High temperature tribological behavior of advanced hard coatings for cutting tools
High temperature tribological behavior of advanced hard coatings for cutting tools
The new generation of hard coatings for cutting tools for modern machining and drilling applications shows excellent wear and abrasion resistance even at high temperatures and in harsh environment . These coatings, based often on AlTiN, are finding their way into many demanding applications. The coatings have very high hardness (up to 35 GPa) and Young’s modulus (up to ~500GPa). Before putting these coatings in real life conditions, their wear and abrasion resistance as well as their hardness and adhesion must be measured. There is a number of tests that simulate the real working conditions which are crucial for end-user.
However, one of the first steps in development of these coatings is standardized pin-on-disk testing because it offers well defined environment and controlled testing conditions (applied load, rotating speed, number of laps, etc.). The pin-on-disk (see Fig. 1 for Anton Paar Pin-on-disk tribometer) tests have been an indispensable part of research and development of many hard and protective coatings in the past.
Figure 1 – Anton Paar pin-on-disk tribometer for testing up to 400°C.
The new generation of extremely wear and abrasion resistant coatings, however, shows such high wear resistance that the common pin-on-disk tribological tests result in extremely low or almost non-measurable wear. For efficient tribological testing and determination of wear resistance of the new hard coatings it is therefore crucial to establish a valid set of room temperature and high-temperature wear test parameters.
This Application bulletin will demonstrate the utility of the Anton Paar THT800 High temperature tribometer for tribological testing of new types of hard coatings. The measurements were done on AlTiN, nanostructured AlCr-based nitride coating and oxynitride coating. All the coatings were deposited using an industrial rotating cathodes arc PVD process on cemented carbide (WC-Co) coupons with 50 mm diameter and 10 mm thickness . The nitrogen in the coating was progressively substituted by oxygen up to 99 at.% to create oxynitride structure in order to avoid oxidation of the coatings at high temperatures. This new type of oxynitride hard coatings is known to withstand extremely high temperatures in dry milling and turning of high-strength materials while at the same time maintaining high wear resistance. However, characterization of their wear resistance by the common tribological tests had proven to be very difficult and new testing procedures therefore had to be established.
The state-of-the-art THT 800 High Temperature Pin-on-Disk Tester was used in preliminary test in order to obtain a valid set of parameters resulting in measurable wear of the coatings. The same instrument was then used for systematic characterization of wear resistance of these new, extremely wear resistant coatings.
The pin-on-disk tests with up to ~4 hours in duration were performed at temperatures up to 800°C and subsequent analyses were used for quantification of the wear resistance of the coatings. The Anton Paar THT800 (and also THT1000 reaching 1000°C) pin-on-disk tribometers are equipped an automatic arm which allows automatic start of measurement when the test temperature is attained. The tangential (frictional) force is measured using a double LVDT sensor on each side of the automatic arm so that the effect of temperature on the tangential force measurement is eliminated. The tribometer is efficiently cooled with water containing closed loop independent cooling circuit. The system has several safety features to ensure flawless function among others safety against overheating and cooling circuit malfunction.
Materials and pin-on-disk tribological test parameters
The protective coatings were deposited using p Technology: LARC® Lateral- and CERC® Central Rotating Arc Cathodes in a deposition system developed by Platit AG (Selzach, Switzerland). The nitride and oxynitride coatings were deposited in N2/O2 atmosphere and bias voltage from -30 V to -100 V using medium frequency. The deposition was done at 550°C on cemented carbide coupons with diameter of 50 mm and thickness 10 mm.
The multilayer (as shown in Fig. 2) was composed of adhesion layer, buffer layer(s) and functional coating. The functional (top) layer, crucial for wear properties, was AlTiN coating, AlCrN nitride coating and AlCrON oxynitride coating. The thickness of the whole coating system was ~5 mm.
Figure 2 – The protective layer structure of the tested coatings.
The pin-on-disk tribological tests were done on Anton Paar THT800 pin-on-disk tribometer (Fig. 3) at temperatures of 24°C (room temperature), 600°C and 800°C. The preliminary tests showed that use of normal load of 7 N (10 N for 24°C tests), alumina ball with 6 mm diameter as a counterbody and linear velocity of 20 cm/s lead to measurable wear on most of the coatings. The wear track on the sample as well as the worn cup on the alumina ball were inspected in optical microscope in order to observe the morphology of the wear track and to measure the diameter of the worn cup on the alumina ball.
The duration of the tribological tests was 32’000 laps for the less wear resistant coatings and up to 40’000 laps for more wear resistant coatings. Depending on the radius of the wear track the test resulted in total duration between ~120 minutes and ~240 minutes. Long duration of the pin-on-disk tests was necessary to model as closely as possible the duration of real milling/cutting times. The performance of the high temperature tribometer which can easily withstand such high temperatures for very long time was therefore crucial for these measurements.
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
One of the main results of pin-on-disk tribological tests is the coefficient of friction (CoF). This coefficient is not only directly related to friction force occurring between the two sliding bodies but also indicates how well the bodies react to mutual contact. If the coefficient of friction is stable, both materials usually remain relatively intact and have good wear resistance. On the other hand, when the coefficient of friction is varying substantially, higher wear usually occurs. In the case of the tested coatings the CoF at room temperature was very stable for all samples whereas the variation of the CoF increased with increasing temperature for most of the samples except for the oxynitride sample.
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.
According to these results the AlTIN and AlCrN performed well at room temperature whereas the variation of the coefficient of friction of these two coatings increased at 600°C and 800°C, indicating severe damage to the coating.
The AlCrON coating performed well up to 600°C and its CoF was very stable and exhibited consistent values of CoF of ~0.5 from room temperature up to 800°C. Slight decrease of coefficient of friction at 800°C can be result of formation of protective layer or tribofilm due to contact with the counterbody at high temperature.
Wear resistance and wear rate
Wear rate is a measure of the wear resistance of the material in a pin-on-disk test. The contact of the static counterbody (alumina ball) with the rotating sample generates damage to the coating which results in material removal and wear of the coating. Wear can be quantified as volume of the material removed from the sample – this volume can be calculated for both the rotating sample and the static counterbody (alumina ball). The wear rate is defined as volume loss normalized by the applied load and test distance. The unit of wear rate is therefore m3/m/N = m2/N. The volume of the material removed from the sample is obtained by measuring of the wear track profile by surface profilometer. In our case we used Taylor Hobson profilometer and on each sample at least six measurements were done along the wear track (see Fig. 5). The wear rate w was calculated according to this formula:
where V is the volume of the material removed, d is the total test distance and P is applied load. In some cases material build-up can be observed: instead of (or in addition to) material removal there was accumulation of material at the surface of the sample. The corresponding measure was then named build-up rate and the same formula (1) was used for the calculation of build-up rate with V being the volume of the accumulated material.
Figure 5 – Schematic illustration of the wear track and surface profilometer measurements (scans).
The wear rate at room temperature was non-measurable for all coatings except the AlTiN coating. This was also related to low variation of CoF of all coatings except for the AlTiN coating.
At 600°C however, AlTIN showed high wear rate and also the AlCrN coating exhibited higher wear rate than at room temperature. Wear rate of the AlCrON at 600°C was still very low At 800°C however, both AlTiN, AlCrN showed severe damage. Only the AlCrON coating remained at 800°C relatively intact with low level of wear.
Figure 6 - Comparison of wear rates at 24°C, 600°C and 800°C for the three tested coatings.
The AlTiN coating showed lower wear rate at 800°C than at 600°C and at room temperature (Fig. 6). The wear track profiles in some areas on the AlTiN and AlCrN coatings at 800°C also did not show typical wear scare profile but rather quite large material build-up. Figure 6 shows relatively low wear rate of the AlTiN coating at 800°C; however, the build-up rate on this coating was very high. The low wear rate (i.e. only shallow wear track) and high build-up rate (i.e. large build-up) at 800°C on AlTiN sample was subject to further studies by scanning electron microscope (SEM), X-Ray Energy Dispersive Analysis (EDX).
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 sample AlTiN after the high temperature (800°C) tests to elucidate the wear behavior with low wear rate and high build-up rate.
After a series of observation of the surface of the wear track of this sample in the SEM it was concluded that the coating failed mainly due to cohesive fracture and coating delamination (see Fig. 8). This resulted in exposition of the substrate and in oxidation of the Co component in the WC-Co substrate material. The growth of the Co oxide then led to the formation of the build-up in the wear track (see Figs. 7 and 8). EDX analysis also confirmed the presence of the Co oxide in the wear track. The apparently low wear rate values as calculated from the surface profilometer measurements were therefore due to oxidation of the substrate. The build-up rate was the highest on the TiAlN coating at 600°C and 800°C.
Figure 8 – Wear track on the AlTiN coating after the 800°C pin-on-disk tests.
The newly developed AlCrON coating, on the other hand, showed only minor wear with very little traces of interaction with the alumina counterpart at 600°C and 800°C. The SEM observation of the wear track on the AlCrON oxynitride coating revealed little damage to the coating (and to the alumina ball as well). The EDX analysis in the wear track did not show any traces of the elements from the underlying materials confirming very good integrity and wear resistance of this type of oxynitride coating.
In most of the pin-on-disk measurements it is convenient to quantify also the wear of the static partner. In our case the static partner was alumina ball with 6 mm diameter; the wear rate of this ball was calculated as the volume of the removed cap, normalized by distance and applied load. The wear rate of the ball was increased with increasing temperature for the AlTiN and AlCrN while it remained very low for the AlCrON coating. This result is in very good agreement with the wear rate (and build-up) rates of the coatings and it confirms that the AlCrON coating has very good wear resistance.
This Application bulletin presents an application of high temperature Anton Paar THT800 pin-on-disk tribometer for characterization of new types of hard coatings designed for use at high temperatures. The THT800 tribometer is particularly suitable for establishing a set of testing parameters enabling to rank the AlCrON coating together with other types of coatings. Thanks to the dual LVDT tangential force measurement and the robust construction of the instrument the coefficient of friction could reliably be measured even during experiments with more than four hours in duration. The tests confirmed very good wear resistant properties and oxidation resistance of the AlCrON coating at temperatures up to 800°C. The THT800 High Temperature Pin-on-Disk Tester was shown to be an indispensable tool for characterization of high temperature tribological properties of hard coatings.
 K.Holmberg, A. Matthews, H. Ronkainen, Coatings tribology-contact mechanisms and surface design. Tribology International 31 (1-3) (1998) 107-120.
 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
Jiri Nohava, PhD, Anton Paar
Marcus Morstein, PhD, Platit
Pascal Dessarzin, Platit
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With its unique dual heating elements layout a THT 1000 °C brings high-temperature tribology testing to a new level of reliability and stability. Differential friction force measurement ensures negligible signal drift at extremely high temperature. Careful design of the static partner and sample holders ensures a relaxing experience for the user – even during tribology tests at 1000 °C.