A silicon nitride sintered body according to an embodiment includes not less than 0.1 mass % and not more than 10 mass % of zirconium when converted to oxide. In XRD analysis (2) of any cross section of the silicon nitride sintered body, 0.01I.sub.35.3/I.sub.27.00.5 and 0I.sub.33.9/I.sub.27.01.0 are satisfied; I.sub.35.3 is a maximum peak intensity detected at 35.30.2 based on -silicon nitride crystal grains; I.sub.27.0 is a most intense peak detected at 27.00.2 based on -silicon nitride crystal grains; and I.sub.33.9 is a most intense peak detected at 33.90.2 based on zirconium nitride.

The present invention provides a silicon nitride wear resistant member comprising a silicon nitride sintered compact containing -Si.sub.3N.sub.4 crystal grains as a main component, 2 to 4% by mass of a rare earth element in terms of oxide, 2 to 6% by mass of Al in terms of oxide, and 0.1 to 5% by mass of Hf in terms of oxide, wherein the silicon nitride sintered compact has rare earth-HfO compound crystals; in an arbitrary section, an area ratio of the rare earth-HfO compound crystals in a grain boundary phase per unit area of 30 m30 m is 5 to 50%; and variation of the area ratios of the rare earth-HfO compound crystals between the unit areas is 10% or less. Due to above structure, there can be provided a wear resistant member comprising the silicon nitride sintered compact having an excellent wear resistance and processability.

A silicon nitride substrate including a phase encompassed of silicon nitride particles, and intergranular phase formed from a sintering aid, wherein a separation layer is formed on the surface of a molded body including silicon nitride powder, sintering aid powder, and organic binder, by using a boron nitride paste containing boron nitride powder, organic binder, and organic solvent; the separation layer and molded body are heated; the organic binder is removed from the separation layer and molded body; subsequently molded bodies stacked with a separation layer therebetween, are sintered. Boron nitride paste contains 0.01 to 0.50% by oxygen mass and 0.001 to 0.5% by carbon mass, and c/a is within range of 0.02 to 10.00, where c is oxygen content in the powder of the boron nitride paste, and a carbon content in the degreased separation layer, which includes 0.2 to 3.5 mg/cm.sup.2 of hexagonal boron nitride powder.

A silicon nitride substrate including a phase encompassed of silicon nitride particles, and intergranular phase formed from a sintering aid, wherein a separation layer is formed on the surface of a molded body including silicon nitride powder, sintering aid powder, and organic binder, by using a boron nitride paste containing boron nitride powder, organic binder, and organic solvent; the separation layer and molded body are heated; the organic binder is removed from the separation layer and molded body; subsequently molded bodies stacked with a separation layer therebetween, are sintered. Boron nitride paste contains 0.01 to 0.50% by oxygen mass and 0.001 to 0.5% by carbon mass, and c/a is within range of 0.02 to 10.00, where c is oxygen content in the powder of the boron nitride paste, and a carbon content in the degreased separation layer, which includes 0.2 to 3.5 mg/cm.sup.2 of hexagonal boron nitride powder.

A silicon nitride substrate including silicon nitride crystal grains and a grain boundary phase and having a thermal conductivity of 50 W/m.Math.K or more, wherein, in a sectional structure of the silicon nitride substrate, a ratio (T2/T1) of a total length T2 of the grain boundary phase in a thickness direction with respect to a thickness T1 of the silicon nitride substrate is 0.01 to 0.30, and a variation from a dielectric strength mean value when measured by a four-terminal method in which electrodes are brought into contact with a front and a rear surfaces of the substrate is 20% or less. The dielectric strength mean value of the silicon nitride substrate can be 15 kV/mm or more. According to above structure, there can be obtained a silicon nitride substrate and a silicon nitride circuit board using the substrate in which variation in the dielectric strength is decreased.

A silicon nitride substrate including silicon nitride crystal grains and a grain boundary phase and having a thermal conductivity of 50 W/m.Math.K or more, wherein, in a sectional structure of the silicon nitride substrate, a ratio (T2/T1) of a total length T2 of the grain boundary phase in a thickness direction with respect to a thickness T1 of the silicon nitride substrate is 0.01 to 0.30, and a variation from a dielectric strength mean value when measured by a four-terminal method in which electrodes are brought into contact with a front and a rear surfaces of the substrate is 20% or less. The dielectric strength mean value of the silicon nitride substrate can be 15 kV/mm or more. According to above structure, there can be obtained a silicon nitride substrate and a silicon nitride circuit board using the substrate in which variation in the dielectric strength is decreased.

The disclosure herein includes methods of preparing ceramic beads, useful as proppant materials, by mixing ceramic precursors, such as slag, fly ash, or aluminum dross, forming bead precursors from the mixture, and heating the bead precursors to drive a chemical reaction between the ceramic precursors to form the ceramic beads. The resultant ceramic beads may be generally spherical particles that are characterized by diameters of about 0.1 to 2 mm, a diametral strength of at least about 100 MPa, and a specific gravity of about 1.0 to 3.0. A coating process may optionally be used to increase a diametral strength of a proppant material. A sieving process may optionally be used to obtain a smaller range of sizes of proppant materials.

A ceramic substrate according to an embodiment includes a ratio A/B of an arc discharge voltage A to a dielectric breakdown voltage B of not less than 0.3 when the arc discharge voltage A (kV) is measured when an arc discharge is detected when applying an AC voltage of 50 Hz or 60 Hz between a front surface and a back surface of the ceramic substrate at a voltage increase rate of 200 V/s, and when the dielectric breakdown voltage B (kV) between the front surface and the back surface is measured according to IEC 672-2.

A cutting tool including a substrate composed of a silicon nitride-based sintered body and a coating layer. The coating layer includes first, second, third and fourth layers. The first layer is located on the surface of the substrate and is composed of TiN having an average crystalline width of 0.1 to 0.4 m. The second layer is located on the first layer and composed of Al.sub.2O.sub.3 having an average crystalline width of 0.01 to 1.5 m. The third layer is located on the second layer and is composed of TiN having an average crystalline width of 0.01 to 0.1 m which is smaller than that of the first layer. The fourth layer is located on the third layer and is composed of Al.sub.2O.sub.3 having an average crystalline width of 0.01 to 1.5 m.

A cutting tool including a substrate composed of a silicon nitride-based sintered body and a coating layer. The coating layer includes first, second, third and fourth layers. The first layer is located on the surface of the substrate and is composed of TiN having an average crystalline width of 0.1 to 0.4 m. The second layer is located on the first layer and composed of Al.sub.2O.sub.3 having an average crystalline width of 0.01 to 1.5 m. The third layer is located on the second layer and is composed of TiN having an average crystalline width of 0.01 to 0.1 m which is smaller than that of the first layer. The fourth layer is located on the third layer and is composed of Al.sub.2O.sub.3 having an average crystalline width of 0.01 to 1.5 m.