There is no "amount of work" or "trying hard" requirement for a PhD.
Instead, typical programs require you to do a few things:
- Pass qualifying exams
- Pass a certain number of required and elective course (usually this happens in the first year or two)
- Maintain a minimum GPA (usually 3.0 or so)
- The big one - complete original research that advances your field.
Notice that none of these requirements has anything to do with the amount of effort you spend. If you are exceptionally smart or talented at taking tests, you could probably "slack" and make it through the first three requirements. In my program there were definitely people who put in more effort than others, sometimes they needed to, and sometimes they didn't. In the end though, all that matters is that you meet the requirements, not that you spent X amount of time or effort on them.
The first three requirements are easy to measure, and the bar is about the same for everyone. However, the last requirement is tricky. This is why some of the other answers are saying it's not a good idea to go in with the idea that you could slack off and get by.
The reason is that judging your success at #4 comes down to the personal opinions of your adviser and your dissertation committee.
Partly it is based on the number and quality of papers you publish, partly it depends on the perceived quality of your thesis, and partly it depends on your adviser's perception of your ability to conduct original research.
A lot of other answers are saying that if you don't excel, then you won't get good recommendations, won't publish much, and won't get a job after graduating. It is true that there are more PhD recipients than there are PhD jobs, and so you definitely need to be on the "really good" end of the spectrum if you want to do well after school.
On the other hand, even though the original research requirement is highly subjective, and can depend a lot on what your adviser's perception of your work ethic is, I think it is only fair to point out that there is no built-in requirement that you work all the time.
As an example, if you took two students, one who worked 40 hours per week and produced 4 or 5 highly respected papers in a field that brought in a lot of research funding, and one who worked 60 hours per week but only produced 1 or 2 papers that weren't considered ground-breaking, I think it would be unrealistic to think that the first student (who some might say was slacking) would have a hard time getting a job.
I think what the other answers are trying to get you to see is that as a general statement, "hard work" is correlated with success in research. Note that I said correlated - in the end, the research is what matters, and hard work doesn't guarantee anything, nor is it necessarily required. Luck and talent play big parts as well. In my experience, breakthroughs in research actually happen when you take time off to think. You need to put the work in to design the experiments, generate the data, and process it. But the real insights come from stepping back and thinking about things differently. You can't force that, and I have a hard time thinking of that part as "work."
I think what really matters is your ability to overcome challenges. Sometimes people equate that with work, in the sense that you have to keep going even when the going gets tough.
If you are considering going into a PhD program, you should accept the fact that for most people, it takes a lot of effort to succeed, and even more to excel. As a general statement, your adviser will expect you to put in as much time as you can and will not like it if you "slack off." However, in the end, all that matters is the quality of your research and the opinions of your adviser, colleagues, and committee. If you can work less and still do groundbreaking, original research, while maintaining the respect of your colleagues, then it can be done.
Don't tell the admissions committee that you want to slack off, though. They won't like that.