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.

The product quality of selective laser melting (SLM) is closely related to the alloy powder characteristics, including the size distribution and the oxygen content. In this work, the 316L stainless steel powder was prepared by a vacuum atomization furnace and sieved into a normal-sized distribution range from 15 to 53 μm with a median diameter of 37.4 μm, and a fine-sized distribution range from 10 to 38 μm with a median diameter of 18.9 μm. Then they were mixed with each other in different proportions. The results show that, under the condition of the same SLM parameters, the SLM part, with adding a large amount of fine-sized powder, has a lower density and strength, as well as more holes and spheroidized particles, compared with the SLM part with adding a small amount of finer-sized powder. Furthermore, the 316L stainless steel powder with a high oxygen content was prepared by a non-vacuum atomization furnace. Although the 316L stainless steel powder with a high oxygen content can be evenly spread in the SLM process, the surface layer of the powder is easy to form an oxide film during the cooling and solidification of powder inside the molten pool. Under the action of thermal stress, the small crack forms and expands along the oxide film, eventually leading to large cracks inside the melt channel.

316L stainless steel and Hastelloy C alloy powders were sprayed by an HVOF apparatus onto mild steel substrates. The microstructure, pore size distribution, composition and corrosion resistance of thus obtained coatings were evaluated experimentally. Corrosion resistance in sea-water was examined by monitoring the impedance and corrosion potential of samples immersed in artificial sea-water at 300 K over a period of more than 3 months and also by polarization measurement. It was found that the stainless coatings composed mainly of plastically deformed particles and some splats which were molten at the impact. By increasing the combustion pressure, the porosity as measured by mercury porosimeter could be reduced to below 1%. In comparison, Hastelloy C deposits sprayed under the standard condition were so dense that its porosity could not be measured by the porosimeter. The polarization curve and the results of impedance monitoring both exemplified that the Hastelloy C coatings possess much superior corrosion resistance to the stainless coatings in sea-water, which was attributed to the higher density and better adhesion of the Ni-base alloy coatings.

In this work, a novel additive manufacturing process was proposed and employed in the production of stainless steel components. The underlying concept is to use selective laser melting (SLM) to fabricate a core structure onto which basic features are added by cold spraying (CS), followed by heat treatment and finish machining. The microstructure and mechanical properties of as-fabricated and heat-treated parts were studied, and interfacial bonding between the SLM core and a typical CS feature was assessed. In the as-fabricated state, it is observed that the CS material has a dendritic structure similar to the feedstock, while the SLM core is characterized by cellular subgrains confined in coarse grain structures. Following heat treatment, interparticle boundaries are less well defined, equiaxed coarse grains and twinning appear, and the extremely fine subgrains in the SLM material are enlarged. Heat treatment is also shown to improve tensile strength in the CS material and interfacial bond strength between the CS features and SLM core.

In this work, a 2D axisymmetric model of gas atomization at unsteady state that accounts for break-up and solidification is used to simulate laser melting of gas atomized powder. With an optimal nozzle width of 0.6-1 mm and a nozzle angle of 30-32°, the yield of fine 15-45 μm stainless steel powder, suitable for selective laser melting, is shown to increase from 20% to 35%. The effect of laser power on the melting channel width, microstructure, and mechanical properties of the sample is also investigated.

Fatigue properties of the Al 2 O3 plasma-sprayed SUS316L stainless steel rod specimens coated on different spraying conditions have been studied in a physiological saline solution (0.9 % NaCl solution) to evaluate the potential of surgical implant application. Fatigue tests were conducted in push-pull loading at the stress ratio of R = -1, and frequency of 2 Hz. Microstructure related with fatigue damage was examined by SEM and TEM. The fatigue strength of Al 2 O 3 plasma-sprayed metals significantly depended on spraying conditions: the effects of spraying on fatigue strength decreased with increasing the applied stress amplitude. As-blasted specimens were higher in fatigue strength than Al2O3 plasma-sprayed specimens. It was found that the plasma spraying had significant effects on fatigue crack growth behavior in the early stage of crack propagation. Fatigue cracks preferentially originated from dents that had been caused on the substrata metal surface subjected to grit-blasting. These results are discussed with both the compressive residual stresses due to the grit blasting which was carried out prior to plasma spraying and the corrosion-resistance of the alumina deposit.

SUS316L coatings were sprayed by a high-velocity air fuel (HVAF) system to reduce oxidation and thereby improve the corrosion behavior of stainless coatings. The effects of powder feed rate and particle size on the microstructure, oxide amount and adhesion strength of the coatings were investigated. The corrosion resistance of the coatings was evaluated by conducting salt spray tests. The oxide amount in the coatings sprayed by the HVAF process is below 7 % and adhesion strength is below 34 MPa. In comparison with those of coatings sprayed by a high velocity oxygen fuel (HVOF) system, the oxide amount and adhesion strength are decreased because the particles heated below the melting point of the alloy and insufficient softened in HVAF process. The coatings deposited are original porous, and they become denser through the impinging effect caused by the following sprayed particles. With the increase of powder feed rate and particle size, there is a tendency of reduction in oxides, and an obvious decrease in adhesion strength. Corrosion resistance of the unsealed coatings is insufficient, and this becomes notable with increasing powder feed rate and particle size. The sealed HVAF coating sprayed with the largest particles shows the best corrosion resistance.

316L stainless steel powder was sprayed by a high-pressure HVOF process. Effects of powder size and the pressure in the combustion chamber on the velocity and temperature of sprayed particles were studied by using an optical instrument firstly at the substrate position. A strong negative correlation between the particle diameter and temperature was found whereas the correlation between the diameter and the velocity was not significant. The pressure in the combustion chamber affected the velocity of sprayed particles significantly whereas the particles' temperature remained largely unchanged. In-situ curvature measurement was employed in order to sturdy the process of stress generation during HVOF spraying. From the measured curvature changes, the intensity of peening action and the resultant compressive stress by HVOF sprayed particles were found to increase with the kinetic energy of the sprayed particles. The results were further used to estimate the stress distribution within the coatings.

The equiaxed microstructure of 316L stainless steel coating was successfully deposited by low pressure plasma spraying (LPPS), which was different from the lamellar microstructure prepared by other thermal spraying technologies. In this article, the effect of substrate temperature during deposition process and post annealing treatment on the lamellar – equiaxed microstructural transition were investigated. The results indicated that the homogeneous equiaxed grains without lamellar boundaries coatings were observed when the deposition temperature was about 900 °C. Completely lamellar microstructural coatings were deposited at the substrate temperature of about 300 °C, and the lamellar microstructure can transform to equiaxed microstructure after annealing treatment. The hardness of equiaxed coating was lower than lamellar coating.

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.

The conventional high-velocity oxy-fuel (HVOF) process has characteristics of high flame velocity and moderate temperature, and is widely used to deposit cements, metals and alloys coatings such as WC-Co, nickel and stainless steel. In this paper, a high pressure HVOF system with combustion chamber pressure up to 3.0MPa, and with characteristics of higher flame velocity and lower temperature was developed. In-flight particle velocity was measured using the DPV-2000 system at combustion chamber pressures from 1.0 to 3.0MPa, and stainless steel 316L powder was deposited at a combustion chamber pressure of 3.0MPa. The influence of spray conditions on the coating microstructure, deposition efficiency and micro-hardness were investigated. It was shown that the combustion chamber pressure has significant influence on particle velocity. Dense coatings composed of unmolten or partially molten particles could be deposited by varying the spray parameters. In the experiment, deposition efficiency up to 90% was achieved at the optimized spray conditions.

Porous materials possess many unique properties. Many processes have been employed to fabricate porous materials. Recently, it was found that a porous deposit can be created through controlling the deposition of semi-molten spray powder particles. In this study, porous 316L stainless steel deposits were prepared by flame spraying under different spray conditions to aim at controlling deposit porosity. During deposition the temperature of the substrate and/or deposit surface was kept at 600°C to ensure bonding formation between the spray particle and underlying particle. To understand the deposition process mechanism, the velocity of spray particles was quantitatively measured and the morphology of deposited isolated particles was characterized by scanning electron microscopy to estimate the melting degree of spray particles before impact. The microstructure of porous 316L stainless steel deposits was characterized by scanning electron microscopy and deposit porosity was estimated from its cross section by image analysis. It was found that pores were uniformly distributed in the deposit. The results showed that porosity of the 316L deposits changed from 20% to 50% with the change of spray particle melting degrees, resulting from the change of spray parameters. The examination showed that 3D through-deposit pore-net structure was created by viscoplastic flowing of the molten fraction of semi-molten particles which forms the bonding between necks of particles. The present results clearly demonstrated that a porous ferrous alloy based material can be formed by spray deposition through controlling the melting degree of spray particles.

In this study, the in-situ corrosion behavior of an Fe-based amorphous coating is investigated in a simulated deep sea environment (80 atm). FeMoCrYCB powder produced by gas atomization was deposited on 316L stainless steel substrates by HVOF spraying. The amorphous iron coatings exhibited greater pitting resistance than stainless steel under high hydrostatic pressures, evidenced by higher pitting potential, longer pitting incubation time, and reduced pitting growth. Passive films that formed on the amorphous coatings were also analyzed and found to be thicker, more uniform, and harder than those that developed on 316L stainless steel, indicating that the former are more difficult to break down and more resistant to Cl- ion penetration.

Stainless steel coatings were produced by supersonic air-gas plasma spraying method. Mixture of air and natural gas was used as a plasma forming gas. Powders of 304L and 316L stainless steels were used for plasma spraying. Thickness of coatings was up to 3 mm. Coating structure was studied. Dependence of oxygen content in coatings upon particle sizes and spraying conditions was established. Investigation of electrochemical properties of coatings was carried out by potentiostatic method. Corrosion potential and corrosion rate in sea water, hydrochloric and sulphuric acid solutions were determined.

To manufacture a protective coating with low thermal conductivity and good frictional wear performance, a Fe 59 Cr 12 Nb 5 B 20 Si 4 coating was designed and produced by high velocity oxygen fuel (HVOF) spraying; the properties and performance of this coating where then compared with those of a commercially available AISI 316L stainless steel coating. In the as-deposited state, both coatings exhibit dense layered structures with porosity below 1% and slight oxidation. The microstructure of the Fe-based coating has an amorphous matrix and some precipitated nanocrystals. The result is that the designed Fe-based coating has a thermal conductivity (2.66 W/m·K) that is significantly lower than that of the 316L stainless steel coating (5.87 W/m·K). Based on its advantageous structure, the Fe-based coating exhibits higher microhardness, reaching 1258±92 HV. The friction coefficient and wear rate of the Fe-based coating show an increase at 200°C followed by a decrease at 400°C, due to the evolution of the wear mechanism at different temperatures. The dominant wear mechanism of the Fe-based coating at room temperature is fatigue wear accompanied by oxidative wear. At 200°C, due to the existence of “third body” abrasive wear, the wear process was accelerated. The large-area oxide layer is likely responsible for the decrease of friction of the coating at 400°C.

To improve the mechanical properties of 316L stainless steel coatings prepared by plasma spraying, post-spray heat treatments were conducted at 600-800 °C for 1-2 hours. The effect on microstructure and hardness was assessed via XRD and SEM analysis and microhardness measurements. The results show that heat treatment at various temperatures improved coating hardness as well as fracture behavior.

Cold spray processing of stainless steel coatings, which represent a cost-effective method for wear and corrosion resistance, has been demonstrated as technically feasible. However, these coatings have very low tensile strength in the as-sprayed condition and may also exhibit a marginally higher wear rate. In this study, the cold spraying of 316L stainless steel coatings was investigated to assess the effect of powder size distribution and post-spray heat treatment on strength and wear properties. Coatings on aluminum and steel substrates were produced with a feedstock powder obtained in three particle size distributions. All coatings were deposited under the same conditions using nitrogen as the propellant gas, and then annealed at the optimum temperature. The microstructure and mechanical properties of both as-sprayed and heat-treated coatings were evaluated and the results are presented in the paper.

Importance of coating adhesion in a corrosive environment was studied experimentally. Tensile adhesion strength of HVOF sprayed 316L stainless steel and Hastelloy C coatings were tested in as-sprayed condition as well as after immersion in seawater. It was found that the adhesion strength of the stainless steel coatings degraded rapidly whereas that of the Hastelloy coatings remained almost intact. Specimens with an artificial defect were also immersed in seawater. The cross sectional observation after the test revealed that the corrosion at the coating-substrate interface proceeded much faster with the stainless steel coating as compared to the Ni-base alloy coating. A model experiment to simulate the galvanic corrosion of a coating-substrate couple was carried out and no significant difference in the galvanic current density was found between the two coatings when coupled with the steel substrate. The tightness of the coating-substrate interface was then tested with a fluorescent dye penetration test. The dye could penetrate the boundary between the stainless steel coating and the substrate whereas the boundary between the Ni-base alloy coating and the substrate was so tight that no penetration occurred. The size of the micro-gaps at the coating-substrate boundary was discussed from the viewpoint of classical Washburn-Ridiel theory. It was concluded that such micro-gaps between the coating and substrate must be eliminated for these barrier-type coatings to be used in corrosive environments. Heat treatment was highly effective for suppressing the preferential corrosion at the coating-substrate boundary.

Arc sprayed coatings are attractive means to protect components from abrasion wear provided they contain enough hard phases. Because of their hardness and toughness 316LTiB 2 cermets were selected as the basis for developing wear resistant coatings. Core wires composed of 304 stainless steel sheaths filled with 10 to 65 wt % TiB 2 , 1 to 15 wt % additives and remaining 316L stainless steel were fabricated and arc sprayed with air. The arc sprayed stainless steel-TiB 2 coatings were submitted to the ASTM G65-B abrasion test and the volume loss was measured with an optical profilometer. As expected, the volume loss decreases and the proportion of TiB 2 increases. However, large differences in volume loss between coatings that contain about the same volumetric proportion of hard phases cannot be explained by a linear reationship. The inverse rule of mixing was proposed. This inverse rule of mixing was found particularly useful for determining the influence of additives. Tin, added in the core as a fugitive liquid transfer agent, was found to improve the wear resistance of coatings. These advanced arc sprayed stainless steel-TiB 2 coatings can be favorably compared with coatings obtained by arc spraying commercially available solid and core wires.

Oxidation of HVOF sprayed 316L stainless steel coatings was studied experimentally. Oxygen content in the sprayed coatings was analyzed and its dependence on several spray parameters such as spraying distance, mixture ratio of fuel to oxygen, and composition of atmospheric gas on the substrate was studied. The oxygen content in the original powder was about 0.03 wt%, which typically increased to 0.3 % in the HVOF sprayed coatings under the standard spraying conditions. Reduction of spray distance significantly increased the oxygen level due to the excessive heating of substrates by the flame. The sprayed deposits were analyzed by XRD and the oxides within the coatings were identified as magnetite Fe 3 O 4 or chromite FeCr 2 O 4 . By using a nitrogen-gas shield attached to the substrate, it was revealed that the oxidation during flight is around 0.2 wt%. Control of oxidation by attaching a gas shroud to the HVOF nozzle has been attempted and oxygen content below 0.15 % has been achieved so far while maintaining deposition efficiency over 73 %.