I am not an expert at materials science. However, when I look at the literature, I read a process called cold spraying. This process is basically spraying a ductile powder (e.g. copper) onto a solid substrate at a high velocity to improve surface ductility of the substrate. Now, what I am wondering is if surface embrittlement is unavoidable if a brittle powder (glass, PMMA, metallic glass, ceramic) can be sprayed onto a solid substrate. I'm preparing a research proposal where any suggestion would be much valuable and appreciated.
It goes without saying that the hard and brittle materials that are applied to a soft surface make the surface hard and brittle. If you hit this brittle surface, the metallic substrate under the layer is plastically deformed and the layer crumbles. However, it does not mean that this surface has to fail if the component is subjected to other loads. If you think that, for example, when the coated rod is elastically bent, the brittle layer always breaks before the metallic base material, then it is not always the case. In this case, not only the ductility of the layer, but also its strength and its modulus of elasticity play an important role. I know of cases where the steel substrate broke earlier than the brittle layer on its surface under cyclic alternating loads (in the elastic range). The "secret" lay in the layer's low modulus of elasticity. So, in other words, the word "brittle" doesn't always mean "bad".
It is the application for which you are aiming. In one of the reciprocating pumps delivery valve, manufacturer supplied with a steel ball with ceramic coating against hastalloy body. Initially, it works nicely, but more wear rate is observed after six months. Then we switched over to steel ball with teflon lining, now it is working continuously for the last 4 years.
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