Plasma Transferred Wire Arc (PTWA) is a well-established thermal spray process that is used in high-volume production by multiple automotive OEMs. Benefits of these PTWA thermal spray coatings include closer bore spacing, improved thermal transfer, lower bore distortion, increased resistance to corrosion and abrasion, reductions in weight and friction, enhanced durability, and product cost savings. For automobiles, this leads to increased fuel economy and lower emissions. Millions of engine cylinder bores per year are coated using the PTWA thermal spray process. To ensure optimal surface coatings, it is vital to monitor the process variables. Although some process monitoring already exists in current production, new technological advancements allow for additional variables to be monitored. Arc voltage is of particular importance as it can be viewed real-time in situ to the PTWA process to determine the curvature of the feedstock wire. Straight wire is ideal for achieving peak system performance. If the wire has excessive curvature, it can lead to out-of-tolerance conditions that detrimentally affect the quality of the surface coating. Therefore, in-situ monitoring of wire curvature is both desirable and necessary for producing the highest quality PTWA thermal spray coatings possible.

Crank cases of modern car-engines are made in general of light metal alloys, mostly aluminium alloys. Due to the low hardness of these materials, the use of cylinder liners, in general made of grey cast iron is required. The use of cylinder liners also leads to several disadvantages, such as the increase of the engines weight. The aim of this work in the long term is to replace these cylinder liners with a thermally sprayed nano-structured composite coating, characterised by high hardness. Therefore in this study a coating process employing a plasma transferred wire arc unit and a cored wire are used.

The application of wear resistant coatings onto the wall of aluminium engine cylinder bores by thermal spray have seen much development over the last 10 years, with regards to both equipment design and also to the material system used. While the technology has been successful in allowing the implementation of aluminium engine blocks into mass production, there is still room for improvement. One area that can be improved is the preparation of the substrate material in order to allow a good adhesion of the thermal spray coating. Grit blasting, a traditional preparation technique, is undesirable for a mass production environment due to the risk of the assembly areas becoming contaminated with grit. Therefore, an alternative method has been sought that can offer a suitable bond strength without introducing potential risks in the manufacture line. In this study, NiAl bond coats were sprayed onto aluminium substrates using the Plasma Transferred Wire Arc (PTWA) coating system developed by Ford Motor Company. A variety of surface preparation techniques, including traditional and some more novel methods, were used and analysed with respect to bond strength, ease of application and cost.

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.