Surface hardening is a procedure that hardens the surface of a metal object and at the same time maintains the softness and toughness of the core metal deeper underneath. The combination of soft interior and hard surface metal is crucial for attaining remarkable resistance to breakage upon impact such as fatigue and high stress and at the same time strengthening the wear protection of the metal surface. For numerous industrial applications, such a unique property is required in parts, such as antifriction bearings or shafts, a ring gear or cam, turbines and automotive componentsi.
Importance of Nanoindentation Mapping for Surface Hardened Metal
Generally, surface hardening can be classified into the following two approaches: (1) buildup or addition of a new layer; (2) surface and subsurface modification without any additional layer. Coatings, thin films, and overlayers are deposited to provide an enhanced surface in the first case. Here, the homogeneity and interfacial bond strength play a key role.
In the second case of direct surface modification such as nitriding, carburizing, and peening, there is usually no clear boundary between the treated surface and the metal underneath. This proves to be a challenge for determining the thickness of the surface layer. Nanoindentation provides exceptionally localized precise hardness and Elastic Modulus measurements which makes it an ideal tool for hardness mapping to ascertain the homogeneity, quality and thickness of the surface hardened layer.
In this article, the Nanovea Mechanical Tester in nanoindentation mode is used to ascertain the hardness distribution of the hardened surface on the cross section of a surface hardened metal. The capability of Nanovea Mechanical Tester in performing nanoindentation mapping with high reproducibility and preciseness is showcased in the course of this article.
Figure 1. Nanoindentation tip on the cross section of the surface hardened metal.
The hardness, H and Elastic Modulus, E, of 120 points along the distance from the treated metal surface on the cross section were measured spaced 50 μm apart for a total distance of 6 mm as represented in Figure 2. In Table 1, the test parameters are summarized.
Figure 2. Diagram of test locations.
Table 1. Test conditions of the hardness measurements.
|Maximum force (mN)
|Loading rate (mN/min)
|Unloading rate (mN/min)
||Berkovich diamond tip
Results and Discussion
Surface hardening strengthens the surface hardness while retaining the interior metal’s tough, soft nature. Hardness distribution measurement along the thickness of the treated surface guarantees the homogeneity and quality of the surface treatment. In Figure 3, the H and E values as a function of the distance from the treated metal surface are plotted. Figure 3 shows that the metal sample possesses the highest H value of ~9 GPa at the surface.
The H value constantly decreases to a value of ~5.5 GPa until a distance of 3 mm from the surface is reached, after which the metal has a consistent hardness of 5.5 ± 0.5 GPa. The Elastic Modulus maintains a homogeneous value of 225 ± 12 GPa for the surface and interior metal despite the hardness enhancement on the surface. The load-displacement curves measured at the surface and interior of the metal is shown in Figure 4.
Figure 3. Hardness and Elastic Modulus vs. Distance from the treated surface on the cross section of the test sample.
Figure 4. Load-displacement curves measured at the surface and interior of the metal.
To sum up, the Nanovea Mechanical Tester in Nanoindentation mode provides reliable measurement of hardness and Elastic Modulus distribution on a surface hardened metal sample. It allows users to assess the quality of the surface treatment and to determine the thickness of the surface layer or coating, which is vital in research and development and quality control for the coatings in a number of industries, such as aerospace, cutting tools, automotive and many others.
All the modules of the Nanovea Mechanical Tester such as the Nano, Micro, or Macro modules include ISO and ASTM compliant indentation, scratch and wear tester modes and provides the widest and most user friendly range of testing available in a single system. Nanovea's unparalleled range is a perfect answer for determining the entire range of mechanical properties of films and substrates, thick or thin, hard or soft coatings, including hardness.
Additionally, optional 3D non-contact profiler and AFM Module are available for high resolution 3D imaging of indentation, scratch and wear track as well as other surface measurements such as roughness.
i Michael J. Schneider, Madhu S. Chatterjee, Introduction to Surface Hardening of Steels, ASM Handbook, Volume 4A, Steel Heat Treating Fundamentals and Processes
This information has been sourced, reviewed and adapted from materials provided by Nanovea.
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