Hardness Query
07-25-2008, 09:41 AM,
#1
Hardness Query
Good morning to everyone who happens to be in the same time zone as me.Big Grin

My name is Sam and I have been reading this forum with interest for some time now and would like to thank Gordon and all the contributers for their excellent input.

I am a University graduate and have been involved with Thermal Spray for about 3 months now. My query pertains to the classification of the term Hardness....let me explain:

Oxides, porosity, etc. are all observable properties of a coating. They all depend on the spraying conditions, preparation conditions, etc. and can be influenced and changed by varying certain spraying parameters.

Now we come to the property of hardness. I have difficulty accepting hardness as a property of a coating in the same way that oxides and porosity are. Perhaps a better term for it would be measurable property? Hardness is a result of the coating microstructure and if we need to increase the hardness, this necessitates a change in the microstructure to some degree. Admittedly, the change in microstructure may show very limited visual difference....

I realise that observation of the microstructure alone can not predict the hardness value with sufficient accuracy, so it is an important property to measure. But I was wondering; looking at a microsection of coating only gives a view of a thin, 2 dimensional section of a coating. Could the hardness test be perceived as an appropriate method for testing a 3 dimensional section of the coating? Is this part of its function and without it, would more microsections at various points and angles throughout the coating be required?

I hope most of that was understandable, not riddled with errors and food for thought. It makes no difference to the process, but as someone who likes to know the why behind things, it seemed like something that may generate some interest on here.

Thanks for reading and apologies if I have made any mistakes on my first post!

Sam
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07-25-2008, 01:27 PM,
#2
RE: Hardness Query
Hi Sam,

What time zone would that be? We're GMT here and its sunny and warmSmile

As we were talking about earlier in the thread "tensile bond strength and met examination" the hardness of the coating can not be judged from the micro structure alone. Indeed I'm not sure that the typical levels of magnification used to assess mirco structure are appropriate to view grain size and shape? Although its been some time since I looked at hardness. I'm just happy if it serves it purpose in the bigger scheme. For me its a means to an end.

As for taking multiple sections its certainly doable but since the first section is taken randomly and the entire test piece has been exposed to the same environment I wouldn’t expect to see a deal of change.


Jim
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07-25-2008, 02:31 PM,
#3
RE: Hardness Query
Hi Jim.

I thought I should mention the time zone due to this forum being poulated by people from many areas of the globe. Personally, I am GMT too, though sunny and warm are perhaps 2 adjectives too far.Happy0193

Thanks for the reply. I realise that for pretty much every end user, the thought of why hardness testing is important don't go to this level of detail. I suppose my real point, condensed into one question is, "Why can't the hardness be predicted simply through analysis of the microstructure?".

Currently I am putting together a document which details common reasons for thermal spray coatings to fail lab analysis. Having covered Oxides, Porosity, etc. I am left with Hardness and it just felt odd in comparison. I have written the following statement, would you consider it to be accurate?

Quote:If the hardness of a coating is found to be out-with the material spec, analysis of the microstructure may reveal clues at to why the hardness is poor due to the relation between microstructure and hardness.

That isn't to say that you definitely will find a clue behind the poor hardness, just that there is a chance.

Sam
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07-25-2008, 09:13 PM,
#4
RE: Hardness Query
Hi Sam

Sign0016 to the Surface Engineering Forum.

Good afternoon from rather warm south of UK (compared to rest of July Sad ).

Interesting question. First, consider what is hardness? "Hardness is the property of a material that enables it to resist plastic deformation, usually by penetration. However, the term hardness may also refer to resistance to bending, scratching, abrasion or cutting." quoted from https://www.gordonengland.co.uk/hardness/.

Engineers like quantitative measures like with Brinell, Vickers and Rockwell. But we need to understand that hardness is not an intrinsic material property dictated by precise definitions in terms of fundamental units of mass, length and time. A hardness property value is the result of a defined measurement procedure.

Hardness from a more subjective and qualitative view point may be say, material A wears or marks less that material B in same conditions, so we say A is harder than B. Unfortunately, this is not always in complete agreement with quantitative testing, as it possible in a few cases where say the Brinell hardness of B is higher than A.

We also need to consider terms macrohardness, microhardness, nanohardness. These terms are somewhat poorly defined, but hardness testing can utilise different forces and indenter size/shape so that the resultant indentations can be in a few millimeters, micrometres or even nanometres in size. Consider a homogeneous material where we might expect hardness values to be relatively in agreement from macro to nano testing. Then compare to say an heterogeneous thermal spray coating containing porosity, relatively weakly bonded particles and a distribution of different phases with very different properties. Macohardness should give us a average hardness including all the effects from porosity, relatively weakly bonded particles and different phases. This usually is lower than we would expect from an equivalent wrought or cast material. When we start using "microhardness hardness testing" the indentation size may still cover many features of the coating and from the coatings point of view is still a macrohardness, but as indentation size gets smaller, individual features will start to effect the hardness reading. Taking many random microhardness readings will show a far greater degree scatter in results than macrohardness. At some point we may be able to actually isolate and test a specific feature, phase or particle (This is microhardness/nanohardness as far as the materials concerned). So if we consider a WC/Co coating, we have extremely hard WC particles embedded in a relatively soft Co matrix, and negative effects from porosity and also "relatively low" inter particle bonding. The macrohardness of these coating types are usually not very impressive and usually somewhat lower than that of a hardened steel. But we associate these coatings as being very much more wear resistant and subjectively and qualitatively much harder. Sorry, rambling on a bit, simply to say "hardness" is not really such a easy concept as one might first image.

Oh, what was the question? Right, the main reason for hardness testing coatings is primarily a quality check for the process as in Jims comment "I'm just happy if it serves it purpose in the bigger scheme. For me its a means to an end." As for predicting hardness from microstructure, well to some extent this is possible. I remember from doing lots of work on chromium oxide coatings, I certainly felt that I could predict the hardness results form microstructure (usual to surprising accuracy) and to some extent predict microstructure (mainly density) from the hardness, Admittedly, this is relatively simply system of ceramic phase and porosity, where density appeared proportional to hardness.
In the case of WC/Co type coatings I think microstructure is more important than hardness test results. For example over heated particles forming a coating can show similar or even elevated levels of macrohardness, while microstructure shows reduction in primary WC particles compared to correctly sprayed coating. In this case less very hard WC, but more and higher hardness matrix material (Co,W,C metastable phase instead of relatively pure Co). So I would say generically no to predicting hardness via microhardness examination, but with detailed study of some specific coating systems a correlation could be found or felt subjectively.

Quote:If the hardness of a coating is found to be out-with the material spec, analysis of the microstructure may reveal clues at to why the hardness is poor due to the relation between microstructure and hardness.
That certainly sound fair enough.

Quote:I realise that observation of the microstructure alone can not predict the hardness value with sufficient accuracy, so it is an important property to measure. But I was wondering; looking at a micro-section of coating only gives a view of a thin, 2 dimensional section of a coating. Could the hardness test be perceived as an appropriate method for testing a 3 dimensional section of the coating? Is this part of its function and without it, would more micro-sections at various points and angles throughout the coating be required?
No, I don't think this a part of the function of hardness testing. A very important point though about metallography being a 2D view. In many cases the direction of cross-sectioning makes little difference, but in the case where you might find artefacts like globular oxide clusters or porosity clusters, then the orientation of cross-section can reveal very different views. Getting an idea of the actual 3D world makes a big difference here (see http://www.gordonengland.co.uk/sef/about...t-104.html for more.
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07-28-2008, 10:03 AM, (This post was last modified: 07-28-2008, 10:14 AM by Sam.)
#5
RE: Hardness Query
Thanks for the detailed reply Gordon. I have to admit, it took a couple of reads before it all sunk in (Ashamed0002) but your post makes good sense.

I have essentially re-written your point and hopefully I have not misinterpreted anything. If there is anything below that is incorrect, I would appreciate it being pointed out.

Quote:Hardness values vary with coating microstructure but this is difficult to predict due to the coating being made up from different phases of material. An increase in hard phase can lead to an increase in hardness to a certain extent, but can also lead to a reduction in hardness as cohesion between particles may reduce. This difficulty is compounded by considering hardness at different scales of magnification from macro right down to nano scales. As the 'sample-space' reduces, we eventually find we are only measuring the hardness of one particular phase of the coating and this leads to a great deal of variation between different measuring points.

Hardness is difficult to predict because it is a property that is defined by many different variables. If all of these variables were thoroughly defined and understood, including the realtions between them, hardness prediction could be accurate. However analysis of a coating down to a nanoscale and beyond is neither practical nor is it required; the appropriate hardness test will tell us what we need to know very quickly and at very low cost.

Could I clarify when a particular hardness scale (i.e. macro, micro, nano) should be chosen? Is it based on the particular coating, desired application for coating or should it be the smallest acceptable scale?

EDIT - You also mention changes regarding relative quantities of WC and matrix material. This is something I have come across with this material, but I wasn't sure which parameters affect this. Does a change in temperature lead to a difference in DE between the different phases?
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07-28-2008, 06:48 PM,
#6
RE: Hardness Query
Sam Wrote:I have essentially re-written your point and hopefully I have not misinterpreted anything. If there is anything below that is incorrect, I would appreciate it being pointed out.

Quote:Hardness values vary with coating microstructure but this is difficult to predict due to the coating being made up from different phases of material. An increase in hard phase can lead to an increase in hardness to a certain extent, but can also lead to a reduction in hardness as cohesion between particles may reduce. This difficulty is compounded by considering hardness at different scales of magnification from macro right down to nano scales. As the 'sample-space' reduces, we eventually find we are only measuring the hardness of one particular phase of the coating and this leads to a great deal of variation between different measuring points.

Hardness is difficult to predict because it is a property that is defined by many different variables. If all of these variables were thoroughly defined and understood, including the realtions between them, hardness prediction could be accurate. However analysis of a coating down to a nanoscale and beyond is neither practical nor is it required; the appropriate hardness test will tell us what we need to know very quickly and at very low cost.

Seems ok. Your opening sentence "Hardness values vary with coating microstructure but this is difficult to predict due to the coating being made up from different phases of material." I would also include porosity (coating density) and the fact that TS coatings consist of an agglomeration of particles where the particle to particle bond is relatively weak ( mainly mechanical rather than metallurgical).

Quote:Could I clarify when a particular hardness scale (i.e. macro, micro, nano) should be chosen? Is it based on the particular coating, desired application for coating or should it be the smallest acceptable scale?
With thermal spray coatings, hardness testing is usual part of a quality system where test pieces are used to qualify the process prior to coating. Hardness testing of actual coated parts is rarely done (usually a destructive test to coatings). When specified macro-hardness is usually done using Rockwell superficial scales on coating surface and Vicikers micro-hardness (usually HV/0.3 kgf) on a coating cross-section. Type of coating material and thickness of coating will tend to dictate method, forces and indenter type. For example macro-hardness testing using Rockwell Superficial:
* abradable coatings - usually macro only using HR-15-Y on about 2 mm thick coating.
* relatively soft metal coatings - maybe HR-45-T
* Hard WC/co, ceramics HR-15-N
(All these tests require the coatings to be over a certain minimum thickness)

From a coating development or R&D perspective, micro-hardness is useful. Again choice of indenter and force will depend mainly on size/hardness of area/micro-structural object you wish to test.

Quote:EDIT - You also mention changes regarding relative quantities of WC and matrix material. This is something I have come across with this material, but I wasn't sure which parameters affect this. Does a change in temperature lead to a difference in DE between the different phases?
Yes, parameters (many) are critical for these types of coating. No, in this case it is about diffusion, oxidation and phase changes. The powder particles are all roughly the same composition. On the other hand a blended powder containing two or more different compositions could be effected in the manner you described. Also, other problems can occur if blending is poor or if powder classification occurs during spraying. I leave you to think about that one Happy0193
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07-29-2008, 12:41 PM, (This post was last modified: 07-29-2008, 12:43 PM by Sam.)
#7
RE: Hardness Query
Thanks again for the excellent reply Gordon.

I have one final question before I promise to leave you alone! I apologise for taking this away from the primary topic, but I would like to ask a question regarding WC/Co coatings.

In our company, we have found that on occasion, low hardness coincides with high levels of carbides within the microstructure. Now having read through this forum it is now my understanding that a high level of carbides is desirable. Generally a low level of carbides means that the powder temperature is high enough to let the WC dissolve into the Co matrix and this leads to a brittle coating.

So, is it desirable for some of the WC to dissolve into the Co, not enough to make it too brittle, but just enough to add some hardness and increase the distance between WC particles? I would imagine that there is very little cohesion between WC particles and the metal matrix is at least partly there to bind them together. Alternatively, could excessively high WC levels point towards a powder-composition issue?

Porosity-wise, both samples were much the same, although the sample with more carbides does appear to be far more porous at the outer surface. I shall check how the hardness values were measured and get back to you because if they were taken straight into the outer, porous section, that would be an obvious explanation....
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08-05-2008, 12:19 PM,
#8
RE: Hardness Query
Hi Sam

Quote:I have one final question before I promise to leave you alone! I apologise for taking this away from the primary topic, but I would like to ask a question regarding WC/Co coatings.

In our company, we have found that on occasion, low hardness coincides with high levels of carbides within the microstructure. Now having read through this forum it is now my understanding that a high level of carbides is desirable. Generally a low level of carbides means that the powder temperature is high enough to let the WC dissolve into the Co matrix and this leads to a brittle coating.

For a specific % carbide grade I think that is correct.

Quote:So, is it desirable for some of the WC to dissolve into the Co, not enough to make it too brittle, but just enough to add some hardness and increase the distance between WC particles? I would imagine that there is very little cohesion between WC particles and the metal matrix is at least partly there to bind them together. Alternatively, could excessively high WC levels point towards a powder-composition issue?

First consider the starting powder, like a sintered and crush product (similar to a sintered carbide cutting tool material crushed up), agglomerated and sintered or spray dried and sintered, these have particles which each consist of WC crystallites in a relatively pure Co matrix (we are not considering blended powders or cast and crush products, that is a different story). The coatings benefit (better ductility and abrasive wear resistance) if these powders are applied with minimal phase changes. Ideally, we want the right balance of heat and velocity to just soften the Co matrix enough to "splat" out to form a dense coating with minimal interaction between phases. Cohesion between WC particles and the metal matrix is very high, well much higher than bulk particle to particle bonding. WC content of the coating will only be as high as the starting powder if we are lucky Happy0193 So WC content will be reliant on the powder composition and the amount of degradation during spraying.

Quote:Porosity-wise, both samples were much the same, although the sample with more carbides does appear to be far more porous at the outer surface. I shall check how the hardness values were measured and get back to you because if they were taken straight into the outer, porous section, that would be an obvious explanation....

This aspect can be confusing. Normally the aim is to produce the densest and hardest coatings generally, but with WC/Co type coatings this does not usually give us the best coatings. OK, slightly improving density and hardness can sometimes be achieved by increasing particle temperatures, but this can be at the expense of loosing ductility and primary carbides.

Hope that makes sense Happy0193
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