Cold Gas Spraying of Solution-Hardened 316L Grade Stainless Steel Powder

Austenitic steels are characterized by their outstanding corrosion resistance. They are therefore suitable for a wide range of surface protection requirements. The application potential of these stainless steels is often limited by their poor wear resistance. In the field of wrought alloys, interstitial surface hardening has become established for simultaneously acting surface stresses. This approach also offers great potential for improvement in the field of coating technology. The hardening of powder feedstock materials promises an advantage in the treatment of large components and also as a repair technology. In this work, the surface hardening of AISI 316L powder and its processing by thermal spraying is presented. A partial formation of the metastable expanded austenitic phase was observed for the powder particles by low-temperature gas nitrocarburizing. The successful deposition was demonstrated by cold gas spraying. The amount of expanded austenitic phase within the coating structure strongly depends on the processing conditions. Microstructure, corrosion and wear behavior were studied. Process diagnostic methods were used to validate the results.

Qualification of the Low-pressure Cold Gas Spraying for the Additive Manufacturing of Copper–Nickel–Diamond Grinding Wheels

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.

Tribological Properties of Cold Gas Spraying FeMnCrSi Alloy

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.

Qualification of the Low-Pressure Cold Gas Spraying for the Additive Manufacturing of Copper-Nickel-Diamond Grinding Wheels

Grinding wheels are usually manufactured by powder metallurgical processes, i.e. by moulding and sintering. Since this requires the production of special moulds 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 a grinding wheel body 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 (8-12 μm). The coatings were examined metallographically in terms of their composition. Afterwards, the grinding wheel was conditioned for the grinding application and the topography was evaluated. This novel process route offers great flexibility in the combination of binder and hard material as well as a costeffective single-part and small-batch production.

Investigation of Agglomerated and Porous Ceramic Powders Suitable for Cold Spray

Cold gas spraying is a solid-state deposition process developed for metallic powders as feedstock materials. For ceramic materials; such low temperature-high velocity kinetic process is still questionable but could have interesting advantages. In the CERASOL project (ANR-19-CE08-0009); the nature and the architecture of porous ceramic powders involving agglomerated sub-micrometric grains are investigated. To that purpose; three oxide ceramics powders (alumina; zirconia and yttria) have been prepared for cold spray. These powders were analyzed in order to assess their architecture (composition; particle size; porosity; density; crystallite sizes…). Preliminary cold spray experiments were carried out implementing velocities measurements for various stand-off distances and spraying of coupons with line experiments. The characteristics of the deposited layers have been examined by SEM and XRD in order to discuss the role of the powder architecture on the impact behavior of the nanostructured agglomerated particles. The role of the gas stream that affects the kinetic and the trajectory of the particles are also discussed.

Ni-Based Coatings for Oil and Gas Industry Fabricated by Cold Gas Spraying

This paper presents the results of the study of nickel-based coatings fabricated by cold gas spraying. In this study, compositions based on Ni, Ni–Cu, Ni–Zn, and Ni–Al2O3/TiC coatings applied to low-alloyed steel bases were investigated. The composition, type of powder (mechanical mix or mechanically alloying), and thickness varied to choose the optimal characteristics for recovery, repair procedures, and specific applications in the oil and gas industry media. The second phase was added to Ni-based coatings to increase corrosion and wear resistance. Pure nickel coatings were also studied as a benchmark. Corrosion resistance was studied by means of electrochemical testing and autoclave testing in simulated oilfield conditions. Hydroabrasive resistance was studied using a unique testing bench. Scanning electron microscopy mappings, microhardness testing, and adhesion testing were used to correlate the results of the tests with the structure, continuity, and porosity of the studied coatings. It was shown that applying mechanical alloying of the powder did not lead to an effective increase of corrosion and hydroabrasive resistance. All the studied coating specimens have a sufficiently high adhesion. Ni–Zn coating has the lowest corrosion resistance and high hydroabrasive resistance. Ni–Cu coatings have high corrosion and the lowest hydroabrasive resistance. Al2O3/TiC additives give ambiguous results in the studied properties. A thickness of 40–60 microns provides sufficient performance of the studied coatings. Thus, varying chemical composition and thickness of coatings allows for obtaining the optimal qualities of Ni-based coatings made by cold gas spraying for use in the oil and gas industry.

Cold Gas Spraying of Nickel-Titanium Coatings for Protection Against Cavitation

Cavitation erosion is a sever wear mechanism that takes place in hydrodynamic systems. Examples are turbine vanes of hydropower plants or components of valves and pumps in hydraulic systems. Nickel-titanium shape memory alloys (NiTi) are attractive materials for cavitation-resistant coatings because of their pronounced intrinsic damping mitigating cavitation-induced erosion. In this work, NiTi coatings were produced by cold gas spraying. The phase transformation behaviors of the powder feedstock and the as-sprayed coatings were investigated. Regarding the obtained transformation temperatures, the measured substrate temperatures during spraying rule out that either the shape memory effect or the pseudoelasticity of NiTi could affect the deposition efficiency under the applied conditions of cold gas spraying. Another potential effect is stress-induced amorphization which could occur at the particle–substrate interfaces and impair particle bonding by stress relaxation. Moreover, also oxide formation can be significant. Thus, the presence of amorphous phases and oxides in the near-surface zone of particles bounced off after impact was investigated. Oxidation could be confirmed, but no indication of amorphous phase was found. Besides, also the evolution of local microstrains implies that the substrate temperatures affect the deposition efficiency. These temperatures were significantly influenced by the spray gun travel speed.

Cold Gas Spraying of a High-Entropy CrFeNiMn Equiatomic Alloy

Cold gas spraying was used to make a coating from an equiatomic CrFeNiMn high-entropy alloy. This four-component alloy was chosen because it is Co-free, thus allowing application in nuclear industries as a possible replacement of currently used stainless steel coatings. The feedstock material was gas atomized powder with a particle size distribution from 20 to 45 µm. A number of parameters were tested, such as the powder feed rate and gas feed pressure, in order to obtain as dense a coating as possible with nitrogen as the process gas. Spraying was performed using a gas preheating temperature of 1000 °C, gas feed pressure ranging from 50 to 60 bar, and two powder feeding rates. The coating thicknesses ranging from 230 to 490 µm and porosities ranging from 3% to 10% were obtained depending on the powder feed rate and gas feed pressure. The hardness of the cross-section of the coating was usually lower than that of the surface. The highest coating hardness obtained was above 300 HV0.3 for both the surface and the cross-section. The as-atomized powder consisted of a face-centered cubic (FCC) phase with a minute amount of body-centered cubic (BCC) phase, which was no longer detectable in the coatings. The microstructure of the coating was highly stressed due to the high degree of deformation occurring in cold gas spraying. The deformation leads to strain hardening and induces a pronounced texture in the coating. The {111} planes tend to align along the coating surface, with deformation and texturing concentrating mainly on particle boundaries. A high-entropy alloy (HEA) coating was successfully sprayed for the first time using nitrogen as a process gas. The coating has the potential to replace stainless steel coatings in nuclear industry applications.