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Improvement of wear resistance of hot work steels by PVD coatings deposition
Journal of Materials Processing Technology 155–156 (2004) 1995–2001
Improvement of wear resistance of hot work steels by PVD coatings deposition L.A. Dobrzanski a,∗ , M. Polok a , P. Panjan b , S. Bugliosi c , M. Adamiak a a
Institute of Engineering Materials and Biomaterials, Silesian University of Technology, Gliwice, Poland b Jozef Stefan Institute, Jamova 39, Ljubljana, Slovenia c Istituto di Scienza e Tecnologia dei Materiali Ceramici, Orbassano, Italy
Abstract The paper presents results of structure, phase composition, tribological and adhesion investigations of wear resistance PVD coatings TiN, TiN/(Ti, Al)N and CrN types deposited in ion plating PVD process onto X37CrMoV5-l type hot work tool steel. The X-ray quantitative phase analysis makes it possible to find out the preferential crystallographic orientation (1 1 1) for the monolayer TiN coating and for the multilayer TiN/(Ti, Al)N one. The diffraction line broadening in case of the CrN coating attests to occurrence of strong internal stresses or to small sizes of its constituent crystallites. It was found that failure mechanism during the scratch test in all investigated coatings begins with multiple spallings located on the scratch edges followed by cracking and total delamination of coating. Regarding to the coating types different location of such damages and loads typical for them can be seen. According to those observations we found that highest adhesion among investigated coating present, CrN monolayer coating and the lowest one for multilayers TiN/(Ti, Al)N coating. The wear resistance was investigated by the pin-on-disk method performed at the room temperature and the one elevated to 500 ◦ C. We found that the lowest wear in to fixed investigation conditions in both room and elevated temperatures show TiN monolayer coating. Additionally, one can see that TiN coatings application improves wear resistance for about five times. © 2004 Elsevier B.V. All rights reserved. Keywords: PVD coatings; Hot work steel; Wear resistance; Pin-on-disk test
1. Introduction Many different development directions of the hot work steels and tools that are made from them have been observed for the last ten years. Metalworking industries have shown interest in the improvement of tooling used in hot-working process: metal die casting, hot extrusion and hot forging. These technologies are characterised by working temperatures higher than 600 ◦ C, very large surface loads and specific tribo-system. Due to the enormous quantities of products in these industries and the relatively short life of the moulds, tools and dies necessary, even small improvements in that field bring the economic effect. In the past hot-work tools were improved by a variety of surface engineering processes such as weld surfacing, thermal spraying, electrodeposition, etc. The PVD coatings have become the extremely important technological materials for several industrial applications; these coatings are successful in
∗ Corresponding author. Tel.: +48 32 2371653; fax: +48 32 2372281. E-mail address: [email protected] (L.A. Dobrzanski).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.04.405
working processes at elevated temperatures. The PVD hard coatings are known for providing surfaces with enhanced tribological properties in terms of low friction and higher wear resistance [1,2]. It has been documented in the literature that PVD TiN, TiN/(Ti, Al)N and CrN coatings can reduce friction in tribological contacts and increase the abrasive wear resistance. High performance in tool applications: drilling and turning has been reported for these coatings. In addition to the enhanced wear resistance, TiN coatings can also provide wear and oxidation resistance, especially at high temperatures [3–5]. The PVD techniques make it possible to extend by 50–100% the life of tools made from hot work steels [6,7]. The paper presents results of the project focused on investigation of the structure and wear resistance of CrN, TiN, TiN/(Ti, A1)N PVD coatings deposited onto X37CrMoV5-l type hot work steels. Several analytical methods such as glow discharge optical emission spectroscopy (GDOES), X-ray diffraction (XRD), surface profilometry, scratch test, pin-on-disk test performed at the room temperature and at 500 ◦ C are described. Investigation results including SEM
1996
L.A. Dobrzanski et al. / Journal of Materials Processing Technology 155–156 (2004) 1995–2001
and LM as well as mechanical characteristics will provide useful information to understanding and applying of these materials.
2. Experimental procedure TiN and CrN coatings were prepared in BALZERS BAI 730 deposition system by ion plating PVD process at 450 ◦ C temperature, while (TiN/TiAlN)3x coating was deposited by reactive sputtering in a Sputron (Balzers) plasma-beam-sputtering apparatus at the temperature 200 ◦ C. All coatings were deposited onto the X37CrMoV5-l type hot work steel substrates. The chemical composition of the investigated steel is presented in Table 1. The samples in the form of disc (diameter 55 mm and thickness 5 mm) were quenched at 1020 ◦ C and tempered at 550 ◦ C to hardness 55 HRC. After thermal treatment these samples were ground and polished to a roughness of Ra = 0.009 m. Hardness tests of the investigated specimens from hot work steel in the heat treated state were made using Rockwell method. Surface roughness of the polished specimens was measured on the Taylor–Hobson Form Talysurf Series 2 profilometer. The parameter Ra was assumed as a quantity describing the surface roughness. Thickness measurements of the deposited TiN, TiN/(Ti, Al)N and CrN coatings were measured using the kalotest method, consisting of measurement of the characteristic parameters of the crater developed on the surface of the investigated coated specimen. A steel ball with a diameter of 11 mm is used in that method to develop a crater. The thickness measurements were made using the light microscope with the scale graduation. Six measurements were made for each of the examined specimens to obtain the average values. The phase composition of the PVD TiN, TiN/(Ti, Al)N and CrN coatings was determined using the Dron 2,0 diffractometer, using the X-ray radiation coming from the Co anode lamp. The measurements were made in the 2Θ angle ranging from 30◦ to 105◦ . The Leco GDOS instrument SDP 750 A glow discharge optical emission spectrometry was used for analysing the coatings compositions. The sputtering parameters were: cathode voltage 700 V, ion current 25 mA. Evaluation of the adhesion of coatings to the substrate was made using the scratch test, the test were made by the CSEM REVETEST scratch tester. The measurements parameters were as follow: table speed 10 mm/min, loading
X37CrMoV5-l
3. Discussion of results 3.1. Structure, phase composition of the investigated PVD coatings on the X37CrMoV5-l hot-work steel The thickness of the investigated PVD: monolayer TiN, multilayer TiN/(Ti, Al)N and monolayer CrN coatings according to the “kalotest” measurements are presented in Table 2. The X-ray quantitative phase analysis confirmed that according to the assumptions, the TiN, TiN/(Ti, Al)N and CrN Table 2 Thickness of the investigated coatings
Table 1 Chemical composition of hot work tool steel Steel type
rate 100 N/min, loading scale 0–100 N and scratch length 10 mm. The critical force at which coating failures appear, called the critical load Lc , was determined basing on the acoustic emission AE registered during the test and coming from the contact point of the indenter and the examined specimen and microscope observations for five critical forces: Lc3 —flaking on the scratch edge, Lc4 —coating partial delamination, Lc5 —coating total delamination and Lc (Ft )—sudden increase of the scratching force. The character of the defects was determined basing on the observation performed on the light microscope LEICA MEF4A. Wear resistance tests with the pin-on-disk method were carried out on the CSEM THT (High temperature tribometer) device at the room temperature and at the temperature of 500 ◦ C. The Al2 O3 corundum ball of the 6 mm diameter was used as a counter-specimen. During the pin-on-disk test carried out at the room temperature for the investigated TiN, CrN, TiN/(Ti, Al)N coatings and the substrate material the stationary ball was pressed with the load of 7.0 N to the disk rotating in a horizontal plane. The rotational speed of the disk with the specimen was 80 cm/s, friction radius was 15 mm, and the number of rotations made by the specimen was 7500. During the test at the temperature of 500 ◦ C, the friction radius was changed from 15 to 18 mm, leaving other test parameters unchanged, as in the test made at the room temperature. The friction coefficient between the ball and disk was measured during the test. Examinations of wear traces developed during the pin-on-disk test at the room temperature and at the elevated one were made on the LEICA MEF4A light microscope at magnification of 100x. Profiles of the wear traces for specimens with the monolayer coatings: TiN, CrN, and the multilayer ones: TiN/(Ti, Al)N, and also for the X37CrMoV5-l hot work steel substrate, were made on the Taylor–Hobson Form Talysurf 120L laser profilometer in four orthogonal directions (every 90◦ ).
Mass concentration of elements [%] C
Mn
Si
Cr
Mo
V
0.37
0.35
1.0
5.0
1.35
0.4
Coating type
Thickness [m]
Number of layers
CrN TiN (TiN/(Ti, Al)N)3x
7.7 3.16 3.24
1 1 6
L.A. Dobrzanski et al. / Journal of Materials Processing Technology 155–156 (2004) 1995–2001
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Fig. 1. Diffractograms of investigated coatings deposited onto X37CrMoV5-l hot work-steel: (a) TiN coatings, (b) (TiN/(Ti, Al)N)3x coatings, (c) CrN coatings.
(Fig. 1) coatings were deposited on the surface of the specimens made from the X37CrMoV5-l hot work tool steel. Reflexes coming from the substrate materials martensite (Fe␣ ) were found out in the X-ray diffraction patterns for all investigated coatings. The monolayer TiN coating and the multilayer TiN/(Ti, Al)N one demonstrate occurrence of the preferential (1 1 1) crystallographic orientation (Fig. 1a and b). Broadening the diffraction line in case of the CrN coating indicates to occurrence of strong internal stresses or to small sizes of its constituent crystallites (Fig. 1c). Examinations on the glow discharge optical spectroscope (GDOS) make it possible to determine the character of concentration changes of elements constituting the investigated PVD coatings (Fig. 2). The analysis of elements’ concentrations for the TiN coating confirming the presence of titanium and nitrogen in the investigated coating, it should be noted that the nitrogen concentration decreases systematically from the maximum value of 60–75 at.% at the surface to 40–50 at.% at the depth of about 1 m, and the titanium concentration increases from 30 to 40 at.% at the surface to 50–60 at.% at the same depth (Fig. 2a). The occurrence of this distribution of both elements attests to the development of the solid secondary solution based on the titanium ni-
tride TiN. Changes of the concentrations of elements for the CrN coating confirm the presence of chromium and nitrogen (Fig. 2b). Chromium concentration increases from 70 to 90 at.% at the surface and decreases systematically to about 70 at.% at the depth of about 4 m, whereas the nitrogen concentration reaches the level of about 30 at.%. Concentration of elements constituting the CrN coating attests to its non-stoichiometric composition and inhomogeneity. The character of concentration changes of the elements constituting the multilayer TiN/(Ti, Al)N coating confirms its laminar structure (Fig. 2c). The (Ti, Al)N layer is the external layer of this coating, after which the TiN laser comes, in this case the aluminium concentration decreases to 0%. 3.2. Adhesive and wear resistance of the investigated PVD coatings Critical loads Lc characterising adhesion of the investigated coatings to the substrate from the hot work tool steel, were determined during the scratch test with the increasing load. The values of the critical loads for the particular coatings are presented in Table 3. The CrN coatings demonstrate a very good adhesion to the substrate, next come the TiN ones, and the least advan-
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Fig. 2. GDOES depth profiles: TiN (a), CrN (b), (TiN/(Ti, Al)N)3x (c) coatings. Table 3 Critical loads for investigated coatings Coating type
CrN TiN TiN/(Ti, Al)N
Type of defect/force [N] Lc (AE)
Lc3
Lc4
Lc5
Lc (Ft )
37.0 25.5 10.7
32.0 19.4 12.8
77.0 39.0 32.5
87.0 82.0 69.0
91.0 80.1 68.6
tageous adhesion was observed for the multilayer TiN/(Ti, Al)N coating. The coating failure mechanism in all cases begins from the numerous spallings on both edges of the developed scratch (Fig. 3). The difference is in the location of these spallings. In case of the TiN/(Ti, Al)N coating, spallings start from the load as low as 13 N. Then cracks and stretches develop at the scratch bottom, and the spalling process at the scratch edge develops further, and finally total coating delamination occurs at the scratch bottom.
Fig. 3. Scratches with critical load Lc4 —partial delamination: (a) CrN coating, (b) TiN/(Ti, Al)N coating.
L.A. Dobrzanski et al. / Journal of Materials Processing Technology 155–156 (2004) 1995–2001
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Fig. 4. Friction coefficient changes vs. wear track length for investigated coating: (a) TiN coating: RT 20 ◦ C, HT 500 ◦ C, (b) CrN coating: RT 20 ◦ C, HT 500 ◦ C.
The analysis of the results justifies a statement that in spite of the big differences in critical loads, depending on the evaluation criterion assumed, the general trend remains the same. Spalling begins with the monolayer TiN coatings at the load value of about 20 N, and with the multilayer CrN ones at about 32 N load value. The multilayer TiN/(Ti, Al)N coatings have the worst adhesion, whereas the best adhesion is characteristic for the monolayer CrN ones. Employing the EDS analyser in the scanning electron microscope makes it possible to find out in the addition that in case of the TiN/(Ti, Al)N coatings delamination of the layers occurs from the titanium sublayer deposited initially. The investigated coatings and the substrate material were examined using the pin-on-disk test to determine their wear resistance; the test was carried out at the room temperature and at the temperature elevated to 500 ◦ C. Changes of the friction coefficient value between the corundum ball and the tested specimen were recorded both for the room temperature and for the temperature of 500 ◦ C (Fig. 4). Analysis of the friction coefficient changes of the examined specimens justifies a statement that in the assumed experiment conditions, the friction coefficient for the specimen made from the heat treated tool steel tested at the room temperature was about 0.4. In case of the specimens coated in the PVD process the friction coefficient changes in the
Fig. 5. Profiles of wear track performed at 500 ◦ C: (a) multilayer TiN/(Ti, Al)N, (b) monolayer CrN.
Fig. 6. Comparison of volume of materials removed during tribological wear, 20 ◦ C.
range from about 0.45 (TiN) to about 0.7 for the TiN/(Ti, Al)N and CrN coatings, in case of the tests carried out at the room temperature. Results of the tests carried out at the temperature of 500 ◦ C revealed that the friction coefficient for the coated specimens was 0.55 for TiN, 0.6 for CrN, and 0.7 for the TiN/(Ti, Al)N coating. Only the TiN coating, as the only one from all tested coatings does not change the friction coefficient value during the entire test. The TiN/(Ti, Al)N and CrN coatings change their friction coefficient values after the initial test stage, which can be connected with their partial or total failure mechanisms, revealing values close to the ones for the uncoated steel in case of the CrN coating, or higher values in case of the partial failure which occurs in case of the TiN/(Ti, Al)N coating. The quantitative evaluation of the surface wear of the examined specimens due to friction was carried out basing on measurements of the wear track profiles for the TiN, CrN, and TiN/(Ti, Al)N coatings deposited on the substrate made
Fig. 7. Comparison of volume of materials removed during tribological wear, 500 ◦ C.
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Fig. 8. SEM micrographs of wear track on the surface of investigated samples (upper row 20 ◦ C, lower row 500 ◦ C): (a) X37CrMoV5-l steel, (b) TiN coating, (c) CrN coating.
from the X37CrMoV5-l hot work tool steel, as well as for the substrate material in four orthogonal directions, every 90◦ . The measured profiles were put together and the average profile was determined for the worn land for each of the examined coatings and for the substrate material. The area of the material removed due to the friction was measured for the average profile determined in this way (area between the profile contour and the zero line-specimen surface). The average volume of the material removed due to the friction of the corundum ball with the specimen surface was calculated using the known wear track length. Fig. 5 presents the exemplary set of profiles measured for wear tracks performed at the temperature of 500 ◦ C for the multilayer TiN/(Ti, Al)N coating (Fig. 5a) and for the monolayer CrN one (Fig. 5b). The average volume of the material removed during the tribological wear was calculated using the formula: V [mm3 ] = P × S
(1)
where V is the average volume of the material worn out due to the friction, P the average area of the removed material [mm2 ], S the track length 2πr [mm]. Measurement and determination of the average profiles and further of the volume of the removed material were carried out both for specimens tested at the room temperature and at the elevated temperature, and the measurement results are presented in Figs. 6 and 7. One can state, basing on the wear measurement results collected for the specimens
examined at the room temperature, that the highest wear resistance has the TiN coating, whereas both the CrN and the TiN/(Ti, A1)N one get worn at an extent comparable to the wear of the X37CrMoV5-l tool steel. The change of the test temperature to 500 ◦ C results, in each case, in the about fivefold increase of the wear intensity. However, the TiN monolayer still demonstrates its highest wear resistance. Wear resistance of the TiN/(Ti, A1)N coating has grown, and the CrN coating was worn in a similar extent like the uncoated material. Measurements of the wear track width were made basing on observations carried out on the scanning electron microscope (Fig. 8); it was found out that the width increases with the test temperature increase and in case of the material with no anti-wear coating, tested at the room temperature, it is about 0.3 mm, and at the temperature of 500 ◦ C it is about 0.60 mm. The deposition of the TiN coating results in decreasing the wear width to about 0.18 mm and about 0.45 mm for the tests carried out at the room temperature and at the elevated one respectively. Results of these measurements correspond with the removed material volume measurements made during the pin-on-disk tests. 4. Conclusion The X-ray quantitative phase analysis makes it possible to reveal the privileged crystallographic orientation (1 1 1) for
L.A. Dobrzanski et al. / Journal of Materials Processing Technology 155–156 (2004) 1995–2001
the TiN monolayer and TiN/(Ti, Al)N multilayer coatings. Broadening the diffraction line in case of the CrN coating attests to the occurrence of the strong internal stresses or to small size of its constituent crystallites. Changes of the concentration of elements present in the investigated coatings, observed during the measurements on the GDOES depth profiling justify the statement that fluctuations of the chemical composition occur in the investigated coatings indicating to origination of the secondary solid solutions. Basing on the out carried investigations one can state that adhesion of the PVD coatings: monolayer TiN, multilayer TiN/(Ti, A1)N, and monolayer CrN to the substrate from the X37CrMoV5-l hot work tool steel changes significantly depending from the assumed criterion; however the general trend remains. The CrN coating has the best adhesion, and the TiN/(Ti, A1)N coating demonstrates the worst adhesion. Measurement of the friction coefficient during the pin-on-disk test makes it possible to note that the TiN coating has the lowest friction coefficient both at the room temperature and at the elevated one; other coatings have their friction coefficients higher than the substrate material. Moreover, one can state that the friction coefficient changes with the development of the coating failure process and after its total removal the values obtained were typical for the substrate material. The TiN coating has the highest wear resistance at the room temperature, whereas both the CrN and the TiN/(Ti, Al)N ones get worn comparably to the X37CrMoV5-l tool steel. The change of the test temperature to 500 ◦ C causes the ca fivefold increase of the wear intensity in each case.
2001
The wear track width grows with the test temperature increase and in case of the uncoated materials, examined at the room temperature it is about 0.3 mm, and at the temperature of 500 ◦ C it is about 0.60 mm. The deposition of the TiN coating results in decreasing the wear track width to about 0.18 mm and to about 0.45 mm for tests made at the room temperature and at the elevated one, respectively.
References [1] L.A. Dobrzanski, M. Adamiak, Structure and properties of TiN and Ti(C, N) coatings deposited in the PVD process on high-speed steels, J. Mater. Process. Technol. 133 (2003) 50–62. [2] B. Navinsek, P. Panjan, F. Gorenjak, Improvement of hot forging manufacturing with PVD and Duplex coatings, Surf. Coat. Technol. 137 (2001) 255–264. [3] B. Navinsek, P. Panjan, I. Uranian, P. Cvahte, F. Gorenjak, Improvement of hot-working process with PVD coatings and duplex treatment, Surf. Coat. Technol. 142–144 (2001) 1148–1154. [4] J.C.A. Batista, C. Godoy, V.T.L. Buono, A. Matthews, Characterization of duplex and non-duplex (Ti, A1)N and CrN PVD coatings, Mater. Sci. Eng. A 336 (2002) 39–51. [5] M. Cekada, P. Panjan, M. Macek, P. Amid, Comparison of structural and chemical properties of Cr-based hard coatings, Surf. Technol. 151–152 (2002) 31–35. [6] T.S. Li, H. Li, F. Pan, Microstructure and nanoindentation hardness of Ti/TiN multilayered film, Surf. Coat. Technol. 137 (2001) 225– 229. [7] J.C.A. Batista, M.C. Joseph, C. Godoy, A. Matthews, Micro-abrasion wear testing of PVD TiN coatings on untreated and plasma nitrided AISI HI3 steel, Wear 249 (2002) 971–979.
Improvement of wear resistance of hot work steels by PVD coatings deposition L.A. Dobrzanski a,∗ , M. Polok a , P. Panjan b , S. Bugliosi c , M. Adamiak a a
Institute of Engineering Materials and Biomaterials, Silesian University of Technology, Gliwice, Poland b Jozef Stefan Institute, Jamova 39, Ljubljana, Slovenia c Istituto di Scienza e Tecnologia dei Materiali Ceramici, Orbassano, Italy
Abstract The paper presents results of structure, phase composition, tribological and adhesion investigations of wear resistance PVD coatings TiN, TiN/(Ti, Al)N and CrN types deposited in ion plating PVD process onto X37CrMoV5-l type hot work tool steel. The X-ray quantitative phase analysis makes it possible to find out the preferential crystallographic orientation (1 1 1) for the monolayer TiN coating and for the multilayer TiN/(Ti, Al)N one. The diffraction line broadening in case of the CrN coating attests to occurrence of strong internal stresses or to small sizes of its constituent crystallites. It was found that failure mechanism during the scratch test in all investigated coatings begins with multiple spallings located on the scratch edges followed by cracking and total delamination of coating. Regarding to the coating types different location of such damages and loads typical for them can be seen. According to those observations we found that highest adhesion among investigated coating present, CrN monolayer coating and the lowest one for multilayers TiN/(Ti, Al)N coating. The wear resistance was investigated by the pin-on-disk method performed at the room temperature and the one elevated to 500 ◦ C. We found that the lowest wear in to fixed investigation conditions in both room and elevated temperatures show TiN monolayer coating. Additionally, one can see that TiN coatings application improves wear resistance for about five times. © 2004 Elsevier B.V. All rights reserved. Keywords: PVD coatings; Hot work steel; Wear resistance; Pin-on-disk test
1. Introduction Many different development directions of the hot work steels and tools that are made from them have been observed for the last ten years. Metalworking industries have shown interest in the improvement of tooling used in hot-working process: metal die casting, hot extrusion and hot forging. These technologies are characterised by working temperatures higher than 600 ◦ C, very large surface loads and specific tribo-system. Due to the enormous quantities of products in these industries and the relatively short life of the moulds, tools and dies necessary, even small improvements in that field bring the economic effect. In the past hot-work tools were improved by a variety of surface engineering processes such as weld surfacing, thermal spraying, electrodeposition, etc. The PVD coatings have become the extremely important technological materials for several industrial applications; these coatings are successful in
∗ Corresponding author. Tel.: +48 32 2371653; fax: +48 32 2372281. E-mail address: [email protected] (L.A. Dobrzanski).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.04.405
working processes at elevated temperatures. The PVD hard coatings are known for providing surfaces with enhanced tribological properties in terms of low friction and higher wear resistance [1,2]. It has been documented in the literature that PVD TiN, TiN/(Ti, Al)N and CrN coatings can reduce friction in tribological contacts and increase the abrasive wear resistance. High performance in tool applications: drilling and turning has been reported for these coatings. In addition to the enhanced wear resistance, TiN coatings can also provide wear and oxidation resistance, especially at high temperatures [3–5]. The PVD techniques make it possible to extend by 50–100% the life of tools made from hot work steels [6,7]. The paper presents results of the project focused on investigation of the structure and wear resistance of CrN, TiN, TiN/(Ti, A1)N PVD coatings deposited onto X37CrMoV5-l type hot work steels. Several analytical methods such as glow discharge optical emission spectroscopy (GDOES), X-ray diffraction (XRD), surface profilometry, scratch test, pin-on-disk test performed at the room temperature and at 500 ◦ C are described. Investigation results including SEM
1996
L.A. Dobrzanski et al. / Journal of Materials Processing Technology 155–156 (2004) 1995–2001
and LM as well as mechanical characteristics will provide useful information to understanding and applying of these materials.
2. Experimental procedure TiN and CrN coatings were prepared in BALZERS BAI 730 deposition system by ion plating PVD process at 450 ◦ C temperature, while (TiN/TiAlN)3x coating was deposited by reactive sputtering in a Sputron (Balzers) plasma-beam-sputtering apparatus at the temperature 200 ◦ C. All coatings were deposited onto the X37CrMoV5-l type hot work steel substrates. The chemical composition of the investigated steel is presented in Table 1. The samples in the form of disc (diameter 55 mm and thickness 5 mm) were quenched at 1020 ◦ C and tempered at 550 ◦ C to hardness 55 HRC. After thermal treatment these samples were ground and polished to a roughness of Ra = 0.009 m. Hardness tests of the investigated specimens from hot work steel in the heat treated state were made using Rockwell method. Surface roughness of the polished specimens was measured on the Taylor–Hobson Form Talysurf Series 2 profilometer. The parameter Ra was assumed as a quantity describing the surface roughness. Thickness measurements of the deposited TiN, TiN/(Ti, Al)N and CrN coatings were measured using the kalotest method, consisting of measurement of the characteristic parameters of the crater developed on the surface of the investigated coated specimen. A steel ball with a diameter of 11 mm is used in that method to develop a crater. The thickness measurements were made using the light microscope with the scale graduation. Six measurements were made for each of the examined specimens to obtain the average values. The phase composition of the PVD TiN, TiN/(Ti, Al)N and CrN coatings was determined using the Dron 2,0 diffractometer, using the X-ray radiation coming from the Co anode lamp. The measurements were made in the 2Θ angle ranging from 30◦ to 105◦ . The Leco GDOS instrument SDP 750 A glow discharge optical emission spectrometry was used for analysing the coatings compositions. The sputtering parameters were: cathode voltage 700 V, ion current 25 mA. Evaluation of the adhesion of coatings to the substrate was made using the scratch test, the test were made by the CSEM REVETEST scratch tester. The measurements parameters were as follow: table speed 10 mm/min, loading
X37CrMoV5-l
3. Discussion of results 3.1. Structure, phase composition of the investigated PVD coatings on the X37CrMoV5-l hot-work steel The thickness of the investigated PVD: monolayer TiN, multilayer TiN/(Ti, Al)N and monolayer CrN coatings according to the “kalotest” measurements are presented in Table 2. The X-ray quantitative phase analysis confirmed that according to the assumptions, the TiN, TiN/(Ti, Al)N and CrN Table 2 Thickness of the investigated coatings
Table 1 Chemical composition of hot work tool steel Steel type
rate 100 N/min, loading scale 0–100 N and scratch length 10 mm. The critical force at which coating failures appear, called the critical load Lc , was determined basing on the acoustic emission AE registered during the test and coming from the contact point of the indenter and the examined specimen and microscope observations for five critical forces: Lc3 —flaking on the scratch edge, Lc4 —coating partial delamination, Lc5 —coating total delamination and Lc (Ft )—sudden increase of the scratching force. The character of the defects was determined basing on the observation performed on the light microscope LEICA MEF4A. Wear resistance tests with the pin-on-disk method were carried out on the CSEM THT (High temperature tribometer) device at the room temperature and at the temperature of 500 ◦ C. The Al2 O3 corundum ball of the 6 mm diameter was used as a counter-specimen. During the pin-on-disk test carried out at the room temperature for the investigated TiN, CrN, TiN/(Ti, Al)N coatings and the substrate material the stationary ball was pressed with the load of 7.0 N to the disk rotating in a horizontal plane. The rotational speed of the disk with the specimen was 80 cm/s, friction radius was 15 mm, and the number of rotations made by the specimen was 7500. During the test at the temperature of 500 ◦ C, the friction radius was changed from 15 to 18 mm, leaving other test parameters unchanged, as in the test made at the room temperature. The friction coefficient between the ball and disk was measured during the test. Examinations of wear traces developed during the pin-on-disk test at the room temperature and at the elevated one were made on the LEICA MEF4A light microscope at magnification of 100x. Profiles of the wear traces for specimens with the monolayer coatings: TiN, CrN, and the multilayer ones: TiN/(Ti, Al)N, and also for the X37CrMoV5-l hot work steel substrate, were made on the Taylor–Hobson Form Talysurf 120L laser profilometer in four orthogonal directions (every 90◦ ).
Mass concentration of elements [%] C
Mn
Si
Cr
Mo
V
0.37
0.35
1.0
5.0
1.35
0.4
Coating type
Thickness [m]
Number of layers
CrN TiN (TiN/(Ti, Al)N)3x
7.7 3.16 3.24
1 1 6
L.A. Dobrzanski et al. / Journal of Materials Processing Technology 155–156 (2004) 1995–2001
1997
Fig. 1. Diffractograms of investigated coatings deposited onto X37CrMoV5-l hot work-steel: (a) TiN coatings, (b) (TiN/(Ti, Al)N)3x coatings, (c) CrN coatings.
(Fig. 1) coatings were deposited on the surface of the specimens made from the X37CrMoV5-l hot work tool steel. Reflexes coming from the substrate materials martensite (Fe␣ ) were found out in the X-ray diffraction patterns for all investigated coatings. The monolayer TiN coating and the multilayer TiN/(Ti, Al)N one demonstrate occurrence of the preferential (1 1 1) crystallographic orientation (Fig. 1a and b). Broadening the diffraction line in case of the CrN coating indicates to occurrence of strong internal stresses or to small sizes of its constituent crystallites (Fig. 1c). Examinations on the glow discharge optical spectroscope (GDOS) make it possible to determine the character of concentration changes of elements constituting the investigated PVD coatings (Fig. 2). The analysis of elements’ concentrations for the TiN coating confirming the presence of titanium and nitrogen in the investigated coating, it should be noted that the nitrogen concentration decreases systematically from the maximum value of 60–75 at.% at the surface to 40–50 at.% at the depth of about 1 m, and the titanium concentration increases from 30 to 40 at.% at the surface to 50–60 at.% at the same depth (Fig. 2a). The occurrence of this distribution of both elements attests to the development of the solid secondary solution based on the titanium ni-
tride TiN. Changes of the concentrations of elements for the CrN coating confirm the presence of chromium and nitrogen (Fig. 2b). Chromium concentration increases from 70 to 90 at.% at the surface and decreases systematically to about 70 at.% at the depth of about 4 m, whereas the nitrogen concentration reaches the level of about 30 at.%. Concentration of elements constituting the CrN coating attests to its non-stoichiometric composition and inhomogeneity. The character of concentration changes of the elements constituting the multilayer TiN/(Ti, Al)N coating confirms its laminar structure (Fig. 2c). The (Ti, Al)N layer is the external layer of this coating, after which the TiN laser comes, in this case the aluminium concentration decreases to 0%. 3.2. Adhesive and wear resistance of the investigated PVD coatings Critical loads Lc characterising adhesion of the investigated coatings to the substrate from the hot work tool steel, were determined during the scratch test with the increasing load. The values of the critical loads for the particular coatings are presented in Table 3. The CrN coatings demonstrate a very good adhesion to the substrate, next come the TiN ones, and the least advan-
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Fig. 2. GDOES depth profiles: TiN (a), CrN (b), (TiN/(Ti, Al)N)3x (c) coatings. Table 3 Critical loads for investigated coatings Coating type
CrN TiN TiN/(Ti, Al)N
Type of defect/force [N] Lc (AE)
Lc3
Lc4
Lc5
Lc (Ft )
37.0 25.5 10.7
32.0 19.4 12.8
77.0 39.0 32.5
87.0 82.0 69.0
91.0 80.1 68.6
tageous adhesion was observed for the multilayer TiN/(Ti, Al)N coating. The coating failure mechanism in all cases begins from the numerous spallings on both edges of the developed scratch (Fig. 3). The difference is in the location of these spallings. In case of the TiN/(Ti, Al)N coating, spallings start from the load as low as 13 N. Then cracks and stretches develop at the scratch bottom, and the spalling process at the scratch edge develops further, and finally total coating delamination occurs at the scratch bottom.
Fig. 3. Scratches with critical load Lc4 —partial delamination: (a) CrN coating, (b) TiN/(Ti, Al)N coating.
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Fig. 4. Friction coefficient changes vs. wear track length for investigated coating: (a) TiN coating: RT 20 ◦ C, HT 500 ◦ C, (b) CrN coating: RT 20 ◦ C, HT 500 ◦ C.
The analysis of the results justifies a statement that in spite of the big differences in critical loads, depending on the evaluation criterion assumed, the general trend remains the same. Spalling begins with the monolayer TiN coatings at the load value of about 20 N, and with the multilayer CrN ones at about 32 N load value. The multilayer TiN/(Ti, Al)N coatings have the worst adhesion, whereas the best adhesion is characteristic for the monolayer CrN ones. Employing the EDS analyser in the scanning electron microscope makes it possible to find out in the addition that in case of the TiN/(Ti, Al)N coatings delamination of the layers occurs from the titanium sublayer deposited initially. The investigated coatings and the substrate material were examined using the pin-on-disk test to determine their wear resistance; the test was carried out at the room temperature and at the temperature elevated to 500 ◦ C. Changes of the friction coefficient value between the corundum ball and the tested specimen were recorded both for the room temperature and for the temperature of 500 ◦ C (Fig. 4). Analysis of the friction coefficient changes of the examined specimens justifies a statement that in the assumed experiment conditions, the friction coefficient for the specimen made from the heat treated tool steel tested at the room temperature was about 0.4. In case of the specimens coated in the PVD process the friction coefficient changes in the
Fig. 5. Profiles of wear track performed at 500 ◦ C: (a) multilayer TiN/(Ti, Al)N, (b) monolayer CrN.
Fig. 6. Comparison of volume of materials removed during tribological wear, 20 ◦ C.
range from about 0.45 (TiN) to about 0.7 for the TiN/(Ti, Al)N and CrN coatings, in case of the tests carried out at the room temperature. Results of the tests carried out at the temperature of 500 ◦ C revealed that the friction coefficient for the coated specimens was 0.55 for TiN, 0.6 for CrN, and 0.7 for the TiN/(Ti, Al)N coating. Only the TiN coating, as the only one from all tested coatings does not change the friction coefficient value during the entire test. The TiN/(Ti, Al)N and CrN coatings change their friction coefficient values after the initial test stage, which can be connected with their partial or total failure mechanisms, revealing values close to the ones for the uncoated steel in case of the CrN coating, or higher values in case of the partial failure which occurs in case of the TiN/(Ti, Al)N coating. The quantitative evaluation of the surface wear of the examined specimens due to friction was carried out basing on measurements of the wear track profiles for the TiN, CrN, and TiN/(Ti, Al)N coatings deposited on the substrate made
Fig. 7. Comparison of volume of materials removed during tribological wear, 500 ◦ C.
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Fig. 8. SEM micrographs of wear track on the surface of investigated samples (upper row 20 ◦ C, lower row 500 ◦ C): (a) X37CrMoV5-l steel, (b) TiN coating, (c) CrN coating.
from the X37CrMoV5-l hot work tool steel, as well as for the substrate material in four orthogonal directions, every 90◦ . The measured profiles were put together and the average profile was determined for the worn land for each of the examined coatings and for the substrate material. The area of the material removed due to the friction was measured for the average profile determined in this way (area between the profile contour and the zero line-specimen surface). The average volume of the material removed due to the friction of the corundum ball with the specimen surface was calculated using the known wear track length. Fig. 5 presents the exemplary set of profiles measured for wear tracks performed at the temperature of 500 ◦ C for the multilayer TiN/(Ti, Al)N coating (Fig. 5a) and for the monolayer CrN one (Fig. 5b). The average volume of the material removed during the tribological wear was calculated using the formula: V [mm3 ] = P × S
(1)
where V is the average volume of the material worn out due to the friction, P the average area of the removed material [mm2 ], S the track length 2πr [mm]. Measurement and determination of the average profiles and further of the volume of the removed material were carried out both for specimens tested at the room temperature and at the elevated temperature, and the measurement results are presented in Figs. 6 and 7. One can state, basing on the wear measurement results collected for the specimens
examined at the room temperature, that the highest wear resistance has the TiN coating, whereas both the CrN and the TiN/(Ti, A1)N one get worn at an extent comparable to the wear of the X37CrMoV5-l tool steel. The change of the test temperature to 500 ◦ C results, in each case, in the about fivefold increase of the wear intensity. However, the TiN monolayer still demonstrates its highest wear resistance. Wear resistance of the TiN/(Ti, A1)N coating has grown, and the CrN coating was worn in a similar extent like the uncoated material. Measurements of the wear track width were made basing on observations carried out on the scanning electron microscope (Fig. 8); it was found out that the width increases with the test temperature increase and in case of the material with no anti-wear coating, tested at the room temperature, it is about 0.3 mm, and at the temperature of 500 ◦ C it is about 0.60 mm. The deposition of the TiN coating results in decreasing the wear width to about 0.18 mm and about 0.45 mm for the tests carried out at the room temperature and at the elevated one respectively. Results of these measurements correspond with the removed material volume measurements made during the pin-on-disk tests. 4. Conclusion The X-ray quantitative phase analysis makes it possible to reveal the privileged crystallographic orientation (1 1 1) for
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the TiN monolayer and TiN/(Ti, Al)N multilayer coatings. Broadening the diffraction line in case of the CrN coating attests to the occurrence of the strong internal stresses or to small size of its constituent crystallites. Changes of the concentration of elements present in the investigated coatings, observed during the measurements on the GDOES depth profiling justify the statement that fluctuations of the chemical composition occur in the investigated coatings indicating to origination of the secondary solid solutions. Basing on the out carried investigations one can state that adhesion of the PVD coatings: monolayer TiN, multilayer TiN/(Ti, A1)N, and monolayer CrN to the substrate from the X37CrMoV5-l hot work tool steel changes significantly depending from the assumed criterion; however the general trend remains. The CrN coating has the best adhesion, and the TiN/(Ti, A1)N coating demonstrates the worst adhesion. Measurement of the friction coefficient during the pin-on-disk test makes it possible to note that the TiN coating has the lowest friction coefficient both at the room temperature and at the elevated one; other coatings have their friction coefficients higher than the substrate material. Moreover, one can state that the friction coefficient changes with the development of the coating failure process and after its total removal the values obtained were typical for the substrate material. The TiN coating has the highest wear resistance at the room temperature, whereas both the CrN and the TiN/(Ti, Al)N ones get worn comparably to the X37CrMoV5-l tool steel. The change of the test temperature to 500 ◦ C causes the ca fivefold increase of the wear intensity in each case.
2001
The wear track width grows with the test temperature increase and in case of the uncoated materials, examined at the room temperature it is about 0.3 mm, and at the temperature of 500 ◦ C it is about 0.60 mm. The deposition of the TiN coating results in decreasing the wear track width to about 0.18 mm and to about 0.45 mm for tests made at the room temperature and at the elevated one, respectively.
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