by Stephen Huse

Outline Abrasion and adhesion description Variables that change the wear rate Variables changed by coatings Surface coating processes Physical Vapor Deposition (PVD) Chemical Vapor Deposition (CVD) Electrodeposition Thermal Spray

Abrasion Physical interference of surface asperities Types: Cutting Cracking (brittle fracture) Ploughing

Adhesion Cold weld of surface asperities Similar materials such as steel on steel Wear removes oxide layer

Variables That Change the Wear Rate Friction Surface roughness Normal Forces Temperature Surface hardness Surface materials Environmental considerations

Variables Changed by Coatings Surface Hardness Reduces abrasive wear May increase brittleness Friction Reduces force for tooling Reduce heat generated Surface Materials Reduces adhesive bonding Example: steel - steel becomes steel - ceramic coating - steel

Physical Vapor Deposition Process Vacuum environment coating powder is melted and transported to substrate Cathodic arc Electron beam Plasma torch Magnetic Coating condenses on substrate NASA plasma spray PVD Advantages High Hardness Low friction Disadvantage: requires line-of-sight

Chemical Vapor Deposition Process Chemically controlled atmospheric environment. Substrate heated to 2000°F Atmosphere chemically reacts at surface Examples: Titanium cloride (gas), nitrogen, and hydrogen → titanium nitride coating Titanium chloride (gas), methane, and hydrogen → titanium carbide coating Superior adhesion of the coating to the substrate Used for synthetic diamond growth

Electrodeposition Example: Chrome plating Resists corrosive wear Increased hardness Reduced friction Thermal Spray Example: Plasma spray Similar to PVD, but no vacuum Substrate is preheated to remove impurities

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