Investigation of properties of novel multilayer Cr/CrN/CrTiN/CrTiAlN hard coating with four bilayers

A novel multilayer Cr/CrN/CrTiN/CrTiAlN hard coating with four bilayers was developed by reactive closed field unbalanced magnetron sputtering (CFUBMS). The substrate temperature was kept in the range of 170 °C to 200 °C. The reactive gas flow was controlled by Optical Emission Monitoring. A thickness of 1.8 μm was determined using a calotester. The morphology and chemical composition were studied by electron microscopy (SEM and EDS). Nanohardness of 34.1 GPa and a modulus of elasticity of 386 GPa were determined using the nanoindentation technique. The scratch test against a diamond Rockwell indenter did not show features indicating poor adhesion. The reciprocating wear test revealed strong resistance to wear, and a coefficient of friction of 0.19 was determined. The developed multilayered hard coating showed optimal mechanical properties for industrial applications on instruments of low-temperature materials.


Introduction
CrN hard coatings deposited at temperatures up to 200 o C have shown good oxidation resistance in industrial practice [1].These hard coatings obtain higher density and smaller surface roughness when Unbalanced Magnetron Sputtering (UMS) is used instead of the conventional magnetron sputtering technique.This is due to the increased ion flux to negatively biased substrates [2].The Cr/CrN-based ternary systems have gained attention due to their good combination of hardness and toughness.Among them, the Cr/CrN/CrTiN deposited by Close Field Unbalanced Magnetron Sputtering (CFUBMS) is structurally unified and has improved mechanical properties compared to the binary TiN and CrN hard coatings [3].With their high hardness and moderate friction coefficient, CrTiN coatings deposited with UBMS have shown improved stamping performance [4].Multicomponent coatings obtained with unbalanced magnetron sputtering using Ti, Cr, and Al targets outperformed the conventional coatings in terms of oxidation resistance due to the formation of dense protective oxide layers of Cr2O3 and Al2O3 retaining its crystalline structure, morphology, wear resistance and hardness up to oxidation at 900 o C [5].Machining tests confirmed the large potential of Cr-based multilayer nitrides and their better adhesion and wear resistance properties, required for a wide range of cutting applications and workpiece materials [6].Multilayer coatings with four bilayers exhibited higher mechanical properties as hardness around 43 GPa and spallation resistance up to 97 N in combination with good wear resistance [7].The primary goal of the current research is to investigate a novel multilayer Cr/CrN/CrTiN/CrTiAlN hard coating.This not-yet-researched structure will be deposited by CFUBMS technique at temperatures under 200 o C. Four bilayers consisting of CrTiN and CrTiAlN sublayers will form the structure.The coating is expected to have well-balanced mechanical parameters and higher oxidation and temperature resistance.This new structure will be suitable for temperature-sensitive tools and devices oriented to the cutting and stamping industry.

Experiment
UDP850/4 equipment installed with one Cr, two Ti, and one Al targets was used for the deposition of the novel multilayer Cr/CrN/CrTiN-CrTiAlN hard coating with four bilayers.The Ti and Cr targets were powered by DC, while the Al target in pulse DC regime.Flat circle high-speed steel coupons (HSS type P6M5) with grinded surface were used as substrates.They were with dimensions: 10 mm height and 30 mm radius.The distance to the targets was approximately 120 mm.The carousel worked in three-fold rotation regime.The substrates were preliminary cleaned and, after that, loaded in the vacuum chamber.The deposition procedure started with driving a working gas Ar and a 30-minute glow discharge ion cleaning.During this step, the bias was set at 500V in a pulse regime (frequency 250 KHz and pulse on period 3500 ns).The deposition of the structure started with the Cr as adhesive layer.Secondly, CrN was deposited as transition layer.Then the power of Ti and Al targets was gradually increased.The multilayered structure of CrTiN and CrTiAlN sublayers was deposited in four bilayers.The composition of the multilayered structure was controlled by the targets power.Optical Emission Monitoring (OEM) system was used for the management of the MFC of the reactive gas.The process parameters are generalized in table 1.The hardness tests were carried out using a nanoindentation tester (Antoon Paar, Graz, Austria) with a sharp three-faced pyramid Berkovich diamond tip and a load range of 0.5 mN to 500 mN.The tip was driven progressively into the material with a 5-second pause before unloading.Τhree indentations were done for each load (10, 20, 30, 40, 50 and 100 mN), and the maximum value was taken in the evaluation process.For each nanoindentation cycle the applied normal load and the displacement of the indenter tip were continuously monitored during the construction of the load-displacement curve.The Oliver and Pharr method was used to determine the elastic modulus and nanohardness.
The scratch tests were conducted with a scratch module from Anton Paar and a sphere-conical diamond indenter with a tip radius of 200 µm.The scratch tests were carried out progressively, with a normal load increasing from 0.5 to 30 N and a scratching velocity of 1 N/min.The length of the scratch was 1 mm.Data for the penetration depth -Pd, the friction force -Ft, normal force -Fn, acoustic emission -AE, and the coefficient of friction -µ are obtained directly from the software of the equipment.Pictures of the track were taken with the optical microscope at magnifications x5.
A calotester CAT2 (Anton Paar) was used to measure the coating thickness.The ball-cratering method was applied.The steel ball had a diameter of 20 mm.A diamond suspension (0.5 μm grain size) was used during the rotation process.The regime was set to velocity of 1200 rpm for 20 sec.
The diameters of the concentric circumferences were determined with an optical microscope, and the corresponding thicknesses were calculated using the equipment software.
Scanning Electron Microscope (SEM) Hitachi SU 5000 was used for the morphology observation of the coating surface.The structure composition was characterised by the Thermo Scientific energy dispersive X-ray appliance.Secondary electron (SE) images were taken to investigate the sample topology.Backscattered electron (BSE) images and Energy-dispersive X-ray spectroscopy (EDS) in point mode were used for element analyses.
The tribological properties were examined by tribometer.Al2O3 ball (d=6.3 mm) was used as counter body.The set parameters were: linear speed of 0.01 m/sec, load 5N and track length 100 m.

Results
The coatings thickness was defined for the total structure, including the adhesion and transition sublayers and the four multilayer CrTiN and CrTiAlN bilayers and was calculated to be 1.8 µm.Figure 2 shows a SEM image of the surface of the CrTiN-CrTiAlN coating.A dense nonuniform granular surface microstructure without any defects or droplets and with some intercolumnar voids is shown.The elemental composition studied by SEM+EDS is given in table 2. The load-displacement curves from the nanoindentation tests are shown in figure 3. The curves are smooth, and no pop-in or pop-out inhomogeneities are seen.The small penetration depth corresponding to 10% of the coating thickness presents the properties of the investigated structure.The results at higher penetration depths are influenced by the substrate mechanical properties.Figure 4 shows the trends of nanohardness H and the Module of elasticity E determined from each measured unloading curve, where E closely follows the variation of H.The exchange of higher and lower values of both parameters represents the multilayer structure of the hard coating and the four bilayers including both sublayers with different mechanical parameters.The measured maximum nanohardness and elastic modulus of the coating layer are 34.1 GPa and 386 GPa.These results exceed previously obtained results of 19-21 GPa for CrTiN coatings [8], of 14.6-22.2GPa for CrTiAlN coatings [9] and of 13.9 GPa and 22 GPa correspondingly [10].
The trends of Ft, Fn, Pd, AE and µ investigated in the scratch test are shown in figure 5.All the graphs are smooth and no sudden changes are observed.Up to a load of 30 N, no critical loads were identified either from the friction force or the microscope investigation of the scratch tracks.The coating structure is characterized by good adhesion, without any cohesive or adhesive failures of the coating-substrate system.The ratios between nanohardness H and effective module of elasticity E * (H/E, H 2 /E* 2 ) are used for indicating the resistance to elastic and plastic deformations and could be used to predict the wear resistance of the coatings.Figure 6 shows the values H/E* (0.083) and H 3 /E* 2 (0.234 GPa) with the coefficient of friction µ (0.11) against the diamond indenter, measured during the scratch test.These values are prevailing the ones measured for CrTiN [8] and CrTiAlN [10] hard coatings.Reciprocating tests on the UMT tribometer were conducted mainly to determine the dynamic change of friction coefficient against Al2O3 counterbody.A representative trend is illustrated in figure 7. The friction coefficient values show a stable trend after the initial load, and its average value was 0.19.This value is improved compared to the properties of the sublayer coatings [8,11].Average wear value of 0.9 microns has been measured from the coating surface using the craters dimensions from the tests.

Conclusions
Novel multilayer Cr/CrN/CrTiN/CrTiAlN hard coating with four bilayers was deposited using a closed field unbalanced magnetron sputtering system.The coating has a granular surface microstructure with a composition of Cr (33.6%),Ti (19.1%),Al (3.8%) and N (40.8%).The coating structure exhibited higher hardness (34.1 GPa) and a lower coefficient of friction (0.19) defined with a tribometer.These results exceed the cited analogues about CrTiN and CrTiAlN hard coatings, indicating that the developed structure is suitable for industrial applications to improve the wear resistance of lowtemperature materials and the extension of their lifetime.The novel hard coating still requires advanced research and detail optimisation of the composition and structure depending on the thickness of the bilayer period.

Figure 5 .
Figure 5. Graph and picture of the trace from the scratch test.Figure 6. H/E*, H 3 /E* 2 and µ.

Figure 6 .
Figure 5. Graph and picture of the trace from the scratch test.Figure 6. H/E*, H 3 /E* 2 and µ.