METHOD FOR FORMING BINDER-FREE REFRACTORY CARBIDE, NITRIDE AND BORIDE COATINGS WITH A CONTROLLED POROSITY

Abstract

The present invention is directed to methods for formation of refractory carbide, nitride, and boride coatings without use of a binding agent. The present invention is directed to methods of creating refractory coatings with controlled porosity. Refractory coatings can be formed from refractory metal, metal oxide, or metal/metal oxide composite refractory coating precursor of the 9 refractory metals encompassed by groups 4-6 and periods 4-6 of the periodic table; non-metallic elements (e.g. Si & B) and their oxides (i.e. SiO.sub.2 & B.sub.2O.sub.3) are also pertinent. The conversion of the refractory coating precursor to refractory carbide, nitride or boride is achieved via carburization, nitridization, or boridization in the presence of carbon-containing (e.g. CH.sub.4), nitrogen containing (e.g. NH.sub.3), and boron-containing (e.g. B.sub.2H.sub.6) gaseous species. Any known technique of applying the refractory coating precursor can be used. The porosity of resultant refractory coatings is controlled through compositional manipulation of composite refractory coating precursors.

Claims

1. A method of forming a refractory coating comprising: applying a refractory coating precursor to a surface using any known method such as spray techniques; introducing a gaseous species; and generating thermal decomposition of the gaseous species.

2. The method of claim 1, further comprising using one of a group consisting of refractory metal, metal oxide, and metal/metal oxide composite as a refractory coating precursor.

3. The method of claim 1, further comprising using one selected from a group consisting of carbon-containing, nitrogen-containing, and boron containing gaseous species.

4. The method of claim 1, further comprising using plasma spray.

5. The method of claim 1, further comprising using cold spray.

6. The method of claim 1, further comprising forming one of a group consisting of refractory carbide, nitride, and boride coatings or any combination thereof.

7. The method of claim 1, wherein the refractory coating precursor comprises a refractory metal selected from a group consisting of titanium, vanadium, chromium, zirconium, niobium, molybdenum, halfnium, tantalum, and tungsten.

8. The method of claim 1, wherein the refractory coating precursor comprises chromium.

9. The method of claim 1, wherein the refractory coating precursor comprises silicon, boron, or a combination thereof.

10. The method of claim 1, further comprising forming Cr.sub.3C.sub.2.

11. The method of claim 1, wherein the gaseous species comprises methane.

12. A method of forming a refractory coating comprising: applying a refractory coating precursor to a surface using any known technique such as thermal spray; introducing a gaseous species; generating thermal decomposition of the gaseous species resulting in the refractory coating defining a porous refractory matrix; and using the porous refractory matrix as a scaffold for the formation of a multi-functional coating.

13. The method of claim 12, further comprising using one of a group consisting of refractory metal, metal oxide, and metal/metal oxide composite as a refractory coating precursor.

14. The method of claim 12, further comprising using plasma spray.

15. The method of claim 12, further comprising using one or more selected from a group consisting of carbon-containing, nitrogen-containing, and boron containing gaseous species.

16. The method of claim 12, further comprising forming one of a group consisting of refractory carbide, nitride, and boride coatings, or any combination thereof.

17. The method of claim 12, further comprising creating the multi-functional coating using ambient-temperature sealing with organic sealants, filling with sol-gel processed inorganic ceramics, liquid metal infiltration or a combination thereof.

18. The method of claim 12, wherein the refractory coating precursor comprises a refractory metal selected from a group consisting of titanium, vanadium, chromium, zirconium, niobium, molybdenum, halfnium, tantalum, and tungsten.

19. The method of claim 12, wherein the refractory coating precursor comprises silicon, boron, or a combination thereof.

20. The method of claim 12, further wherein the porous refractory matrix comprises Cr.sub.3C.sub.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The accompanying drawings provide visual representations, which will be used to more fully describe the representative embodiments disclosed herein and can be used by those skilled in the art to better understand them and their inherent advantages. In these drawings, like reference numerals identify corresponding elements and:

[0018] FIG. 1 illustrates an SEM cross-sectional image of an as-deposited plasma-sprayed Cr coating precursor.

[0019] FIG. 2 illustrates an SEM cross-sectional image of an as-deposited plasma-sprayed Cr.sub.2O.sub.3 coating precursor.

[0020] FIG. 3 illustrates a graphical view of an XRD pattern of an as-deposited plasma-sprayed Cr coating precursor.

[0021] FIG. 4 illustrates a graphical view of an XRD pattern of an as-deposited plasma-sprayed Cr.sub.2O.sub.3 coating precursor.

[0022] FIG. 5 illustrates an SEM cross-sectional image of a 6 h carburized plasma-sprayed Cr coating precursor.

[0023] FIG. 6 illustrates an SEM cross-sectional image of a 6 h carburized plasma-sprayed Cr.sub.2O.sub.3 coating precursor.

[0024] FIG. 7 illustrates a graphical view of an XRD pattern of a 6 h carburized plasma-sprayed Cr coating precursor.

[0025] FIG. 8 illustrates a graphical view of an XRD pattern of a 6 h carburized plasma-sprayed Cr.sub.2O.sub.3 coating precursor.

[0026] FIG. 9 illustrates a graphical view of final total porosity as a function of composition and initial total porosity.

[0027] FIG. 10 illustrates a schematic diagram of a carburization apparatus, according to an embodiment of the present invention.

[0028] FIGS. 11 and 12 illustrate XRD patterns that indicate conversion of plasma-sprayed Cr and Cr.sub.2O.sub.3 coatings to Cr.sub.3C.sub.2 through equations (11) and (12).

[0029] FIG. 13 illustrates an SEM image of a Cr feedstock powder.

[0030] FIG. 14 illustrates an SEM cross-sectional image of a sprayed Cr coating deposited via APS.

[0031] FIG. 15 illustrates an SEM cross-sectional image of a sprayed Cr coating deposited via CGDS.

[0032] FIG. 16 illustrates an XRD comparison of as-deposited sprayed Cr coatings.

[0033] FIG. 17 illustrates an XRD comparison of 6 h carburized sprayed Cr coatings.

[0034] FIG. 18 illustrates an XRD comparison of 12 h carburized sprayed Cr coatings.

[0035] FIGS. 19A-19D illustrate SEM cross-sectional images of carburized plasma-sprayed Cr (6 h), cold-sprayed Cr (6 h), plasma-sprayed Cr (12 h) and cold-sprayed Cr (12 h), coatings respectively.

[0036] FIGS. 20A-20C illustrate EDS X-ray mapping of the 6 h carburized plasma-sprayed Cr coating; Reference SEM cross-sectional image, EDS X-ray mapping of Cr-Kα, and EDS X-ray mapping of O-Kα, respectively.

[0037] FIGS. 21A and 21B illustrate SEM micrographs of feedstock powder morphologies. FIG. 21A illustrates NiCr—Al used in the bond layer and FIG. 21B illustrates Cr.sub.2O.sub.3 used in the top coat.

[0038] FIG. 22 illustrates a graphical view of XRD patterns of the Cr.sub.2O.sub.3 feedstock powder used in this work (top scan) and the resulting plasma-sprayed coating (bottom scan), indicating no appreciable compositional changes as a result of the deposition process.

[0039] FIG. 23 illustrates a Cross-sectional SEM micrograph of plasma-sprayed Cr.sub.2O.sub.3 (top coat), showing a complex pore structure containing globular voids, interlamellar porosity and intralamellar microcracks.

[0040] FIG. 24 illustrates a graphical view of XRD patterns of plasma-sprayed Cr.sub.2O.sub.3 after various reduction times (0.1-0.3 h) at 1000° C., showing coating phase evolution from Cr.sub.2O.sub.3 to binder-free Cr.sub.3C.sub.2.

[0041] FIGS. 25A-25C illustrate cross-sectional SEM micrographs of plasma-sprayed Cr.sub.2O.sub.3 after various reduction times at 1000° C., illustrating characteristics of the mechanism of reduction to binder-free Cr.sub.3C.sub.2: FIG. 25A illustrates 0.1 h, FIG. 25B illustrates 0.2 h, and FIG. 25C illustrates 0.3 h.

[0042] FIG. 26 illustrates an SEM image view of void formation in the Cr.sub.3C.sub.2 phase formed from the reduction of plasma-sprayed Cr.sub.2O.sub.3.

DETAILED DESCRIPTION

[0043] The presently disclosed subject matter now will be described more fully hereinafter with reference to the accompanying Drawings, in which some, but not all embodiments of the inventions are shown. Like numbers refer to like elements throughout. The presently disclosed subject matter may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Indeed, many modifications and other embodiments of the presently disclosed subject matter set forth herein will come to mind to one skilled in the art to which the presently disclosed subject matter pertains having the benefit of the teachings presented in the foregoing descriptions and the associated Drawings. Therefore, it is to be understood that the presently disclosed subject matter is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims.

[0044] The present invention is directed to a method for the formation of refractory carbide, nitride, and boride coatings without the use of a binding agent. The present invention is also directed to a method of creating refractory coatings with a controlled porosity. The refractory coatings can be formed from a refractory metal, metal oxide, or metal/metal oxide composite refractory coating precursor of the 9 refractory metals encompassed by groups 4-6 and periods 4-6 of the periodic table; non-metallic elements (e.g. Si & B) and their oxides (i.e. SiO.sub.2 & B.sub.2O.sub.3) are also pertinent. The conversion of the refractory coating precursor to refractory carbide, nitride or boride is achieved via carburization, nitridization, or boridization in the presence of carbon-containing (e.g. CH.sub.4), nitrogen containing (e.g. NH.sub.3), and boron-containing (e.g. B.sub.2H.sub.6) gaseous species respectively. Any known technique of applying the refractory coating precursor can be used, such as thermal spray. The porosity of resultant refractory coatings is controlled through compositional manipulation of composite refractory coating precursors.

[0045] The present invention in part is directed to the carburization of plasma-sprayed Cr and Cr.sub.2O.sub.3 coating precursors for conversion to Cr.sub.3C.sub.2 coatings. While the example of the carburization of Cr and Cr.sub.2O.sub.3 coating precursors is used herein, this is understood to be merely exemplary, and any suitable combination of refractory coating precursors and gaseous reactants can be used. For example, the refractory coatings can be formed from a refractory metal, metal oxide, or metal/metal oxide composite refractory coating precursor of the 9 refractory metals encompassed by groups 4-6 and periods 4-6 of the periodic table; non-metallic elements (e.g. Si & B) and their oxides (i.e. SiO.sub.2 & B.sub.2O.sub.3) are also pertinent. Reactants for carburization, nitridization, and boridization of refractory coating precursors include carbon-containing (e.g. CH.sub.4), nitrogen-containing (e.g. NH.sub.3), and boron-containing (e.g. B.sub.2H.sub.6) gaseous species respectively.

[0046] FIGS. 1-4 show scanning electron microscopy (SEM) cross-sectional images and X-ray diffraction (XRD) patterns of as-deposited plasma-sprayed Cr and Cr.sub.2O.sub.3 coating precursors. More particularly, FIG. 1 illustrates an SEM cross-sectional image of an as-deposited plasma-sprayed Cr coating precursor and FIG. 2 illustrates an SEM cross-sectional image of an as-deposited plasma-sprayed Cr.sub.2O.sub.3 coating precursor. FIG. 3 illustrates a graphical view of an XRD pattern of an as-deposited plasma-sprayed Cr coating precursor and FIG. 4 illustrates a graphical view of an XRD pattern of an as-deposited plasma-sprayed Cr.sub.2O.sub.3 coating precursor.

[0047] As-deposited plasma-sprayed Cr and Cr.sub.2O.sub.3 coating precursors are then carburized in a horizontal quartz tube furnace for 6 h at 1000° C. in an Ar-20 vol. % CH.sub.4 gas mixture with a volumetric flow rate of 250 cm.sup.3/min. The system is purged for 3 h with 400 cm.sup.3/min Ar prior to the initiation of the thermal cycle. Heating and cooling ramps were carried out at a rate of 400° C./hr and 200° C./hr respectively, with an atmosphere of 200 cm.sup.3/min Ar. Ultra-high purity gas sources were used in conjunction with a Matheson Nanochem Purifilter to further reduce O.sub.2 and H.sub.2O impurity levels (<0.1 ppb O.sub.2 & H.sub.2O). Gas mixture composition was regulated through the use of Alicat Series MC mass flow controllers. The following equations are likely the overall reaction pathways for the conversion of Cr and Cr.sub.2O.sub.3 coating precursors to Cr.sub.3C.sub.2 via carburization with methane-containing gas:

3Cr(s)+2CH.sub.4(g).fwdarw.Cr.sub.3C.sub.2(s)+4H.sub.2(g)  (1)

3Cr.sub.2O.sub.3(s)+13CH.sub.4(g).fwdarw.2Cr.sub.3C.sub.2(S)+9CO(g)+26H.sub.2(g)  (2)

[0048] SEM cross-sectional images and XRD patterns of plasma-sprayed Cr and Cr.sub.2O.sub.3 coating precursors treated by the carburization procedure described above are shown in FIGS. 5-8. More particularly, FIG. 5 illustrates an SEM cross-sectional image of a 6 h carburized plasma-sprayed Cr coating precursor and FIG. 6 illustrates an SEM cross-sectional image of a 6 h carburized plasma-sprayed Cr.sub.2O.sub.3 coating precursor. FIG. 7 illustrates a graphical view of an XRD pattern of a 6 h carburized plasma-sprayed Cr coating precursor and FIG. 8 illustrates a graphical view of an XRD pattern of a 6 h carburized plasma-sprayed Cr.sub.2O.sub.3 coating precursor. Included in FIGS. 7 and 8 is an XRD pattern of a plasma-sprayed commercial Cr.sub.3C.sub.2−7(Ni 20Cr) cermet coating, Metco 82VF-NS, which represents the industry standard for plasma-sprayed Cr.sub.3C.sub.2 coatings for comparison. It is apparent from FIGS. 7 and 8 that Cr.sub.3C.sub.2 coatings formed by the carburization of plasma-sprayed Cr and Cr.sub.2O.sub.3 coating precursors possess a degree of crystallinity which far surpasses that of the industry standard, as shown by the strong matching of peak locations with the Cr.sub.3C.sub.2 reference pattern and the intensities of such peaks relative to the background. Because, only peaks corresponding to Cr.sub.3C.sub.2 phase were detected, the coatings formed by the carburization of plasma-sprayed Cr and Cr.sub.2O.sub.3 coating precursors are binder-free Cr.sub.3C.sub.2 and possess properties intrinsic to the material in its pure state. Conversion of the compositional extremes (pure Cr & pure Cr.sub.2O.sub.3) of a proposed Cr/Cr.sub.2O.sub.3 composite coating precursor has been demonstrated under identical carburization conditions. This strongly implies that Cr/Cr.sub.2O.sub.3 composite coating precursors with compositions intermediate to the two extremes can be successfully converted under the same carburization conditions, with the notion that Cr and Cr.sub.2O.sub.3 behave in an ideal manner thermodynamically in the Cr/Cr.sub.2O.sub.3 composite coating precursor.

[0049] From inspection of FIGS. 5 and 6 it is clear that the carburization of plasma-sprayed Cr and Cr.sub.2O.sub.3 coating precursors produces Cr.sub.3C.sub.2 coatings with vastly different pore structures, which suggests the opportunity to control porosity through compositional manipulation of Cr/Cr.sub.2O.sub.3 composite coating precursors. The following equations give the fractional volume change associated with the conversion of Cr and Cr.sub.2O.sub.3 coating precursors to Cr.sub.3C.sub.2:

[00001]$\begin{array}{cc}\frac{\Delta V}{V_{\mathrm{initial}}}{.Math.}_{\mathrm{Cr}.fwdarw.{\mathrm{Cr}}_3{C}_2}=\frac{V_{{\mathrm{Cr}}_3{C}_2}-V_{\mathrm{Cr}}}{V_{\mathrm{Cr}}}=\frac{n_{{\mathrm{Cr}}_3{C}_2}{V}_{m{,}_{{\mathrm{Cr}}_3{C}_2}}-n_{\mathrm{Cr}}{V}_{m{,}_{\mathrm{Cr}}}}{n_{\mathrm{Cr}}{V}_{m{,}_{\mathrm{Cr}}}}& \left(3\right)\end{array}$$\begin{array}{cc}\frac{\Delta V}{V_{\mathrm{initial}}}{.Math.}_{{\mathrm{Cr}}_2{O}_3.fwdarw.{\mathrm{Cr}}_3{C}_2}=\frac{V_{{\mathrm{Cr}}_3{C}_2}-V_{{\mathrm{Cr}}_2{O}_3}}{V_{{\mathrm{Cr}}_2{O}_3}}=\frac{n_{{\mathrm{Cr}}_3{C}_2}{V}_{m{,}_{{\mathrm{Cr}}_3{C}_2}}-n_{{\mathrm{Cr}}_2{O}_3}{V}_{m{,}_{{\mathrm{Cr}}_2{O}_3}}}{n_{{\mathrm{Cr}}_2{O}_3}{V}_{m{,}_{{\mathrm{Cr}}_2{O}_3}}}& \left(4\right)\end{array}$

where n and V.sub.m are the number of moles and molar volume of each component respectively. If equations (1) and (2) are expressed in terms of the number of moles of refractory coating precursor material:

[00002]$\begin{array}{cc}{n}_{\mathrm{Cr}}\mathrm{Cr}\left(s\right)=\frac{2n_{\mathrm{Cr}}}{3}{\mathrm{CH}}_4\left(g\right).fwdarw.\frac{n_{\mathrm{Cr}}}{3}{\mathrm{Cr}}_3{C}_2\left(s\right)+\frac{4n_{\mathrm{Cr}}}{3}{H}_2\left(g\right)& \left(5\right)\end{array}$$\begin{array}{cc}{n}_{{\mathrm{Cr}}_2{O}_3}{\mathrm{Cr}}_2{O}_3\left(s\right)+\frac{13n_{{\mathrm{Cr}}_2{O}_3}}{3}{\mathrm{CH}}_4\left(g\right).fwdarw.\frac{2n_{{\mathrm{Cr}}_2{O}_3}}{3}{\mathrm{Cr}}_3{C}_2\left(s\right)+& \left(6\right)\end{array}$$3n_{{\mathrm{Cr}}_2{O}_3}\mathrm{CO}\left(g\right)+\frac{26n_{{\mathrm{Cr}}_2{O}_3}}{3}{H}_2\left(g\right)$

the fractional volume change according to the stoichiometry of equations (5) and (6) is obtained by substitution:

[00003]$\begin{array}{cc}\frac{\Delta V}{V_{\mathrm{initial}}}{.Math.}_{\mathrm{Cr}.fwdarw.{\mathrm{Cr}}_3{C}_2}=\frac{n_{{\mathrm{Cr}}_3{C}_2}{V}_{m{,}_{{\mathrm{Cr}}_3{C}_2}}-n_{\mathrm{Cr}}{V}_{m{,}_{\mathrm{Cr}}}}{n_{\mathrm{Cr}}{V}_{m{,}_{\mathrm{Cr}}}}=\frac{\frac{n_{\mathrm{Cr}}}{3}{V}_{m{,}_{{\mathrm{Cr}}_3{C}_2}}-n_{\mathrm{Cr}}{V}_{m{,}_{\mathrm{Cr}}}}{n_{\mathrm{Cr}}{V}_{m{,}_{\mathrm{Cr}}}}=\frac{\frac{V_{m{,}_{{\mathrm{Cr}}_3{C}_2}}}{3}-V_{m{,}_{\mathrm{Cr}}}}{V_{m{,}_{\mathrm{Cr}}}}& \left(7\right)\end{array}$$\begin{array}{cc}\frac{\Delta V}{V_{\mathrm{initial}}}{.Math.}_{\mathrm{Cr}.fwdarw.{\mathrm{Cr}}_3{C}_2}=\frac{\frac{26.95\frac{{\mathrm{cm}}^3}{\mathrm{mol}}}{3}-7.27\frac{{\mathrm{cm}}^3}{\mathrm{mol}}}{7.27\frac{{\mathrm{cm}}^3}{\mathrm{mol}}}=0.24\frac{\Delta V}{V_{\mathrm{initial}}}{.Math.}_{\mathrm{Cr}.fwdarw.{\mathrm{Cr}}_3{C}_2}=0.24& \left(8\right)\end{array}$$\frac{\Delta V}{V_{\mathrm{initial}}}{.Math.}_{{\mathrm{Cr}}_2{O}_3.fwdarw.{\mathrm{Cr}}_3{C}_2}=\frac{n_{{\mathrm{Cr}}_3{C}_2}{V}_{m,{\mathrm{Cr}}_3{c}_2}-n_{{\mathrm{Cr}}_2{O}_3}{V}_{m{,}_{{\mathrm{Cr}}_2{O}_3}}}{n_{{\mathrm{Cr}}_2{O}_3}{V}_{m{,}_{{\mathrm{Cr}}_2{O}_3}}}=\frac{\frac{2n_{{\mathrm{Cr}}_2{O}_3}}{3}{V}_{m{,}_{{\mathrm{Cr}}_3{C}_2}}-n_{{\mathrm{Cr}}_2{O}_3}{V}_{m{,}_{{\mathrm{Cr}}_2{O}_3}}}{n_{{\mathrm{Cr}}_2{O}_3}{V}_{m{,}_{{\mathrm{Cr}}_2{O}_3}}}=\frac{\frac{2V_{m{,}_{{\mathrm{Cr}}_3{C}_2}}}{3}-V_{m{,}_{{\mathrm{Cr}}_2{O}_3}}}{V_{m{,}_{{\mathrm{Cr}}_2{O}_3}}}$$\frac{\Delta V}{V_{\mathrm{initial}}}{.Math.}_{{\mathrm{Cr}}_2{O}_3.fwdarw.{\mathrm{Cr}}_3{C}_2}=\frac{\frac{2*26.95\frac{{\mathrm{cm}}^3}{\mathrm{mol}}}{3}-29.12\frac{{\mathrm{cm}}^3}{\mathrm{mol}}}{29.12\frac{{\mathrm{cm}}^3}{\mathrm{mol}}}=-0.38\frac{\Delta V}{V_{\mathrm{initial}}}{.Math.}_{{\mathrm{Cr}}_2{O}_3.fwdarw.{\mathrm{Cr}}_3{C}_2}=-0.38$

[0050] It is evident that the conversion of Cr to Cr.sub.3C.sub.2 is associated with a volume expansion, while the conversion of Cr.sub.2O.sub.3 to Cr.sub.3C.sub.2 is associated with a volume contraction. This fact allows porosity to be controlled through composition manipulation of Cr/Cr.sub.2O.sub.3 composite coating precursors, as opposed to controlling porosity by the variation of thermal spray deposition parameters, which is highly empirical in nature. The final total porosity (TP.sub.f) in resultant Cr.sub.3C.sub.2 coatings is a function of the composition and initial total porosity (TP.sub.i) of the Cr/Cr.sub.2O.sub.3 composite coating precursor:

[00004]$\begin{array}{cc}{\mathrm{TP}}_f=\frac{\begin{array}{c}\left(\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}-0.24\right)\left(1-x_{{\mathrm{Cr}}_2{O}_3}\right)V_{m,\mathrm{Cr}}+\\ \left(\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}+0.38\right)x_{{\mathrm{Cr}}_2{O}_3}{V}_{m,{\mathrm{Cr}}_2{O}_3}\end{array}}{\begin{array}{c}\left(1+\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}\right)\left(1-x_{{\mathrm{Cr}}_2{O}_3}\right)V_{m,\mathrm{Cr}}+\\ \left(1+\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}\right)x_{{\mathrm{Cr}}_2{O}_3}{V}_{m,{\mathrm{Cr}}_2{O}_3}\end{array}}& \left(9\right)\end{array}$

[0051] The full derivation for equation (9) is provided below in paragraphs [0053]-[0057]. Because, thermally-sprayed coatings typically have an initial total porosity of 20% or less, the final total porosity of Cr.sub.3C.sub.2 coatings resulting from the carburization of Cr/Cr.sub.2O.sub.3 composite coating precursors with initial total porosities ranging from 0-20% has been plotted as a function of precursor composition. FIG. 9 illustrates a graphical view of final total porosity as a function of composition and initial total porosity.

[0052] One important aspect of the control of porosity in resultant coatings is the ability to achieve net-shape conversion. Net-shape conversion occurs when the final total porosity is equal to the initial total porosity, at a composition of x.sub.Cr.sub.2.sub.O.sub.3=0.137. The composition for net-shape conversion has been indicated in FIG. 9. For compositions on the Cr-rich side of the net-shape composition a fully-dense Cr.sub.3C.sub.2 coating with “negative porosity” (in-plane compressive stress) is produced. This may translate to Cr.sub.3C.sub.2 coatings with hardness values exceeding that of the bulk material. While the numerical analysis above is dependent on the molar volumes of Cr, Cr.sub.2O.sub.3 and Cr.sub.3C.sub.2, the derivation of equation (9) is entirely general to porosity control by compositional manipulation of composite refractory coating precursors. Therefore, the above discussion is regarded as a proof of concept for controlling porosity of refractory carbide, nitride and boride coatings.

[0053] The initial total porosity (vol. %) of the Cr/Cr.sub.2O.sub.3 composite coating precursor is defined as follows:

[00005]${\mathrm{TP}}_i=\frac{V_{\mathrm{porosity},i}}{V_{\mathrm{total},i}}=\frac{V_{\mathrm{porosity},i}}{V_{\mathrm{material},i}+V_{\mathrm{porosity},i}}$

[0054] Rearranging for V.sub.porosity,i in terms of TP.sub.i and V.sub.material,i:

[00006]$V_{\mathrm{porosity},i}=\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}{V}_{\mathrm{material},i}{V}_{\mathrm{material},i}=n_{\mathrm{Cr}}{V}_{m,\mathrm{Cr}}+n_{{\mathrm{Cr}}_2{O}_3}{V}_{m,{\mathrm{Cr}}_2{O}_3}{V}_{\mathrm{porosity},i}=\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}\left(n_{\mathrm{Cr}}{V}_{m,\mathrm{Cr}}+n_{{\mathrm{Cr}}_2{O}_3}{V}_{m,{\mathrm{Cr}}_2{O}_3}\right)$$V_{\mathrm{total},i}=n_{\mathrm{Cr}}{V}_{m,\mathrm{Cr}}+n_{{\mathrm{Cr}}_2{O}_3}{V}_{m,{\mathrm{Cr}}_2{O}_3}+\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}\left(n_{\mathrm{Cr}}{V}_{m,\mathrm{Cr}}+n_{{\mathrm{Cr}}_2{O}_3}{V}_{m,{\mathrm{Cr}}_2{O}_3}\right)$$V_{\mathrm{total},i}=\left(1+\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}\right)n_{\mathrm{Cr}}{V}_{m,\mathrm{Cr}}+\left(1+\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}\right)n_{{\mathrm{Cr}}_2{O}_3}{V}_{m,{\mathrm{Cr}}_2{O}_3}$

[0055] The final total porosity (vol. %) of the resultant Cr.sub.3C.sub.2 coating is defined as:

[00007]${\mathrm{TP}}_f=\frac{V_{\mathrm{porosity},f}}{V_{\mathrm{total},f}}$

[0056] Two assumptions are made, the first being that the total volume of the coating is unchanged during the conversion process. Hence:

[00008]$V_{\mathrm{total},f}=V_{\mathrm{total},i}{V}_{\mathrm{total},f}=\left(1+\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}\right)n_{\mathrm{Cr}}{V}_{m,\mathrm{Cr}}+\left(1+\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}\right)n_{{\mathrm{Cr}}_2{O}_3}{V}_{m,{\mathrm{Cr}}_2{O}_3}$

[0057] It is also assumed that any volume changes associated with the conversion of Cr and Cr.sub.2O.sub.3 to Cr.sub.3C.sub.2 are manifest as the creation/annihilation of porosity. Implicit in this assumption is that volume changes associated with the conversion of Cr and Cr.sub.2O.sub.3 to Cr.sub.3C.sub.2 are not accommodated for by elastic/plastic strain in the resulting Cr.sub.3C.sub.2 coating. Accordingly:

[00009]$V_{\mathrm{porosity},f}=\left(-0.24\right)n_{\mathrm{Cr}}{V}_{m,\mathrm{Cr}}+\left(0.38\right)n_{{\mathrm{Cr}}_2{O}_3}{V}_{m,{\mathrm{Cr}}_2{O}_3}+V_{\mathrm{porosity},i}$$V_{\mathrm{porosity},f}=\left(-0.24\right)n_{\mathrm{Cr}}{V}_{m,\mathrm{Cr}}+\left(0.38\right)n_{{\mathrm{Cr}}_2{O}_3}{V}_{m,{\mathrm{Cr}}_2{O}_3}+\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}\left(n_{\mathrm{Cr}}{V}_{m,\mathrm{Cr}}+n_{{\mathrm{Cr}}_2{O}_3}{V}_{m,{\mathrm{Cr}}_2{O}_3}\right)V_{\mathrm{porosity},f}=\left(\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}-0.24\right)n_{\mathrm{Cr}}{V}_{m,\mathrm{Cr}}+\left(\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}+0.38\right)n_{{\mathrm{Cr}}_2{O}_3}{V}_{m,{\mathrm{Cr}}_2{O}_3}$

[0058] Therefore, the final result:

[00010]${\mathrm{TP}}_f=\frac{\left(\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}-0.24\right)n_{\mathrm{Cr}}{V}_{m,\mathrm{Cr}}+\left(\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}+0.38\right)n_{{\mathrm{Cr}}_2{O}_3}{V}_{m,{\mathrm{Cr}}_2{O}_3}}{\left(1+\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}\right)n_{\mathrm{Cr}}{V}_{m,\mathrm{Cr}}+\left(1+\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}\right)n_{{\mathrm{Cr}}_2{O}_3}{V}_{m,{\mathrm{Cr}}_2{O}_3}}{\mathrm{TP}}_f=\frac{\begin{array}{c}\left(\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}-0.24\right)\left(1-x_{{\mathrm{Cr}}_2{O}_3}\right)V_{m,\mathrm{Cr}}+\\ \left(\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}+0.38\right)x_{{\mathrm{Cr}}_2{O}_3}{V}_{m,{\mathrm{Cr}}_2{O}_3}\end{array}}{\left(1+\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}\right)\left(1-x_{{\mathrm{Cr}}_2{O}_3}\right)V_{m,\mathrm{Cr}}+\left(1+\frac{{\mathrm{TP}}_i}{1-{\mathrm{TP}}_i}\right)x_{{\mathrm{Cr}}_2{O}_3}{V}_{m,{\mathrm{Cr}}_2{O}_3}}$

EXEMPLARY IMPLEMENTATIONS

[0059] Exemplary implementations of the present invention are described herein, in order to further illustrate the present invention. The exemplary implementation is included merely as an example and is not meant to be considered limiting. Any implementation of the present invention on any suitable subject known to or conceivable by one of skill in the art could also be used, and is considered within the scope of this application.

[0060] Haynes 230 (H230) alloy substrates were used with the nominal chemical composition provided in Table 1. Preparation of these substrates included abrasive blasting with 36-grit Al.sub.2O.sub.3 media at 80 psi followed by cleaning with compressed air. A plasma spray system (Metco 9 MB plasma gun, Bay State Surface Technologies Model 1200 powder feeder and Yaksawa America Motoman HP20 movement) was used for deposition of Cr and Cr.sub.2O.sub.3. The nominal specifications for the main chemical constituents of the feedstock powders are presented in Table 2 (Sulzer-Metco). The 43VF-NS feedstock powder was used as a bond layer for the Amdry 6420 coating. Deposition conditions for each feedstock powder are shown in Table 3. Substrates were cooled with compressed air during plasma spray deposition.

TABLE-US-00001 TABLE 1 Nominal chemical composition (wt. %) of H230 alloy. Ni Cr W Mo Fe Co Mn Si Al C La B 57 22 14 2 3 5 0.5 0.4 0.3 .10 .02 .015

TABLE-US-00002 TABLE 2 Nominal feedstock powder specifications. Particle Chemical Composition Size Feedstock Powder (wt. %) (μm) Amdry 50357 99.5% Cr −45 43VF-NS 76.3% Ni 19.5% Cr −45 +5 Amdry 6420 99.5% Cr.sub.2O.sub.3 −45 +22

TABLE-US-00003 TABLE 3 APS deposition conditions. Working Traverse Step Feedstock Voltage Current Distance Speed Size Powder (V) (A) (in) (in/sec) (in) Amdry 50357 70 500 5 16.4 0.125 43VF-NS 70 500 4 16.4 0.125 Amdry 6420 70 500 2.75 16.4 0.125

[0061] The plasma-sprayed Cr and Cr.sub.2O.sub.3 coatings were carburized using a flowing, methane-containing atmosphere (80 vol. % Ar with 20 vol. % CH.sub.4; 250 cm.sup.3/min) in a horizontal quartz tube furnace for 6 h at 1000° C. The system was purged for 3 h with 400 cm.sup.3/min Ar prior to the initiation of the thermal cycle. Heating and cooling ramps were carried out at a rate of 400° C./h and 200° C./h respectively, with an atmosphere of 200 cm.sup.3/min Ar. Ultra-high purity gas sources were used in conjunction with an in-line purifier (Matheson Nanochem Purifilter) to further reduce O.sub.2 and H.sub.2O impurity levels (<0.1 ppb O.sub.2 and H.sub.2O). Gas mixture composition was regulated through the use of mass flow controllers (Alicat Scientific Series MC). A schematic illustration of the carburization apparatus used is shown in FIG. 10.

[0062] SEM cross-sectional images of the plasma-sprayed Cr and Cr.sub.2O.sub.3 coatings in FIGS. 1 and 2 show that these coatings are approximately 30 μm and 80 μm thick respectively. It is apparent from FIGS. 1 and 2 that the as-deposited Cr and Cr.sub.2O.sub.3 coatings contain both the globular voids and the interlamellar/microcrack porosity characteristics of thermally-sprayed materials. While an interconnected network of voids is certainly a detriment in the application of molten fluoride corrosion resistance, it may be advantageous in achieving through-thickness coating carburization. The overall reaction pathways for converting Cr and Cr.sub.2O.sub.3 coatings to Cr.sub.3C.sub.2 in a methane-containing atmosphere are likely described by the following equations:

3Cr(s)+2CH.sub.4(g).fwdarw.Cr.sub.3C.sub.2(s)+4H.sub.2(g)  (10)

3Cr.sub.2O.sub.3(s)+13CH.sub.4(g).fwdarw.2 Cr.sub.3C.sub.2(s)+9CO(g)+26H.sub.2(g)  (11)

[0063] XRD patterns shown in FIGS. 11 and 12 indicate conversion of Cr and Cr.sub.2O.sub.3 coatings to Cr.sub.3C.sub.2 through these reactions. The Cr.sub.3C.sub.2 coating derived from the carburization of Cr.sub.2O.sub.3 resulted in an XRD pattern that closely matched that of untextured, polycrystalline Cr.sub.3C.sub.2 indicating that the carbide grains in this coating were randomly oriented. However, the Cr.sub.3C.sub.2 coating derived from the carburization of Cr displayed preferred orientation of the carbide grains evidenced by the relative heights of the peaks associated with diffraction from the (203) and (301) planes. FIGS. 5 and 6 illustrate SEM cross-sectional images of Cr.sub.3C.sub.2 coatings resulting from the carburization of Cr and Cr.sub.2O.sub.3 coatings.

[0064] Beyond texture differences indicated by XRD, inspection of FIGS. 5 and 6 shows that vastly different Cr.sub.3C.sub.2 coating microstructures result from the carburization of Cr coatings compared to the carburization of Cr.sub.2O.sub.3 coatings. This could be attributed in part to volumetric changes that occur during the conversion process. Conversion of Cr to Cr.sub.3C.sub.2 according to Eq. (10) is associated with a volume expansion of 23.57% and the conversion of Cr.sub.2O.sub.3 to Cr.sub.3C.sub.2 according to Eq. (11) is associated with a volume contraction of 38.30%. These considerable differences should result in correspondingly different Cr.sub.3C.sub.2 coating microstructures because coating dimensions are likely constrained by the substrate. The volume expansion associated with the conversion of Cr to Cr.sub.3C.sub.2 reduces the population of microcracks, initially present in the as-deposited Cr coating. Similarly, the highly porous Cr.sub.3C.sub.2 coating shown in FIG. 6 is a consequence of the large volume contraction associated with the conversion of Cr.sub.2O.sub.3 to Cr.sub.3C.sub.2.

[0065] In this work the carburization of plasma-sprayed Cr and Cr.sub.2O.sub.3 coatings on H230 alloy substrates has been demonstrated. Conversion of these coatings to Cr.sub.3C.sub.2 was accomplished by heating the coating-substrate materials in a high-temperature methane-containing atmosphere, allowing for decomposition of the methane and subsequent reaction between carbon and the coating materials. The microstructures of the Cr.sub.3C.sub.2 coatings depended on characteristic volumetric changes that occur during the conversion process. The results presented here suggest that the carburization of thermally-sprayed Cr coatings with engineered pore structures could be a viable pathway for synthesizing dense Cr.sub.3C.sub.2 coatings for applications requiring molten fluoride salt corrosion resistance. Corrosion studies on these coatings will be performed to assess the value of the processing technique presented here for producing coatings to be used in molten fluoride salt environments.

[0066] In another exemplary embodiment, as nickel-based alloys derived from the Ni—Cr—W system are currently considered to be leading candidates for MSFR materials selection and Haynes 230 (H230) exhibited the greatest weight loss per unit area in recent molten fluoride salt corrosion studies, this particular alloy was chosen as a substrate material in the present study. Table 4 shows the nominal chemical composition of H230 alloy.

TABLE-US-00004 TABLE 4 Nominal chemical composition (wt. %) of H230 alloy. Ni Cr W Mo Fe Co Mn Si Al C La B 57 22 14 2 3 5 0.5 0.4 0.3 0.10 0.02 0.015

[0067] H230 alloy was procured in sheet form (per AMS 5878 rev. C) with dimensions of 36″×12″×0.125″ and cut into substrates measuring 3″×1″×0.125″ via waterjet cutting. Substrates were sand blasted with 36 grit Al.sub.2O.sub.3 media and subsequently cleaned with compressed air prior to the deposition of Cr.

[0068] An atmospheric plasma spray (APS) system (Metco 9 MB plasma gun, Bay State Surface Technologies Model 1200 powder feeder and Yaksawa Motoman HP20 robot movement) and a cold gas dynamic spray (CGDS) system (Cold Gas Technology Kinetiks 4000 cold gas gun and powder feeder, Yaskawa Motoman MH50 robot movement) were used for the deposition of Cr. An SEM image and the specifications of the Cr feedstock powder used in this work are provided in FIG. 13 and Table 5 (Sulzer-Metco). APS and CGDS deposition conditions are presented in Tables 6 and 7.

TABLE-US-00005 TABLE 5 Nominal feedstock powder specifications. Chemical Particle Feedstock Composition Size Powder (wt. %) (μm) Morphology Amdry 50357 99.5% Cr −45 Spherical

TABLE-US-00006 TABLE 6 APS deposition conditions. Working Traverse Step Feedstock Voltage Current Distance Speed Size Powder (V) (A) (mm) (mm/sec) (mm) Amdry 50357 70 500 127 416.67 3.18

TABLE-US-00007 TABLE 7 CGDS deposition conditions. He He Working Traverse Step Feedstock Pressure Temperature Distance Speed Size Powder (MPa) (K) (mm) (mm/sec) (mm) Amdry 50357 4 873.15 25.4 100 0.5

[0069] Coatings were carburized using a flowing, methane-containing atmosphere (80 vol. % Ar with 20 vol. % CH.sub.4; 250 cm.sup.3/min) in a horizontal quartz tube furnace for 6-12 h at 1000° C. The system was purged for 3 h with 400 cm.sup.3/min Ar prior to the initiation of the thermal cycle. Heating and cooling ramps were carried out at a rate of 400° C./h and 200° C./h respectively, with an atmosphere of 200 cm.sup.3/min Ar. Ultra-high purity gas sources were used in conjunction with an in-line purifier (Matheson Nanochem Purifilter) to further reduce 02 and H.sub.2O impurity levels (<0.1 ppb O.sub.2 and H.sub.2O). Gas mixture composition was regulated through the use of mass flow controllers (Alicat Scientific Series MC).

[0070] XRD data used for phase identification at various stages in coating development was collected with a Phillips X'Pert Pro MRD equipped with a Cu-Kα X-ray source and a programmable divergence slit. Irradiated length was held constant at 8 mm throughout the full 20 scan range.

[0071] General secondary electron SEM imaging of sample cross sections was performed on a Hitachi TM3000 tabletop microscope. Special care was taken in metallographic preparation of these sample cross sections in order to minimize distortion of the coating pore structure during sectioning and grinding/polishing. Samples were initially encapsulated/vacuum-impregnated with Buehler EpoThin low-viscosity epoxy at −20 in Hg gage pressure. Sectioning and semi-automatic grinding/polishing were then performed on a Buehler IsoMet 1000 and a Buehler EcoMet 3000/Buehler AutoMet 2000 using the Buehler Technical Notes for the metallographic preparation of thermally-sprayed ceramics as a guideline.

[0072] Secondary electron SEM cross-sectional images of free-standing sample cross sections overlaid with qualitative EDS X-ray mapping results were generated with a JEOL JSM-6700F field emission microscope equipped with a EDAX Apollo XL detector.

[0073] SEM cross-sectional images of as-deposited plasma-sprayed and cold-sprayed Cr are shown in FIGS. 14 and 15. A Cr coating of relatively uniform thickness (˜30 μm) was obtained via APS, whereas significant coating thickness variation (˜50-75 μm) was observed in cold-sprayed Cr in regions of increased porosity. Both the plasma-sprayed and the cold-sprayed Cr deposits display globular voids and interlamellar porosity, while intrallamelar microcracks oriented normal to the plane of the coating are only present in plasma-sprayed Cr. As intralamellar microcracking is characteristic of brittle materials deposited via “hot” thermal spray processes, this result suggests substantial oxidation of plasma-sprayed Cr feedstock material prior to splat quenching. Indeed, chromium oxide formation was detected in plasma-sprayed Cr via XRD, as illustrated in FIG. 16. Further confirmation of oxidation is seen in the varying degree of electron contrast in the lamallae of the plasma-sprayed Cr deposit of FIG. 14. No evidence of impurity phases resulting from the cold spray deposition process was identified in cold-sprayed Cr via SEM and XRD, as illustrated in FIG. 15 and FIG. 16.

[0074] Accounting for the presence of Cr.sub.2O.sub.3 in plasma-sprayed deposits, the conversion of sprayed Cr to Cr.sub.3C.sub.2 via carburization with methane-containing gas is likely described by the following overall reaction pathways:

3Cr(s)+2CH.sub.4(g).fwdarw.Cr.sub.3C.sub.2(s)+4H.sub.2(g)  (12)

3Cr.sub.2O.sub.3(s)+13CH.sub.4(g).fwdarw.2Cr.sub.3C.sub.2(s)+9CO(g)+26H.sub.2(g)  (13)

[0075] FIG. 17 illustrates the coating phase evolution after 6 h of carburization. With respect to FIG. 17, an XRD comparison of 6 h carburized sprayed Cr coatings is shown. Both plasma-sprayed and cold-sprayed Cr demonstrate successful conversion to Cr.sub.3C.sub.2 upon carburization, accompanied by a strong preference for growth of carbide grains oriented such that the [203] and [301] crystallographic directions are normal to the plane of the coating. An impurity phase, albeit of differing chemical composition, was also detected in each coating. Interestingly, a broad low-angle peak corresponding to amorphous carbon was identified in carburized plasma-sprayed Cr, with no such peak present in carburized cold-sprayed Cr. In the case of carburized cold-sprayed Cr, a low intensity peak corresponding to unconverted Cr was recognized. The coating phase evolution for a carburization time of 12 h is shown in FIG. 18. With respect to FIG. 18, an XRD comparison of 12 h carburized sprayed Cr coatings is shown.

[0076] No structural change in the carbide coating derived from plasma-sprayed Cr was discerned with additional carburization time. An intensification of the amorphous carbon peak is noted. In contrast, additional carburization of cold-sprayed Cr results in an XRD pattern with relative peak intensities closely matching that of randomly oriented Cr.sub.3C.sub.2, along with a reduction in the intensity of the peak corresponding to unconverted Cr.

[0077] Due to considerable microstructural differences in as-deposited plasma-sprayed and cold-sprayed Cr it was expected that carburization would lead to vastly different resultant carbide microstructures. Transformations in carbide coating microstructure throughout the carburization process are evidenced in the SEM cross-sectional images of FIGS. 19A-19D. FIGS. 19A-19D illustrate SEM cross-sectional images of carburized: FIG. 19A plasma-sprayed Cr (6 h), FIG. 19B cold-sprayed Cr (6 h), FIG. 19C plasma-sprayed Cr (12 h) and FIG. 19D cold-sprayed Cr (12 h).

[0078] As mentioned previously, as-deposited plasma-sprayed Cr contains globular voids, interlamellar porosity and intralamellar microcracks oriented normal to plane of the coating. After 6 h of carburization the resultant carbide microstructure displays significant deviation from the size, shape and distribution of porosity in the as-deposited coating. Qualitatively speaking, an increase in coating porosity volume fraction appears to accompany the conversion of plasma-sprayed Cr to carbide, which is a noteworthy observation when considering Eq. (12) is associated with an increase in specific volume of 23.57%. It is also apparent from inspection of FIG. 19A that a second impurity phase undetected by XRD is present in the carbide coating and that an effect of the carburization process on the grain boundaries of the H230 substrate (˜100 μm penetration depth) has transpired. SEM in conjunction with EDS was implemented to determine the chemical makeup of the second impurity phase, as well as to investigate the origin of the effect of the carburization process on the substrate (FIGS. 20A-20C). FIGS. 20A-20C illustrate EDS X-ray mapping of the 6 h carburized plasma-sprayed Cr coating: FIG. 20A, Reference SEM cross-sectional image, FIG. 20B EDS X-ray mapping of Cr-Kα and FIG. 20C, EDS X-ray mapping of O-Kα.

[0079] Examination of the EDS X-ray mapping results reveals that the second impurity phase is chromium deficient and oxygen rich relative to the carbide phase. Given the lack of chromium oxide peaks in the XRD pattern of 6 h carburized plasma-sprayed Cr (FIG. 17) it is presumed that the second impurity phase is comprised of an agglomerated amorphous mixture of unconverted chromium and oxygen. The abundance of oxygen in the Cr-rich grain boundaries of the H230 substrate was an unexpected, although consistent, result in the case of plasma-sprayed Cr where the as-deposited partially oxidized Cr coating presents a solid-state source of oxygen for diffusion into the grain boundaries of the substrate prior to conversion to carbide. In agreement with the XRD results presented in FIGS. 17 and 18 no significant microstructural changes in the carbide coating originating from plasma-sprayed Cr were observed with additional carburization time (FIG. 19C). It should be noted that the penetration depth of the substrate effect also remained constant with additional carburization time.

[0080] The carburization of cold-sprayed Cr is also accompanied by extensive reorganization of the pore structure in the resultant carbide. For a carburization time of 6 h the interlamellar porosity present in regions of uniform as-deposited coating thickness has been annihilated, presumably by the earlier mentioned increase in specific volume associated with Eq. (12), while pinhole porosity appears to be originating from the coating boundaries (FIG. 19B). As a result, dense mid-layer carbide is formed. In regions of nonuniform as-deposited coating thickness, and necessarily increased porosity, a coalescence of globular voids and interlamellar porosity leads to growth of the globular voids (FIG. 19B). With additional carburization time the pinhole porosity has nearly penetrated through thickness (FIG. 19D). Due to a lack of oxidation in as-deposited cold-sprayed Cr no indication of an effect of the carburization process on the substrate was observed.

[0081] The presence of globular voids and interlamellar porosity in as-deposited cold-sprayed Cr was surprising, as metallic deposits developed from this particular thermal spray technique typically display negligible micron-scale porosity. This finding may be due in part to a somewhat bimodal particle size distribution in the Cr feedstock powder (FIG. 13). The effect of feedstock powder particle size/morphology on cold spray in-flight particle velocity has been studied extensively, and it is conceivable that the spherical Cr feedstock particles approaching the limitations of a −45 μm sieve do not reach the critical particle velocity necessary for adherence to the substrate upon impingement under the present deposition conditions. Conversely, the plasma spray technique, which relies on complete or partial in-flight particle melting for adequate bonding upon substrate impingement, is more forgiving of a loosely controlled particle size distribution. The elimination of globular voids in as-deposited cold-sprayed Cr by exercising conscientious selection of feedstock powder characteristics would certainly increase the density of the carbide resulting from the carburization process. As angular/irregular feedstock powders have been shown to exhibit higher in-flight particle velocity relative to same-sized spherical feedstock powders under identical cold spray deposition conditions, future work will probe the use of angular/irregular Cr feedstock material for reduction of globular voids in as-deposited cold-sprayed Cr. The effect of a reduction in as-deposited porosity on the density of carbide coatings resulting from the carburization process will also be investigated.

[0082] An unorthodox method for the formation of binder-free Cr.sub.3C.sub.2 coatings on nickel-based alloys via the carburization of thermally-sprayed Cr has been considered. Both plasma-sprayed and cold-sprayed Cr displayed successful conversion to Cr.sub.3C.sub.2 upon carburization, with the density and the purity of the resultant carbide being strongly influenced by the degree of oxidation in the as-deposited coating. A potentially beneficial effect of the carburization process on the substrate microstructure has been observed in the case of plasma-sprayed Cr. In the interest of forming binder-free Cr.sub.3C.sub.2 coatings for molten fluoride salt corrosion resistance, the carburization of thermally-sprayed Cr appears to be an extremely promising technique. Due to the investigative nature of this work, significant room for improvement is evident. Nevertheless, the value of the carburization process for forming corrosion resistant carbide coatings will be explored via future molten fluoride salt immersion studies involving the materials developed herein.

[0083] The present invention is also directed to a method of creating refractory coatings with a controlled porosity. The refractory coatings can be formed from a refractory metal, metal oxide, or metal/metal oxide composite refractory coating precursor of the 9 refractory metals encompassed by groups 4-6 and periods 4-6 of the periodic table; non-metallic elements (e.g. Si & B) and their oxides (i.e. SiO.sub.2 & B.sub.2O.sub.3) are also pertinent. The conversion of the refractory coating precursor to refractory carbide, nitride or boride is achieved via carburization, nitridization, or boridization in the presence of carbon-containing (e.g. CH.sub.4), nitrogen containing (e.g. NH.sub.3), and boron-containing (e.g. B.sub.2H.sub.6) gaseous species respectively. Any known technique of applying the refractory coating precursor can be used, such as thermal spray. The porosity of resultant refractory coatings is controlled through compositional manipulation of composite refractory coating precursors.

[0084] The conversion of thermally-sprayed refractory oxides to binder-free carbide, nitride or boride is usually accompanied by a specific volume reduction, which results in a highly microporous, binder-free refractory carbide, nitride or boride coating. These porous refractory matrices can serve as scaffoldings for the formation of multi-functional coatings for a wide range of potential end-use applications. Owing to the relatively high level of microporosity, as well as the relatively uniform pore size and spatial distribution, the pore structure of such refractory matrices can be filled with materials possessing desirable application-oriented properties. Furthermore, refractory carbides, nitrides and borides are well known to exhibit low chemical reactivity at high temperatures. As a result a wide variety of impregnation techniques/materials can be applied to these porous refractory matrices to form multi-functional, permeation-free protective coatings. These include ambient-temperature sealing with organic sealants, filling with sol-gel processed inorganic ceramics, and liquid metal infiltration which requires significantly higher temperatures. Such processes for forming functionally-filled refractory carbide, nitride and boride matrix composites present a unique broad-based business opportunity for customer-driven coatings solutions; as opposed to existing coatings technologies which only seek to serve niche markets.

[0085] In an exemplary implementation of the present invention, that is not to be considered limiting, the reduction of atmospheric-plasma-sprayed (APS) Cr.sub.2O.sub.3 with methane-containing gas has been investigated. Plasma-sprayed Cr.sub.2O.sub.3 coatings were exposed to a flowing, methane-containing atmosphere at 1000° C. to isothermally convert the as-deposited oxide to Cr.sub.3C.sub.2. Mechanisms of reduction as well as the minimum reaction times required for complete conversion to Cr.sub.3C.sub.2 were investigated using microstructural characterization throughout the reduction process. Measurements were carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM) to determine phase morphology and porosity evolution in the coating.

[0086] Due to the temperature range of interest for Cr.sub.2O.sub.3 reduction experiments which exploit methane-containing gas as a reducing agent (800-1200° C.), substrate materials are generally limited to those capable of undergoing short-term, high-temperature exposure without incurring considerable microstructural re-arrangement. These materials include advanced ceramics (e.g. carbon fiber-reinforced carbon) and Ni-based alloys commonly used in high-temperature applications. Since carbon-based materials could act as a supplementary solid-state source of carbon for Cr.sub.2O.sub.3 reduction, a Ni-based alloy substrate was selected for this implementation of the invention—Haynes 230 (H230) (Haynes International, Windsor, Conn. 06095, USA). The nominal chemical composition of this alloy is shown in Table 1. Coupons measuring 76.2×25.4×3.175 mm were machined from a H230 sheet (per AMS 5878C) using waterjet cutting. Surface preparations for plasma spraying involved grit blasting with 36 grit Al.sub.2O.sub.3, followed by cleaning with isopropyl alcohol and compressed air in an effort to remove adherent oxide particles.

[0087] The deposition of Cr.sub.2O.sub.3 by means of APS does not require the use of a metallic binder phase (as is the case of thermally-sprayed Cr.sub.3C.sub.2), but a bond layer is typically applied between ceramic deposits and metallic substrates to reduce thermal-mismatch stresses that can occur in high-temperature applications. In this work, a NiCr—Al bond layer between the Cr.sub.2O.sub.3 top coat and the H230 substrate was used to moderate cracking and spallation of the oxide coating during heating to the reduction temperature (1000° C.). SEM micrographs of the feedstock powders used in this work are shown in FIGS. 21A and 21B and their characteristics are provided in Table 8, below (Metco, Westbury, N.Y. 11590, USA). An APS system consisting of a 9 MB plasma gun (Metco, Westbury, N.Y. 11590, USA), Model 1200 powder feeder (Bay State Surface Technologies, Auburn, Mass. 01501, USA) and HP20 robot movement (Yaskawa Motoman, Miamisburg, Ohio 45342, USA) was used for the deposition of both the NiCr—Al bond layer and the Cr.sub.2O.sub.3 top coat. APS deposition conditions are reported in Table 9, below.

TABLE-US-00008 TABLE 8 Feedstock Powder Characteristics. Chemical Particle Feedstock Composition Size Powder (wt. %) (μm) Morphology Metco 443NS Ni 18.5Cr 6Al −125 +45 Spherical Amdry 6420 99.5+ Cr.sub.2O.sub.3  −45 +22 Irregular

TABLE-US-00009 TABLE 9 Atmospheric Plasma Spray (APS) Deposition Conditions. Working Traverse Step Feedstock Voltage Current Distance Speed Size Powder (V) (A) (mm) (mm/s) (mm) Metco 443NS 75 500 127 416.56 3.175 Amdry 6420 70 500 69.85 416.56 3.175

[0088] Plasma-sprayed Cr.sub.2O.sub.3 coatings were exposed to a flowing, methane-containing atmosphere (80 vol. % Ar with 20 vol. % CH.sub.4; 250 SCCM) in a horizontal quartz tube (60×64×1300 mm) furnace for 0.1-0.3 h at 1000° C. The system was purged for 3 h with 400 SCCM Ar prior to the initiation of the thermal cycle. Heating and cooling ramps were carried out at a rate of 400° C./h and 200° C./h respectively, with an atmosphere of 200 SCCM Ar. Ultra-high purity gas sources were used in conjunction with an in-line Nanochem Purifilter (Matheson, Basking Ridge, N.J. 07920, USA) to further reduce 02 and H.sub.2O impurity levels (<0.1 ppb O.sub.2 and H.sub.2O). The gas mixture composition was regulated through the use of Series MC mass flow controllers (Alicat Scientific, Tucson, Ariz. 85743, USA).

[0089] The coating phase evolution throughout the reduction process was determined using Xray diffraction with an X'Pert Pro Materials Research Diffractometer (Philips, Andover, Mass. 01810, USA) equipped with a Cu-Kα source. A programmable divergence slit was used to the hold irradiated length constant (8 mm) throughout the scan range thereby increasing the signalto-noise ratio for high-angle diffraction peaks.

[0090] Phase morphology and porosity evolution were examined in coating cross sections for various reduction times (0.1-0.3 h) with a TM3000 tabletop SEM (Hitachi, Schaumburg, Ill. 60173, USA). Samples were initially vacuum-impregnated with EpoThin low-viscosity epoxy (Buehler, Lake Bluff, Ill. 60044, USA) at a pressure of 33.6 kPa. Sectioning, grinding and polishing were then performed using the recommended techniques for the metallographic preparation of thermally-sprayed ceramics (ASTM E1920-03). An IsoMet 1000 low-speed precision diamond saw (Buehler, Lake Bluff, Ill. 60044, USA) and an EcoMet 3000/AutoMet 2000 semi-automatic grinder/polisher (Buehler, Lake Bluff, Ill. 60044, USA) were used for sectioning and grinding/polishing respectively.

[0091] Aspects of the Cr.sub.2O.sub.3 reduction process are directly related to the characteristics of the as deposited coating microstructure. XRD measurements of the Cr.sub.2O.sub.3 feedstock powder and the resulting plasma-sprayed coating (FIG. 22) indicate no appreciable compositional changes as a result of the deposition process. FIG. 22 illustrates a graphical view of XRD patterns of the Cr.sub.2O.sub.3 feedstock powder used in this work (top scan) and the resulting plasma-sprayed coating (bottom scan), indicating no appreciable compositional changes as a result of the deposition process. Although thicker coatings were employed for reduction experiments, the SEM cross-sectional micrograph shown in FIG. 23 is representative of the plasma-sprayed Cr.sub.2O.sub.3 used in this work. FIG. 23 illustrates a Cross-sectional SEM micrograph of plasma-sprayed Cr.sub.2O.sub.3 (top coat), showing a complex pore structure containing globular voids, interlamellar porosity and intralamellar microcracks. The as-deposited coating microstructure contains globular voids and interlamellar porosity which are characteristic of a wide range of thermally sprayed materials as well as the intralamellar microcracks that occur in brittle ceramics.

[0092] Despite uncertainty regarding the mechanism of reduction of Cr.sub.2O.sub.3 powder and pressed pellets with methane-containing gas, there is agreement that the conversion to Cr.sub.3C.sub.2 largely proceeds by the following overall reaction:

3Cr.sub.2O.sub.3(s)+13CH.sub.4(g).fwdarw.2 Cr.sub.3C.sub.2(s)+9CO(g)+26H.sub.2(g)  (14)

[0093] In this work, it was found that the complete conversion of plasma-sprayed Cr.sub.2O.sub.3 to Cr.sub.3C.sub.2 occurred over timescales similar to those reported in related work which exploited vapor-phase reducing agents, but differed significantly from those reported for solid-state carbothermal reduction investigations. The XRD patterns in FIG. 24 show the coating phase evolution for reduction times of 0.1-0.3 h at 1000° C. FIG. 24 illustrates a graphical view of XRD patterns of plasma-sprayed Cr.sub.2O.sub.3 after various reduction times (0.1-0.3 h) at 1000° C., showing coating phase evolution from Cr.sub.2O.sub.3 to binder-free Cr.sub.3C.sub.2.

[0094] For a reduction time of 0.1 h, the onset of Cr.sub.3C.sub.2 formation is observable while Cr.sub.2O.sub.3 clearly remains the major phase in the coating. With increased reduction time (0.2 h), it is apparent a substantial volume fraction of the Cr.sub.2O.sub.3 coating is converted to Cr.sub.3C.sub.2. After 0.3 h, the conversion to Cr.sub.3C.sub.2 is complete (to the penetration depth of the X-ray beam), and an XRD pattern with relative intensities closely matching randomly oriented, polycrystalline Cr.sub.3C.sub.2 is achieved. These measurements demonstrate the ability to form binder-free Cr.sub.3C.sub.2 coatings via reduction of plasma-sprayed Cr.sub.2O.sub.3 with methane-containing gas—this is one of the principal results of this work. Previous investigation has shown the initial porosity in sprayed Cr coatings affects the carburization process, and it is conceivable that defects in plasma-sprayed Cr.sub.2O.sub.3 could serve a similar role in achieving through-thickness conversion. The SEM micrographs in FIGS. 25A-25C illustrate SEM image views of the coating phase morphology and porosity evolution throughout the reduction process, and illustrate characteristics of the mechanism of reduction with methane-containing gas when employing a plasma-sprayed Cr.sub.2O.sub.3 coating precursor.

[0095] FIGS. 25A-25C illustrate cross-sectional SEM micrographs of plasma-sprayed Cr.sub.2O.sub.3 after various reduction times at 1000° C., illustrating characteristics of the mechanism of reduction to binder-free Cr.sub.3C.sub.2: FIG. 25A illustrates 0.1 h, FIG. 25B illustrates 0.2 h, and FIG. 25C illustrates 0.3 h. At the earliest reduction time (0.1 h) the onset of Cr.sub.3C.sub.2 formation (visible in areas of increased image brightness relative to the surrounding unconverted Cr.sub.2O.sub.3) appears to be limited to the near-surface region of the coating and to areas containing sizeable microcracks oriented normal to the plane of the coating (FIG. 25A). Considering that the out-of-plane dimensions of these microcracks far exceed splat thicknesses and also that they were not present in the as-deposited Cr.sub.2O.sub.3 coating microstructure (FIG. 23), it is likely they result from thermal-mismatch stresses generated by heating to the reduction temperature (1000° C.). Even though the NiCr—Al bond layer did not prevent microcracking, the enhanced rate of reduction in the vicinity of microcracks indicates that defects in plasma-sprayed Cr.sub.2O.sub.3 facilitate through-thickness conversion. Inspection of FIG. 25B for a reduction time of 0.2 h further suggests that multiple pathways for reduction are acting. While reduction seems to proceed in-plane by means of thermal-stress-generated microcracks, carbide formation also occurs progressively from the surface to underlying unconverted Cr.sub.2O.sub.3 in other regions of the coating microstructure. Although the imaging results for a reduction time of 0.3 h (FIG. 25C) agree with the XRD measurements presented in FIG. 24 (the Cr.sub.3C.sub.2 surface layer thickness exceeds the X-ray beam penetration depth) an underlying portion of oxide remains unconverted in regions of increased Cr.sub.2O.sub.3 thickness. In areas of the microstructure containing thermal-stress-generated microcracks, carbide formation has progressed through the coating thickness. Closer examination of the interface between the Cr.sub.3C.sub.2 surface layer and the underlying unconverted Cr.sub.2O.sub.3 in FIG. 25C reveals a significant difference in porosity between the two phases (FIG. 26). FIG. 26 illustrates an SEM image view of void formation in the Cr.sub.3C.sub.2 phase formed from the reduction of plasma-sprayed Cr.sub.2O.sub.3. Void formation of similar character has been documented in previous work involving the conversion of Cr.sub.2O.sub.3 powder to Cr.sub.3C.sub.2 via reduction with methane-containing gas; however, in this particular instance it is probable that the newly-formed microporous Cr.sub.3C.sub.2 is a combined result of the constraint on coating dimensions imposed by the substrate and the specific volume decrease associated with Eq. 14 (38.1%). Irrespective of the coating location under consideration, a thick, continuous surface layer of adherent, binder-free Cr.sub.3C.sub.2 is formed after a reduction time of 0.3 h.

[0096] Although thermogravimetric or on-line off-gas mass spectroscopy techniques will be needed to quantify the reaction kinetics of the reduction of plasma-sprayed Cr.sub.2O.sub.3 with methane containing gas, a qualitative understanding can be gained from the XRD and SEM results presented in FIGS. 24 and 25A-25C. Because non-catalytic gas-solid reactions are inherently multi-step phenomena, both chemical and physical (diffusional) processes must be considered when identifying the rate-limiting step in the overall reaction. The lack of diffraction peaks belonging to carbon in the XRD patterns for all of the reduction times in this work (FIG. 24) suggests the newly-formed microporous Cr.sub.3C.sub.2 is a non-catalytic surface for methane decomposition. Also, this lack of carbon deposition indicates that the diffusion of reducing agents rather than their chemical reactivity is probably the rate-limiting step in the overall reaction. This notion contrasts with previous modeling involving the reaction kinetics of the reduction of pressed Cr.sub.2O.sub.3 pellets with methane-containing gas, which operated under the assumption that chemical reactivity was the rate-limiting step. However, the low-density pressed pellets were formed from an initially porous Cr.sub.2O.sub.3 powder of high surface area which likely resulted in a physical system that supported such a model. Regardless of the rate-limiting step under consideration, the progression of conversion over time (FIGS. 25A-25C) seems to indicate that the overall reaction extent is non-linear in nature when employing a plasma-sprayed Cr.sub.2O.sub.3 coating precursor. This is in agreement with previous work using Cr.sub.2O.sub.3 powder and pressed pellets which quantified the extent of overall reactions under isothermal conditions. In addition, the timescale over which complete conversion occurs (less than 0.5 h) compares favorably with these previous reports—this was unexpected considering the large differences in density between the powder, pressed pellet and plasma-sprayed Cr.sub.2O.sub.3 preforms.

[0097] Also of significance is the disparity between the timescales required for the complete conversion of plasma-sprayed Cr and Cr.sub.2O.sub.3 to Cr.sub.3C.sub.2 under identical conditions. In this work the complete conversion of plasma-sprayed Cr.sub.2O.sub.3 to Cr.sub.3C.sub.2 is realized in less than 0.5 h, whereas previous investigation involving the carburization of plasma-sprayed Cr suggested that complete conversion required at least several hours. A fundamental difference in the mechanism of conversion is probably responsible for this discrepancy. While microcracking in the Cr.sub.2O.sub.3 coating defect structure facilitates rapid through-thickness reducing agent penetration, subsequent transport is likely occurring by means of methane gas diffusion through microporosity in the newly-formed Cr.sub.3C.sub.2 in order to reach unconverted oxide. Specific models will be needed to confirm a self-assisted conversion mechanism of this nature.

[0098] The expeditious conversion of plasma-sprayed Cr.sub.2O.sub.3 to Cr.sub.3C.sub.2 may point towards directions for the rapid synthesis of other binder-free refractory carbide coatings of industrial importance, such as tungsten carbide (WC). The conversion of tungsten oxide (WO.sub.3) powder to WC via reduction with methane-containing gas has been shown to result in an increase in particle porosity; therefore, it is likely that plasma-sprayed WO.sub.3 would follow a reduction mechanism similar to what has been observed in the instance of plasma-sprayed Cr.sub.2O.sub.3. Future efforts should involve investigating the ability to convert the remaining group IV-VI refractory oxides to binder-free carbide in order to further substantiate the significance of this processing technique.

[0099] The present invention demonstrates a method for the synthesis of binder-free Cr.sub.3C.sub.2 coatings on nickel-based alloys via the reduction of plasma-sprayed Cr.sub.2O.sub.3 with methane-containing gas. A thick, continuous surface layer of adherent, binder-free Cr.sub.3C.sub.2 has been achieved, with the mechanism of reduction being strongly influenced by the coating defect structure at the reduction temperature. Despite large differences in density between oxide preforms, the conversion of plasma-sprayed Cr.sub.2O.sub.3 to Cr.sub.3C.sub.2 reported here occurs over timescales similar to those in related work which used Cr.sub.2O.sub.3 power and pressed pellets. This might be due in part to the formation of microporosity in the newly-formed Cr.sub.3C.sub.2 which appears to expedite the diffusion of methane gas to unconverted Cr.sub.2O.sub.3. Beyond playing a significant role in the mechanism of reduction, microporous Cr.sub.3C.sub.2 could serve as fracture resistant, refractory scaffolding for lubricants in high-temperature abrasive wear applications.

[0100] The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, because numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.