Piston ring coating compatibility
Selecting a suitable piston ring coating is, dare I say it, rather like choosing a wife. Selected in haste, the time could come when you repent at leisure and very much like choosing a mate, under times of great stress, the issue is one of compatibility.
It has long been acknowledged that the piston ring, particularly the top ring, has one of the most arduous tasks in the internal combustion engine. Having to seal the gap between the piston and bore from the passage of unwanted blow-bye gases, transfer huge amounts of heat from the piston crown into the cylinder liner and do all this reliably at mean piston speeds approaching 5000 ft per minute through the tiniest of contact zones, is no mean task. It is hardly surprising therefore, if abused or if the lubrication of this thin line representing the contact point breaks down, then scuffing, scoring or galling, call it what you will, will inevitably occur. But even if this catastrophic situation isn’t reached there is still the less major issue of durability to be addressed.
In any metal-to-metal surface such as this, there are basically four types of potential wear mechanism: adhesive wear (scuffing, galling, cold welding, seizure), abrasive wear (grinding, scratching, polishing or ploughing), corrosive wear (thermo-chemical wear, electro-chemical wear and oxidation), or surface fatigue (pitting, spalling, flaking…..). In the case of the top ring to cylinder bore contact zone, of these four only two are of interest to us - adhesive and abrasive wear. And for most of the time, that factor affecting durability at the top and bottom ring reversal points, it is most probably abrasive wear. Under these conditions, hydrodynamic lubrication ceases and boundary lubrication, with its much higher friction levels, takes over.
Now there are certain rules that engineers can use to mitigate the situation. Clearly if the hardness of each mating surface is the same then wear will eventually take place on both. The harder the surface, the less wear will take place but since in the early stages, soon after start up,
the rings and liner will need to bed into each other, removing those minor imperfections of manufacture, there is a practical limit to the hardness.
But if we know that wear will take place then why not introduce elements of a soft surface as a sacrificial component against the harder surface. And this is exactly what we are doing when a chromium-plated ring is used in a cast iron liner. The soft graphite element in the cast iron liner will wear away much quicker than the ring, adding a degree of lubricity as well. Thus for comparatively soft cast iron liners almost any ring coating material will suffice.
Another way of coping with this inevitable wear is by embedding hard particles in a much softer matrix. Such is the case in the many aluminium liners where nickel ceramic coatings are electrolytically deposited on the bore surface. The ring coatings running over the small embedded hard silicon carbide inclusions in the coating produce very little wear debris and the presence of the oil in the oleophilic material greatly assists. Nitrided steel, titanium nitride or chrome nitride coatings can often be used and even cast iron if high wear rates are acceptable. However for such surfaces chrome is not recommended, nor is the use of DLC. Thus, although friction levels with many of these surfaces can be low, as with the compatibility issue of our high performance lady, great care needs to be taken.
In the end, much like the eventual choice of bride, proceed with caution, and seek advice along the way. When it comes to piston ring coatings you can’t be too careful!
Written by John Coxon.