Would it be feasible to use graphene in the gearbox of a large turbofan engine? What is the possibility or are the drawbacks of substituting graphene for metal in a gearbox of a geared turbofan the size of a Trent 900, given its strength properties(graphene), harder than a diamond, cheaper than a diamond and 10 times the strength of steel?  Previous answers here, on the reasons why large bypass turbofan engines cannot be fitted with gears for efficiency have alluded to torque vs metal gear strength with metal not being strong enough and net energy losses to heat
Well, just because it is stronger than steel does not mean that it is a useful structural material.  For example, the tensile strength of spider silk is higher than steel, but no one is building gearboxes out of spider silk.  For a graphene gearbox, the big challenge as I see it: graphene by definition is one atom thick. You aren't going to build a gearbox which is one atom thick. You can stack multiple sheets together to get something thicker, but the sheets will just slip against each other, so not useful as a structural material.  i.e. you can build a gearbox out of a big chunk of solid aluminum, but nobody tries to build a gearbox by taking 10,000 sheets of aluminum foil and laying them on top of each other.  What you would need is a graphene composite.  E.g. like we make carbon fiber composites today, which consists of carbon fibers held together by a polymer matrix.  Graphene composites are an active area of research, but I don't think they have reached the point where aerospace companies are considering them.