PVD Superlattice Structured Hard Coatings Designed for Dry High Speed Machining.

Transcription

1 PVD Superlattice Structured Hard Coatings Designed for Dry High Speed Machining. W. D. Münz o+, I. J. Smith o, C. Schönjahn +, A. P. Deeming *, S. Clapham *. O Bodycote SHU Coatings Ltd, Matilda Street, Sheffield, S1 4QF, UK. + Sheffield Hallam University, Materials Research Institute, Howard Street, Sheffield, S1 1WB, UK. * Hydra Tools International PLC. Penistone Road, Sheffield, S6 3AW, UK. ABSTRACT The application of the superlattice architecture in the synthesis of PVD hard coatings allows the combination of materials with special properties within a multi layered coating structure which cannot be achieved in monolythically grown coatings. The layered incorporation of Y leads to a substantial increase of the oxidation resistance of TiAlN based coatings. On this basis dry high speed cutting performances may be achieved which clearly out perform conventional TiAlN coatings. However these coatings exhibit a relatively high friction coefficient (0.7 against Al 2 O 3 ). This drawback can be eliminated in superlattice coatings designed from TiAlN / VN multilayers with a periodicity of 3.5µm. In this case the friction coefficient is reduced to 0.4 against Al 2 O 3 and the hardness of the coatings increases from typically HP 36GPa of TiAlCrYN to HP 42GPa of the superlattice, respectively. These superhard coatings show an excellent dry high speed cutting performance not only at high speeds in die steels with hardness values in the range of HR 60 but also in softer steels of typically HR where TiCN is employed presently.both types of coatings have been deposited onto solid carbide 2 flute ball nosed endmills and tested in a high speed milling centre against TiCN and Y-free TiAlN. KEYWORDS PVD Coatings, Superlattice, Dry High Speed Cutting.

2 1. Introduction Dry High Speed Machining (HSM) gains increasingly in importance mainly due to the need to reduce production costs in the manufacturing industry, but also due to environmental reasons caused by the consumption of chemically hazardous lubricants and coolants. The aerospace and the automotive industries are the driving forces in this business. It has already turned out in the very early stage of this development that the well known golden coloured TiN cannot satisfy these demands. In particular, the increase in temperature during cutting, the tendency of alloying of light metals like Al or Ti, the increased abrasive wear when cutting silicate containing Al alloys and the increased friction of the tool piece and the working material results in the rapid deterioration of TiN under these severe conditions. Two possibilities exist to improve the performance of PVD nitride coatings. The historically older approach consists of incorporating additional elements into the TiN coating. Famous examples are the partial replacement of Ti by Al in the lattice positions or N by C in the interstitial sites. TiCN[HAT 83] and TiAlN[MUN 86] are the well known ternary coating materials following the first route of improvement. In the second case multilayer coatings with a superlattice structure have been developed [HEL 87][CHU 92][CHU 93][MUN 00]. In this case very thin layers of two different types of coatings are deposited in alternating sequence comprising a nano-structure with a periodicity of nm of the layer couple. The main advantages of the superlattice approach include the possibility to synthesise coatings consisting of layers with differing individual properties thus generating new materials with new properties, to develop coatings belonging to the superhard category (plastic hardness, HP >40 GPa) and to design coatings with a new wear mechanism when exposed to high mechanical loads. The latter case is briefly explained by figure 1. Conventional hard coatings are deposited following the principle of monolithically growth comprising a columnar microcrystalline structure as outlined in figure 1a. Figure 1b shows schematically a superlattice structured coating with a typical periodicity of the layer-pair of 3 to 4 nm[luo 99]. Now it has been observed that the wear damage, in the case of the monolythically grown coating is caused by plastic deformation of the individual grains which causes under mechanical forces breakages typically nm in depth. In the case of the superlattice coatings the depth of damage is much shallower, typically 6-8nm. Under these conditions it is no surprise that the wear rate of superlattice coatings is much lower in tribological tests[mun 00]. 2. Manufacturing of Superlattice Coatings. To manufacture superlattice coatings multi-target coating equipment is required with, preferably four linear cathodes which can be operated independently and simultaneously to allow sufficient flexibility in material selection[mun 00][DON 97a]. Figure 2 represents a schematic cross section of a four target unit with electromagnetically controlled unbalanced magnetrons which can be used also as steered arc cathode [DON 97a][MUN 92]. Using such dual purpose cathodes coatings can be deposited after the process-step sequence shown in figure 3. In this special case in-vacuo substrate cleaning is carried out by metal-ion etching. The metal ions are extracted from a cathodic arc plasma as generated by a steered cathodic arc electrodes outlined in figure 2. The coatings are usually deposited in the unbalanced magnetron mode. The metal ion bombardment allows excellent adhesion whilst the unbalanced magnetron deposition avoids the formation of droplets during film growth. This deposition method is known by the label of ABS technology derived from Arc Bond Sputtering[MUN 92]. This paper describes the deposition of two superlattice coatings namely, Ti 0.43 Al 0.52 Cr 0.03 Y 0.02 N[DON 97b] and TiAlYN/VN[MUN 00]. The target configuration for the first coating is illustrated by figure 2, whereas in the case of the superlattice coating, two TiAl 50:50 at.% targets

3 are used, but one including 4at.% of Y, combined with two V targets. The metal ion etching steps is performed either with the Cr or with one of the V targets. Typical power dissipation rates are 8kW per TiAl or TiAlY targets and 6kW per V target when used simultaneously in the unbalanced magnetron mode. The Cr magnetron runs during deposition at a very low power in order to keep the target clean from contamination of TiAlN[DON 97b]. The deposition process is carried out in a reactive mode in an Ar and N 2 atmosphere with a total pressure of 3.5 x 10-3 mbar. Further process details are given in [MUN 00][DON 97a][MUN 92][DON 97b]. Figure 4 outlines schematically the superlattice structure of the two coatings. Figure 4a shows the superlattice structure of TiAlCrYN, which is designated as a pseudo superlattice, as the deviation in coating composition is formally only marginal (4 at.% in 96 at.% TiAl) and the coating thicknesses of TiAlN compared with TiAlYN are not equal (the influence of Cr may be neglected). In case of TiAlYN/VN the thickness of the TiAl based portions of the coatings equals those of the VN ones. Finally, Figure 5 shows a TEM cross section image of a TiAlYN/VN superlattice coating. 3. The Adhesion Problem Dry high speed cutting, particularly in die steels, requires a perfect adhesion quality of the deposited coatings. The ABS technology allows optimisation of the film adhesion which is dependent on the material used during the metal ion etch step. Most importantly, it is possible to initiate localised epitaxial growth of the deposited film[pet 97]. Localised epitaxial growth of fcc-nitrides can be achieved on various substrate materials such as high speed steel, stainless steel or cemented carbide[sch 00]. Full evidence of local epitaxial growth of TiAlN on WC in cemented carbide material, which has been exposed to Cr bombardment prior to coating, is given by figure 6 showing SADP s (Selected Area Diffraction Patterns) of the interface region. Figure 6a shows the SADP of f.c.c TiAlN in the vicinity of the interface, whereas figure 6c displays the SADP of the hexagonal WC grain. Figure 6b represents a superimposition of the TiAlN and the simple hexagonal lattice showing exact matching of the two diffraction patterns. The exact match of these patterns is recognised as proof of local epitaxial growth and, therefore, of optimised adhesion. The occurrence and the degree of pattern matching is strongly dependent on the special type of metal ion pretreatment[sch 00] and is simply reflected in the critical load values which can be measured by scratch test experiments. We have observed a wide spread of results starting from Lc = 35N for TiAlCrYN on WC after Ar + etch to >140N after Cr+ etch with ions accelerated by a bias voltage of -1200V.The adhesion performance as expressed by the scratch test directly influences the life time of coated tools. Figure 7 shows the lifetime results achieved whilst dry cutting of A2 steel (HRc = 58) with 2-flute ball nosed end mills cutting (Cutting speed rpm, feed 7mmin-1). The results shown in figure 7 stem from etching with 1200 ev Ar+ ions, 600 ev Cr + ions, 1200 ev Nb + ions, 1200 ev V + and 1200 ev Cr + ions. 4. Film Properties Various physical and tribological properties of the TiAlCrYN and TiAlYN/VN coatings are listed in Table 1. It is obvious that the superlattice coating is distinctly harder, namely 42 GPa in plastic hardness, HP, compared to 36GPa of the pseudo superlattice coating which is not optimised to the maximum hardness value[hel 87]. Surprisingly, the internal stresses of the TiAlCrYN coating is much higher than that of the TiAlYN/VN superlattice coating. Per definition the superlattice coating maybe designated as superhard coating as HP exceeds 40 GPa. Most interesting is the sliding wear coefficient. The wear of the superlattice coating is approximately 2 orders of magnitude lower than

4 that of the monolythically grown coating. The friction coefficient is of comparable value. The addition of Y leads obviously to the increase in friction of the V containing coating. Y-free TiAlN/VN namely exhibits friction coefficient of 0.4 against Al 2 O 3 [MUN 00] whereas the coating investigated here appears to have a friction coefficient of 0.6, more or less identical with the TiAlCrYN film (0.75). This result maybe explained by the fact that Y prevents VN oxidising during wear, thus no V2O5 can be formed which acts as a dry lubricant. Conversely, if one removes the debris generated during the sliding test the friction coefficient for both materials decreases to 0.2. Critical load values are comparable for both coatings. The TiAlCrYN coating seems to be smoother (Ra = 0.04 µm) than the superlattice coating (0.07 µm). This result maybe explained by the special behaviour of Cr during the metal ion etching step[mun 00][DON 97b]. 5. Cutting Tests Cutting test have been performed by using a MAZAK high speed machine tool FJV- 25 which can be operated with 25kW up to rpm. Figure 8 outlines the improvement of ABS - deposited TiAlN compared to state of the art TiCN and to the further improvement that one can achieve by incorporating Y into the TiAlN coating. The further improvement achieved by Y can be attributed to the increase in oxidation resistance and to the fact that Y diffuses at high temperatures to the grain boundaries in the coating thus preventing there the in/out diffusion of working and substrate material[don 97b]. The results presented in figure 8 have been evaluated by dry cutting of A2 steel (HRc=58) using solid 6mm 2-flute ball nosed carbide tools (Hydra Marwin Whispermills ) with rpm and a feed rate of 7mmin -1. It has been shown that the Y containing coating reaches its ultimate performance only when operated under extremely harsh conditions. In moderate conditions the Y-free TiAlN was superior[mun 99]. TiAlYN/VN were examined to determine whether they can overcome this drawback. Figure 9 shows the cutting performance of 8mm 2- flute ball nosed solid carbide tools in EN24 steel with a hardness of HRc = 38. In this case the cutting speed was rpm with a linear feed rate of 3 mmin -1. The new coating clearly outperforms the conventional TiCN coating thus offering an excellent alternative to achieve improved cutting performance. 6. Conclusions The viability of the deposition of nano-multilayered film structures has been industrially proven by Bodycote-SHU Coatings Ltds for more than 2 years. Besides the Y-free TiAlN and an Al-stabilised TiCN coating, TiAlCrYN and TiAlYNVN have been produced successfully under the brand name SUPERCOTE 11 and SUPERCOTE 33. The scope of applicability of these types of coatings is enormous. Among other applications of the new coatings substantial success has been achieved already in enhancing the lifetime of hot forging dies and glass mould[mun 99]. 7. Acknowledgements The research based upon the above described coating technology has been generously supported by the Link Surface Engineering Programme under Serial number GK/K76351 MULTICOAT, and the BRITE-EURAM research project, HIDAM.

5 References [HAT 83]. HATSCHEK, R.L., Am. Mach.127 (3) (1983) 129 [MUN 86]. MÜNZ, W.-D., J. Vac. Sci. Techol., A4 (1986) 2717 [HEL 87] HELMERSON, U., TODOROVA, S., BARNETT, S.A., SUNDGREN, J.-E., MARKERT, L.C., GREENE, J.E., J. Appl. Phys. 62 (2) (1987) 481 [CHU 92] CHU, X., WONG, M.S., SPROUL, W.D., RHODE, S.L.,. BARNETT, S.A., J. Vac. Sci. Technol., A10 (1992) 1604 [CHU 93] CHU, X., BARNETT, S.A., WONG, M.S., SPROUL, W.D., Surf. Coat. Technol., 57 (1993) 13 [MUN 00] MÜNZ, W.-D., DONOHUE, L.A., HOVESPIAN, P.EH, Surf. Coat. Technol., 125 (2000) 269 [LUO 99] LUO, Q., RAINFORTH, W.M., MÜNZ, W.-D., Wear (1999) 74 [DON 97a] DONOHUE, L.A., MÜNZ, W.-D., LEWIS, D.B., CAWLEY, J., HURKMANS, T., TRINH, T., PETROV, I., GREENE, J.E., Surf. Coat. Technol., 93 (1997) 69 [MUN 92] MÜNZ, W.-D., SCHULZE, D., HAUZER, F.J.M., Surf. Coat. Technol., 50 (1992) 169 [DON 97b] DONOHUE, L.A., SMITH, I.J., MÜNZ, W.-D., PETROV, I., GREENE, J.E., Surf. Coat. Technol., (1997) 226 [PET 97] PETROV, I., LOSBICHLER, P., BERGSTROM, D., GREENE, J.E., MÜNZ, W.-D., HURKMANS, T., TRINH, T., Thin Solid Films 302 (1997) 179 [SCH 00] SCHÖNJAHN, C., LEWIS, D.B., MÜNZ, W.-D., PETROV, I., Surface Engineering Vol. 16 No. 2 (2000) [MUN 99] MÜNZ, W.-D., SMITH, I.J., Society of Vacuum Coaters, 42 nd Ann. Techn. Conf. Proceeding (1999) 350 (ISSN )

6 Monolithically grown Superlattice Figure 1 Mechanical failure in monolithically and superlattice grown coatings

7 Figure 2 Schematic cross section of the combined steered CA / UBM deposition system used in these experiments

8 Figure 3 Schematic flow diagram of the coating process

9 Pseudo-superlattice Ti 0.43 Al 0.52 Cr 0.03 Y 0.02 N TiAlYN TiAlN (Cr) TiAlN TiAlYN TiAlN (Cr TiAl ) TiAlYN N TiAl N(Cr) TiAlN TiAlN (Cr) TiAlN 1.7 nm 0.1µm Base Layer Cr-implanted Superlattice TiAlYN/VN VN TiAlYN Figure 4 Schematic cross section through a superlattice hard coatings

10 Figure 5 cross-sectional transmission electron microscopy image of TiAlYN/VN superlattice

11 Figure 6: Local epitaxial growth of TiAlN on tungsten carbide after Cr ion bombardment at U b =- 1200V (a) Selected area diffraction pattern (SADP) of TiAlN grain (b) SADP with SA placed at interface so that the pattern is a superimposition of the TiAlN pattern and tungsten carbide pattern (c) SADP of tungsten carbide grain.

12 45 Life time in min Ar1200V Cr600V_2 Cr600V2 Cr1200V 5min Cr1200V_2 Nb1200V V1200V Lt = Lc R 2 = Cr1200V_ Lc in N Figure 7 Correlation between tool life and critical load values determined from the same coating deposited after various sputter cleaning procedures

13 Figure 8 Comparison of HSC performance of different coatings on solid cemented carbide endmills

14 Tool Life [ Min.] Uncoated TiCN TiAlYN/VN Figure 9 Life-time of differently coated end mills in dry cutting of EN 24 steel

15 Table 1.Mechanical and tribological characteristics of TiAlCrYN and TiAlYN/VN hard PVD coatings. TiAlCrYN Bias, V HP, GPa Stress, GPa Fr.Coeff. m, ( with debris) Fr. Coeff. m, (without debris) Sl. Wear Coeff. m -2 N x Lc, N Ra, mm TiAlYN/VN x