Materials Technology - Surface Treatments and Coatings

Introduction

• Surface treatments modify a part's surface to enhance properties like appearance, corrosion resistance, hardness, fatigue life, and wear resistance.
• Surface treatments modify the surface's structure/composition.
• Coatings involve depositing a different material onto the original material.

Surface Cleaning and Preparation

• Methods include mechanical (abrasive blasting, ultrasonic cleaning) and chemical (alkaline, acid, solvent, emulsion cleaning).
• Factors influencing method choice:
  • Contaminant type (greases, particles, oxides).
  • Required cleanliness.
  • Part size, geometry, material, and reactivity.
  • Production requirements, costs, environmental, and safety factors.
• Typical route: mechanical preparation -> degreasing -> descaling -> treatment/coating.

Surface Treatments for Steels

• Achieve high surface hardness with a tough core.
• Treatments that don't modify composition: flame/induction/laser surface quenching.
• Treatments that modify composition: nitriding, carburizing, carbonitriding.

Flame Surface Quenching

• Surface austenitized by flaming and quenched.
• Suitable for 0.3-0.6% C steels, large parts; layer thickness 1-6 mm.

Induction Hardening

• Rapid heating via high-frequency induced currents (up to 500 kHz).
• Heating time in seconds (800-1000°C), followed by rapid cooling.
• Layer thickness 0.5-4.5 mm, for 0.3-0.6% C steels.

Laser Surface Hardening

• Localized surface hardening via laser heating, local austenization, and cooling via thermal flow.
• Depth controlled by beam power and displacement (250-750 μm).
• Advantages: localized hardening, no external cooling needed.
• Disadvantages: Requires accurate control to avoid melting; suitable for medium to high carbon content steels.

Nitriding

• Nitrogen surface adsorption to improve mechanical properties.
  • High hardness, good corrosion resistance, no quenching needed (heating below A_1), localized hardening, hot hardness up to 500°C, improved fatigue resistance.
• Depth of nitrided layer depends on temperature, time, and composition (0.2-0.7 mm).
• Nitriding temperature: 500-550°C; time: 20-80 hours.
• Properties conferred by formation of nitrides.
• Alloyed steels with 0.2-0.6% C, containing elements like Al, Cr, Mo, Ti, V are employed.

Nitriding Technology

• Initial thermal treatment (quenching + tempering).
• Mechanical treatment and finishing.
• Protection of areas (Pb masks or liquid glass).
• Nitriding process (500-550°C).
• Final finishing.
• Types: Gas (NH3), Liquid (cyanides), Plasma (ionic N).

Carburizing

• Increases surface hardening by adding ~1% C to the surface of low carbon steels (0.08-0.25% C).
• Temperature: 850-1000°C.
• After carburizing: quenching + tempering.
• Depth of cemented layer depends on temperature, time, composition, carburizing agent (up to 4 mm).

Final Thermal Treatments

• Objective: Refine grain size in nucleus and cemented layer.
• Methods: quenching, tempering at 150-200°C. differing quenching types based on required properties.

Carburizing Methods

• Gaseous, solid, liquid, plasma.
• Gaseous: Furnace with carburizing gas (CH4, ethane, propane, butane, C oxides) at 850-950 °C; provides medium thickness layer (0.2-1.5 mm).
• Solid: Parts buried with charcoal + additives at >900 °C; slow cooling.
• Plasma: Ionization of carburizing gas, high local temperatures.
• Liquid: Steel in fused salt baths with C (cyanides).

Carbonitriding

• Modified carburizing; introduces both C and N (from CH4 and NH3).
• Lower temperatures and times; thickness < 0.75 mm; high hardness and less distortion.

Coating Techniques

• Galvanization, electrodeposition, organic coatings, thermal spraying, CVD, PVD.

Galvanization

• Active metallic coatings (anodic) relative to steel.
• Sacrificial protection: Zn is more active than Fe in a sea environment.
• Corrosion rate is slow due to large anodic to cathodic area ratio; provides long service life (up to 40 years).

Techniques

• Hot dipping, continuous (hot), electrolytic, "Sherardizing" (Powder Zn +T), thermal spraying.
• Hot dipping: Degreasing, descaling, cleaning, flux, drying, immersion in Zn bath, cooling, inspection.
• Continuous: More uniform, smaller thickness than hot dip.
• Dry galvanization ("Sherardizing"): Part cleaned and heated with Zn powder at 300-420 °C; diffusion in solid-state.

Electrodeposition

• Electrochemical coating via metallic ion deposition on a cathode (part to be protected).
• Anode: Coating metal (Zn, Cu, Sn, Cr, Ni, Au).

Organic Coatings (Paints)

• Polymers and resins that dry/harden as thin films.
• Components: vehicle (polymer/resin, solvent), pigments, additives.
• Provides corrosion protection and insulation.
• Production methods: dip coating, spraying, electrolytic spraying.

Thermal Spraying

• Coating materials (metals, carbides, ceramics) applied melted or partially melted.
• Provides corrosion, wear, and high-temperature oxidation resistance.
• Techniques: arc, plasma, flame, HVOF (High-Velocity Oxygen Fuel).

Thermal Spraying Methods

• Wire/Rod: Oxygen fuel flame melts wire, low cost.
• Metallic Powder: Similar to wire method.
• Plasma: High energy, T~8300°C, good adhesion, low oxide contamination.

Features of Thermal Spraying

• Versatile, easy to use/automate, portable.
• Produces thick, porous coatings with irregular microstructure.