Compositions: ceramic – Ceramic compositions – Carbide or oxycarbide containing
Patent
1994-11-24
1997-08-12
Marcantoni, Paul
Compositions: ceramic
Ceramic compositions
Carbide or oxycarbide containing
501 89, 501 953, 501 961, 501 972, 501 984, 264640, 264676, 264119, 264120, C04B 3576
Patent
active
056565615
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a pressureless sintering process for forming whisker-reinforced alumina-based ceramic bodies having excellent physical properties.
BACKGROUND OF THE TECHNOLOGY
Whisker-reinforced alumina has found use in structural applications such as heat engines and turbine blades (see, U.S. Pat. No. 4,543,345) and in more severe applications such as cutting tools (see, U.S. Patent Nos. 4,789,277 and 4,961,757). These composites can have up to about 40 vol. % whiskers using a processing technique known as "hot pressing" which produces composites having a density of at least greater than 98% of theoretical. A density of at least about 95% is needed for virtually all engineering applications of ceramics.
In the conventional processes for making whisker-reinforced alumina bodies, hot pressing can also be described as "pressurized sintering" because the process involves the application of uniaxial loads of about 3500-5500 psi. In the process, a powder blend or preformed sample is loaded in a free flowing form into a graphite die and sintered under an inert atmosphere in a high temperature furnace. Pressure is applied to the powder in the die with a hydraulic ram as the furnace temperature is increased to about 1450.degree.-1850.degree. C. and maintained at temperature for an appropriate time. The furnace is then allowed cool. Cycle times of several hours are used, and the process is limited to the production of discrete batches.
The structural limits of the dies generally prohibit the use of applied pressures greater than about 5500 psi. In addition, the hot pressing method generally limits the shape of the formed body to simple geometric shapes such as round or rectangular plates or cylinders unless special complex dies and pressure rams are constructed at great expense. In addition, the properties of such hot pressed bodies are more anisotropic due to the uniaxial loading than those prepared by the pressureless sintering.
Hot pressing is to be contrasted with "green" preform body preparation, "pressureless sintering" and "hot isostatic pressing" (HIPing). Green preform bodies are prepared by mixing ceramic powder blends with conventional sintering aids (e.g., magnesia and/or rare earth oxides) and an organic binder containing a lubricant. The green preform body is then formed by compacting the powder blend at ambient temperature to a density of about 60-70% of theoretical. The theoretical density of a material is the density calculated from the number of atoms per unit cell and the measurement of the lattice parameters. Generally, green bodies are formed as simple geometrical shapes (e.g., a tube, cylinder, or disk) and, if necessary, machined to the desired shape allowing for shrinkage which will occur during subsequent heating steps (i.e., near-net-shape fabrication). Binder material is removed by heating the shaped green body in air in an oven at temperatures up to 500.degree. C. for about 10-15 hours depending on size and shape.
A green preform body is further densified by pressureless sintering in a furnace at 1450.degree.-1850.degree. C. under an atmosphere (e.g., argon or nitrogen) that does not adversely affect either the composite being sintered or the sintering furnace components. The sintering is allowed to continue until essentially complete (generally greater than 94% to 95% theoretical density). If the resulting sintered body forms a closed cell structure (closed porosity), the density may be further increased by hot isostatic pressing.
Hot isostatic pressing (HIPing) is the process of applying high pressure to a sintered body with inert gas typically at 15,000-30,000 psi for about 1-2 hours at a temperature of from about 1500.degree. to about 1700.degree. C. (for alumina oxide) with the goal of producing a body having greater than about 98% theoretical density. The sintered body to be HIPed must exhibit almost completely closed cell structure. More than about 95% of the pores must be closed for HIPing to have a significant effect.
The concentration of clos
REFERENCES:
patent: 4745091 (1988-05-01), Landingham
patent: 5389586 (1995-02-01), Rogers et al.
Rhodes James F.
Rogers William M.
Advanced Composite Materials Corporation
Marcantoni Paul
LandOfFree
Pressureless sintering of whisker reinforced alumina composites does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Pressureless sintering of whisker reinforced alumina composites, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Pressureless sintering of whisker reinforced alumina composites will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-160195