In cold gas spraying, successful bonding occurs when particle impact velocities exceed the critical velocity. The critical velocity formula depends on material properties and temperature upon impact, relying mainly on tabulated data of bulk material. However, rapid solidification of powder particles during gas atomization can result in strengths up to twice that of bulk materials, causing an underestimation of the critical velocity. Thus, a re-adjustment of the semi-empirical calibration constants could supply a more accurate prediction of the requested spray conditions for bonding. Using copper and aluminum as examples, experimentally determined particle strengths for various particle sizes were 43% and 81% higher than those of the corresponding soft bulk materials. Cold gas spraying was performed over a wide range of parameter sets, achieving deposition efficiencies ranging from 2% to 98%. Deposition efficiencies were plotted as functions of particle impact velocities and temperatures, as calculated by a fluid dynamic approach. By using deposition efficiencies of 50%, the critical velocities of the different powders and the corresponding semi-empirical constants were determined. Based on particle strengths, the results reveal slight material-dependent differences in the mechanical pre-factor. This allows for a more precise description of individual influences by particle strengths on critical velocities and thus coating properties. Nevertheless, the general description of the critical velocity based on bulk data with generalized empirical constants still proves to be a good approximation for predicting required parameter sets or interpreting achieved coating properties.

Restoring the damaged shaft parts to extend their service life is an economical and environmentally friendly solution. In recent years, the laser metal deposition (LMD) process has received increasing attention in component restoration. However, the residual stress and deformation inevitably occur due to the heat input, leading to the deflection of the repaired shafts. Therefore, this study aims to minimize the deflection of LMD-repaired shaft parts through parameter optimization. The width and height of the LMD deposit as a function of the laser power and traverse speed were achieved by fitting a series of one-pass experimental results. Based on it, the finite element analysis was conducted to clarify the effect of the repairing conditions (e.g., laser power, traverse speed, and initial substrate temperature) on the deflection and residual stress distribution of the shaft parts after LMD repairing. A 304 stainless steel round bar with a diameter of 6 mm was served as the component to be repaired. The deposit was 316L stainless steel, whose deposition process was realized by the element birth and death technique. The results indicated that the free-end of the specimen experienced complicated deformation during the LMD and cooling process. After cooling off, the substrate presents a residual compressive stress along the axial direction. Moreover, the substrate deflection can be reduced by improving the initial substrate temperature. This study provided an important reference for optimizing the process parameters in repairing the shaft parts.

In particular, eutectic HEAs (EHEAs) are of interest for coating technology. The microstructure of these multiphase systems is determined by the cooling conditions during solidification and the heat treatment condition. High cooling rates can suppress segregation and allow the formation of a supersaturated solid solution microstructure. Therefore, the property profile differs from that of the equilibrium state. The effect of cooling conditions on the functional properties of EHEA coatings has not been investigated so far. In the current study, the microstructure formation and wear resistance of the metastable EHEA Al 0.3 CoCrFeNiMo 0.75 was investigated. Laser metal deposition (LMD) of the inert gas atomized powder forms a directional vertically solidified lamellar structure. A supersaturated solid solution and a metastable BCC and HCP phase was formed. The microstructure resembles a Widmanstätten structure. By spark plasma sintering (SPS), a statistically distributed orientation of the fine lamellae was produced. The highest microhardness and oscillating wear resistance were detected for the ultrafine LMD coating. By increase of the microstructure domain size, the hardness and oscillating wear resistance decrease. This study reveals the great potential of supersaturated solid solutions of ultrafine EHEAs obtained by LMD processing with high cooling rates.

Thermally sprayed WC/CoCr coatings are the most established coatings in the valve industry. However, due to the high wear resistance and as-sprayed surface roughness, the surface post processing costs are very high. Near-net-shaped fine powder coatings have the possibility to reduce the costs effectively. Due to the high specific surface to volume ratio of the powders, undesired phase transformations can occur during the spraying process. To avoid such phase transformations, the novel thermal spraying process Ultra-HVOF (UHVOF) is used in this study. An extensive parameter study is carried out on the influences of the process parameters on microhardness, porosity, as-sprayed surface roughness, phase composition and wear resistance. With suitable process parameters, near-netshaped and almost pore-free coatings can be applied. Compared to a conventional HVOF sprayed WC/CoCr coating, a wear reduction by a factor of three can be achieved in a pin-on-disktest against Al 2 O 3 at a load of F = 15 N. Due to the pore-free and highly wear-resistant coatings, significantly thinner coatings can be used for the protection against corrosion and wear in valves. In addition, the required surface quality of the near-net-shape coatings can be achieved by polishing only. Thus, the novel UHVOF coatings represent a cost-effective alternative to conventionally used valve coatings.

Amorphous alloys have attracted extensive attention due to their unique atomic arrangement and excellent properties. However, the application in practical engineering is seriously limited due to the size, crystallization and other problems. Laser additive manufacturing technology has the characteristics of high heating, cooling rate and point by point melting deposition, which provides a new idea for the preparation of amorphous alloys. Zr 50 Ti 5 Cu 27 Ni 10 Al 8 amorphous alloy was prepared on the surface of pure zirconium substrate by selective laser melting technology. The composition and structure of the samples were characterized. The results show that the samples are mainly composed of amorphous phase, and the crystallization mainly occurs in the superimposed zone of heat affected zone. With the decrease of laser power, the area of crystallization zone and the number of crystallization particles decrease. However, if the laser power is too low, there will be non-fusion defects and cracks, which will seriously affect the forming quality and amorphous rate of amorphous alloy.

Material’s tensile strength can be improved by the presence of a body-centered cubic (BCC) phase, which is essential in highstrength applications and highly corrosive environments. Thus, synthesizing a BCC single-phase, equiatomic AlCoCrFeNi high-entropy alloy (HEA) feedstock particle using a highenergy mechanical alloying (HE-MA) method was investigated. The transient alloy particles were developed using a planetary mill at a constant rotational speed of 580 rpm employing milling times in the range of 4 to 24 hours. During the process, stearic acid of 3 wt.% of the precursor composition was used as a process-controlling agent (PCA). Two HE-MA manufacturing regimes were utilized: i) conventional (milling constituent elements simultaneously), and ii) sequential (progressive milling while adding elements in a certain order). In addition to the conventional method, a sequential regime was employed to develop FeNiCoCrAl, wherein individual elements were added every 4 hours to the starting/milled Fe + Ni mixture. Based on the results, the HE-MA FeNiCoCrAl showed a BCC single-phase formation after 24 hours, with no intermetallic or contamination traceability. Finally, a nanoindentation hardness measurement was carried out to support the observed phase transformation before and after the HE-MA process.

In metal die casting as well as plastic injection molding, controlling the heat balance during the injection and solidification process can lead to fewer defects and a better component quality. An appropriate cooling channel design for the mold can help to control the solidification to a certain extent. But the heat control achievable by cooling channels is limited due to the high effective thermal mass, and therefore near-cavity energy input is of interest. In this paper, a simulation study is performed demonstrating the use of plasma sprayed ceramic coating as a heating coating at the cavity of the mold. The goal is to apply heat faster and locally focused during the solidification process in metal die casting as well as before the injection phase in plastic injection molding. The heat generation of these ceramic coatings is modelled using experimentally measured values and the effects of this approach on defects such as distortion and hot tearing is discussed.

Three different coatings were deposited using the Detonation Gun Spraying (DGS) technology from steel powders alone, and steel powers mixed with Fe3C and SiC particles, respectively. The microstructural characteristics of these coatings were examined and the hardness of each type of coating was studied. The morphology and structure of the feedstock powders were affected by the exposure to high temperature during the spraying process and rapid solidification of steel powders that resulted in the formation of an amorphous structure. The unreinforced steel coating had the highest hardness among the three types of coatings, possibly due to a higher degree of amorphization in the coating compared to the other two samples. The microstructural observation confirmed the formation of dense coatings with a layered structure with good connectivity between layers with minimum defects and porosities in the interfacial regions.

The effect of martensitic phase transformation on cavitation erosion resistance for a deposited layer prepared from a Fe-8Cr- C-1.5Al-Ti flux-cored wire of metastable steel was studied. A reference material of AISI 316L stainless steel was used as a substrate. Cavitation tests were performed using a modified ultrasonic tester. X-ray diffraction was used to examine the phase transformation before and after cavitation tests. Also, the eroded surfaces of specimens were investigated by optical microscope (OM), scanning electron microscope (SEM), and 3D optical profilometer. The cavitation results revealed that the deposited layer exhibited a resistance to cavitation erosion approximately 10 times higher than the AISI 316L steel due to the martensitic phase transformation occurring during the cavitation process. The phase transformation plays a main role to minimize the cavitation damage of specimen. This is due to the fact that it contributes to obstructing movement of dislocations and increasing the hardness as a result of the increased hardening on the surface.

Due to their superior wear resistance and oxidation behavior, Stellite coatings are widely used in industrial applications where they are exposed to high temperature. Common processes for applying Stellite coatings include high-velocity oxyfuel spraying, laser cladding, and plasma transferred arc welding. Although Stellite welding consumables are available, there are few studies on arc-sprayed Stellite coatings in the literature. This work investigates the microstructural characteristics of an arc-sprayed deposit produced using a CoCr-based cored wire with 4.5 wt% W. The deposit is examined both in its as-sprayed state and after high-temperature exposure. Microstructure formation is assessed via SEM and EDX analysis, phase transformation processes are determined by XRD analysis, and friction and wear properties are measured. The findings are presented and discussed and compared with those obtained from conventional CoCr-based coatings.

Thermal barrier coatings are generally produced one of two ways, depending on the thermomechanical loading expected. This study assesses an alternative approach in which the output of an air plasma torch is directed through two chambers connected by an expansion nozzle. In the first chamber, the particles evaporate under high pressure and temperature conditions. The vapor then passes through a supersonic nozzle into a low-pressure chamber where it condenses on the target substrate. A number of models are developed and used in order to assess the effects of process geometry and operating conditions on gas flows, powder vaporization efficiency, and nucleation and growth kinetics. Numerical simulations also informed various design decisions such as the length of the high-pressure chamber and the diameter of the expansion nozzle.

Tin coatings have been successfully applied to polymeric substrates by means of cold spraying. In this work, three low melting point powders, including Sn, Sn-Zn, and Sn-Bi, are cold sprayed onto various polymeric substrates and different combinations of gas temperature and pressure are assessed. Based on the results, the effect of melting points on the cold sprayability of feedstock powders is discussed.

Epitaxial grain growth during the rapid solidification of molten TiO 2 in plasma spraying was studied. The crystallographic structure of the TiO 2 splats deposited on rutile and α-Al 2 O 3 substrates at 150, 300 and 500 °C was characterized by high resolution transmission electron microscopy and electron back scattering diffraction. The results reveal that homoepitaxial and hetero-epitaxial TiO 2 splats can be formed at the deposition temperature of 500 °C. Epitaxial growth is significantly influenced by the crystal orientation. It is easier to form an epitaxial TiO 2 splat with a <001> orientation in the direction perpendicular to the substrate surface. In order to explain the formation of epitaxial splat during plasma spraying, a competition mechanism between heterogeneous nucleation and epitaxial growth was proposed. It was indicated that the face (001) of rutile crystal exhibits the largest growth velocity, which is conducive to form an epitaxial splat for the melt with a largest undercooling degree. In addition, the effect of deposition temperature and crystalline orientation on the epitaxy was simulated. The simulation results are in agreement with the experimental observations.

ZnO films were deposited by solution precursor plasma spray (SPPS) process with different substrate preheating temperatures and torch powers, which were used to study the effects on crystallizations and microstructures. With increasing substrate preheating temperature from 0 °C to 400 °C, ZnO films were always preferential orientation along (002) plane with much higher crystallinity. And more apparent crystallized particles appeared with higher agglomeration degree forming cauliflower-like microstructure under higher preheating temperature. For adjusting hydrogen flow rate, the moderate hydrogen flow rate was the suitable condition for obtaining oriented growth along (002). Besides, all ZnO films under different hydrogen flow rates with a constant preheating temperature as 400 °C were always combined with crystallized particles. Moreover, the increment of torch power makes microstructure becomes denser with less interspace between neighbouring particles. Moreover, it is found that crystallinity and crystallized particles is more dependent on preheating temperature and torch power plays a more important role on densification by two staggered experiments. Taking applications of metal oxides films via SPPS into consideration, choosing moderate substrate preheating temperature and hydrogen flow rate will obtain crystallized particles, unusual preferentially oriented planes and high specific surface area, which is very favourable for optical, electrical, electrochemical properties.

One important trend in thermal spraying is the application of novel Fe-based corrosion/wear protection coating systems. A typical field of application for such corrosion and abrasive wear protection coatings are rotary dryers of paper machines. At the moment, these cylinders are coated by wire arc spraying. A disadvantage of the wire arc sprayed coatings is their high thickness, which has a heat-insulation effect, and their high roughness. Therefore, an expensive post production grinding process is necessary in order to achieve the required surface quality. The goal is to develop a HVAF process that enables the production of thin, dense and near net shape corrosion/wear protection coating systems, which significantly reduce the post-production time and costs. In this study, the HVAF coating process and a novel Fe-based feedstock material are investigated. In the first step the Fe-based powder is analysed thermally using differential scanning calorimetry, to investigate the solidification and melting temperature of the feedstock material. Furthermore, the influence of the spraying distance and the powder feed rate on the microstructure and porosity of the resulting coatings is investigated using light microscopy. Furthermore, the deposition efficiency of HVAF coatings is analysed regarding their economic efficiency.

Environmental barrier coatings (EBC) are currently being investigated to protect ceramic matrix composite (CMC) turbine engine components in water-vapor rich combustion environments. Dense, crack-free, uniform and well-adhered coatings are demanded for this purpose. This paper represents an assessment of different thermal spray techniques for deposition of Yb 2 Si 2 O 7 and silicon (Si) EBC layers. Plasma spraying of refractory silicates is known to be complicated by undesired glass transition due to rapid solidification as well as evaporation of Si-bearing species during spraying. Plasma spraying of low-density Si also requires careful optimizations as it tends to oxidize during spraying, particularly at atmospheric conditions. Bearing these problems in mind, the Yb 2 Si 2 O 7 coatings were deposited by atmospheric plasma spraying (APS), high-velocity oxygen-fuel spraying (HVOF), and plasma-spray physical vapor deposition (PS-PVD) techniques. As-sprayed microstructure, amorphous content and phase composition of the coatings were analyzed. Based on the findings, the advantages and disadvantages of each method over other techniques are discussed with respect to process parameters and material properties.

A high electromagnetic field (0.3 T-Teslas) was applied during the solidification of Ni-based alloyed splats. Ni, NiCr, NiCrAl, NiCrBSiFe powders were deposited over steel polished substrates using a flame spray and a plasma spray torch. A strong electromagnet was used to produce sufficient magnetic field to induce effects over the splats during solidification. A remarkable change in splat morphology and chemical segregation was identified specially in NiCrBSiFe and the other alloys. Optical microscopy, surface profilometry, and SEM images revealed changes in the regular cracking trends, splashing, and thickness of the splats. This experimental study discusses the possible explanations for this phenomena. The adherence of the coating is the main property to be analyzed with the goal of improving the mechanical interlocking, and therefore, adhesion by engineering the applied electromagnetic field.

Thermally sprayed Al 2 O 3 -TiO 2 coatings were in the area of interest over the last decade because they showed improved wear properties over conventional coatings. In this study, flexicord flame spray gun was used to deposit Al 2 O 3 -TiO 2 coatings at different spray parameters. The microstructural morphology variation and phase transformation of coatings were investigated. In addition, as one of the most important properties for ceramic coatings, hardness, solid particle abrasive wear resistance of coatings were measured before and after heat treated condition. Test results show that the higher mechanical properties and wear resistance by the heat treatment at elevated temperatures.

A transition in the flattening behavior of thermally sprayed metals has been observed in previous studies. It has been proposed that ultra-rapid cooled chill structure preferentially formed at the bottom part of the splat may play a role in the generation of disk-shaped metallic splats. The applicability of this hypothesis to other materials was verified experimentally for several ceramic oxides. To accomplish this, Al 2 O 3 , Y 2 O 3 , and YSZ particles were plasma sprayed onto stainless steel substrates and the fraction of disk-shaped splats was measured as a function of substrate temperature. Splat microstructure was also examined. Unique amorphous and chill structures were observed in the bottom portion of Al 2 O 3 and Y 2 O 3 splats, indicating that similar formation mechanisms may be at work. However, only a columnar microstructure was observed in the YSZ splats, which calls for additional study.

In this work, metallic powders are applied to carbon fiber reinforced polymer (CFRP) substrates by low-pressure cold spraying. The coatings as well as the coating-substrate interfaces are characterized and the deposition mechanism is determined. It is shown that gas temperatures above 300°C are required for the continuous deposition of tin. These temperatures bring about partially melting, which facilitates adhesion. Accordingly, a “crack filling” mechanism is proposed to explain the deposition.