Limitations on the Use of Surface Doping for Improving High-Temperature Oxidation Resistance
- PDF / 2,272,585 Bytes
- 5 Pages / 576 x 777.6 pts Page_size
- 58 Downloads / 227 Views
26
cade have included the discovery of a detrimental role of indigenous sulfur on scale adhesion,8"11 a more accurate determination of the RE effect on scale growth mechanisms using 18O tracers,12"18 the detection of RE ions segregated to scale grain boundaries and the metal-oxide interface,141618"25 and the demonstration that RE ions are not statically doping the metal-scale system but are actively diffusing toward the gas interface during oxidation.17'24"26 Like that of most materials phenomena, study of the RE effect has two facets: mechanistic investigations attempting to explain the role of the RE addition, and engineering testing to maximize the RE effect in improving materials performance. In practice, RE additions to alloys are most commonly made as alloy additions (0.05-0.5 at.%) or as RE oxide dispersions (0.2-2 vol%). Surface doping, such as ion implantation (f luences of 1014-1018 ions/cm2) or as an RE oxide coating (5-2/100 nm thickness), has been effective in both scientific and engineering studies. Surface modifications are well-suited to fundamental investigations because of the flexibility in testing a wide range of elements at various concentrations. For applications, they are attractive because, relative to bulk additions, the total amount of dopant is reduced, and any RE-induced problems with mechanical properties or processing are eliminated. As the understanding of the RE effect has advanced, it has become increasingly clear that there are specific limits on the beneficial effects of RE additions and even greater limitations on the use of surface
doping to improve the oxidation resistance of high-temperature alloys. Nevertheless, surface modifications can still be beneficial under the appropriate conditions. More thorough reviews of ion implantation27'28 and RE surface coatings29 can be found elsewhere. This discussion attempts to define the limitations of surface modifications, indicates potential reasons for these limitations, and outlines possible uses of surface modifications in both scientific studies and practical applications.
Observations of Surface Doping Limitations
As outlined in previous reviews,2'20'27'28 the use of ion implantation has been particularly helpful in isolating the role of RE additions in high-temperature oxidation. Various elements can be implanted at a controlled fluence and with a uniform distribution, regardless of the solubility of the alloy additions. It has been shown repeatedly by the implantation of inert or base metal elements that the damage caused by the implantation process itself has only a minimal effect on the hightemperature oxidation behavior and that the observed RE effects are primarily chemical.27'30"33 In general, a fluence of at least 1016ions/cm2 is necessary to achieve a beneficial effect.14'20'21'27'28'34 Depending on the implant energy and ion size, this corresponds to a 3-5 at.% peak concentration at a depth of 20-50 nm. However, there is an envelope where ion implantation is most effective. Depending on the alloy and the implanted fluen
Data Loading...
