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Thermal properties of cutting tool coatings at high temperatures
Thermochimica Acta 539 (2012) 51–55
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Thermal properties of cutting tool coatings at high temperatures J. Martan a,b,∗ , P. Beneˇs c a
Department of Physics, University of West Bohemia, Univerzitní 22, 30614 Plzen, ˇ Czech Republic New Technologies Research Centre, University of West Bohemia, Univerzitní 8, 30614 Plzen, ˇ Czech Republic c Department of Material Science and Technology, University of West Bohemia, Univerzitní 22, 30614 Plzen, ˇ Czech Republic b
a r t i c l e
i n f o
Article history: Received 2 February 2012 Received in revised form 23 March 2012 Accepted 30 March 2012 Available online 5 April 2012 Keywords: Coatings Thermal conductivity Volumetric specific heat Temperature dependence Cutting tools Nitrides
a b s t r a c t Cutting tools with coated inserts are widely used in high-speed cutting and in the cutting of hard-tomachine materials. The thermal properties of the coatings (or thin films) have a major impact on the cutting process and tool life. As there is a lack of data for high temperatures, we are presenting an experimental study of thermal conductivity and volumetric specific heat of different coatings in the range from room temperature to 500 ◦ C. The coatings under investigation were TiN, TiAlCN, TiAlN, AlTiN, TiAlSiN and CrAlSiN. The thermal properties were measured using the pulsed photothermal radiometry method. The thermal conductivity of the coatings under investigation varied from 2.8 to 25 W m−1 K−1 and increased with the rise in temperature. The lowest thermal conductivity was observed for the CrAlSiN coating. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The thermal properties of coatings (or thin films) for coated machining tools have a major impact on the machining process. A coating with low thermal conductivity acts as a thermal barrier for high speed processes, thus removing more heat to the chip. On the other hand, a coating with high thermal conductivity enables the lateral transfer of heat on the surface [1]. Using a coating as a thermal barrier is also associated with some increase in temperature on the tool–chip interface. High speed machining processes also move the temperature upwards. At high temperatures, different process parameters (e.g. force by material softening [2]), friction conditions and tool deformation [3] can take place, and also the coating itself can have different properties (e.g. oxidation rate, hardness, and thermal conductivity). For discontinuous process (e.g. milling), toughness and adhesion between the coating and the substrate are important [4]. Coatings with low oxidation resistance, like TiN, are no longer suitable for today’s demanding applications. A new type of coatings for cutting tools was developed in recent years – a nanocomposite coating. It consists of 3–5 nm nanocrystals of a hard transition metal nitride, such as TiN, (Ti1−x Alx )N and nc-(Al1−x Crx )N attached
∗ Corresponding author at: New Technologies Research Centre, University of West ˇ Czech Republic. Tel.: +420 377634718; Bohemia, Univerzitní 8, 30614 Plzen, fax: +420 377634702. E-mail address: [email protected] (J. Martan). 0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2012.03.029
together by about one monolayer of Si3 N4 . These coatings have high hardness, high oxidation resistance and low thermal conductivity [5]. This type of coating is in our TiAlSiN sample and possibly also in CrAlSiN sample. For high temperature applications, thermal properties at the working temperature have to be considered [3]. Only a few measurements of the thermal conductivity and specific heat of coatings used in cutting tools can be found in literature; measurements up to high temperatures are even rarer. Barsoum [6] presented the thermal conductivity and heat capacity of bulk TiN and Ti4 AlN3 for temperatures up to 1300 K. The thermal conductivity of TiAlN, AlTiN and AlCrN coatings for the temperature range from room temperature (RT) to 430 ◦ C was reported in Ref. [7]. Recently, the thermal conductivity of AlTiN coatings (and multilayer AlTiN/Cu) for the temperature range from RT to 450 ◦ C was measured [8]. The thermal conductivity and volumetric specific heat of CrAlN coatings with different Al contents were measured at RT in [9]. The coatings containing Ti and Al nitrides showed thermal conductivity increase with temperature. For AlCrN, a decrease at higher temperatures was reported. We present the measurements of thermal conductivity and volumetric specific heat up to 500 ◦ C for both recently developed coating materials used on cutting tools as well as materials that have long been in use. 2. Experiment The samples were coatings deposited on polished DIN 30CrMoV9 steel plate substrates with a thickness of 5 mm and
1.5 450 Pi80
Cr, AlSi (88/12)
75, 110
−80
1.3 450 −75 Pi80
Ti, AlSi (88/12)
85, 100
450 Ti, TiAl (33/67) Pl1000
160/180
−160/−20
0.45 0.55 1.0/2.0 450 450 430 Ti Ti, TiAl (75/25) Ti, TiAl (50/50)
50.0 4.5 25.0 20.5
0.7 20.0
32.1 1.7
2.3
2.5
TiAlSiN
CrAlSiN
17.7
6.8 21.3
AlTiN
21.5
C Al
Cr 45.6 43.3 27.5 1 3.5 2.1 TiN TiAlCN TiAlN
Ti
53.3 4.5
50.2
54.4 34.7 51.2
Si
N
15.2
Mono Gradient Multi TiN/TiAlN (1:2) 3x Multi TiN/AlTiN (1:3) 2x Mono, nanocomposite Mono
Pl1000 Pl1000 Pl1000
150 100 210/180
−100 −180 −160/−40
Temperature (◦ C) Substrate bias (V) Target current (A) Targets’ composition (at.%) Layer type Chemical composition (at.%) – EDX Thickness (m) Name
Table 1 Chemical composition, properties and deposition conditions of the investigated coatings.
diameter 25 mm. These coatings are used industrially and were deposited with a vacuum cathodic arc technique using PLATIT technology at LISS a.s. company. The chemical composition, properties and deposition conditions of the coatings are shown in Table 1. The substrates were first cleaned using the Eurocold system. Just before deposition, the surface was cleaned with an Ar glow discharge. A TiN or CrN adhesion layer was first deposited on the substrate. The TiN, TiAlCN, TiAlN and AlTiN coatings were deposited using ARC technology in the PL1000 deposition system from flat targets. The TiAlSiN (nACo) and CrAlSiN (nACRo) coatings were deposited using lateral rotating ARC cathode (LARC) technology [10] in the Pi80 deposition system from cylindrical targets. The chemical composition was measured using energy-dispersive X-ray spectroscopy (EDX) and verified by X-ray fluorescence (XRF) and glow discharge optical emission spectroscopy (GDOES). The crystal structure was determined by X-ray diffraction (XRD) using Cu K␣ radiation at glancing mode with an angle of incidence of 1◦ . The thermal properties of coatings were measured using the pulsed photothermal radiometry method. In the method, a nanosecond-pulsed laser is used to heat the coating surface and the surface temperature is detected by two infrared (IR) detectors. The measurement is done in nanosecond and microsecond time ranges at the same side of the sample in the center of the heated area. From the measured temperature evolution, an apparent thermal effusivity is calculated. An analytical model is fitted to the corresponding curve in order to determine coating thermal properties. The details of the method and measurement system can be found in [11]. The experimental system is schematically represented in Fig. 1. The ArF laser has a wavelength of 193 nm (ultraviolet light – UV) and pulse duration of 12 ns. The laser pulse energy is measured by an energy-meter and a beam splitter. The sample is placed in a vacuum chamber with a BaF2 optical window for transmission of both UV and IR light. Several samples can be placed on the electrically heated rotary sample holder. The heated sample surface emits IR radiation according to its temperature (Planck’s law). This radiation is collected by two optical systems of parabolic mirrors and directed to the two IR detectors (liquid nitrogen cooled photovoltaic photodiodes). By means of these detectors, the surface temperature can be measured without direct contact with the sample. The two detectors have different
Deposition system
Fig. 1. Schematic representation of the experimental system of pulsed photothermal radiometry.
2.5
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Pressure (Pa)
52
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sizes of sensitive area, and therefore different response times (10 and 110 ns) which enables measurement over a long time range (10 ns–200 s). The signals are measured by a digital oscilloscope with sampling frequency 250 MHz. To obtain absolute temperature evolution on the sample surface, the IR detectors need to be calibrated for each sample. Calibration is done by steady state heating of the samples to several different temperatures while noting the detectors’ responses. After obtaining the calibration curve, the measured signal in voltage during and after the laser pulse can be recalculated into the temperature evolution. The surface temperature evolution contains information about thermal properties at different depths of the sample. This is because the heat absorbed on the sample surface diffuses inside the sample. First, it propagates through the coating and then also through the substrate. The samples were coated with a thin opaque layer to ensure opacity of all coatings and so absorption of the laser light on sample surface. Apparent effusivity evolution is obtained from the surface temperature evolution with knowledge of laser pulse energy and sample absorptivity for laser light determined by spectral reflectivity measurement. The measured effusivity evolution over a long time range contains effusivity values for both investigated coating and substrate and also the transition region between them. Knowledge of the position of the transition region in time enables the simultaneous determination of thermal conductivity and volumetric specific heat of the investigated coating [12]. Thermal conductivity and volumetric specific heat are determined by an analytical model by an inverse problem. The coating thermal properties are varied in the model until the calculated effusivity temporal evolution fits the experimental one. The model is a three-layer 1D model with surface absorption of the laser light [13].
AlN
TiN
CrN
TiN
X-ray intensity (a.u.)
TiAlC N
TiAlN
AlT i N
TiAlS i N
CrAlS i N
30
40
50
60
70
80
2Θ Fig. 2. XRD diffraction patterns of the investigated coatings.
TiN coating TiAlC N coating TiAlN coating
TiAlN coat. [7] AlT i N coat. [7]
TiN bulk [14] TiN bulk [15] Ti4 AlN3 bulk [14]
k (W m-1 K-1)
30
3. Results and discussion From the chemical composition measurement (Table 1), the coatings showed the following atomic stoichiometry: Ti0.84 N, Ti0.87 Al0.14 C0.30 N0.70 , Ti0.54 Al0.42 N, Ti0.35 Al0.64 N, Ti0.40 Al0.38 Si0.08 Cr0.01 N and Cr0.50 Al0.41 Si0.09 N. Fig. 2 shows XRD patterns of the investigated coatings. Main peaks in all the patterns are from face-centered cubic (fcc) crystal structure. Coatings of TiN, TiAlCN and TiAlSiN have the structure of TiN. The TiAlN and AlTiN coatings are composed of a solid solution of AlN and TiN closer to AlN. On the diffraction patterns there are also small peaks of TiN from the interlayers. The CrAlSiN coating diffraction pattern shows only the AlN phase, but CrN reference peaks are very close to this. The peaks in TiAlCN are a bit shifted from pure TiN to smaller angles, probably due to the substitution of N by C. AlTiN also contains small peaks of hexagonal AlN phase. The TiAlSiN pattern has wider peaks and a higher background level between 30◦ and 50◦ 2, a sign of a possible amorphous phase, which together indicate a nanocomposite structure. For CrAlSiN sample this structure is probably not present (no sign in XRD spectra), but can be formed for certain stoichiometries [5]. The thermal conductivity and volumetric specific heat of the coatings were measured in a temperature range from 20 to 500 ◦ C. The thermal conductivity values obtained are presented in Figs. 3 and 4 along with literature data for bulk and coating materials of similar compositions. The present data were fitted by linear curves with coefficients shown in Table 2. Error bars in Figs. 3 and 4 are calculated as a standard deviation from three different measurements on different samples (from one deposition lot) with slightly different measurement system configurations. The standard deviation values contain measurement error of the measurement system, potential differences between
53
25 20 15 10 0
100
200
300
400
500
T (°C) Fig. 3. Thermal conductivity of TiN, TiAlCN and TiAlN coatings in dependence on temperature. Symbols – measured values. Solid lines – linear fit to the data. Dash and dash-dot lines and symbols without error bars – literature data (see Refs. [7,14,15]).
different samples and potential influence from different system configuration. In general, the measurement error (combined standard uncertainty) of the system is 13% [11]. Relative standard deviation was found higher for high thermal conductivity coatings (TiN, TiAlCN and TiAlN): 8–28% (11–22% without best and worst Table 2 Linear fit coefficients for thermal conductivity temperature dependence: k (W m−1 K−1 ) = A + B × T (◦ C). Sample
TiN TiAlCN TiAlN AlTiN TiAlSiN CrAlSiN
Linear fit for k A
B
21.90 16.91 11.16 4.64 2.83 2.76
0.00763 0.01545 0.00485 0.00054 0.00277 0.00084
54
J. Martan, P. Beneˇs / Thermochimica Acta 539 (2012) 51–55 AlTiN coating TiAlSiN coating CrAlSiN coating
AlTiN coating [7] AlCrN coating [7] AlTiN coating [8] AlTiN/Cu coating [8]
6.0
k (W m-1 K-1)
5.5 5.0 4.5 4.0 3.5 3.0 2.5 0
100
200
300
400
500
T (°C) Fig. 4. Thermal conductivity of AlTiN, TiAlSiN and CrAlSiN coatings in dependence on temperature. Symbols – measured values. Solid lines – linear fit to the data. Symbols without error bars – literature data (see Refs. [7,8]).
c (*106 J m-3 K-1)
cases) and lower for low thermal conductivity coatings (AlTiN, TiAlSiN and CrAlSiN): 6–15% (6–14%). The overall average of relative standard deviation was 13.7% for thermal conductivity, 8.7% for volumetric specific heat and 7.0% for thermal effusivity (square root of multiplication of thermal conductivity and volumetric specific heat). Low observed effusivity variation confirms supposed higher precision of its measurement as a directly measured quantity. From the relative standard deviation values it can be concluded that the present system is more accurate for measurement of low thermal conductivity coatings. The volumetric specific heat values of the investigated coatings are shown in Fig. 5 along with literature values for similar bulk materials. The obtained volumetric specific heat changes significantly with temperature. For TiN coating, the values and trend found are very close to the bulk values. The lowest volumetric specific heat was measured for AlTiN. The shape of the temperature dependence for CrAlSiN coating is a superposition of the two shapes of CrN and AlN. For the coatings based on Tix Al(1−x) N the volumetric specific heat decreases as the Al content increases (Ti content decreases). This trend is in coherence with bulk values: bulk TiN has higher volumetric specific heat than bulk AlN. But the TiN coating TiAlCN coating
AlTiN coating TiAlSiN coating
TiN bulk [14,15] Ti4 AlN3 bulk [14]
TiAlN coating
CrAlSiN coating
AlN bulk [14] CrN bulk [16,17]
4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6
measured values are not linearly distributed between the bulk values according to the chemical composition. With the change of structure (e.g. from TiAlN to TiAlSiN), no significant change of volumetric specific heat is observed. The thermal conductivity of the investigated coatings increases as the temperature rises and varies greatly for different coatings (Figs. 3 and 4). The TiAlSiN coating has low thermal conductivity, but it rises about 46% when heated to 500 ◦ C. Similar growth is observed for TiAlCN. For other samples, thermal conductivity increases more slowly: 6% for AlTiN and 15–21% for the other coatings. TiN coating has a thermal conductivity close to the values for bulk material; temperature dependences are also analogous. The AlTiN coating values that were measured fall within two sets of values found in literature. Only the slope is lower for our case. The thermal conductivity of coatings based on Tix Al(1−x) N depends strongly on composition. Thermal conductivity decreases sharply (from 18 to 4.6 W m−1 K−1 ) as the Al content increases (Ti content decreases), despite the fact that pure bulk AlN has high thermal conductivity (270–70 W m−1 K−1 for 20–500 ◦ C [14]). With addition of Si (4.5 at.%) the structure of the coating changes (to nanocomposite) and the thermal conductivity decreases. This can be seen on comparison of TiAlN and TiAlSiN. In this case the Al content is similar, but the structure changes and the thermal conductivity decreases 3–4 times. The CrAlSiN coating has the lowest thermal conductivity of the measured coatings and also of the values found in literature for similar wear resistant coatings. Its thermal conductivity is comparable and slightly lower than the AlTiN/Cu nano-multilayer coating [8]. The thermal conductivity of CrAlSiN coating is also lower than thermal conductivity of the Al2 O3 coating used in composite multilayer coatings (e.g. TiC/Al2 O3 /TiN) [18]. A composite coating can be envisaged from the presently measured coatings as: TiAlN layer on the surface with a CrAlSiN (or TiAlSiN) layer underneath for spreading the heat on the surface and low transfer to the substrate. An experimental assessment of the influence of the used coating on heat transfer to the tool was conducted in Ref. [19]. The Al2 O3 coating led to a reduction of heat flux in the tool, whereas the TiN and TiAlN coatings did not have an impact on the tool’s thermal behavior. This is in correlation with our measurements, where TiN and TiAlN coatings have relatively high thermal conductivity and so do not act as thermal barrier. TiAlSiN and CrAlSiN coatings have high hardness and oxidation resistance up to high temperatures [5]. Together with their low thermal conductivity (as measured in this work), they are very promising materials for cutting tools. They can be used as single layers for continuous cutting or in multilayers for discontinuous processes [4]. 4. Conclusion
0
100
200
300
400
500
T (°C) Fig. 5. Volumetric specific heat of coatings in dependence on temperature. Symbols – measured values. Solid curves – literature data for bulk materials (see Refs. [14–17]).
We have presented the thermal conductivity and volumetric specific heat measured for Ti, Al, Si and Cr nitride-based coatings for cutting tools in temperatures ranging from 20 to 500 ◦ C. Thermal conductivity in general increases with the rise in temperature. The increase at 500 ◦ C ranges from 6 to 46% for different coatings. The values vary from 2.8 to 25 W m−1 K−1 . Volumetric specific heat also increases with temperature with values of 2.5–3.1 × 106 J m−3 K−1 at RT and 3.4–4.5 × 106 J m−3 K−1 at 500 ◦ C. The lowest thermal conductivity with low increase with temperature was observed for the CrAlSiN coating. Acknowledgements We wish to thank LISS a.s. for the deposition of the coatings. This work has been supported by research project MSM4977751302
J. Martan, P. Beneˇs / Thermochimica Acta 539 (2012) 51–55
from the Ministry of Education, Youth and Sports of the Czech Republic. The result was developed within the CENTEM project, reg. no. CZ.1.05/2.1.00/03.0088, that is co-funded from the ERDF within the OP RDI programme of the Ministry of Education, Youth and Sports. References [1] L. Braginsky, A. Gusarov, V. Shklover, Surf. Coat. Technol. 204 (2009) 629–634. [2] S.M. Lee, H.M. Chow, F.Y. Huang, B.H. Yan, Int. J. Mach. Tools Manuf. 49 (2009) 81–88. [3] N.A. Abukhshim, P.T. Mativenga, M.A. Sheikh, Int. J. Mach. Tools Manuf. 46 (2006) 782–800. [4] L. Chen, S.Q. Wang, Y. Du, S.Z. Zhou, T. Gang, J.C. Fen, K.K. Chang, Y.W. Li, X. Xiong, Surf. Coat. Technol. 205 (2010) 582–586. [5] S. Veprek, M.J.G. Veprek-Heijman, Surf. Coat. Technol. 202 (2008) 5063–5073. [6] M.W. Barsoum, Prog. Solid State Chem. 28 (2000) 201–281. [7] W. Kalss, A. Reiter, V. Derflinger, C. Gey, J.L. Endrino, Int. J. Refract. Met. Hard Mater. 24 (2006) 399–404.
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[8] G.S. Fox-Rabinovich, K. Yamamoto, M.H. Aguirre, D.G. Cahill, S.C. Veldhuis, A. Biksa, G. Dosbaeva, L.S. Shuster, Surf. Coat. Technol. 204 (2010) 2465–2471. [9] B. Tlili, N. Mustapha, C. Nouveau, Y. Benlatreche, G. Guillemot, M. Lambertin, Vacuum 84 (2010) 1067–1074. [10] M. Jilek, T. Cselle, P. Holubar, M. Morstein, M.G.J. Veprek-Heijman, S. Veprek, Plasma Chem. Plasma Process. 24 (2004) 493–510. ˇ E. Amin Chalhoub, Rev. Sci. Instrum. 81 (2010) 124902. [11] J. Martan, J. Capek, [12] J. Martan, O. Herve, V. Lang, J. Appl. Phys. 102 (2007) 064903. [13] D.L. Balageas, J.C. Krapez, P. Cielo, J. Appl. Phys. 59 (1986) 348–357. [14] MPDB Software – Temperature Dependent Elastic & Thermal Properties Database, 2009. http://www.jahm.com/. [15] H.O. Pierson, Handbook of Refractory Carbides and Nitrides: Properties, Characteristics, Processing and Applications, Noyes Publications, New Jersey, 1996. [16] M. Binnewies, E. Milke, Thermochemical Data of Elements and Compounds, second ed., Wiley-VCH Verlag GmbH, Weinheim, 2002. [17] I.S. Grigoriev, E.Z. Meilikhov, Handbook of Physical Quantities, CRC Press, London, 1997. [18] W. Grzesik, P. Nieslony, Int. J. Mach. Tools Manuf. 44 (2004) 889–901. [19] A. Kusiak, J.L. Battaglia, J. Rech, Surf. Coat. Technol. 195 (2005) 29–40.
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Thermal properties of cutting tool coatings at high temperatures J. Martan a,b,∗ , P. Beneˇs c a
Department of Physics, University of West Bohemia, Univerzitní 22, 30614 Plzen, ˇ Czech Republic New Technologies Research Centre, University of West Bohemia, Univerzitní 8, 30614 Plzen, ˇ Czech Republic c Department of Material Science and Technology, University of West Bohemia, Univerzitní 22, 30614 Plzen, ˇ Czech Republic b
a r t i c l e
i n f o
Article history: Received 2 February 2012 Received in revised form 23 March 2012 Accepted 30 March 2012 Available online 5 April 2012 Keywords: Coatings Thermal conductivity Volumetric specific heat Temperature dependence Cutting tools Nitrides
a b s t r a c t Cutting tools with coated inserts are widely used in high-speed cutting and in the cutting of hard-tomachine materials. The thermal properties of the coatings (or thin films) have a major impact on the cutting process and tool life. As there is a lack of data for high temperatures, we are presenting an experimental study of thermal conductivity and volumetric specific heat of different coatings in the range from room temperature to 500 ◦ C. The coatings under investigation were TiN, TiAlCN, TiAlN, AlTiN, TiAlSiN and CrAlSiN. The thermal properties were measured using the pulsed photothermal radiometry method. The thermal conductivity of the coatings under investigation varied from 2.8 to 25 W m−1 K−1 and increased with the rise in temperature. The lowest thermal conductivity was observed for the CrAlSiN coating. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The thermal properties of coatings (or thin films) for coated machining tools have a major impact on the machining process. A coating with low thermal conductivity acts as a thermal barrier for high speed processes, thus removing more heat to the chip. On the other hand, a coating with high thermal conductivity enables the lateral transfer of heat on the surface [1]. Using a coating as a thermal barrier is also associated with some increase in temperature on the tool–chip interface. High speed machining processes also move the temperature upwards. At high temperatures, different process parameters (e.g. force by material softening [2]), friction conditions and tool deformation [3] can take place, and also the coating itself can have different properties (e.g. oxidation rate, hardness, and thermal conductivity). For discontinuous process (e.g. milling), toughness and adhesion between the coating and the substrate are important [4]. Coatings with low oxidation resistance, like TiN, are no longer suitable for today’s demanding applications. A new type of coatings for cutting tools was developed in recent years – a nanocomposite coating. It consists of 3–5 nm nanocrystals of a hard transition metal nitride, such as TiN, (Ti1−x Alx )N and nc-(Al1−x Crx )N attached
∗ Corresponding author at: New Technologies Research Centre, University of West ˇ Czech Republic. Tel.: +420 377634718; Bohemia, Univerzitní 8, 30614 Plzen, fax: +420 377634702. E-mail address: [email protected] (J. Martan). 0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2012.03.029
together by about one monolayer of Si3 N4 . These coatings have high hardness, high oxidation resistance and low thermal conductivity [5]. This type of coating is in our TiAlSiN sample and possibly also in CrAlSiN sample. For high temperature applications, thermal properties at the working temperature have to be considered [3]. Only a few measurements of the thermal conductivity and specific heat of coatings used in cutting tools can be found in literature; measurements up to high temperatures are even rarer. Barsoum [6] presented the thermal conductivity and heat capacity of bulk TiN and Ti4 AlN3 for temperatures up to 1300 K. The thermal conductivity of TiAlN, AlTiN and AlCrN coatings for the temperature range from room temperature (RT) to 430 ◦ C was reported in Ref. [7]. Recently, the thermal conductivity of AlTiN coatings (and multilayer AlTiN/Cu) for the temperature range from RT to 450 ◦ C was measured [8]. The thermal conductivity and volumetric specific heat of CrAlN coatings with different Al contents were measured at RT in [9]. The coatings containing Ti and Al nitrides showed thermal conductivity increase with temperature. For AlCrN, a decrease at higher temperatures was reported. We present the measurements of thermal conductivity and volumetric specific heat up to 500 ◦ C for both recently developed coating materials used on cutting tools as well as materials that have long been in use. 2. Experiment The samples were coatings deposited on polished DIN 30CrMoV9 steel plate substrates with a thickness of 5 mm and
1.5 450 Pi80
Cr, AlSi (88/12)
75, 110
−80
1.3 450 −75 Pi80
Ti, AlSi (88/12)
85, 100
450 Ti, TiAl (33/67) Pl1000
160/180
−160/−20
0.45 0.55 1.0/2.0 450 450 430 Ti Ti, TiAl (75/25) Ti, TiAl (50/50)
50.0 4.5 25.0 20.5
0.7 20.0
32.1 1.7
2.3
2.5
TiAlSiN
CrAlSiN
17.7
6.8 21.3
AlTiN
21.5
C Al
Cr 45.6 43.3 27.5 1 3.5 2.1 TiN TiAlCN TiAlN
Ti
53.3 4.5
50.2
54.4 34.7 51.2
Si
N
15.2
Mono Gradient Multi TiN/TiAlN (1:2) 3x Multi TiN/AlTiN (1:3) 2x Mono, nanocomposite Mono
Pl1000 Pl1000 Pl1000
150 100 210/180
−100 −180 −160/−40
Temperature (◦ C) Substrate bias (V) Target current (A) Targets’ composition (at.%) Layer type Chemical composition (at.%) – EDX Thickness (m) Name
Table 1 Chemical composition, properties and deposition conditions of the investigated coatings.
diameter 25 mm. These coatings are used industrially and were deposited with a vacuum cathodic arc technique using PLATIT technology at LISS a.s. company. The chemical composition, properties and deposition conditions of the coatings are shown in Table 1. The substrates were first cleaned using the Eurocold system. Just before deposition, the surface was cleaned with an Ar glow discharge. A TiN or CrN adhesion layer was first deposited on the substrate. The TiN, TiAlCN, TiAlN and AlTiN coatings were deposited using ARC technology in the PL1000 deposition system from flat targets. The TiAlSiN (nACo) and CrAlSiN (nACRo) coatings were deposited using lateral rotating ARC cathode (LARC) technology [10] in the Pi80 deposition system from cylindrical targets. The chemical composition was measured using energy-dispersive X-ray spectroscopy (EDX) and verified by X-ray fluorescence (XRF) and glow discharge optical emission spectroscopy (GDOES). The crystal structure was determined by X-ray diffraction (XRD) using Cu K␣ radiation at glancing mode with an angle of incidence of 1◦ . The thermal properties of coatings were measured using the pulsed photothermal radiometry method. In the method, a nanosecond-pulsed laser is used to heat the coating surface and the surface temperature is detected by two infrared (IR) detectors. The measurement is done in nanosecond and microsecond time ranges at the same side of the sample in the center of the heated area. From the measured temperature evolution, an apparent thermal effusivity is calculated. An analytical model is fitted to the corresponding curve in order to determine coating thermal properties. The details of the method and measurement system can be found in [11]. The experimental system is schematically represented in Fig. 1. The ArF laser has a wavelength of 193 nm (ultraviolet light – UV) and pulse duration of 12 ns. The laser pulse energy is measured by an energy-meter and a beam splitter. The sample is placed in a vacuum chamber with a BaF2 optical window for transmission of both UV and IR light. Several samples can be placed on the electrically heated rotary sample holder. The heated sample surface emits IR radiation according to its temperature (Planck’s law). This radiation is collected by two optical systems of parabolic mirrors and directed to the two IR detectors (liquid nitrogen cooled photovoltaic photodiodes). By means of these detectors, the surface temperature can be measured without direct contact with the sample. The two detectors have different
Deposition system
Fig. 1. Schematic representation of the experimental system of pulsed photothermal radiometry.
2.5
J. Martan, P. Beneˇs / Thermochimica Acta 539 (2012) 51–55
Pressure (Pa)
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sizes of sensitive area, and therefore different response times (10 and 110 ns) which enables measurement over a long time range (10 ns–200 s). The signals are measured by a digital oscilloscope with sampling frequency 250 MHz. To obtain absolute temperature evolution on the sample surface, the IR detectors need to be calibrated for each sample. Calibration is done by steady state heating of the samples to several different temperatures while noting the detectors’ responses. After obtaining the calibration curve, the measured signal in voltage during and after the laser pulse can be recalculated into the temperature evolution. The surface temperature evolution contains information about thermal properties at different depths of the sample. This is because the heat absorbed on the sample surface diffuses inside the sample. First, it propagates through the coating and then also through the substrate. The samples were coated with a thin opaque layer to ensure opacity of all coatings and so absorption of the laser light on sample surface. Apparent effusivity evolution is obtained from the surface temperature evolution with knowledge of laser pulse energy and sample absorptivity for laser light determined by spectral reflectivity measurement. The measured effusivity evolution over a long time range contains effusivity values for both investigated coating and substrate and also the transition region between them. Knowledge of the position of the transition region in time enables the simultaneous determination of thermal conductivity and volumetric specific heat of the investigated coating [12]. Thermal conductivity and volumetric specific heat are determined by an analytical model by an inverse problem. The coating thermal properties are varied in the model until the calculated effusivity temporal evolution fits the experimental one. The model is a three-layer 1D model with surface absorption of the laser light [13].
AlN
TiN
CrN
TiN
X-ray intensity (a.u.)
TiAlC N
TiAlN
AlT i N
TiAlS i N
CrAlS i N
30
40
50
60
70
80
2Θ Fig. 2. XRD diffraction patterns of the investigated coatings.
TiN coating TiAlC N coating TiAlN coating
TiAlN coat. [7] AlT i N coat. [7]
TiN bulk [14] TiN bulk [15] Ti4 AlN3 bulk [14]
k (W m-1 K-1)
30
3. Results and discussion From the chemical composition measurement (Table 1), the coatings showed the following atomic stoichiometry: Ti0.84 N, Ti0.87 Al0.14 C0.30 N0.70 , Ti0.54 Al0.42 N, Ti0.35 Al0.64 N, Ti0.40 Al0.38 Si0.08 Cr0.01 N and Cr0.50 Al0.41 Si0.09 N. Fig. 2 shows XRD patterns of the investigated coatings. Main peaks in all the patterns are from face-centered cubic (fcc) crystal structure. Coatings of TiN, TiAlCN and TiAlSiN have the structure of TiN. The TiAlN and AlTiN coatings are composed of a solid solution of AlN and TiN closer to AlN. On the diffraction patterns there are also small peaks of TiN from the interlayers. The CrAlSiN coating diffraction pattern shows only the AlN phase, but CrN reference peaks are very close to this. The peaks in TiAlCN are a bit shifted from pure TiN to smaller angles, probably due to the substitution of N by C. AlTiN also contains small peaks of hexagonal AlN phase. The TiAlSiN pattern has wider peaks and a higher background level between 30◦ and 50◦ 2, a sign of a possible amorphous phase, which together indicate a nanocomposite structure. For CrAlSiN sample this structure is probably not present (no sign in XRD spectra), but can be formed for certain stoichiometries [5]. The thermal conductivity and volumetric specific heat of the coatings were measured in a temperature range from 20 to 500 ◦ C. The thermal conductivity values obtained are presented in Figs. 3 and 4 along with literature data for bulk and coating materials of similar compositions. The present data were fitted by linear curves with coefficients shown in Table 2. Error bars in Figs. 3 and 4 are calculated as a standard deviation from three different measurements on different samples (from one deposition lot) with slightly different measurement system configurations. The standard deviation values contain measurement error of the measurement system, potential differences between
53
25 20 15 10 0
100
200
300
400
500
T (°C) Fig. 3. Thermal conductivity of TiN, TiAlCN and TiAlN coatings in dependence on temperature. Symbols – measured values. Solid lines – linear fit to the data. Dash and dash-dot lines and symbols without error bars – literature data (see Refs. [7,14,15]).
different samples and potential influence from different system configuration. In general, the measurement error (combined standard uncertainty) of the system is 13% [11]. Relative standard deviation was found higher for high thermal conductivity coatings (TiN, TiAlCN and TiAlN): 8–28% (11–22% without best and worst Table 2 Linear fit coefficients for thermal conductivity temperature dependence: k (W m−1 K−1 ) = A + B × T (◦ C). Sample
TiN TiAlCN TiAlN AlTiN TiAlSiN CrAlSiN
Linear fit for k A
B
21.90 16.91 11.16 4.64 2.83 2.76
0.00763 0.01545 0.00485 0.00054 0.00277 0.00084
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J. Martan, P. Beneˇs / Thermochimica Acta 539 (2012) 51–55 AlTiN coating TiAlSiN coating CrAlSiN coating
AlTiN coating [7] AlCrN coating [7] AlTiN coating [8] AlTiN/Cu coating [8]
6.0
k (W m-1 K-1)
5.5 5.0 4.5 4.0 3.5 3.0 2.5 0
100
200
300
400
500
T (°C) Fig. 4. Thermal conductivity of AlTiN, TiAlSiN and CrAlSiN coatings in dependence on temperature. Symbols – measured values. Solid lines – linear fit to the data. Symbols without error bars – literature data (see Refs. [7,8]).
c (*106 J m-3 K-1)
cases) and lower for low thermal conductivity coatings (AlTiN, TiAlSiN and CrAlSiN): 6–15% (6–14%). The overall average of relative standard deviation was 13.7% for thermal conductivity, 8.7% for volumetric specific heat and 7.0% for thermal effusivity (square root of multiplication of thermal conductivity and volumetric specific heat). Low observed effusivity variation confirms supposed higher precision of its measurement as a directly measured quantity. From the relative standard deviation values it can be concluded that the present system is more accurate for measurement of low thermal conductivity coatings. The volumetric specific heat values of the investigated coatings are shown in Fig. 5 along with literature values for similar bulk materials. The obtained volumetric specific heat changes significantly with temperature. For TiN coating, the values and trend found are very close to the bulk values. The lowest volumetric specific heat was measured for AlTiN. The shape of the temperature dependence for CrAlSiN coating is a superposition of the two shapes of CrN and AlN. For the coatings based on Tix Al(1−x) N the volumetric specific heat decreases as the Al content increases (Ti content decreases). This trend is in coherence with bulk values: bulk TiN has higher volumetric specific heat than bulk AlN. But the TiN coating TiAlCN coating
AlTiN coating TiAlSiN coating
TiN bulk [14,15] Ti4 AlN3 bulk [14]
TiAlN coating
CrAlSiN coating
AlN bulk [14] CrN bulk [16,17]
4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6
measured values are not linearly distributed between the bulk values according to the chemical composition. With the change of structure (e.g. from TiAlN to TiAlSiN), no significant change of volumetric specific heat is observed. The thermal conductivity of the investigated coatings increases as the temperature rises and varies greatly for different coatings (Figs. 3 and 4). The TiAlSiN coating has low thermal conductivity, but it rises about 46% when heated to 500 ◦ C. Similar growth is observed for TiAlCN. For other samples, thermal conductivity increases more slowly: 6% for AlTiN and 15–21% for the other coatings. TiN coating has a thermal conductivity close to the values for bulk material; temperature dependences are also analogous. The AlTiN coating values that were measured fall within two sets of values found in literature. Only the slope is lower for our case. The thermal conductivity of coatings based on Tix Al(1−x) N depends strongly on composition. Thermal conductivity decreases sharply (from 18 to 4.6 W m−1 K−1 ) as the Al content increases (Ti content decreases), despite the fact that pure bulk AlN has high thermal conductivity (270–70 W m−1 K−1 for 20–500 ◦ C [14]). With addition of Si (4.5 at.%) the structure of the coating changes (to nanocomposite) and the thermal conductivity decreases. This can be seen on comparison of TiAlN and TiAlSiN. In this case the Al content is similar, but the structure changes and the thermal conductivity decreases 3–4 times. The CrAlSiN coating has the lowest thermal conductivity of the measured coatings and also of the values found in literature for similar wear resistant coatings. Its thermal conductivity is comparable and slightly lower than the AlTiN/Cu nano-multilayer coating [8]. The thermal conductivity of CrAlSiN coating is also lower than thermal conductivity of the Al2 O3 coating used in composite multilayer coatings (e.g. TiC/Al2 O3 /TiN) [18]. A composite coating can be envisaged from the presently measured coatings as: TiAlN layer on the surface with a CrAlSiN (or TiAlSiN) layer underneath for spreading the heat on the surface and low transfer to the substrate. An experimental assessment of the influence of the used coating on heat transfer to the tool was conducted in Ref. [19]. The Al2 O3 coating led to a reduction of heat flux in the tool, whereas the TiN and TiAlN coatings did not have an impact on the tool’s thermal behavior. This is in correlation with our measurements, where TiN and TiAlN coatings have relatively high thermal conductivity and so do not act as thermal barrier. TiAlSiN and CrAlSiN coatings have high hardness and oxidation resistance up to high temperatures [5]. Together with their low thermal conductivity (as measured in this work), they are very promising materials for cutting tools. They can be used as single layers for continuous cutting or in multilayers for discontinuous processes [4]. 4. Conclusion
0
100
200
300
400
500
T (°C) Fig. 5. Volumetric specific heat of coatings in dependence on temperature. Symbols – measured values. Solid curves – literature data for bulk materials (see Refs. [14–17]).
We have presented the thermal conductivity and volumetric specific heat measured for Ti, Al, Si and Cr nitride-based coatings for cutting tools in temperatures ranging from 20 to 500 ◦ C. Thermal conductivity in general increases with the rise in temperature. The increase at 500 ◦ C ranges from 6 to 46% for different coatings. The values vary from 2.8 to 25 W m−1 K−1 . Volumetric specific heat also increases with temperature with values of 2.5–3.1 × 106 J m−3 K−1 at RT and 3.4–4.5 × 106 J m−3 K−1 at 500 ◦ C. The lowest thermal conductivity with low increase with temperature was observed for the CrAlSiN coating. Acknowledgements We wish to thank LISS a.s. for the deposition of the coatings. This work has been supported by research project MSM4977751302
J. Martan, P. Beneˇs / Thermochimica Acta 539 (2012) 51–55
from the Ministry of Education, Youth and Sports of the Czech Republic. The result was developed within the CENTEM project, reg. no. CZ.1.05/2.1.00/03.0088, that is co-funded from the ERDF within the OP RDI programme of the Ministry of Education, Youth and Sports. References [1] L. Braginsky, A. Gusarov, V. Shklover, Surf. Coat. Technol. 204 (2009) 629–634. [2] S.M. Lee, H.M. Chow, F.Y. Huang, B.H. Yan, Int. J. Mach. Tools Manuf. 49 (2009) 81–88. [3] N.A. Abukhshim, P.T. Mativenga, M.A. Sheikh, Int. J. Mach. Tools Manuf. 46 (2006) 782–800. [4] L. Chen, S.Q. Wang, Y. Du, S.Z. Zhou, T. Gang, J.C. Fen, K.K. Chang, Y.W. Li, X. Xiong, Surf. Coat. Technol. 205 (2010) 582–586. [5] S. Veprek, M.J.G. Veprek-Heijman, Surf. Coat. Technol. 202 (2008) 5063–5073. [6] M.W. Barsoum, Prog. Solid State Chem. 28 (2000) 201–281. [7] W. Kalss, A. Reiter, V. Derflinger, C. Gey, J.L. Endrino, Int. J. Refract. Met. Hard Mater. 24 (2006) 399–404.
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