فصلنامه علوم و مهندسی سطح ایران
پیاپی 6 (زمستان 1387)

Surface segregation, whereby one species tend to preferentially move to free surface, is a hindrance phenomenon in obtaining sharp interfaces in nanometer multilayer. In this work, we studied Cu surface segregation during Ni deposition onto Cu substrate by electron beam evaporation in ultra high vacuum (UHV) and also during heat treatment. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and Atomic Force Microscopy (AFM) were used to study the surface in nanometer scale. XPS analysis revealed that the accumulated Cu is uniform over the surface with thickness of one monolayer. By increasing the thickness of Ni deposit, surface segregation decreased and was prevented in thickness of higher than 4 nm. Linear increase of the accumulated Cu was observed by in-situ measurements during heat treatment which is different behavior from micrometer layers. However, when the first accumulated Cu monolayer was completed, the rate of segregation was reduced. In addition, surface energy and surface concentration were computed by measurement of contact angle. Surface energy measurements showed that as the thickness of Ni decreased, surface energy reduced down to the surface energy of Cu.

Thin films of Cadmium Sulfide have been deposited on glass substrates by thermal evaporation technique under the pressure of p= 3 × 10-6 Torr. The rate of deposition and thickness of the films were taken fixed at 3.5nm/s and 550 nm respectively. Some of the CdS specimens were selected for indirect implantation of Copper with different densities.The crystalline structure of the samples was investigated by X- ray diffractometry (XRD). It was revealed that hexagonal structure with [002] plane of reflection at 165 C° is remained the same in CdS films before and after Cu implantation. In other word, doping of Cu atoms do not change the crystal phase, but only enhance the peak intensity at [002] plane. In this paper variation of refractive index and band gap of composites (i.e CdS and CdS:Cu) with substrate temperature and copper densities were studied. It was found that, band gap of samples varies between 2.43 ev to 2.48 ev. Also, that of refractive index variation was between 1.75 to 2.51 ev. The electrical properties of CdS planes un doped and doped with Cu, examined by four point probe in which, the resistivety of CdS from 5.2 × 103 WCm was reduced to 9 × 10-3 WCm by Cu implantation.

The aim of this work was preparation, development and characterization of bioactive glass coating by sol–gel technique for improvement of biocompatibility of 316L stainless steel implant used in dentistry and orthopaedic surgery. Bioglass powder was made by sol–gel technique and thermal properties of the prepared powder were studied using differential thermal analysis (DTA). The prepared bioglass powder was immersed in the simulated body fluid (SBF) solution. Fourier transform infrared spectroscopy (FTIR) was utilized to recognize and confirm of the formation of apatite layer on prepared bioglass powder. Bioactive bioglass coating was performed on 316L stainless steel (SS) substrate by the sol–gel technique. Structural characterization techniques including XRD, SEM and EDX were used to investigate the microstructure and morphology of the coating. Electrochemical potentiodynamic polarization tests were performed in two different types of physiological solutions at 37 ◦C in order to determine and compare the corrosion behavior of the bioglass coated SS and uncoated specimens as an indication of biocompatibility. The formation of apatite layer confirmed the bioactivity of the bioglass powder and tests revealed that all the films signs of bioactivity. It was also found that at sintering temperatures above 900 ◦C, crystalline phase Ca2SiO4 was formed. Crack-free and homogeneous bioglass coatings were obtained with no observable defects. The bioglass coating also improved corrosion resistance of the 316L SS substrates such as the corrosion current density of coated samples in comparison with pristine samples was decreased.It was concluded that the sol–gel bioglass coating can improve the corrosion behavior of dental and orthopedic metallic implants and two goals including improvement of biocompatibility and bone osteointegration can be obtained simultaneously.

Contact angle has an important effect on the dynamic behavior of a droplet during its impact onto a solid surface; this behavior includes deformation, spreading and even rebounding of the droplet from the surface. In this paper, the effect of contact angle during droplet impact is studied and numerically simulated. First, using molecular-kinetic theory and based on the newly available research in this field, a correlation for determining the dynamic contact angle during droplet impact is obtained. This correlation is then implemented in the numerical model used to simulate the process. The results of the numerical model are compared with those of the experiments and those of the previous models (using an equilibrium contact angle; two advancing and receding contact angles). Model predictions agreed well with experimental measurements; this validated the model results for the droplet deformation during the impact. The results show that using the molecular-kinetic theory in modeling contact angle prevents the spreading-and-recoiling oscillations of the droplet that are seen when an equilibrium contact angle is used. Compared to the model with advancing and receding contact angles, the molecular-kinetic theory results in a better prediction of the time and value of the maximum and minimum spread of the droplet on the solid surface. Based on the results of the developed model, a critical maximum spread and two regions of droplet spreading or rebounding from the surface are defined. Next in the paper, an analytical model is presented to study the effect of contact angle on the droplet maximum spread. The results of this model show that for low Capillary numbers (Ca) with an increase of the equilibrium contact angle or a decrease of We number, the maximum spread of the droplet is decreased and becomes less dependent on Re number. In contrast, the maximum spread for high We numbers or low equilibrium contact angles is increased and becomes more dependent on Re numbe

This paper discusses the role of fillers including solid lubricants (graphite and molybdenum disulfide), polytetraflouroethylene and silicon carbide on the tribological behavior of a specific polymer, Phenolic Resin. Polymeric composite material samples are manufactured by compression molding and are evaluated for their friction and wear properties. Using a pin-on-disc apparatus, dry sliding wear behavior under ambient conditions was studied which measured friction coefficient of samples. The counterface was made from hardened steel, AISI 52100. A number of experiments were performed using different filler types and weight percents including: graphite and molybdenum disulfide (10, 15 and 20%), polytetraflouroethylene (5, 10, 15 and 20%) and Silicon Carbide (5 and 10%). In silicon carbide case, two different particle sizes (5, 38) were tested as well. Results showed that addition of fillers with layer structure (graphite and MoS2) decrease the friction coefficient and wear resistance but PTFE powder improves both tribological behaviors. Ceramic filler of SiC exhibited different effect. In this case particle size affects the wear property. Large particles (of about 38) found to protect the polymer matrix better than small particles (of about 5), which partially increase the wear rate.

Co2MnSi Heusler alloy, due to its half metallic properties and its potential applications in spintronic industry has attracted substantial attractions in recent years. Investigation of the interface properties of this alloy with well known semiconductors, such as GaAs, is of special interest. Hence, we studied the electronic and magnetic properties of Co2MnSi/GaAs (001) heterojunction by using spin density functional ab-initio method. The electron core interaction was approximated by pseudo potential method and the exchange correlation energy was calculated within Generalized Gradient Approximation (GGA). We limited our calculation to MnMn-As interface that was constructed by substituting Si with Mn atom. By examining the energy band and density of state curves we found out that a few interface states exist in the minority energy gap. However, the calculated spin polarization is 90%, much higher than the experimental 12% value. Furthermore, the band alignments were extracted for minority and majority spin channels.

In this research the wear behavior of low carbon steel cladded by high Cr- high C electrodes was studied. For this purpose, two types of high Cr- high C electrodes were selected and deposited on the St37 steel by SMAW. Then microstructure, microhardness and wear behavior of the alloyed surfaces was investigated. The results show that high chromium content is not the main reason of wear resistance modifying in clad layer but presence of the optimized composition of chromium and carbide- forming elements would result in the effective increasing of wear resistance. For instance, presence of Nb and Mo in high chromium electrodes, not only produce NbC carbides and (Fe,Cr,Mo)7C3 fish bone type carbides, but also modify the carbides morphology, hardness and wear behavior. The high Cr- high C coatings have outstanding wear resistance and low coefficient of friction under room temperature dry sliding wear test condition.

Thermophysical properties, electrical contact resistance (ECR) and corrosion performance are considered as critical properties of coatings, which are utilized in the satellite manufacturing industry. Chromate conversion coatings (CCCs) for Al alloys are among these coatings.In this study, CCCs were applied on Alclad AA7075-T6 Al Alloy by means of a chromating solution having chemical composition very near to the commercial chromating solution of Alodine 1200S. Influence of coating time on the coatings thermophysical properties including coefficients of solar absorption or Absorptivity (αs) and thermal emission or Emissivity (εT) as well as their ECR values were investigated. The εT was measured directly by Emmisometer device, while αs was determined indirectly by Reflectometer equipment. In coating time from 30 up to 180 seconds αs and εT values were measured in the ranges of 0.7-0.94 and 0.06-0.22, respectively. The results were revealed that different εT and αs as well as desired ECR values according to the specifications of satellite industry are obtainable by control of coating parameters such as coating time. In addition, base on the results of surface coating morphology evaluating conducted by SEM and AFM, it was concluded that developed coatings were formed in nanocrystalized structures.

Aluminum samples were coated by plasma electrolytic oxidation method to achieve 40µm of alumina oxide on their surface. Abrasive behavior with a rotating wheel-type apparatus has been examined with angular alumina and rounded silica abrasives as a function of test conditions, namely wheel-type rubber wheel or steel wheel and dry or wet environment conditions. Water tends to lubricate the contact between the particles and the sample, especially with small and/or rounded particles and thus the wear rate is reduced. With larger particles, the presence of water still affects wear, in that two-body abrasion may be favored, cutting enhanced and particle embedment reduced. The steel wheel tends to produce more fragmentation of abrasives, but in the wet environment, this is reduced as the lubricated contact with the sample results in lower stresses in the particles. The role of water has been shown to be significant in both the rubber and steel wheel tests and affects particle motion and particle fragmentation depending on particle type, shape and size and thus has a strong effect on wear rates and mechanisms observed. The conditions employed in a test used to simulate service conditions must be carefully chosen so as to mimic the latter conditions as closely as possible and the wet or dry environment is a significant parameter that must be considered.