Hardness of Plate

Valid hardness measurements of chromium deposits are difficult to make, and values are dependent on test conditions, so hardness values should not ordinarily be used as quality control specifications and routine criteria. Rather, the as-plated brightness can serve as an indication of hardness.

The hardness of chromium plate cannot be accurately determined by the common hardness testers, such as Brinell and Rockwell, because hard chrome is generally too thin for these tests. The indentation produced in these tests distorts the base metal and is influenced by it, which usually results in low hardness readings.

The most reliable and most widely accepted hardness values are those obtained with the Vickers 136 diamond pyramid indenter or the Knoop indenter. With these, the hardness test must be made on a carefully prepared and polished surface, preferably on a cross section of the plate, to eliminate any possible influence of the base metal on the hardness values obtained.

Cracks in the chromium plate influence the hardness values, depending on the type of indenter used and the load applied. In general, lighter loads are more sensitive to hardness variations and result in higher hardness values. Cracks influence values obtained with heavy loads more than values obtained with light loads. Also, because of the smaller area covered by the Vickers 136 diamond pyramid indenter, hardness values determined with this indenter are influenced less by underlying cracks than values obtained with the Knoop indenter.

When conducting microhardness tests, it is important to make sharp and accurate impressions, particularly when using light loads. Also, when hardness values are reported, the load, type of indenter, and optical system used should be indicated. The importance of stating the load and type of indenter is shown by the following data obtained on chromium plate from one plating cycle (each range or average represents 25 tests):

Load, g

Hardness value

136° diamond pyramid

Knoop indenter

100

950-1110 (1040 avg)

940-1090 (1025 avg)

In an investigation of the scratch hardness of chromium deposits of all types, it was observed that bright or semibright deposits had the best combination of hardness and wear resistance, regardless of plating conditions. In this investigation, the wear resistance was measured by means of a specially constructed abrasion hardness machine. The machine contained a small grinding wheel that revolved at 18 rpm. The number of revolutions required to grind through a 25 pm (1 mil) deposit on steel was an indication of the abrasion hardness. Results are summarized in Table 12, which shows the relation between the appearance of deposits and their hardness and resistance to abrasive wear.

Table 12 Relation between appearance and hardness of hard chromium plate deposited from conventional solutions

Average appearance

Average scratch hardness(a),

HB

Average relative abrasive hardness

Matte (cold bath)

640

25

Milky

830

100

Slightly milky

990

290

Bright

1000

300

Slighty frosty

1005

300

Frosty (smooth)

1020

235

Frosty (rough)

1060

125

Burnt

1165

110

(a) Converted to Brinell scale from values obtained with a Bierbaum microcharacter using a 9 g (0.3 oz) load

The effect of temperature on the hardness of electrodeposited chromium is often a significant factor in applications involving wear resistance. The electrodeposited metal begins to decrease in hardness when it is exposed to temperatures above about 205 °C (400 °F). Hardness decreases progressively with an increase in temperature (Fig. 6). As the hardness of chromium plate decreases, its resistance to wear may be affected adversely. Chromium plate should not be used for wear resistance in applications where service temperature exceeds 420 °C (790 °F).

TOOD

Temperstuie, F 400 SOD 12(H)

Temperstuie, F 400 SOD 12(H)

T

—f-

u

0

200 400 600 SOD Temperature, ^C

200 400 600 SOD Temperature, ^C

Fig. 6 Effect of annealing temperature on the hardness of chromium plate deposited during a single cycle in a mixed-catalyst solution. All data represent 25 measurements of each condition; every readable impression was accepted as valid. Heating cycles of 1 h were used.

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