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.

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.

A typical structure of thermal spray coatings consisted of molten particles, semi-molten particles, oxides, pores and cracks. These factors caused the porosity of sprayed coatings, leading to a great influence on the coating properties, especially their wear-corrosion resistance. In this study, a post-spray sealing treatment of Cr3C2-NiCr/Al2O3-TiO2 plasma sprayed coatings was carried out, then their corrosion properties were evaluated, before and after the treatment. For sealing process, aluminum phosphate (APP) containing aluminum oxide (Al2O3) nanoparticles (~10 nm) was used. The permeability of APP into the sprayed coating was analyzed by scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS). The treatment efficiency for porosity and corrosion resistance of sprayed coatings were evaluated by electrochemical measurements, such as the potentiodynamic polarization and electrochemical impedance spectroscopy. In addition, the wear-corrosion resistance of the sealed coating was examined in 3.5 wt.% NaCl circulation solution containing 0.25 wt.% SiO2 particles. The obtained results showed that APP penetrated deeply through the sprayed coating. The incorporation of Al2O3 nanoparticles into APP sealant enhanced the treatment efficiency of porosity for sprayed coating. The effect of the post-treatment on corrosion protection of the sprayed coating has been discussed.

High-entropy alloys (HEAs) represent an innovative development approach for new alloy systems. These materials have been found to yield promising properties, such as high strength in combination with sufficient ductility as well as high wear and corrosion resistance. Especially for alloys with a body-centered cubic (bcc) structure, advantageous surface properties have been revealed. However, typical HEA systems contain high contents of expensive or scarce elements. Consequently, applying them as coatings where their use is limited to the surface represents an exciting pathway enabling economical exploitation of their superior properties. Nevertheless, processing conditions strongly influence the resulting microstructure and phase formation, which in turn has a considerable effect on the functional properties of HEAs. In the presented study, microstructural differences between high-velocity oxygen fuel (HVOF) and high-velocity air fuel (HVAF) sprayed coatings of the alloy AlCrFeCoNi are investigated. A metastable bcc structure is formed in both coating processes. Precipitation reactions are suppressed by the rapid solidification during atomization and by the relatively low thermal input during spraying. The coating resistance to corrosive media was investigated in detail, and an improved passivation behavior was observed in the HVAF coatings.

Cold spraying is a semi-empirical method in that spray parameters must be optimized experimentally in regard to coating quality and deposition efficiency. In this work, porosity and deposition efficiency are the key parameters in the optimization of corrosion-resistant zinc coatings produced by high-pressure cold spraying. The deposition process is described along with the tests used to assess the morphology, adhesion, and anticorrosion properties of the coatings obtained.

Nowadays combustion engines are the most common way to impel vehicles. Thereby losses occur, due to cooling, exhaust gas and friction. Modern engines roughly dissipate 8% of the chemical energy stored in the fuel because of friction in different tribological systems. The highest potentials for optimisation can be found in the tribological system of inner surface of combustion chamber and piston ring. Besides friction, corrosive stress of inner surface of combustion chamber increases e.g. due to the utilization of auxiliary systems such as Exhaust-Gas-Recovery. In order to save energy, reduce emissions and enhance the lifetime of combustion engines innovative coating material systems need to be developed, especially for inner surface of combustion chamber. This study focuses on the development of innovative iron based materials for combustion chamber application using Plasma Transferred Wire Arc (PTWA) and Rotating Single Wire Arc (RSW) technologies. In order to improve the wear and corrosion resistance boron and chromium are added into the feedstock material. After deposition, different honing topographies are manufactured in order to evaluate their influence on the tribological behavior. Furthermore, electro-chemical corrosion tests are conducted by using an electrolyte simulating the exhaust gas concentrate. In conclusion an optimised coating material deposited by PTWA and RSW and improved surface topographies can be combined.

In this study, hydroxyapatite (HA) and HA-SiO 2 coatings are applied to unalloyed Ti by atmospheric plasma spraying and corrosion resistance is assessed by immersion in Ringer’s solution for 24 h. The results show that the HA coating improves corrosion resistance, which is further improved with the addition of SiO 2 . An analysis based on Scherrer’s equation confirms an observed increase in crystallite size in the coated samples.

Commercially pure aluminum was arc sprayed on low-carbon manganese steel and electrochemical impedance spectroscopy (EIS) was carried on coating samples in a simulated marine immersion environment at 35 °C. A simple pore network circuit model was used to analyze the data and calculate the corrosion rate, which was estimated to be 5-15 μm/y from the charge transfer resistance value. After 9 months of exposure, the actual corrosion rate was found to be ~5 μm/y. The mechanism of protection offered by thermally sprayed aluminum (TSA) coatings is discussed.

The bonding between flattened particles in plasma-sprayed metal coatings dominates their corrosion behavior by influencing the porosity in coatings, especially the porosity connected to the substrate for coatings used in a corrosive environment. Therefore, how to efficiently enhance the lamellar interface bonding in metallic coatings has been an important issue which has not been settled effectively. In this study, a shell-core structured powder particle designing with cladding spherical Ni20Cr powders with refractory molybdenum as alloying element is proposed to limit the evaporation of low melting point elements and subsequently raise particle temperature significantly high enough to cause impact melting. Results show that a dense coating with much low porosity was obtained due to the improved lamellar interface bonding by gas shrouded plasma spraying of the composite NiCr -Mo particles. Electrochemical method was employed to evaluate the polarization behavior of the NiCr - Mo coating to estimate its connected porosity. It was revealed that NiCr-Mo coating of excellent corrosion resistance with low connected porosity can be obtained by designing the shell-core-structured powder.

In the current investigation plasma spray technique was used for depositing hydroxyapatite (HA) and hydroxyapatite – silicon oxide (SiO 2 ) coatings on 316L SS substrate. In HA-SiO 2 coating, 20 wt% SiO 2 was mixed with HA. The feedstock and coatings were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) / energy-dispersive X-ray spectroscopy (EDX) analyses. The corrosion resistance of the uncoated, HA coated and HA + 20 wt% SiO 2 coated 316L SS was investigated by electrochemical corrosion testing in simulated human body fluid (Ringer’s solution). After the corrosion testing, the samples were analyzed by XRD and SEM / EDX analyses. The addition of SiO 2 reduces the crystallinity of the coating. The corrosion resistance of the 316L SS was found to increase after the deposition of the HA + 20 wt% SiO 2 and HA coatings.

In this work, Al and Al-Al 2 O 3 coatings up to 8 mm thick were cold sprayed on AZ91D magnesium alloy substrates. Microstructure, microhardness, bond strength, and corrosion and wear resistance were studied to assess the viability of using these coatings to restore dimensionally degraded parts and protect them from further corrosion and wear.

In this investigation, 5083 aluminum alloy coatings were deposited on substrates of the same material by high-pressure cold spraying. Spray trials were carried out using powders with size ranges of 5-20 µm and 20-44 µm, gas temperatures of 673 K and 773 K, and nitrogen and helium process gases. Coatings and coating-substrate interfaces were evaluated primarily by SEM and EDS, while XRD was used to examine coating stresses and oxidation effects. Corrosion protection was assessed by electrochemical potentiodynamic measurements in synthetic seawater and Knoop indentations tests were conducted as a measure of work-hardening and mechanical integrity of the coatings. Test results are presented and correlated with spray parameters.

In this study, WC-CoCr coatings are deposited on grit-blasted steel substrates by high-velocity airfuel (HVAF) spraying. A cross-sectional image of the feedstock powder shows that the WC grains are evenly distributed in the Co-Cr matrix. As-sprayed coating cross-sections are examined under different levels of magnification, coating hardness is measured, and coating and powder phases are analyzed by XRD. In addition, the corrosion behavior of coated and uncoated substrates is analyzed and compared with a reference hard chrome coating.

Due to the demand for improved fuel economy as well as increased safety features, weight reduction is one of the major aims in the automotive industry. Future lightweight automotive components for the next car generation will probably use lots of magnesium alloy. These will form galvanic couples with other materials and may induce phenomena accelerating the corrosion rate of automotive components. The materials used were magnesium alloy AZ31B and several types of cold sprayed coating. The relative performance of each cold sprayed corrosion preventive compounds (CPC) was assessed in combination with the materials under several different electrochemical and accelerated corrosion tests. Baseline data for AZ31B with no CPC applied was also collected. CPC characteristics and properties are also included and discussed. The studies on bare Mg/Steel couples validated accelerated corrosion but found that CPC cold sprayed coatings mitigate corrosion rates. Thus Mg/Fe interfaces with defect-free cold sprayed coatings CPC can prevent buildup of corrosion products and reduce galvanic corrosion of automotive components.

In this paper the characteristics (microstructure, phase compositions) and electrical insulating properties of thermally sprayed alumina coatings produced by suspension-HVOF (S-HVOF) process and conventional HVOF spray method are compared. The electrical resistance (electrical resistivity) and dielectric strength were investigated using DC-electrical resistance measurements, electrochemical impedance spectroscopy (EIS) and dielectric breakdown test. The electrical resistance was determined at room temperature at different relative air humidity (RH) levels, from 6% RH up to 97% RH. Differences in the electrical insulating properties due to the different coating characteristics are discussed. The suspension-sprayed Al 2 O 3 coatings showed better electrical resistance stability at high humidity levels (> 75% RH), which could be explained by a specific microstructure and retention of a higher content of α-Al 2 O 3 . Nonetheless, the values of dielectric breakdown voltage and dielectric strength recorded for suspension sprayed coatings were lower than those of HVOF coatings.

The corrosion behavior of thermally sprayed Al-Zn alloy coatings immersed in NaCl aqueous solution was determined by using electrochemical impedance spectroscopy. Ohmic resistance and capacitance of the corrosion product film formed on the sprayed coating surface, and the corrosion resistance and electric double layer capacitance at the interface between the sprayed coating and the solution were evaluated. As immersion time in NaCl solution increased, the corrosion rate of the Al-2mass%Zn coating increased, but the corrosion rate for Al-6mass%Zn and Al-15mass%Zn coatings decreased due to the influence of the corrosion product.

The high strength nickel alloys, and in particular Inconel type alloys, are extensively used in several applications, such as aeronautics and petroleum industry, thanks to the combination of their high mechanical properties and their thermal and chemical resistance. In particular Inconel 625 is already used in oil pipelines and pipelines of large thermal plants, and the possibility to replace high cost bulk Inconel parts with Inconel coated steel parts is of great interest. On this context the first topic to allow the use of coated parts instead of bulk Inconel is the capability to provide high corrosion and thermal resistance. The aim of this study is to investigate the capability of Coldspray in the deposition of high strength materials, such as hard nickel alloys, for corrosion protection, and to compare the corrosion behaviour of Coldspray coatings with commercial HVOF deposited coatings. Inconel 625 coatings were deposited by using CGT Kinetic3000 deposition system with nitrogen as carrier gas on AISI316L flat substrates. The coating thickness ranges between 0.3 and 1.0 mm. Different feedstock materials were used and the effect of powder size distribution on the growth capability, as well on coating microstructure and porosity, were evaluated. The corrosion behaviour of Coldspray coatings were studied by electrochemical potentiondynamic analysis and compared with the behaviour of commercially available coatings deposited by HVOF that could be considered as a high-quality benchmark. In particular, the effects of the different coating microstructures due to the different deposition processes were related with the corrosion resistance. Further development and key features are finally outlined in order to candidate the Coldspray as promising technology for the deposition of high-strength nickel alloys.

Cold gas dynamic spraying (CGDS) can be used to deposit oxygen sensitive materials, such as titanium, without significant chemical degradation of the powder and with minimal heating of the substrate. The process is thus believed to have potential for the deposition of corrosion resistant barrier coatings. However, to be effective a barrier coating must not allow ingress of a corrosive liquid and hence must have minimal interconnected porosity. Thus the aim of the present study was to investigate the effects of processing, including a post-spray annealing treatment, on the deposit meso- and microstructures and corrosion behavior. Commercially pure titanium powder was deposited using pre-heated nitrogen as main and powder carrier gas using a CGT Kinetiks 4000 system to produce coatings on stainless steel. Selected coatings were debonded from the substrate, and the resultant free standing deposits heat treated at 1050° C in vacuum for 60 minutes. Changes in microhardness were measured and correlated with microstructural changes. Optical microscopy, scanning electron microscopy, X-ray diffraction (XRD), helium pycnometry and mercury porosimetry were all employed to examine the microstructural characteristics of coatings and free standing deposits, before and after heat treatment. Their corrosion performance was also investigated using potentiodynamic polarization tests in 3.5 wt% NaCl. The influences of heat treatment and corrosion behavior will be analyzed and discussed in terms of pores structure evolution and microstructural changes.

Plasma-sprayed yttria-stabilized zirconia (YSZ) can be potentially employed as electrolyte coating in solid oxide fuel cells (SOFCs). The formation of a convex coating surface with semi-melted particles will increase the specific surface area and subsequently improve the output performance of SOFCs. However, using completely molten YSZ particles during plasma spraying leads to the formation of locally flat surface owing to the spreading of the liquid droplets on impact. In this study, flame spraying was employed to deposit YSZ particles on YSZ substrate using surface-melted particles. 4.5 mol% YSZ powder particles were used as spray materials. The deposition was carried out at different spray distances on the YSZ substrate preheated to 650 °C. The surface morphology of YSZ particles was characterized by SEM. The bonding of YSZ particles with the underlying YSZ substrate was examined from cross section. The electrochemical behavior of a single cell with a structured cathode was analyzed by electrochemical impedance spectroscopy. The results show that spray distance has significant influence on the morphology of deposited YSZ particles. The cathode polarization of a structured cathode was decreased by about 30% compared to a flat cathode.

Moving from a 2-dimensional to a 3-dimensional approach to microstructure and properties has been expected eagerly for a long while to result in a dramatic increase in the knowledge of thermally-sprayed processes and coatings. To meet these expectations, in the present work, microtomography and electrochemical impedance spectroscopy (EIS) were carried out to simulate the microstructure of plasma-sprayed alumina. As-sprayed and excimer laser-processed deposits were studied. Some unexpected but relevant results, e.g. regarding pore orientation in the coatings, could be obtained. EIS simulation led to the establishing of an electrical circuit equivalent to the microstructure which simulated the insulating properties as a function of interfaces and pore interconnection. The latter was studied by microtomography. From this 3-dimensional simulation, a finite element analysis of mechanical properties was developed and compared to experimental measurements. Using this approach to microstructure and properties, excimer laser surface processing was shown to be an innovative process to modify insulating characteristics of plasma-sprayed alumina.