How do you increase Vickers on B50TF56CLA METCO 450

[gejohn]

Hello, hope all is well.

We are spraying Ni-Al (Metco 450) as a bond coat via 9MB plasma gun. The top coat is then applied, B50TF222 CLA METCO 601NS (Al Polyester).
Our standard practice manual requires a Vickers Hardness test (120 HV 0.3 minimum) on the bond coat. We have consistently been failing the metlab test with a Vickers hardness value on the bond coat of about 105-110 HV.
Is there a way to increase the hardness without interfering with the metallographic testing requirments (oxides, laminations...etc.). I am in the process of testing various coupons at various settings, but hopefully your answer will save me some time (and money). We are spraying Inconel 718. Thank you.

John

BOND COAT TOP COAT:
Gun 9MB 9MB
Nozzle GH GP
Powder Port 2 2
Powder Shaft A A
Powder Feed Rate 5 3.2-3.5
Carrier Gas Flow 13 24
Vibrator 10-15 10-15
Air Jet Pressure 30-40 70-80
Air Jet Position PARALLEL PARALLEL
Spray Distance 4 3.5-4
Spray Angle 90 90
Table RPM 100 100
Table Angle A/R A/R
Voltage (Ref.) 63-73 63-73
Amperage 500 500
Pri Gas ARGON PSI 75 75
ARGON FLOW 80 190
Secondary Gas
HYDROGEN PSI 50 50
HYDROGEN FLOW 20 5
Thickness .002 - .005? Total wall & coating thickness = 0.25?

[Gordon]

Hi gejohn

to the Surface Engineering Forum.

Not sure why hardness of a bond coat is important, assuming bond strength and metallography are good. But I suppose you can't argue against an aerospace standard/procedure.

My first concern would be with the actual hardness test. If you are doing a HV/0.3 test on a cross-section through 0.002 -0.005" (50 - 125 micrometre) thickness there could be a problem.

The distance between the centre of any indentation and any edge of the test piece or edge of any other indentation shall be not less than 2.5 times the diagonal of the indentation.

With a hardness range of say 100 - 220 HV/0.3 the indentation diagonal would be around 50 - 75 micrometres (0.002 - 0.003") which does not leave very much room within 0.002 - 0.005" thickness of bond coat, even with support from substrate and top coat. You could test to see if this is a factor by spraying a test piece under very similar conditions, but apply a much thicker bond coat ~ 0.015" (400 micrometres) or more.

If the hardness testing is not at fault, then I'm not really sure what is causing your low hardness coating. Are your spraying parameters dictated by customer standard procedure (set in stone) or can you modify or optimise them? I can only suggest you consider the following:

• Spray desired thickness of bond coat in one pass
  
• Try without any air jet cooling
  
• Extend spray distance from 4" to 6"
  
• Powder feed rate seems low compared to that typically used
  
• Hydrogen secondary plasma gas flow seems higher than typically used
  

Hope that helps.

[gejohn]

Gordon,

Thank you for the fast reply.??It is nice to hear someone who knows the issues we are facing here.??I have argued our point of why a hardness test needs to be performed on a bond coat for months now, but no luck from the French. I have been trying to explain that the minimum thickness per the standard (2.5 times diagnal) is around 0.012" to achieve a harness of 120. In other words, we worked the test backwards and came up with the thickness we would need everytime to achieve 120, with our current settings. My problem is that the actual part is only to receive 0.002"-0.005" bond coat.

We NEVER spray our coupouns with just one pass, we work in "loops" here.??What does spraying the coupon in one pass do???This is very interesting.??Gordon, thanks again..I look forward to learning from everyone.

John

[Gordon]

Hi John

I will quote from a Metco 450 technical bulletin

The best bond coat is obtained if a bond coat 0.004" to 0.006" thick is applied in one pass. Bond coats less than 0.004" thick should not be used to avoid possibility of reduced bond strength. Bond coats more than 0.006" thick do not add to bond strength

Not sure really how valid that info is I will make clear that spraying a test coupon with a much thicker 450 coating purely to enable proper hardness testing obviously should not be done in one pass nor does this apply to the 601 top coat. I think the theory is that 450 is a "synergistic powder which exhibits an exothermic reaction during spraying" this extra heat aids bonding and spraying in a single pass adds to this effect.

[bhellman]

Hi John

I will quote from a Metco 450 technical bulletin

The best bond coat is obtained if a bond coat 0.004" to 0.006" thick is applied in one pass. Bond coats less than 0.004" thick should not be used to avoid possibility of reduced bond strength. Bond coats more than 0.006" thick do not add to bond strength

Not sure really how valid that info is I will make clear that spraying a test coupon with a much thicker 450 coating purely to enable proper hardness testing obviously should not be done in one pass nor does this apply to the 601 top coat. I think the theory is that 450 is a "synergistic powder which exhibits an exothermic reaction during spraying" this extra heat aids bonding and spraying in a single pass adds to this effect.

To fulfill the hardness demand, maybe you could spray a thicker coating on the test coupons to make the hardness test reasonable.
bhe

[gejohn]

Hello Gordon,

Thanks again for great information. I am taking lots of notes. Ok, here is the majic question.

Do you have a "spreadsheet" of how coatings react to certain situations or any information regarding troubleshooting. This would be a dream come true. I'm developing my own data, but I will not be done before I retire I believe.[/font]

Thank you,

John

[Gordon]

Hi John

I'm afraid I don't have a spreadsheet or database info. I tend to just follow a logical path using my knowledge and experience of the processes and coatings to troubleshoot. I suppose I could write a troubleshooting guide to thermal spray coatings, but I could be putting myself out of business

One important piece of advice i will give, which we could take the subject of this thread as an example. Assume you are told that your coating is too soft. I know from experience that some people jump in at the deep end and start changing spraying parameters and maybe achieve an increase in hardness. But, what if the hardness information given to you was flawed, lets say in this case the hardness is in fact really OK. This approach really has only wasted time and resources and in reality only achieved a hardness value that is probably too hard. I have come across this type of scenario many times, particularly when coating metallography has been poorly done or interpreted. Very carefully evaluate your base information and validity before you do anything. Generally, following recommended spraying procedures should give a certain standard of result. If this is not the case, chances are that these procedures have not been followed correctly or equipment/material is at fault. Parameter development should be the last consideration.

[Alexangel1226]

Hi John,

We are spraying this powder meeting many engine standard, and we have no prob.

Gordon has pointed out some anomaly in your parameter, which you could try out if you have not. And I have no comment as I do not use Ar-He set up to spray this powder.

When come to testing of specimen, be it metallurgical, tensile, hardness Rockwell or Vickers, we try to give the same coating thickness as per the part condition for its representativeness, unless the requirements specify the same thickness used to perform the test, else, we will follow Standard practise manual.

For example, tensile, for single coating, we prepare the slugs and coat a thickness of 0.005" with the surface flatness of 0.003", so, if the engine manual call for a thickness of 0.010"~0.015" on the part. Would you prepare the slugs to 0.015"? When we know if we spray to this thickness, it will probably not give a favourable result compared to you spray 0.005". So, it is always better to follow what the standard practice manual specified range to play safe.

And as a standard practise, we give extra coating thickness when a hardness (Rockwell or Vickers) test is required as SPM always only state minimum thickness. The thickness of 0.015" is usually a suitable and safe range for any hardness test to be performed. The whole idea is to ensure the test sample has sufficient coating material coated for the test.

As discuss in my query before by Gordon, you may find useful

http://www.gordonengland.co.uk/sef/coati...-t-62.html

Regards,
Alexangel1226

[Donald]

Though I've never dealt with microhardness for B50TF56A, I have dealt with various other coats and usually the sure fire way to increase hardness is to back off the air pressure (if you're running 100psi, drop it 10psi or so and record the difference) or relocate the air placement (instead of sweeping across the sample, make it so it impinges 90 degrees from the sample and barely sweeps across it...)

The problem you'll run into with this is a higher oxide content (which influences an increase in microhardness, but may fail GE spec for oxides, which I'm assuming you're taking into consideration since you're quoting the GE powder spec for NiAl coating...)

I also feel your amps/volts may be a touch low for working with a ArH2 setup, but if its conforming to photostandard leave as is...

Most microhardness samples should be sprayed to atleast 0.010" for proper 'rainbow' indentation pattern. I don't think you can get a very accurate reading from 002-005" of coating, especially when you have to be x amount of indentations from the substrate and from the top of the coating you're testing.

Hope this helps

[Alexangel1226]

John,

Just to add on...if SPM says so, you always follow SPM specified range of coating thickness to produce specimen with sufficient amount of coating to perform the test. Engine manual specifies coating thickness on the part, which varies from SPM, you should follow engine manual as it is on to the part,. Yet, for specimen , you follow the SPM. This is simply because, you are not doing any destructive evaluation or performing the test (hardness, tensile or micro) onto the part. Also, often enough the engine manual cross ref to SPM when it calls for the repair using this powder type.

As a conclusion, there is no violation or controversy at all, part thickness follow manual, while specimen follow specific test requirement. You are not wrong.

Hope this clarifies your query if thickness is found to be the vital factors in failure.

Just out of suden thought of some funny illustration:
Imagine you are going for a blood test, you must be fasting for 8~10 hours before the blood sample is taken(this is the test requirement for better accurary), however, it is habitual for you to have meal every 8 hrs, and the last meal you have prior to the blood test is 1 hr before. So, for the test, I think it will be wise if you follw the instruction and not your eating habit..

Alexangel1226

[derek]

Gejohn,

The parameter that you used was different for Metco 450 technical bulletin.
Maybe you should check it again.