Mechanical seal

1. A mechanical seal, comprising:

a case-side sealing ring fixed to a seal case, the case-side sealing ring including a case-side sealing ring seal end surface; and

a shaft-side sealing ring held on a rotary shaft via an annular packing made of an elastic material, the shaft-side sealing ring being movable in an axial direction of the rotary shaft, and the shaft-side sealing ring including a shaft-side sealing ring seal end surface;

wherein:

the case-side sealing ring and the shaft-side sealing ring are configured to be rotated relative to each other while the case-side sealing ring seal end surface and the shaft-side sealing ring seal end surface are in contact, thereby forming a shield seal between a liquid area and a gas area, wherein the liquid area is one of an inner peripheral area and an outer peripheral area of the case-side sealing ring seal end surface and the shaft-side sealing ring seal end surface, and the gas area is a different one of the inner peripheral area and the outer peripheral area of the case-side sealing ring seal end surface and the shaft-side sealing ring seal end surface, the gas area different than the liquid area;

the shaft-side sealing ring is formed from a ceramic or cemented carbide;

the shaft-side sealing ring seal end surface is formed at a distal end part thereof, and an inner peripheral part of the shaft-side sealing ring has a sealing ring-side seal surface which is concentric and parallel to a shaft-side seal surface which is an outer peripheral surface of the rotary shaft;

the annular packing is provided between the sealing ring-side seal surface and the shaft-side seal surface so as to be relatively movable in the axial direction in such state that a space between the sealing ring-side seal surface and the shaft-side seal surface is sealed; and

a diamond film is formed on the sealing ring-side seal surface.

2. The mechanical seal according to claim 1 , wherein a diamond film contiguous with the diamond film formed on the sealing ring-side seal surface is formed on a surface portion of the shaft-side sealing ring excluding the shaft-side sealing ring seal end surface, the surface portion being a portion that is in contact with a liquid of the liquid area.

3. The mechanical seal according to claim 1 , wherein:

the shaft-side sealing ring is formed on the inner peripheral part thereof with a latching surface which is an annular surface perpendicular to the sealing ring-side seal surface of the shaft-side sealing ring and is not in contact with the shaft-side seal surface, the latching surface configured to prevent protrusion of the annular packing toward the inner peripheral area from between the sealing ring-side seal surface and the shaft-side seal surface due to a pressure of the outer peripheral area when the outer peripheral area of the shaft-side sealing ring seal end surface and the case-side sealing ring seal end surface is higher in pressure than the inner peripheral area thereof; and

a diamond film contiguous with the diamond film formed on the sealing ring-side seal surface is formed on the latching surface.

4. The mechanical seal according to claim 3 , wherein between the latching surface and the annular packing, a plastic backup ring is fitted to the shaft-side seal surface in such a state that a relative movement thereof is possible in the axial direction.

5. The mechanical seal according to claim 1 , wherein the case-side sealing ring is comprised of a stationary ring, which is fixed to the seal case, and a floating ring, which is linked to the stationary ring in such a state that a relative rotation with the stationary ring is prevented, and which is provided between the stationary ring and the shaft-side sealing ring.

6. The mechanical seal according to claim 1 , wherein the annular packing is a rubber O-ring.

7. The mechanical seal according to claim 1 , wherein the diamond film has a thickness of at least 1 μm.