镁合金表面再制造AlCoTi非晶涂层组织及力学性能研究

采用高速电弧喷涂技术在AZ91镁合金表面制备了高非晶含量AlCoTi涂层,研究了涂层显微组织、力学性能、摩擦磨损及电化学腐蚀性能。结果表明,涂层呈典型的层状结构,其结构紧凑,与镁合金基体结合良好,孔隙率约为1.63%。涂层的组织主要由非晶相、纳米结构的α-Al和Al3Ti相组成。相比于AZ91镁合金,AlCoTi非晶涂层具有更高的显微硬度和耐磨性能：涂层的显微硬度约为511.3Hv0.1,远高于AZ91镁合金(62Hv0.1);在相同的磨损条件下,非晶涂层相对耐磨性约为晶体结构AZ91镁合金的3.9倍,其主要磨损机制为脆性剥落。在0.6 mol/L NaCl溶液中,非晶涂层自腐蚀电位、自腐蚀电流密度和电荷转移电阻分别为-0.696V、0.741 8μA/cm²和33 660?·cm²,明显优于AZ91镁合金的-1.392V、769.3μA/cm²和1 914?·cm²。通过对镁合金表面不同防护涂层的电化学腐蚀性能和显微硬度比较分析,本研究为镁合金提供一种低成本、高性能的涂层材料及再制造关键技术。     

Tribological properties of composite multilayer coating

Abstract

The use of surface coatings is emerging as one of the most important approaches in reducing friction and wear in various tribological applications. Even though single layer coatings have a wide range of applications, the performance of the single layer alone may not always be adequate to meet the desired tribological property requirements. Hence, coatings consisting of multilayers to meet different property requirements in demanding applications are required. In this study, the tribological properties of a graded composite multilayer coating, with a specific layer sequence of MoS₂/Ti–MoS₂/TiBN–TiBN–TiB₂–Ti deposited on tool steel substrate, have been investigated at temperatures of 40 and 400°C respectively. The experimental results from the tests at 40°C have shown that the friction coefficient value ranges between 0·02 and 0·034. It was found that the deposition parameters influenced the friction and durability of the coatings. Higher substrate bias was found to result in higher friction, and the coating deposited at high substrate bias and low N₂ flow showed the lowest durability. The friction coefficient and durability of the coatings were found to be highly dependent on temperature. At high temperature, the friction coefficient increases almost threefold, and the durability decreases significantly.     

Relationship between particle erosion and lamellar microstructure for plasma-sprayed alumina coatings

The lamellar structure determines mechanical properties of a thermal spray coating. A model for the erosion of thermally sprayed ceramic coatings resulting from the debonding of flattened ceramic particles is proposed based on the examination of the erosion mechanism. The relationship between erosion rate and microstructural parameters is established both experimentally and theoretically to reveal main lamellar structural parameters controlling erosion of thermally sprayed ceramic coating. The microstructural parameters include the mean bonding ratio between lamellae and thickness of the lamellae. The erosion rate of plasma-sprayed Al₂O₃ coatings was measured at impact angle of 90° under the fixed erosion test conditions. The correlation of theoretical model with the observed structural parameters and erosion data of alumina coatings was examined. It is revealed that the theoretical relationship agreed well with the observed relation. The results clearly revealed that the erosion of plasma-sprayed ceramic coating was inversely proportional to the mean lamellar bonding ratio. The influences of spray parameters on erosion effected mainly through their influences on the lamellar bonding. The erosion resistance of a thermally sprayed ceramic coating was controlled by coating fracture toughness.     

Corrosive Wear and Mechanical Properties of Ni/Al2O3 Micro- and Nanoparticulates-Reinforced Coatings on Ti-6Al-4V Alloy

Nanocomposite coatings can endow a plated surface with various properties such as wear resistance, high-temperature corrosion protection, oxidation resistance, and self-lubrication. This work studies the corrosion and corrosive wear resistance of electroplated nickel nanocomposite coatings on Ti-6Al-4V alloy in a Hank's solution, adding various concentrations of an Al₂O₃ powder in plating solution, with particle diameters of 20–30 nm and 1 μm for comparisons. The experimental results showed that the content of Al₂O₃ incorporated into the electroplated nickel composite coating increased with the concentration of Al₂O₃ powder in the electroplating solution, and increasing the surface hardness, corrosion, and corrosive wear resistance of electroplated nickel micro- and nanocomposite coatings caused smearing of the nodule boundary and elimination of voids in the deposits. The Al₂O₃ nanoparticulates were embedded and distributed more uniformly than the Al₂O₃ microparticulates in the nickel matrix after a heat treatment of 400°C, producing a more continuous and dense coated composite layer on the Ti-6Al-4V substrate. This phenomenon is responsible for the Ni/Al₂O₃ composite coating with superior surface hardness, providing high corrosion resistance and corrosive wear protection to the Ti-6Al-4V alloy substrate in Hank's solution.     

Resistance of nanostructured titanium dioxide coatings to erosion corrosion

Abstract

Investigations were undertaken to determine the erosion corrosion resistance of nanostructured titanium dioxide coatings in 5 vol.-%–3·5 wt-% NaCl slurry at velocities ranging from 1 to 4 m s^–1 in a recirculating loop. Two types of nanopowders, spray dried and densified (AE 9342) and chemically precipitated and spray dried (AE 9303) were used. The results were compared with a conventional TiO₂ coated samples (SM 102). Specimen AE 9342 showed a higher resistance compared to AE 9303. No localised corrosion on the above specimen was observed. The erosion corrosion was caused by etching of intersplat boundaries. The erosion corrosion is dependent on surface topography. A homogeneous distribution of nanoagglomerates of unmelted, partially melted nanoparticles embedded in coatings, a large area of melted zone and porosity less than 1% enhances the erosion corrosion resistance of nanostructured titanium dioxide coatings.     

Surface performances of PVD ZrN coatings in biological environments

Zirconium nitride (ZrN) thin films were deposited by reactive RF magnetron sputtering on Ti-6Al-4V and Si (100) substrates for potential use in biomedical applications. The tribological behaviour was evaluated against bovine bone in dry condition using a pin-on-disc apparatus. Abrasion is the primary wear mechanism observed in ZrN/bone contact. The corrosion properties were determined through two electrochemical techniques: potentiodynamic polarization and electrochemical impedance spectroscopy. The coatings with reduced oxygen content provided: (i) good resistance against corrosion when exposed to physiological solution and (ii) better anti-bioadhesion against Staphylococcus aureus bacteria.     

Microstructure and properties of Fe–Al intermetallic coatings on the low carbon steel synthesized by mechanical alloying

Fe–Al coating was obtained on low carbon steel substrates using mechanical alloying and subsequent heat treatment. Light optical microscopy and a scanning electron microscope equipped with energy dispersive spectroscopy were used to conduct the microstructure characterization. Mechanical properties of the coatings were evaluated by microhardness measurements and wear tests. The corrosion behavior was determined by potentiodynamic polarization measurements in 3.5 % NaCl solution. The results of the mechanical and corrosion tests showed that the hardness and wear resistance of the coatings decreased with increasing the milling time, while increase in the milling time resulted in a significant increase in the thickness, porosity level, and corrosion resistance.     

Nanometre scale mechanical properties of extremely thin diamond-like carbon films

Abstract

Extremely thin diamond-like carbon (DLC) films are deposited by the filtered cathodic vacuum arc (FCVA) and plasma chemical vapour deposition (p-CVD) methods. The target thicknesses of the extremely thin protective DLC films deposited on a Si (100) surface by FCVA and p-CVD are 0·1, 0·4, 0·8, 1·0, 2·0, 5·0 and 100·0 nm. Nanoindentation hardness and nanowear resistance are evaluated by atomic force microscopy (AFM). The nanoindentation hardnesses of 100 nm thick DLC films deposited by FCVA and p-CVD are 57 and 25 GPa respectively. The nanowear test by AFM clarifies the mechanical properties of extremely thin DLC films. The wear depths of 1 and 2 nm thick FCVA-DLC films are extremely shallow. The wear depths of the 1·0 and 2·0 nm thick p-CVD-DLC films exceed the film thicknesses after five sliding cycles. These results reveal differences in the wear resistance of extremely thin DLC films and the superior mechanical properties of FCVA-DLC thin films.     

Microstructure and performance of multi-dimensional WC-CoCr coating sprayed by HVOF

WC-based coatings deposited by high velocity oxy-fuel (HVOF) spraying have been widely used in many industrial fields, where mechanical components are subjected to severe abrasive wear. Much attention has been especially paid to nanostructured and multimodal WC-based coatings due to their excellent abrasive wear resistance. In this study, a new kind of multi-dimensional WC-10Co4Cr coating, composed of nano, submicron, micron WC particles and CoCr alloy, was developed by HVOF. The microstructure, porosity, microhardness, fracture toughness, and electrochemical properties of the coating were investigated in comparison with nanostructured WC-10Co4Cr coating deposited by HVOF. Abrasive wear resistance of both WC-10Co4Cr coatings was evaluated on wet sand rubber wheel abrasion tester. The results show that the multi-dimensional coating possesses low porosity (0.31 ± 0.09%), excellent microhardness (1126 ± 115 HV_0.3), fracture toughness (4.66 ± 0.51 MPa m^1/2), and outstanding electrochemical properties. Moreover, the multi-dimensional coating demonstrates approximately 36% wet abrasive resistance enhancement than the nanostructured coating. The superior abrasive wear resistance originates from the coating’s multi-dimensional structure and excellent mechanical and electrochemical properties.     

Corrosion-wear behaviour of PVD Cr/CrN multilayer coatings for gear applications

At present, one of the most important problems in automobile engines and transmission components is due to tribological processes (friction and wear) that in many cases come accompanied by corrosion processes due to the environmental conditions to which these materials are exposed during their lifetime. Both mechanisms can be minimized by means of the development and the application of adequate coatings that combine low friction with a high corrosion and wear resistance.The new tendencies in industrial PVD coatings to improve their properties are focused in the development of new multilayer and nanostructured coatings. These structures allow in a relatively simple way enhancing their tribological properties and the corrosion resistance that can not be reached by means of the traditional monolayer coatings. The background of this type of coatings consists of the stacking up of several layers with good individual tribological and mechanical properties, but every individual layer has a thickness that can be from hundreds of nanometres down to only 5-10 nm. The properties of these nanostructured coatings depend strongly on the thickness modulation of every individual layer.Concerning PVD coatings, the chrome nitride coatings have demonstrated to possess excellent wear resistance properties. In this work, multilayer Cr/CrN coatings with different individual layer thickness have been deposited on substrates of steel F1272 and silicon. The deposition has been carried out by means of the cathodic arc method alternating an atmosphere of pure Ar with a reactive mixture of N₂/Ar. The multilayers obtained have been analyzed by means of Glow Discharge Optical Emission Spectroscopy (GD-OES) and in some cases by means of FE-SEM obtaining bilayer (Cr/CrN) periods of the order of 220 and 45 nm. The coating characterization has been complemented with hardness and composition measurements as well as by the performance of several wear and corrosion-wear tests.     

Sliding wear behaviour of ZrN and (Zr, 12 wt% Hf)N coatings

In the present study, the sliding wear resistances of ZrN and (Zr, 12 wt% Hf)N coatings deposited on a hardened AISI D2 tool steel by arc-physical vapor deposition (PVD) technique were examined by a ball-on-disc wear tester. Alloying of ZrN coating with 12 wt% Hf did not change the hardness significantly, but achieved an improvement on adhesion strength and dry sliding wear resistance against steel (AISI 52100-55HRC) and Al₂O₃ balls.     

Structure of Micro-nano WC-10Co4Cr Coating and Cavitation Erosion Resistance in NaCl Solution

Cavitation erosion(CE) is the predominant cause for the failure of overflow components in fluid machinery. Advanced coatings have provided an effective solution to cavitation erosion due to the rapid development of surface engineering techniques. However, the influence of coating structures on CE resistance has not been systematically studied. To better understand their relationship,micro-nano and conventional WC-10Co4 Cr cermet coatings are deposited by high velocity oxygen fuel spraying(HVOF), and their microstructures are analyzed by OM,SEM and XRD. Meanwhile, characterizations of mechanical and electrochemical properties of the coatings are carried out, as well as the coatings' resistance to CE in 3.5 wt % Na Cl solution, and the cavitation mechanisms are explored. Results show that micro-nano WC-10Co4Cr coating possesses dense microstructure, excellent mechanical and electrochemical properties, with very low porosity of 0.26 ± 0.07% and extraordinary fracture toughness of 5.58 ± 0.51 MPaám~(1/2). Moreover, the CE resistance of micro-nano coating is enhanced above 50% than conventional coating at the steady CE period in 3.5 wt % Na Cl solution. The superior CE resistance of micronano WC-10Co4Cr coating may originate from the unique micro-nano structure and properties, which can effectively obstruct the formation and propagation of CE crack. Thus,a new method is proposed to enhance the CE resistance of WC-10Co4Cr coating by manipulating the microstructure.     

Tribological and corrosion behaviors of Al2O3/polymer nanocomposite coatings

In this paper, the tribological and electrochemical corrosion properties of Al₂O₃/polymer nanocomposite coatings were studied by using micro-hardness test, single-pass scratch test, abrasive wear test, and finally electrochemical technique such as potentiodynamic polarization measurement. The coatings containing Al₂O₃ nanoparticles showed improvement in scratch and abrasive resistance compared with that of polymer coating. The improvement in scratch and abrasive resistance is attributed to the dispersion hardening of Al₂O₃ nanoparticles in polymer coatings. Corrosion test results showed that the embedded Al₂O₃ nanoparticles in polymer matrix do not sacrifice the corrosion resistance of the polymer itself.     

Influence of aging treatment on microstructure, wear and corrosion behavior of a nickel base hardfaced coating

In this work nickel based hardfacing alloy (Colmonoy 5) was deposited on 316 L (N) stainless steel substrate to study the effects of aging treatment on coating microstructure, wear and corrosion properties. Coatings, deposited through plasma transferred arc (PTA) welding process, were aged at 923 K for 5000 h. Microstructural characterization studies carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed the coarsening of dendrites and precipitation of Cr₂₃C₆ particles in the aged coating. The wear behavior of the as deposited and aged coatings was compared in room temperature (RT) and high temperature (823 K) under dry sliding wear condition (pin-on-disc configuration). At RT, aged coating experienced more wear loss when compared to as-deposited. At high temperature, the wear loss was almost same with similar operating wear mechanisms (tribo-oxidation) for both as-deposited and aged coating. From pitting corrosion studies, it was found that aged coatings are more prone to pitting when compared to the as-deposited coatings.     

Behaviour of ferritic–martensitic steel and aluminium base coatings under demanding elevated temperature conditions

Abstract

Ferritic–martensitic steels are found in many elevated temperature applications due to their excellent strength properties and thermal conductivity. However, their resistance to elevated temperature corrosion, wear and combination of these is typically not at a desired level. One solution is to improve the surface properties by the application of a coating. In this study, aluminium diffusion coatings were deposited on 9Cr–1Mo steel and characterised in terms of microstructure and elevated temperature corrosion and erosion–corrosion behaviours. The two behaviours are then compared to those of an uncoated steel. The results from the tests indicate that aluminising shows great potential under the studied demanding elevated temperature conditions. The benefits and challenges of the deposition and use of aluminised coatings are discussed in this paper.     

Structural and Tribological Performance of Solid NiCr-WSe2-BaF2·CaF2-Y-hBN and NiCr-WSe2-BaF2·CaF2-Y Lubricant Coatings Produced by Atmospheric Plasma Spray

NiCr matrix WSe₂-BaF₂·CaF₂-Y-hBN and WSe₂-BaF₂·CaF₂-Y powders were prepared by mechanical granulation and crushing, and composite coatings were fabricated by atmospheric plasma spray technology. The microstructures and phase compositions of the powders, as well as the coatings, were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The friction coefficient and the wear behavior of the coatings from ambient temperature to 800°C were evaluated using a ball-on-disk tribometer. From the investigation of the worn surfaces, it was concluded that brittle fracture and delamination were the dominant wear mechanisms of the coatings at low temperature. At higher temperatures, a dense and protective oxide layer (BaCrO₄ and NiO) is generated on the worn surfaces of the coatings. Layered hexagonal BN particles reduce the direct contact and severe adhesion between friction pairs. Thus, the friction coefficient of the NiCr-WSe₂-BaF₂·CaF₂-Y-hBN coating is stable at the evaluated temperatures relative to the non-hBN coating. These fluorides exhibit excellent properties in these coatings over a large temperature range.     

Evolution of residual stresses in micro-arc oxidation ceramic coatings on 6061 Al alloy

Most researches on micro-arc oxidation mainly focus on the application rather than discovering the evolution of residual stresses. However, residual stresses in the surface coatings of structural components have adverse effects on their properties, such as fatigue life, dimensional stability and corrosion resistance, etc. The micro-arc oxidation ceramic coatings are produced on the surfaces of 6061 aluminum alloy by a homemade asymmetric AC type of micro-arc oxidation equipment of 20 kW. A constant current density of 4.4___0.1 A/dm2 and a self-regulated composite electrolyte are used. The micro-arc oxidation treatment period ranges from 10 min to 40 min, and the thickness of the ceramic coatings is more than 20 Bin. Residual stresses attributed to 7-A1203 constituent in the coatings at different micro-arc oxidation periods are analyzed by an X-ray diffractometer using the sin2~u method. The analysis results show that the residual stress in the ceramic coatings is compressive in nature, and it increases first and then decreases with micro-arc oxidation time increase. The maximum stress value is 1 667_＋20 MPa for period of 20 min. Through analyzing the coating thickness, surface morphology and phase composition, it is found that the residual stress in the ceramic coatings is linked closely with the coating growth, the phase composition and the micro cracks formed. It is also found that both the heat treatment and the ultrasonic action release remarkably the residual compressive stress. The heat treatment makes the residual compressive stress value decrease 1 378 MPa. The ultrasonic action even alters the nature of the residual stress, making the residual compressive stress change into a residual tensile stress.     

Microstructure,tribological and mechanical strengthening effect of multiphase Zn/ZrO<Subscript>2</Subscript>-SiC electrodeposited composite coatings

In an attempt to improve the mechanical and thermal resilient properties of mild steel, Zn-ZrO₂-SiC composite coating was fabricated from zinc-based sulphate electrolyte with incorporated composite particles of ZrO₂/SiC at 2.0 A/cm² for 10 min. The effects of particle on the mechanical properties were examined using scanning electron microscope attached with energy dispersion spectroscopy and atomic force microscopy. The micro-hardness and wear resistance behaviour were determined with high diamond micro-hardness tester and three body abrasive MTR-300 testers with dry sand rubber wheel apparatus with 5 N. The fabricated coating was thermally heated at 200 °C for 4 h to evaluate the coating stability. From the results, a modification in the microstructure and topographic orientation as a result of incorporated composite was noticed on the zinc matrix. The mechanical and thermal properties were observed to be considerably improved by the incorporation of the ZrO₂/SiC weight fraction. A significant improvement in wear and hardness properties were also obtained for the multiphase embedded coatings.     

Investigating Corrosion Performance and Corrosive Wear Behavior of Sol–gel/MAO-Coated Mg Alloy

In this study, a hydroxyapatite composite coating was prepared by a sol–gel technique on the micro-arc oxidation (MAO)-coated AZ31 Mg alloy to seal the micro-pores. The composite coating achieved a larger hardness value and two times thickness more than pure MAO coating. The corrosion and wear resistance of the sol–gel/MAO coating in simulated body fluid were investigated compared to MAO coating. It was found that the composite coating presented a positive corrosion potential and a lower corrosion current density than MAO coating. The sol–gel/MAO composite coating could provide more effective barrier against corrosive ions than single MAO coating for AZ31 alloy. In the wear tests, a ball-on-disk tribometer was used to study the effect of loads on the wear properties of the coatings at 37 °C. The wear resistance of sol–gel/MAO composite coatings was apparently superior to MAO coating. The wear mechanisms of abrasion and adhesion in composite coatings are investigated. Finally, two physical models for the corrosion and sliding wear mechanisms of sol–gel/MAO composite coatings are proposed, respectively.     

Influence of BN and B4C particulates on wear and corrosion resistance of electroplated nickel matrix composite coatings

Abstract

Electrodeposition of nickel–boron nitride–boron carbide (Ni–BN–B₄C) composites was carried out from a Watt's solution containing 50 and 10?g L of dispersed boron nitride and boron carbide particles respectively. The corrosion behaviour for both Ni and Ni–BN–B₄C deposits was evaluated by polarisation curves in a 3·5% NaCl solution at room temperature. Wear resistance of the composites was also studied using pin on disc technique. Scanning electron microscopy was extensively used to understand the effect of different electroplating conditions on the concentration and distribution of particles. The results showed that BN and B₄C particle concentration in the Ni–BN–B₄C composite film was sensitively dependent on the electroplating current density and bath temperature. Meanwhile, the Ni–BN–B₄C composite films were much more resistant to corrosion and wear than pure Ni coatings.