Friction properties of composites of the system TiN−AlN. I. Effect of structure and phase composition of friction and wear of materials of the system TiN−AlN

The tribological properties of ceramic composites of the system TiN?AlN are studied in the concentration range 10–90% AlN. It was found that materials which contain 25, 50, and 75% AlN have a low friction coefficient. Their rate of wear when paired with steel is neglible—2.9–6.0 μm/km (the wear rate of a steel-steel 024 couple is 1000 μm/km). Thin oxide films are formed during friction at high velocities and pressures. These films have relatively high adhesion with respect to materials of the TiN?AlN system and relatively low adhesion with respect to steel. The films may act as a solid lubricant, thus reducing the friction coefficient and wear. This is particularly true of the materials 25% TiN?75% AlN and 50% TiN?50% AlN.     

Mechanical properties of vacuum-deposited metal films—II. Nickel films

The mechanical properties of vacuum-deposited polycrystal and single crystal Ni films in the thickness range 2000 to 20,000 Å are presented and discussed. It was found that the average tensile strength of as-deposited polycrystalline Ni films was about 100 kg mm⁻² and this is reduced by a factor two upon annealing, as a result of structural changes involving an increase of the average grain size. Single crystal films have an average strength of 50 kg mm⁻² which is about twice the value for annealed bulk material (32.4 kg mm⁻²). No strength-thickness relationship is observed for any of the investigated Ni films in the above mentioned range of thicknesses. Electron microscopy results are presented which provide some information regarding the grain sizes and structural natures of the different types of specimens investigated.     

On the tensile properties of a fiber reinforced titanium matrix composite—II. Influence of notches and holes

The effects of holes and notches on the ultimate tensile strength of a unidirectionally reinforced titanium matrix composite have been examined. During tensile loading, a narrow plastic strip forms ahead of the notch or hole prior to fracture, similar to that observed in thin sheets of ductile metals. Examination of the fibers following dissolution of the matrix indicates that essentially all the fibers within such a strip are broken prior to catastrophic fracture of the composite. The trends in notch-strength have been rationalized using a fracture mechanics-based model, treating the plastic strip as a bridged crack. The observations suggest that the bridging traction law appropriate to this class of composite is comprised of two parts. In the first, the majority of fibers are unbroken and the bridging stress corresponds to the unnotched tensile strength of the composite; in the second, the fibers are broken and the bridging stress is governed by the yield stress of the matrix, with some contribution derived from fiber pullout. This behavior has been modeled by a two-level rectilinear bridging law. The parameters characterizing the bridging law have been measured and used to predict the notch strength of the composite. A variation on this scheme in which the fracture resistance is characterized by an intrinsic toughness in combination with a rectilinear bridging traction law has also been considered and found to be consistent with the predictions based on the two-level traction law.     

Oxidation resistance and strength of a molybdenum fiber–oxide matrix composite material

The oxidation kinetics of a composite material, which consists of an Al₂O₃–Al₅Y₃O₁₂ matrix and molybdenum fibers and has a high cracking resistance, is studied. The mass loss of the composite material during oxidation is shown to be several orders of magnitude lower than that of molybdenum. Oxidation in quiet air at 1250°C for several hours weakly changes the strength of the composite material at temperatures from room temperature to 1300°C. It is also shown that the strength of the composite material as a function of the oxide matrix composition (Al: Y ratio) changes nonmonotonically. The maximum strength shifts from the Al₂O₃–Al₅Y₃O₁₂ eutectic point toward garnet.     

Modelling of the temperature and the velocity of ceramic powder particles in a plasma flame—II. Zirconia

The model developed in Part I of this study, has been extended to a ZrO₂ particle here. It was found that the temperature and the velocity of a ZrO₂ particle, computed by assuming a constant average plasma temperature, deviate significantly from those calculated by considering the variation of the plasma temperature with the distance from the nozzle, as in the actual case. Again, it was found that the particle size reduction due to vaporisation cannot be neglected for particles moving close to the axis of the flame. The injection distance and the injection velocity were identified as two important spray parameters. It was noted that ZrO₂ particles within a certain size range should be used for developing coatings. The computations also revealed that the particles travelling in the peripheral region of the flame, do not undergo complete melting. It was experimentally verified that the particle size reduces significantly due to vaporisation.     

Growth of spinel particles on alumina thin films—II. Morphology and crystallography of the interface

In the second of two papers on the growth of spinel particles on alumina thin films, the morphology and structure of the heterophase boundary has been examined. The phase boundaries are found to adopt planes which are low-index planes in the alumina and, whenever possible, they also facet parallel to the low-index planes in the spinel. These low-index planes are the planes with large d-spacings. The structure of the phase boundary appears to be such that the misfit along the interface plane is minimized. For some of the phase boundary planes examined, the density of near-coincident lattice points with respect to the oxygen sublattice of the spinel and alumina is particularly high. Each of the five different types of phase boundary produces a specific morphology as described in the companion paper [Acta metall. mater.42, 2729 (1994)]. This morphology is related to the structure of the phase boundary. In the early stage of the thin-film reaction, the spinel growth consists of two stages; the spinel first grows out from the edge of the foil until it reaches a critical length, and then moves into the alumina substrate. It is proposed that the interface segment which moves fastest is the one along which the misfit is higher.     

Computational modeling of metal matrix composite materials—II. Isothermal stress-strain behavior

The mechanical behavior of particulate reinforced metal matrix composites, in particular an SiC reinforced Al-3 wt% Cu model system, was analyzed numerically using the computational micromechanics approach. In this, the second in a series of four articles, the isothermal overall stress-strain behavior and its relation to microstructural deformation is examined in detail. The macroscopic strengthening effect of the reinforcement is quantified in terms of a hardness increment. As seen in the first article for microscale deformation, inhomogeneous and localized stress patterns develop in the microstructures. These are predominantly controlled by the positions of the reinforcing particles. Within the particles stress levels are high, indicating a load transfer from matrix to reinforcement. The higher straining that develops in the matrix grains, relative to the unreinforced polycrystal, causes matrix hardness advancement. Hydrostatic stress levels in the composite are enhanced by constraints on plastic flow imposed by the particles. Constrained plastic flow and matrix hardness advancement are seen as major composite strengthening mechanisms. The latter is sensitive to the strain hardening nature in the matrix alloy. To assess the effects of constraint more fully, simulations using external confining loads were performed. Both strengthening mechanisms depend strongly on reinforcement volume fraction and morphology. In addition, texture development and grain interaction influence the overall composite behavior. Failure mechanisms can be inferred from the microscale deformation and stress patterns. Intense strain localization and development of high stresses within particles and in the matrix close to the particle vertices indicate possible sites for fracture.     

The effect of microstructure and composition on the properties of vapour quenched AlCr alloys—II. Tensile properties

The effect of chromium and iron additions and of annealing and working on the microstructure and tensile properties of vapour quenched AlCr and AlCrFe alloys has been determined. Tensile strengths of the worked AlCrFe alloys were in the range 568–831 MPa. Chromium in solid solution or iron present as iron-rich precipitates increased the yield stress by 44.7 MPa/at.%Cr and 333 MPa/at.%Fe respectively. The contributions to the yield strength of AlCr alloys were solid solution 40% and dislocation density/cell size 60% and to the yield strength of AlCrFe alloys were solid solution 25%, iron-rich precipitates 42% and dislocation density/cell size 33%. Vapour quenching may allow the more efficient use of alloying elements in the strengthening of Al-alloys and greater flexibility in obtaining the desired combination of solute concentration, particle volume fraction and particle size.     

Densification and structure formation in the sintering of composite materials based on nitrides of the IV–V group transition metals. III. Sintering of composite materials in the (VN, TaN)—Cr systems

An investigation was made of densification and structure formation in composite materials in the systems (VN, TaN)—Cr during sintering in argon. It was shown that the shrinkage of these materials during liquidphase sintering is insufficient to provide dense composites (residual porosity was 35–40%). This is attributed to the low thermodynamic stability of VN and TaN, and the rapid evolution from these of nitrogen which accumulates in closed pores. Processes of heterodiffusion and alloy formation also have a negative effect on densification. Exchange reactions between chromium and the nitride-forming metals lead to the formation of a large quantities of intermetallics which embrittle the composite materials. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(405), pp. 13–18, January–February, 1999.     

Some effects of microcracks on the mechanical properties of brittle solids—II. Microcrack toughening

The stress-strain characteristics of a microcracking material are used as the basis for computations of the fracture toughness, by applying a line integral formulation, pertinent to frontal and steady-state microcrack process zones. The calculated toughness is used to predict the trends in toughness with grain size and specimen geometry. The trends are correlated with experimental data.     

The effect of residual stresses on the debonding of coatings—II. An experimental study of a thermally sprayed system

Specimens have been produced by plasma spraying of boron carbide coatings about 1 mm in thickness on to titanium alloy substrates about 3 mm thick. The residual stress distributions in these specimens have been calculated using a numerical process model and also estimated from observed changes in curvature on debonding. Good agreement was observed between the two methods, with both suggesting the average substrate stress to be about + 20 MPa and the average coating stress to be about −60 MPa. In both constituents, there was a significant positive gradient of stress level through the thickness. These specimens were loaded in four point bending until cracks propagated along the interface between substrate and coating. From the load/displacement plots obtained during this testing, and taking account of the effect of relaxation of the residual stresses during debonding, the critical strain energy release rates of the interfaces, G_ic, were estimated to be ∼ 0.2–0.5 kJ m⁻². Substantial errors would have resulted from neglect of the presence of the residual stresses. Also of significance is the effect of the residual stresses on the mode mixity of interfacial loading, as characterised by the phase angle, ψ, since G_ic has been often observed to vary with ψ. The value of ψ for the four point bend test in the absence of residual stress is about 47°, whereas for the specimens tested here it was estimated to cover the complete range from 90° (pure shear) to 0° (pure opening) as the applied load was increased. The quoted values of G_ic were obtained in a regime where ψ ∼ 30°.     

Diffusional coalescence of island films on the real crystal surface in the case of layer-by-layer growth of islands—II. An open system. Undamped sources of deposited atoms

Coalescence of island films on the real crystal surface in the presence of the sources of deposited atoms is studied in case of layer-by-layer growth of islands. Asymptotic distribution functions and critical radii time dependences with different modes of mass-transport are obtained and analyzed. Restrictions in the sources intensity are given.     

On the tensile properties of a fiber reinforced titanium matrix composite—I. Unnotched behavior

An investigation of the ultimate tensile strength and fracture strain of a fiber-reinforced Ti-matrix composite has been conducted. Comparisons have been made between experimental measurements and predictions of two micromechanical models: one assumes that the fibers behave independently of the matrix, i.e. as in a dry fiber bundle, and the other assumes frictional coupling between the fibers and the matrix, characterized by a constant interfacial sliding stress. To conduct such comparisons, a number of constituent properties have been measured, including the fiber strength distribution, the thermal residual stress and the interfacial sliding stress. In addition, the effects of gauge length on the tensile properties of the composite have been studied. The comparisons indicate that the model prediction based on frictional coupling provide a good representation of the experimental results. In constrast, predictions based on the dry fiber bundle approach strongly underestimate both the ultimate strength and the fracture strain and predict a gaunge length dependence that is inconsistent with the experiments.     

Effect of C/Ti ratio on the compressive properties and wear properties of the 50 vol.-% submicron-sized TiCx/2014Al composites fabricated by combustion synthesis and hot press consolidation

In this study, in situ 50?vol.-% TiC_x/2014Al composites with different C/Ti molar ratios (0.6, 0.7, 0.8, 0.9 and 1) were successfully produced by the method of combustion synthesis and hot press consolidation. The microstructure and the mechanical properties of the composites were investigated. Microstructure characterisation of the TiC_x/2014Al composites showed relatively uniform distribution of the TiC_x particles with the particle size in the range of 200–900?nm. With the increase of the C/Ti molar ratio, the yield strength (σ_0.2) and the ultimate compression strength (σ_UCS) increased first then decreased, and the fracture strain (ε_f) increased. The σ_0.2, σ_UCS and the abrasive wear resistance of the 50?vol.-% TiC_x/2014Al composites reached the highest value when the value of the C/Ti molar ratio comes to 0.8. The σ_0.2, σ_UCS and ε_f of the 50?vol.-% TiC_x/2014Al composites with the C/Ti molar ratio of 0.8 are 1094?MPa, 1454 and 6.13%, respectively.     

Effect of gadolinium on the properties of Pr–Dy–Fe–Co–B materials

Sintered (Pr_1–x–y Dy _x Gd _y )_13–14(Fe_1–z Co _z )_balB_6–7 materials (x = 0.18–0.58, y = 0.05–0.33, z = 0.2–0.36) have been studied. The magnetic moments of gadolinium ions and those of the sublattice formed by Fe and Co ions are shown to be ordered antiferromagnetically. It is noted that an increase in the content of gadolinium, which substitutes for dysprosium, leads to an increase in residual induction B _r , a decrease in coercive force H _cJ , and an increase in the absolute value of the temperature coefficient of induction. The opposite effect takes place in the case of substitution of gadolinium for praseodymium in materials with a fixed dysprosium content.     

Effect of Gd on microstructure,mechanical properties and wear behavior of as-cast Mg–5Sn alloy

Effect of minor Gd addition on the microstructure, mechanical properties and wear behavior of as-cast Mg–5Sn-based alloy was investigated by means of OM, XRD, SEM, EDS, a super depth-of-field 3D system, standard high-temperature tensile testing and dry sliding wear testing. Minor Gd addition has strong effect on changing the morphology of the Mg–5Sn binary alloy. Gd addition benefits the grain refinement of the primary α-Mg phase, as well as the formation and homogeneous distribution of the secondary Mg₂Sn phase. The mechanical properties of the Mg–5Sn alloys at ambient and elevated temperatures are significantly enhanced by Gd addition. The wear behavior of the Mg–5Sn alloy is also improved with minor Gd addition. The alloy with 0.8% Gd addition exhibits the best ultimate tensile strength and elongation as well as the optimal wear behavior. Additionally, the worn surface of the Mg–5Sn–Gd becomes smoother in higher Gd-containing alloys. The best wear behavior of alloy was exhibited when Gd addition was up to 0.8%, showing a much smoother worn surface than that of control sample. The improvement of tensile properties is mainly attributed to the refinement of microstructure and the increasing amount and uniform distribution of Mg₂Sn phase. The larger amount of Mg₂Sn phase uniformly distributed at the grain boundary of Mg–Sn–Gd alloys act as a lubrication during sliding, and combined with smaller grain size improve wear behavior of the binary alloy.