The optical coating industry has adopted the US Military Specifications (often referred to as “MIL specs”) as a standard for testing the mechanical properties of coatings. All NOC coatings are adhesion tested and inspected for surface quality to the grade required by the customer. At NOC we can also test against humidity, abrasion and other MIL specs as necessary.
3.4.1.2 Scratches. - Surface scratches (coating and substrate) shall not be in excess of the values specified on the component drawing or procurement document (See 6.2f). Scratches are permissible provided the width does not exceed that specified by the scratch letter. The accumulated length of all maximum scratches shall not exceed 1/4 of the average diameter of the element, The scratch letter and corresponding width are shown in Table I.
| Scratch No. (MIL-PRF-13830B) | Scratch Letter | Scratch Width (mm) | Scratch Width (inch) | Disregard Scratch less than (mm) | Disregard Scratch less than (inch) |
|---|---|---|---|---|---|
| 5 | A | .005 | .00020 | .0010 | .00004 |
| 10 | B | .010 | .00039 | .0025 | .00010 |
| 20 | C | .020 | .00079 | .0050 | .00020 |
| 40 | D | .040 | .00157 | .0100 | .00039 |
| 60 | E | .060 | .00236 | .0100 | .00039 |
| 80 | F | .080 | .00315 | .0200 | .00079 |
| 120 | G | .0120 | .00472 | .0200 | .00079 |
TABLE I, Scratch Identification (See 6.3.1)
3.4.1.3 Digs. - Surface digs (coating and substrate) shall not be in excess of the values specified on the component drawing or procurement document (See 6.2g). Digs are permissible on a surface provided the average diameter does not exceed that specified by the dig letter and no more than (1) maximum size dig occurs in any 20mm (0.8”) diameter circle on the substrate. The dig letter and corresponding average diameter are shown in Table II.
| Dig no. (MIL-PRF-13830B) | Dig Letter | Avg. Dig Diameter (mm) | Avg. Dig Diameter (inch) | Disregard Digs less than (mm) | Disregard Digs less than (inch) |
|---|---|---|---|---|---|
| 5 | A | .05 | .0020 | .010 | .00004 |
| 10 | B | .10 | .0039 | .025 | .00004 |
| 20 | C | .20 | .0079 | .050 | .00004 |
| 30 | D | .30 | .0118 | .050 | .00004 |
| 40 | E | .40 | .0157 | .100 | .00004 |
| 50 | F | .50 | .0197 | .100 | .00004 |
| 70 | G | .70 | .0276 | .200 | .00004 |
| 100 | H | 1.00 | .0394 | .250 | .00004 |
TABLE I, Scratch Identification (See 6.3.1)
6.3.1 Scratch and dig identification. - This is specified by two letters separated by a hyphen (i.e. F-F). The first letter of the pair is the maximum scratch letter. The second letter is the maximum dig letter.
The coated component, or witness piece, shall be subjected to an adhesion test using 1/2" (12.7 mm) wide cellophane tape conforming to type I of L-T-90. Press the adhesive surface of the cellophane tape firmly against the coated surface so as to cover the stained area, then quickly remove it at an angle which is normal to the coated surface. Immediately following the removal of the adhesive tape, the coated surface of the component, or witness piece, shall be examined by reflection using the inspection technique specified in 4.6.4. The coating shall conform to the requirements of 3.4.2.1.1. Subsequent to this test the coated component, or witness piece, shall be subjected to the test in 4.6.8.2.
The coated component, or witness piece, shall be placed into an environmental controlled test chamber and exposed to a temperature of +120° +/-4°F (49° +/-2°C) and 95% to 100% relative humidity for a minimum of 24 hours. Subsequent to this exposure the coated component, or witness piece, shall be removed from the test chamber, cleaned (see 4.6.1), dried, and then subjected to the examinations specified in 4.6.7.1 and 4.6.7.3. The coating shall conform to the requirements of 3.4.2.1.2. The coated component, or witness piece, shall then be subjected to the test specified in 4.6.8.3.
Within one hour after the humidity test of 4.6.8.2 the coated component, or witness piece, shall be subjected to a moderate abrasion be rubbing the coated surface with a 1/4 inch (6.4mm) thick by 3/4 inch (9.5mm) wide pad of clean dry, laundered cheesecloth conforming to CCC-C-440 affixed to an abrasion tester that conforms to drawing D7680606.
The cheesecloth pad shall completely cover the eraser portion of the tester and be secured to the shaft with an elastic band. The cheesecloth pad shall be rubbed across the coated surface from one point to another over the same path for 25 complete cycles (50 strokes) with a minimum force of 1.0 pound (0.45kg) continuously applied. The length of the stroke shall be approximately equal to two widths of the cheesecloth pad when the diameter of the area of the component permits.
The abrasion tester shall be held approximately normal to the surface under test during the rubbing operation. Subsequent to the rubbing operation the component, or witness piece, shall be cleaned (see 4.6.1), dried and then subjected to an examination by reflection using the inspection technique specified in 4.6.4 for evidence of physical damage to the coating. The coating on the component, or witness piece, shall meet the requirements of 3.4.2.1.3.
4.6.9.1 Temperature. - The coated component, or witness piece, shall be subjected to temperatures of -80°F ±2°F (-62°C ±1°C) and +160°F ±2°F (+71°C ±1°C) for a period of 2 hours at each temperature. After each exposure the coated components or witness pieces shall be stabilized at an ambient temperature between +60°F and +90°F (+16°C and +32°C) and subjected to the examination and test specified in 4.6.7.1 and only the adhesion test of 4.6.8.1. (Adhesion) The coating shall conform to the requirements of 3.4.2.2.1.
4.6.9.2 Solubility and Cleanability. - The coated component, or witness piece, shall be immersed, in sequence, in the following solutions maintained at room temperature (+60°F to +90°F): Trichloroethylene; Acetone; and Ethyl Alcohol. The immersion time in each solution shall be a minimum of ten minutes. Upon removal from each solution, the solvent shall be allowed to evaporate to dryness without wiping or forced drying before proceeding to the next solution. Upon removal from the alcohol solution, and after drying any resulting stains on the coated surface shall be removed by wiping the coating to a clean, stain-free condition with an ethyl-alcohol-moistened cheesecloth. Subsequent to this cleaning, the coating shall be subjected to the examinations specified in 4.6.7.1 and 4.6.7.3. The coating shall conform to the requirements of 3.4.2.2.2.
4.6.10.3 Water solubility. - The coated components, or witness piece, shall be immersed for a period of 24 hours in distilled water at room temperature (+60°F to +90°F). Subsequent to this immersion, the coated components, or witness piece, shall be removed from the solution, cleaned (See 4.6.1), dried, then subjected to the examination specified in 4.6.7.1 and 4.6.7.3. The coating on the component, or witness piece, shall meet the requirements of 3.4.2.3.3.