Showing papers in "Journal of Materials Research in 1986"

A method for interpreting the data from depth-sensing indentation instruments

Abstract: Depth-sensing indentation instruments provide a means for studying the elastic and plastic properties of thin films. A method for obtaining hardness and Young’s modulus from the data obtained from these types of instruments is described. Elastic displacements are determined from the data obtained during unloading of the indentation. Young’s modulus can be calculated from these measurements. In addition, the elastic contribution to the total displacement can be removed in order to calculate hardness. Determination of the exact shape of the indenter at the tip is critical to the measurement of both hardness and elastic modulus for indentation depths less than a micron. Hardness is shown to depend on strain rate, especially when the hardness values are calculated from the data along the loading curves.

Indexing of icosahedral quasiperiodic crystals

Abstract: Since the definition of quasiperiodicity is intimately connected to the indexing of a Fourier transform, for the case of an icosahedral solid, the step necessary to prove, using diffraction, that an object is quasiperiodic, is described. Various coordinate systems are discussed and reasons are given for choosing one aligned with a set of three orthogonal two-fold axes. Based on this coordinate system, the main crystallographic projections are presented and several analyzed single-crystal electron diffraction patterns are demonstrated. The extinction rules for three of the five icosahedral Bravais quasilattices are compared, and some simple relationships with the six-dimensional cut and projection crystallography are derived. This analysis leads to a simple application for indexing powder diffraction patterns.

Kinetics of formation of silicides: A review

Abstract: The kinetics of silicide growth are classified into three different categories: (a) diffusion controlled, (b) nucleation controlled, (c) others (reaction rate controlled). These are analyzed with the aim of understanding both the phenomenology of growth and the specific atomic mechanisms of phase formation. Diffusion-controlled growth is discussed with respect to the Nernst-Einstein equation. Stress relaxation is considered as a possible cause of reaction-rate control. The relative merits of two different types of marker experiments are compared. A few silicides are discussed in terms of what can be inferred about diffusion mechanisms. The competition between reaction-rate and diffusion control phenomena is shown to have specific effects on the sequence of phase formation; it is also related to the formation of some amorphous compounds. Reactions between silicon and alloyed metal films are used to illustrate the respective influences of mobility and driving force factors on the kinetics of silicide growth; they can also be used to underline the dominance of nucleation over diffusion in some silicide formation processes.

Plastic properties of thin films on substrates as measured by submicron indentation hardness and substrate curvature techniques

Abstract: Substrate curvature and submicron indentation measurements have been used recently to study plastic deformation in thin films on substrates. In the present work both of these techniques have been employed to study the strength of aluminum and tungsten thin films on silicon substrates. In the case of aluminum films on silicon substrates, the film strength is found to increase with decreasing thickness. Grain size variations with film thickness do not account for the variations in strength. Wafer curvature measurements give strengths higher than those predicted from hardness measurements suggesting the substrate plays a role in strengthening the film. The observed strengthening effect with decreased thickness may be due to image forces on dislocations in the film due to the elastically stiffer silicon substrate. For sputtered tungsten films, where the substrate is less stiff than the film, the film strength decreases with decreasing film thickness.

Formation of LanxideTMa) ceramic composite materials

Abstract: An overview is given of a new process that has been used successfully to make numerous ceramic/metal composite materials by directed oxidation of molten metallic precursors. As an example, the formation of A12O3/A1 composites from Al is discussed in detail.

Nanosecond resolution time-resolved x-ray study of silicon during pulsed-laser irradiation

Abstract: We have used the pulsed time structure of the Cornell High-Energy Synchrotron Source (CHESS) to carry out a nanosecond resolution time-resolved x-ray study of silicon during pulsed-laser irradiation. Time-resolved temperature distributions and interfacial overheating and undercooling were measured on 〈111〉 and 〈100〉 silicon during 25 ns UV laser pulses through the analysis of thermal expansion induced strain. The temperature gradients were found to be > 107 K/cm at the liquid-solid interface and the temperature distributions have been shown to be in agreement with numerical heat flow calculations for these laser conditions. The combined overheating and undercooling (during ∼ 10 m/s melting and ∼ 6 m/s regrowth) was measured to be 110 ± 30 K on 〈111〉 oriented silicon and 50 ± 25 K on 〈100〉 silicon. These values have been interpreted in terms of velocity coefficients of overheating and undercooling.

Solid phase equilibria in the Au-Ga-As, Au-Ga-Sb, Au-In-As, and Au-In-Sb ternaries

Abstract: : The Au-Ga-As, Au-Ga-Sb, Au-In-As, and Au-In-Sb ternaries were surveyed using X-ray powder diffraction to determine which metallic phases exist at equilibrium with the III-V compound semiconductors. In closed, small-volume systems (i.e., formation of gas-phase products was prevented), Au does not react with GaAs but does react with the other III-V's investigated to produce Au-Group III intermetallic compounds and another solid phase containing the group V element. However, each semiconductor formed pseudobinary systems with at least two different intermetallic compounds. The bulk phase diagrams determined in this study provide frameworks within which much of the experimental data in the literature concerning the products of reactions at Au/III-V interfaces can be understood.

Thermodynamic aspects of amorphous phase formation

Abstract: The glass-forming ability of the three alloy systems Co–Zr, Cu–Zr, and Ni–Zr has been analyzed for three distinct production routes: (1) liquid quenching, (2) vapor deposition, and (3) solidstate reaction. Using the free energy and heats of formation curves obtained from the thermodynamic characterization of the respective alloy systems, a satisfactory rationale can be obtained for amorphous phase formation by all three routes. The analysis shows that while amorphous phase formation by quenching from the high-temperature liquid is clearly dependent on factors such as quench rate and the value TG/TM, it is the low-temperature stability of the amorphous phase relative to the other crystalline structures that enables amorphous phases to be formed by both vapor deposition and solid-state reaction. The underlying free energy curves indicate the interesting possibility of a supersaturation sequence in the nucleation of an amorphous phase by solid-state reaction. The principles underlying thermodynamic characterizations are briefly discussed, and a characterization for Co–Zr is presented.

Influence of toughness on Weibull modulus of ceramic bending strength

Abstract: It is demonstrated both theoretically and experimentally that fracture toughness does not directly influence the Weibull modulus of ceramic bending strength for materials that obey the Griffith criterion for crack propagation. Weibull modulus remains unchanged as toughness is increased. However, toughness variations with crack length do affect the Weibull modulus. Thus materials that display R-curve behavior or Dugdale character give an increased Weibull modulus and appear more reliable.

Higher nitrides of hafnium, zirconium, and titanium synthesized by dual ion beam deposition

Abstract: We report the preparation and properties of higher nitrides of Hf, Zr, and Ti synthesized by dual ion beam deposition. For Hf and Zr, evidence is given for the existence of a metastable nitride phase with composition of approximately Hf3 N4 and Zr3 N4. These two materials are insulating and transparent straw colored, in contrast to the well-known mononitrides, which are shiny, gold colored, and highly conducting. For Ti-N we do not reach as high an N content and do not obtain an insulating, transparent phase. The higher nitrides of Hf and Zr are synthesized under energetic nitrogen ion bombardment (200 e V) of a growing film and do not form in the presence of molecular nitrogen gas alone. Several variations of the ion beam deposition process are used to obtain a wide range of film composition and to study the transition from the mononitride to the higher nitride phase. Transmission electron diffraction shows the structure of Hf3N4 and Zr3N4 to be very close to the Bl (NaCl) structure of the mononitrides, but with a slight rhombohedral distortion. Additional evidence from noble gas incorporation (Ne, Ar, and Xe) supports a model of these higher nitrides as containing a large number of vacancies on the metal atom sites.

Alpha-recoil damage in zirconolite (CaZrTi 2 O 7 )

Abstract: Radiation effects in a natural, metamict zirconolite from Sri Lanka that has received an alpha-decay dose greater than 1026 alpha decays/m3 have been studied using x-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), electron paramagnetic resonance spectroscopy (EPR), extended x-ray absorption fine structure spectroscopy (EXAFS), and x-ray absorption near edge structure spectroscopy (XANES). The same techniques were applied to the sample annealed between 1000°and 1100°C. The heat of recrystallization was measured by differential thermal analysis (DTA) with values of 40 to 50J/g. In contrast to previous work [A. E. Ringwood, Am. Scientist 70, 201 (1982); Mineralog. Mag. 49, 159 (1985)], these results demonstrate that there are fundamental differences at the atomic level between the annealed, crystalline, and the natural, fully damaged zirconolite. We suggest that the most likely structure for the fully damaged state is that of a random, three-dimensional network with no atomic periodicity extending beyond the first coordination sphere. Even within the first coordination sphere, there is a reduction in coordination number and an associated decrease in bond length for principal cations (Ca and Ti). Despite this structural modification and the great age of the specimen (∼550 million years), the natural zirconolite shows only minor signs of geochemical alteration. The metamict structure is readily annealed, initially to a disordered, flourite-type structure, and finally, at higher temperatures, to a highly twinned, monoclinic zirconolite structure.

Factors influencing the stability of the tetragonal form of zirconia

Abstract: The p-italicH of the solution that zirconia is precipitated from defines the crystal phase formed after calcination of the material at 400 /sup 0/ to 600 /sup 0/C. A metastable tetragonal form is obtained for either low (less than about 5) or high (greater than about 13) p-italicH. The tetragonal phase formed at high p-italicH is much more stable at the calcination temperature than the material obtained at a low p-italicH is. For a material obtained by precipitation at a low p-italicH, monoclinic crystallites, determined by x-ray line broadening, were obtained that were smaller than the tetragonal crystals that produced them. A crystallite size effect, based on x-ray line broadening, is not responsible for the formation or stabilization of the tetragonal phase.

The effect of lateral crack growth on the strength of contact flaws in brittle materials

Abstract: The effect of lateral cracks on strength controlling contact flaws in brittle materials is examined. Inert strength studies using controlled indentation flaws on a range of ceramic, glass, and single crystal materials reveal significant increases in strength at large contact loads, above the predicted load dependence extrapolated from strength measurements at low indentation loads. The increases are explained by the growth of lateral cracks decohesing the plastic deformation zone associated with the contact from the elastically restraining matrix, thereby reducing the residual stress field driving the strength controlling radial cracks. A strength formulation is developed from indentation fracture mechanics which permits inert strengths to be described over the full range of contact loads. The formulation takes account of the decreased constraint of the plastic deformation zone by lateral crack growth as well as post-contact nonequilibrium growth of the radial cracks. Simple extensions permit the strengths of specimens controlled by impact flaws to be described, as well as those failing under nonequilibrium (fatigue) conditions. The implications for materials evaluation using indentation techniques are discussed and the dangers of unqualified use of strength measurements at large indentation loads pointed out. The work reinforces the conclusion that a full understanding of the residual stress field at dominant contact flaws is necessary to describe the strength of brittle materials.

Transmission electron microscopy of process-induced defects in β-SiC thin films

Abstract: Transmission electron microscopy (TEM) has proven to be an invaluable tool in a multifaceted research program at North Carolina State University, which centers on developing β-SiC as a useful semiconducting material. As such, techniques have been developed for fabricating both plan-view and cross-sectional TEM silicon carbide thin films grown by chemical vapor deposition. The results of the TEM observations are used to determine the effectiveness of specific processing parameters in terms of film quality and defect structure as well as oxidation, ion implantation, and annealing procedures.

Self-propagating high-temperature synthesis of the SiC

Abstract: Self-propagating high-temperature synthesis (SHS), also called combustion synthesis, is useful for fabricating numerous ceramics. In the case of SiC, heat released from the exothermic reaction is not sufficient to completely convert the mixed reactants of constituent elements into SiC in the usual nonadiabatic experimental system. This disadvantage could be overcome by a new ignition process called, the “direct passing method of electric current.” By using this method, stoichiometric fine SiC powder could be obtained rapidly and efficiently with low electric power. This paper also involves the effect of particle size of Si and C initial reactant powders on conversion efficiency into SiC and also on particle size of SiC powder fabricated by this method.

Nucleation behavior of Al–Mn icosahedral phase

Abstract: Electrohydrodynamic (EHD) atomization has been used to rapidly solidify micron and submicron size droplets of Al-14 at. % Mn to study nucleation behavior of icosahedral phase. Icosahedral grain size has been found to decrease continuously with decreasing droplet size. Based on this result, formation of the icosahedral phase is explained by homogeneous nucleation. Extremely low resistance to nucleation of icosahedral phase can be understood if possible topological similarities between liquid and icosahedral quasicrystal are considered. Formation of glass as configurationally frozen liquid in Al-Mn and similar alloy systems is questionable, implying that the reported Al-Mn glass probably has a microquasicrystalline structure.

Review: The nucleation of disorder

Abstract: Four types of phase transformation that involve the conversion of crystalline phases into more disordered forms are reviewed: melting, disordering of superlattices, amorphization by diffusion between crystalline phases, and irradation amorphization. In the review emphasis is placed on evidence for the heterogeneous nucleation of the product phases; in this connection, the role of surfaces, antiphase domain boundaries, dislocations, vacancies, and grain boundaries is specifically discussed. All of these features have been either observed, or hypothesized, to play a role as heterogeneous nucleation sites in one or more of the four transformations. An attempt is made to draw parallels between nucleation mechanisms in the various processes.

Mechanisms of the electron irradiation-induced amorphous transition in intermetallic compounds

Abstract: A buildup of radiation-induced lattice defects is proposed as the cause for lattice instability that can give rise to a crystalline-to-amorphous transition. An analysis of published experiments on intermetallic compounds suggests that, when amorphization takes place, no microstructural evolution based on the aggregation of like-point defects occurs. This observation leads us to suggest that buildup of a different type of defect, which will destabilize the crystal, should occur. We thus propose that an interstitial and a vacancy may form a complex, giving rise to a relaxed configuration exhibiting a sort of short-range order. Two mechanisms of complex formation are analyzed, one diffusionless (limited by the point defect production rate) and the other temperature dependent. The amorphization kinetics as a function of temperature, dose, and point defect sink strength are studied. Theoretical predictions on the amorphization dose as a function of temperature are made for the equiatomic TiNi alloy and compared with available experimental results.

Icosahedral ordering in quasicrystals and metallic glasses

Abstract: Many workers have recently conjectured that the underlying forces responsible for icosahedral clustering in crystals may also explain the origin of metallic glass formation. Critical examination of thermochemical structural diagrams and radial distribution functions of good metallic glasses shows that this assertion is unfounded.

Oxidation of SiC powder by high-temperature, high-pressure H2O

Abstract: The reaction between SiC powder and H2O has been studied at 400°–800 °C under 10 and 100 MPa. Silicon carbide reacted with H2O to yield amorphous SiO2 and CH4 by the reaction SiC + 2H2O→SiO2 + CH4 above 500 °C. Cristobalite and tridymite crystallized from amorphous silica after the almost complete oxidation of SiC above 700 °C. The oxidation rate, as calculated from the weight gain, increased with temperature and pressure. The Arrhenius plotting of the reaction rate based on a Jander-type model gave apparent activation energies of 167–194 kJ/mol. Contrasted with oxidation in oxidative atmosphere, oxidation in H2O is characterized by the diffusion of H2O and CH4 in an amorphous silica layer where the diffusion seemed to be rate determining. Present results suggest that the oxidation of SiC includes the diffusion process of H2O in silica layers when atmospheres contain water vapor.

Entropy-driven loss of gas phase group V species from gold/III-V compound semiconductor systems

Abstract: : Temperature dependent chemical interactions between Au and nine III-V compound semiconductors (III=A1, Ga, In and V=P, As, Sb) have been calculated using bulk thermodynamic properties. Enthalpic considerations alone are insufficient to predict metal/compound-semiconductor reactivities. The entropy of vaporization of the group V elements is shown to be an extremely important driving force for chemical reactions involving the III-V's, since it enables several endothermic reactions to occur spontaneously under certain temperature and pressure conditions. Plots of either Gibb's Free energies of reaction or equilibrium vapor pressure of the group v element versus temperature are used to predict critical reaction temperatures for each of the systems studied. These plots agree extremely well with previous experimental observations of thin film reactions of Au on GaAs.

Temperature dependence of tracer diffusion coefficients in polystyrene

Abstract: The temperature dependence of the tracer diffusion coefficient D* of long deuterated polystyrene (d-PS) chains of molecular weight M>Mc, where Mc is the critical molecular weight for entanglement, diffusing into highly entangled PS matrices, each of molecular weight P = 2×107, is studied using forward recoil spectrometry It is found that the temperature dependence of D*/T, reflected primarily in the monomeric friction coefficient, is accurately described by a Vogel equation The constants that are used to fit these results are independent of M and are the same as those used to fit the temperature dependence of the zero shear rate viscosity of polystyrene

Evidence for a nucleation barrier in the amorphous phase formation by solid-state reaction of Ni and single-crystal Zr

Abstract: In an ultrahigh vacuum (UHV) environment, thin poly crystalline Ni films have been deposited on a Zr (112) single-crystal surface. In contrast to the case of poly crystalline Ni and Zr films, formation of amorphous Ni—Zr is not observed upon annealing at 300 °C for 11.5 h. The possible presence of an oxide or an amorphous phase diffusion barrier is ruled out and therefore the lack of a reaction must be due to a reaction barrier at the single-crystal Zr/ Ni interface. Either ion mixing of the interface with 400 keV, 5 × 1015 Xe+ /cm2, or deposition of a poly crystalline Zr layer in between the Zr single crystal and the Ni overlayer can overcome this reaction barrier. These results indicate that grain boundaries in polycrystalline Zr play an important role in amorphous Ni—Zr formation by a solid-state reaction.

Carbon effects in rapidly solidified Ni 3 Al

Abstract: The effect of carbon on the mechanical properties of ordered, face-center-cubic Ni3Al has been studied. It has been found that carbon provides no ductihzation to the intermetallic compound, but exerts a large solid solution strengthening effect. The strengthening rate measured is Δσy/ΔC∼0.5G per atom percent carbon, where G is the Ni3Al shear modulus. Auger analysis and lattice parameter measurements were also carried out. The results are discussed with respect to the nature of carbon in grain boundary regions and in the bulk.

Amorphization of rapidly quenched quasicrystalline Al-transition metal alloys by the addition of Si

Abstract: The structure and thermal stability of rapidly solidified Al-Cr-Si, Al-Mn-Si, Al-Fe-Si, Al-Co-Si, and Al-Ni-Si alloys have been investigated using x-ray diffraction and thermal analysis measurements. Each series of alloys shows a region of stoichiometry that yields icosahedral symmetry and a region that yields an amorphous phase. Thermal and structural properties of these alloys are reported as a function of stoichiometry and quench rate.

Creep behavior of a polycrystalline nickel aluminide: Ni-23.5 at.% A1-0.5 at.% Hf-0.2 at.% B

Abstract: The creep behavior of a poly crystalline nickel aluminide with the composition Ni-23.5 at.% Al-0.5 at. % Hf-0.2 at. % B has been measured as a function of stress, temperature, and grain size. At high stresses, of the order of 100 MPa, the strain rate is nonlinear in the stress, with a stress exponent greater than two. Below approximately 10 MPa, at 1033 K, the steady-state strain rate is almost proportional to the stress, indicating that diffusional creep is rate controlling. Calculations of expected Nabarro—Herring and Coble creep rates did not answer whether diffusive mass transport through the grains, or along the grain boundaries, is rate controlling. The grain-size dependence of the strain rate, however, indicates predominance of volume diffusion control, i.e., Nabarro—Herring creep, for our experimental conditions.

Growth of tetragonal BaTiO3 single crystal fibers

Abstract: With the increasing use of optical fibers in the telecommunication network, there is need for fiber geometry compatible optical devices such as optical amplifiers, switches, couplers, and isolators. These active devices are based on field-dependent material properties, such as electrooptic and magneto-optic effects, which are stronger in single crystal than in amorphous materials. Single crystal fibers can be grown by the laser heated pedestal growth (LHPG) technique. In this paper we report the growth of single crystal fibers of ferroelectric barium titanate from sintered ceramic rods of stoichiometric barium titanate. Barium titanate is one of the most extensively investigated ferroelectric materials. However, its growth from stoichiometric melt always results in its hexagonal nonferroelectric phase. Using LHPG, single crystal strontium titanate seed, and sintered ceramic barium titanate rods, we have succeeded in growing single crystal fibers (∼ 100 μ m diameter) of pure barium titanate with tetragonal (ferroelectric) crystal structure. This paper discusses growth and characterization of these fibers.

Chemical preparation and properties of high-field zinc oxide varistors

Abstract: Chemical preparation methods were developed for high-field ZnO varistors which used precipitation techniques to prepare precursor powders. Varistors were made by sintering uniaxially pressed pellets in the range of 675°–740 °C in air. Properties of these varistors included electric fields (E) in the 10–100 kV/cm range at current densities (J) of 5 A/cm2, nonlinearity coefficients (α) greater than 30 for 2.5≤J≤5.O A/cm2, and densities in the range of 65%-99% of theoretical depending both on sintering temperature and composition.

Chemical reactions at the Au/InP interface

Abstract: : Chemical reactions at the Au/InP interface were investigated in the temperature range 25 510C by x-ray diffractometry, scanning electron microscopy and energy dispersive X-ray analysis. The samples were prepared by depositing gold films onto clean InP(100) single-crystal substrates under .000000001 torr vaccum. Spots appeared in SEM micrographs of the deposited film of a sample annealed at room temperature for 35 days, indicating that a solid state reaction had occurred. After the sample was heated to 330C under flowing N2 gas, dendritic islands were observed in the film. A 365C anneal turned the color of the Au film from yellow to pink, and Au, dendritic islands and Au2P3 were identified by X-ray diffractometry. In reacting with the InP substrate, dendritic islands produced silver-colored gamma and more Au2P3 at 450C. With respect to InP, gamma was relatively stable, and further reactions were minimal even when the sample was annealed at 510C for 40 min under atmospheric-pressure N2. The ternary phase diagram for the bulk Au-In-P system provides the basis for understanding the sequence of the above results and much of the information in the literature about Au/InP interfacial reactions.

Knoop hardness anisotropy on {001} faces of germanium and gallium arsenide

Abstract: Knoop hardness measurements have been carried out as a function of azimuthal angle and temperature (in the range 20°–440°C) on {001} faces of n-type, p-type, and intrinsic Ge and GaAs. The degree of hardness anisotropy shown increases with increasing temperature and for Ge is undetectable below a certain temperature which depends on doping. In GaAs, asymmetry in hardness between [110] and [1 1 0] directions was found at high temperatures. A new model of hardness anisotropy has been developed, based on detailed modeling of the plastic zone. This relates the hardness to the degree of workhardening in different regions of the plastic zone. Using this model, detailed explanations are given of the hardness anisotropy behavior and of the plastic recovery around indentations.