formed by charge $Q=It$ : $n_{\text{oxide}} = \frac{Q}{6F} \cdot \eta$ where $F=96485\,\mathrm{C/mol}$ and $\eta$ is current efficiency (fraction of charge actually building oxide). Converting to thickness $d$ over area $A$ : $\boxed{d = \frac{M\,Q\,\eta}{6F\,\rho\,A}}$

2. Plasma spraying — building a thick coat from molten droplets

WHY: to deposit refractory ceramics like $\mathrm{ZrO_2}$ that melt at ~2700 °C — no electrolyte or vapour route is practical for thick (100–500 µm) heat-shield layers.

HOW — the energy a particle must absorb. To melt a particle of mass $m$ from $T_0$ to its melting point $T_m$ then fuse it: $Q_{\text{melt}} = m\,c_p (T_m - T_0) + m\,L_f$

Why two terms? First raise temperature ( $c_p\Delta T$ ), then supply latent heat of fusion $L_f$ at constant $T$ to actually melt.

A particle survives only if plasma residence time gives it this energy faster than it cools — the basis of spray-parameter optimisation.

3. Vapour deposition — atom-by-atom thin films

WHY: for thin (0.5–10 µm), extremely hard, smooth films (wear, oxidation, diffusion barriers). The key contrast: PVD = physical change only; CVD = a chemical reaction at the surface.

HOW — deposition rate from flux. The arriving mass flux gives growth rate $r$ : $r = \frac{\Phi M}{N_A \rho}$ where $\Phi$ = atoms hitting per m² per s, $M$ = molar mass, $\rho$ = film density.

Why divide by $N_A\rho$ ? $\Phi M/N_A$ is mass per area per second; dividing by density turns mass-per-area into thickness per second.

Figure — Surface treatments — anodising, plasma spraying, vapour deposition

Recall Feynman: explain to a 12-year-old

Imagine your metal plane part is like a sandwich. The bread (surface) gets stale and mouldy first, not the filling. Anodising = letting the bread grow its own tough crust by zapping it in acid. Plasma spraying = a super-hot spray gun that throws tiny melted ceramic "snowballs" that splat and stack into a heat-blanket. Vapour deposition = painting with single atoms from a cloud — a paint so thin it’s invisible but harder than the metal. All three armour the skin, not the whole sandwich, to stay light.

Connections

Corrosion and Passivation — anodising is engineered passivation
Thermal Barrier Coatings — plasma-sprayed YSZ on turbine blades
Faraday's Laws of Electrolysis — quantifies anodised thickness
Adhesion and Surface Roughness — grit-blasting before spraying
Aluminium Alloys in Aerospace — the substrate being protected
Latent Heat and Phase Change — melting droplets in plasma

Anodising makes the workpiece the

anode (it is oxidised to grow oxide)

Oxidation half-reaction in Al anodising

2Al + 3H_2O \rightarrow Al_2O_3 + 6H^+ + 6e^-

Electrons needed per mole of

Al_2O_3

in anodising

Formula for anodised oxide thickness

d = M I t \eta /(6 F \rho A)

Typical anodised oxide thickness range

~5–25 µm

Why anodised oxide can be dyed

it is porous before sealing

Plasma spray jet temperature

~10,000–15,000 K

Energy to melt a spray particle

Q = m c_p (T_m-T_0) + m L_f

Why two terms in melting energy

heating (

c_p\Delta T

) then latent heat of fusion

L_f

Main bonding mechanism in plasma spraying

mechanical interlocking on roughened surface

One-word difference PVD vs CVD

chemical reaction (CVD reacts, PVD is physical)

Balanced CVD reaction for TiN

TiCl_4 + \tfrac{1}{2}N_2 + 2H_2 \rightarrow TiN + 4HCl

PVD/CVD film thickness range

~0.5–10 µm (thin)

Vapour deposition growth rate formula

r = \Phi M /(N_A \rho)

Which surface treatment gives thickest coat

plasma spraying (100–500 µm)

Which gives thinnest coat

vapour deposition (sub-µm to few µm)

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, aerospace mein part andar se strong ho sakta hai par problems hamesha surface se shuru hoti hain — corrosion, wear aur heat sab skin pe attack karte hain. Isiliye hum surface ko alag se engineer karte hain. Teen main methods hain.

Anodising: aluminium ko acid bath mein daalke uspe current chalate hain, aur part ko anode banate hain (yaad rakho — anode pe oxidation hoti hai). Isse natural oxide layer thick ho jaati hai, 5-25 micron tak, jo metal se hi grow hoti hai. Thickness Faraday's law se nikalti hai: jitna charge ( $It$ ), utna oxide. Common galti — log sochte hain part cathode hai, par nahi, oxide grow karne ke liye oxidation chahiye, isliye anode.

Plasma spraying: ek super-hot plasma gun (~12000 K) powder ko melt karke part pe maarti hai, jahan droplets "splat" hoke thick coating banate hain — jaise turbine blade pe zirconia heat-shield. Particle ko melt karne ki energy do part mein hoti hai: pehle temperature badhao ( $c_p \Delta T$ ), phir latent heat ( $L_f$ ) de ke actually melt karo. Bonding mostly mechanical interlocking hai, weld nahi — isiliye surface ko grit-blast karke rough banana zaroori hai.

Vapour deposition: yahan film atom-by-atom banti hai gas phase se. PVD mein sirf physical change (evaporate/sputter), CVD mein chemical reaction surface pe (jaise $TiCl_4$ se TiN, balanced: $TiCl_4 + \tfrac12 N_2 + 2H_2 \to TiN + 4HCl$ ). Ye films bahut patli (sub-micron se kuch micron) par extremely hard hoti hain. Growth rate slow hota hai ( $r=\Phi M/N_A\rho$ ), isiliye sirf thin coats ke liye use hota hai. Easy memory: thick to thin = Plasma > Anodise > Vapour.

Surface treatments — anodising, plasma spraying, vapour deposition

1. Anodising — growing a protective oxide on aluminium

2. Plasma spraying — building a thick coat from molten droplets

3. Vapour deposition — atom-by-atom thin films

Connections

Concept Map

Hinglish (regional understanding)

Go deeper — visual, from zero

Test yourself — Materials Chemistry (Aerospace)

Connections