8.2 Types of Plasma Spray Coatings

Plasma spray coatings offer diverse materials and applications. From metals and ceramics to composites, these coatings enhance surface properties for various industries. Understanding the types and properties of plasma spray coatings is crucial for selecting the right coating for specific needs.

Functionally graded coatings represent an advanced approach, providing smooth property transitions. These coatings offer improved thermal stress distribution, enhanced bonding, and tailored properties. Thermal, mechanical, wear, and corrosion properties are key factors in coating performance across different applications.

Types of Plasma Spray Coatings

Materials for plasma spray coatings

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• Metallic materials encompass pure metals (copper, aluminum, titanium) and alloys (stainless steel, nickel-based alloys, cobalt-based alloys) that offer excellent thermal and electrical conductivity, ductility, and strength
• Ceramic materials include oxides (alumina, zirconia, chromia), carbides (tungsten carbide, chromium carbide), and nitrides (titanium nitride, silicon nitride) that exhibit high hardness, thermal stability, and chemical inertness
• Composite materials combine the properties of metallic and ceramic materials to achieve tailored performance characteristics
  • Metal matrix composites (MMCs) consist of a metallic matrix reinforced with ceramic particles or fibers, offering improved strength and stiffness
  • Ceramic matrix composites (CMCs) comprise a ceramic matrix reinforced with ceramic fibers, providing enhanced fracture toughness and thermal shock resistance
  • Cermet coatings (ceramic-metallic composites) combine the hardness and wear resistance of ceramics with the toughness and thermal conductivity of metals
• Polymeric materials, including thermoplastics (polyethylene, nylon) and thermosets (epoxy, polyester), offer low density, flexibility, and electrical insulation properties

Types of coatings and applications

• Metallic coatings provide excellent corrosion resistance and wear resistance, making them suitable for applications such as turbine blades (improved efficiency and durability), automotive parts (enhanced wear resistance and reduced friction), and biomedical implants (improved biocompatibility and corrosion resistance)
• Ceramic coatings offer high hardness, thermal stability, and chemical inertness, making them ideal for applications such as thermal barrier coatings (protecting underlying components from high temperatures), wear-resistant coatings (enhancing the durability of cutting tools and machine components), and electrical insulators (providing electrical isolation and protection)
• Composite coatings combine the properties of metallic and ceramic materials, offering tailored properties for specific applications such as aerospace components (lightweight and high-strength), cutting tools (enhanced wear resistance and toughness), and high-temperature environments (improved thermal stability and oxidation resistance)

Concept of functionally graded coatings

• Functionally graded coatings (FGCs) exhibit a gradual change in composition or microstructure across the coating thickness, allowing for a smooth transition between the coating and substrate properties
• Benefits of FGCs include:
  1. Improved thermal stress distribution by minimizing the mismatch in thermal expansion coefficients between the coating and substrate
  2. Enhanced bonding between the coating and substrate, reducing the risk of delamination and coating failure
  3. Tailored properties for specific applications, such as gradual changes in hardness, thermal conductivity, or electrical properties
  4. Reduced risk of delamination and cracking due to the gradual change in properties, minimizing stress concentrations at the coating-substrate interface
• Examples of FGCs include thermal barrier coatings with a gradual change in ceramic composition (yttria-stabilized zirconia to alumina) for improved thermal insulation and bonding, and wear-resistant coatings with a gradual change in hardness (tungsten carbide to cobalt) for enhanced wear resistance and toughness

Properties of plasma spray coatings

• Thermal properties:
  • Thermal conductivity: Ability to conduct heat, which is crucial for applications involving heat transfer (heat exchangers, thermal management systems)
  • Thermal expansion coefficient: Change in dimensions with temperature, important for ensuring compatibility between the coating and substrate under thermal cycling conditions (gas turbine components)
  • Melting point: Temperature at which the material melts, determining the maximum service temperature of the coating (refractory materials for high-temperature applications)
• Mechanical properties:
  • Hardness: Resistance to plastic deformation, essential for wear-resistant coatings (cutting tools, bearings)
  • Toughness: Ability to absorb energy before fracture, important for impact-resistant coatings (aerospace components, armor)
  • Elastic modulus: Measure of stiffness, relevant for coatings subjected to mechanical loading (structural components)
• Wear properties:
  • Abrasion resistance: Resistance to wear caused by hard particles, crucial for coatings in abrasive environments (mining equipment, agricultural machinery)
  • Erosion resistance: Resistance to wear caused by impacting particles, important for coatings in erosive environments (turbine blades, valve components)
  • Sliding wear resistance: Resistance to wear caused by relative motion between surfaces, essential for tribological coatings (bearings, seals)
• Corrosion properties:
  • Corrosion resistance: Ability to withstand chemical attack in various environments (acidic, alkaline, saline), critical for coatings in corrosive applications (chemical processing equipment, marine structures)
  • Oxidation resistance: Resistance to oxidation at high temperatures, important for coatings in high-temperature oxidizing environments (gas turbine components, furnace linings)