Climate change mitigation requires advances in energy efficiency. Thermal barrier coatings (TBCs) are widely employed to improve thermal efficiency in gas turbines and internal combustion engines, and engineering coatings’ porosity is an important route to control the heat transfer to metallic components. In this study, porous gadolinium zirconate (Gd2Zr2O7, GZO) coatings were produced using air plasma spraying (APS) with polyester as a pore-forming agent. A systematic investigation was carried out to evaluate the influence of stand-off distance, polyester particle size, and polyester content on the microstructure, porosity, thermal conductivity, and erosion resistance of the composite coatings. Results demonstrated that porosity increased with spraying distance and higher pore former content, reaching up to 50% porosity at 20 wt.% polyester addition. Enhanced porosity led to a significant reduction, up to 80%, in thermal conductivity compared to dense GZO coatings, with larger pores showing a more pronounced effect. However, erosion tests revealed that while moderate polyester additions improved erosion resistance due to crack-arresting effects, higher porosity levels reduced mechanical integrity and accelerated material loss, particularly after polyester burn-out post-treatment. These findings highlight the balance required between optimizing thermal insulation and maintaining erosion resistance, providing new insights into the design of high-performance porous TBCs.

The quest to find an alternative to Co-based metal binders has been of prime importance within the thermal spray community to develop sustainable, Co-free WC-based cermet coatings. In this study, we investigate dWC-based coatings with FeCrNiMo as a Co-free binder, as an alternative to traditional WC-CoCr coatings. WCFeCrNiMo powders with particle sizes of fine (25/5 lm) and coarse (45/15 lm) were deposited via high velocity air fuel (HVAF) spraying with various nozzle configurations.

For benchmarking, a standard WC-CoCr coating with two particle sizes (45/15 and 30/5 lm) was included in all analyses and testing. The microstructure and mechanical properties of the coatings were thoroughly examined.Performance was evaluated through ball-on-disk sliding wear tests and air jet erosion tests. Microstructural analysis showed dense coatings, and XRD results confirmed that all coatings maintained the main phase composition of the feedstock, demonstrating HVAF’ s efficiency in preserving feedstock integrity. In sliding wear tests, the fine WCFeCrNiMo coating showed a 57% lower wear rate (8.42 9 10-8 mm3/Nm) compared to the coarser WCFeCrNiMo coating (13.23 9 10-8 mm3/Nm). In comparison, the standard WC-CoCr system exhibits an overall lower wear rate (2-3 9 10-8 mm3/Nm), attributed to its better strain hardening.

Although WC-FeCrNiMo coatings had higher wear rates, their values remained within the same order of magnitude (*10-8 mm3/Nm), which is extremely low and suitable for many demanding tribological applications. Under erosion conditions, no significant difference in removal mechanisms was observed; however, the standard WC-CoCr coatings had better erosion resistance than WC-FeCrNiMo coatings. The overall findings from this study convey that WC-FeCrNiMo coatings are promising, offering performance comparable to Co-based binders.

With plasma spraying of metal coatings, severe oxidation leads to coatings with high content of oxide inclusions and, subsequently, limited interlamellar bonding. Recently, the strategy has been proposed to generate oxide-free molten droplets by air plasma spraying in ambient atmosphere through designing metal powders with specific deoxidizer. In this paper, the synchronous effect of the generation of super-hot molten droplets on the formation of oxide-free metal droplets and intersplat bonding by a spread-fusing self-bonding mechanism during APS using deoxidizer-containing powders will be examined. With Ni-based, Cu-based and Fe-based materials, boron can be used as deoxidizer, while with Al-containing alloys such as MCrAlY and NiAl, carbon can be used as deoxidizer. It was confirmed that the generation of molten droplets with a temperature higher than 2400oC fulfills the thermodynamic deoxidizing conditions. By examining the collected powder particles passing through the plasma jet, and the splat morphology and splat-interface on samples prepared by FIB, it was revealed that oxide-free molten metal droplets are achieved through using super-hot deoxidizer-containing powders. Furthermore, the results also show that sufficient metallurgical bonding is achieved between splats within the coatings to yield a tensile adhesion over 150 MPa for NiCrB coatings. Therefore, with spray metal powders design and generation of super-hot droplets, dense metal coatings with high cohesion and low oxide inclusions can be deposited by APS. 

Thermal spray technology, a versatile coating technique, significantly impacts diverse industries and is pivotal in modern manufacturing processes. Like other technologies, it must continuously evolve to address new challenges and market demands. In this context, “hybrid thermal spraying” utilizing distinct feedstocks (such as powders, wire, suspensions, and solution precursors) offers a novel pathway to conveniently combine dissimilar materials at very different length scales to realize coatings with unique properties and enhanced performance.

This approach seamlessly integrates into all thermal spray techniques. Introducing two or more distinct feedstocks simultaneously or sequentially with independent control over each can deposit coatings with varied architectures and novel microstructures. The present-day industry constantly demands enhanced performance and longevity of established wear-resistant coatings, thermal barrier coatings (TBCs), etc., and novel functionalities for emerging fields such as batteries. Hybrid thermal spray can potentially address these needs by elegantly combining established material systems with additional constituents.

This review discusses the different variants of hybrid thermal spraying, and their relevance to practical applications is explored based on a comprehensive assessment of available literature. This review is intended to serve as a bridge between traditional and innovative approaches for inspiring further research to harness the advantages of hybrid thermal spray processes gainfully. It also discusses the challenges and limitations associated with this approach.

A new technique combining wire and powder in the form of a ‘hybrid’ feedstock during arc spraying has been explored in this study. The primary goal of this research was to investigate the feasibility of incorporating a second phase in a Babbitt matrix by spraying wire and powder simultaneously. While Tin-based Babbitt alloys have excellent tribological properties, such as a low friction coefficient and frictional resistance, making them popular materials for sliding bearings, dual-phase/composite coatings have been of interest due to their potential to perform better than corresponding single constituent coatings. A further objective was to assess the effect of copper (Cu), molybdenum (Mo), and tungsten carbide (WC) contents as second-phase reinforcements on composite Babbitt coatings’ microstructure and tribological properties. These effects were systematically investigated through scanning electron microscopy (SEM), microhardness tests, and dry sliding, as well as lubricated wear tests. The study demonstrated that a hybrid arc spray process effectively incorporated Cu, Mo, and WC particles into the coating. The wear test results indicated that adding the second phase enhanced the wear resistance of the coating.

Advanced thermal barrier coatings (TBCs) consisting of suspension plasma sprayed (SPS) topcoat and high-velocity oxy-fuel (HVOF) deposited bond coat were used in this study. Hastelloy-X substrates were coated with NiCoCrAlY bond coat using HVOF and yttrium stabilized zirconia (YSZ) with SPS techniques, respectively. Eight different topcoats were investigated with the standard HVOF bond coat for a lifetime and functional properties evaluation. Diverse microstructural features like interpass porosity, vertical and branching cracks in the coatings revealed a substantial influence on the lifetime and functional properties of the coatings. Thermal cyclic fatigue (TCF) and thermal shock tests were performed to evaluate the lifetime of the coatings and functional properties were assessed using laser flash analysis (LFA), air jet erosion tester, micro-indentation, etc. Specific emphasis on the sintering behavior of the microstructure and the mechanism behind the coating failure was detailed in this work. The findings demonstrate that HVOF-SPS systems offer a promising alternative to conventional coating technologies. 

The quest to find an alternative for Co-based metal binders has been of prime importance within the thermal spray community to develop sustainable WC-metal binder-based coatings. This is mainly driven due to the growing concerns about the potential health hazards and scarcity of cobalt reserves, prompting researchers to explore Co-lean or Co-free metal binders as an alternative to WC-Co-based coatings without compromising coating performance. High-velocity air fuel (HVAF) spraying has become a popular choice for depositing WC-based coatings due to its relatively lower combustion temperature and higher kinetic energy than high-velocity oxyfuel (HVOF) spraying, which effectively minimizes decarburization during deposition. The objective of this work was to explore Fe-based green binders as a potential alternative to Co-based binders. WC-FeCrNiMo powders with varying particle sizes (fine 25/5 µm and coarse 45/15 µm) were deposited using HVAF spraying with different nozzle configurations along with standard WC-CoCr powder as reference. The microstructure and hardness of the deposited coating were thoroughly analyzed. Performance evaluation including ball-on-disk sliding wear test, air jet erosion test and corrosion tests was carried out. The results show that coatings with Fe-based binders look promising and can provide similar performance as the Co-based binders in different operating environments.

Light alloys are being increasingly investigated as alternatives to ferrous-based engineering components, based on weight considerations. However, in-service applications of such light alloy components often require a surface modification step to enhance their wear and corrosion responses for improved functionality. Thermally sprayed cermet coatings offer an enhanced resistance to wear and corrosion. This work investigates WC-CoCr coatings deposited using two different feedstocks comprising fine and coarse powder size distributions on aluminum alloy and steel substrates using high-velocity air-fuel (HVAF) and high-velocity oxy-fuel (HVOF) spray techniques. The WC-CoCr coatings were HVAF sprayed at various parameters to investigate the relationship between the processing conditions, microstructure, and performance. Microindentation, dry sliding wear, dry sand abrasion, cavitation erosion, and corrosion tests were conducted to assess the performance of the coatings. Despite the qualitative similarities in the microstructures of the coatings, the measured microindentation hardness values were observed to vary, and coatings deposited with higher particle impact velocities showed the highest microhardness between 1400 and 1600 HV0.3. For the three categories of wear investigated, the HVAF coatings showed better resistance than the HVOF coating investigated in this study. The estimated average specific wear rate (SWR) due to sliding wear of the HVOF coating was 16.7 ± 4.0 × 10−8 mm3/Nm compared to that of the most resistant HVAF coating, which exhibited a SWR of 1.7 ± 0.6 × 10−8 mm3/Nm. The cumulative mass loss rate due to the abrasive wear on the HVOF coating reached 1.11 mg/min compared to 0.76 mg/min of the most abrasion-resistant HVAF coating. All coatings showed similar corrosion resistances under the investigated conditions. The combination of wear and corrosion performance of the respective coatings could provide insight into the coating selection for intended applications.  

The wear performance of HVAF sprayed WC-based composite coatings with Co-lean or Co-free binders was evaluated as an alternative to conventional WC-CoCr wear-resistant layers. The coatings were characterized for microstructure and tested for their micro and nano hardness. Their tribological behavior was studied for reciprocating sliding wear at room and elevated temperature (~300 ◦C). Using nanoindentation, the hardness of carbide grains was measured to be between 18 GPa and 25 GPa. Nanoindents that fell in the binder phases had a hardness of 15 GPa–18 GPa, which, due to the fine scale of the microstructure, represents some average mixture of the binder and carbide. Using Vickers indentation at higher loads, the hardness values of the coatings at room temperature were found to be between 1000 and 1200 HV0.3, which decreased by ~200 HV0.3 when tested at elevated temperatures. The wear performance of all coatings with alternative binders at room temperature was comparable to that of the reference WC-CoCr. However, the specific wear rates of the coatings tested at elevated temperature was higher by 1–2 orders of magnitude compared to those performed at room temperature. With the exception of WC-FeNiCrMoCu, the other three coatings exhibited comparable high-temperature wear performance. Ripple-like patterns were the prominent feature observed on the room temperature wear tracks. At elevated temperatures, wear tracks showed macro cracking and micro fatigue cracks, as well as pitting and oxide formation on the surface of wear track.

The wear performance of HVAF sprayed WC-based composite coatings with Co-lean or Co-free binders was evaluated as an alternative to conventional WC-CoCr wear-resistant layers. The coatings were characterized for microstructure and tested for their micro and nano hardness. Their tribological behavior was studied for reciprocating sliding wear at room and elevated temperature (~300 ◦C). Using nanoindentation, the hardness of carbide grains was measured to be between 18 GPa and 25 GPa. Nanoindents that fell in the binder phases had a hardness of 15 GPa–18 GPa, which, due to the fine scale of the microstructure, represents some average mixture of the binder and carbide. Using Vickers indentation at higher loads, the hardness values of the coatings at room temperature were found to be between 1000 and 1200 HV0.3, which decreased by ~200 HV0.3 when tested at elevated temperatures.

The wear performance of all coatings with alternative binders at room temperature was comparable to that of the reference WC-CoCr. However, the specific wear rates of the coatings tested at elevated temperature was higher by 1–2 orders of magnitude compared to those performed at room temperature. With the exception of WC-FeNiCrMoCu, the other three coatings exhibited comparable high-temperature wear performance. Ripple-like patterns were the prominent feature observed on the room temperature wear tracks. At elevated temperatures, wear tracks showed macro cracking and micro fatigue cracks, as well as pitting and oxide formation on the surface of wear track.