GB2065714A - Method of manufacturing silicon nitride objects - Google Patents

Abstract

A silicon nitride object is made by hot pressing a powder mixture containing Si3N4 with an SiO2 oxide coating, 4-12 wt.% Y2O3 and 0.5-2.5 wt.% of a nucleating agent such as Al2O3 to 99% theoretical density, the mixture being subjected to additional heat treatment before or after hot pressing to provide a nucleating reaction in secondary phases produced during heating. In one embodiment, the object is heat treated after hot pressing at a temperature in the range of 1,000-1,400 DEG C without mechanical pressure for at least 5 minutes but for a time sufficient to provide the nucleating reaction, the heat treatment taking place either as an interruption of the cooling cycle or as a subsequent reheat cycle after cooling to room temperature. In another embodiment, the mixture is heated to 1000-1400 DEG C prior to hot pressing under a pressure of at least 2,000 psi for a time of at least one minute but sufficient to permit a nucleating reaction to take place in secondary phases.

Description

SPECIFICATION Method of making silicon nitride objects This invention relates to methods of making silicon nitride objects.

Silicon nitride based materials, which have been hot pressed or sintered to a ceramic, have been recognised 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 recognised 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. Patent 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 is less useful to cutting tool applications. More useful phases would be silicon oxynitrides such as Y10Si,O23N4, or Y1Si102N1. These useful secondary phases, which are formed as a result of a chemical reaction between elements of the ternary system Si3N4 Y203 SiO2, 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.

According to the present invention there is provided a method of making a silicon nitride object comprising the steps of hot pressing a uniform mixture of silicon nitride powder containing SiO2 as an oxide surface coating, from 4 to 12% by weight of Y203 powder and at least 0.5% by weight of a nucleating catalyst such as WC, W, Al203, or SiO2 (preferably 0.5 to 2.5% by weight of Al203) at a pressure and for a period sufficient to produce an object having a density of at least 99% of the full theoretical density, characterised in that the mixture is subjected to additional heat treatment under conditions sufficient to produce a nucleating reaction in secondary phases formed in the mixture during heating.

The additional heat treatment, which may be carried out before or after the hot pressing, preferably involves heating the mixture to a temperature of from 1000 to 14000 C. Where the additional heat treatment is carried out before the hot pressing, it is preferably effected at a pressure of 2000 psi and for a period of at least one minute.

Where the additional heat treatment is carried out after the hot pressing, it is preferably effected in the absence of mechanical pressure for a period of at least 5 minutes.

In either case the hot pressing may be carried out at a temperature of from 1680 to 1 7500C.

Pressures of 3500-5000 psi are preferably employed, usually with heating periods of from 5 to 45 minutes, preferably 1 5 to 45 minutes.

When the additional heat treatment is carried out after the hot pressing the method of the invention is preferably carried out as follows:- (a) uniformly mixing silicon nitride powder containing SiO2 as an oxide surface coating, 410% by weight Y203 powder, and 0.50-2.5% Al203, (b) hot pressing the mixture at a temperature of 1680-1 7500C under a pressure and for a period of time sufficient to produce at least 99.0% of full theoretical density in said mixture to form a pressed object, (c) relieving said heating and pressure on said object and cooling, and (d) heat treating said object by holding at a temperature in the range of 1,000--1,4000C without mechanical pressure for at least 5 minutes but for a time sufficient to provide a nucleating reaction in secondary phases formed as a result of hot pressing. The resulting object will contain fully crystallised secondary phases of Si3N4. SiO2 - Y203. The object when shaped as a cutting tool, is particularly useful in the machine cutting of cast iron.

It is advantageous in carrying out this method that: in step (d) the heat treatment be carried out for 10-300 minutes, the silicon nitride powder contain 0.53.5% SiO2, steps (b) to (d) be carried out under a flowing nitrogen atmosphere, and the mixture be precompacted under a light pressure of about 500 psi between steps (a) and (b).

When the additional heat treatment is carried out before the hot pressing, the method of the invention is preferably carried out as follows:- (a) uniformly mixing silicon nitride powder containing SiO2 as an oxide surface coating and 412% by weight Y203 powder, and 0.502.5% Al203, (b) heating the mixture to 1,000-1 ,4000C under a pressure of at least 2,000 psi and for a time of at least one minute but sufficient to permit anucleating reaction to take place in secondary phases formed as a result of heating, and (c) continuing to heat the mixture to a temperature of 1,680-1 ,7500C 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 crystallised secondary phases of 5i3N4.SiO2.Y2O3. 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 this method that in step (b) the pressure be 2,000--5,000 psi and the time period 5-1 5 minutes, and in step (c) the pressure be 3,500-5,000 psi and the time period 5 15 minutes. It is preferred that steps (b) and (c) be carried out under a flowing N2 atmosphere and that the SiO2 be limited to 0.5-3.5% of the mixture, and that the starting powder for Si3N4 be at least 859/0 alpha phase. It is also advantageous if a precompaction step, at light pressures of about 500 psi, is used between steps (a) and (b).

The preferred methods of performing the invention will now be described in detail by way of illustration only, as applied to the manufacture of silicon nitride based cutting tools for metals.

(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 consisting of 4-12% Y203. Alumina in an amount of 0.5-2.5% (preferably 0.75-2.5%) is added by ball milling attrition. It has been found that the addition of up to 2.5% alumina catalyzes a nucleating reaction during subsequent heat treatment to provide the seeds or nuclei for crystallization of secondary phases. This is advantageously added by adapting the milling media to consist essentially of alumina, except for up to 10% SiO2.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 can build up over a predetermined period of time so that the powder mixture will uniformly contain a desired alumina content. The milling time to achieve this specific transfer of alumina, so that it does not exceed 2.5 is determined by routine experimental experience. Such experience has shown that there will be a corresponding milling ball media wear in the range of 0.50-2.5 weight percent.

The content of SiO2 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% 02. It is advantageous if the cation impurities are limited to 0.5% or less, excluding free silicon and 02. 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 yttria powder, such powder is preferably 99.9% pure, and its particle size is 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 Al203 milling media. It is typical to employ a wetting lubricant such as methanol which may be 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 0.5-2.5 (preferably 0.752.5)% of the alumina milling media.

(2) The mixture is then subjected to hot press sintering to effect agglomeration and a density of at least 99.0%, preferably 99.5% or more of full theoretical density. It is preferred that such hot press sintering be carried out by the use of hot pressing equipment comprising a graphite die assembly into which the powder mixture is inserted, the assembly being induction heated to the desired temperature. It is typical to employ a pressing force of about 4,500 psi, although a range of between 3,500 and 5,000 psi is useful when using Y203 as a pressing aid.The temperature to which such silicon nitride mixture is heated is about 1,680-1 ,7500C for a period of time which can be as short as 5 minutes but can be as long as economically justified while achieving substantially full density which is defined herein to mean 99.5% or more of theoretical density advantageously the period may be 15-45 minutes.

It is preferred, in carrying out the second step of the process, that the graphite assembly be air blown to a clean condition and coated with a boron nitride slurry to a coating thickness of about 0.002 inches prior to the insertion of the powder mixture and prior to hot pressing. 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. When the pressure dial indicator stabilizes, the pressure then is advantageously increased to 4,500 psi at approximately the rate of 1,000 psi per miriute.

When this ultimate pressure condition is reached, heat is administered by induction heating which increases the graphite die chamber to a temperature of about 1 ,7400C. Such temperature setting is held throughout the hot pressing run.

The run is continued until the ram movement for the graphite die undergoes no more than 0.002 inches travel during a 15 minute interval and essentially stops.

(3) When ram movement essentially stops, the pressure and heat are relieved and the object is preferably cooled by flowing nitrogen with the open die assembly. Any cooling rate can be utilized as long as thermal shock of the pressed body is avoided.

(4) The object is heat treated in the temperature zone of 1,000-1 ,4000C, either during cooling but before descending to 1 ,4000C or after cooling to room temperature with subsequent reheating. The object is held in said temperature range of 1,000-1 ,4000C without mechanical pressure for at least five minutes, but for a time sufficient to provide a nucleating reaction in secondary phases formed as a result of hot pressing.

(a) In the preferred mode, the heat treating step is preceded by cooling the object to room temperature preferably at a rate of 1000C/minute.

The object is then reheated in a controlled atmosphere furnace, preferably containing N2. The temperature is raised in the furnace until the object experiences a temperature in the range of 1,000-1 ,4000C preferably about 1 ,3000C.

Transient temperature movement within the range of 1,000-1 ,4000C should preferably not be greater than 10 C/minute. The object is held in said temperature range for a time period sufficient to provide a nucleating reaction in secondary phases. This will require a period of at least 5 minutes with a maximum limit imposed by economics. It is preferable to employ a period of 10-300 minutes, and optimally 1 5-45 minutes. This time at which the object is subjected to high temperatures is considerably shorter than with prior art methods wherein 6-8 hours is typical. Here, the hot press sinter time is desirably 1 5-45 minutes; the combined time herein is about 1.5 hours, considerably shorter than the prior art.

A specific example of heat treatment is to heat for 2.5 hours up to a temperature of 1 ,3000C; the object is held an additional 2 hours at 1 ,3000C, and cooled to 1 ,0000C over a period of one hour.

The total time in the zone of 1,000-1 ,4000C was 5 hours.

The hot pressing step (2) will produce a silicon nitride matrix having amorphous (glassy) secondary phases residing in the intergranular boundaries. These glassy phases are converted to crystalline phases as a result of the nucleating reaction of the subsequent heat treatment. The product resulting from the practice of the preferred mode will exhibit a secondary phase constituent which will consist of one or more of three crystallised forms in the final product. Such forms of secondary phase comprise the group consisting of 5Y203 - 4SiO2 - Si3N4; 2Y203. SiO2. Si3N4; and Y203 - SiO2. 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,OS;702N4 and Y,Si,O2N,. 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.

(b) Alternatively, the method may follow steps (1) and (2), but in step (3) the cooling may be arrested when the temperature of the object descends to the zone of 1,400-1 ,0000C. The object is then held within this zone, without pressure, for at least 5 minutes but for a period of time to provide a nucleating reaction in the secondary phases formed as a result of hot pressing. Preferably the time period may be 10-300 minutes and optimally for 1 5-45 minutes.

The secondary phases form an amorphous (glassy) phase during cooling and prior to descending through the 1,400-1 ,0000C range.

The glassy phase, when in the temperature zone is nucleated so that upon subsequent further cooling, substantially 100% crystalline secondary phases reside in the grain boundary of the resulting product.

In an alternative embodiment of the invention, the powder mixture is prepared as in paragraph (1) above. This mixture is then treated as follows:- (2A) The mixture is heated to 1,000-1 ,4000C under a pressure of at least 2,000 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 1,000-1 ,4000C, the powder mixture is under a pressure of 2,000--5,000 psi (advantageously about 4,500 psi) when Al03 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 2,000 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 1 ,000 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 1 OO"C/minute. The time period or excursion in the temperature zone of 1,000-1 ,4000C can be as little as one minute and as long as desired but advantageously is 5-1 5 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 Si3N4, SiO2 and Y203. The higher the bulk density of the powder the lower the applied pressure can be within a given range. The presence of Al203 promotes or catalyzes the formation of nucleii as part of this reaction and thus influence time. The nucleating reaction is necessary to the in situ crystallization of the secondary phases.

As indicated, it has been found that the addition of 0.752.5% Al203 catalyzes the nucleating reaction. Al203 is advantageously added by adapting the milling media to consist of alumina, except for up to 10% SiO2. 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 0.752.5% weight percent.

(3A) After the nucleating reaction has taken place, heat up of the mixture is continued to the temperature of 1,680-1 ,7500C 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 comprising 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/cm3, 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 5Y203 4Si02. Si3N4; 2Y203 5i02. Si3N4; and Y203 SiO2. 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 Y1oSi702N4 and Y1Si102Nl. 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 product resulting from step (4) or (3A) may then be 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 Y203 and 2.5% Al203 was hot pressed at 1 ,7000C with 5,000 psi load applied when the mixture reached 1 ,7000C. 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 (5,000 psi) was applied at room temperature and continuously thereafter, produced a product that showed acceptable tool life. X-ray analysis showed that presence of crystallized yittrium silicon oxynitride compounds in the latter example and showed the absence of these compounds in the former example.

Claims (18)

1. A method of making a silicon nitride object comprising the steps of hot pressing a uniform mixture of silicon nitride powder containing SiO2 as an oxide surface coating, and from 4 to 12% by weight of Y203 powder at a pressure and for a period sufficient to produce an object having a density of at least 99% of the full theoretical density, characterized in that at least 0.5% by weight of a nucleating agent is incorporated in the mixture, and in that the mixture is subjected to additional heat treatment under conditions sufficient to provide a nucleating reaction in secondary phases formed as a result of heating.

2. A method according to Claim 1 wherein the additional heat treatment is effected before the hot pressing.

3. A method according to Claim 2 wherein the additional heat treatment is effected at a temperature of from 1,000 to 1 ,4000C under a pressure of 2,000 psi and for a period of at least one minute.

4. A method according to Claim 2 or Claim 3 wherein the additional heat treatment is carried out under a pressure of from 2,000 to 5,000 psi for a period of from 5 to 15 minutes.

5. A method according to Claim 1 wherein the additional heat treatment is effected after the hot pressing.

6. A method according to Claim 5 wherein the additional heat treatment is effected at a temperature of from 1,000 to 1 ,4000C in the absence of mechanical pressure for a period of at least 5 minutes.

7. A method according to Claim 5 or Claim 6 wherein the object is cooled to room temperature prior to the additional heat treatment.

8. A method according to any one of Claims 5 to 7 wherein the additional heat treatment is effected for a period of from 10 to 300 minutes.

9. A method according to any one of Claims 1 to 8 wherein the hot pressing and the additional heat treatment are effected in a flowing stream of nitrogen.

10. A method according to any one of Claims 1 to 9 wherein the hot pressing is effected for a period of from 1 5 to 45 minutes.

11. A method according to any one of Claims 1 to 10 wherein the uniform mixture is compacted prior to any heating.

12. A method according to any one of Claims 1 to 11 wherein the mixture is hot pressed at a temperature of from 1,680 to 1 ,750OC.

13. A method according to any one of Claims 1 to 12 wherein the mixture is hot pressed at a pressure of from 3,500 to 5,000 psi for at least 5 minutes.

14. A method according to any one of Claims 1 to 1 3 wherein the silicon nitride powder contains from 0.5 to 3.5% by weight of SiO2.

15. A method according to any one of Claims 1 to 14 wherein at least 85% of the silicon nitride powder is in the alpha-phase prior to heating.

16. A method according to any one of Claims 1 to 1 5 wherein the nucleating catalyst comprises WC, W, Al2O3 or SiO2.

1 7. A method according to any one of Claims 1 to 16 wherein the nucleating catalyst is Al203 and is present in an amount from 0.5 to 2.5% by weight.

18. A method according to any one of Claims 1 to 17 further comprising the step of shaping the object to form a finished article.

1 9. A method according to any one of Claims 1 to 1 8 wherein the object and finished article is a cutting tool.