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Systematic Investigation of the Temperature Field in Atmospheric 
Plasma Processing (APP) 
M.Stummer1,*, P.Stögmüller2, T. Eichinger1, N.Enzinger1 
1 Graz University of Technology, Austria 
2 Inocon Technologie GmbH, Austria 
Keywords: Atmospheric Plasma Processing (APP), plasma activation, temperature field 
Abstract. The application of functional layers has increased constantly over the last decades [1]. 
Coating processes like plasma spraying allow efficient processing of metal or oxide particles, and 
have already found their application in various sectors of industries. Ultra fine cleaning, surface 
activation or surface modification with the plasma arc are currently also under investigation. In the 
scope of this work the influence of four different main parameters - current, working distance, feed 
speed and gas flow - on the arc temperature field was investigated. Due to the complex and different 
interactions of these parameters on the temperature field, the temperature distribution in steel and 
aluminium sheets was systematically examined. Furthermore, the relationship between the 
measured surface temperatures and the wettability of the substrates is being discussed.  
To generate the required data, two different experimental setups were used. First, the spatial heat 
distribution of the plasma arc was measured with a special arrangement of thermocouples. Second, 
the temperature fields during the plasma surface treatment of DC01 and Al 6082 substrates was 
measured. In addition to measurements with NiCr-Ni thermocouples an investigation with an IR-
Camera was performed. After the plasma surface treatment, the resulting wettability was 
determined by contact angle measurement.  
The obtained results and especially the measured temperature fields will be used in a next step to 
validate numerical simulations with SYSWELD and ANSYS CFX, which will be used for process 
optimization. 
1 Introduction 
 Besides chemical or mechanical surface treatments, the application of atmospheric plasma 
processing (APP) is a popular pre-treatment in coating and joining technology. During APP the 
substrate is put under a thermal cycle which is performed by a plasma torch. After the pre-treatment 
is completed, the subsequent coating process starts. APP leads to an ultra-fine cleaned surface, 
which significantly improves the layer quality after coating [2]. The pre-treatment reduces pores 
and improves the wettability as well as the morphology of the final layer. To take advantage of 
these benefits, it is important to gain a better understanding of the process parameters and their 
interactions. Besides the local heat input of the plasma 
torch, the surface condition after APP is highly time-
dependent [2]. Heat input and time are crucial factors 
for process control, especially in coating applications. 
To obtain the measurement data, the time dependence 
as well as the interaction of the parameters, variable 
parameters had to be defined. These parameters 
comprise electric current (I), working distance (a), 
feed speed (v) and plasma gas flow (argon flow rate).  
 
Figure 1 shows a schematic sketch of the temperature investigation sequence. The experimental 
investigation is set up in two main sequences. First, the spatial heat distribution of the plasma arc is 
measured. Second, the temperature fields of two different substrate materials during APP are 
investigated. Based on application-oriented coatings for automotive industries the focus of the 
parameter study lies on steel substrates. Additionally, the consequence of APP of steel sheets 
Figure 1 - Sequence of the temperature investigation 
Materials Science Forum
ISSN: 1662-9760, Vol. 879, pp 1870-1875
doi:10.4028/www.scientific.net/MSF.879.1870
© 2017 Trans Tech Publications, Switzerland
concerning changes in wettability is being discussed. The spatial heat distribution, the temperature 
fields of the substrates and the surface energies are important data serving as boundary conditions in 
numerical simulations.  
2 Spatial temperature distribution of the plasma arc 
Inocon Technologie GmbH performs APP with their 
patented plasma torch. To investigate the temperature 
distribution, a spatial arrangement of thermocouples was 
used. A solid block of steel was milled and thermocouples 
were positioned in ceramic cylinders. According to Figure 
2, five positions (Pos1 to Pos5) were chosen to measure 
the transient temperatures at these locations. During all 
measurements two main plasma torch parameters - electric 
current and argon gas flow - were kept constant while two 
parameters were varied systematically. An initial path of 
activation was created with these parameters.  
 
 
2.1 Experimental procedure and results 
The initial path of activation was set on the front 
surface of the ceramic cylinders and in a working distance 
(a) of 25 mm. The tips of the thermocouples were set at 
the face of the ceramic cylinders. To get the spatial results, 
the paths were defined by an offset value (o), which was 
increased in direction of the y-axis (Figure 2), as well as 
by a stepwise reduction of the working distance (a).  
 
 
Table 1 shows the value ranges of the 
variables. Figure 3 shows the results of the 
measurements. 
 
 
Table 1 - Parameter range of the spatial heat distribution 
tests 
 min max 
working distance (mm) 12 25 
offset (mm) 0 0.3 
 
 
 
 
3 Systematic investigation of the temperature field on steel and aluminium substrates 
Based on the previous results the second experiment took place. Therefore, two different 
substrate materials were used. 
Figure 2 - Experimental setup of the spatial 
temperature distribution measurement with 
thermocouples 
Figure 3 - Spatial temperature distribution in 
dependence of working distance and offset (I = 100 A,                             
argon flow rate = 10 l/min) 
Materials Science Forum Vol. 879 1871
3.1 Substrate materials 
The materials used are steel (DC01) and aluminium (Al 6082) sheets with a dimension of 
200x100x1.5 mm. The surface condition of the steel substrates is “as received”. These types of 
material are commonly used in automotive and ship building industries. Due to difficulties while 
applying the thermocouples concerning the natural Al2O3 passive layer, the aluminium sheets were 
pickled. Table 2 shows the nominal chemical composition of the substrate materials. 
 
Table 2 - Chemical composition of the substrate materials 
mat. [3] C P S Mn Ti 
DC01 0.12 0.045 0.045 0.6 - 
 
mat. [4] Si Fe Cu Mn Mg Cr Zn other Ti Al 
Al 6082 0.7-1.3 0.5 0.1 0.4-1.2 0.6-1.2 0.25 0.2 0.15 0.1 balance 
 
 
3.2 Experimental procedure 
To measure the temperature fields 
experimentally, six K-thermocouples were applied on 
the top and two on the bottom surface of the substrates. 
The activation path was set in the centre of the substrate 
sample (Figure 4). 
 
The activation path represents the trajectory of the 
plasma torch. To ensure a stable plasma arc during APP 
and gather reproducible measurement signals, a 
stabilisation period of 5 seconds in a distance of 20 mm 
in front of the substrate was chosen.  
The main goals of the systematic investigation are the 
transient temperature, the interaction of the parameters, 
as well as the peak temperatures during APP. 
 
 
3.3 APP on steel substrates 
For the systematic investigation of temperature fields, a basic parameter set was defined 
(electric current = 100 A, working distance = 20 mm, feed speed = 50 mm/s and argon flow rate = 
10 l/min). In one experiment, three parameters were kept constant while one parameter was 
systematically varied. Table 3 shows the range of the variable parameters for the experiment. Figure 
5 shows the impact of the investigated parameters on the resulting peak temperature in the centre of 
the substrate specimen (Pos2). 
Table 3 – Parameter range of the experiment 
 min max 
electric current (A) 50 200 
working distance (mm) 15 30 
feed speed (mm/s) 25 100 
argon flow rate (l/min) 5 20 
Figure 4 - Thermocouple positions on substrates 
1872 THERMEC 2016
 
Figure 5 - Peak temperatures during plasma activation according to the parameters electric current, working distance, 
feed speed and gas flow 
3.4 APP on aluminium substrates 
 To measure the temperature fields of Al 6082, 
the same experimental setup was used. As mentioned 
before, the aluminium was pickled before applying the 
thermocouples. However, just three K-thermocouples 
were used; two thermocouples on the top (Pos1, Pos2) 
and one on the bottom surface (Pos3). The positions of 
the thermocouples are the same as on the steel substrate. 
Figure 6 shows the peak temperatures of all positions 
measured. 
3.5 Infrared Camera measurements 
Additionally to the thermocouples, the 
temperature was also measured using a FLIR 2102 
Infrared Camera (IRC). The used main parameters of the 
IRC measurement are shown in Table 4. These 
parameters are the emissivity (ε), the distance of IRC to 
the object, the humidity and the ambient temperature. To 
achieve the emissivity value of 0.92 according [5], the 
substrates were blackened with an acetylene burner. In 
Figure 7, the temperatures which were measured by 
using two different methods are compared. 
 
 
 
 
ε distance (m) hum. (%) amb. Temp. (°C) 
0.92 1 27 26 
Figure 7 - Comparison of IR and TC 
measurement during APP (I = 100 A, a = 20 
mm, v = 50 mm/s, argon flow rate  = 10 l/min) 
Figure 6 - Peak temperatures of the aluminium 
substrate (basic parameters) 
Table 4 – IRC parameters 
Materials Science Forum Vol. 879 1873
0
10
20
30
40
50
60
70
80
90
100
co
nt
ac
t a
ng
le
 (°
)
time (min)
deionized water diiodomethane
4 Wettability 
A common experimental procedure to examine 
the wettability, which is highly dependent on the 
previously measured temperature fields, is contact angle 
measurement. The optically measured contact angle 
reflects an APP - dependent surface condition. In order to 
be able to measure the surface energy by determining the 
respective contact angle, two different liquids (deionized 
water and diiodomethane) are necessary [6]. The tests 
were performed at the Institute of Chemistry of Materials 
(Graz University of Technology) on the Drop Shape 
Analyzer 100 under constant conditions. After an 
exposure time of 20 min in air, the time dependence of 
the APP was investigated. The APP parameters used are 
shown in Table 5. Within this data the time-dependent 
contact angle can be plotted (Figure 8) and compared with 
the as received surface condition (t>>). 
Table 5 - APP parameters for contact angle measurement 
electric current (A) working distance (mm) feed speed (mm/s) Argon (l/min) 
100 10 50 10 
5 Discussion 
5.1 Temperature field  
 The investigation of the plasma arc shows a temperature gradient in horizontal and vertical 
direction. An offset of 0.3 mm leads to an absolute temperature reduction of 675 K to 338 K at a = 
25 mm in horizontal direction. The same trend can be observed at all measurement points. In 
vertical direction, the temperature curve shows a bell-shaped behaviour. Due to the melting 
temperature of the thermocouples and a plasma arc length of 12mm, an investigation limit was 
reached with this measurement setup. The highest temperature (x̅ = 1250 °C) was measured at the 
tip of the plasma arc (a = 12 mm; o = 0 mm). 
 
5.2 Temperature field of steel and aluminium substrates 
 The systematic investigation reveal that Varying the electric current and the feed speed leads 
to the biggest changes regarding the peak temperature.  
It was shown that an electric current of 200 A leads to a unstable situation, resulting in a higher 
standard deviation for the measured temperatures. The reason has not been investigated yet.  
Changing the gas flow leads to the lowest temperature changes. The temperature levels on Al 6082 
substrate are significantly higher than on the steel substrates. A good agreement between IRC and 
thermocouple temperature measurements was observed. Therefore, future measurements can be 
performed with the less time consuming IRC setup. 
 
5.2 Wettability of steel substrates 
APP shows a significant improvement of the wetting behaviour compared to the initial 
condition as due to the APP the contact angle of e.g. deionized water could be decreased by 62%. 
The results also show a clear trend concerning the time-dependent behaviour of APP. Therefore, a 
systematic parameter study is under development. 
6 Conclusion 
The temperature distribution of the plasma arc was measured with thermocouples and IRC. The 
resulting temperature fields on steel (DC01) and aluminium (Al 6082) substrates were measured 
Figure 8 – Contact angle in order to discrete 
time steps 
1874 THERMEC 2016
successfully. Contact angles and surface energies could be measured. The contact angle 
measurements clearly showed that APP is a suitable possibility for surface preparation and 
modification, and brings a great benefit regarding wettability. Therefore, APP has a significant 
impact on the subsequent coating processes. The experiments performed showed clear trends and 
results, which can be used to validate numerical simulations for further process development. 
References 
[1] R. Heimann, Plasma Spray Coating. Weinheim: Wiley-VCH Verlag GmbH & Co. KGaA, 
2008. 
[2] S. Tang, O.-J. Kwon, N. Lu, and H.-S. Choi, “Surface characteristics of AISI 304L stainless 
steel after an atmospheric pressure plasma treatment,” Surface and Coatings Technology, vol. 
195. pp. 298–306, 2005. 
[3] S. Stahl, “data sheet - DC01,” 2015. [Online]. Available:  
http://www.bauforumstahl.de/upload/publications/kaltgewalzte_Feinbleche_aus_weichen_St
%C3%A4hlen_DC01.pdf. [Accessed: 07-Dec-2015]. 
[4] Haeuselmann, “data sheet - EN AW 6082,” 2015. [Online]. Available: 
http://www.haeuselmann.ch/webautor-data/23/EN6082.pdf. [Accessed: 07-Dec-2015]. 
[5] R. Marek and K. Nitsche, Praxis der Wärmeübertragung. Leipzig: Carl Hanser Verlag GmbH 
& Co.KG. 
[6] D. Y. Kwok and  a. W. Neumann, Contact angle measurement and contact angle 
interpretation, vol. 81, no. 3. 1999. 
 
Materials Science Forum Vol. 879 1875


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