Cr(N,C) diffusion coating formation on pre-nitrocarburised H13 tool steel

The microstructural processes of Cr(N,C) coating formation by thermoreactive deposition and diffusion (TRD) on pre-nitrocarburised H13 tool steel were studied. Both nitrocarburising and TRD were performed in fluidized bed furnaces at 570 °C. During TRD, chromium was transferred from chromium powder in the fluidized bed, to the nitrocarburised substrates by gas-phase reactions initiated by reaction of HCl gas with the chromium. Addition of 30% H2 to the input inert gas was found to increase the rate of coating formation, although hydrogen reduction resulted in rapid loss of nitrogen to the surface. The reason for the increased rate of coating formation could not be established without further investigation, although several possible explanations have been proposed. It was found that porosity and the formation of an iron nitride ‘cover layer’ during nitrocarburising were the biggest influences on the microstructure of the Cr(N,C) coating. Microstructural characterization of the coatings was performed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and glow discharge optical emission spectroscopy (GDOES).

Characterisation of galvanneal coatings on strip steel

Galvanneal is a form of zinc-coated sheet steel, where steel is dipped in molten zinc, and then heat treated in a furnace to produce a complex iron-zinc coating. Many industries, such as automotive, use galvanneal for components fabricated from sheet steel. The microstructural properties of galvanneal have a significant influence on how well the sheet metal changes shape on stamping. By means of optical microscopy, scanning electron microscopy, and glow-discharge optical emission spectrometry, we present a study of the microstructure of several galvanneal samples, both stamped and unformed, relating the phases and morphology of the coatings to performance in stamping operations. Samples of galvanneal were subjected to different heat-treatment temperatures. The frequency of defects in stamped components was found to be related to the average alloy content in the coatings, which varied with furnace temperature. An increased average iron content in the coatings was related to increased powdering defects in stamping operations that use galvanneal coated sheet steel.

Ammonia dissociation in the fluidised bed furnace

Ammonia dissociation is the controlling reaction for several important thermochemical heat treatment processes; nitriding, nitrocarburising (ferritic and austenitic) and carbonitriding. The fluidised bed furnace is a convenient and widely used medium for all of these treatments, yet understanding of the reaction in a fluidised bed context is minimal. This paper deals with the influence of process parameters on nitrogen activity aN; temperature, fluidising flowrate, ammonia inlet level, carbonaceous gas. Two basic behaviours were observed; inlet NH3-dependant and inlet NHr insensitive, with a transition region at intermediate temperatures. The nitrocarburising response of steel specimens was measured by optical microscopy of the layer thicknesses and glow discharge optical emission spectroscopy (GD-OES) determination of nitrogen depth-penetration profiles. aN was found by gas analysis of the exit stream ammonia with the aid of a dissociation burette.

Interaction between the coating characteristics of galvanneal steel and forming parameters during flat die drawing

Galvanneal steel is considered to be better for automotive applications than its counterpart, galvanized steel, mainly because of its superior coating and surface properties. Galvanneal steel is produced by hot dipping sheet steel in a bath of molten zinc with small, controlled, levels of aluminium, followed by annealing which creates a Fe-Zn intermetallic layer. This intermetallic layer of the coating improves spot weldability and improves subsequent paint appearance. However, if the microstructure of the coating is not properly controlled and forming parameters are not properly selected, wear of the coating could occur during stamping. Frictional sliding of the sheet between the tool surfaces results in considerable amount of coating loss. An Interstitial Free steel with a Galvanneal coating of nominally 60g/m2 was used for the laboratory experiments. Flat Face Friction (FFF) tests were performed with different forming conditions and lubricants to simulate the frictional sliding in stamping. Glow-Discharge Optical Emission Spectrometry (DG-OES) was used to measure the change in the coating thickness during sliding. Optical microscopy was considered for imaging the surfaces as well as an optical method to compare the changes in the coating thickness during the forming. The change to the Galvanneal coating thickness was found to be a function of forming parameters.

Ferritic nitrocarburising of tool steels

Four different tool steel materials, P20, H13, M2 and D2, were nitrocarburised at 570°C in a fluidised bed furnace. The reactive diffusion of nitrogen and carbon into the various substrate microstructures is compared and related to the different alloy carbide distributions. The effect of carbon bearing gas (carbon dioxide, natural gas) on carbon absorption is reported, as well as its influence on compound layer growth and porosity. Partial reduction of Fe3O4 at the surface resulted in the formation of a complex, epsi-nitride containing oxide layer. In H13, carbon was deeply absorbed throughout the entire diffusion zone, affecting the growth of grain boundary cementite, nitrogen diffusivity and the sharpness of the compound layer: diffusion zone interface. When natural gas was used, carbon became highly concentrated in the compound layer, while surface decarburisation occurred with carbon dioxide. These microstructural effects are discussed in relation to hardness profiles, and compound layer hardness and ductility. The surfaces were characterised using glow discharge optical emission spectroscopy, optical and scanning electron microscopy and X-ray diffraction.

Improvement of adhesion of conductive polypyrrole coating on wool and polyester fabrics using atmospheric plasma treatment

In this paper wool and polyester fabrics were pretreated with atmospheric plasma glow discharge (APGD) to improve the ability of the substrate to bond with anthraquinone-2-sulfonic acid doped conducting polypyrrole coating. A range of APGD gas mixtures and treatment times were investigated. APGD treated fabrics were tested for surface contact angle, wettability and surface energy change. Effect of the plasma treatment on the binding strength was analyzed by studying abrasion resistance, surface resistivity and reflectance. Investigations showed that treated fabrics exhibited better hydrophilicity and increased surface energy. Surface treatment by an APGD gas mixture of 95% helium/5% nitrogen yielded the best results with respect to coating uniformity, abrasion resistance and conductivity.

An investigation of the native oxide of aluminum alloy 7475-T7651 using XPS, AES, TEM, EELS, GDOES, RBS.

The native oxide on the rolled aerospace aluminum alloy 7475-T7651 was characterized using a variety of different techniques, including X-ray Photoelectron Spectrometry (XPS), Auger Electron Spectrometry (AES), Transmission Electron Microscopy (TEM), Electron Energy Loss Spectrometry (EELS), Glow Discharge Optical Emission Spectrometry (GDOES), and Rutherford Backscattered Spectrometry (RBS). All techniques revealed that the native oxide layer is magnesium-rich and is probably a mixture of magnesium and aluminum–magnesium oxides.1 The oxide layer was found to be of nonuniform thickness due to the rolling process involved during the manufacture of this sheet alloy; this complicates analysis using techniques which have poor spatial resolution. Direct thickness measurement from cross-sectional TEM reveals an oxide thickness which varies between 125 and 500 nm. This large variation in thickness was also evident from GDOES and AES depth profiles as well asthe RBS data. Both XPS and RBS also show evidence for the presence of heavy metals in the oxide.

Reducing the macroparticle content of cathodic arc evaporated TiN coatings

Cathodic arc evaporation (CAE) is a widely used technique for generating highly ionised plasma from which hard, wear-resistant PVD coatings can be deposited. A major drawback of this technique is the emission of micrometer-sized droplets of cathode material from the arc spot, which are commonly referred to as ‘macroparticles’. In this study, the effect of cathode poisoning was investigated as a method to reduce the number of macroparticles in PVD coatings. While the study focuses on the reduction of macroparticles in titanium nitride coatings, the outcomes and key findings can be broadly applied to the cathodic arc process, in particular, for the reduction of macroparticles in more advanced CAE coatings. The results support earlier findings that have shown that poisoning of the cathode can reduce the number of macroparticles emitted from the arc spot. The results of glow discharge optical emission spectroscopy (GD-OES) showed that the titanium content of the coatings varied little between the respective coatings despite changes in the deposition pressure from 0.1 to 1.2 Pa. The GD-OES results also showed the presence of oxide contamination at the surface of the coatings, which was significantly reduced with increasing deposition pressure. The coatings were also deposited onto high-speed steel twist drills to compare the metal-cutting performance when dry drilling a workpiece of cast iron. The results of the drill tests showed that tool life increased with a reduction in the number of macroparticles.

Tribology of lubricated nitrocarburised and titanium carbonitride surfaces

In the current work, two different coatings, nitrocarburised (CN) and titanium carbonitride (TiCN) on M2 grade high speed tool steel, were prepared by commercial diffusion and physical vapour deposition (PVD) techniques, respectively. Properties of the coating were characterised using a variety of techniques such as Glow-Discharge Optical Emission Spectrometry (GD-OES) and Scanning Electron Microscopy (SEM). Three non-commercial, oil-based lubricants with simplified formulations were used for this study. A tribological test was developed in which two nominally geometrically-identical crossed cylinders slide over each other under selected test conditions. This test was used to evaluate the effectiveness of a pre-applied lubricant film and a surface coating for various conditions of sliding wear. Engineered surface coatings can significantly improve wear resistance of the tool surface but their sliding wear performances strongly depend on the type of coating and lubricant combination used. These coating-lubricant interactions can also have a very strong effect on the useful life of the lubricant in a tribological system. Better performance of lubricants during the sliding wear testing was achieved hen used with the nitrocarburised (CN) coating. To understand the nature of the interactions and their possible effects on the coating-lubricant system, several surface analysis techniques were used. The molecular level investigation of Fourier Transform Infrared Spectroscopy (FTIR) revealed that oxidative degradation occurred in all used oil-based lubricants during the sliding wear test but the degradation behaviour of oil-based lubricants varied with the coating-lubricant system and the wear conditions. The main differences in the carbonyl oxidation region of the FTIR spectra (1900-1600 cm-1) between different coating-lubricant systems may relate to the effective lifetime of the lubricant during the sliding wear test. Secondary Ion Mass Spectrometry (SIMS) depth profiling shows that the CN coating has the highest lubricant absorbability among the tested tool surfaces. Diffusion of chlorine (C1), hydrogen (H) and oxygen (O) into the surface of subsurface of the tool suggested that strong interactions occurred between lubricant and tool surface during the sliding wear test. The possible effects of the interactions on the performance of whole tribological system are also discussed. The study of Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) indicated that the envelope of hydrocarbons (CmHn) of oil lubricant in the positive TOF-SIMS spectra shifted to lower mass fragment after the sliding wear testing due to the breakage of long-chain hydrocarbons to short-chain ones during the degradation of lubricant. The shift of the mass fragment range of the hydrocarbon (CmHn) envelope caries with the type of both tool surface and lubricant, again confirming that variation in the performance of the tool-lubricant system relates to the changes in surface chemistry due to tribochemical interactions at the tool-lubricant interface under sliding wear conditions. The sliding wear conditions resulted in changes not only in topography of the tool surface due to mechanical interactions, as outlined in Chapter 5, but also in surface chemistry due to tribochemical interactions, as discussed in Chapters 6 and 7.

Enhancing the interfacial properties of Quickstep{u2122} cured biocomposites

This research has enhanced the performance of natural fibre composites by optimisation of the cure cycle under Quickstep{u2122} process and treatment time under atmospheric pressure glow discharge plasma. The study has also utilised many characterization methods, theoretical models, and established surface-property relationship to manufacture composites with optimum strength and toughness.

Influence of atmospheric helium plasma on the surface energy of jute fibres and the performance of resulting composites

In this work, an atmospheric pressure glow discharge helium plasma treatment was employed to modify the surface properties of jute fibres. The resulting bio-composites showed an increase in flexural properties and interlaminar shear strength (ILSS) compared to composites produced using untreated jute fibres. To understand the reason behind the ILSS improvement, the acid–base properties of jute fibres were determined by contact angle analysis using the capillary rise method. The results were fitted further to van Oss–Chaudhury–Good (vOCG) and Chang–Qin–Chen (CQC) models to determine the Lifshitz–van der Waals (LW) and acid–base components of surface energy. Surface energy determined by the vOCG model revealed that plasma treatment of jute fibre resulted in a 22% increase in total surface energy, a 19% increase in the LW component and a 24% increase in the acid–base component of surface energy. The increase in the acid–base component is due to the significant increase (69%) in the electron-accepting (γ+S) parameter. On the other hand, the CQC model clearly indicates an amphoteric nature of the fibre surface based on opposite signs of the acid and base principal values (PSa and PSb). Overall, the results indicated that increases in both LW and acid–base components were responsible for improvement in the properties of the composites.

Deposition and wetting characteristics of polyelectrolyte multilayers on plasma-modified porous polyethylene

Hydrophilic and chemically reactive porous media were prepared by adsorbing functional polymers at the surface of sintered polyethylene membranes. Modification of the membrane was accomplished by first exposing the membrane to an oxygen glow discharge gas plasma to introduce an electrostatic charge at the membrane surfaces. Cationic polyelectrolyte polyethylenimine (PEI) was adsorbed from solution to the anionic-charged surface to form an adsorbed monolayer. The adsorption of a second anionic polyelectrolyte onto the PEI layer allows further modification of the membrane surface to form a polyelectrolyte-bilayer complex. The conformation and stability of the adsorbed monolayers and bilayers comprising the modified surface are probed as a function of the polymer structure, charge density, and solubility. Using X-ray photoelectron spectroscopy analysis, we demonstrate that the presence of the polyelectrolyte multilayers drastically increases the density and specificity of the functional groups at the surface, more than what can be achieved through the plasma modification alone. Also, using the wicking rate of deionized, distilled water through the porous membrane to gauge the interfacial energy of the modified surface, we show that the membrane wicking rate can be controlled by varying the chemistry of the adsorbing polyelectrolytes and, to a lesser extent, by adjusting the polarity or ionic strength of the polyelectrolyte solution.

Characterization and tribological performance of Cu-based intermetallic layers

Fluidized bed reactor chemical vapor deposition (FBR-CVD) has been used to enrich the surface of oxygen free high conductivity (OFHC) copper with titanium, silicon and aluminum. This technique enables the production of coherent and adherent intermetallic surface layers of uniform thickness and high hardness. The characterization of the coatings was performed using backscatter scanning electron microscopy (BS-SEM), X-ray diffraction (XRD), glow discharge optical emission spectroscopy (GDOES) and micro-hardness. The tribological properties of the coatings in dry sliding contact with steel were evaluated by pin-on-disc wear testing.

Influence of surface mechanical attrition treatment attrition media on the surface contamination and corrosion of magnesium

Surface mechanical attrition treatment (SMAT) is a mechanical peening process used to generate ultrafine grain surfaces on a metal. SMAT was carried out on pure magnesium using different attrition media (zirconia [ZiO2], alumina [Al2O3], and steel balls) to observe the effect on microstructure, surface residual stress, surface composition, and corrosion. Surface contamination from SMAT was characterized using glow discharge optical emission spectroscopy (GDOES). The SMAT process produced a refined grain structure on the surface of Mg but resulted in a region of elemental contamination extending ~10 μm into the substrate, regardless of the media used. Consequently, SMAT-treated surfaces showed an increased corrosion rate compared to untreated Mg, primarily through increased cathodic kinetics. This study highlights the issue of contamination resulting from the SMAT process, which is a penalty that accompanies the significant grain refinement of the surface produced by SMAT. This must be considered if attempting to exploit grain refinement for improving corrosion resistance.

Comparative study of the microstructures and mechanical properties of direct laser fabricated and arc-melted AlxCoCrFeNi high entropy alloys

High entropy alloys (HEA) are a relatively new metal alloy system that have promising potential in high temperature applications. These multi-component alloys are typically produced by arc-melting, requiring several remelts to achieve chemical homogeneity. Direct laser fabrication (DLF) is a rapid prototyping technique, which produces complex components from alloy powder by selectively melting micron-sized powder in successive layers. However, studies of the fabrication of complex alloys from simple elemental powder blends are sparse. In this study, DLF was employed to fabricate bulk samples of three alloys based on the AlxCoCrFeNi HEA system, where x was 0.3, 0.6 and 0.85M fraction of Al. This produced FCC, FCC/BCC and BCC crystal structures, respectively. Corresponding alloys were also produced by arc-melting, and all microstructures were characterised and compared longitudinal and transverse to the build/solidification direction by x-ray diffraction, glow discharge optical emission spectroscopy and scanning electron microscopy (EDX and EBSD). Strong similarities were observed between the single phase FCC and BCC alloys produced by both techniques, however the FCC/BCC structures differed significantly. This has been attributed to a difference in the solidification rate and thermal gradient in the melt pool between the two different techniques. Room temperature compression testing showed very similar mechanical behaviour and properties for the two different processing routes. DLF was concluded to be a successful technique to manufacture bulk HEA[U+05F3]s.