GB2196956A - Process and apparatus for the production of rapidly solidified powders of high melting point ceramics - Google Patents

Abstract

A centrifugal atomization process is described which uses a non- transferred plasma flame for melting non-conducting ceramic materials and raw materials shaped into rods which are revolved at a high speed to generate centrifugal forces for disintegrating the ceramic melt formed, by plasma melting, on the end surface of said rod. The droplets spun off said rod are allowed to solidify either during their free flight in the atmosphere or on a quench substrate placed in the vicinity of said rod. <IMAGE>

Description

SPECIFICATION Process and apparatus for the production of rapidly solidified powders of high melting point ceramics The present invention relates to a process and an apparatus for the production of rapid solidification processed (RSP) powders of ceramic alloys.

Conventional Processes Rapid solidification processed (RSP) powders of high melting point ceramics may be achieved by laboratory techniques such as hammer-anvil and gun splattering methods with arc, laser or image furnace melting, which techniques produce small quantities of RSP samples. Single roll or twin roll melt spinning processes, melt extraction techniques and flame pressure atomization which uses gas flame melting and water jets for atomization, are also available to produce small to medium quantities of RSP ceramic powders.

Rotating electrode processes, versions of centrifugal atomization, have been developed for producing powders of reactive metals such as Ti, Zr and Hf and their alloys, so as to avoid deleterious reactions between the melt and the atmosphere and contamination from the crucible, which problems are often encountered in conventional atomization processes.

Examples of the rotating electrode processes may be found in Japanese Patent No. 1,260,218 which discloses a process with transferred plasma arc melting and U.S. Patent No. 4,485,031 claiming a similar process. Centrifugal atomization generally gives a narrow size distribution and hence a small interparticle difference in microstructure of an atomized powder.

However, cooling rates typically encountered in centrifugal atomization are normally less than those desired for rapid solidification purposes, unless some special means to enhance cooling, such as forced convection by application of a high pressure quench gas, is employed.

Problems Circumvented by the Present Invention The RSP technique for producing powders of high melting point ceramics results in extreme size refinement of microstructure, extended solid solubilities, chemical homogeneity, non-equilibrium phases and amorphous phases, all of which are unattainable via conventional processing routes. However, the difficulties associated with melting ceramics due to their inherent high melting points and deleterious reactions between the ceramic melt and surrounding materials such as crucibles, have been the barriers against melting a large quantity of high melting point ceramics.Since providing the ceramic melt and hence a stable supply of the molten material to an atomizing apparatus is essential in most atomization processes suitable for mass production, such as gas atomization and melt spinning, the production of RSP ceramic powders is not easy in practice.

The rotating electrode processes described earlier as avoiding crucible-melt contact are only used for atomizing conducting materials due to the restriction imposed by the transferred arc used in those processes.

As previously mentioned, the use of a large amount of quench medium necessitated in some centrifugal atomization processes inevitably causes an increase in production costs.

BRIEF SUMMARY OF THE INVENTION The present invention provides a process for the production of rapid solidification processed (RSP) powders of ceramic alloys which comprises non-transferred plasma flame melting a revolving rod made from premixed ceramic powders to produce, by centrifugal force, fine droplets of ceramic melt which are either allowed to solidify while in free flight through the atmosphere or by quenching on a substrate placed in the vicinity of said rod.

The present invention also provides an apparatus for producing RSP ceramic alloy powders comprising a non-transferred plasma torch, a revolving mechanism with means for holding a raw material rod and a detachable substrate for quenching liquid droplets, with said rod, torch, detachable substrate and holding means for said rod being aligned coaxially in a gas tight container.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic depiction of an apparatus suitable for producing RSP ceramic powders by the present process.

Figures 2 and 4 are charts depicting the X-ray diffraction of the RSP powders of mullite and Al203-43 wt. % ZrO2 ceramic alloys produced by the present process.

Figure 3 shows the DTA curve obtained for a RSP mullite powder.

Figure 5 schematically illustrates a centrifugal atomizer with transferred plasma arc heating of the prior art.

DETAILED DESCRIPTION The present invention circumvents the difficulties associated with conventional atomization processes as applied to RSP of ceramics and enables one to mass produce clean RSP powders of ceramic alloys. The details of the invention are explained below.

Fig. 1 depicts a schematic vertical section of an apparatus suitable to perform the inventive method for producing RSP ceramic powders. The apparatus comprises non-transferred plasma torch 2, raw material rod 3, rod holder 4, revolving mechanism 5 and quench substrate 6 which has a truncated conical shape and is placed coaxially around raw material rod 3 in a gas tight container in which the inside portion is isolated from the outside portion. Raw material rod 3 is made by consolidating a mixture of ceramic powders of a desired composition by an appropriate method so that said rod can endure the mechanical and thermal stresses due to rapid revolution and melting of said rod during atomization.

While raw material rod 3, clamped in holder 4 coaxially with revolving mechanism 5, is rotated at a desired speed, the top end of rod 3 is heated by non-transferred plasma torch 2 to effect melting of the ceramic material. The liquid ceramic so formed flies off to be disintegrated into fine droplets by the centrifugal force exerted by the rotation of said rod and the aerodynamic forces caused by the high relative velocities between said liquid and the atmosphere.

As previously stated, one of the major attributes of the present invention is the use of nontransferred plasma heating which enables melting of high melting point non-conducting ceramics.

Avoidance of melt-crucible contact which causes undesired contamination of the ceramic melt is also assured in the present invention. The droplets so produced are allowed to solidify during their flight in the atmosphere and/or quench substrate 6 placed coaxially around raw material rod 3.

Powders produced by allowing solidification during flight in the atmosphere consist of spherical particles with a relatively identical diameter and are relatively free from satellite particles which are commonly observed for conventionally gas atomized metal powders and hence show an excellent fluidity. However, the quench rates typically obtained for those spherical powders are often insufficient to effect rapid solidification because of the limited heat transfer coefficient at the droplet-gas interface. Furthermore, the spherical shape of the powders makes post atomization comminution difficult. On the other hand, powders produced by substrate quenching of centrifugally atomized droplets consist of flaky particulates.Although, the latter particulates may not be appropriate where fluidity is essential, both uniformity and severity of quench rate as well as ease of reduction of particle size after atomization are assured for those powders.

The cooling rate of atomized droplets can be also controlled through control of droplet diameter. Therefore, the ability to change the revolution speed and to a lesser degree, the diameter of raw material rod 3 provides an additional independent means for controlling the droplet cooling rate.

Performing the above-mentioned centrifugal atomization using a non-transferred plasma arc requires one to maintain the distance between the top end of rod 3 and plasma torch 2 unless, alternatively, the strength of plasma flame is progressively increased to maintain a desired amount of heat input at the top of rod 3 as melting causes shortening of said rod. The latter requirement may be met by either lowering plasma torch 2 or raising rod 3 as melting takes place. Fig. 1 illustrates an example of the former scheme.The upper limit of atomization rate in the present invention (V) may be given approximately, provided that no fracture of rod 3 due to mechanical and/or thermal stresses occurs, by the following equation: V --xl/R2MH where I is the heat input of plasma torch 2, R and M are, respectively, the radius and the density of rod 3, H is the latent heat of fusion and x is a constant measuring the efficiency of plasma heating.

Quench substrate 6 may be either combined with the structure supporting plasma torch 2 or be designed so that it can be moved independently to minimize overlapping of the quenched droplets upon impinging on the substrate and hence to guarantee uniform droplet quenching.

Needless to mention, in principle, the present invention does not necessarily require a vertical axis of rod revolution. For example, provision of an appropriate means of particulate collection makes the use of an apparatus with a horizontal revolution axis possible. Thus, the apparatus shown in Fig. 1 is merely exemplary of an apparatus which is capable of performing the inventive method.

For comparative reference, the rotating electrode apparatus of the prior art, i.e. Japanese Patent No. 1,260,218, is schematically depicted in Fig. 5 wherein rod 9 which is made of the raw material to be employed, is rotated by revolving mechanism 8 and is melted on its top end by transferred plasma arc 10. The molten material, as it is produced, is disintegrated into fine droplets by the centrifugal forces caused by the rapid revolution of rod 9. The pressure inside container 7 is kept between 10-1 and 10-3 torr during atomization. The transferred plasma arc may be stabilized by the use of a solenoid coil placed around rod 9 as needed.

The following examples explain the present invention in more detail.

Example 1 Sintered rods 15 mm. in length of a commercial grade mullite were centrifugally atomized with the apparatus shown in Fig. 1 at revolution speeds ranging from 4,900 to 10860 rpm. The rods are sintered at 1575"C in air prior to atomization. A water cooled quench substrate having a truncated cone shape 170 and 260 mm. in top and bottom diameters, respectively, and 245 mm. high made of a type 304 stainless steel was used. Atomization was performed in air with a 27kw non-transferred plasma flame produced with a mixture of argon and hydrogen at flow rates of 950 and 115 normal cubic centimeters per minute, respectively.

Fig. 2 shows the results of X-ray diffraction analysis performed on the RSP mullite powders so produced. The prominent peaks observed at low revolution speeds are those of crystalline mullite. The mullite peaks become broader and shorter with increasing revolution speed. At 10,860 rpm almost the entire material of the RSP powder is found to be amorphous. The latter formation of an amorphous phase is even more readily achieved when SiO2 content is increased relative to that of mullite. Fig. 2 also shows the X-ray diffraction result obtained for an Al203-50 wt. % ZrO2 ceramic alloy powder produced at 7,000 rpm.

Fig. 3 shows the result of differential thermal analysis done on the RSP mullite powder produced at 9,930 rpm. The sharp exothermic peak at about 985"C and the slight negative deflection at about 915"C indicate, respectively, the crystallization of mullite and glass transition and thus provide further evidence that the RSP material was indeed amorphous, for the most part.

Example 2 Reagent grade powders of Al203 and ZrO2 are mixed to the eutectic composition, i.e., 43% ZrO2 by weight and sintered as rods having the same dimensions as those of the mullite rods given in Example 1. Atomization of the rods was performed in air at 7,000 and 9,700 rpm with the same quench substrate. as described in Example 1. Fig. 4 shows the X-ray diffraction data obtained for the RSP Al203-43% ZrO2 powders. It is clearly noted that increase in revolution speed and hence in cooling rate increases the formation of the metastable tetragonal modification of ZrO2 in preference to monoclinic ZrO2 and that at 9,700 rpm most ZrO2 precipitated as tetragonal Zero2. Alumina was found to be present as corundum.

From the foregoing, it is apparent that the present invention provides a method for producing RSP powders of high melting point ceramics in a large quantity and thus constitutes a breakthrough in the commercial production of such powders. The method provided by the present invention is characterized by avoidance of melt crucible-contact which often causes melt contamination and easy melting of non-conducting high melting point materials made possible by combining centrifugal atomization and non-transferred plasma flame melting. The ability to control the quench rate by independently altering revolution speed and the dimensions and material of the quench substrate is another attribute of the inventive method. In addition, only a small variation in cooling rate between particles of an atomized powder results, since powders produced by allowing liquid droplets to impinge on the quench substrate, have a nearly identical particulate thickness under identical atomization conditions.

Finally, but more importantly, the present invention can provide an impact on R & D activities in the area of high performance ceramics, since the novel RSP microstructures often unattainable via conventional processing routes can be obtained by rapid solidification and such technique can be used to obtain unusual properties of ceramic alloys.

Variations and modifications of the foregoing will be apparent to the art-skilled without departing from the essential teachings herein.

Claims (3)

1. A process for the production of rapid solidification processed (RSP) powders of ceramic alloys which comprises non-transferred plasma flame melting a revolving rod made from premixed ceramic powders to produce, by centrifugal force, fine droplets of ceramic melt which are either allowed to solidify while in free flight through the atmosphere or by quenching on a substrate placed in the vicinity of said rod.

2. An apparatus for producing RSP ceramic alloy powders comprising a non-transferred plasma torch, a revolving mechanism with means for holding a raw material rod and a detachable substrate for quenching liquid droplets, with said rod, torch, detachable substrate and holding means for said rod being aligned coaxially in a gas tight container.

3. Apparatus according to claim 2 substantially as described herein with reference to Fig. 1.