In selecting appropriate coating processes and compositions consider the following:
Bond strength between the coating and base metal is of paramount importance.
• Residual Stresses
Residual stresses are a primary concern for spray coatings. Residual stresses have a significant effect on coating bond strength. Generally, the outer portion of the spray coating is in tension. This reduces the stress required to cause fracture. Thick electroplated chrome coatings, on the other hand, may develop compressive residual stresses, increasing their resistance to cracking.
The density of spray coatings depends on individual particle size and density, degree of oxidation during deposition, and kinetic energy of the impinging particles. Density of electroplating processes depends primarily on plating bath temperature and current density.
• Corrosion Resistance and Porosity
The corrosion resistance of all coatings is determined by chemical composition. In addition, coating porosity and cracking may allow corrosion of the base metal. For lower temperature applications, various epoxy, silicone wax, and vinyl materials may be used to seal coatings. For high-temperature applications, sintering (a heat-treating process) may be needed to seal spray coatings.
• Thermal Properties
Thermal conductivity and coefficient of thermal expansion must be considered when selecting coatings for a particular application. High-pressure, hightemperature applications require coatings which effectively remove heat from the contacting surfaces and are resistant to thermal shock.
• Lubricant Retention
Surface porosity and cracks provide storage voids for lubricant.
Friction depends on the materials, surface roughness, and the lubricant.