Tribological Testing of Hard Coatings for Cutting Tools

The latest range of hard coatings meant for cutting tools used for contemporary drilling and machining applications has excellent resistance against wear and abrasion even in adverse environment and at extreme temperatures1. These coatings are often based on AlTiN and are increasingly being used in many challenging applications. The hard coatings have Young’s modulus (up to ~500GPa) and extremely high hardness (up to 35 GPa). Before these coatings are used in real life conditions, it is necessary to measure their abrasion and wear resistance as well as their adhesion and hardness properties. Several tests are available that mimic the actual working conditions which are important for end-users.

Standardized pin-on-disk testing is one of the first steps involved in the development of hard coatings because it provides a well-defined environment as well as controlled testing conditions (rotating speed, applied load, number of laps, etc.). Earlier, the pin-on-disk (shown in Figure 1 for Anton Paar Pin-on-disk tribometer) tests have been a central part of research and development of various hard and protective coatings.

Anton Paar pin-on-disk tribometer for testing up to 400 °C.

Figure 1. Anton Paar pin-on-disk tribometer for testing up to 400 °C.

However, the latest generation of extremely wear and abrasion resistant coatings shows so much wear resistance that even the standard pin-on-disk tribological tests lead to very low to almost non-measurable wear. Therefore, a valid set of high-temperature and room-temperature wear test parameters should be established to perform efficient tribological testing and determination of wear resistance of the new generation of hard coatings.

This article shows how tribological testing of the new generation of hard coatings is made using the Anton Paar THT800 High temperature tribometer. The measurements were made on AlTiN, oxynitride coating and nanostructured AlCr-based nitride coating. Using an industrial rotating cathodes arc PVD process, all the coatings were deposited on cemented carbide (WC-Co) coupons measuring 10 mm in thickness and 50 mm in diameter 2. The coating contained nitrogen which was gradually replaced by oxygen up to 99 at.% to create oxynitride structure. This is done to prevent the coatings from being oxidized at high temperatures. This new generation of oxynitride hard coatings can endure very high temperatures in dry milling and turning of high-strength materials and at the same time can maintain high wear resistance. Yet, applying the common tribological tests to characterize the wear resistance of these hard coatings has been shown to be highly complicated, making it necessary to establish new testing procedures.

The advanced THT 800 High Temperature Pin-on-Disk Tester was used in a preliminary test so as to achieve a valid set of parameters leading to measurable wear of the coatings. Subsequently, the same instrument was employed for systematic characterization of wear resistance of these highly wear resistant coatings.

The pin-on-disk tests with a duration of up to ~4 hours were carried out at temperatures up to 800 °C and subsequent analyses were used to quantify the wear resistance of the coatings. The Anton Paar THT800, as well as THT1000 reaching 1000 °C, pin-on-disk tribometers are fitted with an automatic arm, which helps initiate automatic measurements as soon as the test temperature is reached. A double LVDT sensor on each side of the automatic arm was used to measure the tangential (frictional) force so that the impact of temperature on the tangential force measurement is removed. Efficient cooling of the tribometer is achieved with water containing closed loop independent cooling circuit. The system has a number of safety features to ensure seamless function among other safety features against cooling and overheating circuit malfunction.

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This information has been sourced, reviewed and adapted from materials provided by Anton Paar GmbH.

For more information on this source, please visit Anton Paar GmbH.


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