Coatings Durability and Mechanical Reliability of PVD - Bright Chrome Coated Aluminum Wheels 2007-01-1530
Commercially available processes for applying coatings by Physical Vapor Deposition (PVD) offer the promise of bright chrome finishes on cast aluminum wheels, without relying on conventional platings, clad coverings, or polishing. The PVD-coated wheels are lighter in weight, lower in cost, require less design and tooling work than either plated or clad wheels, and are produced in an environmentally friendly manner, without use of hexavalent chromium or other toxic chemicals. The commercially available PVD-coating process known as Permastar® produces wheels capable of passing all of the typical coatings durability tests required by OEM automakers. However, the process relies on the use of an epoxy base coat that cures at a relatively high temperature (490°F). This high temperature cure leads to an overaged condition in the aluminum wheel, with accompanying reduction in hardness and strength, which has been of some concern in the wheel community. This paper describes the results from a full battery of durability tests performed on Permastar-coated wheels, and also addresses the issues of mechanical reliability arising from the softer condition of these wheels. Wheels produced with this PVD-coating process pass testing under conditions of radial and rotary fatigue (SAE J328), and lateral impact (SAE J175), as well as meeting the requirements of nut-seat strength (SAE J2315) and lug torque-tension testing (SAE J2316). Laboratory bar fatigue testing demonstrates that these mechanical reliability results are not merely wheel-specific coincidences: the overaged material itself is fully capable of handling the fatigue stresses and cyclic lifetimes that are commonly required for cast aluminum wheels. Furthermore, as an added benefit, the high temperature cure produces a 70% reduction in residual tension, in the spokes.