Sliding member having coating film and method for forming coating film

1. A sliding member having a coating film which comprises a plural number of coating film layers onto a surface of the sliding member using a composition for a sliding member containing at least a binder resin and a solid lubricant,

wherein the plural number of coating film layers is two layers, and a concentration of the solid lubricant in an upper layer of the two-layer coating film layers located distal from the sliding member is higher than a concentration of the solid lubricant in a lower layer of the two-layer coating film layers located proximate the sliding member,

each of the coating film layers contain the same components,

polyamide-imide resin is used as the binder resin,

polytetrafluoroethylene and graphite are used as the solid lubricant in combination,

an amount X of the solid lubricant in the composition of the lower layer is in a range of 5 wt % to 35 wt %,

an amount of the binder resin in the composition of the lower layer is in a range of 65 wt % to 95 wt %, and

an interface between the upper layer and the lower layer is not present.

2. A film forming method which comprises:

coating a composition containing at least polyamide-imide resin as a binder resin and polytetrafluoroethylene and graphite as solid lubricants onto a sliding member to form a first coating film layer,

coating a composition containing at least polyamide-imide resin as a binder resin and polytetrafluoroethylene and graphite as solid lubricants onto the first coating film layer without drying the first coating film layer to any extent to form a second coating film layer, and then drying these to form a coating film, wherein an amount X of the solid lubricant in the composition of the first coating film layer is in a range of 5 wt % to 35 wt %, an amount Y of the solid lubricant in the composition of the second coating film layer is in a range of 35 wt % to 65 wt %, a total value (X+Y) of the amounts of the solid lubricant is 85 or less, an amount of the binder resin in the composition of the first coating film layer is in a range of 65 wt % to 95 wt %, and an amount of the binder resin in the composition of the second coating film layer is in a range of 30 wt % to 65 wt %,

wherein components of the compositions for a sliding member used for the first coating film layer and the second coating film layer are the same.

3. A sliding member having a coating film, which is obtained by the method for forming a coating film according to claim 2 .

4. The sliding member having a coating film according to claim 1 , wherein the amount X of the solid lubricant in the composition of the lower layer is in a range of 5 wt % to 31.4 wt %.

5. The sliding member having a coating film according to claim 1 , wherein an amount of Y of the solid lubricant in the composition of the upper layer is in a range of 35 wt % to 65 wt %, and an amount of the binder resin in the composition of the upper layer is in a range of 30 wt % to 65 wt %.

6. The sliding member having a coating film according to claim 5 , wherein a total value (X+Y) of the amounts of the solid lubricant is 85 wt % or less.

7. The sliding member having a coating film according to claim 1 , wherein the composition for the sliding member further contains a solvent selected from the group consisting of N-methyl-2-pyrrolidone, N, N-dimethylformamide, and xylene.

8. The method for forming a coating film according to claim 2 , wherein the compositions further contain a solvent selected from the group consisting of N-methyl-2-pyrrolidone, N, N-dimethylformamide, and xylene.