FR2607492A1 - PROCESS AND APPARATUS FOR THE PRODUCTION OF HIGHLY SOLIDIFIED CERAMIC ALLOY POWDERS - Google Patents

Abstract

THE INVENTION CONCERNS A PROCESS AND AN APPARATUS FOR THE PRODUCTION OF CERAMIC POWDERS TREATED BY RAPID SOLIDIFICATION. A NON-TRANSFERRED PLASMA TORCH 2 IS USED TO MELT A NON-CONDUCTIVE CERAMIC BAR 3 WHICH IS ROTATED SO THAT THE DROPS OF MELTED CERAMIC ARE PROJECTED AND COOL IN AIR OR ON CONTACT WITH A SUBSTRATE 6 PLACE NEAR THE BAR 3. FIELD OF APPLICATION: PRODUCTION OF CERAMIC ALLOY POWDERS.

Description

The invention relates to a method and an apparatus for

  for the production of powders of ceramic alloys,

  processed by rapid solidification (TSR).

  Fast solidification treated (TSR) powders in high melting point ceramics can be obtained by laboratory techniques such as hammer-anvil and arc furnace fusing, laser or image fusing processes. which techniques produce small quantities of samples of fast-solidified powders or TSR powders. Melt spinning processes, single or double roll, melt extraction techniques and flame-pressure atomization, which utilizes gas flame fusion and water jets for atomization, can also be used for the production of quantities

  low to medium amounts of TSR ceramic powders.

  Rotary electrode methods, which are variants of centrifugal atomization, have been developed for the production of reactive metal powders such as Ti, Zr and Hf and their alloys, in order to avoid harmful reactions between the material.

  melting and the atmosphere and contamination due to

  set, which problems are often encountered in

  conventional atomization methods.

  Examples of rotary electrode methods can be found in Japanese Patent No. 1,260,218 which discloses an arc fusion method.

  transferred plasma, and in the United States patent

  United States of America N 4,488,031 describing a similar process

  lar. Centrifugal atomization generally gives a narrow particle size and thus a small difference between particles with respect to the microstructure

  an atomized powder. However, cooling rates

  dissement generally encountered in atomization

  are normally lower than those desired

  for the purpose of rapid solidification, unless some special means to enhance cooling, such as forced convection by application of high pressure cooling gas, are used. The TSR technique for the production of high-melting ceramic powders gives an extreme dimensional refinement of the microstructure,

  extensive solid solubilities, chemical homogeneity

  non-equilibrium phases and amorphous phases, all impossible to obtain by conventional methods of treatment. However, the difficulties posed by the melting of ceramics because of their naturally high melting points and the harmful reactions between the melted ceramics and the surrounding materials such as crucibles, constitute obstacles to the fusion of a large quantity of ceramic tiles

  high melting point. Since the production of ceramics

  Thus, in most atomization processes capable of mass production, such as gas atomization and extrusion spinning, a substantial amount of molten feed for a spray apparatus is essential. the production

  The use of TSR ceramic powders is not easy in practice.

  The rotary electrode methods previously described as avoiding the contact between the crucible

  and the melted material are only used for atomisa-

  conductive materials due to the restriction

  imposed by the transferred arc used in these processes.

  As mentioned earlier, the use of a large amount of a cooling medium,

  necessary in certain processes of centrifugal atomization

  fuge, inevitably leads to higher costs

of production.

  The present invention provides a process for the production of fast-solidified (TSR) powders in ceramic alloys, which consists in melting the non-transferred plasma flame with a rotating bar consisting of premixed ceramic powders.

  to produce, by centrifugal force, fine droplets

  melted ceramics that are allowed to solidify as they fly freely through the atmosphere, or that cool on a substrate placed

proximity of said bar.

  The present invention also provides a process for the production of alloy powders

  TSR ceramics comprising a non-transmissive plasma torch.

  characterized, a rotation mechanism provided with means for holding a bar of raw material and a removable substrate for cooling liquid droplets, the bar, the torch, the removable substrate and the rod holding means being aligned

  coaxially in a gastight container.

  The invention will be described further in decall with reference to the accompanying drawings by way of non-limiting example and in which: - Figure i is a diagram of an apparatus suitable for the production of ceramic powders TSR by the method of the invention ; FIGS. 2 and 4 are diagrams showing X-ray diffraction of TSR powders of mullite and ceramic alloys of A1 203 and 43% by weight of ZrO 2 produced by the present process; FIG. 3 is a diagram showing the differential thermal analysis curve obtained for a TSR mullite powder; and FIG. 5 schematically illustrates a centrifugal atomizer with plasma arc heating.

transferred from the prior art.

  The present invention avoids the difficulties associated with conventional atomization processes as applied to TSR ceramic powders and allows the production in large quantities of clean TSR powders made of ceramic alloys. The details of the in-

are explained below.

  FIG. 1 is a schematic vertical section

  than an apparatus suitable for carrying out the process of the invention for the production of TSR ceramic powders. The apparatus comprises an untransferred plasma torch 2, a bar 3 of raw material, a bar-holder 4, a rotation mechanism 5 and a

  cooling substrate 6 which is frustoconical in shape

  and which is placed coaxially around the bar 15 of raw material, in a container 1 sealed to

  gas whose inner part is isolated from the outside.

  The bar 3 of raw material is obtained by consolidating

  of a mixture of ceramic powders of a composition

  desired, by a suitable method, so that the bar can withstand the mechanical and thermal stresses due to rapid rotation and fusion

of the bar during atomization.

  While the bar 3 of raw material, clamped in the bar holder 4, coaxially with the mechanism

  5 rotation is rotated at a desired speed.

  The upper end of the bar 3 is heated by the non-transferred plasma torch 2 so that the ceramic material melts. Liquid ceramics as well

  formed is projected and disintegrates

  under the effect of the centrifugal force exerted by the rotation of the bar and the aerodynamic forces due to the high relative velocities between the liquid

and the atmosphere.

  As mentioned earlier, one of the most

  The main features of the present invention are the use of non-transferred plasma heating which melts non-conductive high melting ceramics. The invention also avoids any contact between the melt and the crucible, which contact causes undesired contamination of the molten ceramic. The droplets thus produced are allowed to solidify during their flight in the atmosphere and / or on the cooling substrate 6 disposed

  coaxially around the bar 3 of raw material.

  The powders produced by allowing solidification during flight into the atmosphere are

  consisting of spherical particles of

  and they are relatively free from satellite particles commonly seen in conventionally atomized metal powders.

  a gas and, therefore, they have an excellent

  fluidity. However, the cooling rates generally obtained for these spherical powders are often insufficient to achieve fast solidification due to the limited coefficient of heat transfer at the droplet-gas interface. In addition,

  the spherical shape of the powders makes it difficult to

  after atomization. Moreover, the particles

  the powders produced by cooling on a substrate of atomized droplets by centrifugation are in the form of flakes. Although these last particles may not be suitable in the case o

  fluidity is essential, uniformity and

  the speed of cooling, as well as the

  reduction of particle size after

  are ensured for these powders.

  The cooling rate of g) ute-

  atomized letters can also be set by a

2 607492

  adjustment of the droplet diameter. Therefore, the possibility of varying the rotation speed and, to a lesser degree, the diameter of the bar 3 of

  raw material, gives an additional independent means

  shut up to control the cooling speed

droplets.

  Performing the centrifugal atomization mentioned above using a non-transferred plasma arc requires maintaining the distance between the upper end of the bar 3 and the plasma torch 2 unless, alternatively, the power of the plasma flame is increased gradually to maintain a desired heat input to the top of the bar 3 as the fusion shortens the bar. This latter requirement can be satisfied either by a lowering of the plasma torch 2 or by a rise of the bar 3 as the merger progresses. Figure I illustrates an example of the first mode. The upper limit of the atomization rate in the present invention (V) can be given approximately, provided that no fracture of the bar 3 due to mechanical and / or thermal stresses occurs, by the following equation: V = xI + R2MH where I is the heat input of the plasma torch 2, R and M are respectively the radius and the density of the bar 3, H is the latent heat of fusion and x is a constant giving a measure efficiency

plasma heating.

  The cooling substrate 6 can either be combined with the structure supporting the plasma torch 2, or be designed to be independently movable to minimize overlap of the cooled droplets arriving on the substrate and thus to ensure uniform cooling of the droplets. It is needless to say that the present invention does not require, in principle, a vertical axis of rotation of the bar. For example, the presence of an appropriate means of particle collection makes it possible

  to use an apparatus having a horizontal rotational axis

  tion. Thus, the apparatus shown in FIG. 1 is only given as an example of a device capable of

  implement the method of the invention.

  For comparison, the rotary electrode apparatus of the prior art, that is to say the patent

  Japanese Patent No. 1,260,218, is shown schematically

  5, apparatus in which a bar 9, consisting of the raw material to be used, is rotated by a rotation mechanism 8 and is brought to the molten state, at its apex, by a plasma arc 10 transferred. The melt, as it is produced, is disintegrated into fine particles by the centrifugal forces caused by the rotation

  9. The pressure inside the container

  The neur 7 is maintained at a value between 133 × 10 -3 and 133 × 10 Pa during the atomization. The transferred plasma arc can be stabilized by the use of a coil placed around the bar 9 when this is

necessary.

  The following examples explain more in

detail the present invention.

Example 1

  Sintered bars 15 mm in length, in industrial grade mullite, were atomized by centrifugation using the apparatus shown in FIG. 1, at rotation speeds of 4900 to 10 860 rpm. . The bars are sintered at 1575 C

  in the air before atomization. A cooling substrate

  water-cooled cone shaped 170 and 260 mm upper and lower diameters, respectively, and 245 mm high,

  made of type 304 stainless steel, has been used.

  The atomization was carried out in the air using an untransferred plasma flame of 27 kW produced with

  a mixture of argon and hydrogen at flow rates of

  950 and 115 cm3 / min, respectively, in

normal conditions.

  Figure 2 shows the results of an X-ray diffraction analysis performed on TSR mullite powders thus produced. The prominent ridges observed at low rotational speeds correspond to crystalline mullite. The mullite ridges become wider and shorter as the rotational speed increases. At 10,860 rpm, the

  almost all of the TSR powder turns out to be amorphous.

  The last formation of an amorphous phase is even more easily obtained when the content of SiO 2 is increased relative to that of mullite. FIG. 2 also shows the result of X-ray diffraction obtained for a ceramic alloy powder consisting of equal parts, by weight, of Al 2 O 3 and ZrO 2, produced

at 7000 rpm.

  FIG. 3 shows the result of the differential thermal analysis carried out on the powder of

  mullite TSR produced at 9930 rpm. The exothermic tip

  that frayed at about 985 C and the slight negative deviation at about 915 C respectively indicate the crystallization of mullite and the glass transition and thus still provide clear evidence that the TSR material is actually amorphous in its more

big party.

Example 2

  Reactive quality powders of A1203 and

  ZrO2 are mixed with the eutectic composition, that is,

  that is to say 43% ZrO 2 by weight, and sintered in the form of bars having the same dimensions as those of

  The mullite bars shown in Example 1. The atomisa-

  The rods were run in air at 7000 and 9700 rpm using the same cooling substrate as described in Example 1. Figure 4 shows the X-ray diffraction data obtained.

  for TSR powders of A1203 and 43% of ZrO2. We see clearly

  that an increase in the speed of rotation

  and therefore the speed of cooling raises the

  metastable tetragonal modification of ZrO2 preferentially to monoclinic ZrO2 (M) and that

  9700 rpm, most of the ZrO2 has

  pity in tetragonal ZrO2 (T). Alumina was found

in the form of corundum.

  It follows from the foregoing that the present invention provides a process for the production of TSR powders of high melting ceramics, in large quantities, and therefore constitutes a breakthrough in the industrial production of these powders. The process proposed by the present invention is characterized by the fact that it avoids any contact between the crucible and the melt, which contact often causes contamination of the melt, and by the fact that it facilitates the melting of non-melted materials. High melting point conductors, by combining centrifugal atomization and non-plasma flame

  transferred. The ability to adjust the cooling speed

  Another advantage of the process according to the invention is that of varying the speed of rotation and the dimensions and the material of the cooling substrate independently. In addition, the cooling rate varies only slightly between the particles of the atomized powder, since the powders obtained by allowing liquid droplets to reach 100.degree.

  the cooling substrate have a practical thickness

  equal under identical atomization conditions.

  c. Last but not least, the present invention can have an impact on the R & D activities in the field of high performance ceramics, because the

  new TSR microstructures, that it is often impossible -

  obtainable by conventional treatment methods, can now be obtained by solidification

  fast and this technique can be used to obtain

  retention of ceramic alloys with unusual properties. It goes without saying that many modifications can be made to the described method and apparatus

  and shown without departing from the scope of the invention.

Claims (2)

  1. Process for the production of powders of ceramic alloys, treated by fast solidification (TSR), characterized in that it consists of melting, by flame under non-transferred plasma, a bar   (3) in rotation consisting of ceramic powders premixed   to produce, under the effect of centrifugal force, fine melted ceramic droplets which are allowed to solidify as they fly freely through the atmosphere, or which are cooled on a substrate   (6) placed near the bar.   2. Apparatus for the production of ceramic alloy powders (TSR), characterized in that it comprises a non-transferred plasma torch (2), a rotation mechanism (5) equipped with means (4) intended to carry a bar (3) of raw material, and a   removable substrate (6) for cooling the droplet   liquid letters, the bar, the torch, the removable substrate and the support means of the bar being aligned coaxially in a container (1) sealed to gases.