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.

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.

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.

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.

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.

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 plasma spray-physical vapor deposition (PS-PVD), deposition takes place not only from liquid splats, but also from nanosized clusters as well as the vapor phase. As a result, thin, dense, and porous ceramic coatings can be produced for special applications using this method. In this study, columnar-structured YSZ coatings were deposited by PS-PVD on graphite and zirconia substrates and the effect of substrate temperature on coating microstructure was investigated. A deposition mechanism of heterogeneous nucleation is presented based on the observations and findings of the study.

Tungsten carbide coatings are often applied to improve surface properties such as wear, high temperature degradation, and corrosion resistance. Zirconia coatings have also been used extensively in various industries due to their excellent tribological and insulation properties combined with high stiffness. It is speculated that adding zirconia to tungsten carbide may result in a coating with combination of excellent thermal and mechanical properties of constituents. In the current study, a powder mixture of 50 wt. % WC-Ni and 50 wt. % ZrO 2 -Y 2 O 3 deposited on a low carbon steel substrate using atmospheric plasma spray technique. The microstructural evolution of deposited sample was investigated. Splat boundaries, micro cracks, pore morphology conversion, and grain growth mechanism were elucidated comprehensively. Results indicated a good adhesion between two different major components. No porosity formed due to mismatch between zirconia and tungsten carbide. This study pays special attention to the dependency of the microstructural characteristics to the phase distribution within the coating.

The mist cooling technique was developed and applied to various thermal spraying guns. For example, aluminum-magnesium coatings prepared using a gas flame thermal spraying gun with mist cooling had superior anticorrosion characteristics. Stellite coating thermally sprayed with mist cooling had higher anti-cavitation-erosion characteristics. Next, we endeavored to develop high velocity oxygen fuel guns with mist gas cooling to improve high-temperature toughness.

The effect of substrate template effect on the crystalline structure of plasma sprayed 8YSZ (8mol%Y 2 O 3 ) splats was investigated by high resolution transmission electron microscopy (HR-TEM) examination of FIB-processed splat samples. 8YSZ splats were deposited by the atmospheric plasma spraying (APS) on the polished sintered tetragonal structure substrate (3YSZ) and cubic structure substrate (8YSZ) at different preheating temperatures. The focused ion beam (FIB) was utilized to prepared TEM cross-sectional sample of splats. The crystalline structures of both the splat and the underlying substrate were examined by HRTEM. Results showed that the 8YSZ splats deposited on the polished sintered cubic structure 8YSZ substrate at a substrate surface temperature of 900°C exhibited cubic structure and the epitaxial grain growth was confirmed between the crystalline of splat grain and immediately underlying cubic crystalline substrate grain. Moreover, epitaxial grain growth was confirmed between the crystalline of splat grain and the tetragonal structure substrate when substrate surface temperature was increased to 1200°C. The present results suggest that the crystalline structure formation of 8YSZ splats produced by plasma spraying was affected by the substrate template effect.

Different directions characterize the advances being made in thermal spraying today. On the one hand, spray processes are becoming colder, on the other hand, high-performance systems are being designed which enable a higher powder throughput, thus making production faster and more efficient. The current spectrum of substrate materials is much broader and even more versatile, as can be seen with new materials which prevent thermal stresses from arising during the production process. More and more applications require the use of special cooling methods to increase the cooling efficiency and, in turn, optimize the process. An ongoing objective of the gas industry is to offer the user hardware which not only exploits all the advantages of CO 2 , for example, but which is also suited to new applications.

Alumina-zirconia ceramic material has been plasma sprayed using a water stabilized plasma torch (WSP) to produce free standing coatings. The as-sprayed coatings have very low porosity and are mostly amorphous. The amorphous material crystallizes at temperatures above 900 °C. A spark plasma sintering apparatus has been used to heat the as-sprayed samples to temperatures above 900 °C to induce crystallization while at the same time a uniaxial pressure of 80 GPa has been applied to the their surface. After such post-treatment, the ceramic samples are crystalline and exhibit very low open porosity. The as-sprayed amorphous materials also exhibit high hardness and high abrasion resistance. Both properties are significantly improved in the heat-treated samples whose microstructure is best described as nanocomposite with the very small crystallites embedded in an amorphous matrix.

In this study, a suspension containing Mg-Al-spinel nanopowder was deposited on bond-coated IN738 and stainless steel disks by suspension plasma spraying with and without substrate cooling. Coating surfaces and cross-sections were examined by SEM, EDS, and XRD analysis and thermal cycling tests were performed. SEM images of coatings obtained on cooled stainless steel show a unique columnar microstructure with a cauliflower-like surface. XRD spectra of the nanopowder and coatings revealed evidence of phase changes in the material deposited on cooled substrates. In preparing samples for thermal cycling tests, a YSZ layer was deposited on bond-coated IN738 prior to spraying the suspension. Double-layered Mg-Al-spinel/YSZ thermal barrier coatings produced on cooled substrates exhibited a thermal cycling lifetime of 2000 cycles at 1390°C, compared to 101 cycles for the TBCs sprayed without substrate cooling. The superior performance of the TBCs sprayed with substrate cooling is attributed to the densification of the coatings, revealed by SEM images, and possibly the formation of CaO-6Al 2 O 3 needles and Al 2 O 3 precipitates as identified by EDS measurements.