AU8106587A

AU8106587A - One step sinter/hip processing of ceramics

Info

Publication number: AU8106587A
Application number: AU81065/87A
Authority: AU
Inventors: Andrew C. Nyce
Original assignee: GORHAM ADVANCED MATERIALS INST
Current assignee: GORHAM ADVANCED MATERIALS INSTITUTE
Priority date: 1987-10-08
Filing date: 1987-10-08
Publication date: 1989-05-02

Description

"ONE STEP SINTER/ H IP PROCESSING OF CERAMICS " .

BACKGRODND OF THE INVENTION

The present invention relates to the field of Hot Isostatic Pressing ("HIP") processing of ceramic materials and is characterized by affording combined benefits of elimination of microporosity in the finished products achieved at relatively low cost for such performance enhancement.

There are both physical and chemical (reactive) aspects of sintering a particulate compact while at elevated pressure and this invention deals with both (although in some instances the contribution of reaction mechanisms to sintering may be more or less).

The present invention is applicable to overcoming the foregoing problems in connection with various ceramics including aluminas, zirconias, (both silicon and aluminum) nitrides e.g., Si₃N₄, as well as SiAlON's, silicon carbides, zirconium borides, tungsten and titanium carbides, titanium nitride, and combinations of these internally within the above group and with other ceramic or metal additives and reinforcements such as high strength ceramic whiskers and fibers and ceramic particulates. These combined benef its compare favorably with state of the art processing in sin tering (which does not completely eliminate microcracks or microporosity) , hot pressing, and costly containerized HIP processing and containerless HIP processing. The latter process eliminates microporosity, but involves two full thermal cycles which can adversely affect microstructures and mechanical properties of resultant products.

It is the object of the invention to provide improvements in ceramic processing and resultant products with elimination of microporosity and in a way that enables low cost, high volume production.

SUMMARY OF THE INVENTION

The foregoing object and subsidiary objects of applying the same benefit on a material class by class basis are realized in accordance with the present invention. The invention utilizes a one thermal cycle approach in which green, uncontainerized parts of assembled ceramic powder compacts of net shape or near net shape are placed in a hot isostatic press. This is done after a de-bindering step which removes any binder material from the compact. Prepress processing of the compact may comprise a cold isostatic press step as part of assembling and consolidating. The compacts are sintered in the hot isostatic press to close up any surface-connected porosity within a few tenths of an inch of the surface and to pre-react sinter-aiding additives, if any within the compact, prior to the ultimate sintering. Pressure is then applied in stages at suitable levels of temperature until such combination of combined high pressurehigh temperature and duration where the parts are fully densified and flaw-free. Compared to other approaches, the total cycle time for these processes (called collectively "sinte r-HIP" reactive-sinte red he r ein) are sh ort. Sinter/HIP'd or high pressure reactive sintered parts have fine-grained microstructures, no detectable flaws, and excellent mechanical properties. The process also enables lower manufacturing costs compared to other techniques for achieving similar product results.

The terms used to describe the above-cited current generic ways to produce high density Si₃N₄ ceramics and other ceramics and the new process steps establ ished hereby comprise:

Sintering: This term is to be construed as denoting a high temperature densification process conducted under low to moderate gas pressure (generally N2 for Si3N4 and SiAlON ceramics) . The terms "sintering" and "low press ure sintering" as used herein indicate high temperature densification processing of green powder compacts conducted at between 1-20 atmospheres N₂. under such conditions the function of the gas is primarily to reduce net dissociation of S13N4 and thereby enhance densification. This mode of sintering includes, then, the sorts of processing often referred to in the open research literature as "pressureless sintering" or "overpressure sintering". Th term "high pressure sintering", is reserved to connote sintering of green compacts under higher N₂ pressures, e.g., up to 100 or more atmospheres. The function of the gas pressure in such cases is, theoretically, two fold: to reduce Si₃N₄ dissociation and to modify the sintering kinetics of the test materials by chemically altering their phase composition at elevated temperature and by causing a pressure induced change in liquid phases present in the system. SiAlON and AI₂O₃ other ceramic production is enhanced through the present invention through the use of high reactive gas pressure to enhance sintering kinetics of these material systems.

Hot Pressing : This term means uniaxial hot pressing, where the compressive force is generally mediated through a hydraulically driven ram. This mode of ceramic powder compaction is also referred to in the research literature as "pressure sintering". As noted above high pressure sintering and low pressure sintering in context of this application designate elevated temperature densification processes conducted under gas pressure.

HIP' ing; This term denotes the simultaneous application of elevated temperatures and high isostatic gas pressure, used to densify materials. Pressures in the 15,000-30,000 psi range are typically used in HIP'ing (also known as clad HIP'ing or canned HIP'ing) of Si₃N₄ and SiAlON ceramics involves enclosing powders or preforms within hermetically sealed, gas tight containers made of refractory metals or glasses. The compressive forces exerted by the high pressure gas during processing are transmitted through the container to the enclosed powder or part, while the gas itself is prevented from entering (and equilibrating within the pores of the enclosed material , which would preclude pressing) .

Containerless HIP'ing requires high density, closed porosity parts. Such parts are themselves gas tight; thus the need for enclosing them within sealed, refractory metal or glass gas-barriers is eliminated. Containerless HIP'ing encompasses two different processing approaches designated Sinter + HIP and Sinter/HIP.

Sinter + HIP is a mode of containerless HIP' ing employing discrete sintering and HIP'ing processing cycles. In this approach, green compacts are sintered to high density and closed porosity in a sintering furnace (as used in "pressureless" or "over pressure" sintering) . Following cooling and removal from the furnace, the parts are introduced into a hot isostatic press and then run through a complete HIP processing cycle. Sinter/HIP, on the other hand, is a mode of containerless HIP'ing in which sintering and HIP'ing are combined into a single, continuous processing cycle. Specifically, green compacts are sintered under lowto-moderate gas pressure to high density and closed porosity in the initial phase of the overall cycle; following completion of this initial phase of this process, the system is rapidly pressurized to a suitable HIP pressure, while high temperature is maintained, and the sintered specimens are

HIP'd in situ.

The theories of enhancement of properties through the present invention are usefully stated hereinafter to aid in understanding thereof.

In a first series of experiments carried out between 1981-1983, I observed totally unexpected results. These experiments involved sinter + HIP described above. I placed sintered, closed porosity compacts into a HIP vessel and subjected them along with green compacts f rom the same processing lot as the sintered samples to cycles of the general type described below in Detailed Description. Certain cycles were not effective in containerless HlP'ing previously sintered compacts with closed porosity to near theoretical density. Furthermore, none of the green compacts were densified above 90 percent of theoretical. Certain cycles were effective in densifying both green compacts and compacts previously sintered to closed porosity.

My original explanations of these results were:

For green compacts subjected to the effective cycles, the sintering to closed porosity was accomplished as the samples were pressurized to the HIP'ing pressure and that the pressure acted upon the closed porosity structure not sintered extensively enough to resist the pressure. It was reasoned that recrystalization of the grain boundary phase was not complete after sintering so that the compact could be densified by the applied pressure. It was not apparent that an open porosity compact could be subjected to 15,000 psi and densified because the pressure inside the compact would equal the pressure outside.

For samples sintered to closed porosity (densities greater than 92 percent of theoretical) the pressure acted on the closed porosity structure which was not extensively sintered enough to resist the compressive stresses produced by the gas pressure of 15,000 psi.

These original surmises were not correct. Later work showed that the pre-sintered samples did not begin to densify any further until the pressure of 15,000 psi was achieved and maintained. This result was at first unexpected and confusing. But upon reflection, the potential mechanisms which may be operative became apparent:

(1) The solubility of N₂ gas in Si₃N₄ and SiAlON increases with pressure. It also increases in the SiO₂- Si₃N₄-Al₂O₃ grain boundary phase and in all likelihood reacts with this phase.

(2) Diffusion coefficients of N and Si increase with increasing N content of the SiAlON, Si₃N₄ and grain boundary phases.

(3) Gas pressure above 1 atomsphere changes the phase composition and volume of the phases present in the SiO₂-Si₃N₄-Al₂O₃ system at the grain boundaries. Increased gas pressure to 30 ,000 psi is probably not high enough to materially affect the lattice constants of the SiAlON phase. If a reactive gas is net used, there can still be phase composition and phase changes as pressure is raised.

(4) Increases in temperatures above that used for sintering will change the volume of phases and the type and composition of these phases. Accordingly, a compact sintered at T₁ and temporarily subjected to T₂ where T₂ is at least 120% of T₁, the phases at the grain boundary and their composition can change even in the absence of a reactive gas. Thus, a thermal spike coupled with either a reactive gas or inert gas pressure could result in a compact which is readily deformed, (i.e., densified) by isostatic gas pressure.

Using the mechanisms in (1) through (4) above, one is now in a position to describe what happened in the earlier experimental work.

For a closed porosity compact previously sintered: increasing the pressure affects the viscosity, volume and type of phases at the grain boundaries. The sintered structure is thereby "loosened" and weakened so that applied pressure can cause densification by particle and grain boundary rearrangement and perhaps filling some pores with the SiO₂-Y₂O₃-Al₂O3-Si₃N₄ phases. Increasing the temperature could cause the same result. Atmospheric pressure could assist this process.

For open porosity greenbodies: open porosity green bodies can be both sinter/HIP'ed or high pressure reactive gas sintered depending upon whether the porosity is open or closed when high pressure gas is admitted to the system.

Starting w ith a pre-sintered or green body, and raising the pressure, the sintering of the body to closed porosity is accelerated by the mechanisms of 1 through 4 and at the closed porosity density, high pressure gas is trapped in the pores, but can effectively escape by becoming chemically a part of the SiAlON or of the SiO₂-Si₃N₄-Y₂O₃-AI₂O₃ phase thereby allowing the hot isostatic pressure densification to proceed unencumbered.

For other ceramics such as AI₂O₃, similar mechanisms may be envisioned. Also for other ceramics the reactive gas, grain boundary, and matrix composition can be tailored to allow the mechanisms of 1-4 to operate effectively.

As an example, AI₂O₃ was reactive sintered at 1000 psi and at 15 ,000 psi to densities normally not attained under ambient sintering conditions.

Other objects, features, and advantages will be apparent from the following detailed description of preferred embodiments taken in conjunction with the accompanying drawing in which :

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1 - 5 are graphical representations of ceramic, processing data illustrating the fine distinction between processing within and outside preferred embodiments of the present invention; and

FIGS. 6 and 7 are flow charts of steps for practice of the process of the invention in accordance with preferred embodiments thereof.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Each of FIGS. 1 - 5 shows overlayed pressuretime/termperature-time curves, FIG. 1 illustrating prior art and FIGS. 2 - 4 illustrating preferred examples practice of the present invention in a typi cal case and FIG. 5 illustrating a less preferred embodiments.

One or more ceramic powder compacts are placed in a HIP furnace with heating elements and controllable press urization/depressurization means. In FIGS. 2 - 4 the furnace is operated to attain a sintering temperature of the compacts ( i.e. , to ove r 1000° C) . Only then i s pressurization increased from a low level to isostatic pressing level while temperature is raised to HIP level and in excess of 15000 psi (but well below the 30,000 psi or so required for effective HIP per se) , all within less than a few hours from sinter cycle initiation. Then such high temperature and pressure are held for one one half to several hours; then both are released to ambient conditions. The FIGS. 1 and 5 profiles are more typical of a HIP cycle per se, resulting from initiating HIP too soon in relation to sinter heating.

Pressurization should be initiated at any schedule that allows significant sinter bonding or prereaction of additives to precede it. This initial sinter bonding achieves some densification and an essentially closed pore condition of the compact (at least at surface layers thereof) to avoid penetration by pressurized gas during the later onset of HIP processing. The sintering continues under assistance of simultaneous raising of both temperature and external gas pressure to further densify the compact as both temperature and pressure rise. The plateau hold pressure of HIP process during simultaneous sinter is lower than would be required in the absence of concurrent sintering.

A liquid or otherwise viscous phase is formed in the initial pre-HIP sintering, at grain boundaries within the compact, as temperature raises and is present as HIP pressure is applied to enhance the effective densification of the compact by such pressure through a combination of classic sinter and HIP densification mechanisms and a unique synergistic effect occurring upon proper sinte r/HI P conditions.

FIGS. 2 - 4 processing results in substantially higher final densities of the compact than processing in accordance with FIGS. 1 and 5, and at half the pressure and in less time at high pressure. Graph 4A shows a proposed cycle for high pressure reactive sintering in accordance with the invention.

The materials treated in the FIGS. 1 - 5 processing comprised 90-95%:0-3%:6-8% (weight percent mixture of silicon ni tride (Si₃ N₄) : yttria (Y₂O₃ ) : alumina (Al₂O₃) , respectively. Pressures on the order of 15000 psi are applied for the HIP portion of the cycle. The additional ingredient (s) effects appear to include enhancing response of the basic Si₃N₄ to the sinter HIP/processing conditions. The use of a reactive gas (e.g., N₂ as applied to Si3N₄ systems) also appears to enhance sinter/HIP synergy.

The mixtures are at less than 50% of theoretical density as initially poured into a container and tamped. They are then, slip cast, injection molded, die pressed or cold isostatic pressed to coherent compacts of 55-65% theoretical density. Sinter/HIP processing as described above achieves over 98% of theoretical density of such compacts.

The short time at elevated temperature and rapid cool down and simultaneous depressurization limits grain growth in the compact to micron size.

The data in Table I below relates to FIGS. 1 through 5. The cycles of FIGS. 1 and 5 respectively are conventional HIP cycles which as can be seen from the data were not effective in fully densifying either green or sintered compacts. On the other hand, the FIG. 2 cycle resulted in some apparent improvement in the density of previously sintered compacts. Cycles of FIGS. 3 and 4 are particularly effective in achieving near theoretical density for previously sintered compacts of all compositions containing Al₂O₃. For green samples subjected to cycles of FIGS. 3 and 4 , only those compacts with greater than 3% AI₂O₃ achieved densities near theoretical. For all of the work reflected in the data re FIGS. 1 through 5, there was no attempt to presinter the green compacts, the main thrust in these experiments was to densify the previously sintered compacts by raising the pressure after the compacts were heated to the HIP'ing pressure. In this way, the grain boundary phases would be softened and more likely to yield under pressure according to a then goal of the work.

DENSITY g/cm³

Sintex/HIP or

Composition HIP Cycle Green Sintered

95% Si₃N₄ + 5% Y₂O₃ D 2.12 2.63

92% Si₃N₄ + 8% Y₂O₃ C 2.13 2.63

92% Si₃N₄ + 8% Y₂O₃ D 2.15 2.55

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ B 1.75 3.12

92% Si₃N₄ + 6% Y₂O₃ + 2% Al₂O₃ B 1.83 -

92% Si₃N₄ + 6% Y₂O₃ + 2% Al₂O₃ C 1.85 -

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ D 1.85 -

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ A 2.07 -

92% Si₃N₄ + 6% Y₂O₃ + 2% Al₂O₃ A 2.12 3.19

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ B 2.10 3.18

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ C 2.10 3.08

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ D 2.13 3.09

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ E 2.13 3.09

92% Si₃N₄ + 6% Y₂O₃ + 2% Al₂O₃ C 2.12 3.18

92% Si₃N₄ + 6% Y₂O₃ + 2% Al₂O₃ D 2.15 3.17 DENSITY g/cm³

Sintex/HIP or

Composition HIP Cycle Green Sintered

92% Si₃N₄ + 6% Y₂O₃ + 2% AI2O3 D 2.12 3.11

92% Si₃N₄ + 6% Y₂O₃ + 2% Al₂O3 A - 3.09

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ B - 3.08

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ C - 3.04

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ D - 3.04

92% Si₃N₄ + 6% Y₂O₃ + 2% AI2O3 E - 3.10

91% Si₃N₄ + 6% Y₂O₃ + 3% AI₂O₃ C 2.12 -

91% Si₃N₄ + 6% Y₂O₃ + 3% AI2O3 D 2.12 -

89% Si₃N₄ + 8% Y₂O₃ + 3% AI₂O₃ D 2.10 3.23

89% Si₃N₄ + 8% Y₂O₃ + 3% AI₂O₃ ^C 2.11 -

89% Si₃N₄ + 8% Y₂O₃ + 3% AI₂O₃ D 2.11 -

88% Si₃N₄ + 8% Y₂O₃ + 4% Al₂O₃ A - 3.28

88% Si₃N₄ + 8% Y₂O₃ + 4% Al₂O₃ A 2.18 3.24

88% Si₃N₄ + 8% Y₂O₃ + 4% Al₂O₃ ^B 2.11 3.19

88% Si₃N₄ -1- 8% Y₂O₃ + 4% Al₂O₃ B - 3.27 DENSITY g/cm³

Sintex/HIP or

Composition HIP Cycle Green Sintered

88% Si₃N₄ + 8% Y₂O₃ + 4% AI₂P₃ C 2.21 3.16

88% Si₃N₄ + 8% Y₂O₃ + 4% AI₂O₃ D 2.12 3.09

88% Si₃N₄ + 8% Y₂O₃ + 4% Al₂O₃ D 2.12 3.03

88% Si₃N₄ + 8% Y₂O₃ + 4% AI₂O₃ D 2.12 3.24

88% Si₃N₄ + 8% Y₂O₃ + 4% AI₂O₃ E - 3.28

88% Si₃N₄ + 8% Y₂O₃ + 4% AI₂O₃ E 2.21 3.21

DENSITY g/cm³

Density Change

Composition HIP'ed g/cm³

95% Si₃N₄ + 5% Y₂O₃ 2.89 + 0.25

92% Si₃N₄ + 8% Y₂O₃ 2.70 + 0.12

92% Si₃N₄ + 8% Y₂O₃ 2.82 + 0.27

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ 3.18 + 0.06

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ 3.00 + 1.17

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ 2.60 + 0.75

92% Si₃N₄ + 6% Y₂O₃ + 2% Al₂O₃ 2.93 + 1.08

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ 2.90 + 0.83

92% Si₃N₄ + 6% Y₂O₃ + 2% Al₂O₃ 3.20 + 0.01

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ 3.21 + 0.03

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ 3.24 + 0.16

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ 3.25 + 0.16

92% Si₃N₄ + 6% Y₂O₃ + 2% AI2O3 3.12 + 0.03

92% Si₃N4 + 6% Y₂O₃ + 2% AI₂O₃ 3.27 + 0.09

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ 3.25 + 0.08 DENSITY g/cm³

Density

Change

Composition HIP'ed g/cm³

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ 3.27 + 0.17

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ 3.10 + 0.01

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ 3.18 + 0.10

92% Si3N₄ + 6% Y₂O₃ + 2% AI₂O₃ 3.25 + 0.21

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ 3.27 + 0.23

92% Si₃N₄ + 6% Y₂O₃ + 2% AI₂O₃ 3.12 + 0.02

91% Si₃N₄ + 6% Y₂O₃ + 3% AI₂O₃ 3.24 + 1.12

89% Si₃N₄ + 8% Y₂O₃ + 3% AI₂O₃ 3.27 + 0.04

89% Si₃N₄ + 8% Y₂O₃ + 3% AI₂O₃ 3.18 + 1.07

89% Si₃N₄ + 8% Y₂O₃ + 3% AI₂C₃ 3.17 + 1.06

88% Si₃N₄ + 8% Y₂O₃ + 4% AI₂O₃ 3.28 + 0.00

88% Si₃N₄ + 8% Y₂O₃ + 4% AI₂O₃ 3.24 + 0.00

88% Si₃N₄ + 8% Y₂O₃ + 4% Al₂O₃ 3.26 + 0.07

88% Si₃N₄ + 8% Y₂O₃ + 4% AI₂O₃ 3.27 + 0.00 DENSITY g/cm³

Density Change Composition HIP'ed g/cm³

88% Si₃N₄ + 8% Y₂O₃ + 4% AI₂O₃ 3.27 + 0.11

88% Si₃N₄ + 8% Y₂O₃ + 4% AI₂O₃ 3.24 + 0.15

88% Si₃N₄ + 8% Y₂O₃ + 4% AI₂O₃ 3.25 + 0.22

88% Si₃N₄ + 8% Y₂O₃ + 4% AI₂O₃ 3.26 + 0.02

88% Si₃N₄ + 8% Y₂O₃ + 4% AI₂O₃ 3.26 - 0.02

88% Si₃N₄ + 8% Y₂O₃ + 4% AI₂O₃ 3.24 + 0.03

The data in Table II below provides:

(1) Evidence of high pressure reactive sintering.

(2) Evidence of the need for a packing powder to attain densities near theoretical.

(3) Evidence that by extended prereacting/presintering, the composition 92% Si₃N₄-6% Y₂O₃-2% Al₂O₃ can be fully densified by high pressure reactive sintering. In Exhibit I, we could not densify from the green state.

(4) Evidence that the further increase in density of the 92-6-2 composition did not occur until the pressure was raised to 15,000 psi.

(5) Evidence that the total time at temperature for the complete cycle (2.75 hours) was by itself not sufficient to densify this composition either at 1750 or 1850° C.

(6) These results show that the final density is a function of starting powder and forming method. Die pressed and CIP'd compacts and injection molded and CIP's compacts attained high densities than injection molded except when the injection molder parts were HIP'd.

Table II presents data on sinter/HIP densification kinetics of 92-6-2 SiAlONs. Two types of specimen of each 92-6-2 composition were investigated in this kinetics study:

( 1) Specimens die pressed f rom powder and subsequently cold isostatically re-pressed.

(2) Injection molder 92-6-2 specimens: most of the specimens had been pre-sintered at 1300° C and contained microscopic surface cracks. Cold isostatic re-pressing of pre-sintered specimens did not heal the cracks, while the cracks in specimens which had not been pre-sintered appeared to heal with CIP'ing.

The following observations may be made regarding the data on Table II:

(a) The sintering (under ca. 200 psi) stage of the singer/HIP cycle did not result in densities greater than 3.00g/cc at both 1750 and 1850° C without the use of packing powders.

(b) The application of 15,000 psi N₂ appears to have the greatest influence on achieving densities above 3.00 g/cc without packing powders (high pressure reactive sintering effect) .

(c) LC-12 based 92-6-2 specimens sinter and sinter/HIP more readily than SN502 based 92-6-2 specimens.

(d) All pre-sintered injection molded specimens displayed marked cracking after high temperature processing, the cracks initially detected in the dewaxed, green specimens were exacerbated by sintering and sinter/HIP. Only those injection molded specimens which had been CIP'd immediately after dewaxing without pre-sintering did not crack when sinter/HIP' d.

TABLE II

SUMMARY OF RESULTS OF 92-6-2 SIAION

SINTER/HIP DENSIFICKPION KINETICS - 1st PAGE

Conditions A B C D E F

Sinter @ 1750° C. LC-12 CIP 2.50 2.70 2.62 2.70 and 200 psi N₂ for SN502 CIP 2.17 2.09 2.09 2.31

1 hour LC-12 IM 2.42 2.36 2.53 2.60

Sinter @ 1750° C. LC-12 CIP 2.79 2.72 2.76 2.80 and 200 psi N₂ for SN502 CIP 2.23 2.20 2.21 2.33

1.75 hours LC-12 IM 2.74 2.69 2.71 2.81

LC-12 IM-CIP 2.84 2.89 2.90 2.99

Columns : A = Si₃N₄

B = Forming Method

C = Density (g/cm³) with no packing powder

D = Density (g/cm³) with LC-12 packing powder

E = Density (g/cm³) with 92-6-2 packing powder

F = Density (g/cm³) with p 72 packing powder

Method Terms

CIP = die pressed cold isostatically re-pressed at

60 KPSI IM = injection molded and dewaxed IM+CIP= injection molded, dewaxed and cold isostatically re-pressed at 60 KPSI

40% IC-12 + 8% Y₂O₃ + 4% AI₂O₃ + 48% BN TABLE II SUMMARY OF RESULTS OF 92-6-2 SIAION

SINTER/HIP DENSIFICKPION KINETICS - 2nd PAGE

Conditions A B C D E F

Sinter @ 1750°C. LC-12 CIP 2.80 2.78 2.85 2.92 and 200 psi N₂ for SN502 CIP 2.34 2.28 2.65 2.39

2.75 hours LC-12 IM 2.75 2.69 2.76 2.88

Columns: A = Si₃N₄

B = Forming Method

C = Density (g/cm³) with no packing powder

D = Density (g/cm³) with LC-12 packing powder

E = Density (g/cm³) with 92-6-2 packing powder

F = Density (g/cm³) with p 72 packing powder

Method Terms

CIP = die pressed cold isostatically re-pressed at

60 KPSI IM = injection molded and dewaxed IM+CIP= injectionmolded, dewaxed and cold isostatically re-pressed at 60 KPSI

SINTER/HIP DENSIFICKPION KINETICS - 3rd PAGE

Conditions A B C D E F

Sinter @ 1750° C. LC-12 CIP 2.80 2.78 2.84 2.92 and 200 psi N₂ for SN502 CIP 2.33 2.31 2.63 2.55

1 hour; pressurize to LC-12 IM 2.75 2.68 2.76 2.88

15 KPSI N₂ in 0.75 hour while maintaining 1750° C ; terminate cycle when pressure reached 15 KPSI N₂

Columns: A = Si₃N₄

B = Forming Method

C = Density (g/cm³) with no packing pcwder

D = Density (g/cm³) with LC-12 packing powder

E = Density (g/cm³) with 92-6-2 packing powder

F = Density (g/cm³) with p 72 packing powder

Method Terms

CIP = die pressed cold isostatically re-pressed at

60 KPSI IM = injection molded and dewaxed IM+CIP= inj ection molded, dewaxed and cold isostatically re-pressed at 60 KPSI

SINTER/HIP DENSIFICKPION KINETICS - 4th PAGE

Conditions A B C D E F

Sinter @ 1750° C. LC-12 CIP 3.13 3.11 3.15 3.23 and 200 psi N₂ for SN502 CIP 2.59 2.56 2.68 2.73

1 hour; pressurize to LC-12 IM 2.86 2.82 2.87 2.95

15 KPSI N₂ in 0.75 hour while maintaining

1750° C. ; HIP @ 1750°

C. and 15 KPSI for 1 hour

Columns: A = Si₃N₄

B = Forming Method

C = Density (g/cm³) with no packing powder

D = Density (g/cm³) with LC-12 packing pcwder

E = Density (g/cm³) with 92-6-2 packing pcwder

F = Density (g/cm³) with p 72 packing pcwder

Msthod Terms

CEP = die pressed cold isostatically re-pressed at

60 KPSI IM = injection molded and dewaxed IM+CIP= injectionmolded, dewaxedand cold isostatically re-pressed at 60 KPSI 40% IC-12 + 8% Y₂O₃ + 4% AI₂O₃ + 48% BN TABLE II SUMMARY OF RESULTS OF 92-6-2 SIAION

SINTER/HIP DENSIFICKPION KINETICS - 5th PAGE

Conditions A B C D E F

Sinter @ 1850° C. LC-12 CIP 2.87 2.87 2.92 3.01 and 200 psi N₂ SN502 CIP 2.56 2.50 2.61 2.82 for 1 hour LC-12 IM 2.88 2.86 2.89 2.95

Sinter @ 1850° C. LC-12 CIP 2.95 2.90 2.98 3.10 and 200 psi N₂ SN502 CIP 2.71 2.67 2.83 2.87 for 1.75 hours LC-12 IM 2.88 2.87 2.93 3.02

Columns: A - Si₃N₄

B = Forming Method

C = Density (g/cm³) with no packing pcwder

D = Density (g/cm³) with LC-12 packing powder

E = Density (g/cm³) with 92-6-2 packing powder

F = Density (g/cm³) with p 72 packing pcwder

Method Terms

CTP = die pressed cold isostatically re-pressed at

40% IC-12 + 8% Y₂O₃ + 4% AI₂C₃ + 48% BN TABLE II SUMMARYOF RESULTS OF 92-6-2 SIAION

SINTER/HIP DENSIFICKPION KINETICS - 6th PAGE

Conditions A B C D E F

Sinter @ 1850°C. LC-12 CIP 3.20 3.17 3.18 3.25 and 200 psi N₂ SN502 CIP 2.89 2.78 2.87 2.93 for 1 hour; pressurize to 15 KPSI N₂ in 0.75 hour while maintaining

1850° Co ; HIP @ 1850°

C. and 15 KPSI for 1 hour

Columns: A = Si₃N₄

B = Forming Method

C = Density (g/cm³) with no packing powder

D = Density (g/cm³) with LC-12 packing powder

E = Density (g/cm³) with 92-6-2 packing powder

F = Density (g/cm³) with p 72 packing powder

Method Terms

CTP = die pressed cold isostatically re-pressed at

60 KPSI IM = injection molded and dewaxed IM+CIP= injectionmolded, dewaxedand cold isostatically re-pressed at 60 KPSI

40% IC-12 + 8% Y₂O₃ + 4% Al₂O₃ + 48% BN FIG. 6 shows in block diagram form the process steps of compacting, heating, sinter/HIPing and release of temperat and pressure. These include more specifically:

10-powder additives addition and mixing;

20-mixing of ceramic powder components and bind usually a wax;

30-compact forming by cold isostatic pressing, die pressing or slip casting;

40-dewaxing or other debinderization w ithin or outside the HIP furnace;

50-optional cold isostatic pressing (CIP) at conditions of 40,000 - 200,000 psi, preferably 60,000 psi for defect healing if necessary.

60-(initial) sinter: i.e., temperature rise in a closed chamber furnace established by radiant heaters facing the containerless compact on a pedestal therein -- to achieve a closed surface, i.e., non-porosity of the surface regions of the compact and further a densification to 90-95% of theoretical indicative of sufficiently advancing sinter to allow the next steps;

70-sinter/HIP (S/HIP) through increasing gas pressure in the chamber such gas pressure being applied di rectly to the compact or indi rectly via a sleeve in turn surrounding and containing loose ceramic powder which surrounds the compact;

80-cooling; and

90-pressure drop (depressurizing) .

FIG. 7 shows in block diagram form a variant of the FIG. 6 processing with the same steps 10 - 50 and 80, 90, but with variant steps:

60' - presintering at high enough temperature (1500 - 2000º C) for some sinter bonding, but leaving internal porosity connected, to achieve 78 - 85 percent of theoretical density, preferrably at least 80; and 70' - raising pressure to a level of 3000 - 30,000 psi and holding temperature to achieve target densification.

The mechanism in this instance emphasized reactions occurring at high pressure under a set of reaction kinetics conditions for more favorable than low pressure. Reactive atmosphere and/or additives in the compact provide the necessary reactive feedstock.

In the steps 70 or 70 ' of FIG S. 6 and 7. respectively, it may be useful to induce a thermal spike, a brief increase of the usual S/HIP temperature of about 2000º C held for an hour or so, by at least 20%, e.g. to 2400º for no more than 20% of the plateau hold time (e.g. for 6 - 12 minutes) . This enables effective sinter/HIP of materials that are otherwise insufficiently responsive to the process. The temperature spike condition is easily achieved in the sinter/HIP furnace.

The materials treated through such processing may include those noted above and, additionally

silicon carbide titanium carbide zirconia

tungsten carbide aluminum nitride partially stablized zirconia

zirconium diboride alumina boron carbide

boron nitride titanium diboride

and mixtures of such ceramics with each other and/or preferrably with minor ingredients such as yttria, alumina, silica and various rare earch oxide and precious metal oxide additives which depress the temperature (compared to the major component(s) at which grain boundary liquid phases form. The ceramics may be in particulate forms ranging from low aspect ratio (near spherical) to high aspect ratios (essentially in whisker fibrous or platelet forms).

The invention is preferrably practiced in a single chamber. But elevated temperature and/or pressure processing can be interrupted ty transfer between chambers, provided the drop of temperature or pressure is no more than 5% from the last achieved value.

It will now be apparent to those skilled in the art that other embodiments, improvements, details, and uses can be made consistent with the letter and spirit of the foregoing disclosure and within the scope of this patent, which is limited only by the following claims, construed in accordance with the patent law, including the doctrine of equivalents.

What is claimed is:

Claims

1. Method of densifying silicon nitride-base, mixed powder, ceramic systems to at least 95% of theoretical density at near net shape comprising the steps of:

(a) pre-compacting the powders to a near net shape,

(b) heating the compact so formed in a closed chamber to a level held for sufficient time to achieve a non-porous sinter-bonded structure at a surface layer of the compact,

(c) then without dropping said temperature by more than 5% (and preferrably while increasing the same) , admitting gas to said chamber at a pressure at which the compact is hot-isostatically pressed without significant shrinkage or crack formation and thereby densified at 95%+ of theoretical,

(d) terminating heat soon enough to limit grain growth to micron size range and terminating pressurization.

2. Method in accordance with claim 1 wherein the initial sinter heating (b) is carried out in a reactive ambient of nitrogen, the reactive ambient being maintained as pressurization (c) is applied.

3. Method in accordance with claim 1 wherein the initial compacting step (a) comprises initial consolidation followed by cold isostatic pressing the compact to heal defects in the green body.

4. Method in accordance with claim 3 wherein the consolidation comprises cold isostatic repressing.

5. Method in accordance with claim 1 wherein the silicon-nitride-based system comprises as a first major component a mixture of at 90-95 weight percent Si₃N₄, 4.5 to 9.5 percent aluminum oxide and balance impurities and as a second major component a material selected from the group consisting of boron nitride, yttrium oxide, zirconium dioxide and magnesium oxide and mixtures thereof.

6. Method in accordance with claim 5 wherein the Si₃N₄:Al₂O₃ weight ratio is in excess of 98:2 and the first:second component ratio is at least 20:1.

7. Method in accordance with claim 1 wherein the compact is maintained packed in powder during the steps (b) , (c) to suppress SiO volatilization losses, the packing powder being selected from the group consisting of silicon nitride, aluminum oxide, yttrium oxide, boron nitride, magnesium oxide and mixtures thereof.

8. Method of densifying silicon nitride-base, mixed powder, ceramic systems to at least 95% of theoretical density at near net shape by a high pressure reactive sintering process comprising the steps of:

(a) pre-compacting the powders to a near net shape,

(b) pre-heating the compact so formed in a closed chamber to a level held for sufficient time to achieve an 80%+ of theoretical density sinter-bonded structure with substantial retained intra-compact porosity

(c) then w ithout dropping said temperature by more than 5% (and preferrably while increasing the same) , admitting gas to said chamber at a pressure at which the compact is pressed without significant shrinkage or crack formation and thereby densified at 95%+ of theoretical, with the aid of continuing reaction mechanisms,

9. Method in accordance with claim 8 wherein the initial sinter heating (b) is carried out in a reactive ambient of nitrogen, the reactive ambient being maintained as pressurization (c) is applied.

10. Method in accordance with claim 8 wherein the initial compacting step (a) comprises initial consolidation followed by cold isostatic pressing the compact to heal defects in the green body.

11. Method in accordance with claim 8 and further comprising the addition of a thermal spike process enhancement to step (c) wherein for up to 20% of the step (c) time, temperature is raised to at least 120% of the average value of the step (c) temperature.