Chrome plated parts and chrome plating method

Details Chrome plated parts and chrome plating method Chrome plated parts and chrome plating method

1999-11-02

2001-12-11

Dawson, Robert (Department: 1712)

Stock material or miscellaneous articles

All metal or with adjacent metals

Composite; i.e., plural, adjacent, spatially distinct metal...

C205S104000, C205S170000, C205S179000, C205S283000, C205S290000

Reexamination Certificate

active

06329071

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to chrome plated parts comprising substrates having industrial chrome plating applied on the surfaces thereof. The present invention also relates to a chrome plating method and a production method for obtaining such parts.
Chrome plating, especially hard chrome plating, provides a hard metallic coating (i.e., a chrome layer) having a low coefficient of friction. Therefore, chrome plating has been widely used as industrial chrome plating for parts which are required to have high wear resistance.
With respect to general-purpose hard chrome plating, a chrome layer formed on a metallic substrate contains many cracks reaching the substrate, called channel cracks. Such a chrome layer enables a corrosive material to migrate into the metallic substrate and cause corrosion. This leads to formation of red rust when the substrate is made of steel.
In producing chrome plated parts, generally, a plated substrate is subjected to polishing, such as buffing, so as to provide a smooth surface. It is known that during polishing, cracks in a chrome layer become clogged due to the occurrence of plastic flow over the surface of the chrome layer. Therefore, in producing general-purpose chrome plated parts, after polishing, no special measures have been taken to prevent rusting.
However, when a chrome layer is subject to thermal hysteresis, contraction of the chrome layer occurs. In this case, cracks which have been clogged due to plastic flow in the chrome layer are caused to open. Consequently, parts which are used at temperatures higher than room temperature (for example, at 120° C. for 100 hours or more) are likely to suffer a lowering in corrosion resistance.
As a countermeasure, it has been attempted to conduct nickel plating or copper plating as a pretreatment, to thereby form a lower layer having a thickness almost equal to that of a chrome layer to be formed, and conducting hard chrome plating on the lower layer. However, in this countermeasure, a plating process must be conducted in two steps, leading to low productivity and high process costs.
As another countermeasure, it has been proposed to conduct chrome plating by using two different plating baths, to thereby deposit two chrome layers having different crystal orientations, thus preventing the formation of cracks reaching the substrate [reference is made to, for example, Unexamined Japanese Patent Application Public Disclosure (Kokai) No. 4-350193]. However, this countermeasure also requires a two-step plating process.
Further, there is a method of conducting electro-plating with a pulse current, so-called pulse plating, so as to obtain a crack-free chrome layer [reference is made to, for example, Unexamined Japanese Patent Application Public Disclosure (Kokai) No. 3-207884]. However, the chrome layer formed simply by pulse plating is subject to tensile residual stress. This leads to the formation of large cracks in the chrome layer due to the application of heat.
Further, there is a method of conducting pulse plating in a Sargent bath by application of an irregular pulse current, to thereby obtain a crack-free decorative chrome layer [reference is made to, for example, Examined Japanese Patent Application Publication (Kokoku) No. 43-20082]. The chrome layer obtained by this method has low (or no) stress. However, the obtained chrome layer has a stress gradient (as the thickness of the chrome layer becomes large, the value of stress shifts from a side of compressive stress toward a side of tensile stress). Therefore, average compressive stress in the chrome layer is undesirably low. Consequently, when the above-mentioned chrome layer is used as a lower layer and a cracked chrome layer is formed as an upper layer by plating on the lower chrome layer, the lower chrome layer is subject to tensile stress from the upper chrome layer, so that propagation of cracks through the upper chrome layer to the lower chrome layer occurs. Further, in the chrome plating bath in Kokoku No. 43-20082, average compressive residual stress can be increased only to a level as low as 100 MPa, even by controlling the waveform of an applied pulse current, a bath temperature and a current density.
In view of the above, the present invention has been made. It is an object of the present invention to provide chrome plated parts which maintain excellent corrosion resistance even when the chrome plated parts are subject to thermal hysteresis. It is another object of the present invention to provide a chrome plating method and a production method for efficiently obtaining such chrome plated parts.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a chrome plated part comprising a substrate having a crack-free chrome layer applied on a surface thereof. The crack-free chrome layer has compressive residual stress and is formed by plating.
In the chrome plated part of the present invention in which a crack-free chrome layer having compressive residual stress is formed on a surface of the substrate, due to the compressive residual stress in the chrome layer, no formation of cracks in the chrome layer occurs. Therefore, the chrome layer maintains a crack-free structure. Consequently, the chrome plated part maintains excellent corrosion resistance even when it is subject to thermal hysteresis.
When compressive residual stress in the chrome layer is too low, the compressive residual stress changes to tensile residual stress due to the occurrence of thermal hysteresis. This leads to the formation of cracks in the chrome layer. Therefore, it is preferable for the compressive residual stress in the crack-free chrome layer to be 100 MPa or more.
Generally, when a chrome layer is subject to thermal hysteresis, the formation of cracks is likely to occur due to contraction of the chrome layer. This contraction is affected by the amount of lattice defects present in crystal grain boundaries in the chrome layer. Therefore, contraction of the chrome layer due to thermal hysteresis can be suppressed by suppressing the amount of lattice defects, that is, by increasing a crystal grain size and decreasing the length of a crystal grain boundary (the length of a crystal grain boundary is in inverse proportion to a crystal grain size). Therefore, in the chrome plated part of the present invention, it is preferred that the crystal grain size of the crack-free chrome layer be 9 nm or more.
The crystal grain size of a chrome layer formed by general-purpose hard chrome plating is as small as about 6 nm. The above-mentioned crystal grain size of the chrome layer in the present invention is much larger than this size. Therefore, the chrome layer in the present invention contains no cracks even prior to polishing, and maintains a crack-free structure even when it is subject to thermal hysteresis. Therefore, the chrome plated part has desired corrosion resistance. When the crystal grain size is too large, a crystal structure of the chrome layer changes. Therefore, it is preferable for the crystal grain size of the crack-free chrome layer to be less than 16 nm.
In the chrome plated part of the present invention, the crack-free chrome layer may be a lower chrome layer and the chrome plated part may further comprise a cracked upper chrome layer which is formed or applied on the lower chrome layer by plating. In this case, the hardness of the upper chrome layer can be increased to a maximum level. This improves wear resistance of the chrome plated part. Further, cracks in the upper chrome layer serve as oil sumps for holding lubricating oil, leading to suppression of sliding resistance.
The chrome plated part may further comprise at least one intermediate chrome layer which is formed between the lower chrome layer and the upper chrome layer by plating. When an intermediate chrome layer is provided, direct propagation of cracks through the upper chrome layer to the lower chrome layer can be suppressed. Therefore, corrosion resistance of the chrome plated part can be stably maintained.
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