How does reinforcement particle size affect the properties of cold sprayed composite coatings?

The question is complex, and there is no consensus on this issue, since there is no unambiguous answer. The fact is that the optimal particle size depends on many factors. I will give only a part of them:

The design of the device (length and diameter of the nozzle, power, gas flow rate, gas temperature, etc.). The more powerful the spraying device and the larger its nozzle, the larger the particles of the sprayed powder can be.

Chemical composition of the powder. The higher the tendency of the alloy to oxidize and the lower its melting point, the larger the particles should be.

Requirements for the quality of the coating (uniformity, porosity, amount of oxide phase, etc.). There are no rules at all at this point. The particle size of the powder greatly affects the quality of the coating, but in different directions. For example, a small particle size helps to improve the uniformity of the coating, but at the same time increases the content of the oxide phase in the coating.

Vadim Verlotski

Here it is necessary to distinguish between the size of inclusions of aluminum oxide in the coating and the particle size of corundum in the powder. Of course, the smaller the size of the inclusions of aluminum oxide in the coating, the better (the higher the wear resistance of the coating, the better its mechanical properties, machinability, etc.). However, in the powder, on the contrary, sufficiently large particles of the abrasive component are required, which improve the flowability of the powder mixture and contribute to better adhesion of the metal component of the coating. To overcome this contradiction (large grains of corundum in the powder, but small inclusions of corundum in the coating), it is necessary to ensure that the grains of corundum are broken into small pieces from hitting the surface of the substrate during the spraying process. In order for the abrasive grains to break upon impact, a sufficiently high speed is required, but the properties of the abrasive itself are no less important: the less strong (more brittle) the abrasive grains, the smaller will be its inclusions in the coating. In this sense, corundum is not an ideal abrasive for use in powder mixtures for spraying with low pressure gas-dynamic spraying devices (Dimet and others). It is much more profitable to use more brittle abrasive materials: glass powders, slags, cristobalite etc.

Тут надо различать размер включений оксида алюминия в покрытии и размер частиц корунда в порошке. Разумеется, что чем меньше размер включений оксида алюминия в покрытии, тем лучше (выше износостойкость покрытия, лучше его механические свойства, обрабатываемость и т.д.). Однако в порошке требуются, наоборот, достаточно крупные частицы абразивного компонента, которые улучшают сыпучесть порошковой смеси и способствуют лучшей адгезии металлического компонента покрытия. Чтобы преодолеть это противоречие (крупные зерна корунда в порошке, но мелкие включения корунда в покрытии) необходимо добиться, чтобы зерна корунда разбивались на мелкие куски от удара о поверхность субстрата в процессе напыления. Для того, чтобы зерна абразива разбивались при ударе, необходима достаточно высокая скорость, но не менее важны и свойства самого абразива: чем менее прочны (более хрупки) зерна абразива, тем мельче будут его включения в покрытии. В этом смысле корунд не является идеальным абразивом для применения в порошковых смесях для напыления устройствами газодинамического напыления низкого давления (Димет и прочие). Гораздо выгоднее использование более хрупких абразивных материалов: порошков стекол, шлаков, кристобалита и т.д.

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