Final answer:
The most critical point of lubrication in a gas turbine engine is at the high-speed rotating shaft bearings, where lubricant must withstand extreme temperatures and pressures. It is akin to a substance at its critical point on a phase diagram – beyond this, a substance can't maintain its liquid form irrespective of pressure, just as a lubricant might fail under intense turbine operating conditions.
Step-by-step explanation:
The most critical point of lubrication in a gas turbine engine is where the conditions are extreme and require a lubricant that can remain effective under high temperatures and pressures. In gas turbine engines, this is typically at the bearings that support the high-speed rotating shafts, where the lubricant must maintain its viscosity and endurance against breakdown.
In the context of water’s phase diagram, the analogy would be the engine's operating conditions nearing the critical point. Beyond this critical temperature and critical pressure, a lubricant would lose its characteristics necessary to prevent metal-on-metal contact, much like how water cannot be liquefied irrespective of the pressure applied above its critical point.
It’s essential for the lubricant in the engine to not only reduce friction and wear but also to maintain its properties under these severe conditions. Manufacturers of turbine oils must therefore pay attention to the product's viscosity range and thermal stability to ensure that it remains a liquid and retains its lubricating properties even when exposed to the extreme temperatures found within a gas turbine engine.