D.L. Joslin

Reaction synthesis of FeAl alloys

Abstract Reaction synthesis of several FeAl alloys has been observed in an evacuated quartz tube under a vacuum of 10 −4 Pa. The compositions studied include 25, 30, 35.8, 45, and 54 at.% Al. Direct observation of the progression of the reaction of compacts formed from 45 μm Fe and 10 μm Al powders revealed that an unusual “two-stage” rection exists under these conditions. The occurrence of this two-stage reaction is sensitive to the composition, density, and volume of the powder compact. The first stage, which lasts several seconds, starts at around 650 °C. After it passes through the powder compact, the temperature continues to rise, and the second reaction begins at about 900 °C. The second reaction reaches temperatures between 1250 and 1350 °C. Electron microprobe analysis of samples quenched during the first reaction stage revealed the presence of large amounts of FeAl 3 together with unalloyed Fe and small amounts of an aluminum-rich phase (∼ 95 at.% Al). Samples allowed to react through the second stage contained primarily FeAl and Fe 3 Al, with very small amounts of FeAl 2 . The occurrence of the “two-stage” reaction is explained in terms of thermodynamics and heat transfer.

Ion beam mixing in insulator substrates

Abstract Studies of ion beam mixing involving insulator substrates is reviewed with an emphasis on thermodynamical and thermochemical considerations. Such studies generally have employed the bi-layer configuration of metal/insulator combinations. There is little evidence for long-range material transport. In the case of sapphire (single crystal Al2O3) substrates, only recoil mixing has been detected for both metal and oxide films irradiated at room temperature. “Demixing” of recoil-implanted cations is indicated at temperatures of 825–900°C. Agglomeration of non-wetting metal films suggests that surface diffusion occurs under the influence of the ion beam. Ballistic mixing has been reported for metal films on SiC, Si3N4 and SiO2. Ion beam-induced reactions are observed at the interfaces for systems in which the enthalpy of reaction is favorable. There is evidence that chemical effects in the cascade mixing regime determine the phases present in the mixed zone.

Etching of amorphous Al2O3 produced by ion implantation

Abstract Single crystal Al 2 O 3 (sapphire) is noted for its extreme resistance to attack by most chemical reagents. The reactivity of amorphous Al 2 O 3 has not been reported although it is well known that an amorphous layer can be produced by ion beam irradiation at 77 K or by implantation of certain chemical species at 300 K. In the current study, an amorphous layer was produced by ion beam mixing of Zr/Al 2 O 3 using Kr at 300 K or by implantation with Cr, Zr, Sn or Al plus O at 77 K. The amorphous layer can be completely removed by etching in an acid solution of HNO 3 HFH 2 O. The attack ceases once the amorphous region is dissolved. Crystalline samples containing the same implanted species are not attacked. The amorphous state is necessary for this enhanced chemical reactivity and the results for the stoichiometric implant of Al + O indicate that it is sufficient. The implanted (impurity) species may influence the rate of dissolution.

The hardness and elastic modulus of chromium-implanted silicon carbide☆

Abstract The hardness and elastic modulus of α-SiC single crystals were determined using an ultra-low load micro-indentation hardness tester for implantation conditions that produced both crystalline and amorphous as-implanted surfaces. The effect of fluence for 260 keV chromium implantation was noted as was the effect of substrate temperature. For implantation at room temperature, the relative hardness (implanted/unimplanted) rises to a maximum of 1.10 at a fluence of 1.5 × 1014 Cr/cm2 (260 keV) and then decreases with increasing fluence as amorphization begins. The fully amorphous silicon carbide has a hardness value that is about 40% of the unimplanted value. The elastic modulus of the implanted material is lower for all implanted conditions.

Ion mixing in oxide-sapphire systems

Ion beam mixing of thin oxide films on sapphire substrates has been studied in order to examine any role of equilibrium thermodynamic parameters on the mixing process. Mixing experiments were performed with polycrystalline oxide films deposited on single crystalline {alpha}-Al{sub 2}O{sub 3} substrates. According to the equilibrium phase diagrams, Cr{sub 2}O{sub 3} is completely soluble in {alpha}-Al{sub 2}O{sub 3}, while ZrO{sub 2} is insoluble. The couples were irradiated with Cr ions (160 and 340 keV) or Kr ions to fluences of 4 {times} 10{sup 16} ions/cm{sup 2} at temperatures between 20 and 900{degrees}C. Rutherford backscattering spectrometry, X-ray photoelectron spectroscopy, and transmission electron microscopy were used to analyze samples before and after irradiation to determine the extent and nature of interface modifications. No long-range mixing was detected under any condition studied; the width of the {open_quotes}mixed{close_quotes} region in each case was consistent with recoil mixing. The absence of long-range mixing is rationalized in terms of the different ranges of oxygen ions and cations.

The Structure and Properties of Ion Implanted Ceramics

Ion implantation consists of bombarding a sample’s surface with an electrostatically accelerated beam of ions in a vacuum chamber. Energies of a few tens to several hundred kiloelectron volts are commonly used. Because of their kinetic energy, the ions become embedded to a depth of about one micrometer or less; the depth is determined by the incident ion energy and ion mass for a given substrate material. The ions come to rest by dissipating the kinetic energy through elastic and inelastic (ionizing) collisions with the target atoms (ions). The end result is the production of large numbers of lattice defects and the introduction of an impurity or alloying species. Figure 1 illustrates the distribution of damage energy, i.e., defect production, and distribution of implanted impurity ions for 150 keV Cr implantation into sapphire.