A Variable Hardness PVD Coating - A Paradigm Shift For Running-In

Rings & liners must run-in before reaching optimum performance & oil control. A top softer run-in coating is better for removing asperities and gives optimum surfaces after which a lower harder durable coating is best. So, a dual coating is required. WC/C on CrN is available but not always successful delamination problems. Therefore, engines are often operated with no piston ring coating or a single durable coating which is not optimum. Work by SwRI developed a durable low friction Ti-Si-C-N coating. In a full vehicle fuel economy savings of 0.82% were obtained. The Ti-Si-C-N coating is ‘tuneable’ during the deposition process giving a softer top layer. Being the same coating with different hardness applied in one process removes delamination concerns. Coatings were tested in a tribometer and liner surfaces analysed (3D & SEM) to find the best coating. SwRI worked with Total Seal Piston Ring to install the variable hardness piston rings in a NASCAR engine to observe improved horsepower.
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