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Applied Surface Science 254 (2008) 4552–4556 www.elsevier.com/locate/apsusc
Microstructure and selected properties of hot-work tool steel with PVD coatings after laser surface treatment Marcin Adamiak *, Leszek A. Dobrzan´ski Institute of Engineering Materials and Biomaterials, Silesian University of Technology, 44-100 Gliwice, Konarskiego Street 18A, Poland Received 29 June 2007; accepted 11 January 2008 Available online 29 January 2008
Abstract The paper presents the effect of HPD laser treatment on the microstructure and selected properties of the PVD CrN, (Ti,Al) and Ti(C,N) coatings deposited onto hot-work tool steel substrates. The microstructure and surface topography of the investigated samples are characteristic of the diversified morphology connected with the applied laser beam power. Employment of laser beam at 0.7 kW power to the laser treatment of samples with Ti(C,N) coatings causes clear coating adhesion growth because of the diffusive processes induced by heat release. Because of the higher value of the (Ti,Al)N absorption coefficient one can state that the observed substrate materials change and finally coatings destruction in case of those samples is the most noticeable. The moderate effect of the laser beam treatment of the hot-work tool steel with the PVD coating was observed for CrN coatings. However, for laser beam power above 0.5 kW differences in the thermal expansion coefficients of the substrate materials and coatings generate coating crackings. # 2008 Elsevier B.V. All rights reserved. Keywords: Microstructure; PVD coatings; Laser treatment; Mechanical properties; CrN; TiCN; TiAlN
1. Introduction Hot-work tool steels are used in many industrial branches as the material deciding efficiency, labor demand, and manufacturing process reliability. Tool life is dependant on its quality, and therefore, on the material from which it is made. The appropriate carbon concentration and alloying elements affect directly the structure and phase transformations, occurring during the metallurgical operations, and plastic works or heat treatment. These steels should be characteristic of the abrasion wear resistance and should carry the load without plastic strain. This is dependant on steel hardness. Its increase causes ductility fall and therefore, the maximum hardness value after heat treatment is one of the most important criteria of the appropriate heat treatment process selection. Additionally surface layer, and – in fact – its structure and properties play more and more important roles. Therefore, service properties of tools depend not only on their capability to carry the mechanical loads by the entire element’s transverse section
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[email protected] (M. Adamiak). 0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.01.091
but also by the surface layers. Economical considerations also dictate using surface layers which ensure the required service properties, making it possible to use simultaneously the possibly inexpensive materials for the element’s core, when lower service properties are required from it. Service life of tools made from hot work steels (among others forging tools, moulds for light metals pressure die casting, rolls for copper hot rolling, mandrels, tools for hot cutting) for the sake of their prices is an extremely essential thing in the context of production costs lowering and optimization [1–3]. One of the most frequently applied methods of tool life improvement is PVD technique. Selected PVD coatings have become important for several industrial applications at an elevated temperature. However, application of PVD hard coatings to the relatively soft substrate cannot guarantee the optimal tribological performance. It is possible to enhance the protection efficiency of tools made of hot work steel with combined treatment procedure. One of the possible approaches is duplex treatment combined thermo-chemical treatment of the tool followed by PVD hard coatings deposition. Surface layers obtained in this way display property characteristics of both types of treatment, ensuring simultaneously the quasi-gradient changes of structure and properties of the surface layers of the hot-work tools [4–10].
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Other possibilities are to apply laser source to modify surface layers and their mechanical properties. Laser radiation features currently the state-of-the-art source of heat energy, used to form structure and properties of the surface layer. Thanks to the very precise energy delivery laser radiation that makes it possible to carry out the technological operations better or faster within the framework of the technologies known to date. It also makes possible introduction of the new technologies whose realization is impossible when using the conventional power density. The effect of the process in which the cooling speed is very high is the fine grained structured material with over-cooling phases. Diode lasers have appeared in materials engineering only in 1998 among many lasers used for materials processing. A growing interest is observed in using this energy source since that time. The laser has several unique properties used in the heat treatment of materials’ surfaces. The laser beam’s electromagnetic radiation is absorbed inside the several top atom layers of the opaque materials, like metals. A big advantage is also that the laser beam may be located on the treated element surface exactly where it is required. Surface treatment using laser is attractive because of the possibility to save materials and to improve their surface properties. Such treatment also makes it possible to improve wear resistance, including improving hardness and abrasion wear resistance of surfaces with the relatively small sizes. Employment of the laser surface treatment is justified both from the economical point of view and because the laser treatment, in many cases, ensures obtaining better mechanical properties of the processed surfaces, e.g., tools edges, which could not be attained using the conventional surface treatment methods. The laser heat treatment includes operations which are conducted using the laser beam as the source of energy needed for heating the surface layer of the processed material, to change its structure for obtaining the relevant mechanical, physical, or chemical properties, improving service life of the processed element [11–14]. The goal of this work is to investigate the microstructure of surface layers and their selected property changes after the high power diode laser (HPDL) treatment of hot-work tool steels with CrN, (Ti,Al)N and Ti(C,N) coatings. 2. Experimental The experiments were carried out on specimens made from the X40CrMoV5-1 alloy hot-work tool steel. The investigated steel was molten in the electric vacuum furnace, cast into ingots and was roughed at the temperature range 1100–900 8C into the O.D. 75 mm bars, which were soft annealed. The samples in the form of disc (55 mm diameter and 6 mm thick) were heattreated. The specimens were austenitized on the salt bath furnace for 30 min at the temperature of 1020 8C with the isothermal stops at 650 and 850 8C for 15 min and quenched in hot oil. The specimens were tempered twice for 2 h at the temperature of 550 8C. Surfaces of specimens were sand blasted and machined on magnetic grinder. Next, the thermal treatment samples were grounded and polished to a roughness Ra = 0.08 mm, than the PVD coatings were deposited. CrN
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coatings were prepared in BALZERS BAI 730 deposition system by ion plating PVD process at 450 8C temperature, while (Ti,Al)N and Ti(C,N) coatings were deposited by triode sputtering in Balzers Sputron deposition apparatus at the temperature of 250 8C. Specimens of the X40CrMoV5-1 steel with deposited PVD coatings were fixed in a turntable and laser treated with the Rofin DL 020 high power laser beam (HPDL) with the following parameters: - wavelength of the laser radiation, 808 5 nm - maximum output power of the laser beam (continuous wave), 2500 W. The dimensions of the laser beam focused at spot size on the material surface are 1.8 mm 4.8 mm. The treatment was carried out perpendicularly to the longer side of the focused beam with the multimode energy distribution, which makes it possible to obtain the wide run face. Experiments were carried out at the constant treatment rate, 0.3 m/min changing the laser beam power in the 0.3, 0.5 and 0.7 kW range. The argon blowin with the flow rate of 20 l/min through the f 12 mm circular nozzle oppositely directed with respect to the treatment direction provides antioxidation protection. The thickness of the investigated coatings was measured using the ‘‘ball cratering’’ method. The surface roughness of the polished specimens and roughness of the PVD coatings were measured on the Taylor-Hobson Form Talysurf Series 2 profilometer. The parameter Ra was assumed as a quantity describing the surface roughness. Hardness test of the investigated specimens from hot work steel in the heat treated state has been made using Rockwell C method. Nanoindentation has been performed in order to characterize the coatings hardness and elastic modulus. This test has been performed at room temperature with a Nanoindenter IIs (Nano Instrument, Inc, Knoxville, TN). Detailed nanoindentation experiments used in this characterization comprise of the following steps: approach indenter to sample surface at 10 nm/s; loading step until the total indenter displacement has reached 500 nm; then, load is kept for 10 s to stabilize the measurement. The Oliver and Pharr method has been followed to analyze the obtained load-displacement data, and hence hardness and elastic modulus are determined as a function of the indenter displacement. The evaluation of the adhesion of coatings to the substrate was made using the scratch test with the linearly increasing load, the test was made by the CSEM REVETEST scratch tester. The critical forces at which coating failures appear, called the critical load Lc, were determined based on the acoustic emission Lc(AE) registered during the test and Lc(F t)––sudden increase of the scratching force. The surface topography before and after laser treatment as well as defects of coating created during that treatment was observed on the scanning electron microscope (SEM). The evaluation of the adhesion of coatings to the substrate after laser treatment was made using the Rockwell C method. The character of the defects was determined based on observation performed on the SEM. The examinations of the microstructure of the created surface layers were made in the SEM on the
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Table 1 Summary of the roughness Ra, thickness, hardness, Young modulus and adhesion-critical loads of the investigated PVD coatings Coating type
Roughness Ra (mm)
Thickness (m)
Hardness (GPa)
E modulus (GPa)
Lc(AE) (N)
Lc(Ft) (N)
CrN Ti(C,N) (Ti,Al)N
0.106 0.098 0.101
4.2 2.2 2.7
27.5 32.5 37.5
380 375 390
20 30 41
52 36 60
etched cross-sections. Etching of the specimens was carried out in Nital, at room temperature. 3. Results and discussion As a result of the metallographic examinations made on the SEM, it has been found that the morphology of the investigated PVD coatings deposited onto hot work steel X40CrMoV5-1 type is characterized by a topography homogeneity however the occurrence of several drop-shaped micro-particles and sometimes pits developed by falling out by some of these drops were observed. The results of this investigation correspond with the results of roughness measurements. Deposition of PVD coatings slightly increases the roughness in comparison with the substrate materials after polishing. The roughness of the investigated PVD coatings ranges from 0.089 to 0.319 mm. The results of these measurements correspond with the metallographic examinations made on the SEM. The topography of the coatings influences the roughness, which is characterized by heterogeneity in the forms of cavities and elementary particles as well as a little smoothness of the surfaces of the investigated PVD coatings. The lowest value of Ra parameter was observed for the Ti(C,N) coating. Metallographic examinations of coatings fractures show that investigated coatings have compacted, columnar structure. It has been found out that the investigated PVD coatings according to the assumption are characterized by a uniform thickness. Thickness of these coatings range from 4.2 m for CrN to 2.2 mm for Ti(C,N). The hardness of the investigated PVD coatings ranges from 27.5 GPa for CrN to 37.5 GPa for the (Ti,Al)N one. Whereas Young modulus for that coatings ranges from 375 GPa for the Ti(C,N) to 390 GPa for the (Ti,Al)N. Fig. 1 presents typical results of hardness measurement for the CrN coatings. The
Fig. 1. Hardness changes along the contact depth for the CrN coatings.
results of the roughness measurements, thickness, hardness and Young modulus measurements as well as critical loads of the investigated coatings are presented in Table 1. The critical load values Lc, that are characterized by the adhesion of the investigated PVD coatings to the substrate from the hot work steel are presented in Table 1. It has been found, on the basis of the determined Lc (AE) values, that (Ti,Al)N coatings have very good adhesion to the substrate from the hotwork tool steels, whereas the CrN coatings adhesion reaches the lowest value. The damage of the coatings commences in all cases with the cracks and widespread coating spallation on both edges of the originating scratch. The difference consists in the location of these spalling defects. When friction force is taken to the account the lowest critical loads value reaches Ti(C,N) coatings than CrN and highest (Ti,Al)N. Experiments of laser treatment of the X40CrMoV5-1 hotwork alloy tool steel with PVD coatings indicate to the clear influence of the treatment processes’ parameters, especially of the laser beam power, on the surface topography and microstructure changes of the investigated samples. The laser treatment, in the analyzed power range, causes structural changes occurring both in the coatings and in the substrate material, depending on the coating-type deposited onto the substrate from the hot-work alloy tool steel. One can observe the occurring changes most clearly, at the macroscopic level, in case of the (Ti,Al)N coating, and the least noticeable changes occur in case of the Ti(C,N) one. Figs. 2 and 3 show traces of paths developed on the specimen surfaces laser treated with the 0.7 kW beam for the (Ti,Al)N and CrN coatings. Action of the 0.3 kW laser beam does not cause any essential changes of
Fig. 2. Macrographs of the laser track after 0.7 kW laser beam treatment of the hot-work tool steel with CrN coatings.
M. Adamiak, L.A. Dobrzan´ski / Applied Surface Science 254 (2008) 4552–4556
Fig. 3. Macrographs of the laser track after 0.7 kW laser beam treatment of the hot-work tool steel with (Ti,Al)N coatings.
the surface topography of the examined coatings, and only observations of the microstructure of the transverse sections make it possible to note the tempered martensite structure coarsening. Test pieces with the Ti(C,N) were not changed with the interacting beam just like in case of the macroscopic observations. Laser treatment with the 0.5 kW beam causes development of a network of cracks in the CrN coating and many cracks with spallings for the (Ti,Al)N one. Observations of the transverse sections microstructure revealed its growth again with the clear boundaries of the primary austenite grains (Fig. 4). The examinations also confirmed also occurrences of coating cracks and in case of the (Ti,Al)N one and its delamination from the substrate material. However, test pieces with the Ti(C,N) coating do not display any changes again. Increase of the beam power to 0.7 kW resulted in intensification of the observed changes (Fig. 5) and in case of the Ti(Al,N) ca. 50% of the coating was removed or remelted (Fig. 6). Also the substrate material was remelted with development of the characteristic dendritic structure (Fig. 7). Only employment of
Fig. 4. Microstructure of the transverse section of hot-work tool steel with CrN coatings at the area treated with 0.5 kW laser beam.
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Fig. 5. Surface topography of CrN coating after 0.7 kW laser beam treatment.
Fig. 6. Surface topography of (Ti,Al)N coating after 0.7 kW laser beam treatment.
Fig. 7. Microstructure of the transverse section of hot-work tool steel with (Ti,Al)N coatings at the area treated with 0.7 kW laser beam.
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M. Adamiak, L.A. Dobrzan´ski / Applied Surface Science 254 (2008) 4552–4556
Fig. 8. View of the Rockwell C indent area of the hot-work tool steel with Ti(C,N) coating after treatment with 0.7 kW laser beam.
these laser treatment conditions made it possible to observe changes of the substrate material microstructure in case of the Ti(C,N) coatings, changes similar to those observed 0.3 kW for (Ti,Al)N and 0.5 kW for CrN coatings, however, the coating was still not damaged. Coating adhesion evaluation based on observations of the areas around the Rockwell C indents made it possible to determine that in case of the TiCN coating its adhesion grows significantly after employing the 0.7 kW laser beam (Fig. 8). It is worth noting that adhesion of this coating evaluated with this method for the test pieces that were not heat treated and treated with the beams with powers of 0.3 and 0.5 kW was very low. 4. Summary Based on the carried out research it was found, that it is feasible to develop the surface layers on the X40CrMoV5-1 hot-work tool steel with PVD coatings by laser heat treatment using the HPDL. The microstructure and surface topography of the investigated samples is a characteristic of the diversified morphology connected with the applied laser beam power. This effect is connected with the laser radiation absorption by the test piece surface, thanks to the higher value of the (Ti,Al)N absorption coefficient, the changes and finally destruction observed in case of these samples is the most noticeable.
Employment of laser beam at 0.7 kW power to the laser treatment made it possible to observe changes of the substrate material microstructure in case of the Ti(C,N) coatings, but without coatings damaging. Moreover, this kind of treatment causes clear coating adhesion growth because of the diffusive processes induced by heat release. It is worth noting that adhesion of this coating evaluated with this method for the test pieces that were not heat treated and treated with the beams with powers of 0.3 and 0.5 kW was very low. Laser treatment of the samples with the CrN coating with the power above 0.5 kW causes development of a network of cracks connected with different thermal expansion coefficient of both substrate materials and coatings. Observations of the transverse sections microstructure revealed its growth indicating excess of the Ac3 transformation line. The research results indicate the feasibility and purposefulness of the practical use of laser treatment in carefully selected laser beam power range for making new tools from the X40CrMoV5-1 hot-work tool steel with PVD coatings. References [1] L.A. Dobrzan´ski, M. Bonek, E. Hajduczek, A. Klimpel, Mater. Eng. 3 (140) (2004) 564. [2] L.A. Dobrzan´ski, E. Hajduczek, J. Marciniak, R. Nowosielski, Physical Metallurgy and Heat Treatment of Tool Materials, WNT, Warsaw, 1990 (in Polish). [3] G. Kra´lik, P. Fu¨lo¨p, B. Vero¨, D. Zsa´mbo´k:, Mat. Sci. Forum 414–415 (2003) 251. [4] L.A. Dobrzan´ski, M. Polok, M. Adamiak, J. Mat. Proc. Tech. 164 (2005) 843. [5] L.A. Dobrzan´ski, M. Polok, P. Panjan, S. Bugliosi, M. Adamiak, J. Mat. Proc. Tech. 155 (2004) 1995. [6] L.A. Dobrzan´ski, M. Adamiak, J. Mat. Proc. Tech. 133 (2003) 50. [7] S. PalDey, S.C. Deevi, Mat. Sci. Eng. A 361 (2003) 1. ˇ ekada, R. Kirn, M. Sokovic´, Surf. Coat Tech. 180 (2004) [8] P. Panjan, M. C 561. [9] J.C.A. Batista, C. Godoy, V.T.L. Buono, A. Matthews, Mater. Sci. Eng. A 336 (2002) 39. [10] R.J. Rodriguez, J.A. Garcia, R. Martinez, B. Lerga, M. Rico, G.G. Fuentes, A. Guette, C. Labruguere, M. Lahaye, Appl. Surf. Sci. 235 (2004) 53. [11] J. Kusin´ski, Laser Applications in Materials Engineering, WN Akapit, Cracow, 2000. [12] W.M. Steen, Laser Materials Processing, 2nd ed., Springer Verlag, 1998. [13] L.A. Dobrzan´ski, M. Bonek, E. Hajduczek, A. Klimpel, Appl. Surf. Sci. 247 (2005) 328. [14] L.A. Dobrzan´ski, M. Bonek, A. Klimpel, A. Lisiecki, Mat. Sci. Forum 437–438 (2003) 69.