Vijaya Agarwala

Electroless alloy/composite coatings: A review

Since the inception of electroless coating by Brenner & Riddell in 1946, it has been the subject of research interest and, in the past two decades, emphasis has shifted to the studies of its properties and applications. The co-deposition of paniculate matter or substance within the growing film has led to a new generation of electroless composite coatings, many of which possess excellent wear and corrosion resistance. This valuable process can coat not only electrically conductive materials including graphite but also fabrics, insulators like plastics, rubber etc. The low coating rates with these can provide better reflectivity of plated surfaces and many more applications. Coatings can be tailored for desired properties by selecting the composition of the coating alloy/composite/metallic to suit specific requirements. The market for these coatings is expanding fast as the potential applications are on the rise. In the present article, an attempt has been made to review different electroless alloy/composite coatings with respect to bath types and their composition, properties and applications. Different characterisation studies have been conducted on various electroless nickel-based coatings with emphasis on wear and corrosion properties.

ANALYSIS OF FREQUENCY SELECTIVE SURFACES FOR RADAR ABSORBING MATERIALS

Nowadays, applications of Frequency Selective Surfaces (FSS) for radar absorbing materials (RAM) are increasing, but it is still a challenge to select a proper FSS for a particular material as well as the dimensions of FSS for optimized absorption. Therefore, in this paper an attempt has been made to optimize the dimensions of FSS for microwave absorbing application using Genetic Algorithm (GA) approach. The considered frequency selective surfaces are composed of conducting patch elements pasted on the ferrite layer. FSS are used for flltration and microwave absorption. In this work, selection and optimization of FSS with radar absorbing material has been done for obtaining the maximum absorption at 8{12GHz frequency. An equivalent circuit method has been used for the analysis of difierent FSS, which is further used to design fltness function of GA for optimizing the dimensions of FSS. Eight difierent available ferrite materials with frequency dependant permittivities and permeabilities have been used as material database. The GA optimization is proposed to select the proper material out of eight available materials with proper dimensions of FSS. The optimized results suggest the material from database and dimensions of FSS. The selected material is then mixed with epoxy and hardener, and coated over the aluminium sheet. Thereafter, all flve FSS were fabricated on ferrite coated Al sheets using photolithographic method followed by wet etching. The absorption was measured for all FSS using absorption testing device (ATD) method at X-band. Absorption results showed that signiflcant amount of absorption enhanced with the addition of proper FSS on radar absorbing coating.

The effect of lead content on the wear characteristics of a stir-cast Al-Pb alloy

Abstract The present investigation was carried out to determine the effect of lead content in stireast Al-Pb alloys. It was found that dispersion of about 8–20 wt.% Pb in commercial aluminum modifies the wear characteristics of aluminum by introducing a transition from mild to severe wear with increasing sliding velocity. Beyond 20 wt.% Pb, the bulk wear was found to increase gradually.

Electroless Ni‐P Based Nanocoating Technology—A Review

An advancement of electroless, (EL) Ni‐P based coatings from micro to nano level is being discussed in this present review. This paper provides a coherent understanding of the EL coatings in terms of systems, bath composition for metal/alloy and composite coatings. The elements of the EL composite bath have been discussed. The studies on the second phase nanosized particles dispersed in the Ni‐P matrix have been illustrated. The bonding mechanisms of EL coatings with the substrates have been discussed. The morphology, physical and mechanical properties of the Ni‐P‐X (X the second phase particles added into EL Ni‐P matrix) composite coatings have discussed. A few applications have also been outlined that may reveal the capabilities of the EL Ni‐P based nanocomposite coating technology to the relevant industries with the emphasis on tribological, corrosion, and magnetic behavior.

Friction characteristics of stir-cast Al-Pb alloys

Abstract The coefficient μ of friction of stir-cast Al-Pb alloys with different lead contents and with the same lead content but cast at different agitator speeds against carburized steel has been determined under dry sliding conditions in a Timken wear- and lubricant-testing machine. It is observed that μ remains almost steady with increase in load or sliding velocity for all the stir-cast Al-Pb alloys and commercially pure aluminum but for commercially pure lead it increases sharply with increase in load and sliding velocity. It has also been observed that μ decreases with increase in lead content in Al-Pb alloys until about 20 wt.% but, beyond this composition, it increases sharply. To understand the friction mechanism, the friction tracks are studied by scanning electron microscopy.

Effect of Particle Size of BaFe 12 O 19 on the Microwave Absorption Characteristics in X-Band

Present work deals with the microwave absorption characteristics of BaFe12O19 of interest as radar absorbing material (RAM). There are very few reported works available where particle size has been critically analyzed for absorbing characteristics at microwave frequencies, therefore, in this paper microwave absorption properties of the BaFe12O19 with difierent particle sizes were investigated. The results showed that the particle size had signiflcant in∞uence on the dielectric and absorption properties of the composites in the 8.2{ 12.4GHz frequency range. BaFe12O19 powder of difierent particle sizes were synthesized by varying the annealing time and it was observed that the real part of permittivity of the composite increases from 5.18 (average value) to 7.50 (average value) and imaginary part increases from an average value of 0.20 to an average value of 2.33, whereas the real part of permeability increases from 0.95 (average value) to 1.11 (average value) and imaginary part of permeability was measured in the range of 0.02 to 0.07. These changes in permittivity and permeability afiects microwave absorption application. It is observed that the maximum bandwidth for average particle size of 240nm is 3.02GHz and with the increase in average particle size, microwave absorption properties increased.

DRY SLIDING WEAR AND FRICTION BEHAVIOR OF Ni–P–ZrO2–Al2O3COMPOSITE ELECTROLESS COATINGS ON ALUMINUM

In the present study, dry sliding wear behavior of a newly developed electroless coating, prepared by in-situ co-precipitation of X and its co-deposition, has been investigated both as-coated and heat treated, at different loads on a pin-on-disc wear monitor with constant sliding speed. The cumulative volume loss for increasing sliding distance is observed to be linear and variation in wear rate has been found to increase linearly with normal load following Archards law. As-coated and heat-treated samples showed significantly better wear performance than that of the substrate aluminum (AS) in terms of the wear rate, which was also found to be influenced by substrate hardness. The friction coefficient decreases with increasing normal load for both the un-coated and coated aluminum. However, relatively higher values of friction coefficient has been observed for coated and heat-treated samples compared to that for the substrate.

Characterization of carbon fabric coated with Ni-P and Ni-P-ZrO2-Al2O3 by electroless technique

Ni-P and Ni-P-ZrO2-Al2O3 (Ni-P-X) composite coatings on carbon fiber woven fabric have been attempted by electroless (EL) coating technique. For producing Ni-P EL coating, a sodium hypophosphite is used as reducing agent in alkaline bath whereas for producing composite coating, in the same bath, the co-precipitation of Al2O3 along with ZrO2 by a chemical reaction has been used. The bath used was maintained at pH value of 9 ± 0.25 and a temperature of 90 ± 2°C for all the coatings under investigation. The morphology of coatings has been studied under SEM and the phases have been identified by XRD and TEM. The phases like microcrystalline nickel, amorphous nickel, Ni5P4 and Ni12P5 have been identified to be present. The grain size of these phases has been seen to be in the range of 5–10 nm. The tensile strength of as-coated and heat treated samples of both the types of coatings have been compared with that of uncoated fabric. The UTS values of uncoated carbon fabric used has been found to 3.7 N/mm2 whereas that for heat treated after coating the fabric samples with Ni-P and Ni-P-ZrO2-Al2O3 EL coatings have been observed to be 11.1 and 12.4 N/mm2 respectively.

An efficient use of waste material for development of cost-effective broadband radar wave absorber

Very few literatures are available on the utilization of natural waste composite materials for radar wave absorption. The main objective of this paper was to achieve good absorption with wide bandwidth corresponds to reflection loss (RL) ≤ −10 dB for less absorber layer thickness (≤2.0 mm) for a cost-effective production of radar wave absorber. In this study, mineral dust and beach sand-based waste composite material is critically analyzed for its application as broadband radar wave absorber in the frequency range of 8.2–12.4 GHz. A multilayer approach is applied for obtaining the good absorption, where thickness of different layers is optimized by genetic algorithm. The effective absorption bandwidth for two- and three-layer composite absorber is 3.5 and 2.8 GHz for the optimized thickness 1.9 and 1.8 mm, respectively. The two-layer absorber possesses measured RL of −27.20 dB at 10.8 GHz and for a three-layer absorber, RL reaches up to −32.58 dB at 11.2 GHz. The measured RL values agree quite well with the calculated ones, which show the effectiveness of absorber for various practical EM wave absorption applications.

Design and experimental verification of a thin broadband nanocomposite multilayer microwave absorber using genetic algorithm based approach

The bandwidth-thickness tradeoff of single layer microwave wave absorber has become challenge for researchers. This paper presents experimental results of thin broadband multilayer microwave wave absorbing structures using magnetic ceramic based nano-composites for absorption at X-band. A genetic algorithm (GA) based approach has been used to optimize thickness of different material layers and selection of suitable material to ensure minimum reflection. The parameters optimized through genetic algorithm have been simulated through Ansoft High Frequency structural simulator (HFSS) and experimentally verified through Absorption Testing device (ATD). It has been found that the peak value of reflection loss is −24.53 dB for 1.3 mm absorber layer coating thickness, which shows the effectiveness of absorber for various applications..