Combining Thermal Spraying and Magnetron Sputtering for the Development of Ni/Ni-20Cr Thin Film Thermocouples for Plastic Flat Film Extrusion Processes

Article Combining Thermal Spraying and Magnetron Sputtering for the Development of Ni/Ni-20Cr Thin Film Thermocouples for Plastic Flat Film Extrusion Processes

Wolfgang Tillmann 1, David Kokalj 1,* , Dominic Stangier 1, Volker Schöppner 2 and Hatice Malatyali 2

1 Institute of Materials Engineering, TU Dortmund University, Leonhard-Euler-Straße 2, 44227 Dortmund, Germany; [email protected] (W.T.); [email protected] (D.S.) 2 Kunststoﬀtechnik Paderborn, Paderborn University, Warburger Straße 100, 33098 Paderborn, Germany; [email protected] (V.S.); [email protected] (H.M.) * Correspondence: [email protected]; Tel.: +49-(0)231-755-4866

Received: 26 August 2019; Accepted: 20 September 2019; Published: 24 September 2019

Abstract: In the digitalization of production, temperature determination is playing an increasingly important role. Thermal spraying and magnetron sputtering were combined for the development of Ni/Ni-20Cr thin film thermocouples for plastic flat film extrusion processes. On the thermally sprayed insulation layer, AlN and BCN thin films were deposited and analyzed regarding their structural properties and the interaction between the plastic melt and the surfaces using Ball-on-Disc experiments and High-Pressure Capillary Rheometer. A modular tool, containing the deposited Ni/Ni-20Cr thin film thermocouple, was developed and analyzed in a real flat film extrusion process. When calibrating the thin film thermocouple, an accurate temperature determination of the flowing melt was achieved. Industrial type K sensors were used as reference. In addition, PP foils were produced without affecting the surface quality by using thin film thermocouples.

Keywords: nickel–chromium; thin film thermocouples; physical vapor deposition; flat film extrusion; foil quality

1. Introduction Self-learning machines play a crucial role in today’s and future production. In the 4th Generation Industrial Revolution (Industry 4.0), machines are digitally upgraded and merged to a big data environment [1]. The overall goal is the improvement of product quality and production scheduling by building up and using prediction tools [1]. However, self-learning machines are still far from being implemented in many industries [1]. Data required for these prediction tools can only be generated if the machines are equipped with corresponding sensors, such as temperature-, wear-, distance-, or pressure-sensors, that work exactly. In many areas, the determination of the temperature, in particular, plays an important role. For this purpose, especially thin film thermocouples are suitable, since they reveal a lower mass and thus a faster reaction time in contrast to bulk thermocouples [2]. Multi-functional PVD coatings are successfully used for the temperature measurement in combustion engines [3] or gas turbine engines [4], for example. However, there is also a need for thin film thermocouples in production processes. Therefore, different thin film thermocouples are currently developed for several applications, such as welding [5] or metal [6–9] and polymer processing [10,11]. However, the thin film thermocouples must be adapted to the respective application fields. On the one hand, the materials of the thermocouples need to be selected according to the desired temperature range.

Coatings 2019, 9, 603; doi:10.3390/coatings9100603 www.mdpi.com/journal/coatings Coatings 2019, 9, 603 2 of 17

According to DIN EN 60584, eight different thermocouple pairs are available, whereas Fe–CuNi [12], Cu–CuNi [13], NiCr–Ni [6–9] and Pt-10%Rh/Pt [14] are the most reported thin film thermocouples deposited by means of PVD. Apart from the standardized thermocouples, TiC–TaC [15], Al–Au [11], Ni–Cu [12,16], Ni–Fe [12], Cu–Fe [12], Chromel–Alumel [5,12], and Pt–Pd [4] thin film thermocouples are synthesized. On the other hand, depending on the application, the thin film thermocouples need to be protected against wear or oxidation processes by means of a suitable tribological cover layer. To enhance the wear resistance of thin film sensors, protection layers consisting of Al2O3 [7,14,16,17], AlN [17], or TiN [8,18] are mainly used. For electrical isolation purposes, HfO2 [6] and Al2O3 [16] layers are utilized. In plastic processing, thermoplastics are extruded into semi-finished products that can be processed further (e.g., films). The plastic is plasticized and formed in the die [19]. For a good product quality, the melt needs to flow through the die at constant pressure, throughput and temperature [20]. In particular, an accurate measurement of the melt temperature is preferred and helps to increase the productivity. Temperature sensors are part of every extruder system, so that the time stability and the absolute level of the melt temperature can be monitored. Malfunctions can be detected quickly and the reaction time to undesired temperature changes increases. Nevertheless, state-of-the-art touching sensors cannot be used in the die itself because of the negative impact of the sensors on the product quality. The measurement tip protrudes at different depths into the melt and can disturb the homogeneity in the process itself. Thin film thermocouples offer an alternative measuring method to conventional sensors due to their flat construction and low net weight. In addition, they offer the possibility to measure the temperature even when embedded in the wall. Especially, in the case of embedded sensors, it is important to measure the temperature of the melt and not of the die wall. Therefore, thin film thermocouples are to be thermally insulated from the die walls. Thermal sprayed barrier coatings, such as ZrO2 and Al2O3, are particularly suitable for thermal insulation applications since they reveal pores and a higher thickness compared to sputtered coatings. Therefore, these coatings were applied by means of atmospheric plasma spraying and serve as substrate for the thin film thermocouples [19,20]. In particular, in film extrusion, it is important that the melt flow is not impaired to ensure a homogeneous film quality. Accordingly, the influence of different PVD top coatings on the wall friction and the adhesion of the melt is investigated. Since AlN coatings reveal high electrical resistance and high thermal conductivity, three AlN thin films deposited with different sputter parameters are selected. As an alternative, BCN is used as additional top coat. Finally, the application of the Ni/Ni-20Cr thin film thermocouple is proved in a real operation test und compared to a reference bulk thermocouple.

2. Materials and Methods

2.1. Deposition Process For the deposition of the nitride thin films and the Ni/NiCr thin film thermocouples, the industrial scale magnetron coating system CC800/9Custom (CemeCon AG, Würselen, Germany) was utilized. The AlN and BCN thin films were synthesized on AISI 1045 steel substrates (Ø40 mm), which were prior coated with Al2O3 using atmospheric plasma spraying. Detailed spray parameters of the Al2O3 coating are reported in [21]. The nitride thin films were deposited in medium frequency (MF) mode using one B4C (99.50% purity, Sindlhauser Materials GmbH, Kempten, Germany) and two Al (99.50% purity, CemeCon AG, Würselen, Germany) targets operating with 3150 W and 2 1875 W at a total × chamber pressure of 330 mPa. The frequency was set to 50 kHz with a duty cycle of 50%. Three AlN thin films and one BCN thin film were synthesized using different heating powers and Ar/N2 gas flow rates according to Table1. The heating power was changed to influence the crystallinity and the topography of the AlN thin films. Changing the Ar/N2 gas flow ratio results in a change of the chemical composition of the coating. Therefore, it was possible to analyze which properties influence the melt flow, respectively, the friction behavior. The two-fold rotation system was biased with –150 V in MF Coatings 2019, 9, 603 3 of 17 mode during all deposition processes. The deposition time of all coatings was adjusted to achieve comparable thin film thicknesses of approximately 1200 nm. The thickness was measured analyzing the cross section of the coatings by means of scanning electron microscopy.

Table 1. Deposition parameters of the AlN and BCN thin ﬁlms.

Coating AlN-1 AlN-2 AlN-3 BCN-1 Heating power [W] 5000 7000 3000 7000 Ar/N2 gas ﬂow ratio 6.0 6.0 5.1 26.0

In addition to the reference nitride layers, Ni/Ni-Cr thin film thermocouples were synthesized for the temperature measurement of the plastic melt in flat film extrusion processes. Therefore, a modular tool system was developed. This system consisted of a screw-in bush (AISI 4140), a ceramic insert, and a locking nut. The bush can be integrated into the die by means of a drilling with a thread (M24 1.5). The ceramic insert was coated with the thin film thermocouples. By means of a step × on the one side and a locking nut on the other side, the thin film thermocouple was positioned and fixed in the bush. The ceramic insert was composed of a steel core (AISI 1045), which was coated by atmospheric plasma spraying from all sides. For a sensitive temperature measurement, the insert had to be thermally insulated from the die walls, since the temperature of the on-rushing plastic melt was measured. The thermal insulation was achieved by a thermal sprayed multilayer design consisting of a 120-µm-thick NiCoCrAlTaY (AMDRY 997, Sulzer Metco, Pfäffikon, Switzerland) bond coat, a 530-µm-thick 7Y2O3-ZrO2 (AMPERIT 817.7, HC Starck, München, Germany), and a 680-µm-thick Al2O3 (Metco 6062, Sulzer Metco, Pfäffikon, Switzerland) ceramic coating. After the coating processes, the Al2O3 coating was entirely ground to a thickness of 350 µm, resulting in a total coating thickness of 1000 µm. After the grinding process, the outsides were polished in several steps to a roughness of Ra = 0.212 0.104 µm. ± The step of the ceramic insert was manufactured using a micro milling machine HSPC 2522 (Kern, Eschenlohe, Germany) equipped with a solid carbide end mill, type 910 Marlin, with a corner diameter of 1 mm (Zecha, Königsbach-Stein, Germany). The milled surfaces reveal a roughness of Ra = 0.811 ± 0.116 µm. After processing the final dimensions of the ceramic inserts, they were cleaned of contaminations by means of ethanol in an ultrasound bath. Thereafter, the Ni/Ni-Cr thin film thermocouple was deposited on the ceramic inserts using a steel masking system reported in [22] to synthesize the individual Ni and Ni-20Cr conducting paths. The conducting paths were sputtered in DC mode using a heating power of 1200 W and a Ar/Kr gas flow rate of 2.7 at a chamber pressure of 250 mPa. For the Ni conducting path, two Ni targets (99.99% purity, Sindlhauser Materials GmbH, Germany) were operated at 1875 W in DC mode, whereas for the Ni-20Cr path, a Cr target (99.95% purity, Sindlhauser Materials GmbH, Kempten, Germany) was operated at 1035 W, additionally. The deposition time of both conducting paths was adjusted to reveal similar thicknesses of 1050 35 nm. ± 2.2. Characterization The structural properties of the as-deposited nitride thin films were analyzed by means of X-Ray Diffraction (XRD) utilizing the diffractometer D8 Advance (Bruker, Madison, WI, USA). The thin films were investigated using Cr-radiation (2.29106 Å), whereas the current and voltage were set to 40 mA and 35 kV. To avoid an overlapping of the reflexes from the Al2O3 substrate, detector scans were performed, whereby the tube was set to an incident angle of 5◦. The topography and morphology of the AlN and BCN thin films were investigated using the Scanning Electron Microscope (SEM) JSM 7001F (Jeol, Tokyo, Japan). The roughness Ra of the Al2O3 substrate, as well as the thin films, was measured by the confocal white-light microscope µSurf (NanoFocus, Germany). The mechanical properties hardness (H) and Young’s modulus (E) were examined by means of a nanoindentation test using the nanoindenter G200 (Agilent Technology, Santa Clara, CA, USA). The evaluation was performed in a Coatings 2019, 9, 603 4 of 17 depth between 100 and 400 nm using the equations in accordance to Oliver and Pharr [23]. Thereby, a constant Poisson’s ratio of 0.25 was assumed. The adhesion of the nitride thin films under plastic deformation was tested by a Rockwell indentation test in accordance with DIN EN ISO 26443 [24], using a force of 60 kgf and a dwell time of 4 s. Tribological investigations were carried out using a high-temperature Ball-on-Disc tribometer (CSM-Instruments, Peseux, Switzerland). The velocity was set to 0.1 m/s, the normal force to 10 N, and the radius to 8 mm with a total distance of 20 m per experiment. Counter body balls (Ø6 mm) made of Polypropylene (PP) were used, since this material was used for the operation test of the thin film thermocouples. The tribological investigations were carried out at 85 ◦C (Vicat Softening Temperature) and 153 ◦C (Heat Deflection Temperature) to simulate a non-steady-state operating point of the plastic flat film extrusion process during heating up or cooling down. The adhesion of the PP to the coating surface was investigated by means of SEM. Additionally, the wear of the PP balls was analyzed using a confocal light microscope type InfiniteFocus (Alicona, Raaba/Graz, Austria). By utilizing a High-Pressure Capillary Rheometer (HPCR), the influence of the different coatings on the melt flow was investigated. The measurement was carried out according to DIN 54811. When measuring with the HPCR, first, a purely thermal melting takes place in a pre-chamber. In order to melt the material completely, the definition of a residence time is important. For the investigations with the Moplen HP420 M, a duration of 4 min was specified. After melting, a piston pushed the melt through the tempered capillary. The capillary was preheated to a certain temperature by two thermal sensors. The operating temperature for the measurements was set to 230 and 250 ◦C. The capillary was preheated to the desired temperature by means of two sensors. The piston speed and the pressure loss of the melt flow were recorded simultaneously by means of a pressure sensor. For the operational test, a wide-slot die was used, which allows to measure the melt temperature. The positioning of the thermocouples was important for the accuracy and reproducibility of measuring points. The thin film and industrial thermocouples were inserted near to the melt outlet from the die. The investigations were carried out using an extruder with a diameter of 45 mm and a typical three-zone screw for plasticizing (Battenfeld-Cincinnati Gmbh, Germany). The mass temperature measurements were carried out using a high density Polyethylen, while for the film production, a high flow polypropylene homopolymer (Moplen HP420M, lyondellbasell, Rotterdam, The Netherlands) was selected. Furthermore, for producing films, a chill roll unit was used (Collin GmbH, Maitenbeth, Germany). To compare the thin film sensors with conventional sensors of type K (Gneuss GmbH, Bad Oeynhausen, Germany), sensors with measuring tips (depth length) of 0, 0.5, and 1.5 mm were also utilized to measure the melt temperature and serve as reference. Besides the operational tests, the quality of the produced film was examined for evaluating the influence of the measuring tip of the industrial sensors. The optical properties of the film were determined by measuring the reflection and the transmission. An optical spectroscope HR2000+ (Ocean optics, EW Duiven, The Netherlands) enabled the measurement of the film quality. The measuring setup was in accordance with DIN 5036-3. Optical spectroscopy enabled a fast quality criterion for the evaluation of samples. In this method, a light beam with an intensity I0 irradiated the sample, while the exit intensity was recorded. The radiation sources covered a selected wavelength range, respectively, a certain spectrum. The intensity of the light depends on the wavelength λ and, accordingly, a function I(λ) of the light intensity [25]. The spectrum of the incident intensity was compared over the wavelength with the spectrum of the incident intensity. In the present study, the visual spectrum (VIS), which reveals wavelengths of approximately 380–780 nm, was used. When the light beam hits the matter, the light is distributed to different parts and the following output intensities are generated: The reflected IR, the transmitted IT, the scattered IS, and the absorbed intensity IA. These intensities form in sum I0 [25]. Coatings 2019, 9, 603 5 of 17

3. Results

3.1. AlN/BCN Thin Films Coatings 2019, 9, x FOR PEER REVIEW 5 of 17 3.1.1. Structure and Morphology XRD patterns of the three AlN and the one BCN coatings synthesized with different deposition parametersXRD patterns are presented of the threein Figure AlN 1. and For the the one AlN BCN thin coatings films, the synthesized positions of with the di reflectionsfferent deposition are 2θ ≈ parameters50.2°, 2θ ≈ 54.7°, are presented 2θ ≈ 57.7°, in 2 Figureθ ≈ 77.4°,1. For2θ ≈ the 94.8°, AlN and thin 2θ films, ≈ 106.6°, the positionswhich correspond of the reflections to the hexagonal are 2 θ ≈ AlN50.2◦ phase, 2θ 54.7(JCPDS◦, 2θ 25-1133).57.7◦, 2Theθ 77.4reflections◦, 2θ 94.8of the◦, andthree 2θ AlN106.6 thin◦ ,films which reveal correspond comparable to the intensities, hexagonal ≈ ≈ ≈ ≈ ≈ AlNshowing phase a similar (JCPDS crystallinity. 25-1133). The Nevertheless, reflections of slight the three differences AlN thin can films be detected. reveal comparable The AlN-3 intensities, thin film showsshowing the a similarhighest crystallinity.crystallinity, Nevertheless, which is favored slight by di fftheerences higher can nitrogen be detected. flow Theduring AlN-3 the thin coating film process.shows the At highest constant crystallinity, gas flow, whichthe AlN-2 is favored film shows by the a higher slightly nitrogen higher flow crystallinity during the compared coating process. to the AtAlN-1 constant film, which gas flow, is due the AlN-2to the higher film shows deposition a slightly temperature. higher crystallinity Compared compared to the AlN-3 to the thin AlN-1 film, film, the whichAlN-1 isund due AlN-2 to the thin higher films deposition reveal small temperature. additional Compared peaks at to 2 theθ ≈AlN-3 69.0° thinand film,2θ ≈ the106.4°, AlN-1 which und AlN-2correspond thin filmsto the reveal cubic smallAlN phase additional (JCPDS peaks 46-1200). at 2θ However,69.0◦ and none 2θ of106.4 the ◦AlN, which thin correspond films show toa ≈ ≈ highthe cubic crystallinity, AlN phase which (JCPDS is also 46-1200). shown However, by the broad none ofpeaks. the AlN Some thin of films these show also a overlap high crystallinity, with the whichcomparable is also structure shown by of the the broad substrate, peaks. which Some makes of these accurate also overlap evaluation with thedifficult. comparable The wurtzite-type structure of AlNthe substrate, was already which obtained makes accurateby Aissa evaluation et al. for a di dcfficult. and Thehigh wurtzite-type power impulse AlN magnetron was already sputtering obtained processby Aissa [26]. et al. The for aremaining dc and high reflections, power impulse marked magnetron with a black sputtering circle, belong process to [26 the]. Thecubic remaining (α) and trigonalreflections, (γ) markedAl2O3 phase with of a the black thermally circle, belong sprayed to substrate. the cubic (Concerningα) and trigonal the BCN (γ) Al coating,2O3 phase basically, of the onlythermally peaks sprayed of the substrate substrate. are Concerning visible, which the indicates BCN coating, a nearly basically, amorphous only state peaks of of the the thin substrate film. Only are visible,slight reflections which indicates at 2θ a≈ nearly52.8° and amorphous 2θ ≈ 57.4° state are of theobserved, thin film. which Only coin slightcide reflections with the at position 2θ 52.8 of◦ ≈ rhombohedraland 2θ 57.4◦ areB4C. observed, which coincide with the position of rhombohedral B C. ≈ 4

Figure 1. 1. XRDXRD patterns patterns of the of theAlN-1, AlN-1, AlN-2, AlN-2, AlN-3, AlN-3, and BCN and thin BCN films thin deposited ﬁlms deposited with various with variousparameters. parameters.

Additionally, the nitride thin filmsfilms were analyzed regarding topography and morphology by means of SEM, as shown in FigureFigure2 2.. ItIt cancan bebe seen seen that that the the thermally thermally sprayed sprayed Al Al22O33 coating, which was usedused asas substrate substrate for for the the thin thin films, films, reveals reveals a fine a topographyfine topography accompanied accompanied by a small by a crack small pattern crack patternin the polished in the polished state. The state. AlN-1 The and AlN-1 AlN-2 and thin AlN- films2 possessthin films a coarse possess cauliflower-like a coarse cauliflower-like topography. topography.By contrast, theBy contrast, AlN-3 thin the film AlN-3 shows thin afilm fine shows structured a fine topography.structured topograp Accordingly,hy. Accordingly, a high Ar/N a 2highgas Ar/Nflow ratio2 gas offlow 6.0 ratio and highof 6.0 heating and high powers heating of 5000powers and of 7000 5000 W, and as 7000 is the W, case as foris the the case AlN-1 for andthe AlN-1 AlN-2 and AlN-2 coatings, lead to a coarse topography. The structure of the topography can be correlated with the XRD pattern shown in Figure 1. The AlN-1 and AlN-2 thin films reveal a lower crystallinity compared to the AlN-3 thin film and a small amount of a second phase, which results in the coarser growth behavior. The BCN-1 coating reveals an even finer structure, which is comparable to the polished Al2O3 surface. The cross-sections of all thin films show a glass-like featureless structure.

Coatings 2019, 9, 603 6 of 17 coatings, lead to a coarse topography. The structure of the topography can be correlated with the XRD pattern shown in Figure1. The AlN-1 and AlN-2 thin films reveal a lower crystallinity compared to the AlN-3 thin film and a small amount of a second phase, which results in the coarser growth behavior. The BCN-1 coating reveals an even finer structure, which is comparable to the polished Al2O3 surface. The cross-sections of all thin films show a glass-like featureless structure. Coatings 2019, 9, x FOR PEER REVIEW 6 of 17

FigureFigure 2. 2.SEM SEM imagesimages ofof thethe topographytopography (top)(top) andand morphologymorphology(bottom) (bottom)of of thethe AlN-1,AlN-1, AlN-2,AlN-2, AlN-3,AlN-3,

andand BCN BCN thin thin ﬁlms, films, as as well well as as the the thermally thermally sprayed sprayed Al Al2O2O33coating. coating.

3.1.2.3.1.2. MechanicalMechanical PropertiesProperties UsingUsing nanoindentation, nanoindentation, the the mechanical mechanical properties—hardness properties—hardness and and Young’s Young’s modulus—of modulus—of the thin the films, as well as the Al O coating, were analyzed. As listed in Table2, the Al O coating reveals a thin films, as well as the2 Al3 2O3 coating, were analyzed. As listed in Table 2, the2 Al32O3 coating reveals hardness of 13.8 3.9 GPa and a Young’s modulus of 155.2 31.9 GPa. The AlN-1 and AlN-2 thin a hardness of 13.8± ± 3.9 GPa and a Young’s modulus of 155.2± ± 31.9 GPa. The AlN-1 and AlN-2 thin films reveal the lowest hardness of 6.0 1.6 GPa and 6.2 2.0 GPa, respectively. A higher hardness of films reveal the lowest hardness of 6.0± ± 1.6 GPa and 6.2± ± 2.0 GPa, respectively. A higher hardness 13.6 3.0 GPa is analyzed for the AlN-3 thin films, which reveals a finer structure compared to the of 13.6± ± 3.0 GPa is analyzed for the AlN-3 thin films, which reveals a finer structure compared to the AlN-1 and AlN-2 thin films. The highest hardness of 19.1 2.3 GPa is obtained for the BCN-1 coating. AlN-1 and AlN-2 thin films. The highest hardness of 19.1± ± 2.3 GPa is obtained for the BCN-1 coating. With increasing hardness, the Young’s modulus increases from 98.6 21.2 GPa (AlN-1) to 187.7 20.2 With increasing hardness, the Young’s modulus increases from 98.6± ± 21.2 GPa (AlN-1) to 187.7± ± 20.2 GPaGPa (BCN-1). (BCN-1). AlN AlN thin thin films films deposited deposited on stainless on stainl steeless substrates steel substrates by means by of magnetronmeans of sputteringmagnetron revealsputtering similar reveal hardness similar values hardness between values 6 and between 14 GPa 6 inand dependence 14 GPa in dependence on the bias-voltage on the asbias-voltage reported byas Choudharyreported by et Choudhary al. [27]. The et H al./E ratio,[27]. The which H/E demonstrates ratio, which the demonstrates resistance to the plastic resistance deformation, to plastic is betweendeformation, 0.048 is (AlN-2) between and 0.048 0.102 (AlN-2) (BCN-1). and Since0.102 (BCN a higher-1). valueSince a demonstrates higher value ademonstrates higher resistance a higher to plasticresistance deformation, to plastic thedeformation, BCN-1 and the AlN-3 BCN-1 thin and films, AlN-3 as well thin as films, the uncoated as well Alas2 theO3 coating,uncoated are Al the2O3 bestcoating, choices are asthe top best layer choices for the as applicationtop layer for in the plastic application melt extrusion in plastic dies. melt extrusion dies.

Table 2. Roughness and mechanical properties of the nitride thin ﬁlms, as well as the Al2O3 coating. Table 2. Roughness and mechanical properties of the nitride thin films, as well as the Al2O3 coating. Coating Al O AlN-1 AlN-2 AlN-3 BCN-1 Coating Al2 23O3 AlN-1 AlN-2 AlN-3 BCN-1 Roughness Ra [µm] 0.21 0.10 0.36 0.06 0.30 0.08 0.26 0.06 0.35 0.08 Roughness Ra [µm] 0.21± ± 0.10 0.36 ±± 0.06 0.30 ±± 0.08 0.26 ± ±0.06 0.35 ±± 0.08 Hardness [GPa] 13.8 3.9 6.0 1.6 6.2 2.0 13.6 3.0 19.1 2.3 Hardness [GPa] 13.8± ± 3.9 6.0 ±± 1.6 6.2 ±± 2.0 13.6 ±± 3.0 19.1 ±± 2.3 Young’s modulus [GPa] 155.2 31.9 98.6 21.2 130.4 28.5 167.5 24.1 187.7 20.2 Young’s modulus [GPa] 155.2± ± 31.9 98.6 ±± 21.2 130.4 ±± 28.5 167.5 ±± 24.1 187.7 ±± 20.2 H/E 0.089 0.061 0.048 0.081 0.102 H/E 0.089 0.061 0.048 0.081 0.102

Hence,Hence, thethe thinthin films films were were exposed exposed to to high high pressures pressures in in the the nozzles nozzles without without abrasive abrasive particles particles occurring.occurring. The The adhesion adhesion of of the the thin thin films films was was analyzed analyzed by means by means of a Rockwell of a Rockwell indentation indentation test, which test, representswhich represents the thin filmthe thin behavior film underbehavior plastic under deformation. plastic deformation. The corresponding The corresponding light microscope light imagesmicroscope of the images indents of are the shown indents in Figure are shown3. According in Figure to DIN 3. ENAccording ISO 26443, to DIN the adhesion EN ISO of26443, the thin the filmsadhesion can be of classifiedthe thin films into fourcan dibeff classifiederent classes. into Thereby,four different class 0classes. represents Thereby, a coating class without 0 represents cracks a andcoating local without delamination, cracks andand class local 3, delamination, full delamination and ofclass the coating3, full delamination after indentation. of the The coating thin films after AlN-2indentation. and BCN-1 The arethin classified films AlN-2 as class and 1, BCN-1 since they are revealclassified only as slight class cracks 1, since around they thereveal indent. only AlN-1 slight andcracks AlN-3 around correspond the indent. to class AlN-1 2, showing and AlN-3 local delamination.correspond to Accordingly,class 2, showing the adhesion local delamination. strength of theAccordingly, thin films decreasesthe adhesion with strength decreasing of heatingthe thin power, films decreases respectively, with deposition decreasing temperature. heating power, The respectively, deposition temperature. The lowest adhesion is shown by the AlN-3 thin film, deposited at the lowest temperature and with the highest nitrogen gas flow.

Coatings 2019, 9, 603 7 of 17 lowest adhesion is shown by the AlN-3 thin ﬁlm, deposited at the lowest temperature and with the highestCoatings 2019 nitrogen, 9, x FOR gas PEER ﬂow. REVIEW 7 of 17

FigureFigure 3.3. LightLight microscope microscope images images of of the the Rockwell Rockwell indent indentss of ofthe the AlN-1, AlN-1, AlN-2, AlN-2, AlN-3, AlN-3, and andBCN BCN thin thinfilms. ﬁlms.

3.1.3.3.1.3. Tribological PropertiesProperties

Polymer processing mainly TiN, CrN, TiAlN, Cr1 xAlxN, CrAlON, and DLC coatings deposited by Polymer processing mainly TiN, CrN, TiAlN, Cr− 1−xAlxN, CrAlON, and DLC coatings deposited meansby means of PVD of PVD were were investigated investigated and used and [28used–30 [28–30].]. It was shownIt was thatshown the that addition the addition of Al to CrN of Al increases to CrN theincreases contact the angle contact between angle the surfacebetween and the Polycarbonate surface and melt, Polycarbonate resulting in amelt, lower resulting adhesion. in Therefore, a lower theadhesion. AlN and Therefore, BCN thin the films AlN were and investigated BCN thin films regarding were theirinvestigated tribological regarding properties. their Tribologicaltribological experimentsproperties. Tribological were conducted experiments at 85 and 153were◦C usingconduc PPted counter at 85 balls,and whereby153 °C using the temperatures PP counter are balls, the Vicatwhereby Softening the temperatures Temperature are and the the Vicat Heat Softening Deflection Te Temperature.mperature and These the Heat temperatures Deflection were Temperature. selected to simulateThese temperatures the adhesion were behavior selected of the to PPsimulate melt to the the ad diffhesionerent topbehavior layer-coated of the thinPP melt film to thermocouples the different duringtop layer-coated heating up thin or film cooling thermocouples down processes during of theheating thin up film or extrusion cooling down process. processes Different of the friction thin coefilmffi cientsextrusion between process. the melt Different and the friction thin film coefficients thermocouple, between as well the as themelt uncoated and the extrusion thin film die, canthermocouple, lead to inhomogeneous as well as the film uncoated qualities extrusion due to di diffe,erent can frictionlead to forces.inhomogeneous Since the extrusionfilm qualities die wasdue madeto different of AISI friction H11 (1.2343) forces. Since steel, thisthe extrusion steel was die used was as made reference of AISI for theH11 tribological (1.2343) steel, experiments. this steel was In Figureused as4, reference the friction for coe theffi tribologicalcients of the experiments. AlN-1, AlN-2, In Figure AlN-3, 4, andthe friction BCN-1 thincoefficients films, as ofwell the AlN-1, as the steelAlN-2, substrate AlN-3, andand theBCN-1 Al2O thin3 coating, films, as are well shown as the for steel temperatures substrateof and 85 the and Al 1532O◦3 Ccoating, usingPP are counter shown balls.for temperatures It was observed of 85 that and there 153 °C is nousing temperature PP counter dependency balls. It was of observed the friction that coe therefficient, is no except temperature for the reference steel and the Al O coating. At 85 C, a friction coefficient of 0.52 0.11 is measured for dependency of the friction2 coefficient,3 except ◦for the reference steel and the Al±2O3 coating. At 85 °C, the steel substrate, which drops down to 0.27 0.05 at 153 C. A contrary behavior is observed for a friction coefficient of 0.52 ± 0.11 is measured for± the steel substrate,◦ which drops down to 0.27 ± 0.05 the Al O coating; the friction coefficient increases from 0.42 0.10 at 85 C to 0.66 0.16 at 153 C. at 153 2°C.3 A contrary behavior is observed for the Al2O3 coating;± the friction◦ coefficient± increases from◦ Concerning0.42 ± 0.10 at the 85 thin°C to films, 0.66 ± they 0.16 basicallyat 153 °C. reveal Concerni comparableng the thin friction films, coe theyfficients basically independent reveal comparable from the used deposition parameters. The lowest friction coefficient of 0.28 0.05 is obtained for the AlN-2 thin friction coefficients independent from the used deposition parameters.± The lowest friction coefficient film, which slightly increases in the order AlN-1, AlN-3 up to 0.39 0.07 for the BCN-1 thin film at 85 of 0.28 ± 0.05 is obtained for the AlN-2 thin film, which slightly increases± in the order AlN-1, AlN-3 ◦upC. to In 0.39 a diff ±erent 0.07 for study, the Ball-on-DiscBCN-1 thin film experiments at 85 °C. wereIn a different also used study, to investigate Ball-on-Disc the frictionexperiments coeffi cientwere betweenalso used CrAlN, to investigate respectively, the friction DLC thin coefficien films andt between Polycarbonate CrAlN, respectively, counter balls DLC [31]. thin It was films shown and thatPolycarbonate the friction counter coefficient balls depends [31]. onIt thewas temperature, shown thatand the the friction adhesion coefficient behavior depends is related on to thethe surfacetemperature, energies and of the the adhesion deposited behavior thin films is related and surfaces. to the surface Moreover, energies it was of reported the deposited that the thin friction films coeandffi surfaces.cient depends Moreover, on the it roughness was reported of the that surfaces. the fric Smoothertion coefficient surfaces depends lead to on a higherthe roughness fraction ofof thethe adhesivesurfaces. eSmootherffect on the surfaces friction, lead which to a inhigher turn leadsfraction to aof higher the adhesive coefficient effect of on friction the friction, [32]. In which fact, the in obtainedturn leads friction to a higher coeffi coefficientcients at 85 of◦C friction can be [32]. tendentially In fact, the correlated obtained with friction the roughness. coefficients The at 85 polished °C can steel surface reveals the smoothest surface with a roughness of Ra = 0.006 0.001 µm and the highest be tendentially correlated with the roughness. The polished steel surface± reveals the smoothest frictionsurface coewithffi cient.a roughness The second of Ralowest = 0.006 roughness ± 0.001 µm is shownand the by highest the thermally friction sprayedcoefficient. Al2 TheO3 coating, second whichlowestshows roughness the second is shown highest by the roughness. thermally sprayed Higher roughnessAl2O3 coating, and which lower shows friction the coe secondfficients highest were analyzedroughness. for Higher the thin roughness films. At and 153 ◦lowerC, the friction influence coeffi ofcients the roughness were analyzed on the frictionfor the thin coeffi films.cient At is not153 observed.°C, the influence The melting of the point roughness of PP ison between the friction 160 andcoefficient 165 ◦C, is and not therefore observed. the The mechanical melting point properties of PP ofis PPbetween are already 160 and decreased 165 °C, atand 153 therefore◦C. The surfacethe mechan of theical Al 2propertiesO3 coating of contains PP are defectsalready such decreased as pores, at as153 shown °C. The in surface Figure2 of, which the Al a2Offect3 coating the friction contains behavior, defectsespecially such as pores, in contact as shown with in a softFigure material 2, which as alsoaffect shown the friction by the largebehavior, deviation especially bar of thein contact friction with coeffi acient. soft Therefore,material as the also Al 2shownO3 coating by the reveals large a higherdeviation friction bar coeof ffithecient friction at 153 coefficient.◦C compared Therefore, to the other the thin Al2 films.O3 coating reveals a higher friction coefficient at 153 °C compared to the other thin films.

Coatings 2019, 9, 603 8 of 17 Coatings 2019, 9, x FOR PEER REVIEW 8 of 17 Coatings 2019, 9, x FOR PEER REVIEW 8 of 17

Figure 4.4. Friction coecoefficientfficient of the AlN-1, AlN-2, AlN-3, and BCN-1 thin films,films, as well as thethe steelsteel substrate andand thethe AlAl2O3 coating,coating, at at 85 85 and and 153 153 °CC using using PP PP counter counter balls. balls. substrate and the Al22O3 coating, at 85 and 153 °C◦ using PP counter balls. In addition to the frictional forces, a comparable adhesion of the plastic to the thin film thermocouple In addition to the frictional forces, a comparable adhesion of the plastic to the thin film and the extrusion die is to be ensured. Therefore, the wear of the PP balls after the Ball-on-Disc thermocouple and the extrusion die is to be ensured. Therefore, the wear of the PP balls after the Ball- experimentson-Disc experiments was analyzed, was analyzed, which is an which indirect is an hint indire for thect hint adhesion for the behavior adhesion to thebehavior used coatings. to the used As on-Disc experiments was analyzed, which is an indirect hint for the adhesion behavior 5to the3 used visualized in Figure5, the wear coe cients of the PP balls are between 16.9 1.3 10 mm Nm−5 coatings. As visualized in Figure 5, theffi wear coefficients of the PP balls are between 16.9− ± 1.3 ×/ 10−5 coatings. As visualized in Figure5 5,3 the wear coefficients of the PP balls are between± × 16.9 ± 1.3 × 10 (AlN-3) and 34.3 5.1 10 mm /Nm−5 (AlN-1) for the PVD coated counter parts, whereas a wear mm³/Nm (AlN-3)± and 34.3× ±− 5.1 × 10−5 mm³/Nm (AlN-1) for the PVD coated counter parts, whereas a 5 −53 coewearffi cientcoefficient of 26.7 of 26.73.4 ± 103.4− ×mm 10−5 /mm³/NmNm is analyzed is analyzed for the for uncoated the uncoated steel counterpart steel counterpart at 85 ◦ C.at Using85 °C. wear coefficient of± 26.7× ± 3.4 × 10 mm³/Nm is analyzed for the uncoated steel counterpart5 at 853 °C. theUsing thermally the thermally sprayed sprayed Al2O3 coating Al2O3 coating as a counter as a body,counter a wear body, coe affi wearcient coefficient of 10.6 0.6 of 10.610− ± mm0.6 ×/ Nm10−5 Using the thermally sprayed Al2O3 coating as a counter body, a wear coefficient± of× 10.6 ± 0.6 × 10−5 ismm³/Nm noticed. is Accordingly, noticed. Accordingly, at 85 ◦C, all at counterparts 85 °C, all counterparts cause a comparable cause a comparable wear coefficient wear of coefficient the PP balls, of mm³/Nm is noticed. Accordingly, at 85 °C, all counterparts cause a5 comparable3 wear coefficient of since the maximum difference of the wear coefficient is 23.7 10 mm /Nm−5 within the analyzed the PP balls, since the maximum difference of the wear coefficient× −is 23.7 × 10−5 mm³/Nm within the counter parts. analyzed counter parts.

Figure 5. Wear coefficient of the PP counter balls after the sliding against the AlN-1, AlN-2, AlN-3, Figure 5.5. WearWear coecoefficientfficient of of the the PP PP counter counter balls balls after after the the sliding sliding against against the the AlN-1, AlN-1, AlN-2, AlN-2, AlN-3, AlN-3, and and BCN-1 thin films, as well as the steel substrate and the Al2O3 coating, at 85 and 153 °C. and BCN-1 thin films, as well as the steel substrate and the Al2O3 coating, at 85 and 153 °C. BCN-1 thin films, as well as the steel substrate and the Al2O3 coating, at 85 and 153 ◦C. At 153 °C, an increased wear coefficient of the PP balls is detected, compared to the tribological At 153 ◦°C,C, an increased wear coecoefficientfficient of the PP ballsballs isis detected,detected, comparedcompared toto thethe tribologicaltribological tests at 85 °C. The wear coefficient of the PP balls is increased by the factor 10 to 20, since the teststests atat 85 85◦ C.°C. The The wear wear coe fficoefficientcient of the of PP the balls PP isball increaseds is increased by the factor by the 10 tofactor 20, since10 to the 20, mechanical since the mechanical properties of the PP balls are reduced at elevated temperatures. Analyzing the wear coefficient at 153 °C, the differences between the counter parts become more visible compared to 85

Coatings 2019, 9, 603 9 of 17

Coatings 2019, 9, x FOR PEER REVIEW 9 of 17 properties of the PP balls are reduced at elevated temperatures. Analyzing the wear coefficient at 153 ◦°C.C, theThe di PPfferences balls slid between against the the counter steel surface parts become reveal morea wear visible coefficient compared of 253.7 to 85 ± ◦48.7×C. The 10 PP−5 mm³/Nm, balls slid 5 3 againstwhich is the comparable steel surface to a reveal wear acoefficient wear coeffi ofcient 250.3 of ± 253.741.0 × 1048.7−5 mm³/Nm10− mm for /theNm, Al which2O3 coating. is comparable A lower 5 3 ± × tocoefficient a wear coe offfi 149.9cient ± of 22.9 250.3 × 1041.0−5 mm³/Nm10− mm is obtained/Nm for thefor Althe2O BCN-13 coating. counterpart, A lower coe andffi cienthigher of 149.9wear 5 3 ± × 22.9 10 mm /Nm is obtained for−5 the BCN-1 counterpart, and higher wear−5 coefficients between ±coefficients× − between 467.0 ± 70.5 × 10 mm³/Nm (AlN-3) and 594.8 ± 175.2 × 10 mm³/Nm (AlN-1) for 467.0 70.5 10 5 mm3/Nm (AlN-3) and 594.8 175.2 10 5 mm3/Nm (AlN-1) for the AlN thin the AlN± thin× films.− The used PP counter ball is a ±non-polar× plastic,− revealing a low fraction of polar films.surface The energy. used PPAccordingly, counter ball only is a non-polarthe disperse plastic, fraction revealing of the a lowfree fractionsurface ofenergy polar surfaceinfluences energy. the Accordingly,adhesion behavior only the to dispersethe PP counterpart fraction of [33]. the free Theiss surface et al. energy reported influences that the the work adhesion of adhesion behavior to PP, to thewhich PP is counterpart calculated [based33]. Theiss on the et surf al.ace reported energies, that is thehigher work for of AlN-rich adhesion thin to PP,films which compared is calculated to steel based[34]. Especially, on the surface for thin energies, films grown is higher in the for AlN-richhexagonal thin structure, films compared a high work to steel of adhesion [34]. Especially, is observed for thin[34]. filmsAccordingly, grown in the the higher hexagonal wear structure, of the PP aballs high sl workid against of adhesion the AlN is thin observed films [compared34]. Accordingly, to the steel the highercounter wear body of can the be PP related balls slid to againstthe surface the AlNenergies thin. filmsMoreover, compared this mechanism to the steel counteris overlaid body with can the be relatedsurface toroughness. the surface As energies. listed in Moreover,Table 2, the this used mechanism deposition is parameters overlaid with of thethe surfaceAlN thin roughness. films lead As to listeda change in Table in the2, theroughness. used deposition The roughness parameters Ra varies of the AlNbetween thin films0.26 ± lead 0.06 to µm a change (AlN-3) in and the roughness.0.36 ± 0.06 The roughness Ra varies between 0.26 0.06 µm (AlN-3) and 0.36 0.06 µm (AlN-1). The wear of the µm (AlN-1). The wear of the balls decreases± with decreasing roughness± in the order AlN-1, AlN-2, ballsAlN-3. decreases with decreasing roughness in the order AlN-1, AlN-2, AlN-3. Summarized,Summarized, withinwithin the the AlN AlN thin thin films, films, the deposition the deposition parameters parameters influence influence the friction the coe frictionfficient andcoefficient the wear and coe theffi wearcient coefficient of the PP ballsof the at PP 85 balls◦C, at as 85 well °C, as as 153 well◦ C.as At153 153 °C. ◦AtC, 153 the °C, influence the influence of the depositionof the deposition parameters parameters of the AlNof the thin AlN films thin onfilms the on friction the friction coeffi cientcoefficient is lower is lower compared compared to the to wear the coewearffi cient.coefficient. Concerning Concerning the wear the coewearfficient coefficient at 153 ◦atC, 153 the °C, wear the of wear the PPof the balls PP is balls more is influenced more influenced by the counterby the counter body material body material than the than used the deposition used depositi parameterson parameters of a coating, of a sincecoating, the since difference the difference between thebetween different the materialsdifferent materials is higher thanis higher the di thanfference the difference within the within AlN thin the films.AlN thin films. CorrespondingCorresponding SEMSEM imagesimages ofof thethe adhesions from the PP balls to the didifferentfferent coatingscoatings afterafter the testtest atat 8585 ◦°CC are shown in Figure6 6.. NoNo adhesionadhesion ofof PPPP toto thethe surfacesurface ofof thethe uncoateduncoated 1.2343 1.2343 substrate substrate cancan bebe observed.observed. In addition, no adhesionsadhesions to thethe AlAl22O3,, AlN-3, AlN-3, and and BCN-1 BCN-1 surfaces surfaces have have occurred. InIn contrastcontrast toto that,that, locallocal PPPP accumulationsaccumulations areare observedobserved onon thethe AlN-1AlN-1 andand AlN-2AlN-2 thin films,films, whereaswhereas thethe amountamount adheredadhered toto thethe AlN-1AlN-1 thinthin filmfilm isis slightlyslightly higher.higher. OnOn thethe oneone hand,hand, thethe adhesionadhesion toto thesethese thinthin filmsfilms can be be explained explained by by the the cauliflower-like cauliflower-like surface, surface, resulting resulting in in a ahigher higher surface surface roughness roughness in incontrast contrast to tothe the other other coatings, coatings, as shown as shown in Figure in Figure 2. On2. Onthe theother other hand, hand, the theadhesive adhesive layer layer is only is onlyobserved observed for the for AlN the thin AlN films thin featuring films featuring a small a fracti smallon fraction of a second of a secondphase, as phase, discussed as discussed in chapter in Section3.1.1. 3.1.1.

Figure 6. SEM images of the didifferentﬀerent surfacessurfaces afterafter thethe Ball-on-DiscBall-on-Disc experimentsexperiments atat 8585 ◦°C.C.

SEM images of the different surfaces after the Ball-on-Disc experiments at 153 °C are visualized in Figure 7. In contrast to the surfaces analyzed at 85 °C, at 153 °C for all coatings, no rubbing of the PP balls on the surfaces can be noted, which could be related to the change in properties of the PP

Coatings 2019, 9, 603 10 of 17

SEM images of the diﬀerent surfaces after the Ball-on-Disc experiments at 153 ◦C are visualized in

FigureCoatings7 2019. In, 9 contrast, x FOR PEER to the REVIEW surfaces analyzed at 85 ◦C, at 153 ◦C for all coatings, no rubbing of10 of the 17 PP balls on the surfaces can be noted, which could be related to the change in properties of the PP materialmaterialCoatings withwith2019, 9 increasing,increasing x FOR PEER temperature.REVIEWtemperature. At 85 andand 153153 ◦°C,C, therethere isis nono generalgeneral correlationcorrelation betweenbetween10 of 17 thethe ballball wearwear coecoefficientsfficients andand thethe formationformation ofof anan adhesiveadhesive layer.layer. However, thethe formationformation ofof thethe adhesiveadhesive layerlayermaterial onon thethe with AlN-1AlN-1 increasing andand AlN-2AlN-2 temperature. thinthin filmsfilms At atat 8585 ◦and°CC correlates 153 °C, there withwith is thethe no highesthighestgeneral ballcorrelationball wearwear ratesrates between withinwithin the thethe threethreeball AlNAlN wear thinthin coefficients films.films. and the formation of an adhesive layer. However, the formation of the adhesive layer on the AlN-1 and AlN-2 thin films at 85 °C correlates with the highest ball wear rates within the three AlN thin films.

FigureFigure 7.7. SEM images of the didifferentfferent surfacessurfaces afterafter thethe Ball-on-DiscBall-on-Disc experimentsexperiments atat 153153◦ °C.C. Figure 7. SEM images of the different surfaces after the Ball-on-Disc experiments at 153 °C. Furthermore,Furthermore, thethe influenceinfluence ofof thethe didifferentfferent coatingcoating surfacessurfaces onon thethe meltmelt flowflow waswas investigatedinvestigated atat higherhigher temperaturesFurthermore,temperatures the correspondingcorresponding influence of the toto thedifferentthe extrusionextrusion coatin process.process.g surfaces Therefore,Therefore, on the melt thethe flow coatingscoatings was investigated werewere analyzedanalyzed at inin ahighera High-PressureHigh-Pressure temperatures CapillaryCapillary corresponding RheometerRheometer to the (HPCR).(HPCR). extrusion ByBy process. meansmeans of ofTherefore, coatedcoated surfacessurfaces the coatings ofof speciallyspecially were analyzed designeddesigned in a High-Pressure Capillary Rheometer (HPCR). By means of coated surfaces of specially designed exchangeableexchangeable inserts,inserts, the the influence influence on on the the flow flow properties properties of the of melt the wasmelt investigated. was investigated. Figure 8Figure shows 8 exchangeable inserts, the influence on the flow properties of the melt was investigated. Figure 8 theshows basic the setup basic of setup the capillary of the capillary and the and coated the insertscoated oninserts theexample on the example of the AlN-3 of the thin AlN-3 film. thin film. shows the basic setup of the capillary and the coated inserts on the example of the AlN-3 thin film.

FigureFigureFigure 8. 8.(a )() aSetup Setup) Setup High-Pressure High-Pressure High-Pressure Capillary CapillaryCapillary RheometerRheometer Rheometer andand and ((bb ()b) coated )coated coated exchangeable exchangeable exchangeable inserts inserts inserts with with with AlN-3. AlN-3. AlN-3.

FlowFlowFlow anomalies,anomalies, anomalies, which which which are are are due due due to toto the thethe di ffdifferenterent surface surfacesurface qualities qualities qualities based based based on on theon the the compositions compositions compositions of of the of coatings,thethe coatings, coatings, can becan can detected be be detected detected with with awith so calleda a soso calledcalled “critical” “critica walll”l” shearwallwall shear shear stress. stress. stress. Until Until theUntil criticalthe the critical critical wall wall shear wall shear stressshear stress is reached, the melt flow behavior is like “wall sticking”. Therefore, flow anomalies cannot be isstress reached, is reached, the melt the flow melt behavior flow behavior is like “wall is like sticking”. “wall sticking”. Therefore, Therefore, flow anomalies flow anomalies cannot be cannot detected. be detected. The onset of the throughput jump depends on, among other things, the melt temperature Thedetected. onset The of the onset throughput of the throughput jump depends jump on,depen amongds on, other among things, other the things, melt temperaturethe melt temperature and the and the polymer type. In the transition area above the critical wall shear stress, the throughput and the polymer type. In the transition area above the critical wall shear stress, the throughput multiplies many times over, while the wall shear stress remains constant. This range is also referred multiplies many times over, while the wall shear stress remains constant. This range is also referred to as the limiting wall shear stress. Above a certain shear rate, the wall shear stress increases again to andas the the limiting effect of wallsliding shear occurs. stress. Above a certain shear rate, the wall shear stress increases again and the effect of sliding occurs.

Coatings 2019, 9, 603 11 of 17 polymer type. In the transition area above the critical wall shear stress, the throughput multiplies many times over, while the wall shear stress remains constant. This range is also referred to as the limiting wall shear stress. Above a certain shear rate, the wall shear stress increases again and the Coatings 2019, 9, x FOR PEER REVIEW 11 of 17 effCoatingsect of 2019 sliding, 9, x FOR occurs. PEER REVIEW 11 of 17 FigureFigure9 shows 9 shows for for the the two two operating operating temperatures temperatures of of 230230 andand 250250 °C◦C and and the the obtained obtained values values of of Figure 9 shows for the two operating temperatures of 230 and 250 °C and the obtained values of thethe shear shear rate rate in in dependence dependence of of the the shear shear stress stress usingusing HPCR.HPCR. Both Both diagrams diagrams show show a asteady steady increase increase the shear rate in dependence of the shear stress using HPCR. Both diagrams show a steady increase of theof the shear shear rate rate over over the the wallwall shear stress. stress. A A co comparisonmparison of the of thecoated coated measurement measurement curves curves with the with of the shear rate over the wall shear stress. A comparison of the coated measurement curves with the thereference reference measurement measurement (grey (grey curve curve with with squares) squares) shows shows no no significant significant differences. differences. Minimal Minimal reference measurement (grey curve with squares) shows no significant differences. Minimal deviationsdeviations between between the the curves curves are are found found and and cancan bebe attributedattributed to the the used used setup. setup. From From the the obtained obtained deviations between the curves are found and can be attributed to the used setup. From the obtained values,values, it isit concludedis concluded that that no no particular particular flowflow anomaliesanomalies can be detected detected with with the the surface surface coatings coatings values, it is concluded that no particular flow anomalies can be detected with the surface coatings present.present. A A critical critical wall wall shear shear stressstress cannotcannot bebe detecteddetected at at the the typical typical shear shear rates rates in in the the extrusion extrusion present. .A critical0 wall4 1shear stress cannot be detected at the typical shear rates in the extrusion processprocessγ 𝛾= =10 10 1010 .. Consequently,Consequently, all investigated coatings coatings can can be be used used as as top top layers layers for for the the process 𝛾 = 1010− − s. Consequently, all investigated coatings can be used as top layers for the thermocouples,thermocouples, since since there there is nois dinoff erencedifference in the in meltthe melt flow flow behavior behavior between between theuncoated the uncoated and coatedand thermocouples, since there is no difference in the melt flow behavior between the uncoated and surfacescoated of surfaces the wide-slot of the wide-slot nozzle. nozzle. coated surfaces of the wide-slot nozzle.

FigureFigure 9. 9.Critical Critical wall wall shear shear stress stress at at ( a(a)) 230230 °CC and ( b) 250 250 °C,C, independent independent of of the the thin thin film. film. Figure 9. Critical wall shear stress at (a) 230 °C◦ and (b) 250 °C,◦ independent of the thin film. 3.2.3.2. Insertable Insertable Thin Thin Film Film Thermocouple Thermocouple 3.2. Insertable Thin Film Thermocouple TheThe insertable insertable thin thin film film thermocouple thermocouple isis designeddesigned to be exchangeable, applicable applicable to to plastic plastic film film The insertable thin film thermocouple is designed to be exchangeable, applicable to plastic film extrusionextrusion dies. dies. As As shown shown in in Figure Figure 10 10,, the the unity unity consists consists of of a bush,a bush, which which can can be be screwed screwed into into any any die extrusion dies. As shown in Figure 10, the unity consists of a bush, which can be screwed into any revealingdie revealing an appropriate an appropriate drilling drilling (M24). (M24). The ceramicThe ceramic insert insert contains contains the the thin thin film film thermocouple thermocouple and die revealing an appropriate drilling (M24). The ceramic insert contains the thin film thermocouple canand be insertedcan be inserted into the into bush, the bush, and is and fixed is fixed by a stepby a onstep the on bottomthe bottom side side and and a cap a cap on on the the top top side. side. The and can be inserted into the bush, and is fixed by a step on the bottom side and a cap on the top side. totalThe length total length of the screw-inof the screw-in bush, bush, including including the top the cap, top iscap, 63.5 is 63.5 mm. mm. The total length of the screw-in bush, including the top cap, is 63.5 mm.

Figure 10. Structure of the exchangeable thin film thermocouple. FigureFigure 10. StructureStructure of of the the exchangeable exchangeable thin thin film ﬁlm thermocouple. thermocouple. The Ni and Ni-20Cr conducting paths (width of 0.5 mm), deposited on the ceramic insert using The Ni and Ni-20Cr conducting paths (width of 0.5 mm), deposited on the ceramic insert using a masking system, are shown in Figure 11. On the bottom side (step), the conducting paths are a masking system, are shown in Figure 11. On the bottom side (step), the conducting paths are connected to form the hot junction length of 1.5 mm. The conducting paths run over the lateral surface connected to form the hot junction length of 1.5 mm. The conducting paths run over the lateral surface

Coatings 2019, 9, 603 12 of 17

The Ni and Ni-20Cr conducting paths (width of 0.5 mm), deposited on the ceramic insert using a masking system, are shown in Figure 11. On the bottom side (step), the conducting paths are connected Coatings 2019, 9, x FOR PEER REVIEW 12 of 17 to form the hot junction length of 1.5 mm. The conducting paths run over the lateral surface to the top 2 side. On the top side, the conducting paths end with contact points (2.5 2.5 mm ), as visualized2 in to theCoatings top 2019 side., 9, xOn FOR the PEER top REVIEW side, the conducting paths end with contact× points (2.5 × 2.5 mm12 of), 17 as visualizedFigure 10. Thermocouplein Figure 10. Thermocouple balancing lines balancing can be connected lines can to be these connected points, eitherto these by points, brazing either or spring by 2 brazingprobeto the pins, or top spring to side. conduct probeOn the the pins, top generated toside, conduct the thermovoltage conducting the generated paths to thethermovoltage end measuring with contact system.to thepoints measuring (2.5 × 2.5 system. mm ), as visualized in Figure 10. Thermocouple balancing lines can be connected to these points, either by brazing or spring probe pins, to conduct the generated thermovoltage to the measuring system.

FigureFigure 11. 11. CeramicCeramic insert insert showing showing the the conducting conducting paths paths of of the the Ni/Ni-20Cr Ni/Ni-20Cr thin thin film film thermocouple. thermocouple. Figure 11. Ceramic insert showing the conducting paths of the Ni/Ni-20Cr thin film thermocouple. TheThe multilayer designdesign ofof thethe thermally thermally coated coated insert insert is shownis shown in Figurein Figure 12. 12. Primarily, Primarily, the steelthe steel core coreis coated is coatedThe with multilayer with a bond a bond design coat coat (NiCoCrAlTaY) of (NiCoCrAlTaY)the thermally to coated enhance to enhance insert the is adhesion the shown adhesion in of Figure the of ceramic the 12. ceramicPrimarily, layers layers tothe the steelto steel the core is coated with a bond coat (NiCoCrAlTaY) to enhance the adhesion of the ceramic layers to the steelsubstrate, substrate, especially especially at higher at higher temperatures. temperat Theures. bond The coatbond is followedcoat is followed by a coarse by a 7Y coarse2O3-ZrO 7Y22Ocoating3-ZrO2 coatingto ensuresteel substrate,to the ensure thermal especiallythe insulationthermal at higherinsulation between temperat thebetween thinures. film th Thee thermocouple thin bond film coat thermocouple is andfollowed the die, by and asa coarse well the asdie, 7Y the 2asO steel3 -ZrOwell core 2as theof thecoating steel insert, core to ensure for ofan the accuratethe insert, thermal measurementfor insulation an accurate between of theme temperatureasurement the thin film of of thermocouplethe the plastictemperature melt and variations. ofthe the die, plastic as Especiallywell meltas the steel core of the insert, for an accurate measurement of the temperature of the plastic melt variations.for wall mounted Especially thermocouples, for wall mounted it is reported thermocouples, that the measurement it is reported of temperaturethat the measurement fluctuations of is variations. Especially for wall mounted thermocouples, it is reported that the measurement of temperaturedifficult when fluctuations the thermal is difficult insulation when is not the adequatethermal insulation [10]. The is 7Y not2O adequate3-ZrO2 coating [10]. The is layered 7Y2O3-ZrO by a2 temperature fluctuations is difficult when the thermal insulation is not adequate [10]. The 7Y2O3-ZrO2 coatingdense Al is2 Olayered3 coating, by which a dense improves Al2O3 thecoating, adhesion which of theimproves thin film the thermocouples adhesion of duethe tothin a lowerfilm coating is layered by a dense Al2O3 coating, which improves the adhesion of the thin film thermocouplescontent of defects, due such to a aslower pores. content Moreover, of defects, Al2O3 suchreveals as apores. higher Moreover, electrical Al resistance2O3 reveals compared a higher to thermocouples due to a lower content of defects, such as pores. Moreover, Al2O3 reveals a higher electricalZrO2 to guarantee resistance nocompared electrical to short ZrO2 of to the guarantee thermocouple. no electrical short of the thermocouple. electrical resistance compared to ZrO2 to guarantee no electrical short of the thermocouple.

Figure 12. Cross-section of the multilayer design of the thermally sprayed insulating coatings of the FigureFigure 12. 12. Cross-sectionCross-section of of the the multilayer multilayer design design of of the the thermally thermally sprayed sprayed insulating insulating coatings coatings of of the the ceramic insert. ceramicceramic insert. insert. The extrusion nozzle is made of steel and, therefore, the inserted tool with the thin film TheThe extrusion extrusion nozzle nozzle is is made made of of steel steel and, and, ther therefore,efore, the the inserted inserted tool tool with with the the thin thin film ﬁlm thermocouplethermocouple should should reveal reveal the samethe tribologicalsame tribologic properties.al properties. As shown As byshown the Ball-on-Disc by the Ball-on-Disc experiments, thermocoupleexperiments, shouldall coatings reveal reveal the nosame adhesions tribologic of althe properties. PP counter As ball shown on the bysurface the Ball-on-Discat 153 °C. all coatings reveal no adhesions of the PP counter ball on the surface at 153 ◦C. Concerning the wear of experiments,Concerning all the coatings wear of the reveal PP ball, no similar adhesions values of are the obtained PP counter for the ball steel onsurface the andsurface the thermallyat 153 °C. Concerningsprayed Althe2O wear3 coating. of the High-PressurePP ball, similar Capillaryvalues are Rheo obtainedmeter for experiments the steel surface also anddemonstrate the thermally no sprayedsignificant Al2O 3differences coating. High-Pressurein the melt flow Capillary for the Rheo differentmeter coatings.experiments Therefore, also demonstratethe thin film no significantthermocouple differences was not incovered the meltby an flowadditional for thenitride different layer for coatings. the operation Therefore, test. the thin film thermocouple was not covered by an additional nitride layer for the operation test.

Coatings 2019, 9, 603 13 of 17

the PP ball, similar values are obtained for the steel surface and the thermally sprayed Al2O3 coating. High-Pressure Capillary Rheometer experiments also demonstrate no significant differences in the melt flow for the different coatings. Therefore, the thin film thermocouple was not covered by an additional nitride layer for the operation test. CoatingsCoatings 2019 2019, 9, ,9 x, xFOR FOR PEER PEER REVIEW REVIEW 1313 of of 17 17 3.3. Operation Test 3.3.3.3. Operation Operation Test Test 3.3.1. Validation of the Temperature Measurement 3.3.1.3.3.1. Validation Validation of of the the Temperature Temperature Measurement Measurement Before the operational testing, the calibration curves for the thermocouples have to determined. BeforeBefore the the operational operational testing, testing, the the calibration calibration cu curvesrves for for the the thermocouples thermocouples have have to to determined. determined. In a previous study [20], the procedure was described and the dependence of the thermovoltage InIn a a previous previous study study [20], [20], the the procedure procedure was was described described and and the the dependence dependence of of the the thermovoltage thermovoltage and and andtemperaturetemperature temperature forfor for thethe the thermocouplesthermocouples thermocouples waswas was found.found. found. InIn In thethe the presentpresent present study,study, study, thethe the massmass mass temperaturetemperature temperature waswas was measuredmeasuredmeasured shortly shortly shortly before before before the the the outlet outlet outlet of of of the the the die.die. die. TheTh Thee used used experimental experimentalexperimental setup setup is is is shown shown shown in in in Figure Figure Figure 13. 13. 13 .

FigureFigureFigure 13. 13. 13.Experimental Experimental Experimental setup setup setup showingshowing showing the the array array of ofof th thethee thermocouples thermocouplesthermocouples in in the the the wide-slit wide-slit wide-slit nozzle. nozzle. nozzle.

AfterAfterAfter the the the extrusion extrusion extrusion line line line reached reached reached aa a steadysteady steady state state for for thethe the cylinder cylindercylinder heating heating and and and the the the used used used material, material, material, the the the embeddedembeddedembedded thin thin thin ﬁlm film film thermocouple thermocouple thermocouple (TE) (T (TE)E) waswas was insertedinserted inserted in in the thethe die diedie and and the the recording recording recording of of of the the the measurement measurement measurement signalsignalsignal began. began. began. All All All four four four elements elements elements werewere were insertedinserted inserted at at the the same same time. time.time. Figure Figure 14 1414 shows showsshows the the the heating heating heating process process process ofof theof the the thermocouple thermocouple thermocouple (grey (grey (grey line) line) line) until until until aa a staticstatic static statestat statee was was reached. reached. All All other other temperature temperature sensors sensors sensors were were were installedinstalledinstalled in in advancein advance advance to to to check check check the the the tool tool tool temperature.temperature. temperature. In In In total, total,total, the thethe thermocouple thermocouple required required required 130 130 130 s s until suntil until 90% 90% 90% ofof theof the the ﬁnal final final value value value was was was reached, reached, reached, and and and approximately approximately approximately 10001000 1000 s suntil until the thethe stationary stationarystationary state state was was was reached. reached. reached.

FigureFigureFigure 14. 14. 14.Heating Heating Heating curve curve curve of of of thethe the thinthin thin filmfilm film thermocouple thermocouple in inin the thethe plastic plastic flat flat flat film film film extrusion extrusion extrusion process. process. process.

AfterAfter the the steady steady state state of of the the thermocouple thermocouple was was reached, reached, the the measurement measurement with with all all sensors sensors was was recordedrecorded over over a a time time of of 1.5 1.5 h. h. The The measurement measurement of of the the melt melt temperature temperature at at the the mold mold outlet outlet was was additionallyadditionally determined determined in in 5 5 min min intervals intervals by by mean meanss of of a a thermocouple thermocouple as as a a guideline guideline value. value. The The

Coatings 2019, 9, 603 14 of 17

After the steady state of the thermocouple was reached, the measurement with all sensors was

Coatingsrecorded 2019 over, 9, x FOR a time PEER of REVIEW 1.5 h. The measurement of the melt temperature at the mold outlet14 wasof 17 additionally determined in 5 min intervals by means of a thermocouple as a guideline value. The surface surfacetemperature temperature was also was measured also measured with a laser with to verifya laser the to values.verify the Figure values. 15 shows Figure the 15 temperature shows the temperaturecurve over time. curve over time.

Figure 15. ((aa)) Monitor Monitor conditioning conditioning of of the the melt melt temperature temperature and ( b) influence inﬂuence of the calibration on the measured mass temperature.

During the measuring process, no temperature peaks werewere detecteddetected (Figure(Figure 1515a).a). All sensors displayed a a constant constant measuring measuring signal signal within within the the time time interval. interval. Due Due to the to measuring the measuring tip of tip the of T1.5, the T1.5,the sensor the sensor was placed was placed deeper deeper in the inmelt the and melt measured and measured the melt the temperature melt temperature significantly significantly better. Thesebetter. measured These measured values and values the andmathematical the mathematical equation equationss for the calibration for the calibration curves were curves used were to adjust used accuracyto adjust accuracyof the measured of the measured melt temperatures. melt temperatures. Figure Figure15b shows 15b shows the adjusted the adjusted temperatures. temperatures. All Allthermocouples thermocouples show show relatively relatively similar similar measured measured va valueslues after after calibration. calibration. Compared Compared to to the the mass temperature ofof 200.6200.6◦ °CCobtained obtained by by a a laser laser at at the the nozzle nozzle exit, exit, the the sensors sensors T0 T0 and and T0.5 T0.5 measure measure with with the thelowest lowest deviation deviation after after calibration. calibration. The The deviation deviation of theof the thin thin film film thermocouple thermocouple is 2.52%. is 2.52%. Especially Especially in polymerin polymer extrusion, extrusion, the the determination determination of the of the temperature temperature is important, is important, since sinc evene even small small deviations deviations lead leadto a highto a changehigh change of the shearof the viscosity shear viscosity of the melt of the [35 ].melt However, [35]. However, all used thermocouples, all used thermocouples, including includingthe thin film the thermocouple, thin film thermocouple, can be used can for be the used temperature for the temperature monitoring monitoring of the mass of inthe the mass extrusion in the extrusiondie process die after process recalibration after recalibration regarding theregardin mountingg the place mounting in the nozzle.place in Beforethe nozzle. the recalibration, Before the recalibration,the accuracy ofthe the accuracy measured of the mass measured temperature mass te increasedmperature with increased extent with into theextent melt. into For the obtaining melt. For obtainingdie melt temperature die melt temperature profiles in extrusion profiles processes,in extrusion generally, processes, thermocouple generally, mesh thermocouple arrangements mesh are arrangementsinserted in the are melt inserted flow [10 in]. the However, melt flow this [10]. technique However, can only this betechnique used in testcan conditions,only be used since in test the conditions,melt flow is since disturbed. the melt Using flow many is disturbed. thin film thermocouples,Using many thin a temperaturefilm thermocouples, profile can a temperature be obtained withoutprofile can aff ectingbe obtained the melt without flow. affecting the melt flow.

3.3.2. Foil Quality The optical film film properties were determined by measuring gloss and transmission. Ten Ten samples (dimensions 50 × 100 mm 2)) were were taken taken for for evaluation during fl flatat filmfilm production. The transmission × and thethe gloss gloss of theof the films films were were tested tested based onbased the describedon the described experimental experimental setup. For thesetup. transmission, For the transmission,the determination the determination of the light intensity of the lightI0 is intensity necessary I0 asis necessary reference. as The reference. transmitted The transmitted light intensity lightIT wasintensity measured IT was by measured clamping by the clamping samples the in the samples sample in holder. the sample The diholder.fference The between differenceI0 and between IT is the I0 andtransmission IT is the transmission spectrum. For spectrum. the reflection, For the a reflection white comparison, a white comparison sample is required, sample is which required, completely which completelyreflects the entirereflects light the intensity entire Ilight0. Ocean intensity Optics I0 provided. Ocean Optics white specimens provided forwhite the comparisonspecimens sample.for the Then,comparison the samples sample. were Then, irradiated the samples with thewere light irradiated intensity withI0 at the the light angle intensity of 2◦ defined I0 at the in DINangle 5036-3, of 2° definedand the reflectedin DIN 5036-3, part IR andwas the measured. reflected part IR was measured. Table 3 shows the reflection results for settings 1 and 2 as reflectance [%]. The velocity of the trigger unit was changed between the two settings. While at setting 1, the velocity reaches 1.5 mm/s, at setting 2 the velocity is 1.7 mm/s. For both settings, the values are between 29.91% and 31.88% and, accordingly, there is no significant difference between the different sensors. The wall-flush thermal sensors and the thin film thermocouples have a slightly lower value than the others.

Coatings 2019, 9, 603 15 of 17

Table3 shows the reflection results for settings 1 and 2 as reflectance [%]. The velocity of the trigger unit was changed between the two settings. While at setting 1, the velocity reaches 1.5 mm/s, at setting 2 the velocity is 1.7 mm/s. For both settings, the values are between 29.91% and 31.88% and, accordingly, there is no significant difference between the different sensors. The wall-flush thermal sensors and the thin film thermocouples have a slightly lower value than the others.

Table 3. Results of the reﬂection results in dependence of the used thermocouple.

Setting 1 Setting 2 Thermocouple Reﬂectance [%] Reference 31.387 31.456 T0 30.937 31.228 T0.5 31.485 30.876 T1.5 31.087 31.150 Thin ﬁlm thermocouple 30.964 29.910

Table4 shows the transmission results for settings 1 and 2 as transmittance [%]. There are no significant differences in the settings. Therefore, the geometry of the thermocouples does not influence the transmittance. Only for setting 2, the measured values are 5% higher than setting 1. This effect is to be explained due to the fact that setting 2 produces thinner films, and an increased light intensity penetrates the sample.

Table 4. Results of the transmission results in dependence of the used thermocouple.

Setting 1 Setting 2 Thermocouple Transmittance [%] Reference 77.47 85.03 T0 80.33 85.00 T0.5 79.09 85.32 T1.5 78.90 84.75 Thin ﬁlm thermocouple 79.39 85.52

4. Conclusions

In this study, AlN and BCN thin films were magnetron sputtered on thermally sprayed Al2O3 coatings using different deposition parameters. In the case of the AlN thin films, a high Ar/N2 gas flow ratio of 6.0 and a high heating power led to a coarse structure with a reduced hardness from about 14 to 6 GPa. Since these films are proposed to be used in flat film extrusion nozzles, tribological experiments were conducted. Using a high deposition temperature and high Ar/N2 gas flow ratio for the AlN coatings favor the adhesion of Polypropylen to the thin film surfaces after Ball-on-disc experiments at 85 ◦C. The formation of the adhesive layer is caused by the higher roughness of the AlN thin films, which can be related to the physical structure, and the work of adhesion. In contrast, no adhesions to the steel surface, the thermally sprayed Al2O3 coating, the BCN thin film, or a AlN thin deposited with lower temperature and higher Ar/N2 gas flow ratio were observed. At 153 ◦C, for all coatings, no adhesions emerged, since the mechanical properties of the PP ball were changed with the temperature. The interaction of the thin film thermocouples with the plastic melt was investigated in application tests. Compared to the uncoated tool surface, the thin films reveal no negative effects on the wall shear stress and the wall shear speed. Accordingly, an effect of the thermocouple on the foil quality could not be detected in the samples examined under the spectroscope. Thus, flat films with constant reflection properties could be produced by measuring the melt temperature using thin film thermocouples. Ni/Ni-20Cr thin film thermocouples were deposited on newly developed exchangeable tool inserts and deployed in a flat film extrusion process. The measurement signal of the thin film thermocouple and industrial reference thermocouples were tested using a specially designed die. No temperature Coatings 2019, 9, 603 16 of 17 peaks were detected during the measuring process, demonstrating a stable measurement behavior. At a melt temperature value of 200.57 ◦C, the obtained value by the help of the thin film thermocouple was 205.75 ◦C, which is slightly higher compared to the industrial sensors. The deviation of the thin film thermocouple did not exceed 2.52%, which is within a typical tolerance range of a type K sensor. Within the proposed attempt, PVD thermocouples are a promising approach which can be used for online monitoring. However, the used design can be used to further enhance the performance of the wide-slit nozzle by using wear resistant top layers.

Author Contributions: Conceptualization, D.K., H.M. and D.S.; Methodology, D.K. and H.M.; Validation, D.K., H.M. and D.S.; Investigation, D.K. and H.M.; Writing—Original Draft Preparation, D.K. and H.M.; Writing—Review and Editing, W.T., D.S. and V.S.; Supervision, W.T. and V.S.; Funding Acquisition, W.T. and V.S. Funding: The authors gratefully acknowledge the financial support of project 18627 N of the German Federation of Industrial Research Associations (AiF) and the Forschungskuratorium Maschinenbau e.V. (FKM). Acknowledgments: We acknowledge financial support by Deutsche Forschungsgemeinschaft and Technische Universität Dortmund/TU Dortmund University within the funding programme Open Access Publishing. In addition, the authors thank Dirk Biermann and Eugen Krebs from the Institute of Machining Technology (TU Dortmund University) for machining the ceramic inserts and providing the confocal 3D microscope. Conflicts of Interest: The authors declare no conflicts of interest.

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