USE PULSED LASER DEPOSITION TO DEPOSIT TELLURIUM-DOPED THIN FILMS
STAINLESS STEEL-BASED CERAMIC COATINGS FOR BIOMEDICAL APPLICATIONS
EMPLOY DIAMOND LAYER TO SUPERHARDEN STEEL
CODEPOSIT SILICON AND ALUMINUM ONTO LOW-CARBON STEEL
LOW PRESSURE MOCVD OF ALUMINUM OXIDE FILMS USING ALUMINUM ACETYLACETONATE PRECURSOR

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USE PULSED LASER DEPOSITION TO DEPOSIT TELLURIUM-DOPED THIN FILMS

Since there is immense interest in extending Group III-V nitride blue lasers and light emitting diodes (LEDs) into the UV region, researchers at the University of Virginia at Charlottesville and the University of Tennessee at Knoxville have been studying tellurium (Te)-doped alkaline earth sulfide materials (such as SrS:Te; CaS:Te, and MgS:Te) as alternative sources for UV and deep-UV solid-state light.

Virginia’s James Fitz-Gerald, who is collaborating with Philip Rack at the University of Tennessee-Knoxville’s Department of Material Science, points out that they are "exploring a new material set (alkaline earth sulfides) for emission in the UV, and possibly a different device technology (field emission cathodoluminescence) in contrast to the wealth of research being pursued in the AlGaN light emitting diode and diode laser material programs that are receiving all of the attention."

In their work, Fitz-Gerald and Rack deposited SrS:Te thin films through pulsed laser deposition (PLD) in an argon atmosphere. Subsequently, they determined the luminescent properties of these thin films. Te doping was carried out by means of both ion implantation and Te capping layers deposited through PLD. One notable aspect from this investigation is that there is strong emission of Tes-Tes (420 nm) at low doping concentrations and a rise in the emission of Tes (360 nm) at high Te concentrations, a trend that is particularly evident for the highest doped films. This is also in contrast to findings from previous research, which suggest that there is a gradient in the concentration of the Te dopant.

Fitz-Gerald says there are several applications for this technology in the commercial and defense sectors. He indicates the primary commercial application is in solid-state lighting where "the idea is to make solid-state light emitting sources that have longer lives, better efficiency, and environmentally friendly (that is, no Hg)." Semiconductor-based UV light sources are of great importance for miniaturized UV light sources. Such light sources are gaining increasing usage in areas such as water purification, communications, decontamination, biological agent detection, and white light generation.

Fitz-Gerald adds that in the future they will further "optimize the thin film deposition and doping conditions, as well as power efficiency measurements." He explains that through this they will have a better understanding of the commercial applications when those critical measurements are made.

Finally, he says that the "principal investigators are looking for support to fully explore and exploit the potential that this material set and technology offers."

Details: James M. Fitz-Gerald, Assistant Professor, Department of Materials Science and Engineering, School of Engineering and Applied Science, University of Virginia, 116 Engineer's Way, Charlottesville, VA 22904. Phone: 434-243-8830. Fax: 434-982-5660. E-mail: jmf8h@virginia.edu
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STAINLESS STEEL-BASED CERAMIC COATINGS FOR BIOMEDICAL APPLICATIONS

Orthopedic and biomedical parts and devices such as cardiac valves, wire leads, and aneurysm clips employ metal-based components composed of titanium, titanium alloy, cobalt-chromium alloy or stainless steel of particular grades (AISI 316L type) in ceramic bases (major types of ceramic coatings are summarized in Table 1). Any material for biological use must be both biocompatible and biofunctional. Resistance to corrosion--especially in a high chloride environment; as in the human body--is among the most important attributes for biocompatibility. AISI 316L, an austenitic stainless steel where the austenitic phase is stabilized by nickel, is one of the most popular substances for implant applications. However, even this steel is reported to corrode in vivo, and therefore researchers are trying to find ways to perform surface modification to reduce corrosion.

Armed with the knowledge that ceramic coatings such as alumina and zirconia have the ability to deliver superb biocompatibility and wear resistance, researchers at India’s Defense Metallurgical Research Laboratory in Hyderabad deposited ceramics such as alumina, magnesia-stabilized zirconia (MSZ) and yttria-stabilized zirconia (YSZ) on 316L stainless steel substrates through the air plasma spray (APS) process. This research comprised part of a study to develop a cost-effective and efficient coating material. The ceramic coatings were deposited at varying thickness levels and subsequently characterized using sophisticated analytical tools such as scanning electron microscopy (SEM), X-ray diffraction (XRD), and metallography techniques. To determine corrosion resistance, coatings were analyzed by electrochemical techniques in deaerated Hank’s solution (which, at pH 7.4 and 37 degrees C, simulates body fluid conditions).

It was discovered that the 150-micrometers-thick alumina coating provided the best protection and increased the breakdown potential. Similarly, while all thicknesses of MSZ coating provided superior protection, breakdown potential increased significantly for 150-micrometers-thick coating. Notably, the breakdown potential increased only marginally for the 50-micrometers-thick YSZ coating. In all, among the three types of coatings, the MSZ coating has been determined to provide the best corrosion-resistance for 316L stainless steel and 150 micrometers is the optimum thickness level for affording protection.

Table 1. Major Types of Ceramic Coatings

Details: I. Gurappa, Defense Metallurgical Research Laboratory, Kanchanbagh PO, Hyderabad 500058, India. Phone: +91-40-458-6515. Fax: +91-40-434-1439. E-mail: igpl@rediffmail.com
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Copyright 2003, Frost & Sullivan, San Antonio, TX 78229

EMPLOY DIAMOND LAYER TO SUPERHARDEN STEEL

As part of his doctoral research, University of Nijmegen-based Dutch chemist Ivan Buijnsters has successfully employed diamond to layer a steel substrate and make it superhard. This research is significant in that it opens new vistas for producing wear-resistant tools.

Buijnsters fabricated the diamond layers by facilitating the disassociation of methane gas diluted in hydrogen gas on a hot wire just above the substrate. The carbon atoms present in the methane dropped onto the substrate and formed a thin layer of diamond. During the course of his research, Buijnsters unlocked another key mystery: why a diamond layer cannot be created on some types of steel. During the deposition process the carbon penetrated several micrometers into the metal, where it formed iron carbides, ultimately resulting in graphite. In effect, Buijnsters' technique did not work on a steel substrate since predominantly graphite was formed.

Although it was still strong enough to prevent the formation of a well-sealed diamond coating, this issue was markedly less significant in stainless steel. To overcome this issue Buijnsters searched for something that is compatible with and adhere strongly to both the steel and diamond, be appropriate for diamond growth and could be placed between the layers. So perhaps the secret to the success of this technique lies in the adhesive layer between the steel and the diamond layer.

Silicon--an early choice--was discarded because the carbon atoms diffused through the intermediate silicon layer into the iron causing the steel to weaken. Buijnsters then came across chromium nitride, through which it became easy to apply a good-adhering intermediate layer using a deposition apparatus. (Boron was another good material for developing an intermediate layer on stainless steel).

One of the plusses of Buijnsters method is that the importance of the coefficient of expansion diminishes over time, that is, the difference in expansion between diamond and steel gradually dissipates. For instance, after diamond is produced (at temperatures in the range of 600 degrees C), steel contracts at a much faster rate, and as a result the diamond and steel can separate over time. However, employing boron in Buijnsters method imparts to the external surface of the steel an expansion coefficient that is close to that of diamond. Eventually, the two phases remain intact and attached.

Details: Ivan Buijnsters, Department of Applied Physics, University of Nijmegen, P.O. Box 9010, NL-6500 GL Nijmegen , The Netherlands. Phone: 31-243-653-024. E-mail: buijnste@sci.kun.nl
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CODEPOSIT SILICON AND ALUMINUM ONTO LOW-CARBON STEEL

Corrosion- and oxidation-resistant metals have been codeposited on steel, as well as nickel-based alloys through pack cementation for some time now. For instance, to improve hot corrosion resistance of aluminide coatings, a pack cementation process for simultaneously depositing aluminum and chromium on either steel or superalloys has been explored. While codeposition has usually been accomplished through the use of a masteralloy powder as a pack component, this is rather expensive and inflexible.

Researchers at the Korea Electric Power Research Institute in Taejon, South Korea have reported the successful codeposition of Al and Si onto low-carbon steel through a pack cementation (diffusion coating) process that employs elemental Al and SiO₂ as the sources of Al and Si. The Korean researchers have produced a number of silicon-containing aluminide coatings by tweaking the content of SiO₂ while maintaining fixed Al content at elevated temperature.

In the specimens processed using a pack containing only NaCl as the activator, the Si content at the surface was extremely low while the Al content decreased with an increase in the pack SiO₂ content. It appears that SiO₂ can be reduced to elemental silicon, which was poorly activated by NaCl to be deposited to the aluminide layers. It also appears that the best activator for such purposes was a dual type, NaCl + KBF₄, which exhibited a desirable content of Si at the surface (3-5 wt%). This indicates that SiO₂ in the pack is converted to Si and ultimately activated to silicon fluoride to be deposited with aluminum.

Furthermore, the aluminized low-carbon steels were subjected to cyclic oxidation tests at 900 degrees C in air, and showed good oxidation-resistant properties within approx. 30 hr of the testing period when the aluminide coatings were modified by Si at the surface.

Details: M. T. Kim, Machinery and Materials Group, Korea Electric Power Research Institute, Taejon 305-380, South Korea. Phone: +82-42-865-5233. Fax: +82-42-865-5304. E-mail: mtkim@kepri.re.kr
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Copyright 2003, Frost & Sullivan, San Antonio, TX 78229

LOW PRESSURE MOCVD OF ALUMINUM OXIDE FILMS USING ALUMINUM ACETYLACETONATE PRECURSOR

Because of its attractive deposition characteristics, namely high growth rates, low growth temperature, and uniform coverage of irregular and contorted surfaces, metal organic chemical vapor deposition (MOCVD) is being increasingly used for the deposition of dielectrics and other types of coatings. Under this process, film structure and composition are determined by a function of the type of chemical precursor employed and the deposition conditions (for example, temperature and pressure). Similarly, chemical processes also influence growth, nucleation, and microstructure of films deposited through this process. Thus, to gain comprehensive understanding of properties of CVD films and devices employing these, it is important to know the various factors that influence CVD film nucleation and growth.

Researchers at the Indian Institute of Science in Bangalore, India have, in the absence of an oxidant gas, studied the deposition of aluminum oxide films by low pressure MOCVD from the complex aluminum acetylacetonate. The researchers' study reveals that through this process and employing a metalorganic precursor Al(acac)₃, it is possible to obtain coatings containing crystalline alumina at temperatures as low as 500 degrees C.

Furthermore, morphological analysis of the coatings reveals that they are dense, pore-free and marked by spherulitic features, which indicates that melting is a part of the nucleation and growth processes. The precursor vapors, carried along by argon, when moved to a low-pressure environment undergo a sudden expansion. This sudden expansion results in the formation of sub-micrometer clusters of precursor molecules. Such clusters melt congruently as they land on the hot substrate and provide a carbon-containing matrix where alumina crystallites are liable to grow at relatively low temperatures. Analyses through transmission electron microscopy (TEM), X-ray spectroscopy (XPS) and ellipsometry, confirms the fact that the crystallites are small and embedded in a carbon-rich matrix.

Details: S.A. Shivashankar, Materials Research Center, Indian Institute of Science, Bangalore 560012, India. Phone: +91-80-394-2782. Fax: +91-80-360-0683. E-mail: shivu@mrc.iisc.ernet.in
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