Method of Making Silicon Nitride Base Cutting Tools -II

Silicpn nitride based materials, which have been hot pressed or sintered to a ceramic, have been recognized for their heat resistant qualities useful in structural members and in some cases for use as a cutting tool. However, silicon nitride, as a single substance, is not easily sintered, even under pressure, without the addition of sintering aids. Sintering aids are substances that form secondary compounds with silicon nitride or with silicon oxide present on the surface of the silicon nitride granules, which compounds form an intergranular binder assisting in the achievement of full density and greater strength properties under ambient conditions.

With known processing techniques, it has been recognized that the substances formed by such sintering aids are harmful to high temperature use of the base ceramic since the compounds are amorphous or glassy in nature.

One attempt by the prior art (See U.S. Pat. No. 4,046,580) to eliminate the glassy phase has consisted of stripping or eliminating the silicon oxide coating on the silicon nitride granules thereby forcing any chemical reaction with the pressing aid to be with the silicon nitride. The resulting secondary phase of this attempt tends to be less glassy, but unfortunately makes the process expensive and the phase is less useful to cutting tool applications. More useful phases would be silicon oxynitrides such as Y.sub.10 Si.sub.7 O.sub.23 N.sub.4, or Y.sub.1 Si.sub.1 O.sub.2 N.sub.1. These useful secondary phases, which are formed as a result of a chemical reaction between elements of the ternary system Si.sub.3 N.sub.4.Y.sub.2 O.sub.3.SiO.sub.2, are of more significance since they make processing more economical and promote enhanced strength and thermal shock properties for a ceramic material that is to be used for cutting of cast iron.

The invention relates to a method of making a unitary silicon nitride comprising object and to a method of making a silicon nitride comprising cutting tool. The method of making the object comprising Si.sub.3 N.sub.4 includes the step of : (a) uniformly mixing silicon nitride powder containing SiO.sub.2 as an oxide surface coating and 4-12% by weight Y.sub.2 O.sub.3 powder, and 0.50-2.5% Al.sub.2 O.sub.3, (b) heating the mixture to 1000.degree.-1400.degree. C. under a pressure of at least 2000 psi and for a time of at least one minute but sufficient to permit a nucleating reaction to take place in secondary phases formed as a result of heating, and (c) continuing to heat the mixture to a temperature of 1680.degree.-1750.degree. C. under a pressure and for a period of time sufficient to produce at least 99%, preferably 99.5% or more of full theoretical density in said pressed mixture. The resulting pressed body will contain fully crystallized secondary phases of Si.sub.3 N.sub.4.SiO.sub.2.Y.sub.2 O.sub.3. The pressed body when cooled, can be shaped as cutting tool useful particularly in the machine cutting of cast iron.

It is advantageous in carrying out the method that in step (b) the pressure be 2,000-5,000 psi and the time period 5-15 minutes, and in step (c) the pressure be 3,500-5,000 psi and the time period 5-45 minutes. It is preferred that steps (b) and (c) be carried out under a flowing N.sub.2 atmosphere and that the SiO.sub.2 be limited to 0.5-3.5% of the mixture, and that the starting powder for Si.sub.3 N.sub.4 be at least 85% alpha phase. It is also advantageous if a precompaction step, at light pressures of about 500 psi is used between steps (a) and (b)

An illustrative method mode for carrying out the inventive method of making silicon nitride based cutting tools for metals is as follows:

(1) A uniform powder mixture is prepared comprising essentially alpha phase silicon nitride powder (preferably at least 85% alpha phase) carrying 0.5-1.5% silicon oxide on the surface of the silicon nitride particles and a sintering aid consistng of 4-12% Y.sub.2 O.sub.3. 0.75-2.5% alumina is added by ball milling attrition.

The content of SiO.sub.2 on the silicon nitride powder may be indirectly determined by atomic activation analysis. The major impurities in the silicon nitride powder are preferably controlled in the following manner: less than 0.5% iron, less than 0.01% calcium, less than 0.4% aluminum, and less than 2.0% O.sub.2. It is advantageous if the cation impurities are limited to 0.5% or less, excluding free silicon and O.sub.2.

The average particle size for the silicon nitride powder is preferably controlled or selected to be 2.0-2.5 microns. The particle size is preferably determined by the x-ray sedimentation method. With respect to the yttria powder, such powder is preferably controlled to have a chemistry of 99.9% pure, and its particle size is preferably controlled to about -325 mesh.

It is preferred that the mixture be milled in a ball milling device so that the final average particle size of the mixture will be in the range of 1.0-1.7 microns. To this end, the powder ingredients are placed in a milling device along with the introduction of Al.sub.2 O.sub.3 milling media. The alumina serves as a catalyzer for a nucleating reaction to be described. It is typical to employ a wetting lubricant such as methanol which may be added in the ratio of 1:1 with the silicon nitride powder. The powder mixture is sufficiently ground and milled for a predetermined time, which depends on mill speed, particle size of the starting powders, and the average particle size to be achieved. The mixture is then preferably dried and subjected to a screening operation preferably using a 100 mesh screen. The milled mixture should contain 0.50-2.5% of the alumina milling media. (2) The mixture is then heated to 1000.degree.-1400.degree. C. under a pressure of at least 2000 psi and for a period of time sufficient to permit a nucleating reaction to take place in secondary phases formed as a result of heating. During heat up of the powder mixture, mechanical pressure is applied initially at room temperature and maintained throughout heat up so that when the powder mixture passes through the temperature zone of 1000.degree.-1400.degree. C., the powder mixture is under a pressure of 2000-5000 psi (advantageously about 4,500 psi) when AlO.sub.3 is employed. Preferably, in carrying this out, the mixture is placed in a graphite die assembly for applying pressure. The assembly is air blown to a clean condition and coated with a boron nitride slurry and dried to a coating thickness of about 0.002 inches prior to the insertion of the powder mixture. It is preferred that the powder mixture after having been inserted into the coated graphite dies, be precompacted under a pressure of about 500 psi prior to the introduction of any heat and prior to elevation to pressing pressures in excess of 2000 psi. When the pressure dial indicator stabilizes at 500 psi, the mechanical loading is advantageously increased to 4,500 psi at approximately the rate of 1000 psi per minute. When this latter pressure condition is reached, heat is administered by induction heating which heats up a graphite die chamber in the assembly at a rate which is preferably no less than 100.degree. C./minute. The time period or excursion in the temperature zone of 1000.degree.-1400.degree. C. can be as little as one minute and as long as desired but advantageously is 5-15 minutes. Pressure and time are interrelated variables that together are adjusted under given conditions to achieve a nucleating reaction in the compounds or secondary phases formed between Si.sub.3 N.sub.4, SiO.sub.2 and Y.sub.2 O.sub.3. The higher the bulk density of the powder the lower the applied pressure can be within a given range. The pressure of Al.sub.2 O.sub.3 promotes or catalyzes the formation of nucleii as part of this reaction and thus influence time. The nucleating reaction is necessary to the insitu crystallization of the secondary phases.

As indicated, it has been found that the addition of 0.75-2.5% Al.sub.2 O.sub.3 catalyzes the nucleating reaction. Al.sub.2 O.sub.3 is advantageously added by adapting the milling media to consist of alumina, except for up to 10% SiO.sub.2 Thus, during the ball milling operation, there is a transfer during each particle impact with the milling media of a tiny portion of alumina. These particle transfers build up over a predetermined period of time so that the powder mixture will uniformly contain a desired alumina content. The milling media time to achieve this specific transfer of alumina, so that it does not exceed 2.5% is determined by experimental experience. Such experience has shown that there will be a corresponding milling media wear in the range of 0.75-2.5% weight percent.

(3) After the nucleating reaction has taken place, heat up of the mixture is continued to the temperature of 1680.degree.-1750.degree. C. under a pressure and for a period of time sufficient to produce 99%, preferably 99.5% or more of full theoretical density in the pressed mixture. This step achieves hot press sintering. It is preferred that the applied pressure be 3,500-5,000 psi for at least 5 minutes and for as long as economically justifiable, but advantageously 15-45 minutes.

The product resulting from the above method can be cooled at any rate, even quenched, to form a silicon nitride compirsing object which has a primary matrix phase of silicon nitride and crystallized secondary phases in the grain boundary of the matrix.

With the prescribed powder chemistry and hot pressing technique, the resulting product will possess a density in the range of 3.28-3.35 grams/cm.sup.3, a hardness on the 45-N scale in the range of 86.5-90.0. The secondary phase constituent will consist of one or more of three crystallized forms in the final product. Such forms of secondary phase comprise the group consisting of 5Y.sub.2 O.sub.3.4SiO.sub.2.Si.sub.3 N.sub.4 ; 2Y.sub.2 O.sub.3.SiO.sub.2.Si.sub.3 N.sub.4 ; and Y.sub.2 O.sub.3.SiO.sub.2. Two of these secondary phases are silicon oxynitrides and the other is a silicon oxide. The molecular formulas for each of the two oxynitrides are Y.sub.10 Si.sub.7 O.sub.2 N.sub.4 and Y.sub.1 Si.sub.1 O.sub.2 N.sub.1. These oxynitrides will, in most cases, occupy approximately 80% of the secondary phase present in the resulting product and the silicon oxide will, in most cases, occupy the remaining 20% of the product secondary phase. The silicon nitride in the final product will be of the beta type, the conversion from alpha to beta occurring typically before full density is achieved. In the case where aluminum oxide is used as an intended additive in the range of up to 2.5%, the final product will have 100% beta silicon nitride containing aluminum atoms.

The resulting product is shaped as a cutting tool preferably by diamond sawing. Tool life of this product has been found to be related to the existence of the crystalline secondary phases. For example, wet milled silicon nitride powder containing 7.22 weight percent Y.sub.2 O.sub.3 and 2.5% Al.sub.2 O.sub.3 was hot pressed at 1700.degree. C. with 5000 psi load applied when the mixture reached 1700.degree. C. The hot pressing was carried out for 2 hours. The resultant material, when shaped and used as a cutting tool in a continuous machining operation of grey cast iron showed unacceptable tool life. The same composition hot pressed at identical conditions with the exception that the load (5000 psi) was applied at room temperature and continuously thereafter, produced a product that showed acceptable tool life. X-ray analysis showed the presence of crystallized yittrium silicon oxynitride compounds in the latter example and showed the absence of these compounds in the former example.