Thermal Measurements of Substrate during ... - SLIDEBLAST.COM
Thermal Measurements of Substrate during Spray Processes Oliver Brandt Swiss Federal Laboratories for Materials Testing and Research, Switzerland Michael Wandelt Federal Armed Forces University, Hamburg, Germany
Abstract Most research work about thermal spraying involves the special process itself, the spray powder materials and the coatings. The major aim is to clarify the basic relations between different spray parameters and the coating properties, such as bond strength, porosity, wear resistance and residual stress. This paper presents temperature measurements of the substrate while a spray stream is directed at the surface. The substrate temperature was measured with an infrared camera at the back side of a sample. The camera allows a measuring frequency of 4 Hz using 140 by 140 pixel view field. Basic studies were carried out with a High Velocity Oxygen Fuel (HVOF)- System. Typical HVOF parameters were compared while spraying different tungsten carbide alloys on aluminum substrates. Comparative studies with plasma processes were performed. These results should help to calculate the temperature and thermal expansion of real parts with various structures before the spray process is used to apply a coating.
Theoretical Whenever a thermal spray stream strikes the surface of a part or sample, the generation of heat in the substrate is very local and transient. Local heat distributions in the substrate can generate thermal strain and warpings while building up a coating. The entire spray stream represents a source of heat generated by the hot spray particles and the flame stream. The heat-up of a sample by heat transmission is shown schematically in Figure 1. The heat flow of the spray particles and the hot gas stream is markted as QP. The heat flow of the stray stream (hot gas arround the basic spray stream) is marked as QR. The total heat extent in the substrate is influenced by several physical properties /1,2/ as listed in Table 1.
Fig. 1 Heat up of a sample during thermal spraying Table 1. Properties of the substrate for calculation of heat emission and distribution in the substrate Material thermal conductivity heat transmission coefficient specific heat capacity density thickness temperature of spray stream environmental temperature
λ α c ρ d t t
AlMg 3 130-170 (Wm-1K-1) 5-10 (Wm-2K-1) 0.9-0.96 (kJkg-1K-1) 2.66 (g mm-3) 2 mm (K) (K)
Experimental Procedure The heat flow and heat distribution of the substrate was measured with an infrared camera, while a thermal spray stream was directed at the surface. The advantage of this measuring device is a quick and contactless determination of complex heat distributions. In this study a camera type AGEMA THV 880 LWB /3/ was used with a measuring
Autor: O. Brandt and M. Wandelt Publiziert in: 9th National Thermal Spray Conference - Thermal Spray: Practical Solutions for Engineering Problems, Cincinnati, Ohio, 1996, ISBN/ISSN: 0-87170-583-4 Seite: 799-802
frequency of 4 Hz. The total resolution limit of the view field is 140 by 140 pixels with a cross-section of each pixel of 0.6 mm by 0.6 mm. A total cross section of 80 mm by 80 mm was engaged. The experimental set-up is shown in Figure 2 in principle. The shield in front of the sample can be used in an opened or closed position to start and interrupt the heat flow of the spray stream. Studies in this work were determined with a measuring time of 5 s. The measured heat distributions were recorded and evaluated using a standard personal computer.
Fig. 3: Example of typical picture series, colors are printed in gray scale, WC 17Co powder, agglomerated and sintered.
Calibration and Error Estimation Fig. 2: Experimental set-up used to measure temperature fields and heat distributions during spraying The heat flow of a single reference volume (pixel) element is according to the following equation /4/: t
Q = ∫ VρcdT (1) t0
(Note: use substrate properties, V = volume of pixel) The total heat flow arises from a numerical integration of all pixels of one test run. Examples of a typical test run are shown in Figure 3 spraying a WC 17Co powder using a JET KOTE II /5/ with base line conditions. Different temperatures are printed using gray scale. The real infrared picture provides the different temperatures by colors. The conversion from a single color of a pixel to its corresponding temperature was carried out with a commercial computer program equipped with the infrared camera.
Measurement of temperatures using infrared techniques are influenced by the emissivity of the object to be measured. The emissivity of the surface itself is influenced by the temperature, chemisty and roughness (topography). Before doing measurements the camera needs to be calibrated regarding the non-constant emissivity of the surface. An easy calibration could be done by measuring the temperature of the surface with the camera and an additional thermal couple while the sample heats up. The calibration was carried out in a temperature range from 18° to 270°C in steps of 20°C using a warm air heater as heat source. Measurements of temperatures, described as shown in Figure 2, could be carried out with an absolute exactness of ± 0.5 K. To describe the influence of different spray parameters on the heat flow in the sample, a few simplifications are necessary: (i) the temperature on the side to be sprayed is the same as on the side to be measured, (ii) the measured temperature would be engaged in a real part and (iii) the thickness variation of real part does not influence the heat distribution by thermal conduction. These simplifications are allowed because of the constant boundary conditions for all test runs. The presented datas are average values of 5 tests of each parameter set.
Some Results The first measurements were performed using a Jet Kote II System® for spraying. All test were sprayed with base line conditions using a oxygen/propane flame and commercial agglomerated and sintered powders.
Autor: O. Brandt and M. Wandelt Publiziert in: 9th National Thermal Spray Conference - Thermal Spray: Practical Solutions for Engineering Problems, Cincinnati, Ohio, 1996, ISBN/ISSN: 0-87170-583-4 Seite: 799-802
25.00
20.00 volume fraction (%)
A heat flow of 531 W was measured while the spray stream was directed on the surface without powder feed (without shield). As soon as powder is injected in the spray stream, the heat flow in the sample is influenced. A minimum heat flow of about 410 W was measured with a powder feed rate in the range of 33 - 35 g/min. Both increasing and decreasing powder feed rate yields an increasing heat flow. While the spray stream is focused through the opening in the shield, the measured heat flow decreases by approximately 7-11% as compared to spraying without shield, as shown in Figure 4.
-45 +5µm -45 +10µm -45 +22.5µm
15.00
10.00
5.00
0.00 0.00
20.00
40.00 60.00 80.00 spray powder diameter (µm)
100.00
120.00
Fig. 6: Particle size distribution of three different sieve fractions of WC 17Co powder, agglomerated and sintered, measured with Laser diffraction. Certain applications require coatings of different chemical compositions. Carbide coatings could be varied by concentration or type of the binding matrix. These changes influence the heat flow as shown in Figure 7.
Fig. 4: Heat flow as a function of spray powder feed rate, base line flame conditions, WC 17Co powder, agglomerated and sintered, grain size range -45 +22.5 µm. Spray powders are available in different particle size ranges. The sieve fraction influences the particle size distribution. The measured heat flow is shown in Figure 5 as a function of the particle size range . These tests were sprayed with 3 different sieve fractions of a commercial WC 17Co. Laser diffraction measurements were performed to measure the particle size distribution, as shown in Figure 6. This studies were carried out while spraying with the same powder feed rate.
Fig. 7: Heat flow as a function of the WC concentration in the spray powder, agglomerated and sintered powders, -45 +22.5µm
Discussion and Assessment
µ
µ
µ
Fig. 5: Heat flow as a function of spray powder grain size range, WC 17Co powder, agglomerated sintered.
While constant flame conditions are used to spray with a HVOF system, the spray powder feed rate influences the heat-up of the sample or a part. A minimum heat flow was found in the region of the maximum deposit efficiency. Spraying with those powder feed rates, a maximum amount of heat is transferred by melting the particles in the spray stream, thus resulting in a minimum heat-up of the sample. The impact heat energy of a spray particle increases with an increasing particle diameter while heated up to a similar temperature in the spray stream. Therefore, in these experiments, the resulting heat-up of a sample is maximized with a particle size range of -45 +22.5 µm because of the increased amount of larger particles.
Autor: O. Brandt and M. Wandelt Publiziert in: 9th National Thermal Spray Conference - Thermal Spray: Practical Solutions for Engineering Problems, Cincinnati, Ohio, 1996, ISBN/ISSN: 0-87170-583-4 Seite: 799-802
The measured heat flow yields a minimum while spraying a WC 12Co powder. Additional measurements should clarify the relation between spray powder composition and the heatup of the sample in the spray stream.
sprayed coating as a machine part should be possible in the future. Additional measurements with different parameters are in preparation, as listed in table 2.
Outlook
Table 2: Further parameters to be investigated.
The heat-up rate of parts to be coated is not only affected by the spraying parameters. The properties and dimensions of the substrate are also an important factor influencing the heat-up time and maximum temperature. Correlations of heat flow and the substrate properties (shape, dimension, physical properties) are shown in Figure 8 in principle. In both cases the shaft has the same diameter d and length l, so the surface to be coated is equal. The smaller volume of the drilled shaft results in higher substrate temperature while the same spray parameters are used to apply the coating as for the shaft without drilled hole. It is obvious that the temperature of the substrate is also influenced by the thermal conductivity of the material.
Parameters VPS chamber conditions plasma power plasma gas composition spray powder variations substrate material
HVOF fuel/oxygen flow ratio kind of fuel cooling conditions pass crossing speed clamping conditions
Conclusions The heat up of a sample or part is strongly influenced by the spray parameters. A minimized heat-up was found while spraying with maximum deposition efficiency and a narrow powder sieve classification. Further studies should help to understand the complex subject of heating up of parts while the thermal spray process acts as heat source.
References
Fig. 8: Heat-up of a part during the coating process as a function of substrate volume. Measurements of temperature fields or the heat flow during spraying should help to calculate the possible temperature of the substrate before applying the coating. Precalculations of the warping of the part and the residual stress between coating and substrate should become possible. These informations are important if a connection of a cooling nozzle is impossible, for example while spraying the internal side of a drilling of a large part. As well while using spray processes in a chamber like the VPS process, informations about the heat-up of the substrate are useful to select optimized parameters for an application. Further calculations should help to understand the thermal spray process as heat source regarding the substrate. Calculations of stability of constructions with the thermal
1
Radaj, D., Wärmewirkung des Schweissens Springer Publisher Berlin, Heidelberg New York (1988), (in german)
2
Grigull, U., and H., Sander, Wärmeleitung Springer Publisher Berlin Heidelberg New York (1979), (in german)
3
N.N., AGEMA Thermovision Porating Manual, Dandeeryd, Sweden, 1988
4
Baehr, H.D., Thermodynamik Springer publisher Berlin Heidelberg New York (1989), (in german)
5
Jet Kote is registrated trade name of Stoody Deloro Stellite, Inc.
Autor: O. Brandt and M. Wandelt Publiziert in: 9th National Thermal Spray Conference - Thermal Spray: Practical Solutions for Engineering Problems, Cincinnati, Ohio, 1996, ISBN/ISSN: 0-87170-583-4 Seite: 799-802
Abstract Most research work about thermal spraying involves the special process itself, the spray powder materials and the coatings. The major aim is to clarify the basic relations between different spray parameters and the coating properties, such as bond strength, porosity, wear resistance and residual stress. This paper presents temperature measurements of the substrate while a spray stream is directed at the surface. The substrate temperature was measured with an infrared camera at the back side of a sample. The camera allows a measuring frequency of 4 Hz using 140 by 140 pixel view field. Basic studies were carried out with a High Velocity Oxygen Fuel (HVOF)- System. Typical HVOF parameters were compared while spraying different tungsten carbide alloys on aluminum substrates. Comparative studies with plasma processes were performed. These results should help to calculate the temperature and thermal expansion of real parts with various structures before the spray process is used to apply a coating.
Theoretical Whenever a thermal spray stream strikes the surface of a part or sample, the generation of heat in the substrate is very local and transient. Local heat distributions in the substrate can generate thermal strain and warpings while building up a coating. The entire spray stream represents a source of heat generated by the hot spray particles and the flame stream. The heat-up of a sample by heat transmission is shown schematically in Figure 1. The heat flow of the spray particles and the hot gas stream is markted as QP. The heat flow of the stray stream (hot gas arround the basic spray stream) is marked as QR. The total heat extent in the substrate is influenced by several physical properties /1,2/ as listed in Table 1.
Fig. 1 Heat up of a sample during thermal spraying Table 1. Properties of the substrate for calculation of heat emission and distribution in the substrate Material thermal conductivity heat transmission coefficient specific heat capacity density thickness temperature of spray stream environmental temperature
λ α c ρ d t t
AlMg 3 130-170 (Wm-1K-1) 5-10 (Wm-2K-1) 0.9-0.96 (kJkg-1K-1) 2.66 (g mm-3) 2 mm (K) (K)
Experimental Procedure The heat flow and heat distribution of the substrate was measured with an infrared camera, while a thermal spray stream was directed at the surface. The advantage of this measuring device is a quick and contactless determination of complex heat distributions. In this study a camera type AGEMA THV 880 LWB /3/ was used with a measuring
Autor: O. Brandt and M. Wandelt Publiziert in: 9th National Thermal Spray Conference - Thermal Spray: Practical Solutions for Engineering Problems, Cincinnati, Ohio, 1996, ISBN/ISSN: 0-87170-583-4 Seite: 799-802
frequency of 4 Hz. The total resolution limit of the view field is 140 by 140 pixels with a cross-section of each pixel of 0.6 mm by 0.6 mm. A total cross section of 80 mm by 80 mm was engaged. The experimental set-up is shown in Figure 2 in principle. The shield in front of the sample can be used in an opened or closed position to start and interrupt the heat flow of the spray stream. Studies in this work were determined with a measuring time of 5 s. The measured heat distributions were recorded and evaluated using a standard personal computer.
Fig. 3: Example of typical picture series, colors are printed in gray scale, WC 17Co powder, agglomerated and sintered.
Calibration and Error Estimation Fig. 2: Experimental set-up used to measure temperature fields and heat distributions during spraying The heat flow of a single reference volume (pixel) element is according to the following equation /4/: t
Q = ∫ VρcdT (1) t0
(Note: use substrate properties, V = volume of pixel) The total heat flow arises from a numerical integration of all pixels of one test run. Examples of a typical test run are shown in Figure 3 spraying a WC 17Co powder using a JET KOTE II /5/ with base line conditions. Different temperatures are printed using gray scale. The real infrared picture provides the different temperatures by colors. The conversion from a single color of a pixel to its corresponding temperature was carried out with a commercial computer program equipped with the infrared camera.
Measurement of temperatures using infrared techniques are influenced by the emissivity of the object to be measured. The emissivity of the surface itself is influenced by the temperature, chemisty and roughness (topography). Before doing measurements the camera needs to be calibrated regarding the non-constant emissivity of the surface. An easy calibration could be done by measuring the temperature of the surface with the camera and an additional thermal couple while the sample heats up. The calibration was carried out in a temperature range from 18° to 270°C in steps of 20°C using a warm air heater as heat source. Measurements of temperatures, described as shown in Figure 2, could be carried out with an absolute exactness of ± 0.5 K. To describe the influence of different spray parameters on the heat flow in the sample, a few simplifications are necessary: (i) the temperature on the side to be sprayed is the same as on the side to be measured, (ii) the measured temperature would be engaged in a real part and (iii) the thickness variation of real part does not influence the heat distribution by thermal conduction. These simplifications are allowed because of the constant boundary conditions for all test runs. The presented datas are average values of 5 tests of each parameter set.
Some Results The first measurements were performed using a Jet Kote II System® for spraying. All test were sprayed with base line conditions using a oxygen/propane flame and commercial agglomerated and sintered powders.
Autor: O. Brandt and M. Wandelt Publiziert in: 9th National Thermal Spray Conference - Thermal Spray: Practical Solutions for Engineering Problems, Cincinnati, Ohio, 1996, ISBN/ISSN: 0-87170-583-4 Seite: 799-802
25.00
20.00 volume fraction (%)
A heat flow of 531 W was measured while the spray stream was directed on the surface without powder feed (without shield). As soon as powder is injected in the spray stream, the heat flow in the sample is influenced. A minimum heat flow of about 410 W was measured with a powder feed rate in the range of 33 - 35 g/min. Both increasing and decreasing powder feed rate yields an increasing heat flow. While the spray stream is focused through the opening in the shield, the measured heat flow decreases by approximately 7-11% as compared to spraying without shield, as shown in Figure 4.
-45 +5µm -45 +10µm -45 +22.5µm
15.00
10.00
5.00
0.00 0.00
20.00
40.00 60.00 80.00 spray powder diameter (µm)
100.00
120.00
Fig. 6: Particle size distribution of three different sieve fractions of WC 17Co powder, agglomerated and sintered, measured with Laser diffraction. Certain applications require coatings of different chemical compositions. Carbide coatings could be varied by concentration or type of the binding matrix. These changes influence the heat flow as shown in Figure 7.
Fig. 4: Heat flow as a function of spray powder feed rate, base line flame conditions, WC 17Co powder, agglomerated and sintered, grain size range -45 +22.5 µm. Spray powders are available in different particle size ranges. The sieve fraction influences the particle size distribution. The measured heat flow is shown in Figure 5 as a function of the particle size range . These tests were sprayed with 3 different sieve fractions of a commercial WC 17Co. Laser diffraction measurements were performed to measure the particle size distribution, as shown in Figure 6. This studies were carried out while spraying with the same powder feed rate.
Fig. 7: Heat flow as a function of the WC concentration in the spray powder, agglomerated and sintered powders, -45 +22.5µm
Discussion and Assessment
µ
µ
µ
Fig. 5: Heat flow as a function of spray powder grain size range, WC 17Co powder, agglomerated sintered.
While constant flame conditions are used to spray with a HVOF system, the spray powder feed rate influences the heat-up of the sample or a part. A minimum heat flow was found in the region of the maximum deposit efficiency. Spraying with those powder feed rates, a maximum amount of heat is transferred by melting the particles in the spray stream, thus resulting in a minimum heat-up of the sample. The impact heat energy of a spray particle increases with an increasing particle diameter while heated up to a similar temperature in the spray stream. Therefore, in these experiments, the resulting heat-up of a sample is maximized with a particle size range of -45 +22.5 µm because of the increased amount of larger particles.
Autor: O. Brandt and M. Wandelt Publiziert in: 9th National Thermal Spray Conference - Thermal Spray: Practical Solutions for Engineering Problems, Cincinnati, Ohio, 1996, ISBN/ISSN: 0-87170-583-4 Seite: 799-802
The measured heat flow yields a minimum while spraying a WC 12Co powder. Additional measurements should clarify the relation between spray powder composition and the heatup of the sample in the spray stream.
sprayed coating as a machine part should be possible in the future. Additional measurements with different parameters are in preparation, as listed in table 2.
Outlook
Table 2: Further parameters to be investigated.
The heat-up rate of parts to be coated is not only affected by the spraying parameters. The properties and dimensions of the substrate are also an important factor influencing the heat-up time and maximum temperature. Correlations of heat flow and the substrate properties (shape, dimension, physical properties) are shown in Figure 8 in principle. In both cases the shaft has the same diameter d and length l, so the surface to be coated is equal. The smaller volume of the drilled shaft results in higher substrate temperature while the same spray parameters are used to apply the coating as for the shaft without drilled hole. It is obvious that the temperature of the substrate is also influenced by the thermal conductivity of the material.
Parameters VPS chamber conditions plasma power plasma gas composition spray powder variations substrate material
HVOF fuel/oxygen flow ratio kind of fuel cooling conditions pass crossing speed clamping conditions
Conclusions The heat up of a sample or part is strongly influenced by the spray parameters. A minimized heat-up was found while spraying with maximum deposition efficiency and a narrow powder sieve classification. Further studies should help to understand the complex subject of heating up of parts while the thermal spray process acts as heat source.
References
Fig. 8: Heat-up of a part during the coating process as a function of substrate volume. Measurements of temperature fields or the heat flow during spraying should help to calculate the possible temperature of the substrate before applying the coating. Precalculations of the warping of the part and the residual stress between coating and substrate should become possible. These informations are important if a connection of a cooling nozzle is impossible, for example while spraying the internal side of a drilling of a large part. As well while using spray processes in a chamber like the VPS process, informations about the heat-up of the substrate are useful to select optimized parameters for an application. Further calculations should help to understand the thermal spray process as heat source regarding the substrate. Calculations of stability of constructions with the thermal
1
Radaj, D., Wärmewirkung des Schweissens Springer Publisher Berlin, Heidelberg New York (1988), (in german)
2
Grigull, U., and H., Sander, Wärmeleitung Springer Publisher Berlin Heidelberg New York (1979), (in german)
3
N.N., AGEMA Thermovision Porating Manual, Dandeeryd, Sweden, 1988
4
Baehr, H.D., Thermodynamik Springer publisher Berlin Heidelberg New York (1989), (in german)
5
Jet Kote is registrated trade name of Stoody Deloro Stellite, Inc.
Autor: O. Brandt and M. Wandelt Publiziert in: 9th National Thermal Spray Conference - Thermal Spray: Practical Solutions for Engineering Problems, Cincinnati, Ohio, 1996, ISBN/ISSN: 0-87170-583-4 Seite: 799-802
