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Advantages of depositing multilayer coatings for cutting wood-based products
Surface & Coatings Technology 203 (2009) 3197–3205
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Advantages of depositing multilayer coatings for cutting wood-based products D. Pinheiro a,⁎, M.T. Vieira a, M.-A. Djouadi b a b
ICEMS-Grupo de Materiais, Departamento de Engenharia Mecânica, Faculdade de Ciências e Tecnologia da Universidade de Coimbra, 3030-201 Coimbra, Portugal Institut des Matériaux Jean Rouxel IMN, UMR 6502, Université de Nantes, 2 rue de La Houssinière, BP 32229, 44322 Nantes Cedex, France
a r t i c l e
i n f o
Article history: Received 9 May 2008 Accepted in revised form 27 March 2009 Available online 5 April 2009 Keywords: OSB Particleboard Long period-multilayer Cutting tools W–(Ti–Cr)–N
a b s t r a c t An effective coating for cutting wood-based products, especially during interrupted cutting operations, must have excellent impact resistance, associated with excellent adhesion to the substrate, high hardness and corrosion resistance. At the beginning of the cutting operations, the impact has significant effects on the substrate, leading to variable material loss at the cutting edge. In order to improve the impact resistance of coated tools an optimized long period-multilayer coating was deposited on a cemented carbide substrate. These coatings are based on an alternate sequence of hard layers with softer monolayers, which work like a fuse in an electric circuit; when subjected to high impact stress crack propagation induces the delamination of hard layers, thereby avoiding the collapse of the entire coating, as in bulk laminated macrocomposites. From cutting tests of wood-based products, which used cutting tools coated with multilayer coatings based on Ti–W–N/Ti–W or Cr–W–N/Cr–W with different periods, the best result was achieved by the deposition of the multilayer coatings Cr–W–N/Cr–W (3 layers). This solution increases by 500% the ability to cut through wood-based products. In spite of this, the Ti–W–N (HV = 51 GPa; Lc2 N 80 N) has a slightly superior cutting performance to the Cr–W–N (HV = 43 GPa; Lc2 N 80 N). This study based on real cutting tests and applied using identical cutting parameters to those used in the industry, shows new solutions that can be implemented with success in the industries involved in cutting wood-based products. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The wide-range applicability of thin films, and their patent progress throughout the world, in different scientific fields, have been the reason why this technology has also been investigated for use in components under severe mechanical stress, as occurs in cutting tools. The micro or nanometer scale of the materials composed of stable or metastable phases, in a mono- or multilayer mode, can help to reduce the dimensions of many types of equipment and to amplify the functionality and quality of materials. This notably improves the behaviour of diverse mechanical components, when submitted to different mechanical or chemical applications. The commercialization of coatings for metal cutting tools is already well established. However a lot of basic work still needs to be done in order to bring the same success to the area of wood and wood-based product cutting tools. While there is a general consensus as to the bulk material of cutting tools, with regard to their surface, thin coatings may provide improvements. Unlike the coatings that are very much applied in the cutting of metals and metal alloys since 1980, specific
⁎ Corresponding author. E-mail addresses: [email protected] (D. Pinheiro), [email protected] (M.T. Vieira), [email protected] (M.-A. Djouadi). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.03.052
research still needs to be done into the efficiency of such coatings in the cutting of wood-based products. But, to what extent can the development of coatings for metal cutting tools be used in the cutting of wood-based products? In fact, they are hardly applicable. The differences in the type of tool and wear are obvious factors that make them universally impracticable. However, it is known that there are many companies, which produce cutting tools for wood-based products, that are willing to try new solutions, but only if they are given some performance guarantees. There are a number of monolithic coating solutions that can be pointed out to have some success in the cutting of wood-based products, however, we have chosen the coatings based on Ti–W–N due to their good corrosion resistance [1] and wear performance [2–5], and the coating of Cr–W–N, because the literature points diverse advantages of CrN coatings in relation to the properties offered by the TiN coatings. The resistance to oxidation and corrosion [6–11], and the wear resistance joined with the low friction coefficient and high toughness [12–14] are some of the appealing factors of these thin films in detriment to the TiN coatings. The mixing of the chromium nitride with tungsten is a solution of great interest; therefore, an increase of the hardness motivated by the presence of tungsten, which approaches these coatings to the films based on TiN, linked to a chemical stability that turns them very competitive for applications where such is necessary, as in the case of the cutting of wood-based products, can be presented as an advantage.
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2. Experimental details
Table 2 Chemical composition of the CrW and the ternary monolithic coatings of the Cr–W–N (R = N2/Ar).
2.1. Coating deposition technique In this century long period-multilayer coatings oppose monolithic coatings, such reality have led to the study of both types of coatings when deposited over lapping tungsten carbide-cobalt blades, by a d.c. magnetron industrial sputtering prototype. The deposition pressure, substrate bias and substrate-to-target distance were kept constant at 0.3 Pa, − 70 V and 65 mm, respectively. Prior to deposition, all the substrates were heated and ion etched with an ion gun in an argon atmosphere. In order to create a nitrogen gradient in the first layer from the substrate, Ti–W or Cr–W similar to the target's chemical composition was deposited and then the flow of nitrogen was increased up to the ultimate partial pressure, corresponding to the stoechiometry of the selected nitride. For the deposition of the hard coatings: Ti–W–N or Cr–W–N was used with titanium–tungsten or chromium–tungsten targets and a reactive gas of N2 with a partial pressure of N2/Ar ratio of 0.33 [2]. The multilayers based on Ti–W–N/ Ti–W were deposited using two identical targets of 20 wt.% Ti–80 wt.% W (Ti0.5W0.5), already optimized. For the multilayer coatings of Cr– W–N/Cr–W one target of 10 wt.% Cr–90 wt.% W (Cr0.3W0.7) was used because of their atomic mass. In both cases, the total thickness of the coatings was 3 μm, and the metallic layers were kept constant (≈80 nm) [15]. 2.2. Thin film characterization The chemical composition of the coatings was evaluated by electron probe microanalysis (EPMA) (Cameca SX 50). The morphology of the coatings was observed using Scanning Electron Microscopes (SEM). The hardness and Young's modulus of the coatings were evaluated using depth-sensing-indentation hardness equipment (Fischerscope H100) with a Vickers tip (load varying from 50 to 1000 mN; time of each step = 0.5 s; creep time at maximum and minimum loads of 30 s). The results of hardness tests are presented through the application of the methodology developed by Fernandes et al. [16] and the values of the Young's modulus are a function of the best correlation from the linear or Gao models [17,18] (all methods intend to remove the influence of the substrate). The testing procedure, including the correction of the experimental results for geometric defects in the indentor and thermal drift of the equipment, has been described elsewhere [19]. The adhesion (Lc2 = load corresponding to adhesion failure) of the coatings was evaluated by a commercial CSEM Revetest scratch-tester in the range of 5 to 80 N, scanning speed 10 mm min− 1, load speed = 100 N min− 1.
CrW (R = 0) CrWN (R = 1/3)
Cr(1 − x)WxNy
Cr (at.%)
W (at.%)
N (at.%)
O (at.%)
Cr0.1W0.9 Cr0.2W0.8N0.8
10 14
88 73
0 13
2 0
OSB (specific weight of 700 kg/m3, tensile strength of 0.58 MPa and bending strength of 36 MPa (//) and 16 MPa (⊥)); wood particleboards (specific weight of 686 kg/m3, tensile strength of 0.38 MPa and bending strength of 12.8 MPa). 2.4. Cutting tools The tool used in the cutting experiments (enterprise: Frezite, S.A. — http://www.frezite.pt; cutting tool blade support reference: HMW A257.018.150.20; cutting blade reference: HM 750.106) is a very common tool to cut wood-based products. The sintered tungsten carbide cutting blades, with 4% by weight of cobalt as binder, with a hardness value of 17.6 GPa, rake angle of 0° and clearance angle of 35° (with point angle of 55°), were coated with thin films deposited by reactive magnetron sputtering deposition. The coatings were deposited directly on lapping cutting blades, which exhibited lapping roughness (Ra = 0.06 μm). This procedure was made to avoid an increment in the coating's stress and a preliminary delamination of the coatings due to the roughness of the substrate's surface. 2.5. Cutting conditions Cutting tests were carried out in an industrial wood cutting machine using up milling. The cutting conditions were as follows: n = 18 000 rpm; feed rate = 15 m/min; diameter of the cutting tool = 18 mm; cutting depth (z' axis) = 10 mm; cutting depth (x'y' axis) = 2 mm; Z (number of teeth) = 1; linear cutting speed = 17.0 m/s; Φz (feed/rotation and teeth) = 0.8 mm/rot. 2.6. Wear measuring method The qualitative and quantitative analyses of the cutting tool wear were performed using an optical and electronic scanning microscope. The real cutting wear that occurs at the cutting edge was measured; attention was paid not only to the rake face but also to the clearance face. The results of the wear measurement indicate medium wear values along 10 mm of depth of cut (z' axis); the methodology used is described elsewhere [5,20].
2.3. Wood-based products
3. Results and discussion
The materials used in the cutting tests were Oriented Strand Board (OSB) and wood particleboards. The first material is commercially available and is comparable in strength to solid wood products and the second one is a cheaper material with similar properties to OSB. The strength properties of OSB are higher than wood particleboards or MDF. Compared to solid wood or commodity plywood, OSB is more uniform in its structure and thus interesting for applications in engineered constructions. The main physical and mechanical properties of the materials used in the cutting tests are the following ones:
3.1. Chemical composition
Table 1 Chemical composition of the TiW and ternary monolithic coatings of the Ti–W–N (R = N2/Ar).
TiW (R = 0) TiWN (R = 1/3)
Ti(1 − x)WxNy
Ti (at.%)
W (at.%)
N (at.%)
O (at.%)
Ti0.3W0.7 Ti0.4W0.6N0.9
26 24
69 29
0 46
5 1
The chemical compositions of each type of layer used in the coatings were determined by electron probe microanalysis and revealed different thin film compositions, in accordance with the nitrogen ratio (R) introduced in the deposition chamber (Tables 1 and 2). 3.2. Morphology The thin films based on Ti–W–N or Cr–W–N, such as in monolithic or multilayer mode (Ti–W–N/Ti–W or Cr–W–N/Cr–W) have a very dense morphology designated as featureless. It is possible to verify that the multilayers exhibit a compact aspect that does not show any morphological disruption due to the presence of metallic layers and allows fracturing to progress in steps, with a discontinuous transfer of mechanical forces running along the softer
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layers and across the harder layers. This effect is advantageous, because it increases the capability of the film to withstand fracturing of the cutting edge, provoked by the impact and aggressiveness of the cutting of wood-based products. The manner in which the coating is transversally fractured generates a difference of planes/layers that facilitates the identification of the different layers through the deposited multilayer (Fig. 1). 3.3. Mechanical properties 3.3.1. Hardness In the majority of cases, the introduction of layers of Ti0.3W0.7 (HVmod = 16 GPa) or Cr0.1W0.9 (HVmod = 28 GPa), alternated with
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harder layers of Ti0.4W0.6N0.9 (HVmod = 47 GPa) or Cr0.2W0.8N0.2 (HVmod = 42 GPa), exhibits a reduced total hardness relative to the monolithic bi-metal nitride thin film. The hardness of the multilayers reduces with the indentation depth, which is due to the incorporation of a larger percentage of the bi-metal coating. However, in the first type of multilayers, based on Ti0.4W0.6N0.9 with interlayers of Ti0.3W0.7, it is important to highlight the properties of the thin films with three and nine layers because they maintain hardness values closer to the hardness of the thin monolithic deposited films. Through the application of the model developed by Fernandes et al. [16] it was possible to define each coating's hardness values taking out the influence of the properties of the substrate (Table 3). For the coatings with similar behaviour to the
Fig. 1. Morphology of the multilayer coatings of Ti0.4W0.6N0.9/Ti0.3W0.7 and Cr0.2W0.8N0.2/Cr0.1W0.9, function of the total number of layers (SEM).
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Table 3 Hardness of the monolithic and multilayer coatings (comparison between the values measured through the application of the smallest indentation loads and the calculation based on a model that eliminates the influence of the substrate [16]). HV (GPa)
Ti0.4W0.6N0.9 (monolithic) Ti0.4W0.6N0.9/Ti0.3W0.7 (3 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (5 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (9 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (11 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (21 layers) Cr0.2W0.8N0.2 (monolithic) Cr0.2W0.8N0.2/Cr0.1W0.9 (3 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (5 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (9 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (11 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (21 layers)
50 mN
70 mN
100 mN
HV (GPa) model [16]
Val.
Dev.
Val.
Dev.
Val.
Dev.
Val.
Dev.
45 33 33 40 33 31 44 32 39 35 36 26
6.8 4.3 1.5 2.6 1.9 1.4 2.8 2.5 2.0 1.4 0.3 1.3
43 40 34 39 32 32 42 34 34 30 31 32
3.1 4.1 0.2 2.5 0.7 1.5 2.4 2.4 1.2 1.3 0.8 0.6
38 39 32 36 32 31 38 37 33 30 33 27
1.7 2.3 1.0 1.1 1.7 0.8 0.9 4.3 0.8 0.8 0.9 0.8
47 39 33 38 33 32 42 39 37 36 37 27
0.3 5.1 0.9 2.0 1.1 0.5 0.8 5.2 1.9 0.6 0.2 1.2 Fig. 3. Measured and estimated hardness of the thin multilayer films based on Cr0.2W0.8N0.2 with interlayers of Cr0.1W0.9, function of the number of layers.
laminar composites, the hardness values using the law of mixtures [21] were estimated as a function of the type and thickness of the layers. The experimental values are in general inferior to the estimated ones for the first pair, and are similar for the highest number of layers. In the first pair the drift is attenuated with the increase in the number of layers. On the other hand for the second pair the divergence of values is aggravated (Fig. 2). For multilayer coatings based on Ti0.4W0.6N0.9 with layers of Ti0.3W0.7 it can be emphasized that from nine layers the experimental and estimated values are similar. For the thin films based on Cr0.2W0.8N0.2 with interlayers of Cr0.1W0.9 the drift of the experimental values towards the estimated ones increases with the number of layers (Fig. 3). The experimental values are inferior and exhibit a tendency to approach the hardness value of the monolithic coating of Cr0.1W0.9. 3.3.2. Young's modulus (E) The Young's modulus values are presented in Table 4. The Young's modulus of the multilayers: Ti0.4W0.6N0.9/Ti0.3W0.7 or Cr0.2W0.8N0.2/ Cr0.1W0.9 shows the influence of the number of the lower hardness layers that are integrated in the measurement on the evaluation of this property. If the coatings have a behaviour similar to the laminar composites, and if the effect of the substrate is eliminated, it is possible to use the law of the mixtures to estimate the Young's modulus of the studied thin film. It should be emphasized that the experimental values are
also inferior to the ones from the application of the law of mixtures and they diverge with the increase in the number of layers (Figs. 4 and 5). In general, the studied coatings have good mechanical properties. However, they denote a decrease in the hardness value with the increase in the number of deposited layers. For the multilayer coatings based on Ti0.4W0.6N0.9 with interlayers of Ti0.3W0.7, the experimental values of hardness and Young's modulus are in general lower than the ones resulting from the application of the respective law of mixtures. This means that there was no contamination of the interlayers of Ti0.3W0.7, by the residual nitrogen present in the deposition chamber during the deposition of the multilayers. This behaviour results from the low affinity of the tungsten, the major element, to nitrogen (ΔHf0298 (W2N) = −71.2 kJ/mol). However, with the increase in the number of deposited layers it is possible to observe two different behaviours: - an increase in the divergence of the experimental values from those resulting from the application of the law of mixtures up to a corresponding maximum value of five layers, which is related to the methodology used to evaluate the hardness and the Young's modulus, and indicates heterogeneity in the integrated areas. The introduction of more layers interferes with the experimental values of hardness and Young's modulus, conferring to the thin
Table 4 Young's modulus of the monolithic and multilayer coatings (measured with the lower applied indentation loads and calculated by the extrapolation model — linear or Gao [17,18]). Ec (GPa)
Fig. 2. Measured and estimated hardness of the thin multilayer films based on Ti0.4W0.6N0.9 with layers of Ti0.3W0.7, function of the number of layers.
Ti0.4W0.6N0.9 (monolithic) Ti0.4W0.6N0.9/Ti0.3W0.7 (3 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (5 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (9 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (11 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (21 layers) Cr0.2W0.8N0.2 (monolithic) Cr0.2W0.8N0.2/Cr0.1W0.9 (3 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (5 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (9 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (11 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (21 layers)
50 mN
70 mN
100 mN
E (GPa) model [17,18]
Val.
Dev.
Val.
Dev.
Val.
Dev.
Val.
625 521 536 589 505 496 606 511 570 531 556 497
55.5 29.3 18.1 24.4 24.7 18.7 43.9 34.5 42.4 22.2 11.0 22.7
610 604 544 553 499 536 618 528 526 516 542 541
36.0 48.9 22.8 16.3 12.9 24.4 30.4 29.3 5.4 9.5 13.6 28.7
594 605 551 537 508 516 610 566 540 542 575 508
41.2 34.0 17.7 18.8 21.9 12.3 13.3 65.3 3.2 14.4 4.2 18.8
628 614 547 594 474 470 595 547 529 506 573 508
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deposited using one target (Cr–W), where some nitrogen contamination is expected.
Fig. 4. Measured and estimated Young's modulus of the thin multilayer films based on Ti0.4W0.6N0.9 with interlayers of Ti0.3W0.7, function of the number of layers.
multilayer films inferior values to those that were expected, because of the inferior properties of the deposited low hardness layers. - a decrease in the divergence for numbers above five layers, which have values similar to nine layers, which indicates that the response of the thin film to the indentation is from a homogeneous coating, attenuating the measurement error of the values of hardness and Young's modulus. This behaviour also occurs in the evaluation of the more ductile or softer layers, but is completely hidden by the effect of nitrogen. For the thin films based on Cr0.2W0.8N0.2 and Cr0.1W0.9 it is possible to conclude that the increase in the divergence from the experimental values relative to the law of mixtures, a function of the number of deposited layers, may be due to the increased difficulty in measuring the intended partial nitrogen ratio in the hard layers. The increase in the number of layers implies a reduction in the thickness of the hard layers, so that the global thickness of the multilayer coating can be maintained. This factor also justifies the tendency of the value of the coating with twenty one layers to approach the hardness value of the monolithic coating (Cr0.1W0.9). However, it is important to highlight that this is only valid for Cr0.2W0.8N0.2/Cr0.1W0.9 based coatings
3.3.3. Adhesion Most of the deposited monolithic and multilayer coatings (Table 5) begin to lose adhesion at values (Lc2) greater than the maximum load applied during the test (80 N). However, the Ti0.4W0.6N0.9/Ti0.3W0.7 coatings with eleven and twenty one layers reveal their first adhesive failure at 80 N. It can be pointed out that in both multilayer compositions studied, the highest value of Lc1, related to cohesive failure, occurs in the nine layer coating. Cohesive failure in all the coatings is tensile and when adhesive failure occurs, it is by lateral peeling. The multilayers deposited over lapping tools with softer layers based on Ti–W or Cr–W, exhibit a behaviour favourable to the reduction in wear of cutting tools; there is an increased adhesion between the thin film and the substrate, similar to that evaluated for the hard monolithic coatings. The loads for the first adhesive failures occur, for any investigated situations, always equal to or higher than the applied maximum load (Lc2 = 80 N). Coatings deposited by sputtering have appropriate values of residual stress state (compression), homogeneously distributed over a substrate with a flat and lapping surface [2,22]. However, when it is necessary to deposit a thin film over a true cutting tool, the situation is very different. Coatings are deposited over rough surfaces (with poor superficial finishing, normally lapping), imposed by the manufacturer due to reasons of economy. Moreover, modified geometries, associated with the geometry of the respective cutting tool, contribute to an undesired concentration of stresses, which leads to decohesion or delamination of the coatings. Wiklund et al. [22], studied a representative number of combinations of coating/substrate used in mechanical applications. Using finite elements they highlighted the effect in the residual stresses of different materials and thicknesses deposited on flat surfaces, some “perfectly” polished and others with high roughness. Moreover the study is extended to the effect of geometrical alterations, edges or pores on the stress state. In this investigation it was clearly proved that thinner coatings are less sensitive to failures induced by the residual stresses due to the adjustment of the geometrical conditions in the interface. These authors have also shown that there is a critical film thickness for which the normal stresses (crossing the interface) are enough to induce the delamination of the coating.
Table 5 Adhesion characteristics of the monolithic and multilayer thin films based on the Ti–W– N system or the Cr–W–N system. Coatings Ti0.4W0.6N0.9 (monolithic)
Cr0.2W0.8N0.2 (monolithic)
Fig. 5. Young's modulus (measured and estimated) of the thin multilayer films based on Cr0.2W0.8N0.2 with interlayers of Cr0.1W0.9, function of the number of layers.
Ti0.4W0.6N0.9/Ti0.3W0.7 (3 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (5 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (9 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (11 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (21 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (3 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (5 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (9 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (11 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (21 layers)
Thickness
Lc1 [N]
[μm]
Average
Deviation
Average
2 1 0.5 0.3 2 1 0.5 0.3 3 3 3 3 3 3 3 3 3 3
61.2 67.8 78.7 76.3 50.6 51.6 57.2 74.6 38.8 38.4 41.5 36.8 40.2 43.2 47.3 50.6 27.7 31.4
2.6 1.6 1.1 1.9 4.1 0.7 2.6 0.3 1.6 1.2 1.6 2.5 1.6 0.9 0.5 0.8 0.8 0.5
N80 N80 N80 N80 N80 N80 N80 N80 N80 N80 N80 80 80 N80 N80 N80 N80 N80
Lc2 [N]
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Fig. 6. Average wear area of the cutting tools coated by thin Ti0.4W0.6N0.9 monolithic films (different coating thicknesses) and by Ti0.4W0.6N0.9/Ti0.3W0.7 multilayer coatings, during the cutting of OSB.
With regard to the grade of superficial finishing to be used in the cutting tools for wood-based products, taking into account the thickness of each deposited layer of the multilayer coatings and the roughness degree of the unpolished surfaces, it is essential that the deposition should be carried out on lapped surfaces. 3.4. Cutting efficiency The program of the cutting tests used to compare the monolithic coatings with the new type of multilayer ones was composed of two
sequences of tests; first both types of coatings were compared during the cutting of OSB and secondly during the cutting of wood particleboards (high wear aggressive material). Such comparison confirms that it is possible to extrapolate the results of OSB to wood particleboards or other wood-based products. For the first sequences of tests (during the cutting of OSB), both types of monolithic thin films were tested: Ti–W–N or Cr–W–N, for the thicknesses of 0.3 μm and 1 μm. Concerning the multilayer coatings, only the multilayers with the smallest and the greatest number of layers (three and twenty one layers) were tested; for the
Fig. 7. Average wear area of the cutting tools coated by thin Cr0.2W0.8N0.2 monolithic films (different coating thicknesses) and by Cr0.2W0.8N0.2/Cr0.1W0.9 multilayer coatings, during the cutting of OSB.
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Fig. 8. Average wear area of the cutting tools coated by thin Ti0.4W0.6N0.9 monolithic films (different coating thicknesses) and by Ti0.4W0.6N0.9/Ti0.3W0.7 multilayer coatings, during the cutting of wood particleboards (wear of the zone where the tool cuts the internal layer of the particleboard).
Fig. 9. Average wear area of the cutting tools coated by thin Cr0.2W0.8N0.2 monolithic films (different coating thicknesses) and by Cr0.2W0.8N0.2/Cr0.1W0.9 multilayer coatings, during the cutting of wood particleboards (wear of the zone where the tool cuts the internal layer of the particleboard).
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other multilayers, the coatings, which possess the higher hardness values: Ti0.4W0.6N0.9/Ti0.3W0.7 with nine layers and Cr0.2W0.8N0.2/ Cr0.1W0.9 with five layers, were selected. For the second sequence of tests (during the cutting of wood particleboards) all the monolithic and multilayer coatings were tested. The study of the efficiency of the coatings during the cutting of wood particleboards was achieved by a bipartition of wear evaluation at the cutting edge; in this analysis the wear due to the cutting of the internal and external layer of the wood particleboards was differentiated quantitatively, with very different aggressiveness. Figs. 6–9 show the results of the total cutting tests. Using the methodology developed for the evaluation of the wear of the cutting tools it became possible to evaluate that the wear from the cutting of the internal layer of the wood particleboards is much higher than cutting OSB and the cutting tool wear is much lower for coated than for uncoated tools. Regarding the efficiency of the monolithic films as a function of coating thickness it was possible to observe dissimilar behaviours. It can be inferred that beyond the composition type of the deposited coating, it is also necessary to optimize the coating thickness for cutting tools used in wood-based products. This procedure can improve the wear resistance of the cutting tool. The results demonstrate that, for both monolithic compositions tested during the cutting of OSB, a 1 μm thick coating can increase wear resistance by about 2.5 times relative to uncoated cutting tools. From the results of the average wear area during the cutting of wood particleboards, according to the zones of the blade that only cut the internal layer of the wood particleboards, it is clear that the 0.5 μm thick coating of Ti0.4W0.6N0.9, results in the greatest reduction in the cutting tool's wear, 72% less than that obtained with an uncoated tool (after cutting 426 m). Using a binocular magnifying glass (Fig. 10) it was possible to evaluate the advantages brought by the deposition of the monolithic hard films (1 μm) of Ti0.4W0.6N0.9. However, wear is almost identical to that seen in uncoated tools, when the blade cuts the external layer of the particleboards. With the best of the monolithic thin films, the application of the coating appeared to reduce wear in the initial phases of cutting, but after a hundred meters of cutting, the wear patterns are similar to those of uncoated tools. The most promising results were achieved by the application of the multilayer coatings with the highest values of hardness and adhesion to substrate. From the behaviour of the coatings it can be inferred that the thin multilayer films with fewer layers exhibit better cutting wear resistance. When cutting OSB, the multilayer coatings of Cr0.2W0.8N0.2/Cr0.1W0.9 contribute to a fivefold increase in the resistance of the tool, relative to an uncoated tool. With regard to the cutting of particleboard for the coatings of Ti0.4W0.6N0.9/Ti0.3W0.7, the most efficient multilayers are those with three and nine layers, which allow a reduction of the average wear area by about 70%, after cutting 426 m of wood particleboards. However, for the three layer thin films based on Cr0.2W0.8N0.2/ Cr0.1W0.9 there is still less wear on the cutting tool, increasing its life time. The coated tool presents, after cutting 1065 m of wood particleboard, a cutting increase of about 5 times possible with an uncoated tool. Nevertheless, for the cutting of the external layer of the wood particleboards, the analysis of wear shows that in most cases wear resistance of the coated tools is somewhat similar to that for an uncoated tool. Only three layers of Cr0.2W0.8N0.2/Cr0.1W0.9 contributes, in this zone, to a slight increase in the wear resistance of the cutting tool; but this only confers positive results for a limited number of cutting meters (some hundreds) of wood particleboard. The production of new multilayer coatings based on Ti–W–N or Cr–W–N, alternated respectively with layers of Ti–W or Cr–W, brought benefits to the wood cutting tools based on sintered tungsten carbide. The best deposited coating was: Cr0.2W0.8N0.2/Cr0.1W0.9/Cr0.2W0.8N0.2 (3 layers). The increase in the number of deposited layers led to a decrease in tool wear resistance, due to the decrease in hardness.
Fig. 10. Wear of the cutting edge (sight of the plan parallel to the rake face) after cutting 213 m of wood particleboard, with a depth of cut (zz′) of 10 mm: (a) cutting tool coated by a thin film of Ti0.4W0.6N0.9 with a thickness of 1 μm; (b) uncoated cutting tool.
Fig. 11 shows the features of tool wear in the thin films of Ti0.4W0.6N0.9/Ti0.3W0.7 (21 layers), after cutting 383 m of OSB. In this figure it is possible to see the steps observed in all of the studied multilayer coatings after starting the cutting. The type and extension of the wear is a function of the number of deposited layers. The substrate is only exposed at the cutting edge and there is never a delamination of the first hard layer deposited as in the plane of the rake face and in the plane of the clearance face. This is corroborated by the chemical analysis (Fig. 11), where it is possible to observe titanium metal of the coating in test up to the cutting edge. Different wear mechanisms can be seen in the worn surfaces of the cutting tools coated by multilayers based on Cr0.2W0.8N0.2/Cr0.1W0.9 (11 layers). The fractures result from the contact between the tool and
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Fig. 11. Wear of the multilayers based on Ti0.4W0.6N0.9/Ti0.3W0.7 (21 layers) of the rake face (SEM), after cutting 383 m of OSB; in line chemical pondered analysis (a) Ti; (b) W.
the wood-based material, where the gradual and controlled stepped progression of the wear into the thin films is visible, mainly close to the zones that are fractured, but maintaining the protection of the substrate. Some surfaces also stand out as being aggressively affected and that, apparently, is due to the remotion of the WC particles from the substrate. The fissures in the hard coatings are due to the impact of the interrupted cutting and/or by the high cutting speeds applied.
Acknowledgements The authors would like to acknowledge “Fundação para a Ciência e a Tecnologia (FCT) — Portugal” and “Fundo Social Europeu (FSE)” for the financial support (SFRH/BD/1461/2000) and “FREZITE, S.A. — Portugal” for providing the cutting tool support. References
4. Conclusions The investigation made use of experimentally sustainable real cutting tests to reach the following conclusions. Monolithic coatings can significantly increase wear resistance in cutting tools resulting from the cutting of materials such as wood-based products. Thus, it was possible to verify that the monolithic coatings with the best properties (Ti0.5W0.5N0.9 with 0.5 μm or 2 μm of thickness and Cr0.2W0.8N0.2 with 0.5 μm of thickness) are also the ones that contribute to a significant increase in wear resistance of the cutting tool for wood-based materials. However, the deposition of multilayer coatings, creating a balance between high hardness layers with a softer nanolayer, contributes even more to increasing the number of meters cut (about five times with a multilayer coating of Cr0.2W0.8N0.2/Cr0.1W0.9/Cr0.2W0.8N0.2, with a total number of three layers) when compared to that obtained with an uncoated tool of sintered tungsten carbide (with 4% of cobalt in weight), during the cutting of wood-based products. The production of multilayer coatings with architecture similar to sea urchin teeth improves the performance of the coated tools, without modification of the conventional deposition equipment, and using a set-up equal to that used for monolayer deposition.
[1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
J.C. Oliveira, A. Cavaleiro, C.M.A. Brett, Corros. Sci. 42 (11) (2000) 1881. M.T. Vieira, C.M. Pereira, J.M. Castanho, Surf. Coat. Technol. 131 (1–3) (2000) 417. A. Cavaleiro, B. Trindade, M.T. Vieira, Surf. Coat. Technol. 174–175 (2003) 68. P.N. Silva, J.P. Dias, A. Cavaleiro, Surf. Coat. Technol. 200 (1–3) (2005) 186. D. Pinheiro, “Optimização da Superfície de Ferramentas para o Corte de Aglomerados de Partículas de Madeiraq, PhD Thesis, University of Coimbra, (2007), ISBN: 972-99204-9-4. H. Schulz, E. Bergmann, Surf. Coat. Technol. 50 (1) (1991) 53. A. Leyland, M. Bin-Sudin, A.S. James, M.R. Kalantary, P.B. Wells, et al., Surf. Coat. Technol. 60 (1–3) (1993) 474. P. BaIIhause, B. Hensel, A. Rost, H. Schussler, Mater. Sci. Eng., A 163 (2) (1993) 193. S.B. Sant, K.S. Gill, Surf. Coat. Technol. 68–69 (1994) 152. K. Baba, R. Hatada, Surf. Coat. Technol. 66 (1–3) (1994) 368. T. Kacsich, K.-P. Lieb, Thin Solid Films 245 (1–2) (1994) 4. C. Yuji, O. Toshio, I. Hiroshi, J. Mater. Res. 8 (5) (1993) 1109. T. Sato, Y. Tada, M. Ozaki, K. Hoke, T. Besshi, Wear 178 (1–2) (1994) 95. Z.P. Huang, Y. Sun, T. Bell, Wear 173 (1–2) (1994) 13. J.M. Castanho, M.T. Vieira, Surf. Coat. Technol. 143–144 (2003) 352. J.V. Fernandes, A.C. Trindade, L.F. Menezes, A. Cavaleiro, Surf. Coat. Technol. 131 (2000) 457. J. Menčík, D. Munz, E. Quandt, E.R. Weppelmann, J. Mater. Res. 12 (9) (1997) 2475. H. Gao, C.-H. Chiu, J. Lee, Int. J. Solids Struct. 29 (20) (1992) 2471. J.M. Antunes, A. Cavaleiro, L.F. Menezes, M.I. Simões, J.V. Femandes, Surf. Coat. Technol. 149 (2002) 27. D. Pinheiro, M.T. Vieira, J.P. Dias, M.A. Djouadi, C. Nouveau, 16th IWMS — International Wood Machining Seminar, Aug 24–30, Matsue, Japan, 2003, p. 82. J.B. Wachtman, Mechanical Properties of Ceramics, John Wiley & Sons, inc., 1996. U. Wiklund, J. Gunnars, S. Hogmark, Wear 232 (2) (1999) 262.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Advantages of depositing multilayer coatings for cutting wood-based products D. Pinheiro a,⁎, M.T. Vieira a, M.-A. Djouadi b a b
ICEMS-Grupo de Materiais, Departamento de Engenharia Mecânica, Faculdade de Ciências e Tecnologia da Universidade de Coimbra, 3030-201 Coimbra, Portugal Institut des Matériaux Jean Rouxel IMN, UMR 6502, Université de Nantes, 2 rue de La Houssinière, BP 32229, 44322 Nantes Cedex, France
a r t i c l e
i n f o
Article history: Received 9 May 2008 Accepted in revised form 27 March 2009 Available online 5 April 2009 Keywords: OSB Particleboard Long period-multilayer Cutting tools W–(Ti–Cr)–N
a b s t r a c t An effective coating for cutting wood-based products, especially during interrupted cutting operations, must have excellent impact resistance, associated with excellent adhesion to the substrate, high hardness and corrosion resistance. At the beginning of the cutting operations, the impact has significant effects on the substrate, leading to variable material loss at the cutting edge. In order to improve the impact resistance of coated tools an optimized long period-multilayer coating was deposited on a cemented carbide substrate. These coatings are based on an alternate sequence of hard layers with softer monolayers, which work like a fuse in an electric circuit; when subjected to high impact stress crack propagation induces the delamination of hard layers, thereby avoiding the collapse of the entire coating, as in bulk laminated macrocomposites. From cutting tests of wood-based products, which used cutting tools coated with multilayer coatings based on Ti–W–N/Ti–W or Cr–W–N/Cr–W with different periods, the best result was achieved by the deposition of the multilayer coatings Cr–W–N/Cr–W (3 layers). This solution increases by 500% the ability to cut through wood-based products. In spite of this, the Ti–W–N (HV = 51 GPa; Lc2 N 80 N) has a slightly superior cutting performance to the Cr–W–N (HV = 43 GPa; Lc2 N 80 N). This study based on real cutting tests and applied using identical cutting parameters to those used in the industry, shows new solutions that can be implemented with success in the industries involved in cutting wood-based products. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The wide-range applicability of thin films, and their patent progress throughout the world, in different scientific fields, have been the reason why this technology has also been investigated for use in components under severe mechanical stress, as occurs in cutting tools. The micro or nanometer scale of the materials composed of stable or metastable phases, in a mono- or multilayer mode, can help to reduce the dimensions of many types of equipment and to amplify the functionality and quality of materials. This notably improves the behaviour of diverse mechanical components, when submitted to different mechanical or chemical applications. The commercialization of coatings for metal cutting tools is already well established. However a lot of basic work still needs to be done in order to bring the same success to the area of wood and wood-based product cutting tools. While there is a general consensus as to the bulk material of cutting tools, with regard to their surface, thin coatings may provide improvements. Unlike the coatings that are very much applied in the cutting of metals and metal alloys since 1980, specific
⁎ Corresponding author. E-mail addresses: [email protected] (D. Pinheiro), [email protected] (M.T. Vieira), [email protected] (M.-A. Djouadi). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.03.052
research still needs to be done into the efficiency of such coatings in the cutting of wood-based products. But, to what extent can the development of coatings for metal cutting tools be used in the cutting of wood-based products? In fact, they are hardly applicable. The differences in the type of tool and wear are obvious factors that make them universally impracticable. However, it is known that there are many companies, which produce cutting tools for wood-based products, that are willing to try new solutions, but only if they are given some performance guarantees. There are a number of monolithic coating solutions that can be pointed out to have some success in the cutting of wood-based products, however, we have chosen the coatings based on Ti–W–N due to their good corrosion resistance [1] and wear performance [2–5], and the coating of Cr–W–N, because the literature points diverse advantages of CrN coatings in relation to the properties offered by the TiN coatings. The resistance to oxidation and corrosion [6–11], and the wear resistance joined with the low friction coefficient and high toughness [12–14] are some of the appealing factors of these thin films in detriment to the TiN coatings. The mixing of the chromium nitride with tungsten is a solution of great interest; therefore, an increase of the hardness motivated by the presence of tungsten, which approaches these coatings to the films based on TiN, linked to a chemical stability that turns them very competitive for applications where such is necessary, as in the case of the cutting of wood-based products, can be presented as an advantage.
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2. Experimental details
Table 2 Chemical composition of the CrW and the ternary monolithic coatings of the Cr–W–N (R = N2/Ar).
2.1. Coating deposition technique In this century long period-multilayer coatings oppose monolithic coatings, such reality have led to the study of both types of coatings when deposited over lapping tungsten carbide-cobalt blades, by a d.c. magnetron industrial sputtering prototype. The deposition pressure, substrate bias and substrate-to-target distance were kept constant at 0.3 Pa, − 70 V and 65 mm, respectively. Prior to deposition, all the substrates were heated and ion etched with an ion gun in an argon atmosphere. In order to create a nitrogen gradient in the first layer from the substrate, Ti–W or Cr–W similar to the target's chemical composition was deposited and then the flow of nitrogen was increased up to the ultimate partial pressure, corresponding to the stoechiometry of the selected nitride. For the deposition of the hard coatings: Ti–W–N or Cr–W–N was used with titanium–tungsten or chromium–tungsten targets and a reactive gas of N2 with a partial pressure of N2/Ar ratio of 0.33 [2]. The multilayers based on Ti–W–N/ Ti–W were deposited using two identical targets of 20 wt.% Ti–80 wt.% W (Ti0.5W0.5), already optimized. For the multilayer coatings of Cr– W–N/Cr–W one target of 10 wt.% Cr–90 wt.% W (Cr0.3W0.7) was used because of their atomic mass. In both cases, the total thickness of the coatings was 3 μm, and the metallic layers were kept constant (≈80 nm) [15]. 2.2. Thin film characterization The chemical composition of the coatings was evaluated by electron probe microanalysis (EPMA) (Cameca SX 50). The morphology of the coatings was observed using Scanning Electron Microscopes (SEM). The hardness and Young's modulus of the coatings were evaluated using depth-sensing-indentation hardness equipment (Fischerscope H100) with a Vickers tip (load varying from 50 to 1000 mN; time of each step = 0.5 s; creep time at maximum and minimum loads of 30 s). The results of hardness tests are presented through the application of the methodology developed by Fernandes et al. [16] and the values of the Young's modulus are a function of the best correlation from the linear or Gao models [17,18] (all methods intend to remove the influence of the substrate). The testing procedure, including the correction of the experimental results for geometric defects in the indentor and thermal drift of the equipment, has been described elsewhere [19]. The adhesion (Lc2 = load corresponding to adhesion failure) of the coatings was evaluated by a commercial CSEM Revetest scratch-tester in the range of 5 to 80 N, scanning speed 10 mm min− 1, load speed = 100 N min− 1.
CrW (R = 0) CrWN (R = 1/3)
Cr(1 − x)WxNy
Cr (at.%)
W (at.%)
N (at.%)
O (at.%)
Cr0.1W0.9 Cr0.2W0.8N0.8
10 14
88 73
0 13
2 0
OSB (specific weight of 700 kg/m3, tensile strength of 0.58 MPa and bending strength of 36 MPa (//) and 16 MPa (⊥)); wood particleboards (specific weight of 686 kg/m3, tensile strength of 0.38 MPa and bending strength of 12.8 MPa). 2.4. Cutting tools The tool used in the cutting experiments (enterprise: Frezite, S.A. — http://www.frezite.pt; cutting tool blade support reference: HMW A257.018.150.20; cutting blade reference: HM 750.106) is a very common tool to cut wood-based products. The sintered tungsten carbide cutting blades, with 4% by weight of cobalt as binder, with a hardness value of 17.6 GPa, rake angle of 0° and clearance angle of 35° (with point angle of 55°), were coated with thin films deposited by reactive magnetron sputtering deposition. The coatings were deposited directly on lapping cutting blades, which exhibited lapping roughness (Ra = 0.06 μm). This procedure was made to avoid an increment in the coating's stress and a preliminary delamination of the coatings due to the roughness of the substrate's surface. 2.5. Cutting conditions Cutting tests were carried out in an industrial wood cutting machine using up milling. The cutting conditions were as follows: n = 18 000 rpm; feed rate = 15 m/min; diameter of the cutting tool = 18 mm; cutting depth (z' axis) = 10 mm; cutting depth (x'y' axis) = 2 mm; Z (number of teeth) = 1; linear cutting speed = 17.0 m/s; Φz (feed/rotation and teeth) = 0.8 mm/rot. 2.6. Wear measuring method The qualitative and quantitative analyses of the cutting tool wear were performed using an optical and electronic scanning microscope. The real cutting wear that occurs at the cutting edge was measured; attention was paid not only to the rake face but also to the clearance face. The results of the wear measurement indicate medium wear values along 10 mm of depth of cut (z' axis); the methodology used is described elsewhere [5,20].
2.3. Wood-based products
3. Results and discussion
The materials used in the cutting tests were Oriented Strand Board (OSB) and wood particleboards. The first material is commercially available and is comparable in strength to solid wood products and the second one is a cheaper material with similar properties to OSB. The strength properties of OSB are higher than wood particleboards or MDF. Compared to solid wood or commodity plywood, OSB is more uniform in its structure and thus interesting for applications in engineered constructions. The main physical and mechanical properties of the materials used in the cutting tests are the following ones:
3.1. Chemical composition
Table 1 Chemical composition of the TiW and ternary monolithic coatings of the Ti–W–N (R = N2/Ar).
TiW (R = 0) TiWN (R = 1/3)
Ti(1 − x)WxNy
Ti (at.%)
W (at.%)
N (at.%)
O (at.%)
Ti0.3W0.7 Ti0.4W0.6N0.9
26 24
69 29
0 46
5 1
The chemical compositions of each type of layer used in the coatings were determined by electron probe microanalysis and revealed different thin film compositions, in accordance with the nitrogen ratio (R) introduced in the deposition chamber (Tables 1 and 2). 3.2. Morphology The thin films based on Ti–W–N or Cr–W–N, such as in monolithic or multilayer mode (Ti–W–N/Ti–W or Cr–W–N/Cr–W) have a very dense morphology designated as featureless. It is possible to verify that the multilayers exhibit a compact aspect that does not show any morphological disruption due to the presence of metallic layers and allows fracturing to progress in steps, with a discontinuous transfer of mechanical forces running along the softer
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layers and across the harder layers. This effect is advantageous, because it increases the capability of the film to withstand fracturing of the cutting edge, provoked by the impact and aggressiveness of the cutting of wood-based products. The manner in which the coating is transversally fractured generates a difference of planes/layers that facilitates the identification of the different layers through the deposited multilayer (Fig. 1). 3.3. Mechanical properties 3.3.1. Hardness In the majority of cases, the introduction of layers of Ti0.3W0.7 (HVmod = 16 GPa) or Cr0.1W0.9 (HVmod = 28 GPa), alternated with
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harder layers of Ti0.4W0.6N0.9 (HVmod = 47 GPa) or Cr0.2W0.8N0.2 (HVmod = 42 GPa), exhibits a reduced total hardness relative to the monolithic bi-metal nitride thin film. The hardness of the multilayers reduces with the indentation depth, which is due to the incorporation of a larger percentage of the bi-metal coating. However, in the first type of multilayers, based on Ti0.4W0.6N0.9 with interlayers of Ti0.3W0.7, it is important to highlight the properties of the thin films with three and nine layers because they maintain hardness values closer to the hardness of the thin monolithic deposited films. Through the application of the model developed by Fernandes et al. [16] it was possible to define each coating's hardness values taking out the influence of the properties of the substrate (Table 3). For the coatings with similar behaviour to the
Fig. 1. Morphology of the multilayer coatings of Ti0.4W0.6N0.9/Ti0.3W0.7 and Cr0.2W0.8N0.2/Cr0.1W0.9, function of the total number of layers (SEM).
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Table 3 Hardness of the monolithic and multilayer coatings (comparison between the values measured through the application of the smallest indentation loads and the calculation based on a model that eliminates the influence of the substrate [16]). HV (GPa)
Ti0.4W0.6N0.9 (monolithic) Ti0.4W0.6N0.9/Ti0.3W0.7 (3 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (5 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (9 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (11 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (21 layers) Cr0.2W0.8N0.2 (monolithic) Cr0.2W0.8N0.2/Cr0.1W0.9 (3 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (5 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (9 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (11 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (21 layers)
50 mN
70 mN
100 mN
HV (GPa) model [16]
Val.
Dev.
Val.
Dev.
Val.
Dev.
Val.
Dev.
45 33 33 40 33 31 44 32 39 35 36 26
6.8 4.3 1.5 2.6 1.9 1.4 2.8 2.5 2.0 1.4 0.3 1.3
43 40 34 39 32 32 42 34 34 30 31 32
3.1 4.1 0.2 2.5 0.7 1.5 2.4 2.4 1.2 1.3 0.8 0.6
38 39 32 36 32 31 38 37 33 30 33 27
1.7 2.3 1.0 1.1 1.7 0.8 0.9 4.3 0.8 0.8 0.9 0.8
47 39 33 38 33 32 42 39 37 36 37 27
0.3 5.1 0.9 2.0 1.1 0.5 0.8 5.2 1.9 0.6 0.2 1.2 Fig. 3. Measured and estimated hardness of the thin multilayer films based on Cr0.2W0.8N0.2 with interlayers of Cr0.1W0.9, function of the number of layers.
laminar composites, the hardness values using the law of mixtures [21] were estimated as a function of the type and thickness of the layers. The experimental values are in general inferior to the estimated ones for the first pair, and are similar for the highest number of layers. In the first pair the drift is attenuated with the increase in the number of layers. On the other hand for the second pair the divergence of values is aggravated (Fig. 2). For multilayer coatings based on Ti0.4W0.6N0.9 with layers of Ti0.3W0.7 it can be emphasized that from nine layers the experimental and estimated values are similar. For the thin films based on Cr0.2W0.8N0.2 with interlayers of Cr0.1W0.9 the drift of the experimental values towards the estimated ones increases with the number of layers (Fig. 3). The experimental values are inferior and exhibit a tendency to approach the hardness value of the monolithic coating of Cr0.1W0.9. 3.3.2. Young's modulus (E) The Young's modulus values are presented in Table 4. The Young's modulus of the multilayers: Ti0.4W0.6N0.9/Ti0.3W0.7 or Cr0.2W0.8N0.2/ Cr0.1W0.9 shows the influence of the number of the lower hardness layers that are integrated in the measurement on the evaluation of this property. If the coatings have a behaviour similar to the laminar composites, and if the effect of the substrate is eliminated, it is possible to use the law of the mixtures to estimate the Young's modulus of the studied thin film. It should be emphasized that the experimental values are
also inferior to the ones from the application of the law of mixtures and they diverge with the increase in the number of layers (Figs. 4 and 5). In general, the studied coatings have good mechanical properties. However, they denote a decrease in the hardness value with the increase in the number of deposited layers. For the multilayer coatings based on Ti0.4W0.6N0.9 with interlayers of Ti0.3W0.7, the experimental values of hardness and Young's modulus are in general lower than the ones resulting from the application of the respective law of mixtures. This means that there was no contamination of the interlayers of Ti0.3W0.7, by the residual nitrogen present in the deposition chamber during the deposition of the multilayers. This behaviour results from the low affinity of the tungsten, the major element, to nitrogen (ΔHf0298 (W2N) = −71.2 kJ/mol). However, with the increase in the number of deposited layers it is possible to observe two different behaviours: - an increase in the divergence of the experimental values from those resulting from the application of the law of mixtures up to a corresponding maximum value of five layers, which is related to the methodology used to evaluate the hardness and the Young's modulus, and indicates heterogeneity in the integrated areas. The introduction of more layers interferes with the experimental values of hardness and Young's modulus, conferring to the thin
Table 4 Young's modulus of the monolithic and multilayer coatings (measured with the lower applied indentation loads and calculated by the extrapolation model — linear or Gao [17,18]). Ec (GPa)
Fig. 2. Measured and estimated hardness of the thin multilayer films based on Ti0.4W0.6N0.9 with layers of Ti0.3W0.7, function of the number of layers.
Ti0.4W0.6N0.9 (monolithic) Ti0.4W0.6N0.9/Ti0.3W0.7 (3 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (5 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (9 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (11 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (21 layers) Cr0.2W0.8N0.2 (monolithic) Cr0.2W0.8N0.2/Cr0.1W0.9 (3 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (5 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (9 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (11 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (21 layers)
50 mN
70 mN
100 mN
E (GPa) model [17,18]
Val.
Dev.
Val.
Dev.
Val.
Dev.
Val.
625 521 536 589 505 496 606 511 570 531 556 497
55.5 29.3 18.1 24.4 24.7 18.7 43.9 34.5 42.4 22.2 11.0 22.7
610 604 544 553 499 536 618 528 526 516 542 541
36.0 48.9 22.8 16.3 12.9 24.4 30.4 29.3 5.4 9.5 13.6 28.7
594 605 551 537 508 516 610 566 540 542 575 508
41.2 34.0 17.7 18.8 21.9 12.3 13.3 65.3 3.2 14.4 4.2 18.8
628 614 547 594 474 470 595 547 529 506 573 508
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deposited using one target (Cr–W), where some nitrogen contamination is expected.
Fig. 4. Measured and estimated Young's modulus of the thin multilayer films based on Ti0.4W0.6N0.9 with interlayers of Ti0.3W0.7, function of the number of layers.
multilayer films inferior values to those that were expected, because of the inferior properties of the deposited low hardness layers. - a decrease in the divergence for numbers above five layers, which have values similar to nine layers, which indicates that the response of the thin film to the indentation is from a homogeneous coating, attenuating the measurement error of the values of hardness and Young's modulus. This behaviour also occurs in the evaluation of the more ductile or softer layers, but is completely hidden by the effect of nitrogen. For the thin films based on Cr0.2W0.8N0.2 and Cr0.1W0.9 it is possible to conclude that the increase in the divergence from the experimental values relative to the law of mixtures, a function of the number of deposited layers, may be due to the increased difficulty in measuring the intended partial nitrogen ratio in the hard layers. The increase in the number of layers implies a reduction in the thickness of the hard layers, so that the global thickness of the multilayer coating can be maintained. This factor also justifies the tendency of the value of the coating with twenty one layers to approach the hardness value of the monolithic coating (Cr0.1W0.9). However, it is important to highlight that this is only valid for Cr0.2W0.8N0.2/Cr0.1W0.9 based coatings
3.3.3. Adhesion Most of the deposited monolithic and multilayer coatings (Table 5) begin to lose adhesion at values (Lc2) greater than the maximum load applied during the test (80 N). However, the Ti0.4W0.6N0.9/Ti0.3W0.7 coatings with eleven and twenty one layers reveal their first adhesive failure at 80 N. It can be pointed out that in both multilayer compositions studied, the highest value of Lc1, related to cohesive failure, occurs in the nine layer coating. Cohesive failure in all the coatings is tensile and when adhesive failure occurs, it is by lateral peeling. The multilayers deposited over lapping tools with softer layers based on Ti–W or Cr–W, exhibit a behaviour favourable to the reduction in wear of cutting tools; there is an increased adhesion between the thin film and the substrate, similar to that evaluated for the hard monolithic coatings. The loads for the first adhesive failures occur, for any investigated situations, always equal to or higher than the applied maximum load (Lc2 = 80 N). Coatings deposited by sputtering have appropriate values of residual stress state (compression), homogeneously distributed over a substrate with a flat and lapping surface [2,22]. However, when it is necessary to deposit a thin film over a true cutting tool, the situation is very different. Coatings are deposited over rough surfaces (with poor superficial finishing, normally lapping), imposed by the manufacturer due to reasons of economy. Moreover, modified geometries, associated with the geometry of the respective cutting tool, contribute to an undesired concentration of stresses, which leads to decohesion or delamination of the coatings. Wiklund et al. [22], studied a representative number of combinations of coating/substrate used in mechanical applications. Using finite elements they highlighted the effect in the residual stresses of different materials and thicknesses deposited on flat surfaces, some “perfectly” polished and others with high roughness. Moreover the study is extended to the effect of geometrical alterations, edges or pores on the stress state. In this investigation it was clearly proved that thinner coatings are less sensitive to failures induced by the residual stresses due to the adjustment of the geometrical conditions in the interface. These authors have also shown that there is a critical film thickness for which the normal stresses (crossing the interface) are enough to induce the delamination of the coating.
Table 5 Adhesion characteristics of the monolithic and multilayer thin films based on the Ti–W– N system or the Cr–W–N system. Coatings Ti0.4W0.6N0.9 (monolithic)
Cr0.2W0.8N0.2 (monolithic)
Fig. 5. Young's modulus (measured and estimated) of the thin multilayer films based on Cr0.2W0.8N0.2 with interlayers of Cr0.1W0.9, function of the number of layers.
Ti0.4W0.6N0.9/Ti0.3W0.7 (3 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (5 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (9 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (11 layers) Ti0.4W0.6N0.9/Ti0.3W0.7 (21 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (3 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (5 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (9 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (11 layers) Cr0.2W0.8N0.2/Cr0.1W0.9 (21 layers)
Thickness
Lc1 [N]
[μm]
Average
Deviation
Average
2 1 0.5 0.3 2 1 0.5 0.3 3 3 3 3 3 3 3 3 3 3
61.2 67.8 78.7 76.3 50.6 51.6 57.2 74.6 38.8 38.4 41.5 36.8 40.2 43.2 47.3 50.6 27.7 31.4
2.6 1.6 1.1 1.9 4.1 0.7 2.6 0.3 1.6 1.2 1.6 2.5 1.6 0.9 0.5 0.8 0.8 0.5
N80 N80 N80 N80 N80 N80 N80 N80 N80 N80 N80 80 80 N80 N80 N80 N80 N80
Lc2 [N]
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Fig. 6. Average wear area of the cutting tools coated by thin Ti0.4W0.6N0.9 monolithic films (different coating thicknesses) and by Ti0.4W0.6N0.9/Ti0.3W0.7 multilayer coatings, during the cutting of OSB.
With regard to the grade of superficial finishing to be used in the cutting tools for wood-based products, taking into account the thickness of each deposited layer of the multilayer coatings and the roughness degree of the unpolished surfaces, it is essential that the deposition should be carried out on lapped surfaces. 3.4. Cutting efficiency The program of the cutting tests used to compare the monolithic coatings with the new type of multilayer ones was composed of two
sequences of tests; first both types of coatings were compared during the cutting of OSB and secondly during the cutting of wood particleboards (high wear aggressive material). Such comparison confirms that it is possible to extrapolate the results of OSB to wood particleboards or other wood-based products. For the first sequences of tests (during the cutting of OSB), both types of monolithic thin films were tested: Ti–W–N or Cr–W–N, for the thicknesses of 0.3 μm and 1 μm. Concerning the multilayer coatings, only the multilayers with the smallest and the greatest number of layers (three and twenty one layers) were tested; for the
Fig. 7. Average wear area of the cutting tools coated by thin Cr0.2W0.8N0.2 monolithic films (different coating thicknesses) and by Cr0.2W0.8N0.2/Cr0.1W0.9 multilayer coatings, during the cutting of OSB.
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Fig. 8. Average wear area of the cutting tools coated by thin Ti0.4W0.6N0.9 monolithic films (different coating thicknesses) and by Ti0.4W0.6N0.9/Ti0.3W0.7 multilayer coatings, during the cutting of wood particleboards (wear of the zone where the tool cuts the internal layer of the particleboard).
Fig. 9. Average wear area of the cutting tools coated by thin Cr0.2W0.8N0.2 monolithic films (different coating thicknesses) and by Cr0.2W0.8N0.2/Cr0.1W0.9 multilayer coatings, during the cutting of wood particleboards (wear of the zone where the tool cuts the internal layer of the particleboard).
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other multilayers, the coatings, which possess the higher hardness values: Ti0.4W0.6N0.9/Ti0.3W0.7 with nine layers and Cr0.2W0.8N0.2/ Cr0.1W0.9 with five layers, were selected. For the second sequence of tests (during the cutting of wood particleboards) all the monolithic and multilayer coatings were tested. The study of the efficiency of the coatings during the cutting of wood particleboards was achieved by a bipartition of wear evaluation at the cutting edge; in this analysis the wear due to the cutting of the internal and external layer of the wood particleboards was differentiated quantitatively, with very different aggressiveness. Figs. 6–9 show the results of the total cutting tests. Using the methodology developed for the evaluation of the wear of the cutting tools it became possible to evaluate that the wear from the cutting of the internal layer of the wood particleboards is much higher than cutting OSB and the cutting tool wear is much lower for coated than for uncoated tools. Regarding the efficiency of the monolithic films as a function of coating thickness it was possible to observe dissimilar behaviours. It can be inferred that beyond the composition type of the deposited coating, it is also necessary to optimize the coating thickness for cutting tools used in wood-based products. This procedure can improve the wear resistance of the cutting tool. The results demonstrate that, for both monolithic compositions tested during the cutting of OSB, a 1 μm thick coating can increase wear resistance by about 2.5 times relative to uncoated cutting tools. From the results of the average wear area during the cutting of wood particleboards, according to the zones of the blade that only cut the internal layer of the wood particleboards, it is clear that the 0.5 μm thick coating of Ti0.4W0.6N0.9, results in the greatest reduction in the cutting tool's wear, 72% less than that obtained with an uncoated tool (after cutting 426 m). Using a binocular magnifying glass (Fig. 10) it was possible to evaluate the advantages brought by the deposition of the monolithic hard films (1 μm) of Ti0.4W0.6N0.9. However, wear is almost identical to that seen in uncoated tools, when the blade cuts the external layer of the particleboards. With the best of the monolithic thin films, the application of the coating appeared to reduce wear in the initial phases of cutting, but after a hundred meters of cutting, the wear patterns are similar to those of uncoated tools. The most promising results were achieved by the application of the multilayer coatings with the highest values of hardness and adhesion to substrate. From the behaviour of the coatings it can be inferred that the thin multilayer films with fewer layers exhibit better cutting wear resistance. When cutting OSB, the multilayer coatings of Cr0.2W0.8N0.2/Cr0.1W0.9 contribute to a fivefold increase in the resistance of the tool, relative to an uncoated tool. With regard to the cutting of particleboard for the coatings of Ti0.4W0.6N0.9/Ti0.3W0.7, the most efficient multilayers are those with three and nine layers, which allow a reduction of the average wear area by about 70%, after cutting 426 m of wood particleboards. However, for the three layer thin films based on Cr0.2W0.8N0.2/ Cr0.1W0.9 there is still less wear on the cutting tool, increasing its life time. The coated tool presents, after cutting 1065 m of wood particleboard, a cutting increase of about 5 times possible with an uncoated tool. Nevertheless, for the cutting of the external layer of the wood particleboards, the analysis of wear shows that in most cases wear resistance of the coated tools is somewhat similar to that for an uncoated tool. Only three layers of Cr0.2W0.8N0.2/Cr0.1W0.9 contributes, in this zone, to a slight increase in the wear resistance of the cutting tool; but this only confers positive results for a limited number of cutting meters (some hundreds) of wood particleboard. The production of new multilayer coatings based on Ti–W–N or Cr–W–N, alternated respectively with layers of Ti–W or Cr–W, brought benefits to the wood cutting tools based on sintered tungsten carbide. The best deposited coating was: Cr0.2W0.8N0.2/Cr0.1W0.9/Cr0.2W0.8N0.2 (3 layers). The increase in the number of deposited layers led to a decrease in tool wear resistance, due to the decrease in hardness.
Fig. 10. Wear of the cutting edge (sight of the plan parallel to the rake face) after cutting 213 m of wood particleboard, with a depth of cut (zz′) of 10 mm: (a) cutting tool coated by a thin film of Ti0.4W0.6N0.9 with a thickness of 1 μm; (b) uncoated cutting tool.
Fig. 11 shows the features of tool wear in the thin films of Ti0.4W0.6N0.9/Ti0.3W0.7 (21 layers), after cutting 383 m of OSB. In this figure it is possible to see the steps observed in all of the studied multilayer coatings after starting the cutting. The type and extension of the wear is a function of the number of deposited layers. The substrate is only exposed at the cutting edge and there is never a delamination of the first hard layer deposited as in the plane of the rake face and in the plane of the clearance face. This is corroborated by the chemical analysis (Fig. 11), where it is possible to observe titanium metal of the coating in test up to the cutting edge. Different wear mechanisms can be seen in the worn surfaces of the cutting tools coated by multilayers based on Cr0.2W0.8N0.2/Cr0.1W0.9 (11 layers). The fractures result from the contact between the tool and
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Fig. 11. Wear of the multilayers based on Ti0.4W0.6N0.9/Ti0.3W0.7 (21 layers) of the rake face (SEM), after cutting 383 m of OSB; in line chemical pondered analysis (a) Ti; (b) W.
the wood-based material, where the gradual and controlled stepped progression of the wear into the thin films is visible, mainly close to the zones that are fractured, but maintaining the protection of the substrate. Some surfaces also stand out as being aggressively affected and that, apparently, is due to the remotion of the WC particles from the substrate. The fissures in the hard coatings are due to the impact of the interrupted cutting and/or by the high cutting speeds applied.
Acknowledgements The authors would like to acknowledge “Fundação para a Ciência e a Tecnologia (FCT) — Portugal” and “Fundo Social Europeu (FSE)” for the financial support (SFRH/BD/1461/2000) and “FREZITE, S.A. — Portugal” for providing the cutting tool support. References
4. Conclusions The investigation made use of experimentally sustainable real cutting tests to reach the following conclusions. Monolithic coatings can significantly increase wear resistance in cutting tools resulting from the cutting of materials such as wood-based products. Thus, it was possible to verify that the monolithic coatings with the best properties (Ti0.5W0.5N0.9 with 0.5 μm or 2 μm of thickness and Cr0.2W0.8N0.2 with 0.5 μm of thickness) are also the ones that contribute to a significant increase in wear resistance of the cutting tool for wood-based materials. However, the deposition of multilayer coatings, creating a balance between high hardness layers with a softer nanolayer, contributes even more to increasing the number of meters cut (about five times with a multilayer coating of Cr0.2W0.8N0.2/Cr0.1W0.9/Cr0.2W0.8N0.2, with a total number of three layers) when compared to that obtained with an uncoated tool of sintered tungsten carbide (with 4% of cobalt in weight), during the cutting of wood-based products. The production of multilayer coatings with architecture similar to sea urchin teeth improves the performance of the coated tools, without modification of the conventional deposition equipment, and using a set-up equal to that used for monolayer deposition.
[1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
J.C. Oliveira, A. Cavaleiro, C.M.A. Brett, Corros. Sci. 42 (11) (2000) 1881. M.T. Vieira, C.M. Pereira, J.M. Castanho, Surf. Coat. Technol. 131 (1–3) (2000) 417. A. Cavaleiro, B. Trindade, M.T. Vieira, Surf. Coat. Technol. 174–175 (2003) 68. P.N. Silva, J.P. Dias, A. Cavaleiro, Surf. Coat. Technol. 200 (1–3) (2005) 186. D. Pinheiro, “Optimização da Superfície de Ferramentas para o Corte de Aglomerados de Partículas de Madeiraq, PhD Thesis, University of Coimbra, (2007), ISBN: 972-99204-9-4. H. Schulz, E. Bergmann, Surf. Coat. Technol. 50 (1) (1991) 53. A. Leyland, M. Bin-Sudin, A.S. James, M.R. Kalantary, P.B. Wells, et al., Surf. Coat. Technol. 60 (1–3) (1993) 474. P. BaIIhause, B. Hensel, A. Rost, H. Schussler, Mater. Sci. Eng., A 163 (2) (1993) 193. S.B. Sant, K.S. Gill, Surf. Coat. Technol. 68–69 (1994) 152. K. Baba, R. Hatada, Surf. Coat. Technol. 66 (1–3) (1994) 368. T. Kacsich, K.-P. Lieb, Thin Solid Films 245 (1–2) (1994) 4. C. Yuji, O. Toshio, I. Hiroshi, J. Mater. Res. 8 (5) (1993) 1109. T. Sato, Y. Tada, M. Ozaki, K. Hoke, T. Besshi, Wear 178 (1–2) (1994) 95. Z.P. Huang, Y. Sun, T. Bell, Wear 173 (1–2) (1994) 13. J.M. Castanho, M.T. Vieira, Surf. Coat. Technol. 143–144 (2003) 352. J.V. Fernandes, A.C. Trindade, L.F. Menezes, A. Cavaleiro, Surf. Coat. Technol. 131 (2000) 457. J. Menčík, D. Munz, E. Quandt, E.R. Weppelmann, J. Mater. Res. 12 (9) (1997) 2475. H. Gao, C.-H. Chiu, J. Lee, Int. J. Solids Struct. 29 (20) (1992) 2471. J.M. Antunes, A. Cavaleiro, L.F. Menezes, M.I. Simões, J.V. Femandes, Surf. Coat. Technol. 149 (2002) 27. D. Pinheiro, M.T. Vieira, J.P. Dias, M.A. Djouadi, C. Nouveau, 16th IWMS — International Wood Machining Seminar, Aug 24–30, Matsue, Japan, 2003, p. 82. J.B. Wachtman, Mechanical Properties of Ceramics, John Wiley & Sons, inc., 1996. U. Wiklund, J. Gunnars, S. Hogmark, Wear 232 (2) (1999) 262.