Al2O3-TiO2 Coatings Research Articles

Slurry erosion performance study of HVFS sprayed Ni-20Al2O3 and Ni-15Al2O3-5TiO2 coatings under hydro accelerated conditions

PurposeThis paper aims to cover all the aspects of development, investigation and analysis phases to evaluate the slurry erosion performance of test coatings. The powders having composition of Ni-20Al2O3 and Ni-15Al2O3-5TiO2 were deposited on CA6NM grade turbine steel by using high velocity flame spray (HVFS) technique. The characterization of the coatings was done with the help of SEM/EDS and XRD techniques. Various properties such as micro-hardness and bonding strength of the coatings were also evaluated. Thereafter, these coatings were subjected to an indigenously developed high speed slurry erosion tester at different levels of rotational speed, erodent particle size and slurry concentration. The effect of these parameters on the erosion behavior of coatings was also evaluated. The slurry erosion tests and SEM of the eroded surfaces revealed remarkable improvement in slurry erosion resistance of Ni-15Al2O3-5TiO2 coating in comparison with Ni-20Al2O3 coating.Design/methodology/approachTwo different compositions of HVFS coating were developed onto CA6NM steel. Subsequently, these coatings were evaluated by means of mechanical and microstructural characterization. Further, slurry erosion testing was done to analyze the erosive wear behavior of developed coatings.FindingsThe coatings were successfully developed by HVFS process. Cross-sectional microscopic analysis of sprayed coatings revealed a continuous and defect-free contact between substrate and coating. Ni-15Al2O3-5TiO2 coating showed higher value of bond strength in comparison with Ni-20Al2O3 coating. Under all the testing conditions, Ni-15Al2O3-5TiO2 coatings showed higher resistance to slurry erosion in comparison with Ni-20Al2O3 coatings. Rotational speed, average particle size of erodent and slurry concentration were found to have proportional effect on specific mass loss of coatings. The mixed behavior (brittle as well as ductile) of the material removal mechanism was observed for the coatings.Originality/valueFrom the literature review, it was found that researchers have documented the various studies on Ni-Al2O3, Ni-TiO2 and Al2O3-TiO2 coatings. No one has ascertained the synergetic effect of Alumina and Titania on the slurry erosion performance of Nickel-based coating. In view of this, the authors have developed Ni-Al2O3 and Ni-Al2O3-TiO2 coatings, and an attempt has been made to compare their mechanical, microstructural and slurry erosion characteristics.

Wear and corrosion behaviour of Al2O3-TiO2 coatings produced by flame thermal projection

Evaluated the wear resistance and the coatings corrosion behaviour of Al2O3-TiO2 prepared by thermal spraying by flame on AISI 1020 carbon steel substrates, previously coated with an alloy base Ni. For this purpose, were controlled parameters of thermal spraying and the use of powders of similar but different chemical composition is taken as a variable commercial reference for ceramic coating. SEM images allowed to know the morphology of the powders and coatings. Electrochemical techniques (Tafel) were applied to evaluate the protection against corrosion. Coatings were tested for wear with a tribometer configuration bola-disco. It was determined that the phases present in coatings are directly relate to the behaviour against corrosion and wear them. Keywords: wear, corrosion, thermal imaging.

Influence of microstructure on hardness of plasma sprayed Al2O3–TiO2–MgO coatings with interface diffusion by heat treatment

The effect of heat treatment on the microstructure and mechanical properties of Al2O3–TiO2 coatings doped with 5 wt% MgO was investigated in this paper. The composite coatings were prepared by atmospheric plasma spraying (APS) and heat treated at 1000 °C for 24 h in Ar. The coatings were analyzed using scanning electron microscopy with electron probe x-ray microanalysis and x-ray diffraction. The hardness was determined using a Vickers hardness test on the as-sprayed coatings and after heat treatment. The results showed that the interface diffusion between the Al-rich and Ti-rich layers resulted in mutual pinning within the coating during the heat treatment. The newly formed MgAl2O4 phase promoted cracking-healing behavior within the coating. We conclude that increase of the hardness of the coatings was mainly caused by the mutual pinning interface and crack healing.

Effect of binary rare earth oxide on the properties of plasma sprayed Al2O3/TiO2 coatings

PurposeThis study systematically investigated the effect of the binary rare earth oxide of La2O3 and Sm2O3 on the properties of the Al2O3/TiO2 (AT) coating, including phase transform, wear behavior, etc.Design/methodology/approachAT coatings mixed with different components of binary rare earth oxides of La2O3 and Sm2O3 are prepared by atmospheric plasma spraying. The adhesion strength, micro-hardness, phase transition and tribological behavior of coatings are systematically investigated.FindingsThe X-ray diffraction (XRD) analysis shows that phase transformation is obvious after spraying, and a-Al2O3 is almost translated into γ-Al2O3 when La2O3 and Sm2O3 are doped together. Meanwhile, solid solution generated between rare earth oxide and Al2O3/TiO2 coatings results in disappearance of TiO2 and rare earth oxide phase. The photos under the scanning electron microscope (SEM) indicate that binary rare earth oxide could increase the melting degree of powder and decrease porosity of coatings.The increasing of Sm2O3 rarely affect micro-hardness and adhesion strength, and the coating with 4 per cent Sm2O3 and 1 per cent La2O3 exhibits the best wear resistance and lowest friction coefficient among all the samples.Originality/valueAT coatings mixed with different components of binary rare earth oxide of La2O3 and Sm2O3 are prepared by atmospheric plasma spraying. Binary rare earth oxide could increase the melting degree of powder and decrease porosity of AT coatings.

Trying out spraying modes and properties of wear-resistant flame Al2O3–TiO2 coatings fabricated using a flexible cord material

Samples of Al2O3–TiO2 coatings are fabricated by the flame spraying of a flexible cord. The influence of process parameters and composition of the sprayed material on the structure, composition, and mechanical properties of coatings is investigated. It is shown that an increase in the spraying distance and feed rate of the sprayed material leads to a decrease in their density. An increase in the concentration of the low-melting TiO2 component predetermines a decrease in the coating porosity and has no significant effect on the coating hardness. Being subjected to measuring scratching, Al2O3–TiO2 flame coatings formed with minimal porosity (Π = 3.2%) are characterized by cohesion fracture behavior and no substrate opening under an indenter load of up to 90 N. The friction factor of coatings under study varies from 0.2 to 0.78 after 2800 counterbody revolutions (44 m of the friction path). This is associated with the accumulation of fatigue cracks in the coating material and its subsequent cohesive fracture by the formation of large fragments serving as an abrasive.

Effect of electron beam remelting on the characteristics of HVOF sprayed Al2O3-TiO2 coatings deposited on titanium substrate

The primary objective of this study was to improve the wear resistance of titanium by depositing on its surface a HVOF (high velocity oxygen fuel) sprayed Al2O3-TiO2 coating subsequently remelted by electron beam (EB).The microstructure of the feedstock material, as-sprayed and EB remelted coatings was investigated by scanning electron microscopy (SEM) and X-Ray diffraction techniques (XRD). The effect of EB treatment on the coatings characteristics especially concerning the morphology, the hardness respectively the corrosion and wear behaviour was also analysed.The results indicated that remelting process improved the coating properties in terms of porosity, strength and chemical homogeneity. The coating adherence to the substrate was strengthened by means of metallurgical bonding and interdiffusion processes.The sliding wear resistance and the hardness of the coating were significantly higher in comparison with that of the titanium. The densified material obtained after the EB remelting had the highest sliding wear resistance and its corrosion behaviour was not strongly influenced compared with that exhibited by the substrate material.

The Tribological Behaviors of Different Mass Ratio Al2O3-TiO2 Coatings in Water Lubrication Sliding against Si3N4

ABSTRACTThe tribopairs of water hydraulic plunger pumps are usually operated under severe conditions, due to the poor lubrication of water and silt suspended in natural water. It is essential to identify the desired engineering materials and material combinations for designing water pumps. As the candidate materials of tribopairs, the tribological characteristics of different Al2O3-TiO2 coatings combined with Si3N4 ceramics under silt-laden water and tap water lubrication were investigated. The Al2O3-TiO2 coatings with different weight percentages of TiO2 in a wide range from 3 to 100% were tested. The tribological characteristics of the various couple pairs were researched using a ring-on-ring test rig. The microstructures of the contact surfaces were analyzed with a scanning electron microscope before and after the test to study the wear mechanism. The experimental results indicate that the friction coefficient of the Al2O3-TiO2/Si3N4 tribopairs increases with an increase in the percentage of TiO2 content in the Al2O3-TiO2 coating. However, the TiO2 content does not present a clear relationship with the wear rate. Considering the friction and wear properties, Al2O3-13%TiO2 is the preferred coating to use in water hydraulic pumps when sliding against Si3N4.

Tribological behaviour of alumina-titania nanostructured coatings produced by air plasma spray technique

Nanostructured Al2O3-TiO2 wear resistant coatings were produced by air plasma spray from plasma-densified agglomerated nano-powders. Conventional Al2O3-TiO2 coatings were also deposited for direct comparison of tribological performance. Operating parameters were optimized by a DOE (Design of Experiment) procedure, to produce coherent and well adherent ceramic coatings and to minimize dwell times in the torch in order to reduce the risk of excessive grain growth or complete remelting. Optimal tribological behaviour was selected as the main goal function. Microstructural characterization of APS nanostructured coatings confirmed the presence of fully molten areas combined with unmolten particles still retaining their spherical morphology, and with elongated particles whose microstructure showed initial stages of grain growth and recrystallization processes. Coatings produced with the final set of deposition parameters were tested for micro-hardness, elastic modulus and unlubricated wear resistance against alumina. Results clearly indicate that nanostructured coatings produced from plasma densified nanostructured precursors exhibit considerably improved performances, evaluated in terms of worn out volume and mass loss.

Microstructural and Tribological Properties of Al2O3-13pctTiO2 Thermal Spray Coatings Deposited by Flame Spraying

THe present investigation has been conducted to study the tribological properties of Al2O3-13pctTiO2 (AT-13) ceramic coatings deposited on a low carbon steel type E335 by using a thermal flame spray technique. The microstructure and phase composition of wire and coatings were analyzed by scanning electron microscope, energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Measurements of micro hardness were also performed on the surface of the coatings. The tribological tests were carried out using a pin-on-disk tribometer at different loads. All tests were performed using two disks as counter body, namely Al2O3-ZrO2 (AZ-25) and Al2O3-TiO2 (AT-3) which formed couple 1 and couple 2, respectively, in order to work out the wear rate and friction coefficient. Roughness profiles were also evaluated before and after each test. The SEM showed that the dense microstructure of Al2O3-TiO2 (AT-13) coatings have a homogenous lamellar morphology and complex of several phases with the presence of porosities and unmelted particles. The XRD analysis of the wire before the spray showed a majority phase of α-Al2O3 rhombohedral structure and a secondary phase of Al2TiO5 orthorhombic structure with little traces of TiO2 (rutile) tetragonal structure, whereas the XRD of the coating revealed the disappearance of TiO2 replaced by the formation of a new metastable phase γ-Al2O3 cubic structure. The tribological results showed that the applied contact pressure affects the variation of the friction coefficient with time and that it decreases with the rise of the normal force of contact. It was found also that the couple 2 with nearly chemical compositions of spray-coated (AT-13) and disk (AT-3) exhibited much higher wear resistance than the couple 1 although they have sliding coefficient of friction nearly.

Sliding Wear Behavior of Al2O3–TiO2 Coatings Fabricated by the Suspension Plasma Spraying Technique

The friction and dry sliding wear behavior of alumina and alumina–titania near-nanometric coatings were examined. Coatings were obtained by the suspension plasma spraying technique. Dry sliding wear tests were performed on a ball-on-disk tribometer, with an Al2O3 ball as counterpart material, a normal load of 2 N, a sliding distance of 1200 m and a sliding speed of 0.1 m/s. The effect of including TiO2 in the fabricated coatings on friction coefficient behavior, wear rates and wear damage patterns was determined. The addition of TiO2 to the coatings was found to greatly increase wear resistance by, for example, 2.6-fold for 40 wt% of TiO2. The analysis of the wear surface was correlated with microstructural parameters, mechanical properties and wear rates.

Properties of the thermally sprayed Al2O3–TiO2 coatings deposited on titanium substrate

Based on the fact that titanium and titanium alloys have poor fretting fatigue resistance and poor tribological properties, it is necessary to apply some surface engineering methods in order to increase the exploitation characteristics of these materials. One may either implement some surface treatment technologies or even deposit overlay coatings by thermal spraying.The present study is focused on the achieved properties of the ceramic coatings (Al2O3+13wt.% TiO2) deposited onto a titanium substrate using high velocity oxygen fuel (HVOF) and plasma spraying (APS) respectively.The effect of the deposition method on the microstructure, phase constituents, and mechanical properties of the ceramic coatings was investigated by means of scanning electron microscopy (SEM), X-ray diffraction technique (XRD) and nanoindentation tests. The sliding wear performances of the Al2O3–TiO2 coatings were tested using a pin on disk wear tester.

Effect of the initial particle size distribution on the properties of suspension plasma sprayed Al2O3–TiO2 coatings

Al2O3–TiO2 coatings have been deposited by atmospheric plasma spraying from agglomerated, nanostructured powders showing better properties than those of their conventional (microstructured) counterparts. These nanostructured coatings can be also obtained by suspension plasma spraying however the research on suspension plasma sprayed Al2O3–TiO2 is still scarce. Consequently, it is crucial to study the effect of the suspension characteristics on the coating properties and to optimize the deposition process.In this work, Al2O3–13wt.% TiO2 tribological coatings were successfully deposited by suspension plasma spraying from three different feedstocks: a nanometric suspension and two bimodal suspensions with different solid contents made up of titania nanoparticles and alumina submicron-sized particles. The coating microstructure and phase composition were characterized using scanning electron microscopy and X-ray diffraction analysis. Moreover, nanoindentation technique was used to determine the nanomechanical properties of coatings.The influence of the feed suspension characteristics on the final coating quality was analyzed. Findings showed that similar microstructures and phases were developed after depositing the different feedstocks. In addition suspension feedstock made up of nanoparticles resulted in a coating with better mechanical properties. However the use of submicron-sized particles in the suspension feedstocks gives rise to some technical and economic advantages in the process which should be taken into account when a suspension plasma spraying process is to be set up.

Influence of composite powders’ microstructure on the microstructure and properties of Al2O3–TiO2 coatings fabricated by plasma spraying

Three kinds of alumina–titania composite powders with different microstructure were plasma sprayed to prepare alumina–titania coatings. The influence of composite powders’ microstructure on the microstructure and properties of alumina–titania coatings was investigated. There was typical lamellar structure and columnar grains formed in the conventional micro-composite coating, which were derived from the coarse-grained microstructure and segregated composition of the micro-composite powder feedstock. Comparing with the micro-composite coating, there was not typical lamellar structure and columnar grains in the coatings prepared from nanocomposite powders, which was mainly attributed to the fine-grained microstructure and homogeneous distribution of chemical composition in the nanocomposite powders. There were two types of microstructures formed in the two coatings prepared from nanocomposite powders. Fully-melted regions with splat structure consisting of γ-Al2O3 dissolved with titania and amorphous phase in two nanocomposite coatings were formed by the fully melted nanocomposite powders during plasma spraying. Partially-melted regions with particulate structure consisting of α-Al2O3 and amorphous phase in the nanocomposite coating was formed by the partially melted or unmelted nanocomposite powder with particulate structure, and partially-melted regions with three-dimensional network structure consisting of amorphous intergranular network thin film rich in Ti, Zr and Ce surrounding the α-Al2O3 colonies in the nanocomposite coating were formed by the partially melted or unmelted nanocomposite powder with three-dimensional network structure. The microhardness, crack growth resistance and sliding wear resistance of the coatings prepared from the nanocomposite powders, especially the nanocomposite coating prepared from the nanocomposite powder with three-dimensional network structure, were enhanced comparing with that of the conventional micro-composite coating. The superior properties of the nanocomposite coatings were attributed to the uniform composition distribution in their nanocomposite powders.

Formation and surface characterization of nanostructured Al2O3-TiO2 coatings

One pot synthesis of Al2O3-TiO2 nanoceramic coatings from environment-friendly potassium titanium oxalate (PTO) electrolyte using facile electrochemical anodization has been reported for the first time. Systematic analysis of the anodization parameters such as applied current density and concentration of the PTO electrolyte influence on the morphology of the ceramic coatings was done. The textural properties of the coatings (thickness, growth rate, coating ratio) showed a linear regime with current density and electrolyte concentration decreases up to a certain level and then decreases. The growth process, distribution of chemical elements, phase constitutions and corrosion resistance of the coatings were investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Tafel polarization technique and electrochemical impedance spectroscopy (EIS). The relation between the corrosion resistivity of the anodic coating and the aforementioned anodization parameters is investigated. The mechanisms that are involved in the formation of the ceramic coatings are also discussed. The coatings formed from 30 g/L concentration of PTO and 0.02 A/cm2 current density show good morphology, textural properties and optimum corrosion resistance.

The Formed Mechanism of Pores in Plasma Sprayed Nanostructured Al<sub>2</sub>O<sub>3</sub>/TiO<sub>2</sub> Coatings

Pores are common defects in the coating formed via plasma spraying. In order to investigate the formed mechanism of pores in plasma sprayed nanostructured Al2O3-TiO2 coatings, the microstructure of the feed stock, splat and coating were characterized by means of scanning electron microscopy (SEM). The coating shows a typical lamellar morphology, which contains fully melted (FM) zone and partly melted (PM) zone.The pores in different places of the coating were generated from different sources. The shapes and the size of the pores are also different. Keywords: plasma spray, coating, pores, formed mechanism

Effect of the mechanical properties on drilling resistance of Al2O3–TiO2 coatings manufactured by atmospheric plasma spraying

Al2O3 with 13 and 45wt.% TiO2 microsized powders (6–22 and 13–41μm for each chemical composition) were used as raw materials to coat AISI 1040 steel by atmospheric plasma spraying. The mechanical properties of the coatings were measured by micro-indentation tests, and drilling experiments were carried out using high speed steel (HSS) rotary drill bits of various diameters and varying the load on the drill bits. In order to reduce the effect of the wear on the bit, a new bit was used for each test. According to the results, the drilling test is proposed as a method of determining the mechanical properties of these coatings from the correlation found between coating hardness and drilling resistance.

Characterization of Nanostructured Ceramic and Cermet Coatings Deposited by Plasma Spraying

Industry has a growing need of advanced coatings for a variety of applications (aerospace, special machinery, medicine, etc.). Nanostructured coatings have the potential of providing novel materials with enhanced properties. This paper describes the results of recent research on wear-resistant nanostructured coatings. Cermet (WC–Co) and ceramic (Al2O3–TiO2) coatings were obtained by atmospheric plasma spraying. Coating microstructure and phase composition were characterized using scanning electron microscopy, X-ray energy-dispersion microanalysis, and X-ray diffraction techniques. Vickers microhardness (HV) of the coatings was also measured. The microstructure and properties of the developed nanostructured coating were compared with those of their conventional counterparts. The influence of the substrate on both microstructure end properties was investigated, as the coatings were deposited on two different steels. In the WC–Co coatings, significant decomposition of the WC particles occurred during spraying, which is prejudicial to wear resistance. Nanostructured Al2O3–TiO2 coatings exhibited a bimodal microstructure that retained the initial nanostructure of the powder. The presence of these areas leads to better tribological properties.

Laser remelting of plasma sprayed nanostructured Al2O3–TiO2 coatings at different laser power

Abstract Plasma sprayed nanostructured Al 2 O 3 –13 wt.% TiO 2 coatings were fabricated on a Ti–6Al–4V alloy using as-prepared feedstock. The as-sprayed coatings were remelted by a CO 2 laser. The coatings before and after laser remelting were characterized by using X-ray diffractometer (XRD), scanning electron microscope (SEM), energy-dispersive spectrometer (EDS) and Vickers microhardness tester. The effects of laser power on microstructure and properties of the laser remelted Al 2 O 3 –13 wt.% TiO 2 coatings (LRmC) were investigated. The metastable γ-Al 2 O 3 phase in the as-sprayed coatings was transformed to stable α-Al 2 O 3 during laser remelting. The LRmC possessed much denser and more homogenous structure as well as an excellent metallurgical bonding to the substrate. Microstructural correlation was found in the coatings before and after laser remelting. The microhardness of the LRmC was enhanced to 1200–1800 HV 0.3 , which was much higher than that of the as-sprayed coatings and 3–5 times that of the substrate. The microhardness values were increased with increasing laser power.

Corrosion properties of plasma-sprayed Al2O3-TiO2 coatings on Ti metal

Three different oxides of CrO2-TiO2, Al2O3 and Al2O3-TiO2 were plasma-sprayed on Ti substrate to evaluate the crystal structure and the corrosion properties of the coatings. No phase change of the coatings after corrosion test in 0.5 M H2SO4 solution at 25°C was found regardless of the presence of the NiCoCrAlY bond layer. Electrochemical measurements and SEM results revealed that the single coatings without the bond layer were always effective against corrosion resistance due to lower current density within the passive region. Pitting corrosion of the surface was observed for the Al2O3 coating. It can be concluded that the Al2O3-TiO2 coating without the bond layer may be the best oxide among the oxides investigated due to low porosity (5.4%), smooth surface roughness (4.5 μm), low current density (6.3 × 10‒8 A/cm2) in the passive region, low corrosion potential (Ecorr, ‒0.55 V) and no pitting corrosion.

Wear behavior of flame-sprayed Al2O3–TiO2 coatings on plain carbon steel substrates

In this study, Al2O3–TiO2 powders were sprayed using a flame-spray technique after a NiCrMo bond layer was deposited on plain carbon steel substrate. The produced layers were characterized by X-ray diffraction (XRD), optical microscope, scanning electron microscope (SEM) including energy-dispersive spectroscopy (EDS), surface roughness and microhardness tester. Friction and wear behaviour of the coatings were also evaluated in the present study. The Al2O3–TiO2 coatings were subjected to sliding wear against AISI 303 stainless steel counter body under dry and acidic environments. A pin-on-plate type apparatus was used with normal loads in the range of 49–129 N. Wear resistance of the coatings in acid environment is better than that in dry conditions.