Process for producing body centered cubic (b2) nickel aluminide (nial) coating of controlled thickness on nickle-based alloy surfaces

Abstract

Aluminizing of nickel-base alloys and a process for producing body centered cubic (B2) nickel aluminide (NiAl) coating of controlled thickness on nickel-base alloys surfaces. Importantly, the process of producing the nickel aluminide (B2 NiAl phase) coatings on nickel-base alloys is directed to achieve high hardness of 800-1100 VHN and dimensional tolerance of 80±30 microns coating thickness. The process achieves desired nickel aluminide coatings with minimum environmental emission of toxic chemicals fumes and is also environment friendly. Obtaining the nickel aluminide coating is specifically possible without the need for masking of the unwanted regions of the components and more importantly can be attended both on flat and curved surfaces thereby favouring wide scale utilities of such aluminizing process, facilitate fabrication industries required to carry out surface modifications and engineering (protective coating) on nickel-base alloys desired for high service temperatures such as nuclear industries, aircraft and gas turbine industries.

1 - 10 . (canceled) 
     
     
         11 . A process for producing body centered cubic (B2) nickel aluminide (NiAl) coating of controlled thickness of 50 to 110 microns with high hardness in the range of 800-1100 VHN on nickel-base alloy surfaces with minimum 40 weight % nickel content comprising:
 activating the surfaces to be coated by grit blasting preferably following dimensional check and degreasing;   thermal spraying of the required surfaces with commercial pure aluminium to desired specific thickness in the range of 100 to 200 microns preferably 100-120 microns; and   subjecting the aluminium thus sprayed onto the nickel-base alloy substrate to diffusion heat treatment in vacuum atmosphere at a temperature in the range of 950° C.-1100° C. for a duration of 1.0 to 1.5 hours such that the aluminium sprayed reacts with the nickel-base alloy substrate to form the desired nickel aluminide coating of said controlled thickness and hardness on said nickel-base alloy substrate and subjecting the coated surface to surface cleaning.   
     
     
         12 . A process for producing body centered cubic (B2) nickel aluminide (NiAl) coating according to  claim 11  wherein the said surface activation by grit blasting is carried out using alumina grit of specific size and distribution preferably in the range of 30-120 mesh size at an air pressure of 5.5+/−0.5 kg/cm 2  and more preferably with 50-80 mesh size at an air pressure of 5.0-5.1 kg/cm 2 . 
     
     
         13 . A process for producing body centered cubic (B2) nickel aluminide (NiAl) coating according to  claim 11  wherein said thermal spray parameters comprise (i) aluminium wire diameter in the range of 3.0 to 3.2 mm at a Wire Feed Rate in the range of 2.0-2.5 mm/sec; (ii) Pneumatic pressure in the range of 5.5+/−0.5 kg/cm 2 ; (iii) Oxygen Pressure in the range of 2.5+/−0.3 kg/cm 2 ; and Acetylene pressure in the range of 1.0+/−0.2 kg/cm 2 . 
     
     
         14 . A process for producing body centered cubic (B2) nickel aluminide (NiAl) coating according to  claim 11  wherein said diffusion heat treatment in said vacuum atmosphere is carried out at (10 −5 ) millibar pressure preferably at temperature in the range of 1010±10° C. with an aluminium spray thickness of 100-120 microns with the resultant coating thickness and hardness being 80-90 microns and 980-1030 VHN respectively. 
     
     
         15 . A process for producing body centered cubic (B2) nickel aluminide (NiAl) coating according to  claim 11  wherein the said surface cleaning of the coated surface comprise removing adherent oxide scale using pickling solution following the steps of:
 (i) providing the pickling solution preferably comprising (a) concentrated nitric acid (specific gravity: 1.41 g/cm 3 ) in amounts of 16% (by volume); (b) Hydrofluoric acid (specific gravity 1.61 g/cm 3 ) in amounts of 3.5% (by volume); and (c) Water in amounts of 80.5% (by volume);   (ii) soaking the diffusion treated strips in the solution for a period of about 30 minutes; followed by   (iii) further cleaning the strips involving emery paper.   
     
     
         16 . A process for producing body centered cubic (B2) nickel aluminide (NiAl) coating according to  claim 11  wherein the NiAl aluminide coating is carried out on nickel-base alloys involving selectively flat and/or corrugated surfaces. 
     
     
         17 . A process for producing body centered cubic (B2) nickel aluminide (NiAl) coating according to  claim 11  adapted to produce surface protective coatings such as to improve wear resistance on nickel-base alloys with minimum 40 weight % nickel content for high temperature applications. 
     
     
         18 . A process for producing body centered cubic (B2) nickel aluminide (NiAl) coating of controlled thickness on nickel-base alloy with minimum 40 weight % nickel content surfaces substantially as herein described and illustrated with reference to the accompanying FIGURES.