Piston Ring Museum

Technologies that Assure Ring Function

C-5 Contact Friction & Ring Durability

Vehicle and machine operators share a common wish: to have the best fuel economy and to have the longest possible oil change intervals. The periphery (running face) of a piston ring requires a material that reduces contact friction *1 as much as possible and still allows the ring to perform its sealing function against the cylinder wall. Even though the ring's running face is usually gliding on a very thin layer of oil, the friction force can be large. Minimizing such friction losses is therefore another challenge that faces today's piston ring designer.

Although hard chrome plating and nitriding are well-proven coatings for a piston ring's running face surface, current trends in engine design are driving the development of new piston ring materials and face coatings.

Surface Treatment

Recently developed face coatings have been proven under the most severe engine operating conditions.

a. Top Ring Surface Treatment : IP coating (CrN film by PVD method)

The IP coating is a type of PVD *2 film that offers the following characteristics:

  1. Low coefficient of friction
  2. High hardness
  3. High adhesion to base metal
  4. Excellent scuff resistance

The table below shows various ion plating coatings offered by Riken.

RIKEN lon plating Film Variations
  IP-100 IP-200 IP-251
Composition TIN CrN CrN
Color Gold Metallic Silver Metallic Silver
Hardness 1700~2300 1500~2000 850~1150
File Thickness Difficult to achieve thick film less than 10μm Possible to achieve thick film Up to 40μm Possible to achive thick film Up to 50μm
Application Range 1st ring for high output gasoline engines Oil contorol ring for diesel engines and gasoline engines 1st ring for diesel engine and gasoline engines
b. Second Ring Surface Treatment : Nitriding

This family of processes produces a hard surface layer on the base material. In these processes, nitrogen atoms penetrate and diffuse into the base metal, significantly increasing the surface hardness.

Types and Characteristics of Nitriding Processes
Type Ambient Atmosphere Treatment Temperature Treated Material Characteristics
Salt bath nitriding CN base salt bath 500℃~600℃ Cast iron
Alloy steel
Tool steel
Martensite base SUS
Ostenite base SUS
Cyanogen pollution
No steel type and shape limitations
Washing required after treatment
Gas nitriding NH3+RXガス
500℃~600℃ Cast iron
Alloy steel
Tool steel
(Martensite base SUS)
Ostenite base SUS is difficult.
Martensite base SUS reauires special pre-treatment
Ion nitriding N2+H2+glow discharge
Heating by glow discharge
Cast iron
Alloy steel
Tool steel
Martensite base SUS
Ostenite base SUS
No limitations on the type of steel
Fast nitriding speed
Easy partial nitriding
No shape and size limitations

Refer to "Piston Rings for Vehicles"(Sankaido).

Looking at the layer hardness value as a function of distance from the ring surface, it can be seen in the graph below that the hardness at the surface is over Hv 1,100.

Refer to Piston Rings for Vehicles (Sankaido).

c. Oil Ring and Side Rail(Segment) Surface Treatment

Depending on the application, IP film, nitriding, and chrome plating are all surface treatment options.

Coefficient of Friction

It is desirable to make this number as low as practically possible since it plays a large role in engine wear and efficiency. From the table below it can be seen that IP coating has the lowest coefficient of friction of any of the available surface coatings.

Coefficient of Friction

  • *1 The resistance to movement that the tight-fitting piston ring has as its running face slides along the wall of the cylinder.
  • *2 PVD is short for 'physical vaporized deposit.' This refers to the process whereby a metal is vaporized within a vacuum vessel, which is then ionized and adhered onto a base metal given a negative electric potential.