IL51108A - Method of testing a sinterable material - Google Patents

Abstract

The material made from alumina and titanium carbide has a mean transverse breaking strength of at least 67.9 + 14.7 kg/mm<2>, a mean Rockwell A hardness of 93.8 and a mean Knoop hardness of 3477. It is produced by sintering a powder mixture of 70 % of alumina and 30 % of titanium carbide while the pressure is varied during the heating under specified conditions. The material can be used as refractory material and for the production of cutting tools for metals. [GB1465152A]

Description

pipm*? frr»jn am ri '-ra new Method of testing a sinterable material TIIE BABCOCK & WILCOX COMPANY C: 48246 This invention relates to a method of testing a sinterable material, in particular, to approximately identify the natural rate of densification thereof.

Alumina (AI2O3) and alumina compounds have been used for high temperature and high strength purposes for many years. For example, in refractory applications and in metalworking tools that are subjected to high speeds and great wear, these materials have found widespread industrial acceptance.

It appears, moreover, that the strength of this material is in some manner related to its density and crystal size, the more dense and smaller crystal structures providing stronger and more durable tools. Consequently, there is a great deal of emphasis on producing ceramic cutting tools with these characteristics. When used as a cutting edge, however, alumina occasionally fractures.

In general, these fractures seem to be related to the presence of relatively large alumina crystals, or "grains", in an essentially small crystal or "fine" grain structure. Thus, much of the alumina research effort has been directed to the more specific development of techniques for large-scale production of a high density material with a uniformly fine grain structure.

The crystal growth that occurs when the raw powder material is heated to coalesce (or is "sintered") often is retarded through the addition of magnesium oxide (MgO) in an amount of 0.5% or less. This heating can be accomplished in a vacuum furnace that raises the material temperature to a 1400° to 1550° range. Processes of this sort have been reported to provide a material that has a crystal size of In the interest of efficiency and production economy, it is clear that a reduction in heating time is desirable, especially if the reduced heating time can be coupled with the production of a more uniformly fine grain structure. Because of the tendency for alumina tools to fracture, there also is a need for a technique to produce the even smaller crystal sizes that lead to greater strength. However, in order to ascertain the op.timum operating conditions for a method of producing a refractory material, such as an alumina-carbide material, it is desirable to ascertain the natural rate of densification of the sinter-able material.

According to the present invention, there is provided a method of testing a sinterable material comprising at least a first stage of A) applying a first physical pressure and a first heating rate of a first batch of a degassed sinterable material, to begin sintering of said first batch, applying an increased second physical pressure and a second lower heating rate to said first batch to effect densifi-cation of the material, releasing the pressure and cooling the material prior to bloating, and recording the density of the sintered first batch; and a second stage of: B) applying the first physical pressure and the first heating rate to a second batch of the degassed sinterable material, to begin the sintering of said second batch, applying an increased third physical pressure and a third lower heating rate to said second batch to effect densification of the material, releasing the pressure and cooling the material prior to bloating, and recording the the theoretical maximum density of said material, thereby to approximately identify the natural rate of densifica-tion of the material.

The invention will now be further described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic graph of ram displacement versus time to illustrate a "break away point"; and Figure 2 is an array of graphs that show pressure, temperature density and breakaway point as function of time for a number of materials.

Figure 1 graphically illustrates features of a method of producing a refractory material from a sinter-able powder^ expressed in terms of the movement or displacement of the ram that is used to compress a powdered material which is being sintered, as a function of time. The ram displacement 10 is necessary to "prepress" the powdered mixture in order to enhance sintering and to remove any entrapped gases in the powder between time 0 and t^. After time t^ and before time t2 , the application of heat to the precompressed powder leads to a thermal expansion displacement 12 of the ram. This step in the process is terminated by a "break away point" 13 at the time t . This "break away point" is characterized by a change from the expansion of the precompressed powder to a contraction 14 that commences as sintering begins. The contraction culminates at time t^. The time tj is a time of maximum densifica-tion and coalescense of the sintered powder. The further application of heat after time t3 produces excessive grain growth or "bloating" 16 as indicated by the increase in Figure 2 is a graphic representation of sintered product temperature, density, and "break away points as a function of time for the following materials: Billet Diameter Material V U02 1" A1203 5" A12°3 1" Al203-TiC 5" Al203-TiC For purposes of orientation between Figures 1 and 2 the initial time, zero, of Figure 2 corresponds to the time ti in Figure 1.

The pressure "history" 20 for all of these materials is bonded by straight line segments that identify a pressure increase, a step function from the initial pressure to the maximum predetermined pressure that is maintained throughout the remainder of the process.

The temperature history 22 is bounded by straight line segments. These temperature bounds indicate an increasing temperature in response to the initial heating, followed by a minimum and maximum process temperature range for the remainder of the process.

The theoretical maximum density "history" 24 follow paths to maximum values which are represented by a generalized graph 24. The theoretical maximum density is defined as the closest possible packing of atoms into the crystalline structure of the compound, exclusive of any and all impurities, that will produce a minimum interstitial volume between the packed atoms.

The break away points as a function of time 30 A method of converting a sinterable powder into a ceramic metal oxide will now be described, as an example of a method in connection with which the testing method of the invention can be used.

Alpha alumina powder of less than one micron, preferably less than one tenth of a micron, particle size is worked or ball milled in a dry mill from four to eight hours. Preferably, alumina sold by W R Grace Company of U. S. A. under the name "Grace-KA 210" should be used as the raw material. This alumina powder has a surface area of approximately 9 meters /gram. It is, moreover, of very high purity, although it does contain 0.1% addition of MgO. Other aluminas also can be used, although experimental data, does seem to indicate that best results are achieved with the Grace-KA 210 material.

To maintain powder purity, moreover, the ball mill also should be formed from very pure alumina.

Upon completion of the milling step, the powder is baked for another four to eight hours at 50° to 100°C. Baking the powder at 72°C seems to be a preferred temperature for this step in the process. These ball milling and drying operations appear to have the effect of removing excess surface gases to produce a finer- grained end product. The relation between the surface gas and the grain size of the fully processed material has not been definitely established. It is possible, however, that the surface gas behaves as an impurity phase that causes severe selective grain growth at high temperatures.

After outgassing, to produce a one-inch diameter billet of AI2O3, the powder is screened through a 200 mesh high temperature, high strength die. Typically, a graphite die in an inert, vacuum or reducing atmosphere is suitable for the- purpose. A compacting pressure of 4000 to 8000 pounds per square inch (psi) is applied to the powder within the die. This pressure is applied to initially compact the powder to 30% to 50% of its maximum theoretical density.

For this sample, it has been found that an initial compacting or "prepressing" pressure of 5750 psi leads to the best end product results. The prepress force is then reduced to a range of 500 to 1000 psi. Generally, a reduction in pressure to 1000 psi will produce acceptable results.

The powder and the die are placed in a hot press or other high temperature and high pressure sintering device. A protective atmosphere, moreover, is established in this system in order to preserve the die. A vacuum a helium or other inert atmosphere, or a mixed atmosphere of inert gas and 8% by weight of hydrogen have been found suitable for this purpose. Furthermore, relatively less expensive nitrogen gas may be used for process economy.

Starting then with the reduced pressure on the compacted powder, the temperature of the powder and die is raised by means of an induction heater at a rate that is bounded by 400 to 1000°C per minute. By proper positioning and sizing of the induction heater and the billet generally uniform heating throughout the powder can be established. Within the above range it". appears that the rate of temperature change can be varied in an almost random manner until the onset of shrinkage of "break away point" 13 (Figure 1) is reached without degrading the quality of the final product. the above rate boundaries, of the powder and the die to 760° to 815°C as measured with an optical pyrometer will produce' the desired result. That is, the onset of shrinkage or "break away point" usually commences as the temperature reaches about 800°C. Preferably, while the temperature is being raised to the illustrative 800°C, to commence shrinkage, the reduced pressure of 1000 psi also is applied to the powder billet. This shrinkage may be observed with the aid of a linear variable displacement transducer that is attached to the ram that applies the pressure to the sintering powder.

After the "break away point" is reached, both temperature and pressure are increased in order to promote the rate of. densification that is inherent or natural to the particular material and billet size (further reference is made to this just below). Both pressure and temperature increase rates can be monitored and adjusted to approximate this natural rate. This natural densification rate is identified through a series of tests conducted with sample powders. In each of these tests, pressure and temperature increase rates are varied to identify the ranges of pressures 20 (Figure 2) and temperatures 22 that provide the closest approach to the theoretical maximum density 24. Thus, the natural rate of densification of a sinterable material may be approximately identified by a method of testing that comprises, in a first stage, applying a first physical pressure and a first heating rate to a first batch of the degassed material, to begin sintering of that batch, and then applying an increased physical pressure and a second heating rate, lower than the first is cooled, prior to bloating. The density of the batch is then recorded. In a second stage, a second batch of the same degassed sinterable material is then subjected to the first pressure and first heating rate, to begin sintering of the second batch, whereupon an increased physical pressure different from the first and second pressures is applied to the second batch which is also heated at a third and lower rate, to effect densification of the material. The pressure is released and the material is cooled, prior to bloating. The density of the sintered second batch is then recorded, and the recorded densities of the two batches are compared in order to identify the density that is closest to the theoretical maximum density of the material. This will thus approximately identify the natural rate of densification of the sinterable material. It should be noted in Figure 1 that the natural rate of densification changes as the powder is sintered into its maximum densification as indicated by the minimum billet volume at time t^.

With respect to the above alumina example, the onset of shrinkage is accompanied by an application to the now sintering billet of a physical or ram pressure of 3600 psi. Although this is a preferred maximum process pressure, suitable results are obtained with pressures in the range of 2000 to 6000 psi. This rapid increase in pressure is reflected in the step- function pressure change that characterizes the pressure graph 20.

As the application of this pressure continues, the temperature also is increased, but at a lower rate than that which characterized the initial increase to 800°C. minutes after the earlier 800°C temperature was attained. These higher temperatures also are observed through an optical pyrometer. This maximum temperature and pressure are sustained for two to six minutes, and preferably for three minutes, if a maximum process temperature of 1600°C is achieved. During this time, the alumina is sintering at its "natural" or inherent rate, as referred to above.

The linear change in ram displacement between the times t2 and t3 shown in Figure 1 is a characteristic feature of a billet that is sintering at this natural rate. Other natural sintering rate indices are possible, although ram displacement is a most convenient technique.

The pressure and temperature increase rates that are applied to the sintering billet after the "break away point" 13 has been reached are, generally, adjusted to establish and maintain this natural sintering rate. The natural sintering rate will, of course, vary according to the material that is being processed. This natural rate, moreover, also may vary for different batches of the same material. Consequently, the precise rates of increase of temperature and pressures that should be applied to the sintering billet for any particular material can be determined through a number of tests each performed on a different batch of the material. These tests will identify those conditions that produce the linear ram displacement 14 (Figure 1) , or other indications of the natural sintering rate, for the material under consideration. Once these sintering conditions are identified, subsequent billets can be processed without ram displacement observations and the like. completion of sintering 17. Thus, as shown in the drawing, the rate of ram displacement as a function of time decreases as the sintering billet approaches a condition of maximum densificatio . As this terminal portion of the sintering process is approached, the pressure and temperature applied to the billet is stabilized for two to six minutes to "cure" the now sintered billet.

Care must be exercised to terminate production conditions at this point in order to prevent the development of a "bloated" billet. This "bloating" 16 is characterized by a reduced density billet, as indicated through the greater billet volume which the increasing ram displacement registers.

Turning once more to the completion of sintering 17, it is possible more precisely to promote the natural rate of sintering, which apparently changes as maximum densification is approached, by adjusting the temperature and pressure increase rate that is applied to the sintering billet in a manner that will enable the ram displacement to more nearly approximate the curve illustrated in Figure 1.

After the period of curing, or sustained heating at the maximum process temperature and pressure, the induction heater, or other source of heat, is turned off and the pressure on the alumina within the die is reduced to zero. A cooling period of one to five minutes is sufficient to enable the die (and the now sintered alumina) to cool to room temperature for removal from the press and separation from the die.

The "break away point" graph 30 in Figure 2 illustrates the relation between the diameter of the end higher temperatures and pressures should be applied during processing than these conditions which are mentioned above with re.spect to the one inch diameter billet. It should be kept in mind, however, that a basic feature for all of the materials and billet sizes described herein is the application of an increased process pressure, within described boundaries throughout the sintering process, i.e. after the "break away point" (Figure 1). Moreover, a maximum process pressure, an observed optimum, is identified within the described bounds, obtained by comparing the pressure "history" of the sintering billet with the density of the processed sample, and may be more conveniently applied to the billet to provide the desired closest approach to the theoretical maximum density.

Thus, alumina ceramics manufactured in accordance with the above-described method have a grain structure that is different from those grain sizes that have characterized earlier methods. Crystals of much larger average size, e.g. two or three microns generally occur in alumina made by these earlier methods.

Reference might also be made to our co-pending patent application Serial No. 44318 for an example of a further method of producing a refractory material, in connection with which the testing method of the invention may be used. In that refractory material production method, a refractory carbide, nitride or oxide powder (other than of aluminium) is ball milled and mixed with a refractory metal oxide powder. The mixed powders are compressed for removing entrapped gas, and for achieving a prepressed billet, and the compressed powders are then subjected to - natural rate of densification as identified by the method of testing of the present invention.

Attention is drawn to our co-pending patent application Serial No. 44318 from which the present application was divided and which relates to similar subject-matter.

Claims (1)

WHAT WE CLAIM IS:

1. A method of testing a sinterable material comprising at least a first stage of: A) applying a first physical pressure and a first heating rate to a first batch of a degassed sinterable material, to begin sintering of said first batch, applying an increased second physical pressure and a second lower heating rate to said first batch to effect densification of the material, releasing the pressure and cooling the material prior to bloating, and recording the density of the sintered first batch; and a second stage of: applying the first physical pressure and the first heating rate to a second batch of the degassed sinterable material, to begin the sintering of said second batch, applying an increased third physical pressure and a third lower heating rate to said second batch to effect densification of the material, releasing the pressure and cooling the material prior to bloating, and recording the density of said sintered second batch, to identify which of the densities of the sintered batches is closest to the theoretical maximum density of said material, thereby to approximately identify the natural rate of densification of the material.