Development of New Generation Of Coatings with Strength-Ductility Relationship, Wear, Corrosion and Hydrogen Embrittlement Resistance Beyond the Current Materials
Accomplishments till date As the structural scale reduces to the nanometer range, one accomplishes High strength High interface to-volume ratio Enhancement of the interface-driven processes which will extend the strain to failure and plasticity Mechanical strength is controlled by the Hall-Pitcher relationship σ =kd -1/2 + σ o. A limitation of current in engineering materials is that gain in strength results in a loss of ductility
Accomplishments till date Reducing the structural scale to the nanometer range one can extend the strength-ductility relationship beyond the current materials limit
Accomplishments till date Nanostructured powders with increased strength and ductility have been produced by plasma processing where the reactor vaporizes coarse meat particles; by combustion synthesis where redox reaction takes place at elevated temperature, followed by quenching; and by mechanical alloying with gas atomization Cu/Nb composites (Han at al 1998) showed a complete suppression of the wire brittle fracture. Au-single crystals surfaces have dramatic effects on the yield strength
Accomplishments till date Erb et al. found that the passive current density of nano crystalline Ni is higher than conventional Ni which showed higher susceptibility to localized corrosion Erb et al., synthesized nanocrystalline Ni-Fe with increased hardness, wear resistance and improved corrosion performance in terms of localized corrosion
Objectives for the Next decade Development of the next generation of protective coatings with high corrosion, wear and erosion resistance, integrity under thermal stress and complete inhibition of hydrogen permeation and embrittlement Development of theoretical models which will explain how the shape and the size of the nanostructure affect its properties and optimize the materials surface and bulk properties Development of novel treatment for synthesis of nanostructured materials.
Objectives for the Next decade Development of monolayers and the nanometer range coatings in order extend the strength-ductility relationship beyond the current materials limits Development of procedures capable by using oxide nanoparticles to convert metals into material with wear resistance equal of that of the bets bearing steel. Development of advanced scratch free resistant films, with high strength, wear resistance and ductility (Cu coatings for printing industry).
Why electrodeposition? Multilayer structures with nanometer-scale thickness have been produced by various deposition processes, such as sputtering, molecular beam epitaxy, and chemical vapor deposition. While, versatile, the vacuum deposition techniques require expensive equipment; they cannot be used for fabrication of large structures with complex shapes and in most of the cases are difficult to control. Molecular beam epitaxy is well controlled deposition technique; however, this is not a volume production method.
Why electrodeposition? Multilayer structures with specific textures can also be easily synthesized using the chemical reduction and electrodeposition processes For example, 3D nanostructured, crystallites can be prepared using this method by utilizing the interface of one ion with the deposition of the other. Also, the size of the particles can be controlled precisely by the use of various micellar structures and lyotrophic phases in the solution phase during deposition.
Why electrodeposition? Pulse and pulse reversal deposition of multilayer structures with nanometer scale requires minimal capital investment and can be applied to fabrication of parts of any shape or size. The deposition rates at 10 nm layer level are about 0.05 nm/h, however, the process is non-labor intensive and can run automatically for long periods of time. Multi layer structures with specific textures can also be easily synthesized using the electrodeposition process
A Novel Autocatalytic Reduction Process (ARP) for deposition of nanostructured composites One step process No external current for deposition. Nanosized amorphous layers of Co-P, Ni-P, Co-Ni-P, deposition of amorphous nanostructured multilayers of Ni-Mo-P, Ni-W-P, Ni-Ce-P, Ni-Mo-B can be deposited by controlling the concentration of the electroactive species in the electrolyte and by controlling the factors which control the deposition rate
Factors controlling the deposition rates Substrate pretreatment ph and temperature Concentration of the reducing agent Presence of leveling agents Presence of dendrimers Presence of any of the three liquid crystalline phases exhibited by nonionic surfactant octaethylene glycol monohexadecyl ether (OGME)
A Novel Pulse and Pulse Reversal Plating of Nickel-Iron, Co-Ni, Zn-Ni, Zn-Ni-P alloys and Zn-Ni-SiO 2 Composites Procedures are Under Development at USC Why Pulse or Pulse Reversal Technique? The deposit particle size is proportional to the crystal growth rate while inversely proportional to the nucleation rate. decreases with increasing the nucleation rate. The crystal growth is proportional to the surface adatom concentrations surrounding the site. The nucleation rate is enhanced by increasing the overpotentials. Using Pulse technique, leveling agents, dendrimers and nonionic surfactant the nucleation rate dramatically increases due to increased overpotential
Why Pulse or Pulse Reversal Technique? Since the surface adatom concentration is proportional to the solution concentration in the vicinity of the surface one can expect a controlled pulse of less than milliseconds or micro seconds to deposit in the presence of additives in the electrolyte layers of metals, alloys and composites which have lower growth rate that DC technique. Nanosized layers of Zn and Zn-Ni alloys are deposited by controlling, the average current, the pulse duration, the concentration of Zn and Ni ions in the electrolyte and by controlling the factors which control the deposition rate such as: substrate pretreatment ph and temperature the presence of leveling agents the presence dendrimers, and nonionic surfactant octaethylene glycol monohexadecyl ether (OGME).
Why Pulse or Pulse Reversal Technique? Pulse and pulse reversal technique can be used do deposit multilayer structures composed of hundreds (up to one thousand) layers (5-10 nm) of Ni//Ni-Zn-P//Ni//Ni-Zn-P; Ni- Mo//Ni-Cu-Mo; Ni-Mo-Si//Ni-Cu-Mo-Si; and Ni-Mo-Ti//Ni-Mo-Cu- Ti nanostructured composites The specific objectives should be: to develop coatings with very large interfacial surface area and with superior ductility, strength and hardness, microstructural and mechanical characterizaton and fundamental modeling of crack initiation and propagation. Also, theoretical studies should be carried out which will correlate and tailor both strength (Koehler effect) and elastic modulus by varying the number of layers the layer thickness of nanostructured coatings
Under Potential Deposition of Metals (UPD) UPD occurs with a formation of monatomic layers at potentials more noble the an the reversible Nernst potential UPD has been engineered at USC for Zn, Pb and Bi by using the work functions of these metals and the work functions of the substrates The underpotential shift ( E) in volts when the monatomic layers are formed is determined by the work functions in electron volts of both metals. In situ polarization experiments showed that UPD formed monoatomic layers of Pb, Zn, and Bi on steel surfaces inhibit corrosion, hydrogen penetration and embrittlememnt due to lowering of the binding energy of the hydrogen adatoms on Zn, Pb and Bi adsorbates
Under Potential Deposition of Metals (UPD) Future work is necessary which will Characterize the nature of the deposits plated when pulse and DC technique at overvoltages between UPD potential and Nernst potentials in the presence of leveling agents, dendrimers and nonionic surfactant octaethylene glycol monohexadecyl ether (OGME). With an objective to deposit monolayers of metals or alloys on large surfaces, carbons or carbon nanotubes. to increase the adhesion and the strength of the deposits
Structural Studies In the layered and filamentary nanostructures, the nature of the interfaces has not been studied in details and there is not much information in the literature. The microstructure should be investigated by high resolution TEM, scanning tunneling microscopy (STM) and neutron diffraction techniques The Jeol 100 CX II is a transmission electron microscope capable of accelerating voltages from 20-100kv. It can provide magnification from 100x to 600000x and a resolution of 0.2nm
Structural Studies The microstructural features should include the nature and morphology of grain boundaries and interfaces grain size and morphology the nature of intergrain defects, composition profiles across grains and interfaces and identification of residual trapped species from processing The electrodeposited multi layered nanostructures should be studied in order to evaluate composition profiles across interfaces nature of defects and coherency and thickness of interfaces
Mechanical Characterization Studies Hardness of the deposit defines the abrasion resistance and general wear and tear qualities of the coating. Vickers and Knoop hardness tests are generally used to determine the hardness. Knoop hardness should be used since this is ideally suited for thin electrodeposits. The ductility, the resistance to fatigue damage, abrasion (wear) resistance, porosity, bending and cup impact test will be done using standard methods. Coefficient of Sliding Friction will be determined by measuring the coefficient of sliding friction according to the Coulomb s Law R = µ N The adhesion test should be based on ASTM B571-97 Standard Practice for Qualitative Adhesion Testing of Metallic Coatings.
Mechanical Characterization Studies The ductility, the resistance to fatigue damage, abrasion (wear) resistance, porosity, bending and cup impact test should be done using standard methods. The strength properties of multilayered deposits suggested in this proposal should be evaluated theoretically and experimentally. A mathematical model should be developed which will predict the dislocations as a function of the elastic constants and the thickness of the multi layered nanostructures and the susceptibility to plastic deformation and brittle fracture as a function of the deposit layer thickness