LOW FRICTION LAYERS AND THEIR PROPERTIES. Martina Sosnová

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1 SOUTĚŽNÍ PŘEHLÍDKA STUDENTSKÝCH A DOKTORSKÝCH PRACÍ FST 2004 LOW FRICTION LAYERS AND THEIR PROPERTIES Martina Sosnová 1. ABSTRACT The contribution deals with the analysis of low- friction thin layers. Low friction layers have lower hardness than common wear resistant layers. Their main function is to create the sliding surface. With application of these layers especially on cutting tools, it is possible to achieve e.g. tool s longer lifetime and better quality of machined surface. Thin layer s properties were analysed by scratch and tribology tests. KEY WORDS: Low friction layers, Diamond-like carbon (DLC) layers, scratch test, tribology test. 2. LOW FRICTION LAYERS Main function of low friction layers is to create the sliding surface. With application of these layers especially on cutting tools, it is possible to achieve e.g. tool s longer lifetime and better quality of machined surface. To this group belong soft layers, which are applied only in combination with hard coatings like TiN, TiAlN, TiCN and layers Diamond-like carbon (DLC); which appropriately combine excellent sliding properties with good hardness. There are many varieties of carbon layers in the market. They have in common - low friction coefficient [1]. Diamond-like carbon (DLC) is the name commonly accepted for hard carbon coatings which have similar mechanical, optical, electrical and chemical properties to natural diamond. They are amorphous and consist of a mixture of sp 3 and sp 2 carbon structures with sp 2 -bonded graphite-like clusters in an amorphous sp 3 -bonded carbon matrix. Nowadays these layers undergo its big boom [2]. Fig. 1: Bonding in DLC layers. To produce DLC, the deposition gas should be a hydrocarbon, such as methane, acetylene or propane. Adding to the hydrocarbon other gases such as hydrogen, oxygen, nitrogen or others (doping, alloying) results in a change of the chemical composition and atomic structure of the DLC, and is a very effective means of tailoring its properties such as friction, hardness and elasticity. Diamond-like carbon (DLC) coatings are promising candidates for dry machining of aluminium alloys since in ambient conditions aluminium has much less tendency to adhere to the DLC coating surfaces and the formation of a built-up edge compared to other hard coatings such as TiN, TiAlN and CrN [3]. Low friction layers represent important group of tools industrial surface modification. Their technical benefit in machining and forming is possible to summarize: - better sliding properties - significant decrease in adhesion between layer and workpiece - tool s steady run-in - decrease of cutting forces and their fluent course - limitation of built-up edge effect, especially during machining of non-ferrous materials Diamond-like carbon (DLC) layers are characterized by high wear resistance, low friction coefficients and chemical inertness, thus high-corrosion resistance, appear to be an ideal material for protecting implants, such as hip and knee joints. These properties make the films good candidates as biocompatible coatings for biomedical devices and tools [4].

2 3. EXPERIMENT Two types of systems with thin layers were analysed. First system was layer DLC + adhesion layer (low friction layer), second system was layer TiAlN (common wear resistant layer). Thin layer s properties were analysed by scratch and tribology tests. 3.1 Scratch test Scratch test is nowadays the most commonly method used to evaluate the adhesion of the coating substrate interface. A diamond stylus (Rockwell tip radius 200 µm) is drawn over the sample surface under a continuously increasing normal force until a particular failure of the coating occurs and the coating detaches. The normal load at which this happens is called the critical load Lc expressed in Newton (N). Lc is regarded as a representative of the coating adhesion. To determine the critical load is used light (LM) or scanning electron microscopy (SEM), acoustic emission (AE) and frictional force measurement. Coating detachment at the critical load is a measure for the adhesion. Fig. 2: Scratch tester CSEM Revetest and a detail of indenter. CRITICAL LOAD (fig. 3) Lc [5,6] L C1 [N] represents the load at which the first failure occurs. L C2 [N] represents the load at which the cracks are accompanied by interfacial spallation. L C3 [N] represents the load at which the substrate is firstly revealed. L S [N] represents the load at which the substrate is totally revealed. Fig. 3: Determination of critical load. Table 1. Critical load of analysed layers. Sample L C [N] Layer DLC + L C2 25 adhesion layer L C3 =L S 49 L C1 = L C2 15 TiAlN L C3 44 L S 49 Layer DLC with adhesion layer embodies specific type of failure (fig.4). The top low friction layer was immediately damaged, although there were parts of the layer, which remained in the scratch tract. Therefore L c1 wasn t evaluated. L C2 ~ 25 N. L c3 has in this case same value as Ls total substrate exposure ~ 49 N (see table 1).

3 Fig.4: Failure of the system with low friction layer in the beginning of the scratch track (LM). Fig.5: Sample DLC with adhesion layer. In part of the scratch (area A) takes place transition into the adhesion layer (LM). In case of layer TiAlN, layer behaved differently compared to sample with DLC. First failure occurs after 15 N, when layer is firstly detached, but after that place, coating remains in the scratch track. Layer is buckling in response to the compressive stresses generated ahead of the indenter. Total exposure of the substrate took place at ~ 49N. If we compare these two systems, critical loads of both systems are similar, but system with low friction layer was immediately damaged, therefore system with TiAlN layer embodies better adhesive-cohesive properties. 3.2 Tribological test In recent years it has become clear that in all environments the tribological behaviour of DLC coatings is controlled by an interfacial transfer layer formed during friction. The transfer layer is formed by transformation of the top layer of a DLC film into a material of low shear strength. Clear evidence of the formation of graphitic carbon was detected on the wear debris and also in some parts of the tribolayer formed on the PIN surface (fig.6). The graphite obviously has a major contribution to the low friction behaviour of the DLC coatings.

4 Fig. 6: Schematic representation of the relationship among tribological behaviour, preparation condition and testing environment of DLC layers. Experiment part was divided into three parts. 1) R5mm: v = 12,5cm/s, F=2N, n=5000, ball Al 2 O 3 2) R2mm: v = 5cm/s, F=10N, n=5000, ball Al 2 O 3 3) R3, 5mm: v = 4cm/s, F=2N, n=500, ball PP Experiment was taken at common atmospherical conditions (temperature = 23 C, humidity = 50%), without lubricant. In first experiment load was 2N. This load was chosen, because this friction coefficient is not influenced by high values of contact temperature and contact pressure. As counterpart was chosen ceramic ball from Al 2 O 3. By the sample TiAlN low friction layer was almost immediately damaged. The friction coefficient (fig. 7) after this moment increased up to 0.8. Friction coefficient is unstable and that is caused by frequent pluck up of microscopic asperities. On sample TiAlN there was only mild damage of thin layer and local substrate exposure. The exposure was in places, where roughness of the surface was exserted. Sample with DLC has low and stable friction coefficient. Values are from 0.2 to 0.3. These factors reflect about good quality of low friction layer. On sample with DLC was detected mild wear of thin layer, but this damage was lower than on sample TiAlN. This theory confirms the fact, that the coefficient of friction during the test is very low, corresponding to contact ceramics vs. thin layer DLC. R5mm: v = 12,5cm/s, F=2N, n=5000, ball Al2O3, DLC TiAlN 1,2 1 Friction coefficient 0,8 0,6 0,4 0, Cycles Fig. 7: Course of friction coefficient.

5 Second experiment was done due to reason of detection of wear resistance vs. combination of abrasive and adhesive wear. Main result in this case is monitoring of failure mechanism. This mechanism is researched by LM and SEM. Conditions at this experiment were chosen so that maximum contact pressure and temperature were in the contact. Higher resistance proved sample with low-friction layer. Friction coefficient on sample TiAlN declines after running in. After this period friction coefficient is about This course of friction is accompanied by instability. A transfer layer is created on ceramics counterpart. This phenomenon means that the wear rate is higher than on sample with DLC. Moment of its damage is evident from course of friction (fig.8). On sample TiAlN there was the substrate exposure after 2000 cycle. Friction coefficient on sample with DLC is very low and the value rises up after 3000 cycles. After this time surface low friction layer is definitely removed. After removal of this layer wear rate rapidly increases. Adhesive layer is removed after 5000 cycles. Fig. 8: Course of friction coefficient. In the third experiment was chosen 2N load and as counterbody was chosen ball from polypropylene. This load was chosen, because friction coefficient is not influence by high values of contact temperature and contact pressure. Polypropylene was chosen with respect to final use of thin layer. Experiment was concentrated on monitoring of tribological behaviour on contact of polypropylene counterpart against thin layer. Attention was especially given to local adhered elements from plastic ball on thin layer. In this aspect was sample with DLC layer classified better. Values of friction coefficient are in case of sample TiAlN lower than when ceramic counterpart is used. On sample with DLC is friction coefficient about three times higher. It is possible to think that the main use of system thin layer - substrate on sample with DLC should be in other application than in contact against plastics. Coefficient of friction is on sample TiAlN gently distorted by transfer layer on the sample.

6 R3,5mm: v = 4cm/s, F=10N, n=500, ball PP 0,8 DLC TiAlN 0,7 0,6 Friction coefficient 0,5 0,4 0,3 0,2 0, Cycles Fig. 9: Course of friction coefficient. 4. CONCLUSION From the experiments that have been done so far is clear, that usage of this thin layer for cutting of plastics can be problematic. This conclusion results from high friction coefficient against plastic counterpart. Information about resistance of thin layer against abrasive wear is also very important parameter. This resistance in both samples is average. These types of thin layers should be used at cutting in other kind of materials. First of all in case of sample with DLC layer, friction coefficient against most of materials is low (aluminium, ceramics, steel). From this point of view it is necessary to point out the key role of carbon surface layer (intensive increase of friction coefficient after its delamination). Acknowledgement This contribution was solved under the support of FRVŠ grant no. 1230/2006/G1. LITERATURE Publications in technical paper: [1] Holubář P., Jílek M., Růžička M.: Moderní PVD povlaky pro řezné aplikace a tváření Průmyslové spektrum, 9/2004. [2] Holmberg, K. Matthews, A.: Coating tribology, Elsevier Science Ltd, 1998, [3] Koncaa E., Chengb Y.-T., Weinerb A.M., Daschc J.M., Alpasa A.T.: Elevated temperature tribological behavior of non-hydrogenated diamond-like carbon coatings against 319 aluminum alloy. Surface&Coatings Technology [4] Grill A.: Diamond-like carbon coatings as biocompatible materials-an overview. Diamond and Related Materials Str [5] Jacobs, R. ET AL.: Surface and coatings technology, , 2003, Str [6] Precht, W. Cyzyniewski, A.: Surface and coatings technology, , 2003, Str Ing. Martina Sosnová, University of West Bohemia in Pilsen. Univerzitní 22, Plzeň, tel.: , sosnova@kmm.zcu.cz.