John Smith

Aluminum coatings via kinetic spray with relatively large powder particles

Coatings have been produced by entraining relatively large diameter metal powders in a supersonic airflow. For the first time, most of the particles in the powders have diameters >50 μm. Substantial plastic deformation is involved in the conversion of the particles kinetic energy into heat and strain energy in this kinetic spray process. As suggested by simple estimates and confirmed by coating grain structures, the particles are not melted or thermally softened in this coating process. These coatings have a relatively low oxide content, low thermal stress, high adhesion, low porosity and hardness somewhat higher than those of corresponding bulk materials. Threshold or critical velocities for coating formation are discussed. Critical velocities for the relatively large particles were observed to be substantially less than have been reported earlier for smaller diameter (<50 μm) particles. Coating particle rotation and deformation due to particle impact resulted in a corresponding decrease in porosity. Bond formation, particle deformation and grain deformation were found to be highly anisotropic, depending on the direction of the incident particle velocity. At higher incident velocities, increasing metallic bond formation between particles was observed. This is consistent with a metallic form for stress/strain curves obtained via tensile tests on Al coatings removed from the substrate. The coating elastic modulus was found to be less than half that of bulk Al. Measured ultimate tensile strengths and yield points of Al coatings were comparable to those of bulk Al. This may be due to work hardening resulting from the plastic deformation necessary for coating formation. These tensile test results are consistent with coating cohesive strengths as measured by stud pull tests. Higher powder feed rates produced coatings with higher failure loads in three point bending, higher coating cohesion and lower coating strength anisotropy, presumably due to a peening effect. Four velocity-dependent stages of coating formation are proposed based on observations reported here. Coating properties arise from a competition between these stages. Parallels with models of dynamic (explosive) powder compaction are made. This is the first comprehensive model for kinetic spray coating formation.

The connection between ab initio calculations and interface adhesion measurements on metal/oxide systems: Ni/Al2O3 and Cu/Al2O3

The Ni/Al2O3 and Cu/Al2O3 interfaces have been examined by atomistic, first-principles computations. Relationships have been established with such metallurgical variables as the activity of aluminum and the partial pressure of oxygen. The calculations reveal that the interfaces could be either stoichiometric, or Al-rich, or O-rich, depending on the Al activity. The results are amenable to comparison with available sessile drop and fracture measurements. For conditions applicable to sessile drop experiments performed with Ni(Al) or Cu(Al), the calculations reveal that, as the Al activity increases, initially the work of adhesion increases, reaches a maximum, and finally decreases to that for pure Al. This trend is consistent with the known measurements. Interfaces generated by diffusion-bonding with ‘pure’ Ni or Cu are predicted to be O-rich, with a large work of separation, Wsep. The implication is that the separation process induces substantial plastic dissipation in the metal, consistent with the high interface toughness. For interfaces formed through Al2O3 growth on Ni(Al) alloys, the interface is predicted to be Al terminated, with Wsep several times smaller than for either bulk Ni or Al2O3. This reduction is in accordance with observations that these interfaces fail in a brittle manner with no noticeable plasticity.

Evaluation of coatings produced via kinetic and cold spray processes

An analysis of physical and mechanical properties of coatings produced by kinetic and cold spray processes is presented. Adhesion, hardnesses, porosities, critical velocities, and other properties of aluminum and copper coatings from both spray methods are analyzed and discussed, including scanning electron microscopy and optical micrographs. Similarities and differences between each of the coating methods and their effects on the resulting coatings are presented. A brief history and discussion of the bonding mechanisms for the larger particle coatings produced by the kinetic spray method is provided.

Effects of spray conditions on coating formation by the kinetic spray process

The kinetic spray coating process involves impingement of a substrate by particles of various material types at high velocities. In the process, particles are injected into a supersonic gas stream and accelerated to high velocities. A coating forms when the particles become plastically deformed and bond to the substrate and to one another upon collision with the substrate. Coating formation by the kinetic spray process can be affected by a number of process parameters. In the current study, several spray variables were investigated through computational modeling and experiments. The examined variables include the temperature and pressure of the primary gas, the cross-sectional area of the nozzle throat, the nozzle standoff distance from a substrate, and the surface condition of nozzle interior and the powder gas flow. Experimental verification on the effects of these variables was performed primarily using relatively large-size aluminum particles (63–90 µm) as the feedstock material. It was observed that the coating formation is largely controlled by two fundamental variables of the sprayed particles: particle velocity and particle temperature. The effects of different spray conditions on coating formation by the kinetic spray process can be generally interpreted through their influences on particle velocity and/or particle temperature. Though it is limited to accelerate large particles to high velocities using compressed air or nitrogen as carrier gas, increasing particle temperature provides an additional means that can effectively enhance coating formation by the kinetic spray process.

Surface phase diagram for Cr 2 O 3 ( 0001 ) : Ab initio density functional study

Surface phases of

Temperature dependence of the activity of Al in dilute Ni(Al) solid solutions

{\mathrm{Cr}}_{2}{\mathrm{O}}_{3}(0001)

Some Recollections of Life with Walter Kohn

as a function of ambient oxygen pressure and temperature were computed by ab initio density-functional theory for a solid surface. The surface can be

Universal binding energy curves for metals and bimetallic interfaces

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Process for producing hydrogen

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Kinetic spray coating method and apparatus

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