Surface finishing §

• Nickel plating: 5–50 µm. Corrosion resistance, wear resistance, and a base for other coatings. Electrolytic nickel (ASTM B689) or electroless nickel (phosphorus-containing, self-catalyzed).
• Chromium plating: 8–250 µm hard chrome (wear, corrosion; general engineering 13–50 µm, hydraulic rods 20–100 µm, dimensional restoration 50–500+ µm) or 0.25–1.0 µm decorative chrome (appearance; the nickel undercoat carries the corrosion protection).
• Copper plating: 10–100 µm. A base coat for nickel/chrome on non-ferrous parts; also used for electrical conductivity.
• Gold plating: 0.4–3 µm. Corrosion resistance and solderability for electronics connectors (15–120 µin).

Key parameters: current density (A/dm²), bath composition, temperature, and part geometry (throwing power). Edges and corners receive more current and thus a thicker deposit.

For detailed process physics and electronics-specific parameters, see Deposition.

• Electroless nickel (EN): 1–12 % phosphorus (ASTM B733: Type II 1–3 %, Type III 2–4 %, Type IV 5–9 %, Type V 10 %+). Uniform thickness regardless of geometry. Used for magnetic cores, automotive parts, and chemical equipment.
• Electroless copper: deposited on non-conductive substrates (PCBs, composites) to make them conductive before electroplating.
• Advantage: uniform coverage on complex geometries; no current distribution issues.
• Limitation: slower than electroplating; bath life is limited by decomposition.

For detailed process physics and electronics-specific parameters, see Deposition.

• CVD: a gas-phase reaction deposits a solid film. Examples: diamond-like carbon (DLC) on cutting tools, silicon nitride on turbine blades. Thickness 5–20 µm.
• PVD: a solid source is vaporized (evaporation, sputtering) and condensed on the substrate. Examples: TiN (gold-colored, hard) on drill bits, CrN on molds. Thickness 1–5 µm.
• ALD (atomic layer deposition): a self-limiting surface reaction deposits one atomic layer per cycle. Used for ultra-thin (0.01–1 µm), conformal coatings on high-aspect-ratio structures (semiconductor, MEMS).

These processes are also central to Electronics fabrication. For detailed process physics and electronics-specific parameters, see Deposition.

• Grinding → superfinishing → polishing: a sequence of decreasing abrasive grit. Final Ra < 0.1 µm.
• CMP (chemical mechanical planarization): a slurry and a rotating pad polish a wafer flat. Critical in Electronics for multi-layer interconnects.
• Electropolishing: the part is the anode in an electrolyte; the surface is dissolved uniformly, reducing Ra and removing burrs. Used for stainless steel tubing and surgical instruments.

• Powder coating: a dry powder (epoxy, polyester) is electrostatically applied and cured at 180–220 °C. Thickness 60–300 µm. Used for automotive, appliances, and architectural metal.
• Liquid painting: a solvent- or water-based paint is sprayed, dipped, or flow-coated and cured. Thickness 10–100 µm.
• Conversion coatings: a thin inorganic layer (chromate, phosphate, black oxide) is formed on the surface to improve paint adhesion and corrosion resistance.

• Type II (standard): 5–25 µm, 15–25 µm typical. Used for architectural aluminum, consumer electronics, and general corrosion protection. Specified to ASTM B137.
• Type III (hard): 25–250 µm, 350–550 HV. Specified to MIL-PRF-8625. Used for aerospace structural parts, hydraulic cylinders, and high-wear applications where a thick, hard, wear-resistant oxide is required.
• Process: anodic dissolution in H₂SO₄ (Type II) or oxalic/sulfuric acid (Type III) at 18–22 °C. The porous structure is sealed in hot water or nickel acetate to close the pores and lock in dye.
• Advantage: the coating is integral with the substrate (not a separate layer that can chip); excellent wear and corrosion resistance for aluminum.

• HVOF (high-velocity oxy-fuel): fuel gas and oxygen burn to accelerate molten/semi-molten particles to 700–1200 m/s. Thickness 50–500 µm. Used for turbine blades, pump shafts, and offshore structures. Excellent adhesion and low porosity.
• Plasma spray: an electric arc (10 000–30 000 K) melts ceramic or metal powder; particles are carried by argon/helium. Thickness 100–500 µm (up to several mm). Used for thermal barrier coatings (TBCs) on turbine blades and erosion protection.
• Flame spray: a fuel-oxygen flame melts wire or powder. Thickness 100–500 µm. Lower cost, used for general wear and corrosion protection.
• Cold spray: solid particles are accelerated by a supersonic gas jet and bond kinetically without melting. Thickness 50–500 µm. Used for restoring dimensions and depositing pure metals (Cu, Ni, Ti) without thermal distortion.
• Applications: wear/erosion/corrosion protection on large components where machining or electroplating is impractical (turbine blades, pump shafts, offshore structures, gun barrels).

• Process: immersion or spray in nitric acid (ASTM A967) or citric acid (ASTM A380) at 60–80 °C for 10–30 min. The acid dissolves free iron and any non-protective oxides, leaving a uniform Cr₂O₃ (or TiO₂) layer.
• Thickness: the passive layer is < 1 µm (typically 2–5 nm); it is not a coating but an enhanced native oxide.
• Applications: medical implants, food-grade equipment, pharmaceutical processing, and any stainless steel part where pitting resistance is critical.
• Limitation: only effective on materials that form a stable passive oxide (stainless steel, titanium, nickel alloys). Does not protect carbon steel.

1. Base material: Ti-6Al-4V is already biocompatible and corrosion-resistant. The goal is to reduce surface roughness and remove machining marks.
2. Process sequence:
   • Mechanical polishing: diamond paste (6 µm → 1 µm → 0.25 µm) on a felt wheel.
   • Electropolishing: 10–20 V in a phosphoric/boric acid electrolyte for 30–60 s. Removes the deformed surface layer, reduces Ra to < 0.1 µm, and improves corrosion resistance.
3. Inspection: Surface profilometry to verify Ra < 0.2 µm; Metrology and quality control for dimensional verification.
4. Sterilization: The finished implant is sterilized (autoclave or gamma) before packaging. No further coating is needed.

Why not apply a coating (e.g., PVD TiN) instead?

• Ti-6Al-4V is already biocompatible; a coating adds a potential failure interface (delamination, spalling) that the bare alloy does not have.
• A PVD coating may introduce elements (e.g., nitrogen, carbon) that are not certified for long-term implant contact.
• The coating adds thickness, which may interfere with the implant’s designed geometry and fit.
• Polishing + electropolishing achieves the required Ra and enhances the native TiO₂ passive layer without adding a foreign material. The simplest process that meets the requirement is preferred in medical device design.

1. Base material: Ti-6Al-4V is already biocompatible and corrosion-resistant. The goal is to reduce surface roughness and remove machining marks.
2. Process sequence:
   • Mechanical polishing: diamond paste (6 µm → 1 µm → 0.25 µm) on a felt wheel.
   • Electropolishing: 10–20 V in a phosphoric/boric acid electrolyte for 30–60 s. Removes the deformed surface layer, reduces Ra to < 0.1 µm, and improves corrosion resistance.
3. Inspection: Surface profilometry to verify Ra < 0.2 µm; Metrology / quality control for dimensional verification.
4. Sterilization: The finished implant is sterilized (autoclave or gamma) before packaging. No further coating is needed.