Unless by artificial you mean plastic I wouldn't really recommend it, to be honest even if it was plastic I still wouldn't recommend it!! You would create a very hot, humid environment underneath the grass which eventually would cause the decking to rot, causing it to collapse (we have decking out our back which was untreated and is now totally rotten underneath). Might be a bit of extra work but I'd recommend taking the decking up and putting down topsoil and laying the grass on that. I'm now planning on ripping out all our decking and putting in paving instead.