Abstract Industries developing cold-spray processes aim at producing dense and resistant coatings. Controlling microstructure and inter-particular fracture characteristics of sprayed coatings is essential to improve their properties. To do so; post-spraying heat treatment is a promising approach. This work addresses the development of such heat treatments and focuses on the analysis of recovery and recrystallization. Different heat treatment parameters were explored; namely; holding temperature and time; heating rate; and heating method. This approach revealed a competition between recrystallization and other microstructural evolution mechanisms; such as precipitation and porosity coalescence. An optimized heat treatment; allowing microstructural softening and adequate mechanical properties; was sought after. First; differential scanning calorimetry measurements applied to as-sprayed coatings enabled to identify recovery and recrystallization temperature ranges. Then; a variety of heat treatments was applied; involving long-time isothermal holdings as well as shorter cycles. Microstructure analysis and hardness measurements allowed making a first selection of treatment conditions.

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 costeffective 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 Additive manufacturing processes have been used to produce or repair components in different industry sectors like aerospace; automotive; and biomedical. In these processes; a part can be built by either melted particles as in selective laser melting (SLM) or solid-state particles as in the cold spray process. The cold spray has gained significant attention due to its potential for high deposition rate and nearly zero oxidation. However; the main concern associated with using the cold spray is the level of porosity in as-fabricated samples; altering their mechanical properties. These pores are primarily found in the regions where adiabatic shear instability does not occur. It is worth noting that the deformation of the impacted solid particle plays a vital role in reaching the shear instability. Therefore; for investigating the adiabatic shear instability region; an elastic-plastic simulation approach has been used. For this purpose; it is assumed that an elevated temperature solid Ti6Al4V particle impacts on a stainless-steel substrate surface at high velocity. The results show that increasing particle temperature will significantly enhance particle deformation because of thermal softening. Additionally; they illustrate that a material jet responsible for producing a bonding between particle and substrate by ejecting the broken oxide layer will be formed when the particle has a temperature above 1073 K and substrate remains at room temperature. In the end; it should be noted that increasing particle temperature up to 723 K will not have a significant effect on substrate deformation and final substrate temperature.

Abstract (MnCu)3O4 spinel coatings are good candidates for Cr-positioning protection on stainless steel interconnect. The spinel coatings can be formed by sputtering MnCu metallic coatings followed by a hot oxidation treatment. To understand how the elements diffuse in the MnCu/steel system; a homogenization diffusion-couple model was built with considering Mn oxidation at coating surface. According to the simulation; the diffusion of Fe from the steel substrate to the MnCu coating occurred; while Cr was almost trapped under the MuCu coating. Cu-rich metallic phase was formed under the Mn-oxide layer at the early oxidation time. The solid solubility of Cr in such Cu phase was very low which can function as a Cr blocker so that Cr external oxidation can be inhibited. The inward diffusion of Mn from the coating to the substrate took place with formation of a concentration peak of Mn at the interface. The thermodynamic calculation showed that the formation of Mn peak at the MnCu-steel interface was probably due to a dissolution effect of Mn with Fe and Cr. The thermodynamic behavior was discussed also by considering the surface oxidation.

Abstract In cold spray; high adhesion of soft materials on hard substrates is commonly achieved by using helium as the propelling gas. This is the case of copper coatings on steel where adhesion may reach values as high as 60 to 80 MPa (glue failure); however; helium is a limited; expensive natural resource; and the use of more abundant nitrogen gas is preferred in an industrial setting. Unfortunately; when using nitrogen gas; little to no adhesion is obtained. In order to eliminate the use of helium gas we studied how laser assisted cold spray could lead to an improvement in adhesion of nitrogen sprayed copper coatings. In this work; several laser parameters (e.g.; power and spot size) and process parameters (traverse speed; relative position laser spot vs. gas jet) were varied at a coupon level. Upon optimization; an equivalent adhesion to the coatings prepared with helium was obtained. Furthermore; the cross section of the coatings showed that the copper particles penetrated the steel; similar to what is observed when using helium gas. Optimization of these parameters for application to large diameter (~559 mm) cylinders was also performed. A discussion on the mechanisms which contribute to achieving high adhesion considering the use of helium versus laser assistance is provided.

Abstract A novel powder modification method based on the simultaneous softening and agglomeration of steel powders via heat treatment in a rotary tube furnace has been investigated as a promising way to improve H13 tool steel powder cold sprayability. By adjusting starting powder size and shape as well as heat treatment conditions (maximum temperature; cooling rate and heat treatment atmosphere); cold spray of H13 powder went from virtually no deposition to the production of dense; sound and thick deposits with a powder deposition efficiency of 70%. Powder agglomeration; surface state; microstructure evolution and softening are identified as key factors deter-mining the powder deposition efficiency and resulting deposit microstructure.

Abstract The FeMnCrSi and 316L alloys coatings were obtained by high pressure Cold Gas Spray process on a carbon steel substrate; their microstructure; hardness; and wear resistance were obtained. The ball-on-disk testing; ASTM G99; was used to evaluate the coatings and the substrate sliding wear behavior. The mechanism of wear was the same for both coatings and FeMnCrSi had higher coefficient of friction; but lower volume loss than 316L.

Abstract The influence of plasma spraying parameters on the mechanical properties of coatings has been studied. ZrO2-Y2O3 coatings were sprayed onto stainless steel and aluminium substrates at temperatures of about 75°C and 225°C. An original set of samples, facilitating the measurement of substrate deflection, was used to evaluate the effect of thermal cycling under different spraying conditions. In order to correlate thermal cycling values and mechanical properties to coating microstructure, the coatings were impregnated with low-viscosity resin and examined under a confocal microscope. The results reveal the influence of spraying temperature, substrate properties, and torch-substrate velocity on coating damage.

Abstract Since plasma jets, which have been used as heat sources of thermal plasma spraying process, expand adiabatically under a low pressure environment, the plasma temperatures drastically fell down to 2000K at the nozzle out let at 30Pa chamber pressure. However, the plasma jets still had enough reactivity to form hard nitride layer on the surface of the titanium samples by only a few minutes treatment. In this study, in order to obtain useful information for the practical applications of this plasma as low temperature and high rate surface modification processes, nitriding of nitriding steel and carbon steel using supersonic expanding hydrogen/ nitrogen mixture plasma jets were carried out. Consequently, though surface hardening was occurred slightly in the case of carbon steel, surface hardening was obviously promoted in the case of nitriding steel. In both cases, surface hardening was promoted with increasing hydrogen flow rate and thermal damages of the samples due to heat transfer from plasma jets weren't observed. Besides, according to the results of wear testing, wear mass loss of nitrided samples were much lower than that of non-nitrided samples. From these results, this process was found to have a high potential even in the case of surface modification of steel materials.

Abstract A three-dimensional model of free-surface flows with heat transfer, including solidification, was used to model the build-up of a coating layer in a thermal spray process. The impact of several nickel particles on a stainless steel plate in different scenarios was considered. Particles diameter ranged from 40 to 80 µm and their impact velocity ranged from 40 to 80 m/s. Particles were initially super-heated; their temperature ranged from 1600 to 2000°C. Fast growth of solidification was found to be one cause of particle splashing in thermal spray coatings. Different splat morphologies obtained from the numerical model were comparable with those obtained from the experiments. Simulation of the sequential impact of two nickel particles showed side-flow jetting and particle splashing observed in experiments. The numerical model proved to be capable of simulating different impact scenarios that occur in a thermal spray; this was demonstrated by simulating nine consecutive particles during their impact on the substrate. Several characteristics of a coating layer build-up such as particle splashing and formation of small satellite droplets and rings around the splat could be seen in the numerical results. Particle splashing is one possible cause of porosity formation in thermal spray coatings.

Abstract A numerical model is developed to study the effects of the contact resistance, droplet impacting droplet temperature, and substrate temperature on the droplet solidification rate and temperature of the droplet under the condition when the substrate can melt and re-solidify. Two-dimensional simulations show that the interface velocity is small in the area of poor contact with an irregular solidification interface shape. During the impact of Molybdenum on a steel substrate, Mo solidifies while the steel substrate melts.

Abstract Cold work and heat treatment influence the mechanical properties, residual stress-state, and corrosion resistance of austenitic stainless steels. In this study we have examined changes in the defect substructure and microstructure of Type 304 stainless steel resulting from surface preparation, and deposition of bond coats and thick ceramic coatings using plasma spray methods. The structure of the stainless steel was examined as a function of depth from the coating surface using optical and transmission electron microscopy, and x-ray diffraction. Grit blasting was found to severely cold work the material to a depth of tens of microns, and the amount of cold work varied with measured abrasive particle velocity. The heat input to the surface as a result of depositing a metallic bond coat or thick ceramic coating resulted in substantial annealing of the cold work imparted into the substrate by surface preparation. There was, however, no evidence of change in grain size near the substrate-coating interface that could be attributed to recrystallization or grain growth in the substrate.

Abstract The microstructure of arc sprayed stainless steel 316L coatings appears mainly in bright white matrix, deteriorated layers (grey), and black pores under optical microscopy. The black pores and the chromium-depleted areas in the deteriorated layers are known as the factors for decreasing the ability of protecting substrate under corrosive environments. Results of experiments in this paper suggests, in the condition of this study besides the factors mentioned above, Fe-Cr oxides should be another factor of dominating the corrosion resistance in the coatings. It also describes that the quantity and the distributions of such oxides are great influence on the corrosion behaviors. In this study, two kinds of coatings were used, one with thick deteriorated layers and another with thin deteriorated layers, which were sprayed on mild steel substrate by air atomization and nitrogen atomization respectively. Salt spray test and salt-water dip test were carried out to investigate corrosion behavior in macro and micro view. An effect of sealing treatment on the performance of the coatings was also examined. Results of metallographic examination and image processing analysis are well supported by a detailed investigation of corrosion behaviors of individual phases.

Abstract Individual splats are the building blocks of any thermal spray coating. Near the coating-substrate interface, they affect coating properties like adhesion strength. This article examines the effect of substrate heating on droplet splashing. Nickel powder was plasma-sprayed onto a polished stainless steel substrate at various temperatures and the resulting splats were analyzed. Droplet splashing was observed experimentally for three different cases: low substrate temperature, high substrate temperature, and droplet-splat interaction. Mechanisms for splashing were explained with the help of computer-generated nickel droplet impacts. The article proposes that the jetting of molten metal is not triggered by the formation of a central splat but rather a solidified ring on the periphery of the splat. It was observed that, on substrates below 350 deg C, splashing is triggered by solidification at the edge of the spreading droplet. Interactions with previously deposited splats also cause droplets to splash.

Abstract Molybdenum splats were produced at three plasma conditions on steel substrates preheated to three temperatures. Morphology of splats and corresponding craters formed on substrates were observed; dimensions of splats and craters were measured with an optical non-contact interferometer. It is found that substrate is significantly melted and deformed upon impact of the droplet, which leads to the formation of flower like splats and craters. On average, only about 36 to 53 % of the areas covered by splats were in good metallurgical/mechanical contact with substrate. Normalized crater volume increases with droplet size and the contact is improved for the high particle energy/high substrate temperature condition as compared with low particle energy/medium substrate energy condition. Splat morphology and crater formation is explained based on impinging jet heat transfer model.

Abstract This paper is devoted to the study of alumina splats formation and the adhesion/cohesion of alumina coatings on oxidized 1040 steel substrates. When preheating with the plasma torch, a duplex oxide layer is formed with an upper hematite and an under magnetite. Measurements have been performed on oxide layers with 470 nm average thickness corresponding to preheating times of about 10 min. In these conditions, the relative thickness of each sub-layer can be controlled by monitoring the heating rate and the preheating temperature. When the hematite upper layer is thick, it is mostly broken by the droplet impact and correspondingly the resulting coating has a poor adhesion (? 34 MPa) the rupture occurring within the oxide layer. When the hematite upper layer is thin, the droplet impact breaks only partly the oxide layers and the rupture of the coating occurs both within the oxide layer and the substrate surface (? 40 MPa).

Abstract The focus of this paper is to determine the friction and wear characteristics between conventional metal-metal contacts and cermet-metal contacts. A WC-Co based cermet, applied by the High Velocity Oxy-Fuel (HVOF) process on a carbon steel substrate has been investigated in contact with cast iron pins. That has been compared with conventional carbon steel/cast iron couples. A mineral oil and a mineral oil containing the additive zinc dialkyl dithiophosphate (ZDDP) were used as the lubricants. The formation of a wear film has been shown to vary on the metal and cermet surface. The friction and wear response and the wear film nature are discussed.

Abstract Coatings have been prepared using the Diamond Jet hybrid and JP5000 high velocity oxyfuel (HVOF) systems with the objectives of improving corrosion resistance and reducing costs through increasing deposition efficiency. Models relating deposition efficiency, coating oxygen content and corrosion resistance to process parameters including fuel flow rate, oxygen flow rate and stand-off distance have been developed. A corrosion test cell has been designed and a procedure determined for studying the corrosion behaviour of large numbers of thermally sprayed coatings in an efficient manner. A significant improvement to the corrosion resistance of HVOF sprayed coatings has been achieved by spraying parameter optimisation and investigation of powder size and distribution. The project has also investigated the influence of spray angle on coating performance with a view to future onsite application. Coating materials tested and compared include nickel alloys Hastelloy C276 and 59, cobalt alloy Ultimet, duplex stainless steel S32750 and an experimental iron-based spray-fuse composition.

Abstract Borides are promising materials with good wear and corrosion resistance properties. Boride coatings are expected to perform better where wear and corrosion resistances are simultaneously required. Zirconium diboride is an important emerging material for such applications, due to its high hardness, high melting point, good wear resistance and corrosion as well as high temperature oxidation resistance. Special properties of laser beam like beam directionality, high intensity and spatial resolution makes laser alloying a fast and efficient technique for producing improved wear resistance coatings. In the present work, mild steel was laser alloyed with ZrB2, using "two-stage" technique of laser alloying. These coatings after characterization by optical microscopy, SEM, EDAX and XRD techniques were tested on a "Pin-on-Disk" machine for determining their wear resistance.