Microstructural investigation of plasma-sprayed aluminum coatings

Although plasma-sprayed coatings have been used extensively in industry, limited studies have been carried out on the microstructural details. In this program scanning and transmission electron microscopy, combined with other techniques, were used to examine the grain morphology in plasma-flame-sprayed aluminum coatings.In plasma spraying, the initial particles which impinge onto the cold substrate solidify at rates comparable with those obtained by splat quenching. Since such particles are responsible for the bonding between the coating and the substrate, the first layer has received the central emphasis in this study. Because subsequent layers experience lower cooling rates, a non-uniform grain morphology is obtained which is substantially affected by the particle morphology.The topography of the surface on which the particles impinge is shown to play a decisive role with respect to the orientation of the grains within each particle. These morphological details are considered in the light of models proposed for splat quenching. Another important factor in grain morphology is the position of the particle within the coating. For aluminum, the layers adjacent to the substrate contain highly oriented elongated grains, with the longitudinal axis perpendicular to the substrate. However, because of heating by the flame itself, the particles located away from the substrate are composed of large, equiaxed and randomly oriented grains.Finally, the reaction of the molten droplets with the plasma effluent and the surrounding environment is considered. Oxidation and porosity result when aluminum is sprayed in air. The oxidation of the particles results in a thin layer of oxide which (i) hinders particle-particle interaction and thus results in the formation of interparticle pores and (ii) provides a path for crack propagation and failure, which can occur by delamination. The pore structure of the coating was investigated and correlated with fractography.     

Thermal expansion study of plasma-sprayed oxide coatings

The thermal expansion characteristics of overlay coatings is of considerable importance when the coated parts are exposed to thermal cycling. In this study we investigated the thermal cycling behavior of oxide coatings plasma sprayed onto mild steel. Thermal expansion measurements were carried out on Al₂O₃TiO₂ and zirconia coatings under different conditions of coating-substrate adherence and on coatings that were substrate free. A study was initiated of the effects on the expansion coefficient of variations in composition, porosity, heating rate and thermal cycling. In addition, metallographic and acoustic emission methods were employed to examine the compatibility of these oxide coatings with steel substrates under varying conditions.     

Complete high-performance cache system for the 80386

The 80386 is a high-performance third-generation microprocessor that is now standard in most top-of-the-range PCs. Like all similar processors operating at clock rates above 30 MHz, the 80386 must use cache memory if it is to operate efficiently. Without cache memory, the user must either pay a very high price for very fast RAM or employ slower memory by introducing wait states. This application note describes the 80386 bus interface and demonstrates how it can be interfaced to IDT cache tag RAMs to create a cache system. Although the report describes a relatively basic cache system, it covers all design considerations ranging from system timing to the programming of the PALs needed to implement the interface. A.C.     

Evaluation of NiAl and NiAl-B deposited by vacuum plasma spray

Nickel aluminide, NiAl, has potential for high-temperature structural applications because of its high melting temperature, low density, high elastic modulus, and excellent oxidation resistance. However, inadequate room-temperature ductility and low strength at high temperatures limit the use of NiAl for practical service. An added limitation is the difficulty of forming this intermetallic compound. Plasma spray forming is suggested to produce NiAl in useful shapes and forms. In this study, vacuum plasma spray has been used to produce coatings and free-standing forms of NiAl and NiAl microalloyed with boron (NiAI-B). This research focuses on an investigation of the structures and properties of the as-sprayed deposits. Preliminary analysis reveals that the deposits formed by vacuum plasma spray are very dense and essentially oxide free. The as-received aluminide powders and the sprayed deposits have the same phase structure. Tests also indicate that the mechanical properties achieved by plasma spray are comparable to those achieved by other processing techniques.     

Evaluation of CP Chromo Select Agar for the enumeration of Clostridium perfringens from water

The European Directive on drinking water quality has included mCP agar as the reference method for recovering Clostridium perfringens from drinking waters. In the present study, three media (mCP, TSCF and CP Chromo Select Agar) were evaluated for recovery of C. perfringens in different surface water samples. Out of 139 water samples, using a membrane filtration technique, 131 samples (94.2%) were found to be presumptively positive for C. perfringens in at least one of the culture media. Green colored colonies on CP Chromo Select Agar (CCP agar) were counted as presumptive C. perfringens isolates.