Materials Processing With Thermal Plasmas
- PDF / 3,924,819 Bytes
- 5 Pages / 576 x 777.6 pts Page_size
- 88 Downloads / 281 Views
MRS BULLETIN/AUGUST 1996
Plasma densification of powders, plasma spraying of protective coatings, and plasma production of near-netshape structural components are typical examples of applications in which the plasma is simply used as a high-grade heat source. Worldwide industrial applications of plasma spraying are estimated to be in the billion-dollar range per year, concentrated in the aircraft industry, in medical prosthesis, in mineral processing, and in the chemical industry. Wire-arc spraying is an inexpensive and efficient deposition method for metals and alloys, which is attracting increasing interest in the automotive industry. Although thermal-plasma chemical vapor deposition (TPCVD) is still in the research-and-development stage, its potential for industrial applications is increasingly being realized. It allows one to make high-quality ceramic, diamond, and superconducting films. Another area of actual and potential applications is in the field of plasma synthesis of fine powders, including oxides, carbides, nitrides, borides, and combinations of such powders. Of particular interest today are nanometer-sized particles, which are the key for developing materials with engineered properties. This article will focus on the previously mentioned applications. Because of space limitations, other important developments such as thermal-plasma waste destruction and plasma metallurgy will not be covered here.
Thermal-Plasma Densification of Powders Powder densification is one of the simplest but most successful applications of plasma technology in materials processing. It involves in-flight melting of the material in particulate form, followed by gradual cooling and freezing before be-
ing collected at the bottom of the densification chamber or in an appropriate collection device, depending on the particle density and its size range. Using either direct current (dc) or inductively coupled rf plasma torches, the process has been successfully used for the densification and spheroidization of a large number of materials, ranging from lowmelting-point metals such as copper and nickel to oxide ceramics such as alumina and yttria-stabilized zirconia, and refractory metals such as molybdenum and tungsten. Typical electron micrographs showing the morphology of the individual particles before and after densification in an inductively coupled rf plasma are shown in Figure 1 for alumina, molybdenum, and tungsten powders. These have mostly a mean particle diameter in the range of 45-76 /im.
Thermal-Plasma Coating Technologies Plasma spraying of metals, ceramics, and composites is a well-established technology currently used on an industrial scale for the deposition of protective coatings and near-net-shape parts. Over the past 10 years, an increasing number of applications have emerged in the aerospace and chemical industries. The plasma spraying of yttria-stabilized zirconia as a thermal-barrier coating for turbine blades and the liners of jet-engine combustion chambers are typical examples. Wear-resistant coatings of WC
Data Loading...
