hi all, i have several questions that need to consult experts here,

1. Thermal spray coatings are sprayed layer by layer. If one small area of coating (e.g. TBC) cracks or no bonding due to poor surface preparation, can we re-spray that area instead of rework the whole work-piece? Can we say that the re-spray coating quality is low because of separation of splat (so splat cohesion low) & many porosities?

2. What's the difference between 3MB & 9MB gun?

3. What's the difference between diamond jet gun & HVOF? Can both share the powders?

4. What's difference between Aluminum oxide (e.g. Metco 105NS) and Zirconium oxide (e.g. Metco 204NS-G) in TBC? How to choose between them? If I not wrong, sevice temperature for alumina is 1650C while zirconia is 1010C. Alumina is dielectric.

5. What's difference between Metco 202NS (zirconia 80, yttria 20) & 204NS (zirconia 92, yttria 8)?

6. There are many bond coat, e.g. Amdry 962, Metco 443, Metco 404. How to choose among them?

7. How to choose sand grit (aluminum oxide) size? Depends on process (e.g. HVOF, wire, plasma) or on coating ?

Hi Powerman

to the Surface Engineering forum.

Quote:1. Thermal spray coatings are sprayed layer by layer. If one small area of coating (e.g. TBC) cracks or no bonding due to poor surface preparation, can we re-spray that area instead of rework the whole work-piece? Can we say that the re-spray coating quality is low because of separation of splat (so splat cohesion low) & many porosities?

In the majority of cases I would NOT recommend patch repairs.

Quote:2. What's the difference between 3MB & 9MB gun?

Age mainly. 3MB rated to 40KW, 7/9MB rated to 80KW, very closely related and use very similar parameter settings unlike comparing to say an F1 or F4 gun.

Quote:3. What's the difference between diamond jet gun & HVOF? Can both share the powders?

Diamond Jet is an HVOF process, do you mean comparison like between DJ and JP5000? They very different in design and operation, but similar in that they produce an HVOF jet and high quality coatings. They can share the same powders, but one powder may be more suited to one than the other.

Quote:4. What's difference between Aluminum oxide (e.g. Metco 105NS) and Zirconium oxide (e.g. Metco 204NS-G) in TBC? How to choose between them? If I not wrong, sevice temperature for alumina is 1650C while zirconia is 1010C. Alumina is dielectric.

Pure alumina and zirconia both have poor properties with regard to high temperature thermal cycling or thermal shock. This is due to certain solid state phase changes during heating and cooling which stress the coating severely. A Metco 204 type zirconia coating has stabilising elements added like yttria which modify these phase changes and makes them thermally shock resistant. Alumina is good at very high temperatures, but usually its a one off as in rocket nose cones, it does not like thermal cycling.

Quote:5. What's difference between Metco 202NS (zirconia 80, yttria 20) & 204NS (zirconia 92, yttria 8)?

202NS is whats called a fully stabilised zircomia FSZ and 204NS is a partially stabilised zirconia PSZ. The PSZ tend to show the best thermal spall and shock resisting properties. To explain why is not easy here. You will need to study the zirconia-yttria phase diagram to understand all the different phases and their transformations during heating and cooling.

Quote:6. There are many bond coat, e.g. Amdry 962, Metco 443, Metco 404. How to choose among them?

Many years ago molybdenum was the prime bond coat material until the arrival of the nickel aluminium composites and the so called "one step coatings" or "self bonding coatings" like 442 and 447 etc. Nickel aluminium alloy (not composite) is now very popular. These coatings primarily produce very high bond strengths to the substrate and also provide the right type of surface texture to maximise bonding of the top coat. Another group of so called bond coats do not necessarily improve initial tensile bond strength, but primarily are used as anti-corrosion or thermal oxidation buffer layers. Coatings like NiCr, Hastelloy C, inconel 625, NiCrAl, MCrAlY etc. Generally your NiAl and one steps are good for general purpose high bond strength systems, but tend not to be so good in corrosive or very high temperature oxidation. Your choice depends very much on the environment, substrate and top coat.

Quote:7. How to choose sand grit (aluminum oxide) size? Depends on process (e.g. HVOF, wire, plasma) or on coating ?

Primary requirement is a very sharp etch/texture as opposed to a shot peen like surface. This is probably more important than the actual roughness Ra value obtained. Generally,
the thinner and finer the coating, the finer the grit. I have not found that the commomly held belief that the rougher the blast profile the higher the bond strength is necessarily true.

Hope that helps

hi Gordon, thanks for your reply.

How to explain "not recommend for patch repair" ? Can we say it is because of porosity, impurity inside make the patch coating quality low?

Hi Powerman

Even with good patch repairs, metallography usually reveals a fault line between old and new coating. Patch repair requires considerable skill in removing faulty area of coating, preparing surface to accept new coating, spraying localised area with right amount of overlap and blending new coating with old. Other problems can occur like if a bond coat is required, then the bond coat interface will be exposed on coating surface at the edges of repair when the coating is finished machined/ground. High risk of contamination of old coating during preparation stages before application of repair coating. If you have coating with obvious localised problem, how can you be sure that this is not just the "tip of the iceberg" with more extensive problems going unseen.

Once I watched some guys trying to patch a coating with a tiny crack fault (overly thick area of coating) on a very large component (worth trying I suppose in this situation). They started grinding and chipping out the small faulty area only to find new cracks propagating into adjacent coating areas. After removing nearly a square meter of coating, they seemed happy and started to grit blast the area only to find the coating edges were again cracking and lifting. At this point they realised they were chasing their tails and decided to remove all of the coating an start again.

hi Gordon, thanks for your detailed explanation.

Thanks You.

Hi,
I just wanted to add few points,
1.ZrO2 in pure form transforms from a high temp. cubic form to a room temp. monoclinic structure,which is accompanied by a large volume change and the formation of cracks in the deposit.The high temp. cubic form can be 'stabilized' by adding appropriate quantities of Cao, MgO,Y2O3,or CeO2, and are termed as X-stablisied ZrO2(whereX=stabilising agent).As Gordon has pointed out, further improvement in thermal shock resistance can be achieved by using PSZ( please refer to my book on Surface Wear-Analysis , Treatment, and Prevention,ASM-International p253-254 for detailed explanation)
2.I am not conversant with METCO gun types.However early version of HVOF used to have lower flame velocity and thus suffered from severe limititations with respect to coating thickness particularly for hard carbides and ceramics.Development of higher gas jet velocity(1.5 times than earlier system) HVOF has made it possible to overcome these limitations.
(for further details refer to my second book, p207-212)
ram chattopadhyay

hi ram,

Thanks for your reply.