High-Density Sintered Bodies With High Mechanical Strengths

The present invention relates to a novel sintered body suitable for use as a refractory or abrasive material with its high mechanical strengths at elevated temperatures.

In the prior art, various kinds of sintered bodies are employed for manufacturing certain structural materials suitable for use for rocket housings, turbine blades, high-speed cutting tools and the like, in which high mechanical strengths, e.g. flexural strength and hardness, are essential even at extremely high temperatures. As is well known, a class of such sintered bodies is composed of titanium diboride (TiB.sub.2) as the basic component utilizing its high melting point, hardness and mechanical strengths at elevated temperatures. These TiB.sub.2 -based sintered bodies are usually prepared by sintering a binary powder mixture composed of TiB.sub.2 as the main component and a second component including a powder of a metal such as chromium, molybdenum, rhenium and the like, a metal diboride such as chromium diboride (CrB.sub.2), Zirconium diboride (ZrB.sub.2) and the like, and a nickel phosphide or a nickel-phosphorus alloy (hereinafter denoted as Ni.P).

The above described binary sintered bodies, however, have their respective drawbacks in their performance as well as in their preparation. For example, an extremely high sintering temperature of 2000.degree. C. or higher is required for the sintering of the TiB.sub.2 -metal, e.g. TiB.sub.2 -chromium, TiB.sub.2 -molybdenum and TiB.sub.2 -rhenium, binary sintered bodies giving rise to a very hard difficulty in the production of industrial scale. In addition, these TiB.sub.2 -metal binary sintered bodies suffer from their relatively low flexural strengths in the range of, for example, 40-50 kg/mm.sup.2. The TiB.sub.2 -metal diboride, e.g. TiB.sub.2 -chromium diboride and TiB.sub.2 -zirconium diboride, binary sintered bodies are also subject to the drawbacks of the high sintering temperature and the relatively low flexural strength along with the low relative density, i.e. the ratio of the apparent density to the true density of the sintered body.

The sintering temperature of the TiB.sub.2 -Ni.P binary sintered body, on the other hand, may be as low as ranging from 1000.degree. to 1600.degree. C. and a satisfactorily high flexural strength of around 100 kg/mm.sup.2 is readily obtained with these binary sintered bodies (see, for example, Japanese Patent Disclosure No. SHO 52-106306). The binary sintered bodies of this class have, however, rather poor heat resistance and cannot be used at a temperature exceeding the melting point of the Ni.P, viz. 890.degree. C.

Thus, there have hitherto been known no satisfactory refractory or abrasive material which is a high-density, high-strength and heat-resistant sintered body of TiB.sub.2 as the main component easily manufactured even with a not excessively high sintering temperature.

An object of the present invention is therefore to present a novel sintered body containing titanium diboride (TiB.sub.2) as the main component and suitable for use as a high-temperature refractory material or an abrasive material with excellent mechanical strengths at an elevated temperature but obtained with a relatively low sintering temperature.

Another object of the present invention is to present a ternary sintered body composed of TiB.sub.2, Ni.P and a third component selected from the group consisting of metals of chromium, molybdenum, niobium, tantalum, hafnium, rhenium and aluminum as well as diborides thereof and a method for producing the same.

To be more specific, the Ni.P used in the present invention is an alloy of nickel and phosphorus containing 3 to 25% by weight of phosphorus based on nickel and the amount of Ni.P to be formulated in the ternary mixture is in the range of from 0.5 to 15 parts by weight per 100 parts by weight of TiB.sub.2 and the amount of the third component is in the range of from 1 to 95 parts by weight per 100 parts by weight of TiB.sub.2.

The ternary sintered body of the invention is prepared by the techniques of hot-pressing under a pressure of 50-300 kg/cm.sup.2 at a temperature of 1500.degree.-2000.degree. C. for 10-60 minutes or by sintering a green shaped body of the powder mixture under the above sintering conditions of temperature and time.

The base component of the inventive ternary sintered body as defined above is titanium diboride expressed by the chemical formula TiB.sub.2 which is a well-known refractory material melting at 2980.degree. C. and having a specific gravity of about 4.50 and a very high hardness suitable for use as an abrasive material. There is no specific limitation on the property of this TiB.sub.2 insofar as a satisfactorily high purity is ensured. It is preferable that the TiB.sub.2 has a particle size distribution as fine as possible in order to obtain a uniform blending with the other components.

The second component in the inventive ternary sintered body is a nickel phosphide or an alloy of nickel and phosphorus containing 3 to 25% or, preferably, 5 to 15% by weight of phosphorus based on the nickel content. This component may not necessarily be a ready-prepared Ni.P but, instead, powders of nickel metal and phosphorus can also be used in combination to be blended with the other components. The amount of Ni.P in the ternary mixture is in the range from 0.5 to 15 parts by weight per 100 parts by weight of the TiB.sub.2 since smaller amounts than 0.5 parts by weight result in insufficient mechanical strengths while excessively high amounts over 15 parts by weight lead to a poorer heat resistance of the sintered body.

The third component is a powder of a certain metal exemplified by chromium, molybdenum, niobium, tantalum, hafnium, rhenium and aluminum or a diboride thereof, i.e. CrB.sub.2, MoB.sub.2, NbB.sub.2, TaB.sub.2, HfB.sub.2, ReB.sub.2 or AlB.sub.2. These metal powders or metal borides may be used either singly or as a combination of two or more. The amount of this third component is in the range from 1 to 95 parts by weight per 100 parts by weight of the TiB.sub.2. It is recommended that, when this third component is a powder of the above named metals, the amount is limited to 1 to 10 parts by weight per 100 parts by weight of the TiB.sub.2 while the metal borides are used preferably in an amount from 3 to 95 parts by weight per 100 parts by weight of the TiB.sub.2.

The ternary sintered body of the present invention is prepared by first blending the three components in fine powder forms intimately into a powder mixture with which a mold made of, for example, graphite is packed and subsequently sintering by the techniques of hot-pressing of the powder mixture is conducted in vacuum or in an atmosphere of a reducing gas such as hydrogen under a pressure of 50-300 kg/cm.sup.2 at a temperature of 1500.degree.-2000.degree. C. for 10-60 minutes. Alternatively, a green body shaped by compression molding in advance with the above powder mixture is subsequently subjected to sintering in vacuum or in an atmosphere of a reducing gas at a temperature of 1500.degree.-2000.degree. C. to give a sintered body with almost identical properties as in the hot-pressing.

The combinations of the three components including the cases where the third component per se is a mixture of two or more of the metals or metal diborides are given below as to be exemplary:

TiB.sub.2 -Ni.P-Cr; TiB.sub.2 -Ni.P-Mo; TiB.sub.2 -Ni.P-Ta; TiB.sub.2 -Ni.P-Re; TiB.sub.2 -Ni.P-Nb; TiB.sub.2 -Ni.P-Mo-Ta; TiB.sub.2 -Ni.P-Mo-Re; TiB.sub.2 -Ni.P-Mo-Nb; TiB.sub.2 -Ni.P-Ta-Re; TiB.sub.2 -Ni.P-Ta-Nb; TiB.sub.2 -Ni.P-Re-Nb; TiB.sub.2 -Ni.P-Mo-Ta-Re-Nb; TiB.sub.2 -Ni.P-CrB.sub.2 ; TiB.sub.2 -Ni.P-AlB.sub.2 ; TiB.sub.2 -Ni.P-TaB.sub.2 ; TiB.sub.2 -Ni.P-HfB.sub.2 ; TiB.sub.2 -Ni.P-CrB.sub.2 -AlB.sub.2 ; TiB.sub.2 -Ni.P-CrB.sub.2 -TaB.sub.2 ; TiB.sub.2 -Ni.P-CrB.sub.2 -HfB.sub.2 ; TiB.sub.2 -Ni.P-AlB.sub.2 -TaB.sub.2 ; TiB.sub.2 -Ni.P-AlB.sub.2 -HfB.sub.2 ; TiB.sub.2 -Ni.P-TaB.sub.2 -HfB.sub.2 ; and TiB.sub.2 -Ni.P-CrB.sub.2 -AlB.sub.2 -TaB.sub.2 -HfB.sub.2.

The sintered bodies obtained with the above combinations of the components are excellent in the relative density, mechanical strengths, hardness and heat resistance and suitable as a refractory material and anti-abrasive material as well as a material for high-speed cutting tools.

Following are examples to illustrate the present invention in further detail. In the examples, parts are all given by parts by weight.

Ternary mixtures of TiB.sub.2, Ni.P and a powder of chromium metal in proportions as indicated in Table 1 below were each subjected to sintering by hot-pressing in a graphite mold in vacuum for 15 minutes with the conditions of the sintering temperature and pressure as shown in the table. The apparent density, flexural strength and Vickers hardness of these sintered bodies are set out in the table. The results were almost identical when sintering was carried out in an atmosphere of hydrogen gas.

The same powder mixture as used in Experiments No. 1 to No. 3 in Example 1 above was shaped into a green body by compression molding in cold and the shaped body was subjected subsequently to sintering by heating in vacuum at 1800.degree. C. for 60 minutes. The thus obtained sintered body had an apparent density of 4.50 g/cm.sup.3, flexural strength of 60 kg/mm.sup.2, Vickers hardness at room temperature of 1750 kg/mm.sup.2 and Vickers hardness at 1000.degree. C. equal to about a half of the value at room temperature.

A ternary powder mixture composed of 100 parts of a TiB.sub.2 powder, 1 part of Ni.P containing 8% by weight of phosphorus and 5 parts of a chromium diboride powder intimately blended was subjected to sintering by hot-pressing in a graphite mold in an atmosphere of hydrogen gas under a pressure of 165 kg/cm.sup.2 at 1800.degree. C. for 30 minutes. The resultant sintered body had a relative density of 99.9%, flexural strength of 75 kg/mm.sup.2, Vickers hardness at room temperature of 2500 kg/mm.sup.2 and Vickers hardness at 1000.degree. C. of 2000 kg/mm.sup.2. The results were almost identical when sintering was carried out in vacuum instead of hydrogen atmosphere.

Powder mixtures each composed of 100 parts of TiB.sub.2, 1 part of Ni.P containing 8% by weight of phosphorus and one or more of metal borides selected from chromium diboride, aluminum diboride, tantalum diboride and hafnium diboride in amounts as indicated in Table 2 below were subjected to sintering by hot-pressing in the same manner as in the preceding example. Details of the preparation and the properties of the sintered bodies thus obtained are summarized in the table.

Powder mixtures each composed of 100 parts of a TiB.sub.2 powder, 1 part of the same Ni.P powder as used in Example 3 and one or more of metal powders selected from molybdenum, tantalum, niobium and rhenium in amounts as indicated in Table 3 below were subjected to sintering by hot-pressing under the conditions given in the table. The properties of the resultant sintered bodies are set out in the same table.

A powder mixture composed of 100 parts of a TiB.sub.2 powder, 1 part of the same Ni.P powder as used in Example 3, 5 parts of a powder of chromium diboride and 5 parts of a powder of molybdenum metal intimately blended was subjected to sintering by hot-pressing in a graphite mold in vacuum under a pressure of 165 kg/cm.sup.2 at 1800.degree. C. for 30 minutes. The resultant sintered body had a relative density of 99.9%, flexural strength of 85 kg/mm.sup.2, Vickers hardness at room temperature of 2400 kg/mm.sup.2 and Vickers hardness at 1000.degree. C. of 1630 kg/mm.sup.2.