SURFACE PROCESSING OF THE PM VANADIS 6 STEEL WITH PLASMA NITRIDING AND CrN PVD COATING

Transcription

1 SURFACE PROCESSING OF THE PM VANADIS 6 STEEL WITH PLASMA NITRIDING AND CrN PVD COATING POVRCHOVÉ ZPRACOVÁNÍ PM OCELI VANADIS 6 PLASMOVOU NITRIDACÍ A POVLAKOVÁNIM CrN Peter Jurči a Mária Hudáková b a ECOSOND s.r.o., Křížová 1018, Praha 5, Česká republika b MtF STU Trnava, Pavlínská 16, Trnava, Slovenská republika ABSTRACT The effect of duplex coating with the combination of PVD CrN and plasma nitriding on the structure and wear resistance of PM Vanadis 6 steel was investigated. The sequence of the nitride formation due the plasma nitriding and its effect on main properties of the steel as well as on the adhesion of PVD-layers was determined. Pre-nitrided as well as un-nitrided steel was PVD-coated with the CrN. The beneficial effect of the pre nitriding on the adhesion and wear resistance of coated materials is clearly shown, particularly for the heavily loaded specimens. This is closely connected with the supporting effect of harder nitrided inter layer. 1. INTRODUCTION To meet the requirements of final users, the cold work tool steels should be heat treated to a resulting hardness of about 700 HV. Nevertheless, in order to improve some mechanical properties, mainly the fatigue lifetime and wear properties, a subsequent post heat treatment has to be applied. The PVD coating and its combination with low temperature plasma nitriding, known as the duplex coating, was found as the most convenient post heat treatment technique. The main advantage of the plasma nitriding is the fact that the processing is carried out below the temperature of the tempering cycle (s) so that the structure and mechanical properties remain almost completely unaffected and the process is distortion free. The tools can thus be almost net shape finished before nitriding. The input of nitrogen atoms into the surface leads to the formation of compressive stresses in nitrided region that increases the fatigue strength of tools [1]. Also the ratio of H/E (hardness vs. Young modulus) is increased since the hardness raises considerable but the elastic modulus remains practically constant. The characteristics of nitrided region can very closely be related to the shifted processing parameters, since the devices used for the nitriding enable a very precise adjustment, controlling and regulation of the gas flow, temperature and dwell time. This enables us to avoid several undesirable phenomena, for instance the formation of a compound layer on the surface. As published elsewhere [2], the compound layer can change the constitution and the properties which may have an undesirable effect on the adhesion of PVD layer. The PVD layering with the mixture of chromium and nitrogen is a promising method since the microhardness and wear resistance can be varied in a very wide range [3]. This enables us to modify also the ratio H/E (hardness vs. Young modulus) that is considered as an important criterion for the wear resistance [4-6]. 1

2 2. EXPERIMENTAL PROCEDURES Specimens made from the Vanadis 6 cold work steel (2.1 %C, 7 %Cr, 6 %V, Fe bal.) were net-shape machined and subjected to a heat treatment bringing a hardness of 60 HRC. Plasma nitriding was realised on the RUBIG Micropuls Plasmatechnik equipment at combinations of temperatures 500 and 530 o C and dwell times 60 and 120 min. respectively. Fig. 1 Load arrangement for the PVD coating The PVD layers of thickness 2 and 5 µm were deposited by the arc sputtering of the metal atoms in an appropriate low pressure atmosphere in the device at the workplace of IJS Ljubljana, see Fig. 1. The adhesion of the layers was evaluated by the scratch testing. Hardness and microhardness was tested by the Vickers method with a different loading. The structural analyse were realised using the light microscopy and the scanning electron microscopy. The phase constitution was determined by the X-ray diffraction. The depth chemical composition of thin layer was measured by the GDOES. The wear tests were carried out using the ring-on-plate method (linear contact between sample and counterbody) under non-lubricated conditions at the room temperature. The ČSN (100 Cr 6) steel processed to a hardness of 60 HRC was used as the counterparts. The testing procedure was performed using the loadings of 50 and 150 N. The total sliding distance was 10 km, however, the wear rate, represented by the weight loss of the specimens [g], was also examined after 1, 2.5 and 5 km. 3. RESULTS Recently [9], the optimal processing route for the formation of plasma nitrided inter layer was found to be: Temperature 530 o C, dwell time 120 min, atmosphere composition being represented by the ratio of N 2 : H 2 = 1:3, Fig. 2. The diffusion layer formed by such a way has a thickness of 45 µm, hardness of 1100 HV and consists of the nitrides (Fe 4 N, Fe 3 N, Cr 2 N), matrix formed by the tempered martensite enriched with nitrogen and carbo-nitrides M(C,N) and M 7 (C,N) 3 [7-10]. Unfortunately, the application of plasma nitriding has also undesirable effects on the properties. Due to the regions enriched with nitrogen, the three point bending strength, as a measure for the resistance against the brittle fracture, drops down and this effect becomes 2

3 more evident as the nitrided portion of the material increases [11-13]. The lowering of the fracture toughness is connected with an elevated hardness of the nitrided region, reflected with a cleavage mechanism of the fracture. Therefore, the plasma nitriding must not be applied in the cases when sharp edges on the tools appear or in the similar situations. 2 2 µm 3 2 µm Fig. 2 - SEM micrograph showing the core material Fig. 3 - SEM micrograph showing of the optimized nitrided layer Previous experimental works have shown that the plasma nitriding itself, without any subsequent PVD layering, increases the wear resistance by % [9,10]. This fact is connected with an increased hardness/young modulus ratio, lowered friction coefficient as well as the sensitivity of the material to the plastic deformation of the surface. Examples of duplex coated material (CrN + plasma nitriding) are given in Fig. 4 (overview) and Fig. 5 (detail). At the surface, there is the CrN layer with a real thickness of 5.7 µm. It differs clearly from the nitrided substrate, due to high reflectivity resulting in a high brightness. No inhomogeneities like cracks and pores are presented at the substrate overlay interface µm 5 5 µm Figs. 4, 5 - SEM of duplex layer (CrN 5.7 µm + plasma nitriding 530 o C/120 min.) 3

4 Even if a low loading was applied for the testing, a reduction of the weight loss due to the wear was found, Fig. 6. This can be referred to a lowered friction coefficient (to approx. 0.85) and also to an elevated hardness of the chromium nitride. Moreover, there are significant differences between the specimens processed at different conditions found. Wear resistance was generally better for the specimens with thinner PVD-layer. On the other hand, the role of effective nitriding case depth is not clear because the measurements did not give doubtless results. The use of heavier testing loading highlighted the differences in the wear behaviour of the specimens processed with/without the plasma nitrided inter layer and various thickness of the CrN overlay, Fig. 7. Firstly, a significant reduction of the wear was found for the pre nitrided specimens. The second and general tendency was that the specimens with thicker CrN layer showed a better wear resistance. This is rather surprising on the first sight but it was already established in previous works [13] and referred to higher chromium and lower nitrogen content in thicker PVD-layer, Figs. 8, 9. Higher chromium content led to higher portion of chromium solid solution in the structure of the layers and better resistance against a brittle damage. On the other hard, higher nitrogen content resulted in a formation of a Cr 2 N compound with a higher brittleness. The wear tests confirmed this fact and in many cases, a sudden damage/delamination of thinner coating was observed during the sliding. Also the measurements of the critical load confirmed that for most part of the combinations nitrided layer/crn layer, best wear resistance was for thicker CrN-coatings, Table 1. Duplex coating with CrN, 50 N Weight loss (g) Nitriding 500 C/60 min. Processing route without nitriding, CrN 2.5 mm nitriding 530oC/120 min+crn 2.5 mm nitriding 500oC/120 min+crn 2 mm nitriding 530oC/60 min+crn 2 mm 1 km 10 km 5 km 2.5 km Sliding distance Fig. 6 - Weight loss as a function of sliding distance and surface processing, CrN coated specimens with and without the plasma nitrided layer, loading 50 N 4

5 Duplex coating with CrN, 150 N Weight loss (g) km 5 km km Sliding distance Processing route without nitriding, CrN 2.5 mm without nitriding, CrN 5 mm nitriding 530oC/120 min+crn 2.5 mm nitriding 530oC/120 min+crn 5 mm nitriding 500oC/120 min+crn 2 mm nitriding 500oC/120 min+crn 5 mm nitriding 530oC/60 min+crn 2 mm nitriding 530oC/60 min+crn 5 mm 1 km Fig. 7 - Weight loss as a function of sliding distance and surface processing, CrN coated specimens with and without the plasma nitrided layer, loading 150 N. Processing Lc3 [N] Lc4 [N] Lc5 [N] Nitriding 530 o C/120 min not identified Nitriding 530 o C/120 min Nitriding 500 o C/120 min + 50, - 79, , - Nitriding 500 o C/120 min + 55, 148 not identified 129, 158 Nitriding 530 o C/60 min + 46,135 -, , 138 Nitriding 530 o C/60 min + 47, 52 -,- 98, 134 Table 1 Results of the scratch test measurements 5

6 Plasma nitriding + Plasma nitriding Figs. 8, 9 - Depth profiles of N, Cr, Fe in the PVD layers, 8 2 µm, 9 5 µm. X-ray diffraction measurements of the specimens with nitrided intermediate layer formed at 500 o C/120 min and 530 o C/60 min, respectively, established differences between the phase constitutions of the CrN overlays of a different thickness, Fig. 10. Thicker layers contained more Cr 2 N nitride with higher chromium content. On the other hand, thinner layers exhibited more diffraction peaks of the CrN nitride. Previous experiments [13] also revealed lower nitrogen content in thicker layers, which resulted in a little lower hardness and much better adhesion. Current measurements did not fix as clear difference between adhesion of layered samples but they exist in a some proportion, again in favour of the thicker CrNlayers. Therefore, the statement that the adhesion as well as the wear resistance increases with a thickening of the CrN-overlay (which is connected with increased chromium and decreased nitrogen content) can be considered as general. In addition, the hardness of the PVD overlay does not play an important role neither with respect to the adhesion nor the wear resistance, if the compounds (Cr 2 N or CrN) appear in the PVD layer because there were not relevant differences in hardness observed. The microhardness HV 0.05 exceeded 2200 units for all the specimens. 6

7 Cr 2 N Intensity Intenzita CrN Two Difrakčný theta diffraction uhol 2θ angle Fig X-ray patterns of the layers consisting of the chromium and nitrogen atoms. 4. CONCLUSIONS Plasma nitriding of the surface used as a pre treatment before PVD coating increases the wear resistance and improves the adhesion of the overlay considerable. The CrN coating increases the wear resistance even if low loading is applied, but its beneficial effect is highlighted only under high loading. The wear behaviour of thinner CrN-layer is worse than that of thiocker layers. It is rather surprising on the first sight but evincible through more favourable composition of thicker layers. In selected cases, an excellent critical load (adhesion) was achieved which was reflected in very good wear resistance. Experiments confirmed that the layering with the mixture of Cr and N, combined with the plasma nitriding, is a promising surfacing method for P/M tool steels. ACKNOWLEDGEMENTS The authors wish to thank the Minstry of Education, Youth and Sport for the financial support of the project EUREKA! 3437 PROSURFMET REFERENCES [1] Jurči, P.. Hnilica, F., Stolař, P.: In.: Proceedings of the 10 th Int. Conference CO - MAT - TECH, , Trnava, Slovakia, p. 58. [2] Spies, H.-J., Hock, K., Larisch, B.: HTM 51 (1996) 4, p [3] Rebholz, C. et al.: Surf. Coat. Techn. 115, 1999, p [4] Matthews, A., Leyland, A.: HTM 56 (2001) 1, p. 5. [5] Halling, J.: Proc. Instn. Mech. Engrs., 200, 1986, p

8 [6] Bergmann, E.: Traitements Duplex: principes, technologie et applications, Traitement Thermique, 5, 1996, p. 41. [7] Jurči, P., Hnilica, F., Suchánek, J., Stolař, P.: Structural Features of the Cr-V Ledeburitic Steel Saturated by Nitrogen, Materiali in Tehnologije, 38, 2004, 1-2, p. 13. [8] Jurči, P.: PhD Thesis, MtF STU Trnava (In Czech). [9] Jurči, P., Musilová, A., Stolař, P., Hrubý, V.: In: Proceedings of the Conference Carburizing and Nitriding, November 27-28, 2001, Brno, Czech Republic, p. 81. [10] Jurči, P., Hnilica, F: The Mechanism and Kinetic of the Saturation of Multiphase Cr V Tool Steel Effect on Material Properties, In: Proceedings of the 13 th Int. Conference Metal 2004, May 2004, Hradec nad Moravicí, Czech Republic, CD - ROM [11] Hnilica, P., Čmakal, J., Jurči, P.: Alterations in Fracture Behaviour of the Cr-V Ledeburitic Steel Vanadis 6 Caused by Plasma Nitriding, In.: 11 th Conference on Materials and Technology, book of abstracts, Portorož, Slovenia, Oktober 1-3, [12] Jurči, P., Hnilica, F., Čmakal, J., Pechmanová, J.: In: Proceedings of the 8 th National Conference Degradation of Structural Materials, September 2.-4, 2003, Terchová, Slovakia, p [13] Jurči, P., Panjan, P.: Surface Processing of the PM Vanadis 6 Steel with Plasma Nitriding and CrN PVD Coating, In: Proceedings of the European Powder Metallurgy Congress, Prague, Czech Republic,