Simultaneous Aluminizing and Chromizing of Steels to Form (Fe,Cr)3Al Coatings

A cementation pack involving halide activatorsand elemental Al and Cr powders has been used to achievethe codeposition and diffusion of aluminum and chromiuminto low-alloy steels. A two-step treatment at 925°C and 1150°C yields dense anduniform ferrite coatings of about 400-m thickness,with surface compositions of approximatelyFe₃Al plus several percent Cr. The two stepheating schedule prevents the formation of a blocking chromium carbide atthe substrate surface. An attempt was made to add atrace of Ce to the Al + Cr content of the coating byintroducing Ce oxide into the pack, but there is no evidence that this doping was achieved. Uponcyclic oxidation in air at 700°C, the coated steelexhibits a negligible 0.085 mg/cm² weightgain for 1900 one-hour cycles. Virtually no attack wasobserved on coated steels tested in a simulated boileratmosphere at 500°C for 500 hr. But coatings with asurface composition of only 8 wt.% Al and 6 wt.% Crsuffered limited sulfidation attack in the simulated boiler atmosphere at temperatures higher than500°C for 1000 hr.     

Effect of ZnO coating of whiskers on thermal expansion properties of aluminium borate whisker reinforced aluminium composites

Abstract

The pure aluminium composites reinforced by ZnO coated aluminium borate whiskers were fabricated by squeeze casting. The microstructure of the composite was observed using an optical microscope and the thermal expansion behaviours of the composites were investigated in the range from 50 to 400°C. In addition, the effects of heat treatment and thermal cycling on the thermal expansion behaviours of the composite were also investigated. The results show that the coefficient of thermal expansion of as cast composites decreases with the ZnO coating content increasing. However, heat treatment time and thermal cycling lead to an increase in the CTE of the composite.     

Effects of different metals on adhesive wear behaviour of alloy surface produced by TIG process

Abstract

Low carbon steel surfaces were alloyed with composite powders using the tungsten inert gas welding method. After the alloying process, the effects of cladding surface on the microstructural characteristics and adhesive wear of the alloyed samples were examined. The sliding wear behavior of samples was investigated in a block on ring apparatus under the loads of 20, 40, 60 and 80 N respectively. In the experimental investigation, a low carbon steel surface was alloyed with austenitic stainless steel powder and austenitic stainless steel powder mixed with 4·5% Co, Mo and Ti particles respectively. Following surface alloying, conventional characterisation techniques, such as optical microscopy, scanning electron microscopy, energy dispersive spectrograph and X-ray diffraction, were used to study the microstructure of the alloyed zone. Examination of the microstructure revealed the presence of M₂₃C₆ carbides, solid melt phases and intermetallic phases, such as Ni₃Ti, depending on the alloying element in the composite. As the amount of the reinforcing material increased, the saturation rates for the samples decreased, while their hardness increased. The adhesive abrasion tests conducted revealed that temperature input plays a significant role on the microstructure characteristics, which positively affected the adhesive abrasion values of the samples. Consequently, the tungsten inert gas welding method was successfully used for the surface alloying of low carbon steels.     

Evaluation,degradation and life assessment of coatings for land based combustion turbines

Abstract

Use of metallic and thermal barrier coatings to protect hot section blades and vanes of combustion turbines for power generation has been common practice for the past three and one decades respectively. Because these coatings must be optimised with respect to both different forms of corrosion and modes of operation (base load versus peak load), their performance may be machine specific. Industrial end users generally do not have detailed knowledge of the failure mechanisms of the coatings and the basis for selecting coatings to suit specific requirements, topics the present review seeks to address. The evolution of protective coatings, coating failure mechanisms and a methodology for selecting machine specific coatings are described. The methodology, which can be used to rank and optimise coating systems and to predict the remaining life of coatings, forms the basis of a computer code known as COATLIFE. The ingredients of this methodology, i.e. degradation modelling and thermomechanical fatigue life prediction, are reviewed.     

Adhesion and corrosion resistance properties of coatings obtained from modified low-molecular-weight polystyrenes

In this study, as a continuation of our previous studies, chemical modification of low-molecular-weight polystyrenes (PSs) was carried out with various functional group modifiers: epichlorohydrin (ECH), maleic anhydride (MA) and acetic anhydride (AA), in a single stage using a cationic catalyst. It was determined that the amounts of the functional groups bound to the structure of the polymer depended on the molecular weight of the polymer used, and more functional groups were bound to the lower-molecular-weight PSs. It was found that the coating properties (adhesion properties and resistance to aggressive conditions) of the functional group containing PS to the metal surface depended on the structure and the amount of the functional groups bound to the aromatic ring of the polymer. In addition, it was observed that the PS modified with MA and ECH having carboxyl- and epoxy-groups in their aromatic rings had higher adhesion, as well as higher corrosion resistance properties. Various functional groups bound to the aromatic ring of the polystyrene and their amounts were determined by spectral and chemical analysis methods.     

Improvement of polyurethane adhesion to glass using novolac primer

The adhesion of polyurethane (PU) coatings based on toluene diisocyanate, poly(propylene glycol) (PPG) 2000, polyethylene adipate (PEA) 2000 and castor oil (CO) was studied. The coatings were applied to glass slides with and without novolac primer (due to the high functionality of castor oil, the resultant PU coatings have limited shelf life). Our studies showed that satisfactory adhesion strengths were achievable for immediate bonding. Furthermore, our study also found that the adhesion of polyurethane to glass surfaces was increased by using a thin layer of novolac primer.     

Influence of Yttrium on the Oxidation of Pure Iron at 973 K; Comparison Between Ion Implantation and Sol-Gel Coatings

The influence of yttrium introduced by ionimplantation or by sol-gel coatings on the oxidation ofpure iron has been studied at 973 K underp(O₂) = 0.04 Pa. The oxide scales wereexamined using a wide range of analytical techniques. The addition ofyttrium by ion implantation or by sol-gel coatingssignificantly improved the oxidation resistance of pureiron. This effect depends upon the introduction mode of yttrium on the surface.     

Gold soldering in microelectronic products with tin–indium solder

Abstract

The results of research into the interaction of 0.04 mm diameter gold microwire with STI50 solder have been presented when soldering hybrid integrated circuits and microassemblies on printed boards. The dependence of dissolving of gold wire on the temperature and duration of soldering has been established as well as on holding time at a seam temperature of 100 ° C; the interaction of STI50 solder with gold coating on UHF and HIC boards has been considered when soldering on metallic substrates.     

Studies of the Mechanical Properties and Practical Coating Adhesion on PP Modified by Oxidized Wax

In this study, the influences of polarity and the amount of oxidized polypropylene wax (OPPW) in blends with polypropylene (PP) were investigated by studying their surface properties. OPPW was completely miscible with PP up to 10 wt%. The adhesive strength of a acrylic-based primer coating on PP sheets, containing different wt% of OPPWs, was evaluated by using a direct 'pull off' test method. The results showed that the adhesive strength of the coating improved with an increase of the amount of OPPW in the blend. However, the degree of polarity in the OPPW did not have a significant impact on its adhesive properties. These observations were also supported by the results of ATR–FT-IR spectroscopy and surface energy measurements of the substrate. Furthermore, the results of adhesion test on the coating panels showed a significant enhancement after exposing to heat in an oven prior to the application of coating, e.g., about 230% increase for the blend containing 8 wt% of OPPW. The TGA curves showed a maximum drop of about 10% in thermal stability in comparison with that of the unblended PP. The changes in the mechanical properties of the blends were explained by considering the morphology of the blends and were supported by the changes in blend crystalinity and melting behavior. The elastic modulus remained almost unchanged while elongation and stress to breakpoint experienced a sharp reduction at concentrations of wax content higher than 6 wt%. The study showed a good balance of substrate coatability with its bulk properties at a blend concentration of about 6 wt% of OPPW.     

X-Ray Diffraction Study of Oxide Scales Formed at High Temperatures on AISI 304 Stainless Steel After Cerium Deposition

High-resolution X-ray diffraction has been usedto characterize the structure of oxide scales grownduring oxidation at 1173 K of cerium-modified stainlesssteels. The oxide scales consist of a mixture of Fe₂O₃ andCr₂O₃, as well as spinels. Severaloxidation treatments prior to cerium deposition havebeen applied. New phases, which are cerium-related,appear, depending on the preoxidation treatment. Bycomparing these results with previous ones onlanthanum-modified AISI 304 stainless steel, somepossible explanations for the reactive-element effectare proposed.