Hardness testing

Seal face hardness testing 12.8

The ideal seal face combination is a good grade of carbon/graphite running against a corrosion resistance hard face materials. In another paper I covered the different grades of carbon/graphite, but just what do we mean when we say, “hard face”?

There are a lot of them available that include:

  • Several grades of ceramic along with different grades of ni-resist and stellite
  • Both tungsten and nickel base tungsten carbide
  • Alpha sintered and reaction bonded silicone carbide

In the following paragraphs we’ll be looking at the common methods seal people use to measure the hardness of a seal face and then we will be looking at a chart to learn how to convert from one method to another.

The first method we will look at is called the Brinell test method

  • In this test a hardened steel ball is forced into the material at a given pressure. The width of the depression then becomes the measure of the material’s hardness.
  • Because the ball deforms on very hard surfaces, this test is somewhat limited in its use.

The Moh’s scale is the next one, but I have no illustration to show you. This method compares scratch hardness with ten minerals used as standards. Unlike the other scales mentioned above; in this method the steps are not equal. The difference between #9 and #10 is about as great as the difference between #1 and #9.

The oldest method of testing hardness was to use a hard file on the test piece and see how difficult it is to remove material. One look at the following comparisons and you can learn why this method is seldom used any more, but if you do not have any test equipment it is better than nothing.

Above a number of 350 the standard machining operations of turning, boring, drilling, and tapping become uneconomical. You can compare a Brinell reading to the following file readings :

  • 100 Metal removed easily by the file.
  • 200 Slightly more pressure needed to remove metal.
  • 300 Metal shows resistance to the file.
  • 400 Takes more pressure on the file.
  • 500 File removes almost no metal.
  • 600 Metal cannot be filed.

The next method is called the Rockwell hardness test.

  • This is the most widely used test in the seal business. Hardness is read on two different scales.
  • The most popular is the “C” scale that uses a diamond cone. The less popular “B” scale utilizes a ball similar to that used in the Brinell test.

Mechanical seal faces should read at least 60 on the Rockwell “C” scale. You can consult the following chart to convert this reading to other scales.

Rockwell C
Brinell Scale
72 772 106
70 760 102 8.5
69 755 98
67 725 94
66 712 93
65 699 92
63 672 89 8.0
62 657 86
61 645 85
60 631 84
59 617
58 603 81
57 590
56 577 78 7.5
55 562 75
53 536 73
52 523
51 510 71
49 486
48 473 66
47 462 64 7.0
The last illustration describes the Scleroscope test.
  • Although widely used in industry we seldom find this method used in the seal business. In this test we let a weight with a hard round end fall ten inches (255 mm) through a glass tube. We get our reading from determining how high the weight bounced off the test sample. The harder the material, the higher the bounce.
  • The tube on the scale is marked in 140 increments. On this scale glass would read 130 and hardened steel would record about 110.



  • On February 18, 2018