Sliding member

Abstract

The present invention provides a sliding member, of which the surface cavities do not disappear even after progress of wear of the sliding surface, which permits maintenance of a satisfactory seizure resistance, and has better seizure resistance, wear resistance and strength properties than in a sliding member having a porous chrome plating layer formed thereon in the conventional art. The sliding member of the invention has a laminated hard Cr plating layer comprising a plurality of hard Cr plating layers laminated on the sliding surface. Micro-cracks are formed on each hard Cr plating layer surface, and the micro-cracks of each hard Cr plating layer form micro-pores independent in the film-forming direction. On the cross-section of the laminated hard Cr plating layer, the porosity should be within a range of from 0.5 to 4.5%.

Description

[0001] The present invention relates to a sliding member comprising a lamination of a plurality of hard Cr plating layers on a sliding surface. More particularly, the invention relates to a sliding member suitably applicable to a piston ring for an internal combustion engine.[0002] Along with the recent tendency toward a lighter weight and a higher output of internal combustion engines, the quality requirement is increasing for sliding members such as piston rings. It is the conventional practice for these sliding members such as piston rings for internal combustion engines to apply a wear resistant surface treatment through, for example, hard Cr plating or nitriding to the sliding surface as means for improving durability. Among others, the nitriding treatment, which gives an excellent wear resistance, attracts the general attention as being suitable as a surface treatment for sliding members including piston rings used under severe operating conditions, and is widely used in pract...

AI Technical Summary

Benefits of technology

[0006] The present invention was achieved in view of the aforementioned problems, and has a main object to provide a sliding member in which surface cavities do not disappear even after grinding or in progress of wear of the sliding surface, seizure resistance and wear resistance can be maintained at a satisfactory level, and seizure resistance, wear resistance and strength are higher than in the conventional sliding member having a porous chrome plating layer formed thereon.

[0010] According to the invention, by limiting the porosity and the number of micro-cracks within the above-mentioned ranges, it is possible to obtain a laminated hard Cr plating layer satisfying the seizure resistance, wear resistance and strength requirements, as in the case of above-mentioned porosity on the cross-section. Because many micro-cracks are formed on the hard chrome plating layer, seizure resistance or wear resistance never poses problems.

[0014] In the invention, a plurality of hard Cr plating layers thus having micro-cracks on the surface thereof are laminated, and micro-cracks in each layer form independent micro-pores in the film forming direction. Even when micro-cracks disappear along with progress of wear, new micro-cracks appear on the surface immediately. When applying grinding or using for a long period of time, therefore, presence of micro-cracks serving as lubricant oil sumps always on the surface makes the sliding member free from any problem in seizure resistance or wear resistance. The porosity of the surface resulting from the micro-cracks on the surface of each hard Cr plating layer is within a range of from 0.5 to 5.0%, and the number of these micro-cracks is within a range of from 50 to 1,200 cracks / 10 mm. Cracks on the surface of the hard Cr plating layer in the invention therefore largely comprise many micro-cracks. Since many micro-cracks are formed on the surface of the hard Cr plating layer, no problem is caused in seizure resistance or wear resistance, although the porosity of the surface based on the micro-cracks is within a relatively low range of from 0.5 to 5.0%. Because the porosity on the surface is on such a low level, and cracks are fine in size, strength-related properties such as peeling resistance of the hard Cr plating layer are considerably improved. The present invention have these features is particularly suitably applicable to uses in which the total thickness of the laminated hard Cr plating layer is relatively small as in piston rings of automobile.

[0023] As shown in FIG. 1, the sliding member of the invention comprises a lamination of the hard Cr plating layers having micro-cracks on the surfaces thereof. Therefore, for example, in the case of a sliding member having only a single hard Cr plating layer on the sliding surface, wear causes most of micro-cracks to disappear in a short period of time, thus posing a problem of a decrease in seizure resistance and wear resistance. In the invention, however, a plurality of hard Cr plating layers having such micro-cracks on the surfaces thereof are laminated so that, even when the portion of the surfaces containing micro-cracks disappears as a result of wear or working, micro-cracks (forming micro-pores independent in the film-forming direction) appear on the next surfaces, thus permitting maintenance of satisfactory seizure resistance and wear resistance for a long period of time.

[0026] In the invention, the surface portion of the substrate should preferably be flat. The substrate surface may have fine irregularities to some extent, and after lamination of hard Cr plating layers thereon in a prescribed number, smoothed by wrapping or the like. Presence of such fine irregularities on the substrate surface causes irregularities also in all the hard Cr plating layers. This reduces the possibility of disappearance of fine cracks in a stroke upon progress of wear, and this may improve seizure resistance. As shown in FIGS. 2 and 3, the hard Cr plating layer of the invention laminated by a method described later is observed, varying with plating conditions, to have fine cavities over the entire area in the thickness direction formed thereon. A solid portion free from fine cavities is therefore hard to appear. As a result, a substrate surface even with irregularities within a range of Rz (ten-point average roughness) of from 0.3 to 20 .mu.m is suitably applicable. In a use in which the total thickness of the laminated hard Cr plating layer as a whole is relatively small as in an automotive piston ring, Rz of the substrate surface should preferably be within a range of from 0.3 to 3.0 .mu.m. In a use in which the total length of the entire laminated hard Cr plating layer is relatively large as in a marine piston ring, Rz of the substrate surface should preferably be within a range of from 1 to 20 .mu.m.

[0038] In an aspect of the invention, as recited in claim 4, new findings were obtained that, by causing micro-cracks to serve as such surface cavities, no problem was posed in seizure resistance or wear resistance even with a porosity based on a smaller number of cavities than in an usual case. This aspect of the invention has a marked feature in that it is intended to improve strength of the plating layer itself such as peeling resistance without causing a decrease in seizure resistance or wear resistance.

Problems solved by technology

When micro-cracks disappear along with progress of wear, therefore, another micro-cracks immediately appear on the surface.

Even when grinding is applied or after a long use in service, therefore, there always exist micro-cracks serving as lubricant oil sumps on the surface.

Images