The possible reason could be the oxidation of your Ni-P thin film as your doing annealing at 400 deg C/ 1h in air. You should try to do annealing your Ni-P thin films in inert argon or helium atmosphere. Then you can avoid this problem. You can also alternatively seal your thin film samples in quartz tube and also do annealing, if you don,t have access to inert atmosphere furnaces.

The possible reason could be the oxidation of your Ni-P thin film as your doing annealing at 400 deg C/ 1h in air. You should try to do annealing your Ni-P thin films in inert argon or helium atmosphere. Then you can avoid this problem. You can also alternatively seal your thin film samples in quartz tube and also do annealing, if you don,t have access to inert atmosphere furnaces.

Dear Prof. Piramanayagam: Although the deposits have shiny silvery appearance, nevertheless, the film is possibly not very thick (about 1 micron) and could have resulted in copper phosphate formation at the interface.

Dr. Prasad: We tried heat treatment in controlled environment (Nitrogen flow, or insulation by keeping in covered crucible) but the surface turned reddish brown. Most literature reports heat treatment in air atmosphere.

We'll perform few experiments using thicker coatings and/or in argon and see if there is any improvement.

Thank you very much for your invaluable comments and input!

Dear Mohammad,

I am currently working on annealing of electroless Ni-P/Ni-W-P duplex coating on mild steel substrate. What you are experiencing with that coating is partly oxidation (formation of nickel oxide) and precipitation of Nickel phosphosite (Ni3P); but if you continue heating to 500C in air environment, the color changes to brown and at around 600C the color begins to change to dull grey, when you have more of nanocrytalline Ni3P phase precipitation. However, if you heat treat in Argon, you will not observe the bluish colour but rather complete transformation from amorphous or mixed structure to purely nanocrystalline phase, but you much ensure thorough purging of your (tube) furnace {10 times volume gas pass} before putting in your samples and allow it to cool to very low temperature (say 200Cor below) before removing it from the furnace. If your sample contain addition elements like W, care should be taken to avoid rapid diffusion into the substrate, nevermind, your coating surface morphology becomes finer, porous free with better wear and corrosion resistance. Meanwhile, for optimal performance of electroless Ni-P or its ternary derivatives laser annealing is consider the best, since the substrate mechanical properties is intact. Try this, because it worked for me, ok? All the best Mahommad.

the color of the deposit is most likely due to the formation of nickel oxide

We had the same result when we realized the same treatment in an uncontrolled atmosphere. by cons, when we made these treatments under inert gas, the deposit remained gray

Dear friends,

I deeply appreciate your invaluable response to the question posed. I will do experiment in controlled environment, i.e. under argon flow. We did one run in nitrogen flow but it didn't help. Some time back, I had much thicker coatings that didn't undergo any color change (i.e. oxidation) upon heat treatment. That is why I was a bit surprised to notice a change in color. I agree with Dr. Piramanayagam that it might be oxidation at the interface yet change in surface microstructure indicates surface oxidation as well. I am very thankful to all of you for your insightful inputs.

Dear Mohammad,

You are welcome. I want to thank you for appreciating people's contribution to your area of reseaech, you are unique! Very few people do what you have just done!Anyway thats what friends are for. A very big thanks to RESEARCHGATE (RG) group for connecting all of us together from all part of the globe! Long live RG !!!!!!!!!! cheers men! Keep the flag flying !!!!!!!!!!!!