Ceramic bodies having a plurality of stress zones

Claim 2

A green ceramic body having at least two compositional zones comprising: a first substantially continuous compositional layer composed at least in part of sinterable particles of magnesia; and a second substantially continuous compositional layer substantially coextensive and contiguous with said first layer, said second layer composed at least in part of sinterable magnesia particles wherein at least a portion of the ceramic particles in said second layer are of a composition different from that of the first layer whereby the particles of the second layer, upon sintering and cooling, tend to assume a greater bulk volume than the particles of the first layer, thereby occasioning a compressive stress in said second layer and a tensile stress in said first layer, and said first and second layers contain a sintering aid selected from the class of metal oxides.

Claim 3

The green ceramic body of claim 1 wherein said first layer is interposed between two second layers.

Claim 4

The green ceramic body of claim 1 wherein the composition difference between said first and second layers is a chemical compositional difference.

Claim 5

The green ceramic body of claim 1 wherein the composition difference between said first and second layers is a particle size difference in certain particles susceptible to a phase transformation.

Claim 6

The green ceramic body of claim 3 wherein a significant portion of the sinterable particles of said second layer have the same chemical composition as the particles of said first layer.

Claim 7

The green ceramic body of claim 1 wherein said body is a solid rod, a flat plate, or a hollow tube.

Claim 8

The green ceramic body of claim 1 wherein said second layer contains zirconia or hafnia particles of a crystal phase susceptible of transformation to a crystal phase having a larger volume.

Claim 9

A sintered magnesia ceramic body consisting of two contiguous, coextensive stress layers with each layer having a common interface with the other and with each layer having a surface which is an external surface of the body, one of said layers being in compression and the other layer in tension.

Claim 10

A sintered magnesia ceramic body consisting of two contiguous, coextensive stress layers with each layer having a common interface with the other and with each layer having a surface which is an external surface of the body, one of said layers being in compression and the other layer in tension.

Claim 11

A sintered magnesia ceramic body having more than two contiguous, coextensive stress layers with each layer having a common interface with another layer and two layers having surfaces which are external surfaces wherein at least one of said layers is in compression and at least one of said layers is in tension.

Claim 12

A sintered magnesia ceramic body having more than two contiguous, coextensive stress layers with each layer having a common interface with another layer and two layers having surfaces which are external surfaces wherein at least one of said layers is in compression and at least one of said layers is in tension.

Claim 15

The process of claim 10 wherein at least a significant portion of the sinterable particles in said second layer has the same composition as the particles of said first layer.

Claim 16

The process of claim 10 wherein the sinterable particles of said first layer are of substantially the same composition.

Claim 17

The process of claim 10 wherein said first layer comprises sinterable particles of alumina of which at least a portion of said particles are sinterable magnesia.

Claim 18

The process of claim 13 wherein said second layer has substantially the same composition as said first layer plus zirconia or hafnia particles susceptible of phase transformation to a phase having an increased volume.

Claim 19

The process of claim 10 wherein one said layer is in substantial contact with two of said other layers.

Claim 20

A process for forming a sintered ceramic body having at least one compressive stress zone and at least one tensile stress zone comprising: forming a first layer of sinterable ceramic particles of a composition which undergoes no crystal phase change over a temperature range of about room temperature to the sintering temperature of said particles, wherein at least a portion of said particles are sinterable magnesia; forming in contact with a surface of said first layer a second layer of sinterable ceramic particles having therein at least a minor portion of ceramic particles which are of a composition which form more than one crystal phase over a temperature range of about room temperature to about the sintering temperature of same, whereby a crystal phase formed upon cooling of said sintered particles to room temperature has a bulk volume which is substantially different from the bulk volume of another phase existing at a temperature significantly above room temperature; forming a green body of said first and second layers; sintering said green body to form a sintered body; and cooling said sintered body at a rate such that crystals in said second layer transform to a different crystal structure occupying a different volume than the predecessor crystals.

Claim 21

A process for forming a sintered ceramic body having at least one compressive stress zone and at least one tensile stress zone comprising: forming a first layer of sinterable ceramic particles of a composition which undergoes no crystal phase change over a temperature range of about room temperature to the sintering temperature of said particles, wherein at least a portion of said particles are sinterable magnesia; forming in contact with a surface of said first layer a second layer of sinterable ceramic particles having therein at least a minor portion of ceramic particles which are of a composition which form more than one crystal phase over a temperature range of about room temperature to about the sintering temperature of same, whereby a crystal phase formed upon cooling of said sintered particles to room temperature has a bulk volume which is substantially different from the bulk volume of another phase existing at a temperature significantly above room temperature; forming a green body of said first and second layers; sintering said green body to form a sintered body; and cooling said sintered body at a rate such that crystals in said second layer transform to a different crystal structure occupying a different volume than the predecessor crystals.

Claim 22

The process of claim 16 wherein said composition of said second layer is of a different chemical composition than the first layer whereby said first layer contains a chemical ingredient which retards crystal phase change of certain components within said composition of said first layer.

Claim 23

The process of claim 16 wherein said composition of said second layer is of a different physical composition than said first layer whereby said second layer contains particles having a crystal phase susceptible of transforming to a larger crystal phase, said particles of said second layer being sufficiently larger than the particles of said first layer to obtain phase transformation to a significant extent in said second layer.

Claim 24

The process of claim 17 wherein said second layer contains zirconia or hafnia particles susceptible of transformation to a larger crystal form.