Studies of microhardness and mar resistance using a scanning probe microscope

Scanning probe microscopy (SPM) is in a period of rapid development. It shows great promise for characterizing coating surfaces. This paper describes modification of an SPM so that it can be used to mar the surfaces of coatings under controlled conditions and to characterize the mars. Mar resistance of coatings is analyzed in terms of a ‘three response, two mechanism model.' The three responses (fracture, elastic, and plastic) can be measured quantitatively using the SPM. Of the three responses, only two (fracture and plastic deformation) are marring mechanisms – elastic deformations recover instantaneously. In some cases mars resulting from plastic deformation may recover slowly with time or with immersion in water; this phenomenon is attributed to viscoelastic creep. Microhardness is also measured with the modified SPM. Some thermoset coatings appear to be substantially harder near their surfaces than in the mass of material, and such materials may respond quite differently to stress applied at different levels near the surface. This finding has important implications for all coating properties that are strongly influenced by the surface. A quantity called ‘micro mar resistance' is defined. It may be useful for comparing different coatings under specified conditions of marring. However, there can be no single quantity that expresses ‘mar resistance' of a coating under all conditions.     

高匹配焊接接头裂纹的J积分研究

本文研究了含有纵向裂纹或缺陷的高匹配焊接接头材料不均匀性和焊缝宽度对弹塑性断裂表征参量J积分的影响规律.高匹配焊缝被理想化为含有一平行裂纹的一个硬夹层,其屈服应力高于母材.在平面应力条件下利用弹塑性有限元计算得到数值解.研究结果表明,材料不均匀性和焊缝宽度对J积分值有显著影响,特别当名义应力或名义应变较高时影响更大.因此,估算高匹配接头的裂纹推力时必须对此加以考虑.     

Measurement of mar resistance and study of marring mechanism of polymeric coatings with scanning probe microscope

A scanning probe microscope (SPM) equipped with a custom-made probe, consisting of a diamond tip and rectangular tungsten cantilever, was used to measure the mar resistance of crosslinked polymeric surface coatings at micron and submicron scales. The term “mar” is used to describe the surface damage of coatings, which may not be readily noticeable individually, however, the existence of many mars does degrade the appearance of coatings. Good mar resistance is a requirement for many coatings applications. With the unique high resolution of the SPM, the dimension of the mars can be measured with great accuracy, thus different responses of coatings to the marring stress, i.e., elastic recovery, plastic deformation, and abrasive wear, can be identified quantitatively. In addition, the dynamic process of marring, including viscoelastic creep, strain-hardening, micro-cracking, and surface fatigue, has also been studied with the SPM. Presented at the 77th Annual Meeting of the Federation of Societies for Coatings Technology, on October 18–22, 1999, in Dallas TX. 313 Strong Hall, Ypsilanti, MI 48197.     

Cu/Ta/SiO_2/Si多层结构的纳米压痕研究

利用磁控溅射方法在Si衬底上沉积钽层以及铜层,并利用纳米压痕技术对Cu/Ta/SiO_2/Si多层膜结构进行了硬度和弹性模量的表征,研究发现多层膜结构的硬度随着薄膜厚度的增加而降低,然而弹性模量与膜厚之间并没有这样的关系。利用聚焦离子束（FIB）工艺将纳米压痕区域剖开,并通过透射电子显微镜（TEM）表征发现在纳米压痕过后,钽以及二氧化硅界面有了明显的分层现象,这一点表明层与层之间较弱的键合在相对大的负荷下遭到了破坏。     

Study of tribological properties of coating/substrate system in micrometer and nanometer scales with a scanning probe microscope

We have used a scanning probe microscope equipped with a custom made diamond tip to study tribological properties of an inorganic–organic hybrid Si, O, H, and C coating produced by plasma enhanced chemical vapor deposition (PECVD) on siloxane/acrylic/polycarbonate multilayer substrates and on glass substrates. The micro indentation hardness and micro mar resistance were measured under different normal forces, and the critical loads for cracking, delamination, and chipping were evaluated. The effects of substrate, coating thickness, and interfacial adhesion on tribological properties of the coating/substrate systems are discussed. The results show that increasing coating thickness and strengthening interfacial adhesion can effectively inhibit cracking, delamination, and chipping of the coating/substrate systems under wear. Improving the physical properties of the PECVD coating and substrate, such as enhancing elastic recovery, reducing plasticity and brittleness, and matching the properties of coating and substrate better can improve the wear resistance of the systems further.     

Novel synthesis of carboxy-functional soybean acrylic-alkyd resins for water-reducible coatings

A new process was developed for synthesis of alkyd resins in which a conventional monoglyceride is reacted with a carboxy-functional acrylic copolymer. The novel products are called acrylicalkyd resins. The carboxy-functional acrylic copolymers were synthesized by solution-free radical polymerization. Gelation during alkyd resin synthesis was avoided by: (1) limiting the molecular weight of the acrylic copolymers to M_n 3500–5000 and (2) limiting the number average functionality about 6.5–10 carboxyl groups per molecule. Further, the carboxyl groups were derived from a mixture of acrylic and methacrylic acids (1/1.2 mol ratio) in the expectation that this would help control the process. Three series of acrylic copolymers were prepared from various combinations of acrylic monomers and reacted with a monoglyceride prepared from soybean oil and trimethylol propane. The composition of the acrylic resin was adjusted to minimize phase separation [observed visually and by scanning probe microscopy (SPM)] within cast films. The most satisfactory results were obtained with copolymers of 62–71 wt% of methyl methacrylate, 5–21 wt% of lauryl methacrylate, 7.2 wt% of acrylic acid, and 10.3 wt% of methacrylic acid. In preliminary tests, waterborne coatings made from acrylic-alkyd resins based on these acrylics had excellent stability, with acid numbers changing less than 10% after nine months of storage. Coatings Research Institute Ypsilanti MI 48197 Department of Physics, Ypsilanti, MI 48197.     

QUANTITATIVE EVALUATION OF ADHESION STRENGTH OF INK DEPOSITED ON PLASTIC FILMS WITH A NANO INDENTER AND A SCANNING PROBE MICROSCOPE

As our lifestyles change, the convenience food industry booms. Many convenience food products use flexible, boilable, and microwavable plastic packaging. Almost every package is printed with ink, promoting what is inside the package, listing ingredients and nutrition data, giving preparation instructions, etc. The adhesion of ink on plastic films merits increasing attention to ensure quality packaging. However, this property has not been systematically studied and lacks a scientific method to measure the adhesion strength quantitatively. We are developing a technique of using a Nano-Indenter and a Scanning Probe Microscope to evaluate the adhesion strength of ink deposited on plastic films. It shows promise, and the measurement will be helpful in better understanding the adhesion mechanism, thus giving direction as to how to improve the adhesion.     

High‐solids mar resistant clearcoats prepared from an isophthalate‐based oligoester and a melamine resin. Study and characterization of mar resistance with scanning probe microscopy method

A new series of high‐solids [low‐VOC (volatile organic compound)content] mar resistant clearcoats (CL‐series) were prepared upon crosslinking of a new‐synthesized hydroxyl‐terminated isophthalate‐based liquid oligoester (L‐311) with an hexakis(methoxymethylol)melamine (HMMM) resin, via an acid‐catalyzed etherification reaction. The chemical, physical, and mechanical properties of the CL‐clearcoats were compared to those of a reference clearcoat (CRO). An attempt was made to investigate the effect that the amounts of catalyst and melamine resin had on the clearcoats' physical, chemical, and mechanical properties, and moreover, to correlate these properties to the films chemical structures. The new‐formulated high‐solids clearcoats (CL‐series) presented enhanced processability and higher NVW values (lower VOC content) than the reference clearcoat CRO, and their properties (pencil hardness, knoop hardness, adhesion, impact resistance, solvent and gasoline resistance, mar resistance, and flexibility) were better or comparable to those of the CRO. More specifically, the mar resistance of the CL‐clearcoats series was investigated by applying both the crockmeter test and a novel method that employed a modified scanning probe microscope. In addition, we identified and characterized the different responses of the CRO and certain CL‐clearcoats to marring stress. The experimental data regarding the mar resistance of the new‐formulated clearcoats fully justified our polymer design strategy, verifying our expectations for the possibility of preparing glossy, high‐solids mar‐resistant clearcoats that could present enhanced processability and solvent resistance, relatively high pencil hardness, and at the same time very good elastic recovery to marring stress. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1317–1333, 2002     

Piezoelectric inkjet printing of medical adhesives and sealants

Piezoelectric inkjet printing is a noncontact process that enables microscale processing of biological materials. In this research summary, the use of piezoelectric inkjet printing for patterning medical adhesives and sealants, including a two-component polyethylene glycol hydrogel-based medical sealant, an N-butyl cyanoacrylate tissue adhesive, and a mussel adhesive protein biological adhesive, is described The effect of Fe(III) on mussel adhesive protein structure was evaluated by means of atomic force microscopy. The ability to process microscale patterns of medical sealants and adhesives will provide an improvement in tissue joining, including enhanced tissue integrity, reduced bond lines, and decreased adhesive toxicity. Piezoelectric inkjet deposition of medical adhesives and sealants may be used in wound closure, fracture fixation, and microscale vascular surgery.     

Clutch Shudder Correlated to ATF Degradation through Local Friction vs. Velocity Measurements by a Scanning Force Microscope

Shudder in continuously slipping wet clutch (CSWC) systems is an instability that manifests itself as a vibration of the automobile's drive train. Dynamometer experiments imply that the shudder is connected to degradation of the frictional properties of the clutch friction material-automatic transmission fluid (ATF) system. The poor friction characteristics are revealed in negatively sloped torque vs. slip-speed curves which can result in dynamic friction oscillation instabilities. The authors report local measurements of friction vs. velocity with the use of a scanning force microscope (SFM) on new and glazed wet clutch friction material with either new or degraded ATF acting as a lubricant. The friction material was a commonly used cellulose/phenolic resin material. Results in the very low-speed dynamic regime indicate that the fluid condition plays a critical role in determining the slope characteristics of the friction vs. velocity curve. ATF degraded in service imparts a flat or negative slope to the friction vs. velocity curve, whether on new or glazed (degraded) friction material. A positive slope is measured for fresh ATF. The SFM measurements are therefore consistent with dynamometer and vehicle experiments which measure gross properties of the friction clutch system. The ability of the SFM to reproduce the dynamometer results on a local scale implies that shudder is not due to the gross fluid flow or mechanical effects that are present in a complete torque converter. The frictional characteristics of the clutch plate materials and the ATF are paramount. This technique allows continuous monitoring of ATF fluid degradation in test fleet vehicles without ATF replenishment since only a few ccs of fluid are necessary for these tests.