Development and evaluation of low friction TiSiCN nanocomposite coatings for piston ring applications

• New low friction thick nanocomposite coatings for piston rings
• Liquid precursor HMDSN has been used for reactive magnetron sputtering.
• Plint TE77 tests showed a 10% reduction of friction of the coated piston rings.
• Engine tests validated reduction in frictional and weight losses in piston assembly.

To minimize the frictional losses and wear of piston rings in an automotive engine, thick low friction TiSiCN nanocomposite coatings have been developed. The coatings were deposited by sputtering titanium targets in argon, nitrogen, hexamethyldisilazane (HMDSN), and acetylene (C2H2) using plasma enhanced magnetron sputtering (PEMS). The substrates were AISI 304 stainless steel coupons and piston rings with bore diameters of 137 mm and 86 mm. The elemental composition and microstructure of the coatings were optimized by varying the HMDSN and C2H2 flow rates separately to achieve a combination of excellent adhesion, good mechanical properties, low coefficient of friction (COF) and wear rate. The optimized TiSiCN coatings exhibited a typical nanocomposite structure which showed excellent adhesion and dry COF in the range of 0.17 to 0.2 using a ball-on-disk tribometer. The tribological performance of the coated piston rings was evaluated using Plint TE77 tests in SAE 10W-30 diesel engine oil. The TE77 tests showed a 10% reduction in the COF (0.058) of the optimized coating compared to the uncoated baseline (0.065) at test conditions of 20 Hz, 30 N, and 25 mm stroke length. Finally, the coated rings were evaluated in a single cylinder gasoline engine using SAE 5W-20 engine oil and in a heavy duty diesel engine using 4.1% sooted SAE 10W-30 diesel engine oil. The gasoline engine test showed that the uncoated piston rings contributed 25% to 34% of the frictional loss in two separate baseline engine tests. In contrast, the coated rings contributed to 18% of the total frictional loss in the engine test. The diesel engine durability test showed a 28% and 40% lower ring weight loss for the coated top and second rings, respectively, as compared to the uncoated baseline. In addition, the cylinder liner, which was not coated, showed an average 50% lower wear than that in the uncoated baseline engine test.