STUDY OF DIAMOND-LIKE COATING DEPOSITED ON THE SURFACE OF Nd-Fe-B-TYPE SINTERED MAGNETS

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1 STUDY OF DIAMOND-LIKE COATING DEPOSITED ON THE SURFACE OF Nd-Fe-B-TYPE SINTERED MAGNETS Natalia KOLCHUGINA 1, Aleksandr LUKIN 2, Gennadii BURKHANOV 1, Georgii SPRYGIN 1,, Miroslav KURSA 3, Katerina SKOTNICOVA 3, Aleksandr KOSTYUCHENKO 4 1 Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences,Leninskii pr., 49, Moscow, Russia, natalik@imet.ac.ru 2 JSC SPETSMAGNIT, Dmitrovskoe sh. 58, Moscow, Russia, lukinikul@rambler.ru Abstract 3 Vysoka Skola banska - Technical University of Ostrava, 70833, Czech Republic, Ostrava-Poruba, 17 Listopadu, 15/2172; Katerina.Scotnicova@vsb.cz 4 Voronezh State University, Universitetskaya pl., 1, Voronezh, Russia; avkostuchenko@mail.ru Diamond-like coating (DLC) was deposited on ground and polished surfaces of Nd-Fe-B sintered magnet (prepared with 4% TbH 2 ) using reactive ion-beam synthesis directly from ion-beam of a chemical complex substance (cyclohexane). The film deposited is diamond-like; the fraction of tetragonal carbon-atom bonds is 70%. The composition and thickness of the coating was analyzed by glow-discharge spectroscopy using a GDS 850A (LECO) spectrometer. The thickness of pure carbon (DLC) layer is 0.08 μm. The existence of transition layer μm thick is due to the diffusion (to a lesser degree) and surface roughness. The effect of the coating of surface mechanical characteristics of the magnet was estimated in studying the Vickers hardness of the surface and friction coefficient and by nanoindentation as well. The Nd-Fe-B magnets with applied diamond-like coating demonstrate the lower tendency to mechanical cracking. Dry frictional tests showed that, after wear-in process, the DLC increases substantially the stability of frictional process and decreases the frictional coefficient in the cases of both unchanged and progressively increasing load. The data obtained allow us to predict the possibility of application of the DLC for the increase in wearresistance of sintered Nd-Fe-B magnets (if a thin coating must be used) during their operation in moving abrasive liquid and gas media. Keywords: Nd-Fe-B magnets, diamond-like coating, glow-discharge spectroscopy, frictional tests, nanoindentation 1. INTRODUCTION For the optimum performance of a permanent magnet in special devices, the work conditions, such as the pressure, temperature, medium, vibration, demagnetizing field, other perturbation, and, therefore, their behavior under these conditions must be know. Corrosion of Nd-Fe-B magnets has been an ongoing problem to be resolved. Although various corrosion protection methods have been proposed, the corrosion problem still remains. Usually used electrolytic deposition of coatings (Ni, Cu, Ni/Cu/Ni, Ni/Cr, Zn, Sn, Cu-Sn, Ni-P, Ni-P/TiO 2 composite) is not best since the penetration of atomic hydrogen under coating, its exfoliation and subsequent destruction of magnets are possible. The application of DLC for the protection of Nd-Fe-B magnets was studied in [1]. The DLC coating was deposited by pulsed arc discharge (from methane, coating 500 nm thick) and laser ablation (from graphite target, coating nm thick). The coatings were deposited also on a Ti sublayer formed by evaporation. The authors showed that the DLC had poor adhesion in the absence of buffer (Ti) layer and

2 inadequate corrosion resistance in chlorine (fluorine) media. However, the DLC with Ti buffer can be used for protecting Nd-Fe-B magnets from mechanical impacts. The aim of the present study is to investigate the possibility of application of DLC prepared by reactive ionbeam synthesis for the protecting of sintered Nd-Fe-B magnets. The problem of the protecting of Nd-Fe-B magnets is discussed from the point of view of mechanical characteristics of Nd-Fe-B magnet surface with applied DLC. 2. EXPERIMENTAL The diamond-like coating was deposited on ground and polished surfaces of a sintered Nd-Fe-B magnet (with Dy, Tb, Cu, etc. additions) prepared from a powder mixture containing 4% terbium hydride (TbH 2 ) [1]. The coating was deposited by reactive ion-beam synthesis, which was realized at the Physico-Technological Institute, Russian Academy of Sciences (Moscow). The procedure consists in the deposition of a substance directly from high-intensity low-energy ion beams [2]. The principle of the ion-beam production consists in the ionizing of operating substances both by high-energy electrons, which are accelerated in a specially formed potential well, and in localized crossed electrical and magnetic fields ensured the maximum changes of the electron energy within the anode-cathode space. The ion extraction and beam formation is realized with an accelerating electrode. In our case, the diamond-like film was deposited from cyclohexane ion beams. This technique allows one to (1) control properties of deposited thin films directly in changing the ion-beam energies, (2) to realized the strict direction of beam and, thus, to deposit coatings on shaped elements and to fill deep grooves, and (3) to accurately control the deposition rate of film in varying the current density of beam during deposition. The DLC was deposited using cyclohexane ion-beam (energy of deposited particles is ev; the fraction ion component in the deposited-particle beam is ~100%; the deposition rate at a current density of beam of 1 ma/cm 2 is from 1070 to 900 Å/min at a discharge voltage of from 1.05 to 2.45) [2]. Directly before the deposition, the surface of magnet was cleaned with Ar ion bombardment. Earlier performed studies [2] showed that the films deposited from hydrocarbon ion beams (in particular, cyclohexane ion beams) are diamond-like; the fraction of tetragonal carbon atom bonds is to 70 at%. The element composition and thickness of deposited coating was studied by glow-discharge spectroscopy using a GDS 850A (LECO) spectrometer. The Vickers hardness of the ground surface of sintered Nd-Fe-B magnet with the DLC and free from any coating was studied using a 401/402MVD tester. The tests were performed at a load of 200 gs; time of loading is 12 s. The indentation with diamond indenter (pyramid with a vertex angle of 136 deg) was applied on 2:14:1-phase grains; the indent depth is 1/7 of its diagonal. The mechanical properties of the material were studied by nanoindentation using a Nano-Hardness Tester (CSM Instruments, Switzerland). The construction of NHT comprises a precise hardness tester and an optical microscope, which use a common sample stage with a mechanical drive. The measurements are performed using samples with plane parallel surfaces (seating and studied) no less than 6 mm long in smaller side. The motion in the horizontal plane (positioning) and vertical plane (measuring) are controlled to a high accuracy using a computer and software (CSM). A Berkovich diamond trihedral pyramid is the indenter. Before measuring, parameters of the study (such as Poisson s coefficient of material under study, load, loading rate, holding time, and unloading rate) are preset with a controlling computer program. Data obtained during continuous indentation are processed automatically in accordance with the Oliver-Pharr procedure [4]. Results of the measurements are used to automatically calculate the hardness H, modulus of elasticity E, and fraction of elastic deformation in the indentation work η. Moreover, the maximum depth, the depth of residual indenation at F max, residual depth, projection of contact area, contact rigidity, etc. are recorded from the nanoindentation results. We used linear loading and unloading conditions; the loading time is 45 s. The holding time under the maximum load is 1 s; time of unloading is 30 s.

3 Frictional studies of the sintered magnet free from any coating and with DLC were performed under dry sliding-friction conditions using a CETR UMT Multi-Specimen Test System testing machine and axial loading scheme: finger against disk (with a ShKh15 steel counterbody). The sample in the form of disk 8.3 mm in diameter was mounted at the finger face. The rate of tests was 0.15 m/s. The axial load applied to the finger was 10 N. The sample with DLC was also studied under progressive axial loading at 10, 20, 30, 40, and 50 N. The time of test was 10 min. Before tests, the sample surface was cleaned with an alcohol. The tests were performed in air at 20±1 о С and 60±4% humidity. The frictional force is recorded by a strain gauge; the frictional coefficient is recorded and processed with a computer. 3. RESULTS AND DISCUSSION Figures 1 (a-b) demonstrates results of glow-discharge spectroscopy of the diamond-like coating deposited on the ground surface of Nd-Fe-B magnet. a b Fig. 1 Layer-by-layer analysis of DLC deposited on sintered Nd-Fe-B magnet: depth of analysis (a) 0.4 and (b) 1 μm. It is seen (Fig. 1) that the horizontal portion of the carbon curve finishes at a depth of 0.08 μm. The horizontal portion corresponds to pure carbon (DLC). Then, the increase in the concentration of matrix elements starts. The layer at a depth of μm is transition (Fig. 1). The existence of the layer is due to the diffusion (to a lesser degree) and surface roughness that also explains the disagreement in the coating thickness determined by glow-discharge analysis and from deposition conditions (0.3 μm). The microstructure of ground surface of magnet free from coating and with DLC is given in Fig. 2 (a, b); the determined Vickers hardness is HV 514 and 758, respectively. Fig. 2c demonstrates the microstructure of polished surface of The micrographs indicate the indentations formed during the Vickers hardbess measurements. The micrograph of magnet free from coating demonstrates cracks propagating from the indentation. Figures 3 (a, b) show variations of the frictional coefficient of the sintered magnet free from coating and with DLC during frictional tests under 10 N load.

4 а b c Fig.2 Micrographs of the ground surface of Nd-Fe-B magnet (a) free from coating and (b) with DLC; (c) polished surface with DLC. а b Fig. 3 Variations in the frictional coefficient with time: (a) sintered magnet free from coating and (b) magnet with DLC. As is seen from Fig. 3a, the friction process for the coating-free magnet is unstable. At the beginning stage, the wear-in (destruction of oxide films) takes place, and frictional coefficient increases continuously. At the final stage of the tests, the increase in the frictional coefficient stops. The frictional coefficient is Figure 3b demonstrates that the friction process is stable during the tests. The frictional coefficient increases slightly and reaches Figure 4 shows variations of the frictional coefficient for the sintered Nd-Fe-B magnet with DLC in applying progressively 10-, 20-, 30-, 40-, and 50-N loads. An analysis of the data allows us to conclude the existence of wear-in process at the initial stage of loading. This process manifests itself in the frictional coefficient magnitude (0.2595) and subsequent stability of the

5 frictional process. After wear-in, the frictional process is stable and the spread of the coefficient magnitudes is minimal. The frictional coefficient increases slowly to a stable magnitude of During nanoindentation and determination of hardness by Oliver-Pharr method, the Meyer hardness is measured. According to International Standard ISO , the correct determination of the hardness is realized when the minimum depth of Berkovich indenter penetration exceeds 5% of the average surface roughness (S a ) over the area close to the indentation area. We assume that the height of surface roughness (over 2 2 μm 2 area) for dark regions of coating deposited on the ground surface is comparable with the indentation depth under all loadings used (thus, the effect of roughness on the indentation results is highly probable); for the bright regions of coating deposited on ground surface and coating deposited on polished surface, the roughness is lower substantially than the indentation depth at all loadings used (under 5 mn loading, the roughness relatively weakly affects the indentation results). Fig. 4 Variations of the frictional coefficient (pink curve) and axial loading (blue curve) with time for the Nd- Fe-B magnet with DLC. On the other hand, to exclude the effect of substrate (magnet) on the hardness of coating, the true indentation depth h c (in the case of a hard coating deposited on a soft substrate) must not exceed 10-15% of the coating thickness. The average h с under 5 mn loading is about 85 nm. Thus, for the coating 0.3 μm thick, the more correct hardness measurements were performed for the coating deposited on the polished surface at F max =5 mn. The nanoindentation of the magnet surface free from coating was performed at F max =20 mn. Figure 5 shows load (F) vs indentation depth (h) diagram, which indicates the total character of deformation process during nanoindentation of the DLC deposited on the ground and polished surfaces of sintered Nd- Fe-B magnet and the magnet free from coating at F max = 20 mn. It follows from the diagram that the deformation of samples has the plasto-elastic character. The indentation results (Fig. 5) shows that the hardness (Н) of DLC on the ground surface in bright regions and DLC coating on the polished surface is almost the same at the same F max load (at F max = 10 mn, the hardness is 20 GPa; at F max = 20 мн, the hardness is 19 GPa). The hardness of coating deposited on ground surface is more than 3 and 5 orders of magnitude lower (than the aforementioned magnitudes) for bright and dark regions, respectively. As the load increases, the average Н magnitudes for the coating decreases; for coating deposited on the polished surface, it decreases from 23.7 GPa (F max = 5 mn) to 18.7 GPa (F max = 20 mn). The observed decrease in the hardness is related to the effect of the substrate (Nd-Fe-B sintered magnet) and is typical of the system soft substrate hard coating.

6 20,0 16,0 F, mn ,0 2 8,0 4,0 0, h, nm Fig. 5 Р-h diagrams plotted during nanoindentation of DLC deposited on the (1) polished surface of magnet, ground surface in (2) bright and (3) dark regions, and (4) ground surface free from coating at F max = 20 mn. 3. CONCLUSIONS The DLC coatings were deposited on the ground and polished surfaces of sintered Nd-Fe-B magnet manufactured from TbH 2 -containing powder mixtures. The coatings were prepared by reactive ion-beam synthesis. The Vickers hardness measurements and nanoindentation showed the marked effect of the coating on the surface mechanical characteristics of the sintered magnet. It should be noted the substantial hardening of the ground and polished surfaces of magnet and decrease in tendency to their mechanical failure in using the DLC. The tribological dry-friction tests of the sintered magnet free from coating and with DLC deposited on the ground surface showed that the DLC increases the stability of friction process under static and dynamic loading and decreases substantially the frictional coefficient; in this case, no coating failure and exfoliation are observed. The data obtained allow us to predict the possibility of application of the DLC for the increase in the wear-resistance of sintered Nd-Fe-B magnets (if a thin coating must be used) during their operation in moving abrasive liquid and gas media. ACKNOWLEDGEMENTS This study was performed in the frame of the project No. Cz.1.05/2.1.00/ Regional Materials Science and Technology Center and supported by the Branch of Chemistry and Materials Science, RAS (Program no. 5). LITERATURE [1] KRUUSING, A., PODGURSKI, V., OSVET, A., HERRANEN, M., Diamond-like carbon coatings on NdFeB magnets. Proceedings of Estonian Academy of Science and Engineering, 1998, v. 4., no. 1, pp [2] LUKIN, А.А., KOLCHUGINA, N.B., BURKHANOV, G.S., KLYUEVA, N.E., SKOTNICOVA, K., Role of Terbium Hydride Additions in the Formation of Microstructure and Magnetic Properties of Sintered Nd-Pr-Dy-Fe-B Magnets, Fiz. Khim. Obrab. Mater., 2012, no. 1, pp [3] VALIEV, K.A., MAISHEV, Yu.P., SHEVCHUK, S.L., Reactive Ion-Beam Synthesis of Thin Films Directly from Ion Beams, Fizich. Inzheneriya poverkhnosti, 2003, v.1, no. 1, pp [4] OLIVER, W., PHARR, G. An Improved Technique for Determining Hardness and Elastic Modulus Using Load and Displacement Sensing Indentation Experiments, J. Mater. Res., 1992, v.7, no. 6, pp