Metal Finishing Guide Book

2013

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accounts for the elastic displacement that occurs in both the indenter and the sample. For a test material with elastic modulus EIT it can be calculated by 1 1– ν 2 1– ν i2 + = Er E IT Et Figure 4: Load-displacement curve with defined creep period at maximum load measured on a nanoindentation tester. Here is the Poisson's ratio for the test material, and Ei and i are the elastic modulus and Poisson's ratio of the indenter, respectively. Creep can be used to characterize material behavior at a constant load. Indentation Creep is defined as an increase in penetration depth under constant load. As shown in Figure 4 the selected final load is kept constant for defined time duration and the indentation depth is measured. Indentation Creep, CIT is calculated as ⎛ h – h1 ⎞ . CIT = ⎜ 2 ⎟ 100% ⎝ h1 ⎠ h1: indentation depth at the start of the creep test h2: indentation depth at the end of the creep test Comparing Traditional Hardness Testing to Nanoindentation Hardness As hardness is already being measured for most applications it is important to understand the correlation between these traditional forms of hardness and Instrumented Indentation Hardness. Vickers Hardness vs. Nanoindentation Hardness Surface hardness of hard materials is commonly measured with Vickers or Knoop indenters with traditional microhardness testers. While these tests are still reliable to characterize the hardness of most bulk materials they are not as effective for coatings and thin films. The loads used in traditional microhardness testers are usually too high and results are affected by the properties of the underlying layer. And because the indentation is measured optically, reproducibility and accuracy of the data collected are affected by the quality of optics and user's definition of the diagonals of the residual indent. In nanoindentation the measured depth is used to calculate the area of contact. But there is still a relationship between Instrumented Indentation Hardness and Vickers Hardness as a Vickers geometry indenter is used in both tests. Even the Berkovich geometry indenters that are also used in nanoindentation simulate the same strain rates as a Vickers geometry indenter. Thus, the relationship between Instrumented Indentation Hardness and Vickers Hardness is defined as HV = 0.0945 HIT 5 453

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