Enhanced high-temperature corrosion resistance of (Al2O3–Y2O3)/Pt micro-laminated coatings on 316L stainless steel alloy

A 7-layer (Al₂O₃–Y₂O₃)/Pt micro-laminated coating was successfully prepared on 316L stainless steel alloy by magnetron sputtering. High-temperature cyclic oxidation and hot corrosion tests were adopted to investigate the high-temperature corrosion resistance of the coating. It is revealed that the (Al₂O₃–Y₂O₃)/Pt micro-laminated coating which effectively suppressed the inward diffusion of oxygen and corrosive fused salt to an extremely low level can significantly improve the high-temperature corrosion resistance of alloy substrate. The great mechanical properties of such coating were attributed to the brittle/ductile laminated composite structure by means of multilayer toughening and release mechanisms.     

Processing and mechanical properties of porous 316L stainless steel for biomedical applications

Highly porous 316L stainless steel parts were produced by using a powder metallurgy process, which includes the selective laser sintering（SLS） and traditional sintering. Porous 316L stainless steel suitable for medical applications was successfully fabricated in the porosity range of 40%-50% （volume fraction） by controlling the SLS parameters and sintering behaviour. The porosity of the sintered compacts was investigated as a function of the SLS parameters and the furnace cycle. Compressive stress and elastic modulus of the 316L stainless steel material were determined. The compressive strength was found to be ranging from 21 to 32 MPa and corresponding elastic modulus ranging from 26 to 43 GPa. The present parts are promising for biomedical applications since the optimal porosity of implant materials for ingrowths of new-bone tissues is in the range of 20%-59% （volume fraction） and mechanical properties are matching with human bone.     

Microstrucmre and mechanical properties of Al2O3/TiAl composite

Al2O3/TiAl composites were fabricated by PAXD （pressure-assisted exothermic dispersion） method. The effects of Nb205 content on the microstructure and mechanical properties of the composites were investigated. The results show that the ultimate phases of the composite consist of TiAl, Ti3Al, Al2O3 and a small amount of NbA13. SEM reveals that a submicron γ＋（α2/γ） dual phases structure can be presented after sintered at 1 200 ℃, Furthermore, with the increase of Nb205 content, the ratio of TiAl to Ti3Al phase decreases correspondingly, the grains of the corflposites are remarkably refined, and the produced Al2O3 particles are uniformly dispersed. When 6% Nb205 is added, the composite has the best comprehensive properties. It exhibits a Vickers hardness of 4.77 GPa and a bending strength of 642 MPa. Grain-refinement and dispersion-strengthening are the main strengthening mechanisms.     

Electrodeposition of multi-layer Pd–Ni coatings on 316L stainless steel and their corrosion resistance in hot sulfuric acid solution

Pd–Ni coating shows good corrosion resistance in strong corrosion environments. However, in complex aggressive environments, the performance of the coatings is limited and further improvement is necessary. The effects of the applied plating current density on the composition, structure and properties of Pd–Ni coatings were studied. By changing the current density in the same bath, multi-layer Pd–Ni coatings were prepared on 316L stainless steel. Scanning electronic microscopy, weight loss tests, adhesion strength, porosity and electrochemical methods were used to study the corrosion resistance of the films prepared by different coating methods. Compared with the single layer Pd–Ni coating, the multi-layer coatings showed higher microhardness, lower internal stress, lower porosity and higher adhesive strength. The multi-layer Pd–Ni coating showed obviously better corrosion resistance in hot sulfuric acid solution containing Cl^?.     

Corrosion behaviour of Ti DLC deposition on prenitrided 316L stainless steel and Ti–6Al–4V alloy

Abstract

316L and Ti–6Al–4V are widely used as biomaterials and materials of various mechanical components. In biomedical applications, they are used to manufacture coronary and pulmonary stents, hip prosthesis, screws and external fixations. However, Cr, Al and V are released from the alloys to the body environment and these ions mix into the blood stream. Release of even small amounts of these ions may cause local irritation of the tissues surrounding the implant. This situation may be prevented by applying suitable surface treatments to the biomaterials. The overall objective of the present paper is to examine the corrosion properties of duplex treated (nitrided and with a diamond-like carbon coating) 316L stainless steel and Ti–6Al–4V alloy. Diamond-like carbon films were deposited on nitrided samples using closed field unbalanced magnetron sputtering system. The corrosion behaviour of duplex treated samples was tested using the potentiodynamic method in ringer’s solution at 37°C. The corrosion resistance of duplex treated samples was significantly improved in comparison with the uncoated and single treated samples. In addition, the corroded surfaces were investigated by SEM where small pits were observed on all samples.     

Tribological behavior and mechanism of NiCrBSi–Y2O3 composite coatings

The NiCrBSi–Y₂O₃ composite coatings were prepared on the surface of 45 carbon steel by plasma spray, the microstructure and tribological properties of the coatings were investigated. The results show that the NiCrBSi–Y₂O₃ composite coatings are mainly composed of γ-Ni, CrB, Cr₇C₃ and Y₂O₃. With addition of Y₂O₃, hard phases such as CrB, Cr₇C₃ emerge in composite coating, and the density of the composite coatings also increases. The NiCrBSi–0.5Y₂O₃ composite coating presents excellent tribological properties. Its friction coefficient is 0.175, which is about 37% of that of the pure NiCrBSi coating. The mass wear loss is 1.2 mg, which is reduced by 43% compared with the pure NiCrBSi coating. When the loads are 6–10 N, the NiCrBSi–0.5Y₂O₃ composite coating suffers from slight wear and the wear mechanisms are mainly adhesive wear accompany with slight micro-cutting wear and micro-fracture wear. As the load increases to 12 N, the wear mechanisms are adhesive wear and severe micro-cutting wear.     

Tribological properties of MoSi2–MoS2 coatings coupling with SAE 52100 steel under reciprocating sliding

Tribological properties of MoSi₂–MoS₂ coatings coupling with SAE52100 steel were tested under reciprocating sliding. Effects of normal load, sliding speed and MoS₂ content on the coatings tribological properties were studied. Worn surfaces of the coatings were analyzed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The friction coefficient of the coatings was approximately 0.45 and a little lower than that of the monolithic MoSi_2. The friction did not vary with the sliding time, sliding speed and load. Coating with 12 wt.% MoS₂ had the lowest friction. Wear rate of the coatings increased with the sliding speed and normal load and was higher than that of the monolithic MoSi₂. Wear rate of the coatings did not vary with MoS₂ content. Worn surface of the coatings and the coupling steel ball was covered by a SiO₂ and MoO₃ wear debris layer. Wear mechanism of the coatings was microfracture.     

Effect of hard phase content on the mechanical properties of TiC-316 L stainless steel cermets

The dense TiC-316 L stainless steel cermets with different TiC content were fabricated via conventional powder metallurgy (PM) process. The effects of TiC content on the microstructure and mechanical properties of TiC-316 L stainless steel cermets are studied. Scanning electron microscopy (SEM) is employed to observe the microstructure, fracture morphology, and crack propagation of cermets. The binder mean free path (MFP) and hard phase contiguity are analyzed based on the stereological principles. The relative density, hardness and transverse rupture strength (TRS) of the cermets are measured by the Archimedes' principle, Rockwell scale hardness tester and three-point flexural test, respectively. The average TiC grain size decreases first and then increases with the increase of hard phase content. At a content of 80 wt%, the maximum relative density (99.1%), hardness (88.9 HRA) and TRS (1373 MPa) are obtained. During crack propagation, both intergranular and transgranular fractures exist, of which the intergranular fracture prevail and coarse grains are more prone to transgranular fracture.     

Abrasive wear characteristics and mechanisms of Al2O3／PA1010 composite coatings

The abrasive wear characteristics of Al2O3/PA1010 composite coatings on the surface of quenched and low-temperature temper steel 45 were tested on the tumplate abrasive wear testing machine and the same uncoated steel 45 was used as a reference material. Experimental results showed that the abrasive wear resistance of Al2O3/PA 1010 composite coatings has a good linear relationship with the volume fraction of Al2O3 particles in Al2O3/PA1010 composite coatings, and the linear correlative coefficient is 0.979. Under the experimental conditions, the size of Al2O3 particles (40.5-161.0μm) has little influence on the abrasive wear resistance of Al2O3/PA1010 composite coatings. By treating the surface of Al2O3 parti-cles with a suitable bonding agent, the distribution of Al2O3 particles in matrix PA1010 is more homogeneous and the bonding state between Al2O3 particles and matrix PA1010 is better. Therefore, the Al2O3 particles in Al2O3/PA1010 compos-ite coatings make the Al2O3/PA1010 composite coatings have better abrasive wear resistance than PA1010 coatings. The wear resistance of Al2O3/PA1010 composite coatings is about 45% compared with that of steel 45.     

In-situ sintering reaction of Al2O3–LaAl11O18–ZrO2 composite

Al₂O₃–LaAl₁₁O₁₈–ZrO₂ composites were prepared by in situ sintering reaction of different proportions of Al₂O₃ and La₂Zr₂O₇. The studied batches were uniaxially pressed and pressureless sintered at 1600 °C up to 1725 °C for 1 h. Phase composition study reveals that the only present phases are alumina, lanthanum hexaluminates and zirconia. No other intermediate phases are present. Rodlike LaAl₁₁O₁₈ was observed in the sintered bodies containing more than 25 wt.% LaAl₁₁O₁₈. The effect of rodlike particles on the densification and mechanical behavior was discussed. It was found that increasing the LaAl₁₁O₁₈ content more than 25 wt.% enhances the fracture toughness, but reduces both the bending strength and the hardness of the sintered composites.     

Corrosion protection studies of CeO2–TiO2 nanocomposite coatings on mild steel

Nanocomposite coatings have evolved as corrosion-resistant materials to protect metals and alloys in various environments. The need for development of corrosion-resistant materials for mild steel in marine environment is still in demand. The CeO₂–TiO₂ nanocomposite powders were produced via hydrothermal synthesis and the corrosion resistance behaviour of the nanocomposite coatings were evaluated in 3.5% NaCl solution using Tafel polarisation and electrochemical impedance spectroscopy techniques. The trends of open-circuit potential curves provided clear evidence that the incorporation of CeO₂ in TiO₂ nanostructures is beneficial, as it introduces potential shift towards noble positive potential for nanocomposite coatings. Also, the corrosion resistance was enhanced with increase in the CeO₂ content in TiO₂ nanocomposite coatings. Almost 22 times decrease in the corrosion current densities of mild steel were attained for 15?wt-% CeO₂–TiO₂, which demonstrated the advantage of CeO₂–TiO₂ nanocomposite coatings for corrosion protection of mild steel.     

Electrochemical corrosion behaviour of Mg–Al alloys with thermal spray Al/SiCp composite coatings

The corrosion protection of Mg–Al alloys by flame thermal spraying of Al/SiC particles (SiCp) composite coatings was evaluated by electrochemical impedance spectroscopy in 3.5 wt.% NaCl solution. The volume fraction of SiCp varied between 5 and 30%. The as-sprayed Al/SiCp composite coatings revealed a high number of microchannels, largely in the vicinity of the SiCp, that facilitated the penetration of the electrolyte and the subsequent galvanic corrosion of the magnesium substrates. The application of a cold-pressing post-treatment reduced the degree of porosity of the coatings and improved the bonding at the coating/substrate and Al/SiC interfaces. This resulted in improved corrosion resistance of the coated specimens. The effectiveness of the coatings slightly decreased with the addition of 5–30 vol.% SiCp compared with the unreinforced thermal spray aluminium coatings.     

Effect of Al2O3 addition on properties of non-sintered SiC–Si3N4 composite refractory materials

Preparation of SiC–Si₃N₄ composite refractory materials without sintering entails only low energy consumption and incurs little cost compared with traditional preparation methods. This paper investigated the effect of Al₂O₃ addition on bulk density, apparent porosity, linear shrinkage and oxidation resistance of as-fabricated non-sintered SiC–Si₃N₄ composite refractory materials. Meanwhile, the compressive and flexural strengths both before and after heat treatment were analyzed. The mechanisms of oxidation resistance and cryolite resistance of the SiC–Si₃N₄ composite refractory materials are discussed. Increasing amounts of Al₂O₃ reduced linear shrinkage but increased oxidation resistance and cryolite resistance. Moreover, compressive and flexural strengths initially increased and then decreased, with maximum values achieved at an Al₂O₃ addition of 8% w/w.     

Friction and wear behaviors of Al2O3–13 wt%TiO2 coatings

Nanostructured and conventional Al2O3-13 wt%TiO2 coatings were manufactured by air plasma spray. Friction and wear behaviors of coatings were investigated at room and elevated temperatures using an SRV wear test machine. The nanostructured coating has "two regions" microstructure, while the conventional coating has typical layered microstructure with obvious interfaces among splats. The coefficient of friction decreases with rising of temperature because of the formation of tribo-layer at elevated temperatures. The wear resistance of the nanostructured coatings is higher than that of the conventional coating, and the wear threshold of applied load is 30 N for conventional coating and 40 N for nanostructure coating. The wear resistance difference is related to the "two regions" microstructure of nanostructure coating, which could blunt or branch the cracks propagation. In our test ranges, the wear rates rising are more sensitive with the applied wear load rising than with the temperature rising.     

Microstructure and properties of Al2O3-13%TiO2 coatings sprayed using nanostructured powders

The microstructure and wear performance of M203-13% TiO2 coatings prepared by plasma spraying of agglom- erated nanoparticle powders were investigated. SEM analysis showed that the as-sprayed Al2O3-TiO2 coatings comprise of two kinds of typical region： fully melted region and unmelted/partially melted nanostructured region, which is different than the conventional coating with lamellar structure. It is shown that the microhardness of the nanostructured coatings was about 15%-30% higher than that of the conventional coating and the wear resistance is significantly improved, especially under a high wear load. The nanostructured coating sprayed at a lower power shows a lower wear resistance than the coatings produced at a higher power, because of the presence of pores and microstructural defects which are detrimental to the fracture toughness of the coatings.     

RAFM与316L钢等离子喷涂Al2O3电绝缘涂层研究

在液态金属冷却、磁约束聚变反应堆中,磁流体动力学(MHD)压降是一个不容忽视的问题,包层结构材料内壁涂敷电绝缘涂层是降低MHD压降的最有效方法.本文利用大气等离子喷涂(APS)工艺分别在316L钢和一种新的低活化铁素体/马氏体(RAFM)钢上制备Al<,2>O<,3>电绝缘涂层,研究结果表明:氧化铝涂层表面质量好;涂层的显微硬度较高、致密度较好;涂层与RAFM钢和316L钢基体结合较好,平均结合强度值分别为33.7和27.8 MPa;RAFM钢上涂层抗热震性优良,其原因在于RAFM钢的热膨胀系数与氧化铝陶瓷很好地匹配;两种钢基体上涂层的常温电阻率均为8.0×10<'12>Ω·cm左右.     

Interfacial features of TiAl alloy/316L stainless steel joint brazed with Zr−Cu−Ni−Al amorphous filler metal

TiAl alloy and 316L stainless steel were vacuum-brazed with Zr?50.0Cu?7.1Ni?7.1Al (at.%) amorphous filler metal. The influence of brazing time and temperature on the interfacial microstructure and shear strength of the resultant joints was investigated. The brazed seam consisted of three layers, including two diffusion layers and one residual filler metal layer. The typical microstructure of brazed TiAl alloy/316L stainless steel joint was TiAl alloy substrate/α₂-(Ti₃Al)/AlCuTi/residual filler metal/Cu₉Zr₁₁+Fe₂₃Zr₆/Laves-Fe₂Zr/α-(Fe,Cr)/316L stainless steel substrate. Discontinuous brittle Fe₂Zr layer formed near the interface between the residual filler metal layer and α-(Fe,Cr) layer. The maximum shear strength of brazed joints reached 129 MPa when brazed at 1020 °C for 10 min. The diffusion activation energies of α₂-(Ti₃Al) and α-(Fe,Cr) phases were ?195.769 and ?112.420 kJ/mol, respectively, the diffusion constants for these two phases were 3.639×10^?6 and 7.502×10^?10 μm²/s, respectively. Cracks initiated at Fe₂Zr layer and propagated into the residual filler metal layer during the shear test. The Laves-Fe₂Zr phase existing on the fracture surface suggested the brittle fracture mode of the brazed joints.     

Effects of MgO/Al2O3 ratio on viscous behaviors and structures of MgO–Al2O3–TiO2–CaO–SiO2 slag systems with high TiO2 content and low CaO/SiO2 ratio

The effects of MgO/Al₂O₃ ratio on the viscous behaviors of MgO–Al₂O₃–TiO₂–CaO–SiO₂ systems were investigated by the rotating cylinder method. Raman spectroscopy was used to analyze the structural characteristics of slag and Factsage 7.0 was adopted to demonstrate the liquidus temperature of slag. The results show that the viscosity and activation energy for viscous flow decrease when the MgO/Al₂O₃ ratio increases from 0.82 to 1.36. The break point temperature and liquidus temperature of slag initially decrease and subsequently increase. The complex viscous structures are gradually depolymerized to simple structural units. In conclusion, with the increase of MgO/Al₂O₃ ratio, the degree of polymerization of slag decreases, which improves the fluidity of slag. The variations of liquidus temperature of slag lead to the same changes of break point temperature.