C.J. McHargue

The deformation behavior of ceramic crystals subjected to very low load (nano)indentations

The ultra-low load indentation response of ceramic single crystal surfaces (Al{sub 2}O{sub 3}, SiC, Si) has been studied with a software-controlled hardness tester (Nanoindenter) operating in the load range 2--60 mN. In all cases, scanning and transmission electron microscopy have been used to characterize the deformation structures associated with these very small-scale hardness impressions. Emphasis has been placed on correlating the deformation behavior observed for particular indentations with irregularities in recorded load-displacement curves. For carefully annealed sapphire, a threshold load (for a given indenter) was observed below which the only surface response was elastic flexure and beyond which dislocation loop nucleation occurred at, or near, the theoretical shear strength to create the indentation.

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 effects in silicon carbide

Abstract Results from a program, which has existed for some years, on ion implantation effects in α and β silicon carbide will be summarized. Silicon carbide is easily amorphized by ion implantation at room temperature. Amorphization as determined by Rutherford backscattering spectrometry (RBS) occurs for damage energies of about 20 eV/atom, corresponding to 0.2 to 0.3 displacements per atom (dpa), at room temperature. Implantation at higher temperatures (≈ 500°C or above) does not produce an amorphous region for damage levels as high as 17 dpa. Recovery of damage at the subamorphous damage level is fairly complete by 1000°C. Epitaxial regrowth after amorphization occurs over a very narrow temperature range at ≈ 15000°C in an almost “explosive” fashion. Damage and amorphization are accompanied by swelling of up to 15%. The hardness and elastic modulus values of amorphous SiC are 40 and 70%, respectively, of the unimplanted single crystalline values, but before amorphization, the hardness first increases during the early damage phase and then decreases upon amorphization. The oxidation and chemical rates of the amorphous state are higher than for crystalline material. Amorphization kinetics, annealing kinetics and properly changes are broadly compatible with the idea of a critical accumulation model for amorphization.

Ion implantation and thermal annealing of α‐Al2O3 single crystals

The effects of ion implantation and post‐implantation thermal annealing of α‐Al2O3 have been characterized using ion scattering‐channeling techniques, and correlated with electron paramagnetic resonance (EPR) and microhardness measurements. Although most of the work was done on 52Cr implanted specimens, preliminary results have been obtained also for implanted 90Zr and 48Ti. For Cr implantation, the Al2O3 lattice damage saturates at relatively low doses, but the near‐surface region never becomes amorphous. A preferential annealing behavior begins in the Al sublattice after ∼800 °C annealing, and in the oxygen sublattice, only after 1000 °C annealing. Lattice location measurements show that after annealing to 1500 °C, Cr is greater than 95% substitutional in the Al sublattice. Above 1500 °C, implanted Cr atoms redistribute by substitutional diffusion processes. EPR measurements show that part, if not all, of the implanted Cr is trivalent and substitutional after annealing to 1600 °C. Microhardness measurem...

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 .

Recrystallization of ion-implanted α-SiC

The annealing behavior of ion-implanted α-SiC single crystal was determined for samples implanted with 62 keV 14 N to doses of 5.5X10 14 /cm 2 and 8.0X10 16 /cm 2 and with 260 keV 52 Cr to doses of 1.5X10 14 /cm 2 and 1.0X10 16 /cm 2 . The high-dose samples formed amorphous surface layers to depths of 0.17 μm (N) and 0.28 μm (Cr), while for the low doses only highly damaged but not randomized regions were formed. The samples were isochronically annealed up to 1600°C, holding each temperature for 10 min. The remaining damage was analyzed by Rutherford backscattering of 2 MeV He + , Raman scattering, and electron channeling. About 15% of the width of the amorphous layers regrew cpitaxially from the underlying undamaged material up to 1500°C, above which the damage annealed rapidly in a narrow temperature interval. The damage in the crystalline samples annealed linearly with temperature and was unmeasurable above 1000°C.

The amorphization of ceramics by ion beams

The influence of the implantation parameters fluence, substrate temperature, and chemical species on the formation of amorphous phases in Al/sub 2/O/sub 3/ and ..cap alpha..-SiC was studied. At 300/sup 0/K, fluences in excess of 10/sup 17/ ions.cm/sup -2/ were generally required to amorphize Al/sub 2/O/sub 3/; however, implantation of zirconium formed the amorphous phase at a fluence of 4 x 10/sup 16/ Zr.cm/sup -2/. At 77/sup 0/K, the threshold fluence was lowered to about 2 x 10/sup 15/ Cr.cm/sup -2/. Single crystals of ..cap alpha..-SiC were amorphized at 300/sup 0/K by a fluence of 2 x 10/sup 14/ Cr.cm/sup -2/ or 1 x 10/sup 15/ N.cm/sup -2/. Implantation at 1023/sup 0/K did not produce the amorphous phase in SiC. The micro-indentation hardness of the amorphous material was about 60% of that of the crystalline counterpart.

Structural alterations in SiC as a result of Cr+ and N+ implantation☆

Abstract Ion scattering and channeling techniques were used to study production of disorder and randomization of SiC produced by implantation of Cr + and N + at doses up tp to 3 × 10 16 /cm 2 for Cr + and 8 × 10 16 /cm 2 for N + . Experiments were designed so that the calculated damage energy profiles would be well matched for the two ions species. The results were compared for the degree of effectiveness of Cr + and N + in producing disorder. At higher doses, Cr + was much more effective than N + for a given damage energy using the same calculational method for Cr + as N + . In correlated studies of swelling, both species had about the same effectiveness in producing swelling.

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.