Microstructure and corrosion resistance of the layers formed on the surface of precipitation hardenable plastic mold steel by plasma-nitriding

Plasma-nitriding is used to improve the wear resistance and corrosion resistance of plastic mold steels by modifying the surface layers of these steels. In this study, a precipitation hardenable plastic mold steel (NAK80) was plasma-nitrided at 470, 500, and 530 °C for 4, 8, and 12 h under 25% N₂ + 75% H₂ atmosphere in an industrial nitriding facility. The microstructures of the base material and nitrided layers as well as the core hardness were examined, and various phases present were determined by X-ray diffraction. The corrosion behaviors were evaluated using anodic polarization tests and salt fog spray tests in 3.5% NaCl solution.The results had shown that plasma-nitriding does not cause the core to soften by overaging. Nitriding and aging could be achieved simultaneously in the same treatment cycle. Plasma-nitriding of NAK80 mold steel produced a nitrided layer composed of an outer compound layer constituting a mixture of ?-nitride and γ′-nitride and an adjacent nitrogen diffusion layer on the steel surface. The amount of ?-nitride and total nitrides increased with an increase in nitriding temperature and nitriding time. Corrosion study revealed that plasma-nitriding significantly improved the corrosion resistance in terms of corrosion potential, corrosion and pitting current density, and corrosion rate. This improvement was found to be directly related to the increase in the amount of ?-nitride at the surface, indicating the amount of ?-nitride controlling the corrosion resistance.     

Mechanical properties and wear and corrosion resistance of electrodeposited Ni-Co/SiC nanocomposite coating

Ni-Co/SiC nanocomposite coatings with various contents of SiC nano-particulates were prepared by electrodeposition in a Ni-Co plating bath containing SiC nano-particulates to be co-deposited. The influences of the nanoparticulates concentration, current density, stirring rate and temperature of the plating bath on the composition of the coatings were investigated. The shape and size of the SiC nano-particulates were observed and determined using a transmission electron microscope. The polarization behavior of the composite plating bath was examined on a PAR-273A potentiostat/galvanostat device. The wear behavior of the Ni-Co/SiC nanocomposite coatings was evaluated on a ball-on-disk UMT-2MT test rig. The worn surface morphologies of the Ni-Co/SiC nanocomposite coatings were observed using a scanning electron microscope. The corrosion behavior of the nanocomposite coatings was evaluated by charting the Tafel curves of the solution of 0.5 mol L⁻¹ NaCl at room temperature. It was found that the cathodic polarization potential of the composite electrolyte increased with increasing SiC concentration in the plating bath. The microhardness and wear and corrosion resistance of the nanocomposite coatings also increased with increasing content of the nano-SiC in the plating bath, and the morphologies of the nanocomposite coatings varied with varying SiC concentration in the plating bath as well. Moreover, the co-deposited SiC nano-particulates were uniformly distributed in the Ni-Co matrix and contributed to greatly increase the microhardness and wear resistance of the Ni-Co alloy coating.     

Fretting Wear Behavior of UHMWPE Under Different Temperature Conditions

The fretting wear behavior of ultra-high molecular weight polyethylene (UHMWPE) against a GCr15 steel ball was investigated using an Optimal SRV-IV oscillating reciprocating friction and wear tester (Optimal Corp., Germany). The influence of temperature, stroke size and frequency was studied in detail under a normal load of 10 N. The results showed that the friction coefficients initially decreased and then increased afterward with the increasing of temperature, having the lowest value at 0°C. The wear loss of UHMWPE at a stroke of 100 µm showed a similar tendency as the friction coefficients, but monotonously increased with increasing of temperature for a stroke of 200 µm. According to an analysis of the worn surface, it was concluded that abrasive wear accompanied with local plastic deformation dominated the wear mechanism in the process of the fretting test at ?30°C. The plastic deformation was slightly less at 0°C than that at ?30°C. With the increasing of temperature, the hardness of UHMWPE decreased significantly, plastic deformation and adhesive wear became more severe. In addition, a lubricating transfer film was formed on the steel ball.     

基于光纤光栅光谱分析的混凝土结构钢筋锈蚀监测

混凝土中钢筋发生锈蚀会严重影响混凝土结构的使用功能,文章提出了通过观察长周期光纤光栅(LPFG)的透射谱变化对混凝土结构中的钢筋锈蚀进行监测。这种方法基于长周期光纤光栅的微弯特性,利用其透射光谱随弯曲度增加逐渐变浅、谐振波长处的光功率与LPFG的弯曲度成线性这一特性,将钢筋锈蚀引起的钢筋径向膨胀转化为光栅的弯曲,通过监测光栅的光谱变化获知光栅的弯曲度进而判断钢筋的锈蚀程度,该方法对钢筋直接进行监测,简单易行,并利用光栅的光谱特性从而避免了环境温度、应变、折射率的影响,实验将光栅埋入到混凝土构件中,从已取得的实验结果来看,这种方法测量钢筋锈蚀厚度的准确度可达1.2 μm,可监测到的锈蚀厚度达3 mm,完全能够实现混凝土结构中钢筋锈蚀的早期至中期监测。     

Enhanced corrosion resistance in Nd15Fe77B8 by laser surface amorphization

A thin amorphous layer on a NdFeB ingot produced by surface melting using a KrF excimer laser, and subsequent self-quenching is shown to improve the corrosion resistance properties of the ingot. X-ray diffraction and conversion-electron Mössbauer confirm the amorphous nature of the laser-induced surface layer. Potentiodynamic polarization measurements before and after laser treatment show a shift of Ecorr to a more positive value (ΔEcorr～150 mV) and a reduction in the dissolution current in the passive region. These observations are characteristic of a reduced total corrosion rate.     

Characterization at a local scale of a laser-shock peened aluminum alloy surface

The influence of a laser shock peening mechanical surface treatment on 2050-T8 aluminum alloy has been investigated, mostly using Scanning Kelvin Probe Force Microscopy. Volta potential difference maps around Al(CuFeMn) precipitates were performed before and after laser-shock peening to determine the influence of laser treatment versus galvanic coupling near precipitates, and resulting pit initiations. It has been shown that laser shock peening either preserves or reduces precipitate-matrix Volta potentials gradients, which in this later case, and correlated to recent corrosion electrochemical investigations, could explain corrosion improvement obtained after laser-shock peening treatments of aluminum alloys. The influence of crystal orientation and plastic deformation, and more specifically the effect of laser-induced compressive residual stresses or work-hardening, on the Volta potential values and on the pitting corrosion behavior was also addressed.     

Thermorheological Properties and Thermal Stability of Polyethylene/Wood Composites

The thermal stability, flame retardancy, thermorheological, and mechanical properties of polyethylene/wood flour (PE/WF) composites were characterized. By time–temperature superposition treatment, addition of WF was found to lead to a complexity in the thermorheological behaviors in low-density PE/wood composites. However, high-density PE/wood counterparts showed no obvious thermorheological complexity. The effects of WF and ammonium polyphosphate contents on the thermorheological behavior and thermal stability were also studied. The current work should be of practical significance for the optimization of wood/plastic composite) formulae, as well as for further investigations on correlations between processing and performance of polymer composites.     

Zr-Cu-Ni-Al-Nb大块非晶合金的晶化行为、力学性能及电化学腐蚀行为的研究

利用水冷铜模铸造法成功制备了Zr_65-xCu_17.5Al_7.5Ni ₁₀Nb_x (x=0，2，5)大块非晶合金. X射线衍射、热分析研究结果表 明，Nb的添加显著改变了非晶合金的晶化行为，促进了二十面体准晶相的形成. 各合金的准 静态压缩实验表明，Nb的适量添加有利于提高大块非晶合金的强度和塑性. 其中x=5的大块 非晶合金的抗压强度σ_b和塑性应变量ε_p 关键词： Zr基大块非晶合金 晶化行为 力学性能 耐腐蚀性能     

Effect of Wood Flour on the Curing Behavior,Mechanical Properties,and Water Absorption of Natural Rubber/Wood Flour Composites

The preparation of natural rubber/wood flour (NR/WF) composites and the influence of WF content, modification, and particle size on the vulcanizing behavior, mechanical properties, and water absorption of NR/WF composites are described. Results show that the addition of WF into NR delayed the scorching time and vulcanizing time of NR. The appropriate WF contents can improve the mechanical properties of NR. However, the overloading of WF destroys the mechanical properties of NR. The addition of WF increased the water absorption of NR. The silicone couple agents that were used to modify the WF had little effect on the water absorption of NR/WF composites. Decreasing the WF particle size enhanced the water absorption of NR/WF composites because the water-absorbing surface area increased with decreasing WF particle size. The water absorption of sisal-fiber-filled NR-based composites was larger than that of the WF-filled NR-based composites. A useful equation, w=ktⁿ , was inferred from the water absorption results to calculate the water absorption (w) of the NR/WF composites as a function of time (t), where k was a constant concerning the compounds’ character that was primarily determined by the WF's character and n was the power of time that was related to the NR's inherent character, such as cross-linking density, and primarily determined the water absorption rate.     

Effects of rare earth and tantalum on graphite-like carbon coatings

The graphite-like carbon (GLC) coating with low friction and good wear resistance is appreciable to make further improvement. Ce, Y and Ta are doped into GLC by using mosaic in the target of magnetron sputtering process. The result shows that yttrium composed in chromium interlayer can increase the bonding strength and refine the microstructure of the GLC coating. For the surface layer of GLC, incorporation of Ta improves wear resistance and deposition rate, in the mean time the friction coefficient is decreased. As the modifications of interlayer and top layer are composed together to form complex GLC coating, the specific wear rate is reduced and the hardness, bonding strength and anti corrosion behavior are improved.     

温度和应变速率耦合作用下纳米晶Ni压缩行为研究

本文利用低温力学测试系统研究了电化学沉积纳米晶Ni在不同温度和宽应变速率条件下的压缩行为. 借助应变速率敏感指数、激活体积、扫描电子显微镜及高分辨透射电子显微镜方法, 对纳米晶Ni的压缩塑性变形机理进行了表征. 研究表明, 在较低温度条件下, 纳米晶Ni的塑性变形主要是由晶界位错协调变形主导, 晶界本征位错引出后无阻碍的在晶粒内无位错区运动, 直至在相对晶界发生类似切割林位错行为. 并且, 在协调塑性变形时引出位错的残留位错能够增加应变相容性和减小应力集中; 在室温条件下, 纳米晶Ni的塑性变形机理主要是晶界-位错协调变形与晶粒滑移/旋转共同主导. 利用晶界位错协调变形机理和残留位错运动与温度及缺陷的相关性揭示了纳米晶Ni在不同温度、不同应变速率条件下力学压缩性能差异的内在原因.     

A TiB2 metal matrix composite coating enriched with nitrogen: Microstructure and wear properties

Metal matrix composites containing titanium nitrides or titanium borides raise great interest to researchers due to their high wear resistance and enhanced corrosion properties. In the present investigation composite coatings containing both titanium nitrides/carbonitrides and titanium diborides were produced on plain steel substrates using the plasma transferred arc (PTA) technique with argon-nitrogen mixtures in the plasma and shielding gas. The microstructure of the metal matrix composites (MMC) obtained was thoroughly studied and found to consist of primary titanium diboride particles surrounded by a eutectic matrix containing, apart from ferrite, both titanium diboride and titanium carbonitride particles. The wear behavior of the composite coatings was assessed by pin on disk experiments. The wear rate against both a tool steel counterbody and an alumina counterbody is of the order of 10⁻⁴ mm³/m. The friction coefficient for both the alloyed layer-tool steel system and the alloyed layer-alumina system increases up to sliding speed of 0.30 m/s and then decreases, when the sliding speed increases further. Specifically, the friction coefficients are varied between the values 0.5 and 0.65. The wear mechanism for the tribosystem alloyed layer-tool steel is characterized by plastic deformation and adherence of material coming from the alloyed layer to the surface of the ball, while for the tribosystem alloyed layer-alumina ball, severe plastic deformation and formation of oxide layer are observed.     

Growth and characterization of Mg(OH)2 film on magnesium alloy AZ31

Magnesium-based biomaterials have been proposed as potential candidates for biodegradable implant materials, such as bone screws, bone plates, intraluminal stents and so on. However, the poor corrosion resistance inhibits their applications in surgery. They collapse before the injured tissues are healed. In this paper, Mg(OH)₂ nonstructural film was synthesized on the substrate of AZ31 magnesium alloy by hydrothermal method with NaOH solution as mineralizer to reduce the corrosion rate of magnesium-based materials. The obtained films were characterized by XRD, SEM, and XPS. The results showed that a Mg(OH)₂ film with nanostructure surface can be synthesized by hydrothermal method. It was observed that the thickness of film increased with the holding time. Corrosion rates of the films were studied by immersing the samples in Hank's solution (37 °C). Surface deposits of samples with films soaked in Hank's solution for 31 days were investigated by XRD, SEM, EDS, XPS, and FTIR. It verified that the corrosion rate of the magnesium alloy with grown film was slowed down in the Hank's solution and the behavior of corrosion was inhibited effectively. Amorphous calcium apatite precursor was observed to deposit on the surface of the film during corrosion experiments in Hank's solution. And the tape test revealed a strong adhesion between the film and the substrate.     

Cyclic deformation and fatigue cracking behaviors of Cu–28wt%Ag binary alloy

The cyclic deformation and fatigue cracking behaviors of coarse-grained Cu–28wt%Ag binary alloy were investigated under axial plastic strain amplitudes ranging from 10^?4 to 7.5 × 10^?4. It was found that the cyclic stress of the Cu–Ag alloy increased rapidly in the initial tens of cycles and became saturation with further cyclic deformation. The cyclic saturation stress increased with increasing the plastic strain amplitude. The interfaces are classified into two categories based on the orientations of the eutectic and the dendrites, i.e. type I and type II interfaces. The surface damage morphologies show that fatigue cracks normally nucleated either along the type I interfaces or along the slip bands (SBs), while no cracking occurred along the type II interface. Fatigue striations with different spacings appeared on the fracture surface, and secondary cracks along the striations were also observed. Based on the experimental results, the cyclic deformation and fatigue cracking behavior of the Cu–Ag binary alloy were discussed in detail.     

A Comparative Investigation of Thermal and Tribological Properties of Thermoplastic and Thermosetting Polyimides with Similar Structural Formulae

Polymerization of monomer reactants (PMR)-type polyimide was synthesized and the homogeneous matrix resin solution (30–40% solid) was used to prepare film blocks for tribological tests. Compared with a thermoplastic polyimide with similar molecular formula and similar behavior of weight loss under heating, the high glass transition (Tg) and char yield of the PMR polyimide can be attributed to the self-reaction of phenylethynyl groups to result in a cross-linked structure. Tribological studies on both PMR type thermosetting polyimide and thermoplastic one showed that the friction coefficient and wear rate of the former polyimide were lower than that of the latter one under both similar and even more critical conditions. Scanning electron microscope examinations of worn surfaces and wear debris show that the wear type of the thermoplastic polyimide was adhesive wear and that of the thermosetting one was fatigue wear. In terms of all good tribological properties, this PMR-type thermosetting polyimide, due to its high PV limit, could be a potential candidate for tribo-material in dry sliding against steel under high speed and large load.     

The Effects of Copper and Polytetrafluoroethylene (PTFE) on Thermal Conductivity and Tribological Behavior of Polyoxymethylene (POM) Composites

The effects of copper and polytetrafluoroethylene (PTFE) on thermal conductivity and tribological behavior of polyoxymethylene (POM) composites were investigated by a hot disk thermal analyzer and an M-2000 friction and abrasion testing machine. The results indicated that the incorporation of 3 wt% copper particles into POM had little effect on the thermal conductivity of POM composites, but led to the decreased friction coefficient and wear rate of composites. As the copper content was increased, the thermal conductivity increased and reached 0.477 W m^?1 K^?1 for POM-25% Cu composite, an increase of 35.9% compared with that of unfilled POM, while the friction coefficient and wear rate of composites also increased. The incorporation of PTFE into POM-Cu composites had a negligible effect on the thermal conductivity of composites, but helped in the formation of a continuous and uniform transfer film and resulted in the reduction in the friction coefficient and wear rate of composites. The POM-15% Cu-10% PTFE composite, with a value of wear rate similar to unfilled POM possessed higher thermal conductivity and lower friction coefficient.     

高压、高应变率与低压、高应变率实验的本构关联性

 指出Johnson-Cook（J-C）、Zerilli-Armstrong（Z-A）、Bodner-Parton（B-P）本构方程在一定条件下的适用性,表明对于低压、高应变率实验,单一曲线假定似乎可以采用。通过等效应力、等效应变,可以将不同应力状态下的流动应力函数采用统一的方程描述。然而,这些本构方程的确立,并不包括平面冲击波实验。对适合于平面冲击波实验的Steinberg-Cochran-Guinan（SCG）本构方程,讨论了其方程中所包含的高压与高应变率耦合效应。指出,以剪切模量度量的流动应力具有应变率相关性。基于温度效应的新发现以及直接测量平面冲击波流动应力的新进展,分别用J-C本构及SCG本构方程估算了钨材料在高压、高应变率加载下的流动应力。结果表明,采用J-C本构估算的流动应力仅在压力为10 GPa以下才能与实验数据相近,当压力高于10 GPa时,流动应力只能采用SCG本构估算。也指出了高压、高应变率本构方程与低压、高应变率本构方程所对应的不同物理背景。     

Cr对TiC-VC增强铁基熔覆层耐蚀性及耐磨性的影响

通过X射线衍射、扫描电子显微镜、能谱仪、极化曲线和磨粒磨损实验分析,研究了不同Cr加入量对TiC-VC增强铁基激光熔覆层耐蚀性和耐磨性能的影响。结果表明:熔覆层中物相主要为α-Fe,TiC,VC和TiVC2。随着Cr加入量的增加,伴随有残余奥氏体及Cr3C2的出现,且Cr3C2呈长条状部分聚集、部分单独分布。熔覆层的耐蚀性与耐磨性随Cr加入量的增加呈现先增加后降低的趋势。熔覆粉末中加入适量的Cr元素能显著提高熔覆层的硬度与耐蚀性。当添加质量分数为3.0%的Cr时,熔覆层硬度高达1090HV0.2,且相同磨损条件下熔覆层磨损失重仅约为Q235钢的1/26;当添加质量分数为9.0%的Cr时,所得熔覆层的耐蚀性最好,约为不添加Cr时的3.26倍。     

The effect of heat treatment on the electrochemical corrosion behavior of reactive plasma-sprayed TiN coatings

The effect of heat treatment on the corrosion behavior of reactive plasma sprayed TiN coatings in simulated seawater was investigated by electrochemical methods such as the corrosion potential-time curve (E_corr − t), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and SEM, etc. The results showed that the corrosion potential of TiN coatings increased after heat treatment; the corrosion current of the TiN coatings after heat treatment (be hereafter referred to as HT-TiN) was 13.3% of the untreated coatings (be hereafter referred to as UT-TiN), and the polarization resistance of HT-TiN was 20 times of UT-TiN, which indicated that the heat treatment had significantly increased the corrosion resistance of the coatings. The corrosion behavior of the coatings was mainly local corrosion, and the local corrosion behavior mainly took place at the microdefects (crack and pores) of the coatings. The porosity of the coatings was reduced after heat treatment. The reason was that TiN reacted with O₂ to form TiO₂ and Ti₃O during the heat treating, and volume expansion took place, which led to denser microstructure. The corrosion resistance of the coatings was therefore increased.     

Effect of deformation on the electrochemical behavior of hot-dip galvanized steel sheets

The main purpose of employing pre-coated steel sheets is to minimize corrosion of steel. However, coatings can be severely affected by forming processes. In the forming processes, due to the different modes of deformation, the strain levels are different and so can affect the properties of the coatings to a varying degree. Special attention has to be paid to the influence of deformation conditions on the performance of the coating, as regards protection against corrosion. The adhesion of the coating must remain good, and the surface should not be damaged during forming. Therefore, it is necessary to study the behavior of its corrosion resistance against the deformation.In this work, effect of strain path on the corrosion behavior of hot-dip galvanized steel sheets has been studied. Corrosion behavior of hot-dip galvanized steel sheets at various strain levels has been evaluated under four different strain paths namely, biaxial, plane strain, uniaxial (drawing) and tensile modes. The sheets were deformed by a limiting dome height test (LDH) set-up. A correlation between the degree of deformation and the loss in extent of corrosion protection offered by the coating has been established by carrying out electrochemical studies such as potentiodynamic polarization and impedance spectroscopy (EIS) in 3.5% NaCl solution. The present study shows that increase in deformation increases the extent of delamination of the coating for all the modes of deformation. The severity of deformation on delamination, however, has been found to vary in the order of, tensile < uniaxial < plane < biaxial.