In this work, amorphous Zr-based bulk metallic glass deposit was manufactured by cold spray. The bonding mechanism of metallic glass particles was systematically investigated through studying the deformation behavior of individual particles after deposition or rebound. We revealed two collective particle bonding mechanisms that contributed to the formation of metallic glass deposit, i.e., high-velocity impact induced localized metallurgical bonding at the fringe of interface, and high gas-temperature induced partial melting of particles and resultant annular metallurgical bonding band. Moreover, the dynamic evolution mechanism of amorphous phase into nanocrystal structures at severely deformed interfacial regions during cold spray was carefully investigated. For the first time, we observed different amorphous/nanocrystal structures in cold sprayed metallic glass particles, which can represent different evolution stages in nanocrystallization process. Based on the observation, it is inferred that the nanocrystallization process can be divided into following three stages: composition segregation, the formation of ordered 1D and 2D transition structures, and 3D nanocrystals. The current study provides new insights into bonding mechanisms and the mechanistic nanocrystallization origins in cold sprayed metallic glass.

Bulk metallic glasses (BMGs) are a novel class of metallic materials with disordering atomic structure and excellent mechanical and chemical properties, and are promising for various industry applications. BMGs are usually fabricated by copper-mould casting due to the requirement of fast cooling rate for the obtainment of amorphous structure. However, this casting approach has the limitation for preparation of large size samples (the biggest Fe-based BMG obtained so far is less than 16 mm diameter). In this work, the conventional high velocity oxy-fuel (HVOF) thermal spraying technique was utilized as a novel additive manufacturing route to create large size Fe-based BMGs and BMG composites. It will be reported that a large size of 20x20x20mm BMG (Fe48Mo14Cr15Y2C15B6 (at%) ) and big plate of 100×100×5 mm of Fe-based BMG composites reinforced with 50vol% 316L stainless steel powders was successfully prepared by HVOF thermal spraying. Both BMG and BMG composite showed very dense structure (porosity less than 0.4% ) and good mechanical properties, Especially, BMG composite reinforced with 316 L stainless steel exhibited a yield strength of 1.8 GPa and compressive plastic strain of 2%. More importantly, this Fe-based BMG composite exhibited good toughness of KJ=21 MPa m 1/2 , which is almost 4-times higher than that of as-cast BMG. This present work indicates that HVOF thermal spray can become a versatile technique for preparation of large size of bulk metallic glasses and composites with desired properties.

The practical application of bulk metallic glasses (BMGs) as structural materials is still restricted due to the intrinsic brittleness of BMGs although they have high hardness and strength and even pretty good toughness. As an alternative form of metallic glasses, amorphous coatings based on BMG systems can exactly overcome the drawback of BMGs, but carry forward the superiority in corrosion and wear resistance, thus exhibiting promising applications in surface engineering. However, the monolithic amorphous coatings faces new challenges, such as low adhesion strength and impact toughness. In this presentation, we present some new findings on the design of novel Fe-based amorphous composite coatings by HVOF technique. These include the add stainless steel ductile phase, hard ceramic particles and in-situ carbon phases into the amorphous coating, in addition to form laminar structural coatings. It will be shown that the second phase and their interface structure with amorphous matrix play important role in the resultant mechanical properties of the coatings. The combination of the good mechanical, physical and chemical properties warrants amorphous composite coatings to have extensive applications in industry in near future.

In this work, gas-atomized FeCr powders were deposited on aluminum substrates by HVOF spraying, forming dispersion strengthened coatings with a dense layered structure and low porosity. SEM, TEM, and XRD analyses show that the coatings primarily consist of amorphous matrix (40%) with precipitated nanocrystals and hard boride phases. A number of coating properties, including microhardness, bonding strength, and thermal conductivity, were measured and are correlated with spraying conditions.

The objective of this work is the development of highly amorphous, iron based coatings for thermal barrier applications. Based on the results of previous work, a chemical composition of Fe 72 Si 4 B 20 Nb 4 was selected and modified in order to improve glass forming ability and corrosion resistance. Three metallic glass powder mixtures with different amounts of Cr and Fe were prepared, characterized, and deposited by air plasma and HVOF spraying. Different gas flow rates and standoff distances were used and particle temperatures and velocities were measured during spraying. The deposits were examined, tested, and compared and were found to have good potential for thermal barrier coating applications.

This study investigates the feasibility of forming amorphous iron-based coatings using the cold spray deposition process. Splat tests of cold-sprayed SAM1651 (Fe48Mo14Cr15Y2C15B6 at.%) particles impacting a mild steel substrate were performed using varying gas temperatures and particle diameters. Specimen inspection by scanning electron microscopy revealed splat morphologies that varied from well-adhered particles to substrate craters formed by rebounded particles. Particle flow was analyzed using a finite element model, and impact conditions were predicted using an experimentally validated analytical model, in empirically generating a temperature/velocity window of successful particle deposition as a framework for ongoing work on the formation of cold-sprayed SAM1651 coatings. The results indicate that the unique characteristics of the cold spray process offer a promising means for the formation of metallic glass coatings that successfully retain the amorphous structure, as well as the superior corrosion and wear resistant properties of the feedstock powder.

In this investigation, cold spraying is used to deposit a simple binary amorphous alloy with technical purity. Cu 50 Zr 50 was chosen as the model system due to its glass-forming ability and insensitivity to changes in composition. Critical velocities for coating formation were experimentally determined by systematic variation of spray parameter sets. These values were then used to tune existing bonding models to cold spraying of amorphous Cu 50 Zr 50 powder. It is shown that under suitable conditions, well adhering coatings with the amorphous structure of the powder can be obtained by cold spraying with nitrogen as the process gas.

In this work, tunnel plasma spraying is used to produce Cu 36 Zr 48 Al 8 Ag 8 metallic glass coatings on stainless steel. The results show that cooling gas flow rates play a vital role in oxidation and the formation of intermetallic phases in coating microstructures. Phase formation and microstructural features were evaluated by XRD and SEM-EDX analysis. Coating properties including hardness, sliding wear, and corrosion resistance were measured and the results are compared with the presence of secondary phases. It is shown that an increase in secondary phases improves sliding wear resistance but reduces resistance to corrosion.

As compared to thermal spray techniques, cold spraying allows to retain metastable phases of the feedstock material like amorphous structures, due to lower process gas temperatures. Compared to crystalline metals, metallic glasses are brittle at ambient temperature but viscous at higher temperatures. Therefore, cold spray parameters must be optimized for conditions that allow softening of the amorphous spray material for successfully producing coatings. For this study, a FeCoCrMoBC metallic glass was used that in comparison to others offers advantages with respect to higher hardness, less costly feedstock powder and minimum reactivity with the environment. Necessary impact conditions were investigated to meet the window of deposition. According to calculations and cold spray experiments, neither the glass transition temperature Tg nor the melting temperature Tm can describe required conditions for bonding. Thus, a so called softening temperature between the glass temperature and the melting temperature had to be defined to calculate the critical velocity of metallic glasses. With respect to the bonding mechanism, impact morphologies could prove that a transition to viscous flow gets more prominent for harsher spray conditions. By sufficiently exceeding critical condition for bonding, coatings with rather dense microstructures can be processed at deposition efficiencies of about 70 %. The coatings have a hardness of 1100 HV 0.3, but the results also demonstrate that further work is still needed to explore the full potential for bulk metallic glasses.

In this study, Cu-based bulk metallic glass coatings were deposited by atmospheric plasma spraying with different hydrogen flow rates. The crystallization and oxidation of the coatings is assessed along with corrosion resistance. As thermal energy in the plasma jet increases, the melting fraction and oxidation of particles in the coating increases as does porosity. All of these factors have an effect on the corrosion resistance of Cu-based bulk metallic glass coatings and their relative impact is discussed.

Due to their enhanced mechanical properties and high range applications in severe environments use, there is ongoing research in the area of amorphous metallic glasses and nanostructured materials. The science and mechanisms leading to these exceptional properties are discussed herein. The arc sprayed coatings produced for study in this paper show remarkable performance and are analyzed in-depth using advanced characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Additional analysis of the mechanical properties of the coatings show enhanced strength and hardness, good wear and corrosion properties, along with high temperature corrosion and oxidation resistance.

A brief feasibility study was performed to produce thermal spray coatings using gas atomized powders of Cu47Ti34-xZr11Ni8Six, where x=0 and 1. These alloys have previously been shown to be capable of forming metallic glasses having thick (1-2 cm) cross sections because they can be cooled from the melt at relatively low cooling rates (e.g., 100-102Ks-1). The properties of these metallic glasses include high strength, high elasticity and high fracture toughness. Amorphous plasma arc sprayed coatings were produced which were close in composition to the starting powders, and exhibited comparable glass transition and crystallization behavior. The amorphous structure of the as-sprayed coatings was used as a source for forming a range of partially devitrified and fully crystallized structures. The average hardness of the coatings increased from around 6 GPa to near 10 GPa as the degree of crystallization increased.