Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:firstname.lastname@example.org
Physical Characteristics and Testing of
PART I - HARDNESS AND TESTING
Hardness is defined as resistance to penetration. The hardness of various electroplated and electroless plated deposits has been and continues to be the subject of much discussion and controversy. For example, hard chrome is often reported to have 1000 to 1200 Vickers hardness. Actually the hardness of hard chrome can vary significantly if the plating conditions of temperature and current density are not controlled in certain specific ranges. The hardness can be as low as 670 Vickers hardness number or as high as 1200. It is the greatly dependent on plating conditions. Often hard chrome platers do not know what temperature and current density are required for the highest hardness. There are charts in the literature.
In addition, the hardness test results can vary. If the deposit thickness is insufficient to support the indentation, low readings result. If the gram load is not specified in the report of hardness, the result is meaningless. Most of the early tests were done using a Vickers indenter. ASTM-384, (ASTM is the American society for Testing Materials) an older specification calls for a 100-gram load for hard deposits such as hard chrome and electroless nickel. Some people have used lesser or higher loads. The Vickers indenter is a pyramid-shaped diamond. The thickness must be at least 0.001” ( 1 mil). Thinner deposits are likely to result in incorrect readings. Higher gram loads can cause cracking of the deposit resulting in an error. Lower loading for hard deposits can also result in erroneous readings. Vibration of the indenter during tests can cause errors. Poor specimen preparation such as the specimen is mounted on a slight slant, or the polishing is not done properly.
The most recent ASTM specification calls for the use of the Knoop indenter that uses a rhombic-shaped indenter. It is thinner and longer than the Vickers. Both ASTM and NIST (National Institute of Standards and Technology) specify the Knoop hardness test for hard deposits and Vickers is no longer recommended. The Rockwell indenter is spherical and is not recommended for plated deposits. There are charts in the literature that show the approximate relationship between the various hardness tests. These are not accurate because of the differences in results due to the procedures. Knoop hardness is specified for soft metals but with lower gram loads.
Electroless nickel varies in hardness, similarly to chrome, but for different reasons. The lower the phosphorus in the EN deposit, the harder is the deposit until the phosphorus (P) content is somewhat lower then 4%. The highest as plated hardness is about at about 4% phosphorus and is about 700-750 Knoop, 100-gram load (about the same using Vickers).
The heat-treated hardness is from 970 to 1000 Knoop, 100 gm load (or Vickers 980 to 1010) The heat-treat temperature is best at about 385 degrees C.
The as plated hardness of 10-11% P is about 500 Knoop-100 gm load. These deposits can be heat-treated to a hardness of about 890 Knoop, 100 gm load. The Heat-treat temperature for maximum hardness is about 395 degrees C. The literature usually suggests a heat-treat temperature of 400 degree C (750F) for 1 hour. Note if the oven is already at temperature it really only takes about ½ hour to reach the maximum hardness. Much longer times than 1 hour can lower the hardness somewhat. And higher temperatures above 400 degrees C can soften the deposit.
For Mid Phosphorus electroless nickel deposits 6-8% P are about 900-950 Knoop, 100- gm load. Be sure to prepare the specimen according the recommended practices of ASTM (American Society for Testing Materials) and/or NIST (National Institute of Standards).
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