A piston ring and a method of forming a piston ring. A masking agent is applied to a selective area on an exterior surface of a piston ring. The masking agent inhibits the subsequent application of additional coatings to the selective area on the exterior surface of the piston ring. In one form, the masking agent may inhibit the subsequent application of a nitride layer to the selective area on the exterior surface of the piston ring. A recess may be formed in an outer peripheral side of the piston ring, and a thermal spray coating may be formed within the recess in the outer peripheral side of the piston ring.

A piston ring (2) having increased fatigue resistance includes a plastically deformable material. The piston ring (2) has a running face (4), which is delimited at the top by an upper running face edge (3) and at the bottom by a lower running face edge (1). Compressive stresses are introduced into the upper running face edge (3) and/or into the lower running face edge (1) along at least one part of the circumference, the compressive stresses having been produced by roller burnishing.

A slide member of the present invention is used in a slide unit which is included in a refrigerant compressor for compressing a refrigerant and provided inside a sealed container which reserves lubricating oil therein. The slide member is provided with an oxide coating film on a surface of a base material. The oxide coating film is configured such that (1) when the base material comprises an iron based material, the oxide coating film has a three-layer structure including a first layer comprising Fe.sub.2O.sub.3, a second layer comprising Fe.sub.3O.sub.4, and a third layer comprising FeO in this order from an outermost surface, (2) the oxide coating film has a dense structure having minute concave/convex portions with a height difference which falls within a range of 0.01 m to 0.1 m, or (3) when the base material comprises the iron based material, the oxide coating film has a three-layer structure in which the three layers comprise the iron oxides and are different in hardness.

A seal structure includes: first and second members defining a hollow internal area of a fluid device; and a seal member fixed to the first member for sealing a gap between the first and second members. The seal member includes a sliding contact member being in sliding contact with a surface of the second member and formed of a resin. The second member includes a resin layer and a resin layer holding structure. The resin layer is formed by sliding the second member on the sliding contact member to transfer the resin forming the sliding contact member onto a sliding contact portion of the surface of the second member at which the second member comes into contact with the sliding contact member. The resin layer holding structure is a porous film formed by electric discharge surface treatment and holds the resin layer in the sliding contact portion.