Coating Processes Lecture Notes

Agenda

• Why and when is coating applied?
• Fundamentals of coating
• Coating from the liquid state
• Coating from the granular/powdered state
• Coating by welding
• Coating by soldering
• Coating from the gaseous/vaporous state
• Coating from the ionized state

Examples and Reasons for Coatings

• Examples of coatings:
  • Aluminizing, anodizing, brazing, weld surfacing, vapor deposition, printing, pickling, painting, coating, bleaching, boriding, burnishing, carbonitriding, chlorinating, chromatizing, decating, printing, airbrushing, electropolishing, enamelling, dyeing, hot-dip aluminizing, hot-dip lead coating, hot-dip galvanizing, hot-dip tinning, flame spraying, electroplating, hot stamping, impregnating, in chromizing, calendaring, carbonitriding, laminating, painting, laminating, arc spraying, matting, metal spraying, wet enamelling, wet painting, wet galvanizing, nitriding, phosphating, plasma spraying, cladding, powder coating, cleaning, hot-dip coating, screen printing, filling, spray painting, sputtering, pad printing, dip coating, vacuum vapor deposition, plastering, fulling, roll cladding, fluidized bed sintering
• Reasons for applying coatings:
  • Protection of workpieces against wear, corrosion, heat, etc.
  • Creation of desired surface colors and textures
  • Achievement of specific electrical properties (conductive/non-conductive)
• The geometry of the workpiece is not significantly altered.

Definition

• Coating is manufacturing by applying a shapeless powdery, liquid, or gaseous substance onto a solid body.

When is Coating Applied?

• In manufacturing and repair
• To create defined surface properties
• To supplement missing material
• To reduce wear
  • Increase wear resistance
  • Improve corrosion resistance
  • Provide favorable thermal resistance
  • Repair defects (cracks, open cavities)
  • Create decorative surfaces
  • Produce conductive layers
  • Form thermal insulation layers
  • Generate electrically insulating layers
  • Achieve biochemical activity

Examples per Coating Reason

• Increase wear resistance
• Provide corrosion protection
• Improve thermal resistance
• Repair defects
• Create conductive/electrically insulating layers
• Produce thermal insulation layers
• Create decorative surfaces

Structural Diagram of the Outer Regions of Coated Workpieces

• Schicht (Coating)
  • Chemical composition
  • Structure
  • Layer thickness
  • Porosity
  • Mechanical behavior
  • Electrical behavior
  • Wear resistance
  • Corrosion behavior
• Substrat (Substrate)
  • Chemical composition
  • Bonding
  • Structure and microstructure
  • Mechanical stress state
• Interface (Interaction Zone)
  • Adhesion
• Oberflächeneigenschaften (Surface Properties)
  • Properties and composition of reaction layers
  • Near-surface disturbed zone due to processing
  • Topography

Near-Surface Regions of a Substrate

• Adsorption layer: H2O, organic substances, O2, N2, CO2, etc. (approximately 10 nm)
• Reaction layer: Oxides, sulfides, carbonates, etc. (10 – 100 nm)
• Disturbance field due to processing: Mechanical and thermal processing
• Base material (Substrate): > 10,000 nm

Adhesion Mechanisms

• Chemical Bonding
  • Adhesion through primary valences (chemisorption)
    • Ionic bond
    • Atomic bond
    • Metallic bond
  • Secondary valences (physisorption)
    • Dipole-dipole bonding
    • Hydrogen bonding
    • London or dispersion bonding

Pretreatment/Cleaning Before Coating

• Reinigen (Cleaning)
  • Mechanical: Wiping, Brushing, Sanding, Polishing, Blasting, Roughening, Matting
  • Chemical-physical: Degreasing (Solvents, Alkaline, Electrolytic), Plasma
• Reaktionen in der Randzone (Reactions in the Edge Zone)
  • Mechanical: Abrasion
  • Chemical-physical: Demetallization, Passivation, Diffusion, CVD, Degreasing (Solvents, Alkaline, Electrolytic), Plasma
  • Change in topography

Fundamentals of Coating

• Selection of coating methods and properties depends on:
  • Chemical and physical properties of the substrate material
  • Its near-surface regions
• Coating of metallic substrates is determined by:
  • Heterogeneous structures (multiple phases)
  • Metallic bonding and the presence of displaceable electrons (electron gas)
  • Property changes due to mechanical and thermal processing
• Coating of non-metallic inorganic substrates (e.g., glass and ceramics) is determined by:
  • Ionic or polarized atomic bonding and thus insulating behavior
  • Porosity of ceramic substrates (inclusion of liquids and gases is possible)
  • Presence of a permanent and temporary water film on glasses
• Coating of non-metallic organic substrates is determined by:
  • Low thermal resistance
  • Possible attack by solvents
  • Presence of a heterogeneous system (fillers, fiber reinforcement, polyblends)
• Decisive factors for layer formation from liquid coating material and solid substrate:
  • Wetting
• In layer formation from the gas phase (PVD, CVD):
  • Adsorption.
• Adhesion between layer and substrate occurs through:
  • Chemisorption and physisorption, as well as mechanical anchoring.

Overview of Main Group 5: Coating

• From the liquid state: Dip coating, Painting, Electrostatic coating, Galvanic coating
• From the plastic state: Spackling
• From the pasty state: Cleaning, Plastering
• From the granular or powdery state: Fluidized bed sintering, Thermal spraying
• By welding: Fusion surfacing
• By soldering: Solder surfacing
  • Brazing
  • High-temperature brazing
• From the gaseous/vaporous state: CVD processes
• From the ionized state: PVD processes, Chemical coating

Dip Coating / Hot-Dip Coating

• Coating of three-dimensional workpieces in a liquid bath.
  • All surface areas are wetted during the dipping process.
  • No shadowed areas as with conventional spray methods.
  • Coating through atomic reaction of liquid metal with the component.
  • Air inclusions can lead to explosions (provide boreholes).
• A distinction is made between:
  • Liquid coating materials (paints, impregnating agents, etc.).
  • Materials heated above the melting temperature (hot-dip galvanizing, etc.).

Hot-Dip Coating: Technically Used Hot-Dip Coatings

• Aluminum (Al), Aluminum alloys
  • Al: $659°C$
  • Low-alloy steel, chromium steel, Cr/Ni steel
• Lead (Pb), Lead alloys
  • Pb: $327 °C$
  • Low-alloy steel, zinc, Cu and Al materials
• Tin (Sn), Lead/tin alloys
  • Sn: $323 °C$
  • Low-alloy steel, cast iron, materials made of Al, Ni, Co, Cu, brass, bronze, zinc, lead, silver, gold, platinum
• Zinc (Zn), Zinc alloys
  • Zn: $420 °C$
  • Low-alloy steel, cast iron, bronze, brass
  • Hot tinning (e.g., food cans) replaced by electroplating (high tin price => thinner layers)

Hot-Dip Coating: Process Principle Hot-Dip Galvanizing

• The component is dipped into a zinc melt (approx. $450°C$).
• The surface of the metal reacts with the melt.
• The coating metal diffuses into the near-surface layer of the workpiece => formation of mixed crystals or intermetallic phases with the base metal.
• When the workpiece is pulled out of the bath, a top layer of the coating metal remains adhered to the surface.
• Advantages / Disadvantages:
  • + Good corrosion protection
  • + Zinc layer firmly bonded to the component surface
  • - Component distortion can occur due to heating
• Applications:
  • Screws
  • Small parts
  • Steel girders
  • Car bodies

Hot-Dip Galvanizing

• Formation of iron-zinc alloy layers (Fe + Zn) and a pure zinc (Zn) layer on steel (Fe).

Painting / Coating

• During painting, the coating material can be applied using various techniques:
  • Brushing
  • Rolling
  • Spraying
  • Dipping

Spraying: Process Principle Air/High-Pressure Spraying

• The coating material is atomized with compressed air and sprayed onto the component.
• High-pressure spraying (airless) without compressed air (oil/H2O-free).
• 2-4 bar for compressed air spraying, 100-250 bar for high-pressure spraying.
• Advantages/Disadvantages:
  • Layer thicknesses with high-pressure spraying 60 ≤ 80 µm with better quality
  • Layer thicknesses with compressed air painting 20 ≤ 40 µm
  • Both methods are only suitable for flat components
  • Large material loss
  • Even viscous paints can be processed (airless)
• Applications:
  • Flat components,
  • Tanks,
  • Steel structures,
  • Machine housings.

Spraying: Process Principle Electrostatic Spraying

• Electrostatic charging of the paint droplets in the spray gun.
• Paint droplets move towards the grounded workpiece surface.
• Charge is dissipated, paint adheres evenly.
• Advantages/Disadvantages:
  • Multiple spray sources can be used / the workpiece can be rotated.
  • All-side, even coating of articulated components.
  • Layer thicknesses 60 ≤ 80 µm
  • Environmentally friendly (solvent-free paints)
  • Low material loss
• Applications:
  • Articulated components,
  • Vehicle bodies (car, truck, bicycle, etc.).

Dip Coating: Process Principle Electrostatic Dip Painting

• Immersion of the grounded component in a paint bath to which voltage is applied.
• Charged paint particles migrate to the component (electrical forces).
• Particles remain adhered, layer thicknesses ≥ 50 µm
• Advantages/Disadvantages:
  • Application even in cavities
  • Even application even in hard-to-reach places
  • Complex, highly articulated components can be coated evenly
• Applications:
  • Corrosion protection coating of all car bodies (car, truck, bicycle, etc.).

Powder / Fluidized Bed Sintering / Thermal Spraying

• The most technically important methods of coating from the granular/powdered state include:
  • Powder coating
  • Fluidized bed sintering
  • Thermal spraying methods

Process Principle Powder Coating (Electrostatic Powder Coating is also possible)

• Powdered material (or slurry) is spread or poured onto a workpiece via a vibrating sieve or a needle roller.
• Melting by tempering and drying process.
• Dense, uniform, smooth material coating is created.
• Property profile of the coating => composition of the material powder.
• Coating material (PE), (PA), (PU), as well as ceramics.
• Advantages/Disadvantages:
  • High production capacities
  • High reproducibility
  • Layer thicknesses 150 -300 µm
• Applications:
  • Manufacturing of large-area plastic or ceramic coatings.

Process Principle Fluidized Bed Sintering

• Uniform fluidization of thermoplastic/thermosetting plastic powder by compressed air in a chamber (e.g., polyamide, polyethylene, polyester).
• Components are preheated above the flow-crosslinking temperature of the respective plastic (approx. $200°C$) and immersed in the powder cloud.
• Powder melts / crosslinks on the surface.
• Firm, evenly thick plastic film is created.
• Advantages/Disadvantages:
  • Long service life of the coatings
  • Corrosion protection
  • Layer thickness between 200 µm and one millimeter
• Applications:
  • Dishwasher baskets
  • Water fittings
  • Fence posts

Process Principle Thermal Spraying

• Coating metal (powder or wire)
• Melting in a gun and sprayed onto the component with a hot compressed gas stream.
• A distinction is made according to the type of melting (flame, arc, plasma spraying).
• Layer thicknesses between 30 µm and a few millimeters.
• Advantages/Disadvantages:
  • Any metals, alloys, even ceramics can be processed
  • Mechanical-thermal adhesion of the layer
  • Base material is thermally stressed very little
• Applications:
  • Wear layers,
  • Sliding layers (Mo-, NiCr alloys)
  • Erosion protection (turbine blades, etc.).

Fusion Welding Process Characteristics

• Application of the material by manual arc or MAG welding
• Several beads are applied next to each other
• A closed layer is created
• Advantages/Disadvantages:
  • Application of hard wear layers on tempered components
  • Repairs and value retention of worn components
  • Roughness of fusion-welded layers is large, at 0.5–2 mm
  • Layer thicknesses approx. 1–6 mm
• Applications:
  • Wear-resistant slide rails on machine tools, turbine blades, pump impellers

Overview of Applied Solders

• Typical coating solders are silver-based hard solders, copper-based, brass, and bronze solders.
• Above $900°C$: Nickel-based or precious metal solders based on gold, palladium, or platinum
• Increase the service life of cutting tool edges (e.g., saw blades, …)

Process Principle Solder Application

• Material (fleece) in the size of the area to be coated.
• Solder fleece made of solder and binder, enables penetration into the pores of the component surface.
• Metallic contact surface must be present.
• Metal layer is created after solidification (alloy formation between solder and base material).
• With solder application, layer thicknesses of several millimeters are achieved.
• Advantages/Disadvantages:
  • Application of a wear protection layer made of hard metals.
  • Base material can be inexpensive/softer.
• Applications:
  • Disk cutters
  • Saw blades

General Information about Coating from a Gaseous/Vaporous State

• Deposition from the gas phase is used to apply very thin corrosion or wear protection layers to workpiece surfaces (nano - micrometer range).
• Both pure metals and hard material layers can be applied.
• For corrosion protection, especially in the chemical industry, hard material layers based on titanium such as TiC, TiN, Ti(C,N) have proven very effective.
• A major advantage of this thin coating is that the coated workpieces no longer need to be reworked.
• A distinction is made between gas phase deposition:
  • Chemical Vapor Deposition (CVD)
  • Physical Vapor Deposition (PVD)

Process Principle Physical Vapor Deposition (Vapor Deposition/Sputtering)

• During vapor deposition, vaporized coating material condenses => solid material layer. Uniform coating of complex geometries => difficult!
• During sputtering, negatively polarized coating material is knocked out as molecules or individual atoms (by argon(+) ions of a plasma jet). Process gas and a high-voltage source are required. Condensation on the substrate surface => a very fine metal layer is formed. Layer material is not thermally removed but by impulse transfer.

Process Principle Physical Vapor Deposition (Ion Plating)

• During ion plating, the coating material is positively charged and the workpiece to be coated is negatively charged. Ionized metal particles are accelerated in the electric field towards the component, impinge there and penetrate the surface at high speed. The result is a material coating firmly bonded to the component substrate. During ion plating, deposition rates of up to 0.01 g/cm2s are achieved. All PVD processes: Deposition of very thin metal layers by vapor deposition.

Process Principle Chemical Vapor Deposition

• Gaseous metallic coating material is fed into the reactor chamber.
• The component is heated to $500°C$ - and $1100°C$ (depending on the coating material).
• Layer-forming reaction (decomposition of process gas and reaction with reactive gas).
• Non-directional material transport => complex geometries possible.
• Layers grow approximately 1 μm per minute.
• Process principle possible in low pressure/atmospheric pressure.
• Heating by laser beam possible => precise local application of the pattern.
• After coating, renewed heat treatment is required (hardening, etc.).

Example: Coatings on Cutting Tools

• Advantages:
  • Prevention of contact between tool material and machining material (the cobalt binder phase of the hard metal has a high adhesion tendency towards steel materials).
  • Increased wear resistance due to the significantly higher hardness of the coating compared to the tool material.
  • Higher thermal resistance leads to an insulation barrier and thus reduces diffusion wear.

Example: Coatings on Machining Tools/Layer Structure

• Advantages:
  • With CVD and PVD, monolayer layers as well as multilayer layers can be deposited.
  • Multi-layer layer structure improves the stress conditions (layer, layer/substrate). Conventional hard material layers TiC, TiN, Ti(C,N), Al2O3 are alternately combined.
  • Multilayer coatings can be composed of ten or more layers (each < 0.2 µm).
  • Improve the wear properties at high cutting speeds / temperatures
  • Layer materials differ (thickness, hardness, oxidation resistance, coefficient of friction)
  • The choice of the right coating depends on the machining material / process, as well as the process boundary conditions such as the type of cooling
  • An industrially widespread "universal coating" can be seen on mono- and multilayer coatings based on titanium aluminum nitride (TiAlN) on hard metals.
  • An essential feature of these layers is their sufficiently high hardness and toughness for machining tools at elevated temperatures.

Example: Coatings on Machining Tools/Layer Systems

• TiC - Titanium carbide: hardest and chemically not so stable
• TiN – Titanium nitride: less hard than TiC and more chemically resistant, color gold (yellow)
• TiCN – Titanium carbonitride: color gray-violet
• Al2O3 – Aluminum oxide: brittle, hot hardness, chemically very stable, color black
• TiAlN2 – Titanium aluminum nitride: color dark violet

General Information about Coating from an Ionized State

• Coating from the ionized state is classified according to DIN 8580 into:
  • Galvanic coating
  • Chemical coating

Process Principle Electroplating using the Example of Nickel Plating

• The workpiece is immersed in an electrolyte.
• The cations of the desired coating metal (here: NiSO4) and, if applicable, the coating metal as a soluble anode are located in the electrolyte.
• The workpiece is connected to the cathode, the coating metal to the anode (DC voltage source).
• The metal ions increasingly go into solution and deposit again on the workpiece surface.
• The electroplating bath is constantly renewed, the coating metal anode dissolves.
• Advantages/Disadvantages
  • Electricity is required
  • Cheaper than chemical deposition
• Application
  • Corrosion protection
  • Beautification

Process Principle Galvano-Forming

• Complicated part geometries and precise surface structures can be produced
• Negative casts made of silicone are generated according to CAD data
• Model can contain complex surface geometries such as undercuts
• Manufacture of positive models, made electrically conductive (e.g., Ag layer <1 μm)
• Positive model is hung as a cathode in the electroplating bath
• Deposition of layers of nickel or nickel-copper on the model

Process Principle Chemical Deposition

• Chemical reduction process (metal layers are applied from heavy metal solutions onto a component surface without the flow of an electric current).
• The workpiece is immersed in an electrolyte solution
• This contains a salt of the material to be coated
• The salt usually has a strong reducing agent added,
• which is deposited specifically on the component surface.
• Plastics can thus be provided with an electrically conductive layer
• Advantages/Disadvantages
  • No electricity is required
  • More expensive than electrolytic deposition
• Application
  • Corrosion protection, beautification,
  • Application of hard/lubricating materials

Process Principle Chemical Deposition using the Example of Dispersion Layers

• Solids in very finely divided form (0.1-30 µm particle size)
• Incorporation of these dispersants into the matrix metal
• Hard materials (e.g.: SiC, Al2O3, CBN,…) or solid lubricants (e.g.: PTFE, graphite,…)
• Removing the layer exposes the embedded particles
• Particles then take over/characterize the properties of the layer Wear protection with hard materials / dry lubrication with solid lubricants
• Solid particles must be dispersed and homogeneously distributed in the metal salt solution
• Advantages/Disadvantages
  • Wear protection layer
  • Dry lubrication
• Application
  • Cylinder liners
  • Valve balls

Process Principle Anodizing / Electropolishing

• Anodizing => Electropolishing (Electrolytic Oxidation of Aluminum)
• Component surfaces are provided with a layer of aluminum oxide
• During anodizing, thicker layers are applied in a controlled manner
• Aluminum component (positive pole) is immersed in a bath with acidic sulfuric, chromic, or oxalic acid electrolyte solution (negative pole)
• Electrochem. Corrosion on the Al surface => dense, very hard layer of (Al2O3)
• Post-treatment with hot distilled water or steam
• Advantages/Disadvantages
  • Translucent, hard Al2O3 layer (5-500µm)
  • Protection against mechanical influences
  • Electro polishing color can be applied
• Application
  • Corrosion protection of Al components
  • Beautification of Al components

Process Principle Anodizing / Electropolishing

• Cathodic reaction: $3 O<em>2 + 3 H</em>2O + 6 e^- → 6 OH^-$
• Anodic reaction: $2 Al → 2 Al^{3+} + 6 e^-$
• $6 OH^- + 2 Al^{3+} →Al<em>2O</em>3 + 3 H_2O$

Design Notes Coating

• What needs to be considered in terms of design / manufacturing technology?