Functionally graded alloy coating and method for preparation

Abstract

A method, coated substrate, and coating wherein a metallic substrate is subject to an electrolytic deposition process including an electrolyte with an iron group element and a refractory group element. One or more electrolytic deposition waveform parameters are varied to deposit on the substrate a functionally graded coating with more of the iron group element and less of the refractory group element at an interface between the coating and the metallic substrate to better match the coefficient of thermal expansion of the coating and the metallic substrate and more of the refractory group element and less of the iron group element as the thickness of the coating increases for improving corrosion resistance to salts. The coating may be diffusion bonded to the substrate.

What is claimed is: 
     
         1 . A method of coating a metallic substrate, the method comprising:
 subjecting the metallic substrate to an electrolytic deposition process including an electrolyte with an iron group element and a refractory group element;   varying one or more electrolytic deposition waveform parameters to deposit on the substrate a functionally graded coating with more of the iron group element and less of the refractory group element at an interface between the coating and the metallic substrate to better match the coefficient of thermal expansion of the coating and the metallic substrate and more of the refractory group element and less of the iron group element as the thickness of the coating increases for improving corrosion resistance to salts; and   diffusion bonding the coating to the substrate.   
     
     
         2 . The method of  claim 1  in which the waveform parameters vary from a waveform that preferentially influences the iron group element to transport to the substrate and to deposit on the substrate to a waveform that preferentially influences the refractory group element to transport to the substrate and to deposit on the substrate. 
     
     
         3 . The method of  claim 2  in which varying the electrolytic waveform parameters includes varying the current density of the waveform, the length of time the waveform is applied, and the time between successive waveforms. 
     
     
         4 . The method of  claim 3  in which varying the electrolytic waveform parameters further includes switching between cathodic and anodic waveforms. 
     
     
         5 . The method of  claim 1  further including increasing the roughness of the substrate before deposition. 
     
     
         6 . The method of  claim 1  further including activating the substrate. 
     
     
         7 . The method of  claim 6  wherein the activation is accomplished by nickel strike, acid etch, or plasma treatment. 
     
     
         8 . The method of  claim 1  wherein the substrate is a stainless steel material or tungsten. 
     
     
         9 . The method of  claim 1  wherein the CTE of the functionally graded coating is lower than that of the substrate. 
     
     
         10 . The method of  claim 1  wherein the iron group includes Fe, Co and/or Ni and the refractory group includes Mo, W, and/or Re. 
     
     
         11 . The method of  claim 1  wherein the functionally graded coating adjacent the substrate interface comprises 90+% of the iron group and the functionally graded coating at its surface comprises between 25 and 60% of the refractory group. 
     
     
         12 . The method of  claim 1  wherein the diffusion bonding is accomplished by hot isostatic pressing. 
     
     
         13 . The method of  claim 12  wherein the hot isostatic pressing is conducted for about 3½ hours at about 1250 C and at about 22,000 psi pressure. 
     
     
         14 . A metallic substrate coated with a functionally graded diffusion bonded coating with more of an iron group element and less of a refractory group element at an interface between the metallic substrate and the coating to better match the coefficient of thermal expansion of the coating and the metallic substrate and more of the refractory group element and less of the iron group element as the thickness of the coating increases for improving corrosion resistance to salts. 
     
     
         15 . A coating for a metallic substrate comprising:
 more of an iron group element and less of a refractory group element at a coating interface to better match the coefficient of thermal expansion of the coating and a metallic substrate and more of the refractory group element and less of the iron group element as the thickness of the coating increases for improving corrosion resistance to salts.   
     
     
         16 . A method of coating a metallic substrate, the method comprising:
 subjecting the metallic substrate to an electrolytic deposition process including an electrolyte solution with an iron group element and a refractory group element; and   instead of changing the electrolyte solution, varying one or more electrolytic deposition waveform parameters to deposit on the substrate a functionally graded coating with more of the iron group element and less of the refractory group element at a first portion of the coating more of the refractory group element and less of the iron group element at a second portion of the coating.   
     
     
         17 . The method of  claim 16  in which the first portion of the coating is at an interface between the coating and the substrate and the second portion of the coating is at the surface of the coating. 
     
     
         18 . The method of  claim 17  in which the waveform parameters vary from a waveform that preferentially influences the iron group element to transport to the substrate and to deposit on the substrate to a waveform that preferentially influences the refractory group element to transport to the substrate and to deposit on the substrate. 
     
     
         19 . The method of  claim 18  in which varying the electrolytic waveform parameters includes varying the current density of the waveform, the length of time the waveform is applied, and the time between successive waveforms. 
     
     
         20 . The method of  claim 19  in which varying the electrolytic waveform parameters further includes switching between cathodic and anodic waveforms. 
     
     
         21 . The method of  claim 16  further including increasing the roughness of the substrate before deposition. 
     
     
         22 . The method of  claim 16  further including activating the substrate. 
     
     
         23 . The method of  claim 22  wherein the activation is accomplished by nickel strike, acid etch, or plasma treatment. 
     
     
         24 . The method of  claim 16  wherein the substrate is a stainless steel material or tungsten. 
     
     
         25 . The method of  claim 16  wherein the CTE of the functionally graded coating is lower than that of the substrate. 
     
     
         26 . The method of  claim 16  wherein the iron group includes Fe, Co and/or Ni and the refractory group includes Mo, W, and/or Re. 
     
     
         27 . The method of  claim 17  wherein the functionally graded coating adjacent the substrate interface comprises 90+% of the iron group and the functionally graded coating at its surface comprises between 25 and 60% of the refractory group. 
     
     
         28 . The method of  claim 16  further including diffusion bonding the coating to the substrate. 
     
     
         29 . The method of  claim 28  wherein diffusion bonding includes hot isostatic pressing for about 3½ hours at about 1250 C and at about 22,000 psi pressure. 
     
     
         30 . A method of coating a metallic substrate, the method comprising:
 subjecting the metallic substrate to an electrolytic deposition process including an electrolyte solution with an iron group element and a refractory group element; and   instead of changing the electrolyte solution, varying one or more electrolytic deposition waveform parameters to deposit on the substrate a functionally graded coating with more of the iron group element and less of the refractory group element at a first portion of the coating more of the refractory group element and less of the iron group element at a second portion of the coating in which the waveform parameters vary from a waveform that preferentially influences the iron group element to transport to the substrate and to deposit on the substrate to a waveform that preferentially influences the refractory group element to transport to the substrate and to deposit on the substrate.