This invention relates to cubic boron nitride coated material useful as a tool, such as a cutting tool, a wear resistant tool and the like and an electronic part such as a semiconductor substrate and the like, and the producing method thereof.
Roughly classifying, boron nitride (BN) is present in two types of forms, one being low-density boron nitride and the other high-density boron nitride. Of these, as a representative example of high-density boron nitride, there is cubic boron nitride which is synthesized under special conditions such as high-pressure and high-temperature.
Cubic boron nitride has high hardness, high heat conductivity and high electric insulating property, second to diamond, and moreover, enjoys chemical stability, oxidation resistance, heat resistance and thermal shock resistance all better than diamond. While diamond has high affinity with Iron-group metals, cubic boron nitride has low affinity with them. Therefore, cubic boron nitride is paid attention, for example, as a material for tools used for cutting or grinding Iron-group metal materials. In spite of such excellent properties as mentioned above, cubic boron nitride is brittle and has low sinterability and limitations are hence imposed on its shape and applicaiton.
Thus, it has been attempted to form cubic boron nitride as a coating layer on a surface of a substrate in order to solve the above-mentioned limitations to the shape and application.
Formation of coating layers of boron nitride can generally be carried out, roughly speaking, by chemical vapor deposition method (CVD method), physical depostion method (PVD method) and plasma-assisted CVD method (PCVD method). Of these, CVD method is effected by using, as illustrative, reactant gases comprising a boride such as a boron halogenide or diborane, hydrogen, and ammonia or hydrazine. PVD method includes, for example, the ion beam deposition method, ion injection method, sputtering method and ion plating method as well as combinations of the ion injection method with other PVD methods. In plasma-assisted CVD method, a coating layer of boron nitride is synthesized in a vapor phase in the stream of plasma. As a prior art publication disclosing tools obtained by forming coating layers actually consisting of cubic boron nitride on substrates, Japanese Provisional patent publication No. 95881/1982 has been known.
Among conventional methods for forming coating layers consisting of boron nitride, CVD method is accompanied by a problem that coating layers comprising of hexagonal boron nitride or amorphous boron nitride can only be formed because it is a mere thermal vapor-phase synthesis. On the other hand, when PVD method or plasma-assisted CVD method is relied upon, a coating layer composed of boron nitride having a relatively high hardness is formed. Cubic boron nitiride has been considered to be the component of this coating layer. Coating layers formed by PVD or plasma-assisted CVD method are, however, accompanied by such drawbacks that they contain cubic boron nitride at low contents and due to the overall low hardness of the coating layers and the weak bonding between the coating layers and their corresponding substrates, the resultant coated layers cannot be used for actual applications.
In Japanese Provisional patent publication No. 95881/1982, which discloses tools produced in accordance with these conventional methods, coating layers made of cubic boron nitride are formed directly on surfaces of sintered silicon nitride bodies as substrates. They are produced by conventional CVD, PVD or plasma-assisted CVD method or alternatively by coating the surfaces with hexagonal boron nitride and then heat-treating the coating layers of hexagonal boron nitride to form coating layers consisting of cubic boron nitride. Therefore, the above-mentioned coated tools are accompanied by such drawbacks that even if coating layers of cubic boron nitride are formed on the surfaces of the substrates, the contents of cubic boron nitride in coating layers are very low or otherwise the adhesion between substrates and their corresponding coating layers are poor.