In conventional pin-on-disk testing of the tribological characteristics of two different materials in sliding contact, the main parameters of interest are notably the friction and wear properties of the material pair. However, when two bodies consisting of hard and soft materials respectively are subjected to such testing, the apparition of a transfer film, or third body, which can be a composite mixture of the two, is often observed. This layer is formed through mutual transfer and/or by mechanical mixing of the two materials. Until now the characterisation of transfer films in terms of their mechanical properties has been hampered by their non-homogeneous distribution across a tested surface, their small size, low thickness and the difficulty in accurately positioning a test probe such that the film properties can be measured independently from those of the substrate.
The recent development of nanoindentation techniques, in particular the CSM Instruments Nano Hardness Tester (NHT), has allowed highly localised hardness and modulus measurements to be performed on very small material volumes. This method is ideally suited to tribological transfer films owing to their aforementioned dimensional limitations.
When combined with a surface imaging technique, such as the scanning force microscope (SFM) attachment on the NHT, surface deformation phenomena can also be investigated in and around the indentation area. Some previous work has already shown the net advantages of nanoindentation as a unique tool for the characterisation of tribological transfer films created with a steel/polymer material pair in both ambient air and cryogenic environments. Distinct variations in hardness between the transfer films and their contacting bodies were observed and correlated to the wear behaviour and testing environment.
Mechanical Properties Of Transfer Films
In this study, the mechanical properties of transfer films created between material pairs of significant industrial importance were investigated, namely hard coatings (TiN, TiCN, TiAlN, CrN) for tool applications and bulk ceramics (Al2O3, WC). In the case of high performance cutting tools, the film material must possess high strength at elevated temperatures, good oxidation resistance, a low coefficient of thermal expansion and high thermal conductivity.
The tribological apparatus used consisted of a conventional CSEM pin-on-disk tribometer for room temperature experiments and a high temperature tribometer for experiments up to a maximum of 800°C.
Separate samples of TiCN, TiAlN and CrN were produced by Physical Vapour Deposition (PVD) onto AISI 440C and 100Cr6 steel substrates. Coating thicknesses were measured using the CSEM Calotest method and were all in the range 2-4 μm. Room temperature sliding wear experiments were then carried out with a tungsten carbide (WC) ball of diameter 6 mm acting as the static partner. The wear rate results are shown in Fig. 1 for these samples (TiN data has been added as a reference for comparison purposes).
Figure 1. Disk wear rate plotted as a function of coating type and substrate for TiN, TiCN, TiAlN and CrN combinations, as measured using a pin-on-disk tribometer with a WC ball as static partner, 5 N normal load, 10 cm/s speed and 1.2 μl/4cm2 of HBO 2222 lubricant.
High Temperature Tribometer Measurements
High temperature tribometer measurements were performed on TiN, TiAlCrYN and TiAlN/CrN coatings which had been deposited by PVD (coating thicknesses were measured as 1.9-3.4 μm). The substrate was high speed steel (HSS), the static partner was an Al2O3 ball of diameter 6 mm and the test conditions were 5 N normal load, 6 cm/s speed and the friction coefficient was averaged over a distance of 2000 m. The average roughness (Ra) was measured for the disk wear surface before testing using a three-dimensional stylus profilometer.
Figure 2. SFM images of a TiCN wear track showing 20 mN residual Berkovich nanoindentations made on the coating (a) and the transfer film (b). Note the positioning accuracy of the NHT and the differences in surface morphology between the two areas.
Residual Transfer Films
The residual transfer films which had accumulated on both the ball and disk were analysed using nanoindentation, and their hardness (H) and elastic modulus (E) compared to that of the coating, substrate and ball materials. The maximum force and the loading rate were varied 8-50mN and 16-100 mN/min. respectively. The smallest force value was used when indenting on the transfer films in order to keep substrate effects to a minimum. The optical micrographs of the residual wear tracks (Fig. 3) show the differences in wear behaviour between the various material pairs and test conditions. In the case of the TiCN, small amounts of transfer film are visible along the wear track, these being inhomogeneously distributed and of varying size and thickness. The wear tracks of the TiAlN/CrN and TiAlCrYN were both found to be highly oxidised as a result of high temperature testing.
Figure 3. Optical micrographs of the residual wear tracks on the following measured samples; (a) TiCN (200x), (b) TiAlN/CrN (50x) and (c) TiAlCrYN (50x).
A thick mixture of transfer film and oxide was spread quite homogeneously over the surface of the track. Lateral cracking was visible throughout the transfer layer, probably due to the thermal stresses induced in the sample during the test.
Surface Topographical Characterisation
Surface topographical characterisation of the wear track residual imprints by SFM gave significant additional information about the material response to the indentation as well as to surface morphological changes resulting from the sliding wear process. Fig. 2 shows an image of two 20 mN residual Berkovich nanoindentations made on the TiCN coating and transfer film. Both surfaces are quite obviously made up of sheared particles which can clearly be seen in both images, as well as shallow pits on the surface where fragments have been ripped out during the abrasion process.
It has been shown that, for material pairs consisting of hard coatings in dry sliding with two types of bulk ceramic, an increase in the hardness of the transfer film which is inevitably produced can strongly influence the apparent wear rate. The wear rate itself is dependent on the testing conditions and on the resultant temperature at the ball/coating interface, as well as on the roughness of the mating surfaces. Sliding wear tests at high temperatures have been shown to result in significant amounts of transfer material and the formation of oxides which can strongly influence the measured mechanical properties.
Nanoindentation has been introduced as a valid method for measuring the hardness of ultra-thin transfer films and, by combining it with high resolution SFM, for investigating differences in material response and surface topography, and correlating them to the wear process. Elemental analysis will help to establish whether compositional changes resulting from the wear process have a major contribution to the formation and effect of transfer layers.