In-process exothermic reaction in high-velocity oxyfuel and plasma spraying with SiO<Subscript>2</Subscript>/Ni/Al-Si-Mg composite powder

Reactive thermal spraying, in which thermodynamically stable compounds are formed by expected inprocess reactions, has attracted considerable attention as a result of the wide availability of in situ composite coatings. Such in-process reactions occur differently in high-velocity oxyfuel (HVOF) and plasma spraying because of differences in the flame temperature and speed. In the current study, a composite powder of SiO₂/Ni/Al-Si-Mg was deposited onto an aluminum substrate to fabricate in situ composite coatings by both spraying methods. The coating hardness sprayed with Al-Si-Mg core powder increases with silicon and magnesium content, whereas the coatings by HVOF spraying show higher hardness than those by plasma spraying. In the present reactive spraying, the exothermic reaction of SiO₂ with molten Al-Si-Mg alloy leads to composite materials of MgAl₂O₄, Mg₂Si, and Al-Si matrix. Moreover, a rapid formation of aluminide (NiAl₃), which is introduced by an exothermic reaction of plated nickel with Al-Si-Mg core powder, enhances the reduction of SiO₂, especially in HVOF spraying. A series of in-process reactions proceed mainly during splat layering on a substrate instead of during droplet flight even in plasma spraying. Plasma-sprayed composite coatings become much harder because of the great progress of in-process reactions.     

Microstructure and Thermal Properties of Atmospheric Plasma-Sprayed Yb<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>7</Subscript> Coating

In the present work, Yb₂Si₂O₇ powder was synthesized by solid-state reaction using Yb₂O₃ and SiO₂ powders as starting materials. Atmospheric plasma spray technique was applied to fabricate Yb₂Si₂O₇ coating. The phase composition and microstructure of the coating were characterized. The density, open porosity and Vickers hardness of the coating were investigated. Its thermal stability was evaluated by thermogravimetry and differential thermal analysis (TG-DTA). The thermal diffusivity and thermal conductivity of the coating were measured. The results showed that the as-sprayed coating was mainly composed of crystalline Yb₂Si₂O₇ with amorphous phase. The coating had a dense structure containing defects, such as pores, interfaces and microcracks. The TG-DTA results showed that there was almost no mass change from room temperature to 1200 °C, while a sharp exothermic peak appeared at around 1038 °C in DTA curve, which indicated that the amorphous phase crystallized. The thermal conductivity of the coating decreased with rise in temperature up to 600 °C and then followed by an increase at higher temperatures. The minimum value of the thermal conductivity of the Yb₂Si₂O₇ coating was about 0.68 W/(m K).     

HVOF-Sprayed Coatings Engineered from Mixtures of Nanostructured and Submicron Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript> Powders: An Enhanced Wear Performance

In previous studies, it has been demonstrated that nanostructured Al₂O₃-13 wt.%TiO₂ coatings deposited via air plasma spray (APS) exhibit higher wear resistance when compared to that of conventional coatings. This study aimed to verify if high-velocity oxy-fuel (HVOF)-sprayed Al₂O₃-13 wt.%TiO₂ coatings produced using hybrid (nano + submicron) powders could improve even further the already recognized good wear properties of the APS nanostructured coatings. According to the abrasion test results (ASTM G 64), there was an improvement in wear performance by a factor of 8 for the HVOF-sprayed hybrid coating as compared to the best performing APS conventional coating. When comparing both hybrid and conventional HVOF-sprayed coatings, there was an improvement in wear performance by a factor of 4 when using the hybrid material. The results show a significant antiwear improvement provided by the hybrid material. Scanning electron microscopy (SEM) at low/high magnifications showed the distinctive microstructure of the HVOF-sprayed hybrid coating, which helps to explain its excellent wear performance. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Fabrication and Properties of Plasma-Sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZrO<Subscript>2</Subscript> Composite Coatings

Al₂O₃ /xZrO₂ (where x = 0, 3, 13, and 20 wt.%) composite coatings were deposited onto mild steel substrates by atmospheric plasma spraying of mixed α-Al₂O₃ and nano-sized monoclinic-ZrO₂ powders. Microstructural investigation showed that the coatings comprised well-separated Al₂O₃ and ZrO₂ lamellae, pores, and partially molten particles. The coating comprised mainly of metastable γ-Al₂O₃ and tetragonal-ZrO₂ with trace of original α-Al₂O₃ and monoclinic-ZrO₂ phases. The effect of ZrO₂ addition on the properties of coatings were investigated in terms of microhardness, fracture toughness, and wear behavior. It was found that ZrO₂ improved the fracture toughness, reduced friction coefficient, and wear rate of the coatings.     

HVOF thermal spray deposited Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-stabilized ZrO<Subscript>2</Subscript> coatings for thermal barrier applications

High velocity oxy-fuel (HVOF) thermal spray has been successfully used to deposit yttria-stabilized zirconia (YSZ) for thermal barrier coating (TBC) applications. Adherent coatings were obtained within a limited range of spray conditions using hydrogen as fuel gas. Spray parameters such as hydrogen-to-oxygen ratio, spray distance, and substrate cooling were investigated. Spray distance was found to have a pronounced effect on coating quality; adherent coatings were obtained for spray distances between 75 and 125 mm from the gun exit for the hydrogen-to-oxygen ratios explored. Compared to air plasma spray (APS) deposited YSZ coatings, the HVOF deposited coatings were more fully stabilized in the tetragonal phase, and of similar density, surface roughness, and cross-sectional microhardness. Notably, fracture surfaces of the HVOF coatings revealed a more homogeneous structure. Many theoretical models predict that it should not be possible to melt YSZ in an HVOF flame, and therefore it should not be possible to deposit viable YSZ coatings by this process. The experimental results in the present work clearly contradict those expectations. The present results can be explained by taking into account the effect of partial melting and sintering on particle cohesion, as follows. Combustion chamber pressures (P _o) of ∼3.9 bar (58.8 psi) realized during HVOF gun operation allows adiabatic flame temperature values that are above the zirconia melting temperature. Under these conditions, the Ranz-Marshall heat transfer model predicts HVOF sprayed particle surface temperatures T _p that are high enough for partial melting of small (∼10 μm) zirconia particles, T _p=(1.10−0.95)T _m. Further analysis shows that for larger particles (38 μm), adherent coatings are produced when the particle temperature, T _p=0.59−0.60 T _m, suggesting that sintering may have a role in zirconia particle deposition during HVOF spray. These results suggest two different bonding mechanisms for powders having a broad particle size distribution.     

Microstructure and Thermomechanical Properties of Atmospheric Plasma-Sprayed Yb<Subscript>2</Subscript>O<Subscript>3</Subscript> Coating

In this study, a Yb₂O₃ coating was fabricated by the atmospheric plasma spray technique. The phase composition, microstructure, and thermal stability of the coating were examined. The thermal conductivity and thermal expansion behavior were also investigated. Some of the mechanical properties (elastic modulus, hardness, fracture toughness, and flexural strength) were characterized. The results reveal that the Yb₂O₃ coating is predominantly composed of the cubic Yb₂O₃ phase, and it has a dense lamellar microstructure containing defects. No mass change and exothermic phenomena are observed in the thermogravimetry and differential thermal analysis curves. The high-temperature x-ray diffraction results indicate that no phase transformation occurs from room temperature to 1500 °C, revealing the good phase stability of the Yb₂O₃ coating. The coefficient of thermal expansion of the Yb₂O₃ coating is (7.50-8.67)?×?10^?6 K^?1 in the range of 200-1400 °C. The thermal conductivity is about 1.5 W m^?1 K^?1 at 1200 °C. The Yb₂O₃ coating has excellent mechanical properties and good damage tolerant. The unique combination of these properties implies that the Yb₂O₃ coating might be a promising candidate for T/EBCs applications.     

Superior performance of high-velocity oxyfuel-sprayed nanostructured TiO<Subscript>2</Subscript> in comparison to air plasma-sprayed conventional Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-13TiO<Subscript>2</Subscript>

Air plasma-sprayed conventional alumina-titania (Al₂O₃-13wt.%TiO₂) coatings have been used for many years in the thermal spray industry for antiwear applications, mainly in the paper, printing, and textile industries. This work proposes an alternative to the traditional air plasma spraying of conventional aluminatitania by high-velocity oxyfuel (HVOF) spraying of nanostructured titania (TiO₂). The microstructure, porosity, hardness (HV 300 g), crack propagation resistance, abrasion behavior (ASTM G65), and wear scar characteristics of these two types of coatings were analyzed and compared. The HVOF-sprayed nanostructured titania coating is nearly pore-free and exhibits higher wear resistance when compared with the air plasma-sprayed conventional alumina-titania coating. The nanozones in the nanostructured coating act as crack arresters, enhancing its toughness. By comparing the wear scar of both coatings (via SEM, stereoscope microscopy, and roughness measurements), it is observed that the wear scar of the HVOF-sprayed nanostructured titania is very smooth, indicating plastic deformation characteristics, whereas the wear scar of the air plasma-sprayed alumina-titania coating is very rough and fractured. This is considered to be an indication of a superior machinability of the nanostructured coating.     

Suspension Plasma-Sprayed ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Nanostructured Coatings for ppm-Level Acetone Detection

Zinc ferrite (ZnFe₂O₄) sensitive coatings have been deposited by suspension plasma spraying. The phase constitution of the coatings was characterized by x-ray diffraction while the top surface and cross-sectional morphology of the coatings were inspected by scanning electron microscopy. The response to acetone was tested with the concentration in the range of 25-500 ppm at the working temperature from 175 to 275 °C. The sensors that were deposited at an arc current of 400 A showed better performance than those at 600 A owing to small grain size and high porosity. The sensor response increased with acetone concentration. The optimized sensors showed excellent response/recovery time and selectivity to acetone at 200 °C.     

Preparation and characterization of nanostructured Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-13wt.%TiO<Subscript>2</Subscript> ceramic coatings by plasma spraying

Nanostructured and conventional Al₂O₃-13wt.%TiO₂ ceramic coatings were prepared by plasma spraying with nanostructured agglomerated and conventional powders, respectively. The microstructure and microhardness of the coatings were investigated using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and microhardness measurement. Meanwhile, the friction and wear behaviors were analyzed and compared using a ball-on-disk tribometer. The results show that the conventional coating has lamellar stacking characteristic and has some pores. However, the nanostructured coating shows a bimodal microstructure, which is composed of both fully melted regions and partially melted regions. According to the microstructural difference, the partially melted regions can be divided into liquid-phase sintered regions (a three-dimensional net or skeleton-like structure: Al₂O₃-rich submicron particles embedded in the TiO₂-rich matrix) and solid-phase sintered regions (remained nanoparticles). The microstructural characteristics of the liquid-phase sintered region are formed due to the selective melting of TiO₂ nanoparticles during plasma spraying. On the other hand, the TiO₂ and Al₂O₃ nanoparticles of the solid-phase sintered regions are all unmelted during plasma spraying. Due to the existence of nanostructured microstructures, the nanostructured coating has a higher microhardness, a lower friction coefficient, and a better wear resistance than the conventional coating.     

Effect of Microstructure on the Thermal Conductivity of Plasma-Sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-YSZ Coatings

The microstructures of three atmospheric plasma-sprayed (APS) Al₂O₃-ZrO₂ coatings were investigated using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The differences in the microstructures of the three Al₂O₃-ZrO₂ coatings, including their phase compositions, cracks, pores, grain sizes, and solid solutions, were analyzed in detail. A close relationship was observed between the thermal conductivities of the coatings and the microstructures, and the Al₂O₃-YSZ coatings with more spherical pores, fewer vertical cracks, and finer grains exhibited the lowest thermal conductivity of 0.91 W/m·K. Compared with YSZ coatings, Al₂O₃-YSZ coatings can exhibit lower thermal conductivity, which may be attributed to the formation of an amorphous phase, smaller grains, and Al₂O₃-YSZ solid solution.     

Comparison of ZrB<Subscript>2</Subscript>-MoSi<Subscript>2</Subscript> Composite Coatings Fabricated by Atmospheric and Vacuum Plasma Spray Processes

In this work, ZrB₂-20 vol.% MoSi₂ (denoted as ZM) composite coatings were fabricated by atmospheric plasma spray (APS) and vacuum plasma spray (VPS) techniques, respectively. Phase composition and microstructure of the composite coatings were characterized. Their oxidation behaviors and microstructure changes at 1500 °C were comparatively investigated. The results showed that VPS-ZM coating was composed of hexagonal ZrB₂, tetragonal and hexagonal MoSi₂, while certain amount of ZrO₂ existed in APS-ZM coating. The oxide content, surface roughness and porosity of VPS-ZM coating were apparently lower than those of APS-ZM coating. The mass gain of APS-ZM coating was maximum at the beginning (1500 °C, 0 h) and then decreased with the oxidation time extending, while the mass of VPS-ZM coating gradually increased with increasing the oxidation time. The possible reasons for the different oxidation behaviors of the two kinds of coatings were analyzed.     

Phase Stability of Ce-Modified La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> Coatings and Chemical Compatibility with YSZ

Ce-modified La₂Zr₂O₇ powders, i.e., La₂Zr₂O₇ (LZ), La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3), and La₂(Zr_0.3Ce_0.7)₂O₇ (LZ3C7), were used to produce thermal barrier coatings by atmospheric plasma spray process. The chemical compatibility of the CeO₂-doped La₂Zr₂O₇ with the traditional YSZ was investigated in LZ-YSZ powder mixtures and LZ-YSZ bilayer coatings by x-ray diffraction and scanning electron microscope. The powder mixtures and coatings were aged at 1200 and 1300 °C for 100 h. The results showed that LZ and LZ7C3 presented single pyrochlore structure after the heat treatments at both 1200 and 1300 °C. For LZ3C7, however, fluorite structure was observed at 1300 °C, indicating a poor phase stability of LZ3C7 at the elevated temperature. The results further showed that La₂(Zr_0.3Ce_0.7)₂O₇ reacted with YSZ in the bilayer ceramic coatings due to the diffusion of cerium, zirconium, and yttrium. While for La₂Zr₂O₇(LZ) and La₂(Zr_0.7Ce_0.3)₂O₇, a better chemical compatibility with YSZ was shown.     

Thermal Spray Deposition,Phase Stability and Mechanical Properties of La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript>/LaAlO<Subscript>3</Subscript> Coatings

This paper deals with the deposition of La₂Zr₂O₇ (LZO) and LaAlO₃ (LAO) mixtures by air plasma spray (APS). The raw material for thermal spray, single phase LZO and LAO in a 70:30 mol.% ratio mixture was prepared from commercial metallic oxides by high-energy ball milling (HEBM) and high-temperature solid-state reaction. The HEBM synthesis route, followed by a spray-drying process, successfully produced spherical agglomerates with adequate size distribution and powder-flow properties for feeding an APS system. The as-sprayed coating consisted mainly of a crystalline LZO matrix and partially crystalline LAO, which resulted from the high cooling rate experienced by the molten particles as they impact the substrate. The coatings were annealed at 1100 °C to promote recrystallization of the LAO phase. The reduced elastic modulus and hardness, measured by nanoindentation, increased from 124.1 to 174.7 GPa and from 11.3 to 14.4 GPa, respectively, after the annealing treatment. These values are higher than those reported for YSZ coatings; however, the fracture toughness (K _IC) of the annealed coating was only 1.04 MPa m^0.5.     

Effect of Nano-Si<Subscript>3</Subscript>N<Subscript>4</Subscript> Additives and Plasma Treatment on the Dry Sliding Wear Behavior of Plasma Sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-8YSZ Ceramic Coatings

In this paper, the effect of nano-Si₃N₄ additives and plasma treatment on the wear behavior of Al₂O₃-8YSZ ceramic coatings was studied. Nano-Al₂O₃, nano-8YSZ (8 wt.% Y₂O₃-stabilized ZrO₂) and nano-Si₃N₄ powders were used as raw materials to fabricate four types of sprayable feedstocks. Plasma treatment was used to improve the properties of the feedstocks. The surface morphologies of the ceramic coatings were observed. The mechanical properties of the ceramic coatings were measured. The dry sliding wear behavior of the Al₂O₃-8YSZ coatings with and without Si₃N₄ additives was studied. Nano-Si₃N₄ additives and plasma treatment can improve the morphologies of the coatings by prohibiting the initiation of micro-cracks and reducing the unmelted particles. The hardness and bonding strength of AZSP (Al₂O₃-18 wt.% 8YSZ-10 wt.% Si₃N₄-plasma treatment) coating increased by 79.2 and 44% compared to those of AZ (Al₂O₃-20 wt.% 8YSZ) coating. The porosity of AZSP coating decreased by 85.4% compared to that of AZ coating. The wear test results showed that the addition of nano-Si₃N₄ and plasma treatment could improve the wear resistance of Al₂O₃-8YSZ coatings.     

Mechanical Properties of Layered La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> Thermal Barrier Coatings

Lanthanum zirconate (La₂Zr₂O₇) has been proposed as a promising thermal barrier coating (TBC) material due to its low thermal conductivity and high stability at high temperatures. In this work, both single and double-ceramic-layer (DCL) TBC systems of La₂Zr₂O₇ and 8 wt.% yttria-stabilized zirconia (8YSZ) were prepared using air plasma spray (APS) technique. The thermomechanical properties and microstructure were investigated. Thermal gradient mechanical fatigue (TGMF) tests were applied to investigate the thermal cycling performance. The results showed that DCL La₂Zr₂O₇ + 8YSZ TBC samples lasted fewer cycles compared with single-layered 8YSZ TBC samples in TGMF tests. This is because DCL La₂Zr₂O₇ TBC samples had higher residual stress during the thermal cycling process, and their fracture toughness was lower than that of 8YSZ. Bond strength test results showed that 8YSZ TBC samples had higher bond strength compared with La₂Zr₂O₇. The erosion rate of La₂Zr₂O₇ TBC samples was higher than that of 8YSZ samples, due to the lower critical erodent velocity and fracture toughness of La₂Zr₂O₇. DCL porous 8YSZ + La₂Zr₂O₇ had a lower erosion rate than other SCL and DCL La₂Zr₂O₇ coatings, suggesting that porous 8YSZ serves as a stress-relief buffer layer.     

Fabrication of Oxide Dispersion Strengthened Bond Coats with Low Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Content

Nanoscale oxide dispersions have long been used to increase the oxidation and wear resistance of alloys used as bond coatings in thermal barrier coatings. Their manufacturing via mechanical alloying is often accompanied by difficulties regarding their particle size, homogeneous distribution of the oxide dispersions inside the powder, involving considerable costs, due to cold welding of the powder during milling. A significant improvement in this process can be achieved by the use of process control agent (PCA) to achieve the critical balance between cold welding and fracturing, thereby enhancing the process efficiency. In this investigation, the influence of the organic additive stearic acid on the manufacturing process of Al₂O₃-doped CoNiCrAlY powder was investigated. Powders were fabricated via mechanical alloying at different milling times and PCA concentrations. The results showed a decrease in particle size, without hindering the homogeneous incorporation of the oxide dispersions. Two powders manufactured with 0.5 and 1.0 wt.% PCA were deposited by high velocity oxygen fuel (HVOF) spraying. Results showed that a higher content of elongated particles in the powder with the higher PCA content led to increased surface roughness, porosity and decreased coating thickness, with areas without embedded oxide particles.     

Microstructure of Suspension Plasma Spray and Air Plasma Spray Al2O3-ZrO2 Composite Coatings

Al₂O₃-ZrO₂ coatings were deposited by the suspension plasma spray (SPS) molecularly mixed amorphous powder and the conventional air plasma spray (APS) Al₂O₃-ZrO₂ crystalline powder. The amorphous powder was produced by heat treatment of molecularly mixed chemical solution precursors below their crystallization temperatures. Phase composition and microstructure of the as-synthesized and heat-treated SPS and APS coatings were characterized by XRD and SEM. XRD analysis shows that the as-sprayed SPS coating is composed of α-Al₂O₃ and tetragonal ZrO₂ phases, while the as-sprayed APS coating consists of tetragonal ZrO₂, α-Al₂O₃, and γ-Al₂O₃ phases. Microstructure characterization revealed that the Al₂O₃ and ZrO₂ phase distribution in SPS coatings is much more homogeneous than that of APS coatings.     

Properties of High Velocity Suspension Flame Sprayed (HVSFS) TiO2 coatings

TiO₂ coatings were manufactured by the High Velocity Suspension Flame Spraying (HVSFS) technique using a nanopowder suspension. Their microstructure, nanohardness, tribological properties and photocatalytic activity were studied and compared to conventional atmospheric plasma sprayed (APS) and HVOF-sprayed TiO₂ coatings manufactured using commercially available feedstock. The HVSFS process leaves a fairly large freedom to adjust coating properties (thickness, porosity, anatase content, hardness, etc…) according to the desired objective. Layers with higher anatase content and higher porosity can be produced to achieve higher photocatalytic efficiency, better than conventional APS and HVOF TiO₂. Alternatively, dense protective layers can be deposited, possessing lower porosity and pore interconnectivity and better wear resistance than as-deposited APS and HVOF layers. In all cases, HVSFS-deposited layers are thinner (20 µm-60 µm) than those which can be obtained by conventional spraying processes.     

Electrochemical Corrosion of HVOF-Sprayed NiCoCrAlY Coatings in CO<Subscript>2</Subscript>-Saturated Brine

The effect of pre-oxidation treatment and surface preparation of optimized NiCoCrAlY coatings deposited by high-velocity oxygen fuel (HVOF) spraying and exposed to a low-temperature corrosive environment is reported herein. Coatings with two surface finish conditions (as-sprayed and ground) were heat treated under two different oxygen partial pressures (air and argon). The electrochemical corrosion behavior was evaluated in CO₂-saturated brine via potentiodynamic polarization, polarization resistance, and electrochemical impedance measurements. The results show that the grinding process and pre-oxidation treatment in argon enhanced growth and formation of α-Al₂O₃ scale. The potentiodynamic polarization results show that both pre-oxidation and surface treatment had a positive influence on the corrosion resistance of the coating. The reduction of the porosity and the formation of a dense, uniform, and adherent oxide scale through pre-oxidation treatment led to an increase of the corrosion resistance due to a decrease in active sites and blocking of diffusion of reactive species into the coating. However, according to the results, complete transformation from metastable alumina phases to α-Al₂O₃ in addition to formation and growth of dense α-Al₂O₃ is required to ensure full protection of the coating and base material over long periods.     

Development and Application of Binary Suspensions in the Ternary System Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> for S-HVOF Spraying

Compositions in the system Cr₂O₃-TiO₂-Al₂O₃ are among the most used ceramic materials for thermally sprayed coating solutions. Cr₂O₃ coatings present good sliding wear resistance; Al₂O₃ coatings show excellent insulation behavior and TiO₂ striking corrosion properties. In order to combine these properties, coatings containing more than one oxide are highly interesting. The conventional spraying process is limited to the availability of binary feedstock powders with defined compositions. The use of suspensions offers the opportunity for tailor-made chemical compositions: within the triangle of Cr₂O₃-TiO₂-Al₂O₃, each mixture of oxides can be created. Criteria for the selection of raw materials as well as the relevant aspects for the development of binary suspensions in the Cr₂O₃-TiO₂-Al₂O₃ system to be used as feedstock for thermal spraying are presented. This formulation of binary suspensions required the development of water-based single-oxide suspensions with suitable behavior; otherwise, the interaction between the particles while mixing could lead up to a formation of agglomerates, which affect both the stability of the spray process and the coating properties. For the validation of this formulation procedure, binary Cr₂O₃-TiO₂ and Al₂O₃-TiO₂ suspensions were developed and sprayed using the S-HVOF process. The binary coatings were characterized and discussed in terms of microstructure and microhardness.