ICMCTF1998 Session E3: Coatings Resistant to Erosion, Corrosion and Severe Environments Including Biomedical Environments

A major advance in the study of erosion-corrosion of materials in aqueous conditions in recent years has been the construction of erosion-corrosion maps, showing the transitions between the erosion- corrosion regimes as a function of the main process parameters. Such maps enable the mechanism of damage to be identified, whether erosion- or corrosion-dominated, and provide a basis for optimisation of process parameters in such conditions. However, as there has been little research carried out to study the performance of thin coatings in such environments, the construction of erosion-corrosion maps for such systems has not been attempted.

The objective of this work was to evaluate the effects of velocity and applied potential on the erosion of various coatings, CrN, Ti₂N, TiAlN, Nb, and NbN, deposited by combined steered arc unbalanced magnetron sputtering techniques, in order to establish the erosion-corrosion regime transitions for relatively simple coating systems. (The results were bench marked by experimental data evaluated from commercially available TiN, CrN and TiAlN deposited by cathodic arc technology.) The apparatus used was a rotating cylinder electrode system, the particles used for the experiment were 150 μm alumina and the solution a carbonate/bicarbonate buffer. "Synergistic" and "additive" erosion-corrosion effects were evaluated through precise separation of the erosion and corrosion components, by means of electrochemical monitoring during the erosion-corrosion process.

The results showed that the passivation potentials for the five coatings occurred at different values . Differences between the erosion-corrosion resistance of the coatings, as a function of increasing applied potential, were identified. Possible implications of such trends for the construction of erosion-corrosion "regime" maps for these coating/substrate systems are addressed in this paper.

Five different CVD thin films were examined to improve the erosion resistance of silicon micromachined atomizers. This is a follow up of earlier research which examined the effect of APCVD single crystal SiC coatings on the performance of silicon micromachined atomizers¹. In addition to single crystal SiC, polycrystalline SiC, silicon nitride, silicon dioxide, and diamond-like carbon (DLC) were also examined as coating materials. The coatings were tested by operating the atomizers at 150 psi and room temperature. The test fluid was augmented by adding an abrasive mixture to intensify the erosion process. Quench tests were also performed to subject the atomizers to thermal stresses similar to those found in a combustion environment.

The results of the erosion test demonstrated that the single crystal SiC coating performed better than the other coatings. The polycrystalline SiC was a high stress film that caused the atomizers to crack after an 18 hour erosion test, though the coating morphology and thickness were unaffected. The silicon nitride and silicon dioxide coatings exhibited the tendency to trap erosion particles, which significantly affected the coating morphology. The DLC coating demonstrated a tendency to flake at the edge of the exit orifice, though morphology elsewhere was unaffected. Film thickness measurements showed no decrease in film thickness in any of the coatings as a result of the erosion test. The results of the quench tests were promising for all coatings, with the exception of DLC. Examination of these films under an SEM showed no cracking or delamination as a result of the thermal stresses imposed on the coatings.

The extended paper will include i) details of the deposition and properties of the thin film coatings, ii) details and results of the erosion and wear tests performed on the coatings, and iii) further details and results from the quench tests.

¹Rajan, N., Zorman, C.A., Mehregany, M., DeAnna, R.G., Harvey, R.J. "Performance of 3C-SiC Thin Films as Protective Coatings for Silicon Micromachined Atomizers",Thin Solid Films, in press.

Biomedical researchers have used several strategies to improve the strength of the bond between the implant and growing bone. One advance has been the development of porous coatings, which allow mechanical interlocking between the implant and the forming bone. Another advance has been the use of bioactive coatings to improve the strength of the bone/implant interface. To date, hydroxyapatite (HAP) coatings have shown exceptional promise as bioactive coatings for metallic implant devices. It is believed that the apatite can partially dissolve, intergrow, and become partially incorporated with the apatite in growing bone, forming a coating:bone interface as strong as bone itself. Although significant advances have been made to provide mechanically strong and non-toxic metals and alloys, biological integration of devices into natural tissues remains a problem.

In order to address problems associated with current implant tecnologies, PNNL developed he Surface Induced Mineralization (SIM) and Void Metal Composite (VMC) processes. The VMC process produces porous metal materials which have cylindrical pores of uniform diameter which can completely penetrate the structure of the material. The pore diameter, orientation and interconnectivety are easily controlled for optimum bone ingrowth.

The SIM process uses the idea of nature's template-mediated mineralization by chemically modifying the implant to produce a surface which induces heterogeneous nucleation from aqueous solution. SIM produced bioactive coatings provide 1) control of the thickness and density of the mineral phase, 2) a way to coat porous metals, complex shapes and large objects, 3) the ability to coat a wide variety of materials, 4) potential choice for the phase of the mineral formed.

Surface engineering and, in particular, coatings are being applied in several energy technology systems to achieve a variety of objectives such as improved in corrosion and erosion resistance, protection from elevated temperatures, desired electrical properties, and establishing barriers to the transport of specific elements and/or gases. Corrosion and erosion of metallic structural materials at elevated temperatures in complex multicomponent gas environments that include particulates are potential problems in many fossil energy systems, especially those that are coal-fired. The use of appropriate corrosion-resistant coatings on metallic components can minimize material degradation and extend component life. Similarly, ceramic thermal barrier coatings offer protection to nickel-base superalloys in both aviation and land-based gas turbine systems. Pack diffusion and electrospark-deposited coatings of intermetallic compounds provide protection against sulfur transport and sulfidation corrosion. Highly specialized coatings with electrical insulating characteristics are being sought in fusion energy blanket systems. This paper reviews the requirements and/or performances of coatings in the environments of several energy technologies. The complexity of environments in different systems is discussed, together with coating characteristics needed for acceptable performance.

*Work supported by the U.S. Department of Energy, office of Fossil Energy, Advanced Research and Special Technologies Materials Program, under Contract W-31-109-Eng.38.