Abstract Due to their excellent corrosion resistance, austenitic stainless steels are suitable for surface protection applications. However, the application potential is often limited by the low wear resistance. An interstitial hardening of the surface layer area can solve this problem for massive wrought alloys. Further potential for improvement lies in the transition to surface technology. For this purpose, powder feedstock of the stainless-steel grade AISI 316L was gas nitrocarburized at low temperatures. The formation of a metastable expanded austenitic phase was achieved. Subsequently, the processing was carried out by cold gas spraying. Due to the simultaneously high process kinetics and low thermal load, dense coatings were produced while maintaining the metastable state of the feedstock. When compared to solid reference systems, the scratch resistance saw a marked improvement.

Abstract Manufacturing of steel components is often done at high temperatures (HT) posing a serious challenge to components such as forming tools. Thermal spray coatings provide a cost-effective solution for surface protection under HT, corrosive environments and severe wear conditions. Thermally sprayed coatings based on cubic hard materials such as TiC and TiCN can provide an alternative to widely used Cr3C2-NiCr. While the latter possess a superb oxidation resistance and wear resistance at HT, they are prone to degradation in the presence of Mn, an element commonly alloyed in many modern steel grades such as TWIP (twinning-induced plasticity steel). In this study, a (Ti,Mo)(C,N)-29% Ni hardmetal feedstock powder was prepared by agglomeration and sintering. Coatings were deposited using a high velocity air-fuel (HVAF) spray process. The coating was benchmarked against a standard Cr3C2-NiCr coating obtained with the same spray process. Our work comprises analyses of the feedstock powder along with the resulting coating microstructure after deposition and heat treatment. Further, the HT sliding behavior against TWIP steel using a HT pin-on-disc tribometer at 700°C was investigated. The results showed a clear benefit of the TiCN-based coating, with almost no wear detected, while the Cr3C2-coating showed a significant wear loss. Based on these results, the TiCN-based coating is regarded as potential solution for prospective forming applications of modern high Mn steels, such as TWIP.

Abstract The effect of martensitic phase transformation on cavitation erosion resistance for a deposited layer prepared from a Fe-8Cr- C-1.5Al-Ti flux-cored wire of metastable steel was studied. A reference material of AISI 316L stainless steel was used as a substrate. Cavitation tests were performed using a modified ultrasonic tester. X-ray diffraction was used to examine the phase transformation before and after cavitation tests. Also, the eroded surfaces of specimens were investigated by optical microscope (OM), scanning electron microscope (SEM), and 3D optical profilometer. The cavitation results revealed that the deposited layer exhibited a resistance to cavitation erosion approximately 10 times higher than the AISI 316L steel due to the martensitic phase transformation occurring during the cavitation process. The phase transformation plays a main role to minimize the cavitation damage of specimen. This is due to the fact that it contributes to obstructing movement of dislocations and increasing the hardness as a result of the increased hardening on the surface.

Abstract Pulsed plasma transferred arc surfacing is presently used in many industrial applications to make protective layers against corrosion, temperature exposition, and excessive wear. Increasing wear resistance is especially important in areas of industry where titanium alloys are used, such as aviation and cosmonautics, because the wear resistance of titanium alloys is often weak. One way to increase the wear resistance is to deposit or form a cermet with a titanium matrix (TMC) on the surface of the part. The present study deals with the fabrication and characterization of TMC based on B4C. TMC with B4C was formed by cofeeding Ti6Al4V and B4C powder into a melting pool. It has been found that the deposited, relatively thick layers have homogeneously dispersed B4C grains in the matrix. The deposits are metallurgically connected to the substrate - Ti6Al4V. The TMCs were investigated in terms of microstructure and chemical composition. Wear resistance was determined using the linear pin test.

Abstract Thermally sprayed Al 2 O 3 -TiO 2 ceramic coatings provide exceptional hardness and corrosion and wear resistance, but the high velocities at which they are applied result in an inherently porous structure that requires some type of remediation. This study evaluates the effectiveness of ultrasonic aluminum phosphate sealing treatments on plasma sprayed Al 2 O 3 -40TiO 2 ceramic coatings. The sealants were applied with and without ultrasonication (20-40 kHz) and were assessed using SEM/EDX analysis, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). Test data indicate that optimum sealing, corresponding to the highest values of corrosion protection and erosion resistance, are achieved under ultrasonication at 30 kHz for 5 hours.

Abstract FeMnCrSi alloys have been developed and studied over the past several years with an emphasis on their use as coatings on CA6NM stainless steel hydroturbine components. Much of the work conducted has focused on the optimization of cavitation resistance through chemical composition changes, the use of different thermal spraying (ASP, HVOF, HVAF) and welding (PTA) processes, and post-treatments such as shot-peening, cold working, and PTA remelting. The aim of this current work is to present a compilation of published articles that report on the research that has been done. Among the trends observed is that coating density and cavitation resistance improve with increasing particle velocity, particularly for HVOF-kerosene spraying. In regard to post-treatments, cold working was found to most effective, reducing cavitation mass loss (in PTA FeMnCrSi coatings) by a factor of nearly two.

Abstract The present study compares needed prerequisites for the application of cavitation resistant bronzes by applying different coating techniques, such as cold spraying, HVOF spraying, warm spraying and arc spraying. By optimization to optimum cavitation resistance, the deposited coatings can increase the service life of ship rudders significantly and even serve as repair processes for ship propellers. The given overview aims to support the selection of processes when specifying the target properties to be set with regard to cavitation protection. By using high-pressure warm spraying and cold spraying, properties similar to those of cast nickel aluminum bronze were achieved, however at relatively high costs. In contrast, coatings produced by using HVOF and arc spraying have erosion rates that are only about four respectively three times higher as compared to cast nickel aluminum bronze, while far outperforming bulk shipbuilding steel. Hence, their properties should be sufficient for acceptable service life or docking intervals for ship rudder applications. Propeller repair might demand for better coating properties as obtained by cold spraying. With respect to costs, HVOF and arc spraying in summary might represent a good compromise to reach coating properties needed in application.

Abstract FeMnCrSi and 316L alloy coatings were deposited on carbon steel substrates via high-pressure cold gas spraying and their microstructure, hardness, and wear resistance were obtained. Ball-on-disk testing (ASTM G99) was used to measure sliding wear behaviors. The mechanism of wear was found to be the same for both coatings, although FeMnCrSi had a higher coefficient of friction while 316L had less volume loss.

Abstract Developing effective heating systems to prevent ice accretion on the surface of wind turbine blades and aircraft wings is of great significance for extreme cold environments. However, due to high velocity impingement of water droplets and solid particles on the surface of these components, an appreciable degree of surface material degradation may occur. In this study, nickel-chromium-aluminum-yttrium (NiCrAlY) was chosen as a metal matrix material for a coating-based heating system. Pure ceramic powders, namely, alumina and titania, and a cermet powder, tungsten carbide-cobalt (WC-12Co), were mechanically admixed with NiCrAlY powder and deposited to fabricate reinforced metal matrix composite (MMC) coatings. The powders were deposited on cylindrical low carbon steel bars by using flame spraying. The specimens were placed in a wind tunnel to conduct a comparative investigation of their erosive wear resistance under water droplet impact. A cold spraying unit was used for solid particle impact erosion tests. The erosive wear rates were quantified by measuring mass loss. The experimentally obtained results showed noticeably lower wear rate in NiCrAlY-WC-12Co and NiCrAlY-titania coatings compared to the other coatings. The results suggest that certain MMC coatings could be effectively employed to decrease the erosion rate of coating-based heating elements.

Abstract Hydroelectric turbines are strongly affected by cavitation and the damage it can cause to critical part surfaces and profiles. The study of thermal spray processes and materials is thus relevant to improving turbine performance. The main objective of this work is to evaluate the influence of fuel-oxygen ratio on tungsten- and chromium-carbide cermet coatings deposited by HVOF. Particle velocity and temperature were measured as were coating hardness, porosity, and cavitation resistance. Higher particle velocities were obtained at higher fuel ratios, producing harder, denser coatings with better cavitation resistance. Based on test results, the wear mechanism starts with the nucleation of the cavitation that occurs in the pores, resulting in the formation of craters and the eventual detachment of lamellae as indicated by the smoothness of the surface.

Abstract Two kinds of cermet powders, WC-10Co4Cr and WC-20CrC-7Ni, were deposited on 1040 steel via high velocity air fuel (HVAF) spraying to evaluate resistance in cavitation erosion conditions with additional electrochemical effects. Coating microstructure, phase composition, and microhardness were examined along with the topography of eroded surface layers. The cavitation resistance of the WC-20CrC-7Ni coating was found to be approximately 1.3 times greater than that of the other coating, which can be attributed to its finer grain structure, lower pore density, and the presence of high Cr and Ni content in the feedstock powder which serves to strengthen the matrix.

Abstract High Entropy Alloys (HEAs) are a new class of alloys with multi-principle elements in an equi-atomic ratio that present novel phase structures. HEAs are known for their high temperature microstructural stability, enhanced oxidation and wear resistance properties. Apart from bulk material consolidation methods such as casting and sintering, HEAs can also be deposited as a surface coating. In this work, thermal sprayed HEA coatings are investigated as an alternative bond coat material for a thermal barrier coating system. Nanostructured HEAs that were based on AlCoCrFeNi and MnCoCrFeNi were prepared by ball milling and then plasma sprayed. Splat studies were assessed to optimize the appropriate thermal spray parameters and spray deposits were prepared. Subsequently, the microstructure and mechanical properties of two HEAs coatings of different composition were characterized and compared to conventional plasma spray NiCrAlY bond coats.

Abstract Fiber-reinforced polymer composites (CFRP) are increasingly used in aerospace for weight-sensitive applications. However, they are subjected to degradation from the erosive forces of solid particles and water droplets. This degradation results in a decreased service life of composite components and increased repair costs. A coating can protect the CFRP surface against wear and plasma spraying could be a candidate technique to achieve this coating. However, an issue is the thermal and mechanical damage to the composite surface by the plasma-sprayed particles. Another issue is the coating adhesion, because of the low wettability of polymer surface to liquid metal and ceramic and different atomistic properties between substrate and coating material. A possible solution to both issues is the use of a primary layer deposited by a “softer” technique than thermal spraying. This study deals with the deposition of this primary layer by three methods (magnetron sputtering, air gun spraying and sol-gel) and the deposition of topcoat layer by plasma spraying. The effectiveness of the protection of the CRFP by the primary layer during topcoat plasma spraying is investigated as well as the interfaces of the duplex coatings.

Abstract This study investigates the feasibility of forming amorphous iron-based coatings using the cold spray deposition process. Splat tests of cold-sprayed SAM1651 (Fe48Mo14Cr15Y2C15B6 at.%) particles impacting a mild steel substrate were performed using varying gas temperatures and particle diameters. Specimen inspection by scanning electron microscopy revealed splat morphologies that varied from well-adhered particles to substrate craters formed by rebounded particles. Particle flow was analyzed using a finite element model, and impact conditions were predicted using an experimentally validated analytical model, in empirically generating a temperature/velocity window of successful particle deposition as a framework for ongoing work on the formation of cold-sprayed SAM1651 coatings. The results indicate that the unique characteristics of the cold spray process offer a promising means for the formation of metallic glass coatings that successfully retain the amorphous structure, as well as the superior corrosion and wear resistant properties of the feedstock powder.

Abstract The thicknesses of HVOF-sprayed WC-12Co wear protective coatings usually range between 50 – 500 μm. These thicknesses ensure a homogeneous coating structure as well as sufficient material reserves for an efficient wear protection. For applications which require high shape accuracies, a cost intensive finishing of the thick coatings, in terms of grinding and polishing, is necessary. Thus, smooth and thin coatings are favored. In this study, thin HVOF-sprayed carbide coatings are developed by means of fine sintered and agglomerated WC- 12Co powders (2 – 10 μm). The use of fine powders enables the reduction of the coating thickness down to values below 50 ìm. To influence and adjust the thickness of the coatings the powder flow, the gun velocity and the track pitch were systematically varied. During these tests the hardness and the roughness of the coatings were scrutinized.

Abstract Metal-matrix composite (MMC) coatings consisting of a nickel based metal matrix, providing toughness and tungsten carbides, providing wear resistance, are used in applications subjected to severe abrasive wear conditions. The aim of this work is to map the influence of type, morphology and amount of tungsten carbides in a nickel based matrix on the wear resistance and microstructure of laser cladded MMC coatings. Abrasive wear, evaluated according to ASTM-G65, is mainly influenced by the volume fraction of the tungsten carbides in the coating. Shape and microstructure of the tungsten carbides have a minor impact on this property, for similar degree of melting or dilution from the substrate material and size of the tungsten carbides. Microstructure analysis shows that the dissolution of the tungsten carbides in the melt pool is influenced by the chemical composition of the liquid metal phase, the microstructure and the amount of tungsten carbides selected.

Abstract For coating increasingly complex geometries that require a high accuracy with respect to a near net-shape coating distribution, the use of computer-aided path planning algorithms is mandatory to minimize the need for expensive prototyping experiments. During the planning process, coating simulations are typically evaluated frequently, thus both fast and accurate simulations greatly enhance the potential to find optimal path solutions. In this contribution, an efficient approach for computing the coating thickness on complex workpieces is presented, which makes use of the computational capabilities of mainstream graphics hardware to achieve simulation times well within the single-digit range of seconds for average-sized workpieces. Using a semi-automatic, measurement-based calibration routine, this simulation can easily be adapted to different processes. The simulation has been coupled with a path-optimization approach and was successfully utilized to enhance the deposition accuracy in the context of applying wear-resistant coatings to deep-drawing tools by means of a TWAS process.

Abstract The wear resistance of thermal spray coatings mainly depends on coating properties such as the microstructure, hardness, and porosity, as well as on the residual stress in the coating. The residual stress is induced by a variety of influences e.g. temperature gradients, difference of the thermal expansion coefficient of the coating / substrate materials, and the geometry of the components. To investigate the residual stress, the Impulse Excitation Technique was employed to measure the Young’s and shear moduli. The residual stress was determined by using the hole-drilling method and X-ray diffraction. Pin-on-Disc and Pin-on-Tube tests were used to investigate the wear behavior. After the wear tests, the wear volume was measured by means of a 3D-profilometer. The results show that the value of the residual stress can be modified by varying the coating thickness and the substrate geometry. The compressive stress in the HVOF-sprayed WC-Co coatings has a significant positive influence on the wear resistance whereas the tensile stress has a negative effect.

Abstract Cryomilled Cu-Al powder and the Cu/Al mixture were sprayed onto aluminum substrate using the cold spraying process. This study focused on the wear properties of the nanocrystalline (NC) Cu-Al coating in comparison to its coarse-grained (CG) Cu/Al counterpart. The results showed that the as-sprayed deposit presented a dense microstructure. Investigations on the worn surface of the NC coating revealed that the plastic deformation with grooves and some debris were prominent with no visible cracking. Nanocrystalline Cu-Al coating showed a good wear resistance with a low friction coefficient. The enhancement of the wear properties of the NC Cu-Al was attributed to the grain refinement and the superior hardness.

Abstract Raising demands in industry lead to the request of better and enhanced functional surfaces, also on rolls parts in paper producing machines. Due to the higher usage of post-consumer recycled fibers on paper machines the run ability and machine cleanliness become a major issue. On one hand the contaminates called stickies can be responsible for sheet brakes, holes in the paper, web picking etc. On the other hand more sand containing waste paper enhances the impact on roll surfaces in operation. In this paper the different stickies (micro / macro / primary / secondary) and their impact on the paper production will be explained and the development to avoid stickies and contaminations together with high wear resistant coatings on roll surfaces in the paper machinery will be shown. Therefore a comparison of different materials and technologies regarding surface energy and peel force measurements are explained. Together with results of a pilot scale blade test rig the results in the market regarding anti stick properties wear resistance and corrosion resistance of the developed layer stack designs will be introduced.