High-pressure cold spray deposition of 430 stainless steel and titanium carbide-reinforced 430 stainless steel powders: microstructural and hardness investigation

The present study focuses on the development and characterization of coatings produced by the High-Pressure Cold Spray (HPCS) technique using 430L stainless steel powders and titanium carbide reinforced 430L stainless steel composite powders. The investigation primarily aims to evaluate the influence of TiC reinforcement on the microstructural and hardness properties of the coatings, while also assessing the applicability of the HPCS technique as an effective surface engineering method for advanced material systems. Cold spray, as a relatively recent solid-state coating process, has gained increasing attention in recent years due to its ability to overcome limitations of conventional thermal spray processes. Unlike plasma spraying, flame spraying, or other thermal techniques, cold spray operates at relatively low temperatures, where particles are accelerated to supersonic velocities by means of high-pressure carrier gases, typically nitrogen or helium, and adhere to the substrate upon impact without undergoing significant melting. This unique deposition mechanism eliminates thermal degradation, phase decomposition, and oxidation problems, enabling the fabrication of coatings that retain the original chemical and structural characteristics of the feedstock materials. Such a feature is especially critical when working with materials that are sensitive to oxidation or when reinforcing phases must remain chemically stable during deposition. Stainless steels, particularly ferritic grades such as 430L stainless steel, are widely used in engineering applications due to their corrosion resistance, relatively low cost, and ease of fabrication. However, their relatively moderate hardness and wear resistance often limit their application in environments where surface durability is of prime importance. To address this limitation, reinforcement with ceramic particles such as titanium carbide provides a promising strategy. Titanium carbide is known for its exceptional hardness, high melting point, and excellent chemical stability. When integrated into a metallic matrix, it serves to improve wear resistance, enhance load-bearing capability, and extend service life. Therefore, combining 430L stainless steel with TiC reinforcement through cold spray deposition presents an opportunity to fabricate coatings with improved mechanical and surface properties while retaining the corrosion resistance of stainless steel. The powders used in this study consisted of four different compositions: pure 430 stainless steel, and 430L stainless steel reinforced with 10, 20, and 30 weight percent TiC. Prior to deposition, the powders were characterized using scanning electron microscopy to evaluate particle morphology and distribution. The cold spray process was carried out using optimized parameters to ensure that the particles reached sufficiently high velocities to achieve deposition, while avoiding excessive thermal input. Following deposition, coatings were carefully characterized through multiple techniques. X-ray diffraction was employed to identify the phases present and to determine whether any phase transformations or chemical reactions occurred during deposition. SEM imaging and energy dispersive spectroscopy were used to examine coating cross-sections, identify reinforcement dispersion, and evaluate coating density as well as interface quality. Surface roughness was measured using a profilometer, while microhardness tests were performed using the Vickers method to obtain hardness distributions across the coatings. The results of the study demonstrated that the inclusion of TiC reinforcement had a significant effect on the properties of the coatings. Pure stainless steel coatings produced by cold spray exhibited good adhesion to the substrate and dense microstructures, consistent with the expected characteristics of solid-state deposition. However, their hardness values remained relatively low, highlighting the limitations of stainless steel in terms of mechanical performance when unreinforced. The introduction of TiC reinforcement addressed this limitation by enhancing the hardness of the coatings, with a clear correlation observed between increasing TiC content and improved hardness. The 10% TiC reinforced coatings exhibited noticeable improvement, while the 20% TiC reinforced coatings delivered the most balanced performance, combining high hardness values with a uniform distribution of reinforcement particles and minimal porosity. In contrast, the 30% TiC coatings, although harder than the pure stainless steel coatings, suffered from increased porosity and local agglomerations of ceramic particles, which undermined overall structural integrity. The microstructural analyses confirmed that cold spray successfully embedded the TiC particles within the stainless steel matrix. At moderate reinforcement levels, particularly 20%, TiC particles were evenly distributed, ensuring effective load transfer between the metallic matrix and the ceramic reinforcement. This distribution contributed significantly to the improvement of hardness and the overall performance of the coating. However, at higher reinforcement levels such as 30%, challenges related to particle acceleration and bonding were evident. Due to the relatively high density and brittleness of TiC, some particles did not achieve sufficient velocity for proper embedding, resulting in weakly bonded particles and micro-voids within the coating. This in turn reduced coating homogeneity and compromised the potential benefits of reinforcement at excessive ratios. X-ray diffraction results provided further insights. The analyses revealed that the cold spray process preserved the chemical integrity of the stainless steel and TiC powders, with no significant phase decomposition detected. This is a critical advantage compared to thermal spraying, where high process temperatures often lead to phase instability or oxidation. The retention of original powder phases in cold spray coatings ensures that both the metallic matrix and ceramic reinforcement maintain their intended properties, a factor that directly contributes to the success of the method in fabricating advanced composite coatings. Surface roughness analyses also reflected the impact of TiC reinforcement. Pure stainless steel coatings presented relatively smooth surfaces, while the incorporation of TiC particles led to slight increases in surface roughness due to the harder, angular morphology of ceramic particles. Nevertheless, these increases remained within acceptable ranges and are not expected to pose major drawbacks in practical applications. In fact, slight increases in surface roughness can sometimes benefit coating–substrate adhesion or further coating applications, depending on the service environment The microhardness results established the reinforcing role of TiC conclusively. Pure stainless steel coatings had the lowest hardness values, reflecting the inherent properties of the base alloy. The addition of TiC led to significant increases in hardness, with the 20% TiC coatings once again emerging as the optimal composition. The results highlighted that an appropriate level of ceramic reinforcement can substantially enhance surface performance while excessive levels may have diminishing or even negative effects due to structural inhomogeneity. When compared with similar studies in the literature, the findings of this research are consistent with observations made in other material systems where ceramic reinforcement is added to metallic matrices. Many studies underline the trade-off between reinforcement content and coating quality, emphasizing that optimal performance is achieved at moderate reinforcement levels. The results of this thesis contribute to this broader understanding by specifically demonstrating the behavior of 430L stainless steel reinforced with TiC through cold spray. The implications of these findings extend to several industrial applications. In sectors such as petrochemicals, energy production, and chemical processing, where components are exposed to harsh environments involving wear, corrosion, and mechanical stress, the development of coatings with enhanced surface properties is crucial. The ability to fabricate dense, hard, and well-bonded coatings without subjecting materials to high temperatures makes cold spray a particularly suitable method. Furthermore, the fact that cold spray preserves the corrosion resistance of stainless steel while improving hardness through TiC reinforcement is of notable practical significance. The study also underscores potential future directions. Further investigations could focus on optimizing process parameters for higher reinforcement contents to overcome the porosity and agglomeration issues observed at 30% TiC. Additional characterization, such as wear testing and corrosion performance evaluation, would provide a more comprehensive understanding of the coatings' suitability for industrial use. Moreover, expanding the scope to other types of reinforcements, such as titanium nitride or other carbides, may open new opportunities for tailoring coating properties to specific applications. In conclusion, the research successfully demonstrates that high-pressure cold spray can be effectively used to deposit 430L stainless steel and TiC reinforced 430L stainless steel coatings. The results highlight the role of TiC in enhancing hardness and microstructural characteristics, with 20% reinforcement identified as the most favorable composition under the studied conditions. The work contributes to the growing field of cold spray technology and reinforces the position of this process as a promising surface engineering method for developing advanced composite coatings with improved mechanical performance and durability.