Is it ethical to use already existing material to complement teaching? As background, I have a MSc in Chemical Engineering, and I am currently teaching in one of the first Fachhochschule-type institutes that have been established in my country. This institute stands as the middle ground between high school and university or industry, and I have been appointed as a teacher for one of the mandatory courses.
Due to COVID-19, all lectures are being performed online, and I have prepared the course material, such as slides for lectures and practicals. The lectures I have prepared cover about 80% of all course material.
I have found that one of my former professors' research group has published in their webpage the slides of a university course which comprises the missing 20% material of my course. Moreover, their slides are really well done, and it would take me lots of time (1-2 months) to prepare the same material from scratch. I have the idea of using said material directly in my lectures, by citing very explicitly the source, since it is very well made and clear.
Is it therefore ethical to do it?
If you cite the work you avoid plagiarism, other questions aside.
But the really ethical way to do this is to ask permission first and explain how and why you will use the material. If they have published it online there will probably be no reason to refuse you.
But another issue is how it will be perceived by your own students. Other questions here have revealed that some students feel cheated when this happens. So, make sure that you add value to the slides you borrow and that the value added is clear to your students. You may want to explain the borrowing to your own students as well.
In general, students want to know that you are spending effort on them.