Q:

What is the difference between mechanism and materialism?

Are these two philosophies currently considered synonyms?

A:

In short:  
mechanism
a belief that the world is causal and explanations
of the world are reducible to physics, chemistry
biochemistry & biology.  
materialism
a belief that material substance being all there is
implies that volition and consciousness are the
result of material agency  
Note: I'd change your question to "are these two perspectives synonymous?" It is a grammatical error and misnomer to consider either "a philosophy" as these are both world-views, not respect for obtaining wisdom (read: philosophy). It is etymological fact that philosophy translates from the Greek and through the Latin to "love of wisdom". This is distinct from "love of perspective upon the world". Wisdom obtains - in fact, requires - knowledge. If perspective could obtain knowledge then the poet's claims regarding a rainbow would have epistemic and ontological equivalence to Newton's prism. The former is an example of weltanschauung (an understanding of what is to you), the latter of verstehen (an understanding of what is)
A mechanistic perspective (emphasis my own):

"We ought then to regard the present state of the universe as the
  effect of its anterior state and as the cause of the one which is to
  follow. Given for one instant an intelligence which could comprehend
  all the forces by which nature is animated and the respective
  situation of the beings who compose it - an intelligence sufficiently
  vast to submit these data to analysis - it would embrace in the same
  formula the movements of the greatest bodies of the universe and those
  of the lightest atom; for it, nothing would be uncertain and the
  future, as the past, would be present to its eyes."
  "A Philosophical Essay on Probabilities"
  Pierre Simon, Marquis de LaPlace

As for their overlap, it depends on who is using the terms and how they are using them, so, yes, they can be synonymous but are not necessarily so. For a demonstration of the ways to use the term "materialism" as it relates to consciousness, watch this brief excerpt from a discussion between John Rogers Searle and Robert Lawrence Kuhn (transcription and emphasis my own):

"Well, it depends of course as we like to say, on how you define these
  terms. And if you define materialism as the view that matter exists in
  the sense that all of reality - all of material reality - is all the
  reality there is; and it has only a third person or objective ontology
  where it's equally accessible to any competent observer;... and the
  proposition that consciousness exists is the view that consciousness
  has a first-person ontology (that it's not part of a third person
  reality), then, of course, the existence of consciousness trivially
  would refute materialism...  As always in philosophy you want to get
  behind the surface issues and see what's actually at stake here. The
  intelligent materialism is driven by the conviction that the account
  that we're getting of reality in subjects like physics and chemistry
  and molecular biology and evolutionary biology - that that ultimately
  gives us an account of how the world works and I think that's right.
  If that's materialism, then I'm a materialist, but I also think
  consciousness exists and it has an irreducible subjective ontology. It
  only exists when it's experienced by a human or an animal subject - by
  a conscious agent. But now if you put those two together then my task
  is to make the existence of consciousness in my sense - the real
  thing, not some ersatz or simulation, the real thing - consistent with
  what we know about how the world works. I claim that can be done. So
  consciousness does not refute my version of materialism, but it does
  refute certain traditional versions. What does that tell me? Let's get
  rid of this terminology of materialism and mentalism and so on and
  just describe the facts."
  J.R.S.

