Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-08-28
2004-02-17
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S699000, C428S701000, C428S702000, C428S410000, C428S384000, C428S386000, C428S428000, C427S255130, C427S255360, C427S452000, C427S456000, C076S107100
Reexamination Certificate
active
06692844
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a glaze for coating ceramic superplastic forming (SPF) dies to provide a hard forming surface to increase life.
BACKGROUND OF THE INVENTION
Plasma spraying is becoming more widely accepted for industrial use. The plasma spraying process consists of introducing a powder carried by a gas into a high temperature flow of ionized gas. The powder particles are subsequently melted or partially melted and propelled towards a substrate by the carrier gas. Upon impact with the target surface, the molten particles form lamellae or splats that adhere to the substrate by mechanical bonding to surface imperfections. Many metal, ceramic, and polymer powders are commercially available for plasma spraying and other thermal spray processes. Commercial powders are often spherical or semi-spherical in shape and within a specific size range, primarily to allow for good flowability or feeding characteristics. Plasma spraying can deposit a wide range of materials onto a wide range of substrates. Nearly any material that can be produced in powder form can be deposited by plasma spraying.
Plasma spray coatings made from zircon (ZrSiO
4
) produced either tetragonal or monoclinic zirconia (ZrO
2
) or amorphous silica (SiO
2
). The tetragonal coating particles must have been heated above about 1200° C. while the monoclinic particles never reached 1200° C.
Water-stabilized plasma (WSP) spraying of angular garnet (crystalline) and basalt (amorphous) powders in the range of 56 to 200 &mgr;m found that the sprayed materials were well spherodized, lower in silicon and alkali content, and in the amorphous state. The extraordinarily high temperatures (>10,000° K) reached in such a plasma probably allowed for melting and spheroidization of the powders while volatilizing some constituents. The amorphicity of the coatings was likely influenced by the rapid solidification of the splats, estimated to be on the order of 10 &mgr;s. Most coatings were found to have extremely low spherical or pseudospherical porosity, which ranged from 4.6% to less than 2%, which indicated that the glassy state of the impacting particles allowed for high flowability. The residence time of the powders in the plasma was the dominant factor with respect to coating quality while the spraying distance was determined to be of much less consequence.
Another study focused on the adhesion of plasma sprayed borosilicate glass as a function of the substrate (steel) preheat temperature. Coating thicknesses of 200 to 800 &mgr;m with approximately 5% porosity were obtained by feeding extremely angular powder. Substrate preheat temperatures ranged from 50° C. to 700° C. The plasma plume was scanned across the substrate surface prior to introducing powder into the system. The coating adhesive strength increased with substrate preheating up to T
g
of the glass.
The substrate temperature greatly influenced the adhesion of such coatings by allowing the molten glass particles to remain less viscous for a longer time. From analysis of coating morphologies, the lamellae were better able to completely fill surface anomalies and to bond better mechanically to the substrate. Assuming that the lamellae have a much greater diameter than thickness, a one-dimensional heat conduction model for the coating is described by:
k
/(&rgr;·
C
p
)·∂
2
T/∂x
2
=∂T/∂t
(1)
where T is the coating temperature at a distance x from the substrate at a time t and k, p, and C
p
are the thermal conductivity, density, and specific heat of the droplet. The presumption that preheating the substrate yields increased adhesion is strengthened by finding that greater deposition efficiency, or splat retention, was also obtained. Not only does substrate preheating allow for better glass flow over the substrate surface, but lower thermal gradients and tensile stresses arise across the thickness of the cooling splats.
A simplistic account of the maximum stress, &sgr;
c
, that arises in such a coating is depicted by the relationship:
&sgr;
c
)=&agr;
c
·(
T
g
−T
b
)·
E
c
(2)
where &agr;
c
, T
g
, and E
c
are the coefficient of expansion, glass transformation temperature, and elastic modules of the glass, respectively, and T
b
is the substrate preheat temperature.
Rare-earth alumino-silicates, yttria-alumino-silicates, and the addition of lanthanides to more common glass systems have also been studied. Yttrium is not a rare-earth, but closely emulates true rare-earths when included in alumino-silicates. Rare-earth alumino-silicate glasses are of both scientific and industrial interest due to their relatively high T
g
, high refractive indices (n
d
), high hardness, high elastic moduli, chemical durability, and moderate thermal expansion.
Rare-earth ions do not play a primary role in glass formation, but significantly modify the properties of traditional glasses. Lanthanum additions (as the oxide) to sodium silicate glasses account for increased T
g
, n
d
, and density (&rgr;) while lowering the thermal expansion and electrical conductivity. While lanthanum additions increase T
g
regardless of whether soda or silica is replaced, the coefficient of thermal expansion is more markedly reduced by replacement of soda. The mechanism may involve each trivalent lanthanum ion acting as a modifier by creating three nonbridging oxygens, thus explaining the increase in T
g
. Another option assumes that lanthanum enters a ‘network’ site, likely octahedral rather than tetrahedral, because of its large ionic radius, and simultaneously increases the connectivity of the structure while decreasing the concentration of nonbridging oxygens.
The maximum solubility limit of rare-earth ions that can be incorporated into most rare-earth alumino-silicate glass structures increases with decreasing atomic radii and decreasing atomic number from La to Yb. This trend is a function of lanthanide contraction. In the system xLa
2
O
3
-25Al
2
O
3
-(75-x)SiO
2
, devitrification begins between 25 and 30 mole % La
2
O
3
. All glass compositions are given in mole ratio form unless otherwise noted. Glass transformation temperature, thermal expansion, and n
d
have all been found to be dependent upon ionic radius (in the system 20La
2
O
3
-20Al
2
O
3
-60SiO
2
). The transformation temperature increased with decreasing ionic radius while both thermal expansion and n
d
decreased. These property variations apparently are at least partially the result of rare-earth ion field strength rather than ion size alone.
Although we evaluated only one glass composition (8.25La
2
O
3
-19.25Al
2
O
3
-72.5SiO
2
), Beinarovich et al. discovered that glass formed at approximately 1290° C. in the synthetic batch, about 150° C. lower than in the traditional oxide batch. “Traditional oxide batch” refers to glasses melted from fine raw material oxides while the “synthetic batch” refers to ions suspended in solution. The synthetic batch has higher constituent dispersion and begins with amorphous components.
Karlsson conducted a three-part study on the crystallization behavior of various La
2
O
3
—Al
2
O
3
—SiO
2
glasses and measured some physical properties of the glassy state. The glasses of the first study melted at 1500° C. and were cooled slowly to room temperature, after which large, white crystals were observed within the glass. X-ray analysis showed these crystals to be orthorhombic La
2
O
3
-2SiO
2
grown along the a-axis. From the X-ray patterns the lattice parameters of the devitrified phase were determined as a=13.15 Å, b=10.15 Å, and c=8.64 Å. A density of 2.58 g/cm
3
was calculated for this phase assuming four molecules per unit cell. Its melting temperature was observed to be 1420±10° C.
The second Karlsson study involved devitrification products of glasses slightly higher in alumina content. These glasses were heat treated at 950° C. and 1200° C. after which small, white, crystalline whiskers were observed in the glass. X-ray patterns of these crystals showed the compound to be
Miller F. Scott
Peterson Martin A.
Sanders Daniel G.
Smith Jeffrey D.
Van Aken David C.
Hammar John C.
Jones Deborah
Koppikar Vivek
The Boeing Company
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