Tribological Testing of Advanced 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 temperatures [1]. 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.

Materials and Pin-On-Disk Tribological Test Parameters

The protective coatings were deposited using π Technology: LARC® Lateral- and CERC® Central Rotating Arc Cathodes in a deposition system developed by Platit AG (Selzach, Switzerland). The oxynitride and nitride coatings were deposited in O2/N2 atmosphere and bias voltage from -30 V to -100 V using medium frequency. The deposition was done at a temperate of 550 °C on cemented carbide coupons with a diameter of 50 mm and a thickness of 10 mm.

As shown in Figure 2, the multilayer contains buffer layer(s), adhesion layer and functional coating. The functional (top) layer, vital for wear properties, was AlCrN nitride coating, AlTiN coating and AlTiN coating. The thickness of the whole coating system was ~5 mm.

The protective layer structure of the tested coatings.

Figure 2. The protective layer structure of the tested coatings.

On the Anton Paar THT800 pin-on-disk tribometer (Figure 3), the pin-on-disk tribological tests were performed at temperatures of 24 °C (room temperature), 600 °C, and 800 °C. The preliminary tests revealed that use of normal load of 7 N (10 N for 24 °C tests), linear velocity of 20 cm/s, and alumina ball with 6 mm diameter as a counterbody lead to measurable wear on majority of coatings. The worn cup on the alumina ball and the wear track on the sample were examined in an optical microscope to view the morphology of the wear track and to calculate the worn cup diameter on the alumina ball.

For the less wear resistant coatings, the duration of the tribological tests was 32’000 laps and for more wear resistant coatings the duration was up to 40’000 laps. The test resulted in total duration between ~120 minutes and ~240 minutes, based on the radius of the wear track. To model the duration of real milling/cutting times as closely as possible, long duration of the pin-on-disk tests was required. High temperature tribometer can easily endure such high temperatures for a prolonged period of time. The performance of such a tribometer was thus critical for these measurements.

THT1000 Pin-on-disk Tribometer with heated sample showing the main components for load application and tangential force measurements. The upper heater is used to quickly reach the maximum temperature of 1000 °C.

Figure 3. THT1000 Pin-on-disk Tribometer with heated sample showing the main components for load application and tangential force measurements. The upper heater is used to quickly reach the maximum temperature of 1000 °C.

Pin-on-Disk Tribological Tests Results

Coefficient of Friction

Coefficient of friction (CoF) is one of the key results of pin-on-disk tribological tests. This coefficient is directly related to friction force that occurs between the two sliding bodies and also indicates how well the bodies react to mutual contact. If there is a stable CoF, both materials remain relatively intact and exhibit good wear resistance. However, when the CoF is varying considerably, it usually leads to higher wear. With regard to the tested coatings, the CoF at room temperature was extremely stable for all samples, while the CoF variation increased with increasing temperature for the majority of the samples barring the oxynitride sample.

Comparison of coefficient of friction of the tested coatings at 24 °C, 600 °C and 800 °C. Note large variations of the CoF for the AlTiN coating indicating severe damage while the variation of CoF of the AlCrON remains relatively low even at 800 °C.

Comparison of coefficient of friction of the tested coatings at 24 °C, 600 °C and 800 °C. Note large variations of the CoF for the AlTiN coating indicating severe damage while the variation of CoF of the AlCrON remains relatively low even at 800 °C.

Comparison of coefficient of friction of the tested coatings at 24 °C, 600 °C and 800 °C. Note large variations of the CoF for the AlTiN coating indicating severe damage while the variation of CoF of the AlCrON remains relatively low even at 800 °C.

Figure 4. Comparison of coefficient of friction of the tested coatings at 24 °C, 600 °C and 800 °C. Note large variations of the CoF for the AlTiN coating indicating severe damage while the variation of CoF of the AlCrON remains relatively low even at 800 °C.

Based on these results, the AlCrN and AlTIN coatings performed well at room temperature while the variation of the CoF of both these coatings increased at 600 °C and 800 °C, demonstrating extensive damage to the coating. The AlCrON coating also performed well up to 600 °C and its CoF was extremely stable and showed consistent CoF values of ~0.5 from room temperature up to 800 °C. A slight decrease of CoF at 800 °C can be caused by the formation of tribofilm or protective layer as a result of contact with the counterbody at high temperature.

Wear Resistance and Wear Rate

Wear rate is a measure of the material’s wear resistance in a pin-on-disk test. When the static counterbody (alumina ball) comes into contact with the rotating sample, it causes damage to the coating resulting in material removal and subsequent wear of the coating. Wear can be measured as the volume of material removed from the sample – this volume can be calculated for the static counterbody (alumina ball) as well as the rotating sample. The wear rate is the volume loss normalized by the applied load and test distance. The volume of material removed from the sample is obtained by measuring of the wear track profile by surface profilometer. Here, the Taylor Hobson profilometer was used and on each sample a minimum of six measurements were made along the wear track, as shown in Figure 5.

Schematic illustration of the wear track and surface profilometer measurements (scans).

Figure 5. Schematic illustration of the wear track and surface profilometer measurements (scans).

The wear rate was measured using the following formula:

(1)

Where,
V = Volume of the material removed
d = Total test distance
P= Applied load

The accumulation of material can be observed in some cases: there was material build-up at the sample surface instead of (or in addition to) material removal. The corresponding measure was subsequently termed as build-up rate and the same formula (1) was applied to calculate the build-up rate with V representing the volume of the material build-up.

At room temperature, the wear rate was non-measurable for all coatings barring the AlTiN coating. This also corresponds to low variation of CoF of all coatings excluding the AlTiN coating. However, AlTIN coating exhibited a high wear rate at 600 °C and the AlCrN coating also showed a higher wear rate than at room temperature. At 600 °C, the wear rate of the AlCrON was still extremely low but at 800 °C both AlCrN and AlTiN coatings showed extensive damage. At 800 °C, the AlCrON coating alone remained relatively intact with a low level of wear.

As shown in Figure 6, the AlTiN coating exhibited lower wear rate at 800 °C than at 600 °C and also at room temperature. Further, the wear track profiles in some areas on the AlCrN and AlTiN coatings at 800 °C did not exhibit the typical wear scare profile but rather a relatively large build-up of material. At 800 °C, a relatively low wear rate of the AlTiN coating is seen (Figure 6), but the build-up rate on this coating was observed to be very high. The high build-up rate (i.e. large build-up) and low wear rate (i.e. shallow wear track) at 800 °C on the AlTiN coating sample was subject to more studies by X-Ray Energy Dispersive Analysis (EDX) and scanning electron microscope (SEM).

Comparison of wear rates at 24 °C, 600 °C and 800 °C for the three tested coatings.

Figure 6. Comparison of wear rates at 24 °C, 600 °C and 800 °C for the three tested coatings.

Image of the wear track on the TiAlN coating after the 24 °C and 600 °C tests. Note area with extensive damage to the coating where oxidation of the substrate occurred.

Figure 7. Image of the wear track on the TiAlN coating after the 24 °C and 600 °C tests. Note area with extensive damage to the coating where oxidation of the substrate occurred.

These methods were used on the AlTiN sample following the high temperature (800 °C) tests to explain the wear behavior with high build-up rate and low wear rate.

Following a series of observations of the surface of the wear track of this particular sample in the SEM, it was concluded that the coating predominantly failed because of the cohesive fracture and coating delamination as shown in Figure 8. This resulted in exposition of the substrate and oxidation of the Co component in the WC-Co substrate material. Eventually, the growth of the Co oxide resulted in the formation of build-up in the wear track (Figures 7 and 8). Also, EDX analysis proved that the Co oxide exist in the wear track. The seemingly low wear rate values, as measured from the surface profilometer measurements, were the result of the oxidation of the substrate. The rate of build-up was found to be highest on the TiAlN coating at 600 °C and 800 °C.

Wear track on the AlTiN coating after the 800 °C pin-on-disk tests.

Figure 8. Wear track on the AlTiN coating after the 800 °C pin-on-disk tests.

By contrast, the newly developed AlCrON coating exhibited only slight wear with the least traces of interaction with the alumina counterpart at 600 °C and 800 °C. Moreover, the SEM analysis of the wear track on the AlCrON oxynitride coating showed only slight damage to the coating (and also to the alumina ball). However, the EDX observation of the wear track did not reveal any traces of the elements from the underlying materials, demonstrating very good wear resistance and integrity of this type of oxynitride coating.

Ball Wear

In majority of pin-on-disk measurements, the wear of the static partner can also be quantified easily. In this case, the static partner was a 6 mm diameter alumina ball; the ball’s wear rate was measured as the volume of the removed cap, normalized by the applied load and distance. The ball’s wear rate was increased with increasing temperature for the AlCrN and AlTiN coatings, whereas it remained extremely low for the AlCrON coating. This result agrees well with the wear rate and build-up rates of the coatings, confirming that very good wear resistance is exhibited by the AlCrON coating.

Conclusions

This article describes the application of high-temperature Anton Paar THT800 pin-on-disk tribometer for the characterization of a new generation of hard coatings, specifically designed for use at high temperatures. The THT800 pin-on-disk tribometer is especially suitable for determining a set of testing parameters, thus enabling to rank the AlCrON coating along with other types of coatings. Due to the dual LVDT tangential force measurement along with the powerful instrumental design, the CoF can be reliably measured even during experiments with a duration of more than four hours.

The tests demonstrated that the AlCrON coating at temperatures as high as 800 °C shows very good oxidation resistance and wear resistant properties. Thus, the THT800 High Temperature Pin-on-Disk Tester has been shown to be a crucial tool for the characterization of high-temperature tribological properties of new generation of hard coatings.

Bibliography

[1] K.Holmberg, A. Matthews, H. Ronkainen, Coatings tribology-contact mechanisms and surface design. Tribology International 31 (1-3) (1998) 107-120.

[2] H. Najafi, A. Karimi, P. Dessarzin, M. Morstein, Correlation between anionic substitution and structural properties in AlCr(OxN1-x) coatings deposited by lateral rotating cathode arc PVD. Thin Solid Films 520 (2011) 1597-1602

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

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

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