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Lowdown on Kashima Coat

Story of Kashima Coat’s Development

Anodizing film offers vital wear resistance in addition to providing a protective surface film and industrial grade decoration for aluminum.
In particular, standard anodizing and hard anodizing are utilized for numerous industrial products due to their extremely hard and strong nature.

■Strength of Standard Anodizing and Hard Anodizing

・Good Wear Resistance

Extensively used for the sliding parts of industrial goods. As a sliding material, extremely resistant to abrasive wear※ thanks to excellent hardness.

※Abrasive Wear:wearing out of a surface when a contaminant gets in the space between two friction surfaces.

■Weakness of Standard Anodizing and Hard Anodizing

・Large Friction Coefficient

The fit with the mating friction surface may be less than ideal due to friction conditions. Particularly bad cases may result in locking up and wearing in the form of scoring and seizing.

Wouldn’t the addition of lubrication to anodizing further enhance its advantages?
It was this idea that led to the development of molybdenum disulfide impregnated anodizing.
The answer was Kashima Coat.

Challenging Difficult Concepts

Molybdenum disulfide impregnated anodizing (herein called Kashima Coat) is a compound anodizing where the micropores (diameter of 100Åto several hundred Å, several 100 million/mm2) of the anodizing film are impregnated with solid lubricant in the form of molybdenum disulfide using electrochemical methods.
Combining the advantages of both materials creates a strong, wear resistant, and well lubricated product.
Applicable as material for sliding parts demanding a low friction coefficient, smooth kinetic friction, and long-term wear resistance. It is generally difficult to impregnate the micropores of an anodizing film. Solid substances are especially troublesome to impregnate due to their shape and size.
Development aimed to formulate a method of synthesizing molybdenum disulfide inside these micropores capable of completely impregnating them from the base up.
Complete impregnation to the base of micropores has the distinct advantage of providing lubrication via the molybdenum disulfide until the anodizing layer itself is completely worn away.

Kashima Coat Manufacturing Process

The aluminum or aluminum alloy is first subject to primary electrolysis in an electrolytic bath of sulfuric acid or oxalic acid to form a porous anodizing film.
Next, the product treated with anodizing is put in a solution mainly consisting of the thiosulfate in molybdenum where it becomes an anode in the secondary electrolysis process.
Doing so deposits molybdenum sulfide in the micropores where it solidifies.
This molybdenum disulfide deposition process is inferred in the following way.

  • 1
  • The thiosulfate in molybdenum is disassociated in the secondary electrolysis solution and exists as thiomolybdate ions.
  • 2
  • These negatively charged ions are attracted to the anode and enter the micropores by electrophoretic migration and diffusion. The ions are much smaller than the micropores, allowing them to freely migrate to the bottom.
  • 3
  • After entering the micropores, the thiomolybdate ions are electrolyzed through direct electrolytic reaction or a change in the concentration of hydrogen ions discharged by the anodic oxide of aluminum within the micropores, resulting in the deposit of molybdenum sulfide. Heat treatment of this deposited molybdenum sulfide after secondary electrolysis forms crystals with a graphite structure.

Using an X-ray microanalyser to examine the progress of molybdenum sulfide deposition during secondary electrolysis reveals that initial deposition begins at the base (barrier layer) of the micropores and works its way to the entrance of each pore with time, filling the micropores. It also shows that further electrolysis will result in black deposits on the anodizing film’s surface.

  • Figure 1. shows the analytical results (X-ray microanalyser) obtained from a sample with micropores almost entirely filled with molybdenum sulfide
  • Figure 2. is an hourly voltage curve obtained from secondary electrolysis using the constant current method.

Voltage rises with time in a linear manner as the secondary electrolysis solution is neutral and a barrier layer develops due to the anodic oxide reaction of aluminum during secondary electrolysis. The barrier layer develops in this way as the voltage rises and secondary electrolysis progresses, so the electrolytic reaction progresses sequentially from the barrier layer’s thinnest point. This results in extremely uniform electrodeposition and a thicker barrier layer that enhance corrosion resistance.

  • X-ray microanalyser analysis of cross-sectional direction of anodizing impregnated with molybdenum disulfide reveals distribution of Mo and Al
  • Voltage-time curve during secondary electrolysis

Lubrication Properties

  • The lubrication properties of Kashima Coat’s anodizing film are shown in Fig. 3, Chart 1.
    In this test a disc of each material is placed over the test piece in a taber machine to measure the frictional force when the test piece is rotated. The result of frictional force divided by load is called the frictional coefficient. According to testing, lubrication treatment lowers the friction coefficient of each metal by 1/2-1/3. Moreover, the surface of test pieces after the test remains smooth and virtually free of scoring, confirming good conformability with the mating material.

    Chart 1. Friction Coefficient Variation due to Molybdenum Disulfide Impregnation

    Mating Friction Surface Hardened Steel Hard Steel Brass Hard Chrome Plating
    Anodizing Treatment 0.64 0.66 0.40 0.64
    Kashima Coat Treatment 0.23 0.30 0.24 0.28
    Mating Friction Surface Hardened Steel Hard Steel
    Anodizing Treatment 0.64 0.66
    Kashima Coat Treatment 0.23 0.30
    Mating Friction Surface Brass Hard Chrome Plating
    Anodizing Treatment 0.40 0.64
    Kashima Coat Treatment 0.24 0.28

    Friction coefficient reduced by1/2-1/3

  • Change of Kinetic Friction over Time

Application

The aforementioned characteristics make Kashima Coat a promising wear resistant material utilizing self-lubrication.
Here we will introduce experimental examples of various pulleys, rollers, and guides used on our electrical wire production line that have been trial manufactured for practical evaluation
There is a roller that is slid sideways as it winds up thin electrical wire covered with polyethylene and vinyl chloride. This roller necessitates a low friction coefficient for smooth sliding and durability. The trial roller produced has operated satisfactorily and maintained a low friction coefficient even after two years of use.
For comparative testing, rollers only coated with hard chrome plating or anodizing treatment were tested and immediately became unusable due to an excessive friction coefficient that prevented smooth sliding. Approximately 200μ thick rollers coated with Teflon® resin displayed an extremely low initial friction coefficient which tended to increase as the Teflon® resin layer wore during use. Compared with other parts such as guides and pulleys only coated with anodizing, Kashima Coat has an enhanced friction coefficient and long lifespan. A notable characteristic is the increased difficulty with which copper powder adhered to the lubricating surface when applied to bare copper wire.
Other areas now planned for application include a full range of industrial parts starting with sliding parts in cameras, automobiles, and the electric industry.
The above in-house examples have successfully verified the practical value of Kashima Coat.

Main Characteristics

  • 1
  • Low friction coefficient, good wear resistance.
  • 2
  • Molybdenum disulfide impregnated anodizing has no surface irregularities and can be uniformly treated.
  • 3
  • Good dimensional precision as film thickness is set by thickness of anodizing film.
  • 4
  • Good electric insulation.
  • 5
  • Good corrosion resistance.
  • 6
  • Nonadhesive.
  • 7
  • Superb heat resistance.

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