Inspection of Titanium Nitride Coatings by Tribometer

Hard composite ceramic coatings deposited by PVD/CVD process are widely used in a variety of machine tooling applications, such as razor blades, metal cutting, injection molding and sawing, in order to improve the work efficiency and service lifetime of the metal components. In particular, transition metal nitrides, such as titanium nitride (TiN), TiSiN and TiCN possess superior hardness, chemical stability and wear resistance. They become the best protective coatings under extreme working conditions in aggressive environments.

Wear of the tools in service creates loss of dimensions and functionality of the tools. It has significant influence on the tool life, as well as the surface integrity and dimension accuracy of the finished products. The tribo-mechanical properties of the protective ceramic coatings can substantially enhance the service performance and lifespan of the machine tools. Reliable and accurate quality control of such protective coatings becomes vital to ensure quality performance of the tools.

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The coefficient of friction, COF, and the wear resistance of a batch of Titanium Nitride (TiN) coating samples were evaluated by the NANOVEA Tribometer using the linear reciprocating wear module. An Al2O3 ball (6 mm diameter) was used as the counter material. The test parameters are summarized in Table 1. Wear track 3D profiles were measured by the NANOVEA Optical Profilometer, and the wear track morphology was examined using optical microscope.

The evolution of the COF during the wear tests of the TiN samples is plotted in Fig. 2. The TiN Samples 1, 2 and 3 exhibit a progressively increasing COF from ~0.1 to ~0.5 during the first 600 cycle run-in period. It then gets relatively constant at ~0.5 in the following wear test. In comparison, Sample 4 shows a higher COF of ~0.2 during the run-in period of 400 cycles. Violent fluctuation of the COF takes place as the coating is damaged and the metal substrate is exposed.

Fig. 3 shows the wear tracks of the TiN coatings under the optical microscope after the wear tests. The wear track evaluation is in agreement with the observation of COF evolution: the TiN coating has a rough surface finish with directional parallel texture created during machining of the metal substrate. For Samples 1, 2 and 3, the surface asperities were worn out and flattened during the wear tests across the surface texture, creating a relatively smooth and shallow wear track. The TiN coatings did not fail and they still played a protective role during the wear test. However, the TiN coating of Sample 4 wore out quickly during the wear test and left the steel substrate exposed and unprotected. It resulted in a substantially accelerated wear process which generated a deep wear track on the surface.

Fig. 4 compares the false color view of the wear tracks of the TiN coating Sample 1 (Pass sample) and Sample 4 (Fail sample) after the 5-min wear tests, which can provide more insight in the wear failure mechanism. The wear is restricted in the TiN coating for Sample 1 and it creates a shallow wear track on its surface. The wear process, however, forms a significantly larger wear track on Sample 4 due to the failure of the TiN coating.

Fig. 5 compares the wear scars of the Al2O3 balls after the tests under the optical microscope. Samples 1, 2 and 3 exhibit similar mild wear scar morphology. In comparison, the ball against Sample 4 has a much larger wear scar showing metal and coating debris attached to it.

In this study, we showcased the capacity of the NANOVEA Tribometer in performing fast and accurate inspection of the protective TiN coatings in a controlled and monitored manner. Quality coating enhances wear resistance of the metal substrate and tremendously extends its service lifetime. This study demonstrates an easy and reliable method to evaluate and inspect the quality of the protective coating. By setting up a standard test procedure and conditions, the user can quickly find out the defective coatings during the quality control process.

The NANOVEA Tribometer offers precise and repeatable wear and friction testing using ISO and ASTM compliant rotative and linear modes, with optional high temperature wear, lubrication and tribo-corrosion modules available in one pre-integrated system. NANOVEA’s unmatched range is an ideal solution for determining the full range of tribological properties of thin or thick, soft or hard coatings, films and substrates.