Lehrstuhl für Werkstofftechnologie

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Wetting behaviour of nickel-based brazing alloy BNi-5a on conventionally cast and laser-melted austenitic stainless steel 316L
(2024-10-30) Tillmann, Wolfgang; Bültena, Julia; Wojarski, Lukas; Zajaczkowski, Jonas; Donnerbauer, Kai; Walther, Frank
Laser beam melting is an additive manufacturing process that enables the production of highly complex components. In this process, metal powder is locally melted using a laser beam and built up layer by layer to form a physical component. Due to the layer-by-layer process manufacturing technique of the additive manufacturing process, the microstructure of a laser-melted 316L austenitic stainless steel differs from that of a conventionally cast material. For brazing technology, the different microstructure morphology is important because it affects the known wetting and diffusion behavior with a brazing filler metal, which affects the ability to produce high-strength brazed joints. Brazeability can be determined by examining the wetting of the brazing filler metal with the material to be joined. Therefore, this study investigates the wetting behavior of nickel-base brazing alloy BNi-5a on conventionally cast and laser-melted 316L austenitic stainless steel. Wetting tests were performed to evaluate the spreading area and wetting angle of BNi-5a on both 316L substrates. The wetting tests were performed in a high-temperature vacuum furnace at 1190 °C for 15 min. The results show that the laser-melted 316L stainless steel exhibits enhanced wettability compared to the conventionally cast material. This is related to a higher surface energy and a more pronounced diffusion mechanism called grain boundary grooving on the surface of the material.
Finite element modeling and simulation of vacuum brazing processes with a focus on cycle-dependent component distortion
(2023-08-08) Tillmann, Wolfgang; Henning, Tim; Wojarski, Lukas; Timmer, Christian; Ontrup, Finn
Vacuum brazing is a black box process, so component distortion that occurs during the heat treatment is difficult to prove experimentally. Thus, a novel FE-model was developed in ANSYS Workbench to calculate the time and location resolved component deformation of AISI 316L/B-Ni2 brazing assemblies. In this regard, a new method of radiation and contact modeling was developed that enabled a significant reduction of the calculation times and solved the convergence issue for simulating the distortion of large-scale, thin components. The results showed that the component deformation during heating can be easily kept in the elastic range and can be almost completely eliminated by using a geometry-dependent soaking time. In contrast to this, high cooling rates were found to result in thermally induced stresses well above the elastic yield limit, causing significant component deformation. With further cooling, the deformation decreases significantly, but it depends on the initial stress state, the geometry, and the cooling rate whether the deformation can be completely leveled out during the shrinkage of the component. Thus, the initially high cooling rates were identified to be responsible for the final distortion. Furthermore, this was highly affected by the local position in the heating chamber. The simulation results were used to design a fixture for vertical positioning, which reduced the max. temperature difference in the brazing assembly from 141 to 79 °C, the max. interim distortion from 275 to 31 µm, and the final distortion from 14 to 8 µm.
Influence of the application of ultrasound on the microstructure of cemented carbide/steel joints brazed with Ag49ZnCuMnNi
(2023-08-21) Wojarski, Lukas; Tillmann, Wolfgang; Ulitzka, Henrik; Ulitzka, Tim
The poor wettability of cemented carbides by molten metals as well as the different material properties compared to steel makes thermal joining process challenging. In this regard, innovative ultrasonic-assisted induction brazing is capable of joining the materials quickly, cost-effectively and without using environmentally hazardous fluxes. Instead, the oxide scales are broken up within the joining process due to ultrasonic-induced cavitation in the molten filler alloy and hence promote the wetting. However, since the use of ultrasound in brazing steel/cemented carbide joints is still insufficiently investigated, there is still a need to explore the fundamental influences of ultrasound on the brazing process. In this work, the effect of ultrasound on the microstructure of the brazed joints and the diffusion depth of different elements were analysed. The two ultrasonic parameters activation time and amplitude were varied during the experiments. Acicular or dendritic structures that have formed in the braze metal without ultrasonic stimulation were transformed into globular structures, when ultrasound was applied. In addition, a non-eutectic silver-rich phase formed, which was not observed in joints manufactured without the use of ultrasound. Furthermore, the use of ultrasound led to increased diffusion between the base material and the filler material as well as the dissolution and distribution of oxides in the edge regions of the brazing joints.
Microstructure of conventional/PBF-LB/M 316L stainless steel hybrid joints brazed with nickel-based brazing alloys
(2023-12-13) Tillmann, Wolfgang; Bültena, Julia; Wojarski, Lukas; Crasmöller, Alexander
Due to the additive manufacturing principle, laser-melted materials (PBF-LB/M) such as the austenitic chromium-nickel steel 316L have a different microstructure compared to materials produced by conventional continuous casting. The PBF-LB/M-produced 316L has a thermally metastable, anisotropic microstructure with epitaxially grown grains in which a cellular substructure is located. When brazing hybrid joints from the conventional and additive manufacturing routes with nickel-based brazing alloys, different diffusion mechanisms occur simultaneously in both joining partners. This occurs due to the different microstructural characteristics of the parent materials. The altered diffusion mechanisms lead to a new distinct microstructure in the joining zone, which influences the achievable quality of the brazed joint in a previously unknown way.
Elementmodifizierte Dünnschichtsysteme aus amorphem Kohlenstoff für tribologisch beanspruchte Werkzeugstähle
(2024) Lopes Dias, Nelson Filipe; Tillmann, Wolfgang; Bobzin, Kirsten
Amorphe Kohlenstoffschichten zeichnen sich durch eine hohe Härte, niedrige Reibung und hohe Verschleißbeständigkeit aus und sind daher effektive Schichtsysteme zur Verbesserung des Reib- und Verschleißverhaltens von tribologisch beanspruchten Werkzeugen. Diese kohlenstoffbasierten Dünnschichten weisen allerdings eine niedrige Haftung auf metallischen Substraten auf. Da adhäsive Schichtschädigungen zu einem frühzeitigen Werkzeugausfall führen, ist eine hohe Haftfestigkeit der amorphen Kohlenstoffschichten auf dem Werkzeugstahl entscheidend für die tribologische Leistungsfähigkeit und eine langlebige Einsatzzeit des beschichteten Werkzeugs. Dieser Herausforderung wird mit der Entwicklung einer Substrat/Schichtgestaltung zur Steigerung der Schichthaftung auf den Werkzeugstählen und der Anpassung der tribo-mechanischen Eigenschaften begegnet. Die Prozesskette, bestehend aus der Plasmavorbehandlung der Werkzeugstähle und der nachfolgenden Synthese einer chemisch gradierten Zwischenschicht sowie einer multilagigen Funktionsschicht aus amorphem Kohlenstoff, wird systematisch in Abhängigkeit des verwendeten Werkzeugstahls untersucht. Dabei werden verschiedene Verfahrensvarianten der Magnetron-Kathodenzerstäubung für die Substratvorbehandlung und Schichtsynthese genutzt.
Vacuum brazing of 18MAR300 nickel maraging steel joints based on additively manufactured and conventional material grades
(2023-07-17) Tillmann, Wolfgang; Henning, Tim; Wojarski, Lukas; Bültena, Julia
Laser powder bed fusion (LPBF) processes offer the best possible design options for the production of highly complex components with unique functionalities. Although the development of additive manufacturing processes is progressing impressively and build rates have already been significantly increased, the economic production of high-volume components is still particularly truncated. Comparatively expensive powders, machine-hour rates, and a limited construction space demonstrate a high demand for joining LPBF components to conventionally manufactured parts. Vacuum brazing is an excellent technology for manufacturing and simultaneous heat treatment of such hybrid components. The aim of this study is to investigate the brazeability and the joint properties of ultrahigh-strength nickel maraging steel 18MAR300 (1.2709, X3NiCoMoTi18-9-5) using BVAg-30 (Ag68Cu27Pd5) as brazing filler metal. For this purpose, the effect of nickel-plated surfaces on the wettability and the microstructure of the joint is studied for conv./conv.-, LPBF/conv.-, and LPBF/LPBF joints. Furthermore, the quasi-static and quasi-dynamic joint strength is evaluated. The results prove that nickel-plated surfaces are vital to achieve a sufficient process control. The coating assures a deoxidized and sealed surface which favors the formation of NiCu(Fe,Pd,Ag) phases within the braze metal thus enhancing the joint strength. In addition, LPBF/LPBF joint features have significantly higher tensile strength than conv./conv. joints (825–627 MPa).
Modification of 316L steel powders with bronze using high energy ball milling for use as a binder component in PBF-LB/M printing of diamond-metal matrix composites
(2023-10-17) Tillmann, Wolfgang; Ferreira, Manuel Pinho
For the processing of diamond-metal matrix composites, the powder bed fusion using a laser for metals (PBF-LB/M), represents a new promising method for the additive manufacturing of diamond tools for concrete and rock machining, even with more complicated geometries. Previous research activities show a strong tendency for cracking and delamination during the construction process of the samples. This behavior is caused by thermal residual stresses associated with the embedded diamonds. To control these negative effects on the process side, the volume energy density is reduced accordingly, which, however, led to increased pore formation. This publication deals with an approach on the material side to modify a 316L stainless steel base powder with an addition of 20 wt% bronze via a high energy ball milling (HEBM) process in such a way that a homogeneous solid solution phase is created. A significantly increasing of the melting interval and a decreasing of both solidus and liquidus temperature was observed, which can reduce pore formation in the PBF-LB/M-process. In addition, XRD-diffractometry and SEM/EDS-analysis showed that the homogeneous solid solution phase of this alloyed powder segregates again into Fe- and Cu-rich phases when heated up to the melting point.
Investigation of the applicability of Cu–Fe–Mn–Ni based high entropy and compositionally complex alloys as metal matrix composites for cobalt free hot-pressed diamond tools
(2023-08-29) Timmer, Christian; Tillmann, Wolfgang; Wojarski, Lukas; Ferreira, Manuel Pinho
Due to the rising demand and carcinogenic effect of cobalt, alternative metal matrixes need to be developed for hot-pressed diamond tools. Due to this reason High-Entropy alloys without cobalt were calculated via phase fraction diagrams. Three alloys of the Al–Cu–Fe–Mn–Ni system Al30Cu30Fe5Mn25Ni10, Al11.25Cu35Fe5Mn20Ni28.75 and Al5Cu20Fe25Mn25Ni25 were chosen due to their different crystal structures ranging from pure bcc, eutectic fcc-bcc to pure fcc crystal structure. Cr5Cu20Fe25Mn25Ni25 was chosen to verify the change of one element on the consolidation properties. The alloys were mechanically alloyed and hot-pressed at 800 °C for 3 min without and at 900 °C for 3 min with diamonds. Porosity increased with the fraction of bcc solid solution in the investigated alloys of the Al–Cu–Fe–Mn–Ni system. Samples consisting of Cr5Cu20Fe25Mn25Ni25 showed the lowest porosity, which was attributed to precipitation of a second copper-rich fcc solid solution around the remaining pores. At a process temperature of 800 °C and 3 min isothermal hold the samples featured a porosity of only 2.72%. Within the XRD patterns and SEM images of the hot-pressed samples with diamonds no graphitization or formation of carbides could be observed. Therefore, Cr5Cu20Fe25Mn25Ni25 was identified as a promising cobalt free metal-matrix candidate for diamond tools.
Microstructural and tribo-mechanical properties of arc-sprayed CoCr-based coatings
(2022-07-12) Hagen, Leif; Paulus, Michael; Tillmann, Wolfgang
Due to their superior wear and oxidation resistance, Stellite™ coatings are widely used in industrial applications, where the coatings are exposed to high temperature. Common processes for applying Stellite™ coatings include the high-velocity oxy-fuel spraying, laser cladding, and plasma transferred arc welding. Although Stellite™ welding consumables or similar welding consumables in the form of cored wires (CoCr base without industrial property rights) are commercially available, there are hardly any studies on arc-sprayed Stellite™ coatings available in the literature. In this study, the microstructural characteristics of arc-sprayed deposits were investigated, which were produced using a CoCr-based cored wire with addition of 4.5 wt.% tungsten. The produced deposits were examined in its as-sprayed state as well as after exposed to elevated temperatures. The microstructure was scrutinized by means of electron microscopy, energy-dispersive x-ray spectroscopy, as well as x-ray diffraction analyses using synchrotron radiation. Tribo-mechanical tests were conducted in order to assess the performance of the arc-sprayed coating. The findings were discussed and compared to those obtained from conventional CoCr-based coatings. It was found that the arc-sprayed CoCr-based coating is predominantly composed of Co-rich, Cr-rich lamellae or lamellae comprising a Co(Cr)-rich solid solution interspersed with various oxides between the individual lamellae. Solid solution hardening serves as dominant strengthening mechanism, while precipitation hardening effects are hardly evident. With regard to the oxidation behaviour, the as-sprayed coating mainly contains CoCr2O4 as well as traces of Co3O4. For heating above 550 °C, coating surface additionally consists of Fe2O3 and Co3O4. In dry sliding experiments, the arc-sprayed CoCr-based coating shows a decreased wear resistance compared to CoCr-based coatings processed by HVOF and PTA, whereas the coefficient of friction (COF) sliding against alumina was similar to the COF observed for the HVOF-sprayed CoCr-based coating, but lower than the COF obtained for the CoCr-based hardfacing alloy deposited by PTA.
Characterization of the microstructure and thermomechanical properties of invar 36 coatings deposited by HVOF and cold gas processes
(2022-08-31) Tillmann, W.; Khalil, O.; Baumann, I.
The effect of impact velocity and temperature of invar particles deposited by high-velocity oxygen fuel (HVOF) and cold spray processes on the microstructure and oxidation content of invar coatings is not fully understood. Additionally, the effect of coating thickness on the coefficient of thermal expansion (CTE) of the coated material and the influence of cold working on the coating hardness are also insufficiently investigated. In the present study, invar coatings were deposited at temperatures close to and below the melting point of invar particles to maintain low CTE. It was found that particle impact temperature and velocity strongly affect pore formation and cohesiveness but slightly affect the hardness of invar coatings. Higher particle impact velocities with impact temperatures close to the invar’s melting point enhance highly the cohesiveness of HVOF-invar coatings. Furthermore, invar coatings stabilize the CTE of the coated material up to a temperature of 227 °C. An increment in the coating’s thickness of 150 µm leads to reducing the CTE of the coated material (Al) in the in-plane direction by 7.65%. Applying cold working using 200 kN compression increases the hardness of the treated coatings by 6% while machine hammer peening (MHP) has a slight effect.
An investigation of the influence of integration of steel heat treatment and brazing process on the microstructure and performance of vacuum-brazed cemented carbide/steel joints
(2022-02-28) Tillmann, W.; Ulitzka, T.; Dahl, L.; Wojarski, L.; Ulitzka, H.
Cemented carbides are commonly brazed to transformation hardening tool steels without taking a proper and adequate steel heat treatment into account. This publication shows the limits and possibilities of integrating a steel heat treatment, including a quenching process, into a vacuum brazing process. Therefore, copper-based filler metals are selected to ensure the steel component’s high and homogenous hardness and supply a high joint quality. In this context, the aimed steel hardness was chosen in the range between 400 and 440 HV1 based on industrial experiences. This specific hardness range for the steel component was set to avoid wear of machining tools in subsequent machining steps if the steel hardness is too high and to prevent wear and deformation of the tool itself in case of a steel hardness too low. When using the transformation hardening tool steel 1.2344, the obtained shear strength values did not exceed a threshold of 20 MPa which can be attributed to the required N2-quenching from brazing respectively solution annealing temperature. However, the steel components featured a hardness of 527.1 HV1 for the specimens brazed with pure copper at 1100 °C and 494.0 HV1 for those brazed with a CuGeNi filler metal at 1040 °C. This publication also shows an alternative route to manufacture long-lasting tools with a cemented carbide/steel joint by applying the difficult to wet and not well researched, but for many other reasons very suitable precipitation hardening maraging steel. Especially, the comparable low coefficient of thermal expansion (CTE) and the capability of the lath martensite to compensate large amounts of externally imposed stresses during the austenite-to-martensite transformation as well as the cooling rate independent of the hardening mechanism of the maraging steel and a pre-applied nickel coating including the corresponding diffusion processes are responsible for a sound joint with a shear strength > 300 MPa. Moreover, the subsequent tempering process at 580 °C for 3 h provides the maraging steel joining partner with a hardness of 426.6 ± 6.0 HV1.
Statistical comparison of processing different powder feedstock in an HVOF thermal spray process
(2022-04-12) Tillmann, Wolfgang; Kuhnt, Sonja; Baumann, Ingor Theodor; Kalka, Arkadius; Becker-Emden, Eva-Christina; Brinkhoff, Alexander
Cermet coatings such as WC-Co and Cr3C2-NiCr are frequently applied by means of thermal spray processes to protect highly stressed surfaces against wear. The investigation of the respective spray materials and their coating properties and in-flight particle properties are often carried out in separate experiments. In this study, the coating characteristics (hardness, deposition rate, porosity, thickness) and in-flight particle properties (particle velocity and temperature) of three different WC-based powders and a Cr3C2-NiCr powder processed by means of an HVOF process are investigated as a function of some key process parameters such as kerosene flow rate, lambda, spray distance and feeder disc velocity. These parameters were varied within a design of experiments, whilst all other parameters were fixed. Both the design of experiments plan and the settings of the fixed parameters were defined identically. The in-flight particle properties and coating characteristics are statistically modeled as a function of the process parameters and their influences are compared. A well-selected, limited number of experimental runs using statistical design of experiment (DoE) enable this comparison. The deployed statistical models are generalized linear models with Gamma-distributed responses. The models show that particle velocity and particle temperature mainly depend on kerosene flow rate and spray distance. However, in the case of particle temperature, the model coefficients for Cr3C2-NiCr and WC powders have different signs, reflecting different qualitative behavior.
Structure and tribo-mechanical properties of MoSx:N:Mo thin films synthesized by reactive dcMS/HiPIMS
(2021-12-14) Tillmann, Wolfgang; Wittig, Alexandra; Stangier, Dominic; Thomann, Carl Arne; Debus, Jörg; Aurich, Daniel; Brümmer, Andreas
Modifying MoS2 thin films by additional elements shows great potential in order to adjust the property profile and to meet the increasing requirements regarding high wear resistance and low friction properties of industrial components. Within that context, MoSx:N:Mo thin films were deposited by a reactive hybrid dcMS/HiPIMS process. By systematically increasing the Mo target cathode power, an investigation of the structural and the mechanical properties was conducted to understand the evolution of the tribological behavior. A low Mo target cathode power of 1 kW is related to the formation of the preferential (002) MoS2 basal-plane and thus a low friction with µ = 0.2. With an increasing amount of Mo, the film loses its solid lubricant MoS2 properties and a nitride constitution of the thin film is developing due to the formation of crystalline Mo and MoN phases. Related to this transformation, the hardness and elastic modulus are increased, but the adhesion and the tribological properties are impaired. The film loses its plasticity and the generated film material is directly removed from the contact area during the sliding contact.
Qualification of the low-pressure cold gas spraying for the additive manufacturing of copper-nickel-diamond grinding wheels
(2021-12-05) Tillmann, Wolfgang; Zajaczkowski, Jonas; Baumann, Ingor; Kipp, Monika; Biermann, Dirk
Grinding wheels are usually manufactured by powder metallurgical processes, i.e., by molding and sintering. Since this requires the production of special molds and the sintering is typically carried out in a continuous furnace, this process is time-consuming and cost-intensive. Therefore, it is only worthwhile for medium and large batches. Another influencing factor of the powder metallurgical process route is the high thermal load during the sintering process. Due to their high thermal sensitivity, superabrasives such as diamond or cubic boron nitride are very difficult to process in this way. In this study, a novel and innovative approach is presented, in which superabrasive grinding wheels are manufactured by thermal spraying. For this purpose, flat samples as well as grinding wheel bodies were coated by low-pressure (LP) cold gas spraying with a blend of a commercial Cu-Al2O3 cold gas spraying powder and nickel-coated diamonds. The coatings were examined metallographically in terms of their composition. A well-embedded superabrasive content of 12 % was achieved. After the spraying process, the grinding wheels were conditioned and tested for the grinding application of cemented carbides and the topographies of both the grinding wheel and the cemented carbide were evaluated. Surface qualities of the ground surface that are comparable to those of other finishing processes were reached. This novel process route offers great flexibility in the combination of binder and hard material as well as a cost-effective single-part and small-batch production.
Influence of direct splat-affecting parameters on the splat-type distribution, porosity, and density of segmentation Cracks in Plasma-Sprayed YSZ Coatings
(2021-03-18) Tillmann, Wolfgang; Khalil, Omar; Baumann, Ingor
The integrity and properties of ceramic coatings produced by atmospheric plasma spraying are highly controlled by the splat morphology and splat interconnection. In this study, the influence of selected parameters (spray angle, surface velocity of the spray gun, and substrate temperature) on splat morphology and coating microstructure was investigated. A favorite set of spray gun parameters, of which their effects on splat morphology and coating microstructure have been verified by previous experiments, were used to conduct the experiments for the present work. It was found that depositing fully molten particles on a hot substrate increases the fraction of disk-like splats by about 60% at the expense of the fraction of irregular splats. Preheating the substrate also increases the pore count and level of coating porosity, while it does not influence the density of segmentation cracks. In contrast, the surface velocity of the spray gun does not affect the splat morphology while a slow speed decreases the coating porosity and plays a significant role in generating segmentation cracks. Shifting the spray angle by 15° distorts up to 20% of disk-like splats and slightly decreases the porosity level. However, changing the spray angle does not affect the generation of segmentation cracks.
Internal diameter coating by warm spraying of fine WC-12Co powders (− 10 + 2 µm) with very short spray distances up to 10 mm
(2021-04-23) Baumann, Ingor; Tillmann, Wolfgang; Schaak, Christopher Sven; Schmidt, Katharina; Zajaczkowski, Jonas; Schmidtmann, Gunnar; Götz, Matthäus; Luo, Weifeng
The internal diameter (ID) coating by means of thermal spraying is currently experiencing growing interest in science and industry. In contrast to the well-established plasma- and arc-based spray techniques, there is a lack of knowledge concerning kinetic processes such as HVOF, HVAF and warm spray (WS). A major challenge represents the necessity of short spray distances and the compact design of novel ID spray guns with reduced combustion power. Conventional WC-Co powders (− 45 + 15 µm) are not able to achieve a sufficient heat and momentum transfer. The use of fine powders < 15 µm offers an approach to overcome this drawback as they feature a larger surface-to-volume ratio and a lower mass. However, the processing of fine powders requires suitable spray equipment and a sensitive parameter adjustment. In this study, warm spraying of fine WC-12Co powders (− 10 + 2 µm) with a novel ID spray gun (HVOF + N2) “ID RED” (Thermico Engineering GmbH, Germany) was investigated. First, the flame profile as well as the in-flight behavior of the particles along the spray jet (spray distances SD = 10-80 mm) was analyzed at different nitrogen flows NF = 15-115 L/min to find suitable spray parameter intervals. Subsequently, planar steel samples were coated with SD = 10-50 mm and constant NF = 90 L/min. Analyses regarding the microstructure, the mechanical properties and the phase evolution of the coatings were performed. The aim was to study spraying with the novel ID gun and to scrutinize shortest feasible spray distances. Finally, steel tubes (internal diameter of 81.6 mm and a wall thickness of 10.0 mm) were coated with SD = 20 mm and NF = 90 L/min to investigate in how far the results can be transferred to ID parts. Correlations between the particle behavior, the microstructure and the coating properties were made.
WC decomposition phenomena in ID-HVOF-sprayed WC-CoCr coatings using fine powder feedstock
(2022-01-23) Tillmann, Wolfgang; Hagen, Leif; Baumann, Ingor; Paulus, Michael
Over the last few decades, the high velocity oxygen fuel (HVOF) spraying of WC-CoCr for internal diameter (ID) coating has attracted much interest for hard chrome replacement. Current demands for the ID coating of small cylindrical parts necessitates the use of specialized spray gun equipment and powder feedstocks with small particle size fractions. Due to the limited spray distance inside cylindrical parts with small IDs, the process control, spraying fine WC-CoCr powders, meets new challenges to avoid significant WC decomposition, which increases the risk of mechanical degradation. Within the scope of this study, ID-HVOF spraying using a fine-structured WC-CoCr (−15 + 5 μm) feedstock with a mean WC particle size of 400 nm is examined with respect to the WC decomposition phenomena using X-ray diffraction (XRD). Hence, a statistical design of experiments (DoE) is utilized to systematically analyze various spray parameter settings along with their interaction as part of the WC to W2C conversion.
The effect of argon as atomization gas on the microstructure, machine hammer peening post-treatment, and corrosion behavior of twin wire arc sprayed (TWAS) ZnAl4 coatings
(2021-12-27) Tillmann, Wolfgang; Abdulgader, Mohamed; Wirtz, Andreas; Milz, Michael P.; Biermann, Dirk; Walther, Frank
In the twin wire arc spraying (TWAS) process, it is common to use compressed air as atomizing gas. Nitrogen or argon also are used to reduce oxidation and improve coating performance. The heat required to melt the feedstock material depends on the electrical conductivity of the wires used and the ionization energy of both the feedstock material and atomization gas. In the case of ZnAl4, no phase changes were recorded in the obtained coatings by using either compressed air or argon as atomization gas. This fact has led to the assumption that the melting behavior of ZnAl4 with its low melting and evaporating temperature is different from materials with a higher melting point, such as Fe and Ni, which also explains the unexpected compressive residual stresses in the as-sprayed conditions. The heavier atomization gas, argon, led to slightly higher compressive stresses and oxide content. Compressed air as atomization gas led to lower porosity, decreased surface roughness, and better corrosion resistance. In the case of argon, Al precipitated in the form of small particles. The post-treatment machine hammer peening (MHP) has induced horizontal cracks in compressed air sprayed coatings. These cracks were mainly initiated in the oxidized Al phase.
Heat treatment of binder jet printed 17–4 PH stainless steel for subsequent deposition of tribo-functional diamond-like carbon coatings
(2021-12-06) Tillmann, Wolfgang; Lopes Dias, Nelson Filipe; Stangier, Dominic; Schaak, Christopher; Höges, Simon
Diamond-like carbon (DLC) coatings deposited on additively manufactured steel greatly improve the tribological properties. However, a high substrate hardness is crucial to sustaining high mechanical loads in the tribological contact. Herein, the heat treatment of binder jet printed 17–4 PH enhances the hardness from 24 to 39 HRC. Binder jet printed 17–4 PH substrates are coated by DLC of the types hydrogen-free amorphous carbon (a-C) of ∼23 GPa and hydrogenated amorphous carbon (a-C:H) of ∼20 GPa. The influence of the heat treatment on the tribo-mechanical properties of the DLC coatings is investigated. 17–4 PH demonstrates high friction and wear against steel counterparts, but the wear rate is reduced from 693 ± 43 × 10–6 mm3/Nm to 492 ± 41 × 10-6 mm3/Nm by heat treating the steel. Both a–C and a–C:H are effective in reducing the friction and wear with wear rates below 0.3 × 10–6 mm3/Nm. The a–C and a–C:H coatings demonstrate lower plastic wear on heat treated 17–4 PH due to the higher substrate hardness. Consequently, the heat treatment is an essential process step to ensure maximum tribological functionality of the DLC coating on additively manufactured 17–4 PH steel.
Impact of tungsten incorporation on the tribomechanical behavior of AlCrWxSiN films at room and elevated temperature
(2021-08-27) Tillmann, Wolfgang; Feher, Alexander; Stangier, Dominic
AlCrWxSiN thin films (0 ≤ x ≤ 17.1 at.%) were synthesized by means of a hybrid magnetron sputtering process, merging direct current (DC) as well as tungsten high power impulse magnetron sputtering (HiPIMS) supplies. The influences of increasing the tungsten contents on the structural as well as the friction and wear behavior at room and high temperatures (500 °C) were elaborated. As a reference, a W61.4N38.6 system served to analyze synergetic effects on the oxidation behavior. Increased tungsten contents in AlCrWxSiN resulted in more distinctive (200)-, (202)-, and (311)- crystal orientations. A W/Cr ratio of ~1 could be correlated with a denser film growth, the highest hardness (24.3 ± 0.7 GPa), and a significantly decreased wear coefficient (<0.3 × 10−5 mm3/Nm). Tribological tests performed at room temperature revealed that the coefficient of friction decreased with higher tungsten contents to µ~0.35. In contrast, at elevated temperatures, the coefficient of friction increased with higher W concentrations due to spotty oxidations in the wear track, which resulted in a locally increased surface roughness. Finally, a phase transformation of the WN film to m-WO3 did not contribute to a friction reduction at 500 °C.

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