Abstract
High volume production of a consistent, repeatable coating on the internal diameter of surfaces has been a challenge for engineers for many years. Ford Motor Company and Flame Spray Industries, Inc. have developed a Plasma Transferred Wire Arc (PTWA) process and method of applying a wear resistant thermal spray coating on the internal surfaces of aluminum engine cylinder bores. For this automotive application, many challenges existed, including: achieving coating adhesion capable of passing long term engine durability testing, obtaining optimum materials performance and wear resistance, and developing a low cost thermal spray process that would be capable of high volume manufacturing. A novel fluoride based flux surface preparation technique utilizing a non-hazardous chemical solution was developed. This allows a NiAl bond coat to be applied to a non-roughened aluminum surface, yielding high bond strengths with the consistency necessary for high volume production. Process and materials development produced a cylinder bore coating using inexpensive low carbon steel wire feedstock that resulted in a coating with less than 2% porosity. After completing a detailed materials testing program, it was determined that using low carbon steel wire feedstock with compressed air atomizing gas formed a unique thermal spray coating system consisting of an iron/iron oxide (Fe/FeO) structure that exhibited excellent tribological properties while maintaining compatibility with existing piston ring pack materials. The PTWA thermal spray process was designed to operate in a turnkey environment in excess of 100 hours before requiring maintenance or replacement of consumable components. To fully test this process, Ford completed a production prove out including extensive dynamometer and fleet engine testing. The PTWA process proved to be production capable while producing coated aluminum block engines that accumulated over 3 million miles of fleet vehicle tests with zero coating failures. The test engines also exhibited excellent performance including piston and ring wear, power output, oil consumption, and vehicle emissions.
