Improved Adhesion Strength Research Articles

Cytotoxicity and tribological property of DLC films deposited on polymeric materials

To evaluate the cytotoxicity and the tribological properties for diamond-like carbon films (DLC), DLC films were deposited on high-polymer materials such as polymethylmethacrylate, polycarbonate, polyurethane and polytetrafluoroethylene using a radio-frequency chemical vapor deposition (CVD) system. Adhesion strength between these materials and DLC films is generically poor. To improve adhesion strength of DLC films and high-polymer materials, oxygen plasma pretreatment was performed before DLC films deposited. Evaluation of the tribological properties was performed using ball-on disc system. Especially, in the DLC films deposited on PMMA with oxygen plasma pretreatment, the friction coefficient was significantly reduced from 0.456 to 0.310. Influence of DLC films on cytotoxicity were studied in vitro cell culture by counting cells. Mouse fibroblasts (NIH-3T3) were seeded in 24 well culture plates, and four kinds of high-polymer PMMA, PC, PU and PTFE substrates, DLC-deposited and non-deposited high-polymer substrates, were immersed in each well directly for 24, 48, 72, and 96 h. Multiplication of cell number with DLC-deposited substrates is similar in that with non-deposited substrates. Consequently, these studies reveal that the DLC films have excellent tribological properties and cytocompatibility.

Surface characteristics and adhesive strengths of metal on O 2 ion beam treated polyimide substrate

A polyimide surface was modified by oxygen ion beam bombardment to improve its adhesion to a subsequently deposited NiCr (nickel-chromium alloy) overlayer. The changes in the chemical composition, morphology and adhesion property of the modified polyimide surface were characterized by X-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy and the 90° peel test. The results show that the oxygen ion beam treatment of the polyimide induces the formation of oxygen functional groups and the rearrangement of the imide groups in the polyimide substrate film. An increase in its surface roughness resulted in a substantial improvement in the adhesion strength between the polyimide and NiCr overlayer. The improved adhesion strength, however, was found to be reduced after thermal aging at 150 °C for 168 hrs, which was attributed to the thermal oxidative degradation of the polyimide at the interface. We also observed that the ion beam treatment was effective in retarding the adhesion loss caused by the thermal aging.

Novel sphene coatings on Ti–6Al–4V for orthopedic implants using sol–gel method

Hydroxyapatite (HAp) is commonly used to coat titanium alloys (Ti–6Al–4V) for orthopedic implants. However, their poor adhesion strength and insufficient long-term stability limit their application. Novel sphene (CaTiSiO 5) ceramics possess excellent chemical stability and cytocompatibility. The aim of this study is to use the novel sphene ceramics as coatings for Ti–6Al–4V. The sol–gel method was used to produce the coatings and the thermal properties, phase composition, microstructure, thickness, surface roughness and adhesion strength of sphene coatings were analyzed by differential thermal analysis–thermal gravity (DTA–TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), atom force microscopy (AFM) and scratch test, respectively. DTA analysis confirmed that the temperature of the sphene phase formation is 875 °C and XRD analysis indicated pure sphene coatings were obtained. A uniform structure of the sphene coating was found across the Ti–6Al–4V surface, with a thickness and surface roughness of the coating of about 0.5–1 μm and 0.38 μm, respectively. Sphene-coated Ti–6Al–4V possessed a significantly improved adhesion strength compared to that for HAp coating and their chemical stability was evaluated by testing the profile element distribution and the dissolution kinetics of calcium (Ca) ions after soaking the sphene-coated Ti–6Al–4V in Tris–HCl solution. Sphene coatings had a significantly improved chemical stability compared to the HAp coatings. A layer of apatite formed on the sphene-coated Ti–6Al–4V after they were soaked in simulated body fluids (SBF). Our results indicate that sol–gel coating of novel sphene onto Ti–6Al–4V possessed improved adhesion strength and chemical stability, compared to HAp-coated Ti–6Al–4V prepared under the same conditions, suggesting their potential application as coatings for orthopedic implants.

Adhesion of PBO fiber in epoxy composites

A composite of poly p-phenylene-2,6-benzobisoxazole (PBO) fiber and epoxy resin has excellent electrical insulation properties. However, it is a challenging issue to improve its mechanical properties because of poor adhesion between PBO fiber and matrix. The relatively smooth and chemically inactive surface of PBO fiber prevent efficient chemical bonding in the composite interface. Here, we report the surface modification of PBO fibers by UV irradiation, O2 and NH3 plasma, as well as acidic treatments. We found that the surface free energy and roughness are increased for both sized and extracted fibers after plasma treatments together with maleic anhydride grafting. The sized fiber shows marginal improvement in adhesion strength and no change in fiber tensile strength because of the barrier effect of the finish. For the extracted fiber, however, the tensile strength of the fiber is sensitive to surface treatment conditions and considerable strength reduction occurred, particularly for cases of acidic treatments and UV irradiation. This is because that the treatments increase the surface roughness and introduce more surface flaws. The extracted fiber surface has no adequate wetting and functional groups, which in turn results in coarse interface structures and causes reduction or no apparent variation of the adhesion strength. The fracture surfaces after single fiber pull-out tests exhibit adhesive interfacial failure along the fiber surface, which is further confirmed by similar adhesion strength and interlaminar shear strength values when the fiber was embedded in various epoxy resins with different temperature behavior.

Adhesive bonding using thick polymer film of SU-8 photoresist for wafer level package

【For the application to optic devices, wafer level package including spacer with particular thickness according to optical design could be required. In these cases, the uniformity of spacer thickness is important for bonding strength and optical performance. Packaging process has to be performed at low temperature in order to prevent damage to devices fabricated before packaging. And if photosensitive material is used as spacer layer, size and shape of pattern and thickness of spacer can be easily controlled. This paper presents polymer bonding using thick, uniform and patterned spacing layer of SU-8 2100 photoresist for wafer level package. SU-8, negative photoresist, can be coated uniformly by spin coater and it is cured at $95^{\circ}C$ and bonded well near the temperature. It can be bonded to silicon well, patterned with high aspect ratio and easy to form thick layer due to its high viscosity. It is also mechanically strong, chemically resistive and thermally stable. But adhesion of SU-8 to glass is poor, and in the case of forming thick layer, SU-8 layer leans from the perpendicular due to imbalance to gravity. To solve leaning problem, the wafer rotating system was introduced. Imbalance to gravity of thick layer was cancelled out through rotating wafer during curing time. And depositing additional layer of gold onto glass could improve adhesion strength of SU-8 to glass. Conclusively, we established the coating condition for forming patterned SU-8 layer with $400{\mu}m$ of thickness and 3.25 % of uniformity through single coating. Also we improved tensile strength from hundreds kPa to maximum 9.43 MPa through depositing gold layer onto glass substrate.】

Biomimetic coating of an apatite layer on poly(l-lactic acid); improvement of adhesive strength of the coating

A biodegradable polymer coated with a bonelike apatite layer on its surface would be useful as a scaffold for bone tissue regeneration. In this study, poly(L-lactic acid) (PLLA) was treated with oxygen plasma to produce oxygen-containing functional groups on its surface. The plasma-treated specimen was then alternately dipped in aqueous CaCl(2) and K(2)HPO(4).3H(2)O solutions three times, to deposit apatite precursors onto the surface. The surface-modified specimen then successfully formed a dense and uniform bonelike surface apatite layer after immersion for 24 h in a simulated body fluid with ion concentrations approximately equal to those of human blood plasma. The adhesive strength between the apatite layer and the specimen surface increased as the power density of the oxygen plasma used increased. The maximum adhesive strength of the apatite layer to the specimen was significantly higher than that to the commercially available artificial bone, HAPEX. The resultant bonelike apatite-PLLA composite would be useful as a scaffold for bone tissue regeneration.

Transparent photo‐curable co‐polyacrylate/silica nanocomposites prepared by sol‐gel process

AbstractThis work prepared the highly transparent photo‐curable co‐polyacrylate/silica nanocomposites by using sol‐gel process. The FTIR and 13C NMR analyses indicated that during the sol‐gel process, the hybrid precursors transform into composites containing nanometer‐scale silica particles and crosslinked esters/anhydrides. Transmission electron microscopy (TEM) revealed that the silica particles within the average size of 11.5 nm uniformly distributed in the nanocomposite specimen containing about 10 wt % of Si. The nanocomposite specimens exhibited satisfactory thermal stability that they had 5% weight loss decomposition temperatures higher than 150°C and coefficient of thermal expansion (CTE) less than 35 ppm/°C. Analysis via derivative thermogravimetry (DTG) indicated that the crosslinked esters/anhydrides might influence the thermal stability of nanocomposite samples. The UV‐visible spectroscopy indicated that the nanocomposite resins possess transmittance higher than 80% in visible light region. Permeability test revealed a higher moisture permeation resistance for nanocomposite samples, which indicated that the implantation of nano‐scale silica particles in polymer matrix forms effective barrier to moisture penetration. Adhesion test of nanocomposite samples on glass substrate showed at least twofold improvement of adhesion strength compared with oligomer. This evidenced that the silica and the hydrophilic segments in nanocomposite resins might form interchains hydrogen bonds with the OH groups on the surface of glass so the substantial enhancement of adhesion strength could be achieved. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

Mechanism of enhanced adhesion between hydrogenated amorphous carbon films and tungsten preimplanted steel substrates

Hydrogenated amorphous carbon (a-C:H) films have good mechanical properties but poor adhesion on substrates such as tool steels thereby limiting their applications. Film adhesion can be improved by conducting tungsten preimplantation into the steel substrates before deposition of the carbon film. The enhancement mechanism is investigated in this study by depositing the films on W plasma-preimplanted (20kV, 5×1017ionscm−2) and untreated steel substrates using mixed acetylene and argon plasmas with different flow rate ratios by means of plasma immersion ion implantation and deposition. Preimplantation creates a graded WC interface that mitigates diffusion of the dissociated carbon atoms on the surface. Compared to the films deposited on untreated steel substrates, the ones deposited on the W-implanted steel substrates exhibit improved adhesion strength, especially for the lower acetylene to argon flow rate ratios. Tungsten preimplantation is critical to the successful fabrication of a-C:H films on steel substrates. Furthermore, the flow rate ratio also has a large impact on the film properties due to the variations in the deposition rate causing different internal stress in the film. A nucleation model is proposed to explain the enhancement of the adhesion properties.

Effect of film thickness and interlayer on the adhesion strength of diamond like carbon films on different substrates

The addition of an interlayer is often used to increase the adhesion strength in thin film coating. For diamond like carbon (DLC) films, titanium and chromium are two common interlayer materials to enhance adhesion, especially for metal substrates. In an attempt to explore interlayer effect on nonmetal substrates, plasma enhanced chemical vapor deposition (PECVD) was utilized to deposit DLC with methane on silicon and glass substrates with interlayer titanium and chromium. The film structure and adhesion strength were studied by Raman spectroscopy, optical microscope, nanoindentation and nanoscratch. For DLC on silicon substrates without interlayer, the results show as the film thickness increases, the ratio of I(D)/I(G) increases and the hardness decreases. For DLC on silicon substrate with interlayer, both interlayers do not enhance the adhesion strength. For glass substrate, the chromium showed improved adhesion strength only in small film thickness (200 nm). As the film thickness increases, the peeling of the DLC thin film deteriorated for both interlayers. These phenomena were examined by the bonding structure, hardness, nanoscratch and residual stress. As the film thickness increases, the I(D)/I(G) ratio increases and the fraction of sp 3 decreases. This indicates as the film thickness increases, the bonding is towards graphite structure and reducing hardness. The high sp 2 fraction and low hardness explain the poor adhesion of large film thickness. The reasons for chromium acting as a better interlayer on glass than on silicon are attributed to the low residual stress and preferable carbide formation. The similarity in the magnitude of coefficient of thermal expansion between chromium and glass results in less residual stress. Another possibility is the carbide formation on chromium interface which should enhance the adhesion. This hypothesis is currently under investigation.

Adhesion improvement of TiN film on tool steel by a hybrid process of unbalanced magnetron sputtering and plasma-based ion implantation

An interfacial mixing layer and a titanium (Ti) layer between the titanium-nitride (TiN) film and the substrate material was produced to improve the adhesion of TiN film on tool substrates by a hybrid process of unbalanced magnetron sputtering (UBM) and plasma-based ion implantation (PBII). Before TiN deposition by UBM, the negative high-voltage pulse and DC-bias were applied to the substrate immersed in the Ti plasma, resulting in implantation as well as deposition of Ti ions to the substrate. As a result, a Ti layer and a graded mixing layer of Ti and substrate materials was produced to work as a buffer interface between substrate and TiN film. The adhesion strength of TiN film with the interfacial treatment on tool steel substrates was evaluated by scratch and indentation tests, showing the considerable improvement of adhesion by the formation of the Ti and the interfacial mixing layers. The suitable ion implantation energy for the improvement of adhesion strength was found.

Plasma Surface Modification of Aromatic Liquid Crystal Polymers, and Its Resultant Improvement in Adhesive Strength with Copper Thin Films

Surface modification of aromatic LCP surfaces with plasma treatment was carried out to improve the adhesive strength at LCP/Cu interfaces. Plasma treatment to a polymer surface is known to introduce functional groups and to bring about etching action thereon. This study was intended to investigate which or both functional group introduction and etching action contributes to adhesive strength improvement. The effect on the adhesive strength of ordinary plasma treatment and remote plasma treatment was evaluated. Remote plasma treatment is possible to irradiate mainly radicals on LCP surface. The following three subjects were discussed in this study : (1) plasma treatment increases the adhesive strength at LCP/Cu interfaces ; (2) plasma treatment produces oxygen functional groups and nitrogen functional groups and led to etch LCP surfaces, all of which contribute to adhesive strength ; and (3) the oxygen functional groups, nitrogen functional groups, and etching action work simultaneously to enhance adhesive strength by their synergistic effects.

Effects of Co, Ti Interlayer, and Post-Annealing on the Adhesion Property between TiN Coatings and WC-Co Substrate

In order to improve adhesion strength between TiN coatings and WC-Co substrate, two kinds of interlayer of Co and Ti were pre-deposited before TiN coating process. Thickness and postannealing effects of each interlayer on the adhesion property were investigated through scratch test in this work. Introduction of thin Ti interlayer largely enhanced the adhesion strength between TiN coatings and WC-Co substrate in scratch test. The critical load, Lc2 increased from 64 N without Ti interlayer up to 130N with Ti one of ~ 0.15 thickness. However, post-annealing of TiN/Ti/WCCo system at high temperature of 600 reduced the critical load again. The Ti interlayer caused a deficit of Co content in WC-Co substrate during annealing through diffusion of Co element into Ti interlayer. The reduction of critical load after post-annealing was believed due to diminution in mechanical properties of the substrate derived from the Co deficit in WC-Co substrate. On the other hand, introduction of thin Co interlayer of ~ 0.027 thickness also increased the critical load up to 84 N and improved failure mode, but did not reduce the critical load even after annealing, rather increased it. And, any Co deficit of the substrate was not found after annealing for TiN/Co/WC-Co system.

Improvement of adhesive strength of Ti–Al plasma sprayed coating

In order to improve the adhesive strength of plasma sprayed titanium coating, titanium–aluminum mixed powder was used as a material for plasma spraying. Adhesive strength of the plasma sprayed Ti–Al coatings was examined by changing the Al powder ratio in the Ti–Al mixed powder and input electrical power to the plasma torch. And then, they were discussed from the points of microstructure, hardness and oxygen and nitrogen contents of the coating. The plasma sprayed Ti–Al coating was found to be composed of Al and Ti compounds of TiN 0.3, Ti(N,O) and Ti 3Al, which were synthesized during the powder particles flight in plasma jet. The adhesive strength of the coating was improved by filling cracks and pores of the Ti compounds layers with the Al phase to form a dense microstructure, and also by hardening the Ti compounds and Al phase with the oxygen and nitrogen contained in the coating.

Improvement of adhesion strength of amorphous carbon films on tungsten ion implanted 321 stainless steel substrate

Amorphous carbon (a-C) films have many potential applications due to their good mechanical properties. However, poor adhesion on substrates such as tool steels limits its applications. In the present work, a-C films were deposited on W-implanted (20 kV, 3 × 10 17 ions-cm − 2 ) steel substrates by means of plasma immersion ion implantation and deposition (PIII&D). Compared to the films deposited on untreated steel substrates, the films deposited on the W-implanted steel substrates exhibit improved hardness, surface morphology, and adhesion strength. The modified layer with graded mechanical properties not only provides a structural continuity from the substrate to the film but also acts as the interlayer to mitigate stress concentration under applied loads.

Mechanism for Improvement of Adhesive Strength of TiN Film by Substrate Post Quenching

In our previous studies, it was revealed that the adhesive strength of TiN film deposited on the steel substrate was drastically improved by the substrate quenching after TiN coating. In order to investigate the mechanisms of the improvement of the adhesive strength by such the substrate post quenching, the depth profile of atomic concentrations near the interface between TiN film and the steel substrate was investigated by rf glow discharge optical emission spectrometry (rf-GDOES). It was confirmed that the atomic diffusion at the interface was promoted and the thickness of the diffusion layer increased by the substrate post quenching. A good correlation was recognized between the variation of the adhesive strength with increasing the heating time in the substrate post quenching and the variation of the thickness of the diffusion layer. From these results, it was concluded that the main reason why the adhesive strength was improved was the increase of the thickness of the diffusion layer in the heating process of the substrate post quenching.

ＵＶおよびＴｉＯ２を用いたＡＢＳ樹脂の改質効果

ABS resin has been widely applied as a plating substrate for plastics in applications such as the parts of vehicles and electrical devices. Hexavalent chromium has been applied as an etchant of plating on ABS resin. However, it is required to eliminate hexavalent chromium from the pretreatment process, since the prohibition of these treatments was determined by the EU directive on the End of Life Vehicles (ELV) from July, 2007. Therefore, alternative etching without hexavalent chromium has been investigated by using ultra violet irradiation (UV) and titanium oxide (TiO2) as a photocatalyst. We already achieved adhesion with the strength of about 1.0kNm−1 between ABS resin and deposited metal without hexavalent chromic acid etching. In this study, the mechanism of adhesion strength improvement with photocatalysts has been investigated using ABS specimens and rubber related ABS resin. As the results, the 1,2-polybutadiene distributed in the AS matrix was preferentially reformed and induced hydrophilicity on the reformed surface with the remaining network structure by cross-linking. In addition, we found that the reformed layer was composed a few μm thick and the catalyst penetrated into 30-40nm from the surface. It was concluded that the good adhesion strength between the deposited metal and resin was derived from the nano-anchor effect by the deposition of metal from the inside of 30-40nm of reformed resin.

Strength of Epoxy/Glass Interfaces after Hygrothermal Aging

ABSTRACT The stability of epoxy/glass interfaces subjected to hygrothermal aging was assessed using a fracture-mechanics approach. An epoxy system consisting of diglycidyl ether of bisphenol F cured with 2-ethyl-4-methyl-imidazole was bonded to borosilicate glass adherends that were treated with various types of adhesion promoters to provide a variety of interfaces. Adhesive strength was measured under dry, as-processed conditions and as a function of exposure time to an 85°C/85% relative humidity (RH) environment. As expected, the strain-energy-release rate, G c , dropped significantly with aging time for the bare epoxy/glass interface. The drop in G c is assumed to be due to a loss of interfacial forces. The use of two silane-based adhesion promoters, 3-aminopropyltriethoxysilane (APS) and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane (ECH) resulted in improved adhesive strength both before and after hygrothermal aging. The improvement in adhesive strength can be explained by the introduction of chemical bonds at the interface. The drop in G c is assumed to be due to a loss of interfacial forces and hydrolysis of siloxane bonds. In addition to the use of organosilane-based adhesion promoters, a series of polyhydroxyaminoethers (PHAE) thermoplastic adhesive resins was also investigated as potential adhesion promoters. It was found that 2% PHAE in Dowanol® PM, a hydroxyl-group-containing solvent, was the best system for the PHAE-based adhesion promoters. Interestingly, both the acetic acid concentration in the solvent and maleic anhydride content in the PHAE resin were shown to affect the adhesive strength.

Effect of Carboxymethyl Cellulose on Aqueous Processing of Natural Graphite Negative Electrodes and their Electrochemical Performance for Lithium Batteries

Suspensions of natural graphite particles were prepared in an aqueous medium using carboxymethyl cellulose (CMC) and emulsified styrene-butadiene copolymer latex as part of an environmentally friendly fabrication process for graphite anodes (negative electrodes) intended for application in Li-ion batteries. Suspensions were characterized by adsorption isotherms, electroacoustic measurements, rheology and sedimentation tests, at two different degrees of carboxymethyl substitution (DS) on CMC. A lower DS value (0.7) resulted in greater uptake of CMC on graphite compared with a higher DS value (1.28). This was attributed to attractive hydrophobic interactions associated with the lower carboxymethyl substitution. The greater adsorption for correlates with lower relative viscosity in concentrated graphite suspensions, a higher adhesion strength with a copper substrate, and a greater retention of discharge capacity after cycling. The effect of DS is attributed to differences in the aqueous dispersion properties and stability of graphite suspensions. Based on these results, we fabricated high-capacity graphite negative electrodes characterized by gravimetric and volumetric energy densities of greater than and , respectively. This formulation also led to improved adhesion strength, giving the as-fabricated cell an attractive cycle life greater than 90% of initial discharge capacity after .

Investigation of DLC-Si coatings in large-scale production using DC-PACVD equipment

Probability of practical applications of the DLC-Si coating by DC-PACVD accompanied with the pre-treatment for improving adhesion strength was examined to numbers of small steel specimens with shapes of large-scale production parts. The whole surface of long narrow holes of 15 mm diameter 100 mm long were covered with the DLC-Si coating with enough thickness and high adhesion strength because of the uniform plasma-sheath formation at the inside holes. The coating was done on some hundreds of pins and rings with small spreads of coating thickness, silicon content, and adhesion strength. The process was concluded to have large probability for practical applications in the industry.

Characterization of AlOx/PET Interface by using Sum-Frequency Generation Spectroscopy-Probing Oxide/Polymer Interfaces-

The surface molecular conformation of poly(ethylene terephthalate) (PET) and the interface between alumina and PET has been investigated by sum-frequency generation (SFG). A considerable improvement in adhesion strength between AlOx and PET was achieved by short time Ar plasma modification. The increase in adhesion strength shows good correlation with the increase in SFG peak strength. By depositing AlOx, increase of SFG intensities and appearance of a new peak at 1,685 cm-1 are observed, indicating the formation of a C=O···Al bond at the interface. Adhesion improvement by the Ar plasma treatment is not due to the increase in the C=O···Al chemical species at the interface, but due to the increase in the surface energy followed by the increase in the area of the homogeneous amorphous region of the PET and the creation of the surface hydroxyl groups induced by the plasma pretreatment.