Quasicrystalline Coating Research Articles

Electrical resistivity of coatings of quasicrystalline and crystalline phases of the Ti—Zr—Ni system in the temperature range 4–300 K

The temperature dependence of the electrical resistivity in the temperature range from 4.4 to 300 K for Ti41Zr41Ni18 (at. %) thin coatings of different crystal perfection and phase composition was studied. The coatings were deposited by magnetron sputtering with subsequent vacuum annealing in various modes ensuring the formation of several heterophase combinations, which included 1/1 and 2/1 approximation phases, a quasicrystalline icosahedral phase, a Laves phase (Ti, Zr)2Ni, and an α-Ti (Zr) solid solution. The structure before and after annealing was studied by X-ray diffractometry. It was established that coatings with a predominant content of amorphous, quasicrystalline, or approximant 2/1 phases have a temperature coefficient of resistance less than zero, while samples with a predominance of crystalline phases demonstrate characteristic metallic conductivity. The temperature dependence of the electrical resistivity of the Ti41Zr41Ni18 quasicrystalline coating turned out to be similar to the dependence in bulk samples of the Ti40Zr40Ni20 composition. The electrical resistivity of the approximant 2/1 phase with a low content of the additional Laves phase in the film has an almost constant value in the entire investigated temperature range.

Phase Transformation of High Velocity Air Fuel (HVAF)-Sprayed Al-Cu-Fe-Si Quasicrystalline Coating

Al-Cu-Fe-Si quasicrystalline coatings were prepared by high velocity air fuel spraying to study their phase transformation during the process. The feedstock powder and coating were phase characterized by scanning electron microscopy, X-ray diffractometry, differential scanning calorimetry, and transmission electron microscopy. Results show that Al3Cu2 phase, a small amount of λ-Al13Fe4 phase, quasicrystalline phase (QC), amorphous phase, and β-Al (Cu, Fe, Si) phase were present in the sprayed Al50Cu20Fe15Si15 powder. For a typical flattened powder particle, the splat periphery was surrounded by a 1 µm thick amorphous phase. The inside area of the splat was composed of the QC covered by the Al3Cu2 and Si-rich β-Al (Cu, Fe, Si) phases. Another kind of Cu- rich β-Al (Cu, Fe, Si) phase can be found close to the amorphous area with a similar composition to the original β-Al (Cu, Fe, Si) phase in the powder. Different phases were observed when the periphery and inside area of the splat were compared. This result was caused by the difference in the heating and cooling rates.

Crack Detection in High-Velocity Oxygen-Fuel-Sprayed Al59.2Cu25.5Fe12.3B3 Quasicrystalline Coatings Using Piezoelectric Active Sensors

Quasicrystals have received some attention in recent years regarding their potential usefulness as low friction and wear resistant materials. When quasicrystalline (QC) coatings are exposed to different levels of external loading or are subjected to thermal stresses, crack nucleation, and propagation become life-limiting factors. In this investigation, Al59.2Cu25.5Fe12.3B3 QC coatings were deposited onto AISI-A36 steel substrates using the high-velocity oxygen-fuel (HVOF) technique. The coatings were deposited at different oxygen-to-fuel ratios. To characterize the integrity of the coatings, the active piezoceramic excitation technique was applied to detect the propagation of cracks during three-point bending tests. The piezoelectric transducer signal was processed using wavelet transformation analysis. The results revealed that the coated samples displayed different densities of cracks depending on the oxygen-to-fuel. The crack density was found higher for coatings formed using a slightly oxidizing flame. The opening of the pre-existing cracks in the coating was the main reason of signal attenuation during piezoelectric excitation. The HVOF-sprayed AlCuFeB quasicrystalline coating, with thickness between 150 and 180 μm, withstands a flexural stress as high as 340 MPa. To the best of our knowledge, this study is the first that provides a quantitative assessment of the viability of QC coatings in mechanical applications in spite of their intrinsic brittleness.

The Preparation of Al–Cu–Fe Quasicrystalline Coating by Low Power Plasma Spraying

Al-Cu-Fe with an atomic ratio of 70:20:10 was selected for preparing the quasicrystalline coating by low power plasma spraying. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used for analyzing the phase component and microstructure of the quasicrystalline coating. It was found that the best quality of quasicrystalline coating was obtained under a current of 400A and a voltage of 40V. XRD results show that, using these parameters, a relatively higher content of quasicrystalline phase can be acquired in the coating. SEM results show that the sprayed coating has a low level of porosity.

Structurally complex metallic coatings in the Al-Cu system and their orientation relationships with an icosahedral quasicrystal

Quasicrystals have been identified as alloys possessing unusually low surface energy. This results in poor adhesion properties of quasicrystalline coatings when deposited on metallic substrates, hindering the development of these new materials for technological applications. Here we investigate the possible use of complex Al-Cu metallic phases as interface layers to accommodate the structural and electronic mismatch between a quasicrystalline coating and a metallic substrate and improve adhesion. First, we show that all stable low-temperature phases of the Al-Cu system can be grown as thin films using DC magnetron sputtering. Among the various possible phases, we select the γ-brass γ-Al4Cu9 as a promising candidate for the interface layer. Then the γ-Al4Cu9 phase is grown on the fivefold surface of an icosahedral (i-) Al-Pd-Mn quasicrystal. The interface is investigated by transmission electron microscopy and shows a clear texturing of the film. The grains exhibit rotational epitaxy with the substrate. We find that the interface is mainly composed of a β-phase of unknown chemical composition and sometimes exhibits γ grains in direct contact with the quasicrystalline substrate. Occasionally, we observe a fourth phase at the β/γ interface, identified as β1, possessing a lattice parameter aβ1 equal to 2aβ and 2/3aγ.

Study on the Tribological Properties of Low-Power Plasma-Sprayed Al-Cu-Cr Quasicrystalline Coating

Al-Cu-Cr quasicrystalline coatings were prepared by low power plasma spraying with axially-fed powder systerm. The Al65Cu20Cr15 powders were deposited on AISI 1045 steel substrate at the power ranged from 4.0 to 5.5 kW. The effects of H2/Ar flow ratio on the phase composition, microstructure and tribological properties of the as-sprayed coatings were investigated. The XRD results showed that the feedstock powders and as-sprayed coatings contained a predominant icosahedral quasicrystalline phase I-Al65Cu24Cr11 and three minor crystalline phases, including a body-centered cubic α-Al69Cu18Cr13, a monoclinic θ-Al13Cr2 (i.e. Al83Cu4Cr13) and a hexagonal ε-Al2Cu3. A qualitative analysis on the XRD patterns indicated that the volume fraction of any crystalline phase (α, ε or θ) in the coatings increased, while the quasicrystalline I-phase decreased with the rise of hydrogen (H2) content in the working gas. However, as increasing the plasma power, the friction and wear resistance of the coating were improved under the same dry sliding wear test conditions. The improvementincrease of the wear resistance may be attributed to the high hardness of quasicrystal.

Comparison between Tribological Characteristics of Al-Cu-Fe-B Quasicrystalline and Mo Coatings

Al-Cu-Fe-B quasicrystalline and Mo coatings were obtained on the mild steel and brass substrates by thermal spraying routes for the purpose of replacement of Mo coatings with quasicrystalline ones. Quasicrystalline coatings were prepared by air plasma spraying and/or HVOF (High Velocity Oxygen Fuel) techniques followed by subsequent heat treatment, and Mo coatings, wire flame spraying. For comparative studies of important properties for industrial application, mechanical properties, bonding strength, surface energy, and tribological behavior were investigated based on microstructural characterization. Basic mechanical properties such as hardness, fracture toughness, and elastic modulus of quasicrystalline coating showed comparable values with those of Mo coatings. De-bonding tests of coatings deposited onto brass substrate indicated that the bonding strength of quasicrystalline coatings obtained by HVOF techniques exhibit higher value to Mo coatings. Non-sticking property analogized from surface energy calculation and friction coefficient of quasicrystalline coatings also showed better performance during the tests. It is suggested from this investigation that the quasicrystalline coating can be effectively used as a replacement of the Mo coating, which has shown a recent steep price rise and problems of accidental existence of minor environment harmful elements such as Cr6+, Pb, Cd, and Hg.

Microstructures and Tribological Properties of Plasma Arc Sprayed Al-Fe-Cr-Ti Quasicrystalline Coatings

One of the most important use of thermal spray coatings is for wear resistance. In this work, the tribological properties of plasma prayed Al83.7Fe7Cr6.3Ti3 quasicrystalline coatings have been studied. The quasicrystal powders of Al-base alloys were produced by gas atomization unit in vacuum. The plasma spray process was used to produce quasicrystalline coatings. Then, friction experiments were carried out on a pin-on-disc-type tribometer. The results indicated that the friction coefficient of Al83.7Fe7Cr6.3Ti3 quasicrystalline coating is about 50% lower than that of Al70Fe13Cu10Cr7 quasicrystalline coating.

Hot Corrosion Behavior of AlCuFeCr Quasicrystalline Coating on Titanium Alloy with the Mixture of NaCl and Na<sub>2</sub>SO<sub>4</sub> Deposit

The hot corrosion behavior of titanium alloy and AlCuFeCr quasicrystalline coating on titanium alloy in the presence of a solid mixture of NaCl and Na2SO4 deposit at 700°C was studied. The result shows that weight-gain kinetics for titanium alloy exhibited a linear rate law, while the kinetics of AlCuFeCr quasicrystalline coating displayed parabolic growth rate. The corrosion resistance of the titanium alloy was improved by applying the AlCuFeCr quasicrystalline coating. The corrosive oxide morphology formed on titanium alloy was porous. For AlCuFeCr quasicrystalline coating with the mixture of NaCl and Na2SO4 deposit, the scale formed on the coating surface was compact and uniform. Oxide formed on the surfaces of Al-Cu-Fe-Cr quasicrystalline coatings after hot corrosion consisted of Al2O3.

Effect of the addition of crystalline β-phase in Al–Cu–Fe quasicrystalline coatings on their tribological properties

We investigate tribological properties (friction coefficient, wear, and adhesion force with fretting tests) of both quasicrystalline (Ψ-phase) and dual-phase (Ψ + β-cubic phases) Al–Cu–Fe coatings produced by electron-beam physical vapour deposition. Performed standard pin-on-disk tests (using bear steel pin and a load of 2 N) indicate that both quasicrystalline and dual-phase (Ψ + β) Al–Cu–Fe coatings exhibit close values of the friction coefficient (≈ 0.2–0.3) in vacuum. At the same time, the wear rate of the dual-phase coating is found to be essentially lower than that of quasicrystalline coating. It is demonstrated also that the value of the adhesion between the coating and the steel counterpart under fretting depends on the coating structure. The lowest adhesion is found to be observed for quasicrystalline coatings. Possible origins of the tribological behaviour of the coatings and their potential applications are discussed.

Hot corrosion of AlCuFeCr quasicrystalline coating on titanium alloys with NaCl deposit

The hot corrosion behavior of titanium alloy and AlCuFeCr quasicrystalline coating on titanium alloy in the presence of a solid NaCl deposit at 650, 700 and 750 °C was studied. The result shows that weight-gain kinetics for titanium alloy exhibited a linear rate law, while the kinetics for AlCuFeCr quasicrystalline coating displayed parabolic growth rate. The corrosion resistance of the titanium alloy was improved by applying the AlCuFeCr quasicrystalline coating. The corrosive oxide morphology formed on titanium alloy was porous. For AlCuFeCr quasicrystalline coating with NaCl deposit, the scale formed on the coating surface was compact and uniform. Oxide formed on the surfaces of Al–Cu–Fe–Cr quasicrystalline coatings after hot corrosion consisted of Al 2O 3.

Corrosion of Al-Cu-Fe-Cr Quasicrystalline Coating

Al-Cu-Fe-Cr quasicrystalline coating was deposited on a substrate of stainless steel by low-pressure plasma spraying (LPPS) method. The corrosion behavior of such coating was studied by polarization in 1mol/l H2SO4 and 0.1mol/l NaOH solutions at room temperature. The polarization curve shows that LPPS Al-Cu-Fe-Cr quasicrystalline coating can turn to passive state both in 1mol/l H2SO4 solution and in 0.1mol/l NaOH solution. The corrosion resistance of the coating is poorer than that of bulk quasicrystal in 0.1mol/l NaOH solution. Moreover, in strong acid solution LPPS Al-Cu-Fe-Cr quasicrystalline coating has more corrosion resistance than 1Cr18Ni9Ti in some potential range ranging from -200mVSCE to -35mVSCE, but in strong alkaline solution the corrosion resistance of the coating is poorer than 1Cr18Ni9Ti.

Mechanical spectroscopy of decagonal Al–Cu–Fe–Cr quasicrystalline coatings

Mechanical spectroscopy measurements were performed on decagonal quasicrystalline Al–Cu–Fe–Cr coatings of three different thicknesses deposited on a mild steel substrate. The mechanical loss spectra indicate that the internal friction is mostly caused by the quasicrystalline coating and that the contributions of both the steel substrate and the interface are small. The shear modulus measured in torsion increases with temperature, while the Young’s modulus measured in flexion behaves normally. This shear modulus anomaly is interpreted as being due to solid friction between cracked segments of the quasicrystalline coating. This phenomenon also explains the broad athermal maximum found to occur in isochronal internal friction measurements. A quantitative model successfully reproducing the observed behaviour has been developed. Finally, the reversible high-temperature exponential background was interpreted as being due to the onset of the brittle-to-ductile transition in the quasicrystalline coating. The measured activation enthalpy is similar to the value that was deduced from compression tests performed at high temperatures on icosahedral Al–Cu–Fe bulk material.

Mechanical spectroscopy of Al–Cu–Fe quasicrystalline coatings

Since the bulk Al-Cu-Fe quasicrystals are extremely brittle below 700 K, a quasicrystalline coating deposited on a steel substrate by thermal spraying was used for mechanical spectroscopy measurements at temperatures in the range of 300-800 K. The internal friction of this composite was measured in a forced torsion pendulum and the internal friction of the quasicrystalline coating itself was calculated. An internal friction maximum was observed around 600 K in isochronal measurements at 1 Hz, which is not present in isothermal measurements. This maximum may be associated with the relaxation of the thermal stresses built at the interface between the substrate and the coating. These stresses are also responsible for an anomalous increase of the shear modulus with temperature. The high temperature exponential background is interpreted as the brittle-to-ductile transition in the quasicrystalline coating. (C) 2003 Published by Elsevier B.V.