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The coating of abradable material presents cavities opening out into the surface of the coating, the cavities extending over a fraction of the thickness of the coating and being defined by walls which form a plurality of continuous paths at the inside surface of the annular portion between the axial

The coating of abradable material presents cavities opening out into the surface of the coating, the cavities extending over a fraction of the thickness of the coating and being defined by walls which form a plurality of continuous paths at the inside surface of the annular portion between the axial end faces thereof, and which form an angle of not less than 5° relative to the general direction of end portions of blades that might come into contact with the abradable material.

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1. An annular portion mounted facing a set of rotary gas turbine blades, said annular portion being provided with a coating of abradable material, said coating presenting cavities that open to the surface of the coating, extend over a fraction of the thickness of the coating, and are defined by wall

1. An annular portion mounted facing a set of rotary gas turbine blades, said annular portion being provided with a coating of abradable material, said coating presenting cavities that open to the surface of the coating, extend over a fraction of the thickness of the coating, and are defined by walls which form a plurality of continuous paths on the inside surface of the annular portion between its axial end faces, wherein said cavity walls form an angle in a range between 0° and 20° relative to the general direction of the end portions of the blades liable to come into contact with said abradable material, and the cavities are constituted by polygonal cells quadrangular in section. 2. The annular portion according to claim 1, wherein the cavities are of a depth lying in the range 0.3 mm to 1 mm. 3. The annular portion according to claim 1, wherein the external portion of the coating in which the cavities are formed present a void content lying in the range 40% to 60%. 4. The annular portion according to claim 1, wherein the cavities are constituted by continuous circumferential grooves separated by walls forming continuous circumferential ribs. 5. The annular portion according to claim 4, wherein the ribs follow zigzag lines. 6. A turbine ring for a gas turbine, the turbine ring being in accordance with claim 1. 7. A compressor casing for a gas turbine, the compressor casing including an annular portion in accordance with claim 1. 8. A turbine having an annular portion facing a set of rotary blades, wherein the annular portion is in accordance with claim 1. 9. The annular portion according to claim 1, wherein the cavities are square in section. 10. The annular portion according to claim 1, wherein the angle is in the range between 5° and 15°. 11. An annular portion mounted facing a set of rotary gas turbine blades, said annular portion being provided with a coating of abradable material, said coating presenting cavities that open to the surface of the coating, extend over a fraction of the thickness of the coating, and are defined by walls which form a plurality of continuous paths on the inside surface of the annular portion between its axial end faces, wherein said cavity walls form an angle in a range between 0° and 20° relative to the general direction of the end portions of the blades liable to come into contact with said abradable material, the cavities are constituted by continuous circumferential grooves separated by walls forming continuous circumferential ribs, and the ribs follow undulating lines. 12. A turbine having an annular portion facing a set of rotary blades, the annular portion being provided with a coating of abradable material, said coating presenting cavities that open to the surface of the coating, extend over a fraction of the thickness of the coating, and are defined by walls which form a plurality of continuous paths on the inside surface of the annular portion between its axial end faces, wherein said cavity walls form an angle in a range between 0° and 20° relative to the general direction of the end portions of the blades liable to come into contact with said abradable material, wherein the blades have tips presenting rims that are provided with chamfers. 13. An annular structure disposed opposite to rotary gas turbine blades, said annular structure comprising:a structural member; anda protection coating disposed on a surface of said structural member, said coating comprising cavities extending over a fraction of a thickness of said coating and walls forming a plurality of continuous paths on an inside surface of the annular structure facing said rotary gas turbine blades, wherein the continuous paths form an angle of at least 5° relative to a plane perpendicular to an axis of rotation of the gas turbine and the cavities comprise polygonal cells quadrangular in section. 14. An annular portion mounted facing a set of rotary gas turbine blades, said ann ular portion being provided with a coating of abradable material, said coating presenting cavities that open to the surface of the coating, extend over a fraction of the thickness of the coating, and are defined by walls which form a plurality of continuous paths on the inside surface of the annular portion between its axial end faces, wherein said cavity walls form an angle of at least 5° relative to the general direction of the end portions of the blades configured to come into contact with said abradable material, the cavities are constituted by continuous circumferential grooves separated by walls forming continuous circumferential ribs, and the ribs follow undulating lines. 15. An annular portion mounted facing a set of rotary gas turbine blades, said annular portion being provided with a coating of abradable material, said coating presenting cavities that open to the surface of the coating, extend over a fraction of the thickness of the coating, and are defined by walls which form a plurality of continuous paths on the inside surface of the annular portion between its axial end faces, wherein said cavity walls form an angle of at least 5° relative to the general direction of the end portions of the blades configured to come into contact with said abradable material, the cavities are constituted by continuous circumferential grooves separated by walls forming continuous circumferential ribs, and the ribs follow zigzag lines. 16. A turbine having an annular portion mounted facing a set of rotary gas turbine blades, said annular portion being provided with a coating of abradable material, said coating presenting cavities that open to the surface of the coating, extend over a fraction of the thickness of the coating, and are defined by walls which form a plurality of continuous paths on the inside surface of the annular portion between its axial end faces, wherein said cavity walls form an angle of at least 5° relative to the general direction of the end portions of the blades configured to come into contact with said abradable material and the turbine blades have tips presenting rims that are provided with chamfers.