Surface Technology - riassunti brevi

Surface Technology

Mean free path λ = ^kT⁄_√2π²d²P

Air at room T: λ ≈ 0.1 × 10^-3 cm^-1

Gas flux: Φ = ^mol⁄_cm²s = ^P√M⁄_√2πRT

Φ = ^P√2⁄_√πM = u_mp√ ½

Φ_net (Through Ho opening) = ¹⁄₄Φ₁(m₁ - m₂)   = ^AP₁-P₂⁄_4RT

Knudsen number KN = ^λ⁄_dc

KN > d molecular flow KN < 0.01 viscus flow

Throughput Q = PV = ƒnRT

Throughput across the opening: Q_{= C(P1-P2)} » conductance

C = ^ƒnRT⁄_A × ^J⁄₄ √ ^RT⁄_2πMNA

Pms = ^MecosΘ⁄_4πr2

dS = 

Evaporated mass per area from a surface source:

dM_S = MecosΘcosΘ⁄ƒΘ₂

Film Thickness: d = ^ƒ⁄_ƒdΘ

Atomic fraction of impurity (residue):

X_i = ^A⁄_P ^Mkv⁄ƒƒg

Spattering: ( ^XA⁄_XBUsub_stack  ( ^XA⁄_XBbulk   

Field Thickness: d = ^2ƒ⁄ƒ

Thickness distribution: d ⁄⁄d₀= 

V₀:molecular mass of impurity ƒ S = μννr: d₀ = √ [  √1+v(e/β)

Stoney:

σf = Esd_s / 6R(1-ν_s²)df

Diffusion:

D=Do exp(-Q/RT)

a=g=interatomic distance = grain size

Fraction of time spent diffusing in GBS for an A regime:

f = ℓ/ℓ + 3δ/g

Diffusion along dislocation:

D_e = D_l + qΠS²L Δ (ρ=dislocation density)

Restructuring force per area (pinning):

Pf ≈ 3γ / 2π f=volume fraction γ=grain boundary energy

Velocity:

u ≈ λ₀^3/2 ΔM = Vd Vd=drag term

Approach of two particle centres (2R):

R_i = x₀²_...(ℓ_i)^1/2/a 2a°Ca-m t

Thermal Stress:

σf = (αs-αf)ΔT E/1-νf

Total mean square displacement:

Det = Dt(1-ηt)+Db't

Det = Dt + 3S D S q

Curved boundary: ΔP = γ(1/r₂ + 1/r₁) = σ

Spherical interface: ΔP = 2σ/r

Mean velocity of grain growth:

d² = d² dN = N N₀ d² a=mean grain size Δ = boundary width δγ

Density:

P₀-P = k log(b/ℓ₀)

Power density:

W=swsublime + W_w + W< ... > cond

- Sliding contacts between conformance & surfaces (piston/cylinders, ...): the load is distributed over a large nominal contact area, the contact stresses are localized to the areas of asperity contact

• to minimize the max contact stress on the asperities the coating surface should be as smooth as possible
• most thin coatings inherit the substrate topography - careful polishing - very mild blasting
• a certain roughness can be beneficial for oil retention (sliding bearings) - also recommended to increase adhesion
• high roughness thin coatings - reduce risk of delamination

- Load-carrying capacity:

• a hard thick coating can relieve the substrate by hosting the max shear stress
• a thin high modulus coating can spread the load over a large area
• a soft coating yields, flattening the load distribution

- Coating structures

1. Graded coatings: improve the load-carrying capacity by offering smoother transitions of mechanical properties from those of the hard and stiff coatings to those of the softer and more ductile substrate. The contact load can be distributed over larger areas, which reduces the maximum contact stresses and the stress at the coating-substrate interface.
• Nitride/colourized layers
2. Duplex Coatings: on softer substrate (flame hardened steel, structural ceramics, ...) an intermediate layer acting as a mechanical support for the coating is usually required because coatings are relatively brittle and can be successfully applied only to hard and stiff substrate materials.
3. Multilayer coatings: periodically repeated sequences of layers of two or more materials (few nm - few tenths of um). The layered structure obstructs dislocation glide and also crack propagation - hard and tough coatings
• TiAlN films
4. Superlattice coatings: multilayered coatings with bilayer period in the range of 2-10 nm improve resistance against wear, corrosion, oxidation,...
• TiAlN/VN: low COF, low COF, high wear resistance
• blocking of dislocation motion at the layer interfaces due to difference in the shear moduli and critical shear stress to generate or move dislocation loops
5. Nanocomposite Coatings: nanosized crystal dispersed in an amorphous phase (d=3-8 nm - dislocation glide is made difficult)
• DLC: high hardness > 80 GPa; high wear resistance...

- Hardness:

• Wear resistance only for pure two-body abrasive wear
• Thin films - the extension of the deformed zone -> elastic recovery -> work is recovered -> reduction of the indent size -> overestimate of the hardness value

Nitriding

• T range of diffusion without subsequent heat treatment → less distortions
• Time is longer, core depth is smaller w.r.t. carburizing.
• Endure wear, fatigue
• NH3 when decomposes → nitrogen surface; causes the formation of both nitrides and a microstructure that consists of a compound layer of nitrides and an adjoining diffusion zone.
• N and C are prone to form nitrides and carbonitrides with alloying elements.
• Between nitriding atmosphere and the substrate. J′ nitrides nucleation.
• ε-Nitrides nucleation (compound layer)
• The thickness of the compound layer (more superficial) depends on t, T, steel chemistry, and process gas composition. ε-Nitrides contain porosity (recalculation compound layer).
• Compound layer of alloyed steels (with nitride-forming elements) is thinner than that of a carbon steel (when N → zero diffusion), but is harder.

Determining potential

Determine the degree of dissociation → calculate NK (measuring % of NH3 on H2)

The oxygen partial pressure can be used as an indirect measurement of NK (if present).

• Oxidizing treatment (after the process) → enhances surface characteristics in terms of corrosion resistance.
• Introduction of water vapour/nitrous oxide/air into the process chamber → development on the compound layer of iron oxides.
• Additional effects with carbonizing → better corrosion resistance, increased resistance to adhesive wear.

Plasma Nitriding

1. Cold wall technique: continuous DC power, water circulation jacket around the chamber. Thin wall sections will reach T faster. Sharp rises between thick and thin sections. Risk of arc discharge (low P, high V) on sharp corners → localized overheating reduction in hardness → enlargement of grain size.
2. Active screen Plasma nitriding: the cathodic potential is applied to a meshed screen, which surrounds the work pieces → parts must not be electrically conductive → no arcing → uniform nitriding.
3. Hot wall technique: pulsed DC (3-2000 μΑ) → no possibility for an arc to develop → temperature uniformity → power off: the residual heat at the thin sections is transferred to the thick sections.

Advantages

• Non toxic species (N2, H2, not NH3)
• 315-510 °C
• Reduction of costs (b.t, J′ f ...)
• Control of gas ratios → desired nitrided microstructure (or carbonitrides)
• Cleaning treatment: simple aqueous + additive