Wear-Resistant Peripheral Coatings
Chromium Plated Peripheries
In the tribological system "piston ring / cylinder wall", electrochemically deposited hard chromium coatings on piston ring running faces are noted for their high wear resistance and the low cylinder wear they generate. Such hard chromium coatings are used mainly on rings in the top groove as this is where the mechanical / thermal loads are greatest, and on oil rings as these have a particularly high unit pressure. In truck diesel engines the rings in the second groove are also often coated with hard chromium. These systems are only limited by reduced oil film thicknesses combined with extreme mechanical and thermal loading.
Coatings can suffer damage in early life or after lengthy operating times. Adhesive wear (scuffing) and/or fatigue-related peeling (the "peel and polish" phenomenon) can occur. For these reasons GOETZE developed a special surface topography, the special lap finish, as an addition to the standard lapping process used for hard chromium coatings. Special lapping improves oil film formation in early life and shifts the scuff threshold of hard chromed rings towards higher tolerable loads. The surface structure - plateaus alternating with valleys - eliminates roughness peaks, creates good bearing surfaces for a tight seal and contains oil pockets with the result that critical early-life conditions are better controlled.
With demands on the load levels of modern internal combustion engines ever increasing, there is often a need to improve the thermal and/or mechanical load carrying capacity of piston ring coatings beyond early life. The patented coating system "Hard Chromium Matrix with Hard Particles Embedded in the Microcrack Network" meets such requirements for the whole life (thickness) of the coating.
The variants resulting from the development effort, namely CKS (chromium with aluminium oxide ceramic) and GDC (chromium with microdiamond), have a crack network that extends either part-way or all the way through the thickness of the coating, depending on requirements, and is reinforced with embedded hard particles.
Fig. 24 illustrates the structure of the coating system schematically and shows the repetitive process steps, consisting of chromium deposition, opening of the microcrack network, insertion of the hard particles, integration of the hard particles, closing of the microcracks, and build-up of the next layer of the coating.
The variant CKS® was developed specifically for piston rings in the top groove, predominantly for highly rated diesel engines. This coating has a crack network running through its whole thickness and filled with aluminium oxide ceramic particles. Compared to hard chromium coatings this coating has about a 50% lower wear rate and significantly improved scuff resistance (Fig. 25). An adapted variant is used for the oil control rings, the final layer being entirely in hard chromium to provide mechanical support for the lands.
The variant GDC® was developed for extreme engine loads. This is a hard chromium coating reinforced with minute diamond particles embedded in the extremely fine crack network with substructure. Compared to CKS® the coating wear was halved and scuff resistance further improved without significantly increasing cylinder wear. This coating is also used for oil control rings in an adapted version with a hard chromium final layer.
Rings with chromium, CKS® or GDC® as peripheral wear coatings can be produced either in a full-face or a semi-inlaid design.
Fig. 24: Schematic of Chromium Coatings Reinforced with Hard Particles
Fig. 25: Wear in Engine Operation