Coating Microhardness Query

[Alexangel1226]

Hi Gordon,

I have some doubt pertaining understanding about microhardness, maybe you can help me:

1. What the main 3 variables that determine the microhardness?

2. In a lay man term, the higher the microhardness, does it mean the compactness or density of the coating higher?

3. How does deposition quantity of coating per pass (build up per layer) affect the microhardness of the coating? e.g: I have 1 coating build-up at 0.001" per cycle, compare to 0.002" per cycle, what will be the microhardness difference?

4. How does turn table speed and gun traverse speed could affect the microhardness of a coating?

5. How does oxide concentration affect microhardness? Is there any co-relation? e.g: Is a coating with higher oxide concentration has a higher microhardness?

6. How does a partical travel velocity affect microhardness? The faster the velocity the higher mmicrohardness we will get? Does this mean also coating using HVOF will have a higher microhardness compare to Plasma system?

7. Is Velocity of the partical in the coating stream different from one point to another? E.g: The partical V1 at stand-off 3" compare to V2 at stand-off 6", and compare with V3 at 9". at the same stream of flame.

Thanks in advance on the many question posted.

Regards,
Alex

[Gordon]

Hi Alex

Just to say I've seen your questions, but I'm too short on time now to give any thoughtful responses. I will try to come back tomorrow.

It may be wise to direct your question to all readers of the forum. Some people may have valuable input, but may be put off from responding.

[Gordon]

Hi Alex

First lets define what we mean by microhardness. The terms microhardness and macrohardness can be quite misleading. The hardness of a material is not "micro" or "macro", only the size of the test indentation. Microindentation and macroindentation hardness being possibly better terms.

Microhardness testing is generally accepted as employing indenter force of 1 kgf or less. Using such small indentation enables this test method to be selective and measure localised hardness of microstructural features. The size of test indentation and the nature of the microstructure under test (heterogeneous, multiphase, porosity etc..) will influence the degree of localised or bulk character to the hardness reading obtained. (hope that makes sense!)



Illustration of Vickers diamond indentations (not real). The two smaller diamonds illustrate localised hardness, one being on a relatively oxide free area, the other being high in oxide. The larger diamonds really just show that, as the indentation size goes up, so does the bulk or averaging effect of the hardness reading. More info.

Alex, I assume you use HV/0.3 (300g load) on coating cross-sections, which tends to be the indusry standard for TSCs. This test will be somewhere between localised and bulk hardness testing. Certainly, to some extent you are able to selectively choose the area to test, perhaps avoiding porosity, oxide agglomeration etc., but usually you would randomly do say 10 tests and calculate the mean hardness and note the range. You may have noticed that cross-sectional microhardness very often appear to be relatively higher than say the hardness achieved with Rockwell superficial (bulk hardness).

Now to your specific questions:

Quote:1. What the main 3 variables that determine the microhardness?

• Coating density
• Cohesion between particles/splats
• Metallurgy - phase transitions, oxide, stress etc.
  

Quote:2. In a lay man term, the higher the microhardness, does it mean the compactness or density of the coating higher?

Yes, generally this should apply to all coatings.
No, if you are using very localised hardness tests and testing a specific feature.

Quote:3. How does deposition quantity of coating per pass (build up per layer) affect the microhardness of the coating? e.g: I have 1 coating build-up at 0.001" per cycle, compare to 0.002" per cycle, what will be the microhardness difference?

In many cases difference should be minimal. This assumes temperature and other factors are kept constant. Spraying hotter can influence hardness with some materials.

Quote:4. How does turn table speed and gun traverse speed could affect the microhardness of a coating?

Same as 3 really.

Quote:5. How does oxide concentration affect microhardness? Is there any co-relation? e.g: Is a coating with higher oxide concentration has a higher microhardness?

Many metal coatings get harder with increasing oxide content.

Quote:6. How does a partical travel velocity affect microhardness? The faster the velocity the higher mmicrohardness we will get? Does this mean also coating using HVOF will have a higher microhardness compare to Plasma system?

Generally I would say yes. Denser coatings give higher hardness. Again a few don't follow the rules Silicon aluminium/polyester abradable coatings are harder when plasma sprayed. Reason - HVOF deposits higher proportion of polyester than plasma.

Quote:7. Is Velocity of the partical in the coating stream different from one point to another? E.g: The partical V1 at stand-off 3" compare to V2 at stand-off 6", and compare with V3 at 9". at the same stream of flame.

Yes along with temperature. Also in other plane - radial distance from axis

[Alexangel1226]

Gordon,

Thanks for the detail explaination.

Regards,
Alex