ICMCTF2013 Session G1-1: Innovations in Surface Coatings and Treatments

In the present paper, high density plasma nitriding is proposed as an effective surface treatment for duralumin. This process has a capability to control the RF- and DC-plasmas independently for nitriding. Different from the conventional nitriding processes, the matching between input and output powers is automatically adjusted with the shorter response time less than 1 ms. This enables us to temporally control and describe the plasma state by in-situ plasma diagnosis.

First, this plasma diagnosis was instrumented to search for optimum processing condition to plasma nitriding the duralumin alloys of type A2011 as well as A2014 and A2017. RF-voltage, DC bias, carrier gas pressure and nitrogen-hydrogen gas flow ratio are determined to maximize the peak intensities of activated nitrogen atoms and NH radicals in the spectroscopic measurement. In the case of plasma nitriding where RF-voltage was 250 V, DC-bias, 400 V, pressure, 40 Pa and the gas flow-rate, 80 to 20 %, the surface hardness of A2011 alloy sample increased to 800 Hv even for 7.2 ks at 753 K. Both XRD and SEM were utilized to investigate the formation of nitrided layer with AlN precipitates. Micro-hardness testing was employed to describe the hardness distribution in the cross-section from sample surface to matrix hardness of 100 Hv. Nitriding behavior was discussed on the basis of diffusion theory, assuming that formation of AlN should take place at the nitriding front. Furthermore, XPS was employed to investigate the nitrogen binding state and to discuss the unbound nitrogen atom concentration.

This nitriding process was applied to automotive parts; i.e. a heat sink. Wearing toughness and strength as well as high thermal conductivity were required for this part. After plasma-plasma nitrding for 14.4 ks at 753 K, fins of heat sink with the thickness of 1 mm and the height of 10 mm were homogeneously nitrided.

Because of their unique properties, most recently, thin film metallic glasses (TFMGs) have been studied for various applications. In this study, a Zr_50.3Cu_28.1Al₁₄Ni_7.6 (in at. %) thin film metallic glass is grown by RF magnetron sputtering system on silicon substrate, followed by nanoindentation conducted at room temperature. It is found that indentation of TFMG creates deformation and stress under the indenter. However, the indented area is able to recover to some extents by heating to temperatures within the supercooled liquid region (SCLR) due to surface tension-driven viscous flow as well as thermally-induced structural relaxation. Accordingly, Zr_50.3Cu_28.1Al₁₄Ni_7.6 TFMG is annealed at different temperatures in SCLR and, by using the atomic force microscopy, the indentation depth recovery is evaluated and the results will be discussed in this presentation.

Key Words: Thin film metallic glass, Indentation depth recovery

Growth defects are inherently present in PVD coatings. In an ideal laboratory environment, the growth defect density can be greatly reduced if proper care is taken. First one has to assure that the deposition chamber is clean, without residues from previous depositions. As many defects originate from the substrate, careful substrate preparation suppresses the defect formation considerably. Yet another effective move is to run the deposition at low power, or modest conditions in general, to prevent arcing, overheating and other defect-contributing mechanisms.

In industrial deposition of PVD coatings on the other hand, these measures are limited by technical and economical constrains. Chamber cleaning procedures have to be reduced to an acceptable level. Substrate conditions are up to the customer, which the coater can only partly influence – the substrate material is defined, while the polishing technique is also optimized to an acceptable level. The deposition is conducted at full power thus a certain degree of arcing has to be taken into account.

In summary, the growth defects in industrial PVD coatings are a fact. They are known to deteriorate the corrosion resistance because they can act as shortcuts for the corrosive media to penetrate down to the substrate. The influence on the tribological properties is more dificult to evaluate, however, a ruptured nodular defect is likely to enable cracking and local delamination of the neighbouring coating. For these reasons there are relevant questions regarding growth defects: what is their size, their internal structure, their origin; and how these properties influence the coating lifetime.

While the growth defects are easily visible on SEM, or even on a better optical microscope, it is much more difficult to extract any information on the individual defect properties (except of its size). Cross-section SEM can help, but only if the fracture exactly passes the growth defect, which is quite improbable. In addition, only one slice through the defect – or more likely around the defect is visible. Therefore a targeted cross-sectioning technique is necessary. This aim is well fulfilled by the focused ion beam technique (FIB), which is typically integrated into a SEM. After examination of the coating surface, a suitable growth defect is selected. A slice accross the defect is then made, or a series of slices, followed by standard SEM observation. Another step is to use these SEM micrographs for a 3D reconstruction and visualization of the growth defect. This step will be the topic of the presentation.

The aim of our research was to investigate the influence of different application technologies (PVD or/and PECVD) and process parameters (target voltage, middle frequency voltage, gas mixture, pressure) on the nucleation and rate of defects within DLC coatings.

The coatings have been characterized by means of nano-indentation a scratch testing to address hardness and wear resistance, respectively. This paper is limited to the results concerning corrosion. The corrosion resistance has been analyzed by salt spray testing. By means of metallographic investigations (light microscopy, SEM) the mechanism of the degradation process due to corrosion was investigated. Additional characterization has been performed to assess the ratio of sp² vs. sp³ configuration of the carbon and thus to realize the influence to the corrosion resistance.

From our study is concluded that by combined PECVD and PVD processing, the best performance regarding corrosion resistance could be achieved.

Cold shield is used to improve the image quality and determine the f number of QWIP (Quantum Well Infrared Photodetector) cooled infrared detectors. Cold shield reduces the absorption of photons hitting the highly reflective outer surface; on the other hand, it provides absorption of photons hitting the highly absorbing inner surface. Thus, photons coming from the object to be displayed are directly passed through the opening of the cold shield, placed over the detector; which prevents absorption of the undesirable photons. Cold shields are produced by electroforming method to minimize thermal mass. Production of a 70 μm thickness self-standing cold shield, with the bright gold plated outer surface was aimed in this study.

Very often, the attacks which materials undergo come directly from surface phenomena such as corrosion, wear or other tribological damages. Thus, instead of using very expensive materials or imagining brand new materials, it is interesting to modify the surface properties of materials while maintaining mechanical properties of the bulk.

Among the great number of existing surface treatments, laser surface melting is a current process that keeps on developing because of the recent progress in the technology of lasers. The originality of this process stems from the modification of the chemical composition through the thickness without any weak interfaces, as may be the case with coating processes requiring several powders to obtain material with gradient composition.

This treatment consists in focusing a nanopulsed laser beam on the surface of the material, leading to the rather immediate melting of the surface through a micron depth, immediately followed by an ultra-fast solidification occurring with cooling rate up to 10¹⁰ K/s.

The combination of these processes leads to:

-the elimination of surface defects

-the formation of metastable phases

-the chemical segregation mechanism

-a structural refinement.

The effect of surface melting treatment on the corrosion behaviour of a widespread AISI 304L stainless steel was explored using a nanopulsed laser with high frequency, contributing to the originality of this work. The main goal is to correlate the surface modifications with the laser beam overlap parameter in order to monitor the corrosion resistance of the material. Different techniques such as scanning electron microscopy, X-ray diffraction, glow discharge optical emission spectrometry, and profilometry were used to characterize the global laser-melted surface.

But the laser radius of the beam equals 70 µm and shows a Gaussian energy distribution. Consequently, we would expect a distribution of the physico-chemical properties at the scale of the laser beam. Consequently, the characterization and the study of the distribution of the physico-chemical properties at a micro, or nano-scale are indispensable for understanding the laser-matter interaction which is extremely local and brief. Thus, we chose to characterize physico-chemical modifications by more local investigations such as transmission electron microscopy, micro-raman and X-ray photoelectron spectroscopy.

The best treatment led to an increase of the pitting potential by more than 500 mV, corresponding to a great improvement of the corrosion resistance. It was correlated to chromium enrichment, to the extinction of martensite and ferrite peaks and to the reduction of harmful inclusion density.

Mica, which is chemically inert, insulating, dielectric, hydrophilic, light in weight, elastic/resilient/flexible, reflective, and refractive, is an extremely useful material. Its chemically inert nature facilities it use as an important substrate for epitaxial vapor deposition, including the growth of graphene [1]. Apart from a diverse applications including an antigen for γδ T cells, layered insulators, the space for the possible origin of life, due to its very thin structure, few-layer mica sheet will be used in electrical components, electronics, isinglass, and atomic force microscopy. Furthermore, with its larger surface-to-volume ratio, functionalizing their surface will greatly extend the range of applications. Accordingly, the fabrication of 2D nanosheets with few mica layers will provide enormous interest to the world-wide scientists and engineers community.

Since the thin 2D materials become thermodynamically unstable below a certain thickness, the possibility of preparation of free-standing atomic layers has been intensively disputed so far. Recently, Novoselov et al. have used the mechanical cleavage method to show the separation of single- or few-layered graphene. In this work, we have demonstrated the fabrication of single-layered and few-layered mica [muscovite (KAl₃Si₃O₁₀(OH)₂)] nanosheets not only by means of the mechanical cleavage method, but also by the solvothermal method followed by microwave irradiation. In the solvothermal method, tetrahydrofuran (THF) organic solvent containing potassium hydroxide (KOH) was used for convenient and efficient exfoliation process. Following this, the single-layer or few-layer mica sheets were obtained by the microwave irradiation, which facilitates mass production in a short time with little cost and energy. In addition, in order to reveal the structural/chemical changes upon the exfoliation, we have compared the X-ray diffraction, Raman, and X-ray photoelectron spectroscopy spectra of the expanded mica nanosheets with those of the unprocessed mica powders.