Surface engineering consists of a variety of processes and sub processes. Included for each topic are tribological performances of each process as well as recent research findings. The readers will also benefit from in-depth studies of tribology of thermal sprayed coatings, nano composite films and diamond films for wear resistance, diffusion treated surfaces, hard facing for wear erosion and abrasion, plating for tribology, laser surface modification for protection against wear and surface engineering for biotribology.
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Show search form. Surface engineering for enhanced performance. Laser assisted deposition of high wear resistant overlay on valve seat Laser assisted deposition of high wear resistant overlay on valve seat Laser heat treatment The laser beam can be used as the heat source for the surface heat treatment of some metals. The improvements seen in flux were found to be repeatable, as observed from the systematic investigation of a broad range of power density conditions, on separate modified membranes. Therefore, as all the individual membrane samples subjected to plasma modification have the same fabrication steps, flux and salt rejection changes were consistently altered including improved compared to the control samples.
Surface roughness was analyzed to evaluate the physical impacts of plasma on the morphology of the membranes caused by competing etching and re-deposition mechanisms within different plasma regimes. The modified membranes which showed enhanced flux exhibited a sharp drop in roughness measured by AFM - consistent with the flattening and fusing of protrusions observed during SEM investigations Fig. Each value from roughness represents the mean of three measurements in the sample associated with their estimated standard error.
As seen in Figs 2A and S1, increasing power density resulted in different types and varying levels of surface texturization.
This trend is supported by SEM analysis, where plasma treated membranes were compared to an untreated control membrane. The morphology of the control membrane was found to be irregular and rough that was previously related to the fabrication of these materials, which is carried out by interfacial polymerization IP. During IP of PA, diffusion of the diamine monomer molecules across the nascent thin-film towards the organic phase was shown to produce cross-linked protrusions, giving rise to a micron-scale rough surface morphology, characteristic of these types of membrane An approximate quantification of the level of texturization was obtained from measuring the average roughness Ra for the samples, which were calculated from AFM maps presented in Figure S2.
The calculated values were plotted in Fig. The sharp drop in roughness is consistent with the flattening and fusing of protrusions observed during SEM investigations. The FWHM values provide additional information related to the uniformity of the morphological changes. This broad range of peak heights is indicative of an irregular morphology, as expected for the unmodified control membrane material.
In Fig. Furthermore, the smooth AFM profiles could potentially be associated with surface thinning due to concurrent material removal and re-deposition mechanisms On the other hand, the permeation decline for the longer treatment durations are likely caused by re-deposition mechanisms which could have led to a reduction of the free volume across the PA Therefore, the flux enhancement at low power densities was also shown to be associated with chemical reconfiguration which contributed to surface energy alterations. Although not investigated within the scope of the present work, as well as the increased flux measured in this work, another possible benefit of smoother, more uniform surfaces, could potentially improve permeability by reducing interactions with colloidal matter, as previously observed 3 , Further chemical analyses were therefore performed to better understand the nature of the surface property changes.
Plasma etching typically involves bond scissions and re-deposition mechanisms across materials and may significantly change the surface energy of the top layers of the membranes Chemical analysis using XPS C1s deconvolution was performed to investigate the chemical reconfiguration at the molecular level associated with etching mechanisms across the PA surface structure Carbon bonds are the most vulnerable sites, for instance, argon excited species exhibit high dissociation bond energies in the order of XPS on the control membrane presents the C1s peak at Another peak at The peak at Figure 3A , shows that the peak at It is most likely that C-O bonds were cleaved in the process because these groups are commonly found in the outermost regions and potentially associated with PA by electrostatic bonds, as discussed elsewhere 44 , The native control membranes are multi-layer composite materials and the active top layer is generally primed at the nanoscale with preservative materials These materials, although undisclosed by manufacturers, involve chemistries based on ester or anionic surfactants as determined by FTIR or XPS analysis 44 , The resulting surface reconfigurations may also be associated with rearranged ionized molecular groups from the material etching and re-deposition processes caused by the plasma glow.
These trends suggest that the nascent amide groups at However, the possibility of simultaneous re-deposition mechanisms, which potentially rearranged and re-attached along with vaporized materials, cannot be dismissed. These peaks were found at Furthermore, complex alterations in the aromatic structures were found to occur for the most at Analysis of plasma functionalization mechanisms with XPS C1s high resolution of atomic ratio A peak ATR-FTIR analysis was used to investigate the etching mechanisms occurring during these different plasma regimes by considering the presence and alteration of the nascent preservative materials.
Furthermore, the scope of this analysis is to demonstrate that etching intensity can not only be influenced by plasma parameters, but also by the nature of the substrate. Membranes subjected to plasma treatments were not washed prior to and after plasma treatment in order to preserve initial surface chemistry and the real impact of etching during the plasma process.
This particular band is sharp and well highlighted across the spectra and is easily removed upon washing, indicating the preservative materials. This simple removal, contrasted with the other bands, was therefore used as a control for further testing at different stages of the treatments. Therefore, this result indicates that the etching was not intensive enough to completely remove the as supplied preservative material. Such preservative layers may have shielded the PA internal layers and limited the depth of penetration of the glow.
As the preservative materials were removed upon etching, mechanisms related to re-deposition processes may have led to the incorporation of oxygen containing species - as previously discussed in the C1s deconvolution in the XPS analysis. However, in the FTIR analysis no alterations were detected, particularly in the carboxylate region. Similar results were previously obtained with argon plasma treatment on PES membranes where the XPS C1s deconvolution showed the presence of carboxylic acid groups.
However, the carboxylate region in the FTIR analysis was not necessarily detected The amount of sulphur is normally found at 0. XPS elemental analysis shown in Fig. A strong correlation was found between sulphur increase and salt rejection decline, which can also be seen in Fig. The rate of increase was found to be larger with increasing excitation power suggesting stronger etching of the PA layer at higher power, which could lead to degradation or thinning of the material as previously discussed in morphology analysis.
This was well-correlated to loss in selectivity and is in agreement with XPS C1s findings which demonstrate that at high power density, etching and re-deposition mechanisms are intensified. This correlation indicates physical degradation of PA layer caused by plasma etching which was strong enough to reveal the poly sulfone between the tortuous profile of the PA layer and, therefore, likely local rupture at the nanoscale of the PA layer The displayed data represents the mean of five replicates associated with their estimated standard deviations.
The chemical reconfiguration and etching effects detected by XPS and FTIR analysis also suggests potential alterations of the surface energy, which is caused by functionalization reactions. A pertinent assessment of surface energy is presented in the following section. Streaming potential measurements were performed to characterize the surface charge of membrane surfaces by detecting outer layer functionalization with polar groups and measure the density of hydrophilic ionizable moieties Figure 6 shows an increased negative surface charge, which indicates a shift towards the alkaline pH range for all plasma treated membranes.
Such an effect is commonly achieved during plasma modification of polymers that is attributed to the effect of UV radiation The presence of an IEP point may indicate that a low density of hydrophobic species was also re-deposited due to the plasma treatment Furthermore, the IEP of these membranes was found to lie at 3.
Liquid wettability indicating hydrophilicity and hydrophobicity properties are influenced by chemistry, surface charge and surface morphology. Figure 7 shows the wettability of the plasma modified surfaces was investigated with measurements of contact angle before and after plasma treatment. The hydrophilicity was significantly increased for all plasma treated membranes. The hydrophilicity increase detected by the reduced water contact angles can be attributed to chemical reconfigurations with polar species as previously discussed within the XPS and streaming potential sections However, the hydrophilicity enhancement promoted by plasma treatment, showed to be influenced by smooth surfaces Therefore, enhanced hydrophilicity associated with negative charges also suggests that such chemical modifications may potentially lead to less interaction with hydrophobic contaminants.
For instance, a plasma modified nanofiltration membrane showed lower adsorption of bovine serum albumin BSA compared to an untreated membrane, where improvements were attributed to a decreased contact angle to approximately Although studying the interaction of the membranes with colloidal or organic matter was not considered in this study, a reduction of the interaction between the membranes and such contaminants would be another important benefit of the technique and could be the subject of future studies.
The displayed data represents the mean of three replicates associated with their estimated standard deviations. The stability of the water contact angle values was assessed on the plasma modified membranes after 5 months of storage, and is shown in Fig. The results indicated strong wettability decay at lower power density over time, related to storage and exposure to air. Similar investigations from the literature reported hydrophobicity recovery upon radio frequency RF plasma treatments of polymeric surfaces The reorientation of the polar groups on the surface towards the bulk phase of the polymer is favoured by the cleavage of bonds across the original polymer chains by the plasma, consequently increasing their mobility Wettability decay in plasma treated polymers is well known and the hydrophobicity recovery mechanism is purely chemical However, it is important to consider that, practically, RO membranes would be continuously subjected to water, which would potentially slow down the wettability decay.
Therefore, the combination of negative charges associated with increased hydrophilicity have been attributed as key surface properties for flux improvement in modified membranes 31 , The zeta potential of polymers is also strongly affected by their surface roughness.
Towards Enhanced Performance Thin-film Composite Membranes via Surface Plasma Modification
The roughness value for this membrane was shown the highest compared with all modified membranes Fig. Roughness leads to geometry-induced changes, which can affect electroosmotic flow and zeta potential values This may indicate re-deposition and cross-linking of macromolecules with more hydrophobic characteristics These chemical changes may also contribute to potential anti-fouling properties on the PA material where further long-term tests are required 8 , By examining surface properties obtained after plasma treatment and comparing flux and salt rejection performance, the conclusions have been diagrammatically represented in Fig.
Lower power densities were found to promote chemical changes whilst higher power densities resulted in surface physical alterations. However, the uniformity of chemical changes was not able to be evaluated. Plasma etching and re-deposition mechanisms are intensified with increasing plasma power and the more materials were removed, the more were re-deposited; this was confirmed by XPS C1s, FTIR and elemental XPS analysis;. Surface thinning for flux improvement was also likely due to concurrent material removal and re-deposition mechanisms;. This is also is associated with a potential increase of free volume that physically affects the surface, such as potential thinning or formation of defects across the PA layer, also detected with an increased content of elemental sulphur by XPS analysis.
These etching and re-deposition mechanisms, valid for the different plasma regimes, led to an empirical understanding, in terms of permeation associated with surface morphology and chemical configuration. The present work opens new avenues for modification of membrane surfaces for custom applications in liquid purification and desalination, which could be harnessed beyond the scope of water treatment applications.
For example future work could examine interactions with organic contaminants in solutions for the development of low fouling membrane materials. Milli-Q water was used for the preparation of all aqueous solutions. Plasma activation treatments were performed using a low pressure plasma system from Diener Plasma Surface Technology model Pico-RF-PC that was coupled to a radio frequency RF generator operating at Analytical grade Argon gas was injected into the 7.
The morphology of the modified surfaces was evaluated by scanning electron microscopy. Atomic force microscopy provided high-resolution information on the roughness and other morphological changes induced by the plasma treatments.
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The samples were cut and mounted, using double sided tape, on a magnetic support. The resonance frequency was determined using an automated routine of the control software Igor Pro 6. Data sets were collected using a scan speed of 0.
Water contact angle measurements were acquired on a Biolin Scientific goniometer to evaluate macroscopic variations of both chemical and morphological characteristics of the PA surface layers. Prior to contact angle measurements, the membranes were dried in air overnight.
The surface charge of the modified membranes was evaluated in terms of the streaming potential with a Surpass Anton Paar, electro kinetic analyzer EKA with Visiolab software version 2. The streaming channel gap was set at approximately 0. Both conductivity and pH were systematically recorded to calculate the specific salts adsorption across the materials surfaces as function of pH.
An average value of the zeta potential was calculated based on four repeat measurements obtained by inverting the flow direction of the solution across the cell. Attenuated total reflection - Fourier transform infrared spectroscopy ATR-FTIR tests were performed to investigate the chemical surface groups of the membranes and to analyze the potential surface reactions induced by the plasma treatments. Eight spectra were averaged and analyzed by means of OPUS 7. X-ray photoelectron spectroscopy XPS was utilized for elemental surface characterization. The technique was able to detect elements with a detection limit of 0.
The peak position was calibrated using the C1s peak at In order to investigate the representative desalination performance of the membrane materials, water permeation measurements in model saline water were performed. Salt rejection and water permeation performance were tested accordingly to as reported elsewhere and the manufacture The error bars of flux and salt rejection for both control and plasma modified membranes corresponded to the standard error of the mean. The mean values of the plasma modified membranes were obtained from four replicates of each sample.