Hot gas wetted turbine blade

A turbine blade adapted for use with hot gases comprising a radially inward portion of metal including a core projecting radially outwards on which is supported a ceramic portion of airfoil section enclosing the core. The inner end of the ceramic portion forms a continuous surface contour with the metal inward portion. The ceramic portion extends no more than one-half of the total span of the blade and, preferably, about one-third of the blade span. In a particular embodiment, the wall thickness of the ceramic portion can increase in an radially outwards direction.

FIELD OF THE INVENTION 
This invention relates to a turbine blade which is adapted for being wetted 
with a hot gas and which includes metal and ceramic blade portions. 
PRIOR ART 
German Patent Specification No. 736,958 discloses a gas turbine blade 
having a supporting body of steel and a sleeve of ceramic material, the 
ceramic sleeve extending over the entire blade span and being retained at 
its outer end by means of a retaining plate arranged at the head end of 
the supporting body. Similar composite metal and ceramic blades have been 
disclosed in German Pat. No. 848,883 and U.S. Pat. No. 2,479,057. 
It has been found that blades of this type can develop cracks in the 
ceramic sleeve especially when used as rotor blades in high-speed rotors. 
Such cracks, which lead to subsequent destruction, are attributed to 
substantial compressive stresses arising in the ceramic airfoil at high 
speeds, where said stresses exceed the safe surface compression limits. 
The fracture of ceramic sleeves, when occurring in service, produces 
considerable consequential damage to the turbines. Another problem in the 
known turbine blades is satisfactory retention of the ceramic sleeve in 
the head region of the metal supporting core. The longer the ceramic 
sleeve, the greater the resulting centrifugal forces, and the more 
reliable the retention means at the head of the supporting core must be, 
which requirement often is not sufficiently satisfied for reasons of space 
limitations. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a turbine blade including 
metal and ceramic portions which has high durability for a considerable 
blade span and high centrifugal loads without sacrificing ceramic airfoils 
known for their resistance to high temperature and erosion. 
In accordance with the above and further objects of the invention, there is 
provided a turbine blade adapted for use with hot gases comprising a 
radially inward portion constituted of metal including a metal core 
projecting radially from the inward portion on which a ceramic portion of 
airfoil section is supported to enclose the metal core. 
In this way, the outer portion of the blade is constituted of ceramic 
material and the ceramic portion extends radially less than one-half of 
the total span of the blade and, perferably, about one-third of the blade 
span. 
The reduction in the length of the ceramic airfoil achieved by the present 
invention correspondingly reduces the ceramic mass and, thus, the 
centrifugal forces imposed thereon. This alleviates surface pressures in 
the area of contact between the ceramic airfoil and the supporting core, 
and it reduces the load on the supporting core. The reduction in ceramic 
airfoil compression considerably reduces the risk of cracking and 
subsequent fracture thereof. 
In other respects, the construction in which metal is used for the radially 
inward portion of the blade is acceptable for most applications, since all 
particulate matter in the gas stream will be found in the radially outer 
region of the gas stream, and it is only there that the high resistance to 
erosion afforded by the ceramic material is truly indispensable. 
A blade construction in accordance with the present invention will give 
improved operational reliability to turbomachines also in the case of 
blade fracture, because the small size of the broken component, i.e. the 
ceramic airfoil, will limit the consequential damage inflicted on 
adjacent, especially downstream components. Since a portion of the turbine 
blade is of metal, e.g. steel, some degree of serviceability will still be 
retained even in the event that the ceramic airfoil has been destroyed. 
This will prevent a complete and immediate loss of turbine power. 
According to another advantageous aspect of the present invention, the 
length of the radially outward, ceramic airfoil occupies no more than 
one-half of the full blade span, and, in a preferred arrangement, the 
ceramic airfoil occupies about one-third of the blade span. 
For reasons of good design, the supporting core has, as in a previously 
disclosed arrangement, a mushroom-shaped head at its radially outward end 
for support of the ceramic airfoil.

DETAILED DESCRIPTION 
With reference now to FIG. 1, therein is seen a turbine blade which 
comprises a radially inward portion 1 including blade root 5 which is 
constituted of a metal material such as steel. Integrally formed with the 
radially inward portion 1 of the blade is a single-piece, radially 
outwardly extending contiguous supporting core 2 having a mushroom head 4. 
The mushroom head 4 of the supporting core 2 provides end support for a 
ceramic airfoil portion 3 which encircles core 2. The airfoil portion 3 
has a radially inward end which abuts against the portion 1 and the outer 
surface of the airfoil portion 3 forms a continuous surface contour with 
portion 1. The ceramic airfoil portion 3 has a radial length which is not 
greater than one-half the span h of the glade and is preferably one-third 
the span h of the blade. 
The blade illustrated in FIG. 2 is essentially of the same construction as 
that in FIG, 1, except that the wall thickness of the ceramic airfoil 
portion 3a increases in a radially outward direction towards the end of 
the blade. This enables the supporting core 2a of the blade to be tapered 
in a radially outward direction as illustrated in FIG. 2. In an 
alternative arrangement (not shown) a gap narrowing in a radially outward 
direction can be provided between the ceramic airfoil 3a and a 
constant-section supporting core. The arrangement illustrated in FIG. 2 is 
of particular advantage when the blade surface is subject to aggravated 
erosive wear as in the case of pulverized-coal engines. Aggravated erosion 
will wear away material especially in the radially outward region of the 
ceramic airfoil surface. 
The added thickness of wall section in this region will provide a 
sufficient amount of material over an extended service period. 
As seen from the above, the invention provides for a reduced length of 
ceramic airfoil portion in relation to the blade span i.e. preferably 
one-third thereof and this has the effect of substantially reducing the 
centrifugal forces on the ceramic material and thereby reducing the risk 
of cracking and subsequent fracture. Moreover, the ceramic airfoil portion 
is placed in the region where the particulate matter in the gas stream is 
found so as to confer high resistance to corrosion for the turbine blade. 
Thus, the ceramic material is judicially placed in the most effective 
location without detriment to the overall capability of the turbine blade. 
In the radially inward portion of the blade where there is less 
particulate matter and the centrifugal forces are less, the metal portion 
1 is satisfactory. 
Although the invention has been described in conjunction with specific 
embodiments thereof, it will become apparent to those skilled in the art 
that numerous modifications and variations can be made within the scope 
and spirit of the invention as defined by the attached claims.