I'm using the Johson Matthey Q-5 salt bath for platinum plating. The bath contains 5g/L Q salt, temperature ia 92 Celsius and pH adjusts to 10.5 by sodium hydroxide.
The hardness after electroplating is 470 HV, much higher than platinum itself and has a risk of cracking during high temperature, so I use a 400 C, 3 hours heat treatment to reduce it's hardness, but after the "soft "treatment, it's hardness changes to 480HV.
Can anyone tell me how to reduce its hardness?
Thank you very much for your kind answer.
HI,
I guess that the hardness is closely connected to the grain morpholgy in the coating so a suitable heat treatment for recovery and graingrowth would be the answer for our coatings. Redarding our present condition, it is well known that the plating parameters will affect the morphology so i guess it is worth trying that route as well.
Best wishes
Sten
See www.technology.matthey.com/pdf/178-184-pmr-oct07.pdf. This study relates annealing temperature to rolling strain. The lower the strain, the higher the required annealing temperature.
Hi, I am agree with Johansson. You can reduce hardness by increasing the grain size which can be done by heat treatment ( also know as annealing). Thanks.
Dear Yu Haojun,
Before I can enter this discussion I need some additional information.
1. What is the substrate of the platinum coating?
2. What is the intended thickness of the platinum coating?
3. What is the purity of the salt bath used?
The answers so far suggest that recovery/recrystallization of Platinum determines to a large extent the hardness of the Pt coating; however other factors may be relevant, e.g unforeseen precipitation of intermetallics.
Thanks, Dear P.Van Mourik
1. Substrate I use N5 single crystal superalloy which has a hardness of about 340 HV.
2. The thickness of platinum coating is about 3 to 4 μm
3. The salt we use come from Johnson Matthey, impurities content is very low. Detail I have to check with Johnson Matthey.
The impurity content can been seen in attach.
Thanks again
Dear Yu Haojun
Let us summarize the experimental facts as given so far:
1. Assuming that the quoted coating hardness values really represent a property of the coating itself, its value is larger than that of the substrate and larger than the reference value for Pt.
2. Annealing 3h at 400 oC yielded a hardness increase: from 470 till 480.
Apart form martensitic transformations, metals can increase in hardness via i. dislocation production, ii. precipitation, iii. solid solution hardening, iv. decreasing the average grain size.
For your annealing experiment possibilities i. and iv. can safely be excluded. Hence, precipitation (intermetallics) or solid solution hardening remain.
I would expect that there is a link between the coating's grain size and the current density. However, given the restrictions set by the intended coating thickness I wonder if this effect would be that important to use current density as a parameter for the resulting hardness. Besides, for your annealing experiment the current density was fixed, so possible variations thereof cannot be invoked as an explanation for the observed hardness increase.
Q: What's the relationship between current density and coating grain size?
A: Usually the grain size increases lowering the current density and the voltage.
Q: If I rise the current density to like 5ma/cm2, can I increase the grain size and consequently reduce the hardness?
A: For base plating technology increasing the c.d. you are going in the wrong direction. My suggestion as galvanic chemist are the following:
a) use lower c.d. i.e start with 3 A/square decimeter, then evaluate hardness and then if necessary reduce again
b) Hardness is high with low thickness, maybe increasing thickness if it is not a problem may help you without doing nothing on parameters
c) Change the ratio metal/acid in the sense to increase the metal concentration and lower the acid;
d) If you send me the bath composition and plating parmeters I may help you in a more effective way
my best
Thanks
Dear Luca Patauner from industry.
The reason why I want to reduce the coating hardness is because hardness is a mixed result of growing stress, impurities, defect and many other factors which increase the possibility of cracking when I put the coating into high temperature like 1100C or 1200C. So I use hardness to measure the coating's unti-cracking ability.
For all your kind suggestions.
(a) 0.2A/Square decimeter is the minimum c.d I can use. Any c.d below that I got nothing.
(b) The increasing thickness increase the possibility of local cracking, so it is limited to 5μm.
(c) no acid uses in this bath because it's a mild alkaline bath.
(d) bath contain 5g/L Pt2+(Q salt Pt(NH3) HPO4), 5g/L Na2HPO4*2H2O. NaOH adjust to pH 10.2. Temperature 92 C.
All parameters are fixed except temperature, but the rising temperature increasing the evaporation of liquid, I save it as the last option because I don't want to add water to a bath of boiling poisonous liquid all day.
Kind Regards
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