The relationship between substrate roughness parameters and bond strength of ultra high-performance fiber concrete

The bonding that exists between the old concrete and the new concrete depends largely on the quality of substrate surface preparation. The accurate representation of substrate surface roughness can help determine very precisely the correct bonding behavior. In this work, an experimental investigation was carried out to quantify the normal concrete (NC) substrate roughness parameters and evaluate their relationship with the bonding performance of ultra high-performance fiber concrete (UHPFC), used as a repair material. The bond strength was quantified based on the results of the pull-off test, splitting cylinder tensile test, and the slant shear test. Three types of NC substrate surface preparation were used: as-cast (without surface preparation) as reference, wire-brushed, and sand-blasted (SB); the roughness of which was determined using an optical three-dimensional (3D) surface metrology device (Alicona Infinite Focus). It was observed from the result of the pull-off test that failure occurred in the substrate, even though adequate substrate surface roughness was provided. Moreover, analysis of the splitting cylinder tensile and slant shear test results showed that the substrate surface preparation method had a significant influence in bonding strength between UHPFC and the NC substrate. The composite UHPFC/NC substrate having a SB surface behaved closely as a monolithic structure under splitting and slant shear tests. An excellent correlation (R ²?>?85%) was obtained between the substrate roughness parameters and the results of the splitting cylinder tensile and slant shear tests.     

Long-term hydrophobicity and ice adhesion strength of latex paints containing silicone oil microcapsules

Waterborne anti-icing coatings were prepared by embedding silicone oil microcapsules in latex paints. The long-term hydrophobicity and ice adhesion strength of the coatings were examined with a QUV accelerated weathering tester and a pull-off adhesion tester. The effects of silicone oil content and pigment/binder (PB) ratio on the long-term hydrophobicity and the ice adhesion strength of the coatings were investigated. A higher silicone oil content and a PB ratio close to the critical pigment volume concentration favor long-term hydrophobicity of the coatings. An obvious decrease in ice adhesion strength was achieved for coatings with a PB ratio of 5.0 and a silicone oil content of 4.2%. For coatings with the same surface roughness, a higher water contact angle (WCA) led to lower ice adhesion strength. However, for coatings with different surface roughnesses, the ice adhesion strength was found to be dependent on surface roughness rather than on WCA.     

Pull-off strength of thermoplastic fiber-reinforced composite coatings

The paper aims at presenting the results of pull-off strength tests of fiber-reinforced polymer composite coatings laminated on steel substrates. It contains the measured data on the thickness of manufactured coatings and the substrate surface’s roughness, according to the various methods of surface’s preparation with the means of abrasive blasting. The microstructure analysis of material’s cross-sections and damage analysis of samples after failure were also performed. The highest pull-off strength’s values for composite coatings were obtained for joints with the substrate modified by abrasive blasting with corundum F60 or simply degreased. To establish the compatibility of substrates with coatings the wettability of the chosen materials was tested and work of adhesion was calculated on its base. Concerning the wettability, it was found that the most preferable joints were characterized by the similar thermodynamic work of adhesion and consisted of the coating’s matrix (SBS) and the steel substrate degreased with acetone or modified with corrundum abrasive blasting and then degreased.     

Microstructural analysis of the adhesion mechanism between old concrete substrate and UHPFC

The performance of any repaired concrete structure, and thus its service life, depends on the quality of the interfacial transition zone of the composite system formed by the repair material and the existing concrete substrate. In this work, the properties of the interfacial transition zone between normal concrete (NC) substrate as an old concrete and ultra-high performance fiber-reinforced concrete (UHPFC) as a repair material was investigated. Pull-off and splitting cylinder tensile tests were performed to quantify the bond strength in direct and indirect tensions, respectively. The microstructure of the interfacial transition zone was also studied using scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM/EDS). Different types of NC substrate surface preparation methods were used. An optical three-dimensional surface metrology device was used to estimate the substrate roughness parameters. Based on the results, high interfacial bond strength was achieved on the 3rd, 7th, and 28th days. The pull-off test results revealed that all failures occurred in the substrate, regardless of the substrate surface roughness. The majority of failures in the split tensile test also occurred in the substrate. SEM/EDS proved that the use of UHPFC as a repair material chemically, physically, and mechanically improved the repaired interfacial transition zone to become stronger and denser, as well as more uniform, and durable. Moreover, the use of UHPFC increased the service life of repaired structures and minimized the number and extent of interventions to the lowest possible level.     

An Apparatus to Determine the Pull-Off Force of a Conducting Microparticle from a Charged Surface

A unique experimental apparatus has been developed to determine the pull-off force of a conducting microparticle resting on a conducting surface in the presence of a DC electric field. The apparatus precisely measures the applied electric-field strength at which an individual microparticle of known diameter, which is measured in situ, is removed from the surface. This information is used to determine the adhesion pull-off force required to remove the microparticle from the surface to within an uncertainty of approximately 12% at 95% confidence. In the experiment, the electric field strength between a conducting-surface electrode and a grounded electrode is increased in time using a microcontroller-driven digital-to-analog converter. Both electrodes are semi-transparent, which permits the surface-resident microparticles to be viewed from underneath the surface using microphotography. The microphotography system is operated in conjunction with pixel-intensity, gradient-search software to determine the diameter of each surface-resident microparticle. The apparatus is designed to explore a wide range of operating conditions, including microparticles of diverse composition and sizes, surfaces of differing roughness, and environments of various relative humidity including vacuum. The theory of operation and instrument design are presented in detail. Preliminary results also are given as a proof of concept.     

民机铝合金蒙皮的激光织构化处理

采用波长为1064 nm的激光表面处理设备在2024-T3铝合金表面刻蚀出平行线、正方形和菱形这3种织构表面。采用扫描电子显微镜与激光共聚焦显微镜观察了不同织构表面的微观形貌。通过测量对水和甘油的接触角来评价它们的浸润性。用拉脱法测试了其表面环氧涂层的附着力。结果表明,在单位面积能量密度相同的情况下,表面织构为正方形和菱形的试样表面粗糙度由处理前的1.9μm分别提升至7.6μm和7.9μm,表现出更好的浸润性,环氧涂层的附着力比未处理试样提高了70%左右,而平行线织构表面的涂层附着力只提高了24%。通过金相观察、强度失效分析及硬度测试发现,织构化处理对飞机蒙皮的力学性能基本没有影响。     

纳米颗粒对环氧树脂胶粘剂／钢铁附着强度的影响

采用纳米Al2O3、纳米CaCO3、纳米SiO2三种纳米颗粒,机械混合对环氧树脂胶粘剂进行改性,并对纳米颗粒改性的环氧树脂胶粘试样进行了附着强度的检测。结果发现,通过改变纳米颗粒的种类和含量,环氧胶粘剂／钢铁基体之间的附着强度得到不同程度的提高,其中添加2％纳米Al2O3颗粒的环氧胶粘剂与钢铁基体的附着强度提高了4倍左右。通过断面形貌特征和不同基体粗糙度下对添加纳米颗粒环氧胶与钢铁基体之间附着强度的检测,对此现象产生的原因进行了分析和讨论。     

Influence of environmental moisture on atmospheric pressure plasma jet treatment of ultrahigh-modulus polyethylene fibers

One of the main differences between low-pressure and atmospheric-pressure plasma treatments is that there is little moisture involved in the low-pressure plasma treatment, although moisture could exist at the wall of the vacuum chamber or react with the substrate after plasma treatment, while in the atmospheric-pressure plasma treatment moisture exists not only in the environment but also in any hygroscopic substrate. In order to investigate the influence of environmental moisture on the effect of atmospheric pressure plasma treatment, ultra-high-modulus polyethylene (UHMPE) fibers were treated using an atmospheric-pressure plasma jet (APPJ) with 10 l/min helium gas-flow rate, treatment nozzle temperature of 100°C and 5 W output power. The plasma treatments were carried out at three different relative humidity levels, namely 5, 59 and 100%. After the plasma treatments, the surface roughness increased while the water-contact angle decreased with increasing relative humidity. The number of oxygen containing groups increased as the environmental moisture content increased. The interfacial shear strength of the UHMPE fiber/epoxy system was significantly increased after the plasma treatments, but the moisture level in the APPJ environment did not have a significant influence on the adhesion properties. In addition, no significant difference in single fiber tensile strength was observed after the plasma treatments at all moisture levels. Therefore, it was concluded that the environmental moisture did not significantly influence the effect of atmospheric-pressure plasma treatment in improving interfacial bonding between the fiber and epoxy. The improvement of the interfacial shear strength for the plasma-treated samples at all moisture levels was mainly due to the increased surface roughness and increased surface oxygen and nitrogen contents due to the plasma etching and surface modification effect.     

Influence of surface treatment of carbon fibers on interfacial adhesion strength and mechanical properties of PLA‐based composites

In the present study C/PLA composites with different fiber surface conditions (untreated and with nitric acid oxidation for 4 h and 8 h) were prepared to determine the influence of surface treatment on the interfacial adhesion strength and mechanical properties of the composites. A chemical reaction at the fiber–matrix interfaces was confirmed by XPS studies. Nitric acid treatment was found to improve the amount of oxygen‐containing functional groups (particularly the carboxylic group, —COOH) on carbon fiber surfaces and to increase the surface roughness because of the formation of longitudinal crevices. The treated composites exhibited stronger interface adhesion and better mechanical properties in comparison to their untreated counterparts. There was a greater percentage of improvement in interfacial adhesion strength than in the mechanical properties. The strengthened interfaces and improved mechanical performance have been mainly attributed to the greater extent of the chemical reaction between the PLA matrix and the carbon fibers. The increased surface roughness also has had a slight contribution. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 367–376, 2001     

碳纳米管定向排列增强碳纤维/环氧树脂复合材料制备及力学性能

采用高频电场诱导法制备了碳纳米管定向有序填充的碳纤维/环氧树脂复合材料。研究了电场频率对复合材料力学性能的影响规律,对复合材料的显微形貌进行观察。结果表明:在富树脂区碳纳米管沿着电场方向存在明显的有序排列现象;高频电场诱导后复合材料的层间剪切强度最大提高28.9%,压缩强度提高28.83%,弯曲强度提升15.01%,断口粗糙度增加,树脂与碳纤维的界面结合状态改善。     

Analysis of the Effects of Atmospheric Helium Plasma Treatment on the Surface Structure of Jute Fibres and Resulting Composite Properties

This work investigates the mechanisms involved in the improvement of flexural properties of a jute/polyester composite when the reinforcement material has been atmospherically plasma treated using helium gas. All composites were laid-up by hand and cured using a Quickstep? cure cycle. Surface characterization techniques including scanning probe microscopy (SPM), and surface wettability combined with fabric tensile strength, composite flexural strength and composite Mode-I properties have been used to quantify the effects of plasma modification. Flexural strength and modulus increased with plasma treatment time, reaching a maximum at 25 passes before decreasing. SPM topographical analysis showed that roughness of the fibre decreased as the plasma treatment time increased until 25 passes after which the roughness was found to increase again. The coefficient of friction increased rapidly after only a short plasma treatment time (5 passes) whilst wettability continued to increase until 25 passes after which it remained constant. The fabric tensile strength followed the same trend as the flexural properties of the composites. Decreasing fibre surface roughness is postulated as a reason for decreasing Mode-I interlaminar fracture toughness properties of the composites.     

How Compliance Compensates for Surface Roughness in Fibrillar Adhesion

Fibrillar interfaces play an important role in the ability of many small animals to adhere to surfaces. Surface roughness is generally deleterious to adhesion because it hinders the ability of mating surfaces to make contact, but fibrillar surfaces compensate for surface roughness by virtue of their enhanced compliance. We examine the relationship between roughness and compliance by analyzing the mechanics of detaching an array of fibrils from a substrate. The theory of Johnson, Kendall, and Roberts is used to describe the interfacial adhesion of each fibril, and roughness is modeled by making the fibril length a random variable subject to a probability distribution. We solve for the mean force response of a fibrillar array as a function of the displacement of the entire array. From these results we extract the mean fibrillar pull-off force and work to separate the fibrillar array and substrate. We show how the mean fibrillar pull-off force decreases with increasing roughness-height standard deviation: the relationship is linear for small height standard deviation, and the pull-off force trails off to zero for very rough surfaces. Conversely, the work of separation is shown to be unaffected by small roughness-height standard deviation, although it decreases toward zero for rougher surfaces. The effects of roughness may be offset by increasing fibrillar compliance; for small roughness-height standard deviation, we show that the reduction in pull-off force is inversely proportional to the normalized compliance. We also show that the work of separation increases linearly with the compliance when the compliance is large compared with the roughness-height standard deviation.     

How Compliance Compensates for Surface Roughness in Fibrillar Adhesion

Fibrillar interfaces play an important role in the ability of many small animals to adhere to surfaces. Surface roughness is generally deleterious to adhesion because it hinders the ability of mating surfaces to make contact, but fibrillar surfaces compensate for surface roughness by virtue of their enhanced compliance. We examine the relationship between roughness and compliance by analyzing the mechanics of detaching an array of fibrils from a substrate. The theory of Johnson, Kendall, and Roberts is used to describe the interfacial adhesion of each fibril, and roughness is modeled by making the fibril length a random variable subject to a probability distribution. We solve for the mean force response of a fibrillar array as a function of the displacement of the entire array. From these results we extract the mean fibrillar pull-off force and work to separate the fibrillar array and substrate. We show how the mean fibrillar pull-off force decreases with increasing roughness-height standard deviation: the relationship is linear for small height standard deviation, and the pull-off force trails off to zero for very rough surfaces. Conversely, the work of separation is shown to be unaffected by small roughness-height standard deviation, although it decreases toward zero for rougher surfaces. The effects of roughness may be offset by increasing fibrillar compliance; for small roughness-height standard deviation, we show that the reduction in pull-off force is inversely proportional to the normalized compliance. We also show that the work of separation increases linearly with the compliance when the compliance is large compared with the roughness-height standard deviation.     

Microstructure and properties evolution of silicon-based ceramic cores fabricated by 3D printing with stair-stepping effect control

A ceramic core is the key component in the manufacture of the hollow turbine blades of aeroengines. Compared with the traditional injection molding method, 3D printing is more suitable for manufacturing ceramic cores with a complex geometry at high precision. However, the stair-stepping effect is inevitable in the 3D printing process and affects the surface roughness and strength of the ceramic core. In this study, to explore the influence of nano-silica content on the microstructure and properties of the ceramic core, silicon-based ceramic cores were fabricated with the addition of nano-silica powder by digital light processing and subsequent sintering at 1200 °C. The results showed that the apparent porosity and pore size of the ceramic core gradually decreased as both the nano-silica powder content and bulk density increased. Meanwhile, the printing interlayer spacing was significantly reduced, resulting in a low surface roughness, high flexural strength, and creep-resistance. To simulate the entire casting process of a superalloy blade, the thermal deformation behavior of the ceramic core was observed by heating and cooling cycles performed in a thermal dilatometer at 1540 °C. The total linear shrinkage decreased as the nano-silica powder content increased, which was mainly due to the phase transformation of cristobalite and the densification of the ceramic core sintered at 1200 °C. The low surface roughness and linear shrinkage as well as high flexural strength of the ceramic core can contribute to the excellent quality of cast superalloy blades.     

SURFACE ROUGHNESS EFFECTS ONMICROPARTICLE ADHESION

A new self-consistent model is developed to treat the static contact of a microparticle with a flat barrier in the presence of molecular adhesion and surface roughness. Separation between their mean datum planes is modeled considering the elastic deformation of the microparticle and surface. The contact pressure is computed from the Lennard-Jones law following the Derjaguin approximation. The elastic deflection of the mean datum plane is calculated from the effective pressure by the half-space elastic theory. Roughness is modeled by introducing a Gaussian distribution to the gap between the surfaces. An effective pressure is defined as the statistical average of the contact pressure over the roughness heights. A solution satisfying all of the above conditions gives a self-consistent method of modeling adhesion between the microparticle and the flat barrier. Using collocation methods the equations are discretized as a large system of nonlinear algebraic equations. A continuation method is used to find the multiple numerical solutions for the mean separation and the effective contact pressure. Finally, adhesive contacts of both smooth and rough surfaces are simulated in a comparative manner to elucidate the features of surface roughness in the presence of molecular adhesion. The standard deviation of the Gaussian distribution is used as a parameter to assess the effects of roughness on the pull-off force. It is shown that increasing surface roughness significantly reduces the pull-off force and decreases the tendency for the microsphere to snap-on and snap-off.     

Surface roughness effects onmicroparticle adhesion

A new self-consistent model is developed to treat the static contact of a microparticle with a flat barrier in the presence of molecular adhesion and surface roughness. Separation between their mean datum planes is modeled considering the elastic deformation of the microparticle and surface. The contact pressure is computed from the Lennard-Jones law following the Derjaguin approximation. The elastic deflection of the mean datum plane is calculated from the effective pressure by the half-space elastic theory. Roughness is modeled by introducing a Gaussian distribution to the gap between the surfaces. An effective pressure is defined as the statistical average of the contact pressure over the roughness heights. A solution satisfying all of the above conditions gives a self-consistent method of modeling adhesion between the microparticle and the flat barrier. Using collocation methods the equations are discretized as a large system of nonlinear algebraic equations. A continuation method is used to find the multiple numerical solutions for the mean separation and the effective contact pressure. Finally, adhesive contacts of both smooth and rough surfaces are simulated in a comparative manner to elucidate the features of surface roughness in the presence of molecular adhesion. The standard deviation of the Gaussian distribution is used as a parameter to assess the effects of roughness on the pull-off force. It is shown that increasing surface roughness significantly reduces the pull-off force and decreases the tendency for the microsphere to snap-on and snap-off.     

Fractal analysis of surfaces roughened by grit blasting

Surfaces roughened by grit blasting influence the adhesion strength of plasma-sprayed ceramic coatings. The average surface roughness has been used to evaluate the surface topography of such surfaces. It is well known that the adhesion strength of ceramic coatings reaches a maximum value at a certain substrate surface roughness. However, this result cannot be understood based on only surface roughness. The blasted surface has fractal characteristics. There are two types of fractal surfaces, which are characterized by self-similarity and self-affinity. Using fractal analysis to evaluate the surface topography of substrates, the fractal dimension was measured for the roughened surfaces. The maximum fractal dimension was attained at a blasting angle of 75°, where the adhesion strength also reached approximately its maximum value. It is concluded that the fractal dimension is a more appropriate measure than the average surface roughness for evaluation of the adhesion strength of ceramic coatings.     

The effects of zinc aluminum phosphate (ZPA) and zinc aluminum polyphosphate (ZAPP) mixtures on corrosion inhibition performance of epoxy/polyamide coating

The epoxy/polyamide coating was loaded with different pigment mixtures of the zinc phosphate (ZP), zinc aluminum phosphate (ZPA) and zinc aluminum polyphosphate (ZAPP) pigments. The electrochemical impedance spectroscopy (EIS) and salt spray test were used to investigate corrosion inhibition performance of the coatings. The adhesion strengths of the coatings were measured by a pull-off test. Results revealed lower coating pull-off strength loss when the ZPA and ZAPP pigments were used. A significant decrease in number of blisters together with low pull-off strength loss and best corrosion inhibition properties were observed when the mixture of 80:20 of ZAPP:ZPA was used.     

Surface treatment of wood polymer composites for adhesive bonding

An experimental study was undertaken to evaluate different surface treatment techniques for adhesive bonding of a Wood Polymer Composite (WPC) material. The surface treatment methods were flame, corona discharge treatment (CDT), mechanical abrasion (MA) and combination treatment of MA followed by the CDT. Surface analytical techniques used were contact angle analysis, Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR), Atomic Force Microscopy (AFM) and 2 dimensional profilometry. Adhesion pull-off test was used to assess the effects of surface treatments. The combination surface treatment, i.e. MA+CDT, was particularly effective in improving bonded joint strength. The adhesion pull-off strengths increased from 0.2 MPa for untreated substrate to 3.4 MPa for MA+CDT treated substrates. The carbonyl, hyroxyl and wood indices obtained from the ATR-FTIR spectra were valuable indicators for studying the nature of WPC substrates.     

Study on effects of pre-treatment and surface roughness on tensile-shear strength of 2060 Al–Li alloy adhesive joints

The forces between adhesive and adherend mainly influenced by the pre-treatment technology of the substrates have important effects on the bonding strength. In this paper, the influence of different pre-treatment processes and surface roughness on the tensile-shear strength of 2060 Al–Li alloy adhesive joints as well as related mechanism was investigated. In this perspective, substrates were processed by mechanical abrasion at different levels and by phosphoric acid anodizing, which resulted in different surface topographies that were characterized by means of roughness measurements. Single-lap joints were prepared using a two-component epoxy adhesive. The tensile-shear strength of joints was measured via destructive testing and the failure modes were analyzed to evaluate the quality of bonding. Results showed that with the increase of surface roughness of Al–Li alloy, the tensile-shear strength of the adhesive joints increased and the failure modes changed from interfacial failure to cohesive failure. The groove structures formed during mechanical abrading were regarded as being responsible for this strengthening behavior. Moreover, a rough porous membrane was produced on adherents’ surface by phosphoric acid anodizing, causing a consolidation of adhesion at the adhesive-substrate interface.