Since they are KEY to your searches, try using a Thesaurus or similar tool to get synonyms for your search terms. The more keyword combinations you try the more chances you have to eventually get the right ones that will lead you to the best papers for your topic.

Some suggestions:

• Engineering Village Thesaurus
• IEEE Thesaurus
• Google keyword tool

Even if originally designed for marketing purposes, you will be surprised to see how many keyword ideas the Google keyword tool is actually offering for research purposes!