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Thermal radiative properties of a DLC coating
Cryogenics 48 (2008) 455–457
Contents lists available at ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Short communication
Thermal radiative properties of a DLC coating P. Hanzelka a,*, T. Kralik a, A. Maskova b, V. Musilova a, J. Vyskocil b a b
Institute of Scientific Instruments of the ASCR, v.v.i., Academy of Sciences of the Czech Republic, Královopolská 147, 612 64 Brno, Czech Republic HVM Plasma Ltd., Na Hutmance 2, 158 00 Praha 5, Czech Republic
a r t i c l e
i n f o
Article history: Received 22 January 2008 Received in revised form 18 March 2008 Accepted 27 March 2008
a b s t r a c t Thermal radiative properties of a DLC coating were measured in the range of cryogenic and room temperatures. Both the total hemispherical emissivity and absorptivity are significantly dependent on the radiation temperature and change from the value of 0.032 at 15 K to 0.65 at 300 K. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Thin films C. Heat transfer C. Radiant properties F. Cryostats
1. Introduction DLC (diamond-like-carbon) is used as low friction low wear coating in many industrial branches as automotive or textile industry, optical instruments, biomechanics or electronics [1–3]. Furthermore, applications of DLC layers in spacecrafts and satellites and also in cryogenics were discussed and tested [4–6]. Although some researchers investigated optical properties of specific DLC layers in infrared [7,8], the thermal radiative properties of DLC coatings at cryogenic temperatures were not methodically studied yet. As we propose to use a DLC layer as an antiseizing coating in one of our cryogenic apparatuses, we have measured thermal emissivity and absorptivity of the coating. Interpretation of the obtained data is not the aim of this contribution. 2. Method The method described in [9] was used for measurement. Radiative heat flow QR between two flat discs (radiator and absorber) each of 40 mm in diameter is evaluated from the temperature drop (TATB) on a calibrated thermal resistor (Fig. 1). The mutual emissivity (emissivity factor) eRA of the radiator at temperature TR and the absorber at temperature TA is then calculated from the known relation [10] for the heat transfer between parallel gray surfaces eRA ¼
QR r S ðT 4R T 4A Þ
* Corresponding author. Tel.: +420 541 514 265; fax: +420 541 514 402. E-mail address: [email protected] (P. Hanzelka). 0011-2275/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2008.03.021
ð1Þ
where r is the Stefan–Boltzmann constant and S represents the surface area of the discs. When absorptivity is measured, the sample is used as absorber and is irradiated from radiator at adjustable temperature TR. For the known temperature dependence of total hemispherical emissivity eR(TR) of the radiator, the sample total hemispherical absorptivity aA can be evaluated for individual temperatures TR from the relation [10]: 1=eRA ¼ 1=eR þ 1=aA 1
ð2Þ
For the sample emissivity investigation, the discs are swapped and the radiator emissivity eR is similarly found. As, in general, the absorptivity of a sample also depends on its temperature TA, this temperature should be kept at an approximately constant value during the measurement with variable TR. But the heat flux QR variations (over six orders of magnitude, from 0.11 lW to 350 mW in our measurements) induce a variable temperature drop on the thermal resistor (Fig. 1) and thus cause substantial changes in TA. An adjustable thermal resistor would be the best solution of this problem. Unfortunately, the realization of such resistor which retains high reproducibility and stability would be very difficult. As a compromise, two different hard-set resistors were calibrated and each of them was used in one range of temperature of our measurement. The accuracy of measured heat flux QR is given by the precision of the thermal resistor calibration which is of about 3%. The sensor of the temperature TR was calibrated with an absolute accuracy of 0.25 K. As a result, the mutual emissivity evaluated from the relation (1) is obtained with measurement error of about 6% at TR = 30 K and from 3% to 4% at TR P 100 K. In addition, because a part of radiation leaks out of the space between the radiator and absorber, the actual values of mutual emis-
456
P. Hanzelka et al. / Cryogenics 48 (2008) 455–457 160
80
4.2 K
Measuring chamber
Resultant α ε RA - Resistor 1 ε RA - Resistor 2
70
Radiator holder
140
60
120
50
100
ε RA, α [%]
Heater Radiator
TR
TA
Absorber
TA - Resistor 1 TA - Resistor 2
40
80
30
60
20
40
10
20 0
0
QR
TA [K]
Vacuum
0
50
100
150
200
250
300
TR [K] Fig. 3. Dependences on the radiator temperature TR. Left axis: Mutual emissivity eRA measured with two individual thermal resistors and the resultant absorptivity a of the DLC coating. Right axis: Temperatures TA of the DLC sample registered during the absorptivity measurement with individual thermal resistors.
Thermal resistor
TB
Fig. 1. Simplified scheme of the measuring chamber.
sivity are higher than the values evaluated from the relation (1). We estimated the correction factor not to be higher than 1.03. This systematic error was included into the bars illustrating actual errors of measured absorptivity of DLC layer in Fig. 3. 3. Samples A copper disc, 1 mm in thickness and 40 mm in diameter was used as the substrate for the DLC deposit. It was made of a sheet of technical quality and its surface was polished by fine steel wool. The measured dependence of the substrate absorptivity a on the radiator temperature TR is reported in Fig. 2. The DLC coating was deposited by combined PVD/PACVD method in industrial coating equipment HTC1500 (Hauzer Techno Coating). The first gradient layer Cr/WC:H was deposited by pulsed (40 kHz) magnetron sputtering to achieve good adhesion whereas the top DLC layer was prepared by pulsed plasma (100 kHz) from acetylene. Total coating thickness is of 3.3 lm, where the top DLC layer thickness is of 2.2 lm. Plastic hardness HUpl and indentation
5
100 90 80 70
3
60
α [%]
ε R , α A [%]
4
Epoxy layer 380 μm (emissivity and absorptivity)
50
Copper substrate (absorptivity)
40
2
30 1
20 10
0
0 0
50
100
150
200
250
300
TR [K] Fig. 2. Dependences on the radiation temperature TR. Left axis: total hemispherical emissivity eR and absorptivity aA of the epoxy layer. Right axis: total hemispherical absorptivity a of the copper substrate.
modulus EIT (measured according to ISO 14577) of the coating are 33 GPa and 175 GPa, respectively. The top DLC layer is amorphous hydrogenated carbon structure with hydrogen content of about 15 atomic percent. A copper disc covered by an epoxy layer of 380 lm in thickness was used as the opposite surface in all experiments. Emissivity and absorptivity of this sample is plotted in Fig. 2. We have measured the temperature dependence of mutual emissivity of two identical epoxy surfaces [11] and evaluated their emissivity and absorptivity from the relation (2) under the assumption that absorptivity equals emissivity at the same temperature TR of the radiator. This assumption was justified by the measurement we have published in [11]. 4. Results and discussion The measured values of the mutual emissivity eRA between the epoxy layer used as radiator and the investigated DLC layer as absorber are plotted in Fig. 3. Two different thermal resistors were used successively in two individual experiments. Good coincidence of both overlapped data series is evident. The first measurable value of QR was observed at the radiator temperature TR = 15 K. The resultant values of absorptivity a evaluated from Eq. (2) using the radiator emissivity eR, which is expressed in Fig. 2, do not substantially differ from the values of the mutual emissivity eRA (Fig. 3). This is caused by the fact that in the whole range of the temperatures TR, the epoxy layer is almost black in comparison with the DLC coating. The absorptivity a is strongly dependent on the radiator temperature TR. It increases over one order of magnitude in the range of TR between 15 K and 150 K. On the other hand, only insignificant dependence of a on the sample temperature TA may be expected, although we did not measure this dependence directly. This assumption is based on the fact that while the sample temperatures TA in both experiments with individual resistors are different (Fig. 3), the corresponding emissivities eRA are equal. The dependence of absorptivity a on the temperature TR may be approximated by the sigmoidal fit expression (3). This approximation is of precision of 5% in the temperature range of 20–300 K a ¼ 84=ð1 þ EXPððT R 47:7Þ=32:7ÞÞ þ 64
ð3Þ
The values of the DLC layer emissivity e were obtained by similar procedure when the DLC layer was used as radiator and the epoxy layer as absorber. It is evident from the ratio of both quan-
457
P. Hanzelka et al. / Cryogenics 48 (2008) 455–457 Table 1 The ratio of the measured emissivity e to the absorptivity a of the DLC coating calculated for different radiation temperatures TR TR (K) e/a
20 0.97
30 1.05
40 1.01
50 0.99
60 1.01
80 1.00
tities e/a expressed in the Table 1 for selected temperatures TR that the relative differences between both quantities are lower than 5% and are comparable with the error of the apparatus calibration. Acknowledgement This work was supported by the Academy of Sciences of the Czech Republic, project No. AV0 Z20650511. References [1] Robertson J. Diamond-like amorphous carbon. Mat Sci Eng R 2002;37:129–281. [2] Grill A. Electrical and optical properties of diamond-like carbon. Thin Solid Films 1999;355–356:189–93. [3] Mossner C et al. Characterization of diamond-like carbon by Raman spectroscopy, XPS and optical constants. Thin Solid Films 1998;317:397–401.
100 1.02
120 0.99
140 0.99
180 0.99
200 0.99
240 0.99
260 0.99
300 1.00
[4] Voevodin AA, Zabinski JS. Nanocomposite and nanostructured tribological materials for space applications. Compos Sci Technol 2005;65(5):741–8. [5] Gradt T, Borner H, Schneider T. Low temperature tribometers and the behaviour of ADLC coatings in cryogenic environment. Tribol Int 2001;34(4):225–30. [6] Ostrovskaya YL et al. Friction and wear behaviour of hard and superhard coatings at cryogenic temperatures. Tribol Int 2001;34(4):255–63. [7] Wei Q, Sankar J, Narayan J. Structure and properties of novel functional diamond-like carbon coatings produced by laser ablation. Surf Coat Technol 2001;15(3):250–7. [8] Chiba K et al. Low-emissivity coating of amorphous diamond-like carbon/Agalloy multilayer on glass. Appl Surf Sci 2005;246:48–51. [9] Musilova V, Hanzelka P, Kralik T, Srnka A. Low temperature radiative properties of materials used in cryogenics. Cryogenics 2005;45(8):529–36. [10] Barron RF. Cryogenic heat transfer. Philadelphia: Taylor & Francis; 1999. [11] Kralik T, Hanzelka P, Musilova V, Srnka A. Black surfaces for cryogenic applications. In: The tenth cryogenics 2008 IIR international conference, proceedings, April 2008:CR08-07.
Contents lists available at ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Short communication
Thermal radiative properties of a DLC coating P. Hanzelka a,*, T. Kralik a, A. Maskova b, V. Musilova a, J. Vyskocil b a b
Institute of Scientific Instruments of the ASCR, v.v.i., Academy of Sciences of the Czech Republic, Královopolská 147, 612 64 Brno, Czech Republic HVM Plasma Ltd., Na Hutmance 2, 158 00 Praha 5, Czech Republic
a r t i c l e
i n f o
Article history: Received 22 January 2008 Received in revised form 18 March 2008 Accepted 27 March 2008
a b s t r a c t Thermal radiative properties of a DLC coating were measured in the range of cryogenic and room temperatures. Both the total hemispherical emissivity and absorptivity are significantly dependent on the radiation temperature and change from the value of 0.032 at 15 K to 0.65 at 300 K. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Thin films C. Heat transfer C. Radiant properties F. Cryostats
1. Introduction DLC (diamond-like-carbon) is used as low friction low wear coating in many industrial branches as automotive or textile industry, optical instruments, biomechanics or electronics [1–3]. Furthermore, applications of DLC layers in spacecrafts and satellites and also in cryogenics were discussed and tested [4–6]. Although some researchers investigated optical properties of specific DLC layers in infrared [7,8], the thermal radiative properties of DLC coatings at cryogenic temperatures were not methodically studied yet. As we propose to use a DLC layer as an antiseizing coating in one of our cryogenic apparatuses, we have measured thermal emissivity and absorptivity of the coating. Interpretation of the obtained data is not the aim of this contribution. 2. Method The method described in [9] was used for measurement. Radiative heat flow QR between two flat discs (radiator and absorber) each of 40 mm in diameter is evaluated from the temperature drop (TATB) on a calibrated thermal resistor (Fig. 1). The mutual emissivity (emissivity factor) eRA of the radiator at temperature TR and the absorber at temperature TA is then calculated from the known relation [10] for the heat transfer between parallel gray surfaces eRA ¼
QR r S ðT 4R T 4A Þ
* Corresponding author. Tel.: +420 541 514 265; fax: +420 541 514 402. E-mail address: [email protected] (P. Hanzelka). 0011-2275/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2008.03.021
ð1Þ
where r is the Stefan–Boltzmann constant and S represents the surface area of the discs. When absorptivity is measured, the sample is used as absorber and is irradiated from radiator at adjustable temperature TR. For the known temperature dependence of total hemispherical emissivity eR(TR) of the radiator, the sample total hemispherical absorptivity aA can be evaluated for individual temperatures TR from the relation [10]: 1=eRA ¼ 1=eR þ 1=aA 1
ð2Þ
For the sample emissivity investigation, the discs are swapped and the radiator emissivity eR is similarly found. As, in general, the absorptivity of a sample also depends on its temperature TA, this temperature should be kept at an approximately constant value during the measurement with variable TR. But the heat flux QR variations (over six orders of magnitude, from 0.11 lW to 350 mW in our measurements) induce a variable temperature drop on the thermal resistor (Fig. 1) and thus cause substantial changes in TA. An adjustable thermal resistor would be the best solution of this problem. Unfortunately, the realization of such resistor which retains high reproducibility and stability would be very difficult. As a compromise, two different hard-set resistors were calibrated and each of them was used in one range of temperature of our measurement. The accuracy of measured heat flux QR is given by the precision of the thermal resistor calibration which is of about 3%. The sensor of the temperature TR was calibrated with an absolute accuracy of 0.25 K. As a result, the mutual emissivity evaluated from the relation (1) is obtained with measurement error of about 6% at TR = 30 K and from 3% to 4% at TR P 100 K. In addition, because a part of radiation leaks out of the space between the radiator and absorber, the actual values of mutual emis-
456
P. Hanzelka et al. / Cryogenics 48 (2008) 455–457 160
80
4.2 K
Measuring chamber
Resultant α ε RA - Resistor 1 ε RA - Resistor 2
70
Radiator holder
140
60
120
50
100
ε RA, α [%]
Heater Radiator
TR
TA
Absorber
TA - Resistor 1 TA - Resistor 2
40
80
30
60
20
40
10
20 0
0
QR
TA [K]
Vacuum
0
50
100
150
200
250
300
TR [K] Fig. 3. Dependences on the radiator temperature TR. Left axis: Mutual emissivity eRA measured with two individual thermal resistors and the resultant absorptivity a of the DLC coating. Right axis: Temperatures TA of the DLC sample registered during the absorptivity measurement with individual thermal resistors.
Thermal resistor
TB
Fig. 1. Simplified scheme of the measuring chamber.
sivity are higher than the values evaluated from the relation (1). We estimated the correction factor not to be higher than 1.03. This systematic error was included into the bars illustrating actual errors of measured absorptivity of DLC layer in Fig. 3. 3. Samples A copper disc, 1 mm in thickness and 40 mm in diameter was used as the substrate for the DLC deposit. It was made of a sheet of technical quality and its surface was polished by fine steel wool. The measured dependence of the substrate absorptivity a on the radiator temperature TR is reported in Fig. 2. The DLC coating was deposited by combined PVD/PACVD method in industrial coating equipment HTC1500 (Hauzer Techno Coating). The first gradient layer Cr/WC:H was deposited by pulsed (40 kHz) magnetron sputtering to achieve good adhesion whereas the top DLC layer was prepared by pulsed plasma (100 kHz) from acetylene. Total coating thickness is of 3.3 lm, where the top DLC layer thickness is of 2.2 lm. Plastic hardness HUpl and indentation
5
100 90 80 70
3
60
α [%]
ε R , α A [%]
4
Epoxy layer 380 μm (emissivity and absorptivity)
50
Copper substrate (absorptivity)
40
2
30 1
20 10
0
0 0
50
100
150
200
250
300
TR [K] Fig. 2. Dependences on the radiation temperature TR. Left axis: total hemispherical emissivity eR and absorptivity aA of the epoxy layer. Right axis: total hemispherical absorptivity a of the copper substrate.
modulus EIT (measured according to ISO 14577) of the coating are 33 GPa and 175 GPa, respectively. The top DLC layer is amorphous hydrogenated carbon structure with hydrogen content of about 15 atomic percent. A copper disc covered by an epoxy layer of 380 lm in thickness was used as the opposite surface in all experiments. Emissivity and absorptivity of this sample is plotted in Fig. 2. We have measured the temperature dependence of mutual emissivity of two identical epoxy surfaces [11] and evaluated their emissivity and absorptivity from the relation (2) under the assumption that absorptivity equals emissivity at the same temperature TR of the radiator. This assumption was justified by the measurement we have published in [11]. 4. Results and discussion The measured values of the mutual emissivity eRA between the epoxy layer used as radiator and the investigated DLC layer as absorber are plotted in Fig. 3. Two different thermal resistors were used successively in two individual experiments. Good coincidence of both overlapped data series is evident. The first measurable value of QR was observed at the radiator temperature TR = 15 K. The resultant values of absorptivity a evaluated from Eq. (2) using the radiator emissivity eR, which is expressed in Fig. 2, do not substantially differ from the values of the mutual emissivity eRA (Fig. 3). This is caused by the fact that in the whole range of the temperatures TR, the epoxy layer is almost black in comparison with the DLC coating. The absorptivity a is strongly dependent on the radiator temperature TR. It increases over one order of magnitude in the range of TR between 15 K and 150 K. On the other hand, only insignificant dependence of a on the sample temperature TA may be expected, although we did not measure this dependence directly. This assumption is based on the fact that while the sample temperatures TA in both experiments with individual resistors are different (Fig. 3), the corresponding emissivities eRA are equal. The dependence of absorptivity a on the temperature TR may be approximated by the sigmoidal fit expression (3). This approximation is of precision of 5% in the temperature range of 20–300 K a ¼ 84=ð1 þ EXPððT R 47:7Þ=32:7ÞÞ þ 64
ð3Þ
The values of the DLC layer emissivity e were obtained by similar procedure when the DLC layer was used as radiator and the epoxy layer as absorber. It is evident from the ratio of both quan-
457
P. Hanzelka et al. / Cryogenics 48 (2008) 455–457 Table 1 The ratio of the measured emissivity e to the absorptivity a of the DLC coating calculated for different radiation temperatures TR TR (K) e/a
20 0.97
30 1.05
40 1.01
50 0.99
60 1.01
80 1.00
tities e/a expressed in the Table 1 for selected temperatures TR that the relative differences between both quantities are lower than 5% and are comparable with the error of the apparatus calibration. Acknowledgement This work was supported by the Academy of Sciences of the Czech Republic, project No. AV0 Z20650511. References [1] Robertson J. Diamond-like amorphous carbon. Mat Sci Eng R 2002;37:129–281. [2] Grill A. Electrical and optical properties of diamond-like carbon. Thin Solid Films 1999;355–356:189–93. [3] Mossner C et al. Characterization of diamond-like carbon by Raman spectroscopy, XPS and optical constants. Thin Solid Films 1998;317:397–401.
100 1.02
120 0.99
140 0.99
180 0.99
200 0.99
240 0.99
260 0.99
300 1.00
[4] Voevodin AA, Zabinski JS. Nanocomposite and nanostructured tribological materials for space applications. Compos Sci Technol 2005;65(5):741–8. [5] Gradt T, Borner H, Schneider T. Low temperature tribometers and the behaviour of ADLC coatings in cryogenic environment. Tribol Int 2001;34(4):225–30. [6] Ostrovskaya YL et al. Friction and wear behaviour of hard and superhard coatings at cryogenic temperatures. Tribol Int 2001;34(4):255–63. [7] Wei Q, Sankar J, Narayan J. Structure and properties of novel functional diamond-like carbon coatings produced by laser ablation. Surf Coat Technol 2001;15(3):250–7. [8] Chiba K et al. Low-emissivity coating of amorphous diamond-like carbon/Agalloy multilayer on glass. Appl Surf Sci 2005;246:48–51. [9] Musilova V, Hanzelka P, Kralik T, Srnka A. Low temperature radiative properties of materials used in cryogenics. Cryogenics 2005;45(8):529–36. [10] Barron RF. Cryogenic heat transfer. Philadelphia: Taylor & Francis; 1999. [11] Kralik T, Hanzelka P, Musilova V, Srnka A. Black surfaces for cryogenic applications. In: The tenth cryogenics 2008 IIR international conference, proceedings, April 2008:CR08-07.