We report the effect of graphene (G) allotropic carbon modification in a content range of 0.5–2.0 wt % on the tribological, strength, and structural characteristics of an Al
2
O
3
/G nanocomposite ...produced by a 10-min plasma spark sintering (at a pressure of 50 MPa and a temperature of 1550°C) of a nanopowder mixture, previously subjected to ultrasonic dispersion in organic solvent. Its lubricant free friction and wear are tested at room temperature on a tribometer under a load of 20 N, at a roundabout motion of a ruby ball penetrator on a disk. The nanohardness and elastic modulus of the nanocomposite are determined via kinetic indentation. The fracture surface structure and friction track are monitored using a scanning electron microscope. The microstructure in the bulk of the nanocomposite was probed via dark- and bright-field transmission electron microscopy scanning of thin foils. The thermal stability of graphene was monitored via Raman spectroscopy. The introduction of graphene is shown to improve micro- and nanohardness, elasticity, and wear resistance by two to three orders of magnitude, as well as to slightly decrease the coefficient of friction. A graphene content of 2 wt % alters the mechanism of wear from brittle fracture to viscous shear owing to stronger coupling of matrix grains and the presence of agglomerates. A lack of degradation and the retention of graphene thermal stability are evidenced as well. The morphology of graphene particles reveals their preferential arrangement inside the corundum grains rather than at the grain boundaries.
In this work, five different concentrations of a mixture of TiO2/Al2O3 nanopowders in an alcoholic suspension at 10wt.% solid content were sprayed by Suspension Plasma Spraying on steel discs. The ...influence of the presence of TiO2 at 0, 13, 40 and 75wt.% in Al2O3 was analysed by studying the properties of the sprayed coatings. Microscopy analysis of the projected coatings revealed a homogeneously distributed microstructure, where the densification of the coating increases with TiO2 content, while the original nanostructure is maintained. A nanoindentation study revealed an increment of nanohardness and elastic modulus due to the densifying effect of TiO2. The addition of significant amounts of TiO2 has been revealed as necessary in order to favour the fusion of Al2O3 in the SPS process.
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•Al2O3–TiO2 coatings with retained nanostructure were obtained by SPS.•TiO2 acts as a flux agent in the presence of Al2O3.•Density and hardness of coatings increase with TiO2 content.
This study aims to evaluate the mechanical behavior of nanostructured and conventional alumina–titania coatings by means of combining four-point bend tests monitored with acoustic emission. ...Determination of bending strength, along with information received from acoustic emission signal, such as quantity, amplitude, energy and length of events gives a better understanding of failure mechanisms in coatings. This information has been complemented with optical analysis of fracture surfaces. Failure of coatings has been observed to follow four stages that are clearly defined in the acoustic emission data. This fracture behavior has been observed for all three types of coating tested, but the nanostructured coating presented the best combination of hardness and strength, due to its duplex microstructure.
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 Al
2
O
3
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 TiO
2
in the fabricated coatings on friction coefficient behavior, wear rates and wear damage patterns was determined. The addition of TiO
2
to the coatings was found to greatly increase wear resistance by, for example, 2.6-fold for 40 wt% of TiO
2
. The analysis of the wear surface was correlated with microstructural parameters, mechanical properties and wear rates.
Los recubrimientos de cermet WC-Co proyectados por plasma se utilizan en un amplio rango de aplicaciones industriales, principalmente por su resistencia al desgaste, incluso en medio corrosivo. El ...objetivo de este trabajo es analizar la respuesta mecánica de los recubrimientos de metal duro mediante ensayos de flexión a 3 y 4 puntos aplicando el método de emisión acústica para determinar las tensiones críticas de fallo. Se ha observado el efecto del nivel de carga soportado en el dañado estructural mediante microscopia óptica y electrónica de barrido. La emisión acústica ha permitido relacionar el grado de dañado con el nivel de tensiones y, así, entender el mecanismo de fallo de los recubrimientos.
Electrical conductivity (σ), relative dielectric permittivity (εr) and dissipation factor (D) measured in graphene-alumina composites. Samples obtained by plasma spark sintering (SPS) from a mixture ...of raw powders: δ-alumina (36 nm average particle size) and graphene flakes (3 nm thickness and 2-3 μm length). Graphene content in samples was 0, 1 and 2% by weight. The study carried out for frequencies from 50 Hz to 100 kHz. Both c and εr were higher for Al2O3-2% graphene: up to 90 μS/m and 19 respectively; while alumina with 1% graphene showed similar values to the pure alumina samples: 50 μS/m to electrical conductivity and 16 to relative permittivity. The dissipation factor was similar in the three materials tested. D increased with the frequency, reaching high values (0.7) at 100 kHz. Composites with 1 and 2% graphene content showed a dissimilar dielectric behavior with the frequency. Alumina reflected a classical behavior of the permittivity dependence with the frequency. Graphene composites also show the same behavior at frequencies above 100 Hz. Below this frequency, the presence of graphene increases the relative permittivity to exceed that from pure alumina. The graphene content leads to rise of relative permittivity, which means easier polarizability.
Al2O3-TiO2 coatings were deposited on austenitic stainless steel coupons from nanostructured powders by atmospheric plasma spraying (APS). Commercial suspensions of nanosized Al2O3 and TiO2 particles ...were used as starting materials. Mixtures of these suspensions and of more concentrated suspensions of Al2O3 and TiO2 were then agglomerated into plasma sprayable feedstock. Agglomeration was performed by spray drying, followed by consolidation thermal treatment. These powders were successfully deposited, yielding coatings that were well bonded to the substrates. The coating microstructure thus consisted of semi-molten feedstock agglomerates surrounded by fully molten particles that acted as binders. Agglomerates from suspensions with higher solids contents yielded coatings with lower porosity and fewer semi-molten areas.
The ultrasonic dispersion and spark plasma sintering technique have been used to process Al2O3/Graphene nanocomposite (Al2O3/G) with 0-2wt.% graphene. The topography of fracture, microstructure, ...density, microhardness, Young modulus, electrical and thermal conductivity of Al2O3/G nanocomposite are investigated.
In the present work, the properties of Al
2
O
3
nanocomposite prepared via spark-plasma sintering and reinforced with 0.5–2 wt % graphene are studied. Samples with different graphene contents are ...subjected to measurements of density, microhardness, coefficient of friction of composite–ruby, and frictional wear rate of composite. The fracture and wear track surface are inspected via fractography, and the composite as a whole is examined via X-ray diffraction. The graphene additive is established to increase the microhardness and to decrease the frictional wear rate by two orders of magnitude on account of absence of flaking of grains.
The use of protective coatings on components of machines and mechanisms provides the greatest economic benefit at the lowest additional cost. Plasma spraying is one of the most productive, ...technologically advanced, and efficient methods of producing these coatings. The results of investigations of structures, mechanical properties, and fracture surfaces of ceramic wear resistant coatings produced by plasma spraying have been presented.
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