The invention relates to an imide polymer-based solid film lubricant, a method for producing same, a sliding element comprising same and the use thereof. According to the method, difunctional or cyclized difunctional compounds and optionally functional fillers are added to a non-imidized or partly imidized polyamide acid prepolymer or an imidized short-chain blocked prepolymer in a solvent or solvent mixture and then, depending on the prepolymer, a polymerization reaction or an imidization reaction and in both cases a crosslinking reaction is carried out. The solid film lubricant comprises an imide polymer as the resin matrix and optionally functional fillers, the molecules of the imide polymer comprising groups (R.sub.1) of the difunctional compounds that additionally contribute to the crosslinking.

To provide a sliding member comprising a coating film exhibiting constant and stable chipping resistance and wear resistance and excellent in peeling resistance (adhesion), and the coating film thereof. The above-described problem is solved by a sliding member (10) comprising a coating film (1) on a sliding surface (16) on a base material (11). The coating film (1) has, when a cross section thereof is observed by a bright-field TEM image, a total thickness within a range of 1 μm to 50 μm, in repeating units including black hard carbon layers (B), relatively shown in black, and white hard carbon layers (W), relatively shown in white, and laminated in a thickness direction (Y). In the black hard carbon layer (B) and the white hard carbon layer (W) adjacent to each other, the white hard carbon layer (W) has higher hardness and a larger [sp.sup.2/(sp.sup.2+sp.sup.3)] ratio than the black hard carbon layer (B).

To provide a sliding member comprising a coating film exhibiting constant and stable chipping resistance and wear resistance and excellent in peeling resistance (adhesion), and the coating film thereof. The above-described problem is solved by a sliding member (10) comprising a coating film (1) on a sliding surface (16) on a base material (11). The coating film (1) has, when a cross section thereof is observed by a bright-field TEM image, a total thickness within a range of 1 μm to 50 μm, in repeating units including black hard carbon layers (B), relatively shown in black, and white hard carbon layers (W), relatively shown in white, and laminated in a thickness direction (Y). In the black hard carbon layer (B) and the white hard carbon layer (W) adjacent to each other, the white hard carbon layer (W) has higher hardness and a larger [sp.sup.2/(sp.sup.2+sp.sup.3)] ratio than the black hard carbon layer (B).

A piston ring configured to be disposed in a floating piston for use in a vessel of a thermal energy storage system to separate a hot working fluid from a cold working fluid, wherein the piston ring comprises of a plurality of arc segments, each arc segment interconnected by a joint.

To provide a piston ring excellent in thermal setting resistance, and also excellent in side surface wear resistance even under such a high-temperature high-pressure environment as to be more than 300° C. and up to 400° C., the base metal of the piston ring is a steel containing, by mass %, C: 0.30 to 0.65%, Si: 0.80 to 1.20%, Mn: 0.20 to 0.60%, Cr: 4.50 to 5.70%, Cu: 0.01 to 0.5%, and at least one of Mo, V, W, and Co: 0.2 to 5.4%.

To provide a piston ring excellent in thermal setting resistance, and also excellent in side surface wear resistance even under such a high-temperature high-pressure environment as to be more than 300° C. and up to 400° C., the base metal of the piston ring is a steel containing, by mass %, C: 0.30 to 0.65%, Si: 0.80 to 1.20%, Mn: 0.20 to 0.60%, Cr: 4.50 to 5.70%, Cu: 0.01 to 0.5%, and at least one of Mo, V, W, and Co: 0.2 to 5.4%.

A coated piston ring for a piston is provided. The piston ring includes a running surface, a flank surface, and a transition surface therebetween. The transition surface curves or extends at an angle between the running surface and the flank surface. A running layer is disposed over the running surface and over at least a portion of the transition surface. A flank layer is disposed over the flank surface and over at least a portion of the transition surface. The running layer is applied by physical vapor deposition, and the running layer is applied by galvanic deposition. The running layer is formed of chromium nitride, and the flank layer is formed of chromium. A portion of the flank layer overlaps and is disposed outward of a portion of the running layer. During operation of the piston, the overlapping portion is spaced from both the piston and the cylinder.

A coated piston ring for a piston is provided. The piston ring includes a running surface, a flank surface, and a transition surface therebetween. The transition surface curves or extends at an angle between the running surface and the flank surface. A running layer is disposed over the running surface and over at least a portion of the transition surface. A flank layer is disposed over the flank surface and over at least a portion of the transition surface. The running layer is applied by physical vapor deposition, and the running layer is applied by galvanic deposition. The running layer is formed of chromium nitride, and the flank layer is formed of chromium. A portion of the flank layer overlaps and is disposed outward of a portion of the running layer. During operation of the piston, the overlapping portion is spaced from both the piston and the cylinder.

Provided is a compressor including a sliding material with the wear resistance improved. To solve this problem, a compressor (40) includes: a compression chamber (43) that is formed in a cylinder (41) and compresses gas; and a piston ring 421 and a rider ring 422 that are in contact with the inner wall of the cylinder 41 to form the compression chamber (43). The piston ring 421 and rider ring 422 include: a resin member; and a metal particle that is located within the resin member and includes a surface layer containing an affinity portion having an affinity to the resin member.

Provided is a compressor including a sliding material with the wear resistance improved. To solve this problem, a compressor (40) includes: a compression chamber (43) that is formed in a cylinder (41) and compresses gas; and a piston ring 421 and a rider ring 422 that are in contact with the inner wall of the cylinder 41 to form the compression chamber (43). The piston ring 421 and rider ring 422 include: a resin member; and a metal particle that is located within the resin member and includes a surface layer containing an affinity portion having an affinity to the resin member.