Thermally sprayed coatings have attracted the attention of researchers and a lot of work has been done in this field in recent years. However, less attention has been given to the machinability of these coatings. Main recommendations suggest that grinding is the most suitable process but what about other machining processes? Do you have experience on that? Any recommendations?
Everything depends on what material and by coating its properties: hardness, strength, porosity, bond strength to the substrate and the thermal residual stresses. Put simply, the worse the coating, the more difficult it is to machining, even if it is not too hard. High porosity and low cohesion and adhesion, plus the high residual stresses can cause the coating generally can not be grind. An example of such a thermal barrier coating of YSZ. At the same time, there are quite hard coatings (400-500 HV), which can not only grind, but also sharpen, drill and mill. These coatings have sufficient cohesion and adhesion to withstand machining. The ability of the coating to withstand rough machining - a guarantee of its quality.
honing is another well established process for sprayed coatings for cylinder liners
Hola Diego, como estas?.
Lamentablemente no te puedo ayudar con esto, pero nuevamente se me ocurre que lo contactes a Norberto Lopez de Lacalle. Escribile de mi parte, por favor.
Que espesor pueden tener estos recubrimientos?. Que dureza?. Se pueden aplicar a todo tipo de superficies? (planas, cilindricas, alabeadas)
These coatings are much easier to machine than their substrates. Many processes are suitable. One of the most critical point in any machining process is to avoid bringing a too high tensile load normal to the coating surface. This could lead to coating delamination.
Thank you very much for your answers.
Firstly, let me complete a little bit my question. I'm interested in hard coatings. Let's say WC-Co, Stellite or similar. For instance, as reported by Bonny et al. [2010] the HV hardness (kg/mm2) of the WC-10Co coating is higher than 1100. Thus, the machining of these materials should be carefully done. In particular, I'm interested in milling and turning operations: machining parameters (cutting speed, depth of cut, feed rate...), cutting tools and tool geometry, etc. Have you tried to accomplish the machining of such materials?
Reference: Tribology International 43 (2010) 40–54
Coatings WC-Co-Cr, WC-Co and Cr3C2-NiCr grind only (diamond or SiC) and polishing, sharpen they are too hard (> 900 HV). Stellited and self-fluxing alloys (NiCrBSi) with a hardness of
The answer of Vadim is really good.
With ceramics inserts the cutting speed will be between 100-140m/min, and the feed around 0,1mm/v.
Because this inserts have a negative bisel, is very important the depth of cut in relation with the depth of coating, I think.
Dear Mustafa Al-Azzawi,
thanks for your comment. Can you give more details? What are the expected benefits of laser coating for machining?
Hi Carou,
i read most comments, and understand the point is "how to machining thermally spray coating" , we know that thermal spray has critical thickness with no less of 50 micron, therefore there's researches has been done of sealing process such as EPD, laser sealing process, by using similar powder that used in thermal spray process, laser micromachining that is also could be useful to remove few microns on the PS coatings, some of very special tools where coated with PS machined by Focused Ion beam FIB.
Hope this help you
Mustafa Al-Azzawi
Dear Gregoire Witz,
I'm sorry for the delay but let me return to your comment. Definitively, delamination is a critical issue in the machining of these materials. However, I have not clear what are the best strategies to cope with that. In particular, I don't know if limiting the machining parameters (feed rate, cutting speed and depth of cut) to low values would be enough.
Moreover, I don't undertand why you said that it would be easier to machine the substrate. I think that it was because of the lack of information that I provided, wasn't it?
Thank you. Best regards,
Most of the time hard wear-resistant coatings are deposited by thermal spraying. The thickness of the coating is also not very high (0.5 mm to several mm, at the most). Desirable tolerance and surface finish are quite often requited on these coated surfaces. Grinding seems to be a more suitable technique as compared to conventional machining.
You may refer to some typical papers:
Article High speed and precision grinding of plasma sprayed oxide ce...
Article Precision superabrasive grinding of plasma sprayed ceramic coatings
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