I want to publish some research papers on oral microbiology specifically on root canal treatment, and I want to publish it in a free journal having good impact factor.
It depends on what you mean by "free journal" and "good impact factor". People normally refer to "free journals" to those that do not charge readers for accessing their articles, however the authors must pay for publishing. A more correct category for these journals is "open access" though. Considering the thematic you propose it comes to my mind: Frontiers in microbiology (impact factor around 4) and PloS One (impact factor around 3). I don't know if those are good impact factors for you but I hope that helps. Best regards.