Surface treatments — anodising, plasma spraying, vapour deposition

5.4.10 · Chemistry › Materials Chemistry (Aerospace)

1. Anodising — aluminium pe protective oxide ugana

WHY karte hain? Bare aluminium pehle se hi ~2–3 nm oxide se self-passivate ho jaata hai, lekin wear/corrosion resist karne ke liye yeh bahut p얇a hai. Anodising ise 5–25 µm tak grow karta hai — hard aur porous (taki ise dye karke seal kiya ja sake).

HOW — charge se thickness derive karo. Oxidation half-reaction hai: $2 Al + 3 H_{2} O \to A l_{2} O_{3} + 6 H^{+} + 6 e^{-}$

Toh 6 electrons 1 mole $A l_{2} O_{3}$ banate hain. Faraday's law se charge $Q = I t$ se bane oxide ke moles: $n_{oxide} = \frac{Q}{6 F} \cdot η$ jahan $F = 96485 C/mol$ aur $η$ current efficiency hai (charge ka wo fraction jo actually oxide bana raha hai). Area $A$ pe thickness $d$ mein convert karte hue: $d = \frac{M Q η}{6 F ρ A}$

2. Plasma spraying — molten droplets se thick coat banana

WHY: refractory ceramics jaise $Zr O_{2}$ deposit karne ke liye jo ~2700 °C pe melt hote hain — thick (100–500 µm) heat-shield layers ke liye koi electrolyte ya vapour route practical nahi hai.

HOW — ek particle ko jo energy absorb karni hai. Ek mass $m$ ke particle ko $T_{0}$ se melting point $T_{m}$ tak laane aur phir fuse karne ke liye: $Q_{melt} = m c_{p} (T_{m} - T_{0}) + m L_{f}$

Do terms kyun? Pehle temperature raise karo ( $c_{p} Δ T$ ), phir constant $T$ pe latent heat of fusion $L_{f}$ supply karo taaki actually melt ho.

Ek particle tabhi survive karta hai jab plasma residence time usse yeh energy deta hai cooling se pehle — yahi spray-parameter optimisation ka basis hai.

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

WHY: thin (0.5–10 µm), extremely hard, smooth films ke liye (wear, oxidation, diffusion barriers). Key contrast: PVD = sirf physical change; CVD = surface pe chemical reaction.

HOW — flux se deposition rate. Aane wala mass flux growth rate $r$ deta hai: $r = \frac{Φ M}{N _{A} ρ}$ jahan $Φ$ = atoms per m² per s, $M$ = molar mass, $ρ$ = film density.

$N_{A} ρ$ se kyun divide karte hain? $Φ M / N_{A}$ mass per area per second hai; density se divide karne par mass-per-area thickness per second ban jaata hai.

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

Recall Feynman: 12-saal ke bacche ko samjhao

Socho metal plane part ek sandwich hai. Bread (surface) pehle stale aur mouldy hoti hai, filling nahi. Anodising = bread ko acid mein zap karke apni tough crust khud ugane dena. Plasma spraying = super-hot spray gun jo tiny melted ceramic "snowballs" phenki hai jo splat hokar stack ho jaate hain aur ek heat-blanket banate hain. Vapour deposition = ek cloud se single atoms se painting — itna thin paint ki invisible hai lekin metal se zyada hard hai. Teeno skin ko armour karte hain, poore sandwich ko nahi, taaki halka rahe.

Connections

Corrosion and Passivation — anodising engineered passivation hai
Thermal Barrier Coatings — turbine blades pe plasma-sprayed YSZ
Faraday's Laws of Electrolysis — anodised thickness quantify karta hai
Adhesion and Surface Roughness — spraying se pehle grit-blasting
Aluminium Alloys in Aerospace — jo substrate protect ho raha hai
Latent Heat and Phase Change — plasma mein droplets melt karna

Anodising mein workpiece ko ___ banaya jaata hai

anode (ise oxidise karke oxide grow hota hai)

Al anodising mein oxidation half-reaction

2 A l + 3 H_{2} O \to A l_{2} O_{3} + 6 H^{+} + 6 e^{-}

Anodising mein

A l_{2} O_{3}

ke ek mole ke liye kitne electrons chahiye

Anodised oxide thickness ka formula

d = M I t η / (6 F ρ A)

Typical anodised oxide thickness range

~5–25 µm

Anodised oxide ko dye kyun kiya ja sakta hai

sealing se pehle yeh porous hota hai

Plasma spray jet temperature

~10,000–15,000 K

Spray particle ko melt karne ki energy

Q = m c_{p} (T_{m} - T_{0}) + m L_{f}

Melting energy mein do terms kyun

heating (

c_{p} Δ T

) phir latent heat of fusion

L_{f}

Plasma spraying mein main bonding mechanism

roughened surface pe mechanical interlocking

PVD vs CVD mein ek-word difference

chemical reaction (CVD react karta hai, PVD physical hai)

TiN ke liye balanced CVD reaction

T i C l_{4} + \frac{1}{2} N_{2} + 2 H_{2} \to T i N + 4 H C l

PVD/CVD film thickness range

~0.5–10 µm (thin)

Vapour deposition growth rate formula

r = Φ M / (N_{A} ρ)

Kaun sa surface treatment sabse thick coat deta hai

plasma spraying (100–500 µm)

Sabse thin coat kaun sa deta hai

vapour deposition (sub-µm se kuch µm)