Formation and performance of an Fe-based amorphous-nanocrystalline composite coating by laser cladding and remelting

Abstract

A very important research topic in the area of the surface performance of engineering components, in particular their wear properties, recently has been the application of high quality surface layers on relatively cheap substrates. An Fe-based composite coating with both amorphous and nanocrystalline structures on a mild steel substrate offers a combination of high quality coating and low materials cost, at the same time extending the range of applications of traditional materials. The difficulties posed by preparation of Fe-based amorphous alloys have limited progress for many years. However, the recent development of high power lasers, and of laser material processing technology in general, has made the preparation of a Fe-based amorphous and nanocrystalline composite coatings over a large area a real possibility.     

Effect of deformation temperature on the deformation behaviors of amorphous/crystalline composites

The amorphous/crystalline composite comprising amorphous particles of Cu54Ni6Zr22Ti18 embedded in the crystalline nickel matrix was produced and its temperature dependence of the plastic deformation behavior was studied in the supercooled liquid region (SLR) of the amorphous alloy. The flow stress of the amorphous alloy was quite sensitive to the testing temperature in the SLR. The deformation of the composites was dominated by the flow stress of the amorphous alloy. The deformation behavior of the composite was analyzed by finite element method (FEM) calculations.     

Microstructure evolution and thermal stability of an Fe-based amorphous alloy powder and thermally sprayed coatings

High velocity oxy-fuel (HVOF) thermal spraying has been used to produce coatings of an Fe–18.9%Cr–16.1%B–4.0%C–2.8%Si–2.4%Mo–1.9%Mn–1.7%W (in at.%) alloy from a commercially available powder (Nanosteel SHS7170). X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) were employed to investigate the powder, as-sprayed coatings and annealed coatings which had been heated to temperatures in the range of 550–925 °C for times ranging from 60 to 3900 min. Microhardness changes of the coatings were also measured as a function of annealing time and temperature. The powder was found to comprise amorphous and crystalline particles; the former had a maximum diameter of around 22 μm. The coating was composed of splat like regions, arising from rapid solidification of fully molten powder, and near-spherical regions from partially melted powder which had a largely retained its microstructure. The amorphous fraction of the coating was around 50% compared with 18% for the powder. The enthalpies and activation energies for crystallization of the amorphous phase were determined. Crystallization occurred in a two stage process leading to the formation of α-Fe (bcc), Fe_1.1Cr_0.9B_0.9 and M₂₃C₆ phases. DSC measurements showed that the first stage occurred at 650 °C. Annealing the coating gave a hardening response which depended on temperature and time. The as-sprayed coating had a hardness of 9.2 GPa and peak hardnesses of 12.5 and 11.8 GPa were obtained at 650 and 750 °C, respectively. With longer annealing times hardness decreased rapidly from the peak.     

Corrosion behaviour of Ni-Zr-Ti-Si-Sn amorphous plasma spray coating

Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ amorphous material was deposited by a vacuum plasma spraying technique onto steel and copper substrates in order to investigate their behaviour in a corrosive environment. For comparison, the same alloy was prepared as amorphous ribbons by melt spinning. The amorphous nature of the coatings and ribbons was characterized by XRD, DSC and TEM, while XPS and AES analyses were performed to understand the origin of passivation and mode of corrosion. The corrosion behaviour of the coating was studied in H₂SO₄ and HCl solutions open to air at room temperature. Potentiodynamic polarisation and galvanic coupling tests were carried out on the substrate and the coating. It was found that the formation of Zr-, Ti- and Si-rich passive oxide layers provide a high corrosion resistance in H₂SO₄ solution while the breakdown of the passive layer by chloride ion adsorption was responsible for pitting corrosion of the Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ amorphous ribbons in HCl solution. Galvanic corrosion was the dominant corrosion mechanism for the coating/copper hybrid structure, in contrast to the Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ amorphous coating, which efficiently protected the steel substrate in the corrosive environment.     

Elastic properties of amorphous and crystalline B1−xCx and boron at low temperatures

We present measurements of the internal friction (Q⁻¹) and speed of sound variation (δ

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₀) of amorphous boron (a-B) and amorphous B₉C (a-B₉C). The elastic properties of these materials, which can only be produced as thin films, are consistent with those of other amorphous solids measured to date and exhibit good agreement with the tunneling model (TM) of amorphous solids. The TM parameter

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γ_t²/ρ

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_t² extracted from the elastic data has the same order of magnitude as that observed for all amorphous solids studied to date; a review will be presented. Using the results from the elastic measurements, we calculate the T² thermal conductivity Λ expected in the TM regime (T≤1 K) for a-B. The predicted thermal conductivity falls within the expected range for amorphous solids and agrees with the thermal conductivity of the crystalline icosahedral boride MB_68-δ (M=Y, Gd), which has been previously shown to exhibit glass-like excitations. We have also measured the internal friction and speed of sound variation of bulk polycrystalline c-B_1−xC_x at low temperatures (0.07 K<T<10 K). The elastic properties evolve towards the behavior characteristics of amorphous solids for increasingly carbon-deficient (x<0.20) specimens. The magnitude of the internal friction for the most carbon-deficient crystalline c-B_1−xC_x sample (x=0.1, c-B₉C) is comparable to that for a-B and a-B₉C, thereby confirming the inherent glass-like vibrational properties of carbon-deficient c-B_1−xC_x. Such behavior supports the glass-like character of carbon-deficient c-B_1−xC_x high temperature (T>50 K) thermal transport reported previously and provides the first experimental evidence for the presence of two-level systems (TLS) in these crystalline solids. However, discrepancies with the tunneling model are present; the data for c-B_1−xC_x bear some similarity to those for amorphous metals in which electronic relaxation channels are active, although details are still unclear. Previous studies have shown that the TM quantity C=Pγ_t²/ρ

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_t² (“tunneling strength”) is essentially independent of the material's shear modulus G=ρ

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_t² over a factor of 17. The elastic data presented in this work now extend the observed independence of the tunneling strength, C, over a factor of 70 in shear modulus.     

Characterization of an amorphous–crystalline Mg-based composite

An amorphous–crystalline Mg-based composite with the nominal composition of Mg₆₅Cu₂₀Zn₅Y₁₀ has been formed by casting. X-ray diffractometry, differential scanning calorimetry, backscatter electron imaging, and energy dispersive X-ray analysis were used to characterize the structure of the composite, which is linked to the hardness, modulus, and fracture toughness obtained by Vickers and nanoindentation. Comparisons of the structure–property relationships are also made to composite materials of a similar composition. The effects of the crystalline microstructure on localized deformation through shear banding are discussed and related to the hardness and toughness results.     

Characterizations of cold-sprayed Nickel-Alumina composite coating with relatively large Nickel-coated Alumina powder

Previous studies have shown that the fabrication of metal matrix composites (MMCs) by cold spraying is effective and promising. When light materials, such as SiC and Al₂O₃, were used as reinforcements, it was diffcuclt to obtain a high volume fraction of hard phase in the composite just through the simple powder mixture. Therefore, in this study, a Ni-coated Al₂O₃ powder, which was produced through hydrothermal hydrogen reduction method, was employed aiming at increasing the volume fraction of ceramic particles in the deposited composite coating. It was found that a dense Ni-Al₂O₃ composite coating could be deposited with the Ni-coated Al₂O₃ powder under the present spray conditions. X-ray diffraction analysis indicated that the composite coating had the same phase structures as the feedstock. The volume fraction of Al₂O₃ in the composite was about 29 ± 6 vol.%, which is less than that in the feedstock (nominal: 40-45 vol.%) due to the rebound of some Al₂O₃ particulates upon kinetic impacting. The microhardness of the composite coating was about 173 ± 33Hv_0.2.     

Failure mechanisms of a hydrogenated amorphous carbon coating in load-scanning tests

A diamond-like carbon (DLC) coating with a top layer of pure hydrogenated amorphous carbon (a-C:H) and an interlayer of tungsten-modified hydrogenated amorphous carbon (a-C:H:W) was deposited onto polished cylindrical specimens of a hardened and tempered cold work tool steel. On a load-scanning test rig, tribological–mechanical tests under dry conditions with DLC coated specimens sliding against identical, but uncoated specimens were performed. Additionally, comparative tests with DLC sliding against DLC and tool steel sliding against tool steel were carried out. During each test cycle, the normal load was gradually increased from 13 to 350 N, corresponding to a Hertzian contact pressure of 1.0 to 3.0 GPa. The coefficient of friction was monitored as a function of the normal load, with a significant increase in friction indicating failure of the coating. The tests were repeated and stopped at different total numbers of load cycles. After the tests, a FIB-assisted microscopical analysis in terms of wear and damage of the DLC coating was performed, revealing the (subsurface) failure mechanisms. For DLC sliding against steel, the coating fails within only few load cycles; first tribologically and after that mechanically. Failure is initiated by adherence and subsequent transfer of steel of the counter body. Below adhered steel flakes, tensile cracks form in the a-C:H top layer, with sharp crack edges removing even more steel from the counter body. In following load cycles, coating fragments are being pulled out at these spots, representing the final failure mode. In contrast, for DLC sliding against DLC, no coating failure and also no significant wear are observed, even after a considerably higher number of load cycles.     

Discrepancies in the morphology and physical properties of amorphous and crystalline sprayed lanthanum nickel oxide films

Amorphous LaNiO₃ (a-LNO) and crystalline LaNiO₃ (c-LNO) films were prepared by spraying an aqueous precursor solution of lanthanum and nickel chlorides on hot (450 °C) fused silica substrates followed by annealing at high temperatures (550–850 °C). Thermal analysis of a dried precursor indicated that a stable oxide phase is formed at 560 °C with no distinct crystallization peak. Scanning electron microscopy (SEM) powered with energy-dispersive X-ray spectroscopy (EDX) of as-sprayed films showed rough surfaces with particulate-like deposits and incomplete pyrolysis chloride composition. No chloride contents were detected in annealed films. X-ray diffraction showed that films annealed at 550 °C and 650 °C were a-LNO and those annealed at 750 °C and 850 °C were c-LNO. The c-LNO phase was indexed as a single-phase perovskite structure with (1 1 0) orientation. SEM/EDX showed that a-LNO films have rough surfaces and c-LNO films have uniform crack-free smooth surfaces. Electrical properties measurements showed that c-LNO films have lower resistivity than a-LNO films and both types of LNO films have semiconductor resistant temperature dependence. The activation energy of electric conduction of a-LNO films was found to be much higher than that of c-LNO films. The optical transmittance and reflectance of the films were studied in the UV–visible–near IR range. The optical constants were obtained by modeling the measured transmission and reflection spectra. Because of the discrepancies in the morphology and in the physical properties of a-LNO and c-LNO films, the best fit modeling of transmission and reflection spectra was obtained by using different theoretical models and different geometrical configurations. While the Drude model accounting for larger carrier density was found to be significant for c-LNO, using the Bruggmann model and a configuration of a rough layer on top of a compact film was found to be significant for a-LNO.     

Characterizations of cold sprayed TiN particle reinforced Al2319 composite coating

In this study, a dense Al2319/TiN composite coating was successfully prepared using cold spraying with mechanically blended powders. TiN particles were uniformly dispersed in the coating matrix with a volume fraction of 38.7 vol.%, which is higher than that of 32.7 vol.% in the powder blend. Compared with the pure Al2319 coating, the Al2319/TiN composite coating exhibits a significantly increased adhesive strength. The incorporation of the TiN particles increases the coating hardness from 106 ± 7.8 to 154.5 ± 18.9 Hv_0.2. In addition, compared with the pure Al2319 coating, the composite coating exhibits a significantly improved tribological performance. The results obtained in this work indicated that cold spraying is a promising process to fabricate Al alloy-based composite coatings.     

Formation, properties and microstructure of amorphous/crystalline composite Ag20Cu30Ti50 alloy using miscibility gap

The aim of the work was to produce the amorphous/crystalline composite with uniform distribution of fine crystalline soft phase. Silver–copper–titanium Ag₂₀Cu₃₀Ti₅₀ alloy was prepared using 99.95 wt% Ag, 99.95 wt% Cu, 99.95 wt% Ti that were arc-melted in argon atmosphere. Then the alloy was melt spun on a copper wheel with linear velocity of 33 m/s. Investigation of the microstructure for both arc-melt massive sample and melt-spun ribbons was performed with use of scanning electron microscope (SEM) with EDS, light microscope (LM) and X-ray diffraction. The thermal stability was evaluated by differential scanning calorimetry (DSC). The properties such as Young modulus and Vickers hardness number before and after crystallization of the amorphous matrix were measured with use of nanoindenter. The microstructure was investigated by transmission electron microscope (TEM). It was found, that the alloy has a tendency for separation within the liquid state due to the miscibility gap which resulted in segregation into Ti–Cu–Ag matrix and Ag-base spherical particles after arc-melting. During rapid cooling through the melt spinning the Ag₂₀Cu₃₀Ti₅₀ alloy formed an amorphous/crystalline composite of fcc silver-rich spherical particles within the amorphous Ti–Cu–Ag matrix.     

Effect of annealing and heating/cooling rate on the transformation temperatures of NiFeGa alloy

In this work, Ni₅₅Fe₁₈Ga₂₇ ferromagnetic shape memory alloy was prepared through a suck-casting method. The effects of annealing and heating/cooling rate on the martensitic transformation temperatures were investigated by differential scanning calorimetry (DSC). The results showed that the phase transformation temperatures increase with increasing the annealing temperature, upon the heating and cooling process. However, the start and the finish temperatures (M_s and M_f) of martensitic phase transformation increased firstly and then decreased upon the cooling process with the increase the annealing time at 300 °C. The start and the finish temperatures (A_s and A_f) of inverse phase transformation increased slightly upon the heating process with the increase of the annealing time. The results can be explained by the evolution of the microstructure after heat treatment. It was also found that the phase temperatures show great dependence on the heating/cooling rate of the DSC test, A_s and A_f increased and M_s and M_f decreased with the increase of the heating/cooling rate.     

WC-based composite coatings prepared by the pulsed gas dynamic spraying process: Effect of the feedstock powders

WC-based cermet coatings are typically produced using the HVOF process, due to high particle velocity and the lower heating characteristic of this technique. Despite the effort of optimisation of the coating process, degradation of the feedstock materials such as decarburisation of WC and amorphization of the metallic phase still occurs. It is known that the coating properties do not depend only on the spray process and its parameters, but also on the feedstock powder characteristics such as its chemistry, carbide size, particle morphology and production method. The work presented here is part of a research program aimed at exploring the possible advantages of the Pulsed Gas Dynamic Spray (PGDS) process, as an alternative technique for the preparation WC-based cermet coatings. In this paper, WC-based coatings have been prepared using six different types of cermets powders. In order to study the effects of the feedstock powder on the coatings microstructures and hardness, the selected starting powders differed not only in microstructural features such as size and morphology but also in the chemistry and phases. Using different analysis technique (OM, SEM, XRD, and HV), a detailed comparison of powders and coatings microstructures, phase compositions, and hardness are presented and discussed in detail. It was found that the PGDS process preserves the microstructure of the starting cermet powders in such a way that no significant degradation of the phase composition, even those that show the pre-existence of complex carbides, has been observed. Furthermore, although the same spray parameters were used, the thickness, deposition efficiency, porosity, and micro-cracks within the coatings are different from one type of cermet to another, suggesting that PGDS optimum process parameters are material dependant.     

激光功率对Zr基非晶复合涂层微观组织与耐腐蚀性能的影响

选取单质混合粉末,在纯Zr基板上利用激光熔覆技术制备了Zr-Cu-Ni-Al非晶涂层。采用X射线衍射分析仪(XRD)、扫描电镜(SEM)、显微硬度仪及电化学工作站研究了激光功率对熔覆层显微组织与性能的影响。结果表明:熔覆涂层由非晶相、金属间化合物及部分金属氧化物等共同组成,熔覆涂层树枝晶尺寸随着激光功率的增加而增大,熔覆涂层的硬度随着树枝晶尺寸增大而降低,涂层硬度最高可达(567.1±12.3)HV0.5,是基体硬度的4.2倍;当激光功率为1000 W,扫描速率为800 mm/min时,涂层的耐蚀性能最好,其中自腐蚀电位为-0.182 V,电流密度为5.2×10^-8 A/cm^2。     

退火温度对淬火态Fe-Mn-Si-Cr-Ni合金记忆效应的影响

研究了退火温度对淬火态预先存在热诱发ε马氏体的Fe-14Mn-5.5Si-8.0Cr-5.0Ni合金和无热诱发ε马氏体的Fe-19Mn-5.0Si-8.0Cr-6.0Ni合金记忆效应的影响。结果表明:预先存在热马氏体合金的形状回复率随退火温度的升高,先上升后下降,在500℃附近达到最大值。但无热马氏体存在合金的形状回复率随退火温度的变化却相反,在500℃附近达到最小值;两种合金的Ms温度都随退火温度的升高而下降,在500℃附近达到最低。预先存在热马氏体的合金由于退火后Ms温度的降低,减少了热诱发的马氏体量,因而形状记忆效应得到了提高;而无热马氏体存在的合金由于退火后Ms温度的进一步下降,使得应力诱发马氏体转变更不容易发生,因此形状记忆效应反而下降。     

Phase constituents and mechanical properties of laser in-situ synthesized TiCN/TiN composite coating on Ti-6Al-4V

Laser in-situ synthesis technology at room temperature was applied to obtain TiCN/TiN composite coating. A pulsed Nd:YAG laser (wavelength 1064 nm) was used to melt the mixture of Ti and C powder. Pure nitrogen gas with a pressure of 0.4 MPa was introduced coaxially together with laser beam to the melting pool to react with Ti and C atoms and in-situ synthesize TiCN/TiN composite coating. The coating consists of TiC_0.3N_0.7, TiN and TiN_0.3, but the proportions of these three constituents vary with the laser power density. SEM results revealed that dendrites were oriented in accordance with the heat flow and a metallurgical bonding between the coating and the substrate was achieved. The in-situ synthesized TiCN/TiN composite coating, with a thickness of about 200 μm, increased the hardness and wear resistance compared to the bare Ti-6Al-4V substrate. A remarkable improvement of the average microhardness (3-4 times) and an enhancement of the wear resistance (10-11 times) are observed by laser in-situ synthesizing TiCN/TiN composite coating.     

Microstructure and deuterium resistance of Al2O3/Y2O3 composite coating with different annealing atmospheres

The Al2O3/Y2O3 composite coating with a total thickness of 320 nm was prepared by radio-frequency magnetron sputtering.The annealing treatments in three differe...     

Effect of different cooling rates on thermomechanically processed high-strength rebar steel

The present article is dealing with 0.2% C, 0.1% V and 0.02% Nb steel. Billets with 130 mm × 130 mm cross-section were austenitized and hold at 1080 °C. The billets were hot rolled to 22 mm bar diameter. Hot rolling was finished at 980–1000 °C. The final bars were air-cooled. On a parallel way, an experimental hot deformation investigation, on the same steel, was carried out at deformation temperature range 1200–800 °C with the same amount of deformation (97% reduction in area). However, cooling regimes after deformation were air cooling, water quenching to 600 °C followed by air cooling, and water quenching to room temperature. Microstructure investigation was done using both optical and scanning electron microscopes. Further evaluation was done using mechanical testing. The industrial trial has unsatisfied results with poorer yield strength with higher ultimate strength. Bainitic aggregates are detected in the hard phases islands. Air cooling after pilot hot deformation creates banded ferrite–pearlite microstructure with 9.11 μm ferrite grains. However, quick water quenching to 600 °C followed by air cooling develops tempered and softened coarse bainite phase. On the other hand, water quenching to room temperature develops fine bainite texture. Water quenching to 600 °C followed by air cooling is the best regime creating accepted mechanical properties.