Figure 4. Schematic diagram to illustrate a SiC/C FGM on a carbon-carbon composite (redrawn, with permission, from Ref 23).

Hirai and coworkers (Ref 26) are doing some very interesting work in which CVD is used to fabricate composites. The properties of composites are controlled by the geometric arrangement between the matrix and the dispersion. Moreover, there is good reason to attempt to obtain dispersoids of smaller and smaller size, as well as with controlled shapes. When the size of the dispersoid approaches nanometers, it is very difficult to fabricate composites by mixing powders followed by some type of consolidating process. Because of such problems, it is essential to prepare composites by using an approach whereby phase separation occurs in situ. In situ composite preparation with nanometer dispersoids can be achieved by using a gas (CVD, PVD), a liquid (sol/gel, co-precipitation), a molten alloy (solidification, eutectic reaction), or a solid (phase separation, grain boundary reactions). The CVD method is an excellent method to form nanometer scale deposits as films on different substrates. Hirai and coworkers have prepared a number of nano-scale composites by using CVD. Some typical composites are illustrated in Figure 5.

(a) Spherical particle dispersion (C-B C), C-TIC, C-ZrC, SiC-C, SiN-C, SiN-TiN, Si N-AIN, BN-TIN, BN-Si ̧N).

(b) Disk-type particle dispersion (C-SiC).

(c) Rod-shaped particle dispersion (BN-BN).

(d) Fiber dispersion (SIN-TIN, SIC-TIC).

(e) Thin layer dispersion (Ti Sic2-TiC).

(1) Lamella (Si ̧N-BN).

Figure 5. Nanostructures of CVD

nanocomposites as
presented by Hirai and
Sasaki. Reprinted with
permission from "In Situ
Processing of Inorganic
Composites by Chemical
Vapor Deposition," by
T. Hirai and M. Sasaki, in
Advanced Structural
Inorganic Composites,
edited by P. Vincinzini,
Elsevier Science

Publishers, 1991, p. 541.

In the case of a C-SiC composite having disks of SiC several nanometers in diameter, the carbon and SiC possessed an orientation relationship between each other, and it was reported that this composite had better oxidation resistance than carbon. When

particles of C were dispersed in Si,N, these particles formed a threedimensional network and the addition of 0.2 wt.% of carbon increased the electrical conductivity of the composite very substantially. Other examples have been presented (Ref 26) to show that nanocomposites can be prepared whereby transparency is maintained at elevated temperatures, thermal conductivity is modified, and toughness is improved. Table 2 presents some nanocomposites prepared by CVD and the property altered by using this approach (Ref 27). This technique can also be used to form superconducting oxide films and functionally gradient materials (Ref 26). In Hirai's laboratory, Goto is studying nanocomposites of nonoxide ceramics prepared by CVD. Yamane is working on superconducting oxide films prepared by CVD. Sasaki is preparing functionally gradient materials by using CVD and chemical vapor infiltration (CVI).

There is a very significant effort on composites in Japan. Very great expertise exists in fiber reinforced plastics (FRP). MHI, IHI, and KHI all have very capable workers to produce a variety of components made from these materials including helicopter blades, air brakes, and other airframe parts. In the case of metal matrix and ceramic matrix composites (MMCs and CMCs), the capabilities are not as great as in FRPs but they are still important and at times impressive. At Tokyo University Kagawa is performing studies on the mechanical properties of composites. He is involved with micromechanical modeling and crack growth in carbon/carbon, Ti/SiC, and Si/Al2O3 composites. At the National Aerospace Laboratory experiments are being performed on the resistance of carbon/ carbon plates to ballistic impact. The point to be emphasized is that research and development is being done on MMCs and CMCs in a significant number of laboratories and valuable advancements can be expected.

HIGH TEMPERATURE

CORROSION AND COATINGS RESEARCH

The topics of high temperature corrosion and coatings research will be described first, based upon discussions at the organizations that were visited, and then based upon some of the papers presented at a symposium recently held at the Tokyo Institute of Technology.

Oxidation studies on Ti-Al alloys are being performed at a number of organizations (Ref 28-31). Shida and Anada at Sumitomo Metal Industries have examined the oxidation behavior of Ti-34.5 wt.% Al containing the elements Mn, Mo, Cr, Si, and Y. These elements were added to the base alloy separately and in amounts up to about 3 wt.%. In all cases multilayered scales composed of TiO/AOTO+AO were formed on all of the alloys during oxidation at 900 °C in air. The rates of oxidation were smallest for the alloys with Mo and with Si. The rate of

oxidation was increased when Mn was present in the alloy. The effects produced by these elements in Ti-34.5 Al are proposed to be caused by changes in the compactness of the inner TiO2 + Al2O, mixed oxide layer and by oxygen solubility changes in the alloy adjacent to the oxide scale. As mentioned previously (Ref 32), work is also being performed to develop oxidation resistance in TiAl via preoxidation treatments. Recent results (Ref 31) show that protective scales of Al2O, can be formed via oxidation in gases with low pressures of oxygen. These preformed layers do not provide adequate protection, however, at temperatures higher than 900 °C, but this approach may be important for use at temperatures below 900 °C.

3

Takei and coworkers (Ref 33) at NRIM (Tokyo) are using pack cementation to develop aluminide coatings on titanium alloys as well as TiAl. They on titanium alloys as well as TiAl. They determined that a pack mix of 20 wt.% Al,5% AlF,, and 75% Al2O, produced

strength

the best results. The coatings were deposited by heating in such packs for 5 hours at 1,000 °C. The coating layers on TiAl and titanium alloys were composed of predominantly TiAl,. Such coatings provided protection for Ti-6Al-4V at 900 °C for 12 hours and for less time at 1,000°C. These coatings provided protection to TiAl at these temperatures for longer times but protection was lost when all the TiAl, had been removed from the coating by oxidation and interdiffusion.

Iguchi and coworkers at Tohoku University have been studying the oxidation of SiC and Si,N, prepared by the CVD method. Much of the work on SiC has been performed by Narushima (Ref 34-37). This work is concerned with the active to passive transition during oxidation of SiC and the effects of oxygen pressure and temperature. Passive oxidation occurs when a continuous layer of silica is formed on SiC. Active oxidation, on the other hand, involves the formation of gaseous

products where rates are determined by diffusion in a gaseous boundary layer or chemical reactions at the surface of the SiC. The oxygen pressures for the transition from active to passive oxidation have been determined to increase with increasing temperature and with decreasing gas flow rates. Rates for active oxidation have been determined and described in terms of appropriate rate controlling steps.

Ohmura and coworkers (Ref 38) at Nippon Steel Research and Engineering Center are investigating the oxidation resistance of Fe-20 wt.% Cr-5 wt.% Al alloys containing small amounts of La, Ce, Pr, and Nd. The sum total of these elements was varied between 0 and about 0.15 wt. %. Optimum lives during cyclic oxidation at 1,200 °C in air occurred for alloys with concentrations of these elements between 0.05 and 0.10%. These studies showed that the improved oxidation resistance occurred due to increased adherence

of the Al2O3 scales that formed on these alloys. Furthermore, it was proposed that the better adherence of such oxide scales was due to these elements affecting the growth mechanism of the Al2O3 scale such that less compressive stresses developed during growth. The upper limit for beneficial effects of these elements was explained by proposing that incorporation of oxides of these elements into the AO, scale caused increased oxidation rates due to rapid transport of oxygen through such oxides. Work is continuing to attempt to develop still better alloys of this type for use in automotive exhaust equipment. This research is impressive. In the case of nickel base alloys, many investigators are now proposing that oxygen active elements improve oxide scale adherence by preventing sulfur from segregating to the scale/ alloy interface. The results obtained by Ohmura et al. indicate that other factors are important for at least the Fe-Cr-Al system.

At IHI there is much interest in gas turbine materials, but most of the materials used in the gas turbines parts produced by IHI are under license to companies such as General Electric or Pratt and Whitney. Consequently, while significant expertise is present in fabricating state-of-the-art materials for gas turbines, substantial efforts are not evident to improve these materials. IHI has, however, a great variety of engineering interests, one of which is coal gasification. Kihara and coworkers are concerned with fireside corrosion. Laboratory simulation testing is being performed on highly alloyed stainless steels (Ref 39 and 40). Corrosion resistance is improved when the chromium concentration is greater than 20 wt.%. Iso-corrosion curves have been prepared to give corrosion rates as a function of SO2 pressure in the gas and alkali sulfates in the coal ash.

During the period between 3 December and 7 December 1990, an international symposium on Solid State Chemistry of Advanced Materials was held in Tokyo, Japan. This symposium consisted of two workshops, one on Nonstoichiometric Compounds and the other on High Temperature Corrosion of Advanced Materials and Protective Coatings. The symposium was held at the Tokyo Institute of Technology and was organized by Professor Yasutoshi Saito and Dr. Bulent Onay, both from the institute. The high temperature corrosion workshop was in honor of Professor K. Nishida from Hokkaido University on behalf of his 70th birthday. It consisted of 45 papers of which 27 were by Japanese authors. The time allotted for oral presentations was 15 minutes for invited papers and 10 minutes for the others. Upon conclusion of the day's presentations, the papers were available for more extensive discussions at a poster session period of 2 hours. This procedure proved to be very effective. A large number of papers

were presented and extended discussions took place each day at the poster sessions. Some of the papers presented by Japanese authors will be used to provide additional illustrations of high temperature corrosion research in Japan.

The high temperature corrosion workshop whose proceedings are in press (Ref 40) emphasized the following topical areas.

• Fundamental Studies on High Temperature Corrosion of Advanced Materials

• High Temperature Corrosion of Engineering Alloys

• Hot Corrosion of Engineering Alloys and Corrosion of Nuclear EnergyRelated Materials

• High Temperature Corrosion of Protective Coatings and Intermetallics

• High Temperature Corrosion of Ceramic Materials

Some of the significant papers presented in these areas are described in the following paragraphs.

Sasayama and Kamiya (Nippon Yakin Kogyo) have studied the morphologies of alumina scales formed on aluminum containing ferritic stainless steels at temperatures between 1,123 and 1,473 K. It was found by using x-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses that various forms of alumina could be formed with different surface morphologies. The slowest growing oxide was a-alumina and this oxide was formed as a smooth scale during oxidation above 1,000 °C. During oxidation between 900 and 1,000 °C, however, the scales contained theta and delta alumina, and these aluminas resulted in higher oxidation rates. These results show that

the transient oxidation characteristics of alumina-forming alloys will depend upon the type of Al2O, that is formed during the initial stages of oxidation.

Narita and coworkers (Hokkaido University) have studied the corrosion of iron-chromium alloys in H2S-H2 gas mixtures to identify alloys that could be used in oil-fired boiler tubes. These studies in some cases used preoxidized specimens. A mechanism involving reduction of oxide followed by sulfide formation has been formulated which is proposed to play an important role in the breakdown of Cr2O, scales even when physical defects such as cracks or voids are not present in these scales.

Konno and Furichi (Hokkaido University) examined the use of LaCro, as coatings to protect types 304 and 430

stainless steels from oxidation. These coatings were formed from chromate solutions, followed by a heat treatment and consolidation via CO, laser irradiation. These coatings evidently provided oxidation resistance since small weight gains (0.3-0.5 mg/cm2) were detected after a 1-hour exposure at 1,100 °C.

Yoshiba (Tokyo Metropolitan University) has investigated the interactions between creep processes and hot corrosion attack of the nickel base superalloy IN751. Interactions between creep and hot corrosion were observed as stress-enhanced intergranular attack that resulted in premature fracture. Important characteristics of creep-hot corrosion interactions were identified in mechanistic terms.

Otsuka and Kudo (Sumitomo Metal Industries) are studying the hot corrosion of commercial tube steels in a waste incinerator environment. Their experiments consist of coating specimens with ash deposits followed by exposure to gas mixtures containing SO, O, CO, H,O, HCl, and N, at O2 temperatures between 350 and 450°C. Weight changes of exposed specimens are determined and detailed characterizations of the exposed specimens are

performed. Results are being interpreted by using phase stability diagrams, and acid-base fluxing concepts, to formulate mechanisms for the degradation processes.

Wu and coworkers (Tokyo Institute of Technology) used ac impedance and anodic polarization measurements to monitor the corrosion of alloys in Na2SO-Li2SO, deposits at 700 °C. The corrosion resistance of alloys determined by using these methods was in agreement with the resistance determined by using weight loss techniques.

Nakamori (MHI) has investigated the corrosion behavior of vacuum plasma sprayed NiCrAIY, CoNiCrAlY, and CoCrAlSIY coatings in combustion gas atmospheres to simulate conditions in oil-fired gas turbines. These coatings were deposited on both nickel and cobalt base superalloys and testing was perbase superalloys and testing was performed by using air-cooled specimens at temperatures between 830 and 1,200 °C. The NiCrAlY and CoCrAIY provided adequate resistance for the exposure times used but the CoCrAlSiY performed poorly. This poor behavior was attributed to reaction between SiO2 was attributed to reaction between SiO2 and ash deposits that accumulated on specimens in this test.

Maruyama et al. (Tokyo Institute of Technology) studied the oxidation of silicon-aluminide coatings on molybdenum. These coatings were fabricated by a two step pack cementation process in which silicon was deposited initially (pack mix Si + NH4Cl + Al2O3) followed by an aluminizing treatment (pack mix Al + NH4Cl + Al2O2) at 1,050 °C. The coatings were composed of a layer of Mo,Al, containing precipitates of Mo(Si,Al), and Mo,Si,. Oxidation at about 1,240 °C resulted in the formation of an Al2O3 scale that provided protection for about 20 hours.

Taniguchi et al. (Osaka University) have attempted to preoxidize specimens of TiAl in a Rhines pack containing Cr2O, and chromium. The specimens were then oxidized at 1,023 °C in

oxygen at 1 atm. Preoxidation significantly improved the oxidation resistance of TiAl. Adherent Al2O, scales provided protection. The presence of chromium was also suggested to improve the protectiveness of the preformed Al2O3 scale.

Imai et al. (Japan Atomic Energy Research Institute) formed SiC coatings on graphite by heating specimens in silicon monoxide at 1,300-1,380 °C. These SiC coatings were reported to be resistant to thermal cycling and provided oxidation resistance.

Wada and Yoshiba (Tokyo Metropolitan University) have examined the corrosion of ceramics such as Al2O3, stabilized ZrO„, Si̟N, and SiC at 9001,200 °C in V2O,-Na2SO-NaCl mixtures. The corrosion properties of these ceramics were dependent upon impurities with more corrosion resistance being developed in the most pure materials. The addition of V2O, to the Na2SO-NaCl deposits generally caused increased corrosion.

The high temperature corrosion and coatings research in Japan is impressive. A number of problems are being studied. The work is usually related to some specific problem, but often the approaches are concerned with mechanisms for the observed degradation. The researchers are often young, compared to those in the United States. Significant accomplishments in this area are likely, especially within a period of 5 to 10 years from now.

ADDITIONAL DETAILS CONCERNING MATERIALS RESEARCH IN JAPAN

All of the topics that were discussed and observed during this 2-week visit to materials research laboratories will not be discussed in detail. There is a

very substantial interest in superalloys in Japan by companies such as MHI, KHI, IHI, national laboratories, as well

as universities. As mentioned in a