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Titanium dioxide (TiO2) coatings, renowned for their photocatalytic properties, are of significant interest for environmental and industrial applications. This study investigates the deposition of black titanium oxide (BTO)- containing coatings using the atmospheric plasma spraying (APS) process, employing an argon-hydrogen plasma mixture. The influence of hydrogen flow rate, spray distance, and powder feed rate on the structure and mechanical properties of the resulting coatings was examined. Deposited coatings were examined in terms of their structure, thickness porosity and specific surface area. Examination of the mechanical properties included hardness test and Young's modulus measurement. Presence of partially sintered fine droplets was revealed by SEM observations and mechanism of their formation was discussed. The examination of mechanical properties showed that within the examined parameters, spray distance has the greatest impact on Young's modulus, hardness and porosity of the coatings, while the powder feed rate affects coatings thickness. [ABSTRACT FROM AUTHOR]

Atmospheric plasma spraying (APS) allows deposition of ceramic coatings on metallic substrate, significantly increasing the wear resistance of the component. Coating's microstructure and consequently its properties, depend on heat treatment of the feedstock particles. This property can be controlled by process parameters, especially electric power and spray distance. On the other hand, some plasma torches allow introducing another variable, an injection mode, which could be realized in external or in internal way. In the presented research, wear resistance of Al2O3+13 wt.% TiO2 coatings deposited under various values of spray distance and different injection modes were examined using ASTM G-99 procedure. The impact of spraying parameters on the coating's microstructure was established and connected to the functional properties of the manufactured deposits. Observed differences indicated influence of injection mode and spray distance value. Coatings deposited with internal feedstock injection exhibit lower porosity and slightly higher hardness than these made with external injection mode. In terms of wear resistance for both spray distance the wear factor was lower for internal injection system. Obtained results confirmed assumed thesis, that internal injection mode improves heat treatment and consequently ameliorate wear performance of plasma sprayed alumina-titania coatings. [ABSTRACT FROM AUTHOR]

In this work, the yttria stabilised zirconia ZrO2 + 8 wt% Y2O3 (YSZ) coatings were studied. The coatings were manufactured by using a relatively new method based on liquid feedstock, called suspension plasma spraying (SPS). The main aim of the study was to investigate the influence of one of the fundamental process parameters, stand-off distance, on the YSZ coating mechanical properties, namely adhesion, cohesion, hardness, and dry sliding wear resistance. Moreover, the coating surface morphology and microstructure were investigated. Despite the fact that in the SPS method, the heat flux into the substrate is much higher than in conventional atmospheric plasma spraying (APS), for the stand-off distances as short as 40 mm, the structure has not been damaged by thermal stresses. The results revealed that shorter spray distance leads to obtaining the coatings characterised by higher cohesion and adhesion to the substrate as well as higher hardness and resistance to sliding wear. The wear mechanism of both YSZ coatings relies on the adhesive mode, which is intensified by severe coating material delamination. [ABSTRACT FROM AUTHOR]

The paper briefly describes major thermal spray techniques used to spray functionally graded coatings such as atmospheric plasma spraying, high velocity oxy-fuel spraying, suspension and solution precursor plasma spraying, and finally low and high pressure cold gas spray method. The examples of combined spray processes as well as some examples of post spray treatment including laser and high temperature treatments or mechanical one, are described. Then, the solid and liquid feedstocks used to spray and their properties are shortly discussed. The reviewed properties of functional coatings include: (i) mechanical (adhesion, toughness, hardness); (ii) physical (porosity, thermal conductivity and diffusivity, thermal expansion, photo-catalytic activity), and; (iii) bioactivity and simulated body fluid (SBF) corrosion. These properties are useful in present applications of functionally graded coatings as thermal barriers, the bioactive coatings in prostheses, photo-catalytic coatings in water treatment, coatings used in printing industry (anilox and corona rolls). Finally, some of the future possible fields of functional thermal sprayed coatings applications are discussed, e.g., to coat polymer substrates or to use the cheap technology of low pressure cold gas spray method instead of expensive technology of vacuum plasma spraying to obtain bond coatings. [ABSTRACT FROM AUTHOR]

This paper discusses the influence of 40 wt% TiO 2 addition to Al 2 O 3 and the selection of plasma spray parameters on the morphology, microstructure, microhardness and adhesion of coatings. Mainly, the tribological behavior and cavitation erosion performance of produced coatings were evaluated. First, the two-factorial design of the experiment, with the stand-off distance and torch scan speed as variables, was used to plan coating deposition by means of atmospheric plasma spraying (APS). Then, a process response using a multiple regression method was used to investigate the influence of factors variation on coatings functional properties. Although these properties were already investigated in alumina-titania coatings, there are still some knowledge gaps. Particular attention was given to the composition of Al 2 O 3 + 40 wt% TiO 2 , because, so far, it is the most rarely discussed in the literature in terms of, e.g., cavitation or wear resistance. Increasing the torch scan speed and decreasing the spraying distance densifies the microstructure and increases coating's mechanical properties. A dense and low porosity structure facilitates cavitation erosion resistance and minimizes friction coefficient. Cavitation erosion damage relies on brittle cracking initiated at coatings porosity. Therefore, coatings with decreasing porosity show layer-like removal contrary to uniform material removal stated for porous coatings. The sliding wear mechanism has been dominated by abrasive wear and fatigue cracking. Accumulated wear debris undergoes adhesive smearing and removal. • Alumina with 40 wt% of titania coatings were deposited by APS method. • Selected process parameters after DoE enable to obtain high quality coatings with homogeneous distribution lamellas. • Increasing the spray distance causes better wear resistance and lower friction coefficient. • Increasing torch velocity improves the cavitation erosion resistance. • Mechanisms of wear and cavitation damage were described. [ABSTRACT FROM AUTHOR]

Abstract: The microstructure of ZrO2 +8wt.% Y2O3 coatings has been studied by means of transmission electron microscopy (TEM) including selected-area electron diffraction (SAED) and electron backscatter diffraction (EBSD). The coatings were suspension-plasma-sprayed onto stainless steel substrates. The parameters spray distance and torch scan speed had been varied. Both diffraction methods have confirmed the tetragonal lattice structure of the suspension-plasma-sprayed zirconia. The combination of TEM and EBSD allowed a detailed analysis of the size, shape and orientation of grains. The microstructure of the coatings is mainly lamellar, consisting of columnar grains. Additional microstructural features indicate that powder particles have been subjected to different modifications during spraying. The influence of the variation of spraying parameters on the microstructure is marginal. [Copyright &y& Elsevier]

Plasma generated by the SG-100 torch was applied to spray suspension formulated with the use of ZrO + 8 wt.% YO (8YSZ) and ZrO + 24 wt.% CeO + 2.5 wt.% YO (24CeYSZ) as solid phases. The suspensions were formulated with the use of 20 wt.% solid phase, 40 wt.% water, and 40 wt.% ethanol. The plasma spray parameters were optimized by keeping constant: (a) the electric power of 40 kW and (b) the working gas compositions of 45 slpm for Ar and 5 slpm for H. On the other hand, the spray distance was varied from 40 to 60 mm and the torch linear speed was varied from 300 to 500 mm/s. The coatings were sprayed onto stainless steel substrates, and their thicknesses were in the range from 70 to 110 μm. The coating microstructures were analyzed with a scanning electron microscope. Mechanical properties were tested with the different methods including the indentation and scratch tests. The indentation test, carried out with various loads ranging from 100 to 10,000 mN, enabled to determine elastic modulus and Martens microhardness. Young's modulus of the coatings was in the range of 71-107 GPa for 8YSZ and 68-130 GPa for 24CeYSZ coatings. The scratch test enabled the authors to find the scratch macrohardness. [ABSTRACT FROM AUTHOR]

Abstract: Plasma generated by SG-100 torch was applied to spray suspension formulated with the use of ZrO2 +8wt.% Y2O3 (8YSZ) solid phase. The solids have the mean size of about 4.5μm and were obtained by milling commercial Metco 204NS powder. The suspension was formulated with the use of 20wt.% of solid phase, 40wt.% water and 40wt.% of ethanol. The plasma spray parameters were optimized with the electric power equal to 40kW, working gases composition Ar—45slpm and H2—5slpm, spray distance varying from 40 to 60mm and torch linear speed with regard to substrate varying from 300 to 500mm/s. The coatings of the thickness ranging from 51 to 106μm, were sprayed onto stainless steel substrates. The porosity of the samples was found, with the help of image analysis of metallographically prepared cross-sections of the samples, to be in the range of 8–12%. Thermal diffusivity was measured with the use of the commercial NanoFlash system in the temperature range from room temperature to 523K. The measurements were made with the use of the coatings sprayed on the substrate and the 2-layers numerical model was applied to determine thermal diffusivity of the coatings. The diffusivity was in the range 0.196×10−6 to 0.352×10−6 m2/s in room temperature depending on the spray parameters. The obtained data were then associated with the literature data on density, specific heat and experimental porosity to find thermal conductivity which was in the range of 0.47 to 0.86W/(mK) at room temperature depending on the spray run. [Copyright &y& Elsevier]

Abstract: ZrO2 +8wt.% Y2O3 powder of a mean diameter d VS =38μm was milled to obtain fine particles having mean size of d VS =1μm. The fine powder was used to formulate a suspension with water, ethanol and their mixtures. The zeta potential of obtained suspensions was measured and found out to be in the range from −22 to −2mV depending on suspension formulation. The suspension was injected through a nozzle into plasma jet and sprayed onto stainless steel substrates. The plasma spray experimental parameters included two variables: (i) spray distance varying from 40 to 60mm and (ii) torch linear speed varying from 300 to 500mm/s. The microstructure of obtained coatings was characterized with scanning electron microscope (SEM) and X-ray diffraction (XRD). The coatings had porosity in the range from 10% to 17% and the main crystal phase was tetragonal zirconium oxide. The scratch test enabled to find the critical load in the range of 9–11N. Finally, thermal diffusivity of the samples at room temperature, determined by thermographic method, was in the range from 2.95×10−7 to 3.79×10−7 m2/s what corresponds to thermal conductivities of 0.69W/(mK) and 0.97W/(mK) respectively. [Copyright &y& Elsevier]

Abstract: Home synthesized (HA) powder was formulated with water and alcohol to obtain a suspension used to plasma spray coatings onto titanium substrate. The deposition process was optimized and the resulting coatings were soaked in simulated body fluid (SBF) for the periods of 3, 7, 14, 28, and 60days at controlled temperature of 37°C. The microstructural research enabled to find in the as-sprayed deposits two characteristic zones: (i) dense zone corresponding to the lamellas, observed usually in thermally sprayed coatings; (ii) sintered zone containing fine hydroxyapatite grains corresponding to the fine solids from initial suspension. The sintered zone disappears after soaking in SBF and the pores get filled by the reprecipitated calcium phosphates. The adhesion of the soaked coatings to the substrate was characterized by the critical load in the scratch test and was about 10 to 12N. The Young modules of the coatings were determined with help of depth-sensing indentation test by the use of the technique developed by Oliver and Pharr. The modules were not depending on the time of soaking and their mean values were 15.6 and 28.4GPa, depending on the coating operational parameters. The scratch test enables to determine the hardness of the coatings, which remained fairly constant during the time of soaking in the range of 3 to 5GPa. This hardness was compared to that the Martens microhardness which doubled with time of soaking to reach up to 1GPa. [ABSTRACT FROM AUTHOR]

Thermally sprayed ceramic coatings are applied for the protection of surfaces that are exposed mainly to wear, high temperatures, and corrosion. In recent years, great interest has been garnered by spray processes with submicrometric and nanometric feedstock materials, due to the refinement of the structure and improved coating properties. This paper compares the microstructure and tribological properties of alumina coatings sprayed using conventional atmospheric plasma spraying (APS), and various methods that use finely grained suspension feedstocks, namely, suspension plasma spraying (SPS) and suspension high-velocity oxy-fuel spraying (S-HVOF). Furthermore, the suspension plasma-sprayed Al2O3 coatings have been deposited with radial (SPS) and axial (A-SPS) feedstock injection. The results showed that all suspension-based coatings demonstrated much better wear resistance than the powder-sprayed ones. S-HVOF and axial suspension plasma spraying (A-SPS) allowed for the deposition of the most dense and homogeneous coatings. Dense-structured coatings with low porosity (4 vol.%) and good cohesion to the metallic substrate, containing a high content of α–Al2O3 phase (56 vol.%) and a very low wear rate (0.2 ± 0.04 mm3 × 10−6/(N∙m)), were produced with the S-HVOF method. The wear mechanism of ceramic coatings included the adhesive wear mode supported by the fatigue-induced material delamination. Moreover, the presence of wear debris and tribofilm was confirmed. Finally, the coefficient of friction for the coatings was in the range between 0.44 and 0.68, with the highest values being recorded for APS sprayed coatings. [ABSTRACT FROM AUTHOR]

This paper discusses the influence of using 13 wt% of TiO 2 addition to Al 2 O 3 powder and selection of plasma spray parameters on morphology, microstructure, phase composition, mechanical properties, and, particularly, tribological behaviour and cavitation erosion performance of produced coatings. Two types of powders were used to spray the coatings by means of atmospheric plasma spraying (APS): Al 2 O 3 (AT0) and Al 2 O 3 + 13 wt% TiO 2 (AT13). The stand-off distance and torch scan speed were two spray parameters considered in this work. The aim of the study was to investigate how the addition of titania to alumina in agglomerated and sintered nanostructured powders could improve the coating microstructure and functional properties. The results demonstrated that addition of TiO 2 in the powder provides denser microstructure of the coating, improves the contact between splats, prevents the coating from severe fragmentation and spallation during wear, and prolongs the cavitation lifetime. In general, AT0 coatings exhibit slightly better wear resistance than AT13 equivalents. On the other hand, it was found that AT13 coatings present superior cavitation erosion resistance when compared with AT0. • Al 2 O 3 and Al 2 O 3 -TiO 2 coatings were deopisted by APS method • AT0 and AT13 coatings had a good wear resistance (generally less than 40 x 10-6 mm3/Nm) as well as cavitation erosion resistance • Mechanisms of the sliding wear and cavitation erosion were described. [ABSTRACT FROM AUTHOR]

The Al2O3 + TiO2 coatings are of the interest of surface technology and tribology due to their good wear resistance and enhanced toughness comparing to pure Al2O3 coatings. However, the detailed effect of the used feedstock powder, is often neglected. Here, this work focuses on the deposition of Al2O3 + 13 wt % TiO2 coatings by atmospheric plasma spraying (APS) method as well as on their microstructure, phase composition and selected mechanical properties, in the reference to the route of the powder feedstock preparation. The commercial powder Metco 6221 in agglomerated and sintered form was used as a feedstock material during spraying, due to the fact that, so far, sintered or cladded powders are the most studied ones. The 2k + 1 spray experiment allowed to evaluate the influence of two variables, namely spray distance and torch linear velocity, on the coating microstructure. Afterwards, the coating adhesion was measured by the means of pull-off test. The correlations between Vickers microhardness, fracture toughness (Kc) as well as the coating morphology and phase composition were investigated. Finally, the dry sliding wear resistance was studied by using Ball-on-Disc method. [ABSTRACT FROM AUTHOR]