Silicon carbide fiber having boron nitride layer in fiber surface and process for the production thereof

Abstract

A silicon carbide fiber having a boron nitride layer in a fiber surface and having the following properties of a to c, a. the existent ratio of boron slopingly increases towards the surface of the fiber, b. the existent ratio of boron in the region of from the fiber surface to a depth of 500 nm is 0.5 to 1.5% by weight, c. the existent ratio of boron in a fiber central portion which is a region of a depth of at least 3 μm below the fiber surface is 0 to 0.2% by weight, and a process for the production thereof.

1 . A silicon carbide fiber having a boron nitride layer in a fiber surface and having the following properties of a to c, 
 a. the existent ratio of boron slopingly increases towards the surface of the fiber,    b. the existent ratio of boron in the region of from the fiber surface to a depth of 500 nm is 0.5 to 1.5% by weight,    c. the existent ratio of boron in a fiber central portion which is a region of a depth of at least 3 μm below the fiber surface is 0 to 0.2% by weight.    
     
     
         2 . A silicon carbide fiber according to  claim 1 , wherein the existent ratio of boron is 0.2 to 1.5% by weight based on the fiber as a whole.  
     
     
         3 . A silicon carbide fiber according to  claim 1 , which is formed of a composite phase comprising a silicon carbide phase and a boron nitride phase.  
     
     
         4 . A silicon carbide fiber according to  claim 3 , wherein the amount of the silicon carbide phase is 97% by weight or more based on the fiber as a whole.  
     
     
         5 . A silicon carbide fiber according to  claim 1 , wherein a layer in which the existent ratio of the boron slopingly increases exists in the range of from 5 to 500 nm below the surface of the fiber.  
     
     
         6 . A process for the production of a silicon carbide fiber recited in  claim 1 , which process comprises 
 melt-spinning a modified polycarbosilane obtainable by modifying a polycarbosilane having a main chain structure represented by the formula,                          (in which R is a hydrogen atom, a lower alkyl group or a phenyl group) and a number average molecular weight of 200 to 7,000, with an organic boron compound or melt-spinning a mixture of the modified polycarbosilane and an organic boron compound, to obtain a spun fiber;    infusibilizing the spun fiber; and    sintering the infusible fiber in a nitrogen-containing atmosphere.    
     
     
         7 . A process according to  claim 6 , wherein the organic boron compound is a compound having a basic structure of the formula B (OR′) n or the formula BR″m, in which R′ is an alkyl group having 1 to 20 carbon atoms or a phenyl group, R″ is acetyl acetonate, and each of m and n is an integer of more than 1.  
     
     
         8 . A process according to  claim 6 , wherein at least part of silicon atoms of the polycarbosilane bond to metal atoms selected from the group consisting of Ti, Zr, Hf, Al, V, Mg and Y directly or through oxygen atoms.  
     
     
         9 . A process according to  claim 6 , wherein the organic boron compound bonds to the polycarbosilane as a mono functional polymer.