TASK 70-71-03-700-004
1 . General.
Abradable coatings applied by thermal spraying can be evaluated satisfactorily by the scratch-hardness test. This procedure describes the technique and equipment used for the test, and the method of evaluating it.
2 . Equipment.
Subtask 70-71-03-700-041
A.The following items are required for performing the scratch-hardness test:
(1)Hoffman scratch-hardness tester, Model SG-1610 (modified), obtainable from BYK-Gardner. Refer to the List of Suppliers in Step 4 of 70-80-00.
Subtask 70-71-03-700-044
(2)Pocket comparator C05-112   having 6X magnification equipped with a glass reticle C05-126   having 1X magnification or a comparator equipped with surface illumination capable of reading scratch width within 0.005 inch (0.13 mm) with 10X maximum magnification.
3 . Procedure.
Subtask 70-71-03-700-042
B.Conduct the scratch-hardness test as directed in the following steps:
(1)Preparation of Test Specimen.
The test specimen is a flat plate which has been processed with the production part that it represents, and which is made of the same material. Minimum coating thickness should be 0.035 inch(0.90 mm). Size of the test specimen is not critical; however, the sprayed area should be at least 1 x 3 inches (25 x 76 mm) to permit satisfactory testing. Prepare the specimen for testing as follows:
(a)Test is to be conducted on the as-sprayed surface. Do not file or smooth the coating surface.
(b)Dust surface of thermal sprayed area with fine white talc.
(c)Using a cardboard straightedge, scrape off excess talc to a thin, uniform coating which will provide contrast between the surface and the scratch path.
(d)Place on a flat supporting surface, with the test surface up.
(2)Test Setup.
(a)Prepare the equipment for making the test, as follows:
1Insert the carbide scratching point as deeply as possible into the carriage of the Hoffman scratch-hardness tester, and secure in place.
2Place the beam in the carriage by inserting the 2 notches at the pointed end of the beam into the pivoted stirrup above the scratching tool so that the notches interlock firmly with the pins.
3Set the left edge of the rider weight at number 20 on the beam. This setting places a 2000 gram load on the scratching tool.
(3)Performing the Test.
(a)Make the scratch test in accordance with the following steps:
1Position the carriage of the tester on the prepared surface of the test specimen so that a 2 inch (51 mm) long scratch can be made. Hold the carriage with the right hand so that all 4 wheels are in contact with the coated surface of the test specimen. Simultaneously, raise the beam of the tester with the left hand.
2With the beam still raised, grasp the carriage directly over the pair of wheels nearest the scratching tool with the thumb and first 2 fingers of the right hand. See Figure 1. Hold the carriage so that the wheels are firmly in contact with the test surface during the test.
3Lower the beam gently with the left hand until the scratching tool rests unsupported on the test surface.
4Draw the carriage across the test surface in a straight line for approximately 2 inches (51 mm), making sure that all 4 wheels of the carriage and the scratching tool maintain contact with the test surface.
5Make a total of 3 scratches, employing the technique described in steps (2) through (4) above. The scratches should be approximately parallel, and about 0.250 inch (6.35 mm) apart.
* * * FOR ALL
Figure 1   Hoffman Scratch-Hardness Tester
4 . Evaluating the Test.
Subtask 70-71-03-700-043
C.Get the Hoffman Scratch Hardness number by either of the these methods:
(1) Use the pocket comparator (6k) with reticle in accordance with Subtask 70-71-03-700-044, Equipment:
(a)On each scratch, make three readings approximately 0.50 inch (12.7 mm) apart, reading each to the nearest Hoffman number.
(b)Record the nine Hoffman readings and average them. Report this average as the average Hoffman Scratch Hardness number.
(2) Use the comparator with surface illumination in accordance with Subtask 70-71-03-700-044, Equipment:
(a)On each scratch, make three readings approximately 0.50 inch (12.7 mm) apart, reading each to the nearest 0.005 inch (0.13 mm).
(b)Record the nine scratch width readings and average them. Divide the average by 0.005 inch (0.13 mm) and report this value as the average Hoffman Scratch Hardness number.