NANOSTRUCTURED BULK MATERIALS: SYNTHESIS, PROCESSING, PROPERTIES, AND PERFORMANCE

Bernard Kear, Rutgers University, and
Ganesh Skandan, Nanopowder Enterprises, Inc.
607 Taylor Road
Piscataway, NJ 08854

Introduction

Two different approaches to the fabrication of bulk nanostructured materials are being pursued today: (1) a powder processing route, wherein nanoparticles of the desired material are first synthesized by some convenient chemical or physical method and then consolidated by pressure-less or pressure-assisted sintering, and (2) a spray forming route, wherein nanoparticle synthesis, heating (or melting), and consolidation are combined into a single operation.

Both approaches have their scientific challenges, which must be addressed and overcome. In powder processing, it is essential to synthesize nanoparticles that are nonagglomerated and preferably monodispersed, since these powder characteristics facilitate low temperature sintering, which is a prerequisite for mitigating grain coarsening during the sintering process. When the source material is nanocomposite in nature, such as a ceramic/metal (cermet) powder, liquid phase sintering at high temperatures is required, in which case grain growth inhibitors are essential in order to avoid significant coarsening of the nanodispersed ceramic phase.

In spray forming, the grain coarsening problem is virtually eliminated, and there is little opportunity for contamination of the product material when the entire operation is conducted in a controlled atmosphere of inert gas at low pressure. For these reasons, this approach is now gaining favor. A challenge is the high degree of control of processing parameters needed to achieve a theoretically dense structure. Much of today's research, therefore, is focused on the development of process diagnostic tools, typically nonintrusive laser spectroscopy methods, for elucidating the mechanisms involved in the transformation of the chemical precursor directly into a nanostructured bulk material. Such diagnostics studies are being complemented by extensive work on process modeling. The ultimate goal of these efforts is to develop a computer-integrated process for the production of fully consolidated nanostructured bulk materials, starting from inexpensive chemical precursors.

The following are examples of materials systems that have been produced by these two fundamentally different processing routes.

Nanostructured WC/Co and WC/Co/Diamond

Nanostructured WC/Co is the material of choice for cutting tools, drill bits, and wear parts. Typically, this material is produced by mechanical mixing of powders of the constituent phases, followed by cold compaction and liquid phase sintering. This limits the attainable structural scale of the composite material to about 0.3 microns, so-called "micrograined" WC/Co. Recently, a new chemical process, called Spray Conversion Processing, has been introduced, which is capable of synthesizing "nanograined" WC/Co. The new process involves (1) preparation of an aqueous solution mixture of salts of the constituent elements, (2) spray drying of the starting solution to form an homogeneous precursor powder, and (3) fluid bed conversion (reduction and carburization) of the precursor powder to the desired nanocomposite powder. Using this process, Nanodyne, Inc., is producing industrial-scale quantities of nanophase WC/Co powders, with compositions extending over the range of commercial interest from 3-30 wt% Co. Major advances are also being made in the fabrication of high performance nanostructured sintered parts.

For several years, Rutgers University has been conducting research on the preparation and consolidation of nanophase WC/Co powders. Concurrently, a tribology group at Stevens Institute of Technology has been evaluating the material's friction and wear properties. Highlights of this collaborative effort include (1) synthesis of WC/Co powders with WC grain size controllable down to about 50 nm, (2) densification of powder compacts by liquid phase sintering in vacuum or hydrogen, (3) mitigation of WC grain growth during liquid phase sintering by the use of potent grain growth inhibitor carbide phases, such as VC or Cr3C2, (4) demonstration of hardness in fully sintered nanograined WC/Co (with inhibitor carbide phase) that is twice that of conventional micrograined material, and (5) confirmation of enhanced wear resistance and cutting performance in sintered materials of high hardness.

In tests performed on VC-doped nanophase WC/Co materials, the measured hardness increases with VC concentration up to a maximum of 2190 VHN at 0.8 wt.% VC. These data correlate with a reduced mean free path for the cobalt binder phase (i.e., reduced WC grain size), as determined by magnetic coercivity measurements and transmission electron microscopy. This is striking evidence for the potency of VC as a WC grain growth inhibitor in liquid phase sintering of nanophase WC/Co alloys. Recent measurements show that nanograined materials possess superior hardness at all compositions, without sacrificing fracture resistance.

More recently, the research has shifted towards the synthesis of WC/Co/diamond nanocomposites. Such materials are being produced by chemical vapor infiltration (CVI) of partially sintered nanophase WC/Co with graphitic carbon, followed by high pressure/high temperature (HPHT) conversion of the carbon to diamond. In the critical CVI step of the process, the kinetics of the carbon decomposition may be controlled to develop either a uniform or graded carbon distribution throughout the porous WC/Co preform. After HPHT consolidation, these two types of carbon-infiltrated structures transform into their corresponding triphasic WC/Co/diamond nanocomposites. A functionally-graded superhard material, comprising a nanophase WC/Co core and a diamond-enriched surface, combines high strength and toughness with superior wear resistance, making it an attractive candidate material for applications in percussion tools and roller cone bits.

Nanostructured Ceramics

Silicon-base ceramics, such as SiC and Si3N4, are useful materials for many engineering applications like highly stressed components in heat engines, grinding wheels, and wear parts, because of their excellent high temperature mechanical strength and good oxidation resistance. Silicon carbide is also useful because of its favorable electrical resistance (heating elements) and thermal conductivity (substrate materials). A limitation of today's processing of these materials is the very high sintering temperatures and pressures needed to consolidate powders, which is a consequence of their covalent bonding. Sintering aids can be used, but they frequently degrade properties. An alternative approach is to take advantage of the lower sintering temperatures characteristic of nanostructured powders. This approach is being adopted by many research groups.

Inert Gas Condensation (IGC) is the most versatile process in use today for synthesizing experimental quantities of nanostructured powders. A feature of the process is its ability to generate non-agglomerated nanopowders, which are sinterable at relatively low temperatures. In IGC processing, an evaporative source is used to generate the powder particles, which are convectively transported to and collected on a cold substrate. The nanoparticles develop in a thermalizing zone just above the evaporative source, due to interactions between the hot vapor species and the much colder inert gas atoms (typically 1-20 mbar pressure) in the chamber. Ceramic powders are usually produced by a two-stage process: evaporation of a metal source, or preferably a metal suboxide of high vapor pressure, followed by slow oxidation to develop the desired n-ceramic powder particles.

Recently, we have modified a conventional IGC processing unit for the purpose of synthesizing nanophase ceramic powders from metalorganic precursors. In this new chemical vapor condensation (CVC) process, the original evaporative heating source is replaced by a hot-wall tubular reactor, which decomposes the precursor/carrier gas to form a continuous stream of clusters or nanoparticles exiting from the reactor tube. Critical to the success of the hot-wall CVC process are (1) a low concentration of precursor in the carrier gas, (2) rapid expansion of the gas stream through the uniformly heated tubular reactor, (3) rapid quenching of the gas phase nucleated clusters or nanoparticles as they exit from the reactor tube, and (4) a low pressure in the reaction chamber.

The hot-wall CVC process has been used to synthesize nanophase powders of a variety of ceramic materials, which cannot easily be produced by the IGC process because of their high melting points and/or low vapor pressures. Examples are nanophase SiCxNy powders, for which there are many suitable metalorganic precursors, such as hexamethyl-dilazane (HMDS). In a particular case, the actual composition of the resulting powder is strongly influenced by the choice of carrier gas. Thus, HMDS/H2O, HMDS/H2 and HMDS/NH3 give ceramic nanopowders with compositions close to SiO2, SiC, and Si3N4, respectively.

From our experience with the synthesis of nanoparticle powders by the hot-wall CVC process, it was realized that the key to the high-rate production of powder is the efficient pyrolysis of the precursor/carrier gas stream in the hot zone of the reactor. This can be accomplished by replacing the hot-wall reactor with a combustion-flame reactor. The much higher temperature of the combustion flame ensures a much higher rate of decomposition of the precursor compound. Gases such as hydrogen, methane, or acetylene burned in oxygen may be used to generate a steady state combustion flame. The flame extends a few millimeters out of the burner and uniformly over the entire face, and it provides an intense heat source with a short residence time (fraction of a second) for effective thermal decomposition and reaction of the precursor/carrier gas stream. The burner is capable of operating at pressures as low as 5-25 mbar, which is in the optimum range for the synthesis of non-agglomerated nanoparticles. A variety of nanophase oxide ceramic powders have been produced by the combustion-flame CVC process, including SiO2, TiO2, and SnO2. For high powder production rates, the flat-flame combustor is placed in contact with a rapidly rotating copper chill. On the other hand, when operating in the spray forming mode, a heated substrate is placed in front of the burner so that the superheated nanoparticles sinter as fast as they arrive at the moderately heated substrate. Relatively high deposition rates of fully sintered material over large areas can be realized by this means. Using two or more flat-flame combustors, nanomultilayers with compositionally modulated or continuously graded structures can be produced.

Scientific Issues

Powder processing

  1. Establish the mechanism involved in grain growth inhibition during liquid phase sintering of WC/Co powder compacts.

  2. Having established a new mechanism of displacement reaction processing in the amorphous state for tungsten-base precursors, determine the mechanism involved in the transformation of an amorphous tungsten oxide to an amorphous tungsten carbide. This is a promising route to the synthesis of nanophase WC powders at temperatures as low as ~400oC.

  3. Determine the influence of volume fraction of diamond phase on the abrasive wear properties and fracture toughness of both homogeneous and functionally graded WC/Co/diamond nanocomposites.

Spray forming

  1. Establish the relationship between processing parameters and properties of nanophase spray-deposited materials.

  2. When operating in the multilayer spray deposition mode, characterize the interface structure between the dissimilar phases, determine the internal stress distribution in the nano-multilayered structure, and evaluate properties.

  3. Determine the heat transfer properties of nano-multilayered ceramics, consisting of alternating layers of nanoporous and dense material, and investigate their effectiveness as thermal barrier coatings.

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Published: January 1998; WTEC Hyper-Librarian