Researchers Apply Nanodiamond Nanoreinforced Polymer Composite Coatings by High-Velocity Oxy-Fuel Combustion Spraying

A high-velocity oxy-fuel combustion process can apply nanodiamond polymer composite coatings in  a manner that overcomes previous limitations according to inventors S. Charles Picardi,  Dustin B. Doss, Richard Knight and Antonella Stravato in Drexel University's U.S. Patent Application 20090305031.  Drexel research results indicate significant promise for this new material  system, and should lead to future applications of nanodiamond composite materials. Nanodiamonds may also be used in place of, or in addition to, more conventional reinforcing phases, such as tungsten carbide (WC) or chromium carbide (Cr3C2), where their extreme hardness, low coefficient of friction, high thermal conductivity and modest cost may enable more environmentally friendly and improved wear resistance coatings to be developed.  

High-velocity oxy-fuel combustion ("HVOF") spraying is a thermal spray technique that can be used as an environmentally-friendly solution to deposit polymers and polymer composites. HVOF spraying may also be used to deposit polymer matrix nanocomposites incorporating ceramic phases such as silica, alumina and nanodiamonds. Thermal spraying may offer several advantages: (a) polymer melt-viscosity is not a limitation, as it would be in extrusion or injection molding, (b) thermal spraying does not require the use of volatile organic solvents, (c) thermal spray coatings can be applied in-situ or in the field, (d) high loadings of the nano-diamond reinforcing phase may be achievable, (e) thermal spray coatings are "overlay" deposits, and do not change the properties of the substrate unduly.  

FIG. 9 from Drexel University Patent Application 20090305031 depicts several carbon phases including nanodiamonds, a lesser known member of the nanocarbon family that have applications in creating lubricants and hard coatings. 

Since the discovery of detonation synthesis for ultradispersed diamonds in the 1960s, the full potential of nanodiamonds remains to be explored. Detonation synthesis produces nanosized carbon particles with average diameters of ~5 nm, featuring a diamond core covered by graphitic layers and amorphous carbon. The surface of nanodiamond particles is rich in various functional groups and can be further functionalized. The superior properties of the diamond core combined with a large and chemically tunable surface allow them to be used in a wide variety of ways: initial research performed primarily in the former Soviet Union suggests a broad range of industrial applications. Such uses include chemical and electrochemical codeposition with metals, ultradispersed diamond-polymer composites, polishing, lubrication, and biomedical application. 

Among the possible applications of nanodiamonds are the deposition of wear- and corrosion-resistant metal coatings, additives to cooling fluids and lubricants, and polishing of ultra-flat optical or magnetic components. Among the challenges in processing nanoreinforced polymer composites are achieving a uniform dispersion and distribution of the nano phase within the polymer matrix, and improving the interfacial bonding between the nanoparticles and polymer matrix. 

FIG. 6 depicts optical images of HVOF sprayed (L) pure Nylon-11 coating, and (R) Nylon-11 with 7 wt. % nanodiamond.

FIG. 7 depicts SEM image of the surface of Nylon-11 covered by UD90 (L) as-received, and (R) oxidized and HCl treated.

FIG. 8 depicts several carbon phases including nanodiamonds.

Nanodiamond powder may be produced by detonation synthesis from carbon-containing explosives such as trinitrotoluene and cyclotrimethylenetrinitramine at high pressure under non-equilibrium conditions. FIG. 11 depicts one example of the detonation synthesis of nanodiamonds.

FIG. 13 depicts a carbon structure including embodiment of a nanodiamond.

FIG. 14 is a scanning electron micrograph that depicts a carbon structure including embodiments of a nanodiamond, carbon nano-onions and amorphous nanocarbon at 5 nanometers.  

FIG. 15 depicts polyamide-11 (Nylon-11) particles and nanodiamond particles.  Compositions including polymer particles and nanodiamonds may include polymer particles comprising at least one of poly(ethylene), poly(propylene), thermoplastic poly(urethane), poly(amide), poly(imide), alkylene tetrafluoroethylene, or poly(carbonate). For example, the coating may comprise several polymers, or derivatives of such polymers, such as a polyamide, a polyimide, and a fluoropolymer. For example, the polymer may be polyamide-11, a derivative of polyamide-11, or a mixture of polymers, one of which is polyamide-11

FIG. 16 depicts ball milled polymer particles with nanodiamonds.

Compositions including nanodiamonds may include at least one nanodiamond that is oxidized, for example, one or more --COOH functional groups may be present. For example, the majority of the surface functionalities may be converted to --COOH groups. The carboxyl groups at the surface of nanodiamond may interact with further species, for example the nitrogen atoms present in the amide bonds in the backbone of a Nylon-11 chain through the formation of hydrogen bonds. FIG. 31 depicts powders of polyamide-11 and oxidized nanodiamonds.

FIG. 32 depicts nanodiamonds and oxidized nanodiamond.  Work at Drexel has demonstrated, among other things, the feasibility of producing polymer matrix nanocomposites incorporating 5 nm size diamond as the reinforcing phase via high velocity oxy-fuel spraying. In certain examples, x-ray diffraction and Raman spectroscopy confirmed the presence of nanodiamonds in the sprayed deposits. In further examples, qualitative assessment indicated that coating adhesion was improved through the addition of the nanodiamond to the Nylon-11 matrix.

The feedstock to the thermal spray operation may be varied and optimized, as may techniques for covalently bonding the nanodiamond phase of the polymer matrix, as may techniques for characterizing  the improvement in properties as a function of nanodiamond loading.