Philip S. Sklad

Ion implantation and annealing of crystalline oxides

The technique of ion implantation is being investigated as a general method for altering the near-surface properties of insulating materials. The primary motivation behind these investigations is to develop ion implantation as a practical means of controlling and improving the near-surface mechanical, optical, or electronic properties of insulators. Changes in these properties depend on the microstructures and compositions developed in the material during the ion implantation process and subsequent thermal treatments. In many cases, structures and compositions can be produced by implantation and thermal annealing that cannot be achieved by conventional techniques. In this work, the response of a wide range of crystalline oxides to ion implantation and subsequent thermal processing will be reviewed. The materials treated here include Al 2 O 3 , LiNbO 3 , CaTiO 3 , SrTiO 3 , ZnO, and MgO, as well as the non-oxide materials Si 3 N 4 and SiC. The response of these insulators to ion implantation varies widely and depends on the specific material, the implantation species and dose, and the implantation temperature. Ion implantation produces displacement and other damage in the near-surface region, and in many cases, the surfaces of originally crystalline insulators are turned amorphous. Thermal annealing can often be used to restore crystallinity to the damaged near-surface region, and additionally, metastable solid solutions can be produced. For a number of oxide materials, the annealing behavior has been studied in detail using both Rutherford backscattering-ion channeling techniques and transmission electron microscopy. These studies show that, in some materials, the annealing behavior is quite simple and takes place by solid-phase epitaxial crystallization where the amorphous-to-crystalline transformation occurs at an interface that moves toward the free surface during the annealing process. In such materials, the regrowth kinetics have been measured, and the associated activation energies for crystallization have been determined. The formation of metastable solid solutions during crystallization of the amorphous phase will also be discussed.

Ion implantation and annealing of crystalline oxides and ceramic materials

Abstract The response of several crystalline oxides or ceramic materials to ion implantation and subsequent thermal annealing is described. For both SrTiO 3 and CaTiO 3 single crystals, the near-surface region can be turned amorphous by relatively low doses of heavy ions (Pb, 10 15 /cm 2 , 540 keV). During annealing, the amorphous region recrystallizes epitaxially with the underlying substrate by simple solid-phase epitaxy, and the crystallization kinetics have been determined for both of these materials. In Al 2 O 3 , the amorphous phase of the pure material is produced by a stoichiometric implant at liquid nitrogen temperature. During annealing, the amorphous film crystallizes in the (crystalline) γ phase, followed by the transformation of the γ to the α phase at a well-defined interface. The kinetics characterizing the growth of α-Al 2 O 3 have been determined. Preliminary results are presented on the effect of impurities (Fe) on the nature and kinetics of the crystallization of amorphous Al 2 O 3 .

Microstructural and chemical effects in Al2O3 implanted with iron at room temperature and annealed in oxidizing or reducing atmospheres

Rutherford backscattering (RBS)-ion channeling, transmission electron microscopy (TEM), and conversion electron Moessbauer spectroscopy (CEMS) have been used to determine the structure of {alpha}--Al{sub 2}O{sub 3} implanted with iron at room temperature. Changes produced by post-implantation annealing in oxidizing and reducing atmospheres were followed using the same techniques. Implantation of 160 keV Fe at room temperature produces a damaged but crystalline microstructure for fluences as high as 1{times}10{sup 17} Fe/cm{sup 2}. The iron resides in a variety of local environments: three Fe{sup 2+} components, one Fe{sup 0} component, and two Fe{sup 4+} components. The relative amount of each component varies with implantation fluence. Only the Fe{sup 0} component seems to be associated with second-phase formation. In this case, 2 nm diameter {alpha}-iron particles were detected by TEM studies. Recovery of implantation-induced disorder in the Al- and oxygen-sublattices occurs in two stages for annealing in oxygen and in one continuous stage for hydrogen-annealing. The end state for iron is Fe{sup 3+} for oxygen anneals and Fe{sup 0} for hydrogen anneals. The precipitated phases observed are those to be expected from the equilibrium phase diagrams.

Iron ion implantation effects in sapphire

Single crystals of α-Al 2 O 3 have been implanted at room temperature with 160 or 100 keV 57 Fe + ions and doses ranging from 10 16 Up to 10 17 ions cm −2 . The valence states and the local environment of implanted atoms as well as the damage in the implanted zone have been studied with the conversion electron Mossbauer spectroscopy technique associated with channeling and transmission electron microscopy. It was found that implantation introduces iron in sapphire in three charge states: Fe 2+ , Fe 3+ and metallic precipitates. Annealings in oxidizing or reducing atmosphere at temperature up to 1500°C convert all iron into Fe 3+ or Fe 0 respectively, and the precipitations of small oxide or metallic iron particles are observed correlatively with the rearrangement of the matrix.

Modelling of residual stresses and property distributions in friction stir welds of aluminium alloy 6061-T6

Abstract An integrated thermal–metallurgical–mechanical model is used to analyse and provide insights into the formation of the residual stress and the changes in microstructure and property of Al6061-T6 friction stir welds. The simulations were conducted by means of a three-dimensional finite element model that accounts for the phenomena of frictional heating, weld microstructure and strength changes due to dissolution and reprecipitation of the hardening precipitate particles, and the mechanical workpiece/tool contact during the friction stir welding (FSW) process. The model predictions were confirmed by experimental measurement data from previous studies. For the friction stir welds investigated, it was found that the residual stress distribution is strongly dependent on the welding process parameters and the degree of material softening caused by welding. The recovery of material strength from natural aging does not increase the residual stress in the weld. The failure of friction stir weld under tensile load is controlled by the combination of the reduction in strength and the residual stresses in the heat affected zone (HAZ).

The structure of ion implanted ceramics

Abstract The structure of ion implanted ceramics may be crystalline with large concentrations of point defects, point defect clusters, and dislocations, or it may be amorphous. The details of the implanted microstructure depend upon the implantation parameters including ion species, fluence, and substrate temperature. For a given set of implantation parameters, the as-implanted microstructure depends upon the type of chemical bonding present in the ceramic. A second level of structure is the distribution of the implanted ions between substitutional and interstitial lattice sites and among various residual charge states. The amorphous state may contain different short-range order for different implanted ion species. Recent results for these various effects are reviewed.

Damage accumulation in ceramics during ion implantation

Abstract The damage structures of α-Al2O3 and α-SiC were examined as functions of ion implantation parameters using Rutherford backscattering-channeling, analytical electron microscopy, and Raman spectroscopy. Low temperatures or high fluences of cations favor formation of the amorphous state. At 300 K, the mass of the bombarding species has only a small effect on the residual damage state, but certain ion species appear to stabilize the damage microstructure and increase the rate of approach to the amorphous state. The type of chemical bonding present in the host lattice is an important factor in determining the residual damage state.

Formation of amorphous layers in Al2O3 by ion implantation

Abstract Conditions are described which can be used to turn the near surface region of Al2O3 amorphous by ion implantation. Epitaxial recrystallization of the amorphous region during thermal annealing is demonstrated. Changes in the physical and structural properties as a result of implantation are correlated with changes in the mechanical properties (microhardness).

Thermal annealing of Fe implanted Al2O3 in an oxidizing and reducing environment

Abstract The effects of ion implantation of Al2O3 followed by thermal annealing in a reducing environment have been studied and compared with the results obtained by annealing in an oxidizing environment. For the case of implanted Fe, partially coherent crystalline precipitates are observed after annealing in an oxidizing environment, and evidence is presented that crystalline precipitates of a different nature also form during annealing in a reducing environment. The precipitates which form in an oxidizing annealing environment have a major channeling axis nearly aligned with the Al2O3 〈0001〉 direction. These precipitates are tentatively identified as Fe3O4. Lattice damage recovery is also different in the two annealing environments.

Microstructural and chemical effects in Al 2 O 3 implanted with iron at 77 K and annealed in oxidizing or reducing atmospheres

Implantation of Fe (160 keV) into {alpha}--Al{sub 2}O{sub 3} at 77 K produces an amorphous surface layer for fluences in the range of 10{sup 16} to 10{sup 17} Fe/cm{sup 2}. Measurements of short-range order were made by extended energy loss fine structure analysis (EXELFS). The structure of amorphous Al{sub 2}O{sub 3} produced by implantation of iron at 77 K exhibits short-range order that differs from that produced by stoichiometric (Al+O) implants. This difference is manifested by changes in the Al--O near-neighbor bond length. The local environments of implanted iron were determined from conversion electron Moessbauer spectroscopy (CEMS). The iron resides in several different local environments consistent with the electronic states of Fe{sup 2+}, Fe{sup 4+}, and Fe{sup 0}. The relative amount of each environment depends upon the concentration (fluence) of the implanted iron ions. Regrowth of the amorphous zone during annealing occurs in the sequence amorphous {r arrow}{gamma}--Al{sub 2}O{sub 3}{r arrow}{alpha}--Al{sub 2}O{sub 3}. The kinetics of regrowth and phase separation vary with implanted fluence and with annealing atmosphere. The higher the concentration of implanted iron, the slower the formation of iron-aluminum oxide precipitate phases in oxidizing atmospheres and {alpha}--Fe precipitates in reducing atmospheres.