Surface modification of high temperature iron alloys
The United States Government has rights in this invention pursuant to Contract No. W-31-109-ENG-38 between the U.S. Department of Energy and the University of Chicago. 
    
    
     The invention is concerned generally with a method of modifying a surface region of high temperature iron alloys, such as Fe--Cr alloys. More particularly, the invention is directed to a method of protection against corrosive atmospheres by modifying the surface of an Fe--Cr alloy by forming a silicide based coating on the Fe--Cr surface. 
     Protection against corrosion of structural iron based alloys is of fundamental importance in high temperature materials applications. Typically, corrosion protection is obtained by formation of a chromium oxide scale which acts as a diffusion barrier between the corrosive environment and the iron based alloy. Unfortunately, conventional protective coatings are subject to erosion and spallation under high temperature corrosive atmospheric use. Such iron based alloys have been the subject of substantial unsuccessful research to determine compositions resistant to formation of such unstable coatings. 
     It is therefore an object of the invention to provide an improved article and method of manufacture of iron based alloys. 
     It is another object of the invention to provide a novel article and method of manufacture of a protective coating on iron based alloys. 
     It is a further object of the invention to provide an improved article and method of manufacture of a protective silicide based coating on iron-chromium alloys. 
     It is still another object of the invention to provide a novel article and method of manufacture of a protective amorphous silicon containing coating on an iron based alloy. 
     It is yet a further object of the invention to provide an improved article and method of manufacture of a protective silicon (nitrogen and/or chromium) compound coating on an iron based alloy. 
     It is also an object of the invention to provide a novel article and method of manufacture of an iron-chromium alloy doped with cerium, yttrium or lanthanum with the surface coated with a silicide layer diffused into the alloy surface. 
     It is an additional object of the invention to provide an improved article and method of manufacture of a silicon containing (or silicide), corrosion protection coating which passivates line and point defects near the surface of an iron based alloy. 
     Other objects, features and advantages of the present invention will be readily apparent from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings described below wherein like elements have like numerals throughout the several views. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrate a schematic of a microstructural cross section near the surface of an iron based alloy undergoing ion diffusion; 
     FIG. 2 illustrates another schematic of a microstructural cross section near the surface of an iron based alloy with grain boundary phases containing yttria; 
     FIG. 3 illustrates a chemical vapor deposition apparatus used to apply silicon based coatings to the surface of iron based alloys; 
     FIG. 4A illustrates the weight change arising from corrosion for selected iron based alloys oxidized in a pO 2  atmosphere of 10 -4  atm, at 1000° C. over a 0-40 hour time period; FIG. 4B is a similar plot as FIG. 4A but for a different alloy set oxidized in air at 1000° C. over a 30-day time period; FIG. 4C illustrates weight change arising from corrosion for selected iron based alloys sulfided at 700° C. over a 50-hour time period; FIG. 4D illustrates the same tests of FIG. 4C but extended to 35 days; and FIG. 4E shows the relationship between log pS 2  versus 1/T for 498 ppm H 2  S/H 2 , Cr/CrS and Fe/FeS; 
     FIG. 5A illustrates grain size variation in iron based alloys during oxidation; and FIG. 5B illustrates a comparison of grain size variation in iron based alloys annealed at 1000° C. over time; 
     FIG. 6 illustrates sample weight gain for the chemical vapor deposition process for Fe--25Cr--0.3Y and Fe--25Cr--0.3 Ce; and 
     FIG. 7A illustrates a microstructure of an iron based alloy with silicide diffused into the dark gray areas in and around surface defects, fissures and microcracks; and FIG. 7B shows a magnified portion of the right side of the microstructure of FIG. 7A. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A system 10 for practicing a preferred form of the method of the invention is shown generally in FIG. 3. As shown in FIG. 1 an iron based alloy 12 can undergo corrosion to form an oxide layer 14 which grows by cation diffusion 16 and/or anion diffusion 18 out of, or into, respectively, the iron based alloy 12. In addition to growth and spallation of the oxide layer 14 due to lack of adhesion, the high temperatures of operation (such as, 700°-1000° C.) applied to the iron based alloy 12, can cause enlargement of the size of grains 20 (see FIG. 5A), which also contributes to spallation of the oxide layer 14. This unwanted grain growth can be substantially eliminated by including selected reactive elements, such as Y (0.1-3 wt.%), Ce (0.3-1.0 wt.%), or other such elements like lanthanum as small additions to the iron based alloy 12. Without limiting the scope of any invention which may be claimed herein, it is believed the reactive elements form phases 22 of intermetallic compounds, oxides or stabilized voids at various grain boundary sites as shown in FIG. 2 (the vertically cross hatched phased are, for example, a Fe--Cr--Y phase, the horizontally cross hatched phases are, for example, yttrium oxide and the open circle is a void region). These reactive element containing phases 22 act to stabilize the grain boundaries, prevent any substantial grain growth as can be noted by considering FIG. 5B, and inhibit outward cation diffusion to the surface of the alloy 12. 
     In order to form an oxide layer 24 (see FIG. 2), which is stable (good adhesion and grows slowly) at high temperatures in a corrosive atmosphere, the system 10 shown in FIG. 3 is used to deposit a silicon containing (preferably a silicide, i.e., a silicon compound) composition on the iron based alloy 12. In a preferred method a chemical vapor deposition source 25 is disposed near iron based alloy samples 26 within a chamber 28 positioned in a furnace 30. In this system 10 the vapor deposition source 25 is heated to cause emission of vapors 32 which can be, for example, a powder mixture of NaF--Si--SiO 2 , a silane gas source, or other conventional sources which will produce a silicon containing layer on the surface of the alloy samples 26. In a preferred embodiment, the silicide layer can include silicon and oxygen and/or nitrogen and/or chromium. Preferably, the silicon containing layer is roughly 1-2 microns in thickness. The furnace 30 can be used to perform a post anneal and/or to maintain the alloy samples 26 at an elevated temperature (typically, 700°-1200° C.). Such thermal treatments, preferably maintaining elevated temperature during silicide deposition, enable the deposited silicon containing layer to diffuse into the alloy samples 26 and also fill or compensate selected defects, such as dislocations, cracks, voids, atomistic point defects and the like. 
     In the most preferred embodiment the resulting silicide coated form of the alloy samples 26 were next exposed to an ammonia (NH 3 ) vapor 32 input from ammonia supply 34. Other suitable sources for treating the silicon containing coating can be various nitrogen containing gases, such as NH 4  OH, NH 4  ions or SiN 2 . The ammonia vapor 32 reacts with the silicon containing coating to form a resulting preferred silicon/oxygen/nitrogen coating 36 on the oxide layer 24 (see FIG. 2). In other forms of the invention, the coating 36 is generally a silicon containing layer having at least a partially amorphous structure. These silicide layers, silicon containing layers, and amorphous silicon containing layers are stable under corrosive atmospheres, such as oxidizing or sulfiding atmospheres, and fill defects, such as cracks, voids and other surface defects. If uncompensated, such defects can act as conduits for cation and/or anion diffusion which act to thicken the oxide layer 24 (or sulfide layer if the alloy 12 is exposed to a corrosive sulfur atmosphere). 
     Comparison of examples of corrosion protection between iron based alloys with and without the silicon containing protective layer are shown in FIG. 4. Tests were performed in a thermogravimetric microbalance. In FIGS. 4A and 4B are comparisons of iron based alloys oxidized at 1000° C. As can be noted, the alloys coated with the silicon containing layer were much more stable than the unprotected alloys, including even the iron based alloys having reactive element additions of cerium and yttrium. In FIGS. 4C and 4D are shown data from iron based alloys protected with the silicon containing protective layer and exposed to a sulfiding corrosive atmosphere (498 ppm H 2  S/H 2  gas mixture and see FIG. 4E re partial pressure versus 1/T). The protected alloys are again much more stable than the unprotected alloys. 
     In other forms of the invention a silicon/oxygen/nitrogen and/or chromium layer can be obtained by ion beam deposition of silicon followed by, or preferably accompanied by, introducing ammonia to react with the ion beam deposited silicon to form the silicon and oxygen and/or nitrogen and/or chromium layer. Table I summarizes the resulting properties of ion beam deposited (IBAD) layers and chemical vapor deposition (CVD) layers as measured by X-ray diffraction, Auger spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy. This layer is then treated by annealing at high temperatures during deposition to enable diffusion and compensation to take place as described hereinbefore. The resulting layer has good adhesion to the alloy 12 but does not slow down the oxide layer thickening as well as the chemical vapor deposition process. However, the protective layer does provide protection superior to iron based alloys having no protection. 
     
                       TABLE I                                                     
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FE--25Cr--Re (Re = 1 &amp; 0.3 Y and Ce)                                      
IBAD                                                                      
No Heat        After Heat   CVD                                           
Treatment      Treatment    Annealing                                     
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XRD    Amorphous   Cr.sub.2 O.sub.3                                       
                                Cr.sub.2 O.sub.3                          
                   Fe--Cr-Silicides                                       
                                Fe--Cr-Silicides                          
Auger              Si, N, O                                               
XPS                Si, N, O                                               
SEM    Good Adhesion                                                      
                   Good Adhesion                                          
                                Good Adhesion                             
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     The following nonlimiting example illustrates various exemplary methods of manufacture of articles. 
     EXAMPLE 
     A coating of silicon containing material on an iron-chromium alloy was obtained by chemical vapor deposition using a powder mixture of NaF--Si--SiO 2 . Initially, a 10 gram mixture of NaF--Si--SiO 2  was made having a weight ratio of NaF/Si/SiO 2  =2/1/7. The mixture was ground in a mortar and pestle to refine the powder size. About 2.5 grams of mixed powder was loaded into a high purity alumina boat and then moved into the middle of a quartz chamber (OD-1.5 inch, one end closed and 4 feet long). Coupon samples (0.5 inch ×0.5 inch and 0.05 inches in thickness) of Fe--25Cr--0.3(Y and/or Ce) were placed into the chamber. A liquid ammonia (NH 3 ) container was attached to the chamber. About 1/4 by volume of the chamber was filled with a gas phase of ammonia preceded by filling with high purity Argon gas. The quartz chamber was evacuated with a rotary vacuum pump 50 sitown in FIG. 3. Care was taken not to disturb the powder with slow vacuum pumping at the beginning, and the evacuation process required only several minutes. The furnace was then heated to about 1000° C. over a period of 25 minutes from room temperature. The rotary pump was stopped after 30 minutes heating. About 8 hours later, the ammonia containing chamber was opened slowly and left open for about 2 hours, and the stopcock was closed. The temperature was maintained for several hours, and then the furnace power was turned off. FIG. 6 shows the sample weight gain versus position for the chemical vapor deposition process for Fe--25Cr--0.3Y and Fe--25Cr--0.3Ce. After each run, samples were examined by SEM and by electron energy-dispersive, Auger and X-ray photoelectron spectroscopies. Investigations by SEM, TEM and energy-dispersive spectroscopy, show that the silicon diffused and reacted near the surface of the Fe--25Cr--0.3Y alloy. Silicon has also diffused into and around surface defects, fissures, and micro cracks that are normally the chemically active sites where corrosion begins on the surface of an alloy (see FIG. 7). This sample was polished to remove the silicon containing material from smooth areas of the alloy surface. 
     While preferred embodiments of the invention have been shown and described, it will be clear to those skilled in the an that various changes and modifications can be made without departing from the invention in its broader aspects as set forth in the claims provided hereinafter.